Direct power control of doubly fed induction generator without phase-locked loop in synchronous reference frame during frequency variations

Abstract

This study presents a direct power control strategy for a grid-connected doubly fed induction generator (DFIG) without phase-locked loop (PLL) in a synchronous reference frame. In the proposed control system, a virtual phase angle, produced by a fixed angular speed, is used to replace the actual one of the network voltage for synchronous coordinate transformations, thus, PLL can be eliminated. The steady-state and dynamic behaviours of DFIG active and reactive powers are analysed with the potential grid frequency variations. It is found that the natural components of the flux linkages are the root cause of power and torque pulsations during the transient frequency variations. Thus, an auxiliary demagnetising current control scheme is proposed and designed to damp the natural flux linkages caused by the transient frequency variations. Then, based on the mathematic model, the control system performance is theoretically analysed. Finally, the simulation and experimental results are provided to demonstrate that the developed control system is adaptive to frequency deviations and achieve a decoupling control of both active and reactive powers.