1) AC Cable Versus DC Cable Transmission for Offshore Wind Farms

A large interest in renewable energy systems has been developed to mitigate the green house effect. In Western Europe, the focus is thus, on wind power and especially on offshore locations, due to higher energy production, a longer expected life time and lack of suitable wind turbine sites on land. A DC cable is the preferred choice when connecting large offshore wind farms to the grid. The utilization of the Dc cables was enabled by the new HVDC transmission technology using voltage sourced converters.

The offshore wind farms that are being built today use an AC connection to shore and the distances to shore are also quite moderate. This paper describes three different solutions with a DC connection the shore and compares the cost and losses of a DC system with a traditional AC system that is used as a reference. DC systems will be competitive solutions in ten years and have several advantages compared with an AC system. This paper includes several reasons for selecting a DC cable is the preferred choice when connecting large offshore wind farms, to the grid.

The offshore wind farms that are being built today use an AC connection to shore and the distances to the shore are also quite moderate. This is a well-known technology and a natural step for the first off-shore wind farms. However in ten, years when the experiences from the first offshore wind farms have been evaluated and the next expansion starts, the alternatives with a DC grid will become cost-effective solutions. The future wind farms will becomes larger and will be situated at a farther distance from shore. The connection to the onshore transmission grid must be at a point with high short circuit capacity and the onshore distance, therefore is expected to be greater in the future.

The cost of power electronics will be significantly lower in the future and the losses will continue to decrease, so the advantages will outweigh the disadvantages.