The Effect of Grid Topology on Transient Fault Currents in Multi-Terminal VSC-HVDC Offshore Networks

This paper investigates different network topologies of potential future DC offshore grids with respect to their impact on transient fault currents for permanent single pole-toground faults. Investigations are carried out on a twelve-terminal ±320kV VSC-HVDC network modeled around the physical topology of the North Sea. Results are obtained through EMTP simulations in PSCAD using a detailed, frequency-dependent model of an XLPE-insulated HVDC submarine cable. Four distinct grid topologies are considered. The obtained results suggest that topologies that are advantageous for system redundancy and flexible trading, i.e. meshed networks, may be disadvantageous from a maximum fault current point of view. Densely meshed grids do not only lead to higher fault currents, but also pose a serious challenge in the first few ms due to high discharge currents of cable capacitances. Results are discussed in relation to the performance of modern HVDC steady-state hybrid circuit breakers and mechanical passive resonance breakers. They suggest that the faster interruption speed of the former might lead to lower breaking current requirements.