How to Make Flexible AC Transmitter System Using Thyristor Switch Reactance

FACTS is the acronym for Flexible AC Transmitter System. A Flexible AC Transmission System (FACTS) increase the reliability of AC grids. The IEEE defines FACTS as alternating current transmission systems integrating power electronics based and other static controllers to enhance controllability and power transfer ability. previously we have discussed “Need of FACTS and Types

They improve power quality and transmission efficiency from generation through transmission down to the private and industrial consumers. In this article, we discuss Flexible AC Transmitter System Using Thyristor Switch.

Flexible AC Transmitter System by using TSR

A Flexible AC Transmitter System (FACTS) consists of static equipment which is used for AC transmission of electrical signals. It is used to increase controllability and to increase power transfer capability of an AC transmission system. This project can be enhanced by using firing angle control methodology for smooth control of voltage.

Flexible AC Transmitter System increases the reliability of AC grids and reduces power delivery cost. They also increase the quality of transmission and efficiency of power transmission.

Flexible AC Transmitter System
Flexible AC Transmitter System Block Diagram

This method is used while charging transmission line or when there is a low load at the receiver end. When there are low load or no load, very low current flows through the transmission lines and the shunt capacitance in the transmission line becomes dominant. This causes voltage amplification due to which the receiver end voltage may become double than the sending end voltage.

To compensate this, the shunt inductors are automatically connected across the transmission line. In this system the lead time between the zero voltage pulse and zero current pulse duly generated by a suitable operational amplifier is fed to two interrupt pins of the microcontroller.

Types of Flexible AC Transmitter System Controllers

  • Series Controller
  • Shunt Controller
  • Combined Series-Series Controller
  • Combined Series-Shunt Controller
Types of FACTS Controllers
Types of FACTS Controllers

Thyristor

A thyristor is a four-layered, three terminal semiconductor device. The four layers are formed by alternate p-type and n-type semiconductors. Thus forming a p-n junction device. This device is also called as Silicon Controlled Switch (SCS) because of the silicon semiconductor in it and it is a bistable device.

Thyristor Symbol
Thyristor Symbol

A thyristor is a unidirectional device and can be operated as an open circuit switch or as a rectifying diode. The three terminals of the thyristor are named as the anode (A), the cathode (K) and Gate (G).

The Anode is positive, the cathode is negative and the gate is used to control the input signal. It has two p-n junctions which can be switched ON and OFF at fast rates. The following shows the layers and terminals of the thyristor with its symbol.

Thyristor
Thyristor

Thyristor has three basic states of operation

  • Reverse Blocking
  • Forward Blocking
  • Forward Conducting

Reverse Blocking: In this mode of operation, the thyristor blocks the current in the same direction as that of a reverse bias diode.

Forward Blocking: In this mode of operation, the thyristor blocks the forward current conduction that is normally carried by a forward bias diode.

Forward Conducting: In this mode of operation the thyristor has been triggered into conduction. It continues conducting until the forward current drops below a threshold level called ‘holding current’.

Thyristor Switched Reactor

A thyristor switched reactor is used in electrical power transmission systems. It is a reactance connected in series with a bidirectional thyristor value. The value of thyristor is phase controlled, which allows the value of delivered reactive power to be adjusted to meet changing system conditions.

TSR can be used to limit the voltage rises on lightly loaded transmission lines. The current in TSR is varied from maximum to zero by varying the firing delay angle.

TSR can be used to limit the voltage rises on lightly loaded transmission lines. The current in TSR is varied from maximum to zero by varying the firing delay angle.

The following circuit shows TSR circuit. When the current flows the reactor is controlled by the firing angle of the thyristor. During every half cycle, the thyristor produces the triggering pulse through the controlled circuit.

Thyristor Switched Reactor
Thyristor Switched Reactor

Circuit of TSR

A thyristor switched reactor is a three-phase assembly which is connected in a delta arrangement to provide partial cancellation of harmonics.The main thyristor reactor is split into two halves, with the thyristor valve connected between the two halves.

TSR Circuit
TSR Circuit

This protects the thyristor reactor circuit valve from damages due to flashovers and lightning strikes.

The main thyristor reactor is split into two halves, with the thyristor valve connected between the two halves.This protects the thyristor reactor circuit valve from damages due to flashovers and lightning strikes.

Operating Principle

The current in the thyristor is varied from maximum to zero by varying the firing delay angle (α). It is defined as the delay angle from the point at which the voltage becomes positive to the point at which the thyristor valve is turned on and the current starts to flow.

Maximum current is obtained when the α is 90o. At this point, TCR is said to be in full conduction. The RMS current is given by

Itcr-max = Vsvc/2πfLtcr

Where

Vsvc is the RMS value of the line to line bus bar voltage

Ltcr is the total TCR transducer for phase

The below waveform is the voltage and current of TCR.

TSR operation
TSR operation

Advantages of Thyristor

  • It can handle high current
  • It can handle high voltage

Applications of Thyristor

  • Used in electrical power transmission
  • Used in alternating power circuits to control alternating output power.
  • Used in inverters to convert direct current to alternating current

Applications of FACTS

  • Used to control power flow
  • Damping of power system oscillation
  • Reduces generation cost
  • Steady state voltage stability
  • HVAC (Heating Ventilation and Air Conditioning) application
  • Flicker Mitigation

I hope you have understood the concept of the flexible AC transmission system from the above article. If you have any queries on this concept or on the electrical and electronic projects leave the comments section below.

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