What are FACTS and related technologies?

FACTS (Flexible Alternating Current Transmission System) refers to a power electronics-based system that uses static devices to enhance the power transfer capability and controllability of AC transmission networks.

These power electronic devices are integrated into conventional AC grids to boost key performance metrics, including:

Power transfer capacity of transmission lines
Voltage stability and transient stability
Voltage regulation precision
System reliability
Thermal limits of transmission infrastructure

Before the advent of power electronic switches, issues like reactive power imbalance and stability were addressed using mechanical switches to connect capacitors, reactors, or synchronous generators. However, mechanical switches had critical drawbacks: slow response times, mechanical wear and tear, and poor reliability—limiting their effectiveness in optimizing transmission line controllability and stability.

The development of high-voltage power electronic switches (e.g., thyristors) enabled the creation of FACTS controllers, revolutionizing AC grid management.

Why Are FACTS Devices Needed in Power Systems?

A stable power system requires precise coordination between generation and demand. As electricity demand grows, maximizing the efficiency of all network components becomes essential—and FACTS devices play a key role in this optimization.

Electrical power is categorized into three types: active power (useful/true power for end-use), reactive power (caused by energy-storing elements in loads), and apparent power (vector sum of active and reactive power). Reactive power, which can be inductive or capacitive, must be balanced to prevent it from flowing through transmission lines—uncontrolled reactive power reduces the network’s capacity to transmit active power.

Compensation techniques (to balance inductive and capacitive reactive power by supplying or absorbing it) are therefore critical. These techniques improve power quality and enhance transmission efficiency.

Types of Compensation Techniques

Compensation techniques are classified based on how devices are connected to the power system:

1. Series Compensation

In series compensation, FACTS devices are connected in series with the transmission network. These devices typically act as variable impedances (e.g., capacitors or inductors), with series capacitors being the most common.

This method is widely used in EHV (Extra High Voltage) and UHV (Ultra High Voltage) transmission lines to significantly improve their power transfer capability.

The power transfer capacity of a transmission line without using compensation device;

Where,

V₁ = Sending end voltage
V₂ = Receiving end voltage
X_L = Inductive reactance of transmission line
δ = Phase angle between V₁ and V₂
P = Power transferred per phase

Now, we connect a capacitor in series with the transmission line. The capacitive reactance of this capacitor is XC. So, the total reactance is XL-XC.So, with a compensation device, the power transfer capacity is given by;

The factor k is known as the compensation factor or degree of compensation. Generally, the value of k is lies between 0.4 to 0.7. Let’s assume the value of k is 0.5.

Thus, it is evident that the use of series compensation devices can increase power transfer capacity by approximately 50%.When series capacitors are employed, the phase angle (δ) between voltage and current is smaller compared to an uncompensated line. A smaller δ value enhances system stability—meaning, for the same power transfer volume and identical sending-end and receiving-end parameters, a compensated line offers significantly better stability than an uncompensated one.

Shunt Compensation

In a high voltage transmission line, the magnitude of receiving end voltage depends on the loading condition. The capacitance performs an important role in the high voltage transmission line.

When a transmission line is loaded, the load requires reactive power, which is initially supplied by the line’s inherent capacitance. However, when the load exceeds the SIL (Surge Impedance Loading), the heightened reactive power demand leads to a significant voltage drop at the receiving end.

To address this, capacitor banks are connected in parallel with the transmission line at the receiving end. These banks supply the additional reactive power needed, effectively mitigating the voltage drop at the receiving end.

An increase in line capacitance leads to a rise in the receiving end voltage.

When a transmission line is lightly loaded (i.e., the load is below SIL), the reactive power demand is lower than the reactive power generated by the line’s capacitance. In this scenario, the receiving end voltage becomes higher than the sending end voltage—a phenomenon known as the Ferranti effect.

To prevent this, shunt reactors are connected in parallel with the transmission line at the receiving end. These reactors absorb the excess reactive power from the line, ensuring the receiving end voltage remains at its rated value.