GB2513991A

GB2513991A - High voltage cable

Info

Publication number: GB2513991A
Application number: GB1405411.8A
Authority: GB
Inventors: Peter Worrall; Andrew Norman; Andrew Simmonds
Original assignee: JDR Cable Systems Ltd
Current assignee: JDR Cable Systems Ltd
Priority date: 2013-03-26
Filing date: 2014-03-26
Publication date: 2014-11-12
Also published as: GB201405411D0; GB201305519D0

Abstract

The cable has an inner portion and an outer portion, the outer portion comprising a layer of hygroscopic material 15 and a layer of low diffusion polymer 14 surrounding the layer of hygroscopic material. The layer of hygroscopic material comprises a desiccant, e.g. a zeolite, contained in a polymer material. The hygroscopic material surrounds the inner portion. The low diffusion polymer provides a barrier to moisture passing into the cable and the hygroscopic material provides a moisture absorbing layer to absorb any moisture that does pass through the low diffusion polymer layer. This helps restrict flow of moisture towards the centre of the cable. The low diffusion polymer layer 14 may be replaced by a metal foil layer.

Description

HIGH VOLTAGE CABLE

Field 01 Invention

This invention relates to the design of a novel method of waterproofing dynamic cables particularly for sub-aquatic, high voltage transmission applications.

Background to Invention

The demand for dynamic cables is becoming increasingly significant in offshore, sub-aquatic applications. Examples of such applications include Floating Production, Storage and Offloading (FPSO) systems in oil production, floating power generation systems such as offshore wind turbines and also ocean thermal energy conversion (OTEC) systems. Traditionally, power has been transmitted from offshore power generation installations using Medium Voltage Alternating Current (MVAC). With the development of larger power turbines, it is expected that there will be a greater desire to transmit power using high voltage alternating current (HVAC) to minimise transmission losses. Cables are governed by various standards such as IEC 60502-2; ICEA S-93-639 NEMA WC 74; and ANSI/ICEA S-97-682-2007. Medium voltage typically refers to cables operating at voltages in the range 1kV to 36kV (IEC) or 46kV (ICEA). Cables above these voltages are generally designated High Voltage.

It is usual to design HV cables using an insulation system consisting of super clean polymers, e.g. Cross-linked Polyethylene (XLPE), which may operate at dielectric electrical stress levels »= 8 ky/mm. At these electrical stress levels it is generally considered essential that the dielectric remains free from water vapour or moisture to ensure long term reliability.

Hence, in order to create a HV electrical cable that is suitable for use in sub-aquatic or wet environments, it is normally expected that the cable must possess the ability to prevent water permeating through its outer surface and reaching the critical portions of the cable, i.e. the insulation system, when immersed in water. Traditionally, to prevent water permeating through the cable and reaching the insulation system, the cable or individual power cores has been coated with a waterproof material, forming a hermetic moisture barrier. When water infiltrates a material at a given point, it propagates radially away from the point. The shape of the propagation has been likened to that of a tree, thus the propagation pattern is often referred to as a moisture-tree" or more specifically a "water-tree". Materials that are resistant to water propagation are also referred to as being water-tree retardant".

Until the early 1960's, the only dielectric material available suitable for use with medium voltage (MV) cables was impregnated paper. Such a material was incompatible with moisture. To overcome this issue, cables were typically encased in lead for use in wet conditions with the lead acting as a hermetic moisture barrier. Such cables are referred to as Paper Insulated Lead Coated (PILC) cables. It was proven that a 3mm thick coating of lead was sufficient to allow the cables to be deployed in subsea locations for static applications Whilst suitable for use in static applications, lead coated cables are unsuitable for use in dynamic applications since repeated flexing of the cable will eventually result in the lead layer fracturing. With no reliable, flexible, cost-effective hermetic barrier yet discovered, the use of solid dielectrics made from materials such as polymers was experimented with.

It was not until the late 1970's that polymeric materials were developed that could create a reliable, water-tree retardant seal around a cable core, which had sufficient flexibility to allow use in dynamic applications. The use of such materials removed the need to use hermetic moisture barriers, revolutionising the power cable design industry.

Early attempts to use solid polymeric dielectrics in medium voltage applications were unsuccessful, with early polymers being incompatible with direct immersion in water.

Consequently the use of PILC cables persisted for sub-aquatic applications long after the use PILC cables for land-based applications cables had been become obsolete.

