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Contract No. DE-AC36-08GO28308

Frequency Response Study of a 

U.S. Western Interconnection 

under Extra-High Photovoltaic 

Generation Penetrations

Preprint 

Jin Tan,1 Yingchen Zhang,1 Shutang You,2

Yong Liu,2 and Yilu Liu2,3

1 National Renewable Energy Laboratory

2 University of Tennessee

3 Oak Ridge National Laboratory

Presented at the IEEE Power and Energy Society 

General Meeting

Portland, Oregon

August 5–9, 2018 

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Conference Paper

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August 2018
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1

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Frequency Response Study of U.S. Western 

Interconnection under Extra-High Photovoltaic 

Generation Penetrations

Jin Tan, Member, IEEE, Yingchen Zhang, Senior Member, IEEE, Shutang You, Student Member, IEEE, Yong Liu, Member, 

IEEE, Yilu Liu, Fellow, IEEE

Abstract— Increasing penetration levels of solar photovoltaic 

(PV) and the reduced inertia of inverter-based generation 

technologies have caught the attention of the electric power 

industry and academia in the United States. This paper conducts 

a comprehensive frequency response study of the Western 

Interconnection under high penetration levels of PV, which are up 

to 80% at the interconnection level and 100% at the regional level. 

Based on the 2022 Light Spring case, PV has been projected by 

areas for four PV penetration levels based on collected realistic PV 

distribution information. The overall system frequency response 

has been analyzed under different scenarios. In particular, the 

100% regional PV penetration case has been developed to reveal 

local frequency issues in the Western Interconnection. The 

potential of using existing resources to improve frequency 

response has also been studied. New insights in this paper address

the implications of the rapid deployment of solar generation in the 

Western Interconnection.

Index Terms —Western Interconnection, low inertia, frequency 

response, high PV penetration, frequency response obligation.

I. INTRODUCTION

igh penetration levels of renewable generation will be 

expected in the United States based on well-established 

renewable portfolio standards (RPS) [1]. Among the various 

renewable generation technologies, photovoltaic (PV) is

expected to be a main contributor to these RPS. Hawaii is 

leading with its target to achieve 100% renewable energy 

penetration by 2045. In its 2020 planning case, of that 48% 

renewable capacity, estimated 33% will be utility-scale PV, and 

36% will be distributed PV [2]. In 2017, California lawmakers

begin to push for 100% clean energy by 2045 [3]. Yet a

significant technical barrier is grid stability—managing rapidly

decreasing system inertia and deteriorating frequency response

and determining whether the grid is able to operate safely and 

reliably under such extremely high penetration levels of

renewable generation.

i

Much previous work focused mainly on the impact of PV on

the distribution system [4-6], including voltage-related impacts, 

reverse power flow, and system protection impact; however, in 

these studies, the amount of PV is assumed to be small, and it

has few impacts on the transmission system. In recent years, the 

impact of high-penetration PV on the transmission system has 

caught the attention of industry and academia. The impact of 

increased penetration levels of PV generation on the static 

performance, transient stability, and frequency response of a 

large power system has been studied [7-10]. Most current 

studies reveal that high penetration levels of PV/renewable 

generation can have diverse effects on system frequency 

response, yet these studies could not give further practical 

guidance for real interconnection operation because they are 

generally based on simplified grid models and lack of realistic 

PV distribution information.

To address practical concerns from industry about high 

penetration levels of renewable generation, beginning in 2010 

the National Renewable Energy Laboratory (NREL) and 

General Electric (GE) conducted a series of analyses under the 

Western Wind and Solar Integration Study (WWSIS) based on 

the practical model of the Western Interconnection and Eastern 

Interconnection[11, 12]. These studies preliminarily considered 

the frequency response of real large interconnections in the 

United States; however, the highest renewable penetration level 

was no more than 70%, and wind generation was the dominant 

resource in the wind-solar mix cases. 

