Electrical switching device with multiple nominal contacts and method for manufacturing the electrical switching device

Abstract

 The electrical switching device (1) has at least a nominal contact arrangement, wherein the nominal contact arrangement comprises at least a first nominal contact comprising a plurality of contact fingers (3c) forming a finger cage concentric with respect to a longitudinal axis (z), and at least a mating second nominal contact (3b), wherein at least one of the nominal contacts (3c, 3b) is movable parallel to the longitudinal axis (z) and cooperates with the other nominal contact (3b, 3c) for closing and opening said electric switching device (1). At least a first contact finger (3c) is formed in such a way that it has electrical and mechanical contact with the second nominal contact (3b) in at least a first and a second contacting area (6, 7) of the first contact finger (3c), when the electric switching device (1) is closed. The first and the second contacting area (6, 7) are separated from one another by a third area (8), wherein the third area (8) is not in mechanical contact with the second nominal contact (3b), when the electric switching device (1) is closed.

Description

Technical Field

[0001] The invention relates to the field of medium and high voltage switching technologies and concerns an electrical switching device and a method for manufacturing said electrical switching device according to the independent claims. The electrical switching device is particularly used as an earthing device, a fast-acting earthing device, a circuit breaker or a switch disconnector in power transmission or distribution systems.

Background

[0002] Electrical switching devices are well known in the field of medium and high voltage switching applications. They are e.g. used for interrupting a current, when an electrical fault occurs. As an example for an electrical switching device, circuit breakers have the task of opening contacts and keeping them far apart from one another in order to avoid a current flow, even in case of high electrical potential originating from the electrical fault itself. For the purposes of this document the term medium voltage refers to voltages from 1 kV to 72.5 kV and the term high voltage refers to voltages higher than 72.5 kV. The electrical switching devices, like said circuit breakers, may have to be able to carry high nominal currents of 5000 A to 6300 A and to switch very high short circuit currents of 63 kA to 80 kA at very high voltages of 550 kV to 1200 kV.

[0003] Because of the high nominal current, the electrical switching devices of today require many so-called contact fingers for the nominal current.

[0004] WO 2006/063943 A1 discloses an electrical contact arrangement comprising a first contact point and a second contact point. Adjacent contact regions of the first contact point are staggered in relation to each other. Each contact region is formed by a contact finger. Thus, each contact finger contacts the second contact point in one area which is displaced relatively to the respective contacting areas of adjacent fingers.

[0005] EP 1 973 202 A1 discloses a contact system for low voltage applications with a multiple contact surface. A contact spring is formed in such a way that it contacts the contact surface simultaneously in two different areas.

Description of the invention

[0006] It is an objective of the present invention to improve an electrical switching device in terms of robustness by reducing the risk of damage to its contact fingers, particularly of the contacting area of the contact fingers. This objective is solved by the features of the independent claims.

[0007] An electrical switching device for medium or high voltage circuits is claimed which has at least a nominal contact arrangement. The nominal contact arrangement comprises at least a first nominal contact comprising a plurality of contact fingers forming a finger cage concentric with respect to a longitudinal axis, and at least a mating second nominal contact. At least one of the nominal contacts is movable parallel to the longitudinal axis and cooperates with the other nominal contact for closing and opening said electric switching device. At least a first contact finger is formed in such a way that it has electrical and mechanical contact with the second nominal contact in at least a first and a second contacting area of the first contact finger when the electric switching device is closed. The first and the second contacting area are separated from one another by a third area which is not in mechanical contact with the second nominal contact.

[0008] Furthermore, a method for manufacturing said electrical switching device is claimed, comprising the step of grinding the first and the second contacting area of the contact fingers of the finger cage by means of a cylindrical grinder with an outer diameter being equal to an outer diameter of the second nominal contact.

[0009] High nominal currents or short circuit currents, which occur in medium or high voltage applications, heat up the contacting area between the first and the second nominal contact. Furthermore, the developed heat cannot be conducted away efficiently due to the very small contacting area of the contact fingers. Over time, this contacting area erodes and the result is a damage of the contact finger. By providing at least two separate contacting areas for the first contact finger, the nominal or the short circuit current through the first finger is split, wherein a first part of it travels through the first contacting area and a second part of it travels through the second contacting area, when the switching device is in a closed configuration. Thus, the first and the second contacting area are heated up less as compared to a single contacting area receiving the entire nominal current or short circuit current, because each of them conducts a lower current. This results in a significantly lower damage of the contact surface, thus prolonging the life of the contact arrangement.

