Arrangement, system, and method for detecting a fault in an electric circuit breaker

Abstract

An arrangement for interrupting current comprises a first and a second terminal (11,12) being adapted to electrically connect two sections (100, 200) of a power system, the arrangement comprising a circuit breaker module (10) adapted to interrupt current. The circuit breaker module is provided with a mechanical circuit breaker (1 ), which, during normal operation, carries current, and a resonant circuit (3) adapted be excited to achieve a zero-crossing the current through the mechanical circuit breaker (1 ). By providing an excitation circuit (40) adapted to initiate excitation of the resonant circuit (3) for the purpose of determining whether the mechanical circuit breaker (1) can carry a current as intended, a faulty circuit breaker module can be identified.

Description

ARRANGEMENT, SYSTEM, AND METHOD FOR DETECTING A FAULT IN AN ELECTRIC CIRCUIT BREAKERTechnical field

[0001] This invention relates to electric circuit breakers. In particular, it relates to circuit breakers employing resonant circuits to achieve an artificial zero-crossing in the current through a mechanical circuit breaker whereby a current interruption can be made absent any zero-crossings in the primary current to be interrupted.Background art

[0002] It is known that a circuit breaker for interrupting current when there are no zero crossings (such as in a DC current, for instance) can be designed by using a resonant circuit to impose a resonant current in a mechanical circuit breaker, such as a vacuum interrupter, carrying the line current, see Fig. 1 and Fig. 2. Opening the mechanical circuit breaker by itself will generally not eliminate the current; instead an arc will form inside the mechanical circuit breaker by which the current continues to flow. However, if the resonant current exceeds the line current a zerocrossing occurs whereby the current though the mechanical switch is interrupted. The current is then commutated into an energy absorbing device such as a metal- oxide surge arrester. As this device starts conducting the current it will provide a counter voltage that drives the line current I to zero, whereby the current extinction process is completed.

[0003] One variety of such an active resonant circuit breaker uses a power- electronic voltage source converter to excite the resonant circuit, see Fig. 3 and refer to patent EP3398198 and EP3161846. This offers several benefits. A high resonance frequency can be used, which implies smaller components in the resonant circuit. Furthermore, the resonant process can be precisely controlled by the voltage source converter. The typical procedure for interrupting current using such a circuit breaker is as follows. Initially, the line current I current flows through a mechanical circuit breaker 1 . To start the interruption process, the said circuit breaker is opened, preferably using a fast actuator. The current keeps flowing by way of an arc inside the breaker. Next, an alternating voltage llo is produced bythe voltage source converter 4 at the resonance frequency of the resonant circuit 3. This leads to a resonant current Io flowing through the resonant circuit with increasing amplitude. Notably, the resonant current also flows through the mechanical breaker. When the amplitude eventually exceeds the magnitude of the line current I a zero crossing will occur in the current though the mechanical circuit breaker Isw. This causes the arc to be quenched whereby Isw will stop flowing. As there is no other path for the line current I to flow it will be forced into a surge arrestor 2. The surge arrestor will generally be chosen so as to provide a counter voltage that is significantly higher than the driving voltage in the grid when a current flows through it. Thereby, the line current is forced to zero and the current interruption process is completed.

[0004] An alternative embodiment of the mentioned circuit breaker using a voltage source to excite the resonant circuit is seen in Fig. 4. In this case the placement of the surge arrester has been altered to be connected in series with the inductor of the resonant circuit. Fig. 4 also indicates 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 circuit breaker for high voltage can be designed by series-connecting several circuit breaker modules 10 designed for lower voltage, see Fig. 5. This brings the important benefit that the current interruption is still possible even if one of the circuit breaker modules fail to interrupt the current properly, as long as the combined voltage provided by the other modules is sufficient to bring the line current to zero.

[0006] The mentioned types of circuit breaker do not provide a metallic separation when in the open state, since an energy absorbing device 2 is connected in parallel to the mechanical circuit breaker 1 . This implies that a low current may flow even when the breaker is in the open state. It is therefore common practice to connect a switch in the form of a residual current breaker 5 in series with the main circuit breaker. The residual current breaker is then used to eliminate the current flowing though the energy absorption device and provide agalvanic separation in the path of the main current. The residual current breaker only needs very limited current interruption capability.

