Circuit breaker

Abstract

The circuit breaker according to the invention comprises a power source terminal (A), a load terminal (B), a main current-interrupting mechanical switch (MS) connected between the power source terminal (A) and the load terminal (B), and a commutation path (S1, C, L, S2; D1, L, S1, C2, S2; D1, S1, C2, L2, S2). The commutation path is connected in parallel to the main current-interrupting mechanical switch (MS) and comprises, in series, a capacitor (C; C2), an inductance (L; L2), a first auxiliary current-interrupting mechanical switch (S1) and a second auxiliary current-interrupting mechanical switch (S2). The first and second auxiliary current-interrupting mechanical switches (S1, S2) are connected to each other in the commutation path via at least the capacitor (C; C2) and are actuated simultaneously.

Description

[0001] Circuit breaker

[0002] The present invention relates to a circuit breaker, more particularly to a DC circuit breaker for low, medium or high voltages.

[0003] International patent application WO 2015 / 062644 discloses a circuit breaker comprising a main current-interrupting switch connected in a power transmission line, a commutation path connected in parallel to the main current-interrupting switch and comprising a capacitor, an inductance and an auxiliary currentinterrupting switch connected in series, the commutation path forming a resonant circuit with the main current-interrupting switch, a non-linear resistor connected in parallel to the main current-interrupting switch and to the commutation path and a disconnecting switch connected in the power transmission line, in series with the parallel connection of the main current-interrupting switch, the commutation path and the non-linear resistor.

[0004] The use of a disconnecting switch for galvanically isolating the load in such a circuit breaker has several disadvantages. A first disadvantage is that it must withstand the load current of the power transmission line and the short-time short- circuit current and, in this respect, must meet many requirements. Another disadvantage is that it cannot be actuated to galvanically isolate the load immediately after the interruption of current at the main switch.

[0005] The present invention aims at remedying these disadvantages, at least partially.

[0006] To this end there is provided a circuit breaker comprising:

[0007] - a power source terminal,

[0008] - a load terminal,

[0009] - a main current-interrupting switch connected between the power source terminal and the load terminal, - a commutation path connected in parallel to the main currentinterrupting switch and comprising, in series, a capacitor, an inductance and a first auxiliary current-interrupting switch, characterized in that the commutation path further comprises a second auxiliary current-interrupting switch separated from the first auxiliary currentinterrupting switch by at least the capacitor and controlled synchronously with the first auxiliary current-interrupting switch.

[0010] More particularly, the main current-interrupting switch and the first and second auxiliary current-interrupting switches are mechanical switches and are actuated simultaneously. This provides an intrinsic galvanic isolation of the load.

[0011] The capacitor, the inductance and the first auxiliary switch in the commutation path may be arranged in any order. In general manner, in the present document, when a series connection of elements is mentioned, the elements are not necessarily connected in the order in which they are cited. The inductance may be the self-inductance of the cables of the commutation path supplemented or not by a winding. The capacitor may consist of a plurality of capacitors connected in series and / or in parallel.

[0012] The second auxiliary switch may replace the disconnecting switch of the circuit breaker disclosed in WO 2015 / 062644 and ensure the galvanic isolation of the load in association with the main current-interrupting switch. Since it intervenes only during the current interruption at the main switch and is not in the power transmission line (like the first auxiliary switch), the second auxiliary switch need not meet the requirements of a disconnecting switch. Moreover, the second auxiliary switch can provide galvanic isolation of the load immediately after the current interruption at the main switch because, contrary to a disconnecting switch, it takes part in the current interruption process.

[0013] Another advantage of the presence of both auxiliary switches is that they enable the capacitor to be pre-charged by the voltage of the power source before the making of the main switch without the need for a high-cost insulated external power source.

