On-load tap changers, as the name suggests, permit tap changing and hence voltage regulation with the transformer on-load. Tap changing is usually done on the HV winding for two reasons:

a. Because the currents are lower, the tap changer contacts, leads, etc., can be smaller.

b. As the HV winding is wound outside the LV winding, it is easier to get the tapping connections out to the tap changer. Below shows the connections for an on-load tap changer that operates on the HV winding of the transformer.

The tap changer has four essential features:

Selector Switches
These switches select the physical tap position on the transformer winding and, because of their construction, cannot and must not make or break the load current.

The load current must never be interrupted during a tap change. Therefore, during each tap change, there is an interval where two voltage taps are spanned.

Reactors (inductors) are used in the circuit to increase the impedance of the selector circuit and limit the amount of current circulating due to this voltage difference. Under normal load conditions, equal load current flows in both halves of the reactor windings and the fluxes balance out giving no resultant flux in the core.

With no flux, there is no inductance and, therefore, no voltage drop due to inductance. There will be however, a very small voltage drop due to resistance.

During the tap change, the selector switches are selected to different taps and a circulating current will flow in the reactor circuit. This circulating current will create a flux and the resulting inductive reactance will limit the flow of circulating current.

Vacuum Switch
This device performs the duty of a circuit breaker that makes and breaks current during the tap changing sequence.

Bypass Switch
This switch operates during the tap changing sequence but, at no time, does it make or break load current, though it does make before break each connection.

The operating mechanism for the on-load tap changer is motor driven. Manual operation is used in the event of motor failure.

The sequence of operation is mechanically linked, or interlocked, to ensure that all contacts always operate in their correct order. Any failure of the operating mechanism can result in severe damage to the transformers and tap changers.

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