A selective and staged active current limiting control strategy for fault identification in MMC HVDC power grids

Abstract

The modular multilevel converters (MMC) based high-voltage direct-current (HVDC) grids have a fast rise rate and higher peak value of fault current. The active current limiting control (ACLC) of MMC can effectively reduce the breaking current of DC circuit breakers (DCCB) and reduce the requirement for rapid fault identification in the line protection. However, the current ACLC based on changing the MMC control strategy cannot meet the selectivity requirements. The lack of selectivity may lead to power transmission on non-faulty lines in a multi-terminal DC power grid, thus expanding the range of fault influence. Therefore, this paper proposed a selective and staged ACLC. Firstly, the starting range of ACLC is determined according to the selective requirement and purpose of ACLC in the flexible DC power grid. Secondly, a staged ACLC approach was proposed to balance the speed and selectivity of ACLC. In this approach, the single end and both terminals electrical quantities of the line are fully utilized as the starting criteria for staged ACLC, and corresponding ACLC principles are proposed. Finally, the AC side current and MMC bridge arm current under ACLC were analyzed, and an ACLC scheme was proposed in coordination with DC circuit breakers (DCCB) and fault identification. The simulation results show that the proposed ACLC guarantees the current limiting ability and the MMC does not block during the whole fault period, and has a certain adaptability to the fault type, fault resistance, and noise.