Wind integration: experience, issues, and challenges

Overview

Wind integration: experience,issues, and challengesHannele Holttinen∗

The challenge of wind integration is to make best use of the variable and uncertainpower source while maintaining the continuous balance between consumptionand generation and high level of reliability in the power system. There is alreadyexperience of operating power systems with large amounts of wind power andintegration studies give estimates on wind power impacts. Power systems areequipped to handle variability and uncertainty that comes from the electricityconsumption, the load. Short-term wind forecasting is required to manage largeamounts of wind power. The main impacts of wind integration are investmentsin grid infrastructure and efficiency losses in power plants when following theincreased variations and uncertainty in the power system. Wind power will loweremissions while replacing energy produced by fossil fuels and can also replacesome power plant capacity. However, wind’s lower capacity value compared toconventional power plants is one integration impact of wind power, meaninghigher total installed capacity in power systems with high wind penetration.Managing options for wind integration impacts includes proper wind power plantgrid-connection rules, increasing transmission capacity and increasing flexibilitythat is available from generation plants and demand side. Further development ofmodels and tools is required to study how entire power systems can be operatedduring the hours and days of very high penetration levels covering 60–80% ofload. C© 2012 John Wiley & Sons, Ltd.

How to cite this article:WIREs Energy Environ 2012, 1: 243–255 doi: 10.1002/wene.18

INTRODUCTION

W ind power is a strongly growing renewableelectricity technology that has high technical

potential worldwide and high targets of deploymentin many countries. Integrating variable and uncertainwind power production to the electricity system is onelimiting factor in using the high technical potential.The concerns that wind power brings for system op-eration are how to maintain power system reliabilityand the balance between load and generation.

Integration challenge depends on the penetra-tion level of wind power in the power systems. Thepenetration of wind power can be expressed by vari-ous measures. Usually, either energy or capacity met-rics are used: yearly wind power production as a per-centage of yearly electricity consumption (energy) and

∗Correspondence to: [email protected]

VTT, Technical Research Centre of Finland, Espoo, Finland

DOI: 10.1002/wene.18

installed wind power capacity as a percentage of peakload (capacity). In this paper, the energy penetrationlevel is used.

There is already experience in operating powersystems with 10–20% penetration levels and a lotof studies on the impacts of wind power to powersystems. This paper is about integrating wind powerin larger power systems and is mainly based on threesummary publications on the issue.1–3 For smaller au-tonomous systems where wind provides a large partof the energy for wind–diesel systems, or in small is-land systems see, e.g., Refs 4 and 5.

THE CHALLENGE OF WIND POWERTO POWER SYSTEMS

The goal of power system operations is to maintainthe continuous balance between load and generationwith high reliability. Possible impacts are presented inFigure 1 in two dimensions—impacts in different area

Volume 1, November /December 2012 243c© 2012 John Wi ley & Sons , L td .

Overview wires.wiley.com/wene

FIGURE 1 | Impacts of wind power on power systems, displayedby time and spatial scales relevant for the studies. Primary reserve ishere denoted for reserves activated in seconds (system-wide frequencyactivated reserve; regulation; automatically activated reserve of thebalancing zones). Secondary reserve is here denoted for reservesactivated in 10–15 min (minute reserve, load following reserve,manually activated).

size (locally and systemwise) and impacts in differenttimescales. Impacts can been seen during operation (inseconds to days) and need to be taken into accountwhen planning the adequacy of future generation ca-pacity and transmission grid years ahead.

The integration cost is the additional cost ofthe design and operation of the nonwind part of thepower system when wind power is added to the gen-eration mix. Often integration costs are associatedonly with wind power; however, actually other gen-eration can also cause integration costs as increasedtransmission and increase in contingency reserve toaccommodate a large unit, for example. Integrationcost can sometimes also be defined as difference incost compared to some alternative power productionthat could be added to a system. Cost allocation isanother point—often system costs are allocated to allusers in tariffs. Part of the integration costs may becovered by the wind power producers as connectioncosts and network and imbalance tariffs.

