Devices for interrupting electric current, systems for interrupting electric current, and methods for detecting faults in mechanical circuit breakers.

Abstract

A device for interrupting current comprises first and second terminals (11, 12) adapted to electrically connect two sections (100, 200) of a power system, and the device comprises a circuit breaker module (10) adapted to interrupt current. The circuit breaker module comprises a mechanical circuit breaker (1) that conducts current during normal operation, and a resonant circuit (3) adapted to be excited to achieve zero-crossing of the current flowing through the mechanical circuit breaker (1). A faulty circuit breaker module can be identified by providing an excitation circuit (40) adapted to initiate excitation of the resonant circuit (3) in order to determine whether the mechanical circuit breaker (1) can conduct current as intended.

Description

Technical Field

[0001] The present invention relates to an electrical circuit breaker. In particular, the present invention relates to a circuit breaker that can perform current interruption without a zero-crossing of the primary current to be interrupted by achieving an artificial zero-crossing of the current passing through a mechanical circuit breaker using a resonant circuit.

Background Art

[0002] It is known that a circuit breaker for interrupting current when there is no zero-crossing (such as a direct current) can be designed using a resonant circuit that passes a resonant current through a mechanical circuit breaker such as a vacuum circuit breaker through which the line current flows (see FIGS. 1 and 2). Generally, opening the mechanical circuit breaker itself does not remove the current. Rather, an arc is formed inside the mechanical circuit breaker, so the current continues to flow. However, when the resonant current exceeds the line current, a zero-crossing occurs, so the current passing through the mechanical switch is interrupted. At that time, the current is diverted to an energy absorption device such as a metal oxide surge arrester. When current starts to flow through this device, the device provides a countervoltage that drives the line current I to zero, completing the current arc extinction process.

[0003] One such active resonant circuit breaker uses a power electronic voltage source converter to excite the resonant circuit (see Figure 3 and European Patent Nos. 3398198 and 3161846). This type of breaker has several advantages. The ability to use high resonant frequencies means that the components within the resonant circuit are smaller. Furthermore, the resonant process can be precisely controlled by the voltage source converter. A typical procedure for interrupting current using such a breaker is as follows: First, the line current I flows through a mechanical circuit breaker 1. To initiate the interruption process, this breaker is opened, preferably using a high-speed actuator. The current continues to flow through the arc inside the breaker. Next, an AC voltage Uo is generated by the voltage source converter 4 at the resonant frequency of the resonant circuit 3. As a result, a resonant current Io flows through the resonant circuit, increasing its amplitude. In particular, the resonant current also flows through the mechanical circuit breaker. When the amplitude eventually exceeds the magnitude of the line current I, a zero-cross occurs in the current Isw flowing through the mechanical circuit breaker. This extinguishes the arc and stops the current Isw. Since there is no other path for the line current I to flow, the line current I is forced into surge arrester 2. Surge arresters are generally selected to provide a counter voltage significantly higher than the drive voltage in the grid when the current flows through the surge arrester. This forces the line current to zero and completes the current interruption process.

[0004] Another embodiment of the circuit breaker described above, which uses a voltage source to excite a resonant circuit, is shown in Figure 4. In this case, the arrangement of the surge arrester is modified so that it is connected in series with the inductor of the resonant circuit. Figure 4 also shows how several sets of voltage source converters and resonant circuits can be connected in parallel to achieve a higher resonant current.

[0005] It is also known that a high-voltage circuit breaker can be designed by connecting multiple circuit breaker modules 10 designed for low voltage in series (see Figure 5). A key advantage of this configuration is that even if one of the circuit breaker modules fails to properly interrupt the current, current interruption is still possible as long as the combined voltage provided by the other modules is sufficient to reduce the line current to zero.

[0006] In the case of the type of circuit breaker described above, since the energy absorption device 2 is connected in parallel with the mechanical circuit breaker 1, metallic isolation does not occur when the circuit breaker is open. This means that even when the circuit breaker is open, a low current may flow. Therefore, it is common practice to connect a switch formed by a residual current circuit breaker 5 in series with the main circuit breaker. The residual current circuit breaker is used in this case to remove the current flowing through the energy absorption device and to provide galvanic isolation to the path of the main current. The residual current circuit breaker requires only very limited current interruption capability.

[0007] It is often beneficial to use the aforementioned switch 5 to close the current circuit and allow the line current I to flow. By this method of closing the circuit, the circuit breaker, which is normally in the closed state, conducts, and in the case of a circuit breaker consisting of multiple series-connected circuit breaker submodules 10, all of these submodules are closed and therefore conduct. The voltage is then interrupted by the residual current circuit breaker, which is in the off state. When the command to close the current circuit is given, the residual current circuit breaker closes, and a path for the current is formed. Therefore, the residual current circuit breaker is required to have current-carrying capability.

