TECHNICAL FIELD

The invention relates to a device for inducing a voltage into an electric circuit, a zero- voltage switch, which comprises a connection to a DC grid, a switching device and a switchable auxiliary DC voltage source and a method for operating the device and the zero-voltage switch.

BACKGROUND ART

Generally, the contact voltage between switching contacts of a switching device in case of a switch off operation is a problem because switching contacts are exposed to a switching arc, which is more intense the higher the contact voltage is. For this reason, zero crossing switches have been proposed, which switch during a zero crossing of an AC grid voltage. In this way, deterioration of the switching contacts of a switching device can substantially be limited. However, unfortunately this does not work for DC grids because there are no zero crossings.

DISCLOSURE OF INVENTION

Accordingly, an object of the invention is the provision of an improved device for inducing a voltage into an electric circuit, an improved zero-voltage switch and an improved method for operating the device and the zero-voltage switch. In particular, a solution shall be proposed, which allows to limit the deterioration of the switching contacts of a switching device in DC grids.

The object of the invention is solved by a device for inducing a voltage into an electric circuit, wherein the device comprises: a magnetic core having three legs, wherein first ends of the three legs are interconnected and second ends of the legs are interconnected and wherein a first leg and a second leg of the legs are continuous legs and a third leg of the legs is a broken leg with an airgap, a first coil being wound around the first leg and a second coil being wound

1 around the second leg, wherein a first end of the first coil is electrically connected to a first terminal of the device and a first end of the second coil is electrically connected to a second terminal of the device and wherein second ends of the first coil and the second coil are electrically connected to each other in a way that a current between the terminals causes magnetic fluxes in opposite directions in the magnetic core, a third coil being wound around the third leg and a switchable induction voltage source connected to the third coil.

Moreover, the object of the invention is solved by a zero-voltage switch, which comprises a connection to an electric circuit including a DC grid, a switching device and a switchable auxiliary DC voltage source, wherein the switchable auxiliary DC voltage source is switched in series with the switching device and is designed to generate a counter voltage counteracting a grid voltage of the DC grid in case of a switch off operation of the switching device.

In particular, the object of the invention is solved by a zero-voltage switch where the switchable auxiliary DC voltage source is embodied as a device as proposed above, which device is switched in series with a switching device by means of the terminals connected to the first coil and the second coil, wherein the switchable induction voltage source of said device is designed to generate a counter voltage between the said terminals counteracting a grid voltage of the DC grid in case of a switch off operation of the switching device.

In addition, the object of the invention is solved by a method for reducing the contact voltage between switching contacts of a switching device in case of a switch off operation of the switching device, wherein the method comprises the steps of detecting a switch off demand, switching on a switchable auxiliary DC voltage source, which is connected in series to the switching device, as a consequence of said detection, wherein a counter voltage of the switchable auxiliary DC voltage source counteracts a voltage across the switching contacts of the switching device and subsequently opening the switching contacts of the switching device.

In particular, the object of the invention is solved by the above method, wherein the step of switching on the switchable auxiliary DC voltage comprises switching on the

2 switchable induction voltage source of a device as proposed above, which device is connected in series to the switching device by means of its first terminal and second terminal, as a consequence of said detection, wherein a counter voltage of the induction voltage source counteracts the voltage across the switching contacts of the switching device.

Finally, the object of the invention can also be solved by an electric circuit including a DC grid, which the proposed device or the zero-voltage switch is switched into.

In the on-state of the switching device when there is no switch off demand, the switchable induction voltage source is in its off-state. The current through the first and the second coil then causes a first flux and a second flux of the same strength but in opposite directions. Because the magnetic resistance in the third leg is considerably higher than in the first and second leg based on the air gap, the third flux through the third leg is almost zero in this state.

If a switch off demand is detected, a switch off operation is initiated. For this reason, the switchable induction voltage source is switched on. Accordingly, a current starts to flow through the third coil and in turn a third flux is generated what causes an additional first and second flux (preferably in the same direction as the already existing first and second flux). In turn, voltages are induced in the first and second coil which add up to a total voltage between the first and second terminal.

