NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

NREL Variability and Reserves 

Analysis for the Western 

Interconnect

WECC WebEx

Michael Milligan

Jack King

National Renewable Energy 

Laboratory

October 20, 2011 

NREL/PR-5500-53182

Composite photo created by NREL
National Renewable Energy Laboratory 2 Innovation for Our Energy Future

Wind/Solar Change Operational Requirements
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• Separate from contingency 

reserve;

• Flexibility reserve depends on 

what the wind/solar are doing;

• Conventional production 

simulation is “unaware;”

• Sufficient maneuverability must 

be available to respond over 

different time scales (up/down);

• Objective: ensure sufficient 

operational flexibility, minimizing 

expensive spin but maintaining 

reliability.

Additional Variability and Uncertainty Increase 

Reserve Requirement
National Renewable Energy Laboratory 4 Innovation for Our Energy Future

• Describe the EWITS method which was also 

used in the EIM analysis;

• Suggest how this approach, or other similar 

approaches, could be incorporated into 

WECC’s production simulation;

• Increase the accuracy of the simulation;

• Although we assume ProMod as the simulation 

tool, this reserve method can be adapted to 

other models running on different time steps;

• Over time it is possible that these techniques 

will be implemented directly in the production 

simulation models.

Objective of this Presentation
National Renewable Energy Laboratory 5 Innovation for Our Energy Future

• Developed for the EWITS study.*

• Statistical approach based on 10-minute time 

series wind, solar and load.

• Method that can estimate adequate reserves to 

cover the short-term and hour-ahead forecast error 

based on historical data.

• Predicts requirements based on current hour load 

and wind, solar production.

• Statistically combine with load regulation 

requirements.

• Provide 8760 vector of requirements for the 

production simulations.

*For in-depth discussion see section 5 of the EWITS final report: 

http://www.nrel.gov/wind/systemsintegration/pdfs/2010/ewits_final_report.pdf 

Reserve Calculations
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• Regulation – Fast changes:

– Due to variability and short term forecast errors;

– Faster than re-dispatch period;

– AGC resources required.

• Spinning – larger, slower, less frequent variations:

– Due to longer term forecast errors;

– AGC not required;

– 10-minute response.

• Non-spinning/supplemental:

– Large, infrequent, slow moving events such as 

unforecasted ramps;

– 30-minute response.

Reserve Definitions 
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Overview of the Reserve Calculations
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Short-Term Forecast Error - Regulation 

• Based on persistence 

forecast;

• Wind data is 10 

minute, 10-minute 

delay for forecast;

• Forecast error is 

calculated as 

difference from actual 

to forecast.
National Renewable Energy Laboratory 9 Innovation for Our Energy Future

Short-Term Calculation

• Measure as standard deviation of the forecast error; 

• Forecast error varies with production level;

• Empirical expected error as a function of production 

is quadratic:

– Low variability at low and high wind (cut-in and rated); 

– High variability in mid range at steep part of power curve;

– Solar follows same pattern, more or less.

• Predicts the expected variability for the hour based 

on the intra-hour statistics, current production;

• Assumes fast dispatch, 10 minutes in this case:

– Implication is that economic movement happen at 10- 

minute updates.
National Renewable Energy Laboratory 10 Innovation for Our Energy Future

Calculation of Short-Term Forecast Error

• Calculate and sort error 

by production level;

• Divide production into 

deciles;

• Calculate error sigma in 

each decile; 

• Blue line is calculated 

from data;

• Red line is curve fit;

• Equation of the curve 

shown below.

σSS HHHHHH WWWW

= −6.72E − 06 ∙ (HHHHHH WWWW)2 + 0.0437 ∙ (HHHHHH WWWW) + 26.74
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Calculating the Regulation Requirement
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Flex Reserve

• Calculated without load regulation

• For use when load regulation is handled separately 

from VG related reserves

• RRR RR RRR RRR R WW W WWWW S =

3 ∙ (σSSSSSS HHHHHH WWWW )2 +

(σSSSSSSS HHHHHH S )2

• σSSSSSS HHHHHH WWWW Wind component of reserves

• σSSSSSSS HHHHHH S Solar component 

• Again, 3 sigma to cover 99.7% of all short term 

forecast errors

• Assumed equal up and down

• Calculated for each hour of the study year
National Renewable Energy Laboratory 13 Innovation for Our Energy Future

Hour-Ahead Forecast Errors

• Repeat the short-term forecast procedure with 

hour-ahead forecasting;

• Again, same procedure for wind and solar;

• Load following not included.

