ELECTRIC SWITCHING DEVICE

Abstract: The invention relates to a switching device (1) comprising a switching means with two contact members arranged movably in relation to each other for making or breaking a current path between two cables with an electric conductor (6, 10), an inner semiconductive layer (7, 11) surrounding the conductor, an electrically insulating solid cable body (8, 12) surrounding the inner layer, and an outer semiconductive layer (9, 13) surrounding the cable body. The switching device comprises a field-controlling means surrounding the switching means and comprising at least one conductive or semiconductive field-controlling body (21) connected to a first potential, an electrically insulating solid body (24) surrounding the field-controlling means, and a conductive or semiconductive shield (27) surrounding the insulating solid body (24) connected to a second potential. According to one embodiment of the invention, the field-controlling body (21) is electrically connected to at least one of the inner semiconductive layers (7, 11) of the cables. According to another embodiment of the invention, the shield (27) is electrically connected to at least one of the outer semiconductive layers (9, 13) of the cables.

Description: Electric switching device TECHNICAL FIELD The present invention relates to an electric switching device of the kind described in the preamble to the independent claim 1. More particularly, the invention relates to a diverter switch or a switching device intended to be used in on-load tap-changers for voltages exceeding 1 kilovolt. BACKGROUND ART In an electrical power system it is desirable to provide voltage control in order to maintain the voltage in the system. Normally, voltage control is achieved by changing the ratio of transformers in the system. To this end, a controllable transformer has one or a plurality of regulating windings which, with the aid of on-load tap-changers, are connected to or disconnected from the primary or secon- dary winding of the transformer. In principle, there are two types of on-load tap-changers, namely, tap-changers of selector-switch type and tap-changers or diverter-switch type. In an on-load tap changer of diverter-switch type, the power is broken by means of a special switching device, a so-called diverter switch, and the choice of regulating winding is performed by a separate switching device, a so- called switch. In an on-load tap-changer of selector-switch type, the choice of regulating winding and power breaking is performed in the same operation and in the same component, the so-called selector-switch pole, in which both the diverter-switching function and the switching function are integrated. On-load tap-changers are available in mechanical designs, in which the switching and diverter-switching functions are performed by means of a switching device which closes and opens current paths by means of movable contact members operated by an operating device. On-load tap- changers are also available in completely electric designs, in which the switching and diverter-switching functions are performed by a semiconductor switch which closes and opens current paths by controlling the conductivity of semiconductors in the switching device. From the published PCT application SE97/00875, a transformer with windings consisting of cable with an electrically conductive conductor, a semiconductive inner layer surrounding the conductor, an electrically insulating cable body surrounding the inner layer, and a semiconductive outer layer surrounding the cable body, is previously known. The inner layer is in electrical contact with the conductor and has the same potential as the conductor. The potential of the outer layer is controllable and is normally set at zero by grounding the outer layer . Such a winding has the property of enclosing in the cable body, between the inner and outer layers, the electric field which surrounds the conductor of the cable. Since the outer layer has a constant potential, adjacent winding turns need not be insulated from one another. If, in addition, the potential of the outer layer is connected to ground, the windings need not be insulated from the transformer core and the transformer may operate without any electrically insulating transformer oil, which results in a number of technical and environmental advantages . When using a conventional on-load tap-changer for controlling a cable-wound transformer of the type described above, the inner layer, the cable body and the outer layer of the cables of the regulating windings must be broken when being connected to the on-load tap-changer. The field-enclosing power is thus lost together with many of the advantages of the cable-wound transformer. At the cable ends, expensive cable terminations must be used, and in the on-load tap- changer, insulating oil or large clearances are required to prevent electric flashovers. SUMMARY OF THE INVENTION The object of the present invention is to achieve a field- enclosing switching device comprising a switching means to make or break a current path between a first cable and at least one second cable, which cables have an electric conductor, an inner semiconductive layer surrounding the conductor, an electrically insulating cable body surrounding the inner layer, and an outer semiconductive layer surrounding the cable body. This object is achieved according to the invention with a new type of switching device according to the characteris- tic features described in the characterizing portion of the independent claim 1. The switching device according to the invention comprises: a field-controlling means surrounding the switching means and comprising at least one conductive or semiconductive field-controlling body connected to a first potential , an electrically insulating solid body surrounding the field-controlling means, - a conductive or semiconductive shield surrounding the body and connected to a second potential, and at least two contact members movably