WO2012159675A1

WO2012159675A1 - Vacuum interrupter

Info

Publication number: WO2012159675A1
Application number: PCT/EP2011/058671
Authority: WO
Inventors: Rama Shanker Parashar
Original assignee: Alstom Technology AG
Current assignee: GE Vernova GmbH
Priority date: 2011-05-26
Filing date: 2011-05-26
Publication date: 2012-11-29
Anticipated expiration: 2013-11-26
Also published as: EP2715762B1; ES2540882T3; EP2715762A1

Abstract

A vacuum interrupter comprises a pair of contact electrodes (44) mounted on electrically conductive rods (34,36) inside a vacuum-tight enclosure so as to define opposed contact surfaces, each of the electrically conductive rods (34,36) including an elongate, tubular wall portion (38) supporting a respective one of the contact electrodes (44) at a first end and being connectable at a second end to an electrical circuit (45) and at least one of the electrically conductive rods (34,36) being movable to open or close a gap between the opposed contact surfaces.

Description

VACUUM INTERRUPTER

This invention relates to a vacuum interrupter. 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.

One example of such a vacuum interrupter comprises a pair of cup-shaped, multi-slotted contact coils, each contact coil being brazed to an end of a respective oxygen-free high conductivity (OFHC) copper conductor rod. The conductor rods are movable relative to each other to either bring the contact coils into contact or separate the contact coils to perform current interruption. The shape of the contact coils results in the generation of a magnetic field which aids the current interruption process.

The operating rating of the vacuum interrupter is related to its continuous current rating, short-circuit current rating and voltage rating. As such, it is necessary to increase the upper limits of one or more of these ratings for the vacuum interrupter to be usable at higher current and/or voltage levels When increasing the voltage rating of a conventional vacuum interrupter, it is necessary to extend the overall length of the vacuum interrupter to comply with insulation requirements. Consequently this results in an increase in length of the current-carrying conductor rods. This in turn increases the overall electrical resistance of the vacuum interrupter and thereby increases thermal losses during operation of the vacuum interrupter. It is possible to minimise the increase in electrical resistance by increasing the cross-section of the conductor rods and increasing the contact pressure between the contact coils to reduce the interfacial resistance between the contact coils. This however results in an increase in mass of the conductor rods and thereby an increase in overall weight of the vacuum interrupter. In addition, the increase in length of the conductor rods increases the risk of mechanical deformation resulting from higher mechanical loads.

Operating the vacuum interrupter at higher continuous current ratings not only raises the issue of sufficient heat dissipation, but also leads to an increase in complexity of arc control and thermal management of the vacuum interrupter. An example of such thermal management is the incorporation of heat sinks towards the end of the current carrying conductor rods, which is necessary to enable the continuous current ratings of conventional vacuum interrupters to exceed 5 to 6 kA. This however adds to the overall weight and cost of the vacuum circuit breakers. Moreover, an increase in short-circuit current rating must be accompanied by uniformity and higher peak value of the magnetic field across the entire region of the contact gap. One way of increasing the current and voltage ratings is to use an assembly of multiple vacuum interrupters in series and parallel. Connecting vacuum interrupters in series results in an improved overall dielectric performance, whilst connecting vacuum interrupters in parallel results in an improved continuous current and short-circuit current performance. The increased complexity of designing and installing such assemblies, together with the use of multiple vacuum interrupters, however increases the operational costs of the current interruption process.

According to an aspect of the invention, there is provided a vacuum interrupter comprising a pair of contact electrodes mounted on electrically conductive rods inside a vacuum-tight enclosure so as to define opposed contact surfaces, each of the electrically conductive rods including an elongate, tubular wall portion supporting a respective one of the contact electrodes at a first end and being connectable at a second end to an electrical circuit and at least one of the electrically conductive rods being movable to open or close a gap between the opposed contact surfaces.

A vacuum interrupter employing rods having an elongate, tubular wall portion in the vacuum interrupter has been found to exhibit decreased electrical resistance compared to a conventional vacuum interrupter employing solid rods with similar cross-sectional dimensions. This is because current flow in a solid rod is restricted mostly to its outer cross-sectional diameter, primarily due to the frequency-dependent skin effect. The skin effect causes the majority of the current to flow through part of the cross-sectional area of the solid rod within the limits set by the skin depth. The remainder of the cross-sectional area of the solid rod plays a minimal role in current flow. As such, a conventional vacuum interrupter employing solid rods exhibits a high electrical resistance, which results in high thermal losses. The use of rods having an elongate, tubular wall portion minimises the skin effect and thereby reduces the electrical resistance of the vacuum interrupter when compared to a conventional vacuum interrupter employing solid rods.

