Michael Milligan & Debbie LewNational Wind Technology Center
National Renewable Energy LaboratoryGolden, Colorado USA
Top FAQ’s (Frequently Asked Questions) About Utility Integration of Renewables…
and Answers
The Questions
1) Overview of the grid (interconnections, markets, BAs, etc) 2) How does the utility schedule their resources? 3) What are reserves? 4) Does a wind plant start or stop very suddenly? 5) Does geographic diversity help? 6) What generation is displaced when the wind blows? (How many kWh of
conventional generation is offset by 1 kWh of wind generation? ) 7) Does wind need backup or storage? 8) Does wind have capacity value? How is it measured and what does it mean? 9) What happens back at the utility when the wind stops blowing? 10) How much does it cost to integrate renewables into the grid? 11) How much does forecasting help? 12) Is there a limit to how much wind can be accommodated on the grid? 13) How do wind, concentrating solar power and PV plants impact the grid? 14) How can more wind be accommodated on the grid? 15) What happened in Texas in Feb ‘08 when they had to cut power to certain loads?
1. Overview of the Grid
• Interconnections, markets, BAs
There are 3 interconnections in the U.S.
1 2
3
There are many control areas (balancing areas) in the East and in the
West
General Operating Requirements
• The interconnection must be balanced ∑ loads = ∑ generation *
• Implication for individual balancing areas (BAs)∑ loads = ∑ generation + Imports – Exports
* DC ties can span two interconnections, but these are limited
Markets cover part of the U.S.
NPPD is Joining SPP
2. How does the utility schedule resources?
• Non-market areas (regulated by state PUCs)– Balancing Authority is responsible for
scheduling generation to meet expected loads
– Individual utilities will often provide schedules to BA, based on economics
• Market areas– Similar, except the schedules are induced
by the energy market, not single entity
Economic DispatchLoad Duration Curve
16x103
14
12
10
8
6
4
2
0
MW
80006000400020000Hours
Economic Dispatch16x103
14
12
10
8
6
4
2
0
MW
80006000400020000Hours
Base load generation does not change its output
Intermediate gens change output in response to load
Peaking gens provide capacity, not much energy
Time (hour of day)0 4 8 12 16 20 24
Syst
em L
oad
(MW
)
seconds to minutes
Regulation
tens of minutes to hours
LoadFollowing
day
Scheduling
A Chronological View
Hours to Day ahead: use load forecast to commit units10 minutes to few hours: manually adjust generator set pointsSeconds to minutes: AGC automatically adjusts generator output
3. What are reserves?There are different kinds of reserves• Contingency reserves
– Extra spinning capacity to guard against sudden loss of generator or transmission
– Contingency event is big and fast (wind does not behave this way)
– Contingency reserves are often shared in reserve pools or groups
– Largest hazard– Expensive
• Regulating Reserves• Operating Reserves
3. What are reserves?
• Contingency reserves• Regulating reserves
– Capacity that is reserved for small up-down changes in load
• Other operating reserves
3. What are reserves?• Contingency reserves• Regulating Reserves• Operating Reserves
– Generation that can be used to meet changing or unexpected load conditions
– No universally-accepted, rigorous definition– May be spinning or non-spinning (10-minute availability)– CA study of 33% renewables found increased operating reserves
of 3-7%
4) Does a wind plant start or stop very suddenly?
• Large wind farms have many individual wind turbines
• The turbines are spread over many miles and do not experience the same wind at the same time
• TX event Feb 24, 2007: drop of 1,500 MW over 2 hours is similar to behavior of load
Total WIND
0
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1000
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2500
2/24/07 9:00 2/24/07 9:28 2/24/07 9:57 2/24/07 10:26 2/24/07 10:55 2/24/07 11:24 2/24/07 11:52 2/24/07 12:21 2/24/07 12:50
Total WIND
2 Hours
Typical monthly wind-speed state transition matrix
0.0 0.2 0.4 0.6 0.8 1.0
Rel
ativ
e Fr
eque
ncy
0 6 12 18 24 Velocity(t-1)
18 12
6
Wind-speed State Transition MatrixOctober, 1995, Imperial, NE
Velocity(t)
