Abstract-This article presents a topology for DC grids which enables very robust DC fault protection with moderate costs, good operating flexibility and simple controls. It is postulated that radial DC systems are best suited for limited-size local DC grids. Radial topology enables robust and fast protection selectivity using only local signals and exploiting the advantages of hybrid DC circuit breakers. To enable flexible expansion options to national/international systems, it is suggested to interconnect star points of radial systems using DC/DC converters. DC/DC converters enable inherent isolation of DC faults and provide firewall between radial DC grids. Each interconnecting cable is protected by a DC/DC at one end and a hybrid Circuit Breaker at the other end. The control options for DC/DC converter and the radial grids are analysed. A detailed simulation model of 6 terminal DC grid with 2-star points is presented. The PSCAD simulation results confirm DC fault isolation and good control performance of the proposed topology for a range of DC fault contingencies.

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-This paper assesses 4 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 3 separate 300km offshore HVDCs connecting 3 1GW offshore wind farms with 3 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 case 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 4 topologies. While 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.

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