Transformer with improved liquid cooled disc winding

Abstract

A force cooled electrical transformer with a disc or flat coil winding conventionally has vertical duct walls and horizontal baffling to direct liquid coolant flow in a zig-zag path. The several disc coils in each coil section between horizontal baffles have a graduated, unequal coil spacing which decreases with height, to thereby achieve uniform velocity of the coolant flow radially between the coils for improved cooling.

Description

BACKGROUND OF THE INVENTION

This invention relates to the cooling of electrical transformers, and more particularly to a force cooled transformer with a disc or flat coil winding having a graduated, unequal spacing of the individual coils for improved distribution of the liquid coolant in each winding section.

Force cooled power transformers of the type having a vertically oriented disc coil winding are commonly constructed with an equal spacing between individual coils and with a baffle structure to assure that pumped insulating oil or other liquid coolant is directed radially between the coils as well as upwardly through the winding. Although variations in coil spacing may have been used, these are at random locations to accommodate electrical tappings, etc. In these liquid-cooled transformers, a pair of concentric cylindrical duct walls are mounted within and surrounding the disc winding, thereby defining inner and outer vertical ducts for coolant flow at each side of the coils. According to prior practice, staggered flow baffles completely block the vertical ducts at alternate vertically separated locations inside and outside the coils to thereby circulate the pumped coolant in a zig-zag path through the winding. When the coils in each winding section between adjacent horizontal baffles are all equally spaced, equal cooling of the individual coils in each section is not obtained because the flow distribution among the various horizontal ducts in the winding section is unbalanced. Both by computer calculations and experimentally it has been shown that the spread of coolant velocity among the several horizontal ducts in a section is considerable, being relatively high at the uppermost duct and subnormally low at the lowermost duct. Since the cooling process depends to a large extent upon the coolant velocity distribution, it follows that some of the disc coils are cooled better than expected while other coils are not receiving their share of coolant. For improved cooling it is essential to insure that the coolant flow velocity is uniform for all the horizontal ducts in a winding section, and thus avoid potential local hot spots caused by lower than expected coolant flow.

One solution given by Japanese Utility Model publication No. SHO-46-15364 (U.M. registration No. 951,748 granted Jan. 25, 1972), is to supplement the alternate full blockage baffles, placed at intervals of several equispaced coils to establish a zig-zag flow path, by graduated partial baffle rings in each section to provide more uniform velocity of flow between the disc coils. The horizontal width of the partial baffle rings increases with vertical height in each section to thereby achieve uniform pressure head across the several horizontal ducts and result in an equal balance of coolant flow in the horizontal passages. This baffle arrangement and an alternative embodiment are illustrated in FIGS. 5 and 6, and will be discussed briefly later with its disadvantages. The present invention is directed to an improvement over these force cooled power transformers.

SUMMARY OF THE INVENTION

In accordance with the invention, in a forced liquid cooled electrical transformer with a disc or flat coil winding as previously described, uniform coolant velocity for improved and more uniform cooling is obtained by grouping the disc or flat coils into coil sections wherein the individual coils are vertically unequally spaced with a graduated coil spacing which decreases in the direction from the lowermost to the uppermost coil in each coil section. Preferably, the variable coil spacing is linear with height, but this is not essential. The successive coil sections are bounded in the vertical direction by staggered complete blocking horizontal baffles, either annular plates or peripherally attached rings, which direct the coolant flow horizontally inwardly and outwardly in the successive coil sections in a zig-zag flow path as presently known. This technique for improved forced cooling does not require additional baffling, is easily and inexpensively manufactured, and is compatible with many transformer configurations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic vertical cross section, with parts omitted, through a transformer constructed as herein taught and showing a single magnetic core and disc coil winding assembly immersed in a liquid coolant in a tank with provision for external cooling and pumping of the coolant;

FIG. 2 is a horizontal cross section through the disc coil winding and duct walls of FIG. 1 illustrating an inner flow baffle;

FIG. 3 is a perspective view of a pair of disc coils and the spacer elements between them;

FIG. 4 is a schematic vertical cross section through half of the disc coil winding with graduated, unequal vertical spacing according to a modification wherein baffle rings are used to direct the pumped flow; and

FIGS. 5 and 6 are similar schematic vertical cross sections of two embodiments with additional baffles for uniform coolant flow essentially as disclosed by a prior art Japanese utility model patent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 is shown in diagrammatic form a magnetic core and disc coil winding sub-assembly or assembly such as is used in forced oil cooled power electrical transformers. The subassembly is comprised by a vertically oriented magnetic core element 10 and associated surrounding disc coil winding indicated generally at 11, and is immersed in a suitable liquid coolant 12 such as insulating oil or pyranol contained with a transformer tank 13. A number of cooling and recirculation lines 14 for the liquid coolant are mounted exterior to the tank adjacent to one sidewall or completely around its periphery, each making connection between an outlet manifold 15 near the top of the tank and an inlet manifold 16 near the bottom of the tank. Heated coolant entering the exterior recirculation line 14 is cooled by heat exchangers, forced air cooling using fans, or possibly by radiation cooling, and is then force-pumped to re-enter the tank near the bottom by means of a suitable pump 17 included in the cooling and recirculation system. Force cooled transformers with disc coil or flat coil windings are made in a variety of single phase or multi-phase configurations with different arrangements of the primary and secondary windings. In a three-phase power transformer, for example, there are three such upstanding magnetic core winding legs, all interconnected in a suitable magnetic core structure and immersed together with their windings in a large rectangular tank. Most commonly, several concentric disc coil windings are mounted surrounding the vertical magnetic core in each leg and connected respectively as the primary, secondary, and perhaps tertitary windings. In some transformers, the primary and secondary windings are alternated in a single coil winding. Although described primarily with regard to transformer windings with disc coils, the invention is applicable generally to flat coils of many shapes.