In detail, the switchable induction voltage source of the proposed device is designed to generate a counter voltage between the terminals counteracting a grid voltage of the DC grid in case of a switch off operation of the switching device. Because the counter voltage has the opposite direction of the grid voltage, the total voltage in the electric loop and thus the current in the electric loop, which the proposed device is built into, is limited and in an advantageous design even reduced to zero. In turn, the reduced or zero loop voltage is used to switch off the switching device so as to avoid or at least limit the bad effect of a switching arc between the switching contacts of the switching device. In other words, the proposed device provides the function of a zero crossing or a zero approaching of the loop voltage in case of a switch off operation. This zero crossing or zero approaching in turn is used to switch of the switching device. As said, the switching contacts of the switching device are opened at a point

3 in time when the voltage across the switching contacts of the switching device is zero or at least close to zero.

As a result, deterioration of the switching contacts of a switching device in DC grids is substantially be limited by the proposed measures.

Generally, a “switch off demand” can result of an overcurrent situation, a command of a user or a command of an external control. In case that the zero-voltage switch reacts on an overcurrent situation, it fulfills the function of a circuit breaker. That is why the switching device then is embodied as a circuit breaker and the zero-voltage switch may be termed then as “zero-voltage circuit breaker”. In this case, the switchable induction voltage source is in its off-state if the current through the proposed device and/or the switching device stays below a predefined current limit, and a switch off operation is initiated if said current rises above said limit, i.e. if an overcurrent event occurs. So, the off-state of the switchable induction voltage source in this embodiment is associated with normal operation of the circuit breaker, and the on-state of the switchable induction voltage source in this embodiment is associated with overload situation of the circuit breaker. Nevertheless, a switch off operation may also be initiated by a command of a user or a command of an external control.

In particular, the circuit breaker may be embodied as a hybrid circuit breaker.

It should be noted that “zero-voltage” does not necessarily mean that the voltage across the switching contacts indeed gets zero, but it is sufficient if said voltage gets closer to zero based on the proposed measures. In particular, “close to zero” means a voltage of 10% of a grid voltage. However, in an advantageous embodiment, the voltage across the switching contacts indeed reaches zero or even crosses zero.

Because the proposed device induces a voltage into an electric circuit, it could also be seen and termed as “induction device”.

It should also be noted that care should be taken that the magnetic core of the proposed device does not saturate, neither in the on-state of the switching device, nor when a switch off operation is initiated.

Further advantageous embodiments are disclosed in the claims and in the description as well as in the figures.

4 Beneficially, the switchable induction voltage source is embodied as a capacitor with a first switch in series. In this way, the switchable induction voltage source can be made up with simple and reliable means. Moreover, a capacitor can generate a current impulse through the third coil with a high slew rate because of its low internal resistance. Beneficially, the capacitor can be preloaded by the DC grid or an external voltage source. In particular, the induction voltage source or the capacitor can be connected to the DC grid or the external voltage source by means of second switches and/or resistors. Generally, one switch or two switches or one resistor or two resistors work well for this function.

In particular, the switchable induction voltage source, which is embodied as a capacitor with a first switch in series, can be connected to the grid voltage or the external voltage source and switched away from the third coil in the on-state of the switching device and can be switched away from the grid voltage or the external voltage source and connected in series with the third coil in case of a switch off operation of the switching device.

In this way, the capacitor is ready to generate a current pulse through the third coil all the time.

When resistors are used to charge the capacitor, the resistance of the resistors should be sufficiently high so that the capacitor does not substantially discharge in case the counter voltage is generated. In this embodiment, advantageously a control for the second switches can be omitted.

It is also beneficial, if the first switch is embodied as a semiconductor switch. In this way, switching the induction voltage source to the third coil can take place very fast.

