Flash Point of Transformer Oil is defined as the temperature at which light hydrocarbon present in the transformer oil starts evaporating causing a flash on introduction of source under specified condition. It is an important parameter which must be maintained above 140 degree Celsius.
Light hydrocarbons are already present in the transformer oil and gets evaporated under the influence of heat due to electrical stress. Light hydrocarbons are also generated by the degradation of the oil due to aging / heat / electrical stress.
Minimum Value of Flash Point of Transformer Oil:
Flash Point of new Transformer Oil should be more than 140 degree Celsius. In any case, this temperature should not be allowed to be less than 130 degree Celsius for transformer oil in service.
We know that, the maximum continuous operating temperature of transformer oil is around 105 degree Celsius. However, during fault condition, transformer oil temperature may go as high as 130 degree Celsius. Therefore, it is very much required that the flash point of oil should be maintained above 130 degree Celsius to prevent fire during fault condition. Therefore, if the flash point of a transformer oil is less than 130 degree Celsius then this must be replaced to ensure no fire hazard during fault.
How Flash Point Reduces due to Service Condition or Internal Faults in Transformer?
Flash point is basically decided by the presence of light hydrocarbon present in the oil. It may be noted that, this light hydrocarbon may be present in the transformer oil from the oil refining stage itself. During the refining of crude oil, lighter petroleum product like kerosene, petrol, diesel etc. are removed. The flash point of kerosene is around 50 degree Celsius and that of petroleum is 35 degree Celsius.
Lighter hydrocarbons are also generated due to degradation of oil due to heat generated in the winding. Therefore, the flash point of transformer oil may slightly decrease during service condition. Above all, if the varnish of the transformer winding is mixed in the oil then it may reduce the flash point by around 4 to 5 degree Celsius.
If larger amount of light hydrocarbon is present in the oil then it represents the incipient fault in the transformer. A flash point below 130 degree Celsius may indicate unsatisfactory working condition viz. electrical discharge, excessive internal temperature, core fault etc. In such case, the transformer should be taken out to investigate and subsequent repair.