Abstract
The modular multi-level converter (MMC) has recently gotten great attention due to the installation of large renewable generation farms and high voltage direct current transmission (HVDC). In the same manner as the conventional inverters, MMC operates based on multiple insulated-gate bipolar transistors (IGBTs). Each IGBT is generally combined with an anti-parallel diode in the half-bridge MMC, the most efficient MMC topology. Due to the diode path, the DC fault current, caused by the AC phase voltage differences when its DC-Link bus shorts, flows the MMC freely. This paper proposes a DC-Link fault current interrupting method based on Ultra Fast Disconnectors (UFDs) and bypass thyristor. The proposed method is verified based on electromagnetic transient (EMT) level simulations, including several case studies with various short circuit ratios (SCRs) and reluctance per resistance (X/R) ratios. As a result, the proposed method successfully blocks the DC-Link fault current within at most one cycle. The case studies are conducted using a power system computer aided design and electromagnetic transient including DC (PSCAD/EMTDCTM).
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