EP2180490A1

EP2180490A1 - A switching device, a switchgear provided therewith, and a method of operating a switching device

Info

Publication number: EP2180490A1
Application number: EP08167610A
Authority: EP
Inventors: Pal Kristian Skryten; Robert Espeseth; Tom-Rune Bjortuft; Thor Endre; Ole Granhaug; John-Anders Tveten; Kim Boe
Original assignee: ABB Technology AG
Current assignee: ABB Technology AG
Priority date: 2008-10-27
Filing date: 2008-10-27
Publication date: 2010-04-28
Anticipated expiration: 2028-10-27
Also published as: CN101728112B; EP2180490B1; CN101728112A

Abstract

A switching device (3)comprising an electric conductor (10), a breaker (6) connected to said conductor (10), and a switch (7), said switch (7) comprising a switch element (9,16) that is pivotable around a rotational axis to a first position in which a first end (13) thereof is electrically connected to the breaker (6) and a second end (14) thereof is electrically connected to the electric conductor (10), and to a second position in which said second end (14) thereof is disconnected from the conductor (10) and connected to a grounded element (12), wherein the switch element (9,16) is also movable to a third position in which it is disconnected from the conductor (10) and from the grounded element (12). The third position is a position in which the dielectric strength between the switch element (9,16) and the conductor (10) and grounded element (12) is higher than in any of the pivotation positions of the switch element (9,16) between the first and second position obtained solely by pivotation of said switch element (9,16).

Description

TECHNICAL FIELD

The present invention relates to a switching device comprising an electric conductor, a breaker connected to said conductor, and a switch, said switch comprising a switch element that is pivotable around a rotational axis to a first position in which a first end thereof is electrically connected to the breaker and a second end thereof is electrically connected to the electric conductor, and to a second position in which said second end thereof is disconnected from the conductor and connected to a grounded element, wherein the switch element is also movable to a third position in which it is disconnected from the conductor and from the grounded element.
The invention also relates to a method of operating such a switching device.
Generally, the invention relates to switching devices of the type used in switchgear for the distribution of electric power. Accordingly, the invention also relates to such switchgear comprising a switching device according to the invention. Typically such switchgear is connected to a three phase AC distribution network. Thereby, the above-mentioned conductor of the switching device may define a so-called bus-bar assumed to carry a medium or high voltage when the switchgear is in operation. Medium or high voltage is referred to as 1 kV and above. Typically, but not necessarily, said breaker comprises a vacuum interrupter. The above mentioned switch may be referred to as a selector switch which is used for separating the breaker from the conductor upon activation of the breaker, i.e. upon interruption of the electric connection between components upstream and downstream the breaker by means of the latter. The second position of the switch may be regarded as a safety position, enabling safe repair and service on components such as cables connected to the switchgear. The third position of the switch may be regarded as a test and measurement position in which test and measurements may be performed on components such as cables connected to the switchgear.
Typically, switchgear according to the invention may comprise a grounded, metallic encapsulation inside which the above-mentioned, voltage-carrying components are located. Bushings are provided in the walls of said encapsulation, and electric power cables for conducting the current to or from the switchgear are connected to the bushings. Inside the encapsulation, further conductors connect each bushing with a respective breaker.

