EP4152346A1

EP4152346A1 - Multi-core direct-current submarine cable and method for producing same

Info

Publication number: EP4152346A1
Application number: EP20935489.3A
Authority: EP
Inventors: Hongliang Zhang; Shuhong XIE; Ming Hu; Hongmiao YU; Jianlin XUE; Yan Yan; Zhiyu Yan
Original assignee: Zhongtian Technology Submarine Cable Co Ltd
Current assignee: Zhongtian Technology Submarine Cable Co Ltd
Priority date: 2020-05-14
Filing date: 2020-06-28
Publication date: 2023-03-22
Also published as: CN111554435A; WO2021227212A1; CN111554435B

Abstract

Disclosed are a multi-core submarine cable and a method for producing same. The multi-core direct-current submarine cable comprises: two identical polar electric units (11, 12), an additional electric unit assembly (13), several filling strips (15), a belt (161), a cushion layer (162), an armored layer (163), and an outer protective layer (164). The outer diameter of the cross-section of the additional electric unit assembly is the same as the outer diameter of the cross-section of each polar electric unit; the two polar electric units and the additional electric unit assembly are twisted together, and are sequentially provided with the belt, the cushion layer, the armored layer and the outer protective layer wrapped around same; and the several filling strips are disposed in gaps between the two polar electric units and the belt and between the additional electric unit assembly and the belt. The multi-core direct-current submarine cable is conducive to saving laying costs and laying time, and the problem of roundness of electric units with different outer diameters can be effectively solved. In the method for producing the multi-core direct-current submarine cable, the two polar electric units and the additional electric unit assembly are twisted by means of gravity in a vertical direction, and twisting of the multi-core direct-current submarine cable can be stably and efficiently achieved.

Description

FIELD

The present application relates to the technical field of submarine cables, particularly, relates to multicore DC submarine cables and production method thereof.

BACKGROUND

This part is intended to provide a background or context for the embodiments of the present application stated in claims. The description here is not to be recognized as prior art because it is included in this part.
DC submarine cables are configured for connecting to converter equipment at both ends of a DC transmission system, and supporting submarine cable accessories to build a complete underwater or terrestrial transmission line system. Large capacity and long-distance transmission are often found as features. The required conductor cross-section is large. Therefore, existing DC submarine cables are mostly designed with single core submarine cable structure. The existing submarine cable adopts a large section single core structure, resulting in high project cost, and long project cycle. Additionally, when an optical unit needs to be added in a single core DC submarine cable, the optical unit needs to be in an armored layer of the single core DC submarine cable, or an optical fiber filling layer needs to be added separately in the cable. Placing the optical unit in the armored layer increases the risk of damage to the optical fiber unit. Adding the optical fiber filling layer separately will increase the cost of DC submarine cable raw materials, and reduce the transmission capacity of the submarine cable due to the addition of thermal insulation materials.
Generally, a pseudo bipolar DC submarine cable transmission system consists of two polar submarine cables. If one of the two polar submarine cables fails, the whole circuit will not operate normally. In a true bipolar DC submarine cable transmission system, a return submarine cable is added in the system. The true bipolar DC submarine cable transmission system is composed of three electrical paths to form a circuit. When one polar submarine cable fails, another polar submarine cable can form a circuit with the help of the return submarine cable, maintaining half of the transmission capacity. At present, in order to avoid an overall outage after the failure of one of the two polar submarine cables in the pseudo bipolar DC submarine cable transmission system, another standby polar submarine cable is also added in the pseudo bipolar DC submarine cable transmission system. When a fault occurs in one polar submarine cable, the polar submarine cable can be converted to the standby submarine cable, and the circuit can be reconstructed by the other polar submarine cable and the standby submarine cable.
When building a DC submarine cable transmission system, the main cost includes costs of converter systems, submarine cables, and submarine cable construction. A key content of the submarine cable construction is submarine cable laying. In the conventional single core structure of existing DC submarine cables, two or three DC submarine cables in each circuit need to be laid separately, which will increase the construction period and the cost of the construction ship. In order to solve the problem of high cost, the existing solution in the field is to use a construction ship equipped with two independent turntables to lay two single core DC submarine cables at the same time, bind them at the same time, and complete the construction at one time to save time and cost. However, no successful experience and case for manufacturing and using three single core DC submarine cables is known. Additionally, the bundling and laying scheme has high requirements for the construction ship, which requires the special multi turntable construction ship and the completion of the transformation and upgrading of auxiliary equipment. High costs are involved, which is not conducive to large-scale promotion.