Disc coil winding 11 is comprised by a stack of approximately equal sized individual disc coils or flat coils 11a, grouped into a plurality of coil sections. Each coil section comprises several of the disc coils 11a, the individual coils in a section being unequally, vertically spaced from one another with a graduated coil spacing which decreases in the direction from the lowermost to the uppermost disc coil in the section. Three such coil sections each made up of five disc coils are illustrated in FIG. 1. As is conventional, a pair of concentric vertical cylinders 18a and 18b are mounted inside of and outside of the disc winding 11, respectively, equally spaced from the inner periphery and the outer periphery of the individual disc coils. These concentric cylinders provide parallel cooling duct walls to direct the circulation of pumped liquid coolant 12 through the disc winding, and these duct walls 18a and 18b are made of pressboard or other suitable insulating material. The annular spacing thus defined between the inner periphery of the disc winding 11 and the inner duct wall 18a provides an inner vertical cooling duct 19a for the upward circulation of liquid coolant, while similarly the annular space between the outer periphery of the disc coil and the outer duct wall 18b provides an outer vertical cooling duct 19b. The concentric, vertical cooling ducts for the passage of coolant have approximately equal width in the radial direction. Staggered horizontal baffle plates 20a and 20b are respectively attached to the inner and outer duct walls 18a and 18b between the coil sections, and alternately completely block the inner and outer vertical cooling ducts 19a and 19b. As also shown in FIG. 2, the horizontal baffle plates 20a and 20b are annular in shape and are made of pressboard or other suitable insulating material. With this arrangement of the horizontal baffles between coil sections, the pump liquid coolant is directed to flow generally in a zig-zag path as indicated by the arrows in FIG. 1. In the sucessive coil sections, the horizontal flow between individual coils is alternately radially inwardly and radially outwardly.

Referring also to FIG. 3, each individual disc coil 11a is annular in shape and tightly wound so as to be continuous, and within the coil itself no ducts are provided for the passage of liquid coolant. The electrical connections of the individual coils to form a winding are as known in the art. The individual disc coils 11a are stacked vertically one upon the other using a plurality of spacer members 21 made of a suitable insulating material. Commonly, the spacer members 21 are formed by stacking thin pieces of pressboard or plasterboard, and in this case the graduated, unequal vertical spacing of the disc coils in each coil section is easily implemented by using a different number of laminations. As will be pointed out, the graduated unequal coil spacing in each coil section is preferably linear or proportional to height.

The effect of varying the vertical coil spacing in a graduated manner in each coil section between the horizontal baffling is to achieve a relatively uniform velocity and distribution of the coolant flow in the horizontal spaces or ducts between the various disc coils in each coil section. Since there is a more uniform velocity of the coolant among the coils in each coil section, improved cooling and the avoidance of hot spots are obtained. This is further clarified by reference to FIG. 4 where the zig-zag flow pattern of the pumped coolant through and past the disc coil winding 11 is indicated by the flow arrows. A different form of horizontal baffling to direct the coolant flow radially between the coils is illustrated. According to this embodiment, staggered complete blockage rings 22a and 22b are attached alternately to the inner and outer peripheries of the uppermost coils in the succeeding coil sections. These circumferentially continuous rings 22a and 22b alternately completely block the vertical cooling ducts 19a and 19b and are effective in the same manner as the horizontal baffle plates 20a and 20 b in FIG. 1 to direct the coolant flow radially inwardly and outwardly in the successive coil sections. Each of the coil sections as shown is comprised by five of the vertically unequally spaced disc coils 11a, which are numbered at the left as 1 through 5. The vertical separation between adjacent coils defines a plurality of horizontal cooling ducts 23 in each coil section which establish parallel radial paths for the flow of coolant in the direction toward an unblocked vertical cooling duct.

