integrating windfarms to grid

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TECHNOLOGIES FOR INTEGRATING WIND FARMS TO THE GRID(Interim Report)

Contractor

AREVA T&D Technology Centre (with University of Manchester)

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The work described in this report was carried outunder contract as part of the DTI TechnologyProgramme: New and Renewable Energy, which ismanaged by Future Energy Solutions. The views andjudgements expressed in this report are those of thecontractor and do not necessarily reflect those of theDTI or Future Energy Solutions.

First published 2006-.-


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Acknowledgements:

The authors would like to thank Dr Mark Osborne and Mr Antony Johnson of the NationalGrid and Dr Phillip Cartwright of AREVA T&D, and Dr Olimpo Anaya-Lara and ProfessorNick Jenkins of the DTI Centre for Distributed Generation and Sustainable Electrical

Energy for their comments and suggestions.


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EXECUTIVE SUMMARY

This report discusses possible technologies for enabling the integration of wind farms intothe power supply network. The capabilities of each technology in complying with therequirements set by the Grid Code are analysed. Subsequent reports associated with this

project will establish the effect of a STATCOM to enable a grid connection of wind farms tothe National Grid power supply network.

Background of the work

The Government has set targets for the UK to generate 10% of electrical energy fromrenewable sources by 2010, 15% by 2015, with aspirations of 20% by 2020. By 2050 thereis a further aspirational goal to reduce emissions by 60% against a baseline year of 1990.Drivers behind this policy include the UKs obligations within the United NationsFramework Convention on Climate Change, the Kyoto Protocol, carbon reductionmeasures set by the EU and the enhancement of security of energy supply within the UK.The UK possesses a third of the EUs potential for offshore wind generation, yet in 2000renewables supplied only 1.3% of the UKs electricity compared with 16.7% in Denmarkand 3.2% in Germany. In order to help facilitate the Government targets from renewablesources, the UK Renewables Obligation and the Climate Change Levy together willprovide the renewables industry with 1billion/year of support. To meet the Governmentsaspiration to reduce CO2 emissions, a penetration of 25% to 35% renewables will berequired. As the total installed capacity of wind farms increases, the reliable and secureoperation of the electrical power transmission network needs careful consideration.Hitherto, wind farms were considered essentially as negative loads which did notcontribute to voltage or frequency support. However, for wind energy to become a

substantial quantity within the British generation portfolio, the technical capabilities of windfarms need to be enhanced. Grid Code compliance is a necessity and further research anddevelopment is needed to fully ensure that wind farm designs meet the requirementsspecified by the GB Grid Code.

The work performed

To illustrate the technical challenges of integrating large wind farms into the network, ananalysis of the GB Grid Code was carried out with particular attention paid to:

Voltage control

Fault ride through capability

Fault current contribution Frequency control.

The main three wind generation technologies reviewed were:

Doubly Fed Induction Generators (DFIGs)

Fixed Speed Induction Generators (FSIGs)

Direct drive synchronous generators through back-to-back voltage sourceconverters.


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The four technical architectures for the connection of offshore wind farms to the powersupply network reviewed were:

AC connection

AC connection along with employment of Dynamic Reactive Compensation devices

Voltage Source (VS) HVDC link

Line Commutated (LC) HVDC link.

The GB Grid Code requirements apply to the whole wind farm as seen at the Point ofConnection (PoC) rather than at the turbine terminals. The philosophy behind this decisionavoids discrimination between different wind turbine technologies and promotes flexibilityfor wind farm developers. Since the Grid Code requirements are based on the behaviourof the windfarm at the PoC, a discussion of potentially Grid Code compliant configurationshas to include representative systems from all major generator technologies, and possiblecombinations with available approaches for connecting offshore wind farms into the powersupply network. A qualitative comparison of the technical capabilities of thesecombinations was conducted and is presented in tabular form.

Finally, this report provides an overview of the current state-of-the-art modellingtechniques used for various components of a wind farm. Generic models of a DFIG, aFSIG generator and a STATCOM device are shown through simulation in thePSCAD/EMTDC software modelling package.

Findings

For cases of wind farms connected through AC submarine cables, without asupplementary reactive compensation device, the performance at the PoC depends mainly

on the employed wind turbine generator technology. If a dynamic reactive compensationdevice is used, then this complementary technology has the potential to enhance thebehaviour of the wind farm,especially for fault ride-through, reactive power and voltagecontrol capability. In the case of a DC connection to shore, the performance of the windfarm as seen at the PoC depends mostly on the inherent technical capabilities of theemployed DC technology.

Doubly-Fed Induction Generators (DFIGs) have been extensively researched and thecapability of DFIGs to comply with the requirements for power quality is well documented.In cases where the electrical distance between the wind farm and the PoC is large, the useof a static and/or dynamic reactive compensation device may be necessary in order that

the required reactive power capability of the farm is satisfied, as measured at the PoC.System frequency control has been demonstrated in the Horns Rev wind farm byappropriate blade pitch angle control of the individual wind turbines. Fault ride throughcapability is starting to be demonstrated in the public domain through simulations and sitetests. However, there is a lack of information on the impact of a wind farm comprisingDFIG wind turbines on the existing network under transient and fault conditions.

A wind farm comprised of Fixed Speed Induction Generators (FSIGs) and connected tothe grid with an AC connection without dynamic VAr compensation, cannot meet the newrequirements of the Grid Code. Particular issues are voltage control and the supply ofreactive power to the network during faults, and here the use of a dynamic reactive

compensation device can significantly enhance the wind farms technical capabilities.Frequency control can be provided by a wind farm with FSIG wind turbines when they


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possess blade angle controllers. For transient stability, recent research shows that acombination of improved machine design and supplementary controls during faults can aidfault ride through capability but this must be coordinated with Grid Code requirements forpost-fault real power output recovery. Still, further work is needed for an assessment of theimpact of a STATCOMs rating and electrical point of connection in technical and

economical terms.

Theoretically a wind farm connected to the power supply network through a VS HVDCscheme can meet all the requirements set in the GB Grid Code. Evaluation is necessaryon a site specific basis for the performance of the scheme during faults in the powersupply network, especially to establish whether the scheme can provide active powersupport to the network during voltage sags at the PoC. The principal drawback of VSHVDC to date is the relatively large power losses, and the limited DC voltage (150kV)compared to conventional HVDC links (500kV) based on present technology. It is credibleto consider that VS HVDC will become more attractive in the future, as the technologiesbehind self-commutated switches and multilevel inverters progress, increasing device

power ratings, decreasing losses and reducing capital costs.

