Note: Descriptions are shown in the official language in which they were submitted.
1~77~83 RD 6787
The invention relates to sheet-wound, high-
voltage electrical coils, and more particularly to apparatus
for insulating and cooling such coils.
Conventional high-voltage power transformers
presently can utilize no more than 20% of the core window
area (i.e., the open area in a rectangul.ar core) for
accepting electrically conducting ma-teri.al. Any improve-
ments in this utlization percentage would have great
leverage on reduction in overall size and weight of the
transformers, consequently reducing manufacturing Coslts.
Doubling the utilization o~ the window area would make
posslble production of large power transformers having
roughly one-third the weight and occupying only about
on~~third -the volume oE -the aquivalent design bu:il-t according
to present design practice. Present design practice is based
on wire conductors, with mineral oil and. cellulosic materials
for insulation, and cooling accomplished by a combination
o~ forced and natural convection-circulation of the mineral
oil insulant.
A general approach t.o design oE high-voltage
power -transformers exhibiting the aforementioned improve~
ments may include use of sheet-wound coils, cooled by a
system of sealed, self-contained, annular cooling ductso
Insulation may be provided by polymer film for turn-to-
turn insulation, together with compressed gas insulation
which is advantageously employed in large, high power
coils. Coils of this type may employ conventional core
material~
Another object is to prov.ide a power t.ra:ns-Eormer
of reduced size and weight Eor any given electrical ra-ting~
Another object is to provide a selE-contained
cooling system of simple design for a sheet-wound coil~
'.~.
1~77583 RD 67~7
:`
Briefly, in accordance with a preferred ambodi-
ment of the invention, an electrical coil comprises a
conductive sheet and an insulating sheet overlaying the
conductive sheet. The conductive sheet, together with
the overlaid insulating sheet, is wound continuously
in a plurality of turns about a magnetic core. The
coil is immersed in, and surrounded by, an environment
of gas at high pressure.
The features of the invention believed to be novel
are set forth with particularity in the appended claims.
The invention itself, however, both as to organization and
method of operation, together with further objects and
advantages thereof, may best be understood by reEerence
to the following description taken in conjunction wLth the
accompanying drawings in which:
FIGURE 1 is a longitudinal section view of
transformer coils wound according to the teachings of the
invention;
FIGURE 2 is a magnified view of a portion of the
apparatus shown in FIGURE l;
FIGURE 3A is a cross sectional view along line
3-3 of FIGURE 1 for one embodiment of a sheet-wound
transformer coil; and
FIGURE 3B is a cross sectional view along line
3-3 of FIGURE 1 for another embodiment of a sheet
wound transformer coil.
In FIGURE 1, a sheet-wound power transformer
is illustrated as comprising a low voltage sheet: winding
10 wound about a magnetic core such as an iron core 12,
typically of laminated construction, and a high voltage
sheet winding 11 wound about winding 10. Insulation
means 13 separates the high voltage winding from the low
RD ~787
~77583
voltage winding, and acts as a reactance gap in the trans-
former. Each turn is insulated from the adjacent turn, as
shown in detail in FIÇURE 2 wherein each turn 30 is
separated from the adjacent turn by polymer film insula-
tion 310 The polymer film may typically comprise two
sheets of Mylar, a trademark of E.I~ Du Pont de Nemours
& Company of Wilmington, Delaware, for polyethylene
terephthalate film. The winding material may be comprised
of aluminum.
As shown in FIGURE 1, inlet manifold 14 supplies
a coolant 15 through inlet conduits 16 to cooling ducts
17 extending between adjacent or consecutive turns of the
windings at locations in the windings s~lected to achieve
suEficient coolin~. This does not unduly increas~ th~
volume or weiyht of th~ transformor since the ducts are
typically comprised of stainless steel walls that are
eith~r thin, substantially cylindrical annuli, or a
plurality of closed sectors thereof~ The ducts are
wound into the coil at intervals determined by conventional
temperature rise and heat ~lux calculations, so as to be
spaced apart from each other by one or more intervening
turns oE the coil. The ducts are completely sealed
except where they communicate with inlet and outlet
conduits. Those skilled will recognize that the ducts
may, in the alternative, be comprised o~ any suitable
heat conducting metal.
