DC transmission grids with fault tolerant LCL VSC converters and mechanical DC circuit breakers

The introduction of HVDC light technology has been a significant step towards developing DC transmission grids. However, all voltage source converters (VSC) employed with HVDC are completely defenceless against DC faults, and significant further technical advances are required to achieve security and reliability levels that exist with AC grids. This paper proposes building DC grids using VSC converters based on LCL passive circuits that enable inherent regulation of DC fault currents. The DC grids with LCL AC/DC converters will not have high DC fault currents and do not require very fast DC fault isolation. The paper describes mechanical DC circuit breakers that can be used at the ends of each DC cable in similar manner and with similar protection systems as with AC transmission systems. The advantage and feasibility of this method in DC fault isolation is studied using a 1GW three terminal HVDC test system simulated on PSCAD platform.