Dry-type transformer and method of manufacturing

A dry type transformer has an iron core, high voltage windings embedded in cast resin, and low voltage windings resin encapsulated. The low voltage winding is constructed with flexible sheet conductors. Insulating material includes an apparatus to secure the windings in place during a vacuum and pressure resin impregnation process. The result is a coil that exhibits high short circuit protection due to the tightly bond conductors comparable to completely resin encased molded transformers at a substantially reduced cost.

DESCRIPTION 
RELATED APPLICATIONS 
This application is related to the following, commonly assigned 
applications filed concurrently herewith, entitled "Method of 
Manufacturing a Strip Wound Coil to Eliminate Lead Bulge" Ser. No. 
08/032,371, Our Docket MON-02); "High Strength Strip Wound Coil" Ser. No. 
08/032,218, Our Docket MON-03); "Method of Manufacturing a Strip Wound 
Coil to Reinforce Edge Layer Insulation" Ser. No. 08/032,225, Our Docket 
MON-04); and "Method of Manufacturing a Laminated Coil to Prevent 
Expansion During Coil Loading" Ser. No. 08/032,224, Our Docket MON-05). 
The contents of these applications are expressly incorporated herein by 
reference. 
1. Technical Field 
Applicant's invention relates generally to dry type transformers having an 
iron core, a high voltage winding embedded in cast resin, and a low 
voltage winding, and more particularly to a method of manufacturing the 
low voltage winding. 
2. Background Art 
Dry type transformers with primary voltages over 600 volts have generally 
been constructed using one of three types of techniques, conventional dry, 
resin encapsulated, or solid cast. The conventional dry method uses some 
form of vacuum impregnation with a solvent type varnish on a completed 
assembly consisting of the core and the coils or individual primary and 
secondary coils. Some simpler methods required just dipping the core and 
the coils in varnish without the benefit of a vacuum. The resulting voids 
or bubbles in the varnish that are inherently a result of this type of 
process due to moisture and air, does not lend itself to applications 
above 600 volts. The resin encapsulated method encapsulates a winding with 
a resin with or without a vacuum but does not use a mold to contain the 
resin during the curing process. This method does not insure complete 
impregnation of the windings with the resin and therefore the turn to turn 
insulation and layer insulation must provide the isolation for the voltage 
rating without consideration of the dielectric rating of the resin. The 
solid cast method utilizes a mold around the coil which is the principal 
difference between it and the resin encapsulated method. The windings are 
placed in the mold and impregnated and/or encapsulated with a resin under 
a vacuum, which is then allowed to cure before the mold is removed. Since 
all of the resin or other process material is retained during the curing 
process, there is a greater likelihood that the windings will be free of 
voids, unlike the resin encapsulated method whereby air can reenter the 
windings as the resin drains away before and during curing. Cooling 
channels can be formed as part of the mold. One type of such a transformer 
is manufactured by Square D Company under the trademark of Power-Cast 
transformers. Another example of a cast resin transformer is disclosed in 
U.S. Pat. No. 4,488,134. 
Since the resin coating on solid cast coils results in a solid bond between 
adjacent conductors than is possible with resin encapsulated coils, solid 
cast coils exhibit better short circuit strength of the windings. Because 
the conductors in the coils are braced throughout by virtue of the solid 
encapsulant there is less likelihood of movement of the coils during short 
circuit conditions and short circuit forces are generally contained 
internally. External bracing, foil-wound coils, or selective geometry in 
the shape of the coils must be used in the resin encapsulated method to 
prevent movement of the coils caused by the forces of short circuit 
faults. An added benefit is that by having greater mass, there is a longer 
thermal time constant with the solid cast type coils and there is better 
protection against short term overloads. The resin encapsulated method 
does however have several distinct advantages over solid cast coils. They 
are simpler to manufacture and require less resin and other materials, 
resulting in less weight and lower costs. Additionally, the cast resin 
process requires an epoxy resin which also requires fillers such as glass 
fibers to provide mechanical strength. The epoxy resins generally are 
limited to a 185 deg.C. temperature, whereas resin encapsulated coils can 
utilize polyester resins which can achieve 220 deg.C. ratings. Given these 
advantages, it would be desirable to produce transformers with the resin 
encapsulated method if there were a method to increase the strength of the 
coil windings to prevent movement during short circuits. It would also be 
advantageous to provide better insulation at the top and bottom portions 
of the coils to prevent moisture and other environment contaminants from 
deteriorating the windings. 