BACKGROUND OF THE INVENTION

Switchgears of prior art normally comprises from one to five modules in a housing, or encapsulation and each module comprises at least three bushings (one for each phase of a three phase AC power distribution system), conductors leading from each bushing to a respective vacuum interrupter and selector switch (one for each interrupter), and bus-bars (one for each phase). The bus-bar of each phase extends through all units and are, thus, common for those units. The selector switches are used for connecting the vacuum interrupters to the bus-bars or disconnecting them from the latter. Each selector switch normally comprises a switch knife that is pivotable between positions in which it is connected to or disconnected from the bus-bar.
The vacuum interrupters may be spring loaded and provided to interrupt the electric circuit upon occurrence of specific operational conditions. After such breaking, the selector switches may be manually or automatically disconnected from the respective bus-bar, either to a position in which and end thereof is connected to ground or an open position in which, for example, electric measurements on the components on the breaker side of the switch may be performed. Accordingly, there are three main positions of the switch knives of the selector switches, namely closed (connected), grounded or open. In order to go from closed to grounded position, each switch knife is pivoted a certain angle. The open position is obtained in an angular position between the first and second positions.
In a medium or high voltage application, when e.g. for the purpose of fully disconnecting a load the open position of the switch knife is used, a certain electric field will exist between the switch knife and the adjacent bus-bar as well as between the switch knife and the grounded element to which it is connected in its grounded position. The distance to the bus-bar and to the grounded element respectively should be as large as possible in order to prevent discharges or arcing between the knife and any of said components. Accordingly, an optimal pivotation position between the bus-bar and the grounded element obtained by rotation of the knife is commonly used as the open position. In the future it is conceivable to use switchgear in which gases of less electrically insulating capacity than today's may fill the enclosure in which the switch is located. Designs of prior art may then be unsatisfying from a safety point of view, and may require too much space in order fulfil safety regulations, i.e. in order to prevent the upcoming of discharges or arcs. Accordingly, with regard to sustainable switchgear design, compactness is an essential parameter that necessitates thought through solutions to prevail.

THE OBJECT OF THE INVENTION

It is an object of the present invention to present a switching device as initially defined, the design of which is such that safety against disruptive discharge is assured , in particular when the switch element of the switch is in a third, open position.

SUMMARY OF THE INVENTION

The object of the invention is achieved by means of the initially defined switching device, characterised in that said third position is a position in which the dielectric strength between the switch element and the conductor and grounded element is higher than in any of the pivotation positions of the switch element between the first and second position obtained solely by pivotation of said switch element. Dielectric strength is referred to as the ability of avoiding disruptive discharge or between the switch element and any one of the conductor and the grounded element. For any given situation, depending on the specific voltages of the components involved, i.e. the switch element, the conductor and ground, there will be a pivotation position between first and second positions that is optimal with regard to dielectric strength. The invention teaches that the third position be a position in which the dielectric strength is even higher than in said optimal position obtained only by pivotation of the switch element. As a result of the improved dielectric strength, the need of electrically insulating gas inside an encapsulation in which a switching device is housed may be reduced, and possibly air instead of, for example, SF₆ may be used.
Preferably, said third position is a position obtained either through pivotation of the switch element from the first position beyond the second position or through a longitudinal displacement of the switch element, or a combination of pivotation of the switch element and longitudinal, linear, displacement thereof. Pivotation of the switch element is, preferably, achieved by rotation of a shaft to which the switch element is connected. Longitudinal or linear motion of the switch element is, preferably a motion cross-wise to the rotational axis. Longitudinal motion may, as an example, be achieved by rotation of a second shaft provided with circumferential teeth that engage a rack provided on the switch element, i.e. a rack and pinion gear. Other solutions are also possible and within the scope of the invention, though the given examples are preferred. The third position may also be achieved through a motion of the switch element in the direction of the pivotation axis, either through a linear displacement or pivotation in a plane other than that of the first-mentioned pivotation.
