WO2013071945A1

WO2013071945A1 - A solid direct current (dc) transmission system comprising a laminated insulation layer and method of manufacturing

Info

Publication number: WO2013071945A1
Application number: PCT/EP2011/070045
Authority: WO
Inventors: Rongsheng Liu
Original assignee: Abb Research Ltd
Priority date: 2011-11-14
Filing date: 2011-11-14
Publication date: 2013-05-23

Abstract

The invention relates to a direct current transmission system (1) comprising a conductor layer (2), an inner semiconductive layer (3) covering the conductor layer. An insulation layer is provided on the outer circumference of the semiconductive layer comprising laminated insulation material. The insulation material comprises an inner part (4) from a first radial distance (r0) at a semi-conductive layer/insulation layer interface, and a main part (5) covering the inner part from initial distance (η) to maximum distance (rd), and having midd le rad ial d istance (rm). An outer semiconductive layer (7) covers the insulation layer. The laminated insulation material comprises a thickness gradient. The thickness gradually increases in the radial direction from the first radial distance. The insulation layer may further comprise an outer part (6) circumferentially covering the main part and comprising laminated insulation material having substantially the same thickness as the laminated insulation material in the inner part.

Description

Title:

A SOLID DIRECT CURRENT (DC) TRANSMISSION SYSTEM COMPRISING A LAMINATED INSULATION LAYER AND METHOD OF MANUFACTURING THE FI ELD OF THE I NVENTION

The present invention refers to a solid direct current (DC) transmission system accord ing to the pre-characterized portion of claim 1 and a method for preparing said system according the pre-characterized portion of claim 13.

BACKGROU ND OF THE I NVENTION AND PRIOR ART Insulation for direct current (DC) transmission systems is important for the reliability of a transmission system. The reliability depends on the material used for covering the conductive layers. The geometry of the insulation material around the transmission system is also important.

The amount of power that can be delivered by a DC cable has increased dramatically in the past decades. Further increasing the amount of power that can be delivered by a DC cable can be achieved in several ways as described by Nordberg et al. , Cigre, Session 2000, 21 -302. Examples mentioned are increasing the size of the conductor or alternatively increasing the voltage. The latter has the benefit of lower power losses but necessitates an increase in the thickness of the insu lation in general . This will increase the cables' size and weight. An alternative solution is to increase the maximum allowed conductor temperature or to increase the dielectric strength of the insulation material .

New insulation liquids have been developed , such as gelling liquids described in US 6,383,634, to allow an increase in conduc- tor temperature. Laminated insulation materials have been developed to increase the d ielectric strength of the insu lation material. As explained by Hampton R. , I EEE Electrical I nsulation Magazine, Vol 24, No 1 , 2008, page 5, important parameters for the provision of a relia- ble DC insulation material are electrical resistivity at a range of stresses and temperatures, DC breakdown performance, sensitivity to electrical aging and space charge development. Resistivity is dependent on DC stresses and temperatures as well as on the thickness of the insulation material , whereby the resistivi- ty decreases with increased stress and temperature. Electrical charges that become trapped with in the insulation material (space charge) will also have an effect on the electrical stress performance of the material. The breakdown strength may decrease with time of applied DC stress due to such space charg- es. The geometry of a transmission system such as a cable, cable joints, buses and the like, and the distribution of the temperature are further critical factors for the reliability of the DC transmission system. Hampton also explains the advantage of a homogenous insulation layer and mentions that a laminated in- sulation system may be a source for inhomogeneity, which in turn may affect the quality of the insu lation material. Leakage of current should preferably be prevented . If leakage becomes too high, dielectric heating may occur. This condition may result in melting .

