Thyristor converter module, cascaded converter topology and control method thereof

Abstract

The disclosure provides a thyristor device type modular cascaded novel converter topology which completely solves the commutation failure problem, explores the circuit topology principle of thyristor and capacitor providing controllable commutation support and arrester providing energy transfer branch, constructs a thyristor converter module with controllable output voltage and impedance and automatic capacitor input, and proposes a novel converter topology structure which can completely resist commutation failure in the commutation process. Each converter valve is composed of at least two thyristor converter modules in cascade; the thyristor converter module is composed of four groups of thyristor bridge arms and a forced turn-off part. Each two groups of bridge arms are connected in series, left and right symmetrical, and the capacitor branch is connected in parallel with the branch where the arrester is located, and the whole after parallel connection is connected between the four groups of bridge arms as the forced turn-off part.

Description

Technical Field

[0001] This invention belongs to the field of high-power power electronic converter technology, specifically involving a thyristor-based modular cascaded converter topology and control method that completely solves the problem of commutation failure. Background Technology

[0002] Traditional line-commutated converter high-voltage direct current (LCC-HVDC) transmission systems offer advantages such as long-distance operation, large capacity, and controllable active power, leading to their widespread application globally. In HVDC transmission systems, the converter is a crucial component, enabling DC / AC or AC / DC conversion. Because thyristors lack self-turn-off capability, commutation failures are highly likely in the DC system when a fault occurs in the receiving-end AC system, causing a drop in the provided commutation voltage. With increasingly tight coupling in my country's power grid, the cascading commutation failures in multi-feed DC transmissions are becoming more severe. Commutation failures cause interruptions in active power transmission. If the reactive power compensation devices at the converter station are not promptly disconnected, residual reactive power can occur, potentially leading to transient overvoltages in the system. In cases with a high proportion of renewable energy integration, this can even result in large-scale grid disconnection, severely impacting the safe and stable operation of the power grid.

[0003] In existing technologies, common solutions involve using fully controlled semiconductor devices to construct voltage-type inverters or a hybrid design combining fully and semi-controlled devices. However, this approach, due to the use of power semiconductor devices such as IGBTs and IGCTs, requires consideration of their voltage withstand and current carrying capacity, as well as their maximum turn-off current capability. Therefore, there is a pressing need to develop high-voltage, high-current fully controlled devices, and extensive engineering testing is required, resulting in a complex control system. Furthermore, the static voltage equalization, turn-on frequency, and simultaneous turn-on / turn-off technology of IGBTs are also key considerations for such devices; any deviation can directly damage the devices.

[0004] To address the aforementioned problems, this invention provides a solution that offers cost advantages, eliminates the need for fault detection devices, and ensures high equipment reliability. This invention provides a novel modular cascaded converter topology using thyristor devices that completely resolves the commutation failure problem. This converter maintains the original advantages of LCC (Limited-Capacity Converter) power transmission (high capacity, low loss) while completely eliminating the commutation failure issue. Furthermore, the thyristor converter module offers flexible operation and simple control. Summary of the Invention

[0005] To address the aforementioned issues, this invention proposes a novel modular cascaded converter topology based on thyristor devices that completely resolves commutation failure problems, along with a novel converter module capable of forcibly shutting off the commutation current and a coordinated control method between cascaded modules. The converter topology employs a cascaded converter module structure to form a converter valve, utilizing thyristor devices exclusively, which offers cost advantages while improving converter reliability and simplifying control logic. Furthermore, it completely resolves commutation failure issues during converter operation to adapt to complex fault types in AC / DC systems.

[0006] To achieve the above objectives, the specific solution adopted by the present invention is as follows.

