Simulations and Studies Report
Update
David Piper, P.E.
Operations Planning & Analysis
SCE Grid Control Center
May 22, 2019
1
Fault Voltage impact on inverters
Many inverters momentarily cease current injection
when voltage is outside 0.9-1.1 pu.
3 PH fault can cause widespread voltage depression
below 0.9 pu.
>9,000 MW of impacted inverter based resources for
most impactful faults
System Separation
Area 2
Center
Area 1
Inverter Based Resource
Load
Gen
Load
Gen
VArea 1
VCenter
VArea 2
Loss of inverter based resource can
cause systems to separate.
Distance relays may isolate the two
systems.
Dynamic Voltage Stability
Excessive power swings can cause voltage collapse
Flow on transfer path
Voltage along transfer path
Dangerous first swing voltage/uncontrolled loss of load
(fast voltage collapse followed by overvoltage)
Momentary Cessation=Energy Loss
Frequency Stability
The total energy loss
(MW∙seconds) due to
Momentary Cessation
is proportional to the
change in system
frequency
• Magnitude and
duration of gen loss
matters!
UFLS
W ide Area Studies
Studies sought to identify potential issues:



Instability caused by Momentary Cessation
Limited reactive capability during disturbance
Lack of frequency response capability or headroom
Assumptions included:



Stressed path flows into Southern California
High renewable output
Interconnection-wide summer mid-day (noon)
MC assumptions:



Low Voltage Threshold: 0.9 pu
Recovery Delay: 0.5 s
Active Current Recovery Ramp Rate: 1.0 pu/sec
I nitial Study Results
Two fault scenarios identified potential problems with two
contingencies:



PV did not recover
Voltage along the Pacific AC intertie collapsed within 1.5 seconds
Chicken or the Egg Scenario – Does voltage collapse cause inverters to go
into MC, or does MC cause voltage collapse?
Study Results
Further analysis revealed that DC line blocked during the fault
due to low voltage and remained blocked for 1 second
DC line did not restart due to sustained low voltage associated
with the initiation of voltage collapse.
This response triggers existing RAS actions to protect the Pacific
AC intertie
Sequence of Events
Fault


Momentary Cessation
DC line blocks due to fault voltage
Fault clears normally
PV
Sequence of Events
PACI starts to increase

