Circuit Breakers MCQ Quiz - Objective Question with Answer for Circuit Breakers - 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.

Concept:

The Rate of Rise of Restriking Voltage (RRRV) is a critical parameter in high-voltage circuit breakers. It refers to how quickly the voltage across the circuit breaker contacts increases after the current has been interrupted. If this voltage rises too fast, it can cause the arc to reignite between the contacts, preventing successful interruption.

Importance in Circuit Breakers:

A circuit breaker must withstand the RRRV during the arcing period; otherwise, the arc may restrike, and the fault may not be cleared. The ability to handle high RRRV is a mark of a good breaker design.

Explanation:

Three-Phase Breaker Making Current

Problem Statement: A three-phase breaker is rated at 2000 MVA and 33 kV. The task is to calculate its making current.

Solution:

To calculate the making current of the three-phase breaker, we use the standard formula:

Making Current = √2 × Symmetrical Breaking Current

The symmetrical breaking current can be determined using the formula:

Symmetrical Breaking Current (I_sym) = Rated MVA / (√3 × Rated Voltage)

Given data:

• Rated MVA (S) = 2000 MVA
• Rated Voltage (V) = 33 kV = 33 × 10³ V

Step 1: Calculate the symmetrical breaking current:

I_sym = S / (√3 × V)

Substituting the values:

I_sym = 2000 × 10⁶ / (√3 × 33 × 10³)

We know that √3 ≈ 1.732, so:

I_sym = 2000 × 10⁶ / (1.732 × 33 × 10³)

I_sym = 2000 × 10⁶ / 57.156 × 10³

I_sym = 35,000 A or 35 kA

Step 2: Calculate the making current:

Making Current = √2 × I_sym

We know that √2 ≈ 1.414, so:

Making Current = 1.414 × 35 kA

Making Current = 49.49 kA

Since the making current is typically rounded off to the nearest whole number, the making current is approximately 49 kA.

Correct Answer: Option 3) 49 kA

Important Information

To analyze the other options, let us evaluate them:

Option 1: 70 kA

This value is significantly higher than the calculated making current of 49 kA. A making current of 70 kA would correspond to a much higher symmetrical breaking current or a different breaker rating.

Option 2: 89 kA

This value is even higher than option 1 and is unrealistic for the given breaker ratings. A making current of 89 kA would require a symmetrical breaking current of over 63 kA, which does not match the given MVA and voltage ratings.

Option 4: 35 kA

This value represents the symmetrical breaking current, not the making current. The making current is always higher than the symmetrical breaking current due to the factor of √2.

Option 5: Not Provided

This option is irrelevant as it does not present any numerical value for consideration.

Conclusion:

The making current of the given three-phase breaker is accurately calculated to be approximately 49 kA, as derived from the rated MVA and voltage. Understanding the relationship between the symmetrical breaking current and the making current is crucial for accurately determining the breaker specifications and ensuring proper system protection.

Shortening the arc is NOT a method used for extinction of arc.

Methods for arc quenching of a circuit breaker

Current Zero Method:

• The Current Zero Method is different because it relies on the natural current zero crossing in AC circuits.
• In AC systems, current passes through zero twice per cycle. Circuit breakers designed for AC use this point to interrupt the arc with minimal energy.
• Unlike the other three methods, it does not involve modifying the arc physically but rather utilizing the natural characteristics of AC current.

Cooling the Arc Method:

• The arc is cooled using air, oil, or gas, reducing its temperature and increasing resistance, leading to extinction.

Lengthening the Arc Method:

• The arc path is extended to increase resistance, making it more difficult for current to sustain.
   

Splitting the Arc Method:

• The arc is broken into multiple small arcs using arc splitters, increasing resistance and aiding in extinction.

Differential relay

• A differential relay is defined as the relay that operates when the phase difference of two or more identical electrical quantities exceeds a predetermined value.
• It works on the principle of comparison between the phase angle and magnitude of two or more similar electrical quantities, i.e., vector difference between two or more similar electrical quantities.

Time distance impedance relay

• The relay operates based on impedance measurement, which is directly related to the distance between the relay location and the fault.
• Time grading is applied, meaning the relay operates faster for faults closer to the relay and slower for faults farther away.