[0007] The first aspect of this invention provides a thyristor converter module, which is composed of a forced turn-off section and bridge arms. The bridge arms include a first bridge arm, a second bridge arm, a third bridge arm, and a fourth bridge arm. Each bridge arm is composed of multiple thyristors connected in series with the anode connected to the cathode in a forward direction. The first bridge arm is connected in series with the second bridge arm, and the third and fourth bridge arms are connected in series and then connected in parallel with the first and second bridge arms. One end of the forced turn-off section is connected between the first and second bridge arms, and the other end is connected between the third and fourth bridge arms. The forced turn-off section is composed of a capacitor and a surge arrester connected in parallel.

[0008] According to one embodiment of the present invention, each thyristor is connected in parallel with a voltage equalization resistor.

[0009] The second aspect of the present invention proposes a thyristor-based modular cascaded converter topology, wherein each converter valve of the converter topology is composed of at least two cascaded thyristor converter modules.

[0010] According to one embodiment of the present invention, each thyristor converter module is connected in parallel with a voltage equalization resistor.

[0011] According to one embodiment of the present invention, the converter topology sets the forward charging voltage threshold and reverse voltage threshold of the capacitor based on the principle of capacitor forced turn-off commutation of the thyristor converter module and energy transfer of the surge arrester. The reverse voltage threshold determines the operating voltage threshold of the surge arrester, and finally determines the thyristor control timing of the converter module and the feasible operating range of the trigger pulse for automatic capacitor connection without fault detection.

[0012] According to one embodiment of the present invention, the number of cascaded converter modules is determined by the system voltage level and the capacitor reverse voltage threshold.

[0013] A third aspect of this invention proposes a control method for a novel modular cascaded converter topology based on thyristor devices. This converter topology is used in high-voltage direct current transmission systems. When the system is in normal operation, the method includes the following steps:

[0014] Step 1: After the converter valve changes from the closed state to the open state, when the capacitor voltage is lower than the forward charging voltage threshold, the first bridge arm and the fourth bridge arm in the converter module are triggered. The first bridge arm and the fourth bridge arm of the converter module are continuously turned on, and the system forward charges the capacitor until the capacitor voltage reaches the forward charging voltage threshold. At this time, the second bridge arm and the third bridge arm bear the positive voltage across the capacitor.

[0015] Step 2: After the forward voltage of the capacitor reaches the forward charging voltage threshold, the second bridge arm is triggered, the fourth bridge arm is turned off by the reverse voltage of the capacitor, the first bridge arm and the second bridge arm are in the conducting state, and the forced turn-off part is in the bypass state.

[0016] Step 3: Trigger the third bridge arm at the commutation point when the converter valve is running normally. At this time, the current of the converter valve naturally crosses zero. Although there is a trigger pulse, all four bridge arms are in the blocking state. The forced turn-off part is not engaged. The voltage equalization resistor connected in parallel to the thyristor balances the voltage division of the thyristor on each bridge arm.

[0017] According to one embodiment of the present invention, the converter topology is used in a high-voltage direct current transmission system. When the converter valve is operating in a fault state, the method includes the following steps:

[0018] Step 1: After the converter valve changes from the closed state to the open state, when the capacitor voltage is lower than the forward charging voltage threshold, the first bridge arm and the fourth bridge arm in the converter module are triggered. The first bridge arm and the fourth bridge arm of the converter module are continuously turned on, and the system forward charges the capacitor until the capacitor voltage reaches the forward charging voltage threshold. At this time, the second bridge arm and the third bridge arm bear the positive voltage across the capacitor.

[0019] Step 2: After the forward voltage of the capacitor reaches the forward charging voltage threshold, the second bridge arm is triggered, the fourth bridge arm is turned off by the reverse voltage of the capacitor, the first bridge arm and the second bridge arm are in the conducting state, and the forced turn-off part is in the bypass state.

[0020] Step 3: The third bridge arm is triggered at the commutation point when the converter valve is operating normally. At this time, due to the drop in commutation voltage, the current of the converter valve has not dropped to zero, the third bridge arm is turned on, and the first bridge arm is turned off by the reverse voltage threshold of the capacitor. After that, the current flow path is the second bridge arm, the capacitor, and the third bridge arm, and the forced turn-off part is engaged.

