Vacuum bulb, circuit-breaker pole including such a vacuum bulb, and method to manufacture such devices

Abstract

A vacuum bulb is provided, including a sealed chamber; two electrical contacts, which move relative to one another, the chamber including a cylindrical body of a dielectric material and closed at ends thereof by two metal covers, each of the two metal covers being connected to one of the two electrical contacts: and a dielectric coating, which covers an outer surface of the chamber, and includes at least two layers, including an overmolding layer of a synthetic material and an intermediate layer of silicone, the intermediate layer being interposed between the outer surface and the overmolding layer, the intermediate layer being discontinuous and localized on metal portions of the chamber so as to cover at least partially an outer surface of the metal portions, and the silicone includes compressible hollow bodies having a skin of a thermoplastic material.

Claims

1. A vacuum bulb, comprising: a sealed chamber; two electrical contacts, which move relative to one another, wherein the sealed chamber includes a cylindrical body made of a dielectric material and closed at ends thereof by two metal covers, wherein each of the two metal covers is connected to one of the two electrical contacts; and a dielectric coating, which covers an outer surface of the sealed chamber, wherein the dielectric coating includes at least two layers, including an overmoulding layer made of a synthetic material and an intermediate layer made of silicone, wherein the intermediate layer is interposed between the outer surface of the sealed chamber and the overmoulding layer, wherein the intermediate layer is discontinuous and localised in direct contact with at least a portion of each of the two metal covers, so as to cover at least partially an outer surface of the two metal covers, and wherein the silicone of the intermediate layer includes hollow bodies that are compressible and have a skin made of a thermoplastic material.

2. The vacuum bulb according to claim 1, wherein the hollow bodies are microspheres having an average diameter of between 1 m and 800 m.

3. The vacuum bulb according to claim 1, wherein an interface between the dielectric coating and the outer surface of the sealed chamber is sealed.

4. The vacuum bulb according to claim 1, further comprising at least one protective metal screen positioned inside the sealed chamber and attached to the sealed chamber.

5. The vacuum bulb according to claim 4, wherein the cylindrical body includes at least one first portion and one second portion, the protective metal screen is attached to the sealed chamber by an attachment means interposed between the at least one first portion and one second portion, and the intermediate layer is also localised on the attachment means so as to cover at least an entire outer surface of the attachment means.

6. The vacuum bulb according to claim 1, wherein the cylindrical body is made of a ceramic material.

7. The vacuum bulb according to claim 6, wherein the ceramic material is alumina.

8. The vacuum bulb according to claim 7, wherein the ceramic material is enameled.

9. The vacuum bulb according to claim 1, wherein the overmoulding layer is made of a thermosetting polymer.

10. The vacuum bulb according to claim 1, further comprising a shielding layer positioned on the dielectric coating.

11. A medium-voltage switchgear including at least one vacuum bulb according to claim 1.

12. A circuit-breaker pole including an assembly formed of a vacuum bulb according to claim 1 and of two electrically conductive connections, wherein the assembly is coated by the overmoulding layer.

13. The circuit-breaker pole according to claim 12, wherein the overmoulding layer is coated by a shielding layer positioned on the dielectric coating.

14. The vacuum bulb according to claim 1, wherein the hollow bodies are microspheres having an average diameter of between 10 m and 80 m.

15. The vacuum bulb according to claim 1, wherein the overmoulding layer is made of an epoxide polymer.

16. The vacuum bulb according to claim 1, wherein the two metal covers are provided with elements protruding from the two metal covers and the intermediate layer is localised in direct contact with at least a portion of each of the two metal covers so as to cover at least entirely an outer surface of the elements.

17. The vacuum bulb according to claim 1, wherein the intermediate layer is localised in direct contact with at least a portion of each of the two metal covers so as to cover at least entirely the outer surface of the two metal covers.

18. The vacuum bulb according to claim 1, wherein the intermediate layer is also localised in direct contact with portions of the cylindrical body, at least in an area of an edge of each of the two metal covers that is joined to the dielectric material of the cylindrical body.

Description

BRIEF DESCRIPTION OF THE ILLUSTRATIONS

(1) FIG. 1 represents a diagrammatic view as a longitudinal section of a switchgear, in this case a circuit-breaker pole, including a vacuum bulb in accordance with the invention.

(2) FIG. 2 illustrates the hot thermal cycles to which the vacuum bulbs which were assessed were subjected.

(3) FIG. 3 illustrates the cold thermal cycles to which the vacuum bulbs which were assessed were subjected.

