The present invention relates to a method of electrically separating at least a metallic sheath and an outer semi-conductor of a high voltage mass impregnated direct current (HVDC MI) cable. The application also relates to a HVDC MI cable comprising as least one longitudinal segment with electrically separated outer layers.

Electric power transmission is the bulk transfer of electrical energy, from generating power plants to electrical substations near demand centers. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. Transmission lines, when interconnected with each other, become transmission networks.

Most transmission lines are high-voltage three-phase alternating current (AC), although single phase AC is sometimes used in railway electrification systems. High- voltage direct-current (HVDC) technology is used for greater efficiency at very long distances (typically hundreds of kilometers), or in submarine power cables (typically longer than 50 kilometers). When electrical energy is to be transmitted over very long distances, the power lost in AC transmission becomes appreciable and it is less expensive to use direct current (DC) instead of alternating current (AC). HVDC links are also used to stabilize and control problems in large power distribution networks where sudden new loads and blackouts in one part of a network can otherwise result in synchronization problems and cascading failures.

Since the 1950s, HVDC mass impregnated (MI) cables have been used in large submarine power transmission projects at the highest DC voltage levels. MI cables are rated up to 525 kilovolts (kV) DC, and are currently the only option at this voltage level.

State-of-the-art MI cables technology has a long, established history and an excellent track record of superior reliability. MI cables can transmit voltages of up to 1,000 MW per cable with low losses in a 500 kV DC system, equivalent to 2,000 MW in a bipolar operation.

The central conductor of a typical submarine MI cable can be made of copper or aluminum. Conductor size can be any value up to 2,500 mm and is not limited to the usual cross-section steps. Insulation consists of super clean paper impregnated with a high viscosity compound based on mineral oil. It is safely confined in an extruded lead-alloy sheath, which is normally reinforced with a polymeric and steel enclosure. Submarine MI cables can further be protected by an armor of steel wires, which can be designed to meet all types of protection requirements. For improved corrosion protection, the armoring wires and the outer serving can be soaked with bitumen.

MI cables contain no free oil and are environmentally friendly because in the event of a severe mechanical damage such as rupture, no liquids or gases are released. The electric field around a MI cable is zero even under the most stringent operational conditions because of the metallic sheath. The magnetic field is around the same magnitude as the natural geomagnetic field, and two to three orders of magnitude below the limit values defined by the ICNIRP (International Commission on Non-Ionizing Radiation Protection) for human exposure. Since HVDC cables are usually installed in pairs with counter directional current, the resulting magnetic field is reduced further still. At the end of the product life cycle, the cable may be recovered and recycled.

A challenge for the HVDC MI cable when they extend over long distances is that it is difficult to locate the position of a fault when this occurs in the cable system. Hence, there is a need for a method and a system to easily and cost effective, detect in which part of the cable a failure occurs.

WO2012/116712 discloses a method for joining two HVDC MI cables. The method includes removing at least one protecting layer and the outer semiconducting layer at the cable ends and insulating the joint with impregnated paper. This prior art discloses a jointing technique for jointing to cable ends and does not solve the problem of locating a failure in a HVDC MI cable.

An object of the invention is to provide a method and apparatus in order to ease the fault finding if a failure occurs in a HVDC MI cable.

Another object of the invention is to provide a method and apparatus being able to test different sections of an underground cable system.

Another object of the invention is to provide a method and a cable which allows for separate grounding of the sheath near the transition between land and subsea installation.

Another object of the invention is to provide a method and cable applicable for HVDC MI Internal Return Conductor (IRC), IRC cable where transmission capacity and thermal conditions limits the possibility of using IRC on land sections and the IRC thereby has to be separated into two separate cables.

Yet another object of the invention is to provide a method and apparatus applicable where sections of an existing IRC cable system being replaced by two separate cables (HVDC MI cable and low volt, LV return cable). In the present application, the term "electrically separating" a layer of a cable means providing a gap or an interruption in the layer in the longitudinal direction of the cable such that sub-sections or segments of the layer is formed on either side of the gap, where sub-sections or segments are electrically isolated from each other. In other words, "electrically separating" a layer implies forming an electrical separation or gap in the layer.

In the present application, the term "outer screen" of a cable is referred to the layers of a HVDC MI cable, which comprises the metallic sheath and the outer semiconductor layer.

A goal of the invention is to provide a method and a cable for handling different electrical potentials in the outer screen of HVDC MI cables for testing sections of the cable system or in the HVDC MI IRC cables for separating out the return cable. The problem is solved by electrical separate at least one metallic sheath and an outer semi- conductor layer of a HVDC MI cable, providing adequate insulation thickness, and optionally connecting a connector for a low volt (LV) cable to the at least one metal sheath.

