AU2016202308B2 - Down conductor - Google Patents

Abstract

A cable for at least partly channelling electrical energy generated by a flash of lightning, the cable including a plurality of concentric, annular layers, extending along a length of the cable, the layers including: an electrically conductive first layer, a middle layer surrounding the electrically conductive first layer, the middle layer including: a first at least partially conductive annular layer; a second at least partially conductive annular layer; and a substantially electrically non conductive annular layer positioned between the first at least partially conductive annular layer and second at least partially conductive annular layer, and an electrically conductive second layer surrounding the middle layer wherein the first at least partially conductive annular layer, the substantially electrically non conductive annular layer and the second at least partially conductive annular layer are simultaneously extruded during construction of the cable. 100 102 1068 110lilRE 112 KASL 1l14&\ il6 10 1126 FIGU1 16

Description

102

110lilRE

112 KASL

1l14&\ il6 1068

10

1126

FIGU1 16

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Technical Field

This invention generally relates to down-conductors for use in at least partly channelling electrical energy generated by a flash of lightning to ground, and methods of their construction. Such down-conductors may take the form of at least partially electrically conductive cables.

Background

Lightning flashes represent very rapid electrical discharge events, and involve very high electrical currents. In particular, cloud-to-ground (CG) discharges can be hazardous to animals, equipment, assets and personnel. Some of the specific features of CG lightning discharges that present a hazard to assets include:

• multiplicity (a flash is comprised of multiple "strokes", typically 3-4, but the number varies between 1 and about 30); • multiple ground terminations (MGT's), where strokes in a flash can strike the ground at different points; • charge transfers to ground up to 400 C; • peak "return-stroke" currents up to 200 kA; • high voltages and currents characterised by extremely fast rise times, with peak values being reached in less than a few microseconds; and • continuing currents of 200-500 Amperes, lasting 1-2 seconds.

The probability and severity of damage to assets can be reduced by the use of a lightning protection system, one example of which is illustrated in Figure 3. Such a lightning protection system may consist of an air terminal 302 such as a Franklin rod, a down conductor in the form of a cable 100 and a low impedance earthing system 304. The earthing system may consist of a number of buried earth rods 306 and horizontal conductors 308.

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The air terminal or terminals are placed at strategic points either on or nearby a structure and its assets to be protected. The air terminal is placed so that lightning strikes the air terminal rather than the structure. Once the air terminal is struck by lightning, a very fast flow of charge occurs between the lightning cloud and ground through the down conductor. The lightning current flows through the earthing system and the charge is dissipated safely into the ground. All three components (the air terminal, down-conductor, and earthing system) work together to avoid damage to the structure or its contents.

Conventional down-conductors can be difficult to install, for several reasons. Firstly, many down-conductors are bare (that is, do not have any electrical insulation). The installation of such down-conductors can result in electrification of the structure and hence pose a "touch voltage" hazard to occupants. Secondly, even if the down-conductor is in the form of a cable having some electrical shielding or insulation, it is very difficult to ensure that sufficient "separation distance" is achieved, i.e., so that "side-flashing" from the down conductor cable to a conductive member of the structure such as a water pipe is avoided. Side-flashing occurs when the voltage within a down-conductor exceeds the impulse withstand voltage of the down-conductor. To reduce the probability of side-flashing, conventionally, a blanket approach is used in which many conventional down-conductors are installed to reduce the peak or impulse voltage on each one and hence reduce the risk of side-flashing. However, given the very wide range of lightning parameters such as strokes currents and rise times, it is difficult to design a system that has sufficient numbers of down-conductors at a sufficient spacing to be both effective and economically feasible, in view of the large number of down-conductors and fixing points that would be required. Although the impulse withstand voltage of a down-conductor cable may be increased by increasing the thickness of any insulating or shielding layers, this generally results in a down-conductor cable having a very large minimum radius of curvature, limiting installation flexibility.

