The present invention refers to a direct current (DC) transmission system according 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 conductor 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 distance from the conductor layer. I n 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 rad ial 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 middle 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 middle 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, wh ich is impregnated with a med ium viscosity flu id . 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 middle 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 p ₀ and 0.7p ₀ , where p ₀ 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 midd le 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 radial d istances from the conductor layer. The different parts may comprise the same or d ifferent 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 h igher 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 ultra 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 inner part has a first thickness, the middle part has a second thickness and the outer part has a third thickness, whereby the second thickness is greater than the first thickness and greater than the third thickness, and wherein the laminated polymer material within each one of the parts has a constant thickness and constant thickness ratio of polymer to laminated material, and wherein at least one of the parts has a th ickness ratio of less than 35%.

The breakdown strength depends, among other things, on the th ickness of the material. Th inner material normally has a 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 h igh at locations where it is most likely to break down . The overall dielectric properties of the in- sulation system are therefore improved . The risk of breakdown 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 insulation material is especially useful for (U)HVDC-M I ND transmission systems impregnated with a high viscosity fluid .

In one embodiment, the first thickness of the inner part is substantially between 2 to 20% of the total thickness of the insulation layer, the second thickness of the middle part is substantially between 1 0 to 96% of the total th ickness of the insu lation lay- er, and the third thickness of the outer part is substantially between 2 to 20% of the total thickness of the insu lation layer.

In another embodiment, the total thickness of the insulation layer is between 0,5 and 50 mm. I n a further embodiment, the mid- die part comprises laminated polymer material , wherein the thickness ratio of polymer to laminated material is more than 35%.

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 controlled laminated plastic or rubber films as the insulation material . The thickness arrangement of the laminated insulation material according to the invention provides for insulation material in the inner and the outer part with lower values of volume resistivity and higher breakdown strength compared to the laminated insulation material in the midd le part. Instead of creating a flat resistivity gradient over the insulation material or a part thereof, the new arrangement of insu lation material has reduced resistiv- ity-governed E-stresses close to the semiconductive layer/insulation layer interfaces, while the resistivity-governed E- stresses are higher in the midd le part of the insu lation material .

One effect of th is new arrangement is a decrease in breakdown, especially at the semiconductive layer/insulation layer interfaces. 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 and outer parts improves the control of the resistivity over the insulation layer. It also improves the flexibility to adapt and use the insulation material for different transmission systems under different working cond itions. 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.

The breakdown strength also depends on the material used in the insulation layer. Different transmission systems may have different requirements for the material . For example, the breakdown 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.

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 embodiment, the plastic material is high density polyethylene.

In one embodiment, 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 propylene rubber, or mixtures thereof.

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 liqu id . Preferably, the flu id is not a medium vis- cosity flu id .

In an alternative embodiment, the density of the laminated insulation material in the inner part and the outer part is h igher compared to the density of the laminated insulation material in the middle part. Breakdown strength is improved by increasing the density of the laminated insulation material . This density arrangement will thus further improve the reliability of the transmission system and prevent space charging and aging . The object is also ach ieved by a method , for preparing the transmission system, initially defined accord ing to the pre- characterized portion of claim 13, which is characterized by comprising a first step of provid ing the conductor layer, circum- ferentially covered by the semiconductive layer,

a second step of laminating a plastic or rubber material of the inner part, the middle part and the outer part with paper, a third step of winding the obtained laminated material on the inner semiconductive layer, whereby firstly the inner part, second ly the middle part and thirdly the outer part is wound on the inner semiconductive layer, wherein the inner part has a first thickness, the middle part has a second thickness and the outer part has a third thickness, whereby the second thickness is greater than the first thickness and greater than the third thickness, and wherein the laminated polymer material within each one of the parts has a constant thickness and constant thickness ratio of polymer to laminated material, and wherein at least one of the parts has a thickness ratio of less than 35%, and optionally

a fourth step of removing gases from the obtained product, followed by an optional fifth step of cross-linking the insulation ma- terial, 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 im- pregnating 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 flu id , 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 sys- tern 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 1 as a power cable. Other transmission systemcomponents may be a cable joint as shown in Fig 2. The transmission systems 1 or systemcomponents 1 may also be bushings, insulated buses, bus bars and cable terminations. One embod iment relates to cable terminations. Further transmission systems or systemcomponents 1 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-drain ing (M I ND) transmission systems or systemcomponents 1 .

As shown in Fig 1 , a conductor layer 2 is circumferentially cov- ered 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 divided by an inner part 4, a midd le part 5 and an outer part 6. The inner part 4 is located in the vicinity of the semiconductive layer 3 from a first radial distance r ₀ at a semi- conductive layer 3/insulation layer interface. The inner part 4 is circumferentially covered by a midd le part 5 from an in itial rad ial distance η to a maximum radial distance r _d of the midd le part 5. The midd le part 5 is circumferentially covered by an outer part 6 from the maximum radial distance r _d to the outer radial distance r _r . 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 of lead or metal . This sheath may be further covered by a protection layer that may also have insulation and mechan ical properties such as a plastic or rubber material (not shown). The inner, middle and outer part 4, 5, 6 of the insulation material may comprise sub-parts. The insulation material 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 term "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 differ- ent 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 invention 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 . Other plastic materials may be polyvinyl chloride, polyesters, aramid or polyimide. 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 rubber 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 insu lation material . The material used may be one or mixtures of plastic material or one or mixtures of rubber material. The material used may also be a mixture of plastic and rubber materials. Alternatively, different materials may be used in different parts 4, 5, 6 of the insulation material . 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 outer part 6 compared to the midd le part 5, the electrical resistivity of the film of the laminated 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 mate- rial are lower compared with the resistivity-governed E-stresses in the midd le 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 middle 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 midd le part 5 of the insulation material may be more than 1 0 ¹⁴ Q.m, or more than 10 ^{1 0} Q.m and the re- sistivity p in the inner part 4 and outer part 6 of the insulation material is less than 10 ¹⁴ Q.m or less than 1 0 ^{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 r ₀ such that the material is relatively thin in the vicin ity of the semiconductive layer 3. Because different transmission systems 1 may be used for different applications, the different systems may have different requ irements regarding insulation materials. Therefore, the given thickness and g iven differences in thickness may vary depending on the transmission system or components 1 (e.g . cable or cable joint), the application for the system , the material used , etc.

