WO1998000898A1

WO1998000898A1 - Improved lightning conductor and method

Info

Publication number: WO1998000898A1
Application number: PCT/AU1997/000413
Authority: WO
Inventors: John Richard Gumley
Original assignee: Erico Lightning Technologies Pty. Ltd.
Priority date: 1996-07-02
Filing date: 1997-07-01
Publication date: 1998-01-08
Also published as: ID17512A; AUPO079196A0

Abstract

An improved lightning conductor (1) is provided which has an earthed conductor element (11) surrounded by a rounded conductive surface element (12), a high impedance bleed circuit (17) connected between the conductive surface element (12) and earth, and a triggering electrode (14) positioned adjacent or above the upper end of the conductor element (11) and connected electrically to the conductive surface element (12). The conductor element (11) is preferably a tube with the triggering electrode (14) disposed on the central axis of the tube. An insulating spacer (15) is provided between the tube (11) and the triggering electrode (14). In use, the rounded surface element (12) minimises the produciton of corona and the high impedance bleed circuit (17) delays the production of a triggering arc between the triggering electrode (14) and the earthed conductor (12) until the rapid approach of a lightning down leader. The triggering arc, which occurs at the highest point of the lightning conductor (10), causes a stepped increase in the near electric field above the conductor (10) to provide ideal conditions for upleader initiation and propagation.

Description

IMPROVED LIGHTNING CONDUCTOR AND METHOD The present invention relates to an improved lightning conductor of the passive kind which reduces corona in the quasi static electric fields below an approaching storm cloud, and converts to an active mode by producing a triggering spark on the approach of a lightning down leader. The invention also relates to a method for reducing pre-discharge corona produced by an air terminal whilst maintaining the ability to produce a triggering spark on the approach of a lightning down leader.

Conventional types of lightning conductors provided on buildings for the attachment of lightning fall into several classes. Firstly, there is the Franklin rod which is a sharp pointed conductive rod which is usually located above the highest point of a structure. Secondly, there are derivatives of the Franklin rod which induce additional ionisation supposedly to enhance air conductivity. Proponents of this technology claim that additional ionisation will assist in the production of a streamer which may lead to the lightning attachment process.

Another form of conductor is generally referred to as a Faraday Cage and comprises flat horizontal conductors which reside on parapets of buildings in order to collect and conduct the lightning current to ground.

All of the prior art modified Franklin rods operate on the principle of electric field intensification due to their shape and geometry and the creation of multiple points which, in one way or another, enhance corona production. Their physical placement in a position of advantage on a structure provides for production of a continuous ionisation stream with the aim of improving the air conductivity in their immediate vicinity. A departure from these concepts is disclosed in Australian Patent No 580,698 which in 1986 provided a significant deviation from the prior art. The device of 580,698 is rounded to restrict corona and local ionisation during quasi static conditions in stormy weather but becomes active on approach of a lightning down leaser by producing an ionisating arc at an annular gap between an earthed rod and surrounding rounded floating conductive surface. The up leader initiated by the arc has the advantage of operating in a more linear electric field than one subject to near space charge created by the pre-stroke ionisation of other prior art devices.

It is an object of this invention to provide an improved method and apparatus for enhancing up leader initiation in lightning conductors.

Accordingly, one broad form of the invention provides a method for enhancing up leader initiation at a lightning conductor comprising synchronizing a stepped increase in the near electric field strength at said lightning conductor with a triggering arc at the highest point of the conductor.

Preferably, said method further includes minimising the production of corona from the lightning conductor under the strong quasi static electric fields below thunder clouds.

Preferably, said lightning conductor includes an earthed element surrounded by a conductive floating surface. The conductive floating surface is preferably smoothly rounded and may be provided by a surface element having a substantially continuous curved surface to minimise the production of corona. Examples of suitable shapes of smoothly rounded surface elements are described in Australian Patent No. 580698, the contents of which are incorporated herein by reference, but in a particularly preferred embodiment the surface element is in the form of an oblate spheroid. Preferably a triggering electrode electrically connected to the floating surface is provided substantially adjacent or above the upper end of the earthed element and the triggering arc is established at substantially the highest point of the conductor between the triggering electrode and the earthed element.

