CP TD1 User Manual
62 OMICRON
Figure 6-5: "Hot collar" test
Measured dissipation factor values should be temperature corrected to 20°C before being compared
with reference values which are measured at 20°C.
Temperature correction factors are average values at best, and therefore, subject to some error. The
magnitude of error is minimized if tests are performed at temperatures near the reference temperature
of 20°C. If questionable dissipation factors are recorded at relatively high temperatures then the
bushings should not be condemned until it has been allowed to cool down to near 20°C and repeat tests
have been performed. This also applies to bushings tested near freezing where a large (greater than
1.00) correction may cause the result to be unacceptably high; in this case the equipment should be
retested at a higher temperature. Bushings should not be tested when their temperatures are much
below freezing because moisture may have changed to ice, which has a significantly higher volumetric
resistivity and therefore may be undetected. In the case of bushings mounted in transformers, taking the
average between the ambient and transformer top-oil temperatures approximates the bushing
temperature.
Bushing capacitance should be measured with each power or dissipation factor test and compared
carefully with both nameplate and previous tests in assessing bushing condition. This is especially
important for capacitance-graded bushings where an increase in capacitance of 5 % more over the
initial/nameplate value is cause to investigate the suitability of the bushing for continued service. The
manufacturer should be consulted for guidance on specific bushings.
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