EP0750788A1

EP0750788A1 - Current-limiting switch

Info

Publication number: EP0750788A1
Application number: EP94921596A
Authority: EP
Inventors: David Walter Branston; Jörg KIESER; Werner Hartmann; Reinhard Maier
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1993-07-26
Filing date: 1994-07-25
Publication date: 1997-01-02
Anticipated expiration: 2014-07-25
Also published as: JP3636461B2; DE59405547D1; DE4425330A1; JPH09501003A; US5859579A; EP0750788B1; WO1995003619A1

Abstract

Such switches have current connections for contacts. One contact may be a fixed contact and the other contact may be a mobile contact. A drive opens the mobile contact when a predetermined electric current intensity is exceeded. The electric drive is a thermoelectric drive. For that purpose, the contacts (2, 3) are arranged in a closed housing (1) made of an insulating material and a disk-shaped resistance (4) is arranged between the contacts (2, 3). A switch of this type may also be advantageously designed as a bistable limiter.

Description

description

Current limiting switch

The invention relates to a switch for current limitation, with current connections and contacts, one of which is a fixed contact and the other a moving contact, and with an associated drive for opening the moving contact when a predetermined electrical current is exceeded.

In the event of a short circuit, the current in power distribution networks should be limited as well as possible in order to prevent damage to lines and consumers which can otherwise only be excluded by massive oversizing. Conventional mechanical low-voltage switches draw the mechanical energy required for switching off from a spring accumulator. If necessary, additional support can be provided by the intrinsic magnetic field of the short-circuit current. The opening speed in such switches is limited by the complex mechanical structure on the one hand and the limited spring energy on the other. As a result, the current-limiting effect of such switches is limited.

From WO-A-91/12643 and EP-A-0 487 920 current-limiting arrangements are known as ballasts for switches which specifically use the so-called PTC effect or PTC effect (positive temperature coefficient). High-current resistors are used, which essentially consist of a polyethylene layer filled with soot, which has the PTC effect. In order to ensure the PTC effect in such a high-current resistor that can be used as a protective element, the base of the polymer resistor body should be connected to an electrode, a pressure device being present which applies a pressure perpendicular to the Electrodes and the bases of the resistance body of the conductive polymer layer exerts.

In contrast, the object of the invention is to provide a switch for current limitation which operates on a different physical principle.

The object is achieved according to the invention in a switch of the type mentioned at the outset in that the drive for opening the moving contact is a thermodynamic drive. With such a drive, it is possible to apply sufficient switching energies by thermal means.

In the invention, the contacts are arranged in a closed insulating material housing and a disk-shaped resistance body is arranged between the contacts. There is preferably an expansion volume between the resistance body and the moving contact in the insulating housing. The resistance body can consist of graphite-containing plastic, for example based on polyethylene, or can be formed by a large number of carbon fibers, which are brought into a film-like or felt-like consistency.

In deviation from the prior art, the mechanical one required for contact separation is thus used in the invention

Switching energy applied electrothermally. For this purpose, the current discharge occurring in the event of a short circuit first heats up an enclosed gas volume. The resulting pressure wave acts on a movable piston and performs the mechanical contact opening work on it.

Advantageously, in the invention by the Verwen¬ a large-area resistance body with respect to a metal substantially higher electrical resistance of the electrode-making by the surface distribution of the current river ^'s on the one hand a localized melting prevented. Ande- on the one hand, this promotes a uniform heating of the gas space.

The switch according to the invention is used in energy distribution networks in the low-voltage range. In the event of a fault, especially in the event of a short circuit, parts of the network must be disconnected at higher branches. In order to limit or avoid damage at the location of the fault as well as in the area of the network, the shutdown should take place as soon as possible, especially within the first half-wave concerned. A limitation of the short-circuit current is also often required if the switch-off cannot be recognized quickly enough or if suitable measures can be taken. The limitation of the short-circuit current also becomes a limitation of the amplitudes of the voltage peaks generated during the shutdown due to the inductive load component in the network and at the consumer and thus reduces the risk of further damage due to insulation faults which can be caused by such overvoltages. Particularly in the case of building installations, but also in other cases, the requirements for the required components increase, which must therefore have a high selectivity.

