Transformer Oil Test

Periodic inspection on Transformer for example take oil sample for test may prevent abnormalities from being detected early, resulting in malfunction or deterioration of performance.


Procedure:
1. Use glass receptacles so that if any water present it may be readily observed.


2. Rinse sample containers with dry non-leaded gasoline or dry water-white kerosene until they are entirely clean.

Invert and drain to remove excess solvent, and then wash them with strong soapsuds, rinse throughout with water and dry them preferably in an oven at 105C to 110C.


After drying, store them un-stoppered in a dry, dust free cabinet or compartment at a temperature of not less than 35C.

If they are not stored in a hot cabinet, cork or cap the containers immediately after drying.

3. Take samples when the oil is at least as warm as the surrounding air.

Insulating oil is not hygroscopic but cold oil may condense enough moisture from a humid atmosphere to seriously affect its insulating properties.

Take sample from outdoors transformer on a clear day only, and guard against contamination by wind-blown dust, etc.


4. Before taking samples from the transformer, carefully clean the sampling valve and allow enough oil to run out so that any contamination that may have collected in the valve will be removed.


5. Rinse the container several times with the same oil as the sample to remove any contamination that may have collected in the container.

Do not mix these rinsing with oil sample.


6. Carefully seal the containers to prevent leakage or exposure of the oil to atmosphere. Use glass stoppers, clean corks or screw caps to seal the glass receptacles.

If corks are used or if the screw caps contain cork disc inserts, the cork should be covered with tin or aluminum foil.


Testing For Dielectric Strength.
Since the oil is major insulation of the transformer, its dielectric strength must be kept up to the specific values.

If the oil fails to withstand the minimum breakdown kV specified, it is a sign that impurities, particularly moisture have entered it. In this event the oil no longer safe for use as an insulating medium and must be filtered to remove and bring it back to its original conditions.


Colour Test.
The colour test is not too reliable a test in itself, since darkening of the oil does not indicates any particular conditions with certainty.

Discolouration is a rough indication of oxidation but this should be verified by making an acidity (neutralization) test.

In general, the best indication that a colour test can give is that the condition of the oil is doubtful and further test should be made.


These tables below related with Dielectric strength of oil before oil filing and vacum treatment.















These tables below related with degree of vacuum under oil filling, final check of oil quality, energization and dielectric strength of oil for on load tap changer.














Adopted from Toshiba.


















Adopted from Fortum

More info about transformer life time, please click http://idpowerstation.blogspot.com/2011/10/transformer-life-time.html

Note:
IEEE C57104-1991 is Institute of Electrical and Electronic Engineer (IEEE).
TDCG is Total dissolved Combustible Gas.

More information click this http://www.satcs.co.za/Transformer_Oil_Analysis.pdf