Direct current pre-charging starting control method of bridge arm alternating current converter

Abstract

The invention relates to the technical field of converter starting control, and discloses a direct-current pre-charging starting control method for a bridge arm alternating converter, which comprises the following steps of: firstly, establishing a parallel charging loop, and simultaneously supplying power to a sub-module capacitor and a direct-current support capacitor; when it is detected that the voltage of the sub-module capacitor reaches a rated value, the direction switch is turned off, and a charging loop of the sub-module capacitor is cut off; after the sub-module capacitor is isolated, the direct-current support capacitor is continuously and independently supplied with power until the rated value is reached, and then the bypass switch is closed to complete pre-charging. By adopting a staged charging control strategy, the technical conflict caused by inconsistent rated voltage targets of the two capacitors is solved, the overvoltage risk of the sub-module capacitors is reliably avoided, the direct current support capacitor can be safely and fully charged to the rated value, and the reliability of the direct current support capacitor is improved. And all energy storage elements are ensured to meet operation requirements when the converter is started.

Description

Technical Field

[0001] This invention relates to the field of converter start-up control technology, specifically a DC pre-charge start-up control method for a bridge arm alternating converter. Background Technology

[0002] Bridge-arm alternating current converters, as a novel modular multilevel converter topology, have broad application prospects in flexible DC transmission and high-power AC / DC conversion. Before the converter is put into operation, its internal energy storage components, especially the numerous submodule capacitors within the half-bridge submodules and the DC support capacitors on the DC side, must be pre-charged. If pre-charging is not performed and the converter is started directly, it will be subjected to a huge inrush current from the DC power supply at startup, which will cause permanent damage to critical equipment such as fully controlled devices in the system.

[0003] In existing DC pre-charging technologies, especially in startup scenarios where the AC side is a passive network and the converter can only be powered by the DC side, safely and effectively charging all submodule capacitors and DC support capacitors to their respective rated voltages is a key technical challenge. Existing pre-charging control strategies face an inherent technical conflict when attempting to simultaneously power submodule capacitors and DC support capacitors: the target rated voltage values ​​for these two types of capacitors are not the same, with the rated value of the DC support capacitor being much higher than that of a single submodule capacitor.

[0004] This voltage target mismatch presents a dilemma when using a simple parallel charging method: if charging continues to bring the DC support capacitor to its rated value, the already fully charged submodule capacitor will inevitably be in an overvoltage state, threatening the safe operation of the converter; conversely, if the entire pre-charging process is terminated when the submodule capacitor reaches its rated value, the voltage of the DC support capacitor will be severely insufficient, failing to meet the requirements for converter startup. Therefore, it is necessary to provide a new DC pre-charging startup control method to solve the aforementioned problem of charging target conflict. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a DC pre-charge start-up control method for bridge arm alternating converters, which solves the problem that during the DC pre-charge process, the rated voltage targets of the DC support capacitor and the submodule capacitor are inconsistent, resulting in the inability to simultaneously meet the overvoltage protection of the submodule capacitor and the full-charge start-up requirements of the DC support capacitor.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] This invention provides a DC pre-charge start-up control method for a bridge arm alternating converter, comprising the following steps:

[0008] S1. The control and monitoring unit controls the bypass switch of the bridge arm alternator to disconnect, connects the DC pre-charge current limiting resistor and DC power supply therein, and ensures that the selection switch therein remains in the off state; after the DC power supply is connected, the control and monitoring unit triggers the conduction of the first direction switch and the second direction switch in the three-phase phase unit to establish the charging path of the submodule capacitor. The DC power supply establishes the charging path of the DC support capacitor through the DC pre-charge current limiting resistor.

[0009] S2. The control and monitoring unit detects the voltage of all the sub-module capacitors in real time. After detecting that the voltage of all the sub-module capacitors has reached the rated value, the control and monitoring unit shuts off the first direction switch and the second direction switch in the three-phase phase unit. The control and monitoring unit determines whether the voltage of the DC support capacitor has reached the rated value of the DC support capacitor.

