UPFC-unified power flow controller: theory, modeling, and applications

This paper describes the theory and the modeling technique of a Flexible Alternating Current Transmission Systems (FACTS) device, namely, Unified Power Flow Controller (UPFC) using an Electromagnetic Transients Program (EMTP) simulation package. The UPFC, in this paper, consists of two solid-state voltage source inverters which are connected through a common DC link capacitor. Each inverter is coupled with a transformer at its output. The first voltage source inverter, known as STATic Synchronous COMpensator (STATCOM), injects an almost sinusoidal current, of variable magnitude, at the point of connection. The second voltage source inverter, known as Static Synchronous Series Compensator (SSSC) injects an almost sinusoidal voltage, of variable magnitude, in series with the transmission line. This injected voltage can be at any angle with respect to the line current. The exchanged real power at the terminals of one inverter with the line flows to the terminals of the other inverter through the common DC link capacitor. In addition, each inverter can exchange reactive power at its terminals independently. The functionalities of the models have been verified.