Capacitor, Capacitive Voltage Sensor and Method for Manufacturing a Capacitor

Abstract

A capacitor comprises an electrically conductive cylinder, an electrically conductive or semi-conductive cylindrical shell or shell segment arranged concentrically around the electrically conductive cylinder, and a dielectric arranged between the electrically conductive cylinder and the electrically conductive or semi-conductive cylindrical shell or shell segment. The dielectric comprises at least one dielectric layer having a positive thermal coefficient of relative permittivity, and at least one compensation dielectric layer having a negative thermal coefficient of relative permittivity. The thermal coefficient of relative permittivity is thereby selected such that the capacitance value of the capacitor is constant within a stability margin over a predefined temperature interval.

Claims

1. Capacitor, comprising: an electrically conductive cylinder; an electrically conductive or semi-conductive cylindrical shell or shell segment arranged concentrically around the electrically conductive cylinder; and a dielectric, the dielectric comprising: at least one dielectric layer having a positive thermal coefficient of relative permittivity; and at least one compensation dielectric layer having a negative thermal coefficient of relative permittivity, wherein the dielectric layers are arranged between the electrically conductive cylinder and the electrically conductive or semi-conductive cylindrical shell or shell segment and wherein the compensation dielectric layer is chosen such that it compensates for any changes in relative permittivity of the at least one dielectric layer.

2. Capacitor according to claim 1, wherein the at least one compensation dielectric layer is sandwiched between the at least one dielectric layer and the electrically conductive or semi-conductive cylindrical shell or shell segment.

3. Capacitor according to claim 1, wherein the electrically conductive or semi-conductive cylindrical shell or shell segment forms a ring structure having a recessed trench extending circumferentially in its inner wall.

4. Capacitor according to claim 3, wherein the at least one compensation dielectric layer comprises a sealant embedded within the recessed trench of the electrically conductive or semi-conductive cylindrical shell or shell segment.

5. Capacitor according to claim 1, wherein the at least one compensation dielectric layer is sandwiched between the at least one dielectric layer and the electrically conductive cylinder.

6. Capacitor according to claim 1, further comprising: additional material applied to the dielectric adjoining to the electrically conductive or semi-conductive cylindrical shell or shell segment.

7. Capacitor according to claim 6, wherein the additional material has an opposite thermal coefficient of relative permittivity than the dielectric layer.

8. Capacitor according to claim 6, wherein the additional material is at least partially applied on top of or adjacent to the electrically conductive or semi-conductive cylindrical shell or shell segment.

9. Capacitor according to claim 1, wherein the electrically conductive cylinder forms part of a high-voltage or medium-voltage power conductor in a high-voltage or medium-voltage power network.

10. Capacitor according to claim 1, wherein the electrically conductive or semi-conductive cylindrical shell or shell segment comprises a mechanically rigid metal component, in particular an aluminium, steel and/or copper annulus.

11. Capacitor according to claim 1, wherein the at least one dielectric layer comprises at least one of a resin, an epoxy and/or a polyurethane.

12. Capacitor according to claim 1, wherein the at least one dielectric layer may be filled with high permittivity materials, such as for example titanate such as barium titanate or conductive carbon black powder dispersed in the polymeric material.

13. Capacitor according to claim 1, wherein the at least one compensation dielectric layer comprises plastic or rubber material like for example silicone, ethylene propylene diene monomer (EPDM), high module ethylene propylene rubber (HEPR), polyethylene (PE) and/or polypropylene (PP).

14. Capacitive voltage sensor, comprising: a capacitor according to claim 1 as voltage sensing capacitor; a reference impedance coupled in series between the voltage sensing capacitor and a reference potential; and a voltage measurement circuit configured to measure a voltage drop across the reference impedance.

15. Method for manufacturing a capacitor, comprising the steps of: applying a dielectric to an electrically conductive cylinder; and arranging an electrically conductive or semi-conductive cylindrical shell or shell segment concentrically around the electrically conductive cylinder over the dielectric, wherein the dielectric comprises at least one dielectric layer having a positive thermal coefficient of relative permittivity and at least one compensation dielectric layer having a negative thermal coefficient of relative permittivity.

16. Method according to claim 15, wherein applying the dielectric comprises: moulding a dielectric layer over the electrically conductive cylinder; and applying a compensation dielectric layer over the moulded dielectric layer.

17. Method according to claim 15, wherein arranging the electrically conductive or semi-conductive cylindrical shell or shell segment comprises: applying an electrically conductive ink or coating on the dielectric layer; or cold-shrink tubing an electrically conductive tube on the dielectric layer.

