GB2321348A - Lightning protection for overhead power lines - Google Patents

Abstract

A cross-arm 22 for supporting overhead power lines 6 is at least in part formed as a hollow tube and incorporates a lightning protection device enclosed within the hollow tube. The cross-arm may be earthed or unearthed and made of polymer composite material (Figs.6,7,10,13), or semiconductive material, or of metal (Figs.4,5,8,9,11,12). The power lines 6 may be bare or covered. Lightning protection may be effected by one or more arc gaps 28 and/or by one or more varistor surge arresters 44. Arc gaps may be formed between sections of horizontal or vertical rods (figs.4,5), between vertical rods and a common horizontal rail (38, Fig.6) bolted to the interior of a composite material cross-arm, or between vertical rods and circular apertures (50, Fig.7) in a metal plate (42) disposed horizontally within a composite cross-arm. The ends of the hollow tube cross-arm may be gas transmissive to allow egress of gaseous combustion products.

Description

LIGHTNING PROTECTION DEVICES The present invention relates to lightning protect ion devices for overhead power distribution lines.

Electrical power is often transmitted over distances by overhead lines, which may consist of bare or covered conductors. Typically there is a separate conductor for each of the three power phases. When lightning strikes an overhead power line conductor (or strikes in close proximity to the line) high voltage surges are produced and travel along the conductor(s) in both directions. Such over-voltage surges can reach several 100 kV or even a few MV. These surges can cause damage to overhead line equipment such as insulators, transformers, switchgear etc and also to cable inserts in the line. In order to reduce the damage due to lightning it is necessary to protect the system using devices to divert, reduce or eliminate the over-voltage surge within as short a time as possible, generally of the order of microseconds.

Generally, bare conductors suffer very little damage due to lightning. However the increasing use of covered conductors has greatly increased the likelihood of lightning damage to the conductor. This can be due to the actual strike, but also because arcs generated between the phases do not travel along covered conductors as they do along bare conductors. A stationary arc on a covered conductor will not only damage the sheath, introducing corrosion and reducing the conductor lifetime, but can burn out the conductor totally within a matter of seconds.

Currently, overhead line (OHL) equipment is normally protected by arc gap protection devices (APDs) or surge arresters. Arc gaps divert the over-voltage surge to earth or other suitable 'floating' metal components by allowing an ionic breakdown to occur and an arc to be established. This arc then erodes sacrificial electrodes until extinguished by a circuit breaker operation. Suitable arc gap protection devices (APDs) may make a connection to the metal conductor through the polymer sheath of a covered conductor via an insulation piercing connector (IPC).

Surge arresters operate by changing their initial highly resistive state to a highly conductive state and diverting the current surge. The arrester will limit the voltage across it at currents up to 10 kA.

A third possibility is to use a combined arc gap and arrester where the arc gap is smaller than normal for the overhead line voltage. The arc gap breaks down as normal but is intended to self extinguish as the surge arrester becomes non-conductive after the surge has dissipated. This is designed to avoid circuit breaker operation, and the onset of damaging levels of follow through current.

The operation of arc gaps can lead to metal splatter, causing local damage to insulators and the conductor sheath. There is also the possibility of explosion if a surge arrester is subject to a lightning current well above its rating. Finally APDs and IPCs can be a danger to wildlife as they can have exposed live metal parts.

The present invention provides a cross-arm for supporting overhead power lines, which is at least in part formed as a hollow tube and incorporating a lightning protection device enclosed within said hollow tube. Incorporating the lightning protection device (LPD) within the cross-arm may protect the LPD from the environment, as well as minimizing the damage caused by metal splatter from an arc gap or from explosion of a surge arrester. Further, wildlife is protected from the exposed live metal parts of APDs.

Conveniently the cross-arm may be substantially wholly formed as a hollow tube.

Conveniently the cross-arm may include an insulator mounted on the hollow tube to support an overhead power line, and a conductor enclosed within the insulator, arranged for connection at one end to an overhead power line supported by the insulator and extending at the other end into said tube for connection to the lightning protection device.

