Robust tracking control of a three-phase DC-AC inverter for UPS applications

This paper describes a combination of robust control techniques applied to an offset-free robust tracking control of a three-phase dc-ac inverter with an output LC filter for application to uninterruptible power supply (UPS). The control law, which employs state feedback and integration of the tracking error, is described in a synchronous dq frame and is implemented using a space vector modulation technique. The controller is designed 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. These uncertainties are expressed as possible ranges of the capacitor and inductor values. In the absence of uncertainties, the designed controller will become a deadbeat controller, whereas in the presence of uncertainties, it provides the shortest possible settling time. Thus, the proposed design method provides a systematic tool to combine the robustness to model uncertainties with the deadbeat control. The use of a one-step-ahead predictor is considered to compensate for the computation delay, and an LMI-based method to obtain an optimal gain of the state estimator is also proposed. The efficacy of the proposed approach was experimentally confirmed on a three-phase 10-kVA prototype UPS system.