GB2226448A

GB2226448A - Electrical capacitors protected against over-pressure

Info

Publication number: GB2226448A
Application number: GB8925434A
Authority: GB
Inventors: David R Snowdon
Original assignee: CAMBRIDGE CAPACITORS Ltd
Current assignee: CAMBRIDGE CAPACITORS Ltd
Priority date: 1988-11-12
Filing date: 1989-11-10
Publication date: 1990-06-27
Also published as: GB8826530D0; WO1990005370A1; GB8925434D0

Abstract

A capacitor is composed of a casing (4) containing a capacitor element (1) connected with conductors (6, 7) to terminals (21, 22) on a sealed cover (20). Weakened zones (36) of the conductors (6, 7) act as a cut-out device and are designed to fracture if the pressure rises in the casing (4) and causes an upper region of the casing (4) to deform. To optimise the response of the cut-out device the majority of the casing interior is filled with a particulate electrically insulating material (45).

Description

ELECTRICAL CAPACITORS This invention relates to electrical capacitors.

In a known construction, an electrical capacitor comprises a sealed casing, a 'dry' capacitor element housed in the sealed casing, terminals accessible from the exterior of the casing, conductors connecting the terminals to the capacitor element and pressure sensitive cut-out or interrupter means for disconnecting the capacitor element from the terminals in the event of an increase of pressure within the casing caused as a result of overheating and/or electrical breakdown of the capacitor element. The interrupter means is thus a safety device. Ideally the operation of this device in response to the rise in pressure within the casing should be as rapid as possible to minimise the danger of an explosion and to reduce the risk of damage to associated components if a high current flows through the capacitor for too long.

It is an object of the present invention to provide a capacitor of the type to which the invention relates with a more rapid cut-out action.

In accordance with the invention, the unoccupied volume within the capacitor casing is wholly or partly filled with a pourable, electrically insulating, particulate filler material. The filler material preferably substantially fills the otherwise unoccupied space in the casing but in general the filler material should occupy from 50 to 95% and more preferably 80% of the free volume otherwise present in the casing. The filler material allows the passage of any gas generated in the casing to create the necessary increase in pressure to operate the interrupter means.

Since the rate of rise of pressure is inversely related to the unoccupied volume within the sealed case it will be apparent that a reduction of this volume in accordance with the invention will produce a speedier rise of pressure for a given volume of gas generated as a result of breakdown and/or overheating and hence a more rapid operation of the interrupter means.

The filler can be sand, crushed marble or alumina which is introduced into the casing by pouring during manufacture and assembly.

The casing can be sub-divided interiorly to provide a main cavity in which the capacitor element is located and an upper chamber in which the interrupter means is located. A fold in the wall of the casing can be provided between the main cavity and the upper chamber and this can permit an upper region of the casing around the upper chamber to displace as the pressure rises inside the casing beyond a pre-determined level. Such displacement or deformation then causes the operation of the interrupter means.

In another aspect, the invention provides a method of making a capacitor comprising: forming a sub-assembly by attaching conductors to a preformed capacitor element and fitting the capacitor element to locating means, weakening the conductors over zones which are intended to form a pressure-sensitive interrupter means, introducing the sub-assembly into a casing, forming a folded or collapsed portion around the casing to engage at least part of the location means and hold the sub-assembly in place, introducing a particulate filler material into the casing to substantially fill the free volume therein, attaching a cover with terminals to the casing, sealing the casing to the cover and connecting the conductors to the terminals.

The invention may be understood more readily, and various other features of the invention may become apparent from a consideration of the following description.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a sectional side view of a capacitor constructed in accordance with the invention; Figure 2 is a sectional side view of the capacitor shown in Figure 1 after operation of a safety cut-out device; Figure 3 is a plan view of an assembly used in the capacitor shown in Figures 1 and 2; Figure 4 is a side view of part of the assembly in Figure 1; and Figure 5 is an end view of the part of the assembly shown in Figure 4.

