Coupling controllable LCC converter based on reverse voltage blocking and control method thereof

Abstract

The invention discloses a coupling controllable LCC converter based on reverse voltage blocking and a control method thereof, the coupling controllable LCC converter comprises a half-controlled thyristor module and a full-controlled IGBT device module which are connected in series, and the working mode is switched by controlling the active turn-off of the full-controlled IGBT device module. Comprising a normal operation commutation mode and an AC fault forced commutation operation commutation mode, and when a fault occurs at the AC side of a receiving-end power grid, the reverse voltage provided by the second RC branch accelerates the recovery of the forward blocking capability of the thyristor, so that the commutation failure fault is resisted, the problem that the traditional LCC-HVDC system cannot resist the commutation failure is solved, and the reliability of the system is improved. The stability of a power transmission system is improved and a good application prospect is possessed.

Description

Technical Field

[0001] The present invention relates to the technical field of high voltage direct current transmission, and in particular to a coupled controllable LCC converter based on reverse voltage blocking and a control method thereof. Background Art

[0002] Commutation failure is one of the most common faults in traditional LCC-HVDC projects. It is very easy to occur when a fault occurs in the receiving grid, resulting in a surge in DC current, a decrease in DC voltage and transmission power, and affecting the stable operation of the grid. Therefore, it is of great significance to study new LCC converter topologies that can resist commutation failure.

[0003] The fundamental reason for the commutation failure of the commutation converter of the traditional power grid is that the thyristor device it uses has not been reliably shut down to restore the forward blocking capability. After the commutation process of the two bridge arms of the rectifier is completed, the valve group that was originally subjected to the forward voltage will be subjected to the reverse voltage. At this time, the thyristor device needs a certain amount of time to complete the carrier recombination and restore the forward blocking capability. Usually this period of time is about 400μs, equivalent to an electrical angle of 7.2°. If the forward voltage is reapplied to both ends of the thyristor device before the forward blocking capability is restored, the thyristor device will not be triggered to conduct, resulting in commutation failure. At present, the valve group on the rectifier side will be in a state of reverse voltage for a long time after the commutation is completed.

[0004] Therefore, the commutation failure fault on the rectifier side only occurs when the trigger circuit fails, and most of the commutation failure faults in the HVDC system occur on the inverter side. How to uniformly solve the problem that the traditional LCC-HVDC DC system cannot resist commutation failure is currently in urgent need of resolution. Summary of the invention

[0005] The purpose of the present invention is to overcome the problem of commutation failure in the fault state of the existing commutation converter. The coupled controllable LCC converter based on reverse voltage blocking and the control method thereof provided by the present invention control the active shutdown switching working mode of the fully controlled IGBT device module, including the normal operation commutation mode and the AC fault forced commutation operation commutation mode. When a fault occurs on the AC side of the receiving end power grid, the reverse voltage provided by the second RC resistor-capacitor branch accelerates the thyristor to restore the forward blocking capability, thereby resisting the occurrence of commutation failure, solving the problem that the traditional LCC-HVDC direct current system cannot resist commutation failure, improving the stability of the transmission system, and having good application prospects.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A coupled controllable LCC converter based on reverse voltage blocking, comprising a half-controlled thyristor module and a fully controlled IGBT device module connected in series.

[0008] The half-controlled thyristor module includes a thyristor THY, the positive electrode of the thyristor THY is connected to the commutation bridge arm as the input end of the half-controlled thyristor module, and the negative electrode of the thyristor THY is connected to the input end of the full-controlled IGBT device module as the output end of the half-controlled thyristor module.

[0009] The fully controlled IGBT device module includes an IGBT device with an anti-parallel diode, the collector of the IGBT device is connected to the output end of the half-controlled thyristor module as the input end of the fully controlled IGBT device module, and the emitter of the IGBT device is connected to the ground.

