Ch-16 - CONTENTS CONTENTS CHAPTER $ Introduction to...

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387 387 ±$ C H A P T E R C H A P T E R C H A P T E R C H A P T E R Introduction to Switchgear 16.1 Switchgear 16.2 Essential Features of Switchgear 16.3 Switchgear Equipment 16.4 Bus-Bar Arrangements 16.5 Switchgear Accommodation 16.6 Short-Circuit 16.7 Short-Circuit Currents 16.8 Faults in a Power System General General A great demand for electrical energy is a notable feature of modern civilisation. Most of this energy is needed for light- ing, heating, domestic appliances, industrial elec- trical machinery and electric traction. The im- portance of electric supply in everyday life has reached such a stage that it is desirable to protect the power system from harm during fault condi- tions and to ensure maximum continuity of sup- ply. For this purpose, means must be provided to switch on or off generators, transmission lines, distributors and other equipment under both nor- mal and abnormal conditions. This is achieved by an apparatus called switchgear. A switchgear essentially consists of switching and protecting devices such as switches, fuses, circuit breakers, relays etc . During normal operation, switchgear permits to switch on or off generators, transmission lines, distributors and other electrical equipment. On the other hand, when a failure ( e.g . short circuit) occurs on any part of power system, a heavy cur- rent flows through the equipment, threatening damage to the equipment and interruption of ser- vice to the customers. However, the switchgear detects the fault and disconnects the unhealthy
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388 Principles of Power System section from the system. In this way, switchgear protects the system from the damage and ensures continuity of supply. In this chapter, we shall present the elementary introduction to switchgear. 16.1 16.1 Switchgear The apparatus used for switching, controlling and protecting the electrical circuits and equipment is known as switchgear . The switchgear equipment is essentially concerned with switching and interrupting currents either under normal or abnormal operating conditions. The tumbler switch with ordinary fuse is the simplest form of switchgear and is used to control and protect lights and other equipment in homes, offices etc . For circuits of higher rating, a high-rupturing capacity (H.R.C.) fuse in conjuction with a switch may serve the purpose of controlling and protecting the circuit. However, such a switchgear cannot be used profitably on high voltage system (3·3 kV) for two reasons. Firstly, when a fuse blows, it takes sometime to replace it and consequently there is interruption of service to the custom- ers. Secondly, the fuse cannot successfully interrupt large fault currents that result from the faults on high voltage system. With the advancement of power system, lines and other equipments operate at high voltages and carry large currents. When a short circuit occurs on the system, heavy current flowing through the equipment may cause considerable damage. In order to interrupt such heavy fault currents, auto- matic circuit breakers (or simply circuit breakers) are used.
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