indoor vaccum circuit breaker upload

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Indoor vaccum circuit breaker outdoor circuit breaker

SF6 circuit breaker

This is one of the best circuit breaker amongst differentCircuit Breaker Types, sf6 CircuitBeaker uses sulphur hexaflouride ( SF6 ) gas as an arc quenching medium. The sf6 gas has a

strong tendency to absorb free electrons because it is an electro negative gas in nature . Thecontacts 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 enoughinsulation strength to extinguish the arc. The sf6 circuit breakers are very effective for high

power and high voltage service.

Construction :

As figure shows below the parts of a typical sf6 circuit breaker. It consists of fixed and movingcontacts enclosed in a chamber called arc interruption chamber containing sf6 gas. This chamber

is connected to sf6 gas reservoir. When the contacts of breaker are opened the valve mechanism

permits a high pressure sf6 gas from the reservoir to flow towards the arc interruption chamber.

The fixed contact is a hollow cylindrical current carrying contact fitted with an arc horn. The

moving contact is also a hollow cylinder with rectangular holes in the sides to permit the sf6 gasto let out through these holes after flowing along and across the arc. The tips of fixed contact,

moving contact and arcing horn are coated with copper-tungsten arc resistant material. Since sf6gas is costly, its reconditioned and reclaimed by a suitable auxiliary system after each operation

of the breaker.
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Working :

In the closed position of the breaker the contacts remained surrounded by sf6 gas at a pressure ofabout 2.8 kg/cm2. When the breaker operates the moving contact is pulled apart and an arc is

struck between the contacts. The movement of the moving contact is synchronized with the

opening of a valve which permits sf6 gas at 14 kg/cm2 pressure from the reservoir to the arcinterruption chamber. The high pressure flow of sf6 rapidly absorbs the free electrons in the arc

path to form immobile negative ions which are ineffective as charge a carriers. The result is that

the medium between the contacts quickly builds up highdielectric strengthand causes theextinction of the arc. After the breaker operation the valve is closed by the action of a set of

springs.

Advantages over oil and air circuit breakers :

Gives noiseless operation due to its closed gas circuit. Due to superior arc quenching property of sf6 , such breakers have very short arcing

time. There is no risk of fire as sf6 is non inflammable. Dielectric strength of sf6 gas is 2 to 3 times that of air, such breakers can interrupt much

larger currents.

Low maintenance cost, light foundation requirements and minimum auxiliary equipment. Closed gas enclosure keeps the interior dry so that there is no moisture problem. sf6 breakers are totally enclosed and sealed from atmosphere, they are particularly

suitable where explosion hazard exists.

There are no carbon deposits.Disadvantages :

sf6 gas has to be reconditioned after every operation of the breaker, additional equipmentis required for this purpose.

sf6 breakers are costly due to high cost of sf6 gas.Applications :

sf6 Circuit Breaker have been used for voltages 115kV to 230 kV, power ratings 10 MVA to 20

MVA and interrupting time less than 3 cycles.

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SF6 Circuit Breakers and Current

Transformers for High Voltage Lines

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Heres a typical set-up of circuit breakers (left) in a high voltage switchyard. Thecircuit breaker here is a SF6 (Sodium Hexafluoride) type. SF6 is used as a medium ofthe circuit breaker because of its high dielectric strength (2-3 times than that of air)and approximately 100 times effective than air in quenching spurious arcing. On the

right of the Circuit breakers are current transformers which transform HV line currentinto safe and manageable level for proper relaying and coordination. In the even oftransmission line or equipment fault, the current transformers will sense the highamount of current which will then trigger the circuit breakers to open the circuit.
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Oil Circuit Breaker

a high-voltage AC electrical switch whose main contacts are located in a space filled withmineral (transformer) oil. Upon interruption of the electric circuit, an electric arc forms between

the contacts of the circuit breaker. Because of the high temperature of the arc the oil is

evaporated rapidly and oil vapors are partially decomposed, liberating ethylene, methane, andother gases. A gas bubble is formed in the arcing zone; the pressure in the bubble may be as high

as several dozen meganewtons per sq m. The arc is then extinguished, both because of its

elongation upon parting of contacts and because of intensive cooling by the gases and oil vapor.

In an oil circuit breaker with simple interruption under oil, the duration of arcing is 0.02-0.05

sec. To extinguish the arc more efficiently, arc-quenching chambers are used. In a longitudinalblast chamber the vapors and gases evolved travel upward along the arc, thus cooling it. In

addition, the arc is in contact with the cold oil that fills the annular slots of the chamber, which

also accelerates cooling of the arc. In a transverse blast chamber a drastic pressure increase

within the gas bubble causes a stream of oil and gases to flow across the arc, thus accelerating

the cooling process.

In terms of design, a distinction is made between tank-type oil circuit breakers and oil-minimum,or low-oil-capacity, circuit breakers. In the first type, the main contacts and the arc-quenching

devices are located in a grounded metal tank; in the second type they are in an insulating or

ungrounded metal enclosure filled with oil. Tank-type oil circuit breakers are inferior to othertypes of high-voltage breakers in many regards. However, their low cost and high reliability have

led to their continued use in the USSR, the USA, and Canada.

In the USSR, tank-type oil circuit breakers are manufactured for voltages from 6 to 220 kilovolts

(kV); maximum rated current, 3.2 kiloamperes (kA); breaking current, 50 kA. For voltages of 10

kV or less and breaking currents of 15 kA or less, all three poles of the oil circuit breaker arelocated in the same tank. For higher voltages and breaking currents, each pole is located in aseparate tank. Oil-minimum circuit breakers are used in the USSR, the Federal Republic of

Germany, and France. They are manufactured for 3 to 420 kV; since the late 1960s they have

also been manufactured for higher voltages.


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