DE2548328A1 - SUPPRESSION OF CORONA DISCHARGE IN HIGH VOLTAGE EVOLUTIONS - Google Patents

Description

DIPL.-ING. KLAUS NEUBECKER

Patent attorney
4 Düsseldorf 1 ■ Schadowplatz 9

Dr.-lng. Ernst Strafmann

Patent attorney

Düsseldorf, Oct. 27, 1975

44.249
7564

Westinghouse Electric Corporation
Pittsburgh / Pa., V. St. A.

Corona discharge suppression
with high-voltage windings

The invention relates to the suppression of corona discharge in high-voltage windings, in particular in electrical Generators is intended. In doing so, special emphasis is placed on creating an arrangement around the voltage load and to reduce the corona discharge in the high voltage insulation.

If an electrical conductor that has one or more sharp edges or corners, with a sufficiently high electrical When the potential is applied, a discharge occurs between the edge of the conductor and the surrounding atmosphere Electricity known as corona discharge. This corona discharge occurs at the edges of the conductor because because for a given electrical potential the electrical field strength is greatest at the sharp edge and one Can reach value at which the surrounding air is electrical

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Telephone (0211) 32 08 58 Telegrams Custopat

is broken, while on other parts of the conductor the intensity is still far below this value. The corona discharge therefore limits the potential to which the ladder can be raised without any safety restrictions. These Difficulties are particularly serious with elongated conductors that transmit power at high voltages. Examples of this are a busbar or strip of rectangular cross-section, the larger sides of which are essentially flat and are parallel. Usually the corona discharge is limited by rounding the sharp edges and corners of the conductor and the conductor is surrounded with an insulating material.

Material deterioration due to corona discharge at points of high voltage stress is a major cause of Insulation failure in high voltage applications. The corona discharge itself is relatively harmless. However surrendered serious secondary effects resulting from the development ■ development of strongly oxidizing substances in an intense electric field. Ozone is generated, which does the oxidation accelerated by surrounding organic materials. Nitrogen oxide components, which are generated when the air is ionized, combine with water and form acids, the organic ones and attack inorganic materials. Organic insulation materials, such as paint and cellulose, are used in one strong corona field oxidizes quickly. Mica and glass are produced by the corona discharge and by the oxidants not influenced, since it is an inorganic one which is inert to it

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Have composition.

Corona discharge can be expected in dynamo-electric machines that work with voltages of 6 kV and more. Corona discharge usually occurs in the high voltage stator windings, within the stator slots and at the ends of the stator slots. Such corona discharge occurs within the slots when the gas enters the small voids that exist between the massive insulation materials and the punched holes are ionized. In some high-voltage machines, such a corona discharge has been avoided by the massive insulation a semiconductor surface was given. The semiconducting envelope takes the tension load inside the massive one Isolation and collects the occurring along the coil surface capacitive current and harmlessly discharges it into the core.

"End-of-turn corona discharge" occurs where the coil is located leaves the slots and also between adjacent coils in the areas of the end turns. A concentration The voltage stress occurs at the edge of the core and there can be a corona discharge along high voltage windings occur on the surface of the insulation beyond the core area or at the end of the semiconductor treatment on the slot part of the coils. Corona discharge can also be between the coils occur in end windings, especially between coils of different phases, where the highest voltage

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load results. About the tension along the coil surface To distribute, a surface treatment of much higher resistance has usually been applied to the part of the coils given that just goes beyond the core. This overlaps the less resistant treatment and sufficiently distributes the voltage gradient along the coil surface in these final turns.

After the operating voltages are set continuously higher, it becomes more and more difficult to reduce the axial load by semiconductor treatment to control alone, especially during the overvoltage test attempts. A different method of axial load control was used for these high stresses. According to this method, conductive foils were incorporated into the insulation wall, in each case with or without a semiconductor surface treatment. The radial and longitudinal Loads are distributed more evenly and the thickness of the insulation wall is significantly reduced.

The object of the invention is to create a conductive film structure which maximizes the reduction in stress the insulation for a given number of conductive foils.

According to the invention, the object is achieved in connection with a dynamoelectric machine that has a magnetic core and Has windings within slots in the

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Magnetic core are arranged. The windings comprise a conductor that has planar, intersecting surfaces and thereby forms edges. The edges have a curvature with an arc length which is small compared to the extent of the circumference the flat surfaces. Around the conductor there are concentric insulations, in which (at least) one conductive layer is arranged, namely substantially concentric to the conductor. The conductive layer has a predetermined distance from the edge of the conductor, the distance being substantially the geometric mean of the radius represents the curvature of the conductor edge and the radius of curvature of the curved outer surface of the insulation which surrounds the edge in the curved area.

