A LNG ultra-low temperature DC medium voltage coaxial cable

Abstract

The present invention discloses an LNG ultra-low temperature direct current type medium voltage coaxial cable, comprising an inner conductor, a shield, an insulating layer, an insulating shield, an outer conductor, a copper tape wrap, a wrapping tape, a glass fiber protective layer and an outer sheath arranged in sequence from the inside to the outside, the inner conductor and the outer conductor respectively adopt a compact wire twisted structure, and the outer diameter of the inner conductor is not more than 6.9 mm. The LNG ultra-low temperature direct current type medium voltage coaxial cable provided by the present invention has an inner conductor and an outer conductor that are twisted compactly, and under the condition of equal cross-sectional resistance, the outer diameter is smaller than that of a conventional twisted conductor, the obtained cable has a small outer diameter, low cost, convenient installation and use, a smoother surface and no gaps, the outer conductor and the inner conductor are interconnected inside and outside, and the inner conductor shields and equalizes the electric field strength of the inner conductor, and the cable does not break down in a DC withstand voltage test of 21kV / 5min and does not crack when stretched at an ultra-low temperature of ‑276°C, is not prone to heat overload, has stable electrical properties, and has a long service life, and is suitable for ultra-low temperature occasions.

Description

Technical Field

[0001] The invention belongs to the technical field of cables, and in particular relates to an LNG ultra-low temperature direct current type medium-voltage coaxial cable. Background Art

[0002] With the in-depth application of clean energy LNG (English full name: Liquefied Natural Gas, Chinese full name: liquefied natural gas), the demand for ultra-low temperature DC type medium voltage coaxial cables for some large LNG transport ships, LNG shore-based storage tanks, LNG distribution pump stations, etc. has increased year by year. In ultra-low temperature media environments (-100~ -270℃) such as Freon liquefied natural gas, liquid nitrogen, liquid helium, liquid oxygen, and liquid hydrogen, the temperature range of common low-temperature cables is generally -40℃~-60℃. When applied to ultra-low temperature environments, the insulation layer and sheath layer will become hard, brittle, or even crack, which brings safety hazards. Summary of the invention

[0003] In order to solve the problems in the prior art, the object of the present invention is to provide an LNG ultra-low temperature direct current type medium voltage coaxial cable.

[0004] In order to achieve the above purpose and the above technical effect, the technical solution adopted by the present invention is:

[0005] An LNG ultra-low temperature direct current type medium voltage coaxial cable, which comprises, from the inside to the outside, a coaxially arranged inner conductor, a shield, an insulation, an insulating shield, an outer conductor, a copper tape wrap, a wrapping tape, a glass fiber protective layer and an outer sheath; the inner conductor and the outer conductor respectively adopt a compressed wire twisted structure, the outer diameter of the inner conductor is not more than 6.9 mm, and the outer diameter of the insulated wire core composed of the inner conductor, the shield and the insulating layer is not more than 11.1 mm.

[0006] Furthermore, the inner conductor adopts a 1+6 compressed wire twisted structure, and the inner conductor includes an inner layer and an outer layer from the inside to the outside. The inner layer is a soft round copper wire with a diameter of 2.66 mm, and the outer layer adopts 6 2.1*3.23 mm trapezoidal wires compressed and twisted on the outside of the soft round copper wire. The outer layer is twisted to the left, and the outer layer pitch of the inner conductor is 82~117 mm, ensuring that the inner conductor is twisted more tightly and roundly, with a small single wire gap, and has the characteristics of compact structure, small outer diameter, and low manufacturing cost.

[0007] Furthermore, the shielding is formed by wrapping a polyimide film tape around the outside of the inner conductor and then sintering it, the wrapping overlap rate is not less than 30%, and the outer diameter of the cable after wrapping is not more than 7.1 mm. The polyimide film tape has the characteristics of thin thickness, light weight, low dielectric loss, little influence by temperature and signal frequency changes, stable dielectric constant, excellent high and low temperature resistance, and good weather resistance.

[0008] Furthermore, the thickness of the thinnest point of the insulating layer is controlled at 1.70-1.75 mm, and the average thickness is 1.8-2.0 mm; the thickness of the insulating layer can ensure that the cable has high insulation resistance, withstand voltage strength and low dielectric loss, excellent anti-puncture performance, improve the safety performance of the cable and maintain the stability of cable transmission, and has good physical and mechanical properties.

