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David Piper, PE
Presentation toNERC IRPTF
January 24, 2018
IRPTF-Simulation Update
UpdateThe studies subgroup has completed simulationsSome of the most severe fault/setting combinations have resulted in transient instabilitySystem response characteristics have been grouped for further analysis.
2
3
Momentary Cessation (Stable)1. Fault in Southern California
1. Inverters enter MC2. DC converters may block3. Inertial and Primary
frequency response (mostly from the Northwest)
4. Power swing magnitude depends on energy loss
2. Fault cleared1. DC converters resume2. Inverters resume3. System reaches
equilibrium
Magnitude of Energy Loss Matters
4
• Fault type and location increases the number of inverters that observe depressed voltage
• Vmc, Δsr, and Δrr settings have a direct impact on the total energy loss following a fault
Location of Spin Matters
5
6
Momentary Cessation (Unstable)1. Fault in Southern California
1. Excessive amount of inverters enter MC
2. DC converters block3. Inertial response (mostly from
the Northwest)4. Excessive power swing
2. Fault cleared1. DC converters resume 2. Inverters start to resume3. Voltages decline along
transfer paths4. Voltage collapse and/or
separation
Next StepsAnalysis will be completed on unstable cases to identify cause of instability.Studies subgroup will analyze the impact that a reallocation of spinning reserve may have on stabilityAnalysis of peak load casesProvide recommendation of real/reactive power priority
7
Questions?
8
Simulate Inverter Momentary Cessation
NERC IRPTF Updates
Songzhe Zhu, California ISO
David Piper, Southern California Edison
Ryan Quint, NERC
Modeling Objectives
• Existing inverter based generators Generic models shall be able to simulate the momentary
cessation and match the performance observed in the actual events
• Future inverter based generators Establish inverter performance guidelines
Generic models shall be examined for modeling capability against the inverter design under the performance guidelines
Illustration of Momentary Cessation
Momentary Cessation Modeling Requirement
• Cease active and reactive currents if voltage is below v1 or above v2
• Time delay between voltage is back to range [v1, v2] and recovery starts
• Ramp rate limit of active current during recovery• Control of reactive current during recovery• Priority between active and reactive current during
recovery
MC Modeling Capability of Generic RE Models
• Recommendation was to use reec_a model to simulate momentary cessationMeet most of the modeling requirements by carefully
setting up VDL1 and VDL2 blocks, voltage_dip and thld2
Does not simulate the reactive current recovery delay
Conversion of existing reec_b model to reec_a is a challenge
User-Written Model to Simulate MC
• Replaced in-run epcl with the user-written model for better control of model execution sequence, i.e. when the epcl is executed at each time step of integral.
• Interact with reec, pv1e and wt4e models to simulate zero current injection during cessation and recovery delay At the moment Vt_filt < V1 or Vt_filt > V2, set Imax = 0 At the moment V1 <= Vt_filt <= V2, start and recovery
delay timer. When the recovery delay timer has expired and voltage is still
within normal range, Imax is restored to the value in the dynamic model.
User-Written Model to Simulate MC (Cont.)
• Freeze state variables in reec models during recovery delay. During recovery delay, Iqmax and Iqmin are 0
to achieve zero current injection. S2 and s3 are winding up, which causes numerical distortion when the delay is over. Set all control gains to 0 to prevent this.
User-Written Model to Simulate MC (Cont.)
• Include a lock-out mechanism in the user-written model. After a certain number of successive MCs, the inverters are
tripped.
