This invention relates to the suppression of oscilla¬ tion of an overhead electrical conductor. In particular, though not exclusively,the invention relates to the suppression of oscillation in a substantially vertical mode, known as galloping.

An overhead power conductor is suspended in a catenary shape between two support towers, which may be 400 meters apart. These power transmission lines often extend across exposed areas of countryside and are thus subject to trans¬ verse and vertical oscillation in windy conditions. Their tensioning is such that vertical oscillation, i.e. galloping, especially in extreme conditions, can put undue strain on the conductor and its supports, which is exacer¬ bated if the conductor is also coated with ice or snow. The galloping is believed to be due in extreme climatic con¬ ditions to a build up of snow or ice on the conductor on the leeward side of a prevailing wind. The conductor can be formed into an aerofoil shape that generates lift. As the conductor rises, the angle of attack presented to the wind by the aerofoil changes, and this is usually accompanied by rotation of the conductor. The resultant effect is that the conductor then falls. This motion can occur at different times along the length of the conductor leading in extreme cases to violent damaging movement. Furthermore, when several conductors extend parallel with one another adjacent conductors may clash together, and this can cause a phase- to-phase fault current that disrupts the power supply and may burn the conductors.

In gentler climatic conditions less violent movement can still occur that leads to aeolian vibrations that can cause damage to the physical connection between the overhead con¬ ductor and the support towers.

Furthermore, an overhead conductor can generate acoustic noise under certain wind conditions.

It is known to mount gallop suppression devices on an overhead line, which act by reducing the aerodynamic lift of the conductor. Such devices are typically about 4 meters in length and are formed from rigid electrically insulating polymeric rod. Each end of the rod is given a helical con¬ figuration so that it can be wound on to the conductor and mechanically supported thereby. However, the polymeric rod distorts the electrical field around the conductor and can consequently give rise to discharges that cause erosion of the polymer and also produce electrical interference with neighbouring electrical equipment. Along the major portion of its length the rod may either be tightly wound around the conductor or spaced a significant distance away therefrom. It has been found that the major location of the above- mentioned discharges is around the end regions where the insulating rod is mechanically secured to the conductor, but they also occur elsewhere along its length in the neigh¬ bourhood of points of contact with the conductor, whether it is insulated or not.

In accordance with one aspect of the present invention, there is provided an arrangement comprising an overhead electrical conductor and an elongate oscillation suppression member, the oscillation suppression member comprising an outer electrically insulating component and an inner electrically conductive component which extend longitudi¬ nally thereof, the oscillation suppression member being mounted on the conductor so as to extend therealong and with the conductive component electrically connected to the con¬ ductor at least at one location along its length.

The oscillation suppression member acts not only to prevent oscillation such as galloping and aeolian vibration, but can also reduce acoustic noise, by disturbing the flow of air over the conductor.

Particularly when the oscillation suppression member is mounted away from an end of the conductor, the inner conduc-

tive component may conveniently be electrically connected to the conductor at one or each end of the member. It is not intended however that the conductive component carry any significant amount of current in comparision with that flowing along the overhead line. The electrical connection may be accompanied by direct mechanical connection, or a further conductive component may electrically interconnect the inner component of the suppression member to the conduc¬ tor. Alternatively, the electrical connection may be made partway along the length of the suppression member, and the conductive component may be sealed into the outer insulating component at each end of the oscillation suppression member.

The arrangement of the invention, in ensuring that the conductor and the conductive core of the suppression member are at the same electric potential, thus effectively short circuits any gaps between the oscillation suppression member and the conductor across which discharge activity could otherwise take place.

The major, intermediate portion of the length of the oscillation suppression member may be tightly wound around the conductor, for example helically, or it may extend generally parallel thereto or otherwise around or alongside the conductor at a distance sufficiently far therefrom such that at the operating voltage of the conductor and in the ambient atmospheric conditions substantially no discharges can take place between these two components.

The oscillation suppression member may be a solid elongate member, for example with the insulating component extruded on to the conductive component, or the insulating component may be a hollow tubular component in which the conductive member is located.

The conductive component may form a supporting core for the insulating component, which may be moulded or extruded thereon for example. Preferably the conductive component

comprises a metal, but is may comprise a conductive polymer or a material containing carbon fibre. The conductive com¬ ponent may be in rod form, for small diameter oscillation suppression members, up to about 5mm, or tubular for larger diameter members. A tubular conductive component may be a thin wall metal tube, and this may be filled with polymeric or other, preferably low density, material. The filler may be a foam, a rod or itself a tube, and since it is located in a region that is free of any electrical field its electrical properties are irrelevant.

It will be appreciated that although in general the oscillation suppression member will have a substantially circular cross-section, other cross-sections may be suitable.

The insulating component of the oscillation suppressor is advantageously also electrically non-tracking, and is preferably a polymeric material, for example that disclosed in EP-A-0218461.

One oscillation suppressor, or a plurality of oscilla¬ tion suppressors spaced apart, each of length about 4 metres, or other suitable length, may be mounted on a cate¬ nary length, usually up to about 400 meters, of overhead conductor supported between two towers, in regions where the power transmission line is exposed to adverse, such as windy and/or icy, conditions. Alternatively, a single oscillation suppressor in accordance with the present invention may be mounted so as to extend along substantially the whole length of the conductor, as disclosed in our patent application filed contemporaneously herewith (Our Ref: RK376 and claiming priority from UK Patent Application No. 8817075.8), the entire contents of which are incorporated herein by this reference.

