There are several basic types of faults, which can occur in the transformer:
Arcing or high current break down.
Low energy sparking or partial discharges.
Localized overheating or hot spots and general overheating due to inadequate cooling or sustained overloading.
Each of the faults results in thermal degradation of the oil either alone or in combination with paper insulation. This gives rise to the evaluation of various hydrocarbon gases, hydrogen and oxides of carbon, in quantities depending on the type of fault.
Heavy current arcing is characterized by the evolation of significant quantities of hydrogen and acetylene. If the arcing also involves paper insulation, the oxide of carbon will also be present.
Partial discharge usually results in evolution of hydrogen and lower order hydrocarbons.
Localized heating or hot spot gives rise to methane and ethane in appreciable amount.
Prolonged overloading or impaired heat transfer can cause CO and CO2 to be generated due to overheating paper insulation.
IEC-599 is the guide to the interpretation of dissolved gas analysis in mineral oil. The faults are broadly divided into thermal and electrical type.
Fault of low energy will favor the breaking of C-H molecular bonds. More energy or higher temperature is needed to break the C-C single bonds, C=C double bonds and C≡C triple bonds in ascending order of greater energy or higher temperature. Acetylene gas will require very high temperature of at least 800 to 1200 degree Celsius to form because it has a C≡C triple bond. Ethylene gas will form at a lower temperature of more than 500 degree Celsius because of the C=C double bond. Ethane and methane will form at lower temperature because of the C-C single bond.
For thermal fault T1 of less than 300 degree Celsius, the paper insulation of the transformer will turn brownish. For thermal fault T2 between 300 to 700 degree Celsius, the paper insulation will carbonize. For thermal fault T3 of more than 700 degree Celsius, the oil will carbonize, metal will colorize of fuse.
For electrical fault of partial discharge is nature, wax may form in the oil. Electrical fault of low energy discharge and high energy can be due to discharge through the oil, discharge through the paper insulation or discharge at the surface of the paper insulation, or degradation of the surface of the paper insulation to form conducting paths or small arcs.
