Variable speed AC-generators applied in WECS

Paper in proceeding, 1986

Constant speed operation is at present the most common way of operating wind turbines. This method, however, results in a stiff coupling between the generator speed and grid frequency. To overcome this stiffness, adjustable speed ac drives can be used. Variable speed drives allow the wind turbine to operate at optimal tip-speed ratio and smooth out the torque transients in the drive train.

This paper describes two variable speed ac-drives, the synchronous generator with frequency convertor for large speed range and the rotor cascade induction generator for small variations of speed. Results obtained with transient state calculations using computer models and verifying laboratory tests were performed for the electrical systems. In the synchronous generator system, a frequency convertor with a dc-link is installed between the grid and the generator. The rotor cascade induction generator system, the generator is a standard wound rotor induction machine which rotor circuit is connected to a 6-pulse rectifier. The rectified slip-power can be recovered by feeding it back to the grid via an inverter and a transformer.
A 40 kW, three-bladed, variable pitch wind turbine is used as the test machine at Chalmers Test Station for Windmills. The system with synchronous generator and frequency convertor together with a digital control system has been tested on the windmill.