arranged in relation to each other, wherein one contact member is electrically connected to the conductor of the first cable and the other contact member is electrically connected to the conductor of the second cable, which contacts are operable by means of an operating device between a closed position and an open position. The fact that the field-controlling body and the shield are conductive or semiconductive means in this context that, at room temperature, they have an electrical resistivity of less than 10,000 ohmmeters . With respect to the field-enclosing power, the field- controlling means corresponds to the inner layers of the cables and functions in the switching device, in practice, as a continuation of these layers. In the same way, the insulating body corresponds to the cable bodies of the cables, and the shield corresponds to the outer layers of the cables . The field-controlling body preferably has a potential which essentially corresponds to the potential of the cable conductors, and the shield preferably has a potential which essentially corresponds to the potential of the outer layer of the cables . According to one preferred embodiment of the invention, the field-controlling body is electrically connected to at least one of the inner layers of the cables. According to another embodiment of the invention, the shield is electrically connected to at least one of the outer layers of the cables . The insulating body assumes the voltage difference between the field-controlling body and the shield. The voltage difference between the cables in the open position is assumed in the switching means, for example in air gaps between movable contact members or by power semiconductor devices . BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be explained in greater detail with reference to the accompanying drawings, wherein Figure 1 shows a first embodiment of the invention, Figure 2 shows a second embodiment of the invention, and Figures 3 and 4 show a third embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a first embodiment of a switching device 1 according to the invention, wherein the switching device 1 is connected to a first field-enclosing cable 2 via a first joint means 3 and to a second field-enclosing cable 4 via a second joint means 5. The switching device 1 is rotationally symmetrical and is shown in Figure 1 in a section along its axis. The cable 2 has an electric conductor 6, an inner semiconductive layer 7 surrounding the conductor 6, an electrically insulating cable body 8 surrounding the inner layer 7, and a first outer semiconductive layer 9 surrounding the cable body 8. In the same way, the second cable 4 has an electric conductor 10, an inner semiconductive layer 11 surrounding the conductor 10, an electrically insulating cable body 12 surrounding the inner layer 11, and an outer semiconductive layer 13 surrounding the cable body 12. The outer layers 9 and 13 are connected to ground. The switching device 1 has a mechanical switching means for making and breaking a current path between the cable 2 and the cable 4. The cable means comprises an elongated contact member 14 of a magnetic material. The contact member 14 is movably arranged in a gas-filled switching chamber 15. The contact member 14 is in electrical contact with the conductor 10 via a connection 16 in the joint means 5. Further, the switching means has a stationary contact member 17 in electrical contact with the conductor 6, via a connection 18 in the joint means 3. On the outside of the switching device 1, a first 19 and a second 20 coil are arranged. Upon a closing signal from control equipment (not shown) , the coil 19 generates a magnetic field in the switching chamber 15 in such a way that magnetic forces force the contact member 14 to an end position where it is in contact with the contact member 17 and electrically connects the conductor 6 to the conductor 10. Upon an opening signal from the control equipment, the coil 20 generates a magnetic field in the switching chamber 15 in such way that magnetic forces force the contact member 14 to an end position where it has such a position in relation to the contact member 17 that it is electrically insulated therefrom. The switching device 1 has a field-controlling means in the form of an inner semiconductive layer 21 which surrounds the switching means. At one end, the layer 21 makes contact, via an inner semiconductive connecting layer 22 in the joint means 3, with the inner semiconductive layer 7 of the first cable 2. At its other end, the semiconductive layer 21 makes contact, via an inner semiconductive connecting layer 23 in the second joint means 5, with the inner semiconductive layer 11 of the second cable 4. The layer 21 surrounds the contact member 17 and is in electrical contact therewith along the whole of its length. The layer 21 also surrounds the contact member 14 but is electrically connected thereto along part of its length only, whereupon the inner surface of the layer 21 deviates from the surface of the contact member 14 and forms the radial limiting surface of the switching chamber 15. Outside the layer 21, and making good contact therewith, an electrically insulating body 24 is arranged, which surrounds the layer 21 along substantially the whole of its length. The ends of the body 24 make contact with electrically insulating bodies 25 and 26 in the joint units 3 and 5. Further, the switching device 1 has a shield, arranged outside the body 24, in the form of a semiconductive layer 27 which at one end, via a semiconductive connecting layer 28 in the first joint means 3, makes contact with the outer semiconductive layer 9 of the first conductor 2. At its other end, the layer 27 makes contact, via a second semicon- ductive connecting layer 29 in the second joint means 5, with the outer semiconductive layer 13 of the second conductor 4. The layers 7, 22, 21, 23 and 11 together form a continuous inner semiconductive layer which surrounds all the current- carrying