Rm22-12

2022 California Battery
Energy Storage System
Disturbances
California Events: March 9 and April 6, 2022
Joint NERC and WECC Staff Report
September 2023

RELIABILITY | RESILIENCE | SECURITY

NERC | Report Title | Report Date
I

Table of Contents
Preface ........................................................................................................................................................................... iii
Executive Summary ........................................................................................................................................................ iv
Key Findings and Recommendations .......................................................................................................................... iv
Recommendations for GOs ...................................................................................................................................... v
Chapter 1: Overview of Disturbances ............................................................................................................................. 1
Description of Analysis Process ................................................................................................................................... 1
Pre-disturbance Operating Conditions ........................................................................................................................ 1
Overview of Disturbances involving BESS ................................................................................................................... 1
Chapter 2: Detailed Findings from Disturbance Analysis ................................................................................................ 4
Causes of BESS Abnormal Power Reduction ............................................................................................................... 4
Previously Reported Causes of Tripping .................................................................................................................. 4
Inverter Unbalanced AC Current Tripping ................................................................................................................ 4
Inverter DC Bus Overvoltage .................................................................................................................................... 5
Inverter Half DC Bus Overvoltage ............................................................................................................................ 5
Data Quality ................................................................................................................................................................. 5
Miscoding Inverter Faults ............................................................................................................................................ 5
Mitigations through CAISO Enforcement of Interconnection Agreement................................................................... 5
Appendix A: Detailed Review of Affected BESS Facilities Tripping .................................................................................. 6
Appendix B: Details on Non-BESS Generator Tripping .................................................................................................. 10
Synchronous Generation ........................................................................................................................................... 10
Solar PV Facilities ....................................................................................................................................................... 10
Appendix C: List of Contributors ................................................................................................................................... 14

NERC | 2022 CA BESS Disturbance Report | September 2023
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Preface
Electricity is a key component of the fabric of modern society and the Electric Reliability Organization (ERO) Enterprise
serves to strengthen that fabric. The vision for the ERO Enterprise, which is comprised of the NERC and the six Regional
Entities, is a highly reliable, resilient, and secure North American bulk power system (BPS). Our mission is to assure
the effective and efficient reduction of risks to the reliability and security of the grid.
Reliability | Resilience | Security
Because nearly 400 million citizens in North America are counting on us
The North American BPS is made up of six Regional Entities as shown on the map and in the corresponding table
below. The multicolored area denotes overlap as some load-serving entities participate in one Regional Entity while
associated Transmission Owners/Operators participate in another.

MRO

Midwest Reliability Organization

NPCC

Northeast Power Coordinating Council

RF

ReliabilityFirst

SERC

SERC Reliability Corporation

Texas RE

Texas Reliability Entity

WECC

WECC

NERC | 2022 CA BESS Disturbance Report | September 2023
iii

Executive Summary
NERC continues to analyze disturbances that involve the widespread reduction of inverter-based resources to identify
systemic reliability issues, support affected facility owners, and share key findings and recommendations with industry
for increased awareness and action. Refer to the NERC Quick Reference Guide: Inverter-Based Resource Activities 1 for
more details on all aspects of work in this area. NERC continues to stress the need for industry action in multiple areas
to address the systemic reliability risks posed by inverter-based resource performance issues. Multiple recent
disturbances that involve the widespread reduction of solar photovoltaic (PV) resources have occurred in California,
Utah, and Texas. This report assesses two widespread losses of battery energy storage systems (BESS) caused by
normally cleared faults in the Western Interconnection that occurred on March 9, 2022, and April 6, 2022. These
events are unique in that they are the first major events involving BESS facilities. These events highlight the need to
consider BESS in the same light as any other inverter-based resource, such as solar PV for their systemic reliability
risks. A majority of the inverters involved are made by the same manufacturers that also make solar PV inverters.
The events were each initially categorized as a Category 1a (3/9) and 1i 2 (4/6) events in the NERC Event Analysis
Process due to the magnitude of generation loss and number of affected facilities. 3

Key Findings and Recommendations

Based on the findings of this disturbance report and in the context of past disturbance reports for inverter
performance issues, NERC recommends the following actions:
•

BESS may have the same systemic performance problems as solar PV resources.


