Synchronous generator and frequency converter in wind turbine applications : system design and efficiency

Licentiate thesis, 1994

This report deals with an electrical system for variable-speed wind power plants. It consists of a synchronous generator, a diode rectifier and a thyristor inverter. The aim is to discuss the system design and control, to model the losses and to compare the average efficiency of this variable-speed system with the average efficiency of a constant-speed and a two-speed system. Only the steady state operation of the system is discussed. Losses in the system are modelled, and the loss model is verified for a 50 kVA generator. The proposed simple loss model is found to be accurate enough to be used for the torque control of a wind turbine generator system. The most efficient generator rating is discussed, and it is shown how the voltage control of the generator can be used to maximize the generator and converter efficiency. The average efficiency of the system is calculated. It depends on the median wind speed of the turbine site. It is found that a variable-speed system, consisting of a generator and a converter, can have an average efficiency almost as high as a constant-speed or a two-speed system. Three different control strategies and their effect on the system efficiency are investigated.