To comply with the goal of the U.S. Department of Energy

SunShot Vision Study, which suggests that solar energy could 

supply approximately 27% of U.S. electricity demand by 2050, 

a very high installed capacity and instantaneous power 

penetration level (approximately 70%) of solar will be expected 

for future grid operation. As an extension of the WWSIS, this 

paper investigates the potential impact of extra-high

penetrations of PV generation on the frequency response of the 

Western Interconnection. The major contributions of this paper

include:

1. Based on a realistic model of the Western Interconnection, 

develop extra-high instantaneous PV penetration cases that

are up to 80% of the Western Interconnection and 100% at 

the regional level; neither has been achieved by previous 

studies. 

2. Conduct a comprehensive frequency response study at both

the system and regional levels. This study not only

provides a system-wide evaluation of average system 

frequency response under extremely high PV penetration, 

it also reveals potential local issues brought by 100% 

regional PV penetration. 

3. Examine practical mitigation solutions by using existing 

resources to deal with extra-high PV penetration levels.

H
2

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The rest of this paper is structured as follows: Section II

introduces the extra-high PV penetration scenario development 

and dynamic model; Section III simulates each 

interconnection’s frequency response at different PV 

penetration levels; Section III and IV study and discuss the 

practical solutions available to improve the frequency response 

of U.S. interconnections.

II. MODEL DEVELOPMENT 

A. Scenario Development

To truly reflect the industry needs, a survey was conducted 

with 14 industry participants from electric utilities, national 

labs, and research institutes from among all three U.S. 

interconnections. According to the survey results, four systemlevel scenarios and one regional scenario are suggested as 

shown in Table I. 

TABLE I. SCENARIOS OF PV, WIND, AND RENEWABLE PENETRATION

Scenario

Instantaneous 

PV 

Penetration

Instantaneous 

Wind 

Penetration

Total 

Renewable (PV 

and Wind) 

Penetration

20% Case 5% 15% 20%

40% Case 25% 15% 40%

60% Case 45% 15% 60%

80% Case 65% 15% 80%

Regional Case 100% 0 100%

 As shown in Table I, up to 65% PV and 80% nonsynchronous 

renewable penetration levels will be studied, and 100% regional 

PV penetration level is also suggested to be built up because of

strong interest from some electric utilities.

(a) 5% PV (b) 25% PV (c) 45% PV (d) 65% PV

Figure 1. PV penetration level for each region in different simulation 

scenarios

Additionally, PV is projected by areas/states based on 

collected existing PV resource data; RPS 2022 data; and PV 

technical potential estimation, which is based on the National 

Solar Radiation Database. (A detailed description of this 

practical PV projection method can be found in [13].) The PV 

penetration level for each area in the Western Interconnection 

is shown in Fig. 1.

B. Modeling for Power Flow and Dynamic Studies

A 2022 Light Spring (LSP) planning case of the Western 

Interconnection is chosen as an ideal benchmark for the 

frequency response study because the relatively low level of 

online synchronous power generation in a light-load condition 

might represent a challenge for frequency response.

1) Base Case

The model of the 2022 LSP case is implemented in GE’s 

Positive Sequence Load Flow analysis simulation software. The 

original instantaneous output and penetration level of the total 

generation, load, wind generation, and PV generation are

summarized in Table II. The total generation in the 2022 LSP

case is 117 GW; wind is the largest renewable resource, at

approximately 13.9% instantaneous penetration; and PV is a 

small portion of the renewables, at only 1.1% instantaneous 

penetration.

TABLE II. 

