Advantages and Disadvantages of Oil as an Arc Quenching medium

(A)  Advantages:

1. The energy of arc is utilized for decomposition of oil which produces gases and gas has excellent cooling property.

2. Oil has greater dielectric strength than air so it provides better insulation between live parts and tank. Also clearance between these parts is reduced.

3. Problems of over voltage and current chopping are minimum.

4. Surrounding oil presents cooling surface in close proximity of arc.

5. Air pollution and inflammable gas in atmosphere have little effect on arc extinction.

(B) Disadvantages:

1.    It is inflammable and so there is risk of fire.

2.    The decomposed product of oil is inflammable and if CB fails to break the fault current, the pressure in the tank goes on increasing and explosion may occur.

3.    The carbonization of oil occurs during arcing and also absorption of moisture reduce the dielectric strength. So oil should be replaced after certain number of breaker operation.

4.    Regular maintenance is necessary.

5.    Oil is not a suitable arc quenching medium where breaker operations are frequent e.g. arc furnace, railways etc.

6.    Oil replacement, purification is rather troublesome.

7.    Oil CBs are bulky and hence transportation is difficult.

CIRCUIT BREAKERS USED FOR HT

Circuit Breaker: This circuit breaker uses transformer oil for arc quenching medium. The circuit breaker contacts are immersed in oil. When the CB contacts are opened an arc is struck between them and oil in that region is decomposed because of which a hydrogen gas bubble is formed between the contacts as shown in figure.

                                                 

The hydrogen gas has high conductivity and this helps in cooling of arc. It also produces turbulence that pushes the arc away from contacts. The hydrogen gas occupies almost thousand times the volume of decomposed oil. Thus the arc is cooled and extinguished effectively.

Bulk Oil Circuit Breaker: It uses large quantity of oil. The oil is used for two purposes.

i)        To extinguish the arc.

ii)       To insulate the conducting parts from earthed tank.

Bulk oil C.Bs are of two types:

i)        Plain oil CB

ii)       Arc control oil CBs.

(I) Plain Oil C.B.

Construction: It consists of a whether tight tank. The fixed contact is kept in this tank. The tank is earthed properly. The tank is filled with oil up to a certain level and the contacts are immersed in this. Air is present at the top side. This air acts as a cushion.

There are two moving contacts which are connected to a rod that can slide up-down.

                                                 

Two contacts are provided so that the total transient re-striking voltage is distributed over two contacts. So, half of the transient re-striking voltage appears across each CONTACT. For same distance of separation, the arc length is doubled because arc appears across both contacts. This helps in arc extinction process.

Working: Under normal working conditions, the fixed and moving contacts area closed. On occurrence of fault, the moving contacts come down and an arc is struck between contacts. The oil between the contacts decomposes and hydrogen gas bubble is formed around the contacts. The hydrogen gas cools the arc and the turbulence effect causes lengthening of arc. The deionization of medium between contacts takes place and at some critical length of gap between contacts, the arc is extinguished.

The hydrogen gas bubble produces a very high pressure in the oil. The tank is therefore made strong to withstand a large pressure. The oil is displaced (moves upwards) when hydrogen bubble is formed. The air is present between oil level and tank top. The air acts as a cushion, it absorbs the mechanical shock produced due to upward oil movement is absorbed by this air cushion.

The disadvantages oil plain oil circuit breakers are:

1.    These breakers have long and in consistent arcing time.

2.    These breakers do not permit high speed interruption.

3.    The arc interruption control is obtained only by increasing he length between the contacts i.e. by increasing the arc length only.

These are used up to 11 kV with an interrupting capacity of 250 MVA.

(ii) Arc Controlled Oil C.B.

There are three types of arc controlled oil C.Bs.

a)    Plain explosion pot oil C.B.

b)    Cross jet explosion pot oil C.B.

c)    Self compensated explosion pot oil C.B.

(a) Explosion Pot Oil C.B.

Construction: The gasses produced during arcing are confined in a small volume using a special pressure chamber.

This chamber or pot is made up of insulating material. The fixed and moving contacts are placed in this pot and it is filled up and surrounded by oil.

                                     

Working: When fault occurs, the moving contact moves downwards. An arc is struck between are formed (hydrogen gas). The gas is enclosed in the closed chamber i.e. pot, so the high pressure is developed that forces the oil and gas round the arc to extinguish it. If the arc does not extinguish when, moving contact is inside the pot, it definitely extinguishes when moving contact comes out of the pot. This is because oil and gas at high pressure rushes out from that opening.

Disadvantages:

1.    Cannot be used for very low or very high fault currents. Because very low fault current, the gas formation is less so pressure is less, so arc lasts for a longer time i.e. arcing time is longer.

2.    For very high fault current, tremendous pressure will be developed in the pot, that may cause bursting of pot.

(b) Cross jet Explosion Pot Oil C.B.

Construction: This is the modification of previous type. It consists of a chamber made up of insulating material.

The chamber is split into two parts as shown.

Working: Under normal working conditions the fixed and moving contacts are closed. On occurrence of fault, the moving contact separates and comes down. An arc is formed between moving contact and fixed contact. The arc causes decomposition of oil and hydrogen gas is formed causing production of pressure in the oil.

The decomposed oil at high pressure travels from upper section to lower section as shown in figure. The oil in lower section is fresh oil, it also travels in the same direction due to pressure. Thus, oil flow is at right angle to the arc. Hence, this oil C.B. is called cross jet explosion pot.

The arc is cooled and it is forced into arc splitters, so its length also increases. And finally arc is extinguished.

(c)  Self Compensated Explosion Pot Oil C.B.:

Construction: This is the combination of previous two types. Its upper portion is cross jet explosion pot and lower portion is plain explosion pot.

Working: Under normal working conditions the fixed and moving contacts are closed. On occurrence of fault, the moving contact separates and comes down. An arc is formed between moving contact and fixed contact. The arc causes decomposition of oil and hydrogen gas is formed causing production of pressure in the oil.

When short circuit current is heavy it behaves as cross jet explosion pot. If severity of short circuit current is less it behaves as a plain explosion pot.

Minimum Oil Circuit Breaker (MOCB)

The supporting, current interrupting and top chamber are made up of porcelain. Hence, clearance between live parts and enclosure is small and requires less quantity of oil, hence the breaker is called as Minimum Oil Circuit breaker (MOCB).

The chambers area completely filled with oil. The oil from upper chamber does not come to lower.

The fixed contact is enclosed in the quenching chamber. Moving contact makes sliding contact with lower fixed contacts. The operating rod is operated by operating mechanism; the three poles are operating simultaneously. The voltage ratings are from 3.6 kV to 420 kV.

When the contacts carrying current are separated in the oil the arc is developed. The heat of arc causes decomposition of oil and gases area formed. These gases expand due to heating of ar. The gas flowing near the contact zone causes cooling and splitting of the arc and extinguishes the arc.

After the arc is quenched at last current zero instant, fresh dielectric medium is inserted between the contacts.