[0010] The method for manufacturing the electrical switching device has the advantage that the radial positions, with respect to the longitudinal axis, of the first and the second contacting areas are mutually adjusted. By using a cylindrical grinder having an outer diameter which is equal to the one of the second nominal contact, it is made sure that all first and second contacting areas have a shape which matches the outer shape of the cylindrical second nominal contact. In other words, the surfaces of the first and the second contacting area have the same curvature as the outer surface of the second nominal contact, which normally is a metal rod, thus ensuring an optimum contact of the first contact finger to the second nominal contact.

[0011] In an embodiment, all contact fingers of the finger cage are formed like the first contact finger, i.e. all fingers have the same shape. Thus all contact fingers have at least a first and a second contacting area which are formed in the same way as the respective contacting areas of the first contact finger. This advantageously results in a substantially uniform distribution of the current through all contact fingers.

Short description of the drawings

[0012] Further embodiments, advantages and applications of the invention result from the dependent claims and from the now following description by means of the figures. It is shown in:

Fig. 1 a partial sectional view of a simplified basic embodiment of a high voltage circuit breaker;

Fig. 2 a sectional view of a contact finger of a finger cage of the high voltage circuit breaker of Fig. 1 in a closed configuration of the circuit breaker, according to the prior art;

Fig. 3 to 7 sectional views of different embodiments of a first contact finger according to the invention; and

Fig. 8 a schematic sectional view of a cylindrical grinder according to the invention.

Ways of carrying out the invention

[0013] The invention is described for the example of a high voltage circuit breaker, but the principles described in the following also apply for the usage of the invention in other switching devices, e.g. of the type mentioned above, such as a disconnector, earthing switch, combined disconnector and earthing switch, load-break switch, fast-acting earthing switch, generator circuit breaker, medium voltage circuit breaker, high-voltage circuit breaker, or the like.

[0014] Fig. 1 shows a partial sectional view of a simplified basic embodiment of a high voltage circuit breaker 1 in a closed configuration. In Fig. 1, "partial sectional view" means that only the upper half of the circuit breaker is shown, for reasons of clarity. The device is rotationally symmetric about a longitudinal axis z. Only the elements of the circuit breaker 1 which are related to the present invention are described in the following, other elements present in the figures are not relevant for understanding the invention and are known by the skilled person in high voltage electrical engineering.

[0015] A "closed configuration" as used herein means that nominal contacts of the circuit breaker are closed and are thus conducting a current, in particular a nominal current.

[0016] The circuit breaker 1 comprises a chamber enclosed by a shell or housing 5 which normally is cylindrical around the longitudinal axis z. It further comprises a nominal contact arrangement formed by a first nominal contact comprising a plurality of contact fingers 3a, of which only one is shown here for reasons of clarity, formed as a finger cage around the longitudinal axis z. A shielding 9 is arranged around the finger cage. The nominal contact arrangement further comprises a second mating contact 3b which normally is a metal tube. The contact fingers 3a and the second contact 3b are movable relatively to one another from the closed configuration shown in Fig. 1, in which they are in electrical contact to one another, into an opened configuration, in which they are apart from one another, and vice versa. It is also possible that only one of the contacts 3a, 3b moves parallel to the longitudinal axis z and the other contact 3b, 3a is stationary.

[0017] The contact fingers 3a are attached to or can be a part of a finger support 2, particularly a metal support cylinder 2.

[0018] The circuit breaker 1 furthermore comprises an arcing contact arrangement formed by a first arcing contact 4a and a second arcing contact 4b.

[0019] The first nominal contact 3a and the first arcing contact 4a may or may not be movable and typically are not movable relatively to one another. In the same way, the second nominal contact 3b and the second arcing contact 4b may or may not be movable and typically are not movable with respect to one another. For the explanatory purposes of the present invention it is assumed that only the second nominal contact 3b and the arcing contact 4b are movable and the nominal finger cage and/or the arcing finger cage is or are stationary.

