Electrical System with Frequency Converters for Variable Speed Operation of Wind Turbines

Generation of electrical power with synchronous generators is today the most common way to produce power. The synchronous generator has however a stiff coupling between the generator speed and the grid frequency. If the primary energy is wind energy, the variation of wind speed will give a variation of input power to the generator, which gives high loads, stresses on turbine and not optimal operation at constant speed of the generator. The coupling between the rotational speed and frequency of the grid can be eliminated if a frequency converter is used between the generator and the grid. The converter allows the turbine to be run at variable speed and makes torque control in the generator possible and thereby a reduction of the stresses on the drive train and the gearbox. With a variable speed operation it is possible to operate the wind turbine at optimal speed up to rated wind speed. This will reduce the acoustic noise from the rotor blades, since the optimal speed of rotation decreases with the wind speed and the noise of the turbine is dependent on the rotation speed. In this paper two methods of variable speed operation will be discussed: synchronous generator with a frequency converter and induction generator with a rotor cascade. Transient state computer models have been developed for these systems. The models cover both the generators and the converters. The models were verified by laboratory tests. For the system with synchronous generator and frequency converter the results from calculations, laboratory tests and operation at Chalmers test wind mill show that the system operates well with variable speed of the turbine. The tests also show that it is possible to control the electrical torque and the torque on the drive train which reduces the stresses on the wind mill.