Conventional prior art down-conductor cables have a number of other shortcomings. As indicated above, conventional down-conductor cables designed for carrying lightning currents have limited voltage withstand capability. Not only does this increase the

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probability of side-flashing, it greatly limits the length over which they can be installed and hence taller buildings required multiple down-conductor cables. Secondly, conventional insulated down-conductor cables can be quite heavy, making installation more difficult. Thirdly, lack of concentricity of the down-conductor cable layers gives rise to inconsistent cable performance when testing under lightning impulses and in actual application in the field. Finally, the thickness of the insulation layer, a crucial aspect of the down-conductor cable's electrical performance, can also be inconsistent.

It is desired to address or ameliorate one or more disadvantages or drawbacks of prior art down-conductors, or at least provide a useful alternative.

Summary

In at least one embodiment, the present invention provides a cable for at least partly channelling electrical energy generated by a flash of lightning, the cable including a plurality of concentric, annular layers, extending along a length of the cable. The annular layers include an electrically conductive first layer, a middle layer surrounding the electrically conductive first layer, and an electrically conductive second layer surrounding the middle layer. The middle layer includes a first at least partially conductive annular layer, a second at least partially conductive annular layer, and a substantially electrically non conductive annular layer positioned between the first at least partially conductive annular layer and second at least partially conductive annular layer. The first at least partially conductive annular layer, the substantially electrically non conductive annular layer and the second at least partially conductive annular layer are simultaneously extruded during construction of the cable.

In a further embodiment, the present invention provides a method of manufacturing a cable, the method including the steps of preheating an electrically conductive first layer, extruding a middle layer by simultaneously extruding a first at least partially conductive annular layer, a second at least partially conductive annular layer and a substantially electrically non conductive annular layer (the middle layer being made up of the

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substantially electrically non conductive annular layer sandwiched between the first at least partially conductive annular layer and the second at least partially conductive annular layer), and applying the extruded middle layer to the preheated electrically conductive first layer to make a cable core.

Brief Description Of The Drawings

Some embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is cross-sectional view of one embodiment of the present invention. Figure 2 is a view the embodiment of Figure 1 with layers partially removed. Figure 3 is an illustration of an application of an embodiment of the invention. Figure 4 is a cross-sectional view of an exemplary triple extrusion head for use in manufacturing a middle layer of an embodiment of the invention, and applying it to an electrically conductive first layer of an embodiment of the invention.

Detailed Description

To facilitate the channelling of transient high-current electrical energy which characterises a lightning flash, an embodiment of the present invention is a cable constructed using a series of concentric layers surrounding an inner, central filler core. The layers include at least an electrically conductive layer and an insulation layer. The performance of the cable (characterised by high electrical conductivity, high transient current tolerance and low arc or flashover probability) is improved by the use of an outer metallic screen (or sheath). The insulation layer, although electrically non-conductive, may be positioned within inner and outer electric field limitation screens, to control the electric fields generated by the passage of electrical energy through the down-conductor cable.

Figure 1 shows a cross-section of a cable 100 constructed in accordance with one embodiment of the invention. The central core 102 of the cable 100 consists of a cylindrical filler material that provides some of the mechanical properties of the cable 100.

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The material is selected to resist bending of the cable 100 so as to avoid kinking of the electrically insulating or conductive layers. The central core 102 has a substantially circular cross-section to enable an electrically conductive first layer 104 to be wrapped around it, such that the electrically conductive first layer 104 has a substantially annular cross-section. The central core 102 is preferably manufactured using an extrusion process to ensure that it has a consistent and smooth outer surface and has a substantially constant cross-sectional profile throughout its length.

As indicated above, the electrically conductive first layer 104 has a substantially annular cross-section. This enables the down-conductor cable 100 to better carry high-frequency or transient currents, the annular shape of the conductor lowering the inductance of the cable and catering for the "skin effect" (where the current density is greatest near the surface or "skin" of a conductor).