members of the switching device 1 and the cables 2 and 4. Surrounding this continuous layer, the bodies 8, 25, 24, 26 and 12 form a continuous electrically insulating body, and surrounding this continuous body, the layers 9, 28, 27, 29 and 13 form a continuous outer semiconductive layer . When the switching device 1 is closed, the layer 21 serves as an extension of the inner layers 7 and 11 of the cables. In the same way, the body 24 functions as an extension of the cables bodies 8 and 12, and the layer 27 as an extension of the layers 9 and 13. Preferably, the layers 9, 28, 27, 29 and 13 are connected to ground, whereby a whole field-enclo- sing arrangement is obtained. When the switching device 1 is opened, a voltage difference arises between the conductor 6 and the conductor 10. The end positions of the contact member 14 must thereby be so sepa- rated that no flashover occurs through the switching chamber 15. Along the layer 21 between the contact elements 14 and 17, a voltage gradient arises when the switching device 1 is opened. To this end, the layer 21 must be dimensioned to withstand this gradient. Figure 2 shows a second embodiment of the switching device 1 according to the invention. A field-controlling device in the form of a conductive cylinder 31 substantially surrounds the switching means instead of the semiconductive layer 21 shown in Figure 1. The cylinder has no ability, when the switching device is open, to maintain a voltage gradient in its longitudinal direction, so the cylinder does not make contact with the stationary contact member 17. A gap 32, occupied by the insulating body 24, separates the contact member 17 and the cylinder 31. Figures 3 and 4 show a third embodiment of the switching device according to the invention in the form of a switch. The switch is able to connect, via a first connection ter- minal 41, a first cable (not shown) to either a second cable (not shown) via a second connection terminal 42, or to a third cable (not shown) via a third connection terminal 43. The switch has a switching means in the form of a movable contact member 44 which, via an insulated drawbar 45, is operable by an operating device 46. The switching means is able to make or break a current path between the first cable and the second cable, or between the first cable and the third cable. The switching means is substantially surrounded by a field-controlling means in the form of three conductive field-controlling bodies 47, 48 and 49. The field-controlling bodies are each in electrical contact with a cable via the connection conductors 50, 51, 52. Surrounding the field- controlling means is an electrically insulating body 24, and surrounding the body 24 is a shield in the form of a metal casing 53. The field-controlling bodies 47, 48 and 49 form a continuation of the inner semiconductive layer of the cables, the insulating body 24 forms a continuation of the cable bodies of the cables, and the metal casing 53 forms a continuation of the outer semiconductive layer of the cables .

Claims: CLAIMS 1. A switching device (1) comprising a switching means for making or breaking a current path between a first cable (2) and at least one second cable (3), which cables have an electric conductor (6, 10), an inner semiconductive layer (7, 11) surrounding the conductor, an electrically insulating solid cable body (8, 12) surrounding the inner layer and an outer semiconductive layer (9, 13) surrounding the cable body, characterized in that the switching device comprises: - a field controlling device surrounding the switching means comprising at least one conductive or semiconductive field- controlling body (21, 31, 47, 48, 49) connected to a first potential, - an electrically insulating solid body (24) surrounding the field-controlling means, - a conductive or semiconductive shield (27, 53) surrounding the insulating solid body (24) connected to a second poten- tial, and - at least two contact members (14, 17, 40) movably arranged in relation to one another, where one contact member is electrically connected to the conductor of the first cable (6) and the second contact member is electrically connected to the conductor of the second cable (10) . 2. A switching device according to claim 1, characterized in that the contact members are capable of being operated by an operating device between a closed posi- tion and an open position. 3. A switching device according to claim 1 or 2 , characterized in that the switching means comprises semiconductors which close or open the current path when con- trolling the conductivity of the semiconductors. 4. A switching device according to any of the preceding claims, characterized in that the field-controlling body (21, 31, 47, 48, 49) is electrically connected to any of the inner semiconductive layers (7, 11) of the cables (2, 4). 5. A switching device according to any of the preceding claims, characterized in that the shield (27, 53) is electrically connected to at least one of the outer semiconductive layers (9, 13) of the cables (2, 4). 6. A switching device according to any of the preceding claims, characterized in that the shield (27, 53) is electrically connected to ground. 7. A switching device according to any of the preceding claims, characterized in that a first and a second coil are arranged at the outside of the switching device and generate a magnetic field in the switching chamber which forces the contact members to a predetermined position. 8. A switching device according to any claim 1 to 5, characterized in that the movable contact member (44) is capable of being operated via an insulated drawbar (45) .

Inventors: ASPLUND GUNNAR, JONSSON LARS, LARSSON TOMMY, SVEDJEHED LARS-OLOF, ASPLUND, GUNNAR, JONSSON, LARS, LARSSON, TOMMY, SVEDJEHED, LARS-OLOF
Applicants: ABB AB, ASPLUND GUNNAR, JONSSON LARS, LARSSON TOMMY, SVEDJEHED LARS OLOF, ASPLUND, GUNNAR, JONSSON, LARS, LARSSON, TOMMY, SVEDJEHED, LARS-OLOF
Application year: 2000
Main classification: H01H33
IPC codes: H01F29, H01H9, H01H33