Manual 37137B
ESDR 4 - Current Differential Protection Relay
Testing with cyclical swapping
With the system off, the secondary connections of the CTs should be rotated between phases. Turn the system on
and increase the excitation to gradually increase the load current. .
The resulting displayed values must be in accordance with the following:
L
s
I
2
1
I
⋅
=
L
d
I
3
I
⋅
=
I
s
Restraint current displayed value
I
d
Differential current displayed value
I
L
Line current (measured using a separate device)
During this test it is possible that the threshold value is exceeded causing the relay to trip.
Attention
In the event of an extreme tripping, it must be ensured that the trip does not open the short-circuit
bridge (in this case a tripping of the breaker would lead to large overvoltages).
If these current relationships are correct, the equipment can be shut down and the original condition restored..
Renewed test with the correct connection
Measurement of the maximum differential current: Read the differential current and the restraint current at 20%
generator rated current and compare these with the original measured values. If the measurements agree, the gen-
erator current can be increased. The differential currents should remain close to zero right up till generator rated
current is reached.
The determination of the maximum differential currents during fault-free operation should help in determining
the corner points of the trip characteristic.
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© Woodward
