Customized converter for cost-effective and DC-fault resilient HVDC Grids

•Optimal design of hybrid modular multilevel converters, with reduction on station equipments for weak grids, and additional ancillary services by enhanced active and reactive power converter capabilities.•Cost effective operation of HVDC grids and DC pole-to-ground and pole-to-pole faults handling.•Continuous operation of HVDC grids with slow mechanical DC breakers.•Enhanced energy converter controller. This paper presents a comprehensive study that aims to establish the meaningful range for the ratios of full-bridge cells to the total number of cells per arm, in which the performance of the mixed cells modular multilevel converter (MC-MMC) can be customized by trading the number of FB cells for high value features such as: resiliency to DC faults, reduced capital costs of DC circuit breakers (DCCBs) and running costs of the semiconductor, and continued operation. The MC-MMC design for particular ratio of FB cells to total number of cells per arm to achieve tailored features at system level is termed as an customized modular multilevel converter (CMMC). The primary motivation for the CMMC is to facilitate the use of low-cost and relative slow mechanical DC circuit breakers, with fault isolation times in the order of 8 ms to 12.5 ms. With these objectives to be achieved, interruption of power flows across the DC grid and surrounding AC grids must be minimized. It has been found that after certain ratios of FB cells to total cells per arm, the CMMC starts to exhibit current limiting mode, which helps to reduce DCCBs current breaking capacities and extend critical fault clearance times for remote converters from the fault point. Results of the pole-to-ground and pole-to-pole DC faults, semiconductor loss studies, and extended control range indicate that the presented CMMC is promising for future realization of DC grids.