ABSTRACT
The impedance of two winding transformers is mathematically deducted, the results obtained through the equations verified by tests are applied to real design calculations. Based on it, the calculation methods of impedances between series connected two windings and a third winding, between zigzag and delta or wye windings and between two LV sets and one HV set are introduced, with examples of calculations given. Lead impedance is also discussed, since it has significant impact on a low-impedance unit when its LV winding current is high. The impedances which bring minimum cost of core and coil are listed. Further, zero sequence impedances are discussed in detail. The calculation methods of zero sequence impedance networks of units having no neutral terminal and having one, two or three neutral terminals are given, accompanied with calculation examples.
