WO2015062644A1

WO2015062644A1 - Circuit breaker

Info

Publication number: WO2015062644A1
Application number: PCT/EP2013/072672
Authority: WO
Inventors: Lars Liljestrand; Lars Jonsson; Marco Riva
Original assignee: Abb Technology Ltd
Priority date: 2013-10-30
Filing date: 2013-10-30
Publication date: 2015-05-07

Abstract

The present invention relates circuit breakers for breaking of an electrical current. Disclosed is a circuit breaker which comprises a current-interrupting switch; a resonant circuit connected in parallel with the current-interrupting switch, the resonant circuit comprising a circuit-closing device; and a disconnecting switch connected in series with the parallel connection of the current-interrupting switch and the resonant circuit. The current-interrupting switch and the disconnecting switch are implemented as vacuum interrupters. The circuit breaker can for example be used as a DC circuit breaker, or as an AC fault current limiter.

Description

CIRCUIT BREAKER

Technical field

The present invention relates to circuit breakers for breaking of an electrical current.

Background

In a direct current (DC) system, no natural current zero-crossings occur, and the breaking of a DC current is therefore less straightforward than the breaking of a current in an alternating current (AC) system, where current zero-crossings occur twice every period. In order to generate a current zero-crossing in a DC line, a counter voltage may for example be generated in the circuit breaker; or an oscillating current, which is larger than the system line current, could be injected through the circuit breaker. A DC circuit breaker can for example comprise a mechanical interrupter connected in parallel with a resonant circuit, where the resonant circuit is designed so as to create a counter current through the current interrupter when the current interrupter is opened. An example of such DC circuit breaker comprising a mechanical interrupter and a resonant circuit is disclosed in

WO2012/045360. The resonant circuit disclosed in WO2012/045360 comprises a pre- charged capacitor, a reactor and a switch, where the switch is open during normal operation. When a need to break the current occurs, the interrupter is opened, whereas the switch in the resonant circuit is closed so as to generate an oscillating current through the interrupter. A current zero can thereby be achieved, and the current through the DC circuit breaker can be interrupted.

DC power systems provide a number of advantages over AC power systems in many areas. This is often true for public transportation, energy collection from renewable energy sources such as wind and solar, industrial applications, long distance power transmission etc. For society to be able to efficiently exploit the advantages of DC systems, there is a need for reliable and affordable equipment for operating the DC power systems, including reliable and affordable DC circuit breakers.

Summary

The present technology relates to a circuit breaker comprising a current-interrupting switch, a resonant circuit connected in parallel with the current-interrupting switch, and a disconnecting switch connected in series with the parallel connection of the current- interrupting switch and the resonant circuit. The resonant circuit comprises a circuit- closing device. The current-interrupting switch and the disconnecting switch are implemented as vacuum interrupters. The circuit breaker can for example be used as a DC circuit breaker in a DC power system, or as an AC fault-current limiter in an AC power system.

Although not conventionally used as a disconnecting switch, a vacuum interrupter can advantageously serve the purpose of the disconnecting switch in a circuit breaker application. By incorporating into the circuit breaker the functionality of a disconnector, in the form of a disconnecting switch implemented as a vacuum interrupter, while also implementing the current-interrupting switch a as vacuum interrupter, the performance of the circuit breaker can be improved. For example, a leakage current through the circuit breaker can be interrupted by such disconnecting switch, also for large leakage currents.

Furthermore, the reliability of a closing action of the circuit breaker can be improved by including in the circuit breaker a disconnecting switch in the form of a vacuum interrupter. Since a vacuum interrupter can withstand the current surge which typically appears when a transmission line is closed, the disconnecting switch of the inventive circuit breaker could, if desired, be closed after the current-interrupting switch has been closed, so that the current-interrupting switch can close in a current-less state. By allowing the current- interrupting switch to close in a current-less state, the risk that the circuit breaker will be incapable of breaking the current, in case an opening action is required immediately upon closing, will be greatly reduced.

In one embodiment of the inventive circuit breaker, the circuit-closing device is also implemented as a vacuum interrupter. A vacuum interrupter can efficiently serve the purpose also of the circuit-closing device. For example, the stroke is generally shorter for a vacuum interrupter than for other mechanical interrupters, thus allowing for a faster closing operation. Furthermore, once closed, the vacuum interrupter stays closed until triggered to open, as opposed to e.g. spark gaps. Moreover, the timing of the actuation process of the different switches of the circuit breaker will be simplified by using the same switch type, i.e. the vacuum interrupter type, for the current-interrupting switch and the circuit-closing device. An improved performance of the circuit breaker can thereby be achieved.

In one aspect of this embodiment, the circuit-closing device is arranged to perform, upon actuation of the circuit breaker, a shorter stroke than the disconnecting switch. Hereby, a short opening time for the circuit breaker, as well as adequate electrical insulation of the circuit breaker when in the open state, can be achieved. Different strokes can for example be achieved by using vacuum interrupters of different maximum stroke, or by reducing the stroke of the circuit-closing device by means of an actuator system.

By implementing all three of the current-interrupting switch, the circuit-closing device and the disconnecting switch as vacuum interrupters, an efficient process can be used for the assembly of a circuit breaker. Although not necessarily identical, the vacuum interrupters forming the current-interrupting switch, the circuit-closing device and the disconnecting switch will typically have similar external physical dimensions. This can facilitate the assembly of the components of the circuit breaker. In one embodiment, the external dimensions of the vacuum interrupters forming the current-interrupting switch, the circuit- closing device and the disconnecting switch are the same. In one embodiment, the circuit breaker further comprises a physical structure, which holds the current-interrupting switch, the circuit-closing device and the disconnecting switch. Such structure is here referred to as a common physical structure. In one aspect of this embodiment, only the vacuum interrupters, and possibly their associated actuation system, are held by the common physical structure. In another aspect, the circuit breaker comprises further components of which at least some are also held by said physical structure. Such components can for example be a non-linear resistor, or a capacitor and/or reactor forming part of the resonant circuit. In one aspect, all components of the circuit breaker are held by said physical structure. Oftentimes, the same or a similar design of a physical structure, which is arranged to hold the vacuum interrupters of the circuit breaker, can be used for circuit breakers of different specifications, and a circuit breaker can thus easily be customized. A physical structure, which holds said three vacuum interrupters, could for example be of a design which is also used as the design of a physical structure for holding the three vacuum interrupters of a three-phase AC vacuum circuit breaker. Hereby, much time can be saved in a manufacturing plant, since both three-phase AC vacuum circuit breakers and DC circuit breakers/ AC fault current limiters can be manufactured by use of a similar assembly process.

