Method of making an electrical transformer

A method of forming cooling ducts in the windings of a liquid cooled electrical transformer without adding permanent duct formers to the winding, and without requiring an additional manufacturing step to remove duct formers. Plastic tubes dissolvable in the liquid dielectric are utilized as the duct formers. Thermal siphon flow of the liquid through the tube openings when the transformer is energized dissolves the tubes and enlarges the cooling ducts to the outside dimensions of the tubes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates in general to electrical transformers, and more 
specifically to electrical transformers cooled by a liquid dielectric. 
2. Description of the Prior Art 
Electrical transformers cooled by a liquid dielectric, such as mineral oil, 
have cooling ducts formed through the transformer windings in order to 
direct the liquid as closely as possible to the source of the heat, i.e., 
the conductor turns. It is conventional to place solid spacers between the 
turn layers of transformers insulated primarily with cellulosic 
insulation, with the spacers, which become a permanent part of the 
winding, supporting the turns and creating gaps or ducts for coolant flow. 
In my co-pending application Ser. No. 524,227, filed Aug. 18, 1983, now 
U.S. Pat. No. 4,503,605, entitled "Cellulose-Free Transformer Coil and 
Method", which is a continuation of application Ser. No. 264,151, filed 
May 15, 1981, now abandoned, there is disclosed new and improved 
cellulose-free winding structures, and methods of constructing such 
windings, in which the insulation starts out as a liquid resinous 
insulation and is solidified. The methods disclosed control the formation 
and size of the voids in the resulting winding structure, such as those 
due to polymerization shrinkage. My co-pending application discloses 
constructing an electrical winding in a substantially continuous 
operation, including the steps of applying liquid resinous insulation to a 
substrate, quickly solidifying the liquid resin, and immediately applying 
one or more conductor turns to the just-solidified resin. Cooling ducts 
are formed by placing solid elongated strips of plastic into the 
electrical winding as it is being wound. Prior to placing the winding in a 
transformer tank, the plastic is melted, with the resulting voids forming 
the cooling ducts. 
SUMMARY OF THE INVENTION 
Briefly, the present invention relates to new and improved methods of 
constructing electrical windings, which methods may be used with, or in 
place of, certain of the methods disclosed in my hereinbefore-mentioned 
commonly assigned patent application. The present invention retains the 
advantages of the methods disclosed in my co-pending patent application, 
including the formation of cellulose-free insulation in situ while an 
electrical winding is being constructed on a mandrel or coil former at 
commercial winding speeds. The substantially void-free solid insulation 
produced by such methods possesses a higher and more uniform electrical 
breakdown strength, a greater mechanical strength, and improved thermal 
conductivity. 
The present invention specifically relates to the formation of 
cellulose-free electrical windings having cooling ducts formed therein for 
the flow of a liquid cooling dielectric. Instead of utilizing said solid 
strips of plastic, as disclosed in my co-pending application, plastic 
tubes are strategically placed in the electrical winding as it is being 
wound, with the plastic material being selected such that it dissolves in 
the liquid dielectric without deleteriously affecting the electrical or 
cooling characteristics of the liquid. The opening in each tube is 
selected such that the liquid will flow therethrough by thermal siphon 
when the transformer is energized, and the outside dimensions of each tube 
are selected according to the size of the desired cooling duct. Thus, the 
cooling ducts of the desired cross-sectional dimension are automatically 
formed, without the necessity of an additional manufacturing step. 