The invention discloses a method of electrically separating longitudinal segments of at least one metallic sheath and an outer semi-conductor layer of a high voltage mass impregnated direct current (HVDC MI) cable, the HVDC MI cable comprising, from the centre of the cable;

at least one conductor, at least one mass impregnated insulation layer, the outer semi-conductor layer and the at least one metallic sheath, the method comprising the steps of;

exposing a section of the outer semi-conducting layer by removing the at least one metallic sheath within said section,

within the exposed section of the outer semiconductor layer, exposing a section of the at least one mass impregnated insulation layer by removing the outer semiconductor layer between a first and a second position,

forming at least one insulation tape layer by wrapping a insulating tape onto the cable within at least part of the exposed section of the outer semiconductor layer,

forming at least one semi-conducting tape layer by wrapping a semi-conducting tape from at least one of the first and the second positions in a direction towards the opposite position thereby extending the outer semi-conducting layer,

such that the at least one semi-conducting tape layer in the longitudinal direction overlaps a section of the of the outer semiconductor layer or an extension thereof thereby forming an overlap section and

such that within the overlap section the at least one semi-conducting tape layer is radially electrically separated from the exposed section of the of the outer semiconductor layer or the extension thereof by the at least one insulation tape layer.

The method can further comprise the step of wrapping a further insulating tape layer onto any exposed surface of the outer semiconductor layer or any extension thereof.

The method may further comprise wrapping the tape of the at least one semiconducting tape layer around a length of the exposed outer semi-conducting layer to provide electrical contact with the outer semi-conductor layer over said length.

In the method the at least one insulation tape layer, at least one semi-conducting tape layer and optionally the further insulating tape layer, are arranged within the volume of the removed sections of the at least one metallic sheath and outer semi-conducting layer and fills out said volume.

The method may further comprise applying an insulator mantle around the circumference of the cable at least spanning the exposed section of the outer semiconducting layer.

The method may also further comprise securing said insulator mantle to the HVDC MI cable by securing one metallic flange to each side of the insulator mantle and soldering the metallic flanges to the metallic sheath.

The method can also comprise providing said metallic flanges with electric connection elements. This aspect allows for connection of the two sides to ground, or one side to ground and the opposite side to a voltage potential, or one side to ground and the opposite side to a separate conductor, or one side to one separate conductor and the opposite side to another separate conductor, or one side to one voltage potential and the opposite side to another voltage potential.

The method may further comprise the step of providing at least one of said metallic flanges with an impregnation fluid inlet, and providing impregnation fluid either pressurized or not pressurized into the HVDC MI cable through said fluid inlet.

The tape for the at least one insulation tape layer, and optionally the further insulating tape layer is an impregnated insulation paper and the tape for the at least one semi-conducting tape layer is an impregnated semi-conductive paper. The radial thickness of the insulation tape layer within the overlap section has adequate insulation thickness calculated to the maximum electrical stress of the semiconducting tape layers and semi-conducting layer in the overlap section.

The term "tape" as used herein refers to a screen, cape, coating or layer being wrapped around the HVDC MI cable. In a preferred embodiment according to the invention, the first insulating tape is an impregnated insulation paper and the semiconducting tape is an impregnated semi-conductive paper.

The outside surface shape of the further insulation layer along the separated longitudinal segment of the cable, has advantageous a plane and smooth surface for applying an insulator mantle. The insulator mantle is pulled tight onto the outside surface of the insulation paper. The insulator mantle is made of a dielectric material, such as porcelain, polyoxymethylene (POM), Epoxy, polyvinylidene fluoride (PVDF), etc. The insulator mantle is arranged around the circumference of the cable overlapping at least the longitudinal segment being separated and secured to the cable by connecting metallic flanges to the metallic sheath on each side of the insulator mantle preferably by soldering. The metallic flanges are also connected to the insulator mantle preferably by molding or other mechanical connections. In one aspect of the invention the separation segment is provided with a conic taper and the internal through whole of the insulator mantle is provided with a similar taper to allow for the smooth internal fit between the outer surface of the segment and the internal through whole of the insulator mantle.

As described in prior art, an HVDC MI cable can extend over long distances, and it is difficult to locate the position when a fault occurs in the cable. By separating the outer screen including the metal sheet, according to the present invention, a certain length of cable can be divided into sections for individual testing purposes. Each section can be tested separately, section by section, from the cable end. In a soil or land based cable system, and by separating the outer screen of the HVDC cable, the main insulation of the HVDC cable may be tested either by impulses or steady state voltage. The amplitude of test voltages is dependent on the design of the gap between the separated layers.

