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  1. 1. K.K.deepika, Prof.A.Srinivasa Rao / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 5, September- October 2012, pp.524-527 Transient Analysis Of Wind-Based Doubly-Fed Induction Generator K.K.deepika*, Prof.A.Srinivasa Rao** *(EEE Department, Vignan‘s Institute of Information and Technology, Visakhaptanam-46) **(EEE Department, GITAM Institute of Technology, GITAM University, India)ABSTRACT Demand for wind power has increased demand more reliability to wind power technologies;considerably due to technological advances and therefore, standards with regard to the connection,favourable government policies. As a result, large operation, and maintenance of such power plantswind farms with multi-megawatt capacity are become more restrictive. In this scenario, simulationconnected to sub-transmission and transmission tools such as PSCAD or MATLAB, where thesystems. With high penetrations of wind energy, distributed generation networks can be analyzed inperformance of the overall system is affected by deep, have gained a great importance for designingthe technical impacts introduced by wind turbine advanced functionalities and control strategies togenerators (WTG). Fault current contributions improve the integration of wind energy.from WTGs will have a significant impact on the Wind turbines can either operate at fixedprotection and control of the wind farm as well as speed or variable speed. For fixed speed windthe interconnected system. This paper initially turbines, the induction generator is directly connecteddescribes the modelling aspects of Doubly- Fed to the electrical grid accordingly. Since the speed isInduction Generator (DFIG) during steady state almost fixed to the grid frequency and most certainlyand faulty conditions. Vector decoupling control not controllable, it is not possible to store thestrategy has been adopted to establish the turbulence of the wind in form of rotational energy.mathematical model of DFIG based wind For a variable speed wind turbine thegenerator. Further, a 9 MW wind farm with 6 generator is controlled by power electronicunits of 1.5 MW DFIG is modelled in equipment. There are several reasons for usingMatlab/Simulink and, voltage and current variable-speed operation of wind turbines amongwaveforms are presented and discussed for 3- thoseare possibilities to reduce stresses of thephase fault, phase to phase fault and phase to mechanical structure, acoustic noise reduction andground fault created at the generator terminal the possibility to control active and reactive power.and close to MV bus. These large wind turbines are all based on variable- speed operation with pitch control using a direct-Keywords—Decoupled control, doubly-fed driven synchronous generator (without gear box) or ainduction generator, dynamic performance, doubly-fed induction generator. Today, variable-slip,mathematical modeling, wind power i.e., the slip of the induction machine is controlled with external rotor resistances, or doubly-fed1. Introduction induction generators are most commonly used by the India is a rapidly transforming country. wind turbine industry for larger wind turbines.Due to liberalization and globalization, steady In order to guarantee the safety andgrowth was witnessed in India‘s GDP for the last reliability for wind power integration operation, it istwo decades. Energy is the life line for economic of great significance to establish an appropriate windgrowth. India‘s current primary commercial energy power generator system model and analyze electro-requirements are mostly fed by conventional fuel magnetic transient characteristics.sources such as coal, oil, natural gas, hydro andnuclear that totals to about 520 million tonnes of oil 2. Basic concepts of DFIGequivalent (mtoe) and this is expected to raise to The basic layout of a DFIG is shown in Fig. 1.740 mtoe by end of 12th plan i.e., 2016-17.Currently Inida is importing energy to the 19 mtoeand this has to increase to 280 mtoe to meet theabove demands. This will be highly expensive;hence it is necessary to harness renewable energyresources like solar, wind, biomass, etc. to meet partof the demand. In this paper an attempt is made touse the available technology to tap wind power andgenerate electricity.The transmission system operators (TSOs) currently Fig.1. DFIG connected to a grid 524 | P a g e