WO201 1 /073709 describes a high voltage direct current (HVDC) cable comprising an insulation layer of laminated polypropylene (PP)/Kraft paper. The insulation layer has a constant thickness over the entire insulation layer. The invention relates to de- lamination of the insulation layer during impregnation with an impregnation flu id having a medium viscosity of at least 1000 cSt at 60°C and an air impermeability of at least 100000 Gurley sec^"1. This problem is solved by using special paper in the insulation laminate. US 7,943,852 describes a su perconducting cable that can be used in both DC and alternating current (AC) cables. The cables are housed in a heat-insulated pipe filled with a coolant. The resistivity of the laminated polymer (PP)/paper insulation material can be varied by varying the density, or by adding dicyandi- amide to the paper, or by varying the thickness ratio of polymer to paper in the laminate. The insulation layer has a low resistivity on the inner part close to the conductor layer and a higher resistivity at increasing rad ial d istance from the conductor layer. In the examples, Kraft paper is positioned around the conductor, while laminated polymer/paper is used as insulation material in the rest of the insulation layer. This laminated insulation layer comprises material having an increasing resistivity at increased radial d istance so that the cable also has excellent AC electrical properties.

US 6,399,878 describes insulation material for DC cables that may comprise three different parts, whereby the inner and outer part closest to the semiconductive layers contain paper that has a low resistivity. The main insulation part comprises laminated polymer/paper material having higher resistivity. This layer may be divided in different parts, whereby the different parts have different polymer/paper ratios and whereby the ratios decrease at increasing radial distance from the inner conductor layer. (Fig 8a, 8b, 13 and 14) The resistivity in the main layer thus decreases at increasing radial distance. The insulation material is impregnated with a medium viscosity oil having a viscosity from 10 centistokes and less than 500 centistokes (est) at 60°C. US 6,207,261 describes a laminated polymer/paper insulation material for DC cables, which is impregnated with a med ium viscosity fluid . The thickness of the laminate may be varied by varying the thickness of the paper or the polymer. Nothing is mentioned about variation of thickness of the laminated material within one cable. After lamination , the laminate is being calen- dered or supercalendered . The paper in the laminate has one smooth and one rough surface.

EP 875907 describes insulation material comprising paper at the inner and outer part of the insulation layer, wh ich paper material has low resistivity. The main part comprises laminated polymer/paper material having higher resistivity. The thickness of the paper may be varied to change the resistivity. The aim of the invention is to provide insulation material having a resistivity be- tween 0.1 p₀ and 0.7p₀, where p₀ is the resistivity of the normal Kraft paper, over the whole temperature range. This may be achieved by varying the quality of the materials, or using additives such as amine or cyanoethylpaper. Hata R. SEI Technical review, 62, June 2006, page 3, describes solid DC submarine cable insulated with polypropylene (PP) laminated paper, whereby the inner part of the insu lation layer in the vicinity of the conductor layer comprises paper, which is covered by a layer of laminated PP forming the main part of the insulation material , which is subsequently covered with paper, which forms the outer part of the insulation layer.

US 3,987,239 describes insu lation material , whereby the electrical stress distribution in a high voltage system is improved by provid ing insulation material comprising different parts located at different rad ial d istances from the conductor layer. The d ifferent parts may comprise the same or different insulation material . The effect of the arrangement of layers is that the resistivity gradient in the insulation material from the inner part to the out- er part of the insulation layer is as flat as possible. Fig 9 in US 3,987,239 shows that the resistivity is flat at the inner part of the insulation layer and then decreases at increasing radial d istance from the conductor layer. The plastic material used has an E- stress below 22 kV/m. Modern insulation materials have an E- stress above this value. US 4,075,421 describes insulation paper, whereby the resistivity in the most inner part is higher compared to the resistivity in the outer part of the insulation layer. A limiting factor in the development of DC transmission systems, especially cable joints and cable terminations, is the insulation breakdown strength . Experiments have shown that the breakdown location in a cable is often started from the semiconduc- tive layer/insu lation layer interface.

There is a need for insu lation material , whereby the resistivity is lowered at locations close to the inner and outer semiconducting layers. There is a need for an improved resistivity control in the insulation material , especially at these locations. By improving the electrical field stress distribution , the breakdown stress of the insu lation material can be improved .

Although many improvements have been made to laminated insulation materials for DC transmission systems, there is still a need for improving the electrical performance, increase the transmission capacity, improve the reliability, decrease aging and manufacturing costs for insulated transmission systems. With regard to high and ultra high voltage (UHV) DC and (U)HVDC for mass-impregnated non-draining (M I N D) transmis- sion systems there is a need for improved resistivity control over the entire insulation layer, especially with regards to insulation materials impregnated with h igh viscosity fluids.