The circuit breaker advantageously comprises an actuator system arranged to provide, in an actuation action, a force acting on the vacuum interrupters of the circuit breakers. In the embodiment wherein the circuit-closing device is also a vacuum interrupter, the actuator system could advantageously be arranged to provide, in an actuation action, a force acting on the circuit-closing device in a first direction, and a force acting on the current- interrupting switch and the disconnecting switch in the opposite direction. The actuator system could for example be arranged to, upon closing of the circuit breaker, close the interrupting switch prior to closing the disconnecting switch, thereby achieving that the circuit breaker would be ready to immediately re-open in case of a fault.

A combination of actuation techniques could be used. For example, the actuators operating to actuate the current-interrupting switch and the disconnecting switch, respectively, could be arranged to operate according to different actuation techniques. Different actuation techniques could also be combined for the actuation of one vacuum interrupter, if desired.

Further aspects of the invention are set out in the following detailed description and in the accompanying claims. Brief description of the drawings

Fig. 1 is a circuit diagram illustrating a prior art circuit breaker for interruption of a

DC current.

Fig. 2 is a circuit diagram illustrating another prior art circuit breaker for interruption of a DC current.

Fig. 3 is a circuit diagram illustrating an example of a circuit breaker having a

current-interrupting switch, a disconnecting switch and a circuit-closing device, where the current-interrupting switch and the disconnecting switch are implemented as vacuum interrupters. Fig 4 is a flowchart illustrating an example of a breaker closing action.

Fig 5 is a circuit diagram illustrating an example of a circuit breaker having a

current-interrupting switch, a disconnecting switch and a circuit-closing device, each implemented as a vacuum interrupter.

Fig. 6a is a schematic illustration of an example of a vacuum interrupter in the closed state.

Fig. 6b is a schematic illustration of an example of a vacuum interrupter in the open state.

Fig. 7a is a schematic illustration of an example of an actuator for the actuation of a vacuum interrupter of a circuit breaker.

Fig. 7b is a also a schematic illustration of an example of an actuator for the actuation of a vacuum interrupter of a circuit breaker.

Fig. 8 is an example of a wiring diagram of an embodiment of the circuit breaker shown in Fig. 5.

Fig. 9a-c illustrate different embodiments of a switch assembly comprising three vacuum interrupters and a common physical assembly onto which the vacuum interrupters are arranged.

Fig. 10 is a schematic illustration of a DC switchgear. Detailed description

An example of a prior art circuit breaker 100 for the interruption of a DC current is shown in Fig. 1. The circuit breaker 100 of Fig. 1 corresponds to a circuit breaker disclosed in WO2011/050832. The circuit breaker 100 of Fig. 1 is connected in a power transmission line 103, and comprises a current-interrupting switch 105 connected between two connection points 110a and 110b. A resonant circuit 115 is connected in parallel to the current-interrupting switch 105. Furthermore, a non- linear voltage dependent resistor 120 is also connected in parallel to the current-interrupting switch 105. The resistance of the non- linear resistor 120 decreases with increasing voltage. The current-interrupting switch 105 of Fig. 1 is mechanical, and the resonant circuit 115 of Fig. 1 comprises a capacitor 125 and an inductance 135 connected in series.

During normal operation of the circuit breaker 100, the current-interrupting switch 105 is in a closed state. However, in case of a need to break a current flowing through the current- interrupting switch 105, the current-interrupting switch 105 is opened. An arc is then built up between the breaker contacts of the current-interrupting switch 105. The arc voltage is not entirely constant, since the arc voltage decreases with increasing current, and a current/voltage oscillation is built up by means of the resonant circuit 115. In the current- interrupting switch 105, the oscillating current will be superimposed on the transmission line current. If the resonant circuit 115 is suitably dimensioned, a current zero crossing will occur, at which the arc will be extinguished. The current will then be commutated to the resonant circuit 115. Once the voltage has reached the clamping voltage of the non- linear resistor 120, the resistor 120 will change to a conducting state, and, consequently, the current will start to decrease and the remaining inductive energy stored in the transmission line 103 will be dissipated in the non-linear resistor 120.

A vacuum interrupter is one type of mechanical interrupter which can be used as the current-interrupting switch in a circuit breaker for interruption of a DC current. The use of a vacuum interrupter is advantageous in several respects: Vacuum interrupters are typically capable of interrupting oscillating currents of higher time derivative than mechanical interrupters of other types, and higher oscillation frequencies of the resonant circuit can hence be used. With a higher resonance frequency, the components of the resonant circuit can be of a smaller size. Furthermore, the time duration can be shortened between the onset of the current oscillation and the first current-zero in the current-interrupting switch 105, and fault current can therefore typically be interrupted at an earlier stage.

However, the arc voltage in a vacuum interrupter is low, and the counter voltage produced by the arc, upon opening of a vacuum interrupter, will generally not be sufficient to generate a current-zero in the interrupter. Therefore, an active resonant circuit, by means of which an oscillating current can be injected into the interrupter upon opening of the interrupter, has been used in circuit breakers wherein the current-interrupting switch is a vacuum interrupter.

