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REDUCING LOSSES IN DISTRIBUTION TRANSFORMERS: A TECHNICAL PAPER


A technical paper completed by Juan Carlos Olivares, Member, IEEE, Yilu Liu, Senior Member, IEEE, Jose M. Cañedo, Member, IEEE,Rafael Escarela-Pérez, Member, IEEE, Johan Driesen, Member, IEEE, and Pablo Moreno, Member, IEEE

ABSTRACT
This paper examines three methods of reducing distribution transformer losses. The first method analyzes the effects of using aluminum electromagnetic shields in a distribution transformer. The goal of placing electromagnetic shields in the distribution-transformer tank walls is to reduce the stray losses. A 500-kVA shell-type transformer was used in the experiments.


The overall results presented indicate that stray losses can be considerably reduced when electromagnetic shielding is applied in the transformer tank. In the experiment, the tank walls were lined with aluminum foil. The possibility of reducing the dielectric losses was shown through experiments in the second method. And the third method of this work analyzes the behavior of wound-cores losses in distribution transformers, as a function of joint configuration design parameters. The joint configuration used in this paper is called step-lap joint.

CONCLUSION
This paper presented results of experimental investigations regarding reduction of distribution-transformer losses. The work contains experimental data that will be helpful for practicing engineers in the transformer industry.

Experiments were carried out in well-controlled conditions. First, a load loss test was carried out under three different conditions: (a) tank walls without shield, (b) tank walls with aluminum shield of 1.2 mm of thickness, and (c) tank walls with aluminum shield of 10-mm thickness. The electromagnetic shields of the transformer in this experiment prevented the penetration of the magnetic stray flux in the magnetic materials, where high losses would be induced. In this case, an increase of the stray losses by 20.9% was observed when the aluminum shield of 10 mm was not used.

On the other hand, there were not significant changes in the losses when the 1.2-mm shield was used with respect to unshielded case, since the depth of penetration was larger than the shield thickness and the magnetic flux could reach the carbon steel. The study also demonstrated that the dielectric losses are important in no-load loss in the transformer when the transformer insulations have a high water content.

Until now, little attention has been given to the design of wound transformer cores. In the paper, results for the wound-core distribution-transformer family were presented. It is known that minimum losses occur when the rolling direction coincides with flux magnetic lines, but this condition is not satisfied in the core joints since the joints air gaps cause local disturbances of magnetic flux.

Two experiments were carried out, varying: (a) the overlap length and (b) the number of laminations per step. It is observed that the number of laminations per step does not have much effect on the core losses. This is because as the number of laminations was increased, the overlap length was decreased in order to comply with manufacturing limitations. The test showed that transformers had higher losses when an overlap length of 2 cm is used.

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