[0020] When the closed circuit of Fig. 1 shall be disconnected, the second nominal contact 3b and the second arcing contact 4b are moved parallel to the direction of the z-axis into the direction indicated by the z-arrow, such that the nominal contact arrangement 3a, 3b disconnects first. Thereafter, the current commutates to the arcing contact arrangement 4a, 4b, which is still closed. With further movement of the second nominal contact 3b and the second arcing contact 4b into said direction, the arcing contact arrangement 4a, 4b also disconnects thereby creating an electric arc between the arcing contacts 4a, 4b, which is normally blasted out in a very short time. These principles are known and are therefore not explained in more detail here.

[0021] Fig. 2 shows a sectional view of a nominal contact finger 3a of a finger cage of the high voltage circuit breaker of Fig. 1 in a closed configuration of the circuit breaker. In this known configuration the nominal contact finger 3a of a first nominal contact contacts a second nominal contact 3b in a contacting area 6a of the contact finger 3a. A current I flows from the second nominal contact 3b through the contacting area 6a into the contact finger 3a of the first nominal contact, which current flow is denoted by the two arrows. As can be seen in the figure, the contacting area 6a is relatively small. When a high current I passes through the contacting area 6a the contact finger 3a heats up at and around the contacting area 6a. With regard to the contact heat, it must be made sure that a prescribed maximum value of about 105°C is not exceeded.

[0022] By using multiple contacting areas, as set forth in the following embodiments of the invention, it is easier to meet this requirement.

[0023] Fig. 3 shows a sectional view of a first embodiment of a first contact finger 3c according to the invention. For this embodiment and the following embodiments the second nominal contact 3b can be identical to the one of Fig. 2, and thus it will not be described in more detail. As can be seen, the first contact finger 3c has two arc-shaped contacts which contact the second nominal contact 3b in a first contacting area 6 and a second contacting area 7, respectively. The first and the second contacting area 6, 7 are separated by a third area 8 which has no mechanical and/or electrical contact with the second nominal contact 3b, as illustrated in Fig. 3.

[0024] The two contacts, which refer here to the curvature parts of the first contact finger 3c, have the shape of an arc, or at least a rounded shape, on the side facing the second nominal contact 3b, in order to glide easier over the second nominal contact 3b when the circuit breaker is being closed. This is advantageous, because the finger cage, not shown here, has a slightly smaller diameter than the second nominal contact 3b, which is a metal tube as mentioned above, in order to exert a pressure on the second nominal contact 3b, which pressure is high enough to ensure a good mechanical and electrical contact. Thus, the contact fingers 3c of the finger cage are elastically deformed in a radial direction upon closing the circuit breaker. The first contact finger 3c is formed in such a way, that the electrical and mechanical contact with the second nominal contact 3b occurs first in the first contacting area 6 and subsequently in the second contacting area 7, when the electric switching device, here the circuit breaker, is being closed. When the circuit breaker is being opened, the electrical and mechanical contact with the second nominal contact 3b is lost first in the second contacting area 7 and is subsequently lost in the first contacting area 6. Hence, particularly in case the circuit breaker is being opened, a small electric arc is formed between the second nominal contact 3b and the first contacting area 6 just before the current commutates to the arcing contacts 4a, 4b. Thus, the first contacting area 6 has to be made of a material which is comparatively resistant to electrical commutating arcs. Accordingly, the first contact finger 3c is advantageously made of copper, at least in a section conducting a first current, here denotes by I/2, through the first contacting area 6. Instead of copper other electric conductive materials with similar robustness in the presence of electrical arcs may be used, as well. For example, the first contact finger 3c is made of aluminium at least in a section conducting a second current, here denotes by I/2, through the second contacting area 7. By this, the overall cost of the circuit breaker is reduced, because the cheaper aluminium can be used for the conducting section of the second contacting area 7. However, it is also possible to use copper for the section conducting the second current through the second contacting area 7, as well. An additional measure can be to coat the contact surface of both the first and the second contacting area 6, 7 with silver. The material considerations described above are also valid for all further embodiments of the first contact finger 3c, which will be described in the following.