According to one embodiment said third position is a position obtained by pivotation of the switch element from the first position beyond the second position. Provided that there is enough space in the region beyond the second position, as seen from the first position, to enable an improved dielectric strength between the switch element and the grounded element in relation to that of an optimal privation position between said first and second positions, this solution is preferred since it is simple and will only require a further pivotation of the switch element. Accordingly, the invention teaches that the switch element be pivotable from the first position beyond the second position to such a degree that said improved dielectric strength is achieved.
According to one embodiment, in the third position, the distance between the switch element and the grounded element is larger than in a position of optimal dielectric strength obtainable only by pivotation of the switch element to a position between the conductor and the grounded element.
As already mentioned above, it is preferred that said switch element is displaceable in relation to the rotational axis thereof, preferably cross-wise to direction of the rotational axis. It is also preferred that the switch element be elongated. According to one embodiment, in said first and second positions, the rotational axis is closer to the first end than to the second end of the switch element. In said third position, the second end is displaced towards the rotational axis, as compared to the first and second positions.
According to one embodiment, the switching device comprises a shaft that extends coaxially with said rotational axis, wherein the switch element is connected to said shaft and pivoted through a rotation of the latter. Thereby, pivotation of the switch element is achieved by operation of said shaft. Preferably, the shaft extends through the wall of an encapsulation inside which the switching device is housed, and is manoeuvred from outside said encapsulation.
According to one embodiment the switch element is displaceable in relation to said shaft. In order to permit such displacement the switch element may be slideably arranged on the shaft in a direction cross-wise to the longitudinal direction of the latter. In order to effect displacement of the switch element the device comprises means for displacement of the switch element in relation to said shaft. Such means may include a rack and pinion gear. Preferably, such a rack and pinion gear may comprise a second shaft extending coaxially and inside the first shaft and carrying a pinion that engages a rack on the switch element. Preferably, the rotation of the first shaft is independent of the rotation of the second shaft, such that privation of the switch element does not necessitate longitudinal displacement thereof and vice versa.
According to one embodiment, in said third position, the switch element is electrically disconnected from the breaker. Thereby, a further improved dielectric strength of the total switching device is achieved. Preferably, in said first and second positions, the switch element is connected to the breaker through a contact element. Through the pivotation from the first to the second position, the first end of the switch element is in sliding contact with said contact element. Upon further pivotation, beyond the second position, the first end of the switch element goes free from the contact element, whereby an electrically insulating gap is generated between said elements. In those cases when the pivotation to the third position does not go beyond said second position and longitudinal displacement is used in order to reach the third position, the design of the contact element and/or the first end of the switch element should be such that contact therebetween ends upon such displacement of the switch element. Several solutions to achieve this will be obvious for persons skilled in the art.
According to one embodiment the switch element forms a switch knife.
According to one embodiment said breaker comprises a vacuum interrupter.
According to one embodiment, in each of said first, second and third positions of the switch element, the conductor is arranged so as to carry medium or high voltage. It is preferred that said conductor is a bus bar provided in a switchgear.
The object of the invention is also achieved by means of an electric power distribution switchgear, characterised in that it comprises a switching device according to the invention.
The objective of the invention is also achieved by means of the initially defined method, characterised in that, when the switch element is moved to the third position, it is retracted or moved from the first or second position to a position in which the dielectric strength between the switch element and the conductor and grounded element is higher than in any of the pivotation positions of the switch element between the first and second position obtained solely by pivotation of said switch element.
Preferably, said third position is obtained either through a pivotation of the switch element from the first position beyond the second position or a longitudinal displacement of the switch element, or a combination of pivotation of the switch element and longitudinal displacement thereof.
According to a preferred embodiment the switching device is housed in an encapsulation and the displacement of the switch element between said first, second and third positions is performed from outside the encapsulation by means of at least one shaft connected to said switch element.
Further features and advantages of the present invention will be presented in the following detailed description of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described more in detail with reference to the annexed drawing, on which,