SUMMARY

The present application discloses a multi-core direct-current submarine cable, including a structure in which two identical polar electric units and an additional electric unit assembly are integrated into a cable and are fully armored. An outer diameter of the cross-section of the additional electric unit assembly is the same as an outer diameter of the cross-section of each polar electric unit. Filling strips are arranged between the internal polar electric unit and the additional electric unit assembly, so as to reduce the material cost and the laying cost, and give roundness to the cabling of cores with different size.
The present application also discloses a stranding equipment of the multi-core direct-current submarine cable. The stranding equipment can achieve stable operation for the multi-core direct-current submarine cables by setting counterweights on a turntable.
The present application also discloses a production method of the multi-core direct-current submarine cable. The stranding equipment is used to squeeze together two polar electric units and additional electric unit assembly with the help of gravity, so as to achieve stranding of the multi-core direct-current submarine cable stably and efficiently.
The present application discloses a multi-core direct-current submarine cable, the multi-core direct-current submarine cable includes two identical polar electric units, additional electric unit assembly, several filling strips, a belt, a cushion layer, an armored layer, and an outer protective layer. An outer diameter of the cross-section of the additional electric unit assembly is the same as an outer diameter of the cross-section of each polar electric unit. The two polar electric units are squeezed and twisted together with the additional electric unit assembly, and are sequentially provided with the belt, the cushion layer, the armored layer and the outer protective layer wrapped around same. A plurality of the filling strips is arranged in gaps between the two polar electric units, the additional electric unit assembly, and the belt.
Preferably, the additional electric unit assembly includes an additional electric unit and at least one additional filling strip arranged at a side of the additional electric unit.
Preferably, the additional electric unit assembly includes two semicircular additional filling strips. The two semicircular additional filling strips are arranged on two sides of the additional electric unit, and the two semicircular additional filling strips form a substantially circular overall shape.
Preferably, after the two additional filling strips cooperate with the additional electric unit to give roundness, gaps are formed at an upper end and a lower end.
Preferably, the additional filling strip is located at the side where the additional electric unit is far away from the two polar electric units or between the additional electric unit and the two polar electric units.
Preferably, when the multi-core direct-current submarine cable is used in a pseudo bipolar DC transmission system, and transmission capacity of the additional electric unit assembly and the polar electric unit assembly is required to be the same, the additional electric unit assembly includes the additional electric unit with the same structure as the polar electric unit, and a sectional area of the conductor of the additional electric unit is the same as that of the polar electric unit.
Preferably, when the multi-core direct-current submarine cable is used in a true bipolar DC transmission system, the additional electric unit assembly includes an additional electric unit with the same structure as the polar electric unit, and the sectional area of the conductor of the additional electric unit is the same as the sectional area of the conductor of the polar electric unit.
Preferably, the additional electric unit assembly includes three additional electric units cabled together, and filling strips are arranged inside.
The application discloses a stranding equipment of the multi-core direct-current submarine cables, the stranding equipment is configured for stranding the multi-core direct-current submarine cables. The stranding equipment of the multi-core direct-current submarine cables includes a first platform, a second platform, a counterweight plate, a central shaft, two polar electric unit turntables, an additional electric unit turntable, at least one auxiliary turntable, a first traction wrapping assembly, a first stranding mold, a second traction wrapping assembly, a second stranding mold, and a steering wheel. The two polar electric units are respectively mounted in the two polar electric unit turntables. The auxiliary turntable is arranged at a side of the additional electric unit turntable. The counterweight plate, the first platform, and the second platform are all parallel and arranged from bottom to top. The central shaft is perpendicular to a middle portion of the counterweight plate. The two polar electric unit turntables and the additional electric unit turntable are uniformly arranged on the counterweight plate. The first traction wrapping assembly is arranged on the first platform, and the first stranding mold is arranged at a bottom of the first traction wrapping assembly. The second traction wrapping assembly is arranged on the second platform, and the second stranding mold is arranged at a bottom of the second traction wrapping assembly. The steering wheel is arranged on the second platform, and the steering wheel abuts the second traction wrapping assembly. A plurality of counterweights are arranged on the counterweight plate at a position vertically corresponding to the additional electric unit turntable. A counterweight is arranged on a side of the first platform away from the first traction wrapping assembly.
The production method of the multi-core direct-current submarine cable, applied in the stranding equipment of the multi-core direct-current submarine cable, includes the following steps:

(A) The two polar electric unit turntables and the additional electric unit turntables rotate counterclockwise around the central shaft at angular velocity ω1; the polar electric unit rotary turntables, the additional electric unit turntable, and the auxiliary turntable rotate clockwise synchronously at an angular speed ω2, the first platform rotates counterclockwise synchronously at an angular speed ω3, wherein ω1=ω2=ω3;
(B) Synchronously, the first traction wrapping assembly vertically pulls the additional electric unit or the additional electric unit and the additional filling strip at a speed v 1, the additional electric unit assembly is assembled at the first stranding mold and bound firmly, the second traction wrapping assembly vertically pulls the additional electric unit assembly and the two polar electric units at a speed v2, and the second stranding mold is a point of convergence to form an integral circular structure, wherein v1=v2;
(C) Synchronously, the integral circular structure is filled with and rounded by the filling strips, and the second traction wrapping assembly is configured to be bound firmly to complete the wrapping;
(D) After the belt binding is completed, the production process of the armored layer and the outer protective layer through the steering wheel is entered.

Compared with the prior art, the multi-core direct-current submarine cable and the production method disclosed in the present application have the following advantages: the multi-core direct-current submarine cable saves required raw material costs under the same conductor cross-section and insulation structure design, and have better market value. It is conducive to laying a single circuit DC submarine cable line at one time during the construction of submarine cable project, saving a lot of laying costs. Problem of roundness of electric units with different outer diameters is effectively solved. The stranding equipment of the multi-core direct-current submarine cable improves the stability of the stranding equipment by setting a counterweight turntable and counterweights. The multi-core direct-current submarine cable production method can achieve stable stranding of the multi-core direct-current submarine cable, and can improve the production efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with reference to the following drawings. The drawings in the following description are some embodiments of the present disclosure. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative work.

FIG. 1 is a cross-sectional view of a multi-core direct-current submarine cable of the present application.
FIG. 2 is a cross-sectional view of a polar electric unit of the multi-core direct-current submarine cable in the present application.
FIG. 3 is a cross-sectional view of an additional electric unit assembly of the multi-core direct-current submarine cable in the present application.
FIG. 4 is a cross-sectional view of a first variant of the multi-core direct-current submarine cable of the present application.
FIG. 5 is a cross-sectional view of a second variant of the multi-core direct-current submarine cable of the present application.
FIG. 6 is a cross-sectional view of the third variant of the multi-core direct-current submarine cable of the present application.
FIG. 7 is a schematic view of a stranding equipment of multi-core direct-current submarine cable in the present application.
FIG. 8 is a top view of turntable rotation of the stranding equipment of multi-core direct-current submarine cable of the present application.
FIG. 9 is a top view of turntable rotation of a variant of the stranding equipment of multi-core direct-current submarine cable of the present application.
FIG. 10 is flow chart of the production method of multi-core direct-current submarine cable in the present application.

Description of main components or elements:

Polar electric unit 11, 12
Conductor 111
Inner semiconducting shielding layer 112
Extruded insulating layer 113
Outer semiconducting shielding layer 114
Semiconducting water-blocking layer 115
Metal shielding layer 116
Plastic sheath 117
Additional electric unit assembly 13
Additional electric unit 131
Additional filling strip 132
Optical unit 14, 133C
Filling strip 15
belt 161
Cushion layer 162
Armored layer 163
Outer protective layer 164
First platform 1
Second platform 2
Counterweight plate 3
Polar electric unit turntable 10
Center shaft 20
Additional electric unit turntable 30
Counterweight 301
Twisting center 302
Auxiliary turntable 31
First traction wrapping assembly 32
First stranding mold 321
Counterweight 322
Second traction wrapping assembly 40
Second stranding mold 401
Steering wheel 50.

