An Equivalent Calculation Method for Pole-to-Ground Fault Transient Characteristics of Symmetrical Monopolar MMC Based DC Grid

The pole-to-ground fault is one of the most typical faults in modular multilevel converters (MMCs) based DC grid. Analyzing the transient characteristics of fault voltages and currents is of great significance for fault identification, configuration of protection, and optimization of system parameters. This paper proposes an equivalent transient characteristics calculation method for pole-to-ground faults for symmetrical monopolar MMC based DC grid. Firstly, according to the mechanism of the pole-to-ground faults, the fault network and the steady-state network are formed. Then, the star-delta transformation is performed on modelling the MMCs in the fault network, which overcomes the problem that the number of the inserted sub-modules (SMs) in each arm is time-variant. Based on the discrete adjoint model, the pre-fault and faulted matrices describing the voltages and currents are established, and a general method for solving the transient voltages and currents is proposed. Finally, the proposed method is validated in a four-terminal symmetrical monopolar MMC based DC grid. The results show that the proposed method is of high accuracy, and is much more time efficient than time-domain simulation.