Tap changers on transformers

The modern electrical equipments are designed to operate satisfactorily at one fixed voltage level. Hence, it is important to keep the voltage at consumers within the prescribed limits. The transformer output voltage and hence the consumer’s terminal voltage can be controlled by providing taps either on low-voltage side or high-voltage side. It is difficult to tap the low-voltage winding, since it is placed over the core due to insulation considerations. Moreover on low-voltage side, the tap changing gear would have to handle large magnitude of current. Hence, in general taps are not provided on low-voltage side. The high-voltage winding is placed on high-voltage winding and is easily side. The high-voltage winding and is accessible, therefore the taps are provided on high-voltage side without any difficulty. The principle of regulating the secondary output voltage is based on changing the number of turns in the primary or secondary as shown in fig above.

 

By changing the number of turns in the primary or secondary, the transformation ration (k) can be changed and hence secondary voltage changes. Hence, by selecting proper tappings, the consumers’ terminal voltage can be maintained within the prescribed limits. The tap changing may be done under OFF-load condition. If the tappings are changed with the transformer isolated from the supply i.e. on no-load, it is in the supply i.e. on load, it is called ON-load tap changing.