[0007] It is often beneficial to use the mentioned switch 5 for closing the current circuit and allow the line current I to flow. By this method of closing the circuit, the circuit breaker is normally in the closed state, or in case it consists of several series connected breaker submodules 10 all of these are closed, and thus conducting. The voltage is then blocked by the residual current breaker, which is in the off-state. When the command to close the current circuit is given, the residual current breaker is closed and a path for the current is formed. The residual current breaker thus needs current making capability.

[0008] A problematic fault case when using such a modular high-voltage circuit breaker occurs when one or several of the circuit breaker modules 10 have failed to turn on. If the modular high-voltage circuit breaker is commanded to turn on, and one or several of the modules fail to start conducting current as intended in the on-state, this or these modules will still be forced to carry current which may result in their destruction.Summary of the invention

[0009] An object of the invention is therefore to provide an arrangement and a system for interrupting current comprising one or more circuit breaker modules with a diagnostic function capable of determining whether a mechanical circuit breaker can carry a current or not.

[0010] Another object of the invention is to provide a method for determining that a circuit breaker module is properly in the shorted state and thus does not impede the flow of current when the complete circuit breaker is to be turned on.

[0011] The invention is based on the insight that by temporarily forcing current though the mechanical circuit breaker and observing the voltage across it, or the transient behavior of the current, the conducting capability of the circuit breaker module can be verified.

[0012] According to a first aspect of the invention, an arrangement for interrupting current is provided comprising a first and a second terminal being adapted to electrically connect two sections of a power system, the arrangement comprising a circuit breaker module adapted to interrupt current, wherein the circuit breaker module is provided with a mechanical circuit breaker, which, during normal operation, carries current, and a resonant circuit adapted be excited to achieve a zero-crossing the current through the mechanical circuit breaker, the arrangement being characterized by an excitation circuit adapted to initiate excitation of the resonant circuit for the purpose of determining whether the mechanical circuit breaker is properly operable. Adding the excitation circuit to an existing design simplifies the development of the arrangement, reducing costs.

[0013] In a preferred embodiment, a voltage source converter is provided connected in series with the resonant circuit, wherein the excitation circuit is connected to the voltage source converter, and wherein the excitation of the resonant circuit is achieved by means of the voltage source converter.

[0014] In a preferred embodiment, a current sensor is provided to sense current that flows in a loop containing the mechanical circuit breaker and the resonant circuit.

[0015] In a preferred embodiment, a disconnecting switch is connected in series with the circuit breaker module.

[0016] According to a second aspect of the invention, a system for interrupting current is provided comprising at least two arrangements for interrupting current according to the invention, wherein the at least two arrangements for interrupting current are connected in series. In a preferred embodiment, a disconnecting switch is connected in series with the circuit breaker modules.

[0017] According to a third aspect of the invention, a method of detecting a fault in a mechanical circuit breaker is provided, the method comprising the following steps: exciting a current through the mechanical circuit breaker, preferably sensing the current, determining the behaviour of the current, and depending on thebehaviour of the current, determining the current-carrying capability of the mechanical circuit breaker.

[0018] In a preferred embodiment, a resonant current is excited through the mechanical circuit breaker, and the transient behaviour of the resonant current is observed, for the purpose of determining the current-carrying capability the mechanical circuit breaker.

[0019] In a preferred embodiment, the method comprises a step of sensing and comparing the peak amplitude of at least two subsequent oscillation cycles of the resonant current.

[0020] In a preferred embodiment, the method comprises the step of detecting no current.

[0021] In a preferred embodiment, the current is sensed by means of a current sensor.

[0022] In a preferred embodiment, the current is excited by means of a voltage source converter forming part of a loop in which the current flows.

[0023] In a preferred embodiment, the current is sensed by observing a voltage across a DC link capacitor of the voltage source converter.

[0024] In a preferred embodiment, the method comprises a step of operating a disconnecting switch, before the step of exciting a current through the mechanical circuit breaker.

[0025] In a preferred embodiment, the method comprises a step of synchronizing the excitation to the resonance frequency, preferably by sensing the current, providing an open loop, measuring the voltage, and measuring magnetic fields.Brief description of drawings

[0026] The invention is now described, by way of example, with reference to the accompanying drawings, in which:Figs. 1--5 show prior art arrangements for interrupting current;Fig. 6 shows a diagram of a general embodiment of an arrangement for interrupting current according to the invention; andFig. 7 shows waveforms of the exciting voltage \ / o and the resonant current Io during a procedure of exciting the resonant circuit for the purpose to ascertain the conducting capability of the mechanical circuit breaker.Description of embodiments

[0027] An arrangement for interrupting current according to the invention will now be described, particularly with reference to Figs. 6 and 7. In this specification, the expression that the mechanical circuit breaker “is properly operable” should be interpreted as conducting current in the intended way, i.e. , its current carrying capability.

