european balancing act

90 IEEE power & energy magazine november/december 2007

by Thomas Ackermann,Juan Rivier Abbad, Ivan M. Dudurych,Istvan Erlich, Hannele Holttinen,Jesper Runge Kristoffersen, and Poul Ejnar Sørensen

1540-7977/07/$25.00©2007 IEEE

European Balancing Act

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november/december 2007 IEEE power & energy magazine 91

WWHEN DISCUSSING HIGH WIND POWER PENETRATION IN POWER SYSTEMS, THEpossible impact of variable wind power production on power system balancing and frequencycontrol is typically of concern. There are two dimensions to the problem: an economical onerelated to optimization of the resources and a fair sharing of the cost, and a technical one relatedto security of supply. As Europe has some of the highest wind penetration levels in the world,and high targets to increase the share of wind power, European experience and the approachestoward balancing and frequency control are of general interest. This includes market-basedapproaches, i.e. organization of balancing markets, as well as technical solutions such as usingwind farms to provide balancing and frequency services.

Wind Impact on ReservesPower systems have to deal with uncertainty in both consumption and production. To managethis, reserves are kept in power plants. The reserves are then used for up or down regulation dur-ing the operating hour to keep the consumption and production in balance. Up and down regula-tion is used to maintain system balance in a control or balancing area. As the power systemconsists of thousands of individual consumer and production units, there is a great benefit inoperating the power system so that only the net imbalance needs to be controlled.

Wind energy brings more variability to the power system. Part of this variability can be fore-casted some hours or days ahead. The uncertain part of the variability is managed with reservesin the power system. During the operating hour, the imbalance of wind is merged with all otherimbalances in the power system—wind power does not need dedicated backup.

When large amounts of wind power are planned, the question that arises is how much windpower impacts the reserves in the power system. There are several issues related to reserves thathave to be estimated:

✔ allocation of reserves: how much capacity, e.g., megawatts, needs to be allocated for eachday, a decision that needs to be taken beforehand by the system operator to ensure an ade-quate amount of reserve is kept in power plants

✔ use of reserves: how is the utilization time of the reserves impacted during the operatinghour after wind power is introduced.

Both the allocation and use of reserves can incur increased costs for the power system. Theincrease in the use of reserves is seen first, as the experience in countries with large amounts of

wind power has proven. In many European countries,

wind power imbalances are treat-ed in balance settlement after theoperating hour, like all other pro-duction and consumption.Through the imbalance costs,wind power producers will seethe cost incurred thoughincreased use of reserves (exceptin those countries where trans-

mission system operators (TSOs) cover the imbalance costs, such as Germany and Denmark).However, in most countries the technical costs that have actually been incurred are not directlyallocated to actors, but there are penalties imposed. This means that in some countries, windpower producers are paying more in imbalance costs than they are actually causing for thepower system.

Allocation of ReservesAn increase in the total amount of capacity (MW) that needs to be allocated to reserves does notnecessarily mean an increase in costs of operating the power system. For example, the Nordicpower system (Denmark, Sweden, Norway, Finland) has contracted reserves for disturbances

Digital Object Identifier 10.1109/MPE.2007.906306

Impact of High WindPenetration on Balancingand Frequency Control in Europe

and second-to-second variations (primary reserves). Windpower is not foreseen to impact these reserves. The impact ofwind power is seen in the ten-minute balancing market thatprovides slower reserves for frequency control, which hasbeen organized as a market. All producers bid all their flexi-bility during the operating hour into a common pool. As theNordic systems contain considerable amounts of flexiblehydro power, there is still room for increasing wind power inthe market. This will not result in extra costs to the powersystem. The highest costs will occur when wind power pene-tration in a power system is so high that there are no morereserves available in power plants, and then allocating morereserve means building more flexible capacity.

Use of ReservesThere are different ways of estimating the incrementalincrease in reserves caused by wind power. The megawattamounts of reserves can be estimated by probabilistic meth-ods combining the uncertain load and wind variations.Sometimes also the disturbance or contingency reserveneeds are combined. The cost can be based on technicalcosts or market/opportunity cost.

