EP2806432A1 - Insulation body for providing electrical insulation of a conductor and an electrical device comprising such insulation body - Google Patents

Abstract

The present invention relates to an insulation body (400) for providing electrical insulation of a conductor in an electrical device. The insulation body (400) comprises at least two modules (200[1] - 200[n]), each module having a hole through which the conductor (110) may extend. The at least two modules are arranged so that the holes of the at least two modules are aligned to form a passage for the conductor, and adjacent modules are arranged firmly against each other. Each module comprises at least one insulationg material, and the relative permittivity of the insulation body varies in the axial and/or the radial direction of the insulation body.

Description

Technical field
• The present invention relates to the field of power transmission technology, and in particular to insulation bodies for providing electrical insulation of a conductor in an electrical device such as a bushing, instrument transformer or cable termination.
Background
• Electrical bushings are used for carrying current through a plane which is at a different potential than the current path. Bushings are designed to electrically insulate a conductor, located inside the bushing, from such plane. The plane through which the conductor extends is often referred to as the grounded plane, even though the plane does not need to be grounded - in some applications, the plane is at potential further from ground potential than the conductor. The grounded plane can for example be a transformer tank or a wall.
• In order to obtain a smoothening of the electrical potential distribution between the conductor and the grounded plane, a bushing often comprises an insulation body around the conductor. In an often used bushing design, the insulation body comprises a number of coaxial foils made of a conducting material, where the foils are at a floating potential and separated by a dielectric spacing material. Such insulation body is often referred to as a condenser core. The dielectric spacing material could for example be oil impregnated or resin impregnated paper. An example of a condenser core comprising coaxial conducting foils which are separated by a dielectric spacing material is for example described in WO2008/74166 .
• A condenser core comprising floating-potential coaxial foils in a dielectric spacing material not only provides electrical insulation between the conductor and the grounded plane, but can also provide a desired field grading in a satisfying manner. However, the production of such condenser cores is typically cumbersome and time consuming. Hence, condenser cores which are easier to manufacture, and yet provide sufficient field grading, are desired.
Summary
• An object of the present invention is to provide an alternative design of an insulation body for providing field grading and insulation of a conductor in an electrical device.
• One embodiment provides an insulation body for providing electrical insulation of a conductor in an electrical device. The insulation body comprises at least two modules, where each module has a hole through which the conductor may extend and each module comprises at least one insulating material. The at least two modules are arranged so that the holes of the at least two modules form a passage through the insulation body, and adjacent modules are arranged firmly against each other. The relative permittivity of the insulation body varies in the axial and/or the radial direction of the insulation body.
• By this embodiment is achieved that the production time of an insulation body can be considerably reduced. Furthermore, the customization of an insulation body will be facilitated.
• The modules could, if desired, be pre-fabricated, and the pre-fabricated modules could be assembled to form the insulation body. Alternatively, the modules could for example be molded one on top of the other in order to obtain the desired insulation body.
• In one embodiment, at least one of the at least two modules comprises at least two layers of different materials having different relative permittivity, so that the relative permittivity of at least one module varies in the radial direction of the module. The layers of the modules could for example be arranged so that the innermost layer of each module is formed from the material of the module which has the lowest relative permittivity.
• In one embodiment, the highest relative permittivity of the material(s) of a first of said at least two modules is higher than the highest relative permittivity of the material(s) of a second of said at least two modules, so that the relative permittivity of the insulation body varies in the axial direction of the insulation body. Such permittivity variation serves to gradually allow the equipotential lines to deviate from the direction of the insulation body axis, thus providing grading of the electric field around the insulation body. The variation in permittivity along the axial direction of the insulation body can for example be such that the ratio of the highest permittivity to the lowest permittivity is larger than 3. This ratio will often be higher than 3, for example 5, 10, 20, 30, 50 or even higher.
• In one implementation of this embodiment, the module having the highest relative permittivity is located at a position such that, when the isolation body forms part of an electric device and the electric device is in use, a side of said module is in physical contact with the high stress part of the device, the high stress part being for example a flange, a grounded cable shield or a metering-core cabinet. The modules could for example be arranged such that the highest relative permittivity of each of the at least one modules is lower than, or equal to, the highest relative permittivity of all of the other modules which are located closer to such high stress part.
• In one embodiment, the insulation body comprises an inner aggregate layer and an outer aggregate layer, where an aggregate layer is formed from a sequence of overlapping layers in adjacent modules. The overlapping layers overlap in the radial direction, and an aggregate layer extends through the entire insulation body. In this embodiment, the relative permittivity differs between the inner and outer aggregate layers at at least one location along the axis of the insulation body. The modules could for example be arranged so that the highest permittivity of the inner aggregate layer is lower than, or equal to, the lowest permittivity of the outer aggregate layer. This arrangement allows for an efficient field grading in that the outer aggregate layer of higher permittivity will guide the equipotential lines of the electric field, in the inner aggregate layer. Oftentimes, the inner and the outer aggregate layers are formed from insulating materials having an electric conductivity lower than 1 µS/m.
• In one embodiment, at least one module includes a material having a conductivity which exceeds 1 µS/m. Such conductive materials can be efficiently contribute to the grading of the field, for example in the vicinity of areas of high field exposure such as bushing flanges. In one implementation of this embodiment, a majority of the modules includes at least one conductive layer formed from a conducting material; and the modules in said majority are arranged next to each other in a sequence, so that a sequence of modules comprising conductive layers is formed. Such a sequence could act in a similar manner as conductive foils of a condenser core.
• The invention also relates to a kit of parts for an insulation body for providing electrical insulation to a conductor. The kit of parts comprises at least two modules, each module having a hole through which the conductor may extend and each comprising at least one insulating material. The relative permittivity of the materials forming said at least two modules varies in a manner so that the relative permittivity of an insulation body, formed from said kit of parts, will vary in the axial and/or the radial direction of the insulation body.
• Further aspects of the invention are set out in the following detailed description and in the accompanying claims.
Brief description of the drawings

Fig. 1

shows a prior art bushing having an insulation body in the form of a condenser core.

Fig. 2a

is a schematic illustration of an example of a module having a single layer of insulation material, as seen from a point along the module axis.

Fig. 2b

is a schematic illustration of the module of Fig. 2a, as seen from a point along a line perpendicular to the module axis.

Fig. 3a

is a schematic illustration of an example of a module having two layers of different insulation materials, the materials having different permittivity.

Fig. 3b

is a view of the module shown in Fig. 3a, as seen from a point along a line perpendicular to the module axis.

