Modified Direct Power Control Of VSCF Doubly Fed Induction Wind Power Generation System

With the intensification of the energy crisis and environmental issues, wind power generation, which is the representative of renewable energy, has become the focus of social concern. Double Fed Induction Generator (DFIG) has become the mainstream generator of variable speed constant frequency wind power generation technology because its electrical safety and reliability and the converter of which could be of small capacity. Vector control (VC) strategy has been the main control method of DFIG wind power system for the past few decades, while recently direct power control (DPC) strategy has received more and more attention for its good dynamic performance, simple control structure, and its capability of maintaining both active and reactive power constant at the same time under the voltage unbalance conditions. Overall, this thesis investigated the control strategies of the back-to-back PWM converter fed DFIG theoretically and verified them by model simulation and experimental study.This thesis studied the DFIG based wind power system by controlling of grid-side converter (GSC) and the rotor side converter (RSC) separately. Simple theoretical analysis and specific implementation VC strategy have been investigated ofr both GSC and RSC. Different kind of gird voltage faults has been discussed and the performance of DPC under unbalanced grid voltage conditions has been analyzed.Then, mathematical modeling of GSC and RSC under normal grid voltage conditions in the two-phase stationary frame was established. The theoretical analysis of traditional looking-up-table DPC (LUT-DPC) strategy based on the position of virtual grid flux or stator flux, for GSC and RSC respectively, has been studied. The simulation for LUT-DPC has been carried out and the simulation results showed high level current harmonic and inconstant switching frequency. To obtain better performance, modified DPC strategies have been proposed in the thesis.For GSC, DPC based on Discrete Space Vector Modulation (DSVM-DPC) and DPC based on Effective Time concept(ET-DPC) have been studied. Theoretical analysis of DSVM-DPC was discussed, with principle of DSVM and voltage selection the key points. Also, PWM modulation based on effective time was introduced and principle of ET-DPC was studied. Simulation results of DSVM-DPC and ET-DPC verified their good steady state performance and excellent dynamic performance. The ET-DPC has also been proposed for RSC, the simulation results also show its excellent steady-state, dynamic performance. For RSC, Predictive DPC(P-DPC) and ET-DPC have been studied. Detailed theoretical analyses of P-DPC and voltage vector applying time approach have been discussed; Also, theoretical analyses of ET-DPC has been analyzed. Simulation results of P-DPC and ET-DPC verified their good steady state performance and excellent dynamic performance.Under the conditions of grid voltage unbalance, DPC strategy is still able to maintain the active and reactive power constant, but with serious current harmonics, which is not acceptable in the power generation, therefore, an investigation on modified DPC to improve the system performance under unbalanced grid voltage conditions has been carried out. Firstly, the modeling of GSC and RSC in the two-phase stationary coordinates under unbalanced grid voltage conditions has been done. Then, the performance of DPC has been analyzed under the grid voltage unbalance condition. Here proposed three control targets:to obtain sinusoidal, symmetrical grid current (stator current), to obtain constant reactive power, to obtain constant active power. Then, after analyzing the power compensation for three control objectives to improve direct power control performance, no extracting of the negative sequence current is needed. The simulation has been carried out to verify the correctness and effectiveness of the modified DPC proposed in the thesis.Finally, a 3kVA test platform has been designed, experimental studies have been conducted to verify the correctness and effectiveness of the above mentioned control strategy, the results also show that the DPC strategies have good potential for the wind power industrial applications.