Detailed analysis and experimental results of the control system of a UPFC

The decoupled control of the power flow through a transmission line using a PWM-based unified power flow controller (UPFC) is explained. A dynamic model of a UPFC has been developed using the space-vector representation of instantaneous three-phase variables. The Park´s transformation and the reference frame selected reduce the control of the real- and reactive-power flows to the control of the d- and q-axis currents, respectively. The proposed control scheme produces fast and decoupled response of the real- and the reactive-power flow through a transmission line. It also achieves tight control of the DC-link capacitor voltage through careful coordination between the control of the series and shunt compensators of the UPFC. A prototype has been built to illustrate the main contributions. The prototype includes a UPFC with 750 Hz switching frequency applied to a reduced model of a transmission line. Experimental results agree closely with the theoretical analysis.