[0010] S3. If the control and monitoring unit determines that the voltage of the DC support capacitor has not reached the rated value of the DC support capacitor, then with the selection switch and the first and second direction switches both closed, the DC power supply continues to supply power to the DC support capacitor until the voltage of the DC support capacitor rises to the rated value of the DC support capacitor. Then, the bypass switch is closed to short-circuit the DC pre-charge current limiting resistor to complete the DC pre-charge.

[0011] In a preferred embodiment of the present invention, the bridge arm alternating converter includes the three-phase phase unit, each phase unit including a multiplexing shaping circuit, the multiplexing shaping circuit being composed of cascaded half-bridge sub-modules composed of the sub-module capacitors, and each phase unit being connected to a first direction switch, a second direction switch, a first selection switch and a second selection switch. The bridge arm alternating converter also includes the DC power supply, the DC support capacitor, the DC pre-charge current limiting resistor, the bypass switch and a control monitoring unit.

[0012] In a specific embodiment of the present invention, the hardware configuration and execution details of the above technical solution are further defined as follows:

[0013] The DC precharge current-limiting resistor includes a DC precharge current-limiting resistor connected in series with the positive bus of the DC power supply. and the DC pre-charge current limiting resistor connected in series with the negative bus of the DC power supply. Accordingly, the bypass switch includes those connected in parallel. Bypass switches at both ends and in parallel Bypass switches at both ends The control and monitoring unit collects the voltage of the capacitors in each half-bridge submodule in real time through voltage sensors installed on each half-bridge submodule.

[0014] Based on the above configuration, the specific execution process of step S1 is as follows:

[0015] Before the DC power supply is connected, the control and monitoring unit first confirms the bypass switch. and When in the off state, ensure the DC pre-charge current limiting resistor is in the off state. and It is connected in series in the DC bus circuit. After the DC power supply is connected, the control and monitoring unit sends a conduction trigger signal to all fully controllable devices in the first and second direction switches. This operation establishes two parallel charging paths:

[0016] The charging current path of the submodule capacitor is as follows: it flows out from the positive terminal of the DC power supply, and then passes through the DC pre-charge current limiting resistor in sequence. The first direction switch of the phase that is already turned on enters the multiplexing and shaping circuit. It charges the sub-module capacitor through the anti-parallel diodes of the upper arm switches of each half-bridge sub-module, and then through the second direction switch of the phase that is already turned on and the DC pre-charge current-limiting resistor. It eventually returns to the negative terminal of the DC power supply.

[0017] The charging current path of the DC support capacitor is as follows: it flows out from the positive terminal of the DC power supply, passes through the DC pre-charge current-limiting resistor, and then... The DC support capacitor connected in parallel across the DC bus is charged, and the charging current then passes through the DC pre-charge current-limiting resistor. Return to the negative terminal of the DC power supply.

[0018] To ensure that the bridge arm alternator operates within the full modulation range of modulation M∈(0,1], the rated value of the submodule capacitor voltage is... The following relationship must be satisfied:

[0019] ;

[0020] in, This is the rated value of the capacitor voltage in the submodule;

[0021] This refers to the rated value of the DC support capacitor;

[0022] This represents the number of submodules in each phase.

[0023] The specific execution process of step S2 is as follows:

[0024] The control and monitoring unit continuously monitors the voltage values ​​of all submodule capacitors in real time and compares them with the rated values ​​of the submodule capacitors. When it is determined that the voltage of all submodule capacitors has reached its rated value, the control and monitoring unit immediately sends a shutdown command to the gate drive circuits of all first and second direction switches. After confirming that all direction switches have been reliably shut off and the multiplexing and shaping circuits have been isolated from the DC power supply, the control and monitoring unit then collects and determines whether the real-time voltage value of the DC support capacitor has reached its rated value.

[0025] The specific execution process of step S3 is as follows:

[0026] After the submodule capacitor charging circuit is cut off, the DC power supply passes through the DC pre-charge current limiting resistor. and The DC support capacitor continues to charge independently. The control and monitoring unit continuously monitors the voltage of the DC support capacitor, and when the voltage reaches the rated value, it issues a command to drive the bypass switch. and bypass switch Synchronous closing, limiting the DC pre-charge current resistor and Short circuit, pre-charging process ends.

