Background of the Invention Electrical power distribution. at least in the UK. is effected through the grid using a mix of two systems, being overhead lines carried on pylons and underground cables.

Both systems have advantages and disadvantages. as outlined in a booklet"Overhead or Underground"published by The National Grid Company plc, which operates 7000 km of overhead lines and 600 km of underground cables in the UK. which concludes that overhead lines have significant advantages compared to existing underground cable systems. Thus. overhead lines are considerably cheaper to install (some £0. 5 million per kilometer compared to some £10-12 million for underground) but are prone to failure, particularly in icy conditions ; while the hidden nature of underground cables results in their damage through inadvertence or carelessness during building work or in the installation of other services. Underground cables are also less prone to routine failure resulting for instance from ground movement. water ingress, excessive rates of heating and cooling but, upon failure take <25 times longer to locate and repair the fault than overhead failures. Pylons, whilst relatively simple to erect, are an eyesore and require not only maintenance but also anti-vandal measures, whilst the land in the vicinity of a pylon cannot be farmed and the pylon presents an obstacle to various farming activities.

Underground cables require minimum maintenance and do not inhibit farming but, upon installation, cut a swath across the countryside of <30 m width. Also, a relatively expensive cable construction is required, as these require an aluminium or lead protective sheathing to prevent moisture ingress.

With either system, heat losses during transmission are unavoidable, passing from overhead lines to the surrounding air. and from underground cables sometimes causing excessive heating with resultant expansion/contraction problems.

From the health and safety aspect. overhead lines have in recent years been required to carry ever increasing voltages, e. g. 175.000. then 275. 000. now 400. 000 volts, and in more recent times a possible adverse health effect has been suggested on persons living or working in the vicinity of overhead lines.

Object of the Invention A basic object of the invention is the provision of an improved, underground power cable conduit, a method of installing not only the conduit but electrical power cables, and an electrical power transmission system.

Summary of a First Aspect of the Invention According to a first aspect of the invention, there is provided a power cable conduit for installation underground. comprising: (i) an outer pipe ; (ii) a sleeve of synthetic plastics material located coaxially. or generally so, with respect to the pipe. with the sleeve separated by spacers from the internal surface of the pipe to define a first-annulus :

(iii) a central tube. also located co-axially. or generally so. with respect to the pipe and also of synthetic plastics material, located within the sleeve. with the central tube spaced from the internal surface of the sleeve to define a second annulus: (iv) a plurality of cable tubes of synthetic plastics material located in an array around the external periphery of the central tube: (v) a plurality of electrical power cables each extending, with clearance. along some or all of the cable tubes; (vi) the central tube being adapted to convey a heat transfer medium therealong : and (vii) the cable tubes and/or the second annulus. being adapted to convey a heat transfer medium therealong.

Advantages of the Conduit in accordance with the First Aspect of the Invention The conduit provides for the safe accommodation of say 12 electrical cables in 12 cable tubes. capable of conveying typically 400.000 volts at 1500-2000 amp. with heat transfer medium normally being a cooling medium to remove heat under controlled conditions by. e. g. water and/or air cooling and. if required, with heated water and/or air re-introduced into the conduit, with allowances in the cable tube for cable expansion and contraction.

The safe installation of the cables means that the need for expensive sheathing can be avoided by the cable manufacturers, as water ingress is not the problem, thus reducing cable costs.

The heat emitted during power transmission is no longer uncontrollably lost to ambient air or to the ground. but is retained in the thus heated medium. e. g. water, air etc.. and hence remains available for beneficial use. e. g. the fluid may be conveyed to a heat exchanger means for providing. e. g. greenhouse heating.

Preferred or Optional Features The heat transfer medium for the cable tubes is air, oil. and inert gas, or, if the cables are designed for submersion in water. water.

The heat transfer medium for the central tube is water.

The outer pipe is of a high strength material such as concrete or steel, so as to give protection to, and a warning of. the electrical cables housed within.

The tube. centre core. and cable tubes are of polyethylene, polypropylene or R. T. P.

(reinforced thermo plastic).

The outer pipe is constructed from individual pipe lengths (eg <20 m). with water seals between adjacent ends of adjacent pipe lengths. The sleeve is constructed from individual sleeve lengths (eg < 20 m) secured end-to-end on site by butt-fusion.

The array of central tubes and plurality of cable tubes is. in one embodiment, produced as a single extrusion.

The central tube and cable tubes are produced on site in substantial lengths. eg I kilometre, by on site extrusion butt fusing and de-beading (in the known manner) finite lengths.

In an alternative embodiment. a circumferential cradle is located along the central tube. with the external periphery of the cradle providing a plurality of concave seating recesses matched to the external periphery of each cable tube. whereby a cable tube is seated and secured to each recess.

The cradle is discontinuous. comprising cradle hoops. e. g. at 1. 5m spacing secured. e. g. by electrofusion or by bolts, around the central tube.

The cable tubes are secured to the cradle by banding.

The cradle has twelve concave recess at 30° spacing.

Skids. e. g. three at 120"locations. are secured around the cable tubes by the same

banding.