In estimating the amount of variability of wind power, thesize of balancing area and the time scale of prediction errors areimportant. The amount of uncertainty in variations of windpower decreases when the time horizon for forecasting decreas-es. Larger balancing areas reduce the variability of wind andalso improve the forecasting of wind. This is clearly reflected incomparisons of balancing costs made by the IEA WIND Task25 working group, as shown in Figure 1 and Figure 2.

At wind penetrations of up to 20% of gross demand (ener-gy), system operating cost increases arising from wind variabili-ty and uncertainty amounted to about 1–4 ¤/MWh. This is 10%or less of the wholesale value of the wind energy. It can be seenthat there is considerable scatter in results for different countriesand regions. The following differences have been noted:

✔ Different time scales used for estimating. For the Unit-ed Kingdom, the increased variability to four hoursahead has been taken into account. For the Nordiccountries and Ireland, only the increased variabilityduring the operating hour has been estimated. For theGreennet study, the unit commitment and reserve allo-cation is done according to wind forecasts, but the sys-tem makes use of updated forecasts to adjust theproduction levels three hours before the delivery hour.

✔ For the Greennet-EU27 study and the SEI Irelandstudy, only the incremental increase in operating costshave been estimated, whereas investments for newreserves are also included in some results.

✔ The Greennet-EU27 and Nordic studies incorporate thepossibilities for reducing operation costs through powerexchange with neighboring countries, whereas the Ger-man dena study as well as the Sweden, United King-dom, and Ireland studies analyze the country inquestion without taking transmission possibilities intoaccount. The Nordic 2004 results show the benefit oflarger market areas and control areas in providing bal-ancing. The results for the Nordic market clearly showthat there are significant benefits in balancing windpower in larger areas, which allows for smoothing of

the wind plant output variability.

The Spanish Case Installed wind energy capacity in Spainhas grown at an average rate of around1.5 GW per year in the past few years,reaching 11.6 GW by December 2006.The government would like to reach atotal wind installed capacity of 22 GWby 2010. As a reference, the system peakdemand is currently 44 GW, while theminimum load is 20 GW. AlthoughSpain is connected to the rest of theUCTE system through France, the totalcapacity of interconnection is almostinsignificant (around 2.5%). Today, windranks second in the Spanish generation

figure 1. Comparison of reserve requirement versus wind penetration based ondifferent studies.

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ityThe Spanish wholesale market is organized as a succession ofseveral markets until it reaches real time to provide for correctbalancing of the demand and generation, similarly to other systems.


technology mix, after the combined-cycle gas turbine. The reg-ulatory framework for wind energy is trying to find a correctequilibrium between the incentives to attract more wind in orderto fulfill the objectives of installed capacity and the necessity tobetter integrate it in the system.

Spanish Balancing MarketsThe Spanish wholesale market is organized as a successionof several markets until it reaches real time to provide forcorrect balancing of the demand and generation, similarlyto other systems. The first market is the day-ahead market,where each of the 24 hours of the next day is settled (previ-ous to this market is the derivatives market). After this set-tlement, there are six intraday markets that allow anyparticipant to correct his position. The longest periodbetween two different intraday markets is six hours. Fromreal time to here, four different steps are used to balancethe load, constituting the “balancing markets.” The firststep is the primary frequency control, which is mandatoryfor any “ordinary” generator and is based on an automatic,short-time response (seconds), in order to stabilize the fre-quency. The second step is the secondary reserve, whosefunction is to reestablish the frequency. There is a marketmechanism to determine which generator is available andwith what megawatt range, for both increases and decreas-es in production. The energy that is actually used to recoverthe frequency is dispatched automatically, based on theavailable band. The third step is the tertiary reserve, whichis a market for energy that hasto be available in 15 minutesupon request and maintainedfor at least two hours, andwhose objective is to replacethe secondary reserve. Besidesthese three permanent mecha-nisms, the SO can call upon aspecific “deviations market” incase he detects in advance alarger unbalance than usual fora specific hour.

The penalty for an unbalance isasymmetric: only the unbalancesof the same sign as the systemtotal net unbalance are penalized.The penalty is calculated as theaverage price resulting from thedifferent balancing markets.

