CURRENT LIMITER

Abstract: A current limiter comprising a contact means (1) having at least two cooperating contacts (11, 12), at least one of which (12) being movable, connecting members (15, 16) for connecting the current limiter into a circuit, and at least one pair of preferably parallel, resistive runner rails (21, 22) ar­ ranged adjacent to the contact means, said runner rails be­ ing arranged so that the arc (4) which occurs upon contact opening in case of a short-circuit, is moved away from the contact means with the foot points of the arc running along said rails (21, 22), thereby increasing the resistance in the circuit. According to the invention each runner rail (21, 22) is made of a relatively thin, insulated tape (24) of an electrical­ ly conductive material, which tape is continuously folded to form a solid resistance package. Those surfaces of the two runner rails which confront each other are at least partially uninsulated in order to form running paths for the foot points of the arc. Between the two runner rails there extend two parallel walls (32) of insulating material, which form be­ tween them a narrow gap for the arc. The magnetic field in the gap can be reinforced with the aid of a magnetic core and/or current loops.

Description: The invention relates to a current limiter according to the precharacterising part of claim 1. Proposals have been made in the past to utilize the high migration velocity of an arc to rapidly insert resistance in the form of resistive runner rails into a circuit exposed to short-circuit currents. Designs of this kind, in which the runner rails consist of straight or helically formed stiff bars, are disclosed in DE-C-26 13 378 and DE-C-27 34 395. In order to achieve an efficient current limitation upon a short-circuit in an a.c. circuit with designs operating according to the above-mentioned principle, a considerable resistance must be inserted into the circuit even during the first milliseconds of the short-circuit period. Since the migration velocity of the arc admittedly is high but limited (500-1000 m/s), this rapid resistance insertion is only possible if the runner rails have a sufficiently high resistance per unit of length. At the same time the rails must have a high energy-absorption capacity, since no essential thermal dissipation by cooling is possible because of the rapid process. The designs described in the above-mentioned patent specifications do not fulfill the demands which - in these respects - are placed on current limiters for intermediate and high voltage. Proposals have also been made to design a current limiter with two meander-shaped resistors made of insulated metallic tape and located opposite to each other, said resistors being inserted into the circuit with the aid of a current collector, being displaceable in the gap between the resistors (SE-B-192 481). With such a design it is difficult to achieve a sufficiently rapid insertion of the resistors to obtain efficient current limitation. From the GB-A-548 810 a circuit breaker is known which comprises arcing horns between which the arc is able to travel. Such an arcing horn consists of a blank conductor that is bent with alternately long and small sections into meander-shape, whereby the foot of the arc is allowed to jump intermittently from one short section to the next. Between adjacent long sections are formed open gaps. In order to achieve a solid structure for the meander-shaped conductor tape that side of the tape that faces away from the jumping arc is supported by a solid refractory insulating material. The present invention aims to provide a current limiter of the above-mentioned kind, which is designed for intermediate and high voltages and which, at the same time, fulfills the demands for a high energy absorption capacity and sufficient resistance per unit of length of the runner rails. To achieve this aim the invention suggests a current limiter according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. Further developments of the invention are characterized by the features of the additional claims. By making the runner rails of an insulated metallic tape, which is folded and packed together into a solid resistance package, several advantages are obtained. Since the tape is thin in relation to its width and since antiparallel current paths are formed, the inductance will be extremely low, which enables a fast travelling of the arc. By the choice of thickness and width of the tape as well as the length of folding, an arbitrary resistance per unit of length can be easily obtained for a given amount of energy to be absorbed. The current limiter according to the invention is primarily intended for the intermediate and high voltage ranges (i. e. voltages exceeding 1 kV), but in principle it may be employed for low voltages as well. