Patent Application
20220189659
This application claims the benefit of priority from European Patent Application No. 20 306 579.2, filed on Dec. 15, 2020, the entirety of which is incorporated by reference.
The present invention relates to a lead-free water barrier suited for dynamical submarine high voltage power cables.
The current carrying parts of power cables may need to be kept dry. Intrusion of humidity or water may cause electrical breakdown of the power cable insulation system. The core section of power cables is therefore usually protected by a water barrier arranged radially around the cable core. Up to date, the dominating material in water barriers for power cables is lead since lead has proven to be a reliable and sturdy sheathing material, however with some well-known drawbacks.
One drawback is that lead is a high density materiel adding significant weight to the cable. The heavy weight induces extra costs in the entire value chain from production, under transport, storage, deployment, and when the cable is discarded after reaching its lifetime. Another drawback is that lead has a relatively low fatigue resistance making leaden water barriers less suited for dynamical power cables. Furthermore, lead is a rather poisonous material increasingly meeting environmental regulation restrictions. An environmentally friendly replacement of lead as water barrier in power cables is required.
Capacitive charges and resulting currents may cause problems in power cables if they are not conducted out of the cable. It is therefore advantageous to have water barriers being electrically conductive in radial direction.
EP 2 437 272 discloses a power cable comprising water barrier laminate. The main technical feature of a power cable according to the document is that the water barrier laminate comprising foil made of metal laminated between at least two layers of non-insulating polymer foils constituting a final laminate that is non insulating.
The main objective of the invention is to provide a low weight and lead-free water barrier suitable for high-voltage power cables, which is capable of conducting capacitive currents radially out of the cable thus avoiding breakdown due to induced voltage gradients.
The present invention is based on the discovery that a lightweight and fatigue resilient water barrier having excellent water insulating effect and good capacity for leading capacitive currents radially may be obtained by wrapping a laminate structure comprising a metal foil and a thermoplastic semiconductive polymer around the cable core in one or more layers and then heat treat the laminate to thermally joined the layers of wrapped laminate structure.
Thus, in a first aspect, the invention relates to a water barrier encapsulating a cable core,
wherein
The term “encapsulating a cable core” as used herein means that the water barrier is laid onto and around the cable core to form a watertight layer around the cable core effectively preventing water and/or moisture from penetrating into the cable core.
The term “wrapped around the cable core with at least some overlap between opposite edges” as used herein refers to the need for having the laminate structure covering 100% of the surface of the cable core and to enable forming a watertight enclosure along the seam formed by the laminate structure being partly laid over itself.
The term “metal foil” as used herein, refers to the metal layer in the middle of the laminate structure. The invention is not tied to use of any specific metal/metal alloy or thickness of the metal foil. Any metal/metal alloy at any thickness known to be suited for use in water barriers in power cables by the skilled person may be applied. In one example embodiment, the metal foil is either an A1/A1-alloy such as for example an AA1xxx series, an AA5xxx series or an AA6xxx series alloy according to the Aluminium Association Standard, or a Cu/Cu-alloy such as for example pure Cu, a CuNi-alloy or a CuNiSi-alloy, or a Fe/Fe-alloy, such for example stainless alloy SS316 or S32750. The thickness of the metal foil (shown as curly bracket t2 in
In one example embodiment, the adherence between the metal foil and the first and second thermoplastic semiconducting polymer may be enhanced by applying a first adhesive laid onto and covering less than 100%, such as from 5 to 95% of the lower surface area of the metal foil, and a second adhesive layer laid onto and covering less than 100%, such as from 5 to 95% of the upper surface area of the metal foil. The adhesive layers are laid in-between the metal foil and the thermoplastic semiconducting polymer layers. Thus, the term “adhesive layer laid onto and covering from 5 to 95% of the surface area of the metal foil” as used herein refers to an adhesive applied to enhance the adherence between the metal foil and the semiconducting polymer layer. In general, adhesives have relatively poor electrical conductivities such that a layer of adhesive completely covering the interface between the metal foil and the semiconducting polymer may hamper the electric conductivity in the radial direction. Thus, according to the present invention, the adhesive is to be applied with less than complete coverage of the surface of the metal foil to enable direct contact between the metal foil and the semiconducting polymer. In practice at least 5% of the surface area of the metal foil and correspondingly the semiconducting polymer should be free of adhesive after application of the laminate structure and at least 5% of the surface should be covered with adhesive after application of the laminate structure to ensure sufficient adhesion between the metal foil and the semiconducting polymer, i.e. the adhesive should cover the surface area of the metal foil in the range of from 5 to 95%, preferably from 10 to 90%, more preferably from 15 to 85%, more preferably from 25 to 75%, and most preferably from 50 to 75%. The incomplete covering of the adhesive layer may be obtained by applying the adhesive in a raster pattern or in any other manner known to the skilled person.
