Control of variable speed wind turbines equipped with synchronous or doubly fed induction generators supplying islanded power systems
Abstract:
A control method for variable speed wind turbines (VSWTs) supplying islanded parts of electricalnetworks is presented. Active power/frequency and reactive power/voltage droops are applied in order todetermine the active, reactive power production, thus downscaling to the VSWTs the conventional controlconcepts of the power plants. Two types of VSWTs comprising doubly fed induction generators orsynchronous generators are considered. Electrical, aerodynamic and structural detailed dynamic models weredeveloped and combined with the proposed control strategies ensuring fast regulation of the frequency andthe voltage in the islanded mode of operation. The obtained models are used for the simulation of arepresentative simplified distribution network supplied by VSWTs.
1 Introduction
Renewable energy sources exploitation and distributedgeneration (DG) are among the major policy lines of manycountries, but DG accounts today for a small share of theoverall power supply. Current utility practices considerDGs and consequently wind turbines (WTs) as a negativeload in their planning and operation strategies. Mosttechnical standards, for example, [1], dictate the automaticdisconnection of DGs in the occurrence of faults or otherabnormal situations in the upstream distribution network.Especially, WTs are disconnected immediately after voltagedisturbances. This situation is likely to change in the nearfuture, as the installation of DGs is encouraged in manycountries due to the distinct benefits they can offerregarding reliability and quality of supply, as well asincreased efficiency. In this case, automatic disconnectionof large amounts of DG initiated by network failures woulddrastically reduce the expected benefits of DGs and itneeds to be reconsidered. The continuation of theoperation of islanded parts of the network fed solely byDG under certain conditions, in case of faults in theupstream network is studied in several R&D activitiesworldwide concerning Microgrids [2–4].TherecentdevelopmentonvariablespeedWTs(VSWTs)hasbeenfocusedonthestudyoftheirinteractionwiththepowersystemthrough detailed modelling and on methods for maximumelectrical power production [5–9]. Also, great effort to developVSWTs capable of supporting voltage/frequency and remainconnected to the system during faults is made [10–12], forexample, definition and application of low voltage ride throughrequirements. Moreover, new technical challenges associatedwith the operation and coordinated control are going to emergein the near future [2, 11, 13–15]. In particular, autonomousoperation of power systems supplied by VSWTs is verychallenging due to the stochastic nature of the wind. Islandedoperation of such power systems will become an issue of greatimportanceprovidedtheincreasingwindpowerpenetration.Ensuring stable operation during network disturbancesand maintaining stability and power quality in the islanding96 IET Renew.
mode of operation require the development of sophisticatedcontrol strategies. Traditionally, grids are based on rotatingmasses and these are regarded essential for the inherentstability of the systems. In contrast, VSWTs are interfacedthrough power converters. This approach is technicallydemanding, as the converter control must provide theresponse previously obtained form the directly connectedrotating masses, but it does offer the possibility of moreflexible operation.This paper proposes a control scheme for VSWTsconnected to islanded parts of distribution systems. VSWTsequipped with doubly fed induction generators (DFIGs) orsynchronous generator (SGs) are examined. These two typesof VSWTs are examined, as they are the most commonchoice in WT market due to their distinct advantages.The operation of such a system with time-varying powerloads and wind speed is demonstrated for intentionalislanding. In the proposed control scheme, communicationand extra cabling is avoided, because the controller itselfdetermines its instantaneous set values for active and reactivepower using only local measurements. This is achieved byapplying active power/frequency and reactive power/voltagedroops in order to determine the active, reactive powerproduction of each VSWT. Furthermore, wind speedmeasurements or estimations are taken into account foractive power set-point estimation. Finally, it is shownthrough general case studies that the proposed controlscheme achieves stable operation of an islanded distributionsystem, supplied solely by VSWTs, by maintaining systemfrequency and voltage close to their nominal values.
2 Wind turbine model2.1 Models of aerodynamic andmechanical systems
WT rotor aerodynamics are modelled using the well-knownaerodynamic power coefficient Cp(l, b) asa ¼ vr _ Tw ¼12_ r _ A _ Cp(l, b) _ V 3w (1)Pa is the aerodynamic power, Cp(l, b) the dimensionlessaerodynamic power performance coefficient, l the tip speedratio, b the pitch angle, r ¼ 1.25 kg/m3 the air density,A ¼ pR2 the rotor swept area, Vw the wind speed, vr theblade rotating speed and Tw the aerodynamic torque. Forthe reproduction of the wind speed time series, a Fouriersynthesis method is applied which employs the Von Karmanspectral density function of the wind turbulence [16, 17].
2.2 DFIG model
Information about the model of the mechanical system isgiven in the appendix. The electric part of the VSWTswith DFIG is shown in Fig. 1. A four-quadrant voltagesource power converter is used to decouple rotor’s speedfrom power system frequency and also to control theproduced active and reactive power as shown next. Thefourth-order transient model of the induction generatorexpressed in a reference frame rotating at the synchronousspeed with the q-axis leading the d-axis by 908 is used. Ifthe stator resistance is neglected and it is assumed that thed-axis coincides with the maximum of the stator flux, thenthe set of voltage equations forming the model


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