EP4024412A1

EP4024412A1 - Cable design for high voltage cables and qualification method for rating cables

Info

Publication number: EP4024412A1
Application number: EP20306704.6A
Authority: EP
Inventors: Øyvind IVERSEN; Torunn Lund Clasen
Original assignee: Nexans SA
Current assignee: Nexans SA
Priority date: 2020-12-29
Filing date: 2020-12-29
Publication date: 2022-07-06

Abstract

The present invention relates to power cables and more particular to thickness of insulation layers of medium and high voltage power cables beyond standards in order to decrease the charging current and the electric gradient over the insulation layer to comply with the prequalified electric gradient for the cable. The present invention also relates to a qualification method for rating a power cable based on maximum voltage level for the cable and calculation of electric gradient for the cable in order to keep the electric gradient below the prequalified electric gradient for the cable.

Description

FIELD OF THE INVENTION

The present invention relates to power cables and more particular to insulation layers of medium and high voltage power cables. The present invention also relates to a qualification method for rating a power cable.

BACKGROUND

The growth in the market for offshore and onshore wind farms and offshore oil or gas rigs, wind turbine generators and power supply for direct electric heating (DEH) have increased the demand for more underwater and underground medium - and high-voltage power transmission cables with voltage levels above 36 kV.
Medium- and high-voltage power cables may come into contact with the surrounding dampness during their lifetime. The presence of moisture combined with the presence of an electric gradient and a polymer material promotes the progressive degradation of the cable's insulating properties. This degradation mechanism, well known as "water treeing", can thus lead to the breakdown of the cable concerned and therefore constitutes a considerable threat to the reliability of the energy transmission network with well-known economic consequences caused by power failures.
The dimension and type of insulation system required for a power cable depends on the transmission voltage as higher voltage increase the electric gradient and charging current and thus requiring a more robust insulation to prevent breakdown of the insulation layer of the power cable.
Today, in order to handle draining of charging current and dielectric stress and avoid breakdown of the insulation material, manufacturers of three-phase cables such as medium and high voltage cables increase the cross section area of the conductor and use a metallic water barrier, mainly lead or copper sheaths for discharging of the dielectric stress and to prevent the possibility of getting water into the insulation system and consequently preventing the formation of water trees. Cables comprising a metallic water barrier are often referred to cables having a "dry design".
In an alternative single-phase system, the cross-section area of the conductor is often increased in order to decrease dielectric stress and draining of charging current of the cable.
The use of a metallic water barrier increases the weight and the cost of the cables and leaded cables may soon be forbidden for environmental reasons. Increasing the conductor cross section area also increases the costs of the cables.
Thus, there is a need for alternative and cheaper insulation systems for power cables in particular for long distant underwater or underground power cables of medium and high voltage traditionally having a metallic water barrier and/or over-dimensional conductor cross section area.
Another problem of power cables today, is the rating system that increasing the costs of the cables. Nowadays a power cable is rated for a given voltage range from maximum (Um) to minimum with an associated insulation thickness according to standards such as IEC 60502-2 (power cable with Um 7.2 kV up to 36 kV), IEC 60840 (power cables with Um 36 kV up to Um 170 kV) and IEC 63026 (submarine power cables). If a cable design is just above the maximum voltage for a given voltage range such as 52 kV, 72.5 kV, 123kV, 145kV, 170kV, 245kV, 300kV, it automatically falls into the next voltage range, resulting in an increase in insulation thickness and cost.
Thus, there is a need for qualification system based on voltage level, prequalified dielectric stress and thickness of insulation layer for the cable, and not necessarily based on the actual electrical stress the cable will be exposed to.
The present invention has as its objects to overcome one or more of the disadvantages of the prior art by increasing the dimension of the insulation layer around the conductors beyond the defined standard and industry practice.
In particular, the inventors have surprisingly been able to design medium and high voltage power cables, i.e. cables with maximum voltage level above 36 kV, with a wet design i.e. cables traditionally comprising a metallic water barrier by demonstrating that the cables according to the invention wherein thickness of insulation layer above nominal standards limit charging currents, surprisingly without oversizing the total cable design.
The insulation system of the medium and high voltage power cables according to the invention provides cables with reduced dielectric stress without having to increase the conductor cross section area normally applied for decreasing dielectric stress in power cables.