Even though the use of Line Commutated HVDC links is a mature technology, itsapplication for connecting offshore wind farms to the power supply network has beenproposed only recently. One particular technical issue for wind turbines employing powerelectronic interfaces, is that they may have limited capability to provide reactive powersupport and voltage control to the offshore wind farms DC link rectifier terminal. If the windturbine generators are FSIGs then the issue is more prevalent since the turbines areessentially passive in terms of voltage control. Configurations where a synchronous sourceis used at the wind farm to provide the necessary e.m.f for operation of the rectifierthyristor converters have been proposed. This type of configuration henceforth referred toas Hybrid HVDC, can have a similar behaviour to a VS HVDC link even though additionalauxiliary equipment may be needed. However, research is needed especially in the areasof ride through capability and power system frequency response.


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CONTENTS PAGE

EXECUTIVE SUMMARY (iii)

CONTENTS PAGE (vi)

1. BACKGROUND 1

2. GB GRID CODE CONNECTION REQUIREMENTS 2

2.1 Reactive Power and Voltage Control 22.2 Fault Ride-through Capability 32.3 Frequency Range of Operation and Power Frequency Characteristic 42.4 Frequency control 42.5 Power Quality Harmonics 42.6 Power Quality Flicker 42.7 Further Requirements for Wind Farms 5

3. WIND TURBINE GENERATION TECHNOLOGIES 6

3.1 Fixed Speed Induction Generators (FSIG) Wind Turbines 63.2 Doubly Fed Induction Generators (DFIG) Wind Turbines 63.3 Direct Drive Synchronous Generator Wind Turbine 6

4. MAIN TYPES OF CONNECTION TO THE TRANSMISSION NETWORK 8

4.1 Synchronous connections 84.1.1 AC Connection 84.1.2 AC Connection plus Dynamic Reactive Compensation 9

4.2 Asynchronous connection 104.2.1 Voltage Source HVDC 104.2.2 Line Commutated HVDC 11

5. WINDFARM TECHNICAL CAPABILITIES IN RESPECT TO GENERATORTECHNOLOGY AND TYPE OF CONNECTION

13

6. MODELLING ASPECTS 15

6.1 Offshore Wind Farm 156.2 Mechanical Dynamics of the Wind Turbine Generators 156.3 Power Electronic Converters 16

7. MODEL DEVELOPMENT AND EVALUATION 17

7.1 Wind farm with DFIGs and no STATCOM 217.2 Wind farm with FSIGs+STATCOM 22

7.3 Comparison between average and switched models 238. CONCLUDING REMARKS AND FURTHER WORK 24


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REFERENCES 25

GLOSSARY OF TERMS 29


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1. BACKGROUND

Under present plans, without new-builds, only one nuclear energy plant will still beoperating in the UK by 2025. The 2003 Energy White Paper [1] does not rule out theconstruction of new nuclear plant but points out that its current economics make it an

unattractive option for new, carbon-free generating capacity and there are also importantissues of nuclear waste disposal to be resolved. For coal, the White Paper concludesCoal fired generation will also have an important part to play in widening the diversity ofthe energy mix provided ways can be found to materially reduce its carbon emissions.Although it also notes that: Domestic coal production is likely to continue to decline asexisting pits reach the ends of their geological and economic lives. This leavesrenewables, particularly wind, with an important part to play in the UK energy productionmix to ensure diversity and security of supply.

The DTI and Carbon Trust have recently commissioned an impact study on renewables [2]to assess the impact on the electricity network of Government aspirations. Broadly thefindings were that 72% of the 2010 target can be met by 2006, but significant barriers needto be addressed for further expansion of renewables, particularly planning uncertainty andreinforcement of the transmission and distribution networks. The level of reinforcementrequired to the transmission and distribution systems in Scotland and North-West Englandrequires expenditure in the order of at least 1.4bn for the transmission network and780m for the distribution network [2] to ensure a secure and stable system. Furthermorethe transmission value quoted above is based on power flows and system security butexcludes transient stability and fault levels [3]. Intermittency is not thought to be asignificant issue affecting the development of renewables for present 2010 targets, butbalancing costs may increase substantially for the 2020 target.

Hitherto, wind farms were considered essentially as negative loads which did notcontribute to voltage or frequency support. However, when wind energy becomes asubstantial percentage within the British generation portfolio, the technical capabilities ofwind farms need to be enhanced. As a consequence of this, the three TransmissionLicensees in Great Britain (England and Wales National Grid, Scottish PowerTransmission and Scottish Hydro Electric Transmission Limited) revised the Grid Coderequirements. Grid Code compliance is a necessity and further testing and verification isneeded to fully ensure that wind farm designs satisfy the requirements for fault ridethrough specified by the latest GB Grid Code. A summary of the Grid Code process isgiven in [4].


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2. SUMMARY OF THE GB GRID CODE CONNECTIONREQUIREMENTS

The requirements for new and renewable forms of generation including DC converterswere introduced into the Grid Code on the 1st June 2005. The key issues of the

Connection Conditions of the GB Grid Code [5] specifically concerning wind farms arereviewed. The technical requirements are defined for the whole wind farm as seen at thePoint of Connection to the power supply network and not individually for each generator.

2.1 Reactive Power and Voltage Control

According to CC.6.3.2, CC.6.3.6, and CC.6.3.8 of the Grid Code, wind farms have to beable to provide automatic voltage control at the Point of Connection (PoC) by continuouschanges to their reactive power output, according to a slope characteristic whose exactspecifications will be specified in a site specific bilateral agreement. The general reactive

power capability requirement of a wind farm is defined in CC.6.3.2 and depicted in figure1.This reactive power capability applies when all wind turbines are in service. If fewermachines are operating then the required maximum amount of reactive power isdiminished proportionally with the number of machines running.

