WO2011009482A1

WO2011009482A1 - Shielding assembly

Info

Publication number: WO2011009482A1
Application number: PCT/EP2009/059335
Authority: WO
Inventors: Jonathan Peter Sturgess
Original assignee: Areva T&D Uk Limited
Priority date: 2009-07-21
Filing date: 2009-07-21
Publication date: 2011-01-27

Abstract

An electrical field shielding assembly (34) for placement about electrical equipment (31) comprises at least one primary, electrically conductive, shielding element (33) connected to at least one secondary, electrically conductive, shielding element (35). The or each primary shielding element (33) presents a continuous and uninterrupted outer shielding surface (36). The or each secondary shielding element (35) presents a broken outer shielding surface (37).

Description

SHIELDING ASSEMBLY

The invention relates to an electrical field shielding assembly for use with high voltage equipment.

In the context of this application, references to "high voltage" are intended to refer to voltages above 1000V.

The application of an electrical potential difference between a conductor and some other object results in the creation of an electrical field about the conductor. The existence of this electrical field can lead to electrical breakdown, and is therefore problematic when the conductor is a piece of electrical equipment and particularly problematic when the conductor is a piece of high voltage electrical equipment .

Electrical breakdown occurs when the dielectric strength of the medium surrounding a conductor is exceeded. Partial breakdown initially occurs when the medium surrounding the conductor ionizes and permits the conduction of an electric current. As a result corona discharge occurs in the immediate vicinity of the conductor, which can be observed in the form of plasma. If the electrical stress is high enough complete breakdown occurs, which results in sparking or arcing from the conductor. The electrical discharge caused by either full or partial breakdown results in damage to, or the failure of, the electrical equipment. Edges and pointed features of a conductor are considered critical regions that are particularly prone to electrical breakdown because the electrical stress tends to be higher in the immediate vicinity of these regions.

The problems associated with electrical discharge from electrical equipment are often resolved by shielding the electrical equipment with an electrical field shielding assembly i.e. a corona shield. The purpose of a corona shield is to redistribute the electrical field lines surrounding the electrical equipment across a surface of relatively large curvature, particularly in the vicinity of critical regions including edges and pointed features. This reduces the electrical stress about the electrical equipment and therefore reduces the risk of electrical breakdown.

An example of a conventional electrical field shielding assembly 12 surrounding electrical equipment 10 is shown in Figure 1.

The electrical field shielding assembly 12 is provided in the form of a closed element that surrounds the outer exterior of the electrical equipment 10. The shield 12 presents a continuous and uninterrupted shielding surface 14. The shielding surface 14 is curved and the shield 12 is formed so as to avoid the inclusion of any sharp edges or other abrupt changes in geometric angle, which could otherwise lead to corona discharge and electrical breakdown at the shield itself.

The solid nature of the electrical field shielding assembly 12 shown in Figure 1 results in a structure that is relatively heavy and complex to manufacture, depending on the shape and size of the electrical equipment to be shielded. This weight and complexity often leads to increased costs in terms of the manufacture and transportation of the shielding assembly 12.

The solid nature of the electrical field shielding assembly 12 also means that it can be difficult, if not impossible, to access shielded electrical equipment 10 should access be required for maintenance purposes, for example .

Another example of a conventional electrical field shielding assembly 22 is shown in Figure 2. The electrical field shielding assembly 22 uses an array of elongate tubular elements 24 that are arranged relative to each other, and supported by circular rings 26, to define a broken shielding surface 28 having a similar overall shape to the shielding surface 14 defined by the electrical field shielding assembly 12 shown in

Figure 1.

The use of elongate tubular elements 24 means that less material is required to construct the electrical field shielding assembly 22 when compared with the quantity of material required to construct the electrical field shielding assembly 12 shown in Figure 1. The electrical field shielding assembly 22 is therefore more economical to manufacture and transport.

