SF6 Circuit Breaker: Properties, Operation, Types, Advantages, Disadvantages

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SF6 Circuit Breaker
SF6 Circuit Breaker

Introduction

The SF6 circuit breaker is a type of circuit breaker that utilizes pressurized SF6 gas to effectively extinguish electrical arcs. SF6 is a colorless, non-toxic gas with good thermal conductivity and a density approximately five times that of air (6.14 kg/m3.).It does not react with materials commonly used in high-voltage circuit breakers. Applications for SF6 include gas-insulated transmission lines and gas-insulated power distribution substations.

The SF6 gas is identified as a greenhouse gas, and safety regulations are being introduced in many countries in order to prevent its release into the atmosphere. Oil as an inflammable substance for extinguishing the hot arc releases hydrogen, which, by virtue of its low mass and high velocity, acts as an excellent cooling medium. But modern circuit breakers employ heavy gas SF6 because of its excellent dielectric strength, arch quenching, chemical properties, and other physical properties.

SF6 circuit breakers are developed by different manufacturers for rated voltages of 3.6 to 760 kV. Vacuum circuit breakers and SF6 circuit breakers have better properties in comparison with conventional bulk oil, minimum oil, and air blast circuit breakers. Hence, the SF6 circuit breaker and vacuum circuit breaker are employed in the HV system.

Properties of SF6

Sulfur hexafluoride gas is prepared by burning coarsely crushed sulfur in fluorine gas. The sulfur hexafluoride gas possesses very good insulating properties and outstanding arc-quenching characteristics which make it an ideal medium for circuit interruption.

  • SF6 gas has high dielectric strength
  • It has a high Rate of Rise of dielectric strength after arc extinction.
  • SF6 is a non-toxic gas.
  • SF6 is an inert gas. So in normal operating conditions, the metallic parts in contact with the gas are not corroded.
  • SF6 has high thermal conductivity which means the heat dissipation capacity is more.
  • The gas is quite stable. After the extinction of the arc, the SF6 gas is reformed from the decomposition.
  • SF6 is non-flammable.
  • It is chemically stable up to 500°c and does not react with structural material up to 500°c.
  • It is a strongly electronegative gas which means that free electrons are readily removed from a discharge by the formation of negative ions.
  • The thermal time constant of SF6 is low and as a result, the pressure at which it should be stored and used is relatively smaller as compared to air.

SF6 circuit breaker is equipped with separated poles each having its own gas. Gas pressure is 2 bars. Even if the pressure drops to 1 bar, there will not be any change in the breaking properties of the circuit breaker. During arcing, the circuit breaker maintains a relatively low pressure (max 5-6 bars) inside the chamber and there will be no danger of explosion and spilling of the gas around.

Inside view of the mechanism.

Operation of Circuit Breaker

  • Breakers consist of two main parts:

– the poles and

– the mechanism.

  • The poles consist of

– contact and

– arc-extinguishing devices.

  • The mechanism is the part to open or closes the contacts
  • The closing and opening procedures are performed through springs
  • In the system, the closing springs are first charged. If the “close” button is pressed the opening springs get charged while the contacts get closed. Thus, the circuit breaker will be ready for opening.
  • The mechanical operating cycle of the circuit breaker is

– OPEN – 3 Min – CLOSE/OPEN – 3 Min – CLOSE/OPEN or

– OPEN -0.3 sec – CLOSE/OPEN – 3 Min – CLOSE/OPEN

  • The second cycle is valid when the circuit breaker is used with a re-closing relay. In that case, after the closing operation, the closing springs are charged by the driving lever or by the driving motor.
  • Thus, the circuit breaker will be ready for opening and re-closing.

Arc Extinction

  • When the contacts separate, an arc is established.
  • During the opening operation, the gas contained inside a part of the breaker is compressed by a moving cylinder.
  • This forces the SF6 gas to flow through the interrupting nozzle axially along the arc. The flow of gas attains almost supersonic speed in the divergent portion of the nozzle.
  • The heat is removed from the arc. Consequently, the arc diameter reduces during the decreasing mode of the current wave. The diameter becomes small during current zero. Turbulent flow is introduced around the current zero for extinguishing the arc.
  • If the short circuit current is high, the arc extinction may not occur at the first zero crossing, but the gas pressure will increase sufficiently to blow the arc out.
  • The basic requirement in arc extinction is not primarily the dielectric strength, but a high rate of recovery of dielectric strength. current chopping problems are minimum.
  • SF6 gas regains its dielectric strength rapidly after the final current zero.
  • The electron is absorbed by the neutral gas molecule.
  • The contact space is filled with fresh SF6 gas and its dielectric strength is rapidly regained because of the electronegativity of the gas and turbulent flow of fresh gas in the contact space.

