EP0717876A1

EP0717876A1 - Limiter for current limiting

Info

Publication number: EP0717876A1
Application number: EP94924694A
Authority: EP
Inventors: Fritz Pohl; Wilfried Jaehner
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1993-09-09
Filing date: 1994-08-26
Publication date: 1996-06-26
Anticipated expiration: 2014-08-26
Also published as: US5793278A; DE4330607A1; DE59405539D1; WO1995007540A1; EP0717876B1

Abstract

Limiters for current limiting are distinguished by a controllable high-current resistor designed to assume a low-resistance state in rated service and a high-resistance during a short-circuit disconnect. Prior art limiters can have, for example, a thermoplastic resistor and metal flat electrodes. The invention provides that the metal flat electrodes (10) have a surface profile (15) on their side (11) facing the thermoplastic resistor (1), which has a complementary surface profile (5). The flat electrodes (10) and the resistor (1) are thereby inseparably connected, the surface electrodes (10) and the resistor (1) being pressed together by a pressure force (K). The interlocking of the surfaces (11, 2, 3) results in an especially rapid readjustment of the limiter resistor from its high-resistance state during a short-circuit disconnect back into the low-resistance state in rated service.

Description

description

^• fyi limiter for current limitation

5

The invention relates to a limiter for current limitation, with a thermoplastic resistance body and metallic surface electrodes.

10 low-voltage circuit breakers are often connected in series with so-called limiters in order to greatly increase the short-circuit switching capacity in low-voltage networks and to greatly limit the forward currents. Such limiters are designed so that they quickly in the event of a short circuit

15 transition from a low-resistance to a high-resistance state and contribute to rapid current limitation and switch-off with their voltage requirement.

Specifically for use as a central limiter, which is upstream of these for the short-circuit protection of several branches, it would be desirable for the limiter function to be effective only at high short-circuit currents which, for example, cannot be protected by circuit breakers in the branches, and at moderate or decaying short-circuit currents no limiter function occurs. This would avoid undesired, longer-lasting voltage dips that lead to uncontrolled switching states, e.g. of contactors or relays.

30 Electromechanically simple switching devices are often used as limiters, the contacts of which open dynamically due to current forces and which usually do not have a * switch lock and no release system. Your bow Voltage plateau lies in the area of the mains voltage amplitude. Together with the arc voltage of the circuit breaker connected in series, the short-circuit current is rapidly decayed and the switch-off time is shortened. Arc limiters of this type have problems of contact welding, which can only be solved technically with special contact materials and / or with a special contact mechanism.

Limiters are known from WO-A-91/12643 and EP-A-0 487 920, which specifically utilize the so-called PTC effect or PTC effect (jDOsitive jtemperature c_oefficient). High current resistors are used which essentially consist of a polyethylene layer filled with soot, which has the PTC effect. To ensure the PTC effect in this high-current resistor, which can be used as a protective element, the polymer resistor body should be connected to its base surfaces with electrodes, a pressure device being present which applies pressure perpendicular to the electrodes and the electrodes

Base areas of the resistance body of the conductive polymer layer exerts.

The physical basis of the limiters described above is that the temperature in the event of a short circuit rises above the crystallization temperature of polyethylene as the electrically insulating base material due to ohmic heating, whereby microscopic current paths of the carbon black as an electrically conductive material component break open and the limiter resistance increases by a factor of 100 or more. The

Limiter resistance can mainly be determined here by the volume resistance of the PTC material. According to its switching principle, in the known limiter the surface resistance at the interfaces between the polymer resistance body and the electrodes contributes significantly to the current limiting effect in the event of a short circuit. Since the electrodes and the polymer resistance body are only in contact with one another due to the external compressive force, the heating of the PTC material takes place in a thin layer near the surface, which in the event of a short circuit very quickly changes from a low-resistance to a high-resistance state switches.

After the short circuit has been switched off, this boundary layer limiter requires a reset time of typically about 20 ms for the high electrical resistance to subside. In practice, a parallel resistor is connected to relieve the PTC thermistor element, which carries the major part of the short-circuit current in the high-resistance state of the PTC thermistor material. During the disconnection period of the circuit breaker connected in series, the decaying short-circuit current at the parallel resistor generates a considerable voltage drop. This can be 100 V or more and exist for a period of 3 ms and longer. However, such a long-lasting switching voltage is unsuitable for use as a central limiter, since otherwise long-lasting voltage drops would be caused in branches of the network which are not disturbed per se.

The object of the invention is therefore to provide a limiter which can also be used as a central limiter. For the function of the central limiter, it must be demanded that voltage caused by its switching voltage dips remain limited to a fraction of a half-wave duration, for example to a time period t <1 ms.

