Classification Of Circuit Breakers Used In Power System

Posted on January 4, 2018 in Circuit Breakers, Power System, Substation by With 0 Comment
Classification Of Circuit Breakers Used In Power System

Classification Of Circuit Breakers Used In Power System ( Credit: wolfartistic.deviantart.com )

Circuit breakers can be classified using the different criteria such as, different voltage application, location of installation, their external design characteristics, insulating medium used for arc quenching etc. However, the most general way of classification is on the basis of medium used for arc extinction. Various types of circuit breakers used in power system for smooth working of the Transmission Line.

Following are classification of circuit breakers used in power system:

A. Classification Based on Voltage Range:

  • Low voltage CB (less than 1kv)
  • Medium voltage CB (1-52kv)
  • High voltage CB (66-220kv)]
  • Extra high voltage CB (300-765kv)
  • Ultra high voltage CB (above 765kv)

B. Classification Based on Location:

  • Indoor type
  • Outdoor type

Low and medium switchgears and high voltage gas insulated switchgears are categorized as indoor switchgears, whereas which have air as an external insulating medium, i.e. they are classified as outdoor switchgears.

C. Classification Based on External Design (Only for Outdoor Type CB):

  • Dead tank
  • Live tank type

D. Classification Based on Medium used for ARC Quenching:

  • Air-break circuit breakers
  • Oil circuit breakers
  • Air blast circuit breakers
  • Sulphur Hexa fluoride (SF6) circuit breakers
  • Vacuum circuit breakers

Air-Break Circuit Breakers

These circuit breakers are suitable for high current interruption at low voltage, this type of circuit breaker uses air at atmospheric pressure as a quenching medium. It employs two pairs of contact main contact and the arcing contacts. They have low contact resistance. The main contact carries the current when breaker is at the closed position. When contacts are opened, the main contacts separate first, the arcing contacts remain in closed position. Therefore the current is shifted from main contacts to the arcing contacts. The arcing contacts separate later on the arc is drawn between them. The principle of high resistance is employed for arc interruption; the arc resistance is increased by lengthening, splitting and cooling the arc. The arc interruption is assisted by current zero in case of air break circuit breakers, high resistance is obtained near current zero. These circuit breakers are available in the voltage 400 to 12kv. They are widely used in the low and medium voltage system.

Oil Circuit Breaker

Oil Circuit Breaker

Oil Circuit Breaker

Mineral oil is the best insulator than air and it has good cooling properties. So, this is employed in many electrical equipments as, well as circuit breakers. But these type of circuit breakers are not suitable for heavy current interruption at low voltages due to carbonization of oil.

Some of the types of oil circuit breakers are given below.

Plain–Break Oil Circuit Breakers

Plain-break oil circuit breaker

Plain-break oil circuit breaker

In these circuit breakers the moving as well as fixed contacts are immersed in oil, the metal tank is strong, weather tight and earthed When the contacts are separated there is a severe arc which decomposes the gas into oil. Mainly hydrogen is obtained from the oil. Hence, the oil is pushed away from the arc and gaseous medium surrounds the arc, creating a large gaseous pressure, to withstand this pressure tank is made strong.

Self Generated Pressure Oil Circuit Breaker

In this type of C.B. the arc energy is utilized to generate a high pressure in a chamber known as pot. The contacts are enclosed within the pot and it is made up of insulating material. It is placed inside the tank. They are of several types; some of them are given below.

(i) Plain Explosion Jet

This is the simplest form of explosion pot, when the moving contact separates a severs arc is formed. The oil is decomposed and gas is produced. It generates a high pressure within the pot because there is a close fitting throat at the lower end pot. So, the high pressure developed causes turbulent flow of streams of the gas into the arc resulting in arc-extinction. If the arc extinction does not occur within the pot, it occurs immediately after the moving contact leaves the pot, due to high velocity axial blast of the gas which is released through the throat.

(ii) Cross-Jet Explosion Pot

Cross-jet explosion pot.

Cross-jet explosion pot.

It is suitable for high current interruptions. Arc splitters are used to obtain an increased arc length for a given amount of contact travel. When moving contact is separated from the fixed contact, an arc is formed.

This arc is pushed into the arc splitters as, shown in figure and finally, it’s extinguished, in this oil blast is across the arc and it’s known as cross- jet explosion pot.