(4) FIG. 4 illustrates the alternative thermal cycles to which the vacuum bulbs which were assessed were subjected.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(5) Switchgear

(6) In FIG. 1, a circuit-breaker pole 1 has been represented.

(7) This circuit-breaker pole 1 is formed by the assembly of a vacuum bulb 2 and of two electrically conductive connections, one lower connection 3 and one upper connection 3.

(8) Vacuum bulb 2 includes a sealed chamber 4 in which there is a controlled low pressure of air or of another dielectric fluid, also called a vacuum.

(9) Sealed chamber 4 includes a cylindrical body 5, formed by two portions 5a and 5b made of a dielectric material, advantageously of a ceramic material, notably of alumina, the ceramic material being possibly enamelled. This material of cylindrical body 5 could also be made of glass.

(10) Cylindrical body 5 is closed at its ends by metal covers 6, 6 which are connected in sealed fashion to cylindrical body 5, for example by brazing or by welding.

(11) Metal covers 6, 6 can have protruding edges 6a, 6a extending from their respective outer surfaces.

(12) Chamber 4 also includes two electrical contacts 7, 7 which move relative to one another along the axis of vacuum bulb 1. In a conventional manner, electrical contact 7 is stationary and fixed to metal cover 6, whereas electrical contact 7 moves axially and is connected to metal cover 6. To allow the movement of mobile electrical contact 7 whilst preserving the tightness in sealed chamber 4, a bellows seal 8 is installed.

(13) Sealed chamber 4 also includes a protective metal screen 9 positioned inside sealed chamber 4 and attached to this chamber 4. The function of this protective metal screen 9 is to protect cylindrical body 5 from the liquid metal vapours and from metallic projections from the arc phase produced between electrical contacts 7, 7 when the electrical current is turned off. Protective metal screen 9 is supported by a circular ring 10 attached, for example by brazing, between portions 5a and 5b of cylindrical body 5.

(14) According to the invention, circuit-breaker pole 1 is covered by a dielectric coating 12 including two layers, an intermediate layer 13 and an overmoulding layer 14 made of a synthetic material. Overmoulding layer 14 is positioned on intermediate layer 13, such that no free space remains between this intermediate layer 13 and this overmoulding layer 14. It is said that the interface between dielectric coating 12 and the outer surface of chamber 4 is sealed.

(15) Intermediate layer 13 is a discontinuous layer localised on the metal portions of sealed chamber 4 so as to cover at least partially the outer surface of these metal portions. At least the protruding portions of the metal portions are advantageously fully covered, together with the edges of the said metal portions joined to the dielectric material of cylindrical body 5.

(16) In FIG. 1, intermediate layer 13 covers at least partially the outer surface of metal portions 6, 6 and 10. Intermediate layer 13 is thus localised on the outer surface of metal covers 6, 6 and on the outer surface of circular ring 10 of protective metal screen 9, i.e. on the surfaces or areas of overmoulding layer 14 which are sensitive to cracking.

(17) Indeed, since the metal outer surfaces of sealed chamber 4, and in particular those of protruding portions 6a, 6a of metal covers 6, 6, are covered by intermediate layer 13, they are no longer in direct contact with overmoulding layer 14. The risks of cracking of overmoulding layer 14 by these outer metal surfaces, including those of protruding portions 6a, 6a, are consequently eliminated.

(18) It is also important to note that this intermediate layer 13 is made of a particular silicone. Indeed, this silicone includes hollow bodies which are compressible, and which include a skin made of a thermoplastic material.

(19) Use of such a silicone, which can be qualified as compressible, therefore enables a dielectric coating 12 to be formed, the intermediate layer 13 of which can absorb the thermal expansion variations between the metal elements (covers 6, 6 and circular ring 10) of sealed chamber 4 and overmoulding layer 14, without any expansion of the volume which this intermediate layer 13 occupies within dielectric coating 12. Indeed, the expansion of intermediate layer 13 is in some sense absorbed by the hollow bodies present in the silicone. Consequently, under the effect of the thermal stresses to which circuit-breaker pole 1 can be subject, no formation of cracks in overmoulding layer 14 is observed.

(20) Conversely, use of a silicone which is non-compressible in nature, as described in documents [1] and [2], in such an intermediate dielectric coating layer does not enable the risk of cracking to be eliminated in the overmoulding layer of such a coating. Indeed, under the effect of the same thermal stresses, when the intermediate layer expands it causes thermomechanical stresses not only on the overmoulding layer, but also on the cylindrical body, causing a double risk of cracking, or of fracture, of the overmoulding layer and of the ceramic material of the sealed chamber, with the inherent loss of vacuum.