The use of PILC cables for sub-aquatic applications was deemed acceptable since the majority of sub-aquatic applications were static.

In order for a polymer to be deemed suitable for use in MV applications, it must first be tested according to HD 605 (or BS 7870-2). Cables using such suitable materials have been successfully deployed for many years in various industrial sectors. They have for example been used in the utility sector for transmitting power to islands, the renewable energy sector for connecting offshore wind turbines to the shore and also in the oil and gas industry to transmit power from the shore to offshore gas and oil platforms or to transmit power between main and satellite platforms. Cables using these polymers can be directly immersed in water without the requirement for moisture barriers. Being polymeric, these materials also possess sufficient flexibility for dynamic applications.

Dynamic power cables need to be designed to satisfy operational requirements. By definition, such cables need to be able to withstand numerous flexions. For this reason, conductors in the cores of dynamic cables are often made from stranded copper in preference over solid aluminium due to the superior cyclical loading performance of stranded copper, i.e. superior S-N (stress-number of cycles to failure) 1 0 characteristic.

The magnitude of the flexions experienced by a dynamic subsea cable during use is typically low. For this reason semi-compacted or even compacted class 2 copper strands can be used. This in turn allows the cable to maintain a smooth, circular profile during flexion, a feature that is important for MV applications. A problem with using stranded copper is that it is stiffer than solid aluminium. However, the cumulative benefits of using stranded copper over solid aluminium far outweigh this drawback, making stranded copper the favoured material for dynamic cables.

For dynamic operation, the conducting elements in the core of a cable should be isolated from the dielectric by an extruded conductor screen. This conductor screen should be firmly bonded to the primary dielectric. The conducting elements however should be able to move within the extruded conductor screen and the mechanical bond between the conducting elements and the conductor screen should be minimal. Fully bonded systems are preferred to further mechanically decouple the interface between the primary dielectric and the extruded conductor screen from the interface between the extruded conductor screen and the conducting elements.

Techniques exist to minimise the mechanical bond between the extruded conductor screen and the conducting elements. In low voltage (LV) applications these techniques include: providing the conducting elements in the core with a smooth surface, coating the conducting elements with inorganic materials such as talc or organic lubricants such as a silicone-based process aid and applying separator tapes to the conducting elements to separate the extruded conductor screen and the conductor.

For MV applications, there should be no electrical insulation between the conducting elements and the extruded conductor screen. So to minimise the mechanical bond between the conducting elements and the extruded conductor screen, semi-conducting tape can be applied over the conducting elements. These methods allow free movement between the conducting elements and the extruded conductor screen thereby enabling the cable to bend repeatedly without compromising the bond between the primary dielectric and the extruded conductor screen. In dynamic applications, the extruded conductor screen will typically have the minimum thickness increased to around 1.0mm compared to a typical 0.5mm for static applications.

The polymers used to make the insulation and extruded conductor screens have relatively large elastic limits and can be bent to the required radius of the cable without being mechanically degraded through repeated flexing. Typical polymers used for cable construction include silicones, rubbers and polyethylenes. In order to determine the most suitable material for a particular application, factors such as electrical properties, temperature performance, compatibility, aging performance and cost are considered. Silicon rubbers are commonly used for LV, thermal performance type cables and also in MV terminations where their low elastic modulus properties make them particularly suitable. Their high cost makes them generally unsuitable for use with MV power cables and instead Ethylene Propylene Rubber (ERR) and Cross-Lined Polyethylene (XLPE) are more commonly used.

Whilst ERR and XLRE have similar mechanical and electrical properties, ERR has a lower elastic modulus than XLPE. For large mechanically protected power cables however this difference is of little significance to the overall cable modulus making the use of both materials equally suitable. Both materials have acceptable electrical properties with modern XLRE's and ERR's having an electrical conductivity approaching 1018Sm1 and 1014Sm' respectively. The two materials can be differentiated by their water compatibility properties.

Figure 1 shows a water-tree in ERR created using stained water. Medium voltage ERR's typically include red lead in order to improve their electrical performance. This addition also makes the material more resistant to water-trees. Since the 1970's major chemical companies have improved XLRE's such that both materials now have largely similar resistances to water-trees at MV.

The results of tests of various XLPE materials according to the BS7870 protocol are shown in Figures 2a and 2b. It can be demonstrated that XLPE has a failure probability, for 1000 km of core, of 0.069 % ± 0.035 % per year resulting in a mean failure time of 1,361 ± 396 years per million meters. XLPE is therefore a commonly used material for MV wet applications.

