Flexible AC Transmission Systems | FACTS

What Are Flexible AC Transmission Systems (FACTS)?

FACTS stands for Flexible AC Transmission Systems. IEEE defines the term as AC transmission systems that use power-electronic and other static controllers to improve controllability and power-transfer capability. Depending on the controller, FACTS can regulate voltage, supply or absorb reactive power, change effective line impedance or control power flow. These functions can improve the use and stability of a power transmission system, but equipment ratings and control settings must come from a network study.

Features of Flexible AC Transmission Systems (FACTS)

• Dynamic voltage regulation and reactive-power support
• Higher usable transfer capability when voltage, transient or oscillatory stability is the limiting factor
• Damping of electromechanical power oscillations
• Load flow control through series impedance, phase angle or combined control

The achievable benefit depends on the network limit, controller location, rating and control mode. FACTS cannot raise a conductor’s thermal rating or replace protection and planning studies.
When correctly specified, Flexible AC Transmission Systems (FACTS) can use existing network capacity more effectively, support post-fault voltage recovery and reduce poorly damped oscillations. A device that solves one constraint can introduce control interactions or overvoltage concerns elsewhere, so coordinated simulation and commissioning remain necessary.

Influence of Reactive Power Flow on Power System Voltage

Reactive Power Compensation in Power Transmission System

Loads, lines, cables and transformers exchange reactive power with the network. The associated current contributes to losses and affects voltage. Shunt compensation places reactive support near the bus that needs it. Passive components include shunt reactors that absorb reactive power and capacitors that supply it. Mechanical switching suits slower changes. SVCs use thyristor-controlled or switched branches, while STATCOMs use voltage-source converters for dynamic reactive-current control. Series compensation instead changes effective line reactance and therefore power flow.

Effects of Reactive Power Flow

Reactive power flow affects a transmission system in several ways:

1. Higher conductor current and I²R losses for the same active-power transfer
   • Uses reactive capability in a power plant, inverter or compensator
   • Can reduce active-power headroom in current-limited equipment
2. Voltage rise or drop along network impedance
   • Undervoltage can impair load performance and voltage stability
   • Overvoltage increases insulation stress and may require shunt absorption
3. Reduced transfer margin when current or voltage limits are reached
4. Changes to steady-state, transient and oscillatory stability margins

Shunt and Series Compensation

Shunt device	Short-circuit contribution	Transmission phase angle	Steady-state voltage	Post-rejection voltage	Typical application
	Depends on device and system	Changes indirectly	Raised by capacitive vars	May rise unless switched	Voltage support during heavy load
	Depends on device and system	Changes indirectly	Lowered by inductive vars	Overvoltage is reduced	Voltage control during light load
	Converter current is limited	Changes indirectly	Dynamically controlled	Limited by rating and control	Fast voltage control, reactive-power control and oscillation damping

The table compares common shunt-compensation functions. Exact results depend on the network strength, connection point, controller type and rating.

The power-angle relationship shows why voltage magnitude, phase angle and effective line reactance are useful control variables. Shunt controllers act mainly on voltage, series controllers act mainly on line reactance and combined controllers can coordinate more than one variable.

Protection and Control of FACTS

A FACTS station uses measurement, control, firing, supervision and protection functions. Control loops calculate the required reactive current or impedance command, while valve controls convert that command into semiconductor switching signals. Independent protection must detect abnormal current, voltage, cooling or communication conditions and block or bypass equipment when required. Redundancy is selected from the station’s availability requirements rather than assumed for every installation.

Human-machine interface
The operator interface displays operating mode, setpoints, alarms, trends and equipment status.
An HMI does not replace automatic protection.
Access control, event records, time synchronisation and secure engineering interfaces are also part of the station design.

Hardware for Control and Protection

Control hardware must provide deterministic processing, high-speed I/O, fibre-optic links to high-potential equipment and reliable station communications. The platform may be proprietary, but its functional design still has to satisfy the project’s response time, availability, maintainability and cyber-security requirements. Redundant processors, power supplies, measurement channels or communications can reduce single-point failures when the reliability study requires them.
Disturbance recorders capture time-synchronised analogue values, digital states, controller outputs and protection events. Required sample rate, trigger logic, retention and reporting time depend on the transients that the study needs to reconstruct; one historical value is not a universal FACTS specification.

Power-Electronic Valves for FACTS

LTT – Light-triggered thyristors
Thyristors switch reactor or capacitor branches in SVCs and controlled-series-compensation equipment. A light-triggered thyristor receives its gate command through an optical fibre, reducing the amount of powered trigger electronics at high potential and improving electrical isolation. Optical triggering does not make a valve self-protecting: the complete design still needs voltage grading, overvoltage protection, cooling, monitoring and a coordinated bypass or blocking strategy.
A valve uses multiple devices in series or parallel as required by blocking voltage and rated current. Device diameter, pulse energy, voltage rating and current rating are manufacturer- and generation-specific. Voltage-source-converter FACTS such as STATCOMs instead use turn-off semiconductor devices and different converter topologies.