GB2260027A - Electrical switchgear - Google Patents

Abstract

Electrical switchgear is formed by a pair of main electrodes 7, 17 and a pair of arcing electrodes 8, 18, one electrode 17, 18 of each pair being mounted on a movable assembly 14 and the other being fixed. The fixed 7, 8 and movable 17, 18 electrodes are respectively electrically connected to each other, The movable assembly 14 moves, eg pivotally or linearly, from an ON position where the main electrodes 7, 17 are in contact and the arcing electrodes 8, 18 are not, through an intermediate position where the main electrodes and the arcing electrodes are in contact to an OFF position where none of the electrodes are in contact. In moving from the intermediate position to the OFF position the main electrodes 7, 17 separate before the arcing electrodes 8, 18. The assembly 14 may be moved to an earthing position wherein an earthing electrode 19 electrically connected to the movable electrodes 17, 18 is engaged with a fixed earthing electrode. One of the arcing electrodes may be tubular having an inwardly projecting portion (Fig 4) which is engageable with an outwardly projecting portion on an elongate electrode movable through the tubular electrode. <IMAGE>

Description

Electrical Switchaear This invention relates to electrical switchgear, and particularly to circuit breakers.

Circuit breakers have to stop current flow in a circuit as rapidly as possible in the event of an accident or overload. A problem encountered by circuit breakers, particularly when stopping high current flows, is arcing as the circuit breaker opens and the circuit is broken. Such arcing can cause considerable damage and while the arc exists current continues to flow in the circuit, preventing rapid shut off. One known method of dealing with this problem is to provide separate main and arcing contacts in electrical parallel and to separate the main contacts before the arcing contacts to open the circuit breaker. This ensures that arcing occurs only between the arcing contacts which can be designed to withstand arcing while the main contacts can be designed purely for good current transmission.

There are two main problems with this arrangement, the first is caused by the fact that it is normal to place an arc extinguishing coil adjacent to and in electrical series with the arcing contacts so that the magnetic field generated by the coil helps extinguish the arc. Although the use of such a coil is necessary to ensure rapid arc extinction and has been used in known circuit breakers of this type for safety the presence of a large reactive component in a power supply is electrically undesirable.

The second problem is simply that known designs of this type use very complex mechanical drive systems to ensure that the two sets of contacts move at the correct times, the use of complex mechanical drives is generally undesirable on grounds of cost, but is particularly undesirable in a circuit breaker which must remain in a current transmitting position for long periods of time, generally months to years, and reliably move to a current blocking position in fractions of a second on demand.

Although they are most pronounced in circuit breakers these problems are encountered in electrical switchgear generally, the larger the currents being switched the greater the problems become.

This invention was intended to produce electrical switchgear at least partially overcoming these problems.

In a first embodiment this invention provides electrical switchgear comprising first and second main electrodes and third and fourth arcing electrodes, the first and third electrodes being electrically connected together and attached to a first member and the second and fourth electrodes being electrically connected together and attached to a second member, the second member being able to move relative to the first member between a first ON position and through a second intermediate position to a third OFF position, where in the ON position the main electrodes are in contact and the arcing electrodes are out of contact, in the intermediate position the main electrodes and the arcing electrodes are in contact and in the OFF position all of the electrodes are out of contact such that as the members move from the intermediate position to the OFF position the main electrodes separate while the arcing electrodes are in contact.

This gives the advantage that movement of a single member operates the electrical switchgear.

Preferably the second member moves linearly relative to the second member as this allows for a very simple mechanical arrangement.

Advantageously the switchgear includes an arc extinguishing coil in electrical series with the arcing electrodes and in electrical parallel with the main electrodes to ensure rapid arc extinction, because the coil is in series with the arcing electrodes there will always be a current through the coil to generate an extinguishing magnetic field when an arc exists but because the coil is in parallel to the main electrodes the coil will be out of circuit when the switchgear is in the ON position.

In a second embodiment this invention provides an arcing electrode arrangement for use in electrical switchgear comprising a tubular arcing electrode and an elongate arcing electrode, the tubular electrode having an inwardly projecting conductive portion and the elongate electrode having a outwardly projecting portion and being arranged to move through the tubular electrode so that the two electrodes are in electrical contact only when their respective projecting portions are adjacent.

