An advanced transient protection circuit for enhancing the overvoltage tolerance of MMC submodules

IGBTs are critical switching devices in MMCs, yet their vulnerability to turn-off overvoltage under transient and fault conditions poses a fundamental reliability challenge. This paper presents a novel transient protection topology for MMC submodules, wherein a spark gap is connected in series with a MOV to form a passive, self-triggering clamping branch. Unlike conventional snubber or pure-MOV approaches, the proposed circuit introduces zero steady-state conduction loss by leveraging the gap’s high-impedance blocking characteristic during normal operation. Upon detection of a hazardous overvoltage transient, the gap spontaneously breaks down, instantly engaging the MOV for precision voltage clamping. Comprehensive parameter design guidelines are established and experimentally validated on a 4500 V/1200 A IGBT double-pulse platform. Results confirm that the proposed circuit extends the transient overvoltage withstand capability of MMC submodules to 1.45 p.u., enabling adoption of higher submodule capacitor voltages and improving converter power density.