Spanish Wind Energy RegulationThe uptake of wind energy in Spain has happened when afeed-in-tariff (FiT) was put in place in 1998 (Spanish RoyalDecree 2818/1998). This regulation (R1) includes all thetypes of renewable energy source (RES), although most of itis basically wind energy. The success of this regulation, atleast in terms of installed capacity, was not only due to theeconomic return guaranteed by such regulation but alsobecause the technical conditions were particularly good. Anywind power facility smaller than 50 MW could be connectedto the grid without any regard to the variability of windpower. For each wind farm connected under R1, each kWhproduced was remunerated at a fixed rate. The only signifi-cant technical restriction was to keep the power factor equalto one under all circumstances. Distribution companies had toinclude in their bid to the market their own forecast of thewind power output connected to their network (as a negativeload). They were responsible for the total error of their bid,which included the RES forecast error. Most of the agentsparticipating in the electricity market were using wind fore-casting tools, except the wind power facilities themselves.

To help improve integration, the Spanish government imple-mented a new regulation in 2004 (Spanish Royal Decree436/2004). This second regulation (R2), in place until recently,tried to better integrate RES in the existing system. It introducedseveral additional technical obligations on the RES:

✔ For any wind power installation larger than 10 MW,mandatory prediction of its power output (hourly

figure 2. Comparison of balancing cost versus wind penetration based on differentstudies.

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There are two dimensions to the problem of integration:economical, related to optimization of the resources and a fairsharing of the cost, and technical, related to security of supply.


production for a full day). This prediction had to begiven 30 hours before the beginning of the day of theprediction (18.00 of day D-2). This prediction can bechanged until one hour prior to each intraday market.If the error (absolute difference between productionand prediction in one hour) was greater than 20% (inthe case of wind generators), then the wind farmoperator would have to pay a penalty equivalent to10% of the average estimated total cost (¤/kWh) perkWh of deviation.

✔ RES generators still did not have any obligation interms of voltage control at their point of common cou-pling. But they had an incentive to keep their reactiveenergy production within certain limits that varieddepending on the level of load of the system.

✔ Although there was no mandatory obligation to installfault ride-through (FRT) capability either in new orexisting wind parks, an economic incentive to do sowas put in place.

To complete this switch to a better integration scheme,the new regulation added an alternative to this scheme:instead of receiving the FiT for their production, any RESbigger than 1 MW could integrate in the wholesale marketto sell its energy and participate in the balancing markets.On top of their market income, these generators wouldreceive a fixed premium for each kWh produced. This way,

any generator that had chosen this sec-ond option would be completely inte-grated in the existing system, as theyreceived all the economic signals thatwere in place to manage short-termuncertainty and network costs, throughall the successive electricity markets.These fully included the market costsassociated with the prediction error(deviation costs), with the exception thatRES neither had to participate in nor payfor the secondary reserve band.

An additional difference from a “nor-mal” generator was that any RES installa-tion could access the market through anintermediary that could act as an aggrega-tor of different RES installations. Thisaggregator could play an important role tohelp minimizing the balancing costs asso-ciated with participation in the wholesalemarket. To compensate the RES for being

exposed to market signals, which implies more risk andknowledge, and to recognize the market benefits thisapproach should have, the premium was theoretically calcu-lated so that an average RES would have a slightly greaterincome than if on the standard FiT.

In Spain, since the beginning of this regulatory frame-work, more than 95% of wind farms have chosen to switch tothe market plus premium scheme, following a considerableincrease in the wholesale electricity prices that have greatlyimproved the expected incomes for this option.

Next Steps in SpainOn 26 May 2007, a new regulation was published (Spanish RoyalDecree 661/2007). This third regulation (R3) tries to strengthenthe technical integration of all the wind farms, while at the sametime somehow control the earnings of the “market plus premium”option. There are five main changes from the previous regulationfrom a technical point of view:

✔ All facilities, whether they choose the feed-in-tariff or themarket plus premium remuneration option, have to par-ticipate in the market, which means forecasting theirproduction, making bids in the day-ahead and intradaymarkets (zero value in case of feed-in-tariffs), and pay-ing fully the balancing costs: no more dead band with-out penalty as before. Definitely, the previousexperience with individual forecasting was positive.

figure 3. Development of wind power installation connected to distributionas well as transmission networks in Ireland.

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Because the power balance is unlikely to be maintained within theisland and wind turbines usually do not provide the required controlservices, separation from the grid is recommended.

✔ FRT capability is now mandatory for both new and oldinstallations, unless it is proven that it is impossible toadapt them (only in case of old installations).

