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

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.

Anahtar Kelimeler:

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

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

Keywords:

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

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magnetic steel. The main contribution of this paper is the establishment of the level of reliability of the linear
surface impedance method for calculation of stray losses in solid magnetic steel parts of power transformers. The
conclusions drawn from the experimental data and 3D FEM models can be also applied for other electromagnetic
devices with parts made of solid nonmagnetic or magnetic material that are exposed to sinusoidally varying
magnetic fields. It is possible to find the equivalent linear magnetic properties of the inherently nonlinear
material to reliably calculate the stray losses, which has been confirmed by experiments conducted on samples
inserted into a transformer core model with a coil.
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