WO2016091318A1

WO2016091318A1 - A switching device

Info

Publication number: WO2016091318A1
Application number: PCT/EP2014/077507
Authority: WO
Inventors: Gunnar Johansson
Original assignee: Abb Technology Ltd
Priority date: 2014-12-12
Filing date: 2014-12-12
Publication date: 2016-06-16

Abstract

The present invention relates to a switching device (1) for interrupting a current flowing through a first electric circuit (10) at a current zero-crossing comprising an arc extinguishing chamber having at least two splitters (30, 31) for splitting an arc into partial arcs, wherein the arc is formed upon an interruption and the partial arcs forms a first alternative current path in parallel or series with the current path of the first electric circuit (10), characterized in that the switching device (1) further comprises a second electric circuit (20) including an impedance unit (50) being configured to connect to the at least two splitters (30, 31) such that, upon an interruption, it forms a second alternative current path in parallel with the first alternative current path.

Description

A SWITCHING DEVICE

FIELD OF THE INVENTION The present invention relates to a switching device for switching an electric circuit through which a current is flowing. More particularly, it relates to a switching device that interrupts the current at a current zero-crossing. The switching device comprises an arc extinguishing chamber having at least one splitter for splitting an arc formed during the current interruption into partial arcs.

PRIOR ART

A switching device for switching on or off an electric circuit includes a con- tacting unit including a stationary contact and a movable contact movable between an open position and a closed position. In the closed position, the movable contact is in contact with the stationary contact and the current flows through the contacts. In the open position, a contact-gap is established between the contacts and when a current is interrupted an arc is inevitably generated in the contact-gap. The arc must be distinguished in a very short time so as that current can be eventually interrupted.

To extinguish the arc, an arc-extinguishing chamber is provided and arranged together with the contacting unit. The arc-extinguishing chamber may have at least one arc-extinguishing splitter or a plurality of arc-extinguishing splitters arranged for guiding, cooling and splitting the arc into partial arcs that stand in between the splitters. The partial arcs sustain during a short period after separation of the current carrying contacts, which forms an alternative arcing current path in parallel or series with the current path of the electric circuit. Whether the alternative path is parallel or series with the current path of the electric circuit depends how the splitters are disposed in the arc-extinguishing chamber in relation with the contacts. As long as the arcs are sustained in between the contacts and splitters, the current flowing through the switching device will not be interrupted completely. Thus, for a complete interruption of the current, it is of high importance to extinguish the arcs as quick as possible.

In the case of a varying current, it is known that such a switching device may use a principle of interrupting the current at or as close as possible at its zero crossing. This is because that at the current zero, the arc energy is minimal and thus its current conduction decreases and a dielectric strength of the contact-gap is built up as long as the current remains low. After the current passes the zero-crossing, the voltage across the arc, denoted as arc voltage below, will immediately change its polarity. A so-called instantaneous recovery voltage containing a transient component, denoted as TRV (Transient Recovery Voltage) is built up/increased very rapidly. If the dielectric strength of the contact-gap is increased quickly to an extent that it is faster than the TRV, the arc will be extinguished and the current will be eventually interrupted. Otherwise, the arc will be restrike/reignited in the reverse direction and results that the current starts to flow between the contacts in the reverse direction. The re-ignition increases arcing duration, and consequently decreases interrupting/breaking performance of the switching device.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide a switching device with an improved interrupting/breaking performance.

The object is achieved by a switching device as defined in the preamble of claim 1. The switching device is further characterized in that switching device further comprises a second electric circuit including an impedance unit being configured to connect to at least two of the splitters such that, upon an interruption, it forms a second alternative current path in parallel with the first alternative current path.

The object is also achieved by a switching device as defined in the preamble of claim 2. The switching device is further characterized in that the switching device further comprises a second electric circuit including an impedance unit being configured to connect between the at least one splitter and the first electric circuit such that, upon an interruption, it forms a second alternative current path in parallel with the first alternative current path. Since upon an interruption, the second electric circuit provides an alternative current path in parallel with the arcing current, the rise of a TRV is delayed, which enables the dielectric strength of the contact-gap to rise faster than the TRV. Consequently, a restrike/re-ignition of the arc is prevented and the breaking performance of the switching device is improved.

Due to the arrangement of the second electric circuit, the switching device according to the invention may break a higher current and/or a higher voltage than without this arrangement. Another advantage is that a fewer splitters are required to break a specific current at a specific voltage than with- out this arrangement.

