Liquid metal current switch

Description The present invention generally relates to electrical current switches, in particular to liquid metal current switches. More specifically, it relates to liquid metal current switches for current limiting and/or circuit breaking. It further relates to an electrical switchgear assembly, and to a method for limiting current using a liquid metal current switch.

Related Art

It is known that liquid metal contacts or switches can be used for fault current limiting. The known liquid metal contacts have a channel, the walls of which have a lower portion comprising a pair of electrodes, and an upper portion made of a resistive material. A liquid metal droplet is provided in the lower portion of the channel, thereby providing an electrical contact between the electrodes. If the current flowing through the droplet exceeds a limiting value, the liquid metal droplet is moved into the upper portion of the channel. After this motion, the direct contact between the electrodes is separated. Instead, the electrodes are connected to each other by a series connection of the liquid metal droplet and the resistive material. Such liquid metal contacts are described e.g. in WO 2005/006368, WO 2005/006375, and WO 2005/006373.

In the known liquid metal contacts, a short switching time or contact separation time is needed in order to limit the fault current. In particular, in AC applications a first fault current peak should be limited. More generally, high speeds of the liquid metal droplet are desired. At the same time, the shape and movement of the liquid metal needs to be sufficiently regular at all times, since the moving liquid metal droplet has to carry the fault current and has to provide a good electrical contact to the channel walls. In particular, the liquid metal droplet should not separate from the channel walls or split up. However, it is difficult to control the the liquid metal and its movement at the desired high velocities, because the dynamics of the liquid metal is complex and subject to numerous conditions.

Summary of the invention

The present invention intends to reduce at least some of the above problems. The object is solved by the liquid metal current switch according to claim 1, by the electrical switchgear assembly according to claim 13, and by the method for limiting current according to claim 14.

Further advantages, features, aspects and details of the invention are evident from the dependent claims, the description and the drawings.

According to one aspect of the invention, a liquid metal current switch for current limiting and/or circuit breaking is provided. The liquid metal current switch comprises a liquid metal and at least one' channel for the liquid metal. The liquid metal is moveable between a first position of the channel and a second position of the channel, the first and second position defining, a first switching state and a second switching state of the liquid metal current switch, respectively. In the first switching state, a first current path and a second current path are provided in parallel for a current through the current switch, wherein the first current path is leading at least partially through the liquid metal which is in its first position. In the second switching state, the first current path is interrupted and the second current path is provided for the current through the current switch. The second current path m be called a backup path or a permanent backup path.

According to a further aspect of the invention, there is provided an electrical switchgear assembly, especially a high, medium or low voltage switchgear assembly, comprising the above- described liquid metal current switch.

According to a further aspect of the invention, a method for limiting current by a liquid metal current switch is provided. The liquid metal current switch comprises a liquid metal and at least one channel for the liquid metal, and the liquid metal is positioned in a first position of the channel, thereby defining a first switching state, in which a first current path and a second current path are provided in parallel for a current through the current switch, with the first current path leading at least partially through the liquid metal. The method comprises switching the liquid metal current switch from the first switching state to a second switching state by the following steps:

- the liquid metal is moved from the first position to a second position of the channel,

- the first current path is interrupted by the motion of the liquid metal, and

- the second current path is sustained at least for a period of time after the liquid metal has moved to the second position.

The invention allows to switch from the first switching state to the second switching state by a current commutation process to the second current path. Thereby, no detailed control over the motion of the liquid metal is necessarily required. This removes restrictions on the speed of the liquid metal movement, whereby the response time of the switch may be reduced. Further, in some embodiments arc-less commutation from the first to the second current path is possible.

The invention is also directed to apparatuses for carrying out the disclosed methods and including apparatus parts for performing each described method steps. Furthermore, the invention is also directed to methods by which the described current switch operates. It includes method steps for carrying out every function of the switch or manufacturing every part of the switch.

Description of the drawings

The invention will be better understood by reference to the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

Fig. 1 shows a first embodiment of the invention in a first switching state; Fig. 2 shows the first embodiment of the invention in a second switching state; Fig. 3 shows a second embodiment of the invention in a first switching state; Fig. 4 shows the second embodiment of the invention partially in a first switching state and partially in a second switching state; and

Fig. 5 shows the second embodiment of the invention in a second switching state. In the figures, same or similar parts are assigned the same reference numerals.

Ways to implement the invention

Fig. 1 shows a first embodiment of the invention in form of a liquid metal contact arrangement 1. The liquid metal contact arrangement 1 has leads 2 and 3, between which two liquid metal contacts 10 are connected in series. Each of the liquid metal contacts 10 has a channel 11a, lib for a respective liquid metal droplet 12. The walls of a lower portion of the channels 11a, 1 Ib are defined by respective electrodes 14a, 14b and 14b, 14c, which are made of a low-resistance conductor such as copper or some other electrically conductive metal. One electrode 14a connects the input lead 2 with the left wall of the lower portion of the channel 11a; a second electrode 14b connects .the inner walls of the lower portions of the respective channels 11a and lib, and a third electrode 14c connects the right wall of the lower portion of the channel lib with the output lead 3. The resistance of each of the electrodes is typically less than 50 μOhm and may be less than 20 μOhm or even less than 10 μOhm.