•

All BESS facilities experienced partial plant tripping caused by inverter protection, failing to ride through
normally cleared single-line-to-ground grid faults. The affected inverters tripped on both new causes and
causes previously reported associated with solar PV resources. Facilities also exhibited the previously
identified plant control interaction that delays the return to pre-disturbance output levels. To minimize
the proliferation of systemic risks in BESS resources as occurred in solar PV resources, industry should
quickly eliminate these problems, especially since BESS resources are expected to play a key role in
enhancing reliability in the changing resource mix.

BESS ride-through performance is not adequately assessed during the interconnection process.


Transmission Owners (TOs) and Generator Owners (GOs) should identify unreliable ride-through
performance issues prior to commercial operation. TOs should establish clear ride-through performance
requirements in their interconnection requirements and enforce them throughout the interconnection
process. NERC Project 2020-02 4 deliverables can provide minimum ride-through performance
requirements. Prior to commercial operation, TOs should conduct comprehensive assessments to
determine if their ride-through performance requirements are met. GOs should facilitate such
assessments, including providing necessary detailed plant modeling in electromagnetic transient (EMT)
domain. GOs should perform testing during the trial operation period prior to commercial operation to
validate that the actual performance is reflective of the models used during the study phase. 5

https://www.nerc.com/pa/Documents/IBR_Quick%20Reference%20Guide.pdf
The April event was initially categorized as a 1i event. However, after further review, the loss was determined to be just under 500 MW, and
the event was declared a Category 0 event.
3 NERC Event Analysis Program: https://www.nerc.com/pa/rrm/ea/Pages/EA-Program.aspx
4 https://www.nerc.com/pa/Stand/Pages/Project_2020-02_Transmission-connected_Resources.aspx
5 See Article 24 of the FERC pro-forma Large Generator Interconnection Agreement (LGIA)
1
2

NERC | 2022 CA BESS Disturbance Report | September 2023
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Executive Summary

•

Poor commissioning practices are a significant contributor to the unreliable performance of IBRs.


•

NERC will be conducting a model quality assessment of this event and subsequently issue an alert as
needed.


•

The unreliable inverter ride-through performance, site metering issues, and meter misconfiguration
should have been identified as part of commissioning checks. Transmissions Planners and Planning
Coordinators should conduct ride-through performance assessments 6 as part of their interconnection
studies and confirm that installed equipment, performance, and capabilities match those in the studied
models. The NERC Inverter-Based Resource Performance Subcommittee (IRPS) is actively working on the
whitepaper BPS-Connected IBR Commissioning Best Practices to highlight best practices for
commissioning BPS-connected inverter-based resources to ensure that the appropriate protection,
controls, and monitoring are configured; inverter performance is assessed and verified; and that actual
installed operational capabilities match what were studied with the models. There is an increasing need
for a commissioning standard to ensure installed facilities match what was modeled and studied during
the interconnection process and that the facility was commissioned in a reliable manner.

NERC collected positive sequence and EMT models from the affected GOs and TOs as part of this
disturbance analysis. In addition to this report, which focuses solely on unreliable performance, NERC will
be conducting a detailed assessment of the models of the affected facilities.

Lack of adequate monitoring hinders performance and event analysis.


In addition to not properly assessing the ride-through performance during interconnection study and
configuring the inverters appropriately, all BESS involved did not have fast logging enabled and thus lacked
the needed data to determine their transient dynamic performance, such as sufficient reactive current
injection during the disturbance. They also did not meet California Independent System Operator’s
(CAISO) 10 ms recording data resolution requirements. NERC Project 2021-04 7 is underway to ensure
adequate data is available across the entire ERO Enterprise footprint to allow for thorough analysis of
abnormal performance of IBR to identify root causes and mitigations.



There were other significant data quality issues, including the lack of individual facility metering in CAISO’s
supervisory control and data acquisition and inconsistency between BESS facility data and CAISO data.
Additionally, several facilities had their meter data “freeze” on the last value at the onset of the fault.

