CURRENT LIMITER

Abstract: A current limiter comprising electrodes (11, 12, 13) and at least one electrically conducting polymer-based body (10) with one or more external contact surfaces (100', 100''). An electrode (13) is embedded in the polymer-based body (10) and the polymer-based body is in free contact, by means of its external contact surfaces, with one or more electrodes (11, 12).

Description: Current limiter TECHNICAL FIELD The present invention relates to a current limiter for use in an electric device or installation. The current limiter comprises an electrically conducting polymer-based body and electrodes for electrical connection of the body. The body is in free contact with at least one electrode. BACKGROUND ART A current limiter comprising a polymer-based thermistor with positive temperature coefficient, a PTC element, and two electrodes for connection of the PTC element, is known from European patent document EP 0 487 920. At least one of the contact surfaces of the polymer body is in free contact with one of the electrodes. The fact that the body is in free contact with an electrode means in this patent application that the body is not fixedly connected to the electrode by welding, soldering, fusion, chemical or physical bonding, etc . , but that the electrode and the body are retained by and electrical contact is ensured by means of a clamping force which is applied by external pressure-applying means. Hereinafter in this application this is designated in such a way that the body is in free contact with the electrode whereas a body which is secured to an electrode by welding, soldering, fusion, chemical or physical bonding, etc., is designated as being secured to the electrode. The polymer body and the electrodes are retained by resilient and pressure-applying means . This results in a contact pressure in a contact surface where the polymer body is in free contact with an electrode. Since current transition between electrode and polymer body only takes place at certain contact points, a current displacement occurs at the contact surface, which results in a voltage drop close to the contact surface. By the difference in resistivity between the polymer body and the electrode, this voltage drop occurs substantially in the body which has the highest resistivity, that is, the polymer body. At current intensities up to rated current, a low contact resistance is maintained in a contact surface by the contact pressure which is applied by the pressure-applying devices, which ensures a current transition at a sufficient number of contact points. At short-circuit currents, the temperature at the contact surface is raised. This increase in temperature results in a local melting and/or gasification of the polymer material at certain of the contact points, whereby the resistance is further increased. Finally, a layer of gasified polymer/carbon arises near a contacting contact surface which is in free contact, whereby a strong increase of the resistance across the layer is obtained, that is, the current limiter trips. Since the contact pressure during a trip is maintained by the pressure- applying devices, the original contact and the original contact resistance are essentially restored across the mentioned contact surface in connection with the gas pressure decreasing. The international patent document WO 96/25783 describes a current limiter of the above-mentioned kind, in which one of the contact surfaces of the polymer body makes contact with an electrode, that is, the voltage drop close to and above the contact surface between the electrode and the polymer body has been minimized. According to this document, the polymer body is contacted in that it is arranged with a low resistance surface layer close to ' the electrode. This low resistance surface layer has an increased electrical conductivity relative to the bulk of the polymer body. A contacted contact surface may be in free contact with or be secured to an electrode. A surface which is connected to or is in free contact with an electrode and for which the voltage drop close to and above the contact surface between the electrode and the polymer body has been minimized is referred to hereinafter in this application as being contacted. The object of the invention is to suggest a current limiter which has a simple electrical and mechanical construction with few parts, comprising an electrically conducting polymer body and electrodes for connection to an electric circuit. For its rated current, the current limiter has small dimensions and a reliable electrical insulation. Especially the risk of flashover and external creeping currents between the electrodes has been essentially eliminated. This has been achieved with a current limiter which still exhibits - low resistance at current intensities below rated current and - increased withstand strength against the overvoltages which normally occur in electrical installations, that is, currents with a current intensity which is normally of the order of magnitude of 10 times the rated current or less, as well as - the