Mineral oil has better insulating properties than air. It is this property of oil which prompted Steinmetz to break current under oil. He immersed an ordinary knife switch in oil and investigated the breaking capacity of the arrangement. Reliability, simplicity of construction and relative cheapness are particular virtues of oil breakers. Oil, however, has the following disadvantages:
- It is inflammable and may cause fire hazards.
- There is a possibility of its forming an explosive mixture with air.
- Because of the production of carbon particles in the oil due to heating, periodical reconditioning or replacement is required.
The following are the categories of oil circuit breaker that have developed so far:
- The plain-break oil C.B.
- The controlled break oil C.B or bulk oil C.B.
- Minimum oil C.B.
The first and second categories of breakers are also known as the dead tank construction because the tank is held at earth potential whereas the third category is known as live tank as the tank containing oil is insulated from the ground.
The Plain-break oil Circuit Breaker
The circuit breaker consists of a metal tank containing oil and encloses two or more contacts Fig. Since large energies are to be dissipated within the tank, a large gaseous pressure is developed. To withstand such a large pressure the tank has to be a strong one. It is usual to make the tank and the top plates either of welded sheet steel or boiler plates. The distance between phases and the clearances between the live metal and the earthed metal are a function of the operating voltage.
An air cushion is necessary between the oil surface and tank cover to accommodate the displaced oil when gas forms around the arc. The air cushion also serves to absorb the mechanical shock of the upward oil movement. The breaker tank should be securely bolted to an adequate foundation otherwise it may jump out when interrupting very heavy current.
An ample head of oil above the acing contact is necessary (i) to provide substantial oil pressure at the are; and (ii) to prevent
A gas outlet from the tank is essential and also some form of vent is fitted in the tank cover. The position of the vent is carefully chosen so that the not harm the flash-over to the neighboring equipment
Principle of operation: The plain-break principle involves the simple practical ionized gases which come out of the vent do personnel and also do not cause process of separating the current carrying contacts under oil with no special control over the resulting arc other than the increase in length caused the moving contact. The final arc extinction is obtained when a certain critical gap between the contacts is reached, the length of which depends upon the arc current and the recovery voltage.
At the instant of contact
From practical point of view the speed of the break should be as high as possible because a certain break distance has to be reached before inter-auction is likely to occur and the sooner this is achieved the smaller the energy released in the breaker and the less mechanically strong a breaker will have to be designed The double break arrangement as shown in Fig. is perhaps the most familiar of all oil circuit breakers. The two breaks in series give rapid arc lengthening without the need for a specially fast contact speed, and the total gap distance at the end of stroke can conveniently be made ample. The vertical break principle also permits the use of a cylindrical oil tank requiring relatively low floor area.
The controlled break oil C.B or bulk oil C.B.
The plain-break circuit breakers are used widely on low voltage d.c. circuits and on low voltage distribution a.c. circuits. For higher voltages they become unduly large in size and require huge amounts of transformer oil. Also, it is not suitable for high speed interruption i.e. they cannot be used for auto reclosing.
The primary object of any controlled-break principle is to obtain final extinction consistently. while the contact gap is still short and is approximately equal to the clearance required under oil when in the open position. The arc control pots are shown in Fig.
The contacts are enclosed in a chamber made of insulating material and provided with a series of vents on one side of the chamber. Final arc extinction takes place within the chamber which is secured to the fixed contact. The whole assembly is immersed in the oil. There is a small clearance between the throat and the moving contact. Also, in most types, one or more small bleed holes are provided in the upper wall to prevent air being trapped when the breaker is filled with oil.
The arcing conditions in this breaker are different from those of plain-break breakers. The internal space available to the gases which are produced due to the decomposition of oil little more than that swept out by the moving contact As is said earlier, the mixture of gases occupies a volume about one thousand times that of oil decomposed, large pressure set up between the contacts: As a result the movement of oi is restricted in the chamber and the espauslon of internal gas bubble is liited in the preenure chamber . Also the heated gas is forced out of the .chamber away from the arc. These two conditions allow much better cooling of the arc which results in higher breaking capacities of these breakers as compared to plain-break breakers. The flow of gases through the vents lengthens the arc and the gases flowing around the arc with high velocity give turbulent condition which increases the energy losses. Also because of the large gas pressures, the mean free path of the electrons and ions is reduced which results in effective deionization. It is seen that the pressures are self-regulated in the sense that higher the breaking currents larger will be the pressures generated and these breakers give their best performance at the highest currents within their ratings.
Various improvements in the design of pressure chambers have been suggested to provide
The aim here is to extinguish the second arc quickly by using the gas pressure and the oil momentum due to the first arc. This is done by arresting the intermediate contact at a definite short distance and high gas pressure is achieved by providing a small vent. Thus, fast and high gas pressures are obtained with safety to the chamber.
A modification of this double break oil CB. has been developed to give a similar effect. A common cross-bar carries both sets of moving contacts which are so arranged that contact separation at one of the breaks occurs slightly before that in the other break. The arc in the first break creates gas pressure and oil momentum, which
Minimum Oil Circuit Breakers
One of the important developments in the design of oil C.is has beet reduce the amount of oil needed because the severity of a fire involving an oil switch is to some extent proportional to the volume of oil contained. The other advantages are:
(i) Reduction in tank size
(ii) Reduction in total weight.
(iii) Reduction in cost.
The use of pressure chamber for arc control in the bulk oil C.B. mentioned in the previous article reduced the volume of oil. But it still requires huge amounts of oil for higher voltages. The minimum oil C.B. uses solid materials for insulating purposes and uses just enough oil for arc quenching The bulk oil breakers described in the previous section are of the dead tank type because tank is at earth potential whereas the arc interrupting device is enclosed in a tank of insulating material in case of a minimum oil breaker, the whole of which is at line voltage during normal operation and therefore, these are known as live tank breakers.
The minimum oil C.Bs. can be of self-blast type or external blast type or a combination of the two. In case of self-blast type the gas pressure developed depends upon the current to be interrupted. The higher the current to be interrupted the larger is the gas pressure developed and hence more effective is the breaker for arc quenching. But this puts a limit on the design of the arc chamber for mechanical stresses. With the use of batter insulating materials for the arcing chambers such as glass fiber, reinforced synthetic resins etc., the minimum oil C.Bs. are able to meet easily the increased fault levels of the systems. Most of the minimum oil C.Bs. these days are the self-blast type
There are two different designs of the arcing chambers in terms of the venting provided: (i) axial venting, and (ii) radial venting
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