The research objective of this paper is to find a control method which can obtain maximum energy conversion efficiency for a complicated non-linear time-varyingsystem------variable speed wind energy conversion system(WECs). According to theenergy conversion characteristic of WECs, the maximum wind energy conversion efficiency can be obtained if the ratio of rotor speed and wind speed is kept at a constant, namely the rotor speed of wind turbine can be continuously adapted to the wind speed. For achiev...