In wet HV applications however, where electrical stress levels are 2 to 3 times higher than MV applications at up to 8 ky/mm, it is a functional requirement that the polymeric primary dielectric be kept dry throughout its operating life.

HV cables that operate over 18/30(36) kV are manufactured to lEO 60840. This directive requires the use of a hermetic moisture barrier. Historically lead has been used to form a hermetic water barrier around such cables, whereby a layer of lead, (typically 3 mm thick) is extruded, at acceptably low temperature, directly onto the outer surface of the insulation system of the core. In addition to being a proven hermetic water barrier, additional benefits are also gained through the coating of cables in lead.

The lead provides a low electrical resistance path for routing prospective asymmetric short-circuit currents. Lead is also non-ferritic and so does not cause additional eddy current losses and, in the absence of an insulating core sheath, allows the establishment of a phantom neutral point thereby removing all charging losses. An example of a lead coated cable system is shown in Figures 3a and 3b. The core 1 covered by a XLPE dielectric layer 2. The outer part of the cable is covered by a layer of lead 3 with an XLPE screen layer 4 interposed between the two layers.

Whilst lead has many properties, which make it attractive for use as a water barrier, its use is now discouraged e.g. under the EU "Waste Electrical And Electronic Equipment Directive" (2002/96/EC) and by article 4 of the EU RoHS Directive (2011/65/EU) and its use within a factory environment is controlled under the UK "Control of Lead at Work Regulations" (2002). Lead also possesses properties, which make its use in cables undesirable, particularly in dynamic cable applications. Lead coated cables are heavy and can fracture if flexed repeatedly. They also have higher electrical reactive compensation needs and are more expensive than present inter array cables. Lead cables are also heavy. A limit therefore exists on the maximum length of cable that can be freely suspended.

To overcome some of the problems with the use of lead, one option is to use aluminium instead. Using aluminium can, however be problematic as the aluminium must be extruded at higher temperatures as well as being far less ductile than lead.

This lack of ductility is problematic for dynamic cables whore flexibility is essential. To provide flexibility, aluminium can be applied to the cable in the form of corrugations. An example of the use of corrugated metal in cable construction is shown in Figures 4a and 4b. In these figures it can be seen that aluminium corrugations 5 are inserted between layers of polyethylene 6 to form a reinforced outer skin layer. In this case the outer skin layer is used to enclose a bundle of cables each with cores 7 covered by an XLPE dielectric layer 8. The corrugations 5 can however lead to an increase in the thickness of a cable and also limit its ability to withstand large hydrostatic pressures, thus limiting the depth at which such a cable can be deployed.

An alternative design of cable is shown in Figures 5a and Sb. This design of cable uses aluminium or copper foil laminate 12 to help repel the inflow of water. As shown in Figure 5a, the cable comprises a cable core 9 covered by a XLPE dielectric layer 10.

An XLPE insulating sheath 11 is extruded over the XLPE dielectric layer. A foil laminate 12 is applied to the surface of the XPLE layer 11 and held in place using a glued seam. A final layer of polyethylene 13 is extruded over the foil laminate layer 12 to provide some additional resistance to moisture.

Figure 5b shows an enlarged view of an outer part of the cable shown in Figure 5a showing the seam.

The use of foil laminate significantly reduces the diffusion rate of water into the cable whilst also keeping the weight of the cable low. Despite this, the presence of a glue seam on the foil laminate layer 12 does not guarantee long-term freedom from water diffusion at high hydrostatic pressures. Although it is possible to increase the thickness of the foil to allow the seam to be welded, the limited compliance of the weld raises concerns about performance at high hydrostatic pressures and also tolerance compliance. The increase in thickness of the foil laminate leads to an increase in weight of the cable thus limiting the maximum length of cable that the cable can support under its own weight. Additionally the metallic foil layer when used in this non-corrugated manner may be less suitable for dynamic applications.

With reference to Figures 5a and Sb, the layer of polymer 13 which separates the metallic screen from the water, may not be a complete water barrier. Polyethylene and other non-metallic materials have a small but finite water diffusion gradient. It must therefore be accepted that water may penetrate such materials over time. Unless a cable design is capable of being tolerant to water the cable may fail prematurely.

