Page 91 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

4.20 CHAPTER FOUR

40
A
Good
35 oil

30 Prop A
Interfacial tension, dynes/cm 25 Bad oil Extremely Oil in
Marginal oil

20
Very
bad oil
15
condition
10 bad oil disastrous
B
5

0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.5
Neutralization number, mg KOH/gram of oil
FIGURE 4.12 Plot shown was developed from the results of more than
10,000 IFT/NN tests. If a plot of IFT versus NN for a given oil sample does
not fall within the range shown on either side of the median line (as in the
case of points A and B), further investigation is in order. Additional tests (see
Table 4.2) should be conducted. The results should be evaluated in combina-
tion to get a true picture of the condition of both the oil and the transformer.

Many separate reclamation/desludging treatments are needed if the transformer is in
extremely sludged condition (NN 1.5). It is recommended to consider replacing the
transformer at this stage. Reclamation/desludging should be performed before NN exceeds
1.5 and IFT drops below 24.0, to ensure adequate operation and minimum deterioration of
the insulation. The oil can be reclaimed/desludged while the transformer is online (ener-
gized and in service) by properly equipped maintenance contractors. Therefore, there is no
reason to allow the oil to deteriorate until an expensive treatment is required.
It is essential to have adequate cleanliness and quality control while obtaining the test
results. Dirty containers or laboratory equipment should not be used for the oil sample. It
is very important to protect the sample from light, air, and moisture. The sample should be
taken quickly, sealed in a container, and tested within hours. It must not be exposed to sun-
light. Aging will accelerate if the sample is exposed to sunlight. The results of the sample
could be misleading in this case.

Gas-in-Oil
The analysis of dissolved combustible gases in transformers is highly indicative of possi-
ble trouble. All transformers will develop a certain amount of gases over their lifetime. The
two principal causes of gas formation within an operating transformer are thermal and elec-
trical disturbances. Heat losses from the conductors produce gases from decomposition of

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