Harmonic current detection chain is an important part of the four leg active power filter

and the performance of the PLL determines the detection accuracy and therefore appears to be particularly important.In this paper

the traditional SRF-PLL in the DQ detection method based on transient reactive power is studied and new double frequency phase-locking method based on the improved SOGI is proposed.It can perform correct and effective phase locking in case of not ideal grid voltage

thereby achieve to keep identical phase with the grid

improve the accuracy of harmonic detection and the performance of APF.Finally

the correctness and effectiveness of the proposed method are proved by Matlab simulation and experimental results.