Note: Descriptions are shown in the official language in which they were submitted.
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CLAMPING DEVICE FOR A CORE FORM TRANSFORMER
Field Of The Invention
This invention relates generally to an electrical transformer and, in
particular, to an
improved clamping device for retaining the windings of the transformer under
the
appropriate compressive force throughout the operational lifetime of the
transformer.
Background of the Invention
Core form power transformers generally include a containment tank, a coil and
insulation assembly, a magnetic core, a static clamping system to maintain
compressive
forces on the magnetic core and coil and insulation assembly, and electrical
conduction
assemblies to connect the coils to the exterior of the tank. After assembly of
the major
components, the containment tank is generally filled with an electrical grade
mineral oil
that becomes an integral part of the electrical insulation of the transformer,
as well as a
heat transfer medium.
During operation, the transformer is periodically subjected to electrical and
mechanical transients as a result of electrical operating contingencies that
occur in the
electrical power system to which the transformer is connected. One of the
common
contingencies on the power system is that of short circuit faults. When a
short circuit fault
occurs on the power system, high magnitude currents flow from the various
circuits to the
short circuit location. If a power transformer is connected in the circuit
path of high
current, the high magnitude current flows through the power transformer and
generates
transient mechanical forces within the transformer components.
Under the stress of short-circuit currents, the magnetic interaction between
the
transformer windings increases the forces tending to expand the windings
axially and
radially with respect to the core.
The coil and insulation assembly is one of the components subjected to the
transient
mechanical forces during a short circuit event. The coils are constructed from
highly
conductive copper or aluminum that is insulated with an electrically
nonconductive
material. For several decades, the prominent non-conductive material utilized
in power
transformers has been an electrical grade paper. Many transformers have been
observed as
having loose coil and insulation assemblies which has been attributed to
shrinkage of the
insulating paper. If the coil and insulation assembly is loose and the
transformer is
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subjected to a mechanical transient, the probability of transformer failure is
much higher
than if the assembly were tightly held together. As such, mechanical failure
of a loose coil
and insulation assembly is a common failure mechanism.
As previously mentioned, the transformer includes a static clamping system
which
applies clamping pressure on the coil and insulation assembly. This fixed
clamping system
contacts the ends of the coil and insulating assembly. However, this fixed
clamp does not
perform the clamping function adequately over time because the insulation
shrinks over a
period of time. Thus, the force on the windings brought about by this fixed
clamping
system is reduced, or even nonexistent, such that the windings are no longer
tightly secured
in position.
Spring-loaded devices have also been used in the past to obtain follow-up
pressure
on windings. 1n these devices, the springs are often large to directly resist
the high
deformation forces resulting from short circuit currents. Also, in many
instances, many
springs are needed to perform the function.
Other dynamic clamping systems have been available in the marketplace, such as
the DynaCompTM device manufactured by ABB of St. Louis, MO. However, this
device is
to be installed only at the time the transformer is built and is permanently
attached to the
clamping system. It was not designed for retrofitting existing transformers in
the field.
Further, it is relatively large and requires substantial springs to perform
the desired
function.
Summary of the Invention
The present invention is a spring-loaded dashpot device which may be
retrofitted
into core form transformers to reestablish the mechanical tightness of the
coil and
insulation assembly that was present when the transformer was manufactured.
The
inventive assembly includes a set of Belleville spring washers, a movable
piston, and a
cylinder. A small hole in the movable piston allows the cylinder to fill with
the
surrounding insulating oil when it is filled into the transformer container
under vacuum. A
loading rod assembly is fixed to the moveable piston and engages the fixed
clamping
system of the core form transformer. The cylinder is positioned against the
coil and
insulation assembly. Consequently, the inventive device is sandwiched between
the fixed
clamping system and the coil and insulation assembly.
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During steady-state operation of the power transformer, the spring washers
maintain
compressive forces on the coil and insulation assembly. The spring washers are
designed
so as to maintain pressure throughout the transformer service life even if the
paper
insulation continues to shrink. The movement is slow enough so that the
displacement of
oil from the cylinder is not impeded by the small size of the hole in the
cylinder. Follow-up
expansion is based on the design and compression capability of the device and
can vary up
to approximately one inch.
Another major feature of the device is that it does not compress during the
mechanical transient events. During these transients, it is necessary for the
device to act as
a rigid component rather than as a spring. Because the transient events are
too short in time
duration to allow sufficient oil flow through the small hole in the cylinder,
the oil volume
within the cylinder is an incompressible volume which prevents rapid movement
of the
piston which, in turn, prevents rapid expansion or compression of the spring.