Apparatus employing the invention will now be described by way of example only with reference to the accompanying diagrammatic figures in which; Figure 1 shows a cut away view of a circuit breaker unit according to the invention; Figure 2 shows another cut away view of the circuit breaker unit of Figure 1; Figure 3 shows a perspective cut away view of circuit breaker unit of Figure 1; and Figure 4 shows an alternative electrode arrangements for the circuit breaker unit of Figure 1; similar parts having the same reference numerals throughout.

Referring to Figures 1, 2 and 3 an A.C. circuit breaker unit is shown. This is a single phase unit and one such unit will be required for each phase of the electrical supply being switched, for example a conventional three-phase supply will need three such units.

The entire circuit breaker unit is enclosed within a sealed conductive earthed casing 1 containing sulphur hexafluoride gas. The casing 1 is earthed at a point 2.

Not all of the casing 1 is shown.

In operation, alternating current passes through the circuit breaker unit between a first conductor 3 which is linked to a current source and a second conductor 4 which is linked to a current using circuit. The second conductor 4 passes through the casing 1 inside an insulating bush 5.

The first conductor 3, a contact block 6, a first, main, electrode 7, a third, arcing, electrode 8 an arc extinguishing coil 9 and a support member 10 are all rigidly connected to form a fixed contact assembly. The fixed contact assembly is fixed relative to the casing 1 and is attached to the casing 1 by a first insulating support 11 attaching the conductor 3 to the casing 1 and a second insulating support 12 attaching the support member 10 to the casing 1.

The contact block 6 is in electrical contact with the first conductor 3 and the first main electrode 7 which it supports. The third, arcing, electrode 8 is rotationally symmetrical about an axis 13 and is a substantially cylindrical tube with an end portion 16 of increased radius.

The arc extinguishing coil 9 is wound around the third arcing, electrode 8 and the inner turn of the coil 9 is in electrical contact with the third arcing electrode 8. The outermost turn of the coil 9 is in electrical contact with the contact block 6.

The second conductor 4 is electrically linked to a contact support bar 14 by a flexible connector 15. The contact support bar 14 supports and is electrically connected to a second main electrode 17, a fourth arcing electrode 18 and a fifth earthing electrode 19. The contact support bar 14 and electrodes 17, 18 and 19 form a moving contact assembly.

The contact support bar 14 is attached to an insulating support 20 which is in turn attached at one end to a carrier 21. The carrier 21 has a pair of pins 22 attached to it which pass through a slot 23 in a moulding 24 in the form of a planar sheet.

The insulating support 20 is attached at its second end to a second carrier 21 with pins 22 passing through a second slot 23 in a second moulding 24. This arrangement is identical to that described above and is omitted in Figure 1 for clarity. It is however visible in Figure 2 which shows the circuit breaker unit from the opposite side.

Thus the moving contact assembly is only able to move linearly parallel to the slots 23.

The fourth arcing electrode 18 and fifth earthing electrode 19 are formed by the two ends of a single elongate conductor passing through the contact support bar 14.

The fourth arcing electrode 18 is rotationally symmetrical about an axis 27 parallel to the slots 23 so that as the moving contact assembly moves the fourth arcing electrode 18 moves along the axis 27. The fourth arcing electrode 18 comprises a main body portion 28 adjacent the contact support bar 14 and a head portion 29 having a rounded end 30, the main body and head portions 28 and 29 being separated by a neck portion 31 having a smaller radius than the head and main body portions 28 and 29.

A sixth earthing electrode 25 is fixed to the casing 1 by a bolt 26 and is in electrical contact with the casing 1.

In Figures 1 and 2 the moving contact assembly is shown in solid lines in an OFF position. In Figure 1 the moving contact assembly is also shown in broken lines in an ON position, indicated by suffixes A, and intermediate position indicated by suffixes B and an earthed position indicated by suffixes C. In all three positions shown in dashed lines only key parts of the moving contact assembly are shown to prevent the picture becoming too confusing.

When the moving contact assembly is in the ON position the first main electrode 7 and second main electrode 17A are in contact while the third arcing electrode 8 and fourth arcing electrode 18A are not because the reduced radius neck portion 28A of the fourth arcing electrode 18A is adjacent the third arcing electrode 8 leaving a small clearance.

This small clearance is sufficient because in the ON position the arcing electrodes 8 and 18A are at the same electrical potential. The axis 13 of the third arcing electrode is inclined relative to the axis of the fourth arcing electrode and as a result the main body position 28A of the fourth arcing electrode does not contact the third arcing electrode 8. In the ON position the pins 22 are at first ends of the slots 23 preventing further movement of the moving contact assembly towards the fixed contact assembly.

