INSULATOR ASSEMBLIES

This invention relates to a seal assembly and a vacuum interrupter including such a seal assembly .

Vacuum interrupters are typically used to act as a load break switch or a circuit breaker in medium and high voltage applications. The operation of the vacuum interrupter relies on the mechanical separation of electrically conductive contacts to open the associated electrical circuit. The operation of the vacuum interrupter at higher voltages results in the development of high electrical stress which may lead to electrical breakdown not only between the contacts, but also external to the vacuum interrupter.

Electrical breakdown occurs when the dielectric strength of a medium is exceeded. This may lead to the initiation of internal and/or external arc discharges which can cause degradation of the vacuum interrupter and affect the safety of those working in the immediate vicinity of the vacuum interrupter installation. The separation of the contacts is typically carried out in a high vacuum environment to extinguish the arc initiated across its contacts. It is, however, still necessary to provide external insulation to the vacuum interrupter which not only extends the lifetime of the vacuum interrupter by avoiding damage due to electrical breakdown but also ensures a safe working environment in the immediate vicinity of the vacuum interrupter installation. A known way of electrically insulating a vacuum interrupter is to provide a vacuum-tight enclosure which includes one or more hollow, annular, insulating members having a glass and/or glass-ceramic composition. Glass-based insulating members provide excellent electrical insulation and are highly resistant to corrosion. The resistance to corrosion also removes the need for an external housing when the vacuum interrupter is used outdoors.

In addition, glass-based insulating members of various shapes and sizes require less energy and are cheaper to manufacture compared to, e.g. metallised alumina ceramic insulating members. Moreover, glass- based insulating members can be recycled easily by re- melting at the end of their useful life.

Furthermore the use of glass-based insulating members minimises the use of sulphur hexafluoride, SF ₆ , to provide electrical insulation. As a result it is not necessary to carry out regular inspection and maintenance to prevent leakage of SF ₆ , which is a greenhouse gas, into the atmosphere, and so the complexity of the vacuum interrupter design is reduced .

The use of glass-based insulating members is, therefore, preferable in terms of economy and environmental concern.

A conventional vacuum-tight enclosure, i.e. a conventional insulator assembly, includes one or more metal connecting members which interconnect respective insulating members and couple the insulating members with the electrically conductive contact elements of the associated vacuum interrupter.

Each connecting member has a body portion which directly engages with the corresponding insulating member. In particular, to form a sealing joint between each connecting member and the corresponding insulating member the body portion of each connecting member is moulded directly into one end of the corresponding insulating member using a press tool, or any other suitable mechanical operation, while the glass-based insulating member is in a molten state.

Subsequent fabrication of the insulator assembly and the associated vacuum interrupter often involves multiple high-temperature cycling operations.

This leads to considerable mechanical stress at the interface between each metal connecting member and the corresponding glass-based insulating member. The mechanical stress arises as a result of the glass-based insulating member and the metal connecting members having different coefficients of thermal expansion. An increase in mechanical stress has a detrimental effect on the integrity and reliability of the interface joint.

In addition, operation of, e.g. a vacuum interrupter, leads to high levels of electrical stress at the interface joint between the metal connecting members and the corresponding glass-based insulating member .

If the operating voltage of the vacuum interrupter were increased, even higher levels of electrical stress would result and the interface joint would progressively degrade and eventually fail. It is, therefore, necessary to artificially limit the operating voltage range of conventional vacuum interrupters so as to maintain their reliability. Such limiting is, however, undesirable from a commercial perspective .

There is, therefore, a need for an improved insulator assembly in which the interface between respective insulating members and connecting members has a greater resistance to mechanical and electrical stresses .

According to a first aspect of the invention, there is provided an insulator assembly comprising at least one insulating member sealingly coupled with at least one connecting member, the insulating member defining a hollow insulator body and the connecting member defining a body portion having an innermost body wall and an outermost body wall and an engagement portion having an innermost engagement wall and an outermost engagement wall, the engagement portion being sealingly retained at least partially within the insulator body, and the portions of the innermost and outermost engagement walls that are sealingly retained within the insulator body being further spaced from one another than the innermost and outermost body walls.

