Process for impregnating thermal energy absorbing material into the structure of an electric cable, and respective electric cable

Description

Technical Field

[0001] The present application describes a process for impregnating thermal energy absorbing material into the structure of an electric cable, and respective electric cable.Summary

[0002] According to the invention there is provided a process for impregnating thermal energy absorbing material into the structure of an electric cable as defined in claim 1 and an electric cable as defined in claim 5. Further developments of the invention are the subject of the dependent claims.

[0003] The present application describes a process for impregnating thermal energy absorbing material into the structure of an electric cable. The process comprises the following steps: a) heating a bath of thermal energy absorbing material at a temperature between 50 °C and 100 °C; b) immersing a substrate in the bath of thermal energy absorbing material; c) passing through a dimensional rectification section for attaching the thermal energy absorbing material to the substrate.

[0004] In one embodiment of the process, the substrate is the shielding layer of an electric cable.

[0005] In another embodiment of the process, it comprises the further step of applying a strapping tape for restraining the thermal energy absorbing material when applied to the cavities of the shielding or cavities of the corrugated metal tape or sheath of an electric cable.

[0006] In another embodiment of the process, the substrate is an impregnation tape.

[0007] According to an aspect of the invention, in step b) the substrate passes through the bath of thermal energy absorbing material through a set of impregnation rollers.

[0008] The present application further describes an electric cable comprising a conductive core, a triple insulation layer, composed of two semiconductive layers and a layer of polymer insulation material, a shielding layer comprised of metal wires, at least one or more semiconductive tapes, a metal tape or sheath, either corrugated or not, and a sheath of polymer material; this cable is characterized in that it comprises a thermal energy absorbing material introduced between at least one of the structural sections of the electric cable according to the impregnation process previously described.

[0009] According to an aspect of the invention, the electric cable comprises a thermal energy absorbing material located in the cavities of the shielding layer to increase the energy conveyance capacity of the cable while maintaining its diameter.

[0010] In a particular embodiment of the electric cable, the thermal energy absorbing material is of an organic base having an operating range between 50 °C and 85 °C.

[0011] In a particular embodiment of the electric cable, the thermal energy absorbing material is a phase change material.

[0012] In another particular embodiment of the electric cable, the thermal energy absorbing material comprises a phase change material mixed with short chain hydrocarbons in amounts between 10% and 25% v / v.

[0013] In yet another particular embodiment of the electric cable, the thermal energy absorbing material to be impregnated in the cavities of the shielding - in empty spaces provided by the metal wires or cavities provided by the corrugated metal tape or sheath - is mixed with thickeners in amounts between 25% and 50% v / v.

[0014] In yet another particular embodiment of the electric cable, the thickener is glass fiber.

[0015] In yet another particular embodiment of the electric cable, the thickener is pyrogenic silica.Background art

[0016] The latent concerns about the environment and the depletion of natural resources impose the demand for durable products and more efficient operation, issues that have greatly motivated the innovations in the energy sector.

[0017] The conveyance of energy through high and very high voltage cables is conditioned by Joule-effect losses resulting from heat generation during the passage of electric current in the conductor, which limit the transmission distances since being proportional to the length of the installation. In addition to this limitation, insulated cables, mainly used in underground installations, for having polymer materials in its structure, make the maximum temperature attainable in the conductor a limiting factor for their dimensioning, such temperature being defined by the characteristics of said materials. This means that, despite having the capacity to convey higher currents, the conductor may not do so in the presence of these polymer materials that allow the correct and safe operation of the cable when buried, in order not to degrade said materials.

[0018] In these cables, the conductor is the area of greater temperature generation (as a function of the current passing therethrough), which is dissipated by the subsequent layers. The inner semiconductor and crosslinked polyethylene insulation (XLPE), layers following the conductor, are those subject to higher temperatures. The maximum temperature the XLPE material can undergo, in steady state, is 90 °C, so that the integrity thereof is not compromised. As a result of this restriction of the insulation material, the cables containing it can not convey energy in a steady state that might offer a temperature rise of more than 90 °C in the peripheral area to the conductor, in steady state.

