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Final Substation Primary and Energization and Loading Tests Chapter | 22 367
22.2.1 Synchronizing/Phasing Checks
Before paralleling of two circuits from the same generating station, the volt-
age difference between the phases of the two circuits should be approxi-
mately zero, as follows:
R 1 to R 1 5 0 B 1 to B 2 5 0 Y 1 to Y 2 5 0
These measurements are done in the secondary voltage of the voltage
transformer of the two circuits.
Before paralleling of two circuits from two different generating stations,
in addition to the phasing process as above, synchronization must also be
done as follows:
Check the correct setting on the synchro check relay;
Check the phasing voltage difference as above between the phases of the
two circuits;
Check the synchro check relay will pickup when the synchroscope is in
the 12 o’ clock position;
Energize the first circuit to an unenergized busbar (no circuits are con-
nected on that busbar) then, before connecting the second circuit to the same
busbar, check that the synchroscope pointer rotates (this is rotation of incom-
ing voltage vector) due to the slip frequency between the two generating
stations;
When the rotation of the synchroscope pointer is slow and the phase
angle between the incoming and running voltage becomes within the permis-
sible range, the synchrocheck relay will close the circuit breaker of the
incoming circuit.
22.2.2 Directional Test
This test will verify that the directional protection, such as distance relay, is
looking in the line direction only. This is done by a load test as we simulate
the load current and voltage in the trip direction and check the relay
response, then by reversing the secondary current polarity, and finally check-
ing that the relay will not trip in the reverse direction.
The following points should be taken care of before and after the test:
Before doing the test isolate the tripping contacts of the relay;
After the test ends restore the correct polarity of current transformer (CT)
connections.
22.2.3 On-Load Stability Test
1. Transformer differential protection
Read the differential current on the relay screen on the digital relay
and also measure it inside the relay wires in old electromechanical relays.
This differential current should be near zero by reversing the current