Fig. 2 schematically illustrates a prior art circuit breaker 200 having an active resonant circuit 215 connected in series with a vacuum interrupter 205 and a non-linear resistor 120, as disclosed in WO2012/045360 The active resonant circuit 215 includes a series connection of a capacitor 125, an inductance 135 and a circuit-closing device 230. During normal operation of the circuit breaker 200, the vacuum interrupter 205 will be in the closed state, the circuit-closing device 230 of the resonant circuit 215 will be in the open state and the capacitor 125 will be charged to a suitable voltage. Pre-charging of capacitors is well known and will not be further described here.

In case of a need to break a current flowing through the vacuum interrupter 205, the vacuum interrupter 205 will be opened and the circuit-closing device 230 of the resonant circuit will be closed. When the circuit-closing device 230 is closed, the capacitor 125 will be discharged through the vacuum interrupter 205 and an oscillating current will occur in the resonant circuit 215, as well as through the vacuum interrupter 205. In the vacuum interrupter 205, the discharge current will be superimposed onto the transmission line current, and since the discharge current is of oscillating nature, a zero-crossing of the total current through the vacuum interrupter 205 will be achieved if the amplitude of the oscillating current is larger than the line current. The commutation of the current to the resonant circuit, and then to the non- linear resistor 120, will be similar to the current- commutation in the circuit breaker 100 of Fig. 1, where the resonant circuit 115 is passive.

According to the invention, a circuit breaker for interruption of a DC current is provided. The circuit breaker comprises a current-interrupting switch, as well as a resonant circuit having a circuit-closing device and a current source, which current source can for example be a charged capacitor. The current-interrupting switch and the resonant circuit are connected in parallel. The circuit breaker further comprises a disconnecting switch, which is connected in series with the parallel connection of the interrupter and the resonant circuit. The current-interrupting switch and the disconnecting switch are both implemented as vacuum interrupters.

Fig. 3 schematically illustrates an example of a circuit breaker 300 having a current- interrupting switch 205, a circuit-closing device 230 and a disconnecting switch 340, where the current-interrupting switch 205 and the disconnecting switch 340 are implemented as vacuum interrupters. In Fig. 3, the current-interrupting switch 205 and the disconnecting switch 340 are shown to be in the closed state, while the circuit-closing device 230 is in the open state, the circuit breaker 300 thus being in the closed state.

Although illustrated in Fig. 3 as a switch, the circuit-closing device 230 of Fig. 3 could for example be implemented as a spark gap; a vacuum spark gap; a mechanical switch; a semiconducting switch, or any other suitable device.

A circuit breaker 300 of Fig. 3 could advantageously be used as a DC circuit breaker for the breaking of a DC current. Furthermore, the circuit breaker 300 could be used as an AC fault current limiter arranged to break a fault current in one phase of an AC transmission line at any time, independently of the natural zero crossings which occur in the AC current. In this application of the circuit breaker 300, the transmission line 103 would be one phase of an AC line. If fault-current limitation is desired in a three phase AC system, one circuit breaker 300 would be required for each phase. The breaking of the current in one phase of a transmission line, at a time when no zero-crossing occurs, can, on a short time scale, be seen as breaking a DC current.

The use of disconnectors is known in DC systems, for example in a series connection with the DC breaker of Fig. 1, see WO2011/050832. The purpose of a disconnector is to isolate the two poles of a transmission line 103 from each other once a zero-current state has been reached, and a conventional disconnector is generally not capable of opening while in the current-carrying state. Although not conventionally used as a disconnecting switch, a vacuum interrupter can advantageously serve the purpose of the disconnecting switch in a DC circuit breaker application. For example, situations may arise where the leakage current through the circuit breaker, when the current through the current-interrupting switch has been interrupted, is higher than what a conventional disconnector could interrupt, for example in the order of a few amperes. In such situations, the current-breaking capability of the vacuum interrupter 340 will be highly useful.

Moreover, a disconnecting functionality in the form of a vacuum interrupter 340 has further advantages. For example, the closing action of the circuit breaker can be improved. If a line fault exists already at the closing of the circuit breaker 300, it would be

advantageous if the circuit breaker 300 could re-open immediately after closing in order to avoid that a fault current rises beyond acceptable levels. By providing a disconnecting functionality in the form of a vacuum interrupter 340, this could be achieved. A

transmission line fault 103 could for example originate in a grounding of the transmission line 103, which was made for maintenance purposes while the transmission line 103 was in a disconnected state, and which has accidentally been left in the system; or a line fault could have other causes. In a conventional circuit breaker, the current-interrupting switch 205 is closed only after any disconnector has been closed, in order to allow for the disconnector to close in a current-less state. Hence, the vacuum-interrupter 205 is the last switch to close upon closing of the circuit breaker, and the first switch to open upon opening of the circuit breaker.

Typically, there will be a short time duration after closing a vacuum interrupter during which the vacuum interrupter 205 cannot be re-opened since the actuation system of the vacuum interrupter needs to be re-loaded. Hence, when the current-interrupting switch 205 is the last switch to close upon closing, there will be a time duration after closing of the circuit breaker 300 during which the circuit breaker 300 will not be able to interrupt the current in the transmission line 103.

However, when implementing the disconnecting switch 340 as a vacuum interrupter, the order in which the current-interrupting switch 205 and the disconnecting switch 340 are closed in a closing operation of the circuit breaker 300 can be reversed: In other words, the disconnecting switch 340 could be the breaker-closing switch, i.e. the switch that, upon closing, changes the transmission line 103 from a current-less to a current-carrying state. When the disconnecting switch 340 is the breaker-closing switch, the current-interrupting switch 205 could re-open as soon as a need to open the circuit breaker 300 has been detected. This could for example be even before the moveable contact 610 of the disconnecting switch 340 has reached its final closed-state position. However, if desired, the current-interrupting switch 205 could be the breaker-closing switch also when the disconnecting switch 340 is a vacuum interrupter. Fig. 4 is a flowchart schematically illustrating the sequence of events in a closing operation of the circuit breaker 300 according to an embodiment of the invention. In a first step 400, it is ensured that the circuit-closing device 230 is opened. If the circuit-closing device 230 is a device which is normally open, such as a spark gap, this step can be omitted. Step 405 is then entered, wherein the capacitor 125 is charged to a pre-determined voltage. Step 410 is then entered, wherein the current-interrupting switch 205 is closed. Step 415 is then entered, wherein the disconnecting switch 340 is closed. When step 415 has been performed, the circuit breaker 300 is in the closed state, wherein the transmission line 103 can carry a current. As mentioned above, the closing operation of Fig. 4 is facilitated by using a vacuum interrupter as a disconnecting switch 340, and an advantage of the closing operation of Fig. 4 is that the circuit breaker 300 will be able to break a current

immediately after having been closed. However, the conventional closing operation, wherein step 415 is performed prior to step 410, could alternatively be used also when both the current-interrupting switch 205 and the disconnecting switch 340 are implemented as vacuum interrupters .