[0014] The circuit breaker of the present invention may comprise one or more of the following additional features:

[0015] - the main and auxiliary current-interrupting switches are vacuum interrupters;

[0016] - the first and second auxiliary current-interrupting switches are mechanically actuated by the same actuator;

[0017] - the first and second auxiliary current-interrupting switches and the main current-interrupting switch are mechanically actuated by the same actuator;

[0018] - an energy absorber is connected in parallel to a portion of the commutation path comprising the capacitor;

[0019] - at least one of the first and second auxiliary current-interrupting switches is connected between the energy absorber and the main currentinterrupting switch. This way of connecting the energy absorber enables to avoid the use of a serial disconnector;

[0020] - in the commutation path the capacitor and the inductance are connected between the first auxiliary current-interrupting switch and the second auxiliary current-interrupting switch, and the energy absorber is connected between the first auxiliary current-interrupting switch and the second auxiliary current-interrupting switch;

[0021] - in the commutation path the capacitor and the inductance are connected between the first auxiliary current-interrupting switch and the second auxiliary current-interrupting switch, and the energy absorber is connected between the power source terminal and the second auxiliary current-interrupting switch and is connected in parallel to the series connection of the first auxiliary current-interrupting switch, the capacitor and the inductance. By virtue of these features, the energy absorber is not energized except during the current breaking;

[0022] - the energy absorber is a non-linear resistor;

[0023] - the capacitor is pre-charged;

[0024] - a first resistor and a second resistor are connected to the commutation path so as to enable the capacitor to be pre-charged by a current coming from the power source terminal;

[0025] - the first resistor is part of a first path from the power source terminal to a first terminal of the capacitor, the second resistor is part of a second path from a second terminal of the capacitor to a second power source terminal, and no switch is provided in the first path and no switch is provided in the second path. These features facilitate the pre-charging of the capacitor, which is pre-charged without the need of actuating a switch;

[0026] - a diode (which can be a diode set) is provided, which is connected in parallel to the main current-interrupting switch when the second auxiliary current-interrupting mechanical switch is in a closed state;

[0027] - a diode (which can be a diode set) is provided, which is connected in parallel to a portion of the commutation path comprising the capacitor;

[0028] - at least one of the first and second auxiliary current-interrupting switches is connected between the diode and the main current-interrupting mechanical switch. This way of connecting the diode enables to avoid the use of a serial disconnector;

[0029] - a diode is connected in parallel to the series connection of the first auxiliary current-interrupting switch, the capacitor and the inductance and is connected between the power source terminal and the second auxiliary current-interrupting switch. With this manner of connecting the diode, the diode is not energized except during the current breaking; - a second commutation path is provided, which is connected in parallel to the main current-interrupting switch and comprises, in series, the first and second auxiliary current-interrupting switches, said inductance or a second inductance and a second capacitor. The provision of such an additional commutation path, sharing at least the auxiliary switches with the first commutation path but having its own capacitor, enables to optimize the current breaking depending on the direction of the current in the main current-interrupting switch;

[0030] - said (first) commutation path further comprises a first diode and said second commutation path further comprises a second diode;

[0031] - the second auxiliary current-interrupting switch is connected between the energy absorber and the load terminal and the energy absorber is connected directly to the power source terminal;

[0032] - the second resistor enables the second capacitor to be pre-charged by a current coming from the power source terminal.

[0033] Other features and advantages of the present invention will be apparent upon reading the following detailed description with reference to the accompanying drawings in which:

[0034] - figure 1 is a circuit diagram illustrating a circuit breaker according to a first embodiment of the invention;

[0035] - figure 2 is a diagram illustrating a mechanical operating sequence of three vacuum interrupters of the circuit breaker according to the first embodiment of the invention;

[0036] - figure 3 is a diagram illustrating an alternative mechanical operating sequence of the three vacuum interrupters of the circuit breaker according to the first embodiment of the invention;

[0037] - figure 4 is a circuit diagram illustrating a circuit breaker according to a variant of the first embodiment of the invention; figure 5 is a circuit diagram illustrating the circuit breaker according to the first embodiment of the invention, provided with resistors for charging a capacitor of the circuit breaker;

[0038] - figure 6 is a circuit diagram illustrating a circuit breaker according to a second embodiment of the invention;

[0039] - figure 7 is a circuit diagram illustrating a circuit breaker according to a third embodiment of the invention;

[0040] - figure 8 is a circuit diagram illustrating a circuit breaker according to a variant of the third embodiment of the invention;

[0041] - figure 9 is a circuit diagram illustrating the circuit breaker according to the said variant of the third embodiment of the invention, provided with resistors for charging capacitors of the circuit breaker.