[0009] Furthermore, the insulation shield is formed by wrapping a polyimide film wrapping tape around the outside of the insulation layer and then sintering, the wrapping overlap rate is not less than 30%, and the outer diameter of the cable after wrapping is not greater than 11.2 mm. The insulation shield formed by sintering the polyimide film wrapping tape has a large insulation resistance and high voltage resistance, can effectively isolate the induced current between the insulation layer and the outer conductor, ensure the stability of the cable electrical performance, and improve the stability of the product.

[0010] Furthermore, the outer conductor is formed by tightly twisting 15 0.72*2.65mm wire conductors, with a twisting pitch of 164~190mm. The outer diameter of the cable after twisting the outer conductor is not more than 12.6mm; the outer conductor and the inner conductor are interconnected inside and outside, which not only shields the inner conductor but also homogenizes the electric field strength of the inner conductor. The outer conductor is twisted compactly, has a smooth surface and is round.

[0011] Furthermore, the copper tape wrapping is made of 0.1mm shielding soft copper tape with a gap wrapping rate of -100%, ensuring that the outer conductor is tightly wound, has no skipping, and has a uniform field strength effect.

[0012] Furthermore, the wrapping tape is formed by wrapping a 0.05 mm thin flame-retardant cloth tape, and the wrapping tape is wrapped around the outside of the copper tape wrapping. The outer diameter of the cable after wrapping is not greater than 13.3 mm.

[0013] Furthermore, the glass fiber protective layer is woven from high-strength glass fiber yarns, with a weaving density of not less than 95%, a pitch of 30-40 mm, a weaving angle of 40°-50°, and an outer diameter of the cable after weaving of not more than 13.7 mm. The glass fiber protective layer woven from high-strength glass fiber yarns can effectively protect the normal operation of the cable interior in the event of failure of the outer sheath, thereby improving the stability of the product.

[0014] Furthermore, the outer sheath adopts PFA with a melting index of 6-8g / 10min, the thickness of the outer sheath at the thinnest point is 1.3-1.4mm, the average thickness is 1.5-1.6mm, and the outer diameter of the cable after extruding the outer sheath is not more than 17.0mm.

[0015] The invention discloses a method for preparing an LNG ultra-low temperature direct current type medium voltage coaxial cable, comprising the following steps:

[0016] Step 1: Inner conductor twisting: The inner conductor adopts a 1+6 compressed wire twisting structure, the inner layer of the inner conductor is a soft round copper wire with a diameter of 2.66 mm located in the center, and the outer layer is 6 2.1*3.23 mm trapezoidal wires twisted in the left direction outside the inner layer to form the inner conductor;

[0017] Step 2: Wrapping the inner conductor with a polyimide film tape to form the shield, with an overlap rate of not less than 30%, and then sintering;

[0018] Step 3: Extruding PFA outside the shield using an insulating extrusion die to form the insulating layer;

[0019] Step 4: wrapping the outside of the insulating layer with a polyimide film wrapping tape with an overlap rate of not less than 30% to form an insulating shield, and then sintering;

[0020] Step 5: 15 0.72*2.65mm wire conductors are compressed and twisted outside the shielding layer to form an outer conductor, with a twisting pitch of 164~190mm. At this time, the outer diameter of the cable obtained is not more than 12.6mm; then, 0.1mm shielding soft copper tape is used for gap wrapping, and the gap overlap rate is -100% to form a copper tape wrapping;

[0021] Step 6: wrapping the copper tape with a thin flame-retardant cloth tape outside to form the wrapping, with an overlap rate of not less than 15%, and placing it in a drying room for sufficient drying;

[0022] Step 7: high-strength glass fiber yarn is used to weave the outside of the wrap to form a glass fiber protective layer, the weaving density is not less than 95%, the pitch is 30-40 mm, the weaving angle is 40°-50°, and it is placed in a drying room for full drying;

[0023] Step eight: using an extrusion die to extrude PFA outside the glass fiber protective layer to form the outer sheath, thereby obtaining the desired cable. The obtained cable passes a DC withstand voltage test of 21 kV / 5 min without breakdown, and does not crack when stretched at a low temperature of -276°C.

