HK40060134A - Power cable, method for production and use thereof - Google Patents

Description

Power cable and manufacturing method and application thereof

Technical Field

The present application relates to a power cable, a method of manufacturing a power cable and the use of a power cable in subsea applications.

Background

Over the past decades, accidental failure of subsea High Voltage (HV) power cables has increased. In most cases, such failures appear to be due to the use of cross-linked Polyethylene (PEX), a highly complex material. PEX was originally introduced as a HV cable manufacturing material to cope with variations in the requirements of the terrestrial cable design (to accommodate conductor operating temperatures of up to 90 cables, not up to 70 ℃). In a normally cold subsea marine environment, this temperature requirement seems insignificant, in which environment the ambient temperature hardly exceeds a few degrees above 0 ℃.

From a material point of view, non-crosslinked polymers (such as ethylene, polyethylene and ethylene propylene rubbers) have no reason to be used for HV cables with operating voltages up to 66 kv (especially when the conductor field stress is kept at a low level). However, in order to reduce the electric field stress in the HV cable to an acceptable level, the outer diameter of the conductor must be increased, which in turn increases the cost of the outer cable armouring to an excessive level and entails serious weight disadvantages, while further reducing the operational convenience of the HV cable.

Summary of The Invention

The present invention relates to a power cable including a tension member placed at a center of the power cable; a first insulating layer in which the tension members are embedded; and outer protective skin (sheath); wherein the power cable further comprises one or more first aluminum conductors embedded in the first insulating layer.

The invention also relates to a method for manufacturing the inventive power cable, which method comprises the step of extruding a first polymer insulation layer onto a tension member and one or more conductors in one single step.

Finally, the invention relates to the use of the power cable of the invention in medium to high voltage subsea applications, such as the driving of an offshore windmill cable infrastructure or a subsea pump.

The present invention utilizes aluminum-based conductors that require an increased conductor diameter compared to conventional copper-based conductors. Furthermore, the present invention replaces the conventional outer armor with an inner tension member placed in the center of the power cable. By utilizing the inner tension member, the outer diameter of the conductor is further increased as it now extends radially to accommodate the tension member. With this arrangement, the electric field stress is significantly reduced compared to conventional power cables, and expensive external armouring can be safely omitted. Further, since the electric field stress is reduced, the insulation thickness may be reduced, and a solid (solid), non-crosslinked ethylene, polyethylene, or ethylene propylene rubber material may be used as an insulator, thereby replacing PEX and solving the above-described problems.

Another advantage of providing an inner tension member and omitting the conventional outer armor is a reduction in overall cable diameter, overall cable weight, and cable bending stiffness. The low specific gravity of the power cable according to the invention and its reduced stiffness when immersed in water allows for a low clamping force and improved handling when installing the power cable, for example during crawler (caterpillar) installation. Thus, the power cable according to the present invention is more flexible than conventional cables and is therefore easier to bundle.

Finally, the omission of the conventional outer armouring results in a significant cost reduction, since the outer armouring usually accounts for 40% of the total material cost of the power cable.

Another advantage of the power cable according to the invention is: the aluminum conductor makes semiconductive insulation unnecessary, thereby reducing the number of components required to form the power cable, as well as reducing the overall diameter of the power cable itself.

Finally, the solid insulation material gives the power cable exceptional crush resistance compared to conventional power cables. The solid design and the consequent absence of any voids (such as those present in PEX foam) ensures that the power cable according to the invention is a so-called ultra-dry design. The ultra-dry design implies a true dry construction where at any point during the cable's service life there is no potential risk of voids present in the cable material being filled with water.

Drawings

Fig. 1 is a schematic cross-section of a power cable according to a first embodiment of the invention.

Fig. 2 is a schematic cross-section of a power cable according to a second embodiment of the invention.

Fig. 3 is a schematic cross-section of a power cable according to a third embodiment of the invention.

Fig. 4A and 4B illustrate two multi-core power cable configurations.

Detailed Description

Fig. 1 is a schematic cross-section of a power cable according to a first embodiment of the invention. The power cable comprises tension members 1 placed in the center of said power cable, a first insulating layer 3 surrounding the tension members 1 and protected from environmental influences by a sheath 9. Embedded in the first insulating layer 3 are one or more (preferably three) first aluminium conductors 4. Each first aluminum conductor may have a circular cross-section, wherein the diameter of each conductor is the same. The conductor diameter may be selected according to the desired application of the power cable.

Furthermore, the power cable may comprise a first semi-conductive outer shield (screen)2 surrounding the tension member 1 and a second semi-conductive outer shield 5 surrounding the insulating layer 3. The power cable may optionally comprise a first metal shield 6 and/or a second metal shield 7, wherein the first metal shield and/or the second metal shield may have various functions (e.g. to facilitate fault search). The first and/or second metallic shield is/are wrapped by a semiconductive tape wrapping (tape wrapping) 8.

