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Tab 5: Wind Integration Operational Impacts
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In this section we will explore:
Frequency Control Impacts
Load frequency Control
Governor action and droop control
Wind impacts on frequency regulation
Major Challenges
Load Following Impacts or Economic Dispatch
Definition of economic dispatch
Characteristics of hydro units
Characteristics of wind units
Dispatch of Wind units
Load following
Impact of wind units on assignment of load following units
Unit Commitment Impacts
Wind Integration Operational Impacts
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Introduction
The growth of wind power have complicated the way optimizedpower system operation is carried out
The main activities of power system operations:
Voltage control
Real Time
frequency control (regulation)
Real Time
Load following
Economic dispatch, and
Unit commitment
System Reliability
Resource and Capacity Planning
now involve different behaviors (than conventional generation)more variability and uncertainties
introduced by wind and otherrenewable generation technologies (e.g. solar)
The impact of the variability and uncertainties will increase as
the penetration increases and can lead to substantial integrationcosts if not treated properly.
Fortunately there are ways of dealing with this effects.
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Time Line for Operational Control Activities
Source: NERC IVGTF Report
ELCC
Wind reducedELCC (EffectiveLoad Carrying
Capability)
Winduncertainty
Wind Variability& uncertainty
Wind Variability& controllability
Impact
Impact
Impact
Impact
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Time Line for Operational Control Activities
(continued)
VoltageRegulation
FrequencyRegulation
Load followingand Economic
Dispatch
Scheduling andUnit
commitment
Real time Short time frames tomaintain systembalance
Dispatch of bothenergy and capacity(reserves) services
Ensure sufficientgeneration is therewhen needed
Seconds -
Minutes
Maintain voltageprofiles & PreventVoltage collapse
Initial:
GovernorAction
seconds Several minutes to afew hours Normally Day AheadHours Ahead provideadjustments.
Week, Month, YearAhead ensureavailability.
Then:
Provided bygenerators on AGC
With wind:
Greater regulation, load-following, and quick-start capabilityrequired from the remaining generators.
Function of balancing area size, the size and geographicaldispersion of Wind Plants, the capabilities of the WTG
& the
flexibility of other generators and load.
Greatest source ofcosts. Windforecasting crucial tomanage this and therisk associated with
the uncertainty in theday-ahead time frame
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Frequency Events in Power Systems
When a generator output is lost the frequency immediatelybegins to fall.
The remaining generator governors, to a varying extent, see
the lower frequency and increase the amount of powergenerated via governor action
Some loads are naturally frequency dependent. Simple ACmotors slow down and consume less power. For some types ofloads this power reduction is much more than a directproportion.
A new equilibrium point is reached, in a few seconds, and the
frequency decline is arrested. The system is off-nominalfrequency
When a block of load is suddenly lost, frequency immediatelybegins to rise, governors decrease the generator output,
frequency dependent loads consume more power, and a newequilibrium is reached.
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Frequency Deviation in the U.S.
For the Northeast interconnected system
Frequency generally remains between 59.85 Hz and 60.15 Hz
During major events the deviation is still small
During the blackout of 2003 frequency only deviated within +/-
0.3 Hz.
Loss of the largest single unit in the interconnection (1300 MW)
produces a frequency decline of about 0.04 Hz; No Problem.
Huge Inertia
For the Eastern Interconnection:
Response of about -3300 MW/0.1 Hz, i.e. the loss of 1000 MW ofgeneration will cause a frequency deviation of -0.0303 Hz.
Usually, rate of frequency change is low:
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550
70
500
450
400
140
210
280
Frequency Deviation in the U.S.
An Example
The figure below shows the impact in ERCOT system when WTG (QSE)
almostinstantaneously ramp up and down causing frequency deviations and RapidResponse Reserve (RRS) to be deployed
Eventually, a 10% ramp rate was agreed upon by ERCOT and the wind
community taking into consideration the economical impact a smaller ramprate could have on the WTGs (more on this later)
Impact
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Frequency Control
When there is disturbance the system responds intwo time frames:
Primary Control
System Inertial Response
Load Response
Prime Mover Governing Response
Primary frequency response (10 sec)
Secondary frequency response (30 sec)
Supplementary Control
Speed changer -
local level (Load reference set-point);
Instruction to operators from the control center.