[0010] If the input end of the half-controlled thyristor module is connected to the upper bridge arm of the commutation, the gate of the IGBT device is connected to the corresponding phase line of the bridge arm on the AC side of the power grid as the output end of the coupled controllable LCC converter; if the input end of the half-controlled thyristor module is connected to the lower bridge arm of the commutation, the gate of the thyristor THY is connected to the corresponding phase line of the bridge arm on the AC side of the power grid as the output end of the coupled controllable LCC converter.

[0011] A first RC resistor-capacitor branch is connected in parallel between the positive input terminal and the negative output terminal of the thyristor THY, a second RC resistor-capacitor branch is connected in parallel between the collector and the emitter of the IGBT device, and a lightning arrester is connected in parallel between the input terminal of the half-controlled thyristor module and the output terminal of the fully-controlled IGBT device module.

[0012] Preferably, the first RC resistor-capacitor branch and the second RC resistor-capacitor branch are both used to implement dynamic voltage-sharing buffering, and the first RC resistor-capacitor branch includes a capacitor C connected in series. d1 , resistor R d1 The second RC branch includes an associated capacitor C t , resistor R1.

[0013] Preferably, the arrester is an MOV zinc oxide arrester, which is used to provide voltage support for phase switching in the event of a fault.

[0014] A control method for a coupled controllable LCC converter based on reverse voltage blocking, including a normal operation commutation mode and an AC fault forced commutation operation commutation mode.

[0015] In the normal operation commutation mode, the current flows through the path formed by the half-controlled thyristor module and the fully-controlled IGBT device module. The specific control process includes the following steps:

[0016] Step (A1), the IGBT device is in the non-shutdown stage: during normal operation, the coupled controllable LCC converter starts commutation, at which time, the IGBT device in the fully controlled IGBT device module is turned on;

[0017] Step (A2), the IGBT device is in the start-off stage: during the commutation process of the coupled controllable LCC converter, when the IGBT device connected in series with the half-controlled thyristor module is actively turned off in a timely manner, the current begins to transfer to the second RC resistor-capacitor branch connected in parallel with the IGBT device;

[0018] Step (A3), the IGBT device is in the fully turned-off stage: when the commutation of the coupled controllable LCC converter is about to end, the IGBT device has been completely turned off, and a small current flow path is formed by the second RC resistor-capacitor branch and the half-controlled thyristor module in series, and the turn-off current is about to cross zero;

[0019] Step (A4), the thyristor THY blocking capacity recovery stage: after the coupled controllable LCC converter commutation is completed, the shutdown current passes through zero, a reverse current is formed, and the thyristor THY begins to recover the blocking capacity. During the entire blocking capacity recovery process, the second RC resistor-capacitor branch provides a reverse current flow path until the half-controlled thyristor module completely recovers the forward blocking capacity and completes the normal operation commutation mode.

[0020] The AC fault forced commutation operation mode, when the AC line of the receiving-end power grid fails, the coupled controllable LCC converter operates in the AC fault forced commutation operation mode, and the specific control process includes the following steps:

[0021] Step (B1), the IGBT device is in the non-shutdown stage: when operating in the fault state, the coupled controllable LCC converter starts commutation, at which time, the IGBT device in the fully controlled IGBT device module is turned on;

[0022] Step (B2), the IGBT device is in the start-off stage: during the commutation process of the coupled controllable LCC converter, when the IGBT device connected in series with the half-controlled thyristor module is actively turned off in a timely manner, the current begins to transfer to the second RC resistor-capacitor branch connected in parallel with the IGBT device;

[0023] Step (B3), the IGBT device is in the fully turned-off stage: When the coupled controllable LCC converter is about to end the commutation, the IGBT device has been completely turned off. Due to the increase of DC current in the fault condition, the fault current continues to flow to the capacitor C t During charging, the half-controlled thyristor module has not been turned off and the fault current is still flowing;

[0024] Step (B4), the arrester operates to provide auxiliary commutation voltage stage: when the voltage charged on the capacitor Ct reaches the action voltage of the parallel arrester, the arrester operates to provide auxiliary commutation voltage to compensate for the voltage drop caused by the AC fault. At this time, the fault current flows through the arrester branch;

[0025] Step (B5), the stage of accelerating the recovery of the forward blocking capability of the thyristor THY: the thyristor THY bears the reverse voltage of the capacitor Ct, accelerates the recovery of the forward blocking capability, resists the commutation failure, and completes the work of forced commutation operation due to AC fault.

