Protection of Single-Phase Fault at the Transformer Valve Side of FB-MMC-Based Bipolar HVdc Systems

Although the probability of occurrence of ac grounding faults at the valve side of the interface transformer of a high-voltage dc (HVdc) link is low, they may cause high risks to the converter when compared to grid-side ac faults. This article analyzes the characteristics of valve-side ac single-phase-to-ground faults in full-bridge modular multilevel converters (FB-MMCs)-based bipolar HVdc systems. Overcurrents in the converter arms are analyzed and it is shown that overvoltages in FB submodules occur without an appropriate protection in place. Two strategies are investigated to protect the FB-MMC during the fault and corresponding controllers are designed. The effectiveness of the presented strategies for the prevention of overcurrents and overvoltages, upon nonpermanent and permanent faults, and system postfault restoration is investigated. For completeness, the strategies are also verified by conducting simulations in PSCAD/EMTDC.