Pole-to-ground Fault Analysis of Transmission Lines in VSC Based DC Power Grids

A detailed analysis of the pole-to-ground fault characteristic is of great practical significance for solving such key problems as fault diagnosis and protection design of DC transmission system. In this paper, the pole-to-ground fault process of DC transmission system is divided into three stages, which are DC-side capacitor discharge, grid-side current feedback and voltage recovery. According to system transient characteristic under different grounding impedance, the time-domain expressions of capacitor voltage and discharge current in each stage are derived. By analyzing the oscillation characteristic of the DC voltage in the fault process, the increasing of damping for DC system can avoid the current feedback at grid-side that can make the diode breakdown in this stage. For pole-to-ground fault, the system can operate continuously through the single non-fault-phase, which makes it have the ability of short-time continuous operation during voltage recovery stage to support for fault clearing. In the end, the characteristic of pole-to-ground fault of DC system is testified through a two-terminal VSC-HVDC transmission system.