[0008] A problematic failure when using such modular high-voltage circuit breakers occurs, for example, when one or more of the multiple circuit breaker modules 10 fail to turn on. Even if the modular high-voltage circuit breaker is commanded to turn on, and one or more of these modules fail to turn on and begin to supply the intended current, these one or more modules are still forced to supply current and, as a result, may be destroyed. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] European Patent No. 3398198 [Patent Document 2] European Patent No. 3161846 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] Therefore, an object of the present invention is to provide an apparatus and system for interrupting electric current, the apparatus and system comprising one or more circuit breaker modules having a diagnostic function capable of determining whether or not a mechanical circuit breaker can conduct electric current.

[0011] Another object of the present invention is to provide a method for determining that a circuit breaker module is properly short-circuited and therefore does not obstruct the flow of current when a complete circuit breaker should be turned on. [Means for solving the problem]

[0012] This invention is based on the insight that the conduction capability of a circuit breaker module can be verified by temporarily forcing current through a mechanical circuit breaker and observing the transient behavior of the voltage or current across its terminals.

[0013] According to a first aspect of the present invention, a device for interrupting current is provided, the device comprising first and second terminals adapted to electrically connect two sections of a power system. The device comprises a circuit breaker module adapted to interrupt current, the circuit breaker module comprising a mechanical circuit breaker through which current flows during normal operation, and a resonant circuit adapted to be excited to achieve zero-crossing of the current flowing through the mechanical circuit breaker, the device further comprising an excitation circuit adapted to initiate excitation of the resonant circuit in order to determine whether the mechanical circuit breaker is properly operational. By adding the excitation circuit to an existing design, the development of the device is simplified and costs are reduced.

[0014] In a preferred embodiment, a voltage source converter is provided in series with the resonant circuit, the excitation circuit is connected to the voltage source converter, and the excitation of the resonant circuit is achieved by the voltage source converter.

[0015] In a preferred embodiment, a current sensor is provided to sense the current flowing through the loop, which includes a mechanical circuit breaker and a resonant circuit.

[0016] In a preferred embodiment, the disconnector is connected in series with the circuit breaker module.

[0017] According to a second aspect of the present invention, a system for interrupting current is provided, the system comprising at least two devices for interrupting current according to the present invention, the at least two devices for interrupting current being connected in series. In a preferred embodiment, a disconnector is connected in series with a circuit breaker module.

[0018] A third aspect of the present invention provides a method for detecting a fault in a mechanical circuit breaker, the method comprising the steps of: exciting a current through the mechanical circuit breaker; preferably sensing the current; determining the behavior of the current; and determining the energizing capacity of the mechanical circuit breaker according to the behavior of the current.

[0019] In a preferred embodiment, to determine the energizing capability of a mechanical circuit breaker, a resonant current is excited through the mechanical circuit breaker and the transient behavior of the resonant current is observed.

[0020] In a preferred embodiment, the method includes the step of sensing and comparing the peak amplitudes of at least two subsequent oscillation cycles of the resonant current.

[0021] In a preferred embodiment, the method includes the step of detecting that no current is flowing.

[0022] In a preferred embodiment, the current is sensed by a current sensor.

[0023] In a preferred embodiment, the current is excited by a voltage source converter that forms part of the loop through which the current flows.

[0024] In a preferred embodiment, the current is sensed by observing the voltage across the DC link capacitor of the voltage source converter.

[0025] In a preferred embodiment, the method includes the step of operating the circuit breaker before the step of exciting the current through the mechanical circuit breaker.

[0026] In a preferred embodiment, the method preferably includes the step of synchronizing the excitation to the resonant frequency by sensing the current, providing an open loop, measuring the voltage, and measuring the magnetic field.

Brief Description of the Drawings

[0027] Here, the present invention will be described by way of example with reference to the accompanying drawings. [Figure 1] A diagram showing a prior art device for interrupting current. [Figure 2] A diagram showing a prior art device for interrupting current. [Figure 3] A diagram showing a prior art device for interrupting current. [Figure 4]A diagram showing a conventional device for interrupting electric current. [Figure 5] A diagram showing a conventional device for interrupting electric current. [Figure 6] A diagram showing an overall embodiment of the device for interrupting electric current according to the present invention. [Figure 7] This figure shows the waveforms of the excitation voltage Vo and resonant current Io during the procedure for exciting a resonant circuit to verify the conduction capability of a mechanical circuit breaker. [Modes for carrying out the invention]

[0028] The apparatus for interrupting current according to the present invention will be described with particular reference to Figures 6 and 7. In this specification, the expression "operable" for a mechanical circuit breaker should be interpreted as meaning that it conducts current as intended, i.e., its ability to conduct current.

