When offshore wind farms reach a capacity of several hundred megawatts, alternating current (AC) cables become extremely large and have high losses of the electricity generated. Direct current (DC) systems can operate at higher voltage using smaller cables and have lower power losses.
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AC transmission of power through submarine cables is economical for short distances at lower voltages and low powers. These limitations are cumulative in that they are caused by AC losses due to the capacitance of the AC cable. Shorter distances mean that more power can be transmitted at higher voltages. Longer distances limit the power which can be transmitted without incurring expensive power losses. Using commercially available technology, the limits on AC underwater transmission are about 1,000 megawatts over 50 miles.
Utilities planning large wind farms far from shore have begun to look at high voltage DC transmission as the only viable technology. DC transmission does not suffer from capacitative effects and the length of cable is therefore unlimited. Current technology makes voltages up to 600 kilovolts commercially available for DC transmission resulting results in smaller currents and therefore smaller cable diameters. Only two of these smaller cables are required as opposed to three cables for three-phase AC transmission.
Offshore wind farms being built are small enough or close enough to shore to use AC transmission. For larger wind farms further offshore, high voltage DC (HVDC) will have to be used. While HVDC transmission has a long track record dating back to 1954, the conversion of AC to DC at the offshore end still presents technological challenges. Building the large converter stations at the offshore location will not be easy. Remote monitoring and fault clearing in hostile environments may affect reliability. In view of these difficulties, research on new AC cable technology is proceeding but no better solution has yet been identified (as of March 2011).
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