Recommendations for GOs
•

•

6
7

All GOs should check with their inverter manufacturer to ensure that their inverters are not prone to tripping
on the following causes, in addition to those previously reported, during normally cleared unbalanced grid
faults:


Unexpected, unbalanced ac current



Unexpected dc bus overvoltage tripping



Unstable dc bus voltage

All GOs should ensure both inverter and plant level recording functions are configured to meet the
requirements in the interconnection agreements. Additionally, GOs should perform the following:


Ensure fast logging is enabled



Ensure that meter data does not freeze on the last value when subjected to system transients during grid
faults

Disaggregated plant EMT models are necessary to fully assess potential partial plant tripping and other performance issues.
https://www.nerc.com/pa/Stand/Pages/Project-2021-04-Modifications-to-PRC-002-2.aspx
NERC | 2022 CA BESS Disturbance Report | September 2023
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Chapter 1: Overview of Disturbances
This 2022 California Battery Energy Storage System Disturbances report documents the key findings and
recommendations from analyzing the abnormal loss of BESS resources that occurred in Southern California on March
9 and April 6, 2022. This chapter provides details regarding the initiating event, pre-disturbance conditions, overview
of disturbances involving BESS, and additional relevant details. Chapter 2 provides the key findings and establishes
the basis for the recommendations. Appendix A provides a detailed analysis of the affected BESS facilities. Appendix
B provides brief summaries of the affected facilities that are not BESS. This report focuses solely on the performance
and causes of abnormal power reduction. NERC is working on a separate assessment of the models of the affected
facilities.

Description of Analysis Process

CAISO identified the events on March 9, 2022, and April 6, 2022, by observing the reduction of power outputs across
multiple BESS and solar PV facilities. The events met the criteria for Category 1a and 1i event, respectively, per the
NERC Event Analysis Program based on the total MW losses and number of facilities outaged. NERC and WECC agreed
to develop a disturbance report to share the key findings and recommendations from the analysis with industry. NERC
and WECC decided to focus specifically on BESS due to the unique opportunity of visibility because the first
disturbance occurred after sunset (exclusively BESS, no solar) and the same facilities participated in both disturbances.
CAISO independently solicited requests for information to affected entities and also held follow-up calls with those
entities to gain any additional information needed to perform root cause analysis. NERC and WECC collaborated with
CAISO and prepared this report based on the information provided by CAISO.

Pre-disturbance Operating Conditions

The March disturbance occurred at 18:03:20 Pacific time after the sun had set. The April disturbance occurred at
15:06:05 Pacific time when the solar PV facilities were still producing power. Most of the affected BESS facilities were
located in the vicinity of solar PV facilities. While solar PV facilities were involved in the April event, this report
specifically focuses on the BESS facilities. Table 1.1 shows the generation mix prior to the disturbances.
Table 1.1: Pre-disturbance Resource Mix
Resource Type

March 9

April 6
26,067.0

24,360.0

55.5

12,589.0

Wind

2,567.2

531.7

BESS

769.4

212.1

15,567.9

8,215.2

Internal Net Demand
Solar PV

Synchronous Generation

Overview of Disturbances involving BESS

This report covers the following two events that occurred in March and April of 2022:
•

March 9, 2022: At 18:03:20 Pacific time, a generator bus was faulted when a generator circuit breaker had an
internal failure at a natural-gas-fired, simple-cycle facility in Riverside County, California, causing a C-phaseto-ground fault on the 220 kV system. Generator units relayed, disconnecting the natural gas generators that
were carrying 694 MW. The fault was cleared in approximately 4.5 cycles. In addition, inverter-based
resources from many different facilities also unexpectedly reduced 408 MW, 124 MW of which is attributed
to BESS. The fault therefore resulted in a total loss of 1,102 MW of generation, and system frequency dropped
NERC | 2022 CA BESS Disturbance Report | September 2023
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Chapter 1: Overview of Disturbances

to 59.916 Hz. CAISO, the Balancing Authority, raised regulating unit output to assist in the recovery of the
frequency and ACE. The time from the event start to the frequency nadir was measured at around 12 seconds.
System frequency returned to normal in three minutes.
•

April 6, 2022: At 15:06:05 Pacific time, a B-phase-to-ground fault occurred on a 220 kV bus at a new BESS
plant that was undergoing testing. The fault was cleared in approximately four cycles. The normally cleared
fault resulted in an unexpected reduction of 498 MW from multiple inverter-based resources. The loss of
generation caused the system frequency to fall from around 60.014 Hz to 59.924 Hz. The time from the event
start to the frequency nadir was around seven seconds. The CAISO Balancing Authority raised regulating unit
output to assist in recovery of the frequency and ACE. System frequency returned to normal in one and half
minutes.