The air gap between the high and low voltage coils is dependent on having 
the same geometry between the outer surface of the inner coil and the 
inner surface of the outer coil. A large factor on the shape of the coil 
is the method of attaching the external leads to the winding. For 
non-molded coils, there is generally a distinct bulge at the point where 
this occurs. As a result, the air gap between coils will be uneven. 
Inductive reactance of a transformer is determined by this air gap, along 
with the number of turns in the coil and the physical dimensions of the 
coil. Controlling these factors will result in limiting short circuit 
currents and thus controlling withstand ratings. 
SUMMARY OF THE INVENTION 
Accordingly, the principal object of the present invention is to provide a 
transformer with a high voltage winding utilizing a cast resin method and 
a low voltage winding constructed according to the resin encapsulated 
method which overcomes the above mentioned disadvantages. 
A further objective of the invention is to provide a method for 
manufacturing a transformer winding constructed according to the resin 
encapsulated method which prevents moisture penetration into the windings 
and which will prevent flashovers due to moisture condensation. 
Yet a further objective of the invention is to provide a transformer 
winding constructed according to the resin encapsulated method utilizing 
aluminum strip wound secondary windings which will prevent conductor 
movement during short circuit fault conditions. 
Another objective of the invention is to provide a transformer winding 
constructed according to the resin encapsulated method which will maintain 
shape and dimension integrity, while facilitating thermal conductivity and 
improving dielectric strength. 
In addition, another objective of the invention is to provide a method for 
manufacturing a transformer winding constructed according to the resin 
encapsulated method which produces an essentially circular winding that 
does not have a bulge due to external lead attachments. 
Still another objective of the invention is to provide a method for 
manufacturing a transformer core which will have a constant, uniform 
compression applied throughout the length of the core legs, resulting in 
an improved coil loading procedure, reduced core losses, and reduced core 
audible noises. 
In one embodiment of the invention, the inner or low voltage coil is formed 
on a special cylinder or mandril with a flat surface on a portion of the 
cylinder from which one external lead which is welded to a conductor 
sheet, such as aluminum or copper, will rest on during the start of the 
winding. The flat surface will allow the windings to retain a circular 
shape. Along with the aluminum, a layer of insulating material will be 
including during the winding process. The insulating material will have a 
pattern of thermo-set or B-stage adhesive coated on it that will prevent 
movement of adjacent windings during the resin impregnation process and 
will allow the various windings to retain a circular shape. The resin will 
be able to provide a better bond between windings since the various 
windings are held in place while processing. This bonding will provide 
extra strength to the windings and prevent movement of them under short 
circuit conditions. At a predetermined number of turns, spacers will be 
added to form air channels within the windings and the process will be 
repeated until the desired number of turns has been reached. The end of 
the winding will terminate at another flat surface and the other external 
lead will be attached to maintain the circular shape. 
After the coil is thusly assembled, it will be subjected to a 
vacuum-pressure impregnation (VPI) process. This process starts with the 
coil first being pre-heated in an oven to remove moisture from the 
insulation and the aluminum windings. The coil is then placed in a vacuum 
chamber which will be evacuated, which will remove any remaining moisture 
and gases, and in particular, voids between adjacent windings will be 
essentially eliminated. A liquid resin is then introduced into the 
chamber, still under a vacuum, until the coil is completely submerged. 
After a short time interval which will allow the resin to impregnate the 
insulation, the vacuum is released and pressure is applied to the free 
surface of the resin. This will force the resin to impregnate the 
remaining insulation voids. The coil is then removed from the chamber or 
the resin from the chamber is drained. The coil is then allowed to drip 
dry and then is placed in an oven to cure the resin to a solid. A further 
buildup of resin could be accomplished by repeating the process with 
resins having a higher viscosity to provide the finished coil with a 
conformal coating for a better appearance and greater isolation from 
environmental factors. The completed coil will have superior basic impulse 
level (BIL) protection since there are essentially no voids, short circuit 
withstandability is improved since there is little chance of the 
individual windings moving due to the bonding, and overload capacity is 
increased since heat generated in the windings will transfer to the 
cooling ducts better through a solid mass than if voids were present in 
the windings. 