For a power cable having a circular cross-section and two or more first aluminum conductors 4, the conductors are preferably arranged in a circumferentially equidistant manner. This is illustrated in fig. 1 for an embodiment with three conductors. In medium (up to 1kV) to high (above 1kV) voltage applications, three or more phases are typically required; the power cable comprises a corresponding number of conductors.

Table 1 provides typical mechanical properties of an exemplary power cable according to the first embodiment.

TABLE 1

Fig. 2 is a schematic view of a cross-section of a power cable according to a second embodiment of the invention. Features corresponding to those of the first embodiment are denoted by the same reference numerals. The second embodiment differs from the first embodiment in that a second insulating layer 3 'is additionally provided (preferably surrounded by a third semiconducting outer shield 5'). Said second insulating layer 3' surrounds the first insulating layer 3 and, if present, the second semiconducting outer shield 5. Embedded in the second insulating layer 3 'are one or more second aluminum conductors 4'. Each second aluminum conductor may have a circular cross-section, the diameter of the first aluminum conductor 4 and the diameter of the second aluminum conductor 4' being preferably the same.

For a power cable having a circular cross-section and two or more second aluminum conductors 4', the conductors are preferably arranged in a circumferentially equidistant manner. This is illustrated in fig. 2 for an embodiment with three conductors.

Fig. 2 shows a power cable comprising three first aluminum conductors 4 and three second aluminum conductors 4', which power cable is configured such that the midpoint of each first aluminum conductor 4 is located on a straight line passing through the midpoint of the power cable and the midpoint of exactly one second aluminum conductor 4'. The configuration of fig. 2 may alternatively include two, four, or more first and second aluminum conductors. This configuration allows the power cable to be used to operate two medium to high voltage applications simultaneously. For example, when the power cable is used for AC applications, two subsea pumps may be operated simultaneously, each powered by its own set of aluminum conductors 4 and 4'. Alternatively, when the power cable is used as a DC output cable, the three first aluminum conductors 4 may be used as a DC conductor phase, and the three second aluminum conductors 4' may be used as a ground line. The latter application is particularly relevant for the power output of offshore windmills.

The tension members 1 comprise a high tensile material, such as steel, preferably high tensile steel, a composite material or an aramid (Kevlar) material. Furthermore, the tension member 1 may be solid (e.g., in the form of a rod, a wire, or a bundle of wires). Alternatively, the tension member may be hollow (e.g., in the form of a tube). The tension member 1 may comprise a further element (e.g. a temperature sensor) in its centre.

Fig. 3 shows a schematic cross-section of a power cable according to a third embodiment of the invention. In this embodiment, the tension member 1 (in the form of a wire bundle) is surrounded by one or more first aluminum conductors 4 (in the form of one or more wire loops), both embedded in the first insulating layer 3. A second insulating layer 5 is provided, the second insulating layer 5 being separated from the first insulating layer 3 by a first semi-conductive outer sheet (sheet) 2.

One or more power cables according to the third embodiment may be bundled into a multi-core power cable, a variation of which is shown in fig. 4A and 4B. The multi-core power cable may comprise one or more power cables 10 (optionally one or more weight elements 11 and optionally further functional elements 12) according to the third embodiment. The functional elements may include, for example, fiber optic cables or signal cables. The weight element 11 may comprise zinc or lead. One or more power cables 10, weight elements 11 and functional elements 12 are embedded in an extruded insulation layer 13. An outer semiconducting shield is provided surrounding the insulating layer 13. The multi-core power cable is protected from the environment by an outer sheath surrounding an outer semi-conductive shield.

Fig. 4A and 4B show a configuration with three power cables 10 and one further functional element 12. In fig. 4A two weight elements 11 are provided and in fig. 4C a number of weight elements 11 are provided.

Table 2 provides typical mechanical properties of an exemplary power cable according to the embodiment of fig. 4A; the various cable masses, immersion weights, specific gravity ratios and stiffness values listed in table 1 may naturally vary depending on the number and type of weight elements present.

TABLE 2

The method of manufacturing an electric power cable according to the invention comprises the step of extruding a first insulating layer 3 onto a tension member 1 and one or more first aluminium conductors 4. Thus, the tension member 1 and the one or more first aluminum conductors 4 are embedded in the first insulating layer 3. Furthermore, all of the one or more second aluminum conductors 4' are embedded in the second insulating layer 3. In order to manufacture the power cable according to the second embodiment, a second insulating layer 3 'is extruded onto one or more second aluminum conductors 4' in a further method step. The first method step and the second method step can be performed sequentially (extrusion of the second insulating layer 3' onto the already extruded first insulating layer 3) or simultaneously by co-extrusion.