The reduced electrical resistance leads to a decrease in thermal losses, which permits the current and voltage ratings of the vacuum interrupter to be increased without the need for additional thermal management equipment. This not only improves the efficiency and cost-effectiveness of the vacuum interrupter but also minimises the overall size and weight of the vacuum interrupter. The cross-sectional shape of each rod may vary, depending on the design requirements of the vacuum interrupter, and is chosen to match the parameters of the current interruption process. The cross-section of the wall portion of each rod may, for example, be circular, oval, elliptical, or polyhedral in shape. The wall portion of each rod may present a continuous and uninterrupted outer surface. In such embodiments, the outer cross-sectional diameter of the wall portion rod may be in the range of 40 to 150 mm and the inner cross-sectional diameter of the wall portion of each rod may be in the range of 10 to 100 mm.

Rods having a tubular cross-section have been found to exhibit a decreased electrical resistance when compared with solid rods having the same cross-sectional diameter .

In such embodiments the second end of each rod may be sealed to be vacuum-tight so as to ensure the suitability of the rod for assembly into a vacuum interrupter .

In other embodiments of the invention the wall portion of each rod may be defined by an array of elongate elements arranged so that the outer surfaces of the elongate elements define a broken, or otherwise incomplete, outer surface of the elongate, tubular wall portion . In such embodiments the elongate elements may be uniformly distributed about the cross-sectional circumference of the elongate, tubular wall portion.

The use of an array of elongate elements to define the elongate, tubular wall portion has not only been found to exhibit decreased electrical resistance compared to a solid rod defining the same cross-sectional area, but has also been found to increase the total surface area of the rod. It therefore improves heat dissipation via radiation losses. In addition, the use of elongate elements that are spaced from each other reduces the flow of eddy currents, and thereby minimises increases in the temperature of the rods during the current interruption process. This in turn reduces the electrical resistance of the vacuum interrupter. The number of elongate elements in each rod may be two or more .

In other such embodiments each of the elongate elements may be formed by two or more elongate sub-elements connected end to end.

The sub-elements may include a first sub-element extending generally parallel to the longitudinal axis of the respective rod and may further include a second sub-element extending from an axial end of the first sub-element and being shaped to curve about the longitudinal axis of the respective rod, the second sub-elements of the elongate elements defining the first end of the elongate, tubular wall portion of the respective rod. In such embodiments each second sub- element may be shaped to define a helical curve about the longitudinal axis of the respective rod.

The use of a second sub-element connected to the respective first sub-element permits the formation of a coil structure at a first end of the rod. This allows the flow of current through the second sub-elements during the current interruption process to generate a magnetic field, which improves the performance of the current interruption process. The connection of each second sub-element to a first sub-element improves the peak value and the uniformity of the distribution of the generated magnetic field when compared to a conventional cup-shaped, multi-slotted contact coil. In embodiments of the invention employing second sub- elements, the second sub-elements of the electrically conductive rods may be axially curved in the same or opposite directions about the longitudinal axes of the rods .

The direction of the axial curve of the second sub- elements about the longitudinal axes of the rods determines whether an axial or radial magnetic field is formed between the contact surfaces during the current interruption process.

The cross-section of the sub-elements may vary in shape. Each sub-element may, for example, have a circular, oval, elliptical, or polyhedral cross- sectional shape.

The cross-sectional shape of each sub-element may vary, depending on the design requirements of the vacuum interrupter, and is chosen to match the parameters of the current interruption process. In embodiments where the wall portion of each rod includes two or more elongate elements, each rod may further include a support structure extending along the length of the elongate, tubular wall portion, the support structure being spaced from the elongate elements and a first end of the support structure supporting the respective contact electrode at the first end of the rod. In such embodiments, the support structure may include a support frame located within the elongate, tubular wall portion and may preferably include one or more projections radially extending from an outer surface of the support frame.

In other such embodiments, the support structure may include a tubular support frame located on the outside of the elongate, tubular wall portion so as to enclose the elongate elements.