✔ The incentive/penalty associated with reactive energyin unchanged, but now the SO can ask any facility big-ger than 10 MW to temporarily change their settings,the incentive being large or small in case of compli-ance or non-compliance.

✔ Noncontrollable RES technologies are explicitly for-bidden from participating in the balancing markets.

✔ All the facilities bigger than 10 MW have to be associ-ated with a generation control center that would act asan intermediary between those generators and the SO,both for transmitting real-time information and real-time orders.

The Two DimensionsAs mentioned before, there are two dimensions to theproblem of integration: an economical one related to opti-mization of the resources and a fair sharing of the cost,and a technical one related to security of supply. As windgeneration increases its penetration, it will be asked to actas a balancing technology. We only need the correct mar-ket mechanism to send the needed economical signals,since from a technical point of view the response of thewind energy generation is very fast. The FRT capabilitysolves a technical problem of potential instability. Themandatory forecast (with strong economic incentive toperform well) will also transform a technical problem ofstability due to possible imbalances into an economicalproblem dealt with by markets already in place.

The Irish CaseThe number of wind turbines connected to the Irish electricalpower system has been increasing rapidly over the last num-ber of years, as shown in Figure 3.

By February 2007, 790 MW of wind generation hadbeen connected to the Republic of Ireland’s (RoI) powersystem, equal to about 12% of the total installed capacityin the country. By the end of 2006 about 42.7% of thenationwide installed wind generation capacity was con-nected to the transmission system, which is estimated toreach 47.4% in 2007. The peak demand in the Irish systemis around 5,000 MW, and the minimum demand is around1,800 MW.

Wind power installation in Ireland will continue to for the fol-lowing reasons:

✔ Ireland has signed the Kyoto Protocol in order to con-tribute to meeting its obligations and is supportingwind generation in order to reduce greenhouse gases(one of the biggest emitters of such gases are conven-tional power stations)

✔ Directive 200/77/EC of the European Parliament andthe Council of 27 September 2001 on the proportion ofelectricity produced from renewable energy sources

has set a target for European Union states that 22% ofEurope’s electricity needs should be produced fromrenewable sources. To meet Ireland’s target, 13.2% ofthe primary electricity needs should come from renew-able sources by 2010.

Characteristics of Wind Power Generation in IrelandFigure 4 shows the schematic layout of the wind farmlocations in Ireland. The majority of the wind farm instal-lations are in the west of Ireland, based on wind resourcesavailable.

The rather significant concentration of wind generationin one geographical area reduces the “smoothing effect” ofthe aggregated wind power output that can be observed inother countries with more equal geographic distribution ofwind power over a larger area. Hence, the variability ofwind farm output can have a larger impact on the powersystem behavior than in other countries, in particular, con-sidering the island characteristic of the Irish power systemdue to almost insignificant transmission connections toother power systems.

Grid Integration IssuesIn order to accommodate wind generation on the Irish powersystem, extensive research and analysis work has been car-ried out by the transmission system operator. The followingmain issues have been identified and specified as grid coderequirements for wind power stations (WPSs):

figure 4. The layout of wind farm location in Ireland.


✔ FRT requirements✔ system frequency and frequency

response requirements✔ transmission system voltage and re-

active power capability requirements✔ communication requirements.

FRT RequirementsTo preserve the current level of dynamic sta-bility of the power system, a WPS shouldremain connected to the transmission systemduring voltage dips on any or all phases in thetransmission system, as long as the voltagemeasured at the high-voltage terminals of thegrid-connected transformer of the WPS, or inother words at the common coupling point(CCP), remains above the solid line in Figure5. This figure was obtained as a result ofanalysis of actual faults on the Irish powersystem as well as simulations. These showthat due to low short-circuit levels, voltagedips below 15% occur in large areas duringfaults, especially in regions where a lot ofwind farms are concentrated. The duration ofvoltage dip during which the WPS shouldcontinue to be connected to the transmissionsystem is determined by the largest possiblefault clearance time on the system.

All WPSs connected to the transmissionsystem should comply with FRT require-ments. Figure 6 gives an example of a dou-bly fed induction generator (DFIG)-basedwind farm operation during a three-phasefault at an adjacent 110-kV line that leads to70% voltage dip at the CCP. This particularWPS has separately installed switchablecapacitor banks (CBs) that support voltageat the CCP and enhance FRT capability ofthe wind farm. Without CBs, the wind farmwould have drawn reactive power from thenetwork, bringing voltage in the CCP down.

