GB2070861A - An electrical capacitor - Google Patents

Abstract

An electrical capacitor has a cut-out device 18, 19 surrounded by a foam material 27 and responsive to internal excess pressure. The housing (28; 43) of the capacitor has housing zone (29; 44) which expands (28; 43) under excess pressure. The electrical conductor (15-17; 52) is stressed in the expansion direction of the housing (28; 43) and is rigidly connected to the housing (28; 43) in front of and behind the expansible housing zone (29; 44). The foam material (27) completely surrounds the conductor (15-17; 52) at least over a portion of its length and in the region of its desired breaking point (18, 19; 53). The foam material (27) is formed as a closed core polyethene which can also be provided with additional fire-proofing means, for example antimonytrioxide. <IMAGE>

Description

SPECIFICATION An electrical capacitor The invention originates from an electrical capacitor according to the type set forth in the main claim.

With one known electrical capacitor of this kind the housing is formed by a rigid sealing compound which directly surrounds the capacitor body, cut-out device and foam material. Thus, the foam material must be strongly elastic so that the conductors to the cutout device can deform and break under the pressure of the expanding capacitor body. During the breaking of the conductor, an arc at high temperature is formed between its free ends which lasts for a relatively long time and is only quenched when, due to the melting of the two free ends of the conductor, their distance from one another has increased sufficiently.

Thus, such a cut-out device is very sluggish and requires a relatively long time for disconnecting the capacitor. Thereby, the capacitor is still very strongly heated by the arc, whereby the dielectric of the capacitor body emits gas in an intensified manner and the danger of a capacitor explosion is evoked and on the other hand the high short circuit current in the capacitor circuit is maintained too long, which can lead to damage to other components.

As opposed to this, the electrical capacitor in accordance with the invention comprising the characterising features of claim 1 and/or of claim 8 have the advantage that the current conductivity of the arc which is formed between the free ends of the broken conductor, is suddenly reduced and the arc rapidly quenched thereby. Thus, the cut-out device has an extremely short response time. Thereby, not only is an additional heating of the dielectric by the arc avoided but also the capacitor circuit is interrupted so rapidly that other components can hardly be damaged.

By means of the measures set forth in the characterising portion of claim 1, the current conductivity of the arc is reduced by a sudden increase in the distance between the free ends of the broken conductor. In addition, due to the foam material surrounding the conductor current conducting portions of the conductor are prevented from coming into contact with other parts of the capacitor so that these cannot be brought to a voltage potential or be melted.

With the selection of foam material according to claim 8, the current conductivity of the arc is reduced so that the very hot arc releases from the foam material reaction favouring radicals, so-called charge carrier collectors, which deprive the arc plasma of the charge carriers, electrons or ions due to adsorption.

Moreover, insulating coating are formed on the free ends of the broken conductor also by chemical reactions from elements released from the heated foam material so that the current conductivity of the arc is also additionally reduced thereby.

An especially favourable solution is provided by the combination of the features of claim 1 and claim 8.

Advantageous further developments and improvements of the electrical capacitor are made possible by the measures set forth in the further claims.

The invention is explained in more detail in the following specification with the aid of embodiments illustrated in the drawing. The drawings show: Figure 1 a longitudinal section through an electrical capacitor according to a first embodiment, Figure 2 a plan view of the capacitor of Figure 1, Figure 3 a plan view of a foam material disc of the capacitor of Figure 1, Figure 4 a partial section of the same representation of the capacitor as in Figure 1 however after its cut-out device has responded, Figure 5 a longitudinal section through a capacitor according to a second embodiment, Figure 6 a longitudinal section through a capacitor according to a third embodiment.

The electrical capacitor illustrated in Figures 1 to 4 is made three poled, that is to say it has three individual capacities which can be connected, for example, in starortriangle. The capacitor body 10 con sists of three ci rcu lar windings 11,12, 13 which are arranged one behind the other within the axial length of the capacitor. In known manner, each circu larwindings 11, 12, 13 consists oftwo spirally wound electrodes which are separated from one another by a dielectric. Electrodes and dielectric are not specifically referenced in Figure 1.The dielectric has the property of emitting gases under inadmissibly high heating.

The capacitor has a cut-out device 14 which comprises at least one electrical conductor provided with a desired breaking point. With the three pole arrangement of the capacitor, each circular winding 11, 12, 13 is separately protected. Thus, in this case, the cut-out device 14 has total of three electrical conductors each with a desired breaking point of which the conductor 15 with the desired breaking point 18 and the conductor 16 with the desired breaking point 19 can be seen in Figures 1 and 4 respec- tively. The conductor 17 is obscured in Figures 1 and 4. Only its lower end can be seen. The conductors 15-17 connect the lugs 20-22 of the capacitor to a respective circular winding 11-13.

The conductors 15-17 are completely enclosed by a foam material 27 wherein the latter surrounds the conductors 15-17 ata distance.

