NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency & 

Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Western Wind and Solar 

Integration Study Phase 3 – 

Frequency Response and 

Transient Stability

N.W. Miller, M. Shao, S. Pajic, and R. D’Aquila

GE Energy Management

Schenectady, New York

NREL Technical Monitor: Kara Clark

Link to Executive Summary

Subcontract Report

NREL/SR-5D00-62906

December 2014
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy 

Laboratory

15013 Denver West Parkway

Golden, Colorado 80401

303-275-3000 • www.nrel.gov Contract No. DE-AC36-08GO28308

Western Wind and Solar 

Integration Study Phase 3 – 

Frequency Response and 

Transient Stability 

N.W. Miller, M. Shao, S. Pajic, and R. D’Aquila

GE Energy Management

Schenectady, New York

NREL Technical Monitor: Kara Clark

Prepared under Subcontract No. KAGN-4-23474-01

Subcontract Report

NREL/SR-5D00-62906

December 2014
NOTICE

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Cover Photos: (left to right) photo by Pat Corkery, NREL 16416, photo from SunEdison, NREL 17423, photo by Pat Corkery, NREL 

16560, photo by Dennis Schroeder, NREL 17613, photo by Dean Armstrong, NREL 17436, photo by Pat Corkery, NREL 17721.

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iii

Acknowledgments

The National Renewable Energy Laboratory (NREL) and GE gratefully acknowledge the support 

of Charlton Clark (Wind and Water Power Program) and Venkat Banunarayanan (SunShot 

Initiative) from the U.S. Department of Energy (DOE) Office of Energy Efficiency and 

Renewable Energy for funding this work.

NREL and GE also thank the members of the Technical Review Committee (TRC) for their 

insightful comments and assistance. Participation in the TRC does not imply agreement with the 

project findings. The TRC included:

• Eric Allen, North American Electric Reliability Corporation (NERC)

• Jamie Austin, PacifiCorp

• Sam Baldwin, DOE

• Venkat Banunarayanan, DOE

• Ben Brownlee, Western Electricity Coordinating Council (WECC)

• Neil Burbure, NERC

• Ray Byrne, Sandia National Laboratories

• Tom Carr, Western Governors' Association (WGA)

• Charlton Clark, DOE

• Bob Cummings, NERC

• Tom Duane, Public Service Company of New Mexico (PNM)

• Bob Easton, Western Area Power Administration (WAPA)

• Sara Eftekharnejad, Tucson Electric Power (TEP)

• Steve Ellenbecker, WGA

• Ryan Elliott, Sandia National Laboratories

• Abe Ellis, Sandia National Laboratories

• Joe Eto, Lawrence Berkeley National Laboratory (LBNL)

• Irina Green, California Independent System Operator (CAISO)

• Fred Huang, Electric Reliability Council of Texas (ERCOT)

• Dmitry Kosterev, Bonneville Power Administration (BPA)

• Doug Larson, WGA

• Debra Lew, Independent Consultant

• Yishan Li, WAPA

• Clyde Loutan, CAISO

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
iv

• Jim McCalley, Iowa State University 

• Jeff Mechenbier, PNM

• Mark O'Malley, University College Dublin

• Brian Parsons, Independent Consultant

• Pouyan Pourbeik, Electric Power Research Institute (EPRI)

• Jim Reilly, DOE

• Sandip Sharma, ERCOT

• Cesar Silva-Monroy, Sandia National Laboratories

• David Silver, Xcel Energy

• Charlie Smith, Utility Variable-Generation Integration Group (UVIG)

• Justin Thompson, Arizona Public Service (APS)

• John Undrill, Independent Consultant

• Vijay Vittal, Arizona State University

• Janice Zewe, Sacramento Municipal Utility District (SMUD)

NREL regrets any inadvertent omission of any project participants and contributors.

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
v

Abbreviations and Acronyms

AC alternating current

BL base load

COI California-Oregon Interface

CSP concentrating solar thermal power

DC direct current

DG distributed generation, embedded PV

DSW Desert Southwest region

FIDVR fault-induced delayed voltage recovery

FR frequency response

FRO frequency response obligation

GR governor response

GW gigawatt

Hz Hertz

HS heavy summer

IFRO interconnection frequency response obligation

LSP light spring

mHz millihertz

MVA megavolt ampere

MW megawatt

MWCAP MW capability

NERC North American Electric Reliability Corporation

NW Northwest region

PDCI Pacific DC Intertie

PV photovoltaic solar power, utility-scale photovoltaic power plant

RAS remedial action scheme

ROCOF rate of change of frequency

s second

UFLS under-frequency load shedding

WECC Western Electricity Coordinating Council

WTG wind turbine generator

WWSIS Western Wind and Solar Integration Study

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
vi

Table of Contents

1 Introduction........................................................................................................................................... 1

1.1 Background and Related Work ..................................................................................................... 2

1.1.1 WWSIS-1 and WWSIS-2................................................................................................. 2

1.1.2 Frequency Response Discussion ...................................................................................... 2

1.1.3 Transient Stability Discussion.......................................................................................... 6

1.1.4 Other Related Work ......................................................................................................... 8

1.2 Scope of Work and Task Structure................................................................................................ 8

2 Develop Study Databases and Establish Initial Conditions........................................................... 10

2.1 Selection of Study Cases............................................................................................................. 10

2.1.1 Key Considerations........................................................................................................ 10

2.1.2 Original Cases................................................................................................................ 10

2.1.3 Evolution of Study Cases............................................................................................... 11

2.2 Investigation and Improvement of Original Cases...................................................................... 12

2.2.1 Identification of Generation Type .................................................................................. 12

2.2.2 Dynamic Model Reconciliation ..................................................................................... 18

2.2.3 Composite Load Model.................................................................................................. 18

2.2.4 Dispatch and Commitment Characterization ................................................................. 20

2.2.5 Initial Condition Summary of Base Cases ..................................................................... 22

2.3 Performance and Monitoring Metrics ......................................................................................... 24

2.3.1 Performance Metrics...................................................................................................... 24

2.3.2 Area and Regional Monitoring....................................................................................... 25

2.3.3 Reference Disturbances.................................................................................................. 25

2.4 Renewables Siting....................................................................................................................... 26

2.5 Incremental Commitment and Dispatch...................................................................................... 29

2.5.1 Mining PLEXOS Results from WWSIS-2..................................................................... 30

2.5.2 Initial Condition Summary of Hi-Mix Cases................................................................. 40

2.6 Transmission and Dynamic Model Changes............................................................................... 45

2.6.1 Method for Adding Incremental Transmission .............................................................. 45

2.6.2 Dynamic Models for Renewables .................................................................................. 45

2.6.3 Other Stability Data Refinements .................................................................................. 46

3 Frequency Response Analysis ......................................................................................................... 47

3.1 Frequency Response Obligation.................................................................................................. 47

3.1.1 Focus on Light Load Conditions.................................................................................... 49

3.2 Light Spring Base and Hi-Mix Comparisons.............................................................................. 50

3.2.1 Trip of Two Palo Verde Units........................................................................................ 50

3.3 Frequency Response Summary ................................................................................................... 56

4 Transient Stability Analysis............................................................................................................... 57

4.1 COI Stability Under Heavy Summer Conditions........................................................................ 57

4.1.1 COI Stabilization with Generation Trip Remedial Action Scheme ............................... 60

4.1.2 Northern California Hydro and COI Stress.................................................................... 61

4.2 Local Stability Examples ............................................................................................................ 62

4.2.1 Colstrip Area Fault......................................................................................................... 63

4.2.2 Laramie River Station Fault ........................................................................................... 65

5 Sensitivity Analysis............................................................................................................................ 66

5.1 Load Model Impact on Fault-Induced Delayed Voltage Recovery............................................. 66

5.1.1 Composite Load Model in Base Cases........................................................................... 66

5.1.2 Midway-Vincent Fault with Composite Load Model .................................................... 66

5.1.3 Distributed Generation Voltage Ride-Through Sensitivity............................................ 71

5.1.4 Impact of Distributed PV Recovery ............................................................................... 71

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
vii

5.2 Impact of Widespread Distributed Generation Tripping............................................................. 73

5.3 Extreme Generation Tripping...................................................................................................... 76

5.3.1 Under-Frequency Tripping of Distributed Generation................................................... 76

5.4 Headroom Depletion Analysis.................................................................................................... 77

5.4.1 Evening Net Load Rise and Duck Curves...................................................................... 78

5.4.2 Case Development.......................................................................................................... 79

5.4.3 Impact on Frequency Response...................................................................................... 81

5.4.4 Headroom Depletion Summary...................................................................................... 84

5.5 Effect of Unit De-Commitment on Base Case ............................................................................ 84

5.6 CSP Sensitivity............................................................................................................................ 85

5.6.1 Converting CSP to Utility-Scale PV .............................................................................. 85

5.6.2 Impacts of CSP and System Inertia on Frequency Response......................................... 86

5.7 Light Spring Extreme Instantaneous Renewable Penetration ..................................................... 87

5.7.1 Dispatch and Commitment............................................................................................. 87

5.7.2 Path Loading .................................................................................................................. 91

5.7.3 Frequency Response....................................................................................................... 92

6 Coal Displacement/Retirement and Weak Grid Concerns ............................................................. 95

6.1 Coal Displacement by Wind and Solar Generation..................................................................... 95

6.2 Northeast Region Stability Analysis ........................................................................................... 98

6.3 Coal Plant Conversion to Synchronous Condensers................................................................. 103

6.4 Weak Grid Discussion............................................................................................................... 106

6.4.1 Sensitivity to WTG Models and Controls.................................................................... 106

6.4.2 Sensitivity to Load Models .......................................................................................... 108

6.4.3 Sensitivity to Non-Synchronous Generation................................................................ 108

6.4.4 Fault Current Levels..................................................................................................... 110

6.5 Coal Displacement and Weak Grid Summary........................................................................... 111

7 Mitigation Analysis........................................................................................................................... 112

7.1 Frequency Controls on Wind Plants.......................................................................................... 112

7.2 Frequency Controls on Solar Plants.......................................................................................... 117

7.2.1 Utility-Scale Solar PV Plants....................................................................................... 117

7.2.2 CSP Plants.................................................................................................................... 120

7.3 Energy Storage for Frequency Response .................................................................................. 121

7.3.1 Estimate Energy Storage Rating .................................................................................. 122

7.3.2 Frequency Performance with Energy Storage.............................................................. 123

7.4 Relative Performance of Frequency Controls........................................................................... 126

7.5 COI Stabilization with Transmission Additions........................................................................ 127

8 Summary and Conclusions ............................................................................................................. 130

8.1 Frequency Response.................................................................................................................. 130

8.2 Means to Improve Frequency Response ................................................................................... 132

8.3 Transient Stability ..................................................................................................................... 133

8.4 Means to Improve Transient Stability....................................................................................... 134

8.5 Model Improvement and Further Analysis ............................................................................... 135

8.6 Conclusions............................................................................................................................... 136

9 References ........................................................................................................................................ 139

10 Appendix ........................................................................................................................................... 142

10.1 Supporting Material on WECC Planning Cases........................................................................ 142

10.2 Supporting Material on Base Cases........................................................................................... 146

10.3 Supporting Material for Performance and Monitoring Metrics................................................. 151

10.4 Supporting Material on Wind and Solar Siting ......................................................................... 154

10.5 Supporting Material on Recommitment and Re-Dispatch ........................................................ 159

10.6 Frequency Response and Frequency Response Obligation Discussion .................................... 174

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
viii

10.7 Supporting Material for Frequency Response Investigation..................................................... 177

10.8 Supporting Material for Transient Stability Investigation......................................................... 180

10.9 Supporting Material for Sensitivities ........................................................................................ 182

10.10 Additional Weak Grid Results................................................................................................ 205

10.11 Supporting Material for Mitigation......................................................................................... 206

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
ix

List of Figures

Figure 1. North American electricity grid interconnections. .................................................................. 3

Figure 2. Balance analogy for frequency stability................................................................................... 4

Figure 3. Electricity demand exceeds electricity generation, and frequency drops. .......................... 4

Figure 4. System frequency in response to a large generation trip. ..................................................... 5

Figure 5. Mechanical analogy for transient stability (Vittal 2003).......................................................... 6

Figure 6. Substation voltage in response to transmission system disturbance.................................. 7

Figure 7. Evolution of LSP cases. ........................................................................................................... 11

Figure 8. Generation type in the LSP Base case. .................................................................................. 16

Figure 9. Generation type for HS Base case. ......................................................................................... 18

Figure 10. Composite load model topology. .......................................................................................... 20

Figure 11. Wind and solar generation in the LSP Base case. .............................................................. 23

Figure 12. Wind and solar generation in the HS Base case. ................................................................ 24

Figure 13. All potential renewable sites.................................................................................................. 26

Figure 14. Dispatch sensitivity of CC plants in LSP window. .............................................................. 31

Figure 15. Dispatch sensitivity of coal plants in LSP window. ............................................................ 32

Figure 16. Wind and solar generation in LSP Hi-Mix case.................................................................... 41

Figure 17. Wind and solar generation in HS Hi-Mix case...................................................................... 42

Figure 18. Renewable generation topology in load flow model........................................................... 46

Figure 19. Frequency response to loss of two Palo Verde units – LSP vs. HS Base. ....................... 50

Figure 20. Frequency response to loss of two Palo Verde units – LSP Base vs. Hi-Mix. ................. 52

Figure 21. Generation response to loss of two Palo Verde units – LSP Base vs. Hi-Mix.................. 52

Figure 22. Governor response to loss of two Palo Verde units – LSP Base vs. Hi-Mix. ................... 53

Figure 23. Interface response to loss of two Palo Verde units – LSP Base vs. Hi-Mix...................... 53

Figure 24. Responsive plant count for loss of two Palo Verde units – LSP Base vs. Hi-Mix............ 54

Figure 25. Response of generation in two areas to loss of two Palo Verde units – LSP Base vs. HiMix........................................................................................................................................................ 54

Figure 26. COI flows for PDCI event for HS cases................................................................................. 58

Figure 27. Bus voltages for PDCI event for HS cases........................................................................... 59

Figure 28. Regional frequencies for PDCI event for HS cases............................................................. 59

Figure 29. Frequency response of Hi-Mix for PDCI event with various generation-tripping RAS.... 60

Figure 30. Dynamic voltage collapse avoided by RAS. ........................................................................ 61

Figure 31. 2013 Summer COI – northern California hydro nomogram................................................ 62

Figure 32. Colstrip machine angles for Broadview fault – HS Base vs. Hi-Mix.................................. 64

Figure 33. Colstrip #4 speed – HS Base vs. HS Hi-Mix with Colstrip #1 committed. ......................... 64

Figure 34. Bus voltages for Laramie River Station fault – HS Base vs. Hi-Mix. ................................. 65

Figure 35. Load-induced voltage collapse in HS Base case. ............................................................... 67

Figure 36. Voltage collapse for Midway-Vincent fault – HS Base vs. Hi-Mix. ..................................... 68

Figure 37. Load bus voltages at different distances from fault location. ........................................... 69

Figure 38. Details of load behavior relative to fault proximity. ............................................................ 70

Figure 39. Reactive power depletion due to DG tripping on voltage dip. ........................................... 72

Figure 40. COI destabilization due to DG tripping on voltage dip. ...................................................... 73

Figure 41. Frequency response of LSP Hi-Mix case – DG trip vs. two Palo Verde unit trip.............. 74

Figure 42. Voltage response of LSP Hi-Mix case – DG trip vs. two Palo Verde unit trip................... 75

Figure 43. Load response of LSP Hi-Mix case – DG trip vs. two Palo Verde unit trip. ...................... 75

Figure 44. Frequency response of LSP Hi-Mix to Palo Verde trip – two units vs. three units.......... 76

Figure 45. Frequency of LSP Hi-Mix to trip of three Palo Verde units – DG trip vs. no DG trip........ 77

Figure 46. LSP Hi-Mix representative dispatch showing "duck” curve. ............................................. 78

Figure 47. Headroom by region for all headroom depletion cases. .................................................... 81

Figure 48. WECC frequency in response to two Palo Verde unit trip for all headroom depletion 

cases.................................................................................................................................................... 82

Figure 49. WECC FR metric for two Palo Verde unit trip – all headroom depletion cases. .............. 82

Figure 50. Regional FR metric for two Palo Verde unit trip – all headroom depletion cases. .......... 83

Figure 51. California FR metrics for two Palo Verde unit trip – all headroom depletion cases........ 83

Figure 52. Effect of thermal plant displacement on frequency and headroom for LSP cases. ........ 85

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
x

Figure 53. Frequency response to trip of two Palo Verde units – LSP Base vs. Hi-Mix vs. CSP 

sensitivity. ........................................................................................................................................... 86

Figure 54. ROCOF response to trip of two Palo Verde units – LSP Base vs. Hi-Mix vs. CSP 

sensitivity. ........................................................................................................................................... 87

Figure 55. Extreme renewable case within LSP coal dispatch samples............................................. 88

Figure 56. Wind and solar generation in the LSP Extreme case.......................................................... 90

Figure 57. Frequency response to two Palo Verde unit trip – LSP Base vs. Hi-Mix vs. Extreme..... 93

Figure 58. Regional generation dispatch for LSP and HS cases (no DG shown). ............................. 96

Figure 59. Coal displacement in Desert Southwest and Northeast regions for LSP cases. ............. 96

Figure 60. Coal displacement in Desert Southwest areas for LSP cases........................................... 97

Figure 61. Coal displacement in the Northeast areas for LSP cases. ................................................. 97

Figure 62. Change in dispatch in the California areas for LSP cases. ................................................ 98

Figure 63. Planned transmission map of Aeolus 500 kV vicinity......................................................... 99

Figure 64. Aeolus bus voltage in response to Aeolus fault – LSP Base vs. Hi-Mix vs. Extreme.... 100

Figure 65. Bus voltages in response to Aeolus fault – LSP Base vs. Hi-Mix vs. Extreme. ............. 101

Figure 66. Lange CT speed in response to Aeolus fault – LSP Base vs. Hi-Mix vs. Extreme......... 102

Figure 67. Aeolus bus voltage response – Aeolus line trip with and without bus fault. ................. 103

Figure 68. Dave Johnson bus voltage for Aeolus fault – LSP Base vs. Hi-Mix vs. Extreme vs. 

Extreme with synchronous condenser conversion...................................................................... 104

Figure 69. Bus voltages in response to Aeolus fault for LSP Extreme case – with and without 

synchronous condenser conversion. ............................................................................................ 105

Figure 70. Real and reactive power output from Dave Johnson unit 4 in response to Aeolus fault – 

LSP Base vs. Extreme with synchronous condenser conversion. ............................................. 106

Figure 65. Impact of weak grid WTG controls for Aeolus fault. ......................................................... 108

Figure 72. Synchronous vs. non-synchronous generation commitment in MVA. ........................... 109

Figure 73. Synchronous vs. non-synchronous generation commitment in percent. ...................... 110

Figure 74. Frequency response to two Palo Verde unit trip for LSP Hi-Mix case with three 

combinations of frequency controls on wind plants.................................................................... 114

Figure 75. Wind plant active power response to two Palo Verde unit trip for LSP Hi-Mix case with 

three combinations of frequency controls on wind plants. ......................................................... 115

Figure 76. Frequency response to two Palo Verde unit trip for LSP Hi-Mix – with and without 

governor control on utility-scale PV plants................................................................................... 118

Figure 77. Frequency and power output in response to two Palo Verde unit trip for LSP Hi-Mix with 

governor controls on utility-scale PV plants................................................................................. 119

Figure 78. Malin bus voltage in response to PDCI event – with and without CSP governor control.121

Figure 79. Frequency response to two Palo Verde unit trip for LSP Hi-Mix – with and without 

energy storage.................................................................................................................................. 124

Figure 80. Frequency and power output of an Arizona energy storage plant for LSP Hi-Mix case.125

Figure 81. Path flows in response to PDCI event for LSP Hi-Mix case with and without 

transmission build-out..................................................................................................................... 129

Figure 82. LSP Base case bubble diagram........................................................................................... 143

Figure 83. HS Base case bubble diagram............................................................................................. 144

Figure 84. Composite load model – DG tripping/blocking logic. ....................................................... 150

Figure 85. WECC reliability criteria. ...................................................................................................... 151

Figure 86. WECC key bus locations...................................................................................................... 152

Figure 87. Wind plant sites and ratings from WWSIS-2 raw data. ..................................................... 154

Figure 88. CSP sites and ratings from WWSIS-2 raw data. ................................................................ 155

Figure 89. U.S. PV near best resources from WWSIS-2 raw data. ..................................................... 156

Figure 90. U.S. PV near population centers from WWSIS-2 raw data................................................ 157

Figure 91. Distributed PV sites and ratings from WWSIS-2 raw data................................................ 158

Figure 92. Simple-cycle gas turbines dispatch – LSP window. ......................................................... 159

Figure 93. Hydro dispatch – LSP window............................................................................................. 160

Figure 94. Pumped-storage hydro dispatch – LSP window. .............................................................. 160

Figure 95. Combined-cycle plants – HS window. ................................................................................ 161

Figure 96. Coal plants – HS window. .................................................................................................... 162

Figure 97. LSP Hi-Mix case bubble. ...................................................................................................... 165

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
xi

Figure 98. HS Hi-Mix case bubble. ........................................................................................................ 168

Figure 93. Sum of Pmax in East (all generation). ................................................................................ 173

Figure 94. Pmax in East (non-renewable synchronous generation). ................................................ 173

Figure 95. Pmax in East (all synchronous generation including CSP). ............................................ 174

Figure 102. Bus voltage response to loss of two Palo Verde units – LSP Base vs. HS Base. ....... 177

Figure 103. Selected interface flows to loss of two Palo Verde units – LSP Base vs. HS Base..... 178

Figure 104. LSP frequency response including loads. ....................................................................... 178

Figure 105. Bus response to loss of two Palo Verde units................................................................. 179

Figure 106. Headroom response to loss of two Palo Verde units...................................................... 179

Figure 107. Interface flows for HS PDCI event..................................................................................... 180

Figure 108. Interface flows – PDCI fault Hi-Mix case with RAS.......................................................... 181

Figure 109. Voltages – PDCI fault Hi-Mix case with RAS.................................................................... 181

Figure 110. Voltage collapse sensitivity to fault type. ........................................................................ 182

Figure 111. Voltage collapse sensitivity to fault impedance.............................................................. 183

Figure 112. IEEE 1547 table on voltage tripping.................................................................................. 183

Figure 113. IEEE 1547 table on frequency tripping. ............................................................................ 184

Figure 114. DG tripping on voltage dip................................................................................................. 184

Figure 115. COI destabilization due to DG tripping on voltage dip. .................................................. 185

Figure 110. DG vs. trip of two Palo Verde units on Hi-Mix case – DG. .............................................. 185

Figure 117. DG vs. trip of two Palo Verde units on LSP Hi-Mix case – interfaces. .......................... 186

Figure 118. DG trip sensitivity. .............................................................................................................. 186

Figure 119. DG trip sensitivity. .............................................................................................................. 187

Figure 120. DG trip sensitivity. .............................................................................................................. 187

Figure 121. Extreme generation tripping – voltages. .......................................................................... 189

Figure 122. Extreme generation tripping – volt sensitivity to UF DG tripping.................................. 189

Figure 123. Extreme generation tripping – flow sensitivity to UF DG tripping................................. 190

Figure 124. Headroom depletion experiment – no wind and solar. ................................................... 190

Figure 125. Summer "duck" curve. ....................................................................................................... 191

Figure 126. Extreme "duck" curve. ....................................................................................................... 192

Figure 127. Headroom depletion: Kt – four regions. .......................................................................... 192

Figure 128. Headroom depletion – WECC frequency response......................................................... 193

Figure 129. Headroom depletion – dynamic headroom...................................................................... 193

Figure 130. Number of units with governors (and non-zero headroom)........................................... 194

Figure 131. Interface flows..................................................................................................................... 194

Figure 132. Voltages. .............................................................................................................................. 195

Figure 133. Load response..................................................................................................................... 195

Figure 134. LSP DG only (between Base and Hi-Mix – Case 2A) – renewable conditions. ............. 197

Figure 135. Five identified CSP plants.................................................................................................. 198

Figure 136. Converted CSP plants. ....................................................................................................... 198

Figure 137. HS Hi-Mix WECC frequency and ROCOF. ........................................................................ 199

Figure 138. Hi-Mix WECC initial detail of frequency and ROCOF. ..................................................... 199

Figure 139. LSP Hi-Mix Extreme case – local reinforcements............................................................ 200

Figure 140. LSP Hi-Mix Extreme case bubble diagram....................................................................... 201

Figure 141. LSP Hi-Mix Extreme case headroom. ............................................................................... 204

Figure 142. LSP Hi-Mix Extreme case – number of units with governors......................................... 204

Figure 143. Regional frequency – comparison for Extreme case...................................................... 205

Figure 144. WTG part Thevenin (Type 3) model vs. current source (Type 4) model. ...................... 205

Figure 145. Frequency response with frequency controls on wind plants – HS Hi-Mix case......... 206

Figure 146. WECC frequency response – LSP Hi-Mix – active power control in selected areas. .. 207

Figure 147. CSP governor response mitigation for Palo Verde event............................................... 208

Figure 148. LSP Hi-Mix Extreme case – energy storage..................................................................... 210

Figure 149. LSP Hi-Mix Extreme case – energy storage..................................................................... 210

Figure 144. HS Hi-Mix case flows with transmission build-out.......................................................... 212

Figure 151. Voltages for PDCI event: Hi-Mix case with transmission build-out............................... 213

Figure 152. Voltages for PDCI event: Hi-Mix case with transmission build-out............................... 213

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
xii

List of Tables

Table 1. Generation Mix from Reconciliation Spreadsheet .................................................................. 13

Table 2. Generation Type for the LSP Base Case by Area ................................................................... 15

Table 3. Generation Type for HS Base Case by Area............................................................................ 17

Table 4. Key to Case Summary Metrics.................................................................................................. 21

Table 5. Study Disturbances.................................................................................................................... 25

Table 6. Wind and Solar Generation in WWSIS-2 Hi-Mix Scenarios .................................................... 28

Table 7. Renewable Capacity for LSP Cases ......................................................................................... 34

Table 8. Renewable Dispatch for LSP Cases ......................................................................................... 35

Table 9. Coal and Combined-Cycle De-Commitment............................................................................ 36

Table 10. Re-Dispatch for Coal, Combined-Cycle, and Hydro.............................................................. 37

Table 11. Renewable Averages from HS PLEXOS Screening .............................................................. 38

Table 12. Renewable Capacity for HS Cases ......................................................................................... 39

Table 13. Renewable Dispatch for HS Cases ......................................................................................... 40

Table 14. Key Initial Conditions for Synchronous Units with Governor Response........................... 44

Table 15. WECC IFRO and Approximate Regional and Area FROs ..................................................... 48

Table 16. WECC Frequency Response Metrics for LSP Base and Hi-Mix Cases............................... 55

Table 17. Area Frequency Response Metrics for LSP Base and Hi-Mix cases................................... 55

Table 18. Headroom Depletion Cases..................................................................................................... 80

Table 19. Path Loadings in LSP Cases................................................................................................... 92

Table 20. LSP FR Metrics Including Extreme Case ............................................................................... 93

Table 21. WECC, Region, and Area FR Metrics for LSP Extreme Case............................................... 94

Table 22. Fault Current Levels............................................................................................................... 110

Table 23. Frequency Response Metrics for LSP Hi-Mix Cases with Three Combinations of 

Frequency Controls on Wind Plants............................................................................................... 116

Table 24. FR Metrics for LSP Hi-Mix Case With and Without Governor Control on Utility-Scale PV 

Plants ................................................................................................................................................. 120

Table 25. Estimate of Energy Storage Rating Required to Meet FRO ............................................... 123

Table 26. FR Metrics for LSP Hi-Mix With and Without Energy Storage........................................... 126

Table 27. Selected WWSIS-2 Transmission Additions........................................................................ 128

Table 28. Approved WECC Planning Databases Considered............................................................. 142

Table 29. Wind Plants Modeled as Synchronous Generators (Example).......................................... 145

Table 30. Netted Units (Examples) ........................................................................................................ 146

Table 31. Generation Initial Condition Summary for LSP Base Case................................................ 146

Table 32. Generation Initial Condition Details for LSP Base Case .................................................... 147

Table 33. Generation Summary for HS Base Case .............................................................................. 148

Table 34. Generation Initial Condition Details for HS Base Case ...................................................... 149

Table 35. Transient Stability Performance Requirements .................................................................. 151

Table 36. WECC Key Buses ................................................................................................................... 153

Table 37. LSP PLEXOS Sample Windows ............................................................................................ 159

Table 38. HS PLEXOS Sample Windows .............................................................................................. 161

Table 39. Adding Renewable Capacity Including WWSIS-2 to LSP Base Case................................ 163

Table 40. Incremental Renewable Dispatch – Sample 31775 ............................................................. 164

Table 41. Added Renewable Capacity Including WWSIS-2 for HS Hi-Mix Case ............................... 166

Table 42. PLEXOS Case – Renewable Summary for HS Hi-Mix Case................................................ 167

Table 43. LSP Hi-Mix Case – Initial Condition Metrics ........................................................................ 169

Table 44. Details of LSP Hi-Mix Case Dynamic Initial Conditions ..................................................... 170

Table 45. HS Hi-Mix Case – Initial Condition Metrics .......................................................................... 171

Table 46. Details of HS Hi-Mix Case Dynamic Initial Conditions ....................................................... 172

Table 47. Extreme Generation Tripping – Comparison of Frequency Response............................. 188

Table 48. Case DG Only (Composite Load Model with DG) – Initial Condition Metrics................... 196

Table 49. LSP Hi-Mix Extreme – Mining PLEXOS Case....................................................................... 200

Table 50. LSP Hi-Mix Extreme Case – Initial Condition Metrics......................................................... 202

Table 51. Details of LSP Hi-Mix Extreme Dynamic Initial Conditions................................................ 203

Table 52. Frequency Response Metrics – HS Hi-Mix Case With Wind Frequency Controls ........... 207

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Table 53. Estimate of Required Energy Storage Rating for LSP Hi-Mix Extreme Case................... 209

Table 54. LSP Hi-Mix Extreme Frequency Response with Energy Storage...................................... 211

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1 Introduction

Power system operators and utilities worldwide have concerns about the impact of highpenetration wind and solar generation on electric grid reliability (EirGrid 2011b, Hydro-Quebec 

2006, ERCOT 2010). The stability of North American grids under these conditions is a particular 

concern and possible impediment to reaching future renewable energy goals. Phase 3 of the 

Western Wind and Solar Integration Study (WWSIS-3) considers a 33% wind and solar annual 

energy penetration level that results in substantial changes to the characteristics of the bulk 

power system, including different power flow patterns, different commitment and dispatch of 

existing synchronous generation, and different dynamic behavior of wind and solar generation.

WWSIS-3 evaluates two specific aspects of fundamental frequency system stability: frequency

response and transient stability. 

Frequency response (FR)—the ability of the power system to stabilize and restore grid frequency 

following large, sudden mismatches between generation and load—has always been an 

operational concern. With the recent approval of the revised North American Electric Reliability 

Corporation (NERC) Standard BAL-003-1 Frequency Response and Frequency Bias Setting

(NERC 2012a), individual balancing authorities (BAs) now have specific frequency response 

obligations (FROs). 

Transient stability—the ability of the power system to maintain synchronism between all 

elements following disturbances—is a major operational constraint in many grids, including the 

western U.S. and Texas systems. These constraints primarily impact large-scale bulk power 

system performance, as local transient stability concerns are generally addressed on a plant-byplant basis—but grid-wide stability concerns with high wind and solar penetration have barely 

been addressed. (For clarity, WWSIS-3 is focused on “traditional” fundamental-frequency 

stability issues, such as maintaining synchronism, frequency, and voltage. This work does not 

explore other non-fundamental frequency issues such as sub-synchronous phenomena, 

harmonics, unbalances, transients, small signal analysis, etc.)

The objectives of WWSIS-3 are to:

• Illustrate the FR and transient stability of the U.S. Western Interconnection to large 

disturbances, including generation outages and critical tie-line disturbances, under a 

variety of future system conditions

• Explore the potential impact of substantially increased levels of wind and solar 

generation, and the accompanying displacement of thermal generation resources, on FR

and transient stability

• Test various operational and control options to improve system FR and transient stability

• Examine and test metrics of system conditions intended to provide operational assistance 

in positioning the system for adequate frequency and transient stability performance.

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1.1 Background and Related Work

1.1.1 WWSIS-1 and WWSIS-2 

Phase 1 and 2 of WWSIS (GE Energy 2010, NREL 2013) established the longer-term 

operational and economic performance of the Western Interconnection under various high levels 

of wind and solar power penetration. Those studies aimed to answer the following questions:

• Can the system supply and demand be balanced across an entire year of operation with 

high penetrations of wind and solar? 

• How do high penetrations of wind and solar impact cycling costs and emissions? 

• How do wind and solar impacts compare?

Analysis included detailed examination of reserves for balancing and addressing incremental 

variability, as well as inter-area transmission constraints (both established and future). 

These studies raised additional questions that WWSIS-3 aimed to answer:

• Can system reliability be maintained with high penetrations of wind and solar? 

• How do advanced features in wind and solar technologies impact reliability and stability?

A variety of system conditions, disturbances, locations, and renewable penetration levels were 

evaluated to help draw broader conclusions from an analysis of two specific types of power 

system stability in WWSIS-3: frequency stability and transient stability. A technical definition of 

the different aspects of power system stability is provided in (Kundur et al. 2004). A less 

technical description of both frequency and transient stability is provided in the following 

subsections. 

1.1.2 Frequency Response Discussion

Reliable operation of a large interconnected power grid such as the Western Interconnection 

(shown in Figure 1) requires a constant balancing of electricity generation with electricity 

demand. Electricity must be generated at the same instant it is used, so operating procedures 

have developed to forecast electricity demand, schedule electric generators to meet that demand, 

and ensure sufficient generating reserves are available to respond to forecast errors and system 

disturbances. The measure of success in this balancing act is frequency. In North America, that 

means maintaining system frequency at or very close to 60 Hz, as shown in Figure 2. 

However, disturbances do occur, including large ones that affect overall system frequency (e.g., 

abrupt outage of a large generator or a major transmission line). For example, a transmission line 

outage may disconnect a large industrial customer. As a result, the total electricity generation 

exceeds the total electricity demand, and frequency rises. Because operators generally have more 

control over generation than demand, they can execute a generation reduction to regain the 

balance and return system frequency to near 60 Hz.

A potentially more significant problem is the loss of a large generating plant. As a result of this 

type of disturbance, the total electricity demand exceeds the total electricity generated and 

frequency drops, as shown in Figure 3. In general, a power grid is designed to withstand the loss 

of the single largest generator. However, the loss of multiple generators or plants may cause the 

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frequency to drop significantly such that protective devices act to disconnect customers in order 

to preserve the bulk of the system. It is a serious reliability failure when operators lose the ability 

to supply all the electricity needed to meet demand.

Figure 1. North American electricity grid interconnections.1

 

1 http://www.nerc.com/AboutNERC/keyplayers/Documents/NERC_Interconnections_Color_072512.jpg

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Figure 2. Balance analogy for frequency stability.

Figure 3. Electricity demand exceeds electricity generation, and frequency drops.

Electricity

Generation

Electricity

Demand

Frequency

60 Hz

59.5 Hz 60.5 Hz

Electricity

Generation

Electricity

Demand

Frequency

60 Hz

59.5 Hz 60.5 Hz

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An example of system frequency in response to a large generation trip is shown in Figure 4. The 

system is operating normally, with a frequency of 60 Hz, up to 1 second. At that time, a large 

generating unit is abruptly lost. Load now exceeds generation, so the frequency drops. The speed 

of the initial decline is related to the number of conventional synchronous generators on the 

system. More generators mean more inertia, which retards the frequency decline. At about 10 

seconds, the frequency nadir or minimum is reached. Frequency nadir is one measure of a 

system's frequency stability; it must be above the highest level of under-frequency load shedding

(UFLS). At that point in time, the generators with governor controls have begun to act to 

increase power output, and thus the system frequency begins to recover. By about 60 seconds, 

the system frequency has settled out somewhat below the normal operating frequency of 60 Hz. 

A metric of frequency stability is based on the change in frequency between the nadir and this 

settling frequency, and the change in power within this interval. Measurements are averaged over 

a defined period between the nadir and the settling frequency, and the ratio of change in power to 

change in frequency is calculated. This is called "frequency response" (FR) and is formally 

defined by NERC (NERC 2012a). After 60 seconds, even more generators begin to increase their 

power output, and the frequency returns to normal within about 10 minutes. One part of this 

study focuses on system frequency behavior in the first 60 seconds after an outage. 

Figure 4. System frequency in response to a large generation trip. 

There is general concern among power system operators and utilities regarding the degradation 

of FR in North America over the past two decades. The decline is due to various factors

including the withdrawal of primary or governor response (GR) shortly after an event, the lack of 

in-service governors on conventional generation, and the unknown and changing nature of load 

frequency characteristics. High penetrations of inverter-based, or non-synchronous, generation 

technologies further complicate this issue. Without special operation or controls, wind and solar 

59.6

59.7

59.8

59.9

60

60.1

0 10 20 30 40 50 60

Frequency (Hz)

Time (Seconds)

Frequency

Minimum or Nadir

Settling 

Frequency

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plants do not inherently participate in the regulation of grid frequency. By contrast, synchronous 

machines always contribute to system inertia, and some fraction of the synchronous generation in 

operation at any point has governor controls enabled. When wind and solar generation displace 

conventional synchronous generation, the mix of the remaining synchronous generators changes 

and has the potential to adversely impact overall FR. 

Therefore, one of the primary objectives of WWSIS-3 is to evaluate and better understand the 

impact of high penetrations of wind and solar power on system-wide FR to large generator 

outages in the first minute after the outage occurs.

1.1.3 Transient Stability Discussion

In addition to maintaining the balance between electricity generation and electricity demand, 

power system operators must ensure that the grid can successfully transition from normal 

operation (e.g., all transmission lines and generating units are in service) through a disturbance 

(e.g., abrupt outage of a major transmission line or large generator) and into a new stable 

operating condition in the 10–20 seconds immediately following the disturbance. The ability to 

make this successful transition is called transient stability, and is an even faster phenomenon 

than frequency stability. 

A mechanical analogy for transient stability is illustrated in Figure 5 (Vittal 2003). Imagine a set 

of balls of different sizes connected to each other by a set of breakable elastic strings. The balls 

represent generators of different sizes and characteristics, and the strings represent the 

interconnecting transmission lines. The system is disturbed when one of the balls is hit with a 

stick. The ball begins to swing, and the string connected to the ball also swings. Other strings 

follow suit, and other balls start to swing. As a result of the single disturbance, the entire system 

of strings and balls is moving in response. If the swings die down, and the system comes back to 

rest, then the system is transiently stable. On the other hand, if the swings grow, one or more 

balls may break away from the rest, and the system is transiently unstable.

Figure 5. Mechanical analogy for transient stability (Vittal 2003). 

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An example of both transient stability (blue) and transient instability (red) is shown in Figure 6. 

The system is operating normally, with a transmission substation voltage of 100%, up to 0.5 

seconds. At that time, a disturbance occurs, such as a tree falling on a transmission line. From 0.5

seconds to 0.7 seconds, the voltage is zero because the tree is connecting the transmission line to 

the ground. At about 0.7 seconds, an automatic protection system trips the transmission line, and 

the voltage returns to near normal. But as described above, the system is swinging in response to 

the disturbance. When the swings grow and the system separates, the substation voltage drops 

precipitously and the system collapses at about 1 second. When the swings die down, the 

substation voltage settles back to normal within 5 seconds. The second part of this study focuses 

on system stability in the first 5–10 seconds.

Figure 6. Substation voltage in response to transmission system disturbance. 

As noted, the Western Interconnection has a long history of constraints due to transient stability 

limitations, which vary depending on system characteristics such as the level of electricity 

demand (e.g., peak summer load), the amount of power flowing on the transmission system (e.g., 

heavy flows on critical paths), and the location of generating plants in operation (e.g., remote 

from population centers). One of the primary objectives of WWSIS-3 is thus to evaluate and 

better understand the impact of high penetrations of wind and solar power on the large-scale 

transient stability of the Western Interconnection. The primary measures of transient stability are 

avoiding bulk system separation and individual generator loss of synchronism with the system, 

and meeting various voltage and frequency swing criteria, which vary with the severity of the 

disturbance according to NERC and Western Electricity Coordinating Council (WECC) 

reliability standards.

0

20

40

60

80

100

120

140

0 1 2 3 4 5

Substation Voltage (%)

Time (Seconds)

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While transient stability can be both systemic and local, this study focuses on large-scale events 

that affect the security of the entire interconnection. Large penetrations of inverter-based, or nonsynchronous, wind and solar generation may substantially alter system stability as a result of 

changes in angle/speed swing behavior due to reduced inertia, changes in voltage swing behavior 

due to different voltage control systems, different power flow patterns, and displacement of 

synchronous generation at key locations.

1.1.4 Other Related Work

The frequency behavior of North American grids has recently been the subject of heightened 

concern (Ingleson 2005, Ingleson 2010). Reports by NERC show that FR has been declining 

over the past two decades. 

