tab 5 operational impacts-v3.1

7/26/2019 Tab 5 Operational Impacts-V3.1

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2012 Siemens Industry Inc. All rights reserved

Tab 5: Wind Integration Operational Impacts


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Siemens Power Academy TD-NA Power System Studies for Wind Integration 5-2

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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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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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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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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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


Functions of the ISO


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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


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


These methods and efforts have resulted in a continuedreduction in the forecasting error


Solution


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The Nature of WTG


Larger areas result in a reduced forecasting error for the region.

This results in less impact on scheduling and therefore lower integrationcosts,


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


- Geographical Diversity

- Large Balancing Areas




- Flexible Markets




- 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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However, this value may change season to season as Avistafound:

Source: Avista Wind Integration Study


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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

tab 5 operational impacts-v3.1

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