Fig. 4

is a schematic cross-sectional illustration of an example of an insulation body where the insulation body is formed from n modules, each module having m layers.

Fig. 5

is a schematic cross-sectional illustration of an example of a bushing comprising an insulation body according to an embodiment of the invention.

Fig. 6a-c

show results from simulations of the electric field in and around different bushings which include an insulation body formed from modules and having a variation in permittivity both in the axial and radial direction.

Fig. 7

is a graph of the tangential electric field at the outer surface of different insulation bodies as a function of distance from a grounded plane.

Fig. 8

is a schematic illustration of an insulation body having a module shaped as a truncated cone and further modules of cylindrical shape.

Fig. 9

is a schematic cross section of a set of modules which have a locking system.

Detailed description
• Fig. 1 schematically illustrates a prior art bushing 100 which comprises an elongate insulating housing 105 through which a conductor 110 extends. At each end of the conductor 110 is provided an electrical terminal for connecting the conductor 110 to electrical systems or devices. The ends of the bushing are referred to as connection ends 113. Bushing 100 of Fig. 1 furthermore comprises a condenser core 115.
• The condenser core 115 of Fig. 1 comprises a number of conducting foils 120 which are separated by a dielectric spacing medium 123. The dielectric spacing medium 123 is typically made of an insulating material, such as oil- or resin impregnated paper.
• The bushing of Fig. 1 further comprises a flange 125 which is attached to the insulator 105. The flange 125 can be used for connecting the bushing 100 to a plane 130 through which the conductor 110 is to extend. In the bushing 100 of Fig. 1, the plane 130 is connected to the outermost conductive foil 120 via a connection 135. Plane 130 may be connected to ground, or can have a potential which differs from ground. However, for ease of description, the term grounded plane will be used when referring to the plane 130.
• When the bushing 100 is in use, the conductive foils 120 serve to capacitively grade the electric field within the bushing 100, and the condenser core 115 acts as a voltage divider which distributes the field within the condenser core 115.
• As mentioned above, the manufacturing of a condenser core 115 having conducting foils 120 which are separated by a dielectric spacing medium 123 is typically cumbersome and time consuming. The conductive foils 120 and thin sheets of the dielectric spacing medium 123 are wound to form the condenser core 115. Once wound and dried, the condenser core is typically immersed in a bath of oil or epoxy. When epoxy is used, the epoxy will have to be cured. This post-winding processing of the condenser core in the form of drying/- impregnation/curing often takes several days.
• Hence, there is a strong desire to find bushings which can provide adequate insulation and field grading properties, and which can be manufactured in a less time consuming way. According to the invention, an insulation body for providing electrical insulation of a conductor in an electrical device is provided. The insulation body is formed from at least two modules, each having a hole through which the conductor may extend, and each comprising at least one insulating material. The at least two modules are arranged firmly against each other in a manner so that the holes of the at least two modules form a passage for the conductor through the insulation body. The relative permittivity of the insulation body varies in the axial and/or the radial direction of the insulation body.
• By forming the insulation body from at least two modules, the manufacturing of the insulation body can be much facilitated, and the production time can be considerably reduced. The modules could, if desired, be pre-fabricated, and the pre-fabricated modules could be assembled to form the insulation body. In this way, the production of insulating bodies would be considerably less time consuming than a condenser core which has been wound and impregnated as described above. Furthermore, an insulation body could easily be customized. If desired, different pre-fabricated modules could be kept in stock, so that when an order for an insulation body is received, the insulation body could be assembled from modules already in stock. Alternatively, the modules could for example be molded one on top of the other in order to obtain the desired insulation body.
• By the spatial variation in the permittivity of the insulation body, so that different parts of the insulation body have different relative permittivity, grading of the electric field in the electrical device can be obtained.
• In one embodiment, the highest relative permittivity of the material(s) of a first of said at least two modules is higher than the highest relative permittivity of the material(s) of a second of said at least two modules, so that the relative permittivity of the insulation body varies in the axial direction of the insulation body.
• A module could e.g. be of cylindrical shape, or have the shape of a truncated cone, or other suitable shape. A module could for example be shaped as a circular or elliptical right cylinder, or as a circular or elliptical truncated cone. Fig. 2a and 2b illustrate a module 200 of right circular cylindrical shape. Fig. 2a is view from a point along the cylinder axis, while Fig. 2b is a view form a point along a line which is perpendicular to the cylinder axis. A hole 205 extends through the module 200, the hole 205 being located at the centre of the cylinder and extending along the cylinder axis. The diameter of the hole 205 is denoted Φ.
• The module 200 of Figs. 2a and 2b is formed from one layer 210 of an insulating material having a relative permittivity ε_r. The thickness of the layer 210 is denoted d, while the length of the module is denoted L. The two sides of the module 200 which are intersected by the hole 205 will be referred to as the sides 215, while the outer side will be referred to as the circumferential surface 220.
• In another implementation, a module 200 is formed from two or more layers 210. A module 200 of circular cylindrical shape having two layers 210i and 210ii of different relative permittivity ε_r and of thickness d_i and d_ii, respectively, is shown in Figs. 3a and 3b . If the length L is small compared to the radius of the module, the module 200 could be seen as a disc.
• Fig. 4 is a cross sectional view along the axis of an example of an insulation body 400 comprising a set of n modules 200 of circular cylindrical shape, where each module is made up of m layers 210 of different materials. The layers 210 of a module 200 are concentric with the hole 205. The layer 210 which is closest to the hole 205 will hereinafter be referred to as the innermost layer, and the layer which is furthest away from the hole 205 will be referred to as the outermost layer.
• By use of modules 200 in the manufacturing of an insulation body 400, an insulation body having varying permittivity in the radial and/or the axial direction can be created. All layers 210 of all modules 200 can, in principle, be of different materials having different permittivity. Alternatively, some layers 210 of at least some modules 200 could be made from the same material, having the same permittivity. The relative permittivity of the i^th layer 210 in the j^th module 200 can be denoted ε_i,j. This notation is used in Fig. 4.
• The number of modules 200 in an insulation body 400 is at least two, and the number of layers in each module 200 is at least one. The thickness d of the layers 210 of a module 200 could vary depending on application, and the thickness of different layers in the same module 200 will often not be the same.
