The present invention relates to a capacitive power transmission cable.

US Patent No. 1 ,825,624 describes and claims:

1. In an electrical power transmission system, a source of alternating current, a receiving circuit, a transmission circuit for interconnecting said source and said receiving circuit and a distributed capacitance interposed in series relation with said transmission circuit and having a value sufficient substantially to neutralize the inductive reactance of said transmission circuit for increasing the power limit of said system.

The abstract of US Patent No. 4,204,129 is as follows:

This invention relates to the transmission of electric power and in particular provides an electric power-transmission system having reduced vector regulation, voltage drop, and power loss through the inclusion of capacitance in the cable in series between the generator and load by utilizing electric conductors, i.e., connective links, having capacitance distributed along the length of the cable. Such capacitance is achieved by dividing a conductor into two parts which are separated by dielectric material such that the two conductor parts are in capacitive relation along the length of the cable and by connecting one conductor part to the generator and the other conductor part to the load such that the distributed capacitance is in series with the generator and load.

In WO 2010/026380 there is described, in terms of its abstract and with reference to Figure 1 hereof:

A charge transfer zero loss power and signal transmission cable comprising, eight lengths of an electric conducting material (18), being layered in alignment, one on top of the other, each of which can be electrically jointed to give any required length. Each of the conductive layers is separated from each other by alternate layers of a dielectric material (19). The conductive layers (10-17) are formed into a charging folded closed loop (20) and a discharging folded closed loop (21) with the apex of the fold (22) of each folded closed loops in opposition to each other, being the ends of the cable, are separated from each other by a dielectric material (19), thereby making capacitive contact and is the means to transfer an electric charge from the said charging loop to the discharging loop, thereby transmitting an alternating current from a power supply to a point of transmission, with substantially zero resistance, by the said two charging and discharging loops, thereby transmitting power from a power supply over a given distance, to a point of transmission with zero power loss.

It is surprising that such a capacitive cable is capable of transmitting data and/or power over a long distance with low, if not completely zero, loss. Our tests have confirmed this.

For this cable, the loop formation is taught to be essential. We believe that the loop formation is not essential.

Litz wires and Milliken conductors are known and consist respectively of fine wire strands and thicker wire strands insulated from each other, typically by so called “enamel” which is polymer based as used on magnet wire, and bundled together usually with twisting. They reduce skin effect which would reduce the conductive capacity of a single round conductor with the same amount of conductive material per unit length. In Milliken conductors, the wires are not always insulated from each other, particularly where they are arranged in six segments insulated from each other. The normal extent of insulation of the wires from each other in Milliken conductors is “light” ¹ .

Litz wires and Milliken conductors are not suitable as such since the former are suitable for light duty and Milliken conductors have only light insulation.

In US patent No 3,164,669, there is described a similar cable, of which the strands are of half-hard copper wire for pulling into a pipe. Selected strands/wires are enamelled to reduce the overall skin effect in the cable. The arrangement described is:

¹ http://www.electropedia.org/iev/iev.nsf/display?openform& ;ievref=461-01-15 Thus an effective construction in a 127 -strand conductor might be the following:

Center wire - bare 6-wire layer - all wires enamelled 12-wire layer - alternating bare and enamelled 18-wire layer - all wires enamelled

24-wire layer - alternating bare and enamelled 30-wire layer - all wires enamelled 36-wire layer — alternating bare and enamelled Figure 1 hereof is Figure 2 of this US Patent. The patent emphasises:

In addition to the fact that the polyurethane enamel may be baked at a temperature which is lower than the temperature which would anneal the individual wires, the polyurethane enamel has the important advantage that it will decompose on application of a temperature of the order of molten solder (approximately 600°) and the products of decomposition have a fluxing action. Thus the concentrically stranded enamel conductor may the spliced simply and solder bonds effected with the usual equipment.

Leaving aside the feature of the strands/wires being some bare and some enamelled, we refer to the layer structure of 6,12,18,24,30,36 wires as the “multiples of six layer” structure, that is to say each layer have a multiple of six strands and each successive radially outer layer having another six strands.

In a Modem Power Systems paper entitled “Capacitative transfer promises significant reduction in losses” dated 15 ^th May 2018 and available at https://www.modempowersvstems.com/features/featurecapacitati ve-transfer- promises-significant-reduction-in-losses-6150871 /. there is described:

Figure 2. A cross sectional representation of a Type III cable. Each of the individual bundles comprises 6 upstream electrodes (connected to supply) and 6 downstream electrodes (connected to load). Each bundle is composed of twisted, insulated wires and then the bundles are twisted as a group to form a cable which is then sheathed according to applicable international standards.

