CA1040696A - Corona suppression in high voltage windings - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An arrangement for reducing voltage stress and corona discharge in high voltage insulating material utilizes thin conductive layers embedded within the insulation. Each conductive layer serves as an equi-potential surface which grades the voltage drop across the insulating material, thereby reducing the chance of corona discharge. For insu-lation which encloses a conductor having a curved edge portion lowest electrical stress in the insulation is obtained when the stress at the curved edge portion of the conductor is equal to the stress at the corresponding curved outside surface of the conducting layer. This condition is satisfied in the present invention by making the spacing distance of the conducting layer from the conductor in the curved region equal to the geometric average of the radius of the curved conductor edge and the radial thickness of the insulation.

Description

BACKGROUND OF THE INVENTION
Field of the Invention:
This inventio~ relates to an improved arrangement for reducing voltage stress and corona discharge in high voltage insulation and has particular, though not exclusive, application to the equi-potential grading of high voltage stator winding insulation of dynamoelectric machines.
Description of the Prior Art:
When an electrical conductor having one or more sharp edges or corners is supplied with a sufficiently high electrical potential, a discharge of electriclty known as a corona discharge occurs between the edge of the conductor -1- ~ .' ': :

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~ 44,249 ~04(~696 and the surrounding atmosphere. The corona discharge occurs at the edges of the conductor because of the fàct that, for a given potential, the electrical field intensity is greatest ;
at the sharpest edge and may re~ch a magnitude at which the surrounding air breaks down while the intensity at other parts of the conductor is still far below this magnitude.
The corona discharge thus limits the potential to which ~ :
the conductor can be sa~ely raised. This difficulty is especially serious ln the case of an elongated conductor 10 which carries power at high voltage; i.e.~ a bus bar or a -strip o~ rectangular cross-section, the ma~or sides of which are substantially flat and parallel. It is customary to llmit the corona discharge by rounding the sharp edges and corners of the conductor and enclosing the conductor with ~ 1 an insulating material.
Corona deterioration at points of high voltage stress ls one of the maJor causes of insulation failure in high voltage appllcations. The corona itself is relatively harmless; however, there are serious secondary effects which ;~
result from the production of powerful oxidizing agents in an intense electrical field. Ozone is produced which ac-celerates oxidation of ad~acent organic materials. Nitrogen o-xide components, produced by the ioni~ation of the air, combine with water to form acids that attack organic and inorganic materials. Organic insulation~; such as varnishes and cellulose are rapidly oxidized in a strong corona field.
Mica and glass are unaffected by corona and the oxidizing agents because of their inert, inorganic composition.
Corona may be expected in dynamoelectric machines which operate at levels of 6kv and greater. The corona . . i . : , , ,. , . , , ~

44,249 ~04~)696 discharge usually occurs in the hlgh voltage stator windings within the stator slots and at the ends of the stator slots.
Slot corona occurs within the slots when the gas in the small voids existing between the solid insulation and the punchings ionizes. This has been overcome on some high voltage machines by the application of a semiconducting surface to the solid insulation. This semiconducting sheath contains the voltage stress within the solid insulation, col-lecting the capacitive current from along the coil surface 10 and discharging it harmlessly into the core; An example of ~-A this approach is exemplified by Berg et ~l~Patent 3,210,461, issued October 6, 1965, and assigned to the assignee of the present invention.
End winding corona occurs where the coils leave the slots and also between ad~acent coils in the end-turn regions.
There is a concentration of the voltage stress at the edge of the core and corona may take place on high voltage windings along the surface of insulation beyond the core or at the end of the semiconducting treatment on the slot part of the 20 coils. It may also occur between-coils in-the end windings, particularly-between-coils of-different phases where the highest voltage stresses exlst. In order to grade the voltage along the coil surfaces, it has been-customary to apply a much hi~her resistivity surface treatment to a portion of the coils ~ust beyond the core. This overlaps the lower resistivity treatment an~ satisfact~rily distributes the voltage gradient along-the coil surfaces into the end windings.
As operating voltages become increasingly greater, it becomes more dîfficult to control the axial stre~ses by ~ 30 semi-conducting treatment alone, especially during over-`. ~'. ':' ., " ~ .

