Page 58 - Energize June 2022

TECHNICAL

Colour and appearance: This routine inspection is applied to water content can cause flashover in the unit, resulting in loss of
every oil sample. When an oil sample arrives at the laboratory, the asset.
one of the “tests” is a visual inspection of the oil sample in a
clear vessel to determine the colour, turbidity, and possible Water content (mg/kg at transformer operating temperature):
particle identification. Dark oils might indicate chemical This is a routine test for all classes of electrical equipment, except
degradation or contamination of the oil. When there is a lot class G. The results of this test should always be considered in
of turbidity, it might indicate a high-water content in the oil. conjunction with the breakdown strength. If it is found that the
If the drain valve was not wiped clean by the sampler, the dirt water content is high, and the breakdown strength is low, further
particles in the drain valve might be incorporated into the action needs to be taken. It is recommended that a second sample
sample. If particles are identified as carbon, it might indicate a from the same unit be tested to confirm the results. In the case
possible electrical fault in the unit. The DGA analysis of the oil of switching equipment, where there is no paper present, the
will confirm if this is the case. Clear oils without contamination breakdown voltage is the determining factor.
will indicate a good condition, and no action is recommended. It should be noted that the limits indicated by IEC 60422 Edition
When oils are dark or turbid, further analysis will confirm any 4 apply to transformers with operating temperatures between 40
problems. The oil analysis results will also determine the degree and 70°C. If it is found that the unit’s operating temperature is
and type of action. outside this temperature range, it is best to refer to Annex A of the
standard. When the value obtained through analyses is good, the
Breakdown voltage: This is a routine inspection. Breakdown normal sampling interval can be maintained, requiring no further
voltage will indicate the water content or the presence of action.
foreign particles, or both, in the oil being analysed. As the oil in • When the value returns a fair result, more frequent sampling is
transformers acts as an insulation medium to avoid flashover in recommended.
the unit, the breakdown voltage must be high.
• If the values are “good”, it is recommended to continue with It is also helpful to consider other parameters such as the
the current sample interval action plan. breakdown voltage, particle content and DDF/resistivity, and acidity
• If the values are “fair”, more frequent sampling is to decide on the action to be implemented.
recommended in collaboration with other parameter results • A poor result will require immediate action from the asset
such as the water content, DDF (dielectric dissipation factor), manager.
and acidity.
• If values are “poor” it is recommended to recondition the This might include taking another sample to confirm the results
oil. If alternative tests indicate severe ageing, the oil can be from the first analysis. If it is confirmed that the water content
replaced with new or reclaimed oil. Another option would be is high, the oil can be filtered; this process should remove a
to perform on-site oil reclamation using a reclamation plant. large portion of the moisture from the oil if applied correctly.
Reclamation of oil has the advantage that the colour of the Follow-up samples need to be taken to ensure that the moisture
oil is restored, and the polar components are removed from content is still within the required limits. The reason is that the
the oil. This process removes acid and water as well as some most significant portion of the water is absorbed in the paper
other compounds. of the transformer. This moisture will move from the paper into
the oil under conditions that favour this movement. It might be
Another advantage is that the oil can be re-used, and in most found later that the oil in the water has increased again without
situations, this can be done without switching off the unit, any apparent reason, but the source would be the paper in the
which contributes to cost-saving. If in doubt, switch off the transformer.
unit during this treatment process. If the values are poor, A visual inspection is also recommended to determine if any
it is advisable to act as soon as possible and not delay the water might move into the transformer or electrical equipment
maintenance process. through leaks. This problem might be more severe if the
Excess water in the transformer system decreases the transformer or electrical equipment is outside and not in a covered
projected transformer lifetime significantly; extremely high- area.

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