The use of elongate tubular elements 24 to construct the electrical field shielding assembly 22 also renders it easier to access shielded electrical equipment 20 via openings between the individual tubular elements 24 and improves cooling of the shielded electrical equipment 20.

The openings between the individual tubular elements 24 also however mean that the shielding surface 28 of the electrical field shielding assembly 22 does not allow complete redistribution of the electrical field surrounding the electrical equipment, and is therefore less effective at shielding than a solid electrical field shielding assembly 12 of similar overall dimensions.

According to an aspect of the invention there is provided an electrical field shielding assembly for placement about electrical equipment comprising at least one primary, electrically conductive, shielding element connected to at least one secondary, electrically conductive, shielding element, the or each primary shielding element presenting a continuous and uninterrupted outer shielding surface and the or each secondary shielding element presenting a broken outer shielding surface. The use of both primary and secondary shielding elements reduces the overall weight of the electrical field shielding assembly when compared with a conventional solid electrical field shielding assembly due to the broken nature of the outer shielding surface provided by the or each secondary shielding element.

The use of both primary and secondary shielding elements allows the creation of an electrical field shielding assembly that provides a continuous and uninterrupted outer shielding surface about critical regions of electrical equipment, where the electric field would otherwise tend to be higher, and a broken outer shielding surface about less critical regions of the electrical equipment where the electric field would otherwise tend to be less and a lesser degree of protection is therefore required. It also therefore ensures that an uninterrupted outer shielding surface is located, in use, in locations where it is desirable that the shielding surface has a relatively small radius of curvature, and a broken outer shielding surface is located in locations where the radius of curvature of the shielding surface can afford to be larger.

The broken nature of the or each secondary shielding element also allows for access to the shielded electrical equipment. In embodiments of the invention, the electrical field shielding assembly may include a plurality of primary shielding elements and a plurality of secondary shielding elements, each of the secondary shielding elements interconnecting adjacent primary shielding elements .

This structure allows the creation of an electrical field shielding assembly that shields multiple critical regions, such as corners of the electrical equipment, through the use of multiple primary shielding elements, whilst maintaining the overall weight of the shielding assembly as low as possible through the use of secondary shielding elements to interconnect adjacent primary shielding elements.

In embodiments of the invention, the secondary shielding element may be provided in the form of a mesh-like structure.

In further embodiments of the invention, the secondary shielding element may be provided in the form of a reversed mesh-like structure - that is a structure in which a plurality of discrete elements are spaced from, and arranged relative to, each other in a grid-like arrangement so as to define a grid-like array of openings in the structure.

The or each secondary shielding element may include an array of elongate sub-elements arranged so that the outer surfaces of the sub-elements define the broken shielding surface of the secondary shielding element.

This arrangement is advantageous in that it provides uniform and clearly defined openings via which the shielded electrical equipment may be accessed.

In such embodiments of the invention, each of the elongate sub-elements is preferably shaped to define an at least partially curved cross-section and may be shaped to define a circular, oval, elliptical or polyhedral cross-section or a broken circular, oval, elliptical or polyhedral cross-section, The use of sub-elements having an at least partially curved cross-section ensures that the or each secondary shielding surface may be constructed so as not to present any edges or abrupt changes in geometric angle, which could otherwise lead to corona breakdown and discharge.

Each of the sub-elements may be hollow or solid.

The use of an array of elongate sub-elements means that the elongate sub-elements may be arranged to define a secondary shielding element having a circular, oval, elliptical or polyhedral cross-section, or a broken circular, oval, elliptical or polyhedral cross-section, whichever is most applicable to the electrical equipment and any constraints on the space to be occupied by the shielding assembly. The or each primary shielding element may be provided in the form of a hollow elongate member having a cross- section corresponding in shape and size to the cross- section of the or each secondary shielding element. This helps to minimize the size of any step at joints between the primary and secondary shielding elements. This in turn helps to ensure that the transition in electrical field at joints between the primary and secondary shielding elements is smooth and gradual and there is no sudden change in the electrical field that would otherwise give rise to undesirable stresses.

