GB2089127A - Venting to capacitor cut-out - Google Patents

Abstract

In a capacitor (1) with a pressure-responsive cut-out, the capacitative element (2) is submerged in a solid encapsulant (6) within a fluid-tight container (4); there is provided adjacent to the element, a passage (e.g. closed cell foam 5 or a closed thin-walled tube, which is substantially fluid-tight under normal operating conditions but under fault conditions permits gases, emitted from the element, free access to operate the cutout. When the passage is a layer of closed-cell foamed plastics material (5) contacting the inner surface of the circumferential wall of the container, with a rough surface at least at its interface with the encapsulant, the heat generated under fault conditions softens the plastics foam to allow gas escape. In the second embodiment, the thin-walled tube ruptures or softens. <IMAGE>

Description

SPECIFICATION Capacitors The present invention relates to electric capacitors having pressure-tight containers and pressureresponsive cut-outs to break at least one electrical connection to the element under fault conditions.

More especially (but not exclusively) it relates to metallised-film and other capacitors for use in power circuits, e.g. in circuits supplying fluorescent lamps or electric motors.

In order to avoid deterioration of a capacitor element caused by exposure to air or moisture, the element is often encapsulated and sealed in a suitable plastics material (e.g. polyurethane resin), but this hinders gases discharged from the element during certain fault conditions from reaching and actuating the cut-out device.

The present invention provides a capacitor comprising: a capacitative element submerged in a solid encapsulant material within a fluid-tight container; a pressure-responsive cut-out to break at least one electrical connection to the element, operative at a pressure in the container greaterthan normal but less than the bursting pressure of the container; and, positioned adjacent to the element, passage means which is so sealed as to be substantially fluid-tight under normal operating conditions but under fault conditions permits gases, emitted from the element, free access to the cut-out.

The passage means may be capable of transmitting pressure at high levels by elastic or plastic deformation, or it may be sealed by means that will fail by fracture or otherwise under high pressure differential, or it may be effective to transmit pressure as a result of melting or softening of all or part of it due to temperature rise caused by the fault.

Thus gas, emitted from a fault occuring in the element, can readily pass to, and activate, the disconnector before rupture of the capacitor itself occurs, even if the fault occurs at a part of the element remote from the disconnector.

Preferably the passage means comprises a body of foamed solid material. The foamed material, e.g. a closed-cell foam, may be a layer contacting the inner surface of the circumferential wall of the container, (optionally additionally contacting the inner base surface of the container); the foamed material may be backed with an insulating film of plastics material (optionally the same material in an unfoamed form).

Preferably the foamed layer has a rough surface at least at its interface with the encapsulant thereby providing crevices in which the gas can accumulate.

Preferably the foamed material has a lower softening temperature than that of the encapsulant, typical values of the softening point being about 90"C and 1600C respectively; as a capacitor element during failure typically heats the surroundings to above 95"C, in fault conditions the foamed material will be softened thereby reducing its resistance to compression, while the encapsulant is unaffected.

The foamed material may comprise foamed polyethylene or polyester having bubbles of any suitable inert gas, e.g. Freon (Registered Trade Mark).

As an alternative to the foamed material, the passage means may comprise one or more than one tube with a fuseable and/or frangible seal at one or each of its ends; in one preferable form a single tube is positioned within the bore of a tubular element.

The cut-out may be of any suitable form, examples being the corrugation-chamber type generally described in our British Patent Specification Nos.

1291291 and 1351290 or the double-disc type generally described in our British Patent Specification No.

1539194.

Part of the cut-out may be outside the pressuretight container; for example an external relay circuit can be provided to simultaneously break an electrical connection to the element and connect the electrical supply to another circuit of similar location to that of the element upon activation of the cut-out.

In order that the invention may more readily be understood, a description is now given, by way of example only, reference being made to the accompanying drawing in which Figure 1 is a cross-section (not to scale) of part of a capacitor embodying the present invention; and Figure 2 is a cross-section (not to scale) of part of an alternative embodiment.