Point A is equivalent (in MVAr) to: 0.95 leading p.f. at Rated MW outputPoint B is equivalent (in MVAr) to: 0.95 lagging p.f. at Rated MW outputPoint C is equivalent (in MVAr) to: -5% of Rated MW outputPoint D is equivalent (in MVAr) to: +5% of Rated MW outputPoint E is equivalent (in MVAr) to: -12% of Rated MW output

Figure 1: Required reactive power capability of wind farms

(Figure 1 of Connection Conditions of the Grid Code [5])


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2.2 Fault Ride-through Capability

The required fault behaviour of a wind farm can be summarised into four requirements:

For system faults that last up to 140 ms, the wind farm has to remain connected tothe network. For supergrid voltage dips of duration greater than 140 ms, the wind

farm has to remain connected to the system for any dip-duration on or above theheavy black line of figure 2.

During system faults and voltage sags, a wind farm has to supply maximumreactive current to the Grid System without exceeding the transient rating of theplant.

For system faults that last up to 140 ms, upon the restoration of voltage to 90% ofnominal, a wind farm has to supply active power to at least 90% of its pre-faultvalue within 0.5 sec.For voltage dips of duration greater than 140 ms, a wind farmhas to supply active power to at least 90% of its pre-fault value within 1 sec ofrestoration of voltage to 90% of nominal.

During voltage dips lasting more than 140 ms, the active power output of a windfarm has to be retained at least in proportion to the retained balanced supergridvoltage.

It should be noted that in cases where less than 5% of the turbines are running, or undervery high wind speed conditions where more than 50% of the turbines have been shutdown, a wind farm is permitted to trip.

Figure 2: Required ride through capability of wind farms for supergrid voltage dips ofduration greater than 140 ms (Figure 5 of Connection Conditions of the Grid Code [5])


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2.3 Frequency Range of Operation and Power Frequency Characteristic

According to CC.6.1.3 of the Grid Code wind farms have to be able to operatecontinuously for system frequencies between 47.5 and 52 Hz and for at least 20 sec forsystem frequencies between 47 and 47.5 Hz.

With the generating plant operating at full power output at frequencies between 47 Hz and50.4 Hz, the minimum active power output as a function of system frequency deviationsfrom 50 Hz is described in CC.6.3.3 and is depicted in figure 2 of the Grid CodeConnection Conditions. For frequencies between 50.4 Hz and 52 Hz the power output of agenerator has to decrease at a minimum of 2% of output power for every 0.1 Hz rise ofsystem frequency above 50.4 Hz (Grid Code Reference: Balancing Code 3.7.2).

2.4 Frequency Control

According to CC.6.3.6 (a) and CC.6.3.7, wind farms are required to have the capability to

contribute to frequency control by adjusting their power output. Accordingly, CC.6.3.7obliges wind farms to be fitted with a fast acting proportional frequency control device. Thisis equivalent to a speed governor controller of a synchronous generator in a thermal plant.

When in Frequency Sensitive Mode, generators (both synchronous and non-synchronous) have to provide primary, secondary and high frequency response, which aredefined in CC.A.3.1 and in figuresCC.A.3.2 and CC.A.3.3 of the Grid Code. The minimumlevel of frequency response that generators have to achieve is defined in figureCC.A.3.1.

2.5 Power Quality Harmonics

The harmonic content from a wind farm as a whole has to be limited in accordance withthe values set in Engineering Recommendation G5/4.

2.6 Power Quality Flicker

A specific form of harmonic distortion caused especially by wind farms with fixed speedinduction generators is flicker, i.e. voltage variations of frequency 8 to 10 Hz that cancause light variations from incandescent lamps that are especially annoying to the humaneye and may even trigger epilepsy episodes [6]. Short Term and Long Term FlickerSeverities at the PoC have to be within the limits of Engineering Recommendation P28.


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2.7 Further Requirements for Wind Farms

During phase to earth and phase-phase to earth faults, phase-to-earth voltages can riseup to 140% in England & Wales and 150% in Scotland. This should be taken into accountwhen establishing the protection settings and voltage ratings of wind farm equipment.


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3. WIND TURBINE GENERATION TECHNOLOGIES

3.1 Fixed Speed Induction Generators (FSIG) Wind Turbines

The FSIG wind turbine is a simple squirrel cage induction generator, which can be directly

coupled to the electricity supply network. The frequency of the network determines therotational speed of the stators magnetic field, while the generators rotor speed changesas its electrical output changes. However, because of the well known steep torque-slipcharacteristic of the induction machine, the operating range of the generator is verylimited. The wind turbine is therefore effectively fixed speed. FSIGs do not have thecapability of independent control of active and reactive power, which is their maindisadvantage. Their great advantage is their simple and robust construction, which leadsto lower capital cost. In contrast to other generator topologies, FSIGs offer no inherentmeans of torque oscillation damping which places greater burden and cost on theirgearbox.

3.2 Doubly Fed Induction Generators (DFIG) Wind Turbines

The DFIG is a wound rotor induction generator whose rotor is fed via slip rings by afrequency converter. The stator is directly coupled to the electrical power supply network.As a result of the use of the frequency converter, the network frequency is decoupled fromthe mechanical speed of the machine and variable speed operation is possible, permittingmaximum absorption of wind power. Since power ratings are a function of slip, DFIGsoperate over a range of speeds between about 0.75 and 1.25 pu of synchronousfrequency, which requires converter power ratings of approximately 25% [7]. A greatadvantage of the DFIG wind turbine is that it has the capability to independently control

active and reactive power. Moreover, the mechanical stresses on a DFIG wind turbine arereduced in comparison to a FSIG. Due to the decoupling between mechanical speed andelectrical frequency that results from DFIG operation, the rotor can act as an energystorage system, absorbing torque pulsations caused by wind gusts [8]. Other advantagesof the DFIG include reduced flicker and acoustic noise in comparison to FSIGs. The maindisadvantages of DFIG wind turbines in comparison to FSIGs are their increased capitalcost and the need for periodic slip ring maintenance.

3.3 Direct Drive Synchronous Generator Wind Turbine

An alternative to the much-used induction machine generator is the use of a multipole

synchronous generator, fed through a power electronic AC/DC/AC stage. The excitation ofthe synchronous generator can be given either by an electrical excitation system or bypermanent magnets. The AC/DC/AC converter acts as a frequency converter anddecouples the generator from the Grid. It consists of two back-to-back voltage sourceconverters, usually with IGBT switches, which can independently control the active powertransfer through the dc link and the reactive power output at each converter terminal [9].The speed range is generally similar to that of DFIGs [10]. The multipole construction ofthe synchronous generator leads to a low mechanical rotational speed of the generatorrotor and can permit direct coupling to the wind turbine. The possibility of reducing thenumber of stages in the gearbox or eliminating it completely is often quoted as anadvantage of direct drive synchronous generator wind turbines. However set against this is

the greater VA rating of the power electronic converter compared with DFIGs and thelarger physical generator size.


