Grid-Connected Shunt Active LCL Control via Continuous Sliding Modes

An 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 the high switching frequency of a voltage source inverter (VSI). In order to prevent the spread of 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 this study, the inherently robust sliding mode controller (SMC) is used in the SAF, for reducing the phase-shift effects over the broad bandwidth, while improving robustness to system's disturbances. Furthermore, in order to prevent very high frequency switching of the SMC that can severely hurt the switching elements, two different continuous SMCs are employed and studied. In the first solution, a signum function used in discontinuous SMC is replaced by a continuous sigmoid function. The second proposed solution uses an artificial increase of input-output relative degree that allows designing the SMC in terms of the control derivative, while the actual SMC function becomes continuous by integrating the discontinuous SMC. The output of the continuous SMC is pulse width modulated (PWM) in order to provide a fixed given frequency of control switching, required for the VSI safe operation. The proposed approach was validated on a mathematically modeled conventional nonlinear load and a real textile factory (Aleppo, Syria) via simulation based on real measurements coming from power quality analyzers. A new modeling approach of nonlinear loads is proposed and the efficacy of the proposed controllers for SAF/LCL filter, even under unbalanced conditions, is validated via simulations.