CP TD1 Reference Manual V 1.44
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like the bushings and the transformer tank (flashovers) with the
unscreened part of the high-voltage test cable must be avoided.
• Connect the IN A, e.g. to the low-voltage winding, IN B, e.g. to the tertiary
winding (according to the connection instructions).
• Short circuit all bushing CTs.
• Do not make high-voltage tests on transformers under vacuum.
• The test voltage should be chosen with respect to the rated voltage of the
winding.
• All tests should be made with oil temperatures near 20°C. Temperature
corrections can be calculated by using correction curves, but they depend
a great deal on the insulation material, the water content and many other
parameters. This way the correction has limited accuracy.
6.2.1 Three-Winding Transformer
A transformer contains a complicated insulation system. High- and low-voltage
windings have to be insulated to the tank and the core (ground) and against each
other. All these insulation gaps should be checked regularly. Normally in a two-
winding power transformer, C-Tan-Delta measurements are made for all
insulation gaps: HV to LV, HV to ground, LV to ground.
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