In further embodiments, the support structure may include both a support frame located within the wall portion and a tubular support frame located on the outside of the wall portion

The use of a support structure in the form of one or more support frames provides additional support for the respective contact electrode mounted at the first end of the rod, and structurally reinforces the array of elongate elements. It thereby minimises the risk of mechanical deformation of the elongate elements during the current interruption process, and reduces the risk of failure of the vacuum interrupter.

Preferably a first end of the or each support frame is flared so as to support the respective contact electrode. This provides the or each support frame with greater resilience during the current interruption process . The or each support frame may be constructed from a non-magnetic and electrically conductive material.

One example of such a material is stainless steel, which enhances the structural strength of the support structure and thereby improves the reliability of the support structure.

Preferably the or each support frame has a lower electrical conductivity than the elongate, tubular wall portion. This reduces the size of current flowing through the support frame and thereby minimises the generation of eddy currents in the support frame. This in turn reduces the effect any eddy currents generated within the support structure may have on the peak value and the uniform distribution of the magnetic field that is generated during the current interruption process.

The material used to form each rod depends on the design requirements of the vacuum interrupter, and is chosen to match the parameters of the current interruption process. In embodiments of the invention, each rod may be constructed from copper, aluminium or stainless steel.

In embodiments of the invention, the surface of the elongate, tubular wall portion may be coated with a material chosen to enhance the electrical properties of the wall portion. The wall portion may for example be coated with an electrically conductive material. The surface of the elongate, tubular wall portion may be coated with a specific material that enhances its electrical properties.

Preferred embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings in which:

Figure 1 shows a vacuum interrupter according to a first embodiment of the invention;

Figure 2 shows an annular cross-section of a rod of the vacuum interrupter in Figure 1 ;

Figure 3 illustrates the change in resistance with bore diameter for the rod of Figure 2, where the outer cross-sectional diameter of the rod varies between 40 to 70 mm;

Figure 4 shows a cross-section of an array of six elongate elements spaced equally around a circular circumference of a rod of a vacuum interrupter according to a second embodiment of the invention; Figure 5 illustrates the change in resistance of different arrangements of elongate elements of the rod in Figure 4 ;

Figures 6 and 7 respectively show rods of vacuum interrupters according to third and fourth embodiments of the invention;

Figure 8 shows a conventional solid rod having a conventional cup-shaped, multi-slotted contact coil mounted at its end;

Figure 9 illustrates the distribution of an axial magnetic field generated in a gap between contact electrodes mounted on moving and fixed rods based on the rods shown in Figures 6 to 8;

Figure 10 shows a rod of a vacuum interrupter according to a fifth embodiment of the invention, where the rod includes a support frame;

Figure 11 shows a variation of the support frame shown in Figure 10; and

Figure 12 illustrates the distribution of an axial magnetic field generated in a gap between contact electrodes mounted on moving and fixed rods based on the rods shown in Figures 6 and 10.

A vacuum interrupter according to a first embodiment of the invention is shown in Figure 1.

The vacuum interrupter includes a pair of cylindrical housings 20, first and second end flanges 22,24 and an annular structure 26 assembled to define a vacuum-tight enclosure. Each end flange 22, 24 is brazed to a first end of a respective cylindrical housing 20 to form a hermetic joint. The two cylindrical housings 20 are joined together at their second ends via the annular structure 26. The annular structure 26 includes a central shield 28 that overlaps inner walls of the cylindrical housings 20 to protect the inner walls of the cylindrical housings 20 from metal vapour deposition arising from arc discharge, while each end flange 22,24 includes an end shield 30 to improve the electrostatic field line distribution along the length of the vacuum interrupter.

Each cylindrical housing 20 is metallised and nickel- plated at both ends. The length and diameter of the respective cylindrical housing 20 varies depending on the operating voltage rating of the vacuum interrupter, while the dimensions and shape of the first and second end flanges 22,24 and the annular structure 26 may vary to correspond to the size and shape of the cylindrical housings 20. It is envisaged that, in other embodiments, corrugations may be added to the external diameters of the cylindrical housings to increase the creepage distance along the length of the vacuum interrupter to improve their dielectric performance. The vacuum interrupter also includes a tubular bellows 32 and first and second electrically conductive rods 34, 36.