[0021] Step 4: When the capacitor voltage reverses to the arrester operating voltage, the arrester operates to provide an energy transfer branch and simultaneously limits capacitor overvoltage. During the charging process, each module capacitor in the converter valve is connected in series through the second and third valve arms, and the voltages of multiple module capacitors are directly added together. The reverse voltage threshold of the converter module capacitor is set according to the forced shutdown requirement. When the voltage across the converter valve reaches the AC commutation voltage, the voltage across each converter module is zero, and the current flowing through that valve arm is also zero, thus forcing the bridge arm to shut down.

[0022] Step 5: When the capacitor is reverse charged to the reverse voltage threshold, if the bridge arm is not turned off, in order to prevent the capacitor voltage from exceeding the rated value, the fourth bridge arm thyristor group is triggered, the fourth bridge arm is turned on, the second bridge arm thyristor is turned off under the capacitor reverse voltage, the third and fourth bridge arms are in the on state, and the forced turn-off part of the converter module is in the bypass state.

[0023] Step 6: When the valve arm changes from the off state to the on state, a trigger pulse is applied to the thyristor group of the first bridge arm and the fourth bridge arm. The capacitor discharges in reverse to promote the valve arm to conduct. After the valve arm is conducted, the current flow path is the first bridge arm, the capacitor, and the fourth bridge arm. The capacitor discharges first and then charges in the forward direction.

[0024] According to one embodiment of the present invention, the third bridge arm is triggered at the commutation point when the converter valve is operating normally, and the converter module is turned on by pulse timing control;

[0025] According to the requirement of forced shutdown, the voltage across the converter valve is changed by altering the number of converter modules or the reverse voltage threshold of the module capacitor, thereby achieving forced shutdown of the converter valve.

[0026] According to one embodiment of the present invention, by automatically connecting the capacitor during a fault in the converter module pulse timing control system, the second and third arms of the cascaded converter module are continuously conducting to charge the capacitor before the capacitor voltage amplitude reaches the reverse voltage threshold.

[0027] Based on whether the capacitor charging voltage reaches the arrester's operating voltage, the fault level is automatically determined, and it is decided whether the arrester needs to be activated to provide an energy transfer branch and limit capacitor overvoltage. The two work together to achieve forced shut-off of the converter valve.

[0028] According to one embodiment of the present invention, when the converter valve is cleared of a fault and resumes normal operation, the third bridge arm is triggered. At this time, since the fault has been cleared, the current of the converter valve naturally crosses zero. Although there is a trigger pulse, all four bridge arms are still in the blocking state. The forced shutdown part is not engaged, and the converter valve automatically switches to the normal operation state. The voltage on each thyristor is balanced by the voltage equalizing resistor so that it is within the allowable range of the device.

[0029] The beneficial effects of this invention are:

[0030] 1. The novel modular cascaded converter topology based on thyristor devices proposed in this invention completely solves the problem of commutation failure. It is suitable for traditional high-voltage direct current transmission, can completely solve the problem of commutation failure, and improve the stability of system operation.

[0031] 2. The novel modular cascaded converter topology based on thyristor devices proposed in this invention completely solves the problem of commutation failure. The number of converter modules and the reverse charging threshold of the capacitor are determined according to the voltage level of the system. It can flexibly adapt to DC transmission systems of different voltage levels and meet the requirement of completely solving the problem of commutation failure.

[0032] 3. The thyristor-based modular cascaded novel converter topology proposed in this invention completely solves the problem of commutation failure. All power semiconductor devices of the converter valve are thyristors, which have good pressure bearing and current carrying capacity, thus improving the safety and reliability of the converter.

[0033] 4. The novel thyristor-based modular cascade converter topology proposed in this invention completely solves the problem of commutation failure. When the system is running normally and does not require forced shutdown, the capacitor of the converter module is only in a forward voltage threshold hot standby state. The loss is minimal during steady-state operation, and no fault detection device is required, thus reducing the cost of the converter.