The impacts of wind power can be different fordifferent power systems because power system char-acteristics are not the same. Adding large amounts ofwind power plants will make the power systems morecomplex and requires new tools and operational prac-tices. Variability and uncertainty in wind generationis managed mainly by using flexible resources in thepower systems.6,7 Flexibility refers to the ability tochange power output level or consumption accordingto the system needs when keeping the continuous bal-ance of consumption and generation. In general, sys-tems where the balance needs to be kept in smaller ar-eas with power plants operating with fixed schedules

have more challenges than larger, well-interconnectedpower systems that can use all flexibility options inpower plants and have a strong grid to start with.

Existing Experience on Wind IntegrationThere is already valuable experience of operatingpower systems with high wind penetrations.3,8–11

West Denmark and North Germany have more than30% of yearly load covered by wind generation. Theyare smaller parts of larger systems and operate attimes with wind power covering more than the load(more than 100% wind penetration). Spain, Portu-gal, and South Australia have about 15% of yearlyload covered by wind generation. They are regionsthat are not very well-connected parts of larger sys-tems and have coped with periods of 54–71% windpenetration levels. Ireland is a small power systemwith about 10% penetration level on a yearly basisand currently coping at times with 50% penetrationlevels.

Coping with these levels of penetration is madepossible by online real-time wind-generation data to-gether with constantly updated forecasts of expectedproduction in system operators’ control rooms.1 Incases where wind power plants are from small unitsin the distribution system, decentralized control cen-tres enable collecting online data and possibility tocontrol wind power plants.11,12

Integration Studies: Issues and SetupThe specific issues investigated in the wind integrationstudies vary and the methods applied have evolvedover time, with studies building upon the experiencegained in previous studies. Best practices are emergingand models are being improved.1,13–15 The studiescover different penetrations and systems and show awide range of results. The main issues studied are asfollows:

• Impacts on balancing in different timescales:any increase needed in short-term reserves orramping requirements, scheduling, and effi-ciency of conventional power plants;

• Impacts on grid reinforcement needs and gridstability; and

• Impacts on generation adequacy—how tomeet the peak loads.

Relevant issues to be taken into account whenassessing the impacts of wind power on the powersystem are as follows14:

244 Volume 1, November /December 2012c© 2012 John Wi ley & Sons , L td .

WIREs Energy and Environment Wind integration

BOX 1: WIND TURBINE CAPABILITIES AREIMPROVING

Wind turbines can provide support the grid. Allowing higherinstantaneous penetration levels means that wind powerplant capabilities in providing (part) of the ancillary servicesneed to be improved.One reliability concern has been that large amounts of windpower can trip off the grid because of a short disturbance ofthe grid (voltage drop).24 This problem has been addressedby new grid connection rules (grid codes) requiring fault-ride-through (FRT) capability from wind turbines.11,12,25–27

The grid codes often require wind turbines to provide re-active power and in some regions also to take part in volt-age and frequency control.8,11,12 It should be noted, how-ever, that when providing frequency (active power) control(Figure 2), this will be at the expense of lost productionfrom the turbines. Even if wind power would probably bethe last to provide such services, it is important to have thatoption in the systems during high penetration events.

• What is the main setup for the assessmentor simulation: is wind power replacing otherproduction or capacity? Is the power sys-tem operation or remaining generation mixoptimized when wind power generation isadded?

• What is the wind input used: How well doesthe wind data represent the future distribu-tion of wind power plants in the power sys-tem area? How is wind power simulated,what timescale effects on variability and pre-dictability have been taken into account? Itis important to have the wind data syn-chronous with load data to capture theircorrelation (see also Box 1).

• How is the uncertainty in the wind genera-tion forecasts handled with respect to the loadforecast uncertainty? Are they combined us-ing proper statistical methods? Are updatedforecasts for wind power closer to deliveryhour taken into account?

• What is the level of detail in the simulation—time resolution, assumptions on pricing (mar-ket/technical cost)? What has been taken intoaccount when modeling thermal and hydrounits and transmission possibilities?