Preferably the method further comprises restricting the flow of capacitive displacement currents from said floating surface on the approach of a lightning down leader. The restriction of current flow causes said floating surface to rise in voltage. The rising voltage on the floating electrode acts to reduce the expected field increase observed at the tip of the electrode when the surrounding field is rapidly increasing. This delays the production of the triggering arc to a period when the median electric field has sufficient field strength to sustain leader production.

Preferably, a high impedance bleed circuit is connected between the floating surface and earth to delay the production of the triggering arc in the same manner as in Australian Patent No. 580,698. Preferably, said rising voltage reaches such a high potential that it will initiate the triggering arc, said triggering arc being physically forced by electrode geometry to penetrate the near electric field at the uppermost point of the conductor.

Preferably an insulating spacer is disposed between the trigger electrode and the upper end of the earthed element. The triggering arc, preferably with a few tens of volts in arc voltage, will effectively and simultaneously ground said floating surface thereby causing a stepped increase in the strength of the electric field observed at the uppermost end of the conductor. The stronger electric field plus the photo ionisation from the triggering arc and the absence of space charge from pre-stroke corona, creates ideal conditions for up leader initiation and propagation.

According to a further aspect of the invention there is provided a lightning conductor comprising a conductor element adapted, in use, to be grounded, and surrounded by a conductive surface which is insulated from the conductor element, a high impedance bleed circuit connected between said surface and ground when said lightning conductor is in use, and a triggering electrode adjacent the upper end of the conductor element and spaced therefrom, said trigger electrode being electrically connected to said surface.

Preferably, an insulating spacer is provided between the conductor element and the triggering electrode, said spacer being arranged, in one configuration, such that a triggering arc established between the electrode and conductor element is forced to rise above said upper end of the conductor element and the electrode.

In another configuration, the triggering electrode is raised to be the highest point of the lightning conductor, so that the triggering arc penetrates into the near electric field at the uppermost point of the lightning conductor. Preferably, at least the upper part of the conductor element is a tube and said triggering electrode is located centrally within said tubular upper end.

Preferably, said conductive surface is in the shape of an oblate spheroid and said tube is on the axis thereof, said surface being mounted on said tube by insulating mountings. Preferably, the high impedance bleed circuit is connected between said surface and said tube. The high impedance bleed circuit may be either a resistance or a complex impedance, preferably falling substantially within the range from 1 megaohm to 20 megaohms, the actual value being selected according to the physical parameters of the whole system.

Preferably, the height of the electrode assembly, comprising the triggering electrode and the upper end of the conductor element, above the floating surface may be adjusted to ensure that minimal pre-discharge corona is produced under a range of applications. As an example, when the electric field is at a strength of lOkV/m, an air terminal at 10 meter elevation will cut equipotential lines at lOOkN level.

This will require a higher than normal height of the electrode above the floating surface. If the terminal were to be mounted at a height of 30 meters, then the equipotential lines cut would lie at the 300kN level. The electrode to floating surface height must then be reduced to ensure minimal pre-discharge corona emissions. It is a feature of this invention that the terminal may be readily customised to meet optimum spacings according to the height of the structure being protected.

According to a further aspect of the invention, there is provided a device for attracting lightning comprising an earth conductor having at least a tubular upper part, a generally spherical conductive surface element mounted on the earth conductor by insulating supports, a high impedance bleed circuit connected between the conductive surface element and the earth conductor, and a trigger electrode located within the tubular upper part of the earth conductor and adjacent or above its upper end, the trigger electrode being spaced from the upper end of the earth conductor and electrically connected to the surface element.

