Grid-Connected Shunt Active LCL Control via Continuous SMC and HOSMC Techniques

Abstract

A LCL grid-connected three-phase and three-wire shunt active filter (SAF) is studied and controlled. It is known that SAFs generate distortive components caused by a high switching frequency of a voltage source inverter (VSI). In order to prevent spreading these distortive components to the grid, the LCL filter (usually controlled by a linear control feedback) is used; while a parasitic phase shift/lag between the reference and injected currents emerges and severely deteriorates the filtration quality. In order to overcome phase shift problem, while ensuring good and robust filtration performance of the SAF, linear and nonlinear controllers are studied. An improved linear RST controller ($RST_{imp}$) presents a good filtration solution in continuous time, but technological limitations prevent its discrete-time implementation. This work proves the advantages of sliding mode over the linear controllers by improving tracking accuracy over the entire bandwidth of harmonics. Specifically, an SMC with sigmoid approximation function, an SMC with artificial increase of relative degree (AIRD), and a continuous higher-order sliding-mode controller (C-HOSMC) are studied, in order to prevent a very high-frequency switching of control that can severely hurt the switching elements. The output of the continuous SMCs is pulse-width modulated (PWM) in order to provide a fixed given frequency of control switching, required for the VSI safe operation. A new modeling approach of nonlinear loads in a real textile factory is proposed, and the efficacy of the studied controllers for SAF/LCL filter, even under unbalanced conditions, is validated via simulations based on real measurements coming from power quality analyzers.