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

Voltage regulation can restrict the useful capacity of permanent-magnet generators. For example, a single-phase permanent-magnet AC generator may be represented by an EMF source proportional to speed and hence frequency, with series reactance also proportional to frequency. The maximum power which can be delivered to a resistive load is proportional to speed and may be much less than the thermal rating. This problem became serious during the development of a power conversion system for a variable speed, permanent-magnet, wind turbine generator. The power available from the wind is proportional to the cube of speed but the generator capacity increases only as speed. The machine concerned is modular and intended for use as a direct-coupled, low-speed generator. The stator consists of modules each of which has an E core with a single coil producing a single phase AC output. The outputs are rectified separately and combined at a common DC link. The power limit imposed by the series reactance leads to poor utilisation and leaves little torque margin to cope with transient torque due to wind gusts. Consequently, some form of compensation is needed, particularly at high speed. The approach adopted was to employ a capacitor connected across the generator AC terminals. The capacitor may be considered to be part of a tuned circuit, cancelling some of the series inductance or alternatively it may be thought of as providing additional excitation due to the capacitive current flowing in the stator coils