Improved Two-Phase Stationary Frame EPLL to Eliminate the Effect of Input Harmonics, Unbalance, and DC Offsets

The effect of input harmonics, unbalance, and dc offsets on the two-phase stationary frame enhanced phase-locked loop (α β - EPLL) is theoretically analyzed first. It indicated that the nonideal grid conditions cause the periodic ripples in the estimated amplitude, frequency, and phase angle. An improved α β - EPLL by introducing two pure integrators and moving average filter (MAF) into both the amplitude and frequency estimation loops of α β - EPLL is proposed to eliminate the effect of the aforementioned nonideal grid conditions. An improved implementation of MAF to eliminate the effect of the variation of the grid frequency is proposed as well. The linear model of the improved α β - EPLL is built and the design rule of the control parameters is determined. Detailed experimental results are presented to verify the validity and feasibility of the improved α β - EPLL.