ISSN 1752-1416 Review of grid code technical · PDF filethat is the offshore cable sealing end...

Published in IET Renewable Power GenerationReceived on 31st July 2008Revised on 21st March 2009doi: 10.1049/iet-rpg.2008.0070

ISSN 1752-1416

Review of grid code technical requirementsfor wind farmsM. Tsili S. PapathanassiouSchool of Electrical and Computer Engineering, National Technical University of Athens (NTUA), 9 Iroon Polytechniou str.,Athens 15773, GreeceE-mail: [email protected]

Abstract: This study provides an overview of grid code technical requirements regarding the connection of largewind farms to the electric power systems. The grid codes examined are generally compiled by transmissionsystem operators (TSOs) of countries or regions with high wind penetration and therefore incorporate theaccumulated experience after several years of system operation at significant wind penetration levels. Thepaper focuses on the most important technical requirements for wind farms, included in most grid codes,such as active and reactive power regulation, voltage and frequency operating limits and wind farm behaviourduring grid disturbances. The paper also includes a review of modern wind turbine technologies, regardingtheir capability of satisfying the requirements set by the codes, demonstrating that recent developments inwind turbine technology provide wind farms with stability and regulation capabilities directly comparable tothose of conventional generating plants.

1 IntroductionIncreasing wind power penetration levels to the powersystems of many regions and countries has led to theelaboration of specific technical requirements for theconnection of large wind farms, usually as a part of the gridcodes issued by the transmission system operators (TSOs).These requirements typically refer to large wind farms,connected to the transmission system, rather than smallerstations connected to the distribution network. The newgrid codes stipulate that wind farms should contribute topower system control (frequency and also voltage), much asthe conventional power stations, and emphasise wind farmbehaviour in case of abnormal operating conditions of thenetwork (such as in case of voltage dips due to networkfaults).

Grid code requirements have been a major driver for thedevelopment of WT technology recently and several relevantpublications are already available. In [1], a technical analysisof the main issues associated with the connection of windfarms to the grid is provided, along with an overview of windturbine technologies. A presentation of the requirementsimposed by the former and more recent version of the

German grid code by E.ON Netz is realised in [2, 3],whereas the respective requirements of the Spanish, Irish andCanadian grid codes are commented in [4–7], respectively.The development of national US and regional grid codesconcerning the integration of wind power is described in [8,9]. British grid code requirements are given in [10], alongwith their comparison to the requirements of the Germanand Irish grid code, as well as in [11]. Further overviewof grid codes involves comparative presentation and analysisof wind farm interconnection regulations: the codes ofDenmark, Germany, Scotland, Ireland, UK and CanadianTSO AESO are compared in [12]. The codes of Denmark,Germany, Scotland, Ireland and Sweden are compared in[13, 14]. Reference [15] includes the codes of Denmark,Ireland, Germany, UK, Spain and Italy. Finally [16] includesa presentation of selected features from the grid codes ofDenmark, Ireland, Germany, UK, Spain and USA, whereas[17] focuses on the fault ride through (FRT) requirements ofthe grid codes of Denmark, Ireland, Germany, UK, Spain,Italy, USA and Canada. Reference [18] compares therequirements appearing in the grid codes of Germany,Denmark, Spain, Great Britain, Canada and USA.Critical examination and interpretation of wind turbineconnection requirements concerning the UK and the Danish

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transmission system are included in [19, 20], respectively. Itmust, however, be noted that different criteria are appliedwhen settling grid access in every country [21] and thenational regulatory frameworks are subject to continuouschanges and revisions, raising some difficulties in their directcomparison and the extraction of global conclusions.

The present paper extents the overview to several countries,providing a presentation and comparison of the most recentavailable editions of their grid codes. More specifically, thefollowing codes have been considered:

† The German code from E.ON Netz [22] that applies tonetworks with voltage levels 380, 220 and 110 kV. Itsrequirements are often used as a reference for other codes.E.ON Netz has also issued supplementary requirements foroffshore wind farms [23]. A nominal voltage level of155 kV is specified for the offshore grid connection point,that is the offshore cable sealing end of the gridinterconnection system that also represents the ownershipboundary between the connectee’s facility and that of theTSO. Germany is the only country that has issued separateoffshore grid code.

† The Great Britain code [24], where the requirements forwind farms are presented in combination with requirementsfor other power production units. It applies to networks withvoltage levels 400, 275 and 132 kV (32 kV for Scotland).Notably, an offshore grid code is under development in theUK, aspects of which are included in the paper.

† The Irish code published by ESB National Grid [25](Section WPFS1, Wind Farm Power Station Grid CodeProvisions) that applies to networks with voltage levels 400,220 and 110 kV.

† The Nordic grid code from Nordel [26] that applies to allwind farms connecting to the Nordic grid (the interconnectedsystem of Denmark, Sweden, Norway and Finland).

† The codes of Denmark [27, 28], referring to wind turbinesconnected to grids with voltages below and above 100 kV,respectively. Denmark is the only country belonging to theNordic power system that has issued separate requirementsfor wind turbines connected to grids with voltagesbelow 100 kV and above 100 kV, which are still active.Requirements issued by Sweden and Norway TSOs, in2002 and 2001, respectively, also appear in the relevantbibliography [14]; however, no additional information ontheir applicability after the release of the Nordic grid codeis available.

† The grid code of Belgium [29] issued by the Belgian TSO,Elia, applying to networks with voltage levels 30–70 and150–380 kV.

† The grid codes of two Canadian TSOs, Hydro-Quebec[30, 31], applying to networks with voltages above 44 kV

and Alberta Electric System Operator (AESO) [32] thatapplies to wind farms with rated capacity above 5 MWconnected to networks with voltage 69–240 kV.

† USA rule for the interconnection of wind generatorspublished by Federal Energy Regulatory Commission(FERC) in June 2005 [33] that applies to wind farms withrated capacity above 20 MW.

† Codes from other countries like Spain [34], Italy [35]Sweden [36] and New Zealand [37].

The article focuses on the technical regulations regardingthe connection of large wind farms to the high-voltagetransmission system, including active and reactive powerregulation, voltage and frequency operating limits and windfarm behaviour during grid disturbances. Requirements forsmaller stations, connected to the distribution network, arenot included in the paper, since they concentrate on powerquality, fault level contribution and anti-islandingprotection, which are not key issues for large wind powerstations connected to the transmission system.

Several solutions have been proposed and implemented bywind turbine manufacturers, in order to achieve grid codecompliance. In the second part of the article, a briefpresentation is made of available technologies of modern,commercially available wind turbines, in terms of theirelectrical system configuration, as far as their response togrid disturbances and compliance to grid code requirementsis concerned.

2 Brief overview of common gridcode requirementsBased on the experience from the operation of power systemswith large wind penetration levels, modification of theexisting grid codes for connection and operation of windpower plants in the high-voltage grid have proven necessary[38]. The objective of these provisions is to improve andstabilise wind turbine behaviour, decrease the amounts ofwind power to be lost following system disturbances andprovide the wind power stations with operationalcharacteristics similar to those of the conventional powerplants. The most common requirements include fault ride-through capability, extended system voltage and frequencyvariation limits, active power regulation and frequencycontrol, as well as reactive power/power factor and voltageregulation capabilities. The nature of these requirements arealready discussed in the literature [1–3, 10–17, 21]. Herea brief review is included for the sake of completeness ofthe presentation.

The occurrence of a fault (short-circuit) at some point ofthe network inevitably results in voltage dips in one ormore phases (possibly also to a voltage rise in healthyphases), depending on the type and location of the fault,

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which may be propagated to fairly remote locations of thenetwork, especially in the case of weak grids. The durationof the dips is dependent on the protection system responsetime and may vary between 0.1 s and several seconds, themost usual duration being in the range of a few tenths of asecond. In the event of such dips, generating stations mayencounter stability problems, depending on the type,magnitude and duration of the dip, as well as on the typeand technology of the power station.

