Investigation On The Modified Direct Power Control Of Doubly Fed Induction Generation System

With the growing of energy crisis and environment problems, wind power generation, as the representative of renewable energy, has been paid much attention. Double Fed Induction Generator (DFIG) based wind power generation system has become the mainstream model of variable speed constant frequency wind power generation technology for its merit of small capacity converters, electrical safety and reliability. Vector control (VC) has always been the main control method of DFIG wind power system, recently direct power control (DPC) strategy has become more and more popular for its excellent dynamic performance, simple control structure, and the capability of maintaining both active and reactive power constant at the same time under the voltage unbalance conditions. Therefore, this thesis investigated the theory and control of the back-to-back PWM converter fed DFIG thoroughly, from the theoretical analysis to the model simulation, from the model simulation to the experimental study, under both the normal grid voltage and grid voltage unbalance conditions.This thesis studied the DFIG based wind power system by control of grid-side converter (GSC) and the rotor side converter (RSC). Simple theoretical analysis and specific implementation of GSC and RSC using VC strategy have been investigated. Different gird voltage faults have been discussed and the performance of DPC under unbalanced grid voltage conditions has been analyzed.Then, mathematical modeling of GSC and RSC in the two-phase stationary frame was established under normal grid voltage conditions. 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 has been studied. The simulation for LUT-DPC has been carried out and the simulation results show high level current harmonic and inconstant switching frequency. So the modified DPCs have been proposed in the thesis. For GSC, Predictive DPC (P-DPC) and Sliding Mode Control DPC (SMC-DPC) have been studied. The detailed theoretical analysis of P-DPC has been discussed, the effect of different voltage vector and the applying time has been investigated in-depth, and also, the theory, stability, robustness of SMC-DPC has been analyzed. The simulation results of P-DPC and SMC-DPC verified their good steady state performance and excellent dynamic performance. The SMC-DPC has also been proposed for RSC, the simulation results also show its excellent steady-state, 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.