[0042] An electrical device according to the schematic illustration of figure 1 , which will be able to make or break a current between a power source terminal A and a load terminal B, comprises the following components:

[0043] - MS: a vacuum interrupter connected in the power transmission line and operating as the main switch of the device.

[0044] - ACTMS: actuator for driving the main switch MS.

[0045] - S1 , S2: two vacuum interrupters operating as auxiliary switches, whose operations are fully synchronized and operated preferably by a common actuator ACT.

[0046] - ACT: actuator for driving simultaneously both auxiliary switches S1 and S2.

[0047] - EA: device performing as a line energy absorption and line current decrease. This function can be obtained by a non-linear resistor like a metal oxide varistor (MOV), for example a zinc-oxide varistor, which has also a voltage limiting function. The EA element can be another type of varistor, or another non-linear resistor like a PTC resistor, or a semiconductor type like transorb, or a liquid metal arcing energy absorption element.

[0048] - C: capacitor pre-charged at a specified voltage, in the parallel path, to be discharged during the switching operation.

[0049] - L: serial inductance of the discharge circuit, which can be the cable inductance of the loop MS, S1 , C, S2.

[0050] The loop or circuit MS, S1 , C, L, S2 is a resonant circuit. The path or branch S1 , C, L, S2 is a commutation path which diverts current from the power transmission line when the auxiliary switches S1 , S2 are closed. The second auxiliary switch S2 is preferably directly connected to the load terminal B.

[0051] For the DC current breaking using a vacuum interrupter as a main switch, there is a need to connect on both poles of the main switch MS an additional parallel path comprising at least a capacitor C and an inductance L for transient current commutation, and preferably an energy absorption path comprising an energy absorber EA, across both auxiliary switches S1 and S2.

[0052] Figure 2 shows the mechanical operating sequence and three mechanical positions. At rest, all three vacuum interrupters MS, S1 and S2 are open. The current making to feed the load B is performed whenever closing the vacuum interrupter MS, while S1 and S2 are kept open. The current breaking sequence is obtained by first opening the main switch MS and just after closing simultaneously both auxiliary switches S1 and S2. The simultaneous closing of S1 and S2 will provoke the discharge of the capacitor C across the main switch MS creating an inverse current and forcing the zero crossing of the current through the main switch MS, this will happen when ic equals ii_ inducing a current interruption in the main switch MS path. Once the current through the main switch MS is interrupted, the whole current will flow through the auxiliary switches S1 and S2 and the capacitor C will be charged again but with the opposite voltage. The increase of the voltage across the capacitor C will be limited thanks to the presence of the parallelly connected energy absorber EA. At this point the auxiliary switches S1 and S2 will open, and the current is still flowing through the vacuum arcs in the auxiliary switches S1 and S2. The increase of voltage across the energy absorber EA will make the line current flowing through the auxiliary switches S1 and S2 decrease enough and then fall below a critical value corresponding to the chopping current, which is an intrinsic characteristic of the vacuum interrupters S1 and S2. At this point the current flowing through the device is cleared and moreover the galvanic isolation of the load against the source will be fully restored.

[0053] Figure 4 represents a variant of the embodiment of figure 1 , in which one pole of the energy absorber EA is connected directly to the source terminal A. In this case, the current through the first auxiliary switch S1 is interrupted earlier than in the second auxiliary switch S2, at the moment of current commutation from the capacitor C to the energy absorber EA.

[0054] Once the isolation across the circuit breaker is restored, the capacitor C can be charged again with the required voltage and polarity. Preferably, it is charged by the source voltage through resistors Rhigh and Riow, in which the resistor Rhigh is connected to the source terminal A and the resistor Riow is connected to ground G as shown in Fig 5, assuming G is the grounded pole of the power source.