Significant research in the FR area began in 2010 (Eto 2010, Undrill 2010, Mackin 2010); 

additional research was performed in 2011 (Sharma 2011). Recent work by GE (Miller 2011b)

examined this issue with a specific focus on the performance of California for the California 

Independent System Operator (CAISO). The NREL-sponsored Eastern Frequency Response 

Study (GE Energy (2013) examined the frequency performance of the Eastern Interconnection

with high levels of wind. In that work, various means to improve modeling and system response 

were tested. Finally, NREL and the Electric Power Research Institute (EPRI) have studied the 

application of active power controls on wind plants to support grid reliability (Ela 2014,

Gevorgian 2014).

1.2 Scope of Work and Task Structure

WWSIS-3 was divided into tasks briefly outlined here.

Task 1: Develop Study Databases and Establish Initial Conditions

In this task, NREL and GE solicited inputs from the Technical Review Committee (TRC) to 

jointly define the key study parameters. System conditions in the study databases, generation 

outages, transmission events, performance metrics, and the initial unit commitment and redispatch procedures were specified. This required a number of subtasks, including:

• Transmission Model and Modifications: Select data sets that reasonably represent 

the Western Interconnection in the not-too-distant future, making minimal changes to 

planned system transmission. The selected data sets included a number of substantial 

new transmission projects that are not currently in service.

• Renewable Siting Method: Add renewable generation at specific projects with 

specific interconnection substations in realistic physical locations, but avoid the

detailed local transmission design that normally accompanies each individual project.

• Stability Model and Modifications: Add dynamic model improvements known to 

significantly impact the bulk system FR or inter-area transient stability, including 

incorporation of the latest composite dynamic load model. 

• Commitment and Dispatch Method: De-commitment and re-dispatch thermal 

generation to accommodate the new wind and solar generation based on selected 

hourly results from the production simulation results of WWSIS-2. 

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• Reference Disturbances and Performance Metrics: Select system disturbances for 

evaluation of FR and transient stability. Performance targets are based on WECC 

stability standards and the NERC BAL-003-1 FRO.

Task 2: Evaluate Western Interconnection Frequency Response

The study evaluated the Western Interconnection FR to large generation outages under a variety

of system conditions. This analysis used the existing unit commitment and dispatch procedures, 

as well as standard power plant controls as currently implemented. Performance was tested for 

the NERC BAL-003-1 reference disturbance of a two-unit outage at the Palo Verde nuclear 

power station (NPS) for a loss of about 2,750 MW. 

Task 3: Evaluate Western Interconnection Transient Stability

Transient stability was evaluated with particular emphasis on events that challenge major 

western interties (e.g., California-Oregon Interface [COI] and West of the River limits). Again, 

this analysis used the existing unit commitment and dispatch procedures, as well as standard 

power plant controls as currently used. System performance, in terms of maintaining 

synchronism and meeting other WECC stability criteria (e.g., post-fault voltage swings, etc.), 

was evaluated for various faults. Particular attention was given to blocking the Pacific DC 

Intertie (PDCI), which is a severe transient stability event for the COI.

Task 4: Evaluate Factors Affecting Dynamic Performance

Sensitivity cases were evaluated to provide a deeper examination of factors that could affect FR

and transient stability. Tests examined the following:

• Sensitivity to tripping (deliberate or unplanned) of distributed photovoltaics (PV)

• Dynamics of concentrating solar thermal power (CSP) compared to utility-scale PV

• Comparative impacts of tripping large thermal generation vs. distributed PV

• Impact of exhausting headroom of frequency-responsive generation

• Transient stability impacts of even higher wind and solar generation with extreme 

levels of coal generation displacement. 

Task 5: Evaluate Mitigation Measures

The study evaluated the ability of a limited selection of mitigation measures to improve both FR

and transient stability, including:

• Conversion of coal steam turbine generators to synchronous condensers to address 

"weak grid" concerns

• Inertial and governor controls on wind plants

• Frequency-responsive controls from utility-scale PV and CSP

• Energy storage for frequency control

• Transmission reinforcement for transient stability improvement. 

The following sections of this report generally align with these tasks and provide additional 

detail.

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2 Develop Study Databases and Establish Initial 

Conditions

This section contains definitions and context for the study. Specifically, Section 2.1 discusses the 

selection and improvement of the study cases. Refinement and documentation of the study cases 

are reported in Section 2.2. Performance and monitoring metrics are described in Section 2.3. 

The development of the Hi-Mix (33% annual energy penetration from wind and solar) cases is 

reported in Section 2.4 through Section 2.6. 

2.1 Selection of Study Cases

This section covers the selection of the WECC cases from which the WWSIS-3 study cases were 

developed. The investigation and improvement of the selected cases are discussed in detail. The 

initial condition metrics for new base cases are presented.

2.1.1 Key Considerations

The study databases were selected based on the following requirements: 

• The study case should be credible and meaningful.

• The study case should have a longer horizon, e.g., extend beyond the year 2020. 

• The load flow in the study cases should include major system generation changes—

additions and retirements—for which there is some degree of consensus in WECC. 

• The study case should have a transmission build-out that is realistic and not overly 

optimistic.

• The study case should have high stress from a transient stability perspective. 

• The study case should have high stress from a FR perspective. 

2.1.2 Original Cases

In order to understand options to meet the selection criteria, extensive discussions were held with 

the TRC. The WECC library included nine approved cases (listed in Appendix Table 28) from 

which suitable starting cases were selected.

The 2022 Light Spring (LSP) case included the highest wind and solar build-out. Further, it 

included known plant retirements, with guidance from the WECC System Review Working 

Group. After extensive discussion and with the concurrence of the TRC, two WECC cases were 

selected for WWSIS-3:

• 2022 Light Spring Base case (LSP Original case)

• 2023 Heavy Summer Load case (HS Original case). 

These two cases have different objectives, and therefore different generation, load, and 

transmission topologies. The 2022 Light Spring Base case represents a light spring load 

condition throughout the West. The renewable penetrations in this case are intended to be 

consistent with state renewable portfolio standard (RPS) requirements in 2022. The 2023 Heavy 

Summer Load case, however, is a WECC general 10-year case with typical flows throughout the 

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Western Interconnection. Load flow bubble diagrams for the two cases are included in the 

Appendix as Figure 76 and Figure 77. 

For the rest of this report, “LSP” refers to cases that originated with the 2022 Light Spring Base 

case, and “HS” refers to cases that originated with the 2023 Heavy Summer Load case. The year 

of the data sets provides a useful point of reference for the study and the infrastructure included 

by WECC in the base cases. For the purposes of this study, however, it is the wind and solar 

penetration level that is important—but this is not a prediction that the Western Interconnection 

will reach these high levels of wind and solar by these study years. 

2.1.3 Evolution of Study Cases

The flow chart in Figure 7 illustrates the evolution of the LSP cases. The study started with the 

WECC LSP Original case, i.e., the data as received from WECC. The dynamic data in the LSP 

Original case were improved to create the LSP Base case. The improvement includes fixing bad 

models, adding models for netted units, and adding the composite load model. In the LSP HiMix case, a large number of distributed generators as well as utility-scale wind and solar plants

were added, with commitment and dispatch adjusted for the same system load level as the Base 

case. In general, the system topology, transmission, and existing generation resources were left 

intact. The details of the development of the LSP Hi-Mix case are discussed below. An LSP HiMix Extreme case was developed as a sensitivity and is presented in Section 5.7. 

HS cases have a similar case evolution as the LSP cases, except that no HS Hi-Mix Extreme case 

was developed in this study. Also, because the HS Original case is a WECC 10-year planning 

case, both the power flow and dynamic data were in better shape from a simulation perspective

and needed little refinement other than the addition of the composite load. The details of the case 

development and evolution are discussed in the subsequent sections.

Figure 7. Evolution of LSP cases. 

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2.2 Investigation and Improvement of Original Cases 

2.2.1 Identification of Generation Type

Detailed understanding of the type of generation in the simulations is critical to gleaning insight 

into the effects of increased variable renewable generation. While WECC has good processes for 

maintaining accurate models, the need to keep consistent track of the type (e.g., steam, CSP, 

combined cycle, etc.) of every generating unit is limited in power flow databases. Part of the 

investigation of the selected WECC cases was to try to assign generation type to every unit in the 

system model.

2.2.1.1 Light Spring

The LSP Original case was developed to look at a high-renewable future from the WECC 

planning perspective. Therefore, compared with the HS case, the LSP Original case already has 

high wind and solar generation built into the database. There were many units of different types 

added by WECC members to the cases.

Considerable efforts were made to identify the type and amount of generation mix in the LSP 

Original case. These efforts included:

• Reviewing associated material for two selected WECC cases

• Reviewing other related WECC documents

• Checking Positive Sequence Loadflow software (PSLF) dynamic data (dyd file) for 

comments

• Using the GE Multi Area Production Simulation (MAPS) software database as a 

reference

• Performing Google searches for information from the public domain.

In many instances, the same unit can be identified by more than one source. 

One useful reference for identifying the generation mix is a reconciliation Excel spreadsheet 

provided by WECC as a supporting material for the LSP Original case. The purpose of this 

spreadsheet is to conduct a comprehensive reconciliation of the LSP Original case against the 

Production Cost Model (PCM). One tab named “Generators” in this spreadsheet provides the 

type and primary fuel information for each generation unit in the power flow data. 

The generation information for each generator category from the reconciliation spreadsheet is 

summarized in Table 1. For each category, the number of units, total generation, total generation 

nameplate capacity, and flag of renewables are provided. 

A few highlights from the WECC-wide summary table include: 

1. Renewables in the LSP Original case total 36,436 MW, which is about 31.7% 

instantaneous penetration. 

2. Wind is the largest resource among all renewables, followed by solar PV, geothermal, 

and CSP.

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13

3. The coal units in the West were dispatched at about 86.1% of their capacity.

4. The combined-cycle (CC) and combustion turbine (CT) units were dispatched at 15% and 

14% of total installed capacity, respectively. These low dispatch rates indicate these two 

types of units were already displaced by the high wind and solar generation.

It is important to note that Table 1 only indicates WECC’s intention for generation mix for the 

LSP Original case, not the final reality. The dynamic model for each generator category, 

especially for those wind and solar units, needed close scrutiny. From a FR and transient stability 

perspective, the renewable generation is deemed to be renewable only if its dynamic data were 

correctly represented.

Table 1. Generation Mix from Reconciliation Spreadsheet 

Generator Category # of Units Total Pgen (MW) Total Pmax (MW) Renewable 

Wind 350 22,448 34,778 Yes

Solar PV 373 5317 7,229 Yes

Solar CSP0 22 2,275 2,171 Yes

Solar CSP6 4 585 541 Yes

Biomass RPS 124 1,148 3,047 Yes

Geothermal 113 3,807 4,820 Yes

Small Hydro RPS 122 856 1,759 Yes

Conventional Hydro 607 26,970 65,351 No

Pumped Storage 14 -2,076 3,720 No

Nuclear 8 8,077 9,681 No

Coal 122 31,387 36,470 No

Combined Cycle 418 9,257 61,600 No

Combustion Turbine 466 3,727 26,648 No

Steam 79 582 5,704 No

Negative Bus Load 23 528 528 No

All Others 101 173 1,341 No

Total 2,947 115,061 265,388

Table 2 shows the on-line generation production by type and by area after all the information had 

been considered. This is referred to as the LSP Base case. The areas have been grouped into five 

regions: California, Desert Southwest (DSW), Northeast, Northwest (NW), and Outside.

Generation is summed by type, by area, and by region. Note that the NW region contains only 

the Northwest area. Also, the areas defined in the WECC power flow databases may cover more 

than is implied by their names. For instance, PSCOLORADO includes Platte River Power 

Authority, Tri-State Generation and Transmission Association, Black Hills Corporation, and 

independent power producers, as well as Public Service of Colorado (now Xcel Energy). Only 

977 MW of generation in the West was not identified by any type and was given the label 

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“Other.” These are all modeled as synchronous generation in the stability database. Generation 

type categories were aligned with those in WWSIS-2. Table 2 is repeated as a bar chart in Figure 

8. 

The minimum power levels (Pmin) from the original WECC power flow cases were used in 

WWSIS-3. If no Pmin was in the power flow database, a default value equal to 20% of the

maximum power level (Pmax) was used.

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Table 2. Generation Type for the LSP Base Case by Area

Area Biomass Combined 

Cycle Coal CSP DG Gas CT Geothermal Hydro Nuclear Other PSH PV Steam Wind Total

California 66 2,702 2,060 858 64 2,113 1,958 4,918 4,390 258 -1,277 3,630 1,319 4,405 27,464

IMPERIALCA 49 0 0 0 0 348 533 49 0 224 0 0 0 0 1,202

LADWP 0 180 1,900 0 0 0 0 177 0 0 -149 0 227 190 2,525

PG AND E 0 1,609 66 0 0 1,009 823 3,826 2,240 8 -1,128 1865 1,052 764 12,133

SANDIEGO 0 107 0 0 64 150 0 4 0 1 0 464 0 272 1061

SOCALIF 17 806 94 858 0 607 603 863 2,150 25 0 1301 40 3,179 10,543

DSW 0 3,518 13,849 0 0 1,714 18 2,674 2,756 47 17 162 334 4,003 2,9093

ARIZONA 0 1,841 7,231 0 0 346 0 2,462 2,756 47 17 50 0 350 15,100

EL PASO 0 252 0 0 0 286 0 0 0 0 0 0 92 0 630

NEVADA 0 1,254 0 0 0 0 0 0 0 0 0 60 242 0 1,556

NEW MEXICO 0 0 1,122 0 0 0 18 7 0 0 0 28 0 1,091 2,266

PSCOLORADO 0 171 1,959 0 0 1,004 0 0 0 0 0 24 0 1,822 4,980

WAPA R.M. 0 0 3,538 0 0 78 0 206 0 0 0 0 0 739 4,561

Northeast 0 997 7,607 0 0 310 239 2,138 0 369 0 0 532 2,468 14,659

IDAHO 0 275 2,156 0 0 0 56 1,534 0 0 0 0 53 390 4,464

MONTANA 0 0 2,491 0 0 0 0 299 0 0 0 0 0 390 3,180

PACE 0 613 2,470 0 0 142 36 304 0 0 0 0 106 1,408 5,079

SIERRA 0 109 490 0 0 168 147 0 0 369 0 0 373 280 1,936

NW 0 536 0 0 0 33 49 12,644 1,139 7 -25 17 165 8,357 22,923

NORTHWEST 0 536 0 0 0 33 49 12,644 1,139 7 -25 17 165 8,357 22,923

Outside 0 5,753 4,578 0 0 1,987 155 10,037 0 297 -21 0 263 1,671 24,720

ALBERTA 0 5,039 4,578 0 0 1,987 0 304 0 63 -21 0 131 1,665 13,746

B.C.HYDRO 0 382 0 0 0 0 0 8,634 0 0 0 0 132 6 9,153

FORTISBC 0 0 0 0 0 0 0 1,029 0 0 0 0 0 0 1,029

MEXICO-CFE 0 333 0 0 0 0 155 0 0 234 0 0 0 0 722

WAPA U.M. 0 0 0 0 0 0 0 70 0 0 0 0 0 0 70

Total 66 13,506 28,094 858 64 6,157 2,420 32,411 8,285 977 -1,305 3,810 2,612 20,904 118,860

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
 

16

Figure 8. Generation type in the LSP Base case. 

2.2.1.2 Heavy Summer

The generation type information identified in the LSP Base case can be used for the HS 

Base case. Table 3 and Figure 9 show the generation type information for the HS Base 

case.

-5000

0

5000

10000

15000

20000

25000

IMPERIALCA

LADWP

PG AND E

SANDIEGO

SOCALIF

ARIZONA

EL PASO

NEVADA

NEW MEXICO

PSCOLORADO

WAPA R.M.

IDAHO

MONTANA

PACE

SIERRA

NORTHWEST

ALBERTA

B.C.HYDRO

FORTISBC

MEXICO-CFE

WAPA U.M.

California DSW NorthEast PNW Outside2

WIND

Steam

PV

PSH

Other

NUC

HYDRO

GEO

GasCT

CSP

Coal

CCPP

Bio

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17

Table 3. Generation Type for HS Base Case by Area

Area Biomass

Combined 

Cycle Coal CSP Gas CT Geothermal Hydro Nuclear Other PSH PV Steam Wind Total

California 158 18,439 2,104 355 7,545 1,630 6,646 4,550 11,687 825 1,134 4,608 2,121 61,801

IMPERIALCA 49 0 0 0 346 544 40 0 440 0 0 115 0 1,534

LADWP 0 1,263 1,900 0 297 0 160 0 570 1,079 0 845 1,90 6,303

PG AND E 0 10,074 84 0 4,953 821 5,747 2,399 3,361 -294 701 1,398 787 30,032

SANDIEGO 0 2,233 0 0 449 0 4 0 920 40 388 0 325 4,359

SOCALIF 109 4,869 120 355 1,499 265 695 2,150 6,396 0 45 2,250 819 19,573

DSW 0 17,986 16,447 0 4,961 0 3,947 4,134 3,747 394 113 1,395 769 53,893

ARIZONA 0 8,940 8,117 0 1,198 0 2,822 4,134 1,189 114 0 737 227 27,478

EL PASO 0 704 0 0 454 0 0 0 288 0 0 176 47 1,669

NEVADA 0 4,580 0 0 884 0 0 0 281 0 61 310 0 6,116

NEW MEXICO 0 769 2,044 0 479 0 13 0 0 0 0 153 20 3,478

PSCOLORADO 0 2,554 1,864 0 1,936 0 0 0 1,304 100 52 19 444 8,273

WAPA R.M. 0 439 4,422 0 10 0 1,112 0 686 180 0 0 31 6,879

Northeast 0 2,254 10,446 0 1,061 215 1,887 0 827 0 0 1,281 1,724 19,695

IDAHO 0 261 2,210 0 371 0 1,337 0 0 0 0 25 142 4,346

MONTANA 0 40 2,583 0 0 0 502 0 0 0 0 58 108 3,290

PACE 0 1,750 4,894 0 519 36 49 0 552 0 0 482 1,155 9,435

SIERRA 0 203 760 0 172 179 0 0 275 0 0 716 320 2,625

NW 0 6,025 0 0 520 0 24,096 1,151 1,100 -25 0 402 0 33,269

NORTHWEST 0 6,025 0 0 520 0 24,096 1,151 1,100 -25 0 402 0 33,269

Other 0 5,859 4,659 0 865 322 12,266 0 2,207 -21 0 868 1,000 28,025

ALBERTA 0 4,931 4,659 0 865 0 552 0 750 -21 0 192 899 12,829

B.C.HYDRO 0 112 0 0 0 0 10,573 0 187 0 0 319 101 11,291

FORTISBC 0 0 0 0 0 0 1053 0 0 0 0 87 0 1,140

MEXICO-CFE 0 816 0 0 0 322 0 0 1,270 0 0 270 0 2,678

WAPA U.M. 0 0 0 0 0 0 87 0 0 0 0 0 0 87

Total 158 50,562 33,657 355 14,952 2,167 48,842 9,835 19,568 1,174 1,247 8,554 5,614 196,683

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18

Figure 9. Generation type for HS Base case. 

2.2.2 Dynamic Model Reconciliation

It is well understood in the Western Interconnection that accurate dynamic modeling is important 

to power system dynamic simulation. In the course of long-term planning, sometimes 

approximate models are used—particularly when the specifics of future plants are not firmly 

established. Because the dynamic behavior of wind and solar plants is substantially different 

from that of thermal and hydro plants, it is important that estimated models have the right basic 

dynamic characteristics.

Comparing the identified generation type against the model in the dynamic database enabled 

identification of incorrectly applied models. 182 wind and PV plants with a total generation of 

4,505 MW were modeled as synchronous machines. As an example, Appendix Table 29 shows 

some wind plants that were represented by a synchronous generator model. Dynamic data for 

those 182 wind and PV plants were replaced with appropriate models.

In the LSP Original case, 54 wind, PV, and CSP units with total generation of 2,722 MW were 

netted. Appendix Table 31 lists those netted units. Netting was disabled and appropriate dynamic 

models were added for these units.

2.2.3 Composite Load Model

The original data sets used the standard WECC load model. This consists of roughly 20% 

induction motor and 80% static with voltage dependence, located at the transmission or subtransmission level. At the request of the TRC, the new WECC composite load model 

-5000

0

5000

10000

15000

20000

25000

30000

35000

IMPERIALCA

LADWP

PG AND E

SANDIEGO

SOCALIF

ARIZONA

EL PASO

NEVADA

NEW MEXICO

PSCOLORADO

WAPA R.M.

IDAHO

MONTANA

PACE

SIERRA

NORTHWEST

ALBERTA

B.C.HYDRO

FORTISBC

MEXICO-CFE

WAPA U.M.

California DSW NorthEast PNW Outside

WIND

Steam

PV

PSH

Other

NUC

HYDRO

GEO

GasCT

CSP

Coal

CCPP

Bio

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19

(CMPLDWG) was used (WECC 2012, WECC 2014), which represents the load at the 

distribution level and includes a significantly higher level of induction motor. The parameters for 

the composite load model were based on the WECC Modeling and Validation Working Group 

(MVWG) Load Model Data Tool, which takes into account regional differences in the 

characteristics of the load. The new CMPLDWG model was used to allow for addition of 

distributed PV (as discussed later in this section). This model was applied to most of the loads 

throughout the database. Loads less than 5 MW and loads whose characteristics were not 

identified in the load modeling data tool were modeled as static with voltage dependence. The 

load modeling for the LSP and HS cases is summarized below. 

For LSP: 

• 4,420 composite load models, 95.1 GW total load + distribution losses

• 22.3 GW of load not modified (modeled as static). 

For HS: 

• 4,408 composite load models, 143.9 GW total load + distribution losses 

• 48.2 GW of load not modified (modeled as static). 

Approximately 1.8 GW of large synchronous motor load was explicitly modeled in the dynamic 

data set.

The topology of the composite load, as shown in Figure 10, is intended to give a more realistic 

representation of dynamic load behavior than present practice. The parameters of the four 

equivalent motors are particularly important for dynamics, as the tendency for motor groups to 

stall (or not) during major voltage depressions has a substantial impact on system stability. One 

of the key features of the composite load model is the ability to control whether stalled motors 

trip (by contactors opening) or continue to run and draw starting current. For WWSIS-3, all 

motor tripping in the composite model is disabled because the motor stalling behavior has such a 

major and acutely non-linear effect on stability results. This is conservative, and allows for a 

simpler and more illuminating comparison between dynamic simulation cases. 

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20

Figure 10. Composite load model topology.

The ability of the PV distributed generation (DG) to ride through voltage and frequency 

excursions is handled by a separate logic. The block diagram for this rather complex function is 

shown in Appendix Figure 78. The model allows selection of different levels of voltage and 

frequency excursion that will result in the DG blocking. A further part of the logic allows 

specification of how much DG will recover if the excursion returns within the user input bounds. 

The result is a high level of flexibility for modeling fault ride-through. However, the current 

model does not support user input time delays on the blocking functions, and so is limited in its 

ability to reflect deliberate time thresholds for tripping (e.g., of the type in NERC low-voltage 

ride-through [LVRT] and IEEE 1547 standards). 

The topology of the composite load requires that the load flow initial condition be changed when 

DG is added. Specifically, the load flow active and reactive power load are labeled as PLnet and 

QLnet in the figure, respectively. The actual power consumed by the six constituents (four motor 

equivalents, electronic equivalent, and static equivalent) is less than PLnet by the losses in the 

equivalent feeder. Further, when DG is added, the generated power is consumed by the load, 

reducing the power from the grid. Thus, the addition of DG shows up in the load flow as reduced 

system load. The reduced flow on the equivalent feeder reduces the distribution system losses.

2.2.4 Dispatch and Commitment Characterization

The FR and transient stability of the system are dominated by the amount and type of generation 

committed and how it is dispatched. Throughout this report, care is exercised to distinguish 

classes of generation in accordance with their FR behavior. According to the power flow and 

dynamic data, each of the generators in the study system can be characterized as one of the 

following types:

Governor-responsive units have governor models and positive headroom. These will provide FR. 

Base load units have governors blocked from increasing mechanical power, but can respond to 

over-frequencies. Units with no governor models will be unresponsive regardless of the sign of 

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21

the frequency deviation. Base load and no governor units will not provide FR. All synchronous 

machines will provide inertial response. The default modeling assumption in this study is that 

wind and solar will not provide FR. CSP plants are modeled as base load, and are included in the 

total of unresponsive synchronous generation. 

2.2.4.1 Metrics for Characterization

Throughout this report, tables summarize important aspects of the initial conditions used for 

various cases. These tables are intended to capture the critical characteristics of the generation 

and load as they relate to frequency performance. Table 4 lists the reported metrics, with a brief 

explanation of each. New monitoring was introduced for this study, which allows dynamic 

monitoring of the metrics during the course of dynamic simulation. Monitoring by WECC, by 

region, and by area was included, and selected traces are provided in the report below, as results 

are presented. 

Table 4. Key to Case Summary Metrics

ID Description 

fr Frequency (Hz) calculated from MVA weighted speed of synch 

machines

pg Pgen of units with GR (GW)

mc Capacity of units with GR (GW)

hr Headroom of units with GR (GW)

nu Number of units with GR 

pm Mechanical power of units with GR (GW)

mv MVA rating of units with GR (GVA)

px Pgen of units without GR (GW)

mx Mechanical power of units without GR (GW)

nx Number of units without GR

ps Pgen of all synchronous generators (GW)

qs Pgen of all synchronous generators (GVAr)

pl P load (GW)

ql Q load (GVAr)

pw Pgen – Wind (GW) 

qw Qgen – Wind (GVAR)

pv Pgen – Solar PV (GW) 

qv Qgen – Solar PV (GVAr) 

ps Pgen – CSP (GW)

dg Pgen – Distributed solar PV generation (GW) 

nh Number of units with GR

Kt The ratio of governor-responsive generation to total

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22

The ratio of generation that provides GR to all generation running on the system is used to 

quantify overall system readiness to provide FR. A Lawrence Berkeley National Laboratory 

(LBNL) report (Undrill et al. 2010) introduces this ratio as the metric Kt; the lower the Kt, the 

smaller the fraction of generation that will respond. The exact definition of Kt is not 

standardized. For this report, it is defined as “mc / (mc+px+pw+pv)” or “GR MWCAP / GW 

Capability.” This is the ratio of power generation capability of units with GR to the MW 

capability of all generation units. Power capability is defined as equal to the MW dispatch rather 

than the nameplate rating of nonresponsive generation because these units will not contribute 

beyond their initial dispatch. This is a reasonable definition, but industry discussion of a standard 

definition of Kt is warranted. Non-synchronous resources, including wind, solar, energy storage, 

and responsive loads, complicate the question. In all tables, only synchronous generation is 

included in the calculation of the capacity of units with GR (mc). Therefore, only synchronous 

generation is included in the calculation of Kt. 

Tables documenting the initial conditions of all the study cases are included in the Appendix. At 

the end of this section, after other aspects of the case developments have been explained, a 

WECC-level comparison of the initial state of the most important metrics is presented. 

2.2.5 Initial Condition Summary of Base Cases 

A high-level view of the initial conditions for the LSP Base case is shown in the pie charts and 

table of Figure 5. Notice that there is no DG (i.e., distribution-connected PV) in this case. 

Explicit equivalents that were added by WECC in southern California are included with the 

(utility-scale) PV.

The generation information for the LSP Base case is summarized in Appendix Table 31, using 

the definition presented in Table 4. Appendix Table 32 shows details for all the areas.

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23

Figure 11. Wind and solar generation in the LSP Base case. 

A high-level view of the initial conditions for the HS Base case is shown in Figure 6. This figure, 

when compared to the previous one, emphasizes that the HS Base case assumes a dramatically 

lower level of wind and solar than the LSP case, as intended by WECC. The initial condition 

details by region are shown in Appendix Table 33 and by area in Appendix Table 34. 

Wind 4.4

PV 3.7

CSP 0.9

DG 0.0

Others 

19.9

Wind 

8.4 PV 0.0

CSP 0.0

DG 0.0

Others 

14.6

Wind 2.5

PV 0.0

CSP 0.0

Others DG 0.0

12.3

Wind 4.0 PV 0.2

CSP 0.0

DG 0.0

Others 24.9

Production/Dispatch in GW

WECC California DSW Northeast Northwest

Wind (GW) 20.9 4.4 4.0 2.5 8.4

PV (GW) 3.9 3.7 0.2 0.0 0.0

CSP (GW) 0.9 0.9 0.0 0.0 0.0

Distributed PV (GW) 0.0 0.0 0.0 0.0 0.0

Others (GW) 94.7 19.9 24.9 12.3 14.6

Total (GW) 120.3 28.9 29.1 14.7 22.9

Penetration (%) 21% 31% 14% 17% 36%

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24

Figure 12. Wind and solar generation in the HS Base case. 

2.3 Performance and Monitoring Metrics

2.3.1 Performance Metrics

2.3.1.1 Frequency Response

FR metrics are established by the new NERC BAL-003-1 (NERC 2012a). The primary 

requirement is that the minimum frequency during design-basis disturbances should not cause 

UFLS. In the West, the first stage of UFLS is normally at 59.50 Hz. Some margin above that 

level to account for normal pre-disturbance variation in frequency (24 mHz) is needed for 

simulations.

Wind 2.1

PV 1.1

CSP 0.4

DG 0.0

Others 

64.9

Wind 0.0

PV 0.0

CSP 0.0

DG 0.0

Others 

33.3

Wind 1.7 PV 0.0

CSP 0.0

DG 0.0 Others 

18.1

Wind 0.8 PV 0.1

CSP 0.0

DG 0.0

Others 

53.0

Production/Dispatch in GW

WECC California DSW Northeast NW

Wind (GW) 5.6 2.1 0.8 1.7 0.0

PV (GW) 1.2 1.1 0.1 0.0 0.0

CSP (GW) 0.4 0.4 0.0 0.0 0.0

Distributed PV (GW) 0.0 0.0 0.0 0.0 0.0

Others (GW) 196.2 64.9 53.0 18.1 33.3

Total (GW) 203.4 68.5 53.9 19.8 33.3

Penetration (%) 4% 5% 2% 9% 0%

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25

2.3.1.2 Transient Stability

The transient stability performance requirements are well defined by WECC. Appendix Table 35

specifies the system performance requirements for various categories of events. As an example, 

the WECC rules dictate that a category B disturbance in the system shall not cause a transient 

voltage dip that is greater than 20% for more than 20 cycles at load buses at any time other than 

during the fault.

2.3.2 Area and Regional Monitoring

To make the comparisons of the study results less confusing, all protective relays explicitly 

modeled in the database were switched off. New dynamic models (epcmod) had been developed 

to record metrics in Table 4 for all of WECC, each of the 21 areas defined in the databases, and 

four regions (California, NW, DSW, and Northeast). These metrics were monitored dynamically 

and output to the channel file for each simulation. 

Voltage, frequency, and angle were monitored at key high-voltage buses in the Western 

Interconnection. Appendix Figure 80 and Appendix Table 36 show the detailed information for 

these key buses. Voltage and frequency were also monitored for all buses that are 230 kV and 

above. Flows on all major interfaces were monitored. 

2.3.3 Reference Disturbances

Five disturbances (shown below in Table 5) were selected to evaluate the impacts of high 

penetrations of wind and solar generation on the FR and transient stability. Some of these 

contingencies (e.g., loss of PDCI) normally involve a remedial action scheme (RAS) to improve 

post-contingency system performance. However, the TRC agreed that such non-linear actions 

obscure rather than illuminate the differences caused by added wind and solar. The added 

complexity only makes the results more difficult to interpret. In general, therefore, RAS were not 

simulated. For similar reasons, the relay models, including UFLS, were also de-activated. 

There is an exception: the analysis of the loss of PDCI examined system performance both with 

and without a simple generation-tripping RAS.

Table 5. Study Disturbances

Name Description 

Loss of Two PV Loss of the two Palo Verde nuclear power station units

Loss of PDCI Bi-polar loss of the PDCI 500 kV line

Vincent Fault Three-phase Vincent 500 kV fault and loss of one Midway-Vincent 500 kV line

Laramie Fault Three-phase Laramie 345 kV fault and loss of the Laramie-Story 345 kV line

Aeolus Fault Three-phase Aeolus 500 kV fault and loss of the Aeolus-Anticlin 500 kV line

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26

2.4 Renewables Siting

The wind and solar data used in this study are based on the WWSIS-2 High Mix scenario. This 

scenario defined an equal mix of wind and solar resources totaling approximately 33% energy

penetration for the year 2020 load for the Western Interconnection. Specifically, 16.5% of load 

was supplied by wind resources and 16.5% by various solar technologies including distributed 

PV, utility-scale PV, and CSP. 

Most of the 16.5% wind plants are between 100 MW and 200 MW. There are 276 wind plants 

representing approximately 44,000 MW assigned to the Hi-Mix scenario. Of the 16.5% solar 

resources, 60% were PV and 40% were CSP. The 60% PV was further divided as 40%

distributed PV, 20% utility-scale PV near population centers, and 40% utility-scale PV located at 

best resources. In the WWSIS-2 data, there are a total of 326 utility-scale PV plants (bestresource and population-centered combined) with an average plant size of 72 MW, median of 

52 MW, and maximum of 200 MW. There are 108 CSP plants with an average size of 130 MW, 

median of 105 MW, and maximum of 200 MW. 

Note that most of the wind, PV, and CSP sites from the WWSIS-2 data have a corresponding 

substation (bus) in the LSP Base case and HS Base case, and were therefore candidates to be 

added. Only a few sites are excluded due to a missing bus or isolated bus. Figure 13 shows the 

wind and solar sites that were identified in the WECC case.

The addition of distributed PV was handled differently. As discussed in Section 2.2.3, the 

distributed PV was embedded with the complex load model. The dispatch of these renewables 

varies with the different cases, depending on the instantaneous renewable power penetrations.

Figure 13. All potential renewable sites.

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27

Table 6 shows the actual breakdown by state for the various wind and solar technologies from

the WWSIS-2 Hi-Mix scenario. This breakdown guided the incremental changes made to the 

WECC LSP and HS Base cases in the creation of the Hi-Mix cases for this study, with the intent 

of matching the capacity by area in the Hi-Mix cases for this study to the Hi-Mix capacity in the 

WWSIS-2 cases. As noted above, the mapping between the WWSIS-2 system topology and the 

WECC cases is good, but not perfect, so the capacity numbers do not perfectly align with the HiMix cases of this study.

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28

Table 6. Wind and Solar Generation in WWSIS-2 Hi-Mix Scenarios

Rooftop PV Utility-Scale PV CSP Wind Total

State

Capacity

(MW)

Capacity

Factor

Energy

(GWh)

Capacity 

(MW)

Capacity

Factor

Energy 

(GWh)

Capacity 

(MW)

Capacity

Factor

Energy 

(GWh)

Capacity 

(MW)

Capacity

Factor

Energy 

(GWh)

Capacity 

(MW)

Capacity

Factor

Energy 

(GWh)

Arizona 3,655 19% 6,160 5,394 25% 12,029 9,374 42% 34,385 1,440 30% 4,009 19,863 33% 56,582

California 8,412 18% 12,984 9,592 24% 19,715 3,594 45% 13,884 6,157 27% 16,690 27,754 26% 63,272

Colorado 1,127 18% 1,794 1,653 21% 3,143 169 37% 541 4,396 36% 12,869 7,344 29% 18,346

Idaho 3 17% 5 2 18% 2 1,093 29% 2,787 1,098 29% 2,794

Montana 25 15% 33 34 18% 51 4,288 37% 13,659 4,347 36% 13,742

Nevada 772 19% 1,275 3,282 26% 7,593 562 40% 1,963 1,560 31% 4,373 6,177 28% 15,204

New Mexico 943 20% 1,638 1,280 27% 3,031 298 40% 1,032 3,134 39% 10,520 5,654 33% 16,221

Oregon 101 14% 120 126 20% 231 5,413 25% 12,283 5,640 26% 12,634

South Dakota 4 17% 6 6 20% 10 1,950 36% 6,231 1,960 36% 6,248

Texas 208 20% 361 193 25% 428 401 22% 789

Utah 1,204 17% 1,801 1,216 21% 2,305 683 30% 1,950 3,102 22% 6,056

Washington 405 13% 460 709 18% 1,186 5,762 28% 13,781 6,876 26% 15,427

Wyoming 10 18% 16 18 21% 32 7,244 43% 27,711 7,272 44% 27,759

Total 16,870 18% 26,651 23,504 24% 49,756 13,997 42% 51,805 43,118 34% 126,861 97489 30% 255,073

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29

2.5 Incremental Commitment and Dispatch

A credible method to change commitment and dispatch is critical to the creation of meaningful 

and comparable cases for this study. A one-to-one de-commitment of conventional generation to 

accommodate wind and solar is overly simplistic. Transient stability and FR are dominated by 

the generation initial conditions. Thus, realistic and economically rational initial conditions are 

needed. The load flow that provides the starting point for dynamic simulations is a single 

snapshot in time and not, in itself, an economic tool. Therefore, it is necessary to use economic 

tools to guide the commitment and dispatch process.

The process outlined in this section is based on the WWSIS-2 PLEXOS results. In that study, a 

base level of wind plus solar was compared to the Hi-Mix system with much more wind and 

solar. The result was annual simulations at 5-minute intervals—about 100,000 system-wide data 

points per year per case.

As discussed above, LSP and HS load cases were selected for this study. The previous section 

documented the addition of new wind and solar generation capacity. The overall intent of this 

step is to re-commit and re-dispatch the conventional generation to allow meaningful 

investigation of the impact of adding wind and solar to the system. As wind and solar generation 

pick up, other generation will be displaced in a fashion that results in the lowest operating cost 

that satisfies the system physical and security constraints. The process is more complex than 

might be expected, as the two starting cases do not have the same infrastructure. In other words, 

there are differences in the generation portfolio, the load level, and the resultant commitment and 

dispatch, as well as some minor topological differences. More important, there are significant 

differences between the two power flow base cases and the WWSIS-2 PLEXOS Base case. The 

objective was to get clearer insight into the effective dispatch and commitment rules that apply 

during LSP periods. 

The overall process is as follows:

1. Mine the WWSIS-2 PLEXOS Base case to select sample periods similar to the power 

flow conditions (e.g., LSP morning with high wind and solar production). 

2. Compare the incremental changes between the WWSIS-2 Base and the Hi-Mix scenarios 

for the selected periods. 

3. Determine how the commitment and dispatch of other generation in the West tends to 

change during these time periods by examining the differences between the two WWSIS2 scenarios. 

4. Increase the wind and solar dispatch in the WWSIS-3 LSP and HS Base cases consistent 

with the new capacity added. 

5. Use the trends extracted from the WWSIS-2 incremental changes to guide changes to the 

LSP and HS Base cases in order to create new LSP Hi-Mix and HS Hi-Mix cases. Start 

with unit de-commitments, and then re-dispatch non-wind and non-solar generation.

The rest of this section examines the details of these five steps. Additional supporting material is 

included in the Appendix. 

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30

2.5.1 Mining PLEXOS Results from WWSIS-2 

The WWSIS-2 PLEXOS Base case results were filtered to capture periods of operation that are 

close to that of the WECC LSP case. The 105,120 PLEXOS results (8,760 hour/year x 12 5-

minute periods/hour) were searched for periods that met the following criteria:

• Daytime (PV ≠ 0.0)

• First month of spring (March 21–April 21)

• Load within 10 GW of WECC LSP case: 115 GW 

• Total wind plus solar production > 18 GW in PLEXOS Base (WECC LSP case has 

24.4 GW of wind and solar in the United States). 

A total of 487 5-minute samples (about 0.5% of the full year) met these criteria. The samples 

were spread across nine contiguous windows on nine different days (as listed in Appendix Table 

37). Of those sample periods, the average wind plus solar production was 20.6 GW in the 

PLEXOS Base case and 48.3 GW in the PLEXOS Hi-Mix case. The single highest condition was 

26.5 GW in the Base case and 62.9 GW in the Hi-Mix case. 

2.5.1.1 Mining PLEXOS Spring Samples

In order to understand the general impact of wind and solar generation on dispatch and 

commitment, the dispatch of all of the committed generation is collected and summarized by 

type. As the only change between the PLEXOS Base and Hi-Mix cases is the addition of wind 

and solar, the dispatch reflects both the instantaneous penetration and sequential history of load, 

weather, outages, and, most important, commitment constraints such as minimum downtime, 

startup costs, and marginal heat rates. 

Thus, pairing of the two cases effectively filters out only the impact of wind and solar generation 

for operations under LSP load with high wind and solar availability. 

Previous work in WWSIS-1 and WWSIS-2 showed that displacement of marginal thermal 

generation is a strong function of net load and renewable penetration levels. Figure 14, which 

includes data from all three spring months, reveals important trends and differences between the 

behaviors of CC generation for the two cases. This figure contains four colored clusters of data 

points. The idea is to illuminate how CC dispatch changes as a function of wind and solar 

production, and as a function of system load. Each point is an XY pair of total CC dispatch 

(MW) vs. wind plus solar power, or of total CC dispatch (MW) vs. system load. The key to the 

clusters is as follows:

• Blue: PLEXOS Base case CC Power vs. Wind + Solar Production (MW)

• Red: PLEXOS Base case CC Power vs. System Load (MW)

• Green: Hi-Mix CC Power vs. Wind + Solar Production (MW)

• Purple: Hi-Mix CC Power vs. System Load (MW). 

