Sliding Mode Control of a Four-Leg Dynamic Voltage Restorer in a Natural Reference Frame

This article focuses on designing a sliding mode control (SMC) for a dynamic voltage restorer (DVR) using a four-leg voltage source converter and its output filters in the natural reference frame. The coupling of filter-capacitor voltages through the control inputs of the DVR system leads to cross-coupling in the dynamics of conventional sliding variables with respect to four control input variables, making the design of SMC using Lyapunov stability criteria difficult. To overcome this, a new sliding variable for each converter leg is proposed, which is a function of the respective filter-capacitor voltage and a fictitious voltage that depends on the converter neutral-point voltage (NPV). This allows control of both the compensator voltages and the converter NPV using the proposed scheme. This article also presents the calculation of the optimum sliding coefficient to maximize the sliding existence region for a four-leg DVR. The performance of the proposed control scheme is experimentally validated under various operating conditions using a laboratory prototype of a four-leg DVR.