Research On Control Strategy Of Grid-side Converter For Dfig Under Unbalanced Grid Voltage Conditions

Among the renewable energy that countries around the world are seeking to develop,wind energy received wide concern because of its large reserves, wide distribution andother features. Most of China’s abundant wind energy resources are located far away fromload centers, the grid voltage often occur unbalanced and other states fluctuations.Therefore, the paper is mainly focused on the control strategies of grid-side converter forDFIG under the unbalanced grid voltage conditions.First, this paper introduced the grid synchronization. Grid synchronization is one ofthe key techniques for integration of DFIG wind power generation system into grid, andits accuracy of detection impact on the DFIG wind power system control strategyoperating results directly. Second-order generalized integrator frequency locked loop(SOGI-FLL) was analyzed in detail, and was applied in the detection of the voltagepositive and negative sequences under unbalanced grid voltage conditions. In addition,Double Second-order generalized integrator frequency locked loop were improved byincreasing the phase function, and it were analyzed by simulation and experiment.Secondly, grid-side converter models and control strategies of the DFIG wind powersystem were analyzed. Vector control has always been the main control strategy ofgrid-side converter, recently direct power control (DPC) strategy has achieved wideattention for its excellent dynamic performance, simple control structure, etc. This paperanalized the mathematical model of the grid-side converter and sliding mode control directpower control (SMC-DPC) strategy under balanced and unbalanced grid voltageconditions, and gived a detailed analysis of the design of sliding mode controller. Underunbalanced grid voltage conditions, the different control targets were made according tothe grid-side converter operational status, and rectifier and inverter simulation of converterwere made using SMC-DPC. In addition, the paper also analyzed vector and SMC-DPCby simulation and experimental comparison.Finally, the hardware circuit design of the grid-side converter and dSPACE real-timesimulation system were introduced in detail. Experimental waveforms of the rectifier and inverter experiment were gived under the balanced and unbalanced grid voltage conditions.Experimental studies have been conducted to verify the effectiveness and correctness ofthe above mentioned grid synchronization and control strategy under balanced andunbalanced grid voltage conditions.