Preparation method of polypropylene-based high-voltage direct-current cable

Abstract

The invention discloses a preparation method of a polypropylene-based high-voltage direct-current cable. The preparation method comprises the following steps: preparing a conductive wire core through compressing and twisting; an inner shielding layer is extruded outside the conductive wire core; a modified graphene layer is coated outside the inner shielding layer; an insulating layer is extruded outside the modified graphene layer; an outer shielding layer is extruded outside the insulating layer; a metal shielding layer is coated outside the outer shielding layer; and the metal shielding layer is coated with a cable sheath. The silane coupling agent modified graphene is coated outside the inner shielding layer, so that the bonding surface of the inner semi-conductive layer and the insulating layer is tighter and smoother, air gaps and impurities at the bonding surface are reduced, the accumulation amount of space charges in the insulating layer is reduced, the insulating property of the insulating layer is ensured, and the service life of the cable is prolonged.

Description

Technical Field

[0001] The invention relates to the technical field of high-voltage direct current cables, and in particular to a method for preparing a polypropylene-based high-voltage direct current cable. Background Art

[0002] Compared with AC transmission technology, DC transmission technology has the advantages of larger capacity, higher voltage and longer transmission distance. With the development and application of high-voltage DC transmission technology, the application of high-voltage DC cables has become more extensive and mature. Polypropylene is prepared by polyaddition reaction of propylene monomer. It is widely used as the insulation material of high-voltage DC cables because of its good electrical insulation performance, high heat resistance temperature, thermoplastic recyclability and simple production process.

[0003] However, due to the disordered molecular chains and even a few impurities inside polypropylene, an energy band gap will appear inside it, thus forming a local center of charge. When the energy level of the local center is deep, the charge cannot break free after being captured, and the accumulation gradually increases, causing the electric field to be distorted, which in turn has a significant impact on the insulation performance of the polypropylene insulation layer.

[0004] Therefore, it is urgent to develop a preparation process for polypropylene-based high-voltage DC cables to reduce the accumulation of space charge inside the polypropylene insulation layer. Summary of the invention

[0005] The object of the present invention is to provide a method for preparing a polypropylene-based high-voltage DC cable to solve the technical problems mentioned in the background technology. The object of the present invention is achieved through the following technical solutions:

[0006] A method for preparing a polypropylene-based high-voltage direct current cable comprises the following steps:

[0007] Step S1, preparing a conductive wire core by compacting and twisting;

[0008] Step S2, extruding an inner shielding layer outside the conductive wire core;

[0009] Step S3, coating a modified graphene layer outside the inner shielding layer;

[0010] Step S4, extruding an insulating layer outside the modified graphene layer;

[0011] Step S5, extruding an outer shielding layer outside the insulating layer;

[0012] Step S6, covering the outer shielding layer with a metal shielding layer;

[0013] Step S7, covering the metal shielding layer with a cable sheath.

[0014] Furthermore, step S1 specifically includes:

[0015] Step S11, preparing a stranded wire core by twisting copper wires;

[0016] Step S12, preheating the twisted wire core to 100±3°C;

[0017] Step S13, pressing the twisted wire core tightly to obtain a conductive wire core;

[0018] Among them, the twisted wire core is divided into multiple layers from the inside to the outside, and the outermost layer of copper wire is tinned copper wire.

[0019] Furthermore, the twisting method in step S11 is regular twisting, and the pitch diameter ratio of the twisted wire core is 15-20.

[0020] Furthermore, the inner shielding layer and the outer shielding layer are both semi-conductive shielding materials, and the preparation method of the semiconductor shielding material includes:

[0021] Step S21, weighing low-density polyethylene, acetic acid-vinyl acetate copolymer, antioxidant, cross-linking agent and carbon black according to the proportion;

[0022] Step S22, adding low-density polyethylene, acetic acid-vinyl acetate copolymer, antioxidant and carbon black in turn into a mixing mill and mixing to obtain a mixture;

[0023] Step S23, adding the mixture obtained in step S22 into an internal mixer, adding a cross-linking agent in batches at 90-120° C., and mixing thoroughly to obtain a semiconductor shielding material.