Fig. 1 is a side view of a part of the interior of a switchgear provided with a switching device according to the invention,
Figs. 2-4 are side views of a part of the switching device according to the invention, with switch element in first, second and third positions respectively, and
Fig. 5 is a side view corresponding to those in figs. 2-4 of an alternative embodiment of the switching device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows an electric power distribution switchgear 1 according to the invention. The switchgear 1 comprises a metal encapsulation 2 inside which a number of switching devices 3, only one of which is shown in the figure, are housed. The switching devices not shown in the figure are arranged in parallel with the one shown and are thus either hidden behind the one shown or located in planes in front of the latter and not shown in the figure. A wall of the encapsulation 2 is penetrated by a number of bushings 4, one for each phase of a plural phase system. From each bushing 4 a respective conductor 5, only one of which is shown in the figure, extends to a respective switching device 3. On the outside of the encapsulation 2 the bushings 4 are connected to cables, not shown, that either connect the switchgear 1 to a load or to a medium or high voltage power distribution line.
Each switching device 3 according to the invention comprises a breaker 6 connected in one end to the conductor 5 that extends from a dedicated bushing 4, and in another end to a switch 7. The breaker 6 is a vacuum interrupter that, though not shown in the drawing but as known per se, has a fixed part and a moveable part, wherein the fixed part is connected the conductor 5 and the moveable part is connected to the switch. The switch 7 is a so called safety switch or selector switch which is not adapted to break a medium or high voltage circuit itself, but only to disconnect the breaker from a medium or high voltage line after breaking has been performed by means of the breaker. The switch 7 comprises a contact element 8 permanently connected to the breaker 6, and a switch element 9. The switch element is movable between positions in which it connects the breaker 6 with a further conductor 10 formed by a so called bus bar that extends through the switchgear 1, with ground or an open position in which the breaker is neither connected to the bus bar nor to the ground. These positions are defined as first, second and third positions in this application. The reasons to why these positions are used during operation of the switchgear are well known to persons skilled in the art and will, therefore, not be further described here. It should be understood that a switchgear may comprise a plurality of switchgears, or units, such as the one described above. For each electric phase there is a common bus bar, like the conductor 10 defined above, that extends from unit to unit. The encapsulation may or may not be common for the plurality of switchgears/units. The encapsulation may be filled with an electrically insulating gas or gas mixture, which, preferably, is pressurised. Air-filled encapsulations are also conceived.
A first embodiment of the switch 7 is shown in figs. 2-4. The switching device comprises a shaft 11 onto which the switch element 9 is attached. The shaft 11 is common for the parallel three switches 7 of the three-phase switchgear described so far. The shaft 11 extends through a wall of the encapsulation 2 and is manoeuvrable from outside the encapsulation 2. The bus bar 10 extends horizontally in an upper part of the compartment defined by the encapsulation 2, and there is provided a grounded element 12 on an inner wall of the encapsulation 2, below the level of the bus bar.
Fig. 2 shows the device in the above-mentioned first position in which a first end 13 of the switch element 9 is in physical and electrical contact with the contact element 8 and a second end 14 of the switch element is in physical and electrical contact with the bus bar 10. The switch element 9, which is elongated in a longitudinal direction thereof is generally vertically directed from its rotational axis towards the bus bar 10.
Fig. 3 shows the same device in the second position obtained by a pivotation of the switch element 9 approximately 90°. The second end 14 is now in physical and electrical contact with the grounded element 12. The first end 13 of the switch element 9 is still in contact with the contact element 8 as a result of the design of the latter and the fact that the first end slides on the contact element 12 in the angular range covered between the first and second positions.
Fig. 4 shows the device in the third position. Here the third position is obtained through a further pivotation of the switch element 9 from the first position beyond the second position. In order to obtain an improvement of dielectric strength of the device, i.e. is a position in which the dielectric strength between the switch element and the conductor and grounded element is higher than in any of the pivotation positions of the switch element between the first and second position obtained solely by pivotation of said switch element, the switch element must be pivoted a certain angular distance beyond the second position. This distance may differ from case to case, but as a rule the pivotation angle in relation to the first position is larger than 1,5 times the pivotation angle between the first and the second position.
Fig. 5 shows a second embodiment of the switching device of the invention. This embodiment differs in one main aspect from the one of shown in figs. 2-4, namely in that it comprises a rack and pinion gear 15 by means of which the switch element 16 is longitudinally displaceable cross-wise to the rotational axis, i.e. cross-wise to the axis around which it pivots. As in the previous embodiment the device comprises a first shaft 17 onto which the switch element 16 is attached. However, in this case the switch element is slideably held in relation to the first shaft 17 in order to permit longitudinal displacement thereof in relation to said first shaft 17. A second shaft 18 coaxial with the first shaft 17 extends through the centre of the latter and is provided with teeth 19, or a pinion, that engage a rack 20 provided on the switch element 16. The second shaft 18, likewise the first shaft 17, extends out of the encapsulation 2 and is also manoeuvrable from outside the encapsulation. By means of the provision of the rack and pinion gear 15 formed by the second shaft 18 and the rack 20 of the switch element, a plurality of different alternative third positions may be achieved. Preferably, in order to permit the electric and physical disconnection of the first end of the switch element 17 from the contact element 8 upon transition to the third position by means of longitudinal displacement, the contact surfaces between these components should have a component in a direction cross-wise to the displacement direction, such that these surfaces are separated upon longitudinal displacement of the switch element 17 in the direction towards the third position.