A demand therefore exists for a design of cable, which is lead free, suitable for dynamic applications and also suitable for HV operation. It is therefore an aim of the present invention to overcome or ameliorate some of the problems set out above.

Summary of Invention

Therefore, according to the present invention, there is provided an electrical cable having an inner portion and an outer portion, the outer portion comprising a layer of hygroscopic material surrounding the inner portion and a layer of low diffusion material surrounding the layer of hygroscopic material, wherein the hygroscopic material comprises a desiccant contained in a polymer material.

The layer of hygroscopic material absorbs water that permeates into the cable through the low diffusion polymer layer. No water will pass through the layer of hygroscopic material to the inner portion until the hygroscopic material portion reaches its saturation point. By balancing the water diffusion rate of the layer of low diffusion material and the water capacity of the hygroscopic material portion, it is possible to keep the inner portion of the cable dry for a period of time beyond the desired service life of the cable.

Using a desiccant dispersed within a polymer material, helps to provide a high level of water absorbency without making the cable unduly bulky by having to have a very thick layer of less absorbent material. This design of cable is therefore able to operate in wet conditions for HV applications without being impractically large and whilst still having a satisfactory useable life before the hygroscopic layer becomes saturated.

The cable is preferably made from materials which allow them to withstand numerous, low amplitude flexions. The low diffusion material and layer of hygroscopic material are preferably made from polymers which allows the cable to flex without failure unlike alternative water proofing solutions such as Lead. The use of such materials avoids the need to use lead and allows the cable to be used in dynamic applications.

However, the layer of low diffusion material may be a metallic layer such as a metallic foil layer. This layer may be formed into a laminate structure with a polymer layer on one or other sides or on both sides, such that the metallic layer is embedded between layers of polymer.

The hygroscopic layer is formed from a polymer with a desiccant dispersed within the polymer matrix. The polymer carrier matrix may be a polymer such as a fluoropolymer, HDPE, or another polymer suitable for dosing with desiccant whilst maintaining a suitably high level of flexibility and toughness for use within the dynamic offshore environment.

The desiccant is preferably a zeolite, of natural or synthetic origin, the synthetic compounds also being known as molecular sieves'. The zeolites are preferably in powder form. The zeolites mean porosity can be chosen to maximise the absorption efficiency for the chosen molecule. In this case, water vapour molecule are required to be captured, and so a mean porosity of 3 angstroms is preferred. Alternative embodiments of the design can utilise alternative desiccant materials, including, but not limited to Calcium Oxide.

The concentration of the desiccant in the polymer is preferably chosen to be as high as practical, for example 30% or higher. The amount of desiccant is therefore may be between 30% and 80% by weight, preferably between 40% and 70% by weight, more preferably between 45% and 55% by weight and yet more preferably between 49% and 51%byweight.

The desiccant is preferably mixed with the polymer material as a ground powder. The size of the zeolite particles is preferably less than 10pm. The mean porosity of the zeolite particles is preferably between 2.5 and 3.5 Angstroms and more preferably at or around 3 Angstroms The upper limit of the concentration of the desiccant is affected by the limitations on extruding the desiccant loaded polymer. Too much desiccant will adversely affect the extrusion properties of the polymer and may also be detrimental to the structural properties (for instance maximum strain level achievable) of the polymer/desiccant product. By controlling the particle size of the desiccant material, high concentrations of desiccant can be achieved whilst still allowing good extrusion and structural properties for the polymer.

Optionally, the inner portion of the cable can be provided with a multiplicity of service voids, defined as open channels arranged circumferentially within the inner portion of the cable and which run along the entire length of the cable. The service voids can be connected at one end to a pump and at the other to an extractor. Gas may be may be passed through the service voids using the pump. This allows the interior of the cable to be dried thus potentially extending the service life of the cable indefinitely.

Alternatively or in addition, the gas flow may be used to determine the water saturation level in the cable and more specifically the saturation level of the hygroscopic layer.

This can be used to determine the remaining life of the cable and whether remedial action is required to replace or treat the cable such as by drying using gas passed through the voids.

The service voids may be provided in a service layer between the inner portion and the layer of hygroscopic material. The service voids preferably extend circumferentially around a portion of the circumference of the service layer around a portion of the service layer and generally longitudinally along the cable although they may form a spiral. There may be more than one void running in parallel through the service layer.

Optionally, the inner portion of the cable can be provided with a one of more sensing wires, disposed in the cable and which run along some or all of the length of the cable.