The invention may apply any adhesive known to the skilled person being suited for attaching a polymer layer to a metal surface. Examples of suited adhesives includes, but is not limited, to; epoxy resins, phenolic resins, polyurethane based glues, cyanoacrylates, acrylic glues, polyester based glues, copolymer of ethylene and ethyl acrylate, copolymer of ethylene and ethyl acrylic acid, methacrylic acid, copolymer of ethylene and glycidyl methacrylate or epoxy-based monomer such as 1,2-epoxy-1-butene, and copolymer of ethylene and maleic-anhydride. The above mentioned adhesives may be applied with or without electrically conductive particulates providing the glue an enhanced electric conductivity.
The term “thermoplastic polymer” as used herein means that the polymer material becomes pliable or mouldable at certain elevated temperatures and thereafter solidifies upon cooling. The property of being thermoplastic, eventually combined with the incomplete coverage of the adhesive, provides the advantage that an improved electric contact between the metal foil and the semiconducting polymer may be achieved by a heat treatment causing the polymer to melt and then solidify in intimate contact with non-glued parts of the surface of the metal foil. This effectively reduces the electrical resistance across the metal foil/semiconducting polymer interface in these non-glued area(s). The thickness of the thermoplastic semiconducting polymer (before thermosetting) may in an example embodiment be in one of the following ranges; from 25 to 300 μm, preferably from 35 to 200 μm, more preferably from 40 to 150 μm, more preferably from 50 to 100 μm, and most preferably from 50 to 75 μm.
The invention may apply any thermoplastic semiconductive polymer known to the skilled person being suited for use in power cables. Examples of suited polymers includes, but is not limited, to; a polyethylene-based material constituted of either low density polyethylene (LDPE), a linear low density polyethylene (LLDPE), a medium density polyethylene (MDPE), or a high density polyethylene (HDPE), or a copolymer of ethylene with one or more polar monomers of; acrylic acid, methacrylic acid, glycidyl methacrylate, maleic acid, or maleic anhydride. The polymer is made semiconducting by addition and homogenisation of 20 to 40 weight % particulate carbon in the polymer mass. Examples of suited particulate carbon includes but is not limited to; comminuted petrol coke, comminuted anthracite, comminuted char coal, carbon black, carbon nanotubes, etc.
The term “the laminate structure is thermally joined by a heat treatment” as used herein means that after wrapping the laminate structure around the cable core, the laminate structure is heat treated to a temperature at which the thermoplastic semiconducting polymer melts and then cooled to the solid state. If a polyethylene based polymer is applied, the temperature treatment needs typically to increase the temperature of the laminate structure to 120° C.-130° C. to melt the polymer. In one example embodiment, the heat treatment for thermally joining the laminate structure may be obtained by forming an outer sheathing laid onto the water barrier layer/laminate layer(s) by extrusion of a polymer at an extrusion temperature of around 200° C. The heat from the molten polymer exiting the extruder fuses the semiconducting polymer layers of the laminate structure below so that the adjacent semiconducting polymer layers of overlapping laminate structure edges are fused together and seals the water barrier. In other example embodiments, the heat treatment for thermally joining the laminate structure may be obtained by application of hot air, radiation (e.g. laser, IR) or induction.
