# Stray Loss

6 June 2017

Generally, there is a simplified understanding that the socalled load losses in a transformer (load losses, PL ) depend on the ohmlc resistance of the windings. This Is only partly true, as the losses arising from magnetic leakage flux in various metal parts of a transformer constitute a signlflcant part of the total losses.

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This component of total losses is called 0 stray (additional) loss (Ps ). In addition, experts distinguish between the stray losses In the windings (winding eddy loss: ddy-current loss in conductors, and circulating-current loss in parallel strands) and outside the windings (stray 2 magnetic shield on the tank yoke clamping plate 3 core The company Kon©ar Power Transformers (KPT) has been applying for over 25 years of a systematic approach in the research, control and continuous reduction of stray losses from magnetic leakage flux.

Here are only several segments resulting from the complex and variegated activity, ohmic losses 12R, the calculation of stray losses represents an exceptionally complex process, for which there is still no standardised method of determining them in the ransformer calculation phase. The basis for the calculation of stray losses is the knowledge of the magnetic leakage field in a transformer.

The main tool for solving this problem in KPT is a computer programme for the calculation of the magnetic leakage field in the transformer window, by means of which the magnetic field in a cylindrical coordinate system is determined at any point in or around the winding. Although stray losses, as a rule, are linked with the magnetic leakage field of windings, the high-current loops and their respective magnetic fields should not be neglected. Generally, stray losses appear in all metal parts penetrated y the magnetic leakage field produced by the windings or current losses in structural parts: tank, clamps, etc).

Large transformers, especially autotransformers or transformers with higher short-circuit impedance, can generate stray losses accounting for 30-50% of the ohmic losses 12R, and, exceptionally, even more. The problem of transformer stray losses can be considered from three aspects: how to calculate them as accurately as possible, how to reduce them to a reasonable level, and how to prevent possible dangerous local overheating of transformer parts resulting from the losses. outer package of the core limb oops.

This means that there are many potential locations where stray losses may appear, and the most important ones are those: in winding conductors, due to eddy-currents (skin-effect); in winding parallel strands, due to circulating currents; in tank, due to magnetic leakage flux from the windings; in tank cover, around the high-current bushings; in clamping plates of the core yoke, due to the winding leakage flux; in core sheets at the outer packages of the core limbs, due to the winding leakage flux.

As stray losses are generated at many places in a transformer, it would be extremely difficult to determine ith sufficient reliability the total loss level by calculating every e I emen t individually. Therefore, simplified integral methods, yielding much better results on the basis of several influential parameters and coefficients determined from experience, are preferred. The main influential parameters that are most relevant in calculating stray losses are: a) the value of the magnetic leakage flux of windings, b) transformer tank shielding, and c) high-current loops in transformer.

Stray losses in a transformer are reduced i. e. controlled by taking several appropriate measures, depending on the type and size of a ransformer. Some of the measures use of small-dimensioned conductors for windings; use of the so-called continuously transposed conductors in the case of high currents; optimum transposition of parallel strands; magnetic shielding of the inner tank walls; use of nonmagnetic steel in the areas of strong magnetic fields; maximum possible distance of all large metal parts from the strong magnetic fields; optimum spatial dimensioning of high-current loops; optimum selection of the winding type.

Reduction of the total losses in a transformer results in increasing its efficiency (h), or decreasing the price of the active part. However, in the case of very large transformers, control of high local losses and prevention of local overheating (hot spots) is often a more important problem. For the control of the local losses and overheating KPT has developed several technical procedures and computer programmes. The following controls are frequently used: calculation of the maximum local losses in the windings (e. g. t the ends of the windings) due to eddyocurrents in conductors; calculation of losses and local overheating of core sheets at the outer packages of the core limbs; to magnetic leakage flux of the windings; control of local heating on the ransformer tank cover in the area of high-current bushings. A separate topic are additional losses and local hot-spots in special transformers with high-currents , such The application of transposed conductor is an efficient method for decreasing stray losses due to circulating currents and of skin-effect in high-current windings. ectifier and furnace transformers, where preventing of local overheating is more important problem than the level of additional losses. Additional measures for reducing local losses and overheating are applied in such transformers, for instance: electromagnetic shields made of luminium or copper plates, wooden yoke clamps instead of steel clamps.

An appropriate question would be whether the achieved level of knowledge provides, from the aspect of stray losses and local overheating, sufficient guarantee that even the largest transformers with extremely high magnetic leakage flux will have relatively low values of stray losses and local overheating below the permissible level. The thirty years of experience in the production of large transformers in KPT, strict application of a wide range of technical procedures, together research on physical and mathematical models certainly give such guarantee.

The verified permanent trend of decreasing the level of stray losses in transformers over the several past years speaks in favour of this manufacturer. KPT has developed a very efficient shielding system for the inner tank wall by means of magnetic shield of specific design. Magnetic shielding of the tank represents a standard technique of decreasing stray losses in medium-sized transformers,

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