A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be selected. This means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the output. The tap selection is made via a tap changer mechanism.
If only one tap changer is required, tap points are usually made on the high voltage, or low current, side of the winding to minimize the current handling requirements of the contacts. However, a transformer may include a tap changer on each winding if there are advantages to do so. For example, in power distribution networks, a large step-down transformer may have an off-load tap changer on the primary winding and an on-load tap changer on the secondary winding. The high voltage tap is set to match long term system profile on the high voltage network and is rarely changed. The low voltage tap may be requested to change positions once or more each day, without interrupting the power delivery, to follow loading conditions on the low-voltage network.
To minimize the number of windings and thus reduce the physical size of a transformer, a 'reversing' winding may be used, which is a portion of the main winding able to be connected in its opposite direction and thus oppose the voltage. Insulation requirements place the tap points at the low voltage end of the winding. This is near the star point in a star connected winding. In delta connected windings, the tappings are usually at the center of the winding. In an autotransformer, the taps are usually made between the series and common windings, or as a series 'buck-boost' section of the common winding.