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Aidan Tuohy, [email protected] Manager, Bulk System Renewables/DER Integration ProgramSouth Africa Institute Of Electrical EngineersJune 16, 2020
Integration of Renewable
Energy ResourcesEPRI Program 173: Bulk System
Renewable/Distributed Resource Integration
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Electric Power Research Institute (EPRI)
▪ Founded in 1972 as an independent, non-profit center for public interest energy and environmental research
▪ Collaborative resource for the electricity
sector
– $425M annual R&D funding in 2018
– 1,000+ members in more than 30 countries
– International is ~30% of funding
Independent
Collaborative
Nonprofit
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Market Operations & Design
Long-Term Planning Real-Time Operations
Resource
Adequacy
Transmission
Planning
Operations
Planning
Operations
Scheduling
Real-Time
Operations
System Protection
EPRI Transmission Ops & Planning R&D Program Strategy
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Impacts of Variable and Distributed Energy Resources
Variability/Uncertainty
▪ Output varies across seconds, minutes, hours, days, weeks
▪ Some correlation between resources
▪ Not perfectly predictable or dispatchable
▪ Zero marginal costs
Location
▪ Connected to grid through power electronics
▪ Displaces traditional sources of inertia, active/reactive support, short cct, etc.
▪ Can be controlled to provide quick responses and various services
▪ Can be far from load and require additional transmission and system strength
▪ Can be distributed and provide visibility/controllability issues
▪ Often used to provide multiple services at dist level
Inverter Based
New models, methods and tools have been and are being developed to manage these issues
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Generic wind/PV/storage models and
incl 100% inverter based and weak grid
Bulk System Renewables Integration – Key Activities
System Flexibility and Resource Adequacy
Assessment
Slip rings
Gearbox
Grid
Stator power
Rotor-Side
Converter
Grid-Side
Converter
Rotor power
Step down
transformer
Wind
turbine
Crow-bar
Vdc igir
IL , PL
Modeling: DER in Transmission Planning
and Transmission Hosting CapacityMarket Operations and Design
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Bulk System Renewables Integration – Key Activities
Forecasting: Distributed PV/Load Forecasting
and Advanced Solar Forecasting w/Sky Imaging
Frequency Response and
Reserve Determination Tools
Variability/Uncertainty: Advanced Production
Cost Modeling and Reserve Requirements
Risk Based Transmission Planning
7,2
50
8,0
00
8,7
50
9,5
00
10,2
50
11,0
00
11,7
50
12,5
00
13,2
50
14,0
00
14,7
50
15,5
00
16,2
50
17,0
00
17,7
50
18,5
00
19,2
50
20,0
00
20,7
50
21,5
00
1
10
100
1000
0
800
1,600
2,400
3,200
4,000
4,800
5,600
6,400
Region Load (MW)
Frequency of Occurence
Renewable Output (MW)
100-1000
10-100
1-10
HILF
Events
Average
System
Conditions
HILF
Events
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Renewables
instantaneous
penetration
records in
North America
April 2019: 78%
Jan. 2020: 57.9%
April. 2020: 73.2%
Dec. 2018: 25%
Feb. 2019: 14.65%
March 2019: 10%
2019: 94% energy carbon-free resources
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Checklist/curve for experiences
Interconnection reqts
Models (pos seq, generic)
Forecasting
Reactive and active control
Reserves/flexibility
Coordination across regions
‘Duck curve’
Inertia/frequency
Weak grid
Stability assessment
Seasonal storage
Grid forming
Demand side
Zero to low
Low (few % annual)
Medium (low 10s % annual)
High (instantaneous >50%)
Very high (40%+ annual)
Very few left Many regions ERCOT, SPP, NGUK Denmark, Hawaii islands, etc.
Each system will be somewhat different, general trends in
challenges
Need to plan for these issues well before we reach ops
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Relative Reliability Contributions for Various Resources
• Must Ensure Reliability
when considering new
Resource Mix
• Not all Resources equal
in Reliability Capability
• Synchronous resources
broader & deeper ability
to support reliability
• Reliability is not only
consideration: Diversity,
Economics, Emissions,
and others…
EPRI whitepaper (2015):
Contributions of Supply & Demand
Resources to Required System
Reliability Services (3002006400)
Update coming soon
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Modeling, Planning and Protection
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Planning/Protection Work Streams
Renewables ProtectionDER/Load Models Transmission Hosting Cap.
