Transformer Oil

As already mentioned, transformer oil is the most commonly used liquid dielectric in
power apparatus. It is an almost colourless liquid consisting a mixture of hydrocarbons
which include paraffins, iso-paraffins, naphthalenes and aromatics. When in
service, the liquid in a transformer is subjected to prolonged heating at high temperatures
of about 950C, and consequently it undergoes a gradual ageing process. With
time the oil becomes darker due to the formation of acids and resins, or sludge in the
liquid. Some of the acids are corrosive to the solid insulating materials and metal parts
in the transformer. Deposits of sludge on the transformer core, on the coils and inside
the oil ducts reduce circulation of oil and thus its heat transfer capability gets
considerably reduced. Complete specifications for the testing of transformer oils are
given in IS 1866 (1983), IEC 296 (1969) and IEC 474 (1974).

Electrical Properties
The electrical properties that are essential in determining the dielectric performance
of a liquid dielectric are
(a) its capacitance per unit volume or its relative permittivity
(b) its resistivity
(c) its loss tangent (tan 5) or its power factor which is an indication of the power
loss under a.c. voltage application
(d) its ability to withstand high electric stresses.
Permittivities of most of the petroleum oils vary from 2.0 to 2.6 while those of
askerels vary between 4.5 and 5.0 and those of silicone oils from 2.0 to 73 (see Table
3.1). In case of the non-polar liquids, the permittivity is independent of frequency but in the case of polar liquids, such as water, it changes with frequency. For example, the permittivity of water is 78 at 50 Hz and reduces to about 5.0 at 1 MHz.
Resistivities of insulating liquids used for high voltage applications should be more
than 1016 ohm-metre and most of the liquids in their pure state exhibit this property.
Power Factor of a liquid dielectric under a.c. voltage will determine its performance
under load conditions. Power factor is a measure of the power loss and is an
important parameter in cable and capacitor systems. However, in the case of transformers, the dielectric loss in the oil is negligible when compared to copper and iron losses. Pure and dry transformer oil will have a very low power factor varying between 1(T4 at 2O0C and 10~3 at 9O0C at a frequency of 50 Hz.
Dielectric Strength is the most important parameter in the choice of a given liquid
dielectric for a given application. The dielectric strength depends on the atomic and
molecular properties of the liquid itself. However, under practical conditions the
dielectric strength depends on the material of the electrodes, temperature, type of
applied voltage, gas content in the liquid etc., which change the dielectric strength by changing the molecular properties of the liquid. The above factors which control the breakdown strength and lead to electrical breakdown of the liquid dielectrics are discussed in subsequent sections.