We herein advance a novel, power summation hybrid transmission, which has the ability to convert the variable speed of a wind turbine rotor shaft into the constant speed required at a generator shaft for a whole range of wind speeds, thereby eliminating the need for a frequency converter. The transmission consists of a single one-stage planetary gear train (PGT) with three rotating shafts and a simple control system consisting of a few sensors and a control motor controlled by a microprocessor. One of the PGT shafts is the input, another is the output, and the third is coupled to the control motor as second input. The optimal tip-speed ratio is kept constant at low wind speeds by controlling the speed of the control motor, maximising the capture of energy from the wind. The wind-rotor speed continues to vary above the rated wind speed zone, but the rotor shaft power is kept constant by using the same control system. In this way, a constant electrical power output is achieved without altering the blade pitch, i.e., with the rotor in a fixed geometry. A frame design procedure for the transmission is proposed, efficiency expressions are derived, an example transmission operation is presented and efficiency comparisons to a mainstream variable speed wind turbine are carried out. (C) 2015 Elsevier Ltd. All rights reserved.