Offshore DC Grids as an Interconnection of Radial Systems: Protection and Control aspects

This paper 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 (CBs). 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 CB at the other end. The control options for dc/dc converter and the radial grids are analyzed. A detailed simulation model of six terminal dc grid with two-star points is presented. The power systems computer aided design (PSCAD) simulation results confirm dc fault isolation and good control performance of the proposed topology for a range of dc fault contingencies.