The method according to the invention is in contrast to the manufacturing method of conventional power cables, in which each conductor is first embedded in its own insulating layer, on which a desired number of such insulated conductors are bundled together and fixed in place by a separate outer layer. The method according to the invention thus achieves considerable cost savings and is much simpler to implement than conventional power cable manufacturing methods.

The first 2, second 5 and third 5' semiconducting outer shields comprise a polymer, preferably polyethylene, polystyrene or polyamide.

The first insulating layer 3 and the second insulating layer 3' comprise a non-crosslinked polymer (preferably ethylene, polyethylene or ethylene propylene rubber).

The optional first 6 and second 7 metallic shields comprise copper (preferably annealed copper) or lead. The metallic shield is preferably provided in the form of a strip or sheath. The semiconductive tape wrap 8 comprises polyamide (nylon). Finally, the sheath 9 comprises high density polyethylene, and the sheath 9 may have been extruded onto the underlying layer, or may be wrapped around the underlying layer in the form of a tape.

Although the power cables in fig. 1, 2 and 3 are presented with a circular cross-section, this is for illustrative purposes only and is in no way limiting; other cross-sectional geometries (e.g., oval or rectangular) may be used.

The power cable according to the invention may also be provided with a lead sheath (jack) surrounding the sheath. Such lead sheathing adds weight, which may be desirable for subsea applications. Furthermore, the lead sheath greatly increases the expected service life of the power cable (up to 50 years).

The foregoing embodiments and examples are in no way limiting, and the scope of the invention is defined by the appended claims.

Claims (17)

1. An electrical power cable comprising:

a tension member (1) placed at the center of the power cable;

a first insulating layer (3), in which the tension member (1) is embedded; and

an outer protective skin (9);

wherein the power cable further comprises one or more first aluminum conductors (4) embedded in the first insulating layer (3).

2. A power cable according to claim 1, comprising a first semiconductive outer shield (2) positioned between the tension member (1) and the insulating layer (3) and a second semiconductive outer shield (5) surrounding the insulating layer (3).

3. A power cable according to claim 2, comprising a second insulating layer (3 ') surrounded by a third semiconducting outer shield (5 '), the second insulating layer (3 ') surrounding the second semiconducting outer shield (5), wherein one or more second aluminum conductors (4 ') are embedded in the second insulating layer (3 ').

4. A power cable according to claims 1-3, comprising a circular cross-section, the power cable further comprising two or more first aluminium conductors (4) arranged in a circumferentially equidistant manner.

5. A power cable according to claim 4, comprising two or more second aluminium conductors arranged in a circumferentially equidistant manner such that the midpoint of each first aluminium conductor (4) is located on a straight line passing through the midpoint of the power cable and through the midpoint of exactly one second aluminium conductor (4').

6. An electric power cable according to claim 1, wherein the tension member (1) comprises a wire bundle and wherein the first aluminium conductor (4) comprises one or more aluminium wire loops.

7. A power cable according to claim 6, further comprising a second semiconductive outer shield (5) surrounding the insulating layer (3), a second insulating layer (3 ') surrounding the second semiconductive outer shield (5), and a third semiconductive outer shield (5 ') surrounding the second insulating layer (3 ').

8. An electric power cable according to claim 7, wherein said first and second insulating layers (3, 3') comprise a non-crosslinked solid polymer, preferably ethylene, polyethylene or ethylene propylene rubber.

9. A multicore power cable comprising one or more power cables (10) according to any of claims 6-8, the one or more power cables (10), one or more weight elements (11) and at least one functional element (12) being embedded in the extruded insulating layer (13).

10. A power cable according to any of the preceding claims, further comprising a first metallic shield (6) and/or a second metallic shield (7) wrapped by a semiconductive tape wrap (8) and positioned directly within the sheath (9).

11. A power cable according to claim 10, wherein the first and/or second metallic shield (6, 7) comprises copper or lead, the copper preferably being annealed copper.

12. A power cable according to any of the preceding claims, further comprising a lead sheath surrounding the sheath (9).

13. A method for manufacturing a power cable according to any of the preceding claims, comprising the step of extruding a first polymer insulation layer (3) onto the tension member (1) and the one or more conductors (4) in one single step.

14. A method for manufacturing an electric power cable according to claim 13, wherein a second insulating layer (3 ') is extruded onto the first insulating layer (3) and one or more second aluminum conductors (4').

15. A method for manufacturing an electric power cable according to claim 14, wherein the second insulating layer (3') is co-extruded with the first insulating layer (3).

16. A method for manufacturing an electric power cable according to claim 14, wherein the first insulating layer (3) and the second insulating layer (3') are sequentially extruded.

17. Use of the power cable according to any of the preceding claims in medium to high voltage subsea applications, such as the driving of an offshore windmill cable infrastructure or a subsea pump.