DETAILED DESCRIPTION

In order to better understand the above purposes, features and advantages of embodiments of the application, the application is described below in combination with the drawings and specific embodiments. It should be noted that, in the case of no conflict, the features in the embodiments of the present application can be combined with each other.
Many specific details are described in the following description to understand the embodiments of the application. The described embodiments are only part of the embodiments of the application, not all of them.
Unless otherwise defined, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the technical field belonging to the embodiments of the application. The terms used in the specification of the application herein are only for the purpose of describing specific embodiments, and are not intended to limit embodiments of the application.
The following specific embodiments will further explain the embodiments of the application in combination with the above drawings.
Referring to FIG. 1, a multi-core direct-current submarine cable of the present application includes two identical polar electric units 11 and 12, an additional electric unit assembly 13, several optical units 14, several filling strips 15, a belt 161, a cushion layer 162, an armored layer 163, and an outer protective layer 164. An outer diameter of the cross-section of the additional electric unit assembly 13 is the same as outer diameter of the cross-sections of the polar electric units 11 and 12 to improve roundness after cabling. The two polar electric units 11 and 12 are twisted together with the additional electric unit assembly 13, and are sequentially provided with the belt 161, the cushion layer 162, the armored layer 163, and the outer protective layer 164 wrapped around same. Several optical units 14 are arranged in gaps between the two polar electric units 11 and 12, the additional electric unit assembly 13, and the belt 161, and several filling strips 15 are arranged in gaps between the two polar electric units 11 and 12, the additional electric unit assembly 13, and the belt 161.
Referring to FIG. 2, the polar electric unit 11 includes a conductor 111, an inner semiconducting shielding layer 112, an extruded insulating layer 113, an outer semiconducting shielding layer 114, a semiconducting water-blocking layer 115, a metal shielding layer 116, and a plastic protective layer 117, which are sequentially wrapped from the inside to the outside. The structural dimensions of the polar electric unit 12 and of the polar electric unit 11 are the same. A main function of the polar electric units 11 and 12 is to form a direct-current (DC) transmission circuit, which can transmit electrical energy. When the polar electric units 11 and 12 are in normal operation, the additional electric unit assembly 13 is not required to work.
The additional electric unit assembly 13 includes an additional electric unit 131. In a true bipolar DC transmission system, a main function of the additional electric unit 131 is to transmit a return current for the other polar electric unit when one of the polar electric units fails. In the present application, a conductive sectional area of the additional electric unit 131 (recorded as S _additional) is the same as that of the polar electric unit (recorded as S _polar), that is, S _additional = S _polar. Preferably, a thickness of an insulation layer of the additional electric unit (recorded as d _additional) is 30% of a thickness of an insulation layer of the polar electric unit (recorded as d _polar), that is, d _additional=30%×d _polar. In a pseudo bipolar DC transmission system, a main function of the additional electric unit 131 is to provide functionality of the circuit with the other polar electric unit when one of the polar electric units fails, so as to maintain normal operation of the system. At this time, the design of the conduct sectional area and the thickness of the insulation layer of the additional electric unit 131 depends on the transmission capacity that the system requires of the additional electric unit. When the transmission capacity that the additional electric unit 131 is required to achieve is the same as that of the polar electric unit, S _additional = S _polar, and d _additional =d _polar. At this time, the structural composition and size of the additional electric unit are completely consistent with those of the polar electric unit.
In the pseudo bipolar DC transmission system, when the system sets the transmission capacity reserved by the additional electric unit 131 at less than the transmission capacity of the polar electric units 11 and 12, the size of the additional electric unit 131 is inconsistent with that of the polar electric units 11 and 12, and the additional electric unit assembly 13 needs to add additional filling strips to keep the outer diameter of the cross-section of the additional electric unit assembly 13 the same as the sectional size of the polar electric units 11 and 12. Referring to FIG. 3, the additional electric unit assembly 13 includes two semicircular additional filling strips 132. Two additional filling strips 132 are arranged on two sides of the additional electric unit 131, and the two additional filling strips 132 form an approximately round shape. D, shown in FIG. 3, is the approximate outer diameter, and D is the same as the outer diameter of the cross-section of the polar electric units 11 and 12, so as to cooperate with the two polar electric units to improve the roundness of the entire package submarine cable after cabling. In order to maintain a margin of deformation after adding the two additional filling strips 132, two additional filling strips 132 are designed to cooperate with the additional electric unit 131 to lend roundness, and gaps are formed at an upper end and a lower end, that is, the gap is marked as d in FIG. 3, and preferably d is 10mm. In order to avoid deformation of the additional electric unit 131 caused by excessive pressure when tightening the two additional filling strips 132 on the additional electric unit 131 during the stranding process, a radian inside the additional filling strip 132 is designed to be according to the outer diameter of the additional electric unit 131, and fitting angle α between the inner arc of the additional filling strip 132 on both sides and the outer diameter of the additional electric unit 131 is defined as 120 °.
