Patent Publication Number: US-2017363660-A1

Title: Electrode foil

Description:
The invention relates to electrodes for sensors for medium-voltage (MV) or high-voltage (HV) power cables in power distribution networks like national grids. 
     Such power cables are designed for currents of tens of amperes, and for voltages of kilovolts. These cables typically have an inner conductor for carrying the electrical power, surrounded by a main insulation layer and an outer cable sheath. In order to measure the voltage of the inner conductor versus electrical ground, capacitive sensors are often used. In some of these sensors, a sensing capacitor is formed by the main insulation layer, and potentially other insulation layers, forming the dielectric, by the inner conductor forming the first sensing electrode, and by an electrode arranged on the outer circumference of the main insulation layer forming the second sensing electrode of the sensing capacitor. 
     In the European patent application EP 2608338 B1 a voltage sensor on a power cable is described which comprises a piece of conductive or semiconductive material, arranged on the insulating layer and operable to form an electrode of a sensing capacitor for sensing voltage of the inner conductor. 
     While a single sensing electrode is often sufficient to sense the voltage of the inner conductor, one or two further electrodes may be arranged in the vicinity of the sensing electrode such that they shape the electrical field in a suitable manner, e.g. to provide some degree of symmetry to the electric field, or such that they avoid parasitic capacitances. The presence of one or more of such “field-shaping electrodes” or “guard electrodes” can result in a higher accuracy of the voltage sensing by the sensing electrode. 
     An example of a voltage sensor for a power cable is shown in the international patent application WO 2012/130816 A1. In this disclosure, the measurement apparatus comprises a sensing electrode assembly arranged around the cable, comprising a sensing electrode element connected to a semiconductor layer of the cable, such that a sensing capacitor is formed by the semiconductor layer together with the at least one central conductor and at least the first dielectric layer of the insulated electrical cable. An arrangement for defining an effective sensing area is formed by two additional electrode elements positioned on either side of the sensing electrode element, which are connected to the semiconductor layer of the cable. 
     Generally, the sensing electrode and a field-shaping electrode should be installed on an insulating layer of the cable, e.g. the main insulation layer, with a well-defined axial distance between them and without being distorted. Hence an installation of the sensing electrode and of a field-shaping electrode on an insulating layer of a power cable can be difficult and time-consuming. It is desirable to make the installation process more reliable and quicker. The present invention seeks to address this problem. 
     The present invention provides an electrode foil comprising 
     a) an electrode layer comprising a sensing electrode portion, operable as an electrode of a sensing capacitor for sensing a voltage of an inner conductor of a medium or high voltage power cable, and a first auxiliary electrode portion, electrically isolated from the sensing electrode portion, characterized in that the electrode foil further comprises
 
b) a non-conductive carrier film carrying both the sensing electrode portion and the first auxiliary electrode portion, and
 
c) an adhering surface portion for attaching the electrode foil to an exposed insulation layer of the medium or high voltage power cable.
 
     The arrangement of the sensing electrode and of the first auxiliary electrode on a carrier film, and an adhering surface portion for attaching the electrodes on the insulation layer of an MV/HV cable makes installation of the electrodes quicker and more reliable, because the electrodes do not need to be positioned relative to each other on the insulation. The electrodes can be positioned relative to each other on the carrier film in a factory, potentially in an automated process. This results in a more precise geometry of the arrangement of electrodes, which may be essential for a higher accuracy of the voltage sensor comprising the electrodes. With an electrode foil as described, the installation of electrodes is more reliable because there is no need to assemble the two electrodes in the field under potentially adverse conditions, which may result in undesired geometries of the electrodes relative to each other, like for example edges which are not parallel to each other, or wrinkles in the electrode layer because of improper handling e.g. by an installer wearing gloves. 
     The electrode layer of the electrode foil according to the invention is electrically conductive. The expression “electrically conductive” or “conductive” is meant to exclude the property of electrically insulating, but to include the property of electrically semiconductive. The electrode layer may thus be semiconductive. The electrode layer and/or any portion of it may comprise metal or a layer of metal. The electrode layer and/or any portion of it may comprise a conductive polymer or a layer of conductive polymer. The electrode layer and/or any portion of it may comprise an electrically conductive coating. The coating may be supported by the carrier film. The thickness of the electrode layer and/or any portion of it may be between  1  micrometer and  1  millimeter. The electrode layer and/or any portion of it may be conformable such that the electrode foil is conformable around the circumference of an exposed insulation layer of a MV/HV cable. 
     The electrode layer comprises a sensing electrode portion and a first auxiliary electrode portion. “Portions”, in the context of this disclosure, are areas of the electrode layer which appear arranged sideways from each other when looking perpendicularly at the electrode layer, so that two portions of the electrode layer cannot overlap. 
     The sensing electrode portion and the first auxiliary electrode portion may be arranged in the same plane. The expression “in the same plane” refers to a situation where the electrode foil is flattened and the electrode layer is flat. The electrode layer may thus comprise, in the same plane, a sensing electrode portion and a first auxiliary electrode portion 
     The sensing electrode portion of the electrode layer may have a rectangular shape, when the electrode layer is flat. 
     Generally, the sensing electrode portion may extend, in one direction in the plane of the electrode layer, far enough to extend around the full circumference of the exposed insulation layer of a typical MV/HV power cable. Generally, the sensing electrode portion may extend, in one direction in the plane of the electrode layer, further than the full circumference of the exposed insulation layer, so that it can overlap with itself when applied on the insulation layer. Specifically, it may extend, in one direction in the plane of the electrode layer, between 1 cm (centimeter) and 15 cm, between 0.5 cm and 10 cm, between 0.3 cm and 8 cm or between 0.2 cm and 5 cm. Generally, the sensing electrode portion may have two parallel edges. 
     The first auxiliary electrode portion of the electrode layer is electrically isolated from the sensing electrode portion. This is to say that the first auxiliary electrode portion and the sensing electrode portion are not electrically connected with each other before any wires or electrical contacts or connections are attached to them. 
