WO2020167218A1

WO2020167218A1 - Elastic tubular high-voltage insulating body

Info

Publication number: WO2020167218A1
Application number: PCT/SE2020/050132
Authority: WO
Inventors: Bertil Moritz
Original assignee: Hm Power Ab
Priority date: 2019-02-11
Filing date: 2020-02-10
Publication date: 2020-08-20
Also published as: EP3924984A4; SE1930052A1; EP3924984A1; SE543113C2; CN113412522A

Abstract

Tubular insulating body (1) for use on a high voltage element (8), the insulating body comprises an insulating structure wherein an inner surface of the insulating body is electrically in contact with the high voltage element (8) and an outer surface of the insulating body is connected to ground potential, and wherein a plurality of conductive layers (4) are provided between said outer and inner surfaces. Essentially the entire insulating body comprises elastic or stretchable properties making the insulating body deformable or bendable to a predetermined shape different from the shape in the state of no external force applied to the insulating body.

Description

ELASTIC TUBULAR HIGH-VOLTAGE INSULATING BODY

TECHNICAL FIELD

The present invention concerns high voltage insulation. More precisely the invention concerns a tubular body providing insulation between an inner surface and an outer surface having different electrical potential.

Especially the invention concerns a tubular insulating body having a plurality of conductive layers to control the electric field distribution. In particular the invention concerns an insulating body having tapered ends.

BACKGROUND OF THE INVENTION

An insulating body comprising a plurality of conductive layers forming capacitor elements is most commonly known from electric bushings. Such bushings are devices that carry current at high potential through a grounded barrier such as a transformer tank. In order to decrease and control the electric field condenser bushings have been developed.

Condenser bushings facilitate electrical stress control through insertion of floating equalizer plates which are incorporated in the core of the bushing . The condenser core decreases the field gradient and distributes the field along the length of the insulator. Electric field concentrations are thus avoided resulting in absence of partial discharges and flashover.

Generally the basic principle known is to make a cylindrical insulating structure for use on high voltage element, where one inner surface of insulating structure is electrically in contact with the high voltage element and an outer surface of insulating body is connected to ground potential, and between the said outer and inner surfaces there are several

conductive layers, and the conductive layers have different length in axial direction and the distance in axial direction, between innermost

conductive layer and outermost conductive layer, is several times longer than the distance in radial direction. The purpose is to reduce the electric field at the interface of insulation and ambient air. The reason is that the air has much lower specific electric withstand than solid insulation material.

A condenser core of a bushing is commonly wound from paper or crepe paper as a spacer. The equalization plates are constructed of metallic layers. Metallic layers are typically made of aluminum. These cylindrical plates are located coaxially so as to achieve an optimal balance between external flashover and internal puncture strength. The paper spacer ensures a defined position of the electrodes plates and provide for mechanical stability.

The condenser cores are impregnated either with oil (OIP, oil impregnated paper) or with resin (RIP, resin impregnated paper). RIP bushings have the advantage that they are dry (oil free) bushings. The core of an RIP bushing is wound from paper, with aluminum plates being inserted in appropriate places between neighboring paper windings. The resin is then introduced during a heating and vacuum process of the core.

A bushing serves to insulate conductors that are carrying high voltage current through a grounded enclosure. To safely accomplish such a task without a flashover is a challenge, as the dimensions of the bushing are very small compared with the dimensions of the equipment it is

connecting. Not only electric field stress and thermal stress must be handled by the bushings but also mechanical stress. Therefore, the bushing is made of stiff material to support the conductor inside. The stiff housing of a bushing comprises most commonly porcelain or glass fiber tube. Most commonly the conductive layers are made of aluminum foil.

From US 5227584 a barrier of condenser type for field control in

transformer bushing terminals is previously known. The object of the barrier is to overcome flashover between the transformer and the conductor of the transformer. This is accomplished by a geometric shape of the barrier.

From US 7742676 the production method of a high voltage bushing is previously known. The object of the method is to provide a less time- consuming production of a bushing . This is achieved by using electric layers with openings thus providing the matrix material to penetrate.

SUMMARY OF THE INVENTION

A primary object of the present invention is to seek ways to provide a bendable and very flexible tubular body for insulating a high voltage element/conductor from ground potential.

This object is achieved according to the invention by a tubular insulating body defined by the features in the independent claim 1.