[0024] Furthermore, in step three and step eight, the melt index of the PFA is 6-8g / 10min, and it has excellent processing fluidity, ensuring smooth and stable extrusion without defects such as bamboo knots and sand holes. The insulating layer and the outer sheath are placed in an oven at 120±10°C and dried for 1±0.5h.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1) The inner conductor adopts a 1+6 compressed stranded wire structure, with a 2.66mm diameter soft round copper wire in the center and 6 2.1*3.23mm trapezoidal wires on the outer layer twisted to the left to form the inner conductor, which effectively controls the outer diameter of the inner conductor and makes the outer surface smoother and rounder with basically no gaps between the single wires, ensuring that the shielding wrapping outside the inner conductor is flat, making the sintering smoother and flatter, ensuring that the insulation layer is extruded smoothly and the thickness is uniform, the cold deformation stress inside the insulation layer is small, cracking is prevented, and the processing cost is low;

[0027] 2) The outer conductor adopts a compact twisted structure of 15 0.72*2.65mm wire conductors with a twist pitch of 164~190mm. The outer conductor is twisted compactly, with a round and smooth surface and low processing cost;

[0028] 3) The inner conductor and the outer conductor of the present invention adopt a profiled wire structure. Under the condition of the same cross-section and the same resistance, the outer diameter is much smaller than the outer diameter of the conventional stranded conductor (the outer diameter of the conventional stranded conductor is 7.7 mm). The outer diameter of the prepared LNG ultra-low temperature direct current profiled wire medium-voltage coaxial cable is reduced, less material is used, the cost is low, the installation and use are convenient, and the surface is smoother and has no gaps; the outer conductor and the inner conductor are interconnected inside and outside, which plays a shielding role for the inner conductor and homogenizes the electric field strength of the inner conductor;

[0029] 4) The shielding layer formed by sintering polyimide film tape and polyimide film wrapping tape has high insulation resistance and high voltage resistance, which can effectively isolate the induced voltage between the insulation layer and the outer conductor, ensure the stability of the cable electrical performance, and improve the service life of the product. The copper tape wrapping gap wrapping ensures that the outer conductor is tightly wound, without skipping, and has a uniform field strength effect;

[0030] 5) The durability of the cable is improved by setting a glass fiber protective layer, which can effectively protect the normal operation of the cable even when the outer sheath fails, thereby improving the stability of the product;

[0031] 6) The thickness of the insulation layer can ensure that the LNG ultra-low temperature DC type medium voltage coaxial cable has high insulation resistance, withstand voltage strength and low dielectric loss, excellent anti-puncture performance, greatly improves the safety performance of the cable and maintains the stability of cable transmission, and has good physical and mechanical properties;

[0032] 7) The performance of LNG ultra-low temperature DC type medium voltage coaxial cable in low temperature environment is taken into consideration. The stretching ratio and die matching coefficient of the insulation or sheath extrusion mold are controlled within a reasonable range to ensure that the LNG ultra-low temperature DC type medium voltage coaxial cable will not be broken down in the DC withstand voltage test of 21kV / 5min and will not crack when stretched in an ultra-low temperature environment of -276℃. It is also not prone to heat overload, has stable electrical performance and a long service life. It is a customized product suitable for ultra-low temperature occasions. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a structural schematic diagram of the present invention;

[0034] Figure 2 It is a simplified structural schematic diagram of the extrusion die of the present invention;

[0035] 1. Inner conductor; 2. Shield; 3. Insulation layer; 4. Insulation shield; 5. Outer conductor; 6. Copper tape wrapping; 7. Tape wrapping; 8. Glass fiber protective layer; 9. Outer sheath. DETAILED DESCRIPTION

[0036] The embodiments of the present invention are described in detail below so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0037] like Figure 1-2 As shown, an LNG ultra-low temperature direct current type medium voltage coaxial cable comprises an inner conductor 1, a shield 2, an insulating layer 3, an insulating shield 4, an outer conductor 5, a copper tape wrap 6, a wrapping tape 7, a glass fiber protective layer 8 and an outer sheath 9 which are coaxially arranged from the inside to the outside. The outer diameter of the insulated wire core composed of the inner conductor 1, the shield 2 and the insulating layer 3 is not greater than 11.1 mm. The inner conductor 1 and the outer conductor 5 are interconnected inside and outside, which not only plays a shielding role for the inner conductor 1, but also homogenizes the electric field strength of the inner conductor 1.

