Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROSTATIC G~ADING LAYER FOR THE SURFACE
OF AN ELECTRICAL INSULATION EXPOSED TO
HIGH ELECTRICAL STRESS
. . _ . . _ . .
BACKGROUND OF THE INVENTION
The present invention relates to electrical machines
and, more particularly, to machines in which high electrical
stress can be generated on a surface of an insulator in the
vicinity of an electrical conductor.
In a high-powered electrical apparatus such as, for
example, the rotor of an electrical generator, it is
customary to produce slots in the surface of the generally
cylindrical rotor into which field windings may be placed.
In large machines~ such slots may extend generally parallel
to the axis of the rotor for 20 or 30 feet. Such field
windings are energized during operation to produce elec-
tricity when the rotor is driven.
The rotor ~orging itself is generally maintained at
ground potential whereas the coils or windings in the
slots are maintained at relatively high potential. In
order to protect against arcing from the windings to the
rotor forging and also to protect against physical abrasion
of the windings, it is customary to employ a slot armor
lining the sides and bottom of the slot. Such slot armor
is conventional and may consist of, for example, a fabric
laid up from woven or non-wo~en glass fibers impregnated
with a resin and cured by conventional means.
It is known that a high electrostatic gradient is
.
produced at the radially outermost region of the windings
adjacent to the slot armor lining the sides of the slot.
This electrostatic gradient may be sufficient to produce
ionization of the gaseous medium near the surface of the
slot armor and thus initiate arcing along the armor surface
of the slot armor and thus initiate arcing along the armor
surface which bridges the insulating path from the winding
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to the grounded rotor forging.
This problem has been recognizea and met in the past
by applying a grading layer to the surface of the slot
armor facing the windings. The grading layer has a
resistivity intermediate between that of a good insulator,
in the range of 10 ohms per square or more and that of
a good conductor having a resistivity of 10 l ohms per
square or less. Such a semi-conducting material permits
a limited amount of span-wlse distribution of electrical
charge so that the sharpness of the electrostatic grad-
ient and consequently the likelihood of the production of
a corona followed by flashover is reduced.
One type of semi-conducting material which has been
in commercial use for some time is asbestos which has been
applied to the surface of the slot armor. Asbestos, having
a resistivity of about 10 ohms per square and being sub-
stantially fire resistant, has performed satisfactorily
in this use. The use of asbestos is no longer favored
due to the possibility that it may have an adverse effect
on human health.
Other types of grading materials ha~e included a
plastic material containing conducting materials, such
as disclosed in U.S. Patent No. 2,789,154 to Peterson
dated April 16, 1957, or layers of metal ~oil embedded
within an insulating medium as disclosed in U.S. Patent
2,939,976 to Manni dated June 7, 1970.
Certain classes of semi-conducting materials have
non-linear resistance characteristics in the presence of
high voltage. One such non-linear semi-conducting material
is silicon carbide (SiC). In the presence of a relatively
low voltage gradient, silicon carbide has a relatively high
resistivity. When exposed to a high voltage gradient, on
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17GE-3~24
the order of, for example, several thousand volts per inch,
the resistivity of silicon carbide is substantially reduced.
This effect has been employed in a ceramic version of silicon
carbide for lightning suppression. The use of silicon
carbide in a grading system has been disclosed in U.S.
Patent Nos. 3,066,180 to Virsberg et al dated November
27, 1962 and 3,210,461 to Berg et al dated October 5, 1965.
Another disclosure of the use of non-linear resistive
effects is to be found in U.S. Patent No. 3,670,192 to
Andersson et al dated June 13, 1972, which employs a con-
ducting layer on the ends of coils which is connected to
ground through a variable non-linear resistor.
Silicon carbide, although having desirable non-
linear resistance characteristics in the proper range for
use in a grading system, is well known for being a very
hard abrasive material. If a silicon carbide layer were
incorporated in the surface of a slot armor contacting the
windings, abrasion of the windings is likely to occur
during motion of the windings under ~lectrical and thermal
stress.
OBJECTS AND SUM~.ARY OF T~E INVENTION
Accordingly, it is an object of the present invention
to provide a grading layer which overcomes the drawbacks
of the prior art.
It is a further object of the invention to provide
a grading layer which does not require the use of asbestos.
It is a further object of the invention to provide a
grading layer for the surface of an insulating slot armor
wherein the grading layer has a non-linear resistance
characteristic in the presence of a voltage gradient.
It is a further object of the invention to provide a
grading layer employing silicon carbide wherein the silicon
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carbide is physically separated from the associated coil
windings.
It is a further object of the invention to provide a
grading layer for a slot armor which employs an electrically
continuous layer of silicon carbide particles coated on
one side of a tough high temperature paper which is bond-
able to the surface of a slot armor insulating layer during
fabrication of the slot armor insulating layer.
