DC　fault　is　an　important　issue　of　high　voltage　direct　current　(HVDC)　transmission　based　on　modular　multilevel　converter　(MMC),　especially　the　application　of　flexible　DC　transmission　to　overhead　lines.　This　paper　proposed　a　bipolar　MMC　structure　and　operating　principle　including　full-bridge　MMC　fault　equivalent　circuit　and　the　fast　decay　analysis　formula　of　fault　current.　The　full-bridge　MMC　system　at　DC　pole-to-ground　fault　can　avoid　overvoltage　and　overcurrent　due　to　fast-clearing　characteristic　of　full-bridge　MMC　fault　current.　The　bipolar　HVDC　based　on　MMC　system　is　built　on　the　simulation　platform.　The　correctness　and　effectiveness　of　DC　pole-to-ground　fault　analysis　and　protection　scheme　are　verified　by　simulations.