[0029] Figure 6 shows an overview of the entire apparatus according to the present invention. In Figure 6, two electrical nodes 11 and 12 in a power system are electrically connected via an apparatus comprising a circuit breaker module 10 having three parallel branches. Of the three parallel branches, the first branch comprises a mechanical circuit breaker 1, the second branch comprises a voltage limiting energy absorption device 2, and the third branch comprises a controllable voltage source 4 connected in series with a passive resonant circuit 3. The electrical connection between sections 100 and 200 in the power system serves the purpose of transmitting power between these sections, in which case the main current I flows through the mechanical circuit breaker 1. Sections 100 and 200 may be subsystems of a common power system using DC or AC or separate power transmission systems. Alternatively, each section may represent a power system supplying power to a load, for example, a motor 200 connected to a power source 100.

[0030] When the contacts in the mechanical circuit breaker 1 are separated, an internal arc is established between the contacts, and the main current I continues to flow through the arc. When the mechanical switch operates at high voltage, the arc is extinguished only when the current crosses zero naturally or when it is forcibly extinguished by artificial means.

[0031] In a preferred embodiment of the device for interrupting current, the disconnector 5 is connected in series with the circuit breaker module 10. When the main current I is interrupted by the circuit breaker module 10, no current flows through the voltage limiting energy absorption device 2 because the voltage between terminals 11 and 12 is lower than the protection voltage of the voltage limiting energy absorption device 2. Thus, sections 100 and 200 remain connected only through a branch consisting of a resonant circuit 3 connected in series with the voltage source converter 4. Since this branch includes a small series capacitor, the disconnector 5 can be opened without sustained arc discharge. When opened, the disconnector 5 physically isolates the electrical connection between sections 100 and 200 of the power system.

[0032] As one method for connecting sections 100 and 200 of the power system, a disconnector 5 is used to make the connection. In this case, when the disconnector 5 is opened, the resonant circuit 3 can be discharged, and then the main switch 1 can be closed without a discharge pulse. This prepares the device 10 to immediately interrupt the current when the disconnector 5 is re-closed.

[0033] In another configuration for connecting sections 100 and 200 of the power system, the disconnector 5 is closed before the mechanical main switch 1. In this configuration, the disconnector 5 must be designed to tolerate the current pulses that charge the resonant circuit capacitor generated when the disconnector 5 is closed. Furthermore, in this configuration, the main switch 1 must withstand the discharge pulses that appear when the disconnector is closed.

[0034] In this preferred embodiment, an excitation circuit 40 is provided in the circuit breaker module 10 for exciting the resonant circuit 3 to provide a current flowing through the mechanical circuit breaker 1. Processing means for performing a method of detecting a fault in the circuit breaker module 10, in particular the mechanical circuit breaker 1, are provided in the excitation circuit 40. Transient phenomena of the resonant current can be observed during or after excitation by a current sensor 41 adapted to sense the current Io flowing in the loop including the mechanical circuit breaker 1 and the resonant circuit 3. Information on the sensed current Io is provided to the excitation circuit 40 for evaluation. By sensing this current Io, the conduction capability of the mechanical circuit breaker 1 can be determined, as will be described in detail below. This is possible because the mechanical circuit breaker 1 forms part of the resonant circuit 3. Amplitude damping between oscillation cycles, or other characteristics of this oscillation, can be determined. In particular, unlike in the case of turn-off of the circuit breaker module 10, the mechanical circuit breaker 1 does not operate while the resonant circuit is being excited. Rather, the mechanical circuit breaker 1 remains closed as in the normal case. In other words, the excitation is preferably synchronized to the resonant frequency by sensing the current, providing an open loop, measuring the voltage, and / or measuring the magnetic field.

[0035] In particular, if a resonant circuit 3, excited by a voltage source converter 4, is provided in the circuit breaker module 10, the voltage source converter 4 can be controlled by the excitation circuit 40 to provide an AC voltage over a specific number of cycles and initiate a resonant current of sufficient magnitude to determine the conduction capability of the mechanical circuit breaker 1. One way to determine the conduction capability of the circuit breaker module 10, in other words, the mechanical circuit breaker 1, is to observe the peak resonant current during the subsequent oscillation cycles after excitation. Under normal circumstances, the magnitude of the current is expected to decay slowly over continuous oscillation cycles due to the resistance decay of the resonant circuit 3. The amount of decay under normal circumstances is well known and can be determined by calculation or experiment on a functioning circuit breaker. Thus, the rate of the peak amplitude in the subsequent cycles of resonant oscillation in a functioning circuit breaker can be determined.

[0036] According to a preferred embodiment of the present invention, the voltage source converter 40 operates to excite the resonant circuit 3 by providing a number of cycles of AC voltage, and then the voltage source converter 40 is controlled to short-circuit its output (see Figure 7). At this stage, if unforced resonant oscillations can be expected, transient phenomena of the resonant current are observed. In particular, the peak amplitudes of two or more subsequent oscillation cycles can be measured and compared with the values ​​expected for a fully functional system. A fully functional system is one in which the mechanical circuit breaker 1 has established good electrical contacts.