In both events, the faults occurred in the Southern California area within the CAISO footprint. Multiple BESS and solar
PV facilities were identified as exhibiting unreliable performance. All affected facilities are within or near the Southern
California area (see Figure 1.1 and Figure 1.2).
Figure 1.3 and Figure 1.4 show the reduction in solar PV and BESS resources reported by Southern California Edison
(SCE) Operations for each disturbance. Figure 1.5 shows the Western Interconnection frequency from PMU data at
nearby 500 kV substation for both disturbances.

Figure 1.1: Map of the Fault Location and Affected IBR Facilities—March 9, 2022

Figure 1.2: Map of the Fault Location and Affected IBR Facilities—April 6, 2022
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Chapter 1: Overview of Disturbances

Total SCE Solar PV + BESS
Output (MW)

400
350
300
250
200
150
100
50
0
18:03:27 18:03:45 18:04:02 18:04:19 18:04:36 18:04:54 18:05:11 18:05:28 18:05:46

Time
Figure 1.3: SCE BPS-Connected Solar PV + BESS during March 9 Disturbance [Source: SCE]
Total SCE Solar PV + BESS
Output (MW)

6,200
6,100
6,000
5,900
5,800
5,700

Frequency (Hz)

5,600
15:05:59 15:06:16 15:06:33 15:06:50 15:07:08 15:07:25 15:07:42 15:08:00 15:08:17 15:08:34 15:08:51
Time
Figure 1.4: SCE BPS-Connected Solar PV + BESS during Disturbance on April 6 [Source: SCE]
60.10
60.09
60.08
60.07
60.06
60.05
60.04
60.03
60.02
60.01
60.00
59.99
59.98
59.97
59.96
59.95
59.94
59.93
59.92
59.91
59.90

0

5

10

15

20

25
30
35
Relative Time (sec)

40

45

50

55

March-9

Figure 1.5: WECC System Frequency for Each Disturbance [CAISO]
NERC | 2022 CA BESS Disturbance Report | September 2023
3

60
April-6

Chapter 2: Detailed Findings from Disturbance Analysis
This chapter describes the key findings from this analysis, including corrective actions taken by CAISO and GOs. Refer
to Appendix A for details regarding each affected facility.

Causes of BESS Abnormal Power Reduction

The causes of abnormal active power reduction for each BESS or hybrid facility are listed in Table 2.1. The facilities
saw multiple inverters trip due to multiple causes, making it difficult to break down active power reductions by fault
codes.
Table 2.1: Causes of BESS Abnormal Power Reduction
Cause of Reduction

Plant A

Plant B

Plant C

Plant D

Original Equipment
Manufacturer

Power Electronics

Power Electronics

Power Electronics

Power Electronics

March

March

Total MW Reduction
AC Overcurrent

April

20

N/A*

X

X

AC Overvoltage

X

DC Bus Voltage
Unbalance

X

Unbalanced AC Current

X

X

DC Overvoltage

X

Half Bus DC Vmax

X

April

March

26

April

73

March
30

April
N/A*

X

Not
Involved

Unknown/Misc.

X

X

X

X

Inconclusive
due to poor
metering
data to
support GOs
claims

X

X

X

* Not available due to data quality and availability issues detailed in Appendix A.

Previously Reported Causes of Tripping
Most affected facilities experienced the following causes of inverter tripping that have already been extensively
covered in previous disturbance reports. Therefore, technical details regarding them are omitted in this report:
•

Inverter instantaneous ac overcurrent

•

Inverter instantaneous ac overvoltage tripping

•

Inverter dc voltage unbalance tripping

Other causes of tripping not identified in previous disturbance reports are detailed in the following sections.
Inverter Unbalanced AC Current Tripping
This is the most common cause of tripping observed in both disturbances. The inverter module can trip when an
unbalance has been detected in the module output currents that are above a programmed threshold and remains for
longer than a programmed time delay.
All GOs should check with their inverter manufacturer to ensure that their inverters are not prone to unexpected
unbalanced ac current tripping during normally cleared unbalanced grid faults.
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Chapter 2: Detailed Findings from Disturbance Analysis

Inverter DC Bus Overvoltage
Inverter dc bus overvoltage tripping was observed in two facilities: one while discharging and another while charging.
All GOs should check with their inverter manufacturer to ensure that their inverters are not prone to unexpected dc
bus overvoltage tripping during normally cleared unbalanced grid faults.
Inverter Half DC Bus Overvoltage
This protection is triggered when an inverter module continues to experience unstable dc bus voltage despite being
stopped on dc bus overvoltage fault. All GOs should check with their inverter manufacturer to ensure that their
inverters are not prone to tripping on unstable dc bus voltage during normally cleared unbalanced grid faults.

