Power flow control with UPFC

The UPFC can provide simultaneous control of all basic power system parameters ( transmission voltage, impedance and phase angle). The controller can fulfill functions of reactive shunt compensation, series compensation and phase shifting meeting multiple control objectives. From a functional perspective, the objectives are met by applying a boosting transformer injected voltage and a exciting transformer reactive current. The injected voltage is inserted by a series transformer. Besides transformers, the general structure of UPFC contains also a ”back to back” AC to DC voltage source converters operated from a common DC link capacitor, Figure 1. First converter (CONV1) is connected in shunt and the second one (CONV2) in series with the line. The shunt converter is primarily used to provide active power demand of the series converter through a common DC link. Converter 1 can also generate or absorb reactive power, if it is desired, and thereby provide independent shunt reactive compensation for the line. Converter 2 provides the main function of the UPFC by injecting a voltage with controllable magnitude and phase angle in series with the line via an voltage source, Figure 2. The reactance xs describes a reactance seen from terminals of the series transformer and is equal to (in p.u. base on system voltage and base power)