Aidan TuohyGrid Operations and Planning, EPRI
NERC ERSTF Meeting12/10/2014
Flexibility Assessment Metrics
2© 2014 Electric Power Research Institute, Inc. All rights reserved.
EPRI Project: Strategic and Flexible Planning
• Framework in how to think about flexibility from planning perspective– Multi-Level Analysis– Reliability and economics– Metrics to assess flexibility
requirements and flexibility adequacy
– Tool to perform analysis– Case studies to prove concepts
• Considering multiple aspects– Resource adequacy– Transmission– Energy limited (DR, Storage, Hydro)– System operations
-1000 -800 -600 -400 -200 0 200 400 600 800 100010-2
10-1
100
101
MW ramp
Fre
qu
en
cy (
% o
f tim
e)
no wind1 GW wind2 GW wind
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
0
100
200
300
400
500
600
700
800
900
3© 2014 Electric Power Research Institute, Inc. All rights reserved.
Operational Flexibility in Planning and Operations• Planning for flexibility
– Need to ensure in planning time frame that there is sufficient operational flexibility provided
– Similar to capacity/resource adequacy (though more complex)– Consider different resources and the network, as well as how
the system operates– Trade offs between reliability and economics
• Operations with flexibility– Once you have the resources, how do you operate them?– How do you incentivize resources?– New reserve products, new scheduling techniques, etc
Need to consider operational issue in planning to ensure system reliability
4© 2014 Electric Power Research Institute, Inc. All rights reserved.
Flexibility Considerations & Metrics
• Many Regions (Regulators + ISO+ Utilities) Considering Future Flexibility Needs Now – Planning and Operations time frame
• Other systems experiencing similar needs Germany, Spain, New York, Hawaii etc.
• New flexible resources becoming deployable in the bulk system
California•Flexible Resource Adequacy•Flexi-Ramp Market Product•Long Term Procurement Plan
Ireland• Long Term Flexibility Incentives
Oregon• Integrated Resource Planning Process
MISO• Market Rule Changes to
Incentivize Flexibility
All areas are different, but shared learnings help all improve methods
5© 2014 Electric Power Research Institute, Inc. All rights reserved.
Flexibility Metrics for system planning
• Multi-Level Approach– Levels 1 and 2 screening
metrics– Levels 3 and 4 detailed metrics
• Four detailed metrics for planning time frame:– Periods of Flexibility Deficit – Expected Unserved Ramping – Well-being analysis – Insufficient Ramping Resource
Expectation
• Post-processed metrics based on simulation or historical data
Level 1• Variability Analysis &
Flexibility Requirement
Level2• Resource Flexibility
Calculation
Level 3 • System Flexibility
Metrics
Level 4• Transmission and Fuel
Constrained Flexibility
Results from different case studies presented here
6© 2014 Electric Power Research Institute, Inc. All rights reserved.
Level 1: Variability Analysis – what flexibility is required?
Determine ramping with different time horizons, for demand, wind and net demand
Percentiles used to determine likelihood (99.9 here)
7© 2014 Electric Power Research Institute, Inc. All rights reserved.
Time of Day and Year Example Ramp Rates
1 2 3 4 5 6 7 8 9 101112131415161718192021222324
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
0
200
400
600
800
1000
1200
1400
1600
1800• Max 1 hour up ramps for each hourly interval in a given month
• Based on example data from Northwest US
• Calculate for different time intervals, up and down ramping
Flexibility resources will need to be available at different times of day and year depending on system requirements
8© 2014 Electric Power Research Institute, Inc. All rights reserved.
Level 2: What flexibility is available?
Determine Ramping Available in Each Hour of the YearDifferent time scales, need to make assumptions
about intertie and energy limited resources
9© 2014 Electric Power Research Institute, Inc. All rights reserved.
Need to consider operational aspects…
How you operate the system – including reserves – impacts on the availability of flexible capacity
10© 2014 Electric Power Research Institute, Inc. All rights reserved.
Level 3: What is the net flexibility after ramps?
Examine either net available flexibility or against extreme ramps
11© 2014 Electric Power Research Institute, Inc. All rights reserved.