Examples of suited polymers to be applied in the outer sheathing includes, but is not limited, to; polyolefin based materials such as e.g. HDPE, LDPE, LLDPE, MDPE, polyvinyl chloride (PVC), polypropylene (PP), or thermoplastic polyurethane (TPU), etc. The polymer material of the outer sheathing layer may be either electrically insulating (pure polymer) or be made electrically conductive by addition and homogenisation of from 20 to 40 weight % particulate carbon in the polymer mass. Examples of suited particulate carbon includes but is not limited to; comminuted petrol coke, comminuted anthracite, comminuted char coal, carbon black, carbon nanotubes, etc. The deposited outer sheathing may be cooled in a water bath directly after deposition.
An example embodiment of a typical process for forming an example embodiment of a water barrier according to the invention is schematically illustrated in
The combined effect of the feature of wrapping the laminate structure with some overlap between opposite edges and the heat treatment is to seal the water barrier against potential longitudinal intrusion/migration of water by fusing together the semiconductive polymer layers of the opposite edges overlapping each other. The seal resulting from the fusion of overlapping polymer layers is illustrated schematically in
The interface formed between the overlapping parts being in mechanically contact of the semiconducting polymer layers of the different windings of the laminate structure/tape may constitute a migration route relatively easily penetrated by moisture/water diffusing along the interface to enter space 9 and then into the cable core. However, the thermosetting heat treatment causes the semiconducting polymer layers to melt and become fused together at the overlapping parts, as shown schematically in
In general, the longer diffusion path, length w1, the more watertight the water barrier becomes. In one example embodiment, the overlap between successive layers of the laminate structure may advantageously provide a shortest diffusion path, w1, of at least 10 mm, more preferably at least 15 mm, more preferably at least 20 mm, more preferably at least 25 mm, more preferably at least 30 mm, more preferably at least 35 mm, and most preferably at least 40 mm.
In one example embodiment, the laminate structure may be longitudinally wrapped around the cable core with an overlap of its opposite edges to form a longitudinal seam. In this case, there may advantageously be applied two layers of the laminate structure, one wrapped around the cable core from the bottom and sealed at the top of the cable core, and the other is wrapped from the top and sealed at the bottom. This will make the entire second semiconductive polymer layer of the first “bottom-up” laminate structure to be fused and integrated with the entire first semiconductive polymer layer of the second “top-down” laminate structure causing a diffusion length for penetrating water of at least half the circumference of the cable core. An example of such embodiment is illustrated schematically in
This configuration forms an excellent water barrier when being thermally joined because water barrier comprises a first polymer layer of the first laminate structure, a first metal foil of the first laminate structure, a second polymer layer of the first laminate structure being fused and integrated with the first polymer layer of the second laminate structure, a second metal foil of the second laminate structure, and finally a second polymer layer of the second laminate structure. Thus, since the metal foils are impenetrable for moisture/water, any moisture/water entering the cable core must migrate from the seam of the second laminate structure into and migrate through the fused polymer layers between the second and the first metal foil to the seam of the first laminate structure. This diffusion distance is at least equal to a half circumference of the cable core.
In a second aspect, the invention relates to a power cable, comprising:
The term “current conductor” as used herein encompasses any electric current carrying part of power cables known to the skilled person. The electric conductor is typically made of a metal, often an Al/Al-alloy or a Cu/Cu-alloy but may be of any material known suitable as current conductors. The current conductor may be a single strand of the electrically conductive material or a plurality of strands arranged in a bundle. In the case of applying a current conductor comprising a bundle of strands, the space in-between the strands of the bundle may be occupied by an insulating or a semiconducting filler compound.
The term “electric insulation system” as used herein refers to the electric insulation around the current conductor. The invention is not tied to any specific electric insulation system but may apply any electric insulation system known to the skilled person being suited for electrically insulating current conductors. In one example embodiment, the electric insulation system comprises an isolator layer such as e.g. a polyethylene layer and an electric shielding such as e.g. a semiconducting layer arranged around the current conductor. If the power cable comprises two or more current conductors, each of them will have its own electric insulation system.
