Note: Descriptions are shown in the official language in which they were submitted.
RD-6249
10560Z4
This invention relates to the cooling of electrical
transformers, and more particularly to transformers with disc
or flat coil windings having provision for improved natural
circulation of the liquid coolant.
Liquid-cooled medium and power transformers of the
type having disc coil windings mounted a~out a magnetic core
structure are commonly either force-cooled by pumping the
insulating oil or other coolant through the windings, or are
cooled by natural circulation of the coolant upwardly through
the windings by the free convection mechanism. In these
liquid-cooled transformers, a pair of concentric cylindrical duct
walls is mounted within and surrounding the disc winding, thereby
defining inner and outer axial ducts for coolant flow in the
vertical direction at each side of the coils. In force-cooled
equipment, prior practice has been to use complete barriers in ~`
the vertical cooling ducts at alternate vertically spaced ~;
locations inside and outside the disc coils thereby to circulate
the pumped coolant in a zig-zag path through the winding. Since
there is some coolant flow through the horizontal ducts between
adjacent coils, this arrangement has good heat transfer
characteristics.
As a variation in forced-oil cooled transformers, it
is disalosed in Japanese Utility Model Application No. SHO-~3-
2020 published under Utility Model No. SHO-46-15364 May 28, 1971
that the alternate complete barrier rings or inserts placed at
intervals of several coils to establish a zig-zag flow path can
be supplemented by graduated partial barrier rings or inserts in
each section to provide more uniform velocity of flow between
the disc coils. The horizontal length of the partial barrier
ring increases with ~ertical height in each seciion thereby to
achieve increased resistance to flow in the several horizontal
ducts and result in an e~ual ~alance of flow resistances.
When used in con~unction with the complete ~arriers, hbwever,
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the effect of adding graduated partial barriers alternately
in the vertical cooling ducts is to increase the total flow
resistance. The Japanese utility model configurations are
illustrated and discussed in U.S. patent No. 3,902,146 issued
August 26, 1975 to R. Muralidharan and assigned to the present
assignee.
Transformer disc coils cooled by natural circulation
only must ~e considerably derated in order to avoid excessive
winding temperature rise. Ordinarily, the vertical cooling ducts
are provided without flow barriers to minimize flow pressure drop
and thus maximize vertical coolant flow past the winding. This
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results in poor heat transfer due to the absence of or excessively --
limited coolant flow horizontally between the individual disc
coils. The use of alternate complete flow barriers in the ver~
tical cooling ducts, such as is employed in force cooled systems,
is not desirable in natural circulation arrangements since full -~
barriers result in too low a coolant flow and consequent
excessive temperature rise.
In accordance with the invention, in a natural
circulation cooled electrical transformer with vertically -~
mounted disc or ~lat coil windings as previously described,
it has been found that partial flow barrier inserts or rings,
mounted in staggered relation in the inner and outer vertical
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cooling ducts at either side of the winding, are effective to
force a minor amount o coolant flow horizontally between the
individual disc or ~lat coils ~ithout excessively reducing -
the total coolant flow~ Because of the improvement in h~at trans- ~t~., ,., ,'
fer that is realized, higher winding ratings are made possible,
and this is achieved inexpensively in a manner compati~le with
a ~ide variety of present transformer configurations~ Pre~erably, :
the partial flo~ barrier inserts are attached to the inner and
outer peripheries of selected individual coils, in alternating
fashîon, at a uni$orm vertical spacing of every several coils.
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The resulting coolant flow pattern can be referred to as a
modified zig-zag type flow path. Suitably the amount of
staggered partial blockage is such that the partial flow
barrier inserts have a horizontal width equal to about half to
three-quarters the width o~ the vertical cooling auctS. If
desired, the partial flow barrier inserts can be omitted from
predetermined portions of the transformer winding, such as the
cooler lower part of the winding.
FIG. 1 i~ a diagrammatic vertical cross section, with
part~ omitted, through a transformer ~howing a single magnetic
core and disc coil winding assembly immersed in a liquid coolant
in a tank with provision for external cooling of the naturally
circulated coolant;
FIG. 2 is a horizontal cros9 section through the
disc coil winding and duct walls of FIG. 1 illustrating an
outer partial flow barrier insertt and
FIG. 3 is a schematic vertical cross section through
half of the disc coil winding with arrow3 indicating the coolant
flow path~,
2~ In FIG. l is shown in diagrammatic form a magnetic
core and disc coil wi~ding sub-assembly or assembly such a~ is
used in self-cooled medium and power electrical tran~formers.