Description

[0033] The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles of the invention. Other embodiments of the present invention and many of the intended advantages of the present invention will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts.

[0034] FIG. 1 schematically illustrates a perspective view of a voltage sensing capacitor according to an embodiment of the invention.

[0035] FIG. 2 schematically illustrates an axial cross section view through the voltage sensing capacitor of FIG. 1 along the line A-A according to a further embodiment of the invention.

[0036] FIG. 3 schematically illustrates a radial cross section view through the voltage sensing capacitor of FIG. 1 along the line B-B according to a further embodiment of the invention.

[0037] FIG. 4 schematically illustrates a radial cross section view through a modified voltage sensing capacitor according to another embodiment of the invention.

[0038] FIG. 5 schematically illustrates a radial cross section view through a modified voltage sensing capacitor according to an even further embodiment of the invention.

[0039] FIG. 6 schematically illustrates a radial cross section view through a modified voltage sensing capacitor according to yet another embodiment of the invention.

[0040] FIG. 7 schematically illustrates a perspective view of a sensing electrode of a voltage sensing capacitor with an embedded compensation dielectric material according to yet another embodiment of the invention.

[0041] FIG. 8 schematically illustrates a cut-away view of the sensing electrode of FIG. 7.

[0042] FIG. 9 shows a measurement diagram illustrating the thermal dependence of relative permittivity of a compensation dielectric layer according to yet another embodiment of the invention.

[0043] FIG. 10 shows a measurement diagram illustrating the thermal dependence of capacitance value of a voltage sensing capacitor according to a further embodiment of the invention.

[0044] FIG. 11 schematically illustrates a perspective view of a further voltage sensing capacitor according to a further embodiment of the invention.

[0045] FIG. 12 schematically illustrates a block diagram of capacitive voltage sensor circuitry according to another embodiment of the invention.

[0046] FIG. 13 schematically illustrates stages of a method for manufacturing a voltage sensing capacitor according to another embodiment of the invention.

[0047] In the figures, like reference numerals denote like or functionally like components, unless indicated otherwise. Any directional terminology like top, bottom, left, right, above, below, horizontal, vertical, back, front, and similar terms are merely used for explanatory purposes and are not intended to delimit the embodiments to the specific arrangements as shown in the drawings.

[0048] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Generally, this application is intended to cover any adaptations or variations of the specific embodiments discussed herein.

[0049] A perspective view of a voltage sensing capacitor 10 is schematically illustrated in FIG. 1. The two sectional view along the lines A-A and B-B of the voltage sensing capacitor 10 of FIG. 1 are shown in FIGS. 2 and 3, respectively. The voltage sensing capacitor 10 is generally of cylindrical or tubular shape having a central axis denoted with the reference sign X. Of course, the cylindrical shape does not necessarily need to be straight, but instead bent or curved cylindrical shapes may be equally possible. Particularly, when used for sensing a voltage of a current carrying conductor, the voltage sensing capacitor 10 may be manufactured from generally flexible or elastic materials, allowing the capacitor 10 to be bent at its central axis X to deviate from the purely straight cylindrical shape.

[0050] Along the central axis X an electrically conductive cylinder 2 is arranged to act as an inner electrode of the capacitor 10. The electrically conductive cylinder 2 may particularly form a part of a high-voltage or medium-voltage power conductor in a high-voltage or medium-voltage power network. An electrically conductive or semi-conductive cylindrical shell 1 is arranged concentrically around the electrically conductive cylinder 2, being spaced apart from the electrically conductive cylinder 2. The electrically conductive or semi-conductive cylindrical shell 1 acts as outer electrode or counter-electrode for the inner electrode of the capacitor 10. In between the two electrodes 1 and 2, a dielectric 3 is arranged.

[0051] As can be seen in the drawings of FIGS. 2 and 3, the dielectric 3 generally includes a layered arrangement of at least one dielectric layer 3a and at least one compensation dielectric layer 3b. The dielectric layer 3a and the compensation dielectric layer 3b are stacked on top of each other. Although only two layers 3a and 3b are shown in the drawings, it may also be possible to provide for multiple layers 3a and 3b alternately stacked on top of each other. It is also possible the stack several dielectric layers 3a on top of each other and follow with several compensation dielectric layers 3b. The dielectric layer(s) 3a may have a positive thermal coefficient of relative permittivity. Therefore, upon an increase in temperature, the relative permittivity of the dielectric layer(s) 3a will increase and cause the capacitance value of the capacitor 10 to rise concomitantly.