Each cluster includes (in black) a linear regression with the equation printed on the figure. 

Further, the mean value of each cluster is noted with a balloon. So, for example, examining the 

cluster of blue points shows that the mean value of the grouping is 20,633 MW of CC dispatch 

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31

for 9,028 MW of wind plus solar production. The regression shows that the slope of the line is -

0.5061, which means that for every 1 MW increase in wind plus solar production, about 0.5 MW 

of CC generation will be backed down. In other words, CC plants are balancing much of the 

changes in wind and solar production in the Base case. Having CC generation on the margin and 

responsible for a substantial part of balancing is not surprising at these moderate levels of wind 

and solar. The CC plants are also important in balancing system load in the Base case. The red 

cluster shows that during a LSP day with good wind and solar production, about two-thirds of 

the load following (0.6512) is done by CC plants.

The contrast with the Hi-Mix case is dramatic. The mean CC plant production drops by 83% to 

1,562 MW. The 27,653 MW increase in wind and solar production pushes the majority of the CC 

out of the stack. The remaining CC plants contribute about 5% of the wind and solar balancing, 

and are largely inactive for load following. They are mostly at or near minimum, so the wind and 

solar following is apparently driven by drops in the renewable production. These data do not 

explicitly show the fraction of units that change commitment rather than only changing dispatch. 

The difference between backing down and de-committing has big implications for both FR and 

transient stability. This is addressed below.

Figure 14. Dispatch sensitivity of CC plants in LSP window. 

y = -0.5061x + 19471

y = 0.6512x - 65998

y = -0.0513x + 4041

y = 0.0218x - 946.49

0

5000

10000

15000

20000

25000

0 20000 40000 60000 80000 100000 120000 140000

Gas CC Sensitivity to W+S and Load

Gas CC TEPPC

Gas CC TEPPC

Gas CC HiMix

Gas CC HiMix

Linear (Gas CC TEPPC)

Linear (Gas CC TEPPC)

Linear (Gas CC HiMix)

Linear (Gas CC HiMix)

Mean: 20633, 

9028

Mean: 

48286,1562

Mean: 

115217, 9028

Mean: 

115301, 1562

CC Dispatch vs

W+S

WWSIS II 

Base Points

CC Dispatch vs

System Load

WWSIS II 

Hi-Mix points

WECC LSP Base 

Case: 24418, 8516, 

Base vs. W+S

Base vs. Load

Hi-Mix Points vs. W+S

Hi-Mix Points vs. Load

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32

Figure 15. Dispatch sensitivity of coal plants in LSP window. 

A similar investigation of the coal plants (see Figure 15) shows another fundamental change in 

operations. In the PLEXOS Base case, the coal plants do essentially no load following—they are 

baseload, but they do about one-quarter of the wind plus solar following. There is a substantial 

reduction of coal in the Hi-Mix case, but coal continues to do essentially no load following. The 

coal plant participation in following the variation in wind and solar increases to about 40%. As 

noted, this plot alone does not provide direct data about de-commitments, but subsequent mining 

shows that about 10 GW tends to be de-committed, and on average another 5 GW is dispatched 

down. 

Participation in load following or wind plus solar following varies in other types of generation. 

The simple-cycle combustion turbines (see Appendix Figure 86) are on average insensitive to 

load or wind plus solar. For this analysis, the use of combustion turbines (CTs) is assumed to be 

primarily driven by forecast errors and reserve constraints. Consequently, CTs have been left 

untouched in the de-commitment and re-dispatch work. Hydro (see Appendix Figure 87) is also 

relatively insensitive to wind and solar, but tends to be more sensitive to load. The hydro in the 

Northwest is rather more sensitive, and provides a resource for adjustment to changes in wind 

and solar. Pumped-storage hydro (see Appendix Figure 88) is sensitive to both wind and solar, 

and to load. For this work, it is not addressed except in one of the sensitivities reported later.

This macro investigation gives broad guidance on how to modify the dispatch and commitment 

of the non-variable renewable resources in the Hi-Mix stability load flow. The next step was to 

develop rules for de-committing and re-dispatching generation within each area for periods of 

operation that look like the LSP condition. Because the mapping of individual generation from 

PLEXOS to WECC LSP 2022 is imperfect, the approach focuses on the delta, leaving the LSP 

base case dispatch and commitment unchanged from that provided by WECC. 

y = -0.2466x + 28909

y = 0.0249x + 20953

y = -0.4127x + 29324 y = 0.0591x + 2581.5

0

5000

10000

15000

20000

25000

30000

0 20000 40000 60000 80000 100000 120000 140000

Coal Sensitivity to W+S and Load

Coal TEPPC

Coal TEPPC

Coal Hi Mix

Coal Hi Mix

Linear (Coal TEPPC)

Linear (Coal TEPPC)

Linear (Coal Hi Mix)

Linear (Coal Hi Mix)

Mean: 20633, 

23821

Mean: 

48286,9397

Mean: 115217, 

23821

Mean: 

115301, 9397

WECC LSP ‘22: 

24418, 21911 

Base vs. W+S

Base vs. Load

Hi-Mix Points vs. W+S

Hi-Mix Points vs. Load

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33

Investigation of individual CC and coal plants shows that during this type of operation, the plants 

tend to be either at maximum or minimum. Some plants tend to be rarely de-committed; others

come on- and off-line more often. 

There is a broader question of whether the generation portfolio will meet load under other system 

conditions or at other times of year. However, that question was already addressed by WWSIS-1 

and WWSIS-2. WWSIS-3 focuses solely on specific snapshots in time.

2.5.1.2 Create Light Spring Hi-Mix Case

The LSP Hi-Mix case was created from the LSP Base case using the results of the trend 

investigation. Because the intent was to create a meaningful counterpoint to the LSP Base case, a 

single sample wind and solar data point was required. The mean value of the wind and solar 

production for all the 487 samples would unrealistically smooth the plant outputs, which will in 

reality vary substantially over the geography of the Western Interconnection at any instant of 

time. The sample that closely represents the mean of the sample space is sample #31775 (April 4, 

2012 at 7:50). That sample was selected as the target for the Hi-Mix case. The new LSP Hi-Mix 

dispatch was developed to approximately replicate the PLEXOS sample without changing the 

dispatch (or commitment) of any of the wind or solar generation in the LSP Base case. 

The process of mapping the PLEXOS Base and Hi-Mix cases to the LSP Base power flow case 

included the following steps:

• Map bus numbers, area, and zone information for existing plants

• Map bus numbers, area, and zone information for added renewables

• Disregard PLEXOS Hi-Mix plants without corresponding bus number in the LSP Base 

case

• Extract wind, CSP, and PV dispatches by area (for all 17 U.S. areas) for sample #31775 

• Establish the split of PV between utility-scale PV and distributed PV (DG)

• Add new wind, CSP, and utility-scale PV plants to load flow and stability databases

• Add distribution-connected PV, as DG, to the load flow by modifying Pload and Qload, 

and then adjusting the composite load model to include the correct amount of DG

• Dispatch utility-scale wind and solar toward the target for sample #31755 

• De-commit CC and coal plants that were likely to be displaced by wind and solar under 

this condition

• Re-dispatch remaining CC, coal and, where necessary, hydro, to solve the load flow

satisfactorily.

2.5.1.3 Renewables in Light Spring Base and Hi-Mix Cases 

The final variable renewable capacity for the LSP cases is shown in Table 7. These are 

nameplate values and dictate the ratings of the connecting load flow elements [e.g., generator 

step up (GSU) transformers] and the dynamic models. 

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34

Table 7. Renewable Capacity for LSP Cases 

LSP Base Case LSP Hi-Mix Case

Area CSP UtilityScale

PV

Wind CSP UtilityScale

PV

Distributed

PV

Wind

ALBERTA 0 0 2,707 0 0 0 2,707

ARIZONA 0 971 347 6,879 4,923 3,655 1,435

B.C.HYDRO 0 0 108 0 0 0 108

EL PASO 0 0 0 142 343 368 50

IDAHO 0 0 643 0 0 0 643

IMPERIALCA 0 0 0 188 611 71 917

LADWP 0 0 576 1,043 913 1,961 576

MEXICO-CFE 0 0 0 0 0 15 0 

MONTANA 0 0 707 0 11 21 3,975 

NEVADA 0 64 0 229 556 285 0 

NEW MEXICO 0 100 1,726 156 1,260 758 3,108

NORTHWEST 0 59 8,680 0 869 500 11,655

PACE 0 0 2,384 0 390 1,126 4,111

PG AND E 0 3,232 2,399 0 3,232 1,474 2,399

PSCOLORADO 0 79 2,134 169 1,016 547 2,134

SANDIEGO 0 528 712 0 528 421 712

SIERRA 0 0 432 0 1,504 432 777

SOCALIF 1,436 2,139 4,497 2,814 5,913 4,741 4,497

FORTISBC 0 0 0 0 0 0 0 

WAPA R.M. 0 4 739 0 690 594 8,136

WAPA U.M. 0 0 0 0 0 0 60

Total 1,436 7,164 28,616 11,618 22,747 16,969 47,999

The LSP Base case, which reflects a commitment and dispatch for the LSP case, does not align 

perfectly with the starting point of the WWSIS-2 PLEXOS runs. Consequently, the increment 

between the WWSIS-2 PLEXOS Base and High Mix cases is used to give the increment targeted 

when changing from the LSP Base case to the LSP Hi-Mix case. The word "target" is important. 

Unit ratings, as well as a degree of topological mismatch, influence the ability to reach the target. 

From the perspective of this investigation, the results are desirable. The net capacity, 

commitment, and dispatch of wind and solar generation are greater than those of the WWSIS -2 

High Mix case. 

The target capacity factor of the added wind and solar provides an additional constraint to that of 

maintaining the renewable dispatch in the Base case. In a few areas, the new plants couldn’t be 

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35

dispatched high enough to meet the target (>100%), and so dispatch was limited to rated power. 

In Appendix Table 40, the differences between the WWSIS-2 PLEXOS Base dispatch and the 

High Mix (data point #31775) are listed by area. A comparison of final renewable dispatches for 

the LSP cases is shown in Table 8. 

The WWSIS-2 PLEXOS simulation results, and in particular the difference between the Base 

and High Mix cases, were credibly mapped to the power flow and dynamic model databases.

Table 8. Renewable Dispatch for LSP Cases 

LSP Base Case LSP Hi-Mix Case

Area CSP UtilityScale

PV

Wind CSP UtilityScale

PV

Distributed

PV

Wind

ALBERTA 0 0 1,665 0 0 0 1,665

ARIZONA 0 50 347 6,395 1,407 1,400 775

B.C.HYDRO 0 0 6 0 0 0 6 

EL PASO 0 0 0 142 226 193 2 

IDAHO 0 0 390 0 0 0 390

IMPERIALCA 0 0 0 124 275 30 339

LADWP 0 0 190 490 402 823 190

MEXICO-CFE 0 0 0 0 0 0 0 

MONTANA 0 0 390 0 3 2 1,599

NEVADA 0 60 0 112 374 158 0 

NEW MEXICO 0 28 1,091 156 295 207 999

NORTHWEST 0 17 8,354 0 317 196 8,361

PACE 0 0 1,408 0 86 228 2,662

PG AND E 0 1,861 764 0 1,860 561 764

PSCOLORADO 0 24 1,822 169 605 339 1,822

SANDIEGO 0 528 272 0 528 111 272

SIERRA 0 0 280 0 722 203 625

SOCALIF 858 1,300 3,177 857 2,696 2,182 3,177

FORTISBC 0 0 0 0 0 0 0 

WAPA R.M. 0 0 739 0 425 347 3,328

WAPA U.M. 0 0 0 0 0 0 27

Total 858 3,868 20,895 8,445 10,221 6,980 27,003

2.5.1.4 De-Commitment/Re-Dispatch from WWSIS-2 PLEXOS Base to Hi-Mix

The overall dispatch trends for periods similar to the LSP condition were used to develop rules 

for de-committing and re-dispatching conventional generation. A total of 583 coal and CC units 

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36

were examined in the 487 sample space for patterns of operational change due to increased wind 

and solar.

For each unit, the tendency to be re-dispatched or de-committed for similar conditions was 

examined. Units that were de-committed for more than 30% of the samples were shut down. The 

net results are summarized in Table 9, which gives the plant count and average MW of 

generation dropped. So, for example, 20 individual plants in Arizona are de-committed for an 

expected drop of 1,265 MW in generation. 

These are based on the PLEXOS results; not all the individual units expected to shut down would 

necessarily be on-line in the WECC LSP case, nor would they necessarily be at the dispatch 

suggested by the mean numbers listed.

Table 9. Coal and Combined-Cycle De-Commitment

Area Name Area # # of Units Average Change in P [MW]

ALBERTA 54 6 223

ARIZONA 14 20 1,265

BC HYDRO 50

EL PASO 11

IDAHO 60 5 203

IMPERIALICA 21 1 37

LADWP 26 1 35

MEXICO-CFE 20 2 36

MONTANA 62 3 139

NEVADA 18

NEW MEXICO 10 3 86

NORTHWEST 40 3 229

PACE 65 14 661

PG&E 30 6 497

PSCOLORADO 70 10 555

SAN DIEGO 22 1 35

SIERRA 64 3 156

SOCALIF 24 2 72

FORTISBC 52

WAPA R.M. 73 21 1,226

WAPA U.M. 63

Total 101 5,455

Re-dispatch follows de-committing the available plants. The units that were often re-dispatched 

in the sample space (i.e., more than 50 times in the sample space) were backed down. For the 

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37

majority of plants, this meant that they were dispatched down to their minimum power. 

Displacing thermal plants (other than CTs, per the discussion above), is insufficient to meet the 

wind and solar displacement, so hydro in the Northwest and British Columbia are also 

dispatched back. Hydro units that had ∆P > 0 more than 10 times in the sample space were used. 

The key point of all this detail is that location matters. Displacement by wind and solar is not 

one-for-one in each area. Further, in this sample space, the economic tendency is for more 

downward dispatch than de-commitment. This has stability and FR implications and is explored 

later in this report. 

Table 10. Re-Dispatch for Coal, Combined-Cycle, and Hydro

Area Name Area # # of Units Average Change in P [MW]

ALBERTA 54 3 102

ARIZONA 14 15 3,645

BC HYDRO 50 1 90

EL PASO 11

IDAHO 60 4 719

IMPERIALICA 21

LADWP 26 3 1,234

MEXICO-CFE 20 3 149

MONTANA 62 5 1,113

NEVADA 18 2 164

NEW MEXICO 10 5 1,071

NORTHWEST 40

PACE 65 21 2,254

PG&E 30 16 334

PSCOLORADO 70 6 715

SAN DIEGO 22 3 41

SIERRA 64 3 378

SOCALIF 24 7 253

FORTISBC 52

WAPA R.M. 73 15 1,847

WAPA U.M. 63

Total Coal+CC 112 14,109

BC HYDRO 50 7 1,072

NORTHWEST 40 126 1,797

Total Hydro 133 2,869

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38

2.5.1.5 Mining PLEXOS Summer Samples

A similar process was used to develop the HS Hi-Mix case. Screening PLEXOS base and HiMix data for summer (July 16–August 26, 2020) with high wind and solar conditions resulted in

95 5-minute samples, spread across 10 contiguous windows on 10 different days (listed in 

Appendix Table 38). Sample #68417, August 25 at 13:20, is closest to the mean of the samples. 

Overall, the same approach used for LSP was pursued here. However, the differences between 

the WWSIS-2 systems and the HS Base power flow were more significant given that the 

PLEXOS data represented a 2020 load condition and the HS power flow data represented a 2023 

load condition. Most notably, the HS Base case has a load of ~192 GW, whereas the highest load 

ever in the PLEXOS study was 169 GW. This causes the mapping to be of lower fidelity.

The PLEXOS results were filtered to capture periods of operation that are close to that of the 

WECC HS 2023 case, with characteristics as shown in Table 11.

Table 11. Renewable Averages from HS PLEXOS Screening 

Resource Screening 

Criteria

Base

Average 

Hi-Mix 

Average 

CSP >4,000 MW 4,568 MW 9,745 MW

PV >4,000 MW 4,201 MW 24,887 MW

Wind >3,000 MW 4,513 MW 9,362 MW

Load >150 GW 152.8 GW 152.8 GW

The average wind plus solar production in the PLEXOS samples is 13.3 GW, which is 

substantially higher than the 6.4 GW in the WECC HS data (U.S. only). This makes sense, as the 

HS case was not developed by WECC for specific consideration of a higher-renewable future. 

The Hi-Mix samples have an average renewable generation of 44.0 GW. 

As with the LSP cases, the commitment and dispatch under HS conditions shows that most of the 

load following is done by CC plants (Appendix Figure 89), and to a lesser degree, hydro. There 

is essentially no change in the coal plants, as shown in Appendix Figure 90. 

The process of adding renewable capacity was similar to that used for the LSP Base to Hi-Mix 

development. More wind and solar plants needed to be added, as the HS Base case starts with 

fewer plants than the LSP Base case. A comparison of the total capacity of the two HS cases is 

shown in Table 12. Appendix Table 42 gives the total capacity, including the WWSIS-2 case for 

reference. 

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39

Table 12. Renewable Capacity for HS Cases

HS Base Case HS Hi-Mix Case

Area CSP UtilityScale

PV

Wind CSP UtilityScale

PV

Distributed

PV

Wind

ALBERTA 0 0 1,061 0 0 0 1,061

ARIZONA 0 700 227 7,654 5,202 3,655 1,457

B.C.HYDRO 0 0 237 0 0 0 237

EL PASO 0 0 47 142 350 305 68

IDAHO 0 0 407 0 0 0 564

IMPERIALCA 0 0 0 188 611 71 917

LADWP 270 0 574 837 913 1,961 574

MEXICO-CFE 0 0 0 0 0 15 294

MONTANA 0 0 364 0 27 21 3,989

NEVADA 0 64 0 229 656 324 0 

NEW MEXICO 0 27 396 156 1,157 758 3,120

NORTHWEST 0 0 0 0 1,607 500 11,642

PACE 0 0 2,309 0 409 1,126 4,032

PG AND E 0 2,570 1,033 0 2,570 1,474 1,758

PSCOLORADO 0 79 2,134 169 960 547 2,134

SANDIEGO 0 516 1,562 0 516 357 1,562

SIERRA 0 0 352 0 1,511 432 802 

SOCALIF 822 49 887 2,825 6,100 4,741 3,147

FORTISBC 0 0 0 0 0 0 0 

WAPA R.M. 0 0 139 0 696 594 8,137

WAPA U.M. 0 0 0 0 0 0 60

Total 1,092 4,052 11,680 12,199 23,286 16,882 45,535

The final renewable dispatches for the HS cases are shown in Table 13. The data show that even 

under high wind and solar conditions, the capacity factor in the summer is lower, especially for 

wind. Again, the HS Hi-Mix dispatch is slightly different than the PLEXOS sample, as 

differences in topologies between the cases and maximum powers are respected on the individual 

renewable plants. 

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40

Table 13. Renewable Dispatch for HS Cases 

HS Base Case HS Hi-Mix Case

Area CSP UtilityScale

PV

Wind CSP UtilityScale

PV

Distributed

PV

Wind

ALBERTA 0 0 899 0 0 0 899

ARIZONA 0 0 227 3,162 2,144 1,858 227

B.C.HYDRO 0 0 101 0 0 0 101

EL PASO 0 0 47 0 142 0 68

IDAHO 0 0 142 0 0 0 167

IMPERIALCA 0 0 0 187 385 43 28

LADWP 0 0 190 568 566 1,167 190

MEXICO-CFE 0 0 0 0 0 0 0 

MONTANA 0 0 107 0 14 11 652

NEVADA 0 61 0 197 255 138 0 

NEW MEXICO 0 0 20 0 361 240 102

NORTHWEST 0 0 0 0 245 283 6,869

PACE 0 0 1,154 0 221 596 1,447

PG AND E 0 701 787 0 701 966 787

PSCOLORADO 0 52 444 169 519 295 444

SANDIEGO 0 388 325 0 388 239 325

SIERRA 0 0 320 0 1,073 294 320

SOCALIF 355 45 814 2,359 3,790 2,943 803

FORTISBC 0 0 0 0 0 0 0 

WAPA R.M. 0 0 30 0 390 321 910

WAPA U.M. 0 0 0 0 0 0 0 

Total 355 1,247 5,607 6,642 11,194 9,394 14,339

2.5.2 Initial Condition Summary of Hi-Mix Cases

The system load flow diagram for the LSP Hi-Mix case is included in Appendix Figure 91. With 

the massive increase in solar generation in California and the DSW, both the PDCI and

Intermountain high-voltage direct current (HVDC) line are off. Notice that the “total system 

load” as reported in the load flow and the bubbles is reduced due to the composite load model 

with distributed PV. 

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41

Figure 16. Wind and solar generation in LSP Hi-Mix case. 

The renewable production in the LSP Hi-Mix case is shown in Figure 16 in both pie chart and 

tabular format. With about 52.8 GW of wind and solar total, the case has more than double the 

approximately 25 GW of wind and solar total in the LSP Base case. The totals for the four 

regions listed are for the U.S. WECC only and do not include Canada or Mexico. The wind and 

solar in Canada and Mexico for the Hi-Mix case are the same as in the Base case. Details, by 

region and area, of all the dynamic initial conditions are included in Appendix Table 43 and 

Table 44, respectively. The load flow bubble for the HS Hi-Mix case is shown in Appendix 

Figure 92. 

Wind 4.7

PV 5.8

CSP 1.5

DG 3.7

Others 

15.1

Wind 

8.4

PV 0.3

CSP 0.0

DG 0.2

Others 

11.7

Wind 5.3

PV 0.8

DG 0.4 CSP 0.0

Others 

5.5

Wind 6.9

PV 3.3

DG 2.6 CSP 7.0

Others 11.4

Production/Dispatch in GW

WECC California DSW Northeast NW

Wind (GW) 27.0 4.7 6.9 5.3 8.4

PV (GW) 10.2 5.8 3.3 0.8 0.3

CSP (GW) 8.4 1.5 7.0 0.0 0.0

Distributed PV (GW) 7.0 3.7 2.6 0.4 0.2

Others (GW) 65.7 15.1 11.4 5.5 11.7

Total (GW) 118.4 30.8 31.3 12.0 20.5

Penetration (%) 44% 51% 64% 55% 43%

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42

This HS Hi-Mix case, as shown in Figure 17, has about 41.5 GW of wind and solar—

substantially more than the approximately 7.2 GW of wind and solar (U.S. only) in the Base 

case. This table gives the initial renewable production for the dynamic simulations.

Details of the HS Hi-Mix dynamic initial conditions are included by region in Appendix Table 

45 and by area in Appendix Table 46. 

Figure 17. Wind and solar generation in HS Hi-Mix case. 

A comparison of several key metrics from the four cases is shown in Table 14. Complete details 

for all four cases, for all areas and regions, are included in the Appendix. 

A few points are of particular note. The entire WECC system Kt was 0.46 in the LSP Base case, 

and it declined to 0.42 in the LSP Hi-Mix case. Both levels are well above the nominal 0.30 level 

Wind 2.1

PV 5.8

CSP 3.1

DG 5.4

Others 

54.6

Wind 

6.9

PV 0.2

CSP 0.0

DG 0.3

Others 

29.8

Wind 

2.6 PV 1.3

CSP 0.0

DG 0.9

Others 

15.6

Wind 1.8 PV 3.8

CSP 3.5

DG 2.9

Others 

36.9

Production/Dispatch in GW

WECC California DSW Northeast NW

Wind (GW) 14.3 2.1 1.8 2.6 6.9

PV (GW) 11.2 5.8 3.8 1.3 0.2

CSP (GW) 6.6 3.1 3.5 0.0 0.0

Distributed PV (GW) 9.4 5.4 2.9 0.9 0.3

Others (GW) 162.7 54.6 36.9 15.6 29.8

Total (GW) 204.2 71.0 48.9 20.4 37.2

Penetration (%) 20% 24% 22% 24% 22%

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43

considered to be a warning (Undrill 2010). However, the regional distribution of Kt is important. 

The NW region, with a large amount of responsive hydro generation committed, has Kt above 

60% for both cases. California remains at around a Kt of 1/3, a substantial portion of which is 

hydro in both cases (as explored later in this report). The DSW region sees Kt drop rather 

dramatically, from 0.44 to 0.27, as responsive thermal units, especially gas CC plants, are decommitted. The Northeast sees Kt increase slightly, as responsive units are left committed, but 

dispatched back from maximum power, and some non-responsive units are de-committed. 

In addition, the headroom is a measure of the generation reserves in each study scenario. In the 

LSP Base case, WECC has a total of 20.7 GW of reserves. In the HS Base case, WECC has a 

total of 24.1 GW of reserves. With the addition of significant wind and solar, the WECC-wide 

reserves are 21.9 GW for the LSP Hi-Mix case, and 27.2 GW for the HS Hi-Mix case. 

As discussed above, the displacement of conventional generation to accommodate wind and 

solar is accomplished by a combination of re-dispatching down and de-committing units. In 

comparison to the PLEXOS case, the initial dispatch and commitment of thermal generation in 

the LSP Base case left relatively little room to dispatch down thermal plants. Therefore, most of 

the displacement is accomplished by de-committing units. From a stability perspective, this is 

conservative, as de-commitment is more stressful than re-dispatch down. The mix of frequencyresponsive units and non-responsive units that were de-committed was about 1:2; i.e., about 2 

MW of non-responsive generation were de-committed for each MW of responsive generation decommitted. The 1.2 GW increase in reserves is because much of the conventional generation that 

was re-dispatched down was frequency responsive. 

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44

Table 14. Key Initial Conditions for Synchronous Units with Governor Response

LSP Base WECC California DSW Northeast NW

Pgen (GW) 43.5 6.4 10.4 2.9 12.4

Capacity (GW) 64.2 11.7 15.0 4.2 17.3

Headroom 

(GW)

20.7 5.3 4.6 1.3 4.9

Number (GW) 800 169 128 91 202

Kt 0.46 0.34 0.44 0.26 0.62

Penetration (%) 21.3% 31.0% 14.3% 16.8% 36.5%

LSP Hi-Mix WECC California DSW Northeast NW

Pgen (GW) 34.4 5.4 5.4 2.7 9.5

Capacity (GW) 56.3 10.7 8.7 3.9 17.2

Headroom 

(GW)

21.9 5.3 3.3 1.2 7.7

Number (GW) 768 167 103 89 200

Kt 0.42 0.33 0.27 0.31 0.61

Penetration (%) 44.5% 50.9% 63.6% 54.5% 43.2%

HS Base WECC California DSW Northeast NW

Pgen (GW) 84.8 26.5 17.0 3.3 24.4

Capacity (GW) 109.1 32.3 21.4 4.3 32.6

Headroom 

(GW)

24.1 5.7 4.4 1.0 8.1

Number (GW) 1,135 295 242 91 301

Kt 0.48 0.43 0.37 0.21 0.78

Penetration (%) 3.5% 5.3% 1.6% 8.7% 0.0%

HS Hi-Mix WECC California DSW Northeast NW

Pgen (GW) 83.6 26.4 18.1 3.1 23.4

Capacity (GW) 111.2 31.8 26.4 4.0 31.9

Headroom 

(GW)

27.2 5.1 8.2 0.9 8.4

Number (GW) 1,152 298 267 89 298

Kt 0.50 0.45 0.49 0.19 0.70

Penetration (%) 20.4% 24.0% 22.2% 24.2% 22.2%

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45

2.6 Transmission and Dynamic Model Changes

2.6.1 Method for Adding Incremental Transmission 

Following recommitment and re-dispatch of the Hi-Mix study cases, they were tested for static 

thermal and voltage violations. In practice, the recommitment and re-dispatch was done in steps 

of several thousand MW until the final Hi-Mix condition was achieved. This step-by-step 

process was needed to obtain satisfactory load flow solutions with the massive generation shift 

from Base to Hi-Mix. 

The overall approach approved by the TRC was to add minimal transmission. As noted above, 

the topology of the Western system, as provided by WECC in the selected data sets, includes a 

number of substantial transmission additions that are not currently in service. Improvements to 

allow satisfactory load flow solutions for the Hi-Mix cases did not include the addition of any 

further major transmission.

Adjustments to phase angle regulator or phase-shifting transformer schedules, HVDC schedules,

and other minor adjustments were used to avoid overloading major transmission lines and 

exceeding path ratings. The path ratings observed were according to the 2013 rating catalog. 

Whether these path ratings will still apply for the future system is outside the scope of this study. 

Equally important, it is possible that path ratings would be affected by the addition of wind and 

solar. By observing the present limits, the study has made it possible to observe problems that 

might arise with the current limits. However, WWSIS-3 is not a system planning study, and it 

does not include the necessary but enormous effort required to validate and update all the path 

ratings. 

In order to achieve satisfactory static initial conditions, about 4.5 GVAr of shunt capacitors were 

added to the LSP Hi-Mix case. No shunt capacitors were added to the HS Hi-Mix case. The large 

amount of solar added in central Nevada stressed the system there. In addition to localized shunt 

support and transformer upgrades, a synchronous condenser was added at Frontier. 

Overall, the minimally enhanced transmission used for the Hi-Mix case gives a valuable 

reference, as both FR and transient stability concerns will tend to improve with added 

transmission. If the system has acceptable dynamic performance, new transmission might still 

make economic sense, but will not be needed to maintain stability and adequate FR even with 

high levels of wind and solar. 

2.6.2 Dynamic Models for Renewables

The load flow topology is modified to accommodate the new utility-scale plants. Wind plants 

and utility-scale PV plants have two transformations, one for the substation transformer and an 

equivalent for the unit transformer (from collector voltage to inverter voltage) with an 

intervening equivalent of the collector system. CSP plants have the same electrical topology as 

other steam plants, and only need a single GSU transformer. The two arrangements are shown in 

Figure 18. The hundreds of new renewable plants add many buses to the load flow. 

For dynamic modeling, the CSP plants are modeled as synchronous machines, with a standard 

WECC model without GR. The utility-scale PV plants are modeled with full four-quadrant 

dynamic models (based on the Type 4 wind turbine generator [WTG] model) with voltage 

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46

regulation and LVRT (gewtg and wtgfc). All new wind plants use the Type 3 GE WTG model 

with voltage regulation and LVRT (gewtg, wtg, wndtge). The distributed PV embedded with the 

load is modeled separately in the composite load model as discussed above.

Figure 18. Renewable generation topology in load flow model. 

2.6.3 Other Stability Data Refinements 

During the data checking, 60-second no-disturbance simulations were run. This time period is 

relatively long for typical stability studies, and it uncovered two problems. Unstable power 

oscillations were observed after 30 seconds. Root cause analysis revealed these oscillations were 

caused by three units with a total MVA rating of about 100 MVA. Several attempts to fix the 

data, including using default dynamic data and removing the exciter and turbine model, did not 

correct the issue. The final solution for this oscillation was to net these three units. In addition, 

low-level power oscillations were observed in the Northeast after ~35 seconds. These were found 

to be due to initial conditions exceeding part of the dynamic model rating on a small plant. The 

plant dispatch was slightly reduced.

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3 Frequency Response Analysis

The investigation of FR presented in this section focuses on the impact of higher levels of wind 

and solar generation on the system. 

3.1 Frequency Response Obligation

The WECC interconnection frequency response obligation (IFRO) is given in Table 15 and 

throughout the report as 840 MW/0.1 Hz. Part of the NERC BAL-003-1 standard that sets the 

obligation includes periodic update of the IFRO. Consequently, this is only a reference point, 

rather than a static and absolute statement of obligation. The other FROs in that table are

estimates based on the HS Base case initial conditions, using the generation and load from that 

condition as an approximation for the peak generation and load levels dictated by the standard.

All of these figures are for reference only. FRO is assigned by BA and is in proportion to the 

relative size of the individual BA compared to the entire interconnection. This calculation is only 

an approximation and should not be used to determine whether any BA is in compliance. The 

Appendix includes a more complete discussion of FR (NERC 2012c).

Later, when the FR is calculated and compared to the FRO, the WECC totals always include the 

contribution of resources in WECC Canada and Mexico. Only U.S. resources are included in the 

regional and area levels. 

There is also a locational aspect of FRO. There is no rule that says that the BAs have to meet 

their FRO with their own resources. However, it is not clear that practice has evolved yet to 

allow individual BAs to procure FR from other entities. A formal contractual arrangement is 

required. This is new ground for the industry. Throughout this report and investigation, the 

results are reported based on how regions and areas meet the estimated FROs. This is not a 

statement that BAs need to meet their FRO with their own resources; rather, these are metrics on 

how much the regions and entities contribute.

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Table 15. WECC IFRO and Approximate Regional and Area FROs

ID Name Generation 

(GW)

Load 

(GW)

FRO 

(MW/0.1 Hz)

1 WECC 204 185 840

By Region

2 CALIFORNIA 68.8 67.9 296

3 DSW 53.9 47.8 220

4 NORTHEAST 19.7 18.0 82

40 NORTHWEST 33.3 27.2 131 

By Area

14 ARIZONA 27.5 20.8 104

11 EL PASO 1.7 2.3 9 

60 IDAHO 4.3 3.9 18

21 IMPERIALCA 1.5 0.3 4 

26 LADWP 6.3 7.2 29

62 MONTANA 3.3 1.9 11

18 NEVADA 6.1 6.8 28

10 NEW MEXICO 3.5 3.1 14

65 PACE 9.4 9.9 42

30 PG AND E 32.4 29.0 133

70 PSCOLORADO 8.3 8.1 36

22 SANDIEGO 4.4 5.5 21

64 SIERRA 2.6 2.3 11

24 SOCALIF 24.2 25.9 108

73 WAPA R.M. 6.9 5.8 27

63 WAPA U.M. 0.1 0.0 0 

The FR performance of the interconnection and the individual entities is given by the ratio of the 

change in power resulting from a disturbance-induced change in frequency. For this metric, the 

frequency change is assumed to be uniform across the interconnection. In the work presented 

here, the only power change measured and included in the calculations is that of the turbine 

power of the responsive generation. Load response is not considered in the calculation of FR. 

Load modeling and losses are included as discussed in Sections 3.1 and 3.3.3. This study focuses 

on system-wide FR, as measuring the frequency at a single node in the grid following a 

disturbance can be confusing and misleading. In this study, an MVA-weighted sum of 

synchronous machine speeds is calculated and used as a composite frequency. Details of the 

calculation are included in the Appendix. 

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49

3.1.1 Focus on Light Load Conditions

The system-wide concerns for FR tend to be greatest under light load conditions, as has been 

observed in several investigations (NERC 2012b, Eto 2010). In basic terms, under light load 

conditions, much less generation is operating, but the size of the design-basis disturbance 

remains the same. Thus, the upset is proportionally larger and has more significant 

consequences.

For WECC, the design-basis event, per NERC (NERC 2012b), is the largest category C (N-2) 

event, i.e., the simultaneous trip of two of the Palo Verde NPS units for a total instantaneous loss 

of 2,756 MW of generation. The IFRO (see discussion in Appendix) is based on this event. 

UFLS action or violation of other stability criteria (e.g., separation or extreme voltage swing) for 

this event is not acceptable.

In the WECC Base cases (both LSP and HS), two Palo Verde NPS units are on-line and 

dispatched near their maximum net power. In all the cases presented in this report, these two 

units are tripped, resulting in the loss of 2,756 MW of generation. Figure 19 presents the WECC 

frequency in response to the Palo Verde trip for the LSP (red trace) and HS (blue trace) Base 

cases. The calculation of system frequency is discussed in the Appendix. Appendix Figure 96

and Figure 97 show selected bus voltage and interface flow swings for the event; these voltages 

and power swings are relatively benign.

As expected, the LSP FR is much worse than that of the HS case. The depth of the frequency 

nadir for the LSP case is below 59.7, at 59.668, compared to a nadir of 59.839 for the HS case. 

Note that the event (as measured by frequency nadir) is twice as severe for the LSP case, even 

though the load is more than 50% of the heavy load (~111 GW vs. ~185 GW). The relationship 

between load level and FR is a strong function of generation responsiveness and headroom. 

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Figure 19. Frequency response to loss of two Palo Verde units – LSP vs. HS Base. 

3.2 Light Spring Base and Hi-Mix Comparisons 

In this section, the FR of the Base and Hi-Mix systems is discussed. 

A few of the key system-wide metrics compare as follows (LSP Base vs. Hi-Mix, respectively):

• Kt: 0.46 vs. 0.42

• Headroom: 20.7 GW vs. 21.9 GW

• Number of responsive units on-line: 800 vs. 740. 

Two of the three metrics are such that the expected FR for the Base case should be somewhat 

better than that of the Hi-Mix case.

3.2.1 Trip of Two Palo Verde Units

The design-basis generation trips for the two cases result in quite similar performance. As 

suggested by the initial condition metrics, the performance of the Base case is slightly better than 

the Hi-Mix case. The WECC frequency swings are shown in Figure 20, with the exact details of 

the system FR performance being summarized in Table 16 and Table 17. The frequency nadir 

drops from 59.668 Hz to 59.646 Hz, a 22 mHz (7%) degradation, and the FR drops from 

1,352 MW/0.1 Hz to 1,311 MW/0.1 Hz, a 3% degradation. The performance in both cases meets 

WECC and NERC criteria at the system level. Note that the initial rate of change of frequency 

(ROCOF), i.e., the slope of the frequency trace at the instant following the generation trip, is 

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51

greater in the Hi-Mix case. This is the effect of reduced system inertia. As the Western 

Interconnection generally does not rely on ROCOF-based protection, the increased ROCOF is of 

limited system-wide consequence. However, the combination of the rate of frequency drop and 

the opposing action of primary FR does dictate the frequency nadir—and therefore the margin 

above UFLS. Note also that the PSCOLORADO area, which covers more than just Public 

Service of Colorado (now Xcel Energy), has a negative FR. This occurs because several 

generators in this area have supervisory load controls modeled that override the GR. 

Comparative details of the system performance are shown in the subsequent figures. Figure 21

shows the electrical (solid traces) and mechanical (dotted traces) power excursions of the 

responsive generation. The characteristics of the two cases are similar, with the reduced initial 

generation from responsive units causing the roughly 8 GW difference between the cases.

Appendix Figure 98 shows load swings that have slightly different starting points, due to 

different methods of accounting for losses in the complex load model and the treatment of the 

distributed PV. This is not significant to the results, but the amplitude of the load power 

deviation has some impact on the frequency swing and ROCOF. The depression of system load 

voltages during the initial stages of the disturbance affects the Base case more than the Hi-Mix

case, resulting in slightly more load relief. This helps reduce the severity of the frequency swing. 

Load effects, while not dominant, do play an observable role in system FR. Specifically, load 

voltage sensitivity will be shown later to have a significant impact on FR. 

Figure 22 shows how the GR is geographically distributed across the West. Notice that the 

predominantly thermal generation in the regions outside of the NW is faster and leads the system 

response. However, as the event continues, the predominantly hydro resources in the NW

become the dominant contributor to FR. Thus, the thermal generation has a somewhat more 

significant impact on the frequency nadir than the FR metric (as reported in Table 16 and Table 

17) suggests. The relative time separation of the regional GR impacts the interregional power 

swings, as is reflected in the flows of Figure 23. The slow drift of the flows over the course of 

the 60-second simulation are due to the hydro generation slewing into steady-state response. 

System voltages, a sampling of which is plotted in Appendix Figure 99, are similar and well 

behaved for the two cases. In this case, the timing difference has limited impact on system 

performance. The headroom of the responsive generation, shown in Appendix Figure 100, 

mirrors the mechanical power swing. The headroom drops and returns everywhere but the hydrorich NW, which supports the observation about the difference in speed of response between the 

different types of resources. Figure 24 shows the dynamic count of generation supplying GR. 

The figure shows some units saturating during the swing; these are units that have insufficient 

headroom to sustain the requested power increase for this event. For example, in California, 

about 15 units or about 10% of the total responsive units saturate. Other units had more 

headroom than was useful for this event. This saturation has implications for other frequencyresponsive resources that could be used to maintain stability and to meet FRO. This is discussed 

further in Section 1. 

Details for two areas, PG&E and Arizona, are shown in Figure 25, which illustrates the diversity 

of the response. The effects of apparent governor withdrawal in the Arizona traces can also be 

seen. As noted in other work [NERC (2012c), Eto (2010)], this is a source of concern.

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Figure 20. Frequency response to loss of two Palo Verde units – LSP Base vs. Hi-Mix. 

Figure 21. Generation response to loss of two Palo Verde units – LSP Base vs. Hi-Mix. 

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Figure 22. Governor response to loss of two Palo Verde units – LSP Base vs. Hi-Mix. 

Figure 23. Interface response to loss of two Palo Verde units – LSP Base vs. Hi-Mix. 

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Figure 24. Responsive plant count for loss of two Palo Verde units – LSP Base vs. Hi-Mix. 

Figure 25. Response of generation in two areas to loss of two Palo Verde units – LSP Base vs. HiMix. 