Therefore,
the device becomes a rigid component during transient mechanical events and
the
compressive force is maintained on the coil and insulation assembly.
The device is relatively small and occupies a volume less than about 150 cubic
inches while delivering up to 60,000 lbs. of force. A device delivering 15,000
lbs. of force
will occupy less than about 35 cubic inches. The device incorporates
compression retaining
bolts that hold the spring washers under the appropriate pressure when
assembled. After
installation in the transformer, the compression bolts are removed such that
the force of the
spring washers is applied to the windings. Thus, the field installation of the
present
invention is quite simple. In summary, this invention provides an economical
solution for
re-establishing the appropriate compression on the coil and insulation
assembly of core
form power transformers, maintaining compression, and dampening axial
movement.
Brief Description Of The Drawings
In the accompanying drawings:
FIG. 1 is a cross-sectional view through the dynamic coil clamping device of
the
present invention;
FIG. 2 is a top view of the piston of the dynamic coil clamping device of FIG.
1;
FIG. 3 is a side view of the piston of the dynamic coil clamping device of
FIG. 1;
FIG. 4 is a top view of the cylinder of the dynamic coil clamping device of
FIG. 1;
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FIG. 5 is a side view of the cylinder of the dynamic coil clamping device of
FIG. 1;
FIG. 6 illustrates a side view of a typical core form transformer; and
FIG. 7 illustrates the transformer of FIG. 6 with the inventive coil clamping
device
of FIGS. 1-5 installed therein.
Detailed Description Of The Drawings
Refernng initially to FIGS. 1-5, a dynamic coil clamping device 10 includes a
cylinder 12 and a piston 14 positioned within the cylinder 12. The lower
surface of the
piston 14 and the lower interior surface of the cylinder 12 define a chamber
15. At the base
of the cylinder 12 and within the chamber 15, a plurality of spring washers 16
are disposed.
The spring washers 16 (e.g. disc springs) contact the lower inner surface of
the cylinder 12
and also the lower surface of the piston 14. An alignment pin 18 is positioned
within the
cylinder 12 and is the structure around which the spring washers 16 are
placed. While the
alignment pin 18 is shown as a separate piece, it could be integrally formed
with the
cylinder 12.
A loading rod assembly 20 is located at the upper end of the coil clamping
device
10. The loading rod assembly 20 includes a threaded rod 22 around which an
upper nut 24
and a lower nut 26 are threadably engaged. Due to the inherent vibration (e.g.
60 Hz) that
is present in a transformer, each of the nuts 24 and 26 includes a set screw
28 that is
tightened after the coil clamping device 10 is installed in the transformer as
will be
described in more detail with reference to FIGS. 6 and 7. The threaded rod 22
could also
be integrally formed on the piston 14.
Because of the necessity of having each electrically conductive component of a
transformer grounded, a spring 30 is located between the alignment pin 18 and
the piston
14 to ensure that the alignment pin 18 maintains mechanical contact with
either the piston
14 or the cylinder 12. The spring 30 is a steel compression spring. The
cylinder 12 and
piston 14 are also generally made of steel, as is the alignment pin 18.
Additionally, the
components of the loading rod assembly 20 are also preferably made of steel.
The coil clamping device 10 can be made relatively small while providing a
substantial amount of force, for example, up to 60,000 lbs. of force. The
outer diameter of
the cylinder 12 is, for example, approximately 7 to 8 inches. The height of
the cylinder 12
is approximately 4 inches. Thus, it occupies about 150 cubic inches. The
distance that the
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loading rod assembly 20 extends above the surface of the piston 14 in such a
device is
approximately two inches. A device 10 delivering 15,000 Ibs. of force would
have a
cylinder 12 with an outer diameter of about 4 inches and a cylinder height of
2-3/4 inches.
Thus, the device 10 delivering about 15,000 Ibs. occupies about 35 cubic
inches. Its
5 loading rod would have a height of about one inch.
As shown best in FIGS. 2 and 3, the piston 14 includes an alignment recess 42
that
receives the alignment pin 18. The alignment recess 42 is also the internal
structure of the
piston 14 that receives the spring 30.
A rod recess 44 is located in the uppermost surface of the piston 14 so as to
receive
the loading rod assembly 20. The rod recess 44 of the piston 14 is primarily
used to center
the loading rod assembly 20 on the piston 14. As shown, the rod recess 44 is
not threaded.
However, the rod recess 44 could be threaded to securely hold the threaded rod
22 of the
loading rod assembly 20.