ability to rapidly and reliably limit short-circuit currents and other large fault currents. Further, with a current limiter according to the invention, it is possible to predict where tripping occurs. In this way, constructive measures may be suggested, which further reduce the resistance across the electrically conducting body, increase the withstand strength against brief and limited overcurrents and improve the ability to rapidly and reliably limit the short- circuit currents. SUMMARY OF THE INVENTION For a current limiter comprising electrodes and at least one electrically conducting polymer-based body, this is achieved by at least one electrode being embedded in the polymer-based electrically conducting body and by one or more of the contact surfaces of the body being in free contact with at least one external electrode. The embedded electrode is preferably secured to the polymer body and fixed to the polymer body while at the same time pressure-applying resilient devices retain the parts included in the current limiter. In one embodiment, several of the external contact surfaces of the electrically conducting polymer body are adapted to be in free contact with a plurality of external electrodes. The external electrodes are electrically connected to one another and electrically separated from the embedded electrode by the electrically conducting polymer-based body. In certain applications, however, an alternative embodiment, in which only one outer contact surface of the electrically conducting polymer body is in free contact with an electrode, has proved to be advantageous . Other contact surfaces present on the body have been covered with an electrically insulating body or an electrically insulating coating. The retaining and pressure-exerting members retain the parts included in the current limiter while at the same time achieving a contact pressure, acting in the contacting contact surfaces being in free contact, which is essentially directed in the normal direction of these surfaces. In a preferred embodiment, the electrically conducting polymer body is contacted close to the embedded electrode, that is, the voltage drop across and close to the contact surface has been minimized for the embedded electrode. To ensure good contacting, the polymer body has been arranged with low resistance layers or zones adjacent to this contact surface. The shape of the electrode may also been chosen on the basis of the criterion to ensure a large effective contact surface between the embedded electrode and the polymer body. By effective contact surface is meant in this application the actual surface in which current transition between electrode and polymer body takes place. Commonly occurring shapes of the embedded electrode are a plate, a net or a web, a plurality of electrically and mechanically inter- connected needles or pins which are preferably contacted by means of low resistance layers or zones close to the contact surface. An enlargement of the contact surface for reduced current displacement and improved contacting may also be obtained with open structures or a large area/volume ratio of the electrode, such as wire gauge electrodes, pins and the surface structure of the electrode. These shapes which are based on an enlarged contact surface presuppose that the polymer-based material penetrates into the open structure of the electrode and is bonded to the electrode with good adhesion. In addition, measures must be taken to avoid gas or air pockets as well as precipitations in or adjacent the contact surface since the occurrence of such inhomogeneities disturb the current transition or creates current displace- ment. A flat current limiter with an embedded electrode according to the invention comprises, according to one embodiment, a flat metal electrode of copper, nickel or an alloy based on any of these metals. The electrode is preferably wholly or partially coated with a surface layer of an alloy, which has very good conductivity, such as a layer of silver or a silver-based alloy. The electrode has been embedded into a polymer body by placing two polymer-based electrically con- ducting plates, which are wider and longer than the electrode, on either side of the flat electrodes. Those parts of the major surfaces of the polymer plates which are in contact with the electrode are arranged with a surface layer or surface zones with reduced resistivity, for example a layer of the silver alloy mentioned above, or a polymer-based coating containing silver. The polymer plates and the electrode have then been compressed under heating. During the compression, the edges of the polymer plates have been welded together such that the electrode has been embedded into and essentially surrounded by the flat polymer body formed, while at the same time the polymer body has been caused to make contact with the electrode. The embedded electrode may be connected to an insulated conductor or by the fact that part of the embedded electrode has been left open and uninsulated for connection. Electrodes have then been adapted to be in free