A Unified Power Flow Controller , commonly known as a UPFC , is an advanced Flexible AC Transmission System controller used to regulate voltage and control power flow in high-voltage AC transmission networks. The operating principle of a UPFC is based on two back-to-back Voltage Source Converters connected through a common DC-link capacitor. One converter is connected in shunt with the transmission system, while the second converter is connected in series with the transmission line. The series converter injects a controllable AC voltage into the line, while the shunt converter regulates the common DC-link voltage and can provide independent reactive-power support to the connected bus. This coordinated arrangement enables the UPFC to control: Transmission-line voltage Effective line impedance Voltage phase angle Active-power flow Reactive-power flow Bus voltage Power-system oscillations The UPFC combines many of the operating capabilities of a Static Synchronous Compensator, or ST...