A Sliding Mode-Control for a Three-Phase SAPF Using a Nonlinear Transformation

This paper introduces a novel switching strategy for a three-phase two-level shunt active power filter (SAPF) based on sliding mode control (SMC). Unlike traditional SMC implemented in the natural reference frame, which suffers from cross-coupling among phase-leg controllers due to the neutral point voltage and results in sliding losses, the proposed method achieves full axis-decoupling. To overcome this limitation, a nonlinear transformation maps the voltages and currents into a new two-dimensional reference frame. Within this transformed frame, a nonlinear model is developed, and two sliding-mode surfaces are implemented using hysteresis band comparators. The resulting switching strategy removes sliding losses, allows third-harmonic injection in the control signals, and extends the effective control range by approximately 15% without saturating the controller. Experimental results validate the theoretical framework and show that the proposed method reduces the grid-current THD from 2.839% with conventional SMC to 1.138%. The method also preserves stable DC-link regulation and sinusoidal grid currents under load-step, voltage-sag, and distorted-grid-voltage conditions.