• The user-written model is invoked in the dyd file:
epcmod 10000 "12ST TAP" 46.00 "1 " : #9 "blockinv_epcmod.p" 8 "vblk" 0 "delay" 0 "rrpwr" 0 "lockout" 0
Output of the MC Model
• Report MC events
TIMEMODEL
TYPE BUS NOBUS
NAME ID EVENT EVENT DETAIL V
0.52917 reec_b xxxxx xxx 1 entering block state voltage outside of range 0.9-2 0.897255
0.570838 reec_b xxxxx xxx 1 entering delay state delay timer is 1 seconds 0.907237
1.570851 reec_b xxxxx xxx 1entering recovery state
unit will fully recover in 1.111111 seconds 1.036536
2.683346 reec_b xxxxx xxx 1entering normal state voltage inside of range 0.9-2 1.010558
BUS NO BUS NAME ID MC COUNTxxxxx xxx 1 1
Simulation Results – Pg of Solar PV
High Voltage ContextNERC Inverter-Based Resource Performance Task Force
Matthew Richwine
Dustin Howard
Min Lwin
Jason MacDowell
GE Energy Consulting
Schenectady, NY USA
Austin, TX
January 23-25, 2018
GE Energy Consulting
High Voltages - Classification
2
• For the sake of discussion, it’s separated into two groups:
• Fast Switching Transients
• “Spikes” lasting less than 1 cycle of the fundamental
• Typical of switching events
• Temporary Over-Voltages (TOV)
• Brief periods of high voltage lasting longer than 1 cycle to
~seconds
• Typical of fault-clearing
GE Energy Consulting
Fast Switching Transients
3
• Relevant factors in such events:
• Capacitors (both series and shunt)
• Point-on-wave timing
• Transformer saturation, capacitive coupling
• Grounding and transformer connection groups
• Often, there are many (typically damped) electrical resonances in
a system
• These resonances may be excited by switching events, causing
significant distortion in the voltage waveforms
• Control actions are not intended to operate in these time frames
• Equipment protection functions should be designed to ride
through these
GE Energy Consulting
Temporary Over-Voltages
4
• On fault recovery, voltage may overshoot for many cycles
• Degree and duration of overshoot is related to
• System characteristics – system strength (SCR after clearing),
X/R ratio, effective grounding (X0/X1, R1/X1)
• Equipment control actions
• Exacerbated by shunt capacitance, relative to equipment size
• Exacerbated by transformer saturation
GE Energy Consulting
Example Simulation – Line-Line Fault
5
~ Zeq GZcoll
System Equivalent
L-L Fault
Series CapStation Xfmr
Collector
Unit XfmrPlant
High Voltage Bus Voltages
Medium Voltage Bus Voltages
Time (50ms / division)Fault Onset
Fault Cleared
GE Energy Consulting
Example Simulation – Capacitor Switching
6
~ Zeq GZcoll
System Equivalent
Shunt Cap Station XfmrCollector
Unit XfmrPlant
High Voltage Bus Voltages
Medium Voltage Bus Voltages
Time (50ms / division)Cap switched in
(worst case point-on-wave)
GE Energy Consulting
Aspects of a specification
7
• Phase-phase, phase-ground, positive sequence voltages?
• Minimum or maximum of these?
• Duration?
• Applicable bus?
GE Energy Consulting
Conclusions
8
• Faults and switching events can excite existing resonances in the
system, which may produce significant voltage waveform
distortion and brief voltage spikes
➢May be mitigated through arrestors and plant design
• High voltages lasting many cycles to seconds often upon fault
clearing
➢May be mitigated through controls and plant design
Equipment protection functions must be designed to account for
fast switching transients and TOV to achieve selectivity and security
GE Energy Consulting
Grounding and System Overvoltages
Effectively grounded vs. isolated systems
Vo
lta
ge
(p
u)
Voltage within reasonably safe range
when system is effectively grounded
System is effectively
grounded when 0<[X0/X1]<3
© 2017 Tesla Inc. | Proprietary & Confidential
© 2017 Tesla Inc. | Proprietary & Confidential
O U R M I S S I O N
Accelerate the world’s transition to sustainable energy
2
© 2017 Tesla Inc. | Proprietary & Confidential
Location
Hornsdale, South Australia
Project Size100 MW | 129 MWh
Distributed Energy Resource
Wind
ApplicationsEnergy Arbitrage
Frequency Regulation
Contingency Reserves
CommissionedDecember 2017
1 . S O U T H A U S T R A L I A G R I D - S C A L E S T O R A G E
A ) O V E R V I E W
© 2017 Tesla Inc. | Proprietary & Confidential
1 . G R I D - S C A L E S T O R A G E
B ) P S S / E A N D P S C A D A N A L Y S E S
• Tesla has developed detailed user-defined models that are a direct replica of the firmware running on the inverter and site controller
• The models have been validated against lab test results and actual site measurements
• Transmission level analyses studied for South Australia:
• PCC voltage control response time and setpoint change command tests
• Short duration, long duration, shallow, and deep single-phase, double-phase, and three-phase to ground fault ride