Energization of the transformer during test and/or during actual usage 
thereof, will heat the liquid dielectric and start its flow through the 
openings in the tubes. The heated liquid dissolves each tube, starting at 
its inner wall and progressing outwardly, until the complete tube is 
dissolved, leaving a cooling duct of the desired cross-sectional 
configuration and dimensions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a perspective view, with parts cut away, of an exemplary 
distribution type transformer 10 having liquid cooled windings constructed 
according to the new and improved methods of the invention. Transformer 10 
includes a core-coil assembly 12 which includes a coil 13 comprising high 
and low voltage windings disposed in inductive relation with a magnetic 
core 18. The core-coil assembly 12 is disposed in a tank 20, and it is 
immersed in a liquid cooling medium 22. Transformer oil may be used for 
the liquid cooling medium, but if the windings are of the type which do 
not require oil for electrical insulating purposes, other liquids selected 
primarily for their cooling characteristics may be used. The high voltage 
winding is connected to a high voltage bushing 24 for energization by a 
source 26 of electrical potential, and the low voltage winding is 
connected to low voltage bushings, such as bushing 28, for connection to a 
load 32. Each winding of coil 13 may be constructed in sections, which are 
electrically connected together, or only one section per winding may be 
used, as desired. While it has been conventional for the low voltage 
winding to be physically located next to the magnetic core 18, in a 
low-high (L-H) arrangement or in a low-high-low (L-H-L) arrangement, it is 
to be understood that the high and low voltage windings may be in any 
desired order. The low voltage winding may be constructed of sheet 
conductor, such as aluminum sheet insulated with a thin resinous layer of 
insulation on each side thereof, or it may be formed of wire commonly 
called strap. The high voltage winding may be constructed of flattened 
round wire, pre-insulated with a suitable insulating material such as 
enamel, but other cross-sectional configurations may be used, such as 
round or rectangular. 
FIG. 2 is a perspective view illustrating a first step of a method of 
forming cooling ducts in a winding of transformer 10. For purposes of 
example, the invention will be described relative to the construction of a 
winding 32 of coil 13 constructed of wire, and it will be described with 
reference to a rotating mandrel or coil support 36 which has a rotational 
axis 38. It would also be suitable for the mandrel 36 to be stationary, 
with supply stations rotating about the mandrel. The rotational axis of 
mandrel 36 is coaxial with the center line of the resulting coil 13. 
Coil 13 requires ground wall insulation 42 which will be disposed between 
the innermost winding, which will be winding 32 in this example, and the 
magnetic core 18 shown in FIG. 1. The ground wall insulation 42 may be 
provided by disposing a pre-manufactured winding tube on mandrel 36, or it 
may be built up of a plurality of thin layers of liquid insulation, with 
each layer of insulation being applied and quickly solidified, such as by 
ultra-violet light, before the next layer is applied, as described in 
detail in my hereinbefore-mentioned commonly assigned patent application 
Ser. No. 524,227. In order the simplify the description, application Ser. 
No. 524,227 is hereby incorporated into the present application by 
reference, and the details of forming a non-cellulosic transformer will 
not be described. If ground insulation 42 is formed in situ as mandrel 36 
is rotated, it may be formed of the same liquid resin used to form the 
insulation for winding 32. A suitable mold release material may be sprayed 
or otherwise applied to the mandrel 36 prior to the building up of the 
ground wall insulation 42 with liquid resinous insulation. When the 
plurality of thin layers of solidified resinous insulation have been 
applied to achieve the desired thickness of ground wall insulation 42, 
winding 32 may be wound on insulation 42. Mandrel 36 is rotated in the 
direction of arrow 44, about is rotational axis 38. A conductive strand 
46, such as copper or aluminum wire suitably insulated with enamel 48, 
hereinafter referred to as wire 50, is used to construct winding 32. Wire 
50 may have any desired cross-sectional configuration, such as round or 
rectangular, with flattened round wire being excellent because of its good 
space factor. Wire 50 is placed into position on the insulative substrate, 
i.e., the insulation 42 in the present example, and it is suitably secured 
adjacent one axial end of mandrel 36. Mandrel 36 is then rotated about its 
axis 38 to draw wire from a supply reel (not shown) to form conductor 
turns 52, as shown in FIG. 2. Turns 52 are formed side-by-side until a 
layer of turns have been completed. Another layer of turns may be formed 
directly upon the first layer, with the turns of the next layer usually 
progressing in the opposite axial direction from the turns of the 
preceding layer. 
