Abstract
Electrode foil (6) comprising a) an electrode layer (60) comprising—a sensing electrode portion (100), operable as an electrode of a sensing capacitor for sensing a voltage of an inner conductor (20) of a medium or high voltage power cable (10),—a first auxiliary electrode portion (200), electrically isolated from the sensing electrode portion. The electrode foil further comprises b) a non-conductive carrier film (71) carrying both the sensing electrode portion and the first auxiliary electrode portion, and c) an adhering surface portion (90) for attaching the electrode foil to an exposed insulation layer (30) of the power cable.
Claims
1. 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 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.
2. Electrode foil according to claim 1, wherein the sensing electrode portion and the first auxiliary electrode portion are arranged on one major surface of the carrier film.
3. Electrode foil according to claim 1, wherein the electrode foil is conformable such that it can be attached circumferentially to the exposed insulation layer of the medium or high voltage power cable.
4. Electrode foil according to claim 1, wherein an edge of the sensing electrode portion and an opposite edge of the first auxiliary electrode portion are separated by a non-conductive gap, and wherein said edge of the sensing electrode portion and said opposite edge of the first auxiliary electrode portion are parallel to each other.
5. Electrode foil according to claim 1, wherein the electrode layer further comprises a second auxiliary electrode portion, electrically isolated from the sensing electrode portion.
6. Electrode foil according to claim 5, wherein the second auxiliary electrode portion is arranged in the same plane as the first auxiliary electrode portion.
7. Electrode foil according to claim 6, wherein the sensing electrode portion is arranged between the first auxiliary electrode portion and the second auxiliary electrode portion.
8. Electrode foil according to claim 1, further comprising an adhesive layer comprising the adhering surface portion.
9. Electrode foil according to claim 8, wherein the carrier film is arranged between the adhesive layer and the electrode layer.
10. Electrode foil according to claim 8, wherein a portion of the carrier film is not covered by the adhesive layer.
11. Electrode foil according to claim 8, wherein the electrode layer is arranged between the carrier film and the adhesive layer.
12. Electrode foil according to claim 11, wherein a portion of the electrode layer is not covered by the adhesive layer.
13. Electrode foil according to claim 1, wherein the sensing electrode portion and the first auxiliary electrode portion are arranged on a first major surface of the carrier film, wherein a conductive low-voltage capacitor layer is arranged on an opposed second major surface of the carrier film, and wherein at least a portion 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 capacitor layer can be operated as a low-voltage capacitor of a voltage divider for sensing a voltage of the inner conductor.
14. (canceled)
15. Medium or high voltage power cable comprising an inner conductor and an insulation layer surrounding the inner conductor, further comprising an electrode foil according to claim 1, wherein the electrode foil is attached to the insulation layer.
Description
[0053] The invention will now be described in more detail with reference to the following Figures exemplifying particular embodiments of the invention:
[0054] FIG. 1 Perspective view of a portion of a stripped power cable with two electrode foils according to the present invention;
[0055] FIG. 2 Perspective view of a third electrode foil according to the present invention;
[0056] FIG. 3 Perspective view of a fourth electrode foil according to the present invention;
[0057] FIG. 4 Perspective view of a fifth electrode foil according to the present invention;
[0058] FIG. 5 Perspective view of a sixth electrode foil according to the present invention, applied on a stripped power cable;
[0059] FIG. 6 Cross-section of the sixth electrode foil and the power cable;
[0060] FIG. 7 Perspective view of a seventh electrode foil according to the invention;
[0061] FIG. 8 Perspective view of an eighth electrode foil according to the invention, not comprising an adhesive layer;
[0062] FIG. 9 Cross-section of the eighth electrode foil applied on a stripped power cable; and
[0063] 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.
[0064] 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.
[0065] 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.
[0066] 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.
[0067] 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.
[0068] 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.
[0069] 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.
[0070] 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.
[0071] 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).
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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).
[0076] 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.
[0077] 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.
[0078] 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.
[0079] 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.
[0080] 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.
[0081] 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.
[0082] 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.
[0083] 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.
[0084] 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.
[0085] 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.
[0086] 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.
[0087] 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.
[0088] 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.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] The first capacitor, i.e. the high-voltage capacitor, indicated as electrical symbol C1 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 C1. The second capacitor of the voltage divider, the low-voltage capacitor, indicated by electrical symbols C2 and C3, 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 C2, C3 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.
