Page 59 - Energize June 2022

TECHNICAL

Acidity (mgKOH/g oil) neutralization number: This is a The structure of the oil is damaged, in effect that the chemical
routine test for all classes except F and G. The acids in oils are bonds between the molecules have broken down, and even with
formed due to chemical reactions between the oil, water, and filtration,the recommended dielectric values can’t be achieved.
paper. Higher temperatures or load increases will assist in the
formation of the acids. Because acids are polar compounds, this Resistivity (GΩm) at 20 or 90°C: This is not a routine test - DC
will adversely affect the insulation properties of the oil and will resistivity of the oil is one of the key parameters to assess the
increase paper degradation. If left untreated in transformers, transformer insulation condition; this is based on the fact that DC
this can lead to sludge formation, usually around the lower resistance is sensitive to oil degradation.
parts of the transformer core. The sludge will eventually form a • When the result is fair, more frequent sampling and checking
semi-solid substance that is extremely difficult to remove. additional parameters is recommended.
• If the result is good, the regular sampling interval can continue. • When the result is poor, reclamation or an oil change is
• In case of a fair result, the sampling interval should be decreased recommended.
to fit the situation. Future analysis should include a visual
inspection of the oil for sediment and sludge.
• If the result is poor according to the prescribed values in IEC
60422 Edition 4.0, the asset manager may decide to reclaim
the oil or replace it with new or reclaimed oil, whichever option
might suit their requirements the best.

Dielectric Dissipation factor at 40 to 60 Hz at 90°C: This is Inhibitor content percentage: This test is restricted to oils with this
a routine test for all classes of electrical equipment, except additive. It would be advisable to contact the oil supplier to verify
classes F and G. The dielectric dissipation factor or tan delta the details regarding additives. The two most common oxidation
of this test provides information regarding the extent of the inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol
dielectric losses in transformer oil. This test measures the (DBPC) and 2,6-di-tertiary butyl-phenol (DBP). The purpose of
inefficiency of insulating material. When oil ages, we have the the inhibitor is to prevent oxygen from reacting with the oil. This
formation of polar compounds, leading to phase displacement significantly slows the ageing process in the oil and the solid
and dielectric losses. Other impurities that might influence the insulation.
dissipation factor include water, dissolved insulating resin and • If the result is fair, it is advised to top up the inhibitor level to the
paper. prescribed level as per supplier instructions. It is advisable to use
• When the result is fair, more frequent sampling and checking a field professional trained in the procedure to perform this task.
additional parameters is recommended. • If the result obtained is poor, the recommendation for this
• When the result is poor, reclamation or an oil change is scenario would suggest that the end user continues to use the
recommended. oil “uninhibited,” but this may lead to more rapid degradation of
both the liquid and solid insulation.

It should be noted that some transformers already have a built-
in oil preservation system which is designed to keep dissolved
oxygen at levels below 1000 ppm. This would be in the form of a

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