SUMMARY OF GENERATION AND LOAD IN WECC 2022 LIGHT SPRING CASE

Capacity

(MVA)

Rate

(%)

Pgen

(MW)

Rate

(%)

System 

Total Gen. 312,438 117,238

Load N/A 112,990

Wind 

Gen. 26,323 8.4 16,464 13.9

PV Gen. 1,838 0.6 1,347 1.1

2) Power Flow Model

This study does not specifically address transmission 

planning, which might be brought on by high renewable 

penetration levels. Therefore, we replace current synchronous 

generators with PV power plants to avoid transmission 

upgrades. The replacement philosophy is to replace existing 

synchronous generators for each area in the order of coal power 

plants, gas generators, then hydropower plants, and nuclear 

power plants. With the displacement of traditional synchronous 

generators by PV power plants, the capacity of the online units 

with governors and the headroom of governor-responsive 

generators decreases approximately linearly with the increase 

in renewable penetration level, as shown in Fig. 2. 

 

Figure 2. Governor-enabled units in the Western Interconnection

3) Dynamic Model 

The 2022 LSP case is modeled at a very detailed level with 

more than 4,000 generators and more than 19,000 buses. Each 

power plant has detailed dynamic models, including a

generator, exciter, and turbine governor models that can vary 

widely as a result of the diversity in power plant configurations.

Control devices such as high-voltage direct current controls and 

protective systems such as underfrequency load shedding 

(UFLS) and line protection are included in the Western 

Interconnection dynamic database as well. 

To better illustrate the dynamic behavior of full-converterbased PV units, newly added PV plants are approximately 

modeled with the GE Type 4 wind power plant model, as 

suggested in [14]; this includes the converter model, active and 

reactive power control model, and low-voltage power logic.
3

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Replacing synchronous generators with PV generation

reduces the number of online synchronous generators as well as 

the system inertia. In the developed cases, the system equivalent 

inertia will linearly reduce along with the penetration levels, as 

shown in Fig. 3.

Figure 3. System equivalent inertia at different renweable penetration levels

III. FREQUENCY RESPONSE ANALYSIS

A. Frequency Response Metrics

To align with the North America, Electric Reliability 

Corporation’s (NERC’s) frequency response initiative, 

previous studies have proposed various frequency response 

metrics to improve frequency response performance tracking, 

as summarized in [13]. In this study, the following frequency 

metrics are adopted to evaluate the frequency response of the 

interconnection and region: (1) rate of change of frequency 

(ROCOF), which mainly reflects the inertia response; (2) 

frequency nadir, which is decided by both inertia and primary 

frequency response; (3) and settling frequency, which mainly 

reflects the primary frequency response of the system. 

Frequency response obligation (FRO) is designed by NERC 

and updated annually based on measurements. FRO ensures 

that the interconnection is capable of delivering enough power 

(primary frequency response) during the first couple of seconds 

after an event happens to avoid declining frequency and 

triggering the first stage of UFLS. For the Western 

Interconnection, FRO is updated as 906 MW/0.1 Hz, which 

means that the power output from all generators should increase 

by at least 906 MW for a frequency drop of 0.1 Hz.

B. Interconnection Frequency Response Analysis

1) Generation Trip Scenario

According to the target resource contingency protection 

criteria of the Western Interconnection [15], the loss of the two 

largest generating units, which are in the Palo Verde nuclear 

power plant and total 2,625 MW in the 2022 LSP case, was 

chosen as the largest N-2 contingency. As shown in Table III, 

the Western Interconnection contingency events included the 

largest N-2 contingency, a typical N-2 contingency, and a 

typical N-1 contingency.

TABLE III. SIMULATED CONTINGENCIES FOR THE WESTERN 

INTERCONNECTION

Contingency Description Unit and Location MW 

Capacity

1 The largest N-2 

contingency

Loss of the two largest 

generating units in the 

Palo Verde nuclear 

facility

2,625

2 A typical N-2 

contingency

Loss of the two units in 

the Colstrip coal power 

plant

1,514

3 A typical N-1 

contingency

Loss of one unit in the 

Comanche generating 

station

804

2) Response to Generation Loss Contingency

To truly show the severity of the frequency drop as a result 

of the different renewable penetration levels, all the protection 

models are disabled in this simulation, including the UFLS. The 

frequency response to different generation losses under 

different renewable penetration levels is shown in Fig. 4. (For 

space reason, only two contingencies are shown in Fig.4)

(a) 2,625-MW generation loss (b) 1,514-MW generation loss

Figure 4. WECC Frequency responses under high PV scenarios

The frequency response metrics of these three contingencies are 

calculated and summarized in Fig. 5.

