US3845436A

US3845436A - Power transformer having shields for shaping the electric field in the major insulation spaces

Info

Publication number: US3845436A
Application number: US00426390A
Authority: US
Inventors: H Robin
Original assignee: Westinghouse Electric Corp
Current assignee: ABB Inc USA
Priority date: 1973-12-19
Filing date: 1973-12-19
Publication date: 1974-10-29
Anticipated expiration: 1991-10-29
Also published as: JPS5352008Y2; JPS5090112U

Abstract

A power transformer having at least one shield positioned in the insulation structure of the transformer and located between the high-voltage and the low-voltage windings. The shield includes a conducting member which shapes the electric field in the insulation structure in a manner which permits more efficient utilization of the insulating members in the insulation structure. The shield may be connected to an intermediate tap on the high-voltage winding in order to acquire a fixed potential thereon.

Description

United States Patent [191 Robin 1 Oct. 29, 1974' POWER TRANSFORMER HAVING SHIELDS FOR SHAPING THE ELECTRIC FIELD IN THE MAJOR INSULATION SPACES [75] Inventor: Harral T. Robin, Muncie, Ind.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Dec. 19, 1973 21 Appl. No.2 426,390

52 U.S.'C1. .Q. 336/84 [51] Int. Cl. H01t 15/04 [58] Field of Search 336/69, 70, 84

[56] References Cited UNITED STATES PATENTS Biermanns 336/70 FOREIGN PATENTS OR APPLICATIONS 1,513,911 4/1969 Germany 336/84 415,414 0/1934 Great Britain 336/70 1,474,535 2/1967 France 336/70 1,213,911 4/1966 Germany 336/84 Primary Examiner-Thomas .1 Kozma Attorney, Agent, or FirmJ. R. Hanway [57] ABSTRACT A power transformer having at least one shield positioned in the insulation structure of the transformer and located between the high-voltage and the lowvoltage windings. The shield includes a conducting member which shapes the electric field in the insulation structure in a manner which permits more efficient utilization of the insulating members in the insulation structure. The shield may be connected to an intermediate tap on the high-voltage winding in order to acquire a fixed potential thereon.

1 Claim, 8 Drawing Figures .PATENIEBUBTZQ 191 4 3.845.436 SHE 10$ 3 PRI FIG.

FIG. 2

FIG. 4

POSlTlON ON HIGH-VOLTAGE- WINDING STRUTURE PATENTEDnm 29 I974 SHEEIZOF 3 llllllllllllllllllllllllllllll III PATENTEDnm 29 I974 samura mvmw- POSITION ON VERTICAL BARRIER SURFACE FIG.8

POSITION ONHORIZONTAL FIG 6 BARRIER SURFACE O O O O O EIEJVIIOA H OBI HJJM (SW8 HONl/AN) S53E18 dI-THO BACKGROUND OF THE INVENTION 1. Field of the Invention:

This invention relates, in general, to electrical inductive apparatus and, more specifically, to power transformers having shields for shaping the electric field within the insulation structure of the transformer.

2. Description of the Prior Art:

Large power transformers, particularly of the shellform type, require a considerable amount of insulating material positioned between the primary and secondary windings of the transformer and between the winding structures and the magnetic core. The use of solid insulating material, such as pressboard, is desirable due to its excellent dielectric properties and the ease with which it may be assembled into the transformer. However, since cooling oil must circulate through the insulation structure to cool various components of the transformer, oil spaces or channels between the pressboard members must be provided. Therefore, some portions of the insulation structure have oil spaces or voids which do not contain any solid insulating material. Other portions of the'insulation structure contain regions which are completely occupied by solid insulating material since the stresses in these regions may become high enough to cause insulation failure if an appreciable oil space is present.

According to the prior art, the pressboard insulating members are arranged in a manner consistent with the electric field which may exist in the insulation structure. As a result thereof, some regions of the insulation structure contain more than one thickness of pressboard material, some of the oil spaces are larger than others, and some of the regions of the insulation structure contain substantially a solid mass of pressboard insulating members. Because of the relatively rigid nature of the pressboard material and the general shape of the winding structure in large shellform power transformers, the insulation structure usually consists of numerous pressboard sheets mounted either horizontally or vertically within the winding structure. Since this type of construction technique must normally be used, the arrangement of the insulation structure to provide the necessary dielectric strength is very time consuming and a surplus of solid insulating material in some regions of the insulating structure usually occurs.

