Preventive DC-side Decoupling: A System Integrity Protection Scheme to Limit the Impact of DC Faults in Offshore Multi-terminal HVDC Systems

To create synergies between offshore wind integration, trading capacities, and operational flexibility, interconneting HVDC links to multi-terminal networks is highly desired. However, its technical realisation remains a major challenge – especially in a multi-vendor environment. In particular, it is crucial to prevent DC faults from leading to an intolerable loss of power infeed to the connected AC grid(s). To restrict this loss of power infeed, this paper proposes a concept based on state-of-the-art equipment only – i.e., without dependence on DC circuit breakers (DCCB) – for linear HVDC networks. In this concept, the DC-side interconnection is preventively decoupled via DC high-speed switches (DC-HSS) whenever the cumulative wind infeed exceeds the frequency containment reserve of the adjacent onshore AC grid. At all other times, the multi-terminal network is operated in a coupled manner. The decoupling sequence is realised via the converters’ ( V / P )-droop controls and a supervisory control. Both the decoupling sequence and the resulting DC fault behaviour in decoupled state are validated via EMT simulations of a four-terminal network, and challenges as well as the expandability to larger networks are discussed. The proposed concept could help to de-risk and accelerate the development of first offshore multi-terminal HVDC networks at reasonable costs.