Protection Measures for Modular Multilevel Converters in Case of DC Short-Circuit Faults

The Modular Multilevel Converter (MMC) technology brings several advantages, although its operation and protection are very different compared to conventional Voltage Source Converters (VSC). Short-circuit faults at the DC-link transmission systems of MMCs that use the Half-Bridge Submodule (HB-SM) are still a major concern due to potentially high fault current magnitudes. The sequence of the pole-to-pole DC short circuit of an HB-MMC is analyzed and modeled through respective equivalent circuits. A simplified calculation is presented that allows estimating the current magnitudes after the DC-fault appearance. Finally, several protection measures for the HB-MMC are described, and a new protection method that uses an AC crowbar is introduced and analyzed.