Torque Ripple Reduction in a DFIG-DC System by Resonant Current Controllers

A DFIG connected to a dc bus by a diode rectifier and a unique reduced-power PWM converter is considered in this paper. With respect to the traditional ac-grid connected DFIG, such a layout avoids the presence of the grid-side PWM converter and is an interesting solution to integrate the DFIG in a dc micro-grid together with other generating units, loads as well as storages. The peculiarity of the DFIG, which allows the control of the rotor current space vector independently of the mechanical position, offers two important benefits when the stator is connected to a constant voltage dc grid by a diode bridge: it avoids the need to boost the flux amplitude at low speed, and it allows to considerably reduce the torque ripple due to the diode commutation. This last issue is developed in the present paper by using a field oriented control scheme based on Proportional-Integral and Resonant Controllers. The proposed control is validated through simulation and experiments.