Enhancing the three-phase synchronous reference frame PLL to remove unbalance and harmonic errors

This paper presents a modified three-phase phase locked loop, consisting of a multi-block adaptive notch filter (ANF) integrated into a conventional three-phase synchronous reference frame phase-locked loop (SRFPLL). The addition of the ANF to the system allows it to become frequency adaptive. Also, since the ANF consists of multiple blocks which are in parallel with one another, the system is able to remove multiple input signal distortions. Thus, the proposed system is able to eliminate the double frequency ripple that is normally caused in the conventional three-phase SRFPLL by input unbalance, as well as harmonic errors, despite the presence of frequency variations in the input signal. The proposed system is compared to a conventional three-phase SRFPLL, as well as a three-phase SRFPLL with a simple notch filter, and the advantages of the proposed system are discussed.