[0055] Important features of the device of figures 1 , 4 and 5:

[0056] • Galvanic isolation of the load side against the source side is obtained immediately after current interruption due to both vacuum gaps of the main switch MS and the second auxiliary switch S2 without the need of an additional serial disconnector.

[0057] • Neither the first auxiliary switch S1 nor the second auxiliary switch S2 must withstand the permanent thermal current flowing through the circuit breaker. Moreover, the auxiliary switches S1 , S2 do not have to withstand the short- time short-circuit current, only the main switch MS must withstand such a current. If a serial disconnector was implemented, it would have to withstand those currents. • Due to the presence of both auxiliary switches S1 and S2 the capacitor can be charged by the source voltage as shown in figure 5 without the need of an additional high-cost insulated high-voltage external DC power source, and can be ready to use even before the making of the main switch MS.

[0058] • The embodiment of figure 4 gives rise to additional interesting features. Most of the time, the energy absorber EA is not under bias, which eliminates the well-known risk of thermal runaway of the energy absorber EA. Because when an energy absorber like EA is permanently biased with a voltage close to its rating, a leakage current will flow through it, which heats up the energy absorber in its idle state. At that point, the excess of temperature of the energy absorber will dramatically decrease the amount of line energy that it can absorb. This must be paid by choosing a bigger size energy absorber. Moreover, with a permanent leakage current, the aging effect is increased, and the lifetime of the energy absorber is drastically decreased. The solution shown in this embodiment gives the possibility to choose an energy absorber EA whose voltage limit is not too high and moreover of smaller size.

[0059] • In case of fast-switching requirement it is well known that it is possible to implement a Thomson coil as the actuator for the main switch MS. Here it is also possible to implement a Thomson coil as the actuator for the auxiliary switches S1 and S2 for fast-switching operation.

[0060] • The common actuator ACT of the auxiliary switches S1 and S2 ensures a synchronized brief closing and immediate opening of these two components.

[0061] • The actuator ACT in another embodiment like in figure 3 can also operate simultaneously the main switch MS in opposite direction to the auxiliary switches S1 and S2. In this case three different positions are also required: middle position (rest position), where all the switches MS, S1 and S2 are open; making position, where the main switch MS is closed to feed the load B, and the auxiliary switches S1 and S2 are further opened; breaking position, where the auxiliary switches S1 and S2 are closed, and the main switch MS is largely open. The last position is a brief operation during switching and is always immediately followed by the opening of the auxiliary switches S1 and S2 and bringing back all the switches to the middle position (rest position).

[0062] As said in the above description, the current flowing through the main switch MS will be cleared at zero crossing, that is when ic = i , and more accurately said, when the current through the main switch MS is below the chopping current level ichop characteristic of such vacuum interrupter, that is when II i - ic II ichop. It is well known by the specialists of vacuum interrupters that for some reasons the plasma (arc) in the interrupter may fail to extinguish, leading to current increase in the opposite direction in the vacuum interrupter MS, which in such a case will likely provoke a breaking failure. This failure can be easily observed when increasing the di / dt at zero crossing of the current in the vacuum interrupter MS.

[0063] Given the explanation of this behavior, at current zero crossing, the decay of the plasma density and temperature inside the interrupter needs some time to meet the current interruption conditions and restore the electrical isolation. In case such a condition is not met, a reignition of the plasma after current zero crossing may occur.

[0064] The classical means to prevent such unwanted situation is to decrease the di / dt characteristic of the capacitor discharging loop, by decreasing the resonant frequency of the LC circuit, but this must be paid in terms of a higher energy capacitor C.

