Control Strategy Of Doubly Fed Induction Generator System Under Unbalanced AndDistorted Power Grid

As the increasing development of wind power generation technology and the increasing implementation of wind power generation system, the quality of wind power delivered to the power grid, as well as the safe and reliable operation of power grid draws increasing attention. Since the wind power generation system is always connected to the end of the large scale power grid, thus it is highly likely that the point of common coupling is the micro grid or weak grid condition, which includes the non-linear loads, three phase unbalanced loads, and the single phase distributed generation system such as PV inverter. This micro grid or weak grid may always contain three phase unbalance and harmonic distortion, thus the quality of the wind power produced may be jeopardized, and the power grid operation safety and reliability can be deteriorated. In order to ensure the safe and reliable operation of the power grid and the wind power generation system, it is required according to the Chinese Standard that the connected wind power generation system should be able to remain grid-connected operation when the power grid unbalance and harmonic distortion occurs, and the quality of wind power injected should also be accordance to the standard. This dissertation focuses on the improved control strategy of Doubly Fed Induction Generator (DFIG) based wind power generation system under the three phase unbalance and harmonic distortion, theoretical analysis, simulation validation and experiment validation have been implemented to investigate the proposed control strategy in detail, and several conclusions and achievements have been obtained.1. When the DFIG system operates under the low order harmonic distorted (5th and 7th) grid voltage condition, the proportional integral and resonant (PIR) regulator is adopted to regulate the rotor current. The optimized parameter design of the PIR current regulator is investigated concerning the closed-loop control system gain margin of around-4--6dB and phase margin of around 30°～60°. The digital closed-loop control delay caused by the voltage and current AD sample and PWM modulation and update is also taken into consideration. The variation of bandwidth parameter is also considered, so that the control gain parameter can be adjusted to ensure the same magnitude response when different bandwidth parameter is applied.2. When the DFIG system operates under the low order harmonic distorted (5th and 7th) grid voltage condition, the proportional integral and resonant (PIR) regulator and vector proportional and integral (VPI) regulator are adopted to regulate the rotor current respectively. The 300Hz ac error signal regulation capability of both regulators are compared with several perspectives, i.e., closed-loop operation stability, steady state regulation precision. It is validated by theoretical analysis and experiment results that the VPI regulator has better regulation capability than PIR regulator in the terms of closed-loop stability and steady state precision.3. When the DFIG system operates under the low order harmonic distorted (5th and 7th) grid voltage condition, the VPI regulator is applied to implement the direct power control (DPC), with the control target of smooth stator output active and reactive power. The steady power tracking precision, fast dynamic performance and closed-loop operation stability of the proposed DPC strategy are theoretically analyzed and proved experimentally. The proposed DPC strategy also shows an excellent disturbance rejection ability against grid voltage harmonic components and back EMF harmonic components.4. The modularized control strategy of doubly fed induction generator (DFIG) system, including the grid-side converter (GSC) and rotor-side converter (RSC), under unbalanced and harmonic grid voltage is investigated, with the control target of 1) smooth active and reactive power injected into the power grid; 2) balanced and sinusoidal current injected into the power grid. The sequence decomposition process and complicated control reference calculation can be avoided in the proposed control strategy. The 3rd harmonic current component, DC link voltage fluctuation and electromagnetic torque pulsation under the different control targets are theoretically analyzed.5. The control strategy of doubly fed induction generator (DFIG) using bandwidth based repetitive control (BRC) under generalized harmonic grid voltage is investigated, with the control target of eliminating the DFIG stator current 6n±1 harmonic components. Considering that the frequency deviation always occurs in the practical grid, the BRC regulator is designed on the basis of conventional repetitive control (RC) regulator with the introduction of control bandwidth. Besides, the magnitude response compensation unit with larger magnitude response at the higher control frequency is proposed to more effectively restrain the stator current higher harmonic sequence. Moreover, under the circumstance of grid voltage fundamental unbalance and harmonic distortion unbalance, the RC regulator with the fundamental control frequency of 100Hz is adopted to eliminate the stator current fundamental negative component, as well as both positive and negative harmonic distorted components, thus the balanced and sinusoidal DFIG stator current can be ensured.