Andrea Tilli, Lorenzo Marconi, Christian Conficoni
In this chapter some results related to Shunt Active Filters (SAFs) and obtained by the authors and some coauthors are reported. SAFs are complex power electronics equipments adopted to compensate for cur-rent harmonic pollution in electric mains, due to nonlinear loads. By using a proper "floating" capacitor as energy reservoir, the SAF purpose is to inject in the line grid currents canceling the polluting har-monics. Control algorithms play a key role for such devices and, in general, in many power electronics applications. Moreover, systems theory is crucial, since it is the mathematical tool that enables a deep understanding of the involved dynamics of such systems, allowing a correct dimensioning, beside an effective control. As a matter of facts, current injection objective can be straightforwardly formulated as an output tracking control problem. In this fashion, the structural and insidious marginally-stable internal/zero dynamics of SAFs can be immediately highlighted and characterized in terms of sizing and control issues. For what concerns the control design strictly, time-scale separation among output and internal dynamics can be effectively exploited to split the control design in different stages that can be later aggregated, by using singular perturbation analysis. In addition, for robust asymptotic output tracking the Internal Model Principle is adopted.