Page 88 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

TRANSFORMER COMPONENTS AND MAINTENANCE 4.17

The Neutralization Number Test
The NN test (ASTM D-974) determines the acid content of the oil. An oil sample of known
quantity is titrated with the base potassium hydroxide (KOH) until the acid in the oil has
been neutralized. The NN is expressed as the amount of KOH in milligrams required to
neutralize 1 g of oil. A high NN indicates high acid content.
The acid formation in the transformer begins as soon as the oil is placed in service. Figure
4.11 illustrates the increase of NN with time and temperature. Electromechanical vibration,
mechanical shock, and especially heat will accelerate the normal deterioration of the oil. Even
minute amounts of water will enhance the oxidation process and the formation of acids. The
copper or copper alloys in the windings will act as a catalyst for this reaction.

The Interfacial Tension Test
The ASTM D-971 test for interfacial tension determines the concentration of sludge. In this
test, a platinum ring is drawn through the interface between distilled water and the oil sam-
ple. A delicate balance (Cenco DuNuoy Tensiometer) is used to draw the ring. The test
results are expressed in dynes per centimeter. This test gives good indication about the pres-
ence of oil decay products. The IFT of new oil is more than 40 dyn/cm. The IFT of badly
deteriorating oil is less than 18 dyn/cm.

The Myers Index Number
A high IFT indicates that the oil is relatively sludge-free. Therefore, it will be purer than an
oil with low IFT. Conversely, when the oil has a high acid content and bad deterioration, it

NN, mg KOH/gram of oil 70°C Critical NN = 0.25 <60°C

60°C

Time
FIGURE 4.11 Typical pattern of increase in NN as a func-
tion of time is exhibited by curve for transformer oil operated
at 60°C. The exponential rise in NN at the critical point results
from the catalytic action of acids and the depletion of the oxi-
dation inhibitors. Heat is the greatest accelerator of oil deteri-
oration; deterioration is most marked above 60°C. Beyond
60°C, the rate of deterioration approximately doubles for each
10°C increase.

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