Thyristor circuit and thyristor protection method

Abstract

The thyristor circuit (100) comprises at least one branch including at least one thyristor (10, 20), a thyristor control circuit (50), and a current detector (60, 61). The current detector (60, 61) is configured to detect a current value representative of a current flowing through the thyristor and input the detected current value to the thyristor control circuit. The thyristor control circuit is configured to determine a fault condition when the detected current value exceeds a predetermined current threshold based on a degraded blocking capability of the thyristor. In response to the determination result, the thyristor control circuit triggers the thyristor to a conductive state.

Description

[Technical field]

[0001] The present disclosure relates to a thyristor circuit having at least one branch comprising at least a thyristor, and a thyristor protection method for protecting the thyristor in the at least one branch. [Background technology]

[0002] In high voltage, high current applications, electronic devices often use thyristors as power electronic components. Upon receiving a trigger current at its gate, the thyristor begins to conduct current through its stack of semiconductor junctions in a path from its anode terminal to its cathode terminal. In the literature, triggering is also called firing or gating. Unless triggered, the thyristor is in a non-conducting or blocking state.

[0003] Thyristors are therefore used for current control in a variety of applications such as crowbars, high power rectifiers, etc. When a thyristor or a stack of several series-connected thyristors are connected in an anti-parallel fashion, they can be used to control current in both directions in AC applications such as AC furnaces.

[0004] In a typical application, a fault current condition may occur. In this case, the thyristor involved in conducting the fault current is conventionally blocked. If the temperature of the junction, i.e. the junction temperature, rises to a high value, the blocking ability of the thyristor begins to deteriorate, leading to a high resistance electrical path through the junction. This may therefore accelerate the rise of the junction temperature further under blocking conditions and in the case of a fault current condition, which may lead to failure or thermal destruction of the thyristor.

[0005] It is desirable to protect one or more thyristors in a thyristor circuit from thermal destruction under fault current conditions.

[0006] No. 3,611,043 A describes a protection circuit for a power system that includes a thyristor switch and a backup circuit breaker in series with the thyristor switch, the protection circuit causing the backup circuit breaker to trip in the event that the thyristor switch becomes disabled. Summary of the Invention

[0007] It is an object of the present disclosure to provide a thyristor circuit with improved fault current behavior. This object is achieved by the subject matter defined in the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

[0008] According to an aspect related to the thyristor circuit, the thyristor circuit comprises at least one branch, the branch comprising at least one thyristor. The thyristor circuit further comprises a thyristor control circuit and a current detector. The thyristor control circuit is for selectively triggering the thyristor. The current detector is configured to detect a current value representative of a current flowing through the thyristor. The current detector is further configured to input the detected current value to the thyristor control circuit. The thyristor control circuit is configured to determine a fault condition in which the detected current value exceeds a predetermined current threshold based on a degraded blocking capability of the thyristor. The thyristor control circuit is further configured to trigger the thyristor into a conductive state in response to a result of the determination.

[0009] A fault condition, as used herein, typically refers to an event or period of time when a particular electrical value of a thyristor, or of the branch in which the thyristor is located, exceeds a limit value. At the thyristor or at the branch in which the thyristor is located A certain current It may represent a value.

[0010] In some aspects, the fault condition may be an excessive current, i.e., an overcurrent, occurring in one or more branches. For example, the fault condition may be a branch fault, e.g., excessively high current in one branch. In other examples, the fault condition may be a multi-branch fault, e.g., excessively high current in multiple branches.

[0011] Excessively high current, as used herein, typically refers to a current magnitude or amplitude that would lead to destruction of the thyristor within a short period of time, such as a time period of less than 500 ms, or less than 100 ms, or less than 10 ms.

[0012] According to an aspect related to the thyristor protection method, a thyristor protection method for protecting a thyristor in at least one branch of a thyristor circuit comprises detecting a current value representative of a current flowing through the thyristor, determining whether the current value exceeds a predetermined current threshold based on a degraded blocking capability of the thyristor, and triggering the thyristor into a conductive state in response to a result of the determination.

