Electrical transformers usually have their “under tension” parts immersed in a dielectric oil to insure good insulation and heat evacuation. When aging, this oil is accumulating particles such as water, dust… leading to a premature wear and a decreasing efficiency of the transformer. Having a periodical check of these oil is the best way to avoid problems and expensive materials replacement.
Several parameters can be analyzed, each of them informing on a specific potential problem of the transformer:
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Dielectric strength (KV)
Informs about the breakdown voltage of the transformer. If the value is too low, water of other particles might be present in the transformer which can ultimately lead to a failure of the equipment.
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Presence of water (mg / Kg of oil)
Informs about the quantity of water presents in the oil and by extrapolation the quantity of water in the cellulose insulation around the transformer winding. It will also confirm the result of the dielectric strength. When the temperature is rising, the water is moving from the cellulose insulation to the oil reducing its isolating power which can ultimately lead to the explosion of the equipment.
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Acid number
Informs about the age of the oil which oxidize in the presence of oxygen, cooper, etc.. This acidity will damage the isolation around the winding and increase the risk of short-circuits (also known as “ignition”). Those problems usually appears with voltage dips and lightning chocks and will lead to a premature wear of the transformer.
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Dissolved gases (ppm)
These gases (hydrogen, oxygen, azote, carbon dioxide; etc…) are naturally present in the transformer but abnormally high values can inform about an air leakage or a deterioration of the winding isolation. The risk is an explosion of the transformer.
Maintainers tend to perform this kind of tests every 5 years.
Greensolver strongly advises to request such oil analysis every 2 to 3 years and to keep track of the results, as the evolution of the different tests is even more important than their absolute value.