(21) Dielectric coating 12 can itself be covered by an electrically conductive layer, called a shielding layer (not illustrated).

(22) Method for Manufacturing a Vacuum Bulb in Accordance with the Invention

(23) A method for manufacturing a vacuum bulb will now be described, where this method is in accordance with the invention.

(24) A pre-assembled vacuum bulb is used, of reference Schneider Electric VG3-I, which is commercially available.

(25) Such a vacuum bulb includes a sealed chamber, two electrical contacts and a protective metal screen, but has no dielectric coating. The sealed chamber of this vacuum bulb is formed from a cylindrical body including two portions made of ceramic material, and closed by two metal covers with protruding edges. The sealed chamber also includes a cylindrical metal ring connected with the two portions made of ceramic material, where this ring constitutes the bracket of the protective metal screen.

(26) After a possible prior cleaning of the vacuum bulb, for example using isopropanol, in order to eliminate every residual trace of foreign bodies (greasy substances, dust, etc.), beads or strips of a silicone rubber called high compressibility silicone rubber, sold by the company Wacker with the commercial name ElastosilRT 713, are deposited on the outer surface of the metal covers of the chamber and on the outer surface of the cylindrical metal ring constituting the bracket of the protective metal screen. This deposition is accomplished such that the entire outer surface of the metal covers, together with the entire outer surface of the cylindrical metal ring, are covered by the silicone rubber. These silicone beads have the form of a truncated toroid, the radius of which is greater than or equal to 3 mm.

(27) Such a deposition on the metal surfaces of the sealed chamber enables all the metal areas or portions of the outer surface of this sealed chamber to be covered, and by so doing, enables the coating of any protruding edges of the metal covers, and also of the triple point of the ceramic material, where the triple point is the joining area between the two portions made of ceramic material of the cylindrical body and the cylindrical metal ring.

(28) It is perfectly possible to envisage that the metal portions could be only partially covered by the intermediate layer. In particular, there is no imperative requirement for the metal areas or portions without protruding corners to be coated.

(29) Through such a localised deposition, are prevented any incipient rupture which might be caused, in the overmoulding layer, by the stresses exerted by the metal elements of the chamber, and notably by any protruding edges of the covers, if these metal elements were overmoulded directly with an epoxide polymer, or if a non-compressible silicone were used.

(30) The deposition can advantageously be accomplished such that the areas or portions of the outer surface of the cylindrical body adjoining these metal surfaces formed by metal covers and by the metal cylindrical ring are also covered by this silicone rubber. Although such a hypothesis is conceivable, there is however no advantage in depositing a continuous intermediate layer on the entire outer surface of the cylindrical body, notably for economic reasons.

(31) The vacuum bulb coated with beads of silicone rubber is then cleaned once again, for example using isopropanol, to eliminate the foreign bodies, and by this means to improve the subsequent adherence of the overmoulding layer. It is then placed in a furnace at a temperature of between 160 C. and 170 for 2 hours, to allow the cross-linking of the silicone rubber.

(32) After it is removed from the furnace, the vacuum bulb coated with silicone beads or strips is placed in a mould which is then closed, and the temperature of which is raised to and then maintained at 150 C. throughout the moulding cycle; the mould dimensions are such that filling the space remaining between the vacuum bulb and the mould with the chosen material enables a compact overmoulding layer to be obtained, of the desired thickness.

(33) Injection moulding is then undertaken, preferably using automatic pressure gelation, to form the overmoulding layer.

(34) To accomplish this, a blend including epoxide monomers, a hardening agent and a mineral filler, which blend is sold by the company Huntsman, with the commercial name AralditeCY 225/HY 225 (hardening agent)/silica flour, and in which the compounds are in the respective proportions by weight of 100/80/270, is injected at an injection pressure of between approximately 1 bar and 1.5 bar. A pressure called a gelation pressure of 6 bar maximum is then applied for a cycle time of 22 min., before the mould is opened and the vacuum bulb is extracted. Post-curing of the overmoulded vacuum bulb including the chamber, the electrical contacts and the dielectric coating formed of the intermediate layer and of the overmoulding layer is accomplished by heating the mould at 145 C. for 220 min., and then at 130 C. for 44 min., and then finally at 80 C. for 44 min.