This significantly improves the state of the art. This state of the art corresponds in particular to such methods of causing a current limitation or current interruption in the event of a short circuit in low-voltage networks. The most widespread means for this purpose is the circuit breaker, which, however, always carries the current as a zero-point switch for at least one half-wave and is therefore not suitable for current limiting and quick disconnection. Due to the relatively high masses that are moved in circuit breakers, rapid disconnection cannot be achieved with reasonable effort. Fast switches for high currents require very high acceleration forces to move the moving masses Bring electrode systems in milliseconds to distances of several millimeters in short times. This is generally not possible with conventional spring energy stores, so that correspondingly powerful drive mechanisms are necessary.

Technical solutions for the latter purpose have hitherto been, for example, the pyrotechnic drive with chemical drive means which are ignited electrically, and the so-called Thomson drive. However, due to disadvantages inherent in the system, neither method has been widely used.

Such disadvantages are already eliminated in the current limiter according to the invention. However, the work area is restricted as a purely passive component, so that a correspondingly large variety of types is required to adapt to the respective area of use. An active tendon cutoff coupled with early short-circuit detection is not possible due to the passive nature of the function.

With a further invention, a bistable limiter can also be created by appropriate design of the switch. Such a switch according to the invention can be locked in the closed state and can be set by a suitable force accumulator to a response threshold which is above the maximum current to be expected in the high-load range. When locked, the volume resistance is so low that the nominal current losses are negligible. As a result, the self-response threshold of the limiter is in the order of the prospective short-circuit current. In the unlocked state, the contact resistances and thus the energy consumption in the area of the switch contacts increase. In the same way, the self-response threshold drops to a value in the nominal current range. The unlocking can advantageously be triggered by an electronic short-circuit early detection. This behavior is advantageously supported by the fact that a second energy store is used, which is tensioned in the locked state and can be constructed in such a way that in the unlocked state it leads to the mechanical opening of the moving electrode of the limiter. When the limiter is mechanically closed and locked, this second energy accumulator is automatically tensioned.

With a corresponding design of the operating parameters, the current limiter according to the invention is even able not only to limit the current but also to interrupt it completely, i.e. so to work as an opening switch. In this case, it is particularly advantageous if a second locking unit is provided which locks the limiter operating as a quick switch in the open state. The limiter is thereby prevented from automatically returning to the closed state after a successful power interruption. A real bistable behavior is thus achieved.

In the switch according to the invention, a passive circuit with RLC elements can ensure that no harmful overvoltage peaks are generated in the event of the current being separated during the current half-wave. For this purpose, voltage-limiting elements, such as zener diodes, varistors, surge arresters or the like, can also be present.

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

1 shows an electrothermal switch for low-voltage applications, FIGS. 2 to 4 show three alternative developments of such a switch for designing bistable limiters and

5 shows a current-time diagram to illustrate the advantageous switching behavior.

In the figures, the same or equivalent parts are provided with the same reference numerals.

The figure denotes an insulating material housing, which, for example, forms a rotationally symmetrical hollow cylinder about an axis I. The hollow cylindrical insulating material housing 1 is closed by a flange 6.

A fixed contact 2, which has a current connection 2a in the axial direction, is fitted into the insulating housing 1 rotationally symmetrically to the axis I. At a distance from this, a moving contact 3, the current connection 3a of which also extends in the axial direction I, is fitted into the insulating housing 1 so as to be longitudinally movable. A disk-shaped resistance body 4 is arranged between the fixed contact 2 and the moving contact 3 such that it rests on the surface of the fixed contact 2 without a space. For this purpose, the outer contour of the resistance body 4 is precisely fitted into the insulating housing 1.

Between the surface of the resistance body 4 facing away from the fixed contact 2 and the moving contact 3 there is a separating surface 8 which characterizes a variable space. In addition, an expansion volume 9 is provided all around the wall of the insulating material housing 1.

The moving contact 3 is connected with its current connection 3a through a bellows 5 to the separating surface 8 of the resistance body. pers 4 pressed. The spring body 5 defines a mechanical pretension, when the movement contact 3 is overcome, it is displaced in the horizontal direction. The displacement can be limited by a ring 10 encircling the insulating housing 1, a suitable stroke distance d being able to be specified by appropriate dimensioning of an annular attachment part 3b on the moving contact 3 on the one hand and of the encircling ring 10 on the other hand.