through for faults at the PCC and other grid buses
• Active and reactive power reference change
• Operation under different grid’s rate of change of frequency events
• Operation under weak grid conditions
• Multiple successive faults ride though
© 2017 Tesla Inc. | Proprietary & Confidential
1 . G R I D - S C A L E S T O R A G E
C ) R E S P O N S E T O F R E Q U E N C Y E V E N T S
© 2017 Tesla Inc. | Proprietary & Confidential
1 . G R I D - S C A L E S T O R A G E
C ) R E S P O N S E T O F R E Q U E N C Y E V E N T S
© 2017 Tesla Inc. | Proprietary & Confidential
1 . G R I D - S C A L E S T O R A G E
C ) R E S P O N S E T O F R E Q U E N C Y E V E N T S
© 2017 Tesla Inc. | Proprietary & Confidential
1 . G R I D - S C A L E S T O R A G E
C ) R E S P O N S E T O F R E Q U E N C Y E V E N T S
© 2017 Tesla Inc. | Proprietary & Confidential
1 . G R I D - S C A L E S T O R A G E
C ) R E S P O N S E T O F R E Q U E N C Y E V E N T S
© 2017 Tesla Inc. | Proprietary & Confidential
1 . G R I D - S C A L E S T O R A G E
D ) R E S P O N S E T O F R E Q U E N C Y E V E N T S
Source: http://reneweconomy.com.au
October 9 2017 Canyon 2 Solar Loss Disturbance
Rich BauerIRPTF updateJanuary 23, 2018
RELIABILITY | ACCOUNTABILITY2
Event
220 kV line to line fault voltages
RELIABILITY | ACCOUNTABILITY3
Event
500 kV line to line fault voltages
RELIABILITY | ACCOUNTABILITY4
Event
WI frequency after 2nd fault
RELIABILITY | ACCOUNTABILITY5
Event
Solar PV Response during Canyon 2 Fire
RELIABILITY | ACCOUNTABILITY6
• October 9, 2017 Canyon 2 Fire Smoke induced 2 transmission phase to phase faultso 220 kV line fault at 12:12:16 – normal clearing time ~3 cycleso 500 kV line fault at 12:14:30 – normal clearing time ~2.5 cycles
~682 MW solar reduction on first faulto No frequency tripping observedo ~773 MW Momentary Cessationo ~133 MW voltage tripping
~937 MW solar reduction on second faulto No frequency tripping observedo ~652 MW Momentary Cessationo ~400 MW voltage tripping
Event
RELIABILITY | ACCOUNTABILITY7
• No erroneous frequency tripping• Continued use of Momentary Cessation• Ramp rates on return from Momentary Cessation• Voltage tripping based on PRC-024 ride-through curve• Tripping on voltage transients• Voltage signal used for voltage protection measurement is
unfiltered• Recommend Alert 2…
Key Findings
RELIABILITY | ACCOUNTABILITY8
• Phase Lock Loop Synchronization Issues• DC Reverse Current Tripping• Transient Interactions and Ride-Through Considerations
Key Findings
RELIABILITY | ACCOUNTABILITY9
Various facilities output response
Take this out
RELIABILITY | ACCOUNTABILITY10
Time [ms]
1.0
1.5
2.0
1.6
1.7
1.8
1.9
1.4
1.3
1.2
1.1
1.6 3.0 16.66 200.0
IEEE P1547 Transient Overvoltage LimitsFundamental Frequency Overvoltage Ride ThroughTransient Overvoltage Ride ThroughMomentary cessation is allowed in this region, and actually preferredRide-through under these conditions allows for momentary cessationRide-through under these conditions does not allow momentary cessationRide-through under these conditions does not allow momentary cessation
**
1.0
1.5
2.0
1.6
1.7
1.8
1.9
1.4
1.3
1.2
1.1
POI R
MS
Volta
ge [p
u]
Inve
rter
Ter
min
al V
olta
ge
(Per
Uni
t of N
omin
al In
stan
tane
ous P
eak
Base
)**
****
**
*
No Trip Zone
No Trip Zone
May Trip Zone*
Technical PointsRed Curve (Instantaneous) Blue Curve (RMS)
• Represents sub-cycle timeframe (not intended to be captured by PRC-024-2) less than 16.66 ms
• Clarifications• Inverse-time characteristic• Momentary cessation only allowed for very short,
high magnitude transient V• Protects IGBTs
• Inverter manufacturers have stated that 1.3-1.4 pu for less than 1 cycle is reasonable ride-through requirement
• Recent disturbances have shown 1.5-1.6 pu for 1/10th of a cycle (< 3 ms) – need to be able to ride through these types of fault voltages
• Peak/crest V above 1.7 pu for > 3 ms doesn’t seem all that feasible for sinusoidal AC voltage waveform…
• Mirrors PRC-024-2 after 16.66 ms• Clarifications
• Fundamental frequency voltage• Should be well filtered
• Inherently requires some period over the AC waveform
• Should be RMS quantity, not instantaneous
IRPTF Stability StudiesWestern Interconnection Resource Loss Protection CriteriaNERC IRPTF Update to NERC RS
NERC RS/IRPTF MeetingJanuary 2018
RELIABILITY | ACCOUNTABILITY2
• The Blue Cut Fire Disturbance Report Recommendation 2ci states: “…Once a more detailed quantification of potential impact has been
determined, including the amount of momentary cessation and/or tripping as well as the inverter-based resources’ return from these conditions, the NERC Resources Subcommittee should consider whether any adjustments to the resource loss protection criteria are needed to protect interconnection frequency stability.”
