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A novel topology of a modular per-phase uninterruptible power supply (UPS) system based on reduced-switch-count configuration is proposed in this paper. The proposed power conversion methodology offers active front-end filtering capability which ensures unity input power factor (reactive power compensation) and low input total harmonic distortion (harmonic power compensation). In addition, this UPS system provides dynamic voltage sag compensation capability, which consequently eliminates any series transformer or dc-dc boost converter that is typically used in conjunction with traditional stand-alone UPS systems. Furthermore, it has the desirable characteristics of making seamless transition from normal to backup mode during power failures and vice versa, as well as providing controlled voltage charging at the dc bus link. The proposed UPS is also impervious to load variations, which enables it to operate under nonlinear load condition. Lastly, the circuit structure is conceived from a commonly used three-leg six-switch building block, thereby making it simple and cost-effective, and offering improved power conversion efficiency as compared to a conventional line-interactive or on-line UPS schemes. A complete set of simulation and sample experimental results based on a 1-kW test prototype of the proposed UPS system are presented in this paper to demonstrate its viability and efficacy.