Explain the Tap Changing Transformers?

This is the most popular form of voltage control at all voltage levels. It is based on changing the turns ratio of the transformer, hence the voltage in the secondary circuit is varied and voltage control is obtained. Figure shows a schematic diagram of an off-load tap changer which requires the disconnection of the transformer when the tap setting is to be changed. Most transformers have on-load tap changers which is shown in a basic form in Figure.

In the position shown, the voltage is at minimum and the current divides equally in the two halves of the coil L resulting in zero resultant flux and minimum impedance. To increase the voltage, S1 opens and the total current passes through the other half of the reactor. Selector switch A then moves to the next contact and S1 closes. A circulating current now flows in L superimposed on the load current. Then, S2 opens and B moves to the next tapping; S2 then closes and the operation is complete. To avoid large voltage disturbances, the voltage change between taps is normally small; about 1.25 % of the nominal voltage. The total range of tapping varies with the transformer usage, a typical figure for generator transformers is +2% to -16% in 18 steps.