(a) ROCOF (b) Frequency nadir

(c) Settling frequency (d) Settling time

Figure 5. WECC Frequency response metrics change as a result of

increased PV penetration

The frequency responses of three contingencies have the 

same trend: the higher the PV penetration rate is, the worse 

frequency response becomes because both inertia and governor 

response decreases with increasing renewable penetration 

levels. When the penetration level increases to 80%, as in the 

case with a 2.6-GW generation loss, the frequency drops below

59.4 Hz, which is lower than the first stage of the UFLS, 59.5 

Hz. This means that the Western Interconnection is unable to 

afford 80% renewable penetration because of the low inertia 

and limited frequency response it can provide.

C. 100% PV Regional Analysis 

To study the potential local impact of 100% PV regional 

penetration, a 100% PV penetration case was developed in the 

0 5 10 15 20

Time/s

59.2

59.3

59.4

59.5

59.6

59.7

59.8

59.9

60

Frequency/Hz

2.6 GW Contingency

Base case

20% Renewable

40% Renewable

60% Renewable

80% Renewable

UFLS=59.5 Hz

0 5 10 15 20

Time/s

59.6

59.65

59.7

59.75

59.8

59.85

59.9

59.95

60

60.05

Frequency/Hz

1.5 GW Contingency

Base case

20% Renewable

40% Renewable

60% Renewable

80% Renewable

0 10 20 30 40 50 60 70 80 90 100

Renewable penetration(%)

0

0.05

0.1

0.15

ROCOF(Hz/s)

2.6GW Contingency

1.5GW Contingency

0.8GW Contingency

0 10 20 30 40 50 60 70 80 90 100

Renewable penetration(%)

59.3

59.4

59.5

59.6

59.7

59.8

59.9

60

Frequency nadir(Hz)

2.6GW Contingency

1.5GW Contingency

0.8GW Contingency

0 10 20 30 40 50 60 70 80 90 100

Renewable penetration(%)

59.55

59.6

59.65

59.7

59.75

59.8

59.85

59.9

59.95

60

Settling frequency(Hz)

2.6GW Contingency

1.5GW Contingency

0.8GW Contingency

0 10 20 30 40 50 60 70 80 90 100

Renewable penetration(%)

20

22

24

26

28

30

32

34

36

38

40

Settling time(s)

2.6GW Contingency

1.5GW Contingency

0.8GW Contingency
4

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Los Angeles region (A26). Assume that partial PV generation 

in A26 was lost as a result of the extreme weather by applying 

a 2.3-GW generation loss under the 20% case. 

Figure 6. WECC 100% PV penetration region and interconnection frequency 

response

Fig. 6 shows that when the generation loss happens in Area 

26 with no inertia, the regional frequencies in or near this area 

(A26, A21, A24) drop very fast at first 0.1 s; then, it is brought 

back and follows the frequencies of other areas. This happens 

because for a large-scale interconnection (such as the Western 

Interconnection), the frequency disturbance will propagate 

from one local area to another in the speed of an electrometrical 

wave [16]. In this case, there is a lack of frequency response 

capability in A26, and the synchronous generators in other areas 

could not provide immediately power support unless they detect 

the local frequency deviation. This might result in some local 

issues. For example, some protection might be triggered as a 

result of the big ROCOF; and the innovative adaptive UFLS 

module, which is based on local measurement of ROCOF, 

might overshed the load as a result of the wrong estimation of 

generation loss.

Note that regional issues are strongly related to the grid 

structure, transmission corridor, and types of generation-mix in 

the neighboring areas. Specific regional analysis should be 

carried out in accordance with specific conditions.

