A robust control technique for parallel operation of uninterruptible power supply inverters

A robust control technique applied to modular UPS inverters operating in parallel is presented. 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, achieving an excellent distribution of currents between modules. 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. Experimental results on a 1 kVA modular UPS system confirm the viability of the proposed scheme.