Technical Field

The invention relates to a bendable and thus flexible tubular electrical insulator designed for receiving at least one electrically uninsulated electrical conductor segment for forming a high voltage power transmission line, a method for producing such a tubular electrical insulator as well as a high voltage power transmission line comprising such a tubular electrical insulator. The high voltage power transmission line might be used for an installation on land, e.g. like an underground cable, or like as a sub-sea cable.

Background Art

The development of technologies for long distance power transmission or distribution is required for connecting remote renewable power generation facilities, e.g. large solar parks, several connected wind parks or large hydropower plants and the like to a hub. In case of wind parks, for example, several generators are feeding power to a hub from where it is directed to shore for further use. In prior art, electric power transmission for capacities higher than 3 GW is in general limited to overhead lines for economic reasons.

Current bulk power high voltage transmission for capacities exceeding 2 GW is limited to overhead lines as the only economically affordable solution. However, high voltage overhead transmission lines cause a visual, electric and magnetic pollution where they pass close to or inside densely populated areas. Due to the growing public opposition it became almost impossible to build new overhead high voltage transmission lines (both AC and DC) in most areas of Europe and the USA. Utilities face the problem of not getting permissions for the required right of way, e.g. the required corridor is too large or of extremely high land prices in densely populated areas. Changes in the regulatory framework regarding the planned extension of high voltage systems have intensified the interest on underground transmission lines as an alternative solution to overhead lines. In addition, future intercontinental high capacity transmission will require new subsea solutions. Existing underground and subsea technologies are dominated by cable technologies and scaling up is challenging due to limits of conductor size, insulation thickness and transportation limitations.

For that reason there is a rising demand for underground and submarine power cables for transmitting the electric power. US 5043538 for example describes a water impervious cable construction which includes a central electrical conductor, an overlayer of insulation material such as a plastic, a shield layer formed of a plurality of individual conductors, wherein the shield layer is embedded in a layer of semiconducting material, a layer of overlapped moisture barrier metal foil material, and a further overlayer of insulating material.

Unfortunately a mere scaling-up of such power cables to higher transmission capacities required in particular for high voltage applications of several hundred kilovolts at high currents is limited by inner conductor size and cable insulation thickness rendering such a cable extremely bulky and heavy rendering it almost useless in handling. Higher voltages require an increase of the thickness of the electric power cable insulation if the electric field shall be kept within sound boundaries in order to allow a fair handling. Higher currents typically require an increase of the overall cross-section of the actual conductor area of the electric power cable's inner electrical conductor or require a material with higher conduc- tivity for the inner electrical conductor and a thicker cable insulation.

Said bulkiness and weight limitations of such a scaled-up conventional power cable would lead to an increased number of cable segments required for establishing a power line between a point A and a point B. The increased number of cable segments requires a high number of cable joints which are not only labori- ous but also detrimental for establishing an economic transmission line between point A and B.

The inventors of the present invention developed an economic solution for achieving an electric power transmission for capacities above 3 GW by a high voltage power transmission line that requires no excessive number of joints be- tween point A and point B compared to conventional power cable systems. In brief, the solution resides in that the tubular electrical insulator is produced separately from the electrical conductor segment first, transported to the actual installation site separately of one another second and third assembled at the actual installation site to a high or medium voltage power transmission line by insertion of the at least one electrically uninsulated electrical conductor segment into the tubular electrical insulator such as disclosed in PCT/EP2014/052390 filed by the same applicant. The whole content of PCT/EP2014/052390 is incorporated by reference herein, in particular the disclosure portions relating to the separate production of the tubular electrical insulator and the conductor segments as well as their separate transportation to the installation site where they are assembled to form a high voltage power transmission line (e.g. coiled on a drum for transportation). In other words, the separation of the electric insulation member from the actual conductor member overcomes transportation limitations such as weight or size allowing for a higher conductor's cross section and eventually higher transmission capacity, without compromising the overall segment length.

This allows an increased cross-section area of the conductor and consequently high energy transmission capacity that can reach 3 GW per line (6 GW in bipolar operation) or even higher.

However, one of the main challenges of the inventive concept disclosed in PCT/EP2014/052390 resides in the manufacturing of a suitable flexible insulation tube. Shortly recalling the principle of PCT/EP2014/052390 can be summa- rized as follows:

1 . Increasing the current transmission ability compared to conventional high voltage power transmission cables by increasing the conducting cross sectional area and keeping the high voltage "moderate",

2. Separating the solid tubular electrical insulation from the conductor strand in their respective manufacturing state, thereby achieving comparative long segments of more than 300 meters, and thereby reducing the number of joints of a high voltage power transmission line per transmitted GW/km.

The bending properties of the tubular electrical insulator allow for a transportation on a drum with a segment length of up to some hundred meters. The same holds true for the at least one conductor segment accordingly. Such coiling on a drum requires a well-defined degree of the flexibility with reference to the maximum conventional size of standard cable transportation drums.

The tubular electrical insulator according to the PCT/EP2014/052390 application is to be understood as a tubular electrical insulation device for forming a high voltage power transmission line arrangement once a plurality of electrical conductor segments are inserted into the tubular electrical insulator, wherein the tubular electrical insulator is flexible, flexible in terms of bendability, and comprises an inner circumferential electrically conductive layer for establishing an electric contact to the electrical conductor segments once inserted into the tubular electrical insulator. Further the tubular electrical insulator can electrically insulate the electrical conductor segments from the exterior of the tubular electrical insulation device, e.g. from ground potential, once inserted into the tubular electrical insulator.