The sensing wires can be used to detect the condition of the cable and particularly the hygroscopic layer. The condition of the cable can be monitored to determine whether it is still serviceable or if there has been any degradation of the cable such as by saturation of the hygroscopic layer.

The sensing wires may, for example, be electrical. In this arrangement, the sensing wires could be provided as a pair of conductive wires, which may be connected at either end of the cable to electrical analysis equipment to measure the resistance between them to monitor the internal resistance of the cable thereby allowing the moisture level of the cable to be measured, even whilst the cable is in use. The conductive wires may be formed within the service layer or between the layers.

The sensing wires may also use other sensing techniques. For example, the sensing wires may be optical fibres which monitor the condition of the cable without the need for a conductive electrical wire, for example, by detecting changes in temperature due to saturation, or light conductivity.

The present invention also provides a method of forming an electrical cable comprising forming an inner portion and forming an outer portion around the inner portion, the outer portion being formed by providing a layer of hygroscopic material surrounding the inner portion and a layer of low diffusion material surrounding the layer of hygroscopic material, wherein the hygroscopic material comprises a desiccant contained in a polymer material.

Brief Description of the Drawings

A specific embodiment of the invention will now be described in detail by reference to the attached drawings in which: Figure 1 shows the propagation of water-trees in Ethylene Propylene Rubber (ERR); Figure 2a show a cumulative probability chart for failure of a cable with variation in voltage density carried by the cable; Figure 2b shows a cumulative probability chart for failure of a cable with age of the cable; Figure 3a shows a transverse cross-section through a lead coated cable with an XLPE dielectric; Figure 3b shows a detailed view of an outer portion of the cable shown in Figure 3a; Figure 4a shows a transverse cross-section of a cable system incorporating wire bundles with XLRE dielectrics enclosed by a corrugated aluminium layer; Figure 4b shows an axial cross-section of the cable shown in Figure 4a; Figure 5a shows a cross-section through a cable system incorporating an XLPE dielectric and toil laminate moisture barrier; Figure 5b shows a detailed view ot an outer portion ot the cable of Figure 5a; Figure Ba shows a cross-section through a cable system according to the present invention: Figure 6b shows a detailed view of the outer portion ot the cable shown in Figure 6b; and Figure 7 shows a transverse cross-section through a cable system of an alternative embodiment of a cable system according to the present invention.

Detailed Description of Invention

An embodiment of a cable according to the invention is shown in Figures 6a and 6b.

The cable consists ot a plurality of outer polymer layers. These outer layers seal the conducting elements ot the core 19 of the cable.

In the embodiment, the conducting elements ot the core 19 are covered by a conductor screen layer 18, which is typically made of a semiconducting material. This is surrounded by a primary dielectric layer 17 which in turn is surrounded by an insulation screen layer 16, which is also typically made ot a semiconducting material.

Surrounding this is a hygroscopic medium layer 15 covered by an outer layer 14.

The outer layer 14 ot the cable is made from a polymer (although it may also be a toil barrier -see below). As indicated above, all polymers will allow water to permeate at a low, usually known rate. When the cable is immersed in water, the water may permeate through the polymeric outer layer 14 to the hygroscopic layer 15. These hygroscopic materials are otten referred to as a getter". The hygroscopic layer 15 is made using a base polymer material (tor example a tluoropolymer or HDPE, or any other polymer suitable tor a high dosing of desiccant) with a desiccant dispersed within the polymer matrix.

The desiccant is mixed in a powder form with the base polymer powder during a melt/extrusion process to create desiccant dosed pellets. By forming the pellets in this way, the desiccant is evenly distributed through the polymer. The pellets may be prepared separately to the main extrusion process to be used at a later time or used immediately. Equally, the desiccant loaded polymer may be provided directly into the cable extrusion process.

In this embodiment, pre-prepared pellets are provided to the cable extrusion process.

The pellets are melted and extruded into hygroscopic layer 15 with the desiccant evenly dispersed within the layer. The concentration of desiccant is rnaximised whilst maintaining mechanical properties with suitably high level of flexibility and toughness for use within the dynamic offshore environment.

In this embodiment, a desiccant concentration of 50% is used to provide an acceptable balance between the mechanical properties of the finished cable/extrusion material and the required absorbency of the layer.