[0065] Another way to do in the present invention, is to insert a diode D in the discharging circuit as shown in figure 6 to limit the voltage across the interrupter when the capacitor current becomes higher than the line current, i.e. when ic > ii_. The diode D may consist of a plurality of diodes connected in series and / or in parallel. Obviously, the diode D when starting conduction will prevent its voltage from growing over its own forward drop voltage. Since the diode is directly connected in parallel with the main switch MS when the second auxiliary switch S2 is closed, it will limit the voltage increase of the plasma to a very low value. The forward voltage of the diode (some volts) is much smaller than the arc voltage of the interrupter (around 20 V), that will force the plasma to cool down and collapse in the interrupter.

[0066] The upgraded setup of figure 7 with two diodes D1 and D2 and two capacitors C1 and C2 gives rise to the possibility to force a clear zero voltage, this as soon as the line current ii_ becomes smaller than the corresponding capacitor current id or iC2 having an opposite direction to ii_. In this configuration we do not only limit the voltage across the main switch MS as stated before, but we force a zero voltage across the main switch MS, thanks to the zero sum of the opposite forward voltages drops of both diodes D1 and D2 applied directly on the main switch MS when the second auxiliary switch S2 is closed. Moreover, at the breaking operation, it will work always like that, regardless of the direction of the line current ii_. In this embodiment, the commutation path is formed by D1 , L, S1 , C2 and S2 if the current II in the power transmission line is flowing from the power source to the load (as illustrated in figure 7), and is formed by C1 , S1 , L, D2 and S2 if the current in the power transmission line is flowing from the load to the power source.

[0067] Important features of the device of figures 6 to 9:

[0068] • The improvement brought by one or two diodes gives the possibility to increase the LC resonant frequency by decreasing the inductance L, making it possible to break higher currents than without diodes, without increasing the capacitor size. • Galvanic isolation of the load side against the source side is also obtained immediately after current interruption without the need of an additional disconnector, thanks to the switches MS and S2.

[0069] • In these setups the thermal current and the short-time short circuit current are still withstood only by the main switch MS.

[0070] • Another embodiment of the two diodes setup is to implement different sets C1 -L1 and C2-L2 according to different breaking requirements on positive ii_ and on negative ii_ as shown in Fig 8. For example, if the breaking requirement for negative ii_ is smaller compared to the breaking requirement for positive i , there is the opportunity to install a smaller and cheaper capacitor on the C1 side than on the C2 side, the opposite statement being obviously true as well.

[0071] • Due to the presence of both auxiliary switches S1 and S2, both capacitors C1 and C2 can be charged by the source voltage as shown in figure 9 without the need of a high-cost insulated high-voltage external power source.

[0072] The present invention is not limited to the embodiments disclosed in the above description. In particular, any combination of the embodiments of figures 1 to 9 forms part of the present disclosure.

Claims

CLAIMS1 . A circuit breaker comprising:- a power source terminal (A),- a load terminal (B),- a main current-interrupting mechanical switch (MS) connected between the power source terminal (A) and the load terminal (B), and- a commutation path (S1 , C, L, S2; D1 , L, S1 , C2, S2; D1 , S1 , C2, L2, S2) connected in parallel to the main current-interrupting mechanical switch (MS) and comprising, in series, a capacitor (C; C2), an inductance (L; L2), a first auxiliary current-interrupting mechanical switch (S1 ) and a second auxiliary current-interrupting mechanical switch (S2), wherein the first and second auxiliary current-interrupting mechanical switches (S1 , S2) are connected to each other in the commutation path via at least the capacitor (C; C2) and are actuated simultaneously.

2. The circuit breaker of claim 1 , wherein the main current-interrupting mechanical switch (MS) and the first and second auxiliary currentinterrupting mechanical switches (S1 , S2) are vacuum interrupters.

3. The circuit breaker of claim 1 or 2, wherein the first and second auxiliary current-interrupting mechanical switches (S1 , S2) are mechanically actuated by the same actuator (ACT).

4. The circuit breaker of claim 1 or 2, wherein the first and second auxiliary current-interrupting mechanical switches (S1 , S2) and the main currentinterrupting mechanical switch (MS) are mechanically actuated by the same actuator (ACT).

