How Transformer Tap Changer actually control the voltage??
The on-load transformer tap-changer has to provide uninterrupted current flow during the transition operation from one tap to the other. The current flow must be maintained uninterrupted without partial short-circuiting of the tapped winding. The operation of an on-load tap-changer can be understood by two identifiable functions.
It implies a switching device that transfers the throughput power from one tap of the transformer to an adjacent one. During this operation the two taps will be connected through fitted transition impedance. In this phase the two taps will share the load current.
Thereafter the connection to the former tap will be interrupted and the load has been transferred to the new tap. The device that performs this switching is called diverter switch.
The connections to the two taps that involve the diverter switch are maybe transferred one position along the series of physical taps of the regulating winding for each operation. This is the tap selector function. The tap selection is conducted without any current rupture.
Transformer Taps are generally provided on HV winding, as it is easier to work with low current compare to LV winding. Transformer tap position control the secondary side output voltage.
Tap changer operation:
V2/V1 = N2/N1
V2 = (N2/N1)*V1
Since, V1 and N2 are constant, V2 can be regulated by change in N1.
It means that, If LV Voltage is increased, to decrease the same, Tap Position should be lowered and vice-versa. Lowering the primary side taps will increase secondary voltage and raising the primary taps will lower the secondary voltage.