Although the central core 102 may be made from polyvinylchloride (PVC) or other similar thermoplastics, it has been found that solid polyethylene provides sufficient resistance to bending while still allowing some flexibility and being lighter than PVC. The combination of limited flexibility and reduced weight enable embodiments of the invention to be easily installed in an efficient and correct manner.

The electrically conductive first layer 104 may be constructed from any suitable electrical conductor. For example, it may be constructed from a plurality of copper strands, where the total cross-sectional area is at least 50 mm . Preferably, it is constructed from a 2

plurality of aluminium strands with a total cross-sectional area of at least 50 mm , which 2

are sufficiently electrically conductive, but are much lighter than copper strands. The electrically conductive first layer 104 is constructed to have a sufficiently high total cross sectional area to minimise electrical resistance and to avoid excessive temperature rise, which could damage other parts of the cable 100.

Where copper or aluminium strands are used to construct the electrically conducting first layer 104, the outer surface of the first layer 104 will be uneven and inhibit the application

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to the first layer 104 of additional layers having an annular cross-section. To provide a smooth surface upon which additional layers may be added, binder tape 106 may be wrapped around the outer surface of the electrically conducting first layer 104.

Surrounding the electrically conductive first layer 104 is a middle layer 108. As indicated above, binder tape 106 may be applied to the electrically conductive first layer 104 to assist in the application and positioning of the middle layer 108.

The primary function of the middle layer 108 is to electrically insulate the electrically conductive first layer 104. It is preferably constructed from a substantially electrically nonconductive annular layer 112 (an insulating layer) sandwiched between a first at least partially conductive annular layer 110 and a second at least partially conductive annular layer 114.

Surrounding this middle layer is an electrically conductive second layer 116, also referred to as a "screen". The electrically conductive second layer 116 may consist of any suitable electrically conductive material. For example, the conductive second layer 116 may include a plurality of strands of an electrically conductive metal, such as copper or aluminium. In other embodiments, it may be desirable to construct cable 100 with a very thin conductive second layer 116, including tape made from an electrically conductive metal, such as copper. In other embodiments, an electrically conductive metal mesh may be used.

This electrically conductive layer 116 operates to electrically shield the inner components of the cable 100, including the electrically conductive first layer 104. An outer sheath 118 surrounds and physically protects electrically conductive second layer 116, and is the outermost layer of the cable 100. This outer sheath 118 may be made from any suitably flexible electrically nonconductive material, such as PVC. Alternatively, the outer sheath 118 may be made from, or include, a partially conductive material. Where the outer sheath 118 includes a partially conductive material, the conductivity of the material may be controlled by, for example, varying the amount of carbon black added.

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In addition to being flexible, the material from which the outer sheath 118 is made should also be able to withstand harsh environmental conditions such as direct and sustained UV radiation, water, temperature extremes and the like, as the cable 100 may be installed on the exposed outer surface of a structure (as illustrated in Figure 3) anywhere in the world.

As is clear from the above, a "radial field design" is used for the cable, wherein the electrically conductive first layer 104 (together with the first at least partially conductive annular layer 110) and the electrically conductive second layer 116 form a long, concentric cylinder and the dielectric forming the substantially electrically nonconductive annular layer 112 is electrically stressed exclusively cylinder-symmetrically when looked at macroscopically, in accordance with:

where U, is the applied voltage, R and r are the external and internal radii of the substantially electrically nonconductive annular layer 112 and x is the continuous coordinate.

To ensure the radial field remains undisturbed, even in the areas close to the electrically conductive first layer 104 and the electrically conductive second layer 116, the first at least partially conductive annular layer 110 and the second at least partially conductive annular layer 114 (which may also be referred to collectively as voltage stress-relieving layers) are provided between these elements and the substantially electrically nonconductive annular layer 112 respectively. The electrically conductive second layer 116 provides effective electric screening of the cable 100, so the cable environment is free of electric fields.