[0038] The inner conductor 1 adopts a compressed stranded wire structure, including an inner layer and an outer layer. The inner layer is a soft round copper wire or its stranded conductor located in the center. A plurality of trapezoidal wires are compressed and twisted outside the inner layer to form an outer layer. The outer layer is twisted to the left and the pitch of the outer layer is 82~117mm. After twisting, the outer diameter of the inner conductor 1 is not greater than 6.9mm.

[0039] The shield 2 is made of a polyimide film tape wrapped around the outside of the inner conductor 1 and then sintered. The wrapping overlap rate is not less than 30%, and the outer diameter of the cable after wrapping is not more than 7.1mm. The polyimide film tape has the characteristics of thin thickness, light weight, low dielectric loss, little influence by temperature and signal frequency changes, stable dielectric constant, excellent high and low temperature resistance, and good weather resistance.

[0040] The thickness of the thinnest point of the insulating layer 3 is controlled at 1.70-1.75mm, and the average thickness is 1.8-2.0mm. The thickness of the insulating layer 3 can ensure that the cable has high insulation resistance, withstand voltage strength and low dielectric loss, excellent anti-puncture performance, improve the safety performance of the cable and maintain the stability of cable transmission, and has good physical and mechanical properties.

[0041] The insulating shield 4 is formed by wrapping a polyimide film wrapping tape around the outside of the insulating layer 3 and then sintering. The outer diameter of the cable after wrapping is not more than 11.2 mm. The insulating shield 4 formed by sintering the polyimide film wrapping tape has a large insulation resistance and a high withstand voltage strength. It can effectively isolate the induced current between the insulating layer 3 and the outer conductor 5, ensure the stability of the cable electrical performance, and improve the stability of the product.

[0042] The outer conductor 5 is formed by twisting a plurality of profiled wire conductors tightly together, the twisting is compact, the surface is smooth and round, the twisting pitch is 164-190 mm, and the outer diameter of the cable after twisting the outer conductor 5 is not greater than 12.6 mm.

[0043] The outer conductor 5 is wrapped with a copper tape wrap 6. The copper tape wrap 6 is wrapped with a gap of soft copper tape for shielding, and the gap overlap rate is -100%, ensuring that the outer conductor 5 is tightly wound, has no skipping, and has a uniform field strength effect.

[0044] The wrapping tape 7 is made of 0.05 mm thin flame retardant cloth tape, and the outer diameter of the cable after wrapping is not greater than 13.3 mm.

[0045] The glass fiber protective layer 8 is woven from high-strength glass fiber yarns, with a weaving density of not less than 95%, a pitch of 30-40 mm, a weaving angle of 40°-50°, and an outer diameter of the cable after weaving not more than 13.7 mm.

[0046] The outer sheath 9 is made of polytetrafluoroethylene PFA with a melting index of 6-8g / 10min. The thickness of the thinnest point of the outer sheath 9 is controlled at 1.3-1.4mm, and the average thickness is 1.5-1.6mm. The outer diameter of the cable after extruding the outer sheath 9 is not more than 17.0mm.

[0047] The extrusion principle of the outer sheath 9 is similar to that of the insulating layer 3. The polytetrafluoroethylene with a melt index of 6-8g / 10min is extruded to form the outer sheath 9 or the insulating layer 3 using a corresponding extrusion die (such as an insulating extrusion die, a sheath extrusion die). The polytetrafluoroethylene with a melt index of 6-8g / 10min has excellent processing fluidity, ensuring smooth and stable extrusion without defects such as bamboo knots and sand holes. The structural schematic diagram of the extrusion die is shown in FIG. Figure 2 As shown, the mold configuration parameters of the extrusion mold include the stretch ratio K and the mold matching coefficient δ. The calculation formulas of the stretch ratio K and the mold matching coefficient δ are:

[0048] K = (D 大 2 -D 小 2 ) / (d 大 2- d 小 2 ) (1)

[0049] δ = (D大 / d 大 ) / (D 小 / d 小 ) (2)

[0050] Among them, D 大 D is the diameter of the extrusion die sleeve, in mm; 小 d is the outer diameter of the die center of the extrusion die, in mm; 大 d is the diameter of the insulated wire core, in mm; 小 is the conductor diameter, unit: mm; the value range of K is 1~50, and the value range of δ is 1.01~1.1.