Data Quality

None of the facilities involved met CAISO’s requirement of 10 ms data recording resolution. No inverter-level
oscillography data was available from any of the facilities involved because they did not have the required fast logging
enabled. CAISO reported that these deficiencies have been corrected.

Miscoding Inverter Faults

While one facility had legacy inverters that utilize momentary cessation, GOs of other affected facilities have miscoded inverter tripping lasting tens of seconds as momentary cessation. Momentary cessation is a controlled behavior
during certain grid conditions and is vastly different from an inverter tripping inadvertently and reconnecting after a
short time.
One facility only reported high level inverter fault codes, which do not provide any useful insights. The inverters
involved in these events are made up of multiple modules, and inverters can experience loss of all or some of the
modules (partial trip). The actual trip mechanism, as part of self-protection design, occurs within the modules and
thus the module fault codes are more meaningful. NERC recommends all GOs log and report module fault codes.

Mitigations through CAISO Enforcement of Interconnection Agreement

Based on the findings from the two BESS events, CAISO identified that the affected facilities did not meet the
performance-related requirements of the Generator Interconnection Agreement (GIA). 8 Subsequently, CAISO issued
notice of breach letters to affected GOs advising that the terms and conditions of Appendix H of the LGIA are not
being met by their facilities. The notice highlighted the provisions related to the voltage ride-through and minimum
transient data recording requirements. The letter required GOs of affected BESS facilities to develop and implement
mitigation plans to eliminate the unexpected causes of tripping and make changes to recording capabilities to meet
the interconnection agreement requirements. The following summarizes the corrective actions taken by CAISO and
GOs:

8

•

CAISO reported that all but one facility has been enhanced with inverter software upgrades. The software
upgrades increased the threshold for dc bus unbalance tripping and implemented faster activation of stronger
dc balancing and faster activation of low voltage ride-through mode. CAISO reviewed evidence of the
effectiveness of the mitigation plans provided by the GOs working with the inverter manufacturer. Evidence
includes the lab test results from the inverter manufacturer that showed the performance of the inverter
before and after the software upgrade.

•

These enhanced facilities performed better during an event in February 2023 that involved an unbalanced
grid fault. This suggests some evidence of the efficacy of the changes implemented post 2022 events.

•

CAISO updated technical requirements of their pro forma LGIA, requiring the plant controller to be
coordinated with the inverters so that the plant controller will not restrict inverter reconnection following the
clearance of a low voltage transient.

https://www.caiso.com/planning/Pages/GeneratorInterconnection/InterconnectionAgreement/Default.aspx
NERC | 2022 CA BESS Disturbance Report | September 2023
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Appendix A: Detailed Review of Affected BESS Facilities Tripping
This appendix describes the causes of abnormal performance at each affected BESS facility that reduced power
output unexpectedly during the Cat 1a and 1i disturbances in 2022.
Table A.1 provides an overview of the BESS facilities involved in the March event followed by additional details for
each specific facility.
Table A.1: Review of BESS Facilities Involved in 3-9-2022 Event
Facility
ID

Capacity
[MW]

Reduction
[MW]

POI
Voltage
[kV]

InService
Date

Cause of Reduction

Plant A

66.7

20

230

June
2021

Inverter ac overcurrent tripping; unbalanced ac
current tripping.

Plant C

250

73

230

June
2021

Inverter ac overcurrent tripping; unbalanced ac
current tripping.

Plant D

70
(35 MW
BESS)

31

230

June
2021

DC bus voltage overvoltage; unknown for some.

