Digital frequency-adaptive PLL technique for three-phase grid-connected inverter

Aiming at the low accuracy of traditional PLL(Phase Locked Loop) under the conditions of seriously distorted grid voltage and abrupt frequency variations, an all-digital PLL algorithm with frequency-adaptation function is presented for the three-phase grid-connected inverters, which adopts the adaptive second-order low-pass filter to effectively suppress the line negative-sequence fundamental and harmonic components and to quickly track the line positive-sequence fundamental phase with little distortion. The proposed method is compared with existing open-looped and closed-looped methods, and the simulative and experimental results show its correctness and feasibility.