Multiple Reference Frame-Based Torque Ripple Reduction in DFIG-DC System

In this article, a multiple reference frame (MRF)-based approach is proposed to mitigate the torque ripple of the DFIG-dc system. The DFIG-dc system uses a diode bridge rectifier between the stator winding of DFIG and dc link. A reduced-power three-phase pulsewidth modulation converter is connected between the rotor winding of DFIG and dc link. Due to the diode commutation, the stator current and stator flux linkage are distorted with harmonics. To reduce the resulting torque ripple, the rotor currents need to track a pulsating reference signal to compensate the distortion of the stator flux. To improve the current tracking performance, MRF-based estimators are used to calculate the main harmonics in both the command value and actual value of the rotor current. Then, an MRF-based regulator is implemented to achieve an accurate tracking for the harmonics of the rotor current command. Both simulation and experimental results demonstrate the effectiveness of the proposed scheme in reducing the torque ripple. The robustness of the proposed algorithm against deviation of stator frequency is also supported by the simulation and experimental results.

I. Introduction

The fast development of wind energy and the increasing adoption of dc power system (e.g., dc micro-grid [1], HVdc transmission [2], [3], etc.) demand a reliable and cost-effective interface that connects the variable speed wind power generator to the dc power system. The so-called doubly fed induction generator (DFIG)-dc system studied in this article was first investigated in [4]. A diode bridge rectifier is used to connect the stator winding of the DFIG to the dc link. The dc terminal of the three-phase pulsewidth modulation (PWM) rotor-side converter (RSC) is connected to the dc link directly. Therefore, the grid-side converter (GSC) that is indispensable in the conventional DFIG-ac topology can be eliminated.