SE543113C2

SE543113C2 - Elastic tubular high-voltage insulating body

Info

Publication number: SE543113C2
Application number: SE1930052A
Authority: SE
Inventors: Bertil Moritz
Original assignee: Hm Power Ab
Priority date: 2019-02-11
Filing date: 2019-02-11
Publication date: 2020-10-06
Also published as: CN113412522B; SE1930052A1; EP3924984A4; WO2020167218A1; CN113412522A; EP3924984A1

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 structure is electrically in contact with the high voltage element (8) and an outer surface of the insulating structure 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 structure comprises elastic properties making the insulating structure deformable to a predetermined shape different from the shape in the state of no external force applied to the insulating structure.

Description

An insulating body comprising a plurality of conductive layers formingcapacitor elements is most commonly known from electric bushings.Such bushings are devices that carry current at high potential througha grounded barrier such as a transformer tank. In order to decreaseand control the electric field condenser bushings have been developed.Condenser bushings facilitate electrical stress control through insertionof floating equalizer plates which are incorporated in the core of thebushing. The condenser core decreases the field gradient anddistributes the field along the length of the insulator. Electric fieldconcentrations are thus avoided resulting in absence of partial discharges and flashover.

Generally the basic principle known is to make a cylindrical insulatingstructure for use on high voltage element, where one inner surface ofinsulating structure is electrically in contact with the high voltageelement and an outer surface of insulating body is connected to groundpotential, and between the said outer and inner surfaces there areseveral 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 severaltimes longer than the distance in radial direction. The purpose is toreduce 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 orcreped paper as a spacer. The equalization plates are constructed ofmetallic layers. Metallic layers are typically made of aluminum. Thesecylindrical plates are located coaxially so as to achieve an optimalbalance between external flashover and internal puncture strength. Thepaper spacer ensures a defined position of the electrodes plates and provide for mechanical stability.

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

A bushing serves to insulate conductors that are carrying high voltagecurrent through a grounded enclosure. To safely accomplish such atask without a flashover is a challenge, as the dimensions of thebushing are very small compared with the dimensions of the equipmentit is connecting. Not only electric field stress and thermal stress mustbe handled by the bushings but also mechanical stress. Therefore thebushing is made of stiff material to support the conductor inside. Thestiff housing of a bushing comprises most commonly porcelain or glassfiber tube. Most commonly the conductive layers are made of aluminum foil.

From US 5227584 a barrier of condenser type for field control intransformer bushing terminals is previously known. The object of thebarrier is to overcome flashover between the transformer and theconductor of the transformer. This is accomplished by a geometric shape of the barrier.

From US 7742676 the production method of a high voltage bushing ispreviously known. The object of the method is to provide a less timeconsuming 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 abendable 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 tubularinsulating body defined by the features in the independent claim 1.

Preferred embodiments are described in the dependent claims.

According to the invention the tubular insulating body is made of anelastic and stretchable insulating material comprising conductive layerscontaining carbon powder. In an embodiment the conductive layers areformed in the stretchable insulating material. The stretchable insulatingmaterial may comprise an elastic compound as well as a plasticcompound. In an embodiment the insulating material comprises anelastomer, silicone rubber or EPDM rubber. By the expression elasticmust best be understood a rubbery material. In an embodiment of theinvention the flexible tubular body comprises a first tapered end and asecond 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 fieldconcentrations may thus be avoided which otherwise may cause partial discharges.

The field stress at the end of the conductive layer is high. The objectiveis 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 voltagecable terminations is a so called stress cone. Basically the insulationthickness is increased at the high stress area, allowing the electric fieldto become lower when reaching the boundary between insulation and air.

The invention resolves the requirements to reach all the objectives, bycombining the stress control using very few conductive layers withsection of increased thickness of insulation material outside the endingsof conductive layers. The invention also resolves the problem to adaptthe shape to another shape without destroying the insulationproperties. 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 madeas a straight body and then formed to fit a curved conductor. In case ofa bendable conductor the insulating body is threaded onto theconductor whereafter the conductor and the insulating body are benttogether. 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 shortest axiallength is at the inner diameter and the longest layer is at the outerdiameter. This design is applicable to cable terminations and cable joints.