The outer coil or high voltage coil is a cast resin coil and is also 
fabricated using a VPI process, with the chief difference being that the 
resin is poured into a mold containing the coil, allowing the curing to 
take place inside the mold. The transformer is then assembled by inserting 
the inner coil over an iron laminated core and then inserting the outer 
coil around the inner coil. The resultant assembly is then secured with 
appropriate clamps and mounting feet, along with terminal means for 
external connections. 
Other features and advantages of the invention will be apparent from the 
following specification taken in conjunction with the accompanying 
drawings in which there is shown a preferred embodiment of the invention. 
Reference is made to the claims for interpreting the full scope of the 
invention which is not necessarily represented by such embodiment.

DETAILED DESCRIPTION 
Although this invention is susceptible to embodiments of many different 
forms, a preferred embodiment will be described and illustrated in detail 
herein. The present disclosure exemplifies the principles of the invention 
and is not to be considered a limit to the broader aspects of the 
invention to the particular embodiment as described. 
FIG. 1 illustrates a typical three phase transformer 1 constructed 
according to the preferred embodiment. Although a three phase transformer 
is shown, it is to be understood that the invention is not to be limited 
to three phase construction. A high voltage coil 2 surrounds a low voltage 
coil 4. The high voltage coil 2 is constructed using a VPI cast resin 
process, the details of which are well known and are therefore not an 
object of this invention. U.S. Pat. No. 4,523,171 discloses one such 
method. The low voltage coil 4 is constructed using a VPI resin 
encapsulated process which will be discussed later. A core 6 is formed in 
the shape of a cruciform from laminated straps of iron for ease of 
manufacturing. A core locking strap 7 is added to the top of the stack. 
Previously, after the core legs 6 were stacked, a series of banding straps 
were used to keep the core legs compressed. During the loading of coils 2, 
4, the bands were cut as they are lowered into position. This causes the 
core legs to expand, interfering with the procedure. The expanded core 
legs result in increased core noise and losses. A fiber glass tape could 
be wound around the core legs and then coated with a type of epoxy tape, 
but this increases manufacturing time and costs. To improve the method, 
instead of banding straps, core compression and stabilization is 
accomplished with the use of a heat shrink film material 8 with an elastic 
property that will hold the core leg in a constant uniform compression. 
The heat shrink material 8 Such as Dupont Mylar is wound around the core 
legs 6 and then heated to shrink the material 8 tightly around the the 
core legs. An alternative to the heat shrink material 8 is to use some 
other type of film material or narrow tape having elastic properties and 
wrapping the material under tension around the core legs 6 to keep them 
under compression. After the core legs are thusly secured, an epoxy type 
paint is applied to exposed areas for environmental protection. An upper 
core yolk 10 is secured to the core 6 by mating strap 11 with core locking 
strap 7 after the low voltage coils 4 and high voltage coils 2 have been 
inserted over the three legs of the core 6. Lower core clamp 12 holds and 
secures core 6 with mounting hardware 18. Upper core clamp 20 holds and 
secures upper core yolk 8 similarly with mounting hardware 22. Upper 24 
and lower 26 mounting blocks support high voltage coil 2 and low voltage 
coil 4. Tab 28 of mounting blocks 24, 26 maintains an air gap 30 between 
the coils 2, 4. Mounting feet 32 can be attached for stability. Terminal 
blocks 34 allow for high voltage connections and have provisions for 
selected various voltage taps for a wide selection of input and output 
voltages. Terminals 36 provide the means for low voltage connections. A 
transformer thus assembled can accommodate input voltages up to 36 kV, 
with a power rating between 112.5-10,000 kVA. 