[0028] The general form of an arrangement according to the invention is outlined in Fig. 6, where two electrical nodes 11 , 12 in a power system are electrically connected through an arrangement comprising a circuit breaker module 10 with three parallel branches, the first comprising a mechanical breaker 1 , the second comprising a voltage limiting, energy absorbing device 2 and the third comprising 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 transferring electrical power between said sections, in which case a main current I flows through the mechanical breaker 1 . The sections 100 and 200 may be subsystems of a common power system or separate electrical power transmission systems using de or ac. Alternatively, the sections may represent an electrical power system feeding a load, e.g., a motor 200 connected to a power source 100.

[0029] At contact separation in the mechanical breaker 1 an internal arc will be established between the contacts and the main current I will continue to flow through the arc. If the mechanical switch operates at high voltage, the arc will only extinguish if a current zero cross-over, natural or forced by artificial means, occurs.

[0030] In the preferred embodiment of an arrangement for interrupting current, a disconnecting switch 5 is connected in series with the circuit breaker module 10 When the main current I has been interrupted by the circuit breaker module 10, the voltage limiting, energy absorbing device 2 does not conduct any current as the voltage between the terminals 11 , 12 is lower than its protective voltage. Consequently, the sections 100, 200 remain connected only through the branch consisting of the resonant circuit 3 in series with the voltage source converter 4. This branch contains a small series capacitor and therefore the disconnecting switch 5 can be opened without any sustained arcing. When open, the disconnecting switch 5 provides a physical separation in the electrical connection between the sections 100, 200 of the power system.

[0031] In one scheme for operating the connection between the sections 100, 200 of the power system, the disconnecting switch 5 is used to close the connection. In this case the resonant circuit 3 can be discharged once the disconnector 5 has been opened, and the main switch 1 can then be closed without any discharge pulse. The arrangement 10 thereby becomes ready to perform current interruption immediately at reclosing of the disconnector 5.

[0032] In another scheme for operating the connection between the sections 100, 200 of the power system, the disconnecting switch 5 is closed before the mechanical main switch 1 . The former then must be designed to allow the current pulse that charges the resonant circuit capacitor that occurs, when it closes. Moreover, in this case the main switch 1 must withstand the discharge pulse that appears when it closes.

[0033] In this preferred embodiment, the circuit breaker module 10 is provided with an excitation circuit 40 to excite the resonant circuit 3 to provide a current through the mechanical breaker 1 . The excitation 40 is provided with processing means for performing a method of detecting a fault in the circuit breaker module 10 and particularly in the mechanical circuit breaker 1 . The resonant current transient can be observed during or after the excitation by means of a current sensor 41 adapted to sense the current Io that flows in a loop containing the mechanical circuit breaker 1 and the resonant circuit 3. Information relating to thesensed current Io is provided to the excitation circuit 40 for evaluation. By means of sensing this current Io, the conducting capability of the mechanical breaker 1 can be determined, as will be explained in detail below. This is possible since the mechanical breaker 1 forms part of the resonant circuit 3. The magnitude decay between oscillation cycles, or other characteristics of this oscillation, can be determined. Notably, unlike the case of a turn-off of the circuit breaker module 10, the mechanical circuit breaker 1 is not operated during this excitation of the resonant circuit. Instead, it stays closed as is normally the case. In other words, the excitation is preferably synchronized to the resonance frequency, preferably by sensing the current, providing an open loop, measuring the voltage, and / or measuring magnetic fields.

[0034] In particular, if the circuit breaker module 10 is provided with a resonant circuit 3 excited by a voltage source converter 4, this voltage source converter 4 can be controlled by the excitation circuit 40 to provide an ac voltage for a certain number of cycles to initiate a resonant current of sufficient magnitude to determine the conducting capability of the mechanical circuit breaker 1 . One way to determine the current carrying capability of the circuit breaker module 10, in other words, the mechanical circuit breaker 1 , is to observe the peak resonant current during subsequent oscillation cycles after excitation. In the normal case, it can be expected that the current magnitude should be decaying slowly with successive oscillation cycles due to the resistive damping of the resonant circuit 3. The amount of damping in the normal case is well known and can be determined by calculation or experiments on a functioning circuit breaker. Therefore, the quotient of the peak amplitudes in subsequent cycles of the resonant oscillation in a functioning circuit breaker can be determined.

