A study on the electrical system arrangement for offshore wind turbines and factors influencing the voltage level choice

The downtime of an offshore wind turbine is very significant in an event of a failure, and results in loss of generation and revenue. Different topologies are evaluated, including a comparison between AC and DC star networks. Star network topologies are suggested for secure connection and to centralise power electronic components, which have high failure rates, so it is more accessible for maintenance and repair. Transformers can be eliminated, because their failure result in a long downtime, but without it, the voltage level of the network should be as low as possible to reduce the cost and voltage rating of the converter but then it is restricted by the losses in the cable. A simple cable study on 5 MW turbines found that the minimum voltage level can be reduced to as low as 3.5 kV rms for an AC star network, and 4.6 kV for a DC star network. A three level converter is recommended to connect to a network at this voltage level without a transformer.