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doubley fed induction motor

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  • 1. Proceedings of the 2009 IEEE International Conference on Mechatronics and Automation August 9 - 12, Changchun, China Doubly-Fed Induction Generator Control for Variable-Speed Wind Power Generation System YAO Xing-Jia ,LIU Shu,WANG Chang-chun and XING Zuo-xia Xiao-Dong, GUO JIANG Hong-Liang Wind Energy Institute of Technology Shenyang University of Technology Shenyang, Liaoning Province, China Vision Automatic Identification Technology Research Institute Shenyang,Liaoning Province, China jhljhw@163.com lsjhl@163.com Abstract- This paper presents control of doubly fed induction II. VARIABLE SPEED OPERATION generator for variable speed wind power generation. The control scheme uses stator flux-oriented control for the rotor side converter and grid voltage vector control for the grid side converter. A complete simulation model is developed for the control of the active and reactive powers of the doubly fed generator under variable speed operation. Several studies are performed to test its operation under different wind conditions. A laboratory test setup consisting of a wound rotor induction generator by a variable speed de motor is used to validate the software simulations, Index Terms: Doubly-fed induction generator; Wind generation; Control; Simulation Fixed speed wind electric conversion systems (WECS) generally use squirrel cage induction generators with direct grid connection so as to maintain a fixed speed that matches the electrical frequency of the grid. In order to operate the fixed speed systems at low and high wind speeds efficiently, pole changing is generally employed. Smaller number of pole pairs is used at high wind speeds and higher number at lower wind speeds. This allows the generator to operate at a different mechanical speed without affecting its electrical frequency. The advantage is that a cost-effective aerodynamic control like stall control can be used. However, the drawbacks in fixed speed systems are: • it cannot optimally use the available wind power due to constant speed operation; • since there is no inherent reactive power control method in this configuration, it must use capacitor banks instead of drawing the reactive power from the grid; • since the generator is made to run at a constant speed in spite of fluctuations in wind speed, it will result in fluctuation of generated voltage as well as output power. In variable speed systems, the turbine rotor absorbs the mechanical power fluctuations by changing its speed. So the output power curve is smoother which greatly enhances the quality of power. However, since variable speed operation produces a variable frequency voltage, a power electronic converter must be used to connect to the constant frequency grid. I. INTRODUCTION Wind power generation is an important alternative to mitigate this problem mainly due its smaller environmental impact and its renewable characteristic that contribute for a sustainable development. Doubly-fed induction generator (DFIG) is able to supply power at constant voltage and constant frequency while the rotor speed varies, that makes it suitable for variable-speed wind power generation. Another major advantage of the DFIG-based system is that the power electronic equipments only need to handle a fraction of the total system power, resulting the reduction of the power losses and the cost of the power electronic equipments. Most wind turbine manufacturers now equip their power generating units with induction generators. These machines are operated either at fixed speed or variable speed. Generators driven by fixed speed turbines can be directly connected to grid. However, variable speed generators need a power electronic converter interface for interconnection with the grid. Variable speed generation has better energy capture than fixed speed generation. There are several other advantages of using variable speed generation such as mechanical stress reduction of turbine and acoustic noise reduction. With recent developments in power electronic converters, variable speed generation looks entirely feasible and cost effective. The paper characterizes the performance of a double-fed induction generator (DFIG) for variable speed wind power generation. 978-1-4244-2693-5/09/$25.00 ©2009 IEEE III .COMPOSITION OF THE WIND GENERATION SYSTEM The induction generator converts the power captured by the wind turbine into electrical power and transmits it to the grid. The ac/dc/ac converter consists of the rotor-side converter ( Crotor) and the grid-side converter (Cgrid). Both Cgrid and C grid converters are voltage-sourced converters using forced commutated power electronic devices to synthesize an ac 855