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Table 16. WECC Frequency Response Metrics for LSP Base and Hi-Mix Cases

LSP

Base Hi-Mix

Generation (GW) 204 204

Load (GW) 185 185

FRO (MW/0.1 Hz) 840 840

FR (MW/0.1 Hz) 1,352 1,311

FR Margin (MW/0.1 Hz) 512 471

Frequency Nadir (Hz) 59.668 59.646

Nadir Time (s) 7.77 7.19

Settling Frequency (Hz) 59.839 59.844

Kt 0.46 0.42

Table 17. Area Frequency Response Metrics for LSP Base and Hi-Mix cases 

LSP Base Case LSP Hi-Mix

FRO FR FR Margin FR FR FR Margin FR

Name (MW/

0.1 Hz)

(MW/

0.1 Hz)

(MW/

0.1 Hz)

% of 

WECC

Kt (MW/

0.1 Hz)

(MW/

0.1 Hz)

% of 

WECC

Kt

WECC 840 1,352 512 100.0 0.46 1,311 471 100.0 0.42

By Region

CALIFORNIA 296 305 9 22.6 0.34 312 16 23.8 0.33

DSW 220 215 -5 15.9 0.44 119 -101 9.1 0.27

NORTHEAST 82 61 -20 4.5 0.26 47 -34 3.6 0.30

NORTHWEST 131 434 303 32.1 0.62 483 353 36.9 0.62

By Area

ARIZONA 104 69 -35 5.1 0.37 50 -55 3.8 0.29

EL PASO 9 4 -5 0.3 0.42 4 -5 0.3 0.30

IDAHO 18 21 3 1.6 0.38 22 4 1.7 0.68

IMPERIALCA 4 14 10 1.0 0.24 14 10 1.1 0.16

LADWP 29 31 1 2.3 0.60 30 1 2.3 0.42

MONTANA 11 10 -1 0.7 0.17 10 -1 0.8 0.23

NEVADA 28 44 16 3.2 0.72 34 6 2.6 0.52

NEW MEXICO 14 50 35 3.7 0.49 2 -13 0.1 0.03

PACE 42 23 -19 1.7 0.22 8 -34 0.6 0.19

PG AND E 133 190 57 14.1 0.40 197 64 15.0 0.46

PSCOLORADO 36 -14 -50 -1.1 0.34 6 -30 0.4 0.25

SANDIEGO 21 7 -14 0.5 0.15 7 -14 0.6 0.16

SIERRA 11 7 -3 0.6 0.23 7 -4 0.5 0.18

SOCALIF 108 63 -45 4.7 0.23 63 -45 4.8 0.21

WAPA R.M. 27 63 35 4.6 0.67 24 -3 1.8 0.19

WAPA U.M. 0 3 3 0.2 0.94 3 3 0.2 0.76

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3.3 Frequency Response Summary

There are no obvious FR-related stability problems that become apparent as the system moves to 

the Hi-Mix condition for the NERC design-basis event in the West. Doubling the wind and solar 

production to more than 50 GW leaves the characteristics of the system response to this large 

generation trip event essentially unchanged. There is a 7% degradation in the frequency nadir

(i.e., the difference between nadir and nominal is 332 mHz in the LSP Base case and 354 mHz in 

the LSP Hi-Mix) and a 3% degradation in the FR for these conditions (i.e., from 1,352 MW/0.1

Hz to 1,311 MW/0.1 Hz). There are significant locational impacts on FR, with some regions and 

areas providing substantially less response from resources within their own areas than are 

indicated by the NERC standard.

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4 Transient Stability Analysis

4.1 COI Stability Under Heavy Summer Conditions

As discussed in Section 2.3.3, a block or shutdown of both poles of the PDCI was identified by 

the TRC as one of the most severe transient stability events in the Western Interconnection. The 

loss of PDCI should not result in UFLS or any cascading outages. 

To simulate this event, all power transfer on both poles of the PDCI is stepped to zero. The 

resultant loss of power transfer from the NW to California causes a substantial incremental 

power flow on the Pacific AC intertie across the COI, as well as increased loading on an 

aggregate of transmission paths farther east. This event has long been subject to scrutiny, and 

under some conditions requires a relatively complex RAS that can include trip of generation and 

switching of reactive devices. In this investigation, the focus is primarily comparative. For 

simplicity and clarity, only the PDCI power is blocked, and in this sequence of comparisons

there is no further RAS of any type in this simulation. 

The change in system dispatch with the added wind and solar results in the COI and PDCI 

loading being initially much higher in the Hi-Mix compared to the Base case. The Base case 

loading is 3,589 MW and 2,527 MW, respectively, considerably lower than the path rating. 

Conversely, the Hi-Mix COI loading is slightly greater than 4,800 MW, which is the path rating. 

This rating is further subject to constraints due to northern California hydro, which is discussed 

below in Section 4.1.2. The PDCI is loaded to about 3,000 MW in the Hi-Mix case. As system

stability for this event is a function of the loading on the AC and DC interties, a third case was 

added. The Base case was modified by de-committing the two San Onofre units in southern 

California, and increasing hydro generation in the NW by both committing additional units and 

dispatching up other units. The San Onofre units were in service in the original WECC cases. 

The resulting initial condition, named Base-COI in the plots, has the COI AC flow and PDCI 

flow within 10 MW of the Hi-Mix case. The wind and solar generation remains the same as for 

the Base case.

The active power flow on the COI is shown for the three cases in Figure 26. The Base case (blue 

trace) is stable. Both the Hi-Mix case (red trace) and the Base-COI case are unstable. The BaseCOI case is somewhat worse, as the separation occurs faster. Flows on other interfaces are 

included in Appendix Figure 101. 

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Figure 26. COI flows for PDCI event for HS cases. 

Selected bus voltages from throughout the Western Interconnection are plotted in Figure 27. The 

precipitous drop in voltage at Malin is characteristic of a system separation in that vicinity. The 

system separation is also evident in Figure 28, which shows the four regional frequencies. In the 

Hi-Mix case, the frequencies in the generation-rich northern areas become unstable due to high 

frequency, and those to the south experience a frequency drop. In the Base-COI case, all areas 

become unstable due to high frequency.

As noted earlier, these simulations are deliberately kept simple, with no RAS and with no 

protective relaying action. The intent here is to observe the difference in performance, not to 

determine whether or how the system might tolerate a separation. The rate at which separation 

occurs is an indication of the degree of instability, but separation is not allowed for this event. 

With the COI and PDCI at the same loading, the system is less stable with lower wind and solar 

generation for this particular study condition. 

Heavy Summer Base 

case

Heavy Summer Base 

case with high COI 

flows

Heavy Summer Hi-Mix 

case with high COI 

flows

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59

Figure 27. Bus voltages for PDCI event for HS cases. 

Figure 28. Regional frequencies for PDCI event for HS cases. 

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4.1.1 COI Stabilization with Generation Trip Remedial Action Scheme

As noted earlier, system separation for the PDCI event is not allowed. As the Base-COI case 

showed, system separation is a risk today under conditions that include high flow levels on the 

COI. Present practice is to trip generation in the NW for the PDCI event, depending on the initial 

COI and other conditions. A complex set of rules is used to enable specific generation tripping 

for specific operating conditions. This exercise does not attempt to replicate the details of the 

current practice; rather, this is an investigation to understand if the currently accepted practice of 

generation tripping will be effective at mitigating the separation observed in the Hi-Mix case. 

A sequence of three tests, tripping about 1,000 MW, 2,000 MW, and 2,800 MW of generation, 

results in a stable system with relatively small voltage swings. The introduction of deliberate 

generation tripping raises concern about the impact on system frequency. The system frequency 

for this sequence of cases is shown in Figure 29. A case with no generation-tripping RAS is 

shown in blue, along with the three cases with increasing levels of unit tripping. 

The result is not surprising: the more generation tripped, the deeper the nadir and the lower the 

settling frequency. Also, the FR for tripping about 2,800 MW is similar to that for the Palo Verde 

event on the HS Hi-Mix case described earlier. While tripping this level of generation is overkill 

for this event, the frequency is still within limits. This suggests that use of a standard generationtripping RAS with the Hi-Mix level of wind and solar does not carry appreciable frequency risk 

for this condition. There is no obvious need to be overly timid with tripping. 

The plots of Appendix Figure 102 and Figure 103 show interface flows and voltages for this 

sequence of cases. The differences when the system is stabilized are relatively small.

Figure 29. Frequency response of Hi-Mix for PDCI event with various generation-tripping RAS. 

The sequence presented above shows that tripping about 1,000 MW of generation is effective at 

stabilizing COI. This raises the question of how much generation is enough. In this exercise, a 

No generation tripping

1,000 MW generation tripping

2,000 MW generation tripping

2,800 MW generation tripping

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further search for the minimum generation trip that will stabilize the system was pursued. As the 

amount of generation tripping is reduced, the difference in voltage swings becomes more 

apparent. 

Figure 30 shows the voltages along the AC intertie. Shown on the left is the unstable Hi-Mix 

case with no RAS. The separation occurs at about 4 seconds, with Malin and Captain Jack shown 

at the bottom. On the right, a case is shown in which two 138 MW generators (John Day units) 

are tripped. Notice that separation is avoided. The voltage swing at Malin and Captain Jack 

meets WECC criteria. Tripping one unit of this size is stable, but it does not bring the voltage 

swing into compliance with WECC voltage criteria.

This exercise shows that heavy loading on the AC intertie is more stressful than a high wind and 

solar condition. High flows on the COI currently require a generation-tripping RAS. This 

exercise suggests that this practice can continue, and that the transient stability of the system for 

one of the well-known and critical events, while somewhat improved, is not fundamentally 

changed by high wind and solar generation. This conclusion is not a statement that the system 

behaves identically. It is possible, and perhaps likely, that the system dynamics are sufficiently 

different to require somewhat different levels of generator tripping or different arming criteria. A 

complete evaluation of the current practice would be prudent. There is, however, nothing in this

exercise that indicates that the system dynamics have fundamentally changed and that radically 

different means to ensure stability for this event are required.

Figure 30. Dynamic voltage collapse avoided by RAS. 

4.1.2 Northern California Hydro and COI Stress

The loading on the COI has a further constraint that is dictated by the northern California hydro 

(NCH) generation. An operating nomogram that applies during the summer is shown in Figure 

31. Under conditions of high (i.e., >70%) NCH generation, the COI transfer limit is reduced. 

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62

Figure 31. 2013 Summer COI – northern California hydro nomogram. 

The NCH is divided into five groups. The dispatch of this hydro has been left essentially 

untouched as renewables were added moving from the Base cases to the Hi-Mix cases. A review 

of the four cases shows NCH generation as follows: 

• HS Base and Hi-Mix: 81% 

• LSP Base and Hi-Mix: 47%. 

Consequently, the actual limit on the COI because of the thermal limit to the south is 128 MW

lower than was enforced in HS. This is noted on the nomogram. Therefore, the HS Hi-Mix cases

are at slightly higher stress than thermal limits would allow. Stability performance would be 

expected to improve if system stress was relieved by re-dispatching to lower the COI loading. It 

is interesting to note that the COI is overloaded 128 MW, which is on the order of half the 

amount of generation needed to be tripped by a RAS to stabilize the case and meet voltage swing 

criteria. It is entirely possible that no generation-tripping RAS would be necessary at this lower 

transfer level.

4.2 Local Stability Examples

There are many localized stability limits in the West. The addition of substantial wind and solar 

generation has the potential to alter the system dynamics of the events that dictate these limits. 

The two examples included here are intended to illustrate possible impacts on more localized 

limits. While interesting, they are not a substitute for thorough system engineering. 

4672 MW 

limit at 

81.2% NCH

HS’23 Hi-Mix 

case at 

4800MW

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4.2.1 Colstrip Area Fault 

One well-known stability concern in the West is maintaining the synchronism of the Colstrip 

thermal plants. Over the years, the 500 kV backbone running east from Colstrip and the 

surrounding 230 kV systems were designed and tuned to give satisfactory stability results for 

Colstrip. With the addition of substantial amounts of wind, and some solar, in the region, the 

stability of that part of the system could be compromised.

For this investigation, a three-phase fault at the Broadview 500 kV bus is applied. It is cleared 

after three cycles by tripping one 500 kV line: Broadview – Town – Garrison. This fault was 

chosen because it is just downstream of a connection to the underlying 230 kV system with many 

new wind power plants in the vicinity. The angle swings of the four Colstrip units are shown in 

Figure 26. There are three interesting aspects of these results. First, one of the Colstrip units is 

de-committed in the Hi-Mix case. This makes economic sense, as the wind and solar will 

displace at least some of the coal generation in the West, even under heavy load conditions. 

Second, the initial angle of the three remaining Colstrip units is greater in the Hi-Mix case. This 

is evidence that this part of the system—and probably the entire northeastern part of the Western 

Interconnection—is exporting heavily. The implication is that the exporting transmission is 

loaded with wind and solar, as well as the usual thermal power exports from the region. Third, 

the net angle swing of the Colstrip units is smaller in the Hi-Mix case. This suggests a higher 

level of stability. The angle change shows that the acceleration of the units is lower in the HiMix case, a significant consideration because of the specialized type of stability protection used 

there.

The results raise the question of the causality of the lower acceleration: is it because one of the 

units at the plant is off-line, or because of some other dynamic characteristic associated with the 

higher wind and solar? In Figure 33, an additional case is reported in which the de-committed

Colstrip unit is returned to service at the same dispatch as in the Base case. The plot is for the 

speed of the four Colstrip units. The blue trace is for the Base case, and the green trace is for this 

new sensitivity: Hi-Mix with all four Colstrip units committed. Interestingly, the speed increase 

appears to be the same, with the maximum speed—a good proxy for the maximum 

acceleration—still lower in the Hi-Mix case. This result suggests that stability is not degraded in 

the Hi-Mix case, and arguably is slightly improved for this particular system condition. It is an 

interesting data point, but it does not show conclusively that the stability will inevitably improve. 

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Figure 32. Colstrip machine angles for Broadview fault – HS Base vs. Hi-Mix. 

Figure 33. Colstrip #4 speed – HS Base vs. HS Hi-Mix with Colstrip #1 committed. 

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4.2.2 Laramie River Station Fault

Figure 34 shows a comparison between a pair of HS transient stability cases with a three-phase,

four-cycle fault applied at the Laramie River 345 kV substation. After the fault is cleared by trip 

of the Laramie River – Archer TS – Story line, the system recovers in both cases. The recovery is 

faster in the Hi-Mix case, apparently because of the newly added renewables nearby. 

Figure 34. Bus voltages for Laramie River Station fault – HS Base vs. Hi-Mix. 

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5 Sensitivity Analysis

The objective of the WWSIS-3 sensitivity analysis was to define a band of potential responses, 

indicate general impact (e.g., better or worse), and provide insight into specific dynamic 

performance questions raised by the TRC.

This section reports on a number of investigations developed to provide specific insights into the 

impact of key elements related to FR and transient stability under conditions of high wind and 

solar generation.

5.1 Load Model Impact on Fault-Induced Delayed Voltage Recovery 

The impact of load behavior is explored in the set of sensitivity investigations reported here. The 

impacts of load modeling assumptions and the fault tolerance or ride-through capability of 

distributed PV are also tested. The phenomena of fault-induced delayed voltage recovery 

(FIDVR) (NERC 2009) is of particular concern.

5.1.1 Composite Load Model in Base Cases 

In both original WECC cases, load is modeled with induction motor (motorw) and static

components. Specifically, for the HS original case, 5,848 loads are modeled as 20% induction 

motor (~36 GW) and static (~156 GW). The static or ZIP (constant impedance [Z], current [I], 

power [P]) components vary by area, but the majority use constant impedance for reactive load 

and constant current for real load. The total load is about 185 GW. 

The composite load model, as discussed in Section 2.2.3, has a complex topology and flexible 

load characteristics. It accommodates DG and a variety of tripping characteristics. In all results 

presented in this report, stalled motor tripping is not allowed, giving a conservative view of 

stability impacts. About three-quarters of the total load is converted to the composite load in the 

base cases.

5.1.2 Midway-Vincent Fault with Composite Load Model

The results of a three-phase fault at Vincent 500 kV in California for the HS Original and Base 

cases are shown in Figure 35. The fault is cleared by tripping two Midway-Vincent lines in six 

cycles. In the figure, the original case (blue trace) with WECC standard load modeling (20%

induction motor, 80% static) exhibits fast voltage recovery and a stable response. The Base case 

(red trace), with 143.9 GW composite load, fails to recover. There is a dynamic voltage collapse 

about 3 seconds after the fault clears. The simulation after that point is meaningless, and the plot 

is truncated. This is an extreme case of FIDVR. The difference shows that load behavior 

dominates system response for this event. No protection was modeled.

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Figure 35. Load-induced voltage collapse in HS Base case. 

To examine the relative impact of renewables, and particularly the DG, a comparison of the HS 

Base case (blue trace) and the HS Hi-Mix case (red trace) is shown in Figure 36. Both cases 

show dynamic voltage collapse, but the Hi-Mix case collapses about twice as fast.

Two contributing factors to the faster collapse in the Hi-Mix case were considered. One 

contribution is the low-voltage blocking of the distributed PV inverters during the voltage 

depression. The inverters are modeled to maintain full output down to 80% voltage; between 

80% and 70% they begin to block. Below 70%, they fully block, recovering only when the 

voltage recovers. Modeling this behavior is a proxy for the fact that smaller inverters generally 

cannot continue firing during deep voltage depressions. In this context, blocking is distinct from 

tripping. Tripping removes the inverter from operation so that it will not return when the voltage 

recovers. Tripping could be due to inverter limitations, or it could be deliberate, such as in 

response to default requirements of IEEE 1547. In this case, about half of the PV (4.4 GW of 

9.3 GW) blocks during the fault. The vast majority of that generation is near the fault, in the Los 

Angeles Department of Water and Power and Southern California Edison areas. Blocking the 

local current injection of the PV exacerbates the stalling behavior of the motors, making the 

event somewhat more severe. 

A second factor is that the displacement of synchronous generation by renewables was expected 

to reduce the system strength in the vicinity of the fault. Reduced system strength is known to 

aggravate FIDVR. A good proxy for system strength is short-circuit current level. However, 

Heavy Summer with standard load model

Heavy Summer with composite load model

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when examined, the fault current level was found to be nearly identical for the two cases. This 

suggests that the PV blocking, which causes additional current to be drawn from the bulk power 

system, is the dominant factor in the difference between the cases. 

Figure 36. Voltage collapse for Midway-Vincent fault – HS Base vs. Hi-Mix. 

The depth of the voltage depression that causes the motors in the composite load model to 

decelerate is a function of the location and severity of the fault. A sequence of tests was run to 

determine sensitivity to fault severity. A case in which the Midway fault impedance is set to

.00005 + j.005 to approximate a single-phase fault results in no FIDVR. The voltage (as shown 

in Appendix Figure 104) and the load recovered within 50 ms of fault clearing. The DG blocking 

was reduced to about 900 MW. With this single-phase fault equivalent, the difference in the 

characteristic of the successful recovery between the Base case and Hi-Mix case was small. The 

Hi-Mix recovery is slightly faster. In a further test of sensitivity to fault severity, the fault 

impedance for the successful single-phase event was progressively reduced, until FIDVR 

occurred. This sequence, which includes a case where the system exhibits classic FIDVR for 

about 30 seconds before recovering, is shown in Appendix Figure 105. 

5.1.2.1 Details of Composite Load Model Behavior

The dynamics of the composite load model are considerably more complex than those of the 

standard WECC load model, and as shown, dominate the system behavior for some faults. The 

details of the behavior of two of the composite loads for the unstable Midway-Vincent fault are 

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shown in Figure 37. Two buses are shown: one in southern California that is close to the faulted 

bus, and another in Arizona, remote from the fault. The voltage never recovers for the bus near 

the fault. The remote bus finally collapses as the entire system becomes unstable at around 2 

seconds. The simulation after that point is meaningless, and the plot is truncated. In Figure 38, 

the details of these two loads are shown, with the red trace being the load near the fault. The 

dynamic part of the load, as represented by one of the four motor equivalents, is ill-mannered, 

with an aggressive but failed attempt to recover on fault clearing driving up the reactive power 

consumption. Some load reduction is observed in the static and electronic components, but it is 

insufficient to save the system. The drop-out of the PV, shown in the bottom left plot, 

exacerbates the problem. When the fault clears, the voltage does not recover enough for the 

distributed PV to restart. Had some of the motor load been allowed to trip (as discussed above), 

the system would likely recover. That behavior would, however, make the impact of the 

incremental wind and solar extremely difficult to discern.

Figure 37. Load bus voltages at different distances from fault location. 

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Figure 38. Details of load behavior relative to fault proximity. 

The conclusion to be drawn from this sequence of tests is that with this load modeling, the

system is on the edge, regardless of renewable generation. The behavior of the system for deep 

faults is completely dominated by the load model, and more specifically by the trip vs. stall

behavior assumed for the motor models. Drop-out of the distributed PV during the voltage 

depression exacerbates the problem, as does the fact that voltage stays low enough to prevent the 

DG from restarting after the fault clears. This is an extraordinarily complex issue for planning 

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and for research. However, this stability risk is not primarily one of renewable integration on the 

bulk power system. Overall, the utility-scale renewables have relatively little impact on voltage 

recovery here, especially compared to the sensitivity and uncertainty of the load modeling. The 

ride-through and recovery characteristics of the embedded PV contribute to the motor stall as 

modeled, which suggests that PV controls designed to address this behavior could be beneficial.

Further investigation of the load behavior, the motor tripping, and the behavior of the PV DG is 

warranted. In the next section, some aspects of DG tripping are investigated.

5.1.3 Distributed Generation Voltage Ride-Through Sensitivity

The operation of inverter-based DG, including PV, during and following system disturbances is 

governed by two factors: equipment physical capability and deliberate and/or mandated 

reactions. 

The physical constraints on inverter-based generation are substantially different from those on 

synchronous machines. During system disturbances, fast, high-bandwidth controls must measure 

and react to depressed, occasionally distorted, and unbalanced voltage waveforms while 

respecting the sensitive current-carrying capability of the power electronics. Therefore, it is a 

non-trivial challenge for PV to continue operation through disturbances. As was shown in the 

preceding example, inverters may block (i.e., stop carrying current) briefly or for extended 

periods.

For utility-scale generation, fault ride-through capability has been mandated by FERC and 

NERC rules (FERC 2005, NERC 2013), and wind and utility-scale PV suppliers have designed 

equipment that will continue operation through disturbances. However, for distributionconnected generation, such requirements have not been imposed, so the ride-through behavior of 

the embedded PV currently in operation is not well known. Further, IEEE Standard 1547

addresses the obligation of DG to trip in order to avoid inadvertent islanding. That standard 

provides ranges of voltage and frequency depression depth and duration for which DG must trip 

(see Appendix Figure 106 and Figure 107). The standard has been recently modified, with the 

intent to provide a mechanism by which the must-trip behavior can be modified. The issues and 

details of the tension between the desire to avoid inadvertent islanding and the desire to maintain 

bulk power system reliability are complex. 

5.1.4 Impact of Distributed PV Recovery

5.1.4.1 Midway-Vincent Fault

For the single-phase fault at Midway presented above, about 825 MW (out of 9.4 GW) of 

distributed PV blocked during the fault and then recovered following the fault. A sensitivity case 

in which the blocked PV does not recover was run. This simulates DG tripping due to physical 

limitations (i.e., a sympathetic trip) or deliberately (i.e., to comply with IEEE 1547 or other 

objectives).

Even with the loss of this DG, the system is stable and voltages recover. The aggregate behavior 

of the DG active power is shown in Appendix Figure 108. The majority of the lost active power 

is picked up by responsive generation. Voltage recovers slightly slower to a level 1.2% lower 

with tripped DG. More dramatically, the tripping of the DG has a large impact on the reactive 

power balance. The total reactive power generation from all generation is plotted in Figure 39. 

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The blue trace is for full recovery (temporary blocking), and the red trace is for no recovery 

(trip). In the post-fault-clearing condition, the 825 MW of tripped DG “costs” the system about 

3,100 MVAr – almost 4 MVAr/MW. This is indicative of a significant level of system stress. 

This result suggests that inadvertent or deliberate tripping of DG during system disturbances may 

have significant voltage stability effects. These effects could be more problematic than frequency 

impacts. 

Figure 39. Reactive power depletion due to DG tripping on voltage dip. 

5.1.4.2 PDCI Event

The results of the PDCI block event were presented above. For the HS Hi-Mix case, RAS 

tripping of a single NW hydro plant was shown to stabilize the power swing across the COI, but 

it does not quite meet the WECC voltage swing criteria.

That event behaves quite differently if there is a significant tripping of DG due to the postdisturbance voltage swing. In this exercise, the DG is modeled with a pessimistic under-voltage 

tripping characteristic. The DG only maintains full current output down to 88% voltage. Below 

83%, all the current is stopped and is not allowed to recover. This is allowed under IEEE 1547, 

but normally a time delay of up to 2 seconds would be imposed before the DG is deliberately 

tripped.

The result of DG tripping during the power swing is to exacerbate the voltage stress and cause 

the system to lose synchronism, as shown in Figure 40. Other voltages are shown in Appendix

Figure 109. 

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Figure 40. COI destabilization due to DG tripping on voltage dip. 

5.2 Impact of Widespread Distributed Generation Tripping

The results presented above give rise to the question: what is the impact of tripping a large 

amount of small, widely distributed DG? How does that compare to the impact of a single large 

event such as the Palo Verde outage? A test was run in which a fraction of all the DG in the 

system was tripped, for a total amount approximately equal to the generation of the two units

tripped in the Palo Verde event.

In Figure 41, the system frequency plots show that the DG event results in a less severe 

frequency nadir and a better settling frequency. The difference is relatively small and is primarily 

due to two factors. First, the loss of DG partially starves the loads for locally generated power. 

Additional MW must be drawn from the grid, not only through the bulk power system but down 

through the distribution system to the load (see the composite load topology in Figure 10). The 

result is depressed voltages at the load buses, as can be seen in Figure 42. This causes the net 

load to drop in the DG case, as shown in Figure 43. The effect is especially pronounced in the 

California and DSW regions, where most of the DG resides. This load relief helps the system 

frequency. A second relatively minor effect is that the DG that trips is ultimately a few hundred 

MW smaller than the 2,756 MW of the Palo Verde event. This is partly due to voltage effects on 

the tripping logic in the composite load model, as can be observed in Appendix Figure 110. 

Broadly, the location of the generation tripping is not terribly significant compared to the amount 

of generation that is tripped. The impact on loads due to voltage change is a different face of the 

effect observed in other cases: namely, post-disturbance flows tend to be different and can have 

Pessimistic approximation to 

worst case 1547 UV tripping (88% 

and no delay) takes down WECC

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substantial impacts on load voltages. Dropping load voltage results in some beneficial load relief

due to load voltage sensitivity. Conversely, improved voltages can aggravate the frequency drop. 

This result tends to reinforce the conclusion that load voltage sensitivity is a more important 

consideration for FR than for load frequency sensitivity. 

Figure 41. Frequency response of LSP Hi-Mix case – DG trip vs. two Palo Verde unit trip. 

Trip of two Palo Verde units

Trip of equivalent DG 

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Figure 42. Voltage response of LSP Hi-Mix case – DG trip vs. two Palo Verde unit trip.

Figure 43. Load response of LSP Hi-Mix case – DG trip vs. two Palo Verde unit trip. 

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5.3 Extreme Generation Tripping

The FR results presented thus far have focused primarily on the loss of two Palo Verde NPS 

units, as this is the design-basis event for FR in WECC. Here, simulations on the LSP Hi-Mix 

case are presented showing the design-basis event (trip of two Palo Verde units) and a more 

severe event (trip of all three Palo Verde units). 

The WECC frequency for these two events is shown in Figure 44. Notice that the frequency 

nadir drops below 59.5 for the three-unit outage—which is the frequency at which UFLS would 

occur. The UFLS is deliberately not modeled here, so the relative performances can be 

compared. Some selected voltages are shown in Appendix Figure 115. Note that the decrease in 

frequency nadir is nearly proportional to the size of the event. The WECC FR response metrics, 

1,311 MW/0.1 Hz vs. 1,265 MW/0.1 Hz, are similar. This extends to the individual regions and 

areas, as shown in Appendix Table 47. The FR behavior for this substantially larger event is 

close to linear, showing only a slight degradation due to the larger-sized event and more 

governor controls saturating. This is reassuring from a robustness perspective, though obviously 

an event this severe will still cause UFLS-triggered interruptions, just as it does today.

Figure 44. Frequency response of LSP Hi-Mix to Palo Verde trip – two units vs. three units. 

5.3.1 Under-Frequency Tripping of Distributed Generation

The rather extreme frequency excursion shown for the trip of three Palo Verde units raises an 

interesting question about the deliberate or inadvertent under-frequency tripping of DG. 

In this sensitivity, all of the PV DG is modeled with high sensitivity to under-frequency. The 

settings for under-frequency trip modeling (as described in Section 2.2.3) are such that some DG 

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starts to drop out at 59.5 Hz and all of it is tripped by 59.4 Hz. This frequency tripping is 

instantaneous and latching. 

This representation is deliberately pessimistic. Inverter systems are relatively tolerant of smooth 

frequency deviations of this type. If this sensitivity is viewed in the context of tripping to meet 

compliance with IEEE 1547, the representation is also pessimistic in that there is no delay 

between hitting the tripping threshold and actual tripping.

The results are rather dramatic. A comparison of system frequency between the three-unit Palo 

Verde trip case (blue traces) and this sensitivity with DG tripping on under-frequency is shown 

in Figure 45. Appendix Figure 116 and Figure 119 show voltages and interface flows, 

respectively. When the grid frequency drops below 59.5 Hz, the resultant tripping of DG causes 

a further drop in frequency. While the system does stabilize below 59.2 Hz, there is no reason to 

think that the Western system would actually tolerate such a violent swing. As the reference case 

(blue trace) drops below 59.5 Hz for a number of seconds, under-frequency tripping would be 

likely even with the minimum 2-second time delay allowed by IEEE 1547. A much longer delay 

(e.g., >100 seconds) would have largely decoupled the anti-islanding tripping from the frequency 

excursion. This is uncertain territory. While UFLS action is allowed for a severe event like this, 

cascading failure is not. Consequently, widespread DG tripping for moderately severe frequency 

excursions is unwise. 

Figure 45. Frequency of LSP Hi-Mix to trip of three Palo Verde units – DG trip vs. no DG trip. 

5.4 Headroom Depletion Analysis

This investigation examines growing concern over emerging net load profiles with strong 

midday solar generation. These so-called “duck curves” first appeared in Europe, with net load 

profiles in Germany and Italy being particularly affected. The midday depression of net load 

presents some particular challenges to the system. Based on discussions with CAISO, there is 

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concern that commitment and dispatch to accept high solar can leave the system short of 

responsive resources in the evening when solar drops off and before wind picks up. 

A particular concern is that that FRO will not be met, especially for some individual BAs. The 

investigation presented in this section aims to develop a better understanding of the FR (and 

transient stability) consequences of possibly extreme measures for following the rapid net load 

rise.

5.4.1 Evening Net Load Rise and Duck Curves

In order to illustrate this concern, a single-day dispatch chart is shown in Figure 46 from April 20 

of the WWSIS-2 Hi-Mix PLEXOS runs. The contribution of the solar (both PV and CSP) is 

outlined in green to show the “duck” curve. 

The LSP Hi-Mix case was developed to investigate a high instantaneous penetration in the 

morning. The case has a total load of 111 GW, with wind and solar at 52.8 GW for 48% 

penetration. The total headroom is 21.9 GW. The wind and solar production was from a high 

wind and solar spring morning and is highlighted with the vertical blue line (7:50 on April 4, 

2020). 

Figure 46. LSP Hi-Mix representative dispatch showing "duck” curve. 

A key point here is that the wind and solar output, load, generation commitment, and dispatch in 

the morning are similar to those in late afternoon (shown with the vertical red line). That means 

that the LSP Hi-Mix condition is a good starting point to examine the impact of the relatively 

rapid drop in PV production that occurs in the afternoon. The same-day dispatch with no wind or 

solar is shown in Appendix Figure 118. For comparison, the HS and Extreme LSP cases are

shown in Appendix Figure 119 and Figure 120. 

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5.4.2 Case Development

A primary concern about the evening drop-off of solar production is that the committed units 

need to be dispatched up to compensate for the combinations of lost solar production and 

evening load rise. The timing makes it a challenge for system operators to commit more units. As 

a result, generation headroom tends to be used up rather than saved to meet FRO. This

investigation aims to examine the consequences of that headroom depletion. The investigation 

followed this sequence:

1. Hold commitment constant

2. Decrease solar PV production

3. Dispatch up committed units

4. Recalculate headroom, Kt, and other metrics

5. Run Palo Verde two-unit event

6. Observe frequency and calculate FR performance

7. Return to step 2 and repeat until solar PV is off. 

Freezing the commitment is a conservative approach. The objectives were to see how far the 

system can go without committing more generation, determine how much quick-start capability 

needs to be kept ready, and determine whether additional commitment and dispatch constraints

are needed to hold headroom. This also provides some insight into the consequences of making 

mistakes (e.g., due to poor forecasts). In the longer term, this type of investigation could drive 

changes in the generation portfolio in the future.

A sequence of cases is summarized in Table 18, with a shorthand name provided in the left 

column and notes on the re-dispatch and other changes needed to accommodate the drop in PV in 

the second column. As the table notes, re-dispatch to cover the drop in distributed PV was 

accomplished using units identified by the PLEXOS dispatch sensitivity. As the utility-scale PV 

dropped, other committed units were dispatched upwards. The only hydro that was changed was 

in the NW. 

More than 30 intermediate steps were required to develop the first three cases (Hi-Mix, DGoff, 

PVoff) shown in the table. Changing more than 13 GW of solar generation required trimming 

voltages, monitoring flows, and many minor adjustments to create satisfactory initial conditions 

for meaningful stability simulations. The final steps of the re-dispatch squeezed remaining online resources. 

In addition to the need to trim the system load flows, many steps were made in reducing the PV 

because it was the expectation that the system would indeed fail. As will be shown below, the 

system did not fail in the sense of losing stability or hitting UFLS. The FR relative to FRO 

dropped further, and in California the contribution of hydro, mostly in the PG&E area, was 

critical. This last point is important, and was the motivation for the additional five cases shown 

in the table. In the Hi-Mix, DGoff, and PVoff cases, there is 3.2 GW of headroom on the 

California hydro. The contribution of that hydro to FR in these cases is critical, especially for 

California’s FRO. In the additional cases in this sequence (e.g., case “1.2BL”), the California 

hydro headroom is removed by turning the governors to base load (BL), thereby removing their 

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under-frequency responsiveness. This is a modeling proxy for the possibility that these plants 

might not be able to provide extra power (e.g., because of lower water levels and gates wide 

open).

The impact of the re-dispatch on regional headroom is summarized in Figure 41. The three bars 

on the left in each chart are for the Hi-Mix, DGoff, and PVoff cases. The five bars to the right 

only affect California headroom. Note that the headroom in the eastern regions is relatively 

unaffected. This is because the generation available for re-dispatch under this condition is not 

responsive, so dispatching closer to maximum has no impact on FR. However, the headroom in 

the NW is reduced by about half, and the California headroom drops about 1 GW. Figure 121 in 

the Appendix shows Kt.

Table 18. Headroom Depletion Cases

Case Description 

Hi-Mix 2022 LSP Hi-Mix case

DGoff Starting from Hi-Mix case, DG is turned completely off (≈7.2 GW); based on 

PLEXOS results, units were dispatched up; hydro in NW (responsive), and coal 

and CC (mostly unresponsive). 

PVoff Starting from DGoff case, utility-scale PV is switched off (≈6.4 GW); pumped 

storage units switched to generating mode; hydro units in CA dispatched up; the 

rest of PLEXOS list units dispatched up. Phase shifters (BLGS PHA, RMRK PHA, 

and CROS PHA) adjusted and limit increased. Valmont capacitor bank added and 

BLACKWTR DC increased from 50 MW to 200 MW. 

1.2BL 1.2 GW of California hydro units with governor switched to base load units

1.8BL 1.8 GW of California hydro units with governor switched to base load units

2.1BL 2.1 GW of California hydro units with governor switched to base load units

2.8BL 2.8 GW of California hydro units with governor switched to base load units

3.2BL 3.2 GW (all) of California hydro units with governor switched to base load units

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Figure 47. Headroom by region for all headroom depletion cases.

5.4.3 Impact on Frequency Response

The impact of declining headroom on overall frequency is shown in Figure 48. The intermediate 

cases between PVoff and all CA hydro unresponsive (3.2BL) are not plotted to make the figure 

more legible. 

The impact of reduced headroom on the frequency nadir and settling frequency is substantial. 

However, the frequency does not drop below 59.5 Hz, where UFLS would act, even for the most 

extreme case with all CA hydro unresponsive. The nadir of 59.58 Hz gives some margin. The

regional frequencies, dynamics of the GR, and various network voltages and flows are shown in 

the Appendix starting with Figure 124. The dynamics of the loads are interesting: in the last case 

(3.2BL), there is several hundred MW of load response due to voltage changes that did not occur 

in the reference case. 

The overall WECC FR is shown in Figure 49. Note that WECC meets the IFRO, even with all 

PV off and no new generation committed (PVoff case). As the California hydro drops out, the 

WECC FR declines below the IFRO. At the regional level, the FR shown in Figure 50 illustrates

the decline associated with re-dispatch, with the major impact being in the NW as hydro 

dispatches up. It is interesting to note that the FR there decreases roughly in proportion to the 

drop in headroom. Figure 51 shows, as expected, that most of the decline in FR from disabling 

hydro in California occurs in the PG&E area.

~21GW 

dispatch

~26GW 

dispatch

~28GW 

dispatch

~12GW 

dispatch

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Figure 48. WECC frequency in response to two Palo Verde unit trip for all headroom depletion 

cases. 

Figure 49. WECC FR metric for two Palo Verde unit trip – all headroom depletion cases. 

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Figure 50. Regional FR metric for two Palo Verde unit trip – all headroom depletion cases. 

Figure 51. California FR metrics for two Palo Verde unit trip – all headroom depletion cases. 

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5.4.4 Headroom Depletion Summary

The rather extreme case of shutting down all PV generation in the West, while allowing no

additional generation to be committed, does not create a catastrophic failure in the system 

performance. No dramatic non-linearities in performance trends were observed as the PV 

dropped out (i.e., there was no cliff). Rather, the degradation of FR is steady and monotonic.

However, once the PV output has reached zero, the overall WECC FR is marginal, even with 

significant contribution from CA hydro. The FR for CA is below the approximate FRO. As the 

CA hydro becomes unresponsive, the overall WECC FR drops. WECC fails to meet the IFRO,

with about half of the CA hydro unresponsive. 

As the system re-dispatches, there are many local stress points (e.g., poor voltage) as PV output

drops. This suggests that the need to commit/recommit units could be driven as much by local 

constraints as by overall stability. The details will be important as the system stress builds. 

Further, there are minor local stress-related stability issues. For example, individual units were 

observed to have low dynamic voltage capability, and some units dispatched at their maximum

lost synchronism. This further suggests that locational issues may drive some constraints on FR. 

5.5 Effect of Unit De-Commitment on Base Case

In creating the LSP Hi-Mix case, there was an intermediate step in which only the distributed PV 

was added. This is a substantially different case, which embeds about 7 GW of distributionconnected PV with the load. 

To balance the added DG, thermal units were de-committed. Only de-commitment was used in 

this step; no units were dispatched down. The net result is a case in which Kt drops and 

headroom does not increase. The renewable profile and initial condition metrics are shown in 

Appendix Table 48. 

This is unrealistic in the sense that the economics are unlikely to cause only de-commitments. 

However, the stability results are of some interest. Therefore, a simulation of the two Palo Verde 

unit outage was run. Figure 52 shows the WECC frequency of this case (green trace) compared 

to that of the Base case (blue trace) and Hi-Mix case (red trace). De-commitment of governorresponsive units causes a slight degradation of frequency nadir. This case has poorer frequency 

performance than when the rest of the utility-scale renewables are added to create the Hi-Mix 

case. However, while the trends of this case are consistent with the preceding headroom 

depletion exercise, the correlation with headroom does not prove causality. Changes in flow 

patterns and other factors could also contribute. The result illustrates that specific details of 

commitment and dispatch impact overall system performance, as well as increasing renewable 

generation. 

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85

Figure 52. Effect of thermal plant displacement on frequency and headroom for LSP cases. 

5.6 CSP Sensitivity

About 10 GW of CSP capacity is added to the Hi-Mix cases. Recent development trends in the 

West appear to be shifting to utility-scale PV instead of CSP. The Hi-Mix cases therefore benefit 

from having this well-mannered CSP synchronous generation in Arizona and California, and the 

stability results for the Hi-Mix cases may be optimistic. This investigation explores the possible 

impact of replacing CSP synchronous generation with utility-scale PV. 

5.6.1 Converting CSP to Utility-Scale PV

The LSP Base case includes about 900 MW of CSP plants. The Hi-Mix case increases that by 

7.5 GW to about 8.4 GW. For this sensitivity, only 3.3 GW of CSP was retained in the Hi-Mix 

case. All other CSP plants were converted to PV. 

