GB2178239A - Circuit breakers - Google Patents

Abstract

A circuit breaker comprises first and second main contacts (15, 16) and first and second arcing contacts (31, 32). The first arcing contact (31) ia annular, and the second arcing contact (32) lies in the space within the open centre of the annular contact. An arc-driving coil (19) surrounds the annular contact (31) and is connected in series therewith in the arc current path. The second arcing contact (32) and a conductor assemblyconnected thereto, which assembly forms part of the arc current path, are substantially symmetrical with respect to the radial mid-plane of the coil (19). <IMAGE>

Description

1 b.

GB2178239A 1 SPECIFICATION

Circuit breaker This invention relates to circuit breakers. 70 Rotating arc circuit breakers are known wherein an arc is drawn between separating contacts in an electronegative medium such as sulphur hexafluoride (SIFJ, and is maintained between a central electrode and an annular arcing electrode which is coaxial with an arc driving coil through which the arc current flows. The magnetic field induced in the coil drives the arc around the annular electrode and the arc is extinguished at a current zero. 80 Many proposals have been made for circuit breakers operating on this principle, but all are defective in the degree of control that they are able to apply to the arc, and so result in high energy arcs requiring high volumes of in- 85 sulating medium for successful arc extinction.

Many circuit breakers of this type strike the arc between a fixed contact and a movable contact and then transfer at least one root of the arc onto an arcing electrode. This causes 90 difficulties in that the arc has a tendency to be driven from its optimum transfer path, and the geometry of such devices is often such that this tendency is accentuated by magnetic loop forces generated by the current flowing to and in the arc. The coils in such systems are not energised until the arc has been estab lished, and accordingly the arc roots are rela tively free to wander until the arc becomes controlled by the magnetic flux. In order to accommodate this, coils of considerable axial length are required in order to ensure that the arc is brought under control. Inevitably, the resulting arc is long and thus has high energy.

Typical of such systems is that described in 105 GB-A-2038100.

In other systems, one example of which is shown in GB-A-2052160 an arc current path is established in parallel with a main current path, the main current path including first and second main contacts which are opened be fore contacts in the arc current path open.

Arc current thus flows in the coil before the arc is established, and the arc is struck di rectly between the arcing electrodes. Arc po sitioning can thus be improved, but the geo metry of the device is again such that the magnetic loop forces generated by the current flowing to and in the arc will drive the arc from its optimum path. Long, high energy arcs again result.

The present invention aims to improve arc control, with a view to reducing arc energy and thus rendering the arc capable of being extinguished more easily.

According to the invention a circuit breaker comprises first and second main contacts rela tively movable between closed and open posi tions, and, when closed, forming part of a main current path through the circuit breaker; first and second arcing contacts forming part of an arc current path through the circuit breaker, and means for effecting contact movement so that during opening of the breaker the main contacts separate and an arc is located between the first and second arcing contacts; in which the first arcing contact is annular, the second arcing contact lies in the space within the open centre of the annular contact, an arc-driving coil surrounds the annular contact and is connected in series therewith in the arc current path, and the second arcing contact and a conductor assembly connected to the second arcing contact and forming part of the arc current path are substantially symmetrical with respect to the radial mid-plane of the coil.

The crux of the invention is that the second arcing contact and the current path thereto are substantially symmetrical with respect to the radial mid-plane of the coil, leading to the result that current is introduced into the arcing contact system in such a way that magnetic loop forces imposed on the arc by the current flow in the conductors to the arcing contacts are effectively cancelled. This result arises since substantially equal current will flow to the second arcing contact from each side of the mid-plane of the coil, the magnetic loop forces induced by the current flowing in a first axial direction being cancelled by those induced by the current flowing in the second axial direction. This significantly reduces wandering of the arc roots and allows a coil of much reduced axial length to be employed in comparison to the coils needed in prior art systems. Furthermore, the cancellation of the loop forces will ensure that the arc rapidly stabilises substantially in the radial mid-plane of the coil and rotates in that plane. This is advantageous in that such mid-plane is the plane of maximum flux density thus maximising angular velocity of the arc and assisting in extinguishing the arc at current zero.

