A novel FRT strategy for transformerless MMC to improve current differential protection performance under single-phase grounding faults

Abstract

AC/DC distribution networks are interconnected by converters such as modular multilevel converters (MMCs). In AC/DC distribution networks, single-phase grounding (SPG) faults can cause enormous challenges in relay protection. Additionally, traditional MMCs’ fault ride-through (FRT) strategies cause further degradation of the protection performance. Furthermore, under SPG faults, the voltages of the nonfaulted phases swell beyond the high-voltage ride-through (HVRT) capabilities of MMCs using traditional FRT strategies, which can lead to their shutdown. Therefore, this manuscript has proposed a novel FRT strategy for transformerless MMC to solve these two problems. For the first problem, this manuscript has proposed a positive-sequence (PS) and zero-sequence (ZS) current injection method aimed at improving the sensitivity of current differential protection without requiring adjustments to the relay parameters. The optimal phase angles for the injected PS and ZS currents are derived based on the proposed interconnected sequence network. In addition, since the line impedances may not be accurately measured during fault conditions, the appropriate phase angle ranges for PS and ZS current injections have been studied. For the second problem, this manuscript has proposed the self-cooperative FRT method to promote MMC’s HVRT capability. This approach enables MMCs to withstand severe and minor SPG faults. What’s more, the PS and ZS current references for the FRT strategy have been discussed. The effectiveness of the proposed FRT strategy has been validated in MATLAB/Simulink.