What is purpose of Tapping on transformer winding?

Why Tapping are provided on transformer winding? And what is benefit of Tapping

Regulating voltage is required in power system due to following reason

1) to supply required voltage to load
2) to compensate voltage drops due to           load fluctuations
3) to compensate input voltage supply to load
Most of electrical loads, equipment are designed to work satisfactorily on constant  voltage. So it is necessary to keep consumer end  terminal voltage levels in prescribed limits.
Voltage control is performed by changing transformer turn ratio. And this is achieved by providing Tapping on the transformer winding. Tapping are either provided on  primary or on secondary winding of transformer.

Voltage per turn is very high in large transformer, even changing single turn at LV  can achieve large voltage change in secondary.

Why tap changer is connected to on HV side winding of power transformer?

Low voltage winding (LV)  being inner winding in core type transformer so it difficult to access for tapping purpose. Hence tapping are provided on HV winding on core type transformer.
Providing taps to changes voltage levels called as tap changing of transformer

Tap changing of transformer are two type
1) No-load tap changing
2) On load tap changing
In No-load tap changing transformer load thrown off before tap changing done and other hand in on load tap changing load remains on transformer while performing tap changing

How tap changers works in transformer?

The principle of changing output voltage is base on number of  turn in primary or secondary
Let discuss V1 N1 and V2, N2 are primary and secondary quantities
If N1 decreases, emf per turn in primary increases hence secondary voltage becomes  (V1/N1) *N2 increases on other hand in  N2 increased secondary emf  V2 increased and secondary emf equal to V1/N1 * N2 increases
Hence decreasing primary number of turn have same effect as increasing secondary numbers of turn