EP0717876B1 - Limiter for current limiting - Google Patents

Description

The invention relates to a limiter for current limitation, the one with overload from a low resistance value changes to a high resistance value with a thermoplastic resistance body and metallic Surface electrodes for making contact.

Low voltage circuit breakers are often used with so-called Limiters connected in series to the short-circuit switching capacity to increase significantly in low-voltage networks and greatly limit the forward currents. Such Limiters are designed to be quick in the event of a short circuit change from a low-resistance to a high-resistance state and with their tension needs to a quick Current limitation and shutdown contribute.

Specially for use as a central limiter, which for Short-circuit protection upstream of several branches it would be desirable that the limiter function only at high short-circuit currents, for example in cen Do not master branches of existing circuit breakers, becomes effective and with moderate or decaying short-circuit currents no limiter function occurs. This would prevent unwanted longer lasting voltage drops avoided, the uncontrolled switching states, e.g. from Contactors or relays.

Electromechanically simple structures are often used as limiters Switchgear used, whose contacts through Electricity forces open dynamically and usually via none Switch lock and no release system. Your arch tension plateau lies in the range of the mains voltage amplitude. Together with the arc tension of the series connected Circuit breaker becomes the short circuit current to brought rapid decay and shortened the switch-off time. Arc limiters of this type have problems with contact welding, which only with special contact materials and / or with a special contact mechanism technically can be solved.

From WO-A-91/12643 and EP-A-0487920 limiter are known that specifically utilize the so-called PTC thermistor or PTC effect (p ositive T emperature oefficient c). 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 base of the polymer resistance body is to be connected to electrodes, a pressure device being present which exerts a pressure perpendicular to the electrodes and the base areas of the resistance body of the conductive polymer layer.

Furthermore, from DE-A-37 07 494 is a resistance component with two metal electrodes, between which one Material having PTC properties is arranged, known in which by electrolytic deposition Roughening of the electrode surfaces is generated. Here may have irregular peaks from 0.5 µm to 500 µm. When such a PTC element works becomes a permanent, unchanging connection provided the electrode surfaces with the PTC material, constant resistance values of the component depending on the temperature. Finally, from US-A-43 77 541 a method for Manufacture of varistors from polycrystalline metal oxide known for low voltage applications where a press is used in which the press dies have projections have a regular structure. This means that powder metallurgical Making a profile in the varistor disc introduced to a low breakdown voltage to achieve. Such varistors are intended specifically for Small power applications with low currents and low voltages, with a non-linearity the electrical conductivity of the metal oxide is exploited. The non-linearity of the varistor material is based on its semiconductor properties and differentiates fundamentally different from the course of the electrical properties of the PTC material of suitable for high currents Limiters that consist of a mixture of a non-conductive Carrier material, for example polyethylene, and one electrically conductive material, such as carbon black, consists.

Physical basis of the limiters mentioned above is that the temperature in the event of a short circuit is caused by ohmic Heating above the crystallization temperature of the polyethylene increases as an electrically insulating base material, whereby microscopic current paths of the soot as break up electrically conductive material component and itself the limiter resistance is increased by a factor of 100 or more. The limiter resistance can mainly be determined by the Volume resistance of the PTC material can be determined.

According to its switching principle, the known limiters the surface resistance at the interfaces between the polymer resistance body and the electrodes essential to the current limiting effect in the event of a short circuit at. By placing the electrodes and the polymer resistance body due to the external pressure only in contact are together, the heating of the PTC material in a thin layer close to the surface instead, which in the event of a short circuit very quickly by a low impedance switches to a high-resistance state.

After the short circuit has been switched off, this boundary layer limiter is required for the decay of the high electrical Resistance to the low initial value a reset time typically around 20 ms. In practice this becomes Relief of the PTC element a parallel resistor connected, which is in the high-resistance state of the PTC thermistor material the major part of the short-circuit current wearing. During the shutdown period of the series Circuit breaker generates the decaying short-circuit current a considerable voltage drop across the parallel resistor. This can be 100 V or more and during exist for a period of 3 ms and longer. For the application as the central limit is such a long one Switching voltage intolerable, otherwise in itself undisturbed branches of the network correspondingly long-lasting Voltage dips would be caused.

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

According to the invention, the task is as a whole of the features specified in independent claim 1 solved. Advantageous further developments are in the dependent Claims specified.