TOTAL

124

Plant A
Plant A is a 66 MW facility connected to the 230 kV network that went into commercial operation in June 2021. The
plant reduced output from 57 MW to 37 MW during the event (see Figure A.1). The plant consists of a total of 29
inverters; 15 inverters experienced instantaneous ac overcurrent tripping, and 12 inverters experienced unbalanced
ac current tripping.
To mitigate these issues, the GO worked with the inverter original equipment manufacturer to perform software and
configuration updates not only at this facility but proactively at other facilities that use the same inverter make and
model.
60
40
30
20
10
0
-10
3:03:38 PM
3:03:39 PM
3:03:41 PM
3:03:43 PM
3:03:45 PM
3:03:46 PM
3:03:48 PM
3:03:50 PM
3:03:52 PM
3:03:53 PM
3:03:55 PM
3:03:57 PM
3:03:58 PM
3:04:00 PM
3:04:02 PM
3:04:04 PM
3:04:05 PM
3:04:07 PM
3:04:09 PM
3:04:11 PM
3:04:12 PM
3:04:14 PM
3:04:16 PM
3:04:17 PM
3:04:19 PM
3:04:21 PM
3:04:23 PM
3:04:24 PM
3:04:26 PM
3:04:28 PM

Power

50

MW

Figure A.1: Plant A Active and Reactive Power at POI on March 9
NERC | 2022 CA BESS Disturbance Report | September 2023
6

MVAR

Appendix A: Detailed Review of Affected BESS Facilities Tripping

Plant C
Plant C is a 250 MW facility connected to the 230 kV network that went into commercial operation in June 2021. The
plant reduced output by 73 MW during the event (see Figure A.2). The plant consists of a total of 88 inverters. Of
these, 27 inverters experienced instantaneous ac overcurrent tripping, and 35 inverters experienced unbalanced ac
current tripping. GO was unable to provide any useful information regarding the cause of tripping from 12 additional
inverters that reduced power.

220

9

200

7

180

5

160

3

18:03:49,

140

1

120

-1

100

-3

Active Power MW

Reactive Power (MVAR)

11

18:03:39,
229.0699997

18:01:45
18:02:08
18:02:31
18:02:54
18:03:17
18:03:40
18:04:03
18:04:26
18:04:49
18:05:12
18:05:35
18:05:58
18:06:21
18:06:44
18:07:07
18:07:30
18:07:53
18:08:16
18:08:39
18:09:02
18:09:25
18:09:48
18:10:11
18:10:34
18:10:57
18:11:20
18:11:43
18:12:06
18:12:29
18:12:52
18:13:15
18:13:38
18:14:01
18:14:24
18:14:47

Active Power (MW)

240

Reactive Power MVAR

Figure A.2: Plant C Active and Reactive Power at POI on March 9
Plant D
Plant D is a 70 MW hybrid PV + BESS facility connected to the 230 kV network. It went into commercial operation in
June 2021. The plant consists of 13 BESS inverters that are rated for 35 MW total. The plant reduced output from
30.7 MW to 0 MW during the event (see Figure A.3). Inverter logs show 12 inverters tripped and 1 inverter was
already off-line before the event. Two inverters tripped on high dc voltage protection four seconds before another
11 inverters tripped for an unknown reason as their fault code merely indicated that all internal modules were offline. The plant partially recovered after 48 minutes.
40
30

10
0
-10

Figure A.3: Plant D Active Power at POI on March 9
NERC | 2022 CA BESS Disturbance Report | September 2023
7

18:04:36

18:04:33

18:04:30

18:04:26

18:04:23

18:04:19

18:04:16

18:04:12

18:04:09

18:04:05

18:04:02

18:03:58

18:03:55

18:03:52

18:03:48

18:03:45

18:03:41

18:03:38

18:03:34

18:03:31

18:03:27

18:03:24

18:03:20

18:03:17

18:03:14

-20
18:03:10

Power

20

Active Power MW

Appendix A: Detailed Review of Affected BESS Facilities Tripping

Table A.2 provides an overview of the BESS facilities involved in the April event followed by additional details for each
specific facility. Unlike the March event, this disturbance occurred during solar production, so the power output
reduction can be attributed to unexpected inverter tripping in both the solar PV plants and BESS. The facilities
involved in the March event also participated in the April event.
Table A.2: Review of BESS Facilities involved in 4-6-2022 Event
Facility
ID

Capacity
[MW]

Reduction
[MW]

