Sulfur Hexafluoride (SF6) circuit breaker

 

A circuit breaker in which SF6 under pressure gas is used to extinguish the arc is called SF6 circuit breaker. SF6 (sulphur hexafluoride) gas has excellent dielectric, arc quenching, chemical and other physical properties which have proved its superiority over other arc quenching mediums such as oil or air. The SF6 circuit breaker is mainly divided into three types

• Non-puffer piston circuit breaker
• Single- puffer piston circuit breaker.
• Double-puffer piston circuit breaker.

The circuit breaker which used air and oil as an insulating medium, their arc extinguishing force builds up was relatively slow after the movement of contact separation. In the case of high voltage circuit breakers quick arc extinction properties are used which require less time for quick recovery, voltage builds up. SF6 circuit breakers have good properties in this regards compared to oil or air circuit breakers. So in high voltage up to 760 kV, SF6 circuit breakers is used. 

Construction of SF6 Circuit Breaker:

SF6 circuit breaker is filled with SF6 gas at pressure 5Kaf/cm^2. Sulphur hexafluoride breaker consists of two-part mainly interrupter unit and gas unit.

1) The Interrupter Unit: 

This consists of fixed contacts which comprise a set of current carrying fingers and an arcing probe. When the breaker is in the close position, the fingers make contact round the circumference of the moving contact which has the arcing probe enclosed within its hollow end. The contacts are surrounded by interrupting nozzles and a blast shield which controls are displacement and the movement of the hot gas. The moving contact is in the form of hollow nozzle sliding in a second set of spring loaded fingers. Side vents in the moving contact allow the high pressure gas into the main tank. As soon as the moving contact is withdrawn from the fixed finger contacts an arc is drawn between the moving nozzle and the arcing probe. As the contacts move further apart, the arc is extended and attenuated. It is finally extinguished by the gas flow from the high pressure to the low pressure systems.

2) Gas System:

The closed circuit gas system used in the SF6 C.Bs. Since the gas pressure is very high, lot of care is to be taken to prevent gas leakages at joints by providing perfect scaling. The low and high pressure system are fitted with low pressure alarms and a set of lock-out switches which give a warning the moment the gas pressure drops below a certain value, because otherwise there will be reduction in the dielectric strength and arc quenching ability of the breaker is endangered. 

       If the danger limit is reached the safety devices immobilise the breaker. The over-riding safety devices see to it that a fault in the control circuit does not permit the compressor to build up excessive pressure in the high pressure reservoir or continue to pump gas into the atmosphere in the event of a major leak. The gas is stored in the high pressure chamber at 16 atmospheres whereas the gas pressure on the low pressure side is 3 atmospheres. The temperature is 20°C. In order to prevent liquefaction of the gas in the high pressure chamber at low temperature, a heater is fitted in the high pressure chamber. A thermostat is set to switch on when the ambient temperature falls below 16°C

Working of SF6 Circuit Breaker:

The working of an SF6 circuit breaker is similar to the working of a normal circuit breaker we discussed in previous articles.

In the closed position of the breaker, the contacts remain surrounded by sulphur hexafluoride gas (SF6) gas at a pressure of about 2.8 kg/cm2.

When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts.

The high-pressure flow of sulphur hexafluoride gas (SF6) rapidly absorbs the free electrons in the arc path. It forms immobile negative ions which are ineffective as charge carriers.

The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc.

After the breaker operation (i.e. after arc extinction in circuit breaker), the valve is closed by the action of a set of springs.

Vacuum Circuit Breaker

VCB stands for Vacuum Circuit Breaker, which is a type of electrical switchgear used in power systems to control and protect electrical equipment. A vacuum circuit breaker is a kind of circuit breaker where the arc quenching takes place in a vacuum medium. The operation of switching on and closing of current carrying contacts and interrelated arc interruption takes place in a vacuum chamber in the breaker which is called a vacuum interrupter.

A vacuum that is used as the arc quenching medium in a circuit breaker is known as a vacuum circuit breaker because vacuum gives high insulating strength due to superior arc quenching properties. This is suitable for most standard voltage applications because, for higher voltage, vacuum technology was developed however not commercially feasible.

