Differential protection can also detect and clear insulation faults in the transformer
windings. The principal cause of these faults is arcing inside the bushings
and faults in the tap changer. This type of protection responds not only to phase-tophase
and phase-to-earth faults but also in some degree to faults between turns.
However, phase-to-phase faults between the windings of a three-phase transformer
are less common. An internal fault that does not constitute an immediate danger is
designated an incipient fault and, if not detected in time, could degenerate into a
major fault. The main faults in this group are core faults, caused by the deterioration
of the insulation between the laminations that make up the core.