Technical field

[0001 ] The invention relates to fast-acting circuit-breakers that include

mechanical current interrupters, and in particular to actuators that drive the movable contact in such current interrupters with run-time control.

Background art

[0002] When load current flows through the mechanical contacts in a

conventional, mechanical circuit-breaker for alternating current (AC) and the contacts are being separated, the load current continues to flow in an arc between the mechanical contacts until a current zero cross-over occurs. As such current zero-crossing is not created by the circuit-breaker, it is a prerequisite for proper function of the circuit-breaker that the external circuit brings about a current cross- over before the maximum allowed arcing time has elapsed. In AC applications with 50 or 60 Hz nominal frequency such current zero-crossings naturally appear with a periodicity of 100 or 120 Hz.

[0003] Circuit-breakers with capability to interrupt even non-zero current, i.e. performing current interruption independent of the external network, are requested for applications in direct current (DC) networks and in current-limiting circuit- breakers in AC networks. Both these applications require very short operating time, typically current growth at a short-circuit must be neutralized already after a few milliseconds.

[0004] Mechanical current interrupters, like vacuum interrupters, together with auxiliary circuits that create artificial current zero cross-over in the arc flowing between the contacts at contact separation, can be used to interrupt the load current in such fast-acting circuit-breakers.

[0005] The position of the movable contact in such current interrupter is controlled by an actuator that includes a force generating device and a mechanical drive train that connects the force generating device, i.e. the actuator, to the movable contact. Fig. 1 shows an outline of a prior art current interrupter 1 comprising a contact arrangement, generally designated 10, adapted to interrupt current ii _oad flowing in a current-carrying line 2. The contact arrangement 10 comprises a first, fixed contact 12 and a second, movable contact 14. The movable contact 14 is mechanically connected to a mechanical drive train 20 which may comprise components such as an over-travel mechanism 22 and a bi- stable mechanism 24. In the end opposite to the end connected to the current interrupter 10, the mechanical drive train 20 is mechanically connected to an actuator, i.e. , a force-generating device 30.

[0006] The actuator 30 performs both opening and closing operations on the contacts 12 and 14. Operating time for opening operations is critical and the system for opening operations will be discussed in the following text.

[0007] The force-generator in the actuator 30 typically is an electro-magnetic device, most often based on the Thomson coil principle, see Fig. 2. According to this, a Thomson coil consists of a flat coil 32 facing a flat metallic armature disc 34, which is arranged concentrically close to of the flat coil 32. When a current is forced through the coil 32, a mirror current is induced in the armature disc 34 and a strong repulsive force is created between the coil 32 and the armature disc 34. The current driven into the coil 32 normally comes from a current driver 40 with an energy storage 42 in the form of a charged capacitor connected to the coil 32 through a switch 44, typically a semiconductor switch such as a thyristor, which controls the start of the discharge of the energy storage. When the start of the discharge has been initiated, the discharge current from the capacitor 42 cannot be controlled in any way. A second coil 32’, placed below the disk 34, may be provided to close the contacts 12 and 14.

[0008] The short operating time in the current interrupter calls for extremely fast acceleration and retardation, ranging up to thousand times gravity, of the movable contact 14 in the contact arrangement 10. Accordingly, very high force, causing extreme stress of the mechanical components, is exerted on the actuator 30 and the mechanical drive train 20 pushing and pulling the movable contact 14. [0009] Applying a design philosophy where the actuator 30 always creates and applies the full force that is needed in a worst-case scenario leads to short life span of the circuit-breaker due to the wear and tear caused by the extreme stress applied on the actuator, the drive train and the current interrupter. The stress is further worsened when various tolerances, like initial capacitor voltage, initial distance between Thomson coil and armature, coil and capacitor resistance etc., shall be considered. Furthermore, the use of semiconductor switches that only control the turn-on instant, makes it impossible to influence the driving current once the course has been initiated.