The desiccant is preferably a zeolite, of natural or synthetic origin, the synthetic compounds also being known as molecular sieves'. Preferably in powder form, the zeolites mean porosity can be selected to maximise the absorption efficiency for the chosen molecule. In this case the water vapour molecule is required to be captured, and so a mean porosity of 3 Angstroms is preferred.

The size of the zeolite particles is preferably less than 10pm. Selecting smaller particles of desiccant in this size range helps to maintain good extrusion and mechanical properties of the polymer matrix at higher concentrations. Alternative embodiments of the design can utilise alternative desiccant solutions, including, but not limited to Calcium Oxide.

The hygroscopic layer 15 is capable of absorbing large amounts of water, using the desiccant contained within a polymer. Water reaching the hygroscopic layer 15 will not pass through to the inner surface until it is saturated. Before the hygroscopic layer 15 reaches saturation, the polymeric XLPE outer insulation screen layer 16 and thus the XLPE primary dielectric layer 17 will be kept dry.

The all polymer construction allows the cable to provide a number of advantages compared to lead cables and other alternative metal sheath cables. Polymer cables are generally lighter than equivalent lead cables. The required tolerances are also generally easier to achieve with polymer cables. Polymers are more flexible and less likely to fracture under repeated flexing than lead and aluminium cables. Polymer cables are also less restricted by hydrostatic pressure limitations. However, the present invention may still be used in conjunction with a cable comprising a metallic layer. In this way, the hygroscopic layer 15 may be formed around a cable comprising a metallic foil layer.

A consequence of the cable structure of the present invention is that the cable is not hermetically sealed and so water will normally eventually permeate the cable and reach the inner components. In use, the outer layer 14, being made of polymer, will eventually allow water to permeate through it. The rate at which the water permeates can however be predicted to determine the amount of water that will permeate in a given time. Once the water passes through the outer layer 14, it reaches the hygroscopic layer 15, which will initially absorb the arriving water without allowing any to pass through to the screen layer 16. This effectively provides a dry environment for the screen layer 16 and all the components of the cable within it. This feature is particularly important for HV operation, where it is normally necessary for the primary dielectric to be kept dry during operation. In this way, the cable provides an acceptable environment for the transmission of HV power.

Once the hygroscopic layer 15 is saturated, water will start to emerge out of the hygroscopic layer 15 and wet the screen layer 16. Water will eventually penetrate the insulation screen layer 16 wetting the primary dielectric layer 17, cable core screen 18 and ultimately the cable core itself 9. Once this occurs, the cable may ultimately fail.

The present invention relies upon the water barrier provided by the limited diffusion of water through the outer layer 14 and the capacity of the hygroscopic layer 15 controlling the time before the water reaches the inner components such that the time exceeds the working lifetime of the cable. In this way, it is possible to retard the permeation of water through the outer layers of the cable to such an extent that the primary dielectric layer 17 can be kept dry for a period of time approximating to the desired operational life of the cable.

For example, if the operation lifetime of a cable is 50 years then by adjusting the water absorbing capacity of the hygroscopic layer 15 and the rate of permeation of water through the outer layer 14, it is possible to ensure that the water will only saturate the hygroscopic layer 15 after 50 years, although this may be chosen to be higher to allow a safety margin, for example for variations in the various parameters.

The water absorbing capacity of the hygroscopic layer 15 can be controlled by varying the thickness (and hence volume) of the layer but also by selection of the material used. Again the rate of permeation of the outer layer 14 can be similarly adjusted by varying the thickness and materials used. Other additional measures may still be used to restrict the flow of water into the cable.

A second embodiment of the invention is shown in Figure 7. This embodiment is a modification of the first embodiment with the basic structure being largely identical.

Reference numerals from Figures 6a and 6b have been carried over onto Figure 7.

The two embodiments differ by the inclusion of a plurality of open channels or service voids 20 disposed within the insulation screen layer 16. The service voids 20 are arranged circumferentially within the insulation screen layer 16 and run along the entire length of the cable.

As water begins to permeate into the insulation screen layer 16 it will appear at the surfaces of the service voids 20. The water will then evaporate into the gas (usually air) in the voids. By moving the gas along the channels formed by the service voids 20, gas and the water vapour in the gas can be extracted from the cable. This can be used either as a way of extracting the water from the cable or simply to allow measurement of the water level present to aid in determining the viability of the cable.

