Improved virtual quadrature-coordinate EPLL considering input harmonics and DC offset

The two-phase quadrature-coordinate enhanced phase-locked loop (QC-EPLL) possesses a better transient performance than the standard single-phase EPLL. In this paper, a virtual QC-EPLL (VQC-EPLL) based on signal delay method is proposed to realize using QC-EPLL to estimate the synchronization information of the single-phase grid. Furthermore, the effect of the input harmonics and DC offset on the VQC-EPLL is originally derived and it reveals these nonideal conditions causes the unexpected periodical ripple in the estimated amplitude, phase angle and frequency. Furthermore, the improved approach, including the DC offset estimator and finite impulse response filter is proposed to eliminate these effect. Detailed experimental results verify the correctness and feasibility of the theoretical analysis and proposed improved approach.