Voltage along PACI starts to decrease
PV
Sequence of Events
RAS activates

Alleviates loading on Pacific AC
Intertie
Gen
PV
Sequence of Events
Voltage recovers
DC Line Restarts
PV generation recovers
Gen
No Voltage Collapse if M C is Elim inated
Bus voltage along Pacific AC Intertie, with momentary
cessation causing voltage collapse without RAS action
simulated
Prelim inary Conclusions
Unintended interactions between MC and existing RAS
Further evidence that MC can have a negative impact on system
performance
References
Reliability Guideline – PPMV Inverter-Based Resources
https://www.nerc.com/comm/OC_Reliability_Guidelines_DL/PPMV_for_Inverter-Based_Resources.pdf
Reliability Guideline – Inverter Based Resource Performance Guideline
https://www.nerc.com/comm/OC_Reliability_Guidelines_DL/Inverter-Based_Resource_Performance_Guideline.pdf
Reliability Guideline – Modeling Distributed Energy Resources in Dynamic Load Models
https://www.nerc.com/comm/PC_Reliability_Guidelines_DL/Reliability_Guideline_-_Modeling_DER_in_Dynamic_Load_Models_-_FINAL.pdf
Reliability Guideline – Integrating Inverter-Based Resources into Low Short Circuit Strength Systems
https://www.nerc.com/comm/PC_Reliability_Guidelines_DL/Item_4a._Integrating%20_Inverter-Based_Resources_into_Low_Short_Circuit_Strength_Systems_-_2017-11-08-FINAL.pdf
Reliability Guideline – Power Plant Model Verification Using PMUs
https://www.nerc.com/comm/PC_Reliability_Guidelines_DL/Reliability%20Guideline%20-%20Power%20Plant%20Model%20Verification%20using%20PMUs%20-%20Resp.pdf
BPA Presentation - Power Plant Model Verification Using PMUs
https://www.naspi.org/sites/default/files/2016-09/01%20-%20PPMV_NASPI%20_March_2015.pdf
NERC Alert
Momentary Cessation Assessment for
Existing Solar Generation Resources
John Schmall
ERCOT Transmission Planning
ROS Meeting
January 10, 2019
NERC Alert
• NERC Alert issued in May 2018 to recommend assessment of potential
reliability risks due to adverse characteristics of solar PV resources
– Assess the use and impact of momentary cessation
– Assess model updates to eliminate/reduce momentary cessation
• Momentary Cessation
– Inverter does not inject current under voltage conditions where the
resource is not allowed to trip (e.g. “no trip” zone specified in Nodal
Operating Guide, Section 2.9.1)
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TRE Recommendations
• In order to efficiently address NERC Alert (with recommendations
addressed to Transmission Planners, Planning Coordinators,
Transmission Operators, and Reliability Coordinators):
– ERCOT survey solar resources
– ERCOT assess models and perform studies
– Share results with Transmission Planners and Transmission Operators
• Minimum 5-day comment period (November 16-28, 2018)
– Address all comments and finalize report by December 7, 2018
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NERC Alert Survey Response
• ERCOT sent out the survey to 16 solar resources (~1400 MW total)
and all 16 resources responded to survey
– Three resources (~200 MW total) indicated the use of momentary
cessation
– One resource does not have plans to eliminate momentary cessation
• ERCOT staff reviewed models for all 16 solar resources
– Worked with Resource Entities to confirm appropriate model response
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Model Review
• Individual model performance tested
– Voltage profiles per ERCOT Nodal Operating Guide, Section 2.9.1
• Model adjustments confirmed with Resource Entities as necessary
– Expected response with respect to momentary cessation
– Protection relay model settings
– Specific model parameter tuning
• All reviewed models required some adjustment
– Most updates reflect data corrections
– Most updates not directly associated with momentary cessation
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Grid Impact
• ERCOT DWG 2020 HWLL case
– All sixteen solar generation projects were modeled at full output
– Applied a nine cycle 3-phase fault at each POI
• Simulation results did not indicate instability, cascading, or uncontrolled
separation associated with the use of momentary cessation
• Updated models for two solar resources exhibited suspect behavior
contributing to over-voltage conditions and subsequent resource
tripping (~300 MW)
– ERCOT will work further with the Resource Entities to improve this
undesirable response
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Questions?
PUBLIC
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Modeling Improvement Efforts
NERC IRPTF Meeting
May 22, 2019
Songzhe Zhu
California ISO
ISO Public
NERC Alert II Generator Owner Data Submission
Total MW
7198
No MC
Existing
MC
1233
5965
MC Can Be
Eliminated
3250
MC
Remaining
2714
• Data submission from 65 solar plants
• 13 submission included dynamic models for
existing momentary cessation settings
– All have deficiencies
• No dynamic models for proposed momentary
cessation settings
ISO Public
Page 2
NERC Alert Data Follow-up
