Oxidation Stability of Insulating Oil

The insulating oil reacts with oxygen and generates organic acids, sludge, water, and other matter. These contaminants significantly degrade the electrical characteristics of the insu­lating oil. The organic acid and the moisture advance corro­sion and other deterioration of materials in contact with the oil, and sludge lowers its cooling efficiency. Therefore, oxida­tion stability of the insulating oil is important for the life and reliability of oil-filled electrical equipment. It is known that the copper used for conductors in electrical equipment can catalyze the oxidative deterioration of the insulating oil (5,6).

The oxidation stability of the insulating oil is evaluated from the amount of sludge, total acid number, and electrical characteristics after heating the insulating oil in contact with excess oxygen and copper. Test methods are described in IEC Publications 74, 474, 813; ASTM D 1313, 1934, 2112, 2440; BS 148; DIN 51554; and JIS C 2101. Tests are carried out at 100°C to 120°C. Since ASTM D 1313 uses no catalyst, that test is done at the highest temperature (140°C).

The oxidation stability of mineral oil is influenced by its degree of refinement. However, higher refinement does not necessarily mean higher stability. Research on improved oxi­dation stability of insulating oil is often done from the view­point of optimum aromaticity.

tan 8 of Insulating Oil in Relation to Oxidative Deterioration

The insulating oil of oil-immersed electrical devices that are equipped with oxidation deterioration prevention devices does

Operation (year)

0 5 10 15 20 25

Figure 4. tan S behavior for actual transformer oil in the field (A) and insulating oil in laboratory data (B).

Aging time (h)

not come in direct contact with the atmosphere. Therefore the oxidative deterioration of the insulating oil is slow. As shown in Fig. 4, the temperature dependence of tan S for insulating oil shows peculiar behavior.

In Fig. 4, the band marked A shows the deterioration of tan S in the insulating oil in a nitrogen-enclosed transformer and in a sealed transformer, at 80°C, gathered from many transformers over years of operation. The time dependence of tan S yields an N-shaped curve. A peak is observed at 5 to 7 years after the start of operation. This behavior is confirmed in laboratory experiments. It implies that oxygen and copper play an important role.

Curve B shows laboratory data on the deterioration of in­sulating oil where the copper surface area was 44.8 cm2 per 100 mL of oil, the oxygen volume was 5 mL per 100 mL of oil, and the oil temperature was 95°C (7).

From comparison of curves A and B, one hour of deteriora­tion as accelerated in the laboratory is seen to be equivalent to about one year in operation.

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