  • 2. voltage from a de voltage source. A capacitor connected on the de side acts as the de voltage source and a coupling inductor L is used to connect the grid-side converter to the grid. The three-phase rotor winding is connected to Crator by slip rings and brushes and the three-phase stator winding is directly connected to the grid. The pitch angle command and the voltage command signals Vr and Vgc for C rotor and Cgrid converters, Fig. 2. Crot control loop. The control system for the double fed induction generator, operating under maximum power extraction, is based on a flux orientated control of the induction machine, in which the dq current and voltage values are referred to the reference frame aligned with air-gap flux. respectively, are generated by the control system driving the power of the wind turbine, the de bus voltage and the voltage at the grid terminals . The ac/dc/ac converter consists of the rotor-side converter (Crotor) and the grid-side converter (Cgrid). Both Crotor and Cgrid converters are voltage-sourced converters using forced commutated power electronic devices to synthesize an ac voltage from a de voltage source. A capacitor connected on the de side acts as the de voltage source and a coupling inductor L is used to connect the grid-side converter to the grid. The three-phase rotor winding is connected to Crotor by slip rings and brushes and the three-phase stator winding is directly connected to the grid. The pitch angle command and the voltage command IV. CONTROL OF DFIG A schematic diagram of the overall system is shown in Fig. 3. Back-to-back PWM converters are connected between the rotor of 2[MW] DFIG and the grid utility. The DFIG is controlled in a rotating d-q reference frame, with the d-axis aligned along the stator-flux vector as shown in Fig. 4. For the stable control of the active and reactive power, it is necessary to independently control them. The stator active and reactive power of the DFIG is controlled by regulating the current and voltage of the rotor windings. Therefore the current and voltage of the rotor windings need to be decomposed into components related to the stator active and reactive power. signals Vr and Vgc for Crotor and Cgrid converters, respectively, are generated by the control system driving the power of the wind turbine,the de bus voltage and the voltage at the grid terminals. The control system for the double fed induction generator ,operating under maximum power extraction, is based on a fluxorientated control of the induction machine , in which the dq current and voltage values are referred to the reference frame aligned with air-gap flux. The control system is composed of two parts : in the first part the wind turbine output power and the voltage at the grid terminal are controlled by means of Crot; in the second part the voltage at de bus capacitor is controlled by means of Cgrid . The general scheme is illustrated in Fig. I while the control loop of the Crot, modeled as a voltage source, is shown in Fig. 2. - Turbine Wind .. Pow er System Fig.3. Basic configuration of DFIG wind turbine L =Sta tor Induction Generator Th ree-ph ase Grid ~~h.n~e --- - - ---- ---- _· Fig.4.Vector diagram for stator flux-oriented control A. Stator-Flux Oriented Control Fig. 1. Wind turbin e and the doubly fed induction generator system 856
  • 3. For the stator active and reactive power control, a d-q reference frame synchron ized with the stator flux is chosen. The stator flux: vector is adjusted to be aligned with the daxis. The flux linkages of the stator and rotor are expressed as: As = Ads = Lmi ms = L si ds + Lmidr (3) Rs : stator resistance; Be : synchronous frame angle . B. Grid side converter control The grid-side converter serves to meet two purposes keeping the DC-link voltage constant regardless of the magnitude and direction of the rotor power;joining the reactive power composition .The scheme of the grid-side converter is shown in Fig. 5. In the stationary three-phase coordinates,the state equation of the converter is: (4) (5) (12) (6) Where Lm : Where Land R are the inductance and resistance of the line reactors, respectively; p=dldt is derivative operator. To facilitate independent control of the active and reactive power flowing between the grid and the grid-side converter, transformed into a dq coordinates with its q-axis fixed on the grid voltage vector position, expressed as. magnetizing inductance; L, : stator self-inductance; L, : rotor self-inductance; }'ds ' }'qs : stator d-q flux linkage; }'dr'}'qr : rotor d-q flux linkage; ims ,ids,idr : magnetizing, stator and rotor d-axis urrents. Rotor voltages in d-q reference frame can be expressed as a function of rotor and magnetizing currents 