The term “cable core” as used herein refers to a single current conductor comprising its electric insulation system and the water barrier according to the invention. In case the power cable comprises two or more current conductors and thus two or more cable cores, the laminate structure according to the invention is to be laid separately around each of the cable cores. In one example embodiment, each of the cable cores applied in the power cable may further comprise an outer polymer sheathing laid onto its water barrier. The outer polymer sheathing may e.g. be made of a polyethylene, such as a low density polyethylene (LDPE), a linear low density polyethylene (LLDPE), a medium density polyethylene (MDPE), or a high density polyethylene (HDPE). The outer sheathing may be insulating or made conductive by containing/having incorporated from 20 to 40 weight % carbon black in the polyethylene phase.
The term “power cable” as used herein encompasses any known power cable having one or a plurality of the above defined cable cores. In one example embodiment, the power cable is an intermediate to high current carrying power cable intended for outdoor and/or subsea use. The power cable may in an example embodiment further comprise optical fibres, umbilical tubes, distancing profiles arranging the cable cores conductors in a cross-section, and any other component known to be present in a power cable.
The term “mechanical protection system” as used herein refers to outer layers of the power cable intended to mechanically protect the cable core(s) and other eventually present components of the power cable from potentially detrimental mechanical strains imposed on the cable under handling, use and storage. The mechanical protection system may comprise any layer/part known to the skilled person suited for mechanically reinforcing and/or protecting the power cable. In one example embodiment, the mechanical protection system comprises an armouring made of e.g. steel cords. The term “laid around the at least one cable cores as a group” as used herein means thus that the mechanical protection system is, contrary to the water barrier not to be separately laid around each cable core, but is to be laid as a layer surrounding the group of one or more cable cores applied in the power cable.
In a further example embodiment, the mechanical protection system may comprise an oversheath, an outermost polymer layer defining the interface towards the surrounding environment of the power cable. The invention may apply any oversheath known to the skilled person suited for being used as the outer mantle of power cables, such as e.g. a polyethylene polymer such as e.g. chlorosulphanated polyethylene (CSP), polypropylene yarn with bitumen, HDPE, LLDPE etc.
The cable core with its water barrier and the mechanical protection system are the typical minimum of components required to make a functional power cable with comparable high electric power transferring capacity.
The water blocking effect of the water barrier according to the invention is verified by simulation of water intrusion through the water barrier. The simulation applied an embodiment of laminate structure comprising an adhesive layer between the metal foil and the first and second thermoplastic semiconducting polymer layers. The simulation is based on determination of the diffusion of water through the semiconducting polymer layer and the adhesive layer of the laminate structure. The metal foil was assumed impenetrable for water.
The calculations were made on an example embodiment shown in
The simulations are based on Fick's law of diffusion and Henry's law to determine the saturation and diffusion of moisture through the outer sheathing 21, adhesive layers 6 and the semiconducting polymer layer 5, 7, and the inner domain 20. The simulation method and the diffusion and solubility parameters applied in the calculations are taken from reference [1].
The diffusion coefficient D was calculated using the Arrhenius parameters D0 and ED. D0 and ED was 3.30·10−1 m2/s respectively 55.7 kJ/mol for the adhesive layers 6 and the polymeric parts of the laminate structure 5,7 as well as the inner domain 20.
The corresponding parameters for the outer sheathing 21 were 1.40·102 m2/s and 81.37 kJ/mol.
The solubility coefficient S was calculated using the Arrhenius parameters S0 and ES. S0 and ES was 1.80·10−7 kg/(m3Pa) respectively 9.90 kJ/mol for the adhesive layers 6 and the polymeric parts of the laminate structure 5,7 as well as the inner domain 20. The corresponding parameters for the outer sheathing 21 were 7.21·10−11 kg/(m3Pa) and −35.92 kJ/mol.
The temperature was assumed to be 40° C.
With these assumptions and parameters, the calculations gave that the time needed for 1 gram of moisture entering into the low-density polyethylene insulation layer 20 of the cable core was 320 years with an overlap (length w1) of 10 mm, 768 years with an overlap of 25 mm, and 1216 years with an overlap of 40 mm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20 306 579.2 | Dec 2020 | EP | regional |