Referring to FIG. 4, a first variant of the multi-core direct-current submarine cable in the application is shown. The difference is that the additional electric unit assembly 13A includes an additional electric unit 131A and an additional filling strip 132A. The additional filling strip 132A is arranged at a side of the additional electric unit 131A away from the polar electric units 11 and 12. The additional electric unit 131A is in contact with the additional filling strip 132A and two polar electric units 11 and 12, and being held at three points the position is stable. There is no mandatory requirement for the contact area and angle between the additional filling strip 132A and the additional electric unit 131A. Both sides of the additional filling strip 132A are required to be tightened and pressed under by the adjacent filling strip 15 to ensure no large displacement. The first variant simplifies the structure of the additional electric unit assembly 13A, and uses a single side filling strip to replace two semi-circular composite filling strips to improve the overall cabling roundness.
Referring to FIG. 5, a second variant of the multi-core direct-current submarine cable in the present application is shown. The difference is that the additional electric unit assembly 13B includes an additional electric unit 131B and an additional filling strip 132B. The additional filling strip 132B is arranged between the additional electric unit 131B and the polar electric units 11 and 12. Both sides of the additional filling strip 132B must be tightened and pressed under by the adjacent filling strip 15 to ensure there is no large displacement. The second variant simplifies the structure of the additional electric unit assembly 13A, and has the advantage that the center of gravity of the submarine cable is closer to the geometric center after cabling, which is conducive to later storage and construction process.
Referring to FIG. 6, a third variant of the multi-core direct-current submarine cable in the present application is shown. The difference is that the additional electric unit assembly 13C is a three core pre-cabling structure. The additional electric unit assembly 13C includes three additional electric units 131C after cabling, and the filling strip 132C is set inside. After that, the additional electric unit assembly 13C conducts secondary cabling with two polar electric units 11 and 12. The sum of the sectional areas of the conductors of the three additional electric units 131C is the same as that of the polar electric units 11 and 12. Several optical units 133C can also be added to the additional electric unit assembly 13C, increasing optical fiber monitoring and communication channels. The third variant divides the separate additional power unit into three independent power units, which can flexibly control the number of connected cores in coordination with the transmission system, and adjust the load capacity of the additional power unit as required, making adjustment flexible and convenient.
Referring to FIG. 7 and FIG. 8, the present application discloses a stranding equipment of multi-core direct-current submarine cable, including a first platform 1, a second platform 2, a counterweight plate 3, a central shaft 20, two polar electric unit turntables 10, an additional electric unit turntable 30, two auxiliary turntables 31, a first traction winding assembly 32, a first traction winding mold 321, a second traction winding assembly 40, a second winding mold 41, and a steering wheel 50. The polar electric units 11 and 12 are respectively mounted in two polar electric unit turntables 10, the additional electric unit 131 is mounted in the additional electric unit turntable 30, and two additional filling strips 132 are respectively mounted in two auxiliary turntables 31. The counterweight plate 3, the first platform 1 and the second platform 2 are parallel and arranged from bottom to top and are positioned coaxially, the central shaft 20 is arranged to be perpendicular to a middle portion of the counterweight plate 3. The two polar electric unit turntables 10 and the additional electric unit turntable 30 are uniformly arranged on the counterweight plate 3 and surround the central shaft 20. The two auxiliary turntables 31 are symmetrically arranged on both sides of the additional electric unit turntable 30, the first traction wrapping assembly 32 is arranged on the first platform 1, and the first twisted mold 321 is arranged at the bottom of the first traction wrapping assembly 32. The second traction wrapping assembly 40 is arranged on the second platform 2, and the second stranding mold 41 is arranged at the bottom of the second traction wrapping assembly 40. The steering wheel 50 is arranged on the second platform 2, and the steering wheel 50 abuts the second traction wrapping assembly 40.
In the structure of the multi-core direct-current submarine cable in the present application, the weight per unit length of the additional electric unit 131 will be less than the weight per unit length of the polar electric units 11 and 12, causing uneven weight distribution on the counterweight plate 3. Therefore, a plurality of counterweights 301 are arranged on the counterweight plate 3 at a position perpendicular to the additional electric unit turntable 30, and the counterweight 301 is weighted according to the difference in weight between the sum of the weights of the additional electric unit turntable 30, the auxiliary turntable 31, and the load, and the polar electric unit turntable 10 and the load, to avoid the problem of the center of gravity not being in the geometric center during the cabling process, this also ensures balance in the vertical rotation of the stranding equipment. For the same reason, the counterweight 322 is arranged on a side of the first platform 1 where the first traction wrapping assembly 40 is not set, to further maintain balance of the stranding device. It is worth noting that the weight difference are recalculated and the counterweight possibly adjusted after production of each kilometer of stranding.
For the first and second variants of multi-core direct-current submarine cables, it is only necessary to install additional filling strip 132A or 132B in the auxiliary turntable 31 on one side.
For the third variant of the multi-core direct-current submarine cable, the stranding equipment of the multi-core direct-current submarine cable is shown in FIG. 9, which includes three additional electric unit turntables 30A. The three additional electric unit turntables 30A need to first rotate counterclockwise around the stranding center 302A to complete a primary stranding. During the primary stranding process, each additional electric unit 131C rotates clockwise to eliminate axial torsion of the cable itself. In synchronization, the twisted additional electric unit 131C and the two polar electric units 11 and 12 rotate counterclockwise around the central shaft 20 to complete a secondary stranding. During the secondary stranding process, each polar electric unit rotates clockwise prevent axial torque of the cable, wherein ω1 = ω2. Other working processes of the third variant are the same as the original scheme.
Referring to FIG. 10, the present application also discloses a production method of multi-core direct-current submarine cable, which is configured to prepare the multi-core direct-current submarine cable, and is applied in the stranding equipment of multi-core direct-current submarine cable, the production method includes the following steps:

(A) The polar electric unit turntables 10 and additional electric unit turntable 30 rotate counterclockwise around the center shaft 20 at an angular velocity ω1; at the same time, the polar electric unit turntables 10, the additional electric unit turntable 30 and the auxiliary turntable 31 are synchronized and rotate clockwise at an angular velocity ω2, to prevent axial torque of cable core; at the same time, the first platform 1 rotates counterclockwise synchronously at an angular velocity ω3; wherein ω1=ω2=ω3 ;
(B) Synchronously, the first traction wrapping assembly 32 exerts a vertical pull on the additional electric unit 131 or on the additional electric unit and the additional filling strip at a speed v1, the additional electric unit assembly 13 shown in FIG. 3 is assembled at the first stranding mold 321 and bound firmly; at the same time, the second traction wrapping assembly 40 vertically pulls the additional electric unit assembly 13 and two polar electric units 11 and 12 after wrapping at a speed v2, and the second stranding mold 41 is the point of convergence, to form an integral circular structure; wherein v1=v2;
(C) Synchronously, the integral circular structure is filled with and rounded by filling strip 15, and the wrapping assembly 40 is configured to bind the integral circular structure firmly to complete the wrapping.
(D) After the belt binding is completed, the subsequent production process of the armored layer and the outer protective layer through the steering wheel 50 is entered.

The above embodiments are only used to describe the technical solution of the embodiments of the application, not the limitations. Although the embodiments of the application have been described in detail with reference to the above preferred embodiments, ordinary technicians in the art should understand that the technical solution of the embodiments of the application can be modified or replaced equivalently, which should not be divorced from the spirit and scope of the technical solution of the embodiments of the application.