This arrangement creates an insulation structure, which reduces the voltage load and suppresses corona discharge in the high-voltage insulation, and also nor the necessary thickness of insulation for a non-circular conductor operating at a certain voltage should, is significantly reduced. To achieve this, one or more conductive layers or foils are placed inside embedded in the insulation structure at predetermined radial thicknesses. Each conductive layer has a uniform one predetermined distance from the conductor, so that the electrical load on the surface of a conductor is equal to the electrical Load is on the corresponding outer surface of each conductive layer. With an insulated conductor with one or more curved edges will meet this condition

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thereby realized in an isolation structure by placing the radius of a conductive layer in a curved area equal to the geometric mean of the radius of curvature of the conductor edge and the radius of curvature of the outer surface of the Isolation is made. When two or more conductive layers are used, the state becomes uniform Potential is realized where the ratio of the radii of successive conductive layers is in the same ratio relate to one another, like the radius of the first conductive layer to the radius of the curved conductor edge. Every so arranged conductive layer acts as an area of equal potential, whereby the voltage drop is uniformly distributed over the insulation material and thereby the necessary Insulation thickness is reduced and corona discharge is reduced or completely eliminated.

Further details, advantages and possible applications of the invention emerge from the accompanying illustration of exemplary embodiments as well as from the following description.

It shows:

Fig. 1 partially in perspective view of a stator slot section from which two isolated extend electrical conductor windings;

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FIG. 2 is a cross-sectional view of the electrical conductor windings of FIG. 1;

3 shows a cross section of an insulated conductor winding with a single conductive one Layer disposed within the insulation; and

4 shows a circuit illustration equivalent to the electrical conductor winding shown in FIG. 3.

In the following description, like reference numbers refer to like elements in all figures of the drawings.

Fig. 1 shows a perspective view of part of a stator arrangement 10 of a dynamo-electric machine (not shown), the stator having a rectangular stator slot 12 extending axially along the stator and radially from the stator bore. The stator assembly 10 is made from stacks of metal sheets that have openings that are axially aligned with one another, so as to form the axially extending Form stator slot 12 in which the electrical windings and components of an inventive Isolation system are located. The stator assembly 10 includes electrical conductor windings 20. The windings 20 are against each other isolated and also isolated from the stacked laminations of the stator assembly 10. The windings 20

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are within the stator slot 12 with the help of a slot wedging 14 that are made of mica, asbestos, glass fibers or similar inorganic materials with a synthetic resin bond is composed. A winding separator 16, which is made of Micanit (phenolic resin with mica additive) is between the electrical line windings 20 arranged. A surface 29 of the insulated conductor windings 20 is made semiconducting within the stator slot 12 and a certain distance outside the slot, preferably by applying a load-distributing silicone carbide composition. However, other coating materials can also be used be used.

In the embodiment shown, the electrical conductor windings have a rectangular cross-section with rounded ones Edge. Although a solid inner conductor 22 is shown with rounded corners with a radius of curvature "r" is, the conductor is usually divided into a large number of mutually insulated strands of wire around the Reduce skin effect losses. The inner conductor 22, however, is treated as if it were a solid rail with a rectangular shape Cross-section with rounded edges with a fixed radius of curvature "r" and made of highly conductive metal consists, such as copper or aluminum, this conductor electrical currents from a point within the dynamoelectric Machine is to be transferred to another point at high voltage. Usually the dimensions are similar Heads in high numerical ratios, like that

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that the length is much greater than the width. If not the sharp edges of such a conductor are slightly rounded corona discharge can occur if the conductor is operated at a high potential. The outline of the components the insulating structure for the electrical conductor windings 20 is shown in the cross-sectional view of FIG. 2, 3 and 4 reproduced. In Fig. 3 a cross section of the electrical conductor winding 20 is shown, while in Fig. 4 a approximately equivalent circuit of the conductor arrangement is shown. The inner conductor 22 is made up of a first layer 24 of insulating material surrounded to a radial thickness "x" on each edge of the electrical conductor winding is being built. This layer preferably consists of spliced mica tape or mica paper.

According to the invention, the high electrical load on the Edges of the conductor reduced in that one or more conductive layers are embedded in the insulation. Corresponding is a conductive layer 26 between the first insulation layer 24 and the second insulation layer 28 arranged in concentric relation to the inner conductor 22. The conductive layer 26 is spaced from the surface of the electrical conductor 20 arranged in a manner to be described. The conductive layer 26 is intimate Contact with the first insulating layer 24 and the second insulating layer 28 completely surrounds the insulating layer 24 and the inner conductor 22 and extends axially at least as far as the outer semiconducting layer 29

