Low-THD, Fast-Transient, and Cost-Effective Synchronous-Frame Repetitive Controller for Three-Phase UPS Inverters

This paper presents a novel synchronous-frame repetitive controller for three-phase UPS inverters. Distinguished from conventional repetitive control techniques, the proposed synchronous-frame approach minimizes the repetitive control time delay to one-sixth of the fundamental period such that the dynamic response is significantly improved. In order to overcome the harmonic distortions under severe load conditions (e. g., unbalanced and nonlinear), in this paper, three synchronous rotating frames are deliberately selected, in each of which the repetitive controller is incorporated. Resultantly, the (6n +/- 1)th harmonics as well as the triplen harmonics are compensated. Moreover, a high-performance fourth-order linear phase infinite-impulse-response filter is applied to further enhance the accuracy of steady-state tracking. The proposed controller is programmed on the 16-bit fixed-point digital signal processor (TI TMS320LF2407) and eliminates high-resolution current sensors for cost effectiveness. Simulations and experimental tests have been carried out based on an 18-kW three-phase UPS system. Low total harmonic distortion (<2%) has been achieved under heavily distorted nonlinear load and unbalanced load. Fast dynamic response has been demonstrated during step load transients.