     The first auxiliary electrode portion is electrically conductive. The first auxiliary electrode portion may comprise the same or a different material than the sensing electrode portion. It may have the same or a different thickness than the sensing electrode portion. 
     The first auxiliary electrode portion may have a rectangular shape, when the electrode layer is flat. Generally, the first auxiliary electrode portion may extend, in one direction in the plane of the electrode layer, far enough to extend around the full circumference of the exposed insulation layer of a typical MV/HV power cable without overlapping itself. Specifically, it may extend, in one direction in the plane of the electrode layer, between 1 cm (centimeter) and 15 cm, between 0.5 cm and 10 cm, between 0.3 cm and 8 cm or between 0.2 cm and 5 cm. Generally, the first auxiliary electrode portion may have two parallel edges. Alternatively, the first auxiliary electrode portion may extend, in one direction in the plane of the electrode layer, further than the full circumference of the exposed insulation layer, so that it can overlap with itself when applied circumferentially on the insulation layer. 
     The electrode foil of the present disclosure comprises a non-conductive carrier film, which carries both the sensing electrode portion and the first auxiliary electrode portion. The carrier film may have two major surfaces, a first major surface and an opposed second major surface. The carrier film may provide mechanical stability to the electrode foil. It may serve as a carrier or support for the electrode layer. It may comprise the adhering surface portion. It may serve as a carrier or support for an adhesive layer comprising the adhering surface portion. The carrier film may comprise a polymeric material. It may comprise, for example, polyester, polyvinylchloride, polyethylene, polypropylene, EPDM (ethylene propylene diene monomer), paper and/or non-conductive silicone. The carrier film may comprise a plurality of layers. Each of the layers may comprise a polymeric material, like for example polyester, polyvinylchloride, polyethylene, polypropylene, EPDM, or non-conductive silicone, and/or it may comprise paper. Generally, the carrier film may be conformable such that it can conform around the circumference of an exposed insulation layer of a MV/HV power cable. The thickness of the carrier film may be, for example, between 0.1 micrometer and 1 millimeter. Specifically, it may be between 1 micrometer and 100 micrometers. 
     A layer or a plurality of layers may be arranged between the carrier film and the sensing electrode portion. A layer or a plurality of layers may be arranged between the carrier film and the first auxiliary electrode portion. The carrier film may comprise heat stabilizers, so that it can withstand elevated temperatures of a MV/HV cable for long times, e.g. for longer than a year or for longer than 10 years. 
     Generally, an electrode foil according to the present invention may be attachable to an exposed insulation layer of a MV/HV power cable. Specifically, it may be attachable to an exposed main insulation layer of a MV/HV power cable. 
     An electrode foil according to the present invention may comprise an adhesive layer. The adhesive layer may comprise the adhering surface portion. Providing an adhesive layer may be a particularly cost-effective way of allowing the electrode foil to be attached to an insulation layer of a power cable. Specifically, the adhesive layer may be arranged on the carrier film. While one side of the adhesive layer faces the carrier film, the opposed side of the adhesive layer is exposed and provides the adhering surface portion. The adhering surface of the adhesive layer, or only a portion of it, can be brought into contact with the insulation layer of the power cable. Depending on the type of adhesive, an appropriate amount of pressure or heat makes the adhering surface stick to the insulation layer, whereby the electrode foil is attached to the insulation layer. 
     The adhesive layer may comprise, for example, a pressure-sensitive adhesive or a heat-activatable adhesive. The thickness of the adhesive layer may be, for example, between 0.1 micrometer and 500 micrometers, specifically it may be between 1 micron and 100 microns. Generally, the adhesive layer may be conformable such that it can conform around the circumference of an exposed insulation layer of a MV/HV power cable. The adhesive layer may comprise a material that is selected appropriately to adhere well to the surface material of an exposed insulation layer of a MV/HV power cable, such as XLPE, EPR, PE. 
     The adhesive layer may be arranged directly on a major surface of the carrier film. Alternatively, a layer or a plurality of layers may be arranged between the carrier film and the adhesive layer. In particular the electrode layer may be arranged between the carrier film and the adhesive layer, as is described in more detail below. Generally, the adhesive layer may comprise heat stabilizers, so that it can withstand elevated temperatures of a MV/HV cable for long times, e.g. for longer than a year or for longer than 10 years. 
     The adhesive layer may comprise a single portion or a plurality of separate portions. Separate portions of an adhesive layer comprising a plurality of separate portions may be separated by areas having no adhesive or by areas comprising non-adhesive material. A separate portion of the adhesive layer may be co-extensive with the sensing electrode portion. A separate portion of the adhesive layer may be co-extensive with the first auxiliary electrode portion and/or with a second auxiliary electrode portion. 
     The sensing electrode portion and the first auxiliary electrode portion may be arranged on one side, i.e. on the side of one major surface, of the carrier film. They may be arranged on one major surface of the carrier film, e.g. directly on one major surface of the carrier film. This arrangement facilitates manufacturing of the electrode foil, because the sensing electrode portion and the first auxiliary electrode portion may then be applied, e.g. coated, in one single step. This arrangement also may allow for the sensing electrode portion and the first auxiliary electrode portion to lie in a same plane, when the electrode foil is flat, so that these portions have the same distance to the inner conductor of the power cable when the electrode foil is attached to the exposed insulation layer of the power cable. This, in turn, may under specific circumstances result in a higher accuracy of the voltage sensor. 
     The electrode foil may be conformable such that it can be arranged circumferentially on the exposed insulation layer of a medium or high voltage power cable. The electrode layer and the carrier film and the adhering surface portion may be conformable such that the electrode foil can be arranged circumferentially on the exposed insulation layer of a medium or high voltage power cable. A conformable electrode foil is quicker and easier to attach circumferentially around the circumference of the exposed insulation layer. Circumferential arrangement of the electrode foil and in particular of the sensing electrode portion may increase the sensing accuracy of the voltage sensing. Circumferential arrangement of the first auxiliary electrode portion may be better suitable for shaping the electrical field in the vicinity of the sensing electrode portion and thereby increase the sensing accuracy of the voltage sensing. 