Typical electrical fields in large shell-form power transformers have equipotential lines which pass through the insulation structure and form an oblique angle with the surfaces of the pressboard insulating members. Due to the angle of the equipotential lines of the electric field, voltage differences between points on the surface of the pressboard members and between the opposite sides of the pressboard members are developed. Thus. the pressboard is susceptible to both creepage and puncture failure if the electrical stresses are too large. In order to make the most efficient use of the pressboard insulating members, it is desirable to have the equipotential lines aligned parallel with the surface of the pressboard member. Thus, it is desirable, and it is an object of this invention, to provide a means for shaping the electric field in the insulation structure of a power transformer, with the resultant shape of the field permitting a more convenient and efficient use of the solid insulating material in the insulation structure of the transformer.

SUMMARY OF THE INVENTION There is disclosed herein a new and useful arrangement for shaping the electric field within the insulation structure of a power transformer. At least one shield, consisting of an electrical conducting member, is located within the insulation structure of the transformer and positioned between a high-voltage winding and a low-voltage winding of the transformer. The conducting member may be connected to a potential within the high-voltage winding structure of the transformer. Due to the conducting nature of the shield, the equipotential lines in the portion of the insulating structure between the high-voltage winding and the low-voltage winding acquire a substantially straight shape which conforms to the orientation of the pressboard insulating members contained in this region of the insulation structure. The shield extends sufficiently into the portion of the insulation structure which is located between the high-voltage winding and the magnetic core of the transformer in order to shape the equipotential lines existing in this portion of the insulation structure. These equipotential lines extend generally in a perpendicular direction from the shield and are substantially parallel to the horizontal insulating members in the in sulation structure. The overall result of the shaping of the electric field is that the equipotential lines assume a more rectangular shape than equipotential lines in a insulation structure constructed according to the prior art. Thus, since the solid insulating members are substantially positioned in the insulation structure in a rectangular fashion, more efficient use thereof is made when the shield of this invention is used.

BRIEF DESCRIPTION OF THE DRAWING Further advantages and uses of this invention will become more apparent when considered in view of the following detailed description and drawing, in which:

FIG. 1 is a schematic representation of high-voltage and low-voltage windings in a shell-form power transformer constructed according to the prior art;

FIG. 2 is a schematic representation of the highvoltage and low-voltage windings in a shell-form power transformer constructed according to this invention;

FIG. 3 is a partial cut-away view of a shell-form power transformer constructed according to this invention;

FIG. 4 is a graph illustrating the difference in the puncture stresses near the high-voltage winding structure of a transformer constructed according to the prior art and of a transformer constructed according to this invention;

FIG. 5 is a graph illustrating the difference between the creep stresses at Hz on a horizontal barrier surface of a transformer constructed according to the prior art and of a transformer constructed according to this invention;

FIG. 6 is a graph illustrating the difference between the creep stresses at 180 Hz on a vertical barrier surface of a transformer constructed according to the prior art and of a transformer constructed according to this invention;

FIG. 7 is a graph illustrating the difference between the creep stresses due to an impulse voltage on avertical barrier surface of a transformer constructed according to the prior art and of a transformer constructed according to this invention; and

FIG. 8 is a graph illustrating the difference between the creep stresses due to an impulse voltage on a horizontal barrier surface of a transformer constructed according to the prior art and of a transformer constructed according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the following description, similar reference characters refer to similar elements or members in all the figures of the drawing.

Referring to the drawing, and to FIG. 1 in particular, there is shown schematically a shell-form power transformer 10 constructed according to the prior art. The primary or high-voltage winding 12 and the secondary or low-voltage windings l4 and 16 contain coil sections, such as the coil section 18, which are positioned in inductive relationship with each other. The coil sections are disposed around a portion of the magnetic core 20 which is not illustrated in FIG. 1. The shellform type magnetic core 20 also extends around the ouside of the coil sections. A static plate 22 is positioned adjacent to the coil section 18 and improves the impulse voltage distribution across this end of the highvoltage winding 12.

When a voltage is applied to the high-voltage winding 12, an electric field develops between the high-voltage winding 12 and the low-voltage windings l4 and I6, and between the high-voltage winding 12 and the magnetic core 20. The

dashed lines

24, 26, 28, 30 and 32 indicate the position of equipotential lines which exist between the high-voltage winding 12 and the lowvoltage winding 14 and the magnetic core 20 when a low-frequency voltage, such as a 60 or I80 H2 voltage, is applied to the high-voltage winding 12. Although shown as lines in the two dimensional diagram of FIG. 1 for simplicity, the equipotential points located within an actual transformer 10 would form a three dimensional surface which extends around the high-voltage winding structure 12. The electric field pattern existing in the transformer 10 is unique from other types of transformers due to the difference in size of the highvoltage coil sections.