Frequency ResponseRequirementsWPS should have the capability to:

✔ operate continuously at normalrated output at transmission sys-tem frequencies in the range of49.5 Hz to 50.5 Hz

✔ remain connected to the transmis-sion system at frequencies within therange of 47.5 Hz to 52.0 Hz duringsystem disturbances. Such require-ments do not differ from thoseapplied to a conventional plant.

figure 6. DFIG-based wind farm operation during three-phase fault at adja-cent 110-kV line.

figure 7. Frequency response capability of wind farm.

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figure 5. Fault ride-through capability of wind farms according to grid code.

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IEEE power & energy magazine november/december 200796

The WPS should becapable of operating eachWTG at a reduced level, ifthe WPS’s active poweroutput has been curtailedby the TSO for systemsecurity reasons.

The frequency responseof WPS with maximumexport capacity (MEC) ofmore than 5 MW shouldhave the capabilities as dis-played in Figure 7. Undernormal transmission systemfrequency ranges, the windpower station should operatewith an active power outputas set by the line “A”–“B.” Ifthe transmission system fre-quency falls below point “A,”then the frequency responsesystem should act to ramp upthe WPS’s active power out-put, in accordance with thefrequency/active power char-acteristic defined by the line“B”–”A.” Where the systemfrequency is below/above thenormal range and is recover-ing back toward the normalrange, the frequency responsesystem must act to rampup/down the wind power sta-tion’s active power output inaccordance with the frequen-cy/active power characteristicshown in Figure 7. The set-tings (points A and B in thisfigure) may be different foreach WPS depending on sys-tem conditions and WPSlocation.

Currently eight WPSsconnected to the high-volt-age system and eight WPSsconnected to the distribu-tion system, representing

figure 8. Frequency fluctuations versus wind power output fluctuations for (a) high windpenetration and (b) moderate wind penetration.

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Research activities are focusing on identifying anddeveloping/testing new control structures for wind generators thatare best suited to provide frequency support during a disturbance.


56% of total installed wind generation, are equipped withfrequency response features and their settings are controlledfrom the National Control Centre.

Wind Power Output and Frequency ControlDue to the island characteristic of the RoI power system andthe high concentration of wind power in one area, frequencycontrol issues are studied in particular for low load/high windsituations. In Figure 8(a), there is an example showing thedirect effect of wind power variation on frequency variation inIreland. Here the wind power output was between 50% and70% of the total installed wind power capacity. For compari-son, wind power output versus frequency profile is shown inFigure 8(b) for the same day in the previous week, when windpower output was moderate. It can be seen that frequencyvariations increase in situations with high wind penetration. Atpresent such changes force power system operators to moveresponsive plants more frequently then previously. Increasingwind power penetration levels can, during low load/high windconditions, potentially lead to unwanted underfrequency pro-tection operation, spurious load shedding, and operatingreserve actuation.

The German CaseThe overall installed wind power generation capacity inGermany at the end of 2006 stood at 20,621 MW. In 2006,wind power provided about 6% of the electricity demandand it is envisaged to rise to at least 12.5% by 2012, and toat least 20% by 2020. Preliminary studies foresee furthersignificant increases in the timeframe extending to 2050.Consequently, in addition to the current large number ofonshore sites, many offshore sites are in the planning orimplementation stages. The installed capacity of the off-shore plants in the North and Baltic seas is expected toreach 2–3 GW by 2010. This figure is projected to rise to20–25 GW by 2030.

Grid Integration IssuesThe predicted increase of the already high wind penetrationin Germany results in the need to involve wind turbines in theoverall reactive power generation, voltage control, and fre-quency control processes. Depending on the level of windgeneration, wind plants might be required to supply reactivepower over a wide operating range in response to the chang-ing load flow configuration.

figure 9. Definition of FRT requirements in Germany.

