Power conversion system for a modular, direct-drive,permanent-magnet wind turbine generator

摘要

Voltage regulation can restrict the useful capacity ofnpermanent-magnet generators. For example, a single-phasenpermanent-magnet AC generator may be represented by an EMF sourcenproportional to speed and hence frequency, with series reactance alsonproportional to frequency. The maximum power which can be delivered to anresistive load is proportional to speed and may be much less than thenthermal rating. This problem became serious during the development of anpower conversion system for a variable speed, permanent-magnet, windnturbine generator. The power available from the wind is proportional tonthe cube of speed but the generator capacity increases only as speed.nThe machine concerned is modular and intended for use as andirect-coupled, low-speed generator. The stator consists of modules eachnof which has an E core with a single coil producing a single phase ACnoutput. The outputs are rectified separately and combined at a common DCnlink. The power limit imposed by the series reactance leads to poornutilisation and leaves little torque margin to cope with transientntorque due to wind gusts. Consequently, some form of compensation isnneeded, particularly at high speed. The approach adopted was to employ ancapacitor connected across the generator AC terminals. The capacitor maynbe considered to be part of a tuned circuit, cancelling some of thenseries inductance or alternatively it may be thought of as providingnadditional excitation due to the capacitive current flowing in thenstator coils