POI Voltage
[kV]

In-Service
Date

Cause of Reduction

66.7

Inconclusive

230

June 2021

Inverter ac overcurrent tripping; unbalanced
ac current tripping; dc bus unbalance; half
bus dc voltage max; dc bus overvoltage; high
ac voltage

115

26

230

April 2021

Unbalanced ac current tripping; Unknown for
some

Plant C

250

Inconclusive

230

June 2021

Unknown

Plant D

70

Inconclusive

230

June 2021

Unknown

Plant A

Plant B

Plant A
The facility metering failed a few minutes before the event, so the data from the GO was inconclusive. CAISO data
aggregates output from these facilities with several others, so the total facility output power reduction due to BESS
inverter tripping could not be determined. However, inverter logs show that 14 out of 29 inverters tripped from a
variety of different inverter protections: dc inverter ac overcurrent, unbalanced ac current, dc bus unbalance, half
bus dc voltage max, dc bus overvoltage, and high ac voltage. All but one inverter remained off-line for an extended
period of time.
The GO identified and implemented a process for collecting data from a secondary source as a mitigation for future
events.
Plant B
Plant B is a 115 MW facility connected to the 230 kV network that went into commercial operation in April 2021. The
plant output dropped by 26 MW during the event from the pre-disturbance level of 36 MW (see Figure A.4). The
plant consists of a total of 45 inverters. GO reported that five inverters tripped (three tripped on unbalanced ac
current protection; cause of trip unknown for the rest). However, the loss of five inverters does not add up to the
total 26 MW lost, suggesting that there is a data quality issue.

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Appendix A: Detailed Review of Affected BESS Facilities Tripping

14:58:34 15:00:00 15:01:26 15:02:53 15:04:19 15:05:46 15:07:12 15:08:38 15:10:05 15:11:31 15:12:58
5
0

Power

-5
-10
-15
-20
-25
-30
-35
-40

Active Power MW

Reactive Power MVAR

Figure A.4: Plant B Active and Reactive Power at POI on April 6

Power

Active Power MW

15:08:17

15:08:04

15:07:51

15:07:38

15:07:25

15:07:12

15:06:59

15:06:46

15:06:33

15:06:20

15:06:07

15:05:54

15:05:41

2
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20

15:05:28

Plant C
The GO reported there was no inverter fault and attributed the abnormal power response to the facility charging at
a lower power level than the set point to manage the battery nearing its full state of charge. However, data from GO
(Figure A.5) indicated that the facility metering was interrupted right around the disturbance due to the log recording
the same value for a few seconds. When the recording resumed, the plant charging power level dropped, and the
reactive power consumption increased simultaneously, suggesting that a few inverters were lost. The results are
inconclusive.

Reactive Power MVAR

Figure A.5: Plant C Active and Reactive Power at POI on April 6
Plant D
This facility is part of a multi-phase project. Since CAISO’s data is an aggregate for multiple phases, metering data for
the individual facility was not available. Therefore, no power output plot can be produced. However, the GO-supplied
logs indicate that 9 out of 13 inverters faulted for unknown reasons
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Appendix B: Details on Non-BESS Generator Tripping
This section briefly describes the power reduction observed in facilities that are not BESS during the same events
described above.

Synchronous Generation

On March 9, a generator breaker failed at a natural-gas-fired facility in Riverside County, causing a phase to ground
fault on the generator 220 kV bus. Consequentially, the generator units relayed as expected, causing a loss of 694
MW. The fault was cleared in approximately 4.5 cycles.
On April 6, a steam unit and a natural gas unit tripped, causing a loss of 123 MW and 167 MW, respectively. The cause
of the natural gas unit tripping was attributed to a transformer bank differential protection scheme current
transformer being left shorted during past scheduled work.
The issue was corrected on April 11, 2022.

