A UPFC for Voltage Regulation in LV Distribution Feeders With a DC-Link Ripple Voltage Suppression Technique

The large-scale integration of distributed photovoltaic generation causes several power quality issues in low voltage (LV) distribution networks. Network voltage profile variations severely affect the LV distribution networks. The four-leg unified power flow controller (4L-UPFC) has series and shunt converters that can address the power quality issues. However, instantaneous power theory shows that second-order harmonic voltage (2ωHV) appears at the dc-link capacitor of the 4L-UPFC during any unbalanced operations. This article proposes control strategies for series and shunt converters that will simultaneously regulate the load voltages of a distribution feeder while suppressing the 2ωHV term on the dc-link of the UPFC. A controlled negative sequence current from the shunt converter is used to suppress the 2ωHV term on the dc-link. The active suppression of the 2ωHV term allows electrolytic capacitors to be replaced with small long life ceramic or film capacitors, and this does not require additional passive compensation. Stability analysis of the control loops demonstrates the overall stability of the converter system. The proposed control methods have been implemented on a Texas DSP (F28377D). An experimental demonstration on a laboratory scale prototype shows that the proposed control methods can effectively regulate the load voltages at LV distribution feeders and suppress the 2ωHV on the dc-link of UPFC during unbalanced loads and supply conditions.