GB2160011A

GB2160011A - Electrical conductors

Info

Publication number: GB2160011A
Application number: GB08513633A
Authority: GB
Inventors: William Brian Stark
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK
Priority date: 1984-06-05
Filing date: 1985-05-30
Publication date: 1985-12-11
Also published as: GB8513633D0

Abstract

An electrical conductor to suppress an electrical surge in the conductor, such as that resulting from a lightning stroke, having one or more conductive elements enveloped in a second element of lower conductivity in which about 60% or more of higher frequency components of current in the conductor are caused to flow. The one or more conductive elements may be of Cu or Al strengthened with steel or aluminium wires. The second element may be steel or an amorphous magnetic alloy and wound as a tape or foil around the first element. Adjacent turns of the winding may overlap or have gaps.

Description

SPECIFICATION Electrical conductors This invention relates to the construction of electrical conductors having the ability to suppress an electrical surge.

It is an object of the invention to provide an improved electrical conductor construction with an ability to suppress surges, particularly those induced in such a conductor.

According to the invention there is provided a surge-suppressing electrical conductor of a conductive first element to carry a load current and a conductive enveloping second element thinner than the classical skin effect thickness and of lower conductivity than the first element in conductive contact with at least part of the first element to provide, in operation, a conductor of increased overall resistance in which about 60% or more of the higher frequency components of a current in the conductor are caused to flow in a path of higher impedence.

Preferably the second element is of a magnetic material.

Conveniently the first element is of the conventional conductor materials such as copper and aluminium and may include arrangements of steel and aluminium wires to strengthen the cables in known manner.

The second element may be of ferromagnetic material such as steel, amorphous magnetic material and like alloys.

The second element may be of high-permeability material such as those alloys used for high-frequency transformer cores, e.g. Mu-metal and the like proprietary alloys. This material is conveniently in the form of a foil or tape which can be wound or otherwise formed around the first element and be in conductive connection therewith. The conductive connection can be continuous, that is over substantially the whole surface of the first element.

Embodiments of the invention will now be described, with reference to the waveforms in Figures 1 and 2.

A length of copper conductor, cross-section 12 mm x 1.5 mm, was wrapped with tightly wound turns of high permeability material (Mu-metal RTM) of lower conductivity than the copper. The material was in the form of a tape about 0.0025 mm (0.001 inch) thick and 12 mm (0.5 inch) wide. The turns were side by side. The turns are preferably placed without any gaps or overlap in a layer. Gaps can be left by spacing the turns. This reduces the effect achieved per unit length of conductor and can be useful to produce a specific performance on a long conductor. Tests were conducted with one and more layers. Subsequent layers were placed to cover the abutment of turns in a lower layer.

By the use of a Q meter the effect of one and more layers of tape on the dynamic resistance of the arrangement was measured.

The dynamic resistance was found to increase with the number of layers of tape and the electrical frequency at which the measurement was made. Estimates of the dynamic resistance of a copper conductor of the above-mentioned 12 x 1.5 mm size wrapped with three layers of high permeability tape based on these measurements produce values of about 6000 ohm/Km at 0.55 MHz and 9000 ohm/Km at 0.8 MHz. As the copper conductor has a dynamic resistance of only a hundred or so ohms per kilometer at 1 MHz the effect of the wrapping is very significant for any high-frequency component of the current in the conductor. (The d.c. resistance of such a copper conductor is about 1 ohm/Km.While the d.c. resistance of the assembly is higher than a copper conductor of the same overall cross-section.) To examine the effect of the wrapping on a surge in a conductor a current surge with a rise-time of some 8 microseconds was applied to a length of copper conductor. The addition of one or more wrappings of high permeability conductive material had a very noticeable effect. The surge approximates to the notional lighting stroke.

Figures 1 and 2 show waveforms which indicate the action of the wrapping. A test circuit was set up by which a surge of current could be passed through a length of conductor. A capacitor at the input (IN) and another at the outlet (OUT) provide measuring points. Figure 1 shows the effect of passing current surge through a length of bare copper conductor 4 mm x 1 mm x 600 mm long. The voltages (IN and OUT) on the capacitors at the ends of the conductor are almost identical. (The waveform sense is reversed for convenience of measurement and is actually a rise of voltage with time). When the same conductor is wrapped with one layer of abutting turns of the Mu-metal tape C mentioned above the waveforms in Figure 2 are obtained. These show the significant slowing of the rate of rise of the outlet voltage by the action of the wrapping. Additional layers, say three, increase the effect.