     Alternatively, the electrode foil may be non-conformable, e.g. stiff or rigid. Such a non-conformable electrode foil may be pre-formed such as to have a shape that corresponds to a shape of an outer surface of an insulation layer of a power cable, so that it can be attached on that insulation layer. 
     The sensing electrode portion and the first auxiliary electrode portion may be separated by a non-conductive gap. The sensing electrode portion may have an edge, e.g. a straight edge, adjacent to the gap. The first auxiliary electrode portion may have an edge, e.g. a straight edge, adjacent to the gap, opposite to the edge of the sensing electrode portion. The edge of the sensing electrode portion and the opposite edge of the first auxiliary electrode portion may be separated by the non-conductive gap. The edge of the sensing electrode portion and the opposite edge of the first auxiliary electrode portion may be parallel to each other. Generally, an edge of the sensing electrode portion and an opposite edge of the first auxiliary electrode portion may be separated by a non-conductive gap, and said edge of the sensing electrode portion and said opposite edge of the first auxiliary electrode portion may be parallel to each other. 
     The electrode layer may further comprise a second auxiliary electrode portion, electrically isolated from the sensing electrode portion. The second auxiliary electrode portion may have the same chemical, physical and geometric properties as described above for the first auxiliary electrode portion. A second auxiliary electrode portion may help in shaping the electric field in the vicinity of the sensing electrode portion even better than would be possible with the first auxiliary electrode portion alone. A more symmetric field shape may increase the accuracy of the sensing of the voltage of the inner conductor even further. The presence of a second auxiliary electrode portion may reduce parasitic capacitance and thereby increase the accuracy of the voltage sensor. 
     Generally, and independent of other features, the second auxiliary electrode portion may extend, in one direction in the plane of the electrode layer, further than the full circumference of the exposed insulation layer, so that it can overlap with itself when applied on the insulation layer. 
     Alternatively, an electrode foil according to the invention, comprising a sensing electrode portion and a first auxiliary electrode portion on one side of the carrier film, may comprise a second auxiliary electrode portion which is arranged on the other, i.e. on an opposite, side of the carrier film. In other words, the sensing electrode portion and the first auxiliary electrode portion may be arranged on a first major surface of the carrier film, while the second auxiliary electrode may be arranged on a second, opposed major surface of the carrier film. 
     Independent of other features, the first auxiliary electrode portion and the second auxiliary electrode portion may be electrically connected with each other. This may provide for a particularly efficient field-shaping, and/or may facilitate a simpler electrical setup. Auxiliary electrode portions connected with each other may also reduce parasitic capacitance and thereby increase the accuracy of the voltage sensor. Alternatively, the first auxiliary electrode portion and the second auxiliary electrode portion may be electrically isolated from each other. This may allow the electrode portions being on different electrical potential, which in turn may allow for greater control in field shaping. 
     The second auxiliary electrode portion may be arranged in the same plane as the first auxiliary electrode portion. It may be arranged in the same plane as the first auxiliary electrode portion when the electrode layer is flat. Arrangement of the two auxiliary electrode portions in the same plane may allow for a more efficient shaping of the electric field in the vicinity of the sensing electrode portion, which may result in higher accuracy of the voltage sensing. Also, an electrode foil in which both auxiliary electrode portions are arranged in the same plane may be more cost-efficient to manufacture than other electrode foils, because both portions can be applied in a single manufacturing step, e.g. in a single coating step. 
     The first auxiliary electrode portion, the second auxiliary electrode portion and the sensing electrode portion may be arranged in the same plane when the electrode foil is flat. This may facilitate more cost-effective production, because three portion of the electrode layer can be manufactured, e.g. coated, in a single step. Arranging all three electrode portions in a single plane may also be beneficial for the accuracy of the voltage sensing, because all three electrode portions may then have the same distance to the inner conductor of the power cable, when the electrode foil is applied circumferentially to the insulation layer of the power cable. 
     In an electrode foil having a second auxiliary electrode portion, the sensing electrode portion may be arranged between the first auxiliary electrode portion and the second auxiliary electrode portion. Such an arrangement may allow for a symmetric shaping of the electric field in the vicinity of the sensing electrode portion, which may result in even higher accuracy of the voltage sensing. This arrangement may also allow to reduce parasitic capacitance, which in turn may increase the accuracy of the voltage sensor. 
     The sensing electrode portion and the second auxiliary electrode portion may be separated by a non-conductive gap. The sensing electrode portion may have an edge, e.g. a straight edge, adjacent to the gap. The second auxiliary electrode portion may have an edge, e.g. a straight edge, adjacent to the gap, opposite to the edge of the sensing electrode portion. The edge of the sensing electrode portion and the opposite edge of the second auxiliary electrode portion may be separated by the non-conductive gap. The edge of the sensing electrode portion and the opposite edge of the second auxiliary electrode portion may be parallel to each other. Generally, an edge of the sensing electrode portion and an opposite edge of the second auxiliary electrode portion may be separated by a non-conductive gap, and said edge of the sensing electrode portion and said opposite edge of the second auxiliary electrode portion may be parallel to each other. 
     In an electrode foil according to the invention, comprising an adhesive layer comprising the adhering surface portion, the carrier film may be arranged between the adhesive layer and the electrode layer. This arrangement may facilitate cost-effective manufacturing of the electrode foil, because it may allow to obtain the electrode foil by applying the electrode layer on the non-adhesive side of an existing adhesive foil, e.g. self-adhesive, foil. Also, this arrangement may be beneficial in that it avoids direct contact between the electrode layer and the adhesive layer, thereby preventing any undesired chemical interaction between them. 