[0013] In the above aspects relating to the thyristor circuit or thyristor protection method, when the thyristor control circuit determines that a fault condition has occurred, it issues a trigger current to the gate of at least one thyristor, which causes the thyristor to conduct. The fault condition is detected as an overcurrent condition, not an overvoltage condition. The fault condition comprises a high current condition, i.e., an overcurrent condition, typically due to a fault external to the device containing the thyristor circuit, e.g., a converter external fault.

[0014] If the thyristor control circuit determines that no fault condition has occurred, it may perform normal operation of the thyristors, i.e., selective triggering of the thyristors according to a normal operation application scheme. Such a normal operation application scheme may also be performed by another device different from the thyristor control circuit. As non-limiting and illustrative examples, the respective device may trigger the thyristors according to a crowbar scheme, e.g., in a crowbar application, may trigger the thyristors according to a rectification scheme, e.g., in a rectification application, or may trigger the thyristors according to a current and / or power control scheme, e.g., in an AC arc furnace.

[0015] The current detector may be any suitable type of current detection device, and may be, as a non-limiting example, an inductively coupled current detector. The current threshold may be predetermined and set in advance, i.e., prior to operation. The current threshold may be set, for example, taking into account the nominal or extended operating range of the thyristor. The nominal operating range, as used herein, comprises at least the current that the thyristor can withstand permanently, e.g., by design, without degrading or destroying beyond the design limit. The extended operating range, as used herein, comprises at least the current that the thyristor can withstand for a limited period of time. For example, the current threshold may be set just above the maximum nominal operating current of the thyristor, or may be set at, for example, 105%, 110%, or 115% of the nominal operating current. In a further example, the current threshold may be set just above the extended operating range.

[0016] A conductive state to be achieved by triggering a thyristor, as used herein, is typically the state for which a conducting current valve behavior is intended. In the conductive state, the electrical resistance across the junctions of the thyristor is typically significantly lower than during unintended current flow that flows in a degraded blocking state, e.g., at a high junction temperature.

[0017] By intentionally triggering the at least one thyristor into a conductive state when a fault condition is detected, degradation of the blocking capability can be inhibited, thereby preventing a detrimental increase in junction temperature due to fault current passing through the at least one thyristor.

[0018] According to an embodiment, triggering the thyristor into a conductive state comprises successively triggering the thyristor into a conductive state. An arrangement may thus be for successively triggering the thyristor into a conductive state, for example depending on the outcome of the determination. Successively triggering, as used herein, means that the conductive state of the thyristor is maintained by the triggering action without allowing the thyristor to return to a blocked state, at least as long as it is determined that a fault condition prevails.

[0019] In an embodiment, successively triggering the thyristors into a conductive state may include successively triggering the thyristors for at least 50 ms, preferably at least 100 ms, more preferably at least 130 ms, and even more preferably at least 160 ms.

[0020] In a further embodiment, the thyristor circuit further comprises a circuit breaker, e.g. for a branch current, whereby when triggering the thyristors in succession, a control of the circuit breaker is performed to interrupt a current, e.g. a branch current. As used herein, the term branch current may refer to all branch currents, if multiple branches are provided.

[0021] Circuit breaker operation involves a certain delay, or time lag, which mainly results from the breaker actor performing the mechanical action to interrupt the lines carrying the branch current. In embodiments involving circuit breakers, the control described herein by the thyristor control circuitry may effectively bridge the time gap between the occurrence of a fault condition and the safe interruption by the circuit breaker.

[0022] In an embodiment, sequentially triggering the thyristors into a conductive state may include sequentially triggering the thyristors for at least as long as it takes a circuit breaker to interrupt a line carrying the branch current.

[0023] In further embodiments, the thyristor circuit comprises multiple branches, for example, but not limited to, three branches in a three-phase system. Each branch comprises at least one thyristor. According to an embodiment, the thyristors in each branch are controlled in a similar manner, for example, by a thyristor control circuit. For example, the thyristors in each of the branches are triggered into a conductive state depending on the result of a determination, as described herein.

[0024] In embodiments using multiple branches, a circuit breaker as described herein may be configured to interrupt the current in each of the branches, e.g., a common circuit breaker for all branches, e.g., the circuit breaker is controlled to interrupt the branch current in all branches, e.g., upon sequential triggering of one or more thyristors.

