Preventive DC-side Decoupling: A System Integrity Protection Scheme to
Limit the Impact of DC Faults in Offshore Multi-terminal HVDC Systems
Abstract
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’
(VDC/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.