Robust model-following control of parallel UPS single-phase inverters

This paper presents a robust control technique applied to modular UPS inverters operating in parallel. When compared to conventional PI control, the proposed technique improves the response of the output voltage to load steps and to high distorted output currents. Furthermore, an excellent distribution of currents between modules is achieved, resulting in a fine power equalization between the inverters on stream. The crossover frequency of the different loop gains involved is moderate, so that robustness to variations of the operation point and to tolerances of the output LC filter components values is achieved. A comparative study with a two-loop, conventional PI control scheme is presented. Experimental results on a 1 kVA modular UPS system confirm the viability of the proposed scheme.