Key Issues Research On Paralleled Single-phase UPS System

For the purpose of better output voltage characteristics and h igher capacity, UPS inverters are usually connected in parallel. UPS inverters with nonlinear load and controlled with conventional control strategies are difficult in resisting output voltage distortion, sharing power and anti-interference. In order to realize the demand of development, for instance: high quality power, high output power factor, high reliability, the research of paralleled single-phase UPS system control strategies was carried out in this paper.The basic operation principles of inverter with conventional double-loop control strategy and paralleled inverters with instantaneous current-sharing control strategy are researched. Furthermore, the conventional strategies are simulated.The output voltage of single-phase UPS inverter is disturbed by harmonic component when the load of inverter is nonlinear load. Thus, a modified Lyapunov-function based control strategy is designed. The switch function is consisted of steady-state component and perturbed component, so that, the state variable can be tracked accurately. The global stability of modified Lyapunov-function is proved. The result was proved by simulation.In order to improve power sharing effect of paralleled multi module, in this paper, a visual impedance based instantaneous current-sharing control strategy was designed to realize high response and low circulating current. By means of the added of visual impedance, the circulating current caused by the difference of parameters between each module is restrained. A high robustness to parameter difference is realized, and proved by simulation.A paralleled UPS system experiment platform is preliminarily completed. Furthermore, simple experiments of conventional double-loop control strategy for single-phase UPS and instantaneous current-sharing control strategy for paralleled inverters is designed to certify principle analysis. In addition, a good dynamic response characteristics of modified Lyapunov-function control strategy is proved by simulation.