• Two layers 210 in adjacent modules 200, which are both located so as to include a point at a particular distance from the conductor, are here referred to as corresponding layers. Thus, two corresponding layers 210 overlap with each other in the radial direction. A sequence of corresponding layers 210 which runs through the entire insulation body 400 will here be referred to as an aggregate layer 405. Although corresponding layers 210 are often of the same thickness and located at the same distance from the conductor 110 (cf. Fig. 4), this is not always the case, and a particular layer 210 could therefore be part of more than one aggregate layer. A layer 210 of a particular module 200 can have more than one corresponding layer on each side 215 in the axial direction, and a particular corresponding layer on one side 215 could, but does not have to, overlap the corresponding layer(s) on the other side 215.
• An aggregate layer 405 thus extends, in the axial direction, through the entire insulation body 400 (possibly at a varying distance from the conductor 110). In one embodiment of the insulation body 400, at least one layer 210 of a first module will have a permittivity which differs from all corresponding layers 210 of an adjacent module 200. In the embodiment of Fig. 4, this can be described as ε_i,j≠ε_i,j+1 for at least one value of i and j. The modules of an insulation body 400 can for example be selected so that for at least one aggregate layer 405, a corresponding layer 210 forming part of the module 200 which is closest to the area where the highest electric field is expected, is the corresponding layer 210 of the aggregate layer 405 which has the highest permittivity. For example, in a bushing, the module 200 including the material of highest permittivity could be located closest to the flange 125.
• In one embodiment, the layer 210 of a module 200 will either have a permittivity which is the same as a corresponding layer in the adjacent module 200 which is closer to the area where the highest electric field is expected, or have a permittivity which is lower than the permittivity of the corresponding layer in this adjacent module which has the highest permittivity. In the denotation introduced in relation to Fig. 4, this can be described as ε_i,1 ≥ε_i,2≥ε_i,3...≥ε_i,n, where the 1^st module is the module which is closest to the area of high electric field and the n^th module 200 is the module 200 which is at the greatest distance from the area of high electric field, where i denotes the i^th aggregate layer 405. Such a variation in the permittivity between different modules 200 will here be referred to as an overall decreasing permittivity of an aggregate layer 405. By selecting the modules 200 forming an insulation body 400 in a manner so that an overall decrease of the permittivity of an aggregate layer 405 is achieved, an efficient field grading in the axial direction of the insulation body 400 can be obtained.
• The insulation body 400 could advantageously form part of an electric device for providing electrical insulation of a conductor as well as field grading around the conductor. Examples of such electric devices include bushings, instrument transformers and cable terminations.
• Fig. 5 is a schematic cross sectional view of an example of an electrical device comprising an insulation body 400. The example of Fig. 5 is an embodiment of a bushing 500 comprising an insulation body 400 and a flange 125 arranged to be connected to a grounded plane 130. The flange 125 of Fig. 5 is a schematic flange only, and in an implementation, the shape of the flange 125 would typically be smoother in order to smoothen the electric field around the flange 125.
• The axis of the bushing 500 coincides with the axis of the insulation body 400. In Fig. 5, only the part of bushing 500 which extends on one side of the grounded plane 130 is shown. On the other side of the grounded plane 130, further modules 200 will be arranged to form a further part (not shown) of the insulation body 400. In one implementation, the bushing 500 is symmetric around the grounded plane 130, so that the same number of modules 200 will be arranged on both sides of the grounded plane 130, the modules 200 on one side being a mirror image of the modules 200 on the other side. In another implementation, the bushing 500 is asymmetric, so that the modules on one side differ from the modules on the other side of the grounded plane 130.
• In the bushing of Fig. 5, the flange 125 is arranged to be in physical contact with the circumferential surface 220 of a first module 200₁. Furthermore, the flange 125 of Fig. 5 is in physical contact with the side 215 of the module(s) 200₂ which are adjacent to said first module 200₁, where the module(s) adjacent to the flange 125 are of larger thickness than the module 200₁ of which the circumferential surface 220 is in physical contact with the flange 125. Hence, the adjacent module(s) 200₂ extend along the flange 125 in the radial direction.
• Such adjacent module(s) 200₂ advantageously has at least two layers 210, of which the outer layer has a higher permittivity than the inner layer, so that the outer, higher-permittivity layer 210 limits the number of equipotential lines allowed to deviate from the axis into the radial direction of the bushing.
• In the embodiment of Fig. 5, the first module 200₁, whose circumferential surface 220 is in physical contact with the flange 125, includes a single layer 210. In another embodiment, the first module 200₁ includes further layers 210. In one implementation, the first module 200₁ includes a conducting outer layer 210, to which the flange 125 will be arranged to be in electrical contact. Such conducting outer layer 210 can be seen as an extension of the flange 125. In this implementation, it would be advantageous to arrange the adjacent module(s) 200₂ such that an outer high-permittivity layer 210 extends beyond the conductive layer 210 in the radial direction, while the total thickness of adjacent module(s) 200₂ could, if desired, be the same as, or smaller, than the thickness of module 200₁.
• Furthermore, in the embodiment of Fig. 5, the width of the flange 125 corresponds to the width of the first module 200₁. In another embodiment, the width of flange 125 is different than the width of the first module 200₁. For example, if the width of the flange 125 is smaller than the width of the first module 200₁, the first module 200₁ could have a groove in the circumferential surface 220, so that one end of flange 125 is surrounded by the first module 200₁. At least one inner side 215 of first module 200₁ will in this embodiment be in physical contact with the flange 125. The layer 210 of which an inner side 215 is in physical contact with the flange 125 could advantageously have a permittivity which is higher than the permittivity of an inner layer 210 of the first module 200₁.
• In general, a module 200, of which a side 215 is in physical contact with the flange 125, or with a conductive layer 210 which extends the flange 215 into a first module 200₁, could advantageously have an outer layer 210 and an inner layer 210 arranged so that the permittivity of the outer layer is higher than the permittivity of the inner layer.
• A bushing 500 could, if desired, include an insulating housing 105 to protect the bushing 500 from rain, dirt etc. Such housing 105 could be separately formed, or could be formed from the outer layer 210 of the modules 200. The outside of such housing 105 could, if desired, have protrusions to extend the creepage distance along the surface of the bushing, the modules 200 in this embodiment being shaped so that the circumferential surface of the insulation body will have protrusions.
• In Figs. 6a-6c , results from simulations of the electric field surrounding a modular bushing 500 are shown. A cross section of a part of the bushing is shown in each drawing, where the shown part is delimited by the central axis 600 and the grounded plane 130. The electric field has been indicated in Figs. 6a-6c by means of equipotential lines 605. In each of Figs. 6a-6c, the part of the modular bushing 500 for which simulations were made included six modules 200, which are denoted modules 200₁,...,200₆. In each of the simulations, module 200₁ is a single layer module of relative permittivity ${\epsilon }_r^{}$ ${}_1^{}$

