Control Strategy And Simulation Study Of VSC-HVDC For Separate Control Of Real And Reactive Power

VSC-HVDC is a kind of new DC transmission technology developed from the traditional HVDC, with the IGBT based Voltage Source Converter (VSC) as its major components. Resorting to its superior characteristics, VSC-HVDC has been put into practical application successfully. But there are still many problems that need to be solved in theoretical and practical aspects. Compared with the traditional HVDC, a very important feature of VSC-HVDC is that the real and reactive power can be controlled separately and simultaneously. Therefore, how to effectively realize the decoupling control for separate adjusting of real and reactive power has become an important research topic.First, the application of the SVPWM in the VSC-HVDC is investigated in detail. Analysis shows that SVPWM is difficult to be implemented with traditional method. So, the equivalent implementation method is discussed and a new equivalent scheme, which is more easily implemented, is proposed. Based on the steady state model of VSC and through coordinate transformation, the intermediate control variables are introduced, and the VSC's power transmitting equations in rectangular coordinates is derived, so the impact of the trigonometric function is eliminated. The function relationships between the intermediate control variables and controlled variables(dc voltage,real power,reactive power,ac voltage) as well as between the intermediate control variables and controlling variables(voltage phase angle and magnitude) are determined with the theory of inverse system. After analyzing the control modes and requirements respectively for VSC-HVDC linked to active and passive systems, the corresponding control strategy is presented. So the VSC-HVDC control system is designed for different controlled variables, this control system can realize the separate adjusting of real and reactive power.PSCAD/EMTDC software is employed to simulate VSC-HVDC operation for active as well as passive inverter operating mode. Through repeatedly simulating, the controlled variable waveforms, for steady-state operation, setting point changing and load flow reversing respectively, are obtained. Simulation results show that the proposed VSC-HVDC control strategy can achieve sufficient response speed and good control effect.