  2. 2. K.K.deepika, Prof.A.Srinivasa Rao / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 5, September- October 2012, pp.524-527A doubly-fed induction generator is a standard t=0 sec and faults are created at t=0.02sec. Faults arewound rotor induction machine with its stator cleared after 0.02 sec at t=0.04 sec. Fault resistancewindings directly connected to grid and its rotor has been kept constant at 0.001Ω.windings connected to the grid through anAC/DC/AC converter. AC/DC converter connected 3.1Three Phase faultto rotor winding is called rotor side converter and 3.1.1 At the generator terminalanother DC/AC is grid side converter. Doubly-fed A three phase fault is created at theinduction generator (DFIG), is used extensively for generator terminal at t=0.02s and cleared at t=0.04s.high-power wind applications. DFIG‘s ability to Fig.3 shows the voltage and current waveforms at thecontrol rotor currents allows for reactive power generator terminal and close to MV bus.control and variable speed operation, so it can operate During the fault, voltages of all three phases reachat maximum efficiency over a wide range of wind very low values as shown in Fig.3. Ideally thisspeeds. The aim of this paper is to develop a control should reach to zero, but in practice, there will bemethod and analysis of dynamic performance of some fault resistance and hence will have a veryDFIG‘s rotor control capabilities for unbalanced small magnitude voltage values. Generator terminalstator voltages, grid disturbances and dynamic load current will suddenly increase at the instant of faultcondition. initiation followed by a rapid decay as determined by the transient impedance of the generator.Fig.2.Control structure of DFIG In modern DFIG designs, the frequencyconverter is built by self-commutated PWMconverters, a machine-side converter, with an Fig. 3 Voltages and currents at generator terminal andintermediate DC voltage link. Variable speed MV bus for a 3-phase fault at generator terminaloperation is obtained by injecting a variable voltage Voltage drop on the MV bus is marginal wheninto the rotor at slip frequency. The injected rotor compared to the voltage drop at the generatorvoltage is obtained using DC/AC insulated gate terminal. This is due do the positive sequencebipolar transistor based voltage source converters generator step-up transformer impedance, positive(VSC), linked by a DC bus. By controlling the sequence impedance of the collector cable sectionconverters, the DFIG characteristics can be adjusted and the fault as to achieve maximum of effective power During the fault period, current at MV busconversion or capturing capability for a wind turbine jumps to a high value and will not experience anand to control its power generation with less appreciable decay as shown in fig.3. This is due tofluctuations. Power converters are usually controlled the feeding from the other generators and from theutilizing vector control techniques, which allow power system.decoupled control of both active and reactive power. 3.1.2 At MV Bus3. Case study A three phase fault is created at the medium The purpose of this study is to understand voltage bus at t=0.02s and cleared at t=0.04s. Voltagethe behaviour of a wind farm with DFIGs, under and current waveforms at generator terminal and MVdifferent faulty conditions. A 9 MW wind farm with Bus are shown in Fig.4. There is voltage collapse atsix DFIG based wind turbine generators, each having the instant of the fault initiation and a small residuala capacity of 1.5 MW has been considered in this voltage is present during the fault period. This maystudy. Various symmetrical and asymmetrical faults be due to the additional fault loop impedancehave been created at the generator terminal and at the introduced by step up transformers and cables.MV Bus. Time domain voltage and current Results are similar to a three phase fault at thewaveforms at generator terminal and at the MV bus generator terminal.are observed for 0.02 seconds. Simulations start at 525 | P a g e
  3. 3. K.K.deepika, Prof.A.Srinivasa Rao / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 5, September- October 2012, pp.524-527 Fig. 6.Voltages and currents at generator terminal and MV bus for a Phase-phase fault at MV Bus. 3.3 Phase- ground fault 3.3.1 Phase- ground fault at generator terminal A phase to ground fault is created atFig. 