[0026] Preferably, in step (B5), the thyristor THY accelerates the recovery of the forward blocking capability stage: the thyristor THY withstands the capacitor C t The reverse voltage accelerates the recovery of the forward blocking capability and resists commutation failure. Whether commutation failure occurs is determined by the turn-off angle γ of the half-controlled thyristor module, as shown in formula (1):

[0027]

[0028] Where β is the triggering advance angle of the half-controlled thyristor module; X r is the equivalent commutation reactance of the half-controlled thyristor module; I d is the DC current flowing through the half-controlled thyristor module; U2 is the effective value of the winding line voltage on the converter transformer valve side;

[0029] Assuming that the angular frequency ω of the coupled controllable LCC converter remains unchanged, the critical turn-off angle γ0 is determined by the turn-off characteristics of the thyristor THY, as shown in formula (2):

[0030] γ0=ω(t rr +t gr ) (2)

[0031] Among them, t rr is the reverse blocking recovery time of thyristor THY; t gr is the forward blocking recovery time of the thyristor THY; the turn-off angle difference △γ is as shown in formula (3):

[0032] △γ=γ-γ0 (3)

[0033] When △γ<0, it is determined that commutation failure occurs, because t rr and t gr Affected by the reverse external voltage u during the turn-off process of the thyristor THY R , the valve current zero crossing point and the valve current change rate factors. Therefore, under different shutdown conditions, γ0 will change to a certain extent, affecting the accurate judgment of the system commutation failure. The thyristor THY bears the reverse voltage of the capacitor Ct, which can accelerate the recovery of the thyristor THY blocking ability, thereby resisting the occurrence of commutation failure.

[0034] The beneficial effects of the present invention are as follows: the coupled controllable LCC converter based on reverse voltage blocking and the control method thereof of the present invention, by controlling the active shutdown switching working mode of the fully controlled IGBT device module, including the normal operation commutation mode and the AC fault forced commutation operation commutation mode, can transfer the fault current when the commutation voltage drops due to a bus fault at the receiving end of the high-voltage direct current transmission system (a fault occurs in the AC side of the receiving end power grid), and accelerate the thyristor to restore the forward blocking capability through the reverse voltage provided by the second RC resistor-capacitor branch, thereby resisting the occurrence of commutation failure faults, solving the problem that the traditional LCC-HVDC direct current system cannot resist commutation failure, and improving the stability of the transmission system. Moreover, the topology transformation scheme of the present invention is a series transformation scheme, which is easy to implement in actual engineering applications, and has a low transformation cost, and has a good application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a topological structure diagram of a coupled controllable LCC converter based on reverse voltage blocking of the present invention;

[0036] Figure 2 The invention is a coupled controllable LCC converter based on reverse voltage blocking.

[0037] Body installation position

[0038] Schematic diagram;

[0039] Figure 3 is a current flow diagram of a normal operating commutation mode of a coupled controllable LCC converter of the present invention;

[0040] Figure 4 It is a current flow diagram of the AC fault forced commutation operation commutation mode of the coupled controllable LCC converter of the present invention;

[0041] Figure 5 : is the inverter side AC voltage and current waveform of the coupled controllable LCC converter in the verification experiment of the present invention;

[0042] Figure 6 It is the voltage waveform at both ends of the thyristor THY of the coupled controllable LCC converter in the verification experiment of the present invention. DETAILED DESCRIPTION

[0043] The present invention will be further described below in conjunction with the accompanying drawings.

[0044] like Figure 1 As shown, the coupled controllable LCC converter based on reverse voltage blocking of the present invention comprises a half-controlled thyristor module and a fully-controlled IGBT device module connected in series.