SUMMARY OF THE I NVENTION

The object of the invention is to provide a DC transmission system with improved resistivity control in the insulation material . It is also an object to provide a DC transmission system with im- proved electrical field stress distribution . Another object is to provide a DC transmission system with excellent electrical per- formance and increased transmission capacity. The DC transmission system preferably has a decreased resistivity at the semiconductive layer/insulation layer interface. It is also an object to provide a DC transmission system, which is reliable. An- other object is to provide a DC transmission system, which is less sensitive to aging . It is a further object to provide a DC transmission system, which can be adapted and used for different transmission systems under different working conditions. It is also an object to provide a DC transmission system, which can be manufactured at low cost. The above mentioned objects are preferably achieved in a high or u ltra high voltage direct current ((U)HVDC) system for mass-impregnated non-draining transmission system (M I N D). Said systems should preferably be impregnatable with a flu id that has a high viscosity at working temperatures below 65 or 80°C and a low viscosity at processing temperatures of 100°C or more.

The objects are achieved by the DC transmission system initially defined according to the pre-characterized portion of claim 1 , which is characterized in that the laminated insulation material of the insulation layer comprises a thickness gradient, whereby the thickness of the laminated insulation material gradually increases in the radial direction from the first radial distance at the semiconductive layer/insulation layer interface, and wherein at least the thickness ratio of polymer to laminated material in a part of the inner part is less than 35% .

The breakdown strength depends, among other things, on the thickness of the material . Thinner material has normally higher breakdown strength . By arrang ing the thinner layer of the insulation material close to the inner and outer semiconductive layers, the breakdown strength will be high at these locations, where it is most likely to break down . The overall dielectric properties of the insulation system are therefore improved . The risk of break- down of the transmission system decreases. The new transmission system is thus more reliable and will last longer than the transmission systems used today. The new arrangement of insu- lation material is especially useful for (U)HVDC-M I ND transmission systems impregnated with a high viscosity flu id .

In one embodiment, the thickness of the laminated insulation material gradually increases from the first radial distance to the maximum radial distance of the main part.

In another embodiment, the thickness of the laminated insulation material gradually increases from the first radial distance to the midd le radial distance of the main part, whereafter the thickness gradually decreases to the insulation layer/semiconductive layer interface.

In a further embodiment, the insulation layer further comprises an outer part circumferentially covering the main part and comprising laminated insulation material having su bstantially the same thickness as the laminated insulation material of the inner part. In one embodiment, the thickness of the laminated insulation material gradually increases from the initial radial distance of the main part and whereby the thickness of the insulation material of the inner part and optionally the outer part is substantially the same as or less than the minimum thickness of the insula- tion material of the main part.

The inventors have found that the electric field stress in a DC transmission system can be reduced at the semiconductive layer/insulation layer interface(s) by introducing thickness con- trolled laminated plastic or rubber films as the insulation material . The thickness gradient arrangement of the laminated insulation material according to the invention provides for insulation material in the inner and optionally the outer part with lower values of volume resistivity and higher breakdown strength com- pared to the laminated insulation material in the main part. I nstead of creating a flat resistivity gradient over the insulation material or a part thereof, the new arrangement of insulation material has reduced resistivity-governed E-stresses close to at least one of the semiconductive layer/insulation layer interface, while the resistivity-governed E-stresses are higher in the main part of the insu lation material . Preferably, the resistivity-governed E-stresses gradually increase from the inner part to the main part of the insulation material . Alternatively, the resistivity-governed E-stresses gradually increase from the inner part towards the main part and decrease again at the outer part of the insulation material . This decrease may start in the middle of the main part.

One effect of th is new arrangement is a decrease in breakdown, especially at the semiconductive layer/insulation layer interface. This increases the reliability of the DC transmission system. It is expected that the new insulation material is less sensitive to space charges or aging .

Further, the use of laminated insulation materials in the inner part and optionally the outer part improves the control of the re- sistivity over the insulation layer. It also improves the flexibility to adapt and use the insulation material for different transmission systems under d ifferent working conditions.

