Essential Features of 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.
With the advancement of power system, lines and other equipment 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, automatic circuit breakers(or simply circuit breakers) are used.
A circuit breaker is a switchgear which can open or close an electrical circuit under both normal and abnormal conditions
Principles of circuits breakers, the various types of circuit breakers and their applications as control devices in power systems.The essential features of switchgear are :
i. Complete reliability: When fault occurs on any part of the power system, the switchgear must operate to isolate the faulty section from the remainder circuit.
ii. Absolutely certain discrimination: When fault occurs on any section of the power system, the switchgear must be able to discriminate between the faulty section and the healthy section. In other words, it should isolate the faulty section from the system without affecting the healthy section. This is necessary to ensure continuity of supply.
iii. Quick operation: When fault occurs on any part of the power system, the switchgear must operate quickly so that no damage is done to generators, transformers and other equipment by the short-circuit currents. If fault is not cleared by switchgear quickly, it is likely to spread into healthy parts, and consequently the complete shutdown of the system.
iv.Provision for manual control: A switchgear must have provision for manual control. In case the electrical (or electronics) control fails, the necessary operation can be carried out through manual control.
v.Provision for instruments. There must be provision for instruments which may be required. These may be in the form of ammeter or voltmeter on the unit itself or the necessary current and voltage transformers for connecting to the main switchboard or a separate instrument panel.
A circuit breaker is a piece of equipment which can
i. make or break a circuit either manually or by remote control under normal conditions
ii. break a circuit automatically under fault conditions
iii. make a circuit either manually or by remote control under fault conditions
A circuit breaker incorporates manual (or remote control) as well as automatic control for switching functions. The latter control employs relays and operates usually under fault conditions.
A circuit breaker essentially consists of fixed and moving contacts, called electrodes. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit.
The relay detects the abnormal conditions in electrical circuits by constantly measuring the electrical quantities which are different under normal and faulty conditions (e.g., current, voltage , frequency ,etc.). When the fault is detected the relay operates to complete the trip circuit which result in te opening of the circuit breaker and subsequently the disconnection of the faulty circuit. It should be noted that the contacts can be opened manually or by remote control whenever desired. Basic operation of Circuit Breaker
When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them as shown Arcing Phenomenon in Circuit Breakers
The current is thus able to continue until the discharge ceases. The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself. Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value. In the next section we will discuss the arcing phenomenon.