This invention relates to the suppression of oscilla¬ tion of an overhead supply line, which may be an electrical power conductor, or a telephone line. In particular, the invention relates to the suppression of oscillation in a substantially vertical mode known as galloping.

An overhead power conductor, for example, 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 sub¬ ject to transverse and vertical oscillation in windy con¬ ditions. 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 exacerbated if the conductor is also coated with ice or snow. The galloping is believed to be due in extreme clima¬ tic conditions to a build up of snow or ice on the conductor on the leeward side of a prevailing wind. The conductor 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 conductor to and support towers.

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

It is known to mount gallop suppressors at discrete loca¬ tions along an overhead line but this is a time consuming operation even though gallop suppression may need to be applied over only an exposed part of the route of the transmission line. In some instances the supply has to be switched off and the conductor lowered to the ground for such devices to be fitted. Some devices of about 4 meter length are fitted on to the conductor by having helical end portions that are twisted on to the conductor, but invariably gaps exist between the device and the conductor which can give rise to undesirable electrical discharges.

Cables of eccentric configuration are known, see for example US Patents 3 659 038 and 3 624 276, for use in reducing vibration of an overhead line, but such dampening can only be provided for during manufacture of the original cable, and gives rise to complex manufacturing processes.

US Patent 3 991 550 and Japanese Patent Publication No. 63-129806 disclose the addition of a helically wound member on to a line or cable to reduce "strumming" and noise respectively, but in each case the component that is added on to the conductor is of small lateral dimension (for example diameter) compared with that of the conductor.

It is an object of the present invention to provide suppression of the oscillation of an overhead line in a con¬ venient manner.

In accordance with one aspect of the present invention, there is provided an arrangement comprising an overhead supply line that extends in a catenary between two support members, and an elongate member substantially helically wrapped around substantially the whole length of the supply line, wherein the diameter of the elongate member is at least 50% of the diameter of the supply line, the elongate member being arranged to suppress oscillation, and par¬ ticularly galloping, of the supply line.

Preferably, the diameter of the elongate member is at least 75% of the diameter of the supply line, and may be 100% or up to about 120% of the diameter of the supply line.

It will thus be appreciated that the elongate member has a diameter that is at least an appreciable proportion of that of the conductor. This is significant in preventing the formation of an aerofoil, by the accretion of ice for example, and thus preventing galloping. The addition of an elongate member of relatively small diameter, as the speci¬ fications referred to above, on the other hand would not, at least to any significant effect, prevent the accumulation of ice in the lee of the conductor from forming an aerofoil configuration. Unlike the addition of a relatively small diameter fibre optic cable or the provision of one strand of a stranded conductor as a significantly larger strand than the others, the elongate member of the present invention is not required to serve any function other than to prevent oscillation of the conductor. That is to say it is not used for transmission of information or current along the con¬ ductor. The weight of the elongate member should therefore be minimised, whilst being of such a size as to perform its sole function. To this end, the elongate member is advan¬ tageously made of a polymeric material, preferably low den¬ sity and for example polyethylene, and may be of rod or hollow tubular configuration.

The supply line may generally be of circular cross- section, and in the case of the supply line being an electrical power conductor, may well be of stranded con¬ figuration; its "diameter" is then understood to be the diameter of a closely enveloping circle therearound. Although the elongate member used in the present invention advantageously has a circular cross-section, it may have any other appropriate cross-section that assists in preventing galloping. The "diameter" of a non-circular elongate member is then understood to be its lateral dimension extending substantially radially away from the supply line.

The oscillation suppression member is supported by the overhead line, but may also be secured to the support members.

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.

In accordance with a further aspect of the present invention, there is provided a method of suppressing oscillation, and particularly galloping of an overhead supply line that extends in a catenary between two support members, wherein an elongate member is wrapped substantially helically around substantially the whole length of the supply line, the supply line and the elongate member being of generally circular cross-section and the diameter of the elongate member being selected to be at least 50% of the diameter of the supply line.

It will be appreciated that the elongate member can, in accordance with the present invention, advantageously be mounted on to the conductor after the conductor is in its overhead position, suspended between the support members.