The conductor segments and the tubular electrical insulation are transported individually to the actual installation site of the high voltage power transmission line. Thereby the individual unassembled parts transport is lighter, and the transport weight limitation (e. g. 32 tons for road transport) is reached at much longer segment length.

If for example a power of 3 GW is to be transmitted with conventional cable tech- nique at a voltage of 320 kV it would require five high voltage cables in parallel in order to obtain the sufficient conductive area. This power transmission line of this example can replace five conventional cables, rendering the novel power transmission line particularly suitable for a bulk power transmission in urban areas where partial undergrounding is likely to become economical in the near fu- ture and where conventional extruded cables have dissatisfactory limited transmission capacity.

Thus not only the production costs for establishing the power transmission line are reduced, but also the required space for the cable trench for the power transmission line according to PCT/EP2014/052390. In case that the power transmis- sion capacity between a point A and a point B should exceed the maximum power transmission capacity of a single transmission line a plurality of power transmission lines may be provided alongside one another in the cable trench. General disclosure of the invention

A first object to be solved by the present invention resides in providing a suitable flexible and bendable tubular electrical insulator for establishing a high voltage power transmission line. The term 'flexible' is not to be understood as merely being versatile in terms of a use but in terms of bendability.

A second object resides in promoting a method for manufacturing a tubular electrical insulator forming a vital element for forming such a high voltage power transmission line.

A third object resides in promoting a high voltage power transmission line comprising such a suitable tubular electrical insulator.

The term 'high voltage' shall be understood hereinafter as a rated or nominal voltage of more than at least 1 kV in an operating state of the power transmission line although the term 'high voltage' is frequently used for a nominal voltage above 50kV whereas the term 'medium voltage' is frequently used for a nominal voltage between 1 kV and 50kV.

The present invention is particularly economic for high voltage power transmission lines having a nominal voltage of at least 320kV. The term 'high voltage power transmission line' shall not be understood to be limited to a power transmission line that can provide a durable and reliable electric insulation from live potential applied to its interior surface to ground potential applied to its exterior surface, for example but also to encompass power trans- mission lines carrying high currents such as 5000 Amperes, for example.

The term 'power transmission' shall be understood as the ability to transport electric power in the form of an AC or a DC current permanently, i.e. over a comparatively long period of time and not just instantaneously, over a short period of time, e.g. for discharging a lightning stroke, a fault current or the like.

The first object is achieved by a bendable and thus flexible tubular electrical insulator for receiving an electrical conductor having the following features. In a most basic embodiment the tubular electrical insulator is bendable and thus flexible along a longitudinal axis defined by its tubular overall shape and comprises an electrically conductive carrier tube that is covered by a first electrically con- ductive layer, an electrically insulating layer and a second electrically conductive layer. The first conductive layer and the insulating layer are co-extruded onto the carrier tube. The second electrically conductive layer is applied radially outside the electrically insulating layer such that the insulating layer is arranged in between the first conductive layer and the second conductive layer in a radial di- rection with respect to the longitudinal axis and such that the first conductive layer adheres to the electrically insulating layer and such that the electrically insulating layer adheres to the second conductive layer. The first conductive layer is electrically connected to the carrier tube.

The insulation material for the electrically insulating layer has to withstand the at least maximal electrical field located between the first and second conductive layers. The electrical field is not constant in radial direction with respect to the longitudinal axis. The electrical field is maximal at the radial joint of the first conductive layer to the electrically insulating layer and minimal at the radial joint of the second conductive layer to the electrically insulating layer due to the decreas- ing joint curvature. In this context the term 'radial joint' is understood as the radial connection of the layers with respect to the longitudinal axis. The average electrical field is a value in between said two radial joints. For example a 550 kV extruded cable has a maximal field of 33kV/mm, a minimal field of 14kV/mm, and the average field is about 21 kV/mm.

Since the radial joint at the radially outer, second electrically conductive layer is less sensitive to inhomogeneity's resulting from impurities and voids it is possible to allow a time delay between the application of the second electrically conductive layer and the co-extrusion of the first conductive layer and the electrically insulating layer.

The term 'co-extruded' shall not be interpreted narrowly such that the extrusion of the first conductive layer and the insulating layer needs to be simultaneously at the very same moment in time in a rather strict manner, The term 'co-extruded' shall be understood functionally in that it encompasses also tubular electrical insulators whose first conductive layer and insulating layer have been extruded consecutively on top of one another but within a limited amount of time allowing for a very clean and steady process. The requirements in terms of cleanness and the maximal number of voids is a function of the nominal voltage the high voltage power transmission line is intended for. If the high voltage power transmission line shall be used for carrying nominal voltages of less than say 50 kV the time delay for applying the electrically insulating layer onto the first conductive layer is way larger than if the high voltage power transmission line shall be used for carrying nominal voltages of more than say 300 kV. As a result, the electrically insulating layer may be applied by a separate extrusion process onto the first conductive layer in the above case for a nominal voltage of less than 50 kV whereas it is highly advisable to reduce the time delay in between the application of the electrically insulating layer onto the first conductive layer in the above case for a nominal voltage of 300 kV as low as possible, for example below 10 seconds, for meeting the insulation requirements for such a voltage.