According to an aspect of the present invention, there
is provided a grading layer for a slot armor of an electric
machine, of the type having electrical windings in a slot,
comprising an outer layer of an inert web having a first
side facing the windings, a layer of semiconducting
material on a second side of the inert web, and means for
bonding the grading layer to the slot armor.
According to a further aspect of the present invention,
there is provided a slot armor for an electric machine
of the type having electrical windings in a slot, com-
prising an insulating layer lining the slot, a grading
layer covering a surface of the insulating layer facing
the windings, the grading layer including a first layer
of semi-conducting material adhered to the insulating
layer and a second layer of a web on the first layer, the
web being contiguous to the windings, the semi-conducting
material being silicon carbide powder in a cured resin,
and the web being an aramid paper.
According to a feature of the present invention,
there is pro~ided a method of forming a slot armor with
a grading layer on a surface -thereof, comprising coating
a first surface of a web of aramid paper with a uniform
mixture of semi-conducting particles in a first heat cur-
able resin, drying the mixture, laying up the coated web
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L7r~E-3o24
on a first mold plate with a second uneoated surface of
the aramid paper in contaet with the first mold plate and
the first surface faeing away from the first mold plate,
laying at least one layer of a fabric pre-impregnated with
a second heat cllrable resin in contact with the first
surfaee, the first and second heat curable resins being
mutually compatible and being eurable under the same eon-
ditions of time and temperature, plaeing a seeond mold
plate in eontaet with the at least one layer of a fabrie,
and curing the first and second resins while forming the
slot armor between the first and seeond mold plates.
Aeeording to a further feature of the present
invention, there is provided a method of redueing high
voltage gradients at a surfaee of an insulator com~rising
plaeing an insulating layer contiguous to the surfaee and
eapaeitively eoupling an eleetrostatic field through the
insulating layer to a semi-conducting layer at an inter-
face between the insulator and the insulating layer, the
semi-conducting layer being effeetive to reduce the
voltage gradients.
The above, and other objects, features and advantages
of the present invention will become apparent from the
following description read in eonjunetion with the
aceompanying drawings, in which like referenee numerals
designate the same elements.
BRIEF DESCRIPTION OF THE DR~WINGS
Figure lA is a eross seetion of a slot in a generator
stator eontaining a slot armor having a grading layer
according to an embodiment of the invention.
Figure lB is a plot of voltage along the surface
of a slot armor of Figure lA.
Figure 2 is a cross seetion of a grading layer and
17G~-8024
a portion of slot armor insulation taken along II-II of
Figure lA.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figure lA, there is shown a cross
section of a slot 10 in a rotor 12 oE a dynamo electric
machine. Rotor 12 is a metal forging and is normally
maintained at ground potential.
Windings of conventional rotors 12 in generating
equipment are customarily cooled by flowing gas there-
through at atmospheric or elevated pressures. In somegenerators, cooling air at atmospheric pressure enters
the slots and flows radially past the windings. In other
systems, flowing hydrogen gas at elevated pressure such
as, for example, about 45 to 75 psig provides the cooling.
Although for a given pressure, air resists corona and
flashover at higher voltages than does hydrogen, pressuri-
zation of the hydrogen maintains the flashover and corona
voltage in an acceptable range when used in conjunction
with the present invention. Since full disclosure of gas
cooling and the apparatus for circulating the gas through
the windings is described in issued patents, a detailed
description of them is omitted.
Slot 10 extends radially inward from a tangential
surface 14 of rotor 12 and customarily extends in an
axial direction from one end of rotor 12 to the other.
In a large generator, this length dimension of rotor
12 may be quite great such as, for example, 14 to 27
feet or more.
An insulating bottom cover 18 is laid at a bottom
30 16 of slot 10. Bottom cover 18 is preferably of an
insulating material such as, for example, a glass fiber
matrix in a cured resin. A pair of generally L-shaped
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insulating slot armor elements 20 and 22 line the sides
24 and 26 continuously along the full length o~ slot 10
with short arms 28 and 30 facing each other and preferably
touching.
A first winding 32 rests in the bottom of the cavity
formed by slot armor elements 20 and 22. An insulating
layer 36 is interposed between winding 32 and a next
winding 34. Insulating layer 36 may be of any con-
venient material such as, for example, a conventional
polyester glass insulation.
The remainder of slot 10 is filled with alternating
windings and insulating layers to a last, or radially
outermost winding 38. Finally, a creepage block 40
consisting of a glass fiber matrix in a cured resin
binder covers the top of slot 10 above radially outer-
most winding 38.
A machined metal wedge 41 is inserted into dovetails
43 in the sides of slot 10 to hold the aforementioned
parts in place against the large centrifugal force they
experience during rotation of rotor 12.