A vacuum interrupter could also efficiently serve the purpose of the circuit-closing device 230 of the resonance circuit 515. An example of an embodiment of the circuit breaker 300 wherein the circuit-closing device as well as the disconnecting switch 340 and the current- interrupting switch 205 are vacuum interrupters is schematically shown in Fig. 5. A circuit-closing device in the form of a vacuum interrupter will hereinafter be referred to as circuit-closing device 530 or circuit-closing switch 530. A resonance circuit 215 wherein the circuit-closing device is a vacuum interrupter 530 will be denoted by reference numeral 515.

Figs. 6a and 6b are schematic illustrations of an example of a vacuum interrupter 205, 340, 530 in the closed and open states, respectively. The vacuum interrupter of Figs. 6a and 6b is shown to include a vacuum bottle 600 which houses a fixed contact 605 as well as a moveable contact 610. The vacuum interrupter of Figs. 6a and 6b is further shown to comprise a first external terminal 620a connected to the moveable contact 610 via a flexible electrical connection 625, as well as a second external terminal 620b connected to the fixed contact 605. In Fig. 6b, the fixed contact 605 and the moveable contact 610 are shown to be at a maximum distance from each other, this maximum distance being referred to as the maximum stroke, S_max, of the vacuum interrupter. The direction of movement of the moveable contact upon opening and closing of the vacuum interrupter will here be referred to as the stroke direction 615 and is indicated in Fig. 6a by an arrow 615. The vacuum interrupter design shown in Fig. 6a and 6b is one example only, and vacuum interrupters of other designs may alternatively be used. For example, in Figs. 6a and 6b, the contact surface of the external terminals 620a,b both face the same direction, which is perpendicular to the stroke direction 615. In other designs, the external terminals 620a,b may be arranged in a different manner.

A circuit breaker 300 having the current-interrupting switch 205, the disconnecting switch 340 and the circuit-closing switch 530 implemented as vacuum interrupters shows similarity with a three-phase AC vacuum breaker in that both breakers include three vacuum interrupters: a three-phase AC vacuum breaker also comprises three vacuum interrupters, one for each phase. However, since natural zero-crossings occur in an AC system, there is no need for a resonance circuit or a non-linear resistance in an AC breaker, but a three-phase AC vacuum breaker is typically simply formed of three vacuum interrupters and an associated actuator system.

The vacuum interrupters in a three-phase AC vacuum breaker are typically identical, since the task of the three interrupters in a three-phase AC vacuum breaker is identical. In the circuit breaker 300 for interrupting a DC current, on the other hand, the requirements on the current-interrupting switch 205, the circuit-closing switch 530 and the disconnecting switch 340 are different in some respects. For example, regarding insulation properties, the disconnecting switch 340 will typically have to withstand the full system voltage over a longer period of time, while the current-interrupting switch 205 and the circuit-closing switch 530 are protected against over-voltages by the non-linear resistor 120. Hence, the insulation requirements are typically less strict for the current-interrupting switch 205 and the circuit-closing switch 530 than for the disconnecting switch 340. Time duration and timing of the opening process, on the other hand, is of more importance for the current- interrupting switch 205 and the circuit-closing switch 530, in order to allow for the breaking of a fault current at an early stage, while being of less importance for the disconnecting switch 340.

Although the requirements on the vacuum interrupters of the circuit breaker 300 are different in some regards, the current-interrupting switch 205, the circuit-closing switch 530 and the disconnecting switch 340 could, if desired, still be implemented as identical vacuum interrupters. Alternatively, at least one of the circuit-closing device 530 and the disconnecting switch 340 can differ from the current-interrupting switch 205, for example in terms of rating and/or stroke. In an embodiment wherein the circuit-closing device 230 is not a vacuum interrupter (cf. Fig. 3), the current-interrupting switch 205 and the disconnecting switch 340 could be different, or identical.