[0019] CN103745772 (A) discloses a power cable consisting of a plurality of shielded conductors individually grouped and strapped by a polyester layer onto which a collective shielding is applied, and an outer sheath. For controlling temperature, this solution has two areas where a phase change material (PCM) is incorporated: one area between the polyester strapping and the collective shield, characterized by a higher melting point and another area in spaces between the several shielded conductors, characterized by a lower melting point. Based on the definition of PCM (thermal absorption stabilizing the temperature) and once applied in layers around the cable, PCM in this concept absorbs heat in its solid-solid transition phase stabilizing at a certain characteristic temperature of this material while maintaining the solid feature thereof. After this transition phase, PCM changes to its liquid state and here, due to gravity, the core ceases to be concentric with these PCM layers, making its operation inefficient (thermal diffusion is radially uniform). In its solid-solid transition phase, PCM has less thermal absorbing capacity than in its solid-liquid transition phase, which may lead to the need for larger quantities of material. The use of cables with more than two conductors allows, in the first place, to have an empty space available without requiring an increase in the final volume thereof, which in the case of a conductor cable would result in a volume increase that could be significant, thus having a direct impact on the entire production and installation process. Secondly, the low melting temperature PCM being housed in the empty spaces between conductors, when reaching its liquid state, the set of conductors acts as a structural element containing the PCM in the target areas, this meaning that in the case of a single conductor cable the required concentricity would disappear.

[0020] CN204178800 (U) has a three-pole high voltage cable, with a core consisting of the 3 insulated conductors, surrounded by a phase change composite, followed by an outer protective layer. The phase change composite is intended to improve current conveying capacity, overload capacity, thermal stability of the cable and useful life thereof. The proposed solution for this phase change composite involves a mixture of organic paraffin (also designated PCM) with polyethylene and expanded graphite. The temperature control in the cable occurs with the solid-liquid transition of the PCM, that is, PCM goes from its solid state to its liquid state. In this disclosure, PCM is on a support structure giving rise to the phase change composite. The application of this composite results in the reduction of PCM in the cable structure, a material that contributes to temperature control. Once this phase change composite has expanded graphite in its constitution significantly improves the electric conductivity of the material, however it does not contribute to the thermal absorption, feature allowing temperature control.

[0021] CN203983937 (U) discloses a utility model for a temperature-controlled high voltage cable connector by application of a phase change composite. The connector is a concentric tube structure reinforced with at least three structural fins, which contains a filler between its walls, which filler is a phase change composite. The components of the connector structure are metal materials and the phase change composite is also metal based. Temperature control is achieved by the phase change composite when it reaches its solid-liquid transition phase. This connector, when applied in the joint areas of high voltage cables, allows to control temperature rise in these areas, an optimization of the temperature control being inexistent in the cable since this connector is restricted to this operation area.

[0022] CN106531335A discloses a power transmission cable having an anti-icing function. The power transmission cable comprises a high temperature phase change material layer and a low temperature composite phase change material layer for slowing down heat escape and avoiding snow melting.General Description

[0023] The present application describes a process for impregnating thermal energy absorbing material into an electric cable, and respective electric cable. The technology herein developed has the purpose of maximizing the intensity of electric current conveyable by the cable and reducing Joule-effect losses over time allowing to increase its useful life. The technology developed involves the inclusion within the structure of the electric cable of a thermal energy absorbing material. The effect of the presence of the thermal energy absorbing material makes use of the latent heat in phase change, which could be that of the solid-liquid or solid-solid transition, for thermal energy storage and, therefore, reduces the temperature of the surrounding materials that make up the cable.

[0024] In this particular case, the electric cable consists of the following structure: a conductive core, covered by a triple insulation layer, inner semiconductor, reticulated polyethylene (XLPE) insulation and outer semiconductor, which in turn is enclosed by a shield, consisting of metal wires, which may be, for example, made of copper, aluminum or bimetallic material, semiconductive tapes and corrugated or non-corrugated metal tape or sheath, thereafter receiving a polymer sheath. The new developed electric cable structure introduces at least one layer of thermal energy absorbing material between said layers of the cable.