D. Interconnection FRO Analysis

Based on Section III B, the frequency response of the 

Western Interconnection for three different contingencies at 

four renewable penetration levels are calculated and shown in 

Table IV. 

TABLE IV. FREQUENCY RESPONSE VALUE OF WECC

Frequency Response (MW/0.1 Hz)

Senario Contingency 1 Contingency 2 Contingency 3

20% Case 1,398 1,638 1,841

40% Case 1,235 1,464 1,643

60% Case 954 1,136 1,249

80% Case 599 692 724

Table IV shows that the frequency response of the Western 

Interconnection reduces with increasing penetration level of 

PV, and the interconnection-wide frequency response meets its 

obligation (906 MW/0.1 Hz) at 20%, 40%, and 60% penetration

levels; however, the values of the 80% renewable penetration 

scenario are less than the interconnection FRO. This means that 

the frequency response of the Western Interconnection with an

80% renewable penetration level might not comply with the 

current BAL-003-1 standard. The shortage of the frequency 

response would affect the safety of the power system operation. 

Traditionally, NERC determines a balancing authority’s 

obligation by first determining the obligation for the 

interconnection as a whole and then assigning a share to each 

balancing authority based on its share of the total generation 

and load of the interconnection. Because some areas might have 

an extremely high renewable penetration level, it might be hard 

to fulfill the individual obligations of these areas. In that case, 

new methodology for dispatching area FRO is suggested to be 

considered from an independent system operator (ISO) 

perspective.

E. Using Existing Resources to Improve Frequency Response

To take advantage of the online available synchronous 

generators, governor settings (deadband and droop) are

modified to help improve system frequency response after 

generation loss contingencies. The loss of 2.6 GW of generation 

is applied in all simulations, and UFLS is enabled.

1) Deadband

A governor deadband is designed to prevent excessive 

governor actions caused by small load variations, but it also 

delays the response of governors to contingencies. In this case, 

two governor deadbands (16.7 mHz and 36 mHz) are simulated 

and compared.

(a) 20% renewable (b) 80% renewable 

Figure 7. WECC frequency responses with different governor deadbands 

(2.6 GW of generation loss)

Fig. 7 shows that that a narrow governor deadband can 

improve the WECC frequency nadir and settling frequency 

slightly because it makes the governor kick in earlier than the 

one with a wide deadband; however, this improvement was 

negligible compared to the large frequency deviation caused by 

the generation loss contingency. Therefore, decreasing the

governor deadband cannot meaningfully improve the WECC 

frequency response after large contingencies.

2) Droop 

The recommended droop setting for WECC is in the range 

from approximately 3%–6%. To check the impact of different 

governor droops, the governor droops in WECC are set to 3% 

and 5%, respectively.

Fig. 8 shows that a 3% droop can clearly better improve the 

frequency nadir and settling frequency compared to the 5% 

droop. In particular, in the 80% case, the 3% droop case has a 

higher frequency nadir, so less load is dropped by the UFLS, 

therefore resulting in a lower settling frequency. This case 

0 5 10 15 20 25 30

Time/s

59.7

59.75

59.8

59.85

59.9

59.95

60

60.05

Frequency/Hz

5 5.2 5.4 5.6 5.8 6

59.88

59.9

59.92

59.94

59.96

59.98

60

A21

A24

A26

0 5 10 15 20 25 30 35 40 45 50 59.4

59.5

59.6

59.7

59.8

59.9

60

Time/s

Frequency/Hz

20% Renewable

16.7 mHz governor deadband

36 mHz governor deadband

UFLS 59.5 Hz

0 10 20 30 40 50 59.4

59.5

59.6

59.7

59.8

59.9

60

Time/s

Frequency/Hz

80% Renewable

16.7 mHz governor deadband

36 mHz governor deadband

UFLS 59.5 Hz
5

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illustrates that employing a 3% governor droop instead of the 

original 5% droop can significantly improve the overall primary 

frequency response of the Western Interconnection and reduce 

the load-shedding amount in the 80% case.

