Medium voltage fire resistant stranded conductor, power cable and method of forming same

Abstract

This invention discloses a medium-voltage fire-resistant stranded conductor, a power cable, and its forming method, relating to the field of power facility technology. The cable includes, from the inside out, a conductor, an extruded semi-conductive conductor shielding layer, a cross-linked polyethylene insulation layer, an extruded semi-conductive insulation shielding layer, a wrapped semi-conductive tape, and a metal shielding layer. The extruded semi-conductive conductor shielding layer and the insulation shielding layer eliminate air gaps between the conductor surface burrs and the insulation layer interface, achieving uniform electric field distribution and preventing partial discharge. The wrapped semi-conductive tape not only acts as a filling and buffering "cushion," preventing the metal shielding layer from puncturing or damaging the internal XLPE insulation layer during bending or thermal expansion, but also ensures electrical continuity between the insulation shielding layer and the metal shielding layer through its semi-conductive properties. This reliably guides fault current or induced voltage into the grounding system, significantly improving the safety margin and lifespan of the cable under long-term operation and overvoltage conditions.

Description

Technical Field

[0001] This invention relates to the field of power facility technology, specifically to a medium-voltage fire-resistant stranded conductor, a power cable, and a method for forming the same. Background Technology

[0002] Medium-voltage fire-resistant cables differ from traditional low-voltage fire-resistant cables. Traditional low-voltage fire-resistant cables use inorganic mica tape wrapped around the conductor. Inorganic mica tape is a composite of mica layer and fiberglass base cloth, and is a non-combustible material. Even if the external materials catch fire in a fire, it can ensure the cable's safe and reliable operation for a certain period of time. However, considering the electrical performance requirements, medium-voltage fire-resistant cables cannot be designed using this method of wrapping the conductor with mica tape. The wrapping of mica tape will make the conductor surface rough, leading to uneven electric field distribution inside the cable. Existing technology uses a semi-conductive shielding layer, an insulation layer, and a metal shielding layer wrapped around the conductor. However, when the cable is bent, heated, or subjected to mechanical impact, the metal shielding layer is prone to deformation, and its edges or burrs may pierce or compress the soft inner insulation shielding layer and insulation layer, causing insulation damage and thus triggering discharge faults. In severe cases, this can cause the cable to fail before or in the early stages of a fire.

[0003] Therefore, it is necessary to develop and design medium-voltage fire-resistant stranded conductors, power cables and their forming methods to avoid insulation failure due to damage to the metal shielding layer, while also possessing reliable fire-resistant properties. This is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a medium-voltage fire-resistant stranded conductor, a power cable, and a method for forming the same, which avoids insulation failure due to damage to the metal shielding layer while also possessing reliable fire-resistant properties.

[0005] To achieve the above objectives, the present invention provides the following solution: A medium-voltage fire-resistant stranded conductor includes, from the inside out, a conductor, an extruded semi-conductive conductor shielding layer, a cross-linked polyethylene insulation layer, an extruded semi-conductive insulation shielding layer, a wrapped semi-conductive tape, and a metal shielding layer.

[0006] Preferably, the conductor includes a circular core wire located in the middle and at least three fan-shaped core wires arranged in the circumferential direction of the circular core wire to match the curvature of the circular core wire, and the at least three fan-shaped core wires sequentially wrap around the circular core wire to form a circular structure.

[0007] Preferably, the wrapped semiconductive strip is configured as two layers, the thickness of the wrapped semiconductive strip is 0.20mm, and the wrapping overlap rate of the two layers of the wrapped semiconductive strip is 40%~50%.

[0008] Preferably, the metal shielding layer is a copper strip shielding layer, and the overlap rate of the copper strip shielding layer is 5%.

[0009] The present invention also discloses a medium-voltage fire-resistant power cable, comprising at least three medium-voltage fire-resistant stranded conductors, and further comprising, from the inside out, PP filler rope, PET wrapping tape, a first low-smoke halogen-free oxygen barrier layer, a second low-smoke halogen-free oxygen barrier layer, a double-sided mica tape layer, and a low-smoke halogen-free outer sheath, which are sequentially wrapped around the at least three medium-voltage fire-resistant stranded conductors.