The first end flange 22 includes a hollow bore dimensioned to accommodate the tubular bellows 32, while the second end flange 24 includes a hollow bore dimensioned to accommodate the second rod 36 within its hollow bore. The tubular bellows 32 also includes a hollow bore for retention of the first rod 34. Each rod 34,36 includes an elongate, tubular wall portion 38. The elongate, tubular wall portion 38 has an annular cross-section 40 surrounding an axially extending bore 42, and presents a continuous, uninterrupted outer surface to define a cylindrical tube, as shown in Figure 2.

The vacuum interrupter further includes a pair of contact electrodes 44. The elongate, tubular wall portion 38 of each rod 34,36 supports a respective one of the contact electrodes 44 so that each contact electrode 44 is mounted on at a first end of the respective rod 34,36. The first and second rods 34,36 are respectively retained within the hollow bores of the tubular bellows 32 and the second end flange 24 so that the first ends of the rods 34,36 are located inside the vacuum-tight enclosure and the second ends of the rods 34, 36 are located outside the enclosure. As such, the contact electrodes 44 mounted onto the first ends of the rods 34,36 are located inside the vacuum-tight enclosure. The longitudinal axes of the rods 34, 36 are aligned so that the contact electrodes 44 define opposed contact surfaces. Corrugated walls of the tubular bellows 32 allow the tubular bellows 32 to undergo expansion or contraction so as to increase or decrease the tubular length of the tubular bellows 32. This allows the first rod 34 to move relative to the second rod 36 between a first position where the opposed contact surfaces are kept in contact and a second position wherein the opposed contact surfaces are separated by a gap. The second rod 36 is kept at a fixed position.

In use, a second end of the first rod 34 is connected to a terminal of an AC electrical circuit 45, while a second end of the second rod 36 is connected to the other terminal of an AC electrical circuit 45. The second ends of the first and second rods 34, 36 are sealed to be vacuum-tight.

The first and second rods 34,36 may be fabricated from, for example, oxygen-free high conductivity (OFHC) copper while the contact electrodes 44 may be fabricated, for example, from copper-chromium to enhance the short-circuit current rating of the vacuum interrupter . In the event of a fault resulting in a high fault current flowing in the connected AC network, the current must be interrupted in order to prevent the high fault current from damaging components of the AC network. Interruption of the fault current permits isolation and subsequent repair of the fault in order to restore the network to normal operating conditions. The current interruption process is initiated by controlling the tubular bellows 32 to move the first rod 34 away from the second rod 36 so as to separate the opposed contact surfaces of the contact electrodes 44. The separation of the opposed contact surfaces results in the formation of a gap between the contact electrodes 44, which leads to the formation of an arc in this gap. The arc consists of metal vapour plasma, which continues to conduct the AC current flowing between the contact electrodes 44.

When the AC current of the AC electrical circuit 45 reaches zero, the arc between the contact electrodes 44 is extinguished, which allows full dielectric recovery and successful current interruption to take place.

SLIM 2D magneto-dynamic finite element model calculation was carried out to compare the electrical resistance of the wall portion 38 in Figure 2, and the electrical resistance of a conventional solid rod, where the outer cross-sectional diameter of the wall portion 38 is equal to the cross-sectional diameter of the conventional solid rod. The following assumptions were used in the calculations:

(i) The resistivity of copper is taken as 0.177 x 10^~7 (Qm) .

(ii) The skin depth at an operating frequency of 50 Hz is equal as 9 mm. The air boundary (or the surface area) is 3 times the diameter of the conductor.

The length of each rod is equal to 50mm.

Calculations were carried out at an operating AC frequency of 50 Hz.

An AC voltage with a peak voltage of IV was applied across the ends of the conductor.

The results of the SLIM 2D magneto-dynamic finite element model calculation is seen in Figure 3, which illustrates the variation of the electrical resistance of the wall portion 38 of different outer cross- sectional diameters as a function of varying diameter of the axially extending bore.

When the wall portion 38 has an outer cross-sectional diameter of 70 mm, the lowest electrical resistance achieved is 0.457 μΩ, which is approximately 6.4% lower than the electrical resistance of the solid rod of 0.488 μΩ and is achieved with a bore diameter of 40 mm.

When the wall portion 38 has an outer cross-sectional diameter of 60 mm, the lowest electrical resistance achieved is 0.549 μΩ, which is approximately 5.5% lower than the electrical resistance of the solid rod of 0.581 μΩ and is achieved with a bore diameter of 30 mm.