The carrier tube provides for sufficient mechanical dimensional and form stability of the interior of the tubular electrical insulator when exposed to external me- chanical forces acting on the high voltage power transmission line once installed. The carrier tube serves not only as a mechanical armoring for protection the conductor segments against undesired mechanical impacts from the environment of the tubular electrical insulator but also ensures by its form stability that a sufficient cross-section is guaranteed for inserting the at least one electrically uninsulated conductor segment inserted into the tubular electrical insulator after manufacturing of the tubular electrical insulator at the time of forming the actual high voltage power transmission line. Maintaining its initially given form and cross-section of the carrier tube is advantageous form since it eliminates any need to reshape its geometry at the assembling site of the power transmission line and thus eases the insertion of the at least one conductor segment. The term 'conductor segment' shall not be understood as being limited to a particular geometry, for example a segment of a full circle when seen in a cross-section of the overall conductors being inserted into the tubular electrical insulator. Contrary thereto the term conductor segment is understood to be an element or member of the overall nominal conductor for transmitting the high voltage power. Since the bendability of conductor segment decreases with increasing its cross- section the present power transmission line will in most of its embodiments comprises a plurality of conductor segments running in essentially the same longitudinal direction defined by the power transmission line for ensuring the desired degree of flexibility in turns of bendability ratio to the outer diameter of the tubular electrical insulator.

When inserting the at least one electrically uninsulated conductor segment into the tubular electrical insulator it also serves as a guiding means to the at least one conductor segment. In an operation state of the high voltage power line the carrier tube establishes also a reliable and durable electrical connection to the at least one conductor segment located within its interior cavity.

The first conductive layer ensures that the electric field emerging from the at least one inserted electrical conductor segment is smoothened and uniformed in an operating state of the high voltage power transmission line while the electrically insulating layer provides for a sufficient electric insulation of the at least one electrically uninsulated conductor segment inserted into the tubular electrical insula- tor after manufacturing of the tubular electrical insulator against the exterior or environment of the tubular electrical insulator, for example against ground potential.

The second conductive layer ensures that the electric field within the electrically insulating layer is further smoothened and uniformed in an operating state of the high voltage power transmission line. Thus the first and the second conductive layers contribute substantially to controlling and distributing of the electric field in the high voltage power transmission line in an operating state of the latter. The electric field inside the insulating layer and at the radial joint with the second conductive layer is lower compared to the electric field inside the insulating layer and at the radial joint with the first conductive layer, due to the lower curvature of the radial joint.

By applying the first electrically conductive layer and the electrically insulating layer onto the carrier tube by way of co-extrusion it is ensured that a first radial joint in between the first conductive layer and the insulating layer is essentially free of any voids. Although voids are known to be particularly detrimental to the insulation quality of any power transmission lines avoiding such voids is particularly critical to any high voltage power transmission line because of enhanced partial discharge requirements and resistance, respectively. The avoidance of voids is especially crucial at the radial joint between first electrical conductive layer and insulating layer where the highest fields are present.

Moreover the co-extrusion allows for a particularly intimate adhesion in terms of an excellent bonding in between the two neighboring layers, in particular between the first conductive layer and the insulating layer and - if required - also the second electrically conductive layer extending circumferentially around the carrier tube as well one another with respect to the longitudinal axis each. Having such an intimate bonding in between the layers prevents the layers from delam- ination during bending of the tubular electrical insulator and thus prevents the formation of voids, too. Moreover the co-extrusion process ensures that there are no impurities such as dust particles present in the sensitive interfaces in between the different layers, in particular the first conductive layer and the insulating layer.

In the lower voltage range of the said high voltage power line, the first conductive layer and the insulation layer are co-extruded, whereas the second conductive layer, thus including the joint where the field is minimal if compared to the interface between first conductive layer and insulation, could be extruded in a subsequent process.

For achieving an even better and more homogeneous shielding of the electric field than only with the first electrically conductive layer it is highly recommended that the second electrically conductive layer is extruded on the insulating layer.

For achieving the superior insulation quality of the high voltage transmission line for a high voltage applications having a nominal voltage of more than 300 kV AC, it is advisable to apply the second electrically conductive layer on the electrical insulating layer by way of a triple extrusion process. That kind of process ensures for an about simultaneous extrusion of the first conductive layer, the insulating layer and the second electrically conductive layer.

A superior partial discharge resistance quality as required for high voltage appli- cations having a nominal voltage of more than 300 kV AC, for example, is difficult to achieve by way of a conventional sequential application of the first electrically conductive layer and the electrically insulating layer onto a carrier tube such as done in the conventional tubular protector such as disclosed in US201 1/041944A1 for example, as a conventional sequential application inevi- tably implies the formation and presence of voids. Suppressing any formation of voids is particularly critical at the radial joint of the first conductive layer and the insulation where the local electrical field is highest in an operating state of the high voltage power transmission line.

Particularly advantageous embodiments of the tubular electrical insulator are achievable if the first electrically conductive layer and the electrically insulating layer and in a preferred option also the second electrically conductive layer are not only tightly physically connected to one another but also chemically connected to one another each. A chemical connection between two neighboring layers may be achieved by a crosslinking as it is known from polyethylene's, for example. In other words, the tubular electrical insulator has an insulating layer comprising a polymer and at least one of the first conductive layer and the second conductive layer comprises also a polymer. Said latter polymer can be a similar or even the same polymer as the one selected for the electrically insulating layer, for example the same polymer with 30 %-wt of carbon black to enable a resistivity below 500 Qm could be used.

As insulation material, cross-linked low density polyethylene (XLPE or PEX), synthetic rubbers (e.g. ethylene propylene rubber (EPR) and ethylene propylene diene monomer (EPDM)), silicones, flexibilized thermoplastics (e.g. isotactic polypropylene (iPP) flexibilized by a plasticizer such as mineral oil), and other olefin- based thermoplastic elastomers can be used (e.g. blends of an isotactic propyl- ene polymer (i-PP) and a propylene-ethylene copolymer (PEC) or random and block copolymers of various monomeric units such as ethylene, propylene, butadiene or styrene).

