Current status and future prospects of continuously variable speed wind turbines: A systematic review

•Reviewing current status and future prospects of continuously variable speed wind turbines.•Personal sophisticated discussions and critical comments are provided to existing systems.•Analyzing the high potential of using hydro-viscous transmission in megawatt wind turbines. Continuously variable speed wind turbines (CVSWTs) without using fixed speed gearboxes continue being as viable options for modern wind power industry. CVSWTs have an infinite number of transmission ratios in contrast to specific gear ratios of a standard wind turbine. This transmission flexibility allows the generator to maintain a constant speed regardless of highly turbulent wind speeds. The development of CVSWTs has made great progress in recent years due to the high demand for renewable energy sources. Therefore, this paper serves to provide a systematic review of the CVSWTs including mechanical, hydraulic and hydro-mechanical type CVSWTs. The operating principles, design topologies and basic characteristics are described and analyzed. The comparisons and new trends of the CVSWTs are also discussed, which indicate the feasibility of applying CVSWTs in wind power industry and the high potential of using hydro-viscous transmission in megawatt CVSWTs.