Direct Power Control of Doubly Fed Induction Generator Without Phase-Locked Loop Under Harmonically Distorted Voltage Conditions

This paper proposes an improved direct power control (DPC) strategy of the doubly fed induction generator without phase-locked loop (PLL) under harmonically distorted voltage conditions. The proposed DPC scheme is implemented in a virtual synchronous reference frame, so that the potential instability problems caused by PLL can be eliminated. A second-order vector integrator is integrated with DPC to suppress the harmonic components directly, so that three different control targets of smooth stator active and reactive powers, sinusoidal stator current and constant electromagnetic torque, can be achieved. The sequential separations and the complex calculations of the power compensating items can also be eliminated. Meanwhile, a simple grid frequency estimation scheme is proposed, which can make the control strategy self-adaptive to the frequency variation in the practical situation. Finally, the experimental results validate the availability of the proposed DPC strategy.

I. Introduction

Wind power generation systems based on the doubly fed induction generators (DFIGs) have been widely employed due to the variable-speed constant-frequency operation, four quarter active and reactive power regulation and smaller converter rating, compared to the fixed speed induction generators and the permanent magnet synchronous generators [1], [2]. Several control strategies have been proposed to improve the DFIG operation performance, which can be mainly divided into the vector control (VC) and direct power control (DPC). Compared with VC schemes, DPC strategy can avoid the complex configuration of the commanded current calculation and achieve the direct power regulation and fast dynamic response, with less dependence on generator parameters [3].