Research On Control Strategy Of Wind Farm Grid Integration Using VSC-HVDC

Voltage source converter based HVDC(VSC-HVDC) is a kind of new DC transmission technology developed from the traditional HVDC. Besides the advantages of traditional HVDC, VSC-HVDC is also possible to control reactive power and active power independently, to compensate the AC bus reactive power dynamically to stabilize AC bus voltage, to feed AC systems with low short circuit power or even passive networks, to form multiterminal system easily and so on. It is more suitable for wind farm grid integration. The power relations, voltage equations and flux equations in stator and rotor side of DFIG are derived systematically in the paper, and a kind of control strategy based on stator flux oriented is presented, which laid the foundation for the connection of wind farm to grid using VSC-HVDC. The principle and control mode of VSC-HVDC are introduced, and the mathematic model of VSC-HVDC is studied. Then a kind of control strategy which is applied to the connection of wind farm to grid using two terminal VSC-HVDC system is proposed. VSC-MTDC is applied to the wind farm grid integration, and a multi-terminal control strategy with DC voltage slope characteristics is proposed. Simulation results show that the control strategy can realize the proportional distribution of active power between two grids without high speed communication has good dynamic response to the step change of wind speed and the favorable fault recovery characteristic after the fault of AC side. The mathematical models and power balance relation of VSC-HVDC under unbalance AC condition are in-depth studied. By the analysis of the power balance relation, the control method which aims to suppress the negative sequence current can also reduce 2nd harmonic ripple. Based on the mathematical model and the power relations, a kind of negative sequence voltage compensating control strategy was proposed to suppress the negative sequence current. To provide accurate and quick tracking of the generated active and reactive current trajectories, deadbeat control was applied to the positive sequence current inner loop control. The simulation results show that the proposed control strategy not only can suppress the negative sequence current effectively, but also mitigate 2nd harmonic ripple.