In the ON position a current path exists from the first conductor 3 through the contact block 6, the first main electrode 7, the second main electrode 17A, the contact support bar 14A, and the flexible connector 15A to the second conductor 4.

In order to break the current path the moving contact assembly is moved along the slots 23 away from the fixed contact assembly towards the OFF position, en route it passes through the intermediate position.

In the intermediate position the head portion 29B of the fourth arcing electrode 18 brushes against the third arcing electrode 8 while the first and second main electrodes 7 and 17B are still in contact. As a result two parallel current paths exist from the first conductor 3 to the second conductor 4. The first current path passes from the contact block 6 through the first and second main electrodes 7 and 17B to the contact support bar 14 [Not shown in this position], the second current path passes from the contact block 6, through the coil 9, and the third and fourth arcing electrodes 8 and 18 to the contact support bar 14. The inner lip of the third arcing electrode 8 is rounded off in order to provide a good contact between the third and fourth arcing electrodes 8 and 18 and to minimise erosion due to arcing.

As the moving contact assembly continues its movement towards OFF position the second main electrode 17 separates from the first main electrode 7, however no arcing will occur between the first and second main electrodes 7 and 17 because they are at the same potential because a current path still exists through the coil 9 and third and fourth arcing electrodes 8 and 18B.

As the moving contact assembly moves further towards the OFF position the head position 30 of the fourth arcing electrodes 18 separates from the third arcing electrode 8 because of the axis of the third arcing electrode 8 is inclined to the direction of movement of the fourth arcing electrode 18. As the third and fourth arcing electrodes 8 and 18 separate an arc will be struck between them. The separation of the third and fourth arcing electrodes continues to increase until the moving contact assembly stops in the OFF position. In the OFF position the head portion 29 of the fourth arcing electrode 18 is positioned on the axis 13.

The arc between the third and fourth arcing electrodes 8 and 18 is extinguished by the atmosphere of sulphur hexafluoride within the casing 1 and by the arc rotating magnetic field produced by the coil 9. Both of these methods of arc extinction are well known and need not be discussed in detail here.

When the moving contact assembly reaches the OFF position the head portion 29 of the fourth arcing electrode 18 is situated on the axis 13 of the third arcing electrode 8 and coil 9, giving a symmetrical arrangement to give good arc rotation to ensure rapid arc extinction.

The larger radius end portion 16 of the third arcing electrode 8 and the rounded end 30 of the head portion 29 minimise the damage done by the arcing between the third and fourth arcing electrodes 8 and 18.

In the OFF position there is no current path between the first and second conductors 3 and 4.

From the OFF position the moving contact assembly can be moved still further away from the fixed contact assembly into an earth position where the fifth earthing electrode 19C is in contact with the sixth earthing electrode 25, in the earthing position the pins 22 are at the end of the slots 23 and the moving contact assembly is at its farthest possible position from the fixed contact assembly. This provides an earthing current path for the conductor 4 by way of the flexible connector 15, contact support bar 14, the fifth and sixth earthing electrodes l9C and 25 and the casing 1, earthing the external circuit supplied by the conductor 4.

The mechanism used to move the moving contact assembly is shown in Figure 2. A driving shaft 31 bears an arm 32 to which one end of a first link member 33 is pivotally connected. The opposite end of the first link member 33 is pivotally connected to an arm 34 which is pivotally linked to a fixed supporting structure 35 at one end and to a second link member 36 at its other end.

The point at which the first link member 33 is connect to the arm 34 lies between the points at which the arm 34 is linked to the support structure 35 and the second link member 36. The second link member 36 is pivotally linked to one of the pins 22 on the opposite side of the moulding 24 to the rest of the circuit breaker unit.

Rotation of the drive shaft 31 causes the moving contact assembly to move linearly along the slots 23. The circuit breaker unit shown is for use in a three phase system so three such units are situated side by side, all operated simultaneously by a single drive shaft 31.

The drive shaft 31 is rotated by a servomechanism (not shown) to place the circuit breaker unit in the ON, OFF or earthed positions as necessary. Although the circuit breaker could be moved by hand, for safety a servomechanism able to move the circuit breaker from the ON position to the OFF position in 40 milliseconds is used.

