Analysis of DC-side Single Pole Grounding Fault in MMC-HVDC System Considering the Influence of Control Strategy

Currently, there are few studies on DC-side single-pole fault in flexible DC transmission system giving adequate consideration to factors such as converter control. In view of this phenomenon, a method to analyze the transient characteristic of single-pole fault in MMC-HVDC system considering the impact of control strategy is proposed. First, according to the grounding mode of neutral point, the flow path model of short circuit current after DC-side single-pole grounding fault occurs is constructed. And then, by analyzing the effect of the nearest level control strategy (NLC) on different physical parameters in the fault path, the variation pattern of the number of sub-modules switched on in the bridge arms connected with the fault pole is revealed. On this basis, the formulas of short circuit current and sub-module capacitance voltage are derived, thus the transient process of single-pole fault in MMC-HVDC system can be depicted accurately. Finally, RTDS simulation results verify the correctness of the derived expressions of short circuit current and capacitance voltage.