The operation of current-carrying contacts & related arc interruption take place within a vacuum chamber of the breaker, which is known as a vacuum interrupter. This interrupter includes a steel arc chamber within the center of symmetrically placed ceramic insulators. The maintenance of vacuum pressure within a vacuum interrupter can be done at 10– 6 bar. The vacuum circuit breaker performance mainly depends on the material used for current-carrying contacts like Cu/Cr.

Construction of VCB:

Vacuum Chamber: The heart of a VCB is the vacuum interrupter, which is a sealed chamber that contains contacts and a vacuum medium. It is responsible for interrupting the electrical current when the breaker operates. The vacuum inside the interrupter ensures efficient arc extinction.

Contacts: A VCB consists of two sets of contacts, namely the fixed contact and the moving contact. The fixed contact is connected to the stationary part of the circuit breaker, while the moving contact is connected to the moving mechanism. When the VCB operates, the moving contact separates from the fixed contact to create an air gap, thus interrupting the current flow.

Operating Mechanism: The operating mechanism is responsible for opening and closing the contacts of the VCB. It usually consists of a spring mechanism and a tripping mechanism. The spring mechanism provides the energy to open and close the contacts, while the tripping mechanism controls the operation of the breaker based on external conditions such as overcurrent or fault detection.

Insulating Enclosure: The entire VCB assembly is housed within an insulating enclosure to provide electrical insulation and protect the components from environmental factors. The enclosure is typically made of high-quality insulating materials such as porcelain or composite materials.

Control Panel: A control panel is provided to monitor and control the operation of the VCB. It includes various indicators, switches, and relays for monitoring parameters such as current, voltage, and fault conditions. The control panel also facilitates manual and remote control of the circuit breaker.

Auxiliary Systems: VCBs may incorporate additional systems for enhanced functionality and safety. These can include features like current transformers (CTs) for accurate current measurement, voltage transformers (VTs) for voltage sensing, and protection relays for detecting faults and initiating breaker operations.

Working of VCB:

When a fault occurs in the electrical system, high fault current flows through the circuit breaker contacts, resulting in the formation of an electric arc. The VCB's primary function is to interrupt this arc.

The VCB consists of two main contacts, namely the fixed contact and the moving contact. When the fault current exceeds a predetermined threshold, the trip mechanism of the circuit breaker is activated, causing the moving contact to rapidly separate from the fixed contact.

As the contacts separate, an arc is initiated between them due to the ionization of the surrounding medium. In the case of a VCB, the medium is a high-quality vacuum. The vacuum has excellent insulating properties and high dielectric strength, allowing it to withstand and extinguish the electric arc.

The vacuum inside the circuit breaker creates a medium with high dielectric strength, which prevents the arc from restricting between the separated contacts. The electric field strength across the contacts is rapidly increased, and the electrons in the arc are accelerated, leading to the dissipation of energy and cooling of the arc.

Due to the high dielectric strength of the vacuum, the arc is extinguished when the current passes through zero during the AC cycle. The rapid increase in voltage across the open contacts helps in deionizing the medium and preventing re-ignition of the arc.

Once the arc is successfully extinguished, the VCB allows the contacts to close again to restore the normal flow of current in the electrical system. The reclosing operation can be done manually or automatically, depending on the design and control mechanism of the VCB.

Below are the protective relay which are used for VCB safety:

1) Earth Faulty Relay:

Earth fault relay is used to detect earth leakage in power line & trip the breaker.  To detect earth leakage here CBCT is used which continuously measures the current of phase & neutral which is zero. When earth leakage happens then there will be difference found in phase & neutral current. This condition is detecting CBCT & it provide the signal to earth leakage relay and on this basis, ELR trip the breaker.

2) Master Trip Relay:

Master trip relay receives the signal from protection relays and outputs tripping command to trip coil of a circuit breaker. Thus, it provides isolation between the protection relays and the circuit breaker’s trip coil. If we directly wire protection relays to the trip coil of a circuit breaker and a fault occurs in a trip coil, then it may cause burning of protection relay contact. The protection relays are expensive therefore direct interfacing of the protection relay with the trip coil is not advisable. A master trip relay is thus used for providing isolation between the protection relays and the breaker’s trip coil.

