Performance Enhancement of QT1-PLL by using cascaded filtering stage

As one of the state-of-the-art grid voltage synchronization techniques, Quasi-type-1 structure based phase locked loop (QT1-PLL) achieves an excellent detection performance when only odd-order harmonics needs to be eliminated by Moving average filter (MAF). Nonetheless, when it is necessary for MAF to eliminate the fundamental negative sequence component, more delay will be introduced to the PLL, reducing its stability. The transient response speed of QT1-PLL needs to be reduced to ensure stability, which may be unacceptable for grid-connected power converters. To deal with this situation, this paper proposes a cascaded filtering stage with small delay. The suggested filtering stage is composed of second-order lead compensator, notch filter, first-order lead compensator and MAF, which is integrated in QT1-PLL. Compared with the traditional QT1-PLL, the proposed PLL has faster transient response speed and higher detection accuracy. The simulation and experimental results verify the effectiveness of the suggested filtering stage.