~he sub-assembly i8 comprised by a vertically oriented magnetic
core element 10 and associated surrounding di~c coil winding
indicated generally at 11, and is Lmmersed in a suitable liquid
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B coolant 12 such as ln~ulating oil or pyranol contained within
a transformer tank 13. Although described primarily with regard ~
to a disc coil configuration, the invention is applicable ~- -
generally to ~lat coils of any shape. A number of cooling and
recirculation line 14 ~or the liquid coolant are mounted
exterior to the tank adjacent to one side wall or completely
around its periphery, each making connection between an outlet
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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 lines 14 is cooled by radiation cooling
to the atmosphere o~ by forced air cooling using fans, and
naturally circulates downwardly to re-enter thetank near the
bottom. Self-cooled transformers with disc coil or flat coil
windings are made in a variety of single-phase and multi-phase
configurations with different arrangements of primary and
secondary windings. In a three-phase power transformer, for
example, there are three such magnetic core winding legs all
interconnected in a ~uitable magnetic core structure and
immer~ed together with their windings in a large rectangular
tank Mo~t 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 tertiary windings. In some transformers, the primary
and secondary winding~ are alternated in a ~ingle disc coil
winding.
Disc coil windin~ 11 is comprised by a large number
of individual disc coils or flat coiLs lla assembled vertically -
~with an equal spacing between individual coils~ Each disc coil ~ - -
lla (see also FIG. 2) is annular in shape and tightly wound so
a~ to be continuous. Within the coil itself, no ducts are
provided for a passage of liquid coolant. m e individual disc ~ -
coils lla are stacked vertically one upon the other using
spacer member~ (not shown) made of pre~sboard or other suitable
insula~ing material. The electrical connections of the
individual coils to form a winding are also not shown. As is
conventional, a pair of concentric vertical cylinders 17a and
17b are mounted inside of and outside of the disc winding 11,
respectively equally spaced from the in~er periphery and the
outer periphery of the individual disc coils. These concentric
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cylinders provide parallel cooling duct walls to constrain the
circulation o~ liquid coolant 12 upwardly through the disc
winding by the free convection mechanism. The duct walls 17a
and 17b are also made of pressboard or other suitable insulating
material. The annular space thus defined between the inner
periphery of the disc winding 11 and the inner duct wall 17a
provides an inner vertical cooling duct 18a, while similarly
the annular space between the outer periphery of the di~c coil
and the outer duct wall 17b provide~ an outer vertical cooling
duct 18b. ~he concentric or parallel vertical cooling ducts
for the passage of coolant have approximately equal width in
the radial direction, although equal width is not essential.
Horizontal cooling ducts 19 of approximately the same height
are defined between the horizontal major coil surfaces of
adjacent individual disc coils lla.
In accordance with the invention, a plurality of
staggered, ;nner and outer partial flow barrier inserts or
rings 20a and 20b are mounted on the disc winding 11 extending
into the vertical cooling ducts 18a and 18b to improve the
natural circulation of coolant through the cooling duct and the --
disc winding 11~ The blockage provided by the flow barrier
inserts 20a and 20b ~ small enough to prevent exces~ive
natural circulation flow reduction and large enough to force a
small fraction of the coolant flow through the horiæontal cool-
ing ducts 19, in a modified zig-zag type pattern for better
coolingO The properly placed, partial coolinq duct flow
re~trictions result in attaining a higher natural circulation
cooled winding rating. The partial ~low barrier in~erts 20a
and 20b are peripherally continuous (see FIG. 2) and are -
preferably attached to the vertical face or periphery of an
individual disc coil lla, normally having the same height as
the disc coil. In terms of horizontal or radial width, the
partial flow barrier inserts are sized to block about one-half
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to three-quarters of the width of the vertical cooling ducts
18a and 18b. The inserts 20a and 20b are mounted alternately
in staggered fashion on the outside and inside of every several
disc coils lla. Preferably, they are regularly spaced in the
axial direction, such as every fifth or eighth disc coil, but
can be omitted in the cool parts of the winding toward the
bottom to still further limit the total coolant flow reduction.