Conveniently, said conductor may extend through a side of the hollow tube opposite to the insulator for use as an earthing point.

Said conductor may be a rod.

Conveniently, there may be clamping means for connecting said power line to said conductor.

Said hollow tube may be made at least in part from conducting material, and the lightning protection device may be connected by means of said hollow tube to earth or to a second overhead power line.

Advantageously, at least one end of the hollow tube perpendicular to the longitudinal axis of said tube may be gas transmissive, allowing the passage of gaseous combustion products from the cross-arm.

The lightning protection device may include an arc gap or a plurality of arc gaps.

The hollow tube may form an electrode of the arc gap or of each of the plurality of arc gaps.

The lightning protection device may include a surge arrester or a plurality of surge arresters.

A number of embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a drawing of a steel cross-arm with integral lightning protection; Figure 2 is a schematic of a clamp for a bare conductor; Figure 3 is a schematic of a clamp for a covered conductor; Figures 4a and 4b are cross-sectional views of a metal cross-arm for unearthed pole tops with integral arc-gap protection in accordance with an embodiment of the present invention; Figures 4c and 4d are views corresponding to Figures 4a and 4b for earthed pole tops; Figures 5a and 5b are cross-sectional views of a metal cross-arm for earthed pole tops with integral arcgap protection in accordance with a further embodiment of the present invention; Figures 6a and 6b are respectively side and underneath cross-sectional views of a composite crossarm with integral arc gap protection in accordance with an embodiment of the present invention; Figures 6c and 6d are assembly views of the crossarm of Figures 6a and 6b; Figures 7a and 7b are respectively side and underneath cross-sectional views of a composite crossarm with integral arc gap protection in accordance with a further embodiment of the present invention; Figures 7c and 7d are assembly views of the crossarm of Figures 7a and 7b; Figures 8a and 8b are cross-sectional views of a metal cross-arm for unearthed pole tops with integral combined arc gap and surge arrester protection in accordance with an embodiment of the present invention; Figures 8c and 8d are views corresponding to Figures 8a and 8b for earthed pole tops; Figures 9a and 9b are cross-sectional views of a metal cross-arm for earth pole tops with integral combined arc gap and surge arrester protection in accordance with a further embodiment of the present invention; Figures 10a and 10b are respectively side and underneath cross-sectional views of a composite crossarm with integral combined arc gap and surge arrester protection in accordance with an embodiment of the present invention; Figures 10c and 10d are assembly views of the cross-arm of Figures 10a and 10b; Figures lla and llb are cross-sectional views of a metal cross-arm for unearthed pole tops with integral surge arrester protection in accordance with a further embodiment of the present invention; Figures llc and lld are views corresponding to Figures lla and llb for earthed pole tops; Figures 12a and 12b are cross-sectional views of a metal cross-arm for earthed pole tops with integral surge arrester protection in accordance with a further embodiment of the present invention; Figures 13a and 13b are respectively side and underneath cross-sectional views of a composite crossarm with integral surge arrester protection in accordance with an embodiment of the present invention; and Figures 13c and 13d are assembly views of the cross-arm of Figures 13a and 13b.

A range of embodiments of the present invention with various combinations of cross-arm material and protection devices will now be described with reference to accompanying figures 1 - 13. These include metal and polymer composite cross-arms which may be earthed or unearthed, in conjunction with simple arc-gap, surge arrester and combined surge arrester and arc-gap LPDs.

It is believed that all of these proposed embodiments would be suitable for use with both bare conductors and covered conductors.

