Collection grid topologies for off-shore wind parks

Off-shore wind parks are expected to he a growing market for the future wind power industry. Today, some off-shore wind parks are erected, especially around the coast of Denmark, and future expansions will be seen in Denmark and in the rest of northern Europe as well as in other parts of the world. The existing wind parks are in sizes up to about 200 MW, covering areas up to 20-30 km2. The sizes of the erected wind turbine generators, WTG are about 1-2 MW and the distances between the towers are from 600-700 m almost up to 1000 m. The collection grid of an off-shore wind park interconnects all the WTG, into switchgear and step-up transformers on a platform if long distances from shore or, at short distances, in a land-based switchyard. The off-shore platform is then connected to the main grid on shore by subsea transmission cables. As the wind parks cover large areas, it requires a quite lot of sub-sea cable to interconnect all the WTG in a wind park. The total sub-sea cable length of a collection grid of a size of 150 MW wind park is assumed to be as much as 50 km or maybe more. This paper discusses system grounding, switching overvoltages, and reliability of supply for a number of alternative configurations of off-shore collection grids. (5 pages)