Page 371 - Practical Power System and Protective Relays Commissioning
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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
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