[0051] The present invention also discloses a method for preparing an LNG ultra-low temperature direct current type medium voltage coaxial cable, comprising the following steps:

[0052] Step 1: preparing an inner conductor 1;

[0053] Step 2: Wrap the inner conductor 1 with a polyimide film tape, with an overlap rate of not less than 30%, and then sinter it to form a shield 2;

[0054] Step 3: Use an adapted extrusion die to extrude polytetrafluoroethylene with a melt index of 6-8g / 10min outside the shield 2 to form an insulating layer 3. Before extrusion, place the polytetrafluoroethylene raw material in an oven at 120±10℃ for 1±0.5h to fully dry it for standby use; the extrusion body temperature of the extrusion die is controlled at 320℃~340℃, 340℃~360℃, 360℃~380℃, and 380℃~400℃ from the first zone to the fourth zone; the flange temperature is controlled at 380℃~400℃, and the head temperature is controlled at 375℃~410℃; the discharge surface is uniform, smooth, and has good fluidity, and the insulating layer 3 produced thereby has the advantages of ultra-low temperature resistance, not easy to breakdown, not easy to crack, stable electrical properties, and long service life;

[0055] The calculation formulas for the stretch ratio K and die matching coefficient δ of the extrusion die are:

[0056] K = (D 大 2 -D 小 2 ) / (d 大 2- d 小 2 ) (1)

[0057] δ = (D 大 / d 大 ) / (D 小 / d 小 ) (2)

[0058] Among them, D 大 D is the diameter of the die sleeve of the extrusion die, in mm; 小 d is the outer diameter of the die core of the extrusion die, in mm; 大 is the diameter of the insulated wire core consisting of the inner conductor 1, the shield 2 and the insulation layer 3, in mm; d 小 is the conductor diameter, in mm; the value range of K is 1~50, and the value range of δ is 1.01~1.1;

[0059] Step 4: A polyimide film wrapping tape is wrapped around the outside of the insulating layer 3 and then sintered to form an insulating shield 4. The wrapping overlap rate of the polyimide film wrapping tape is not less than 30%, and the outer diameter is not greater than 11.2 mm. The polyimide film wrapping tape has the advantages of large insulation resistance, high voltage resistance, and can effectively isolate the induced voltage between the insulating layer 3 and the outer conductor 5, ensure the stability of the electrical performance of the cable, and improve the service life of the product;

[0060] Step 5: A plurality of wire conductors are compressed and twisted outside the insulating shield 4, with a twisting pitch of 164-190 mm and an outer diameter of no more than 12.6 mm, to form an outer conductor 5; a shielding soft copper tape is gap-wrapped outside the outer conductor 5, with a gap overlap rate of -100%, to form a copper tape wrap 6;

[0061] Step 6: Use a thin flame-retardant cloth tape to wrap the outside of the copper tape wrap 6 to form a wrapping tape 7, with an overlap rate of not less than 15%, and place it in a drying room for full drying;

[0062] Step 7: Use high-strength glass fiber yarn to weave a glass fiber protective layer 8 on the outside of the wrapping tape 7, with a weaving density of not less than 95%, a pitch of 30-40 mm, a weaving angle of 40°-50°, and place it in a drying room for full drying.

[0063] Step 8: Use an appropriate extrusion die to extrude polytetrafluoroethylene PFA with a melt index of 6-8g / 10min outside the glass fiber protective layer 8 to form an outer sheath 9, and finally obtain the desired product, which can ensure that the cable passes the DC withstand voltage test of 21kV / 5min without breakdown, and does not crack when stretched at a low temperature of -276°C;

[0064] The calculation formulas for the stretch ratio K and die matching coefficient δ of the extrusion die are:

[0065] K = (D 大 2 -D 小 2 ) / (d 大 2- d 小 2 ) (1)

[0066] δ = (D 大 / d大 ) / (D 小 / d 小 ) (2)

[0067] Among them, D 大 D is the diameter of the sleeve of the sheath extrusion die, in mm; 小 d is the outer diameter of the die core of the sheath extrusion die, in mm; 大 d is the diameter of the insulated wire core, in mm; 小 is the conductor diameter, unit: mm; the value range of K is 1~50, and the value range of δ is 1.01~1.1.

