Hybrid learning control strategy for three-phase uninterruptible power supply

This study is concerned with the control of three-phase uninterruptible power supply (UPS) systems based on a hybrid controller. A UPS must be able to maintain regulated output voltage in the face of all line/load disturbances. In order to achieve this, the control system of a UPS must have a fast transient response and a low steady-state error. Fast controllers cannot always maintain the required steady-state accuracy. Therefore in this study, a hybrid control solution is proposed. In the proposed method, a fast, yet simple, controller based on deadbeat (DB) control law is used to achieve the fast transient response required for UPS output voltage control. Noting that non-linear loads that tend to distort UPS output voltage in steady-state have periodic waveforms in nature, a type of learning controller known as a repetitive controller is added to the DB controller to improve steady-state performance of the UPS system. This results in distortion-free output voltage along with a fast error convergence. The system modelling and controller design for the proposed structure are presented in this study. A brief stability analysis is given. Both simulations and experimental results are shown for verification of the theoretical analysis.