In order that the invention may be more readily understood, particular embodiments will now be described with reference to the accompanying drawings wherein:

Figure 1 is a schematic sectional view of a lightning conductor in accordance with the invention,

Figure 2 is an enlarged section through the upper electrode assembly of the lightning conductor of Figure 1;

Figure 3 is a sectional view of another embodiment of a lightning conductor in accordance with the invention; and

Figure 4 is an enlarged section through the upper electrode assembly of the lightning conductor of Figure 3.

Figure 5 is a schematic diagram showing equipotential lines about a device in accordance with the invention in a quasi static field when the floating surface and trigger electrode are effectively grounded. These lines would represent a field strength of around lOkN/m.

Figure 6 shows the equipotential lines at a time when the electric field is in the dynamic phase and the trigger electrode and floating surface are at an elevated potential with the instantaneous field strength near lOOkV/m.

Figure 7 shows the equipotential lines when the electrode assembly has reached arc over voltage, and both trigger electrode and floating surface become effectively grounded in the now high electric field.

In Figures 1 and 3, the lightning conductor 10 consists essentially of a conductor element 11 which in use is connected to earth and which supports a conductive floating surface 12 which surrounds the rod 11 and is mounted thereon by means of insulating ring supports 13. The conductive surface 12 is preferably shaped as an oblate spheroid, being generally spherical in shape, flattened at the poles, although other smoothly curved shapes, such as described in Australian Patent No. 580,698 can provide the desired corona reducing properties.

An electrode assembly 19 comprises a triggering electrode 14 which is arranged centrally within the upper end region of the conductor element 11 which is in the form of a tube of circular cross-section. The triggering electrode 14 is mounted at the upper end of the conductor element 11 by a mounting member 22 made of an insulating material, such as Delrin. In one embodiment, the upper end of the triggering electrode 14 is disposed essentially in the same plane as the upper end of the tubular conductor element 11. In a second embodiment, as shown in Figures 3 and 4, the triggering electrode 14 is raised to be the highest point of the lightning conductor 10. An annular insulating spacer 15 surrounds the triggering electrode 14 in each embodiment and projects the arc 18 upwardly beyond the upper end of the conductor element 11. In the embodiment of Figures 1 and 2, the insulating spacer 15 extends above the upper ends of the tubular conductor element 11 and the triggering electrode 14. In the embodiment of Figures 3 and 4, a conducting conical extension 20 is mounted in the upper end of the tube 11 with the insulating spacer 15 mounted between the extension 20 and the electrode 14. The triggering electrode 14 is electrically connected to the conductive surface 12 by means of electrical connection 16 which extends through an opening 21 in the wall of the tubular conductor element 11, and a high impedance bleed circuit 17 is connected between the conductive floating surface 12 and the conductor element 11. In this manner, in use, both the conductive surface 12 and the triggering electrode 14 are connected to earth via the high impedance bleed circuit 17.

In the present invention, the concepts described in earlier Australian Patent No. 580,698 are enhanced in a novel manner by an improvement in capacitive coupling to an approaching lightning down leader by use of an architecturally lower profile oblate spheroid, and the raising of a spark discharge between the triggering electrode and the conductor rod, to the highest elevated point which is concurrent with the point of highest electric field strength.

The arrangement of the present invention causes a stepped increase in the near electric field by the triggering arc effectively grounding the floating conductive surface 12 on the rapid approach of a lightning down leader. In prior art devices, such a stepped increase in the field strength would not provide any significant advantage because the triggering arc is located a significant distance below the point of optimum field strength. This synchronism of the triggering arc with the effective grounding of the floating conductive surface simultaneously provides four beneficial effects. The arc is now in an optimum physical position in respect of the median field; the arc has created its own increase in the electric field into which it extends; the arc has achieved a primary function of liberating electrons, creating photoionisation; and the whole scenario takes place in an environment devoid of the pre-existing space charge which reduces the median field strength.