The respective walls of the upper portions of the channels 11a, lib are made of an insulator 18 or a very highly resistive material. The very highly resistive material typically has a resistance of more than 100 mOhm; it may have a resistance of more than 1 Ohm or even of

more than 3 Ohm, depending on the application. Further, commutation layers 16 are provided between the electrodes 14a, 14b, 14c and the insulator 18. The commutation layers 16 can be made of a highly resistive material such as graphite, a metal such as platinum or tungsten, and/or a conductive ceramic. The input lead 2, the second electrode 14b, and the output lead 3 are in addition connected via at least one resistor 30. Preferably, at least one resistor 30 is provided per channel 11a, lib for parallel connection and bridging the respective channel 11a, lib.

In Fig. 1, the liquid metal contacts 10 are shown in a "switched-on" or "closed-contacts" state, in which the liquid metal droplets 12 are located in the lower portions of the channels 11a, lib. The droplets 12 provide a direct electric contact between the electrodes 14a, 14b and between the electrodes 14b, 14c on either side of the respective channel 11a, lib, thereby bridging the channels lla, lib. Thus, a first or low-resistance current path 40, which leads through both liquid metal droplets 12, is provided between the leads 2 and 3. Further, a second or high-resistance path 42, which leads through the resistor 30, is provided in parallel to the low-resistance path 40. In the presence of the low-resistance path 40, the high-resistance path

42 has a negligible influence on the total resistance of the liquid metal contact arrangement 1.

In Fig. 2, the liquid metal contacts are shown in a "switched-off" or "open-contacts" state, in which the liquid metal droplets 12 are located in the upper portions of the respective channels lla, lib. The direct electric contact between the electrodes 14a, 14b and between the electrodes 14b, 14c on either side of the respective channel lla, lib, which was shown in

Fig. 1, is now interrupted. Thus, the low-resistance current path 40 previously provided between the leads 2 and 3 is now interrupted. However, the parallel high-resistance path 42, which leads through the resistors 30, is still present. Furthermore, in the case that the insulator

18 is replaced by a high-resistance material (not shown), another high-resistance current path leading through this high-resistance material and the liquid metal droplets 12 may be present.

Typically, the liquid metal contact arrangement 1 of Figs. 1 and 2 is used for current limiting and/or circuit breaking. Thus, the liquid metal droplet 12 is normally in the "switched-on" state of Fig. 1, and is moved by a driving mechanism to the "switched-off ¹ state of Fig. 2 in the case of a fault current.

The driving mechanism can be an electromagnetic driving mechanism. Here, in the "switched- on" state and in the presence of a magnetic field B, the liquid metal droplet 12 experiences a

Lorentz-force when current passes through the bridge or contact 10. If the current exceeds a

limiting value, the Lorentz force exceeds a counter-force (typically the gravitation force), and the liquid metal droplet 12 starts to move into the upper portion of the channel lla, 1 Ib, such that the liquid metal contact 10 is switched to the "off' state. The magnetic field B is dimensioned such that the limiting current value is exceeded in the case of a fault current. The magnetic field B can be generated e.g. by a permanent magnet, by an electromagnet, or by the fault current itself. An electromagnetic driving mechanism is described in further detail in WO 2005/006373, the disclosure of which is herewith incorporated. The driving mechanism can also be a mechanical drive, a piezoelectric drive, and, in particular, a drive using a dielectric fluid. The counter-force can be due to a pressure difference, a capillary force, or a compensating electromagnetic force. Some of these driving mechanisms are described in further detail e.g. in WO 2005/006368, the disclosure of which is herewith incorporated.

Thus, by the motion of the liquid metal drops 12 from the lower portion ("switched-on" state) to the upper portion ("switched-off" state) of the channels lla and/or lib, it is possible to increase the total resistance of the device 1 for limiting the fault current.

The high resistance path 42 has a sufficiently high resistance in order to limit or suppress a fault current to a sufficient degree. It is preferred that the second current path 42 has at least three times, preferably 10 times, more preferred 500 times or even most preferred 1000 times the resistance of the first current path 40. Further, due to the presence of the high resistance path 42, the switching off can be achieved relatively smoothly. Then, the resistance of the liquid metal contact arrangement 1 increases continuously as a function of time during the switching process, such that the fault current is not switched off abruptly and completely. The continuous increase makes it possible that a minimal arcing voltage of the channels lla, 1 Ib (i.e. the voltage above which arcing may occur, typically 10 V - 20 V) is never exceeded, such that arcing can be avoided. This can be achieved e.g. by dimensioning the resistance of the second current path 42 for arc-free switching from first switching state to second switching state. The arc-free switching condition is defined under operating conditions of the liquid metal current switch 1, i.e. at expected nominal and/or fault currents. A typical fault current is 140 kA.