The cable tube skids incorporate a hydrophilic expansion agent so that. once in place the assembly of central tube and cable tubes can be locked to the sleeve.

The spacers of the sleeve are also skids, e. g. three at 120°, four at 90° or more. secured by banding.

Low voltage cables, telecommunication cables, sensors and information cables are located in the first annulus.

The electrical power cables have a profiled external surface provided by blisters or ribs for example. to ensure maintenance of an air space.

Temperature sensors are provided at suitable intervals to assist in determining the cooling medium flow rates required for satisfactory temperature control, particularly for controlled cooling following a period of high power demand and resultant high temperature rise of the cables.

Summary of A Second Aspect of the Invention According to a second aspect of the invention. there is provided a method of installing an underground power cable conduit comprising : (i) installing pipe lengths at a suitable depth underground with sealed joints between adjacent ends of adjacent lengths to define an outer pipe; (ii) hauling a sleeve of synthetic plastics material in substantial lengths <1 kilometre. coaxially through and along the outer pipe using centralising spacer skids; (iii) hauling an assembly of central tube. and cable tubes coaxially through and along the sleeve. using centralising spacer skids : and (iv) hauling electrical power cables through and along some or all of the cable tubes.

Whilst step (i) can. if pipe jacking is employed, be effected over 400m runs. steps (ii) (iii) and (iv) can be effected over lengths of typically 1 kilometre, provided service stations are excavated at 1 kilometre distances for housing cable joints, pumps for cooling medium, inlet and outlet systems for cooling medium etc. Step (i) can alternatively be effected by open cut if this is more suitable for particular site conditions, which alternative is not restricted to 400m runs.

Preferred or Optional Features of the Second Aspect of the Invention Typical dimensions for the pipe lengths are 1500 mm O/D. 1300 mm I/D. installed in known manner either by open cut with suitable material compaction above. or by pipe jacking with a prior micro-tunnelled hole. e. g. over 400 m runs. between a launch pit at one end of the run, and a receiver pit at the other end of the run, typically to a depth of 3.5 m. to give 2 m ground cover.

Finite lengths to create the sleeve 3 may be supplied on site and butt-fused end-to-end. and progressively hauled along the pipe 2.

On-site extrusion or butt-fusion is effected to create the central tube and the cable tubes as an on-going operation. as the assembly is hauled through and along the sleeve.

Typically the sleeve is of 1000 mm O/D, 885mm I/D, the central tube is of 400 mm diameter, and the cable tubes are of 160mm O/D. 142mm I/D., which tubes can accommodate cables <133mm O/D.

Summary of a Third Aspect of the Invention According to a third aspect of the invention. there is provided an underground electrical power transmission system. comprising: (i) a length of underground conduit in accordance with the first aspect extending

between minor excavations spaced at a first distance. typically 1 km apart.

(ii) heat exchanger means at each excavation to extract heat from the cooling medium of the cable tubes and to transfer such heat to the cooling medium of the central tube, and (iii) major excavations spaced at a second distance, typically 10 km apart : and (iv) heat exchanger means located in or at each major excavation. to extract heat from the cooling medium of the central tube.

Advantages of the Third Aspect of the Invention The cables may be produced economically in. e. g. 1 km lengths, thereby avoiding the need for jointing except at the first distance, e. g. 1 km. with a first heat transfer step from cable tube cooling medium to central tube heating medium every 1 km, and with transfer from the cooling medium of the central tube to a beneficial use. e. g. the heating of farm buildings. greenhouses. domestic dwellings etc.. rather than the heat being dumped and atmosphere being affected every 10 km.

Clearly. connections are required at the minor and major excavations between the electrical cables and also between the central tube etc to heat exchanges, pumps etc following standard engineering principles.

Brief Description of the Drawings One embodiment of underground power cable conduit in accordance with the first aspect of the invention. adapted to be installed in accordance with the second aspect of the invention. and adapted for use in the power transmission system in accordance with the third aspect. is shown in the accompanying diagrammatic drawings. in which:

Figure 1 is a sectional view through a conduit in accordance with the first aspect, and Figure 2 shows the method of installation in accordance with the second aspect ; and Figure 3 shows the power transmission system in accordance with the third aspect.

Detailed Description of the Drawings In Figure 1 is shown a power cable conduit 1 for installation underground.

The conduit 1 comprises an outer pipe 2 of steel or concrete, typically of 1.5 m O/D. and 1. 3 m I/D. produced in unit length such that they may be installed by the well known pipe jacking technique. after excavating a so-called micro-tunnel to 1. 5m diameter by well known micro tunnelling techniques.

A sleeve 3 of synthetic plastics material, such as polyethylene, polypropylene or R. T. P.. is located coaxially. or generally so. with respect to the pipe 2, with external surface 4 of the sleeve 3 separated by three 120 °. or four 90 ° spacers 5 from internal surface 6 of the pipe 2 to define a first-annulus 7, preferably with at least some of the spacers 5 incorporating a hydrophilic agent.