5. Does Geographic diversity help?
6. Which generation is displaced when the wind plant generates power?
0
20
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100 P
erce
nt o
f Pea
k
1 3 5 7 9 11 13 15 17 19 21 23Time of Day
Nuclear Other Wind Gas/Oil Hydro CT's
P+: Peakday SimulationUtility U1, High Plains Wind
0
20
40
60
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erce
nt o
f Pea
k
1 3 5 7 9 11 13 15 17 19 21 23Time of Day
Nuclear Other Gas/Oil Hydro CT's
P+: Peakday SimulationUtility U1, No Wind
• Wind displaces the most expensive (marginal) generation, absent dispatch or other constraints
• Reduced CT and gas-oil generation in this scenario (your mileage will vary)
Do other generators reduce output by 1kWh when wind/solar produces 1kWh?
• Yes – if not in this balancing area, then another• (unless generation is charging a storage system)• System must be balanced
∑ loads + reserves = ∑ generation
• Unit or combination of units run at reduced output– May be a heat-rate penalty (generators are less efficient at partial
output)– Some units could perhaps be de-committed
• CA study– Mostly combined cycle units were displaced (40% CA and 60%
WECC)– 7500 MW wind and 1900 MW solar reduced NOx ~1500 tons and
SOx ~725 tons
7. What happens in the control center when the wind stops generating
power?• Operating reserves (other generation) is
deployed– Committed, spinning resource in short time
frame, in merit order subject to ramp constraints
– Units may be started over longer time frame if economic
– Import/export from neighboring Balancing Area
8. Does wind need backup or storage?• Increased operating reserves may be
necessary, but not dedicated backup• Although new storage has value, it may
not be cost effective• There is typically already storage on the
system– Natural gas in the pipeline or storage
facility– Controllable hydro
• A recent study by Xcel/PSCO found – Using existing pumped storage to offset
variability provided $1.30/MWh reduction in wind integration cost
– Enlarging existing gas storage facility was economic at large wind penetration
Wind Penetration 10% 15% 20% $/ MWH Gas Impact No Storage Benefits $2.17 $2.52 $3.66 $ / MWH Gas Impact With Storage Benefits $1.26 $1.45 $2.10
9. Does wind have capacity value? How is it measured and what does it
mean?• System Resource Adequacy: is there
enough installed generation to meet load?• Reliability methods applied to power
systems– Loss of load probability (or related metric)– Loss of load expectation (LOLE) and reliability
target such as 1 day/10 years LOLE
Effective Load Carrying Capability (ELCC)
0.12
0.11
0.10
0.09
0.08
0.07
0.06
Loss
of L
oad
Exp
ecta
tion
(day
s/ye
ar)
11.0x10310.510.09.59.08.58.0Load (GW)
Original Reliability Curve Reliability Curve after Adding New Generation Target Reliability Level
<-400 MW->
Each generator added to the system helps increase the load that can be supplied at all reliability levels
Gi Gi+1 Gi+2 Added Generators
What is ELCC?
• What ELCC is Not – a minimum generation value– a schedule or forecast for wind
• ELCC is– Measure of wind (or other resource’s)
contribution to overall system adequacy– Decomposition of the generator’s
contribution to adequacy
10. What is wind’s impact on power systems operation, and is there a
cost?• Wind integration studies can help answer this
question• Two primary impacts: variability and
uncertainty• Data requirements
– High fidelity wind power data set– Detailed system specifications (generator
characteristics, loads)• System simulation and statistical analysis
Each relevant time frame for system operations is analyzed to determine
wind’s impact• Evaluate operating
impacts and associated costs– Regulation– Load Following– Unit Commitment
• Evaluate reliability impacts – Effective Load
Carrying Capability/Loss of Load Probability
Time (hour of day)0 4 8 12 16 20 24
Syst
em L
oad
(MW
)
seconds to minutes
Regulation
tens of minutes to hours
LoadFollowing
day
Scheduling
10. What is wind’s impact on power systems operation, and is there a
cost?