When the wall portion 38 has an outer cross-sectional diameter of 50 mm, the lowest electrical resistance achieved is 0.687 μΩ, which is approximately 3.4% lower than the electrical resistance of the solid rod of 0.711 μΩ and is achieved with a bore diameter of 30 mm.

When the wall portion 38 has an outer cross-sectional diameter of 40 mm, the lowest electrical resistance achieved is 0.92 μΩ, which is approximately 0.5% lower than the electrical resistance of the solid rod of 0.925 μΩ and is achieved with a bore diameter of 10 mm. In general, the electrical resistance of the wall portion 38 has generally been found to be lower than the solid rod having the same cross-sectional diameter. The diameter of the axially extending bore of the wall portion 38 has been found to be a factor in determining the electrical resistance of the wall portion 38. It was found that in order to improve the electrical resistance of the wall portion 38, the effective radius needs to be at least 12.5 mm which is approximately 3.5mm bigger than the skin depth of the copper material, where the effective radius is equal to the difference between the outer cross-sectional diameter of the wall portion 38 and the cross-sectional diameter of the axially extending bore of the tube. The electrical resistance of the wall portion 38 starts to increase when the effective radius is less than 12.5mm.

On the basis of the results shown in Figure 3, an increase in outer cross-sectional diameter of the wall portion 38 of Figure 2 permits a larger decrease in electrical resistance at an optimum size of the axially extending bore 42. The vacuum interrupter in Figure 1 therefore exhibits a decreased electrical resistance when compared to a conventional vacuum interrupter employing solid rods having a cross-sectional diameter that is equal to the outer cross-sectional diameter of the wall portion 38 shown in Figure 2.

It is envisaged that, in other embodiments, the outer cross-sectional diameter of wall portion 38 of Figure 2 may be in the range of 40 to 150 mm and the cross- sectional diameter of the axially extending bore 42 of the tube of Figure 2 may be in the range of 10 to 100 mm.

A rod of a vacuum interrupter according to a second embodiment of the invention is shown in Figure 4. The vacuum interrupter of the second embodiment of the invention is identical to the vacuum interrupter of Figure 1 except that, in the vacuum interrupter of the second embodiment of the invention, each rod is defined by an array of elongate elements 46 arranged so that the outer surfaces of the elongate elements 46 define a broken outer surface of the elongate, tubular wall portion of the respective rod.

Each elongate element 46 is formed by a first sub- element having a circular cross-section and extending generally parallel to the longitudinal axis of the respective rod. The elongate elements 46 are uniformly distributed at 60 degree intervals about a circular orbit having a radius, R, where R is the distance between the centre of the circular orbit and the centre of the cross- section of each elongate element. The elongate elements 46 are equally spaced from each other by a spacing, D.

The ends of the elongate elements 46 support a respective one of the contact electrodes so that each contact electrode is mounted on the first end of the respective rod.

SLIM 2D magneto-dynamic finite element model calculation was carried out to compare the electrical resistance of the rod of Figure 4, and the electrical resistance of a conventional solid rod, where the total cross-sectional area of the elongate elements 46 is equal to the cross-sectional area of the conventional solid rod. The following assumptions were used in the calculations:

(i) The resistivity of copper is taken as 0.177 x 10^~7

(Qm) .

(ϋ) The skin depth at an operating frequency of 50 Hz is equal as 9 mm.

(iii) The air boundary (or the surface area) is 3 times the diameter of the conductor.

(iv) The length of each rod is equal to 50mm.

(v) Calculations were carried out at an operating frequency of 50 Hz. (vi) An AC voltage with a peak voltage of IV was applied across the ends of the conductor.

The results of the SLIM 2D magneto-dynamic finite element model calculation is seen in Figure 5, which illustrates the change in resistance of different arrangements of elongate elements of the rod in Figure 4 as a function of Rn/Ro. Ro is the radius of the circular orbit of the array of elongate elements when the spacing, D, is set at 0.2 mm. Rn is the radius of the circular orbit of the array of elongate elements for each finite model calculation where the spacing, D, increases with the radius of the circular orbit. The ratio of Rn/Ro ranges from 1 to 4.