contact with the major surfaces of the flat polymer body obtained. Alternatively, a flat current limiter with an embedded metal electrode may be manufactured by extruding an electrically conducting polymer composition around a metal band. The strip obtained comprises a metallic strip surrounded by two polymer layers and has then been cut in suitable lengths into flat polymer bodies. Metal in the cutting surfaces, exposed during the cutting, has then been coated such that the electrodes have been surrounded essentially completely. The embedded electrode may be connected to an insulated conductor or by leaving part of the embedded electrode open and uninsulated for connection. Finally, the major surfaces of the body have been arranged to be in free contact with external electrodes. In certain embodiments, only one of the major surfaces is in free contact with an electrode whereas the other major surface is electrically insulated. Preferably, as described in the foregoing, the embedded electrode is metallic but in alternative embodiments it is polymer-based with a reduced resistivity relative to the PTC element. Preferred metallic electrodes are silver, copper, nickel or alloys based on any of these metals. Copper- or nickel-based electrodes are often coated with a low-resistance chemically resistant silver alloy. A polymer-based electrode comprises a conducting polymer or an electrically conducting polymer composition, an electrically conducting plastic, based on a polymer such as a thermoplastic resin, an elastomer or a thermosetting resin to which an electrically conducting powdered material has been added. As powder additive there is preferably chosen a metallic powdered material based on a metal with good electrical conductivity such as silver, gold, copper, nickel or aluminium. Other feasible powdered materials are soot, carbon black and other graphite-based materials, ceramic powders based on, for example, borides such as titanium diboride, zirconium dibo- ride, carbides such as tantalum carbide, silicon carbide, tungsten carbide, zirconium carbide, nitrides such as zir- conium nitride, titanium nitride, oxides such as vanadium trioxide, titanium oxide, or mixtures of two or more of these powders. Decisive for the choice of powdered material are its conductivity and its ability, under the conditions prevailing in the electrode layer when using and manufacturing the current-limiting device, to withstand oxidation or other reactions. The polymer-based electrode has a reduced resistivity in relation to the polymer-based body. In an additional alternative embodiment of the invention, the polymer-based electrically conducting body and the polymer- based electrode have been jointly extruded. Like the polymer- based electrode mentioned in the preceding paragraph, the jointly extruded polymer-based electrode is preferably chemically bonded to the polymer-based electrically conducting body and has a reduced resistivity in relation to the electrically conducting polymer-based body. During the extrusion, the two electrically conducting polymer compositions have been jointly extruded into a layered strip, whereby the polymer composition which has the lowest resistivity is surrounded by the polymer composition which has a higher resistivity. After the extrusion, the extruded strip has been cut into suitable lengths. The electrode exposed in the cutting surfaces during the cutting has then been coated such that the electrodes have been surrounded essentially completely. The embedded electrode may be connected to an insulated conductor or by leaving part of the embedded electrode open and uninsulated for connection. Finally, the major surfaces of the body have been adapted to be in free contact with external electrodes. In certain embodiments, only one of the major surfaces is in free contact with an electrode whereas the other major surface is electrically insulated. In an alternative embodiment, an electrically conducting polymer body with a polymer-based electrode has been formed by simultaneous injection moulding of two electrically conducting polymer-based materials . The material which has the lowest resistivity has been arranged as an embedded electrode surrounded by the other polymer material which has a higher resistivity. The obtained polymer body with the embedded electrode is then adapted to be in free contact with at least one external electrode. A current limiter according to the embodiments of the invention described in the foregoing is simple, compact and has a reliable electrical insulation while at the same time it still exhibits a high withstand strength in relation to overcurrents which are of the order of magnitude of 10 times the nominal current or less and a rapid and very reliable current limitation in case of short-circuit currents . In addition, a device according to the invention provides a possibility of predicting where tripping occurs. In this way, constructive measures may be taken to further reduce the resistance across the electrically conducting body, increase the withstand strength against brief and limited overcurrents and improve the ability to rapidly and reliably limit short- circuit currents. Examples of such measures are to arrange the external electrodes with means for absorbing heat which is generated in the polymer-based electrically conducting body close to these and/or means for emitting the heat which is absorbed by the electrodes. For example, the electrodes are arranged as heat-absorbing and/or heat-emitting bodies such as cooling bodies of aluminium with or without cooling flanges. Alternatively, the electrodes are arranged in thermal contact with such cooling bodies. