Low Frequency AC transmission for offshore wind power: A review

Offshore wind farm integration is providing substantial technical and economic challenges in the medium term and the trend for farther shore development in the future is focusing research and industry attention on cost effective transmission alternatives to existing technologies. In 2014, offshore wind capacity in Europe consisted of 8045 MW, with the majority of this connected to onshore grid via High Voltage AC (HVAC) transmission. Wind farms commissioned further offshore ( > 80 km) utilise High Voltage DC (HVDC) transmission for grid interconnection. The deployment and operation of Voltage Sourced Converter (VSC) HVDC substations in harsh offshore environments is still a major challenge for the offshore wind industry. Recently, research studies to reduce the complexity of the offshore network have been undertaken both in industry and in academia, with the primary motivation of reducing cost and increasing reliability. This paper provides a review of Low Frequency AC (LFAC) transmission, which is of significant interest for offshore wind farm integration at a range of 80-180 km. LFAC is an adaptation of HVAC transmission, operated at lower frequency, typically 16.7 Hz. The key advantage of LFAC compared to HVDC is the elimination of the requirement for an offshore converter station, thereby reducing offshore complexity and potentially increasing the operational life of the offshore wind farm. Design challenges are introduced surrounding the design of the 16.7 Hz transmission transformer and associated offshore platform for this potential transmission technology. A comprehensive review of existing research conducted on LFAC and a discussion centreing on the design considerations for offshore LFAC transmission components is presented. The frequency changing converter onshore, offshore LFAC substation and the wind turbine considerations are evaluated in detail. LFAC multi-terminal offshore grids are also considered, as this alternative to HVDC multi-terminal grids may reduce the requirement for multiple offshore converter stations. (C) 2015 Elsevier Ltd. All rights reserved.