The cross-sectional diameter of each elongate element was set at 20.42 mm, 24.496 mm, 28.58 mm and 32.66 mm so that the corresponding total cross-sectional area of the elongate elements is respectively identical to the cross-sectional area of a conventional solid rod having a cross-sectional diameter of 50 mm, 60 mm, 70 mm and 80 mm. In Figure 5, it was found that the electrical resistance of the array of elongate elements decreases with increasing Rn/Ro, up to a drop of approximately 35%. It was also found that the electrical resistance of each array of elongate elements is less than the electrical resistance of the corresponding conventional solid rod having the same cross-sectional area. Current flow in a solid rod is restricted mostly to its outer cross-sectional diameter mainly due to the frequency-dependent skin effect. The skin effect causes the majority of continuous current to flow through part of the cross-sectional area of the solid rod within the limits set by the skin depth. The remainder of the cross-sectional area of the solid rod plays a minimal role in current flow. As such, a vacuum interrupter employing solid rods exhibits a comparatively high electrical resistance, which leads to high thermal losses .

The use of the rods in Figures 2 and 4 however minimises the skin effect and thereby reduces the electrical resistance of the vacuum interrupter when compared to a conventional vacuum interrupter employing solid rods with similar total cross-sectional dimensions. The reduced electrical resistance leads to decreased thermal losses, which permits the current and voltage ratings of the vacuum interrupter to be increased without the need for additional thermal management equipment. This not only improves the efficiency and cost-effectiveness of the vacuum interrupter but also minimises the overall size and weight of the vacuum interrupter.

The reduction in electrical resistance and the decrease in thermal losses also renders such a vacuum interrupter compatible with higher current and voltage operating levels. The use of an array of elongate elements 46 in the rod of Figure 4 also has been found to increase the total surface area of the rod. It therefore improves heat dissipation via radiation losses. In addition, the use of elongate elements 46 being spaced from each other reduces the flow of eddy currents and thereby minimises increases in the temperature of the rod during the current interruption process. This in turn reduces the electrical resistance of the vacuum interrupter.

It is envisaged that, in other embodiments, the rod shown in Figure 4 may be employed in other types of switchgear applications to enhance the continuous current rating or reduce thermal losses.

Figure 6 shows a rod of a vacuum interrupter according to a third embodiment of the invention. The vacuum interrupter of the third embodiment of the invention is identical to the vacuum interrupter of the second embodiment of the invention except that, in the vacuum interrupter of the third embodiment of the invention, each elongate element 46 is formed by first and second sub-elements 48,50 connected end to end.

Each first sub-element 48 has a rectangular cross- section and extends generally parallel to the longitudinal axis of the respective rod. The first sub-elements 48 are uniformly distributed at 60 degree intervals about a circular orbit. The elongate elements 46 are equally spaced from each other .

Each second sub-element 50 extends from an axial end of the respective first sub-element 48. Each second sub- element 50 has a circular cross-section and is shaped to define a helical curve about the longitudinal axis of the respective rod, the second sub-elements 50 defining a circular coil 52 at the first end of the respective rod.

The second sub-elements 50 support a respective one of the contact electrodes so that each contact electrode is mounted on the first end of the respective rod.

Figure 7 shows a rod of a vacuum interrupter according to a fourth embodiment of the invention. The vacuum interrupter of the fourth embodiment of the invention is identical to the vacuum interrupter of the second embodiment of the invention except that, in the vacuum interrupter of the fourth embodiment of the invention, each rod is defined by an array of ten elongate elements 46. Each first sub-element 48 has a rectangular cross- section and extends generally parallel to the longitudinal axis of the respective rod.

The first sub-elements 48 are uniformly distributed at 36 degree intervals about a circular orbit. The elongate elements 46 are equally spaced from each other .

Each second sub-element 50 extends from an axial end of the respective first sub-element 48. Each second sub- element 50 has a quadrilateral cross-section and is shaped to define a helical curve about the longitudinal axis of the respective rod, the second sub-elements 50 defining a circular coil 52 at the first end of the respective rod.

The second sub-elements 50 support a respective one of the contact electrodes so that each contact electrode is mounted at the first end of the respective rod.

