SF6 Gas Circuit Breaker MCQ Quiz - Objective Question with Answer for SF6 Gas Circuit Breaker - Download Free PDF

Explanation:

SF6 (Sulphur Hexafluoride) Circuit Breaker

Definition: An SF6 circuit breaker is a type of circuit breaker that uses sulphur hexafluoride (SF6) gas as the insulating and arc-quenching medium. SF6 gas possesses excellent dielectric properties, making it highly effective in interrupting high-voltage electric currents. These breakers are widely used in high-voltage applications due to their high reliability, compact design, and ability to handle large fault currents.

Working Principle: In an SF6 circuit breaker, the arc generated during the interruption of current is extinguished by the SF6 gas. When the contacts of the circuit breaker separate, an arc is formed between them. The arc heats up the SF6 gas, causing it to decompose into ions and other reactive components. These components absorb energy from the arc, cooling and extinguishing it rapidly. The gas then recombines into its original form, ready for subsequent operations.

Components of SF6 Circuit Breaker:

• Fixed Contact: The stationary part of the breaker where the moving contact makes and breaks the circuit.
• Moving Contact: The movable part of the breaker that moves away from the fixed contact to interrupt the circuit.
• Arc Quenching Chamber: The chamber where the arc is extinguished using SF6 gas.
• SF6 Gas Reservoir: The storage for SF6 gas under high pressure.
• Operating Mechanism: The mechanism that controls the movement of the contacts.

Correct Option Analysis:

The correct option is:

Option 4: A hollow cylinder contact with an arc horn.

The fixed contact in an SF6 circuit breaker is typically a hollow cylindrical contact equipped with an arc horn. This design is crucial for the effective operation of the circuit breaker. The hollow cylindrical shape allows the SF6 gas to flow uniformly around the contact area, ensuring efficient arc quenching. The arc horn serves to guide the arc and facilitate its extinction by ensuring that the arc is stretched and cooled effectively. This design ensures that the arc is extinguished quickly, minimizing damage to the contacts and ensuring reliable operation of the breaker.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: A spherical contact with an insulating shield.

This option is incorrect. While spherical contacts may be used in some types of electrical devices, they are not typically used as fixed contacts in SF6 circuit breakers. The spherical shape does not facilitate the effective flow of SF6 gas around the contact area, which is critical for efficient arc quenching. Moreover, an insulating shield is not a standard feature of the fixed contact in SF6 circuit breakers.

Option 2: A solid cylindrical contact with no additional fittings.

This option is also incorrect. While a solid cylindrical contact may provide a basic connection, it lacks the arc horn, which is essential for guiding and stretching the arc for efficient extinction. The absence of additional fittings like an arc horn would compromise the performance of the circuit breaker, making it less effective in handling high fault currents.

Option 3: A flat plate contact with thermal insulation.

This option is incorrect as well. A flat plate contact is not suitable for SF6 circuit breakers because it does not allow for the optimal flow of SF6 gas around the contact area. Additionally, thermal insulation is not a standard feature of fixed contacts in SF6 circuit breakers. The design of the fixed contact must prioritize efficient arc quenching, which is not achievable with a flat plate contact.

Conclusion:

In conclusion, the fixed contact in an SF6 circuit breaker is a hollow cylindrical contact equipped with an arc horn. This design ensures efficient arc quenching by allowing the SF6 gas to flow uniformly around the contact area and effectively cool the arc. The arc horn plays a crucial role in guiding and stretching the arc, facilitating its rapid extinction. The other options provided do not align with the design and operational requirements of SF6 circuit breakers, making option 4 the correct choice.

Transportation of SF6 Gas:

Sulfur hexafluoride (SF6) is a man-made gas used primarily in electrical equipment as an insulating and arc-quenching medium. It is known for its high dielectric strength and thermal stability, which makes it an excellent electrical insulator. However, due to its potent greenhouse gas properties, its handling and transportation are subject to stringent regulations.