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 a schematic side view of a first embodiment of a current limiter according to the invention, Figure 2 a section taken along the line II-II of Figure 1, Figure 3 a schematic perspective view of a runner rail included in the current limiter, Figure 4 a schematic side view of the central part of a second embodiment of a current limiter according to the invention, Figure 5 a section taken along the line V-V of Figure 4, Figure 6 a section taken along the line VI-VI of Figure 4, Figure 7 a schematic side view of an embodiment of a current limiter according to the invention with two series-connected runner rail pairs, Figure 8 in a similar view as in Figure 7 a current limiter according to the invention having six series-connected runner rail pairs. The current limiter shown in Figures 1 and 2 comprises a contact means 1 with a fixed contact 11 and a contact 12 which is rotatable about an axis. The contacts 11 and 12 are each connected to a connection member 15 and 16, respectively, for connecting the current limiter into a circuit. The connection between the movable contact 12 and the connection member 16 takes place via a flexible conductor 14. From the contact means 1 there extends a pair 2 of runner rails consisting of two elongated parallel runner rails 21 and 22. At that end of the rails which is positioned near the contact means 1, each rail is fixedly connected to one of the connection members 15 and 16. At their other end, the rails are fixedly interconnected by means of a rail 23. The runner rails 21, 22 are manufactured from an insulated tape 24 (Figure 3) of electrically conductive material, for example copper, brass or the like, which has been folded and packed into a solid package of rectangular cross-section. The thickness t and width b of the tape as well as the length of folding s are chosen in view of the system voltage and the material of the tape so, that a desired resistance per unit of length of the rails is achieved. The thickness t of the tape may, for example, lie between 0,1 and 2 mm, the width b between 15 and 100 mm and the length of folding s between 30 and 200 mm. However, these values are stated as examples only and do not at all constitute any limiting minimum or maximum values. For current limiters for intermediate voltage a resistance of, for example 4 mΩ/cm may be suitable, which can be attained by using a copper tape with a thickness of about 0,3 mm, a width of about 20 mm and a length of folding of about 50 mmm. The length of the rails may be of the order of magnitude of 1 m. The runner rails may, for example, be oriented in such a way that confronting surfaces of the two rails are formed by the curved portions of the tape, as shown in Figure 1. The insulation on these surfaces is then removed at least along the mid-portion of these surfaces so as to form running paths for the foot points of the arc. Between the two rails 21,22, two parallel walls 31,32 of insulating material extend, which form between them a gap 33 for the arc, as will be clear from Figure 2. By making the gap 33 relatively narrow, a considerable amount of energy will be transferred from the arc to the wall while at the same time the velocity of the arc is increased. As material in the walls, different inorganic insulating materials, such as mica glass or aluminum oxide, are feasible. The good thermal conductivity of aluminum oxide is advantageous in this connection. To improve the coefficient of heat transfer between the arc plasma and the walls, the inwardly-facing surfaces of the walls can be made rough or be enlarged, for example by providing them with grooves or the like. Also certain organic insulating materials, which may possibly give off gas when being heated, may come into question as materials in the walls 31,32. When a short-circuit occurs in the circuit equipped with the current limiter, the contact means 1 is immediately opened by the action of an automatically acting operating device (not shown). The arc 4 which is struck between the contacts 11 and 12, is moved under the influence of the magnetic field generated by the current, into the gap 33 where the foot points of the arc are rapidly moved along the runner rails 21,22, the resistance of the rails thereby being successively connected into the circuit. Since the tape 24 is thin in relation to the width and forms antiparallel current paths, as shown by arrows in Figure 1, the inductance of the rails 21,22 will be very low, which contributes to a rapid arc travelling. The arc 4 approaches the outer ends of the guide rails, which ends are short-circuited by the connecting rail 23 , and extinguishes. The total resistance of the runner rails will then be inserted into the circuit, thus achieving a considerable limitation of the short-circuit current and a reduction of the phase displacement between current and voltage. The short-circuit current thus limited can then easily be broken by a circuit-breaker arranged in series with the current limiter. To increase the magnetic field generated by the current in the gap 33 in order to achieve a more rapid arc travelling, a magnetic core 5 can be arranged around one of the runner rails, as shown in Figure 4 and 5. In order further to reinforce the magnetic field, the current conductor 17 between the connection members 15 and 