[0045] The half-controlled thyristor module includes a thyristor THY, the positive electrode of the thyristor THY is connected to the commutation bridge arm as the input end of the half-controlled thyristor module, and the negative electrode of the thyristor THY is connected to the input end of the full-controlled IGBT device module as the output end of the half-controlled thyristor module.

[0046] The fully controlled IGBT device module includes an IGBT device with an anti-parallel diode, the collector of the IGBT device is connected to the output end of the half-controlled thyristor module as the input end of the fully controlled IGBT device module, and the emitter of the IGBT device is connected to the ground.

[0047] like Figure 2 As shown, the specific installation position of the coupled controllable LCC converter based on reverse voltage blocking, if the input end of the half-controlled thyristor module is connected to the upper bridge arm of the commutation, the gate of the IGBT device is connected to the corresponding phase line of the bridge arm on the AC side of the power grid as the output end of the coupled controllable LCC converter; if the input end of the half-controlled thyristor module is connected to the lower bridge arm of the commutation, the gate of the thyristor THY is connected to the corresponding phase line of the bridge arm on the AC side of the power grid as the output end of the coupled controllable LCC converter,

[0048] A first RC resistor-capacitor branch is connected in parallel between the positive input terminal and the negative output terminal of the thyristor THY, a second RC resistor-capacitor branch is connected in parallel between the collector and the emitter of the IGBT device, and a lightning arrester is connected in parallel between the input terminal of the half-controlled thyristor module and the output terminal of the fully-controlled IGBT device module.

[0049] Preferably, the first RC resistor-capacitor branch and the second RC resistor-capacitor branch are both used to implement dynamic voltage-sharing buffering, so as to improve the stability and reliability of the entire circuit. The first RC resistor-capacitor branch includes a capacitor C connected in series. d1 , resistor R d1 The second RC branch includes an associated capacitor C t , resistor R1.

[0050] Preferably, the arrester is an MOV zinc oxide arrester, which is used to provide voltage support for phase switching in the event of a fault.

[0051] The control method of the coupled controllable LCC converter based on reverse voltage blocking of the present invention includes a normal operation commutation mode and an AC fault forced commutation operation commutation mode.

[0052] In the normal operation commutation mode, the current flows through the path formed by the half-controlled thyristor module and the fully controlled IGBT device module, combined with Figure 3 , the specific control process includes the following steps,

[0053] Step (A1), the IGBT device is in the non-shutdown stage: during normal operation, the coupled controllable LCC converter starts commutation, at which time, the IGBT device in the fully controlled IGBT device module is turned on;

[0054] Step (A2), the IGBT device is in the start-off stage: during the commutation process of the coupled controllable LCC converter, when the IGBT device connected in series with the half-controlled thyristor module is actively turned off in a timely manner, the current begins to transfer to the second RC resistor-capacitor branch connected in parallel with the IGBT device;

[0055] Step (A3), the IGBT device is in the fully turned-off stage: when the commutation of the coupled controllable LCC converter is about to end, the IGBT device has been completely turned off, and a small current flow path is formed by the second RC resistor-capacitor branch and the half-controlled thyristor module in series, and the turn-off current is about to cross zero;

[0056] Step (A4), the thyristor THY blocking capacity recovery stage: after the coupled controllable LCC converter commutation is completed, the shutdown current passes through zero, a reverse current is formed, and the thyristor THY begins to recover the blocking capacity. During the entire blocking capacity recovery process, the second RC resistor-capacitor branch provides a reverse current flow path until the half-controlled thyristor module completely recovers the forward blocking capacity and completes the normal operation commutation mode.