All the CSP in the LSP Base case was retained and matched to NREL's CSP database where 

possible.

2 Then, the NREL and other data were used to identify high-probability CSP plants that 

were under construction or fully financed. Five CSP plants (Ivanpah, Mojave, Tonopah, Genesis,

and Solana) were identified. New CSP plants in the Hi-Mix case that roughly aligned with these 

projects were retained. All other new CSP plants were converted to utility-scale PV. Several 

figures in the Appendix, starting with Figure 129, show this mapping. The net result was that 70 

of 81 plants totaling about 5,100 MW were converted from CSP to utility-scale PV.

 

2 Thanks for support from Mark Mehos at NREL for source material/CSP database. 

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86

5.6.2 Impacts of CSP and System Inertia on Frequency Response

The new CSP sensitivity case (green trace) is shown with the Base (blue trace) and Hi-Mix (red 

trace) cases in Figure 53. The impact on FR of conversion of the CSP to PV is directionally as 

expected, with the biggest impact on ROCOF: the PV has no inertia, and so loss of the inertia 

from the CSP would be expected to cause a more rapid initial frequency decline. The initial 

ROCOF, shown in detail in Figure 54, was 0.096 Hz/s in the LSP Base case, and 0.113 Hz/s with 

the Hi-Mix case. This is an 18% increase in ROCOF. For the CSP sensitivity case, the initial 

ROCOF rises further to 0.118 Hz/s; this is 4% lower than the Hi-Mix case and 23% lower than 

the Base case.

The impact of this increased ROCOF on the system stability is nearly invisible in terms of FR. 

Both the nadir and the settling frequency are essentially unchanged. These levels of ROCOF, on 

the order of 0.1 Hz/s, are quite small. Some smaller systems around the world have ROCOF 

concerns primarily driven by the use of ROCOF relays. In the United States, ROCOF relays are 

not in significant use. Similar, but less severe, results were observed for the HS case, as shown in 

Appendix Figure 131 and Figure 132. 

This exercise further reinforces the observation that for large U.S. interconnections, the loss of 

system inertia associated with increased wind and PV is of little consequence for up to 50% 

instantaneous levels of penetration.

Figure 53. Frequency response to trip of two Palo Verde units – LSP Base vs. Hi-Mix vs. CSP 

sensitivity.

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87

Figure 54. ROCOF response to trip of two Palo Verde units – LSP Base vs. Hi-Mix vs. CSP 

sensitivity. 

5.7 Light Spring Extreme Instantaneous Renewable Penetration

The LSP Hi-Mix case represents a system condition with high instantaneous wind and solar 

penetration likely to present operational challenges—especially for FR. The evolution of the 

commitment and dispatch of the generating resources from the Base case to the Hi-Mix case is 

described in detail in Section 2.5. The Hi-Mix condition was deliberately selected to be both 

challenging and representative, rather than an extreme outlier or the absolute worst possible 

condition. In this section and in Section 6, such an extreme condition is developed and analyzed. 

5.7.1 Dispatch and Commitment

The coal dispatch figure used to describe the process of changing commitment and dispatch on 

the coal plants is shown again in Figure 55 (this figure was first shown as Figure 9 and is 

described in detail in the accompanying text in that section). In the Hi-Mix case, a specific

sample of wind and solar generation near the middle of the cloud of green points was used to 

dispatch the wind and solar. That sample (# 31775 on April 4, 2020, at 7:50) had about 48 GW 

of wind and solar production, representing a point close to the mean of the sample space. In this 

version of the figure, the point with the absolute highest wind and solar generation of that 

sampling of 5-minute points is highlighted with a vertical green line. That point, with about 62

GW of wind and solar production (sample # 30692 on April 16, 2020, at 13:35), is used for this 

Extreme case. 

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88

Figure 55. Extreme renewable case within LSP coal dispatch samples. 

To develop this extreme case, the production of the wind and solar plants in the Hi-Mix case was 

changed (usually—but not always—increased) to the levels of the Extreme sample. The same 

relatively minor differences between the WWSIS-2 Hi-Mix renewable capacity and that in the 

final Hi-Mix case for this study were taken into account.

During the commitment and dispatch to accommodate the added wind and solar production, it 

became apparent that the difference in the system load level between this LSP case and the

WWSIS-2 PLEXOS model was important. Specifically, the load level for this case (about 

111 GW) is about 10 GW lower than the smallest load in any sample of the WWSIS-2 PLEXOS 

cases. This difference in load level was easily handled in the development of the Hi-Mix case

and resulted in a rather conservative initial condition, in the sense that the system dynamics 

would be more challenging than the WWSIS-2 system would suggest. However, for this extreme 

condition, it proved challenging to reduce generation by another 12 GW to accommodate more 

wind and solar. The net result is truly extreme for the 33% wind and solar energy scenario: 

extremely high wind and solar generation combined with extremely low system load.

The details of the difference between the Hi-Mix and Extreme PLEXOS samples are shown in 

Appendix Table 49. The totals for DG, PV, and wind each increase more than 3 GW. The CSP is 

essentially unchanged because CSP production is substantially less volatile, especially with the 

thermal energy storage assumed for these plants from WWSIS-2. The topological differences 

between the WWSIS-2 wind and solar capacity and the capacity in this case results in another

1 GW difference. Thus, the total additional wind and solar production is 12 GW. 

y = -0.2466x + 28909

y = 0.0249x + 20953

y = -0.4127x + 29324 y = 0.0591x + 2581.5

0

5000

10000

15000

20000

25000

30000

0 20000 40000 60000 80000 100000 120000 140000

Coal Sensitivity to W+S and Load

Coal TEPPC

Coal TEPPC

Coal Hi Mix

Coal Hi Mix

Linear (Coal TEPPC)

Linear (Coal TEPPC)

Linear (Coal Hi Mix)

Linear (Coal Hi Mix)

Mean: 20633, 

23821

Mean: 

48286,9397

Mean: 115217, 

23821

Mean: 

115301, 9397

WECC LSP ‘22: 

24418, 21911 

Base vs. W+S

Base vs. Load

Hi-Mix Points vs. W+S

Hi-Mix Points vs. Load

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89

The de-commitment and re-dispatch approach applied to the coal and CC plants was similar to 

that described in Section 2.5, but there were some differences. For the Hi-Mix case, changes in 

commitment and dispatch were made to units that the production simulation showed as likely to 

change with the added wind and solar. Because this case is extreme, a reasonable assumption 

was that any unit that PLEXOS showed as de-committed between the Base and Hi-Mix cases for 

any sample pair would be de-committed. Obviously, many units were de-committed to evolve 

from the Base case to the Hi-Mix case, so the remaining candidates had a total of 3,580 MW

available. This is only the first step in the process of displacing 12 GW of other generation to 

make room for the incremental wind and solar. Other CC plants that are in the WECC case but 

not in the WWSIS-2 PLEXOS data set (and were therefore invisible to the difference-based 

selection process) were de-committed. This provided about 5 GW of reduction. A further 2 GW

of hydro plants in the NW and in Canada were de-committed. Any peaking CTs that had been 

committed were retained, in order to reflect their likely participation in secondary FR or 

regulation. 

The net result is a dispatch and commitment that is credible for this condition but does not have 

the degree of economic rigor applied to the Hi-Mix cases. It is likely that the overall profile 

includes some deviations from normal practice. 

A summary of the regional dispatch is shown in Figure 56. For quick comparison, the three LSP 

cases have wind and solar totals of:

• ~25.6 GW in the Base case

• ~52.7 GW in the Hi-Mix case

• ~64.6 GW in the Extreme case. 

The WECC-wide and regional summary of the FR initial condition is given in Appendix Table 

50, with details given in Appendix Table 51. A key point is that the overall WECC Kt is 0.38. Kt 

was 0.42 in the Hi-Mix case. The regions to the east have Kt less than 0.30, and there is only 

1.0 GW of headroom in the Northeast. This is evidence of stress on the system.

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Figure 56. Wind and solar generation in the LSP Extreme case. 

The difference in commitment and dispatch from the Hi-Mix to the Extreme case results in some 

substantial changes in flow patterns—and equally important, the removal (by de-commitment) of 

important sources of voltage support. In order to obtain successful load flow solutions and 

achieve reasonable voltage profiles, the process of increasing the wind and solar production and 

reducing the other generation was carried out in many relatively small steps. In the intermediate 

steps, the displaced generation was typically dispatched down to zero, but left in service to 

regulate voltage to get an interim load flow solution. Then, the displaced generation was decommitted, thereby eliminating voltage support. At each step of the way, various locations 

experienced poor voltages. This was especially true in the East and in more geographically

distributed parts of the system. In a few locations, transformers (e.g., between 138 kV and 

230 kV) experienced overloads. These local voltage problems were generally addressed by the 

addition of shunt capacitors, and in a few cases by adding a parallel transformer. Appendix 

Wind 4.5

PV 7.5

DG CSP 2.2

5.7

Others 

12.4

Wind 

8.3

PV 0.2

CSP 0.0

DG 0.1

Others 

9.3

Wind 7.0

PV 0.7

CSP 0.0

DG 0.7

Others 

4.1

Wind 10.9

PV 5.1

CSP

6.1

DG 3.8

Others 9.1

Production/Dispatch in GW

WECC California DSW Northeast NW

Wind (GW) 32.4 4.5 10.9 7.0 8.3

PV (GW) 13.5 7.5 5.1 0.7 0.2

CSP (GW) 8.3 2.2 6.1 0.0 0.0

Distributed PV (GW) 10.4 5.7 3.8 0.7 0.1

Others (GW) 56.6 12.4 9.1 4.1 9.3

Total (GW) 121.2 32.3 35.1 12.6 17.9

Penetration (%) 53% 62% 74% 67% 48%

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91

Figure 133 gives a sense of the areas and sequence of these additions. It should be emphasized 

that these reinforcements are not the result of the rigorous process that would be needed to 

identify robust and economic mitigation of local voltage and thermal problems. Rather, they are 

reinforcements to allow reasonable initial conditions for the transient stability and FR

investigations. In order to correct these types of problems—which are inevitable under these

extreme conditions—much more detail of individual wind and solar projects, and of local 

transmission, is needed. The basic system engineering required to accommodate a high wind and 

solar build-out is well established and is not in the scope of this work. Appendix Figure 134

shows the bubble diagram for the Extreme case. 

5.7.2 Path Loading

As noted above, the re-dispatch to achieve the wind and solar penetration of the Extreme case 

was challenging. Table 19 shows a comparison of the loadings on some key paths in the Western 

Interconnection. In the Extreme case, a few of the paths are over their catalog ratings; these are 

highlighted in red. These paths are inside California and in the vicinity of particularly highdensity wind in eastern Wyoming and Colorado. Even though the results presented to this point 

in the study have not identified radical changes in system dynamics, the acute changes in system 

flow patterns suggest that rigorous analytical checks of the events that dictate the path ratings 

(and any new events) are warranted. Certainly, all the regional paths in the high wind parts of the 

eastern part of the Western Interconnection will change as transmission is added to accommodate 

the new plants.

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Table 19. Path Loadings in LSP Cases

Path 

Number Path Name

Path Loading (MW)

Path Rating 

(MW)

LSP 

Base

LSP

Hi-Mix

LSP

Extreme

10 West of Colstrip 2,025 297 -193 2,598

15 Midway-Los Banos 1,467 4,545 5,997 4,800–5,400

22 Southwest of Four Corners 1,829 -339 485 2,325

26 Northern-Southern California 1,140 -2,654 -4,181 4,000

30 TOT 1A 414 154 875 650

37 TOT 4A 357 259 1,088 810

43 North of San Onofre 664 1,018 -682 2,440

46 West of Colorado River (WOR) 4,204 7,126 7,365 10,623

48 Northern New Mexico (NM2) -18 -357 -1,790 -1,970

49 East of Colorado River (EOR) 3,100 3,588 4,062 9,300

66 California-Oregon Interface 

(COI)

2,346 -267 -1,593 4,800/-3,675

5.7.3 Frequency Response

A comparison of the WECC frequency for the new Extreme case (red trace) to the Base (blue 

trace) and Hi-Mix (green trace) results for the Palo Verde event are shown in Figure 57. The 

frequency nadir decreases to 59.613 Hz, and the FR decreases to 1,055 MW. 

Note that even for this extreme condition, the frequency nadir is above UFLS and the WECCwide FR meets NERC criteria. The summary of individual area FR in Table 21 shows that about 

half of the areas are deficient in terms of meeting their estimated FRO with reserves within their 

areas. The DSW region has less than half of the approximate FRO. Appendix Figure 135 shows 

the regional headroom being dynamically depleted. Appendix Figure 136 shows the governorresponsive units saturating. The impact is incremental, and in many regards it looks similar to the 

headroom depletion cases presented in Section 5.4. 

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Figure 57. Frequency response to two Palo Verde unit trip – LSP Base vs. Hi-Mix vs. Extreme. 

Table 20. LSP FR Metrics Including Extreme Case

LSP

Base Hi-Mix Extreme

Generation (GW) 204 204 204

Load (GW) 185 185 185

FRO (MW/0.1 Hz) 840 840 840

FR (MW/0.1 Hz) 1352 1311 1055

FR Margin (MW/0.1 Hz) 512 471 215

Frequency Nadir (Hz) 59.668 59.646 59.613

Nadir Time (s) 7.77 7.19 7.38

Settling Frequency 

(Hz)

59.839 59.844 59.814

Kt 0.46 0.42 0.38

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94

Table 21. WECC, Region, and Area FR Metrics for LSP Extreme Case

LSP Base Case LSP Extreme Case

FRO FR FR Margin FR FR FR Margin FR

Name (MW/

0.1 Hz)

(MW/

0.1 Hz)

(MW/

0.1 Hz)

% of 

WECC

Kt (MW/

0.1 Hz)

(MW/

0.1 Hz)

% of 

WECC

Kt

WECC 840 1,352 512 100.0 0.46 1,055 214.7 100 0.38

By Region

CALIFORNIA 296 305 9 22.6 0.34 295 -0.6 28 0.32

DSW 220 215 -5 15.9 0.44 97 -123.4 9 0.22

NORTHEAST 82 61 -20 4.5 0.26 51 -30.8 5 0.27

NORTHWEST 131 434 303 32.1 0.62 280 149.5 27 0.53

By Area

ARIZONA 104 69 -35 5.1 0.37 45 -59.7 4 0.27

EL PASO 9 4 -5 0.3 0.42 3 -5.6 0 0.32

IDAHO 18 21 3 1.6 0.38 21 3.2 2 0.68

IMPERIALCA 4 14 10 1.0 0.24 14 10.2 1 0.18

LADWP 29 31 1 2.3 0.60 29 -0.1 3 0.33

MONTANA 11 10 -1 0.7 0.17 11 -0.3 1 0.16

NEVADA 28 44 16 3.2 0.72 19 -9.3 2 0.45

NEW MEXICO 14 50 35 3.7 0.49 2 -12.7 0 0.01

PACE 42 23 -19 1.7 0.22 11 -30.5 1 0.13

PG AND E 133 190 57 14.1 0.40 189 55.8 18 0.47

PSCOLORADO 36 -14 -50 -1.1 0.34 6 -29.8 1 0.25

SANDIEGO 21 7 -14 0.5 0.15 7 -14.4 1 0.16

SIERRA 11 7 -3 0.6 0.23 7 -3.2 1 0.21

SOCALIF 108 63 -45 4.7 0.23 56 -52.3 5 0.19

WAPA R.M. 27 63 35 4.6 0.67 23 -4.5 2 0.12

WAPA U.M. 0 3 3 0.2 0.94 3 2.5 0 0.61

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95

6 Coal Displacement/Retirement and Weak Grid 

Concerns

The growth of renewable generation in the West, coupled with other economic and societal 

factors, raises the possibility of retirement of coal generation. Consideration of all the factors 

involved is outside of the scope of WWSIS-3. However, the displacement of coal, as discussed in 

the previous section and illustrated in Figure 55, provides an opportunity to investigate the 

transient stability implications of not only temporary de-commitment of coal generation but also 

the impact of possible coal plant retirements. The specific point is that evaluation of transient 

stability is always based on snapshots of operation. It matters little whether a specific resource, 

including a coal plant, is off-line because it was de-committed for this particular operating 

condition or because it was retired. Aside from the possibility of changes in overall system 

commitment and dispatch associated with a retirement, the system dynamics will be the same. In 

short, any coal plants de-committed in a snapshot power flow could be considered retired.

This section explores the impact of this on system performance, with particular attention to 

changes to transient stability. 

6.1 Coal Displacement by Wind and Solar Generation

As described above, the evolution from the LSP Base case to the LSP Hi-Mix case and then to 

the LSP Hi-Mix Extreme case displaces thermal generation, especially in the Northeast and 

DSW regions. 

The following sequence of figures illustrates the changes in dispatch with increasing wind and 

solar production. Figure 58 shows the regional dispatches by generation type for all five cases. 

Because the distributed PV appears as a load modifier, it is not included in these tables. This 

reflects the reality facing the grid operators, as the embedded DG will likely remain largely 

outside of operator controls, and absent monitoring and estimation systems may be nearly 

invisible to operators as well. The differences between the HS cases, while substantial, are much 

less dramatic than for the LSP cases. The next plot, Figure 59, shows the same information, but 

only for the LSP cases in the DSW and Northeast regions. This figure shows the dramatic 

reduction in coal dispatch (dark red bar) as the wind and solar generation increase from the Base 

case to the Hi-Mix case, and then a further drop in the Extreme case. Figure 60 and Figure 61

show a further breakdown for those two regions, with the individual area dispatch plotted. The 

differences are substantial. For example, essentially 100% of the coal generation in Idaho, 

Montana, and Arizona is off-line in the Extreme case. By comparison, coal is a relatively minor 

component of the change in dispatch in California (shown in Figure 62). 

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96

Figure 58. Regional generation dispatch for LSP and HS cases (no DG shown). 

Figure 59. Coal displacement in Desert Southwest and Northeast regions for LSP cases. 

-10000

0

10000

20000

30000

40000

50000

60000

70000

HS Base

HS HiMix

LSP Base

LSP HiMix

LSP HiMixXtrm

HS Base

HS HiMix

LSP Base

LSP HiMix

LSP HiMixXtrm

HS Base

HS HiMix

LSP Base

LSP HiMix

LSP HiMixXtrm

HS Base

HS HiMix

LSP Base

LSP HiMix

LSP HiMixXtrm

California DSW NorthEast PNW

WIND

Steam

PV

PSH

Other

NUC

HYDRO

GEO

GasCT

CSP

Coal

CCPP

Bio

0

5000

10000

15000

20000

25000

30000

35000

LSP Base LSP HiMix LSP HiMixXtrm LSP Base LSP HiMix LSP HiMixXtrm

DSW NorthEast

WIND

Steam

PV

PSH

Other

NUC

HYDRO

GEO

GasCT

CSP

Coal

CCPP

Bio

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97

Figure 60. Coal displacement in Desert Southwest areas for LSP cases. 

Figure 61. Coal displacement in the Northeast areas for LSP cases. 

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

ARIZONA EL PASO NEVADA NEW MEXICO PSCOLORADO WAPA R.M.

DSW

WIND

Steam

PV

PSH

Other

NUC

HYDRO

GEO

GasCT

CSP

Coal

CCPP

Bio

0

1000

2000

3000

4000

5000

6000

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

IDAHO MONTANA PACE SIERRA

NorthEast

WIND

Steam

PV

PSH

Other

NUC

HYDRO

GEO

GasCT

CSP

Coal

CCPP

Bio

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98

Figure 62. Change in dispatch in the California areas for LSP cases. 

6.2 Northeast Region Stability Analysis

A major new 500 kV transmission project across the Northeast portion of the Western 

Interconnection is included in the WECC planning databases. It terminates in Eastern Wyoming 

at a new 500/230 kV substation named Aeolus. This project is currently under construction, and 

what is ultimately built may not match what was included in the data set. 

The new project provides a significant path for power from both thermal and variable renewable 

generation in Eastern Wyoming to be exported toward the western part of the system. The flow 

on the two 500 kV lines out of Aeolus is 1,159 MW, 1,051.7 MW, and 1,787 MW for the LSP, 

LSP Hi-Mix and LSP Hi-Mix Extreme cases, respectively. A disturbance on the new 500 kV

system thus presents a new and potentially significant stress to the grid. This section examines a 

four-cycle, three-phase to ground fault at the Aeolus 500 kV bus, cleared by tripping one of the 

500 kV circuits to Anticline. A segment of the WECC planned transmission additions map3

 is 

shown in Figure 63 below, with a few added notations. The new Aeolus bus is called out, and the 

approximate location of the fault is indicated. Other elements discussed below are also 

highlighted. 

 

3 http://www.wecc.biz/committees/StandingCommittees/PCC/Shared%20Documents/2014-

10yrmap%20Model%20(1).pdf

-2000

0

2000

4000

6000

8000

10000

12000

14000

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

IMPERIALCA LADWP PG AND E SANDIEGO SOCALIF

California

WIND

Steam

PV

PSH

Other

NUC

HYDRO

GEO

GasCT

CSP

Coal

CCPP

Bio

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99

Figure 63. Planned transmission map of Aeolus 500 kV vicinity. 

The dispatch summaries presented in the previous section show that the majority of coal 

generation in the eastern regions in the LSP Hi-Mix and Hi-Mix Extreme cases is off-line. This 

represents a radically different operating condition than the historic norm in which many of the 

coal plants in the region are base-loaded even under light load conditions. The difference in 

dynamic response to large disturbances in the vicinity has the potential to be substantial. 

Figure 64 shows the voltage at Aeolus for the three system conditions in response to the Aeolus 

fault. The Base case (blue trace) and Hi-Mix case (red trace) show an acceptable recovery from 

the fault. The traces for these two cases included some numerical noise, which has been filtered 

out here and in some of the following plots. The Hi-Mix Extreme case (green trace) fails to 

recover. The simulation is meaningless after that point, so the plot was truncated. 

LANGE

Aeolus 

500kV

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Figure 64. Aeolus bus voltage in response to Aeolus fault – LSP Base vs. Hi-Mix vs. Extreme. 

The voltage behavior during the fault is of considerable interest in this case. In Figure 65, the 

voltages during the fault are shown at several 230 kV buses near and to the north of the Aeolus 

bus. These buses, except Windstar, are visible in Figure 63. At every node, the voltage drops as 

the fault is applied, then continues to decay. This decay is rapid, but not uniform. Notice that the 

voltages of the more northerly buses at Wyodak and Lookout decay faster and reach zero before 

the fault is cleared. This suggests that the system around Rapid City, South Dakota, cannot 

export all the wind power, resulting in a localized system separation. The fact that the voltages 

are better at Dave Johnson and Windstar, which are electrically closer to the fault, is significant. 

This is evidence that the collapse is driven by resources between the point of voltage collapse 

and the actual fault. The system dynamics in this part of the system are dominated by the wind 

plants. The system voltage behavior is also quite ill-mannered because of the composite load 

model dynamics and the fact that they do not allow motor tripping. In that sense, these

simulations are similar to those presented in Section 5.1 and are almost certainly pessimistic. 

However, from a comparative performance perspective, the poor voltages at Wyodak and Dave 

Johnson are undoubtedly exacerbated by the fact that all the units at those two plants are off-line 

in this case. The question of whether system separation might be avoided by tripping motor load, 

rather than letting it stall, is examined below.

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Figure 65. Bus voltages in response to Aeolus fault – LSP Base vs. Hi-Mix vs. Extreme. 

The impact on the remaining relatively small synchronous machines in the area can be observed 

in Figure 66, which shows the machine speed of the Lange CT (highlighted in Figure 63). The 

acceleration of Lange during the fault is dramatically higher for the Hi-Mix case (which it 

survives) and even more so for the Extreme case (which is a failure).

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Figure 66. Lange CT speed in response to Aeolus fault – LSP Base vs. Hi-Mix vs. Extreme. 

The observation that the stability problem is primarily due to acceleration during the fault is 

reinforced by the simulation captured in Figure 67. The red trace shows the 500 kV line opened 

without an initiating fault. The line-open case is well behaved, indicating that the stability 

problem is substantially due to the voltage depression during the fault. The voltage depression 

has multiple effects, including reducing synchronizing strength, driving the complex load 

towards motor stall, and aggravating swings of the Type 1 wind turbines in the vicinity. This was 

explored further by experimenting with different fault impedances. The results confirm that as 

the fault becomes progressively more severe, with decreasing fault impedance as a proxy for 

severity, the stability degrades. The stability limit without any mitigation is for a fault impedance 

between 0.05 pu and 0.06 pu. 

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Figure 67. Aeolus bus voltage response – Aeolus line trip with and without bus fault. 

6.3 Coal Plant Conversion to Synchronous Condensers

The Northeast portion of the Western Interconnection evolved with the expectation that the large 

coal plants would be base-loaded, providing an anchoring effect on system voltage. The poor 

voltage behavior for the Extreme case suggests a need for additional system strength. For this 

experiment, three of the de-committed coal plants were converted to synchronous condensers as 

follows:

• Dave Johnson, unit 4 – 400 MVA (65445 DAVEJON4)

• Wyodak, unit 1 – 402.3 MVA (66730 WYODAK 1)

• Laramie River, unit 1 – 690 MVA (73129 MBPP-1). 

In order to convert these machines to synchronous condensers in the model, the load flow is 

changed so that the unit status is on and dispatch is set to 0 MW. This causes the unit to regulate 

voltage and initialize to a non-zero reactive power. In the dynamic model, the governor is 

removed, and the inertia constant is changed to 1.0 pu. This is an estimate based on the need to 

remove the turbine. For steam plants, conversion to synchronous condenser operation is a 

significant and irreversible step. Thus, this conversion only makes sense for a unit that is to be 

permanently retired. There are several such conversions recently completed or underway in the 

United States, including units at the Cherokee plant in Colorado, the Eastlake plant in Ohio, and 

the Turkey Point plant in Florida.

The voltage depression further suggests that additional dynamic response from the generation 

that is synchronized in the case could be beneficial. Thus, in addition to the three synchronous 

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condensers, shunt capacitors (beyond those discussed in Section 5.7.1) were added to free up 

dynamic range on generation as follows:

65420 DAVEJOHN 230 kV 100 MVAr

65425 DAVEJOHN 115 kV 100 MVAr

70122 COMANCHE 230 kV 60 MVAr

70821 CEDARCRK 230 kV 150 MVAr 

76400 PUMPKIN BTS 230 kV 100 MVAr

In Figure 68, the Dave Johnson 230 kV bus voltage is shown for the three LSP cases plus the 

new sensitivity case with synchronous condensers (pink trace). With the condensers added, the

Extreme system is now stable. 

Figure 68. Dave Johnson bus voltage for Aeolus fault – LSP Base vs. Hi-Mix vs. Extreme vs. 

Extreme with synchronous condenser conversion. 

In Figure 69, the same bus voltages that were displayed in Figure 65 are shown for the Extreme 

case, with and without the synchronous condensers added. Notice that the rapid decay of voltage 

during the fault is greatly improved by the addition of the condensers. The system recovers in an 

orderly fashion when the fault is cleared.

The behavior of the Dave Johnson generator unit 4 is shown in Figure 70. Two cases are plotted: 

the LSP Base case, in which the unit is committed and running as a generator, and the new 

Extreme case with the synchronous condenser conversions. Recall that for the other two cases 

(Hi-Mix and Extreme), the unit is off-line. The initial and average active power of the 

synchronous condenser are, of course, zero. The active power swings as the machine inertia 

interacts with the rest of the system dynamics. The reactive power output swings following the 

Light Spring Base

Light Spring Hi-Mix 

Light Spring Extreme

Light Spring Extreme 

with synchronous 

condenser conversion

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fault clearing. The frequency of the reactive power swings in the first half-second after fault 

clearing is relatively high. This is partly due to the reduced inertia of the synchronous condenser, 

but closer inspection of the simulation results suggests that this rather fast swing in post-fault 

voltage is primarily due to the dynamics of nearly 1 GW of Type 1 induction generator wind 

turbines in the vicinity. A few spot checks suggest that this oscillation is quite sensitive to the 

model for these wind plants. Further, the tripping behavior of these machines could be an 

important stability consideration for this and similar events. With most of the large thermal units 

de-committed, the dynamic characteristics of the wind plants become important. This is a 

significant observation; extra care will likely be needed in the future to ensure that dynamic 

models of previously relatively unimportant plants are up to WECC standards for accuracy.

Another important observation is that the system dynamics and the support provided by the 

synchronous condensers during the fault are the critical element in maintaining system stability.

The support from electromagnetic synchronous devices (i.e., generators and synchronous 

condensers) is instantaneous and inherent to the equipment. This is evident in the difference in 

the amplitude of the voltage drop immediately following the application of the fault. This 

inherent response applies for real-world complexities that accompany violent grid faults: 

unbalance, distortion, saturation, etc. Creating functionally similar performance with powerelectronics-enabled devices is challenging. The voltage collapse of these simulations occurs 

within about two cycles of fundamental frequency (i.e., about 32 msec)—an extraordinarily short 

timeframe from the perspective of closed-loop controls. The design and verification of power 

electronics devices in this timeframe and for this type of disturbance requires analytical tools 

other than standard fundamental frequency, positive sequence, and dynamic stability programs 

like PSLF or Power System Simulator for Engineering (PSS/e). This is an important aspect of the 

“weak grid” concerns discussed next.

Figure 69. Bus voltages in response to Aeolus fault for LSP Extreme case – with and without 

synchronous condenser conversion. 

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Figure 70. Real and reactive power output from Dave Johnson unit 4 in response to Aeolus fault – 

LSP Base vs. Extreme with synchronous condenser conversion. 

6.4 Weak Grid Discussion 

The rapid voltage collapse and system separation observed during the fault described above is 

representative of so-called “weak grid” issues. Systems at or approaching very high levels of 

wind penetration, such as those in west Texas, Australia, and Brazil, are challenged to provide

fast, confident control during faults as wind and solar generation becomes the dominant 

generation resource in a particular portion of the grid. 

Modern wind turbines and all PV systems depend on power electronics that must behave well 

during grid disturbances. Until recently, this concern has primarily focused on fault ride-through 

capability. The wind industry has largely addressed the ride-through questions, and the solar

industry is following suit. (Questions of PV ride-through in distribution applications are

discussed in Section 5.1.) However, no commercially available wind or utility-scale solar PV 

generation is capable of operation in a system without the stabilizing benefit of synchronous 

machines. There is a point at which the amount of short-circuit strength provided by synchronous 

machines becomes insufficient and operation becomes impossible with known, current 

technology. "Weak grid" is a generic term that describes operating near that point. The factors 

that dictate that point are complex, evolving, and not fully understood. The rest of this discussion 

addresses some aspects revealed by this study work. 

6.4.1 Sensitivity to WTG Models and Controls

The behavior observed in these study cases is representative of these "weak grid" concerns. The 

failure of the Extreme case is largely due to the forced injection of active power current into a 

part of the system that cannot accept the power. There are two factors at play in this 

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phenomenon: modeling and equipment controls. Both models and actual controls of WTGs have 

been advancing and evolving for the past decade. The model used in these cases is the result of 

this evolution and is a compromise between model simplicity, numerical behavior, accuracy, and 

functionality. In general, development of the model tended to be conservative, with the model 

structured to underestimate desirable performance. The intent is to help ensure that planning 

decisions based on simulations are conservative with respect to system reliability. Further, the 

models evolved for the applications most commonly under investigation by planners that can be 

meaningfully evaluated with fundamental frequency stability simulation tools. 

Stable and reliable operation of WTGs in weak grids requires dedicated designs of various 

controls. Some of these controls have bandwidths of more than a few cycles and can be reflected 

accurately in stability simulation tools (e.g., PSLF, PSS/e). There are also control loops in WTGs 

with sub-cycle bandwidth that could cause instabilities and trips if not properly designed or 

adjusted. Detailed representation of these very fast control loops is outside the realm of stability 

simulation tools. Hence, the simulation work described here addresses important interaction 

aspects of WTGs and the system within the bandwidth allowed by a stability tool. In specific 

wind plant applications, confirmation from the relevant vendors that designs are compatible for 

weak grid conditions may be required to complement the simulation work.

This philosophy applies to the next sensitivity case, in which all of the new WTGs have been 

changed from the part-Thevenin model used for Type 3 machines to the pure current injection 

model used for Type 4 machines. In this sensitivity case, the system is unstable at initialization, 

with no grid disturbance applied. Roughly 5 Hz oscillations start spontaneously shortly after the 

simulation begins. Appendix Figure 138 shows a comparison of the Extreme case with the 

synchronous condenser fix for the two different WTG models. 

Note the choice of words here: the pure current source model is initially unstable. This is almost 

certainly a modeling issue, and not evidence that Type 4 WTGs are necessarily less stable than 

Type 3 machines. However, this behavior is indicative that the system is weak.

To further investigate sensitivity to wind plant controls, another variation of the wind turbine 

model was tested. This model was updated to be more realistic, i.e., less pessimistic, and to 

reflect recent improvements in turbine controls for weak grids. Figure 65 shows system response 

to the Aeolus fault with the synchronous condenser fixes for two cases. The blue trace is for the 

Extreme case with the standard model. The red trace is for the new model and controls. Notice 

that the severity of the dynamic voltage decline during the fault is dramatically reduced. This 

rather encouraging result is further evidence that the controls and models of the WTGs are 

critical in this high-stress condition.

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Figure 71. Impact of weak grid WTG controls for Aeolus fault. 

6.4.2 Sensitivity to Load Models

Additional investigations were made on the impact of load modeling on the Extreme case failure. 

A case with the standard WECC load model was stable and did not exhibit the dynamic voltage 

collapse and separation. However, a case in which the default composite load motor tripping

settings were restored, allowing the motors to trip as voltage collapses, still fails. Had the Type 1 

WTGs tripped, it is possible that the system would survive the event.

6.4.3 Sensitivity to Non-Synchronous Generation

The weak grid investigation is representative of growing industry concern about operating with a

low fraction of total generation coming from synchronous generation. One leader in this area is 

Ireland. The Irish grid operator EirGrid monitors SNSP, or “system non-synchronous 

penetration” (EirGrid 2011a). For EirGrid, non-synchronous resources consist primarily of wind 

generation and their large HVDC interconnections with Great Britain. Ireland is on a trajectory 

to have the highest annual energy penetration of wind power of any major power system in the 

world. At 40% (EirGrid 2014), Ireland's target exceeds the 33% energy penetration for this 

study. In order to reach these annual energy targets, the instantaneous penetration of wind 

generation will occasionally need to be very high. However, concerns about FR and a weak grid 

resulted in SNSP being limited to less than 50%. In the near-term future, EirGrid expects to raise 

that limit to 75%. In other systems, including those in west Texas, Brazil, and Australia, weak 

grid concerns have also emerged.

The bar charts of Figure 72 show the mix of synchronous and non-synchronous generation for 

each of the four regions and for the five main cases. The y-axis is not dispatch (MW), but the 

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capacity, in MVA, of the committed resources. A primary weak grid concern is the relative size 

of the electronic devices that depend on the stabilizing influence of the synchronous resources.

Note that added condensers have been shown in green. These would increase the blue bar.

Figure 73 shows exactly the same information, but displayed as the relative fraction of 

synchronous and non-synchronous (inverter) resources. The red bars (inverters) are analogous to 

the EirGrid SNSP metric.

Figure 72. Synchronous vs. non-synchronous generation commitment in MVA. 

It is interesting to note that in the Northeast, where the Aeolus disturbance is investigated, the 

system is well behaved at SNSP (red bar) of about 56% in the LSP Hi-Mix case, but shows poor 

behavior at ~66% SNSP for the LSP Extreme case. Further, the SNSP improves to about 61% 

with 1,600 MVA (green bar) of condensers added, and the system behavior greatly improves.

condensers

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Figure 73. Synchronous vs. non-synchronous generation commitment in percent.

6.4.4 Fault Current Levels

Short-circuit levels decline significantly in the 230 kV system when the majority of coal 

generation is de-committed, as shown in Table 22. The lower fault levels are related to the 

increased electrical distance between the wind generator and synchronous generator discussed in 

previous sub-sections. The industry often uses indicators based on the ratio of fault levels to 

rating of power electronic generation to quantify how weak the grid is for an application or for a 

region with several wind plants. There are variations on where to calculate the short-circuit 

power and how to aggregate multiple wind plants. As a reference, the short-circuit levels at Dave 

Johnson and Populus for the base and the extreme scenarios are shown in the table below. These 

values are calculated without considering the contributions from the wind plants, as a measure of 

the system strength seen by the wind plants. The difference between scenarios is significant. 

Review of system protection is recommended.to ensure proper behavior at reduced short-circuit 

levels 

Table 22. Fault Current Levels

Location LSP Base LSP Hi-Mix Extreme

Dave Johnson 230 kV 19.7 kA/7,730 MVA 10.7 kA/4,260 MVA

Populus 345 25.1 kA/15,000 MVA 20.1 kA/12,000 MVA

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6.5 Coal Displacement and Weak Grid Summary

A high de-commitment of coal does not obviously over-stress the system. A greater than 80% 

reduction of coal commitment from the LSP Base case in the Northeast region exhibited 

acceptable dynamic behavior for the limited tests performed.

The regional transmission system evolved with the coal plant at “anchor” nodes. Displacement 

by wind and solar that is more dispersed results in those nodes being less supported. 

Consequently, increasing wind and solar dispatch, and de-committing and re-dispatching thermal 

generation, resulted in localized voltage and thermal problems. These problems were particularly 

acute in the evolution from the Hi-Mix case to the Extreme case. These problems were “fixed” 

mainly with added shunt capacitors and a few 230 kV line upgrades or transformer additions

(i.e., fixed in the sense that the results were satisfactory for the conditions investigated, but 

questions of voltage stability and “brittleness” need closer investigation). These improvements 

are not in any sense optimized, which would require much more precise information on the 

specifics of individual projects. The solutions used are relatively straightforward reinforcements.

Ohm's Law still applies, and good planning practices need to be followed.

System dynamics for the extra-high-voltage Aeolus fault were stable for the Hi-Mix case with 

about an 80% reduction in coal commitment from the LSP Base case. The extreme decommitment further stresses the system, with a greater than 90% reduction of coal commitment. 

Additional shunt compensation and some rotating dynamic reactive compensation stabilized the 

Aeolus fault. Conversion of some coal plants to synchronous condensers (assuming retirement) 

works well to stabilize the system. Improved wind and solar plant controls may be sufficient to 

mitigate problems with extreme coal de-commitment.

Results suggest that substantial displacement of coal can likely be tolerated, with the application 

of good utility practice for local transmission mitigation. The last few steps (i.e., de-committing 

the final 1–2 GW in the eastern half of the Western Interconnection), while possible, will need to 

be carefully examined. 

Results suggest that when wind and solar are the predominant source of generation throughout 

the region, care to use good-fidelity dynamic models is critical. WECC has a longstanding best 

practice to keep dynamic models up to date. Wind and solar plant modeling need to be held to 

the same level of accountability in a very-high-penetration future. The fidelity of the load model 

assumes increased importance with the increase in power-electronics-based generating resources.

This single sequence of sensitivities is illuminating, but in no way complete or conclusive. More 

analysis is needed, but in summary, these results suggest:

• Displacement of 80% of the coal generation is possible. 

• Displacement of 90% of the coal generation is possible with some mitigation. 

• Good-fidelity dynamic models are critical. 

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7 Mitigation Analysis

WWSIS-3 investigated mitigation strategies for any adverse impact on transient stability and FR 

due to high levels of wind and solar. With the exception of the Extreme case, the results showed 

no loss of stability or other fundamental changes in system behavior due to high wind and solar 

penetration. Therefore, this investigation focuses on the potential performance improvement 

achievable with various means of mitigation, rather than mitigation per se. While individual 

areas do not meet their NERC FRO, the current NERC rules do not mandate that each entity 

meet their obligation with their own resources. Depending on how practice and rules evolve, and 

as long as the overall system meets the IFRO and other performance criteria, the results so far do 

not require mitigation of individual BA FR shortfalls with physical (as opposed to contractual) 

means.

However, system performance does change with high wind and solar penetration. This section 

presents a variety of investigations that could broadly be characterized as mitigation, in that they 

tend to improve system performance. These investigations are focused on changes to the 

renewables or to the system that might affect the findings reported above. In general, the 

investigations reported here look at the same initial conditions and events, with changes in the 

assumptions about the renewables or other infrastructure. 

7.1 Frequency Controls on Wind Plants

The use of frequency-sensitive controls on wind plants is the subject of considerable interest in 

the United States and has been analyzed in WECC and California. Requirements for this 

functionality started in Ireland, but are rapidly gaining traction in North America. The Electric 

Reliability Council of Texas (ERCOT 2010) and the Alberta Electric System Operator (AESO 

2010), at least, require wind plants to have primary response (governor) functionality. HydroQuebec continues to be the only North American system that requires inertial controls, but again,

considerable interest is growing.

In this section, the two Hi-Mix cases (LSP Hi-Mix and HS Hi-Mix) are tested with the primary 

frequency control or governor function (active power control [APC] is the GE PSLF 

implementation of this function), inertial control (WindINERTIA [WI] is the GE PSLF

implementation of this function), and a combination of both (Miller 2011a, Miller 2009). 