The power connection 3a points in the figure on its outer

End a notch 7a, which can engage an associated pawl 7b. Means for locking the moving contact in the open state are thus realized.

The switch described can be switched on in conventional energy distribution networks. The current flows through the current connection 2a, the fixed contact 2, the resistance body 4 onto the moving contact 3 and from there via the current connection 3a further into the network. In the event of a short circuit, the high-current discharge initially heats up the enclosed gas volume over the surface via the fixed contact 2 and the disk-shaped resistance body attached to it. The resulting pressure wave displaces the moving contact 3 until it stops, locking in the open state via the pawl 7a and notch 7b.

The response threshold of the monostable switch shown in FIG. 1 is thus determined by the pressure force of the spring 5. This makes the switch self-triggering, but not controllable.

In FIG. 2, however, the contact pressure spring 5 is fastened to an axially movable part 6b of the housing cover 6 with the parts 6a and 6b. The part 6b acting as a spring abutment is locked in position a via a locking mechanism 11a and 11b, so that the spring 5 is preloaded and generates the pressing force of the moving electrode 3 on the resistance body necessary for the closed state. At the same time, a spring 12 provided to accelerate the opening process is biased. When the latch 11 is unlocked by an actuator 13, the spring abutment 6b is accelerated in the axial direction away from the housing 1 by the springs 5 and 12, so that the contact force between the moving electrode 3 and the resistance body 4 drops to very low values within a very short time. This increases the contact resistance very strongly and the response threshold of the electrothermal drive drops to a value within the nominal current range of the switch.

In the embodiment according to FIG. 2, the electrothermal drive is triggered and the switch limits and interrupts or opens the current within a very short time, i.e. far below the prospective short-circuit current. In the fully open state, the mechanism 3a and 7a and 7b locks the moving electrode 3 and thereby prevents the switch from being inadvertently closed again. The actuator 13 is activated and triggered, for example, by electronic short-circuit detection.

In another embodiment according to FIG. 3, the opening spring 12 ^" acts directly on the moving electrode 3 and thus supports the opening by direct mechanical acceleration. This further accelerates the opening process and more strongly limits the current The same effect can be achieved if the opening spring 12 does not attach to the electrode 3, but rather to the guide element 3b or to the power supply 3a mechanically coupled to the moving electrode 3. Specifically in FIG Replaces actuator 14, which is actuated simultaneously with the unlocking actuator 13. When actuated, the actuator 14 reduces its length, so that the contact pressure force is already reduced. is moved before the moving electrode 3 is moved by the opening force memory.

In the embodiment according to FIG. 4, the piezoelectric actuator 14 is arranged parallel to the opening spring 12 and is lengthened when activated. As a result, the contact pressure force of the pressure spring 5 is briefly overcompensated and the spring action is supported in the initial phase of the opening process.

5 shows the opening behavior of the new switch. Curve 51 describes the time course of the prospective short-circuit current. Curve 52 describes the current through the uncontrolled limiting element of conventional design, the value A indicating the fixed response threshold. Curve 53 describes the current through the new bistable limiter or rapid switch, the response threshold B of the locked limiter being at or even above the prospective short-circuit current maximum. The threshold C of the ent unlocked limiter is within the _nominal Nennεtrombereiches I, so that a very early initiation can occur at hazardous current values. It is triggered by active unlocking via the short-circuit early detection electronics and the unlocking actuator 13.

The short-circuit early detection electronics detect short circuits within a few microseconds after zero current. Due to the small moving masses in the actuator 13 and in the locking mechanisms 11a and 11b, a very early

Current limitation and opening in the switch are reached, so that the currents actually occurring are limited to harmless values within the nominal current range. The selectivity is achieved by the threshold value setting of the short-circuit early detection and is therefore for a single electromechanical switching element of the "BISTABILER" type LIMITER "adjustable within wide limits.

In FIGS. 2 to 4, the actuator 13 used for unlocking can be designed as an electromechanical or electromagnetic actuator. However, it can also be designed as a piezoelectric or piezostrictive element to accelerate the unlocking process due to reduced accelerated masses. Furthermore, an actuator with a magnetostrictive element can be used as an active component.

It has been shown that the bistable limiters described, which operate as current limiters, can be combined with early electronic short-circuit detection. Likewise, in this combination the limiter can operate as a current-limiting quick switch. The suitable circuits for early detection of short circuits are known from the prior art.

