Fault blocking capability in the DC-MMC with reduced number of sub-modules

The capability of DC-DC converters to block DC faults is an important issue for the development of HVDC grids. To include such feature, converters are generally oversized in terms of components by adding more semiconductors than needed in normal operation. This paper proposes to use a main switch instead of adding sub-modules in the topology to provide fault blocking capability. A converter control method is proposed to open the switch at zero current; thus, no breaking capability is required. An analysis on the impact on the converter design and a comparison with the classical solution are done. The operation of the proposed solution is verified through transient simulations. In the analysis, the requirements in terms of opening time for the switch were determined to be around 1 ms to 6 ms, which is feasible with a fast disconnector. However, the required control to keep the current on the switch at zero amps adds constraints to the lower arms of the topology in terms of installed capacitance and current withstanding.