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in greater detail in the following and be exemplified by means of a preferred embodiment with reference to the accompanying Figures la, lb and lc, wherein Figure la shows a view of the current limiter, Figure lb a section A-A through the current limiter and Figure lc a section B-B through the same current limiter. DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiment of the invention shown in Figures la, lb and lc comprises a polymer-based electrically conducting body in the form of a plate 10 with an electrode 13 embedded in the body and external electrodes 11, 12 with which the two major surfaces 100', 100" of the plate are in free contact. The figures show and describe an embodiment with two electrodes, but of course one of the electrodes may be replaced by an electrically conducting disc or an electrically insulating coating, or, alternatively, additional electrodes be arranged. In the embodiment shown the electrodes 11, 12 are flat bodies . By a flat body or a plate is meant in this patent application a body, the thickness of which is essentially smaller than its length and width. The plates included in the current limiter are retained in the embodiment according to the figures by one or more elastically resilient elements 14, 15 which engage the flat body 10 of the current limiter and the electrodes 11, 12, 13 and retain them by applying a resilient clamping force to act in the major surfaces 100', 100" essentially perpendicularly to the plane of the flat bodies. The polymer-based plate 10 is preferably an electrically conducting resin based on a polymer such as a thermoplastic resin, an elastomer or a thermosetting resin, to which an electrically conducting powdered material has been added. As powder additive there is preferably chosen carbon, carbon black or another carbon-based electrically conducting powder. In alternative embodiments, a metallic powdered material based on a metal with good electrical conductivity, such as silver, gold, copper, nickel or aluminium, has been used as well as ceramic powders based on, for example, borides such as titanium diboride, zirconium diboride, carbides such as tantalum carbide, silicon carbide, tungsten carbide, zirconium carbide, nitrides such as zirconium nitride, titanium nitride, oxides such as vanadium trioxide, titanium oxide, or mixtures of two or more of these powders. Decisive for the choice of powdered material are its conductivity and its ability, under the conditions prevailing in the current limiter when using and manufacturing it, to withstand oxidation or other reactions. The embedded electrode 13 is shown in the figures as a plate, which is centrally arranged in the bulk of the polymer-based electrically conducting plate 10 and oriented in a plane which is essentially parallel to the external electrodes 11, 12 and the major surfaces 100', 100" of the polymer plate 10. The embedded electrode 13 is preferably a metallic plate, for example a silver, copper or nickel plate. To improve the contacting, in many embodiments the polymer-based body is arranged with low-resistance and chemically resistant layers or zones close to the embedded electrode. Although the embedded electrode 13 shown in the figures is a plate, the shape may be arbitrary with an expressed ambition to raise the effective contact surface between the embedded electrode 13 and the polymer body 10 through the shape of the elec- trode . Electrode shapes by which a large contact surface is achieved are nettings, webs, interconnected needles or pins, perforated plates, plates with a modular surface structure. It is also important that measures be taken such that no air or gas pockets remain at the boundary between embedded electrode and polymer bulk, 13/10. Likewise, precipitations in or close to this boundary must be avoided. Gas pockets, gas-filled pores as well as precipitations reduce the effective contact surface and hence raise the contact resistance. As an alternative to metallic electrodes, plates, webs, etc., of graphite or other electrically conducting unorganic materials may be used as well as electrically conducting polymer-based materials with a resistivity lower than the polymer-based body 10. Polymer-based electrodes may comprise electrically conducting polymers with an intrinsic electrical conductivity or electrically conducting resins which have received their electrical conductivity through electrically conducting particular fillers .