With the voltage on the low-voltage winding 14 and the core 20 being at substantially ground potential, a low-frequency voltage applied to the high-voltage winding 12 provides an equipotential line 24 which represents 10% of the voltage on the static plate 22. The equipotential line 26 represents percent of the voltage, the line 28 represents 50 percent of the voltage, the line 30 represents 75 percent of the voltage, and the line 32 represents 90 percent of the voltage. From the equipotential lines illustrated in FIG. I, it can be seen that by using substantially rigid and straight insulating members in the insulation structure, it would be impractical to provide an insulation structure which makes the most efficient use of the solid insulating material.

FIG. 2 is a schematic representation of the transformer shown in FIG. 1 with a field shaping shield disposed therein as taught by this invention. In this specific embodiment, the transformer 34 includes the electrically conducting shields 36 and 38 as shown in FIG. 2. The shields 36 and 38 are positioned between the high-voltage winding 12 and the low-voltage winding 14 and are orientated substantially perpendicular to the axis of the winding structure. The shields 36 and 38 in this embodiment are constructed similar to the static plate 22, but with larger outside dimensions. Details of the construction of static plates to which the shields 36 and 38 may be similar are described in US. Pat. Nos. 3,376,53I and 3,643,196, both of which are assigned to the same assignee as is this invention. In general, the shields exhibit a hollow rectangular shape. It is within the contemplation of this invention that other arrangements for providing the conducting element in the shields 36 and 38 may be used.

The shield 38 is electrically connected by the lead 40 to the coil section 42 at position 44. The equipotential line 46 extends from one end of the shield 38, through the coil section 42, to the other end of the shield 38. Since a constant potential exists on the conducting element of the shield 38, the electric field in the region of the shield 38 is substantially straight and is orientated vertically according to the direction illustrated in FIG. 2.

In addition to straightening the electric field in the region adjacent to the shield 38, the electric field in the insulation structure above and below the high-voltage winding 12 is straighter than the field in the same region of the transformer shown in FIG. 1. Control of the shape of the field in these regions is determined by the outside dimensions of the shield 38, its spacing from the high-voltage winding 12, and the potential to which it is connected. In the specific embodiment illustrated in FIG. 2, the shield 38 is substantially the same size as the coil section 42 to which it is connected.

The shield 36 is connected by the lead 48 to the coil section 50 at position 52. The shield 36 is larger than the shield 38 and shapes the equipotential line 54 between the shield 36 and the high-voltage coil section 50. As can be seen from an overall view of FIG. 2, the equipotential lines exhibit a more rectangular shape with the placement of the shields 36 and 38 in the portion of the insulation structure which is located between the high-voltage winding 12 and the low-voltage winding 14. Thus. efficient and convenient use of the solid insulating members may be made in the transformer insulation structure. More shields than the two indicated, or only one shield, may be used without departing from the scope of the invention.

By properly selecting the position at which the shields are located, the leads connecting them to the proper coil section in the high-voltage winding 12 may be tapped to an outside turn of the coil section for tapping convenience. Thus, the potential on the shield would be substantially equal to the potential at the outside of the coil section to which it is connected. In addition, it is possible to shape the electric field in the insulation structure without connecting the shields, such as the shields 36 and 38, to a potential within the highvoltage winding 12. With such a construction, a potential develops on the shield which is related to the position of the shield between the high-voltage and the lowvoltage windings and the insulating material therebetween.

FIG. 3 is a partial cut-away view of a shellform power transformer constructed according to this invention with one electric field shaping shield. The magnetic core 20 is separate from the high-voltage winding structure 12 by the portion of the insulation structure which includes the horizontal pressboard members 54. The low-voltage windings would normally be positioned on both sides of the high-voltage winding structure 12 and would be separated therefrom by the vertical pressboard members 56. The coil section 18 is electrically connected to the static plate 22 by a lead which is not shown in FIG. 3. A shield 58 is positioned between the high-voltage winding 12 and the low-voltage winding which is not shown, and is oriented substantially in parallel with the vertical pressboard members 56.

The structure shown in FIG. 3 makes it possible to make the most efficient use of the pressboard insulating members contained in the insulation structure. Since the puncture and creep stresses on the pressboard members are reduced in many portions of the insulating structure, less solid insulating material may be used and still provide sufficient electrical insulation. Thus, by conforming the electrical field to convenient and standard insulating structures, a more economical insulating structure may be constructed than by trying to conform the insulation structure to the electric field normally exhibited by such transformers. By using less insulating material, the overall size of the transformer may be reduced.

