DC Line-to-Ground Fault Analysis for VSC Based HVDC Transmission System

Voltage Source Converter based HVDC (VSC- HVDC) transmission technology as a kind of new dc transmission, is attracting more and more research. VSCs are susceptible to transmission line to line fault and line to ground fault. This paper focuses on the transient characteristics of electrical quantities in a VSC-HVDC system after the occurrence of line to ground fault. Equivalent circuit and equation is given to calculate the voltage and current in transmission line. Simulations are undertaken in PSCAD. The behaviors of DC voltage and DC current in faulty transmission line after the line to ground fault is studied. According to the characteristics of the fault current circuit when the line to ground fault occurs, the three stages of fault process were presented in detail. Firstly, DC-side capacitor discharge stage and the voltage of capacitor were derived. Secondly, the state equation of grid-side current feeding stage. Thirdly, the distribution of DC-side capacitor voltage in voltage recovery stage was analyzed. This paper also present a propose protection scheme for transmission line in VSC - HVDC system.