Note: Descriptions are shown in the official language in which they were submitted.
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EARTH COOLED DISTRIBUTION TRANSFORMER
SYSTEM AND METHOD
TECHNICAL FIELD
The present invention relates to an earth cooled
distribution transformer, . system and method wherein the
transformer is buried directly in the ground with the
transformer having its primary and secondary windings casted
in a solid insulating material and its steel magnetic core
exposed to provide heat dissipation in the surrounding soil,
and further wherein the supply cable and feed power cables
may also be buried in the ground, and further wherein a
plurality of these transformers may form part of a
distribution system buried in ground to supply electrical
power to an entire consumer load community.
BACKGROUND ART
Distribution transformers are usually mounted in a
protection casing which is provided with oil to act as a
thermal carrier whereby heat generated by the windings is
dissipated through the casing in contact with the oil. The
use of oil and casing has proven to be problematic and also
increases the physical size of the distribution transformer,
making them difficult to directly bury them in the ground.
Attempts have been made to bury distribution transformers in
the ground as well as their cables which service the homes
whereby to eliminate pole-mounted transformers or pad-
mounted transformers. However, these buried transformers
still required cooling when positioned underground and
therefore the transformers were mounted in housings and the
oil in the transformer or else the housing was cooled by a
system of cooling pipes, also buried in the ground. An
example of such distribution transformers construction is
disclosed in U.S. Patent 3,212,563 and 4,009,418.
With the advent of solid or dry transformers the
size of the transformer has been greatly reduced but the
cooling of such transformer is usually affected by heat
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exchange with air or a liquid circulated within the
transformer structure or solid heat sinks in contact with
ambient air. Some of the difficulties encountered with such
solid or dry transformers are that heat dissipation through
a solid dielectric material has proven to be poor and the
result of thermal build-up can create hot spots or high
thermal gradients which can crack the solid dielectric
material. This can lead to 5terious breakdowns which require
the replacement of the transformer. Also, most of these
transformers require a grounded enclosure to remove any
electrical shock hazard. Such enclosures are typically
metal cages having dimensions much larger than the
transformer itself, thus making installation space
requirements difficult.
Recently, and with reference to U.S. Patent
5,656,984, improved solid insulating transformers have been
developed but these also require the casting of the entire
transformer into a surrounding shell or housing. Such are
usually mounted in very large manholes whereby to
accommodate the cooling equipment which circulates the
cooling liquid through the inner coils and core of the
transformer to extract heat. Accordingly, such
installations are fairly bulky and expensive. Recently,
distribution transformers have also been developed wherein
the transformer is fully protected by fiberglass and
vinylester resin or else the transformer is casted in
concrete. Such transformers are usually mounted in manholes
and require some form of cooling medium.
U.S. Patent No. 4,349,801 describes a single-phase
transformer wherein the windings are cast in resin and
wherein the wound cores are also protected from corrosion
and mechanical damage in a suitable encapsulation of shock
proof construction whereby to permit the sealed transformer
to be buried in the soil. Waterproof connector sockets are
provided to connect underground cable thereto. Figure 3 of
the patent illustrates such a buried-type construction.
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These transformers are costly to fabricate, are bulky and
develop the problems associated with overheating.
SUMMARY OF THE INVEN,~'ION
It is a feature of the present invention to
provide a power distribution transformer structure which can
be buried directly in ground without the use of a cooling
agent, whereby the surrounding earth provides the cooling
medium to dissipate heat generated by the transformer
windings.
Another feature of the present invention is to
provide an earth cooled distribution transformer wherein the
primary and secondary windings are casted in a solid
insulating material with the steel magnetic core remaining
relatively exposed and providing for the conduction of heat
generated by the windings to dissipate the heat in the
surrounding earth.
Another feature of the present invention is to
provide an earth cooled distribution transformer which is
buried in ground and wherein the high voltage supply cable
as well as the power distribution cable of the secondary
winding are also buried in ground to form an underground
power distribution network to feed electrical power to a
plurality of consumer loads.
Another feature of the present invention is to
provide an earth cooled distribution transformer which is
buried in ground adjacent a manhole enclosure and wherein
the connections to the transformer are provided through
terminals mounted in the manhole and connected directly into
the buried transformer adjacent to the manhole.
Another feature of the present invention is to
provide a method of cooling a distribution transformer by
burying the transformer in ground with the primary and
secondary windings of the transformer being encapsulated in
a solid insulating material and the core ,being exposed to
the surrounding earth to provide heat dissipation into the
earth.