Beneficially, the proposed device comprises a current sensor between the first terminal and the second terminal which is connected to and designed to control the switchable induction voltage source. In particular, the current sensor is connected to and designed to control the first switch of the switchable induction voltage source. Alternatively, the current sensor may also be arranged in the electric circuit, which the proposed device is switched into. In particular, the current sensor may be part of a switching device in said electric circuit. More particularly, the current sensor may be

5 part of a zero-voltage switch, which the proposed device is part of. In any case, the switchable induction voltage source is reliably switched on in case of an overload situation. It should be noted at this point that a control circuit for controlling the switchable induction voltage source can be integrated into the current sensor, but such a control circuit may also be embodied as a separate part. For example, the current sensor together with the switchable induction voltage source may act as a voltage source which is switched off below a first threshold current and which is switched on above a second threshold current. The first and the second threshold currents may be the same, or a hysteresis may be provided between the first and the second threshold current as the case may be. It should be noted that the proposed device beneficially is used in combination with a circuit breaker which also reacts on an overload event.

Advantageously, the counter voltage of the switchable auxiliary voltage source substantially equals the grid voltage of the DC grid. Nonetheless, the voltage of the switchable induction voltage source can be different because of the ratio between the count of windings of the third coil and the total number of windings of the first and second coil. In detail, the voltage of the switchable induction voltage source substantially equals the grid voltage multiplied by the ratio between the total number of the first and second winding and the count of windings of the third coil. In more mathematical notation that means wherein Usi is the voltage of the switchable induction voltage source, UG is the voltage of the grid, m is the number of windings of the first coil, r2 is the number of windings of the second coil and is the number of windings of the third coil. It should be noted at this point that if the aforementioned equation is fulfilled, the loop voltage usually has a zero crossing because of the voltage across a load in the electric loop, which has the same sign as the counter voltage, normally is unequal zero.

Beneficially, the switching device is embodied as vacuum interrupter. Vacuum interrupters can switch very fast because the dielectric distance, which is necessary

6 to avoid an arc between the switching contacts, is very short. So, the zero crossing of the loop voltage or its approaching to zero, which is just a transient phenomenon, can be utilized in an advantageous way.

Finally, it is of advantage if a single switchable auxiliary DC voltage source is switched in series with a plurality of switching devices, wherein said switchable auxiliary DC voltage source is designed to generate a counter voltage counteracting a grid voltage of the DC grid in case of a switch off command for any of these switching devices. In this way, a single switchable auxiliary DC voltage source can be used to generate a zero crossing or zero approaching of the loop voltage for a plurality of switching devices. Said arrangement can be seen as a zero-voltage switch with a plurality of outputs or, in particular if the switchable auxiliary DC voltage source and the plurality of switching devices are distributed over a larger area, as an electric circuit having said features.

It should be noted that the various embodiments and the advantages resulting thereof which have been presented for the proposed device, the zero-voltage switch or the method are interchangeable as the case may be. That means, that an embodiment or advantage, which has been presented for the proposed device may equally apply to the zero-voltage switch and so on.

BRIEF DESCRIPTION OF DRAWINGS

The invention now is described in more detail hereinafter with reference to particular embodiments, which the invention however is not limited to.

Fig. 1 shows a schematic view of a proposed device for inducing a voltage into an electric circuit in an idle state;

Fig. 2 shows a schematic view of the device of Fig. 1 in an acitve state;

Fig. 3 shows a zero-voltage switch with a switching device and a device as shown in Figs. 1 and 2 which acts as a switchable auxiliary DC voltage source;

7 Fig. 4 shows an example where a single switchable auxiliary DC voltage source or a single proposed device is switched in series with a plurality of switching devices;

Fig. 5 shows an example where a capacitor of the switchable induction voltage source is charged by a switched external voltage source;

Fig. 6 shows an example where such a capacitor is charged by the switched grid voltage;

Fig. 7 shows an example where such a capacitor is connected to the grid voltage by means of resistors and

Fig. 8 shows a more generalized example of the proposed principles.

DETAILED DESCRIPTION

Generally, same parts or similar parts are denoted with the same/similar names and reference signs. The features disclosed in the description apply to parts with the same/similar names respectively reference signs. Indicating the orientation and relative position is related to the associated figure, and indication of the orientation and/or relative position has to be amended in different figures accordingly as the case may be.