The first at least partially conductive annular layer 110 and the second at least partially conductive annular layer 114 may form semi-conductive layers that sandwich the

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substantially electrically nonconductive annular layer 112. These semi-conductive layers 110, 114 principally fulfil two functions:

(a) they equalise and reduce the electric stress in the substantially electrically nonconductive annular layer 112 by preventing local electric field enhancement in non-homogeneous areas, such as individual wires of the electrically conductive first layer 104.

(b) they prevent of the formation of gaps or voids between the voltage-carrying components of the cable 100 (that is, the electrically conductive first layer 104) and the substantially electrically nonconductive annular layer 112 due to mechanical stress, e.g., bending of the cable 100 or differential expansion of the various materials under thermal stress. The solid bond between the semi conductive layers and 110, 114 and the substantially electrically nonconductive annular layer 112 (created by the triple extrusion process described below) effectively prevents the occurrence of partial discharges.

The semi-conductive layers 110, 114 may be made from a thin layer of sufficiently electrically conductive cross-linked polyethylene (XLPE) or sufficiently flexible electrically conductive material (including other cross-linked polymers). For example, the XLPE may be made conductive by doping it with a sufficient amount of carbon black. Stabilisers, additives and peroxides may also be used to achieve the optimum balance of conductivity, physical properties and manufacturability.

The thickness of the substantially electrically nonconductive annular layer 112 should be selected taking into account the competing considerations of minimising impedance and maximising withstand voltage. The thickness of this layer is preferably between 2.5 mm and15 mm, more preferably between 4 mm and 7 mm, and even more preferably 5 mm and 6 mm, assuming that the layer is made of XLPE.

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In some embodiments, the voltage stress-relieving semi-conductive layers 110, 114 may be made from the same material, and be of the same thickness. However, in other embodiments, the thickness and/or material of semi-conductive layer 110 may be different to that of semi-conductive layer 114.

The overall diameter of the cable 100 is preferably between 30 mm and 65 mm, and even more preferably between 34 mm and 40 mm. In one preferred embodiment, the overall diameter of the cable is 36 mm.

As described above, the middle layer 108 is constructed from three components - a first at least partially conductive annular layer 110, a substantially nonconductive annular layer 112 (an insulating layer), and a second at least partially conductive annular layer 114. The insulating layer 112 is preferably made from a single material (for example, naturally non polar thermoplastic polyethylene, XLPE, ethylene-propylene rubber or other flexible non conductive material with a high dielectric strength) in a single layer. A method of manufacturing the middle layer 108 and applying it to the electrically conductive first layer 104 will now be described.

To reduce imperfections in the middle layer 108 and the possibility of contamination, voids, and other manufacturing defects, and to facilitate a smooth and consistent interface between the substantially nonconductive annular layer 112 and the adjacent at least partially conductive annular layers 110 and 114, the middle layer 108 is "triple extruded". The "triple extrusion" manufacturing process involves simultaneously extruding each of the three components of the middle layer 108. The partially conductive annular layers 110 at 114 are welded solidly to the substantially nonconductive annular layer 112 during production. This manufacturing process may be undertaken in an integrated extrusion plant employing a continuous vulcanising process line with dry curing.

Preferably, the middle layer 108 is triple extruded in a fully enclosed process in which the partially conductive annular layer 110, substantially nonconductive annular layer 112 and partially conductive annular layer 114 are applied simultaneously to the electrically

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conductive first layer 104, which is preheated to facilitate bonding of the electrically conductive first layer 104 to the middle layer 108 (more specifically, the partially conductive annular layer 110).

The result of this process, in which the middle layer 108 is triple extruded and applied to the pre-heated electrically conductive first layer 104, is a cable core. Generally, the cable core consists of the central core 102, the electrically conductive first layer 104 and the middle layer 108. The central core 102, which may be made from XLPE, is preferably extruded and cross-linked in a previous process.