The invention is particularly though not exclusively applicable to the suppression of oscillation of a high voltage conductor, that is to say one carrying current at a voltage in excess of lkV, and typically at 132 kv or 275 kV, whether insulated or uninsulated. In such cases it is particularly advantageous that the elongate member has at least its outer surface of electrically insulating, and preferably non- tracking material, thereby to minimise electrical discharge activity on its surface, and preferably also has a low average permittivity. It is envisaged, however, that the material of the elongate member may have a volume resisti¬ vity that is lower than that of an insulating material (i.e. of the order about lO******** ohm-cm and above) and may be a semi- conductive material, having a volume resistivity down to

- D -

about the order 10^ ohm-cm. Advantageously, the elongate member is formed as a substantially homogeneous rod, pre¬ ferably of polymeric material and of low density, and may comprise a foam. The elongate member however may have internal reinforcement, and thus may be fibre reinforced. Accordingly, the elongate member can be arranged to have a high tensile strength combined with a low mass per unit length. The polymeric component can be chosen for its electrical properties and also for its resistance to weathering (water, acids, salts, ultra-violet radiation etc), whilst the tensile strength can be ov.! * -_.<\ primarily by the reinforcement. Advantageously, the polymeric material is a silicone-based material as described in EP-A-0218461 and the reinforcing material is KEVLAR. The reinforcing member or component of the elongate oscillation suppression member may advantageously be secured at each end to the overhead line support members, thereby conveniently to retain the elongate member in position on the overhead line.

Although the elongate oscillation suppression member may extend along the supply line as it passes over its support members, it is envisaged that either it would be removed from the line to form a detour at each support member, or it would be secured to, and terminated on the line a short distance away from each support member.

It is envisaged that the elongate member, extending substantially continuously between the support members, may comprise an outer insulating material as described herein and an inner conductive core. When the supply line is a conductor, bare or insulated, the conductive core of the oscillation suppression member may be terminated thereto for suppression of electrical discharges as described in our copending patent application filed contemporaneously herewith (Our Ref: RK377 and claiming priority from UK Patent Application No. 8817076.6), the entire contents of which are incorporated herein by this reference.

The addition of such a comparatively large diameter elongate member to an overhead supply line significantly reduces the aerodynamic uplift that is experienced in adverse windy conditions, especially where ice is involved, and thus tends to suppress galloping of the line. It will be appreciated that oscillation of the line may still occur, but the object of the invention is to suppress this so as to prevent the build-up of galloping to a level that would damage the line or its supports. The disruption of the air flow around the overhead line caused by the presence of the elongate member may also suppress the build up of ice on the conductor. The elongate member is thus required purely for a mechanical purpose; it is not required to carry electrical current or information.

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.

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 fibre optic cable in such an arrangement is not large enough with respect to the conduc¬ tor, typically being less than 25% of its diameter, to act effectively as a galloping suppressor. Furthermore, 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 diameter of the gallop suppressor in accordance with the present inven¬ tion has to be of at least twice this magnitude, that is to say, to be at least 50% of the diameter of the supply line, in order to suppress galloping of the supply line, espe¬ cially in adverse environmental conditions.

The typical diameter of an overhead conductor that would need gallop-suppression is between 19mm and 50mm, and for example may be 20mm for a 132 kV line. Typically the length of the oscillation suppression member would be up to about 400 metres.

An oscillation suppression arrangement in accordance with the present invention will now be described by way of example with reference to the accompanying drawing in which

Figure 1 shows schematically an oscillation suppression member mounted on a conductor of 275kV overhead electric supply system; and

Figure 2 shows a cross-section through the suppression member and the conductor of Figure 1.

Referring to the drawing, a stranded overhead power conductor 2 is supported in a catenary form by metal towers 4 adjacent each end. The conductor 2 is lead by jumper lines 6 past each earthed tower 4 and tensioned by means of insulators 8. A flexible, solid rod 10 of insulating and non-tracking material is helically wound tightly around the conductor 2 for the whole of its length between the jumper lines 6. The rod 10 has a circular cross-section that has a diameter 50% of the effective diameter of the stranded con¬ ductor 2. The rod acts to supress oscillation of the con¬ ductor 2 in adverse weather conditions.

The rod 10 is wound on the conductor 2 by a modified cable winding machine that is mounted on the conductor 2 at one of the supports 4 and then pulled along the conductor from the ground level by means of a tow rope. The winding

machine is arranged to rotate about the axis of the conduc¬ tor as it advances from one tower to the other.

As shown in Figure 1, the oscillation suppresser 10 is mounted only on one section of the conductor 2 but other sections along the path of the overhead line may be suppressed, wherever there is a danger of galloping or other oscillation.