In smaller systems, the studies can and needto be more detailed as frequency control is morechallenging.16–18 When integration studies are made

to areas that are part of larger areas, it is chal-lenging to simulate possibilities that transmission ca-pacity to neighboring areas can have in a way thatwould not under- or overestimate the system flex-ibility in managing variability.19 Larger area stud-ies capture the impacts of wind power taking intoaccount the possibilities for cross-border trade andbalancing.20–23

Most studies so far have concentrated on thetechnical costs of integrating wind into the power sys-tem The benefit when adding wind power to powersystems is reducing the total operating costs and emis-sions as wind replaces fossil fuels. To set the costsin scope, integration costs of wind power should becompared to something, such as the production costsor market value of wind power, or integration costof other generation forms.1 A fair comparison shouldkeep system reliability in the same level and some-how take into account any changes in costs and emis-sion savings. Cost–benefit analysis has been used inIreland.18

WIND IMPACTS ON BALANCING ANDRESERVES

Power systems have controllable generation to handlesignificant variability and uncertainty in loads overtimescales from seconds to days. The generation isscheduled first based on forecasted demand, then re-dispatched closer to delivery with updated informa-tion on forecasts, and, finally, systems carry operat-ing reserves that will respond either automatically ormanually on 5–15 min notice to keep the balance ofdemand and generation at all times. System operatorsfollow the change in total demand, not the variationfrom a single generator or customer load.

The variations of wind power do not correlatewith how the load varies from one time step to an-other. Adding this new component of variability to apower system will not result in just adding up the totaland extreme variability of both, because the extremevariations are not likely to coincide. Power systemscarry reserves to handle rare events occurring, for ex-ample, once a year. Carrying reserve for even moreimprobable events such as the extreme of wind andload and a large outage occurring all at the same in-stant would mean extra costs for reserve that wouldnot be used in practice. ‘Backup’ generating plantsdedicated to wind plants, or to any other generationplant or load, are not required, and would be quitecostly use of power-generation resources.2,3 Exampleof the variability of load and the net load, the load



FIGURE 2 | Outline of possible active power control functions from wind power plants. The plots show the possible power and the actualachieved power when different control functions are activated (Reproduced by permission of Jesper Runge Kristoffersen.)

FIGURE 3 | Wind power will add to the variability that powersystems experience. One week of hourly data from West Denmark(January 10–16, 2005) that has 24% wind penetration level on yearlybasis, showing the variability of load and wind (upper graph) andresulting net load: Net load = load–wind power (lower graph). Sourceof data: http://www.energinet.dk.

minus the wind power produced, is shown from realdata from West Denmark that has 24% wind pene-tration level in a yearly basis (Figure 3).

The Variability and Uncertainty ofWind PowerThe relative variability of wind will decrease with theaggregation of more wind power plants (Figure 4).Aggregating wind generation over larger geographic

areas decreases the number of hours of zero output.One wind power plant can have zero output for morethan 1000 h in a year, whereas the output of aggre-gated wind power in a very large area is always greaterthan zero. Aggregation also means that the full totalinstalled capacity will not be reached at any instant,as the wind will not blow hard enough over largeareas of several hundred kilometers simultaneously.Also, as the technical availability of single turbinesis of the order of 95% of time, there will always besome turbines standing still when a fleet of hundredsor thousands of turbines is considered.

The variability also decreases as the timescaledecreases. The second and minute variability of large-scale wind power is generally small. Over severalhours, however, there can be great variability evenfor distributed wind power: low wind and high winddays can still clearly be seen (Figure 4).

Storm events can result in extreme variationfrom wind power: when wind speeds are high enoughto require wind turbines to shut down from fullpower, to protect the wind turbine. These events arequite rare: usually one or two times in 1–3 years de-pending on location. Large storm fronts take 4–6 hto pass over several hundred kilometers so, aggrega-tion of wind capacity turns the sudden interruption ofpower into a multihour downward ramp.2 Short-termforecasts of wind power are critical in managing thesesituations. Large wind-power plants can be requiredto operate at partial loads during storm events to pre-vent large ramps. The impact can also be reduced bychanging the controls of wind turbines: preventing allturbines from shutting down during the same minute,and by reducing the output more slowly as winds in-crease over cutout wind speeds.

