Robust Tracking Control of a Three-Phase DC-AC Inverter for UPS Application under Unbalanced Load Conditions

This paper describes a robust control technique for three-phase uninterruptible power supply (UPS) with output LC filter under unbalanced load conditions. The proposed control scheme consists of a combination of two robust controllers for the positive- and negative-sequence in order to provide balanced sinusoidal output voltages by eliminating the negative-sequence which caused by unbalanced loads. The gains of controller are determined by a linear matrix inequality (LMI)-based optimization so that the convergence rate to the steady state is maximized in the presence of the uncertainties in the LC filter. The uncertainties of the system are expressed as the possible ranges of inductor and capacitor values. The effectiveness of the proposed control technique has been validated and illustrated by the simulation result under unbalanced load conditions.