HOW TO MAINTAIN SF6 CIRCUIT BREAKER

1 International Journal of Scientific Research Engineering &Technology (IJSRET) Volume 1 Issue1 pp 030-035 March 2012 www. ISSN 2278 - 0882 HOW TO MAINTAIN SF6 CIRCUIT BREAKER 1 Snigdha Sharma, 2 Hemant bharadwaj, 1 Faculty of EE Department, SIT, Meerut, India 2 Faculty of EE Department, SIT, Meerut, India AbstractIndia s growing economy needs an efficient power transmission system to meet the increasing demand for reliable and affordable power.

2 CIRCUIT breakers play an important role in protection system of electrical power transmission networks. A CIRCUIT BREAKER is an automatically operated electrical switch which detects a fault condition and interrupt immediately to discontinue electrical flow. So its maintenance deserves special considerations in order to prevent the equipments and transmission system from damage. This paper deals with high voltage SF6 CIRCUIT BREAKER . This paper shows construction and operation of SF6 CIRCUIT BREAKER , properties of SF6 CIRCUIT BREAKER , importance of its maintenance, rating and testing and gas handling.

3 The aim of this paper is to make a review of maintenance of SF6 CIRCUIT : Arc extinguishes, CIRCUIT BREAKER , compressed SF6 gas, Electric CIRCUIT BREAKER is a switching device which can open or close a CIRCUIT allowing interrupting the circulation of current. Previously oil and air CIRCUIT breakers were used commonly but SF6 CIRCUIT BREAKER is most widely used CB now a days for high voltage applications worldwide. Some of the outstanding properties of SF6 making it desirable to use in power applications such as high dielectric strength, unique arc-quenching ability, excellent thermal stability and good thermal conductivity.

4 The only industrial process currently in use is the synthesis of sulfur hexafluoride by allowing fluorine obtained by electrolysis to react with sulfur according to the exothermic reaction:S + 3F2 SF6 + 262 kcalSF6 is a gaseous dielectric used in high voltage electrical equipment as an insulator or arc quenching medium. The separation of the CB contacts which are carrying current give rise to an arc. During arcing the CIRCUIT BREAKER maintains a relatively low pressure inside the chamber and there will be no danger of explosion and spilling of gas around. Any leakage from the chamber will not create a problem since SF6 can undergo considerable decomposition in which toxic products may stay inside the chamber in the form of white dust.

5 Normally at least once a year or after every 500 operations, the CIRCUIT BREAKER must be maintained. Maintenance of CIRCUIT breakers at appropriate time is important as these are used to protect power network from short circuits and other problems environmental problems. However, it has its limitations while maintaining. This paper will contribute in studying that why is it necessary to MAINTAIN CIRCUIT and Operation of SF6 CIRCUIT breakerIt consists of two parts The interrupter unit: It consists of fixed contacts which comprise a set of current carrying fingers and an arcing probe.

6 When the BREAKER is in close position, the fingers make contact round the circumference of the moving contact which has the arcing probe enclosed within its hollow end. The contacts are surrounded by interrupting nozzles and a blast shield which controls are displacement and the movement of hot gases. The moving contact is in the form of hollow nozzle sliding in the second set of spring loaded fingers. Side vents in the moving contact allow the high pressure gas into the main tank. As soon as the moving contact is withdrawn from the fixed finger contact an arc is drawn between the moving nozzle and the arcing probe.

7 As the contacts move further apart, the IJSRET @ 2012 International Journal of Scientific Research Engineering &Technology (IJSRET) Volume 1 Issue1 pp 030-035 March 2012 www. ISSN 2278 - 0882arc is extended and attenuated. It is finally extinguished by the gas flow from the high pressure to the low pressure The gas system:Since the gas pressure is very high, lot of care is to be taken to prevent gas leakage at joints by providing perfect scaling. The low and high pressure systems are fitted with low pressure alarms and set of lock out switches which give a warning the moment the gas pressure drops below a certain value, because otherwise there will be reduction in the dielectric strength and arc quenching ability of the BREAKER is endangered.

8 If the danger limit is reached the safety devices immobilize the BREAKER . The over riding safety devices see to it that a fault in the control CIRCUIT does not permit the compressor to build up excessive pressure in the high pressure reservoir or continue to pump gas into the atmosphere in the event of major leak. The gas is stored in the high pressure chamber at 16 atmospheres whereas the gas pressure on low pressure side is 3 atmospheres. The temperature is 20 C. in order to prevent liquefaction of a gas in the high pressure chamber at low temperature; a heater is fitted in the high pressure chamber.

9 A thermostat is set to the switch on when the ambient temperature falls below 16 Importance of adequate maintenanceThe maintenance of CIRCUIT BREAKER is necessary because of their importance for routine switching and for protection of other equipments. Electric transmission system breaks up and equipment destruction can occur if a CIRCUIT BREAKER fails to operate due to lack of preventive maintenance. Breakers that remain idle for 6 months or more should be made to open and close several times in succession to verify proper operation and remove any accumulation of dust or foreign material on moving parts and Properties of SF6 Toxicity: SF6 is odorless, colorless, tasteless, and nontoxic in its pure state.

10 It can, however, exclude oxygen and cause suffocation. If the normal oxygen content of air is re-duced from 21 percent to less than 13 percent, suffocation can occur without Physical properties:SF6 is one of the heaviest known gases with a density about five times the density of air under similar conditions. SF6 shows little change in vapor pressure over a wide temperature range and is a soft gas in that it is more compressible dynamically than air. The heat transfer coefficient of SF6 is greater than air and its cooling characteristics by convection are about times air. Its density at 20 degree centigrade is Dielectric strength:SF6 has a dielectric strength about three times that of air at one atmosphere pressure for a given electrode spacing.