[0025] According to a further embodiment, the fault condition is determined when the detected current value exceeds a predetermined current threshold in at least one of the plurality of branches. For example, but not by way of limitation, when the detected current value exceeds a predetermined current threshold in only one or two of the three branches of the three-phase system, a fault condition is determined and one or more thyristors, preferably thyristors in all branches, are triggered into a conductive state. According to yet a further embodiment, the thyristor control circuit is configured to trigger at least one thyristor in each branch into a conductive state in response to the fault condition.

[0026] According to a further embodiment, at least one branch comprises a stack of thyristors. The stack of thyristors comprises two or more thyristors. In an aspect, the number of thyristors in the stack is selected depending on the voltage level to which the stack is connected or to which the stack is exposed. The voltage level is, for example, a rated voltage level or a maximum expected voltage level.

[0027] In an exemplary configuration, the stack of thyristors comprises 10 or more thyristors, preferably 20 or more thyristors, and even more preferably 24 thyristors. The stack typically comprises a first series connection of thyristors and a second series connection of thyristors. The first and second series connections are arranged in an anti-parallel connection, i.e., each carrying current in one branch direction and in a branch direction opposite to the one branch direction.

[0028] In embodiments using stacks of thyristors, the thyristor control circuits are configured to trigger all thyristors of the stack in each branch into a conductive state in response to a fault condition.

[0029] In a further embodiment using stacks of thyristors, where the stacks of thyristors are arranged in multiple branches, the thyristor control circuit is configured to trigger all thyristors of each stack in all branches into a conductive state in response to a fault condition.

[0030] According to an embodiment, the thyristor circuit may further comprise a network interface for connecting at least one of the thyristor control circuit and the current detector to a data network, in particular a global data network. The data network may be a TCP / IP network such as the Internet. The thyristor control circuit and / or the current detector are operatively connected to the network interface for executing commands received from the data network. The commands may include control commands for controlling the thyristor control circuit, for example, a setting command for setting a current threshold, or a circuit breaker operation command. In this case, the thyristor control circuit is adapted to perform a task in response to the control command. The commands may include a status request. In response to the status request or without a prior status request, the thyristor control circuit and / or the current detector may be adapted to send status information to the network interface, which is then adapted to send the status information through the network. The commands may include an update command including update data. In this case, the thyristor control circuit and / or the current sensor are adapted to initiate an update in response to the update command and using the update data. According to an embodiment, the network interface is configured to transmit and receive digital signals between the thyristor control circuit and / or the current detector on the one hand and a data network on the other hand, the digital signals including operating commands, typically set values ​​for current thresholds, and / or information about the thyristor control circuit and / or the current detector or the network.

[0031] The subject matter of the present disclosure will now be described in more detail with reference to exemplary embodiments illustrated in the drawings, in which: [Brief description of the drawings]

[0032] [Figure 1] 1 illustrates a schematic diagram of a thyristor circuit according to an embodiment; [Diagram 2] 4 illustrates diagrammatically a thyristor circuit according to a further embodiment; [Diagram 3] 1 shows a flow chart of a thyristor protection method according to an embodiment described herein.Throughout the drawings, the same or similar parts are given the same reference numerals and the description thereof will not be repeated. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] FIG. 1 shows a thyristor circuit 100 having a branch with a series circuit, which consists of a snubber reactor 80 and a pair of anti-parallel connected thyristors 10, 20. A parallel reactor 81 is electrically connected in parallel to the series circuit. In FIG. 1, upstream of the power supply line 75, the number "3" indicates a three-phase system in which a thyristor circuit 100 is provided in each of the three branches. However, the present disclosure is not limited to a three-phase system, and one, two or four or more branches may also be provided. A three-phase circuit breaker 70 is provided for electrically interrupting the power supply line 75 upon receiving a circuit breaker interruption signal.