In a further embodiment, the DC transmission system is selected from a cable, a cable joint, bushings, insulated buses, bus bars and cable terminations. The new insulation material is less susceptible to breakdown and thus especially suited to be used in cable joints and cable terminations. In one embodiment, the laminated insulation material comprises a plastic material laminated with a paper. In another embodiment, the plastic material is selected from polyolefins selected from polyethylene, low density polyethylene, which is linear or not, medium density polyethylene, high density polyethylene, cross-linked polyethylene, and polypropylene, polyvinyl chloride, polyester, aramid and polyimide, or mixtures thereof. In an alternative embod iment, the laminated insulation material comprises a rubber material laminated with a paper. I n one embodiment, the rubber material is selected from silicone rubber, ethylene propylene diene monomer rubber and ethylene propyl- ene rubber, or mixtures thereof.

The breakdown strength depends also on the material used in the insulation layer. Different transmission systems may have different requirements for the material . For example, the break- down strength for polyethylene or polypropylene is higher than 200kV/mm at a thickness of 100 μιη, while the breakdown strength for cross-linked polyethylene can be below 65 kV/mm at a th ickness of 9 mm. Preferably, the insulation material comprises paper that has been calendered before lamination with the plastic or rubber material. The laminated paper may be smooth on both surfaces.

In another embodiment, the transmission system is impregnated with a gas or a liquid . Preferably, the flu id is not a medium viscosity flu id .

In another embodiment, the total thickness of the insulation layer is between 0,5 and 50 mm.

In one embodiment, the density of the laminated insulation material in the inner part and optionally the outer part is higher compared to the density of the laminated insulation material in the main part. Breakdown strength is improved by increasing the density of the laminated insulation material. This will thus improve the reliability of the transmission system and prevent space charging and aging .

The object is also achieved by a method for preparing the transmission system initially defined according to the pre- characterized portion of claim 13, which is characterized by comprising a first step of providing the conductive layer, circum- ferentially covered by the semiconductive layer, a second step of laminating a plastic or rubber material of the inner part, the main part and optionally the outer part,

a third step of winding the obtained laminated material on the inner semiconductive layer, whereby firstly the inner part, sec- ondly the main part and optionally thirdly the outer part is wound on the inner semiconductive layer, whereby the laminated insulation material of the insu lation layer comprises a thickness gradient, whereby the th ickness of the laminated insulation material gradually increases in the radial direction from the first rad ial distance at the semiconductive layer/insulation layer interface, and wherein at least the thickness ratio of polymer to laminated material in a part of the inner part is less than 35%, and optionally

a fourth step of removing gases from the obtained product, fol- lowed by an optional fifth step of cross-linking the insulation material, and

a final step of circumferentially covering the insulation layer with the outer semiconductive layer. In one embodiment, the method comprising a further step of impregnating the insu lation material with a gas or a liqu id , wh ich is solid below 80°C.

In another embodiment, the liquid is selected from a mineral oil and/or an ester fluid , and the gas is selected from sulfur hex- afluoride, compressed air and/or nitrogen .

The new transmission system is easy to prepare. The manufacturing costs are low.

BRI EF DESCRI PTI ON OF TH E DRAWI NGS

The invention will now be explained more closely by means of a description of various embod iments and with reference to the drawings attached hereto. Fig 1 shows a schematic view of a DC transmission system as a power cable.

Fig 2 shows a schematic view of a cable joint insulated with the new insulation material .

DETAI LED DESCRI PTI ON OF VARIOUS EMBODI MENTS THE I NVENTION Fig 1 shows a direct current (DC) transmission system as a power cable. Other transmission systems may be a cable joint as shown in Fig 2. The transmission systems 1 or systemcom- ponents 1 may be bushings, insulated buses, bus bars and cable terminations. One embodiment relates to cable terminations. Further transmission systems may be any electrical DC device that has insulation . The invention also relates to solid DC transmission systems. Another embodiment relates to high and ultra high voltage DC ((U)HVDC) transmission systems, preferably (U)HVDC systems or systemcomponents for mass- impregnated non-draining (M I ND) transmission systems or systemcomponents 1 .