As shown in Figures 1 and 2, a capacitor is composed of a tubular capacitor element 1 mounted within a main cavity 48 inside a tubular casing 4 made, for example, of aluminium. The casing 4 adjoins a cover 20 which is separately formed and rests on a depression 19 in the casing 4. A pair of external terminals 21,22 are provided on the cover 20. An optional discharge resistor 23 is connected between the terminals 21,22 inside the cover 20. The cover 20 is fixed and sealed to the casing 4 with the aid of a folded rim 9. The capacitor element 1 is made up from multiple layers of a metallised plastics film, for example polypropylene, wound in a roll in a manner known per se or dry capacitors. The ends of the element 1 have metallised surfaces 24,25 which provide electrical connectors for the element 1.The capacitor element 1 is held coaxial within the casing 4 by means of insulating mounting components 2,3 with locating pegs 50,51 projecting into the element 1 from above and below.

The components 2,3 are fitted at upper and lower end regions of the casing 4. The upper component is in the form of a plate 3 perforated with one or more holes 26 while the lower component 2 is of cup-like hollow configuration.

An annular space 44 between the inner wall surface of the casing 4 and the centralised capacitor element 1 is substantially filled with an electrically insulating particulate filler material 45 which is flowable or pourable.

A cavity or chamber 40 is formed at the upper region of the casing 4 by a folded intermediate portion or rill 5 in the wall of the casing 4. The chamber 40 is formed in the casing 4 between the upper mounting plate 3 and the cover 20. A safety cut-out device 41 is provided in this chamber 40. As illustrated the safety device 41 takes the form of frangible zones 36 of conductors 6f7 interconnecting the terminals 21,22 t9 the connectors of the element 1. The zones 36 of the conductors 6,7 can be rendered fragible by structural weakening by any suitable process. The conductors 6,7 are bent into bows below the weakened zones 36 and these bows extend through slots 8 in a boss 12 on the upper side of the plate 3 as shown in Figure 1 and through holes 15,16.The bows of the conductors 6,7 are held in the slots 8 by another plate 11 fitted over the boss 12 and fixed to the latter by welding, preferably by ultrasonic means. The conductors 6,7 pass through holes 15,16 in the plate 3.

The conductor 6 extends through the peg 50 and the capacitor element 1 to emerge below the latter for connection with the lower connector 25. The conductor 7 makes connection with the upper connector 24.

A method of manufacturing the capacitor shown in the drawing will now be described. Initially, the conductors 6,7 are attached to the connectors 24,25 of the preformed capacitor element 1 and the element is fitted between the mounting components 2,3 with the conductors 6,7 passing through the holes 15,16. The conductors 6,7 are then bent and positioned in the slots 8 and the plate 11 is introduced to hold the conductors 6,7 in place.

The plate 11 is then fixed to the boss 12 by heating e.g.

by ultrasonic energy. The conductors 6,7 are then weakened over the zones 36 by any suitable process such as cold forming, machining or laser cutting. The subassembly 1,6,7,2,3 is then introduced into the casing 4 and the casing 4 is treated to form the folded or collapsed portion 5 to hold the sub-assembly in place.

The casing 4 is further treated to provide the depression 19 below the upper rim 9 and the filler material 45 is poured into the casing 4 and passes through the holes 26 in the plate 3 to substantially fill the space 44 and then the chamber 40 above the plate 3. The cover 20 with the preformed terminals 21,22 is then placed onto the casing 4 to engage on the depression 19 with the conductors 6,7 passing into the terminals 21,22. The cover 20 is fixed in place by forming the in-turned rim 9. The casing 4 can be sealed by spinning or curling the rim 9 of the casing onto the upper face of the cover 20 and by simultaneously using a sealant between the casing 4 and the cover 20. Finally, the conductors 6,7 are soldered to the terminals 21,22.

The cut-out device 41 can take other forms but serves to disconnect one or both terminals 21,22 from the capacitor element 1 in the event of increased pressure within the casing 4. This can be appreciated by comparing Figures 1 and 2.

Figure 1 shows the capacitor in its normal operating state with the terminals 21,22 connected via the conductors 6,7 to the capacitor element 1. In the event of a fault, the capacitor element 1 can dissipate excessive heat which generates gas or vapour, principally hydrogen, which represents a safety hazard in the event of arcing. To preclude this any increased pressure by gas emission within the casing will act on the cover 20 to cause the upper portion of the casing 4 to displace by deformation of the folded portion 5. This in turn subjects the conductors 6,7 to strain and results in fracture of the conductors 6,7 as shown in Figure 2.