Transport in Liquid Form:

• SF6 gas is typically transported in liquid form within cylinders. This method of transportation is efficient because SF6 can be liquefied under pressure at room temperature. This allows a large quantity of the gas to be stored and transported in a smaller volume, maximizing efficiency and safety.
• The gas liquefies at a pressure of about 2.2 MPa at room temperature. This relatively low pressure makes it feasible to store and transport in standard gas cylinders designed to handle high-pressure gases.
• Transporting SF6 in liquid form also reduces the risk of accidental release and minimizes the volume of gas that would be emitted in the event of a cylinder leak.

Why not Air Cylinders or Solid Form?

• Transporting SF6 in air cylinders is not feasible because SF6 is denser than air and requires special containment to maintain its properties. Air cylinders are not designed to handle the pressures needed to keep SF6 in a liquefied state.
• SF6 does not exist in a solid form under normal transportation conditions. It transitions from a gas to a liquid at room temperature and under high pressure, and it would require extremely low temperatures to solidify, which is impractical for transport purposes.

Conclusion:

• Therefore, the correct method of transporting SF6 gas is in liquid form within cylinders, as it ensures safe, efficient, and regulated transportation.

Concept:

SF6 circuit breaker:

• The SF6 circuit breaker is a type of circuit breaker that uses pressurized SF6 gas to extinguish the arc.
• It is a dielectric gas having superior insulating and arc quenching properties far better than air or oil.
• It is used for arc quenching in high-voltage circuit breakers up to 800 kV in power stations, electrical grids, etc.

Advantages of SF6 circuit breaker:

• The gas is inflammable and chemically stable.
• These breakers give noiseless operations. 
• It requires less maintenance and costly compressed air, systems are required.
• Due to the superior quenching property of SF6, such breakers have a very short arcing time.
• There are no carbon deposits.

Disadvantages of SF6 circuit breaker:

• The entrance of moisture level in the SF6 breaker tank is very harmful to the breaker, and it causes several parts failures. 
• SF6 breakers are costly due to the reason for the high cost of SF6.

Concept:

SF6 circuit breaker:

• The SF6 circuit breaker is a type of circuit breaker that uses pressurized SF6 gas to extinguish the arc.
• It is a dielectric gas having superior insulating and arc quenching properties far better than air or oil.
• It is used for arc quenching in high-voltage circuit breakers up to 800 kV in power stations, electrical grids, etc.

Advantages of SF6 circuit breaker:

• The gas is inflammable and chemically stable.
• These breakers give noiseless operations.
• It requires less maintenance and costly compressed air, systems are required.
• Due to the superior quenching property of SF6, such breakers have a very short arcing time.
• There are no carbon deposits.

Disadvantages of SF6 circuit breaker:

• The entrance of moisture level in the SF6 breaker tank is very harmful to the breaker, and it causes several parts failures.
• SF6 breakers are costly due to the reason for the high cost of SF6.

Concept:

Sulphur hexafluoride possesses very good insulating and arc-quenching properties. These properties are

• It is a colourless, odourless, non-toxic, and non-inflammable gas.
• SF6 gas is extremely stable and inert, and its density is five times that of air.
• It has high thermal conductivity better than that of air and assists in better cooling current carrying parts.
• SF6 gas is strongly electronegative, which means the free electrons are easily removed from discharge by the formation of negative ions.
• It has a unique property of fast recombination after the source energising spark is removed. It is 100 times more effective as compared the arc quenching medium.
• Its dielectric strength is 5 times that of air and 30% less than that of the dielectric oil. At high pressure the dielectric strength of the gas increases.
• Moisture is very harmful to the SF6 circuit breaker. Due to a combination of humidity and SF6 gas, hydrogen fluoride is formed (when the arc is interrupted) which can attack the parts of the circuit breakers.

• In SF6 circuit breaker, Sulphur hexafluoride gas is used as the arc quenching medium.
• Due to its low gaseous viscosity and high molecular weight, the SF6 gas can efficiently transfer heat by convection.
• It has high insulating properties and highly electronegative properties.
• Pure Sulphur hexafluoride gas is inert and thermally stable.
• SF6 has a very high dielectric strength (breakdown strength). It has 100 times more effective than air and oil as interrupting mediums.
• Its dielectric strength is 2.5 times that of air and 30% less than that of the dielectric oil.

Additional Information

Circuit breaker:

A circuit breaker is a switching device which can be operated manually and automatically for controlling and protecting an electrical power system.