SUMMARY OF THE INVENTION

The present inventors have solved the above-mentioned need by providing in a first aspect a power cable comprising an electric conductor, an inner semi-conducting screen surrounding said conductor, an insulating layer surrounding said inner semi-conducting screen and an outer semi-conducting screen surrounding said insulating layer wherein the cable has a predefined maximum voltage level (Um) above 36 kV and wherein the insulation layer has an outer diameter (dy) and the inner diameter (di) defining the thickness of the insulation layer
wherein the cable has a wet design.
In one embodiment according to the first aspect the outer diameter (dy) is defined according to equation $E_{x} = \frac{2 \cdot U}{d_{x} \cdot \ln (\frac{d_{y}}{d_{i}})} [kV / mm]$

E_x = is the electric gradient over the insulation layer at any diameter x
x = is any diameter of the insulation layer from di to dy
U = U_m /√3 voltage over insulation
d_y = outer diameter of the insulation layer
d_i = inner diameter of the insulation layer
- the thickness of the insulation layer is dimensioned in order to provide an electrical gradient (E_x) over the insulation layer from 1.5 kV to 12 kV/mm and provide a decrease of Ex in order to keep Ex equal to or below a prequalified Ex for said cable.

In one embodiment of the first aspect the electric gradient (Ex) is the electric gradient (Ei) measured at di.
In one embodiment of the first aspect predefined maximum voltage level (Um) is from 36 kV to 700 kV.
In one embodiment of the first aspect predefined maximum voltage level (Um) is from 36 kV to 650 kV.
In one embodiment of the first aspect predefined maximum voltage level (Um) is from 36 kV to 300 kV.
In one embodiment of the first aspect predefined maximum voltage level (Um) is from 36 kV to 245 kV.
In one embodiment according to the first aspect the power cable is a medium voltage power cable.
In one embodiment according to the first aspect the power cable is a high voltage power cable.
In one embodiment of the first aspect the cable does not comprise a metallic water barrier.
In one embodiment of the first aspect the electric gradient (Ei) at the inner diameter (di) of the insulation layer range from 1.5 kV/mm to 3.5 kV/mm for a power cable having a predefined maximum voltage level, Um of 36 kV.
In one embodiment of the first aspect the electric gradient (Ei) at the inner diameter (di) of the insulation layer range from 1.5 kV/mm to 4.5 kV/mm for a power cable having a predefined maximum voltage level, Um of 52 kV.
In one embodiment of the first aspect the electric gradient (Ei) at the inner diameter (di) of the insulation layer range from 1.5 kV/mm to 5.0 kV/mm for a power cable having a predefined maximum voltage level, Um of 72 kV.
In one embodiment of the first aspect the electric gradient (Ei) at the inner diameter (di) of the insulation layer range from 1.5 kV/mm to 6.5 kV/mm for a power cable having a predefined maximum voltage level, Um of 123 kV.
In one embodiment of the first aspect the electric gradient (Ei) at the inner diameter (di) of the insulation layer range from 1.5 kV/mm to 10 kV/mm for a power cable having a predefined maximum voltage level, Um of 170 kV.
In one embodiment of the first aspect the electric gradient (Ei) at the inner diameter (di) of the insulation layer range from 1.5 kV/mm to 12 kV/mm for a power cable having a predefined maximum voltage level, Um of 245 kV.
In one embodiment of the first aspect insulating layer is a polymer layer based on a crosslinked polyolefin.
In one embodiment of the first aspect the crosslinked polyolefin is selected from the group consisting of crosslinked polyethylene (XLPE), a crosslinked ethylene-propylene, a crosslinked ethylene-propylene-diene elastomer (EPDM), polypropylene (PP) and any combination thereof.
In one embodiment of the first aspect the power cable is three-phase cable.
In one embodiment of the first aspect the power cable is single-phase cable.
In one embodiment of the first aspect the power cable is an Alternating Current (AC) power cable or a Direct Current (DC) power cable.
In one embodiment of the first aspect the power cable is a Direct Electric Heating (DEH) cable system.
In one embodiment of the first aspect the cable is for use under water, under ground or in the air.
In one embodiment of the first aspect the cable is for use under water.
In one embodiment of the first aspect the insulation layer is dimensioned with an outer diameter (dy) in order to provide a decrease in charging current.
In a second aspect the present invention provides a qualification method for rating a power cable comprising an electric conductor, an inner semi-conducting screen surrounding said conductor, an insulating layer surrounding said inner screen and an outer semi-conducting screen surrounding said insulating layer wherein the cable has a predefined maximum voltage level (Um), a wet design and the insulation layer has an outer diameter (dy) and the inner diameter (di) defining thickness of the insulation layer, wherein the method comprises:

a) assessing an electric gradient (Ex) for the power cable; and
b) comparing the assessed Ex for the power cable with a prequalified Ex for the cable where the cable is qualified if the measured Ex is equal to or below the prequalified Ex for said cable.

In one embodiment of the second aspect the thickness of the insulation layer is dimensioned to provide a decrease of Ex in order to keep E_x equal to or below a prequalified Ex for said cable.
In one embodiment of the second aspect Ex range from 1.5 kV to 12 kV/mm.
In one embodiment according to the second aspect the Ex is defined according to equation $E_{x} = \frac{2 \cdot U}{d_{x} \cdot \ln (\frac{d_{y}}{d_{i}})} [kV / mm]$

E_x = is the electric gradient over the insulation layer at any diameter x
x = is any diameter of the insulation layer from di to dy
U = U_m /√3 voltage over insulation
d_y = outer diameter of the insulation layer
d_i = inner diameter of the insulation layer

In one embodiment of the second aspect the electric gradient (Ex) is the electric gradient (Ei) measured at di.
In one embodiment of the second aspect the cable has a predefined maximum voltage level (Um) from 7.2 kV to 36 kV.
In one embodiment of the second aspect the cable has a predefined maximum voltage level (Um) above 36 kV.
In one embodiment of the second aspect the insulation layer is dimensioned to provide a decrease in charging current.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 depicts a cross section of a conductor and insulation layer according to the invention.

DETAILED DESCRIPTION

In the following description, various examples and embodiments of the invention are set forth in order to provide the skilled person with a more thorough understanding of the invention. The specific details described in the context of the various embodiments and with reference to the attached drawings are not intended to be construed as limitations.
Where a numeric limit or range is stated, the endpoints are included. Also, all values and subs range within a numerical limit or range are specifically included as if explicitly written out.

Definitions:

The term "wet design" is defined herein as a cable without a metallic water barrier and the outer semi-conducting layer of the electric power cable is in direct contact with water or its surroundings. The term also includes a cable with polymeric jacket(s) over the outer semi-conducting layer to limit rate of water vapour ingress.
The term "dry design" is defined herein as a cable with a continuous hermetic metallic water barrier such as lead extrusion or any other form of metal extrusion such as copper, or longitudinally welded metallic sheath. The term also includes a cable with a metallic foil radial water barrier with a longitudinal glued overlap and bonded to an outer polymeric jacket. The term may also include a hybrid version with lapped metallic tapes where the intersection which is filled with polymers for water migration to occur, cf. CIGRE TB 722.
The medium and high voltage power cables according to the present invention does not comprise a "dry design", i.e. is without a metallic water barrier.
With "insulated conductor" it is meant an electrical conductor 1 surrounded by an insulating system comprising, an inner semiconducting layer 2 surrounding the conductor, an insulating layer 3 surrounding the inner semiconducting layer, and an outer semiconducting layer 4 surrounding the insulating layer as depicted in figure 1.
The thickness or dimension of the insulation layer is defined according to the present invention as the difference between the outer diameter (dy) of the insulation layer and the inner diameter of the insulation layer (di).
The term "electric gradient" or "dielectric stress" is maximum at the inner diameter of the insulation layer (di) and minimum at outer diameter of the insulation layer (dy). The dielectric stress decreases from the inner diameter of the insulation layer (di) to the outer diameter of the insulation layer (dy). A non - uniform distribution of dielectric stress leads to insulation break down in the cable. To avoid this insulation break down, the dielectric stress is distributed throughout the dielectric material.
A dielectric material is an electrical insulator that can be polarized by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor but only slightly shift from their average equilibrium positions causing dielectric polarization.
The term "dielectric stress" is defined herein as the maximum voltage required to produce a dielectric breakdown through the insulation material and is expressed in terms of Volts per unit thickness, for example kV/mm.
The inner diameter of the insulation layer (di) 6 (figure 1) is predefined by the conductor of the cable.
The outer diameter of the insulation layer (dy) 5 (figure 1) is defined according to the following equation $E_{x} = \frac{2 \cdot U}{d_{x} \cdot \ln (\frac{d_{y}}{d_{i}})} [kV / mm]$
wherein

E_x = is the electric gradient over the insulation layer at any diameter x
x = is any diameter of the insulation layer from di to dy
U = U_m /√3 voltage over insulation
d_y = outer diameter of the insulation layer 5 (figure 1)
di = inner diameter of the insulation layer 6 (figure 1)

A skilled person knows how to perform prequalification tests of medium and high voltage direct current or alternating current power transmission cable for example according to CIGRE TB 496, CIGRE TB 722, IEC 60840 or IEC 62067.
As mentioning above, the present inventor has surprisingly found that by providing in a first aspect a power cable comprising an electric conductor, an inner semi-conducting screen surrounding said conductor, an insulating layer surrounding said inner semi-conducting screen and an outer semi-conducting screen surrounding said insulating layer wherein the cable has a predefined maximum voltage level (Um) above 36 kV and wherein the insulation layer has an outer diameter (dy) and an inner diameter (di) and thickness of the insulation layer is defined as the difference between the outer diameter (dy) and the inner diameter (di) wherein the cable has a wet design; and in an embodiment

the thickness of the insulation layer is dimensioned in order to provide an electrical gradient (E_x) from 1.5 kV to 12 kV/mm and provide a decrease of Ex in order to keep Ex equal to or below a prequalified Ex for said cable.

In one embodiment of the first aspect the outer diameter of the insulation layer (dy) is defined according to equation $E_{x} = \frac{2 \cdot U}{d_{x} \cdot \ln (\frac{d_{y}}{d_{i}})} [kV / mm]$

E_x = is the electric gradient over the insulation layer at any diameter x
x = is any diameter of the insulation layer from di to dy
U = U_m /√3 voltage over insulation
d_y = outer diameter of the insulation layer
di = inner diameter of the insulation layer