According to yet another embodiment of the invention, the arc extinguishing chamber comprises a plurality of splitters and the second circuit further includes at least one additional impedance unit configured to connect a dif- ferent pair of the splitters, wherein at least one of the two splitters is a different one to which the first impedance unit is connected. Preferably, the second circuit may include a plurality of impedance units, each of the impedance units is configured to connect two adjacent splitters. According to one embodiment of the invention, the impedance unit is any of resistance, capacitance, or a combination thereof. Moreover, the resistance of the resistor is in a range of ΙΟΟΓΤΙΩ to 1kQ while the capacitance of the capacitor is in a range of 1nF to 100mF. According to one embodiment of the invention, a resistive material is adapted as the second circuit, which may be fastened and electrically connected along sides of the splitter plates so that the splitter plates are connected to each other by the resistive material. The resistivity of the material may be in a range of 0.001 to 1000 ΩΓΤΙ. It is advantageous to use a resistive material as the second circuit from both production and cost perspectives. It is easy to produce without wiring an electric circuit including an impedance unit. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Fig.1a illustrates a schematic view of a switching device including a second circuit connecting two splitters, according to a first example of the invention. Fig.1b illustrates a schematic view of a switching device including a second electric circuit connecting at least one spillers and first electric circuit, according to a second example of the invention.

Fig.2a illustrates a schematic view of a switching device including a plurality of splitters and a second circuit connecting one of the splitters and the first electric circuit, according to a third example of the invention.

Fig.2b illustrates a schematic view of a switching device including a plurality of splitters and a second circuit connecting at least two pair of the splitters, according to a fourth example of the invention.

Fig.2c illustrates a schematic view of a switching device including a plurality of splitters and a second circuit including a plurality of impedance units, each of the impedance units connecting two adjacent plurality of splitters, according to a fifth example of the invention. illustrates a schematic view of a switching device, an extended example of Fig.2c, according to a six example of the invention.

Fig.3 illustrates a partial schematic view of an arc extinguishing chamber of a switching device, wherein the second circuit is made of a resistive material according to a yet another example of the invention. illustrate schematic views of three impedance units, each of them may form either an entire or part of the second electric circuit, according to some of examples of the invention.

Fig.5a-d illustrate schematic views of an arc-extinction process during the interruption of a current, according to one example of the invention.

Fig.6 illustrates a schematic view of a switching device including a second electric circuit connecting a plurality of splitters, according to yet another example of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1a illustrates a schematic view of a switching device 1. The switching device 1 is configured to be operated for controlling and protection of an electric power system comprising an electric power and a load (not shown in the figure) in an electrical power line which is a first electric circuit according to the invention. There are different types of switching devices, for example circuit breaker, contactor, a switching disconnector and miniature circuit breaker. Such a switching device can be used for both an AC and a DC application, wherein a current is interrupted at zero-crossing, for DC applications additional means are normally required in order to produce artificial zero-crossing(s).

The switching device is further provided with a movable contact 3 and stationary contact 2, each of the contact is provide with a contact tip that form a contact area.

With reference to Figure 5a-e, the principle of the invention is explained in details as follows.

The switching device is closed at a normal working condition so that there is a current flowing through the first electric circuit, see Fig.5a. When a current shall be broken, the switching device is switched off to interrupt the current, the contacts move apart and the contact area decreases rapidly until finally the contacts are physically separated. When the contact area decreases to a very small spot, the contact resistance increases considerably while the flowing current becomes highly concentrated and cre- ates a plasma with a temperature in the range of 5,000 to 20,000 K, within switching technology this is called an arc. The arc has a low resistance, through which the current will flow so there will not be any sudden interruption of the current. This results in an arcing current across the contacts, see Fig.5b. To provide the contact-gap an opportunity to change from a current carrier to an insulator, the arc must be extinguished completely for a period of time.

The arc may be lengthened and/or cooled and/or split so that the total arc voltage increases and thereby forces the current to decrease. Splitting the arc into partial arcs will also increase the total dielectric strength that is built up at the current zero crossings and can facilitate to prevent the arc from reigniting in the other direction after a current zero crossing. For this purpose, the switching device 1 comprises an arc extinguishing chamber having at least two splitters 30, 31 for splitting an arc into partial arcs.

These partial arcs forms a first alternative current path in parallel or series with the current path of the first electric circuit, Fig.5c shows that the first alternative current path is in series with the current path of the first current circuit while Fig.6 shows that a part of the first alternative current path is in parallel with the current path of the first current circuit. The cooled and split partial arcs increase the total arc voltage and also increase the dielectric strength that is built up at current zero crossing. The first alternative current path may also called as arcing current path. To delay the rise of TRV in relation to increase of the dielectric strength of the contact-gap when the switching devices 1 is configured to interrupt a current at its zero crossing, the switching device 1 is further provided with a second electric circuit 20 including an impedance unit 50. The impedance unit 50 is configured to connect to a first and a second splitters 30, 31. Thus, upon an interruption, it forms a second alternative current path in parallel with the arcing current path comprising arcs B. The arcs indicated by A will remain without any second alternative current path and after current interruption the remaining gaps will give full galvanic isolation. By shunting the arcing current path with the second electric circuit, the resistance of the whole current path is decreased, which decreases the rate of rise of the TRV, i.e, dV/dt. An experiment has shown that a delay of TRV about δμβ achieves a higher value of the dielectric strength, which prevents the TRV from re-striking and consequently the arc from re-ignition, the arc is then eventually extinguished and the current is interrupted, see Fig.5d.

