Analysis of Converter Valve-Side Single-Phase-to-Ground Faults in Symmetrical Monopolar MMCs

Converter valve-side single-phase-to-ground (SPG) faults are among the most critical issues affecting the secure operation of modular multilevel converters (MMCs). However, the fault characteristics of these faults in symmetrical monopolar MMCs have not been thoroughly explored. This paper aims to fill this gay by theoretically investigating SPG faults in such systems. The analysis considers various influencing factors, including MMC grounding schemes and dc line types. The findings demonstrate that valve-side SPG faults cause dc voltage oscillations and therefore, large discharging currents from the distributed capacitors of dc lines. These scenarios are examined through mathematical analyses and verified through simulations of a generic MMC-HVDC system using PSCAD/EMTDC. The results accurately reveal the fault behaviors under different system conditions, providing valuable insights for the design of protection systems and insulation coordination for internal ac grounding faults in MMC stations.