The load flow and initial dispatch are identical in all cases. For the case with the governor 

function, 5% of each new wind plant’s output is held for primary FR. This assumes a higher 

wind speed than in the original cases for plants operating at less than 95% of rated output. The 

net result is that about 300 MW is held in reserve on about 19 GW nameplate of new wind 

plants. About 350 MW of new plants are running at rated power, so they do not participate. The 

curtailed power is headroom that is used to increase power output when the frequency drops 

below the dead band. This ~300 MW is 5% of the 6.3 GW of production on the new plants, or 

about 1% of the total wind production of all plants. 

A comparison of the system frequency for four cases is shown in Figure 74. The corresponding 

power response of the new wind plants is shown in Figure 75. The power traces have some 

numerical noise filtered out. This causes the initial increase in power to be slightly larger than 

expected, but there is no practical impact on the results. The Hi-Mix case with no wind plant 

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frequency controls is shown in blue. With only GR, as shown by the green trace, the frequency 

nadir is slightly improved and the settling frequency—which is critical to the calculation of FR—

is noticeably improved. The active power of the wind plants, again in green, shows that the GR

of the wind plants is relatively slow, rising to steady state about 20–30 seconds into the event. 

The total change in wind power output is about 310 MW by this time. Both the speed and time of 

response are important. The control modeling assumptions drive this behavior, not the 

fundamental physics of the wind turbines. First, the gain between power and frequency deviation 

(i.e., the frequency droop) is set at 5%, the NERC standard. Qualitatively, this means that change 

in MW output is proportional to frequency error once the frequency deviation exceeds the dead 

band, regardless of the loading of the wind plant.

4 This means that a 0.25% frequency error

(150 mHz + 36 mHz dead band) is needed to create a 0.05 pu increase in power. In this case, 5% 

of the available power is kept in reserve. When the plant is running at a fraction of its rating, then 

the frequency error necessary to “use up” the reserved power is lower. The average loading of 

the new wind plants in this case is about 33%, so a relatively small sustained frequency error 

(about 86 mHz) is enough to saturate the controls. Because the settling frequency in the APC 

cases is about 59.86 Hz (i.e., 140 mHz error), the response is saturated. Thus, essentially all of 

the power kept in reserve is contributing to the FR in this case. Second, the speed of response 

roughly matches the speed of response of typical hydro machines. In general, wind turbines can 

respond faster if so required, though there are physical limits on increasing the mechanical 

torque on the turbine. One significant limit is that the turbine blades, while relatively quick, do 

take time to move (pitch in). This leads to the introduction of inertial controls. 

Unlike the governor function, which relies on capturing more wind power through pitching the 

turbine blades, inertial control relies primarily on fast control of the electrical torque to 

temporarily extract inertial energy from the turbine drive train and deliver it to the grid. This 

energy loss slows down the turbine, thus the control is only a short-term measure, and the energy 

must be returned to the drivetrain. The control objective of the inertial control shown here (GE’s 

WI function) is to help reduce the severity of the frequency nadir, and it is not intended to 

improve FR as defined by NERC. The impact of just the inertial control is shown in the red 

traces of the figures. The control makes a significant—45 mHz or 13%—improvement in the 

frequency nadir. The power trace shows that the control has exhausted its contribution by about 

10 seconds and can then be seen to be recovering energy back from the grid. This tends to stretch 

out the frequency depression, but in this case, a substantial improvement in the margin above 

UFLS is realized. Unlike the governor function, the inertial control has no opportunity or lost 

energy cost. The two controls work well together, as can be observed in the pink traces. The 

frequency nadir is improved, but the excursion is not as stretched out in time, and the settling 

frequency improves.

The detailed FR metrics for the four cases are reported in Table 23. Overall, this particular GR

improves the WECC FR by about 300 MW/0.1 Hz, from 1,311 MW/0.1 Hz to 

1,610 MW/0.1 Hz, as shown in both the APC cases. The FR of the combined governor and 

inertial control case (APC+WI) is not quite as good as APC alone: the inertial term greatly 

 

4 This is in distinct contrast to the Irish grid code, which makes power change proportional to available wind power, 

not the nameplate. There is substantial potential for confusion in using the models and interpreting results between 

the two approaches.

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improves the frequency nadir, but at a slight penalty in the early part of the 20–52 second 

window during which FR is calculated.

The governor control raises the FR above the 1,352 MW/0.1 Hz for the Base case. The regional 

improvement in DSW and the Northeast are rather more striking, with both regions going from a 

substantial deficit to a positive margin with the governor control. 

As one measure of efficacy, the contribution of the governor function to FR is 299 MW/0.1 Hz

per 310 MW of function, or 0.96 MW per MW per 0.1 Hz (an admittedly odd unit). Similarly, 

with both controls, the 47 mHz improvement in nadir works out to about 0.15 mHz/MW of 

control.

Figure 74. Frequency response to two Palo Verde unit trip for LSP Hi-Mix case with three 

combinations of frequency controls on wind plants. 

Light Spring Hi-Mix

Light Spring Hi-Mix with governor control

Light Spring Hi-Mix with inertial control

Light Spring Hi-Mix with both controls

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 Figure 75. Wind plant active power response to two Palo Verde unit trip for LSP Hi-Mix case with 

three combinations of frequency controls on wind plants. 

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Table 23. Frequency Response Metrics for LSP Hi-Mix Cases with Three Combinations of Frequency Controls on Wind Plants

LSP Hi-Mix

(MW/0.1 Hz)

Governor 

Control 

(MW/0.1 Hz)

Inertial Control

(MW/0.1 Hz)

Governor and Inertial 

Controls

(MW/0.1 Hz)

FRO

(MW/0.1 Hz)

FR FR 

Margin

FR FR 

Margin

FR FR 

Margin

FR FR 

Margin

WECC 840 1,311 471 1,610 770 1,323 483 1,571 731

By Region

CALIFORNIA 296 312 16 335 39 315 19 334 38

DSW 220 119 -101 240 20 111 -109 215 -5

NORTHEAST 82 47 -34 140 59 40 -41 129 47

NORTHWEST 131 483 353 528 397 507 376 528 397

By Area

ARIZONA 104 50 -55 67 -38 48 -56 63 -41

EL PASO 9 4 -5 5 -4 3 -5 4 -4

IDAHO 18 22 4 23 5 22 4 23 5

IMPERIALCA 4 14 10 27 23 13 9 24 21

LADWP 29 30 1 31 1 30 1 31 2

MONTANA 11 10 -1 53 42 5 -7 44 33

NEVADA 28 34 6 34 7 34 7 35 7

NEW MEXICO 14 2 -13 9 -5 8 -6 9 -5

PACE 42 8 -34 58 16 6 -36 55 14

PG AND E 133 197 64 205 72 198 66 205 72

PSCOLORADO 36 6 -30 6 -29 6 -29 6 -29

SANDIEGO 21 7 -14 7 -14 7 -14 7 -14

SIERRA 11 7 -4 7 -4 7 -3 7 -4

SOCALIF 108 63 -45 64 -44 65 -43 66 -43

WAPA R.M. 27 24 -3 118 91 11 -17 98 70

WAPA U.M. 0 3 3 4 4 3 3 4 3

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The results of a similar investigation for the HS Hi-Mix case are shown in Appendix Table 52

and Figure 139. The new controls have some beneficial impact on the system, but it already had

a substantial FR margin. The controls on the wind plants have largely displaced the response of 

other plants.

7.2 Frequency Controls on Solar Plants

7.2.1 Utility-Scale Solar PV Plants

Compared to wind plants, there is even less industry experience with providing under-frequency 

regulation services with solar PV plants. Conceptually, the process is similar to that for wind: an 

amount of power can be kept in reserve for regulation by curtailing the power output of the 

installation. And like wind, there is substantial opportunity cost associated with the lost energy 

production. However, unlike wind power, there are no rotating components, and consequently 

there is no inherent mechanism to provide inertial response. However, fast FR controls could 

operate in the inertial time frame of 5–10 seconds post-disturbance.

For this investigation, all new utility-scale PV plants are assumed to be equipped with frequencyresponsive controls, and every plant is assumed to be curtailed by 5% of rating. This causes 

about ~820 MW curtailment on ~16 GW nameplate, leaving ~10.2 GW production for all solar 

(of which about 6.4 GW is from the new PV plants).

Unlike the governor controls for wind plants presented in the previous section, the control used 

here for PV is aggressive: the response is fast, with gains and time constants selected to saturate 

relatively quickly once the system frequency is outside of the 36 mHz dead band. From an 

equipment perspective, there is essentially no mechanical risk or penalty for fast response, as 

there are significant mechanical stresses associated with rapidly changing power output. Further, 

from the perspective of this specific investigation, it is illuminating to show the potential impact 

of aggressive governor control compared to the relatively slower response modeled for the wind 

plants.

Figure 76 and Figure 77 show the impact of this level of governor control on the LSP Hi-Mix

system for the trip of two Palo Verde NPS units. The impact on both the frequency nadir and the 

settling frequency is substantial. The frequency nadir is improved by 110 mHz, from 59.646 Hz

to 59.752 Hz. The improvement in FR is about 765 MW/0.1 Hz, to 2,065 MW/0.1 Hz. The perunitized improvement in FR is 0.93 MW per MW per 0.1 Hz, and 0.13 mHz/MW on the 

frequency nadir. By this measure, the efficacy of the PV plant controls is essentially the same as 

for the wind plants. The FR metrics are detailed in Table 24. 

Figure 77 shows the aggressive response of the PV to the frequency drop. This non-linear 

response raises some interesting questions. The control is deliberately set to provide as much 

support to the grid as possible for this design-basis event. Philosophically, the idea is to take 

maximum advantage of this highly responsive resource—there is essentially no penalty on the 

PV resource for aggressive response, and it reduces the duty on synchronous generating 

resources. Had the event been somewhat less severe, the control would still have saturated, 

providing proportionally greater response—so the FR metric would show a greater benefit. 

Conversely, had the event been even larger, the FR metric would not be as good. Other resources 

for FR, such as triggered demand response and energy storage, will likely exhibit such nonlinearities. With the inevitable addition of non-linear frequency-responsive resources, practice 

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will need to evolve to adapt to the reality that overall system FR will itself be a function of the 

size of the disturbing event. This is not necessarily a problem, but it certainly adds complexity to 

the overall FR issue. The deliberately aggressive control in this case is almost certain to have 

other undesirable side effects not evident in this particular case. The objective of this 

investigation is to place an upper bound on the benefit of potentially fast frequency controls.

Figure 76. Frequency response to two Palo Verde unit trip for LSP Hi-Mix – with and without 

governor control on utility-scale PV plants. 

Light Spring Hi-Mix

Light Spring Hi-Mix with governor 

controls on utility-scale PV plants

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Figure 77. Frequency and power output in response to two Palo Verde unit trip for LSP Hi-Mix with 

governor controls on utility-scale PV plants. 

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Table 24. FR Metrics for LSP Hi-Mix Case With and Without Governor Control on Utility-Scale PV 

Plants

LSP Hi-Mix LSP Hi-Mix with 5% Frequency 

Control on Utility-Scale PV

FRO FR FR Margin FR FR FR Margin FR

Name (MW/

0.1 Hz)

(MW/

0.1 Hz)

(MW/

0.1 Hz)

% of 

WECC

(MW/

0.1 Hz)

(MW/

0.1 Hz)

% of 

WECC

WECC 840 1,311 471 100.0 2,065 1,225 100.0

By Region

CALIFORNIA 296 312 16 23.8 562 266 27.2

DSW 220 119 -101 9.1 475 255 23.0

NORTHEAST 82 47 -34 3.6 135 54 6.6

NORTHWEST 131 483 353 36.9 537 407 26.0

By Area

ARIZONA 104 50 -55 3.8 237 133 11.5

EL PASO 9 4 -5 0.3 21 13 1.0

IDAHO 18 22 4 1.7 23 5 1.1

IMPERIALCA 4 14 10 1.1 42 38 2.0

LADWP 29 30 1 2.3 71 42 3.5

MONTANA 11 10 -1 0.8 9 -3 0.4

NEVADA 28 34 6 2.6 56 28 2.7

NEW MEXICO 14 2 -13 0.1 55 41 2.7

PACE 42 8 -34 0.6 26 -16 1.3

PG AND E 133 197 64 15.0 202 70 9.8

PSCOLORADO 36 6 -30 0.4 49 13 2.4

SANDIEGO 21 7 -14 0.6 7 -14 0.4

SIERRA 11 7 -4 0.5 76 65 3.7

SOCALIF 108 63 -45 4.8 240 131 11.6

WAPA R.M. 27 24 -3 1.8 56 29 2.7

WAPA U.M. 0 3 3 0.2 3 3 0.1

7.2.2 CSP Plants

In all of the cases presented up to this point, the CSP plants are modeled as base load steam 

plants. This assumption is based on the expectation that, like other steam generation, there is an 

efficiency penalty associated with throttling steam to provide fast GR. In this section, the 

potential contribution of CSP steam turbines to FR is examined by enabling GR on all the new 

CSP plants. This is accomplished by turning off the base load flag in the turbine governor model.

It is important to note that the CSP dynamic models are based on plants currently in operation. In 

the WWSIS-2 work, it was assumed that all CSP had significant amounts of thermal storage. It is 

possible that such plants could be designed to have the ability to relatively rapidly increase 

power output. Details of such a design are outside the scope of this study.

The frequency nadir is improved by 13 mHz, and the settling frequency is also improved for the 

HS Hi-Mix case. The frequency traces are shown in Appendix Figure 141. 

As discussed in Section 4.1, the PDCI event can cause system separation at the COI in the HS

Hi-Mix condition, unless some generation-tripping RAS is enabled. Figure 78 shows a different 

potential benefit of enabling CSP GR. In this case, the PDCI event is imposed with the CSP GR

enabled (blue trace). The figure shows that the system is stabilized without resorting to a RAS. 

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This is because the beneficial contribution of CSP is geographically advantageous for this event. 

Most of the CSP is to the south of COI, so the transient increase in power output from these 

plants is such that power swing from north to south on the COI is slightly reduced and eliminates

the voltage collapse along the corridor. Thus, the beneficial CSP contribution to FR also has a 

positive locational aspect. 

 Figure 78. Malin bus voltage in response to PDCI event – with and without CSP governor control. 

7.3 Energy Storage for Frequency Response

The FR results presented thus far have shown that the WECC-wide system FR meets NERC 

criteria, and the system avoids UFLS for all cases examined.

However, many individual areas are short of FR. As previously noted, all of the FROs assigned 

to individual areas are estimates for illustration. The NERC rules apply specifically and 

exclusively to individual BAs. There is no requirement that BAs meet their FRO with resources 

within their Bas; however, a formal contractual arrangement is required to use resources outside 

their own area.

Energy storage has the potential to help individual BAs meet their FRO with resources that could 

be deployed within their territories. In this investigation, inverter-based energy storage systems 

are added to areas short of FR. The model used for this investigation is deliberately independent 

Heavy Summer Hi-Mix (no RAS)

Heavy Summer Hi-Mix with CSP 

governor control (no RAS)

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of the storage medium (e.g., batteries, flywheels, super conducting magnetic energy storage 

[SMES], etc.). It is assumed that the medium has sufficient energy to supply the nominal power 

rating for 60 seconds, and that it has the dynamic capability necessary to follow the change in 

power required for the control. Again, this is focused solely on FR, and only concerned with the 

first minute following a disturbance. There is neither a requirement nor a prohibition that the 

energy storage provide other services, such as balancing, secondary control, arbitrage, etc. 

In this investigation, two cases are considered: the LSP Hi-Mix and Extreme response to the loss 

of two Palo Verde units. 

7.3.1 Estimate Energy Storage Rating 

An estimate of the amount of energy storage required to bring up the FR in those areas that are 

short of their FRO is summarized in Table 25. The FR from the LSP Hi-Mix case is shown in the 

middle. Those areas with negative FR margin are short. The last column shows an estimate of the 

energy storage power rating sufficient to make up the deficit for this settling frequency. 

The math is as follows, using the Arizona area as an example:

• The actual FR = 49.7 MW/0.1 Hz. 

• The estimated FRO = 104.4 MW/0.1 Hz. 

• The shortfall in FR is therefore = 54.7 MW/0.1 Hz. 

• The settling frequency deviation from 60 Hz for this case = -0.156 Hz. 

• Therefore, the extra response needed = 54.7 MW x 0.156/0.1, which is 85.3 MW. 

An estimated total of 316 MW of energy storage deployed in 10 areas should bring these areas to 

approximately 100% FRO. As noted earlier, the system response is non-linear, so this method is 

necessarily approximate.

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Table 25. Estimate of Energy Storage Rating Required to Meet FRO 

LSP Hi-Mix

FRO FR FR Margin Estimated 

BESS 

Rating

Name (MW/0.1 Hz) (MW/0.1 Hz) (MW/0.1 Hz) (MW)

WECC 840 1,311 471 0.0

By Region

CALIFORNIA 296 312 16 0.0

DSW 220 119 -101 157.3

NORTHEAST 82 47 -34 53.6

NORTHWEST 131 483 353 0.0

By Area

ARIZONA 104 50 -55 85.3

EL PASO 9 4 -5 7.6

IDAHO 18 22 4 0.0

IMPERIALCA 4 14 10 0.0

LADWP 29 30 1 0.0

MONTANA 11 10 -1 1.6

NEVADA 28 34 6 0.0

NEW MEXICO 14 2 -13 19.6

PACE 42 8 -34 52.6

PG AND E 133 197 64 0.0

PSCOLORADO 36 6 -30 46.7

SANDIEGO 21 7 -14 22.0

SIERRA 11 7 -4 5.6

SOCALIF 108 63 -45 69.9

WAPA R.M. 27 24 -3 5.1

WAPA U.M. 0 3 3 0.0

A similar exercise for the Extreme case is summarized in Appendix Table 53. For this more 

challenging condition, a total of 412 MW of energy storage is estimated to be required.

7.3.2 Frequency Performance with Energy Storage 

The system frequency for the LSP Hi-Mix case tripping the two Palo Verde NPS units is shown 

in Figure 79. The energy storage results in improved frequency nadir and settling frequency. The 

nadir improves by 45 mHz to 59.690 Hz. FR metrics, as reported in Table 26, improve by 

201 MW/0.1 Hz at the WECC level. Per-unitized, the benefit on the nadir is 0.14 mHz/MW of 

energy storage, and the benefit on FR is 0.63 MW per MW per 0.1 Hz. 

More important for this investigation, the individual areas that were short of FR are brought up 

approximately to compliance. The reader’s attention is directed to the FR margin columns: the 

reference in blue and with energy storage in pink. The areas with added energy storage are 

highlighted in yellow. Notice that FR margins are close to zero, having changed from 

significantly negative. As noted above, the rating method is approximate, so having the FR 

exactly equal to the FRO is not expected. 

It is of interest to examine the performance of one of the energy storage devices in more detail. 

In Figure 80, the power output of the Arizona energy storage system is plotted with the Arizona 

frequency. The case with no energy storage is shown for reference (blue trace). In this case, the 

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frequency gain and control time constant are selected to have a less aggressive rise than was used 

in the PV case (above), but more aggressive than the wind plant controls, and to just saturate for 

this frequency nadir. The output of the Arizona energy storage reaches maximum after 2–3 

seconds, well before the frequency nadir, and slightly declines after about 28 seconds, when the 

frequency is recovering. This decline, coupled with the dynamic impact of the fast-responding 

resource on the other conventional governors, makes the efficacy in terms of impact on FR per 

MW of energy storage look worse than that for the wind and solar controls. However, this 

combination of fast initial response, to benefit the nadir, and less aggressive sustained response, 

to facilitate coordination with other responsive resources, is arguably better.

Figure 79. Frequency response to two Palo Verde unit trip for LSP Hi-Mix – with and without 

energy storage. 

Light Spring Hi-Mix

Light Spring Hi-Mix with energy 

storage

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Figure 80. Frequency and power output of an Arizona energy storage plant for LSP Hi-Mix case.

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Table 26. FR Metrics for LSP Hi-Mix With and Without Energy Storage

LSP Hi-Mix LSP Hi-Mix with Energy 

Storage

FRO FR FR Margin Estimated 

BESS 

Rating

FR FR Margin

Name (MW/0.1 Hz) (MW/0.1 Hz) (MW/0.1 Hz) (MW) (MW/0.1 Hz) (MW/0.1 Hz)

WECC 840 1,311 471 0.0 1,513 672.6

By Region

CALIFORNIA 296 312 16 0.0 369 73.5

DSW 220 119 -101 157.3 224 4.0

NORTHEAST 82 47 -34 53.6 85 3.8

NORTHWEST 131 483 353 0.0 487 356.4

By Area

ARIZONA 104 50 -55 85.3 105 1.0

EL PASO 9 4 -5 7.6 9 0.3

IDAHO 18 22 4 0.0 22 4.0

IMPERIALCA 4 14 10 0.0 14 10.4

LADWP 29 30 1 0.0 30 0.4

MONTANA 11 10 -1 1.6 10 -1.0

NEVADA 28 34 6 0.0 34 6.0

NEW MEXICO 14 2 -13 19.6 14 -0.6

PACE 42 8 -34 52.6 40 -1.7

PG AND E 133 197 64 0.0 197 64.2

PSCOLORADO 36 6 -30 46.7 33 -2.1

SANDIEGO 21 7 -14 22.0 21 -0.7

SIERRA 11 7 -4 5.6 10 -0.3

SOCALIF 108 63 -45 69.9 103 -5.1

WAPA R.M. 27 24 -3 5.1 26 -1.6

WAPA U.M. 0 3 3 0.0 3 2.5

Similar results for the system response in the Extreme case with 412 MW of energy storage are 

shown in Appendix Figure 142 and Figure 143, and in Appendix Table 54. Qualitatively, the 

results are similar to the Hi-Mix case. The FR improves by 252 MW/0.1 Hz from 

1,055 MW/0.1 Hz to 1,307 MW/0.1 Hz. The per-unitized benefit is 0.61 MW per MW per 

0.1 Hz, essentially the same as that for the Hi-Mix case. The frequency nadir improves 56 mHz, 

from 59.614 Hz to 59.670 Hz, or 0.14 mHz/MW. This linearity reflects the fact that the energy 

storage is sized and controlled with the same objectives and constraints.

7.4 Relative Performance of Frequency Controls

Primary response from fast energy storage (e.g., batteries, flywheels, etc.) is effective at 

improving both nadir and settling frequency. Relatively small amounts (total ~400 MW for all 

WECC) “fixed” all area FRO deficiencies in these cases. The benefits are similar to those 

observed from aggressive control of solar PV. In comparison, the impact of controls on wind 

plants varied depending on the speed of response. The fast response of the inertial control 

improved the frequency nadir; the sustained governor output improved the FR metric. The CSP 

response example highlighted a locational aspect that, in this case, was beneficial to transient 

stability.

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The combined findings suggest that the best use of rapidly responding power-electronics-enabled 

resources for FR is non-linear. The medium (i.e., wind or solar or energy storage or a 

combination) is less important than the control philosophy employed. Other technologies, such 

as fast control of loads that deliver similar dynamic response, would likely produce similar 

improvements. The results illustrate that improvement in frequency nadir and beneficial impact 

on system security are not necessarily perfectly aligned with the FR metric. In the past, the 

primary available control variable was gain, in the form of governor droop, and speed of 

response was not readily adjusted. Practice to coordinate droops and to ensure adequate response 

were more straightforward. With the added flexibility of new resources that have the ability to 

easily adjust speed of response, there is the potential for FR that equals or exceeds that of the 

present system. However, the frequency control problem gains dimensionality and complexity, 

as both control flexibility and the necessity to consider energy as well as power constraints, are 

introduced. As in the past, locational aspects may prove important in some cases.

7.5 COI Stabilization with Transmission Additions

In this study, a minimalist approach has been used, adding only the reinforcements necessary to 

relieve performance problems for the specific cases under consideration.

In this mitigation exercise, a portion of the WWSIS-2 transmission additions are made to the HS 

Hi-Mix case to examine their impact on stability. 

Table 27 is adopted from transmission build-out work from WWSIS-2, which used 500 MW

increments on PLEXOS inter-area paths. Path capability for that type of model does not 

necessarily provide simple mapping to actual new physical lines. Rather than postulate new 

corridors or attempt to replicate specific project proposals, this exercise was limited to 

duplicating, and thereby doubling, existing circuits in the dataset. The work explicitly did not 

consider other potential transmission projects. 

The original WECC case included significant new transmission. One project is a major new 

500 kV line from central Oregon at Grassland to outside Las Vegas at Harry Allen, via a route 

well east of the Sierras through the Hemingway, Midpoint, and Robinson substations. The three 

highlighted improvements in the table provide an approximate proxy for the build-out of this 

test, which is to make this new line a double circuit. The addition of this path provides an 

additional north-to-south transfer path that decreases flow across the COI and increases the 

synchronizing strength of the system. COI flow drops about 400 MW. Appendix Figure 144

shows the bubble chart, with some of the major flow changes noted.

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Table 27. Selected WWSIS-2 Transmission Additions

Transmission Path Upgrades 

(MW)

WWSIS-2 Interface Initial 

Rating

TEPPC

($10 Hurdle 

Rate)

Hi Mix

($10 Hurdle 

Rate)

AZ to CA so 1,600 1,000 1,000

AZ to CO 200 0 500

AZ to IID 195 500 500

AZ to DWP 468 500

AZ to UT 250 500

CA no to NV no 100 500 1,500

CA no to SF 1,272 500 1,500

ID to NV no 350 500 500

MO to NW 2,000 1,000

NV no to NW 300 1,000 1,000

UT to WY 2,100 500

Total 4,000 9,000

The added transmission stabilizes the system without a generation-tripping RAS for the PDCI 

event. Figure 81 shows the interface flow power swings for the unstable LSP Hi-Mix case and 

the stabilized Hi-Mix case with added transmission. Appendix Figure 145 and Figure 146 show 

voltages for the event, which comfortably meet WECC voltage criteria. 

The improvement in system stability due to transmission additions is not surprising. The key 

observation from this test is that transmission additions that are economically justified for their 

reduction in overall variable operating cost (from WWSIS-2) will tend to improve system 

transient stability as well. 

Again, this raises the question of whether the interface limits should change. It is possible that 

this improved parallel path increases the COI limit, but it is also possible that there are other 

consequences. A complete system evaluation is needed to assess the impact of a major 

transmission change like this on path limits. 

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Figure 81. Path flows in response to PDCI event for LSP Hi-Mix case with and without 

transmission build-out. 

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8 Summary and Conclusions 

8.1 Frequency Response 

Frequency response (FR) is the overall response of the power system to large, sudden 

mismatches between generation and load, and it is measured according to NERC BAL-003 

(NERC 2012a). The primary concern is that the minimum frequency, or nadir, during designbasis disturbances should not cause under-frequency load shedding (UFLS). In the West, the first 

stage of UFLS is normally at 59.5 Hz. The NERC standard also provides a specific definition of 

the quantitative metric “frequency response.” It is this metric that is compared to the frequency 

response obligation (FRO) to determine compliance. 

WWSIS-3 focused on light spring conditions, as the relatively low level of generation may 

present a challenge for FR. Similarly, the analysis focused on the single largest design-basis 

generation outage in the Western Interconnection. According to BAL-003-1, this design-basis 

event is the trip of two fully loaded Palo Verde nuclear power station units for a loss of about 

2,750 MW. The subsequent frequency excursion is severe; however, all cases avoid UFLS relay 

action and meet the interconnection frequency response obligation (IFRO), which is 840 MW/0.1

Hz for this study. To help understand the system frequency performance, estimates of regional 

FROs were made. Actual FROs are assigned to individual balancing authorities (BAs) and are 

updated annually, so these estimated obligations are for reference only and should not be used to 

determine individual BA compliance with the NERC standard. As noted above, the WECC-wide 

FR meets its obligation in all cases, with some margin. Portions of the system that rely primarily 

on thermal generation, however, tend to be short of meeting their approximate FRO with their 

own generation resources, especially in the Hi-Mix case (e.g., the DSW and Northeast regions). 

This occurs because that thermal generation was displaced by wind and solar, which do not 

provide FR unless equipped with specific controls. Other regions, particularly the NW, far 

exceed their approximate FRO due to high levels of responsive hydro. 

As expected, the WECC-wide response is substantially improved for the HS condition compared 

to the LSP condition. 

Extreme Generation Loss

The FR to an extreme event (i.e., loss of three Palo Verde units) compared to that of the designbasis event (i.e., loss of two Palo Verde units) is close to linear and showed a slight degradation 

due to the larger-sized event and more governor controls saturating. This is reassuring from a 

robustness perspective, though obviously a severe event will still cause UFLS-triggered 

interruptions, just as it does today.

While UFLS action is allowed for a severe event like this extreme generation tripping, cascading 

failure is not. One sensitivity case in which the embedded PV DG was pessimistically assumed 

to have aggressive under-frequency tripping resulted in an acute frequency depression and would 

have likely caused widespread outages.

Distributed vs. Central Station Generation Tripping

System performance in response to a large distributed generation outage was compared to a large 

central station outage. The DG event results in a less severe frequency nadir and a better settling 

frequency. The difference is relatively small and is primarily due to two factors. First, the loss of 

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locally generated power depresses the load voltage and causes the net load to drop. This load 

relief helps the system frequency. A second factor is that the tripped DG is less than the 

2,750 MW of the Palo Verde event, due to voltage effects on the tripping logic in the composite 

load model. The post-disturbance voltages tend to be different, however, which can have 

substantial impact on load active power. This result tends to reinforce the conclusion that load 

voltage sensitivity is a more important consideration for FR than load frequency sensitivity.

Broadly, the location of the generation tripping is not as important as the amount of generation 

that is tripped. The mechanisms for widespread DG tripping are complex, so it may be possible 

for more DG to trip than was used in this sensitivity case. 

System Inertia

Much has been said about the possible impact of loss of system inertia due to the displacement of 

synchronous generation by inverter-based resources. Between the LSP Base case and the Hi-Mix 

case, the initial rate of change of frequency (ROCOF) increases about 18%. The impact of this 

increased ROCOF on the system stability is nearly invisible in terms of FR: both the nadir and 

the settling frequency are essentially unchanged. It should be noted that these levels of ROCOF, 

on the order of 0.1 Hz/s, are quite small compared to some of the smaller systems around the 

world that have ROCOF concerns primarily driven by the use of ROCOF relays. This reinforces 

other results that suggest that the loss of system inertia associated with increased wind and solar 

generation is of little consequence for up to at least 50% levels of instantaneous penetration for 

the Western Interconnection as long as adequately fast primary frequency-responsive resources 

are maintained.

Headroom Depletion

The effects of headroom depletion due to a relatively rapid afternoon decline in solar PV 

generation—the so-called “duck curve”—is a growing concern. In an effort to bound the 

problem, an extreme case was simulated where all PV generation in the Western Interconnection 

was shut down, mimicking sunset, while no other generation was committed. This does not 

create a catastrophic failure in the system performance for the given commitment and dispatch. 

No dramatic changes in performance were observed (i.e., there was no cliff) as the PV dropped 

out. Rather, the degradation of FR is steady and monotonic, while transient stability was 

maintained.

However, once the PV output is reduced to zero, the overall WECC FR is marginal, even with 

significant contribution from California hydro. The FR for California is below the approximate 

statewide FRO. To test the impact of having less responsive hydro, governors were removed as a 

proxy for low water levels or other constraints on the hydro generators' ability to provide more 

power. As the California hydro becomes less responsive, the overall WECC FR drops, and 

eventually WECC fails to meet the IFRO.

As PV output drops, the system re-dispatches, creating many local stress points (e.g., poor 

voltage). This suggests that the need to commit/recommit units could be driven as much by local 

constraints as overall stability. The details will be important as the system stress builds. This 

further suggests that locational issues may drive some constraints on how system operators strive 

to maintain adequate FR.

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8.2 Means to Improve Frequency Response

Current operating practice uses traditional approaches (e.g., commit conventional plants with 

governors) to meet all FR needs. Selected non-traditional frequency-responsive controls on wind, 

solar PV, CSP plants, and energy storage were examined in this study.

Frequency-Responsive Controls on Wind Plants

This study examined two types of frequency-responsive controls for wind plants. The inertial 

control helps improve the nadir, but the energy recovery tends to stretch out the frequency 

depression. A substantial improvement in the margin above UFLS is realized. Unlike the 

governor function, the inertial control has no opportunity or lost energy cost. There is, however, 

a capital cost associated with the controls.

The governor or active power control (APC) alone greatly improved the settling frequency and 

the FR, but had little impact on the nadir. The combination of the governor and inertial control 

improved the frequency nadir, settling frequency, and FR. Note that the wind governor controls 

were set to emulate those on conventional generation. Both the wind governor and inertial 

controls could be made more aggressive, as is examined with the utility-scale PV and energy 

storage controls below.

Frequency-Responsive Controls on Solar PV Plants

Primary FR from utility-scale solar PV generation is effective at improving both nadir and 

settling frequency. Unlike the governor controls for wind plants, the control used here is 

aggressive: the response is fast, with gains and time constants selected to saturate relatively 

quickly once the system frequency is outside of the dead band. The PV plant response to the 

design-basis event (i.e., loss of two Palo Verde units) is so fast that it is essentially a step 

response. Had the event been somewhat less severe, the control still would have saturated and 

provided proportionally greater response. Thus, the FR metric would show a greater benefit. 

Conversely, had the event been even larger, the FR metric would be worse. 

Frequency Response from Energy Storage 

This study shows that the WECC-wide FR meets NERC criteria, and the system avoids UFLS 

for all cases examined. However, many of the individual areas are short of FR. Note that the 

FROs assigned to individual areas in this study are estimates. The NERC rules apply specifically 

and exclusively to individual BAs; there is no requirement that BAs meet their FRO with 

resources within their BAs.

In this investigation, inverter-based energy storage systems are added to areas short of FR. The 

model used for this investigation is deliberately independent of the storage medium (e.g., 

batteries, flywheels, super conducting magnetic energy storage [SMES], etc.). It is assumed that 

the medium has sufficient energy to supply the nominal power rating for 60 seconds, and that it 

has the dynamic capability necessary to follow the change in power required for the control. The 

storage systems for each area were sized specifically to meet that area's estimated FRO, and an 

aggressive governor control was applied to each system. Primary FR from fast energy storage is 

effective at improving both nadir and settling frequency. A total of about 400 MW of energy 

storage for all of the Western Interconnection allowed the individual areas that were short of FR

to meet their approximate FROs with resources in their areas. The NERC requirements allow 

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individual areas to contract with others for sufficient frequency-responsive resources to meet 

their obligation.

8.3 Transient Stability

During heavy load conditions, the addition of high levels of wind and solar generation increases 

the heavy loading on the Pacific AC and DC Interties to about their present path ratings. High 

flows on the COI are well known to be stressful and to require a generation-tripping remedial 

action scheme (RAS). This investigation suggests that this practice can continue, and that the 

transient stability of the system for one of the well-known and critical events for the Western 

Interconnection is not fundamentally changed by the high wind and solar generation. One 

sensitivity case in which the Base case had the same PDCI and COI loading had slightly worse 

performance than the Hi-Mix case. This conclusion is not a statement that the system behaves 

identically. It is possible, and perhaps likely, that the system dynamics are sufficiently different 

to require somewhat different levels of generator tripping or different arming criteria. A 

complete evaluation of the current practice to check for refinements would be prudent. There is, 

however, nothing in this analysis to indicate that the system dynamics have fundamentally 

changed and that radically different means to ensure stability for this event are required.

Local Stability 

There are many localized stability limits in the West. The addition of substantial wind and solar 

generation has the potential to alter the system dynamics of the events that dictate these limits.

For the limited examples, the local system stability is slightly better in the Hi-Mix case. 

Distributed Generation Fault Ride-Through 

The deliberate or sympathetic trip of distributed PV during disturbances results in a slower 

recovery and lower sustained voltages. Local reactive power balance can be disrupted, with 

reactive power demand increasing many times the amount of active power tripped. In one test, 

with a pessimistic approximation to a worst-case under-voltage tripping, the loss of the DG

causes a system collapse.

A number of cases showed adverse consequences, up to and including system collapse, from 

widespread tripping of embedded PV distributed generation. Consequently, both prudence and 

existing reliability rules would argue against widespread, common-mode DG tripping for 

moderately severe voltage dips or frequency excursions. 

IEEE Standard 1547a, published in May 2014, revises three existing requirements for 

interconnection of DG with electric power systems. It now has significantly wider mandated 

ranges for DG allowable trip settings in response to utility abnormal voltages and frequency with 

different specified settings allowed via mandatory mutual agreement between the power system 

and DG operators. However, when the grid experiences a large disturbance, if the 1547 default 

trip settings are used for all DG sites, that may result in widespread DG tripping at the same 

time. The draft IEEE Standard 1547.8 further permits additional DG functions to support the 

grid, again via mutual agreement. Although this draft has initially been approved by IEEE 

balloters, it is undergoing final revisions before publication.

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The system-wide impact of common mode tripping of significant amounts of DG, regardless of 

the mechanism, requires more study. 

Coal Displacement and Weak Grid in the Northeast Region

A high de-commitment of coal did not overstress the system. A greater than 80% reduction of 

coal commitment in the Northeast region, compared to the LSP Base case, resulted in acceptable 

dynamic behavior for the limited tests performed. System dynamics were stable for an extrahigh-voltage fault at Aeolus, in the heart of the high wind area of the Northeast. The system 

non-synchronous penetration (SNSP) was driven to 56% in the Hi-Mix case.

The regional transmission system was designed based on the size and location of the large coal 

power plants, which thus became critical nodes in the network. As a result, the transmission 

system operators have historically counted on those plants to provide the voltage and reactive 

power support needed for reliable operation. Displacement of those central plants by more 

dispersed wind and solar generation results in those nodes being poorly supported. Not 

surprisingly, local voltage and thermal problems occur, and good planning practices need to be 

followed.

The de-commitment in the Extreme case further stresses the system, with a greater than 90% 

reduction of coal commitment from the LSP Base case. This gives an SNSP of about 66%. There 

was a rapid voltage collapse and system separation during the fault representative of so-called 

“weak grid” issues. Systems with very high levels of inverter-based generation are challenged to 

provide fast, confident control during faults and other disturbances. No commercially available 

wind or utility-scale solar PV generation is capable of operation in a system without the 

stabilizing benefit of synchronous machines. Therefore, the conversion of some coal plants to 

synchronous condensers and the addition of mechanically switched shunt compensation were 

needed to stabilize the Aeolus fault with the pessimistic load and wind plant modeling used. The

synchronous condenser conversion (assuming retirement) works well to stabilize the system. The 

system recovers in an orderly fashion when the fault is cleared.

8.4 Means to Improve Transient Stability

Transmission Additions

The mitigation investigation in WWSIS-3 included a portion of the WWSIS-2 transmission 

additions to examine their impact on the transient stability of the COI. The added transmission 

stabilizes the system without a generation-tripping RAS. This reinforces the need to analyze 

whether the interface limits should change. 

Frequency-Responsive Controls on CSP Plants

The CSP plants are modeled as base load steam plants under most conditions. However, the 

potential contribution of CSP steam turbines to FR was examined by enabling GR on all the new 

CSP plants. 

The PDCI event can cause system separation at the COI unless some generation-tripping RAS is 

enabled. However, there is a potential benefit of having CSP GR when the PDCI event is

imposed: the system is stabilized without resorting to a RAS. This is because the beneficial 

contribution of CSP is geographically advantageous for this event. Most of the CSP is to the 

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135

south of COI, so the transient increase in power output from these plants is such that power 

swing from north to south on the COI is slightly reduced and eliminates the voltage collapse 

along the corridor. Thus, the beneficial CSP contribution to FR also has a positive locational 

aspect that benefits transient stability. 

8.5 Model Improvement and Further Analysis

Load Model 

Changing the load model had a greater impact on system performance than did changing the 

level of renewable generation. The behavior of the system for deep faults is completely 

dominated by the load model, and more specifically by the tripping vs. stalling behavior assumed 

for the motor constituents of the composite load. The motor stalling behavior is exacerbated by 

blocking or tripping of embedded PV. This is an extraordinarily complex issue for planning and 

for research. This stability risk is not primarily one of utility-scale renewable integration. Further 

investigation of the load behavior, the motor tripping, and the behavior of the embedded PV DG 

is warranted.

Wind and Solar Models 

When wind and solar are the predominant source of generation throughout the region, it will be 

important to have appropriate dynamic models. WECC has a longstanding best practice to keep 

dynamic models up to date. Wind and solar plant models needs to be held to the same level of 

accuracy in a high-penetration future. Adoption of wind plant controls designed for weak grids 

greatly improved system stability. Further, the results were extremely sensitive to the 

assumptions about load modeling, as described above.

Frequency-Responsive Control Philosophy 

The combined findings of the wind, solar, and energy storage investigations suggest that the best 

use of rapidly responding power-electronics-enabled resources for FR is non-linear. The 

medium—wind or solar or energy storage or a combination —is less important than the control 

philosophy employed. Other technologies that deliver similar dynamic response, such as fast 

control of loads, would likely produce similar improvements. 