The capacitor body 10 formed by the circularwindings 11-13, the cut-out device 14 and the foam material 27 are enclosed by a housing 28 which has an expansible zone 29. This expansible housing zone 29, which is provided by expansion folds 30 in the housing wall, permits a definite expansion of the housing 28 along its longitudinal axis underthe effect of excess pressure in the interior of the housing 28 produced by the formation of gas. The electri cal conductors 15-17 of the cut-out device 14 am mounted in the expansion direction of the housing 28 and are rigidly connected to the housing 28 in front of and behind the expansible housing zone 29.

With the capacitor according to Figures 1 and 4, this rigid connection takes place by means of two insulating plates 31,32 which are mounted on the housing 28. Both insul3tir s s 3; and 32 are supported on an i',. fold 30. The outer insulating plate 32 cvenng the end of the housing 28 is additionally retained by a rim 33 of the housing 28. The three electrical conductors 15-17 of the cutout device 14 are clamped between the two insulating plates 31 and 32 and at one of their ends are soldered to lugs 20-22 arranged on the insulating plate 32 and at the other of their ends are soldered to connecting wires 24-26.

The foam material 27 is arranged between the two insulating plates 31 and 32 in the form of a foam disc 23 (Figure 3) the sides of which engage the respective insulating plates 31, 32. The foam disc 23 has three passages 34-36 and a centraily arranged pressure equalising bore 37. Each of the electrical conductors 15-17 of the cut-out device 14 passed through a respective passage 34-36. The foam disc 23 is made of closed pore polyethylene which can be obtained, for example, commercially under the brand name "Alveolit 3000". In the heated condition this foam material may be pressed into suitable shapes which remain when it is cooled under pressure. The foam material then completely fills the space available to it on further heating.In addition, the foam material can also be provided with fire resistant means, for example antimony trioxide.

When the dielectric of the capacitor body 10 is heated to an inadmissible extent then it emits gas which leads to an excess pressure in the interior of the housing 28. This gas excess pressure is applied to the inner surface of the insulating plate 32 and moves the latter outwards whilst opening the expansion folds 30 axially (Figure 4). The tension existing on the conductors 15-17 thereby leads to their rupturing at the desired breaking points 18, 19.

Thus, the current supply from the lugs 20-22 to the circular windings 11-13 of the capacitor body 10 is interrupted. An arc is formed between the free ends of the conductors 15-17 formed by rupturing. The current conductivity of the arc is rapidly reduced by two operations which proceed independently of one another so that the arc is again extinguished immediately after it is formed. On the one hand, the gas excess pressure in the interior of the housing 28 moves the insulating plate 32 further outwards so that the distance between the two free ends of the ruptured conductors 15-17 is very rapidly increased.

Secondly, so-called charge carrier collectors are set free from the foam material 27 by the hot arc plasma which withdraw the charge carriers, electrons and ions from the arc plasma by adsorption. In addition, the elements set free from the foam material 27 by the hot arc react with one another and are deposited on the free ends of the ruptured conductors 15-17 as insulating coatings.

Due to the heating of the foam material 27, the foam disc 23 is expanded and completely fills the space between the two insulating plates 31 and 32 enlarged by the opened expansion folds 30. At the same time, this prevents the arc from engaging other voltage conducting, earthed or heat sensitive components of the capacitor or even the ruptured ends of the conductors 15-17 from coming into contact there-,^ 5th. At the same time, the existence of - g - apa e s in rife rio r ofthe housing of the capacitor in the vicinity of the ruptured ends of the conductors 15-17 in which arcs can easily form or can freely flash over to other parts, is prevented thereby.

Due to the above-described construction of the capacitor, both a reliable and extremely rapid disconnection of the capacitor is ensured on being heated too strongly at the instant when the cut-out device breaks.

In the capacitor accord'snug to Figure 5, the capacitor body 40; Ili3 on one single cylindrical capacitor winding 41 likewise made hollow inside. In a simiiar manner, the capacitor housing 43 surrounding the capacitor winding 41 and a cut-out device 42 has an expansible housing zone 44 which is likewise provided by expansion folds 45 or expansion flanges in the housing wall. The capacitor body 40 is supported at its respective ends by caps 46 and 47 of insulating material on the one hand at the base of the cup-like housing 43 and on the other hand against an expansion fold 45 and is therefore axially non-displaceably held within the housing 43.The capacitor body 10 of the capacitor according to Figures 1 to 4 also has such caps of insulating material but which have not been referred to specifically.

The end of the capacitor housing 43 is covered with a plate 48 of insulating material at a distance from the end of the capacitor body 40 facing the expansible housing zone 44. Moreover, the plate 48 of insulating material is rigidly clamped between an expansion fold 45 and a rim 49 of the housing 43.