[0035] According to the preferred embodiment of the invention, the voltage source converter 40 is activated to excite the resonant circuit 3 by providing a number of cycles of an alternating voltage, after which the voltage source converter 40 is controlled to short-circuit its output, see Fig. 7. At this stage, when an unforced resonant oscillation can be expected, the resonant current transient is observed. In particular, the peak amplitude of two or more subsequent oscillationcycles can be measured and compared to the mentioned values expected for a fully functioning system, in which the mechanical circuit breaker 1 has established a good electrical contact.

[0036] Notably, in an extreme case where the mechanical circuit breaker 1 has completely failed to close the circuit, no resonant current will flow when the resonant circuit is excited, which can be easily detected by the proposed method.

[0037] For the case where the circuit breaker module 10 is provided with a resonant circuit 3 excited by a voltage source converter 40, it is also possible to detect the presence of a resonant current by observing the voltage across a DC link capacitor of the voltage source converter. In case there is a resonant current rising, this voltage can be expected to decrease in a predictable fashion as energy is transferred from the DC link capacitor to the resonant circuit. Therefore, an alternative embodiment of the invention is based on measuring the DC capacitor voltage. It may also be of value to ascertain the correct operation of the voltage source converter, for instance by measuring the output voltage llo, or by detecting the correct switching function of its semiconductor valves. For this purpose, additional circuitry may be added to the gate drive units of the voltage source converter.

[0038] Preferred embodiments of an arrangement and a system for interrupting current as well as a method of detecting a fault in an electric circuit breaker have been described. Although the arrangement described with reference to Fig. 6 only comprises a single circuit breaker module 10, it will be appreciated that several circuit breaker modules 10 can be serially connected, as shown in Fig. 5.

[0039] In the embodiment described above, the conducting capability of the mechanical breaker is determined by sensing the current Io by means of a current sensor. It will be appreciated that there are alternative ways of synchronizing the excitation to the resonance frequency, such as providing an open loop, measuring the voltage, measuring magnetic fields etc.

Claims

CLAIMS1 . An arrangement for interrupting current comprising a first and a second terminal (11 ,12) being adapted to electrically connect two sections (100, 200) of a power system, the arrangement comprising a circuit breaker module (10) adapted to interrupt current, where the circuit breaker module is provided with a mechanical circuit breaker (1 ), which, during normal operation, carries current, and a resonant circuit (3) adapted be excited to achieve a zero-crossing the current through the mechanical circuit breaker (1), characterized by an excitation circuit (40) adapted to initiate excitation of the resonant circuit (3) for the purpose of determining whether the mechanical circuit breaker (1) is properly operable.

2. The arrangement according to claim 1 , 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 wherein the excitation of the resonant circuit (3) is achieved by means of the voltage source converter (4).

3. The arrangement according to claim 1 or 2, comprising a current sensor (41 ) adapted to sense current (Io) that flows in a loop containing the mechanical circuit breaker (1) and the resonant circuit (3).

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

5. A system for interrupting current comprising at least two arrangements for interrupting current according to any of claims 1-3, wherein said at least two arrangements for interrupting current are connected in series.

6. A system according to claim 5, comprising a disconnecting switch (5) connected in series with the circuit breaker modules (10).

7. A method of detecting a fault in a mechanical circuit breaker (1 ), the method comprising the following steps:- exciting a current (Io) through the mechanical circuit breaker (1 ),- determining the behaviour of the current (Io), and- depending on the behaviour of the current (Io), determining the currentcarrying capability of the mechanical circuit breaker (1 ).

8. The method according to claim 7, comprising a step of sensing the current (Io) before determining the behaviour 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 ), and the transient behaviour of the resonant current is observed, for the purpose of determining the currentcarrying capability the mechanical circuit breaker (1 ).

10. The method according to claim 9, comprising a step of sensing and comparing the peak amplitude of at least two subsequent oscillation cycles of the resonant current (Io).11 . The method according to claim 8 or 9, comprising the step of detecting no current (Io).

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

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

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

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

16. The method according to any one of claims 7-15, comprising a step of synchronizing the excitation to the resonance frequency, preferably by sensing the current, providing an open loop, measuring the voltage, and measuring magnetic fields.

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