Determination of electromagnetic properties of steel for prediction of stray losses in power transformers

Authors: LEONARDO STRAC, DAMIR ZARKO

Abstract: This paper introduces a method for determination of equivalent linear electromagnetic parameters (constant complex permeability and electrical conductivity) of nonlinear magnetic steel, which can be used in a time-harmonic finite-element simulation to yield the same losses in the volume of that material as the measured ones. The conductivity and the static hysteresis loop of the steel have been measured, from which complex permeability as a function of flux density has been extracted. The indirect measurement of losses in various samples of nonmagnetic and magnetic steel has been carried out using a physical model of a transformer core with a coil. The 3D model of the core has been made with finite-element software and combining it with evolutionary optimization the equivalent constant complex permeability and conductivity of each sample have been found, which yield the equality of measured and calculated losses in the sample. Thus, calculated equivalent material parameters have been implemented in 3 finite-element models of transformers of various power ratings in order to determine the share of hysteresis losses in the total amount of losses in structural parts of transformers. The results have been compared with the measurement and the reasons for discrepancies have been explained.

Keywords: Transformers, steel, magnetic losses, eddy currents, hysteresis, permeability, finite element methods, optimization

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