Understanding Protective Relays: Definition, Detection, and Operation, Quizzes of Low Power Electronic Systems

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Why does any fault in the system need to be

cleared quickly?

because if the fault is not cleared quickly, it may cause unnecessary interruption of service to the customers disconnecting the faulted apparatus limits the damage to it and prevents the effects from spreading into the system TERM 2

How can the detection and disconnection of a

faulty section or apparatus be achieved?

DEFINITION 2 by the use of relays or fuse Fuse performs both detection and interruption functions automatically, but its use is limited for the protection of low voltage circuits only For high voltage circuits, relays and circuit breakers are used for the desired function of protective gear TERM 3

Protective Relay

DEFINITION 3 Is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the system TERM 4

How does a relay detect abnormal conditions

in electrical circuits?

DEFINITION 4 By constantly measuring the electrical quantities which are different under normal and fault conditions TERM 5

What are the electrical quantities that may

change under fault conditions?

DEFINITION 5 Voltage, current, frequency, phase angle

What are the operation steps of a Relay

Circuit?

When a short circuit occurs at point F, the current flowing in the line increases to an enormous value This causes a heavy current to flow in the relay coil, causing the relay to operate by closing its contacts this in turn operates the trip circuit which opens the circuit breaker, disconnecting the faulty section from the rest of the system TERM 7

What are the requirements of a protective

Relay?

DEFINITION 7 Sensitivity Reliability Speed Selectivity Economy TERM 8

Selectivity

DEFINITION 8 A certain amount of overlap is required because even though more circuit breaker than the minimum required will be opened in case of fault, if there was no over lap, any fault in any region between to adjacent zones would not lie in any region thus, no breaker would be opened TERM 9

Speed

DEFINITION 9 The Relay system should disconnect the faulty section as fast as possible for the following reasons: Electrical Apparatus may be damaged if they are made to carry the fault current for too long A failure on the system leads to a great reduction in the line voltage, this small voltage may shutdown consumers' motors and generators may become unstable The high speed Relay system decreases the risk of development of one type of failure to the other more severe type TERM 10

Sensitivity

DEFINITION 10 It is the ability of the relay system to operate with a low value of actuating quantity Sensitivity of the relay is a function of volt-amperes input to the coil of the relay necessary to cause its operation Thus relays can not be operated directly from the primary circuit, instead they're connected to the secondary winding of a voltage or current transformer

Reliability

It is the ability of the relay to operate under pre- determined conditions Relay must operate only for the faulted for which the relay is designed to operate TERM 12

Economy

DEFINITION 12 The most important factor in the choice of a particular protection scheme is the economic aspect Sometimes it is economically unjustified to use an ideal scheme of protection. as a rule, the protective gear should not cost more than 5% of the total cost However when the apparatus to be protect is of utmost importance (Generator, motor, transformer...), the economic considerations often become subordinate to the reliability TERM 13

What are the main principles of operation of

electro-mechanical relays?

DEFINITION 13 Electromagnetic attraction Electromagnetic induction TERM 14

Electromagnetic attraction

relays

DEFINITION 14 Electromagnetic attraction relays operate by virtue of arms being attracted to poles of an electromagnet or a plunger being drawn into a solenoid TERM 15

Types of Electromagnetic attraction Relays

DEFINITION 15 Attracted armature type relay Solenoid type relay