  

  and thickness d₁, while the other modules 200₂-200₆ are of thickness d₂, d₂>d₁. The flange 125 is in physical contact with the circumferential surface 220 of module 200₁. In the simulations presented in Figs. 6a-6c, module 200₆, closest to the connection end 113, is a single layer module of thickness d₂ and relative permittivity ${\epsilon }_r^{}$ ${}_6^{}={\mathrm{<figure-callout\; id="1"\; label="\epsilon \; r"\; filenames="imgf0002.png,imgf0006.png"\; state="\{\{state\}\}">\epsilon </figure-callout>}}_r^{},$

  

  while the intermediate modules 200₂ - 200₆ each has two layers 210, referred to as the inner layer and the outer layer, respectively. The inner layers of modules 200₂-200₅ each have a thickness d₁ and relative permittivity ${\epsilon }_r^{}$ ${}_{1,j}^{}={\epsilon }_{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="imgf0002.png,imgf0006.png"\; state="\{\{state\}\}">r</figure-callout>}}^1,$

  

  so that an inner aggregate layer 405 of thickness d₁ and a homogenous relative permittivity ${\epsilon }_r^{}$ ${}_1^{}$

  

  is formed by the modules 200₁,... 200₆. The outer layers of modules 200₂-200₅, on the other hand, have a relative permittivity ${\epsilon }_r^{}$ ${}_{2,j}^{}$

  

  which is greater than ${\epsilon }_{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="imgf0002.png,imgf0006.png"\; state="\{\{state\}\}">r</figure-callout>}}^1,$

  

  j=2,...,5. The relative permittivity of the outer layers of modules 200₂-200₅ varies, the outer layers 200₂-200₅, together with single layer module 200₆, thus forming an aggregate layer 405₂ exhibiting a permittivity variation in the axial direction. The single layer module 200₆ forms part of two aggregate layers 405. The outer layers 210 of modules 200₂,... 200₅ are of thickness d₃, where d₃=d₂-d₁, the radius of the insulation bodies 400 for which simulations have been performed thus being constant along the length of the insulation body 400. In the simulations performed, the distance between the flange 125 and the connection end 113 of the bushing 500 was set to 600 mm, divided between 6 modules of length 100 mm.
• The simulations of Figs. 6a and 6b have been made for two bushings 500 having modular insulation bodies 400 of the same geometrical dimensions, but where the material of the outer aggregate layer were of different relative permittivities. The simulation of Fig. 6c was made for a modular bushing 500, for which the relative permittivity of the layers were the same as in Fig. 6b, but the thickness of the inner and outer aggregate layers 405 were larger. The thicknesses of the aggregate layers in Figs. 6a-6c are given in Table 2. Table 2. The thickness d₁ of the inner aggregate layer 405₁ and the thickness d₃ of the outer aggregate layer 405₂ for the simulations of Figs. 6a-6c. The radius of the conductor was set to 25 mm in all simulations.

  	Fig. 6a	Fig. 6b	Fig. 6c
  d1	20 mm	20 mm	30 mm
  d
  3	15 mm	15 mm	40 mm

• The relative permittivity of the material of the inner aggregate layer 405₁ was ${\epsilon }_{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="imgf0002.png,imgf0006.png"\; state="\{\{state\}\}">r</figure-callout>}}^{1,j}=3$

  

  in each simulation, while the permittivities of the outer layers 210 of modules 200₂,...,200₅ was altered between the simulations, the permittivity gradient of the outer aggregate layer 405₂ thus being different for the different simulations.
• The relative permittivities of the outer layers of the different modules in the simulations illustrated in Figs. 6a-6c were as follows:

  	Fig. 6a	Fig. 6b	Fig. 6c
  ${\epsilon }_{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="imgf0002.png,imgf0006.png"\; state="\{\{state\}\}">\; <figure-callout\; id="2"\; label="r"\; filenames="imgf0002.png,imgf0006.png"\; state="\{\{state\}\}">r</figure-callout>\; </figure-callout>}}^{2,2}$	60	15	15
  ${\mathrm{<figure-callout\; id="2"\; label="\epsilon \; r"\; filenames="imgf0002.png,imgf0006.png"\; state="\{\{state\}\}">\; <figure-callout\; id="3"\; label="\epsilon \; r"\; filenames="imgf0006.png"\; state="\{\{state\}\}">\epsilon </figure-callout>\; </figure-callout>}}_r^{}$	60	15	15
  ${\mathrm{<figure-callout\; id="2"\; label="\epsilon \; r"\; filenames="imgf0002.png,imgf0006.png"\; state="\{\{state\}\}">\; <figure-callout\; id="4"\; label="\epsilon \; r"\; filenames="imgf0006.png"\; state="\{\{state\}\}">\epsilon </figure-callout>\; </figure-callout>}}_r^{}$	30	8	8
  ${\mathrm{<figure-callout\; id="2"\; label="\epsilon \; r"\; filenames="imgf0002.png,imgf0006.png"\; state="\{\{state\}\}">\; <figure-callout\; id="5"\; label="\epsilon \; r"\; filenames="imgf0006.png"\; state="\{\{state\}\}">\epsilon </figure-callout>\; </figure-callout>}}_r^{}$	10	5	5
  ${\mathrm{<figure-callout\; id="6"\; label="\epsilon \; r"\; filenames="imgf0006.png"\; state="\{\{state\}\}">\epsilon </figure-callout>}}_r^{}$	3	3	3

  Table 1. Permittivies of the outer layer of the different modules in the simulations presented in Figs. 6a-6c . The permittivity of the inner aggregate layer was constant and set to ${\epsilon }_{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="imgf0002.png,imgf0006.png"\; state="\{\{state\}\}">r</figure-callout>}}^1=\mathit{3.}$