• Minnesota 25% wind energy penetration (by energy) causes an increase in variability that must be met by power system operators and the non-wind generation fleet
14x103
12
10
8
6
4
2
Load
(MW
)
0
5-Minute Periods for Approximately 9 Months
1000500
0-500
-1000
5-m
inut
e ch
ange
s
6,600 Hours of 5-minute Electric Load14x103
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Load
(MW
)
0
5-Minute Periods for Approximately 9 Months
-1000
-500
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500
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5-m
inut
e ch
ange
s
6,600 Hours of 5-minute Electric Load and WindLower minimum load with wind
Impact on Uncertainty
• Increase in regulating and operating reserves• Operating reserve will depend on wind generation
in real-time• Change in unit commitment (wind forecasts can
help – more later)
Comparison of Cost-BasedU.S. Operational Impact Studies
* 3-year average; total is non-market cost** highest integration cost of 3 years; 30.7% capacity penetration corresponding to 25% energy penetration; 24.7% capacity penetration at 20% energy penetration*** found $4.37/MWh reduction in UC cost when wind forecasting is used in UC decision
Date Study Wind Capacity Penetra-tion (%)
Regula-tion Cost ($/MWh)
Load Following Cost ($/MWh)
Unit Commit-ment Cost ($/MWh)
GasSupplyCost($/MWh)
Tot Oper. Cost Impact($/MWh)
May ‘03 Xcel-UWIG 3.5 0 0.41 1.44 na 1.85
Sep ‘04 Xcel-MNDOC 15 0.23 na 4.37 na 4.60
June ‘06 CA RPS 4 0.45* trace na na 0.45
Feb ‘07 GE/Pier/CAIAP 20 0-0.69 trace na*** na 0-0.69***
June ‘03 We Energies 4 1.12 0.09 0.69 na 1.90
June ‘03 We Energies 29 1.02 0.15 1.75 na 2.92
2005 PacifiCorp 20 0 1.6 3.0 na 4.60
April ‘06 Xcel-PSCo 10 0.20 na 2.26 1.26 3.72
April ‘06 Xcel-PSCo 15 0.20 na 3.32 1.45 4.97
Dec ‘06 MN 20% 31** 4.41**
Jul ‘07 APS 14.8 0.37 2.65 1.06 na 4.08
11. How much does wind forecasting help?
• Wind forecasts are derived from weather prediction models
• Wind forecast accuracy is improving
• Several wind forecasting firms in U.S. Courtesy: WindLogics, Inc. St. Paul,
MN
Wind forecasting benefits for NYISO
• 10% penetration, 2005 operating costs with wind forecasts
Forecasting benefits for CAISO
12. Is there a limit to how much wind can be accommodated on the grid?
• Current studies in the U.S. have analyzed up to 25% of all electric energy from wind
• Based on work done so far, the question is not whether wind can be accommodated at high penetrations, the question is how and at what cost of integration
International Energy Agency Report: Wind Integration Studies
0102030405060708090
100
W D
enmark
Irelan
d ESBNG UK
Portuga
l
US Minn
esota
2006
Netherl
ands
Spain
2011
Irelan
d ESBNG
Mid Norw
ay S
intef
German
y 201
5 dena
Irelan
d SEI
Sweden
Irelan
d SEI
Nordic
2004
Nordic+
Germany
Gree
nnet
US Minn
esota
2004
US Colo
rado
Finland
2004
US New
York
Perc
ent P
enet
ratio
n
Peak Energy
• Recent International Energy Agency Report: Design and operation of power systems with large amounts of wind power