The use of a master trip relay reduces the complexity of wiring. If the master trip relay is not used, then we have to wire all trip contacts of the protection relays to the circuit breaker’s trip coil. The output of the protection relays has 110 VDC and thus the DC voltage routes in the breaker for each protection relay. Thus, there are more chances of DC voltage leakage. It reduces the reliability of the protection system. Moreover, a lot of wiring cause more cost. If we use a master trip relay, we need to carry two wires to the tripping coil.

Master relay has multiple contacts that can be used for interlocking for tripping other breakers, annunciation, and signaling to PLC or SCADA. A master trip relay is a high-speed auxiliary tripping relay and it immediately issues a trip command to the circuit breaker’s trip coil. The operating time of the relay is 10 milliseconds nominal, at rated voltage.

We cannot directly wire the protection relays with a circuit breaker’s trip coil that does not have anti pumping circuit. The anti-pumping circuit interrupts the tripping command once the breaker gets tripped. Master trip relay releases a single pulse for tripping and thus, no anti-pumping circuit is required in a circuit breaker.

3) Supervision Relay:

As shown in the diagram, the CB trip coil will energize when the trip relay is operated. When the CB is open, the position of the S1 & S2 contacts are NO & NC respectively and when the CB is closed, the position of S1 & S2 contacts are NC & NO.

DC(+Ve) of the trip circuit supervision relay is permanently connected and through the trip coil and S1/S2 contact, DC (-Ve) is extended to the relay. Under CB open condition, DC (-ve) is extended through S2 contact, and under CB close condition, DC(-ve) is extended through S1 contact.

At both the CB open & close position, DC (-ve) is extended to the trip supervision relay and it will not operate. Due to the failure of the trip coil or any other issue, DC (-ve) will not be extended to the supervision relay, and under that condition, the relay will operate.

 4) Short Circuit Relay:

The working of this circuit is based on the principle that “Current always try to flow from the path of least resistance”. The circuit is normally open and Red LED Glows when we connect a power source to the input terminal of this circuit. Red LED indicates a short circuit and Green LED indicates that output power is ON. When we press the push button coil of the relay becomes active and it switches from normally close to normally open contact. You can see that even after leaving the push button the relay stays in a latched condition. The current required to keep that relay turned on is coming from a normally open terminal. This is how the circuit comes in on state. We can connect any load to the output terminal the circuit will work.

When overloading or short circuit occurs or when we short the output terminals of load, a huge current flow through the circuit. The voltage across coil terminals becomes nearly zero, entire current tries to flow from the least resistive path. Relay immediately Switches from normally open terminal to normally closed terminal protecting our power supply or battery. This is how Short Circuit Protection Using Relay works.

5) Shunt Relay:

Basically, this relay can also be called as power relay for air circuit breaker. Shunt coil relay is an automatically controlled relay, which can be operated remotely to open or close a circuit breaker. Just like a PLC operates a relay, this shunt coil relay can also be operated (on or off) by it. The operation of this relay directly opens or closes the circuit breaker.

6) Closing Relay:

Basically, it is same as shunt coil relay. It can also be controlled remotely via PLC, and it will automatically open or close the circuit breaker.

Oil Circuit Breaker

 

        An oil circuit breaker is a traditional type of circuit breaker. For arc extinguish in this transformer used oil so it is called oil transformer. It has separate contact. The primary function this contact is to separate insulating oil. When fault or problem occurs, this includes good comparable properties to air, which opens in the lower part of the breaker contact oil.

        The oil in oil circuit breakers (OCBs) serves two purposes. It insulates between the phases and between the phases and the ground, and it provides the medium for the extinguishing of the arc. When electric arc is drawn under oil, the arc vaporizes the oil and creates a large bubble that surround the arc. The gas inside the bubble is around 80% hydrogen, which impairs ionization. The decomposition of oil into gas requires energy that comes from the heat generated by the arc. The oil surrounding the bubble conducts the heat away from the arc and thus also contributes to deionization of the arc.

            When the arc strikes between the two breaking contacts, the heat from the arc dissolves the oil around it, and the high pressure separates some gaseous hydrogen. The unique feature of this circuit breaker is its low cost, reliable operation, and ease of use.

Construction:

This type of circuit breaker is simple to build. It has current-carrying contacts that are encased in a sturdy metal tank. The tank is filled with transformer oil here. The transformer oil acts as an insulator and arc extinguisher between the existing element and the earth.