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FRT RequirementsFRT requirements are described basedon a time voltage diagram (see Figure9), which does not contain characteris-tic voltage behavior but border lines.According to the proposed require-ments, wind turbines have to stay onthe grid at voltages within areas 1 and2 shown in Figure 9. However, if windturbines face overloads, stability, orother kinds of technical problems inarea 2, they can interrupt the connec-tion to the grid if resynchronizationtakes place within 2 s [short-term inter-ruption (STI)]. They must also be ableto increase the active power afterresynchronization by gradients of atleast 10% of the nominal power persecond. Wind turbines with muchfaster STI cycles can already initiateSTI at higher voltage levels under thefollowing conditions:

✔ the interruption time is ITSTI � 2 s✔ reactive current in-feed continues during the interrup-

tion period; e.g., by the grid side front-end converter orby other equipment.

Wind turbines remaining on grid during faults have toreturn to total active power in-feed with gradients of about20% IPnom/s. Often, active power generation is reduced bythe converter control temporarily during the low-voltage peri-od. This allows the further increase of reactive power genera-tion. After the fault period, a fast return to normal activepower generation is essential to ensure power balance withinthe grid, and thus frequency stability.

Grid faults resulting in voltage sags in area 3 affect windturbines considerably. Therefore, a short disconnection fromthe grid is allowed in this area. However, within the next 2 s,resynchronization is always required. When the voltageremains low for longer than 1.5 s, stepwise tripping of windturbines is allowed.

Frequency Response RequirementsAccording to the 2006 E.on grid code, wind turbines nowhave to stay connected to the grid within a frequency rangefrom 47.5 Hz to 51.5 Hz. Beyond these limits, separationwithout any time delay is required. However, wind turbineshave to begin reducing their power output at frequencies ofabout 50.2 Hz, as shown in Figure 10.

When the wind turbine terminal voltage increases up to120% of the maximum permanently allowed terminal volt-age, disconnection with a time delay of 100 ms is necessary.When the voltage falls below 85% of the grid nominal volt-age and the reactive power flow is directed to the wind farm,i.e., the wind farm is consuming reactive power, and the windturbines have to be disconnected after 0.5 s delay.

The conditions of this rule refer to the wind farm con-nection points. However, disconnection has to be made atthe wind turbines directly in order to ensure fast restoration.Taking into account the direction of reactive power flow, theconditions also provide for monitoring of the voltage sup-port requirements. Assuming that the voltage at the windturbine terminal nodes falls below 80% of the minimumpermanently allowed voltage, disconnection of wind tur-bines is required in time steps of 1.5 s, 1.8 s, 2.1 s, and 2.4s. In each time step, 25% of the wind farm units have to betripped if the voltage doesn’t increase again to about 80%.

Figure 11 provides an overview of the voltage and fre-quency monitoring and protection functions, respectively.

After disconnection due to violation of voltage and fre-quency limits, resynchronization can take place once the volt-age increases again to about 105 kV in 110-kV-networks, toabout 210 kV in 220-kV-networks, and to 370 kV in 380-kV-networks. In this case the maximum power gradient allowedis about 10% per minute of the contracted grid capacity.

In the subsequent protective switching actions, windfarms might remain separated from the grid. However, stableoperation of islands presupposes the balance between gener-ation and consumption is maintained, as well as the voltageand frequency control capabilities of the remaining generatorunits. Because the power balance is unlikely to be main-tained within the island and wind turbines usually do notprovide the required control services, separation from thegrid is recommended. Wind turbines as a rule will be trippedby voltage and frequency relays due to violations of the cor-responding limits. However, when the circuit breakers con-necting the wind farm to the grid trip, shut-down signalshave to be sent to the wind turbines, too. Then, island opera-tion has to be terminated within 3 s.

figure 10. Frequency characteristic of wind power generation.

fNet

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at 50.2 Hz ≤ fNet ≤ 51.5 Hz

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Wind Turbine Control Options for Frequency ControlThe upcoming utility scale offshore wind farm developmentsand their integration into the interconnected power system arebound to add a new dimension to the challenges to beaddressed by power engineers. Hence, research activities arefocusing on identifying and developing/testing new controlstructures for wind generators that are best suited to providefrequency support during a disturbance.

For the evaluation of different wind turbine controlapproaches supporting frequency control, a 137-bus test sys-tem was used. The results of the simulations are summarizedin Figure 12. In all cases, the loss of generation is assumed tobe 2.5% of the total installed generation, and the steady-statefrequency drop for this amount of loss of generation, with nowind generators connected to the system, is 1,000 MHz. Thecomparison is between a situation purely based on conven-tional power plants and a situation based on 51.9% wind gen-eration. At lower wind penetration levels, the impact ofwind-power-supported frequency control is less obvious.