[0027] This invention provides a DC pre-charge start-up control method for a bridge arm alternating converter. It has the following beneficial effects:

[0028] 1. This invention establishes a charging path for the submodule capacitor by controlling the monitoring unit to trigger and activate the first and second direction switches in the three-phase phase unit during the initial pre-charging stage. Simultaneously, the connection of the DC power supply establishes a charging path for the DC support capacitor. This enables synchronous pre-charging of two types of key energy storage components using the system's built-in DC power supply without additional charging power equipment, providing the necessary voltage support for the initial startup of the converter.

[0029] 2. This invention continuously monitors the voltage of all submodule capacitors in real time through a control monitoring unit. Once the voltage reaches the rated value of the submodule capacitor, it precisely executes the operation of turning off the first direction switch and the second direction switch, isolating the multiplexing shaping circuit from the DC power supply. This effectively avoids the overvoltage risk caused by prolonged charging time of the submodule capacitor or the difference between the rated value of the DC support capacitor and the submodule capacitor, thus enhancing the safety of converter operation.

[0030] 3. This invention, after cutting off the charging circuit of the submodule capacitor, continues to independently power the DC support capacitor by utilizing the existing DC support capacitor charging path, which is independent of the direction switch, until its voltage rises to the rated value of the DC support capacitor. Finally, the bypass switch is closed to remove the DC pre-charge current limiting resistor, ensuring that all energy storage components meet the operating voltage requirements when the converter starts up. Attached Figure Description

[0031] Figure 1 This is a topology diagram of the AAC of the present invention.

[0032] Figure 2 This is a flowchart of the DC pre-charge start-up control strategy of the present invention.

[0033] Figure 3 This is a schematic diagram showing the charging of the AAC submodule capacitor and DC support capacitor of the present invention.

[0034] Figure 4 This is a schematic diagram of the charging of the AAC DC support capacitor of the present invention.

[0035] Figure 5 The simulation results show the DC support capacitor voltage during the AAC pre-charging process of this invention.

[0036] Figure 6 The simulation results show the capacitor voltage of each phase submodule during the AAC pre-charging process of this invention. Detailed Implementation

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] See attached document Figure 1 The DC pre-charge start-up control method for a bridge arm alternating converter provided in this embodiment of the invention is executed on this system structure.

[0039] The bridge arm alternator system comprises three symmetrical phase units, each with an identical circuit structure. Taking any one phase (e.g., phase j, j=a,b,c) as an example, it includes a multiplexed shaping circuit (MSC) and a first selection switch connected to the multiplexed shaping circuit. Second selection switch First direction switch Second direction switch .

[0040] The multiplexing and shaping circuit MSC is composed of It consists of cascaded half-bridge submodules. Each half-bridge module contains an upper bridge arm switch. and its anti-parallel diode Lower bridge arm switch tube and its anti-parallel diode and a submodule capacitor Switching transistor and It is a fully controllable device, usually an IGBT.

[0041] Both the selector switch SS and the direction switch DS are... It consists of a series of fully controlled devices (such as IGBTs) with anti-parallel diodes. The two ends of the multiplexing shaping circuit MSC are connected, one end via a first selection switch. Connect to the AC side port and via the first direction switch Connect to the positive port on the DC side; the other end is connected via a second selector switch. Connect to the AC side port and via the second direction switch Connect to the negative port on the DC side.

[0042] The system has a DC power supply on its DC side. A DC support capacitor And a current-limiting circuit for pre-charging. This current-limiting circuit includes a DC pre-charging current-limiting resistor connected in series with the DC positive bus. and the DC pre-charge current limiting resistor connected in series with the DC negative bus. .

[0043] To short-circuit the current-limiting resistor after pre-charging is complete, the current-limiting resistor... A bypass switch is connected in parallel at both ends. Current limiting resistor A bypass switch is connected in parallel at both ends. During normal operation of the converter, the bypass switch... and Keep the circuit closed and short-circuit the current-limiting resistor.

[0044] In addition, the system also includes a control and monitoring unit, which is connected to each switch in the system and a sensor for collecting voltage signals, and is used to execute the DC pre-charge start-up control method of the present invention.

