Analysis and Dynamic Control of a Dual-Stator BDFIG-DC System Supplying DC Grid With Minimized Torque Ripple Through Harmonic Current Injection

Due to inherent high reliability and low maintenance cost, the brushless doubly fed induction generator (BDFIG) has become a promising candidate in offshore wind energy generation systems and dc/ac/hybrid microgrids. This paper proposes a novel BDFIG based system targeting at dc power generation (BDFIG-DC), which features interfacing the power winding (PW) and the dc grid through an uncontrolled rectifier and common dc link. Compared to conventional BDFIG systems fed by ac grid, the BDFIG-DC system offers an alternative to integrate the BDFIG into the dc transmission system with a simpler structure and lower cost. Due to the constraint imposed by the dc-side diode bridge, torque ripple reduction emerges as an urgent issue as a result of the distorted flux and current. To achieve the smooth dc-connected operation of the BDFIG, an improved vector control strategy is proposed in this paper, where the control of the system is divided into three parts: torque ripple reduction, frequency regulation, and average torque control. Based on the investigations into the electromagnetic torque reduction and small-signal model of the PW flux linkage, average torque and PW frequency are successfully controlled with torque ripple effectively suppressed. The effectiveness of the proposed system, along with the associated control strategy, is validated by the experimental results of a 10-kW 2/1 dual-stator BDFIG (DS-BDFIG) prototype.