In brief, the tubular insulating body for use on a high voltage element comprises an insulating structure, wherein an inner surface of the insulating body/structure is electrically in contact with the high voltage element and an outer surface of the insulating body/structure is connected to ground potential. A plurality of electrically conductive layers are provided between said outer and inner surfaces and separated by layers of electrically insulating material. Electrically conductive granules or powder material is embedded on a molecular level in a matrix material which is essentially the same molecule as the insulating material, whereby the insulating body comprises elastic properties making the insulating body deformable to a predetermined shape different from the shape in the state of no external force applied to the insulating body/structure.

Preferred embodiments are described in the dependent claims. According to the invention the tubular insulating body is made of an elastic and stretchable insulating material comprising conductive layers containing carbon powder or other conductive powder or grains. In an embodiment the conductive layers are formed in the stretchable insulating material. The stretchable insulating material may comprise an elastic compound as well as a plastic compound. In an embodiment the insulating material comprises an elastomer, silicone rubber or EPDM rubber. By the expression elastic must best be understood a rubbery material. In an embodiment of the invention the flexible tubular body comprises a first tapered end and a second tapered end. The tapering may differ depending on whether the conductor ends in the atmosphere or in a fluid. By the tapered ends the electric field gradient may be smoothly distributed.

Electric field concentrations may thus be avoided which otherwise may cause partial discharges.

The field stress at the end of the conductive layer is high. The objective is to reduce the electrical field level lower than the flashover withstand in the air at the insulation boundary.

Another objective is to reduce the number of conductive layers to a minimum of cost and manufacturing reasons.

One common way to achieve electric stress control for high voltage cable terminations is a so-called stress cone. Basically, the insulation thickness is increased at the high stress area, allowing the electric field to become lower when reaching the boundary between insulation and air.

The invention resolves the requirements to reach all the objectives, by combining the stress control using very few conductive layers with section of increased thickness of insulation material outside the endings of conductive layers. The invention also resolves the problem to adapt the shape to another shape without destroying the insulation properties. The reasons to change the shape by applying external forces are to make either or both manufacturing and assembling easier.

In an embodiment of the invention the bendable tubular body is made as a straight body and then formed to fit a curved conductor. In case of a bendable conductor the insulating body is threaded onto the conductor where after the conductor and the insulating body are bent together. In case of a curved solid electrode the stretchable insulating body is threaded onto the curved structure.

In an embodiment the layers are inverted, meaning that the shortest layer in axial direction is at the inner diameter and the longest layer is at the outer diameter of the insulating body. This design is applicable to cable terminations and cable joints.

In a further embodiment of the invention the outer insulation comprises flanges to increase creepage distance. The flanges are located just outside the endings of the conductive layers to allow the electric field level to be reduced at the insulation/air boundary.

In a further development of the invention the bendable insulating body constitutes an integral part of a current transformer for high voltage use. The current transformer comprises a bendable core forming a ring with an opening to be clamped around a high voltage conductor. It should be pointed out that the openable ring comprises one opening only and lack joints. The bendable insulating body surrounds part of the core and carries the secondary winding. Hence the secondary winding receives ground potential and the current may be read at ground level.

In one aspect of the invention, the object is achieved by a tubular insulating body for use on a high voltage element, the insulating body comprises an insulating structure wherein an inner surface of the insulating structure is electrically in contact with the high voltage element and an outer surface of the insulating structure is connected to ground potential, and between said outer and inner surfaces several conductive layers are provided, wherein essentially the entire insulating structure material is preferably homogeneous and comprises elastic or stretchable properties making the insulating structure deformable or bendable to a predetermined shape different from the shape in the state of no external force applied to the insulating structure.

Preferred embodiments and features of the invention are listed as follows below.

- the insulating body/structure comprises an elastic material with rubbery properties,

- the conductive layers comprise granules or powder of carbon or other electrically conductive material dispersed in elastic material,

- the conductive layers have essentially the same elastic properties as the material of the non-conductive material, i.e. insulating structure,

- the conductive layers have different lengths in axial direction, between innermost conductive layer and outermost conductive layer, and the distance in axial direction is longer, preferably several times longer, than the distance in radial direction,

- the length in axial direction of the innermost conductive layer is longer than the length of the outermost conductive layer, or vice versa,

- the lengths in axial direction of the conductive layers increases

successively or stepwise from said outer surface to said inner surfaces, or vice versa, - the lengths in axial direction of the conductive layers are essentially equal and the conductive layers are axially displaced in relation to each other from said inner surface to said outer surfaces, or vice versa.

