Direct Stator Current Vector Control Strategy of DFIG Without Phase-Locked Loop During Network Unbalance

This paper proposes a direct stator current vector control strategy of a doubly fed induction generator (DFIG) without phase-locked loop (PLL) during network unbalance. In the proposed control strategy, a virtual phase angle produced by the nominal frequency is used for coordinate transformations instead of the actual voltage one estimated by the PLL. Since the calculation of the commanded stator currents is based on the instantaneous power theory regardless of the generator parameters, it can greatly reduce the parameter dependence of the control system. Then, for a satisfactory grid synchronization, an improved synchronization method is proposed. It can smoothly produce the stator voltage with no overshoot voltage and no parameter dependence. During network unbalance, a reduced-order vector integrator is used to directly regulate the pulsating components without the complex calculations of the commanded current values and the sequential separations of the voltages and currents. Meanwhile, four alternative control targets can be achieved for the DFIG during network unbalance. Furthermore, the control performance, including the control system stability, the robustness against frequency disturbances and parameter deviations, and the rejection of voltage unbalance, are fully analyzed. Finally, the experimental results are given to validate the effectiveness of the proposed control strategy.