Theoretically, 1mm can withstand lOOkv 1.2 / 50μβ surge.

The invention can be designed to maintain a continuous outer screen all along the cable length during normal operation and that, in a fault finding situation, the outer screen can be separated at a desired section. Further, according to the invention, the metallic flanges on each side of the insulator mantle, are provided with electric connection units. The purpose is to connect the two sides to ground, or one side to ground and the opposite side to a voltage potential, or one side to ground and the opposite side to a separate conductor, or one side to one separate conductor and the opposite side to another separate conductor, or one side to one voltage potential and the opposite side to another voltage potential. In this way, even when the HVDC MI cable has a separated outer screen, the cable can maintain a continuous outer screen all along the cable length during normal operation, and in the case of a fault finding situation, the metallic flanges can be connected or reconnected for the intended purpose.

Further, at least one of the metallic flanges, which are arranged on each side of the insulator mantle, is provided with an impregnation fluid inlet for providing impregnation fluid to the tape layers. The insulator may be pressurized by an external oil reservoir if necessary or fully self contained by the cable impregnation compound itself. The insulator may be fitted with connectors for Internal Return Conductor (IRC) and separate Low Volt return cable or ground cables to Earth Link Boxes.

According to the invention the semi-conducting tape layer is separated from the outer semi-conducting layer in an overlap section, or separated from an extension of outer semi-conductive layer in an overlap section with adequate insulation thickness (gap) calculated to the maximum electrical stress of the semi-conducting tape layers. The insulating gap between the semi-conductive layers must withstand the maximum electrical stress caused by propagated atmospheric or operating impulses. Typically lightning and switching impulses may induce a sheath current in which will create a rise in potential over the insulated gap. The design of the insulated gap must be calculated to withstand such increase in potential.

As described above, the present invention provides a method that makes it possible to divide a HVDC MI cable into individual sections for testing purposes. Each cable section is during normal operation connected to the next section. In a fault finding situation (in the main insulation) a test equipment (known to the skilled person in the art) is typically connected to the cable termination at one end for testing the whole length of the cable. By implementing the cable according to the present invention in an

underground cable length, the sectioning of the cable length can be done stepwise from the remote apparatus and toward the cable termination in which the test equipment is connected. By reconnection of each individual section, one by one, backwards to the testing equipment, the faulty section can be found by elimination method. Testing of an insulating oversheet can be performed locally at each separated longitudinal segment. Each separated longitudinal segment, according to the invention, can be located at desired locations along the cable route. The number of separated longitudinal segments along the cable route is not technically limited by any means. One or more separation segments in a cable according to the present invention can typically be located in connection with an underground cable joint and thereby the sections are given by the cable route and sub sections of the installation of a cable system.

The electrically separated longitudinal segment of a HVDC MI cable, according to the invention can be installed in connection with a submarine/underground cable joint. The method can then be used to separate the outer screen between a submarine cable and an underground cable in which make it possible to test the underground cable section separately. By this means it can be determined whether a fault is present in the submarine cable or the underground cable by elimination. The amplitude of test voltages is dependent on the design of the gap between the screens. The cable also makes it possible to perform routine testing of an insulation oversheet where a semi-conductive sheet of an underground cable section is connected to a semi-conductive sheet in a submarine cable section. By disconnecting the outer screen of the submarine cable section from the underground section the insulating oversheet can be routine tested by voltage amplitude dependent on the design of the gap between the outer screens.

Another object of the invention is to provide a method and apparatus applicable for HVDC MI Internal Return Conductor (IRC), IRC cable where transmission capacity and thermal conditions limits the possibility of using IRC on land sections and the IRC thereby has to be separated into two separate cables. The internal return conductor of an IRC cable will have an increasing voltage potential from the earth point dependent on the conductor resistance. It is thereby necessary to implement the apparatus where the need for separating the IRC to a separate return conductor due to the voltage potential to ground at the separation position.

Separating the IRC into a separate return conductor may be necessary due to thermal conditions as the HVDC cable conductor and the temperature drop of the insulation may exceed allowable limits. The apparatus is then used to separate a grounded outer screen (including a metal sheet of a cable) of a for instance separate underground cable from an energized outer screen (including the metal sheet) of for instance an IRC submarine cable. The invention also discloses a HVDC MI cable comprising from the centre of the cable; at least one continuous conductor, at least one continuous mass impregnated insulation layer, an outer semi-conductor layer with a longitudinal electrically separation and at least one metallic sheath with a longitudinal electrically separation, where the electrical separations are arranged in a longitudinal segment of the cable.