The resistance body 4 can be made of conductive plastic, for example the known electrically conductive polyethylene. For example, graphite can be used for filling. The resistance body 4 can also be formed by graphite fibers, which have been brought into a foil or felt-like consistency by appropriate processing.

In deviation from this, a defined, conductive, non-organic material can also be used instead of the previously made conductive organic material, such as soot-filled polyethylene. However, highly doped semiconductor materials, such as, in particular, polycrystalline silicon carbide, can also be used as resistance bodies.

In further embodiments, the spatial shape can deviate from the rotational symmetry and, for example, be rectangular with flat resistance bodies. It can several resistance bodies can also be connected in series. Corresponding means for ventilating the interior of the housing from the insulating housing 1 can also be provided.

Claims

claims

1. Switch for current limitation, with current connections and contacts, one of which is a fixed contact and the other a moving contact, and with an associated drive for opening the moving contact when a predetermined electrical current is exceeded, characterized in that the drive is a is thermoelectric drive.

2. Switch according to claim 1, characterized in that the contacts (2, 3) are arranged in a closed insulating housing (1) and that between the contacts (2, 3) there is a disk-shaped resistance body (4) .

3. Switch according to claim 2, so that an expansion volume (9) is present between the resistance body (4) and moving contact (3) in the insulating material housing (1).

4. Switch according to 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 resistance body (4) consists of graphite-containing plastic, for example based on polyethylene.

5. Switch according to claim 2, d a d u r c h g e k e n n ¬ z e i c h n t that the resistive body (4) is formed by a plurality of carbon fibers, which are brought into a film or felt-like consistency.

6. Switch according to one of the preceding claims, since ¬ characterized in that Isolier¬ material housing (1), contacts (2, 3) and resistor body (4) each have a rotationally symmetrical cross section.

7. Switch according to claim 6, d a d u r c h g e k e n n ¬ z e i c h n e t that insulating material housing (1), contacts (2, 3) and resistor body are arranged rotationally symmetrically and colinearly to the power connections (2a, 3a).

8. Switch according to one of the preceding claims, g e ¬ k e n n z e i c h n e t by means (5) for adjusting the spring force of the moving contact (3).

9. Switch according to one of the preceding claims, g e ¬ k e n n z e i c h n e t by means (5) for stroke limitation and locking of the moving contact.

10. Switch according to one of the preceding claims, g e - k e n n z e i c h n e t by means for ventilating the interior of the housing.

11. Switch for current limitation, with current connections and contacts, one of which is a fixed contact and the other a moving contact, and with an associated drive for opening the moving contact when a predetermined electric current is exceeded, the drive being a thermoelectric drive, characterized by a bistable design of the switch.

12. Switch according to claim 11, g e k e n n z e i c h n e t by means (6b, 11a, 11b) for locking the switch in the closed state.

13. Switch according to claim 12, dadurchge ¬ indicates that the means (6b, 11a, lib) for locking a force memory (5) is assigned, which is adjustable to a current response threshold which is above the currents to be expected in the high load range.

14. Switch according to claim 13, d a d u r c h g e ¬ k e n n z e i c h n e t that the energy accumulator is a pressure spring (5).

15. Switch according to one of claims 11 to 14, so that the unlocking can be triggered by an electronic early detection of a short circuit.

16. Switch according to claim 15, d a d u r c h g e ¬ k e n n z e i c h n e t that the means for unlocking a second energy store (12) is assigned.

17. Switch according to claim 16, d a d u r c h g e - k e n n z e i c h n e t that actuators (13, 14) are available for unlocking.

18. Switch according to one of claims 11 to 17, a ¬ d u r c h g e k e n n z e i c h n e t that the actuators are piezo actuators (14).

19. Switch according to one of claims 11 to 18, so that the resistor body (4) consists of a highly conductive, non-organic material, for example polycrystalline silicon carbide.

20. Switch according to one of claims 11 to 19, g e ¬ k e n n z e i c h n e t by a passive circuit with RLC elements.

21. Switch according to one of the preceding claims 11 to 20, g e k e n n z e i c h n e t by a circuit with voltage-limiting elements, for example Zener diode, varistor, surge arrester or the like.