A Two Degrees of Freedom Resonant Control Scheme for Voltage-Sag Compensation in Dynamic Voltage Restorers

This paper presents a two degrees of freedom (2DOF) control scheme for voltage compensation in a dynamic voltage restorer (DVR). It commences with the model of the DVR power circuit, which is the starting point for the control design procedure. The control scheme is based on a 2DOF structure implemented in a stationary reference frame ( $\alpha \text{--}\beta$), with two nested controllers used to obtain a passband behavior of the closed-loop transfer function, and is capable of achieving both a balanced and an unbalanced voltage-sag compensation. The 2DOF control has certain advantages with regard to traditional control methods, such as the possibility of ensuring that all the poles of the closed-loop transfer function are chosen without the need for observers and reducing the number of variables to be measured. The use of the well-known double control-loop schemes that employ feedback current controllers to reduce the resonance of the plant is, therefore, unnecessary. A simple control methodology permits the dynamic behavior of the system to be controlled and completely defines the location of the poles. Furthermore, extensive simulations and experimental results obtained using a 5-kW DVR laboratory prototype show the good performance of the proposed control strategy.