Note: Descriptions are shown in the official language in which they were submitted.
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GECAN 3144
MVLTI LA~ER INSIJLATION FOl~ WIND~G ELEMENTS
OF DYNAMOELECTRIC MAC~NES ~I).E.M.s)
This application relates to a method of insulating electrical
coils of a dynamoelectric machine using insulation layers of insulating
tape having differing insulating characteristics and qualities arid more
particularly layers having differing corona withstand capabilities.
The cost of the layers of insulation applied to the coils or half coils of
dynamoelectric machines varies in accordance with the cost of
production of each variety of insulation, and as expected, the
insulating mediurn which offers the most attractive capabilities from
an insulation point of view is generally the costliest. At the present
o tirne, most manufacturers who are attempting to provide an effective
insulating layer on large AC dynamoelectric machine winding
elements would probably defer to a composite corona resistant
polyimide insulation which is loaded with a corona resistant material,
such as for instance, finely divided al~lmim]m oxide, which is bonded
1~ or somehow joined to a mica tape layer to form a unitary insulating
medium in the ~orrn of a sandwich tape.
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While such insulating tapes are superior performers, it has
been found that these expensive layers of insulating material may be
used in combination with less costly layers of other types of
insulation without impairing the overall performance of the insulation
applied to the winding element.
BACKGRO~JND OF THE INVENTION
The designers of insulation systems for large high voltage AC
dynamoelectric machines have f'aced a constant challenge over the
last century, and that is to provide a machine which will operate at
0 higher and higher voltages whilst keeping the layers of insulating
material on the critical electrical conducting parts to a minimum
thickness. To meet the challenge, designers have utilized mica in a
variety of forms from large flake dispersed on a backing material, to
the product known as rnica paper which is a product made from tiny
1~ mica flakes which are incorporated into a product which nearly
resembles paper~ and in fact is made by a process which is very
similar to a process used for m~king paper from pulp fibers.
Mica by its very nature has physical properties which make it
intractable for use in an insulation system. However, its superior
~o corona breakdown resistance has provided the incentive that
insulation system designers needed to overcome the obstacles created
by its unattractive physical qualities. Today, mica paper enjoys an
unchallenged position as being one of the most corona discharge
resistant materials kno~ to mankind.
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Mica paper has an inherently low tensile strength and the tiny
mica flakes composing the paper.tape used in most mica paper
insulation tend to flake from the body of the tape as it is wound on
the winding elements, which will subsequently become a part of an
AC machine. As a result, insulation designers currently bond the
mica paper to another insulating medium, traditionally glass fibers
which will improve the tensile strength of the mica paper and the
backing tends to prevent the shedding of mica flakes from the mica
tape during a winding operation.
0 At the present time, a composite insulation which has
exceptional insulation qualities and good corona discharge resistance
is a CR KAPTON~) (trademark of DuPont) insulating film, which is
used as a backing on a rnica paper, glass fiber composite tape. The
addition of enhanced corona resistant materials yields an overall
insulation system which is electrically more robust than standard
systems. Experimentation has shown that by strategically locating
the highly corona resistant materials in the high electrically stressed
locations, the resultant hybrid system becomes dielectrically superior
(higher volt per mil capability) than either a fully corona resistant
~o system or a standard system. The resulting system has the added
advantage of being less costly than a fi~lly corona resistant enhanced
system.
S~JMMARY OF THE INVENTION
This invention relates to an insulation system which combines
an expensive enhanced corona resistant composite insulating tape
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with a much less inexpensive but, however less corona resistant
composite insulating tape such that the expensive, enhanced corona
resistant composite insulating tape may be utilized in areas of high
voltage stress and the less expensive corona resistant composite tape
may be utilized in areas where the voltage stress is somewhat
limini .~hed.
This insulation system satisfies the current movement toward a
reduction ~n overall insulation thickness on the conductive
components of a machine, which is ultimately subjected to an
increasing dielectric stress. If the final build thickness on the
conductive parts can be successfully reduced, the machine efficiency
may be increased. This invention seeks to strike a cost benefit
balance between the utilization factor resulting from the dielectric
characteristics of both component insulation systems.