The piston 14 includes a circumferential recess 45 that receives a seal 46.
The seal
46 (FIG. 1) inhibits fluid from flowing between the outer circumference of the
piston 14
and the inner wall of the cylinder 12.
Around its circumference, the piston 14 also includes a plurality of tapered
recesses
47. Each of the tapered recesses 47 receives a compression retaining bolt 48
as shown in
FIG. 1. The compression retaining bolts 48 are threaded through
correspondingly aligned
threaded bores 52 in the cylinder 12, as shown in FIGS. 4 and 5. The
compression
retaining bolts 48 are used during the assembly process to hold the coil
clamping device 10
in a steady-state force condition prior to installation. Thus, when the
appropriate
compression force of the spring washers 16 is achieved, the compression
retaining bolts 48
are threadably inserted through the cylinder 12 and engage the piston 14 to
hold the
assembly in a suitable position for storage and transportation.
The piston 14 also includes a fluid conduit 60 that includes a large region 62
and a
small region 64. Because this coil clamping device 10 is installed into a
transformer that
has insulating fluid throughout its inner workings, the coil clamping device
10 will be
exposed to this insulating fluid. As such, when the transformer is vacuum oil
filled with
the insulating fluid (e.g. mineral oil), fluid is allowed to flow from the
exterior of the piston
14 through the conduit 60 into the chamber 15 where the spring washers 16 are
located.
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The insulating fluid would also flow up along the alignment recess 42 into the
region
occupied by the spring 30. Including an incompressible fluid within the
chamber 15 and
providing the small region 64 in the conduit 60 (e.g. 0.06 inch in diameter)
allows the coil
clamping device 10 to act as a rigid member when the transformer is subjected
to transient
mechanical shocks.
The spring washers 16 may include multiple sets of washers. For example, as
shown in FIG. l, the spring washer 16 includes a first set of Belleville
washers 72 and a
second set of Belleville washers 74 positioned around the alignment pin 18.
The
dimensions of the second set of Belleville washers 74 are such that they fit
within the
annularly-shaped gap defined by the alignment pin 18 and the first set of
Belleville washers
72 after the first set of Belleville washers 72 has been placed in the
cylinder 12. Also, the
spring washers 16 can be replaced by standard springs.
Referring now to FIG. 6, a core form transformer 110 is illustrated which
includes a
container 112 and a coil assembly 114 which is comprised of a plurality of
windings of an
electrically conductive wire and its associated insulation. A top coil support
116 is located
above the coil assembly 114 and a support block 118 is located above the top
coil support
116. Because the coil assembly 114 has a round cross-sectional shape, as does
the top coil
support 116, the support blocks 118 are usually placed in a circumferentially
symmetric
fashion around the top coil support 116. It should be noted also that the
transformer 110
includes a bottom coil support similar to the top coil support 116. However,
in the
drawings of FIG. 6, only the top coil support 116 is shown.
The entire coil assembly 114 is held in compression by a static clamping
system 120
when it is manufactured. The static clamping system 120 includes a top clamp
122 and a
corresponding bottom clamp that is not shown in FIG. 6. The top and bottom
clamps are
connected via an axially-extending tie rod. During the assembly of a typical
transformer,
the static clamping system 120 is positionally adjusted so as to place the
coil assembly 114
under a predetermined amount of force. To accomplish this, an appropriately
sized support
piece 124 is positioned on the support block 118 below the top clamp 122.
Thus, the static
clamping system 120 provides a known amount of force on the coil assembly 114.
The
static clamping system 120 is also used to compress a yoke assembly of the
magnetic core
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that is not shown in FIG. 6. It should be noted that in some transformers,
jack screws are
used instead of support pieces 124.
Because of the electromagnetic nature of the transformer 110, the top coil
support
116 and the support blocks 118 are made of an electrically insulative
material. Examples of
such material include wood, SpaulditeTM manufactured by the Spaulding
Corporation,
LebaniteTM, high density pressboard manufactured by Weidmann, or PermawoodTM
manufactured by Pernali Corporation. Additionally, the support piece 124 is
often a piece
of wood. And as previously mentioned, the windings of the coil assembly 114
are metallic
structures having an insulative material therearound. Also, the coil assembly
114 may be
comprised of multiple sets of concentric windings. Between those concentric
windings, the
coil 114 would include axially extending electronically insulative material
that is
commonly referred to as "fill material." Also, the entire contents of the
transformer 110
located within the container 112 are surrounded by an insulative fluid 130.
