Dear Suman,

If the secondary side has capacitive load, the voltage gets higher. Please refer to the vector diagram of a capacitive loaded transformer. OR if primary side has some switching curcuit, at this point again sec. voltage will get higher than its rated values. Best regards, Okan

This generally happens at light load, as the secondary winding impedance-drop reduces at light load, while the secondary induced EMF has remained same, this can be taken care by provided, tape changer in transformer. Over voltage also happen some times with large unbalance in  three phase transformer, as  due to unbalance load, the neutral(high) current path to ground, gives considerable voltage difference between ground to neutral, which will rise the voltage of lightly loaded phase, and reduces the voltage of heavily loaded phase. Switch in or lightening over voltages are transient over voltages which exists only for few cycles.

Also the distribution transformers may have off-load tap changers that may be set above nominal voltage ratio so that secondary voltage is maintained within codes under heavy loading. As stated above this means voltages will be higher under light loads.

Apart from the concept of tap-changer position, light loads mean that secondary winding is loaded somehow and this will not yield to voltage increase in sec. winding. The only way to increase to sec. voltage is to add some capacitive load to the sec. windings. In 3 phase TR. example, it one of the phase is short circuited to the ground, it may also have voltage increase at sec. windings. All the best, Okan 

Distribution transformer does not have ON load tap changer, thus when  the load at the secondary end varies and the nature of load is capacitive, the secondary end voltages increases.. For your interest you may also read about ferro-resonance.

The industrial/domestic loading, are mainly Inductive (induction machines/furnaces/equipment), to maintain the power factor, capaciter bank used, may give adverse effect(capacitive load, see vector diagam) of rise of voltage(E2) at light load. 

over voltage also happen some times with large unbalance in  three phase transformer as  due to unbalance load the neutral(high) current path to ground,

difference between ground to neutral, which will rise the voltage of lightly loaded phase