[0034] 5. The thyristor-based modular cascaded novel converter topology proposed in this invention completely solves the problem of commutation failure. The converter module bridge arm uses thyristors as the path, so the capacitor can be selected as a high-voltage device type, which greatly reduces the number of converter modules and has great advantages in terms of device cost and control simplicity.

[0035] 6. The thyristor-based modular cascaded converter topology proposed in this invention completely solves the commutation failure problem and enables the capacitor to self-charge without the need for an external power supply.

[0036] This invention discloses a novel modular cascaded converter topology based on thyristor devices that completely solves the problem of commutation failure. It explores the circuit topology principle of thyristors and capacitors providing controllable commutation support and surge arresters providing energy transfer branches. A thyristor commutation module with controllable output voltage and impedance and automatic capacitor connection is constructed. A novel converter topology is proposed, featuring flexible and variable impedance structure, rapid controllability of voltage, time, and area during commutation, and the ability to completely resist commutation failure. Each commutation valve consists of at least two cascaded thyristor commutation modules; each thyristor commutation module consists of four sets of thyristor bridge arms and a forced turn-off section. Every two sets of bridge arms are connected in series, symmetrically arranged. The capacitor branch is connected in parallel with the surge arrester branch, and the entire parallel connection serves as the forced turn-off section connected between the four sets of bridge arms. This invention also discloses a novel converter module capable of forcibly shutting off commutation current and a control method for cascaded modules. Based on the principles of forced commutation shutdown by module capacitors and energy transfer by surge arresters, the control timing of the thyristors in the converter module and the feasible operating range of the trigger pulse for automatic capacitor activation without fault detection are determined. Valve group balancing control after cascading multiple modules is studied to ensure that the capacitors provide sufficient commutation voltage support and that the surge arresters provide a commutation energy transfer path. The novel converter can completely solve the commutation failure problem, is simple and easy to control, and has high operational reliability. Attached Figure Description

[0037] The accompanying drawings illustrate exemplary embodiments of the invention and, together with the description thereof, serve to explain the principles of the invention. These drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification.

[0038] Figure 1 This is a topology diagram of the novel modular cascaded converter based on thyristor devices of the present invention.

[0039] Figure 2 This is a topology diagram of the transistor commutation module of the present invention.

[0040] Figure 3 This describes the operating mode of the thyristor converter module of the present invention under normal system conditions.

[0041] Figure 4 This describes the operating mode of the thyristor converter module of the present invention under system failure conditions.

[0042] Reference numerals in the attached diagram: 31, first bridge arm of the thyristor converter module; 32, second bridge arm of the thyristor converter module; 33, third bridge arm of the thyristor converter module; 34, fourth bridge arm of the thyristor converter module; 35, capacitor of the thyristor converter module; 36, surge arrester of the thyristor converter module. Detailed Implementation

[0043] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be noted that, for ease of description, only the parts relevant to the present invention are shown in the accompanying drawings.

[0044] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. The technical solution of this invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0045] Unless otherwise stated, the exemplary embodiments / exemplifications shown are to be understood as providing exemplary features of various details that provide ways in which the technical concept of the invention can be implemented in practice. Therefore, unless otherwise stated, the features of the various embodiments / exemplifications may be additionally combined, separated, interchanged and / or rearranged without departing from the technical concept of the invention.

[0046] The terminology used herein is for the purpose of describing particular embodiments and is not restrictive. As used herein, unless the context clearly indicates otherwise, the singular forms “a” and “the” are intended to include the plural forms as well. Furthermore, when the terms “comprising” and / or “including” and variations thereof are used in this specification, it indicates the presence of the stated features, integrals, steps, operations, parts, components, and / or groups thereof, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, parts, components, and / or groups thereof.