Automatic Generation Control (AGC) -
system level
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Initial Line of Defense
Inertial Response / Load Response
Inertia comes from thelarge mass in therotors and turbines of
conventionalgenerators.
Load can contribute
significantly to arrest theinitial drop in frequencyworking together with thesystem inertia.
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Next Line of Defense
Response of Machines Governors
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But
Frequency Response is Affected by WTG Impac
t
WTG have less inertia and traditionally did notparticipate in frequency control: e.g. rotor speed
constant philosophy for the DFIG
This produced:
Increased rate of change of system frequency for the samedisturbance (drop large generator unit)
Need to increase the up-regulation / down-regulationspinning reserve in the system
More acute for relatively small isolated systems: increased
risk of under-frequency load shedding and cascadingoutages
Gets worse for greater levels of penetration
WTG could ramp up and down very rapidly makingthe situation worse.
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However
Modern WTG Generation Addressed This
Modern Larger WTG
have greater inertia
The inertia stored in the rotating mass tends to increase slightlymore than linear with the rated power of the turbine
Deliberately use the control to extract stored inertial energyby decelerating the machine rotor
Provide incremental energy contribution during the firstseveral seconds of grid events
Do not go too far down with the rotor speed to avoid stalling awind turbine
Can provide Governor like response by spilling
wind if thisis the most economic way of providing regulation
Have ramp controls.
Solution
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Modern WTG Generation Addressed ThisSiemens Netconverter
The output of the WTG is controlled so it is frequency
sensitive.
The energy is extracted from both slowing the turbine and thewind that is spilled.
This gives time for the system to recover.
Solution
Turbines at reduced power
Increase in turbine output power
Reduction in turbine output powerTurbines at reduced power
Increase in turbine output power
Reduction in turbine output power
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ERCOT LOAD SHED LEVELS
Three steps are considered in ERCOT
A Typical response shown
Frequency Threshold Load Relief
59.3 Hz 5% of the ERCOT System Load(Total 5%)
58.9 Hz An additional 10% of the ERCOT System Load
(Total 15%)
58.5 Hz An additional 10% of the ERCOT System Load
(Total 25%)
Recorded, Initial, Adjusted
59.2
59.3
59.4
59.5
59.6
59.7
59.8
59.9
60
60.1
60.2
60.3
0 5 10 15 20 25
Seconds
Hz
recorded
initial
adjusted
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Modern WTG Generation Addressed ThisGE WindINERTIA
1
WindInertia increases electric power during the initial stages ofa significant downward frequency event by issuing a commandfrom the special frequency control
Usually it is enough to provide response within several secondsuntil governors of larger conventional machines start their work
WTG electric power drops to allow recovery of rotational speed
Solution
Source: N. Miller et al., GE Energy. CanWEA, Vancouver, BC, October 20, 2008
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Modern WTG Generation Addressed ThisRamp Control
ISOs are imposing limits on the ramping of WTG (e.g. 10%/minfor ERCOT)
Modern WTG have the capability to respond.
Below to examples one from Siemens and another from GE
Solution
Source: N. Miller et al., GE Energy. CanWEA, Vancouver, BC, October 20, 2008
Ramp Rate Control
0
50
100150
200
250
300
350
0 5 10 15 20
t, min
P,MW
Output Available Output
Curtailment to 100 MW
@ 15 MW/min
Curtailment released;
ramp to max power @
10 MW/min
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Final Line of Defense
AGC
Governor control is proportional and will leave an error inthe frequency.
As shown before the final line of defense after a major
perturbation is the Automatic Generation Control (AGC).
Units on AGC can change outputquickly to track minute byminute load variations and anyvariations in generation.
Units in AGC are usually themarginal units in the network
(e.g. GTs. CCPs, etc) or storage
hydro.
WTG are never the marginal unitand would not be on AGC.
