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Transient stability of differentelectrical concepts for wind farms

UpWind Work Package 9, D 9.4.1

E.J. WiggelinkhuizenJ.T.G. Pierik

(ECN Wind Energy)

ECN-E11-003

Transient stability of different electrical concepts for wind farms

AbstractAs part of the EU 6th FP project "UpWind" Work Package 9.3 the dynamic response of twodifferent electrical designs of offshore wind farms has been evaluated for symmetrical onshoregrid faults. The first design uses an HVDC connection to shore based on voltage source con-verters and the second design uses an HVAC connection. In both cases the wind farm consistsof variable speed turbines, with a with permanent magnet generator and full-power voltagesource converter, grouped into 5 feeders connected to a single bus.

The simulation results of a 3-phase fault in the onshore grid have been compared for differentminimum grid voltages, for different wind speeds and for different power ratings of the dc-linkbraking resistor.In all cases the HVAC and HVDC connected wind farms did ride-through the 3-phase onshoregrid fault while supporting the grid voltage during and shortly after the fault. The appliedmethod of active power reduction of the HVDC connected wind farm by a fast decrease of theac-voltage in the wind farm showed to be effective. Also it does not rely on communicationlinks and also the WT converter control is very similar as in the HVAC connected wind farm.On the WT generator side the response to the applied faults was similar in the HVAC connectedWF and in the HVDC connected WF as the full-rated WT convertereffectivly decoupled thegenerator side from the WF grid side. The drive-train oscillations only depend a little on theoperating conditions and are rapidly damped during the rampup of the active power.

Due to the limitations of the models several aspects are not or not accurately represented inthe results, such as harmonics, unbalance and the response of protection systems. However,the models can be used to design evaulate wind farm configurations and wind farm controlsystems. In order to make the models better applicable they should be validated with measure-ments or with other models.

Acknowledgement Project funded by the European Commission under the 6th (EC) RTDFramework Programme (2002- 2006) within the framework of the specific research and tech-nological development programme "Integrating and strengthening the European Research Area"and by the Ministry of Economic Affairs of the Netherlands as ECN Programmafinanciering.

Contract No.: 019945 (SES6)Project title: UpWindWork Package: WP9: Electrical gridTask: 3: Reliability and electrical and control concept of wind farmDeliverable: D9.4.1ECN project number: 7.9466

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Contents

1 Introduction 51.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2 Choice of configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 51.3 Structure of the report . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 6

2 Model description 72.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2 Wind farm with HVDC-VSC connection . . . . . . . . . . . . . . . . . . .. . 72.3 Wind farm with HVAC connection . . . . . . . . . . . . . . . . . . . . . .. . 82.4 Wind turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.5 Wind turbine control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 82.6 HVDC control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.7 Model parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

3 Simulations 153.1 Case descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 153.2 Results for HVDC connected wind farm . . . . . . . . . . . . . . . . .. . . . 153.3 Results for HVAC connected wind farm . . . . . . . . . . . . . . . . .. . . . 223.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Conclusions and recommendations for future work 25

ECN-E11-003 3

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1 Introduction

In the last five years offshore wind energy has taken off, withthe installed capacity reaching2GW in 2009, 3.5GW under construction and a target of 40GW in 2020 [9]. With this in-creasing number and scale of offshore wind farms the influenceon the power balance becomesmore significant, especially in case of failures, grid abnormalities or extreme wind events.Grid abnormalities normally affect all the wind turbines ina large wind farm simultaneouslyand may therefore lead to large power drops or even damage of components in the wind farm.The consequences of grid abnormalities such as voltage dips,outages and unbalance, dependon the control capabilities of the wind turbines and of the electrical transmission system of thewind farm to "ride-through" these disturbances. Also the wind farm control is important as itshould provide adequate grid support during the recovery phase.Therefore most wind-specific connection requirements, or grid codes, include so-called "FaultRide Through" (FRT) requirements for the behaviour of wind turbines during grid faults [11].These requirements specify typical profiles of voltage dips for which the wind turbines shouldstay connected for a certain period of time and support the grid voltage. After the fault hasbeen cleared the wind turbines should support the voltage restoration and in case of trippingthe conditions for reconnection are specified, for example a certain ramp-up rate for the ac-tive power feed-in. Compliance of wind farms should be measured at the Point of CommonCoupling (PCC) where the functional performance requirements apply, supplemented by in-formation gained in certifcation process for single wind turbines.

1.1 Objectives

The objective of this study, which is part of the EU 6th FP project"UpWind" Work Package9.3, is to investigate the design requirements of electrical systems of wind turbines and windfarms due to the need for reliability and controllability ofwind farms in power systems duringgrid transients.The approach is chosen to compare simulations of different electrical designs of offshore windfarms for onshore grid faults, also in relation to results available through literature. As oneof the evaluation criteria compliance with Fault-Ride-Through requirements in grid codes isused, for example the E.ON and REE grid codes [8], [23]. Anothercriterion is to comparethe levels of electrical and mechanical stress resulting from grid transients. Other aspects forstudying are the sensitivity for parameter variations and disturbances or other capabilities andlimitations of the technology.A number of simplifications are made in the models to reduce thecomputation time so thatmore complex wind farms can be simulated. The converter models, for example, are non-switching controlled voltage sources and a very simple gridmodel has been used. Thereforea detailed analysis of harmonics, protection systems and component losses is out of the scopeof this study.The chosen electrical parameters in this study do not refer topractical systems and also thesimulation results have not been validated with measurements. Finally, only a limited numberof designs can be evaluated in detail, therefore only two basic designs have been chosen whichare representative for offshore wind farms and which are notfully investigated yet.

1.2 Choice of configurations

While the size of wind farms is steadily increasing as well asthe distance to shore, HVACtechnology will reach its technical and economical limits and connection through HVDC willbecome more attractive. Therefore a wind farm with HVDC connection is studied and com-pared to an HVAC connected system. Two basic HVDC technologies exist, namely HVDCwith Line Commutated Converters and with Voltage Source Converters. HVDC-LCC hasbeen applied for many years already, but is not well suitablefor offshore wind energy, becauseof its large footprint and its limited controllability [4],[1], [26]. Therefore HVDC-VSC, whichis better suitable because of its good controllability and small footprint, has been selected forthis study. Several strategies for FRT of HVDC-VSC connected wind farms are explained in[13]. Recently several studies simulating HVDC connected wind farms have been performed

ECN-E11-003 5

Transient stability of different electrical concepts for wind farms

or have just started, cf. [10, 24, 28]. Most studies apply more detailed converter modelsincluding switching with less detailed wind turbine models, cf. [5].The type of wind turbine that is modelled is a variable speed pitch turbine with full-ratedconverter, as recently the share of this turbine type with its superior control capabilities isincreasing [14], [6]. However it would be relevant to also study different types of wind turbinesin future. For instance, by applying simple and robust wind turbines with directly coupledinduction generators in combination with HVDC-VSC connection, the major difficulties withgrid connection of this type of wind turbines can be overcome, e.g. [3].The grid is represented as a single synchronous generator with series impedance and the faultis represented as a symmetrical 3-phase shunt resistance. Response to unsymmetrical faults,cf. [16], [2] as well as to dc-faults and other events leadingto transients, such as tripping ofwind turbines or lines and tap changer activation, has not been considered. The model doesnot include negative and zero sequence control and protective equipment such as groundingtransformers and circuit breakers.For the wind farm model the following configurations are studied and compared:

Size: 60 wind turbines of 6MW

Turbine electrical system: Variable Speed collectine Pitch

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