The large increase in the installed wind capacity intransmission systems necessitates that wind generationremains in operation in the event of network disturbances[39, 40]. For this reason, grid codes issued in the currentdecade invariably demand that large wind farms (especiallythose connected to HVQ1 grids) must withstand voltage dipsdown to a certain percentage of the nominal voltage (0% insome cases) and for a specified duration. Such requirementsare known as FRT or low voltage ride through (LVRT)requirements and they are described by a voltage againsttime characteristic, denoting the minimum requiredimmunity of the wind power station to dips of the systemvoltage. Fig. 1 depicts a typical FRT limit curve. In case ofdips above the limit line of Fig. 1, wind turbines mustremain in operation, whereas they can disconnect in theevent of dips below this limit. The voltage prescribed inFig. 1 generally corresponds to the voltage at the gridconnection point and the voltage dip may either besymmetric or correspond to the maximum of the phasevoltages at this point, depending on the particular coderequirements. FRT curves are similar to Fig. 1, althoughtheir quantitative characteristics vary among different systems.

FRT requirements also include fast active and reactiveoutput power restoration to the pre-fault values, after thesystem voltage returns to normal operation. Certain codesimpose increased reactive current generation by the windturbines during the disturbance, in order to support thesystem voltage, in much the same way as a conventionalsynchronous generators increases its excitation during faultsvia AVRQ2 action.

Wind power plants can actively take part in grid operationand control by regulation of their output power. All gridcodes currently impose requirements on the regulationcapabilities of the active power of wind farms, taking theform of several different modes of control, illustrated inFig. 2. Within the constraint of the primarily available activepower (i.e. the prevailing wind conditions), output powercan be regulated to a specific value (Fig. 2a) or to bear afixed relationship to the available power, such as maintaininga specified reserve, either in MW or as a percentage of theavailable power (Fig. 2b). Additional requirements include

Figure 1 Typical limit curve for FRT requirements

Figure 2 Typical active power constraints [14, 28]

a Absolute power constraintb Delta production constraint (active power reserve)c Power gradient constraint


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the limitation of the rate of change of the output power(Fig. 2c). Ramp rates are possible for power increases, butoperation with a power reserve is necessary to becomeeffective when the output power decreases.

A direct consequence of the active power controlpossibilities is the provision of ancillary services from thewind farm, such as the participation in frequency control orthe provision of spinning reserve, as already mentioned. Asshown in Fig. 3, frequency response may be requested in theover-frequency region (reduction of active power in relationto the positive frequency deviation), but also for underfrequency. In the latter case, the wind farm must operatewith a power reserve when the frequency is in the normaloperating range, which inevitably has economic implications.

Reactive power regulation capabilities are requested by manygrid codes. This is effected either by externally providing aspecific reactive power value or by a specific power factor.Further, the reactive power regulation capability may beexploited for voltage control at the wind farm connectionpoint to the system, or at a more distant node. Figs. 4 and 5show typical requirements for the power factor regulationrange, depending on the terminal voltage and the activeoutput power of the wind farm, respectively.

In addition to the response, regulation and controlcapabilities briefly outlined, grid codes demand also thatwind power stations operate over an extended range ofsystem voltage and frequency deviations from the nominalvalues. When the deviations are large, a reduction of theoutput power may be allowed, or operation for a limitedperiod may be foreseen.

3 Presentation of grid codetechnical requirementsThe present section presents the requirements encountered in themajority of grid codes concerning wind farm interconnection.

3.1 FRT requirements

3.1.1 Germany: According to the E.ON grid code [22],the FRT requirements are given in Fig. 6 for symmetricalnetwork faults. The code defines the following:

† Three-phase short circuits or fault-related symmetricalvoltage dips must not lead to instability above limit line 1or to disconnection of the wind farm. After fault clearance,Figure 3 Typical power–frequency response curve

Figure 4 Typical requirements for power factor variationrange in relation to the voltage

Figure 5 Typical requirements for power factor variation range in relation to the active power



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the active power in-feed must increase with a rate of 20% ofthe rated power per second.

† Voltage drops within the area between limit line 1 andlimit line 2 should not lead to disconnection, but in case ofwind turbine instability, short-time disconnection isallowed. The resynchronisation must take place within upto 2 s and active power in-feed must increase with a rate of10% of the rated power/s after fault clearance.

† Below limit line 2 disconnection of the wind turbines isallowed.

According to Fig. 6, wind farms must withstand voltagedrops down to 0% of the nominal voltage at the connectionpoint, for durations up to 150 ms (7.5 cycles). Thisrequirement differs from the FRT curve of the previousversion of E.ON code (2003) which defined that windfarms must withstand voltage drop down to 15% forapproximately 600 ms (30 cycles), whereas the maximumvoltage dip duration extended to 3 s (150 cycles) [2]. Therespective characteristic is illustrated in Fig. 7 and has beenadopted by the current versions of the Irish, AESO(Canada) and FERC (USA) grid codes.

The automatic protection system mentioned in Fig. 6operates in the following situations. If the voltage at theconnection point falls and remains below 85% of the nominalvoltage, with a simultaneous reactive power absorption, thewind farm must disconnect with a time delay of 0.5 s. If thevoltage on the low voltage side of each generator transformerfalls and remains below 80% of the lower value of the voltageband (e.g. 690 V � 0.95 � 0.8 ¼ 525 V) the generators

must disconnect from the grid in four groups, after 1.5 s,1.8 s, 2.1 s and 2.4 s, respectively. If the voltage on thelow voltage side of each generator transformer rises andremains above 120% of the upper value of the voltageband (e.g. 690 V � 1.05 � 1.2 ¼ 870 V), the generatoraffected must disconnect itself from the grid with a time delayof 100 ms.

3.1.2 UK: Great Britain’s LVRT requirements apply tonetworks with a voltage level above 200 kV. The grid codedivides voltage dips in two categories (Fig. 8):

† Those lasting less than or equal to 140 ms (seven cycles),caused by symmetrical or unsymmetrical network faults. Inthis case, the wind farm must stay connected for voltagedrops down to 0%.

Figure 6 Limit curve for FRT requirements of the E.ON grid code [22]

Figure 7 Limit curve for FRT requirements adopted by theIrish, AESO (Canada) and FERC (USA) grid codes [25, 32, 33]


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† Those lasting more than 140 ms (seven cycles), caused bysymmetrical faults. In this case, disconnection is not allowedabove the curve shown in Fig. 8.

After restoration of the voltage, the active power must berestored to at least 90% of the level available before the dip,within 1 s.

The UK grid code, unlike other codes, defines that theprofile of Fig. 8 is not an rms voltage–time responseenvelope that would be obtained by plotting the transientvoltage response at a point on the transmission systemagainst time. Rather, it is clarified that each point on thecharacteristic represents a combination of voltage level andassociated time duration, which the connected wind powerstation must ride through.

As mentioned in the Section 1, an offshore grid code iscurrently being drafted in the UK [41]. The FRTprovisions, as well as other technical requirements, resemblein principle those applicable to onshore stations, using asgrid connection point the interface point to the onshoretransmission system. Hence, HVDCQ3 interconnection ofoffshore wind farms is directly catered for. It is interestingto note that such a rationale is not explicitly present in theEO.N offshore code [23], where system voltage andfrequency are measured at the grid connection point,defined as the offshore end of the interconnectingsubmarine cables, formally belonging to the TSO.

3.1.3 Ireland: The LVRT curve specified by the Irish gridcode is illustrated in Fig. 7, corresponding to voltage dips inone or more phases at the point of connection of the windfarm to the system (high-voltage side of the connectiontransformer). It is required that the wind farm providesactive power during the disturbance in proportion to thevoltage level. It is also defined that after the restoration ofvoltage to normal operation levels, the active power mustbe restored to at least 90% of the level available beforethe dip within 1 s at the latest, as in the case of the UKgrid code.

