Q15Power Quality and FACTS

Classification of FACTS Controllers

FACTS (Flexible AC Transmission Systems) controllers are power-electronic-based devices used to enhance the controllability and power transfer capability of AC transmission systems. Based on their connection to the network, they are broadly classified into series controllers, shunt controllers, combined series-series controllers, and combined series-shunt controllers.

Series Controllers

Series controllers inject a voltage in series with the transmission line, effectively modifying the line's impedance or inserting a controllable voltage source. TCSC (Thyristor Controlled Series Capacitor) varies the effective series reactance using a thyristor-controlled reactor in parallel with a fixed capacitor, providing continuous compensation control. SSSC (Static Synchronous Series Compensator) uses a VSC to inject a controllable voltage in quadrature with line current, independent of line current magnitude, giving it superior dynamic control of power flow and damping.

Shunt Controllers

Shunt controllers inject current into the system at the point of connection, effectively behaving as a controllable reactive power source or sink. The SVC (Static VAR Compensator) uses thyristor-switched/controlled capacitors and reactors (TSC/TCR) to provide variable reactive power. STATCOM uses a VSC to synthesize a controllable voltage source, providing faster and more compact reactive support with better performance during low-voltage conditions than SVC.

Combined Series-Series and Series-Shunt Controllers

Combined series-series controllers, such as the Interline Power Flow Controller (IPFC), consist of two or more series VSCs sharing a common DC link, connected to different transmission lines, enabling active power transfer between lines for overall power flow management across a multi-line corridor. Combined series-shunt controllers, most notably the Unified Power Flow Controller (UPFC), integrate a shunt VSC and a series VSC on a common DC bus. The shunt converter provides reactive power support and DC bus voltage regulation, while the series converter injects a voltage that controls active and reactive power flow on the line, giving UPFC simultaneous control of voltage, impedance, and phase angle.

• Series: TCSC, SSSC - control line impedance/power flow
• Shunt: SVC, STATCOM - control bus voltage and reactive power
• Series-series: IPFC - coordinated power flow control across multiple lines
• Series-shunt: UPFC - comprehensive independent control of P, Q, and voltage