• The NERC Resources Subcommittee (RS) requested the NERC Inverter-Based Resource Performance Task Force (IRPTF) investigate these impact via study
Background
RELIABILITY | ACCOUNTABILITY3
• NERC IRPTF created a “Modeling and Studies” sub-group consisting of the following: WECC affected entities – Transmission Planners from throughout WECC
region Simulation and modeling experts Software vendors NERC and WECC staff
Background
RELIABILITY | ACCOUNTABILITY4
Momentary Cessation
• Vmc: voltage threshold where momentary cessation occurs
• Δtsr: delay after voltage recovers before recovery in current injection
• Δtrr: ramp rate of recovery in current injection
RELIABILITY | ACCOUNTABILITY5
• The IRPTF is recommending in its draft Reliability Guideline that momentary cessation…
1. Should not be used for future resources (resources should continue injecting current during ride-through events)
2. Should be mitigated, or minimized to the greatest extent possible, for existing resources
To Be Clear on Momentary Cessation…
RELIABILITY | ACCOUNTABILITY6
PV Active Power Response
Δtrr
Vmc
Δtsr
RELIABILITY | ACCOUNTABILITY7
• Current generic models used in planning cases do not accurately represent all aspects of momentary cessation IRPTF developing recommendations and action plan to address this Some “modification” to off-the-shelf models required to perform this
assessment
• Modeling approach taken: Leave all as-provided models as-is to greatest extent possibleo These models do accurately represent the other aspects of the resources well
Develop user-defined model that “hooks into” these models and controls only the momentary cessation aspectso Rest of the model still in full control
• User-defined model controls commanded Ip and Iq by controlling specific parameter values only when MC occurs
Modeling Momentary Cessation
RELIABILITY | ACCOUNTABILITY8
“Default” Momentary Cessation Settings
RELIABILITY | ACCOUNTABILITY9
• Minimum load level case – ~85,000 MW Based on WECC historical data and planner input
• Transfers within known operating limits• All wind dispatched at 60-65% output• Solar dispatched at 95% output• Resource mix ~14.4 GW wind ~13.7 GW solar ~59.7 GW synchronous
Powerflow Setup
RELIABILITY | ACCOUNTABILITY10
• For the purposes of these studies… Spinning Reserve: the amount of “unloaded generation that is
synchronized and ready to serve additional demand” that is modeled in the steady-state powerflow case. This generation may or may not be frequency responsive.
Online Frequency Responsive Reserve: the amount of unloaded generation that is synchronized and to the grid and responsive to changes in frequency. The model uses a baseload flag to disable, or block, governors response for units that are not responsive to frequency.
Online Frequency Responsive Reserve to UFLS: the amount of online frequency responsive reserve that would be deployable prior to reaching underfrequency load shedding (UFLS).
Online Contingency (Spinning) and Frequency Responsive Reserves
RELIABILITY | ACCOUNTABILITY11
Online Contingency (Spinning) and Frequency Responsive Reserves
• All areas analyzed very closely
• Areas with higher HR% have must-run reqs
• Interconnection-wide: Spin = 7010 MW Spin % = 8.26% Headroom = 2538 Headroom % = 2.99% Based roughly on BAL-002-
WECC-2o Contingency Reserve > (3%
of Load + 3% of gen)
RELIABILITY | ACCOUNTABILITY12
• FERC asked that the case be set up at absolute minimum demand level and reserve level 85,000 MW demand 3% online frequency responsive reserve
• This case DOES NOT reflect actual system operation and dispatch (under these conditions or other conditions) Entities expressed that the modeled reserve levels to not reflect historical
minimum reserve levels
• Therefore, IRPTF is calling this case “minimum requirements” case and not “expected minimum” case
Notes on Case and Reserves
RELIABILITY | ACCOUNTABILITY13
• Dynamics data files as-provided• User-defined model added for momentary cessation• Composite load model included• UFLS relays disabled• Contingencies 2 Palo Verde 2 Diablo Canyon 3-Phase Normally Cleared Critical Contingencies
Dynamics Setup
RELIABILITY | ACCOUNTABILITY14
“Critical Contingencies”
RELIABILITY | ACCOUNTABILITY15
“Critical Contingencies”
RELIABILITY | ACCOUNTABILITY16
“Critical Contingencies”
• Initial screening - each area with wind/PV penetration
• Key BPS substations that can trigger large momentary cessation
• Potential impacts upwards of 12,000 MW momentary cessation NOT tripping
RELIABILITY | ACCOUNTABILITY17
Problem Statement: • Identify if the impacts of momentary cessation result in a more
severe frequency excursion than the Resource Loss Protection Criteria (RLPC) for the Western Interconnection.
