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Principles of Differential Relay
Differential relaying systems are based on the premise that under normal conditions current in
equals current out (no source or sinks)
...
These could result due to line charging current, CT mismatching, the transformer tap
changer, etc
...


For faults inside the protected zone (F1):
I1’ + I2’ =IF1’ (KCL for the node at the point of fault)
I1 ’ ≠ - I2 ’
I1 ≠ - I2
I1 + I2 ≠ 0 → Differential relay trips
...
This means that a
relay should trip if Idiff > Ip (and not when Idiff >
0) , where Ip is the pickup differential current
(maximum differential current that is tolerated
without tripping)
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Example
A differential relay protects a transformer in a single-phase two-port power system, as shown in the
figure below
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Assume that the fault currents flow into the point
of fault and are of the same phase
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(a) Calculate the differential current and the restraint current, as seen by the relay, if a fault occurs inside the
protected zone at point F1
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Does the relay operate for
faults external to the protected unit?

33kV/11kV

1500:5

500:5

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~

A

F1

Differential
relay

F2

B



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