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As a result of the increased mechanical stresses experienced by FSIG wind turbines atpresent there is a practical limit to the rating of commercial models of this technology. Allpresent commercial models for multi-MW wind turbines in the range above 3MW are eitherDFIGs, or synchronous generators coupled to the network through back-to-back

converters.


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4. CONNECTION OPTIONS TO THE TRANSMISSION NETWORK

The following section describes the technical issues associated with various types ofconnection for offshore wind farms and existing transmission and distribution technology.

Connection options of an offshore windfarm to the Grid can be divided into two categories: AC connection options

The wind turbines are connected to the power supply network through ACsubmarine cables. The electrical field of the generators rotates synchronously withthe power supply network.

DC connection optionsThe wind turbines are connected to the power supply network through a DCtransmission scheme. The offshore wind farm is decoupled electrically from thepower supply network. The electrical frequency of the offshore wind farm isdetermined by the control of the DC scheme. As a consequence the electrical fieldof the generators may run with a frequency different to the frequency of the powersupply network.

4.1 AC connection options4.1.1 AC Connection

For a simple AC connection, the behaviour of the wind farm is defined solely by theinherent technical capabilities of the wind turbines. In this report the term AC connectiondenotes this case.

Referring to GB standardised voltage levels, the two principal options for the AC

connection of large offshore wind farms are the use of multiple 33 kV links, which is thecheapest option for distances of a few kilometres, or voltages of 132 kV. The latter optionenables a reduction in power losses [11, 12] but will necessitate an offshore platform toaccommodate the step-up transformer(s) and circuit breaker(s). The presently favouredstrategy for relatively large offshore wind farms [13] appears to be the use of 33kV cable tointerconnect the offshore wind farm arrays, feeding to a higher voltage centraltransmission link for connection to the shore. The final choice of voltage level(s) forconnection however is not specified by the GB Grid Code and belongs to the wind farmdeveloper.

The major problem of connection by AC submarine cables is their capacitance. Cables

present a high shunt capacitance in comparison to overhead lines. The capacitivecharging currents increase the overall current of the cable and thus reduce the powertransfer capability of the cable (it is thermally limited). In addition transient voltage stressesare present requiring suitable switchgear. As the length of the cable increases, so does thecapacitance of the cable and the charging currents. As a consequence, a single cable canonly transmit a certain amount of power for a given distance, after which more cables inparallel are required. One way of addressing this issue would be to compensate thecapacitive losses along the length of the cable. This is difficult for submarine cables so thenext best solution is to compensate losses from both ends of the cable. The compensationis usually achieved by fixed shunt inductors. However, even this form of compensation willonly provide a limited improvement to the cable rating. A further limiting factor to be

considered is the voltage drop between sending and receiving end, which in most caseshas to be contained in the range of 10% - 15% [14].


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Another issue that should be taken into consideration at the design phase of an ACconnection for an offshore wind farm is the potential danger of resonance between theshore step-up transformer and the capacitance of the cable at low harmonic frequenciesinherent in the power system [14]. Resonance phenomena can cause unacceptably high

harmonic currents at the critical frequencies, which will exert high stresses in thesubmarine cable(s). Also the amplification of the harmonic currents may in turn causeamplified harmonic voltages at the offshore wind farm [15].

The overall costs of AC transmission cables are higher than the costs of DC transmissioncables because they can transfer less power per cable core. However, an advantage ofAC transmission is that it requires the smallest and thus the cheapest offshore platformwhich represents a major capital cost component. Furthermore, one should include thehigher investment and maintenance costs and operational losses of power converters inthe lifetime cost of DC transmission. If transmission requirements are fulfilled through ACconnection, then an AC connection would appear to be preferable over a HVDC link [14].

4.1.2 AC Connection plus Dynamic Reactive Compensation

Until now Dynamic Reactive Compensation devices in conjunction with wind farms havebeen used mainly for voltage control and mitigation of flicker [16, 17]. However, byspecification, these devices can enhance transient stability of the wind turbine generatorsand may also contribute to fault current [5, 18, 19]. Consequently, Dynamic ReactiveCompensation devices, along with the inherent capabilities of the generators, willdetermine the steady-state and dynamic behaviour of the wind farm as a whole.

There are three main types of Dynamic Reactive Compensation devices:

Static Var Compensators (SVCs)

STATic COMpensators (STATCOMs)

Synchronous CondensersThis report will concentrate on STATCOMs which provide the most capabilities forenhancing the steady-state and especially the dynamic behaviour of an offshore windfarm.

A STATCOM is a shunt voltage source inverter coupled to the network through atransformer or an inductor. A STATCOM can provide or absorb reactive power in acontinuous range. Its output does not depend on the value of the dc link capacitor.

Theoretically the dc link capacitance could have a very small value, however in practice itsvalue should be large enough in order that dc voltage ripple is small and potentialresonances with the coupling reactance are avoided [20, 21].

There are two main classifications of STATCOM, depending on the switching techniquethat is used. A Pulse Width Modulation technique (e.g. Space Vector PWM) permitsindependent control of both the amplitude and the angle of the output AC voltage of theSTATCOM, while 180 degree conduction mode or fixed Selective Harmonic EliminationModulation techniques lead to a dependent relationship between amplitude and angle ofthe output AC voltage. The first technique has higher switching losses and was avoided forlarge ratings in the past, but the progress of technical innovation and the reducing cost of

IGBTs makes the use of PWM switching techniques progressively more promising. Theadvantage of higher switching frequencies is that smaller harmonic filters can be used. In


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addition, if a form of energy storage is incorporated to the STATCOM, PWM switchingallows independent control of its real and reactive power output [22-24], enhancing itscapabilities significantly. Possible applications of the combination of STATCOM andenergy storage could include power oscillation damping [24] and load-levelling [23]. If asufficient amount of energy storage was incorporated, the latter approach could have an

application for the planning uncertainty inherent in weather forecasting. However, technicalchallenges for the incorporation of energy storage into a practical STATCOM remain, andthe current cost of energy storage devices is presently restrictive. The widespread use ofSTATCOMs and energy storage for load levelling is therefore unlikely in the near future.