[0047] This invention proposes a novel modular cascaded converter topology based on thyristor devices that completely solves the commutation failure problem, such as... Figure 1 As shown, in the converter topology of the present invention, each converter valve is composed of at least two thyristor converter modules cascaded in the same direction; the thyristor converter module consists of four sets of thyristor bridge arms and a forced turn-off section, the capacitor branch is connected in parallel with the branch where the surge arrester is located, and the whole after parallel connection is connected between the four sets of bridge arms as the forced turn-off section.

[0048] Figure 2 This invention relates to a novel modular cascaded converter topology converter module based on thyristor devices.

[0049] In this example, the converter valve capacitor branch and the surge arrester branch are connected in parallel as the forced turn-off part of the thyristor converter module. This part, together with the four bridge arms, forms a converter module, in which each thyristor has a parallel voltage divider resistor.

[0050] All the power semiconductor devices constituting the converter module are thyristors. Each bridge arm is composed of multiple thyristors connected in series with the anode connected to the cathode. The first bridge arm is connected in series with the second bridge arm, and the third and fourth bridge arms are connected in series and then connected in parallel with the first and second bridge arms. One end of the forced turn-off section is connected between the first and second bridge arms, and the other end is connected between the third and fourth bridge arms.

[0051] This novel modular cascaded converter topology, based on thyristor devices, can forcibly shut off the current of the corresponding converter valve during system faults, thus achieving normal commutation. The number of converter modules is determined by the system voltage level and the capacitor reverse voltage threshold 2.

[0052] In this example, the rated DC voltage is 500kV, the number of thyristor converter modules is 10, the capacitor reverse charging threshold 2 is calculated to be 30kV according to the forced turn-off command requirements, the forward charging voltage threshold 1 is 2kV, the surge arrester operating voltage threshold 3 is 15kV, the capacitor rated voltage is 30kV, and each thyristor bridge arm uses 6 thyristors with a voltage level of 7.2kV connected in series to form each bridge arm thyristor; the cathode of the first bridge arm is connected to the anode of the second bridge arm, and the cathode of the third bridge arm is connected to the anode of the fourth bridge arm; the anode of the first bridge arm is connected to the anode of the third bridge arm, and the cathode of the second bridge arm is connected to the cathode of the fourth bridge arm; one end of the forced turn-off section is connected between the first and second bridge arms, and the other end is connected between the third and fourth bridge arms.

[0053] In this example, when the system is in normal operation, the thyristor converter module proposed in this paper has a symmetrical structure. Figure 3 In the module's operating states, modes 1, 2, and 3 form one group, while modes 4, 5, and 6 form another group. These modes are symmetrical and operate alternately. Due to space limitations, only one group of operating modes will be described in detail; the other operating modes operate on the same principle. By monitoring the converter module's capacitor voltage, after the converter valve transitions from the off state to the on state, when the capacitor voltage falls below the forward voltage threshold 1, the first and fourth bridge arms in the module are triggered, the capacitor charges, and the module operates as follows: Figure 3 Mode 1 is shown. When the module's capacitor voltage exceeds the forward voltage threshold 1, a trigger pulse signal is sent to the second bridge arm. The fourth bridge arm is turned off under the reverse voltage of the capacitor, while the first and second bridge arms are in the on state. The forced shutdown part of the module is in the bypass state. This process is as follows: Figure 3 Mode 2 is shown. At the commutation point during normal operation, Δton = 120° + tμ after the converter valve transitions from the off state to the on state, a trigger pulse signal is sent to the third bridge arm. In normal operation mode, the converter valve current naturally crosses zero at this time. Although there is a trigger pulse, the third bridge arm will not conduct, and the first and second bridge arms will naturally turn off. This process is as follows: Figure 3 Pattern 3 is shown.