Referring to Figure 4 an alternative design for the main and arcing electrodes and a different design of arcing electrodes is shown. A fixed contact assembly is formed by the first conductor 3, contact 6, first main electrode 7, coil 9, third arcing electrode 37 and support member 10 and is attached to the casing 1 by a first insulating member 12 and an insulating support member 11 as before.

In Figure 4 the moving contact assembly is shown in solid lines at the intermediate position and in dashed lines at the ON and OFF positions, the suffixes A and B are used to designate the various positions as before.

The moving contact assembly is formed by inter-alia the contact support bar 14, the second main electrode 17 and a fourth arcing electrode 38.

The third arcing electrode 37 is formed by a cylindrical conductive tube having a radially inwardly projecting ridge 39 extending through a full circle midway along its length. The ridge 39 extends perpendicularly to the inner surface of the conductive tube and has a tubular inner face. The third arcing electrode 37 is rotationally symmetrical about an axis 40. The coil 9 is wound around the third arcing electrode 39 and its inner turn is in electrical contact with it as before. An insulating layer 41 covers the inside and end faces of the third arcing electrode leaving only the inner face of the ridge 39 exposed, the insulating layer extends beyond the third arcing electrode 37 to cover the end faces of the coil 9 as well. The insulating layer 41 forms a continuous smooth surface with the inner face of the ridge 39.

The fourth arcing electrode 38 has a main cylindrical conductive body portion 42 surrounded by a tubular insulating shroud 43, at the end of the body portion 42 is a conductive head portion 44 of larger radius than the body portion 42. The head portion 44 has a rounded end and has the same radius as the insulating shroud 43 so that the insulating shroud 43 and the head portion 44 form a smooth continuous surface. The fourth arcing electrode 38 is symmetrical about the axis 40 like the third arcing electrode 37 and has the same outer radius as the inner radius of the ridge 39.

In this embodiment the moving contact assembly moves linearly parallel to the axis 40, so that the fourth arcing electrode 38 moves along the axis 40.

When the unit is in the ON position the first and second main electrodes 7 and 17A are in contact while the conductive head portion 44A of the fourth arcing electrode is adjacent the insulating layer 41, as a result the only current path is through the first and second main electrodes 7 and 17A.

As the moving contact assembly moves towards the OFF position it passes through an intermediate position where the first and second main electrodes are still in contact and the conductive head portion 44B of the fourth arcing electrode is adjacent the ridge 39, because the outer radius of the head portion 44B is the same as the inner radius of the ridge 39 the head portion 44B brushes against the inner surface of the ridge 39 so that there are two current paths, one through the first and second main electrodes 7 and 17B and the other through the third and fourth arcing electrodes 37 and 38B.

As the moving contact assembly continues to move towards the OFF position the first and second main electrodes 7 and 17 separate while the third and fourth arcing electrodes 37 and 38B are still in contact without arcing because they are at the same potential because a current path still exists through the third and fourth arcing electrodes. Then the conductive head portion 44 of the fourth arcing electrode 38 separates from the ridge 39 of the third arcing electrode 37 and an arc is formed. This arc is constrained to only between the conductive portion 44 and the ridge 39 because the other surfaces of the third and fourth arcing electrodes 37 and 38 are shielded by insulators 41 and 43.

This arrangement has a number of advantages, because the fourth arcing electrode 38 moves along the axis 40 of the coil 9 and the arc is constrained to contact the ridge 39 situated at the mid point of the third arcing electrode 37 and the coil 9 the arc must enter the region where the magnetic field generated by the coil 9 is strongest and the third and fourth arcing electrodes are always symmetrically arranged about the axis 40, ensuing the best arc rotation extinction conditions. Also because the arc cannot transfer from the ridge 39 to a closer part of the third arcing electrode 37 as the fourth arcing electrode 38 moves the arc length is increased more rapidly than before as the moving contact assembly moves away from the fixed contact assembly.

A further advantage is that because the forth arcing electrode 38 is always on the axis of the coil 9 any magnetic forces exerted radially on the electrode 38 by the coil will cancel themselves out due to the symmetry of the arrangement.

In the switchgear of Figures 1 to 3 the moving contact assembly could pivot about an offset pivot point between its positions, but a linear movement is preferred.

Any other electronegative gas could be used to replace the sulphur hexafluroide.

In the switchgear of Figures 1 to 3 the flexible connector 15 could be replaced by a sliding or pivoting contact arrangement.