As is shown in Figure 1, common to all of the described embodiments are insulating units 2 positioned on the top of a cross-arm 4. These units may consist of an outer insulating polymeric weathershed material covering a vertical metal core which may make an electrical contact with an overhead power line conductor 6 at the top and may attach to a LPD within the crossarm. The outer insulating weathershed may be of varying creepage length depending on the nature of the cross-arm below (i.e. whether it is of metal or composite construction) Attachment to the conductor at the top of the unit may vary depending upon the nature of the conductor. As is shown in Figure 2, for the case of bare conductors 8 attachment may be achieved using a split circular metal clamp 10A, 10B with an internal radius approximately the same as the conductor. The lower portion of the clamp 10A shall be continuous with the vertical metal core 12, with the upper portion of the clamp 10B used to clamp the conductor in place on the insulation unit 2 using two bolts equipped with wing nuts 14, or other fastening devices such as clamps or compression fittings.

As is shown in Figure 3, in the case of covered conductors 16, the conductor can be located onto the top of the insulation units 2 by a groove moulded into the insulation. Metal spikes 18 extending from the top of the metal insulator core 12 pierce the insulating sheath of the covered conductor enabling the core to again become energised to line voltage. The covered conductor 16 may be held in place and extra electrical insulation provided by a cover clamp 20 made of insulating material e.g. cross-linked polyethylene (XLPE), held in place as before with two wing-nuts 14. The unit may be greased to restrict moisture ingress.

The external connection of the conductor to the top of the insulator can be of the penetrating type as described above, or alternatively use a stripped section of conductor. The latter would be more efficient but also more time consuming to install and subject to corrosion problems.

Figures 4 and 5 illustrate two embodiments utilising a metal box section cross-arm assembly incorporating arc gap protection devices.

In Figures 4a and 4b the vertical metal rod 12 connecting through the insulator 2 to the conductor 6 is extended out through the bottom side of a metal box section cross-arm 22 and through a further insulation unit 24, as shown, to an earthing ring. This then enables easy access for earthing operations prior to working on lines using this construction. The rod may have an external loop or other suitable connecting device. Attached to each of these vertical metal rods are horizontal rods 26 enabling an arc-gap 28 to be created between each of the phases. These assemblies could be installed by means of narrow slots cut through the upper surface of the box section cross-arm and sealed after assembly. To prevent small animals or birds entering the centre of the metal box section end caps 30 may be installed at each end. These end caps could be a wire grating of slotted metal or composite construction to allow flash-over combustion products to escape. This embodiment would be most suitable for unearthed cross-arms. Figures 4c and 4d show a further embodiment for use with earthed cross-arms in which the earthing rings beneath the cross-arm are omitted.

Figure 5 shows a simpler embodiment where the vertical metal rods 12 descending from each of the conductors 6 are terminated an appropriate distance from the tips of further vertical rods 32 projecting upwards from the base of the cross-arm and thus forming an arc gap 28. Alternatively, the arc gap may be formed by the vertical metal rods 12 descending from each of the conductors 6 being terminated an appropriate distance from the bottom internal surface of the cross-arm.

Unlike the previous embodiment, this simplified embodiment would not require slots to be machined into the upper surface of the cross-arm for installation purposes. This simplified embodiment would be suitable for both earthed and un-earthed cross-arms, enabling lightning protection to be rendered either by a flash directly to earth or by a double flash using the cross-arm to conduct the flashover potential to an adjacent phase.

Due to the non-conductive nature of a composite cross-arm a number of novel solutions for lightning protection within the cross-arm are possible. Described below are two embodiments for lightning protection using arc-gaps within a composite cross-arm. In both of these embodiments the vertical metal rods which extend down from each of the conductors are extended through the base of the composite box-section cross-arm and bolted into place. This enables easy access for earthing operations prior to work on the lines In both cases an arc-gap of an appropriate distance is created between the vertical energised rods and a horizontal electrode isolated from earth passing each of them.

Figures 6a to 6d illustrate the construction and assembly method of an embodiment appropriate for a composite cross-arm 40 where a horizontal electrode consists of a rail 38 bolted into the side wall of the composite cross-arm. The rail passes each of the vertical metal rods 12 such that the nearest point to each corresponds to the appropriate arc-gap 28.

Lightning protection is achieved by a double arc-gap flash-over from phase to phase.