[0024] Furthermore, step S3 specifically includes:

[0025] Step S31, preparing modified graphene coating;

[0026] Step S32, heating the inner shielding layer to 50-80° C.;

[0027] Step S33, spraying the modified graphene coating on the outside of the inner shielding layer;

[0028] Step S34, drying the modified graphene coating.

[0029] Furthermore, the preparation method of the modified graphene coating comprises:

[0030] Step S311, weighing graphene powder, placing the graphene powder in concentrated nitric acid, slowly adding concentrated sulfuric acid, reacting for 1 hour, and then centrifuging and washing;

[0031] Step S312, immersing the graphene powder obtained in step S311 in a xylene solution and fully dispersing it at a temperature of 75-85° C.;

[0032] Step S313, adding a silane coupling agent to the dispersion obtained in step S312, and fully reacting for 5-7 hours to obtain a modified graphene coating.

[0033] Furthermore, the insulating layer is graft modified polypropylene, and the preparation method of the graft modified polypropylene includes:

[0034] Step S41, weighing a polypropylene fraction and adding it into a reactor, replacing the air with nitrogen, adding benzoyl peroxide and styrene and stirring evenly;

[0035] Step S42, heating the reactor to 55-65°C and continuing the reaction for 1.5-2.5h;

[0036] Step S43, heating the reactor to 85-95°C and continuing the reaction for 0.5h under stirring;

[0037] Step S44, after filtering, blowing and drying with nitrogen at 70-80° C. to obtain graft-modified polypropylene.

[0038] Furthermore, the metal shielding layer is braided from red copper wires or tinned copper wires.

[0039] The technical solution provided in the embodiments of the present application has at least the following technical effects or advantages:

[0040] 1. By coating graphene modified by silane coupling agent on the outside of the inner shielding layer, the bonding surface between the inner semi-conductive layer and the insulating layer is made tighter and smoother, reducing the air gap and impurities at the bonding surface, reducing the amount of space charge accumulation inside the insulating layer, ensuring the insulation performance of the insulating layer, and extending the service life of the cable;

[0041] 2. By designing the outer layer of the conductive core to be tinned copper wire, the outer wall of the conductive core is smoother after compression, avoiding the burrs generated during the twisting process to cause tip discharge, while improving the smoothness of the inner surface of the inner shielding layer and improving the shielding performance of the inner shielding layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The drawings in the specification, which constitute a part of this application, are used to provide a further understanding of the present invention. The illustrative embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0043] Figure 1 This is a flow chart for preparing a DC cable according to an embodiment of the present application. DETAILED DESCRIPTION

[0044] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the drawings and specific implementation methods of the specification. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] like Figure 1 A method for preparing a polypropylene-based high-voltage DC cable is shown, comprising the following steps:

[0046] Step S1, preparing a conductive wire core by compacting and twisting.

[0047] Specifically include:

[0048] Step S11, twisting copper wires to obtain a twisted wire core.

[0049] The stranded wire core is made by twisting the single copper wire in layers in a regular twisting method. The pitch-diameter ratio of the stranded wire core is 15-20, and the pitch-diameter ratio of the inner copper wire is greater than that of the outer copper wire. The copper wire has the advantages of high conductivity and high tensile strength. Among them, the outermost copper wire is tinned copper wire, and the thickness of the tin layer of the tinned copper wire is 14um.

[0050] Step S12, preheating the twisted wire core to 100±3°C.

[0051] Heating the twisted wire core can remove moisture from the surface of the twisted wire core and avoid residual moisture between the twisted wire core and the inner shielding layer; at the same time, it also reduces the hardness of the copper wire, making subsequent pressing operations easier.

[0052] Step S13, compacting the twisted wire core to obtain a conductive wire core.

[0053] The twisted wire core is pressed tightly by three pairs of pressing wheels: vertical, horizontal and vertical, to obtain a conductive wire core with a smooth surface and no burrs.

[0054] By designing the outer layer of the conductive core to be tinned copper wire, the outer wall of the conductive core is smoother after compaction, avoiding tip discharge caused by burrs generated during the twisting process, and at the same time improving the smoothness of the inner surface of the inner shielding layer, thereby improving the shielding performance of the inner shielding layer.

