What is a flexible AC transmission system (FACTS)?

FACTS can be described as the modification of the AC transmission system by using power electronics or with the help of other static controllers. There are many limitations for the large AC system and some of them are voltage stability problems, reactive power problems, steady-state stability, transient stability, frequency control, etc. So in this system, there will be electronic devices used with the static equipment in order to control the power line transmission and thus quantity and quality of the AC power transmission can be improved. So by using this in an AC transmission system the power transfer capacity, stability, and reliability can be improved a lot.

What are the major problems that can be seen in power transmission?


The overhead line should have the proper thermal capability, the thermal capacity is the ability to handle the temperature, wind conditions, conductor conditions and this can be achieved with the help of the FACT system.


The transmission lines could increase the operating voltage over time and this can be controlled with the help of FACTS.


These are the major problems that could arise in a transmission system and this would widely affect the performance of a transmission system. The stability-related problems which can be seen in a transmission system could be dynamic stability, steady-state stability, transient stability, etc. So all of the stability issues can be controlled by using the FACT system.

What are the major functions of FCATS?

  • It would do the correction of power factor by the continuous voltage injection
  • Load balancing can be done for the interconnected distribution network
  • Capacitive and reactive power requirements can be satisfied
  • Power flow can be controlled
  • Harmonic distortion can be decreased by active filtering
  • It would help to handle the changes in the transmission system and thus the power transmission is done properly
  • The capability to transfer the power can be increased

How does a flexible AC transmission system operate?

The FACTS can be described as a collection of controllers like phase shifters, static var compensator, etc. The major operation of the FACT system is by varying the apparent impedance of a particular transmission line so that the flow of power can be improved over the required path. So by controlling the impedance of the transmission line with the help of the electronic devices in the FACT system we could achieve a constant power flow in the required path. The power flow can be maintained in case of any variations in the load levels in the AC network and also in case of any contingencies. Mostly we can use FACT devices to do the dynamic control of the voltage, it can also control the impedance and phase angle of high voltage AC transmission lines.

What are the types of FACTS devices?

The facts device falls under certain types of facts controller and they are


This type of controller would have capacitors or reactors and this would impose voltage in the line and the voltage will be injected in series with the line. This type of controller is a very powerful controller and it can do the active power control and also can improve the transient stability of the power system.


This can be described as a variable impedance connection to the line, so this type would inject the current into the system and the dynamic stability can be increased. By doing so the capacitivity of the transmission line can be decreased, the current which is injected will be in phase with the line voltage.


This is a series controller combination and both these controllers would be capable to do the series compensation, and the real power transfer can be done smoothly. This would control the amount of real and reactive power flow in a transmission line and thus the transmission system utilization can be increased.


This is a combination of separate series and shunt controllers and these controllers will be coordinated in a proper manner. The series controller would inject current in series and the shunt controller would inject voltage in the system too. So because of these two controller combinations, the real power exchange can be easily done, the unified power flow control is an example of this.



This device would do the regulation of the voltage by controlling the amount of reactive power and this will be done by adding or removing the reactive power. This device consists of fixed capacitor banks and it would be connected in parallel to the reactor. The reactor will be controlled by the thyristor switch and the capacitor bank would do the dynamic shunt compensation. The SVC is a type of parallel controller, this device would impose or add capacitive or inductance reactance to the line. So if the system voltage is low then reactive power is generated with the help of capacitor banks and if the voltage of the system is high then with the help of reactors it can be decreased.

Advantages of SVC

  • Power transfer capability is increased
  • Grid voltage control is improved
  • Power factor improvement
  • Harmonics can be removed and also the voltage distortion can be reduced too

Applications of SVC

  • Power oscillation damping
  • Power quality is improved by flicker mitigation and voltage balancing
  • Transient stability of the synchronous machine can be improved


This is very similar to the SVC and the only difference is that the capacitive and the inductive output current can be controlled independently of the system voltage. The damping ability of the STATCOM is really better than the SVC, the STATCOM could be a current-based or voltage-based converter. We can use this FACT device in electrical networks that have very poor power factors and voltage regulation. This device can be used for voltage stability and also to provide better reactive power support in case of low AC voltages


This device is almost similar to the STATCOM, it can be described as a simple series version of the STATCOM. This device would inject a synchronous voltage source into the line and the voltage source will be injected in series with the line. This device would increase or decrease the voltage drop in the transmission line according to the requirement. SSSC has a voltage source inverter and this is connected to the transmission line in series. This type of FACT device would require an energy source so that the DC voltage can be provided for the capacitor.


This device can control the capacitive reactance of the line, continuous power control in a transmission line can be achieved by this device. In this device, many compensators could be connected in series in order to achieve the required voltage rating and also the operational requirements. The TCSC is composed of a series capacitive bank and it will be shunted by a rector which is thyristor controlled so that proper variable capacitive reactance can be achieved.

Advantages of TCSC

  • Steady-state control of the power
  • Improvement in transient stability
  • Line impedance control
  • Transmission losses in power lines can be reduced

What are the advantages of FACTS?

  • It would help to increase the power transfer without the need for adding new transmission lines
  • Reduction in power transmission cost
  • Steady-state and dynamic voltage control
  • Stability of the system and also the quality of the voltage is increased too
  • Oscillation damping
  • We can utilize the assets in the existing transmission system properly
  • Transmission system availability and reliability can be increased
  • Stability and reduction of the loop flow
  • Supply quality improvement
  • Quick voltage regulation
  • Power transfer can be increased over long AC lines
  • The machines which are connected to the system can be utilized properly

What are the applications of FACTS?

  • Load flow can be controlled in the meshed system
  • The FACT controllers can be used for steady-state applications
  • This device is used to do the congestion management
  • This device is used for dynamic applications, in this application the FACT device would improve the transient stability, and also the voltage stability
  • It is also used in the deregulated environment
  • Power system interconnection can be achieved with the help of FACT devices
  • It can be also used for synchronous elimination


post-graduate in Electronics & communication.

Related Articles

Back to top button