The technique involves transfer of the capacitively induced voltage on the floating conductive surface 12 to a central triggering electrode 14 surrounded by an earthed cylindrical conductor rod 11 with a spark gap 18 such that the spark transits over insulating spacer 15 to the point of highest electric field strength. The stepped downward movement of the voltage on the conductive surface 12 due to the occurrence of the triggering arc ensures an instantaneous rise in the strength of the near electric field observed at the top of the electrode assembly. This ensures an increased realisation of the electric field energy requirements for successful streamer/leader development. Prior to the creation of the triggering arc, the high impedance bleed circuit 17 ensures that there would be no potential rise of the conductive surface 12 in the quasi static state of high electric fields existing below thunderstorms. On the approach of a down leader, the conductive surface 12 rises in voltage due to restriction of the flow of displacement currents by the high impedance bleed circuit 17. The raising of the potential of the conductive surface 12 acts to reduce the strength of the near electric field at a time when the median electric field is rapidly increasing due to the approaching down leader. The combination of electrode shape and electrode potentials in both static and dynamic phases ensures the minimisation of pre-stroke corona and resulting space charge.

Figure 5 shows the field distribution in the static state before the approach of a leader. In Figure 6 the leader is approaching and both the floating surface 12 and triggering electrode 14 have increased in potential. The figure shows that the triggering electrode and floating surface have created field lines that have acted to reduce the near median field above the electrode assembly 19. This effect is in such direction to reduce corona production from the electrode during the phase of rapid field increase. When the electrode assembly reaches the arc over level, the whole assembly reverts to a grounded state, but now at electric field strengths that will support leader propagation. The leader can now propagate into high electric fields that have not been modified by a space charge formed by prior corona activity.

More specifically, the triggering arc has the dual role of liberating electrons and creating photoionisation in the strongest electric field and in the absence of corona and the consequent space charge which acts to reduce the energy of the field. The resulting streamer is thus free to propagate by absorbing energy from a substantially unmodified electric field.

The method and geometry of raising the triggering arc to the highest elevation is also important. In this invention, a hollow rounded cylindrical conductor tube is the lightning attachment point and has adequate cross-sectional area to conduct lightning currents of the highest energy content. The triggering electrode lies in the centre of the earthed tube where it has influence to initiate the required arc but plays little part in the subsequent conduction of the main discharge. As mentioned above, a further feature is the inclusion of an insulating spacer 15 between the triggering electrode 14 and the earthed element 11. In one embodiment, as shown in Figure 1, this is positioned so that the arc from the triggering electrode must rise over the upper edge of the spacer 15 to reach the main conductor element 11. In so doing, the arc extends into the strongest section of near electric field where conditions are most favourable for its development into a streamer and subsequent up leader.

In the second embodiment of Figure 2, the trigger electrode 14 is raised to be the highest point of the electrode assembly.

Whilst two particular embodiments have been described above, it will be readily apparent that modifications can be made without departing from the scope of the invention. For example it is conceivable that the conductor element 11 may be a solid conductor rod with an annular triggering electrode around the outer perimeter of the rod at the top end. An annular insulating spacer would be located between the electrode and the solid rod and would extend above the top of the rod. Furthermore, the shape of the conductive surface 12 may be varied from that described above.

Since modifications within the spirit and scope of the invention may be readily effected by persons skilled in the art, it is to be understood that the invention is not limited to the particular embodiments described, by way of example, hereinabove.

Claims

1. A lightning conductor comprising a conductor element adapted, in use, to be connected to earth, a conductive surface element surrounding the conductor element and insulated from the conductor element, a high impedance bleed circuit adapted to be connected between the surface element and ground when the lightning conductor is in use, and a triggering electrode adjacent the upper end of the conductor element and spaced therefrom, the triggering electrode being electrically connected to the surface element.

2. A lightning conductor according to claim 1 wherein an insulating spacer is provided between the conductor element and the triggering electrode.

3. A lightning conductor according to claim 2 wherein the spacer is positioned such that a triggering arc established between said electrode and the conductor element is forced by the insulating spacer to rise above the upper end of the conductor element into the zone of highest electric field strength.