Figures 7a to 7d illustrate the construction and assembly method of a further embodiment intended for use with insulating or composite cross-arms 40. Internally, a moulded section of the composite construction has a groove 48 or other means to hold an internal metal plate 42. This plate has circular holes 50 through which the metal rods 12 are allowed to pass. The radial distances from each of the vertical rods to the edge of the holes are equal and correspond to an appropriate arc-gap 28.

The unit may have the central metal rod terminating in an external attachment as an earthing point.

Again, lightning protection is achieved by a double arc-gap flash-over from phase to phase. In use, the over-voltage may break down from one metal rod to the plate and then to the adjacent metal rod, the arc transferring then to a rod-rod discharge. There may thus be annular arc gaps of typically 20 mm for 11 kV operation. The use of substantial rods and the rapid transit of the arc root across the plate may ensure constant gap sizes and minimal erosion problems.

The above system could also be adapted for use with metal cross-arms, for example by using the cross-arm as the second electrode. It is proposed that this embodiment would be most suitable for un-earthed poles.

Below are described three embodiments of the invention relating to the construction of cross-arms with integral combined arc-gap/ surge arrester protection means. The arc-gap distance is envisaged to be considerably smaller than is normal for an arc-gap device alone. The advantage of a small arc gap combined with the arrester is that it disconnects the arrester from other line fault conditions (e.g. ferro-resonance, switching surges) which could damage the arrester. The disadvantage of the arc gap/arrester is that the onset of protection may be delayed by around 0.5 s, which could be critical in avoiding phase-phase breakdown.

These units would not be recommended for use at transformer poles, or OHL/cable junctions.

Figures 8 and 9 illustrate two embodiments where combined arc-gap/ surge arrester protection means 46 are used within a metal box-section cross-arm 22. It can be immediately seen that the basic concept used here mirrors that proposed for those of arc-gap protection only within a metal cross-arm, as shown in Figures 4 and 5. In this case however, it is proposed that appropriate surge arresters 44 should be placed in series with the arc-gaps 28 in order to form the combined arc-gap/ surge arrester means 46.

Figures 10a to 10d illustrate the construction and assembly method to be used for the manufacture of a composite cross-arm 40 incorporating combined arc-gap/ surge arrester protection means 46. As with the embodiment which uses a metal cross-arm with combined protection, it is proposed that combined protection means 46 within a composite cross-arm shall be achieved by placing an arc-gap 28 and surge arrester 44 in series between each of the phases. In this way lightning protection may be achieved from one phase to another via an arc-gap and arrester combination.

Below are described three embodiments relating to the construction of cross-arms with integral surge arrester protection means only. The first two of these embodiments would utilise metal box section cross-arm construction, with the third utilising a polymer composite box cross-arm construction. In all cases the design concept used mirrors those proposed for the construction of integral cross-arms with combined arc-gap and surge arresters as described above. In this instance however, the arc-gaps shall be removed and replaced with a continuous metal rod giving lightning protection by surge arrester means only.

Figures 11 and 12 illustrate embodiments where surge arrester protection devices 44 are used within the metal box-section cross-arm 22. The arrangements of Figures lla and lib and Figures 12a and 12b are suitable for unearthed poles, and the arrangement of Figures llc and lld is suitable for earthed pole tops. Figure 13a to 13d illustrates the construction and assembly method to be used for the manufacture of a composite cross-arm 40 incorporating surge arrester protection devices 44 only. These figures show an arrester arrangement whereby inter-phase protection only is provided. The arresters are mounted horizontally between the metal rods of the adjacent phases. The arrester units here may or may not have insulating finned sleeves. If an earthing point is desired, the rods may be continued to exit from the cross-arm with a suitable termination.