     In an electrode foil comprising an adhesive layer comprising the adhering surface portion, in which the carrier film is arranged between the adhesive layer and the electrode layer, a portion of the carrier film may not be covered by the adhesive layer. Such an adhesive-free portion of the carrier film may serve as a flap. A flap may facilitate handling of the electrode foil before, during or after application to the insulation layer of the power cable. It may also make it easier to establish an electrical contact with the electrode layer, because an adhesive-free portion of the carrier film can be folded over such that the electrode layer is easier accessible. 
     Alternatively, in an electrode foil according to the invention, comprising an adhesive layer comprising the adhering surface portion, the electrode layer may be arranged between the carrier film and the adhesive layer. This arrangement may allow the carrier film to act as a protective layer for the electrode layer, when the electrode foil is applied to the insulation layer of the power cable via the adhesive layer. Also, this arrangement may avoid the carrier film to be located between the electrode layer and the inner conductor of the power cable, which in turn may prevent electrical interference of the carrier film with the function of the electrode layer. Specifically, it may reduce or eliminate an electrical influence of the carrier film on the properties of the dielectric of the sensing capacitor, which is formed by the sensing electrode portion as a first sensing electrode and the inner conductor as a second sensing electrode. 
     In an electrode foil in which the electrode layer is arranged between the carrier film and the adhesive layer, a portion of the electrode layer may not be covered by the adhesive layer. In particular, a portion of the sensing electrode portion of the electrode layer may not be covered by the adhesive layer. Such an adhesive-free portion of the electrode layer may serve as a contact portion, which can be folded back such that a part of the electrode layer is accessible and can be electrically contacted, while other portions, not folded back, are protected under the carrier film. 
     An electrode foil according to the invention may comprise a carrier film which comprises the adhering surface portion. The adhering surface portion may thus be a portion of a surface of the carrier film. Such a carrier film may provide advantages in manufacturing cost, because a dedicated adhesive layer may not be required. An adhering surface portion may be a surface portion that can adhere to other surfaces. The adhering surface portion may be a surface portion that can adhere to other surfaces in an adhesiveless manner, e.g. through static forces, or capillary forces. 
     The adhering surface portion may be a self-adhering surface portion. A self-adhering surface portion is a surface portion of the carrier film that can adhere to other surface portions of the carrier film without requiring adhesive, such as, for example, a surface of a self-fusing silicone material. Such self-fusing silicone material can adhere to itself (i.e. one portion of a layer of the material can adhere to another portion of the layer or to a portion of another layer of the material), if applied under tension and/or pressure. By making the adhering surface portion of the carrier film adhere to another surface portion of the carrier film, the two portions of the carrier film can be attached to each other, such that the carrier film can be arranged circumferentially around the insulation layer of the power cable in a belt-like manner, and held in place by friction between the carrier film and the insulation layer. 
     The electrode foil according to the present disclosure may be adapted such, that it comprises a low-voltage capacitor of a capacitive voltage divider circuit, which is operable to sense a voltage of the inner conductor of the cable. This can be achieved by arranging the sensing electrode portion on one major surface of the carrier film, and by arranging a further conductive layer, the “low-voltage capacitor layer” or “LV capacitor layer”, on the opposite major surface of the carrier film in a way that the sensing electrode portion and the low-voltage (LV) capacitor layer can be operated as a capacitor, the “low-voltage capacitor” or “LV capacitor”. For that purpose, at least a portion of the LV capacitor layer may be arranged opposite to at least a portion of the sensing electrode portion. Such an arrangement may allow the LV capacitor layer and the sensing electrode portion to form the LV capacitor, with a portion of the carrier film forming the dielectric, or a portion of the dielectric, of the LV capacitor. The adhering surface portion may then be provided on the sensing electrode portion such that the sensing electrode portion is arranged radially between the adhering surface portion and the carrier film. 
     The resulting electrode foil can be attached, via the adhering surface portion, to an exposed insulation layer of the cable. The inner conductor of the cable and the sensing electrode portion can then form a first capacitor, a high-voltage capacitor, of a capacitive voltage divider, and the sensing electrode portion and the LV capacitor layer can form a second capacitor, the LV capacitor, of the voltage divider. This concept may be very advantageous, because it may avoid the need to provide a separate low-voltage capacitor for the voltage divider, thereby saving cost and increasing reliability. 
     So generally, in one aspect of the invention, the sensing electrode portion and the first auxiliary electrode portion are arranged on a first major surface of the carrier film. A conductive low-voltage capacitor layer may be arranged on an opposed second major surface of the carrier film. At least a portion of the of the low-voltage capacitor layer is arranged opposite to at least a portion of the sensing electrode portion such that the sensing electrode portion and the low-voltage (LV) capacitor layer can be operated as a low-voltage capacitor of a voltage divider for sensing a voltage of the inner conductor. A portion of the carrier film may be operable as dielectric of the low voltage capacitor. The adhering surface portion may be arranged on, or radially outward of, the sensing electrode portion such that the sensing electrode portion is arranged radially between the adhering surface portion and the carrier film. 
     An electrode foil comprising a LV capacitor layer may comprise a carrier layer consisting, for example, of a polyethylene foil or generally of a polymeric film. In such an electrode foil, the sensing electrode portion may comprise a metal layer or a metallized layer. The LV capacitor layer may comprise a metal layer or a metallized layer. The adhering surface portion may comprise a pressure-sensitive adhesive. 
     Alternatively, the carrier film may comprise an electrically insulating self-fusing rubber material. The sensing electrode portion and/or the LV capacitor layer may comprise a conductive self-fusing rubber material. A surface portion of self-fusing rubber material adheres to another surface portion of self-fusing rubber material without requiring dedicated adhesive. 
     In one specific aspect of the invention, the carrier film may be adapted or selected such that with changing temperature, its permittivity changes in the same direction or in the same sense as the permittivity of the insulation layer of the cable, so that the divider ratio of the capacitive voltage divider (formed by the sensing capacitor and the LV capacitor) remains constant, or changes less, when the temperature of the cable changes. 
