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 αβ 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.
 

Phase locked loop; distorted and unbalanced grid; discrete Fourier transform