[0025] As can be seen in a comparison of Fig. 2 and Fig. 3, the current I coming from the second nominal contact 3b is now split into two parts, one flowing into the first contact finger 3c via the first contacting area 6 and the other part flowing into the first contact finger 3c through the second contacting area 7. Thus, the current amount flowing through each of the two areas is reduced, with the result that less heat is produced at the contact surfaces, whereas still the same amount of nominal current is transferred between the first nominal contacts 3c and the second nominal contacts 3b.

[0026] Fig. 4 shows a sectional view of a second embodiment of a first contact finger 3c according to the invention. In this embodiment the first contact finger 3c has at least a first subfinger 11a and a second subfinger 11b. The first contacting area 6 is formed on the first subfinger 11a and the second contacting area 7 is formed on the second subfinger 11b. The first contact finger 3c is typically relatively stiff, such that a problem may arise that the second contacting area 7 has a better contact to the second nominal contact 3b than the first contacting area 6, or vice versa. Thus, particular attention has to be paid to ensuring that all contacting areas 6, 7 along the first contact finger 3c are contacting the second nominal contact 3b with sufficient force in order to reliably conduct their respective transitional currents, here each denoted as 1/2. For this, the length of the subfingers 11a, 11b is chosen in such a way that the stiffness of the first contact finger 3c is reduced, and in particular that the stiffness of the first subfingers 11a and the second subfingers 11b are reduced and their contact forces are made more equal. Another additional or alternative measure to ensure a good contact of the subfingers 11a, 11b to the second nominal contact 3b is to adjust the angle between the first and the second subfinger 11a, 11b depending on the geometry of the finger cage and of the second nominal contact 3b. If e.g. the contact fingers are arranged in a cone configuration, said angle has to be wider than in a cylindrical configuration, because in the cone configuration the second subfinger 11b is farther away from the second nominal contact 3b than the first subfinger 11a. Another additional or alternative measure to ensure a good contact of the subfingers 11a, 11b to the second nominal contact 3b is to adjust the thickness of each subfinger 11a, 11b according to its position as seen in longitudinal direction. Thus, by choosing an appropriate length of the subfingers 11a, 11b, and/or adjusting the angle between the first and the second subfinger 11a, 11b and/or adjusting the thickness of the subfingers 11a, 11b it is possible to achieve a substantially equal contact force for all contacting areas 6, 7.

[0027] Fig. 5 shows a sectional view of a third embodiment of a first contact finger 3c according to the invention. In this embodiment the first contact finger 3c forms an arcuate section 10 between the first and the second contacting area 6, 7. The arcuate section 10 has a current path which is longer than a distance (i.e. direct or shortest distance) between the first and the second contacting area 6, 7. Regarding said stiffness of the first contact finger 3c, in this embodiment the length and/or shape of the arcuate section 10 is adjusted to decrease the stiffness of the first contact finger 3c. The longer the arcuate section 10 is chosen, the more flexible is the first contact finger 3c. Additionally or alternatively, the distance between the first and the second contacting area 6, 7 is kept as small as possible.

[0028] Fig. 6 shows a sectional view of a fourth embodiment of a first contact finger 3c according to the invention. In this embodiment the first contact finger 3c has electrical and mechanical contact with the second nominal contact 3b in the first, the second and a fourth contacting area 6, 7, 7a of the first contact finger 3c when the electric switching device is closed. The first, the second and the fourth contacting area 6, 7, 7a are separated from one another by two third areas 8. The first, second and fourth contacting area 6, 7, 7a correspond to a first, a second and a fourth arc-shaped contact 13a, 13b, 13c respectively. In the shown embodiment the contacts may be attached to or be a part of the first contact finger 3c. However, they may also be arranged in a way similar to the second embodiment of Fig. 4, that is, they may each be part of a subfinger of the first contact finger 3c. This alternative embodiment is herwith disclosed, as well. This arrangement with three contacting areas 6, 7, 7a further reduces the current I/3 conducted by each contacting area 6, 7, 7a. In general, the current travelling through each contacting area is calculated as the total current I divided by the number of contacting areas, provided that the current paths through each subfinger or contact have the same electric impedance.