The outer semi-conductor layer at the longitudinal electrically separation can comprise a longitudinal overlap section wherein the two electrically separated ends of the outer semi-conductor layer or extensions thereof overlap in the longitudinal direction and are radially separated by at least one insulation tape layer.

In the cable at least one of the separated ends of the outer semi-conductor layer can be extended by a semi-conducting tape layer to provide for the overlap in the longitudinal direction.

The cable may comprise at least one further insulation tape layer providing the cable in the longitudinal segment with a diameter equal to the cable diameter of the cable outside of the longitudinal segment.

The cable may comprise an insulator mantle arranged around the circumference of the cable in the longitudinal segment. The insulator mantle may comprise two metallic flanges secured to each the longitudinal ends of the insulator mantle, and the insulator mantle may be secured to the cable by said metallic flanges preferably by soldering to the metallic sheath.

The metallic flanges may be provided with electric connection units. At least one of the said flanges may further comprise an impregnation fluid inlet for providing impregnation fluid to the longitudinal segment.

The insulation tape layer can be made of an impregnated insulation paper and the at least one semi-conducting tape layer can be made of an impregnated semi-conductive paper.

The radial thickness of the at least one insulation tape layer radially separating the semi-conductor ends within the overlap is advantageously an adequate insulation thickness calculated to the maximum electrical stress of the semi-conducting layers.

The cable can maintain a continuous outer screen all along the cable length during normal operation, and in the case of a fault finding situation, any electrical current in the metallic sheath can be measured separately on each side of the longitudinal segment. In a preferred embodiment, the semi-conducting tape layer(s) are applied by wrapping from the respective ends of the separated semi-conductor layer, and the semiconducting tapes are insulated (over and/or under) by wrapping an insulation tape, creating an insulation thickness or a gap between the semi-conducting layers.

In an embodiment of the invention, the outer semi-conducting layer at the first position is not extended, but the outer semi-conducting layer at the second position is extended. Wherein, the extended outer semi-conducting layer in form of the second semi- conductive tape layer is overlapping the outer semi-conducting layer at the first position by a predetermined length Fl . Bringing the second semi-conducting tape layer into electrical contact with the outer semi-conductor layer at the second position, and bringing the second semi-conducting tape layer electrically isolated from but overlapping the outer semi-conductor layer at the first position.

In another embodiment, both the outer semi-conducting layer at the first position and the second position are extended towards each other and overlapping each other by a predetermined length Fl . Bringing the first semi-conducting tape layer into electrical contact with the outer semi-conductor layer at the first position, and bringing the second semi-conducting tape layer into electrical contact with the outer semi-conductor layer at the second position, and bringing the first and second semi-conducting tape layer electrically isolated from each other, but overlapping one another by a length Fl .

In other words, the invention discloses a HVDC MI cable with an electrically separated longitudinal section of at least a metallic sheath and an outer semi-conductor layer.

Further, an insulator mantle, in form of an Epoxy insulator mantle, or made from any other dielectric material such as porcelain, POM, PVDF,etc, is arranged around the circumference of the cable overlapping at least the segment being separated and secured to the cable by soldering metallic flanges to the metallic sheath on each side of the insulator mantle.

According to an embodiment, the metallic flanges are provided with electric connection units. Further, at least one of the flanges is provided with an impregnation fluid inlet for providing impregnation fluid to the wrapped layers. After connecting the insulator to the metallic sheath at each side the insulator may be filled with impregnation fluid to remove any residual air inside the insulator. According to the invention, the insulating gap between the semi-conductive layers must withstand the maximum electrical stress caused by propagated atmospheric or operating impulses. Typically lightning and switching impulses may induce a sheath current in which will create a rise in potential over the insulated gap. The design of the insulated gap must be calculated to withstand such increase in potential.

In order to be able to handle different electrical potentials in the outer screen of HVDC MI cables, it is necessary to electrically separate the metallic sheath and the outer semi-conductor of the HVDC MI cable. Separation of the outer screen can be used in the following applications:

- HVDC MI underground cable systems where High Voltage (HV) testing of different cable sections is demanded.

HVDC MI Internal Return Conductor (IRC) cable design where the IRC has to be separated into a separate Low Voltage (LV) return cable and a HVDC MI underground cable.

- HVDC MI IRC cable design where parts of the IRC are to be replaced by a separate LV return cable.

The advantages of the invention is that the present invention offer flexibility with regards to the following:

Ease the fault fining and be able to test individual sections of an underground cable system in case of a fault occurs in the cable system. By use of this invention in connection with a transition joint between submarine cable and an underground cable, the user can easily determine whether a fault is present in the land section or the submarine section of the cable system by disconnecting the outer screen and test the sections separately.