Referring to FIG. 2, a partial cross sectional view of the low voltage coil 
4 is illustrated., constructed according to the present invention, which 
in turn is surrounded by a cast resin high voltage coil of the type 
depicted in the transformer of FIG. 1. An air gap 40 separates the core 
leg 6 from the low voltage coil 4. The low voltage coil 4 is composed of 
multiple windings 42, 44, 46 of flex sheet conductors such as copper or 
aluminum, with formed air channels 43, 45 to provide a means of cooling 
during operation of the transformer 1. Air gap 30 separates the low 
voltage coil 4 from the high voltage coil 2 with the distance of the gap 
being determined by the tab 28 on mounting blocks 24, 26 previously 
mentioned. High voltage coil 2 consists of wire conductors 48, 49, with 
molded air channels 50. The distance 52 between the top of the conducting 
materials in coil 2 and the top yolk 10 is chosen to meet high voltage to 
frame clearances. Likewise, the distance 53 between the top of the 
conducting materials in coil 4 and the top yolk 10 is chosen to meet the 
low voltage to frame clearances. Air gap 54 provides isolation between 
voltage phases. 
A more detailed view of section C--C of FIG. 2 is shown in FIG. 3 to 
illustrate a means for reinforcing the top and bottom edges of the 
windings 42, 44, 46 of the low voltage coil 4. The low voltage coil 4 is 
composed of multiple laminations of flex sheet conductors. The description 
for winding 44 will also hold true for the other two windings 42 and 46. 
Film insulation sheets such as Nomex form an excellent winding layer 
insulation system. This layer 60 is extended beyond the edge of the sheet 
conductors 62, as designated by the distance X for obtaining the necessary 
creep strength requirements. When winding the insulating layers 60 with 
the sheet conductors 62, the edges of the layers 60 can collapse due to 
the soft texture of the material, which could result in blockage of the 
cooling ducts, limiting the cooling characteristics of the coil. Outside 
barriers 64 which extend a distance Y beyond the edge of the insulating 
layers 60, provide the stiffness to prevent this collapse and are selected 
based on the voltage class of the transformer. For a minimum of a basic 
impulse level (BIL) of 10 kV, common for an isolation rating between the 
core 6 and the low voltage coil 4, the inside barrier 63 will be one 
thickness of 0.031 inch sheet insulation such as a product trademarked 
Glastic plus two pieces of another insulator, 5 mil thick, such as a 
product trademarked Nomex. For a minimum BIL of 95kV, common for an 
isolation rating between the high voltage coil 2 and the low voltage coil 
4, the outside barrier 65 will be two thicknesses of 0.031 inch sheet 
insulation. The space between the insulating layers 60 is packed with a 
glass mat or felt edge material 66 to control the movement of the sheet 
conductors 62 during short circuit conditions. The glass felt edge 
material 66 could be any type of porous dielectric characterized by high 
temperature rating and stability. The dielectric constant must be greater 
than air to maintain proper voltage gradients between the core or frame 
and the high voltage conductors. Examples of such a material 66 are Nomex 
411, Cequin or other types of glass fibrous material. This material 66 
functions to provide protection to the sheet conductors 62 against water 
entry or other contaminants and to provide electrical insulation 
properties for withstanding high voltage transients, in addition to 
providing the mechanical rigidity of the ends of the coil for mechanical 
clamping and short circuit withstand forces. The material 66 must allow 
the sheet conductors to be impregnated with a suitable electrical 
insulating resin during the VPI process. 
The insulating layers 60 are coated with a diamond pattern 
B-staged-thermoset adhesive as shown in FIG. 4. A variation of this type 
of arrangement have been used with oil-filled distribution type 
transformers to facilitate the oil impregnation process. The short circuit 
strength of a strip wound coil can be greatly increased by bonding 
together the layers of the sheet conductors 62 during the VPI process with 
a heat cured resin adhesive. During this process however, the shape of the 
coil can become distorted due to thermal stresses. Use of the thermoset 
adhesive allows the layers to become bonded during a preheating process 
before the VIP process. The diamond pattern will create sufficient bonding 
between the sheet conductors 62 to retain the shape of the coil during the 
VPI process and still provide sufficient unbonded areas for the resin to 
impregnate the body of the coil during the VPI process. The resultant coil 
will have greater short circuit withstandability and improved radial heat 
conduction due to bonding throughout the body of the coil. The type of 
resin is chosen to provide a suitable temperature index for the intended 
temperature rise of the coils. In addition it must be able to fill the 
voids and improve the thermal conduction between the sheet conductors 62 
and the heat dissipating surfaces, and lastly, prevent contaminants such 
as water, oils, acids, and industrial fumes from entering and 
contaminating the coils. One such resin is tradenamed PD George 70 red 
color resin. After VPI processing the completed coil is then baked in an 
oven at 350 deg. F. for two hours. An air dry resin is then applied in the 
void 68 to contour the ends of the windings, eliminating voids, and 
facilitating moisture run-off. 
