Abstract
Due to the topology of MMC base MTDC grid, the fault transient current has these characterisitcs of fast rise rate and high peak value after a DC side fault. In order to limit the fault current, the current-limiting reactors are generally installed at both ends of the DC line in the flexible DC grid, thus providing the boundary for DC line protection. The analysis shows the polarity characteristics of the reactor voltages between the faulted line and the normal line are different for a pole-to-ground fault. In case of a pole-to-pole fault, the reactor voltage characteristics between the faulted line and the normal line on the same DC bus are different. Therefore, identification criterion of pole-to-ground fault is constructed based on the polarity relation of voltage integral of current-limiting reactors. Identification criterion of pole-to-pole fault is proposed based on the ratio of voltage difference between the positive pole and negative pole current-limiting reactors on different lines of the same DC bus. Accordingly, the whole protection scheme is designed. Finally, a three-terminal symmetrical monopole DC grid model based on MMC is constructed, and the simulation results verify the validity of the proposed protection scheme.
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