With reference to figure 1 a power cable according to the present invention comprises an electric conductor 1, an inner semi-conducting screen 2 surrounding said conductor, an insulating layer 3 surrounding said inner screen and an outer semi-conducting screen 4 surrounding said insulating layer.
The electrical insulation is chosen according to the conductor material of the electrical medium and high voltage power cable. If the conductor material of the power cable is copper, then the electrical insulation will be an insulation material which is suitable for a copper conductor. If the conductor material of the power cable is aluminium, or aluminium alloy then the electrical insulation will be an insulation material which is suitable for an aluminium conductor.
In one embodiment, the medium or high voltage power cable according to the invention can be an electric direct current (DC) power transmission cable or an electric alternating current (AC) power transmission cable.
The power cable according to the invention may be a Direct Electric Heating (DEH) cable system. DEH is a flow assurance technology developed to safeguard the well stream through the pipeline to the platform. The pipe is heated by running alternating or direct current through the steel in the pipe.
The power cable according to the present invention may also be one of many elements such as multiple power cables, fiber optical cables or tubes bundled together in a single slender structure. Subsea power cables may also include one or more dedicated load bearing armoring elements in the shape of steel wires or another material if needed.
Power cables comprise one or more cores or conductors, usually disposed within outer layers of insulating materials.
Power cables, particularly high voltage power cables, typically comprise one or more insulated conductors. One insulated conductor for single-phase power transmission, three insulated conductors for 3-phase power transmission.
Cables with a single insulated conductor are also referred to as "single-core" cables, while cables with more than one insulated conductor are also referred to as "multi-core" cables; for example, cables with three insulated conductors are referred to as "3-core" cables.
The power cables according to the invention may be cables submerged in sea or fresh waters, submarine water cables or land cables. A power cable according to the invention is preferably a power cable submerge in sea or fresh water, i.e. under water.
To make a power transmission cable the conductors are normally surrounded by an electric insulation system to cover the conductor. An electric insulation system 3 (figure 1) may comprise one or more insulation layers.
The electrically insulating layer may be a polymer layer based on a crosslinked polyolefin, such as a crosslinked polyethylene (XLPE) or a crosslinked ethylene/propylene or ethylene/propylene/diene elastomer (EPDM) polypropylene (PP) and any combination thereof.
Considerable dielectric losses are encountered in electrical transmission lines, due to their tremendous length. The dielectric losses are directly proportional to the electrical capacitance existing between the coaxial conductors. Therefore, such losses can be decreased by either decreasing the dielectric constant of the insulation between the coaxial conductors or by increasing the radial distance there between. Since the dielectric strength across the insulation must be sufficient to preclude electrical breakdown between the conductors, the dielectric losses may be minimized when the insulation has an optimized dielectric constant and radial thickness.
As described above, there is a need for qualification system based on voltage level U and prequalified dielectric stress for the cable, and not based on the actual electrical stress the cable will be exposed to.
Thus, in a second aspect the present invention provides a a qualification method for rating a power cable comprising an electric conductor (1), an inner semi-conducting screen (2) surrounding said conductor (1), an insulating layer (3) surrounding said inner screen and an outer semi-conducting screen (4) surrounding said insulating layer wherein the cable has a predefined maximum voltage level Um, a wet design and the insulation layer has an outer diameter (dy) and an inner diameter (di) defining thickness of the insulation layer, wherein the method comprises :

a) assessing an electric gradient (Ex) for the power cable;
b) comparing the assessed Ex for the power cable with a prequalified Ex for the cable where the cable is qualified if the measured Ex is equal to or below the prequalified Ex for said cable.

In one embodiment of the second aspect the thickness of insulation layer is dimensioned to provide a decrease of Ex in order to keep E_x equal to or below a prequalified Ex for said cable and provide a decrease in charging current.
A skilled person knows how to perform prequalification tests for direct current (DC) or alternating (AC) current power transmission cable for example according to CIGRE TB 496, CIGRE TB 722, IEC 60502-2, IEC 60840 or IEC 62067.
In one embodiment of the second aspect Ex range from 1.5 kV to 12 kV/mm.
In one embodiment according to the second aspect the Ex is defined according to equation $E_{x} = \frac{2 \cdot U}{d_{x} \cdot \ln (\frac{d_{y}}{d_{i}})} [kV / mm]$

E_x = is the electric gradient over the insulation layer at any diameter x
x = is any diameter of the insulation layer from di to dy
U = U_m /√3 voltage over insulation
d_y = outer diameter of the insulation layer (5)
di = inner diameter of the insulation layer (6)

In one embodiment of the second aspect the electric gradient (Ex) is the electric gradient (Ei) measured at di.
In one embodiment of the second aspect the cable has a predefined maximum voltage level (Um) from 7.2 kV to 36 kV.
In one embodiment of the second aspect the cable has a predefined maximum voltage level (Um) above 36 kV.
The above described method qualification of a power cable prevents the unnecessary and expensive method.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples. The examples illustrate the properties and effects of the of the dielectric fluid according to the invention, and are provided herein for purposes of illustration only, and are not intended to be limiting.

