Proposal of transformerless UPS employing three state switching cell based converters

This work proposes two non-isolated online UPS systems, a 127V and a 220V input output voltage version, that use a high voltage gain non-isolated boost converter based on three state switching cell (TSSC) to raise the battery bank voltage up to the working DC bus voltage of the inverter. These converters present characteristics of continuous input current that improves batteries lifetime, the maximum voltage across the controlled switches is a fraction of the DC bus voltage, and voltage equalization across the dc-link capacitors is intrinsic. The adopted control strategy uses digital control for rectifier and inverter, and a hybrid control for the step-up converter. The battery charger is based on a flyback type converter, featuring lower cost and flexibility to change the battery bank capacity without resizing the main on-line converters specification. In order to verify the feasibility of this topology, experimental waveforms are shown for a 2kVA assembled prototypes for both input output voltages specification.