Colour
An observation of the darkening of the oil by comparing it to previous samples of oil from the
same transformer.
Viscosity
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Total Acid Number
IEC 62021-1
The TAN value indicates the potential of corrosion problems within a system. As the acid number increase (usually due to oxidation of the oil) the insulating quality of the oil (and paper) decreases.
Flash Point
Flash point is a physical property of the oil. Transformer oil has to operate safely within the transformer environment. When standard design practices began to incorporate higher hot spot temperatures, the specification limit for flash point of the oil had to be increased to maintain a margin of safety.
Moisture Content
IEC 60814
Moisture can reduce transformer oil’s effectiveness as an insulator.
A transformer with less than one percent moisture content in the paper could typically last 40 years, whereas a moisture content of four percent or higher could reduce the anticipated life to 10 – 15 years.
Dielectric breakdown
voltage –
IEC 60156/1995-05
A measure of the ability of the oil to withstand electric stress (voltage) without failure. Oils with high breakdown voltage are deemed good electrical insulators. Contaminants such as water, sediment and conducting particles (dirt) reduce the breakdown voltage of insulating oil.
Dissolved Gas Analysis – ASTM D3612
The amounts and types of gases found in the oil are indicative of the severity and type of fault occurring in the transformer. The types of gases we are concerned with are hydrogen, methane, ethane, ethene, acetylene, carbon dioxide and carbon monoxide.
This indicates the following types of faults:
Dissipation & Resistivity Factor – IEC 60247
The dissipation test measures the leakage current through oil, which is the measure of the contamination or deterioration. The test is not specific in what it detects i.e. more a screening test. Resistivity is a reliable indicator of the condition of transformer oil, low resistivity values being a sign of oil, which contains particulate contaminants and oxidative products.