[0054] In this example, when a system fault occurs, module modes 1 and 2 are the same as in normal operation. When the converter valve changes from the on state to the off state and a system fault occurs at this time, after the third bridge arm is triggered, the converter valve current does not decrease to zero, the thyristor of the third bridge arm conducts, the first bridge arm is turned off under the action of the capacitor reverse voltage, and the DC current flow path is the second bridge arm and the third bridge arm, forcing the capacitor of the turned-off part to be connected, and the module operates as follows. Figure 4 Mode 3 is shown. This control allows for automatic capacitor activation after a converter valve failure without the need for additional fault detection devices. Once the module capacitor voltage discharges from the forward voltage threshold 1 to 0kV, it begins reverse charging. If the fault is severe, the capacitor voltage reaches the surge arrester's operating voltage threshold 3, triggering the surge arrester to provide an energy transfer path and limit capacitor overvoltage. During charging, each module capacitor in the converter valve is connected in series through the second and third bridge arms of the thyristor, and the voltages of multiple module capacitors are directly added. When the capacitor is reverse charged until the voltage across the converter valve reaches the AC commutation voltage, the voltage across each thyristor converter module becomes zero, forcing the bridge arm to turn off naturally. The module then operates as follows: Figure 4 Mode 4 is shown; when the capacitor is reverse-charged to the voltage threshold 2, if the bridge arm is not turned off, to prevent capacitor overvoltage, the fourth bridge arm is triggered, the fourth bridge arm is turned on, and the second bridge arm is turned off due to capacitor reverse voltage. At this time, the DC current flow path is the third and fourth bridge arms, and the forced turn-off part in the module will be bypassed. At this time, the module operates in mode 4. Figure 4 Mode 5 is shown; when the valve arm changes from the closed state to the open state, the first bridge arm is triggered, and the first and fourth bridge arms are turned on. The capacitor discharges, promoting the opening of the valve arm, and the module operates in mode 5. Figure 4 The working state corresponding to Mode 6 shown.

[0055] After the system fault is cleared, the third bridge arm is triggered at the commutation point when the converter valve is operating normally. Since the fault has been cleared, the current of the converter valve naturally crosses zero. Although there is a trigger pulse, the four bridge arms and the thyristor converter module's forced shutdown part are still in the blocking state, and the converter valve automatically switches to the normal operating state.

[0056] Those skilled in the art should understand that the above embodiments are merely for illustrating the present invention and are not intended to limit the scope of the invention. Those skilled in the art can make other changes or modifications based on the above disclosure, and these changes or modifications still fall within the scope of the present invention.

Claims

1. A control method for a novel modular cascaded converter topology using thyristor devices, characterized in that: The converter topology described herein is used in a high-voltage direct current (HVDC) transmission system. Each converter valve in the converter topology is composed of at least two cascaded thyristor converter modules. Each thyristor converter module consists of a forced turn-off section and a bridge arm combination. The bridge arm includes a first bridge arm, a second bridge arm, a third bridge arm, and a fourth bridge arm. Each bridge arm is composed of multiple thyristors connected in series with the anode connected to the cathode in a forward direction. The first bridge arm is connected in series with the second bridge arm, and the third and fourth bridge arms are connected in series and then in parallel with the first and second bridge arms. One end of the forced turn-off section is connected between the first and second bridge arms, and the other end is connected between the third and fourth bridge arms. The forced turn-off section is composed of a capacitor and a surge arrester connected in parallel. Each thyristor is connected in parallel with an equalizing resistor. When the system is in normal operating condition, the method includes: After the converter valve changes from the off state to the on state, when the capacitor voltage of the thyristor converter module is lower than the forward charging voltage threshold, the first bridge arm and the fourth bridge arm in the converter module are triggered. The first bridge arm and the fourth bridge arm of the converter module are continuously turned on, and the system forward charges the capacitor until the capacitor voltage reaches the forward charging voltage threshold. At this time, the second bridge arm and the third bridge arm bear the positive voltage across the capacitor. When the forward voltage of the capacitor reaches the forward charging voltage threshold, the second bridge arm is triggered, the fourth bridge arm is turned off under the reverse voltage of the capacitor, the first bridge arm and the second bridge arm are in the conducting state, and the forced turn-off part is in the bypass state. The third bridge arm is triggered at the commutation point when the converter valve is operating normally. At this time, the current of the converter valve naturally crosses zero. Although there is a trigger pulse, all four bridge arms are in the blocking state. The forced shutdown part is not engaged, and the voltage equalization resistor balances the voltage division of the thyristors on each bridge arm.