Being able to stabilise the arc in the plane of maximum flux density enables coils to be made with fewer turns and of weaker construction so leading to cost advantages; conversely a given coil will be able to handle a higher fault current in a circuit breaker according to the invention than would be the case in prior art circuit breakers.

Preferably the first and second arcing contacts are relatively movable between a closed position wherein the second arcing contact engages the inner circumference of the annular first contact and an open position wherein the second arcing contact is spaced from the inner circumference of the annular first contact, and move from the closed to the open position only after the main contacts separate. Thus the arc is struck in the optimum radial mid-plane location, and will immediately start to rotate in that plane as the coil was ener- gised before striking the arc.

2 GB2178239A 2 Preferably the conductor assembly corn prises an axial section extending through the open centre of the annular contact, the sec ond arcing contact is located at the centre of the axial section, and two side arms extend substantially radially of the annular contact, one lying to each side of the annular contact and electrically connecting the axial section of the conductor assembly to the main current path.

Desirably the second arcing contact is circu lar when seen in a plane extending radially of the annular contact, and when in the fully open position the second arcing contact is substantially concentric with the annular con- 80 tact. Concentricity is not essential, and it may be found that an arc can be extinguished be fore the second arcing contact reaches its fully open position. There is advantage in hav ing the difference between the external dia meter of the second arcing contact and the internal diameter of the annular contact as small as possible, in order to keep the arc short and therefore of low energy. Preferably the second arcing contact includes two sec tions resiliently biased one toward the other and engageable with opposite axial sides of part of the annular contact. This ensures good electrical contact between the two arcing con tacts. In an alternative arrangement the annular 95 contact may have sections resiliently biased towards each other so that the second arcing contact may be engaged between the sec tions. However, because of the more complex structure of the annular arcing contact the first 100 arrangement is preferred.

Any suitable drive arrangement may be pro vided for moving the first and second main contacts between their closed and open posi tions and for moving the first and second arc- 105 ing contacts between their closed and open positions. Thus, pivotal movement, linear movement or a combination of the two may be applied either to the main contacts or to the arcing contacts. A preferred arrangement 110 is one in which one of the main contacts is carried by a first pivot arm, one of the arcing contacts is carried by a second pivot arm, and the two pivot arms are each mounted for pivotal movement about an axis parallel to the 115 axis of the annular arcing contact. The two pivot axes may be coincident or may be spaced one from the other.

The drive system for both the main con- tacts and the arcing contacts will be designed according to the mounting arrangement that is used and there are many possible arrangements. As the main contacts are required to break before the arcing contacts, there will need to be a lost motion arrangement in the drive path to the arcing contacts if a single driving input is applied to the assembly. It is advantageous if during closing of the circuit breaker, the main contacts close before the arcing contacts, and means may be included to control a drive linkage in order to achieve this effect. This eliminates the need for a more robust coil construction as the coil is not subjected to the high closing peak cur- rents.

Embodiments of circuit breakers in accordance with the invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a cross-section through a first embodiment of three phase circuit breaker according to the invention; Figure 2 is a cross-section on the line 11-11 Figure 1 showing the circuit breaker in the closed position; Figures 3 and 4 show the mechanism embodied in Figure 2 in two other stages of operation; Figure 5 is a cross-section on the line V-V of Figure 4; Figure 6 is a cross-section on the line VINI of Figure 4; and Figures 7 to 9 each show schematically parts of alternative embodiments of circuit breaker in accordance with the invention.