In a further embodiment of the invention the outer insulation comprisessheaths to increase creepage distance. The sheaths are located justoutside 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 bodyconstitutes an integral part of a current transformer for high voltageuse. The current transformer comprises a bendable core forming a ringwith an opening to be clamped around a high voltage conductor. Itshould be pointed out that the openable ring comprises one openingonly and lack joints. The bendable insulating body surrounds part of thecore 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 tubularinsulating body for use on a high voltage element, the insulating bodycomprises an insulating structure wherein an inner surface of theinsulating structure is electrically in contact with the high voltageelement and an outer surface of the insulating structure is connected toground potential, and between said outer and inner surfaces severalconductive layers are provided, wherein essentially the entire insulatingstructure material is preferably homogeneous and comprises elastic orstretchable properties making the insulating structure deformable orbendable 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 structure comprises an elastic material with rubbery properties, - the conductive layers comprise carbon powder 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 increasessuccessively or stepwise from said outer surface to said inner surfaces, or vice versa, - the lengths in axial direction of the conductive layers are essentiallyequal 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 betweenthe conductive layers have different thicknesses between each conductive layer, - essentially the same matrix molecule is provided in essentially theentire insulation structure or body making the insulating structure orbody deformable more than about 10 % elongation in any direction without causing any separation and/or void in the structure, - the insulating structure or body comprises a tapered shape at at least one end portion, preferably at both end portions, - the insulating structure or body further comprises additionalinsulation material, such as radially extended sheaths or dishes, located axially at the end portions of the conductive layers, - the insulating structure is tapered similarly in both end portions toreduce the electric field at the surface of the insulating structure onboth sides of a mid-section having different electric potential than the inner conducting element, - the insulating structure is tapered in one end where the conductivelayers have a successively longer extension the 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 beprovided for insulating a high voltage conductor from ground potentialcomprising an insulating structure containing a plurality of coaxiallyoriented conductive layers to control the electric field distribution,wherein the insulating structure comprises elastic or stretchableproperties making the insulating body deformable or bendable toassume 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 insulatingstructure comprising a plurality of coaxially oriented conductive layersto control the electric field distribution and a secondary winding carriedby the insulating body, wherein the insulating structure compriseselastic 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 jointmay be provided for high voltage cables comprising of a tubularinsulating body (11) comprising an insulating structure comprising aplurality of coaxially oriented conductive layers (4) to control theelectric field distribution, wherein the insulating structure compriseselastic 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 ofeach conductive layer. The voltage between each layer is proportionalto the capacitances. If the thickness of insulation between theconductive layers vary in inverse proportion to the length of the layer,the voltage distribution can be equal between all layers. The inventionmakes 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 whenthe entire body is deformed, i.e. for example bent, stretched and/orsqueezed. The invention meets this objective using essentially thesame molecule in the entire body. The carbon powder is integrated inthe matrix of this molecule and both the insulation between layers andthe outer insulation have essentially the same molecule. When the finalcuring is made, the cross-linking between all interfaces create one single giant molecule. All mechanical stresses during deformation do not result in any internal separations, voids or gaps. The insulation properties are maintained also after deformation.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will becomemore 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 containingthe 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 isshown Fig 1. The insulating body 1 is made of an elastic material andcomprises an insulating structure comprising insulating mid portions 2and conductive layers 4. A hollow passage for containing a conductor ofa high voltage system is arranged in the centre of the insulating body.The insulating body 1 comprises a first conductive layer forming thepassage way. This layer will be in contact with the conductor to bereceived in the hollow passage. The insulating body further comprises asecond conductive layer defining the outer surface of the insulatingbody. In the embodiment shown the insulating body comprises several intermediate conductive layers 4 cylindrically oriented in the insulating body between the first conductive layer and the second conductivelayer. The outermost conductive layer is shorter than the innermost conductive layer.

The insulating body is made of an elastic material and thus comprisesstretchable feature. By an elastic material should be understood arubber like or rubbery material. The stretchable capacity permits theinsulating body to be bent to assume a curved structure. Therefore, theconductive layers should also be stretchable and thus cannot be solid.In the embodiment shown the conductive layers comprise carbonpowder or grains. In an embodiment the carbon powder is introducedin a polymeric material similar to the insulting material. In an embodiment the polymeric material comprises silicone rubber.

The conductive material. e.g. carbon powder is embedded on amolecular level in a matrix material which is essentially the samemolecule as the insulation material 2 in between the conductive layers4. When fully cured the insulation body has cross-linking to form onesingle molecule and can be deformed without causing any void or gaps internally.

The conductive layer is very thin. In an embodiment the conductinglayer is less than 0.05 mm and the insulating layer is in the order of several mm.