[0055] Step S2, extruding an inner shielding layer outside the conductive wire core;

[0056] The inner shielding layer is mainly used to homogenize the electric field. The inner shielding layer is a semi-conductive shielding material. The preparation method of the semiconductor shielding material includes:

[0057] Step S21, weighing low-density polyethylene, acetic acid-vinyl acetate copolymer, antioxidant, cross-linking agent and carbon black according to the proportion, drying and setting aside;

[0058] Step S22, adding the dried low-density polyethylene, acetic acid-vinyl acetate copolymer, antioxidant and carbon black to an open mill in sequence and mixing them evenly to obtain a mixture;

[0059] Step S23, adding the mixture obtained in step S22 into an internal mixer, adjusting the chamber of the internal mixer to 90-120° C., adding a crosslinking agent in small batches, and mixing thoroughly to obtain a semiconductor shielding material.

[0060] Step S3, coating a modified graphene layer outside the inner shielding layer;

[0061] Specifically include:

[0062] Step S31, preparing modified graphene coating;

[0063] The preparation method of the modified graphene coating comprises:

[0064] Step S311, weighing graphene powder, placing the graphene powder in concentrated nitric acid, slowly adding concentrated sulfuric acid, reacting for 1 hour, and then centrifuging and washing to obtain graphene powder containing active functional groups;

[0065] Step S312, immersing the graphene powder obtained in step S311 in a xylene solution, and fully dispersing it at a temperature of 75-85° C. to obtain a graphene dispersion;

[0066] Step S313, adding a silane coupling agent to the dispersion obtained in step S312, and fully reacting for 5-7 hours to obtain a modified graphene coating.

[0067] Silane coupling agent can significantly increase the number of active sites that react on the surface of the inner shielding layer, the surface of the insulating layer and the surface of the graphene, thereby improving the tightness of the connection between the inner shielding layer and the insulating layer, reducing the air gap between the bonding surfaces of the inner shielding layer and the insulating layer, and avoiding the accumulation of space charges.

[0068] Step S32, heating the inner shielding layer to 50-80° C.;

[0069] Heating the inner shielding layer can remove moisture from the surface of the inner shielding layer and improve the volatilization efficiency of the solvent in the subsequent modified graphene coating.

[0070] Step S33, spraying the modified graphene coating evenly on the outside of the inner shielding layer;

[0071] Step S34, drying the modified graphene coating.

[0072] The inner shielding layer sprayed with modified graphene is dried with clean hot air at 147-176°C to form a modified graphene layer on the surface of the inner shielding layer. Drying can also increase the temperature of the inner shielding layer to avoid temperature shock affecting the extrusion of the insulation layer.

[0073] Step S4, extruding an insulating layer outside the modified graphene layer;

[0074] The insulating layer is mainly used to prevent current leakage and protect personal safety. The insulating layer is grafted modified polypropylene, and the preparation method of the grafted modified polypropylene includes:

[0075] Step S41, weighing a polypropylene fraction and adding it into a reactor, replacing the air in the reactor with nitrogen, adding benzoyl peroxide and styrene and stirring evenly; wherein the mass fraction of styrene is 5% of the polypropylene powder.

[0076] Step S42, raising the temperature of the reactor to 55-65° C. and continuing the reaction for 1.5-2.5 hours to allow styrene and benzoyl peroxide to fully swell and enter the polypropylene powder;

[0077] Step S43, heating the reactor to 85-95°C, adding an appropriate amount of pure water, and continuing the reaction for 0.5h under stirring;

[0078] Step S44, filtering the reaction product obtained in step S43, and blowing and drying it with nitrogen at 70-80° C. to obtain graft-modified polypropylene.

[0079] Preferably, the reaction is further carried out in ethyl acetate for 18 hours to remove unreacted materials.

[0080] Step S5, extruding an outer shielding layer outside the insulating layer;

[0081] The outer shielding layer is mainly used to homogenize the electric field. The outer shielding layer is a semi-conductive shielding material. The preparation method of the semiconductor shielding material is the same as the preparation method of the semi-conductive shielding material of the inner shielding layer in step S2.

[0082] Preferably, the outer shielding layer and the insulating layer are extruded onto the outer side of the modified graphene layer by double-layer co-extrusion.

