Most transmissions for wind turbines are set up by multiple consecutively arranged planetary gear sets and/or normal gear sets. Therefore these transmissions have a constant ratio. In order to feed the electricity produced by the wind turbines into the grid, an electric conversion to a constant frequency of 50 Hz is necessary. FZG developed a new concept for transmissions of wind turbines based on a planetary gear. By superposition of a small electric engine the transmission ratio is continuously variable. This makes an electric conversion unnecessary and thereby increases the efficiency of the wind turbine.