Renewables Weak Grid Model Grid Strength AssessmentRenewables Dynamic Models
M
M
M
Electronic
UVLS
69 kV115 kV138 kV230 kV
UFLS
1-F AC
Static
R-DER
U-DER
Composite Load Model
12.5 kV13.8 kV
Slip rings
Gearbox
Grid
Stator power
Rotor-Side
Converter
Grid-Side
Converter
Rotor power
Step down
transformer
Wind
turbine
Crow-bar
Vdc igir
IL , PL
173.09173.03
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Modeling Challenges – Transmission Connected
Renewables
PV
To
system m
od
el
R, X, B Equivalent Feeder Model
EquivalentGenerator
Step-up Transformer
SubstationTransformer
SubstationMSCs
Reality – many PV array inverters connected through a collector system
Simulation – simplified equivalent representation.Generic models are commonly used for simulation
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RENEWABLE ENERGY SYSTEM MODELS
• The most recent publicly available renewable energy system (RES) models are
the 2nd generation generic models developed through the Western Electric
Coordinating Council (WECC) Renewable Energy Modeling Task Force
(REMTF) effort
• They allow for modeling:
• Wind Turbine Generators (WTG)
• Photovoltaic (PV) Generation
• Battery Energy Storage Systems (BESS)
• Complex plants RES plants
• IEC TC88 WG27 standard models are very similar, but slightly more detailed;
the plant-level models not yet completed
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VALUES OF GENERIC MODELS
• Validation: numerous validation cases demonstrated
• Portability across software platforms: implemented and tested in major
commercial tools, and consistent across the tools
• Transparency & Documentation: standard, generic, public and open
with documentation/specifications that are available to all
• Publicly Available: avoid this issue of being able to share models.
• Modeling the Future: generic models are useful for performing futuristic
studies where the actual equipment to be used is not yet known
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LIMITATION OF GENERIC MODELS• Positive-sequence: cannot be used for accurate assessment of unbalanced conditions.
• Typical range of stability studies: consider dynamics in the typical range of stability
studies (0.1 – 3 Hz); all other models are only truly good within this range of frequencies.
The control loops within the models (e.g. closed-loop voltage control) may also consider
frequencies ranging up to 10 Hz.
• Constant wind speed/ solar irradiation: the generic models assume that wind speed
(and solar irradiation) is constant during a stability simulation.
• Weak Systems: not intended for detailed local studies associated with control tuning and
design for the interconnection of wind/PV plants to very weak systems (i.e. short-circuit
ratios below approximately 2 or 3)
• Specialized Studies: The models cannot be used for detailed studies that relate to
phenomena such as potential torsional interactions between the wind turbine generator
and the electrical power system (e.g. nearby series capacitor).
