Research On Control Strategyand Fault Protection Of Voltage Source Converter Based Multi-terminal High Voltage Direct Current System

With the fast development of the city, there are more and more sensitive load inthe power grid which has become a big challenge for the AC power grid. Studieshave shown that Voltage Source Converter based High Voltage Direct Current（VSC-HVDC） has advantages of increasing transmittable power and improvingpower quality. Therefore, VSC-HVDC has extensive application space in windpower integration, power supply of isolated island, interconnection of asynchronouspower grid and city infeed.This paper applied Voltage Source Converter based Multi-terminal HighVoltage Direct Current（VSC-MTDC）to city infeed and did research mainly on fourterminal parallel HVDC system based on VSC. The structure and modulation modeof VSC were selected to be three-phase two-level structure and Sinusoidal PulseWidth Modulation （SPWM）. And the steady-state models under abc and dqcoordinate systems were estabilshed respectively. According to the mathematicalmodels, the local controllers were estabished to realize decoupled control of activeand reactive power. System performance was analyzed by using MATLAB to provethat the system stability and transient performance met the requirements. Then didsimultion of VSC-HVDC to prove the validity of the design of local cotrollers.Control strategy of VSC-MTDC was investigated. Voltage margin controlstrategy was proposed and master controller was designed. When fault occurred,VSC could change control strategy automatically by using margin control switch.Master controller would redistribute active power to keep balance of the activepower. PSCAD/EMTDC was used to simulate the steady-state operation of fourterminal parallel HVDC system based on VSC. The simulaion results proved thevalidity of voltage margin control strategy and the design of master controller.Fault protection of VSC-MTDC was investigated and simulations based onPSCAD were done. The simulation results proved that VSC-MTDC could ensurelong-term reliable operation by cutting off fault VSC or fault cable.Coordinated operation and long-term reliable operation were achieved bystudies on control strategy of and fault protection of VSC-MTDC. These wouldprovide theoretical basis for practical application of VSC-MTDC.