[0052] The compensation dielectric layer(s) 3b are therefore chosen to have a negative thermal coefficient of relative permittivity. The negative thermal coefficient of relative permittivity of the compensation dielectric layer(s) 3b is selected such that the capacitance value of the capacitor is constant within a stability margin over a predefined temperature interval. In other words, the negative thermal coefficient of relative permittivity of the compensation dielectric layer(s) 3b is used to compensate for the positive thermal coefficient of relative permittivity of the dielectric layer(s) 3a so that the effective relative permittivity of the dielectric 3 as a whole will remain stable with a change in temperature or will slightly change with changing temperature within the stability margins mentioned above. Another option is, that the dielectric layer(s) 3a has a negative thermal coefficient of relative permittivity and the compensation dielectric layer(s) layer 3b has a positive thermal coefficient of relative permittivity. The temperature compensation works equivalently.

[0053] The stability margin may be predefined according to quality standards of the voltage sensing capacitor when used in a capacitive voltage sensor. Some quality standards, e.g. IEC 60044-7 (1999), define a stability margin equal to or less than +/3.0% of the capacitance value of the capacitor 10, alternatively a stability margin equal to or less than +/0.5% of the capacitance value of the capacitor 10, or alternatively a stability margin equal to or less than +/0.2% of the capacitance value of the capacitor. The material, dimensions and positioning of the dielectric layer 3a and the compensation dielectric layer 3b may thus be selected to be able to keep the capacitance value within the desired stability margin.

[0054] The temperature interval may be equally predefined by quality standards, e.g. IEC 60044-7 (1999) or the intended application. In some applications, capacitive voltage sensors are employed in warmer regions of the earth, such as for example Northern Africa or Central America. In those regions, the temperature intervals of interest may be higher than in colder regions of the earth, such as for example Scandinavia or Canada. The predefined temperature interval may for example lie between 40 C. and +70 C. or between 40 C. and +40 C. or between 25 C. and +40 C.

[0055] FIGS. 4, 5 and 6 each schematically illustrate modified voltage sensing capacitors 10 in radial cross section view similar to the view in FIG. 3. The voltage sensing capacitors 10 of the FIGS. 4, 5 and 6 are each modified in different details and various features with respect to the voltage sensing capacitor 10 as shown and explained in conjunction with FIGS. 1, 2 and 3. Of course, the modification of the details for one of the voltage sensing capacitors 10 of the FIGS. 4, 5 and 6 may equally be applied to respective other ones of the voltage sensing capacitors 10 and the modification possibilities are not limited to the ones explicitly and exemplarily depicted in those drawings.

[0056] In the exemplary arrangement of FIGS. 2 and 3, the at least one compensation dielectric layer 3b is sandwiched between the at least one dielectric layer 3a and the electrically conductive or semi-conductive cylindrical shell 1 or shell segment. Of course, the stacking of the layers 3a and 3b may also be reversed as shown exemplarily in FIG. 5.

[0057] The outer electrode in FIGS. 2 and 3 is depicted as closed ring structure, i.e. a shell 1 surrounding the full 360 of the circumference of the cylindrical capacitor shape. However, as exemplarily depicted in FIG. 4, it may be equally possible to use an electrically conductive or semi-conductive cylindrical shell segment 1 that only surrounds a fraction S2 of the circumference of the cylindrical capacitor shape. The fraction S2 may be chosen according to the intended application and may generally take on any value between 0 and 360. It may further be possible to use more than one electrically conductive or semi-conductive cylindrical shell segment 1 as outer electrode so that a plurality of electrically conductive or semi-conductive cylindrical shell segments 1 may be equidistantly spaced around the circumference of the cylindrical capacitor shape.

[0058] As shown in FIG. 6, it may be possible to provide a screening electrode 5 arranged concentrically with and around the electrically conductive or semi-conductive cylindrical shell 1. The screening electrode 5 may be connected to a reference potential, for example to ground G. A further dielectric 4, for example being made of the same material as the dielectric layer 3a may be provided between the electrically conductive or semi-conductive cylindrical shell 1 and the screening electrode 5. The capacitor formed by the electrically conductive or semi-conductive cylindrical shell 1 as inner electrode, the dielectric 4 and the screening electrode 5 may also be used as reference capacitor in a series connection with the screened voltage sensing capacitor 10. That way, some of the thermal expansion effects affecting the screened voltage sensing capacitor 10 may apply equally to the reference capacitor as well, so that the dividing ratio of the voltage divider is kept approximately constant.