An actuator system for actuation of a circuit breaker 300 for breaking of a DC current is arranged to, upon actuation of a circuit breaker 300, provide an actuation force in a first direction for the actuation of the current-interrupting switch 205 and the disconnecting switch 340, and, when the circuit-closing device 220 is a vacuum interrupter, an actuation force in the opposite direction for the actuation of the circuit-closing switch 530: Upon opening of the circuit breaker 300, the actuator system will open the current-interrupting switch 205 and the disconnecting switch 340, while the circuit-closing device 530 will be closed. Upon closing of the circuit breaker 300, the actuator system will close the current- interrupting switch 205 and the disconnecting switch 340, while the circuit-closing device 530 will be opened. As discussed above in relation to Fig. 4, the actuation of the vacuum interrupters can be staggered in time, so that the actuation of vacuum interrupters of a circuit breaker 300 will, in the same actuation operation of the circuit breaker 300, be actuated at slightly different points in time. An advantage of implementing the disconnecting switch 340 as a vacuum interrupter is that the actuation of the disconnecting switch 340 can be performed also when the transmission line 103 is in a current-carrying state, as opposed to conventional disconnectors. For example, as discussed in relation to Fig. 4, the circuit breaker 300 could be arranged so that, in a closing operation of the circuit breaker 300, the current- interrupting switch 205 is closed before the closing of the disconnecting switch 340, so that the closing of the disconnecting switch 340 renders the transmission line 103 to carry a current. Alternatively, the disconnecting switch 340 could be closed first, and the current- interrupting switch 205 could be closed afterwards. In an opening operation of the circuit breaker 300, the current-interrupting switch 205 is typically opened first, and the disconnector is conventionally not opened until the transmission line 103 is in a basically current-less state. However, by use of a disconnecting switch 340 in the form of vacuum interrupter, the opening of the disconnecting switch 340 can be performed while the transmission line 103 carries a current, and could even be initiated at the same time as the opening of the current-interrupting switch. Oftentimes, however, it would be advantageous to delay the opening of the disconnecting switch until the current level has decreased to the level of the leakage current of the non-linear resistor 120. The timing of the actuation process of the different switches of the circuit breaker 300 will be simplified by using the same switch type, i.e. the vacuum interrupter type, for the current-interrupting switch 205 and the circuit-closing device 230. During opening of the circuit breaker, a situation where the circuit-closing device 230 closes, before the interrupting switch 205 is not yet open enough to withstand the recovery voltage, should typically be avoided. At the same time, in order to achieve a fast opening operation of the circuit breaker 300, it is typically advantageous to close the circuit-closing device 230 as soon as possible after the interrupting switch 205 has opened enough to withstand the recovery voltage. Hence, a higher precision in the timing of the actuation of the current- interrupting switch 205 and the circuit-closing switch 530, achieved by implementing the circuit-closing device 230 as a vacuum interrupter 530, will improve the performance of the circuit breaker 300. When the circuit-closing device of the resonant circuit is a vacuum interrupter 530, the circuit-closing device 530 will typically be triggered to close, in an opening operation of the circuit breaker 300, at the same (or approximately the same) point in time as the current-interrupting switch 205 is triggered to open.

In one embodiment, the moveable contact 610 of the current-interrupting switch 205 and that of the circuit-closing switch 530 are of approximately the same mass, so as to better predict the actuation times of the current-interrupting switch 205 and the circuit-closing switch 530.

An actuator for the actuation of a vacuum interrupter 205, 340, 530 will typically comprise a force provision system and a transmission link, where the transmission link is connected to the moveable contact 610 of the vacuum interrupter. An actuation force is provided by the force provision system and transmitted to the moveable contact 610 via the

transmission link. The actuator typically further comprises a control system arranged to trigger the force provision system upon a detected need to open or close the circuit breaker. The transmission link of an actuator which is based on electromagnetic actuation typically comprises an armature, which is made from a material which interacts with a strong magnetic field generated by the force provision system comprising a coil, so that the armature is attracted or repelled when a magnetic field is generated. A part of a transmission link which is arranged to interact with a mechanical force provision system will here be referred to as a mechanical armature.

An example of an actuator 700 for the actuation of one of the vacuum interrupters 205, 340, 530 of the circuit breaker 300 is schematically illustrated in Fig. 7a. An example of an actuator system for the actuation of a circuit breaker 300 having three (two) vacuum interrupters typically comprises three (two) actuators 700, one for each vacuum interrupter.

For illustration purposes, an actuator 700 based on electromagnetic actuation is used as an example in Fig. 7a, while other actuation techniques may be contemplated. The actuator 700 of Fig. 7a comprises a force provision system 705 in the form of conductive coils connected to a current source (not shown), and a transmission link 710 comprising an armature 715 and a drive rod 720, where the armature 715 is mechanically connected to the moveable contact 610 of the vacuum interrupter via the drive rod 720. In the embodiment shown in Fig. 7a, the armature 715 is connected to the moveable contact 610 in a stiff manner. The drive rod 720 could advantageously be of an insulating material, such as epoxy or para-aramid, so as to allow for a difference in electric potential between the armature 715 and the contacts of the vacuum interrupter. An actuating system could further include a control system (not shown in Fig. 7a) arranged to initiate the actuation of the circuit breaker 300. Such control system could for example include a processor, and the processor could for example be arranged to execute, when closing the circuit breaker 300, the method illustrated in Fig. 4. Different kinds of actuation techniques are known in the art, such as different kinds of electromagnetic actuation techniques based on e.g. eddy current repulsion (Thomson coils), attraction or repulsion of permanent magnets, or ferromagnetic attraction; or different kinds of mechanical actuation techniques, such as spring-loaded actuation, hydraulic actuation or pneumatic actuation. Any suitable actuation techniques can be used for the actuation of the inventive circuit breaker. If desired, different actuation techniques could be used for the actuation of different vacuum interrupters within the circuit breaker 300. For example, in one embodiment, the force provision system 705 for the current- interrupting switch 205 and for the circuit-closing switch 530, if any, could be based on electromagnetic actuation, while the force provision system 705 for the disconnecting switch 340 could be based on mechanical actuation. Typically, this would be cost efficient, since the actuation needs of the functions that the different vacuum interrupters perform can be individually met - typically, the current-interrupting switch 205 requires a more costly actuation technique than the disconnecting switch 340. Combinations of different actuation techniques can also be used, if desired, for the actuation of the same vacuum interrupter, or for the actuation of different vacuum interrupters within the circuit breaker 300.

In one example of the circuit breaker 300, the actuation techniques used for the current- interrupting switch 205 and a circuit-closing device implemented as a vacuum interrupter 530 are of the same type, and the moveable contact of the current-interrupting switch 205 and that of the circuit-closing switch 530 are of approximately the same mass, so as to better predict the actuation times of the current-interrupting switch 205 and the circuit- closing switch 530.

The same actuation technique can, if desired, be used for the three vacuum interrupters 205, 340, 530 of the circuit breaker 300. The actuators can be similar to the actuators of a conventional, single-pole operated three-phase AC vacuum breaker. However, the operation of the actuators will be different. For example, the three vacuum interrupters in a three-phase AC breaker are actuated in the same direction, as opposed to the actuation of the circuit breaker 300, wherein the circuit-closing switch 530 is opened when the interrupting switch 205 and the disconnecting switch 340 are closed, and vice versa.

