Abstract
A short-circuit current limiter has a vacuum switch in a main current path and a current-limiting unit connected in parallel therewith in a commutation current path. The vacuum switch has switching contacts. A material of contact pieces of the switching contacts is selected such that a mean value of the chopping current is at least 8 amperes.
Claims
1-5. (canceled)
6. A short-circuit current limiter, comprising: a vacuum switch with switching contacts connected in a main current path; a current-limiting unit connected in parallel with said vacuum switch in a commutation current path; said switching contacts having contact pieces formed with a material selected such that a mean value of a chopping current is at least 8 amperes.
7. The short-circuit current limiter according to claim 6, wherein the material of said contact pieces of said switching contacts is selected such that the mean value of the chopping current is at least 10 amperes.
8. The short-circuit current limiter according to claim 6, wherein the material of said contact pieces of said switching contacts is a pure metal.
9. The short-circuit current limiter according to claim 8, wherein the material of said contact pieces is a metal selected from the group consisting of copper, tungsten, chromium, nickel, molybdenum, titanium, and zirconium.
10. The short-circuit current limiter according to claim 6, wherein the material of said contact pieces of said switching contacts is selected from the group consisting of a copper-bismuth alloy and a tungsten-copper-lithium alloy in a ratio of 69.76/30/0.24 in mass percent.
11. The short-circuit current limiter according to claim 6, wherein the material of said contact pieces of said switching contacts is a copper-chromium composite with additives of 0.1 to 20 percent by weight of additional materials configured to increase the chopping current.
12. The short-circuit current limiter according to claim 11, wherein the additional materials are metals selected from the group consisting of tungsten, nickel, molybdenum, titanium, and zirconium.
Description
[0021] The above-described properties, features and advantages of this invention and the manner in which they are achieved become more clearly and distinctly comprehensible by way of the following description of the drawings. In the drawings, in a schematic illustration that is not true to scale:
[0022] FIG. 1 shows a short-circuit current limiter;
[0023] FIG. 2 shows the design of the vacuum switch illustrated in FIG. 1 with closed contact-position; and
[0024] FIG. 3 shows the vacuum switch of FIG. 2 with open contact-position.
[0025] FIG. 1 shows a short-circuit current limiter 2. This comprises a rated current path 12, which can be interrupted by an interrupter unit 4. The interrupter unit 4 is a vacuum switch. In addition, the current limiter 2 has a parallel current path 14, in which is arranged a current-limiting unit 6, which mainly comprises an overcurrent protection element 8, preferably in the form of a fuse. In the event of a short circuit, the rated current path 12 is opened by means of the vacuum switch 4, producing an arc in the process. The arc voltage brings about complete commutation of the current into the parallel current path 14 containing the overcurrent protection element 8, whereat the arc is extinguished and the overcurrent protection element 8 starts to fuse. The overcurrent protection element 8 does not start the current limiting until the rated current path 12 is definitely isolated by the interrupter unit 4. In order that the commutation from the rated current path 12 into the parallel current path 14 can take place, the impedances of the parallel current path 14 and the rated current path 12 must be matched to one another. A further challenge when selecting the overcurrent protection element 8 and the resultant impedance is to prevent the current that flows through the overcurrent protection element 8 from getting too high in rated operation, hence preventing the overcurrent protection element 8 from being destroyed by fusing already during rated operation. This gives rise to a trade-off between as high an impedance as possible in the parallel current path 14 relative to the rated current path 12 so as not to overload the overcurrent protection 8, and as low an impedance as possible relative to the rated current path 12 in the event of a short circuit occurring, in order to get the current to commute into the parallel current path 14.
[0026] FIG. 2 shows the schematic design of the vacuum switch 4 illustrated in FIG. 1, with closed contact-position. A switching contact 41, which is slidably mounted in a hermetic bearing arrangement 44, for instance a sliding bearing, and a fixed switching contact 42 lie coaxially opposite one another in a vacuum enclosure 43. The moveable switching contact 41 is electrically connected to an electrical conductor of the rated current path 12. The fixed switching contact 42 is likewise electrically connected to the electrical conductor of the rated current path 12. The switching contacts 41, 42 have schematically represented contact pieces 410 and 420, which, in the exemplary embodiment shown, have a larger diameter than the respectively adjoining connecting studs 411 and 421, which penetrate the vacuum enclosure 43 hermetically. The contact pieces 410 and 420 can also have the same or a smaller diameter than the respectively adjoining connecting studs 411 and 421. The connecting studs 411 and 421 are electrically connected outside the vacuum enclosure 43 to the rated current path 12 in such a way that the vacuum switch 4 allows a current flow through the rated current path 12 for the closed contact-position, and the vacuum switch 4 interrupts the rated current path 12 for the open contact-position.
[0027] FIG. 3 shows the vacuum switch of FIG. 2 with open contact-position. After detection of an overcurrent, the moveable switching contact 41 has been moved by a drive (not shown) from the closed contact-position shown in FIG. 2 into the open contact-position shown in FIG. 3, in which position an arc gap 45 separates the two contact pieces 410 and 420 from one another.
[0028] According to the invention, the material of the contact pieces 410, 420 of the switching contacts 41, 42 is selected such that the chopping current between the two contact pieces 410 and 420 is greater than 8 amperes, in an advantageous development greater than 10 amperes. For example, the material of the contact pieces 410 and 420 is selected as a tungsten-copper-lithium alloy having a mixing ratio W/Cu/Li 69.76/30/0.24 percent by weight.
[0029] Unlike conventional vacuum switches, in the present short-circuit current limiter, a chopping current greater than 8 or 10 amperes assists the current commutation and/or the current interruption in the main current path 12. The induced overvoltages that arise in conventional applications do not lead to damaging overvoltages in the grid in this application by virtue of the parallel commutation path 14.
