Mitigation of 2ω-Ripple in DC-Link Voltage of PWM Rectifiers Under Unbalanced Grid Conditions

In PWM rectifiers, cascade control scheme is commonly employed to maintain a constant dc-link voltage, where an outer loop controller takes the responsibility of regulating the dc-link voltage by generating the required command for an inner loop controller. In this article, the inner loop is designed with the objective of controlling the power exchange between the grid and the PWM rectifiers, while the outer voltage loop generates the required power command for maintaining the dc-link voltage at a desired reference. Under balanced grid voltages, asymptotic regulation of the dc-link voltage is achieved with a constant active power flow. However, under unbalanced grid voltages, ripple-free dc-link voltage requires modifying the power command to include double fundamental frequency oscillations around a dc component. This requirement mandates using an inner loop controller capable of achieving asymptotic tracking of sinusoidal references with non-zero dc component to eliminate the ripples in the dc-link voltage under unbalanced grid voltages. This is achieved, in this article, by combining a state-feedback control law with a disturbance observer to form the inner loop controller. The main goal of the disturbance observer is to estimate constant and sinusoidal disturbances, which can then be canceled by the state feedback control law to ensure zero tracking error. Various experimental tests were conducted to validate the effectiveness of this control method. The obtained results confirmed the capability of the controller to achieve accurate tracking of sinusoidal power references with non-zero dc components, which allows maintaining ripple-free dc-link voltage under unbalanced grid voltages.