• The CAISO sent letters to the owners of the solar PV
plants that can eliminate or reduce use of momentary
cessation.
– Requested the GO to provide updated models when
the changes are completed.
• The CAISO received updated models for 5 plants (1400
MW).
– The models are being validated.
ISO Public
CAISO Modeling Enhancement Efforts
• Solar PV plants operational on or before September 1st,
2018
– Model requests were scheduled in phases
– The schedule and model request were posted on the
CAISO website
• Solar PV plants achieving commercial operation after
September 1st, 2018
– As-build models are required within 120 days of
achieving commercial operation
• Periodical updates on the test reports and models
ISO Public
Page 4
Model Submission Categories
• Category 1
– Connecting to BES
– Individual resource > 20 MVA or aggregate resource >
75 MVA
• Category 2
– Connecting to 60kV and above
– Individual resource > 10 MVA or aggregate resource >
20 MVA
ISO Public
Page 5
Model Submission Category
• Category 3
– Connecting to BES or 60kV and above
– Individual resource <= 10 MVA and aggregate
resource <= 20 MVA
• Category 4
– Connecting to below 60kV
– Modeled explicitly in the planning base case
• Category 5
– Connecting to below 60kV
– Aggregated in the planning base case
ISO Public
Page 6
General Model Requirements for Category 1 through 4
• Steady state electrical characteristics and operating
parameters
– One-line diagram, power flow model, etc
• Dynamic models
– Generating units, control devices, protections, etc
• Short circuit models
– Subtransient reactance, transient reactance,
sequence impedance, grounding data, fault currents,
etc
ISO Public
Page 7
Dynamic Model Acceptance Criteria
• Initialize without error;
• A no-disturbance simulation results in negligible
transients; and
• A disturbance simulation results in the models exhibiting
positive damping and reasonable expected performance.
ISO Public
Page 8
Specific Model Requirements for Category 1 & 2
• Test reports for volt/var control and active
power/frequency control less than 10-year old
• Active and reactive capability test report less than 5-year
old
• EMT model for resources close to a series compensated
transmission line
• Geo-magnetic disturbance data
• Ride-through performance compliant with PRC-024
ISO Public
Page 9
Model Submission Schedule for IBRs
Due Date
Phase 1
Phase 2
Phase 3
Phase 4
May 31, 2019
Oct 30, 2019
Mar 30, 2020
Aug 30, 2020
Type
MW
Solar PV
2730
Wind
2530
Solar PV
2070
Wind
410
Solar PV
320
Wind
760
Solar PV
1550
Wind
1090
ISO Public
Page 10
Issues with TOV & Phase Jump in WTGs
In TX close proximity to series compensation
Background
• Type 4 WTGs are located in TX with close proximity of heavy series compensation
• Highlights the issue of WTG tripping due to
•
High TOV
•
DC over voltage resulting from phase jump and ROCOF
Issue with high TOV
It is 690V (~563V phase to ground) system,
one can see the instantaneous phase to
voltage reached close to 1000V (~1.8pu) at
~5.07s. Voltage wave form is very distorted.
This has triggered instantaneous DC voltage
protection and tripped WTG ~5.158s
Fault is applied at 5s with a clearing time of
100ms
One can see even after WTG is
tripped the voltage wave form is
very distorted and reaching voltage
close to 1000V
Underlying cause of Trips
DC Over voltage Trips
Same WTGs but two different
converters (manufacturers)
Phase Jump (Unbalanced L-L fault)
Phase Angle
Protection Flag
One can see two aspects of phase jump:
•
Rapid change
•
Magnitude of change
•
Trip did not just depend on pre-contingent and post contingent phases but
rapidly changing phase during transient at WTG terminal which is very different
from POI phase information, something TOs can provide
In this simulation the trip signal was disabled
to force the simulation to continue. One can
see following:
• Converter was able to manage the initial
phase jump and rapid rate of change
• But couldn’t survive next one
• Protection flags were raised in repeated
manner when the phase kept changing
over time even though the magnitude is
relatively lower
Frequency Estimation (erroneous)
Phase Angle
Phase Angle
Frequency estimation based on
phase angle is erroneous
Frequency
Protection Flag
Unbalanced Fault (L-L)
No tripping should be allowed
Instantaneous on:
1. Frequency estimated by
converter
2. Or ROCOF
Frequency
Protection Flag
Balanced Fault (3Ph-G)
Talking Points
• In WTGs there is no protection on PLL phase jumps or ROCOF
• However such phase jumps or ROCOF may very well results in
• DC over voltage due to energy imbalance
• Sometime AC over voltage
• As shown in previous slide, phase jump changes rapidly at WTG terminal which is
responsible for the consequential trip
• TOs can provide worst case phase jump at POI in post contingent condition, however:
•
They won’t be able do that at WTG’s terminal
•
They won’t be able to predict max phase angle variation during transient at WTG terminal
Thanks
Simulations and Studies Report
Update
NERC IPRTF Meeting
May 22, 2019
