Variable structure control of UPS inverters

The authors present a novel variable-structure control (VSC) technique for UPS inverters. Unlike other VSC approaches in which all control objects are achieved by a single switching surface function, the proposed inverters employ multiloop control in which the regulation of output voltage as well as inductor current is in terms of its own VSC. In addition, the smooth function is adopted to prevent infinite switching of comparator and alleviate the chattering effect. It follows that the inverters are capable of being current-limited and switched with constant frequency. Owing to the robustness of VSC, the output voltages of the inverters have the features of low distortion, fast dynamic regulating speed and insensitivity to load and input voltage variations. A 2 kVA single-phase system is implemented, and both simulated and experimental results are provided to prove the effectiveness of the proposed approach. Applicability to three-phase systems, demonstrated by simulated results, is also included.