This paper proposes a new method to detect voltage imperfections, such as balanced/unbalanced voltage sag/swell, voltage flicker and voltage disturbance in both single and three-phase electrical distribution systems with excellent performance characteristics. The proposed method is based on the single-phase synchronous d-q reference frame transformation (SPSRF). In contrast to the previous detection methods which need a measurement of the threephase voltage for sag detection and fails to detect the voltage sag lower than a definite depth, the proposed method employs the series compensator of the unified power quality conditioner (UPQC) to inject a compensation voltage in-phase/out of phase with the source current during voltage sag/swell. The performance of the two structures of the UPQC, the right-shunt UPQC (RUPQC) and the left-shunt UPQC (L-UPQC,) were tested under different operating conditions based on the proposed controller to improve the power quality of the single feeder distribution system. The simulation analysis and compensation performance of both UPQC topologies was implemented on a PSCAD/EMTDC platform. © 2013 Praise Worthy Prize S.r.l.-All rights reserved.