With reference to Fig.4a-c, the impedance unit is any of resistance, capacitance, or a combination thereof. In the case of resistor, resistance of the resistor is in a range of ΙΟΟΓΤΙΩ to 1kQ. In the case of capacitor, capacitance of the capacitor is in a range of 1nF to 100mF. A combination can have any of these values.

The same invention concept may be achieved by another example as shown in Figure 1b. In this example, a switching device according to the invention has a stationary contact provided with two contacts, a movable contact arm actuated by an actuator is provided two contacts, which together with the responding stationary contacts form two pair of contacts. For each of the pairs, an arc extinguishing chamber is provided together with each of the contact pairs. The second electric circuit connects to at least one splitter arranged in each of the chambers at one end and the first electric circuit at another end, which forms another alternative current path in parallel with the first arcing current path.

Figure 2a-d illustrate respectively schematic views according to four other examples of the invention.

The example of Figure 2a is an extension of the second example, wherein the arc-extinguishing chamber comprises a plurality of splitters and an impedance unit includes a resistor and a capacitor and is connected to the first electric circuit.

Figure 2b further illustrates another possible example wherein a switching device including a plurality of splitters and a second circuit including at least another impedance unit 51 so that at least two pairs of the splitters are connected by two impedance units. In this example, one of the splitters in the two pairs is in common. However, it is possible to connect any two different of the splitters by each of the impedance units.

In example of Figure 2c. a plurality of impedance units are provided by the second electric circuit to connect each of two splitters adjacent to each other and each of the impedance units includes a resistor. It is advantageous to connect each of two adjacent splitters so that the distribution of resistance is equal between the splitters and the rise of TRV is evenly distributed. While the example of Figure 2d is a further extension of the example show in Figure 2c, wherein the second electric circuit is further connected to the first electric circuit.

In yet another preferred example, the second circuit may be made of a re- sistive material as shown in Figure 3. In this example, the resistive material 60 is electrically connected with the splitters 30 of an arc-distinguishing chamber 40. The resistivity of the resistive material 60 is in a range of 0.001 to 1000 ΩΓΤΙ. This example shows the splitters 30 are U-shaped plates. It is understood by skilled person in the art that a splitter may have a different shape and form and, that the resistive material may be connected to some of the splitters rather all of them. Nevertheless, the construction that resistive material is connected to each of the splitters provides an equal distribution of the resistance between the splitters. It should be understood that Figure 1a-b, 2a-d, 3 and 6 present a few of possible examples. However, a person skilled in the art will appreciate that the technology presented herein is not limited to the examples disclosed in the accompanying drawings and the above description, which are merely presented for purpose of illustrations. The invention may be implemented in a number of different ways as long as in the scope of claims.

Claims

1. A switching device (1) for interrupting a current flowing through a first electric circuit (10) at a current zero-crossing comprising an arc extinguish- ing chamber having at least two splitters (30, 31) for splitting an arc into partial arcs, wherein the arc is formed upon an interruption and the partial arcs forms a first alternative current path in parallel or series with the current path of the first electric circuit (10), characterized in that the switching device (1) further comprises a second electric circuit (20) including an im- pedance unit (50) being configured to connect to the at least two splitters (30, 31 ) such that, upon an interruption, it forms a second alternative current path in parallel with the first alternative current path.

2. A switching device (1') for interrupting a current flowing through a first electric circuit (10') comprising an arc extinguishing chamber having at least one splitter (30') for splitting an arc into partial arcs, wherein the arc is formed upon an interruption and the arcs forms a first alternative current path in parallel or series with the current path of the first electric circuit (10), characterized in that the switching device (1') further comprises a second electric circuit (20') including an impedance unit (50') being configured to connect between the at least one splitter (30') and the first electric circuit (10') such that, upon an interruption, it forms a second alternative current path in parallel with the first alternative current path.

3. Switching device according to claim 1 and 2, wherein the arc extinguishing chamber comprises a plurality of splitters (30) and the second circuit further includes at least one additional impedance unit (51) configured to connect a different pair of the splitters.

4. Switching device according to any of claims 1-3, wherein the impedance unit is any of resistance, capacitance, or a combination thereof.

5. Switching device according to claim 4, wherein a resistance of the resistor is in a range of ΙΟΟΓΤΙΩ to 1kQ.

6. Switching device according to claim 4, wherein a capacitance of the capacitor is in a range of 1nF to 100mF.

7. Switching device according to any of the preceding claims, wherein a resistive material is adapted as the second circuit and fastened along sides of the splitters so that at least two of the splitters are connected to each other by the resistive material.

8. Switching device according to claim 7, wherein a resistivity of the ma- terial is in a range of 0.001 to 1000 Qm.