Hitherto, in capacitors of the type to which the invention relates, any pressure rise has taken some time to build up because the volume of gas needs substantially to fill the entire free volume in the interior of the casing 4. Consequently there can be some delay before the safety device 41 operates. By reducing the free volume with the particulate material filler 45 which nevertheless permits the gas to permeate, the safety device 41 is able to respond more quickly and reliably.

It is preferable for the particulate material 45 to occupy from 50% to 95% and more preferably 80% of the free volume in the capacitor casing 4. Examples of suitable filler materials 45 are sand, alumina and crushed marble.

A number of capacitors generally constructed in accordance with the invention but with and without the presence of a particulate material filler 45 in the form of 'Chelford 50' peat-free sand supplied by British Industrial Sand Limited, Macclesfield, Cheshire, were tested by providing an excess voltage of 3kV dc to the terminals 21,22 to break down the capacitor element 1 and then subjecting the capacitor to 500 V a.c.

The results were as follows: CAPACITOR 1. 6 microfarad 250V a.c. working Casing 35mm diameter and 78mm long WITHOUT FILLER WITH FILLER Casing did not expand and 12 samples were tested.

the capacitor went open The casings deformed and circuit but the cut-out the cut-out devices 41 device 41 had not operated operated in all cases within a time period of 4 to 10 seconds after application of the excess voltage CAPACITOR 2. 20 microfarad 250V a.c working casing 35mm diameter and 78mm long WITHOUT FILLER WITH FILLER Casing expanded and the 12 samples were tested.

cut-out device 41 operated The casings deformed and some 10 seconds after the cut-out device 41 operated application of the in all cases between 3 and 6 of the excess voltage seconds after application of the excess voltage.

As appears, the introduction of the filler material 45 improves the response characteristis and reliability of the cut-out device 41.

Claims

1. A capacitor comprising a sealed casing, a capacitor element disposed in the casing, terminals, accessible from the exterior of the casing, conductors within the casing connecting the terminals to the capacitor element, pressure-sensitive interrupter means for disconnecting the capacitor element from one or more of the terminals in the event of increased pressure within the casing and an electrically insulating particulate filler material which substantially fills the unoccupied space in the casing but allows the passage of any gas generated within the casing therethrough to create the increased pressure to operate the interrupter means.

2. A capacitor according to claim 1, wherein the filler material occupies 50% to 95% of the free volume within the casing.

3. A capacitor according to claim 2, wherein the filler material occupies 80% of the free volume within the casing.

4. A capacitor according to claim 1, 2 or 3 wherein the filler material is sand, crushed marble or alumina.

5. A capacitor according to any one of claims 1 to 4, wherein the interior of the casing is sub-divided to form a main cavity in which the capacitor element is disposed with an annular space therearound substantially filled with the filler material and an upper chamber in which the interrupter means is disposed.

6. A capacitor according to claim 5, wherein the interrupter means is constituted by weakened zones of the conductors in the upper chamber connecting the terminals to the capacitor element and the casing has a folded or collapsed portion between the main cavity and the upper chamber which permits an upper region of the casing around the upper chamber to be displaced relative to the remainder of the casing as the pressure inside the casing rises to fracture the weakened zones of the conductors.

7. A capacitor according to claim 6, wherein the capacitor element is located between a pair of mounting components one of said components abutting said folded portion of the casing and being perforated to permit the filler material to be poured into the main cavity.

8. A capacitor substantially as described with reference to, and as illustrated in, any one or more of the Figures of the accompanying drawings.

9. A method of making a capacitor comprising forming a sub-assembly by attaching conductors to a preformed capacitor element and fitting the capacitor element to locating means, weakening the conductors over zones which are intended to form a pressure sensitive interrupter means, introducing the sub-assembly into a casing, forming a folded or collapsed portion around the casing to engage at least port of the location means and hold the sub-assembly in place, introducing a particulate filler material into the casing to substantially fill the free volume therein, attaching a cover with terminals to the casing, sealing the casing to the cover and connecting the conductors to the terminals.

10. A method of making a capacitor substantially as hereinbefore described.