A grading layer 42 lines the inner surfaces 44 and
46 of slot armor elements 20 and 22, respectively.
Along the radial extent of slot 10, very little voltage
gradient is experienced at inner surfaces 44 and 46 of
slot armor elements 20 and 22 except in the vicinity of
the radially outer portion of radially outermost wind-
ing 38 adjacent to slot armor elements 20 and 22. In
this region, a relatively high voltage gradient occurs
due to the discontinuity in voltage. That is, in the
vicinity of the windings within slot 10, a relatively
uniform voltage exists. However, radially beyond radially
outermost winding 38, the voltage drops to zero. In this
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region of sharp voltage change, a severe voltage
gradient can occur.
Referring to Figure lB, the voltage with respect
to ground along the inner surface 44 or 46 of the slot
armor elements 20 or 22 is shown. In the absence of the
grading layer, -the voltage gradient is steep, as represen-
ted by the solid line 48 and its value may exceed the
critical gradient for flashover of the surrounding gas
in which case damaging corona may be produced. This is
particularly true during high voltage proof testing
when overvoltage, several times the level of operating
voltage, is applied to the winding. Addition of the
grading layer 42 in Figure lA changes the span~wise
distribution of voltage due to the non-linear characteri-
stics of the grading material such that voltage gradients
are reduced to an acceptable level as shown by the dashed
line 50. It would be clear to one skilled in the art
that the higher voltage gradient 48 produced in the
absence of a grading layer would have a greater tendency
to produce a corona and strike a leakage path than would
the less steep voltage gradient 50 produced with a
grading layer.
Referring now to Figure 2, grading layer 42 is seen
to consist of an outer layer 52 of a smooth, hard, tough,
flexible material capable of withstanding elevated tem-
perature without charring. One suitable material for
this application is an aramid paper produced by the
Dupont Corporation under the trademark Nomex. A uniform
dense layer 54 of silicon carbide powder is bonded to the
rear surface 56 of outer layer 52. Layer 54 is bonded to
the insulating body of slot armor element 22 along a bond
line 58. An outer surface 60 of outer layer 52 faces the
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windings in slot 10 (Figure 1~).
Referring to the insert in Figure 2, silicon carbide
particles 62 are bonded in a cured plastic matrix 64 so
that sufficient electrical contac-t is achieved between
adjacent particles 62 to make layer 54 electrically
continuous.
Although layer 54 is showr. bonded to slot armor
element 22 along a bond line 58, layer 54 and slot armor
element 22 contain compatible resins such as, for example,
epoxy resins, which are cured during the final stages of
the formation of slot armor element 22 and, in fact,
unite to produce uniform fusion therebetween without voids
and other discontinuities which could permit the buildup
of voltage stress and subvert the action of grading layer
42.
It would be clear to one skilled in the art that,
since layer 54, containing the semi-conducting silicon
carbide, is separated from the windings in slot 10 by
an insulating aramid paper, direct physical contact can-
not be relied on for coupling charges from the windingsto layer 54. Thus, capacitive coupling must be relied
on to couple the field strength from the windings to
layer 54 wherein the charges are resistively dissipated
and spread in a span-wise direction to reduce the voltage
gradient.
The resin systems of slot armors 20 and 22 and the
grading layer 42 are chosen to be compatible, that is,
they cure under the same time and temperature conditions,
final adhesion between them is good and neither inhibits
the cure of the other. Further, the resin formulations
and surrounding materials are such that the grading
material particles remain in electrical contact with
17GE-'024
each other and are not dispersed while resin materials
are in a liquid state during the curing process. The
armor material complete with grading layer 42 is molded
between L-shaped mold plates, after being laid up between
them with apprcpriate mold release materials between the
armor and the plates, in a sealed apparatus such as an
autoclave in which appropriate conventional vacuum and
a pressure/temperature cycles can be applied.
The insulation portions of slot armor elements 20
and 22 are preferably built up using several plies of
glass fiber and most preferably plies of non-woven glass
fiber. In the preferred embodiment, a layer of cross ply
non-woven pre-preg glass fiber is laid atop grading layer
42. A further layer of unidirectional non-woven pre-
preg glass fiber cloth is laid over the first layer and
a final layer of cross ply non-woven pre-preg glass fiber
cloth is laid up over the unidirectional layer. Non-
woven glass fiber is preferred for maximum strength
in this application.
It would be clear that the layup process is not
limited to the above sequence in which grading layer
42 is placed in position first. Instead, the reverse
sequence can be used in which the sandwich is formed
with grading layer 42 on top.
Having described specific preferred embodiments
of the invention with reference to the accompanying
drawings, it is to be understood that the invention
is not limited to those precise embodiments, and that
various changes and modifications may be effected therein
by one skilled in the art without deparing from the scope
or spirit of the invention as defined in -the appended
claims.
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