Wind energy forecasting can be used to predictwind energy variability in advance through a varietyof methods based on numerical weather-predictionmodels and statistical approaches. Wind forecast-ing has been developed since the 1990s and is stilldeveloping.28 The overall shape of wind generationcan be predicted most of the time, but significant



FIGURE 4 | Variability of wind power will smooth out with aggregation of wind power plants. Real data from Germany where the data arefrom the same time period and are normalized to the mean output of each group of wind turbines. (Reproduced by permission of ISET.)

FIGURE 5 | Predictability of wind power is better for shorter timehorizons and for larger areas/several sites. Example of averageabsolute prediction error for a single wind-power plant and fourdistribute wind power plants when forecasting horizon is from one to36 h ahead.

errors can occur in both the level and in the timingof wind generation. Wind forecast accuracy improvesfor shorter time horizons (Figure 5). There is a strongaggregation benefit for wind forecasting, as shown inFigure 6, aggregation over a 750 km region reducesforecasting error by about 50%. Typical wind fore-cast errors for representative wind power forecasts fora single wind project are 10–15% of installed wind

FIGURE 6 | Decrease of forecast error of prediction for aggregatedwind power production due to spatial smoothing effects. Errorreduction = ratio between root-mean-square error (rmse) of regionalprediction and rmse of single site, based on results of measured powergeneration of 40 wind farms in Germany. (Reproduced by permissionof Energy & Meteo Systems).

capacity but drop to 6–8% for day-ahead wind fore-casts for a single control area and to 5–7% for day-ahead wind forecasts for all of Germany [measuredas a root-mean-square error (RMSE)]. Combining dif-ferent wind forecasting models into an ensemble windforecast also can improve wind forecasting accuracyby up to 20%.1,28,29



FIGURE 7 | Results for the increase in reserve requirement due to wind power, presented as percent of installed wind capacity, for differentwind penetration levels.

Both accuracy and uncertainty of short-termforecasts, are important information for system oper-ators, when allocating the reserves needed to managethe real-time operation.

Frequency Control and ReservesThe reserve requirement addresses the more short-term flexibility for power plants that can follow un-predicted net load variations. Wind power will alsoincrease the need for flexibility for power plants thatcan follow the scheduled net load. Operational prac-tices, such as markets scheduling on hourly level orscheduling at 5–15 min, will also have an impacton how much reserves are needed during the op-erating hour. Experience shows that when reachingpenetration levels of 5–10%, an increase in the useof short-term reserves is observed, especially for re-serves activated on 10–15 min timescale. So far, nonew reserve capacity has been built specifically forwind power.8,30 In Portugal and Spain, new pumpedhydro is planned to be built to increase the flexibil-ity of the power system, and this is mainly driven bywind power to enable more than 15% penetrationlevels.11 In the highest wind penetration countries—Denmark, Spain, and Portugal—no significant fre-quency impacts have been observed that are the resultof wind-power variation.31

In wind integration studies, the increase inshort-term reserve requirement is mostly estimatedby comparing the reliability of the system before andafter the addition of wind. A basic approach is to com-bine the variability or forecast errors of wind powerwith that of load and in some cases also power plantoutages, and to investigate the increase in the largest

variations seen by the system. The range of resultsfor reserve requirement increase due to wind poweris wide (Figure 7): increased reserve capacity shouldbe 1–15% of installed wind power capacity at 10%penetration and 4–18% of installed wind power ca-pacity at 20% penetration.1 Timescales used in theestimation explain much of the differences in resultsas can be seen from German results from 2010 thatcalculate the reserve requirement based on either day-ahead, 4 h ahead or hour-ahead uncertainties.32 Ger-man Dena estimates only show the average day-aheaduncertainty (for up and down reserves separately).26

In Minnesota33 and California,34 day-ahead uncer-tainty has been included in the estimate. UK study35

combines the 4 h ahead variability of wind to loaduncertainty–using wind forecasting would give lowerresults.1 For others, the effect of variations during theoperating hour is considered,36 with Ireland37 andSweden38 including the 4 h ahead uncertainty sepa-rately. All studies show increasing trend of reserverequirements as wind penetration increases. Even ifthe aggregation benefits of wind power can decreasethe reserve requirements, in the studies it is often as-sumed that the smoothing effect reaches its maximumalready at lower penetration levels, and adding windin the same area will no longer decrease the variabilitynor forecast error levels.

