Abstract:
In arc interrupters having fixed and movable contacts and a fixed annular electrode surrounded by an arc-driving coil, the problem of electrode collapse owing to the magnetic forces generated in the electrode and the coil is reduced or obviated providing the electrode with at least one flange which has radial slots extending from the outer periphery of the flange through a substantial part of the width of the flange.

Description:
BACKGROUND TO THE INVENTION 
     The invention relates to arc interrupters. 
     To assist in extinguishing arcs in arc interrupters, which operate in an insulating fluid, for example sulphur hexafluoride (SF 6 ) gas under pressure, it has been proposed to rotate the arc about an axis. Examples of such arc interrupters are described in U.K. Patent Applications Publication Nos. 2038100A, 2044538A, 2052160A and 2092385A and in U.K. Pat. No. 2119573B (equivalent to U.S. Pat. No. 4503,302). Typically, such arc interrupters have fixed and movable contacts, an annular arcing electrode and an arc-driving coil connected in series with the electrode. In such interrupters, the arc, once fully established, extends between the electrode and one of the contacts and is driven by the coil about the central axis of the electrode. 
     To avoid problems associated with insulating the arc-driving coil from the arc, it is preferred to locate the coil externally of the electrode. 
     However, as the rating of the interrupters (for example as described in U.K. Pat. No. 2110573B) are increased, it was found that plain cylindrical electrodes tended to collapse owing to the increased magnetic fields generated by the currents induced in the electrode by the magnetic fields generated by the coil, which magnetic fields are in opposition to one another. 
     Although the problem can be alleviated to some extent by increasing the cross-section of the electrode at lower ratings, it is not a practical solution at higher ratings owing to the increased costs of material and increased weight and size of the electrode. 
     It is possible to increase the strength of the electrode by providing radially-extending flanges. Some such electrode configurations are shown in the above-mentioned puslished Applications. However, in those instances, the flanges appear to have been provided for other reasons, for example to provide an arc runner or to protect the coil ends from the arc (which is initally struck in the immediate vicinity of the coil ends). 
     It has been found, however, that the electrode can distort owing to the generation in the flanges of magnetic fields, which oppose the fields generated by the coil, by currents induced in the electrode by the fields generated by the coil. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an arc interrupter in which the afore-mentioned problems are reduced or obviated. 
     According to the invention, an arc interrupter comprises fixed and movable contacts, a fixed electrode having at least one flange divided by radial slots extending from the outer periphery thereof through at least a substantial part of the width of said flange, first and second coaxial arcing surfaces separated by an annular gap provided, respectively, by one of said contacts and said electrode, said first arcing surface being closer to the common axis of said arcing surfaces than said second arcing surface, and an arc-driving coil surrounding said electrode, said coil being coaxial with said surfaces and being electrically connected at one end to said electrode whereby said coil is included in series with said arcing surfaces in an arc current path at least during a later part of the opening of a main current path formed by said contacts in a make position. 
     Preferably, said movable contact is mounted for angular movement about a pivot axis between said make position and a break position. 
     Preferably, said first arcing surface is provided by said fixed contact and in said make position, said movable contact extends from said pivot axis towards said common axis and is in engagement with said fixed contact and, in said break position, said movable contact is disengaged from said fixed contact and the least distance between said contacts is greater than said gap. 
     Preferably, said slots extend through the whole width of said flange. 
     The invention includes an electric switch comprising at least one arc interrupter according to the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An electric switch will now be described to illustrate the invention by way of example only with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic longitudinal section through the fixed contact of an arc interrupter according to the invention; 
     FIG. 2 is a view on arrow II in FIG. 1 of the fixed electrode shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The electrode switch (see FIGS. 1 and 2) is of the type described and claimed in U.K. Pat. No. 2119573B and reference should be made to that document for details concerning the construction of such switches. 
     Briefly, however, the switch has a housing 10 of metal for example defining an enclosure which is filled with an insulating medium for example SF 6  gas under pressure. A bushing 12 insulates a main copper conductor 14 from, and enables it to pass in sealed relationship, through the housing 10. A second main conductor (not shown) is similarly mounted relative to the housing 10 at a location remote from the conductor 14. The two main conductors carry one phase of the current supplied through the switch. 
     An arc interrupter 16 forms part of an operable main current path between the two main conductors. The conductor 14 and the interrupter 16 are coaxial with one another on the common axis 18. 
     The interrupter has a fixed contact 20 formed on the end of the conductor 14 coaxially with the axis 18. The fixed contact 20 has a first cylindrical part 22, from which a segment has been removed to leave a slot 24. A second barrel-shaped part 26 is resiliently mounted (not shown) on the part 22 whereby a movable contact 58 located in the slot 24 is engaged by the parts 22, 26 under pressure. The part 26 has a first arcing surface 28 coaxial with the axis 18. 
