ABSTRACT
After basic relations of voltage and current and the definition of co-ratio are given, the advantages and disadvantages of autotransformers compared with two-circuit transformers are discussed. Regulating winding electrical locations which have more options than two-circuit units are listed. The options of winding physical dispositions are also discussed regarding material economy, followed by an example. Information of auxiliary transformers, such as preventive autotransformers and series transformers, is provided. Like wye–wye connected units, autotransformers often have delta-connected winding for neutral stabilization purpose, and one-third of main winding power rating is often assigned to stabilizing winding as a rule of thumb; this practice has been questioned in the industry for its necessity and accuracy. The method to calculate the stabilizing winding rating is presented here with an example. Besides stabilizing the neutral, the delta-connected winding sometimes delivers the power to external load, the power distributions to loads which have difference power factors are calculated for step-up and step-down operation modes. Finally, the losses used for the temperature rise test of such a unit are given.