As shown in Fig 1 , a conductive layer 2 is circumferentially covered by an inner semiconductive layer 3. An insulation layer is provided on the outer circumference of the semiconductive layer 3. The insulation layer comprises parts of insulation material and can be d ivided by an inner part 4, a main part 5 and optionally an outer part 6. The inner part 4 is located in the vicinity of the semiconductive layer 3 from a first radial distance r₀ at an semiconductive layer 3/insulation layer interface. The inner part 4 is circumferentially covered by a main part 5 from an initial rad ial distance η to a maximum radial distance r_d of the main part 5. The main part 5 has a middle radial distance r_m su bstantially in the middle of the main part 5. The main part 5 may be circum- ferentially covered by an outer part 6. An outer semiconductive layer 7 is provided on the outer circumference of the insulation layer and provides an insulation layer/semiconductive layer 7 interface. The outer semiconductive layer 7 may be covered by a sheath 8 of lead or metal. This sheath 8 may be further covered by a protection layer that may also have insulation and mechanical properties such as a plastic or rubber material (not shown).

The insulation material 4, 5, 6 is laminated material and may comprise a flat film or sheet of polymer material laminated with paper. The polymer material may be plastic material or rubber material. The terms "laminated material" , "laminated sheet" and "laminated polymer material" refer to a sheet comprising polymer and paper.

The paper used may differ and any paper used in the art may be suitable. For example, cellulose paper may be used . In one embodiment, Kraft paper is used . This Kraft paper may have different resistivities in or within the different parts 4, 5, 6 of the insulation layer. The paper may be calendered before being laminated . Normally, the paper had two smooth surfaces, but the inven- tion is not limited to this. The paper may have one smooth and one rough surface.

The plastic and ru bber material may be any material used in the art, which has insulation properties. The material used may be different depending on the application of the transmission system, e.g . low voltage, medium voltage or high voltage systems. Examples of plastic materials, but not limited thereto, may be one of polyolefins such as polyethylene, which may be low density polyethylene (linear or not), medium density polyethylene, high density polyethylene, cross-linked polyethylene, or polypropylene and polybutylene. I n one embodiment polyethylene is used . I n another embodiment high density polyethylene is used . In yet another embodiment polypropylene is used . Other plastic materials may be polyvinyl chloride, polyesters, aramid or poly- imide. Alternatively, mixtures of plastic materials may be used .

Examples of rubber materials, but not limited thereto, may be one of silicone rubber, ethylene propylene diene monomer rub- ber and ethylene propylene ru bber. Alternatively, mixtures of rubber materials may be used .

The insulation material of the present invention may comprise one or more than one insulation material. The material used may be an one plastic material or one rubber material . The material used may also be a mixture of plastic materials or a mixture of rubber materials or a mixture of plastic and rubber materials. Alternatively, different materials may be used in different parts 4, 5, 6 of the insulation material . Furthermore, both mixtures of materials and different materials in different parts 4, 5, 6 may be used . Although the resistivity-governed E-stresses differ in the inner part 4 and optionally outer part 6 compared to the main part 5, the electrical resistivity of the film of laminated insulation material may be the same or different in the different parts 4, 5, 6 of the insulation material .

The materials or mixture of materials in the three parts 4, 5, 6 may have different densities such that the resistivity-governed E-stresses in the inner 4 and outer part 6 of the insulation material are lower compared with the resistivity-governed E-stresses in the main part 5 of the insulation material . The density of the laminated insulation material in the inner part 4 and the outer part 6 may be higher compared to the density of the laminated insulation material in the main part 5. The different densities may be provided by using paper and/or plastic or rubber material having different densities.

The resistivity p in the main part 5 of the insulation material may be more than 10¹⁴ Q.m , or more than 10^{1 0} Q.m and the resistivity p in the inner part 4 and outer part 6 of the insulation material is less than 10¹⁴ Q.m or less than 10^{1 0} Q.m.