The thickness of the insulation depends on the voltage rating of the transmission line in an operating state of the high voltage power transmission line and the selected insulation material for extruding the insulating layer. An exemplary embodiment of the tubular electric insulator for a high voltage application with 300 kV nominal AC voltage may have a wall thickness of the insulating layer being in a range of 20mm to 35mm, in particular in a range of 25 to 35mm measured in a radial direction with respect to the neutral geometric fibre defined by the tubular overall shape of the tubular electric insulator. This way an overall wall thickness of the tubular electric insulator of about 32mm is achievable for an AC high voltage power transmission line with a nominal voltage of 420kV, for example. If the tubular electrical insulator shall serve as an electric insulator for a DC power transmission line even wall thicknesses of less than 25mm are achievable. If the nominal voltage rating of an exemplary embodiment of the transmission line is only about 50kV AC, a wall thickness of the insulating layer with less than 20mm is achievable. The skilled reader will recognize that the wall thickness depends on the quality of the tubular electric insulator.

The inner and the outer semiconductive layers can be of the same or similar polymer as the insulation containing high filler contents of a conductive additive (e.g. carbon black). In the case of a carbon black based semiconductive layers the filler content could for example lie within the range of 15 - 35 wt.%. This percentage can be significantly lower, i.e. as low as 1 wt.%, if other high aspect ratio conductive filler is used (e.g. carbon nanotubes, graphene). The electrical conductivity of the semiconductive layers could for example lie within the range of 10 ^"7 - 10 ^"1 S/cm. A typical thickness of the semiconductive layers is in the range of 1 - 3 mm.

In one embodiment the insulation and the semiconductive materials are the following:

Borealis LE4253 (insulation), a crosslinkable polyethylene homopolymer containing bis (α,α-dimethylbenzyl) peroxide at a concentration < 2.5 wt.%.

Borealis LE0550 (semicon), a crosslinkable semiconductive compound (elastomer modified polyethylene copolymer containing carbon black and [1 ,3(or 1 ,4)-phenylenebis(1 -methylethylidene)]bis[tert-butyl] peroxide at a concentration < 2.0 wt.%).

The insulation can also contain low amounts of other additives such as scorch retarding agents (e.g. 2,4-diphenyl-4-methyl-pentene-1 ) and antioxidants (e.g. a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionicacid and thiodiglycol), if required.

The above insulation system would allow an average electric field in operation typically below 15 kV/mm for AC and typically below 20 kV/mm for DC. Another formulation could allow even higher electric fields up to 30 kV/mm.

Since the tubular electrical insulator shall serve as the basic electrical insulation in a high voltage power transmission line the carrier tube has an inner diameter of at least 80 mm for receiving at least one electrically uninsulated conductor segment whose cross-section must be able to withstand an ampacity according to the specification of the high voltage power transmission line in an exemplary embodiment. The term 'inner diameter' is also referred to as the clear diameter or the maximum clearance diameter of the tubular electrical insulator. The inner diameter of the carrier tube is mainly a function of the intended nominal current rating defining a minimal overall cross-section of the conductor. In an exemplary embodiment a wall thickness of the insulation layer is maybe 27mm for a 420 kV AC current at 5000 A resulting in an achievable inner diameter of the carrier tube of a little as 140 mm.

Depending on the requirements of the application and the bendability of the power transmission line to be established the carrier tube of the tubular electrical insulator is a corrugated tube, for example a corrugated tube as defined by norm code ISO 10380.

Corrugated metal tubes are manufactured from a cylindrical, thin walled tube formed from rolled strip and welded at the seam (Figure 3a). The tube can have either an annular or a spiral profile, closed or open pitch depending on the spacing among the corrugations, or even an omega shaped profile when greater flexibility is needed.

In another embodiment the carrier tube is a so-called strip-wound metal hose or tube. It is manufactured by spirally winding a preformed continuous strip of metal so that the edges are interlocked to form a tube.

Depending on their flexibility, strip-wound tubes are differentiated to interlocked or loose wound (more flexible) and fully-interlocked (having an additional groove that makes them tighter). The interlocking profile can be of various shapes with the most common ones being T-, U-, S- or Z. To facilitate easier pulling of the conductor during the on-site installation an inner lining of strip metal can be also used or a loose wound tube with a low profile cross section. In one embodiment the EXTRAFLEX square locked loose wound galvanized steel flexible tube supplied by the "Universal Metal Hose" company can be used.

Eventually, both the corrugated tube as well as the strip-wound metal tube are flexible (bendable) metallic tubes. Several types of flexible metallic tubes can be used made of different metals such as steel (grades 304, 316, 321 , and monel), galvanized steel, brass, bronze, copper, and aluminum (including any alloy thereof), with thicknesses in the range of 0.25 to 3.50 mm.

Where required, a material such as cotton, synthetic rubber, buna-N, various types of fibres or even metallic wires can be introduced within the interlocks to provide certain tightness (packed carcass) during the forming process of the strip-wound tube. In a preferred embodiment of the tubular electrical insulator a conductive packing material is present. Suitable packing materials are either a polymer (e.g. rubber) containing a conductive filler (e.g. carbon black) or a con- ductive fibre (e.g. carbon fibre) for instance. For making the tubular electrical insulator suitable to be used in a high voltage power transmission line its electrical insulation capacity of the tubular electrical insulator is such that it can withstand maximum electric field levels typically up to 28 kV/mm in AC, and up to 35 kV/mm in DC.

