On-load tap changers (or circuit tap changers), permit tap changing and hence voltage regulation with the transformer on-load. Tap changing is usually done on the HV(High voltage) winding for two reasons:
- Because the currents are lower, the tap changer contacts, leads, etc., can be smaller.
- As the HV winding is wound outside the LV(Low voltage) winding, it is easier to get the tapping connections out to the tap changer.
An on-load tap-changer(oltc) gear comprises two major components, the tap selector and the diverter switch.
The tap selector selects the tapping and its electrical contacts are designed to carry the rated current of the transformer but not to make or/and break this current.
The essential feature of all methods of tap changing under load is that circuit continuity must be maintained throughout the tap stepping operation. The general principle of operation used in all forms of on-load tap changer is that, momentarily at least, a connection is made simultaneously to two adjacent taps on the transformer during the transition period from one tap to the next. Impedance in the form of either resistance of inductive reactance is introduced to limit the circulating current between the two tappings.