Transformer oil testing

Transformer oil, a type of insulating and cooling oil used in transformers and other electrical equipment, needs to be tested periodically to ensure that it is still fit for purpose. This is because it tends to deteriorate over time. Testing sequences and procedures are defined by various international standards, many of them set by ASTM. Testing consists of measuring breakdown voltage and other physical and chemical properties of samples of the oil, either in a laboratory or using portable test equipment on site.

The transformer oil (insulation oil) of voltage- and current-transformers fulfills the purpose of insulating as well as cooling. Thus, the dielectric quality of transformer oil is essential to secure operation of a transformer.

As transformer oil deteriorates through aging and moisture ingress, transformer oil should, depending on economics, transformer duty and other factors, be tested periodically. Power utility companies have a vested interest in periodic oil testing since transformers represent a large proportion of their total assets. Through such testing, transformers' life can be substantially increased, thus delaying new investment of replacement transformer assets.

Recently time-consuming testing procedures in test labs have been replaced by on-site oil testing procedures. There are various manufacturers of portable oil testers. With low weight devices in the range of 20 to 40 kg, tests up to 100 kV rms can be performed and reported on-site automatically. Some of them are even battery-powered and come with accessories.

To assess the insulating property of dielectric transformer oil, a sample of the transformer oil is taken and its breakdown voltage is measured. The lower the resulting breakdown voltage, the poorer the quality of the transformer oil.

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