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2nd Generation Renewable Energy System (RES) Models
Generator
Id
Iq
Vt
Drive-Train
wt1g
wt1t
Pitch-Control
wt1p_b
w
Pm
Generator
Id
Iq
Vt
Drive-Train
wt2g
wt1t
Pitch-Control
wt1p_b
w
Pm
External
Resistor
Control
wt2e
Rext
Generator/
Converter
Model
Iq
Ip
Iqcmd
Ipcmd
Current
Limit
Logic
Vt
Pqflag
= 1 (P priority)
= 0 (Q priority)
Q Control
P Control
Iqcmd’
Ipcmd’
Qref
(or Qext)
Qgen
Pref
(or PExt)
Drive-Train
spd
reec_a
regc_a
wtgt_a
Pord
Torque
Control
wtgtrq_a
Pe
Pref0
Pitch-Controlwtgpt_a
wref
qAero
wtgar_a Pm
Plant Level Control
repc_a
Vref/Vreg or
Qref/Qgen
At plant levelFreq_ref/Freq and
Plant_pref/Pgen
Generator/
Converter
Model
Iq
Ip
Iqcmd
Ipcmd
Current
Limit
Logic
Vt
Pqflag
= 1 (P priority)
= 0 (Q priority)
Q Control
P Control
Iqcmd’
Ipcmd’
Qref
(or Qext)
Qgen
Pref
(or TExt)
Plant Level Control
reec_a
regc_a
repc_a
Vref/Vreg or
Qref/Qgen
At plant levelFreq_ref/Freq and
Plant_pref/Pgen
Generator/
Converter
Model
Iq
Ip
Iqcmd
Ipcmd
Current
Limit
Logic
Vt
Pqflag
= 1 (P priority)
= 0 (Q priority)
Q Control
P Control
Iqcmd’
Ipcmd’
Qref
(or Qext)
Qgen
Pref
(or TExt)
Plant Level Control
reec_b
regc_a
repc_a
Vref/Vreg or
Qref/Qgen
At plant levelFreq_ref/Freq and
Plant_pref/Pgen
WTG Type 1 WTG Type 2
WTG Type 3
WTG Type 4
PV
https://www.epri.com/research/prod
ucts/000000003002006525
Model User Guide for Generic
Renewable Energy System Models
•Latest renewable energy
models available in
commercial platforms for
dynamic studies (positive
sequence)
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Wind Inertial-Based Fast-Frequency Response Model
(WTGIBFFR_A) – Technical Specifications
▪WTGs can provide “emulated” inertial response using auxiliary controls without maintaining any reserve (add-on feature)
▪Auxiliary Control Model in 2nd generation RES models
▪Model technical specifications initially proposed in 2018 (PID: 3002013641)
▪2019 Work: –Finalize model specifications –Develop prototype model for testing
–Engagement with vendors through WECC REMTF for model implementation commercial platforms
–Pursue validation using field measurements
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Inverters and Weak Grids
▪Wind/Solar interconnections in weak systems are becomingmore common
▪Contingencies or scheduled outages (e.g. lines undermaintenance) may reduce system strength below typical levelsexpected under normal operating conditions.
▪Additionally, if such a network situation coincides withramping up of the plant, the location can become even weaker
▪Converter controller instabilities might occur under weak gridconditions
Source: “Deploying Utility-Scale PV Power Plants in Weak Grids”, First Solar, 2017
PES General Meeting, Chicago, IL, July 2017
Source: “Integrating Variable Energy Resources into Weak Power Systems”, NERC, June 2017
▪ No clear industry standard on metrics & associated thresholds to identify a weak area of the system
▪ Low SCR doesn’t necessarily imply converter control instability
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Description of the Grid Strength Assessment Tool…
Value
Provide insights on
• Network locations/conditions which could be susceptible to issues related to weak grid conditions
• The need for detailed studies
Evaluates
• Generic SCR
• Weighted SCR
• Composite SCR
• Advanced EPRI metric
Compatibility
• Siemens PTI PSS®E v33 and v34
• GE-PSLF™ v19 and v21
• DIgSILENT PowerFactory 2017 SP7, 2018 (SP1, SP5, SP6), 2019 (SP1, SP2, SP3)
▪ EPRI’s advanced metric
– Completely analytical, and no requirement of a dynamic run
– Uses few dynamic data values (e.g. controller gains, time constants) of the inverter-based resource model to identify potential converter instability
– Is expressed as critical clearing time before converter instability
▪ Critical clearing time is from the perspective of converter controller and NOT of a synchronous machine.
The advanced metric can be used along with clearing time of existing protection schemes to determine system stability
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Generic Model REGC_C
▪Representation of PLL and inner current control loop
▪Suggested to be used for weak grid studies before conducting detailed three-phase EMT studies
▪Model approved by WECC REMTF
REGC_C parameter values can be used as inputs for GSAT Advanced Metric Calculation
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Inverter Based Resource Integration
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Relatively high average penetrations can result in very high
instantaneous
Need to be able to manage ramping/variability, but also issues like frequency and voltage
Source: Drake Bartlett, PSCO, 2018
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Inverter Interface Resource Considerations
System Protection
• Reduced short circuit
• Different fault contribution
Transmission Reliability
• Displaced inertia/PFR
• Inverter controls/capability
• Dynamic behavior
-- disturbance response
-- voltage/freq ride-through
Transmission Planning
• Validated dynamic models
• Modeling DER in Trans Plan
E, Ela et al., Active Power Control from Wind Power: Bridging the Gaps, NREL Technical Report, December 2013.