[0034] The thyristor control circuit 50 is configured to selectively trigger the thyristors 10, 20. Selectively triggering, as used herein, may include the thyristor control circuit 50 controlling each thyristor 10, 20 independently of one another. Selectively triggering, as used herein, may also include the thyristor control circuit 50 controlling multiple thyristors 10, 20 together, for example a common group of thyristors 10, 20 or a stack of thyristors 10, 20 (described below). The thyristor control circuit 50 may be connected to the gates of the respective thyristors 10, 20 via gate trigger lines 55. In the configuration shown in FIG. 1, the thyristor control circuit 50 is also connected to a three-phase circuit breaker 70 via a circuit breaker trigger line 56 to control the circuit breaker to interrupt a phase of the power supply line 75.

[0035] A current sensor assembly including an induced current sensor 61 and a current value output circuit 60 coupled thereto is connected to the thyristor control circuit 50 via a current signal line 65. The current sensor 61 detects a current flowing in a branch including the thyristors 10 and 20. The current value output circuit 60 calculates a current value from the output of the current sensor 61 and inputs the current value to the thyristor control circuit 50 as a current signal.

[0036] In the thyristor control circuit 50, a value of the current threshold is preset. The thyristor control circuit 50 determines whether the current in the branch, input as a current signal via the current signal line 65, exceeds the current threshold. Exceeding, as used herein, includes exceeding the absolute value of the current, i.e. exceeding is met when the positive sign current is greater than the maximum value or the negative sign current is less than the minimum value. In a typical embodiment, the maximum value is the inverse sign representation of the minimum value, i.e. the absolute value of the current is taken into account.

[0037] The current threshold is typically selected to be representative of a branch fault, e.g. a low resistance current or a short circuit current; in other words, the current threshold is typically representative of an overcurrent condition that may lead to destruction, such as thermal destruction of a thyristor.

[0038] The current threshold in this embodiment is pre-determined and set in advance, i.e., before operation, where the current threshold is set taking into account the nominal operating range of the thyristor, i.e., the maximum current in this case. The maximum current may be determined according to the blocking capability shifting, i.e., deteriorating, when the junction temperature increases due to the current flowing through the thyristor. The maximum current may also be determined to be the current that the thyristor can withstand permanently without being destroyed.

[0039] When the thyristor control circuit 50 determines that a fault condition exists, it issues a gating trigger signal to the gates of the thyristors 10, 20 via a gating trigger line 55. A gating trigger signal, as used herein, is a signal that ensures the firing or passing of a thyristor from its anode to its cathode, for example, via sufficient gate current flowing in the gate. The gating trigger signal is typically a pulse signal, i.e., a pulse train, having a pulse width of at least 50 ms, preferably at least 100 ms or at least 130 ms or at least 160 ms.

[0040] In conjunction with issuing the gate trigger signal, for example simultaneously with issuing the gate trigger signal, the thyristor control circuit 50 issues a circuit breaker trigger signal via a circuit breaker trigger line 56. The circuit breaker trigger signal commands the circuit breaker to be operated to its interrupted or open line position.

[0041] In conventional applications, when a high current that meets the conditions for a fault flows through a branch that includes a thyristor 10, 20, any thyristor 10, 20 that is still operated in a blocked state (i.e., non-firing state) may degrade in its blocking capability due to the increase in temperature caused by the high current. As a result, the temperature of the pn junctions in the thyristors 10, 20 increases further. Even if a circuit breaker such as circuit breaker 70 detects a fault condition and is operated, it may take tens to hundreds of milliseconds for the circuit breaker to operate. Blocking during this time period, or time gap, when the junction temperature in the thyristor is high, may be sufficient to permanently degrade or destroy the thyristor.

[0042] In contrast, according to the present disclosure, the thyristors 10, 20 are operated to be in a conductive state when a fault condition is detected. While the fault current, i.e., excessively high current, flows through the thyristors 10, 20, the pn junctions do not block, thus limiting the temperature rise during the aforementioned time periods to a lower value.

[0043] In other words, the current through the thyristor valves 10, 20 is measured. If a fault occurs and the current exceeds a certain level, the triggering of the thyristors is immediately switched from a controlled and phase-sequenced triggering to a continuous triggering of all phases. This allows the fault current to flow without any reduction, but the continuous firing protects the thyristors 10, 20 since they do not have to block, i.e. they do not have to deal with any overvoltages.

