Megawatt box transformer uses ladder monofilament tight pressing twisted super flexible aluminum alloy cable and preparation method

Abstract

This invention relates to the field of cables and discloses a trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-level transformer substations. From the outside to the inside, it comprises an insulation layer and a conductor. The insulation layer covers the outside of the conductor. The conductor is formed by stranding Al-Fe-Cu based aluminum alloy trapezoidal monofilaments and undergoing a three-stage gradient homogenization stress-relief annealing treatment. After the three-stage gradient homogenization stress-relief annealing treatment, the conductor's soft-state properties meet the following requirements: DC resistivity ≤ 0.028264 Ω·mm at 20℃. 2 The cable has a conductivity ≥61% IACS, tensile strength 100-159MPa, elongation at break ≥12%, and a minimum bending radius of 7D. This solution reduces the minimum bending radius to 7D through a combination of trapezoidal monofilament stranding and gradient annealing processes. This allows for flexible bending and laying in compact substation spaces without requiring large-radius laying space, simplifying construction and significantly improving on-site wiring efficiency. Furthermore, the increased tensile strength and elongation at break enhance the conductor's flexibility and creep resistance, reducing long-term safety hazards.

Description

Technical Field

[0001] This solution relates to the field of cables, specifically to trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cables for megawatt-class transformer substations and their preparation methods. Background Technology

[0002] Currently, most megawatt-level transformer substations use 0.6 / 1kV-YJV copper core cables for internal connections. Although copper core cables offer stable performance, the high price of copper keeps costs high. To reduce costs, the industry is shifting towards 0.6 / 1kV-YJLHV aluminum alloy cables, using aluminum alloy to replace copper and lower costs.

[0003] However, the bending radius of conventional aluminum alloy cables on the market is usually 15-20D (D is the outer diameter of the cable), which is difficult to adapt to the narrow space inside the transformer substation. In addition, due to the poor flexibility and insufficient creep resistance of the internal conductor of aluminum alloy cables, overheating caused by loose joints is likely to occur during long-term operation, which poses a safety hazard. Summary of the Invention

[0004] The present invention aims to provide a trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-level box-type substations and its preparation method. By improving the flexibility and creep resistance of the internal conductor of the aluminum alloy cable, the safety hazards caused by joint loosening and heat generation during long-term operation are reduced.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a trapezoidal single-wire tightly stranded ultra-flexible aluminum alloy cable for megawatt-level transformer substations, comprising, from the outside to the inside, an insulation layer and a conductor. The insulation layer covers the outside of the conductor. The conductor is formed by stranding Al-Fe-Cu based aluminum alloy trapezoidal single wires and undergoing a three-stage gradient homogenization stress-relief annealing treatment. After the three-stage gradient homogenization stress-relief annealing treatment, the conductor's soft-state performance meets the following requirement: DC resistivity ≤ 0.028264 Ω·mm at 20℃. 2 / m, conductivity ≥61%IACS, tensile strength 100-159MPa, elongation at break ≥12%, minimum bending radius of cable is 7D.

[0006] The beneficial effects of this solution are as follows: The combination of trapezoidal monofilament stranding and gradient annealing reduces the minimum bending radius of the cable to 7D, which is far superior to the 20D of conventional copper and aluminum alloy cables. It can be flexibly laid in the compact space of the transformer substation without reserving space for large-arc laying, making construction operations labor-saving and convenient, and greatly improving the efficiency of on-site wiring. Furthermore, through the three-stage gradient homogenization stress-relief annealing treatment, the tensile strength of the conductor in the soft state is 100-159MPa and the elongation at break is ≥12%, which improves the flexibility and creep resistance of the conductor and reduces the safety hazards caused by joint loosening and heat generation during long-term operation.

[0007] Preferably, the composition of the Al-Fe-Cu aluminum alloy trapezoidal monofilament, by mass percentage, is: Fe 0.3%-0.8%, Cu 0.15%-0.25%, Si≤0.1%, Mg≤0.05%, Zn≤0.05%, B≤0.04%, rare earth 0.00%-0.02%, single impurity content ≤0.03%, and the balance being aluminum. The first stage involves heating at 100℃ / h to 500℃ and holding for 1-1.5h to eliminate internal stress during wire drawing and stranding. The second stage involves cooling at 50℃ / h to 425℃ and holding for 0.5-1h to homogenize the alloy phase and repair structural defects. The third stage involves cooling at 50℃ / h to 350℃ and holding for 0.5-1h to achieve complete recrystallization of the conductor. After each stage, the conductor is cooled to room temperature in steps of 50-80℃ / h to complete the hard-to-soft state modification. Through customized alloy formulation and precise heat treatment, the conductivity is ≥61% IACS and the current carrying capacity is ≥410A, exceeding that of conventional aluminum alloy cables and small-diameter copper core cables of the same specification; the creep resistance is greatly improved, and there are no conductor loosening or joint overheating problems during long-term high-temperature operation, completely solving the safety hazards of loose connections and overheating, and ensuring the long-term stable operation of equipment.