An additional electrically conductive layer can be arranged in between the carrier tube and the first conductive layer for several reasons. First it forms a smoother surface for the co-extrusion and thus for a more continuous material flow out of the extrusion head because the material is prevented from entering the space between two neighboring ridges of the carrier tube compared to a corrugated or a strip-wound tube. Second it prevents the space between two neighboring ridges of the carrier tube from being filled in part or as a whole with material forming the first electrically conductive layer. Keeping the spaces free of such material provides for a smaller bending radius of the tubular electrical insulator compared to a solution without said additional electrically conductive layer. In an exemplary embodiment the additional conductive layer is established by helically winding a conductive tape about the carrier tube.

The additional layer has to be electrically conductive because it has to establish a durable and reliable electrical connection between the carrier tube and the first electrically conductive layer.

If the tubular electrical insulator shall have superior insulation properties also in regard of humidity it is advantageous to select an additional electrically conductive layer having these desired properties such that it can act as a moisture barrier.

If required at least one further layer is applied radially outside the second con- ductive layer with respect to the longitudinal axis. Said at least one further layer may be a shield comprising a plurality of metal wires that may be arranged one displaced to another or interwoven with one another to form a mesh-like shield structure. If required, said at least one further layer may be formed as moisture barriers, metallic screens as well as a mechanical protector such as polymeric cover.

Depending on the intended use several of these further layers having different properties might be applied on the tubular electrical insulator disclosed above for conferring the tubular electrical insulator and the power transmission line with additional properties such as for EMV shielding, rodent protection, abrasion pre- vention, for example. As an illustrative example the further layer is formed a protective electrically conductive mesh for carrying short currents or dissipating the energy of a lightning strike.

The blanket term 'further layer' comprises also a PE layer for protecting the high voltage transmission line in an underground application, for example. Alternatively or in addition additional further layers like a wire screen and/or PE sheets, further semiconductive layer and the like may be required for an underground application of the tubular electrical insulator and the power transmission line formed therewith. These layers might be established by using a semiconductive tape.

For maintaining the flexibility in installation of the high voltage power transmission line the tubular electrical insulator has a bending radius about a bending axis extending transversally to the longitudinal axis (referred to a neutral fibre above) being in a range of 4 to 20 times the outer diameter of the tubular elec- trical insulator. The specified range for the bending radius represents a bending property that is optimized for the purpose of the present invention in consideration on one hand the transport aspect and on the other hand other practical aspects such as the stability. The possible bending radius corresponds to the radius to which the tubular electrical insulation can be bent without using excessive force for wounding at that radius without significant deformation, cracks or other defects.

In terms of flexibility of the electrical insulation line expressed in D:d, where D is the bending diameter and d is the outer diameter of the insulation's line, ratios down to 10 could be reached.

If the tubular electrical insulator shall serve also as a carrier for transmitting signals, it can further comprise an additional conductor line provided radially outside the second conductive layer with respect to the longitudinal axis. Technically less favorable, but practically possible is to provide the additional conductor line below the first conductive layer in the neighborhood of the carrier tube.

If required for some reason it is also possible to provide the additional conductor line between the electrical insulator layer and the inner wall surface. In an exemplary embodiment the additional conductor line is embedded in between the electrical insulator layer and the first conductive layer. In a further embodiment the additional conductor line is embedded in between the electrical insulator layer and the second conductive layer. Although such embodiments are presently regarded as technically less favorable as the additional conductor line is less shielded against the electrical field arising from the uninsulated electrical of conductor segments in a radial direction with respect to the longitudinal axis than in an embodiment where the additional conductor line is arranged radially outside the electrically conductive outer layer, such an embodiment may still suffice for establishing a reliable communication line used for monitoring the high voltage transmission line and/or elements thereof.

An advantage of such embodiments having an additional conductor line resides in that no separate signal line needs to be laid along with the power transmission line. Depending on the intended use the additional conductor line can be a signal wire or an optical fiber intended for monitoring for example the temperature of the joints or for allowing communication together with bulk power transmission in one go.

A suitable method for manufacturing the most basic embodiment of the tubular electrical insulator disclosed earlier on comprising the steps.

a) Providing an electrically conductive carrier tube;

b) feeding the carrier tube through a nozzle system in a direction of a longitudinal axis defined by the tubular overall shape of the carrier tube;

c) co-extruding a first electrically conductive layer and an insulating layer onto the carrier tube and

d) applying a second electrically conductive layer such that the insulating layer is arranged in between the first conductive layer and the second conductive layer in a radial direction with respect to the longitudinal axis and such that the first conductive layer is electri- cally connected to the carrier tube, wherein the first conductive layer adheres to the electrically insulating layer and wherein the electrically insulating layer adheres to the second conductive layer.

The first electrically conductive layer and the insulating layer require the provision of two extruders, one for each layer. Besides these two extruders and the co-extrusion nozzle system a complete production line for the tubular electrical insulator consists of additional systems for handling the metallic carrier tube as well as heating and/or cooling zones and applying the second conductive layer. The metallic carrier tube may be preheated (e.g. to about 100°C in the case that the insulation is XLPE), on the one hand to reduce the required thermal energy that has to be transferred in the subsequent cross-linking section resulting in shorter cross-linking sections and/or higher production speeds, and on the other hand to avoid shock cooling of the molten mass of the first semi-conductive layer onto the outer shell surface of the carrier tube.

Both the semi-conducting and the insulating compounds which are typically supplied as granular materials are fed into the corresponding extruders to gently melt as they pass through the various heating zones of the production line. The molten polymer mass is then blended and homogenized by the rotating extruder's screws. Precise temperature control (e.g. around 130 to 135°C for XLPE) is critical since the temperature must be high enough in order to achieve a soft, easily formable molten mass, but not too high as then the peroxide cross-linking pro- cess would begin already in the extruder prior to forming in the extrusion head. The molten XLPE is continuously extruded directly onto the flexible, bendable carrier tube or onto an intermediate layer (e.g. a further semiconductive layer formed by a conductive tape wound around the carrier tube and thus forming a foil for acting as a moisture barrier). The aforementioned description of a produc- tion line can be even more complex taking into account any other inner/outer additional layers such as moisture barriers, additional sheaths and the like, if required.

