A sinewave interface for variable-speed wind turbines

There are a number of major benefits from operating wind turbines at variable speed including improved energy capture, reduced mechanical stress and lower levels of audible noise in light winds. A number of variable speed systems have been operated with varying degrees of success. These include AC-DC-AC link inverters for synchronous or capacitor excited asynchronous generators and also slip-recovery schemes with cycloconverters. The latter has the disadvantage of slip rings whilst the former recover energy through line commutated inverters with variable power factor and poor current waveform. The IGBT may be used to provide a power interface that can synthesise good quality current waveforms in both generator and the power network. In this solution, a generator side step-up power converter produces a constant DC link voltage from the variable frequency and voltage generator. This is then inverted through a high-frequency interface to produce near sinusoidal current at unity power factor. Results show overall efficiencies around 90% with total harmonic distortion as a % of fundamental not exceeding 10%