This paper proposes a new method to detect voltage imperfections such as balanced/unbalanced voltage sag/swell, voltage flicker, and voltage disturbance in a single or three-phase electrical systems with excellent performance characteristics. The proposed method is based on single-phase synchronous d-q reference frame transformation. In contrast to the previous methods which need measurement of three phase voltage for sag detection in which it fails to detect the voltage sag lower than a definite depth. The proposed method employs the series compensator of Unified Power Quality Conditioner (UPQC) to inject compensation voltage in-phase or in-quadrate with source current during voltage sag. The performance of two structures of UPQC, right-shunt (R-UPQC) and left-shunt (L-UPQC) are tested under different operating conditions based on proposed controller to improve power quality of a single feeder distribution system. The simulation analysis and compensation performance of both UPQC topologies is implemented on PSCAD/EMTDC platform. © 2012 IEEE.