     The electrode foil comprising a LV capacitor layer as described in the previous paragraphs may be adapted such that the sensing electrode portion can extend around a portion of the circumference of the exposed insulation layer of the cable, or around the full circumference of the exposed insulation layer. 
     Alternatively, the sensing electrode portion may extend, in one direction in the plane of the electrode layer, further than the full circumference of the exposed insulation layer, so that it can overlap with itself when applied on the insulation layer. Generally, the electrode foil may extend, in one direction in the plane of the electrode layer, further than the full circumference of the exposed insulation layer, so that it can overlap with itself when applied on the insulation layer. An electrode foil comprising a low-voltage capacitor layer may be wound, in a first turn, around the exposed insulation layer of the cable and, in a further turn, or in a plurality of further turns, around the first turn and upon itself. An electrode foil comprising a low-voltage capacitor layer may comprise an electrically insulating cover layer arranged on, or radially outward of, the low voltage capacitor layer, so that the low voltage capacitor layer is arranged radially between the cover layer and the carrier film. This may allow winding the electrode foil around the exposed insulation layer and over itself without creating a short between the sensing electrode portion and the low-voltage capacitor layer. 
     The present invention also provides the use of a foil comprising 
     a) an electrode layer comprising two electrode portions, electrically isolated from each other;
 
b) a non-conductive carrier film carrying the two electrode portions, and
 
c) an adhering surface portion
 
as an electrode assembly of a voltage sensor, for arrangement on an exposed insulation layer of a medium or high voltage power cable.
 
     This new use of such a foil facilitates the integration of a sensing electrode (for sensing the voltage of the inner conductor of the cable in a capacitive manner) and of a first auxiliary electrode (for shaping the electrical field in the vicinity of the sensing electrode) into a MV/HV power cable. One of the electrode portions of the foil is operable as one electrode of a sensing capacitor for sensing a voltage of the inner conductor of the MV/HV power cable, the other electrode of the sensing capacitor being the inner conductor of the cable. The second electrode portion on the foil is operable as a field-shaping or guard electrode. The second electrode portion may be connected to electrical ground and/or to the grounding layer or shielding layer of the HV/MV power cable. The two electrode portions of the foil may be arranged in the same plane when the foil is flat. 
     The present invention also provides a medium or high voltage power cable comprising an inner conductor and an insulation layer surrounding the inner conductor, further comprising an electrode foil as described above, wherein the electrode foil is attached to the insulation layer. Such a cable may be prepared for quick and easy connection of electric or electronic components, which in conjunction with the electrode foil form a highly accurate capacitive voltage sensor for sensing the voltage of the inner conductor of the cable versus electrical ground. 
    
    
     
       The invention will now be described in more detail with reference to the following Figures exemplifying particular embodiments of the invention: 
         FIG. 1  Perspective view of a portion of a stripped power cable with two electrode foils according to the present invention; 
         FIG. 2  Perspective view of a third electrode foil according to the present invention; 
         FIG. 3  Perspective view of a fourth electrode foil according to the present invention; 
         FIG. 4  Perspective view of a fifth electrode foil according to the present invention; 
         FIG. 5  Perspective view of a sixth electrode foil according to the present invention, applied on a stripped power cable; 
         FIG. 6  Cross-section of the sixth electrode foil and the power cable; 
         FIG. 7  Perspective view of a seventh electrode foil according to the invention; 
         FIG. 8  Perspective view of an eighth electrode foil according to the invention, not comprising an adhesive layer; 
         FIG. 9  Cross-section of the eighth electrode foil applied on a stripped power cable; and 
         FIG. 10  Cross-section of a ninth electrode foil according to the invention, comprising a low-voltage capacitor layer, applied on a stripped power cable. 
     
    
    
     In the Figures, like elements are provided with the same reference numbers. Some dimensions are exaggerated for greater clarity. Some elements of the Figures are not to scale. 
       FIG. 1  shows, in a perspective view, a medium-voltage power cable  10 . The cable  10  comprises an inner conductor  20 , which has a diameter of about 3 cm, and which is surrounded by a main insulation layer  30 . The cable  10  is shown partially stripped, so that the main insulation layer  30  is exposed and accessible. In an unstripped portion of the cable  10 , the cable sheath  40  is visible. The cable  10  defines axial, radial and circumferential directions. 
     Two electrode foils  1 ,  2  according to the invention are attached to the exposed main insulation layer  30 , namely to its radially outer surface  31 . Each electrode foil  1 ,  2  provides two electrodes (not visible in  FIG. 1 ) for sensing, in a capacitive manner, the voltage of the inner conductor  20  versus ground and shaping the electrical field. The construction of the electrode foils  1 ,  2  will be described below. Both electrode foils  1 ,  2  are attached along the circumference of the main insulation layer  30 , i.e. they are arranged circumferentially on the main insulation layer  30 . The first electrode foil  1  extends along almost the full circumference of the main insulation layer  30 , leaving only a small gap  50  between its opposed ends. The second electrode foil  2  extends only to a smaller portion of the circumference of the main insulation layer  30 . However, because it follows the circumference of the main insulation layer  30 , it is also considered to be arranged circumferentially with respect to the main insulation layer  30 . 
     The first electrode foil  1  is operable as part of a capacitive voltage sensor (not shown), just as the second electrode foil  2  is operable as part of a capacitive voltage sensor (not shown). In reality, however, two voltage sensors will rarely be installed on the same cable  10  at such a short distance from each other. 
       FIG. 2  shows, in a perspective view, a further, third electrode foil  3  according to the present invention. It can be applied to the main insulation layer  30  of a cable  10  just like the electrode foils  1 ,  2  shown in  FIG. 1 . The electrode foil  3  is shown with a slight curvature, so that it becomes evident which side of it will be facing radially inward towards the inner conductor  20 , and which side will face radially outward, when the electrode foil  3  is applied on the main insulation layer  30 . The actual curvature may be different when the electrode foil  3  is applied to the main insulation layer  30  of a specific cable  10 . The electrode foil  3  comprises three layers: an electrode layer  60 , a carrier film  70 , and an adhesive layer  80 . The electrode foil  3 , however, is conformable and can lie flat. 