[0025] This introduction is achieved by an impregnation process, the thermal energy absorbing material being able to be applied between different layers constituting the inner structure of the cable, depending on its final application and on the gains intended. For example, the thermal energy absorbing material may be applied: in the empty (cavities) spaces of the shield, provided by the metal wires or in the cavities provided by the corrugated metal tape or sheath; between the semiconductor and shielding insulation layers; between the shielding and metal tape or sheath layers; in the conductor, possibly being placed on the surface, inside, or between the metal wires; and / or within the sheath.

[0026] The application of this material in the structure of a cable can be carried out in one of the areas above or in combination, as described in Table 1. Table 1: possible combinations for introducing the thermal energy absorbing material into the cable structure.CombinationSubstrateConductorTapes inside the shieldingCavities in the shielding and / or corrugated sheathTapes outside the shieldingsheath1X----2XX---3XXX--4XXXX-5XXXXX6-X---7-XX--8-XXX-9-XXXX10--X--11--XX-12--XXX13---X-14---XX15----X

[0027] The thermal energy absorbing material has the particularity of using its latent heat, responsible for its state change (for example, solid-liquid state change), to absorb or restore heat to known temperatures, allowing the cable thereby not to increase in temperature, thus increasing the amount of energy conveyed. In this way, the electric cable is able to capture and store the thermal energy released by the conductor when subjected to certain values of current intensity, as long as the thermal energy absorbing material reaches temperatures corresponding to its phase change temperature.

[0028] The introduction of such material is associated with a reduction of the maximum temperature of the conducting core and, consequently, with an increase in the current intensity that the cable can convey. In cases where the installation design contemplates a gradual increase in the current intensity conveyed annually, this solution, when compared to that using standard cables, allows to reduce the annual value of energy losses. Considering that this solution (use of phase change materials) for the same intensity of conveyed current, reduces the temperature in the insulation material, it has the effect of increasing the useful life of the cable.

[0029] With the structure herein proposed, the latent fusion heat of the phase change of the thermal energy absorbing material (e.g., solid-liquid or solid-solid) is used for heat storage and, therefore, for the reduction of the temperature in materials that make up the cable. In fact, the introduction of this material between one or more layers - in particular in the cavities of the shielding or upon its impregnation in semiconductive or non-semiconductive impregnation tapes, applicable before or after the shielding - allows increasing the energy conveyance capacity of the cable with smaller amounts of applied material, without increasing its conductor section, i.e. maintaining the cable diameter.

[0030] Regardless of the various locations for introducing the thermal energy absorbing material into the electric cable structure, the production process passes through its impregnation on a substrate (see Table 1). To this end, the substrate passes through a bath of thermal energy absorbing material followed by a dimensional control section, which ensures the proper amount of material applied to the cable. This amount is defined according to the following parameters: type of current cycle applied, location of the material within the cable structure, i.e. the substrate used, and section of the cable.

[0031] In the case where the thermal energy absorbing material is applied to the cavities of the shield, the substrate - formed by the cable structure up to the shielding - passes through a heated bath of thermal energy absorbing material and will attach to the substrate in its liquid state; and its attaching capacity increases significantly once it is exposed to room temperature. Thereafter, the substrate, already with the thermal energy absorbing material applied, passes through the rectification / dimensional control section composed of a spinneret (cylindrical or conical body) with rectified diameter of the desired section. While passing through this section the dimension of the cable is rectified by eliminating excess material and guaranteeing an uniform thickness of material that has attached to the impregnated substrate. In this section, the application of the quantity of thermal energy absorbing material required to fulfill the intended function by the definition of the diameter of the shielding wires is ensured.

[0032] When the impregnation process of the thermal energy absorbing material occurs on semiconductive tapes - substrate - the tape also passes through a bath allowing it to absorb the required amount of material. In order to guarantee the adequate quantity, this impregnation tape then passes through the dimensional control section, consisting of a set of rollers, which by pressure and temperature control allow to eliminate any excess material that may exist, that is also carried out by rectification of the final thickness. In this sequence the tape is applied over the cable by winding thereon, the amount of thermal energy absorbing material being defined by the number of tapes to be accommodated along the radius of the cable.

