Control Strategy of Single-Phase UPQC for Suppressing the Influences of Low-Frequency DC-Link Voltage Ripple

The single-phase unified power quality conditioner (UPQC) is widely used to improve the power quality of sensitive end-users. However, the inherent instantaneous power difference between parallel and series converter will generate a significant low-frequency dc-link voltage ripple and degrade the compensation performance of UPQC. The generation mechanism of dc-link voltage ripple and its influences on compensation voltage and current are analyzed in this paper. To suppress the influence, a control strategy is proposed for the single-phase UPQC. For parallel converter, a notch filter is introduced in the outer voltage loop to prevent the ripple from entering the control loop, the specific order harmonic compensation is proposed in the inner current loop to reduce the grid current harmonics. For series converter, the dc-link voltage feedback is adopted to suppress the influence of voltage ripple on the compensation performance. Simulation and experimental results show that the grid current total harmonic distortion (THD) can be reduced effectively with the fixed dc-link capacitance compared with the traditional control strategy. The load voltage THD has also been reduced by more than half by the dc-link voltage feedback control. Moreover, with the smaller dc-link capacitance, single-phase UPQC can maintain better performance under the proposed control strategy.