Transformerless single-phase three-wire line-interactive uninterruptible power supply

This study proposes a novel transformerless single-phase three-wire line-interactive uninterruptible power supply (UPS). This UPS comprises a three-leg power converter with series-connected DC capacitors, a decoupling circuit, a filter inductor set, a filter capacitor set, a battery set, a switch set and a digital signal processor. The first and the second power electronic legs are controlled to generate a set of mixed currents, containing DC and AC components, to perform the functions of charging/discharging battery set, active power filter (APF) and an inverter. The third power electronic leg is controlled to generate an AC current to serve as an APF and an inverter. The series-connected DC capacitors act as the energy buffer and establish the DC bus voltage for the power converter. The power flow of the power converter is bidirectional. When the utility is in the normal condition, the power converter will charge the battery set and serve as an APF. When the utility fails, the power converter will generate a sinusoidal voltage to supply the uninterrupted power to the critical loads. A prototype is developed and tested to verify the performance of the proposed UPS. The experimental results show that the proposed UPS has the expected performance.