(35) Assessment of the Cracking Resistance and of the Dielectric Properties of Vacuum Bulbs before and after Thermal Stresses

(36) The tests which were undertaken are intended to assess the cracking resistance of the overmoulding layer and of the sealed chamber of three vacuum bulbs, one of which is in accordance with the invention, together with the dielectric properties of these vacuum bulbs before and after accomplishment of various thermal cycles.

(37) To assess the cracking resistance of the overmoulding layer made of an epoxide polymer, and of the sealed chamber of these three vacuum bulbs, previously assembled vacuum bulbs were used, of reference Schneider Electric VG2, which are available commercially.

(38) These vacuum bulbs have two metal covers, on which were deposited, in succession, a discontinuous intermediate layer localised according to the characteristics of the invention, followed by an identical overmoulding layer made of epoxide polymer. Although for these three vacuum bulbs the overmoulding layer is identical both in terms of composition and of thickness, the intermediate layer of the same thickness, for its part, was made from three different silicones.

(39) The materials used to produce the intermediate and overmoulding layers are as follows:

(40) Intermediate Layer

(41) compressible silicone of reference ElastosilRT 713 from the company Wacker, having the reference Silicone-1 in the tables below, for production of a vacuum bulb in accordance with the invention, non-compressible silicone of reference RhodorsilRTV 3428 from the company Rhodia, a silicone having the reference Silicone-2 in the tables below, for production of a vacuum bulb according to the prior art, non-compressible silicone of reference SilicometAS 310 from the company Henkel, a silicone having the reference Silicone-3 in the tables below, for production of a vacuum bulb according to the prior art.
Overmoulding Layer

(42) The blend formed of 100 pp of AralditeCY 5824-CI resin sold by the company Huntsman Advanced Materials, of 80 pp of hardening agent AradurHY 5924-CI, also sold by the company Huntsman Advanced Materials, and of 300 pp of SilbondW12EST silica, sold by the company Quartzwerke Gruppe, was used.

(43) The measurements made and the associated settings, where these parameters are defined in an IEC standard, are as follows: a voltage of 44 kV is applied for 60 s at a frequency of 50 Hz, which corresponds to the value called the Power Frequency Withstand value, noted PFW in tables 3 to 5 below; the partial discharge level is measured, noted PD in tables 3 to 5 below, at 20 kVrms at the time of the 44 kV descent: the values obtained in pC (pico Coulombs) are shown in table 1; and the distance between the earthed metal plate and the axis of the poles is 110 mm.

(44) Various thermal cycles were undertaken. The variations of temperature (T) as a function of time (t) applied at the rate of 2 C. per minute are given in tables 1 and 2 below and illustrated in FIGS. 2 and 3, respectively, where the ambient temperature (Tamb) is also stipulated in these figures.

(45) TABLE-US-00001 TABLE 1 Hot cycles Duration (h) Observations Cycle at 50 C. 8 OK Cycle at 70 C. 16 OK Cycle at 90 C. 8 OK Cycle at 110 C. 16 OK

(46) In the column Observations, the expression OK means that none of the tested vacuum bulbs was degraded under the effect of the thermal cycles called hot thermal cycles, corresponding to rises in surrounding temperature, in accordance with table 1 above, and illustrated in FIG. 2.

(47) TABLE-US-00002 TABLE 2 Cold cycles Duration (h) Observations Cycle at 10 C. 8 OK except for Silicone-3 Cycle at 20 C. 16 OK Cycle at 30 C. 8 OK Cycle at 40 C. 16 OK

(48) It is observed that, except for the vacuum bulb the intermediate layer of which is produced with Silicone-3, which cracked during the first cycle at 10 C., the vacuum bulbs including Silicone-1 and Silicone-2 did not degrade under the effect of the cold thermal cycles, corresponding to drops in surrounding temperature, in accordance with table 2 above, and illustrated in FIG. 3.

(49) Alternative thermal cycles were also undertaken. Temperature variations between 40 C. and 90 C. are illustrated in FIG. 4.

(50) It is noted that, on conclusion of 4 successive alternative cycles, the vacuum bulb the intermediate layer of which is produced with Silicone-2 has cracks, which is not the case with the vacuum bulb according to the invention, which has an intermediate layer of Silicone-1.