[0057] The AC fault forced commutation operation mode, when the AC measurement of the receiving end power grid fails, the coupled controllable LCC converter works in the AC fault forced commutation operation mode, combined with Figure 4 , the specific control process includes the following steps,

[0058] Step (B1), the IGBT device is in the non-shutdown stage: when operating in the fault state, the coupled controllable LCC converter starts commutation, at which time, the IGBT device in the fully controlled IGBT device module is turned on;

[0059] Step (B2), the IGBT device is in the start-off stage: during the commutation process of the coupled controllable LCC converter, when the IGBT device connected in series with the half-controlled thyristor module is actively turned off in a timely manner, the current begins to transfer to the second RC resistor-capacitor branch connected in parallel with the IGBT device;

[0060] Step (B3), the IGBT device is in the fully turned-off stage: When the coupled controllable LCC converter is about to end the commutation, the IGBT device has been completely turned off. Due to the increase of DC current in the fault condition, the fault current continues to flow to the capacitor C t During charging, the half-controlled thyristor module has not been turned off and the fault current is still flowing;

[0061] Step (B4), the arrester operates to provide auxiliary commutation voltage stage: when the voltage charged on the capacitor Ct reaches the action voltage of the parallel arrester, the arrester operates to provide auxiliary commutation voltage to compensate for the voltage drop caused by the AC fault. At this time, the fault current flows through the arrester branch;

[0062] Step (B5), the stage of accelerating the recovery of the forward blocking capability of the thyristor THY: the thyristor THY bears the reverse voltage of the capacitor Ct, accelerates the recovery of the forward blocking capability, resists the commutation failure, and completes the work of forced commutation operation due to AC fault.

[0063] Preferably, in step (B5), the thyristor THY accelerates the recovery of the forward blocking capability stage: the thyristor THY withstands the capacitor C t The reverse voltage accelerates the recovery of the forward blocking capability and resists commutation failure. Whether commutation failure occurs is determined by the turn-off angle γ of the half-controlled thyristor module, as shown in formula (1):

[0064]

[0065] Where β is the triggering advance angle of the half-controlled thyristor module; X r is the equivalent commutation reactance of the half-controlled thyristor module; I d is the DC current flowing through the half-controlled thyristor module; U2 is the effective value of the winding line voltage on the converter transformer valve side;

[0066] Assuming that the angular frequency ω of the coupled controllable LCC converter remains unchanged, the critical turn-off angle γ0 is determined by the turn-off characteristics of the thyristor THY, as shown in formula (2):

[0067] γ0=ω(t rr +t gr ) (2)

[0068] Among them, t rr is the reverse blocking recovery time of thyristor THY; t gr is the forward blocking recovery time of the thyristor THY; the turn-off angle difference △γ is as shown in formula (3):

[0069] △γ=γ-γ0 (3)

[0070] When △γ<0, it is determined that commutation failure occurs, because t rr and t gr Affected by the reverse external voltage u during the turn-off process of the thyristor THY R , the valve current zero crossing point and the valve current change rate factors. Therefore, under different shutdown conditions, γ0 will change to a certain extent, affecting the accurate judgment of the system commutation failure. The thyristor THY bears the reverse voltage of the capacitor Ct, which can accelerate the recovery of the thyristor THY blocking ability, thereby resisting the occurrence of commutation failure.

[0071] The coupled controllable LCC converter based on reverse voltage blocking of the present invention transfers the fault current through the action of the lightning arrester in the event of a fault. At this time, the reverse voltage of the capacitor Ct is connected in series with the thyristor THY, thereby compensating for the voltage drop caused by the AC bus fault, accelerating the recovery of the blocking capacity of the thyristor THY to resist commutation failure, and completing fault crossing by offsetting the fault current through the capacitor current under a DC fault.

[0072] The coupled controllable LCC converter based on reverse voltage blocking and the control method thereof of the present invention are verified by the following experiment: the receiving-end LCC converter of the conventional LCC-HVDC direct current transmission system is replaced with the coupled controllable LCC converter of the present invention, and a single-phase short-circuit grounding fault is applied. The simulation waveform is as follows: Figure 5 As shown. Figure 5 It can be seen from the figure that in the case of a fault at the receiving end, the coupled controllable LCC converter can actively shut down the IGBT device to transfer the fault current to the arrester branch, transfer the large fault current through the arrester, and establish an action voltage to enhance the commutation voltage, so that the current is continuous and the commutation failure is successfully prevented. At this time, the voltage waveform at both ends of the thyristor THY is as follows Figure 6 As shown, it can be seen that during the commutation period, the thyristor still has enough arc extinction angle margin to restore the forward blocking capability.