The provision of an engagement portion having innermost and outermost engagement walls sealingly retained within the insulator body which are further spaced from one another than the innermost and outermost body walls of the body portion increases the external surface area of the connecting member in contact with the insulator body compared to the amount of external surface area that is in contact with the insulator member when the body portion is directly retained within the insulator body.

Increasing the amount of external surface area of a connecting member that is in contact with a corresponding insulator member increases the interface area between the connecting member and the corresponding insulator member, and so increases the mechanical bond strength at the said interface. An increase in mechanical bond strength improves the resistance of the interface to mechanical stress.

In addition, having a larger interface area means that a given degree of electrical stress generated between the connecting member and the corresponding insulating member is distributed over a greater area, and so results in lower electrical stress per unit area.

Preferably the engagement portion of the or each connecting member includes an inner leg defining the innermost engagement wall and an outer leg defining the outermost engagement wall, the outer leg being resiliently biased relative to the inner leg.

The provision of inner and outer legs allows one leg to move relative to the other leg, so as to accommodate differences in the extent of temperature induced expansion and contraction between the connecting member and insulating member, while resiliently biasing the outer leg relative to the inner leg helps to ensure that the outer leg remains in sealing engagement with the insulator body.

Optionally the inner leg and the outer leg are connected to one another by a deformable coupling. The inclusion of a deformable coupling maintains a desired spacing between the inner and outer legs while providing a necessary degree of resilient biasing between the inner and outer legs .

In a preferred embodiment of the invention the deformable coupling defines a smooth transition between the innermost and outermost engagement walls.

Having a smooth transition between the innermost and outermost engagement walls minimises the concentration of any mechanical or electrical stress along the interface between the connecting member and the corresponding insulating member.

The inner and outer legs and the deformable coupling may be integrally formed from the same material .

Integrally forming these elements from the same material helps to ensure that the engagement portion is readily manufacturable while maintaining the hermetic nature of the sealing and structural integrity of the engagement portion.

Preferably the engagement portion is further integrally formed with the body portion. Such an arrangement further assists with manufacture of the connecting member and helps further maintain the hermetic nature of the sealing and structural integrity of the engagement portion. In another preferred embodiment of the invention the outer leg includes a curved end portion extending towards the inner leg.

The inclusion of a curved end portion provides the exterior of the connecting member with a passive formation which unlikely to cause injury to an installer or maintenance operative handling the insulator assembly of the invention. Such a feature also helps to deter the ingress of contaminants between the inner and outer legs.

Optionally the insulator assembly includes a first insulator member defining a first hollow insulator body having first and second ends, and first and second connecting members, the engagement portion of the first connecting member being sealingly retained at least partially within the first end of the first insulator body, and the engagement portion of the second connecting member being sealingly retained at least partially in the second end of the first insulator body.

Such an arrangement provides a desired vacuum-tight enclosure in which the interface between constituent connecting and insulating members has an increased resistance to mechanical and electrical stress.

An insulator assembly according to a still further preferred embodiment of the invention includes first and second insulating members defining respective first and second hollow insulator bodies having first and second ends, and first, second and third connecting members, each of the first and second connecting members including a single engagement portion and the third connecting member including first and second engagement portions extending substantially opposite one another, the engagement portion of the first connecting member being sealingly retained at least partially within the first end of the first insulator body, the engagement portion of the second connecting member being sealingly retained at least partially within the second end of the second insulator body, and the third connecting member interconnecting the first and second insulator bodies with the first engagement portion of the third connecting member being sealingly retained at least partially within the second end of the first insulator body and the second engagement portion of the third connecting member being sealingly retained at least partially within the first end of the second insulator body.

Such an arrangement provides a scalable vacuum-tight enclosure in which the interface between constituent connecting and insulating members exhibits a desired increase in resistance to mechanical and electrical stress.

Optionally the third connecting member further includes a shield member located inboard of the first and second engagement portions, the shield member extending over each of the first and second insulator bodies. The inclusion of such a shield member helps to maintain the dielectric strength of the insulator assembly during, e.g. separation of opposed contact surfaces in an associated vacuum interrupter, and thereby helps to extend the lifetime of the associated vacuum interrupter.

