The frequency of transformer oil examination depends on:

  • Nominal voltage of the transformer,
  • Power transformer
  • Service and maintenance conditions
  • The results of previous oil analysis,
  • Type of oil used.

The table presents recommendations for the frequency of transformer oil examination for the most commonly encountered operating conditions depending on the rated voltage according to IEC 60442 norms.  In planning subsequent test dates it is important to take the results of previous tests into consideration.  The frequency of such tests should be increased in the event of unsatisfactory data.

Assessment of the presence of overheating or partial and arc discharge as well as thermal degradation is carried out based on the content of gases dissolved in the oil. The type of gas produced in oil depends on the nature and temperature of the phenomenon, the general principle is shown in following figure.

Oil TestRated voltage of transformer (kV)

acid count, breakdown voltage appearance, water content, dielectric loss factor, resistance
< 72,5 kV> 72,5 - 170 kV> 170 kV

2 - 6 years

1 - 4 years

1 - 2 years
Supplementary examination:

surface voltage

Necessary to carry out when acidity content of dielectric loss factor or tone are below recommended requirements

Specialist testing:

corrosive sulfur, PCB, viscosity, density, combustion temperature

Necessary to carry out in case of special circumstances occurring