This document introduces UPQC-S, a new concept for unified power quality conditioning that provides simultaneous voltage sag/swell compensation and reactive power compensation using the series inverter of UPQC. It begins with introductions of power quality, problems like voltage sag/swell, the basic UPQC diagram and operation. It describes types of UPQC including UPQC-P, UPQC-Q and the new UPQC-S. UPQC-S optimally utilizes the series inverter to reduce the burden on the shunt inverter allowing it to use a minimum KVA rating. In conclusion, UPQC-S provides optimal control of apparent power flow through coordinated control of the series inverter.