If the in the current branches in series to the central limit switched circuit breaker at high short-circuit currents have a sufficiently high switching speed, can advantageously be achieved with the invention, that after the limit voltage has decayed, the switch-off process without delay with a sufficiently high bow tension is continued. The function of only with high instantaneous currents activated limit voltage is thus by reached a boundary layer limiter, in which In the event of a short circuit, a partial non-positive contact contact the electrodes on the resistor body. As a characteristic property of such a limiter have the electrodes and the resistance body a positive or negative, i.e. complementary surface profile, with which they are mechanically tight to each other be liable.

To implement the limiter according to the invention, the thermoplastic resistance bodies, for example in Fcrm a rectangular plate, pressed between profiled electrodes and at least on the contact surfaces heated to its softening temperature. That flows Resistance material in the profile recesses of the metal electrodes and there is a complementary surface profile of the resistance body. Stick after this process the electrodes firmly on the resistance body from which they only with mechanical damage to the profile layer can be separated again.

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 compressive force between 50 and 100 N / cm ² .

FIG. 1

einen Limiter in Schnittdarstellung,

FIG. 2

die Draufsicht auf einen Limiter gemäß FIG. 1,

FIG. 3

die Ausbildung einer Flächenelektrode,

FIG. 4

in zwei Teilfiguren den Vergleich der Widerstandskurven des Limiters gemäß den FIG. 1 bis 3 mit dem Stand der Technik, die

FIG. 5

in Teilfiguren das Beispiel einer Kurzschlußabschaltung einer Reihenschaltung durch den Limiter gemäß den FIG. 1 bis 3 und die

FIG. 6

in Teilfiguren die Schaltphasen eines Limiters gemäß den FIG. 1 bis 3.

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 sectional view,

FIG. 2nd

the top view of a limiter according to FIG. 1,

FIG. 3rd

the formation of a surface electrode,

FIG. 4th

in two partial figures the comparison of the resistance curves of the limiter according to FIG. 1 to 3 with the prior art, the

FIG. 5

in partial figures, the example of a short-circuit shutdown of a series connection by the limiter according to FIG. 1 to 3 and the

FIG. 6

the switching phases of a limiter according to FIG. 1 to 3.

The figures are partially described below together.

In the FIG. 1 to 3 1 means a thermoplastic Resistor body with surfaces 2 and 3, between two similar surface electrodes 10 pressed together becomes. For this purpose, a pressure force K is applied. Such The arrangement is in principle from the older German Patent application P 42 28 297.7 known.

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

The resistance body has a complementary profile 5 1 on both surfaces 2 and 3. About the profiles 5 and 15 are the resistance body 1 and the surface electrodes inextricably linked.

In a different training than FIG. 3 can be the rectangular profile also an angle of inclination against the surface of the surface electrode Have 10. The surface profile 15 can advantageously a different section Have direction. Also a conical shape the surface profile 15 is possible.

The limiter described in this way is a known Circuit breaker upstream. The limiter is off the resistance body 1 and the surface electrodes 10 Resistor element connected in parallel. The resistance element is, for example, an ohmic resistance of 100 mΩ. It can also be a nonlinear voltage dependent Be resistance, its resistance with the applied Tension drops. In both cases, the current can be the appropriate one Commute time.

In FIG. 4b is the limiter resistance to a short-circuit shutdown at I _k = 40 kA with a series connection of the limiter according to FIG. 1 to 3, to which a constant resistance of 100 mΩ is connected in parallel, and a power switch is shown and the time course is shown: The limiter resistor according to Graph 42 begins to rise slightly with the onset of the short-circuit current from its initial value R ₀ = 4 mΩ and reaches after about 300 µs a first plateau value of about 8 mΩ. 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 mΩ for about 300 µs. About 900 µs after the start of the short circuit, the limiter resistance returns to a low-resistance value of about 15 mΩ and finally drops to its initial value.

FIG. 4a shows the time profile of the Limiter resistor according to the prior art as Graph 41 shown.

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. 4 described resistance behavior manifests itself in such a way that the limiter generates a voltage pulse of approx. 450 V and a duration of approx. 300 µs approximately 600 µs after the beginning of the short circuit. In this phase, the short-circuit current drops from its maximum value i _max = 6.7 kA to approximately 3 kA, an increasing proportion of the current being borne by the parallel resistor (100 mΩ). After its voltage pulse has subsided, 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 the FIG. 4 and 5 can be explained phenomenologically, what is shown in FIG. 6 clarifies: In the initial state of the limiter according to FIG. 6a takes care of that Surface electrodes vertical pressure force K for a positive contact perpendicular to the pressure force running profile partial surfaces. In contrast is the Force effect between those running parallel to the pressure force Profile partial surfaces 12 much less, because after the thermal manufacturing process, the profile layer of thermoplastic resistance material due to the compared to metals much higher coefficients of thermal expansion a dimensional shrinkage of 1 to 2% experiences.