If a homogeneous shielding shall be achieved by the second electrically conductive layer it is recommended to apply the second electrically conductive layer onto the insulating layer by way of extrusion, too.

A suitable and economic way of co-extrusion for high voltage applications having a nominal voltage above a few hundred kilovolts, e.g. 300kV is achievable by a triple extrusion nozzle system, for example a triple extrusion head. The first electrically conductive layer, the insulating layer and the second electrically conduc- tive layer are produced about simultaneously in time allowing for a sufficiently clean and almost void less radial joint in between the three above-mentioned layers.

In the production of conventional power cables three types of installation are common for production lines where continuous vulcanization is applied: cate- nary, vertical and horizontal lines. A full production line can occupy more than 100 m in length. Having a vertical production line proves advantageous also to the method for manufacturing a tubular electrical insulator according to the present invention as it ensures a geometrically central positioning of the metallic carrier tube and an exact roundness of the insulating layer since no gravitational forces act on the tubular electrical insulator such that the properties of the tubular electrical insulator are the very same all over the circumference of the tubular electrical insulator. So the carrier tube is fed through the co-extrusion nozzle in a direction extending in the direction of earth gravity.

The extrusion process or processes ensures that the electrical insulating layer is not only seamless in a longitudinal direction defined by the overall tubular shape of the tubular electrical insulation device but also homogeneous in the circumferential direction which is decisive for a use as a power transmission line for carrying a current over a long period of time, e.g. for years.

The co-extrusion process is continuous therefore there is no theoretical length limitation of the produced segments of the tubular electrical insulator. Taking into account transportation limitations, segment lengths in the range of 700 m are envisaged for a line intended for an underground (land) power transmission line. For sub-sea installations of the power transmission line much higher lengths in the km range can be expected as the drums on whom the tubular electrical insulator are coiled for transportation on the installation site are far less limited in diameter than for trucking on conventional roads.

If certain desired properties such as a better moisture protection for example are required an additional electrically conductive layer can be applied onto the carrier tube prior to step b).

Often curing of the tubular electrical insulator will be desired after extrusion in step c). The step of curing follows on directly from the extrusion process referring as continuous curing or continuous vulcanization. For the latter, the temperature is raised (e.g. to around 200°C for XLPE) using cylindrical electrically heated ovens. A homogeneous transfer of heat is assured using an inert gas (nitrogen) under pressure, in a so-called dry-curing process. Typical lengths for heating zones are roughly 15 to 50 m. These are followed by cooling sections of at least the same length in which the insulation is cooled down to ambient temperature at a definite slow rate.

At the conductor insertion process the plurality of conductor segments may be bundled together, e.g. by tape, wire or the like for simplifying the insertion of the conductor segments into the tubular electrical insulator.

Tests revealed that the overall free interior cross-section of the tubular electrical insulator may be filled up to 70% with uninsulated conductor segments easily which contributes essentially to establishing high voltage power transmission lines economically. With an overall conductive cross sectional area of the at least one conductor segment of more than at least 10000 mm ² allowing for a continuous power transmission for example of a rated AC current with a currents of 5000 Amperes and a nominal voltage of 420kV, respectively. The advantages of the present inven- tion are most pertinent when the conductive cross sectional area is above 10000 mm ² , in particular above 20000 mm ² . The invention may even be applied for conductive cross sectional areas up to twice and even five times that figure.

Once the at least one electrically uninsulated conductor segment is inserted into the tubular electrical insulator after the aforementioned manufacturing of the tub- ular electrical insulator a superior high voltage power transmission line is achievable. The insertion has to be done such that the at least one conductor segment is touching an inner wall surface of the tubular electrical insulator such that the carrier tube is on the same electric potential as the at least one conductor segment in an operating state of the high voltage power line.

The conductor segments may be made of copper, aluminum or an alloy comprising any one of these elements as well as any other suitable electrically conductive material or material composition.

In particular for establishing long distance connections between a point A and a point B it is preferred to have a high voltage power transmission line whose flex- ibility in turns of bendability is rather high. An embodiment of the high voltage power transmission line has a bendability ratio being better or at least similar to a bendability of a conventional high voltage power cable. Such an embodiment would be suitable for replacing an existing high voltage power transmission cable, for example.

The present high voltage power transmission line may not only be used for establishing long distance connections between a point A and a point B and thus form an economic advantageous alternative to a plurality of conventional high voltage cables provided parallel to one another but also for establishing comparatively short distance connections from a first portion of a gas insulated substa- tion to a second portion of a gas insulated substation, e.g. for electrically connecting two portions of a retrofitted substations extending on both sides of a motorway or over different caverns or different basements in an existing building. The present high voltage power transmission line forms thus an economically attractive alternative to so-called gas insulated lines (GIL) also known as gas insulated bus ducts (GIB).

In an exemplary use the present high voltage power transmission line may be employed for replacing a GIL or a GIB. Gas insulated lines (GIL) or gas insulated bus ducts (GIB) are commonly used to connect gas insulated switchgear with overhead lines, typically routed over-ground and over hundreds of meters leading to a large & pressurized volume of SF6 insulation gas mixture. The most common voltage level for higher voltages is 420kV AC and at least 4000 Amperes. Sulfur hexafluoride (SF6) is an extremely potent greenhouse gas, and thus subject to increased regulations and bans. Thus an increasing number of grid-operators is demanding for alternative insulation solutions in order to minimize the volume of pressurized SF6 insulating gas. Replacing a GIL/GIB with conventional extruded cables is not economic for most cases as one single GIL/GIB would need to be replaced with several conventional cables that are installed in parallel to one another due to the high current rating.

