Continuous SM Controller with Artificial Increase of Relative Degree for Grid-Connected Shunt Active/LCL Filter

We consider in this article the control of LCL grid connected three phases and three wires shunt active filter (SAF). It is generally agreed that the voltage source inverters (VSIs) of the SAFs generate some distortive components, caused by a switching frequency. In order to prevent the spread of these components to the grid side, a LCL output filter is often used. The use of a linear controller (Roots Locus/RST controller in this case) in a LCL output filter for tracking identified harmonics, causes a phase shift in the closed loop system, between the identified and the injected currents, which steeply deteriorates the filtration quality. Therefore, we propose an inherently robust traditional sliding mode controller (SMC) to overcome the phase shift effects, over the entire bandwidth, for the shunt active filter while improving robustness to system's uncertainties. In addition, in order to prevent very high frequency switching of the SMC that can severely damage the switching elements, we employ a SMC design using artificially increased relative degree. In this case, a continuous control is obtained by integrating discontinuous SMC. The output is pulse-width modulated (PWM) in order to allow VSI to operate at the fixed frequency that is easily filtered by the predesigned LCL filter. The efficiency 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.