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Background of Other Operational Concepts
Load Following
Load following takes place in the same time frame as economicdispatch discussed next, usually defined in the intra-hour timeframe.
Load following is defined as the response to changes in systemload and as we will see to changes in wind output.
The composition of units assigned to load following depend onboth of utility company operational practices, economicdispatch regime and on the physical capabilities of generators
(such as ramp rates) that follow load.
Larger control areas typically have several units that provide a
fraction of total output and
that can be called upon to eitherincrease or decrease output.
When this load following capability is spread among several units,
the ramp rate becomes less of a constraint
than if only a smallnumber of units adjust to changing load conditions.
It is generally nor economic to use WTG to provide loadfollowing.
WTG is modeled as a reduction in load in operational studies.
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Background of Other Operational Concepts
Economic Dispatch (ED)
Problem Definition
Given a set of generating units, synchronized and underdispatch control, and given the total net Load demand to besupplied by the units;
Determine the output of each unit so that the totalproduction cost is minimized
and the sum of the outputs
equals the total net demand. Also
no unit can violate its minimum or maximum dispatch
limit.
The flows in the system cannot violate transmission limits
andcreate potentially unsecure conditions (security constrained
solution)
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Background of Other Operational Concepts
Generator Models for Economic Dispatch
Thermal Units
Input-Output characteristics of a thermal generation unit is defined in termsof gross input: Heat rate (H) in (MBtu/hr) versus net generator MW output(P).
Heat Rate
(H)
Is the amount of heat added, usually in Btu per hour, to produce a unit amount of work kWh. Heat
rate thus has units of Btu/kWh.
Hydro Units
Production function of flow, head and efficiency
Value of water is a function of expected (stochastic) futureproduction cost savings versus producing today
Value is zero if the plant is spilling water or is a run-of-river
Value is very high if levels are low and it is likely that in the future veryhighly priced thermal (or load rationing) would have to be incurred.
Wind Units
Function of the Cp and cube of the wind speed.
Detailed hour by hour mesoscale wind models used to defineproduction
Modeled usually as a negative load
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Background of Other Operational Concepts
Economic Dispatch Solution
The goal is to dispatch all the available synchronized units tomeet the load demand at minimum cost.
For a a system with N thermal units, this is achieved under the
condition that at optimal dispatch all units have equalincremental (marginal cost), where F(Pi
) is the Fuel input
power output characteristic of each unit.
All units are either at itsmaximum/minimum output or aremarginal units (within unconstrainedareas)
Tool: Security Constrained EconomicDispatch (SCED)
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Background of Other Operational Concepts
Unit Commitment (UC)
Why Unit Commitment?
Cyclic nature of power system loads.
Load changes and the operation of an electric power system.
Committing enough units and having them on line is one ofeconomics and security (reserves)
Many constraints can be placed on the unit commitmentproblem such as;
Generating unit constraints and status
Reserve constraints
Minimum up-time and down-time
Crew constraints
Ramp rate limits
Transmission Constraints
Generation-Load balance constraint
Tool: Security Constrained Unit Commitment (SCUC)
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The ISOs functions include the administration of the UCand ED according to various timeframes
Day-ahead security-constrained
unit commitment once aday
Real-time (short-term) security-constrained unitcommitment every 15-minutes
with a 30-minute reactiontime
Real-time security-constrained economic
dispatch every 5minutes
with a 10-minute reaction time
Automatic generation control every 6 seconds
Supplementary, computer-assisted manual intervention as
needed
Other functions of the ISOs are
Operating the high-voltage transmission system and
Administer, monitor, and settle the electricity markets
Background of Other Operational Concepts
Functions of the ISO
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Background of Other Operational Concepts
Spinning Reserve Constraints
Spinning reserve
is the term used to describe the total amountof generation available from all units synchronized (i.e.,spinning) on the system, minus the present load and losses
being supplied.
Spinning reserve must be carried so that the loss of one or more
units does not cause too far a drop in system frequency. If one
unitis lost, there must be ample reserve on the other units to make up
for the loss in a specified time period.
