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Optimal control methodology is applied to the design of the feedback loops of a multi-inverter uninterruptible power supplies system. The control signals thus obtained will minimize a performance index, which is a function of the output voltage error, the inductor currents of all inverters and the reference signals. This enables the controller to achieve the desired objectives like minimization of the circulating current and reduction of error of the output voltage. The robustness of the controller has also been investigated. Through frequency-domain analysis, it is shown that variations in the number of paralleled inverters in the system have only small influence on the performance of the system. Therefore, the optimal controller designed for a fixed number of inverters can also work well in systems with variable number of paralleled inverters. Three experimental 110 Vac/l.1 KVA inverters are built and paralleled to verify the analysis and theoretical predictions. Index Terms--Inverters, optimization, power electronics, statespace methods.