High-Voltage Winding for Core-Form Power Transformers

The present invention relates generally to an improved construction and winding method for a core-form power transformer. More particularly, two different specific conductor configurations are provided in a single coil to minimize eddy current losses.

Modern transformer windings are fabricated using a wide variety of methods. In high power applications, a rectangular shaped conductor strip is generally spirally wound about a core to form a coil. Often, the conductive strip itself is composed of a plurality of strands arranged side by side in a roll. The strands themselves may be rectangular to both increase strength, and to provide a more compact transformer.

There are several factors that influence transformer efficiency. Two of the most notable losses are caused by eddy currents and circulating currents within the windings. It has been realized that eddy currents are dependent to a large extent on the dimensions of the conductors. Specifically, eddy current losses may be significantly reduced by reducing the dimensions of the conducting strands. Experiments have shown that conductor bundles comprised of a large number of finely stranded conductors have several advantages over prior conductor constructions, particularly in reducing eddy currents in portions of the transformer that are subject to large eddy current losses.

Conventional core-form coils have two distinct magnetic flux situations about the length of the coil. Specifically, a substantially uniform axial field exists along most of the vertical height of the coil. In contrast, the top and bottom ends of the coil are subject to divergent fields. In core-form coils that utilize tap connectors for a deenergized tap changer (DTC), divergent fields are found about the tap connectors as well. Eddy current losses are particularly prevalent in regions that have divergent fields.

Therefore, it is a primary objective of the present invention to provide an improved high-voltage winding for core-form power transformers that utilize finely stranded conductors to improve transformer efficiency.

A more specific objective is to provide an improved high-voltage winding for core-form.

To achieve the foregoing and other objects and in accordance with the purpose of the present invention, a winding for a core-form induction transformer is disclosed having distinct body and end sections. The body section of the transformer is wound with a first elongated conductor bundle having a plurality of insulated conductor ribbons arranged in a side by side relation. The conductor bundle is spirally wound in a multiplicity of turns to form the body portion of the coil. The end sections of the transformer are each formed from a second elongated conductor bundle having at least one bundle section comprised of a multiplicity of elongated conductor strands arranged in side by side relation. Each of the conductor strands has a substantially rectangular cross section with a pair of spaced apart substantially parallel contact surfaces that are joined by a minor axis. The thickness of each conductor strand along its minor axis is less than approximately 40 mils and the multiplicity of the conductor strands are placed side by side such that the respective contact surfaces abut. The second conductor bundles, like the first, are spirally wound in a multiplicity of turns to form the end portions of the coil.

In embodiments of the transformer that include tap connectors, the winding preferably also includes a tap section that is wound about the tap connectors with a conductor bundle having at least one bundle section formed from a multiplicity of elongated tap connector strands arranged similarly to the end conductor strands discussed above.

In a preferred embodiment, the end and tap conductor strands are in the range of 60 to 90 mils wide and have a thickness in the range of 20 to 40 mils and the conductor ribbons and strands are insulated with a material such as enamel.

The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a conventional core-form transformer.

FIG. 2 is a vertical sectional view of a core form transformer wound in accordance with the present invention.

FIG. 3 is a diagrammatic cross sectional view through two turns of a body section conductor bundle as seen in FIG. 2.

FIG. 4 is a diagrammatic cross sectional view of a portion of the body section with the transformer highlighting the spacers that create cooling ducts.

FIG. 5 is a cross sectional view of an end conductor bundle section.

FIG. 6 is a diagrammatic cross sectional view of through two turns of a end section conductor bundle.

FIG. 7 is a representative transposition pattern for the end section conductor bundle.

As illustrated in the drawings, the present invention comprises a novel high voltage core-form transformer coil that uses two different conductor configurations within the same coil. Referring initially to FIG. 1, a typical coil winding for use in core-form transformers is shown for illustrative purposes. It will be appreciated that any other coil construction would be equally operative for the purpose of this disclosure. The winding 5 is comprised of an elongated conductor bundle 7 spirally wound about a winding tube 9 to form a plurality of layers or turns. The turns may be separated into a plurality of groups 11 that are separated by cooling ducts 12 that facilitate cooling the winding. Axially extending spacing members 13 are provided to maintain the dimensions of the cooling ducts. One or more taps 15 having tap connectors 17 may be provided to maintain the concentricity of the winding.

Two magnetic flux situations exist within core-form coils. First, there is a substantially uniform axial field that extends most of the vertical height of the coil. The section of the coil having the uniform axial field will be referred to herein as the body section 20 of the winding. Divergent fields occur at the opposite end sections 22 and 24 of the coil. Additionally, a divergent field will occur adjacent to that scope tap connectors 17 in coils which incorporate such structures. The region of the winding adjacent to that scope a tap connector 17 that induces a divergent field is referred to herein as the tap section 26.