Spinning reserve
must be allocated to obey certain rules,usually set by regional reliability councils (in the united States)that specify, how the reserve is to be allocated to various units.
Typical rules specify
that reserve must be a given percentage of forecasted peakdemand, or
that reserve must be capable of making up the loss of the mostheavily loaded unit in a given period of time.
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Background of Other Operational ConceptsSpinning Reserve Constraints (continued)
Beyond spinning reserve, the unit commitment problem may involve
various classes of scheduled reserves
or off-line
reserves.
These include:
quick-start diesel or
gas-turbine units
hydro-units
pumped-storage hydro-units
that can be brought on-line, synchronized, and brought up to fullcapacity quickly.
As such, these units can be counted
in the overall reserve assessment,as long as their time to come up to full capacity is taken into account;Rapid Response Reserves (RRS)
Finally, Reserves must be spread around the power system;
to avoid transmission system limitations (often called bottling
of reserves)and
to allow various parts of the system to run as islands, should they becomeelectrically disconnected.
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Background of Other Operational ConceptsResource Adequacy
This refers to the procurement of enough installed generation in
the system to be able to attend the forecasted peak load,schedule the required maintenances and account for forced
unavailability while maintaining a desired level of reliability;
usually measured in LOLP in % or LOLE in days per ten years
One key component to represent the value of a unit is its ELCC or
Expected Load Carrying Capability.
For conventional generation it is approximately equal to Maximum
Demonstrated Capacity (MW) * (1-FOR)
For WTG we will see later.
Yearly the ISO must ensure that there is enough capacity in thenetwork including firm imports less firm exports.
Now we will see how WTG affects all of this!
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The Nature of WTG
Variability & Uncertainty
Variability
are changes in the WTG output due to weatherpatterns and are unavoidable, but can be mitigated.
Geographic Dispersion reduces Variability
Wind mesoscale models demonstrate that it is not credible that in alarge area all wind speed will experiment the same variation.
The larger the geographical dispersion of Wind Plants the smaller thevariability of the generation
Impact
Source: DOE 2008 Report
Solution
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The Nature of WTG
Variability
The size of the Wind Generation reduces the variability
More WTG imply that it is less likely that they all see simultaneouslythe same change in wind speed resulting in soothing effects
More WTG generation is likely to also imply greater geographical
diversity.
Also, shorter time frames are subject to less variability.
Sudden changes in wind speed are less likely and thing localized.
Solution
Source: DOE 2008 Report
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The Nature of WTG
Variability (continued)
Based on the observations above we conclude that makingthe balancing areas larger
reduce variability and as we willsee also the uncertainty.
Virtual arrangements though reserve sharing, ACE sharing anddynamic scheduling of wind plants from smaller to larger areas.
Source: Avista Wind Integration Study
Solution
Increase inGeographicaldiversity
Cost is reduced as
the Balancing Areasincreases (largerareas.
Wind resources were evaluated in the Columbia Basin, in Eastern Montana, as a 50%/50% mix of Columbia Basin and EasternMontana wind, and as a multi-state diversified
mix with many smaller sites combined.
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The Nature of WTG
Uncertainty
Uncertainty are errors in the estimation of production due toerror in the wind speed forecast
The errors in wind speed are magnified in the power output due
to the cube dependency.
10% error in wind speed results in 33% error in power.
Costs can be substantial, particularly for over commitments of capacity.
Impact
Source: Avista Wind Integration StudyLevel ofPenetration
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The Nature of WTG
Uncertainty (continued)
The errors in wind speed are magnified as the time horizonincreases. This means that on one extreme day ahead unitcommitment it has the greatest impact and on the other loadfollowing has the least.
Impact
Source: NERC IVGTF Report 041609
This has an impact onmarkets, the sooner theday ahead closes
the
greater the error in theforecast and the higher thecost
Same applies for hour
ahead and
This is a case for lateclosing of day aheadmarkets and intra-hour
markets.Solution
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The Nature of WTG
Uncertainty (continued)
Wind forecasting technology has improved substantially over theyears, lead by countries like Germany that have large levels ofwind integration.