Fig. 1 shows a schematic view of a device 1 for inducing a voltage into an electric circuit. The device 1 comprises magnetic core 2 with three legs a, b, c, wherein first (upper) ends of the three legs a, b, c are interconnected and second (lower) ends of the legs a, b, c are interconnected. Further on, a first leg a and a second leg b of the legs a, b, c are continuous legs and a third leg c of the legs a, b, c is a broken leg with an airgap G. In simple words this means that the magnetic core 2 is double ring core with the middle leg broken.

In addition, the device 1 comprises a first coil L1 , which is wound around the first leg a, and a second coil L2, which is wound around the second leg b. Furthermore, a first end of the first coil L1 is electrically connected to a first terminal T 1 of the device 1 and a first end of the second coil L2 is electrically connected to a second terminal T2 of the device 1. Additionally, second ends of the first coil L1 and the

8 second coil L2 are electrically connected to each other in a way that a current I between the terminals T1 , T2 causes magnetic fluxes F1 , F2 in opposite directions in the magnetic core 2 like this is indicated in Fig. 1.

Further on, the device 1 comprises a third coil L3 being wound around the third leg c and a switchable induction voltage source 3, which is connected to the third coil L3.

In this embodiment, the switchable induction voltage source 3 is embodied as a capacitor C with a first switch S1 in series. In particular, the first switch S1 can be embodied as a semiconductor switch.

Finally, the device 1 comprises an optional current sensor 4 between the first terminal T1 and the second terminal T2 which is connected to and designed to control the switchable induction voltage source 3. In detail, the first switch S1 of the switchable induction voltage source 3 is controlled by the current sensor 4 in this example. It should be noted at this point that the current sensor 4 may comprise a control circuit, which is not depicted in detail in Figs. 1 and 2, but such a control circuit may also be embodied as a separate part. For example, the current sensor 4 together with the first switch S1 may act as a threshold switch, wherein the first switch S1 is open below a first threshold current and wherein the first switch S1 is closed above a second threshold current. A hysteresis may be provided between the threshold currents as the case may be, but the threshold currents may also be the same.

Alternatively or in addition, the switchable induction voltage source 3 can be controlled by another control. For example, such a control could receive a switch off command from a user or an external device and could be initiate a switch off operation based on that command. It is also possible that the current sensor 4 is omitted and that the switchable induction voltage source 3 is controlled only by such other control.

The function of the device 1 now is as follows:

In normal operation when no overload situation occurs what is depicted in Fig. 1 , the first switch S1 is held open by the current sensor 4 (in detail by its control circuit) and the capacitor C is loaded (see Figs. 5 to 7 how this may work). The current I causes

9 a first flux F1 and a second flux F2 of the same strength but in opposite directions like this is indicated in Fig. 1. Because the magnetic resistance is the third leg c is considerably higher than in the first leg a and the second leg b based on the air gap G, the third flux F3 through the third leg c is almost zero in this state.

If an overload situation occurs, what is depicted in Fig. 2, the first switch S1 is closed by the current sensor 4 and hence the capacitor C is switched to the third coil L3. Accordingly, a current starts to flow through the third coil L3 and in turn a third flux F3 is generated in the direction depicted in Fig. 2. The third flux F3 causes an additional first flux F1 and second flux F2 in the direction depicted in Fig. 2. Accordingly, a first voltage U1 is induced in the first coil L1 and a second voltage U2 is induced in the second coil L2. The first voltage U1 and the second voltage U2 add up to a total voltage between the first terminal T1 and the second terminal T2. The use of this total voltage is now explained by use of the example shown in Fig. 3.

Fig. 3 shows a zero-voltage switch 5, comprising a connection to an electric circuit including a DC grid 6, a switching device 7 and a device 1 as explained hereinbefore which device 1 acts as a switchable auxiliary DC voltage source. The device 1 is switched in series with the switching device 7 by means of the terminals T1 , T2 connected to the first coil L1 and the second coil L2, and the switchable induction voltage source 3 of said device 1 is designed to generate a counter voltage UC between the said terminals T1 , T2 which counteracts a grid voltage UG of the DC grid 6 in case of a switch off operation of the switching device 7. So, in case of overcurrent, the first switch S1 is closed by the current sensor 4 as explained above thus generating the counter voltage UC. Because the counter voltage UC has the opposite direction of the grid voltage UG the total voltage in the electric loop shown in Fig. 3 is limited and in an advantageous design even reduced to zero. To make this happen, the counter voltage UC should substantially equal the grid voltage UG. In other words, the voltage of the capacitor C and the turns of the third coil L3 beneficially are adapted to the grid voltage UG and the turns of the first coil L1 and the second coil L2.