http://www.uwig.org/IEA_Annex25-State_of_the_Art_Report.pdf
12. Is there a limit to how much wind can be accommodated on the grid?
Denmark has access to large export marketsLennart Söder,KTH, Sweden, presented at UWIG, Oct 23-25, 2006
13. How do wind, concentrating solar power, and PV plants impact the grid?
• Investigation in process in Western Wind & Solar Integration Study– 30% Wind, 3.5% CSP, 1.5% PV by
energy in study footprint
• Our prediction:– CSP (with thermal storage) will help
the grid– Wind will have load following
impacts– Distributed PV will have little
impact, but centralized PV could have impacts on regulation and possibly load following
LEGEND
WestConnect LinesCalifornia LinesLADWP LinesDC Lines
Control areas:APSEl PasoNevada PowerPNMSierra PacificSRPTristateTucsonXcelWAPA
Need for Subhourly PV Data and Analysis
Source: Tom Hansen, Tucson Electric Power
Wind power output
PV and CSP output on sunny dayConfidential Data
Wind output (113 MW Storm Lake, Iowa)
PV and CSP on a windy cloudy dayConfidential Data
Wind output (113 MW Storm Lake, Iowa)
PV and CSP on a cloudy day with some windConfidential Data
Wind output (113 MW Storm Lake, Iowa)
14. How can more wind be accommodated on the grid?
• Utility balancing areas can combine or cooperate – large electricity markets (example: Denmark/Europe)
14. How can more wind be accommodated on the grid?
• Utility balancing areas can combine or cooperate – large electricity markets
• Example: Ramping, or changing output of generators that can be eliminated with larger balancing areas -1000
-500
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Ram
p (M
W/h
r)
53045298529252865280Hour of Year (one day)
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Exce
ss R
ampi
ng a
nd R
amp
Pena
lty (M
W/h
r)
Operating separate balancing areas causes extra ramping compared to combined operations. Blue: up-rampGreen: down-rampYellow: combined ramp
Ramping that could be eliminated by combining operations
Some areas are ramping up nearly 1000 MW/hr while other areas are ramping down nearly 500 MW/hr
14. How can more wind be accommodated on the grid?
• Power system operations practices and wind farm control/curtailment
• Integration of wind forecasting and real time measurements into control room operations (WindLogics/EnerNex/UWIG/ Xcel study underway in Minnesota)
• Hydro dispatch, pumped hydro• Longer term: other storage
and markets (plug-hybrid electric vehicles, hydrogen)
14) How can more wind be accommodated on the grid? What
about Storage?• Storage can have
significant benefits to the power system
• Storage may help integrate wind, but storage is not necessary or economic based on results in the U.S. at low-moderate penetrations
• Event was forecasted but ERCOT’s forecasting system had not yet been integrated into system operations
15. What happened in TX, Feb 2008?
15. What happened in TX, Feb 2008?• A frequency drop resulted in a call upon reserves including
loads that had voluntarily signed up for a Load-Acting-As-Resource program in which they are paid to curtail.
• There were 3 major contributors:– Wind generation dropped from 2000 to 360 MW in 3.5 hours– A conventional unit with 370 MW capacity tripped offline– The load forecast was wrong
• The wind ramp (500 MW/hour) is not the same as a reliability event with instantaneous loss of gen or transmission. – Conventional contingencies require dedicated, expensive spinning
reserve and immediate response. – Multi-hour wind ramps allow for use of less expensive resources.
Lessons Learned
• Load forecast failed to predict the large ramp up in demand
• The accurate wind energy forecast was not used in scheduling (this has been rectified)
• LAaR was very effective in economically reducing demand
• Wind event was a ramp event, not a contingency event (similar to 2007 event)
Total WIND
0
500
1000
1500
2000
2500
2/24/07 9:00 2/24/07 9:28 2/24/07 9:57 2/24/07 10:26 2/24/07 10:55 2/24/07 11:24 2/24/07 11:52 2/24/07 12:21 2/24/07 12:50
Total WIND
2 Hours
www.osei.noaa.gov
Old stuff below
10 FAQ’s about Wind
1) How much wind is currently installed in the US?