[0045] Pre-charging the submodule capacitors before the converter enters steady-state operation is a necessary condition to ensure the safe and stable operation of the converter; otherwise, a huge current will be generated at the moment of converter startup, damaging the equipment in the converter. The control method provided in this embodiment of the invention aims to pre-charge all submodule capacitors. With DC support capacitor Charge to their respective rated voltages.

[0046] To ensure that the bridge arm alternator operates within the full modulation range of modulation M∈(0,1], the rated value of the submodule capacitor voltage is... The following relationship must be satisfied:

[0047] ;

[0048] in, This is the rated value of the capacitor voltage in the submodule;

[0049] This refers to the rated value of the DC support capacitor;

[0050] This represents the number of submodules in each phase.

[0051] See attached document Figure 2 , Figure 2 This is a flowchart of a DC precharge start-up control strategy according to an embodiment of the present invention. The DC precharge start-up control method provided by the present invention may specifically include the following steps:

[0052] S1, disconnect the bypass switch , Connect current limiting resistor , The DC power supply triggers the first and second direction switches in the three-phase unit to conduct, and the DC power supply simultaneously supplies power to the submodule capacitor and the DC support capacitor.

[0053] S2, after detecting that the voltage of the submodule capacitor in the bridge arm alternator reaches the rated value of the submodule capacitor, turn off the first and second direction switches in the three-phase phase unit, and determine whether the voltage of the DC support capacitor reaches the rated value of the DC support capacitor.

[0054] S3. If the DC support capacitor voltage does not reach the rated value of the DC support capacitor, the DC power supply continues to supply power to the DC support capacitor while the selector switch and direction switch remain off, until its voltage rises to the rated value of the DC support capacitor, and then the bypass switch is closed. , A short-circuit current-limiting resistor is used to complete the DC pre-charging.

[0055] The following will be combined with the appendix Figure 3 and attached Figure 4 The specific implementation process and working principle of each of the above steps are explained in detail.

[0056] In step S1, the converter undergoes a parallel pre-charge phase. (See attached diagram.) Figure 3 , Figure 3 AAC submodule capacitor in one embodiment of the present invention With DC support capacitor Simultaneous charging circuit diagram.

[0057] S101, Perform system initialization configuration. Before connecting the DC power supply, the control monitoring unit confirms that the system is in the initial pre-charge state. This initial state includes: all bypass switches. and Both are in the off state to ensure the DC pre-charge current limiting resistor is in the off state. and It is reliably connected in series in the DC bus circuit. Simultaneously, all selector switches... and Both should remain off to isolate the AC side circuit.

[0058] S102 executes the parallel charging command. After initialization configuration confirmation, connect the DC power supply. The control and monitoring unit controls all first-direction switches in the three-phase unit. With the second direction switch The fully controllable devices (such as IGBTs) send a turn-on trigger signal. Upon receiving the signal, these devices turn on, thereby establishing a charging path from the DC power supply to the energy storage capacitors inside the converter.

[0059] S103 forms a parallel charging loop. With all directional switches turned on, the system forms a parallel charging topology, with DC power supplied through current-limiting resistors and simultaneously to numerous submodule capacitors. and DC support capacitor Charging is performed. This parallel topology specifically includes the following two parallel charging circuits:

[0060] One is a three-parallel submodule capacitor charging circuit. For any one of phases j (j=a,b,c), the charging current path is: flowing out from the positive terminal of the DC power supply, passing through the DC pre-charging current-limiting resistor. The first directional switch that is already conducting in this phase The current enters the multiplexing and shaping circuit (MSC) of that phase. Inside the MSC, as the current flows through each half-bridge submodule, it passes through the upper bridge arm switching transistor. anti-parallel diode , is the capacitor of this submodule Charging. After the current is collected, it passes through the second directional switch that is already conducting in that phase. Then it flows through the DC pre-charge current-limiting resistor. It eventually returns to the negative terminal of the DC power supply.

[0061] The second is the DC-supported capacitor charging circuit. The charging current path of this circuit is as follows: it flows out from the positive terminal of the DC power supply, passes through the DC pre-charge current-limiting resistor, and then... For the DC support capacitors connected in parallel across the DC bus Charging is performed, and the charging current then passes through the DC pre-charge current-limiting resistor. The circuit returns to the negative terminal of the DC power supply. Through the parallel circuit described above, synchronous charging of the two key energy storage components in the system is achieved, and the voltage of all capacitors rises steadily from the initial state.