[0073] In summary, the coupled controllable LCC converter based on reverse voltage blocking and the control method thereof of the present invention, by controlling the active shutdown switching working mode of the fully controlled IGBT device module, including the normal operation commutation mode and the AC fault forced commutation operation commutation mode, can transfer the fault current when the commutation voltage drops due to a bus fault at the receiving end of the high-voltage direct current transmission system (a fault occurs in the AC side of the receiving end power grid), and accelerate the thyristor to restore the forward blocking capability through the reverse voltage provided by the second RC resistor-capacitor branch, thereby resisting the occurrence of commutation failure faults, solving the problem that the traditional LCC-HVDC direct current system cannot resist commutation failure, and improving the stability of the transmission system. Moreover, the topology transformation scheme of the present invention is a series transformation scheme, which is easy to implement in actual engineering applications, and has a low transformation cost, and has a good application prospect.

[0074] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A coupled controllable LCC converter based on reverse voltage blocking, characterized in that: Including half-controlled thyristor modules and fully controlled IGBT device modules connected in series. The half-controlled thyristor module includes a thyristor THY, the positive electrode of the thyristor THY is connected to the commutation bridge arm as the input end of the half-controlled thyristor module, and the negative electrode of the thyristor THY is connected to the input end of the full-controlled IGBT device module as the output end of the half-controlled thyristor module. The fully controlled IGBT device module includes an IGBT device with an anti-parallel diode, the collector of the IGBT device is connected to the output end of the half-controlled thyristor module as the input end of the fully controlled IGBT device module, and the emitter of the IGBT device is connected to the ground. If the input end of the half-controlled thyristor module is connected to the commutation upper bridge arm, the gate of the IGBT device is connected to the corresponding phase line of the bridge arm on the AC side of the power grid as the output end of the coupled controllable LCC converter; If the input end of the half-controlled thyristor module is connected to the lower commutation bridge arm, the gate of the thyristor THY is connected to the corresponding phase line of the bridge arm on the AC side of the power grid as the output end of the coupled controllable LCC converter. A first RC resistor-capacitor branch is connected in parallel between the positive input terminal and the negative output terminal of the thyristor THY, a second RC resistor-capacitor branch is connected in parallel between the collector and the emitter of the IGBT device, and a lightning arrester is connected in parallel between the input terminal of the half-controlled thyristor module and the output terminal of the fully-controlled IGBT device module.

2. The coupled controllable LCC converter based on reverse voltage blocking according to claim 1, characterized in that: The first RC RC branch and the second RC branch are both used to implement dynamic voltage-sharing buffering. The first RC branch includes a capacitor Cd1 and a resistor Rd1 connected in series, and the second RC branch includes an associated capacitor Ct and a resistor R1.

3. The coupled controllable LCC converter based on reverse voltage blocking according to claim 1, characterized in that: The arrester is a MOV zinc oxide arrester, which is used to provide voltage support for phase switching in the event of a fault.

