Analysis and Protection Scheme of Station Internal AC Grounding Faults in a Bipolar MMC-HVDC System

The symmetrical bipolar configuration shows a great advantage in modular multilevel converter (MMC) based high-voltage dc-current (HVDC) transmission. In a bipolar MMC-HVDC system, station internal ac grounding faults cause sub-module (SM) capacitors to discharge violently. These faults do great harm to MMCs and require special protection schemes. In this paper, a single-phase-to-ground fault is taken as an example to investigate transient characteristics of station internal ac grounding faults in a bipolar MMC-HVDC system. A calculation theory of fault current based on the improving RLC model and second-order differential equation with variable coefficients is introduced. A novel transient zero-mode current based protection criterion with better reliability, quickness, and ability to endure fault resistances is proposed to detect internal ac grounding faults. The fault clearing strategy applicable to three types of grounding faults is also designed. The correctness of the proposed calculation theory and protection schemes is verified through simulations performed in PSCAD/EMTDC.