[0044] A fault condition, i.e., an overcurrent condition, does not necessarily require that an overvoltage condition also exists. Due to increased junction temperature, the thyristors 10, 20 may not be able to block certain voltages in this situation that they might have blocked without the increase in temperature.

[0045] In a conventional setup, the thyristors 10, 20 would, for example, be controlled to a blocking state, which would result in them overheating. In the case of an overheated thyristor, the blocking voltage capability is significantly reduced due to the increased junction temperature. In a conventional setup without the present technique, the thyristors 10, 20 would be controlled to further block, which would result in failure and / or destruction.

[0046] The fault current flowing through the thyristors 10, 20 heats the junction and reduces the blocking voltage capability of the semiconductors, but the thyristors 10, 20 are protected by the circuit breaker 70 disconnecting the power supply line 75 after tens to hundreds of milliseconds so that they no longer need to block voltage. A fault, i.e. an overcurrent, is not prevented, but the blocking of the thyristors 10, 20 in the event of a fault condition, which would destroy the semiconductors when they had to carry this fault current, is prevented.

[0047] It should be noted that in accordance with the present technique, it is the high current, not the voltage, that is monitored, which is used by the thyristor control circuit 50 to determine the conditions for issuing a trigger command. The high current, or overcurrent, results from a fault external to the converter.

[0048] By using the present technique, for example, a series reactor for limiting fault currents can be omitted. Any such reactor would increase cost and increase operating costs due to electrical losses. It would add a component that could fail. This component is no longer needed by using the present technique.

[0049] FIG. 2 shows a schematic representation of a thyristor circuit according to a further embodiment. In the embodiment of FIG. 2, instead of a single thyristor 10, 20, a stack 30 of series-connected thyristors and an anti-parallel connected stack 40 of series-connected thyristors are provided. Each stack 30, 40 comprises two or more thyristors connected in series, i.e. stacked. In a typical application, each stack 30, 40 comprises at least 10 or at least 20 thyristors, for example, each stack 30, 40 comprises 24 thyristors, but is not limited to these numbers. Similar to the configuration shown in FIG. 1, in the embodiment of FIG. 2, three branches are configured as shown and a three-phase circuit breaker 70 is provided to interrupt the power supply line 75 upon receiving a circuit breaker signal. Again, a parallel reactor 81 is provided for each phase. It should be noted that three phases is merely an example and the present disclosure is not limited to three-phase applications, but is applicable to single-phase applications or multi-phase applications having a number of phases different from three.

[0050] The main operation method of the other components including the thyristor control circuit 50 is mainly as described above in the embodiment of FIG.

[0051] In the embodiment of FIG. 2 using stacks 30, 40, when a fault condition is determined to exist in one or more branches, it is preferable that all of the thyristors in the stacks 30, 40, or all of the thyristors in each stack 30, 40 in the branch or branches in which the fault condition occurs, are triggered, preferably sequentially, into a conductive state.

[0052] In the embodiment of FIG. 2, the configuration may be such that when a fault condition is determined to exist in one or more branches, all thyristors of each stack 30, 40 in all branches are triggered, preferably sequentially, into a conductive state.

[0053] 3 shows a flow chart of a thyristor protection method according to an embodiment described herein. The method is applicable, for example, to the exemplary configuration shown in FIG. 1 or the exemplary configuration shown in FIG. 2, but may be applied to each appropriate configuration having at least some of the components described above. The method is a thyristor protection method for protecting thyristors 10, 20 in at least one branch of a thyristor circuit 100, and possibly in thyristor stacks 30, 40.

[0054] In Fig. 3, the method starts at 1000. Processing proceeds to 1001 where a current value representative of the current flowing through the thyristor 10, 20 is detected. At 1002, it is determined whether the current value exceeds a predefined current threshold. The current threshold is preset, the current threshold being set taking into account the nominal operating range of the thyristor, i.e. in this case the maximum current that the thyristor can withstand permanently without being destroyed.