[0010] Preferably, the PET wrapping tape is configured as two layers, and the two layers overlap wrapping, with the wrapping overlap rate of the two layers of PET wrapping tape being 40%~50%.

[0011] Preferably, both the first low-smoke halogen-free oxygen barrier layer and the second low-smoke halogen-free oxygen barrier layer are extruded from low-smoke halogen-free materials with an oxygen index of not less than 40%.

[0012] Preferably, the thickness of the double-sided mica tape layer is 0.14 mm, and the double-sided mica tape layer is configured as two overlapping wrapping layers, with the wrapping overlap rate of the double-sided mica tape layer being not less than 15%.

[0013] Preferably, the low-smoke halogen-free outer sheath is formed by extrusion of a low-smoke halogen-free material with an oxygen index of not less than 32%.

[0014] This invention also discloses a method for forming a medium-voltage fire-resistant power cable, which utilizes the medium-voltage fire-resistant power cable described above and includes the following steps: Conductors are formed through a conductor stranding process; The extruded semi-conductive conductor shielding layer, the cross-linked polyethylene insulation layer, and the extruded semi-conductive insulation shielding layer are formed through a three-layer co-extrusion process. Complete the wrapping of the semiconducting strip and the metal shielding layer; The PP filler rope is used to fill the cable until it is rounded. PET wrapping tape; The low-smoke halogen-free oxygen barrier layer extrusion process is carried out sequentially to form the first low-smoke halogen-free oxygen barrier layer and the second low-smoke halogen-free oxygen barrier layer. The wrapping process forms a double-sided mica tape layer; The low-smoke halogen-free outer sheath is formed by the extrusion process.

[0015] The present invention achieves the following technical effects compared to the prior art: By constructing a complete fully shielded voltage equalization system from the conductor to the metal shielding layer, electrical reliability and structural safety are significantly improved. Among them, the extruded semi-conductive conductor shielding layer and the insulating shielding layer eliminate the air gap between the conductor surface burrs and the interface of the insulating layer, realizing the uniform distribution of the electric field and effectively preventing partial discharge. The wrapped semi-conductive tape located between the insulating shielding layer and the metal shielding layer not only plays the role of a "cushion" for filling and buffering, preventing the metal shielding layer (such as the edge of the copper tape or the corrugated aluminum sheath) from puncturing or damaging the internal XLPE insulation layer when bending or thermally expanding, but also ensures the electrical continuity between the insulating shielding layer and the metal shielding layer through its semi-conductive properties. This allows fault current or induced voltage to be reliably introduced into the grounding system, greatly improving the safety margin and life of the cable under long-term operation and overvoltage conditions. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Appendix Figure 1 This is a schematic diagram of the overall structure of the medium-voltage fire-resistant power cable disclosed in this invention; Appendix Figure 2 This is a schematic diagram of the overall conductor structure in the medium-voltage fire-resistant stranded wire disclosed in this invention; Among them, 1. Conductor; 2. Extruded semi-conductive conductor shielding layer; 3. XLPE insulation layer; 4. Extruded semi-conductive insulation shielding layer; 5. Wrapped semi-conductive tape; 6. Copper tape shielding layer; 7. PP filler rope; 8. PET wrapping tape; 9. First low-smoke halogen-free oxygen barrier layer; 10. Second low-smoke halogen-free oxygen barrier layer; 11. Double-sided mica tape layer; 12. Low-smoke halogen-free outer sheath; 13. Round core wire; 14. Fan-shaped core wire. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] The purpose of this invention is to provide a medium-voltage fire-resistant stranded conductor, a power cable and its forming method, which avoids insulation failure due to damage to the metal shielding layer, while also possessing reliable fire-resistant properties.