EXAMPLE

As depicted in table 1 below the present inventors have demonstrated that a medium or high voltage power transmission cable with a maximum voltage level above 36 kV can have a wet design and preventing the use of a metallic water barrier by providing a dimension of the insulation layer around the conductors beyond the defined standard and industry practice.
The dimension of the insulation layer and thus the thickness of the insulation layer is defined by an outer diameter of the insulation layer and an inner diameter of the insulation layer wherein the dimension of the insulation layer is providing a reduction charging current and of the electric gradient over the insulation layer in order to keep the electric gradient equal to or below the prequalified electric gradient for the cable.
In order to calculate the thickness of the insulation layer (ti) as shown in table 1 below the following calculations was performed:
Calculating the outer diameter of the insulation layer (dy) according to equation $E_{x} = \frac{2 \cdot U}{d_{x} \cdot \ln (\frac{d_{y}}{d_{i}})} [kV / mm]$

E_x = is the electric gradient over the insulation layer at any diameter x
x = is any diameter of the insulation layer from di to dy
U = U_m /√3, voltage over insulation
d_y = outer diameter of the insulation layer (5)
di = inner diameter of the insulation layer (6)

Ex is defined as the electric gradient equal to or below the prequalified electric gradient for the cable.
Um is the maximum voltage for a cable and can be defined in a 3-phase system as Um = √3 ^∗ U.
Calculating the thickness of the insulation layer (ti) by calculating the difference between the outer diameter of the insulation layer and the inner diameter.
With reference to table 1 below it is clear that by increasing the dimension of the insulation layer from 17 mm that is use (based on internal design rules and according to IEC 8040) to 22 mm and at the same time remove the lead water barrier on the three conductors, the electrical stress decreases below the dielectric stress prequalified for said cable. Thus, showing that by increasing the dimension of the insulation layer from 17 mm to 22 mm prevents the use of a metallic water barrier even for a cable that would normally have been designed with a metallic water barrier according to IEC 8040.

With reference to table 2 below it is clear that by increasing the dimension of the insulation layer for the cable with Um=72.5 from 11 mm that is use (based on internal design rules and according to IEC 8040) to 19 mm and at the same time remove the lead water barrier on the three conductors, the electrical stress decreases below the dielectric stress prequalified for said cable according to the electrical stress as for a cable according to IEC 60502-2 (where a maximum electrical stress is 3.3 kV/mm and Um=36 kV).

Table 1

		3x1000mm² 145 kV
		Conventional design		New design
Inner diameter of the insulation layer:	di	41.0	mm	41.0	mm
Thickness of insulation:	t_in	17.0	mm	22.5	mm
Outer diameter of insulation layer:	dy	75.0	mm	85.0	mm
Total diameter of cable:	dtot	212	mm	226.7	mm
Weight of cable		87.7	kg/m	73.8	kg/m

				Inner gradient Ei	Outer gradient Ey	Inner gradient Ei	Outer gradient Ey
Rated voltage:	U₀	76	kV	6.14	3.36	5.08	2.45
Maximum voltage:	U_m	145	kV	6.76	3.70	5.51	2.63
Operating voltage:	U	130	kV	6.06	3.31	4.49	2.36
Lightning voltage (CAR):	U_p	650	kV	52.50	28.70	43.49	20.98

Table 2

		3x6300mm² 72 kV
		Conventional design		New design
Inner diameter of the insulation layer:	di	32.5	mm	32.5	mm
Thickness of insulation:	t_in	11.0	mm	19	mm
Outer diameter of insulation layer:	dy	54.5	mm	70.5	mm
Total diameter of cable:	d_tot	164.7	mm	189	mm
Weight of cable		58.8	mm	52.4	kg/m