  
• The equipotential lines shown in Figs. 6a-6c illustrate the situation where a voltage of 50 kV was applied to the conductor 110, and the flange 125 was grounded. The difference between two adjacent equipotential lines is 1 kV.
• As shown in Figs. 6a-6c, the spatial variation of the permittivity of the bushing in Fig. 6 gives rise to an efficient grading of the electric field. In the inner aggregate layer 405₁, the equipotential lines follow the axis of the conductor 110, while the higher permittivities of the outer aggregate layer 405₂ grades the field so as to limit the number of the equipotential lines allowed to deviate from the axis into the radial direction of the bushing. The high permittivity works by steering away part of the equipotential lines from the region near the flange 125, making the equipotential lines more evenly distributed when crossing the outer surface of the insulation body 400. Since the permittivity of the outer aggregate layer 405₂ displays a decrease from the flange end towards the connection end 113, the equipotential lines are fairly evenly distributed, and areas of very high electric field, which would be present in the area around the flange 125 in the absence of the insulation body 400, can thus be avoided.
• As can be seen in a comparison of Figs. 6a and 6b, a more efficient field grading is achieved for a larger permittivity-variation in the outer aggregate layer 405₂ - the strength of the electric field around the flange 125 is lower in Fig. 6a, where the permittivity span is ε_r=60 to ε_r=3 in the outer aggregate layer 400₂, i.e. a permittivity ratio of 20; than in Fig. 6c, where the permittivity span is ε =15 to ε_r=3, i.e. a permittivity ratio of 5. However, also for a permittivity ratio of 5, the field grading obtained is significant.
• Furthermore, a comparison between Figs. 6b and 6c shows that in general, a more efficient field grading is achieved for a larger thickness of the modules 200. In particular, an increase in the thickness of the layer(s) which exhibits a larger ratio between the high and low permittivity materials results in a stronger grading of the electric field.
• In Fig. 7 , results from the simulations are presented in an alternative way. The tangential electric field, E_tan, at the outer surface of the insulation bodies 400 is plotted as a function of distance z from the grounded plane 130 for the simulation scenarios of Fig. 6a-c, as well as for a further scenario wherein the geometrical dimensions are the same as in Figs. 6a-6b and the highest permittivity of the outer aggregate layer is 30. As expected, the tangential electric field at the edge of the flange 125 (at z = 100 mm) is lower when a material of higher relative permittivity is used near the flange 125, if the geometry is kept constant. However, the simulation wherein the lowest value of the tangential electric field is obtained is the one where a highest permittivity of only ε_r=15 is used, but where the thickness of the outer aggregate layer 400₂ is larger: 40 mm instead of the 15 mm layer used in the other simulations presented in Fig. 7. A lower ratio of the permittivity within the aggregate layer(s) 405 of varying permittivity can thus be compensated for by a larger layer thickness.
• The modules of Figs. 6a-6c are all cylinders. However, as mentioned above, modules of other shapes could also be used, for example modules in the shape of truncated cones or other solids of revolution, so that the module radius will be larger at some locations than in other locations of the insulation body 400. A larger radius could for example be useful near the flange 125 of a bushing, or at other locations where high field stress is expected. In one embodiment, a module 200 shaped as a circular truncated cone is placed closest to the flange 125 with the base end of the cone facing the flange 125, while the other modules 200 are of circular cylindrical shape, the cylinder radius corresponding to the radius at the top end of the conical module, so that a smooth outer surface of the insulation body 400 is obtained. An example of a bushing according to this embodiment is shown in Fig. 8 . In another embodiment, all modules 200 are shaped as truncated cones, where the top radius of a first truncated cone corresponds to the base radius of an adjacent truncated cone in order to obtain a smooth outer surface of the insulation body 400. In yet another embodiment, two or more, but not all, modules 200 are of truncated conical shape. Typically, the base end of a module 200 which is shaped as a truncated cone is facing a high stress region (e.g. the flange 125 in a bushing 500), while the top end of the cone shaped module is facing a low stress region (e.g. the connection end 113 of a bushing).
• In one embodiment, the insulation body 400 has at least one aggregate layer 405 which is formed by at least two modules 200 and which has a varying permittivity in the axial direction of the insulation body 400. An insulation body according to this embodiment could also have further aggregate layers 405, where the permittivity of such further aggregate layers 405 could be constant or varying. An insulation body 400 could for example have an inner aggregate layer 405, located closer to the conductor 110 than the at least one aggregate layer 405 of varying permittivity, where the inner aggregate layer 405 has a permittivity equal to, or lower than, the lowest permittivity of the outer aggregate layer 405 of varying permittivity (cf. Figs. 6a-6c). Such inner aggregate layer will here be referred to as low-permittivity inner layer 405₁. A low-permittivity inner layer 405₁ could for example form the innermost aggregate layer 405 of the insulation body 400.
  The insulation bodies 400 of the bushings 500 of Figs. 6a-6c each has two aggregate layers, of which the outer aggregate layer has varying permittivity and the inner aggregate layer 405₁ has a homogenous permittivity which is equal to the permittivity at the connection end 113 of the outer aggregate layer 405₂.
• An outer layer having a varying permittivity, and for which the permittivity is higher than the permittivity of a low-permittivity inner layer 405₁, will here be referred to as a high-permittivity outer aggregate layer 405₂ of varying permittivity, or high-permittivity outer layer 405₂ for short. By the combination of a low-permittivity inner layer 405₁ and a high-permittivity outer layer 405₂, where the high-permittivity end is located close to an area of high electric field, the equipotential lines will be guided by the high-permittivity material to follow the low-permittivity inner layer 405₁, away from the flange area. As the permittivity of the high-permittivity outer layer 405₂ decreases at locations further away from the flange, more equipotential lines will be allowed to deviate from the direction of the axis, into the radial direction of the bushing. By applying a decrease in the permittivity of a high-permittivity outer layer 405₂ along the axial direction of the insulation body, from a relative permittivity which is higher than that of an inner low-permittivity layer in the region which would experience high field stress in the absence of insulation body 400, the electric field will thus be graded.
• The relative permittivity of a low-permittivity inner layer 405₁ does not have to be completely homogenous: the relative permittivity could vary, but the highest permittivity should not be too high. For example, the relative permittivity of a low-permittivity inner layer could vary within the range of 1 and the lowest permittivity of a high-permittivity outer layer 405₂.
• In an embodiment comprising a low-permittivity inner layer 405₁ and a high permittivity outer layer 405₂ of varying permittivity, the thickness of these aggregate layers 405 is typically of the same order of magnitude, so that the ratio of these thicknesses fall within the range of 0.1-10. In one embodiment, the thickness d₁ of a low-permittivity inner aggregate layer 405 could be within the range of 0.2 t₃ to 5 t₃, where t₃ is the thickness of a high-permittivity outer layer 405 of varying permittivity. Oftentimes, the thickness d₁ of a low-permittivity inner layer 405₁ falls within the range of 0.5-2 times the thickness d₃ of a high-permittivity outer layer 405₂ of varying permittivity.
• In an embodiment comprising at least one aggregate layer having a permittivity variation in the axial direction of the insulation body 400, the permittivity of an aggregate layer of varying permittivity does not have to exhibit an overall decrease along the axis of the insulation body 400, but for one or a few pairs of adjacent corresponding layers 210, a module closer to the flange 125 could be of lower permittivity than a module closer to the connection end 113. The permittivity near the flange area would typically be higher than the average permittivity, and the permittivity near the connection end 113 would be lower than the average permittivity for at least one aggregate layer 405. In many implementations, the module of highest permittivity will be located closest to the area wherein the field stress would be expected to be the highest in the absence of insulation body 400 (e.g. near the flange 125 of a bushing 500).
• In an embodiment comprising at least one aggregate layer having a permittivity variation in the axial direction of the insulation body 400, the relative permittivity of the material having the highest and lowest permittivity of an aggregate layer 405 can be denoted ${\epsilon }_r^{\mathit{high}}$