A central tube 8. also located co-axially. or generally so, with respect to the pipe 2 and also of synthetic plastics material, such as polyethylene, polypropylene or R. T. P.. is located within the sleeve 3. with its centre 9 water filled, and spaced (as will be described later) from the internal surface 10 of the sleeve 3 to define a second annulus 11.

A plurality of twelve cable tubes 12 of synthetic plastics material, such as polyethylene. polypropylene or R. T. P., are located in an array at 30° spacing around the surface 9 of the central tube 8. and in at least some of the tubes 12 is housed an electric power cable 13 each extending with clearance 14 along some or all of the cable tubes 12.

Around the central tube 8 are fitted a plurality of circumferential cradle hoops 15.

typically at 1.5 m spacing. along the central tube. with the external periphery of each cradle providing a plurality of concave seating recesses 16 matched to the external periphery 17 of each cable tube 12. whereby a cable tube 12 may be seated and secured to each recess 16. The cradle hoops 15 are secured to external surface 9 of the central tube 8 by electrofusion or by bolts.

The cable tubes 12 are secured to the cradle in their recess 16 by banding 18.

Twelve skids 19 at 30 ° locations, are secured around the cable tubes 12 by the same banding 18. at least some of the skids 19 incorporating a hydrophilic expansion agent so that, once in place the assembly of central tube 8 and cable tubes 12 can be locked to the 3.

Lengths to form the sleeve 4 are secured together end-to-end by butt-fusion. on site.

The central tube 8 and cable tubes 12 are produced on site in substantial lengths, eg 1 kilometre, by on site extrusion butt fusing and de-beaded (in the known manner) finite lengths.

Low voltage cable 20. telecommunication cables, sensors and information cables 21 are located in the first annulus 7 in tubes 22 and 23, whilst the electrical power cables 14 have a profiled external surface provided by blister or ribs 24 for example, to ensure maintenance of the clearance 14 for air flow.

With a multi-kilometre stretch of conduit 1, a service station 25 is excavated at 1 kilometre spacing, each station 25 housing fluid pumps, and other ancillary equipment. It follows that cables 13 can be drawn out, and replaced, from excavation to excavation.

Fluid, eg water. is pumped along the centre 9 of tube 8 at a suitable flow rate and pressure eg 10 bar. with industry-standard temperature sensors (not shown) provided at suitable intervals to assist in determining the cooling medium flow rates required for satisfactory temperature control, particularly for controlled cooling following a period of high power demand and resultant high temperature rise of the cables.

A method of installing an underground power cable conduit 1 is indicated in Figure 2.

The outer pipe 2 is produced from a plurality of individual pipe lengths-typically industry-standard concrete or steel pipe lengths. < 20 m and are installed by the well known pipe jacking method-briefly a pit is dug to receive hydraulic rams and a pilot tunnel is firstly produced by well known techniques. Then pipe lengths are lowered successively into the pit and jacked forwards with suitable water seals between the trailing end of the preceding pipe length and the leading end of the next pipe length. Pipe jacking can be effected typically over 400 m, so that a pit must be excavated every 400 m.

However. the pipe 2 could be installed by open cut should site conditions indicate this to be preferable.

Once the pipe 2 has been installed for 1 K. a service station 25 is excavated, and similarly at every 1K distance. Within the service stations 25 are located pumps, manifolds etc.

At a first station 25. the sleeve 3 is introduced into the pipe 2. The sleeve 3 is supplied on site in unit lengths, which are secured end-to-end by the well known butt-fusion technique.

The sleeve 3 is hauled along the pipe 2 on its skids 5 by attaching one or more wire ropes to the leading end of the sleeve 3. and installing a winch in the next service station 25.

Also at the first station 25. on site extrusion or butt-fusion is used to produce the central tube 8 and the cable tubes 12, to bond or bolt the cradle hoops 15 around the external periphery 9 of the central tube 8, and to apply the bands 18 to create an assembly which is again rope hauled through the sleeve 3 on its skids 19 by a winch located in the next service station 25.

It only then remains necessary to haul the required number of power cables 13 through cable tubes 12. and to haul the ancillary tube and cables 20, 21,22.

Clearly. at each station 25. it is necessary to joint the power cables 13 etc.

In addition. the central tube 8 is connected to a manifold of a water pump whereby cooling water is pumped along the interior 9 of the central tube 8 at a flow rate determined by

the rate of cooling required, with temperature sensors to control water pumps 26.

The cable tubes 12 housing a cable 13 are also connected at each service station 25 to an air pump 27. whereby cooling air may be drawn along the cable tubes 12. again under the control of temperature sensors, whilst a heat exchanger is indicated at 28.

From the service station 25. heated water may be supplied to farm buildings etc or may be dumped to a reservoir, a river etc.

Figure 3 shows the power transmission system in accordance with the third aspect. whereby every 1 K is excavated a (minor) service station 25A housing a heat exchanger 28A to extract heat from the air of the cable tubes 12 and to transfer the extracted heat to the cooling water of the central tube 8, whilst every 1 OK is excavated a (major) service station 25A housing a heat exchanger 28A to extract heat from the cooling water of the central tube 8, again with warm/hot water being available for distribution for beneficial uses from the station 25A.