Voltage variation effect on transformer?

Under-voltage operation does not have significant impact on transformer, but due to under-voltage condition current drawn by the loads increases. If this increase in load current can create an over current condition. Transformer can be operated upto 110% at no load and 105% at full load without any harm. Further increase in voltage can results in the saturation of magnetic core thereby increasing the magnetizing current and losses. Continued operation in over excited region can result in failure of unit.