With high wind penetration, it will be beneficialto allocate reserve requirements dynamically. If allo-cation is estimated once a day for the next day insteadof using same reserve requirement for all days, the lowwind days will induce less requirements for the systemand thus the reserve allocation can be increased onlyfor days when wind variability and uncertainty are athighest.15,30



Increasing reserve requirement is usually calcu-lated for the extreme cases. Even if there is increase inreserve requirements, this does not necessarily meannew investments for reserve capacity, rather genera-tors that were formerly used to provide energy couldnow be used to provide reserves.1

Electricity MarketsThere is good experience from Denmark, Spain, Ire-land, and New Zealand with balancing wind powervariations through forecasting and liquid day-aheadand balancing markets.39 For West Denmark, the bal-ancing cost from the Nordic day-ahead market hasbeen 1.4–2.6 €/MWh for a 24% wind penetration ofgross demand.1

There is already some experience on how windpower impacts the day-ahead electricity market pricesduring hours with a lot of wind, the market pricesare lowered as wind energy displaces power sourceswith higher marginal costs.40–42 At high wind penetra-tions, wind power will increase the volatility in mar-ket prices as wind energy will not always be availableto displace higher marginal cost generators.43 In thelong run, however, the average effect of wind energyon wholesale electricity prices is not as clear becausethe relationships between investment costs, operationand maintenance costs, and wholesale price signalswill begin to influence decisions about the expansionof transmission interconnections, conventional gener-ator retirement, and the type of new generation thatis built.10,40

BALANCING COSTS OF WIND POWER

Balancing costs reflect increased use of reserves andless efficient scheduling of conventional power plants.Impact on efficiency of conventional power plantsneeds simulations of power system scheduling anddispatch and they are mostly based on comparingcosts of system operation without wind and addingdifferent amounts of wind. The studies show a signif-icant reduction of operational costs (fuel usage andcosts) due to wind power even at higher penetrationlevels so the integration effort will not offset the emis-sion savings of wind power.44 To capture the integra-tion cost means capturing the difference of full creditfor operating cost reduction compared with cost forsystem operation with efficiency penalties due to in-creased variability and uncertainty. One way of cap-turing cost of variability is by comparing simulationswith flat wind energy to varying wind energy.33,45

However, the two simulated cases can also result inother cost differences than just the variability cost.46

Increase in balancing costs at wind penetrationsof up to 20% amounted to roughly 1–4 €/MWh ofwind power produced.1,33,35–37,45,47–51 If intercon-nection capacity is allowed to be used also for balanc-ing purposes, the balancing costs are lower comparedto the case where balancing is made only in the area—increasing area size will have aggregation benefits ofwind power and also add more balancing power. Thetwo points for Greennet Germany and Denmark45 atthe same wind penetration level reflect that balancingcosts increase when neighboring countries get morewind52 (Figure 8).

Not all case studies presented results quantifiedas MW of increase in reserve requirements or mone-tary values for increase in balancing costs. The IrishAll Island Grid Study shows that the net benefit for thepower systems, going from 2 to 6 GW wind, will be€13/MWh, as the operational costs of the electricitysystem fall compared to the base case.18

CURTAILMENTS OF WIND POWERGENERATION

Challenging situations seen in system operation sofar are from high wind-power generation during low-load situations, when wind power penetration lev-els exceed 50%.11 Wind is usually last to be cur-tailed. However, when all other units are alreadyat minimum (and some shut down) system opera-tors sometimes need to curtail wind power to controlfrequency.8 Denmark has solved part of the curtail-ments by increasing flexible operation of combinedheat and power plants and by lowering the minimumgeneration levels used in the thermal plants.1