     The electrode layer  60  is electrically conductive and is applied directly on the radially outer surface of the carrier film  70 . It has two distinct portions, which are arranged side by side to each other, in the same plane: a sensing electrode portion  100  and a first auxiliary electrode portion  200 . The sensing electrode portion  100  and the first auxiliary electrode portion  200  are electrically isolated from each other. They are made by coating respective portions of the carrier film  70  with a thin copper layer. A non-conductive gap  150  separates the sensing electrode portion  100  and the first auxiliary electrode portion  200 . 
     The carrier film  70  is continuous. It is electrically non-conductive and mechanically conformable in order to allow attachment of the electrode foil  3  around the circumference of the main insulation layer  30  of a power cable  10  as shown in  FIG. 1 . 
     The adhesive layer  80  covers the entire radially inner major surface of the carrier film  70 . It is thus arranged on the side of the carrier film  70  which is opposite to the side on which the electrode layer  60  is arranged. The exposed surface (the lower surface, in  FIG. 2 ) of the adhesive layer  80  provides an adhering surface  90 , by which the electrode foil  3  can be attached to the outer surface  31  of the main insulation layer  30 . The adhesive layer  80  comprises a pressure-sensitive adhesive, suitable for attaching the electrode foil  3  to the outer surface  31  of the main insulation layer  30 . Before use, the adhering surface  90  is protected by a liner (not shown). 
     Once the electrode foil  3  is applied on the main insulation layer  30 , the sensing electrode portion  100  and the inner conductor  20  of the power cable  10  can be operated as electrodes of a sensing capacitor for sensing the voltage of the inner conductor  20  versus electrical ground. A portion of the main insulation layer  30  and a portion of the adhesive layer  80  form the dielectric of this sensing capacitor. The sensing capacitor may be operated as the high-voltage capacitor in a capacitive voltage divider, which further comprises a low-voltage capacitor, electrically arranged between the high-voltage capacitor and ground. A voltage picked up between the high-voltage capacitor and the low-voltage capacitor is indicative of the voltage of the inner conductor  20  versus electrical ground. 
     In order to electrically connect the high-voltage capacitor to the low-voltage capacitor, an electrical contact to the sensing electrode portion  100  of the electrode layer  60  is required. Such a contact can be established, for example, by a pressure contact to the sensing electrode portion  100 , or by glueing a contact to the sensing electrode portion  100  with an electrically conductive glue, or by soldering a contact to the sensing electrode portion  100 . 
     Since the sensing electrode portion  100  is not on electrical ground when the cable  10  is in use, the shape of the electric field in the vicinity of the sensing electrode portion  100  is not as cylindrically-symmetric as it would be in a portion of the cable  10  which has a cylindrical grounding electrode layer surrounding the inner conductor  20  and the main insulation layer  30 . In general, however, the more cylindrically-symmetric the electric field in the vicinity of the sensing electrode portion  100 , the more accurate the voltage sensing. In order to shape the electric field such as to be more cylindrically-symmetric in the vicinity of the sensing electrode portion  100 , a field-shaping or guard electrode is desirable. The first auxiliary electrode portion  200  can be operated as such a field-shaping electrode. It may, for example, be electrically connected to ground, either by having a grounded semiconductive layer of the cable  10  be in contact with the first auxiliary electrode portion  200 , or by a separate grounding wire, in electrical contact with the first auxiliary electrode portion  200 . The electrical field will be more cylindrically-symmetric if the first auxiliary electrode portion  200  surrounds the inner conductor  20  around a full circumference. In that regard, the electric field will be more cylindrically-symmetric in the electrode foil  1  than in the electrode foil  2 , shown in  FIG. 1 . 
     While the first auxiliary electrode portion  200  enhances the symmetry of the electric field on one side of the sensing electrode portion  100  on the electrode foil shown in  FIG. 2 , an even more symmetric field can be obtained if a further, second auxiliary electrode portion  300  is arranged on the electrode foil. A part of such an electrode foil  4  according to the invention is shown, in a perspective view, in  FIG. 3 . While the adhesive layer  80  with the adhering surface  90  and the carrier film  70  are identical to the corresponding layers  80 ,  70  of the electrode foil  3  shown in  FIG. 2 , the electrode layer  60  comprises the sensing electrode portion  100 , the first auxiliary electrode portion  200  and a second auxiliary electrode portion  300 . The second auxiliary electrode portion  300  lies in the same plane as the first auxiliary electrode portion  200  and the sensing electrode portion  100  (when the electrode foil  4  is flat). 
     The second auxiliary electrode portion  300  is physically equal to the first auxiliary electrode portion  200 . Axially, it is arranged such that the sensing electrode portion  100  is located symmetrically between the first auxiliary electrode portion  200  and the second auxiliary electrode portion  300 . Two non-conductive gaps  150 ,  151  of equal width separate the sensing electrode portion  100  from the first auxiliary electrode portion  200  and the second auxiliary electrode portion  300 , respectively. This arrangement results in a very symmetric electric field in the vicinity of the sensing electrode portion  100 , when the first auxiliary electrode portion  200  and the second auxiliary electrode portion  300  are put on electrical ground. In a specific embodiment, the first auxiliary electrode portion  200  and the second auxiliary electrode portion  300  are electrically connected with each other, e.g. outside the electrode foil  4 . 