Besides the specifications concerning the active powerrestoration rate (ramp rate) during and after fault clearance,the Irish code describes general guidelines for this rate forall kinds of operating conditions (start up, shut down andnormal operation). According to these guidelines, tworamp rate settings are prescribed, the first one applying tothe MW ramp rate average over 1 min and the second oneapplying to the MW per minute ramp rate average over10 min. Both settings may vary independently between 1and 30 MW/min and are agreed with the TSO.

3.1.4 Nordel: The Nordic grid code from Nordel [26]applies to all wind farms connecting to the Nordic grid(the interconnected system of Denmark, Sweden, Norwayand Finland). The first edition of the Nordic grid code toissue specific requirements on wind turbine connection wasreleased in 2006, followed by the most recent edition of2007. As stated in [26], the code outlines the minimumtechnical requirements that new wind turbines togetherwith their supplemental installations have to fulfil at theconnection point to the transmission network, in order toprovide for adequate safe operation and reliability of theinterconnected Nordic power system. The Nordic TSOsmay publish connection codes for the electricity systemwithin their responsibility having additional requirements.

The Nordic grid code defines the LVRT requirement ofFig. 9. Wind farms must withstand voltage drops at theconnection point down to 0% of the nominal voltage for250 ms (12.5 cycles), followed by an increase to 95%within the next 0.5 s (25 cycles). No further specificationsfor active power production during or after the fault areprovided.

3.1.5 Denmark: As mentioned in Section 1, Denmark isthe only country belonging to the Nordic system that hasissued requirements for wind farms, other than those in theNordic grid code. The Danish requirements [27, 28] applyto grids with voltages below 100 kV and above 100 kV andconcern both parts of the Danish system, belonging toUCTE and Nordel. Fig. 10 illustrates the LVRT

Figure 9 LVRT requirements of the Nordic grid code [26]

Figure 8 LVRT requirements of the British grid code [24]



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requirements for wind farms connected to voltage levelsbelow 100 kV according to the Eltra and Elkraft gridcodes. For voltages below 100 kV, apart from the LVRTrequirement of Fig. 10, wind farms should disconnect ifthe voltage increases above 1.2 p.u.

Additional requirements for voltages below 100 kV definethat a wind turbine must remain connected after the faults orsequence of faults in the distribution system listed below.

† Three-phase short circuit lasting 100 ms (five cycles)

† Two-phase short circuit with/without earth lasting100 ms (five cycles), followed by a new fault 300–500 mslater (15–25 cycles), lasting also 100 ms (five cycles)

† At least two two-phase short circuits within 2 min

† At least two three-phase short circuits within 2 min

† At least six two-phase short circuits at 5-min intervals

† At least six two-phase short circuits at 5-min intervals.

For system voltages above 100 kV, the grid code definesthat a wind turbine must remain connected after the faultsor sequence of faults in the transmission system listed below:

† Three-phase short circuit lasting 100 ms (five cycles)

† Two-phase short circuit with/without earth lasting100 ms (five cycles), followed by a new fault 300–500 mslater (15–25 cycles), lasting also 100 ms (five cycles)

† One-phase short-circuit for up to 100 ms (five cycles)followed after 300–500 ms (15–25 cycles) by a new shortcircuit, lasting also 100 ms (five cycles)

† At least two one-phase short-circuits within 2-mininterval

† At least two two-phase short-circuits within 2-min interval

† At least two three-phase short-circuit within 2-mininterval

Additionally, sufficient energy reserves (emergency power,hydraulics and pneumatics) are demanded for the followingthree independent sequences:

† At least six one-phase earth faults with 5-min intervals

† At least six two-phase short circuits with 5-min intervals

† At least six three-phase short circuits with 5-min intervals.

3.1.6 Belgium: The grid code of Belgium distinguishesbetween two kinds of voltage disturbances, namely voltagedips of ‘limited’ and ‘important’ magnitude, as shown inFig. 11 (the respective LVRT curves refer to the voltage atthe wind farm connection point). During each kind ofdisturbance, the wind farms must remain connected and beable to operate at their full operating range, as long as thevoltage at the connection point remains in the shaded area ofthe LVRT diagrams. The distinction between limited andimportant magnitude of voltage dips is applicable only if thecurves of Fig. 11 are considered as envelopes of the voltagetime variation and not as a voltage–time response curves. Nofurther specifications for active power production during orafter the fault are provided.

3.1.7 Canada: In Canada, several interconnectionrequirements have been developed [42] by different TSOs(e.g. BC Hydro, AESO, SaskPower, Manitoba Hydro,Hydro One and Hydro-Quebec). Alberta and Quebec havedrafted wind-specific interconnection standards, followingproposals made by the Canadian Wind Energy Association(CanWEA) [31], with the Quebec document being asupplement to its general transmission level interconnectionrequirements.

Fig. 12 provides a comparison of the LVRT limit curvesspecified by Hydro Quebec and AESO. Hydro Quebec

Figure 10 LVRT requirements of the Danish grid code forwind turbines connected to grids below 100 kV (Eltra andElkraft) [27]

Figure 11 LVRT requirements of the Belgian grid code [29]


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prescribes LVRT requirements for three-phase andunsymmetrical faults that occur on the transmission system(including the high-voltage side of the grid connectiontransformer). It also sets out requirements for remotesymmetrical and unsymmetrical faults with larger clearancetimes. Moreover, in case of overvoltages up to 1.4 p.u. itspecifies a minimum interval within which the wind farmsmust not trip. The curve of Fig. 12 corresponds to ‘thepositive sequence voltage on the high-voltage side of theswitchyard’. AESO grid code requires the wind farms towithstand voltage drop down to 15% for 0.625 s (37.5cycles) (following the requirements of the old version of theE.ON code, Fig. 7) and mentions that in case of voltagerise above 1.1 p.u. wind turbines must trip.

3.1.8 USA: According to the current FERC rule, issued inJune 2005, the wind power plants ‘shall be able to remainonline during voltage disturbances up to the time periodsand associated voltage levels’ shown in Fig. 7 (formerE.ON requirement). Also, the ‘wind generating plant mustbe able to operate continuously at 90% of the rated linevoltage, measured at the high-voltage side of the windplant substation transformer’.

3.1.9 Other countries: According to the Spanishregulation, the wind turbines must remain connected duringfaults for a voltage profile as shown in Fig. 13. The

respective requirement by the Italian grid code, addressingwind power installations with a rated power over 25 MW, isdepicted in Fig. 14. The Swedish code issued by theSwedish TSO Svk distinguishes the LVRT requirements forwind farms of rated power less or above 100 MW, asdepicted in Fig. 15, with stiffer requirement for the latterones, since their disconnection affects significantly powersystem stability.

According to the Spanish grid code, during balancedthree-phase faults, and later, in the voltage recovery periodafter clearance of the fault, the wind farms will not absorbreactive power. However, reactive power absorptions can beadmitted during a period of 150 ms (7.5 cycles) after thebeginning of the fault, and a period of 150 ms (7.5 cycles)after fault clearance (provided that it does not exceed 60%of rated power). In terms of active power, during the faultand in the voltage recovery period after the clearance, thewind farm will not absorb active power, at the point ofinterconnection with the transmission system. However,active power absorption can be admitted during a period of150 ms (7.5 cycles) after the beginning of the fault, and aperiod of 150 ms after fault clearance. Additionally, duringthree-phase faults, active power consumption is admittedwith a maximum value of 10% of the installation ratedpower [43].

Figure 12 LVRT requirements set by the Hydro Quebec andAESO codes [30, 32]

Figure 13 LVRT requirements of the Spanish grid code [17]

Figure 14 LVRT requirements of the Italian grid code [17]

Figure 15 LVRT requirements of the Swedish grid code [13]



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The specifications of New Zealand (referring to connectionsat 110 kV) link the LVRT curve with the voltage profilecorresponding to three-phase faults that may occur at severalpoints of the network (faults at the medium voltage feedersare also considered). These profiles are used for the extractionof a ‘combined voltage profile’ for all faults, corresponding tothe ‘maximum actual range’ of Fig. 16. However, thespecified LVRT curve corresponds to a more relaxedrequirement, described by the ‘minimum design target’ ofFig. 16.

