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 em ploy 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 funct ion is adopted to prevent infinite switching of comparator and alleviate th e chattering effect. It follows that the inverters are capable of being cur rent-limited and switched with constant frequency. Owing to the robustness of VSC, the output voltages of the inverters have the features of low disto rtion, fast dynamic regulating speed and insensitivity to load and input vo ltage variations. A 2kVA single-phase system is implemented, and both simul ated and experimental results are provided to prove the effectiveness of th e proposed approach. Applicability to three-phase systems, demonstrated by simulated results, is also included.