     In the electrode foils  3  and  4 , adhesive layer  80  and electrode layer  60  were arranged on different sides, i.e. on different major surfaces, of the carrier film  70 . It may be considered to arrange the electrode layer  60  and the adhesive layer  80  on one side of the carrier film  70 , that is, over one and the same major surface of the carrier film  70 . An example of such an electrode film  5  according to the invention is shown in  FIG. 4 , in a perspective view. In the thickness direction (the direction of a normal on a major surface of the electrode foil  5 ) of the electrode foil  5 , the electrode layer  60  is arranged between the carrier film  70  and the adhesive layer  80 , which comprises the adhering surface  90 . Except for their arrangement relative to each other, the adhesive layer  80  and the carrier film  70  are similar to the corresponding elements in  FIG. 3 . The electrode layer  60  comprises three electrode portions: a sensing electrode portion  100 , a first auxiliary electrode portion  200  and a second auxiliary electrode portion  300 . The sensing electrode portion  100  is arranged between the first auxiliary electrode portion  200  and the second auxiliary electrode portion  300 . All three electrode portions  100 ,  200 ,  300  lie in the same plane, i.e. they lie in the same plane when the electrode foil  5  is flat. The auxiliary electrode portions  200 ,  300  are separated from the sensing electrode portion  100  by respective gaps  150 ,  151 . The adhesive layer  80  is continuous and extends over the three electrode portions  100 ,  200 ,  300  and the gaps  150 ,  151 . 
     When the electrode foil  5  is attached, via the adhesive layer  80 , circumferentially to a cylindrical main insulation layer  30  of a power cable  10 , all three electrode portions  100 ,  200 ,  300  have about the same distance to the inner conductor  20  of the cable  10 . 
     In all embodiments described so far, when attached, the adhesive layer  80  is the radially innermost layer of the electrode foils  1 ,  2 ,  3 ,  4 ,  5 . In the embodiment shown in  FIG. 4  the carrier film  70  constitutes the radially outermost layer and thereby provides mechanical protection for the radially inner layers, in particular for the electrode layer  60 . Generally, it should be noted, however, that the carrier film  70  and the electrode layer  60  do not need to be directly adjacent to each other. One or more further layers may be arranged between them in the thickness direction, for example, a layer of adhesive. Similarly, the adhesive layer  80  and the electrode layer  60  do not need to be directly adjacent to each other. One or more further layers may be arranged between them in the thickness direction. 
     While the arrangement of the carrier film  70  radially outwards of the electrode layer  60  in the electrode foil  5  in  FIG. 4  is advantageous in that the carrier film  70  provides mechanical protection, an electrical contact to the sensing electrode portion  100  is more difficult to establish than in embodiments where the electrode layer  60  is the outermost layer, when the electrode foil is attached to the main insulation layer  30  of a cable  10 . This is because the carrier film  70  covers the entire surface of the sensing electrode portion  100 . 
       FIG. 5  shows, in a perspective view, a further electrode foil  6  according to the invention, which addresses this difficulty. The electrode foil  6  has the same sequence of layers as the electrode foil  5  shown in  FIG. 4 , i.e. an electrode layer  60  is arranged between a carrier film  71  and an adhesive layer  80  comprising an adhering surface  90 . The carrier film  71  is drawn transparent, so that the electrode portions are visible, namely a sensing electrode portion  100 , arranged between, and in the same plane as, a first auxiliary electrode portion  200  and a second auxiliary electrode portion  300 . The electrode foil  6  is attached circumferentially, via the adhesive layer  80 , on the outer surface  31  of a main insulation layer  30 , which surrounds an inner conductor  20  of a medium-voltage power cable  10 . 
     The electrode foil  6  is conformable and is shaped such as to form a flap  160 . In the area of the flap  160 , the sensing electrode portion  100  forms a protrusion and extends further, in circumferential direction, than the first auxiliary electrode portion  200  and also further than the second auxiliary electrode portion  300 . The sensing electrode portion  100  of the electrode layer  60  is arranged on the carrier film  70 , also in the portion forming the flap  160 . However, the adhesive layer  80  does not extend into the area of the flap  160 . There is thus no adhesive on the flap  160  that might attach the flap  160  to the main insulation layer  30 . 
     This is shown in more detail in  FIG. 6 , which shows the electrode foil  6  of  FIG. 5 , applied on the main insulation layer  30 , in cross section through the sensing electrode portion  100 . The adhesive layer  80  does not cover the entire surface of the electrode layer  60 . In particular, the adhesive layer  80  does not cover the entire surface of the sensing electrode portion  100 . The absence of the adhesive layer  80  on the flap  160  prevents the flap  160  from sticking to the main insulation layer  30  and allows the flap  160  to be easily folded back, whereby the electrode layer  60  becomes accessible, in particular, a part of the sensing electrode portion  100  of the electrode layer  60  becomes accessible. The accessible part of the sensing electrode portion  100  facilitates establishing an electrical contact to the sensing electrode portion  100 , e.g. by a surface contact, by soldering, or through an electrically conductive adhesive. 
     In an electrode foil, in which the electrode layer  60  is arranged between the adhesive layer  80  and the carrier film  70 , not only the sensing electrode portion  100  on the carrier film  70  may form a flap  160 , but also the first auxiliary electrode portion  200  on the carrier film  70  may form a flap (not shown) which is free of adhesive, i.e. onto which the adhesive layer  80  does not extend. Similarly, the second auxiliary electrode portion  200  on the carrier film  70  may form a flap (not shown) onto which the adhesive layer  80  does not extend. Such flaps would allow the corresponding parts of the electrode foil  6  to be folded back and thereby to be electrically contacted more easily. Flaps may allow the first and/or the second auxiliary electrode portions  200 ,  300  to be more easily electrically connectable to a conductive or semiconductive layer  170  (shown in  FIG. 5 ) of the cable  10 , which is mostly held on electrical ground. 
     Alternatively to an adhesive-less flap as described above, it is contemplated that the adhesive layer  80  may be present on a flap  160 , but be deactivated and made non-adhesive, e.g. by covering it with a non-adhesive material, before applying the electrode foil to a main insulation layer  30  of a cable  10 . 
