Topology assessment for 3 + 3 terminal offshore DC grid considering DC fault management

This study assesses four DC grid topologies where each of the terminals can exchange power with any other terminal and a DC fault on any DC cable can be isolated. The DC grids are built using the following components: hybrid DC circuit breakers (CB), mechanical DC CBs, DC/DC converters and DC hubs. The aim is to compare DC fault performance, technical feasibility/readiness, power transfer security, expansion and costs. The base case for comparison are three separate 300 km offshore HVDCs connecting 3 1 GW offshore wind farms with three onshore VSC terminals. The rating of DC CBs is limited by the state of technology and costs which introduces limitation on the length of DC cables (because of communication delays) and on the DC bus fault level (limiting the number of connecting DC lines). DC/DC converters inherently block propagation of DC faults and their rating is not sensitive to DC fault level. DC hubs have considerable cost advantages over multiple DC/DC converters in cases where multiple DC systems are connecting at the same DC station. The cost assumptions for all major components are analysed, including the offshore platform costs. The study concludes that overall DC grid costs are similar (within 8%) for all four topologies. Although power security is similar for all technologies, the expansion options are best with DC/DC converters or DC hubs. DC hubs nevertheless suffer from highest on-state losses.