Three-phase on-line uninterruptible power supply with isolated battery charger

A conventional UPS consists of a diode-bridge rectifier, a DC link capacitor, an inverter, and a battery system. In most on-line UPS systems, the batteries are charged during normal operation from the grid and discharged during emergency situations. Since one of the most important components in a UPS is the battery system, for safety issues, many battery chargers are required to have electrical isolation. In this paper a three-phase on-line UPS system with isolated bidirectional battery charger is proposed. A suitable dual feedback control strategy is developed for the inverter to reduce grid voltage fluctuations. Isolated battery charger would increase UPS flexibility and ensures required safety issues. Furthermore, a simple soft charger circuit is utilized to mitigate DC link inrush peak current. The basic principles of the developed system and control algorithms have been discussed in detail. UPS efficient performance is confirmed using system simulation in MATLAB/Simulink software.