Figure 1 shows a three phase circuit breaker comprising a cast resin housing 1 divided by internal partitions 2, 3 into three chambers, one for each phase. Each phase has an input conductor 4 and an output conductor 5 each cast in situ in an integral extension 6, 7 of the housing. The circuit breaker is operated by way of a drive shaft 8 common to all three phases, the shaft being supported by bearing assemblies 9 and 10, the bearing assembly 9 incorporating sealing means so that the interior of the housing is sealed from the atmosphere. The housing is closed by a cover plate 11, bolted to the housing. The cover plate 11 again has associated sealing means and will generally be connected to be at earth potential. In use, the free space within the housing is filled with an electronegative medium, for example sulphur hexafluoride (SFJ gas under pressure. It will be understood that although in the example shown the housing is moulded from insulating resin material it is perfectly feasible to use other insulating materials, or to form the housing so that the outer surface thereof includes conductive material connected to earth.

Each of the three phases has an individual circuit breaker mechanism 12 to 14 respec- tively, and since each of the three mechanisms are identical only the mechanism 13 will be described in detail.

Each mechanism includes a fixed main contact 14 bolted and electrically connected to the respective conductor 4 and carrying a plurality of spring fingers 15 biased towards each other in order to receive therebetween a movable contact 16. The movable contact 16 is mounted for pivotal movement about a shaft 17, as will be described in more detail later, of 4 3 GB2178239A 3 r A the shaft 17 being supported by and in electrical contact with, a mounting 18 bolted and electrically connected to the conductor 5. When the contact 16 is in the closed position as shown in Figure 2 it will be seen that a main current path is established through the circuit breaker, from conductor 4 through main contact 14, fingers 15, contact 16, shaft 17, and mounting 18, to conductor 5.

A coil assembly shown generally as 19 is secured to the fixed main contact 14 by two identical fixing arrangements 20, one of which is shown in cross-section in Figure 6. Each fixing assembly incorporates two interlocking insulating spacers 21, 22 which engage with each other through a circular hole 23 formed in the contact 14. Insulating side plates 24, 25 are secured to the spacers by a bolt and nut 26, 27. A coil 28 is held between the plates 24, 25 and one end of the coil windings is electrically connected by a conductor 29 to the main contact 14. An annular arcing contact 30, which will usually be of copper, is secured to the plates 24, 25 and the other end of the coil winding is electrically connected to this annular contact. The contact 30 has a central radially projecting rib 31 of arc resistant material, such as tungsten copper.

A second arcing contact 32 lies within the open centre of the annular contact 30 and includes two sections 33, 34 each made from arc resistant material such as tungsten copper, and engageable with opposite sides of the rib 31 of the contact 30. Each contact section is carried at the end of a copper sleeve 35, 36 respectively, the sleeves passing through respective insulating bushes 37 and 38. The two sleeves 35, 36 are slidably supported on a shaft 39 of nonmagnetic metallic material such as stainless steel, the shaft being formed with threaded ends 40, 41. Each sleeve 35, 36 has a cup formation 42, 43 respectively at the end thereof, and nuts 44, 45 retain compression springs 46, 47 in the respective cup formations. The springs bias the contact sections 33, 34 inwardly towards each other to a limit position wherein they are spaced apart by an insulating ring 48 secured to the shaft 39, in which position the distance between the contact sections is less than the overall axial thickness of the rib 31. The springs also ensure good electrical contact between the various conductive elements. The contact sections 33, 34 have tapered faces 48, 49 en- gageable with the rib 31 as the contact 32 moves radially into engagement therewith.

The second arcing contact 32 is carried by insulating side plates 50, 51 which serve to mount the contact for pivotal movement about -the axis of the shaft 17. The shaft 17 is again of stainless steel and has threaded ends 52, 53 which retain compression springs 54, 55 housed in cupshaped ends of copper sleeves 56, 57 respectively. The springs ensure good electrical connection between the conductive elements. The inner ends of the sleeves 56, 57 are biased by the springs to bear against axially outer surfaces of two plates 18a, 18b, forming part of the mounting 18, the main contact blade 16 being sandwiched between and bearing against the axially inner surfaces of those plates, and being pivotally movable between those plates about the axis of the shaft 17. The copper sleeves 35 and 56 are electrically connected by a copper strap 58 lying externally of the insulating plate 50, while the copper sleeves 36 and 57 are electrically connected by a similar copper strap 59 lying externally of the insulating plate 51.