σH H − aa HHHHHH WWWW

= −2.985E − 05 ∙ (HHHHHH WWWW)2 + 0.1895 ∙ (HHHHHH WWWW) + 103.2 
National Renewable Energy Laboratory 14 Innovation for Our Energy Future

Spin and Non-Spin Calculation

• One sigma allocated to Spinning category

SS RRR RRR R HHH − w f f f

1 ∙ σh− aa (P P HHHH WWWW)

– Covers 68%* of all movements less than 1 hour

– With regulation, covers approximately 98% of 30 minute 

movements

• Two sigma allocated to Non-spin category

N − RRR RRR R HHHH − w f f f =

2 ∙ σh− aa (P P HHHH WWWW)

– With spin, covers 99.7%* of all movements less than 1 hour

*assuming movements are normally distributed which is slightly optimistic 
National Renewable Energy Laboratory 15 Innovation for Our Energy Future

Verification of Reserve Coverage

• Shows coverage of 

VG intra-hour 

movements;

• Actual ramp data 

from footprint EIM;

• Red line – Actual 

average reserve 

calcs from footprint 

EIM;

• Probability lines are 

z% of all ramps at x 

minutes are less 

than y MW.
National Renewable Energy Laboratory 16 Innovation for Our Energy Future

Reserves Provided to E3 for Phase 2 Study

• NREL provided reserve calculations to E3 for phase 

2 study;

• Slight variation in calculation of regulation 

component – load not included in E3 Flex;

• Does not include contingency reserves;

• Flexibility reserves are dynamic.

NREL E3 Response Notes

Regulation Flex AGC

E3 flex reserve does not 

contain load

Spin Spin <10 minutes

Non-spin Supplemental <30 minutes Terminology change
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EIM-Wide Reserves, Low-Load Period

*TEPPC 

operational 

reserve 

requirement
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EIM-Wide Reserves, High-Load Period

*TEPPC 

operational 

reserve 

requirement
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Sample Area with High Penetration

*TEPPC 

operational 

reserve 

requirement
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Low Penetration at High Loads

*TEPPC 

operational 

reserve 

requirement
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Low Load Week with Low Penetration
National Renewable Energy Laboratory 22 Innovation for Our Energy Future

Including Flexibility Reserves in PROMOD

• Produce a software tool to calculate reserves:

– Execute using wind/solar inputs; produces time-series 

of reserves that are input to PROMOD; 

– Straight-forward, well-defined process;

– Can evolve as operating experience with VG dictates;

– Each aggregation requires separate calculations;

– Requires historical wind and solar databases.

• PROMOD does not directly support dynamic 

reserve requirements or multiple categories of 

reserves;

• Workaround has been developed to model most 

of the reserves:

– Add the maximum dynamic reserve to fixed 

contingency reserve;

– Model a transaction that carries the difference 

between maximum and the hourly value.

• Regulation and spin are combined;

• Non-spin is ignored or added as a single 

number.

PIX 17245 
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Relationship to Planning Reserves

• Planning reserve established based on LOLE or 

PRM.

• Operation of some generation changes at some 

times to provide flex reserve instead of energy.

0

2000

4000

6000

8000

10000

12000

1 2

MW

Wind

Unloaded Gen

Conv Gen
National Renewable Energy Laboratory 24 Innovation for Our Energy Future

Planning Reserve vs. Flexibility Reserve

• The traditional focus of resource 

adequacy has been “how much 

installed capacity is needed”.

• With increasing generation from 

wind and solar, the additional 

question is “what type of capacity 

is needed.”

• Wind/solar do not cause need for 

additional capacity, but may 

require a different kind of 

capacity (more flexible capacity).

Methods to Model Calculate Capacity Contributions of Variable Generation for Resource Adequacy Planning, NERC, March 2011. 