Load Shedding!
More realistic Governor Participation
60%
50%
40%
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Concept of System Non Synchronous Penetration
▪ Ireland’s All Island Grid Study in 2008/2009 showed significant penetration levels could be technically possible from transmission planning and production cost perspective
▪ Further studies commissioned by TSOs to study stability issues – Facilitation of Renewables study (2010)
▪ Report concluded that that the two key issues that may limit increased instantaneous penetration
– Frequency stability after the loss of the largest infeed
– Frequency and transient stability after a severe network fault
▪ Developed System Non Synchronous Penetration metric
𝑆𝑁𝑆𝑃 =𝑊𝑖𝑛𝑑(𝑜𝑟 𝑜𝑡ℎ𝑒𝑟 𝑖𝑛𝑣𝑒𝑟𝑡𝑒𝑟 𝑏𝑎𝑠𝑒𝑑) + 𝐼𝑚𝑝𝑜𝑟𝑡𝑠
𝐷𝑒𝑚𝑎𝑛𝑑 + 𝐸𝑥𝑝𝑜𝑟𝑡𝑠
Originally limited to 50%, now at 65%
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Interaction Between Synchronous Inertia, FFR, FCR, RR
In Ireland, new inertial response measures have been added to EIRGrid’s reserve planning scheme, as part of the coordinated plan to increase the penetration of renewable resources.
EirGrid: Frequency Response System ServicesSIR: Synchronous Inertial ResponseFFR: Fast Frequency ResponsePOR: Primary Operating ReservesSOR: Secondary Operating ReservesTOR1: Tertiary Operating Reserves, Level 1TOR2: Tertiary Operating Reserve, Level 2RR: Replacement ReserveRamping: Ramping Margin
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ERCOT – recent developments with high penetrations of non-
synchronous resources
▪ Single synchronous system covering most of Texas
▪ Peak demand of 70 GW, minimum 25 GW
▪ Over 20 GW of wind and 1 GW of utility scale solar by end 2017
▪ Significant buildout of new transmission to West Texas reduced wind curtailment and improved integration of renewables
▪ Inertia is increasingly a concern and part of planning and operations activities
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Trends in inertia in recent years
▪ ERCOT examined actual inertia online over past few years
▪ Clear downwards trend
▪ Wind seems to be part of the trend, with higher wind resulting in lower inertia
▪ Online nuclear units, responsive reserves and private networks provide approximately 78 GW.s so may reach situations in future where additional generators need to be brought online
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ERCOT
• Inertia floor of 100 GWs
• Online monitoring
• Proposed ancillary service market redesign, SIR, FFR, PFR Ireland
• Inertia floor 23 GWs
• Minimum unit constraint
• RoCoF upgrade project to 1 Hz/s
• Ancillary services market redesign with SIR
• Auction-based market for SIR
NG UK
• Inertia floor of 135 GWs
• RoCoF upgrade project to 1 Hz/s
• Ancillary service market redesign with EFR similar to FFR (storage)
Australia
• Inertia floor of 6.2 GWs
• Minimum unit constraint
• Online monitoring, inertia, and stability
NORDIC
• Inertia floor of 120 GWs
• Online monitoring
• Redispatch design contingency nuclear unit for inertia constraint
Worldwide Systems with Inertia Constraint Characteristics
Systems with
Inertia Constraint
Recent public reportsEPRI IDs: 3002015131, 3002015132
https://youtu.be/ZCa2LHxq9C8
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Variability and Uncertainty Issues
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Variability and Uncertainty Considerations
Scheduling & Dispatch
• Increase operating reserve
• Masking of load (DER)
Resource Adequacy
• Planning reserve margin
• Dispatchable Gen revenue
• Operational flexibility
Ops Planning & Real-Time
• Outage scheduling
• Changing flows & SOLs
Transmission Planning
• Which power flow cases?
