A fast non-unit line protection strategy for the MMC-based MTDC grid

Abstract

The topology of DC power grid based on modular multi-level converter is lattic or ring. When DC side fault occurs, the sudden change of the electrical quantity can be detected instantaneously at each end of the lines, which puts forward high requirements for line protection. At present, the conventional DC line protection methods do not fully adapt to the structural characteristics and fault characteristics of the multi-terminal flexible DC grid. Therefore, a novel DC line non-unit protection scheme is proposed. Analysis shows that for different types of faults on the DC side, there are obvious feature differences between the positive and negative abrupt current of the same side line, as well as between the fault line and the normal line on the same DC bus. Thus, a fault identification criterion based on the difference between the ratio of poles' current differential is constructed, and the faulted pole selection is completed according to the difference of the positive and negative voltage of the line; the identification criteria for lightning disturbance is proposed; and the fault isolation scheme based on DC circuit breaker is designed. Finally, the simulation results of the three-terminal flexible DC network verify the effectiveness of the proposed protection.