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Circuit Breakers

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Ally Raxa

on 9 March 2013

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Transcript of Circuit Breakers

Circuit Breaker A circuit breaker is defined as a piece of equipment which can do any one of the following tasks:
Makes or breaks a circuit either manually or by remote control under normal conditions for controlling and protection of electrical power system
Breaks a circuit automatically under fault conditions 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.

The oil circuit breakers can be classified into the following types:

Bulk Oil Circuit Breaker

Minimum Oil Circuit Breaker 1. It requires smaller space for installation.

2. Maintenance problems are reduced.

3. Requires less quantity of oil.

4. Risk of fire is reduced. 10 EL 129 Arc Phenomenon Sources Book Circuit Breakers Ali Raza Memon - Presentation Designer Under the normal operating conditions, these contacts remain closed and are not open automatically until and unless the system becomes faulty.

The contacts can be opened manually or by remote control whenever desired.

When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit. When the contacts of a circuit break are separated under fault conditions, an arc is struck between them.

The arc depends upon the following factors:
Degree of Ionization
Length of the Arc
Cross-section of the Arc

Arc extinction methods are:
By lengthening the gap.
Cooling the arc.
Inserting medium of high dielectric strength. In such circuit breakers, some insulating oil (i.e., transformer oil) is used as an arc quenching medium.

It is designed for 11Kv -765Kv.

The contacts are opened under oil and an arc is struck between them. The heat of the arc evaporates the surrounding oil and dissociates it into a substantial volume of hydrogen gas at a high pressure.

This large volume of the hydrogen gas pushes the oil away from the arc. ADVANTAGES 1. Slower operation , takes about 20 cycles for arc quenching.

2.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. DISADVANTAGES Air Blast Circuit Breakers These circuit breakers employ a high pressure air-blast as an arc quenching medium.

It was employed in earlier days for voltages ranging from 11 to 1100 Kv.

At high voltages this type of circuit breakers are most suitable.

The contacts are opened in a flow of air-blast established by the opening of the blast valve. The air-blast cools the arc and sweeps away the arcing products of the atmosphere.

Consequently, the arc is extinguished and flow of current is interrupted.

It will require only one or two cycles for arc quenching.

It is sometimes called as compressed air circuit breaker. 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. ADVANTAGES 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. DISADVANTAGES Vacuum Circuit Breakers In such circuit breakers, the vacuum (degree of vacuum being in the range from 10−7 to 10−5 torr) is used as the arc quenching medium.

It is designed for medium voltage range (3.3-33kv).

The vacuum circuit breaker takes the advantage of non-sustainability of electric arc in vacuum, and employs the principle of contact separation under vacuum where there is no ionization due to medium.

The initial arc caused by field and thermionic emissions during the contacts separation, will die away soon, as there is no further ionization because of vacuum.

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). 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. ADVANTAGES 1. Vacuum circuit breakers are very expensive .Production in small quantities is
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 . DISADVANTAGES Sulphur Hexafluoride (SF6) Circuit Breakers In these circuit breakers, Sulpher hexafluoride gas (SF6) is used as the arc quenching medium.

The SF6 is an electronegative 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 services. 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. ADVANTAGES 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. DISADVANTAGES Conclusion Therefore, we conclude that Circuit Breaker is the most essential part of the Electrical Networks as it protects every device from damage.

It helps us to detect the fault & area affected by it.

Nowadays vacuum and SF6 Circuit Breakers are widely used due to their reliable and fast operations. 10 EL 102 Junain Kazi 10 EL 87 Dileep Kumar 10 EL 106 Vinod Kumar - Group Leader Presentation Circuit Breakers The breakers are classified according to its arc quenching media.

Oil Circuit Breakers
Air Blast Circuit Breakers
Vacuum Circuit Breakers
Sulphur Hexafluoride (SF6) Circuit Breakers APPLICATIONS This type of circuit breaker has been used earlier for open terminal HV applications, for voltages of 245 Kv, and 400 Kv up to 765 Kv, especially where faster breaker operation was required.

They are preferred for arc furnace duty and traction systems because they are suitable for repeated duty. APPLICATIONS For a country like Pakistan, where distances are quite long and accessibility to remote areas are difficult, the installation of such outdoor, maintenance-free circuit breakers should prove a definite advantage.

Vacuum circuit breakers are being employed for outdoor applications ranging from 22 Kv to 66 Kv. Even with limited rating of say 60 to 100 MVA, they are suitable for a majority of applications in rural areas. APPLICATIONS A typical SF6 circuit breaker consists of interrupter units each capable of dealing with currents up to 60 KA and voltages in the range of 50— 80 Kv.

A number of units are connected in series according to the system voltage.

SF6 circuit breakers have been developed for voltages 115 kV to 230 kV, power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles. Circuit Breakers Ratings There are three ratings for breakers as: Breaking capacity is defined as the r.m.s. current that a circuit breaker is capable of breaking at given recovery voltage and under specified conditions (i.e. power factor, rate of rise of restriking voltage).

It is a common practice to express the breaking capacity in MVA by taking into account the rated the rated breaking current and rated service voltage.

Thus if I is the rated breaking current in Amperes and the rated service voltage is V in volts, the breaking capacity for three-phase circuit is:
Breaking Capacity= √3×V×I×10^(-6) MVA Breaking Capacity It is the peak value of current(including d.c. component) duringthe first cycle of current wave afterthe closure of circuit breaker.

Mathematically stated, making capacity is given as:

Making Capacity= 2.55× Symmetrical Breaking Capacity Making Capacity Making Capacity Short-time Rating The period for which the circuitbreaker is able to carry faultcurrent while remaining closed is known as short-time rating.

The short-time rating of a circuitbreaker depends upon its ability towithstand:

1.The electromagnetic force effects

2.The temperature rise Short - Time Rating Breaking Capacity Book Types of Circuit Breakers
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