ELECTRODE UNIT FOR A VACUUM CAPACITOR AND A VACUUM CAPACITOR

Abstract

The present invention relates to an electrode unit (10, 20) for an electric vacuum capacitor comprising a band-shaped capacitor plate (11, 21) with a height H, wherein the band-shaped capacitor plate (11, 21) is wound in a spiral with a maximum diameter D.sub.max and a constant distance between successive turns, wherein the band-shaped capacitor plate (11, 21) comprises a first longitudinal edge (11a, 21a) attached to a supporting part (12) and a second longitudinal edge (11b, 21b), the second longitudinal edge (11b, 21b) being free, wherein at the outer extremity of the spiral, the first longitudinal edge (11a, 21a) and the second longitudinal edge (11b, 21b) are connected by an inclined edge (11c, 21c) such that the first longitudinal edge (11a, 21a) is longer than the second longitudinal edge (11b, 21b), wherein the inclined edge (11c, 21c) forms with the longitudinal axis (B) of the band-shaped capacitor plate (11, 21) an angle α less than or equal to an angle α.sub.max=(45°.Math.π/180°). The invention relates also to a vacuum capacitor (30) comprising at least one electrode unit (10, 20) according to the present invention.

Claims

1. Electrode unit (10, 20) for an electric vacuum capacitor comprising a band-shaped capacitor plate (11, 21) with a height H, wherein the band-shaped capacitor plate (11, 21) is wound in a spiral with a maximum diameter D.sub.max and a constant distance between successive turns, wherein the band-shaped capacitor plate (11, 21) comprises a first longitudinal edge (11a, 21a) attached to a supporting part (12) and a second longitudinal edge (11b, 21b), the second longitudinal edge (11b, 21b) being free, characterized in that, at the outer extremity of the spiral, the first longitudinal edge (11a, 21a) and the second longitudinal edge (11b, 21b) are connected by an inclined edge (11c, 21c) such that the first longitudinal edge (11a, 21a) is longer than the second longitudinal edge (11b, 21b), wherein the inclined edge (11c, 21c) forms with the longitudinal axis (B) of the band-shaped capacitor plate (11, 21) is an angle a less than or equal to an angle α.sub.max=(45°.Math.π/180°).

2. Electrode unit (10, 20) according to claim 1, wherein angle α is greater than or equal to an angle α.sub.min=γ.Math.tan.sup.−1(2H/(π.Math.D)), wherein γ is a real number greater than or equal to 1.0.

3. Electrode unit (10, 20) according to claim 1 or 2, wherein the supporting part (12) is made of copper, silver, aluminum or an alloy thereof and the band-shaped capacitor plate (11, 21) is made of stainless steel, carbon steel, molybdenum, tantalum, tungsten, nickel, Inconel®, titanium, chromium or an alloy thereof.

4. Electrode unit (10, 20) according to any one of the claims 1 to 3, wherein α.sub.max is equal to (35°.Math.π/180°).

5. Electrode unit (10, 20) according to any one of the claims 1 to 3, wherein am ax is equal to (23°.Math.π/180°).

6. Electrode unit (10, 20) according to any of the preceding claims, wherein the inclined edge (11c, 21c) is radiused at the transition to the first longitudinal edge (11a, 21a) and/or the second longitudinal edge (11b, 21b).

7. Electrode unit (10, 20) according to any of the preceding claims, wherein the supporting part (12) comprises a spiral-shaped guiding edge (13) arranged in the space formed by two successive turns of the band-shaped capacitor plate (11, 21) and wherein the width of the guiding edge (13) corresponds to the distance between two successive turns of the band-shaped capacitor plate (11, 21).

8. Electrode unit (10, 20) according to any of the preceding claims, wherein the band-shaped capacitor plate (11, 21) is attached to the supporting part (12) by means of brazing.

9. Vacuum capacitor (30) comprising an enclosure (31) to contain a vacuum dielectric medium (32), a first electrode unit (10, 20) and a second electrode unit (10, 20) separated by said vacuum dielectric medium (32), the enclosure (31) comprising a first conductive collar (33) in electrical contact with the first electrode unit (10, 20) and a second conductive collar (34) in electrical contact with the second electrode unit (10, 20), the first conductive collar (33) and the second conductive collar (34) being separated by an insulating element (35) of the enclosure, wherein at least one of the first electrode unit (10, 20) or the second electrode unit (10, 20) is an electrode unit (10, 20) according to any of the claims 1 to 8.

10. Vacuum capacitor (30) according to claim 9, said vacuum capacitor comprising two mating and coaxially oriented electrode units (10, 20) according to any of the claims 1 to 8, wherein the spirals of the band-shaped capacitor plates are chiral.