In the rods of Figures 6 and 7, the number of second sub-elements 50 is equal to the number of first sub- elements 48. In the third and fourth embodiments of the vacuum interrupter, the second sub-elements 50 of the first and second electrically conductive rods are axially curved in the same direction about the longitudinal axes of the rods so that the current flows in the same direction in both circular coils 52 and thereby creating predominantly an axial magnetic field in the gap between the contact electrodes located at the first end of the respective rod. In other embodiments, it is envisaged that the second sub-elements of the first and second electrically conductive rods may be axially curved in opposite directions about the longitudinal axes of the rods so that the current flows in opposite directions in both circular coils and thereby creating predominantly a radial magnetic field in the gap between the contact electrodes

The generation of a magnetic field between the contact electrodes improves the current interruption capability of the vacuum interrupter.

The efficiency of the circular coils 52 is defined by the peak value and uniformity of the axial magnetic field generated on the opposed contact surfaces or in a gap between the contact electrodes. It is desirable to have a high magnitude of the generated axial magnetic field distributed over a high percentage of the cross- sectional area of the circular coil 52. The rods in Figures 6 and 7 were analysed to determine the distribution of the axial magnetic field generated in the gap between the contact electrodes over the cross-sectional area of the respective circular coil 52. The results of the analysis were compared in Figure 9 to the result of the analysis of a conventional cup- shaped, multi-slotted contact coil 54 mounted at an end of a conventional solid rod 56, as shown in Figure 8.

In Figure 9, the axial magnetic fields generated by the circular coils 52 of the rods in Figures 6 and 7 have a higher peak magnitude and an increased distribution of the peak magnitude of the axial magnetic field across the width of the cross-sectional area of the circular coil 52 when compared to the conventional cup-shaped, multi-slotted contact coil 54 of Figure 8.

The connection of each second sub-element 50 to a respective one of the first sub-elements 48 therefore improves the peak value and the uniformity of the distribution of the generated magnetic field when compared to a conventional cup-shaped, multi-slotted contact coil. An increase in axial length of the second sub-elements 50 results in an increase in the generated axial magnetic field but also results in an increase in overall electrical resistance of the respective rod. The optimal axial length of each second sub-element 50 is therefore dependent on the required short-circuit current interrupting performance of the respective vacuum interrupter. It is envisaged that in other embodiments, the first and second sub-elements may be dimensioned to generate an axial magnetic field with a peak value of between 3 to 10 mT/kA and a substantially uniform distribution across the area of the circular coil.

A rod of a vacuum interrupter according to a fifth embodiment of the invention is shown in Figure 10. The vacuum interrupter of the fifth embodiment of the invention is identical to the vacuum interrupter of the third embodiment of the invention, except that, in the vacuum interrupter of the fifth embodiment of the invention, each rod further includes a support frame 58 extending along the length of the wall portion.

The support frame 58 includes an elongate member 60 and a flange 62 at a first end of the elongate member, the flange 62 extending radially from the elongate member 60. It is envisaged that, in other embodiments, the support frame 58 may further include one or more additional projections 64 extending radially from the outer surface of the elongate member 60 and located along at various positions along the length of the elongate member 60 as shown in Figure 11.

The longitudinal axis of the elongate member 60 is coaxially aligned with the longitudinal axis of the rod so that the support frame 58 is surrounded by and radially spaced from the array of elongate elements 46. The flange 62 of the support frame 58 is flared and is dimensioned to support the contact electrode mounted at the first end of the rod. As such, the support frame 58 provides additional support to the respective contact electrode mounted at the first end of the rod and structurally reinforces the array of elongate elements 46. It thereby minimises the risk of mechanical deformation of the elongate elements 46 during the current interruption process and thereby minimises the risk of failure of the vacuum interrupter.

The support frame 58 is made from stainless steel to enhance the structural strength of the support frame 58 and thereby improve the reliability of the support frame 58.

The provision of a support frame 58 therefore permits the current rating of the vacuum interrupter to be extended without compromising the mechanical strength of the rods employed in the vacuum interrupter.

During operation of the vacuum interrupter, a portion of the interrupter current flows through the support frame 58. The lower electrical conductivity of stainless steel minimises the size of current flowing through the support frame 58 and thereby minimises the generation of eddy currents in the support frame 58. This in turn reduces the effect any eddy currents generated within the support frame 58 may have on the peak value and the uniform distribution of the generated magnetic field during the current interruption process.

Figure 12 illustrates the distribution of an axial magnetic field generated in a gap between contact electrodes mounted on moving and fixed rods based on the rod without a support frame shown in Figure 6 and the rod with a support frame 58 shown in Figure 10.

