Capacitor-assisted excitation of permanent-magnet generators

Voltage regulation can restrict the useful capacity of permanent-magnet generators. Wind power applications are acutely affected because of the cubic variation of power with speed. The solution adopted for a new form of modular permanent-magnet wind turbine generator was to employ a capacitor connected across the generator AC terminals providing additional excitation due to the capacitive current flowing in the stator coils. The resulting circuit displays unusual characteristics and its analysis must take account of nonlinearities introduced by rectifier loading, magnetic saturation and saliency of the permanent-magnet excitation system. The paper describes the circuit configuration adopted and its analysis by methods ranging from a linearised first approximation to a complete numerical simulation. The theoretical studies are supported by experimental results. Part of the analysis is arranged to assist the designer in the selection of capacitance.