In the past, the primary control variable was gain, in the form of governor droop, and speed of 

response was not readily adjusted. Coordinating droops and ensuring adequate response were 

more straightforward. With the added flexibility of easily adjustable speed of response, and the 

necessity to consider energy as well as power constraints, the frequency control problem also 

gains complexity. With the addition of non-linear frequency-responsive resources, practice will 

need to evolve and incorporate FR that is a function of the size of the disturbing event. As in the 

past, locational aspects may prove important in some cases.

Path Rating Analyses

The majority of the results presented in this study used the transmission topology directly from 

the WECC planning databases. These WECC cases included significant new transmission that is 

not currently in service. With the exception of some local patches, no further major 

reinforcements were added. In general, the study did not identify dramatic changes in system 

dynamics. However, the large changes in system flow patterns suggest that a rigorous analysis of 

individual paths is needed to ensure reliability. Certainly, all the regional paths in the high wind 

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parts of the Northeast region will change as transmission is added to accommodate the new 

plants. A complete system evaluation is needed to assess the impact of such a major transmission 

change on path limits. 

Coal Displacement Analysis

The sequence of coal displacement sensitivities in this study is illuminating, but in no way 

complete or conclusive. It provides an opportunity to investigate the transient stability 

implications of not only temporary de-commitment of coal generation, but also the impact of 

possible coal plant retirements. The key point is that transient stability analysis is always based 

on snapshots of operation. From a system dynamics perspective, whether a specific generating 

resource is off-line because it was de-committed for a particular operating condition is 

indistinguishable from a permanent retirement. The displacement by wind and solar was based 

on the economic analysis of WWSIS-2, rather than an arbitrary, plant-by-plant choice. 

The system appears to behave well for the Hi-Mix case when almost all of the large coal plants 

in the eastern regions are off-line. However, displacing even more coal units and pushing SNSP 

above 60% appears to cause problems. This small sample suggests that care must be exercised in 

driving the system from a high level of coal displacement to an extreme level of coal 

displacement. More analysis is needed.

8.6 Conclusions

This work did not identify any fundamental reasons why the Western Interconnection cannot 

meet transient stability and FR objectives with high levels of wind and solar generation. 

However, good system planning and power system engineering practices must be followed. At a 

minimum, local voltage and thermal problems will inevitably require some transmission system 

improvements. The dynamic behavior of distributed PV generation was shown to have the 

potential to substantially impact the bulk power system. Distribution is not decoupled from 

transmission and will impact bulk power system operation. Mechanisms are needed to allow BAs 

to both share frequency-responsive resources and to ensure that they have adequate frequencyresponsive resources within their control. From a transient stability perspective, the system 

appears to tolerate substantial displacement of thermal generation. However, care will be needed 

in the event that the system, especially the DSW and Northeastern regions, is driven to near zero 

commitment of coal plants. Note that this investigation is not a substitute for thorough system 

planning studies. The conclusions of WWSIS-3 are provided in bullet format below.

Transient Stability Conclusions

For the conditions studied:

• System-wide transient stability can be maintained with high levels of wind and solar 

generation if local stability, voltage, and thermal problems are addressed with traditional 

transmission system reinforcements (e.g., transformers, shunt capacitors, local lines). 

With these reinforcements, an 80% reduction in coal plant commitment, which drove 

SNSP to 56%, resulted in acceptable transient stability performance.

• With further reinforcements, including non-standard items such as synchronous 

condenser conversions, a 90% reduction in coal plant commitment, which drove SNSP to 

61%, resulted in acceptable transient stability performance. 

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137

• Additional transmission and CSP generation with frequency-responsive controls are 

effective at improving transient stability.

Frequency Response Conclusions

For the conditions studied:

• System-wide FR can be maintained with high levels of wind and solar generation if local 

stability, voltage and thermal problems are addressed with traditional transmission 

system reinforcements (e.g., transformers, shunt capacitors, local lines). 

• Limited application of non-traditional frequency-responsive controls on wind, solar PV, 

CSP plants, and energy storage are effective at improving both frequency nadir and 

settling frequency, and thus FR. Refinements to these controls would further improve 

performance.

• Individual BA FR may not meet its obligation without additional FR from resources both 

inside and outside the particular area. As noted above, non-traditional approaches are 

effective at improving FR. Current operating practice uses more traditional approaches 

(e.g., commit conventional plants with governors) to meet all FR needs.

• Using new, fast-responding resource technologies (e.g., inverter-based controls) to ensure 

adequate FR adds complexity, but also flexibility, with high levels of wind and solar 

generation. Control philosophy will need to evolve to take full advantage of easily 

adjustable speed of response, with additional consideration of location and size of the 

generation trip. 

• For California, adequate FR was maintained during acute depletion of headroom from 

afternoon drop in solar, assuming the ability of California hydro to provide FR. 

Other Conclusions

• Accurate modeling of solar PV, CSP, wind, and load behavior is extremely important 

when analyzing high stress conditions; all of these models had an impact on system 

performance. 

• Attention to detail is important. Local and locational issues may drive constraints on both 

FR and transient stability.

• The location of generation tripping, e.g., DG vs. central station, is not as important as the 

amount of generation that is tripped. However, widespread deliberate or common-mode 

DG tripping after a large disturbance has an adverse impact on system performance. It is 

recommended that practice adapt to take advantage of new provisions in IEEE 1547 that 

allow for voltage and frequency ride-through of DG to improve system stability.

• Further analysis is needed to determine operational limits with low levels of synchronous 

generation, to identify changes to path ratings and associated remedial action schemes, 

and to quantify DG impact on transmission system performance.

• Because a broad range of both conventional and non-standard operation and control 

options improved system performance, further investigation of the most economic and 

effective alternatives is warranted. This should include consideration of the costs and 

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138

benefits of constraining commitment and dispatch to reserve FR, as well as the capital 

and operating costs of new controls and equipment.

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139

9 References

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LBNL-4143E, December 2010.

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This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
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10 Appendix 

10.1 Supporting Material on WECC Planning Cases

Table 28. Approved WECC Planning Databases Considered 

a. 2013 LW

b. 2013 HW

c. 2013 HS

d. 2018 HS

e. 2022 LSP

f. 2016 LS

g. 2023 HS

h. 2013 HS

i. 2013 LS

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Figure 82. LSP Base case bubble diagram.

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Figure 83. HS Base case bubble diagram. 

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Supporting Material on Improvements to WECC Dynamic Data

Table 29. Wind Plants Modeled as Synchronous Generators (Example) 

 BusNum BusName BuskV id MVA GenModel Data Owner Unit Type

54251 GARDNG2 25 1 30 genrou Wind

54255 CHINOOK9 34.5 1 13.5 gentpj Wind

54265 US WIND9 34.5 1 13.5 gentpj Wind

54641 TURNIPG2 0.69 2 85 genrou Wind

54789 WINTG1 0.69 G1 21 genrou Wind

54846 WINTG4 0.69 G4 21 genrou Wind

54848 WINTG5 0.69 G5 21 genrou Wind

54852 WINTG6 0.69 G6 21 genrou Wind

55328 MACLEOD4 1 G2 39.8 genrou Wind

55733 WR1LV19 12 1 21 genrou Wind

56155 OLDEL_C1 0.69 G1 80 genrou Wind

56156 OLDEL_C3 0.69 G3 80 genrou Wind

56294 TOTHILL2 0.69 G1 35.6 genrou Wind

56328 MACLEOD5 1 G1 27.5 genrou Wind

56338 SUMV4 0.69 1 46 genrou Wind

56355 OLDEL_C2 0.69 G2 80 genrou Wind

56356 OLDEL_C4 0.69 G4 80 genrou Wind

56389 HILLRIDG 25 1 30 genrou Wind

56402 KETTLES2 34.5 1 1.8 genrou Wind

56402 KETTLES2 34.5 2 14.4 genrou Wind

56402 KETTLES2 34.5 3 1.8 genrou Wind

56402 KETTLES2 34.5 4 12.6 genrou Wind

56464 WR2WF7 12 4 21 genrou Wind

56465 WR2WF12 12 9 21 genrou Wind

56632 WR1LV16 12 1 19.5 genrou Wind

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Table 30. Netted Units (Examples)

10.2 Supporting Material on Base Cases

Table 31. Generation Initial Condition Summary for LSP Base Case

WECC CALIFORNIA DSW NORTHEAST NORTHWEST

Pg 43.5 6.4 10.4 2.9 12.4

mc 64.2 11.7 15.0 4.2 17.3

hr 20.7 5.3 4.6 1.3 4.9

nu 800 169 128 91 202

pm 43.6 6.4 10.4 2.9 12.5

mv 65.5 12.4 16.5 4.5 16.1

px 52.1 14.4 14.6 9.3 2.1

mx 52.3 14.5 14.6 9.4 2.1

nx 754 295 72 128 62

pg 95.6 20.8 24.9 12.3 14.6

qg 5.1 -0.7 2.2 0.8 0.8

pl 111.2 33.1 24.3 11.2 19.0

ql 31.7 8.0 7.0 3.5 4.2

pw 20.9 4.4 4.0 2.5 8.4

qw -1.0 -0.4 0.4 -0.2 -0.7

pv 3.9 3.7 0.2 0.0 0.0

qv -0.2 -0.2 0.0 0.0 0.0

ps 0.9 0.9 0.0 0.0 0.0

dg 0.0 0.0 0.0 0.0 0.0

nh 800 169 128 91 202

Kt 0.46 0.34 0.44 0.26 0.62

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Table 32. Generation Initial Condition Details for LSP Base Case

Pg mc hr nu pm mv px mx nx pg qg pl ql pw qw pv qv ps dg nh Kt

WECC 43.5 64.2 20.7 800 43.6 65.5 52.1 52.3 754 95.6 5.1 111.2 31.7 20.9 -1.0 3.9 -0.2 0.9 0.0 800 0.46

CALIFORNIA 6.4 11.7 5.3 169 6.4 12.4 14.4 14.5 295 20.8 -0.7 33.1 8.0 4.4 -0.4 3.7 -0.2 0.9 0.0 169 0.34

DSW 10.4 15.0 4.6 128 10.4 16.5 14.6 14.6 72 24.9 2.2 24.3 7.0 4.0 0.4 0.2 0.0 0.0 0.0 128 0.44

NORTHEAST 2.9 4.2 1.3 91 2.9 4.5 9.3 9.4 128 12.3 0.8 11.2 3.5 2.5 -0.2 0.0 0.0 0.0 0.0 91 0.26

ALBERTA 2.0 3.3 1.4 67 2.0 3.5 10.1 10.1 80 12.1 1.8 13.4 6.1 1.7 -0.1 0.0 0.0 0.0 0.0 67 0.22

ARIZONA 4.1 6.4 2.2 56 4.1 6.5 10.6 10.6 41 14.7 1.2 10.4 3.3 0.3 0.0 0.1 0.0 0.0 0.0 56 0.37

B.C.HYDRO 8.4 11.1 2.8 118 8.4 10.9 0.8 0.8 107 9.1 0.0 8.7 2.3 0.0 0.0 0.0 0.0 0.0 0.0 118 0.93

EL PASO 0.2 0.3 0.1 3 0.2 0.4 0.4 0.4 7 0.6 0.1 1.1 0.2 0.0 0.0 0.0 0.0 0.0 0.0 3 0.42

IDAHO 1.4 1.9 0.5 29 1.4 1.8 2.7 2.7 36 4.1 0.2 3.2 0.6 0.4 -0.1 0.0 0.0 0.0 0.0 29 0.38

IMPERIALCA 0.2 0.3 0.1 2 0.2 0.3 1.0 1.0 35 1.2 0.0 0.6 0.2 0.0 0.0 0.0 0.0 0.0 0.0 2 0.24

LADWP 1.5 1.8 0.3 9 1.5 1.9 1.0 1.0 2 2.5 0.2 4.6 0.9 0.2 -0.1 0.0 0.0 0.0 0.0 9 0.60

MEXICO-CFE 0.4 0.7 0.3 7 0.4 0.8 0.3 0.3 2 0.7 0.0 1.1 0.5 0.0 0.0 0.0 0.0 0.0 0.0 7 0.69

MONTANA 0.3 0.6 0.3 28 0.3 0.6 2.5 2.5 17 2.8 0.4 1.5 0.5 0.4 -0.1 0.0 0.0 0.0 0.0 28 0.17

NEVADA 1.0 1.4 0.4 7 1.0 1.8 0.5 0.5 3 1.5 0.1 2.5 0.6 0.0 0.0 0.1 0.0 0.0 0.0 7 0.72

NEW MEXICO 0.8 1.4 0.5 6 0.8 1.6 0.3 0.3 2 1.1 0.2 1.7 0.4 1.1 0.0 0.0 0.0 0.0 0.0 6 0.49

NORTHWEST 12.4 17.3 4.9 202 12.5 16.1 2.1 2.1 62 14.6 0.8 19.0 4.2 8.4 -0.7 0.0 0.0 0.0 0.0 202 0.62

PACE 0.8 1.2 0.4 27 0.8 1.4 2.8 2.8 24 3.7 0.3 5.1 1.9 1.4 0.0 0.0 0.0 0.0 0.0 27 0.22

PG AND E 3.4 6.7 3.2 124 3.5 7.0 7.3 7.3 193 10.8 -0.7 12.6 3.6 0.8 0.0 1.9 0.0 0.0 0.0 124 0.40

PSCOLORADO 1.5 1.7 0.2 9 1.5 2.1 1.6 1.6 4 3.1 0.5 5.2 1.3 1.8 0.1 0.0 0.0 0.0 0.0 9 0.34

SANDIEGO 0.1 0.2 0.1 1 0.1 0.2 0.2 0.2 16 0.3 0.0 2.7 0.6 0.3 0.0 0.5 0.1 0.0 0.0 1 0.15

SIERRA 0.4 0.5 0.1 6 0.4 0.6 1.3 1.3 50 1.7 0.0 1.4 0.3 0.3 -0.1 0.0 0.0 0.0 0.0 6 0.23

SOCALIF 1.2 2.8 1.6 33 1.2 2.9 4.9 5.0 49 6.1 -0.2 12.5 2.7 3.2 -0.3 1.3 -0.3 0.9 0.0 33 0.23

FORTISBC 0.6 0.8 0.2 18 0.6 0.8 0.4 0.4 8 1.0 0.2 0.5 0.2 0.0 0.0 0.0 0.0 0.0 0.0 18 0.67

WAPA R.M. 2.7 3.8 1.1 47 2.7 4.2 1.2 1.2 15 3.8 0.1 3.5 1.2 0.7 0.2 0.0 0.0 0.0 0.0 47 0.67

WAPA U.M. 0.1 0.1 0.0 1 0.1 0.1 0.0 0.0 1 0.1 0.0 -0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0 1 0.94

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Table 33. Generation Summary for HS Base Case

WECC CALIFORNIA DSW NORTHEAST NORTHWEST

Pg 84.8 26.5 17.0 3.3 24.4

mc 109.1 32.3 21.4 4.3 32.6

hr 24.1 5.7 4.4 1.0 8.1

nu 1135 295 242 91 301

pm 85.0 26.5 17.0 3.3 24.4

mv 112.3 34.8 24.4 4.6 29.8

px 111.7 38.8 36.0 14.7 8.9

mx 111.9 38.8 36.1 14.8 8.9

nx 1251 509 236 145 129

pg 196.5 65.2 53.0 18.1 33.3

qg 22.6 8.8 7.9 1.9 2.3

pl 184.6 67.9 46.9 18.0 27.2

ql 59.2 20.8 14.6 6.0 8.6

pw 5.6 2.1 0.8 1.7 0.0

qw -0.6 -0.1 -0.2 -0.2 0.0

pv 1.2 1.1 0.1 0.0 0.0

qv 0.1 0.1 0.0 0.0 0.0

ps 0.4 0.4 0.0 0.0 0.0

dg 0.0 0.0 0.0 0.0 0.0

nh 1135 295 242 91 301

Kt 0.48 0.43 0.37 0.21 0.78

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149

Table 34. Generation Initial Condition Details for HS Base Case

Pg mc hr nu pm mv px mx nx pg qg pl ql pw qw pv qv ps dg nh Kt

WECC 84.8 109.1 24.1 1135 85.0 112.3 111.7 111.9 1251 196.5 22.6 184.6 59.2 5.6 -0.6 1.2 0.1 0.4 0.0 1135 0.48

CALIFORNIA 26.5 32.3 5.7 295 26.5 34.8 38.8 38.8 509 65.2 8.8 67.9 20.8 2.1 -0.1 1.1 0.1 0.4 0.0 295 0.43

DSW 17.0 21.4 4.4 242 17.0 24.4 36.0 36.1 236 53.0 7.9 46.9 14.6 0.8 -0.2 0.1 0.0 0.0 0.0 242 0.37

NORTHEAST 3.3 4.3 1.0 91 3.3 4.6 14.7 14.8 145 18.1 1.9 18.0 6.0 1.7 -0.2 0.0 0.0 0.0 0.0 91 0.21

ALBERTA 1.1 1.9 0.9 51 1.1 2.0 10.9 10.9 85 12.0 1.3 12.9 5.7 0.9 -0.1 0.0 0.0 0.0 0.0 51 0.14

ARIZONA 5.7 7.6 1.9 73 5.7 7.9 21.6 21.6 118 27.2 4.1 20.8 6.6 0.2 0.0 0.0 0.0 0.0 0.0 73 0.26

B.C.HYDRO 10.5 13.9 3.3 127 10.6 13.7 0.7 0.7 124 11.2 -0.1 8.2 2.7 0.1 0.0 0.0 0.0 0.0 0.0 127 0.95

EL PASO 1.1 1.3 0.2 14 1.1 1.8 0.5 0.5 4 1.6 0.2 2.3 0.5 0.0 0.0 0.0 0.0 0.0 0.0 14 0.69

IDAHO 1.2 1.6 0.4 28 1.2 1.6 3.0 3.0 29 4.2 0.3 3.9 1.3 0.1 0.0 0.0 0.0 0.0 0.0 28 0.34

IMPERIALCA 0.4 0.6 0.2 9 0.4 0.7 1.1 1.1 34 1.5 0.1 0.3 0.2 0.0 0.0 0.0 0.0 0.0 0.0 9 0.36

LADWP 3.8 4.6 0.8 25 3.8 5.1 2.3 2.3 14 6.1 1.2 7.2 2.4 0.2 0.0 0.0 0.0 0.0 0.0 25 0.65

MEXICO-CFE 1.5 2.0 0.5 11 1.5 2.2 1.2 1.2 14 2.7 0.3 2.6 0.7 0.0 0.0 0.0 0.0 0.0 0.0 11 0.63

MONTANA 0.5 0.7 0.3 30 0.5 0.8 2.7 2.7 18 3.2 0.6 1.9 0.6 0.1 0.0 0.0 0.0 0.0 0.0 30 0.21

NEVADA 1.4 1.8 0.4 14 1.4 2.3 4.7 4.7 52 6.1 1.6 6.8 2.1 0.0 0.0 0.1 0.0 0.0 0.0 14 0.27

NEW MEXICO 1.5 1.6 0.1 9 1.5 1.8 2.0 2.0 14 3.5 0.3 3.1 0.6 0.0 0.0 0.0 0.0 0.0 0.0 9 0.44

NORTHWEST 24.4 32.6 8.1 301 24.4 29.8 8.9 8.9 129 33.3 2.3 27.2 8.6 0.0 0.0 0.0 0.0 0.0 0.0 301 0.78

PACE 0.9 1.1 0.2 23 0.9 1.3 7.4 7.4 51 8.3 0.8 9.9 3.3 1.2 0.0 0.0 0.0 0.0 0.0 23 0.11

PG AND E 13.1 16.2 3.1 169 13.2 17.2 17.8 17.8 322 30.9 2.9 29.0 9.7 0.8 -0.1 0.7 0.1 0.0 0.0 169 0.46

PSCOLORADO 3.7 4.9 1.2 75 3.7 5.7 4.0 4.0 21 7.8 1.1 8.1 2.8 0.4 -0.2 0.1 0.0 0.0 0.0 75 0.52

SANDIEGO 2.1 2.5 0.4 31 2.1 2.9 1.5 1.5 24 3.6 0.3 5.5 1.5 0.3 0.0 0.4 0.0 0.0 0.0 31 0.52

SIERRA 0.6 0.7 0.0 8 0.6 0.8 1.7 1.7 47 2.3 0.1 2.3 0.6 0.3 -0.1 0.0 0.0 0.0 0.0 8 0.25

SOCALIF 7.0 8.3 1.3 61 7.0 8.9 16.0 16.0 115 23.0 4.2 25.9 7.0 0.8 0.0 0.0 0.0 0.4 0.0 61 0.33

FORTISBC 0.6 0.8 0.2 17 0.6 0.8 0.5 0.5 9 1.1 0.2 0.9 0.3 0.0 0.0 0.0 0.0 0.0 0.0 17 0.59

WAPA R.M. 3.6 4.2 0.6 57 3.6 4.9 3.2 3.2 27 6.8 0.5 5.8 2.1 0.0 0.0 0.0 0.0 0.0 0.0 57 0.56

WAPA U.M. 0.1 0.1 0.0 2 0.1 0.1 0.0 0.0 0 0.1 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 2 1.00

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150

Supporting Material on Complex Load Model

The blocking functions, vrflag for voltage and frflag for frequency, allow for the inverters to be 

blocked temporarily when the voltage drops, or to trip, i.e., not recover after the voltage or 

frequency returns to within the allowed region. The trapezoidal shape for the two functions 

allows for the user to account for the distributed nature of the resources and for the nonuniformity of tripping. Further, the fraction of devices that block vs trip can be set anywhere 

between 0 and 1.

Figure 84. Composite load model – DG tripping/blocking logic.

÷

vterm

N

D

×

÷

pinit

0.01

N

D

×

vt 0 vt 1 vt 2 vt3

1

0

vrflag

fterm

ft 0 ft 1 ft 2 ft3

1

0

frflag

ip

iq

Proposed

PV model for 

CMPLDW

qinit Iqord’

Current 

Limit Logic

ialim

ipord

iqord

iaord = (ipord’2

 + iqord’2

)

1⁄2

If( iaord > ialim )

ratio = ialim / iaord

ipord = ratio * ipord’

iqord = ratio * iqord’

ipord’ 

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151

10.3 Supporting Material for Performance and Monitoring Metrics

 Table 35. Transient Stability Performance Requirements

Disturbance 

Class

Outage Frequency 

Associated with 

Performance 

Category 

Transient Voltage 

Dip

Minimum 

Transient 

Frequency 

Standard 

Post Transient 

Voltage Deviation 

Standard 

A NA NERC NERC NERC

B ≥ 0.33 Not to exceed 25% 

at load buses or 

30% at non-load 

buses. Not to 

exceed 20% for 

more than 20 cycles 

at load buses.

Not below 59.6 Hz 

for 6 cycles or 

more at a load bus.

Not to exceed 5% at 

any bus

C 0.033-0.33 Not to exceed 30% 

at any bus. Not to 

exceed 20% for 

more than 40 cycles 

at load buses.

Not below 59.0 Hz 

for 6 cycles or 

more at a load bus.

Not to exceed 10% 

at any bus

D <0.033 NERC NERC NERC

Figure 85. WECC reliability criteria. 

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152

.

Figure 86. WECC key bus locations.

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153

Table 36. WECC Key Buses

Bus # Bus Name Sub Name kV Bus # Bus Name Sub Name kV

40323 CUSTER W Custer 500 40145 BOUNDARY Boundary 230

50704 KLY500 Kelly Lake 500 79021 CURECANT Curecanti 230

54158 LANGDON2 Langdon 500 24804 DEVERS Devers 230

40687 MALIN Malin 500 62071 GT FALLS Great Falls 230

26048 MCCULLGH McCullough 500 40551 HOT SPR Hot Springs 230

14002 MOENKOPI Moenkopi 500 22356 IMPRLVLY Imperial Valley 230

40809 OSTRNDER Ostrander 500 40621 LAGRANDE La Grande 230

40869 RAVER Raver 500 30970 MIDWAY Midway Peaker 230

30015 TABLE MT Table Mt. 500 65975 MINERS Miners 230

30040 TESLA

Tesla 

Peaker 500 14221 PNPKAPS Pinnacle Peak APS 230

73012 AULT Ault 345 79057 RIFLE CU Rifle 230

11093 LUNA Luna 345 66335 SHERIDAN Sheridan 230

60235 MIDPOINT Midpoint 345 26078 TOLUCA Toluca 230

66225 PINTO Pinto 345 54135 SUNDANM9 Sundance 240

66340 SIGURD Sigurd 345 14231 WESTWING Westwing 230

66510 TERMINAL Salt Lake Ci 345 62019 WILSALL Wilsall 230

64130 VALMY North almy 345 62114 RATTLE S Rattlesnake 100

10369 WESTMESA West Mesa 345 66278 RANGELY Rangley 138

64006 AUSTIN Austin 230 64107 SUMMIT 1 Summit 120

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154

10.4 Supporting Material on Wind and Solar Siting

Figure 87. Wind plant sites and ratings from WWSIS-2 raw data. 

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155

Figure 88. CSP sites and ratings from WWSIS-2 raw data. 

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156

Figure 89. U.S. PV near best resources from WWSIS-2 raw data. 

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157

Figure 90. U.S. PV near population centers from WWSIS-2 raw data. 

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158

Figure 91. Distributed PV sites and ratings from WWSIS-2 raw data. 

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159

10.5 Supporting Material on Recommitment and Re-Dispatch

Supporting Material on Recommitment and Re-Dispatch Method

Table 37. LSP PLEXOS Sample Windows

March 23 10:35 – 11:45

April 1 9:10 – 15:40

April 8 9:45 – 18:10

April 9 6:50 – 13:45

April 13 12:55

April 16 10:55 – 14:10

April 17 12:45 – 14:50

April 20 7:10 – 17:35

April 21 8:20 – 12:00

Figure 92. Simple-cycle gas turbines dispatch – LSP window. 

y = -0.0093x + 1087.9

y = 0.0012x + 753.06

y = -0.013x + 1455.6

y = 0.0019x + 604.3

0

200

400

600

800

1000

1200

1400

1600

1800

0 20000 40000 60000 80000 100000 120000 140000

SCGT Sensitivity to W+S and Load

Gas CT HiMix

Gas CT HiMix

Gas CT HiMix

Gas CT HiMix

Linear (Gas CT HiMix)

Linear (Gas CT HiMix)

Linear (Gas CT HiMix)

Linear (Gas CT HiMix)

Mean: 

20633, 896

Mean: 

48286,826

Mean: 115217, 

896

Mean: 

115301, 826

Base vs. W+S

Base vs. Load

Hi-Mix Points vs. W+S

Hi-Mix Points vs. Load

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160

Figure 93. Hydro dispatch – LSP window.

Figure 94. Pumped-storage hydro dispatch – LSP window. 

y = 0.1801x + 25647

y = 0.1907x + 7393 y = 0.0395x + 26800

y = 0.2194x + 3415.5

0

5000

10000

15000

20000

25000

30000

35000

0 20000 40000 60000 80000 100000 120000 140000

Hydro Sensitivity to W+S and Load

Hydro TEPPC

Hydro TEPPC

Hydro HiMix

Hydro HiMix

Linear (Hydro TEPPC)

Linear (Hydro TEPPC)

Linear (Hydro HiMix)

Linear (Hydro HiMix)

Mean: 20633, 

29364

Mean: 48286, 

28707

Mean: 115217, 

29364

Mean: 115301, 28707

Base vs. W+S

Base vs. Load

Hi-Mix Points vs. W+S

Hi-Mix Points vs. Load

y = 0.01x - 480.45

y = 0.0743x - 8838.2

y = -0.1158x + 2367.4

y = -0.0041x - 2754.2

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

0 20000 40000 60000 80000 100000 120000 140000

PSH Sensitivity to W+S and Load

Storage TEPPC

Storage TEPPC

Storage HiMix

Storage HiMix

Linear (Storage TEPPC)

Linear (Storage TEPPC)

Linear (Storage HiMix)

Linear (Storage HiMix)

Mean: 20633, -273

Mean: 48286, -3222

Mean: 115217, -273

Mean: 115301, -3222

Base vs. W+S

Base vs. Load

Hi-Mix Points vs. W+S

Hi-Mix Points vs. Load

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161

Figure 95. Combined-cycle plants – HS window.

Table 38. HS PLEXOS Sample Windows

July 16 13:45 – 14:40 12 samples

July 17 13:55 – 14:30 8 samples

July 21 13:25 – 13:30 2 samples

July 30 13:20 – 13:3 3 samples

August 11 14:05 – 14:15 3 samples 

August 12 12:20 – 13:10 8 samples

August 13 11:35 – 12:55 17 samples

August 14 13:00 – 14:20 16 samples

August 25 12:55 – 13:25 7 samples

August 26 12:05 – 13:35 19 samples

y = 0.0655x + 43454

y = 0.6629x - 56998

y = -1.0533x + 67924

y = 0.764x - 95185

0

10000

20000

30000

40000

50000

60000

0 20000 40000 60000 80000 100000 120000 140000 160000 180000

Summer Peak: Gas CC Sensitivity to W+S and Load

Gas CC

Gas CC

Gas CC

Gas CC

Linear (Gas CC)

Linear (Gas CC)

Linear (Gas CC)

Linear (Gas CC)

Mean: 13283, 

44324

Mean: 44003, 

21577

Mean: 152840, 

44324

Mean: 

152840, 

21577

HS’23: 33868, 

6355

HS’23: 33868, 

192000

Base vs. W+S

Base vs. Load

Hi-Mix Points vs. W+S

Hi-Mix Points vs. Load

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162

Figure 96. Coal plants – HS window. 

y = -0.4879x + 33839 y = -0.029x + 31787

y = -0.023x + 27584

y = 0.1799x - 924.67

20000

21000

22000

23000

24000

25000

26000

27000

28000

29000

30000

0 20000 40000 60000 80000 100000 120000 140000 160000 180000

Summer Peak: Coal Sensitivity to W+S and Load

Coal

Coal

Coal

Coal

Linear (Coal)

Linear (Coal)

Linear (Coal)

Linear (Coal)

Mean: 13283, 

27359

Mean: 44003, 

26574

Mean: 152840, 

27359

Mean: 

152840, 

26574

HS’23: 30509, 

192000

HS’23: 30509, 

6355

Base vs. W+S

Base vs. Load

Hi-Mix Points vs. W+S

Hi-Mix Points vs. Load

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163

Supporting Material on Process of Incremental Commitment and Dispatch

Table 39. Adding Renewable Capacity Including WWSIS-2 to LSP Base Case

Area CSP PV Wind CSP PV Wind CSP PV DG Wind

ALBERTA 0 0 2707 0 0 0 0 0 0 2707

ARIZONA 0 971 175 7654 5120 1440 6879 4923 3655 1435

B.C.HYDRO 0 0 108 0 0 0 0 0 0 108

EL PASO 0 0 0 142 343 50 142 343 368 50

IDAHO 0 0 643 0 0 569 0 0 0 643

IMPERIALCA 0 0 0 188 611 917 188 611 71 917

LADWP 0 0 576 1043 913 0 1043 913 1961 576

MEXICO-CFE 0 0 0 0 0 294 0 0 15 0

MONTANA 0 0 707 0 27 3988 0 11 21 3975

NEVADA 0 64 0 229 642 0 229 556 285 0

NEW MEXICO 0 100 1726 156 1160 3084 156 1260 758 3108

NORTHWEST 0 59 8680 0 817 11642 0 869 500 11655

PACE 0 0 2384 0 409 4082 0 390 1126 4111

PG AND E 0 3232 2399 0 1740 1799 0 3232 1474 2399

PSCOLORADO 0 79 2134 169 937 1720 169 1016 547 2134

SANDIEGO 0 516 712 0 275 0 0 516 421 712

SIERRA 0 0 432 0 1511 821 0 1504 432 777

SOCALIF 1436 2139 4497 2813 6118 3149 2814 5913 4741 4497

FORTISBC 0 0 0 0 0 0 0 0 0 0

WAPA R.M. 0 4 739 0 696 8149 0 690 594 8136

WAPA U.M. 0 0 0 0 1 60 0 0 0 60

Total 1436 7164 28616 12393 21322 41762 11618 22747 16969 47999

LSP Base Case WWSIS2-HiMix LSP Hi-Mix Case

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164

Table 40. Incremental Renewable Dispatch – Sample 31775

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165

Figure 97. LSP Hi-Mix case bubble. 

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166

Table 41. Added Renewable Capacity Including WWSIS-2 for HS Hi-Mix Case

Area CSP PV Wind CSP PV Wind CSP PV DG Wind

ALBERTA 0 0 1061 0 0 0 0 0 0 1061

ARIZONA 0 700 227 7654 5120 1440 7654 5202 3655 1457

B.C.HYDRO 0 0 237 0 0 0 0 0 0 237

EL PASO 0 47 0 142 343 50 142 350 305 50

IDAHO 0 0 407 0 0 569 0 0 0 564

IMPERIALCA 0 0 0 188 611 917 188 611 71 917

LADWP 270 0 574 851 913 0 837 913 1961 574

MEXICO-CFE 0 0 0 0 0 294 0 0 15 294

MONTANA 0 0 364 0 27 3988 0 27 21 3989

NEVADA 0 64 0 229 642 0 229 656 324 0

NEW MEXICO 0 27 396 156 1160 3084 156 1157 758 3120

NORTHWEST 0 0 0 0 1607 11642 0 1607 500 11642

PACE 0 0 2309 0 409 4082 0 409 1126 4032

PG AND E 0 2570 1033 0 1740 1799 0 2570 1474 1758

PSCOLORADO 0 79 2134 169 937 1830 169 960 547 2134

SANDIEGO 0 516 1562 0 275 0 0 516 357 1562

SIERRA 0 0 352 0 1511 821 0 1511 432 802

SOCALIF 822 49 887 2813 6118 3149 2825 6100 4741 3147

FORTISBC 0 0 0 0 0 0 0 0 0 0

WAPA R.M. 0 0 139 0 696 8149 0 696 594 8137

WAPA U.M. 0 0 0 0 1 60 0 0 0 60

Total 1092 4052 11680 12201 22112 41871 12199 23286 16882 45535

HS Base Case WWSIS2-HiMix HS Hi-Mix Case

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167

Table 42. PLEXOS Case – Renewable Summary for HS Hi-Mix Case

Area name Area # CSP WWSIS PV WWSIS_wind Area # CSP WWSIS PV DG ratio DG PV WWSIS_wind Area # CSP WWSIS PV DG PV Wind

ALBERTA 54 54 54

ARIZONA 14 351 389 326 14 3182 4608 0.42 1919 2689 111 14 2831 4219 1919 2300 -215

BC HYDRO 50 50 50

EL PASO 11 21 11 359 0.47 169 190 21 11 0 359 169 190 0

IDAHO 60 120 60 0 0 167 60 0 0 0 0 47

IMPERIALICA 21 155 32 21 187 432 0.10 45 387 27 21 187 277 45 232 -5

LADWP 26 1147 259 26 1147 1770 0.68 1207 563 26 0 1510 1207 304 0

MEXICO-CFE 20 5 20 10 5 20 10

MONTANA 62 64 62 25 0.44 11 14 762 62 0 25 11 14 698

NEVADA 18 116 18 197 464 0.31 142 321 18 81 464 142 321 0

NEW MEXICO 10 53 92 4 10 631 0.40 249 382 106 10 -53 540 249 290 102

NORTHWEST 40 6064 40 770 0.38 292 477 6908 40 0 770 292 477 844

PACE 65 226 65 838 0.73 615 223 1448 65 0 838 615 223 1222

PG&E 30 28 30 2175 0.46 997 1177 102 30 0 2175 997 1177 74

PSCOLORADO 70 186 265 37 70 186 827 0.37 305 522 60 70 0 562 305 257 23

SAN DIEGO 22 22 437 0.57 247 190 22 0 437 247 190 0

SIERRA 64 119 31 64 1372 0.22 305 1067 95 64 0 1253 305 948 64

SOCALIF 24 2396 1825 89 24 3095 6963 0.44 3040 3923 73 24 698 5137 3040 2097 -16

FORTISBC 52 52 52

WAPA R.M. 73 480 363 73 720 0.46 331 389 873 73 0 240 331 -92 510

WAPA U.M. 63 63 1 0.00 0 1 0 63 0 1 0 1 0

-1 -1 251 506 72 -1 1508 1476 132 -1 1256

Total 4500 4090 7480 9502 23877 9875 12516 10890 Total 5001 18817 9875 8932 3349

TEPCC WWSIS II @ 68417 HiMix @ 68417 Difference

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168

Figure 98. HS Hi-Mix case bubble. 