Solar PV Facilities

On March 9, since the event was after sunset, the solar PV facilities were not producing power. The following solar
PV facilities experienced unexpected power reductions during the April 06 event.
Plant E is a 300 MW facility connected to the 230 kV network that went into commercial operation in 2014. The plant
experienced a 104 MW power reduction due to momentary cessation. It is an older facility where momentary
cessation could not be disabled. Its power recovery has improved since the last event it was involved in. One inverter
tripped on dc overcurrent. See Figure B.1.
300

Active Power [MW]

250
200
150
100
50
0
14:52:48 14:55:41 14:58:34 15:01:26 15:04:19 15:07:12 15:10:05 15:12:58 15:15:50 15:18:43 15:21:36
Figure B.1: Plant E Active Power at POI on April 6

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Appendix B: Details on Non-BESS Generator Tripping

Plant F is a 100 MW facility connected to the 230 kV network that went into commercial operation in 2022. During
the event, the plant output dropped from 78 MW to 0 due to 26 inverters tripping on ac overcurrent. See Figure B.2.
90
80

Active Power [MW]

70
60
50
40
30
20
10
0
-10
-20
15:04:19

15:05:46

15:07:12

15:08:38

15:10:05

15:11:31

15:12:58

15:14:24

Figure B.2: Plant F Active Power at POI on April 6
Plant G is a 250 MW facility connected to the 230kV network that went into commercial operation in 2016. During
the event, the plant output dropped from 239 MW to 60 MW due to 192 inverters tripping on low line-to-line voltage
and 2 inverters of a different manufacturer tripping for unknown reasons as the inverters did not record fault codes.
A few months prior to the event, the inverter trip settings had been revised from being set directly on PRC-024 notrip boundaries, but the inverter still tripped. See Figure B.3.
300

Active Power [MW]

250
200
150
100
50
0
15:08:47 15:08:56 15:09:04 15:09:13 15:09:22 15:09:30 15:09:39 15:09:48 15:09:56 15:10:05
Figure B.3: Plant G Active Power at POI on April 6

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Appendix B: Details on Non-BESS Generator Tripping

Plant H is a 125 MW facility connected to the 230 kV network that went into commercial operation in 2020. During
the event, the plant output dropped from 123 MW to 43 MW due to 23 inverters tripping and 3 inverters experiencing
partial loss of modules. The causes of tripping included high voltage ac, unbalanced dc bus, and ac overcurrent. See
Figure B.4.
140

Active Power [MW]

120
100
80
60
40
20
0
15:05:54

15:06:03

15:06:12

15:06:20

15:06:29

15:06:37

15:06:46

15:06:55

15:07:03

Figure B.4: Plant H Active Power at POI on April 6
Plant J is a 125 MW facility connected to the 230 kV network that went into commercial operation in 2020. During
the event, the plant output dropped from 125 MW to 92 MW due to 26 inverters experiencing partial module loss
and 1 inverter tripping completely. The causes of tripping included unbalanced dc bus and unknown. See Figure B.5.
140

Active Power [MW]

120
100
80
60
40
20
0
15:05:54

15:06:03

15:06:12

15:06:20

15:06:29

15:06:37

15:06:46

Figure B.5: Plant J Active Power at POI

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15:06:55

15:07:03

Appendix B: Details on Non-BESS Generator Tripping

Plant K is a 250 MW facility connected to the 230 kV network that went into commercial operation in 2014. During
the event, the plant output dropped from 40 MW to 37 MW due to momentary cessation. No inverter tripping was
reported. See Figure B.6.
40.5
40

Active Power [MW]

39.5
39
38.5
38
37.5
37
36.5
14:45:36

14:52:48

15:00:00

15:07:12

15:14:24

Active Power
Figure B.6: Plant K Active Power at POI on April 6

NERC | 2022 CA BESS Disturbance Report | September 2023
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15:21:36

Appendix C: List of Contributors
This disturbance report was published with the contributions of the following individuals. NERC gratefully
acknowledges WECC, CAISO, and the affected GOs and GOPs. Coordination between all affected entities was crucial
for the successful analysis of this disturbance and publication of this report. NERC would also like to acknowledge the
continued engagement and support of the inverter manufacturers to ensure that the mitigating measures being
developed are pragmatic and implemented in a timely manner. Lastly, members of the NERC IRPS continue to support
NERC in its mission to ensure reliable operation of the BPS with rapidly increasing levels of inverter-based resources.
Name
Rich Bauer
Ryan Quint
Alex Shattuck
Aung Thant
James Hanson
Curtis Holland
Scott Vaughan

Company
North American Electric Reliability Corporation
North American Electric Reliability Corporation
North American Electric Reliability Corporation
North American Electric Reliability Corporation
WECC
WECC
CAISO

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