Instead of using the dry resin, other coil finishing treatments and 
extensions can be employed in the void 68. A moisture cured silicone RTV, 
an epoxy resin having suitable cure characteristics for the application, 
or a filled polyester resin could be substituted for the dry resin. 
Another option requires a woven or braided fibrous rope being placed in 
the void 68 before the coil is subjected to the VPI process. The rope 
could be made of glass fiber, Nomex, or other heat resistant material. 
Supporting the outside layers next to the air channels 43, 45 of the 
multiple windings 42, 44, 46 with the outside barriers 64 results in 
increasing the overall radial dimensions of the windings and therefore the 
overall dimensions of the completed transformer 1. This extra thickness 
translates into extra material requirements for the core and coil 
material, including the conductors, insulating film, and resin used to 
encapsulate the windings. An alternative solution is to provide a 
reinforcing material along the edges of the outer insulating layers 60 
next to the air channels 43, 45, for the distance Y, that will provide the 
stiffness to prevent this collapse of the edges. Thus, FIG. 5 illustrates 
the use of Cequin strips 70 or reinforcing nylon strands 72 which will 
maintain the circular shape of the completed coil during the VPI 
processing and prevent the collapse over the air channels 43, 45. The end 
result will be a finished coil that will have a smaller diameter than one 
manufactured using the traditional glastic material, using less material 
and therefore having lower cost. 
The cross sectional view of FIG. 6 provides a more detailed illustration of 
the preferred embodiment of the low voltage coil 4 construction of the 
present invention. The outer or high voltage coil 2 is separated from the 
low voltage coil 4 by the air gap 30. The essentially circular shape of 
the low voltage coil 4 allows the air gap 30 to remain constant throughout 
its entirety which will reduce susceptibility to voltage impulses and will 
help control impedance changes during short circuit conditions. Air gap 40 
separates the cruciform core leg 6 from the low voltage coil 4. The low 
voltage coil 4 is composed of multiple windings 42, 44, 46 of flex sheet 
conductors such as copper or aluminum, with formed air channels 43, 45 to 
provide a means of cooling during operation of the transformer 1. Dogbone 
spacers 76, 78 are staggered and strategically placed and sized so as to 
enable the final exterior shape at the air gap 30 is circular. The spacers 
76, 78 are protruded glass reinforced polyester. Spacing between adjacent 
spacers 76, 78 varies from 1.5 inches to 2.5 inches on center. This 
spacing is critical since air flow in the created air ducts 43, 45 will be 
restricted if they are too close together, resulting in poorer cooling 
characteristics. If the spacing is too far, voids could be created between 
the insulating layers 60 and the sheet conductors 62 that make up the 
windings 42, 44, 46. This could result in localized hot spots and decrease 
the mechanical rigidity of the over coil 4, which could reduce the short 
circuit withstandability. 
The coil is wound from flexible sheet conducting material start at a flat 
surface 80. Multiple laminations of flex sheet conductor lead are used to 
form the external leads 36, 36' which are welded to the sheet conductor 
62. The leads 36, 36' are deformed during assembly to allow the high 
voltage coil 2 to be inserted around the coil during final assembly of the 
transformer 1 and reshaped appropriately after assembly for external 
connects. Leads 36, 36' are insulated with a creep and strip barrier 
composed of Nomex or other suitable flexible sheet insulation. This 
insulation is to prevent voltage breakdown between the low voltage winding 
4 and the core 6 or other grounded surfaces. The combination of the flat 
surfaces 80, 82, and duct stick 84 allow the leads 36, 36' to be contained 
inside the low voltage coil 4 with no apparent bulge. In addition the 
leads 36, 36' are bonded to the body of the low voltage coil 4. A glass 
rope or other suitable material, running parallel to the lead from top to 
bottom along its major axis is sufficiently porous to absorb resin during 
the VPI process to provide lead support and reinforcement, preventing 
movement of the lead from short circuit forces. 
While the specific embodiments have been illustrated and described, 
numerous modifications are possible without departing from the scope or 
spirit of the invention.