Enhanced Repetitive Control Strategy for Shunt Active Power Filter

This paper presents a discrete-time repetitive control strategy for a three-phase, four-wire (3P4W) shunt active power filter (SAPF). In typical power electronic control systems, a dual-loop structure is often employed—comprising a fast-acting inner current loop and a slower outer voltage regulation loop. However, due to the periodic nature of the current reference, conventional PI controllers often struggle to accurately track it. Repetitive controllers (RCs) are widely recognized for their ability to effectively follow periodic signals, offering high gain across all frequency components. Despite this advantage, the elevated gain in the higher frequency range can potentially cause system instability. To address this, the proposed research includes a modification where the sensitivity function of the standard RC is squared. Additionally, a modified RC based controller is integrated into the system to improve the overall power quality. This approach results in a sensitivity function characterized by moderate amplitude responses, with pronounced attenuation (notches) at low to mid frequencies and reduced impact at higher frequencies. The proposed control method has been tested and implemented on a 3P4W SAPF setup. Index Terms— Modified Repetitive Control, Active Power filter, harmonic compensation, Power Quality, 3P4W, Load Compensation, Non-linear load, Unbalanced Load.