It is not however essential that the or each primary shielding element is hollow and, in other embodiments, the or each primary shielding element may be provided in the form of a solid elongate member having a cross- section corresponding in shape and size to the cross- section of the or each secondary shielding element.

In embodiments of the invention, the primary and secondary shielding elements are connected in an end- to-end configuration so as to define a closed shield for placement about the circumference about the outer exterior of electrical equipment.

The redistribution of the electric field lines around a piece of electrical equipment to be shielded relies on movement of surface charge in the electric field shielding assembly. It is therefore a requirement that the outer shielding elements are electrically- conductive .

In order to ensure that the primary and secondary shielding elements are electrically-conductive the primary and secondary shielding elements may be formed from an electrically-conductive material.

It is also envisaged that in other embodiments the primary and secondary shielding elements may be formed from an electrically insulating material coated with or bonded to, on either an inner surface or an outer surface, an electrically-conductive or semi-conductive material in order to obtain the necessary electrical conductivity.

It is not however essential that the primary and secondary shielding elements present electrically conductive surfaces. It is therefore envisaged that the primary and secondary shielding elements may in other embodiments be formed from an electrically-conductive or semi-conductive material that is painted or otherwise coated in an outer layer of non-conductive paint or material.

As outlined above, the presence of an electrical field shielding assembly will cause redistribution of the electrical field lines so long as the shielding elements are capable of carrying current. A coating or outer layer of electrically-conductive or semi- conductive material applied onto an electrically insulating material therefore has the same effect as an electrically conductive material when used to construct the electric field shielding assembly. Similarly an electrically-conductive or semi-conductive material coated with a non-conductive material will also have the same effect.

In order to ensure that the electrical field shielding assembly is capable of withdstanding high voltages, so as to be compatible with high voltage electrical equipment, the electrically-conductive material used in the production of the primary and secondary shielding elements is preferably capable of conducting current at high voltages.

Preferred embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings in which: Figure 1 shows a prior art electrical field shielding assembly;

Figure 2 shows another prior art electrical field shielding assembly;

Figure 3 shows an electrical field shielding assembly according to an embodiment of the invention; and

Figure 4 shows the juncture between a primary shielding element and a secondary shielding element of the electrical field shielding assembly of Figure 3. An electrical field shielding assembly 34 according to an embodiment of the invention is shown in Figure 3.

The electrical field shielding assembly 34 includes at least one primary shielding element 33 connected to at least one secondary shielding element 35 and, in the embodiment shown in Figure 3, includes a plurality of primary shielding elements 33 and a plurality of secondary shielding elements 35, each of the secondary shielding elements 35 interconnecting adjacent primary shielding elements 33.

The primary shielding elements 33 present continuous outer shielding surfaces 36 and the secondary shielding elements 35 present broken outer shielding surfaces 37.

The use of a plurality of primary shielding elements 33 interconnected by means of a plurality of secondary shielding elements 35 allows the construction of an electrical field shielding assembly 34 in which the primary shielding elements 33 are located so as to shield critical regions of a piece of electrical equipment and secondary shielding elements 35 are located so as to shield less critical regions of the electrical equipment.

This ensures that continuous shielding surfaces 36 are located, in use, in locations where the electric field would otherwise tend to be higher, and broken shielding surfaces 37 are located in locations where the electric field would otherwise tend to be less, and a lesser degree of protection is required.

It also therefore ensures that continuous shielding elements 36 are located, in use, in locations where it is desirable that the shielding surface has a relatively small radius of curvature, and broken shielding surfaces 37 are located in locations where the radius of curvature of the shielding surface can afford to be larger.