Distance type impedance relay

• The relay continuously monitors the voltage (V) and current (I) at a given location. It calculates the impedance (Z = V/I).
• If the impedance falls below a preset value, it indicates a fault within the protected zone, and the relay triggers a trip signal.

Directional overcurrent relay

• The relay measures current magnitude using a Current Transformer (CT). It also measures voltage using a Potential Transformer (PT) to determine the fault direction.
• If both conditions (overcurrent + correct direction) are met, the relay trips the circuit breaker.

Recovery Voltage:

The RMS voltage that appears across the circuit breaker contacts after final arc interruption (when breaker opens) is called “recovery voltage”

Restriking Voltage:

It may be defined as the voltages that appears across the breaking contact at the instant of arc extinction

Active Recovery Voltage:

It may be defined as the instantaneous recovery voltage at the instant of arc extinction

Arc Voltage:

It may be defined as the voltages that appears across the contact during the arcing period, when the current flow is maintained in the form of an arc. It assumes low value except for the point at which the voltage rises rapidly to a peak value and current reaches to zero.

The correct answer is option 1

Concept :

Rated voltage of circuit breaker :

• For below 400 kV systems, the circuit breaker is designed to withstand 10% above the normal system voltage.
• For an above or equal to 400 kV system, the insulation of the circuit breaker should be capable of withstanding 5% above the normal system voltage.
• The voltage rating of the circuit breaker is normally from 1.05 to 1.10 times more than the normal operating voltage.
• this is because during no load or small loads condition voltage level of the power system is allowed to rise up to the highest voltage rating of the system

• 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

 

Circuit breaker	Arc Quenching medium	Rating	Used
SF6 circuit breaker	SF6 gas	Up to 760 kV	Used in a complete range of medium voltage and high voltage electrical power system
Oil circuit breaker	Insulating Oil	Up to 132 kV	It is mainly used for interrupting low current at high voltage
Vacuum circuit breaker	Vacuum	Up to 66 kV	It is mainly used for high current at low voltage Used for remote and rural area
Air circuit breaker	Air	up to 15 kV	Used for railway traction purpose

 

A triple pole with a Neutral (TPN) Miniature Circuit Breaker:

• It is used to protect the circuits from short circuit faults. It is used in the place of the fuse.
• When the amount of electricity is increasing in wire, the MCB turns off and the result is it breaks the circuit.
• It prevents from burning of home appliances.
• The current rating of single-pole Triple pole with Neutral (TPN)  MCB main switches available in the local market is 63 A.

Additional Information

A miniature circuit breaker (MCB):

• It automatically switches off the power supply during overload and faults.
• This function of automatic switching is accomplished by a bimetallic strip.
• Whenever continuous overcurrent flows through MCB, the bimetallic strip is heated and deflects by bending.
• This deflection of the bimetallic strip releases a mechanical latch.
• As this mechanical latch is attached to the operating mechanism, it causes to open the miniature circuit breaker contacts, and the MCB turns off thereby stopping the current to flow in the circuit.

Current chopping in circuit breaker is defined as a phenomenon in which current is forcibly interrupted before the natural current zero.

We can derive the voltage across the capacitor during current chopping

Current Chopping is mainly observed in the vacuum circuit breaker and air blast circuit breaker. There is no such phenomenon in oil circuit breaker.

• A circuit breaker is a switching device that interrupts the abnormal or fault current
• It is a mechanical device that disturbs the flow of high magnitude (fault) current and in additions performs the function of a switch
• The circuit breaker is mainly designed for closing or opening of an electrical circuit, thus protects the electrical system from damage
• When the circuit breaker is closed, it acts as short switch; Thus, before interruption ideal circuit breaker should offer zero impedance
• When the circuit breaker is open, it acts as open switch; Thus, after interruption ideal circuit breaker should offer infinite impedance

Isolator:

• An isolator is a manually operated mechanical switch that separates a part of the electrical power.
• This separates a part of the system from the rest for safe maintenance works.
• Isolators are used to open a circuit under no load (No current condition).
• Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while the current is flowing in the line.
• Isolators are generally used on both ends of the breaker so that repair or replacement of circuit breaker can be done without any danger.

In a circuit breaker, the main contacts are usually made up of copper and conduct current in closed positions. Circuit breakers have low contact resistance and they are silver plated. The arcing contacts are solid, resistant to heat and are made up of copper tungsten alloy.