Preferably, the transmission system 1 is able to deliver voltages in an amount of over 500 kV, preferably at and/or over 800 kV. The E-stress of the insulation material is preferably above 22 kV/m. The insulation layer is arranged at the semiconductive layer 3/insulation layer interface at radial distance r₀ such that the material is relatively thin in the vicin ity of the semiconductive layer 3. Because different transmission systems may be used for different applications, the different systems may have different requ irements regarding insulation materials. Therefore, the given thickness and g iven gradual increase in thickness may vary depending on the transmission system (e.g . cable or cable joint), the application for the system , the material used , etc. As an example, but not limited thereto, the material at radial distance r₀ may be a sheet having a thickness between 1 and 500 μιη, or 5 and 100 μιη, or 100 and 500 μιη or 1 and 50 μιη or 1 and 10 μιη. Starting from the semiconductive layer 3 at a first radial distance r₀, the insulation sheet is wound around the conductive layer 2. The thickness of the sheet material gradually increases, for example by 0.005 to 1 00 μιη , or 0.1 to 50 μιη , or 1 to 25 μιη every time the sheet is wound around the conductive layer 2 (for every loop at increasing rad ial d istance). At the max- imum radial distance r_d of the main part 5 or at the outer part 6, i.e. at the insulation layer/semiconductive layer 7 interface, the sheet of insulation material may have a thickness between 25 and 10000 μιη. The amount by which the thickness increases may vary between the different parts 4, 5, 6.

The th ickness of the inner part 4 may be between 1 to 20%, or 5 to 15%, preferably about 10% of the total thickness of the insulation layer. The th ickness ratio of polymer to laminated material (polymer and paper) in the laminated sheet in the inner part 4 may be between 1 to 50%, preferably below 35% , or below 30%. For the sake of clarity, "a ratio of 30%" means that 30% of the laminated material contains the polymer.

The thickness of the main part 5 may be between 10 to 95%, or 15 to 85%, preferably about 80% of the total th ickness of the insu lation layer. The thickness ratio of polymer to laminated material in the laminated sheet in the main part 5 may be between 2 to 99%, or 50 to 90%, preferably more than 35%, or 40%, or 50%.

The amount by which the thickness increases may even vary within one or more parts 4, 5, 6 of the insulation material . In one embodiment the thickness gradually increases from the first rad ial distance r₀, or alternatively the initial radial distance η , toward the middle radial distance r_m of the main part 5. Than , the th ickness gradually decreases from the middle rad ial d istance r_m to the insulation layer/semiconductive layer 7 interface, or to the maximum rad ial distance r_d of the main part.

The thickness may be gradually increased by increasing the thickness of the plastic or ru bber material or by increasing the thickness of the laminated material such as the paper. Alternatively, the thickness of both plastic or rubber material as well as the laminated material may gradually increase to increase the thickness of the laminated sheet. The increase in thickness of the two laminated materials in the sheet may even be alternat- ed . Important is that the thickness of the sheet of the insulation material gradually increases by increasing radial distance from r₀. In one embodiment, the thickness of the sheet is gradually increased by increasing the thickness of the plastic or rubber material . In another embodiment, the thickness of the sheet is gradually increased by increasing the thickness of polyethylene or polypropylene, which is laminated with Kraft paper.

The outer part 6 comprises preferably thin laminated sheet of insulation material. The th ickness of the sheet in the outer part 6 may be substantially the same as the thickness in the inner part 4. The loops around the conductive layer are larger in the outer part 6 compared to the inner part 4 and the gradual increase in thickness of the th in starting material of the outer part 6 may be the same or different as in the inner part 4 to obtain resistivity- governed E-stress in the outer part 6, which is substantially the same as that in the inner part 4. I n one embodiment the thickness of the laminated insulation material in the inner part 4 and optionally the outer part 6 is the same as or less than the mini- mum thickness of the laminated insulation material in the main part 5. The thickness of the outer part 6 may be between 1 to 20%, or 5 to 15%, preferably about 1 0% of the total th ickness of the insulation layer. The thickness ratio of polymer to laminated material in the laminated sheet in the outer part 6 may be between 1 to 50%, preferably below 35%, or below 30%.