[0033] Both processes for introducing thermal energy absorbing material into the cable structure may occur in line with the manufacturing process of the electric cable or in parallel.

[0034] Since the thermal energy absorbing material must have a phase change profile consistent with the temperature that the cable in that area can reach, and with its ability to attach to the substrate, it is selected taking into account its location and expected temperature profiles. For the present application the thermal energy absorbing material is of organic base with operating ranges, i.e. its phase change temperature, comprised between 50 °C and 85 °C. The selected material is of an organic base, avoiding a set of possible situations not suitable for electric cables: those of inorganic base - metallic - despite their high thermal conductivity, would considerably increase the final weight of the cable in addition to being corrosive and susceptible to over- or undercooling; the hydrated salts would place water molecules within the cable upon liquefaction being still corrosive and having inadequate melting properties through the generation of two phases segregation; the eutectics, although having high energetic density, lack further studies on physicochemical properties. In addition, the organic base materials are commercially abundant, cover a wide range of phase change temperatures, are non-corrosive and are safe to use.

[0035] Where modifying the temperature of the thermal energy absorbing material is desirable, having regard to the target location between the layers of the cable structure, blends with short chain hydrocarbons (Vaseline or mineral oil) may be undertaken in varying amounts between 10% and 25% (v / v), which allow the temperature reduction of the mixture. On the other hand, very low viscosity is a characteristic of organic-based thermal energy absorbing materials (in solid-liquid transition) which may result in a reduction on the ability to attach to the substrate. In these situations, a mixture with thickeners such as glass fibers or pyrogenic silica can be carried out in amounts ranging from 25% to 50% (v / v), increasing the viscosity and the attaching ability of this mixture to the substrate to be impregnated. The application of this type of blend is desirable for the location where the thermal energy absorbing material is applied to the cavities of the shield.Description of the Figures

[0036] For an easier understanding of the present application, figures are herein attached, which represent preferred embodiments which however are not intended to limit the art herein disclosed. Figure 1: Schematic presentation of the impregnation of the thermal energy absorbing material on the cavities of the cable shield, wherein reference numbers refer to: 12 - thermal energy absorbing material; 14 - tub; 15 - rectification / dimensional control section. 16 - cable up to the shielding area. Figure 2: Schematic presentation of the impregnation of the thermal energy absorbing material on the impregnation tape, wherein reference numbers refer to: 12 - thermal energy absorbing material; 14 - tub; 15 - rectification / dimensional control section; 17 - impregnation rollers; 18 - impregnation tape. Figure 3: Schematic representation in axial section of an electric cable currently available, without thermal energy absorbing material, wherein reference numbers refer to: 1 - cable; 2 - conductive core; 3 - semiconductive tape; 4 - semiconductive polymer layer; 5 - polymer insulation material layer; 6 - shielding; 7 - metal wires; 8 - continuity metal tape; 9 - strapping tape 10 - metal tape or sheath; 11 - sheath. Figure 4: Schematic representation in axial section of an electric cable, with insertion of thermal energy absorbing material in the cavities of the shielding, wherein reference numbers refer to: 1 - cable; 2 - conductive core; 3 - semiconductive tape; 4 - semiconductive polymer layer; 5 - polymer insulation material layer; 6 - shielding; 7 - metal wires; 8 - continuity metal tape; 9 - strapping tape 10 - metal tape or sheath; 11 - sheath; 12 - thermal energy absorbing material. Figure 5: Schematic representation in axial section of an electric cable, with insertion of thermal energy absorbing material in the cavities of the metal tape or sheath, wherein reference numbers refer to: 1 - cable; 2 - conductive core; 3 - semiconductive tape; 4 - semiconductive polymer layer; 5 - polymer insulation material layer; 6 - shielding; 11 - sheath; 12 - thermal energy absorbing material; 13 - corrugated metal tape or sheath. Figure 6: Schematic representation in axial section of an electric cable, with insertion of thermal energy absorbing material between the shielding and the metal tape, wherein reference numbers refer to: 1 - cable; 2 - conductive core; 3 - semiconductive tape; 4 - semiconductive polymer layer; 5 - polymer insulation material layer; 6 - shielding; 7 - metal wires; 8 - continuity metal tape; 9 - strapping tape; 10 - metal tape or sheath; 11 - sheath; 12 - thermal energy absorbing material. Figure 7: Schematic representation in axial section of an electric cable, with insertion of thermal energy absorbing material after the conductor, wherein reference numbers refer to: 1 - cable; 2 - conductive core; 3 - semiconductive tape; 4 - semiconductive polymer layer; 5 - polymer insulation material layer; 6 - shielding; 7 - metal wires; 8 - continuity metal tape; 9 - strapping tape 10 - metal tape or sheath; 11 - sheath; 12 - thermal energy absorbing material. Figure 8: Schematic representation in axial section of an electric cable, with insertion of thermal energy absorbing material inside the sheath, wherein reference numbers refer to: 1 - cable; 2 - conductive core; 3 - semiconductive tape; 4 - semiconductive polymer layer; 5 - polymer insulation material layer; 6 - shielding; 7 - metal wires; 8 - continuity metal tape; 9 - strapping tape 10 - metal tape or sheath; 11 - sheath; 12 - thermal energy absorbing material. Description of the embodiments