[0068] Example 1

[0069] like Figure 1-2 As shown, an LNG ultra-low temperature direct current type medium-voltage coaxial cable comprises an inner conductor 1, a shield 2, an insulating layer 3, an insulating shield 4, an outer conductor 5, a copper tape wrap 6, a wrapping tape 7, a glass fiber protective layer 8 and an outer sheath 9 which are coaxially arranged from the inside to the outside. The outer diameter of the insulated wire core composed of the inner conductor 1, the shield 2 and the insulating layer 3 is not greater than 11.1 mm. The outer conductor 5 is interconnected with the inner conductor 1 inside and outside, and plays a shielding role for the inner conductor 1 while homogenizing the electric field strength of the inner conductor 1. The copper tape wrap 6 is wrapped with gaps. The LNG ultra-low temperature direct current type medium-voltage coaxial cable is not easy to break down, crack or overheat in an ultra-low temperature environment, has stable electrical properties and a long service life.

[0070] The inner conductor 1 adopts a 1+6 compressed twisted structure. The inner conductor 1 includes an inner layer and an outer layer. The center is a soft round copper wire with a diameter of 2.66mm, which is the inner layer of the inner conductor 1. Six 2.1*3.23mm trapezoidal wires are compressed and twisted on the outside of the inner layer to form the outer layer. The outer layer is twisted to the left, and the outer layer pitch is 82mm. After twisting, the outer diameter of the inner conductor 1 is not more than 6.9mm.

[0071] The shield 2 is formed by wrapping and sintering a polyimide film tape, and the outer diameter of the cable after wrapping is not greater than 7.1 mm.

[0072] The insulation layer 3 has a thickness of 1.72 mm at its thinnest point and an average thickness of 1.9 mm, which ensures the cable's anti-puncture performance.

[0073] The insulating shield 4 formed by wrapping and sintering a polyimide film wrapping tape has a wrapping overlap rate of not less than 30%, an outer diameter of not more than 11.2 mm, high insulation resistance and high voltage resistance, and can effectively isolate the induced current between the insulating layer 3 and the outer conductor 5, thereby ensuring the stability of the cable electrical performance and increasing the service life of the product.

[0074] The outer conductor 5 is formed by tightly pressing and twisting 15 0.72*2.65mm wire conductors, with a twisting pitch of 164mm. The outer diameter of the cable after twisting the outer conductors is not greater than 12.6mm.

[0075] The outer conductor 5 is wrapped with a copper tape wrap 6. The copper tape wrap 6 is wrapped with a gap of 0.1 mm soft copper tape for shielding, and the gap overlap rate is -100%, ensuring that the outer conductor 5 is tightly wound, has no skipping, and has a uniform field strength effect.

[0076] The wrapping tape 7 is made of 0.05 mm thin flame retardant cloth tape, and the outer diameter of the cable after being wrapped with the wrapping tape 7 is no more than 13.3 mm.

[0077] The glass fiber protective layer 8 is woven from high-strength glass fiber yarns, with a weaving density of not less than 95%, a pitch of 30 mm, a weaving angle of 45°, and an outer diameter of the cable after weaving not more than 13.7 mm.

[0078] The outer protective layer 9 adopts polytetrafluoroethylene PFA with a melt index of 7g / 10min, which has the characteristics of ultra-low temperature resistance, acid and alkali corrosion resistance, oil resistance, weather resistance, etc. The thickness of the thinnest point of the outer protective layer 9 is controlled at 1.35mm, the average thickness is 1.5mm, and the outer diameter of the outer protective layer 9 after extrusion is not more than 17.0mm.

[0079] A method for preparing an LNG ultra-low temperature direct current type medium voltage coaxial cable comprises the following steps:

[0080] Step 1: The inner conductor 1 adopts a 1+6 compressed stranded wire structure, with a soft round copper wire with a diameter of 2.66mm in the center, and the outer layer of the soft round copper wire is 6 2.1*3.23mm trapezoidal wires compressed and twisted, the outer layer twist direction is left, the outer layer pitch is 82mm, and the outer diameter of the inner conductor 1 after twisting is not more than 6.9mm.

