A low cost three-phase transformerless online UPS

This paper describes the structure, control and performance analysis of an alternative configuration of a low cost three-phase transformerless online UPS. The proposed UPS avoids the use of galvanic isolation and consists of a half-bridge rectifier, a battery bank, a half-bridge inverter, a split DC bus and an auxiliary circuit. The auxiliary circuit is composed of an inductor and a small rating switching leg, and it is employed to charge the battery bank in normal mode and additionally to balance the voltage across the DC bus capacitors in backup mode. This entire configuration provides the reduction of overall system size, weight and cost. Detailed circuit operation, control design as well as simulation results are presented to verify its behavior and feasibility.