Transformer Oil Maintenace

Transformer oil is used to insolate and cool the heated parts of the  electric power transformer. The main performance characteristic of this dielectric oil product is viscosity and viscosity is greatly affected by temperature.  If the temperature remains above 0ºС/32ºF, oil becomes less viscous(thinner).  Otherwise, as the temperature falls below the freezing point of water, the product becomes more viscous or thicker.  The thicker the oil becomes due to low temperature extremes,  the ability to flow and exchange heat is significantly reduced. In consideration of these factors, the selection of the correct viscosity should be made before the equipment is filled with transformer oil.  Equipment operators should monitor the the following indexes and not allow the oil’s performance characteristics to fall below the ones that are defined in the table below.

Transformer Class Voltage, kV

up to 15 15-35 60-220 330-500 750
Breakdown voltage, kV






The acid number

Not more than 0.08 mg KOH/g oil

The content of water-soluble acids and alkali

Not more than 0.014 mg KOH/g oil for transformers with the capacity of 630 kVA and more and for air-tight oil-filled bushings

The contents of the mechanical impurities


The temperature fall of  the oil flash in transformes

Not more than 5 ºC as compared with the previous analysis

Dielectric loss tangent for transformers and bushings at 70 ºC

Not more than 7% for oil in the equipment with a capacity of up to 220 kV