Alternatively, the inner part 4 and outer part 6 comprise laminated insulation material , which does not gradually increase or de- crease in thickness at increasing radial distance from the conductive layer 2. I n this embodiment, only the laminated insulation material in the main part 5 increases gradually at increasing radial distance from the conductive layer 2. The gradual increase in thickness in the main part 5 may start at the initial ra- dial d istance η toward the maximum radial distance r_d, or gradually increase from the initial radial distance η toward the midd le radial distance r_m, whereafter the thickness of the laminated sheet gradually decreases to the maximum radial distance r_d. The insulation material may be impregnated with a liquid or a gas. Liquids may be any liquids used in the art such as mineral oils and/or ester fluids. Gases may be selected from sulfur hex- afluoride, compressed air and/or nitrogen . Preferably, the insulation material is impregnated with a high viscosity fluid , which is solid at working temperatures below 65°C, preferably below 80°C. The viscosity of the fluid is at least more than 501 , or 1000, or 5000, 10,000 centistokes (cts) at 65°C, or at 80°C. For processability, the flu id may have a low viscosity above 100°C, or above 1 10°C .

A suitable insulating flu id is T201 5 (H&R ChemPharm Ltd . (UK) , which is based on mineral oil with about 2% by weight of a high molecular weight polyisobutene as viscosity increasing agent. T2015 has a viscosity at 100°C of about 1200 est. Other examples of suitable insulating fluids are gelling compositions such as those disclosed in US 6,383,634, which is hereby incorpo- rated by reference. These gelling compositions may comprise an oil and a gelator and have a thermo-reversible liquid-gel transition at a transition temperature T_t, wherein the gelling composition at temperatures below T_t has a first viscosity and at tem- peratures above T_t a second viscosity, which is less than the first viscosity. The composition comprises molecules of a polymer compound having a polar segment capable of forming hydrogen bonds together with fine dielectric particles having a particle size of less than 1000 nm. Further details concerning the composition are provided in claims 1 to 31 of said patent.

The present invention also relates to a method for preparing the transmission system described above. In one embodiment the method comprises a first step of providing the conductive layer 2, which is circumferentially covered by the semiconductive layer 3. In a second step, the plastic or ru bber material of the inner part 4, the main part 5 and optionally the outer part 6 is laminated with , for example, paper. The paper may have been calen- derad before and not after being laminated . I n a third step, the obtained laminated material is wound on the inner semiconductive layer 3, whereby firstly the inner part 4, secondly the main part 5 and optionally thirdly the outer part 6 is wound on the inner semiconductive layer 3. Alternatively, the laminated layer comprising the three parts 4, 5, 6 is first prepared as one piece and subsequently wound on the semiconductive layer 3. Optionally, gases are removed in a fourth step from the obtained product. This step is optionally followed by a fifth step, whereby the insulation material is cross-linked . In a final step the insulation layer is circumferentially covered with the outer semiconductive layer 7 and sheath 8.

An add itional step may be the impregnation of the insu lation material with a liqu id or a gas, preferably a high viscosity fluid , which is solid below 65°C, preferably below 80°C and has a low viscosity at working temperatures of 1 00°C, or 1 10°C . The term "conductor layer" as used herein , means a conductor as well as a conductive layer and superconductive layer.

The present invention is not limited to the embodiments dis- closed but may be varied and mod ified within the scope of the following claims.

Claims

CLAI MS

1 . A direct current (DC) transmission system ( 1 ) comprising an electrical conductor layer (2),

an inner semiconductive layer (3) circumferentially covering the conductor layer (2),

an insulation layer provided on the outer circumference of the semiconductive layer (3) comprising laminated insulation material having a thickness, a thickness ratio of polymer to laminated material and impregnated with a high viscosity fluid , which is solid below 65°C, and comprising

an inner part (4) in the vicin ity of the inner semiconductive layer (3) from a first radial distance (r₀) at a semiconductive layer (3)/insulation layer interface, and

a main part (5) circumferentially covering the inner part (4), from an initial radial distance (η) to a maximum radial distance (r_d) , and having a midd le radial distance (r_m) in a su bstantially middle of the main part (5),

an outer semiconductive layer (7) circumferentially covering the insulation layer and providing a insulation layer/semiconductive layer (7) interface,

characterized in that

the laminated insulation material of the insulation layer comprises a thickness gradient, whereby the thickness of the laminated insulation material gradually increases in the radial direction from the first radial distance (r₀) at the semiconductive layer

(3) /insulation layer interface, and wherein at least the thickness ratio of polymer to laminated material in a part of the inner part

(4) is less than 35%.