It is shown in Figure 12 that the peak value and the distribution of the generated axial magnetic field in the rod of Figure 6 is largely unchanged from the peak value and the distribution of the generated axial magnetic field in the rod of Figure 10. This means that the addition of a support frame 58 has minimal effect on the peak value and distribution of the generated axial magnetic field.

Claims

1. A vacuum interrupter comprising a pair of contact electrodes mounted on electrically conductive rods inside a vacuum-tight enclosure so as to define opposed contact surfaces, each of the electrically conductive rods including an elongate, tubular wall portion supporting a respective one of the contact electrodes at a first end and being connectable at a second end to an electrical circuit and at least one of the electrically conductive rods being movable to open or close a gap between the opposed contact surfaces.

2. A vacuum interrupter according to Claim 1 wherein the cross-section of the wall portion of each rod is circular, oval, elliptical, or polyhedral in shape.

3. A vacuum interrupter according to Claim 1 or Claim 2 wherein the wall portion of each rod presents a continuous and uninterrupted outer surface.

4. A vacuum interrupter according to Claim 3 wherein the outer cross-sectional diameter of the wall portion is in the range of 40 to 150 mm.

5. A vacuum interrupter according to Claim 3 or Claim 4 wherein the inner cross-sectional diameter of the wall portion is in the range of 10 to 100 mm.

6. A vacuum interrupter according to any of Claims 3 to 5 wherein the second end of each rod is sealed to be vacuum-tight .

7. A vacuum interrupter according to Claim 1 or Claim 2 wherein the wall portion of each rod is defined by an array of elongate elements arranged so that the outer surfaces of the elongate elements define a broken outer surface of the elongate, tubular wall portion.

8. A vacuum interrupter according to Claim 7 wherein the elongate elements are uniformly distributed about the cross-sectional circumference of the wall portion.

9. A vacuum interrupter according to Claim 7 or Claim 8 wherein each of the elongate elements is formed by two or more sub-elements connected end to end.

10. A vacuum interrupter according to Claim 9 wherein the sub-elements include a first sub-element extending generally parallel to the longitudinal axis of the respective rod and further includes a second sub- element extending from an axial end of the first sub- element and being shaped to curve about the longitudinal axis of the respective rod, the second sub-elements of the elongate elements defining the first end of the elongate, tubular wall portion of the respective rod.

11. A vacuum interrupter according to Claim 10 wherein each second sub-element is shaped to define a helical curve about the longitudinal axis of the respective rod .

12. A vacuum interrupter according to Claim 10 or Claim 11 wherein the second sub-elements of the electrically conductive rods are axially curved in the same or opposite directions about the longitudinal axes of the rods.

13. A vacuum interrupter according to any of Claims 9 to 12 wherein each sub-element has a circular, oval, elliptical, or polyhedral cross-sectional shape.

14. A vacuum interrupter according to any of Claims 7 to 13 wherein each rod further includes a support structure extending along the length of the elongate, tubular wall portion, the support structure being spaced from the elongate elements and a first end of the support structure supporting the respective contact electrode at the first end of the rod.

15. A vacuum interrupter according to Claim 14 wherein the support structure includes a support frame located within the elongate, tubular wall portion.

16. A vacuum interrupter according to Claim 15 wherein the support frame includes one or more projections extending radially from an outer surface of the support frame .

17. A vacuum interrupter according to any of Claims 15 to 16 wherein the support structure includes a tubular support frame located on the outside of the elongate, tubular wall portion so as to enclose the array of elongate elements.

18. A vacuum interrupter according to any of Claims 15 to 17 wherein a first end of the or each support frame is flared so as to support the respective contact electrode.

19. A vacuum interrupter according to any of Claims 15 to 18 wherein the or each support frame is constructed from a non-magnetic and electrically conductive material.

20. A vacuum interrupter according to any of Claims 15 to 19 wherein the or each support frame has a lower electrical conductivity than the elongate, tubular wall portion.

21. A vacuum interrupter according to any preceding claim wherein each rod is constructed from copper, aluminium or stainless steel.

22. A vacuum interrupter according to any preceding claim wherein the surface of the elongate, tubular wall portion of each rod is coated with a material chosen to enhance the electrical properties of the wall portion.