       FIG. 7  is a perspective view of a further, seventh electrode foil  7  according to the invention. Its sequence of layers is identical to the sequence of layers of the electrode foil  3  shown in  FIG. 2 , i.e. the adhesive layer  80  is the radially innermost layer, once the electrode foil  7  is applied circumferentially on the main insulation layer of a power cable. The carrier film  70  is arranged between the adhesive layer  80  and the electrode layer  60 . The electrode layer  60  comprises a sensing electrode portion  100  and a first auxiliary electrode portion  200 , electrically isolated from each other. The first auxiliary electrode portion  200  is arranged on a surface of the carrier film  70  such that it surrounds the sensing electrode portion  100 . This arrangement is useful for rendering the electric field between the sensing electrode portion  100  and the inner conductor of the power cable more symmetric. This arrangement also reduces parasitic capacitances in the vicinity of the sensing electrode portion  100  and thereby makes the voltage measurement more accurate. 
     The concept of arranging electrodes for a power cable sensor on an electrode foil according to the invention may be furthered. In addition to a sensing electrode portion  100 , a first auxiliary electrode portion  200  and a second auxiliary electrode portion  300 , an electrode foil according to the invention may comprise a fourth electrode portion, e.g. for forming a third auxiliary electrode portion. Any of the auxiliary electrode portions may serve a particular purposes, like for example field shaping, backup sensing electrode, or energy harvesting. Even five or more electrode portions may be provided. 
       FIG. 8  is a perspective view of an eighth electrode foil  8  according to the invention. It comprises an electrode layer  60  and a carrier film  70 . The carrier film  70  carries a sensing electrode portion  100 , a first auxiliary electrode portion  200  and a third auxiliary electrode portion  300  of the electrode layer  60  on one of its major surfaces. The electrode portions  100 ,  200 ,  300  have the same function as the corresponding electrode portions described before. Specifically, the sensing electrode portion  100  is operable as an electrode of a high-voltage capacitor in a capacitive voltage divider for sensing the voltage of the inner conductor  20  of the power cable. Unlike the previously shown electrode foils, this electrode foil  8  does not comprise an adhesive layer. Rather, the carrier film  70  comprises a self-fusing silicone material, which has the ability to adhere to itself when applied appropriately. Specifically, the material is self-adhering in that its upper surface  90  (in  FIG. 8 ) can adhere to its lower surface. The upper surface of the carrier film  70  thus comprises an adhering surface portion  90  which is suitable for attaching the electrode foil  8  circumferentially to an exposed insulation layer of a power cable. The electrode portions  100 ,  200 ,  300  comprise an electrically conductive, self-fusing silicone material, applied as a paint on a major surface of the carrier film  70 . 
     This is shown in  FIG. 9 , in which the eighth electrode foil  8  is shown, in cross section through the sensing electrode portion  100 , attached circumferentially to an insulation layer  30  of a power cable. The upper surface  90  of the electrode foil  8  forms the radially inner surface of the electrode foil  8 , when the electrode foil  8  is attached circumferentially on the insulation layer  30 . Where the electrode foil  8  overlaps with itself, the adhering surface portion  90  on the radially inner surface of the carrier film  70  adheres to the radially outer surface of the carrier film  70 , so that the electrode foil  8  forms a closed sleeve around the insulation layer  30 . The electrode foil  8  is applied under tension in circumferential direction, so that after arrangement around the insulation layer  30 , the electrode foil  8  is held in place on the insulation layer  30  by friction. The electrode foil  8  does not actually adhere to the insulation layer  30 . 
       FIG. 10  is a cross section, through the sensing electrode portion  100 , of a ninth electrode foil  9  according to the present disclosure, attached circumferentially to an insulation layer  30  of a power cable. The ninth electrode foil  9  comprises a low-voltage capacitor layer  210  (or LV capacitor layer  210 ) as described above. The LV capacitor layer  210  is a metallized layer, coated on the radially outer major surface of the polyethylene carrier film  70 . The opposed major surface of the carrier film  70  supports the electrode layer  60  comprising the sensing electrode portion  100 . An adhering surface portion is formed by an adhesive layer  80 , arranged at the circumferential end portion of the electrode foil  9 , by which the electrode foil  9  is fixed upon itself and thereby on the exposed insulation layer  30 . 
     The electrode foil  9  is wound, in a first turn, around the exposed insulation layer  30  of the cable and, in a further turn, around the first turn and upon itself. The electrode foil  9  comprises as its radially outermost layer (in  FIG. 10 ) an electrically insulating cover layer  220 . The presence of the cover layer  220  allows winding the electrode foil  9  over itself without creating a short between the electrode layer  60 , specifically between the sensing electrode portion  100 , and the LV capacitor layer  210 . 
     The first capacitor, i.e. the high-voltage capacitor, indicated as electrical symbol C 1  in  FIG. 10 , of the voltage divider for sensing the voltage of the inner conductor  20  is formed by the inner conductor  20  and the sensing electrode portion  100  of the electrode layer  60 , the cable insulation layer  30  forming its dielectric. Only the first turn or winding of the electrode foil  9  is relevant for determining the capacitance of the high-voltage capacitor C 1 . The second capacitor of the voltage divider, the low-voltage capacitor, indicated by electrical symbols C 2  and C 3 , is formed between the sensing electrode portion  100  (i.e. the radially inner conductive layer, in the Figure, of the electrode foil  9 ) and the LV capacitor layer  210  (i.e. the radially outer conductive layer of the electrode foil  9 ). The carrier film  70  forms the dielectric of that LV capacitor. The capacitance of the LV capacitor increases with the circumferential length of the electrode foil  9 , assuming constant width (the width being an extension in a direction perpendicular to the plane of the drawing). Greater length, i.e. more turns, results in a greater capacitance. This is why two electrical symbols C 2 , C 3  were drawn. Non-wound portions of the electrode foil  9  contribute to the capacitance of the LV capacitor, too. Hence, by selecting a suitable length and a suitable width of the electrode foil  9 , the capacitance of the LV capacitor and thereby the divider ratio of the voltage divider can be adjusted to a desired value.