(a) 20% renewable (b) 80% renewable

Figure 8. WECC frequency responses with different governor droops (2.6 

GW of generation loss)

IV. DISCUSSION

In the future, some areas might no longer satisfy their 

approximate FRO with extremely high penetrations of 

renewable energy (e.g., California Independent System 

Operator) as a result of the large amounts of synchronous 

generators replaced by solar and wind generation. The ISO 

might need to seek a way to meet the new requirement from

NERC and therefore ensure fulfillment of the interconnection 

FRO. On one hand, the grid could directly benefit from the 

frequency response capabilities of emerging technologies such 

as wind, solar, and energy storage devices; on the other hand, 

new market mechanisms will be required.

A. Frequency Response from PV Power Plant

Compared to synchronous generators, converter-based 

resources can be even more effective thanks to their fast power 

output regulation capabilities. It is understandable that a portion 

of their capacities needs to be reserved for such services, and 

some production curtailments are inevitable. Therefore, the 

synthetic inertia, governor response, and other frequency 

regulation capabilities from PV power plants will significantly 

benefit system operation. However, to avoid conflict among the 

different scattered generation technologies, the controller needs 

to be carefully designed. In addition, unlike synchronous 

generators, the maximum output of PV is variable, and 

determining the proper portion of PV reserve is still a challenge.

B. Market Perspective

Currently, ISOs do not explicitly procure frequency response 

resources. Considering other regions, particularly the Pacific

Northwest—which far exceeds its approximate FRO because of

high levels of responsive hydropower—could provide 

frequency support for other regions. This means that new policy 

and market mechanisms about trading frequency response 

capabilities need to be built up for future grid operation. 

Mechanisms are also needed to allow balancing authorities to 

both share frequency-responsive resources and ensure that they 

have adequate frequency-responsive resources within their 

control. Further, ancillary service markets should be established 

to compensate those generation or load resources providing 

different fast frequency response services. Corresponding grid 

reliability requirements should also be updated.

V. CONCLUSIONS

Extra-high PV penetration rates in U.S. interconnections are 

expected to occur in the next several decades. The impacts of

extra-high PV penetration on the Western Interconnection were 

evaluated in this paper. The effectiveness of potential solutions 

were also evaluated and discussed. The simulation results 

revealed that the Western Interconnection is able to 

accommodate high PV penetration up to 45%; when the 

penetration level of renewable is as high as 80%, some 

additional frequency support regulations need to be engaged. 

This paper provides guidance for future PV penetration in large 

interconnections. It is also encouraging to see how renewable 

energy can contribute to frequency and transient response, and

how the flexible frequency market can contribute to support the 

FRO, resulting in a more reliable electric grid.

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6

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

[16] S. You, M. Gonzalez, X. Zhang, Y. Liu, "U.S. Eastern Interconnection (EI) 

Electromechanical Wave Propagation and the Impact of High PV Penetration 

This work was supported by the U.S. Department of Energy under Contract 

No. DE-AC36-08GO28308 with Alliance for Sustainable Energy, LLC, the 

Manager and Operator of the National Renewable Energy Laboratory. Funding 

provided by U.S. Department of Energy Office of Energy Efficiency and 

Renewable Energy Solar Energy Technologies Office.

Jin Tan, and Yingchen Zhang, are with the National renewable energy 

laboratory (NREL) Golden, Colorado, 80401, USA. (E-mail: 

jin.tan@nrel.gov). 

on Its Speed," the 2018 IEEE PES T&D Conference & Exposition, Denver, 

2018.

Shutang You, Yong Liu, and Yilu Liu are with are with the Department of 

Electrical Engineering and Computer Science, the University of Tennessee, 

Knoxville, TN, 37996, USA. Yilu Liu is also with Oak Ridge National 

Laboratory, Oak Ridge, TN, 37831, USA.