Better data collection / larger areas / faster computers
Use of multi-model approaches for forecast; statistical models using forexample Artificial Neural Networks, mixture of experts, nearest neighborsearch and support vector machines
Use of mutischeme forecasting based on different assumptions/methods in
numerical weather models
Source: Predicting the Wind, IEEE
Solution
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The Nature of WTG
Uncertainty (continued)
These methods and efforts have resulted in a continuedreduction in the forecasting error
Source: Predicting the Wind, IEEE
Solution
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The Nature of WTG
Uncertainty (continued)
Larger areas result in a reduced forecasting error for the region.
This results in less impact on scheduling and therefore lower integrationcosts,
Source: Predicting the Wind, IEEE
Solution
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Modeling Wind as a Load Resource
for System Operations
WTG is normally treated as a negative load resource and the netload forecast
is used to commit and dispatch non-windgeneration:
Since wind has no fuel cost per se, it is always called to run, whenever
available in the dispatch; same as load
It is treated as non dispatchable unit
by most utilities and electricity markets;same as loads
Wind generation is variable and has errors in its forecast; same
as loads
Load and WTG are independent and the combination of errors is :
Example if the load forecast error for a 5,000 MW peak load is 2% = 100 MWand the WTG error for a total of say 1000 MW, with an output of 500 MWduring the peak is 25% = 125 MW, then the total expected error is 160 MW..
Wind forecast error and variability increases ancillary services for
regulation control
2
generationwind
2
loadtotal
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Wind Impacts on Load Following
Wind generation canhave significant Impacton load following.
Wind adds variability tothe to the rampingrequirements.
The figure to the left
shows the increase inrequirements for Xcelenergy North, with andwithout wind
Impact
Source: Grid Impacts of Wind Power Variability: RecentAssessments from a Variety of Utilities in the UnitedStatesB. Parson et. al.
Ramp uprequirementsincreased bywind
Ramp down
requirementsincreased bywind
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Unit Commitment of Wind Generation
The use of wind generation, due to the variable nature of wind,can lead to operational uncertainty:
How much generation will be necessary
to serve the net load (load
WTG)?
How much spinning reserve will be necessary
to maintain reliabilitylevels?
What load following reserves and ramping capabilities
are
required?To allow regulation, i.e. the minute by minute changes insystem load
Some utilities when committing or dispatching wind
generation, use a generation output that is discounted by acertain percentage of the forecast value
to hedge againstuncertainties in wind forecasting.
Remember that errors in the forecast have maximum impact the
longer the time
are maximum in the day ahead unitcommitment.
Impact
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Effect of the Nature of Other Generation
The impact of the variability and uncertainty of wind generation
is minimizedwhen the rest of the generation is flexible and the LOAD!
SolutionImp
act
Technology Impact
Nuclear Power
Plants Very inflexible slow to change
Run-of-river hydro Limited flexibility without wasting water
Large Coal FiredSlow to start and ramp, better at base
load
Combined Cycle
Plants
Faster to start, fast to ramp; high
efficiency at close to nominal
Conventional CT Fast to start and ramp; costly
Advanced CCPFast to start and ramp; very efficient at
many load levels FlexPlant 10
Pump storageFaster to start, fast to ramp; need low
cost energy
Storage HydroFast to start and ramp; can virtual
store
worst
better
Best
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Summary of Impacts and Solutions
The table below summarizes the main operation impacts and theirmitigations/solutions
Not all systems are the same with respect of the quality of the WTG,the flexibility of the generation and the level of penetration but this is a
good indication
Impact
INCREASING COSTS
Regulation
Load following
Economic Dispatch
Unit Commitment
short term
Unit Commitment
Day Ahead
ImpactVery LowSmallest errors in
forecast
Limited Variability
LowSmaller errors in forecast
Subject to Variability
MediumSome errors in forecast
Important Variability
HighSignificant errors in
forecast
Important Variability
Mitigation- WTG Controls
- Flexible Generation
- WTG Controls
- Flexible Generation
- Geographical Diversity
- Large Balancing Areas
- Flexible Generation
- Geographical Diversity
- Large Balancing Areas
- Flexible Markets
- Flexible Generation
- Geographical Diversity
- Large Balancing Areas
- Flexible Markets- Improved forecasting
Effectiveness High High Improving Need to improve
Activi ty
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Summary of Impacts and Solutions
(continued)
The table below summarizes the results of various integration studiesto date.
