Sampling And Testing Oil For Dielectric Strength - Megger OTS60SX Guia Del Usuario

Sampling and Testing Oil for Dielectric Strength

The following notes should be read in conjunction with the relevant
oil testing specifications.
Causes of Bad Oil
Transformers and switchgear oil may be rendered unsuitable for
further use due to four main reasons:-
1. Low dielectric strength.
2. High acidity.
3. High sludge content.
4. Excessive free water content.
Low dielectric strength may result from many causes, the most
common of which is foreign particles or fibres and water in
combination. Individually their effect may be relatively small, but
together a contamination of only a few parts in a million can cause
considerable lowering of the breakdown voltage of the oil. High
acidity, sludge and free water should not be tolerated, but will not
necessarily reduce the dielectric strength below acceptable or
specified levels.
Appearance of the Sample
Only an experienced person can judge the condition of an
insulating oil from its appearance, but a general guide may be
obtained from the following observations:-
1. A cloudy appearance may indicate that sludge has
been formed.
2. A dark yellow colour could be a sign of overheating.
3. A blackish colour often results from an arc having taken
place with either carbonisation of the oil or of the
insulation within the equipment.
4. A green colour may be due to copper salts dissolving
in the oil.
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Cleanliness of the Apparatus
The necessity for scrupulous cleanliness in the apparatus and
during the process of sampling, cannot be over emphasised. The
measurement of the dielectric strength of the sample is as
dependent on the cleanliness of the test cell and the sampling
apparatus as the condition of the oil itself.
After cleaning the apparatus, it should under no circumstances be
wiped, dried or even handled with a dusty or fibrous cloth as loose
dust or fibres are liable to cause contamination.
Precautions Necessary During Sampling
Since the oil taken from a drain cock will inevitably contain an
excess of settled out solid impurities, and will not necessarily be
representative of the bulk of the oil, it should be run to waste until
clear.
Always run off a quantity of oil into a glass bottle or a test tube for
an initial check on the appearance of the oil. Attention to this point
may prevent contamination of a clean test vessel.
If necessary, thoroughly clean and dry the sample point using a
suitable solvent. Any cloth used should be lint free. Open the
sample cock and drain to waste enough oil to ensure that the
sample cock is fully flushed and the sample is representative of
the bulk of oil to be tested. This quantity will depend upon the size
of the transformer or main container. About 2 litres is usually
suitable.
When sampling from a drum or supply container, the oil should
flow at a steady rate into the test vessel and after being swilled
around the sides it should be discarded. Without altering the rate
of flow of the oil the quantity required for the test should then be