| With the rapid development of human society, the energy crisis and environmental problems have become increasingly serious. In recent years, more and more countries realized the importance of renewable energy, and the development and application of renewable energy has been greatly studied, especially the wind power as a representative, which increased at30%annual growth rate since1990. As the mainstream model of variable speed constant frequency wind power generation technology, Double Fed Induction Generator (DFIG) based wind power generation system were widely studied for its merit of small capacity converters, electrical safety and reliability. The direct power control strategy (DPC) has become more and more popular, due to its excellent dynamic performance and simple control structure. DPC control strategy directly choose active and reactive power as control variables, and can maintaining both active and reactive power constant at the same time even under the voltage unbalance conditions, has great potential for application in DFIG wind power system. However, the traditional Looking-Up-Table DPC (LUT-DPC) which is presence of disadvantages of power fluctuations, current rich in harmonics and unfixed switching frequency, needs to be improved. Therefore, this thesis investigated the basic theory of DFIG wind power generation systems and DPC control strategies in theoretical analysis, simulation modeling and experimental studies, both under an ideal balance grid voltage as well as unbalance situation.Due to the nature DFIG back to back double PWM converter, the DFIG wind power system grid side converter (GSC) and rotor side converter (RSC) were studied separately, and a coordinated control of DFIG’s RSC and GSC was studied afterwards. First, the GSC and the RSC were mathematical modeling under ideal balance grid condition. For control strategy research reasons, the model expressions both in two-phase stationary frame, and mathematical two-phase synchronous rotating frame were given. To overcome the disadvantages of LUT-DPC, the low-complexity model predictive direct power control strategy (LCMP-DPC) and the Back-Stepping method based direct power control strategy (BS-DPC) were proposed. The detailed theoretical analysis and applying method of LCMP-DPC has been discussed. This control strategy can reduce power fluctuations while ensuring fast dynamic performance. Then a brief introduction of Back-Stepping method was given. BS-DPC combined Back-Stepping with DPC by using SVM technology to solve the unfixed switching frequency problem. Both of the two methods were carried out with detailed theoretical analysis. The simulation and experiment results proved the correctness and effectiveness of two control strategies. To demonstrate the effectiveness of the control strategy, the simulation and experimental results of both strategies were compared with LUT-DPC and traditional vector control (VC).Secondly, mathematical modeling of GSC and RSC under unbalanced grid conditions was established. The Influence of unbalanced grid conditions to traditional control strategy was investigated. Improved control objectives were proposed. Simulation and experimental studies have shown the accuracy and effectiveness of the improved DPC strategy.Thirdly, for the DFIG entire system, predictive direct power control (P-DPC) was applied to realize the coordinated control of DFIG’s RSC and GSC. The simulation and experimental results were compared with the VC. The results show that the strategy achieved good steady-state performance and excellent dynamic performance.Finally, a brief introduction of the constitution of the experiment hardware platform of a3kVA DFIG, and the software design control algorithm were given. The experimental results verified the correctness and effectiveness of control strategies. The results also show that the DPC strategies have good potential for the wind power industrial applications. |
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