Level 3: Metric 1: Periods of Flexibility Deficit
30%
40%
50%
60%
70%
80%
90%
100%
0 2 4 6 8 10 12 14 16
Perio
ds o
f Fle
xibi
lity
Def
icit
(%)
Time Horizon (Hours)
Example for extreme ramping requirements (97th percentile)
12© 2014 Electric Power Research Institute, Inc. All rights reserved.
Level 3: Metric 2. Expected Ramping Deficit
Expected value of ramping deficit values observed in each direction and time horizon
∑=
=−+>−+ =
8760
1/,0||/, 8760
1 i
iDCit i
DeficitERD
13© 2014 Electric Power Research Institute, Inc. All rights reserved.
Example Conclusions from Studies
• “Flexibility shortages seen over 60 minute interval”– Higher resolution data would be beneficial
• “Peak at 540 minutes indicates that the system may need to add additional capacity”– Unlikely that system leaves long start units offline when available– In that case the system had interties assumed inflexible
• Assumptions on intertie flexibility may alleviate issue
• Frequency of shortages for <1 hour shows that it may be operational issue driving results– Operational changes may be sufficient to meet ramping
requirements reserve etc.– Longer term mitigation measure: intertie operation or new capacity
14© 2014 Electric Power Research Institute, Inc. All rights reserved.
Ongoing work
• Impact of transmission constraints on system flexibility adequacy– Working with Ecco International to demonstrate how metrics can
consider flexibility and how transmission can be a flexibility resource
• Flexibility from Energy Limited Resources– How do demand response, storage, hydro etc provide flexibility?
• Continued Tool Development– Adding new functions (some based on existing algorithms, some new)– Improving interface/ease of use
• Case Studies to demonstrate framework– Detailed case study w/BPA – Screening case studies for TVA & Southern
15© 2014 Electric Power Research Institute, Inc. All rights reserved.
Conclusions
• Many Applications of such an approach– Reliability assessments– Wind and solar integration studies– Flexibility assessment for resource planning – May be applied in the operational planning time frame
• E.g. maintenance scheduling, hydro scheduling
• Flexibility metrics have been or will be made public through various journal and conference publications– White paper available on epri.com – “Metrics for Quantifying Flexibility in Power Systems”
• InFLEXion flexibility assessment tool available to project funders
• EPRI is engaging with a wide variety of parties in this area, e.g. PG&E collaborative effort, IEEE, LBNL, NWPCC
16© 2014 Electric Power Research Institute, Inc. All rights reserved.
Together…Shaping the Future of Electricity
1
WWSIS - 3: Western Frequency Response and Transient Stability Study
GE EnergyNicholas W. Miller (PM)Miaolei ShaoSlobodan PajicRob D’Aquila
NRELKara Clark (PM)
NERC ERSTF BriefingAtlantaDecember 10-11, 2014
The draft report is under review by the TRC and by DOE.
Therefore, all of the results and statements in this presentation MUST be regarded as preliminary and subject to further review and modification.
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Key Points
What: Stability – for the 1st minute after a big disturbance –is critically important limitation in the West
Why: Widespread worry that lots of wind and solar, especially combined with lots of coal diretirements will irreparably disrupt grid stability.
In the context of ERSTF: will essential reliability services be affected (i.e. depleted, altered, enhanced...)
What we learned: The Western Interconnection can be made to work well with both high wind and solar and substantial coal displacement, using good, established planning & engineering practice and commercially available technologies.
2
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Team….