2) What are the benefits of wind energy to the power system?
3) How can wind’s variability be incorporated into power system operations
4) Does wind plant output start/stop suddenly?
5) Can wind be predicted?
10 FAQ’s about Wind6) Can the power system be reliably operated
with wind energy?7) Does wind need backup or storage?8) Is there a limit to how much wind can be
accommodated on the grid?9) Can wind power plants be controlled?10) Can wind energy make effective use of
transmission lines?11) Bonus Question: How can more wind be
accommodated on the grid?
1) How much wind is currently installed in the US?
1) How much wind is currently installed in the US?
Colorado/Xcel: Approx 20% wind penetration
(wind capacity/system
peak)
Iowa: Approx 16% wind
penetration (wind
capacity/system peak)
MW Installed End 2006 Installed 2007
End 2007
Total EU-12 419 263 675
Total EU-15 47,651 8,291 55,860
Total EU-27 48,069 8,554 56,535
1b) How much wind is currently installed in Europe?
2) What are the benefits of wind energy to the power system?
• Wind energy displaces– Fuel– Emissions; carbon
• Wind provides a hedge against rising fuel prices (natural gas, coal)
• Wind is an energy source with limited capacity contribution other generation is also required
• Wind can be cost-competitive with other forms of generation and may reduce electricity cost
8) Is there a limit to how much wind can be accommodated on the grid?
• Recent International Energy Agency Report:
Design and operation of power systems with large amounts of wind power
http://www.uwig.org/IEA_Annex25-State_of_the_Art_Report.pdf
9) Can wind power plants be controlled?
• New low-voltage ride-through (LVRT) grid codes in the U.S. will help wind turbines contribute to grid reliability
• Wind turbines can be controlled but not to the extent that conventional generation can be controlled– Ramp rate limits– Up-regulation (operate below potential so that wind output can be
increased if needed)– Curtailment, if necessary and economic, at low-load/high-wind
conditions
10) Can wind energy make effective use of transmission lines?
• Conditional-firm transmission tariff (recent FERC ruling)
• Wind does not need transmission all of the time
• Most transmission paths have some open capacity most of the time
• Adding wind can result in more efficient usage of existing transmission
Increasing Attention in North America
• IEEE Transactions on Power Systems (2007)
• IEEE Power Engineering Society Magazine, November/December 2005
• Updated in 2007• Wind Power Coordinating
Committee Wind Super-Session, Summer 2008
• Utility Wind Integration Group (UWIG): Operating Impacts and Integration Studies User Group
www.uwig.org
Texas Event Feb 26, 2008
• 15:00 – Wind generation output at 2000 MW and begins a 3.5 hour ramp down to 360 MW at 18:30. The down ramp was 2 hours sooner and somewhat faster (8 MW/minute vs 5 MW/minute) than forecast the day ahead
• 17:10 – Evening load ramp begins, increasing 3800 MW in 90 minutes, (42 MW/minute). The evening load ramp-up began 25 minutes earlier than the short-term hour-ahead load forecast predicted
Texas Event Feb 26, 2008
• 17:44 – 150 MW conventional unit trips offline
18:28 – ERCOT calls on non-spin service to come on-line
18:33 – 328 MW of Responsive (spinning) Reserve deployed
18:41 – ERCOT calls for EECP step 2 18:49 – ERCOT instructs all available
LaaRs to reduce consumption
Texas Event Feb 26, 2008
• 18:59 – 1108 MW of LaaR was reduced within 10 minutes (1200 MW within 12 minutes)
18:56 – Spinning reserve deployment ends
20:08 – ERCOT ends step 2 and enters step 1
21:40 – EECP terminated
Large Transmission Investments Have Very Small Retail Bill Impacts
Average Monthly Retail Bill $ / 1,000 kWhs
Total $65.75 Total $66.62
Transmission 6%
Generation 74%
Distribution 20%
Impact from $12.6 billion increase
(20%) in Transmission Infrastructure
Source: FERC