3.1.10 Comparison of FRT requirements: Fig. 17presents in the same graph all LVRT requirements cited in

Section 3.1. Table 1 summarises the main characteristics ofthe curves of Fig. 17. The requirements depend on thespecific characteristics of each power system and theprotection employed and they deviate significantly fromeach other.

More demanding appear to be the requirements of theGerman, UK, Nordic, Danish, Belgian, Hydro-Quebec,Swedish and New Zealand grid codes, which stipulate thatwind farms must remain connected during voltage dips downto 0%. However, it must be noted that these requirementsapply for the connection point to the network, generally atHV level. Taking into account the typical impedance valuesfor the step-up transformers and interconnecting lines, arelatively simple calculation indicates that the correspondingvoltage dip at lower voltage levels, near the WT terminals, arelikely to be somewhat above 15% [10], facilitating complianceto the LVRT requirements. Specifications may vary accordingto the voltage level or the wind farm power, for example windfarms connected to the Danish grid at voltages below 100 kVare required to withstand less severe voltage dips than theones connected at higher voltages, in terms of voltage dipmagnitude and duration. Similar differences can be observedin the regulation governing the connection of wind farmsbelow and above 100 MW in the Swedish transmissionsystem. Apart from the FRT curve, the codes of Denmarkand Hydro-Quebec define specific kinds of faults(or sequences of faults, in the Danish code) that thewind farm must withstand (including remote faults inthe case of the Hydro-Quebec code, cleared by slow

Figure 16 LVRT requirements of the New Zealand gridcode [37]

Figure 17 LVRT requirements of various grid codes


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protective devices). These more detailed requirements could beattributed to the isolation of the Hydro-Quebec transmissionsystem, which has no synchronous link to neighbouringsystems [7].

Another important difference lies in the active powerrestoration rates specified by the German and British/Irishgrid codes, whereas the British code requires immediaterestoration (at 90% in 0.5 s after voltage recovery), E.ONNetz requires restoration with a rate at least equal to 20%of the nominal output power (reaching 100% in 5 s aftervoltage recovery). The less severe requirement of theGerman code may be attributed to the physical location ofthe German grid and its strong interconnection to theUCTE system, as opposed to the weakly interconnectedBritish system, where the need for active power restorationto the pre-fault values is more crucial for system stability.

3.1.11 Requirements for reactive current supplyduring voltage dips: Some grid codes prescribe thatwind farms should support the grid by generating reactivepower during a network fault, to support and restore fastthe grid voltage.

E.ON requires that wind farms support grid voltage withadditional reactive current during a voltage dip, as shown in

Fig. 18, as well as via increased reactive power consumptionin the event of a voltage swell. The voltage control musttake place within 20 ms (one cycle) after fault recognitionby providing additional reactive current on the low-voltageside of the wind turbine transformer, amounting to at least2% of the rated current for each percent of the voltage dip.A reactive power output of at least 100% of the ratedcurrent must be possible if necessary. The above applies

Table 1 Characteristics of fault ride-through curves in various grid codes

Grid code Fault duration(ms)

Fault duration(cycles)

Min voltage level (% ofVnom)

Voltage restoration(s)

Germany (Eon) 150 7.5 0 1.5

UK 140 7 0 1.2

Ireland 625 31.25 15 3

Nordel 250 12.5 0 0.75

Denmark (,100 kV) 140 7 25 0.75

Denmark (.100 kV) 100 5 0 10

Belgium (large voltagedips)

200 10 0 0.7

Belgium (small voltagedips)

1500 75 70 1.5

Canada (AESO) 625 37. 5 15 3

Canada (Hydro-Quebec) 150 9 0 1

USA 625 37.5 15 3

Spain 500 25 20 1

Italy 500 25 20 0.8

Sweden (,100 MW) 250 12.5 25 0.25

Sweden (.100 MW) 250 12.5 0 0.8

New Zealand 200 10 0 1

Figure 18 Reactive output current during voltagedisturbances according to the E.ON grid code [22]

Note: In case of offshore installations, the +10% dead band isreduced to +5% [23]



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outside a +10% dead band around nominal voltage. In thecase of offshore wind farms the dead band is reduced to+5%. The characteristic refers to the voltage at the gridconnection point, that is at the wind farm side of thesubmarine interconnection.

According to the Spanish grid code, the wind power plantsare required to stop drawing reactive power within 100 ms(five cycles) of a drop voltage and to be able to injectreactive power within 150 ms (7.5 cycles) of grid recoveryas shown in Fig. 19.

Finally, Great Britain and Ireland specify in their gridcodes that wind farms must produce their maximumreactive current during a voltage dip caused by a networkfault.

3.2 Active power and frequency control

These requirements refer to the ability of wind farms toregulate (usually, but not exclusively, reduce) their poweroutput to a defined level (active power curtailment), eitherby disconnecting wind turbines or by pitch control action.In addition, it is required from wind farms to providefrequency response that is to regulate their active outputpower according to the frequency deviations.

The grid codes of the following countries demand thewind farms to have the ability of active power curtailment:

† Germany, with a ramp rate 10% of grid connectioncapacity per minute

† Ireland, with a ramp rate 1–30 MW/min

† Nordic grid code, with a ramp rate 10% of rated power perminute

† Denmark, with a ramp rate 10–100% of rated power perminute.

According to the German code when frequency exceedsthe value 50.2 Hz wind farms must reduce their active

power with a gradient of 40% of the available power of thewind turbines per Hz. In case of offshore wind farms,active power must be reduced with a gradient of 98% perHz and at a rate of 25% per second, based on the activepower available at the moment. The knee point forproviding frequency response is 50.1 Hz, instead of 50.2 Hz.

The British code requires from wind farms to have afrequency control device that can supply primary andsecondary frequency control as well as over-frequencycontrol. It is remarkable that it also prescribes tests, whichvalidate that wind farms indeed have the capability of thedemanded frequency response. The UK offshore grid codeworking group stipulates real-time communication of thesystem frequency measurement to the wind farm, toaddress the case of HVDC interconnections, where thewind farm and onshore system sides of the HVDC linkmay operate at different frequencies (a requirement notexplicitly stated in the EO.N offshore code).

The Irish code demands a frequency response system,which will control active power according to response curveshown in Fig. 20. Under ‘normal’ frequency operation, thewind farm shall operate with an active power output as setby line ‘B’– ’C’ in Fig. 20. If the frequency falls below point‘B’, then the frequency response system shall act to rampup the wind farm active power output, in accordance withthe droop characteristic defined by line ‘B’– ‘A’. Once thefrequency rises to a level above the point ‘C’, the frequencyresponse system shall act to ramp down the wind farm’sactive power output in accordance with the frequency-activepower characteristic defined by line ‘C’– ‘D’– ‘E’. Atfrequencies greater than or equal to ‘D’– ’E’, there shall beno active power output from the wind farm.

The Danish grid codes also require the active power tobe controlled according to frequency, as shown in Fig. 21.

According to the Hydro-Quebec grid code, wind farmswith rated power greater than 10 MW must have afrequency control system that helps reduce large (.0.5 Hz)and short-term (,10 s) frequency deviations in the powersystem. As a general remark, it is clear that most grid codes

Figure 19 Reactive output current during voltagedisturbances according to the Spanish grid code [17]

Figure 20 Example power – frequency response curveaccording to the Irish grid code [25]


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require wind farms (especially those of high capacity) toprovide frequency response, that is to contribute to theregulation of system frequency. It should be emphasisedthat the active power ramp rates must comply with therespective rates applicable to conventional power units.

3.3 Voltage and frequency operatingrange

Wind farms must be capable of operating continuously withinthe voltage and frequency variation limits encountered during

normal operation of the system where they connect to.Further, they should remain in operation even in case ofvoltage and frequency excursions outside the normaloperation limits, albeit for a limited time and in some casesat reduced output power capability.