  

  and ${\epsilon }_r^{\mathit{low}},$

  

  respectively. Figs. 6b and 6c relate to simulations of the electric field in a bushing 500 wherein the ratio of the highest permittivity to the lowest permittivity of the outer aggregate layer was set to 5, while this ratio was set to 20 in Fig. 6a, and to 10 in the further simulation discussed in relation to Fig. 7. These values represent realistic ratios between ${\epsilon }_r^{\mathit{high}}$

  

  and ${\epsilon }_r^{\mathit{low}}\mathrm{.}$

  

  In an insulation body 400 including an aggregate layer with permittivity variation in the axial direction, the ratio of ${\epsilon }_r^{\mathit{high}}$

  

  to ${\epsilon }_r^{\mathit{low}}$

  

  often exceeds 3 for at least one aggregate layer, while in some implementations, this ratio may be as high as 20 or higher. The higher this permittivity ratio, the more efficient will the achieved field grading be for an insulation body 400 of particular dimensions.
• In embodiments wherein a low-permittivity inner layer 405₁ as well as a high-permittivity outer layer 405₂ of varying permittivity, are included in the insulation body 400, the highest permittivity of the low-permittivity inner layer 405₁ can be denoted ${\epsilon }_r^{\mathit{inner},\mathit{high}},$

  

  while the highest permittivity of the high-permittivity outer layer 405₂ can be denoted ${\epsilon }_r^{\mathit{outer},\mathit{high}}\mathrm{.}$

  

  In the simulations discussed above in relation to Fig. 6a-c and Fig. 7, the ratio of ${\epsilon }_r^{\mathit{outer},\mathit{high}}$

  

  to ${\epsilon }_r^{\mathit{inner},\mathit{high}}$

  

  takes the values 5, 10 and 20. This simulated values represent realistic values of the ratio of ${\epsilon }_r^{\mathit{outer},\mathit{high}}$

  

  to ${\epsilon }_r^{\mathit{inner},\mathit{high}}\mathrm{.}$

  

  The permittivity ratio of ${\epsilon }_r^{\mathit{outer},\mathit{high}}$

  

  to ${\epsilon }_r^{\mathit{inner},\mathit{high}}$

  