In Ireland, some curtailments have been due toconcerns of low inertia53 and consequently suscep-tibility to instability in the system because of highinstantaneous wind penetration and low system load.Currently, the issue of low inertia is unique to smallsystems such as Ireland and Crete in Greece.3 In Ire-land, they are working on solutions to go up to 75%penetration levels.17

Does Wind Integration Need Storage?Storage is nearly always beneficial to the grid, but thisbenefit must be weighed against its cost. All variationand uncertainty in power systems is being handledin power system level. This is because of lower costswhen variability is aggregated before being balanced.Storage is most economic when operated to maximizethe economic benefit for the entire system. Additionalwind generation could increase the value of energy



FIGURE 8 | Results from estimates for the increase in balancing and operating costs due to wind power for different wind penetration levels.The currency conversion used here is 1 € = 0.7 £ and 1 € = 1.3 US$. For UK, according to a 2007 study, the average cost is presented here, therange in the last point for 20% penetration level is from 2.6 to 4.7 €/MWh.35

storage in the grid as a whole, but storage wouldcontinue to provide its services to the grid.2

There are many studies that specifically lookedat the cost effectiveness of electricity storage to as-sist in integrating wind.54–58 These studies found thatfor wind penetration levels up to 30% the cost effec-tiveness of building new electricity storage is still lowfor other options than hydropower with large reser-voirs and pumped hydro. With higher wind penetra-tion levels the extra flexibility that energy storage canprovide will be beneficial for the power system op-eration, provided it is economically competitive withother forms of flexibility, e.g., from thermal powerplants and demand side.

WIND IMPACTS ON TRANSMISSIONGRID

Grid planning for future wind energy targets bringsneeds to reinforce the grid as well as building newlines, both inside the country/region as well as inter-connection to neighboring countries/regions. Trans-mission is important both for enabling transfer ofthe generated electricity to loads but is also requiredto gain aggregation benefits in variability and uncer-tainty of wind power. To assess the impacts of windpower to the transmission grid involves steady-stateload flow and transient stability simulations of thenetwork for specific snapshots situations. Networkcontingency situations are studied to meet the criteriaof power system operation and safety established bythe system operator.

Wind power is normally not the only drivingforce for grid investments but it is a major fac-tor (Ireland,59 Germany,60 Europe,61 and the UnitedStates,62).24 The cost of grid reinforcements dueto wind power is very dependent on where thewind power plants are located relative to load andgrid infrastructure. Portugal reported million 145 €

(70 €/kW) increase in grid infrastructure investmentsin the period 2004–2009 for increasing wind pene-tration from 3% (1400 MW) to 13% wind energypenetration (3500 MW).24 In several studies grid re-inforcement costs roughly vary from 0 €/kW to 270€/kW reflecting different systems, countries, grid in-frastructure, and calculation methodologies.1

A challenge for transmission planning is to re-solve the scheduling conflict where wind plants canbe permitted and constructed in 2–3 years and itmay take 5–10 years to plan, permit, and constructa transmission line. Some transitional solutions canallow wind power plants to connect to the existinggrid even if there will be times when the grid is notstrong enough to transmit all generation produced.3

Dynamic line ratings, taking into account the coolingeffect of the wind together with ambient temperaturein determining the transmission constraints, can in-crease transmission capacity and delay the need fornetwork expansion. By curtailing the generation incritical situations, grid equipment such as overheadlines or transformers can be protected from over-loads. As wind power will produce most of the timeat part load, the critical situations often result in onlysmall production losses and in these cases it can be



FIGURE 9 | Capacity credit of wind power, results from eight studies.1,26,33,66,67 The Ireland estimates were made for two power systemconfigurations, with 5 GW and 6.5 GW peak load.

cost effective to curtail and thus lower the connectioncosts.63 However, these transitional solutions are in-sufficient for large amounts of wind power and canresult in high curtailments, such as in Texas with 17%of all potential wind energy generation curtailed in2009.64

Wind Power Impacts on StabilityThere will be technical and operational implicationsfor the power system at times of high shares ofwind power. The power system should sustain dis-turbances, such as the loss of largest power plant orline so that the frequency and voltage remain stable.Wind power is asynchronous generation that does nothave the same inherent, physical support to the powersystem inertia as synchronous machines. In the smallisland system of Ireland, the issues to enable morethan 30% wind penetration level has been studied.17

The issues that could be mitigated were power bal-ancing with instantaneous reserves, voltage stability,transient, and small-signal stability. The fundamen-tal issues regarding frequency stability when losinga large unit or power line as well as large amountsof wind power tripping during network faults needfurther analyses.

