A new fast peak detector for single or three-phase unsymmetrical voltage sags

In this paper, a new fast peak detector for single or three-phase unsymmetrical voltage sags is proposed. The proposed detector is modified from a single-phase digital phase-locked loop based on a d-q transformation using an all-pass filter (APF). The all pass filter generates a virtual phase with 90° phase delay but the virtual phase cannot reflect a sudden change of the grid voltage in the moment of voltage sag, which causes a resulted peak value to be significantly distorted and settle down slowly. Specially, a settling time of the peak value is too long when voltage sag occurs around zero crossing such as phase 0° and 180°. This paper describes operating principle of the APF problem and proposes a modified all-pass filter (MAPF) to mitigate the inherent APF problem. In addition, a new fast peak detector employing the MAPF is proposed and the detector can calculate peak value within 0.5 msec even when voltage sag occurs around zero crossing. The proposed fast peak detector is compared with the conventional detector using APF, and shows faster detection time in the whole range of phase. Furthermore, the proposed fast peak detector can be effectively applied to unsymmetrical three-phase voltage sags. Simulation and experimental results verify the advantages of the proposed detector and MAPF.