Transformer Oil - Testing and Oil Quality

Testing and Oil Quality

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. As a result the original chemical properties of transformer oil changes gradually, rendering it ineffective for its intended purpose after many years. Hence this oil has to be periodically tested to ascertain its basic electrical properties, make sure it is suitable for further use, and ascertain the need for maintenance activities like filtration/regeneration. These tests can be divided into:

1. Dissolved gas analysis
2. Furan analysis
3. PCB analysis
4. General electrical & physical tests:

• Color & Appearance
• Breakdown Voltage
• Water Content
• Acidity (Neutralization Value)
• Dielectric Dissipation Factor
• Resistivity
• Sediments & Sludge
• Interfacial Tension
• Flash Point
• Pour Point
• Density
• Kinematic Viscosity

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.