[0062] In step S2, the converter's status is monitored and the charging circuit is switched. While the parallel charging phase of step S1 is underway, the control monitoring unit continuously monitors the voltage status of each energy storage element within the converter. To achieve this monitoring function, voltage sensors are installed in each submodule and on the DC bus. These voltage sensors are used to collect real-time data on the capacitance of each submodule. Voltage and DC support capacitor The analog voltage value is transmitted to the control and monitoring unit.

[0063] S201 performs real-time monitoring and comparison of the submodule capacitor voltages. The control and monitoring unit, such as a digital signal processor or field-programmable gate array (FPGA), receives signals from each voltage sensor, processes them, and obtains the real-time voltage values ​​of all submodule capacitors. The control and monitoring unit has preset rated values ​​for the submodule capacitors. And continuously compare the real-time voltage value of each submodule capacitor with the target value. Compare them.

[0064] S202, execute the charging circuit switching operation. When the control monitoring unit determines that the voltage of all sub-module capacitors has reached the rated value of the sub-module capacitors... When the switching conditions for the charging circuit are met, the control and monitoring unit immediately generates and sends a shutdown command to the first direction switch that constitutes all three-phase phase units. With the second direction switch The gate drive circuit.

[0065] This switching operation is a key technical feature of this method. Its technical effect lies in actively and precisely disconnecting all three-phase multiplexing and shaping circuits (MSCs) from the DC power supply, thereby terminating the connection to all submodule capacitors. The charging process. The direct purpose of this operation is to solve the technical problem caused by the mismatch in charging rates and the difference in rated voltage target values ​​between the submodule capacitor circuit and the DC support capacitor circuit, and to reliably avoid the risk of overvoltage caused by the submodule capacitor continuing to charge after reaching the rated voltage.

[0066] S203, determine the status of the DC support capacitor voltage. After confirming that all directional switches are reliably turned off, the control monitoring unit collects and judges the DC support capacitor voltage. The real-time voltage value. If this voltage value has reached the rated value of the DC support capacitor. This indicates that all energy storage components of the converter have been fully charged, and the entire DC pre-charging process is complete. If this voltage value does not reach the rated value of the DC support capacitor... Then the control flow enters step S3.

[0067] In step S3, the DC support capacitor is independently supplemented with charge, completing the entire pre-charge process. (See attached document.) Figure 4 , Figure 4 An AAC DC support capacitor in one embodiment of the present invention A schematic diagram of an independent charging circuit.

[0068] S301, perform independent charging of the DC support capacitor. After the switching operation in step S2 is completed, all selection switches and direction switches of the system are in the off state. In this state, all three-phase multiplexing and shaping circuits (MSCs) and their internal submodule capacitors are isolated from the DC power supply, thus being in a protected state.

[0069] At this point, the charging circuit is reconfigured and simplified, and the DC power supply provides DC support capacitors through only one independent path. Continue supplying power. The current path of this charging circuit is as follows: it flows out from the positive terminal of the DC power supply, through the DC pre-charge current-limiting resistor. DC support capacitor Charging, followed by charging current passing through the DC pre-charge current-limiting resistor. Returning to the negative terminal of the DC power supply. This independent charging stage constitutes the latter half of the phased charging strategy of this invention. Its technical advantage lies in the ability to safely and independently increase the voltage of the DC support capacitor to its rated value without affecting the submodule capacitor that has already reached its rated voltage. .

[0070] S302 completes pre-charging and short-circuites the current-limiting resistor. During independent charging, the control monitoring unit continuously monitors the DC support capacitor. The voltage. When the voltage value is monitored to steadily rise to the rated value of the DC support capacitor. At that time, the control and monitoring unit determined that the objectives of the entire DC pre-charging process had been achieved.

[0071] Subsequently, the control and monitoring unit issued the final command to drive the bypass switches respectively. and Close. After the bypass switch closes, the DC pre-charge current-limiting resistor connected in parallel with it will be closed. and Short circuit. At this point, the entire DC pre-charge start-up control method has been completed, and all sub-module capacitors and DC support capacitors inside the converter have reached their respective rated voltages, thus meeting the start-up requirements of the converter.