        The air can be filled within the tank that acts as a pad to maintain the transferred oil on the formation of gas in the region of the arc at the peak of the transformer oil. It helps to absorb the mechanical shock of rising oil movement. The oil tank in this breaker would be bolted to absorb the vibration caused by interrupting the high current flow. This includes a gas outlet installed in the oil tank cover for gas elimination.

Working Principle:

When the oil circuit breaker contacts are opened under oil and an arc is formed between them. The arc’s heat evaporates the surrounding oil, dissociating it into a large volume of gaseous hydrogen at high pressure.

In normal operation of this circuit breaker, the contact in the breaker will be closed as well as carries the current. Once the fault happens within the system, then the contacts will move apart & an arc will be struck among the contacts.

                        

Because of this arc, a huge amount of heat will be released & high temperature can be achieved to vaporize the nearby oil to gas. So this gas will be surrounded by the arc & its unstable increase around it will move the oil violently.

            The arc will be turned off once the space between the contacts like fixed & moving arrives at a certain critical value. It mainly depends on the recovery voltage & arc current. The operation of this circuit breaker is extremely reliable & cheap. The main feature of this circuit breaker is, there is no particular devices are used while controlling the arc which is caused by moving contact.

Oil Circuit Breaker are generally of two types and they are

1) Bulk Oil type or the Dead tank type (OCB)

2) Minimum Oil Circuit Breaker (MOCB) 

Bulk Oil type or the Dead tank type (OCB):

                 Bulk Oil type or the Dead tank type (OCB) The bulk oil circuit breakers are those which use large volumes of oil for two specific purposes. These oils need large tanks which solve the purpose of arc quenching chamber.

The oil is used for

a) Providing insulation from the live current carrying contacts

b) Serving as a medium of arc quenching.

The tank, that houses the current carrying contacts, is at ground potential. When the moving contacts open to initiate the circuit breaking process, an arc is struck inside the oil medium. Due to oil heating, it vaporizes to produce hydrogen and other hydrocarbon gases. These gases generate high pressure in the vicinity of the arc and cool it. Subsequently, the arc can’t sustain itself and gets quenched.

 Minimum Oil Circuit Breaker (MOCB):

        These types of circuit breakers utilize oil as the interrupting media. However, unlike bulk oil circuit breaker, a minimum oil circuit breaker places the interrupting unit in insulating chamber at live potential. The insulating oil is available only in interrupting chamber.

The features of designing MOCB is to reduce requirement of oil, and hence these breakers are called minimum oil circuit breaker. As the volume of the oil in bulk oil circuit breaker is huge, the chances of fire hazard in bulk oil system are more. For avoiding unwanted fire hazard in the system, one important development in the design of oil circuit breaker has been introduced where use of oil in the circuit breaker is much less than that of bulk oil circuit breaker.

        It has been decided that the oil in the circuit breaker should be used only as arc quenching medium, not as an insulating media. In this type of circuit breaker, the arc interrupting device is enclosed in a tank of insulating material which as a whole is at live potential of system. This chamber is called arcing chamber or interrupting pot.  

        The gas pressure developed in the arcing chamber depends upon the current to be interrupted. Higher the current to be interrupted causes larger the gas pressure developed inside the chamber, hence better the arc quenching. But this puts a limit on the design of the arc chamber for mechanical stresses. With use of better insulating materials for the arcing chambers such as glass fiber, reinforced synthetic resin etc, the minimum oil circuit breaker are able to meet easily the increased fault levels of the system.

 

In a minimum oil circuit breaker, the arc is drawn across the current carrying contacts is contained inside the arcing chamber. Hence the hydrogen bubble formed by the vaporized oil is trapped inside the chamber. As the contacts continue to move, after its certain travel an exit vent becomes available for exhausting the trapped hydrogen gas. There are two different types of arcing chambers, available in terms of venting. One is axial venting and other is radial venting. In axial venting, gases (mostly Hydrogen), produced due to vaporization of oil and decomposition of oil during arc, will sweep the arc in axial or longitudinal direction.

Let’s have a look on working principle Minimum Oil Circuit Breaker with axial venting arc chamber. The moving contact has just been separated and arc is initiated in MOCB. The ionized gas around the arc sweeps away through upper vent and cold oil enters into the arcing chamber through the lower vent in axial direction as soon as the moving contact tip crosses the lower vent opening and final arc quenching in minimum oil circuit breaker occurs.

