Three characteristics generally provide means for detecting transformer internal faults . These
characteristics include an increase in phase currents, an increase in the differential current, and gas
formation caused by the fault arc [2,3]. When transformer internal faults occur, immediate disconnec-
tion of the faulted transformer is necessary to avoid extensive damage and=or preserve power system
stability and power quality. Three types of protection are normally used to detect these faults: over-
current protection for phase currents, differential protection for differential currents, and gas accumu-
lator or rate-of-pressure-rise protection for arcing faults.