Air Blast Circuit Breakers Advantages Disadvantages

In air blast circuit breakers, compressed air at a pressure of 20-30 kg/cm² is employed as an arc quenching medium. Air blast circuit breakers are suitable for operating voltage of 132kV and above. Switching resistors and equalizing capacitors are generally connected across the interrupters. The switching resistors reduce the transient over voltages and helps arc interruption. Capacitors are employed to equalize the voltage across the break. The number of breaks depends on the system voltage (eg: for 132kV 2 to 4 breaks and for 220kV 2 to 6 breaks)

 Some of the advantages and disadvantages of air blast circuit breakers are discussed below

Advantages of Air Blast Circuit Breaker:

• Cheapness and free availability of the interrupting medium, chemically stable and inertness of air
• Air blast circuit breakers have advantage of high speed operation
• In air blast circuit breaker fire hazard is eliminated unlike in oil circuit breakers
• Short and consistent arcing time and therefore, less burning of contacts
• Air blast circuit breakers require less maintenance
• They are suitability for frequent operation
• Facility for high speed re-closure

Disadvantages of Air Blast Circuit Breaker:

• An air compressor plant has to be installed and maintained
• In air blast circuit breaker upon arc interruption, the air blast circuit breaker produces a high level of noise when air is discharged to the open atmosphere
• In air blast circuit breaker current chopping problem exists
• There is a problem of re-striking voltage

Circuit Breakers Interview Questions Answers

What is meant by Current Chopping?

Answer: Current Chopping is a phenomenon of current interruption before the natural current zero is reached.

Why Isolation switches are provided along with the circuit breakers?

Answer: Isolation switches are provided for separating the disconnected element from the live portion of the system for the purpose of safety. 

Why asymmetrical breaking current is higher than the symmetrical breaking current in a circuit breaker?

Answer: Asymmetrical breaking current is the rms value of the combined sum of the dc and ac components of the short circuit current at the instant of separation of the breaker contacts while symmetrical breaking current is the rms value of the ac component alone. Thus asymmetrical breaking current is higher than the symmetrical breaking current. 

What is recovery voltage in circuit breaker?

Answer: Recovery voltage in circuit breakers is defined as the power frequency rms voltage which appears across the breaker contacts after the transient oscillations die out and the final extinction of the arc has resulted in all the poles of the circuit breaker.

It is the voltage developed after the arc extinction is competed 

When does an arc extincts?

Answer: If the rate of rise of re-striking voltage (RRRV) is less than rate of rise of dielectric strength between the breaker contacts  arc will not sustain and extincts

What is re-striking voltage?

Answer: The Re-striking voltage may be defined as the resultant transient voltage which appears across the breaker contacts at the instant of arc extinction

For an Resistive Load and Inductive load considered for switching operation of circuit breaker, Switching of which load is difficult and Why?

Answer: In case of switching resistive load, current and voltage will be in phase i.e,when during current zero voltage also will be zero in case of resistive load. Hence the voltage at the breaker terminals during arc extinction is zero or minimal. On the other hand, for switching inductive load during current zero voltage across the breaker will be maximum. This results in re-striking of arc once again. Therefore switching inductive load will be difficult

Advantages Disadvantages of Oil Circuit Breakers

Oil circuit breakers are the oldest type of circuit breakers. The separating contacts of the oil circuit breakers are made to separate within the insulating oil which has better insulating properties than air. On the occurrence of the fault as the breaker contacts open under the oil, an arc is struck between the breaker contacts and the heat of the arc evaporates the surrounding oil and dissociates into a substantial volume of gaseous hydrogen (hydrogen gas with a small percentage of methane, ethylene and acetylene) at high pressure. 

Oil Circuit breakers have the virtues of reliability, simplicity and relative cheapness. In order to determine the advantages of oil circuit breakers, advantages and disadvantages of oil as a arc quenching medium should be understood

Advantages:

• Arc energy is absorbed in decomposing of oil
• The gas formed which is mainly hydrogen, has high diffusion rate and high heat absorption in changing from diatomic to mono-atomic and thus provides good cooling properties
• The oil has high dielectric strength and provides insulation between the contacts after the arc has been finally extinguished and there has been time for the oil to flow into the gap between contacts
• Cooling oil presents the cooling surface in close proximity to the arc
• The oil used (such as transformer oil) is a very good insulator and allows smaller clearance between live conductors and earth components

Disadvantages: 

• Oil may be flammable and can cause fire hazards, if a defective oil circuit breaker should fail under pressure and cause an explosion
• There is a risk of formation of explosive mixture with the air
• Due to the decomposition of the oil in the arc, the oil becomes polluted by carbon particles, which reduces its dielectric strength. Hence periodical maintenance and replacements are required

Circuit Breaker Interview Questions Answers

What is a Circuit Breaker?

Answer: Circuit Breaker is a mechanical device designed to close or open contact members, thus closing or opening an electrical circuit under the normal or abnormal conditions

 How does a Circuit Breaker different from Switch?

Answer: Switch is just a device when can be able to open and close the circuit during normal operation. Whereas on the other hand circuit breaker has the ability to open and close the contacts during abnormal or fault conditions. Thus circuit breaker has the potential to break and make heavy short circuit currents. Auto-reclosures in the circuit beaker has the ability to re-close after certain designed duration to verify whether the short circuit was cleared

What is meant by making capacity of the circuit breaker?

Answer: The making capacity of the circuit breaker when closed on a short circuit is the peak value of the maximum current wave (including dc component) in the first cycle of the current after the circuit is closed by the circuit breaker

Why current chopping is not common in oil circuit breakers?

Answer: Current chopping is not common in oil circuit breakers because in most of the oil circuit breakers the arc extinguishing power is proportional to the magnitude of current to be interrupted

Name the materials used for the contacts of vacuum circuit breakers?

Answer: Copper-Bismuth, Copper-lead, Copper-tellurium, Silver-bismuth, Silver-lead and Silver-tellurium are some of the alloys employed as contact materials in the vacuum circuit breakers

Why current chopping considered as serious drawback in a circuit breaker?

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Answer: The current chopping is considered as serious drawback because it sets up high voltage transient across the breaker contacts.

For EHV applications which type of circuit breaker is mostly preferred?

Answer: For Exta High Voltage (EHV) applications SF6 gas insulated switchgear is preferred

What are the duties of Circuit Breakers?

Answer: Some of the duties of the circuit breakers are listed below:

• Interruption of small inductive currents
• Switching of unloaded transmission lines and unloaded cables
• Switching of capacitor banks and reactors
• Interruption of terminal faults
• Interruption of short line faults
• Asynchronous switching