The use of 66kV technology for offshore wind demonstration sites

Near-to-shore wind farms, such as the UK's Round 1, connect their generation to shore at 33kV. This is also currently the standard distribution voltage (or collection voltage) for interarray networks. Larger, further-from-shore wind farms normally use offshore substations to step-up generation to a higher voltage, e.g.132kV/170kV before transmission back to shore. Recent research shows that a submarine cable capable of being used for 66kV networks, i.e. a “wet-type” cable, could be type-tested and made available. The Carbon Trust is currently progressing work to qualify and type-test this cable, which presents opportunities for standard collection voltages to increase from 33kV to 66kV. This paper presents the benefits of utilising 66kV for near-shore and medium distance offshore wind farms, both for UK and international demonstration wind farm projects. The benefits include the potential to reduce the amount of submarine cabling required, reduced losses in the connection at 66kV versus 33kV, and the potential to eliminate offshore substations in some cases. The costs and benefits have been quantitatively and qualitatively assessed and detail is provided for 33kV/66kV equipment cost comparisons (switchgear, cable and transformers), cost reduction/benefit impacts of substation and cabling rationalisation, and full-life difference in losses and hence revenue from generation. The paper specifically addresses the feasibility of developing a system implementing 66kV at the Blyth Offshore Demonstration site. Also presented are potential enablers and barriers to 66kV market uptake such as supply chain readiness, the feasibility of demonstrating the proposed 66kV technology at dedicated sites in the UK and outline design and feasibility considerations for certification at 66kV with an emphasis on Low Voltage Ride-Through (LVRT) certification. It is concluded that moving to 66kV could lead to capital cost and full-life benefits compared with the present standard voltage of 33kV for many near and medium distance-to-shore wind farms. However, steps to accelerate market uptake should also be considered by exploring enablers (testing, demonstration, certification, etc.) and solutions to related barriers.