Who:
– Project Co-funded by DOE Wind and Solar Programs
– Project Management by NREL: Kara Clark
– Subcontract to GE Energy Consulting
Technical Review Committee:
• North American Electric
Reliability Corporation
• PacifiCorp
• Public Service of New
Mexico
• Western Area Power
Administration
• Tucson Electric Power,
• Western Electricity
Coordinating Council,
• California ISO
• Xcel Energy
• Sacramento Municipal
Utility District
• Arizona Public Service,
• Bonneville Power
Administration
• Western Governors
Association
• Electric Reliability
Council of Texas
• Utility Variable-
generation Interest
Group
• DOE
• Electric Power Research
Institute
• Sandia NL
• Lawrence Berkeley NL
• Iowa State University
• University College
Dublin
• Arizona State University
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Critical Disturbances in the West
Pacific DC
Intertie
Selected by Technical Review Committee:
• Palo Verde Nuclear Plant(2 of 3 units for ~2,750 MW)
• Pacific DC Intertie (Maximum north-to-southpower flow ~3,100 MW)
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
WECC-Wide Summary(1)
Light Spring Base
(2)
Light Spring High Mix
Light Spring Extreme Sensitivity
Wind (GW) 20.9 27.2 32.6
Utility-Scale PV (GW) 3.9 10.2 13.5
CSP (GW) 0.9 8.4 8.3
Distributed PV (GW) 0 7.0 10.4
Total (GW) = 25.7 52.8 64.8
Penetration(3)
(%) = 21% 44% 53%
Light Spring Load Study ScenariosBase Case High Mix Case
(1) Western Electricity Coordinating Council includes parts of Canada and Mexico, (2) Provided by WECC, (3) Penetration is % of total generation for this snapshot.
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Heavy Summer Load Study ScenariosBase Case High Mix Case
WECC-Wide Summary(1)
Heavy Summer Base
(2)
Heavy Summer High Mix
Wind (GW) 5.6 14.3
Utility-Scale PV (GW) 1.2 11.2
CSP (GW) 0.4 6.6
Distributed PV (GW) 0.0 9.4
Total = 7.2 41.5
Penetration(3)
(%) = 4% 20%
(1) Western Electricity Coordinating Council includes parts of Canada and Mexico, (2) Provided by WECC, (3) Penetration is % of total generation for this snapshot.
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Frequency Response Analysis
7Preliminary Results: Not for Further
Distribution or Citation
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Frequency Response with High Renewables
Interconnection frequency response > 840 MW/0.1Hz threshold in all cases.No under-frequency load shedding (UFLS).
Disturbance: Trip 2 Palo Verde units (~2,750MW)
3
2
Light Spring Base
Light Spring High Mix
Light Spring Extreme
23
1
1
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Wind Plant Frequency Responsive Controls
• Inertial control responds– to frequency drops only
– in 5-10 second time frame
– uses inertial energy from rotating wind turbine to supply power to system
– requires energy recovery from system to return wind turbines to nominal speed
– more responsive at higher wind speeds
– ERSTF: this is Fast Frequency Response, NOT System Inertial Response
• Governor control responds– to both frequency drops and increases
– in 5-60 second time frame
– requires curtailment to be able to increase power
– ERSTF: this is either Fast Frequency Response, or Primary Frequency Response (depending on aggresiveness of the control)
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Frequency Control on Wind Plants
Light Spring High MixLight Spring High Mix with governor control*Light Spring High Mix with inertial control*Light Spring High Mix with both controls
Disturbance: Trip 2 Palo Verde units (~2,750MW)
40% of wind plants (i.e., new ones) had these controls, for a total of 300 MW initial curtailment out of 27GW production.
1
2
3
4
1
2
34
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Frequency Control on Utility-scale PV Plants
Light Spring High Mix
Light Spring High Mix with governor controls on
utility-scale PV plants
~80% of utility-scale PV plants (i.e., new ones) had these controls, for a total of 820 MW initial curtailment out of 10.2 GW production.
Disturbance: Trip 2 Palo Verde units (~2,750MW)
1
2
2
1
ERSTF: 820 MW of Fast Frequency Response
FRO Base Hi-Mix Wind
Governor
Control
Wind
Inertial
Control
Wind
Governor
and
Inertial
Controls
Utility-
scale PV
Governor
Control
Energy
Storage
with
Governo
r Control
Extreme
Hi-Mix
WECC 840 1352 1311 1610 1323 1571 2065 1513 1055
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Fault Ride Through Needed with High Levels of DG
Pessimistic
Pessimistic approximation to worst case 1547 under-voltage tripping(88%, no delay)
Pacific DC Intertie trips
Widespread, commonmode tripping of DG (i.e. distributed solar PV results in system collapse
DG with LVRTDG without LVRT
Disturbance: Trip Pacific DC Intertie
1
2
1
2
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Frequency Response Conclusions
For the conditions studied, system-wide frequency response can be maintained with high levels of wind and solar generation with both traditional and non-traditional approaches.