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169

Supporting Material on Dynamic Initial Conditions of Hi-Mix cases 

Table 43. LSP Hi-Mix Case – Initial Condition Metrics

WECC CALIFORNIA DSW NORTHEAST NORTHWEST

Pg 34.4 5.4 5.4 2.7 9.5

mc 56.3 10.7 8.7 3.9 17.2

hr 21.9 5.3 3.3 1.2 7.7

nu 768 167 103 89 200

pm 34.4 5.4 5.5 2.7 9.5

mv 56.7 11.4 9.5 4.1 16.0

px 39.8 11.2 12.9 2.7 2.1

mx 40.3 11.3 13.3 2.7 2.1

nx 740 284 93 107 62

ps 74.2 16.6 18.4 5.5 11.7

qs 3.6 -0.2 2.7 0.6 -1.2

pl 109.3 32.0 23.5 11.1 19.1

ql 29.8 7.6 5.5 3.3 4.3

pw 27.0 4.7 6.9 5.3 8.4

qw -2.0 -0.1 0.3 -0.6 -1.3

pv 10.2 5.8 3.3 0.8 0.3

qv -1.3 -0.3 -0.9 0.1 -0.2

ps 8.4 1.5 7.0 0.0 0.0

dg 7.0 3.7 2.6 0.4 0.2

nh 768 167 103 89 200

Kt 0.42 0.33 0.27 0.31 0.61

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170

Table 44. Details of LSP Hi-Mix Case Dynamic Initial Conditions

Pg mc hr nu pm mv px mx nx ps qs pl ql pw qw pv qv ps dg nh Kt

WECC 34.4 56.3 21.9 768 34.4 56.7 39.8 40.3 740 74.2 3.6 109.3 29.8 27.0 -2.0 10.2 -1.3 8.4 7.0 768 0.42

CALIFORNIA 5.4 10.7 5.3 167 5.4 11.4 11.2 11.3 284 16.6 -0.2 32.0 7.6 4.7 -0.1 5.8 -0.3 1.5 3.7 167 0.33

DSW 5.4 8.7 3.3 103 5.5 9.5 12.9 13.3 93 18.4 2.7 23.5 5.5 6.9 0.3 3.3 -0.9 7.0 2.6 103 0.27

NORTHEAST 2.7 3.9 1.2 89 2.7 4.1 2.7 2.7 107 5.5 0.6 11.1 3.3 5.3 -0.6 0.8 0.1 0.0 0.4 89 0.31

ALBERTA 2.0 3.3 1.4 67 2.0 3.5 9.6 9.6 78 11.6 1.7 13.4 6.1 1.7 -0.3 0.0 0.0 0.0 0.0 67 0.23

ARIZONA 3.3 5.4 2.1 51 3.3 5.3 11.2 11.5 70 14.5 1.9 10.1 2.9 0.8 -0.1 1.4 -0.5 6.4 1.4 51 0.29

B.C.HYDRO 8.4 11.1 2.8 118 8.4 10.9 0.5 0.5 106 8.9 -0.3 8.6 2.3 0.0 0.0 0.0 0.0 0.0 0.0 118 0.95

EL PASO 0.2 0.3 0.1 3 0.2 0.4 0.4 0.5 6 0.7 0.0 1.1 0.2 0.0 0.0 0.2 -0.1 0.1 0.2 3 0.30

IDAHO 1.4 1.9 0.5 29 1.4 1.8 0.5 0.5 31 1.9 0.2 3.2 0.6 0.4 -0.1 0.0 0.0 0.0 0.0 29 0.68

IMPERIALCA 0.2 0.3 0.1 2 0.2 0.3 1.1 1.1 36 1.3 0.0 0.6 0.2 0.3 0.0 0.3 0.0 0.1 0.0 2 0.16

LADWP 0.5 0.8 0.3 8 0.5 0.9 0.6 0.6 12 1.1 0.1 4.2 0.5 0.2 -0.1 0.4 -0.1 0.5 0.8 8 0.42

MEXICO-CFE 0.3 0.5 0.2 6 0.3 0.5 0.3 0.3 2 0.6 0.1 1.1 0.4 0.0 0.0 0.0 0.0 0.0 0.0 6 0.59

MONTANA 0.3 0.6 0.3 28 0.3 0.6 0.4 0.4 13 0.7 0.0 1.3 0.5 1.6 -0.4 0.0 0.0 0.0 0.0 28 0.23

NEVADA 0.8 1.0 0.3 6 0.8 1.5 0.6 0.6 5 1.4 0.3 2.4 0.4 0.0 0.0 0.4 -0.1 0.1 0.2 6 0.52

NEW MEXICO 0.0 0.0 0.0 3 0.0 0.0 0.2 0.2 1 0.2 0.0 1.4 -0.2 1.0 0.1 0.3 -0.2 0.2 0.2 3 0.03

NORTHWEST 9.5 17.2 7.7 200 9.5 16.0 2.1 2.1 62 11.7 -1.2 19.1 4.3 8.4 -1.3 0.3 -0.2 0.0 0.2 200 0.61

PACE 0.7 0.9 0.2 25 0.7 1.1 1.0 1.0 14 1.6 0.1 5.3 1.8 2.7 -0.3 0.1 0.0 0.0 0.2 25 0.19

PG AND E 3.4 6.6 3.2 123 3.4 7.0 5.2 5.2 167 8.6 -0.5 12.0 3.7 0.8 0.0 1.9 0.0 0.0 0.6 123 0.46

PSCOLORADO 0.9 1.0 0.1 4 0.9 1.2 0.5 0.5 3 1.4 0.4 5.3 1.3 1.8 0.3 0.6 -0.1 0.2 0.3 4 0.25

SANDIEGO 0.1 0.2 0.1 1 0.1 0.2 0.1 0.1 12 0.2 0.0 2.7 0.6 0.3 0.0 0.5 0.1 0.0 0.1 1 0.16

SIERRA 0.4 0.5 0.1 6 0.4 0.6 0.8 0.8 48 1.2 0.3 1.4 0.3 0.6 0.1 0.7 0.2 0.0 0.2 6 0.18

SOCALIF 1.2 2.8 1.6 33 1.2 2.9 4.3 4.4 57 5.5 0.2 12.5 2.6 3.2 0.0 2.7 -0.2 0.9 2.2 33 0.21

FORTISBC 0.6 0.8 0.2 18 0.6 0.8 0.4 0.4 8 1.0 0.2 0.5 0.2 0.0 0.0 0.0 0.0 0.0 0.0 18 0.67

WAPA R.M. 0.2 0.9 0.7 36 0.2 1.1 0.1 0.1 8 0.3 0.1 3.3 0.8 3.3 0.1 0.4 0.0 0.0 0.3 36 0.19

WAPA U.M. 0.1 0.1 0.0 1 0.1 0.1 0.0 0.0 1 0.1 0.0 -0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0 1 0.76

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Table 45. HS Hi-Mix Case – Initial Condition Metrics

WECC CALIFORNIA DSW NORTHEAST NORTHWEST

Pg 83.6 26.4 18.1 3.1 23.4

mc 111.2 31.8 26.4 4.0 31.9

hr 27.2 5.1 8.2 0.9 8.4

nu 1152 298 267 89 298

pm 84.0 26.6 18.2 3.1 23.5

mv 113.7 33.9 29.2 4.2 29.2

px 85.7 31.2 22.4 12.5 6.3

mx 85.9 31.3 22.4 12.6 6.4

nx 1094 467 158 143 94

ps 169.3 57.7 40.5 15.6 29.8

qs 29.1 11.9 8.2 2.8 4.4

pl 186.2 68.8 47.4 18.0 27.4

ql 60.4 21.4 14.9 6.0 8.7

pw 14.3 2.1 1.8 2.6 6.9

qw -2.6 0.3 -0.8 -0.4 -1.6

pv 11.2 5.8 3.8 1.3 0.2

qv -0.3 0.4 -0.5 0.0 -0.2

ps 6.6 3.1 3.5 0.0 0.0

dg 9.4 5.4 2.9 0.9 0.3

nh 1152 298 267 89 298

Kt 0.50 0.45 0.49 0.19 0.70

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Table 46. Details of HS Hi-Mix Case Dynamic Initial Conditions

Pg mc hr nu pm mv px mx nx ps qs pl ql pw qw pv qv ps dg nh Kt

WECC 83.6 111.2 27.2 1152 84.0 113.7 85.7 85.9 1094 169.3 29.1 186.2 60.4 14.3 -2.6 11.2 -0.3 6.6 9.4 1152 0.50

CALIFORNIA 26.4 31.8 5.1 298 26.6 33.9 31.2 31.3 467 57.7 11.9 68.8 21.4 2.1 0.3 5.8 0.4 3.1 5.4 298 0.45

DSW 18.1 26.4 8.2 267 18.2 29.2 22.4 22.4 158 40.5 8.2 47.4 14.9 1.8 -0.8 3.8 -0.5 3.5 2.9 267 0.49

NORTHEAST 3.1 4.0 0.9 89 3.1 4.2 12.5 12.6 143 15.6 2.8 18.0 6.0 2.6 -0.4 1.3 0.0 0.0 0.9 89 0.19

ALBERTA 1.1 1.9 0.9 51 1.1 2.0 10.9 10.9 85 12.0 1.3 12.9 5.7 0.9 -0.1 0.0 0.0 0.0 0.0 51 0.14

ARIZONA 7.8 14.0 6.1 116 7.9 14.5 13.0 13.0 72 20.8 4.4 21.1 6.7 0.2 0.0 2.1 -0.1 3.2 1.9 116 0.48

B.C.HYDRO 10.0 13.2 3.2 125 10.0 13.1 0.7 0.7 124 10.7 -0.3 8.2 2.7 0.1 0.0 0.0 0.0 0.0 0.0 125 0.95

EL PASO 1.1 1.3 0.2 14 1.1 1.8 0.5 0.5 4 1.6 0.2 2.2 0.5 0.1 0.0 0.1 -0.1 0.0 0.0 14 0.64

IDAHO 1.2 1.6 0.4 28 1.2 1.6 2.4 2.4 28 3.6 0.6 3.9 1.4 0.2 0.0 0.0 0.0 0.0 0.0 28 0.39

IMPERIALCA 0.5 0.7 0.1 10 0.5 0.7 0.9 0.9 30 1.4 0.2 0.3 0.1 0.0 0.1 0.4 0.0 0.2 0.0 10 0.34

LADWP 3.9 4.6 0.6 28 4.0 5.1 2.1 2.1 12 6.0 2.0 7.4 2.5 0.2 0.0 0.6 0.0 0.6 1.2 28 0.62

MEXICO-CFE 0.9 1.2 0.3 7 0.9 1.3 1.2 1.2 14 2.0 0.4 2.6 0.6 0.0 0.0 0.0 0.0 0.0 0.0 7 0.50

MONTANA 0.5 0.7 0.3 30 0.5 0.8 2.4 2.4 17 2.9 0.5 1.9 0.6 0.7 -0.1 0.0 0.0 0.0 0.0 30 0.20

NEVADA 1.3 1.6 0.3 11 1.3 1.9 1.9 1.9 32 3.2 1.2 6.9 2.2 0.0 0.0 0.3 0.0 0.2 0.1 11 0.42

NEW MEXICO 1.5 1.6 0.1 9 1.5 1.8 1.2 1.2 9 2.7 0.4 3.1 0.6 0.1 -0.3 0.4 -0.1 0.0 0.2 9 0.49

NORTHWEST 23.4 31.9 8.4 298 23.5 29.2 6.3 6.4 94 29.8 4.4 27.4 8.7 6.9 -1.6 0.2 -0.2 0.0 0.3 298 0.70

PACE 0.9 1.1 0.2 23 0.9 1.3 6.4 6.4 48 7.3 0.6 9.9 3.3 1.4 -0.2 0.2 -0.1 0.0 0.6 23 0.12

PG AND E 12.2 15.1 2.9 158 12.2 15.9 15.8 15.9 305 28.0 3.5 29.3 10.0 0.8 0.0 0.7 0.1 0.0 1.0 158 0.47

PSCOLORADO 2.8 3.8 0.9 60 2.9 4.3 3.3 3.4 17 6.2 1.2 8.3 3.0 0.4 -0.1 0.5 -0.1 0.2 0.3 60 0.47

SANDIEGO 1.5 1.8 0.3 27 1.5 2.0 0.7 0.7 20 2.2 0.5 5.6 1.5 0.3 0.1 0.4 0.1 0.0 0.2 27 0.56

SIERRA 0.4 0.4 0.0 6 0.4 0.5 1.4 1.4 50 1.7 1.1 2.3 0.6 0.3 -0.1 1.1 0.1 0.0 0.3 6 0.12

SOCALIF 8.4 9.6 1.2 75 8.5 10.2 11.8 11.8 100 20.1 5.6 26.2 7.3 0.8 0.1 3.8 0.2 2.4 2.9 75 0.37

FORTISBC 0.6 0.8 0.2 17 0.6 0.8 0.5 0.5 9 1.1 0.2 0.9 0.3 0.0 0.0 0.0 0.0 0.0 0.0 17 0.59

WAPA R.M. 3.6 4.2 0.6 57 3.6 4.9 2.5 2.5 24 6.1 0.8 5.8 2.1 0.9 -0.4 0.4 -0.1 0.0 0.3 57 0.53

WAPA U.M. 0.1 0.1 0.0 2 0.1 0.1 0.0 0.0 0 0.1 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 2 1.00

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Additional Comparative Results of Scenario Dispatches

Figure 99. Sum of Pmax in East (all generation). 

Figure 100. Pmax in East (non-renewable synchronous generation). 

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

ARIZONA EL PASO NEVADA NEW MEXICOPSCOLORADO WAPA R.M. IDAHO MONTANA PACE SIERRA

DSW NorthEast

WIND

Steam

PV

PSH

Other

NUC

HYDRO

GEO

GasCT

CSP

Coal

CCPP

Bio

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

ARIZONA EL PASO NEVADA NEW MEXICOPSCOLORADO WAPA R.M. IDAHO MONTANA PACE SIERRA

DSW NorthEast

Steam

PSH

Other

NUC

HYDRO

GEO

GasCT

Coal

CCPP

Bio

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174

Figure 101. Pmax in East (all synchronous generation including CSP). 

10.6 Frequency Response and Frequency Response Obligation 

Discussion

Definition of Frequency Response 

FR of the entire interconnection is calculated as (NERC 2012b):

FF = ∆P

∆f � MM

0.1HH�

where ∆P is the change of power by all resources in response to a grid disturbance,5 and ∆f is the 

change in frequency.

This change in power normally results mostly from primary GR of synchronous generation. It 

also includes contributions from loads and other resources (e.g., energy storage devices) that are 

under frequency control. This investigation is specifically concerned with disturbances that result 

in loss of generation. For this calculation, ∆P is averaged over a time period of 20 seconds to 52 

seconds after the event, and ∆f is the change of frequency averaged over the time period from 20 

seconds to 52 seconds. This is indicative of the primary response. (More discussion of the 

rationale for this definition is given in NERC [2012b]; discussion of the measurement of 

frequency is below in this Appendix.) 

 

5 For a system to reach equilibrium following loss of generation, the ∆P of the response must equal the amount lost, 

but the “response” distinction is important because it is a measure of how the resources react to the change in 

frequency, regardless of the cause.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

LSP Base

LSP HiMix

LSP HiMixXtrm

ARIZONA EL PASO NEVADA NEW MEXICOPSCOLORADO WAPA R.M. IDAHO MONTANA PACE SIERRA

DSW NorthEast

Steam

PSH

Other

NUC

HYDRO

GEO

GasCT

CSP

Coal

CCPP

Bio

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Definition of Frequency Nadir

This is the lowest frequency in the event, labeled Point C in the illustrative frequency event of 

NERC (2012b). 

Definition of Frequency Nadir Time

This is the time it takes for the response to reach its nadir. 

Definition of Settling Frequency

For results presented throughout this report, this is defined as the average frequency between 20

seconds and 52 seconds after the event starts. This is Point B in the illustrative frequency event 

of NERC (2012b), but was refined to the average value over this period in NERC (2012b). The 

intent of this metric is to capture the frequency after the autonomous controls (mainly governors) 

have acted, but before centralized control (mainly automatic generation control) acts. In practice, 

these behaviors overlap, so it is difficult to assign a specific post-disturbance time to make a 

single measurement. 

Definition of Frequency Response Obligation

An IFRO is established in Frequency Response and Frequency Bias Setting Standard: 

Supporting Document (NERC 2012b). The rationale and development of the obligation is 

described in detail in Frequency Response Initiative Report (NERC 2012c). The IFRO for 

WECC is set in these documents and was used in this study. It is calculated as the amount of 

generation lost in the criteria contingency divided by the maximum change in frequency. The 

WECC FRO used in this study is 840 MW/0.1 Hz according to the November 30, 2012, version 

of this document, which was subsequently ratified. Broadly, the intent is that WECC should 

always have a FR that meets or exceeds this minimum. 

This obligation is based on avoiding the first stage of UFLS in WECC at 59.5 Hz (NERC 2012c, 

Table B).6 It takes into account the statistical expectation that the system frequency may be as 

much as 24 mHz lower (NERC 2012c, Table A) leading into an event, and the relative difference 

between the frequency nadir and the settling frequency (NERC 2012c, Table D).

Definition of Balancing Authority Frequency Response Obligation

Each BA within an interconnection is obligated to provide its share of the total IFRO. The 

distribution of the obligation is based on the relative size of each BA. This is calculated as 

follows:

FFFBB = I �

PPPPBB+PPPPPBB

PPPPEE+PPPPPEE

�,

where:

FROBA is the BA FRO. 

IFRO is the interconnection FRO. 

 

6 As of this report, FRCC includes UFLS settings of 59.7 Hz.

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176

PgenBA is the annual BA generation. 

PloadBA is the annual BA load. 

PgenEI is the annual interconnection generation. 

PloadEI is the annual interconnection load. 

In this work, an approximate assignment of FRO by study region and area was based on the 

power flow condition. 

BA Frequency Response

This is the performance of each BA, which is expected to meet or exceed the obligation at all 

times. It is calculated as follows:

FFBB = ∆PBB

∆f ,

where:

FRBA is the BA FR. 

∆PBB is the change in BA power. 

The frequency change is uniform across the interconnection. In the work presented here, the only 

power change measured and included in the calculations is that of the turbine power of the 

responsive generation. Load and loss impacts are not considered, beyond load modeling. In this 

work, the change in power for each study area is used to create a FR that can be reported. As 

noted above, while the FRO applies only to BAs, reporting FR by study area is a mechanism to 

illuminate regional behavior.

Frequency Calculation

This study focuses on system-wide FR, as measuring the frequency at a single node in the grid 

following a disturbance can be confusing and misleading. A system equivalent frequency, f, was 

developed and calculated as:

f = ∑ (MM i∗ωi) n

i=1

∑ MM i n

i=1

 , 

where:

MVAi is the megavolt ampere rating for machine i.

ωi is the speed for machine i.

n is the number of synchronous machines in the system. 

This is the MVA-weighted average speed of synchronous machines in the system. It filters out 

the local swings to give a clearer measure of the system performance of concern in this study. It 

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177

can be regarded analytically as the common mode of the system. Six system equivalent 

frequencies for WECC and each study region and area were calculated in this study. 

10.7 Supporting Material for Frequency Response Investigation

Figure 102. Bus voltage response to loss of two Palo Verde units – LSP Base vs. HS Base. 

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178

Figure 103. Selected interface flows to loss of two Palo Verde units – LSP Base vs. HS Base. 

Figure 104. LSP frequency response including loads. 

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179

Figure 105. Bus response to loss of two Palo Verde units. 

Figure 106. Headroom response to loss of two Palo Verde units. 

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180

10.8 Supporting Material for Transient Stability Investigation

Additional COI Stability Results

Figure 107. Interface flows for HS PDCI event.

The details of the sequence of progressively more aggressive RAS are as follows: 

• Group 1 (gr1), tripping approximately 1,108 MW (JDA 01 through 08)

• Group 2 (gr2), tripping approximately 2,076 MW (group 1 + JDA 09 through 08)

• Group 3 (gr3), tripping approximately 2,776 MW (group 1 + 2 + MCN 01 through 04 + 

LMN 2 through 4). 

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181

Figure 108. Interface flows – PDCI fault Hi-Mix case with RAS. 

Figure 109. Voltages – PDCI fault Hi-Mix case with RAS. 

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182

10.9 Supporting Material for Sensitivities

Additional Load Impact Sensitivity Results

The plot below is for the HS Hi-Mix case, one-phase vs. three-phase fault at Midway, for the 

Midway-Vincent event. It shows that it is the during-fault behavior that drives the voltage 

collapse.

Figure 110. Voltage collapse sensitivity to fault type.

This sequence shows the impact of increasing the severity of the fault (by reducing the fault 

impedance). The traces are as follows:

• Trace a) original MV3p

• Trace b) with fault impedance Z/4

• Trace c) with fault impedance Z/2

• Trace d) with fault impedance 3Z/4

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183

• Trace e) with fault impedance Z (single-phase proxy impedance). 

Specifically, the fault impedance is reduced from .005 to 0.0 in 0.00125 pu steps.

The fourth case is interesting, in that it exhibits classic FIDVR, with the recovery taking more 

than 30 seconds.

Figure 111. Voltage collapse sensitivity to fault impedance.

The following figures are from the IEEE 1547 draft tables for interconnection system default 

response to abnormal voltages and frequencies, with notation to emphasize that the composite 

load does not have built-in time delays.

Figure 112. IEEE 1547 table on voltage tripping. 

Legend:

GreenTrace a) original zero impedance 3phase fault

Red Trace b) with fault impedance Z/4

Blue Trace c) with fault impedance Z/2

Brown Trace d) with fault impedance 3Z/4

Purple Trace e) with fault impedance Z

(single-phase proxy impedance)

 

 

 

 

 

 

 

 

 

... Default settings a

Voltage range (% of base voltage b) Clearing time(s) Clearing time: adjustable 

up to and including (s)

V<45 0.16 0.16

45≤V<60 1.00 11

60<V<88 2.00 21

110≤V<120 1.00 13

V≥120 0.16 0.16

a Under mutual agreement between the EPS and DR operators, other static or dynamic 

voltage and clearing time trip settings shall be permitted

b Base voltages are the nominal system voltages stated in ANSI C84.1-2011, Table 1.

Composite load model 

with DG does NOT 

have time delays for 

tripping

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184

Figure 113. IEEE 1547 table on frequency tripping. 

Figure 114. DG tripping on voltage dip. 

Default settings Ranges of adjustability

Functio

n

Frequency 

(Hz)

Clearing 

time (s)

Frequency 

(Hz)

Clearing time (s) 

adjustable up to 

and including

UF1 57 0.16 56 – 60 10

UF2 59.5 2 56 – 60 300

OF1 60.5 2 60 - 64 300

OF2 62 0.16 60 – 64 10

Composite load model 

with DG does NOT 

have time delays for 

tripping

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185

Figure 115. COI destabilization due to DG tripping on voltage dip. 

Additional Widespread DG Tripping Results

Only about 2,500 MW (rather than 2,740 MW) is tripped because of voltage sensitivities in the 

complex load model logic. 

Figure 116. DG vs. trip of two Palo Verde units on Hi-Mix case – DG. 

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186

Figure 117. DG vs. trip of two Palo Verde units on LSP Hi-Mix case – interfaces. 

Figure 118. DG trip sensitivity. 

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187

Figure 119. DG trip sensitivity. 

Figure 120. DG trip sensitivity. 

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188

Additional Extreme Generation Tripping Results 

FR for two events—trip of two Palo Verde units and trip of three Palo Verde units—is shown in 

the following table. 

Table 47. Extreme Generation Tripping – Comparison of Frequency Response

Trip 2 

Palo 

Verde 

Units

Trip 3 

Palo 

Verde 

Units

 FRO FR FR 

 Name [MW/0.1Hz] [MW/0.1Hz] [MW/0.1Hz]

 WECC 840 1311 1265

 CALIFORNIA 296 312 302

 DSW 220 119 107

 NORTHEAST 82 47 52

 ARIZONA 104 50 42

 EL PASO 9 4 2

 IDAHO 18 22 24

 IMPERIALCA 4 14 14

 LADWP 29 30 30

 MONTANA 11 10 14

 NEVADA 28 34 33

 NEW MEXICO 14 2 2

 NORTHWEST 131 483 463

 PACE 42 8 7

 PG AND E 133 197 191

 PSCOLORADO 36 6 5

 SANDIEGO 21 7 7

 SIERRA 11 7 7

 SOCALIF 108 63 59

 WAPA R.M. 27 24 23

 WAPA U.M. 0 3 3

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189

Figure 121. Extreme generation tripping – voltages. 

Figure 122. Extreme generation tripping – volt sensitivity to UF DG tripping. 

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190

Figure 123. Extreme generation tripping – flow sensitivity to UF DG tripping. 

Additional Headroom Depletion Results

Figure 124. Headroom depletion experiment – no wind and solar.

No wind or 

solar

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191

Figure 125. Summer "duck" curve. 

1:20 pm

No wind or 

solar

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192

Figure 126. Extreme "duck" curve. 

Figure 127. Headroom depletion: Kt – four regions. 

PLEXOS: 

58GW/120GW

13:40

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193

Figure 128. Headroom depletion – WECC frequency response. 

Figure 129. Headroom depletion – dynamic headroom.

Most of California’s 

headroom and 

frequency response 

in this case comes 

from hydro

Only ~1 GW 

headroom from 

non-hydro

Initial Hi-Mix case 

already has very 

limited headroom

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194

Figure 130. Number of units with governors (and non-zero headroom).

Figure 131. Interface flows. 

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Figure 132. Voltages. 

Figure 133. Load response. 

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Additional Base Case De-Commitment Results

Table 48. Case DG Only (Composite Load Model with DG) – Initial Condition Metrics

WECC CALIFORNIA DSW NORTHEAST NORTHWEST

Pgen of units with governors (GW) pg 42.9 6.6 9.3 2.8 12.5

Capacity of units with governors (GW) mc 65.3 12.8 14.0 4.0 17.8

Headroom on units with governors (GW) hr 22.3 6.1 4.7 1.2 5.2

Number of units with governors nu 843 192 126 95 208

Mechanical power of unit with governors (GW) pm 42.9 6.7 9.4 2.9 12.6

MVA rating of unit with governors (GVA) mv 66.2 13.3 15.5 4.3 16.5

Pgen of units w/o governors (GW) px 46.8 13.6 10.5 8.6 2.1

Mechanical power of units w/o governors (GW) mx 47.0 13.7 10.5 8.6 2.1

# units w/o governors nx 787 315 65 124 68

Pgen of all synchronous generators (GW) pq 89.7 20.2 19.8 11.4 14.7

Qgen of all synchronous generators (GVAr) qg -1.2 -2.4 -1.0 -0.1 0.2

P load (GW) pl 113.2 33.6 24.6 11.7 19.1

Q load (GVAr) ql 31.9 8.2 6.9 3.3 4.3

Pgen – Wind (GW) pw 21.6 5.1 4.0 2.5 8.4

Qgen – Wind (GVAR) qw -1.8 -1.1 0.3 -0.2 -0.7

Pgen – Solar PV pv 3.3 3.1 0.2 0.0 0.0

Qgen – Solar PV qv -0.3 -0.3 0.0 0.0 0.0

Pgen - CSP (GW) pc 0.6 0.6 0.0 0.0 0.0

Pgen- DG (solar PV) dg 7.0 3.7 2.6 0.4 0.2

Kt Kt 0.45 0.33 0.45 0.26 0.62

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Figure 134. LSP DG only (between Base and Hi-Mix – Case 2A) – renewable conditions. 

Supporting Material for CSP Sensitivities

The following sequence shows the mapping of the CSP plants to be retained from the NREL 

planning data to the Hi-Mix build-out. The five plants that were identified as real, but not in the 

Base case, as shown in Figure 135. Eleven “new” Hi-Mix CSP plants that roughly correspond in 

location and rating are shown in blue in the next figure. These were kept as CSP; the other 

plants, shown in grey, were converted to utility-scale PV.

WECC CALIFORNIA DSW NORTHEAST NORTHWEST

Wind (GW) 21.6 5.1 4.0 2.5 8.4

PV (GW) 3.3 3.1 0.2 0.0 0.0

CSP (GW) 0.6 0.6 0.0 0.0 0.0

DG (GW) 7.0 3.7 2.6 0.4 0.2

Others (GW) 57.2 7.7 13.0 8.5 6.1

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Figure 135. Five identified CSP plants. 

Figure 136. Converted CSP plants. 

o Ivanpah - 3*130 MW - 35d33'8.5"n, 115d27'30.97"w

o Mojave - 180 MW - 35d1'n 117d20'w

o Tonopah - 110 MW - 38d14'n x 117d22' w

o Genesis - 250 MW - 33d40', 114d59w

o Solana - 250 MW - 32d55'n 112d58'w

All the other new CSP 

HiMix CSP plants, 70 of 

81 (grey circles), with 

total generation of 

~5100 MW were 

converted to utility 

scale PV plants

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199

Figure 137. HS Hi-Mix WECC frequency and ROCOF. 

Figure 138. Hi-Mix WECC initial detail of frequency and ROCOF. 

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Supporting Material for LSP Hi-Mix Extreme case

This table shows difference between the two sample days from the PLEXOS Hi-Mix case that 

were selected for the LSP Hi-Mix and LSP Extreme cases. Notice that the CSP actually dropped 

between these two particular samples: the Extreme case was selected based on the high totals. 

Table 49. LSP Hi-Mix Extreme – Mining PLEXOS Case

The following figure gives a sense of how the stress increased as the load flow transitioned from 

the Hi-Mix case to the Extreme case. The “pass” circles indicate the general area and sequence in 

which voltage and thermal problems cropped up as the wind and solar increased. 

Figure 139. LSP Hi-Mix Extreme case – local reinforcements.

Area # CSP DG PV Wind Area # CSP DG PV Wind Area # CSP DG PV Wind

54 54 54 0 0 0 0

14 6375 1471 2061 560 14 5477 2418 3387 411 14 -898 947 1326 -149

50 50 50

11 143 202 227 2 11 156 163 183 2 11 13 -39 -44 -1

60 0 150 60 0 152 60 0 0 0 2

21 124 32 277 337 21 28 50 428 56 21 -96 18 152 -281

26 493 864 403 0 26 806 1369 638 0 26 313 505 235 0

20 20 20

62 0 2 3 1593 62 0 2 3 2989 62 0 0 0 1396

18 113 166 376 0 18 127 278 627 0 18 14 112 252 0

10 162 217 333 27 10 172 428 655 1200 10 10 211 323 1173

40 0 204 334 4551 40 0 125 205 4116 40 0 -79 -129 -435

65 0 239 87 2772 65 0 646 235 3716 65 0 407 148 944

30 0 584 690 30 30 0 1022 1206 80 30 0 438 517 50

70 178 355 608 594 70 186 354 607 954 70 8 0 0 360

22 0 116 89 0 22 0 253 195 0 22 0 137 106 0

64 0 208 728 661 64 0 132 461 530 64 0 -76 -267 -131

24 1378 2287 2951 2746 24 1933 3303 4263 186 24 555 1016 1312 -2560

52 52 52

73 0 364 428 3346 73 0 377 442 6309 73 0 13 15 2963

63 0 0 27 63 0 0 59 63 0 0 0 33

Total 8967 7312 9592 17395 Total 8885 10920 13536 20759 Total -81 3608 3944 3364

2022 LSP Hi-Mix 2022 Extrem case Delta

HiMix @ 31775 Extreme @ 30692 Difference

1st pass

2nd pass

3rd pass

4th pass

5th pass

We had to inch up on last 

10GW of wind and solar.

Local fixes mounted with 

each successive step

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Figure 140. LSP Hi-Mix Extreme case bubble diagram.

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202

Table 50. LSP Hi-Mix Extreme Case – Initial Condition Metrics

WECC CALIFORNIA DSW NORTHEAST NORTHWEST

Pg 30.7 5.2 4.6 2.5 7.2

mc 48.9 10.2 7.6 3.6 11.8

hr 18.1 5.0 3.0 1.0 4.6

nu 699 166 94 87 144

pm 30.7 5.2 4.6 2.5 7.2

mv 48.9 10.9 8.1 3.7 10.5

px 34.2 9.5 10.7 1.6 2.1

mx 34.7 9.6 10.9 1.6 2.1

nx 711 272 87 100 59

ps 64.9 14.7 15.2 4.1 9.3

ps 8.5 -0.2 1.8 0.7 2.5

pl 111.6 32.7 24.5 11.4 19.0

ql 29.7 7.7 5.7 3.2 4.2

pw 32.4 4.5 10.9 7.0 8.3

qw 0.3 0.2 0.9 0.4 -1.0

pv 13.5 7.5 5.1 0.7 0.2

qv -1.7 -0.4 -1.0 -0.2 -0.1

ps 8.3 2.2 6.1 0.0 0.0

dg 10.4 5.7 3.8 0.7 0.1

nh 699 166 94 87 144

Kt 0.38 0.32 0.22 0.28 0.53

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Table 51. Details of LSP Hi-Mix Extreme Dynamic Initial Conditions

Pg mc hr nu pm mv px mx nx ps qs pl ql pw qw pv qv ps dg nh Kt

WECC 30.7 48.9 18.1 699 30.7 48.9 34.2 34.7 711 64.9 8.5 111.6 29.7 32.4 0.3 13.5 -1.7 8.3 10.4 699 0.38

CALIFORNIA 5.2 10.2 5.0 166 5.2 10.9 9.5 9.6 272 14.7 -0.2 32.7 7.7 4.5 0.2 7.5 -0.4 2.2 5.7 166 0.32

DSW 4.6 7.6 3.0 94 4.6 8.1 10.7 10.9 87 15.2 1.8 24.5 5.7 10.9 0.9 5.1 -1.0 6.1 3.8 94 0.22

NORTHEAST 2.5 3.6 1.0 87 2.5 3.7 1.6 1.6 100 4.1 0.7 11.4 3.2 7.0 0.4 0.7 -0.2 0.0 0.7 87 0.28

ALBERTA 1.9 3.3 1.3 66 1.9 3.5 9.2 9.2 77 11.1 2.2 13.4 6.1 1.7 -0.3 0.0 0.0 0.0 0.0 66 0.23

ARIZONA 2.7 4.7 2.0 44 2.7 4.4 9.2 9.4 65 11.9 1.3 10.4 3.0 0.6 -0.2 2.7 -0.6 5.5 2.3 44 0.27

B.C.HYDRO 8.4 11.1 2.8 118 8.4 10.9 0.5 0.5 106 8.9 1.1 9.0 2.2 0.0 0.0 0.0 0.0 0.0 0.0 118 0.95

EL PASO 0.2 0.3 0.1 3 0.2 0.4 0.4 0.5 6 0.7 0.0 1.1 0.2 0.0 0.0 0.2 -0.1 0.1 0.2 3 0.32

IDAHO 1.4 1.9 0.5 29 1.4 1.8 0.5 0.5 31 1.9 0.2 3.2 0.6 0.4 -0.1 0.0 0.0 0.0 0.0 29 0.68

IMPERIALCA 0.2 0.3 0.1 2 0.2 0.3 1.0 1.0 36 1.2 0.1 0.6 0.2 0.1 0.0 0.4 0.0 0.0 0.0 2 0.18

LADWP 0.5 0.8 0.3 8 0.5 0.9 0.9 0.9 12 1.4 -0.1 4.6 0.9 0.2 -0.1 0.6 -0.2 0.8 1.3 8 0.33

MEXICO-CFE 0.3 0.5 0.2 6 0.3 0.5 0.3 0.3 2 0.6 0.1 1.1 0.3 0.0 0.0 0.0 0.0 0.0 0.0 6 0.59

MONTANA 0.3 0.6 0.3 28 0.3 0.6 0.0 0.0 10 0.3 0.1 1.5 0.5 3.0 0.5 0.0 0.0 0.0 0.0 28 0.16

NEVADA 0.5 0.7 0.2 4 0.5 1.0 0.3 0.3 4 0.8 0.1 2.4 0.4 0.0 0.0 0.6 -0.1 0.1 0.3 4 0.45

NEW MEXICO 0.0 0.0 0.0 3 0.0 0.0 0.2 0.2 1 0.2 0.0 1.5 -0.1 2.2 0.2 0.6 -0.1 0.2 0.4 3 0.01

NORTHWEST 7.2 11.8 4.6 144 7.2 10.5 2.1 2.1 59 9.3 2.5 19.0 4.2 8.3 -1.0 0.2 -0.1 0.0 0.1 144 0.53

PACE 0.5 0.5 0.1 23 0.5 0.7 0.3 0.3 10 0.7 0.2 5.4 1.8 3.0 0.1 0.2 -0.1 0.0 0.6 23 0.13

PG AND E 3.4 6.6 3.2 123 3.4 7.0 4.8 4.8 157 8.2 0.0 12.3 3.4 0.8 0.0 1.9 -0.1 0.0 1.0 123 0.47

PSCOLORADO 0.9 1.0 0.1 4 0.9 1.2 0.5 0.5 3 1.4 0.3 5.3 1.3 1.8 0.3 0.6 -0.1 0.2 0.3 4 0.25

SANDIEGO 0.1 0.2 0.1 1 0.1 0.2 0.1 0.1 12 0.2 0.0 2.7 0.6 0.3 0.1 0.5 0.1 0.0 0.2 1 0.16

SIERRA 0.4 0.5 0.1 6 0.4 0.6 0.8 0.8 48 1.2 0.1 1.4 0.3 0.5 -0.1 0.5 -0.1 0.0 0.1 6 0.21

SOCALIF 1.0 2.3 1.3 32 1.0 2.4 2.7 2.8 55 3.7 -0.2 12.5 2.7 3.2 0.2 4.0 -0.3 1.4 3.2 32 0.19

FORTISBC 0.6 0.8 0.2 18 0.6 0.8 0.4 0.4 8 1.0 0.2 0.5 0.2 0.0 0.0 0.0 0.0 0.0 0.0 18 0.67

WAPA R.M. 0.2 0.9 0.7 36 0.2 1.1 0.1 0.1 8 0.3 0.1 3.8 0.8 6.3 0.6 0.4 0.0 0.0 0.4 36 0.12

WAPA U.M. 0.1 0.1 0.0 1 0.1 0.1 0.0 0.0 1 0.1 0.0 -0.1 0.1 0.1 0.0 0.0 0.0 0.0 0.0 1 0.61

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Figure 141. LSP Hi-Mix Extreme case headroom. 

Figure 142. LSP Hi-Mix Extreme case – number of units with governors. 

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10.10Additional Weak Grid Results

Figure 143. Regional frequency – comparison for Extreme case. 

Figure 144. WTG part Thevenin (Type 3) model vs. current source (Type 4) model. 

Pure current source model is initially 

unstable....this is almost certainly a 

modeling issue, but might be indicative of 

weak grid stability issues

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206

10.11Supporting Material for Mitigation

Additional Wind Frequency-Responsive Controls Results

Figure 145. Frequency response with frequency controls on wind plants – HS Hi-Mix case. 

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207

Table 52. Frequency Response Metrics – HS Hi-Mix Case With Wind Frequency Controls

ID Name ID

 FRO 

[MW/0.1Hz] FR FR margin FR FR margin FR FR margin FR FR margin 

1 WECC WE 840.0 2671 1831 3171 2331 2663 1823 3163 2323

2 CALIFORNIA CA 295.7 767 471 765 469 753 457 766 471

3 DSW DS 220.0 674 455 764 544 676 456 756 536

4 NORTHEAST NE 81.5 53 -28 128 46 65 -16 120 38

14 ARIZONA AZ 104.4 382 278 389 284 382 278 389 285

11 EL PASO EL 8.6 33 24 36 27 33 24 36 27

60 IDAHO ID 17.9 23 5 31 13 29 11 31 13

21 IMPERIALCA IV 4.0 14 10 16 12 13 9 16 12

26 LADWP LA 29.3 75 46 77 48 75 46 77 48

62 MONTANA MT 11.3 14 3 52 40 13 2 47 35

18 NEVADA NV 27.9 60 32 60 32 60 32 60 33

10 NEW MEXICO NM 14.3 33 19 49 34 38 24 48 34

40 NORTHWEST NW 130.8 824 693 1155 1024 818 687 1160 1029

65 PACE PC 41.8 11 -30 41 -1 19 -23 38 -4

30 PG AND E PG 132.7 470 338 476 344 470 337 478 345

70 PSCOLORADO CO 35.5 79 44 80 44 79 44 80 44

22 SANDIEGO SD 21.3 55 34 56 34 56 34 56 35

64 SIERRA SP 10.6 4 -6 5 -6 4 -6 5 -6

24 SOCALIF SC 108.4 152 44 140 31 139 30 140 32

73 WAPA R.M. WR 27.4 87 60 151 124 84 56 143 115

63 WAPA U.M. WU 0.1 3 3 3 3 3 3 3 3

Governor Control (APC) Inertial control (WI) Inertial and Governor 

The following comparison showed what happened when frequency controls were applied only to 

only new wind plants in areas that were short of FRO. It turns out that new wind plants were 

generally added to areas short of FR, so there is essentially no difference. 

Figure 146. WECC frequency response – LSP Hi-Mix – active power control in selected areas. 

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Additional Solar Frequency-Responsive Controls Results

Figure 147. CSP governor response mitigation for Palo Verde event. 

• Red: 23’HS HiMix, CSP w GR

• Green: 23’HS HiMix, CSP w/o 

GR

• Nadir frequency improved 

by 13 mHz

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Additional Energy Storage for Frequency Response Results

Table 53. Estimate of Required Energy Storage Rating for LSP Hi-Mix Extreme Case 

ID Name ID

1 WECC WE

2 CALIFORNIA CA

3 DSW DS

4 NORTHEAST NE

54 ALBERTA AL

14 ARIZONA AZ

50 B.C.HYDRO BC

11 EL PASO EL

60 IDAHO ID

21 IMPERIALCA IV

26 LADWP LA

20 MEXICO-CFE MX

62 MONTANA MT

18 NEVADA NV

10 NEW MEXICO NM

40 NORTHWEST NW

65 PACE PC

30 PG AND E PG

70 PSCOLORADO CO

22 SANDIEGO SD

64 SIERRA SP

24 SOCALIF SC

52 FORTISBC FB

73 WAPA R.M. WR

63 WAPA U.M. WU

 FR 

[MW/0.1Hz]

 %of WECC 

FR

 FR margin 

[MW/0.1Hz]

Pbess for 

LSP Ext

1054.7 100.0 214.7 0.0

295.1 28.0 -0.6 1.2

96.6 9.2 -123.4 230.5

50.7 4.8 -30.8 57.3

0.0 0.0 0.0 0.0

44.7 4.2 -59.7 111.3

0.0 0.0 0.0 0.0

3.1 0.3 -5.6 10.4

21.1 2.0 3.2 0.0

14.2 1.3 10.2 0.0

29.1 2.8 -0.1 0.2

0.0 0.0 0.0 0.0

10.9 1.0 -0.3 0.6

18.6 1.8 -9.3 17.5

1.7 0.2 -12.7 23.6

280.4 26.6 149.5 0.0

11.3 1.1 -30.5 56.8

188.5 17.9 55.8 0.0

5.7 0.5 -29.8 56.0

7.0 0.7 -14.4 26.7

7.4 0.7 -3.2 5.9

56.1 5.3 -52.3 95.0

0.0 0.0 0.0 0.0

22.9 2.2 -4.5 8.4

2.7 0.3 2.5 0.0

412.4

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Figure 148. LSP Hi-Mix Extreme case – energy storage. 

Figure 149. LSP Hi-Mix Extreme case – energy storage.

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Table 54. LSP Hi-Mix Extreme Frequency Response with Energy Storage

 FRO FR FR margin BESS FR FR margin FR

 Name [MW/0.1Hz] [MW/0.1Hz] [MW/0.1Hz] rating [MW] [MW/0.1Hz] [MW/0.1Hz] % of WECC

 WECC 840 1055 215 0.0 1513 672.6 100.0

 CALIFORNIA 296 295 -1 0.0 369 73.5 24.4

 DSW 220 97 -123 230.5 224 4.0 14.8

 NORTHEAST 82 51 -31 57.3 85 3.8 5.6

 ARIZONA 104 45 -60 111.3 105 1.0 7.0

 EL PASO 9 3 -6 10.4 9 0.3 0.6

 IDAHO 18 21 3 0.0 22 4.0 1.4

 IMPERIALCA 4 14 10 0.0 14 10.4 1.0

 LADWP 29 29 0 0.0 30 0.4 2.0

 MONTANA 11 11 0 0.0 10 -1.0 0.7

 NEVADA 28 19 -9 17.5 34 6.0 2.2

 NEW MEXICO 14 2 -13 23.6 14 -0.6 0.9

 NORTHWEST 131 280 150 0.0 487 356.4 32.2

 PACE 42 11 -30 56.8 40 -1.7 2.7

 PG AND E 133 189 56 0.0 197 64.2 13.0

 PSCOLORADO 36 6 -30 56.0 33 -2.1 2.2

 SANDIEGO 21 7 -14 26.7 21 -0.7 1.4

 SIERRA 11 7 -3 5.9 10 -0.3 0.7

 SOCALIF 108 56 -52 95.0 103 -5.1 6.8

 WAPA R.M. 27 23 -5 8.4 26 -1.6 1.7

 WAPA U.M. 0 3 3 0.0 3 2.5 0.2

LSP Hi-Mix Extreme

LSP Hi-Mix Extreme with Energy 

Storage

412 MW energy storage gives ~250 MW/0.1 Hz FR improvement. 

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Additional COI Stabilization by Transmission Addition Results

Figure 150. HS Hi-Mix case flows with transmission build-out. 

1850 MW 

in HiMix

case

4836 MW 

in HiMix

case

2192 MW 

in HiMix

case

-424 MW 

in HiMix

case

1789 MW 

in HiMix

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213

Figure 151. Voltages for PDCI event: Hi-Mix case with transmission build-out. 

Figure 152. Voltages for PDCI event: Hi-Mix case with transmission build-out. 

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