Control of Grid-Connected Shunt Active/LCL Filter: Continuous Sliding Mode Control Approach

Abstract

LCL grid connected three phases and three wires shunt active filter (SAF) is studied and controlled. It is known that SAFs generate distortive components caused by a high switching-frequency voltage source inverter (VSI). A LCL output filter is often used to prevent spreading these distorting signals to the grid side. A usually used linear controller in a LCL output filter unavoidably yields a phase shift between the reference and injected currents that severely deteriorates the filtration quality. In this work, inherently robust conventional/classical sliding mode controller (SMC) is used in the shunt active filter for reducing the phase shift effects over the broad bandwidth, while improving robustness to system's perturbations. In order to prevent inherent very high frequency switching of the SMC that can severely hurt the switching elements, two different continuous SMC are studied. In the first solution, a “sign” function used in discontinuous SMC is approximated by a continuous sigmoid function. The second proposed solution uses an artificial increase of input-output relative degree that allows designing SMC in terms of the control derivative, while the actual SMC function becomes continuous due integrating the discontinuous control derivative. 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 efficacy of the considered electric power system driven by the proposed control algorithms was demonstrated via the simulations, carried out by Matlab, Simulink, and Simscap-Sim_Power_System code.