ppt on Study of circuit breakers

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Study of circuit breakers:

Study of circuit breakers Mini project

sub – station layout :

sub – station layout




WHAT IS CIRCUIT BREAKER?? A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit Circuit breaker layout

cause for Arc formation :

cause for Arc formation Equivalent circuit Arc phenomenon

Arc phenomenon:

Arc phenomenon when a short circuit occurs, the contacts of circuit breaker begin to separate The p.d between the contacts is sufficient to ionize the air or oil or gas. This ionized air acts as conductor and ARC is struck between them even though contacts of breaker are opened


ARC INTERRUPTION METHODS: High Resistance Interruption The resistance of arc is increased by 1.Cooling of arc 2.Increasing the length of arc 3.Reducing the cross sectional area of arc 4.Spliting of arc 1.Energy Balance Theory If rate of heat generation is greater than rate of heat dissipation between the contacts arc will be extinguished. 2.Voltage Rate Theory if the dielectric strength increases more rapidly than the restricking voltage arc is extinguished . Low Resistance Interruption

Types of Circuit breakers:

Types of Circuit breakers based of insulating medium employed in the circuit breakers , CB’s are 4types 1. Oil Circuit Breakers 2. Air Blast Circuit Breakers 3. Vacuum Circuit Breakers 4. Sulphur Hexafluoride (SF6) Circuit Breakers

Oil Circuit Breakers :

Oil Circuit Breakers The arc extinction is facilitated mainly by two processes: Firstly, the hydrogen gas has high heat conductivity and cools the arc, thus aiding the de-ionization of the medium between the contacts. Secondly, the gas sets up turbulence in the oil and forces it into the space between contacts, thus eliminating the arcing products from the arc path. The result is that arc is extinguished and circuit current interrupted.

Types of oil circuit breakers:

Types of oil circuit breakers which use a large quantity of oil. The oil has to serve 2 purposes. 1. it extinguishes the arc during opening of contacts and , 2. it insulated the current conducting parts from one another and from the earthed tank. (a) Plain break oil CB (b) Arc control oil CB Bulk oil circuit breakers Cross sectional view Of bulck oil CB

Types of oil circuit breakers:

Types of oil circuit breakers which use minimum amount of oil. In such circuit breakers, oil is used only for arc extinction; the current conducting parts are insulated by air or porcelain or organic insulating material . Low oil circuit breakers

PowerPoint Presentation:

1. It requires smaller space for installation. 2. Maintenance problems are reduced. 3. Requires less quantity of oil. 4. Risk of fire is reduced. 1. The degree of carbonization is increased due to less quantity of oil. 2. Difficulty of removing the gases from the contact space in time. 3. Possibility of explosion. 4. The dielectric strength of the oil deteriorates rapidly due to high degree of carbonization. Advantages: Disadvantages:


AIR BLAST CIRCUIT BREAKERS Compressed air between 20-30 kg/cm^2 is used for arc extinction in this type of breakers. A compressor is needed for maintaining the air preserving . Arc is blown away by the blast of air under pressure. After the arc extinction the arcing chamber is filled with compressed air again to prevent restricting of the arc. The air gap (-1.75cm) is very small between the contacts after separation.

There are 3 types of AIR BLAST CB’S :

There are 3 types of AIR BLAST CB’S Axial-blast type Cross-blast type In this type of breaker the air blast is directed along the arc path. The fixed and moving contacts are held in the closed position by spring pressure under normal condition In this type of breaker an air blast is directed at right angles to the arc. The cross blast lengthens and forces the arc into a suitable chute (Arc splitters) for arc extinction.

PowerPoint Presentation:

1.The risk of fire is eliminated. 2.Cheap compared to oil circuit breaker 3.Arcing time is very less thereby less burning of contacts DisAdvantages Advantages Radial-blast type 1 .The air blast circuit breakers are very sensitive, to the variations in the rate of rise of restriking voltage . 2 .Maintenance is required for compressor plant which supplies air blast.


VACCUM CIRCUIT BREAKERS(VCB) In such breakers, vacuum degree of vacuum being in the range from (106-7 torr ) is used as the arc quenching medium. Since vacuum offers the highest insulating strength, it has far superior arc quenching properties than any other medium. As soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum .

PowerPoint Presentation:

When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The arc is quickly extinguished because the metallic vapors, electrons and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members and shields. Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum breaker occurs with a short contact separation ( say 0.0625 cm ).