There are four types of circuit breaker

• Oil circuit breaker
• Air circuit breaker
• SF6 circuit breaker
• Vacuum circuit breaker

Concept:

• SF6 gas has excellent insulating property and it has a high electronegativity. 
• Due to high electronegativity, the recovery of dielectric strength after the arc interruption in the SF6 circuit breaker is very fast.
• It has a high affinity for absorbing free electrons.
• It absorbs electrons from a negative ion. The attachment of electron with SF6 molecules may occur in two different reactions.
• Ions are obviously much heavier than a free electron and the overall mobility of charged particles in SF6 gas is much less compared to the other common gases.

Advantages of SF6 Circuit Breaker:

• High dielectric strength
• Take minimum time in installation
• The gas is non-inflammable and chemically stable; The decomposition products are non-explosive
• It gives noiseless operation it does not make a sound like an air-blast circuit breaker during operation
• Minimum maintenance
• The same gas is re-circulated into the circuit thereby reducing the requirement of SF6 gas.

Disadvantages of SF6 Circuit Breaker:

• The SF₆ gas is identified as a greenhouse gas, safety regulation is being introduced in many countries in order to prevent its release into the atmosphere.
• Puffer type design of SF₆ CB needs high mechanical energy which is almost five times greater than that of the oil circuit breaker.

Current chopping: 

• Current Chopping in a circuit breaker is defined as a phenomenon in which current is forcibly interrupted before the natural current zero. 
• Current Chopping is mainly observed in Vacuum Circuit Breaker and Air Blast Circuit Breaker.
• There is no such phenomenon in Oil Circuit Breaker.
• To minimize the current chopping tendency, the SF6 gas is used at low velocity and low pressure.

​SF₆ gas is used due to following reasons:

1.) SF₆ has excellent heat transfer properties as it has a high molecular weight and low gaseous viscosity enabling it to transfer heat by connection more effectively than the common gases. Thus arc can be better interrupted by slowing down the electrons by cooling in the case of SF₆.

2.) Also, the thermal line constant of SF₆ is low, and the pressures at which it should be stored and used are relatively smaller compared to air.

Concept:

• SF6 gas has excellent insulating property and it has a high electronegativity. 
• Due to high electronegativity, the recovery of dielectric strength after the arc interruption in the SF6 circuit breaker is very fast.
• It has a high affinity for absorbing free electrons.
• It absorbs electrons from a negative ion. The attachment of electron with SF6 molecules may occur in two different reactions as shown below,
• Ions are obviously much heavier than a free electron and the overall mobility of charged particles in SF6 gas is much less compared to the other common gases.

Advantages of SF6 Circuit Breaker:

• High dielectric strength
• Take minimum time in installation
• The gas is non-inflammable and chemically stable; The decomposition products are non-explosive
• It gives noiseless operation it does not make a sound like an air-blast circuit breaker during operation
• Minimum maintenance
• The same gas is re-circulated into the circuit thereby reducing the requirement of SF6 gas.

Disadvantages of SF6 Circuit Breaker:

• The SF6 gas is identified as a greenhouse gas, safety regulation is being introduced in many countries in order to prevent its release into the atmosphere.
• Puffer type design of SF6 CB needs high mechanical energy which is almost five times greater than that of the oil circuit breaker.
• SF6 itself is not considered toxic, it can decompose under extreme conditions involving high heat, such as an electric arc or spark, and produce toxic by-products such as sulfur dioxide (SO2) and hydrogen fluoride (HF). These decomposition products are hazardous and can pose health risks if inhaled. Additionally, exposure to these by-products can irritate the skin, eyes, and respiratory system.

Gases for insulation:

• Dielectric materials are basically basic and pure electrical insulators. By applying a sensible electrical field, the dielectric gases can be polarised.
• Vacuum, Solids, Liquids, and Gases can be dielectric materials.
• A dielectric gas is also called an insulating gas. It is a dielectric material in a gaseous state which can prevent electrical discharge.
• Dry air, Sulphur hexafluoride (SF₆), etc are examples of gaseous dielectric materials.
• Due to high molecular weight and low gaseous viscosity SF6 gas has excellent heat transfer properties.