Available at http://www.nerc.com/docs/pc/ivgtf/IVGTF1-2.pdf

Flexibility Requirements and Metrics for Variable Generation, NERC, Aug 2010. Available at 

http://www.nerc.com/docs/pc/ivgtf/IVGTF_Task_1_4_Final.pdf

Lannoye, E.; M.; Milligan, M.; Adams, J.; Tuohy; Chandler, H.; O’Malley; Integration of Variable Generation: Capacity Value and 

Evaluation of Flexibility. IEEE Summer PES meeting, Minneapolis, MN 2010.

Keane, A.; Milligan, M.; Dent, C.J.; Hasche, B.; D'Annunzio, C.; Dragoon, K.; Holttinen, H.; Samaan, N.; Soder, L.; O'Malley, M.; 

Power Systems, IEEE Transactions on Volume: 26 , Issue: 2 Digital Object Identifier: 10.1109/TPWRS.2010.2062543 Publication Year: 

2011 , Page(s): 564 - 572

0

2000

4000

6000

8000

10000

12000

1 2

MW

Wind

Unloaded

Gen

Conv Gen

How flexible 

should this 

capacity be?
National Renewable Energy Laboratory 25 Innovation for Our Energy Future

Going Forward

• NREL is willing to assist 

WECC and 

stakeholders to 

incorporate flexibility 

reserves into production 

simulation.

PIX 11070
National Renewable Energy Laboratory 26 Innovation for Our Energy Future

Conclusions 

• Provided a method for calculating 

additional reserve requirements due 

to wind and solar production;

• Method is based on statistical 

analysis of historical time series 

data;

• Reserves are dynamic, produced for 

each hour; 

• Reserve time series are calculated 

from and synchronized to simulation 

data;

• PROMOD can not model directly, but 

workarounds exist for regulation and 

spin;

• Other production modeling packages 

have varying capability for reserves 

modeling. PIX 11277
National Renewable Energy Laboratory 27 Innovation for Our Energy Future

• Ela, E.; Milligan, M.; Kirby, B. (2011). Operating Reserves and Variable Generation. A comprehensive 

review of current strategies, studies, and fundamental research on the impact that increased 

penetration of variable renewable generation has on power system operating reserves. 103 pp.; NREL 

Report No. TP-5500-51978. http://www.nrel.gov/docs/fy11osti/51978.pdf (presentation)

• Milligan, M.; Kirby, B.; King, J. (2011). NREL Variability Analysis for the Western Interconnect 

(Presentation). NREL (National Renewable Energy Laboratory). 29 pp.; NREL Report No. PR-5500- 52430. (presentation)

• Milligan, M.; Kirby, B.; King, J. (2011). Allocating Reserve Requirements (Presentation). NREL (National 

Renewable Energy Laboratory). 14 pp.; NREL Report No. PR-5500-52432. 

http://www.nrel.gov/docs/fy11osti/52432.pdf

• Milligan, M.; Kirby, B.; King, J.; Beuning, S. (2011). Operating Reserve Implication of Alternative 

Implementations of an Energy Imbalance Service on Wind Integration in the Western Interconnection: 

Preprint. 10 pp.; NREL Report No. CP-5500-51343. http://www.nrel.gov/docs/fy11osti/51343.pdf

• Milligan, M.; Kirby, B.; King, J.; Beuning, S. (2010). Benefit of Regional Energy Balancing Service on 

Wind Integration in the Western Interconnection of the United States: Preprint. 10 pp.; NREL Report No. 

CP-5500-49076. http://www.nrel.gov/docs/fy11osti/49076.pdf

• Kirby, B.; Milligan, M. (2009). Capacity Requirements to Support Inter-Balancing Area Wind Delivery. 29 

pp.; NREL Report No. TP-550-46274. http://www.nrel.gov/docs/fy09osti/46274.pdf

• Kirby, B.; Milligan, M. (2008). Examination of Capacity and Ramping Impacts of Wind Energy on Power 

Systems. 26 pp.; NREL Report No. TP-500-42872. http://www.nrel.gov/docs/fy08osti/42872.pdf

References
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Questions? 