This “Figure 2” is reproduced herein as Figure 2. Please note that: • the numbering in this Figure 2 is 1 to 12 of the strands in each bundle of strands;

• this document is referred to below as “Document 6150821”.

In our PCT/GB2019/051593, which is unpublished at the priority date of this application, we have described and claimed:

A capacitive power transmission cable comprising at least two sets of conductive strands, the sets of strands being insulated from each other and in capacitive relationship, the one with the other.

In a paper entitled “Capacitive Transfer Cable and Its Performance in Comparison with Conventional Solid Insulated Cable” given by Drs Yang Yang and Danvish at the IEEE conference in Calgary in June 2019, it is stated:

“ Figure 3 presents another type of CTS cable with multilayer structure. Grey strands are the input wires. Blue strands are the output wires. Other layers are the same as traditional cables. ”

Figure 3 referred to here is Figure 3 of the accompanying drawings. In it the latter, the named annotation has been replaced as follows:

Outer sheath A

Metallic sheath B

Semi-conductive layer C

Insulation D

Semi-conductive layer E

Input Strand conductor F

Dielectric/enamel layer G

Output strand conductor H

The object of the present invention is to provide an improved capacitive, power transmission cable.

According to the invention there is provided a capacitive power transmission cable, comprising: • at least two sets of conductive strands, the sets of strands being insulated from each other and in capacitive relationship, the one with the other; wherein:

• the strands are laid in alternating layers of all one set and then all of another set;

• all the strands of both sets being bare and

• inter-layer insulation is provided for insulating radially adjacent layers from each other.

Preferably, the conductive strands are laid at least substantially in a multiples of six layer structure, with substantially equal numbers of strands of both sets.

To aid the identification of the strands of the two sets for their connection, the strands of one set can be plain copper and those of the other can be tinned. Thus one set will be copper coloured and the other the colour of tinning.

Whilst the two sets of strands can be wound with differing helical angles from one layer to the next, preferably the helical angles of one layer are equally and opposite that of the next.

Normally the layers will laid in a multiples of six layer structure around a single central former. This could be of a reinforcing material such as steel where the other strands are of copper or aluminium. Alternatively, where cable is of adequate strength with needing reinforcement, the central strand can be of polymeric material, typically polyester.

In certain layers the multiple of six structure may result in slight gaps in the strands. This is because in the absence of inter-layer insulation the circumference of the layers increases with their diameter, but in the presence of interlayer insulations, the increase in diameter from one layer to the next is in proportion to the insulation thickness in addition to be the wire diameter, whereas the circumference is occupied by wires of their diameter alone. To accommodate this, certain layers may be provided with compensatory additional strands beyond their strict multiples of six layer structure number. This can be expected to provide a small order only difference in capacitance per unit length of the cable.

To help understanding of the invention, two embodiments and a variant thereof will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 is Figure 2 of US Patent No 3,164,669;

Figure 2 is Figure 2 of Document 6150821;

Figure 3 is Figure 3 of the above referenced 2019 IEEE paper;

Figure 4 is a view similar to Figure 1 of a capacitive power transmission cable of the invention without external sheathing;

Figure 5 is a full end view of the cable of Figure 3 with external sheathing.

Referring to the drawings, a capacitive cable 1 comprises five layers 2, 3, 4, 5, 6, of two set of copper strands 8,9, which alternate from one layer to the next layer.

The layers are laid around an inner of polyester 10, having a diameter such that twelve, bare copper strands 8 of layer 2 are able to be wound around it tightly abutting each other. The strands of the two layers are at opposite helical angles a to each other. Typically the strands be of 13AWG - 1.82mmOD. Around the strands of this layer are wound insulating tape of polyester or PET for interlayer insulation 11.

This is provided to be 0.2mm thick.

Next eighteen strands of tinned copper strands 9 of the same gauge are wound as layer 3, with another 0.2mm insulating layer of the same type. The strands of the two layers 2,3 are at opposite helical angles a to each other. The successive layers 4- 6 each have six more strands more strands than the last, with their material alternating plain, tinned, plain. The same insulation is provided between each pair of layers.

The resultant capacitance per unit length of this cable is 17nF/m. Outside the outer sixth layer, there will normally be the usual insulating, protective and outer layers 15 of underground power cable.

There may be one or more fewer or additional layers of conductive strands for lesser or great power capacity.