[0055] If, at 1002, it is determined that the current value does not exceed the predetermined current threshold, processing returns to 1001. If, at 1002, it is determined that the current value does exceed the predetermined current threshold, processing continues to 1003. At 1003, the thyristors 10, 20 are triggered into a conducting state, and preferably continuously triggered into a conducting state. Processing continues to 1004, where the method ends.

[0056] In FIG. 3, at 1003, by way of example, triggering the thyristor into a conductive state typically comprises triggering the thyristor continuously for at least 50 ms, or at least 100 ms, or at least 130 ms, or at least 160 ms.

[0057] With triggering at 1003, the method typically provides for issuing a triggering action to open a circuit breaker, such as three-phase circuit breaker 70 of FIGS.

[0058] While the present disclosure illustrates certain embodiments and aspects in detail with reference to the drawings and foregoing description, any such illustration and description should be considered exemplary or illustrative and not restrictive. The invention is not limited to the disclosed embodiments.

[0059] Other variations to the disclosed embodiments will be apparent to those skilled in the art. In the claims, the term "comprise" does not exclude other elements or steps, and the indefinite article "a" / "an" does not exclude a plurality. A single processor or controller or other unit may fulfill the functions of several items in the description or claims. For example, the current controllers 60, 61 may be a single unit. For example, the current controllers 60, 61, or parts thereof, may be integrated with the thyristor control circuit 50. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be interpreted as limiting the scope. The following is a summary of the claims as originally filed: [1] A thyristor circuit (100), at least one branch comprising at least one thyristor (10, 20); a thyristor control circuit (50) for selectively triggering the thyristors; a current detector (60, 61) configured to detect a current value representative of a current flowing through the thyristor and input the detected current value to the thyristor control circuit; wherein the thyristor control circuit (50) is configured to determine a fault condition in which the detected current value exceeds a predetermined current threshold based on a degraded blocking capability of the thyristor, and to trigger the thyristor into a conductive state in response to the determination. [2] The thyristor circuit (100) according to [1], wherein the degraded blocking ability of the thyristor depends on a junction temperature of the thyristor. [3] The thyristor circuit (100) of [1] or [2], wherein triggering the thyristor into the conductive state comprises continuously triggering the thyristor into the conductive state. [4] The thyristor circuit (100) according to any one of [1] to [3], wherein the current threshold is set based on a junction temperature characteristic of the thyristor. [5] The thyristor circuit (100) of any one of [1] to [4], further comprising a circuit breaker (70), wherein the thyristor control circuit is further configured to control the circuit breaker when it successively triggers the thyristors, e.g. to interrupt the current. [6] The thyristor circuit (100) of any one of [1] to [5], wherein triggering the thyristor into the conductive state comprises continuously triggering the thyristor for at least 50 ms, preferably for at least 100 ms, more preferably for at least 130 ms, and even more preferably for at least 160 ms. [7] The thyristor circuit (100) according to any one of [1] to [6], wherein the thyristor circuit has a plurality of branches, each having at least one thyristor, typically three phase branches, each having at least one thyristor. [8] The thyristor circuit (100) of [7], wherein the thyristor control circuit is configured to determine the fault condition when the current value detected in at least one of the plurality of branches exceeds the predetermined current threshold. [9] The thyristor circuit (100) of [7] or [8], wherein the thyristor control circuit is configured to trigger the at least one thyristor in each branch into the conductive state in response to the fault condition.

[10] The thyristor circuit (100) according to any one of [1] to [9], wherein the at least one branch comprises a stack of thyristors (30, 40).

[11] The thyristor circuit (100) of

[10] , wherein the thyristor control circuit is configured to trigger all thyristors of the stacks (30, 40) in each branch into the conductive state in response to the fault condition.

[12] The thyristor circuit (100) of

[10] , wherein each of the branches comprises a stack (30, 40) of thyristors, and the thyristor control circuit is configured to trigger all thyristors of each stack (30, 40) in all branches into the conductive state in response to the fault condition.

[13] The thyristor circuit (100) according to any one of [1] to

[12] , further comprising a network interface for connecting at least one of the thyristor control circuit (50) and the current detector (60, 61) to a network, the network interface being configured to transmit and receive digital signals between the thyristor control circuit (50) and / or the current detector (60, 61) and a data network, the digital signals including operation commands, typically set values ​​for the current thresholds, and / or information about the thyristor control circuit (50) and the current detector (60, 61) or the network.