5. The circuit breaker of any of claims 1 to 4, further comprising an energy absorber (EA) connected in parallel to a portion of the commutation path comprising the capacitor (C; C2).

6. The circuit breaker of claim 5, wherein at least one of the first and second auxiliary current-interrupting mechanical switches (S1 , S2) is connected between the energy absorber (EA) and the main current-interrupting mechanical switch (MS).

7. The circuit breaker of claim 5 or 6, wherein in the commutation path the capacitor (C) and the inductance (L) are connected between the first auxiliary current-interrupting mechanical switch (S1 ) and the second auxiliary current-interrupting mechanical switch (S2), and wherein the energy absorber (EA) is connected between the first auxiliary currentinterrupting mechanical switch (S1 ) and the second auxiliary currentinterrupting mechanical switch (S2).

8. The circuit breaker of claim 5 or 6, wherein in the commutation path the capacitor (C) and the inductance (L) are connected between the first auxiliary current-interrupting mechanical switch (S1 ) and the second auxiliary current-interrupting mechanical switch (S2), and wherein the energy absorber (EA) is connected between the power source terminal (A) and the second auxiliary current-interrupting mechanical switch (S2) and is connected in parallel to the series connection of the first auxiliary currentinterrupting mechanical switch (S1 ), the capacitor (C) and the inductance (L).

9. The circuit breaker of any of claims 5 to 8, wherein the energy absorber (EA) is a non-linear resistor.

10. The circuit breaker of any of claims 1 to 9, wherein the capacitor (C; C2) is pre-charged.

11. The circuit breaker of any of claims 1 to 10, further comprising a first resistor (Rmgh) and a second resistor (Riow) connected to the commutation path so as to enable the capacitor (C; C2) to be pre-charged by a current coming from the power source terminal (A).

12. The circuit breaker of claim 11 , wherein the first resistor (Rhigh) is part of a first path from the power source terminal (A) to a first terminal of the capacitor (C; C2), the second resistor (Riow) is part of a second path from a second terminal of the capacitor (C; C2) to a second power source terminal (G), and wherein no switch is provided in the first path and no switch is provided in the second path.

13. The circuit breaker of any of claims 1 to 12, further comprising a diode (D; D2) which is connected in parallel to a portion of the commutation path comprising the capacitor (C; C2).

14. The circuit breaker of claim 13, wherein at least one of the first and second auxiliary current-interrupting mechanical switches (S1 , S2) is connected between the diode (D; D2) and the main current-interrupting mechanical switch (MS).

15. The circuit breaker of claim 7, further comprising a diode (D) connected in parallel to the series connection of the first auxiliary current-interrupting mechanical switch (S1 ), the capacitor (C) and the inductance (L) andconnected between the power source terminal (A) and the second auxiliary current-interrupting mechanical switch (S2).

16. The circuit breaker of any of claims 1 to 12, further comprising a second commutation path (S2, D2, L, S1 , C1 ; S2, D2, S1 , C1 , L1 ) connected in parallel to the main current-interrupting mechanical switch (MS) and comprising, in series, the first and second auxiliary current-interrupting mechanical switches (S1 , S2), said inductance (L) or a second inductance (L1 ) and a second capacitor (C1 ).

17. The circuit breaker of claim 16, wherein said commutation path (D1 , L, S1 , C2, S2; D1 , S1 , C2, L2, S2) further comprises a first diode (D1 ) and said second commutation path (S2, D2, L, S1 , C1 ; S2, D2, S1 , C1 , L1 ) further comprises a second diode (D2).

18. The circuit breaker of claim 16 or 17 when dependent on claim 5, wherein the second auxiliary current-interrupting mechanical switch (S2) is connected between the energy absorber (EA) and the load terminal (B) and the energy absorber (EA) is connected directly to the power source terminal (A).

19. The circuit breaker of any of claims 16 to 18 when dependent on claim 11 or 12, wherein the second resistor (Riow) enables the second capacitor (C1 ) to be pre-charged by a current coming from the power source terminal (A).

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