Interconnecting subsea DC collection systems into a high reliability DC grid

This paper presents a subsea DC collection grid with robust control and protection scheme and flexible expansion capability. By virtue of the DC-DC converter, different sections of the subsea grid are functionally interconnected and in the meantime decoupled in the event of DC faults. The faults are selectively isolated and system recovery from the faulty DC intertie branch is achieved by a coordinated operation on DC-DC converters and DC disconnectors (or standard AC circuit breakers). A test system is presented which consists of two 100 MW renewable energy sources and DC collection/transmission grids. The simulation results confirm the N-1 security of the presented topology and good control performance over DC faults.