Flexible AC Transmission System (FACTS)

Flexible AC Transmission System

Introduction of Flexible AC Transmission System

FACTS devices used in the AC transmission system. These devices used to improve the stability and controllability of the transmission line. The Flexible AC Transmission System FACTS is a static device, which uses the power electronics equipment.

The power transfer capacity fixed for any transmission network. If we transfer power beyond this limit, equipment may get damaged. We can increase power transfer capacity by changing the layout of the power system but this is not a feasible way. So, in the actual power system, we use FACTS devices to increase power transfer capacity.

In simple words, Flexible AC Transmission System (FACTS) is a variable impedance device like inductors and capacitors. In an ideal transmission system, the active power is equal to the apparent power. It means that the power factor is unity. The power factor defined as the ratio of the active power to the apparent power. This is the main role of FACTS devices, it is trying to keep power factor as near as possible to unity.

The active power known as true power and it is useful for work in the system. Reactive power is not useful for the load. The reactive power caused by the energy storage equipment. If we want to achieve the maximum power transfer capacity, we have to reduce the amount of reactive power.

The reactive power reduces the power transfer capacity. The techniques used to reduce reactive power is known as the compensation technique.

In order to improve the efficiency of the power system, the compensation technique is supplying the inductive and reactive power to the power system. Through these techniques, we can improve the quality of power, stability, voltage regulation, power factor.

Classification of Flexible AC Transmission System 

According to the type of connection, there are two types of compensation techniques; series compensation, and shunt compensation.

Series compensation

FACTS devices connected in series with the power system network. By series compensation, we can improve the stability and dynamic response of the system. Generally, the capacitors connected in series with the transmission network. The amount of compensation controlled by power electronics switches. For better control, more than one capacitor (capacitor bank) connected.

Example of series compensation

  • Thyristor Controlled Switched Capacitor (TCSC)
  • Static Synchronous Series Compensator (SSSC)
  • Fixed Series Capacitor (FSC)
  • Thyristor Switched Series Capacitor (TSSC)
  • Thyristor Switched Series Reactor (TSSR)

Shunt compensation

In this type of compensation technique, FACTS devices connected in parallel or shunt with the transmission network. The current injected into the system from the FACTS devices and in this way the reactive component reduced.

This technique used to improve voltage regulation and reduce losses of the power system. There are two types of shunt compensation; shunt capacitive compensation and shunt inductive compensation. The shunt capacitive compensation technique used to improve the power factor. The shunt inductive compensation technique used to charging the transmission line in low load condition or no-load condition.

Example of Shunt compensation

  1. STATic Synchronous COMpensator (STATCOM)
  2. Static VAR Compensator (SVC)
    • TCR – Thyristor Controlled Reactor
    • TSR – Thyristor Switched Reactor
    • TSC – Thyristor Switched Capacitor

Advantages of FACTS

  • The FACTS devices used to control the power flow of the transmission line. It ensures the optimum condition of the flow of power and also used to increase the power transfer capacity of the line.
  • It increases the reliability and flexibility of the power transmission network.
  • FACTS devices increase the loading capacity of lines.
  • It reduces the flow of reactive power, thus the transmission line can carry more amount of active power.
  • FACTS device used to increase system security by raising the transient stability limit and limiting short circuit current. It managing cascading blackout and damping electromechanical oscillations of a power system.
  • It reduces power delivery costs.

Disadvantages of FACTS

  • If the current increase more than a certain level, the power electronics equipment gets damaged due to overheating.
  • It transmits a limited amount of power.
  • The cost of the FACTS device high. Which increases the initial costs and repair costs.

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