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] refer to Figures 1-2 The medium-voltage fire-resistant stranded conductor disclosed in this embodiment of the invention includes at least, from the inside out, a conductor 1, an extruded semi-conductive conductor shielding layer 2, a cross-linked polyethylene insulation layer (hereinafter referred to as XLPE insulation layer 3), an extruded semi-conductive insulation shielding layer 4, a wrapped semi-conductive tape 5, and a metal shielding layer. By constructing a complete fully shielded voltage equalization system from the conductor 1 to the metal shielding layer, the electrical reliability and structural safety are significantly improved. Among them, the extruded semi-conductive conductor shielding layer 2 and the extruded semi-conductive insulation shielding layer 4 eliminate the air gap between the burrs on the surface of the conductor 1 and the interface of the XLPE insulation layer 3. This achieves a uniform electric field distribution, effectively preventing partial discharge. The wrapped semiconductive tape 5, located between the extruded semiconductor insulating shield layer and the metal shield layer, not only acts as a "cushion" for filling and buffering, preventing the metal shield layer (such as the edge of the copper tape or the corrugated aluminum sleeve) from puncturing or damaging the internal XLPE insulating layer 3 when bent or thermally expanded, but also ensures the electrical continuity between the insulating shield layer and the metal shield layer through its semiconductive properties. This allows the fault current or induced voltage to be reliably introduced into the grounding system, greatly improving the safety margin and lifespan of the cable under long-term operation and overvoltage conditions.

[0022] It should be noted that the material of the extruded semiconducting conductor shielding layer is a thermosetting semiconducting material composed of ethylene-vinyl acetate copolymer (EVA) or low-density polyethylene (LDPE) as the base resin, with the addition of highly conductive carbon black, crosslinking agent (such as dicumyl peroxide DCP), and various additives. The material of the extruded semiconducting insulating shielding layer is similar to that of the extruded semiconducting conductor shielding layer, which is also a crosslinkable semiconducting shielding material. However, there are some key differences in the formulation design and performance requirements to adapt to its functional positioning on the outside of the insulation layer.

[0023] refer to Figures 1-2In one embodiment, conductor 1 includes a circular core wire 13 with a circular cross-section located in the middle. At least three fan-shaped core wires 14 with fan-shaped cross-sections are arranged around the circumference of the circular core wire 13. The curvature of the fan-shaped core wires 14 where they meet the circular core wire 13 is consistent with the curvature of the circular core wire 13. Thus, when the at least three fan-shaped core wires 14 meet the circular core wire 13, a circular cross-section is formed. By setting the circular core wire 13 and attaching at least three fan-shaped core wires 14 with matching curvatures around it, a stable and highly rounded circular cross-section is formed. This structure not only eliminates the gap in the center of the traditional conductor 1, making the conductor 1 compaction coefficient reach 98%, avoiding core slippage and deformation, but also provides an ideal base surface for the uniform extrusion of the subsequent shielding and insulation layers, thereby effectively improving the electric field distribution and suppressing partial discharge. At the same time, maximizing the fill factor reduces the amount of material used and the outer diameter of the cable, achieving a synergistic improvement in electrical performance, structural strength and economy.

[0024] In addition, the structure of the circular core wire 13 and the fan-shaped new conductor 1 has less cold deformation process during stranding. Therefore, the increase in DC resistance of conductor 1 caused by work hardening is smaller than that of circular single wire stranded circular conductor 1. Under the condition of achieving the same DC resistance of conductor 1, the cross-section of conductor 1 can be reduced by about 1% to 2%, so as to achieve the purpose of energy saving, environmental protection, and low carbon emission reduction.

[0025] The material of conductor 1 is copper, but it can also be conductor 1 made of other materials, such as aluminum.

[0026] It should be noted that the fan-shaped core wires 14 are arranged in at least two layers from the inside out. The fan-shaped core wires 14 on the outer layer have the same curvature as those on the inner layer. The inner fan-shaped core wires 14 provide a continuous and curvature-matched support base for the outer layer, enabling the outer fan-shaped core wires 14 to be precisely bonded and evenly arranged. This achieves a tight stacking and high roundness of the multi-layer conductor 1 structure without the need for additional filler material. At the same time, the consistent curvature of the inner and outer layers ensures uniform stress distribution among the core wires, avoiding local gaps or mechanical stress concentration caused by abrupt changes in curvature, and significantly improving the structural stability and bending fatigue resistance of the conductor 1. In addition, this structure allows for flexible adjustment of the number of layers and the size of the single filament under the same conductor 1 cross-sectional area, effectively suppressing the skin effect and reducing AC resistance, thereby improving the current carrying capacity and transmission efficiency of the cable.