The insulation material of the inner part 4 at radial distance r ₀ may be a laminated sheet having a first thickness between 0, 1 and 500 μιη, or 1 and 200 μιη, or 20 and 150 μιη . The thickness 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 thickness ratio of polymer to laminated material (polymer and paper) in the laminated sheet in the inner part 4 may be be- tween 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 insulation material of the middle part 5 between radial dis- tances η and r _d may be a laminated sheet having a second th ickness between 1 and 1000 μιη, or 25 and 500 μιη, or 50 and 200 μιη . The thickness of the middle part 5 may be between 1 0 to 95%, or 15 to 85%, preferably about 80% of the total thickness of the insulation layer. The thickness ratio of polymer to laminated material in the laminated sheet in the middle part 5 may be between 2 to 99%, or 50 to 90%, preferably more than 35%, or 40% , or 50%.

The insulation material of the outer part 6 between a radial dis- tance r _d to r _r may be a laminated sheet having a third thickness between ^" 0, 1 and 500 μιη, or 1 and 200 μιη, or 20 and 150 μιη . The thickness of the outer part 6 may be between 1 to 20%, or 5 to 15%, preferably about 10% of the total thickness 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%. The thickness of the different parts 4, 5, 6 is preferably varied by varying the ratio of polymer to paper in the laminated sheets. The th ickness of the paper or the th ickness of the plastic or rub- ber material may be varied . Preferably, on ly the thickness of the plastic or rubber material in the laminated sheet is varied .

The thickness of the laminated material does not vary within each part 4, 5, 6 or at least the thickness does not vary within each su b-part within each part 4, 5, 6. I n other words, in each part or sub-part, the thickness of the laminated sheet is constant. Also, the ratio of polymer to laminated material does not vary within the parts 4, 5, 6, or sub-parts thereof. I n other words, in each part or sub-part, the ratio of polymer to laminated material is constant. The thickness and the ratio may on ly be different between the parts 4, 5, 6, or between the su b-parts thereof. I n one embod iment, the inner part 4 and the outer part 6 comprise laminated material having the same thickness and/or the same ratio of polymer to laminated material . The thickness and/or the ratio of the laminated material in the middle part 5 is preferably greater than the thickness and/or the ratio of the laminated material in the inner part 4 and the outer part 6.

For example, the inner and outer part 4, 6 may comprise of lam- inated material having a thickness of 1 to 1 0% of the total th ickness of the insulation layer and a thickness ratio of polymer to laminated material between 5 and 25%, while the middle part 5 may comprise of laminated material having a thickness of 40 to 85% of the total th ickness of the insulation layer and a thickness ratio of polymer to laminated material between 40 and 85%. The intervals mentioned in th is example may be replaced by any values mentioned above or any value falling within the intervals mentioned above. In one embod iment, only the middle part 5 may comprise one or more su b-parts 5a having different resistivities. These different sub-part can be used to further improve the resistivity control in the insulation layer. The su b-parts 5a may for example have d ifferent ratios of plastic material or rubber/paper. The su b-parts 5a together form the second thickness. The thickness and the ratio within each sub-part 5a are constant and do not vary within the sub-parts 5a.

In another embodiment, also the inner part 4 and the outer part 6 may comprise one or more su b-parts 4a, 6a having different resistivities, whereby the thickness and the ratio within each sub-part 4a, 6a is constant and does not vary with in the su bparts 4a, 6a.

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 T2015 (H&R ChemPharm Ltd . (UK) , which is based on mineral oi l 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 exam- pies of suitable insulating fluids are gelling compositions such as those disclosed in US 6,383,634, which is hereby incorporated 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 composi- tion at temperatures below T _t has a first viscosity and at temperatures 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. I n one embodiment, the method comprises a first step of providing the conductor layer 2 , which is circumferentially covered by the semiconductive layer 3. In a second step, the plastic or rubber material of the inner part 4, the midd le part 5 and the outer part 6 is laminated with paper. The paper may have been calenderad before and not after being laminated . In a third step, the obtained laminated ma- terial is wound on the inner semiconductive layer 3, whereby firstly the inner part 4, secondly the midd le part 5 and th ird ly 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 from the obtained product in a fourth step. This step is optionally followed by a fifth step, whereby the insulation material is cross-linked . I n a final step the insulation layer is circumferentially covered with the outer semiconductive layer 7 and sheath .

An add itional step may be the impregnation of the insu lation material with a liqu id or a gas, preferably a high viscosity flu id , 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 su perconductive layer.

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