Improved PI Controller for DC Link Voltage Stability in MMC PV Shunt Filter

The increasing presence of power electronic-based loads has made it challenging to maintain high power quality in modern power systems. To reduce total harmonic distortion in line voltage and currents at the point of common coupling (PCC), active power filters are commonly employed. This paper presents an adaptive DC-link voltage strategy for using a Modular Multilevel Converter (MMC) as a shunt active power filter (SAPF) in grid-connected photovoltaic (PV) systems. Initially, a small-signal analysis evaluates DC-link stability and then the interaction between MMC energy and PV, and the impact of harmonic distortion on the DC-link voltage is carried out. Building on this analysis, an adaptive DC-link controller is proposed for the MMC used as an SAPF. The proposed control algorithm effectively addresses issues of DC voltage fluctuation caused by frequent reference variations, thereby improving harmonic compensation performance under various balanced and unbalanced non-linear load conditions. A key innovation is using an adaptive proportional-integral control with anti-windup to manage DC-link voltage. This algorithm offers notable improvements, including: (i) keeping source current total harmonic distortion (THD) within IEEE-519 recommended limits (ii) delivering robust dynamic performance with adaptive PI control and anti-windup for the DC-link capacitor voltage. Comparative analyses with previous studies underscore the benefits of the proposed control algorithm. The approach’s feasibility and effectiveness are confirmed through simulations and hardware experiments.