[0027] It should be noted that conductor 1 can also be formed by twisting multiple Z-shaped core wires together. The adjacent Z-shaped core wires are staggered. The unique geometry of the Z-shaped core wires enables precise interlocking between adjacent single wires, forming an "interlocking" effect during the twisting process. This fundamentally eliminates the triangular or wedge-shaped gaps that inevitably exist when twisting traditional circular single wires, making the filling coefficient of conductor 1 theoretically close to 100%, thus achieving a truly gapless and compact structure.

[0028] refer to Figures 1-2 In one implementation, the wrapped semiconductive strip 5 is configured as two layers with a thickness of 0.20 mm. The overlap rate of the two layers of wrapped semiconductive strip 5 is 40%~50%. The thickness of a single layer of 0.20 mm ensures that the semiconductive strip has sufficient mechanical strength and electrical continuity, while the two-layer wrapping structure provides dual protection: it increases the overall thickness and compression resilience in the radial direction, effectively absorbing the mechanical stress generated by the metal shielding layer and preventing it from damaging the internal insulation shielding layer; at the same time, the 40%~50% wrapping overlap rate ensures that a tight electrical path is formed between the two layers of semiconductive strip and at the overlap within each layer, eliminating the risk of floating potential or partial discharge that may be caused by wrapping gaps; in addition, this overlap rate range achieves the optimal balance between material usage and shielding effect while ensuring the feasibility of the wrapping process. It avoids shielding discontinuity caused by too low an overlap rate, and also prevents material waste and cable outer diameter increase caused by too high an overlap rate. Thus, it significantly improves the partial discharge suppression capability and long-term operational reliability of medium-voltage cables in an economical way.

[0029] It should be noted that the wrapped semi-conductive tape 5 is one of semi-conductive terylene tape, semi-conductive nylon tape, or semi-conductive polyester nonwoven tape.

[0030] refer to Figures 1-2 As a preferred approach, the metal shielding layer is a copper tape shielding layer 6 with an overlap rate of 5%. Compared to the traditional overlap rate design of 15% to 25%, the low overlap rate of 5% significantly reduces the amount of copper tape material used, lowers the cable manufacturing cost and outer diameter, and avoids poor heat dissipation or local electric field distortion caused by excessive overlap. This overlap rate, combined with appropriate wrapping tension, can ensure that the copper tape layer maintains a continuous electrical path under dynamic conditions such as cable bending and thermal expansion, without slippage or detachment. Thus, it achieves the triple functions of electric field shielding, fault current transmission and mechanical protection of medium-voltage cables in an economical and efficient manner.

[0031] It should be noted that single-core cables use a single copper tape shielding layer 6 with a nominal thickness of 0.12mm, while three-core cables use a single copper tape shielding layer 6 with a nominal thickness of 0.10mm. The single-core cable uses a 0.12mm copper tape shielding layer 6 because the induced current on the copper tape is large and generates significant heat during operation, and it lacks mechanical support from adjacent cores. The thicker copper tape provides higher current carrying capacity to withstand the induced current, while also enhancing the mechanical strength of the single-core cable against copper tape breakage or loosening during bending and laying. The three-core cable uses a 0.10mm copper tape shielding layer 6 because the combined magnetic field is smaller during three-phase balanced operation, and the induced current and heat generation of the copper tape are significantly lower than those of the single-core cable. Furthermore, the three cores are tightly packed and mutually supportive after cabling, so a thickness of 0.10mm is sufficient to meet the shielding and fault current carrying requirements. This differentiated design avoids overheating or mechanical failure caused by excessively thin copper strips in single-core cables, and also prevents material waste and increased outer diameter caused by excessively thick copper strips in three-core cables, thus achieving reliable shielding and long-term operational safety for all types of cables in the most economical way.