An advantage of using a vacuum interrupter as the circuit-closing device 530 is that the stroke is generally shorter for a vacuum interrupter than for other mechanical interrupters, thus allowing for a faster closing operation than if another type of mechanical switch were used. Furthermore, once closed, the vacuum interrupter stays closed until triggered to open, as opposed to e.g. spark gaps, which could alternatively be used as the circuit-closing switch 530. As further discussed below, the stroke S of a circuit-closing switch 530 could, in order to further speed up the breaking process, be shorter than the maximum stroke of the vacuum interrupter used as the circuit-closing switch 530.

If desired, a circuit breaker 300 having a circuit-closing device 530 in the form of a vacuum interrupter 530 could be arranged so that the stroke of the circuit-closing switch 530 is shorter than the stroke of the disconnecting switch 340. A shorter stroke of the circuit-closing switch 530 facilitates for a faster opening action of the circuit breaker 300: The closing time of the circuit-closing switch 530 will be faster, and thereby, the time period between triggering of the circuit breaker 300 and completed current injection will be reduced. By providing a disconnecting switch 340 having a stroke S which is larger than that of the circuit-closing switch 530, a high electrical insulation of the disconnecting switch 340 can still be obtained. A difference in strokes between the circuit-closing switch 530 and the disconnecting switch 340 can for example be obtained by using vacuum interrupters of different maximum strokes, or by using, for the circuit-closing switch 530, an actuator 700 which limits the stroke, for example by means of a stop partway through the maximum stroke. An example of such stop 725 is schematically illustrated in Fig. 7b, where stop 725 is attached to a mechanical structure which does not move when the vacuum breaker is opened, e.g. to a physical structure holding the vacuum interrupter 530. A limited stroke could alternatively be achieved without an assigned stop 725 by adjusting other parameters of the actuator, such as for example the distance between the coils of an electromagnetic actuator.

If desired, the current-interrupting switch 205 could also have a shorter stroke than the disconnecting switch 340.

As mentioned above, a DC circuit breaker 300 which comprises a current-interrupting switch 250, a circuit-closing switch 530 and a disconnecting switch 340 implemented as vacuum interrupters 205, 530 and 340, show similarities with a three-phase AC vacuum breaker, for example in that both breaker types include three vacuum interrupters. By re- using parts of the design of a conventional AC vacuum circuit breaker, the manufacturing of the circuit breaker 300 can be simplified, as will be further described below.

The use of vacuum interrupters for the interrupting switch 205, the closing switch 530 and the disconnecting switch 340 facilitates for easy assembly of the circuit breaker 300. The different vacuum interrupters of the circuit breaker 300 will typically have similar physical dimensions. This renders the assembly of the components of the circuit breaker easier. A common physical structure can, if desired, be used for holding the three vacuum interrupters 205, 340, 530. Such common physical structure, together with the vacuum interrupters 205, 340, 530 and possibly their associated actuator system, form a switch assembly. The same or similar design of the physical structure, which is arranged to hold the vacuum interrupters, can if desired be used for circuit breakers 300 of different specifications, and a circuit breaker 300 can thus easily be customized. Fig. 8 is a wiring diagram showing an example of an implementation of the circuit breaker 300 of Fig. 5, where the three vacuum interrupters which make up the current-interrupting switch 205, the circuit-closing switch 530 and the disconnecting switch 340, respectively, are shown to be located in a common physical structure 800. The common physical structure, together with the vacuum interrupters 205, 340, 530 and possibly their associated actuator system, make up a switch assembly 805.

In the embodiment shown in Fig. 8, the common physical structure 800 comprises a support 810 onto which the current-interrupting switch 205, the circuit-closing switch 530 and the disconnecting switch 340 are attached, for example by means of nuts and bolts. Other designs of the common physical structure 800 can be contemplated. The common physical structure 800 could for example be a board onto which the vacuum interrupters are attached, a frame, a rack, a housing, or any other suitable physical structure. The common physical structure 800 is often grounded. The actuator system for actuation of the vacuum interrupters 205, 340, 530 is often also held by the common physical structure 800. In Fig. 8, the actuator system (not shown) is arranged under the support 810. This allows for easy grounding of parts of the actuator system, as desired, although other locations for the actuator system may also be contemplated.

Fig. 9a schematically illustrates a different embodiment of a switch assembly 805. The switch assembly 805 of Fig. 9a could for example be used in a circuit breaker 300 for outdoor use. The switch assembly 805 comprises a common physical assembly 800 in the form of a support 800, onto which the vacuum interrupters 205, 340, 530 are linearly arranged and attached via a respective insulator 900. Each of the vacuum interrupters 205, 340, 530 is enclosed in a further insulator 903. The switch assembly 805 could further include an actuator system 700 (not shown), located for example in the support 800 and/or in the insulators 900. A side view of a yet further example of a switch assembly 805 is schematically illustrated in Fig. 9b. The common physical structure 800 of Fig. 9b comprises a support 810 as well as a closed housing 905, which encloses all parts of the vacuum interrupters 205, 340, 530 except at least two external terminals 620a,b. At least two external terminals 620a,b protrude through the housing wall for connection to the transmission line 103, and possibly to other components of the circuit breaker 300, if not inside the housing. Such housing 905 could, if desired, be filled with an insulating gas, such as SF6; or filled with air. In many implementations, the vacuum interrupters 205, 340, 530 are individually insulated by epoxy or other solid insulating material in a conventional manner. In one implementation of the switch assembly 805, the common physical structure 800 comprises a housing 905 filled with a solid insulating material, such as epoxy.

In one example, the vacuum interrupters 205, 340, 530, and possibly their associated actuator system, are the only components which are held by the common physical structure 800, as shown in Figs. 8, 9a, 9b and 9c. This typically facilitates for the use of the same design of the common physical structure for circuit breakers 300 of different ratings. In another embodiment, other components of the circuit breaker 300, such as a capacitor 125, a non-linear resistor and/or reactor 135, are also held by the common physical structure 800. The use of a common physical structure which also holds other components can e.g. be advantageous where a large number of circuit breakers 300 of the same rating are manufactured by use of the same manufacturing process.