[0081] Step 2: Shield 2 is wrapped with a polyimide film tape with an overlap rate of not less than 30%, and then sintered.

[0082] Step 3: The insulating layer 3 is made of fresh PFA with a melting index of 7g / 10min, which is placed in an oven at 120±10℃ for 1±0.5h before extrusion to be fully dried and set aside;

[0083] In this embodiment 1, an extrusion mold such as a fluoroplastic 65 extruder is used to produce the insulating layer 3, and the extrusion body temperature is controlled at 320°C~340°C, 340°C~360°C, 360°C~380°C, and 380°C~400°C from the first zone to the fourth zone; the flange temperature is controlled at 380°C~400°C, and the head temperature is controlled at 375°C~410°C; the discharge surface is uniform, smooth, and has good fluidity, and the insulating layer 3 produced thereby has the advantages of ultra-low temperature resistance, not easy to break down, not easy to crack, stable electrical properties, and long service life;

[0084] The calculation formulas for the stretch ratio K and the die matching coefficient δ of the fluoroplastic 65 extruder are:

[0085] K = (D 大 2 -D 小 2 ) / (d 大 2- d 小 2 ) (1)

[0086] δ = (D 大 / d 大 ) / (D 小 / d 小 ) (2)

[0087] Among them, D 大 D is the diameter of the die sleeve of the fluoroplastic 65 extruder, in mm; 小 d is the outer diameter of the die core of the fluoroplastic 65 extruder, in mm; 大 d is the diameter of the insulated wire core, in mm; 小 is the conductor diameter, unit: mm; the value range of K is 1~50, and the value range of δ is 1.01~1.1.

[0088] Step 4: Wrap the insulating layer 3 with a polyimide film wrapping tape and sinter it to form an insulating shield 4. The wrapping overlap rate of the polyimide film wrapping tape is not less than 30%, and the outer diameter is not more than 11.2 mm. It has the advantages of high insulation resistance, high voltage resistance, and can effectively isolate the induced voltage between the insulating layer 3 and the outer conductor 5, ensure the stability of the cable electrical performance, and improve the service life of the product.

[0089] Step 5: Fifteen 0.72*2.65mm wire conductors are compressed and twisted outside the insulating shield 4, with a twisting pitch of 164mm and an outer diameter of no more than 12.6mm, to form an outer conductor 5; 0.1mm shielding soft copper tape is gap-wrapped outside the outer conductor 5, with a gap overlap rate of -100%, to form a copper tape wrap 6.

[0090] Step 6: Use a thin flame-retardant cloth tape to wrap the outside of the copper tape wrap 6 to form a wrapping tape 7, with an overlap rate of not less than 15%, and place it in a drying room for full drying;

[0091] Step 7: Use high-strength glass fiber yarn to weave a glass fiber protective layer 8 on the outside of the wrapping tape 7, with a weaving density of not less than 95%, a pitch of 30 mm, a weaving angle of 45°, and place it in a drying room for full drying.

[0092] Step 8: Use a sheath extrusion die to extrude polytetrafluoroethylene PFA with a melt index of 7g / 10min outside the glass fiber protective layer 8 to form an outer sheath 9, and finally obtain the desired product, which can ensure that the cable passes the DC withstand voltage test of 21kV / 5min without breakdown, and does not crack when stretched at a low temperature of -276°C;

[0093] The calculation formulas for the stretch ratio K and the die matching coefficient δ of the sheath extrusion die are:

[0094] K = (D 大 2 -D 小 2 ) / (d 大 2- d 小 2 ) (1)

[0095] δ = (D 大 / d 大 ) / (D 小 / d 小 ) (2)

[0096] Among them, D 大 D is the diameter of the sleeve of the sheath extrusion die, in mm; 小 d is the outer diameter of the die core of the sheath extrusion die, in mm; 大 d is the diameter of the insulated wire core, in mm; 小 is the conductor diameter, unit: mm; the value range of K is 1~50, and the value range of δ is 1.01~1.1.

[0097] Parts not specifically described in the present invention may adopt existing technologies and will not be described in detail here.