            The cold oil occupies the gap between fixed contact and moving contact and the minimum oil circuit breaker finally comes into open position. Whereas in case of radial venting or cross blast, the gases sweep the arc in radial or transverse direction. The axial venting generates high gas pressure and hence has high dielectric strength, so it is mainly used for interrupting low current at high voltage. On the other hand, radial venting produces relatively low gas pressure and hence low dielectric strength so it can be used for low voltage and high current interruption. Many times, the combination of both is used in minimum oil circuit breaker so that the chamber is equally efficient to interrupt low current as well as high current. These types of circuit breaker are available up to 8000 MVA at 245 KV.

Window AC

A window air conditioner (often abbreviated as window AC or window unit) is a self-contained cooling system designed to be installed in a window or a slot in a wall. It is a popular choice for cooling individual rooms or small spaces, such as bedrooms, apartments, or offices.

A Window AC or any other AC working principle is same. All ACs are working on HVAC cycle. Window AC is basically installed in window or in gap of wall. It is very small AC as compared to other ACs. This AC don't have separate indoor & outdoor. Both indoor & outdoor section are placed in one location. This AC take small space for installation.

Working of Window AC:

As shown in above figure AC is installed in wall or in window. AC's front side is indoor which is inside the room & outdoor is outside the room. 

The working of Window is same but as mentioned in HVAC system. 

The compressor is compress the refrigerant gas at high temprature & high pressure after that refrigerant gas goes into condenser coil. In condenser coil heat is exchanged through outdoor fan (condenser fan) and the refrigerant gas become liquid with high pressure. This liquid state refrigerant is entered into expansion valve. Expansion valve spred liquid refrigerant in evaporator coil & then refrigerant is at liquid level with low pressure. When indoor blower blow air on evaporator coil then heat exchanging happens and cooled air enter into room. 

This blower also suck the return air from room which was hot air and blow this hot air again blow on evaporator coil & air temprature is get doen. This thing is continuously repeated. Once temprature maintained then AC will run on FAN mode. 

To maintain air quality in room AC take fresh air from outside through damper. This air damper is placed between indoor & outdoor. 

There are two controls in this AC:

1) Master Control:

It controls the compressor, compressor motor, evaporator & condenser fan motor. 

When switch is on cool mode then all motors & compressor is start and complete unit is work as air conditioning unit. 

When switch is on FAN mode then only evaporator fan motor is turn ON and complete unit is work as FAN. 

2) Thermostat Control:

Thermostat basically sense the temprature. It is placed in the path of return air. When the temprature of return air is same as set point then AC will be automatically switched OFF. 

Note: Either single motor is used for evaporator & condenser fan or seprate motor is used for evaporator & condenser fan. 

Maintenance of Windows AC: 

During maintenance of AC below points need to be check

1) Compressor:

• Check for any undue noise & vibration. 
• Anti - Vibration mounting to be checked. 
• Check the compressor current & voltage. 

2) Condenser & Evaporator:

• Condenser & Evaporator coils ro be washed with proper jet spray. 
• Condenser fan blade to be checked. 
• Evaporator blower to be checked. 
• Check fan & blower motor operation for free movement. 
• Check for any undue noise & vibration from motor. 
• Check outdoor anti vibration pads. 
• Check the current & voltage of both fans. 

3) Control or PCB:

• Check thermostat for proper operation. 
• Selector switch to be checked. 
• Supply air (temp) to be checked.

4) General:

• Check and tighten all terminal.
• Clean the air filter otherwise replace it if it damaged.
• Check the area around the AC unit and remove any leaves, branches, or other debris that may have accumulated. This ensures proper airflow and prevents blockages.
• Window ACs have a drainage system to remove condensation. Inspect the drainage channels or holes for any clogs and clean them if necessary. Ensure that water can flow freely without any obstruction.
• The seal between the window and the AC unit should be intact to prevent air leaks. Check for any gaps or cracks and use weatherstripping or caulk to seal them if needed. This improves energy efficiency.

HVAC System

 

HVAC is stand for heating, ventilation and air conditioning. HVAC system is mainly used to control & maintain the temperature, humidity and indoor air quality of indoor space. 

HVAC system is used residential site, commercial, corporate sector and also used industry area to provide comfort and maintain health environment. 

