Use of surface impedances in a 3D FEM linear model for the calculation of zero‐sequence magnetising impedances of three‐phase core‐type transformers

Abstract

This article shows results of using surface impedances ( Z SURFACE ) in 3D FEM linear model to calculate zero-sequence magnetising impedances ( Z 0M ) of 3-phase core-type transformers. Initially, results with and without Z SURFACE are compared to obtain certainty about model validity. Compared results are magnetising reactances and active power losses in tank during Z 0M tests. Subsequently, results of 3D models with Z SURFACE are compared with Z 0M measurements at different currents, to consider nonlinearity of tank steel. An equivalent permeability for tank steel is estimated for each test point, and accurate results from model are obtained for magnetising reactances. An approximate curve for this equivalent permeability is also found, as in a previous 2D method, and both curves are similar from each other. It is shown that active power losses in tank cannot be properly computed using these 3D linear models. This fact cannot be observed by previous 2D models and it indicates that active power losses in tank are influenced by other phenomena, which are not considered by these FEM models. Main contributions are the application of Z SURFACE as a powerful tool to compute Z 0M , and an analysis of active power losses in tank which could not be performed by previous 2D models.