The result clearly presents two contrasting pictures. In theinitial phase of the disturbance, the high wind power share

based on DFIGs leads to a much quicker frequency dip.However, the kink in the frequency characteristic occursmuch earlier and the frequency generally settles at a highersteady-state value. With only conventional power plants,however, the frequency drop goes on longer and a steady-state deviation of 1 Hz is reached after approximately 60 s.

Providing Balancing and Frequency SupportFor the majority of wind turbines developed in the last centu-ry, the automatic control of wind power installations wasimplemented in the individual wind turbines, and the mainaim of the wind turbine controllers was to ensure maximumproduction, minimum mechanical stress, and to meet noiseemission limits. Remote control and wind farm monitoringsystems were developed at an early stage, but the main aimwas to monitor the wind turbines and provide remote manualcontrol access to shut-down and start-up functions.

In the current century, the wind turbine and wind farmcontrol systems have been equipped with several new fea-tures supporting the grid integration of the wind power. Theindividual wind turbine controllers now have FRT controlcapabilities that enable the wind turbines to stay connected


figure 11. Overview of system monitoring and system protection functions.

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Conditions:

• Reactive Power Flow Directed to Wind Farm

• Voltage Dip Below 85%

Tripping of WT within 100 ms

Frequency Exceeds Limits 47.5 Hz – 51.5 Hz

Power Reduction About 50.2 Hz

Fault Ride Through

Contribution to Voltage Maintenance

Contribution to Frequency Maintenance

Connection Point

Conditions: Voltage About 120% or

during and after grid faults in the power transmission system.For normal conditions, new features have been added to windturbine controllers. The wind turbines have active and reac-tive power set-points available for external control. These set-points are used by wind farm controllers to support the powerbalancing and frequency control functions in the power sys-tem. The most significant step inthis development so far is thewind farm controller for the firstlarge offshore wind farm, theDanish Horns Rev wind farm.The Horns Rev wind farm con-sists of 80 Vestas V80 (2 MW)wind turbines with the DFIGtechnology.

An example of a “normal” dayof operation of the Horns Revwind farm is illustrated in Figure13. At about 1:10, the frequencycontrol is activated with a spin-ning reserve that can be used laterin case of underfrequency. Thiscauses the actual power genera-tion of the wind farm to decreasebelow the theoretical possiblepower production. At about 1:40,a manual balance order is issued,

causing the power to be reduced to 20 MW. After a few minutesa new order is sent, and shortly after that the balance control iscancelled again. The frequency control still keeps about 10 MWin reserve for the next hours. The dip of power at about 2:10 isactually caused by the frequency control reducing the powerbecause a fast frequency rise occurred at that time. At about

figure 13. Balance control and reservation for frequency control at the same time.

figure 12. Comparison of the frequency deviations after a 2.5% loss of generation.

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3:30, a new set of balance ordersis issued. From about 4:00 andtwo hours beyond, the power isreduced significantly because ofoverproduction in the grid. Thenthe balance control and frequen-cy control are cancelled, and thewind farm returns to normaloperation.

The power producer Elsamowned and operated the HornsRev wind farm from the com-missioning in 2001 to 2006,when 60% of the wind farm wastaken over by Vattenfall. In thatperiod, the wind farm main con-troller was integrated withElsam’s power balancing con-trol side-by-side with conven-tional power plants, asillustrated in Figure 13. Based


figure 15. Use of Horns Rev wind farm main controller to reduce the power balanceerror and thus meet the order from the transmission system operator.

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The Horns Rev wind farm main controller has operated as an integrated part of the central control, ensuring the powerbalance in the system.

figure 14. Elsam’s power balance control based on conventional power stations and Horns Rev wind farm main controller.

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on the difference between agreed and actual cross-borderflow, and the frequency in the UCTE system, the Danishtransmission system operator Energinet.dk provided Elsamwith a correction that was added to Elsam’s scheduled pro-duction. This sum provided the set-point for the central pro-duction control, which adjusts the power plant and windfarm set-points in a closed-loop control.