~ 0406~6 potential testing. For these high voltages an alternate ~ethod of controlling the axial stress has been used. -According to this method, conducting foils are incorporated in the insulation wall, with or without semiconducting surface treatment. ~y proper choice of foil spacing and foil length, the radial and longitudinal stresses can be evened out and the inæulation wall thickness reduced ap-preciably. It is, therefore, a principal ob~ect of the present invention to provide a conducting foil structure which achieves maximum reduction in insulation voltage ~tre~s for a given number of conducting foilæ.
SUMMARY OF THE INVENTION
In accordance with this invention, an insulation structure is provided which relieves voltage stress and suppresses corona discharge in high voltage insulation while substantially reducing the insulation thickness required for a non-circular conductor operating at a ~peci~ied potential. For this purpo~e, one or more con-ductive layers, or foils, are embedded within an insulation structure of predetermined radial thickness. Each conductive layer is uniformly spaced a predetermined distance from the conductor so that the electrical stress at the surface of -the conductor is equal to the electrical stress at the cor-responding outside surface of each conducting layer. For an insulated conductor having one or more curved edges, this condition is realized in an insulation structure wherein the spacing distance of a conducting layer irom the conductor in the curved region of the insulation surrounding an edge portion conductor edge and the radial thickness con-ductor edge and the radius of the curved outer surface ' :
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~ . . .; . . .
, ~ . . . ~ . . . -o~ the insulation in the curved region. If two or more conductlng layers are employed, the equi-potential condition is realized where the ratio o~ the ~pacing distances o~ 8UC- :
cessive conduc~ing layers are ln the same ratio as the spa-cing distance of the first conducting la~er is to the radius;
of the curved conductor edge. Each conductlng layer so di~-posed serves as an equi-potent~al surface which grades the -voltage drop across the ln8ulatlng materlal, thereby xe-duclng requlred lnsulation thickness ~or a given operatlng potential and al80 reduclng or ellminatlng corona.
BRIEF DESCRIPTION OF THE DRAWINGS
ffl e lnventlon wlll be more fully understood irom the ~ollowing detalled descrlption, taken ln connection with the accompanylng drawlng, ln whlch:
Flgure 1 18 a partlal isometric view of a stator slot section ~rom which two insulating electrical conductor windlngs proJect, Figure 2 18 a cross sectlon view of the electrical conductor windings of Flgure l;
Flgure 3 1~ a cross section o~ an insulated con-ductor windlng havlng a single conductive layer disposed withln the insulatlon;
Flgure 4 is an approximate equivalent circuit representatlon of the electrlcal conductor windings o~
Figure 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
m roughout the descrlption whlch ~ollows, like reference characters refer to like elements on all ~lgures of the drawing.
: ~

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, ~ . .. .~ , , . ~ , . . .. ..
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~1l,249 ~04~696 Referring now to the drawing, Figure 1 is an isometric view of a portion o~ a stator assembly 10 of a dynamoelectric machine (not shown) having a rectangular stator slot 12 extending axially along the stator and -radially from the stator bore. The stator assembly 10 is comprised of stacks of metal laminations having openings aligned axially relative to each other so as to form the axlally extending stator slot 12 in which electrical windings and components of an insulating system in accordance with ~ -`
the present invention are placed. The stator assembly 10 includes electrical conductor windings 20 whlch are neces~
sary to obtain operation of dynamoelectric machines, which ~ -is well known in the art. The eIectrical conductor windings 20 are insulated relative to each other and are insulated . . .
relative to the stacked laminations of the stator assembly 10 in accordance with the present invention. The electrical conductor windings 20 are confined within the stator slot 12 by means of a slot wedge 14 which is preferably composed Fi6~ 9/ R 55 (~ t ~ 4J~ ~ ~r k) -:
of mica, asbestos, fibcrglasa or similar inorganic materials with resin binding. A ^winding separator 16, preferably micanite, is shown disposed between the electrical conductor windings 20. The surface 29 of the insulated conductor windings 20 within the stator slot-l~ and for a predetermined 1~ .
distance outside the slot is made semiconducting, preferably by applying a stress-grading compositi~n-of silicon carbide.

¦ However, other coating materials may be employed.
The electrical conductor windings 2~-are shown to have rectangular cross section with rounded corners. Al-though a solid inner conductor 22 having rounded corners of radius "r" is depicted, the conductor is commonly subdivided ,~

;..... . .... . . . .. . ... . . . . ... .

