Low Frequency AC Transmission for Offshore Wind Power

The low-frequency AC transmission is a promising technology applicable for offshore wind power transmission, power grid interconnection and integration of large scale renewable generation. Low frequency can increase the static stability limit power of transmission system and permissible current rating of cable, and it is easy to change voltage and break current compared with DC transmission. In addition, the voltage source converter (VSC) provides the low-frequency AC transmission system with a flexible control similar to VSC based high voltage direct current transmission. The AC, DC and low frequency AC transmission schemes for medium to long distance offshore wind power is compared. The effects of decrease frequency on cables and over-head lines are studied. Depending on the capacity and distance of the wind farm, an optimal frequency of 20Hz is proposed. The impact of decreased frequency on main equipment such as converters, transformers and circuit breakers is discussed in detail. With some modifications, the existing equipment are able to perform their functions in a low frequency system. Three actual projects are introduced. The first project verifies the feasibility of transmitting low frequency wind power at small scale. The second project verifies the reliability of main equipment at high voltage and large capacity. The third project is aim to deliver large scale offshore low frequency wind power.