The oscillation suppression member of the arrangement of the present invention achieves the aerodynamic effect of

suppressing oscillation, and particularly galloping, without the penalty of appreciable weight being added to the conduc¬ tor, since its configuration inherently allows tubular mem¬ bers and/or foamed material and/or other material to be used as or within the conductive member which without such electrical screening protection would be unable to withstand the large electric field intensity associated with overhead power distribution lines operating at say 132 or 275 kv.

In accordance with another aspect of the present inven¬ tion, there is provided a method of suppressing the oscilla¬ tion of an overhead electrical conductor wherein at least one elongate oscillation suppression member is mounted thereon, said member comprising an outer electrically insu¬ lating component and an inner electrically conductive com¬ ponent which extend longitudinally thereof, wherein the oscillation suppression member is mounted on the conductor such that the conductive component is electrically connected to the conductor at least at one location along its length.

The method preferably employs an arrangement as described herein.

It is to be understood that whether the oscillation suppression member is a discrete component over a larger or smaller length of the overhead conductor, or whether it extends along the entire length of the conductor, it can conveniently be mounted on to the overhead line not only after manufacture of the conductor itself, but also after the conductor has been tensioned overhead on its tower supports.

The elongate member of the arrangement of the invention is flexible to the extent that it can conveniently be wound around the supply line and can be drawn as a tight fit into contact along substantially its whole length with the supply line. Furthermore, the diameter, or other transverse dimen¬ sion, of the inner conductive component is advantageously as

large as possible, to minimise discharge activity associated with a fine wire configuration, consistent with it being wrapped around the conductor.

The elongate member may be helically wound on to the overhead line by a machine similar to that used for attaching fibre optic communication cables to overhead lines as described in Distribution Developments, September 1982 at page 3 et seq. for example. The flexibility of the elongate member of the arrangement of the invention is such that it can conveniently be wound on to a drum for subsequent unwinding in a helical manner on to the overhead supply line. Such a machine is used to wind a fibre optic cable having a non-tracking and insulating outer jacket along an overhead conductor, and such an arrangement is disclosed in EP-A-0112163. However, the oscillation suppression member of the present invention does not contain any optical fibres, and so is more robust and needs no special care during installation.

The flexibility of the oscillation suppressor of the present invention is used to advantage in that it can con¬ veniently be mounted on a very wide range of diameters of supply line, unlike the known rigid oscillation suppressors which can be fitted to a line of one diameter only. Furthermore, the conductive component may be helically wound around the overhead line at one, and preferably each, end of the oscillation suppression member not only to provide the necessary electrical connection, but also to secure the member mechanically to the line and prevent any longitu¬ dinal movement therealong.

One embodiment of the present invention will now be described by way of example with reference to the accom¬ panying drawing, which shows one end of a gallop suppression member 2 secured to an overhead high-voltage conductor 4. The suppressor 2 has an insulating and non- tracking polymeric outer sheath 6 on a metal core 8. Along

the major portion of its 4 metre length, the suppressor 2 is helically wound as a close fit around the conductor 4. It will be appreciated that in any event small gaps will always exist between part of the suppressor 2 and the conductor 4 when the conductor is stranded, and that larger gaps between the suppressor 2 and the conductor 4, whatever its con¬ figuration, may occur in time due to slippage between them. At each end (only one of which is shown) where the suppressor 2 naturally tends to curl away from the conduc¬ tor, the conductive core 8 emerges from the suppressor 2 and is wound tighly around the conductor to ensure good electrical and mechanical connection thereto. Electrical contact of the core 8 with the conductor 4 is enhanced by removing some of the polymer sheath 6 radially of the core where it emerges, as can be seen in the enlarged sectional view A. Accordingly, any electrical discharge activity in any air gap 10 that may exist between the end of the suppressor 2 and the conductor 4, and between any other spaced apart portions of the suppressor 2 and conductor 4, is effectively prevented.

In a laboratory experiment, two samples of oscillation suppressor were compared. Each had an outer diameter of about 10mm and an outer component of the same insulating elastomer. One sample had an inner component that consisted of 5mm diameter aluminium rod, whilst the reference sample had an inner component of an insulating fibrous material. Both samples were mounted on a smooth copper tube of 22.2mm diameter (representing the overhead conductor). The one sample had its metal core connected to the metal tube. The voltage applied to the copper tube was increased and the onset voltage at which visible electrical discharges occurred around the polymeric surfaces of the suppressors was noted. The experiment was repeated with two further identical samples mounted on a copper tube of 28.0mm diameter. The metal-cored samples gave significantly better results in each experiment. On the 22.2mm and 28.0mm copper

tubes, the discharge onset voltage for the metal-cored sample was 14% and 13% higher respectively than for the all- polymeric references samples. In practice and very signifi¬ cantly, this would mean that the sample in accordance with the invention could be used on a 28.0mm conductor of a 275 kV power transmission system whilst the all-polymeric sample could not.