Claims

1. A switching device (3)comprising

- an electric conductor (10),

- a breaker (6) connected to said conductor (10),

- and a switch (7),

- said switch (7) comprising a switch element (9,16) that is pivotable around a rotational axis to a first position in which a first end (13) thereof is electrically connected to the breaker (6) and a second end (14) thereof is electrically connected

to the electric conductor (10), and to a second position in which said second end (14) thereof is disconnected from the conductor (10) and connected to a grounded element (12), wherein the switch element (9,16) is also movable to a third position in which it is disconnected from the conductor (10) and from the grounded element (12),
characterised in that said third position is a position in which the dielectric strength between the switch element (9,16) and the conductor (10) and grounded element (12) is higher than in any of the pivotation positions of the switch element (9,16) between the first and second position obtained solely by pivotation of said switch element (9,16).

2. A switching device according to claim 1, characterised in that said third position is a position obtained either through pivotation of the switch element (9,16) from the first position beyond the second position or a longitudinal displacement of the switch element (9,16), or a combination of pivotation of the switch element (9,16) and longitudinal displacement thereof.

3. A switching device according to claim 1 or 2, characterised in that said third position is a position obtained by pivotation of the switch element (9,16) from the first position beyond the second position.

4. A switching device according to claim 3, characterised in that, in the third position, the distance between the switch element and the grounded element is larger than in a position of optimal dielectric strength obtainable only by pivotation of the switch element to a position between the conductor and the grounded element.

5. A switching device according to any one of claims 1-4, characterised in that said switch element (9,16) is displaceable in relation to the rotational axis thereof.

6. A switching device according to claim 5, characterised in that said switch element (9,16) is displaceable in a direction cross-wise to the direction of its rotational axis.

7. A switching device according to claim 5 or 6, characterised in that, in said third position, said switch element (9,16) is displaced in relation to said rotational axis.

8. A switching device according to any one of claims 1-7, characterised in that it comprises a shaft (11) that extends coaxially with said rotational axis, wherein the switch element (9,16) is connected to said shaft and pivoted through a rotation of the latter.

9. A switching device according to claim 8, characterised in that it comprises means (18, 19, 20) for displacement of the switch element (9,16) in relation to said shaft (11).

10. A switching device according to any one of claims 1-9, characterised in that, in said third position, the switch element (9,16) is electrically disconnected from the breaker (6).

11. A switching device according to any one of claims 1-10, characterised in that the switch element (9,16) forms a switch knife.

12. A switching device according to any one of claims 1-11, characterised in that said breaker (6) comprises a vacuum interrupter.

13. A switching device according to any one of claims 1-12, characterised in that, in each of said first, second and third positions of the switch element (9,16), the conductor (10) is arranged so as to carry a medium or high voltage.

13. An electric power distribution switchgear, characterised in that it comprises a switching device according to any one of claims 1-12.

14. A method of operating a switching device, wherein said device comprises

- an electric conductor (10),

- a breaker (6) connected to said conductor (10),

- and a switch,

- said switch comprising a switch element (9,16) that is pivotable around a rotational axis to a first position in which a first end (13) thereof is electrically connected to the breaker (6) and a second end (14) thereof is electrically connected to the electric conductor (10), and to a second position in which said second end (14) thereof is disconnected from the conductor (10) and connected to a grounded element (12), wherein the switch element (9,16) is also movable to a third position in which it is disconnected from the conductor (10) and from the grounded element (12),

characterised in that, when the switch element (9,16) is moved to the third position, it is retracted or moved from the first or second position to a position in which the dielectric strength between the switch element (9,16) and the conductor (10) and grounded element (12) is higher than in any of the pivotation positions of the switch element (9,16) between the first and second position obtained solely by pivotation of said switch element (9,16).

15. A method according to claim 14, characterised in that said third position is obtained either through a pivotation of the switch element (9,16) from the first position beyond the second position or a longitudinal displacement of the switch element (9,16), or a combination of pivotation of the switch element (9,16) and longitudinal displacement thereof.

16. A method according to claim 15, characterised in that the switching device is housed in an encapsulation and that the displacement of the switch element (9,16) between said first, second and third positions is performed from outside the encapsulation by means of at least one shaft connected to said switch element (9,16).