[0037] In particular, in the extreme case where the mechanical circuit breaker 1 completely fails to close the circuit, the proposed method can easily detect that no resonant current flows when the resonant circuit is excited.

[0038] If a resonant circuit 3, excited by a voltage source converter 40, is provided in the circuit breaker module 10, the presence of a resonant current can also be detected by observing the voltage across the DC link capacitor of the voltage source converter. If the resonant current rises, this voltage can be expected to decrease in a predictable manner as energy is transferred from the DC link capacitor to the resonant circuit. Therefore, other embodiments of the present invention are based on the measurement of the DC capacitor voltage. For example, it may also be useful to verify the correct operation of the voltage source converter by measuring the output voltage Uo or by detecting the correct switching function of its semiconductor valve. For this purpose, additional circuitry may be added to the gate drive unit of the voltage source converter.

[0039] Preferred embodiments of devices and systems for interrupting current, as well as methods for detecting faults in electrical circuit breakers, have been described. While the device described with reference to Figure 6 comprises only a single circuit breaker module 10, it will be understood that several circuit breaker modules 10 can be connected in series, as shown in Figure 5.

[0040] In the above embodiment, the conduction capability of the mechanical circuit breaker is determined by sensing the current Io with a current sensor. It will be understood that there are alternative methods for synchronizing the excitation to the resonant frequency, such as providing an open loop, measuring voltage, or measuring magnetic field.

Claims

1. A device for interrupting current, the device comprising first and second terminals (11, 12) adapted to electrically connect two sections (100, 200) of a power system, and the device comprising a circuit breaker module (10) adapted to interrupt current, The circuit breaker module is provided with a mechanical circuit breaker (1) through which current flows during normal operation, and a resonant circuit (3), the resonant circuit being adapted to be excited to achieve zero-crossing of the current flowing through the mechanical circuit breaker (1), The excitation circuit (40) is adapted to start the excitation of the resonant circuit (3) in order to determine whether the mechanical circuit breaker (1) is able to operate properly. A device characterized by the following features.

2. The apparatus according to claim 1, further comprising a voltage source converter (4) connected in series with the resonant circuit (3), wherein the excitation circuit (40) is connected to the voltage source converter (4), and the excitation of the resonant circuit (3) is achieved by the voltage source converter (4).

3. The apparatus according to claim 1 or 2, further comprising a current sensor (41) adapted to sense the current (Io) flowing in a loop including the mechanical circuit breaker (1) and the resonant circuit (3).

4. The apparatus according to any one of claims 1 to 3, comprising a disconnector (5) connected in series with the circuit breaker module (10).

5. A system for interrupting an electric current, wherein the system comprises at least two devices for interrupting an electric current as described in any one of claims 1 to 3, and the at least two devices for interrupting an electric current are connected in series. A system characterized by the following features.

6. The system according to claim 5, further comprising a disconnector (5) connected in series with the circuit breaker module (10).

7. A method for detecting a malfunction in a mechanical circuit breaker (1), wherein the method is: The steps include: exciting a current (Io) via the mechanical circuit breaker (1); A step of determining the behavior of the current (Io), The steps include determining the energizing capacity of the mechanical circuit breaker (1) according to the behavior of the current (Io), and including, A method characterized by the following:

8. The method according to claim 7, comprising the step of sensing the current (Io) before determining the behavior of the current (Io).

9. The method according to claim 7 or 8, wherein a resonant current (Io) is excited through the mechanical circuit breaker (1) to determine the current carrying capacity of the mechanical circuit breaker (1), and the transient behavior of the resonant current is observed.

10. The method according to claim 9, comprising the step of sensing and comparing the peak amplitudes of at least two subsequent vibration cycles of the resonant current (Io).

11. The method according to claim 8 or 9, further comprising the step of detecting that no current (Io) is flowing.

12. The method according to any one of claims 7 to 11, wherein the current (Io) is sensed by a current sensor (41).

13. The method according to any one of claims 7 to 12, wherein the current (Io) is excited by a voltage source converter (4) that forms part of the loop through which the current (Io) flows.

14. The method according to claim 13, wherein the current (Io) is sensed by observing the voltage across the DC link capacitor of the voltage source converter (4).

15. The method according to any one of claims 7 to 14, further comprising the step of operating a disconnector (5) before the step of exciting the current (Io) flowing through the mechanical circuit breaker (1).

16. Preferably, the method according to any one of claims 7 to 15, comprising the steps of synchronizing the excitation to the resonant frequency by sensing the current, providing an open loop, measuring the voltage, and measuring the magnetic field.

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