SF₆ Circuit Breaker(Sulphur Hexafluoride)

Introduction

SF6 is a colorless non-toxic gas, with good thermal conductivity and 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, safety regulation are being introduced in many countries in order to prevent its release into atmosphere.oil as a inflammable substance for extinguishing the hot arc release es hydrogen, which by virtue of its low mass and high velocity acts as an excellent cooling medium.But the modern circuit breaker employ heavy gas SF6 because of its excellent dielectric strength,arch quenching,chemical and other physical properties. SF6 circuit breaker are developed by different manufacture for rated voltages 3.6 to 760kv. vacuum circuit breaker and SF6 circuit breakers have better properties in comparison with conventional bulk oil,minimum oil as well as air blast circuit breakers.Hence the SF6 circuit breaker and vacuum circuit breaker are employed in HV system.

Properties of SF₆

sulphur hexafluoride gas is prepared by burning coarsely crushed sulphur in fluorine gas.The sulphur 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 high Rate of Rise of dielectric strength after arc extinction.

• SF₆ is non toxic gas.

• SF₆ is an inert gas. So in normal operating condition 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 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 are 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 close the contacts

• The closing and opening procedures are performed through springs

• In the system, the closing springs are first charged. If “close” button is pressed the opening springs get charged while the contacts get closed. Thus, 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 re-closing relay. In that case, after the closing operation, the closing springs are charged by the driving lever or by 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 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 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 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 fixed piston the SF6 gas inside the cylinder is compressed.The compressed SF6 gas flows through the nozzle and over the electric arc in 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 restored quickly as fresh SF6 gas fills the space.While arc quenching, small quantity of SF6 gas is broken down to some other fluorides of sulphur 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 kgf per sq cm.At higher pressure the dielectric strength of the gas increases. But at higher pressure the SF6 gas liquefy at higher temperature 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. Theses 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, first Arcing Contact will close. Thereafter main contact will close.Similarly while opening, 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 open first then during opening of main contact there will be arcing and as discussed main contacts are not meant to withstand arcing.

Interrupter

An interrupt is a chamber which encloses the breaker contacts, insulating nozzle, 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 breaker.

• Single break SF6 circuit breaker is used for 220 kV applications.

Double Break SF₆ Circuit Breaker

• In such type of breaker, there are two set of moving and fixed contacts connected in series.

• Therefore, to enclose two set of contacts, there must be two interrupt unit in series.

• This type of breaker is used in 400 kV applications.

Multi Break SF₆ Circuit Breaker

• In multi break circuit break, more than two set of fixed and moving contacts are used. Such 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 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 the maintenance once in four to ten years.

• No contamination by moisture, dust etc; sealed construction

• Same gas is circulated in the circuit; no frequent refilling of gas

• No over voltage problem; arc is extinguished without current chopping

• Superior heat transferability so current carrying capacity of conductor is high

Disadvantages

• Sealing problem; leakage may take place

• The SF6 gas is identified as a greenhouse gas, safety regulation are being introduced in many countries in order to prevent its release into atmosphere.

• Puffer type design of SF6 CB needs a high mechanical energy which is almost five times greater than that of oil circuit breaker.

• Special facilities are required for 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

Refrences:

1.power system by JB gupta

2.Electrical Power Systems By C L Wadhwa