An advantage of using a bendable and thus flexible high voltage power transmission line according to the present application resides in that such a power connection can be established also in situations where there would be insuffi- cient space available for establishing a conventional bus duct, for example in case of an uprating of an existing substation or switchyard.

A further advantage of a bendable and thus flexible high voltage power transmission line according to the present application compared to gas insulated bus ducts resides in that no compensator components for balancing thermal expan- sions are required.

Furthermore the high voltage power transmission line according to all embodiments disclosed in the present application is more advantageous than common gas insulated bus ducts because it is SF6 free, it is way easier to install (more tolerant in view of orientation and location tolerances), it is way quicker to install and to maintain (no gas evacuation and gas monitoring required). All this advantages allow for lowering the overall costs for establishing a short distance connection with the high voltage power transmission line according to the present application compared to a common gas insulated bus duct

The high voltage transmission line according to the present invention can be conferred with an additional functionality in that it further comprises an auxiliary conductor for transmitting signals, wherein the auxiliary conductor is inserted into the tubular electrical insulator, too. Since the fill-factor is far less than a 100% there is some space left for optionally include additional functionality inside of the insulating tube. Optical fibres as an exemplary embodiment of an auxiliary con- ductor can be laid alongside the uninsulated conductor segments. That auxiliary conductor may be used for monitoring the temperature along the axial direction, for detecting partial discharges, or for observe any changes in the moisture content of the power transmission line. The electrical insulator tube is suitable for sheltering additional lines, e.g. auxiliary lines/conductors made from reinforced glass fibres for communication purpose. Alternatively the auxiliary conductor may be a copper-based conductor. In any case the auxiliary conductor may be protected against damage and/or electrically or against excessive electrical fields by an insulation of its own, if required.

The high voltage power transmission line according to all embodiments can be used in an AC or DC energy transmission system operated at high or medium voltage levels. A fully functional systems comprises the following basic components:

a) At least at least two tubular electrical insulator segments;

b) At least one power terminations.

The joints can be prefabricated joints in the case of land installations and flexible or so-called factory joints in the case of subsea installations. In an exemplary embodiment, the power terminations can be bushings to connect with adjacent overhead lines and connections to a gas insulated module such as a GIS. The joints and the terminations are achievable by scaling up known cable joints or oil-free cable terminations.

The above high voltage power transmission lines can be used for transmitting an AC or a DC current having more than 1 kV, in particular more than 50kV. Economically particularly advantageous solutions compared to known power cable solutions are achievable for nominal voltages of the system of at least 320kV. Brief description of the drawings

The description makes reference to the annexed drawings, which are schematically showing in

Fig. 1 a high voltage power line comprising a tubular electrical insulator according to a first embodiment where a plurality of electrically uninsulated conductor segments are inserted after manufacturing of the tubular electrical insulator;

Fig. 2 a close-up comprising a sectional breakout of a first embodiment of a carrier tube in the form of an interlocked strip wound tube with an unpacked profile;

Fig. 3 a detail close-up of a second embodiment forming a variation to the first embodiment of a carrier in the form of an interlocked strip wound tube with a packed profile; Fig. 4 a third embodiment of a carrier tube in the form of a corrugated tube; Fig. 5 a close-up comprising a sectional breakout of a first manufacturing method of high voltage power line;

Fig. 6 a close-up comprising a sectional breakout of a second manufacturing method of high voltage power line;

Fig. 7 a further embodiment of a high voltage power line in partial broken view; and

Fig. 8 a sectional view through the wall of the further embodiment of a high voltage power line according to fig.7.

In the drawings identical parts, currents and voltages are given identical reference characters.

Ways of working the invention:

The high voltage power transmission line 14 shown in fig. 1 comprises a first embodiment of a tubular electrical insulator 1 and a nominal conductor compris- ing seven electrically uninsulated conductor elements 2 for transmitting high voltage power load. The display of the elements of the power transmission line 14 shown in fig. 1 is not shown in a single cross-section but in a staggered fashion with respect to a longitudinal axis 3 also referred to a neutral fibre further up in this disclosure and which is defined by the tubular overall shape of the tubular electrical insulator 1 irrespective of its bending radius. The staggered display of the cross-section allows for a better understanding of the set-up of the high voltage power transmission line 14.

The tubular electrical insulator 1 comprises a carrier tube 4 that is flexible in turns of its bendability transversally to the longitudinal axis 3 wherein a bending radius is about 20 times of an outer diameter 12 of the tubular electrical insulator 1 .

A set of three consecutive set of layers is co-extruded onto the carrier tube 4. Said set of layers comprises an electrically conductive carrier tube 4 that is covered by a first electrically conductive layer 5, an electrically insulating layer 6 and a second electrically conductive layer 7. The first conductive layer 5, the insulat- ing layer 6 and the electrically conductive layer 7 are co-extruded to the carrier tube 4 by way of triple extrusion such that the insulating layer 6 is arranged in between the first conductive layer 5 and the second conductive layer 7 in a radial direction with respect to the longitudinal axis 3. The first electrically conductive layer 5 is in electric contact with the carrier tube 4 in a radial direction with respect to the axis 3. As an option and thus displayed in dashed lines, an auxiliary conductor 30 for transmitting signals is inserted into the tubular electrical insulator 1 along with the electrically uninsulated conductor elements 2.