  typically exceeds 3, and could for example be as high as 20 or higher.
• Suitable high permittivity insulating materials for use in the modules 200 include for example composites of a thermoplastic or thermoset matrix and filler particles of a material having high permittivity. Examples of suitable high permittivity filler particle materials include: TiO₂, ZnO, BaTiO₃, BaTi₄O₉, Ba₂Ti₉O₂₀, MgTiO₃, Mg₂TiO₄,CaTiO₃, ZrTiO₄, Ba3Ta2MgO9, Ba₃Ta₂ZnO₉, Al₂O₃, BaZrO₃, etc. The filler particle content in a composite material could for example be less than 50 vol%, and in many implementations, the filler particle content lies within the range of 15 vol% - 50 vol%. Particle sizes could for example lie within the range of 0.1 µm - 100 µm, and in many implementations, particle sizes within the range of 0.1-10 µm are used. However, materials of other filler particle contents and particle sizes can also be used.
• Examples of thermoplastic materials which could be used in the matrix of a composite high-permittivity material include polyethylene terephthalate (PET), polyethersulfone (PES), polysulfone (PSU), polyphenyl ether (PPE), polyphenylene sulfide (PPS), polyether imide (PEI), etc. Examples of thermoset materials which could be used in the matrix include epoxy, polyurethane (PU), silicon rubber etc. By adding different amounts of filler particles, and/or using different filler particle materials, materials of different relative permittivity may be achieved. Composite materials having a thermoplastic or thermoset matrix generally have suitable mechanical and thermal properties. However, other materials of high permittivity could also be used.
• For further information on some materials having a high relative permittivity see e.g. Super high dielectric constant carbon black-filled polymer composites as integral capacitor dielectrics", by J. Xu, M. Wong and C.P. Wong, ; " Dielectric Materials" by TRAK Ceramics Inc., Maryland, USA (http://www.magneticsgroup.com/pdf/p18-25%20Dielectr.pdf); and patent application WO010/116031.
• Suitable low-permittivity materials include thermoset materials such as epoxy, polycarbonate and silicon rubber, as well as thermoplastic materials such as PET, PES, PSU, PPE, PPS, PEI, etc. A material used as a matrix in a high-permittivity material in a layer of varying permittivity could for example be used as the low-permittivity material in a low-permittivity layer. Other materials than the matrix material of the high-permittivity-layer material could also be used. Low-permittivity materials having a relative permittivity in the range of 1 - 5 could for example be used - in some implementations, a low permittivity material of higher permittivity could be used.
• If desired, materials having field grading properties, i.e. materials for which the electric conductivity shows a dependency on applied electric field, could be used, either as a low-permittivity material, a high-permittivity material or both. Materials, the conductivity of which does not vary with electric field, are also suitable as both low- and high-permittivity material.
• At least one aggregate layer 405 of the insulation body 400 can advantageously be formed from insulating materials having a conductivity which is lower than 1 µS/m. Oftentimes, the entire insulating body 400 is made from such insulating materials. The field grading can then be achieved by applying high-permittivity materials in at least one layer of modules 200 located in the vicinity of the flange 125, and using modules of lower-permittivity material close to the connection end 113, as discussed for example in relation to Figs. 6a-6c and Fig. 7.
• However, an insulation body 400 could, if desired, also include materials of higher conductivity, such as metallic or semiconducting materials having a conductivity larger than 1 µS/m. For example, if desired, a module 200 could include a radial plate or sheet of a metallic material such as Al or Cu, or one or more modules 200 could have one or more layers 210 which is conducting. A conducting layer 201 could for example be useful near the flange 125 of a bushing, implemented for example by a conducting layer in one, two, three or more modules. In one embodiment, a majority (for example all, or all but one, two or more) of the modules include at least one conductive layer 210, the conductive layers 210 being located in adjacent modules 200 so that a conductive stretch is formed which extends along the axial extension of the insulation body. Such conductive stretch would act to grade the field in a similar manner as a conductive foil 120 of a conventional condenser core 115. In one implementation of this embodiment, some modules include at least two conductive layers so that at least two concentric conductive stretches are formed.
• In an embodiment where a conductive layer 210 in most modules form a conductive stretch, the insulating layer(s) could, if desired, be of the same relative permittivity in each module 200, so that there is no variation in the permittivity in the axial direction of the insulation body 400.
• A module 400 could, if desired, include one or more locking protrusions and/or locking recesses, for co-operation with corresponding locking recesses/protrusions of an adjacent module 200, in order to reinforce the joint between two modules. An example of a set of modules having a locking system based on protrusions/recesses is schematically illustrated in Fig. 9 , which is a cross sectional view where only the cross sectional surface which is to the right of the axis 600 is shown. The set shown in Fig. 9 includes three modules 200_i, 200_i+1 and 200_i+2, each having a locking protrusion 900 and a locking recess 905 for cooperation with a corresponding locking recess 905/locking protrusion 900 of an adjacent module 200. Only the cross sectional surface of a part of each module 200 is shown in Fig. 9, the axis 600 The locking protrusions 900 of Fig. 9 are shaped as rectangular locking tongues, while the locking recesses 905 are shaped as rectangular locking grooves. Other shapes, for example protrusions shaped as hooks with recesses of corresponding shapes, can also be contemplated.
• In Fig. 9, the location of the locking protrusion 900 of a module 200 is shifted in the radial direction in relation to the location of the locking recess 905, as well as in relation to the location of the locking protrusion 900 of an adjacent module 200. However, no such shift in radial location between different corresponding protrusion/recess pairs is necessary, but a module could have, on one side 215, a locking protrusion/recess which is located at the same radial distance from the conductor 110 as a protrusion/recess on an opposite side 215. However, by such location shift in the radial direction is achieved that, at the location of a protrusion 900, the length L of the module 400 is locally made larger. This can for example be useful if an overlap of a particular material in the axial direction is desired between two adjacent modules 200. In Fig. 9, a layer 910 runs from the protrusion 900 through the module 200 in the axial direction of the module 200, and the shift in radial location between different sets of corresponding protrusion/recess pairs results in an overlap in the axial direction of the layer 910 in one module with the layer 910 in an adjacent module 200. If desired, such overlapping layers 910 could be of a conducting material, and could for example be located near the flange 125, or forming a conducting stretch extending across the entire axial length of the insulation body 400. In one embodiment, one or more modules 200 include two or more overlapping layers 910 of conductive materials, in order to form conductive foils 120. A locking system with shifted protrusions could then be useful, so that the conductive layers of adjacent modules overlap.
• In the modules 200 shown in Fig. 9, the layer 910 is of a smaller thickness d than the width w of the protrusion/recesses. This is an example only, and a protrusion/recess could be wider than, or have the same width as the thickness d of a layer 210 which forms part of the protrusion/recess.
• As mentioned above, an insulation body 400 can for example be formed from a set of pre-fabricated modules 200. Pre-fabricated modules 200 could be of different design, for example: having different number of layers; the modules being of different length L; corresponding layers being of different materials of different permittivity; the layers being of different thickness d; the holes 205 being of different diameters Φ, etc. Hence, by use of different pre-fabricated modules 200, customized insulation bodies 400 can easily be achieved. Here, the term pre-fabricated means that the modules 200 are manufactured in a first step, and that assembly of the modules 200 into an insulation body 400 is performed in a second step. The manufacturing of the modules 200 could be made upon order, or modules of different types could be kept in stock, so that when an order for an insulation body 400 is received, the insulation body 400 can quickly be assembled. Manufacturing of the modules 200 could for example be made by casting, extrusion, moulding, winding of thin sheets, etc.
• In the above, it has been assumed that the permittivity is constant within a layer 210 of a module 200. However, modules 200 having a permittivity variation in the axial direction of the bushing could also be used. If such module 200 has more than one layer 210, the variation in the axial direction could apply to one or more layers. The permittivity variation could be continuous, or the variation could occur in steps, forming radial sheets of constant permittivity within a layer 210. If all modules 200 of an insulation body 400 have a layer 210 with a continuous permittivity variation in the axial direction of the module, an aggregate layer 405 with a truly continuously varying permittivity could be achieved, i.e. an aggregate layer 405 where the permittivity shows a gradient not only at boundaries between modules 200, but also within the modules 200.
• A module 200 could also have a continuous permittivity variation in the radial direction of the module, over parts or all of the radial extension of the module. A module having a more or less continuous variation in permittivity in the radial direction could be seen as a module having a large number of thin layers 210. The permittivity of such module could for example increase continuously towards the circumferential surface 220 of the module.
• In one embodiment, all pre-fabricated modules 200 are circular cylinders of the same length L. In one implementation of this embodiment, the length L is small compared to the radius of the modules, the modules 200 thus being disc shaped. In another implementation, the length of each module 200 corresponds to a considerable part of the length of the insulation body 400.
• In some applications, it is desired to have a stretch of the insulation body wherein the electric properties are constant. In order to achieve such stretch of a particular length, one module of the particular length could be used to form this stretch; or, two or more modules, the lengths L of which add up to the particular length, could be used. A stretch of the insulation body 400 which has constant electric properties in the axial direction will here be referred to as a section. Hence, a single module, or two or more adjacent modules 200 which have the same properties, can be used to form a section of an insulation body 400.
• In Figs. 4, 5 and 6a-c, all modules of the illustrated insulation bodies 400 are of the same length L. However, if desired, modules of varying lengths could be used, as in the example shown in Fig. 8.
• Adjacent modules 200 could be bonded to each other, for example by use of an adhesive substance, such as for example epoxy, polyurethane and/or acrylic. Bonding could also be achieved by welding, for example by ultrasonic welding, vibration welding and/or hot plate welding, or by other means. When a module 200 includes a conductive material which is to be in electrical contact with a conductive material of an adjacent module 200, soldering could for example be used.
• In one implementation of the invention, the manufacturing of the modules 200 is performed such that the modules 200 are moulded one on top of the other, so that the production of the modules takes place at the same time as the production of the insulation body 400, and the bonding between different modules 200 is achieved upon production of the modules 200. Molding techniques, such as e.g. injection molding or Resin transfer molding (RTM) could be used.
• The centre of the holes 205 of the different modules 200 are typically aligned. Furthermore, the diameter of the hole 205 is often the same for each module in the insulation body 400. The diameter Φ of the hole 205 will often correspond to the diameter of the conductor 105 which is to extend through the hole, so that a firm mechanical connection is achieved between the conductor 110 and the insulation body 400. The connection between the conductor 110 and the insulation body 400 could, if desired, be enhanced by use of an adhesive substance such as epoxy, polyurethane or acrylic. By applying an adhesive substance between the conductor and the modules 200, improved mechanical stability can be achieved. However, in an alternative design, the diameter of the hole 205 will be larger than the diameter of the conductor 110, so that a space is created between the conductor 110 and the insulation body 400. This space would for example be filled with transformer oil, SF6 gas, epoxy or any other suitable insulating substance. When an insulating fluid is used to fill such space, mechanical stability could for example be achieved by bonding the modules 200 to each other, and mechanically fix the bonded modules to a bushing housing 105, or to the conductor 110 at selected locations.
• If desired, a shielding body of smooth contours could be arranged at the connection end 113 of the bushing 500, e.g. a spherical or toroidal conducting body, in order to further grade the field and shield the connection to a cable or other device.
• The above description has mainly been made in terms of bushings 500. However, the inventive insulation body 400 would be beneficial also in other electrical devices wherein insulation of a conductor 105 is desired, such as for example instrument transformers or cable terminations, for grading of the electric field at locations which would experience a high field stress in the absence of an insulation body 400. In an instrument transformer, the high field stress is expected in the vicinity of the cabinet with the metering core, which is typically grounded. In a cable termination, the high field stress is expected in the vicinity of the edge of the grounded cable shield. In terms of field stress and field grading, the metering-core cabinet of an instrument transform and the grounded cable shield of a cable termination correspond to the flange 125 of a bushing 500. Similarly, the field situation at the connection ends of an instrument transformer and at the floating end of a cable termination corresponds to the field situation at the connection ends 113 of a bushing 500.
• In the following, such part of an electrical device where the highest field stress is expected in the absence of an insulation body, such as a flange 125, a metering-core cabinet or an edge of a grounded cable shield, will be referred to as the high stress part of the device.
• The insulation body is useful in both AC and DC applications.
• Although various aspects of the invention are set out in the accompanying claims, other aspects of the invention include the combination of any features presented in the above description and/or in the accompanying claims, and not solely the combinations explicitly set out in the accompanying claims.
• One skilled in the art will appreciate that the technology presented herein is not limited to the embodiments disclosed in the accompanying drawings and the foregoing detailed description, which are presented for purposes of illustration only, but it can be implemented in a number of different ways, and it is defined by the following claims.

