Improved phase-locked loop for robust and fast tracking of three phases under unbalanced electric grid conditions

Modern electric power transmission and distribution systems need accurate phase information on three-phase grid voltages and/or currents. Conventional three-phase synchronous reference frame-based phase-locked loop (SRF-PLL) is generally sufficient for phase tracking during balanced operating conditions. However, SRF-PLL becomes inadequate when unbalance exists in the three-phase input signal. Because, unbalance in input signal set causes a double-frequency ripple and as a result, an ideal output signal cannot be obtained. Previous studies have focused on improving the SRF-PLL performance by removing this ripple with using delayed signals methods such as delayed signal cancellation (DSC)-PLL methods. Although these methods give ideal output signals, a certain time delay is occurred. In this study, an effective method DIF-PLL based on adding another signal, which has reverse polarity ripple is proposed for obtaining ideal output signal and fast tracking of phase angle. Performance of the DIF-PLL is compared with the performances of SRF-PLL and DSC-PLL methods. The performances of these methods are also examined for the presence of the harmonics. The results show that the proposed method has a better three-phase tracking performance than the others under balanced and unbalanced electric grid conditions.