ELEVANT P~IOR ART
Canadian Patents 714,637 July 27, 1965
1,329,519 May 17, 1994
U.S. Patents 4,399,190 August 16, 1983
~-,/60,296 July26, 1988
~o BRIEF DES(~RIPTION OF TEIE DRAWINGS
FIGURE lA shows the cross section of a typical stator bar for
a large AC dynamoelectric machine;
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FIGURE lB shows the cross section of a typical stator coil for
a large AC dynamoelectric machine;
FIGURE 2A shows a plot of the equipotential surfaces
surrounding the stator bar as shown in FIGURE lA;
FIGURE 2B shows the plot of the equipotential surfaces
surrounding the stator coil of FIGURE lB;
FIGURE 3A shows an insulating system for the stator bar of
FIGURE lA using the insulating system of this invention;
FIGURE 3B shows an insulating system for a stator coil of
FIGURE lB using the insulating system of this invention.
DESCRIPTION OF T~E PREFERRED EMBODIMENT
~IGURE 1 shows a cross section of a typical stator bar 10 for
a large AC dynamoelectric machine. Bar 10 is composed of a large
number of insulated conductors such as 12 which are insulated from
each other by the skand insulation 14.
The conductors 12 are formed into a group after having strand
insulation 14 applied thereto to provide the necessary isolation. The
top and bottom surfaces of the conductor group are filled with an
insulating material 13 generally referred to as a transposition filler.
~o The group of insulated conductors 12 are next wrapped with a
groundwall insulation material 16. The number of layers of insulating
tape mal~ng up insulation may be from 7 to 16 layers of a mica tape
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insulation wound in half lap fashion, depending on the level of
operating voltage to which the conductors 12 are being subjected.
For high voltage applications, that is for voltages above 4000
volts, the preferred gro~mdwall insulation 16 would be layers of a
composite m~ca tape comprising a corona discharge resistant
polyimide bonded to a mica type paper tape. This tape provides a
good layer of insulation, and because of its corona resistant
properties, provides long service life because of the resistance to
corona discharge. The mica paper composites and tapes used in
0 these hybrid systems contain a high percentage of a serni-cured resin
(resin rich) which may or may not contain a corona resistant material.
The wrapped bar is heated and compressed, in an autoclave or press,
to allow the resin to temporarily liquefy so as to evacuate any
entrapped air and elim~nate any voids. Heat and pressure are
m~int~ined on the bar ~mdergoing keaknent so that the resin
contained in the insulation is driven to gelation, bonding the
insulation system together. The surface of the cured bar may next be
coated with suitable materials to assure that the entire exposed
surface of the bar will form an equipotential surface during machine
operation.
The cured bar manufactured with the tape types as described
above will ~mction acceptably well w~thin the design parameters of
the machine for a predetermined period of time.
FIGURE lB shows the cross section for a typical coil 106. In
~5 this instance, stands 12b of copper (six shown) are grouped together
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so that although strands 12 are separated from each other by the
presence of strand insulation 14b, the six strands grouped into the
tum, must be insulated from the other turns of the coil 10b by means
of turn insulation 1 Sb. The turn package is ultimately covered with
groundwall insulation 16b.
FIGURE 2A shows a partial section of the stator bar of
FIGURE lA and the equipotential lines as they exist at the various
distances from the tllrn conductor bundle. It will be noted that the
voltage stress concentration is much greater in the area nearest the
conductor bundle and is especially intense at the corner of the bundle
(typically 135 volts/mil at inside corner and 65 volts/rnil at outside).
FIGURE 2B shows a partial section of the stator coil of
FIGURE lB with the equipotential lines illustrating the dielectric
stress. Note the similarity with FIGURE 2A.
