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• All the tools or strange objects have been removed from the transformer.
• Any bolted connection i.e., H.V. bushings flange, etc are properly tightened.
• No oil leakages or stains are present
• The oil level is the correct one.
• All levels, oil and temperature are correct.
• All the electric connections are properly tightened.
• General cleaning is made and especially the bushings porcelain (see maintenance
chapter).
• The bushings are in good condition (without blows or fissures).
• People are not near to the three phase pad-mounted transformer previous to be
energized.
If the transformer has been stored more than six months it is recommended to run the
following field tests:
• Verify the isolation resistance of each winding to ground and between windings.
• Verify power factor of each winding to ground and between windings.
• Turn ratio shall be determined for all taps and for the full windings.
• Verify that the dielectric strength of the insulating liquid remains with values superiors to
30kV/mm.
7.5 Recommended Torque
It should be assured, mainly in the maintenance tasks that the torque used on the
transformer screws fulfill that indicated in this section. Fulfill this point will contribute to avoid
oil leakages or the entrance of humidity into the transformer.
Most of transformers have a plate in which the following table has been recorded.
Recommended torque
Screw
Torque(N.m)
Screw
Torque(N.m)
M8 16 M22 370
M10 30 M24 480
M12 50 M27 720
M14 95 M30 860
M16 130 M33 1030
M18 195 M36 1230
M20 240 M39 1500
User Manual 7 Service Preparation
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