Transformer oil testing: what BDV, moisture and DGA tell you
Insulating oil is the cheapest diagnostic sample a transformer gives you. Here is what the common tests measure and what their results mean.
Trafo Power engineering team2 min read
The oil in a transformer insulates, cools and carries evidence. A small sample, taken correctly and tested regularly, shows how the insulation is ageing and can reveal developing faults long before they trip the unit.
## Breakdown voltage (BDV)
BDV is the voltage at which oil between two test electrodes breaks down. It falls sharply with moisture and particles, so a low or falling BDV is an early sign that the oil needs filtration or the breather and gaskets need attention. The test method is defined in IEC 60156.
## Water content
Moisture is measured in parts per million by Karl Fischer titration (IEC 60814). Water in oil migrates to and from the paper insulation with temperature, and wet paper ages faster and lowers dielectric strength. Trend the result together with the oil temperature at sampling.
## Acidity and interfacial tension
As oil oxidises it forms acids and sludge. Rising acidity (neutralisation number) and falling interfacial tension indicate oxidation, which accelerates paper ageing and can block cooling ducts with sludge.
## Dissolved gas analysis (DGA)
Faults inside a transformer break down oil and paper into characteristic gases. DGA measures them and IEC 60599 gives methods to interpret the pattern:
- Hydrogen points to partial discharge.
- Methane, ethane and ethylene indicate thermal faults, with ethylene rising at higher temperatures.
- Acetylene signals arcing or very high-temperature faults and needs prompt attention.
- Carbon monoxide and carbon dioxide show that paper insulation is involved.
The rate of change matters as much as the absolute value, so compare each result with the unit's own history.
## How often
A common practice is BDV and moisture at least yearly for distribution transformers, and DGA yearly or more often for power transformers and critical units, with extra samples after a through-fault, a protection trip or a sudden change in a trend.
General maintenance guidance. Test limits depend on the voltage class, the oil type and the applicable standard.