[0059] If the electrically conductive or semi-conductive cylindrical shell 1 or shell segment comprises a mechanically rigid metal component, for example an aluminium, steel and/or copper annulus, the compensation dielectric layer 3b may be implemented as embedded dielectric material in the inner wall of the rigid metal component. FIGS. 7 and 8 schematically illustrate a perspective view and a cut-away view, respectively, of a sensing electrode to be used in a voltage sensing capacitor 10 of one of the FIGS. 1 to 6. The sensing electrode is formed as an electrically conductive or semi-conductive cylindrical shell 1 shaped in ring form having an outer wall 1c and a recessed trench 1b extending circumferentially in its inner wall. The recessed trench 1b is bordered by an edge portion 1a of the ring structure that prevents the material from flowing out of the trench 1b. The compensation dielectric layer 3b may be implemented as mastic or sealant or silicone that is embedded within the recessed trench 1b of the electrically conductive or semi-conductive cylindrical shell 1 or shell segment.

[0060] FIG. 11 schematically illustrates yet another modification of the voltage sensing capacitor 10 as explained in conjunction with FIGS. 1 to 8. The capacitor 10 of FIG. 11 further includes a stress control material 6 that is applied to the dielectric 3 adjoining to the electrically conductive or semi-conductive cylindrical shell 1 or shell segment. The stress control material 6 may form a layer on top of the dielectric 3 that at least partially reaches over the electrically conductive or semi-conductive cylindrical shell 1. It is also possible to arrange the stress control material 6 adjacent to the shell 1. The stress control material 6 replaces the dielectric compensation layer 3b. Depending on the thermal coefficient of relative permittivity of the dielectric layer 3a, the stress control material 6 may comprise a material having the opposite thermal coefficient of relative permittivity than layer 3a. For example the dielectric layer 3a may have a positive thermal coefficient of relative permittivity and the stress control material 6 may comprise a negative thermal coefficient of relative permittivity.

[0061] The electrical stray field extends outside of the area defined between the electrically conductive or semi-conductive cylindrical shell 1 and the electrically conductive cylinder 2, so that a stress control material 6 adjoining the electrically conductive or semi-conductive cylindrical shell 1 lies at least partially within this stray field. With a positive thermal coefficient of relative permittivity, the stress control material 6 may aid in compensating for the negative thermal coefficient of relative permittivity of the dielectric layer 3a and vice versa. Since stress control may be needed anyway, the additional relative permittivity compensation may be added without additional manufacturing steps. To that end, the length of the material may be selected so that the amount of relative permittivity compensation keeps the capacitance value of the capacitor 10 overall within the predefined stability margins.

[0062] The dielectric layer 3a may for example comprise at least one of a resin, an epoxy and/or polyurethane. It may also be filled with high permittivity materials, such as for example titanate such as barium titanate or conductive carbon black powder dispersed in the polymeric material If the compensation dielectric layer 3a comprises a resin, an epoxy or a polyurethane resin, the compensation dielectric layer 3b may comprise any plastic or rubber material like for example silicone, ethylene propylene diene monomer (EPDM), high module ethylene propylene rubber (HEPR), Polyethylene (PE), Polypropylene (PP), particularly a cold-shrink silicone tube.

[0063] FIG. 9 shows a measurement diagram illustrating the thermal dependence E1 of capacitance value of a compensation dielectric layer 3b including barium titanate (BaTiO.sub.3) particles dispersed in polymeric material. As can be seen, the compensation dielectric layer 3b has a relative permittivity increasing with temperature. Such a compensation dielectric layer 3b may be used for forming a stress control material 6 of FIG. 11 as well.

[0064] FIG. 10 shows a measurement diagram illustrating the thermal dependence C1, C2 of capacitance value C of a voltage sensing capacitor 10 for various combinations of materials of the dielectric layer 3a and the compensation dielectric layer 3b. The measurement curve C1 depicts the capacitance value C vs temperature for a 6.5 mm dielectric layer 3a consisting of the epoxy Scotchcast Electrical Resin 250 available from 3M, U.S.A filled with 45vol % quartz-sand powder, in combination with a 2 mm compensation dielectric layer 3b consisting of a track-resistant silicone tube available from 3M, U.S.A. The measurement curve C2 depicts the capacitance value C vs temperature for a 7 mm dielectric layer 3a consisting of the epoxy Scotchcast Electrical Resin 250 available from 3M, U.S.A. mixed with 15 vol % glass bubbles IM16K available from 3M, U.S.A. in combination with a 3.5 mm compensation dielectric layer 3b consisting of Elastosil 4305/40 silicone available from Wacker Chemie, Germany. As can be seen, the relative deviation AC from a reference capacitance value for each of the combinations does not exceed 0.5% over temperature interval of at least 20 C. to +70 C.