[0037] Referring to the drawings, some particular embodiments shall now be described in more detail, which are not intended, however, to limit the scope of the present application.

[0038] The present application describes an electric cable (1) with high efficiency - according to experimental tests and depending on the type of conductive material, the increase in current intensity can be up to 10% - developed based on a conventional cable composed of a conductive core (2), surrounded by a semiconductive tape (3) wherein a triple insulation layer is then extruded: composed of an inner semiconductive layer (4), a layer of polymer insulation material (5), and a semiconductor layer (4) on the insulation. In this sequence, a semiconductive tape (3) is applied which is subsequently covered by a shielding (6) containing: metal wires (7) and which may further include a continuity metal tape (8) followed by a strapping tape (9). The latter is applied with a metal tape or sheath, which may be smooth or corrugated. Finally, a sheath (11) for protection against the surrounding environment is extruded onto the shielding (6).

[0039] This structure is added with at least one layer of thermal energy absorbing material (12) between the identified cable sections. In the context of the present application, a thermal energy absorbing material (12) may be a phase change material (PCM) or a mixture of a PCM material with short chain hydrocarbons or with thickeners.

[0040] Based on the structure of the electric cable (1) shown, the thermal energy absorbing material (12) may be located in the conductive core (2) or in the cavities of the shielding (6) or between the semiconductive tape (3) and shielding (6) or before the metal tape or sheath (10), or inside the sheath (11).

[0041] According to an aspect of the invention, the thermal energy absorbing material (12) impregnates the cavities of the shielding (6). After winding the metal wires (7) of the shielding (6), the cable - substrate - passes through a heated bath of thermal energy absorbing material (12), which in this case consists of a mixture of PCM material with thickeners, such as glass fibers or pyrogenic silica, in varying amounts between 25% and 50% (v / v). This heated bath is contained in a tub (14) in which the thermal energy absorbing material (12) will be at a minimum height allowing to cover the cable and a length such as to allow the residence time of immersion capable of accommodating the material, associated with a process speed defined by the speed of production of the cable, which is a function of the productive line in which it is inserted and of the type of cable under construction. To ensure such impregnation in the cable, said tub (14) has to be heated in order to maintain the thermal energy absorbing material (12) in its liquid state, whereby its temperature can vary from 50 °C to 100 °C, depending on the type of thermal energy absorbing material (12) in application. The later will occupy the cavities therein, attaching to the cable when it crosses the rectification / dimensional control section (15), at which point its solidification temperature is reached. This process is followed by the application of a semiconductive or non-semiconductive strapping tape (9), which in addition to its regular function will aid in restraining the thermal energy absorbing material (12) in the shielding.