[0008] Preferably, the insulation layer is a modified flame-retardant polyvinyl chloride insulation layer prepared by in-situ polymerization, and its performance indicators meet the following requirements: flame retardant rating not lower than Class B, oxygen index ≥30%, tensile strength before aging ≥16.0 MPa, elongation at break ≥150%, and volume resistivity at 20℃ ≥1.0×10⁻⁶. 12 Ω·m, thermal stability time ≥180min, long-term allowable operating temperature is 90-105℃.

[0009] Preferably, the finished cable performance meets the following requirements: maximum DC resistance ≤ 0.164Ω / km at 20℃, rated current carrying capacity ≥ 410A, and unit weight ≤ 0.76kg / m.

[0010] Preferably, it also includes a wiring terminal, which is disposed at the end of the cable and is formed by pressing in one step using a crimping mold.

[0011] The preparation method of trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class box-type substations includes the following steps: S1. Preparation of aluminum alloy conductor monofilament: Alloy billets are prepared by melting according to the alloy composition ratio, and aluminum alloy rods are made by continuous casting and rolling. Then, trapezoidal monofilaments with regular cross-sections are obtained by precision wire drawing. S2. Tightly twisted irregular monofilaments: The trapezoidal monofilaments are twisted in layers according to the concentric structure of "1+6+9+14". The twisting direction and pitch parameters of each layer are strictly controlled, and the hydraulic compaction process is used simultaneously to make the conductor compaction coefficient ≥0.97. S3. Gradient homogenization-stress relief annealing: The stranded conductor is placed in a fully automatic temperature-controlled annealing furnace and subjected to three-stage gradient heat treatment. The total annealing time is 2-4 hours. After the treatment is completed, the conductor is cooled to room temperature in a stepwise manner to complete the hard-to-soft state modification of the conductor. S4. Insulation layer extrusion: A modified flame-retardant polyvinyl chloride insulation layer is extruded on the outside of the annealed conductor to ensure uniform insulation layer thickness. S5. Customized terminal crimping: The insulation layer at the end of the cable is peeled off and matched with the appropriate terminal. The terminal is formed in one step using a special crimping mold. The terminal is a customized DTL-185 copper-aluminum terminal.

[0012] Preferably, the specific parameters of the three-stage gradient heat treatment in step S3 are as follows: in the first stage, the temperature is increased to 500℃ at a rate of 100℃ / h and held for 1-1.5h; in the second stage, the temperature is decreased to 425℃ at a rate of 50℃ / h and held for 0.5-1h; in the third stage, the temperature is decreased to 350℃ at a rate of 50℃ / h and held for 0.5-1h.

[0013] Preferably, the DTL-185 copper-aluminum terminal in step S5 includes a copper end and an aluminum end. The copper end is made of T3 copper conforming to the GB / T5231-2001 standard, and the aluminum end is made of L2 industrial pure aluminum conforming to the GB / T 3190-1996 standard. The special crimping die used has a crimping area that is 15%-20% larger than that of a conventional die.

[0014] Preferably, it also includes S6, finished product inspection and customized processing: cut to a fixed length according to installation requirements, complete all performance tests, and the finished product is obtained after passing the test.

[0015] Preferably, in step S6, the full performance test items of the finished product include DC resistance at 20℃, current carrying capacity, bending performance, flame retardant performance and creep resistance, and all test indicators meet the requirements of the national standard GB / T 31840.1-2025. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the 1+6+9+14 stranded structure of the aluminum alloy stranded conductor trapezoidal monofilament in an embodiment of the present invention; Figure 2 A schematic diagram of the DTL-185 copper-aluminum terminal structure dimensions is provided for embodiments of the present invention. Figure 3 This is a schematic diagram comparing the effects of one-time crimping of the terminal block with conventional crimping in an embodiment of the present invention.