4. A lightning conductor according to any one of the preceding claims wherein the triggering electrode is arranged to be the highest point of the lightning conductor.

5. A lightning conductor according to any one of the preceding claims wherein at least the upper part of the conductor element is of tubular form and the triggering electrode is located within the tubular upper part of the conductor element.

6. A lightning conductor according to claim 5 wherein the electrical connection between the surface element and the triggering electrode extends through an opening in a wall of the conductor element.

7. A lightning conductor according to any one of the preceding claims wherein the conductive surface element has a smoothly rounded surface.

8. A lightning conductor according to any one of the preceding claims wherein the conductive surface element is generally spherical.

9. A lightning conductor according to claim 8 wherein the conductive surface element is in the shape of an oblate spheroid.

10. A lightning conductor according to claim 8 or claim 9 wherein the conductor element comprises a tube located on a substantially vertical central axis of the conductive surface element.

11. A lightning conductor according to any one of the preceding claims wherein the conductive surface element is mounted on the conductor element by insulating mountings.

12. A lightning conductor according to any one of the preceding claims wherein the high impedance bleed circuit is connected between the conductive surface element and the conductor element.

13. A lightning conductor according to any one of the preceding claims wherein the value of the impedance of the bleed circuit falls substantially within the range from 1 megohm to 20 megohms.

14. A device for attracting lightning comprising an earth conductor having at least a tubular upper part, a generally rounded conductive surface element mounted on the earth conductor by insulating supports, a high impedance bleed circuit connected between the conductive surface element and the earth conductor, and a triggering electrode located within the tubular upper part of the earth conductor and adjacent or above its upper end, the triggering electrode being spaced from the upper end of the earth conductor and electrically connected to the surface element.

15. A device according to claim 14 wherein an annular insulating spacer is provided around the triggering electrode.

16. A device according to claim 15 wherein the upper end of the annular insulating spacer is disposed at a level at least as high as the level of the upper end of the earth conductor.

17. A device according to claim 15 or claim 16 wherein the insulating spacer projects upwardly beyond the upper end of the earth conductor.

18. A method of enhancing up leader initiation at a lightning conductor comprising synchronizing a stepped increase in the near electric field strength at the lightning conductor with a triggering arc at the highest point of the conductor.

19. A method according to claim 18 further including minimizing the production of corona from the lightning conductor under the strong quasi static electric fields below thunder clouds.

20. A method according to claim 18 or claim 19 wherein the lightning conductor includes an earthed element surrounded by a conductive floating surface.

21. A method according to claim 20 wherein a triggering electrode electrically connected to the floating surface is provided adjacent the upper end of the earthed element, the triggering arc being established between the triggering electrode and the earthed element at substantially the highest point of the conductor.

22. A method according to claim 21 wherein at least the upper part of the earthed element is of tubular form and the trigger electrode is provided within the tubular upper end part of the earthed element and terminating substantially adjacent or above the upper end of the earthed element.

23. A method according to claim 21 or claim 22 wherein an insulating spacer is disposed between the trigger electrode and the upper end of the earthed element to cause the triggering arc to penetrate the near electric field at the uppermost point of the conductor.

24. A method according to any one of claims 20 to 23 further including restricting the flow of capacitive displacement currents from the floating surface on the approach of a lightning down leader to allow the floating surface to rise in voltage.

25. A method according to claim 24 wherein a high impedance bleed circuit is connected between the floating surface and earth.

26. A method according to claim 24 or claim 25 wherein when the rising voltage reaches such potential to initiate the triggering arc, the triggering arc penetrates the near electric field and simultaneously grounds the floating surface thereby causing a stepped increase in the strength of the near field to produce a stronger electric field strength for up leader propagation.

27. A method according to any one of claims 20 to 26 including providing a generally spherical surface element as the floating surface.

28. A method according to claim 27 wherein the surface element comprises an oblate spheroid.