The hydro-viscous transmission is potentially attractive for large scale wind turbines and is capable of significantly improving the turbine controllability and reliability due to the direct drive-train speed control. The hydro-viscous transmission based maximum power extraction control is presented in this paper. The system design, basic dynamic characteristics, thermal losses and management are presented and thoroughly analysed. The system model and the maximum power extraction control loop are also presented and designed. By using the designed control loop and controlling the hydro-viscous transmission, the turbine rotating speed can be continuously adapted to the incoming wind speed to maintain the optimum power points. Simulation results of a 2.0 MW wind turbine demonstrate that the hydroviscous transmission based wind turbine has increased power capture due to the continuously variable speed feature as compared to a variable speed wind turbine with merely converter control. The hydro-viscous transmission works well enough and has an enhanced overall efficiency than the conventional system with fixed gear ratio. (C) 2015 Elsevier Ltd. All rights reserved.