So, in a beneficial embodiment, the voltage of the switchable induction voltage source or the capacitor C substantially equals the grid voltage UG multiplied by the ratio between the total number of windings of the first coil L1 and the second coil L2

10 and the count of windings of the third coil L3. In more mathematical notation that means wherein Usi is the voltage of the switchable induction voltage source or the capacitor C, UG is the voltage of the grid, m is the number of windings of the first coil L1 , n2 is the number of windings of the second coil L2 and is the number of windings of the third coil L3.

The reduced loop voltage, which advantageously gets zero, is used to switch off the switching device 7 so as to avoid or at least limit the bad effect of a switching arc between the switching contacts of the switching device 7. In other words, the device 1 provides the function of a zero crossing or zero approaching of the loop voltage in case of an overcurrent. This zero crossing or zero approaching in turn is used to switch of the switching device 7.

It should be noted that the current sensor 4 is not necessarily part of the device 1. It can also be arranged out of the device 1 which is depicted in Fig. 3 by means of the current sensor 4’ (see the dashed control line). It can also be arranged within the switching device 7 (see the dash dotted control line).

The above explanation beneficially applies to applications where the switchable induction voltage source 3 reacts on an overload event and where the switching device 7 is embodied as a circuit breaker. However, the above explanation equally applies to applications where the switchable induction voltage source 3 alternatively or additionally reacts on a switch off command from a user or an external device. In that case, the switch off command for the switchable induction voltage source 3 additionally or alternatively comes from said user or said external device. So, in more general words, the device 1 provides the function of a zero crossing or zero approaching of the loop voltage in case of a switch off operation.

Concluding, the method for reducing the contact voltage between switching contacts of a switching device 7 in case of a switch off operation of the switching device 7, comprises the steps of

11 detecting a switch off demand (based on a user command, on a command from an external control and/or based on an overcurrent situation), switching on the switchable induction voltage source 3 of the device 1 as a consequence of said detection, wherein a counter voltage UC counteracts a voltage across the switching contacts of the switching device 7 and subsequently opening the switching contacts of a switching device 7.

As said, the switching contacts of the switching device 7 are opened at a point in time when the voltage across the switching contacts of the switching device 7 is zero or at least close to zero.

It should be noted that if the aforementioned equation is fulfilled, the loop voltage usually has a zero crossing because the voltage UL across the load RL, which has the same sign as the counter voltage, normally is unequal zero.

Moreover, it should be noted that the switching device 7 beneficially is embodied as vacuum interrupter. Vacuum interrupters can switch very fast because the dielectric distance, which is necessary to avoid an arc between the switching contacts, is very short. So, the zero crossing of the loop voltage which is just a transient phenomenon can be utilized in an advantageous way.

Fig. 4 now shows an example where a single switchable auxiliary DC voltage source or a single device 1 is switched in series with a plurality of switching devices 7a..7c. Here, the device 1 is designed to generate a counter voltage UC counteracting a grid voltage UG of the DC grid 6 in case of a switch off command for any of these switching devices 7a..7c.

Further on, Fig. 5 shows a first example, how the capacitor C can be loaded. In detail, the capacitor C is preloaded by an external voltage source UE. In more detail, the capacitor C, is connected to the external voltage source UE and switched away from the third coil L3 in the on-state of the switching device 7, 7a..7c and is switched away from the external voltage source UE and connected in series with the third coil L3 in case of a switch off operation of the switching device 7, 7a. 7c.

12 (Note that the switching device 7, 7a..7c is not explicitly shown in Figs. 5 to 7.)