Claims

A multi-core direct-current submarine cable, characterized in that, comprising two identical polar electric units, an additional electric unit assembly, several filling strips, a belt, a cushion layer, an armored layer, and an outer protective layer, an outer diameter of the cross-section of the additional electric unit assembly is the same as an outer diameter of the cross-section of each polar electric unit; the two identical polar electric units are squeezed and twisted together with the additional electric unit assembly, and are sequentially provided with the belt, the cushion layer, the armored layer and the outer protective layer wrapped around same; a plurality of the filling strips is arranged in gaps between the two identical polar electric units, the additional electric unit assembly, and the belt.
The multi-core direct-current submarine cable of claim 1, characterized in that, the additional electric unit assembly includes an additional electric unit and at least one additional filling strip arranged at a side of the additional electric unit.
The multi-core direct-current submarine cable of claim 2, characterized in that, the additional electric unit assembly includes two semicircular additional filling strips, the two semicircular additional filling strips are arranged on two sides of the additional electric unit, and the two semicircular additional filling strips form a substantially circular overall shape.
The multi-core direct-current submarine cable of claim 3, characterized in that, after the two additional filling strips cooperate with the additional electric unit to give roundness, gaps are formed at an upper end and a lower end.
The multi-core direct-current submarine cable of claim 2, characterized in that, the additional filling strip is located at the side where the additional electric unit is far away from the two polar electric units or between the additional electric unit and the two polar electric units.
The multi-core direct-current submarine cable of claim 1, characterized in that, when the multi-core direct-current submarine cable is applied to a pseudo bipolar DC transmission system, and transmission capacity of the additional electric unit assembly and the polar electric unit assembly is required to be the same, the additional electric unit assembly includes the additional electric unit with the same structure as the polar electric unit, and a sectional area of the conductor of the additional electric unit is the same as that of the polar electric unit.
The multi-core direct-current submarine cable of claim 1, characterized in that, when the multi-core direct-current submarine cable is applied to a true bipolar DC transmission system, the additional electric unit assembly includes an additional electric unit with the same structure as the polar electric unit, and the conductor sectional area of the additional electric unit is the same as the sectional area of the conductor of the polar electric unit.
The multi-core direct-current submarine cable of claim 1, characterized in that, the additional electric unit assembly includes three additional electric units cabled together, and filling strips are arranged inside.
A stranding equipment of multi-core direct-current submarine cables, configuring for stranding the multi-core direct-current submarine cables as claimed in any one of the claims 1 to 7, the stranding equipment of multi-core direct-current submarine cables comprising a first platform, a second platform, a counterweight plate, a central shaft, two polar electric unit turntables, an additional electric unit turntable, at least one auxiliary turntable, a first traction wrapping assembly, a first stranding mold, a second traction wrapping assembly, a second stranding mold, and a steering wheel; the two polar electric units are respectively mounted in the two polar electric unit turntables; the auxiliary turntable is arranged at a side of the additional electric unit turntable; the counterweight plate, the first platform and the second platform are all parallel and arranged from bottom to top; the central shaft is perpendicular to a middle portion of the counterweight plate; the two polar electric unit turntables and the additional electric unit turntable are uniformly arranged on the counterweight plate; the first traction wrapping assembly is arranged on the first platform, and the first stranding mold is arranged at a bottom of the first traction wrapping assembly; the second traction wrapping assembly is arranged on the second platform, and the second stranding mold is arranged at a bottom of the second traction wrapping assembly; the steering wheel is arranged on the second platform, and the steering wheel abuts the second traction wrapping assembly; a plurality of counterweights are arranged on the counterweight plate at a position vertically corresponding to the additional electric unit turntable; a counterweight is arranged on a side of the first platform away from the first traction wrapping assembly.
A production method of multi-core direct-current submarine cable, applied in the stranding equipment of multi-core direct-current submarine cable as claimed in claim 9, the production method comprising the following steps:
(A) the two polar electric unit turntables and the additional electric unit turntables rotate counterclockwise around the central shaft at angular velocity ω1; the polar electric unit rotary turntables, the additional electric unit turntable and the auxiliary turntable rotate clockwise synchronously at an angular speed ω2, the first platform rotates counterclockwise synchronously at an angular speed ω3, wherein ω1=ω2=ω3;

(B) synchronously, the first traction wrapping assembly vertically pulls the additional electric unit or the additional electric unit and the additional filling strip at a speed v1, the additional electric unit assembly is assembled at the first stranding mold and bound firmly, the second traction wrapping assembly vertically pulls the additional electric unit assembly and the two polar electric units at a speed v2, and the second stranding mold is a point of convergence to form an integral circular structure, wherein v1=v2;

(C) synchronously, the integral circular structure is filled with and rounded by the filling strips, and the second traction wrapping assembly is configured to be bound firmly to complete the wrapping;

(D) after the belt binding is completed, the production process of the armored layer and the outer protective layer through the steering wheel is entered.