In an embodiment where the vacuum interrupters 205, 340, 530 and their associated actuator system 700 are the only components held by the common physical structure 800, the common physical structure 800 could, if desired, be arranged onto another structure which holds other components of the circuit breaker 300. Such other structure could for example be a board, a frame, or a housing. Hereby, is achieved that the process for assembling the switch assembly 805 can be streamlined, while allowing for a compact design of the entire circuit breaker 300. Alternatively, such common physical structure 800 will in another implementation not have any mechanical connection to the other components of the circuit breaker 300, so that the switch assembly 805 will be a freestanding unit which is only connected to the other components of the circuit breaker by means of electrical connections. Fig. 9c schematically illustrates an embodiment of a circuit breaker 300 wherein the common physical structure 800 of a switch assembly 805 is arranged onto an insulating board 910, onto which the non- linear resistor 120, the capacitor 125 and the reactor 135 are also arranged, via a respective insulator 915.

In one embodiment, at least some of the external terminals 620a,b of the vacuum interrupters 205, 340, 530 are plug-in connectors, which are arranged to co-operate with corresponding connectors of a switch-gear cubicle. The common physical structure 800 could then for example be of dimensions corresponding to those of the inner dimensions of the switch-gear cubicle, in order to facilitate for easy assembly of the vacuum interrupters 205, 340, 530 into the circuit breaker 300. In an implementation where all components of the circuit breaker 300 are arranged on a common physical structure 800/910, a plug-in DC circuit breaker 300 could be achieved if at least the external terminals forming connection points to the DC line 103 are of plug-in type. These external terminals are typically the terminal of the current-interrupting switch 205 which faces one side of the DC line 103 (cf. connection point 110a in Figs. 3 and 5), and the terminal of the disconnecting switch 340 which faces the other side of the DC line 103 (cf. connection point 345 in Figs. 3 and 5). Examples of suitable plug-in connectors include spring-loaded contacts; pin and socket contacts; blade contacts; and other contacts of plug-in type which provide, in their final position, a pre-defined contact force. Typically, the vacuum interrupters 205, 340, 530 are arranged so that the stroke direction 615 is more or less in the vertical direction. In Fig. 8 and Figs. 9a-c, the vacuum

interrupters 205, 340, 530 are placed side by side in the common physical structure 800 in a linear arrangement, so that the vacuum interrupters 205, 340, 530 are parallel and are placed along a line, i.e. the stroke direction 615 of the three interrupters are parallel and form part of the same (imagined) plane. In the illustrated examples, the external terminals 620a,b of the three vacuum interrupters face the same vertical plane. In another

implementations, the vacuum interrupters are placed in a different pattern, such as in a delta configuration, the external terminals 620a,b facing outwards. Other configurations could also be contemplated.

The common physical structure 800 can for example have three pre-defined positions for the three vacuum interrupters, in the form of pre-drilled holes for bolts to be attached; in the form of three hollows in a board; or in any other suitable form. Alternatively, the attachment of the vacuum interrupters onto the common physical structure 800 is performed without the guidance of pre-defined positions. The common physical structure 800 is typically of metal in order to allow for grounding of the structure, although other materials could alternatively be used. In one embodiment, a physical structure of a design, which is normally used for holding the vacuum interrupters of a three-phase AC breaker, is used to hold the vacuum interrupters 205, 340, 530 of a circuit breaker 300.

Other implementations of the common physical structure 800 than those discussed in relation to Figs. 8 and 9a-c can be contemplated.

In the wiring diagram of Fig. 8, the capacitor 125 is shown to be formed by a plurality of capacitors, and the non-linear resistor 120 is shown to be formed by a plurality of nonlinear resistors 120. These implementations of the non-linear resistor 120 and the capacitor 125 are shown for illustration purposes only, and any suitable design of the capacitor 125 and non- linear resistor 120 could be used. A non- linear resistor 120 can for example be implemented as a surge arrester, such as a metal oxide varistor, e.g. a zink oxide or silicon carbide resistor, or in any other suitable way. Depending on the inductance requirements, the inductance 135 could be implemented as one or more reactors, or, if the self- inductance of the resonant circuit 115 is sufficient, no separate reactor is required, and the self- inductance can serve as the inductance 135.

The resonant circuit 215/515 would typically be connected in parallel with the current- interrupting switch 205. In Figs. 3, 5 and 8, the non-linear resistor 120 is connected in parallel with the series connection of the circuit-closing device 230/530, the capacitor 125 and the inductance 135. In another implementation, the non-linear resistor 120 could be connected in parallel with the capacitor 125 only; or with the series connection of the circuit-closing device 230/530 and the capacitor 125 while the inductance 135 is connected in series with such parallel connection. Furthermore, the circuit-closing device 230/530 could be connected at any position in the series-connected resonant circuit 515, as could the other components of the resonant circuit 515.

Although a single vacuum interrupter is often sufficient to form each of the current- interrupting switch 205, the circuit-closing device 530 and the disconnecting switch 340, two or more vacuum interrupters connected in series, and/or in parallel, could be used for one or more of the switches 205, 340, 530. Hence, at least one vacuum interrupter is used to form such switch current-interrupting switch 205, the disconnecting switch 340 and, where applicable, the circuit-closing switch 530. When referring, in the above, to a switch being implemented as a vacuum interrupter, then at least one vacuum interrupter is used to form such switch.

The above described circuit breaker 300 can advantageously be used as a DC circuit breaker for the interruption of a fault current in a DC transmission line 103, or in an AC fault current limiter arranged to break a fault current in one phase of an AC transmission line 103 at any time, independently of the natural zero crossings which occur in the AC current. The present circuit breaker technology could for example be useful in a medium voltage DC or AC systems, e.g. in the voltage range of 1- 36 kV. However, the inventive circuit breaker could also be used in other voltage ranges, such as for example in the voltage ranged of 36 - 1000 kV, or higher; or in a lower voltage range, such as 500 - 1000 V.

