Research On Grid-connected Wind Power System Based On VSC-HVDC Technology

With the development and utilizing of wind energy, research of variable speed constant frequency (VSCF) grid-connected wind power system and grid connection technology has improved greatly. Most wind power systems are directly connected to the grid with AC power transmission, in this way, the stability requirements of wind farms and grid system are much higher, and the application of the project costs more. In this paper, grid-connected wind power system based on VSC-HVDC technology is designed, the specific following researches are developed:This paper describes the VSC-HVDC technology, which is based on voltage source converter uses self-turnoff power electronic device-IGBT (Insulated-Gate Bipolar Transistor) and PWM (Pulse-width Modulation) technology. The active and reactive power of the system can be independently controlled and AC and DC voltage fluctuations can be quickly adjusted, thus it's suitable for the grid-connected wind power system.Overview of the development status and technical characteristics of VSC-HVDC technology is present, some practical application projects are introduced; details of the basic operation principle and system control strategy of the VSC-HVDC system are present, and this paper analyses the basic structure and basic theory of grid-connected wind power system with the application of VSC-HVDC. With comparative analysis of several VSC-HVDC-based wind power systems, a new type of structure of VSC-HVDC-based grid-connected wind power system is put forwart:wind farm side contains many direct drive permanent magnet generators, power transmission is centralized to the DC bus after dispersed rectifiers, VSCs of generators are made up of multiple cascade of three phase PWM rectifiers; grid side uses using multiple parallel structures in converter stations, VSCs of converter stations use H-bridge cascaded multilevel inverters. The paper analyses the system control structure of both sides.According to VSC-HVDC system requirements and the design scheme, the characteristics of the high-power VSCs of wind farm side and grid side are studied. This paper describes common structures of high-power converters. VSCs of generators that made up of multiple cascaded of three phase PWM rectifiers can raise the output voltage and power, control strategy is analyzed, it uses hysteresis current control technique in three phase PWM rectifiers, the current can be stable rapidly with less harmonic wave. Structure and control strategy of VSCs of H-bridge cascaded multilevel inverters are present, the paper researches carrier phase shift PWM control strategy of cascaded multilevel inverters, it can obtain a higher voltage rank and a better power quality. VSC simulation results of rotor side and grid side are given by MATLAB/Simulink software, and results show that the proposed design of the structure and control strategy is correct.