Also we are using HVAC system in our home, hospital, hotel etc but it is in small amount. 

Here are three keys or components are present in HVAC system:

1.Heating: HVAC system includes various heating options such as boiler, furnace to increase the temperature in cold atmosphere. Heating system is mainly used in snowy region.

2.Air Conditioning (Cooling) : This system is used to decrease the temperature of indoor area. There are various ACs are available which maintain temperature as well as humidity. 

3. Ventilation: This system mainly used in closed area like office, kitchen and hotel etc. The purpose of this system is to maintain fresh or good air in space. Also remove the air which is not pure. Also it helps AC to maintain temperature ASAP. 

In facility management there are various types of HVAC. It is the technical team responsibility to operate HVAC system & maintenance will did on time for zero breakdown. 

Mostly air conditioning is used in facility management. So we will understand what is air conditioning. 

First we understand HVAC cycle.

Above fig show the HVAC or AC cycle. It contains mainly 4 parts which are most important. 

1. Compressor:

HVAC cycle start from compressor, compressor is the equipment which compress the refrigerant at gas. When refrigerant gas enter in compressor then it was at low pressure and low temperature. Once compressor active & compress the refrigerant then it out at high pressure & high temperature. 

2. Condenser:

Condenser is the basic heat exchange equipment. Condenser is basically located at outdoor unit. It also have fan whose called outdoor fan. That outdoor fan blow the air on the condenser and the heat from that gas will eliminate in atmosphere. 

         The refrigerant gas which is  out from compressor it is at high pressure and high temperature. Then it entered into condenser or condenser coil and outdoor fan helps to remove heat from refrigerant gas. After removing heat from refrigerant gas then refrigerant change from gas to liquid state. This refrigerant liquid is at high pressure. 

3. Expansion Valve:

Expansion valve is used to release the refrigerant in evaporator. Expansion valve spread the refrigerant in evaporator at lower temperatures & low pressure. Expansion valve have thermostat which detect room temperature according to that expansion valve release or spread refrigerant liquid in evaporator. 

4. Evaporator:

Evaporator or Evaporator coil is the second heat exchanger in HVAC cycle and work like condenser. This is the end of HVAC cycle. Evaporator is provide what you want that is cooling. When low temperature liquid & low pressure refrigerant enters into evaporator then indoor blower blow air on the evaporator and the heat exchanged and cooling maintained into room or space. 

After doing this, refrigerant go to compressor and repeat the cycle again. 

There are various types of HVAC system. 

1. Window AC

2. Split AC or Hi Wall AC

3. Cassette AC

4. Ductable AC

5. Package AC

6. Precision AC

7. Chiller

If you have any questions or query then mail us on abhijack143@gmail.com we will help you. 

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Air Blast Circuit Breaker

In this air blast circuit breaker, it uses a high-pressure air as the arc quenching medium. In this type of circuit breaker when the contacts are separated, high-pressure air is forced on the arc through a nozzle.

It cools the arc and the arcing products are blown away by the blast of the air. The chamber is filled with high-pressure air after the extinction of arc which prevents restrike.

  

In the air blast circuit breakers, the arc interruption takes place to direct a blast of air, at high pressure and velocity, to the arc. Dry and fresh air of the air blast will replace the ionized hot gases within the arc zone and the arc length is considerably increased. Consequently, the arc may be interrupted at the first natural current zero. 

    In air blast circuit breakers, the contacts are surrounded by compressed air. When the contacts are opened the compressed air is released in forced blast through the arc to the atmosphere extinguishing the arc in the process. 

    Compressor plant is necessary to maintain high air pressure in the receiver. The air blast circuit breakers are especially suitable for railways and arc furnaces, where the breaker operates repeatedly. 

Working of Air Blast Circuit Breaker:

When a breaker receives a signal either pneumatic or electrical the rod connected to valves gets opened. During tint instant, the air enters into the hallow insulator column and further enters into arc extinction chambers. Because of high pressure in are extinction chamber, the pressure increases on the moving contact and becomes more than the spring pressure. Hence. the contacts are separated. Because of high pressure on moving contact, it travels a short distance against the spring pressure and closes the air outlet valves. As a result-the high-pressure air in the arc extinction chamber is not let to go out However. at the arcing period the air goes out through the air outlet valves with all ionized gases.