Fig. 22 provides a comparison of operating frequency limits(the scale is only indicative of the duration that wind powerplants are required to remain in operation) in countries with50 Hz power systems (therefore Canada and USA limits arenot included, as well as those of New Zealand, where therequirements are different in the north and southern part ofthe country). Where possible, voltage limits in relation tofrequency limits appear, as well (they are not included in thecase of Germany, although they are available, becausedifferent % values are specified for the three transmissionsystem nominal voltage levels). Spain and Italy are also notincluded in Fig. 22, due to lack of relevant information. It isalso noted that, in the case of Germany, the E.ON offshorecode [23] prescribes an extended frequency range foroffshore wind farms, stipulating limited time operation up to10 s for frequency excursions in the ranges 51.5–53.5 Hz or47.5–46.5 Hz, as depicted in Fig. 22. Upon reaching afrequency at the grid connection point of less than 46.5 Hzor greater than 53.5 Hz, offshore wind farms mustdisconnect from the grid with a time delay of 300 ms.

Figure 21 Power – frequency response according to theDanish grid code [27]

Figure 22 Comparison of operating frequency limits imposed by various grid codes



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It is obvious that the most extreme frequency limits are46.5 and 54 Hz. In countries like Ireland, characterised byan isolated power system with weak interconnections,frequency limits are expectedly wider. It is remarkable thatNew Zealand’s grid code prescribes a frequency range of45–55 Hz. The strictest continuous operation frequencylimits appear in the British code (47.5–52 Hz), whereasthe strictest continuous operation voltage limits appear inthe Danish code (90–105% nominal voltage).

In Table 4, the frequency limits of Hydro-Quebec andAESO grid codes are shown in more detail.

3.4 Reactive power control and voltageregulation

Voltage regulation in power systems is directly related to thecontrol of reactive power. The recent grid codes demand

from wind farms to provide reactive power controlcapabilities, often in response to the power system voltage,much as conventional power plants. The reactive powercontrol requirements are related to the characteristics of eachnetwork, since the influence of the reactive power injectionto the voltage level is dependent on the network short-circuit capacity and impedance [44]. Some codes prescribethat the TSO may define a set-point value for voltage orpower factor or reactive power at the wind farm’s connectionpoint. Reactive power control is an important issue for windfarms, because not all wind turbine technologies have thesame capabilities, whereas wind farms are often installedin remote areas and therefore reactive power has to betransported over long distances resulting in power losses.

Fig. 23 compares the permissible wind farms power factorrange (based on rated power) in relation to grid voltage,according to the German (grey line) and British (light grey

Table 3 Values applying to Fig. 21 [27]

Setting range Defaultvalue

lower frequency limit for the control range during under frequency ( fn) 50.00. . .47.00 Hz 48.70 Hz

upper frequency range for the control range during over frequency ( fu) 50.00. . .52.00 Hz 51.30 Hz

lower frequency limit for the deadband during under-frequency ( fd2) 50.00. . .52.00 Hz 50.15 Hz

upper frequency limit for the deadband during over-frequency ( fdþ) 50.00. . .52.00 Hz 50.15 Hz

control factor for the production applying to frequencies in the rangefn . . .fd2 and fdþ . . .fu

Over frequency:

1�f � fdþ

fu � fdþ

� �

(Control factor ¼ 1 corresponds to max. possible production–or to power set point,if specified)

Under frequency:

1�f � fd�

fn � fd�

� �

regulating speed calculated from exceeding a limit value to completed control action 10% of the rated power persecond

Table 2 Frequency and active power ranges for Fig. 20 [25] Q4

Frequency (Hz) Active power (% of the available active power)

MEC . 10 MW 5 MW , MEC � 10 MW

FA 47.0–51.0 PA 50–100 100

FB 49.5–51.0 PB 50–100 100

FC 49.5–51.0 PC

FD 50.5–52.0 PD 20–100 20–100

FE PE 0 0

MEC ¼ maximum export capacity


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line) grid codes. The nominal voltages are 380, 220, 110 kVfor onshore wind farms and 155 kV for offshore wind farmsin Germany and 400, 275 kV for Great Britain. The Britishcode refers to wind farms with rated power above 50 MW.The German code specifies that wind farms may functionin lagging or leading power factor in case of overvoltages.According to the British code, power plants must be ableto provide their full reactive power at voltages +5% aroundthe nominal, for voltage levels 400 and 275 kV.

According to the German code, the operating point for thesteady-state reactive power can be defined in terms of powerfactor or reactive power level (Q in Mvar) or voltage level (Uin kV). Fig. 24 describes additional specifications imposed bythe E.ON offshore grid code concerning the reactive powerof generating units, within a voltage range of +5% aroundnominal (at the terminals of the generating unit). The

Table 4 Frequency limits according to Hydro-Quebec and AESO grid codes [30, 32]

Hydro-Quebec AESO

Frequency (Hz) Minimum time without tripping Frequency (Hz) Minimum time without tripping

f , 55.5 disconnection f , 57 disconnection

55.5 � f , 56.5 0.35 s 57 � f , 57.3 0.75 s

56.5 � f , 57.0 2 s 57.3 � f , 57.8 7.5 s

57.0 � f , 57.5 10 s 57.8 � f � 58.4 30 s

57.5 � f � 58.5 1.5 min 58.4 , f � 59.4 3 min

58.5 , f � 59.4 11 min 59.4 < f � 60.6 Q5continuous operation

59.4 < f � 60.6 continuous operation 60.6 , f � 61.6 3 min

60.6 , f � 61.5 11 min 61.6 , f � 61.7 30 s

61.5 , f � 61.7 1.5 min f . 61.7 disconnection

f . 61.7 disconnection

Figure 23 Requirements for power factor variation range in relation to the voltage, according to the German and British gridcodes [10]

Figure 24 EO.N offshore requirement [23]: minimumuseable P/Q-operating range of a generation unit, withina +5% range around the nominal voltage (at thegeneration unit)



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operating points are defined either by agreement of a value orof a schedule, or by online (remote) set-point determination.The British code requires the high-capacity wind farms tohave an automatic system, which regulates the voltage atthe connection point (or the grid interface point, in thecase of offshore plants). The Irish code further stipulates avoltage regulation system, which will accept a set-point(reference) value for the voltage at the connection point.

The reactive power variation capability according to theBritish and the Irish codes is shown in Fig. 25, where

i. Point A is equivalent (in Mvar) to: 0.95 leading powerfactor at rated MW output

ii. Point B is equivalent (in Mvar) to: 0.95 lagging powerfactor at rated MW output

iii. Point C is equivalent (in Mvar) to: 25% of rated MWoutput

iv. Point D is equivalent (in Mvar) to: þ5% of rated MWoutput

v. Point E is equivalent (in Mvar) to: 212% of rated MWoutput

The above points apply to the British code. The Irish coderequires a power factor equal to 0.835 lagging or leading, atactive power output levels below 50% of the rated.

The methodology used in the UK offshore grid code draft[41] proposes that the reactive power range is expressed interms of power factor, in order to represent more effectivelythe active power exchange between the offshore andonshore TSO.

The Nordic grid code demands that wind farms are able tocontrol their reactive output power, in order to regulate the

voltage at the connection point. According to the Danishcode, the reactive power must be limited within the bandshown in Fig. 26 (where Prated corresponds to the totalrated power of the wind farm). The red line is equivalent to0.995 power factor. Reactive power control can be carriedout by each turbine individually, or centrally at wind farmlevel. The Belgian code mentions that wind farms with arated power above 25 MW must be able to produce orabsorb reactive power from 210% to 45% of their ratedcapacity. For slow voltage variations the following factor aeq

is defined, which must have a value between 18 and 25

aeq ¼ ��DQnet=(0:45xP)

DUnet=Unorm, exp

(1)

where Qnet is the reactive power measured in the high-voltageside of the grid connection transformer, P is the wind farmrated power, Unet is the voltage measured at the high-voltage side of the grid connection transformer andUnorm.exp is the nominal grid voltage.