For example, at the one end of the cable an air pump may be used to pass dry air through the service voids 20. At the other end of the cable, the service voids 20 can be connected to an extractor for removing the air. The air pump causes dry air to flow through the service voids 20. This flow of air will allow water on the surface of the insulation screen layer 16 adjacent to the service voids 20 to evaporate. As the air passes through the service voids 20, this water vapour flows downstream with the air.

The air and water vapour eventually flow out of the cable at the extractor. Through this mechanism, water can be removed from the interior of the cable, allowing the cable to be dried and regenerated. Varying the airflow rate through the service voids 20 will vary the water removal rate from the insulation screen layer 16. Ideally the airflow rate through the service voids 20 will be sufficient to remove water from the insulation screen layer 16 at a rate greater than or equal to that which water permeates into the insulation screen layer 16 from the neighbouring saturated hygroscopic layer 15.

Under such conditions, the insulation screen layer 16 could be continuously regenerated preventing water from reaching the primary dielectric layer 17. The cable would therefore have an indefinite operational life. Even if water permeates into the outer insulation screen 16 from the saturated hygroscopic layer 15 at a rate greater than water can be removed from the insulation screen layer 16, such that water eventually permeates through the insulation screen layer 16 and into the primary dielectric layer 17, the removal of water via the airflow will still help. In this case, the drying effect caused by the blowing of air or other gas through the service voids 20 will act to extend the operational life of the cable.

The air may be passed continuously through the cable during its operational life or it may be only passed through intermittently, or as required. Furthermore, the flow may be very low for most of the time, to simply allow monitoring of the moisture level within the cable but increased in flow to also dry the cable when needed. Different gases may be used for monitoring than for drying or the temperature of the gas may be varied to adjust the drying effect.

The service voids preferably run along the length of the cable to allow monitoring and drying of the whole cable but the cable may have separate voids each running along part of the cable. Gas may be fed into the service voids at intermediate points along the cable and possibly also extracted at intermediate points.

In the above embodiment, the service voids are provided in the service layer 16 but they may alternatively or in addition be provided within the hygroscopic layer 15 or between the service layer 16 and the hygroscopic layer 15. Furthermore, additional or alternative service voids may be provided in the outer layer 14.

In the above embodiments, the outer layer 14 is made from a polymer. However, the outer layer can act as a barrier layer using a foil layer similar to the foil layer 12 shown in figures Sa and Sb. This provides a layer that acts a barrier to water ingress but does not need to be completely water tight to allow satisfactory operation of the cable. In other words, even if water is able to pass through the foil layer, the hygroscopic layer will still provide an absorbent function to absorb any water passing through. The rate of ingress of the water compared to the water absorbing capacity of the hygroscopic layer provides an estimate of the life of the cable before the water saturates the hygroscopic layer. Again, the drying function of the service voids 20 may allow the extension of the life of the cable by drying the hygroscopic layer.

The invention provides a hygroscopic layer and a barrier layer surrounding it. In the above described embodiments, as shown in figures 6a and 6b, the inner components include the cable core, a dielectric layer 17 and an insulation screen layer 16. These inner components could take different forms depending on the application. For example, the inner components may include additional layers. For example, the inner component may include additional barrier layers such as a foil layer similar to the layer 12 shown in figures 5a and Sb. For example the inner components may include all the layers shown in the structure of figure 5a with the hygroscopic layer 15 and the outer barrier layer 14 provided in addition. This would allow the cable of the present invention to be based upon existing cables with the addition of the extra layers where desirable. In this way, the basic structure does not need to be redesigned but simply modified to incorporate the inventive features.

In the above embodiments, the insulation screen layer 16 may additionally include co-extruded conductive wires. The moisture level of the insulation screen layer 16 can be measured by detecting the electrical resistance along the length of the insulation screen layer 16. To achieve this, the conductive wires are used as electrodes to pass electrical current through the insulation screen layer 16. The electrical resistance of the insulation screen layer 16 can then be measured to provide an indication of the permeation of water into the layer 16. This method provides a continuous assessment of the remaining operational life of the cable, even when the cable is in use.

A person skilled in the art will appreciate that the technique of using an outer layer 14 made from a low diffusion polymer in conjunction with a hygroscopic layer 15 and an insulation screen layer 16 as demonstrated in both embodiments can be used to provide a permanent or semi-permanent water free environment for a wide variety of cables for applications other than HV power cables and can be used for cables with constructions other than those described in the specific embodiments. In the described embodiments, the invention was used to waterproof a specific target layer in the cable construction, namely the primary dielectric layer 17. The invention can equally be used to waterproof other target layers in other cable designs, which are not primary dielectrics.