The switching off may be further smoothened due to a commutation layer 16, which is optionally arranged, as shown in Fig. 1 and 2, between any of the insulators 18 and any of the electrodes 14a, 14b, 14c. The commutation layer 16 is arranged to be in contact with the liquid metal 12 during tihe motion of the liquid metal 12 between the "on" position and the "off ¹ position. The commutation layer 16 provides an intermediate resistance current path during the upward motion of the liquid metal droplet 12. The intermediate resistance current path allows

reducing the current and hence the voltage drop upon current commutation to the resistors 30. The resistance and the thickness of the commutation layer 16 is chosen such that an arcing voltage of the channels 11a, 1 Ib is not exceeded during fault current limitation. Furthermore, the material of the commutation layer 16 is advantageously chosen such that the commutation layer 16 has a higher minimal arcing voltage than the electrodes 14a, 14b, 14c, e.g. of more than about 15 V. In this manner, arcing can further be suppressed or the resistors 30 can have a higher resistance. It is possible to modify the liquid metal contact arrangement 1 of Figs. 1 and 2 without departing from the scope of the invention. In particular, the number of liquid metal contacts 10 (i.e. the number of channels 11a, lib) can be varied. For example, only one contact, three contacts, or ten contacts can be provided in series. Furthermore, multiple contacts can be provided in parallel. The advantage of the invention is that such matrix-like arrangements of liquid metal switches 1 comprising liquid metal switches 1 according to invention arranged in series and/or in parallel are now much simpler controllable, because the switching and current commutation process in the novel switches 1 of this invention is much more tolerant against variations in the liquid metal dynamics during switching than in previously known liquid metal switches. In particular, the matrix of liquid metal switches 1 can be equipped with at least one common resistor 30 arranged in parallel to the whole matrix of liquid metal switches 1 by connecting the resistor(s) 30 between an input lead and output lead of the matrix.

For deciding how many contacts should be arranged in parallel and in series and how the contacts should be dimensioned, the following aspects shall be considered: - The resistance of main current path 40 and the corresponding temperature rise during operation, i.e. under normal currents, should be tolerable;

The total resistance of the device in the resistive current path 42, which determines the current limitation behaviour, should be appropriate for fault current limitation;

- Arcing should be avoided, hence in each contact the current and the consequent voltage rise during the current commutation process should be lower than the minimum arcing voltage;

- The contact separation time should be appropriate.

In further modifications of the embodiment of Fig. 1, the commutation layer 16 can be omitted. Furthermore, the resistors 30 can be connected only to the leads 2 and 3, i.e. the electrical connection between the middle electrode(s) 14b and the resistors 30 can be omitted.

The liquid metal droplets 12 can also be replaced by liquid metal columns. The columns would be in contact with a liquid metal reservoir. The height (or depth) of the columns could be adjusted by pumping or driving liquid metal in or out of the channels Ha ₃ lib by a suitable pumping or driving mechanism. La Figs 1 and 2, the liquid metal columns would extend downwardly from the top of the channels lla, lib. In the "on" state, the liquid metal columns would extend downwardly from the top to the bottom portion of the channels lla, lib, i.e. to a portion that allows contact with the electrodes 14a, 14b, 14c. hi the "off ¹ state, the liquid metal columns would extend downwardly from the top only to an upper portion of the channels lla, lib or be completely removed from the channels Ha ₅ l ib, such that they are not in contact with the electrodes 14a, 14b, 14c.

Furthermore, the liquid metal contact arrangement 1 can generally be designed as described in WO 2005/006373 and WO 2005/006375, the disclosure of which is herewith incorporated, except that an additional parallel high-resistance path 42 should be provided as shown in Fig. 1 and 2 of the present application. Additionally, the upper portions of the channels lla, 1 Ib for the liquid metal 12 may be made insulating.

In addition, switch means for continuously or discretely interrupting the second current path 42 can be provided. Since the resistance of the second current path 42 is generally high, these switch means may provide arc-free interruption of this current path using known simple methods.

An advantage of the above arrangement is that the liquid metal 12 does not have to carry a (fault) current when the arrangement is in the second switching state. Instead, the fault current is carried by the resistors 30. Consequently, for switching to an "off ¹ state no control of the liquid metal motion is needed apart from removing it from contact with the electrodes 14a, 14b, 14c. Arcing can nevertheless be suppressed by the parallel high-resistance path 42.