Properties:

A good dielectric gas should have the following properties.

• High dielectric strength
• High thermal stability
• Chemical inertness against the construction materials used
• Non-flammability
• Low toxicity
• Low boiling point
• Good heat transfer properties
• Low cost

• In SF6 circuit breaker, Sulphur hexafluoride gas is used as the arc quenching medium.
• Due to its low gaseous viscosity and high molecular weight, the SF6 gas can efficiently transfer heat by convection.
• It has high insulating properties and highly electronegative properties.
• Pure Sulphur hexafluoride gas is inert and thermally stable.
• SF6 has a very high dielectric strength (breakdown strength). It has 100 times more effective than air and oil as interrupting mediums.
• Its dielectric strength is 2.5 times that of air and 30% less than that of the dielectric oil.

Additional Information

Circuit breaker:

A circuit breaker is a switching device which can be operated manually and automatically for controlling and protecting an electrical power system.

There are four types of circuit breaker

• Oil circuit breaker
• Air circuit breaker
• SF6 circuit breaker
• Vacuum circuit breaker

Sulphur Hexaflouride (SF₆) Circuit Breakers:

• In these circuit breakers, sulphur hexaflouride (SF₆) gas is used as the arc quenching medium.
• The SF₆ is an electro-negative gas and has a strong tendency to absorb free electrons.
• The contacts of the breaker are opened in a high-pressure flow of SF₆ gas and an arc is struck between them.
• The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions.
• This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc.
• The SF6 circuit breakers have been found to be very effective for high power and high voltage service.
   

Advantages:

• Due to the superior arc quenching property of SF₆, such circuit breakers have a very short arcing time.
• Since the dielectric strength of SF₆ gas is 2 to 3 times that of air, such breakers can interrupt much larger currents.
• The SF6 circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to the atmosphere, unlike the air blast circuit breaker.
• The closed gas enclosure keeps the interior dry so that there is no moisture problem.
• There is no risk of fire in such breakers because SF₆ gas is non-inflammable.
• The size of SF6 breaker is smaller than a conventional circuit breaker of the same rating.
• There are no carbon deposits so that tracking and insulation problems are eliminated.
• The SF6 breakers have low maintenance costs, light foundation requirements, and minimum auxiliary equipment.
• Problems connected with current chopping are minimum.
• Since SF6 breakers are totally enclosed and sealed from the atmosphere, they are particularly suitable where explosion hazard exists e.g., coal mines.
   

Disadvantages:

• SF₆ breakers are costly due to the high cost of SF₆.
• Since SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose.

The correct answer is 'option 3'

Solution:

The voltage levels of different circuit breakers are given below:

Additional Information

Advantages of SF₆ circuit breaker: ​

• The SF₆ gas is  non-flammable and chemically stable.
• Their decomposition products are non-explosive and hence there is no risk of fire or explosion.
• Electric clearance is very much reduced because of the high dielectric strength of SF₆.
• It gives noiseless operation.
• There is no over-voltage problem because the arc extinguishes at natural current zero.
• There is no reduction in dielectric strength because no carbon particles are formed during arcing.
• It requires less maintenance and no costly compressed air system is required.
• SF6 performs various duties like clearing short-line faults, switching, opening unloaded transmission lines, transformer reactor, etc. without any problem.

SF6 circuit breaker:

• SF6  circuit breaker is a type of circuit breaker in which sulfur hexafluoride gas or SF6 gas is used as the arc quenching medium.
• SF6  circuit breaker may suffer from gas leakage problem. Because of that, the pressure of the gas inside the extinguisher chamber decreases.
• The arc quenching capability of the breaker may be weaker if the pressure of the gas decreases. to avoid this, regular checking/maintenance of an SF6 CB. 
• The current chopping phenomenon is less in SF6 circuit breaker and so high reliability is obtained.

Additional Information

​Circuit breaker:

A circuit breaker is a switching device that can be operated manually and automatically for controlling and protecting an electrical power system.

There are four types of circuit breaker

• Oil circuit breaker
• Air circuit breaker
• SF6 circuit breaker
• Vacuum circuit breaker