[0098] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. An LNG ultra-low temperature direct current medium voltage coaxial cable, characterized in that: It includes an inner conductor, a shield, an insulating layer, an insulating shield, an outer conductor, a copper tape wrap, a wrapping tape, a glass fiber protective layer and an outer sheath which are coaxially arranged from the inside to the outside. The inner conductor and the outer conductor respectively adopt a compressed wire twisted structure. The outer diameter of the inner conductor is not more than 6.9 mm, and the outer diameter of the insulated wire core composed of the inner conductor, the shield and the insulating layer is not more than 11.1 mm. The method for preparing the LNG ultra-low temperature direct current type medium voltage coaxial cable comprises the following steps: Step 1: Inner conductor twisting: The inner conductor adopts a 1+6 compressed stranded wire structure. The inner conductor includes an inner layer made of soft round copper wire and an outer layer formed by compacting and twisting a trapezoidal wire on the outside of the soft round copper wire. The outer layer is twisted to the left and the pitch of the outer layer is 82~117mm. The inner layer of the inner conductor is a soft round copper wire with a diameter of 2.66mm located in the center, and the outer layer is 6 2.1*3.23mm trapezoidal wires twisted in the left direction outside the inner layer; Step 2: Wrap the inner conductor with a polyimide film tape with an overlap rate of not less than 30%, and then sinter it to form a shield. The outer diameter of the cable after wrapping is not more than 7.1mm; Step 3: Using an insulating extrusion die, extrude polytetrafluoroethylene with a melt index of 6-8g / 10min outside the shield to form an insulating layer, wherein the thickness of the insulating layer at the thinnest point is controlled to be 1.70-1.75mm, and the average thickness is 1.8-2.0mm; Step 4: Wrap the outer part of the insulation layer with a polyimide film wrapping tape, with an overlap rate of not less than 30%, and then sinter it to form an insulation shield. The outer diameter of the cable after wrapping is not more than 11.2mm; Step 5: 15 0.72*2.65mm wire conductors are compressed and twisted outside the insulation shield to form an outer conductor with a twisting pitch of 164~190mm. The outer diameter of the cable obtained at this time is not more than 12.6mm; then, 0.1mm shielding soft copper tape is used for gap wrapping with a gap overlap rate of -100% to form a copper tape wrapping; Step 6: Form a wrapping tape on the outside of the copper tape wrapping, with an overlap rate of not less than 15%, and place it in a drying room for full drying; Step 7: Form a glass fiber protective layer on the outside of the tape, with a weaving density of not less than 95%, a pitch of 30~40mm, a weaving angle of 40°~50°, and place it in a drying room for full drying; Step 8: Using an extrusion die, extrude polytetrafluoroethylene (PFA) with a melt index of 6-8g / 10min outside the glass fiber protective layer to form an outer sheath to obtain the desired cable. The obtained cable passes the DC withstand voltage test of 21kV / 5min without breakdown, and does not crack when stretched at -276°C; In step 3 and step 8, the insulating layer and the outer protective layer are placed in an oven at 120±10°C for drying for 1±0.5h respectively; The calculation formulas for the stretch ratio K and die matching coefficient δ of the insulation extrusion die are: K=(D 大 2 -D 小 2 ) / (d 大 2- d 小 2 ) (1) δ=(D 大 / d 大 ) / (D 小 / d 小 ) (2) Among them, D 大 D is the diameter of the mold sleeve of the insulation extrusion mold, in mm; 小 d is the outer diameter of the die core of the insulation extrusion die, in mm; 大 It is the diameter of the insulated wire core consisting of the inner conductor, shield and insulation layer, in mm; d 小 is the inner conductor diameter, unit: mm; K ranges from 1 to 50, and δ ranges from 1.01 to 1.

1.

2. The LNG ultra-low temperature direct current medium voltage coaxial cable according to claim 1, characterized in that: The outer diameter of the cable after being wrapped with the tape is no greater than 13.3 mm.

3. The LNG ultra-low temperature direct current medium voltage coaxial cable according to claim 1, characterized in that: The outer diameter of the cable after the glass fiber protective layer is woven is no more than 13.7 mm.

4. The LNG ultra-low temperature direct current medium voltage coaxial cable according to claim 1, characterized in that: The outer sheath has a thickness of 1.3-1.4 mm at the thinnest point and an average thickness of 1.5-1.6 mm. The outer diameter of the cable after extruding the outer sheath is no more than 17.0 mm.

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