Songzhe Zhu
California ISO
ISO Public
Use NERC Alert Data in the Study – Current MC
Settings
14,560 MW Solar PV
Yes (7,198 MW)
Yes (5965 MW)
Current MC
Setting
Use MC
currently?
Submitted
Data?
No (1,233 MW)
No (7,362 MW)
Default MC
Setting
Dyd Model
from WECC
Database
ISO Public
Page 2
Use NERC Alert Data in the Study – Proposed MC
Settings
ISO Public
Result Summary
• Instability were found under P1 and P6 contingency
conditions previously with default MC settings for all
solar PV resources.
• The P1 contingency is stable in both current MC setting
and proposed MC setting scenarios.
• The P6 contingency is unstable in the current MC setting
scenario but stable in the proposed MC setting scenario
– P6 contingency results in isolating ~1500 MW wind
generation from the grid.
ISO Public
Inverter Control Comparison
Degradation of V as Ip takes priority
over Iq
Slower recovery in P due to inverter
controls and recovery of V
Post-fault overvoltage. Plant level V
control & local coordinated Q/V
control, dominated by kvi = 40; iqcmd
rises to 1.3 during fault;
Ipcmd drops as Iqcmd ramps up to
provide reactive support. This is from
Kvp parameter, with no Kqv enabled.
Hence, slower type response.
Q-Priority Control with Imax = 1.3,
Kqv disabled
P-Priority Control with Imax = 1.3,
Kqv disabled
ISO Public
Page 5
Inverter Control Comparison
No post-fault overvoltage. Kqv loop
has fast proportional gain response
to clamp voltage down by responding
quickly with change in Iqcmd. Kqv=2
preventing overshoot since Iqcmd
only rises to 0.5 during fault.
Ipcmd able to rise, and only slightly
clamped since voltage drop is not all
that severe.
Fast recovery in Pg since Iqcmd able
to quickly respond and provide
additional voltage support.
Post-fault overvoltage. Plant level V
control & local coordinated Q/V
control, dominated by kvi = 40; iqcmd
rises to 1.3 during fault;
Ipcmd drops as Iqcmd ramps up to
provide reactive support. This is from
Kvp parameter, with no Kqv enabled.
Hence, slower type response.
Q-Priority Control with Imax = 1.3,
Kqv disabled
Q-Priority Control with Imax = 1.3,
Vdip = 0.9, Kqv = 2
ISO Public
Page 6
Study Report Updates
• The study report is being drafted to address
– Modeling of IBR
– Reliability assessment of IBR impacts
• Reliability concerns
• Scenario development
• Case studies
• Mitigation of reliability risks
– Tuning of IBR to support the grid reliability need
– Target date of completion: end of July
ISO Public
Page 7
Unexpected Wind Farm Voltage
Ride Through Performance
Shun Hsien (Fred) Huang, Jeff Billo
ERCOT Transmission Planning
NERC IRPTF Meeting
May 21-22, 2019
Background
• During transmission faults, the neighboring wind
generation resources experienced partial wind turbines
tripped due to various causes.
• The recorded transmission voltage stayed within voltage
ride through (VRT) profile required by ERCOT Operating
Guide.
• ERCOT is working with the affected Resource Entities to
identify the root of causes.
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Events Description
Event
Fault
Total
WGR
Tripped
(MW)
Direct
Connect
WGRs
(MW)
Partial Tripping
in the
Neighboring
WGRs (MW)
Potential Causes of
Partial Tripping
Wind
Type
1
SLG, 138 kV
636
395 MW
241 MW
Smart crowbar, high
tower vibration
3
2
SLG, 138 kV
382
0 MW
382 MW
Smart crowbar, UPS,
frequency ride
through, unknown
3, 4
3
SLG, 138 kV
800
391 MW
409 MW
Smart crowbar, GSU
setting, high tower
vibration, false alarm
due to voltage spike,
unknown
3
Voltages stayed within the Voltage Ride Through limits in these events
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Faulted Line and Tripped Turbines in Event 1
Legend:
Partial Trips
Disconnected
--- Unit
Other Wind Units
Series
compensated
345 kV lines
Faulted 138 kV
Line
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Event 1: Voltage Response (345 kV)
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Event 1: Recorded Frequency and Wind Generation
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Issues With Smart Crowbar and UPS Failures
• Unit is unable to operate outside of the voltage range 0.9
p.u. - 1.1 p.u.
• Lack of VAR support to quickly recover from voltage
deviations
• Smart crowbar/UPS function does not affect normal
operations, and therefore is not prioritized as a
maintenance issue
– Some turbines that tripped offline had known
malfunctioning components
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Corrective Actions Identified
• Increasing Crowbar and UPS battery inspection to 12
months
– Turbines that had recently gone through maintenance
did not fail
• Prioritizing Smart/Active Crowbar and UPS battery repairs
• A single resource indicated there may be a software issue
with the Active Crowbar function
– The owner reaching out to the manufacturer
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Questions?
• Contact:
– Jeff Billo, Jeff.Billo@ercot.com
– Shun Hsien (Fred) Huang, shuang@ercot.com
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