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ANALYSIS OF STRAY LOSSES IN POWER TRANSFORMERS BY 3-D MAGNETIC FIELD SIMULATION TECHNICAL PAPER


Completed and Authored by: Chetan C. Adalja M.L. Jain, Technology Department, EMCO Limited, Thane India

INTRODUCTION
The stray losses in a transformer comprise winding stray losses, viz. eddy loss and circulating current loss; the loss in the edge stack (smallest packet of the core limb); and the loss in structural parts, viz. frame, flitch plate and tank. Core loss at the impedance voltage being insignificantly low, is not considered in the present analysis. In case of large generator transformers, stray losses due to high current carrying leads also become significant.


As the total stray losses with shielding measures in large rating transformers are of the order of 20-25% of the total load losses, it is imperative to estimate stray losses accurately as control over these gives a competitive advantage. Measures like using judiciously designed magnetic shunts help reduce the stray losses effectively. The estimation of stray losses in structural parts of transformer at design stage is generally carried out by using empirical formulae covering wide range of design variants and complicated asymmetrical geometries. These formulae therefore inherently suffer form unpredictable inaccuracies, which would be actually known only at final testing stage. However, with the availability of high speed and accurate computational tools and software programs it is possible to simulate complex geometries for 3-D electromagnetic fields mapping and precise estimation of stray losses at drawing-board stage.

CONCLUSIONS
1. Stray losses in a transformer can be precisely estimated using EDMAG-3D software program that is a powerful tool to aid fairly accurate 3-D field mapping of complex transformer asymmetries.

2. The loss in the bottom frames is higher than the top frames because of its close proximity with bottom edge of the winding. It was observed that lowering of the bottom frame height resulted in reduced frame losses. This is attributed to its reduced interaction with the leakage field returning to the bottom yoke.

3. The stray loss in edge stack is significant, leading to localized hotspot. Division of the edge stack effects
substantial reduction in loss as well as the temperature.

4. Choosing appropriate material for flitch plate and judicious slot dimensioning could effect reduction in stray
losses in the flitch plates. 

FURTHER WORK
Precise estimation of stray losses is a subject in itself. It may not be prudent to attempt very precise simulation for computation of stray losses in routine designs disregarding the economic considerations. However, application of modern high speed and accurate computation tools offer deep insight into the complex field phenomena in asymmetric transformer geometries. There is a wide scope to exploit these tools for development of new cost-effective designs, exploring possibilities for improvements in certain areas like shunt materials, use of yoke shunts, use of widthwise wall shunts etc.

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