This cable core is inspected using in-line inspection apparatus, which irradiates the cable core with x-rays. Based on the radiation re-radiated from the elements of the cable core, the dimensional accuracy of the elements of the cable core is monitored.

An exemplary triple extrusion head 400 which may be used in the construction of the cable core is illustrated in Figure 4. As is clear from Figure 4, each of the elements of the middle layer 108 is simultaneously extruded to form a single cable core, the central core 102 surrounded by the electrically conductive first layer 104 flowing through the centre of the head 400.

A cable as described above, manufactured using a triple-extrusion process also as described above, has a substantially improved voltage withstand capability, without substantially adding to the weight of the cable if at all. This enables fewer down conductors to be more easily installed to protect a structure. The concentricity of the cable layers, and consistency of the insulation layer, in addition to the other cable features described above, provide a down-conductor cable having a more reliable and predictable performance, facilitating lightning protection system design.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

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The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims (14)

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A cable for at least partly channelling electrical energy generated by a flash of lightning, the cable including a plurality of concentric, annular layers, extending along a length of the cable, the layers including: an electrically conductive first layer, a middle layer surrounding the electrically conductive first layer, the middle layer including: a first at least partially conductive annular layer; a second at least partially conductive annular layer; and a substantially electrically non conductive annular layer positioned between the first at least partially conductive annular layer and second at least partially conductive annular layer, and an electrically conductive second layer surrounding the middle layer wherein the first at least partially conductive annular layer, the substantially electrically non conductive annular layer and the second at least partially conductive annular layer are simultaneously extruded during construction of the cable.

2. A cable as claimed in claim 1, wherein the electrically conductive first layer includes aluminium.

3. A cable as claimed in claim 2, wherein the electrically conductive second layer includes copper or aluminium.

4. A cable as claimed in any one of the preceding claims, wherein the first at least partially conductive annular layer or the second at least partially conductive annular layer are made from partially conductive cross-linked polyethylene.

5. A cable as claimed in claim 4, wherein the first at least partially conductive annular layer or the second at least partially conductive annular layer are made from a cross linked polyethylene doped with carbon black.

6. A cable as claimed in any one of the preceding claims, wherein the cable further includes a substantially cylindrical central core around which the electrically conductive first layer is positioned, the central core being formed from solid polyethylene.

7. A cable as claimed in claim 6, wherein the central core is manufactured using an extrusion process.

8. A cable as claimed in any of the preceding claims, wherein the electrically conductive second layer includes at least one of: a plurality of strands; a tape; and a mesh made from an electrically conductive material.

9. A cable as claimed in any of the preceding claims, further including an outer sheath surrounding the electrically conductive second layer, wherein the outer sheath is either electrically non-conductive, or partially electrically conductive.

10. A cable as claimed in any of the preceding claims, wherein the substantially electrically non-conductive annular layer is made from one of: cross-linked polyethylene; and ethylene-propylene rubber.

11. A method of manufacturing a cable as claimed in any one of the preceding claims, the method including the steps of: (a) preheating the electrically conductive first layer; (b) extruding the middle layer by simultaneously extruding the first at least partially conductive annular layer, the second at least partially conductive annular layer; and the substantially electrically non conductive annular layer; (c) applying the extruded middle layer to the preheated electrically conductive first layer to make a cable core.

12. A method of manufacturing a cable as claimed in claim 6 or 7, the method including the steps of: (a) preheating the electrically conductive first layer; (b) extruding the middle layer by simultaneously extruding the first at least partially conductive annular layer, the second at least partially conductive annular layer; and the substantially electrically non conductive annular layer; (c) applying the extruded middle layer to the preheated electrically conductive first layer to make a cable core.

13. A method as claimed in claim 12, further including the step of extruding the central core.

14. A method as claimed in any of claims 11 to 13 further including the step of inspecting the cable core, the inspection including the step of irradiating the cable core with x-rays.