4. A control method for a coupled controllable LCC converter based on reverse voltage blocking according to any one of claims 1 to 3, characterized in that: Including normal operation commutation mode and AC fault forced commutation operation commutation mode. In the normal operation commutation mode, the current flows through the path formed by the half-controlled thyristor module and the fully-controlled IGBT device module. The specific control process includes the following steps: Step (A1), the IGBT device is in the non-shutdown stage: during normal operation, the coupled controllable LCC converter starts commutation, at which time, the IGBT device in the fully controlled IGBT device module is turned on; Step (A2), the IGBT device is in the start-off stage: during the commutation process of the coupled controllable LCC converter, when the IGBT device connected in series with the half-controlled thyristor module is actively turned off in a timely manner, the current begins to transfer to the second RC resistor-capacitor branch connected in parallel with the IGBT device; Step (A3), the IGBT device is in the fully turned-off stage: when the commutation of the coupled controllable LCC converter is about to end, the IGBT device has been completely turned off, and a small current flow path is formed by the second RC resistor-capacitor branch and the half-controlled thyristor module in series, and the turn-off current is about to cross zero; Step (A4), the thyristor THY blocking capacity recovery stage: after the coupled controllable LCC converter commutation is completed, the shutdown current passes through zero, a reverse current is formed, and the thyristor THY begins to recover the blocking capacity. During the entire blocking capacity recovery process, the second RC resistor-capacitor branch provides a reverse current flow path until the half-controlled thyristor module completely recovers the forward blocking capacity and completes the normal operation commutation mode. The AC fault forced commutation operation mode, when the AC line of the receiving-end power grid fails, the coupled controllable LCC converter operates in the AC fault forced commutation operation mode, and the specific control process includes the following steps: Step (B1), the IGBT device is in the non-shutdown stage: when operating in the fault state, the coupled controllable LCC converter starts commutation, at which time, the IGBT device in the fully controlled IGBT device module is turned on; Step (B2), the IGBT device is in the start-off stage: during the commutation process of the coupled controllable LCC converter, when the IGBT device connected in series with the half-controlled thyristor module is actively turned off in a timely manner, the current begins to transfer to the second RC resistor-capacitor branch connected in parallel with the IGBT device; Step (B3), the IGBT device is in the fully turned-off stage: When the coupled controllable LCC converter is about to end the commutation, the IGBT device has been completely turned off. Due to the increase of DC current in the fault condition, the fault current continues to flow to the capacitor C t Charging, the half-controlled thyristor module has not been turned off and the fault current still flows; Step (B4), the arrester operates to provide auxiliary commutation voltage stage: when the voltage charged on the capacitor Ct reaches the action voltage of the parallel arrester, the arrester operates to provide auxiliary commutation voltage to compensate for the voltage drop caused by the AC fault. At this time, the fault current flows through the arrester branch; Step (B5), the stage of accelerating the recovery of the forward blocking capability of the thyristor THY: the thyristor THY bears the reverse voltage of the capacitor Ct, accelerates the recovery of the forward blocking capability, resists the commutation failure, and completes the work of forced commutation operation due to AC fault.

5. The control method of the coupled controllable LCC converter based on reverse voltage blocking according to claim 4, characterized in that: Step (B5), the thyristor THY accelerates the recovery of forward blocking capability: the thyristor THY withstands the capacitance C t The reverse voltage accelerates the recovery of the forward blocking capability and resists commutation failure. Whether commutation failure occurs is determined by the turn-off angle γ of the half-controlled thyristor module, as shown in formula (1): Among them, β is the triggering advance angle of the half-controlled thyristor module; Xr is the equivalent commutation reactance of the half-controlled thyristor module; Id is the DC current flowing through the half-controlled thyristor module; U2 is the effective value of the winding line voltage on the converter transformer valve side; Assuming that the angular frequency ω of the coupled controllable LCC converter remains unchanged, the critical turn-off angle γ0 is determined by the turn-off characteristics of the thyristor THY, as shown in formula (2): γ0=ω(trr+tgr) (2) Among them, t rr is the reverse blocking recovery time of thyristor THY; t gr is the forward blocking recovery time of the thyristor THY; the turn-off angle difference △γ is as shown in formula (3): △ γ＝γ-γ 0 (3) When △γ<0, it is determined that commutation failure occurs. Since trr and tgr are affected by the reverse external voltage uR, the valve current zero crossing point and the valve current change rate during the shutdown process of the thyristor THY, γ0 will change to a certain extent under different shutdown conditions, affecting the accurate determination of the system commutation failure. The thyristor THY withstands the reverse voltage of the capacitor Ct, which can accelerate the recovery of the blocking ability of the thyristor THY, thereby resisting the occurrence of commutation failure.

Images