SE1930052A1

SE1930052A1 - Elastic tubular high-voltage insulating body

Info

Publication number: SE1930052A1
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: 2020-08-12
Also published as: WO2020167218A1; EP3924984A4; SE543113C2; 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

Electric insulating body TECHNICAL FIELD The present invention concerns high voltage insulation. More precisely theinvention concerns a tubular body providing insulation between an innersurface and an outer surface having different electrical potential.Especially the invention concerns a tubular insulating body having aplurality 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 formingcapacitor elements is most commonly known from electric bushings. Suchbushings are devices that carry current at high potential through agrounded barrier such as a transformer tank. In order to decrease andcontrol 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 fieldalong 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 insulatingstructure for use on high voltage element, where one inner surface ofinsulating structure is electrically in contact with the high voltage elementand an outer surface of insulating body is connected to ground potential,and between the said outer and inner surfaces there are severalconductive layers, and the conductive layers have different length in axialdirection and the distance in axial direction, between innermostconductive layer and outermost conductive layer, is several times longerthan 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 2 air has much lower specific electric withstand than solid insulation material.

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

The condenser cores are impregnated either with oil (OIP, oil impregnatedpaper) or with resin (RIP, resin impregnated paper). RIP bushings havethe advantage that they are dry (oil free) bushings. The core of an RIPbushing is wound from paper, with aluminum plates being inserted inappropriate 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 voltagecurrent through a grounded enclosure. To safely accomplish such a taskwithout a flashover is a challenge, as the dimensions of the bushing arevery small compared with the dimensions of the equipment it isconnecting. Not only electric field stress and thermal stress must behandled by the bushings but also mechanical stress. Therefore thebushing is made of stiff material to support the conductor inside. The stiffhousing 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 intransformer bushing terminals is previously known. The object of the barrier is to overcome flashover between the transformer and the 3 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 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 tubular insulatingbody 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 elastic mustbest be understood a rubbery material. In an embodiment of the inventionthe flexible tubular body comprises a first tapered end and a secondtapered end. The tapering may differ depending on whether the conductorends in the atmosphere or in a fluid. By the tapered ends the electric fieldgradient may be smoothly distributed. Electric field concentrations may thus be avoided which otherwise may cause partial discharges. 4 The field stress at the end of the conductive layer is high. The objective isto 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 cableterminations is a so called stress cone. Basically the insulation thickness isincreased 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, bycombining the stress control using very few conductive layers with sectionof increased thickness of insulation material outside the endings ofconductive layers. The invention also resolves the problem to adapt theshape to another shape without destroying the insulation properties. Thereasons 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 asa straight body and then formed to fit a curved conductor. In case of abendable conductor the insulating body is threaded onto the conductorwhereafter the conductor and the insulating body are bent together. Incase 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 outer diameter. 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 voltage use.The current transformer comprises a bendable core forming a ring with anopening to be clamped around a high voltage conductor. It should bepointed out that the openable ring comprises one opening only and lackjoints. The bendable insulating body surrounds part of the core and carriesthe 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 voltage elementand an outer surface of the insulating structure is connected to groundpotential, and between said outer and inner surfaces several conductivelayers are provided, wherein essentially the entire insulating structurematerial is preferably homogeneous and comprises elastic or stretchableproperties making the insulating structure deformable or bendable to apredetermined 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, 6 - 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, betweeninnermost conductive layer and outermost conductive layer, and thedistance 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 between the conductive layers have different thicknesses between each conductive layen - essentially the same matrix molecule is provided in essentially the entireinsulation structure or body making the insulating structure or bodydeformable 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, 7 - the insulating structure or body further comprises additional insulationmaterial, 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 on bothsides 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, whereinthe insulating structure comprises elastic or stretchable properties makingthe 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 beprovided for use on a high voltage power line comprising a power lineenclosing core, a tubular insulating body comprising an insulatingstructure comprising a plurality of coaxially oriented conductive layers tocontrol the electric field distribution and a secondary winding carried bythe insulating body, wherein the insulating structure comprises elastic orstretchable 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 of a tubular insulating body 8 (11) comprising an insulating structure comprising a plurality of coaxiallyoriented conductive layers (4) to control the electric field distribution,wherein the insulating structure comprises elastic or stretchable propertiesmaking the insulating body deformable or bendable to assume a predetermined shaped structure.