It is largely in agreement with the observations made
Impact
INCREASING COSTS
Date Study WindCapacity
Penetration(%)
RegulationCost
($/MWh)
LoadFollowing
Cost($/MWh)
UnitCommit-
mentCost
($/MWh)
Gas SupplyCost
($/MWh)
TotOper.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
2005 PacifiCorp 20 0 1.6 3.0 na 4.60
April 06 Xcel-PSCo 10 0.20 na 2.26 1.26 3.72April 06 Xcel-PSCo 15 0.20 na 3.32 1.45 4.97
Jul 07 APS 14.8 0.37 2.65 1.06 na 4.08
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A larger number of Studies
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Using Economic Dispatch Simulation to Evaluatethe Cost Impact of Wind Generation
The costs impacts of sub hourly load following that includedispatched wind generation can be evaluated by running aSecurity Constrained Economic Dispatch (SCUD) simulationsoftware .
With wind profiles / mesoscale models / forecasts calculate wind
productionby plant in 15 minutes increments and its expected error (introduced asprobability)
Run the SCUD and calculate the dispatch for other units and reserves.
For comparison purposes a case with uniform capacity and energy equal tothat produced by the wind resources can be assessed.
This cost incurred due to using wind during this period, islargely due to factors such as;
Sudden loss or increase of wind power
Slow changes in wind power output (i.e. ramp rates)
Units at their minimum.
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Using Unit Commitment Simulation to Evaluate theEconomic Impact of Wind Generation (continued)
The economic impact of wind generation in the unitcommitment can be done as follows;
With wind profiles / mesoscale models / forecasts calculatewind production by plant in hourly increments. Expectedcase can be used or historical series. Account for errors inthe forecast.
Run the SCUC and calculate the commitment for other unitsand reserves. Calculate the cost.
This cost can be compared with a benchmark case that has a
unit commitment with no wind capacity scheduled, butinstead an uniform block of generation that produces thesame energy of the wind is included.
A case where wind and corresponding load is removed from
the runs.
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Summary of Operational Impacts of Wind Integration
Carrying of additional reserves,
both spinning andnon spinning to meet intra hour load following.
Additional energy costs incurred for intra hour loadfollowing due to greater variability
Possible additional costs of frequency regulationresources or regulation reserves
Found by evaluating the impact of wind fluctuations on thestandard measures such as area control error etc.
Increased reserves for safe operation.
Important for older WTG less so with new designs
Additional cost for Unit commitment costs
due toforecast inaccuracies
in Wind generation outputs
The cost of rescheduling generation to account for short fall
of wind output
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A Case Study of a WPG in High Penetration Denmark(19.3%)
A typical day on the Horns Rev Offshore wind plant in Denmark
Plant spilling windso it has spinningreserve available
Manual orders
to reduceoutput
Fast Reductionto counteractfrequencyincrease
Source: European Balancing Act, IEEE
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The Capacity Value of Wind
While wind is largely an energy resource it does have somecapacity value; i.e. Effective Load Carrying Capability (ELCC)
There are several methods for determining this value for WTG
Source: M. Milligan and K. Porter, Determining the capacity value of wind: A survey of methods and implementation,
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The Capacity Value of Wind (continued)
As can be observed in the figure below in average about 25%ELCC is assigned to WTC across different companies.
Source: Wind Plant Integration-
Cost Status & Issues IEEE
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The Capacity Value of Wind (continued)
However, this value may change season to season as Avistafound:
Source: Avista Wind Integration Study
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The Capacity Value of Wind (continued)
At the end of the day the ELCC of wind is the additional loadthat one can serve when wind is added without degrading thetarget reliability level
Source: Avista Wind Integration StudySource NREL The Capacity Value of Wind