Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
2001-01-24
2002-08-06
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C338S0220SD
Reexamination Certificate
active
06429766
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrical device with a PTC polymer element. Namely the present invention relates to such PTC polymer elements in which a constriction in terms of a reduction of the cross-sectional area perpendicular to a main current direction is formed, wherein that constriction defines a web. The term web relates to a portion of the PTC polymer element extending in that main current direction over a length wherein a minimum cross-sectional area of that constriction is included in that web and that web essentially maintains said minimum cross-sectional area over that length of extension.
2. Prior Art
EP 0 655 760 A2 shows that electrical devices comprising PTC polymer elements can effectively be used for overcurrent limitation purposes. In this document the PTC polymer element is connected in series with a load interrupter. It is intended that the PTC polymer element limits overcurrents to current values that can easily be interrupted by the load interrupter.
The high-voltage capabilities of the PTC polymer elements can be improved according to the teaching of U.S. Pat. Nos. 5,313,184 and 5,414,403. These documents propose to combine PTC polymer elements and varistor elements or linear resistor elements in order to avoid too high local electrical fields within the PTC polymer material during switching. A nonlinear response behaviour of the PTC polymer material can be distributed in this way. It is to be noted that the term PTC polymer element as used within this description and in the claims also covers such PTC polymer elements which besides the mere PTC polymer material comprise filler materials or elements with linear resistance characteristics or resistance characteristics being nonlinear with temperature, electrical field strength (varistors), pressure etc.
What is also known in the prior art is to provide PTC polymer elements having constrictions in the current carrying cross-sectional area, i.e. in the cross-sectional area perpendicular to the main current direction. Such constrictions can be found e.g. in EP 0 038 715 B1. This document intends to achieve a very rapid tripping of the PTC polymer material within the constriction by this specific design. What is also shown in this document, namely in
FIG. 5C
, is a constriction defining a web, that web extending in the direction between two electrodes, consequently in the main current direction, over a certain length.
SUMMARY OF THE INVENTION
Based on the above cited prior art the technical. problem underlying the present invention is to provide a novel electrical device with a PTC polymer element with improved electrical performance.
This problem is solved by an electrical device according to claim
1
. In this electrical device, a web of a constriction in the PTC polymer element extends over a length of at least 5 mm. Even more preferred values are lengths of at least 7, 10, 15 or even 20 mm.
With these values for the length of the web, the switching capability in terms of voltage of the PTC polymer element, and thus of the complete electrical device, can strongly be improved compared to the prior art. E. g. the above named EP 0 038 715 does not mention such long webs at all. Now, the inventors have found that the voltage resistivity of a single trip zone within the PTC polymer material can much be improved by using a long web, that web, by carrying the highest current density within the PTC polymer element, being predetermined to include the trip zone. By avoiding a movement of the borders of the trip zone out of the web, a favourable combination of accelerated trip dynamics on the one hand and high voltage-withstand capability on the other hand can be secured. Thus, for a fast electrical device with voltage switching capabilities in a voltage range of e.g. 690 Vrms, the electrical device according to the invention can preferably be used without any series connection of trip zones.
Thus, the invention is used for protecting an electrical circuit of overcurrent and short-circuit current, preferably at a system voltage of 690 Vrms and more. Therein, preferably, the whole range of fault scenario from only small overcurrents to prospective short-circuit currents of e.g. up to 50-100 kA should be safely limited and, in a preferred combination with a load switch, switched off.
An important advantage of the invention in its combination of fast and voltage stable current limitation lies in the fact, that fast tripping reduces the let-through energy which can, if too large, damage the PTC polymer element. Thus, a fast enough current limitation also in cases of high voltages across the electrical device means that its action can be repetitive, e.g. at least five times for heavy short-circuit limitation. Therefore, the typical time to suppress a short-circuit current to zero should be much smaller than a quarter period of the respective system, e.g. much smaller than 5 ms.
In view of a sufficient current-carrying capacity, the effect of cooling of the web by the rest of the PTC polymer volume can be important. Relatively high current loads necessitate such cooling in order to avoid a tripping at high but permissible current values. This aspect of the invention has to be considered together with the fact that a very high voltage capacity can best be reached by very long webs. For the system voltage range between e.g. 500 Vrms and 12 kVrms, it is therefore preferred, to use webs not longer than 150 mm. Depending on whether the voltage-withstand capability or the current-carrying capability is more important, also webs not longer than 80, 40 or even not longer than 30 mm can be preferred.
However, this does not generally exclude to use very long webs in the area of for example 120 mm (for e.g. 12 kVrms) or even 360 mm (e.g. for 36 kVrms), according to the invention.
Further, in order to improve the thermal conduction to webs of considerable length, holes adjacent to such webs can be filled by an electrically insulating material instead of air.
A second aspect important for the above named cooling effect of the PTC polymer volume for the constriction and the web is the aperture angle of the constriction, as seen from the web. This aperture angle is defined in one longitudinal sectional plane containing the main current direction. At least in one of these planes, that aperture angle should preferably be at least 1000 in total. As can be taken from the embodiments, this total angle is to be regarded as a sum of a right-hand aperture angle and a left-hand aperture angle having their respective apex points separated from each other. Essentially, these apex points are located at the right and the left side, respectively, of the web as seen in the main current direction. It is not necessary that these two partial aperture angles are identical, but it is preferred. Further, the line segments on both side of the constriction being angled by the aperture angles with respect to the main current direction do not necessarily have to be regularly shaped, as appears from the definition of an angle. It is sufficient, if a line segment can be defined as a mean value in order to define an aperture angle. However, essentially straight inclined flanks on both sides of the web are preferred.
Even more preferred lower limits for the total aperture angle are 105°, 110°, 115° and 120°. As already stated above, these minimum aperture angles relate to at least one longitudinal sectional plane through the constriction. All the longitudinal sectional planes through the constriction can show different aperture angles, but preferably they are the same. However, also a two-dimensional constriction, wherein only one longitudinal sectional plane shows the above defined aperture angle, and a second longitudinal sectional plane perpendicular to the first one shows no constriction at all, is possible. Usually, this form is easier to manufacture.
In any case, the combination of high current-carrying capacity and fast tripping dynamics can be achieved. Especially, large aperture angles mean s
Glatz-Reichenbach Joachim
Strümpler Ralf
ABB Research Ltd.
Burns Doane Swecker & Mathis L.L.P.
Easthom Karl D.
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