[14] A thyristor protection method for protecting a thyristor in at least one branch of a thyristor circuit, the method comprising: detecting a current value representative of a current flowing through the thyristor (1001); determining whether the current value exceeds a predetermined current threshold based on a blocking capability dependent on a junction temperature of the thyristor (1002); triggering (1003) the thyristor into a conductive state in response to a result of the determination; A method comprising:

[15] The method of

[14] , wherein triggering the thyristor into the conductive state comprises continuously triggering the thyristor into the conductive state.

Claims

1. A thyristor circuit (100), comprising: at least one branch comprising at least one thyristor (10, 20); a thyristor control circuit (50) for selectively triggering said thyristors; a current detector (60, 61) configured to detect a current value representative of a current flowing through the thyristor and input the detected current value to the thyristor control circuit; Equipped with the thyristor control circuit (50) is configured to determine a fault condition in which the detected current value exceeds a predetermined current threshold based on a degraded blocking capability of the thyristor, and trigger the thyristor into a conductive state in response to the determination; The thyristor circuit (100), further comprising a circuit breaker (70), and the thyristor control circuit is further configured to control the circuit breaker to interrupt the current when it sequentially triggers the thyristors.

2. The thyristor circuit (100) of claim 1, wherein the degraded blocking ability of the thyristor is dependent on a junction temperature of the thyristor.

3. 3. The thyristor circuit (100) of claim 1 or 2, wherein triggering the thyristor into the conductive state comprises successively triggering the thyristor into the conductive state.

4. The thyristor circuit (100) of any one of claims 1 to 3, wherein the current threshold is set based on a junction temperature characteristic of the thyristor.

5. The thyristor circuit (100) of any one of claims 1 to 4, wherein triggering the thyristor into the conductive state comprises triggering the thyristor continuously for at least 50 ms.

6. The thyristor circuit (100) according to any one of claims 1 to 5, wherein the thyristor circuit comprises a plurality of branches each comprising at least one thyristor, typically three phase branches each comprising at least one thyristor.

7. 7. The thyristor circuit (100) of claim 6, wherein the thyristor control circuit is configured to determine the fault condition when the current value detected in at least one of the plurality of branches exceeds the predetermined current threshold.

8. 8. The thyristor circuit (100) of claim 6 or 7, wherein the thyristor control circuit is configured to trigger the at least one thyristor in each branch into the conductive state in response to the fault condition.

9. The thyristor circuit (100) according to any one of the preceding claims, wherein said at least one branch comprises a stack (30, 40) of thyristors.

10. 10. The thyristor circuit (100) of claim 9, wherein the thyristor control circuitry is configured to trigger all thyristors of the stacks (30, 40) in each branch into the conductive state in response to the fault condition.

11. 10. The thyristor circuit (100) of claim 9, wherein each of the branches comprises a stack (30, 40) of thyristors, and the thyristor control circuit is configured to trigger all thyristors of each stack (30, 40) in all branches into the conductive state in response to the fault condition.

12. The thyristor circuit (100) according to any one of claims 1 to 11, further comprising a network interface for connecting at least one of the thyristor control circuit (50) and the current detector (60, 61) to a network, the network interface being configured to transmit and receive digital signals between the thyristor control circuit (50) and / or the current detector (60, 61) and a data network, the digital signals including operation commands, typically set values ​​for the current thresholds, and / or information about the thyristor control circuit (50) and the current detector (60, 61) or the network.

13. 1. A thyristor protection method for protecting a thyristor in at least one branch of a thyristor circuit, the method comprising the steps of: detecting a current value representative of a current flowing through the thyristor (1001); determining whether the current value exceeds a predetermined current threshold based on a blocking capability dependent on a junction temperature of the thyristor (1002); triggering the thyristor into a conductive state in response to the result of the determination (1003); interrupting the current when the thyristors are successively triggered; A method comprising:

14. The method of claim 13 , wherein triggering the thyristors into the conductive state comprises sequentially triggering the thyristors into the conductive state.

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