Coolant is removed ~rom ducts 17 through outlet
conduits 18 which discharye into an outlet manifold 20.
Thus the cooling ducts, conduits and manifolds constitute
a completely sealed, self-contained cooling system. A
cooling duct 17 and outlet condult 18 carrying coolant
15 are illustrated in yreater detail in FIGURE 2.
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~77583 RD-6768
.
Region 21 above and below windings 10 and 11 and reactance
gap 13, and bounded at its extreme ends by a dielectric-
coated shield 22, is occupied by compressed gas insulation
which also fills the vessel (not shown) containing the
transformer. In FIGURE 2, insulation 32 is illustrated
at the ends of turns 30 between layers of turn insulation
31. However, in some applications insulation 32 is
unnecessary, as where the windings are self-supporting
by winding the turns with high tension so that friction
between the individual turns maintains the windings in
position. Conduits 16 and 18, as shown in FIGURE 1, need
not pass through the core window, but advantageously
are directed essentially parallel to the co:Ll axes and
outside of the core window.
As the coil is wound, innumerable small voids
are created within the completed structure. To obviate
any potential problems that might thus be created, the
entire core and windings of the transformer are immersed
in compressed gas such as sulfur hexafluoride in the
pressure range of about 6-8 atmospheres absolute. Since
the coil configuration is such that it may be successfully
impregnated with compressed gas, voids of gas at low
pressure pose essentially no problems because any voids
within the insulation itself are thus small in thickness
compared to the solid insulation thickness. The insulation,
therefore, is effective since the electric stress, or
potential gradient, which can be supported across an insulating
gap increases as the gas void size diminishes; that is,
because the gas voids are small in thickness compared to
the solid insulation, most of the vol-tage across -the
insulating gap is supported across the solid insulation.
Electrical stresses in the voids, though higher than in
1~77583 RV 6787
voids, though higher than in the solid material, can
nevertheless be successfully supported. The cooling ducts,
pipes and manifolds do not carry the compressed gas and
hence comprise a system entirely separate from the compressed
gas-containing volume of the transformer housing or tank
(not shown).
Alternatives to the techniques of interwound ~ilm
insulation exist. For example, the coil, after being
wound with means for spacing adjacent turns of conductor,
could be impregnated with a liquid which is subsequently
processed to form a solid insulation.
Inlet and outlet conduits 16 and 18, respectively,
must be capable oE insulating the voltage existincl between
either winding of the transEormer and inlet and o~ltl~t
mani~olds 14 and 20, re~pectively, which are at grountl
potential. At least a portion of conduits 16 and 18 are
therefore made of an electrical insulating material such
as polytetrafluoroethylene, a ceramic, or rubber, and
careful attention is given to forming a smooth joint
2Q between the insulating conduit and the metal duct structure
to which it is attached. This is necessary since electric
field~ at these critical junctions must be kept as low as
possible. Likewise, the coolant liquid itsel must be a
high voltage insulator in order to withstand the electrical
stress across inlet and outlet conduits 16 and 18 if they
are restricted to very short lengths. The coolant known
as Freon 113, a trademark of E.I. Du Pont de Nemours &
Company, is a suitable coolant for use in the apparatus
for the invention~ In applications of below approximately
50, ooa volts, highly purified water may be a suitable coolant
because of the relatively low electrical stress across
the inlet and outlet conduits under these conditions.
RD 67~7
~77583
In designing sheet-wound coils to withstand
radial forces, the sheet windings themselves are treated
as making no contribution to mechanical strength of the
winding. The coils are typically wound as tightly as
possible on an inner cylinder 23 of high strength ma-terial
such as epoxy-fiberglass and the outer cylindrical surface
is covered with another tightly-fitting cylinder 2~ of
high strength material such as epoxy-fiberglass. Reactance
gap 13 may be comprised of a plurality of turns of the same
polymer film used for insulating adjacent winding -turns
from each other, in sufficient number to provide adequate
insulation between the low and high voltage windings.