[0017] The reference numerals in the accompanying drawings include: conductor 100, trapezoidal monofilament 101, copper end 201, aluminum end 202, and wiring hole 203. Detailed Implementation

[0018] Example 1 Megawatt-class box-type transformer substation trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable and its preparation method. The cable consists of a conductor and an insulation layer from the inside out. The conductor is formed by stranding Al-Fe-Cu aluminum alloy trapezoidal monofilaments and undergoing a three-stage gradient homogenization stress-relief annealing treatment. The insulation layer covers the outside of the conductor.

[0019] like Figure 1 As shown, in this embodiment, the conductor uses trapezoidal monofilaments in a "1+6+9+14" concentric compact stranding pattern. Specifically, the stranding direction is set as follows: the first layer to the left, the second layer to the right, and the third layer to the left. The pitch of each layer is precisely controlled: the first layer is 198-211mm, the second layer is 264-286mm, and the third layer is 287-317mm. The compaction coefficient of the trapezoidal monofilaments is ≥0.97, forming a high-density compact conductor, which reduces the outer diameter while significantly improving bending flexibility. After the conductor undergoes three-stage gradient homogenization stress-relief annealing, the soft core performance is: DC resistivity ≤0.028264Ω·mm at 20℃. 2 / m, conductivity ≥61%IACS, tensile strength 100-159MPa, elongation at break ≥12%, number of repeated bending breaks of a single filament ≥28 times, minimum bending radius 7D, and creep resistance is more than 40% higher than that of conventional aluminum alloy conductors.

[0020] The insulation layer is prepared using in-situ polymerization. Through optimization of the compounding of flame retardants, plasticizers, and anti-aging agents, the molecular structure of PVC is controlled to achieve a synergistic effect of high flexibility and high flame retardancy. Key performance indicators: Flame retardancy rating ≥ B, oxygen index ≥ 30%, tensile strength before aging ≥ 16.0 MPa, elongation at break ≥ 150%, volume resistivity at 20℃ ≥ 1.0 × 10⁻⁶. 12 Ω·m, thermal stability time ≥180min, long-term allowable operating temperature 90℃, excellent aging resistance, moisture resistance and salt spray resistance.

[0021] Key specifications of the finished cable: maximum DC resistance ≤0.164Ω / km at 20℃, rated current carrying capacity of approximately 410A, unit weight of approximately 0.76kg / m, lightweight design without armor, all performance indicators fully meet the standards, and it is suitable for the high current carrying capacity and long-cycle operation requirements of megawatt-level transformer substations.

[0022] The specific preparation method includes the following steps: S1. Preparation of aluminum alloy conductor monofilament: According to the above alloy composition ratio, the alloy billet is prepared by melting in a medium frequency induction furnace, and then processed into a Φ9.5mm standard aluminum alloy rod by continuous casting and rolling. After multiple precision wire drawing, the trapezoidal monofilament is made with a regular cross section and a smooth surface. The size and angle of the monofilament are precisely matched to the subsequent tight-stranding process to eliminate scratches and burr defects.

[0023] S2. Irregularly shaped monofilament compaction and stranding: Trapezoidal monofilaments are stranded in layers according to a 1+6+9+14 concentric structure using a fully automatic stranding equipment. The stranding direction and pitch parameters are strictly controlled, and a hydraulic compaction process is adopted simultaneously to ensure that the monofilaments are tightly bonded and the compaction coefficient is stable at ≥0.97, forming a stranded conductor with uniform outer diameter and dense structure.

[0024] S3. Gradient Homogenization-Stress Relief Annealing: The stranded conductor is placed in a fully automatic temperature-controlled annealing furnace and subjected to a three-stage gradient heat treatment for a total duration of 2-4 hours. Specific parameters are as follows: In the first stage, the temperature is increased to 500℃ at a rate of 100℃ / h and held for 1-1.5 hours to eliminate the internal stress of the stranding process; in the second stage, the temperature is decreased to 425℃ at a rate of 50℃ / h and held for 0.5-1 hours to homogenize the alloy phase and repair structural defects; in the third stage, the temperature is decreased to 350℃ at a rate of 50℃ / h and held for 0.5-1 hours to achieve complete recrystallization of the conductor; after each stage, the conductor is cooled to room temperature in steps at a rate of 50-80℃ / h to complete the hard-to-soft state modification.

