Page 87 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

4.16 CHAPTER FOUR

TABLE 4.2 The Eight Most Important ASTM Tests for In-Service Transformer Oil
ASTM test method Criteria for evaluating test results Information provided by test

D-877, Dielectric New oil should not break down Free water present in oil
breakdown strength at 30 kV or below
D-974, Neutralization Milligrams of potassium Acid present in oil
(or acid) number (NN) hydroxide required to neutralize
1 g of oil (0.03 or less of new oil)
D-971, Interfacial Dynes per centimeter (40 or higher Sludge present in oil
tension (IFT) for new oil)
D-1524, Color Compared against color index scale Marked change from one year to
of 0.5 (new oil) to 8.0 worst case next indicates a problem
D-1298, Specific Specific gravity of new oil is Provides a quick check
gravity approximately 0.875
D-1524, Visual Good oil is clear and sparkling, Cloudiness indicates presence of
evaluation of not cloudy moisture or other contaminants
transparency/opacity
D-1698, Sediment None/slight/moderate/heavy Indicates deterioration and/or
contamination of oil
D-924, Power factor Power factor of new oil is 0.05 Reveals presence of moisture,
or less resin varnishes, or other prod-
ucts of oxidation in oil, or of
foreign contaminants such as
motor oil or fuel oil

Note: For comparative purposes, specifications of new insulating oil can be obtained
from ASTM D-3487-77, Standard Specifications for Mineral Insulating Oil in Electrical
Apparatus, available from the American Society for Testing and Materials, 1916 Race St.,
Philadelphia, PA 19103.

Causes of Deterioration

Oxygen, heat, and moisture have adverse effects on oil. Oxygen is derived from the air that
entered the transformer and from the transformer oil. Oxygen is liberated in some cases by
the effect of heat on cellulose insulation. The natural oxygen inhibitors in new insulating
oil deplete gradually with time. Thus, the oxidation rate of the oil increases steadily while
the oil is in service.
Pure hydrocarbons do not oxidize easily under normal conditions. The American
Society for Testing and Materials (ASTM) has established that oil oxidation results gener-
ally from a process that starts when oxygen combines with unstable hydrocarbon impuri-
ties. The metals in the transformer act as a catalyst for this combination. Acids, peroxides,
alcohols, and ketones are the product of oxidation. The oxidation process results in contin-
uous detrimental action on the insulating materials of the transformer. Sludge will eventu-
ally form. Greater damage will be caused by sludge formation. This is due to the inability
of the sludge to circulate and remove the heat buildup.
The most important tests of the eight ASTM tests listed in Table 4.2 are neutralization
number (NN) and the D-971 for interfacial tension (IFT). This is so because these tests deal
directly with the acid content and the presence of sludge. The two tests provide a quantita-
tive description of the condition of the oil.

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