In the embodiment shown in Figure 3, for example, primary shielding elements 33 are provided to shield corners 31 of a piece of electrical equipment 30 and secondary shielding elements 35 are provided to shield the sides 32 of the electrical equipment 30 where electrical stress is less.

The use of both primary and secondary shielding elements 33,35 ensures that the electrical field shielding assembly 34 is lighter and therefore less expensive to manufacture and transport than an electrical field shielding assembly that provides a continuous outer shielding surface about the entire circumference of a piece of electrical equipment.

Each of the secondary shielding elements 35 includes an array of hollow elongate sub-elements 39 arranged so that the outer surfaces of the sub-elements 39 define the broken outer shielding surface 37 of the secondary shielding element 35. In other embodiments it is envisaged that each of the elongate sub-elements 39 may be provided in the form of solid elements. The provision of a broken outer shielding surface 37 permits access, in use, between the sub-elements 39 to the piece of electrical equipment shielded by the electrical field shielding assembly 34. The elongate sub-elements 39 are shaped to define a circular cross-section, which ensures that each secondary shielding surface 37 does not provide any edges or abrupt changes in geometric angle, which could otherwise lead to corona breakdown and discharge.

In other embodiments it is envisaged that each of the sub-elements 39 may be shaped to define an at least partially curved cross-section and may therefore be shaped to define a circular, oval, elliptical or polyhedral cross-section or a broken circular, oval, elliptical or polyhedral cross-section.

In each of the secondary shielding elements 35, the elongate, tubular sub-elements 39 are supported by means of supports 40. The shape and size of each of the supports 40 is dependent on the cross-sectional shape of each of the secondary shielding elements 35.

In the embodiment shown in Figure 3, the cross- sectional shape of each of the secondary shielding elements 35 is circular and each of the supports 40 is therefore circular. It is envisaged that in other embodiments the cross-sectional shape of each of the secondary shielding elements 35 may be oval, elliptical or polyhedral in cross-section and each of the supports 40 may be shaped accordingly.

It is also envisaged that in other embodiments the cross-sectional shape of each of the secondary shielding elements 35 need not be continuous, and may be an incomplete section of a circle, oval, ellipse or polyhedron, such as C-shaped for example.

The primary shielding elements 33 are provided in the form of hollow tubular members whose cross-sectional shape and size corresponds to the shape and size of the cross-section of the secondary shielding elements 35.

The use of tubular primary and secondary shielding elements 33,35 results in curved outer shielding surfaces 36,37 to redistribute electrical field lines originating from the electrical equipment 30 located, in use, within the electrical field shielding assembly 34. It is envisaged that in other embodiments, the primary shielding elements 33 may be provided in the form of solid tubular members whose cross-sectional shape and size corresponds to the shape and size of the cross- section of the secondary shielding elements 35. The abutment of a continuous shielding surface 36 and a broken shielding surface 37 at the junction between each of the primary shielding elements 33 and the secondary shielding elements 35 connected thereto results in an arrangement having sharp edges, as shown in Figure 4. Conventionally rounded features instead of edged or pointed features have been preferred in electrical field shielding arrangements because the presence of sharp edges normally results in high electrical fields at those points, which is undesirable. The array of elongate sub-elements 39 in each secondary shielding element 35 however acts as a local shield for the sharp edges of the continuous shielding surfaces 36 of the primary shielding elements, and thereby prevents any sudden increase in the electric field at these junctures.

The use of primary shielding elements 33 whose cross- section corresponds in shape and size to the cross- section of the secondary shielding elements 35 helps to minimize the size of any step at joints between the primary and secondary shielding elements 33,35. This in turn helps to ensure that the transition in electrical field at joints between the primary and secondary shielding elements 33,35 is smooth and gradual and there is no sudden change in the electrical field that would otherwise give rise to undesirable stresses.

Ensuring that the primary and secondary shielding elements 33,35 have cross-sections that correspond in shape and size also helps to ensure that the primary and secondary shielding elements can be interconnected easily in an end-to-end configuration in order to define a closed shield for placement about the outer exterior of the electrical equipment 30.

