Control system for doubly-fed induction machine |
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申请号 | EP11173259.0 | 申请日 | 2011-07-08 | 公开(公告)号 | EP2544358A1 | 公开(公告)日 | 2013-01-09 |
申请人 | ABB Oy; | 发明人 | Niiranen, Jouko; Seman, Slavomir; Virtanen, Reijo; | ||||
摘要 | A method and arrangement for controlling a doubly-fed induction machine by a frequency converter comprising a rotor side converter (INU) connected to a rotor circuit of a doubly-fed induction machine (DFIG) and having a control system with rotor flux as a feedback variable, a grid side converter (ISU) connected to an AC power network, and a direct voltage intermediate circuit (DC) connected between the rotor side converter (INU) and the grid side converter (ISU). The method comprises forming a rotor flux reference (ψ r,reƒ ), forming a damping signal ( ψ reƒ,D ), summing the damping signal and the rotor flux reference for obtaining a modified rotor flux reference (ψ reƒ ), and feeding the modified rotor flux reference to a controller of the rotor side converter (INU) for damping sub-synchronous resonances. | ||||||
权利要求 | |||||||
说明书全文 | The present invention relates to doubly-fed machines, and especially to damping of sub-harmonic oscillations of an AC power network by control of doubly-fed machines. Doubly-fed induction machines are used in various generator and motor drives. One such electric machine drive comprising converters is a doubly-fed slip-ring generator configuration whose rotor circuit comprises two converters having a direct voltage intermediate circuit therebetween. One converter is situated electrically between the direct voltage intermediate circuit and a rotor while the other converter is situated electrically between the direct voltage intermediate circuit and an electrical network to be supplied. Such doubly-fed slip-ring generators are commonly used in wind turbines. Sometimes many of these kinds of wind turbines are located in places where the connection to an electrical network requires a long transmission line. The long line has a considerable inductance that tends to limit transmission of the generated power. It is a common practice to alleviate this problem by using capacitance connected in series with the transmission line. This series compensation of the inductance has, however, the drawback that the capacitor with the inductances of the line and the supplying network form a resonance circuit that has a resonance frequency lower than the nominal frequency (50 or 60 Hz) of the grid. Excitation of this sub-synchronous resonance causes a rapid variation of the voltage magnitude and phase, which causes mechanical and electrical stress to the devices connected to the network, sometimes leading to permanent damage. Excitation of the sub-synchronous resonance by doubly-fed generators is described for example in Jindal, A., Irwin, G. and Woodford, D.: Sub-Synchronous Interactions with Wind Farms Connected Near Series Compensated AC Lines. Proceedings of 9th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission Networks for Offshore Wind Power Plants, Oct. 18-19, 2010, Québec City, Canada. However, no clear means for damping the resonance is given in the paper. In An object of the present invention is to provide a method and an arrangement for implementing the method so as to solve the above problems in connection with doubly-fed induction generators. The objects of the invention are achieved by a method and an arrangement which are characterized by what is stated in the independent claims. Preferred embodiments of the invention are disclosed in the dependent claims. The invention is based on the idea of modifying a rotor flux reference in the control system controlling the rotor flux. The modified rotor flux reference is obtained by summing a damping signal to a signal that is typically used as a rotor flux reference. The modified rotor flux signal is then used in the control as the reference signal. The damping signal is preferably formed on the basis of an estimated machine torque or another signal having similar characteristics. An advantage of the method and arrangement is that it damps effectively sub-synchronous oscillations so that the network can be used securely and the production of power is stable. A further advantage of the invention is that it can be used throughout the whole speed range for stabilizing the operation. The method and the apparatus of the invention require minimal changes to the current control structure, thus the invention is easily implemented. In the following, the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which
An example of a conventional prior art direct torque controlled system is disclosed for example in In Stator currents isA, isB and voltages VsA, VsB are measured, and these values are fed to a torque and flux estimator block 21. The torque and flux estimator block 21 receives a further calculated stator flux vector The torque and rotor flux estimator block 21 estimates the torque of a machine Te1, Te2 and the rotor flux In conventional DTC control, the flux reference ψreƒ is obtained directly from the torque and rotor flux estimator block 21. However, in the present invention, the flux reference supplied to a DTC-modulator is a modified flux reference which is obtained by summing the reference ψr,reƒ from the torque and rotor flux estimator 21 with a damping signal from a damping block 29. According to the embodiment of The two-level hysteresis control block 24 and the three-level hysteresis control block 26 also receive input from a hysteresis band control block 27. The hysteresis band control block 27 controls the width of the hysteresis bands and thereby affects the switching frequency. Therefore, the hysteresis band control block receives input from a switch state logic and switching frequency calculation block 28 which controls the switching frequency fsw. The hysteresis control blocks output a flux bit and torque bits for the switch state logic block 28 which also receives information on the sector of the rotor flux from the flux sector identification block 23. The switch state logic and switching frequency calculation block 28 outputs switching instructions Sa, Sb, Sc to the rotor side converter. As mentioned above, the modified rotor flux reference is used in the modulator. If necessary, a suitable phase shift adjustment may also be used in addition to amplification adjustment to further enhance the damping. The phase shift adjustment can be incorporated into the filtering section. Once the modified rotor flux reference is used instead of one obtained directly from the torque and rotor flux estimator, the oscillations are greatly reduced. The modified rotor flux reference is proportional to the torque oscillations which are due to the voltage oscillations in the grid. Instead of using torque for generating the damping signal as shown in The invention is described above in connection with The estimated torque Te in The torque can thus be estimated using stator and rotor fluxes ψstator , ψrotor , stator and rotor currents is, ir, and magnetizing inductance Lm in different ways. The estimated torque used by the damping block 29 and the estimated torque fed to the torque controller 26 may be calculated differently from each other. The torque used for torque control may preferably be calculated using only the positive sequence component of the flux and/or current. In Using the damping means according to the invention ensures a stable operation of the AC power network even during and after faults as can be seen in In It is obvious to one skilled in the art that the basic idea of the invention may be implemented in many different ways. The invention and its embodiments are thus not restricted to the above-described examples but may vary within the scope of the claims. |