[0010] Use of such Thomson coil based actuator for circuit breakers is known from the patent publication EP 0 563 904 B1 , and particularly in the case of DC circuit breaker from WO 2012045360 A. A magnetic flux concentrator is known from EP 3143631 A1 for improved performance.

Summary of invention

[0011 ] An object of the present invention is to provide a circuit breaker having long life span thanks to improved control of the opening and closing operation of the current interrupter.

[0012] According to a first aspect of the invention, there is provided a current interrupter for a circuit-breaker comprising: a contact arrangement adapted to interrupt current and comprising a first contact and a second, movable contact; a mechanical drive train mechanically connected to the movable contact; an actuator mechanically connected to the mechanical drive train and comprising an electro- magnetic force-generating device; a current driver comprising an energy storage connected to the actuator for energizing the electro-magnetic force-generating device; and a current reference generator adapted to provide a current reference signal to the current driver; which is characterized in that the current driver comprises a current controller comprising semiconductor devices having turn-off capability and being adapted to provide switched mode control of discharge current from the energy storage in accordance with the received current reference signal from the reference generator, wherein the current reference signal from the reference generator (50) is derived from conditions measured in run-time.

[0013] In a preferred embodiment, the electro-magnetic force-generating device comprises a Thomson coil.

[0014] In a preferred embodiment, the electro-magnetic force-generating device comprises a first coil for opening the contacts and a second coil for closing the contacts wherein the first and second coils share a common armature disc and each coil has a corresponding current controller.

[0015] In a preferred embodiment, the electro-magnetic force-generating device comprises a double-coil arrangement.

[0016] In a preferred embodiment, the switched mode control comprises pulse- width-modulation control.

[0017] In a preferred embodiment, the current reference signal from the reference generator is derived from conditions measured in run-time.

[0018] According to a second aspect of the invention, a circuit-breaker comprising a current interrupter according to the invention is provided.

[0019] According to a third aspect of the invention, a system of circuit-breakers comprising a plurality of current interrupters according to invention is provided, wherein the electro-magnetic force-generating devices of several actuators are connected to a common energy storage.

According to a fourth aspect of the invention, a method of controlling the mechanical stress on the mechanical drive train and the actuator in a current interrupter according to the invention is provided, which is characterized by the steps of: adapting the current reference signal from the current reference generator, using the current controller to provide a current requested by the current reference generator, thereby controlling current provided from the energy storage to the electro-magnetic force-generating device according to a switched- mode control scheme in dependence of detected conditions, wherein the current reference generator (50) receives input signals from conditions measured in run- time to provide the reference signal to the current controller (46)..

[0020] In a preferred embodiment, the current reference generator receives input signals from conditions measured in run-time to provide the reference signal to the current controller.

[0021 ] In a preferred embodiment, the step of detecting a condition comprises detecting any of the actual level of the current and the derivative of the current to be interrupted and the type of operation to be executed.

[0022] In a preferred embodiment, the detected conditions comprise internal conditions inside the actuator, at least one of the following: distance between coil and armature in the electro-magnets force-generating device; and acceleration, speed and position of the movable contact.

[0023] In a preferred embodiment, the switched-mode control scheme comprises pulse-width-modulation control.

[0024] In a preferred embodiment, an on-going operation, preferably an opening operation, is inhibited before it has been completed and that the movable contact is returned to its start position by controlling the currents to the first and second coils.

Brief description of drawings

[0025] The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Figs. 1 and 2 show a current interrupter according to prior art.

Fig. 3 shows schematically a current interrupter according to the invention.

Fig. 4 shows an embodiment of a current interrupter according to the invention comprising a Thomson coil. Fig. 4a is a perspective view of an electro-magnetic force-generating device in the form of a Thomson coil and Fig. 4b is a corresponding view of an electro-magnetic force-generating device in the form of a double-coil arrangement.

Fig. 5 is a diagram showing the acceleration force for different actuator control schemes.

Fig. 6 shows a PWM control scheme.

Figs. 7 and 8 illustrate current control in the embodiment shown in Fig. 3.