28

PIX 13367
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Results of EIM Reserve Calculations

Appendix 
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Model Area

WALC

BPA

PACW

EPE

CHPD

GCPD

DOPD

TPWR

SCL

TEP

SRP

AZPS

TID NEVP

SMUD

PNM

PSCO

SPP

PACE

WACM

NWE

IPC

PGE

AVA

BCTC

PSE

IID

• Based on TEPPC PC 0;

• Areas of the WI not 

already in a market 

structure (CAISO, 

Alberta);

• LADWP not included;

• Analysis at BAA 

granularity;

• Generation only BAAs 

not considered;

• 28 load and generation 

BAAs retained – The 

study ‘Footprint;’

• Regions are:

– Columbia Grid – Orange;

– NTTG – Blue;

– West Connect – White.
National Renewable Energy Laboratory 31 Innovation for Our Energy Future

• Wind Data:

– NREL WWSIS wind dataset for 2006;

– 10-minute resolution;

– Sites identified by TEPPC 2020 PC 0;

– 29,085 MW in Western Interconnect;

– Approximately 8% of WI 2020 load;

– 18,272 MW nameplate in analysis footprint.

• Solar Data:

– NREL WECC Solar dataset for 2006;

– 10-minute resolution;

– Sites identified by TEPPC 2020 PC 0;

– 14,300 MW in WI;

– Approximately 3% of WI 2020 load;

– 4,568 MW nameplate in analysis footprint.

Variable Generation Data
National Renewable Energy Laboratory 32 Innovation for Our Energy Future

Cases

• Complete analysis includes:

– Footprint EIM

• All 28 BAAs participating 

– ‘Regional’ EIM implementations

• Columbia Grid

• Northern Tier Transmission Group

• WestConnect

– EIM implementations without BPA and/or WAPA 

participation

– Comparison to the Business As Usual (BAU) case – 

No EIM in place
National Renewable Energy Laboratory 33 Innovation for Our Energy Future

Footprint EIM Regulation

• Detail of the individual 

regulation components;

• Bars are average 

values, whiskers are 

min and max;

• 3 regional results in 

smaller reductions 

compared to footprint.

Reduction over BAU

Footprint Regional

MW

% 

Reduct. MW

% 

Reduct.

Load Reg 211 18% 178 16%

Wind Reg 372 53% 231 33%

Solar Reg 49 21% 46 19%

Total Reg 570 36% 419 26%
National Renewable Energy Laboratory 34 Innovation for Our Energy Future

Reserve Details for Footprint and Regional EIM

• Up to 42% (2000 MW) 

reduction in total 

reserve requirement 

for footprint EIM.

• 26% for regional EIMs.

Reduction over BAU

Footprint Regional

MW % Reduct. MW % Reduct.

Total Reg 570 36% 419 26%

Spin 481 45% 271 25%

Non-spin 963 45% 542 25%

Total 2014 42% 1233 26%
National Renewable Energy Laboratory 35 Innovation for Our Energy Future

Ramp Demand Reduction – Footprint EIM

• Based on hourly ramps;

• Shows ramp reduction 

from EIM over BAU;

• Duration plot shows 

hours per year for 

reduction level;

• Reduction in net ramp is 

greater than 1000 MW 

for 251 hours per year 

and averages about 260 

MW.
National Renewable Energy Laboratory 36 Innovation for Our Energy Future

Regional EIM – Columbia Grid (CG)

• Average reserves 

for a CG EIM;

• BPA dominates the 

wind so saving less 

than other regions;

• 8070 MW Wind.

Reserve Reduction

MW % Reduc. 

Load Reg 37 14%

Wind Reg 39 15%

Solar Reg 0 0%

Total Reg 85 21%

Spin 38 12%

Non-spin 76 12%

Total 199 15%
National Renewable Energy Laboratory 37 Innovation for Our Energy Future

Regional EIM – WestConnect (WC)

• Average reserves;

• Substantial saving 

from wind and solar;

• ~5700 MW Wind and 

4550 MW solar.

Reserve Reduction

MW % Reduc.

Load Reg 113 22%

Wind Reg 105 41%

Solar Reg 46 20%

Total Reg 253 34%

Spin 154 27%

Non-spin 307 27%

Total 714 29%
National Renewable Energy Laboratory 38 Innovation for Our Energy Future

Regional EIM – WestConnect (WC)

• Average reserves;

• Substantial saving 

from wind and solar;

• ~5700 MW Wind and 

4550 MW solar.

Reserve Reduction

MW % Reduc.

Load Reg 113 22%

Wind Reg 105 41%

Solar Reg 46 20%

Total Reg 253 34%

Spin 154 27%

Non-spin 307 27%

Total 714 29%