Application of Oil in Power Apparatus

Oil is normally used for providing insulation between the live parts of different phases and between
phases and the grounded enclosure containing the oil and the main parts of the apparatus. Also it
provides cooling effect to the apparatus placed within the enclosure. Besides providing insulation, the
oil helps the C.B. to quench the arc produced between the breaker contacts when they begin to separate
to eliminate the faulted section from the healthy section of the system.

In an oil circuit breaker, the heat of the oil decomposes the oil which boils at 658 K. The gases
liberated are approx. (i) Hydrogen, 70%, (ii) Acetylene, 20%, (iii) Methane, 5% and (iv) Ethane, 5%.
(the abbreviation for these gases could be used as HAME).
The temperature about the arc is too high for the three last-named gases to exist and the arc itself
runs into a mixture of hydrogen, carbon and copper vapour at temperature above 6000 K. The hydrogen
being a diatomic gas gets dissociated into the atomic state which changes the characteristics of the arc
on account of its associated change in its thermal conductivity. The outcome of this is that the discharge
suddenly contracts and acquires an appreciably higher core temperature. In certain cases, the thermal
ionization may be so great that the discharge runs with a lower voltage which may stop the ionization
due to the electric field strength. The transition from the field ionization to thermal ionization is most
marked in hydrogen and, therefore, in oil circuit breakers.
The separation of the C.B. contacts which are carrying current gives rise to an arc without
changing much the current wave form. Initially when the contacts just begin to separate the magnitude
of current is very large but the contact resistance being very small, a small voltage appears across them.
But the distance of separation being very very small, a large voltage gradient is set up which is good
enough to cause ionization of the particles between the contacts. Also it is known that with the copper
contacts which are generally used for the circuit breakers very little thermal ionization can occur at
temperature below the melting point. For effective field emission, the voltage gradient required is 106
V/cm. From this it is clear that the arc is initiated by the field emission rather than the thermal ioniza-
tion. This high voltage gradient exists only for a fraction of a micro-second. But in this short period, a
large number of electrons would have been liberated from the cathode and these electrons while reach-
ing anode, on their way would have collided with the atoms and molecules of the gases. Thus, each
emitted electron tends to create others and these in turn derive energy from the field and multiply. In
short, the work done by the initially-emitted electrons enables the discharge to be maintained. Finally,
if the current is high, the discharge attains the form of an arc having a temperature high enough for
thermal ionization, which results in lower voltage gradient. Thus, an arc is initiated due to field effect
and then maintained due to thermal ionization.