[0032] In addition, the existing technology sets flame-retardant wrapping tape between adjacent low-smoke halogen-free oxygen barrier layers. The wrapping process of flame-retardant wrapping tape increases the production process and equipment investment, and reduces product turnover efficiency. Microscopic gaps or cavities inevitably exist at the overlap of the wrapping tape and the interface between layers. These gaps may become channels for oxygen penetration during a fire, which weakens the oxygen barrier effect. refer to Figures 1-2 This invention also discloses a medium-voltage fire-resistant power cable, comprising at least three medium-voltage fire-resistant stranded conductors, and further comprising, sequentially from the inside out, PP filler rope 7, PET wrapping tape 8, a first low-smoke halogen-free oxygen barrier layer 9, a second low-smoke halogen-free oxygen barrier layer 10, a double-sided mica tape layer 11, and a low-smoke halogen-free outer sheath 12, all wrapped around the at least three medium-voltage fire-resistant stranded conductors. After the at least three stranded conductors are cabled, the PP filler rope 7 ensures the roundness and structural stability between the conductor cores, providing a uniform support base for subsequent functional layers; the PET wrapping tape 8 serves a binding and fixing function, while... With its high heat resistance (melting point 260℃), it maintains structural integrity in the early stages of a fire. The double-layer design of the first low-smoke halogen-free oxygen barrier layer 9 and the second low-smoke halogen-free oxygen barrier layer 10 releases water of crystallization and forms a dense carbonized layer when heated, effectively preventing oxygen from diffusing into the interior, blocking heat conduction, and delaying the temperature rise of the insulation layer. The double-sided mica tape layer 11 serves as a key fire-resistant barrier, forming a hard ceramic-like insulating shell at high temperatures, ensuring that the cable maintains its electrical integrity even when exposed to flames. The outer low-smoke halogen-free sheath produces extremely low smoke and does not release toxic hydrogen halides during combustion, ensuring the safety of personnel evacuation. This multi-layered synergistic protection structure allows the cable to safely supply power for more than 90 minutes under flames of 750℃~950℃, while meeting environmental protection requirements such as low smoke, halogen-free, and flame retardant properties. It is particularly suitable for places with extremely high fire safety requirements, such as subways, high-rise buildings, and nuclear power plants.

[0033] In addition, in this embodiment, a first low-smoke halogen-free oxygen barrier layer 9 and a second low-smoke halogen-free oxygen barrier layer 10 are sequentially provided. After the first oxygen barrier layer is extruded, the second oxygen barrier layer can be continuously extruded immediately without waiting for auxiliary processes such as wrapping and binding. This not only simplifies the production process and shortens the manufacturing cycle, but also reduces the investment and maintenance costs of wrapping equipment, and realizes rapid product turnover. The two layers of low-smoke halogen-free materials are directly and tightly bonded during the hot extrusion process, completely eliminating the gaps between the layers. This follows the basic principle that "combustion must have oxygen." The flame cannot penetrate into the cable through the non-existent gaps, thus cutting off the physical path of oxygen intrusion from the structural design.

[0034] refer to Figures 1-2 As one implementation method, the PET wrapping tape 8 (polyethylene terephthalate wrapping tape) is set as two layers, and the two layers are overlapped. The overlap rate of the two layers of PET wrapping tape 8 is 40%~50%. This 40%~50% overlap of PET wrapping tape 8 (polyethylene terephthalate) replaces the traditional flame-retardant wrapping tape. The PET wrapping tape 8 has a hollow expansion structure similar to a sponge, and is rich in static air. Utilizing the principle that air has an extremely low thermal conductivity (approximately 0.026 W / m·K), this... The hollow structure forms a highly efficient heat insulation barrier, which can effectively block the conduction of external fire heat to the inside of the cable, ensuring that the insulation layer remains within the normal operating temperature range under flame burning. By utilizing the hollow low-density structure of PET wrapping tape 8 and the double-layer overlapping wrapping process, a thicker heat insulation layer is constructed with a lighter material weight, breaking through the limitation of traditional flame-retardant wrapping tape that "increases in thickness and increases in weight". It achieves synergistic optimization in the three mutually restrictive dimensions of heat insulation effect, material consumption and cable weight, significantly improving the economic efficiency and technical competitiveness of medium-voltage fire-resistant cables.

[0035] It should be noted that the weight of PET wrapping tape 8 is 170±20g / m. 2 The PET wrapping tape 8 is made by bonding fibers onto a 0.05mm polyester tape. It has a fibrous hollow structure, which provides excellent heat insulation. The bottom 0.05mm polyester tape provides a stable substrate for the fibrous fibers, ensuring the tensile strength and dimensional stability of the material during the wrapping process. It prevents the fiber layer from tearing or becoming uneven in thickness due to excessive tension, thus ensuring the reliability and consistency of the wrapping process.

