Introduction
Mineral oil performs two important functions in transformers. It cools the transformer and provides electrical
insulation, as well. Therefore any deterioration in the oil can lead to premature failure of the equipment. When the mineral oil is subjected to high thermal and electrical stresses, it decomposes and, as a result, gases are
generated. Different types of faults will generate different gases, and the chemical analysis of these gases, performed through a procedure called DGA (Dissolved Gas Analysis), will provide useful information about the condition of the oil, and help to identify the type of fault in the transformer. DGA requires the removal of an oil sample from the transformer, and this can be done without de-energising it. Th e oil sample is analysed in the laboratory using gas chromatography.