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Another feature of the present invention is to
provide a method of distributing electrical power to a
plurality of consumer loads through distribution
transformers which are buried directly into the ground and
wherein the primary and secondary windings of the
transformers are insulated in a solid insulating material
with their cores being exposed to provide heat dissipation
within the surrounding earth.
According to the above features, from a broad
aspect, the present invention provides an earth cooled
distribution transformer system which comprises a
distribution transformer having a steel magnetic core
defining core legs about which primary and secondary
windings are respectively wound. Each of the windings is
insulated by a solid insulating material. The distribution
transformer is buried at a predetermined depth in ground
with the steel core exposed to the ground whereby to
dissipate heat generated by the windings in the ground to
cool the transformer by the surrounding earth. A high
voltage supply cable is connected to the primary winding and
extends at least partly in the ground. A power distribution
cable is connected to the secondary winding and extends at
least partly in the ground to feed power to one or more
consumer loads.
According to a further broad aspect of the present
invention there is provided a directly buried distribution
transformer having encapsulated windings, feed and
distribution electrically insulated cables, and an exposed
core. The transformer is buried in soil at a desired depth
with the core in contact with the soil whereby to dissipate
heat in the soil which acts as a cooling medium.
According to a still further broad aspect of the
present invention there is provided a method of cooling a
distribution transformer and which method comprises
providing a distribution transformer having a steel magnetic
core defining core legs about which primary and secondary
windings are respectively wound. The windings are insulated
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in a solid insulating material and the steel core remains
exposed. A supply cable is attached to the primary winding.
One or more power distribution cables are attached to the
secondary winding. The distribution transformer is buried
at a predetermined depth in ground with the steel magnetic
core exposed to earth whereby to dissipate heat generated by
said windings in said ground to cool said transformer by
cool surrounding earth. At least part of the supply cable
and distribution cables are also buried in the ground.
According to a still further broad aspect of the
present invention, there is provided a method of cooling a
directly buried distribution transformer comprising the
steps of providing a distribution transformer with
encapsulated windings, feed and distribution electrically
insulated cables and an exposed core. The transformer is
directly buried in soil at a desired depth with the core in
contact with the soil whereby to dissipate heat in the soil
which acts as a cooling medium.
According to a still further broad aspect of the
present invention there is provided a method of distributing
electrical power to a plurality of consumer loads. The
method utilizes the above-mentioned distribution transformer
buried in ground and the distribution cable of the secondary
winding feeds a plurality of consumer loads. A plurality of
distribution transformers may also be interconnected in an
underground network of directly buried transformers to
supply an entire community of consumer loads.
BRIEF DESCRIPTION OF DRAWINGS
The preferred embodiments of the present invention
will now be described with reference to the accompanying
drawings in which
FIG. 1 is a perspective view showing an
installation of the earth cooled distribution transformer of
the present invention directly buried in the ground adjacent
a manhole enclosure;
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FIG. 2 is a schematic illustration of the power
distribution transformer buried in ground to dissipate heat
in the surrounding cool earth; and
FIG. 3 is a schematic illustration showing a
distribution network consisting of a plurality of directly
buried distribution transformers feeding a plurality of
consumer loads.
DESCRIPTION OF PRFFE~RED EMBQJDIMBNTS
Referring now to the drawings and more
particularly to Figure 1, there is shown generally at 10 the
earth cooled distribution transformer of the present
invention which is directly buried in the ground 11 and as
hereinshown adjacent a manhole enclosure 12. As also shown
in Figure 2, the distribution transformer 10 has a steel
magnetic core 13 which defines core legs 14 and 15 about
which a primary winding 16 and a secondary winding 17 are
wound. Each of the windings 16 and 17 are insulated by a
solid insulating resin material 18 as well known in the art.
The steel magnetic core 13 remains exposed.
As shown in Figure 2, the power distribution
transformer 10 is buried in the ground 11 at a predetermined
depth "d" at which the surrounding ground temperature is at
approximately 8°C which is ideal for cooling the
transformer. Because the steel core 13 is exposed to the
surrounding earth 19 heat generated by the coils 16 and 17
is transmitted through the core 13 and dissipated into the
ground 19. Accordingly, the surrounding ground provides the
cooling medium for the transformer and the core acts as a
heat sink.
A high voltage supply cable 20 is connected to the
primary winding 16 and a power distribution cable is
connected to the secondary winding 17. Both these cables
extend at least partly in the ground 11. The power
distribution cable may be connected to an underground or
above-ground terminal whereby to supply power to a plurality
of consumer loads usually four or six residential homes.