The response time of STATCOMs is less than one cycle [25], making it the fastestDynamic Reactive Compensation device available. Compared with SVCs which behavelike a variable susceptance, a STATCOM effectively appears as an emf on the AC side.Hence its reactive current injection to the system does not depend on the bus voltage andit can provide, in principle, a sustained fault current during disturbances depending on therating of the DC capacitor. Its main disadvantage is the increased capital cost and losses

due to the use of self-commutated power electronic switches. If the application permits, agood balance between costs and operational behaviour can be achieved with the use of aSTATCOM in conjunction with an SVC [26].

4.2 DC connection options4.2.1 Voltage Source HVDC

Progress in power electronic switches, especially in IGBTs, and the advent of multilevelinverter configurations has made direct current transmission possible based on voltagesource converters. Under current technology, Voltage Source HVDC (VS-HVDC) linkshave a maximum rated power of approximately 500 MW for DC voltages of 150kV [27].As more sophisticated configurations of voltage source converters are developed, alongwith advances in self-commutated switches, the maximum voltage rating of VS HVDC linkswill increase, and consequently their maximum power ratings.

VS-HVDC is inherently bipolar, with each cable carrying half the power [28]. For theconnection of an offshore wind farm to the power supply network, two DC substationswould be required; one onshore and the other at an offshore platform to house the powerelectronic converter at the wind farm along with its step-up transformer and harmonicfilters. The use of DC power transmission through submarine cables offers a number ofadvantages.

Firstly, when HVDC transmission is employed there is no potential danger of resonancebetween the submarine cables and the onshore step-up transformer, since thecapacitance of the cables and the inductance of the transformer are electrically decoupled.Secondly, submarine cables do not have capacitive losses with DC current transmission.The power handling capability of DC cables is larger than that of AC cables for the sameinsulation level, however one must take into account the additional AC/DC converterpower losses which are not present in an AC system.

In contrast to a classical Line Commutated HVDC scheme a Voltage Source HVDC linkhas the capability of independent control between the active power that it transfers and the

reactive power that it absorbs or injects at each terminal [28]. The reactive powercapability at each terminal will depend on the VA rating of the respective converter. VS-


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HVDC links can also be connected to weak or even completely passive networks withoutthe need of additional plant, since they employ self-commutated active switches and maycontribute to short circuit power [29]. In comparison to Line Commutated HVDC, VS-HVDCrequires smaller harmonic filters and the substation has an overall smaller footprint. Forthe case of offshore wind farms this is important, because of the high construction cost of

offshore platforms. It should be noted however that a VS-HVDC offshore substationplatform will generally still be larger than an AC platform.

The drawback of a VS HVDC link is the increased substation capital cost and the losseswhich, when capitalised, represent the bulk of the operating cost. The higher substationcapital cost is attributable to the present price of self-commutated switches. A VS-HVDCsubstation is 10 times more expensive than an AC substation [28]. Using presenttechnology, relatively high losses at the two voltage source converters can result in lossesbetween 5% and 7% [14]. In comparison, the losses of Line Commutated HVDC, includingtransformer losses, have been reduced down to approximately 1.2% [30]. Also, the copperlosses are higher in the case of VS-HVDC, because such schemes must operate with

lower DC voltage than Line Commutated HVDC, resulting in a higher current. Finally, dueto its bipolar nature, VS-HVDC always needs two cables, whereas monopolar LineCommutated HVDC may use one integrated return cable, reducing cable laying costs.

4.2.2 Line Commutated HVDC

A conventional Line Commutated HVDC (LC HVDC) connection based on thyristorconverters is an alternative to Voltage Source HVDC. Line Commutated HVDC is a wellproven technology for the transmission of bulk power over long distances both on land andthrough submarine cable [31]. With present technology LC HVDC links have a maximumrating of 1300-1500 MW at 500kV dc voltage [14].

LC HVDC can be either mono-polar or bi-polar. The mono-polar link has one high voltagedc cable, with the return path provided physically by ground or water, or by using a metallicreturn path [31]. The use of a metallic return path is necessary when earth restistivity ishigh or electromagnetic interference created would be beyond acceptable limits. Themetallic return path is typically a 24kV XLPE cable [14], however cables with an integratedreturn path have recently been developed [32] for which case just one cable is requiredreducing the cable laying cost. The bi-polar configuration has two converters, one haspositive and the other has negative polarity with respect to ground, and they are of equalrating connected in series on the dc side. Two high voltage dc cables are used, each

carrying half the power. If each group of converters and cables is designed to carry fullpower, and stray currents are within acceptable limits (or a metallic return path has beenused), then in the case of a fault in one group, a switch-over is possible and the schemecan operate as a fully rated mono-polar link. This will increase the availability at theexpense of increased capital cost.

Drawbacks of the technology are that it needs an active source of voltage at both endsand LC-HVDC converters always draw reactive power into both terminals. Reactive powerconsumption depends on the transmitted real power and it is generally about 50% of thereal power flow [31], but this is largely compensated for by the capacitive harmonic ACfilters. Line Commutated HVDC on its own cannot feed passive systems or be connected

to weak networks and is prone to commutation failure due to voltage drops.


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De Oliveira [33] and Andersen [34] describe how Line Commutated HVDC may be used toconnect wind farms to the grid. It is necessary to connect a supplementary voltage sourceat the farm end in order to provide the necessary commutation voltage for the thyristorvalves and to provide reactive power compensation during normal operation. However, thesupplementary voltage source may only need to be small in rating, in the case of DFIGs or

PMSGs having their own voltage source converters. The additional voltage source can bea rotating synchronous condenser or a static compensator.

Comparing the two possible HVDC technologies, LC-HVDC is at present probably the bestsolution for transmission of very large powers and/or over long distances. ContemporaryVoltage Source HVDC technology has a maximum capacity up to around 500 MW fordistances approximately up to 500 km [14, 27]. For connection of larger wind farms and/orlonger distances, multiple cables and converters have to be used, which increases thecost prohibitively [35].


