Page 335 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

3 Aluminium rotating disc

4 Percentage fault quantity time reference dial
5 Tap setting plug
6 Input fault quantity (VONI)
7 Contact plug terminals

Fig 1 Fig 2

An induction type over current relay giving inverse time operation with a definite minimum time characteristic
is in Fig 1. It consists essentially of an ac energy meter mechanism with slight modification to give required
characteristics. The relay has two electromagnets. The upper electromagnet has two windings, one of these is
primary and is connected to the secondary of a CT in the line to be protected and is tapped at intervals.
The tappings are connected to a plug setting bridge by which the number of turns in use can be adjusted, thereby
giving the desired current setting. The plug bridge is usually arranged to give seven sections of tappings to give
over current range from 50% to 200% in steps of 25%. If the relay is required to response for earth fault the steps
are arranged to give a range from 10% to 70% or 20 to 80% in steps of 10%. The values assigned to each tap are
expressed in terms of percentage of full-load rating of CT with which the relay is associated and represents the
value above which the disc commences to rotate and finally closes the trip circuit.
Thus pick-up current equals the rated secondary current of CT multiplied by current setting. For example suppose
that an over current relay having a current setting of 150% is connected to a supply circuit through a CT of 500/5A.
The rated secondary current of CT is 5 A and, therefore, the pick-up value will be 1.5 x 5i.e., 7.5 A. It means that
with above current setting, the relay will actually operate for a relay current equal to or greater than 7.5 A.
Similarly for current settings of 50, 100 and 200% the relay will operate for relay currents of 2.5A, 5 A and 10 A
respectively. Adjustment of current setting is made by inserting a pin between the spring loaded jaws of the bridge
socket at the tap value required. When the pin is
withdrawn for the purpose of changing the setting value while the relay in service, the relay automatically adopts
higher setting, thus the CT’s secondary is not open-circuited.

Time multiplier setting
This setting is helps the relay to shorten the time selected without change of any other settings made in the relay.
Time multiplier helps the relay to activate fast the breaker in case the fault quantity is more than 50% of the fault
quantity selected by the tap setting.
Differential protection relay
Differential protection is a very reliable method of protecting generators, transformers, busbar and transmission
lines from the effects of internal faults. In normal operating conditions the current through the CTs is the same. So
the relay sense no differential current. This is also the case for external faults. Differential protection can be used

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CITS : Power - Electrician & Wireman - Lesson 106-116