[0072] To verify the effectiveness and feasibility of the DC pre-charge start-up control method proposed in this invention, based on the appendix... Figure 1 The bridge arm alternating converter system topology shown was used to build a corresponding simulation model.

[0073] In the simulation model, the key parameters of the system were set, as shown in Table 1.

[0074] Table 1: Simulation Performance Indicators and System Parameters

[0075]

[0076] Appendix Figure 5 The charging process of the DC support capacitor voltage is demonstrated. In the initial stage of pre-charging, the DC support capacitor voltage rises smoothly and synchronously with the submodule capacitor voltage. After the control strategy proposed in this invention executes a switching operation to disconnect the charging circuit of the submodule capacitor, the DC support capacitor voltage continues to rise independently in the second stage until it reaches the rated value of the DC support capacitor, 200kV, and remains stable.

[0077] Appendix Figure 6 The charging process of the submodule capacitor voltage for each phase is demonstrated. After pre-charging begins, the submodule capacitor voltage for each phase rises steadily. When the submodule capacitor voltage reaches its total rated value of 164kV (i.e., the individual submodule capacitor voltage reaches its rated value of 2.05kV), its charging circuit is cut off, and the voltage stabilizes at the rated level. This result verifies the precise control of the submodule capacitor charging process by the proposed method.

[0078] The simulation results above demonstrate that the DC pre-charge start-up control method provided by this invention can raise the voltage of the submodule capacitor and the DC support capacitor to their rated values. This result verifies the effectiveness of the method, meets the start-up requirements of the converter, and thus lays the foundation for the normal operation of the converter.

Claims

1. A DC pre-charge start-up control method for a bridge arm AC converter, characterized in that, The bridge arm alternating converter includes a control and monitoring unit, and the method is executed by the control and monitoring unit, including the following steps: S1. The control and monitoring unit controls the disconnection of the bypass switch of the bridge arm alternator, connects the DC pre-charge current limiting resistor and the DC power supply therein, and ensures that the selection switch therein remains in the off state; after the DC power supply is connected, the control and monitoring unit triggers the conduction of the first direction switch and the second direction switch in the three-phase phase unit to establish the charging path of the submodule capacitor. The DC power supply establishes the charging path of the DC support capacitor through the DC pre-charge current limiting resistor. S2. The control and monitoring unit detects the voltage of all the sub-module capacitors in real time. After detecting that the voltage of all the sub-module capacitors has reached the rated value, the control and monitoring unit shuts off the first direction switch and the second direction switch in the three-phase phase unit. The control and monitoring unit determines whether the voltage of the DC support capacitor has reached the rated value of the DC support capacitor. S3. If the control and monitoring unit determines that the voltage of the DC support capacitor has not reached the rated value of the DC support capacitor, then with the selection switch and the first and second direction switches both closed, the DC power supply continues to supply power to the DC support capacitor until the voltage of the DC support capacitor rises to the rated value of the DC support capacitor. Then, the bypass switch is closed to short-circuit the DC pre-charge current limiting resistor to complete the DC pre-charge.

2. The DC pre-charge start-up control method for a bridge arm alternating converter according to claim 1, characterized in that, The bridge arm alternating converter includes the three-phase phase unit, each phase unit including a multiplexing shaping circuit, the multiplexing shaping circuit being composed of cascaded half-bridge sub-modules composed of the sub-module capacitors, and each phase unit is connected to the first direction switch, the second direction switch, the first selection switch and the second selection switch. The bridge arm alternating converter also includes the DC power supply, the DC support capacitor, the DC pre-charge current limiting resistor, the bypass switch and the control monitoring unit.

3. The DC pre-charge start-up control method for a bridge arm alternating converter according to claim 2, characterized in that, The S1 step specifically includes: S101, before the DC power supply is connected, the control monitoring unit confirms that the bypass switch is in the open state, the DC pre-charge current limiting resistor is connected in series in the DC bus circuit, and the first selection switch and the second selection switch remain in the closed state. S102, after the DC power supply is connected, the control and monitoring unit sends a conduction trigger signal to all the fully controlled devices that constitute the first directional switches and the second directional switches in the three-phase phase unit, so that the fully controlled devices are turned on; S103, the DC power supply charges the submodule capacitor and the DC support capacitor simultaneously via the charging circuit formed by the DC pre-charge current limiting resistor.