Figure 14 illustrates how the central control shown in Fig-ure 13 is using the wind farm main controller to reduce thepower balance error. At 4:50, the production error increases,and the wind farm main controller is activated to reduce thewind farm power. Since the wind farm power control is veryfast, the error can be reduced immediately this way. Later, at5:50, the wind farm production is reduced again. This time, itis not the balance power error that causes this reduction, butperhaps the need to avoid reducing the power on the thermalpower stations just before the morning peak load starts.

Conclusions and Recommendations Wind power integration into power systems has two dimen-sions: an economical one related to optimization of theresources and a fair sharing of the cost, and a technical onerelated to security of supply. The economical dimension isfirst observed in the allocation and use of reserves that canincur increased costs for the power system operation. Theactual impact of adding wind generation in different bal-ancing areas can vary depending on local factors. Compar-ing European studies, some general aspects to reduceintegration costs were identified, such as aggregating windplant output over large geographical regions, larger balanc-ing areas, and operating the power system closer to thedelivery hour. In regard to the technical dimension, appro-priate grid codes, in particular FRT and frequency controlrequirements, are essential to allow high wind penetrationlevels (>15%).

Furthermore, the experience with the Horns Rev offshorewind farm demonstrates that power and frequency controlfunctions provided by a wind farm are very useful tools tosupport the daily operation and control of the Danish powersystem. Thus, the Horns Rev wind farm main controller hasoperated as an integrated part of the central control, ensuringthe power balance in the system. It is expected that suchfunctionality will be inevitable in future power systems withlarge-scale wind power. For instance, in the present Danishsystem wind power produces almost 20% of the electricity,but according to government plans, this number is indicatedto increase to 50% by 2025.

For Further ReadingT. Ackermann, Ed., Wind Power in Power Systems. New York:Wiley, 2005 (see also http://www.windpowerinpowersystems.info).

J.L. Rodríguez-Amanedo, S. Arnalte, and J.C. Burgos,“Automatic generation control of a wind farm with variablespeed wind turbines,” IEEE Trans. Energy Conv., vol. 17, pp.279–284, June 2002.

J.R. Kristoffersen, “The Horns Rev wind farm and the oper-ational experience with the wind farm main controller,” inProc. Copenhagen Offshore Wind 2005, Oct. 2005. [CD-Rom].

E.on Netz (2006 April), “Grid code, high and extra highvoltage,” [Online]. Available: http://www.eon-netz.com/.

I. Erlich and H. Brakelmann, “Integration of wind powerinto the German high voltage transmission grid,” in Proc.IEEE PES General Meeting, Tampa, FL, pp. 1–8, June 2007.

Commission for Energy Regulation (2004, July), Windgenerator connection policy. [Online]. Available:http://www.eirgrid.com.

I.M. Dudurych, M. Holly, and M. Power, “Wind farms inthe Ireland’s power system: Experience and analysis,” inIEEE St Petersburg Power Tech Proc., St Petersburg, paper376, June 2005.

H. Holttinen, P. Meibom, C. Ensslin, L. Hofmann, A.Tuohy, J.O. Tande, A. Estanqueiro, E. Gomez, L. Söder, A.Shakoor, J.C. Smith, B. Parsons, and F. Van Hulle, “State-of-the-art of design and operation of power systems with largeamounts of wind power: Summary of IEA wind collabora-tion,” in Proc. Eur. Wind Energy Conf., EWEC2007, Milan,May 2007. [CD-Rom].

BiographiesThomas Ackermann is CEO of Energynautics GmbH,Denmark.

Juan Rivier Abbad is currently a research fellow and anassistant professor at the Universidad Pontificia Comillas,Madrid, Spain.

Ivan M. Dudurych is with EirGrid, Denmark.Istvan Erlich is professor and head of the Institute of

Electrical Power Systems at the University of Duisburg-Essen/Germany.

Hannele Holttinen is with VTT Technical Research Cen-tre of Finland.

Jesper Runge Kristoffersen is with Vattenfall Research &Development as a senior R&D engineer.

Poul Ejnar Sørensen is a senior scientist at Risø NationalLaboratory in Roskilde, Denmark.

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Due to the island characteristic of the RoI power system and thehigh concentration of wind power in one area, frequency controlissues are studied in particular for low load/high wind situations.

european balancing act

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