Construction

SF6 mainly consists of two parts:

  1. the interrupter unit
  2. the gas system

This breaker has a cylinder and piston arrangement. The piston is fixed but the cylinder is movable. The cylinder is tied to the moving contact so that for opening the breaker the cylinder along with the moving contacts moves away from the fixed contact. But due to the presence of a fixed piston, the SF6 gas inside the cylinder is compressed. The compressed SF6 gas flows through the nozzle and over the electric arc in the axial direction. Due to heat convection and radiation the arc diameter reduces gradually and the arc is finally extinguished at current zero.

The dielectric strength of the medium between the separated contacts increases rapidly and is restored quickly as fresh SF6 gas fills the space. While arc quenching, a small quantity of SF6 gas is broken down to some other fluorides of sulfur which mostly recombine to form SF6 again. A filter is also suitably placed in the interrupter to absorb the remaining decomposed byproduct. The gas pressure inside the cylinder is maintained at around 5 kg per sq cm.

At higher pressure the dielectric strength of the gas increases. But at higher pressure, the SF6 gas liquefies at higher temperatures which is undesired. If the SF6 gas will liquefy then it loses the ability to quench the arc. SF6 Circuit Breaker has fixed contact as well as moving contact. These fixed and moving contacts are known as MAIN CONTACT. There exists one another contact which is known as ARCING CONTACT. Arcing Contact is part of fixed contact. Basically, Arcing contacts are only designed to withstand arcing. It is not designed for carrying load current. In spite, main contacts are designed to carry load current and not the arcing.

Therefore, it can be said that, while closing of SF6 circuit breaker, the first Arcing Contact will close. Thereafter the main contact will close. Similarly while opening, the first main contact will open and then arcing contact will open. This is because, while opening if the main contact opens first, there will not be any arcing as the current is getting path through the arcing contact. But if the arcing contact opens first then during the opening of the main contact there will be arcing and as discussed main contacts are not meant to withstand arcing.

Interrupter

An interrupt is a chamber that encloses the breaker contacts, insulating nozzle, and SF6 gas chamber. Interrupter is made of porcelain.

pic sources: Electrical Concepts

Main and arcing contact opening sequence

pic sources: Electrical Concepts
Pic sources: Electrical system(puffer Type circuit breaker)

Types of SF6 Circuit Breaker

  • There are mainly three types of SF6 CB depending upon the voltage level of application-

– Single interrupter SF6 CB applied for up to 245 KV(220 KV) system.

– Two interrupter SF6 CB applied for up to 420 KV(400 KV) system.

– Multi interrupter SF6 CB applied for up to 800 KV(715 KV) system.

 Single Break SF6 Circuit Breaker

  • In Single Break Circuit Breaker, only one moving and fixed contacts are present. This means that there will only be one interrupter unit in such a breaker.
  • Single break SF6 circuit breaker is used for 220 kV applications.

Double Break SF6 Circuit Breaker

  • In such a type of breaker, there are two sets of moving and fixed contacts connected in series.
  • Therefore, to enclose two sets of contacts, there must be two interrupt units in series.
  • This type of breaker is used in 400 kV applications.

Multi Break SF6 Circuit Breaker

  • In multi-break circuit breaks, more than two sets of fixed and moving contacts are used. Such a type of breaker is used in 765 kV applications.

Advantages

  • Excellent insulating and arc extinguishing property
  • The gas is non-inflammable; no risk of fire or explosion.
  • Electrical clearances are smaller because of their high dielectric strength.
  • Comparatively cheaper in cost, simple in construction, maintenance-free, and compact.
  • Performance is not affected by atmospheric conditions.
  • Noiseless operation compared to air blast circuit breakers
  • Contact erosion is less; no frequent contact replacement
  • No reduction in dielectric strength; no carbon formation during arcing
  • Minimum maintenance.Requires maintenance once in four to ten years.
  • No contamination by moisture, dust etc; sealed construction
  • The same gas is circulated in the circuit; no frequent refilling of gas
  • No over-voltage problem; the arc is extinguished without current chopping
  • Superior heat transferability so the current carrying capacity of the conductor is high

Disadvantages

  • Sealing problem; leakage may take place
  • The SF6 gas is identified as a greenhouse gas, and safety regulations are being introduced in many countries in order to prevent its release into the atmosphere.
  • Puffer type design of SF6 CB needs a high mechanical energy which is almost five times greater than that of an oil circuit breaker.
  • Special facilities are required for the transportation of gas

APPLICATION

  • SF6 circuit breakers are preferred for rated voltages of 3.3 kV to 760 kV.
  • Indoor application:3.3 kV to 36 kV
  • Outdoor application:36 kV to 760 kV
  • SF6 GIS:3.3 kV to 760 k

References

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