| Abstract |
HVDC DC grids composed of half-bridge modular multi-level converters (HB MMC) can be protected against DC faults using DC circuit breakers (DCCBs). However, the rapid increase of DC fault currents during DC pole-to-pole faults on symmetric monopolar HVDC systems or DC pole-to-ground faults on bipolar HVDC systems poses a challenge on the design of DCCBs due to the requirement on handling very large amount of energy during the DC fault clearing process.Blocking of the MMC converters immediately after the DC fault stops the fault current contribution of the HB MMC submodules, and reduces the design requirement of the DCCBs (energy absorption and interruption capability), but at the cost of disrupting continuous operation of the Hybrid AC/DC grid and potentially deteriorating stable post fault system recovery.This paper introduces a method that helps to reduce the DC fault current contribution of the HB MMC submodule capacitors using the converter controls of the MMCs within the HVDC grid in order to reduce the requirements of DCCB designs. The proposed method achieves this during a DC pole-to-pole fault on a symmetric monopolar HVDC system or a DC pole-to-ground fault on bipolar HVDC system through the use of modified circulating current controls of the MMC HVDC. The parameters of the proposed controller are selected taking into consideration the operational limits of the converters. |
