DC Circuit Breaker Feasibility Study – Protection System Design

Abstract

The potential for offshore wind in UK waters is substantial, with a goal to expand from 12 GW of installed capacity to 50 GW by 2030 and 100 GW by 2050[1]. The increasing scale and distance from the shore necessitate a shift from AC systems to DC connections [2]. Existing offshore wind farms typically have a point-to-point (PtP) connection with the onshore network via AC or DC cable. In either case, proven AC circuit breaker (ACCB) technology is used to disconnect in case of fault. As more DC connections are introduced, coastal communities are impacted by more AC/DC converter stations and cables. It is costly to install and maintain many converter stations at different locations, which increases the overall cost of electricity. The alternative is to connect multiple wind farms in a meshed DC network with fewer onshore converter stations.

Doing this at scale requires DC circuit breakers (DCCB), an innovative technology untested in the UK and European markets. DCCBs are at a technology readiness level (TRL) of 9 in China but are at a TRL of between 6 and 7 in Europe, based on results from the Horizon 2020 EU-funded PROMOTioN project [3]. Without DCCBs, the choice in network topology supporting wind targets would be reduced, costs associated with delivering Net Zero increased, timescales lengthened, and the outcome could be less resilient networks.

This paper outlines the findings from the alpha phase of the “Network DC” project, focusing on the design of a protection system that utilizes DCCBs in the United Kingdom. The central case study involves the development of multi-terminal DC (MTDC) connections situated around an established DC hub. This hub is located at the DC Switching Station (DCSS) in Peterhead, Northern Scotland. Within the framework of the “Network DC” project, it is presumed that the DC hub is pre-existing. The project’s approach involves progressively integrating a separate substation equipped with DCCBs into this existing setup. The integration of DCCBs to the existing DCSS provides greater fault isolation flexibility and allows the connection of a greater capacity of offshore wind generation by avoidance of common DC side fault risks that would lead to losses above the maximum power infeed allowed within the GB system. Starting from the use case, the choice of busbar configuration is made considering system constraints, component reliability and techno-economic analysis (TEA). The presented results are a recommendation based on an optimum where the TOTEX (TOTal EXpenditure) of the switching station with DCCBs is minimal, given a certain level of system reliability and respect for the AC system requirements.

Domagoj Hart, Colin Foote, Amjad Mouhaidali, Suresh Rangasamy, Alberto Bertinato, Benjamin Marshall

Presented at CIGRE 2024.
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