Hydro-Quebec requires (especially for wind farms with arated power above 10 MW) the existence of an automaticvoltage regulation system that operates within power factorlimits of at least 0.95 (based on the available rated power)leading or lagging. It is emphasised that wind farms haveto contribute to voltage regulation in normal but also inabnormal and dynamic operating conditions. AESOprescribes that voltage regulation and reactive powerperformance of a wind farm will be assessed at the low-voltage side of the grid connection transformer. Fig. 27illustrates the reactive power requirements, which aredivided in continuous reactive capability and dynamicreactive capability. The minimum range for the former is0.9 cap. to 0.95 ind. power factor, whereas for the latter itis 0.95 cap. to 0.985 ind. power factor, based on the MWoutput.

Figure 25 Reactive power in relation to active power according to the British and Irish codes [10]


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In addition, the AESO code demands that wind farmsmust have a continuously acting voltage regulation system,which will operate only in the voltage set-point controlmode. The system shall be calibrated such that a change inreactive power will achieve 95% of its final value no soonerthan 0.1 s and no later than 1 s following a step change involtage. FERC rule mentions that wind farms must be ableto operate with power factor 0.95 lagging to 0.95 leading ifthe TSO judges that this is necessary for the transmissionsystem operation.

A comparison of the above reactive power requirements isshown in Fig. 28 which includes all available active power-reactive power curves imposed by national grid codes.

3.5 Other requirements

The Eltra and Elkraft grid codes [27, 28] prescribe a virtual‘type test’ that verifies the behaviour of the wind turbinesduring voltage dips. It consists in the simulation of a three-phase short circuit, with the wind turbine operating at rated

Figure 26 Reactive power requirements against active output power, according to the Danish code [27]

Figure 27 Reactive power requirements of the Canadian AESO grid code. Area 1 defines the anticipated continuous reactivecapability and area 2 the anticipated dynamic reactive capability [32]



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power, nominal rotor speed and with full compensation. Thecalculation is conducted with the use of the equivalent circuitof Fig. 29a. The system is represented by a Theveninequivalent with a 0.1þ j1.0 V impedance (at the 10 kVlevel). The RMS value of the Thevenin internal voltagemust vary as shown in Fig. 29b. The result is successful ifthe wind turbine reaches its rated power 10 s at most afterthe voltage recovers and withstands two three-phase shortcircuits (with an interval of 2 min between them). Nofurther details for the wind turbine model are specified,

apart from the requirement to submit the software used forthe calculation.

In addition, the Danish grid code for system voltages above100 kV demands additional simulations, proving that thewind farm is able to withstand the impacts from thefollowing asymmetric faults in the grid, without requiringdisconnection of wind turbines in the wind farm:

† Two-phase fault on a line in the transmission grid withunsuccessful reclosure.

† Single-phase fault on a line in the transmission grid withunsuccessful reclosure.

It is also defined that, if the connection point is on thesecondary side of a transformer belonging to the transmissiongrid, the vector group and phase shift in the transformer(typically YNd11) will be taken into consideration at theexamination of asymmetric faults.

Most grid codes prescribe data exchange between wind farmsand system operators, that is signals that the wind farm mustsupply through a telecommunication or SCADA Q6system tothe TSO. Usually these include data concerning voltage,current, active and reactive power output, set-point values, aswell as wind speed and operating status signals for the mainswitchgear and protection systems.

4 Wind turbine technologies andgrid requirementsIn this section, a brief review is presented of wind turbinetechnology aspects, associated with grid code compliance.A basic categorisation of wind turbines, related to theelectrical schemes used and their behaviour in case of griddisturbances, is in constant and variable speed machines.

Figure 29 Specifications for the simulation of the windturbine behaviour during a three-phase short circuit,according to the Danish grid code [27]

a Thevenin equivalentb Voltage profile for the voltage source in (a)

Figure 28 Comparison of reactive power requirements imposed by national or regional grid codes


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Constant-speed wind turbines are equipped with squirrelcage induction generators directly connected to the grid(Fig. 30a). The rotational speed of the rotor is practicallyfixed, since they operate at a slip around 1%. Since theinduction machine absorbs reactive power from the grid,connection of compensating capacitor banks at the windturbine (or wind farm) terminals is necessary. Theiraerodynamic control is based on stall, active stall or pitchcontrol.

A variation of this scheme utilises a wound rotor inductiongenerator and electronically controlled external resistors tothe rotor terminals [45]. This configuration permits a verylimited variation of speed (typically up to 10% above thesynchronous), basically for stress alleviation and powerquality improvement.

In variable speed wind turbines the rotor speed variesconsiderably, depending on the prevailing wind conditions.Two basic configurations for this type of machines areavailable, as illustrated in Figs. 30b and c. The first utilises adoubly-fed induction generator (DFIG) and a rotor convertercascade of reduced rating, whereas the second employs asynchronous or induction generator, the stator of which isinterfaced to the grid via a full converter cascade. The

aerodynamic control of variable speed machine is based onblade pitch control (although stall operation is in principlepossible, but not preferred in practice).

In case of DFIGs, the generator’s stator is directly connectedto the grid whereas the rotor is connected through a cascade oftwo voltage source converters (VSCs) (rectifier-inverter,connected back-to-back, as in Fig. 31). The rotor-sideconverter excites the generator rotor winding at variablefrequency, thus permitting variation of its rotational speed. Atthe same time, the converter regulates the torque and reactivepower developed by the generator. The grid-side converteroperates in synchronism to the grid, transferring active andpossibly reactive power. In this scheme, the power handled bythe rotor circuits and converters varies in direct proportion tothe operating slip. Hence, for a slip variation typically of theorder of +30%, the converter is dimensioned atapproximately 30% of the rated wind turbine power.

Wind turbines with full converter, as in Fig. 30c, use eithera synchronous or an asynchronous generator, whose stator isconnected to the grid via an AC/DC/AC converter cascade.In this case, however, the converter handles the totalgenerator power to the grid and therefore no sizeeconomies are possible. On the other hand, this scheme

Figure 30 Basic wind turbine technologies

a Constant speed wind turbine with squirrel cage induction generatorb Wind turbine with DFIGc Wind turbine with full power converter



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permits full variation of the generator speed and therefore awider speed control range. The most common alternative isthe application of a wound-field synchronous generator (inwhich case the generator-side converter is considerablysimplified) but permanent magnet excitation is also possibleand gains momentum lately. If an asynchronous generatoris employed, then a squirrel cage machine is used, much asin the case of industrial motor drives.

As described in the previous sections, the grid codes requirethat wind farms must remain connected during and after severegrid disturbances, ensure fast restoration of active power to thepre-fault levels, as soon as the fault is cleared, and in certain casesproduce reactive current in order to support grid voltage duringdisturbances. Depending on their type and technology, windturbines can fulfil these requirements to different degrees, asexplained in the following.

Starting with constant speed wind turbines, their low-voltage behaviour is dominated by the presence of the grid-connected induction generator. In the event of a voltage dip,the generator torque and power reduces considerably (by thesquare of its terminal voltage) resulting in the acceleration ofthe rotor, which may in turn result in rotor instability, unlessthe voltage is restored fast or the accelerating mechanicaltorque is rapidly reduced below the levels of the availableelectromagnetic torque of the generator. Further, operationof the machine at increased slip values results in increasedreactive power absorption, particularly after fault clearanceand partial restoration of the system voltage. This effectivelyprevents fast voltage recovery and may affect otherneighbouring generators, whose terminal voltage remainsdepressed. Since the dynamic behaviour of the inductiongenerator itself cannot be improved, possible measures toenhance the fault ride-through capabilities of constant speedwind turbines are the following:

† Improvement in the response of the wind turbineaerodynamic control system, in order to perform fastlimitation of the accelerating mechanical torque, to preventrotor over speed. Physical limitations of the blades and thepitch regulation mechanism impose a limit on theeffectiveness of such an approach.