• IRPTF to recommend to the RS, based on simulation results, whether any potential changes may need to be made
Reiterate problem statement
RELIABILITY | ACCOUNTABILITY18
Benchmark Case2 Palo Verde + RAS
NOTES: • Due to min reserves, 2PV contingency
falls below UFLS (59.5 Hz)• Regardless, this is “benchmark” for all
momentary cessation simulations• No momentary cessation in 2PV
contingency
RELIABILITY | ACCOUNTABILITY19
Sanity Check with 2 Diablo Canyon Comparison
NOTES: • No momentary cessation
with 2DC contingency• 2PV more severe than 2DC
RELIABILITY | ACCOUNTABILITY20
“Default” Momentary Cessation Settings Results
0 2 4 6 8 10 12 14 16 18 20
Time [s]
59.4
59.5
59.6
59.7
59.8
59.9
60
60.1
Freq
[Hz]
FMED
NOTES: • With Delay = 0, all
simulation results stable• All frequency nadir
higher than 2PV• No need to change RLPC
RELIABILITY | ACCOUNTABILITY21
“Default” Momentary Cessation Settings Results
0 2 4 6 8 10 12 14 16 18 202000
4000
6000
8000
10000
12000
14000SOIP
RELIABILITY | ACCOUNTABILITY22
“Default” Momentary Cessation Settings Results
0 2 4 6 8 10 12 14 16 18 20
Time [s]
59.2
59.3
59.4
59.5
59.6
59.7
59.8
59.9
60
60.1Fr
eq [H
z]FMED
NOTES: • With Delay = 0.5s, all
stable cases have freqnadir higher than 2PV benchmark case
• Two cases unstable
RELIABILITY | ACCOUNTABILITY23
“Default” Momentary Cessation Settings Results
0 2 4 6 8 10 12 14 16 18 202000
4000
6000
8000
10000
12000
14000SOIP
RELIABILITY | ACCOUNTABILITY24
• Frequency stability problem = system-wide energy loss issue Inability of system to dynamically balance gen and load sufficiently Declining frequency to the point of instability
• Transient stability problem = Δδ, Δω, ΔP issue Locational changes in power flow and angles Power swings and inter-area angular separation
Instability Drivers
0 1 2 3 4 5 6 7 8
2000
4000
6000
8000
10000
12000
SOIP
0 1 2 3 4 5 6 7
Time [s]
59
59.2
59.4
59.6
59.8
60
Freq
[Hz]
FMED
RELIABILITY | ACCOUNTABILITY25
Sensitivity StudiesUnderstanding Instability
NOTES: • Reduction of MW results in lower
frequency nadir – frequency stability issue
Settings sensitivity for MC ≠ 0.9
RELIABILITY | ACCOUNTABILITY26
Sensitivity StudiesUnderstanding Instability
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
2000
4000
6000
8000
10000
12000
14000SOIP
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time [s]
58.8
59
59.2
59.4
59.6
59.8
60
60.2
Freq
[Hz]
FMED
NOTES: • Initial drop of Solar P significantly larger• Unstable cases lose stability before
frequency declines – transient stability
Settings sensitivity for MC = 0.9
RELIABILITY | ACCOUNTABILITY27
• Momentary cessation should not be used by inverter-based generating resources moving forward Use of momentary cessation should be eliminated for future resources Use of momentary cessation should be mitigated to the greatest extent
possible for existing resources
• Under current penetration levels, momentary cessation not expected to cause a frequency excursion more severe than the 2 Palo Verde RLPC in the Western Interconnection
Draft Findings and Recommendations
RELIABILITY | ACCOUNTABILITY28
• Identified instability for “conservative expected” momentary cessation settings for certain contingencies – transient stability issue rather than lack of primary frequency response IRPTF still exploring simulated solutions – e.g., redispatch, increasing
spinning reserves in the South, operating limits, etc. These issues do not appear to be a BAL-003 issue – other related BAL
standards and potentially rethink other standards (TOP/FAC/etc.)
Draft Findings and Recommendations
RELIABILITY | ACCOUNTABILITY29
RELIABILITY | ACCOUNTABILITY30
• AC Machine : Power :: Inverter : Current We refer to response of inverter-based resources in terms of their current
injection
• While P is the focus of frequency response and stability, I is the focus on inverter controls Power is simply V*I
Current Control