[0036] refer to Figures 1-2As an implementation method, both the first low-smoke halogen-free oxygen barrier layer 9 and the second low-smoke halogen-free oxygen barrier layer 10 are extruded from low-smoke halogen-free materials with an oxygen index of not less than 40%. The first low-smoke halogen-free oxygen barrier layer 9 and the second low-smoke halogen-free oxygen barrier layer 10 are made of the same material, which makes production more convenient, shortens production time, avoids material waste caused by material change and energy waste caused by equipment heating, and achieves the purpose of energy saving, environmental protection, and low carbon emission reduction. At the same time, the first low-smoke halogen-free oxygen barrier layer 9 and the second low-smoke halogen-free oxygen barrier layer 10 are tightly bonded to prevent air from entering due to gaps caused by wrapping tape, thus accelerating combustion.

[0037] In addition, the oxygen index of the first low-smoke halogen-free oxygen barrier layer 9 and the second low-smoke halogen-free oxygen barrier layer 10 is not less than 40%, which means that the material is difficult to spontaneously combust in air (the oxygen concentration in conventional air is about 21%). Even when placed in a flame, it has a very strong self-extinguishing ability. This characteristic makes the two oxygen barrier layers themselves a highly efficient flame-retardant barrier, which can actively inhibit the spread of combustion in a fire.

[0038] It should be noted that the materials of the first low-smoke halogen-free oxygen barrier layer 9 and the second low-smoke halogen-free oxygen barrier layer 10 are low-smoke halogen-free flame-retardant polyolefin (LSZH). Specifically, it is a special cable material composed of polyolefin resin (such as EVA, PE, etc.) as the base material, with the addition of inorganic flame retardants (such as aluminum hydroxide, magnesium hydroxide) and a variety of functional additives.

[0039] refer to Figures 1-2 As one implementation method, the thickness of the double-sided mica tape layer 11 is 0.14mm. The double-sided mica tape layer 11 is set as two overlapping wraps with an overlap rate of not less than 15%. The two overlapping wraps form a double fire-resistant insulation barrier. Even if the outer mica tape is damaged in a fire due to mechanical impact or local overheating, the inner layer can still maintain the insulation integrity independently, ensuring that the cable continues to supply power under flame burning. The single-layer thickness of 0.14mm takes into account both mechanical strength and wrapping flexibility. The actual thickness after the two layers are superimposed can reach more than 0.56mm, forming a thick and dense fire-resistant shell. An overlap rate of no less than 15% ensures that the mica tape can continuously cover the cable under dynamic conditions such as cable bending and thermal expansion and contraction, without any exposed gaps, thus eliminating weak points in insulation caused by wrapping gaps. At the same time, both sides of the double-sided mica tape are reinforced with reinforcing materials, resulting in high tensile strength. This prevents mica powder from falling off or the substrate from tearing during wrapping and subsequent processes, ensuring the integrity of the refractory layer and the reliability of the production process. It also helps to tightly wrap the second low-smoke halogen-free oxygen barrier layer 10 after carbonization, preventing it from cracking and falling off, thereby forming a carbonized refractory layer and enhancing the fireproofing effect.

[0040] refer to Figures 1-2As an implementation method, the low-smoke halogen-free outer sheath 12 is extruded from a low-smoke halogen-free material with an oxygen index of not less than 32%. When burning, the low-smoke halogen-free outer sheath 12 produces extremely low smoke (light transmittance of not less than 60%) and does not release toxic and corrosive gases such as hydrogen halides. In a fire, it provides a clear visual environment and relatively safe breathing conditions for personnel evacuation and fire rescue. In addition, the outer sheath and the inner double-layer low-smoke halogen-free oxygen barrier layer (oxygen index ≥ 40%) form a gradient flame retardant system from the outside to the inside. The outer sheath initially blocks the flame, and the inner oxygen barrier layer further isolates oxygen and heat. The synergistic effect of the various fire-resistant functional layers significantly improves the overall fire resistance performance and fire safety level of the cable.