[0083] Step S6, covering the outer shielding layer with a metal shielding layer;

[0084] The metal shielding layer is mainly used for grounding protection to prevent discharge from the internal structure of the cable. The metal shielding layer is made by weaving red copper wire or tinned copper wire on the outside of the outer shielding layer.

[0085] Step S7, covering the metal shielding layer with a cable sheath.

[0086] The cable sheath is mainly used to prevent moisture, corrosion and mechanical damage, and prevent electromagnetic interference. The selection of the cable sheath is based on different laying environments.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a polypropylene-based high-voltage direct current cable, characterized in that: The following steps are involved: Step S1, preparing a conductive wire core by compacting and twisting; Step S2, extruding an inner shielding layer outside the conductive wire core; Step S3, coating a modified graphene layer outside the inner shielding layer; Step S4, extruding an insulating layer outside the modified graphene layer; Step S5, extruding an outer shielding layer outside the insulating layer; Step S6, coating the outer shielding layer with a metal shielding layer; Step S7, covering the metal shielding layer with a cable sheath.

2. The method for preparing a polypropylene-based high-voltage direct current cable according to claim 1, characterized in that: Step S1 specifically includes: Step S11, preparing a stranded wire core by twisting copper wires; Step S12, preheating the twisted wire core to 100±3° C.; Step S13, pressing the twisted wire core to obtain the conductive wire core; The twisted wire core is divided into multiple layers from the inside to the outside, and the copper wire of the outermost layer is tinned copper wire.

3. The method for preparing a polypropylene-based high-voltage direct current cable according to claim 2, characterized in that: The twisting method in step S11 is regular twisting, and the pitch diameter ratio of the twisted wire core is 15-20.

4. The method for preparing a polypropylene-based high-voltage DC cable according to claim 1, characterized in that: The inner shielding layer and the outer shielding layer are both made of semi-conductive shielding materials, and the preparation method of the semiconductor shielding material comprises: Step S21, weighing low-density polyethylene, acetic acid-vinyl acetate copolymer, antioxidant, cross-linking agent and carbon black according to the proportion; Step S22, adding low-density polyethylene, acetic acid-vinyl acetate copolymer, antioxidant and carbon black in turn into a mixing mill and mixing to obtain a mixture; Step S23, adding the mixture obtained in step S22 into an internal mixer, adding a cross-linking agent in batches at 90-120° C., and mixing thoroughly to obtain a semiconductor shielding material.

5. The method for preparing a polypropylene-based high-voltage direct current cable according to claim 1, characterized in that: Step S3 specifically includes: Step S31, preparing modified graphene coating; Step S32, heating the inner shielding layer to 50-80° C.; Step S33, spraying the modified graphene coating on the outside of the inner shielding layer; Step S34, drying the modified graphene coating.

6. The method for preparing a polypropylene-based high-voltage direct current cable according to claim 5, characterized in that: The preparation method of the modified graphene coating comprises: Step S311, weighing graphene powder, placing the graphene powder in concentrated nitric acid, slowly adding concentrated sulfuric acid, reacting for 1 hour, and then centrifuging and washing; Step S312, immersing the graphene powder obtained in step S311 in a xylene solution and fully dispersing it at a temperature of 75-85° C.; Step S313, adding a silane coupling agent to the dispersion obtained in step S312, and fully reacting for 5-7 hours to obtain a modified graphene coating.

7. The method for preparing a polypropylene-based high-voltage DC cable according to claim 1, characterized in that: The insulating layer is graft modified polypropylene, and the preparation method of the graft modified polypropylene includes: Step S41, weighing a polypropylene fraction and adding it into a reactor, replacing the air with nitrogen, adding benzoyl peroxide and styrene and stirring evenly; Step S42, heating the reactor to 55-65°C and continuing the reaction for 1.5-2.5h; Step S43, heating the reactor to 85-95°C and continuing the reaction for 0.5h under stirring; Step S44, after filtering, blowing and drying with nitrogen at 70-80° C. to obtain graft-modified polypropylene.

8. The method for preparing a polypropylene-based high-voltage direct current cable according to claim 1, characterized in that: The metal shielding layer is braided from red copper wires or tinned copper wires.

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