It is a general practice to have some means of adjustment to maintain constant voltage at the output terminals by compensating for the variations of the input voltage. This is done by tapping out or adding turns to the primary or input winding and maintaining the volts per turn, and thus the output voltage.
This operation is usually performed when the transformer is de-energized; this is called off-circuit tap changing. In dry type transformers, the usual method is to bring out the tap terminals on the outer surface of the coil or on a terminal board, where the linking to obtain the required turns is done manually with the unit de-energized.
It is possible, though not usual, to have tap switches similar to those used in liquid- filled units. Until recently, dry-type transformers were never supplied with under-load-tap-changing equipment. This was due to the fact that under-load tap changing involves breaking of load current at full voltage, thereby requiring switching equipment with capabilities comparable to those of circuit breakers.
To do this in air was cumbersome, bulky, and extremely expensive. But with the increased capacities and voltages of dry- type transformers, the demand for such equipment has increased, and recently voltage regulators became commercially available.
Two different approaches are used to provide underload voltage regulation. One takes the traditional approach of the liquid-filled units by providing motor-driven selector switches combined with a spring activated vacuum diverter switch.
The other approach uses a separate regulator winding feeding a buck/boost transformer connected in series with the primary winding. Voltage regulation is achieved by means of low-voltage vacuum contactors that modify the tap settings of the regulating winding of the buck/boost transformer, circumventing high-voltage switching equipment.
The contactors are usually controlled by programmable logic controllers (PLC). In cases where high speed response is required, the second approach has successfully used thyristors in place of vacuum contactors, thereby achieving a cycle switching.