    Later, if the valves are closed. then the air in the insulator column is let into the atmosphere, due to which the pressure on the moving contact is dropped to atmospheric pressure. Hence. the moving contact closes over the fixed contact by virtue of its spring pressure; in the applications of high voltage.

Types of Air Blast Circuit Breaker:

Depending on the type of flow of blast of compressed air around the breaker contacts the air blast circuit breakers are classified into three types. They are,

·         Axial blast air circuit breaker

·         Cross blast air circuit breaker

·         Radial blast air circuit breaker

• Axial Blast Air Circuit Breaker:

In axial blast type air circuit breakers, the air flows at high speed axially along the arc. The below figures show the axial blast air circuit breaker. Whenever it is required to open the contacts of the circuit breaker, high-pressure air is entered into the arcing chamber which pushes the movable contact against spring pressure.

As soon as the contacts get separated an arc is drawn between them and it is subjected to high-pressure air. Due to the forced convection, a considerable amount of heat is taken away from the arc periphery. So, the arc diameter reduces, and the core temperature increases.

Hence, the steepness of the temperature gradients established in the arc will increase and so the heat losses also increase. Near the current zero instant, the arc diameter will become narrow and finally, the arc will be completely extinguished at the current zero instant. The gap between the contacts is filled with fresh air. Meanwhile, the movable contact closes the exit hole.

Hence, the air pressure inside the arc chamber increases. This high-pressure air inside the arc chamber will have high dielectric strength and hence it will withstand the transient restriping voltage.

• Cross Blast Air Circuit Breaker:

In cross blast type circuit breaker, the air flows at high speed across the arc. So the arc is subjected to high pressure. The below shows the schematic diagram of the cross-blast air circuit breaker. Fixed contacts shown in the figure above consist of copper fingers surfaced with silver placed between the insulating blocks. The moving contact consists of a copper blade that is connected near the arc splitter plates. The capacity of the circuit breaker depends on the number of splitter plates used.

When the arc is forced on the arc splitter plates its length increases, due to this appreciable resistance is introduced in the arc. So, there is no need for resistance switching in this type of circuit breaker. This type of breaker is used to interrupt high currents.

 

• Radial blast air circuit breaker:

In this radial type, the flow of air is longitudinal along the arc. Breaking by employing a double blast is called a radial blast circuit breaker. The air blast flows radially into space between the contacts.

The contacts are separated and an arc is formed between them. The air flows at a great speed axially along the arc. It causes the removal of heat and the arc is reduced to a value at current zero.  Thus, the arc is extinguished. The flow of fresh air builds the dielectric strength between them.

Protective Devices of Air Blast Circuit:

Pressure Relief Device: Air blast circuit breakers are equipped with pressure relief devices, such as rupture discs or relief valves, to prevent excessive internal pressure buildup. If the pressure inside the breaker exceeds a certain threshold, these devices release the excess pressure to protect the equipment from damage.

Arc Chutes: Arc chutes are designed to guide and cool the arc produced during circuit interruption. They are typically made of materials with high arc-quenching properties. Arc chutes create a longer path for the arc, facilitating effective cooling and reducing the energy and temperature of the arc.

Compressed Air System: The compressed air system provides the necessary air pressure to extinguish the arc. It includes components like compressors, air receivers, and control valves. The system should be properly designed and maintained to ensure an adequate and reliable air supply.

Overcurrent Protection: Air blast circuit breakers have built-in overcurrent protection to safeguard the power system against excessive currents. This protection can be achieved through various devices, such as current transformers and protective relays, which detect abnormal current levels and send signals to trip the circuit breaker.

Trip Circuit Supervision: To ensure proper operation, air blast circuit breakers employ trip circuit supervision. This feature continuously monitors the integrity of the trip circuit, which includes the control wiring, trip coils, and associated devices. Any faults or abnormalities in the trip circuit are detected and can prevent the breaker from closing or tripping unintentionally.

Mechanical Interlocks: Mechanical interlocks are used to prevent incorrect operations and ensure proper sequencing of circuit breaker operations. They can prevent the breaker from being closed or opened when certain conditions are not met, helping to maintain the safety and integrity of the power system.

If you have any questions or query then mail us on abhijack143@gmail.com we will help you. 

Knowledge is only spread, it does not vanish.