Over time, the insulative material associated with the windings of the coil
assembly
114, the insulative material of the top coil support 116, and/or the material
comprising the
support block 118 may begin to shrink. Further, adjacent turns of the
windings, which are
rectangular in cross-section, may shift slightly relative to each other over
time. This
shifting can cause additional shrinkage in the coil assembly 114. Any
shrinkage will result
in less material being compressively held which inherently reduces the
compressive force
on the coil assembly 114. When minimal shrinkage occurs, the support piece 124
may lose
contact with the top clamp 122 as shown generally at line 132 (distance not to
scale) such
that the coil assembly 114 is no longer being held under the compressive force
of the static
clamping system 120. The gap left between the top clamp 122 and the support
piece 124
due to shrinkage (i.e. the distance from line 132 to the top clamp 122) is
usually about
0.002 inch to about 0.003 inch. Consequently, any electrical transients that
occur may
cause a high magnetic force in the coil assembly 114 and result in
displacement of the coil
assembly 114. Such an occurrence will inevitably result in damage to the
transformer 110.
The present invention as described previously with respect to FIGS. 1-5
remedies
this situation when installed into the transformer 110. In particular, the
coil clamping
device 10 can be retrofitted into existing transformers after the
aforementioned shrinkage
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occurs, which usually takes several years after the transformer 110 has been
placed into the
field.
When retrofitting is needed, a field service technician places a jack or jacks
between
the top clamp 122 and the top coil support 116 after the fluid 130 has been
drained from the
transformer 110. It may be possible to remove the support pieces 124 even
prior to the
utilization of the jacks. However, if the support pieces 124 are still being
held in place, the
actuation of the jacks creates a force that tends to separate the top coil
support 116 from the
top clamps 122, thereby allowing the support pieces 124 to be removed with
ease.
Referring now to FIG. 7, once the support pieces 124 are removed and the coil
assembly 114 is being compressively held by the jacks, the technician then
develops a
generally cylindrical shallow recess 140 in each support block 118. The
shallow recess 140
has a diameter slightly larger than the diameter of the cylinder 12 and serves
to hold the
dynamic clamping device 10 in place on the support block 118. The technician
may
develop these recesses 140 with a simple cutting tool such as a chisel.
At this point, the dynamic clamping devices 10 are placed in the recesses 140.
The
loading rod assembly 20 is then positioned in the rod recess 44 on the piston
14. The lower
nut 26 is rotated so as to have its lower surface engage the upper surface of
the piston 14.
The upper nut 24 is rotated so that its upper surface is in tight contact with
the top clamp
122. The set screws 28 (FIG. 1 ) are tightened to hold the lower and upper
nuts 26 and 24 in
place. This process is then repeated for each dynamic clamping device 10 in
the
transformer 110. Finally, the compression retaining bolts 48 are removed while
reducing
the jack pressure such that the force of the spring washers 16 is now on the
coil assembly
114. Once the force is entirely on the coil assembly 114, the jacks are
removed from the
transformer 110. After the installation process is complete, the container 12
of the
transformer 110 is refilled under vacuum with fluid 130 which also enters the
chambers 15
of each device 10 via its conduit 60 (FIG. 3).
Because the dynamic clamping device 10 will be used on a variety of types of
transformers, each of which has its own unique recommended pre-load
manufacturing
force, the present invention contemplates providing a kit that includes
several sets of
dynamic clamping devices 10. For example, a first set may have four devices
10, each of
which provides 40,000 Ibs. of force. Thus, this set could be used on
transformers with four
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support posts 118 that require about 160,000 lbs. of force on their coil
assembly 114. The
device 10 allows for up to about one inch of expansion.
A second set may have four devices, each of which delivers 30,000 lbs. of
force.
Together, this second set could deliver 120,000 lbs. of force. The devices of
a third set may
each have 25,000 lbs. of force. The invention further contemplates appropriate
labeling,
whether by color coding or simple alpha-numerical statements, that reflects
the available
force for any specific device 10.
Because of the use of the compression retaining bolts 48, the dynamic clamping
device 10 can be preloaded for easy transportation. Thus, the device 10 can be
shipped to
manufacturers of transformers for easy placement into new transformers, in
addition to the
retrofitting application previously referenced. The dynamic clamping device 10
can be
installed between the static clamp and top coil support, as described
previously without
being fixedly attached to either structure. In other words, the dynamic
clamping device 10
would be sandwiched between the two structures in the newly manufactured
transformer.
While the present invention has been described with reference to one or more
preferred embodiments, those skilled in the art will recognize that many
changes may be
made thereto without departing from the spirit and scope of the present
invention which is
set forth in the following claims.
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