11. Vacuum capacitor according to claim 9 or 10, wherein the band-shaped capacitor plate (11, 21) and the supporting plate (12) of the at least one electrode unit (10, 20) are attached to each other with a first braze filler material (38) and the first conductive collar (33) and the second conductive collar (34) are each attached to opposite ends of the insulating element (35) of the enclosure (31) with a second braze filler material (39), wherein the first braze filler material (38) and the second braze filler material (39) are selected to have non-overlapping melting ranges.

12. Vacuum capacitor according to claim 11, wherein the first braze filler (38) material has a higher melting temperature as the second braze filler material (39).

13. Vacuum capacitor according to claim 9 or 10, wherein the band-shaped capacitor plate (11, 21) and the supporting plate (12) of one electrode unit (10, 20) are attached to each other with a first braze filler material (38), wherein the band-shaped capacitor plate (11′, 21′) and the supporting plate (12) of the other electrode unit (10′, 20′) are attached to each other with a third braze filler material, wherein the first conductive collar (33) and the second conductive collar (34) are each attached to opposite ends of the insulating element (35) of the enclosure (31) with a second braze filler material (39), wherein the first braze filler material, the second braze filler material and the third braze filler material are selected to have mutually non-overlapping melting ranges.

14. Vacuum capacitor (30) according to claims 9 to 13, further comprising a bellows and a moving system for allowing to change the position of at least one of the two electrode units (10, 20).

15. Vacuum capacitor (30) according to claims 9 to 13, wherein the surface overlap of the band-shaped capacitor plates of the two electrode units (10, 20) is fixed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 is a perspective side view of an electrode unit according to a first embodiment of the present invention;

[0037] FIG. 2 is a side view of an electrode unit according to the first embodiment of the present invention;

[0038] FIG. 3a is a top view of two mating electrode units according to the first embodiment of the present invention;

[0039] FIG. 3b illustrates the orientation of two mating electrode units according to the present invention;

[0040] FIG. 3c illustrates the insertion of o mating electrode units according to the present invention;

[0041] FIG. 4 is a first side view of an electrode unit according to a second embodiment of the present invention;

[0042] FIG. 5 is a second side view of an electrode unit according to the second embodiment of the present invention; and

[0043] FIG. 6 is, a section view of a vacuum capacitor according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0044] FIG. 1 shows a perspective view of an electrode unit 10 according to a first preferred embodiment of the present invention. The electrode unit 10 comprises a band-shaped capacitor plate 11 that is wound in a spiral around the rotation axis A of a supporting part 12. The spiral has a maximum diameter D.sub.max and successive turns are equidistant to each other as shown also in FIG. 3a. The distance between two successive turns exceeds advantageously the thickness of the band-shaped capacitor plate 11. This allows for using two mating electrode units 10 wherein the band-shaped capacitor of a first electrode unit is located in the space between successive turns of a second electrode unit. Two mating electrodes units 10 and 10′ are illustrated in FIG. 3a The electrode units 10 and 10′ differ in that the spirals of the band-shaped capacitor plates 11 and 11′ are chiral. This allows, as illustrated in FIGS. 3b and 3c, to insert the band-shaped capacitor plate of one of the electrode unit into the space between successive turns of the band-shaped capacitor plate of the other electrode unit and by that changing the surface overlap of the capacitor plates.

[0045] The band-shaped capacitor plate 11 comprises a first longitudinal edge 11a attached to the supporting plate 12 and a second longitudinal edged 11b that is free. At the outer extremity of the spiral, the band-shaped capacitor plate 11 exhibits an inclined edge 11c that connects the first longitudinal edge 11a with the second longitudinal edge 11b.

[0046] The supporting part 12 comprises advantageously a spiral-shaped guiding edge 13 arranged in the space formed by two successive turns of the band-shaped capacitor plate 11. The width of the guiding edge 13 corresponds to the distance between two successive turns of the band-shaped capacitor plate 11. The guiding edge 13 have the advantage of guaranteeing that the successive turns of the capacitor plate 11 remains equidistant during the assembly of the band-shaped capacitor plate 11 with the supporting part 12.

[0047] As mentioned earlier, the vacuum enclosure of vacuum capacitors must comprise an insulating piece, most often a cylindrical ceramic which is brazed to the two metallic collars. When trying to replace the copper band-shaped capacitor plate of electrode unit known in the art with a band-shaped capacitor plate made of a high melting point material, such as for instance stainless-steal, it has been observed by the inventors that the band-shaped capacitor electrodes collapse, i.e. the distance between successive turns changes, during the brazing process.