				Inner gradient Ei	Outer gradient Ey	Inner gradient Ei	Outer gradient Ey
Rated voltage:	U₀	36	kV	4.29	2.56	2.86	1.32
Maximum voltage:	U_m	72.5	kV	4.95	2.95	3.3	1.52
Operating voltage:	U	60	kV	4.12	2.46	2.75	1.27
Lightning voltage (CAR):	U_p	650	kV	38.69	23.07	25.83	11.91

Claims

A power cable comprising an electric conductor (1), an inner semi-conducting screen (2) surrounding said conductor (1), an insulating layer (3) surrounding said inner screen and an outer semi-conducting screen (4) surrounding said insulating layer wherein the cable has a predefined maximum voltage level (Um) above 36 kV and wherein the insulation layer has an outer diameter (dy) and an inner diameter (di) and thickness of the insulation layer is defined as the difference between the outer diameter (dy) and the inner diameter (di)
characterized in that the cable has a wet design.
The power cable according to claim 1, wherein dy is defined according to equation $E_{x} = \frac{2 \cdot U}{d_{x} \cdot \ln (\frac{d_{y}}{d_{i}})} [kV / mm]$

E_x = is the electric gradient over the insulation layer at any diameter x

x = is any diameter of the insulation layer from di to dy

U = U_m /√3 voltage over insulation

d_y = outer diameter of the insulation layer (5)

di = inner diameter of the insulation layer (6)
and the thickness of the insulation layer is dimensioned in order to provide an electrical gradient (Ex) over the insulation layer from 1.5 kV to 12 kV/mm and provide a decrease of Ex in order to keep Ex equal to or below a prequalified Ex for said cable.
The power cable according to claim 1 or claim 2, wherein the electric gradient (Ex) is the inner electric gradient (Ei) measured at di.
The power cable according to any one of claims 1 to 3, wherein the cable does not comprise a metallic water barrier.
The power cable according to any one of claims 1 to 4, wherein insulating layer is a polymer layer based on a crosslinked polyolefin.
The power cable according to claim 5, wherein the crosslinked polyolefin is selected from the group consisting of crosslinked polyethylene (XLPE), a crosslinked ethylene-propylene, a crosslinked ethylene-propylene-diene elastomer (EPDM), polypropylene (PP) and any combination thereof.
The power cable according to any one of claims 1 to 6, wherein the power cable is three-phase cable.
The power cable according to any one of claims 1 to 6, wherein the power cable is single-phase cable.
The power cable according to claim 7 or claim 8, wherein the power cable is an Alternating Current (AC) power cable or a Direct Current (DC) power cable.
The power cable according to claim 8, wherein the power cable is a Direct Electric Heating (DEH) cable system.
The power cable according to any one of claims 1 to 10, wherein the cable is for use under water, under ground or in the air.
A qualification method for rating a power cable comprising an electric conductor (1), an inner semi-conducting screen (2) surrounding said conductor, an insulating layer (3) surrounding said inner semi-conducting screen and an outer semi-conducting screen (4) surrounding said insulating layer wherein the cable has a predefined maximum voltage level (Um), a wet design and the insulation layer has an outer diameter (dy) and an inner diameter (di) defining the thickness of the insulation layer, wherein the method comprises:
a) assessing an electric gradient (Ex) for the power cable; and

b) comparing the assessed Ex for the power cable with a prequalified Ex for the cable where the cable is qualified if the measured Ex is equal to or below the prequalified Ex for said cable.
The qualification method for rating an electrical power cable according to claim 12,
wherein Ex range from 1.5 kV to 12 kV/mm.
The qualification method for rating an electrical power cable according to claim 12 or claim 13, wherein the cable has a predefined maximum voltage level (Um) from 7.2 kV to 36 kV.
The qualification method for rating an electrical power cable according to claim 12 or claim 13, wherein the cable has a predefined maximum voltage level (Um) above 36 kV.