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For the integration of large amounts of offshore wind power, the interconnection of HVDC links towards multi-terminal networks is highly desired. However, its technical realisation remains a major challenge - especially in a multi-vendor environment. In addition to the challenges on the DC-side, it is crucial to guarantee that DC faults do not lead to an intolerable loss of power infeed to the connected AC grid(s). To restrict this power loss, a control and operation concept is presented in this paper which utilises state-of-the-art control and switchgear only: When the total wind power infeed exceeds the primary control reserve of the adjacent AC grid, the DC-side interconnection can be preventively decoupled via DC-side load commutation switches. At all other times, the multi-terminal network is operated in a coupled manner. Using (VDC/P)-droop controls and a DC grid controller, a decoupling sequence is implemented and validated via EMT simulations. The resulting DC fault behaviour is discussed along with technical requirements and limitations. The proposed concept could not only save costs, but also de-risk and accelerate the development of first offshore multi-terminal HVDC networks considering missing DC grid codes and a lack of coherent functional requirements for HVDC protection.
Düllmann et al. (Mon,) studied this question.