Thursday, 19 December 2019

OIL CIRCUIT BREAKER

                                                 OIL CIRCUIT BREAKER


Mineral oil 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, however, has the following disadvantages:

           1. It is inflammable and may cause fire hazards.
           2. There is a possibility of its forming an explosive mixture with air.
           3. 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 breakers

           1. The plain-break oil c.b.
           2. The controlled break oil c.b. or bulk oil c.b.
           3. 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
. 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.

principle of operation:the plain-break principle involves the simple process of separating the current carrying contacts under oil with on special control over the resulting arc other than the increase in length caused by 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 separation an arc is established between them. Initially, the separation is very small and a high voltage gradient between the contacts ionizes the oil. the gas obtained from the oil is mainly hydrogen which cannot remain in molecular form and is dissociated in its atomic form releasing lots of heat. with this, the arc core attains a temperature of 5000'k. the mixture of gases occupies a volume about one thousand times that of the oil decomposed. the oil is, therefore, pushed away form the arc and an expanding gas bubble surrounds the arc region based on energy balance principle, final extinction of arc takes place at a current zero when the power input to the arc is less then that dissipated between the contacts.

     form 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 interruption 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. 1.1 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. he vertical break also permits the use of a cylindrical oil tank requiring relatively low floor area.


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