Radial DC transmission grid for integration of large scale Scottish offshore wind energy

Scottish Government has approved 27GW of offshore wind farm contracts recently, and this has accelerated studies for large scale offshore wind integration. It is becoming evident that a coordinated offshore DC transmission grid will be needed. This study evaluates the options for DC grid topology in
the Scottish context, considering state of technology, required timeframe, interoperability requirements and market aspects. It is recommended to adopt a radial ±525 kV, bipolar DC grid with a single, central DC substation located onshore in Scotland. In this topology, each terminal is connected only to the
central DC substation (there is no terminal-to-terminal connection). The paper elaborates on the most important benefits of such system including:
• A central DC substation located onshore in Scotland would provide a fixed landing point, and in this way simplify planning and consenting for offshore wind farms and all terminals.
• Radial topology is simple, highly flexible, and expandable, while it provides opportunity for all terminals to trade power with any other terminal.
• Control and protection principles are known and have been demonstrated on simulators. Robustness is better than with other topologies.
• It is expected that interoperability issues will be minimal since there is no direct interaction (neither electrical nor communications) between terminals.
• The ownership and responsibilities are simple, allowing for staged and long-term development.
• The integration in the UK electricity/power market will need further studies, although benefits are evident in terms of elimination of congestion and locational charging.