In the case of the on the basis of FIG. 4 and 5 short circuit shutdown continues with the beginning of the current rise due to the electrical power loss in the boundary layer increasing warming and associated with it Expansion of the profile layer 5 of the resistance body 1 with a significant reduction in the electrical contact area according to FIG. 6b. Due to the thermal expansion of the Thermoplastic profile bars become the metal electrodes 10 of the recessed partial profile surfaces of the Resistor body 1 lifted off. This sets in current limiter resistance, which is determined by the FIG. 4b shown first plateau value. At the same time, the current density almost doubles the profile end faces of the resistance body 1 and the electrical power loss leads to rapid heating to the decomposition temperature of the resistance material. This is shown in FIG. 6c also on these partial profile surfaces the mechanical-electrical contact largely interrupted and it forms between the profile surfaces a distributed, electrical discharge with high burning voltage out.

During the phase of high limit voltage, the profile layer 5 further heated and partially decomposed material, which builds up a significant gas pressure. With the decrease in gas pressure due to the decaying Short-circuit current and due to the thermal expansion the thermoplastic profile webs are formed according to FIG. 6d between the partial profile surfaces running parallel to the compressive force 12 of the electrodes 10 and the resistance body 1 contact surfaces, their effective contact force with the temperature of the resistance material and its thermal expansion increases and therefore at the end of Limiter voltage pulse to the observed low limiter resistance of about 15 mΩ. This process will supported by the fact that the parallel resistance of the Limiters temporarily takes over the total current and thereby the material decomposition on the resistance body 1 ended.

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

It is therefore essential in the case of the limiter described the arrangements known from the prior art, that there are no free contact areas, but that the Surface electrodes and the resistance body over their each other complementary surface profiles cannot be solved are interconnected.

Claims (13)

1. Limiter for current limitation which, upon overload, changes from a low resistance value to a high resistance value, having a thermoplastic resistance body and metallic surface electrodes for contact making, characterized by the combination of the following features:

   the metallic surface electrodes (10) have at their side (11) facing the thermoplastic resistance body (1) a surface profile (15) where partial profile surfaces extend in part parallel to the plane of the surface electrode (10),

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

   the surface electrodes (10) and the resistance body (1) are mechanically connected to each other in such a way that they cannot be detached from each other,

   the surface electrodes (10) and the resistance body (1) are pressed together by a pressure force (K) which has different effects depending on the orientation of the partial profile surfaces,

   whereby

   upon overload, between the contact-making partial profile surfaces of the surface electrodes (10) and the resistance body (1), in particular at the partial profile surfaces extending parallel to the plane of the surface electrode (10), a gas discharge is produced, and

   upon the dying down of the overload the contact making temporarily extends exclusively across the partial profile surfaces which do not extend parallel to the plane of the surface electrode.
2. Limiter according to claim 1, characterized in that the surface profile (15) of the resistance body (1) occurs by means of thermal moulding to the metallic surface electrodes (10).
3. Limiter according to claim 1 or claim 2, characterized in that the surface profile (5, 15) has a rectangular structure.
4. Limiter according to claim 3, characterized in that the segment width (b) and segment height (h) of the surface profile (5, 15) lie between 0.1 mm and 1 mm, preferably at values of 0.3 to 0.6 mm.
5. Limiter according to one of claims 1 to 4, characterized in that the surface profile (15) has an angle of inclination with respect to the surface (11).
6. Limiter according to claim 1 or claim 2, characterized in that the surface profile (5, 15) has a conical shape.
7. Limiter according to claim 1 or claim 2, characterized in that the resistance body (1) consists of thermoplastic material with a conductive material component.
8. Limiter according to claim 7, characterized in that the thermoplastic material is polyethylene and the conductive material component is graphite.
9. Limiter according to one of claims 1 to 8, characterized in that the surface electrodes (10) consist of material which has good electrical and thermal conductivity.
10. Limiter according ta claim 9, characterized in that the surface electrodes (10) consist of copper with a silver-coated surface.
11. Limiter according to one of the preceding claims, characterized in that a resistance element is provided which is connected parallel to the thermoplastic resistance body (1) and the surface electrodes (10).
12. Limiter according to claim 11, characterized in that the resistance element is an ohmic resistance.
13. Limiter according to claim 11, characterized in that the resistance element is a non-linear, voltage-dependent resistance, the resistance of which decreases with the applied voltage.

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