Claims: CLAIMS 1. A current limiter comprising electrodes (11, 12, 13) and at least one electrically conducting polymer-based body (10) with external contact surfaces (100', 100"), characterized in that an electrode (13) is embedded in the polymer-based body (10) and that the polymer-based body is adapted to be in free contact, by means of its external contact surfaces (100', 100") , with one or more electrodes (11, 12) . 2. A current limiter according to claim 1, characterized in that the polymer-based body is in free contact with one external electrode (11, 12) only. 3. A current limiter according to claim 1, characterized in that the polymer-based body is in free contact with a plurality of external electrodes (11, 12). 4. A current limiter according to claim 3 , characterized in that the external electrodes (11, 12) are electrically connected to each other and electrically separated from the embedded electrode (13) by the electrically conducting polymer-based body (10) . 5. A current limiter according to any of claim 1, 2, 3 or 4 characterized by pressure-applying resilient devices (14, 15) adapted to retain the polymer body (10) and the electrodes (11, 12, 13) . 6. A current limiter according to any of claims 1 to 5 , characterized in that the embedded electrode (13) is secured to and fixed in the polymer body (10) . 7. A current limiter according to any of claims 1 to 6 , characterized in that the polymer body (10) is contacted with the embedded electrode (13) . 8. A flat current limiter according to any of the preceding claims, characterized in that an electrode (13) is embedded in a polymer-based plate (10) and that the polymer-based plate (10) is adapted to be in free contact, by means of its two major surfaces (100', 100"), with at least one flat electrode (11, 12). 9. A flat current limiter according to any of the preceding claims, characterized in that polymer-based plate (10) is adapted to be in free contact, by means of its two major surfaces (100', 100"), with a flat electrode (11, 12) whereas the other major surface is coated with an electrically insulating coating or an electrically insulating disc. 10. A flat current limiter according to any of claims 1-8, characterized in that polymer-based plate (10) is adapted to be in free contact, by means of its two major surfaces (100', 100"), with a plurality of flat electrodes (11, 12). 11. A flat current limiter according to any of the preceding claims, characterized in that the embedded electrode (13) is of metal. 12. A flat current limiter according to claim 11, characterized in that it comprises - an electrically conducting polymer-based and flat body (10), - a flat metal electrode (13) embedded in the flat polymer body (10) which is wider and longer than the electrode in order to surround the electrode, - low-resistance layers arranged on the polymer body close to the embedded electrode, - external electrodes with which the major surfaces (100', 100") of the polymer body are in free contact. 13. A current limiter according to claim 12, characterized in that the electrode (13) is embedded in the polymer (10) by the polymer being formed around a metal strip by extrusion, that the obtained strip comprising a metallic strip embedded in the polymer is cut in suitable lengths into flat polymer bodies, that the electrode exposed during the cutting is essentially surrounded by polymer, whereafter the major surfaces (100', 100") of the flat polymer bodies obtained are adapted to be in free contact with electrodes (11, 12). 14. A current limiter according to any of claims 1 to 10, characterized in that the embedded electrode (13) is polymer- based. 15. A current limiter according to claim 14, characterized in that the electrically conducting polymer body (10) comprises two electrically conducting polymer compositions of different resistivity, whereby the polymer composition which has lowest resistivity is placed in the centre as an embedded electrode (10) surrounded by the polymer composition which has highest resistivity and that the major surfaces (100', 100") of the flat layered polymer body are adapted to be in free contact with electrodes (11, 12). 16. A current limiter according to claim 15, characterized in that an electrically conducting layered body comprising two polymer compositions of different resistivity is manufactured by injection moulding into a polymer body, whereby the polymer composition which has lowest resistivity is adapted to constitute an embedded electrode essentially surrounded by the second polymer composition and that the external contact surfaces of the body are in free contact with at least one electrode . 17. A current limiter according to claim 15, characterized in that an electrically conducting layered body comprising two polymer compositions of different resistivity is manufactured by joint extrusion into a layered polymer strip, whereby the polymer composition which has lowest resistivity is adapted to constitute an embedded electrode surrounded by the second polymer composition, that the extruded layered strip is cut in suitable lengths into flat polymer bodies, that the electrode which is exposed during the cutting is essentially surrounded by polymer, whereby the major surfaces (100', 100") of the flat polymer bodies obtained are adapted to be in free contact with electrodes (11, 12). 18. A current limiter according to any of claims 1 to 10, characterized in that the embedded electrode (13) is of graphite .

Inventors: BAANGHAMMAR LARS, BIJLENGA MARIA, JEPPSSON OLA, BAANGHAMMAR, LARS, BIJLENGA, MARIA, JEPPSSON, OLA
Applicants: ABB RESEARCH LTD, BAANGHAMMAR LARS, BIJLENGA MARIA, JEPPSSON OLA, ABB RESEARCH LTD., BAANGHAMMAR, LARS, BIJLENGA, MARIA, JEPPSSON, OLA
Application year: 1998
Main classification: H01C7
IPC codes: H01C7