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As hereinshown, the high voltage supply cable,
which usually carries approximately 25 KV is connected
directly to the primary winding as well as the distribution
cable 21. Accordingly, there is no need to provide bushings
on the transformer thus resulting in a cost saving. Also,
because the transformer is directly exposed to the
surrounding earth, there is no need to provide a casing nor
an insulating medium. This results in a substantial cost
saving in the construction of such power distribution
transformers and the size is reduced as there is no housing
nor cooling devices.
As shown in Figure 1, when the power distribution
transformer 10 is buried adjacent a manhole enclosure, such
as the enclosure 12 illustrated herein, the high voltage
supply cable 20 is fed directly into the manhole and
connected to the high voltage terminal 22 through a fault
detector device 23. A primary supply cable 20'
interconnects the primary winding 16 to the terminal 22 and
a current limiting fuse 24 is preferably connected to this
cable. Another section of the high voltage cable 20 is also
connected to the terminal 22 and out of the manhole whereby
to supply a further manhole or to connect directly to
another distribution transformer, depending on the
installation.
As shown in Figure 1, the manhole is also provided
with a junction box 25 where the 120/240 volt supply to the
consumer loads is provided. The junction box supplies power
to a plurality of consumer loads, as illustrated in Figure
3.
Referring again to Figure 2, it is pointed out
that the steel magnetic core 13 may be provided with a
thermally-conductive coating 26 so as to retard oxidation of
the core when exposed to earth. Also, as shown in Figure 1,
a grounding rod 27 may be secured in the ground adjacent the
buried transformer 10 and the steel magnetic core may be
connected to this grounding rod by a suitable conductor 28,
as hereinshown. Additionally, as shown in Figure 2, the
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primary and secondary windings may both be provided with a
conductive paint such as a carbon paint 29 on the outer
insulated surface thereof to provide better thermal
conductivity into the surrounding earth. A manhole cover 30
provides access to the interior of the manhole enclosure for
installation and servicing.
With reference to Figure 3, it can be seen that
the earth cooled distribution transformer system of the
present invention has many advantages. As shown in Figure
3, a complete underground direct buried power distribution
system may be provided whereby to feed an entire community
of consumer loads. As hereinshown, a plurality of
transformers 10 are buried under the ground 11 and each
feeds a plurality of consumer loads 31. The primary and
secondary windings of the transformer may be provided with
suitable terminals to provide interconnection of the
transformers as well as the consumer loads directly to the
transformers. Alternatively, underground or above-ground
junction boxes may be provided in suitable safety
enclosures. However, when the distribution system is
entirely underground, the ground also provides electrical
insulation of the distribution system and safety to the
people above ground. To locate the power distribution
transformers a suitable indicating means would be secured
into the ground surface directly above the transformer. The
life expectancy of such directly buried transformers and
cables is expected to be at least 40 years which is quite
suitable as very little or no maintenance will be necessary
for such installations as there are no underground bushings
or cooling mediums or power~d equipment necessary to
circulate a cooling medium into the transformers. Thus, the
transformer is self-sustaining with the cooling being
effected by the natural condition of the surrounding earth.
In the event of explosion the earth will absorb the shock
and prevent debris projectiles. The transformer will also
not contaminate the earth.
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The present invention can be summarized as
providing a method of cooling a distribution transformer
which comprises the provision of the transformer 10 with its
steel magnetic core 13 being exposed to surrounding earth
when buried in ground and with its windings 16 and 17 being
encapsulated in solid insulating material. A supply cable
and a power distribution cable is secured to the primary and
secondary windings respectively and also laid entirely or
partly underground. The transformer is buried at a
predetermined depth where it is known that the temperature
surrounding the transformer will be sufficient to cool the
transformer and this temperature has been found to be
adequate when at approximately 8°C. The depth that the
transformer is buried varies from location to location
depending on the composition of the earth in the ground and
other factors such as geographical locations. The
transformers may also be buried adjacent manhole enclosures
as shown in Figure 1 although this is not necessary. It is
also conceivable that a single manhole enclosure may be
associated with one or more buried power distribution
transformers 10 of the present invention.
Those skilled in the art will appreciate that the
conception, upon which this disclosure is based, may readily
be utilized as a basis for the designing of other
underground distribution networks while using the same
method and system as hereinabove described. It is
important, therefore, that the claims be regarded as
including such equivalent construction insofar as they do
not depart from the spirit and scope of the present
invention.
