Page 120 - Practical Power System and Protective Relays Commissioning

118 Practical Power System and Protective Relays Commissioning

has the ability to measure fault currents exceeding 400 kA peak.
Additionally both AC and DC currents can be measured to this accuracy
level.

10.9 CURRENT TRANSFORMER COMMISSIONING TESTING

10.9.1 Visual Checks
The following visual checks should be carried out:

1. Check the ratings on the CT nameplate are as per the approved drawings
and specifications.
2. Inspect the CT mechanically for any damage or defects.
3. Check that the CT connections are as per schematic drawings.
4. Check the clearance between the primary circuits and the secondary cir-
cuits on the CT especially for if there are any gas-insulated CTs.
5. Check the tightness of all CTs’ secondary terminations at the local con-
trol cubicle (LCC) panels, at relay panels, and at metering panels.
6. Check that the CT is grounded in a single point at LCC panels or on pro-
tection panels as per drawings.
7. Check that the CT has the correct shorting terminal blocks and the
ground cable has the correct cross-section area.

10.9.2 Commissioning Electrical Tests
The following electrical tests should be carried out.

10.9.2.1 Insulation Resistance Test
This test is performed to ground of the CT and wiring at 1000 V DC. There
should also insulation resistance tests and dielectric-withstand tests on the
primary winding with the secondary winding grounded. The voltage will be
applied as follows:
Between primary and secondary connected to the ground.
Between secondary to primary connected to ground.
Between secondary cores to cores if the CT has different cores.
With voltage of 1000 V DC applied for 30 seconds first, then up to
60 seconds as mentioned above; investigate any measurements less than
100 M Ohms.
The voltage withstand test is performed at 2000 V AC 50 Hz for 30 sec-
onds between the primary and secondary windings of the CT.