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is shown on the conductor 20. As shown in Fig. 3, the 'entire radial thickness of two insulating layers including the conductive layer thickness in the curved portions ¹¹ T "is referred to. The conductive layer 26 divides the bulked thickness of both insulation layers at the flat surfaces in the same proportion as around the curved edges of the inner conductor 22, that is:

r + 'χ _ r' + T.
rr + χ

In Fig. 3 are on the cross section of the electrical conductor winding 20 capacitors C1, C2, C3 and C4 are shown. One approximately equivalent circuit of the arrangement with the connections to these capacitors is shown in FIG. C1 represents the distributed capacitance between the conductive layer 26 and the planar surfaces of the inner conductor 22, C2 is the distributed capacitance between conductive layer 26 and the flat outer surface of insulation 28, C3 is the distributed capacitance between conductive layer 26 and the edges of inner conductor 22 and C4 is the distributed Capacitance between the conductive layer 26 and the outer edges of the insulation 28. The symbol R is the resistance of the conductive layer 26 between the flat areas and the curved edge areas. The symbols V1 and V2 represent the potential of the conductive layer 26 at its flat areas or at its curved areas, the outer insulation surface serves as a reference point.

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As mentioned earlier, the voltage stress on the edges of the inner conductor 22 is for a given applied voltage higher than the uniform load in the flat, level areas of the insulation. Without the conductive layer that would be The voltage drop across C3 is therefore greater than the voltage drop across C1, i. H. V1 would be greater than V2. By the arrangement a conductive layer 26 having a resistance "R" between the planar areas and the curved edge areas, the is low compared to the reactances of the four capacitances, the two voltages will be almost identical and there C1 is greater than C3 and C2 is greater than C4, the two voltages will be closer to the initial value of V1 than to the initial one Value of V2. The result is that the stress over C3 and therefore the stress over the first insulating Layer in the curved area has been reduced.

An example will now be given. Let the flat surfaces given above be two and three inches wide (5.08 and 7.62 cm), while the radius of the conductor edge r = 0.03 inch (0.762 mm) and the insulation thickness T = 0.3 inches (7.62 mm). The conductive layer is at a distance of χ = 0.10 inches (2.54 mm) from the surfaces of the conductor. If a value of 4 is assumed as the dielectric constant (the choice does not affect the result), there is a capacity per 1-inch conductor length of:

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C ₁ = 0.2248 10 χ 4 = 89.92 pF ¹ 0.100

C = 0.2248 10 χ 4 = 44.96 pF ^Δ 0.200

C- = 0.6137 χ 4 = 3.85 pF ^ό log 0.130
0.030

⁼ 0/6137 x 4 = 6.06 pF, log 0.330
0.130

In calculating C3 and C4, as well as in the following calculations, it was assumed that the edge of the insulating surface and the edge of the conductive layer are part of one concentric circular cylinder.

If 30 kV are applied to the conductor (average load 100 V / m) and there are no conductive layers, the following values result for V1 and V2:

V ₁ = 30 ^C 1 = 20 kV

C ₊ C

V ₉ = 30 ^C 3 = 11.65 kV

The inner third of the insulation is therefore subject to an average load of:

18350 = 183.5 V / m. 100

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From standard tables, the ζ. B. by J.D. Cockcroft in the article "The Effect of Curved Boundaries on the Distribution of Electrical Stress of Round Conductors," J. Inst. Elect. Engrs., Volume 66, April 1928 on pages 385-409, the load on the edge of the conductor is:

2.57 κ 100 = 257 V / m.

In the case of a highly conductive layer, such as layer 26, the voltage of the conductive layer results in:

V = 30 ^C 1 ^{+ C} 3 = 19.43 kV.

c ₁ + c ₂ + c ₃ + c ₄

The inner third of the insulation now has an average load of only:

= 105.7 V / m,

the maximum load on the edge of the conductor is now only:

1.80 χ 105.7 = 190.3 V / m.

It should be noted that these stresses are somewhat higher when it is taken into account that the conductive layer possesses a certain resistance, however, the differences are small as long as the resistance of the layer is compared with the impedance of the largest capacitance of the system is small.

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In the example above, the impedance of C1 for an AC current of 60 Hz is:

¹ _r yj = 1.9 χ 10 ⁷ ohms.

377 χ 89.92 χ 10

When the conductive layer 26 continues to the inner conductor 22
is shifted, the stresses on the edge of the inner conductor 22 decrease, while at the same time the stresses on the outer surface of the conductive layer 26 are reduced
rise in the curved area. The lowest maximum load in the insulation results when these two loads are equal, which is according to known principles of
Electrostatics occurs when the radius of the conductive
Layer in the curved edge area is equal to the geometric mean of the radius of the conductor edge and the radius of the outer surface of the insulation. The effect of the conductive layer is that the average electrical load for the different parts of the insulation is made the same while the insulation is shared by the conductive layer 26
will.