The redistribution of the electrical field lines, in use, around a piece of electrical equipment relies on the movement of surface charge in the electrical field shielding assembly 34. Consequently the primary and secondary shielding elements 33,35 must be electrically-conductive .

In the embodiment shown in Figure 3, the primary and secondary shielding elements 33,35 are formed from an electrically-conductive material.

It is envisaged that in other embodiments, the primary and secondary shielding elements 33,35 may be formed from an electrically insulating material coated with or bonded to an electrically-conductive or semi-conductive material in order to provide the required electrical conductivity.

Similarly it is also envisaged that in yet further embodiments, the primary and secondary shielding elements 30,35 may be formed from an electrically- conductive or semi-conductive material coated with or bonded to an electrically insulating material. However the primary and secondary shielding elements 33,35 are formed in order to provide the required electrical conductivity, the electrically-conductive material is preferably capable of conducting current at high voltages to ensure that the electrical field shielding assembly 34 is compatible with high voltage electrical equipment.

Claims

1. An electrical field shielding assembly for placement about electrical equipment comprising at least one primary, electrically conductive, shielding element connected to at least one secondary, electrically conductive, shielding element, the or each primary shielding element presenting a continuous and uninterrupted outer shielding surface and the or each secondary shielding element presenting a broken outer shielding surface.

2. An electrical field shielding assembly according to Claim 1 wherein the shield includes a plurality of primary shielding elements and a plurality of secondary shielding elements, each of the secondary shielding elements interconnecting adjacent primary shielding elements .

3. An electrical field shielding assembly according to Claim 1 or Claim 2 wherein the secondary shielding element is provided in the form of a mesh-like structure .

4. An electrical field shielding assembly according to Claim 1 or Claim 2 wherein the secondary shielding element is provided in the form of a reversed mesh-like structure .

5. An electrical field shielding assembly according to Claim 1 or Claim 2 wherein the or each secondary shielding element includes an array of elongate sub- elements arranged so that the outer surfaces of the sub-elements define the broken outer shielding surface of the secondary shielding element.

6. An electrical field shielding assembly according to Claim 5 wherein the or each elongate sub-element is shaped to define a circular , oval, elliptical or polyhedral cross-section cross-section.

7. An electrical field shielding assembly according to Claim 5 wherein the or each elongate sub-element is shaped to define a an incomplete circular, oval, elliptical or polyhedral cross-section.

8. An electrical field shielding assembly according to any of Claims 5 to 7 wherein the or each array of elongate sub-elements is arranged to define a secondary shielding element having a circular, oval, elliptical or polyhedral cross-section.

9. An electrical field shielding assembly according to any of Claims 5 to 7 wherein the or each array of elongate sub-elements is arranged to define a secondary shielding element having an incomplete circular, oval, elliptical or polyhedral cross-section.

10. An electrical field shielding assembly according to Claim 8 or Claim 9 wherein the or each primary shielding element is an elongate member having a cross- section corresponding in shape and size to the cross- section of the or each secondary shielding element.

11. An electrical field shielding assembly according to Claim 10 wherein the primary and secondary shielding elements are interconnected in an end-to-end configuration to define a closed shield for placement about the outer exterior of electrical equipment.

12. An electrical field shielding assembly according to any preceding claim wherein each of the primary and secondary shielding elements is formed from an electrically-conductive material .

13. An electrical field shielding assembly according to Claim 12 wherein each of the primary and secondary shielding elements is coated with an electrically insulating material.

14. An electrical field shielding assembly according to any of Claims 1 to 11 wherein each of the primary and secondary shielding elements is formed from an electrically insulating material coated with or bonded to an electrically-conductive material.

15. An electrical field shielding assembly according to any of Claims 12 to 14 wherein the electrically- conductive material is capable of conducting current at high voltages.