Fig. 9 shows an embodiment of a current interrupter according to the invention with open and close coils.

Fig. 10 illustrates a circuit-breaker incorporating a current interrupter according to the invention.

Fig. 11 shows several current controllers 46 sharing a common energy storage 42.

Fig. 12 presents a set of diagrams showing the voltage across a capacitor in the energy storage 42 when it is shared by several current controllers 46.

Description of embodiments

[0026] In the following, a detailed description of a current interrupter according to the invention will be presented.

[0027] The term“electro-magnetic force-generating device” will be used herein to describe devices supplied with current to generate a mechanical force, such as Thomson coils and double-coil arrangements. When the term“Thomson coil” is used herein, it should be construed as the arrangement including both a flat coil 32 and an armature disc 34, unless otherwise stated.

[0028] A schematic diagram of a current interrupter according to the invention is given in Fig. 3. The general design is similar to the prior art current interrupters shown in Figs 1 and 2. Thus, it comprises a contact arrangement 10 with a fixed contact 12 and a movable contact 14 adapted to interrupt current ii _oad flowing in a current line 2. The movable contact 14 is mechanically connected to a mechanical drive train 20 and in the end opposite to the end connected to the current interrupter 10, the mechanical drive train 20 is mechanically connected to an actuator, i.e. , an electro-magnetic force-generating device 30.

[0029] Fundamentally, actuator control aims to manipulate the movement of the moving contact 14. In prior art this target has been achieved by selecting the initial voltage of one or several energy capacitors that are being discharged through the Thomson coil. The final goal of such voltage selection is to safeguard that the current provided to the electro-magnetic force-generating device 30 exceeds the minimum value required to create sufficient force to separate the contacts 12 and 14 in the allowed operating time. The initial capacitor voltage must be selected, taking the variation of certain parameters in the electrical circuit in the electro- magnetic force-generating device 30 into account. Such parameters are e.g. the resistance in the coil and the rest of the circuit, capacitance variation in the energy storage capacitors, inductance variations etc.

[0030] Design, based on selecting the initial voltage of capacitor 42 to be discharged through the electro-magnetic force-generating device 32, therefore in average will suffer from“extra” current caused by the voltage margin added to cover the maximum mismatch between the circuit parameters. For example, if the initial voltage on the capacitor 42 is at the positive tolerance limit and

simultaneously the inductance in the coil 32 is at the negative tolerance limit, the actuator current and the mechanical force may be higher than necessary. This kind of mechanical overstress, caused by this“extra” current, is eliminated just by the very introduction of current control.

[0031 ] An embodiment of a current interrupter according to the invention is outlined in more detail in Fig. 4. The actuator 30 comprises a Thomson coil with a flat coil 32 and an armature disc 34, see also Fig. 4a, showing a perspective view of a Thomson coil. The winding of the coil 32 is connected to the current driver 40 and more specifically to the outputs of a current controller 46 therein. The current controller 46 is provided between the energy storage in the form of a capacitor 42 and the Thomson coil 32, 34. The current controller 46 comprises semiconductor devices having turn-off capability, such as GTOs, IGBTs and MOSFETs, being adapted to provide switched-mode control of discharge current from the energy storage 42.

[0032] As an alternative to a Thomson coil, a double-coil arrangement could be used, see Fig. 4b showing a perspective view of such windings with a first fixed flat coil 132 and a second moving coil 134. The moving armature coil 134 is

electrically connected in series with the fixed coil 132 through flexible connections causing adjacent currents in the two coils to flow in opposite directions thereby producing a strong repulsive force.

[0033] Furthermore, the current driver 40 may receive a control signal from a reference generator 50 connected thereto. This reference generator receives inputs related to run-time conditions, such as e.g. the amplitude and derivative of the actual line current to be interrupted, other information related to the system wherein the circuit-breaker is connected, measured initial position of the armature disk 34 or measured position of the armature disk throughout the whole open or close operation. Other information may be the required speed of operation at the upcoming operations as well as the type of operation to be executed, i.e. , opening or closing operation of the contacts 12, 14. The obtained run-time conditions can be used to adapt the current sent into the electro-magnetic force-generating device, e.g. the Thomson coil 32, 34, so that unnecessary use of the maximum actuator force, accompanied by unnecessary loss of life time, is avoided.

