Low THD, fast transient, and cost-effective synchronous-frame repetitive controller for three-phase UPS inverters

Abstract

This paper presents a synchronous-frame repetitive controller for three-phase UPS inverters. The proposed control algorithm can eliminate odd-harmonic periodic distortions caused by the nonlinear and unbalanced load, and simultaneously achieve very fast transient response. Distinguished from the conventional repetitive control in the stationary frame, the proposed approach incorporates the repetitive controller into three deliberately selected synchronous frames. Resultantly, the repetitive control time delay is reduced to 1/6 of the fundamental period such that the error convergence rate and transient tracking accuracy are both significantly improved. To further enhance the steadystate performance, a high performance 4th-order linear-phase infinite-impulse-response (IIR) filter Q(z) is synthesized and incorporated into the repetitive controller. The proposed controller is implemented on the Texas Instruments fixed-point digital signal processor (TMS320LF2407) and eliminates all current sensors for cost effectiveness. Experiments have been carried out on an 18 kW three-phase UPS prototype. Low THD (<2%) has been achieved under heavily distorted nonlinear load and unbalanced load. Fast error convergence (< one fundamental period) is demonstrated during step load transients.