It will be seen that when the movable arcing contact 32 is in the closed position, as shown in Figure 2, it establishes an arc current path through the circuit breaker from the conductor 4, through main contact 14, the conductor 29 (Figure 6), the coil 28, the annular arcing contact 30, 31, the movable arcing contact 32, the copper sleeves 35, 36, the copper straps 58, 59, the copper sleeves 56 and 57 and the mounting 18 to the conductor 5. This cur- rent path is in parallel with the main current path, and the electrical conductors forming it are symmetrical about the radial mid-plane AA of the coil 28.

The drive shaft 8 has an arm 61 secured to rotate therewith, the arm being pivotally connected to a pair of parallel links 62, 63 the other ends of which are pivotally connected to the main contact 16. The arm 61 also carries a rod 64 pivotally connected thereto, the other end of the rod passing slidably through a bore in a shaft 65 rotatably mounted between ends of the insulating plates 50 and 5 1. Lock nuts 66 are fitted on the end of the rod 64, and a compression spring 67 sur- rounds the rod between the pivot with arm 61 and the shaft 65.

The spring 67 may be formed from a stacked series of cup springs in order to give the required spring characteristics, one of which is that the electrode 32 is maintained in good electrical contact with the electrode 31 during initial opening of the main contacts.

Operation of the circuit breaker will now be described. Figure 2 shows the circuit breaker in the closed position, and as already explained a main current path for each phase is established between the main contacts 14 and 15, while a parallel arc current path that includes the coil 28 is established between the arcing contacts 30 and 32. If the circuit is to be opened, whether due to the occurrence of a fault condition or otherwise, the drive shaft 8 is controlled in any suitable manner to rotate anti-clockwise from the position shown in Figure 2. The first part of this movement, from the Figure 2 position to the Figure 3 position, causes the contact 16 to be separated from the fingers 15 on the main contact 14, and so breaks the main current path through the circuit breaker. As a result of this 4 GB2178239A 4 1 the full current is caused to flow through the coil 28, so establishing a magnetic field within the open centre of the annular contact 30.

During the movement from the Figure 2 to the Figure 3 positions the rod 64 has passed freely through the bore in the shaft 65, but on further anti-clockwise movement of the shaft 8 from the Figure 3 to the Figure 4 position the lock nuts 66 engage the shaft 65 and thus cause anti-clockwise movement of the in- 75 sulating plate assembly 50 and 51 around the pivot formed by shaft 17. The movable arcing contact 32 thus separates from the annular arcing contact 30 and moves to the position shown in Figure 4 where the contact 32 lies 80 substantially concentric with the annular con tact 31. As the contact 32 separates from the contact 31 an arc is struck between the two contacts, and the magnetic field induced by the current flowing through the coil 28 causes 85 the arc to rotate around the contact 32 and within the annular contact 31. It will be seen that the arc is struck immediately in the plane of rotation, and that the plane of rotation will be the radial mid-plane A-A of the coil, i.e.

the plane of maximum flux density. It will fur ther be seen that because of the symmetrical arrangement of conductors to the arcing con tact 32 the arc current will be shared equally between the current path from the mounting 95 18 to the section 33 and the current path from the mounting 18 to the section 34. The magnetic loop forces induced at sections 33 and 34 by the arc current will thus effectively cancel each other out. The arc is therefore immediately constrained to the radial mid plane of rotation and any tendency of the arc to leave that plane is overcome. The are is kept at minimum length, and thus has low energy. Accordingly, when a current zero is 105 reached, the arc will be extinguished due to the presence of the electronegative medium within the space between the two arcing elec trodes.

When the circuit breaker is to be closed it is merely necessary to rotate the drive shaft clockwise from the position shown in Figure 4 so that the arcing contacts and the main contacts will each close to re-establish their re- spective current paths.