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5. WINDFARM TECHNICAL CAPABILITIES IN RESPECT TOGENERATOR TECHNOLOGY AND TYPE OF CONNECTION

Generator Technology

FSIG DFIG, PMSG

RP CS CS

FRT NC C

FC C C

GridCode

compliance

PQ CS C

AC

(Note:Therecanbe

switchedshuntelements)

Designconsiderations

Smaller offshore platform than DC schemesConventional connection option

Avoid possibility of harmonic resonance at design stageIncreased losses at the submarine cable

RP C C

FRT C C

FC C CGridCode

compliance

PQ C C

AC

+DynamicReactive

Compensation


Smaller offshore platform than DC schemes, but could belarger than standard AC designs without dynamic reactive

compensationConventional connection option

Avoid possibility of harmonic resonance at design stageIncreased losses at the submarine cable

RP C C

FRT FR FR

FC FR FRGridCode

compliance

PQ C C

VoltageSource

HVDC


Smaller offshore platform than LC HVDC but larger than ACNew technology

Large conversion losses

RP C C

FRT FR FR

FC FR FRGridCode

compliance

PQ C C

TypeofConnection

LineCommutated

HVDC


Lower lossesUntested technology for connection of offshore windfarms

Larger offshore platformNeeds emf sources at both ends

Additional reactive compensation at PoC may be necessary


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KEY

C: Can be made Compliant (depending on engineering design)

CS: Case specific (depending on grid and connecting cable)

FC: Frequency control

FR: Further research is neededFRT: Fault ride through capability

NC: Non Compliant

PQ: Power quality

RP: Reactive power and voltage control


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6. MODELLING ASPECTS

Computer simulation is a most powerful tool to investigate the means and capabilities ofdifferent technologies for integrating medium and large offshore wind farms to the powernetwork. The following is a discussion on the key issues in the literature relating to how the

various components of a wind farm should be modelled.

6.1 Offshore Wind Farm

For stability studies the most pessimistic scenario corresponds to rated mechanical inputto every wind turbine. Representation of the whole wind farm as a coherent machine isadequate for transient stability studies under fault conditions [10, 36]. Hence, transientstudies with one equivalent machine and rated mechanical input are sufficient fordetermining the ride-through capability of the wind farm. For steady state studies adetailed representation of the wind farm at the design stage is necessary for determiningthe required reactive compensation for complying with the Grid Code. Having determinedthe exact layout of the wind farm and of the reactive compensation, it is possible torepresent the wind farm as a coherent machine connected to the power supply networkthrough an equivalent submarine cable. The parameters of the equivalent submarine cableis chosen in such a way that the simplified model of the wind farm with the one coherentmachine and the detailed representation of the wind farm have, at the PoC, the samesteady state behaviour [36]. Since the major aim of this project is to draw generalguidelines for Grid Code compliance of offshore wind farms, rather than the design of aspecific offshore windfarm, a coherent machine representation of the wind turbines isadopted. This study approach is considered reasonable both for transient studies underfaults and reactive power capability.

6.2 Mechanical Dynamics of the Wind Turbine Generators

In many studies, the mechanical dynamics are not considered and the wind turbine rotor,mechanical drive train, gearbox and generator rotor are modelled as a lumped mass withan equivalent inertia. It is common for this modelling approach to be adopted forsynchronous machines in stability studies [31]. However, wind turbines have very differentmechanical characteristics than conventional plant. They show large inertias and low shaftstiffness. The interaction between the wind turbine and electrical generator could give riseto low frequency oscillations that can limit the transient stability of the farm. Representingthe mechanical system as a lumped mass may give optimistic results especially for FSIGs

for which a two-mass representation of the drive train is typically necessary [37]. ForDFIGs and Direct Drive Synchronous Generators, mechanical and electrical frequency aredecoupled and torsional oscillations can be damped by a controller. Therefore a lumpedmass representation is reasonable for DFIGs and Direct Drive Synchronous Generators.Consequentlya two-mass representation of the mechanical system is adopted for FSIGs,while lumped mass model is adopted for DFIGs.


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6.3 Power Electronic Converters

Typically power electronic converters have been modelled in system studies usingaverage time invariant models [35]. Even though time variant representation is feasible inelectromagnetic simulation software like PSCAD/EMTDC, such models increase the

simulation time. One other disadvantage is that switching noise obscures the slower time-constant response of the machine and control dynamics which are the more importantfeatures for the examination of the wind farm behaviour. Time invariant models of voltagesource converters are based on their representation as controllable ideal voltage sources.The respective equations have already been analysed for DFIG [38, 39], Voltage SourceHVDC [35] and STATCOM [21]. Average time invariant models of the back-to-back VSIs ofa DFIG and of the VSI of a STATCOM are adopted.


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7. MODEL DEVELOPMENT AND EVALUATION

Qualitative validation of a generic model of a wind farm with DFIGs (figure 3) and a modelof a wind farm with FSIGs+STATCOM (figure 4) has been undertaken in PSCAD/EMTDC.This has been based on an aggregated wind farm model with assumed parameters, as

given in Tables 7.1, 7.2 and 7.3. For the evaluation of the model the distance to the powersupply network was 20km. In both cases the aggregated rated power of the wind turbinesis 504MW. The Short Circuit Level of the power supply network is 5000MVA and the X/Rratio is 14.3.

o865000

Figure 3: Generic model of a 504MW wind farm with DFIGs and without a STATCOM

o865000

Figure 4: Generic model of a 504MW wind farm with FSIGs+250MVAr STATCOM

Table 7.1 FSIG dataElectrical generator data

Rating 3MVAStator Voltage (L-L, Rms) 0.69kV

Ls 0.1413puLr 0.05puLm 4.134puRs 0.006puRr 0.007pu

Mechanical system dataEquivalent turbine-blade inertia

(referred to generator side)3sec

Generator rotor inertia 0.5secShaft stiffness 0.6pu/electrical rad

Step-up transformer dataRating 3MVA

Vsecondary (L-L, Rms) 0.69kVVprimary (L-L, Rms) 33kV

L 0.06puNo load losses 0.2%Copper losses 0%

Fixed power factor capacitors (installed at 0.69kV terminals)3.33 pu


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Table 7.2 STATCOM dataVoltage Source Inverter data

