Sampling And Testing Oil For Dielectric Strength - Megger OTS 60AF/2 Guide D'utilisation

Sampling and Testing Oil for Dielectric Strength

The following notes are mainly intended for the guidance of those whose experience in the subject may
be limited. The 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.
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 run off while taking extreme care to prevent the ingress
of atmospheric dust, cloth fibres or moisture. Do not use a syphon. In the absence of a useable sample
point, use a 'thief'.
Shield the sample from direct light until ready to be tested.
Turbulence and air bubbles should be avoided when pouring the oil. Relevant national specifications
should be observed.
Particular care to prevent contamination is necessary when a test is to be made on a bulk delivery
tanker, and on all occasions when a high value of dielectric strength is anticipated. The procedure
suggested above should be rigorously applied.
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