[0029] Fig. 7 shows a sectional view of a fifth embodiment of a first contact finger 3c according to the invention. In this embodiment, like in the second embodiment according to Fig. 4, the first contact finger 3c has at least a first subfinger 11a and a second subfinger 11b. The first contacting area 6 is formed on the first subfinger 11a and the second contacting area 7 is formed on the second subfinger 11b. In this embodiment the first contact finger 3c is formed in such a way that the first contacting area 6 contacts a radial surface of second nominal contact 3b and the second contacting area 7 contacts an axial end surface 12 of the second nominal contact 3b. The radial surface of the second nominal contact 3b is herein defined as the outer surface of the cylinder forming the second nominal contact 3b, that is, a surface arranged at a substantially constant, in particular constant, radius from the longitudinal axis z. The axial surface 12 is herein defined as a surface extending from an end of the radial surface of the second nominal contact 3b towards the longitudinal axis z, that is a surface arranged at a substantially constant, in particular constant, axial position along the longitudinal axis z.

[0030] Fig. 8 shows a schematic sectional view of a cylindrical grinder G according to the invention. In a manufacturing step the grinder G, which advantageously has the same diameter like the second nominal contact 3b, is inserted into the finger cage and rotates with respect to the longitudinal axis z in order to grind the first and the second contacting areas 6, 7. By this step said contacting areas 6, 7 are equalized and it is thus ensured that they have the same diameter like the second nominal contact 3b. As mentioned, the first and the second contacting areas 6, 7 have the same curvature like the second nominal contact 3b after grinding. This advantageously improves the conductivity performance by ensuring a maximal contact surface between the first contacting area 6 and the second nominal contact 3b and between the second contacting area 7 and the second nominal contact 3b. Even though the contact fingers 3c are relatively stiff, as mentioned above, they still have a certain elasticity, which is desired in order to ensure a pressing force on the second nominal contact 3b when it is inserted into the nominal contact finger cage 3c. Thus, in order to ensure that the first and the second contacting areas 6, 7 of the contact fingers 3c are uniformly grinded it may be preferred to provide means for keeping the contact fingers fixed throughout the grinding procedure. Said means may for example be supporting means forming a sleeve (not shown) around the finger cage and supporting outer surfaces 14 of the contact fingers 3c against a radial force acting radially away from the longitudinal axis z and being exerted by the grinder during the grinding process.

[0031] It is noted that in all mentioned embodiments of the invention a cylindrical second nominal contact 3b has been assumed. However, the second nominal contact 3b may also have another shape, for example a cone shape. The same applies to the nominal contact finger cage 3c of the invention. The tubular shape of the nominal contact finger cage 3c can be derived e.g. from Fig. 6, where the first contact finger 3c is parallel to the second nominal contact 3b. A cone-shaped nominal contact finger cage 3c can be derived e.g. from Fig. 4, in which the first contact finger 3c is inclined with respect to the second nominal contact 3b.

[0032] The present invention is directed to the implementation of modified contact fingers 3c of a finger cage of a first nominal contact. Contrary to a shape of fingers known in the prior art, the modified contact fingers of the present invention have each multiple contacting areas 6, 7, 7a in order to divide the current path I between the nominal contacts 3b and 3c, thus reducing the transitional currents flowing through each contacting area 6, 7, 7a. Furthermore, the overall contacting area, which is a sum of the separate contacting areas 6, 7, 7a, is increased, resulting in a lower electrical resistance and thus better thermal conductivity and heat removal. Not only is the heat generation during current flow through the contacting areas 6, 7, 7a transferred or removed faster, but also the amount of heat generation is decreased. As a result, the lifetime of the nominal contact fingers 3c and 3b is prolonged.

[0033] While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practised within the scope of the following claims. In other words, throughout this application, terms like "preferable", "preferred", "advantageous", "favourable" and the like shall designate embodiments or exemplary features only, that are thus disclosed to be optional only.