The object is achieved by the entirety of the features specified in the claim. If the circuit breakers connected in series in the current branches to the central limiter have a sufficiently high switching speed at high short-circuit currents, it can be achieved according to the invention that after the limit voltage has subsided the switch-off process is continued without delay with a sufficiently high arc voltage. The function of a limiter voltage activated only at high instantaneous currents is thus achieved by a boundary layer limiter, in which a partial non-positive contact of the electrodes on the resistance body takes place even in the event of a short circuit. As a characteristic property of such a limiter, the electrodes and the resistance body have a positive or negative, i.e. complementary surface profile with which they adhere mechanically to one another.

To implement the limiter according to the invention, the thermoplastic resistance body, for example in the form of a rectangular plate, is pressed together between profiled electrodes and is heated to its softening temperature at least on the contact surfaces. The resistance material flows into the profile recesses of the metal electrodes and a complementary surface profile of the resistance body is created. After this process, the electrodes adhere firmly to the resistance body, from which they can only be separated again with mechanical damage to the profile layers. As in the prior art, the adhesive force on the profiled surfaces between the electrodes and the resistance body is not sufficient to achieve a low limiter resistance. In order to achieve the low-resistance nominal resistance of the limiter defined for nominal operation, the surface electrodes are typically pressed against the resistance body with a pressure force of between 50 and 100 N / cm ² .

Further details and advantages of the invention result from the following description of the figures of exemplary embodiments with reference to the drawing. They show a schematic representation

FIG. 1 a limiter in a sectional view,

FIG. 2 the top view of a limiter according to FIG. 1, FIG. 3 the formation of a surface electrode, FIG. 4 the comparison of the resistance curves of the limiter according to FIGS. 1 to 3 with the prior art, the

FIG. 5 in partial figures the example of a short-circuit disconnection of a series connection by the limiter according to FIG. 1 to 3 and the FIG. 6 in partial figures the switching phases of a limiter according to FIG. 1 to 3.

Some of the figures are described below together.

In the FIG. 1 to 3 1 means a thermoplastic resistance body with surfaces 2 and 3, which is pressed together between two similar surface electrodes 10. For this purpose, a pressure force K is applied. One of the- Like arrangement is known in principle from the older German patent application P 42 28 297.7.

According to FIG. 3, both surface electrodes 10 have a profile 15 which, for example, has a rectangular structure with a web width b and a web height h. The web width b can be between 0.1 and 1 mm and the web height can also be between 0.1 and 1 mm. In particular, web width b and web height h have the same order of magnitude, preferably between 0.3 and 0.6 mm. Specifically in FIG. 3, both dimensions are, for example, about 0.4 mm.

The resistance body 1 has a complementary profiling 5 on both surfaces 2 and 3. The resistance body 1 and the surface electrodes are permanently connected to one another via the profiles 5 and 15.

In a different training than FIG. 3, the rectangular profile can also have an angle of inclination towards the surface of the surface electrode 10. The surface profile 15 can advantageously have a different direction in sections. A conical shape of the surface profile 15 is also possible.

The limiter described in this way is connected upstream of a circuit breaker in a known manner. A resistor element is connected in parallel to the limiter from the resistance body 1 and the surface electrodes 10. The resistance element is, for example, an ohmic resistance of 100 m. It can also be a non-linear voltage-dependent resistor, the resistance of which decreases with the applied voltage. In both cases, the current can commutate at the appropriate time. In FIG. 4b is the limiter resistance to a short-circuit shutdown at I. = 40 kA with a series connection of the limiter according to FIG. 1 to 3, to which a constant resistance of 100 is connected in parallel, and a power switch are shown and the time profile is shown: The limiter resistor according to graph 42 begins to rise and reaches the short-circuit current from its initial value R _Q = 4 after about 300 μs a first plateau value of about 8 mfl. While the short-circuit current continues to rise and a value of 5 kA is reached 500 μs after the start of the short-circuit, the resistance curve at this point in time rises steeply and remains at resistance values which are substantially greater than 100 for about 300 μs mü .. About 900 μs after the start of the short circuit, the limiter resistance returns to a low-resistance value of about 15 mfl and finally drops to its initial value.

FIG. 4a shows the time course of the limiter resistance according to the prior art as a graph 41 for comparison.

In the current-voltage oscillogram according to FIG. 5 shows graph 51 the total current, graph 42 the PTC thermistor current, graph 53 the associated PTC thermistor voltage and graph 54 the voltage on the switching device used. The resistance behavior described 4 manifests itself in such a way that the limiter generates a voltage pulse of approx. 450 V and approx. 300 μs duration approximately 600 μs after the beginning of the short circuit. In this phase, the short-circuit current drops from its maximum value i = 6.7 kA to about 3 kA, max, with an increasing current component being borne by the parallel resistor (100 m_Ω). After his chip has subsided voltage pulse, the limiter carries the reduced short-circuit current, which is caused by the partial electrical-mechanical contact of the surface electrodes on the resistance body. The sufficiently high arc voltage of the circuit breaker at this point prevents the current from rising again and the short circuit is switched off after a total duration of 3 ms.