DC link capacitance (Cdc): 3.5 puGrid side converter coupling inductance (Lgrid): 0.15 puGrid side converter coupling resistance (Rgrid): 0.02 pu

AC output voltage (L-L, Rms) 20 kVCoupling transformer data

RatingEqual to the rating of the

VSIVsecondary (L-L, Rms) 20kVVprimary (L-L, Rms) 400kV

L 0.18puNo load losses 0.5%Copper losses 0%

Table 7.3 DFIG dataElectrical generator dataRating 4.5MVA

Stator Voltage (L-L, Rms) 1kVLs 0.09241pu

Lr 0.09955puLm 3.95279puRs 0.00488puRr 0.00549puH 3.5sec

Range of optimum power absorption 0.75-1.25 pu

Equation of optimum torque2ropt 56.0T =

Maximum capacitive reactive power capability 33% of rating3-winding transformer data

Rating 4.5MVAV1 (L-L, Rms) 1kVV2 (L-L, Rms) 0.4kVV3 (L-L, Rms) 33kV

L12 0.08puL23 0.08puL12 0.001pu

No load losses 0.35%Copper losses 0%

Frequency converter dataC 3.5pu

Vdc 0.7kVLcoupling 1 pu (per 0.4kV base)

Grid side VSIRcoupling 0.017pu (per 0.4kV base)Lcoupling 0.2pu (per 0.69kV base)

Rotor side VSIRcoupling 0pu (per 0.69kV base)

The DFIG generator is controlled according to the scheme presented by the DTI Centre forSustainable Electricity and Distributed Generation [7]. According to the scheme, the rotor


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19

side Voltage Source Inverter (VSI) controls the rotor speed and the reactive power outputof the machine in a decoupled manner. The grid side VSI is reactive power neutral and itsmain purpose is to keep the dc link voltage at its nominal value, establishing the powerbalance between the two back-to-back converters. The control strategy for the rotor speedmaintains optimal power extraction from the wind. Reactive power can be controlled either

to keep a constant power factor at the terminals of the machine or to control the voltage ata specific electrical point. In the examined study case, reactive power is controlled in orderto regulate the PoC voltage. In addition, the DC link of the DFIG is equipped with abreaking resistor for overvoltage protection.

The control strategy of the STATCOM follows the work of Schauder and Mehta [22]. ASTATCOM with IGBT switches and a SV-PWM switching technique is considered. Thisallows independent control of the amplitude and the phase of the STATCOM injected ACvoltage. The reactive power output of the STATCOM is controlled in order to regulate thePoC voltage.

The qualitative validation procedure is based on: Comparison of the behaviour of the models with previous research.

Comparison between the results obtained with the developed average timeinvariant models (average models) and the results obtained with more detailedmodels of the VSIs where the power electronic switches are represented(switched models).

The simulation scenario for both wind farms comprises a step change in real power from0.4pu to 0.8pu at 5sec, a step change in the PoC voltage command from 1pu to 1.02pu at15sec, and a 3-phase bolted fault at 20sec at the PoC lasting 140ms. The results are

shown in figures 5 and 6, and are discussed in the next sections

The goal of the study cases at this stage was to evaluate the operation of the main systemcomponents: FSIG and DFIG wind turbine models and STATCOM models. These modelswill be used to assess windfarm behaviour against a test-matrix of Grid Coderequirements. Architectures comprising FSIG turbines with a STATCOM, DFIG turbinesonly and DFIG turbines with a STATCOM will be considered in these subsequent studies.


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0 5 10 15 20 25 300.8

0.9

1

1.1

1.2

Time (sec)

w(pu)

0 5 10 15 20 25 300

0.5

1

Time (sec)

PoC

Voltage(pu)

0 5 10 15 20 25 30-200

0

200

400

600

Time (sec)

Pfarm(MW)

0 5 10 15 20 25 30-500

-250

0

250

Time (sec)

Qfarm(MV

Ar)

0 5 10 15 20 25 300

50100150200250

Time (sec)

RotorCurrent(%)

0 5 10 15 20 25 3050

100

150

200

Time (sec)DClinkVoltag

e(%)

average

switched

Figure 5: Wind farm with DFIGs and without a STATCOM

0 5 10 15 20 25 300.9

1

1.1

Time (sec)

w(pu)

0 5 10 15 20 25 300

0.5

1

Time (sec)

PoCVoltage(pu)

0 5 10 15 20 25 30-200

0200400600800

Time (sec)

Pfarm(MW)

0 5 10 15 20 25 30-800-600-400-200

0200400

Time (sec)

Qfarm(MVAr)

0 5 10 15 20 25 30-200

0

200

400

Time (sec)Qstatcom(MVAr)

0 5 10 15 20 25 3050

100

150

Time (sec)DClinkVoltage(%)

average

switched

Figure 6: Wind farm with FSIGs+STATCOM


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21

7.1 Wind farm with DFIGs and without a STATCOM

In this study case with the assumed DFIG parameters, the windfarm can meet the GridCode reactive power capability requirement without the employment of a DynamicReactive Compensation device. This, however, may change if different submarine cable

lengths and/or reactive power capability of the machine are considered.

For the equivalent DFIG wind turbine, the capability of independent control of real andreactive power output is validated in figure 5. The change of the real power output att=5sec does not affect the reactive power output, and the change in the reactive power att=15sec does not affect the real power output. When the step-change in the mechanicalinput occurs at t=5sec, it takes approximately 10sec in order that electrical output reaches0.8pu, indicating that the dynamic performance of the speed control is dominated by thecombined turbine and machine inertia [7]. A transient DFIG rotor overcurrent occurs as aresult of the sudden rotor flux change at fault occurrence [7, 41-44]. As the stator voltageis collapsed during the fault, the grid side converter cannot remove the power that is fed

into the dc link by the rotor side converter, resulting in a dc link overvoltage [45]. Thebraking resistor keeps the DC link voltage below the threshold of 150%. A magnified viewof the behaviour of the farm during the fault is presented in figure 7.