Fig. 3 shows a liquid metal contact arrangement 1 being a second embodiment of the invention. Similarly to the first embodiment, the liquid metal contact arrangement 1 has leads 2 and 3, between which two liquid metal contacts 20 are connected in series. Each of the liquid metal contacts 20 has a channel 21a, 21b for a liquid metal droplet 22, and electrodes 24, which are made of a low-resistance conductor, and which define a lower portion of the walls of the channels 21a, 21b. The walls of the upper portion of the channels 21a, 21b are made of a highly resistive material 32, such as graphite, a metal such as platinum or tungsten, or a conductive ceramic. An insulating layer 26 separates the electrodes 24 and the resistive

material 32. The insulating layer 26 avoids short circuiting the resistive material 32 by the electrodes 24.

The electrodes 24 connect, with relatively low resistance, the input lead 2 with the left wall of the lower portion of the channel 21a of the first contact (first electrode 24); the inner walls of the respective lower portions of the channels 21a and 21b (second electrode 24); and the right wall of the lower portion of the channel 21b with the output lead 3 (third electrode 24). The resistive material 32 (first to third resistive material portion) connects, likewise, the corresponding upper portions of the channels 21a, 21b, albeit with relatively high resistance. The resistance of the resistive material 32 corresponds to the resistance of the resistor(s) 30 of Fig. 1.

In Fig. 3, the liquid metal contact arrangement 1 is shown in an "on" state. Similarly to the embodiment of Fig. 1, a low-resistance current path 40 is defined that leads, in series, through the electrodes 24 and through the liquid metal droplets 22. Further, a parallel high-resistance current path, denoted by 42 in Fig. 5, is provided. Different from the arrangement of Fig. 1, the high-resistance current path 42 leads, in series, through the high-resistance material 32 and through the liquid metal droplets 22. Like in the embodiment of Fig. 1 and 2, there is provided an electrically isolating region, namely the insulating layer 26, separating a portion of the first current path 40 and a portion of the second current path and being preferably arranged in a plane substantially parallel to at least one of the separated portions.

In Fig. 5, the liquid metal contact arrangement 1 of Fig. 3 is shown in the "off' state after motion of the liquid metal 22 into the upper portion of the channels 21a, 21b. Here, the electrodes or leads 2 and 3 are connected to each other only by the high-resistance current path 42, i.e. by the series connection of the liquid metal droplets 22 and the resistive material 32.

Fig. 4 shows the liquid metal contact arrangement 1 of Fig. 3 in a possible intermediate switching state, wherein the commutation process occurred not simultaneously in all liquid metal contacts 20. Instead, the first liquid metal contact 20 (on the left) is in an "off ¹ state, and the second liquid metal contact 20 (on the right) is in an "on" state, thus defining an intermediate current path 41. In order to avoid interrupting the high-resistance path 42 in this situation, the height of the liquid metal droplets 22 in the "off' state is chosen to reach into the high-resistance material 32 (see Fig. 3). Thus, there is always a current path present between the leads 2 and 3, whereby arc ignition can be suppressed or at least reduced.

It is possible to combine some of the features of the first and the second embodiment. In particular, it is possible to equip the second embodiment with a further high-resistance current path that corresponds to the current path 42 of Fig. 2 and that, in particular, does not lead through the liquid metal 12, 22. Such a current path 42 can e.g. be obtained by electrically connecting the high-resistive material 32 across the channels 21a, 21b by a hard-wired electrical connection. This would then correspond to the resistor 30 current path 42. Independently of the shown embodiment, it is generally preferred that the second current path 42 does not lead through the liquid metal 12, 22, and in particular that it is a hard-wired current path, and/or is leading only through resistive material 30, 32 in the solid state. In particular, the second current parfh 42 leads through at least one discrete resistor 30 arranged in parallel to the current switch 1 and/or through resistive material 32 arranged inside the current switch 1. Furthermore, similar modifications as described for the first embodiment in Fig. 1-2 can also be applied to the second embodiment of Figs. 3-5.

The above embodiments illustrate that the problem of controlling the dynamics of the liquid metal droplet 12, 22 can be avoided, because a simple current commutation is indirectly effected by means of a simple, possibly less coordinated liquid metal movement.

Reference Numerals

Liquid metal current switch, liquid metal contact arrangement with backup electrical path First lead Second lead

0 Liquid metal contact Ia, 1 Ib Channel for liquid metal droplet 2 Liquid metal droplet 4a, 14b, 14c Low-resistance conductor, electrodes 6 Commutation material 8 Insulator

0 Liquid metal contact 1a, 21b Channel for liquid metal droplet 2 Liquid metal droplet 4 Low-resistance conductor, electrodes 6 Commutation material, insulating layer

0 Resistors 2 Resistive material

0 Fist current path (low resistance) 1 Intermediate current path 2 Second current path (high resistance)