[0048] The inventors have figured out that the value of angle α, the angle that is formed by the longitudinal axis B of the capacitor plate 11 with the inclined edge 11c, is critical to permit to have an electrode unit 10 with supporting part 12 made of copper, silver, aluminum, an alloy thereof or the like and the band-shaped capacitor plate 11 made of a high melting point material, such as stainless steel, carbon steel, molybdenum, tantalum, tungsten, nickel, Inconel®, titanium, chromium or an alloy thereof, that is not collapsing during the thermal cycling of the brazing process used in the manufacturing of vacuum capacitor. Simulations have shown that with proper values of a Von Mises stresses due to temperature variations, that are the cause of failure of the electrode unit known in the art, can be avoided or at least greatly diminished. The inventors have determined that angle a shall be less than or equal to an angle α.sub.max=(45°.Math.π/180°. An austenitic stainless steel which is non-magnetic, such as alloy 1.4301 can preferably be used to avoid or to minimize parasitic effects due to the strong electro-magnetic field present in typical application environments of the vacuum capacitor. These parasitic effects could lead to overheating or to otherwise poor capacitor performance in power applications.

[0049] As explained above, it is critical that angle a is smaller than an upper value α.sub.max. It is also advantageous that angle α is greater than or equal to an angle arm α.sub.min=γ.Math.tan.sup.−1(2H/(π.Math.D)), wherein H is the height of the band-shaped capacitor plate 11. The reason for the advantageous lower limit aα.sub.min is to ensure that before reaching half a turn, the outermost spiral extends to the full height H of the other windings and ensures a satisfactory mating with a counter electrode unit.

[0050] The electrode unit 10 of FIG. 2 exhibits, for instance, an angle of (45°.Math.π/180°) which is the upper allowable limit for stainless steel. It is, nevertheless, possible that the upper limit α.sub.max is slightly different, lower or higher, for other materials. A skilled person, now knowing that the value of angle a is crifical, will have no difficulty to find the exact upper limit α.sub.max for the material of choice for the band-shaped capacitor plate.

[0051] Advantageously, the inclined edged 11c has radiused portion 11d at the transition to the first longitudinal edge 11a. While in FIGS. 1 and 2 such a radiused portion is provided only at the first longitudinal edge 11a, it could also be provided at the transition to the second longitudinal edge 11b. The one or more radiused portion have the advantage to avoid forming sharp edges or spikes that could favor vacuum breakdown.

[0052] FIGS. 4 and 5 show side views of an electrode unit 20 according to a second preferred embodiment of the present invention. The electrode unit 20 differs from electrode unit 10 solely by the shape of the band-shaped capacitor plate 21. As illustrated in FIG. 4, the inclined edge 21c of the capacitor plate 21 form with its longitudinal axis an angle a smaller than in electrode unit 10. To be precise angle α of electrode unit 20 amounts for (22.5°.Math.π/180°).

[0053] FIG. 5 shows a section view of a vacuum capacitor 30 according to a preferred embodiment of this aspect of the present invention. The vacuum capacitor 30 comprises an enclosure 31 to contain a vacuum dielectric medium 32; the enclosure comprising a first conductive collar 33 and a second conductive collar 34 separated by an insulating element 35 of the enclosure 31. Commonly the insulating element 35 is made of a ceramic material in the shape of a cylinder. For the manufacturing process the insulating element 35 is brazed to the metallic collars 33, 34 with a second braze filler material 39. During this process a temperature of approximately 800T is required. Such a temperature, if not using an electrode unit according to the present invention, leads to the collapse of the band-shaped capacitor plates 11,21 of the electrode units 1020. As shown in FIG. 6, the electrode units 10,20 positioned inside the vacuum dielectric 32 are in electric contact with conductive collars 33 and 34 respectively, and the vacuum capacitor can be a variable vacuum capacitor with adjustable electrode unit surface overlap by using a moving mechanism 36 and expandable bellows 37. Because the band-shaped capacitor plate 11, 21 and the supporting plate 12 of the electrode units 10, 20 are brazed together by means of a first braze filler material 38 and the ceramic cylinder 35 needs also to be brazed with the metallic collars 33, 34 with the second braze filler material 39 in a vacuum tight manner, the braze filler materials 38 and 39 employed for the successive brazing steps are preferably chosen to have non-overlapping melding ranges. It can also be advantageous to provide for two different braze filler materials for the brazing step of the two electrode units 10, 20.

[0054] Finally, the foregoing has outlined pertinent non-limiting embodiments. It will be clear to those skilled in the art that modifications to the disclosed non-limiting embodiments can be carried out without departing from he spirit and scope thereof. As such, the described non-limiting embodiments ought to be considered merely illustrative of some of the more prominent features and applications. Other beneficial results can be realized by applying the non-limiting embodiments in a different manner or modifying it in ways known to those familiar with the art. It is especially important to notice that is not possible to describe here all possible embodiments, in particular all possible angle o that permits to attain the goal of the present invention, Nevertheless, a person skilled in the art will know how to adapt angle a to the material of choice for the capacitor plate.