Fig. 2 is a close-up comprising a sectional breakout of a first embodiment of a carrier tube in the form of an interlocked strip wound tube 4 made of stainless steel with an unpacked profile when seen in the partial longitudinal cross-section breakout shown in the left hand portion of fig. 2. Since the upper and the lower portions of the carrier tube shown in fig. 2 are not exact halves here there has been a longitudinal cut line drawn in fig. 2. For the sake of simplicity said cut lines are tagged with reference numeral 3 as well.

The reader is asked to pay attention on the cross-section of the wall of the first embodiment of the carrier tube 4 shown in section 15 in order to recognize its distinctive set-up compared to a design variation shown in fig. 3 as explained below.

Fig. 3 a detail close-up of a second embodiment of a carrier tube 40 forming a variation to the first embodiment of a carrier tube 4 in the form of an interlocked strip wound tube with a packed profile in the same section 15 as shown and explained with respect to fig. 2 above. The packing 16 of the stainless carrier tube 40 is achieved by rubber (displayed in black in fig. 3). Said packing 16 com- prises carbon black as an electrically conductive filler.

A third embodiment of the carrier tube is shown in fig. 4. The carrier tube 400 is a corrugated tube made of stainless steel.

Fig. 5 shows a close-up of a triple-extrusion head 13 and a first embodiment of the tubular electrical insulator 4 during its manufacturing process in a vertical production line where the flow of the extrusion is extending in the same direction as the direction of earth gravitation, i.e. in the direction of axis 3 which is the same like a feeding direction 21 of the carrier tube 4. On the left of axis 3 all elements of the tubular electrical insulator 1 as well as of the triple-extrusion head 13 are shown in cross-section whereas the same elements are shown in plain view on the right of axis 3. The triple-extrusion process is employed for achieving the required superior joint quality in between the layers 5, 6, 7 in terms of impurities and voids as required for establishing a reliable high voltage power transmission line suitable for carrying a nominal voltage of more than a 300kV.

The triple-extrusion head 13 comprises a first nozzle 17, a second nozzle 18 and a third nozzle 19 dedicated for extruding the first electrically conductive layer 5, the electrically insulating layer 6 and the second electrically conductive layer 7, respectively. The feeding direction of a first molten mass 22, a second molten mass 23 and a third molten mass 24 for forming the first electrically conductive layer 5, the electrically insulating layer 6 and the second electrically conductive layer 7, respectively, are indicated by arrows.

In this embodiment of the manufacturing method the first conductive layer 5 is allowed to enter the helical interstice 20 between two neighboring coils of the carrier tube 4 to some degree.

Fig. 6 shows a close-up of a triple-extrusion head 13 and a second embodiment of the tubular electrical insulator 10 during its manufacturing process. The sec- ond manufacturing process is similar to the one shown and described with respect to fig. 5. Thus only differences to the manufacturing method shown and described with reference to fig. 5 will be described. Bodily or functional identic elements are given the same reference numerals in fig. 6 as in fig. 5.

The manufacturing method shown in fig. 6 comprises the additional step of ap- plying an additional electrically conductive layer 8 onto the carrier tube 4 prior to inserting it into the triple-extrusion head 13. The application of an electrically conductive tape forming said additional semiconductive layer 8 onto the outer surface of carrier tube 4 is performed by winding in a winding direction such that two neighboring edges of the tape overlap.

Different to tubular electrical insulator 1 , the interstices 20 of the tubular electrical insulator 10 are not penetrated by material of the first conductive layer 5 since the additional electrically conductive layer 8 forms a barrier to the flow of molten mass 22. Keeping the helical interstice 20 between two neighboring coils free from any molten mass 22 forming the first conductive layer 5 confers the tubular electrical insulator 10 with better bending properties compared to the tubular electrical insulator 1 explained above.

Please note that using a corrugated carrier tube 400 instead of carrier tube 4 or carrier tube 40 is possible, too.

Fig. 7 shows together with fig. 8 a further embodiment of a high voltage power transmission line 100 in partially broken view. Again the carrier tube 4 received a set of consecutive layers 5, 6, 7 applied by co-extrusion as explained above. This embodiment of a high voltage power transmission line 100 comprises further layers 9 in the form of a wire shield 9 comprising a plurality of copper wires extending helically around the axis 3 as well as a polyethylene layer forming a jacket layer rendering the high voltage power transmission line suitable for an underground use where the high voltage power transmission line is buried in soil. As an option and thus displayed in dashed lines, an additional conductor line 29 is provided radially outside the second conductive layer 7 with respect to the longitudinal axis for transmitting signals. These further layers 9 are applied after the triple-extrusion process consecutively to one another.

List of reference numerals:

1 , 10, 100 Tubular electrical insulator

2 Electrical conductor segments

3 longitudinal axis

4, 40, 400 Carrier tube

5 first electrically conductive layer

6 electrically insulating layer

7 second electrically conductive layer

8 Additional electrically conductive layer

9 further layer (blanket term for e.g. an armoring, a metallic sheath, a jacket layer, a wire shield)

1 1 PE layer

12 Outer diameter of tubular electrical insulator

13 extrusion nozzle system / co-extrusion head / triple extrusion head

14 high voltage power transmission line

15 Section of the carrier tube

16 Packing

17 First nozzle

18 Second nozzle

19 Third nozzle

20 interstice

21 Feeding direction

22 First molten mass

23 Second molten mass

24 Third molten mass

26 Winding direction of tape 8

27 PE layer

28 Wire shield

29 Additional conductor line

30 Auxiliary conductor line