Any lightning stroke induced surge would be likely to be decaying again before the peak is reached as a result of the action of the enhanced dynamic resistance of the wrapping on the higher frequency components flowing in the stem of the conductor.

It will be apparent from the above that the construction according to the invention can be arranged to produce a required degree of action by choice of material and dimensions. The relation

is believed to apply, where Rw is the dynamic resistance in isolation of the wrapping of resistivity pw and permeability llw and Rc is the dynamic resistance in isolation of the conductor of resistivity pc.

The design of a conductor for a particular application will depend on the choice of material for the main conductor and the size required for the eventual conductor. The material of lower conductivity must not be so thick as to cause skin-effect loss at power frequencies. One possible guide to design is to make the skin thick enough to cause about 60% or more of a highfrequency component to flow in the lower conductivity material. The thickness of the lower conductivity material can then be determined from the values of resistivity-and the well-known skin-effect relationship. A thinner skin is usually required for a given effect when made of a magnetic material such as steel or amorphous magnetic material and an even better effect may be obtained with high permeability materials.The skin is to be thinner than the "classical" skin effect thickness, ie that at which the current density is reduced to about 30% of the surface value, to economise on the special materials used. The resistance of the conductor will be higher than that of an equivalent copper or aluminium conductor.

Conventional overhead conductors are of a number of strands such as steel and/or aluminium wires.

The outer layer of such wires may be designed or modified to exert the skin effect. The outer layer of wires can be of a material of the required impedance characteristic or the wires in the outer layer can be coated with such a material.

It is, of course, possible to make a conductor wholly or a material such as Mu-metal so that the skineffect is made use of to reduce the effect of a current surge but this would only be economic for short distances. As the skin may be conductive materials such as plastics or rubber insulators loaded with ferrite are not suitable.

The wrapping is to be only a small fraction of the diameter of the conductor so that the "skin-effect" is fully expoloited and the main part of the high-frequency currents flows in the lower conductivity material.

Surge-suppressing conductors as described above have various applications and can be included in cables. In long lengths they can be used for overhead transmission lines, with or without insulation and are particularly suitable for use in 11KV distribution networks. They can be used in shorter lengths in areas liable to lightning strike and at switch stations and like installations. They can be used for computer power supply connections and other localised protection.

The techniques described above can clearly be put into practice in various ways as will be apparent to those skilled in the art now given the above guidance by way of example only.

Claims

1. According to the invention there is provided a surge-suppressing electrical conductor of a conductive first element to carry a load current and a conductive enveloping second element thinner than the classical skin effect thickness and of lower conductivity than the first element in conductive contact with at least part of the first element to provide, in operation, a conductor of increased overall resistance in which about 60% or more of the higher frequency components of a current in the conductor ar caused to flow in a path of higher impedence.

2. A conductor according to Claim 1 in which the second element is of a magnetic material.

3. A conductor according to Claim 2 in which the second element is of ferromagnetic material, including steel, amorphous magnetic material and like alloys.

4. A conductor according to Claim 2 in which the second element is a high permeability material, such as Mu-metal.

5 A conductor according to any one of the preceding claims in which the second element is a foil or a tape.

6. A conductor according to Claim 5 in which the second element is wrapped around the first element.

7. A conductor according to any one of the preceding claims in which the second element is a plurality of layers in conductive contact.

8. A conductor according to any one of the preceding claims in which the second element extends along the whole length of the first element.

9. A conductor according to any of the preceding claims in which the second element is in continuous conductive contact with the first element.

10. A conductor according to Claim 1 in which the first element is a plurality of strands of conductive material.

11. A conductor according to Claim 10 in which the second element is a coating on at least some of said strands.

12. A surge-suppressing electrical conductor substantially as herein described.

13. An electrical cable including a surge-suppressing conductor according to any one of the preceding claims.