Transformer oils are subject to electrical and mechanical stresses while a transformer is in operation. In addition there is contamination caused by chemical interactions with windings and other solid insulation, catalyzed by high operating temperature. The original chemical properties of transformer oil change gradually, rendering it ineffective for its intended purpose after many years. Oil in large transformers and electrical apparatus is periodically tested for its electrical and chemical properties, to make sure it is suitable for further use. Sometimes oil condition can be improved by filtration and treatment.
Tests can be divided into:
Dissolved gas analysis
General electrical & physical tests:
Color & Appearance
Acidity (Neutralization Value)
Dielectric Dissipation Factor
Sediments & Sludge
The details of conducting these tests are available in standards released by IEC, ASTM, IS, BS, and testing can be done by any of the methods. The Furan and DGA tests are specifically not for determining the quality of transformer oil, but for determining any abnormalities in the internal windings of the transformer or the paper insulation of the transformer, which cannot be otherwise detected without a complete overhaul of the transformer.
Suggested intervals for these test are:
General and physical tests - bi-yearly
Dissolved gas analysis - yearly
Furan testing - once every 2 years, subject to the transformer being in operation for min 5 years.