0 Vdr = R rldr or di + CJLr - dr - or Figure.5 Structure of the supply-side PWM converter 0 (7) 0 lIJ stCJ rlqr L dt Where lIJ e is the synchronous rotating speed of the gridvoltage vector; subscripts d.q indicate the vector components in the rotating(d-q) reference coordinates. where Vdn Vqr : rotor d-q voltages; Assuming and R; : rotor resistance; lIJ sl : eUXf slip angular frequency . =- lIJLid ,(13) can be rewritten as : The stator flux angle is calculated as follows: (14) (9) Ads = f(VdS - R sids)dt (10) With feed forward control scheme for the grid voltage eqd and compensation control scheme for the cross-coupling terms (II) where a superscript "s" represents quantities reference frame and In ewq,wd' i.e. only considering the impedance voltage vq and v~ ,the plant for the current control loops is given by (15) : stationary 857
  • 4. converter-switching logic, which makes the converter look like a current controlled, current source converter. A . Rotor side converter control The rotor side converter control begins with the stator and rotor current transformation to the d-q reference frame followed by both currents being transformed to the stator fluxoriented frame. Since the objective is to capture the maximum energy available in the wind, the active power reference is always made equal to the available wind turbine power. The reactive power reference value was derived from the active power reference and the desired value of the power factor. The control uses the principle that in the stator flux-oriented frame, the rotor current variations will reflect in stator current variations and hence, by controlling the rotor current, the stator active and reactive powers can be controlled. A reference current i rx •rej was derived from the error between the active power reference and the actual active power by tuning a PI controller, as shown in Fig. 7. Similarly, a reference current i rx •rif was obtained from the error between the reactive power reference and the actual reactive power, as shown in Fig.8. Then, both reference currents were transformed to their natural reference frame that is the rotor frame. These rotor current references, after a dq-to abc transformation, were used for implementing the hysteresis modulation on the rotor side three-phase converter. (15) Neglecting the loss and the voltage drop of inductors and considering the power balance between the AC-side and DCside of the converter,the DC-link voltage can be approximated by : dVdc = ~(.J3 m i - id ) (16) dt C 2 q c Where m is the SVPWM modulation index . From (16), it can be found that the DC-link voltage is controlled by the current iq .Therefore the output of the voltage control loop corrector is selected as the reference of the q - axis component of the AC currents. The reference of the d - axis component is dependent on the optimal reactive power splitting.The space vector control scheme for the gridside converter is shown in Fig.6. Figure.6. The control structure for grid-side converter V. IMPLEMENTATION FF THESIMULATION Iry.ref There are many popular simulation tools such as ACSL, MATLAB/Simulink and PSCAD that can be used to simulate electric drives, power systems, power electronic circuits or a combination of those. However, as is the case with any software, there are certain advantages and disadvantages associated with any one of the above mentioned software and discretion has be used while choosing one of those depending on the type of application it is used for. PSCAD was selected for the simulations effort in this study as it already has models for wind turbine , wind turbine governor, and the wind resource in addition to having a powerful simulation engine that is suitable for simulating time domain instantaneous responses . The main circuit developed in PSCAD contains a wound rotor generator shaft connected to a wind turbine shaft by means of a gearbox as modeled in PSCAD. The stator terminals are directly connected to the grid and the rotor terminals are connected to the grid by means of a back-to back power converter bridge. The pitch angle adjustment for the turbine is done by a wind turbine governor. The rotor side power converter controls the stator active and reactive powers and the grid side converter maintains the de link voltage. To simulate these control schemes two subsystems were developed-c-one for the rotor side converter control and the other for the grid side converter control. A hysteresis modulation strategy was used to implement the Fig. 7. Obtaining quadrature axis rotor reference current in stator flux-oriented frame Fig. 8. Obtaining direct axis, quadrature axis rotor reference current in stator flux-oriented frame. B. Grid side converter control The grid side converter control begins with transforming the grid voltages to the stationary reference frame to obtain the voltage vector angle as given by Eq. As seen before, the de link voltage can be controlled by control of the direct axis current ix in the voltage vector-oriented reference frame. Thus, a reference current i rx,rej was derived from the de link voltage error of the converter bridge by tuning a second PI controller, as shown in Fig. 9. The current i rx,rej was forced to zero so as to make the displacement equal to zero. 858
