Oil circuit breakers is such type of circuit breaker which used oil as a dielectric or insulating medium for arc extinction. In oil circuit breaker, The contacts of the breaker is made to separate within insulating oil. When the fault occurs in the system the contacts of the circuit breaker are open under the insulating oil, and an arc is develop between them and the heat of the arc is evaporate in the surrounding oil. The oil circuit breaker is divide into two categories.
- Bulk Oil Circuit Breaker
- Low Oil Circuit Breaker
Construction of Oil Circuit Breaker
Oil circuit breaker is very easy in construction. It consists of current carrying contacts enclose in a strong, weather-tight earth metal tank and the tank is fill with transformer oil. The oil is both acts as an arc extinguishing medium and as an insulator between the live part and earth.
At the top of the oil, air is filled in the tank which acts as a cushion to control the displaced oil on the formation of gas around the arc and also to absorb the mechanical shock of the upward movement of oil. The breaker tank is securely bolted for carrying out the vibration caused on interrupting very high current. Oil circuit breaker consists gas outlet which fits in the tank cover to remove the gases.
Operation of Oil Circuit Breaker
The operation of oil circuit breaker is quite simple let’s have a discussion. When the current carrying contacts in the oil are separate, An arc is establish between the separate contacts.
Actually, when separation of contacts has just started, distance between the current contacts is small as a result the voltage gradient between contacts becomes high. This high voltage gradient between the contacts ionized the oil and consequently initiates arcing between the contacts. This arc will produce a large amount of heat in surrounding oil and vaporizes the oil and decomposes the oil in mostly hydrogen and a small amount of methane, ethylene and acetylene. The hydrogen gas cannot remain in molecular form and its is broken into its atomic form releasing lot of heat.
The arc temperature may reach up to 5000oK. Due to this high temperature the gas is liberate surround the arc very rapidly and forms an excessively fast growing gas bubble around the arc. Moreover, the mixture of gases occupies a volume about one thousand times that of the oil decomposed. From this figure we can assume how fast the gas bubble around the arc will grow in size.
If this growing gas bubble around the arc is compress by any means then rate of de – ionization process of ionized gaseous media in between the contacts will accelerate which rapidly increase the dielectric strength between the contacts and consequently the arc will be quenched at zero crossing of the current cycle. This is the basic operation of oil circuit breaker. In addition to that cooling effect of hydrogen gas surround the arc path also helps, the quick arc quenching in oil circuit breaker.
Bulk Oil Circuit Breakers
The basic construction of bulk oil circuit breaker is quite simple. Here all moving contacts and fix contacts are merge in oil inside close iron vessel or iron tank. Whenever the current carrying contacts are being open within the oil the arc is produce in between the separated contacts. The large energy will be dissipate from the arc in oil which vaporizes the oil as well as decomposes it. Because of that a large gaseous pressure is develop inside the oil which tries to displace the liquid oil from surrounding of the contacts. The inner wall of the oil tank has to withstand this large pressure of the displaced oil.
Thus the oil tank of bulk oil circuit breaker has to be sufficiently strong in construction. An air cushion is necessary between the oil surface and tank roof to accommodate the displaced oil when gas forms around the arc. That is why the oil tank is not totally fill up with oil. It is fill up to certain level above which the air is tight in the tank. The breaker tank top cover should be securely bolted on the tank body and total breaker must be properly locked with foundation otherwise it may jump out during interruption of high fault current.
In these type of equipment, where expansible oil is enclose in an air tight vessel (oil tank) there must be a gas vent fitted on the tank cover. Naturally some form of gas vent always is provided on the cover of bulk oil circuit breaker tank. This is very basic features for construction of bulk oil circuit breaker.
Arc Quenching in Bulk Oil Circuit Breakers
When the current carrying contacts in the oil is separate an arc is establish in between the separated contacts.
This arc will produce rapidly growing gas bubble around the arc. As the moving contact move away from fix contact the length of arc is increase as a result the resistance of the arc increases. The increased resistance causes lowering the temperature and hence reducing the formation of gasses surround the arc.
The arc quenching in bulk oil circuit breaker takes place when current passes through zero crossing. If we go through the arc quenching phenomenon more thoroughly we will find many other factors effects the arc quenching in bulk oil circuit breaker. As the gas bubble is enclose by the oil inside the totally air tight vessel, the oil surround it will apply high pressure on the bubble, which results highly compressed gas around the arc. As the pressure is increase the de – ionization of gas increases which helps the arc quenching. The cooling effect of hydrogen gas also helps in arc quenching in oil circuit breaker.