List of reference numerals

[0034] 

1

= basic circuit breaker

2

= finger support

3a

= contact finger of first nominal contact (prior art)

3b

= second nominal contact

3c

= contact finger of first nominal contact (according to the invention), nominal contact finger cage

4a

= first arcing contact

4b

= second arcing contact

5

= shell, housing

6

= first contacting area

6a

= contacting area according to prior art

7

= second contacting area

7a

= fourth contacting area

8

= third area

9

= shielding

10

= arcuate section

11a

= first subfinger

11b

= second subfinger

12

= axial end surface

13a

= first contact

13b

= second contact

13c

= third contact

14

= outer surface

I

= total current

I/2

= current through one contacting area (2 contacting areas)

I/3

= current through one contacting area (3 contacting areas)

z

=longitudinal axis

G

= grinder

Claims

1. Electrical switching device (1) for medium or high voltage circuits,
having at least a nominal contact arrangement, wherein the nominal contact arrangement comprises
a) at least a first nominal contact comprising a plurality of contact fingers (3c) forming a finger cage concentric with respect to a longitudinal axis (z), and
b) at least a mating second nominal contact (3b),
wherein at least one of the nominal contacts (3c, 3b) is movable parallel to the longitudinal axis (z) and cooperates with the other nominal contact (3b, 3c) for closing and opening said electric switching device (1),
characterized in that at least a first contact finger (3c) is formed in such a way that it has electrical and mechanical contact with the second nominal contact (3b) in at least a first contacting area (6) and a second contacting area (7) of the first contact finger (3c) when the electric switching device (1) is closed, wherein the first and the second contacting area (6, 7) are separated from one another by a third area (8), wherein the third area (8) is not in mechanical contact with the second nominal contact (3b).

2. Electrical switching device according to claim 1, wherein all contact fingers of the finger cage have the same shape as the first contact finger (3c).

3. Electrical switching device according to claim 1 or 2, wherein the first contact finger (3c) has electrical and mechanical contact with the second nominal contact (3b) in a first contacting area (6), a second contacting area (7) and a fourth contacting area (7a) of the first contact finger (3c) when the electric switching device (1) is closed, wherein the first, the second and
the fourth contacting area (6, 7, 7a) are separated from one another by two third areas (8).

4. Electrical switching device according to one of the preceding claims, wherein the first contact finger (3c) is formed in such a way that the electrical and mechanical contact with the second nominal contact (3b) occurs first in the first contacting area (6) and subsequently in the second contacting area (7), when the electric switching device (1) is being closed.

5. Electrical switching device according to one of the preceding claims, wherein the first contact finger (3c) has at least a first and a second subfinger (11a, 11b), wherein the first contacting area (6) is formed on the first subfinger (12a) and the second contacting area (7) is formed on the second subfinger (11b) .

6. Electrical switching device according to one of the claims 1 to 4, wherein the first contact finger (3c) has an arcuate section (10) between the first and the second contacting area (6, 7), wherein the arcuate section (10) has a current path which is longer than a direct distance between the first and the second contacting area (6, 7).

7. Electrical switching device according to one of the preceding claims, wherein the first contact finger (3c) is formed in such a way that the first contacting area (6) contacts a radial surface of second nominal contact (3b) and the second contacting area (7) contacts an axial end surface (12) of the second nominal contact (3b).

8. Electrical switching device according to one of the preceding claims, wherein the first contact finger (3c) is made of copper at least in a section
conducting a first current (I/2; I/3) through the first contacting area (6).

9. Electrical switching device according to one of the preceding claims, wherein the first contact finger (3c) is made of aluminium at least in a section conducting a second current (1/2; I/3) through the second contacting area (7).

10. Electrical switching device according to one of the preceding claims, wherein a contact surface of the first contacting area (6) is silver-coated.

11. Electrical switching device according to one of the preceding claims, wherein a contact surface of the second contacting area (7) is silver-coated.

12. Electrical switching device according to one of the preceding claims, wherein said contact fingers (3c) are elastically deformed in a radial direction upon closing said switching device (1).

13. Method for manufacturing an electrical switching device (1) according to one of the preceding claims, comprising the step of grinding the first and the second contacting area (6, 7) of the contact fingers (3c) of the finger cage by means of a cylindrical grinder (G) with an outer diameter being equal to an outer diameter of the second nominal contact (3b).

14. Use of the electrical switching device (1) of any of the claims 1 to 12 as an earthing switch, a fast-acting earthing switch, a disconnector, a combined disconnector and earthing switch, a load-break switch, a generator circuit breaker, a medium-voltage circuit breaker, or a high-voltage circuit breaker.

Drawing