4. The DC pre-charge start-up control method for a bridge arm alternating converter according to claim 2, characterized in that, The DC precharge current limiting resistor includes a DC precharge current limiting resistor connected in series with the positive bus of the DC power supply. and the DC precharge current limiting resistor connected in series with the negative bus of the DC power supply ; The bypass switch includes a component connected in parallel to the DC precharge current-limiting resistor. Bypass switches at both ends and connected in parallel to the DC precharge current limiting resistor Bypass switches at both ends ; In step S1, disconnecting the bypass switch specifically means disconnecting the bypass switch. and the bypass switch .

5. The DC pre-charge start-up control method for a bridge arm alternating converter according to claim 4, characterized in that, In step S1, the DC power supply simultaneously supplies power to both the submodule capacitor and the DC support capacitor, specifically including: The DC power supply is connected via the DC precharge current limiting resistor. and the DC precharge current limiting resistor The circuit formed charges the submodule capacitor and the DC support capacitor.

6. The DC pre-charge start-up control method for a bridge arm alternating converter according to claim 5, characterized in that, For any one phase of the three-phase unit, the charging current path of the submodule capacitor is as follows: The current flows out from the positive terminal of the DC power supply and passes through the DC pre-charge current-limiting resistor connected in series with the positive bus. The first direction switch of the phase that is already turned on enters the multiplexing and shaping circuit of that phase. Inside the multiplexing and shaping circuit, the capacitor of the sub-module is charged through the anti-parallel diode of the upper arm switch of each half-bridge sub-module. Then, it passes through the second direction switch of the phase that is already turned on, and then flows through the DC pre-charge current limiting resistor connected in series with the negative bus. Finally, it returns to the negative terminal of the DC power supply.

7. The DC pre-charge start-up control method for a bridge arm alternating converter according to claim 5, characterized in that, The current path through which the DC power supply powers the DC support capacitor is as follows: The current flows out from the positive terminal of the DC power supply and passes through the DC pre-charge current-limiting resistor connected in series with the positive bus. The DC support capacitor connected in parallel across the DC bus is charged, and the charging current then passes through the DC pre-charge current-limiting resistor connected in series with the negative bus. Return to the negative terminal of the DC power supply.

8. The DC pre-charge start-up control method for a bridge arm alternating converter according to claim 2, characterized in that, The S2 step specifically includes: S201, the control and monitoring unit continuously monitors and processes the voltage values ​​of all the sub-module capacitors in real time, and compares the real-time voltage values ​​of the sub-module capacitors with the rated values ​​of the sub-module capacitors; S202, when the control and monitoring unit determines that the voltage of all the sub-module capacitors has reached the rated value of the sub-module capacitors, the control and monitoring unit sends a shutdown command to the gate drive circuit that constitutes the first direction switch and the second direction switch in all the three-phase phase units. S203, after confirming that both the first directional switch and the second directional switch have been reliably turned off, the control monitoring unit collects and determines whether the real-time voltage value of the DC support capacitor has reached the rated value of the DC support capacitor.

9. The DC pre-charge start-up control method for a bridge arm alternating converter according to claim 4, characterized in that, The S3 step specifically includes: S301, in the state where the multiplexing and shaping circuit of the three-phase phase unit is isolated from the DC power supply, the DC power supply continues to charge the DC support capacitor independently through the DC pre-charge current limiting resistor. S302, continuously monitor the voltage of the DC support capacitor, and when the voltage reaches the rated value of the DC support capacitor, close the bypass switch to short-circuit the DC pre-charge current limiting resistor.

10. The DC pre-charge start-up control method for a bridge arm alternating converter according to claim 9, characterized in that, Step S302 is executed by the control and monitoring unit, and specifically includes: The control and monitoring unit continuously monitors the voltage of the DC support capacitor; When the voltage of the DC support capacitor reaches its rated value, the control monitoring unit issues a command to drive the bypass switch. and the bypass switch Close the DC pre-charge current limiting resistor. and the DC precharge current limiting resistor Short circuit.

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