- the insulating material portions or insulating mid portions between the conductive layers have different thicknesses between each conductive layer,

- the same or essentially the same molecule and polymer matrix is provided in both insulating layers and conductive layers making the insulating body or structure deformable, preferably more than about 10 % elongation in any direction, without causing any separation and/or void, neither within nor between layers, between insulating and conductive layers.

- the insulating structure or body comprises a tapered shape in at least one end portion, preferably in both end portions,

- the insulating structure or body further comprises additional insulation material, such as radially extended flanges or discs, located axially at or near the end regions of the conductive layers,

- the insulating structure is tapered similarly in both end portions to reduce the electric field at the surface of the insulating structure on both sides of a mid-section having different electric potential than the inner conducting element,

- the insulating structure is tapered in one end where the conductive layers have a successively longer extension closer to the inner high voltage element, and inversely tapered in the opposite end. In another aspect of the invention, a tubular insulating body may be provided for insulating a high voltage conductor from ground potential comprising an insulating structure containing a plurality of coaxially oriented conductive layers to control the electric field distribution, wherein the insulating structure comprises elastic or stretchable properties making the insulating body deformable or bendable to assume a predetermined shaped structure, such as a predetermined curved structure.

In a further aspect of the invention, a current transformer may be provided for use on a high voltage power line comprising a power line enclosing core, a tubular insulating body comprising an insulating structure comprising a plurality of coaxially oriented conductive layers to control the electric field distribution and a secondary winding carried by the insulating body, wherein the insulating structure comprises elastic or stretchable properties making the insulating body deformable or bendable to assume a predetermined shaped structure.

In a further aspect of the invention, a cable termination or cable joint may be provided for high voltage cables comprising a tubular insulating body comprising an insulating structure comprising a plurality of coaxially oriented conductive layers to control the electric field distribution, wherein the insulating structure comprises elastic or stretchable properties making the insulating body deformable or bendable to assume a predetermined shaped structure.

Another important aspect is the capacitive distribution of voltage of each conductive layer. The voltage between each layer is proportional to the capacitances. If the thickness of insulation between the conductive layers vary in inverse proportion to the length of the layer, the voltage

distribution can be equal between all layers. The invention makes it possible to optimize both the axial length and the thicknesses to achieve best possible use of insulation material. Finally, the objective is to maintain the electric withstand even when the entire body is deformed, i.e. for example bent, stretched and/or

squeezed . The invention meets this objective using the same or

essentially the same molecule in the entire body. The carbon powder or other electrically conductive material is integrated in the matrix of this molecule, and both the insulation material between conductive layers and the outer insulation have the same or essentially the same molecule.

When the final curing is made, the cross-linking between all interfaces create one single giant molecule. Any mechanical stress applied during shaping/deformation will not result in internal separations, or formation of voids or gaps. The insulation properties are maintained also after shaping or deformation.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become more apparent to a person skilled in the art from the following detailed

description in conjunction with the appended drawings in which:

Fig 1 is a cross-section of an insulating body according to the invention,

Fig 2 is a cross section of a current transformer containing the insulating body according to the invention, in a straight shape before it is bent to a full circle,

Fig 3 is a perspective view of a current transformer according to the invention, and

Fig 4 is a cross section of a cable termination according to the invention mounted on a cable.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An insulating body 1 according to an embodiment of the invention is shown Fig 1. The insulating body 1 is made of an elastic material and comprises an insulating structure comprising insulating mid portions 2 and conductive layers 4. A hollow passage for accommodation of a conductor of a high voltage system is arranged in the center of the insulating body. Any type of conductor passing through a hole having different voltage than the conductor, such as a transformer bushing, may apply to the invention.

