ABSTRACT
Transformers are used in power systems to change the voltage levels for economics, safety, and convenience. The principal types are power, distribution, current, voltage, and auxiliary, primarily with two windings. However, there are also many three-winding and autotransformers. Shunt capacitors are widely used in distribution systems to regulate voltage and reactive power flow. For individual single-phase transformers, the transformer impedance is specified on rated single-phase kilovolt-ampere and the rated winding kilovolt voltages. The zero-sequence impedance of a group of three transformers is the same as the positive- and negative-sequence impedances or is infinite except in some types of construction, such as the three-phase core types. Power transformers with three windings on the same core are encountered frequently in power systems. The determination of the equivalent circuit and the sequence interconnections are the same as for the three-winding transformer. The major difference between three-winding transformers and autotransformers is in the determination of the actual currents flowing in the autotransformer windings.
