Disturbance-Observer Based control for PWM Rectifiers Under Unbalanced Grid Conditions

This paper presents the design and experimental validation of a current control technique for Pulse-width modulated (PWM) rectifiers under both balanced and unbalanced grid voltages. In particular, unbalanced grid voltages can cause double fundamental frequency oscillations in the DC-link voltage. These DC-link voltage oscillations can be removed by setting the grid current to accurately track a specific current reference. In this paper, the proposed controller utilizes a cascaded control scheme comprising of an outer voltage loop and an inner current loop. The outer loop controller is designed to generate the specific current reference needed to maintain the DC-link voltage constant and at its reference. The inner current loop consists of a state-feedback controller coupled with a disturbance observer to ensure accurate tracking of the current reference. The current controller is designed in the $\alpha\beta$ reference to ensure accurate regulation of both balanced and unbalanced grid currents needed to suppress the DC-link voltage oscillations. To validate the effectiveness of the proposed controller, experimental tests were conducted and showcased that the control scheme is able to achieve the desired control objectives with excellent results.