19.5 20 20.5 21 21.5 221.1

1.15

1.2

Time (sec)

w(pu)

19.5 20 20.5 21 21.5 220

0.5

1

Time (sec)

PoCVoltage(pu)

19.5 20 20.5 21 21.5 22-200

0

200

400

600

Time (sec)

Pfarm(MW)

19.5 20 20.5 21 21.5 22-500

-250

0

250

Time (sec)

Qfarm

(MVAr)

19.5 20 20.5 21 21.5 220

50100150200250

Time (sec)

RotorCurrent(%)

19.5 20 20.5 21 21.5 2250

100

150

200

Time (sec)DClinkV

oltage(%)

average

switched

Figure 7: Fault behaviour of windfarm with DFIGs and without a STATCOM


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7.2 Wind farm with FSIGs+STATCOM

In this study case a STATCOM of 250 MVA with a 130% overload capability for 1 sec hadto be used in order to ensure dynamic stability. Note that it may be possible with furthersystem and control development to reduce the STATCOM rating, but the main purpose

here was to develop the study case and the machine and STATCOM models.

At t=5sec when the step change in the mechanical power input occurs and at faultclearance, the equivalent FSIG wind turbine shows oscillations in speed and real power.The observed low frequency power oscillations are caused by the stiff coupling betweenmechanical system and electrical behaviour of Fixed Speed Induction Generators and arecharacteristic of FSIG wind turbines [37, 45]. The STATCOM regulates the voltage at thePoC effectively, as can be seen in Figure 6 when at t=15sec the voltage commandchanges. During the fault and after fault clearance the STATCOM provides maximumreactive current, compensating the large amounts of reactive power that the equivalentFSIG draws as a result of its overspeeding [20]. During the fault the PoC voltage collapses

and STATCOM cannot draw enough real power to compensate for its losses, resulting in aDC link voltage drop. At fault clearance an overshoot is observed in the DC link voltage ofthe STATCOM, which however is not dangerous. This overshoot is characteristic of PIcontrollers and can be ameliorated with integrator anti-windup [47].

19.5 20 20.5 21 21.5 220.9

1

1.1

Time (sec)

w(pu)

19.5 20 20.5 21 21.5 220

0.5

1

Time (sec)PoCVoltage(pu)

19.5 20 20.5 21 21.5 22-200

0200400600800

Time (sec)

Pfarm(MW)

19.5 20 20.5 21 21.5 22-800-600-400-2000

200400

Time (sec)

Qfarm(M

VAr)

19.5 20 20.5 21 21.5 22-200

0

200

400

Time (sec)

Qstatcom(MVAr)

19.5 20 20.5 21 21.5 2250

100

150

Time (sec)DClinkVoltage(%)

average

switched

Figure 8: Fault behaviour of windfarm with FSIGs+STATCOM


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7.3 Comparison between average and switched models

For both wind farms, the agreement between the results with average and switchedmodels is very good, indicating that the average models give a sufficient representation.Under steady state conditions these studies demonstrate that the main discrepancies are

in the real power output in the case of DFIGs. In the case of the STATCOM in theFSIG+STATCOM studies the main discrepancy is in reactive power. These are shownmagnified for the period between 25 and 30sec in Figure 9. Studies showed that when theswitching frequency was increased, the discrepancies reduced, as expected since theswitching function then more nearly approximates the average model. Using an averagemodel also reduces secondary effects such as noise coupled to the PLL control block,which otherwise cause further discrepancies. The additional noise produced when theconverters are represented as non-averaged, time-variant devices is evident in figure 9.

During faults, the agreement between the average and switched model is very good forDFIGs. The main observed difference when the switching function is represented, is the

additional noise in the dc link and the rotor current. For the STATCOM, the maindiscrepancy is the pronounced overshoot of the DC link voltage at fault clearance. Still, thedifference is small and the results produced lie on the conservative side. The discrepancycan be attributed to the higher losses in the VSI as a result of the switching harmoniccurrents, which are represented only in the switched model, and introduce additionaldamping in the DC link voltage loop.

25 25.5 26 26.5 27 27.5 28 28.5 29 29.5 3080

85

90

95

Time (sec)

Qstatocm(MV

Ar)

25 25.5 26 26.5 27 27.5 28 28.5 29 29.5 30380

385

390

395

Time (sec)

Pfarm(MW

)

average

switched

Figure 9: Main discrepancies between average and switched models


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8. CONCLUDING REMARKS AND FURTHER WORK

This report has summarised the technical options for connecting an offshore windfarm tothe mainland transmission and distribution network. Each combination has benefits anddrawbacks depending on the size, technology, location and architecture of the windfarm.

Given the scale of any such project, the balance of initial project costs, lifetime, operatingand maintenance costs need to be evaluated

Preliminary investigations showed that for the case of an offshore windfarm with DFIGsand an AC connection to shore, cable length, PoC connection voltage, short circuit level ofthe system and the machine characteristics in terms of reactive power limits and steadystate voltage limits have a significant impact in determining whether additional reactivepower compensation is needed at the PoC. Such issues are important for ensuring thewind farm can comply with the Grid Code requirements for reactive power capability. Forthe case of an offshore windfarm with FSIGs+STATCOM and an AC connection to shore,the STATCOM rating is mainly determined for meeting the Grid Code requirement on faultride-through, not the steady state reactive power capability obligation. However, therequired size of the STATCOM depends considerably on cable length, PoC connectionvoltage level, the systems strength at the PoC and the electrical and mechanicalcharacteristics of the wind turbine generators.

Subsequent studies will concentrate on qualitative aspects of STATCOM performance andthe required rating to achieve Grid Code compliance, on the sensitivity associated withvarying:

System fault level

Connection voltage at the PoC

Wind farm rating Cable length

STATCOM placement.


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GLOSSARY OF TERMS

BC Balancing Code

CC Connection Conditions

DFIG Doubly Fed Induction GeneratorDTI Department of Trade and Industry

FSIG Fixed Speed Induction Generator

HVDC High Voltage Direct Current

IGBT Insulated Gate Bipolar Transistor

LC HVDC Line Commutated HVDC

NGC National Grid Company plc.

PCC Point of Common Coupling

p.f. Power factor

PWM Pulse Width Modulation

PLL Phase Locked LoopSHEM Selective Harmonic Elimination Modulation

SHETL Scottish Hydro-Electric Transmission Limited

SPT Scottish Power Transmission Limited

STATCOM Static Compensator

SVC Static Var Compensator

T&D Transmission & DistributionVS HVDC Voltage Source HVDC

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