Control of a variable speed, fault-tolerant permanent magnet generator

The control of power from a variable speed permanent magnet generator on the low pressure shaft of a gas turbine aero-engine is discussed. The application requires a fault tolerant generator (a high impedance design) to limit the fault current to the full load current at all speeds. The maximum power generation is constant over the whole speed range of the generator. The proposed system seeks to minimise the current rating of the converter and generator. The converter voltage is set at the minimum speed and then held constant as the speed increases. At higher speeds, reactive power is circulated between the generator and the converter to hold down the converter output voltage. This allows the design of the generator to be changed to increase the EMF, decreasing the current rating. At zero power demand, current still flows between the generator and the converter, except at the very lowest speeds. Under fault conditions the generator terminals must be short-circuited to limit the terminal voltage.