[0034] This arrangement allows, throughout the opening or closing operation of the current interrupter, fast and precise control on the force applied to the mechanical drive train 20 acting on the movable contact 14, and accordingly controls the resulting trajectory of the latter.

[0035] Using continuous control of the current through the coil 32 in the actuator 30 makes it possible to optimize the force applied to the armature disc 34 during the whole operation time. It is for example possible to limit the instantaneous peak current experienced when only a thyristor switch is used, as in the prior art. Furthermore, using continuous control of the current through the force generating device in the actuator makes it possible to achieve controlled and reproducible position transfers of the movable contact in the current interrupter.

[0036] The current pulse obtained with a prior art thyristor-controlled discharge of a capacitor, as illustrated in Fig. 2, exhibits a high initial current amplitude followed by a decreasing current level. This current is illustrated in Fig. 5 with a solid line. This means that the mechanical system suffers from a very high impulse due to the fact that the energy in the capacitor is not optimally utilized. With an actuator according to the invention, a better performance of the actuator can be obtained if the current waveform is controlled with a current controller. Thus, the energy in the capacitor can be better utilized by applying a control scheme and one preferred waveform is shown in Fig. 5 with a dashed line.

[0037] Thus, the invention also relates to a method of controlling the mechanical stress on the mechanical drive train and the actuator in the current interrupter, which is part of a fast-acting circuit-breaker for DC or AC. This method involves the steps of detecting a condition during the operation of the current interrupter 1 and then controlling current provided from the energy storage 42 to the Thomson coil 32, 34 in dependence of the detected condition. This regulation is conducted continuously or intermittently during the operation of the current interrupter 1.

[0038] The control scheme is used to limit the stress in the mechanical system that executes the change of the movable contact position. The method implies that the exciting current brought into force generating device of the actuator is controlled by the current driver 40 and that the current reference to the controller is created in run-time taking into account the specific conditions prevailing at each actual interruption.

[0039] The allowed operating time normally refers to short-circuit conditions, but many, if not most, operations are performed with load currents with normal load current. At these current levels the operating time can be extended to reduce the force applied to the mechanical system. Accordingly, the current reference may be reduced when this condition is at hand. [0040] The allowed operating time normally refers to short-circuit conditions, where it is assumed that the highest allowed short-circuit current derivative acts during the full time interval, from trip command until sufficient contact separation has been achieved. The operation time can be extended if measured current derivative is lower (remote fault) than expected.

[0041 ] Current control per se reduces the stress on the mechanical system because the current is limited to the design values irrespective of tolerance deviations in the circuit. Such deviations appear due to several reasons, e.g.

variation in initial voltage on the storage capacitor capacitance variations inductance variations, e.g. due to varying initial gap between the Thomson coil and the armature disc resistance variations due to temperature variation in the environment.

[0042] Current control can be realized by a switched-mode-controlled power electronic controller. Fig. 6 shows an example with a main circuit in the form of a half-bridge voltage source converter (VSC) performing pulse width modulation (PWM). The measured current through the electro-magnetic force-generating device is continuously compared with the current reference and the deviation is amplified in a proportional-integral (PI) amplifier that determines the duty-cycle of the VSC. The switching pattern finally is obtained by subtracting an internally created triangular wave and bringing the difference to a comparator. A large number of alternative main circuits (full-bridge, choppers, etc.) as well as many different control and modulation schemes can be used.

[0043] Switched-mode modulation schemes with high switching frequency may be utilized as the operating time for the actuator is short, typically only a few milliseconds. Cooling is not required in most cases.

