Quick answer: A FACTS device is a power electronic system used in an AC power grid. FACTS stands for Flexible AC Transmission System. These devices control the voltage, the line impedance, or the power angle of a transmission line. This lets the grid carry more power in a safe and stable way, without building new lines.
In this guide, we will look at what FACTS devices do, the main types, and why power companies use them. The language is kept simple, so it is easy to follow even if the topic is new to you.
Why the grid needs FACTS devices
An AC transmission line cannot carry unlimited power. It has limits. If we push too much power through it, the voltage can drop, the line can overheat, or the system can become unstable.
The power that flows on a line depends on three main things. We can see this in a simple formula:
P = (V₁ × V₂ / X) × sin(δ)
Here is what each part means:
- V₁ and V₂ are the voltages at the two ends of the line.
- X is the reactance (a type of impedance) of the line.
- δ (delta) is the angle difference between the two ends.
So there are three ways to change the power flow. We can change the voltage, the impedance, or the angle. A FACTS device does exactly this. It adjusts one or more of these values.
In the past, engineers used mechanical switches and fixed capacitors to do this job. But these are slow. They cannot react to fast changes in the grid.
A FACTS device is different. It uses power electronics, such as thyristors and IGBTs. These are fast electronic switches. They can act in milliseconds. This makes the control smooth, fast, and automatic.
The three ways to control power flow
Before we look at the types, keep these three control targets in mind. Every FACTS device works on at least one of them:
- Voltage control – keep the bus voltage at a safe level.
- Impedance control – change the effective reactance of the line.
- Angle control – change the phase angle to steer the power flow.
FACTS devices are also grouped by how they connect to the line. Some connect in parallel (shunt). Some connect in series. A few use both.
Types of FACTS devices
1. Shunt devices (connected in parallel)
Shunt devices connect between the line and the ground. Their main job is to control voltage. They do this by adding or absorbing reactive power. Adding reactive power raises the voltage. Absorbing it lowers the voltage.
SVC (Static VAR Compensator) The SVC is an older type of FACTS device. It uses thyristors to switch reactors and capacitors in and out. This gives a variable amount of reactive power. The SVC is reliable and widely used. But it has one weakness. When the grid voltage falls very low, the support from an SVC also falls.
STATCOM (Static Synchronous Compensator) The STATCOM is a newer type. It uses a voltage source converter, which is a more advanced power electronic circuit. The STATCOM acts like a controllable source of reactive power. Its big advantage is low-voltage performance. It can keep giving support even when the grid voltage is low. It is also faster and smaller than an SVC. However, it usually costs more.
2. Series devices (connected in the line)
Series devices connect in line with the conductor. The current flows through them. Their main job is to control the line impedance or inject a series voltage.
TCSC (Thyristor Controlled Series Capacitor) The TCSC is a series capacitor with a thyristor-controlled reactor beside it. It changes the effective reactance of the line. By reducing the reactance, it lets the line carry more power. It also helps to damp power oscillations, which makes the system more stable.
SSSC (Static Synchronous Series Compensator) The SSSC uses a voltage source converter in series with the line. It injects a controlled voltage into the line. It can do this independently of the line current, which gives it more flexible control than a TCSC.
3. Combined devices
UPFC (Unified Power Flow Controller) The UPFC is the most powerful FACTS device. It joins a series unit and a shunt unit together. Because of this, it can control all three values at once: voltage, impedance, and angle. This means it can control the real power and the reactive power on a line at the same time. It is very flexible, but it is also complex and expensive.
| Device | Connection | Main control | Key point |
|---|---|---|---|
| SVC | Shunt | Voltage / reactive power | Reliable, but weaker at low voltage |
| STATCOM | Shunt | Voltage / reactive power | Strong at low voltage, faster, costlier |
| TCSC | Series | Line impedance | Boosts power flow, damps oscillations |
| SSSC | Series | Series voltage | Flexible series control |
| UPFC | Series + Shunt | All three | Most flexible, most complex |
Benefits of FACTS devices
Why do power companies spend money on these devices? Because they solve real problems:
- More power on existing lines. The grid can carry more power without new towers or cables.
- Better voltage support. The voltage stays steady, even when the load changes fast.
- Improved stability. The system can recover better after a fault or a sudden change.
- Damped oscillations. FACTS devices calm down power swings that can harm the grid.
- Fewer new lines needed. This saves money, land, and time.
Where are FACTS devices used?
FACTS devices are common in these places:
- On long transmission lines, where voltage and stability are hard to manage.
- In weak grids that have trouble holding a steady voltage.
- Near large industrial loads that change quickly.
- At points where wind farms and solar plants connect to the grid. These renewable sources need strong voltage support.
Key takeaways
- FACTS means Flexible AC Transmission System.
- These devices use power electronics to control voltage, impedance, or angle.
- Shunt devices (SVC, STATCOM) mainly control voltage.
- Series devices (TCSC, SSSC) mainly control line impedance.
- The UPFC controls all three at once.
- The main goal is to move more power in a safe and stable way.
Learn more
Want to go deeper on the two most common shunt devices? Read our detailed comparison: SVC vs STATCOM: What Is the Difference?
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