[0025] S4. Insulation layer extrusion molding: For high insulation requirements, a thin cross-linked polyethylene transition layer is first extruded. Under normal working conditions, a modified flame-retardant PVC insulation layer is directly extruded. The extrusion temperature is controlled at 150-180℃. Custom extrusion molds are used to ensure that the insulation layer is uniform in thickness, without any off-center core, bubbles, or damage, and is tightly bonded to the conductor.

[0026] Before use, the cable should be cut to a fixed length of 2.5-3.5 meters according to project requirements, and terminals should be installed at the ends. Differentiated color pre-insulated sleeves should also be fitted at the ends for easy identification during on-site installation. Subsequently, a full range of performance tests covering DC resistance, current carrying capacity, bending performance, flame retardancy, creep resistance, and salt spray resistance should be carried out in accordance with GB / T 31840.1-2025. When all indicators meet the GB / T 31840.1-2025 standard, the finished product is considered qualified.

[0027] Example 2 Example 2 is basically the same as Example 1, except that, in order to reduce the workload of on-site installation and finished product testing, terminals are also provided at both ends of the cable, such as... Figure 2 As shown, the terminal block includes a copper end 201 and an aluminum end 202. The copper end 201 uses GB / T 5231-2001 standard T3 copper, and the aluminum end 202 uses GB / T 3190-1996 standard L2 industrial pure aluminum. The aluminum end 202 is coaxially provided with a wiring hole 203. The diameter of the wiring hole 203 is adapted to the outer diameter of the cable conductor to ensure the compatibility between the cable conductor and the terminal block. During production, it can be precisely produced in lengths of 2.5-3.5 meters. The end is adapted to a customized DTL-185 copper-aluminum terminal.

[0028] The manufacturing process also includes S5, custom terminal crimping: precisely stripping the insulation layer from the cable end, and mating it with the custom DTL-185 copper-aluminum terminal through the wiring hole 203 set in the aluminum end 202; during mating, a special crimping mold can be used, that is, a crimping mold with a crimping area 15%-20% larger than that of a conventional mold, to crimp the outer wall corresponding to the wiring hole 203, so as to achieve one-time crimping forming, such as... Figure 3 As shown, by using customized DTL-185 copper-aluminum terminals and special crimping molds to connect the terminals, the crimped parts can be made flat and burr-free, without any loose connections, ensuring stable and controllable contact resistance.

[0029] S6. Finished Product Inspection and Custom Processing: Cut to length according to installation requirements, complete all performance tests, and the finished product is qualified. The finished product is tested for all performance items including 20℃ DC resistance, current carrying capacity, bending performance, flame retardancy and creep resistance. All test indicators meet the requirements of GB / T 31840.1-2025 national standard.

[0030] Unless otherwise specified, all raw materials and processing equipment used in this invention are commercially available conventional industrial-grade products. All process parameters and performance testing methods strictly comply with relevant national standards such as GB / T 31840.1-2025 "Extruded Insulated Power Cables and Accessories with Rated Voltages of 1kV (Um=1.2kV) to 35kV (Um=40.5kV) Part 1: Cables with Rated Voltages of 1kV (Um=1.2kV) and 3kV (Um=3.6kV)" and GB / T 3956 "Conductors of Cables" to ensure the standardization and industrial applicability of the technical solution.

[0031] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that the technical means for solving problems in the above embodiments of the present invention can be used in combination to solve multiple technical problems simultaneously. For those skilled in the art, several modifications and improvements can be made without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class transformer substations, characterized in that: From the outside in, it consists of an insulation layer and a conductor. The insulation layer covers the outside of the conductor. The conductor is formed by stranding trapezoidal monofilaments of Al-Fe-Cu aluminum alloy and undergoing a three-stage gradient homogenization stress-relief annealing treatment. After the three-stage gradient homogenization stress-relief annealing treatment, the conductor's soft-state properties meet the following requirements: DC resistivity ≤ 0.028264 Ω·mm at 20℃. 2 / m, conductivity ≥61%IACS, tensile strength 100-159MPa, elongation at break ≥12%, minimum bending radius of cable is 7D.