[0044] Fig. 7 illustrates an embodiment wherein the load current ii _oad to be interrupted is monitored and when a command is given to the circuit-breaker in which the current interrupter 1 is provided, the actual level of current to be interrupted is used to determine the acceptable time of interruption. This means that the current reference generator 50 provides a current reference signal to the current driver 40 and that the control is performed in accordance with the received reference signal from the reference generator. At low current to be interrupted the operation time is prolonged, thereby reducing the stress on the mechanical system and extending the life span of the circuit-breaker.

[0045] Fig. 8 illustrates an embodiment wherein the load current derivative is monitored and the available time before the current reaches the current

interrupting capability of the circuit-breaker is calculated. At low current

derivatives, i.e. at remote faults the operation time can be prolonged and the stress on the mechanical system can be reduced.

[0046] Fig. 9 illustrates an embodiment wherein the actuator 30 comprises two coils: a first coil 32 for opening and a second coil 32’ for closing the contacts 12,

14 of the current interrupter 1. The two coils 32, 32’ share the same armature disc 34 and each coil has a corresponding current controller 46 and 46’, respectively. If the current through the first coil 32 driving the movable contact 14 from its closed position is being brought to zero and, simultaneously, the current in the second coil 32’ driving the movable contact 14 towards its closed position is energized, the movable contact 14 can be stopped and returned to its initial closed position. In this way a commanded opening command may be disabled without disturbing the load current. In other words, if the current interrupter is equipped with two

Thomson coils, one for opening and one for closing the contacts, the two Thomson coils sharing a common armature disc, it is possible to inhibit an ongoing operation before the equilibrium position has been passed by current control of mutually counter-acting force generating devices.

[0047] Fig. 10 illustrates a circuit-breaker for high voltage applications comprising a current interrupter according to the invention. Such a circuit-breaker shall operate very fast, in the range of a few milliseconds and to that end it is equipped with a fast actuator as has been described above.

[0048] When the fast actuator has separated the contacts in the contact arrangement 10, the load current continues to flow in an arc between the contacts in 10, and at this time a current zero-crossing in the arc current in the interrupter must be created to interrupt the current. To this end a parallel branch 64 has been provided and it comprises means to create such a current zero-crossing. In Fig. 10 a resonant passive circuit excited by a low-voltage VSC according to Swedish patent 529392 or EP 3 161 846 is shown. After current extinction the load current becomes commutated into an energy-absorbing, voltage-limiting branch 62, mainly comprising a metal-oxide varistor (MOV). A circuit-breaker of this kind typically also comprises a residual breaker 60 controlled by a control device 61 , which provides isolation once the current has been eliminated by 1.

[0049] Use of control of the current sent into the electro-magnetic force- generating device, e.g. the Thomson coil 32, 34, eliminates the critical

dependence on the voltage across the energy storage 42. Therefore, several actuators, for example the two coils 32, 32’ in the embodiment of Fig. 9 or individual phase breakers in a three-phase breaker, using individual current controllers 46 can utilize a common energy storage 42, see Fig. 11. Likewise, repetitive operations with current controllers 46 can be performed from one energy storage 42, with the same output current fed into the electro-magnetic force- generating device, as the impact of the varying voltage in the energy storage will be compensated by the current controller 46. Fig. 12 shows the time course of the energy storage capacitor voltage (lower diagram) when a common energy storage is shared for current controllers providing current to the open coil (upper diagram) and the closing coil (middle diagram) in a circuit-breaker at a reclosing operation. The current in the actuator 30 is strongly connected to the force from the force generating device in the actuator, i.e. the Thomson coil 32, 34. The arrangement of current control allows, throughout the opening or closing operation of the current interrupter, fast and precise control on the force applied to the mechanical drive train 20 acting on the movable contact 14, and accordingly controls the resulting trajectory of the latter.

[0050] Preferred embodiments of a current interrupter and a method of controlling this current interrupter according to the invention have been described.

It will be appreciated that these may be varied within the scope of the appended claims. The contact arrangement has been described as comprising a first, fixed contact and a second, movable contact. It will be appreciated that also the first contact may be movable.—