† Supply of reactive power through static compensationdevices at the wind turbine or wind farm terminals, such asSVCs or STATCOMs. Such a device would provide highamounts of reactive power during faults, to effectivelysupport the terminal voltage and therefore limit themagnitude of the voltage dip experienced by the windturbines. Nevertheless, FACTS are complicated and costlydevices, whereas there is an obvious limitation to thevoltage correction they can achieve, particularly in the eventof nearby system faults. Further, this is a solution to beexamined on a per case basis and tailored to thecharacteristics of each specific installation [46–49].

Other grid code requirements, such as active powerregulation and frequency response, are possible to be metvia the aerodynamic control systems of the wind turbines.More challenging are the reactive power regulation tasksthat require the installation of suitably size FACTS devices.

Variable speed wind turbines, on the other hand, presentthe distinct advantages of direct generator torque andreactive current control and the possibility to endure largerotor speed variations without any stability consequences.For this reason, grid disturbances affect much less theiroperation and, generally speaking, they are capable ofmeeting the most stringent grid code requirements.

In case of voltage disturbances, rotor over speed becomesan issue of much smaller significance, since a limitedincrease of speed is possible (e.g. 10–15%) without anyconsequences, the rotor inertia acting as an energy bufferfor the surplus accelerating power, until the pitch regulationbecomes effective. In case of severe voltage dips, an energysurplus may occur within the electrical part of the machine,with the potential to cause a dc capacitor over voltage. Thisis dealt with via proper redesign of the convertercontrollers, increase of the local energy storage capacity(e.g. capacitor size) or even by providing a local powerdissipation means (typically a dc chopper and breakingresistor, connected in parallel to the dc capacitors).

However, even with variable speed wind turbines there stillexist LVRT issues affecting their response. In the case of

Figure 31 Configuration of the electrical part of a DFIG wind turbine


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DFIG wind turbines, the direct connection of the generatorstator to the grid inevitably results in severe transients in caseof large grid disturbances. Hence, the stator contributes ahigh initial short circuit current, whereas large currents andvoltages are also induced in the rotor windings, as aconsequence of the fundamental flux linkage dynamics ofthe generator. Furthermore, the depressed terminal voltagereduces accordingly the power output of the grid sideconverter in the rotor circuits, leading to an increase of thedc bus capacitor, possibly to dangerous levels.

To protect the power converters from over voltages andover currents, DFIGs are always equipped with a deviceknown as the crowbar, which short circuits the rotorterminals as soon as such situations are detected. Once thecrowbar is activated, the rotor-side converter is deactivatedand the DFIG behaves like a conventional inductionmachine, that is control is lost over the generator. Notably,crowbar activation may occur not only at the instant of avoltage dip, but also in case of abrupt voltage recovery, afterclearance of a fault. Conceptually, two crowbar options areavailable:

† The passive crowbar, utilising a diode rectifier or a pair ofantiparallel thyristors to shorten the rotor terminals. Thedisadvantage of this option is the lack of control on thedeactivation of the crowbar, leading to sustained operationwith short-circuited rotor, much as a squirrel cageinduction machine.

† The active crowbar that uses IGBT switches to short therotor. This enhances considerably the operation of thedevice, with a faster elimination of the rotor transients(typically within 100 ms) and therefore faster regain ofcontrol. After deactivation of the crowbar, fullcontrollability over the wind turbine behaviour is resumed.

Hence, although voltage dips inevitably cause torque andpower transients in the DFIG wind turbine, which excitethe rotor crowbar protection for a limited time, the variousimplementations of the active crowbar can improve thestability of the wind turbine and its response to suddenvoltage changes [50].

Another important improvement can be achieved by theaddition of a stator switch, as shown in Fig. 31 [3]. Thestator is disconnected for a short period (short-terminterruption – STI) through the stator switch and the rotoris demagnetised. After that, the generator side inverter isrestarted, the stator is reconnected and the operation isresumed. During the stator disconnection, the grid sideconverter stays active and feeds reactive power to the grid.This implementation results in the limitation of transients,both in magnitude and duration, and permits to keep fullcontrol over the generator during the greatest part of thedisturbance interval. Moreover, with proper controlstrategies of the rotor and grid-side converters, the DFIGcan satisfy requirements for reactive power injection during

faults, as the ones imposed by the Spanish grid code [51].Improved control algorithms of DFIG turbines cansignificantly reduce the mechanical stresses, especially inthe case of the predominantly asymmetrical grid faults [52].The inclusion of energy storage at the dc bus significantlyenhances the fault ride-through capability of the WT,whereas several other improvements to the generatorconfiguration and control during the fault and recoveryperiods are proposed (e.g. [53–55]).

Variable speed wind turbines with full power converters,Fig. 30c, presents the distinct advantage that the convertertotally decouples the generator from the grid. Hence, griddisturbances have no direct effect on the generator, whosecurrent and torque variations during voltage dips are muchlower compared with the DFIG and the respectivetransients fade out faster [56]. The converters, on the otherhand, are almost (but not entirely) immune to gridtransients, due to the high bandwidth of the PWM currentcontrollers. The only essential issue that still remains is theimbalance between the generator power, injected to the dcside, and the output power to the grid, which may bedrastically reduced, leading to overcharging of the dc buscapacitor. This can be resolved with fast pitch control andlimited rotor over speed, to reduce the generator power, aswell as via increased storage and possibly power dissipationmeans at the dc link. From the point of view of the reactiveoutput power, the grid side converter has the ability toproduce reactive current during the voltage dip, up to itsrated current capacity, exhibiting a behaviour superior tothat of the DFIG wind turbines, particularly when dealingwith stringent reactive power support requirements duringfaults, such as those of the E.ON grid code. It is notedthat this wind turbine type may exhibit better voltagecontrol capabilities than conventional synchronousgenerators [57, 58]. Hence, they can provide grid support(improvement of the voltage level and faster voltagerecovery), with a positive impact on nearby connected stallwind turbines, reducing the probability of tripping forthose turbines [59]. Moreover, oversizing of the grid sideconverter provides enhanced active and reactive powerinfeed capability, as shown in [60]. Another notableadvantage compared with the DFIG-based wind turbines isrelated with the behaviour of the latter in case ofunbalanced disturbances. In such situations, the lownegative sequence impedance of the induction generatormay give rise to large rotor currents, whose frequency liesoutside the controllers’ bandwidth, resulting in theactivation of the crowbar (or the disconnection of thestator) until the disturbance is cleared.

Wind turbines can control their active power output bypitch control, whereas variable speed wind turbines havethe additional capability for such control via variation oftheir rotor speed. Hence, power curtailment, ramp ratelimitations and contribution to frequency regulation ispossible, even for constant speed machines. In the lattercase, however, the grid frequency is directly related to the

20 IET Renew. Power Gener., pp. 1–25& The Institution of Engineering and Technology 2009 doi: 10.1049/iet-rpg.2008.0070

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generator slip and hence a change in frequency willtransiently affect the active power produced by the windturbine. In the case of variable speed machines, on theother hand, the generator torque and power is regulated bythe converters and therefore their primary frequencyresponse is entirely adjustable via proper design of thecontrol systems. For instance, in [61], the wind farmcapability of providing short-term active power reserveutilising the kinetic energy of the turbine rotors, isquantified on a multi-megawatt commercial variable speedwind turbine. Reference [62] examines the frequencyresponse of constant speed and variable speed windturbines equipped with synchronous generators or DFIGsand concludes that the best frequency responsecharacteristics are achieved by synchronous generator-basedmachines (full-power converter scheme), then by constantspeed wind turbines and last from DFIG wind turbines.