44,2L19 into a large number of mutually insulated conductor strands in order to reduce the skin-effect losses. For convenience of reference in this discussion, however, the inner conductor 22 will be treated as if it were a solid bar of rectangular cross section having rounded corners of specified radius "r".
The inner conductor 22 is preferably composed of a highly conductive metal such as copper or aluminum and is intended to pass electrical currents from one point to another within the dynamoelectric machine at high voltages. As a rule, the dimensions of such conductors stand in high numerical ratios to each other, the length being much greater than the width. -~
Unless the sharp corners of such a ~onductor be somewhat ~ -rounded, a corona discharge may take place when the con- `
ductor is operated at a high potential.
A general outline of the components of the in-sulating structure for the electrical conductor windings -20 as disclosed by thls invention is illustrated in the end view shown in Figure 2 and can be better understood by refer-ring to Figures 3 and 4. A cross section of the electrical conductor winding 20 is shown-in-Figure 3, and an approximate ~ :
equivalent circuit of the conductor assembly is shown in Figure 4. Surrounding the inner conductor 22 is a first layer 24 of insulating material which is built up to a radial thickness "x" at each corner of the electrical conduc-tor winding. This layer is preferably composed o~ mica split-ting tape or mica paper. ~ -In the present invention, a decrease in the high electrical stresses at the edges of the conductor is achieved by embedding one or more conducting layers within the insula~
tion. Accordingly, a conducting layer 26 is disposed between -, . ... , - , , . ,: . ., : .. . . ., . - , ., . ~ . ; ~, , : . . . . .. . ; .

... . . ~ , , ;, ,. , , . , . . ., .,,.
., . . . ., . , - ., ..... . , . . . ,.. . ~ . , . ... , :

44,249 iO4~96 the first insulation layer 24 and a second insulation layer 28 in a concentric relationship with inner conductor 22.
The conducting layer 26 is spaced from the surface of elec-trical conductor 20 in a manner which will be described be-low. The conducting layer 26 is in intimate contact with the first insulation layer 24;and the second insulation layer 28, completely surrounds the insulation layer 24 and the in-ner conductor 22, and extends axially at least as far as the outer semiconducting surface 29 on the conductors 20. As 10 shown in Figure 3, the total radial thickness of both in- ~ -sulating layers including the conducting layer thickness ln ~-the curved regions is designated "T". The conducting layer 26 divides the combined thickness of both layers of insulation ~ -ln the same ratio at the flat surfaces as around the curved edges of the inner conductor 22, i.e., r + x = r + T -r r + x In Figure 3, oapacitors Cl, C2, C3 and C4 are shown superimposed upon a cross section of electrical conductor winding 20. An approximate equivalent circult of the as-~0 sembly which shows the interconnection of these capacitors is shown in;Figure 4. Cl represents the distributed capac- -itance between the conducting layer 26 and the flat surfaces of the inner conductor 22, C2 is the dlstributed capacitance between the conducting layer 26 and the flat outer surfaces of the insulation 28, C3 is the distributed capacitance between the conducting layer 26 and the edges of the inner conductor 22, and C4 is the distributed capacitance between the con-ducting layer 26 and the outer edges of the insulation 28.

The symbol R is the resistance of the conducting layer 26 Z
., 44, 2119 104~)696 ~
between the flat areas and the curved edge areas. The ~ .
symbols Vl and V2 represent the po.tentlal of the conducting ; :
layer 26 at its flat portions and its curved portions, res~
pectively, with the outer insulation surface as reference.
As mentioned above, the voltage stress at.. the . ;
edges of the inner conductor 22 for a given applied voltage is higher than the uniform.stress in the flat parts of the insulation. With no conducting layer present, the voltage drop over C3 will there~ore be larger than the 10 voltage drop over Cl, i.e., Vl> V2. With the presence .
of a conducting layer 26 having a resistance "R" between the flat portions and the curved edge areas that is low :
compared with the reactance of either of the four capaci- :~ -~
tances, the two voltages will be almost identical and be- ~ ~ .
cause Cl is greater than C3, and C2 is greater than.C4, the ~ ~:
two voltages will be closer to the initial value of Vl than :;
to the inltial value of V2. The result is that the stress .
over C3, and consequently the stress over the first insula-tion layer in the curved region, has been relieved. ~ ~ -An example.will now be given... Let the flat sur~
faces previously referred to be two inches and three inches :.
wide, respectlvely and the radius of the conductor.edge r = 0.03 inches, the insulation thickness T = 0.30 inches, and the conducting layer located at a distance of x = 0.10 `."
inches from the surfaces of the con~uctor. If the dielectric : ~`
:~ constant is assumed to be 4 (the choice does not effect the result), the capacitances per inch of conductor length will be~
C = 0.2248 10 x 4 = 89.92 pF
0 . 100 C2 = 0.2248 10 x 4 = 44.96 pF
0 200 :.

