Page 350 - Practical Power System and Protective Relays Commissioning

Protective Relays Testing and Commissioning Chapter | 20 345

FIGURE 20.5.1 Differential relay test connection.

This test is performed to determine the minimum current required to only
operate the relay, the normal limit for pickup value is 6 5% of the nominal
pickup value.
a. Inject a current in one side of the transformer CT in different phases,
then check the pickup value operation to an output contact on the relay for
phases R, Y, B, RY, YB, and BR, and complete a table with this value.
2. Operation characteristic slope test

This test is used to measure the percentage of unbalance that must occur
for the percentage slope set on the differential relay to operate.
The test is performed by gradually increasing the amount of unbalanced
current applied to the relay. The point at which the relay operates is com-
pared to the relay setting. This is done by injecting a current on both sides of
the relay [HV side and low voltage (LV) side] and comparing the measured
point for operating and blocking around the relay characteristic to the
calculated point from the equation of the relay characteristic given by the
relay manufacturer; most transformer differential relays have characteristics
as shown in Fig. 20.5.2.
About 12 points should be chosen around the relay characteristic curve to
be checked to determine the exact operating point of the relay, and they
should have an accuracy normally of about 6 5%.