Although normally attached to the disc winding 11, the partial
flow barrier inserts 20a and 20b can, if desired, be attached
to the cylindrical cooling duct walls 17 and 17b.
The modified natural circulation flow patterns created
by partially blocking the vertical cooling ducts 18a and 18b in
a staggered manner is illu~trated by the flow arrows in the ~--
FIG. 3 diagram, It will be apprsciated that cooled li~uid
coolant 12 entering at the bottom of the disc winding 11 i~
; heated by exposure to the hot winding, ri~ing in temperature
; and changing density as the heated coolant rises in the cooling
duct due to the thermal siphon effect. As was previously
mentioned, in the absence of the partial flow barrier inserts
20a and 20b, most of the coolant flow is vertically in the
vertical cooling ducts 18a and 18b and there is relatively
little horiæontal 1OW in the horizontal cooling ducts 19.
That is, the flow pressure drop between the inner vertical
cooling duct 18a and the outer vertical cooling duct 18b is
minimized~ By employing the staggered partial flow barrier
in~erts 20a and 20b in the vertical cooling ducts, the effect
is to alternatively change the coolant flow pres~ure gradient
in each duct by a predetermined amount. This predetermined
gradient will alternatively force a small fraction of the
vertical coolant flow from one side radially through to the
other side of the disc coil. In each section of the disc coil ~ -
between the partial flow barrier inserts 2~a or 20b, coolant
flow in the vertical cooling duct belwo the partial restriction
is diverted horizontally betwwen the individual disc coils 11a
to the unrestricted vertical cooling duct. The modified zig-zag
natural circulation flow pattern that is crrated is illustrated
by the flow arros and need no further comment. By causing a
small radial or horizontal flow, the horizontal major coil
surfaces 21 are no longer covered by stagnang coolant layers
as woudl be the case if there were no such partial flow barrier
inserts. Only a small radial flow in the horizontal cooling
ducts 19 wil cause a significant improvement in disc coil
cooling as the result of the improved heat transfer charachter-
istics. The partial flow barrier inserts 20a and 20b, when
properly dimensioned produce a sufficienlty small vertical
flow blockage that there is still a net gain in cooling
performance.
The optimum amount of partial blockage, i.e., the
horizontal dimension of the inserts 20a and 20b, can be
determined by computer calculations. The factors involved
are the radial dimensions of the coils, the disc coil separation
in the vertical direction, and the frequency of blocking. It
has alrady been pointed out that full blockage of the vertical
cooling duct, as would be obtaine by extending the rings 20a
and 20b all the way to the duct wall is not desirable in a
liquid-immersed natural circulation cooled transformer since
the coolant flow rate is reduced substantially with a consequent
excessive winding temperature rise. By using partial blockage
of the vertical cooling duct, rather than full blockage, the
flow resistance in the total disc coil winding is reduced.
Therefore, the flow rate is increased as compared to the full
blockage cause and excessive heating of the coolant and resulting
excessive winding temperature rise does not occur. An analogy
can be made to an electric circuit in which the amount of
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resistance is reduced 50 that the current flow is conse~uently
increased.
The improvement in natural circulation of liquid
coolant to enable higher transformer ratings is obtained at
little expense with a relatively small modification of existing
transformer designs. In the event that there is more than one
disc coil winding surroundinq a selected magnetic core, such - -
as low voltage and high voltage windings, the partial flow
barrier inserts 20a and 20b are used with each such winding
and function in essentially the same manner. The partial flow
barrier inserts or rings can be made ~f inexpensive in~ulating
material, such as pressboard, are easily and inexpensively
attached to the disc coils, and small variations in size due to
manufacturing tolerances do not significantly change the amount
of horizontal flow produced between the disc coils or flat
coils. Accordingly, it is evîdent that the addition of partial
flow barrier in~erts to a wide variety of existing electrical
transformers with disc or flat coil windings is an inexpensive
and universal technique for improving the heat transfer from
natural circulation cooled coils without exce~sively reducing
the coolant flow rate. ~ -
While the invention has been particularly shown and
described with reference to a preferred embodiment thereof~ it
will be understood by those skilled in the art that various
change~ in f~rm and detail~ may be made therein without departing
from the spirit and scope of the invention.