Traditional transmission system reinforcements to address local stability, voltage, and thermal problems include:
• Transformers • Shunt capacitors, (dynamic reactive support)• Local lines
Traditional approaches to meeting frequency response obligations are to commit synchronous generators with governors and to provide all response within an individual balancing authority area
Non-traditional approaches are also effective at improving frequency response including:
• Sharing frequency response resources• Frequency-responsive controls on inverter-based resources
• Wind• Utility-scale PV • CSP• Energy storage, (demand response) There are caveats in report
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Transient Stability Analysis
14Preliminary Results: Not for Further
Distribution or Citation
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Heavy Power Transfer Affects Response More than High Wind and Solar
Disturbance: Trip Pacific DC Intertie… NO RAS enabled
Heavy summer Base
Heavy summer Base with high COI flows
Heavy summer High Mix with high COI flows
High power transfer drives performance in both Base case and High Renewables case.
1
2
3
2
3
1
California Oregon Interface Power Flow (MW)
4,800 MW
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Transient Stability in Northeastern WECC
L
Aeolus
500kV
Large Coal Plants
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
0
5000
10000
15000
20000
25000
30000
35000
LSP Base LSP HiMix LSP
HiMixXtrm
LSP Base LSP HiMix LSP
HiMixXtrm
DSW NorthEast
WIND
Steam
PV
PSH
Other
NUC
HYDRO
GEO
GasCT
CSP
Coal
CCPP
Bio
Light
Spring
Base
Coal Displacement in Light Spring Scenarios
Ge
ne
rati
on
pro
du
ctio
n (G
W)
PV=photo voltaic, PSH=pumped storage hydro, NUC =nuclear, GEO=geothermal, GasCT=gas fired combustion turbine, CSP=concentrating solar power, CCPP=combined cycle power plant, Bio=biomass
Light
Spring
High
Mix
Light
Spring
Extreme
Sensitivity
Desert Southwest
Light
Spring
Base
Light
Spring
Extreme
Sensitivity
Light
Spring
High Mix
Northeast (of the West)
Co
al
Co
al
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
System Non-Synchronous Penetration (SNSP)
• Percent of non-synchronous generation (i.e.,
inverter-based generation like wind and solar)
compared to synchronous generation in a system
• EirGrid (Irish grid operator) presently has 50% cap
on the amount of non-synchronous generation
allowed at any time
• ERSTF: a SNSP cap is similar to a SIM, but reflects
restrictions on short-circuit strength as well as
inertia
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Synchronous vs. Non-synchronous
OK
Fail
Inverter-based MVA
Synchronous MVA
80% drop in Coal Dispatch. This
case passes Aeolus fault test.
90% drop in Coal Dispatch. This case
needs further reinforcement
Ge
ne
rati
on
co
mm
itm
en
t (b
ase
d o
n M
VA
ra
tin
g)
Heavy Summer Base
Heavy Summer Base
Heavy Summer Base
Heavy Summer Base
Heavy Summer High Mix
Heavy Summer High Mix
Heavy Summer High Mix
Heavy Summer High Mix
California Desert Southwest Northeast Northwest
Light Spring Base
Light Spring Base
Light Spring Base
Light Spring Base
Light Spring High Mix
Light Spring High Mix
Light Extreme Sensitivity
Light Spring High Mix
Light Extreme Sensitivity
Light Extreme Sensitivity
Light Extreme Sensitivity
Light Spring High Mix
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Dave Johnson Voltage
Light Spring Base
Light Spring High Mix
Light Spring Extreme
Light Spring Extreme with
synchronous condenser
conversion
Synchronous Condenser Conversion Results in
Acceptable Performance in Extreme Sensitivity
Reinforcements for Extreme sensitivity: 3 condensers total ~1700MVA plus ~500 MVArshunt banks.