In many of the embodiments described above the metal rod and cross-arm can be quite substantial, and so it is unlikely that any major erosion will take place when the arc gap operates. Present arc-gaps in the UK system use electrodes of only 6-8 mm diameter. Also, especially in the case of the radial electrode arc-gap embodiment, the gap may be maintained at, say, 20 mm (for an 11 kV system) at each operation as the electrodes will not suffer 'burn-back' as the present duplex arc gaps do. Finally, the arc gap as envisaged will operate substantially faster than the APDs situated on the conductors. APDs generally have gaps of over 0.5 m. Even APDs with power arc devices (PADs) use gaps in excess of 90 mm.

In some of the embodiments the process of arc discharge may occur initially between one metal rod and the cross-arm, transferring eventually to a discharge between the metal rods alone. The rods can be substantial (e.g. 25 mm diameter) to reduce erosion /heating problems compared with transformer arcing horns of only 6 mm in diameter. The cross-arm is unlikely to suffer erosion damage as the arc will travel quickly across its internal surface. All arc debris may be contained within the cross-arm.

Several of the embodiments use a surge arrester protected from the environment and it is anticipated that insulated fins will not be required on the arrester in these embodiments. This eliminates the end seal problems of arresters and should increase their reliability. The cross-arm may be earthed or unearthed.

If earthed, the arresters should be of a suitable value to provide protection e.g. 12 kV or 15 kV (for an 11 kV system). If unearthed, the arresters are being used only as inter-phase protection and should be of a lower value, e.g. 6 kV or 9 kV. The cross-arm can be a totally metallic box-section construction, with louvered ends to restrict bird access or of an insulating, semi-conductive or composite material. In this case, an internal metallic connecting link must be provided between the lower ends of the arresters. If only surge arresters are used, the cross-arm may be completely sealed.

It is anticipated that the cross-arm assembly may be factory produced as a single unit, requiring only to be clamped in position by a linesman.

Although the above embodiments envisage either an insulation piercing connector or a clamp, it does not exclude the use of a compression fitting. It is envisaged however that the stability of the cross-arm mounting may reduce any vibrational problems that IPC mounted on the live conductor would have. Although the diagrams show pin insulators, the units can be adapted for use with section/terminal poles with tension insulators by using jumper leads to attach to suitable pin insulators or clamps on the cross-arm.

Similar embodiments to those described in detail above could, of course, be applied to cross-arms made of other types of conducting, semi-conductive or insulating material.

Claims (12)

1. A cross-arm for supporting overhead power lines, which is at least in part formed as a hollow tube and incorporating a lightning protection device enclosed within said hollow tube.

2. A cross-arm as claimed in claim 1, which is substantially wholly formed as a hollow tube.

3. A cross-arm as claimed in either claim 1 or claim 2, including an insulator mounted on the hollow tube to support an overhead power line, and a conductor enclosed within the insulator, arranged for connection at one end to an overhead power line supported by the insulator and extending at the other end into said tube for connection to the lightning protection device.

4. A cross-arm as claimed in claim 3, wherein said conductor extends through a side of the hollow tube opposite to the insulator for use as an earthing point.

5. A cross-arm as claimed in either claim 3 or claim 4, wherein said conductor is a rod.

6. A cross-arm as claimed in any of claims 3 to 5, including clamping means for connecting said power line to said conductor.

7. A cross-arm as claimed in any of the preceding claims, wherein said hollow tube is made at least in part from conducting material, and the lightning protection device is connected by means of said hollow tube to earth or to a second overhead power line.

8. A cross-arm as claimed in any of the preceding claims, wherein at least one end of the hollow tube perpendicular to the longitudinal axis of said tube is gas transmissive.

9. A cross-arm as claimed in any of the preceding claims, wherein the lightning protection device includes an arc gap or a plurality of arc gaps.

10. A cross-arm as claimed in claim 7 and claim 9, wherein said hollow tube forms an electrode of the arc gap or of each of the plurality of arc gaps.

11. A cross-arm for supporting overhead power lines as claimed in any of the preceding claims, wherein the lightning protection device includes a surge arrester or a plurality of surge arresters.

12. A cross-arm for overhead power lines substantially as herein described with reference to the accompanying drawings.