[0042] In another particular embodiment, the thermal energy absorbing material (12) impregnates the impregnation tape (18). In this case, the material may be composed exclusively of thermal energy absorbing material or be a mixture between thermal energy absorbing material and short chain hydrocarbons (Vaseline or mineral oil) in amounts varying between 10% and 25% (v / v). This substrate passes through a heated bath of thermal energy absorbing material (12) with a set of impregnation rollers (17) followed by compacting rollers - dimensional control section (15) - for controlling the amount of thermal energy absorbing material (12) inserted into the cable structure. Also in this embodiment the bath temperature shall depend on the type of thermal energy absorbing material (12) applied, i.e., it shall be between 50 °C and 100 °C.

[0043] The present application applies to the conveyance of energy at high or very high voltage, for example in underground single conductor cables and provides a maximization of the allowable current intensity, i.e. the introduction of the thermal energy absorbing material (12) allows reducing the temperature inside the cable, whose limit is imposed by the XLPE that covers the conductive core wherein the limit is of 90 °C, in steady state. In addition, this temperature reduction allows Joule-effect losses to be reduced over the time of cable use, and by reducing the operating temperature of the insulation, it can increase the useful life of the cable.

[0044] The present description is of course in no way restricted to the embodiments presented herein and a person of ordinary skill in the art may provide many possibilities of modifying it without departing from the scope as defined in the claims. The preferred embodiments described above are obviously combinable with each other. The following claims further define preferred embodiments.

Claims

1. A process for impregnating thermal energy absorbing material (12) into the structure of an electric cable (1), comprising the steps of: a) heating a bath of thermal energy absorbing material (12) at a temperature between 50 °C and 100 °C; b) immersing the substrate of the electrical cable (1) in the heated bath of thermal energy absorbing material (12); and c) passing through a dimensional rectification section (15) for fixing the thermal energy absorbing material (12) to the substrate; characterized by the immersion comprising passing the substrate of the electrical cable (1) though a set of impregnation rollers (17) submerged within the thermal energy absorbing material (12).

2. Process according to claim 1, characterized in that the substrate is the shielding layer (6) of an electric cable (1).

3. Process according to any of the preceding claims, characterized in that it comprises the further step of applying a strapping tape (9) for restraining the thermal energy absorbing material (12) when applied to the cavities of the shielding (6) or cavities of the corrugated metal tape or sheath (13) of the electric cable (1).

4. Process according to any of the preceding claims, characterized in that the substrate is an impregnation tape (18).

5. Electric cable (1) comprising a conductive core (2); a triple insulation layer, covering the conductive core (2), composed of two semiconductive layers (4), an inner and an outer semiconductor, and a layer of polymer insulation material (5) comprised in between, the triple insulation layer being enclosed by a shielding layer (6) comprised of metal wires (7), at least one semiconductive tape (3), and a metal tape or sheath (10); and a sheath of polymer material (11); said cable being characterized by comprising a thermal energy absorbing (12) material located in the cavities of the shielding layer (6) to increase the energy conveyance capacity of the cable (1) while maintaining its diameter.

6. Electric cable (1) according to claim 6, characterized in that the metal tape or sheath (10) is corrugated.

7. Electric cable (1) according to any of the preceding claims 6 or 7, characterized in that the thermal energy absorbing material (12) is of an organic base having an operating range between 50 °C and 85 °C.

8. Electric cable (1) according to claim 8, characterized in that the thermal energy absorbing material (12) is a phase change material.

9. Electric cable (1) according to any claims 8 or 9, characterized in that the thermal energy absorbing material (12) comprises a phase change material mixed with short chain hydrocarbons in amounts between 10% and 25% v / v.

10. Electric cable (1) according to any of the preceding claims, characterized in that the thermal energy absorbing material (12) to be impregnated in the cavities of the shielding (6) - in empty spaces provided by the metal wires (7) or cavities provided by the corrugated metal tape or sheath (10) - comprises a mixture with thickeners in amounts between 25% and 50% v / v.

11. Electric cable (1) according to claim 11, characterized in that the thickener is glass fiber.

12. Electric cable (1) according to claim 11, characterized in that the thickener is pyrogenic silica.

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