Disturbance: Aeolus bus fault and line trip
1
3
2
4
1
2
3
4ERSTF: this is
transient voltage
collapse.
Pessimistic
dynamic load
model plays a
key role
ERSTF: 80%
reduction in coal
dispatch still
stable
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Transient Stability Conclusions
For the conditions studied, system-wide transient stability can be maintained with high levels of wind and solar generation with both traditional and non-traditional approaches.
Traditional transmission system reinforcements to address stability, voltage, and thermal problems include:
• Transformers • Shunt capacitors, (dynamic reactive support)• Local lines
Non-traditional approaches are also effective at improving transient stability including:
• Synchronous condenser conversions• New wind and solar controls
There are caveats in report.
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Study Conclusions
The Western Interconnection can be made to work
well in the first minute after a big disturbance with
both high wind and solar and substantial coal
displacement, using good, established planning and
engineering practice and commercially available
technologies.
The following detailed conclusions were word-
smithed by Technical Review Committee and include
the appropriate caveats.
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Frequency Response ConclusionsFor the conditions studied:
• System-wide FR can be maintained with high levels of wind and solar generation
if local stability, voltage, and thermal problems are addressed with traditional transmission
system reinforcements (e.g., transformers, shunt capacitors, local lines).
• Limited application of non-traditional frequency-responsive controls on wind,
solar PV, CSP plants, and energy storage are effective at improving both frequency
nadir and settling frequency, and thus FR. Refinements to these controls would further
improve performance.
• Individual BA FR may not meet its obligation without additional FR from resources
both inside and outside the particular area. As noted above, non-traditional approaches
are effective at improving FR. Current operating practice uses more traditional approaches
(e.g., committing conventional plants with governors) to meet all FR needs.
• Using new, fast-responding resource technologies (e.g., inverter-based controls) to
ensure adequate FR adds complexity, but also flexibility, with high levels of wind and solar
generation. Control philosophy will need to evolve to take full advantage of easily
adjustable speed of response, with additional consideration of the location and size of the
generation trip.
• For California, adequate FR was maintained during acute depletion of headroom from
afternoon drop in solar production, assuming the ability of California hydro to provide FR.
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Transient Stability Conclusions
For the conditions studied:
• System-wide transient stability can be maintained with high levels of wind and solar generation if local stability, voltage, and thermal problems are addressed with traditional transmission system reinforcements (e.g., transformers, shunt capacitors, local lines). With these reinforcements, an 80% reduction in coal plant commitment, which drove SNSP to 56%, resulted in acceptable transient stability performance.
• With further reinforcements, including non-standard items such as synchronous condenser conversions, a 90% reduction in coal plant commitment, which drove SNSP to 61%, resulted in acceptable transient stability performance.
• Additional transmission and CSP generation with frequency-responsive controls are effective at improving transient stability.
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Other Conclusions• Accurate modeling of solar PV, CSP, wind, and load behavior is extremely important when
analyzing high-stress conditions, as all of these models had an impact on system
performance.
• Attention to detail is important. Local and locational issues may drive constraints on both FR
and transient stability.
• The location of generation tripping, e.g., DG vs. central station, is not as important as the
amount of generation that is tripped. However, widespread deliberate or common-mode DG
tripping after a large disturbance has an adverse impact on system performance. It is
recommended that practice adapt to take advantage of new provisions in IEEE 1547 that
allow for voltage and frequency ride-through of DG to improve system stability.
• Further analysis is needed to determine operational limits with low levels of synchronous
generation in order to identify changes to path ratings and associated remedial action
schemes, as well as quantify the impact of DG on transmission system performance.
• Because a broad range of both conventional and non-standard operation and control
options improved system performance, further investigation of the most economic and
effective alternatives is warranted. This should include consideration of the costs and
benefits of constraining commitment and dispatch to reserve FR, as well as the capital and
operating costs of new controls and equipment.
Western Wind and Solar Integration Study, Phase III : Transient Stability and Frequency Response. Subject to Final Review and Approval by DOE
Thank [email protected]