Review of Cabling Techniques and Environmental Effects Applicable

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Cable types and installation techniques
Connection voltage – Offshore wind farm projects need to be connected to
regional distribution networks, rather than to the national transmission system.
In England and Wales, these distribution networks currently include 132kV
and 33kV systems. Some of the early Round 1 offshore wind farm projects
have made connections at 33kV while several others have connections at
132kV. It is probable that all Round 2 sites will secure connections at 132kV.
Cable design – Three-core subsea cables using solid insulation (ERP or XLPE)
are typically used for operation at voltages up to 132kV. Higher voltage cables
that use oil as an insulating medium are not deemed to be environmentally
acceptable owing to the potential risks associated with oil leakage in the
near shore environment. The cables are typically designed to give a power
transmission capacity of up to 40MW for a single 33kV cable and 160MW for
a single 132kV cable.
Turbine size – Offshore wind farm developers are continually looking to the
turbine manufacturers to develop larger and more energy efficient turbines.
The early UK Round 1 developments such as North Hoyle, used 2.5MW wind
turbine generators (WTGs). Current offshore wind farm developments are
planning to use 3.0MW and 3.6MW machines and plans for a 5.0MW machine
are already well advanced.
Distance from shore – The use of a single 132kV cable to shore provides a
cost-effective alternative to the use of three or four 33kV cables. The installed
cost (per kilometre) of a single 132kV cable is considerably lower than the
installed cost of three 33kV cables, but this solution requires an offshore
substation in order to step up to 132kV from the wind farm collection voltage
(usually 33kV). Some of the current projects under consideration have more
than one 132kV export cable to link the offshore wind farm to shore.
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