According to a second aspect of the invention there is provided a vacuum interrupter including an insulator assembly as described hereinabove .

The vacuum interrupter shares the advantages of the corresponding features of the insulator assembly, as mentioned above.

There now follows a brief description of preferred embodiments of the invention, by way of non- limiting examples, with reference being made to the accompanying figures in which:

Figure 1 shows a schematic view of an insulator assembly according to a first embodiment of the invention;

Figure 2 shows an enlarged portion of the insulator assembly shown in Figure 1 ;

Figure 3 shows a finite element model of mechanical stress arising from a temperature change in a portion of the insulator assembly shown in Figure 1 ;

Figure 4 shows a finite element model of mechanical stress arising from an identical temperature change in a portion of a conventional insulator assembly;

Figure 5 (a) shows a finite element model of electrical stress in the insulator assembly shown in Figure 1;

Figure 5 (b) shows an enlarged portion of the finite element model shown in Figure 5 (a) ; Figure 6 (a) shows a finite element model of electrical stress in a conventional insulator assembly;

Figure 6 (b) shows an enlarged portion of the finite element model shown in Figure 6 (a) ; and

Figure 7 shows a comparison of the electrical stresses shown in Figures 5 and 6.

An insulator assembly according to a first embodiment of the invention is designated generally by the reference numeral 10.

The insulator assembly 10 includes first and second insulating members 12, 14 which are sealingly coupled with first, second and third connecting members 16, 18, 20.

The first insulating member 12 defines a first hollow insulator body 22 which has first and second ends 24, 26, and the second insulating member 14 defines a second hollow insulator body 28 that has first and second ends 30, 32.

In the embodiment shown the insulator bodies 22, 28 are identical annular rings and each has a glass-based composition. The glass-based composition may include a glass or a glass-ceramic material. In other embodiments of the invention (not shown) the insulator bodies 22, 28 may have different shapes, and/or different proportions according to the configuration in which the insulator assembly is to be used .

Each of the first and second connecting members 16, 18 includes a body portion 34, which has an innermost body wall 36 and an outermost body wall 38. Each of the first and second connecting members 16, 18 also includes a first engagement portion 40 that has an innermost engagement wall 42 and an outermost engagement wall 44.

The third connecting member 20 includes an interconnecting body portion 46 and first and second engagement portions 40, 48. The first and second engagement portions 40, 48 extend substantially opposite one another, and each engagement portion 40, 48 includes respective innermost and outermost engagement walls 42, 44. In other embodiments of the invention (not shown) the first and second engagement portions 40, 48 may extend relative to one another in a different manner.

In the embodiment shown each of the connecting members 16, 18, 20 is formed from a metal material. In other embodiments of the invention (not shown) the connecting members 16, 18, 20 may be formed from a different material.

The first engagement portion 40 of the first connecting member 16 is sealing retained partially within the first end 24 of the first insulator body 22, and the first engagement portion 40 of the second connecting member 18 is sealingly retained partially within the second end 32 of the second insulator body 28.

The third connecting member 20 interconnects the first and second insulator bodies 22, 28. In particular, the first engagement portion 40 of the third connecting member 20 is sealingly retained partially within the second end 26 of the first insulator body 22 and the second engagement portion 48 of the third connecting member 20 is sealingly retained partially within the first end 30 of the second insulator body 28.

Each of the engagement portions 40, 48 is sealingly retained within the corresponding insulator body 22, 28 by forcing the engagement portion 40, 48 into the respective end of the corresponding insulator body 22, 28 while the insulator body 22, 28 is in a molten state.

The portions of each of the innermost and outermost engagement walls 42, 44 that are sealingly retained within the corresponding insulator body 22, 28 are spaced from one another to a greater extent than the innermost and outermost body walls 36, 38 of the body portion 34.

In the embodiment shown each engagement portion 40, 48 includes an inner leg 50 which defines the innermost engagement wall 42 and an outer leg 52 that defines the outermost engagement wall 44. The outer leg 52 is resiliently biased relative to the inner leg 50.

