CP TD1 Reference Manual V 1.44
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used to test bushings in apparatus when the three tests are either inapplicable
or impractical, such as, with SF6 bushings. Perform a hot-collar test at every
third skirt on SF6 bushings. Hot-collar tests are effective in locating cracks in
porcelain, deterioration, or contamination of insulation in the upper section of a
bushing, low compound or liquid level, or voids in compound often before such
defects are noticeable with the previous tests.
Figure 7-6 Bushing "hot collar" test
In tables of the appendix, typical dissipation factors and dissipation factor the
manufacturers initially published limits. However, the typical or initial dissipation
factor of many bushings is listed on the nameplate. In such cases, field
measurement, particularly UST, should compare with the nameplate dissipation
factors. In general, any bushing that exhibits a history of continuing increase in
dissipation factor should be questioned and scheduled for removal from service.
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
Mounting flange
Paper insulation
Main conductor
GST
IN A
IN B
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