This work presents a novel variable speed generator system, i.e. double-generator system for the hydropower plants. It consists of two synchronous machines, a superposition gearbox and an inverter. One of the synchronous machines is coupled directly to the grid and the other is connected to the inverter. The stabilization of the system and the regulation of the turbine desired speed are achieved by the control of the latter synchronous machine. Due to its specific construction the investment of hydropower plants can be reduced. The dynamic behavior e.g. the step responses characterizes the system properties. Firstly, the equations, which describe the total system, are presented. Then these equations are linearized at an operating point, so that the controller can be designed. Finally, the stability of the system and the control behavior are examined with the method of state space representation.