This invention seeks to insulate the first several layers of
insulating medium surrounding the conductor bundle with a polyirn~de
film backed mica tape wherein the polyimide is loaded with a corona
discharge resistant material such as fumed alllmin~lm oxide and sold
as CR KAPTON(~ (a trademark of DuPont).
~o FIGURE 3A shows the cross section of a stator bar insulated
in accordance with the teachings of this invention. Here the
conductor bundle is composed of individual conductors 22 separated
by turn to turn insulation 24 similar to that as previously shown in
FIGURE lA. The conductor bundle is next wound with several
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layers of a composite tape comprising a rnica tape (or some other
acceptable material) backed with a corona discharge resistant
polyimide to form layer 26 to build up to the required thickness to be
present in areas where the voltage stress concentration is the greatest.
For most stator bars, the layer 26 will have a thickness of about one
third of the overall thickness of the groundwall insulation medium.
The two layers will generally carry a resin impregnant sirnilar to that
described earlier in FIGIJR E l A in association with groundwall
insulation 16.
o The balance of the groundwall may be layers of half lapped
tape composed of a composite such as mica paper backed on a glass
tape backing to forrn layer 28. A suitable resin impregnant may be
present in the mica paper. This standard tape has an excellent
voltage withstand capability but suffers from a corona discharge
resistance which is inferior to the tape forrning insulation layer 26. If
the intense voltage stress is concentrated on layer 26, the layer 28
will adequately serve to provide the protection from the dielectric
stress required by the groundwall insulation system and because the
voltage stress concentration has been dealt with by layer 26, the
~o overall groundwall insulation system is still quite functional and much
less expensive than if insulation layer 26 had been employed
exclusively for the groundwall insulation system. The insulation of
FIGURE 3 is functional and cost ef~lcient and thus provides a more
effective and ef~1cient groundwall insulation layer.
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The groundwall insulation comprising layers 26 and 28 may be
subjected to press curing or an autoclaving curing process to
eliminate any voids in the insulation layers 26 and 28 and to
subsequently drive the resin impregnant to gelation.
s Suitable surface coatings may be applied to the external
surface of insulation layer 28 once it is cured.
FIGURE 3B shows the composite groundwall insulation as it
applies to coil 20 composed of three turns. In this instance, the
copper conductors 22b are surrounded by strand insulation 24b. The
0 turn insulation 25b is applied to each turn and the initial layer of
groundwall insulation 26b containing the same constituents as layer
~6 in PIGURE 3A is applied. Finally, the layer of outer groundwall
insulation 28b is applied. With the exception of the presence of the
turn insulation 2~b, the insulation systems of FIGURES 3A and 3B
are very sirnilar.
In surnmary, a groundwall insulation is disclosed which utilizes
a pair of insulating materials in a most efficient manner. The material
which has excellent voltage withstand capability for the long and
short terrn (i.e. good corona discharge resistance) has been chosen to
~o be present where the voltage stress is most critical. A preferred
insulating tape for this layer will definitely have a corona resistant
layer such as CR KAPTON(~' layer in its constituents. The tape
envisaged for this layer would be a rnica tape which is resin rich,
where the resin is preferably ~llled with a corona resistant material
such as silicon or alllminllm oxide. An additional layer of glass fiber
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material may be incorporated into the composite tape (corona
resistant material plus mica layer) in the inner layer 26. The outer
layer such as layer 28 of FIGU~E 3A will not have the expensive
component such as CR KAPTON(~) in its composite layer. This
composite tape will have a resin rich mica tape (where the resin may
or may not contain a corona resistant filler) bonded to a glass fiber
backing in a woven or mat form.
As those skilled in the art are aware, alterations in the
components of the tapes are to be expected, but the presence of the
0 corona resistant layer in the tape employed in the inner most layer of
the groundwall insulation system is essential to this invention.
Applicant has been able to elimin~te this costly constituent from the
insulation applied to the outer- layer of the groundwall system without
any signiflcant compromise in the performance of the overall
groundwall insulation system.