  • 5. has proven that such a strategy will work well under wind conditions. The simulation model can also be used to test the controller 's performance under network disturbances. A single lineto-ground (SLG) fault will result in a dip in the de link voltage, but the controller should be able to recover and continue the tracking of stator and rotor currents after the fault is removed. However, the line-line (LL), a double lineto-ground (LLG) or a three-phase fault is likely to have a catastrophic effect on the controller as it may lose tracking of the current thereby resulting in a possible collapse of the de link voltage. Induction generators, in general, cannot sustain an appreciable fault current for a fault at their terminals for a long time due to the collapse of excitation source voltage during the fault. However, they will inject a large amount of current for a short transient period of time and this can impact the power system. The results obtained from the laboratory setup have shown that the real power output of the induction generator can be varied by controlling the power handled at the rotor. This factor is helpful in optimally trapping the maximum amount of wind energy available in an efficient manner. Moreover, the power factor and hence the reactive power of the generator can be controlled . Near unity power factors have been achieved. The sub- and super-synchronous modes of operation could be easily combined to provide a continuous operation of the system at various speed ranges. The laboratory results also validate the results obtained from the software simulations. Similar rotor and stator power variations may be seen in both results. The reference currents in the grid voltage vector-oriented frame were then transformed to their natural frame of reference - the stationary frame. An inverse transformation was used to obtain the reference currents as phase currents . With the reference currents for both rotor side and grid side converters, hysteresis modulation may then be implemented for both converters. I x.ref Fig. 9. Obtaining tbe reference current VI. SIMULATION RESULTS Simulation studies for various wind conditions were performed and the control response observed for each. Only the following conditions are shown in this paper Fig. I0: l';f : iJ ': " ':' ' ':' ': ' f :~ ~ ., :E " : " c : J j j j j j : ..(J3.75 '~ ': ': '~ ' ' '~ ':' ':' '~ '" I= ": .: f ~ : u ':' ':' ':' ':' S ':1 '~ ':' ': ~ ' e I! : ': ' " f-0,:j < : '~ ': ':' ':' ' , 10° : " '" " : : : u " " r.a us REFERENO:S 1 25 [I] Tolmasquim, M. T, Szklo, A S, Soares, J. B "Potental Use For Alternative : Energy Sources in Brazil", Annua l Petrobras Conference 2002, Oxford, . : Inglaterra. " 'MfV.fV.fV.'Vv'VvVVV'-./"-./"./'../'./'-/'-/i [2] Forsyth, T. and Tu, P. " Economics of grid-connected Small Wind Turbines :~,~~ ~ :k " ~' ':"" u" in the domestic Market" AWEA WindPower '99 Conference Burlington, '??] Vermont, June 1999. ~ i~'''';:'''':;-·"~w;'':;-~:,'y'';'''''~ [3] theUnited State: Market and Research Update" European Wind Energy . :~~~~~.~,~~~;md -2 0 Cl2 O~ oe '" , 12 ,. 16 II Goldman, P. R. Thresher, S. W. and Hock, S.M. " Wind Energy in Conference, Nice, France, April 1999. 2 [4] E. Muljadi, K. Pierce, P. Migliore, Control strategy for variable speedstall Fig. 10. Response to step increase in wind speed. regulated wind turbines, in: Proceedings of tbe American VII . CONCLUSION ControlConference, vol. 3, 24-26 June, 1998, pp. 1710-1714. Poor power factors in some fixed speed wind machines may result in the wind generator drawing high amounts of reactive power resulting in unnecessary stress on the transmission network. With the variable speed strategy and by use of a power converter in conjunction with a DFIG, the reactive power becomes controllable . Thus, the wind generator can be operated at near unity power factor or any desirable power factor. By software simulations, this paper [5] P. Carlin, A.S. Laxson, E. Muljadi, History and state of the art of variablespeed wind turbine technology, NREL Tech. Rep. TP-500-28607 , 2001. [6] The MathWorks, SimPower Systems For Use with Simulink, User's GuideVersion 4 . [7] L.. Zhang ,CO Watthansam and W.Shehered," The 4 th International PEMC Conf. VoI.2,pp.886 -890,2004 . 859