The insulating body 1 comprises a first conductive layer forming the passage. This layer will be in contact with the conductor to be received in the hollow passage. The insulating body further comprises a second conductive layer defining the outer surface of the insulating body. In the embodiment shown the insulating body comprises several intermediate conductive layers 4 cylindrically or coaxially oriented in the insulating body between the first conductive layer and the second conductive layer. The outermost conductive layer is shorter than the innermost conductive layer. The insulating body is made of elastic material and comprises stretchable feature. By elastic material should be understood a material with a pronounced capacity for elongation and/or compression, such as a rubber like or rubbery material. The stretchable capacity permits the insulating body to be bent to assume a curved structure. Therefore, the conductive layers should also be stretchable and thus cannot be solid. Albeit shown in the drawings as solid lines, the conductive layers 4 comprise carbon or other electrically conductive powder or grains. In an embodiment the electrically conductive material is introduced in a polymeric material similar to the insulating material. In an embodiment the polymeric material comprises silicone rubber.

The conductive material, e.g. carbon powder, is embedded on a molecular level in a matrix material which is the same or essentially the same molecule as the insulation material 2 in between the conductive layers 4. As used herein, "embedded" means that the grains of carbon are

immovably fixed between molecules in the matrix of the elastomeric compound. When fully cured, cross-linking between conductive and insulating layers form one single elastic molecule which can be shaped or deformed without causing any void or gap between the conductive and insulating layers.

A conductive layer may be considerably thinner than an insulating layer. For example, a conductive layer may have a thickness ranging from about 0.2 mm to about 0.02 mm, whereas the thickness of an insulating layer may be in the order of several mm such as from about 0.5 mm to about 5 mm, e.g. In an embodiment, the thickness of the conductive layer is between 0.05 mm and 0.02 mm.

Additional insulation material 6 is filling a shape with thicker insulation close to the end portions of the conductive layers. In an embodiment this additional insulation material may be in the form of radially extended flanges or discs 7, located axially at or near the end regions of the conductive layers. This arrangement extends the creepage distance along the interface insulation-to-air. The electric field strength in air will also in this way become essentially lower due to the fact that the field, where it is as highest, can be reduced at the conductive end portions out to air.

Fig 2 shows an embodiment where the insulating body 1 is molded into a complete insulating structure to accommodate a magnetic core 8 and a secondary winding 10. The outer contour is formed by insulation 6 and flanges 7. The secondary winding wires exit in a cylindrical body 9. The layers of insulation material and conductive material layers are cross linked after curing to form the insulation body 1. In this connection it can be noted that curing, a process that is well known in the art of

polymerization, usually involves heating with or without the presence of a catalyst. In particular, the entire insulation body is bendable to assume a

predetermined shaped structure. This shape may comprise an arbitrary design but is most conveniently a bend or curve. The body can also be stretched or expanded in radial direction. The bending capacity may include formation of an angle between a first and a second angular leg. The first angular leg comprises a line from one end point to a midpoint of the insulating body. The second angular leg comprises a line from the other end point to the midpoint of the insulating body. This intermediate angle may be in the order of at least 45 degrees.

In a development of the invention the insulating body is used as a part of a current transformer. Fig 3 shows the current transformer mounted on a high voltage line 5. The magnetic core 8, which can be realized in the form of a thread or tape, e.g., assumes the same potential as the high voltage line and the secondary winding assumes ground potential. The insulating body is molded in original straight shape. An advantage of molding the insulation body in a straight shape is that it is much easier to insert straight electric sheet core. Also, the secondary winding is much easier to wind on a straight cylindrical surface. The entire current transformer is then possible to bend in any form to finally make the ends connected magnetically.

In use, the current transformer is hung onto a high voltage conductor. Thereby also the core receives high voltage potential. To isolate the secondary winding, the core is dressed with an insulating body according to the invention. The insulating body follows the bended shape (curve) of the core.

Fig 4 shows an embodiment of the invention for cable terminations. The insulation body 11 differs from insulation body 1 only in terms of shape. The steps of layers are inverse at inner diameter. Left and right sides of the insulation body are mirrored. This may be the base shape which can be applied for cable terminations. The electric field grading is similar.

However, this embodiment can have almost the same axial length on each layer, resulting in even voltage distribution if each layer has the same thickness.

The conductive layer 12 of the high voltage cable is peeled off. The conductor 14 and the cable insulation 15 extend beyond the cable conductive layer 12. The integrated insulation body, 11 and 6, has in relaxed state a smaller inner diameter than the cable insulation 15. The result is that the insulation body, 11 and 6, can squeeze the cable enough to expel any air and exclude formation of any void in between. A

conductor 13 is connected between the cable conductive layer 12 and the outer conductive layer of the insulation body 11. In a similar way a conductor 13 connects the conductor 14 and the innermost conductive layer of the insulation body 11.