[0041] It should be noted that the low-smoke halogen-free outer sheath 12 is made of the same material as the first low-smoke halogen-free oxygen barrier layer 9 and the second low-smoke halogen-free oxygen barrier layer 10.

[0042] This invention also discloses a method for forming a medium-voltage fire-resistant power cable, which utilizes the medium-voltage fire-resistant power cable described above and includes the following steps: Conductor stranding step: Conductor 1 is formed through a conductor stranding process; Three-layer co-extrusion process: Through the three-layer co-extrusion process, an extruded semi-conductive conductor shielding layer 2, a cross-linked polyethylene insulation layer and an extruded semi-conductive insulation shielding layer 4 are sequentially formed on the outer layer of conductor 1. Shielding wrapping steps: On the outside of the extruded semi-conductive insulating shielding layer 4, the wrapping of the semi-conductive tape 5 and the metal shielding layer are completed in sequence. Filling and cabling steps: Fill the outside of the metal shielding layer with PP filler rope 7 to fill the cable into a round shape, forming a round cable core; PET wrapping steps: Wrap PET wrapping tape 8 around the outside of the filled and rounded cable core; Oxygen barrier extrusion step: On the outside of the PET wrapping tape 8, the low smoke halogen-free oxygen barrier extrusion process is performed in sequence to form the first low smoke halogen-free oxygen barrier layer 9 and the second low smoke halogen-free oxygen barrier layer 10. Mica tape wrapping step: On the outside of the second low-smoke halogen-free oxygen barrier layer 10, a double-sided mica tape layer 11 is formed by wrapping. Outer sheath extrusion step: On the outside of the double-sided mica tape layer 11, a low-smoke halogen-free outer sheath 12 is formed by an extrusion process to obtain a medium-voltage fire-resistant cable.

[0043] The following is a comparison of the thickness and weight of each layer in the present invention and the comparative examples: (1) Thickness and material usage of each layer in the present invention (the material layers whose thickness is not listed in the table are the same as the material layers with the same thickness in common medium-voltage fire-resistant cables) Table 1 Dimensions of each layer in this invention

[0044] Table 2. Material usage of each layer and total cable weight of the present invention.

[0045] The total thickness of the medium-voltage fire-resistant cable in this invention is calculated as follows: PET wrapping tape thickness + first low-smoke halogen-free oxygen barrier layer thickness + second low-smoke halogen-free oxygen barrier layer thickness + double-sided mica tape layer thickness + low-smoke halogen-free outer sheath thickness = 14.6 mm. This structure has a thicker total thickness for both insulation and fire resistance, resulting in better insulation. Furthermore, the use of PET wrapping tape further enhances the insulation effect. This structure has passed fire resistance tests and fully meets the requirements for medium-voltage cable fire resistance testing.

[0046] (2) Thickness of each layer and material usage of common medium-voltage fire-resistant cables Table 3. Dimensions of each layer of common medium-voltage fire-resistant cables

[0047] Table 4. Material usage of each layer and total weight of common medium-voltage fire-resistant cables.

[0048] The total weight of the cable with this structure is 488.7 kg less than that of the cable in this patent (14481.8 - 13993.1). However, the total thickness of the insulation and fire resistance of this structure is 12.4 mm (first flame-retardant wrapping layer thickness + first low-smoke halogen-free oxygen barrier layer thickness + second flame-retardant wrapping layer thickness + second low-smoke halogen-free oxygen barrier layer thickness + third flame-retardant wrapping layer thickness + outer sheath thickness). The total thickness of the insulation and fire resistance is 2.2 mm less than that of this patent (14.6 mm - 12.4 mm). The insulation effect is significantly worse than that of this patented technology. Furthermore, the fire resistance performance of this structure has been tested and verified, showing a low pass rate in fire resistance tests, posing a greater risk to cable use. To improve the insulation effect of this structure, the only solution is to increase the thickness between the first flame-retardant wrapping layer and the outer sheath.