2. The control method for the thyristor-based modular cascaded novel converter topology as described in claim 1, characterized in that: When the system is in a fault state, the method includes the following steps: After the converter valve changes from the closed state to the open state, when the capacitor voltage is lower than the forward charging voltage threshold, the first bridge arm and the fourth bridge arm in the converter module are triggered. The first bridge arm and the fourth bridge arm of the converter module are continuously turned on, and the system forward charges the capacitor until the capacitor voltage reaches the forward charging voltage threshold. At this time, the second bridge arm and the third bridge arm bear the positive voltage across the capacitor. When the forward voltage of the capacitor reaches the forward charging voltage threshold, the second bridge arm is triggered, the fourth bridge arm is turned off under the reverse voltage of the capacitor, the first bridge arm and the second bridge arm are in the conducting state, and the forced turn-off part is in the bypass state. The third bridge arm is triggered at the commutation point when the converter valve is operating normally. At this time, due to the drop in commutation voltage, the current of the converter valve has not dropped to zero, the third bridge arm is turned on, and the first bridge arm is turned off by the reverse voltage threshold of the capacitor. After that, the current flow path is the second bridge arm, the capacitor, and the third bridge arm, and the forced turn-off part is engaged. When the capacitor voltage reverses to reach the surge arrester's operating voltage, the surge arrester operates to provide an energy transfer branch and simultaneously limit capacitor overvoltage. During the charging process, each module capacitor in the converter valve is connected in series through the second and third valve arms, and the voltages of multiple module capacitors are directly added together. The reverse voltage threshold of the converter module capacitor is set according to the forced shutdown requirement. When the voltage across the converter valve reaches the AC commutation voltage, the voltage across each converter module is zero, and the current flowing through that valve arm is also zero, thus forcing the bridge arm to shut down. When the capacitor is reverse charged to the reverse voltage threshold, if the bridge arm is not turned off, in order to prevent the capacitor voltage from exceeding the rated value, the fourth bridge arm thyristor group is triggered, the fourth bridge arm is turned on, the second bridge arm thyristor is turned off under the capacitor reverse voltage, the third and fourth bridge arms are in the on state, and the forced turn-off part of the converter module is in the bypass state. When the valve arm changes from the off state to the on state, a trigger pulse is applied to the thyristor group of the first bridge arm and the fourth bridge arm. The capacitor discharges in reverse to promote the valve arm to conduct. After the valve arm is conducted, the current flow path is the first bridge arm, the capacitor, and the fourth bridge arm. The capacitor discharges first and then charges in the forward direction.

3. The control method for the thyristor-based modular cascaded novel converter topology as described in claim 2, characterized in that: The third bridge arm is triggered at the commutation point when the converter valve is operating normally, and the converter module is turned on by pulse timing control. According to the requirement of forced shutdown, the voltage across the converter valve is changed by altering the number of converter modules or the reverse voltage threshold of the module capacitor, thereby achieving forced shutdown of the converter valve.

4. The control method for the thyristor-based modular cascaded novel converter topology as described in claim 3, characterized in that: During a fault in the pulse timing control system, the converter module automatically connects the capacitor. Before the capacitor voltage amplitude reaches the reverse voltage threshold, the second and third bridge arms of the converter module are continuously conducting to charge the capacitor. Based on whether the capacitor charging voltage reaches the surge arrester's operating voltage, the fault level is automatically determined, and it is decided whether the surge arrester needs to be activated to provide an energy transfer branch and limit capacitor overvoltage. The two work together to achieve forced shut-off of the converter valve.

Images