Claims (15)

1. An insulation body for providing electrical insulation of a conductor in an electrical device, characterized in that
   the insulation body comprises at least two modules, each module having a hole through which the conductor may extend and each comprising at least one insulating material;
   the at least two modules are arranged so that the holes of the at least two modules are aligned to form a passage for the conductor, and adjacent modules are arranged firmly against each other; wherein
   the relative permittivity of the insulation body varies in the axial and/or the radial direction of the insulation body.
2. The insulation body of claim 1, wherein
   at least one of the at least two modules comprises at least two layers of different materials having different relative permittivity, so that the relative permittivity of at least one module varies in the radial direction of the module.
3. The insulation body of claim 2, wherein
   the innermost layer of each module is formed from the material of the module which has the lowest relative permittivity.
4. The insulation body of claim 1 or 2, wherein
   the highest relative permittivity of the material(s) of a first of said at least two modules is higher than the highest relative permittivity of the material(s) of a second of said at least two modules, so that the relative permittivity of the insulation body varies in the axial direction of the insulation body.
5. The insulation body of claim 4, wherein
   the variation in permittivity along the axial direction of the insulation body is such that the ratio of the highest permittivity to the lowest permittivity is larger than 3.
6. The insulation body of claim 4 or 5, wherein
   the module having the highest relative permittivity is located at a position such that, when the isolation body forms part of an electric device and the electric device is in use, a side (215) of said module is in physical contact with the high stress part of the device, the high stress part being for example a flange, a grounded cable shield or a metering-core cabinet.
7. The insulation body of claim 6, wherein
   the highest relative permittivity of each of the at least one modules is lower than, or equal to, the highest relative permittivity of all of the other modules which are located closer to the position of the insulation body which will experience the highest electric field when in use.
8. The insulation body of any one of claims 1-7, comprising
   an inner aggregate layer (405₁) and an outer aggregate layer (405₂), where an aggregate layer is formed from a sequence of overlapping layers (210) in adjacent modules, which overlapping layers overlap in the radial direction and wherein an aggregate layer extends through the entire insulation body; and
   the relative permittivity differs between the inner and outer aggregate layers at at least one location along the axis (600) of the insulation body.
9. The insulation body of claim 8, wherein
   the highest permittivity of the inner aggregate layer (405₁) is lower than, or equal to, the lowest permittivity of the outer aggregate layer (405₂).
10. The insulation body of claim 8 or 9, wherein
    the inner and the outer aggregate layers are formed from insulating materials having an electric conductivity lower than 1 µS/m.
11. The insulation body of any one of the above claims, wherein
    at least one module includes a conducting material having a conductivity which exceeds 1 µS/m.
12. The insulation body of claim 13, wherein
    a majority of the modules includes at least one conductive layer (210) formed from a conducting material; and
    the modules in said majority are arranged next to each other in a sequence, so that a sequence of modules comprising conductive layers is formed.
13. The insulation body of any one of the above claims, wherein
    at least one pair of adjacent modules of said at least two modules comprises a locking system, wherein:

    a first module of said pair comprises a locking protrusion (900);

    a second module of said pair comprises a locking recess (905), wherein said first and second modules are arranged so that the locking protrusion cooperates with the locking recess.
14. An electrical device comprising an insulation body according to any one of the preceding claims.
15. A kit of parts for an insulation body for providing electrical insulation to a conductor, the kit of parts comprising:

    at least two modules, each module having a hole through which the conductor may extend and each comprising at least one insulating material; and

    the relative permittivity of the materials forming said at least two modules varies in a manner so that the relative permittivity of an insulation body, formed from said kit of parts, will vary in the axial and/or the radial direction of the insulation body.

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