(51) The PFW (power frequency withstand), SA (ignition threshold), SE (extinction threshold) and PD (partial discharge) measurements taken, before application of the different thermal cycles, are shown in table 3 below, the measuring conditions being as follows: temperature 23.6 C., pressure 1024 mbar and relative humidity 32.7%:

(52) TABLE-US-00003 TABLE 3 Position of the electrical contacts Open position Open position Voltage applied to the Voltage applied to the stationary electrical contact mobile electrical contact Closed position Silicone PFW, SA, SE PD (pC) PFW, SA, SE PD (pC) PFW, SA, SE PD (pC) Silicone-1 PFW: OK <5 PFW: OK <5 PFW: OK <5 (according to SA > 44 kVrms SA > 44 kVrms SA = 28 kVrms the invention) SE > 44 kVrms SE > 44 kVrms SE = 22 kVrms Silicone-2 PFW: OK <5 PFW: OK <5 PFW: OK <5 (according to the SA > 44 kVrms SA > 44 kVrms SA = 29 kVrms state of the art) SE > 44 kVrms SE > 44 kVrms SE = 29 kVrms Silicone-3 PFW: OK <5 PFW: not OK(*) <5 PFW: OK <5 (according to the SA = 39 kVrms SA = 41 kVrms SA = 30 kVrms state of the art) SE = 36 kVrms SE = 29 kVrms SE = 26 kVrms (*)In this position the PFW measurement for the vacuum bulb including the intermediate layer made of Silicone-3 can be 41 kVrms maximum. Above this value initiation occurs in the vacuum bulb which does not impair the electrical insulation of the overmoulding layer.

(53) The PFW, SA, SE and PD measurements made, after application of the hot and then cold thermal cycles according to the profile represented in FIGS. 2 and 3, i.e. 8 h at 50 C., 16 h at 70 C., 8 h at 90 C., 16 h at 110 C., and then 8 h at 10 C., 16 h at 20 C., 8 h at 30 C., 16 h at 40 C., are shown in table 4 below, the measuring conditions being as follows: temperature 22.6 C., pressure 999.6 mbar and relative humidity 31.9%:

(54) TABLE-US-00004 TABLE 4 Position of the electrical contacts Open position Open position Voltage applied to the Voltage applied to the stationary electrical contact mobile electrical contact Closed position Silicone PFW, SA, SE PD (pC) PFW, SA, SE PD (pC) PFW, SA, SE PD (pC) Silicone-1 PFW: OK <5 PFW: OK <5 PFW: OK <5 (according to SA > 44 kVrms SA > 44 kVrms SA = 23 kVrms the invention) SE > 44 kVrms SE > 44 kVrms SE = 23 kVrms Silicone-2 PFW: OK <5 PFW: OK <5 PFW: OK <5 (according to the SA > 44 kVrms SA > 44 kVrms SA = 25 kVrms state of the art) SE > 44 kVrms SE > 44 kVrms SE = 20 kVrms Silicone-3 Cracked vacuum bulb (according to the state of the art)

(55) It is observed that only the vacuum bulbs including the intermediate layer made of Silicone-1 and Silicone-2 resisted cracking, and had satisfactory dielectric properties after the hot and then cold thermal cycles.

(56) The PFW, SA, SE and PD measurements made, after application of the alternative thermal cycles according to the profile represented in FIG. 4, are shown in table 5 below, the measuring conditions being as follows: temperature 23.7 C., pressure 1019.2 mbar and relative humidity 35.8%:

(57) TABLE-US-00005 TABLE 5 Position of the electrical contacts Open position Open position Voltage applied to the Voltage applied to the stationary electrical contact mobile electrical contact Closed position Silicone PFW, SA, SE PD (pC) PFW, SA, SE PD (pC) PFW, SA, SE PD (pC) Silicone-1 PFW: OK <5 PFW: OK <5 PFW: OK <5 (according to the SA > 44 kVrms SA > 44 kVrms SA = 28 kVrms invention) SE > 44 kVrms SE > 44 kVrms SE = 24 kVrms Silicone-2 Cracked vacuum bulb (according to the state of the art) Silicone-3 Cracked vacuum bulb (according to the state of the art)

(58) It is observed that only the vacuum bulb in accordance with the invention including the intermediate layer made of Silicone-1 resisted cracking, and had satisfactory dielectric properties after the alternative thermal cycles.

BIBLIOGRAPHY

(59) [1] EP 0 866 481 A2 [2] U.S. Pat. No. 5,917,167 [3] U.S. Pat. No. 5,750,581 [4] EP 0 971 369 A1 [5] EP 1 571 685 A1 [6] WO 2009/106731 A2