4. Voltages and currents at generator terminal generator terminal at t=0.02s and cleared at t=0.04s.and MV bus for a 3-phase fault at MV Bus The voltage and current waveforms are obtained at4.2. Phase- Phase fault generator terminal and MV bus as shown in Fig. At the generator terminal Current waveforms at generator terminal for the A phase to phase (phase ‗a‘ to phase ‗b‘) single phase to ground fault on generator terminal isfault is created at generator terminal at t=0.02s and shown in fig. 8. Ideally, phase ‗b‘ (green color) andcleared at t=0.04s. phase ‗c‘ (red color) currents should drop to zero, and phase ‗a‘ (blue color) should experience an over current. A marginal increase in phase ‗a‘ (blue color) is observed and a reduction in phase ‗b‘ and ‗c‘ voltages can be seen in Fig.7. Fig.7.Voltages and currents at generator terminal and MV bus for a Phase-ground fault at generatorFig.5 Voltages and currents at generator terminal and terminal.MV bus for a 3-phase fault at generator terminal C.2 Phase- ground fault close to MV BusCurrent going through phase ‗a‘ should return A phase to ground fault is created close tothrough phase ‗b‘ during the fault condition. After MV Bus at t=0.02s and cleared at t=0.04s. Thethe fault initiation, this can be seen as Ia=-Ib, phase voltage and current waveforms are obtained at‗a‘ current and phase ‗b‘ current going in opposite generator terminal and MV bus as shown in Fig.8.directions as shown in Fig. 5. Ideally, phase ‗c‘current should decay to zero. Initially there is amarginal drop of phase ‗c‘ current and afterwards atransient condition results. This is due to the feedingfrom other wind generators and from the powersystem4.2.2 At MV Bus A phase to phase (phase ‗a‘ to phase ‗b‘)fault is created at MV bus at t=0.02s and cleared att=0.04s. Voltage and current waveforms are obtained Fig.8 Voltages and currents at generator terminal andat generator terminal and MV bus as shown in Fig.6. MV bus for a Phase-ground fault at MV Bus 526 | P a g e
  4. 4. K.K.deepika, Prof.A.Srinivasa Rao / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 5, September- October 2012, pp.524-527Conclusion selecting proper instrument transformers, switchgear This paper presents a study of the dynamic and control gear, and in designing effectiveperformance of variable speed DFIG coupled with protection systems.wind turbine and the power system is subjectedvarious symmetrical and unsymmetrical faults REFERENCESlocated at various locations in the power system. The [1] G.H. Li, B.H. Zhang, Z.G. Hao, J. Wang anddynamic behaviour of DFIG under power system Z.Q. Bo, David Writer, Tony Yip, ―disturbance was simulated using MATLAB/ Modelling of DFIG based wind generatorSIMULINK platform using vector control concept. and transient characteristics analysis‖,Accurate transient simulations are required to Proceedings of EEEIC,2011, pages 1-4investigate the influence of the wind power on the [2] W. Leonhard, Control of Electrical Drives,power system stability. Springer-Verlag, Berlin, 1990. The DFIG considered in this analysis is a [3] Alvaro Luna, Francisco Kleber de Araujowound rotor induction generator with slip rings. The Lima, David Santos, Pedro Rodríguez,stator is directly connected to the grid and the rotor is Edson H. Watanabe, and Santiago Arnaltes :interface via a back to back partial scale power Simplified Modeling of a DFIG forconverter (VSC). Power converter are usually Transient Studies in Wind Powercontrolled utilizing vector control techniques which Applications. IEEE TRANSACTIONS ONallow the decoupled control of both active and INDUSTRIAL ELECTRONICS, VOL. 58,reactive power flow to the grid. NO. 1, JANUARY 2011 [4] Erlich, H. Wrede and C. Feltes : DynamicFault Variations of quantities Behavior of DFIG-Based Wind Turbines near generator close to MV during Grid Faults, 1-4244-0844- Locati terminal Bus X/07/$20.00 ©2007 IEEE pages 1195 toType 1200 on Voltag Curre Volta Curre es nts ges nts [5] Perdana, O. Carlson, and J. Persson : Dynamic Response of Grid-Connected Wind -70% +300 -50% +300 X Turbine with Doubly Fed Induction3 phase % % Generator during disturbances, NORDICfault -50% +250 -100% +500 WORKSHOP ON POWER AND Y % % INDUSTRIAL ELECTRONICS.Phase( X -20% +100 -50% +300 TRONDHEIM – 2004.A)- % % [6] P. Kundur, Power System Stability andphase( -50% +100 -100% +400 Control, McGraw-Hill, 1994.C) Y % % [7] P.C. Krause, O. Wasynczuk and S.D.fault Sudhoff, Analysis of Electric Machinery and - +400 -50% +150 Drive Systems, Wiley, 2002. 50%(A % (A,B, % [8] Matlab/Simulink/SimPpowerSystemsTM ),- (A), C) (A), Ver.2009a, user‘s guide and reference. X 20%(B +100 +20%phase( ,C) % (B,C)A)- (B,C)ground -20% +100 -100% +200fault (A) % (A) (A), % No (A), Y chang +90% e( (B,C) B,C)TABLE 1. Effect of various faults created atlocations X and Y within the power system.Location X,Y denote at generator terminal and at MVbus respectively. In this paper, a 9 MW wind farm ismodelled and simulated for symmetrical andasymmetrical faults at generator terminal and at MVbus in the power system. Voltage and currentwaveforms are presented and compared with thoseunder ideal fault conditions. Authors conclude thatunderstanding fault current behaviour will help in 527 | P a g e