     A fixed assembly 30 is secured to the housing 10 through insulated mounts (not shown). The assembly 30 comprises a tubular arcing electrode 32 of copper which is mounted coaxially with the axis 18 and is surrounded by, and electrically connected to (not shown) one end of, an arc-driving coil 34. The other end (not shown) of the coil 34 is electrically connected to the second main conductor. To enhance the effect of the coil 34 on the arc, particularly at low currents, ferromagnetic material 36 such as mild steel is located around the coil 34. 
     The electrode 32 is frusto-conical in shape and has, at the end thereof adjacent the contacts 20, 58, an outwardly-extending flange 38 or flange portion and, at its opposite end, an inwardly-extending flange 40. The shape of the electrode 32 together with the flanges 38, 40 improves the ability of the electrode 32 to resist the forces tending to collapse the electrode radially inwardly as compared with a plain cylindrical electrode. 
     The forces arise, during arc interruption, owing to the magnetic field generated by the arcing current flowing in the coil 34 inducing an opposing current in the electrode 32 and that current generating a magnetic field opposing the coil magnetic field. 
     The induced current flows in the flanges 38, 40 as well as in the axial extent of the electrode 32. The magnetic field generated by the induced current particularly affects the flange 38 owing to is proximity to the end of the coil 34 and to the ferromagnetic material 36 which concentrates the magnetic fields. At high currents the strengths of the opposed fields are sufficient to distort the flange 38. 
     To counteract such effects, the flange 38 has spaced-apart radial slots 68 (see FIG. 2). The slots 68 disrupt the annularly-circulating induced currents in the flange 38 thereby minimising the strength of the magnetic field which opposes the coil magnetic field. 
     In a modification, the flange 40 is similarly slotted. 
     The ferromagnetic material 36 is provided in the form of a cylinder 42 which has an inwardly-extending flange 44 which is recessed at 46 to receive the flange 38. The flange 38 is bolted or otherwise secured at 48 to the flange 44 to be supported thereby against the effects of teh opposing fields. 
     The ferromagnetic material 36 also consists of an annular plate 50 which is bolted to the cylinder 42. In a modification, the flange 40 can be bolted to the plate 50 which is then shaped to accommodate the flange 40 as shown in ghost outline at 52 in FIG. 1. 
     The cylinder 42 and the plate 50 also support the coil 34 in position, the coil 34 being insulated from the electrode 32 and the ferromagnetic material 36 by insulating material 54. 
     The electrode 32 has a main portion which includes an internal surface which forms a second arcing surface 56 coaxial with the axis 18 and spaced from and positioned substantially opposite to the arcing surface 28 so that an annular gap exists between the arcing surfaces 28 and 56. 
     A movable contact 58 is located adjacent the assembly 30. 
     The contact 58 is made of copper and is arcuate in shape as seen along the axis 18. The contact 58 is mounted for angular movement about a pivot axis 60 parallel to the axis 18 by a pivot block 62 mounted on the assembly 30 by flanges 64 extending, but electrically insulated (not shown), from the cylinder 42 of the ferromagnetic material 36. The lower end of the pivot block 62 is electrically connected to the second main conductor by a bar 66, for example. 
     The movable contact 58 is angularly movable by mechanism (not shown) between a make position (see FIG. 1) in which the contact 58 extends from the pivot axis 60 towards the axis 18 and engages the fixed contact 20 and a break position in which the least distance between the contact 58 and the fixed contact 20 is greater than the gap between the arcing surfaces 28 and 56. 
     OPERATION 
     The interrupter 16 is shown in the make position. The main current path is through the main conductor 14, the fixed contact 20, the movable contact 58, the pivot block 62, the bar 66 and the second main conductor. 
     Actuation of the operating mechanism causes the movable contact 58 to move to the break position. As the contact 58 disengages from the contact 20, an arc is struck between the contacts 20, 58. As the contact 58 passes over the electrode 32, the arc is transferred to the electrode 32 which brings the arc-driving coil 34 into an arc current path. The magnetic field generated by the coil causes the arc to rotate about the axis 18. 
     At an appropriate current zero the arc is extinguished. 
     The interrupter is closed by reverse operation of the operating mechanism which causes the contact 58 to return to its make position. 
     The switch described with reference to the accompanying drawings has a normal rating of 12 kilovolts (kV), 1.25 kiloamps (kA) and a fault rating of 25 kA. 
     Similar considerations can be applied to the flange 44 (or to the plate 50 especially as shown at 52 when the flange 40 is slotted also) and it is preferred to provide radial slots 70 in the flange 44 also, the slots 70 registering with the slots 68 in the flange 38. 
     It will be appreciated that interrupters according to the invention can have fixed electrodes selected from a wide range of geometries and the interrupter described with reference to the accompanying drawings is intended to be exemplary and not limiting. 
     Typically, switches having interrupters in accordance with the invention can have normal ratings of up to 36 kV, 2 kA and fault condition ratings of up to 40 kA.