4~,24g 1~40696 C = 0.6137 x 4 = 3.85 pF
3l 0---130 0.030 ~ -C4 = 0.6137 x 4 = 6.o6 pF
log 0.330 0.130 In the determination of C3 and C4 as well as the following calculations, it has been assumed that the edge of the insulation surface and the edge of the conducting layer are parts of concentric, circular cylinders.
With 30kv applied to the conductor (average stress lOOvpm), and with no conducting layers, the following values for Vl and V2 are obtained: - -~ , .
Vl = 3 Cl = 20~kv lOC1+C2 V2 = 30 3 = 11.65 kv The inner thlrd of the insulation at the edges has therefore an average stress of:

1`~8350 = 183.5 vpm, 100 . :: :
and from standard charts, e.g., that published by J. D.
Cockcroft in "The Effect of Curved Boundaries on the Dis~
tribution of Electrical Stress of Round Conductors," J. Inst.
Elect. Engrs., Vol. 66, April 1928, pp. 385-409, the stress at the edge of the conductor is:

2.57 x lO0 = 257 vpm With a highly conducting layer such as conducting ;; layer 26, the voltage of the conducting layer becomes V = 30 Cl+C3 = 19.43 kv l 3 The inner third of the insulation at the edges now ~ -has an average stress of only:
-10- ;

10570 = 105.7 vpm, 100 ~,. . .
and the maximum streæs at the edge of the conductor i8 now only:
1.80 x 105.7 = 190.3 vpm It should be noted that these stresses will be slightly higher when lt i~ taken into account that the con- -ducklng layer has a certain resistlvity, but the difierence wlll be small as long as the reslstance of the layer is small compared with the impedance of the largest capacitance of the sy~tem. In the example above, the impedance of Cl with 60 cycle excitation is:

377 x 89.92 x 10 12 = 2 9 x 107 Q
If the conductlng layer 26 is moved further in to-wards the inner conductor 22, the stresses at the edge of inner conductor 22 wlll decrease, while at the same time, I the stresses at the outside surface of the conductlng layer 1 26 in the curved region will increase. The lowest maximum I stress in the ln~ulation will exist when these two stresses are the same, which accordlng to known principles of electro-- 20 Qtatic~ will occur when the ~pac~ng distance from the inner conductor of the conducting layer in the curved edge area is equal to the geometric average of the radius of the conductor edge~and the radial thickness of the insulation. The effect o~ the conductlng layer is to make the average electric stre8s the 8ame for the various parts of the ln~ulatlon as it i8 dlvlded by the conducting layer 26.
If two or more conducting layers are utillzed~ the maxlmum stress ln a given paxt of the insulation between two . .-i` ' ' J ~' ' ' \

"

104~)696 adjacent conducting layers will depend upon the ratio of the spacing distances of these layers, and wlll increase as this ratlo increases. In order to keep the h~ghest electric stress in the insulation as low as possible with a given number o~ conductinglayers, the hlghest ratio between the spaclng distances oi two ad~acent conducting layeræ should be as low as po~sible. This ls accomplished by locating the conductlng layers such that all the ratios of spacing dis-tances of ad~acent layers are the same; i.e., lOr + x r + x2 r ~ xi r + T
1 = = ,, = = =
r r + xl r + Xi-l r + xn where r = radius of conductor;
T = radial thickness of insulation ln curved region;
Xi = distance from conductor sur~ace to the ith conducting layer;
n = nu~ber o~ conducting layers (a posltive lnte-ger); and 1 = a posltive integer less than or equal to n.
From thls relatlon it can be shown that n+l xl = l rn~l-i )i - r, where n~i o. I~ only one conducting layer i8 u~ed, n - l and Xl = ~ - r.
The ~ormer equation may be expressed in the ~ollowing form:
(Xi + r) n+l ~ rn+l-i (r~
m e conductlng layer 26 is easily applied if the insulatlon is built from multiple layers Or tape, as has been suggested, because it may be conveniently inserted , . : ~ , , .. :