A DC circuit breaker 300 according to the invention could for example be used for breaking the current in a single DC line 103 (cf. Figs. 3, 5, 8). A DC circuit breaker 300 according to the invention could also be used in a DC switchgear 1000, as schematically illustrated in Fig. 10. This would for example be useful in medium voltage DC systems, but also in other voltage ranges. DC switchgear 1000 of Fig. 10 comprises an incoming feeder line 1003, which is connected to a set of six outgoing feeder lines 1006 via an incoming DC circuit breaker 1004 and a busbar 1005. The DC switchgear 1000 of Fig. 10 is given as an example only, and the number of incoming feeder lines, m, and the number of outgoing feeder lines, n, could take any values for which (n+m) > 3.

In one embodiment, each of the outgoing feeder lines 1006 has an outgoing DC circuit breaker 1007, so that each outgoing feeder line 1006 is connected to each incoming feeder line via an incoming DC circuit breaker 1004 and an outgoing DC circuit breaker 1007, where at least one of the incoming DC circuit breaker 1004 or the outgoing DC circuit breaker 1007 has a current-interrupting switch 205 implemented as a vacuum interrupter and a disconnecting switch 340 implemented as a vacuum interrupter, and possibly also a circuit-closing device 530 implemented as a vacuum interrupter. The DC switchgear 1000 of Fig. 10 is a single busbar switchgear and is given as an example only. The inventive DC circuit breaker 300 is suitable for use in a DC switchgear of any configuration, for example a one-and-a-half-breaker switchgear; a two-breaker/two- busbar switchgear; a two-main-busbar/single-breaker switchgear; a one-main- and-one- auxiliary-busbar/single-breaker switchgear; a two-main-and-one-auxiliary-busbar/single- breaker switchgear; a double-bus-bar-selection switchgear; or a ring bus switchgear.

Although various aspects of the invention are set out in the accompanying claims, other aspects of the invention include the combination of any features presented in the above description and/or in the accompanying claims, and not solely the combinations explicitly set out in the accompanying claims.

One skilled in the art will appreciate that the technology presented herein is not limited to the embodiments disclosed in the accompanying drawings and the foregoing detailed description, which are presented for purposes of illustration only, but it can be

implemented in a number of different ways, and it is defined by the following claims.

Claims

1. A circuit breaker (300) comprising

a current-interrupting switch (205);

a resonant circuit (215; 515) connected in parallel with the current-interrupting switch, the resonant circuit comprising a circuit-closing device (230; 530); and

a disconnecting switch (340) connected in series with the parallel connection of the current-interrupting switch and the resonant circuit; wherein

the current-interrupting switch (205) is implemented by means of at least one vacuum interrupter; and

the disconnecting switch (340) is implemented by means of at least one vacuum interrupter.

2. The circuit breaker (300) of claim 1, further comprising:

an actuator system (700) for actuation of the opening and/or closing of the circuit breaker (300), wherein

the actuator system is arranged to, upon closing of the circuit breaker, close the current-interrupting switch (205) prior to closing the disconnecting switch (340).

3. The circuit breaker (300) of claim 1 or 2, wherein

the circuit-closing device (530) is implemented by means of least one vacuum interrupter.

4. The circuit breaker (300) of claim 3, wherein

the circuit breaker comprises an actuation system (700) arranged to provide, in an actuation action, a force acting on the circuit-closing device (530) in a first direction, and a force acting on the current-interrupting switch (205) and the disconnecting switch (340) in the opposite direction.

5. The circuit breaker (300) of claim 3 or 4, wherein

the circuit-closing device (530) is arranged to perform, upon an opening or closing operation of the circuit breaker, a shorter stroke (S) than the disconnecting switch (340).

6. The circuit breaker (300) of any one of claims 3-5, further comprising a physical structure (800), wherein

said physical structure holds the current-interrupting switch (205), the circuit- closing device (530) and the disconnecting switch (340) to form a switch assembly (805).

7. The circuit breaker (300) of claim 6, wherein

said physical structure (800) is of a design which is also used as the design of a physical structure for holding three vacuum interrupters of a three-phase AC vacuum circuit breaker.

8. The circuit breaker (300) of any one of the above claims, wherein

the resonant circuit (515) comprises a capacitor (125);

the circuit breaker further comprises a non-linear resistor (120) connected in parallel with the capacitor; and

the current-interrupting switch (205), the circuit-closing device (530) and the disconnecting switch (340) are held by a physical structure (800) which does not hold at least one of the capacitor and the non-linear resistor (120).

9. The circuit breaker (300) of any one of the above claims, wherein

the actuator system comprises a first actuator (700) for actuating the current- interrupting switch and a second actuator for actuating the disconnecting switch, wherein the first and second actuators are arranged to operate according to different actuation techniques.

10. The circuit breaker (300) of any one of the above claims, wherein

the current-interrupting switch (205) is arranged to perform, upon an opening or closing operation of the circuit breaker, a shorter stroke (S) than the disconnecting switch (340).

11. The circuit breaker (300) of any one of the above claims, wherein

said vacuum interrupters are of the same voltage and/or current rating.

12. The circuit breaker (300) of any one of the above claims, wherein the circuit breaker is an AC fault current limiter.

13. The circuit breaker (300) of any one of claims 1-11, wherein

the circuit breaker is a DC circuit breaker.

14. A DC switchgear (1000) comprising

at least one DC circuit breaker (300; 1004, 1007) according to claim 14.

15. A power system comprising the circuit breaker of any one of claims 1-14.

16. A method of operating a circuit breaker (300), wherein

the circuit breaker comprises:

a current-interrupting switch (205);

a resonant circuit (215; 515) connected in parallel with the current- interrupting switch, the resonant circuit comprising a circuit-closing device (230; 530);

the current-interrupting switch (205) and the disconnecting switch (340) are vacuum interrupters, the method comprising:

closing (410), upon closing of the circuit breaker, the current-interrupting switch prior to closing (415) the disconnecting switch.