When two or more conductive layers are used, the maximum stress in a given part of the insulation between two adjacent conductive layers depends on the ratio of the curvatures of these layers and will increase as this ratio increases. To the highest electrical
Load in the isolation for a given number of
should keep conductive layers as low as possible

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the highest ratio between two curvatures of two adjacent conductive layers as low as possible. This is achieved in that the conductive layers in be arranged such that all ratios between the curvatures of adjacent layers are equal. That means, that:

r + X ₁ r + x ₂ r +

where r = radius of the conductor;

T = radial thickness of the insulation in the curved area;

x. = distance from the conductor surface to the i-th conductive layer;

η = number of conductive layers (positive integer); and

i = a positive integer that is less than or equal to η.

From this relationship it can be seen that

n + 1 f X ₁ = ~ V ^r

n + 1-i.

(r + T) ¹ -r,

where n> i> 0. If only one conductive layer is used becomes, η - 1 and

(r + T) - r.

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The penultimate equation can also be expressed in the following way;

(X ₁ + r) ^{n + 1} = r ¹¹ + ¹ " ¹ (r + T) ¹ .

The conductive layer 26 can be arranged in a simple manner if the insulation is made up of multiple layers of tape, such as has already been proposed. The conductive layer can then be conveniently placed between two layers of tape, where the best value of χ is given by the above equations. If z. B. for a conductor with an edge radius 0.03 "(0.762 mm) with an insulation thickness of 0.145" (3.683 mm) is used, the lowest value is the maximum stress is achieved when the conductive layer is 0.042 inches (1.0668 mm) from the conductor surface is located away. In a fully loaded insulation system, a metal foil can act as a conductive layer can be used, but for an insulation system to be impregnated in a vacuum, the conductive layer must also be permeable be. A conductive impregnable tape is available that has been tested as an external tie tape and is also suitable for use as a conductive layer in vacuum-impregnated insulation systems.

Reference is again made to FIG. 1, in which the conductive layer 26 and the insulation layers 24 and 28 are shown in this way are that they extend along the inner conductor 22 with the conductor protruding from the core piece 12. the

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Conductive layers extend from the end of a semiconductive outer surface 29 on conductor 20 to a predetermined one Distance to an axial load distribution on the surface of the conductor insulation and in its outer To achieve layers, in addition to the radial load distribution.

Claims ;

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Claims (8)

Patent a η s ρ r ü c h e: Dynamo-electric machine having a magnetic core and one disposed within slots in the magnetic core Winding, characterized in that the winding (20) comprises a conductor (22) which has flat surfaces which intersect and thereby form an edge region which is curved and has an arc length, which is small compared to the circumferential extension of the flat surfaces, and by one around the conductor (22) concentrically arranged insulation (24, 28) in which a conductive layer (26) is essentially concentric Relationship with respect to the conductor (22) is embedded, the conductive layer (26) from the edge part of the conductor (22) has a predetermined distance which is substantially the geometric mean the radius of curvature of the conductor edge and the radius of curvature of the curved outer surface of the insulation is in the curved area surrounding the edge part. 2. Machine according to claim 1, characterized in that the conductor edge part has a radius of curvature r, the insulation (24, 28) has a radial thickness T in the area surrounding the edge part and the distance between the conductive Layer of the edge part χ, where x, r and T obey essentially the following relationship: r + χ _ ' r + T
rr + χ 609819/0371 2bA8328 3. Machine according to claim 1 or 2, characterized by several conductive layers which are concentric in the insulation and are spaced from one another, the ratio of the radii of curvature being more successive conductive layers is essentially the same as the ratio of the radius of curvature of the conductive layer, closest to the conductor to the radius of curvature of the curved edge of the conductor. 4. Machine according to claim 3, characterized in that the conductor edge part has a radius of curvature r, the insulation has a radial thickness T in the curved region surrounding the edge part that the number of conductive layers corresponds to the positive integer η and that the removal of the i-th conductive layer of the edge part x. where i is a positive integer and that x., r, T, η and i are essentially of the following Obey the equation: r + X ₁ r + X ₂ r + X _{1 r} + T r + X ₁ r + X _1-1 ^{r + x} n where η> i ~> 0. 5. Machine according to one of the preceding claims 1-4, characterized in that the insulation consists of several Layers of insulation tape consists. 609819/0371 6. Machine according to claim 5 / characterized in that the conductive layer is made of metal foil of predetermined radial thickness which is small compared to the radial thickness of the insulation. 7. Machine according to claim 6, characterized in that the conductive layer consists of conductive impregnable tape. 8. Machine according to one of the preceding claims 1 - 7, characterized in that the conductor (22) and insulation (24, 28) extend axially from the core, wherein the conductive layer extends from the core a predetermined distance for axial compensation to achieve the electrical load for the axially extending insulation. ES / hs 5 609813/0371