Claims

CLAIMS1. An electrical cable having an inner portion and an outer portion, the outer portion comprising a layer of hygroscopic material surrounding the inner portion and a layer of low diffusion material surrounding the layer of hygroscopic material, wherein the hygroscopic material comprises a desiccant contained in a polymer material.
2. An electrical cable according to claim 1, wherein the desiccant is a zeolite.
3. An electrical cable according to claim 1 or 2, wherein the proportion of desiccant by weight in the polymer is between 30% and 80%.
4. An electrical cable according to any one of the preceding claims, wherein the proportion of desiccant by weight in the polymer is between 40% and 70%.
5. An electrical cable according to any one of the preceding claims, wherein the proportion of desiccant by weight in the polymer is between 45% and 55%.
6. An electrical cable according to any one of the preceding claims, wherein the proportion of desiccant by weight in the polymer is between 49% and 51%.
7. An electrical cable according to any one of the preceding claims wherein the desiccant is held in the polymer as particles having a particle size of less than 10pm.
8. An electrical cable according to any one of the preceding claims wherein the outer portion further comprises a service layer between the inner portion and the layer of hygroscopic material, the service layer comprising one or more voids extending longitudinally along the cable.
9. An electrical cable according to claim 8 wherein the voids extend circumferentially around a portion of the service layer.
10. An electrical cable according to claim S or 9 wherein the voids extend along the cable to connection points to allow gas to be passed through the voids.
11. An electrical cable according to any one of the preceding claims further comprising providing a plurality of conductive wires extending along at least a portion of the cable, to allow measurement of the internal resistance of the cable.
12. An electrical cable according to any one of claims 8 to 11 wherein the service layer is a semiconductor insulation layer.
13. An electrical cable according to any one of the preceding claims wherein the inner portion comprises a high voltage core.
14. An electrical cable according to any one of the preceding claims wherein the layer of low diffusion material includes a metal layer.
15. A method of forming an electrical cable comprising forming an inner portion and forming an outer portion around the inner portion, the outer portion being formed by providing a layer of hygroscopic material surrounding the inner portion and a layer of low diffusion material surrounding the layer of hygroscopic material, wherein the hygroscopic material comprises a desiccant contained in a polymer material.
16. A method of forming an electrical cable according to claim 15, wherein the desiccant is a zeolite.
17. A method of forming an electrical cable according to claim 15 or 16, wherein the proportion of desiccant by weight in the polymer is between 30% and 80%.
18. A method of forming an electrical cable according to any one of claims 15 to 17, wherein the proportion of desiccant by weight in the polymer is between 40% and 70%
19. A method of forming an electrical cable according to any one of claims 15 to 18, wherein the proportion of desiccant by weight in the polymer is between 45% and 55%.
20. A method of forming an electrical cable according to any one of claims 15 to 19, wherein the proportion of desiccant by weight in the polymer is between 49% and 51%.
21. A method of forming an electrical cable according to any one of claims 15 to 20, further comprising providing particles of desiccant having a particle size of less than 10pm.
22. A method of forming an electrical cable according to any one of claims 15 to 21 wherein a service layer is formed around the inner portion and the layer of hygroscopic material is formed around the service layer, the method further comprising forming one or more voids in the service layer, the voids extending longitudinally along the cable.
23. A method of forming an electrical cable according to claim 22, wherein the voids are formed circumferentially around a portion of the service layer.
24. A method of forming an electrical cable according to any one of claims 15 to 23, wherein the voids are formed to extend along the cable to connection points to allow gas to be passed through the voids.
25. A method of forming an electrical cable according to any one of claims 15 to 24 further comprising co-extruding a plurality of conductive wires extending along at least a portion of the cable, to allow measurement of the internal resistance of the cable.
26. A method of forming an electrical cable according to any one of claims 22 to 25 wherein the service layer is a formed from a semiconductor material.
27. A method of forming an electrical cable according to any one of claims 15 to 26 wherein the inner portion comprises a high voltage core.
28. A method of forming an electrical cable according to any one of claims 15 to 27 wherein the layer of low diffusion material includes a metal layer.
29. An electrical cable substantially as described herein with reference to and as shown in figures 6 and/or 7 of the attached drawings.
30. A method of forming an electrical cable substantially as described herein with reference to and as shown in figures 6 and/or 7 of the attached drawings.