DC-link voltage ripple reduction for a transformerless modular wind generator system

An important issue in the single phase converter system is to deal with the pulsating power which is the twice of the fundamental frequency. The pulsating power may shorten the lifetime of the dc-link capacitor and affect the system control performance. This paper has presented some methods to reduce the dc-link voltage ripple caused by the pulsating power in a modular converter for a multi-coil direct-drive permanent magnet wind generator system. At the generator side, the generator coils with 90 degree phase shift are rectified through power factor correction (PFC) circuit and connected in parallel to counteract the pulsating power to achieve the stable dc-link power. At the grid side, the pulsating power frequency (100Hz or 120Hz) will be twice of the grid frequency, which can be compensated by adjusting the wind generator output power, thus reducing the required dc-link capacitor value. A proportional-integral-resonant (PIR) controller is introduced in the dc-link voltage control loop and generator coil current control loop to generator the required pulsating power to compensate the grid-side pulsating power. The corresponding impact of the current harmonics to the generator torque is analyzed in the paper. Meanwhile, the required dc-link capacitor value is also given in the paper. The simulation results validate the proposed control scheme.