16 is wound two turns around the magnetic core 5 in this embodiment. These turns 17a, 17b are wound in such a direction that the magnetic field generated by the current therethrough cooperates with the field generated by the current through the runner rails. This is clear from Figure 5, in which the current direction in the magnetizing turns and the runner rails have been indicated by dots and crosses in the conventional manner. It is also possible to bring about a reinforcement of the magnetic field without the use of a magnetic core by moving the current conductor in a number of turns along the insulating walls 31,32. This results in the advantage of the current limiter becoming considerably lighter in weight. In the embodiment according to Figures 4-6, in contrast to the embodiment according to Figure 1, the runner rails are oriented in such a way that the confronting surfaces of the rails are formed of one of the longitudinal edge surfaces of the folded tape. This embodiment is simpler to manufacture than the embodiment according to Figure 1, since it is easier to achieve even running surfaces if these are formed of the unbroken longitudinal edge surface 25 (Figure 3) of the folded tape. Current limiters for higher system voltages are suitably made with several series-connected runner rails. Figure 7 shows as an example an embodiment having two and Figure 8 an embodiment having six series-connected runner rail pairs 2a-2f. The runner rail pairs extend from a common contact means comprising a rotatable contact 12 and two fixed contacts 11,13 cooperating with contact 12 and being connected to the connection means 15 and 16, respectively. The three runner rail pairs on either side of the contact means in Figure 8 need not necessarily be arranged perpendicular to each other, as shown in the figure, but may also be arranged, for example, in parallel with each other, which considerably reduces the space requirement.

Claims: A current limiter comprising a contact means (1) having at least two cooperating contacts (11,12), at least one of which (12) being movable, connecting members (15,16) for connecting the current limiter into a circuit, and at least two substantially parallel, resistive runner rails (21,22) arranged adjacent to the contact means, said runner rails being arranged so that the arc (4) which occurs upon contact opening when a short circuit current flows in the circuit, is moved away from the contact means under the influence of the magnetic field generated by the current, with the foot points of the arc running along said rails (21,22), thereby increasing the resistance in the circuit, characterized in that each runner rail (21,22) is manufactured from an insulated tape (24) of electrically conductive material, which has continuously been folded and narrowly packed together into a solid resistance package, and that those surfaces of the two runner rails which confront each other are at least partially uninsulated in order to form running paths for the foot points of the arc. A current limiter according to claim 1, characterized in that between the two confronting sides of the rails (21,22) a gap (33) extending perpendicular to these two sides is formed by means of walls (31,32) of insulating material for containing the arc. A current limiter according to claim 2, characterized in that those surfaces of the insulating walls (31,32) which are facing the gap (33) are rough for improving the heat transfer from the arc to the walls. A current limiter according to any of the preceding claims, characterized in that the material in the tape (24) consists, at least for the main part, of copper. A current limiter according to any of the preceding claims, characterized in that members (5,17a,17b) for reinforcing the magnetic field in the arc gap (33) are arranged adjacent to said gap. A current limiter according to claim 5, characterized in that said magnetic field-reinforcing members comprises a magnetic core (5) surrounding one (21) of the runner rails. A current limiter according to claim 5 or 6, characterized in that said magnetic field-reinforcing members comprise one or more current loops (17a,17b) extending along the runner rails (21,22), said current loops being adapted to be traversed by the current in the circuit into which the current limiter is connected. A current limiter according to any of the preceding claims, characterized in that the confronting surfaces of the two runner rails (21,22) are formed of one of the longitudinal edge surfaces (25) of the folded tape (24) (Figure 4). A current limiter according to any of claims 1 - 7, characterized in that the confronting surfaces of the two runner rails (21,22) are formed of the curved portions of the folded tape (24) (Figure 1). A current limiter according to any of the preceding claims, characterized in that it comprises a plurality of runner rail pairs (2a-2f) extending from a common contact means (11,12,13).

Inventors: SCHREURS, EMILE, TOADER, STEFAN, VALDEMARSSON, STEFAN
Applicants: ASEA AB
Application year: 1986
Main classification: H01H9
IPC codes: H01H9