A frequency controller for induction generators in stand-by minihydro power plants

Induction generators working in minihydro power plants separated from the main network have to maintain constant frequency for every load condition. The equilibrium between the torque generated by the turbine and the load torque has to be reached for the speed corresponding to the requested frequency. In the small power range plants (up to 500 kW), the solution with turbines without regulation can be economically attractive. However, an additional variable dissipative system in parallel to the main loads supplied by the induction generator has to be provided. A variable dissipative system controlled with continuity and not step by step can be realized utilizing a thyristor converter with resistive load. The thyristor firing signals have to be generated by a PID frequency controller. In order to avoid the problems of unbalanced voltage due to single phase loads or unsymmetrical three phases loads, a new digital controller for the variable dissipative load is proposed. The authors deal with the design of the overall system for a practical application