The inner and outer legs 50, 52 are connected to one another by a deformable coupling 54 which defines a smooth transition between the innermost and outermost engagement walls 42, 44.

In the embodiment shown the inner and outer legs 50, 52 and the deformable coupling 54 are integrally formed from the same material, i.e. a metal. In particular the legs 50, 52 and deformable coupling 54 are formed from a single sheet of metal which is press-formed into the desired shape. In the embodiment shown each engagement portion 40, 48 is also integrally formed with the corresponding body portion 34. In other embodiments of the invention (not shown) the engagement portion may be formed separately from the corresponding body portion.

Each outer leg 52 includes a curved end portion 56 that extends towards the inner leg 50. In other embodiments of the invention (not shown) the end portion 56 may have a different shape, e.g. straight, but still extends towards the inner leg 50 so as to at least partially close off the gap between the inner and outer legs 50, 52.

In the embodiment shown the third connecting member 20 additionally includes a shield member 58 which is located inboard of the first and second engagement portions 40, 48. The shield member 58 has an annular configuration and extends over each of the first and second insulator bodies 22, 28.

In the embodiment shown the insulator assembly 10 is arranged within a vacuum interrupter 60.

The vacuum interrupter 60 includes first and second contact elements 62, 64 each of which is secured to an end of a respective first or second conductor rod 66, 68.

The first conductor rod 66 is moveably coupled with the first connecting member 16 via a set of tubular bellows 70. The second connector rod 68 is fixedly secured to the second connecting member 18.

In use the first conductor rod 66 is moveable relative to the insulator assembly 10 so as to separate the first and second contact elements 62, 64 from one another (and hence break the circuit within which the conductor rods 66, 68 are electrically connected) .

The greater resistance to mechanical and electrical stress of the interface between insulating and connecting members in the insulator assembly described above, as compared to the interface in a conventional insulator assembly, is illustrated in Figures 3 to 7.

Figure 3 shows a finite element model to calculate the distribution of mechanical stress in the second end 32 of the second insulator body 28 of the invention when the second insulator body 28 and associated second connecting member 18 are heated over a temperature range of 20°C to 800°C.

Figure 4 shows a finite element model to calculate the distribution of mechanical stress in the same region a conventional insulator assembly under the same heating conditions.

The results of the aforementioned calculations show that the maximum mechanical stress per unit area is significantly lower in the insulator body 28 within the insulator assembly 10 of the invention .

Figures 5 (a) and 5 (b) show the distribution of electrical stress during operation in service of the insulating assembly 10 of the invention, while Figures 6 (a) and 6 (b) show the distribution of electrical stress in a conventional insulating assembly during the same operation in service. Figure 7 shows a comparison of the electrical stress distribution in the insulator assembly 10 of the invention and the conventional insulator assembly.

First and second plots Invi, Inv ₂ show the electrical stress distribution adjacent the first and second ends 30, 32, respectively, of the second insulator body 28 of the insulator assembly 10 of the invention .

Third and fourth plots Coni, Con ₂ show the electrical stress distribution adjacent to first and second ends of an insulator body in the conventional insulator assembly.

Comparing the first and third plots Invi, Coni from adjacent the first, i.e. moveable, end 30 of each insulator body it is clear that the maximum electrical stress in the second insulator body 28 of the invention (as shown in plot Invi) is less than half the maximum electrical stress in the same region of the conventional insulator assembly (as shown in plot

Similarly, when considering the second, i.e. fixed, end 32 of each insulator body, the maximum electrical stress in the second insulator body 28 of the invention (as shown in plot Inv ₂ ) is again less than half the maximum electrical stress in the same region of the conventional insulator assembly (as shown in plot Con ₂ ) .

In other embodiments of the invention (not shown) the insulator assembly may include less than or more than two insulator bodies 22, 28, e.g. depending on the operating voltage and the resulting amount of separation required between opposed contact elements in a vacuum interrupter.

In still further embodiments of the invention the insulator assembly may be incorporated within a high voltage electrical bushing, an insulator for transmission lines, an insulating rod for switchgears to provide coupling between the operation mechanism and the switching element, or a high voltage cable gland end terminator.