The plate 48 of insulating material carriers two tags 50 and 51 each of which is electrically connected to one end of the capacitor winding 41. The electrical conductor 52 provided with a desired breaking point 53 is arranged between one end ofthe capacitor winding 41 and the tag 51. Moreover, the conductive passes through the inner hollow space 54 of the capacitor body 40 or of the capacitor winding 41. The inner hollow space 54 is almost completely filled with the same foam material 27 which has also been used in the disc 23 of foam material in the capacitor according to Figures 1 to 4. Moreover, this foam material 27 is preferably made of polyethylene and can be provided with fire-proofing additives such as for example antimonytrioxide.

The behaviour of the capacitor according to Figure 5 when the dielectric is heated too strongly corresponding exactly to the behaviour of the capacitor according to Figures 1 to 4 already thoroughly described so that reference can be made to the explanations concerning figures 1 to 4.

The construction of the capacitor illustrated in Figure 6 is known perse and is thoroughly described for example in German Patent Specification 23 50 271. A capacitor winding 60 carries on its outside a cut-out device 61 which consists of a plate 62 of insulating material provided with a desired breaking point and an electrical conductor in the form of a metallic coating 63. The cut-out device 61 is covered by a body 64 of foam material. The capacitor winding 60, the cutout device 61 and the body 64 of foam material are embedded in a rigid mastic compound 65 which completely fills a housing 66.

In this embodiment, the body 64 of foam material is provided of the same foam material 27 as with the previously described two embodiments of a capacitor.

If the capacitor is heated too strongly the gas pressure prevailing within the capacitor winding 60 then cambers the winding on the side of the cut-out device 61. In so doing, the plate 62 of insulating material of the cut-out device 61 ruptures at the desired breaking point and with it also the metallic coating 63. The arc formed between the two free ends of the metallic coating 63 heats the foam material 27 very strongly so that the latter released charge carrier collectors and chemical elements reacting with one another as already described in accordance with the embodiment of Figures 1 to 4. As already described, charge carriers are withdrawn from the arc plasma thereby. In addition, insulating coverings are formed at the free ends of the metallic cotating 63. Both of these conditions lead to a rapid reduction in the conductivity of the arc and thus to its rapid extinction.

Thus, in this capacitor, only the arc quenching property of the body 64 of foam material made from polyethylene is used.

Claims (10)

1. An electrical capacitor comprising a capacitor body which has a dielectric which releases gas under inadmissibly high heating, a cut-out device which has at least one electrical conductor provided with a desired breaking point which is separated on the occurrence of an excess pressure brought about by the formation of gas and thus interrupts a current flow conducted thereby, a foam material partially filling the interior of the capacitor in the region of the conductor and a housing surrounding the capacitor body, the cut-out device and the foam material, characterised in that, the housing has at least one expansible zone permitting a definite expansion of the housing on the occurrence of excess pressure, that the conductor is stressed in the expansion direction of the housing and is rigidly connected to the housing before and after the expansible housing zone and that the foam material completely surrounds the conductor at least in the region of the desired breaking point and at least over a portion of its length, preferably with a gap.

2. A capacitor according to claim 1 characterised in that a plate of insulating material is retained in front of and behind the expansible housing zone and that the electrical conductor is stressed between the two plates of insulating material.

3. A capacitor according to claim 2 characterised in that a disc of foam material which has a passage receiving the electrical conductor is arranged between the plates of insulating material.

4. Acapacitor according to claim 3 characterised in that the plates of insulating material are arranged at one end of the capacitor body spaced one behind the other in an axial direction of the housing (28).

5. A capacitor according to claim 1 characterised in that a plate of insulating material is retained in the housing at one end of the capacitor body and spaced therefrom, that the expansible housing zone lies between the plate of insulating material and the capacitor body and that the electrical conductor is stressed between the plate of insulating material and the capacitor body held axially non-displaceably in the housing.

6. A capacitor according to claim 5 provided with an internally hollow capacitor body, preferably a hollow cylindrical capacitor winding, characterised in that the electrical conductor passes through the interior hollow space of the capacitor body and the interior hollow space is at least partially filled with the foam material.

7. A capacitor according to one of claims 1 to 6 characterised in that the expansible housing zone has expansion folds and/or expansion flanges.

8. A capacitor provided with a capacitor body which has a dielectric which releases gas due to inadmissibly high heating, comprising a cut-out device which has at least one electrical conductor provided with a desired breaking point which is separated on the occurrence of an excess pressure produced by the formation of gas and thereby interrupts a current flow conducted thereby, a foam material partially filling the interior of the capacitor in the region of the conductor and a housing surrounding the capacitor body, the cut-out device and the foam material, especially according to one of claims 1 to 7, characterised in that the foam material is a closed pore polyethylene.

9. A capacitor according to one of claims 1 to 8 characterised in that the foam material has fire-proof means (for example antimonytrioxide).

10. A capacitor substantially as hereinbefore described with reference to the accompanying drawings.