FACTS Devices

Flexible AC Transmission Systems (FACTS) are power electronic-based devices inserted into AC transmission networks to enhance controllability and increase power transfer capability. Traditional AC transmission networks have limited ability to control the flow of power on individual lines or to dynamically manage voltage profiles. FACTS devices solve this by providing fast, continuously variable reactive power compensation and impedance control, making the transmission network more flexible, stable, and efficient.

Core Concept of FACTS

In an AC transmission system, the active power flow between two nodes depends on the voltage magnitudes at the two ends, the impedance of the line, and the phase angle difference between the two voltages. The expression is P = (V1 * V2 * sin(delta)) / X, where delta is the angular difference and X is the line reactance. Under normal conditions, none of these three variables is easily controllable in real time using conventional equipment such as tap-changing transformers or fixed capacitors. FACTS devices make all three controllable using power electronics.

Shunt FACTS devices are connected in parallel with the transmission line. They primarily control the bus voltage by injecting or absorbing reactive current at the point of connection. A Static Var Compensator (SVC) achieves variable reactive power using thyristor-controlled reactors (TCR) in parallel with fixed or thyristor-switched capacitors (TSC). A STATCOM (Static Synchronous Compensator) uses a voltage source converter (VSC) with a DC capacitor to generate or absorb reactive current independently of the bus voltage, giving it superior performance at low voltage conditions compared to SVC.

Series FACTS devices are inserted in series with the transmission line. They modify the effective impedance of the line to control power flow. A Thyristor Controlled Series Compensator (TCSC) uses a thyristor-controlled inductor in parallel with a fixed capacitor. By varying the firing angle of the thyristors, the effective impedance seen by the line current is varied, allowing control of power flow and also providing sub-synchronous resonance damping. An SSSC (Static Synchronous Series Compensator) uses a VSC to inject a series voltage in quadrature with the line current.

UPFC: The Most Powerful FACTS Device

The Unified Power Flow Controller (UPFC) is the most versatile FACTS device. It consists of two VSCs sharing a common DC bus. One VSC is connected in shunt (like a STATCOM) and the other is connected in series with the transmission line (like an SSSC). The shunt VSC supplies the real power needed by the series VSC from the AC system. The series VSC injects a voltage with controllable magnitude and phase angle in series with the line. By varying this injected voltage, the UPFC can simultaneously control the line impedance, the bus voltage, and the power angle, giving it full control over all three parameters that determine power flow.

Mathematical Expression

The power flow equation that FACTS devices manipulate is the fundamental starting point. For a lossless transmission line between bus 1 (V1 at angle delta) and bus 2 (V2 at angle 0), the transmitted active power is P = (V1 * V2 / X) * sin(delta). FACTS devices modify X, V1, or delta to control P. An SVC modifies V1 by injecting reactive current Iq such that V1 = V_ref. A TCSC modifies the effective series impedance from X to (X - Xc_eff), changing the power flow for the same angle difference.

Given:
Transmission line: V1 = V2 = 1.0 pu, X = 0.4 pu
Power angle delta = 30 degrees
TCSC reduces effective X to 0.28 pu

Why this formula applies:
Active power flow between two buses depends on V, X, and delta.

Formula:
P = (V1 * V2 / X) * sin(delta)

Without TCSC:
P = (1.0 * 1.0 / 0.4) * sin(30)
P = 2.5 * 0.5 = 1.25 pu

With TCSC (X reduced to 0.28 pu):
P = (1.0 * 1.0 / 0.28) * sin(30)
P = 3.571 * 0.5 = 1.786 pu

Final Answer:
TCSC increases power transfer from 1.25 pu to 1.786 pu, an increase of 42.9% for the same voltage and angle conditions.

Exam Tip: In GATE, remember that SVC and STATCOM are shunt devices for voltage control; TCSC and SSSC are series devices for power flow control; UPFC controls all parameters simultaneously. STATCOM performs better than SVC at low voltage because its output current is independent of terminal voltage. SVC output current is proportional to voltage.

Mechanism Explained

• SVC uses Thyristor Controlled Reactors (TCR) to absorb variable reactive power and Thyristor Switched Capacitors (TSC) to supply discrete steps of reactive power. Net reactive output is continuously variable.
• The fundamental limitation of SVC is that its reactive output is proportional to the square of the terminal voltage (Q = BV^2). During voltage collapse, when voltage drops, SVC output reduces sharply exactly when it is most needed.
• STATCOM produces reactive current independent of the terminal voltage. Even at very low voltage, a STATCOM can supply rated reactive current, making it far superior for voltage support during faults and voltage collapse scenarios.
• TCSC controls effective line impedance by varying thyristor firing angle, changing power flow distribution between parallel transmission corridors without changing generation dispatch.
• UPFC injects a series voltage of controllable magnitude and angle, enabling simultaneous control of real power, reactive power, and bus voltage at one device, making it the most powerful single FACTS device.
• FACTS devices improve transient stability by increasing synchronizing torque, improve small-signal stability by providing damping, and improve voltage stability by supplying dynamic reactive power.

Quick Revision

• FACTS devices provide fast, controllable reactive power and impedance adjustment in AC networks using power electronics.
• Shunt devices (SVC, STATCOM): control bus voltage by injecting or absorbing reactive current at connection point.
• Series devices (TCSC, SSSC): control power flow by modifying effective line impedance in series with the line.
• UPFC: series-shunt combined device. Controls voltage, impedance, and angle simultaneously. Most versatile FACTS device.
• Power flow formula: P = (V1*V2/X)*sin(delta). FACTS controls V, X, or delta to control P.
• GATE trap: STATCOM output is independent of bus voltage (constant current source behavior). SVC output drops with voltage squared. STATCOM is superior during low-voltage or fault conditions.
• TCSC also provides sub-synchronous resonance (SSR) damping, which is an additional benefit beyond power flow control.