Up to now, wind turbine compatibility to the various gridcode requirements is established only through specific tests orsimulations that are performed by the manufacturers or otherindependent laboratories, upon demand of system operators.Standardised-type tests have not been developed yet, due tothe diversity of requirements appearing in grid codes andthe relatively limited time they have been in force.Moreover, testing the actual behaviour of wind turbinesduring system faults presents significant difficulties, sinceon site tests on installed machines are necessary, whichwould involve variation of power system variables, such asvoltage and frequency. A first reference to such type testsappears in the draft of the IEC 61400-21, 2nd Ed. [63],which is under development. According to this standard,the response of the wind turbine to the temporary voltagedips specified in Table 5 is to be measured. The statedresponse includes time series of active power, reactive powerand voltage at wind turbine terminals for the time shortlyprior to the voltage dip and until the effect of the voltagedip has abated. The test should be carried out for the windturbine operating at 20% and 100% of rated power. The

test can be carried out using the set-ups outlined inFig. 32. Notably, the test is basically for providing a basisfor wind turbine numerical simulation model validation,although optional tests may be carried out for assessingcompliance with specific grid code requirements. In thedraft standard [63] it is recommended that the test iscarried out on the complete wind turbine rather than on itsdrive train only, since other reasons for cut-out may exist,besides the electrical drive train.

In the case of offshore wind farms, the configuration of theinterconnection to the transmission system is also an issue ofinterest in terms of grid code compliance. Interconnectioncan be realised either with HV AC submarine cables,typically including dynamic VAR compensation, or withHVDC schemes, based on VSC [14]. The VSC-basedHVDC interconnection demonstrates superior dynamicperformance, providing fault ride-through capabilities evento wind farms utilising simple asynchronous generator-based wind turbines [64].

The installation of properly sized centralised FACTSdevices, with the capability to provide wind turbine powerfactor control and voltage regulation and enhanced LVRTcapability may provide grid-code compliance even for windfarms comprising turbines which are partly compliant tocode requirements [65]. In general, the requirements onreactive power compensation/regulation for large windfarms may exceed the reactive power capabilities of the windturbines themselves, in which case external reactive

Figure 32 Short circuit emulator for testing wind turbineresponse to temporary voltage dips [63]

Table 5 Specification of voltage dips according to [63]. The specified magnitudes, durations and shapes apply for voltage dipsoccurring when the wind turbine under test is not connected

Case Magnitude of phase to phase voltage(fraction of voltage before the dip)

Magnitude of voltage(positive sequence)

Duration(s)

Shape

symmetrical three phasevoltage dip 1

0.90 + 0.05 0.90 0.5 + 0.05


0.50 + 0.05 0.50 0.5 + 0.05


0.20 + 0.05 0.20 0.2 + 0.05

two phase voltage dip 1 0.90 + 0.05 0.95 0.5 + 0.05




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compensation systems are employed, consisting of FACTSdevices (SVCs or inverter-based dynamic compensators) andpossibly also multiple-switched capacitor banks to meet theoverall reactive power requirements for large wind farms [66].

5 Critical view on grid codedevelopmentThe grid code overview provided in Section 3 reveals that therequirements imposed on wind farms differ considerablyfrom one country to another, which inevitably opens thediscussion on the need for harmonisation of the grid-codeprovisions. To a certain extent, differences are reasonableand inevitable, since the requirements are directly related tothe particular technical and operational characteristics ofthe systems, as already discussed in Section 3. For instance,it is reasonable that primary frequency responserequirements will be increased in a weakly interconnectedsystem, such as Ireland, compared to a large and stronglyinterconnected system belonging to UCTE.

On the other hand, the diversity of the existing requirementsposes a disproportionate burden on WT manufacturers, whichare constantly challenged to adapt the design of their turbines tothe latest requirements of the network operators and strive todevelop wind turbines suited for specific markets, rather than‘universal’ products. Manufacturer difficulties start withinterpreting the underlying meaning of the various codedocuments, managing the differences in grid code formats(terminology and definitions, designation, parameterisationetc.), understanding the particularities of each power systemand then move to the development of hardware and softwaresolutions for the specific requirements of each code [67–69].

Besides the harmonisation issue, another point of disputebetween system operators and the wind industry is theextent and the strictness of the requirements. The windindustry argues that the requirements imposed on windturbines in some cases exceed those for conventional powerstations, while the time granted for meeting them has beenminimal. Further, there is also a view that the demand forprovision of ancillary services from wind farms lies beyondthe actual needs of today’s systems. TSOs, on the otherhand, claim that wind power penetration levels in severalsystems are high enough to warrant a treatment of windfarms similar to that of conventional stations. Further, evenif certain grid-code provisions are indeed ahead of theirtime, retrofitting installed wind farms for their satisfactionmay not be possible in the future and thus they need to bein place from today.

In this context, a discussion has already started on the issueof further development and harmonisation of wind farm-related grid code requirements. For example, EWEA hasproposed the development of a ‘Generic European WindGrid Code’, built on a technical basis elaborated jointlyfrom TSOs and the wind power industry. If such a

proposal could be introduced at European level, it wouldset a strong precedent for the rest of the world [68]. Itwould also enhance the European internal electricitymarket, where national networks are interlinked and mustbe operated as part of an integrated European grid toenable the necessary cross-border exchanges. However, itmust be stressed that a complete European technicalharmonisation is not practical in the short term and couldonly lead to the implementation of the most stringentrequirements from each nation. Currently, it is realistic toexpect a harmonisation on the major categories ofcapabilities and the circumstances in a power system thatthese may become necessary. Exact values for parameters,limits, set-points etc. needed not be standardised, sincethey depend on system characteristics (for instance, a fasterfrequency response will be necessary for systems with a lowinertia).

In any case, there exist certain issues and principles to betaken into account in the future grid code development.Codes should be comprehensive, transparent and explicit toavoid misinterpretation. Clear and commonly sharedterminology and definitions are necessary in all aspects, windfarm or power system related. The requirements imposedshould reflect an optimum balance between cost and technicalperformance. A reasonable amount of development time isalso needed, by planning grid code requirements in advance.Specific areas that require additional consideration include thelarge offshore installations and their extended HVDCnetworks, already starting to be addressed in certain countries(Germany, UK). Proper consideration should also be given toregional networks belonging to larger interconnected systems,such as the UCTE network. In such cases, major events (e.g.faults) may be experienced by several neighbouring TSOs andtherefore the grid code provisions should ensure a coordinatedoverall system response. Obviously, the collaboration amongTSOs and UCTE is necessary to this end [70]. Finally, grid-code compliance verification remains a major open issue forwind turbines and wind farms. This concerns specific windpower plant capabilities and will require the development ofstandards for testing, from the level of the component up tothe entire wind power plant.

6 ConclusionsIn this paper, the grid code technical requirements werepresented for the connection of wind farms to the powersystems, basically at the HV level. A comparative overviewand analysis of the main requirements was conducted,comprising several national and regional codes from manycountries where high wind penetration levels have beenachieved or are expected in the future. The objective of theserequirements is to provide wind farms with the control andregulation capabilities encountered in conventional powerplants and necessary for the safe, reliable and economicoperation of the system. Current wind turbine technology,and particularly its development over the last 5–10 years,has been heavily influenced by these requirements. Modern



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wind turbines are indeed capable of meeting all requirementsset, with the exception of the constant speed machines, whichare practically not produced anymore.

7 AcknowledgmentThis work has been financially supported by the RegulatoryAuthority for Energy (RAE) of Greece.

8 References

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RPG57167Author Queries

M. Tsili, S. Papathanassiou

Q1 Please provide the full form of “HV”.

Q2 Please provide full form of “AVR”.

Q3 Please provide full form of “HVDC”.

Q4 Tables 2 and 3 are not cited in text. Please check.

Q5 Please provide significance of bold characters in table 4.

Q6 Please provide full form of “SCADA”.

Q7 Please provide author names for reference [10, 22–38, 41, 68–71].

Q8 Reference [71] is not cited in text. Please check.

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Dr. Stavros Papathanassiou Electrical & Computer Engineering NTUA 9, Iroon Polytechniou st. Zografou Athens Greece

Reference: Item Number 0070U Ref. Id: RPG-2008-0070 A review of grid code technical requirements for wind farms IET– PROOFS Please check the enclosed proofs carefully. The ultimate responsibility for the accuracy of the information contained within the paper lies with the author. However, only essential corrections should be made at this stage and the proofs should not be seen as an opportunity to revise the paper. Please return all corrections to this office by the date on the proof. Late corrections may not be included.

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