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Switch gear protection
Prof. N. Visali Dept. of EEE, JNTUA College of Engineering, Kalikiri Chittoor District, A P, India
Prof. N. Visali, Dept. of EEE, JNTUA College of Engineering, Kalikiri, Chittoor District, A P, India

Outline of Presentation
Introduction Circuit Breakers Operation of circuit breakers Types of circuit breakers Relays Types of relays Key points on distant relays Previous GATE questions Work to students
Prof. N. Visali, Dept. of EEE, JNTUA College of Engineering, Kalikiri, Chittoor District, A P, India

Introduction
A protection system is designed for continuous monitoring of power system and it should ensure maximum continuity with minimum damage to life, equipment, property and reliability of service. The objectives of power system are: speed of operation, sensitivity, selectivity, reliability, simplicity and economical. The components of protection system are: i. Circuit breakers ii. Relays iii. Lightning arresters( for over voltages ) iv. Fuses ( for over currents )
Prof. N. Visali, Dept. of EEE, JNTUA College of Engineering, Kalikiri, Chittoor District, A P, India

Circuit breakers
Circuit breakers are used to break the circuit or to make the circuit. To break the circuit: During fault conditions and under full load for load shedding. To make the circuit: under no-load for maintenance, during full load, during fault (for auto recloser operation to know whether the fault is permanent or transient). Always circuit breaker is designed with respect to maximum current as if breaker is able to close at maximum current or open at minimum current then the breaker is able to operate at any current. Among all faults three phase faults have maximum current. So, it is taken as reference for designing the circuit breaker. Prof. N. Visali, Dept. of EEE, JNTUA College of Engineering, Kalikiri, Chittoor District, A P, India

Circuit Breakers working
Circuit breaker essentially consists of fixed and moving contacts. These contacts are touching each other and carrying the current under normal conditions when the circuit is closed. During the normal operating condition, the arms of the circuit breaker can be opened or closed for a switching and maintenance of the system. To open the circuit breaker, only a pressure is required to be applied to a trigger. Whenever a fault occurs on any part of the system, the trip coil of the breaker gets energized and the moving contacts are getting apart from each other by some mechanism, thus opening the circuit.
Prof. N Visali, Dept. of EEE, JNTUA College of Engineering, Kalikiri, Chittoor District, A P, India

Operation of Circuit Breaker:
During separation of fixed and movable contacts in a circuit breaker, arc is produced between them, which must be extinguished in order to stop the flow of current. There are three stages of operation in a circuit breaker: i. Arc initiation ii. Arc maintenance iii. Arc interruption
Prof. N. Visali, Dept. of EEE, JNTUA College of Engineering, Kalikiri, Chittoor District, A P, India

Arc initiation

When the current carrying contacts are separated, arc is produced between them. The figure demonstrates the closed and open contacts. When the contacts are closed, high current density, high resistance and high thermal pressure exists at the terminals.

When the contacts are

opened, the pressure decreases

and the surrounding air gets

ionized. The current keeps on

flowing between the contacts as

long as the medium is not

de-ionized.

Fig: Arc initiation

Prof. N. Visali, Dept. of EEE, JNTUA College of Engineering, Kalikiri, Chittoor District, A P, India

Arc maintenance
Arc is maintained between the circuit breaker contacts due to thermal ionisation because of excessive heat energy released between contacts due to high magnitude fault current.
Arc interruption methods:
There are two methods for interrupting the arc. They are: i. High resistance method ( DC bus bar ). ii. Current zero interruption method (AC bus bar ) or Low resistive
method.
Prof. N visali, Dept. of EEE, JNTUA College of Engineering, Kalikiri, Chittoor District, A P, India

Arc interruption methods(contd....)
i. High Resistance method: If fault current is reduced to a value which is not
sufficient to maintain the arc, then arc extinguishes. So, the arc resistance has to be increased sufficiently.
Rarc  l a
The resistance is increased by: a. Cooling the arc b. Lengthening the arc c. Decreasing the cross-sectional area of the arc d. Splitting the arc e. Forced blast.
Prof. N Visali, Dept. of EEE, JNTUA College of Engineering, Kalikiri, Chittoor District, A P, India

Arc interruption methods(contd....)
ii. Low Resistive method or Current zero method :  Applicable only for AC circuit. Because in AC circuits only there occurs natural zero.  At every current zero, rate of ionisation is minimum.  If rate of ionisation < rate of charges removed, arc will not sustain.  There are 2 methods to kill the free electrons: a. Recovery rate theory b. Energy balance theory.
Prof. N Visali, Dept. of EEE, JNTUA College of Engineering, Kalikiri, Chittoor District, A P, India

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Switch gear protection