When the desired number of turn layers has been formed, mandrel 36 is 
stopped momentarily for the provision of cooling ducts. Instead of 
applying solid plastic strips to the outer surface of a turn layer of coil 
32, as described in my co-pending application, non-collapsible plastic 
tubing 52 is used. The non-collapsible plastic tubing 54 is selected such 
that it will dissolve in the liquid 22 when the coil 12 is immersed in the 
liquid and electrically energized. In addition to dissolving in liquid 22, 
the plastic selected must not adversely affect the cooling or electrical 
insulating characteristics of liquid 22. If liquid 22 is transformer oil, 
polyethylene tubing is excellent. A plastic tube of any cross-sectional 
configuration which will not collapse during coil winding and which will 
provide a flow path for the liquid coolant may be used. For example, 
polyethylene tubing having an O.D. of 0.25 inch (0.635 cm) and a 0.040 
inch (0.102 cm) wall thickness is excellent. The tubing may be thermally 
flattened on two sides, if desired, so that the maximum radial dimension 
added to the coil layers or any one grouping of plastic tubes is about 
0.125 inch (0.317 cm). The actual outside dimensions of the flattened 
round tubing was 0.13 inch .times.0.031 inch (0.33 cm.times.0.787 cm). The 
flattened tubing defines an opening for liquid coolant flow having a 
dimension of 0.050 inch .times.0.23 inch (0.127 cm .times.0.584 cm). A 
plurality of such tubes, pre-cut to the desired length, may be melt bonded 
or ultrasonically bonded to a thin, e.g., 0.005 inch thick (0.0127 cm) 
plastic ribbon 56 (FIG. 3), which may also be formed of polyethylene, to 
facilitate quick and accurate placement and spacing of the tubes 54 where 
coolant ducts are required. The tubes 54 may have a predetermined spacing 
formed between them, which spacing will be filled with liquid resin which 
is solidified during the winding process, or the tubes may be placed in 
contacting side-by-side relation to eventually create a single elongated 
cooling duct. Bonding of the resin used to insulate winding 32 to the 
material of tubing 54 is not necessary or important, as the plastic tubing 
will not become a permanent part of coil 13. 
After the plastic tubes 54 have been secured to the winding layer, as shown 
in FIG. 3. the winding process continues as shown in FIG. 4, with one or 
more turn layers of winding 32 being applied over the plastic tubes 54. 
Additional cooling ducts may then be formed between turn layers, if 
desired, by repeating the steps shown in FIG. 3. 
When coil 13 has been completed and assembled with magnetic core 18 to 
provide the core-coil assembly 12 shown in FIG. 1, the assembly 12 is 
disposed in tank 20 and immersed in the liquid 22. When the transformer 10 
is energized by source 26 and connected to load 32, either during testing 
in the factory or during actual usage, the temperature of the windings of 
coil 13 will start to rise, thermal gradients will be produced in liquid 
22, and the warm liquid will rise and the cooler liquid will fall, 
creating a thermal siphon flow of liquid 22 about coil 13 and also through 
the openings defined by the plastic tubes 55. 
FIG. 5A is an enlarged cross-sectional view of coil 13, taken between 
arrows V--V in FIG. 1, illustrating the tubes 54 before dissolving in 
liquid 22. When liquid 22 reaches the point of solubility, which is 
approximately 75.degree. C. for polyethylene, the tubes 54 dissolve in 
liquid 22 and the duct size increases to the outside dimensions of the 
tubes, forming the cooling ducts 60 shown in FIG. 5B. FIG. 5B is similar 
to the view shown in FIG. 5A except the tubes 54 have been dissolved to 
create the larger cooling ducts 60. The insulation 62 which defines the 
walls of the cooling ducts 60 is the cellulose-free liquid resinous 
insulation which has been applied in thin layer upon thin layer, with each 
thin layer being solidified before the next layer is applied as described 
in my incorporated co-pending application. 
In summary, there has been disclosed new and improved methods of forming 
cooling ducts in the winding of an electrical transformer cooled by a 
liquid dielectric, which greatly simplifies the formation of such ducts 
without adding any permanent duct formers to the winding, and without 
adding a separate manufacturing step to remove duct formers. The duct 
formers which are utilized create relatively small flow paths for the 
liquid dielectric when the transformer is initially energized, and the 
flow paths increase in cross section as the flowing liquid dissolves the 
tubing, to enlarge the coolant ducts or flow paths to the outer dimensions 
of the tubes.