[W-TR-15] Loss Evaluation of a Transformer Shield Based on the Homogenization Method

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1. Introduction

For power transformers, stray loss generated in the surrounding structures due to leakage flux originating from windings is a common issue. Because people may make direct physical contact with the tank, measures have been taken, such as placing a shied inside the tank. Because shields often use magnetic materials such as electromagnetic steel sheets, leakage flux primarily passes through the shield as a magnetic path. This results in magnetic flux linking to the tank being suppressed, and not only heat in the tank drastically reduced, but it can be expected that all stray loss including the shield is reduced.
This leads to the following question: quantitatively, to what extent does the shield have an effect on loss reduction? For stray loss generated in the entire transformer, evaluations can be performed by taking physical measurements, but loss evaluations for each part, such as tank stray loss reduction depending on if there is a shield or not and loss occurrences in the shield, require an evaluation using a magnetic field simulation based on the finite element analysis (FEA). The primary cause of losses resides in the leakage flux that is difficult to examine in advance, which makes simulations even more necessary. FEA-based electromagnetic field simulations realizes a high accuracy analysis using a mesh model to model the object as it is and applying a Maxwell equation.
FEA-based electromagnetic field simulations can be performed with as much detail as needed by raising the detail level of the model. However, the shield inside the transformer has a thin lamination structure when electromagnetic steel sheets are used. Calculating the mesh model accounting for the degree of freedom inside the steel sheet is no longer realistic, when considering the aspect ratio with the size of the entire transformer. This requires some sort of approximation method.
In this paper, each electromagnetic steel sheet of the shield is not modeled; by applying the homogenization method which replaces magnetically and electrically equivalent materials, a realistic mesh model is proposed and an analysis was performed, which is reported below.

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