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The “Duck Curve” is Real – and all over the world
Pumped hydro and exports very helpful in managing rampFlexibility from conventional generation would be exhausted, relying on others
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Operating Reserves and Other Essential Reliability
Services
EPRI developing tools for ensuring sufficient reserves, while minimizing costs
*Inertia is not a reserve but part of the instantaneous event correction process.
Planning Reserve
ICAPFlexible Capacity
Volt/Reactive Control Reserve
Static Dynamic
Black Start
Restoration
Short Circuit
Contribution
Op
era
tin
g R
eserv
e
Event
Contingency Reserve
Inertia*
Fast Freq. Response
Primary
Secondary
Tertiary
Ramping Reserve
Secondary
Tertiary
Non-Event
Flexibility Reserve
Regulating Reserve
Instantaneous events (e.g., contingencies)
Reduce ROCOF
Stabilize frequency
Return frequency to
nominal and/or ACE to zero
Bring back to N-1 secure
stateLonger duration events
Return frequency to
nominal and/or ACE to zero
Bring back to secure state
Correct the anticipated ACE
Manual (part of optimal dispatch)
Correct the current ACE
Automatic (within optimal dispatch)
Reduce nadir, avoid UFLS
Recent project in Hawaii showed
significant reduction in costs when
optimizing reserves ($20m/year)
https://eprijournal.com/a-win-win-for-grid-
operators-and-customers/
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Traditional "Generic Capacity" Metrics New "Flexible Capacity" Metrics
What “Type” of Capacity is Needed?
Resource adequacy methods and tools need to be adopted
LOLEGENERIC-CAPACITY
Traditional metric to capture events that occur due to
capacity shortfalls in peak conditions
LOLEMULTI-HOUR
New metric to capture events due to system ramping
deficiencies of longer than one hour in duration
LOLEINTRA-HOUR
New metric to capture events due to system ramping
deficiencies inside a single hour
20,000
30,000
40,000
50,000
1 5 9 13 17 21
41,500
42,500
43,500
44,500
45,500
10:00 10:10 10:20 10:30 10:40 10:50 11:00
0
10,000
20,000
30,000
40,000
50,000
60,000
1 3 5 7 9 11 13 15 17 19 21 23
MW
Hours
Load Generation
Source: California Public Utility
Commission Workshop
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Flexibility Will Become More Valuable
EPRI working on flexibility tools and metrics to assess long term resource adequacy impacts
▪ Increasing variability and uncertainty will require flexibility on all time scales and at different spatial scales
▪ Different resources may contribute– DER, storage and inverter
based resources may provide some of the needed flexibility services
– Retrofits and altered operational practices
▪ Wind/PV flexibility (with or without storage) increasingly important
More Information:
Metrics for Quantifying Flexibility in Power
System Planning, 3002004243, 2014 (EPRI)
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Recent week in Ireland – up to 65% penetration
Forecasts, control of renewables and flexible system all important
0%
10%
20%
30%
40%
50%
60%
70%
0
1000
2000
3000
4000
5000
6000
5/31/2020 6/1/2020 6/2/2020 6/3/2020 6/4/2020 6/5/2020 6/6/2020 6/7/2020
Inst
anta
neo
us
Pen
etra
tio
n
Dem
and
/Gen
erat
ion
/Win
d (
MW
)
Date
ACTUAL DEMAND(MW) FORECAST WIND(MW) ACTUAL WIND(MW)
ACTUAL GENERATION(MW) Instantaneous Data from Eirgrid.com
Difference between generation and
demand is import/export
Limited renewables to 65% of generation
(and curtailed when over this, resulting in
forecast error)
Average penetration of 35% of generation,
40% of demand
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Integration can Improve Reliability, Increase Efficiency, Create
New Opportunities, and Expand Customer Choice (ien.epri.com)
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Together…Shaping the Future of Electricity