Application considerations for variable volts per turn power transformers

Power transformer tap changing, either under load or when the transformer is de-energized, can be achieved by tapping the winding other than that which is experiencing the voltage change to be compensated. Thus the transformer has a variable volts per turn or variable excitation. The design technique is well known to transformer manufacturers but the authors experience is that it is not widely understood, nor are its advantages appreciated by transformer users. A search for papers revealed a surprising lack of material on this subject. The intent of this tutorial paper is to familiarize transformer users with the principles of designing variable volts per turn transformers and with performance differences from the usual constant volts per turn design, and with the economic advantages of reduced losses and reduced initial cost that is often possible with a variable volts per turn transformer design