[0049] Example 1: Increase the thickness of the first flame-retardant wrapping tape

[0050] Example 2: Increase the thickness of the first low-smoke halogen-free oxygen barrier layer

[0051] Example 1 uses 15470.7 - 14481.8 = 988.9 kg more material than this invention; Example 2 uses 15376.1 - 14481.8 = 894.3 kg more material than this invention; In summary, this invention has the characteristics of good fire resistance and heat insulation performance, low material consumption, simple production, and relatively low production energy consumption, thus achieving the goals of energy conservation, environmental protection, and low carbon emission reduction.

[0052] It should be noted that, for those skilled in the art, it is obvious that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A medium-voltage fire-resistant stranded conductor, characterized in that, It includes, from the inside out, a conductor (1), an extruded semi-conductive conductor shielding layer (2), a cross-linked polyethylene insulation layer, an extruded semi-conductive insulation shielding layer (4), a wrapped semi-conductive tape (5), and a metal shielding layer.

2. The medium-voltage fire-resistant stranded conductor according to claim 1, characterized in that, The conductor (1) includes a circular core wire (13) located in the middle position, and at least three fan-shaped core wires (14) arranged in the circumferential direction of the circular core wire (13) to match the arc of the circular core wire (13). The at least three fan-shaped core wires (14) are sequentially attached to wrap the circular core wire (13) to form a circular structure.

3. The medium-voltage fire-resistant stranded conductor according to claim 1, characterized in that, The wrapped semiconductive strip (5) is configured as two layers, the thickness of the wrapped semiconductive strip (5) is 0.20mm, and the wrapping overlap rate of the two layers of the wrapped semiconductive strip (5) is 40%~50%.

4. The medium-voltage fire-resistant stranded conductor according to claim 3, characterized in that, The metal shielding layer is a copper strip shielding layer (6), and the overlap rate of the copper strip shielding layer (6) is 5%.

5. A medium-voltage fire-resistant power cable, comprising at least three medium-voltage fire-resistant stranded conductors as described in any one of claims 1-4, characterized in that, It also includes PP filler rope (7), PET wrapping tape (8), first low smoke halogen-free oxygen barrier layer (9), second low smoke halogen-free oxygen barrier layer (10), double-sided mica tape layer (11), and low smoke halogen-free outer sheath (12), which are sequentially wrapped around at least three medium-pressure fire-resistant stranded conductors from the inside out.

6. The medium-voltage fire-resistant power cable according to claim 5, characterized in that, The PET wrapping tape (8) is configured as two layers, and the two layers overlap wrapping is adopted. The wrapping overlap rate of the two layers of PET wrapping tape (8) is 40%~50%.

7. The medium-voltage fire-resistant power cable according to claim 5, characterized in that, The first low-smoke halogen-free oxygen barrier layer (9) and the second low-smoke halogen-free oxygen barrier layer (10) are both extruded from low-smoke halogen-free materials with an oxygen index of not less than 40%.

8. The medium-voltage fire-resistant power cable according to claim 5, characterized in that, The thickness of the double-sided mica tape layer (11) is 0.14 mm. The double-sided mica tape layer (11) is configured as two overlapping wrapping layers, and the wrapping overlap rate of the double-sided mica tape layer (11) is not less than 15%.

9. The medium-voltage fire-resistant power cable according to claim 5, characterized in that, The low-smoke halogen-free outer sheath (12) is formed by extrusion of low-smoke halogen-free material with an oxygen index of not less than 32%.

10. A method for forming a medium-voltage fire-resistant power cable, applied to the medium-voltage fire-resistant power cable as described in any one of claims 6-8, characterized in that, Includes the following steps: The conductor is formed by the conductor stranding process (1); The extruded semi-conductive conductor shielding layer (2), the cross-linked polyethylene insulation layer and the extruded semi-conductive insulation shielding layer (4) are formed through a three-layer co-extrusion process. Complete the wrapping of the semiconductive strip (5) and the metal shielding layer; Fill the cable with PP filler rope (7) to make it round; PET wrapping tape (8); The low-smoke halogen-free oxygen barrier layer extrusion process is carried out sequentially to form the first low-smoke halogen-free oxygen barrier layer (9) and the second low-smoke halogen-free oxygen barrier layer (10). The wrapping process forms a double-sided mica tape layer (11). The low-smoke halogen-free outer sheath (12) is formed by the extrusion process.

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