Reference is next made to FIG. 2, which illustrates the embodiment of the invention chosen for the purposes of illustration. Within the body section 20 of the transformer, where there is a uniform axial field, the eddy current losses are a function of the radial dimensions of the individual ribbons that form the conductor bundle. Therefore, as can be seen in FIGS. 2 and 3, within the body section, the conductor bundle 7 is formed of several wide rectangular insulated conductor ribbons 8. The conductor bundle, in effect forms a turn and the actual width of the conductor bundle, as well as the actual dimensions of the various conductor ribbons are selected to provide a turn having the area and total width dictated by a particular coil design. By way of example, in many high voltage transformer applications, a winding width in the range of 2 to 4 inches would be appropriate.

The large width of the conductor bundles 7 insures that the series capacitance of the coil will be very high and that the impulse voltage distribution will be essentially uniform. Thus, the conductor bundle insulation can be much thinner than that provided on conventional continuous coils. A heavy enamel coating 30 on the conductor ribbons 8 provides adequate turn-to-turn insulation. However, to increase the mechanical strength of the coil 5, a sheet of adhesive coated paper 32 having the same width as the conductor bundle 7, is wound in between turns. By way of example, the adhesive paper 32 may take the form of 3-7 mil thick paper coated on both sides with a heat-curing adhesive. Heavy enameled coated wires having dimensions in the range of 30-96 mils by 280-580 mils would be appropriate to form the conductor ribbons 8. Appropriate enamel coatings for the conductor ribbons are in the range of 1.2 mils to 2.2 mils per side, with the most preferred being approximately 2 mils per side.

In a coil section 2-4 inches wide as described above, the conductor bundle edges do not have sufficient area to provide adequate cooling. In such designs, vertical cooling ducts 12 formed in the regions between vertical spacers 13 may be provided as shown in FIGS. 1 and 4 to cool the winding. When needed, cooling ducts may be placed in the end and tap coil sections in the same radial locations as in the body sections.

In the tap and end sections of the coils which are subject to divergent fields, the eddy current losses are largely determined by the vertical dimension of the individual strands. Referring next to FIGS. 5 and 6, the conductor bundle 40 in these regions may be divided into a pair of side by side bundle sections 42. To minimize the eddy current losses, each bundle section 42 is comprised of a large number of extremely small rectangular conductor strands 45 as shown in FIG. 5. Each conductor strand 45 is enamel coated and will generally be in the range of 60 to 90 mils high and less than 40 mils thick. By way of example, an appropriate thickness would be approximately 30 mils. For the purpose of this description, each of the substantially rectangular strands 45 will be defined as having a major axis and a minor axis. The major axis (M) is defined as the cross sectional height, while the minor axis (m) is defined as the cross sectional width.

The rectangular conductor strands 45 are laid side by side and may be bonded together using a solvent-activated adhesive over the enamel insulation. The bundle section 42 is then taped with an adhesive paper 57 as shown in FIG. 5. Preferably two layers of the adhesive paper 57 will be wrapped about the conductor strands 45 to form a bundle section 42.

The thickness of the strands 45 across the width of the turn largely determines the magnitude of the eddy current losses due to the direction of the magnetic flux. Thus, the thickness of the strands 45 along their minor axis, (i.e., the 30 mils) and not their major axis height will determine the magnitude of the eddy current losses.

To eliminate circulating currents, transpositions may be made as needed. An appropriate transposition pattern is shown in FIG. 7.

To ensure good conductor strength, the conductor bundle sections 42 should not exceed a width of two inches. In a preferred embodiment, the width of the conductor bundles 40 that form the winding in the region of the end and tap sections will be identical to the width of the conductor bundle 7 in the body section of the coil. In the described embodiment, the conductor bundles are between two and four inches wide. In such an embodiment, the conductor bundle 40 in the end and tap regions may be formed of a pair of side by side bundle sections. However, it should be appreciated, that the width of the conductor bundle 40 may be widely varied within the scope of the invention and that when the design of a particular transformer dictates, the conductor bundle 40 may be formed from a single bundle section, or more than two bundle sections.

Finely-stranded conductors formed into bundle sections that are inches wide yet only a small fraction of an inch thick have several advantages in addition to reducing eddy current losses. For example, continuous windings formed in such a manner have the advantage of greatly improving impulse voltage distribution which permits a significant reduction in turn-to-turn, section-to-section and section-to-ground insulation clearances. Further, circulating currents within the winding may be virtually eliminated since the conductor bundle in the end and tap regions may be nearly equivalent to continuously transposed conductors. Additionally, the overall size of the transformer may be reduced significantly since the number of section-to-section ducts may be reduced.

Although only one embodiment of the present invention has been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, the actual construction of the transformer may be widely varied. For example, the need for cooling ducts and tap connectors will be entirely dependant upon the transformer design requirements. It should also be appreciated that the dimensions of the conductor bundles and bundle sections, as well as the dimensions of its individual conductor strands could be varied beyond the exemplary ranges provided within the scope of the present invention. This is particularly true for the dimensions of the conductor bundle and ribbons within the body section. The transposition scheme may also be widely varied within the scope of the present invention. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.