Page 84 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

TRANSFORMER COMPONENTS AND MAINTENANCE 4.13

trical testing. Therefore, advanced deterio-
ration would have already occurred by the
time the evidence appeared in the test data.
A suitable solution involves continuous
dehydration of the transformer during nor-
mal operation. A less expensive solution
involves servicing the transformer every 3
to 5 years.
The amount of moisture in the paper of
a transformer is expressed as percent mois-
ture by dry weight (%M/dw). The aging
factor is controlled by controlling the
%M/dw. The upper limit of 0.5 %M/dw FIGURE 4.8 Water in microscopic droplets that
cause the Kraft paper to dissolve from oil decay
should be specified when a transformer is products.
selected. If this limit is not specified, the
moisture content could be as high as 1.5 to
2.0 %M/dw.
Figure 4.9 illustrates the aging factor versus %M/dw. As shown, moisture weakens the
Kraft paper. This weakening is measured and expressed by the aging factor (AF). Notice the
significant increase of the AF with %M/dw. Figure 4.10 illustrates the significant decrease in
transformer life expectancy with %M/dw.
Most transformers fail at the bottom due to high %M/dw. This is caused by the fact that
paper has up to 3000 times higher affinity for water than oil does at lower temperatures.

TRANSFORMER OIL

The oil acts as insulation and a cooling medium for the transformer. The mineral oil used
in transformers normally has these features:
1. It has a high flash point to minimize fire hazard.
2. It is nonvolatile at operating temperatures to avoid evaporation losses.
3. It has a low pour point.
4. It remains as a liquid at the lowest ambient temperatures expected.
5. It is stable and inert.
6. It resists oxidation which increases acidity and formation of sludge.

Table 4.1 lists all the oil tests and when they are normally performed. The private indus-
try usually performs the following tests only: dielectric breakdown, neutralization number
(NN), interfacial tension (IFT), specific gravity, color, and visual exam. The remaining
tests may be done when necessary.
The visual examination consists of checking the color of the oil. The color varies from
the clear (like water), with acidity of 0.01 to 0.03 mg KOH/g and interfacial tension of 30
to 45 dyn/cm; to black with acidity of 1.01 mg KOH/g and higher and interfacial tension of
6 to 9 dyn/cm. The acceptable conditions are clear to amber (acidity: 0.15 mg KOH/g; inter-
facial tension: 22 dyn/cm).

Testing Transformer Insulating Oil
The functions of the oil in the transformer are to

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