2. The trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class transformer substations according to claim 1, characterized in that: The composition of the Al-Fe-Cu aluminum alloy trapezoidal monofilament by mass percentage is: Fe 0.3%-0.8%, Cu 0.15%-0.25%, Si≤0.1%, Mg≤0.05%, Zn≤0.05%, B≤0.04%, rare earth 0.00%-0.02%, single impurity content≤0.03%, and the balance is aluminum. The parameters of the three-stage gradient homogenization stress-relief annealing treatment are as follows: the first stage is to heat up to 500℃ at 100℃ / h and hold for 1-1.5h to eliminate the internal stress of wire drawing and stranding; the second stage is to cool down to 425℃ at 50℃ / h and hold for 0.5-1h to homogenize the alloy phase and repair structural defects; the third stage is to cool down to 350℃ at 50℃ / h and hold for 0.5-1h to achieve complete recrystallization of the conductor; after each stage, the conductor is cooled to room temperature in steps of 50-80℃ / h to complete the hard-to-soft state modification.

3. The trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class transformer substations according to claim 1, characterized in that: The insulation layer is a modified flame-retardant polyvinyl chloride insulation layer prepared by in-situ polymerization, and its performance indicators meet the following requirements: flame retardant rating not lower than Class B, oxygen index ≥30%, tensile strength before aging ≥16.0MPa, elongation at break ≥150%, and volume resistivity at 20℃ ≥1.0×10⁻⁶. 12 Ω·m, thermal stability time ≥180min, long-term allowable operating temperature is 90-105℃.

4. The trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class transformer substations according to claim 1, characterized in that: The finished cable performance meets the following requirements: maximum DC resistance ≤ 0.164Ω / km at 20℃, rated current carrying capacity ≥ 410A, and unit weight ≤ 0.76kg / m.

5. The trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class transformer substations according to any one of claims 1-4, characterized in that: It also includes wiring terminals, which are located at the ends of the cable and are formed by pressing in one step using a crimping die.

6. The method for preparing a trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class transformer substations according to claim 5, characterized in that: Includes the following steps: S1. Preparation of aluminum alloy conductor monofilament: Alloy billets are prepared by melting according to the alloy composition ratio, and aluminum alloy rods are made by continuous casting and rolling. Then, trapezoidal monofilaments with regular cross-sections are obtained by precision wire drawing. S2. Tightly stranded irregular monofilaments: Trapezoidal monofilaments are stranded in layers according to a concentric structure of "1+6+9+14". The stranding direction and pitch parameters of each layer are strictly controlled, and a hydraulic compaction process is used simultaneously to make the conductor compaction coefficient ≥0.

97. S3. Gradient homogenization-stress relief annealing: The stranded conductor is placed in a fully automatic temperature-controlled annealing furnace and subjected to three-stage gradient heat treatment. The total annealing time is 2-4 hours. After the treatment is completed, the conductor is cooled to room temperature in a stepwise manner to complete the hard-to-soft state modification of the conductor. S4. Insulation layer extrusion: A modified flame-retardant polyvinyl chloride insulation layer is extruded on the outside of the annealed conductor to ensure uniform insulation layer thickness. S5. Customized terminal crimping: The insulation layer at the end of the cable is peeled off and matched with the appropriate terminal. The terminal is formed in one step using a special crimping mold. The terminal is a customized DTL-185 copper-aluminum terminal.

7. The method for preparing a trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class transformer substations according to claim 6, characterized in that: The specific parameters for the three-stage gradient heat treatment in step S3 are as follows: in the first stage, the temperature is increased to 500℃ at a rate of 100℃ / h and held for 1-1.5h; in the second stage, the temperature is decreased to 425℃ at a rate of 50℃ / h and held for 0.5-1h; in the third stage, the temperature is decreased to 350℃ at a rate of 50℃ / h and held for 0.5-1h.

8. The method for preparing the trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class transformer substations according to claim 7, characterized in that: The DTL-185 copper-aluminum terminal mentioned in step S5 includes a copper end and an aluminum end. The copper end is made of T3 copper conforming to the GB / T5231-2001 standard, and the aluminum end is made of L2 industrial pure aluminum conforming to the GB / T 3190-1996 standard. The special crimping die used has a crimping area that is 15%-20% larger than that of a conventional die.

9. The method for preparing a trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class transformer substations according to claim 8, characterized in that: It also includes S6, finished product inspection and customized processing: cut to the required length according to installation requirements, complete all performance tests, and the finished product is obtained after passing the test.

10. The method for preparing the trapezoidal monofilament tightly stranded ultra-flexible aluminum alloy cable for megawatt-class transformer substations according to claim 9, characterized in that: In step S6, the full performance test items of the finished product include DC resistance at 20℃, current carrying capacity, bending performance, flame retardancy and creep resistance. All test indicators meet the requirements of the national standard GB / T 31840.1-2025.

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