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Power Cable Monitoring: Subsea Electrical Cables, Greater Gabbard Wind Farm UK (Omnisens®)
Topic: FOSA Case Studies
Owner: Tom Cohen
Date: 2017-04-12
Subtopic: Power

Built on shallow sandbanks 23 km from the south eastern coast of the UK, Greater Gabbard offshore wind farm (GGOWL) is capable of generating 500 MW electrical power, producing 1,750 GWh, enough to power more than 500 000(1) homes. The project is a 50:50 joint venture between RWE npower renewables and Scottish and Southern Energy.
Three buried HV AC 132 kV (800 mm2) export cables, each 45 km long, (supplied by Prysmian Cables and Systems), transmit the electrical energy ashore to a substation adjacent to the existing 400 kV line near Sizewell. A fourth 16 km cable connects the Inner Gabbard and Galloper offshore substations. The subsea cables are in shallow water, at most 37 m deep, in a tidal area.
Many of the challenges facing subsea cables are factored into the project’s design, but the environment poses many natural and man-made risks, among which are:
• Scour resulting in the exposure of the buried cables.
• Changes in the seabed morphology and environment (eg: underwater seismic activity and rising sea levels). • Tidal currents which redistribute sediment and migrating sand waves. • Dropped objects, dragged anchors, fi shing tackle. The export cables are designed to handle the 500 MW load of Greater Gabbard. If a fault occurs on one cable, the electrical energy can be rerouted to one of the others. Knowing the temperature of the cables under different loads helps the operator calculate how much can be safely switched and for how long. About 7% of an offshore wind farm’s capital expenditure is spent on export cable(2) but if a cable fails, the ability of the wind farm to produce is drastically reduced and the costs of remedial action are expensive and time consuming, dependent not only on the availability of a range of services to repair the cable but also fi nding a ‘window’ in the weather to carry out the work.
How does the operator know what challenges the cables are facing or when they are compromised, before the cable fails?

Download File   en_cs-007_gb2_ru.pdf