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Appendices
Transformer Commissioning
23
5.0 COMMISSIONING
5.1 PURPOSE
5.1.1
After the satisfactory completion of installation, the following pre-
commissioning checks and tests on instruments must be performed before
putting the transformer into service. Prior to commissioning work, for specific
help and information on the accessories supplied, refer to the manufacturer’s
instruction booklet / product catalogue, etc., furnished with the handing-over
documents.
5.2 COMMISSIONING TESTS
5.2.1
Insulation Resistance (IR) Test:
a) Before starting this test all the power terminal bushings should be
thoroughly cleaned with a dry clean piece of cloth.
b) During IR test, no external power lines / cables, lightning arresters,
neutral earthing, etc., should be in the power circuit. And ensure that
transformer is completely isolated at HV & LV sides and all non-current
carrying conductors are earthed.
c) At all the tap positions, IR values of windings to earth & between
windings shall be measured with designated insulation tester of
suitable ratings and readings noted.
Between HV Winding and Earth use 5000V or 2500V Insulation
Tester (Megger)
Between HV and LV Winding
use 5000V or 2500V Insulation
Tester (Megger)
Between LV Winding and Earth use 1000V or 500V Insulation
Tester (Megger)
d) IR values obtained should be similar to those indicated in the
manufacturers’ test report, furnished with the handing-over documents.
In humid weather, IR values obtained may be lower due to
condensation on the terminal bushings.
e) If IR values are very low and unacceptable, then it may be necessary
to filter the oil / dry-out the winding till the insulation reaches
satisfactory values.
5.2.2
Break-Down Voltage (BDV) Test:
a) Oil samples from tank bottom, tank top, radiator, etc. shall be carefully
taken and tested for BDV value, as per clause 4.3 of this manual.
b) BDV value of oil should be more than 50kV(rms) for 1 minute in
standard test cell.
c) If BDV value is very low and unacceptable (30kV(rms) or less for 1
minute, then it may be necessary to dry-out & clean the oil till the
insulation reaches satisfactory values.
Note: For very low IR values and low BDV values, it is recommended
to contact suppliers or manufacturer for suitable recovery procedure
based on available facilities at site.
