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Differential relay :
A differential relay is defined as the relay that operates when the phaser
difference of two or more similar electrical quantities exceed by a predetermined
amount. Suppose that current ‘I’ flows through the primary circuit to external fault. If
the two CTs have the same ratio and are connected as shown, no current will flow
through the relay and it will remain un-operational.
If now an internal fault occurs and if current flows to the fault from both sides, the
current flow through the relay will be I1 & I2. It may be noted that the fault current
need not necessary flow to the fault from both sides to cause current flow in the relay.
There may be flow of one side only or even same current flowing out of one side,
while a large current entering the other side will cause a different current.
This type of differential relay is likely to operate inaccurately with heavy load
through faults (external load). suppose identical CTs may not have identical
secondary current due to constructional feature and errors or under severe fault
condition CTs may saturate and cause unequal secondary currents and its difference
of secondary currents may approach the pick-up value of relay. This disadvantage is
overcome in percentage differential relay.
gregorymasila answered the question on February 22, 2018 at 16:43
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