If the arcing contacts close before the main contacts, as could be the case with this embodiment, it will be seen that there is a potentially high closing peak current that can flow through the coil 28, so requiring a more robust coil construction than would otherwise be the case. It would accordingly be advantageous if the main contacts were to close before the arcing contacts, and Figure 7 illus- trates a modification whereby this may be achieved. As shown in Figure 7, the shaft 65 that extends between the insulating plates 50 and 51 has a dashpot arrangement 71 associ ated therewith. The dashpot arrangement comprises a fluid filled cylinder 72 in which is130 arranged a piston 73 biased to an extended position by a compression spring 74. The cylinder 72 has a bleed hole 75 in a side wall thereof, and due to the presence of the spring the piston 73 will normally be in its extended position, with the cylinder being filled by the pressurised insulating medium present within the chamber. Thus, on closing movement from the Figure 4 position the shaft 65 will engage the piston, and further movement will be delayed by the spring 74 and the slow exhaust of fluid through the restricted bleed hole 75. This will delay pivotal movement of the arms 50 and 51 and so also delay closing movement of the arcing contacts 32, the delay being dependent on the rate at which fluid is allowed to bleed from the cylinder 72. The bleed rate can readily be designed to ensure that the main contacts are fully closed before the arc contact 32 touches the annular contact 30. Alternative arrangements for achieving the same result will be readily apparent to those skilled in the art.

The embodiment described shows arc con- tact 32 mounted for pivotal movement within a fixed annular contact and coil. In an alternative embodiment as shown in Figure 8 the coil construction 81 and annular contact 82 may be carried between insulating plates, one of which is shown at 83, those plates being mounted about a pivotal axis 84 coincident with the pivotal axis of the movable main contact 85. The second arcing contact 86 is supported by an arrangement of conductors 87 from the fixed main contact 88, the arrangement of conductors being substantially symmetrical with respect to the radial mid-plane of the annular arcing contact 81. The arcing contacts are shown in the closed position; it will be appreciated that operation is similar to that described with reference to Figures 1 to 4, and that the arcing contacts will be opened by an anti-clockwise movement of the plates 83 about the pivot axis 84. In this arrangement the conductors for the central arcing contacts 86 may be buried in the insulating material of the chamber walls and partitions, and it may be possible to produce a more compact arrangement with a shorter distance between centres of individual phases.

For reasons of space it will generally be preferred to use pivotal movement for both the main contacts and the arcing contacts, although it will be understood that linear move- ment may be used for either or both of these. One such arrangement is schematically shown in Figure 9. In this arrangement a fixed main contact 91 is connected by a conductor 92 to a coil and annular arcing contact assembly 93.

A conductor 94 carries a movable main contact 95 engageable with the contact 91, and a movable arcing contact 96 engageable with the inner circumference of the annular contact 93. As with the other embodiments, the arrangement of conductors to the arcing contact j GB2178239A 5 Y 9 96 is symmetrical with respect to the radial mid-plane of the annular arcing contact 93. An insulating drive arrangement 97 is capable of moving the conductor to the right as shown in Figure 9, the contacts 95 and 91 separating during the first stage of such movement. A pin and slot arrangement 98 connects the arcing contact 96 to the conductor 94, providing lost motion so that the main contact separa- tion occurs before the arcing contact 96 is separated from the annular contact 93. The main contact 94 may be connected to an output 99 by a shunt, a sliding contact or any other suitable means. Means such as a com- pression spring 100 may be included to ensure good contact pressure both between the main contacts and between the arcing contacts.

Many of the described embodiments may be further adapted by providing for movement of the movable main contact from its open position to a position where it engages an earth contact, for example a star point contact. It is necessary that the movable arcing contacts remain in the open position during this additional movement of the main contact, and it is also desirable that the additional main contact movement should be effected through the common drive shaft. Modifications of the link- age that will make this additional movement possible will be apparent to those skilled in the art.