CAPACITY VALUE OF WIND POWER

Power system planning includes determining genera-tion capacity needs for the future. Wind power is oftenconsidered as an energy source, but it can also provide

some capacity to be relied upon during peak loads.Ensuring power adequacy is typically done using reli-ability analysis, which is based on loss of load proba-bility (LOLP) or loss of load expectation (LOLE). Theuse of these approaches allows the system planner todetermine the power adequacy level and also the con-tribution that each generating plant makes towardresource adequacy, called capacity credit or capacityvalue.2,65 The availability of high-quality chronolog-ical synchronized data that captures the correlationwith load data is of paramount importance and therobustness of the calculations is highly dependent onthe volume of this data.

So far, wind power has been built as additionalgeneration to power systems, and thus no problemswith having adequate power capacity to cover peakloads have been reported.8

The results from studies estimating the capac-ity value for wind power range from 5% to 40%of wind-rated capacity (Figure 9). The wide range ofcapacity credit assigned to wind reflects the differ-ences in the timing of wind energy delivery (when thewind blows) relative to system peak loads.66 Aggre-gating larger areas benefits the capacity credit of windpower.1,67

In some reports the term ‘capacity cost’ is used.The meaning of this is the cost for the differencebetween lower capacity credit for wind power andhigher capacity credit for a conventional power plant.It is not straightforward to calculate how a ‘re-duced value’ transforms to a cost for wind power.It is important to use the lowest investment cost



generating capacity as back-up not to overestimatethis cost.68

CONCLUSION AND OUTLOOK

The natural variability of wind power makes it dif-ferent from other generating technologies. This raisesquestions about how wind power can be successfullyintegrated into the grid. Power systems are equippedto handle variability and uncertainty that comes fromthe load. There is already a lot of experience of op-erating power systems with large amounts of windpower and integration studies have also offered valu-able insights into how high wind penetrations can besuccessfully achieved. Model and tool development isstill required to study how entire power systems canbe operated with occasionally high penetration levelsat 60–80% of load.

Wind integration means investments in grid in-frastructure and increased use of balancing powerthat can lead to efficiency losses in power plants.There will be significant reduction of operationalcosts (fuel usage and costs) due to wind power even athigher penetration levels so integration effort will not

offset the emission savings of wind power. The capac-ity value of wind power is less than for conventionalpower plants and will reduce at higher penetrationlevels.

Reaching high wind penetration levels chal-lenges generation owners and transmission operatorsto better utilize technology and existing assets to pro-vide flexibility. More flexibility from the nonwindgeneration fleet would include reduced minimum gen-eration levels, greater ramp rates, quicker start times,and designs that allow frequent cycling without in-creasing material fatigue or reducing component life-times. This will require incentives or requirements andmarkets and tariffs also need to be designed to rewardincreased flexibility. Integrating wind power genera-tion into power systems can be aided by enlargingbalancing areas and moving to subhourly scheduling,which enable grid operators to access a deeper stackof generating resources and to take advantage of thesmoothing of wind power output due to geographicdiversity. The on-going developments on smart grids,demand response, and plug-in hybrids as well as con-tinued improvements in new conventional-generationtechnologies will give new opportunities for wind in-tegration.

ACKNOWLEDGMENTS

This work is part of International Energy Agency Implementing Agreement for Wind Energy(IEAWIND) research collaboration Task 25 Design and Operation of Power Systems with LargeAmounts of Wind Power http://www.ieawind.org/AnnexXXV.html. All participating membersare greatly acknowledged for providing information on wind integration studies and experience.Juha Kiviluoma is acknowledged for his comments on the text.

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Wind integration: experience, issues, and challenges