FIG. 4 is a graph illustrating the decrease in the puncture stress adjacent to the coil sections of the highvoltage winding structure 12 by the addition of a stress shaping shield. Data collected for this graph was obtained from a transformer constructed with a shield positioned in the mid-point of the high-low space, or half way between the high-voltage winding 12 and the lowvoltage winding 14 as shown in FIG. 2. The shield was also connected to a potential within the winding structure 12 which was equal to the voltage at the position of the shield during low-frequency tests without any field shaping shield present. Curve 62 represents the puncture stress in a transformer having a sheild for shaping the electric field. Curve 60 represents the puncture stress in a similar transformer without such a shield. At different positions within the high-voltage winding structure. the puncture stress exhibited by the transformer having the shield was always lower than the puncture stress in a similar transformer without such a shield. The positions A through L designated in FIG. 4 refer to the coil sections of the high-voltage winding as they progress in a direction away from the static plate.

FIG. 5 is a graph representing the creep stress on a horizontal barrier surface or pressboard member in transformers with the application of a 180 Hz voltage. Curve 64 represents the creep stress on a horizontal barrier surface in a transformer with the shield, and curve 66 represents the creep stress at the same position in a transformer without the shield. As shown in FIG. 5, the creep stress at low frequency voltages on the horizontal barrier surfaces is considerably less with the transformer containing the electric field shaping shield.

FIG. 6 illustrates the creep stress under similar conditions on vertical barrier surfaces, or pressboard members. Curve 68 represents the creep stress as measured in the transformer containing the electric field shaping shield and curve 70 represents thecreep stress at the same position without the shield.

FIG. 7 indicates the creep stress on a vertical barrier surface, or pressboard member, when an impulse voltage is applied to the test transformers. The creep stress is expressed in percent of the applied impulse voltage. Curve 72 represents the creep stress for the transformer with the shield placed therein and curve 74 represents the creep stress without the shield. It can be seen from FIG. 7 that, with an impulse voltage applied to the high-voltage winding, the creep stress on the vertical barrier surface is substantially less in the transformer having the electric field shaping shield.

FIG. 8 represents the difference in creep stress on a horizontal barrier surface as a percent of the applied impulse voltage. Curve 76 represents the creep stress in the transformer with the shield and curve 78 represents the creep stress in the transformer without the shield.

FIGS. 4 through 8 are typical of curves measured at other positions within the winding structure of the transformer and illustrate a considerable reduction in the electrical stress to which the solid insulating members of the insulation structure are subjected. It is emphasized that the shield placed between the highvoltage and the low-voltage winding structures of the transformer do not necessarily grade the potential be tween the winding structures, but actually shape the electric field in the portions of the insulation structure which are located between the high-voltage winding structure and the magnetic core. It is also emphasized that the shields do not act primarily as capacitor plates in grading electrical stresses between and on sides of the plates, but act as an electrical means for shaping the electric field surrounding the high-voltage windings, particularly the field between the high-voltage winding and the magnetic core. This is accomplished by increasing the height of the equipotential lines as they enter the insulation region between the high-voltage winding and the magnetic core.

Since numerous changes may be made in the above described apparatus, and since different embodiments of the invention may be made without departing'from the spirit thereof, it is intended that all of the matter contained in the foregoing description, or shown in the accompanying drawing, shall be interpreted as illustrative rather than limiting.

I claim as my invention:

1. A shell-form power transformer comprising:

a shell-form magnetic core;

a primary winding structure and a secondary winding structure inductively coupled to said magnetic core, said primary winding structure including a plurality of substantially rectangular coil sections disposed at different axial positions, with the coil section closest to said secondary winding structure having a smaller number of turns than the other coil sections;

a non-magnetic static plate positioned adjacent to the coil section of the primary winding structure which has the smaller number of turns;

a first non-magnetic, electrical conducting shield positioned substantially parallel to said static plate and between said static plate and said secondary winding structure, said shield having a larger outside dimension than said static plate; and

a second non-magnetic, electrical conducting shield positioned substantially parallel to said first shield and between said first electrical conducting shield and said secondary winding structure, said second shield having a larger outside dimension than said first shield, with said first and second shields being electrically connected to potentials within the primary winding structure.

Claims

1. A shell-form power transformer comprising: a shell-form magnetic core; a primary winding structure and a secondary winding structure inductively coupled to said magnetic core, said primary winding structure including a plurality of substantially rectangular coil sections disposed at different axial positions, with the coil section closest to said secondary winding structure having a smaller number of turns than the other coil sections; a non-magnetic static plate positioned adjacent to the coil section of the primary winding structure which has the smaller number of turns; a first non-magnetic, electrical conducting shield positioned substantially parallel to said static plate and between said static plate and said secondary winding structure, said shield having a larger outside dimension than said static plate; and a second non-magnetic, electrical conducting shield positioned substantially parallel to said first shield and between said first electrical conducting shield and said secondary winding structure, said second shield having a larger outside dimension than said first shield, with said first and second shields being electrically connected to potentials within the primary winding structure.