2. The transmission system (1 ) according to claim 1 , characterized in that the thickness of the laminated insulation material gradually increases from the first radial d istance (r₀) to the maximum radial distance (r_d) of the main part (5).

3. The transmission system (1 ) accord ing to claim 1 , characterized in that the thickness of the laminated insulation material gradually increases from the first rad ial distance (r₀) to the mid- die radial distance (r_m) of the main part (5) , whereafter the thickness gradually decreases to the insulation lay- er/semiconductive layer (7) interface.

4. The transmission system (1 ) accord ing to any one of claims 1 to 3, characterized in that the insulation layer further comprises an outer part (6) circumferentially covering the main part (5) and comprising laminated insulation material having substantially the same thickness as the laminated insulation material of the inner part (4).

5. The transmission system (1 ) accord ing to any one of claims 1 to 4, characterized in that the thickness of the laminated insulation material gradually increases from the initial radial dis- tance (η) of the main part (5) and whereby the thickness of the insulation material of the inner part (4) and optionally the outer part (6) is substantially the same as or less than the minimum thickness of the insulation material of the main part (5).

6. The transmission system (1 ) accord ing to any one of claims 1 to 5, characterized i n that the DC transmission system (1 ) is selected from a cable, a cable joint, bushings, insulated buses, bus bars and cable terminations.

7. The transmission system (1 ) accord ing to any one of claims 1 to 6, characterized in that the laminated insulation material comprises a plastic material laminated with a paper.

8. The transmission system (1 ) according to claim 7, character- ized in that the plastic material is selected from polyolefins selected from polyethylene, low density polyethylene, which is linear or not, medium density polyethylene, high density polyethylene, cross-linked polyethylene, and polypropylene, polyvinyl chloride, polyester, aramid and polyimide, or mixtures thereof.

9. The transmission system (1 ) accord ing to any one of claims 1 to 6, characterized in that the laminated insulation material comprises a ru bber material laminated with a paper.

10. The transmission system (1 ) according to claim 9, characterized i n that the rubber material is selected from silicone rubber, ethylene propylene diene monomer rubber and ethylene propyl- ene rubber, or mixtures thereof.

1 1 . The transmission system (1 ) according to any one of claims 1 to 10, characterized in that the total thickness of the insulation layer is between 0,5 and 50 mm.

12. The transmission system (1 ) according to any one of claims 1 to 1 1 , characterized in that the density of the laminated insulation material in the inner part (4) and optionally the outer part (6) is higher compared to the density of the laminated insu lation material in the main part (5).

13. A method for preparing the transmission system (1 ) comprising

an electrical conductor layer (2),

an insulation layer provided on the outer circumference of the semiconductive layer (3) comprising laminated insulation material having a thickness, a th ickness ratio of polymer to laminated material and comprising

a main part (5) circumferentially covering the inner part (4), from an initial first radial distance (η) to a maximum radial distance (r_d), and having a middle radial distance (r_m) in a substantially midd le of the main part (5), and optionally

an outer part (6) circumferentially covering the main part (5), an outer semiconductive layer (7) circumferentially covering the insulation layer, characterized by

comprising a first step of provid ing the conductive layer (2), circumferentially covered by the semiconductive layer (3) , a second step of laminating a plastic or rubber material of the inner part (4), the main part (5) and optionally the outer part (6), a third step of winding the obtained laminated material on the inner semiconductive layer (3), whereby firstly the inner part (4) , secondly the main part (5) and optionally thirdly the outer part (6) is wound on the inner semiconductive layer (3), whereby the laminated insulation material of the insulation layer comprises a th ickness grad ient, whereby the th ickness of the laminated insulation material gradually increases in the radial direction from the first radial distance (r₀) at the semiconductive layer

(4) is less than 35%, and optionally

a final step of circumferentially covering the insulation layer with the outer semiconductive layer (7).

14. The method according to claim 13, characterized by comprising a further step of impregnating the transmission system (1 ) with a gas or a liqu id , which is solid below 80°C.

15. The method accord ing to claim 14, characterized in that the liquid is selected from a mineral oil and/or an ester fluid , and the gas is selected from sulfur hexafluoride, compressed air and/or nitrogen .