[0065] Typical axial lengths of the cylindrical capacitors 10 are around at least 20 mm to 500 mm preferably between 30-60 mm. Any other length is possible as well and strongly depends on the application.

[0066] Instead of epoxy, a polyurethane resins such as for example Scotchcast Flame-Retardant Compound 2131 available from 3M, U.S.A. may be selected for the dielectric layer 3a as well. In particular, materials having high shore hardness may be advantageously chosen to largely avoid thermal expansion problems.

[0067] FIG. 12 schematically illustrates a block diagram of capacitive voltage sensor circuitry 20 employing a capacitor 10 as a voltage sensing capacitor CS of a voltage divider. A reference impedance, such as for example a reference capacitor CM, is coupled in series between the voltage sensing capacitor CS and a reference potential such as for example ground G. A voltage measurement circuit 7 that is connected to a node NM between the two capacitors CS and CM on one hand and to a node NG connected to the reference potential on the other hand is configured to measure a voltage drop across the reference capacitor CM. The measured voltage drop may then be output to a control processor 8 that produces a corresponding voltage signal. The control processor 8 may for example be additionally equipped with electronic compensation means that further enhance the accuracy of the voltage signal, for example by taking into account temperature induced variations of the electric properties of the capacitors CS and CM. The reference capacitor CM may for example be installed on a PCB in the vicinity of the voltage sensing capacitor CS to provide for a compact voltage sensor assembly 20.

[0068] FIG. 13 schematically illustrates stages of a method M for manufacturing a capacitor, for example a voltage sensing capacitor 10 as explained in conjunction with FIGS. 1 to 11. The method M may be specifically used for manufacturing a capacitor that may be used as voltage sensing capacitor CS in a capacitive voltage sensor assembly 20 as illustrated in conjunction with FIG. 12.

[0069] In a first step M1, a dielectric 3 is applied to an electrically conductive cylinder 2. The dielectric is comprised of at least one dielectric layer 3a having a positive thermal coefficient of relative permittivity and at least one compensation dielectric layer 3b having a negative thermal coefficient of relative permittivity. The application in step M1 may for example be performed by first moulding a dielectric layer 3a, e.g. an epoxy layer, over the electrically conductive cylinder 2 in a sub stage M1a. Then, in a following optional sub-stage M1b, the epoxy of the dielectric layer 3a may be cross-linked at a predefined cross-linking temperature and post-cured at a predefined post-curing temperature.

[0070] After moulding the epoxy layer, cooling the epoxy and removing the epoxy from the mould, a compensation dielectric layer 3b may be cast on top of the epoxy. For example, a silicone tube may be used as compensation dielectric layer 3b. The application of the compensation dielectric layer 3b may in particular involve a sub stage M1c, cold-shrink tubing the silicone tube over the moulded epoxy layer. Cold-shrink tubing involves sliding a rubber sleeve, made for example from elastomeric silicone and pre-stretched over a removable support tube, over the component onto which the sleeve shall be installed and retracting the removable support tube to cause the sleeve to contract tightly over the underlying component.

[0071] Then, in a stage M2, an electrically conductive or semi-conductive cylindrical shell 1 or shell segment is arranged concentrically around the electrically conductive cylinder 2 over the dielectric 3. This may for example be done by applying an electrically conductive ink or coating on the compensation dielectric layer 3b, or, alternatively, cold-shrink tubing an electrically conductive tube, for example made from silicone, on the compensation dielectric layer 3b. For electrically contacting the electrically conductive or semi-conductive cylindrical shell 1 or shell segment a copper tape may be applied on top of the electrode.

[0072] In the foregoing detailed description, various features are grouped together in one or more examples or examples with the purpose of streamlining the disclosure. It is to be understood that the above description is intended to be illustrative, and not restrictive. It is intended to cover all alternatives, modifications and equivalents. Many other examples will be apparent to one skilled in the art upon reviewing the above specification.

[0073] The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. In the appended claims and throughout the specification, the terms including and in which are used as the plain-English equivalents of the respective terms comprising and wherein, respectively. Furthermore, a or one does not exclude a plurality in the present case.