The switching properties of the on the basis of FIG. 1 to 3 shown limiters and the associated measurement curves according to FIG. 4 and 5 can be phenomenologically explained what is shown in FIG. 6 illustrates: In the initial state of the limiter according to FIG. 6a, the pressure force K, which is perpendicular to the surface electrodes, ensures a frictional contact of the partial profile surfaces which are perpendicular to the pressure force. In contrast, the force effect between the partial profile surfaces 12 running parallel to the compressive force is considerably less, since after the thermal production process the profile layer of the thermoplastic resistance material experiences a dimensional shrinkage of 1 to 2% due to the much higher coefficient of thermal expansion than metals.

In the case of the on the basis of FIG. 4 and 5, the short-circuit shutdown shown begins with the beginning of the current increase due to the electrical power loss in the boundary layer, increasing warming and, associated therewith, an expansion of the profile layer 5 of the resistance body 1 with a considerable reduction in the electrical contact area according to FIG. 6b. Due to the thermal expansion of the thermoplastic profile webs, the metal electrodes 10 are removed from the recessed profile partial surfaces of the Resistor body 1 lifted off. A current limiter resistance is established, which is determined by the 4b shown first plateau value. At the same time, the current density on the profile end faces of the resistance body 1 almost doubles and the electrical power loss leads to rapid heating to the decomposition temperature of the resistance material. This is shown in FIG. 6c, the mechanical-electrical contact is also largely interrupted on these partial profile surfaces, and a distributed electrical discharge with a high operating voltage is formed between the profile surfaces.

During the phase of high limiter voltage, the profile layer 5 is further heated and material is partially decomposed, as a result of which a considerable gas pressure is built up. With the reduction of the gas pressure due to the decaying short-circuit current and due to the thermal expansion of the thermoplastic profile webs, according to FIG. 6d between the profile partial surfaces 12 of the electrodes 10 and the resistance body 1 running parallel to the pressure force, the effective contact force of which increases with the temperature of the resistance material and its thermal expansion and therefore at the end of the limiter voltage pulse to the observed low limit resistance of about 15 mil leads. This process is supported by the fact that the parallel resistance of the limiter temporarily takes over the total current and thereby ends the material decomposition on the resistance body 1.

During the cooling time of the thermoplastic resistance body 1 of up to a few 100 ms, its surface profile is formed under the force of the Pressed K profiled metal electrodes 10 and the limiter resistance returns to its initial value. -

It is therefore essential in the limiter described compared to the arrangements known from the prior art that there are no free contact surfaces, but that the surface electrodes and the resistance body are not releasably connected to one another via their complementary surface profiles.

Claims

claims

1. Limiter for current limitation, with a thermoplastic resistance body and metallic surface electrodes with the following features:

- The metallic surface electrodes (10) have a surface profile (15) on their side (11) facing the thermoplastic resistance body (1),

the thermoplastic resistance body (1) has a complementary surface profile (5) on its sides (2, 3) facing the metallic surface electrodes (10),

- The surface electrodes (10) and the resistance body (1) are not releasably connected to each other, - The surface electrodes (10) and the resistance body (1) are pressed together by a compressive force (K).

2. Limiter according to claim 1, so that the surface profile (15) of the resistance body (1) takes place by thermal molding on the metallic surface electrodes (10).

3. Limiter according to claim 1 or claim 2, so that the surface profile (5, 15) has a rectangular structure.

4. Limiter according to claim 3, dadurchge ¬ indicates that web width (b) and web height (h) of the surface profile (5, 15) are between 0.1 mm and 1 mm, preferably at values from 0.3 to 0, 6 mm.

5. Limiter according to one of claims 1 to 4, because - characterized in that the rectangular profile (15) has an angle of inclination towards the surface (11).

6. Limiter according to claim 1 or claim 2, d a ¬ d u r c h g e k e n n z e i c h n e t that the surface profile (5, 15) has a conical shape.

7. Limiter according to claim 1 or claim 2, that the resistance element (1) consists of thermoplastic material with a conductive material component.

8. Limiter according to claim 7, d a d u r c h g e - k e n n z e i c h n e t that the thermoplastic

Material is polyethylene and the conductive material component is graphite.

9. Limiter according to one of claims 1 to 8, that the surface electrodes (10) are made of electrically and thermally well-conducting material.

10. Limiter according to claim 9, so that the surface electrodes (10) consist of copper with a silver-plated surface.

11. Limiter according to one of the preceding claims, characterized in that a resistance element is provided which is connected in parallel to the thermoplastic resistance body (1) and the surface electrodes (10).

12. Limiter according to claim 11, d a d u r c h g e ¬ k e n n z e i c h n e t that the resistance element is an ohmic resistance.

13. Limiter according to claim 11, so that the resistance element is a non-linear, voltage-dependent resistance, the resistance of which decreases with the applied voltage.