Page 90 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

TRANSFORMER COMPONENTS AND MAINTENANCE 4.19

TABLE 4.4 Correlation of Neutralization Number and Interfacial Tension to
Transformer Sludging*

Neutralization number, mg KOH/g of oil Number of units found sludged
0.00–0.10 0
0.11–0.20 190
0.21–0.60 360
0.60 or higher 500
Interfacial tension, dyn/cm Number of units found sludged
24 or higher 0
22–24 150
20–22 165
18–20 175
16–18 345
14–16 425
14 or less 500
*Study conducted by ASTM from 1946 to 1957 on 500 transformers that had been in
service for some time. Study is described in ASTM Special Publication No. 218, Evaluation
of Laboratory Tests as indicators of the Service Life of Uninhibited Electrical Insulating
Oils (1957).

tests are needed. Degradation products released by arcing can cause this odor. However, the
odor of arc-over products can be masked by the odor of acid if the oil has a high acid con-
tent. The oil should be tested by gas chromotography if there is a reason to suspect that dis-
solved combustible gases are present.

Methods of Dealing with Bad Oil

There are two options when tests indicate the oil is in proposition A (second classification)
range or worse: replacement or reclamation of the oil. The reclamation process involves
these steps:
1. Dissolve the sludge on the internals of the transformer.
2. Purge the sludge from the transformer.
3. Filter the sludge from the oil to restore it to “like new” condition.
The sludge deposited on the internal components of the transformer will not be removed
when the oil is replaced. Simple replacement will only put new oil into a contaminated con-
tainer. There are also handling and disposal problems with the replacement option.
The same process and same equipment are used to reclaim the oil regardless of the
degree of deterioration. Reclamation is relatively simple if the oil is in the proposition A
range. If the oil has deteriorated further, a more extensive treatment will be required.
The reclamation process is performed in a closed loop. Special equipment is used to
continuously heat the oil, filter it through absorbent beds, and recirculate it in the trans-
former. The heated oil is maintained at its aniline point (82°C) during the process. The hot
oil acts as a strong solvent for decay products. The sludge is removed usually by 6 to 10
recirculation cycles. Twenty recirculations may be required to desludge the transformer if
it is badly sludged (oil with NN 0.3 and IFT 18).

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