Another important aspect is the capacitive distribution of voltage of eachconductive layer. The voltage between each layer is proportional to thecapacitances. If the thickness of insulation between the conductive layersvary in inverse proportion to the length of the layer, the voltagedistribution can be equal between all layers. The invention makes itpossible 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 theentire body is deformed, i.e. for example bent, stretched and/orsqueezed. The invention meets this objective using essentially the samemolecule in the entire body. The carbon powder is integrated in the matrixof this molecule and both the insulation between layers and the outerinsulation have essentially the same molecule. When the final curing ismade, the cross-linking between all interfaces create one single giantmolecule. All mechanical stresses during deformation do not result in anyinternal 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 become moreapparent to a person skilled in the art from the following detaileddescription in conjunction with the appended drawings in which: Fig 1 is a cross-section of an insulating body according to the invenüon, 9 Fig 2 is a cross section of a current transformer containing theinsulating body according to the invention, in a straightshape before it is bent to a full circle, Fig 3 is a perspective view of a current transformer accordingto 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 2 andconductive layers 4. A hollow passage for containing a conductor of a highvoltage system is arranged in the centre of the insulating body. Theinsulating body 1 comprises a first conductive layer forming the passageway. This layer will be in contact with the conductor to be received in thehollow passage. The insulating body further comprises a secondconductive layer defining the outer surface of the insulating body. In theembodiment shown the insulating body comprises several intermediateconductive layers 4 cylindrically oriented in the insulating body betweenthe 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 an elastic material and thus comprisesstretchable feature. By an elastic material should be understood a rubberlike or rubbery material. The stretchable capacity permits the insulatingbody to be bent to assume a curved structure. Therefore, the conductivelayers should also be stretchable and thus cannot be solid. In theembodiment shown the conductive layers comprise carbon powder or grains. In an embodiment the carbon powder is introduced in 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 a molecularlevel in a matrix material which is essentially the same molecule as the insulation material 2 in between the conductive layers 4. When fully curedthe insulation body has cross-linking to form one single molecule and can be deformed without causing any void or gaps internally.

The conductive layer is very thin. In an embodiment the conducting layeris 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 embodiment thisadditional insulation material may be in the form of radially extendedsheaths or dishes 7, located axially at the end portions of the conductivelayers. This arrangement extends the creepage distance along the surfaceinsulation-air. The electric field strength in air will also become essentiallylower due to 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 acomplete insulating structure to accommodate a magnetic core 8 and asecondary winding 10. The outer contour comprises of insulation 6 andsheaths 7. The secondary winding wires exit in a cylindrical body 9. Theinsulation 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 bendable capacity may comprise an angle between a first and a second angular leg. 11 The first angular leg comprises a line from one end point to a mid point ofthe insulating body. The second angular leg comprises a line from theother 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 part ofa current transformer. Fig 3 shows the current transformer mounted on ahigh voltage line 5. The magnetic core 8 assumes the same potential asthe high voltage line and the secondary winding assumes groundpotential. The insulating body is molded in original straight shape. Anadvantage of molding the insulation body in a straight shape is that it ismuch easier to insert straight electric sheet core. Also, the secondarywinding is much easier to wind on a straight cylindrical surface. The entirecurrent 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. Therebyalso the core receives high voltage potential. To isolate the secondarywinding the core is dressed with an insulating body according to theinvention. The insulating body follows the bended shape (curve) of the COFe.

Fig 4 shows an embodiment of the invention for cable terminations. Theinsulation body 11 differs from insulation body 1 only in terms of shape.The steps of layers are invers at inner diameter. Left and right side of thebody are mirrored. This may be the base shape which can be applied forcable terminations. The electric field grading is similar. However, thisembodiment 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 12 conductive layer 12. The integrated insulation body, 11 and 6, has inrelaxed state, a smaller inner diameter than the cable insulation 15. Theresult is that the insulation body, 11 and 6, can squeeze the cable enoughto avoid air and void in between. A conductor 13 is connected between thecable conductive layer 12 and the outer conductive layer of the insulationbody 11. In a similar way a conductor 13 connects the conductor 14 and the innermost conductive layer of the insulation body 11.

Although favorable the scope of the invention must not be limited by theembodiments 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 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 theconductive layers (4) comprise carbon powder dispersed in elastic material.

4. Tubular insulating body according to any of claims 1-3, wherein theconductive 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.

5. Tubular insulating body according to claim 4, wherein the length inaxial direction of the innermost conductive layer is longer than the length of the outermost conductive layer, or vice versa. 10. 11. 14 . 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 thelengths 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 insulting 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).