PowerPoint Presentation:

1. VCB are compact, reliable and have longer life . 2. They have low arc energy. 3. Risk of fire is eliminated. 4. Maintenance is less. 5. There is no generation of gas during and after operation. 6. They can withstand lightning surges. 1. Vacuum circuit breakers are very expensive .Production in small quantities is uneconomical. 2. For above 36KV VCB `s are not used. 3. They require high technology for production. 4. In case of loss of vacuum, the entire breaker is useless . Advantages Disadvantages


SULPHUR HEXAFLUORIDE (SF6) CB SF6 has excellent insulating strength because of its affinity for electrons. whenever a free electron collides with the neutral gas molecule to form negative ion, the electron is absorbed by the neutral gas molecule. This property of SF6 is approximately 100 times as effective as air in quenching arcs. SF6 has excellent heat transfer by convection more effectively than the common gases .

PowerPoint Presentation:

SF6 consists of 2parts mainly: 1) The interrupter unit This consists of fixed contacts which comprise a set of current carrying fingers and an arcing probe. As the contacts move further apart, the arc is extended and attenuated. It is finally extinguished by the gas flow the high pressure to the low pressure systems. 2) The gas system . 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

PowerPoint Presentation:

1 . Due to superior arc quenching property, SF6 circuit breakers have very short arcing time. 2. Since the dielectric strength of SF6 is 2 to 3 times than that of air , these breakers can interrupt larger currents 3. The SF6 circuit breaker gives noiseless operation due to its closed gas circuit . 4. Risk of fire is eliminated because SF6 gas is non-flammable. 1. SF6 breakers are costly due to high cost of SF6 . 2. Sealing problems arise , Imperfect joints give rise to leakage of gases. 3. After every operation SF6 gas has to be reconditioned, so additional equipment is required. Advantages Disadvantages


CIRCUIT BREAKER RATINGS The rating of a circuit breaker is given according to the duties that are performed by it. The circuit breaker is required to perform following major duties: 1.A circuit breaker must be capable of breaking the circuit and isolating the faulty section in case of faults. This is described as Breaking capacity of C.B. 2.It must be capable of being closed under faulty condition. This is described as Making capacity of C.B. 3.It must be capable of carrying fault current for a short period of time. This is described as Short time capacity .

PowerPoint Presentation:

For circuit breakers up to 245 KV, spring operating mechanism is used. The closing spring gets charged by geared motor. During closing stroke the closing spring is discharged. It closes the breaker and also charges the opening springs. The C.B mechanism is basically very simple since it compresses a motor reducer and two springs. OPERATING MECHANISM Spring mechanism The hydraulic cylinder is mounted at the base of interrupter unit and the hydraulic rod is pushed up during closing stroke and pushed down during opening stroke. The hydraulic oil at atmosphere pressure from the low pressure tank is pumped into accumulators at high pressure oil is used for opening and closing operations. Hydraulic mechanism

Pneumatic mechanism:

Pneumatic mechanism The pressure of operating air produced by the self enclosed motor compressor unit is supervised by a pressure gauge and controlled by a pressure switch. This type of system is providing for each pole of 420KV and 765KV. In EHV OCB, compressed air is used for closing and a spring is used for opening. The spring is charged during the closing stroke. In ABCB, compressed air is used for both closing and opening. In some cases of ABCB, the moving contact is held in closed position by a spring. When compressed air enters the arc chamber and its pressure exceeds the spring force, the contacts are opened. The contacts automatically come in closed position by the spring action when the supply of compressed air is stopped. Therefore, the supply of compressed air must be maintained till the auxiliary CB switch is opened.


CONCLUSION We have abbendent resources for power generation. Besides generation we have to protect our transmission , distridution and household equipment mainly we have to protect our lifes from electrical hazards. So, for the protection circuit breaker is a major and immportant used device for protecting and intterrupting the power supply from the fault conditions…

PowerPoint Presentation:

We express our heartful thanks to Dr. G. SubbaRao , Principal , GIST Mr. S.Sridhar , H.O.D (EEE) Mr.B . Prasad , A.D.E (shift) of 220KV substation, Nellore. for their encouragement, support, valuable guidance, moral support and motivations in completing our project . BY N.YAMINI P.RAJESH S.SUNEEL KUMAR J.PRIYANKA V.SUDHEER KUMAR

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