Application of a novel soft phase-locked loop in directly-driven permanent magnet wind power generation system

In directly-driven permanent magnet synchronous generator (PMSG) system, a fast, robust and precise phase angle detector is most important to calculate feedback variables for control the power factor and the flow of active,reactive power. Because the grid side converter of directly-driven PMSG system is connected to the power network directly, the angle information can be easily corrupted by the distorted utility voltage such as voltage unbalance and harmonics in waveform. In this paper, a new phase locked loop (PLL) method with decoupling network using all pass filter (APF) in a synchronous reference frame to extract the positive sequence components of the grid voltage under unbalance grid condition is proposed. Thus a better stability of directly-driven PMSG system can be achieved. Proposed method does not cause harmonics in the reference signal by adequately eliminating unbalance components and high-order harmonic terms in the utility voltage. The proposed PLL is completely implemented in software without any filters. The effect of PLL applied in PMWG system is verified by simulations and experiments.