Three-Phase PLL Based on Vector DFT under Distorted and Unbalanced Grid

For large distributed generation systems, like wind or solar, where the grid suffers from non-idealities such as unbalance and harmonics, conventional Synchronous Reference Frame-Phase Locked Loop (SRF-PLL) cannot works perfectly, that means there will be steady-state errors in the final output. Therefore, there is a need for a strategy that can accurately detect and track the phase angle of the grid voltage under these non-ideal conditions. Combing the vector discrete Fourier transform (VDFT) with SRF-PLL, the VDFT-PLL is proposed in this paper. In the case of unbalanced and distorted three-phase grid, VDFT can not only retain the ability to decouple the positive and negative sequence components of the fundamental wave and filter out all odd harmonics in the
$\alpha\beta$
frame, but also shorten the dynamic time of fundamental wave extraction from 1 to 1/2 fundamental cycle. Thereby, the steady-state error of output phase angle can be eliminated. Finally, the effectiveness of VDFT-PLL is verified by simulation results.