Additional insulation material 6 is filling a shape with thicker insulationclose to the end portions of the conductive layers. In an embodimentthis additional insulation material may be in the form of radiallyextended sheaths or dishes 7, located axially at the end portions of theconductive layers. This extends the creepage distance along the surface insulation-air. The electric field strength in air will also become 11 essentially lower due to 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 intoa complete insulating structure to accommodate a magnetic core 8 anda secondary winding 10. The outer contour comprises of insulation 6and sheaths 7. The secondary winding wires exit in a cylindrical body 9.The insulation 6 is cross linked after curing to the insulation body 1. Inparticular, the entire insulation body, 1 and 6, is bendable to assume apredetermined shaped structure. This shape may comprise an arbitrarydesign but most conveniently a bent curve. The body can also bestretched in radial direction. According to the invention the bendablecapacity may comprise an angle between a first and a second angularleg. The first angular leg comprises a line from one end point to a midpoint of the insulating body. The second angular leg comprises a linefrom the other end point to the mid point of the insulating body. This angle should be in the order of at least 45 degrees.

In a development of the invention the insulating body is used as a partof a current transformer. Fig 3 shows the current transformer mountedon a high voltage line 5. The magnetic core 8 assumes the samepotential as the high voltage line and the secondary winding assumesground potential. The insulating body is molded in original straightshape. An advantage of molding the insulation body in a straight shapeis that it is much easier to insert straight electric sheet core. Also, thesecondary winding is much easier to wind on a straight cylindricalsurface. The entire current transformer is then possible to bend in any form to finally make the ends connected magnetically.

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 12 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 ofshape. The steps of layers are invers at inner diameter. Left and rightside of the body are mirrored. This may be the base shape which canbe applied for cable terminations. The electric field grading is similar.However, this embodiment can have almost the same axial length oneach 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. Theconductor 14 and the cable insulation 15, extend beyond the cableconductive layer 12. The integrated insulation body, 11 and 6, has inrelaxed state, a smaller inner diameter than the cable insulation 15.The result is that the insulation body, 11 and 6, can squeeze the cableenough to avoid air and void in between. A conductor 13 is connectedbetween the cable conductive layer 12 and the outer conductive layerof the insulation body 11. In a similar way a conductor 13 connects theconductor 14 and the innermost conductive layer of the insulation body11.

Although favorable the scope of the invention must not be limited bythe embodiments presented but contain also embodiments obvious to aperson skilled in the art. For instance any type of conductor passingthrough a hole having different voltage than the conductor e.g. a transformer bushing may apply to the invention.

Claims

1. Tubular insulating body (1, 11) for use on a high voltage element (8, 14), the insulating body comprises an insulating structure wherein aninner surface of the insulating structure is electrically in contact withthe high voltage element (8, 14) and an outer surface of theinsulating structure is connected to ground potential, and wherein aplurality of conductive layers (4) are provided between said outerand inner surfaces, characterized in that conductive material isembedded on a molecular level in a matrix material which isessentially the same molecule as an insulation material (2) inbetween the conductive layers (4), whereby essentially the entireinsulating structure comprises elastic properties making theinsulating structure deformable to a predetermined shape differentfrom the shape in the state of no external force applied to the insulating structure.

2. . Tubular insulating body according to claim 1, wherein the insulating structure 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, betweeninnermost conductive layer and outermost conductive layer, and thedistance in axial direction is longer, preferably several times longer, than the distance in radial direction. 15 9. 20 10. 11. 2 . 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. . Tubular insulating body according to claim 4, wherein the lengths in axial direction of the conductive layers (4) increases successively or stepwise from said inner surface to said outer surfaces, or vice versa. . Tubular insulating body according to any of claims 1-3, wherein the lengths in axial direction of the conductive layers (4) are essentiallyequal and the conductive layers (4) are axially displaced in relationto each other from said inner surface to said outer surfaces, or vice VeFSa . . Tubular insulating body according to any of the preceding claims, wherein the insulating material portions (2) between the conductive layers (4) have different thicknesses between each layer. Tubular insulating body according to any of the preceding claims,wherein essentially the same matrix molecule is provided inessentially the entire insulation structure making the insulatingstructure deformable more than about 10 % elongation in any direction without causing any separation and/or void in the structure. Tubular insulating body according to any of the preceding claims,wherein the insulating structure comprises a tapered shape at both end portions (3). Tubular insulating body according to any of claims 1-8, wherein theinsulating structure further comprises additional insulation material,such as radially extended sheaths or dishes (7), located axially at the end portions of the conductive layers (4).