Capacitor 1 comprises an element 2 formed of metallised film wound onto a core 3 and supported in an extruded aluminium can 4which has a 2 mm thick layer of foam 5, formed of polyethylene into which Freon (Registered Trade Mark) has been dissolved to form bubbles, contacting the inner circumferential wall of can 4. Element 2 is submerged in polyurethane resin 6 which acts as a barrier against air or moisture.

Foam 5 provides a path of lower resistance to gas, emitted upon failure of element 2, than any path through the resin. Thus even when a failure occurs on a part of element 2 (e.g. at A) remote from the cut-out (not shown in the Figures) the resultant gas readily passes through foam 5 and activates the cut-out. Without foam 5, gas from a fault at A could not pass through the entire body of resin 6 and would accumulate to form one or more pockets of gas with localised high pressure which could eventually cause splitting of the resin and/or explosion of the capacitor.

It normal operation of the element, the foam 5 is sufficiently rigid to transmit clamping forces exerted by the can 4 on the element, thereby providing it with a "corseting" effect which restricts thermal expansion and reduces void formation and consequent ionisation under electrical stress.

When a fault occurs at a point on element 2, a substantial amount of gas is created which, when it reaches the foam, compresses the bubbles thereby deforming foam 5 to produce a passageway towards the cut-out along the interface with resin 6. Part of the gas can permeate through the foam 5 itself.

The deformation of the foam can be further aided if heat dissipated at the fault raises the temperature of the foam above its softening point, which is about 90"C, thereby substantially lower than that of the resin at about 16000. In some cases this softening effect may only be localised.

The surface of the foam at its interfaces with can 4 and with resin 6 are rough compared with the surfaces where can and resin contact directly. This roughness can improve the movement of gas along the foam as it enables the gas to be readily trapped and accumulate in the crevices at the interface with resin 6; moreover crevices at the interface with can 4 act as if they were bubbles, thereby increasing pliability of the foam under pressure of gas.

There is no significant permeation of resin into the bubbles which might increase the foam's resistance to compression.

In Figure 2, where the same numerals are given to features identical to that already described, capacitor 7 has a thin-walled polyethylene (with a softening point at about 90"C) tube 8 sealed at its upper and lower end which provides a passageway through resin 6. Tube 8 is positioned in the bore of core 3 with its sealed lower end at the level of the bottom of element 2. When a fault occurs in element 2, passage of gas into tube 8 is made easier by softening of the polyethylene material oftube 8 and/or by rupturing of the seal due to dissipation of heat from the fault.

Claims (11)

1. Acapacitorcomprising:- a capacitative element submerged in a solid encapsulant material within a fluid-tight container; a pressure-responsive cut-out to break at least one electrical connection to the element, operative at a pressure in the container greater than normal but less than the bursting pressure of the container; and, positioned adjacent to the element, passage means which is so sealed as to be substantially fluid-tight under normal operating conditions but under fault conditions permits gases, emitted from the element, free access to the cut-out.

2. A capacitor according to Claim 1, wherein the passage means comprises a body of foamed solid material.

3. A capacitor according to Claim 2 wherein the foamed material is a layer contacting the inner surface of the circumferential wall of the container.

4. A capacitor according to Claim 2 or Claim 3, wherein the foamed material is a layer contacting the inner base surface of the container.

5. A capacitor according to any one of Claims 2 to 4, wherein the foamed material is a closed-cell foam.

6. A capacitor according to any one of Claims 2 to 5, wherein the foam layer has a rough surface at least at its interface with the encapsulant.

7. A capacitor according to any one of Claims 2 to 6, wherein the foam layer has a lower softening temperature than that of the encapsulant.

8. A capacitor according to any one of Claims 2 to 7, wherein the foam layer comprises foamed polyethylene.

9. A capacitor according to Claim 1, wherein the passage means comprises one or more than one tube with a fusible and/or frangible seal at each of its ends.

10. A capacitor according to Claim 9 comprising a single tube positioned within the bore of a tubular element

11. A capacitor substantially as hereinbefore described with reference to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.