There are of course many alternative mechanical constructions which could be used in the switchgear.

Claims (16)

Claims

1. Electrical switchgear comprising first and second main electrodes and third and fourth arcing electrodes, the first and third electrodes being electrically connected together and attached to a first member and the second and fourth electrodes being electrically connected together and attached to a second member, the second member being able to move relative to the first member between a first ON position and through a second intermediate position to a third OFF position, where in the ON position the main electrodes are in contact and the arcing electrodes are out of contact, in the intermediate position the main electrodes and the arcing electrodes are in contact and in the OFF position all of the electrodes are out of contact such that as the members move from the intermediate position to the OFF position the main electrodes separate while the arcing electrodes are in contact.

2. Electrical switchgear as claimed in claim 1 where the second member moves linearly relative to the first member.

3. Electrical switchgear as claimed in claim 1 or claim 2 and further comprising fifth and sixth earthing electrodes, the fifth electrode being electrically connected to the second and fourth electrodes and attached to the second member and the sixth electrode being earthed and fixed relative to the first member and the second member being able to move to a fourth earthing position where the earthing electrodes are in contact and the main and arcing electrodes are out of contact.

4. Electrical switchgear as claimed in claim 3 where the second member can move from the ON position to the OFF position and further in the same sense to the earthing position.

5. Electrical switchgear as claimed in any preceding claim and further comprising an arc extinguishing coil in electrical series with the arcing electrodes and in electrical parallel with the main electrodes.

6. Electrical switchgear as claimed in claim 5 in which when the second member is in the OFF position the fourth electrode is on the axis of the coil.

7. Electrical switchgear as claimed in any preceding claim where the third electrode is tubular and the fourth electrode is elongate and at an angle to the third electrode and comprises a head portion at one end adjacent a neck portion narrower than the head portion, arranged so that when the second member is in the ON position the fourth electrode is situated within the third electrode but not in contact with it and when the second member is in the intermediate position the head portion of the fourth electrode is in electrical contact with the third electrode.

8. Electrical switchgear as claimed in claim 7 where the third electrode is a cylindrical tube.

9. Electrical switchgear as claimed in any preceding claim where the switchgear is a circuit breaker.

10. An arcing electrode arrangement for use in electrical switchgear comprising a tubular arcing electrode and an elongate arcing electrode, the tubular electrode having an inwardly projecting conductive portion and the elongate electrode having a outwardly projecting portion and being arranged to move through the tubular electrode so that the two electrodes are in electrical contact only when their respective projecting portions are adjacent.

11. An arrangement as claimed in claim 10 in which the tubular electrode is a substantially cylindrical tube and the elongate electrode is substantially cylindrical.

12. An arrangement as claimed in claim 11 where the outer radius of the head portion of the elongate electrode is the same as the inner radius of the inwardly projecting conductive portion of the tubular electrode.

13. An arrangement as claimed in any of claims 10 to 12 where the neck portion of the elongate electrode is surrounded in an insulating layer.

14. An arrangement as claimed in any of claims 10 to 13 where the inner surface of the tubular electrode is coated in an insulating layer such that the inwardly projecting ridge is the only exposed conductive part.

15. An arrangement as claimed in any of claims 10 to 14 where the arrangement is used in a circuit breaker.

16. An arcing electrode arrangement substantially as shown in or as described with reference to Figure 4 of the accompanying drawings.

16. Electrical switchgear substantially as shown in or as described with reference to Figures 1 to 3 of the accompanying drawings.

17. An arcing electrode arrangement substantially as shown in or as described with reference to Figure 4 of the accompanying drawings.

Amendments to the cfahrs hwe been fled ss follows:;

11. An arrangement as claimed in claim 10 in which the tubular electrode is a substantially cylindrical tube and the elongate electrode is substantially cylindrical.

12. An arrangement as claimed in claim 11 where the outer radius of the head portion of the elongate electrode is the same as the inner radius of the inwardly projecting conductive portion of the tubular electrode.

13. An arrangement as claimed in any of claims 10 to 12 where the neck portion of the elongate electrode is surrounded in an insulating layer.

14. An arrangement as claimed in any of claims 10 to 13 where the inner surface of the tubular electrode is coated in an insulating layer such that the inwardly projecting conductive portion is the only exposed conductive part.

15. An arrangement as claimed in any of claims 10 to 14 where the arrangement is used in a circuit breaker.