For the insulating body 1 and 11, a material with the properties of rubber or synthetic rubber is thus preferred in both electrically conductive and electrically insulating layers. Among the synthetic rubbers, silicone rubber (based on crosslinked polysiloxanes or polydimethylsiloxanes) and EPDM rubber (ethylene propylene diene monomer rubber) are mentioned as preferred compounds for their mechanical properties in general terms of tensile strength, temperature and weather resistance and, of course, their insulating capacity and elastomeric properties. Other synthetic rubbers may however be considered as alternatives, such as butadiene rubber, neoprene or chloroprene rubber, or nitrile rubber, e.g., and others not mentioned.

It is further preferred that the electrically conductive and insulating layers are the same or essentially the same molecule and polymetric matrix.

That is, as long as the compounds are compatible to form crosslinking between layers upon curing, a slight difference in composition may be accepted without leaving the scope and spirit of the invention. As a non- limiting illustration to the expression "same or essentially the same" molecule and polymetric matrix, a reference can be made to elastomeric compounds and synthetic rubbers comprising polymers of either of butadiene, CH2=CH-CH=CH2, isoprene or methyl butadiene, CH2=C(CH3)- CH=CH2, and dimethyl butadiene CH2=C(CH3)-C(CH3)=CH2, e.g., which would be considered the same or essentially the same molecule and polymetric matrix for the purpose of forming the insulating body, if appropriate.

Electrically conductive polymers are available commercially and usually doped with carbon (C) to the extent that contact is made between the grains of carbon. The technology for dispersing carbon grains in a polymer material is developed and well established by companies in the trade. However, elements of other conductivity than carbon, such as copper or aluminum, e.g., may be utilized in the present invention, if appropriate. Although favorable, the scope of the invention is not limited to the embodiments presented but covers also other embodiments which may become natural to a person skilled in the art after reading the present disclosure.

Claims

1. Tubular insulating body (1, 11) for use on a high voltage element (8, 14), the insulating body comprises an insulating structure, wherein an inner surface of the insulating body is electrically in contact with the high voltage element (8, 14) and an outer surface of the insulating body is connected to ground potential, and wherein a plurality of electrically conductive layers (4) are provided between said outer and inner surfaces and separated by layers (2) of electrically insulating material, characterized in that electrically conductive granules or powder material is embedded on a molecular level in a matrix material which is essentially the same molecule as the insulating material, whereby the insulating body (1, 11) comprises elastic properties making the insulating body deformable to a predetermined shape different from the shape in the state of no external force applied to the insulating body.

2. Tubular insulating body according to claim 1, wherein the insulating body (1, 11) comprises an elastic material with rubbery properties.

3. Tubular insulating body according to claim 1 or 2, wherein the

conductive layers (4) comprise carbon powder dispersed in elastic material.

4. Tubular insulating body according to any of claims 1-3, wherein the conductive layers have different lengths in axial direction, between innermost conductive layer and outermost conductive layer, and the distance in axial direction is longer, preferably several times longer, than the distance in radial direction.

5. Tubular insulating body according to claim 4, wherein the length in axial direction of the innermost conductive layer is longer than the length of the outermost conductive layer, or vice versa.

6. Tubular insulating body according to claim 4, wherein the length in axial direction of the conductive layers (4) increases successively or stepwise from said inner surface to said outer surfaces, or vice versa.

7. Tubular insulating body according to any of claims 1-3, wherein the lengths in axial direction of the conductive layers (4) are essentially equal and the conductive layers (4) are axially displaced in relation to each other from said inner surface to said outer surfaces, or vice versa.

8. Tubular insulating body according to any of the preceding claims, wherein the insulating material layers (2) between the conductive layers (4) have different thicknesses between each layer.

9. Tubular insulating body according to any of the preceding claims, wherein the same or essentially the same molecule and polymer matrix is provided in both insulating layers (2) and conductive layers (4) making the insulating body (1, 11) deformable more than about

10 % elongation in any direction without causing any separation and/or void between insulating and conductive layers.

10. Tubular insulting body according to any of the preceding claims,

wherein the insulating body comprises a tapered shape at both end portions (3).

11. Tubular insulating body according to any of the preceding claims, wherein the insulating body further comprises additional insulation material, such as radially extended flanges or discs (7), located axially at or near the end regions of the conductive layers (4).