(iii) Double Break Oil Circuit Breaker

To obtain high arc interruption, particularly at low currents is provided by the double break oil circuit, it employs an intermediate contact between the fixed and moving contact. When the moving contact separates, the intermediate contact also follows it. The arc first appears between the fixed contact and the intermediate contact. Soon after, the intermediate contact stops and second arc is extinguished quickly by employing gas pressure and oil momentum developed by first arc.

Air Blast Circuit Breakers

In the air blast circuit breakers, compressed air at pressure of 20-30kg/cm2 is employed as, an arc quenching medium. Air blast circuit breakers are suitable for operating voltage of 132kv and above. The main advantage of using them is their cheapness and free availability of the interrupting medium, chemical stability and inertness of air, high speed operation.

They are further classified into: (i) Cross-blast circuit breaker (ii) Axial-blast circuit breaker

(i) Cross-Blast Circuit Breaker

In this type, a high pressure blast of air is directed perpendicularly to the arc for its interruption. The arc is forced into a suitable chute. Sufficient lengthening of the arc is obtained, resulting in the introduction of appreciable resistance in the arc itself. Therefore, resistance switching is not common in this type.

(ii) Axial Blast Circuit Breaker

In this type of circuit breaker, a high pressure of air is directed longitudinally, i.e. in the line with the arc. These are suitable for EHV and super high application. This is because interrupting channels can be fully enclosed in porcelain tubes. The number of breaks depends upon the system voltage, for Example 4 at 220 kv and 8 at 750kv. They have also been commissioned to 1100 kV system.

SF6 Circuit Breaker

These type of circuit breakers have good dielectric strength and excellent arc quenching property. It is an inert, non-toxic, non-flammable and heavy gas. As circuit breakers are totally enclosed and sealed from atmosphere so it is very careful where explosion hazards exist. At atmospheric pressure, its dielectric strength is about 2.35 times that of air.

At normal conditions it is chemically inert, these properties of sf6 has made it possible to design circuit breakers with smaller overall dimensions, shorter contact gaps, which help in the constructions of outdoor breakers with fewer interrupts and evolution of metal clad. It is suitable for the range 3.3kv to 765kv. They are preferred for voltages 132kv and above.

Double Pressure Type SF6 Circuit Breaker

In double pressure type circuit breaker, the system employed is double pressure system in which the gas from a high pressure compartment is released into low pressure compartment through a nozzle during the arc extinction process. This type of circuit breaker has become obsolete.

Single Pressure Type SF6 Circuit Breaker

In this type the gas is compressed by the moving cylinder system and is released through the nozzle during arc extinction. This is most popular type of design of sf6 circuit breaker.

Properties of SF6 Circuit Breaker

Physical properties:

  • Colorless, odorless, non-toxic and non inflammable gas.
  • Pure gas is not harmful to health.
  • It has excellent heat transfer property.

Chemical properties:

  • It’s chemically stable at atmospheric pressure and temperature.
  • It is chemically inert.
  • It’s non-corrosive on all metals at ambient temperatures.

Electrical properties:

  • It has high Di-electric strength.
  • High corona inception voltage.
  • Dielectric constant of SF6 is independent of the frequency of the applied voltage.
  • Arc-interrupting capacity.

Vacuum Circuit Breaker

The dielectric strength and interrupting ability of high vacuum is superior to those of porcelain, oil, air and SF6 at atmospheric pressure. Its construction is very simple as, compared to other circuit breakers. When contacts are separated in high vacuum, an arc is drawn between them. The arc does not take place on the entire surface of the contacts but only a few spots. The contact surface is not perfectly smooth. It has certain micro projections. At the time of contact separation, these projections form the last point of separation.

The current flows through these points of separation resulting in the formation of a few hot spots, these spots emit electrons and act as cathode spots. Its enclosure is made up of insulating material such as, glass, porcelain or glass fiber reinforced plastic. The vapour condensing shield is made up of synthetic resin. Vacuum CB is now very popular for voltage rating up to 36kv. The main advantage of vacuum CB is it’s suitability for repeated operations, least maintenance silent operation, long life etc.

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Hello, I'm Kalpesh, An Electrical Engineer and the founder of Substation System.

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