A Hybrid Parallel Active Filter / Off-Line UPS Unit for Computer Loads

A hybrid parallel active filter / off-line uninterruptible power supply (UPS) system is proposed here. The hybrid system contains a single inverter, which acts as a parallel active filter during the normal mode of operation (when the AC supply is available) and also supplies the load with energy from the UPS back-up battery during the back-up mode of operation (when the AC supply is not available, or is beyond the preset tolerance). In both modes of operation, the inverter is controlled in such a way that the line current is a pure sinusoid. This is to improve the input power factor and to eliminate the current harmonics in the first mode and to remove any stress on the back-up battery caused by the discharging during the second mode of operation. Full simulation results and an experimental setup are presented together with a closed loop control algorithm.