Simulation and analysis of fault characteristics of the MMC-HVDC system under bipolar connection mode

Two terminal DC transmission technology based on modular multi-level converter (MMC), which takes advantages of both MMC technology and flexible DC transmission technology, has broad application prospects thanks to the high quality of output voltage and low transmission loss. The current researches for MMC-HVDC are focused on fault characteristics analysis on the pseudo bipolar systems, while only a few is studying it under the true bipolar system. When a fault occurs in the AC side of a converter station, the differences in the topology and operation mechanism lead to different fault characteristics between the pseudo and true bipolar system. On the basis of true bipolar system, the mathematical model is constructed under steady state operation. This paper performs research on the faults of single-phase grounding on AC system side and the valve side on converter, proposes an equivalent circuit for the occurrence of transient fault, and analyzes the fault transient process of the true bipolar system. In addition, in fault process, the operation mechanism of true bipolar system is expounded, and the effect of AC fault on the system operation is explained considering whether the neutral point of DC side should be grounded. Finally, the system model of MMC-HVDC true bipolar is built in RT-LAB, the correctness of fault analysis is verified by theoretical analysis and simulation results.