~ 4~)696 between those two tape layers which will give the be~t value of x as determined from the equations given above. For axample, in the single conducting layer ~tructure of Figure 3, for a conductor ha~ing edge radius of 30 mil8, and insulatlon thickness of 0.145 inches, the loweæt value of the maximum stress i8 obtained when the conductlng layer is located 42 mils from the conductor surface. In a fully loaded insulation system, a metal foil can be used for the con-ductlng layer, but for an lnsulation system that i8 to be vacuum impregnated, the conductlng layer must also be im-pregnable. A conducting, impregnable tape i8 available, -~
has been tested as an outer binder tape, and ls also suitable for use as a conducting layer ln a vacuum impreg- ~ -nated insulation system.
Referring agaln to Figure 1, the conducting layer 26 and lnsulation layers 24 and 28 are shown extending along the inner conductor 22 as lt pro~ects out of core slot 12. The conducting layer~ are extended out from the end of a sem1conductlng outer sur~ace 29 on the conductor 20 a pre-determlned distance in order to provide axlal grading on thesur~ace of and in the outer layers o~ the conductor insula-tion in addition to radial gradlng.
It i8 therefore apparent that the structure des-crlbed above 18 an improved arrangement for reducing radial voltage stress in high voltage insulation wlth or without slmultaneous reduction of axlal voltage stress. The struc- :
ture reduces or elimlnates lnternal corona di~charge while substantially reducing the lnsulatlon thlckness requlred for a specified operating potential. In addition to stator wind-39 ings~ the gradin8 principle described herein can be used for the lnsulation o~ any current carrylng metallic part having .. . . . , ; . ..

44,249 ~4~)696 a non-circular cross section. It should be understood that various modifications, changes and variations may be made -in the arrangements, operations and details of construction of the structure disclosed herein without departing from the spirit and scope of the present invention.

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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a dynamoelectric machine having a magnetic core and a winding disposed within slots in said core, said winding including a conductor having planar surfaces which intersect to define an edge portion, said edge portion being curved and having an arc length which is small as compared with the peripheral extent of said planar surfaces, and insulation concentrically disposed about said conductor, the improvement which comprises:
a conductive layer disposed within said insulation in substantially concentric relationship with respect to said conductor, said conductive layer being spaced at a predetermined distance from said edge portion of said con-ductor, said spacing distance being substantially the geometric average of the radius of curvature of said conductor edge portion and the radial thickness of said insulation in the curved region surrounding said edge portion.

2. The combination defined in claim 1, said conductor edge portion having a radius of curvature r, said insulation having a radial thickness T in the curved region surrounding said edge portion, the spacing distance of said conductive layer from said edge portion being x, wherein x, r, and T are substantially in the relation:

.

3. The combination defined in claim 1 including at least one additional conductive layer, said at least one additional conductive layer being concentrically disposed within said insulation, the plurality of said conductive layers being spaced apart one from another at predetermined spacing distances relative to said edge portion, the ratio of the spacing distances of successive conductive layers being substantially in the same ratio as the spacing distance of the conductive layer next adjacent to said conductor is to the radius of curvature of the curved edge of said conductor.

4. The combination defined in claim 3, said conductor edge portion having a radius of curvature r, said insulation having a radial thickness T in the curved region surrounding said edge portion, the number of said conductive layers being a positive integer n, the spacing distance of the ith said conductive layer from said edge portion being xi, i being a positive integer, where xi, r, T, n, and i are substantially in the relation:

and n i 0.

5. me combination defined in claim 1, said insulation being composed of multiple layers of an insulating tape.

6. The combination defined in claim 1, said conduc-tive layer comprising a metal foil of predetermined radial thickness which is small as compared to the radial thickness of said insulation.

7. The combination defined in claim 1, said conductive layer comprising conductive impregnable tape.

8. The combination defined in claim 1 wherein said conductor and said insulation project axially from said core, said conductive layer being extended from said core for a predetermined distance to provide axial grading for said axially projecting insulation.