It will be realized that heat transfer takes place largely from the horizontal major surfaces 24 of the disc coils, and thus the flow distribution in the several horizontal ducts 23 is of importance. Normal coolant temperature rise from the bottom to the top of a force cooled winding is relatively small (for example, about 2.degree.C) and almost all of the permitted temperature rise (for example, 40.degree.C) of a winding hot spot above the liquid inlet temperature takes place along the boundary layer between the coolant and the coil surface. The convective heat transfer rate at the disc coils in a force cooled, directed flow transformer is usually proportional to about the one-third power of liquid coolant velocity. Therefore, uniform cooling is achieved by making the velocity of radial coolant flow substantially or relatively uniform in the several horizontal ducts between adjacent disc coils in each coil section. The graduated, unequal vertical spacing of the disc coils in a section decreasing preferably linearly with height achieves this result. The actual spacings for a particular transformer winding are calculated by computer program and depend upon such factors as the number of coils in a section, the direction of the flow within a section, whether radially outwards or inwards, the maximum rating of the transformer, etc. The minimum coil spacing, of course, is determined by electrical considerations. The graduated, unequal vertical spacing of the disc coils in a section results in balancing the flow resistances in the several parallel horizontal passages to the pressure head available across the passages and thus achieves a uniform velocity distribution.

The advantages of the invention are appreciated and understood by comparison with other selected prior art. In a structure as shown in FIG. 4, assuming that the individual disc coils 11a are now all equally spaced, it can be shown analytically and experimentally, as was previously mentioned, that the horizontal coolant velocity increases greatly with height. The spread of coolant velocity among the several horizontal ducts within a coil section can be as high as -25 percent to +200 percent of the design value. As a result, the upper disc coils in a section are cooled better than expected while the lower disc coils are not receiving their share of liquid coolant. The variation in coolant flow from passage to passage within a coil section, in the equi-spaced arrangement, is due to the variation in the pressure head available across the several passages caused by non-uniform friction, momentum and buoyancy effects in the inlet and outlet manifold paths associated with the individual horizontal passages. The actual horizontal passage resistances are all uniform, however. As herein taught, correction of the imbalances in coolant flow rates among the several passages is obtained by correcting the imbalance in the passage pressure heads by varying the spacing between the disc coils to counteract the manifold effects. Thus, the disc coils are spaced progressively closer from the bottom to the top in each coil section in a manner to counteract the increasing pressure head available with height across the horizontal flow passage in a section.

Uniform cooling of the several disc coils in a coil section is also achieved by the two embodiments of Japanese Utility Model publication No. SHO46-15364, shown in FIGS. 5 and 6. In these figures, components similar or identical to those in FIG. 4 are identified by the same numerals. In these disc winding configurations, the coils are equally spaced by the same coil spacing "a." Uniform coolant velocity and flow distribution is achieved in the several equal height horizontal ducts 23' in a coil section by the use of the graduated partial barrier rings 25 attached either to the inner or outer peripheries of the intermediate coils in the coil section. The graduated partial barrier rings also balance the flow resistances in the several horizontal ducts. In practice, however, it is difficult in view of the type of inexpensive insulating material used and the usual range of manufacturing tolerances, to manufacture and assemble the graduated barrier rings with sufficient precision to obtain the desired uniform flow distribution. Accordingly, it is readily seen that the transformer winding with graduated, unequally spaced disc coils as herein taught is more easily manufactured and is less expensive, while yet achieving the desirable results of uniform coolant velocity in the horizontal ducts of a coil section. The alternative embodiment of the Japanese patent shown in FIG. 6 also uses equally spaced disc coils. The additional baffling to achieve uniform coolant velocity and flow distribution among at least several horizontal ducts takes the form of vertically oriented wedge shaped baffles 26 mounted in the vertical cooling ducts on the adjacent duct wall. This configuration requires additional baffling and is disadvantageous for reasons similar to those already given for the embodiment in FIG. 5.

While the invention has been particularly shown and described with reference to several preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

The invention claimed is:

1. A force cooled electrical transformer comprising

a tank containing liquid insulating coolant in which is immersed at least one vertically oriented magnetic core element and associated surrounding flat coil winding,

said flat coil winding being comprised by a stack of approximately equal sized flat coils grouped into a plurality of coil sections, each coil section comprising several of said flat coils unequally vertically spaced from one another with a graduated coil spacing which decreases in the direction from the lowermost to the uppermost flat coil in each section, to thereby obtain a relatively uniform velocity of coolant flow between said coils,

a pair of continuous duct walls respectively mounted inside and outside said flat coil winding to define inner and outer vertical cooling ducts, and staggered horizontal baffle means alternately completely blocking said inner and outer vertical cooling ducts between said coil sections, and

means for recirculating and pumping said coolant to flow upwardly through said vertical cooling ducts and horizontally between the several unequally spaced flat coils in each coil section respectively inwardly and outwardly in successive coil sections as directed by said staggered horizontal baffle means.

2. An electrical transformer according to claim 1 wherein said flat coils are disc coils, and the graduated coil spacing between the several unequally spaced disc coils in each coil section is approximately linear.

3. An electrical transformer according to claim 2 wherein said staggered horizontal baffle means is comprised by a plurality of annular horizontal plates alternately attached to said duct walls between the uppermost coil of one coil section and the lowermost coil of the succeeding coil section.

4. An electrical transformer according to claim 2 wherein said staggered horizontal baffle means is comprised by a plurality of complete blockage rings alternately attached to the inner and outer peripheries of the uppermost coils in the successive coil sections.