These cylinders provide essentially the entire radlal
strength o~ the coll. rrho radial strength requircmerlt
on the coil structure itself is, therefore, that the ~*~
winding not be crushed nor the outer winding burst by the
largest anticipated radial stress. This requirement is
readily met, even Eor severe ~ault conditions (i.e., for
induced radial stresses up to 1,000 pounds per square inch).
In the case of sheet windings, mechanical stresses
are continuously distribu-ted, and are not concentrated into
individual current-carrying filaments as in conventional
wire-wound coils. Although it may appear tha-t the cooling
ducts might be collapsed by the mechanical stresses arising
under severe fault conditions, the ducts, beiny filled
with liquid as are, also, all their connecting manifolds
and tubes, actually experience negligible compression by
the~e forces for the duration of the maximum AC fault wave
which would be expected. Consequently the radial stress
problem is much simpler to accommodate than in a conventional
coil with its concentrated filaments of current in layers
of wire wound on discrete spacers.
- 1~77583 RD 6787
FIGURE 3A is a cross qectional view of shset-
wound transformer coil 11 along lina 3-3 of FIGURE 1,
illustrating the location, in one embodiment, of a cooling
duct 17 carrying coolant 15 within the winding. The locations
of inlet conduits 16 are also shown. The locations of
outlet conduits 18, not visible in FIG~ 3A, are typically
directly above inlet conduits 16, respectively. In this
embodiment, electrically-conductive duct 17 itself constitutes
a single turn of coil 11 by being joined at either end,
as by welding, to the axially-interrupted sheet-wouncl
conductor of coil 11.
FIGURE 3B is a cross sectional view of sheet-
wound transormer coil 11 along llne 3-3 of E'IGURE 1,
illustrating the location, in another embodiment, o~ a
cooling duct between consecutive windings, split into
segments 17A. 17B, 17C and 17D. Each segment communicates
with at least one inlet conduit 16, as shown, and a
corresponding outlet conduit, not visible in FIGURE 3B,
typically directly above each inlet conduit 16, respec-
tively. In this embodiment, the windings and cooling ducts
are electrically insulated from each other.
There exist certain additional mechanical and
electrical advantages to the invention disclosed herein.
Greater utilization of the core window area is made
pos~ible by the sheet-wound construction since the sheet
conductor makes better use of available space by being
continuous and uninterrupted in the axial direction.
Additionally, the nearly perfect plane-to-plane insulating
geometry achieved in the radial direction by virtue of
the sheet-wound construction allows closer spacing
between adjacent turns for any given voltage between
the turns. Use of comprssed gas insulation enables
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RD S787
~7~S83
the transformer to withstand higher electrical stresses
across its major spacings, such as the separation between
a coil and the core material, so that these spacings can
be held to small value~. Moreover, the self-contained
cooling system of the invention is far more efficient
than conventional oil cooling since the coolant need not
perform any insulating function within the coils them-
selves. This is because the metallic duct effectively
shield~ the coolant from any electrical stress. Hence
the coolant can be selected solely for its cooling capa-
bilities; indeed, the self-contained cooling system
allows coolant flow rates that are an order of magnitude
greater than the forced flow velocity of insulating oil
in conv~ntional transformers, thus enhancing cooling
ef~iciency~ A8 a result of these advantages, core
window utilization in sheet-wound transformers can be
doubled to 40% and transformer weight reduced by a factor
of three over conventional wire-wound transformers. A
concomitant volume reduction is also realized. In addi-
tion to the foregoing advantages, superior performance is
exhibited under impusle voltages since the extraordinarily
high turn-to-turn capacitance of sheet windings provides
a high degree of uniformity in distributing impulse
voltages through the coil.
The foregoing describes a sheet-wound coil
useful for high voltage applications. Use of the coil in
power transformers or reactors allows such apparatus to
be constructed of reduced size and weight for any given
electrical rating. The invention further includes a self-
contained cooling system of simple design for a sheel-
wound coil.
While only certain preferred features of the
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1~77S83 RD 6787
invention have been shown by way of illustration, modifi-
cations and changes will occur to those skilled in the art.
It is, therefore, to be understood that the appended claims
are intended to cover all such modifications and changes
as fall within the true spirit of the invention.
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