Modelling of multi-winding transformers for short-circuit calculations in the power system – Modelling accuracy and differences in equivalent circuits

Abstract

The theory related to the modelling of two-winding and three-winding transformers is well known and widely used in short-circuit analysis. The simplest equivalent circuit, and at the same time accurate, used for the above-mentioned transformer units is the star diagram. For transformers with more than three windings, it is simplified as it does not account for all short-circuit impedances between the individual windings. Therefore, in order to more accurately model multi-winding transformers, e.g. four- or five-winding ones, apart from star diagrams, other equivalent circuits, such as complete polygons, star polygons or chain diagrams, should be used. Which equivalent to use depends on the required accuracy of the calculations. Some of the diagrams mentioned (complete polygons, star polygons) are accurate, but require access to detailed data that are sometimes difficult to obtain from manufacturers, and determining all the parameters of the scheme is laborious and time-consuming. The remaining diagrams (star or chain ones) are created on the basis of certain simplifying assumptions: they do not require all detailed data and it is not difficult to determine their parameters. The article presents and describes various equivalent diagrams that allow to model multi-winding transformers. On the basis of exemplary data, for a four- and five-winding transformer, their parameters were determined for each scheme and sample short-circuit analyses were performed, indicating the similarities and differences in the modelling method and their impact on the calculation results.