Research On Direct Power Control Strategy Of Dfig Wind Energy Conversion System Under Grid Faults

In the recent years, many countries have published a series of rules related to ride-through capability and reactive power compensation capability of wind turbine during grid faults. So the fault ride-through(FRT) technology has become the greatest challenge of wind power generation technology at the moment. Especially for DFIG, due to its sensitivity to grid faults and the limited rotor-side converter(RSC) capability, the control problem of DFIG under grid fault conditions has been the focus of academic circles. Therefore, the research on control technique of DFIG WECS under grid fault conditions is significant to the theoretical research and practical applications.In this paper, FRT control technique based on direct power control under grid faults of DFIG WECS is intensively studied from two aspects: improve steady-state and transient-state operation performance of DFIG. In steady-state case, the direct power control strategy based on sliding-mode control and the novel optimization method to compensate power under unbalanced grid voltage conditions are proposed, which can enhance the steady-state control capability of DFIG WECS under mild grid faults. In transient-state case, the novel demagnetization control scheme based on direct power control is proposed, combined with hardware protection circuit based on the stator-side series dynamic breaking resistor(SDBR), which can restrain the transient fault current of DFIG under various grid faults. Conclusively, the main research contents can be outlined as follows:(1) Direct power control strategy of DFIG WECS is investigated under ideal grid conditions. The mathematical models of DFIG and GSC are established in two-phase stationary α-β reference frame, with instantaneous active and reactive power as their state variables. The direct power control strategies based on sliding-mode control of GSC and RSC are designed, and maximum power point tracking of DFIG using the proposed control strategy has been successfully implemented. In particular, a novel double-loop control strategy of GSC is presented to improve dynamic response performance of the system, which consisted of the outer-loop based on sliding-mode control and the inner-loop based on direct power control.(2) Direct power control strategy of DFIG WECS under mild asymmetrical grid faults is emphatically studied, to strengthen steady-state operation capability of DFIG during grid faults. In view of GSC, the influences of unbalanced grid voltage on instantaneous active and reactive power are analyzed in detail;A novel optimization method to compensate power based on direct power control without the sequence components extraction of current is proposed, which can realize the following GSC control objects: to maintain a constant reactive power, and to get sinusoidal and symmetric grid current. In consideration of the GSC AC-side impedance, an improved optimization method to compensate power based on direct power control is presented according to the internal power flow of GSC, which can eliminate fluctuations of the DC-bus voltage by compensating power oscillating components of the AC-side impedance.(3) Direct power control strategy of DFIG is investigated under mild asymmetrical grid faults. The influences of unbalanced grid voltage on instantaneous active and reactive power, and electromagnetic torque of DFIG are analyzed; The proposed optimization method to compensate power for GSC is applied to rotor-side converter, which can realize the following control objects: to maintain a constant stator active power, to cancel oscillating components at twice grid frequency of the stator reactive power and electromagnetic torque, and to get sinusoidal and symmetric stator current. According to the internal power flow of DFIG WECS analysis under unbalanced grid voltage, a coordinated control scheme is proposed for DFIG’s both GSC and RSC, which can cancel oscillating components at twice grid frequency of the electromagnetic torque, maintain a constant total active power of DFIG WECS, get sinusoidal current, and suppress the oscillation of the DC-bus voltage. In addition, the saturation problem of RSC output voltage caused by unbalanced grid voltage is discussed,and the efficiently operating range of RSC is obtained under unbalanced grid voltage conditions.(4) The transient-state characteristics of DFIG are analysed in detail under various grid faults to reduce the negative effect of DFIG electromagnetic transient process on fault ride-through, the essential laws and action mechanisms of DFIG transient components are revealed. The transient response of DFIG using direct power control strategy under mild grid fault conditions has been studied. A novel demagnetization control strategy based on direct power control is proposed, which realizes stator transient-state flux linkage damping and restrains the overcurrent of stator and rotor. Hardware protection circuit based on the stator-side SDBR is investigated. A FRT combined control scheme based on the proposed demagnetization control strategy and stator-side SDBR is presented, which can enhance the FRT and reactive power compensation capability of DFIG WECS.(5) An experimental platform of DFIG measurement and control based on dSPACE real-time simulation system has been designed and developed, the architecture and main parameters of the platform are introduced in detail. DFIG WECS is simulated based on the experimental platform under laboratory conditions. Many experiments are conducted to validate the correctness and effectiveness of the proposed control strategy based on direct power control under balanced and unbalanced grid voltage conditions.