Fig. 6 shows an embodiment, which is very similar to the embodiment shown in Fig. 5. In contrast, the capacitor C is preloaded by the grid voltage UG. In detail, the capacitor C, is connected to the grid voltage UG and switched away from the third coil L3 in the on-state of the switching device 7, 7a..7c and is switched away from the grid voltage UG and connected in series with the third coil L3 in case of a switch off operation of the switching device 7, 7a..7c.

In the examples of Fig. 5 and 6, second switches S2, S2’ are used to switch the capacitor C to and from the external voltage source UE or the grid voltage UG. Of course, a single second switch S2, S2’ works as well.

Fig. 7 shows an example where the capacitor C is connected to the grid voltage UG by means of resistors R1 , R2. Flere the capacitor C is slowly charged via the resistors R1, R2. The resistance of the resistors R1, R2 should be sufficiently high so that the capacitor C does not substantially discharge in case the counter voltage UC is generated. Advantageously, a control for the second switches S2, S2’ can be omitted in this embodiment. Of course, the capacitor C can be connected to the external voltage source UE by means of resistors R1 , R2 in a similar way. It should also be noted that a single resistor R1 , R2 works as well for connecting the capacitor C to the grid voltage UG or to the external voltage source UE.

Fig. 8 finally shows a more generalized example of the principles presented hereinbefore. The example shows a zero-voltage switch 5’, comprising a connection to a DC grid 6, a switching device 7 and a switchable auxiliary DC voltage source 8. The switchable auxiliary DC voltage source 8 is switched in series with the switching device 7 and is designed to generate a counter voltage UC counteracting a grid voltage UG of the DC grid 6 in case of a switch off operation of the switching device 7. The switchable auxiliary DC voltage source 8 can be embodied as a device 1 as shown in the Figs. 1 to 7 but it can also be designed in another way. In other words, the device 1 is a special embodiment of a switchable auxiliary DC voltage source 8.

13 A method for reducing the contact voltage between switching contacts of a switching device 7 in case of a switch off operation of the switching device 7 generally comprises the steps of: detecting a switch off demand, switching on a switchable auxiliary DC voltage source 8, which is connected in series to the switching device 7, as a consequence of said detection, wherein a counter voltage UC of the switchable auxiliary DC voltage source 8 counteracts a voltage across the switching contacts of the switching device 7 and subsequently opening the switching contacts of a switching device 7.

It should be noted that the example of Fig. 8 additionally shows a line inductance LL, an arc flash AF and a third switch S3, which is used to mitigate the arc flash AF in case of fault by short circuiting the connection to the grid.

Again, the switchable auxiliary DC voltage source 8 is used to provide a loop voltage, which is close to zero or is even zero. Again, this point in time is used to close the third switch S3 and to open the switching device 7. In an optimal case, the third switch S3 may even be omitted because the loop voltage based on the switchable auxiliary DC voltage source 8 breaks down what can mitigate the arc flash AF anyway.

It is noted that the invention is not limited to the embodiments disclosed hereinbefore, but combinations of the different variants are possible. In reality, the device 1 and the zero-voltage switch 5, 5’ may have more or less parts than shown in the figures. Moreover, the description may comprise subject matter of further independent inventions.

It should also be noted that the term "comprising" does not exclude other elements and the use of articles "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

14 LIST OF REFERENCE NUMERALS

1 device

2 magnetic core

3 switchable induction voltage source

4, 4’ current sensor

5, 5’ zero-voltage switch

6 DC grid

7, 7a. 7c switching device

8 switchable auxiliary DC voltage source a first leg of magnetic core (continuous) b second leg of magnetic core (continuous) c third leg of magnetic core (broken)

AF arc flash

C capacitor

F1 first magnetic flux

F2 second magnetic flux

F3 first magnetic flux

G air gap

I current

L1 first coil

L2 second coil

L3 third coil

LL line inductance

R1 , R2 resistor RL load

S1 first switch

15 S2, S2’ second switch S3 third switch

T1 first terminal

T2 second terminal

U1 first voltage

U2 second voltage

UC counter voltage

UE external voltage source

UG grid voltage

UL load voltage

16