- The invention is applicable for HVDC MI IRC cable where the transmission capacity and thermal conditions limits the possibility for using IRC on land sections and the IRC has to be separated into two separate cables.

Further, the invention is applicable where sections of an excisting IRC cable system shall be replaced by two separate cables (HVDC MI cable and LV return cable).

The description above, as well as further objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the preferred embodiment which should be read in conjunction with the accompanying drawings in which: Fig. 1 - shows a sectional view of the HVDC MI cable with different layers.

Fig. 2 - shows a sectional view of the separated longitudinal segment of the HVDC MI cable.

Fig. 3 - shows a sectional view of an embodiment according to the invention where there is no extension at the first position of the outer semi-conducting layer.

Fig. 4 - shows a sectional view of another embodiment according to the invention where there is an extension at the first position of the outer semi-conducting layer, Z > 0.

Figure 1 shows the upper half portion of a HVDC MI cable seen from a sectional view. The HVDC MI cable 1 comprises several layers that are overlaid or wrapped over each other. The HVDC MI cable 1 comprising from the centre of the cable: at least one conductor 2, at least one mass impregnated insulation layer 3, an outer semi-conductor layer 4 and at least one metallic sheath 5.

Figure 2 shows a sectional view of the separated longitudinal segment LI of the HVDC MI cable 1. The HVDC MI cable 1 has an electrically separated longitudinal segment of at least a metallic sheath 5 and an outer semi-conductor layer 4. The separated longitudinal segment comprises a first section LI with a first Eland a second E2 annular edge of the at least one metallic sheath 5. A second section L2 located within the first section LI with a first and a second position PI, P2 of the at least one outer semi-conducting layer. As seen on Figure 2, the at least one mass impregnated insulation layer 3 and the at least one conductor layer 2 are not separated in the longitudinal direction.

Figure 3 shows a shows a sectional view of the HVDC MI cable 1 with two insulating tape layers 101 and 102 and one semi-conducting tape layer 201. The outer semi-conductor layer 4 at the first position PI has no extension. In that Z = 0. An insulating tape layer 101 is wrapped onto the second section L2 (defined on figure 2) and overlapping the outer semi-conductor layer 4 at the first position PI and further to the first annular edge El . As seen in Fig. 3, the insulating tape layer 101 as an increasing diameter from the second position P2 towards the first position PI . A semi-conducting tape layer 201 is wrapped onto the surface of the insulating tape layer 101 from the second position P2 towards and overlapping the first position PI by a predetermined first length Fl . The second semi-conducting tape layer 201 follows the curve of the second insulating tape layer 101. The semi-conducting tape layer 201 is brought into electrical contact with the outer semi-conductor layer 4 at the second position P2 and over a length from P2 to the edge E2. The semi-conducting tape layer 201 is electrically isolated from but overlapping the outer semi conductor layer 4 at the first position PI .

Figure 3 shows that the outside surface shape of the second 101 and the further insulation layer 102 has advantageous a plane and smooth surface for applying an insulator mantle 103, preferably an Epoxy insulator layer. After separating the outer semi-conductor layer 4 the insulator is pulled tight onto the outside surface of the insulation layers 101, 102. The insulator mantle 103 is arranged around the circumference of the cable 1 overlapping at least the longitudinal segment including the separation and is secured to the cable 1 preferably by soldering metallic flanges 104 to the metallic sheath 5 on each side of the insulator mantle 103. The metallic flanges 104 are also connected to the insulator mantle 103 preferably by molding or other mechanical connections.

Figure 4 shows a sectional view of another embodiment according to the invention where there is an extension Z of the outer semi-conducting layer. In this case, an insulation tape 100 layer is wrapped onto the surface of the exposed at least one mass impregnated insulation layer 3 from the first position PI and along the second section L2 towards the second position P2. Further, a first semi-conducting tape layer 200 extends the semi-conducting tape at least from the first position PI and towards and overlapping the first insulating tape layer 100 along the length Z.

A second insulating tape layer 101 is wrapped along a section of the second section LI fully overlapping the first semi-conducting tape layer 200.

Further, a second semi-conducting tape layer 201 is wrapped onto the surface of the second insulating tape layer 101 at least from the second position P2 in the direction towards the first position PI and overlapping the extended outer semi conductor layer at by a predetermined length in the section Fl . The first semi-conducting tape layer 200 is in electrical contact with the outer semi-conductor layer 4 at the first position PI, and the second semi-conducting tape layer 201 is in electrical contact with the outer semiconductor layer 4 at the second position P2, and the first 200 and second 201 semiconducting tape layers are electrically isolated from each other, but overlapping one another.