In each of the embodiments described there is relative movement between the two arcing electrodes in order to form the arc. In an alternative arrangement both arcing electrodes may be fixed, and the main current path may include a movable finger that engages one of the arcing electrodes when in the closed posi- tion. On opening the movable finger an arc will strike between that finger and the arcing electrode and the geometry of the structure is such that the arc root will commutate from the finger to the other arcing electrode. The arrangement is not as effective as the preferred constructions which form the arc immediately in the optimum location, but is a possible construction.

The specific description has been of a three- phase circuit breaker, but it will be evident that the principles of the invention may equally well be applied to single phase circuit breakers. It will also be apparent that there are other contact arrangements that can be used that will ensure a symmetrical disposition of the conductors to the central arcing contact, and also that there are many alternative drive arrangements that can be employed. Finally, it should be mentioned that the invention is not limited to a multi-phase breaker wherein the coil arrangements for the three phases are coaxial, although this may lead to particularly advantageous results from a space consideration. It is, however, equally possible for the coils of the three phases to be arranged so that their axes are parallel, the coils lying in a linear, triangular, or any other arrangement when viewed perpendicular to their axes. In such alternative arrangements the mid-planes of the coils of the different phases may be coplanar or may lie in more than one plane.

Claims (11)

1. A circuit breaker comprising first and second main contacts relatively movable between closed and open positions, and, when closed, forming part of a main current path through the circuit breaker; first and second arcing contacts forming part of an arc current path through the circuit breaker, and means for effecting contact movement so that during opening of the breaker the main contacts separate and an arc is located between the first and second arcing contacts; in which the first arcing contact is annular, the second arcing contact lies in the space within the open centre of the annular contact, an arc-driving coil surrounds the annular contact and is connected in series therewith in the arc current path, and the second arcing contact and a conductor assembly connected to the second arcing contact and forming part of the arc cur rent path are substantially symmetrical with respect to the radial mid-plane of the coil.

2. A circuit breaker according to claim 1 in which the first and second arcing contacts are relatively movable between a closed position wherein the second arcing contact engages the inner circumference of the annular first contact and an open position wherein the second arcing contact is spaced from the inner circumference of the annular first contact, and move from the closed to the open position only after the main contacts separate.

3. A circuit breaker accordng to claim 2 in which the conductor assembly comprises an axial section extending through the open centre of the annular contact, the second arcing contact is located at the centre of the axial section, and two side arms extend substantially radially of the annular contact, one lying to each side of the annular contact and electrically connecting the axial section of the conductor assembly to the main current path.

4. A circuit breaker according to claim 2 or claim 3 in which the second arcing contact is circular when seen in a plane extending radially of the annular contact, and when in the fully open position the second arcing contact is substantially concentric with the annular contact.

5. A circuit breaker according to any one of claims 2 to 4 in which the second arcing contact includes two sections resiliently biased one towards the other and engageable with opposite axial sides of part of the annular contact.

6. A circuit breaker according to any one of claims 2 to 5 in which one of the main con- tacts is carried by a first pivot arm, one of 6 GB2178239A 6 the arcing contacts is carried by a second pivot arm, and the two pivot arms are each mounted for pivotal movement about an axis parallel to the axes of the annular arcing contact.

7. A circuit breaker according to claim 6 in which the pivot axes of the first and second pivot arms are coincident.

8. A circuit breaker according to claim 6 or claim 7 in which the contact moving means comprises a drive arm connected through a rigid linkage to the first pivot arm and through a lost motion linkage to the second pivot arm.

9. A circuit breaker according to claim 8 in which the lost motion linkage includes a spring for applying a force tending to bias the arcing contacts into engagement when in the closed position.

10. A circuit breaker according to any one of claims 2 to 9 in which the contact moving means is effective during closing of the circuit breaker to close the main contacts before closing the arcing contacts.

11. A circuit breaker assembly for substan- tially simultaneously interrupting current flowing in each phase of a multi-phase supply, the assembly comprising, for each phase of the supply, a circuit breaker according to any one of the preceding claims, the circuit breaker be- ing mounted in a common housing, and opening and closing of the breaker for each phase being controlled by a drive means common to all phases.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8817356, 1987. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.

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