A hybrid fast pre-charge method for modular multilevel matrix converter

Abstract

The application discloses a hybrid fast pre-charging method of a modular multilevel matrix converter, and the method comprises the following steps: short-time charging is performed on all sub-module capacitors by using an uncontrolled rectification mode, and the charging is performed until the supply voltage threshold required by an auxiliary circuit is reached; each bridge arm sub-module is divided into several groups, and open-loop voltage boosting charging is sequentially performed on each group, so that the capacitor voltage is raised to a preset initial voltage; on the basis of the initial voltage, closed-loop charging control is performed on all sub-modules, so that the capacitor voltage is quickly and smoothly raised to a rated voltage; a target value of the closed-loop charging control is set as a voltage slope target value with an adjustable slope, and the voltage rising rate is adjusted according to a preset charging time; meanwhile, a charging current feedforward value matched with the slope rising rate is introduced, and dynamic lag in the slope tracking process is compensated.

Description

Technical Field

[0001] This invention belongs to the field of modular multilevel matrix converter control technology, specifically a hybrid fast pre-charging method for modular multilevel matrix converters. Background Technology

[0002] In offshore wind power transmission technology, low-frequency AC transmission (LFAC) demonstrates good techno-economic efficiency because it avoids the capacitance effect problem of high-voltage AC transmission and eliminates the need for expensive offshore converter stations. Modular multilevel matrix converters (MMMCs), as a topology capable of direct three-phase AC / AC conversion, eliminate the DC link and some bridge arms, resulting in a simpler structure. More importantly, MMMCs can effectively adapt to low-frequency, high-power operating scenarios by adjusting the power balance between bridge arms, thus their application potential in LFAC systems has attracted attention.

[0003] In actual operation, before the MMMC is put into operation, the capacitors of each submodule need to be pre-charged to raise their voltage to near the rated value. If pre-charging is not completed, the initial capacitor voltage of the submodule is low, which can easily generate a large inrush current at startup, thereby damaging the switching devices and related components. Depending on the source of charging energy, pre-charging methods are mainly divided into external power supply charging and grid-side self-charging. External power supply charging requires an additional auxiliary power supply, which is more expensive; grid-side self-charging directly uses the grid voltage to complete the charging, which is simpler in structure, lower in cost, and more suitable for engineering applications.

[0004] In traditional pre-charging methods, uncontrolled rectification charging is simple in structure and easy to implement; however, it has limited charging capacity and slow charging speed, making it difficult to reach the rated capacitor voltage. Furthermore, the entire charging process lacks active regulation capability. Closed-loop charging utilizes closed-loop control logic to raise the capacitor voltage according to a preset target value, offering advantages such as controllable charging process and adjustable charging time. However, this method requires a certain initial voltage base; otherwise, a large charging inrush current can easily occur during charging, affecting the stability of the charging process. Additionally, in transmission systems, each arm of the MMMC has a large number of submodules. When charging power is limited, a simultaneous charging approach restricts the capacitor charging speed. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the defects of the prior art and provide a hybrid fast pre-charging method for modular multilevel matrix converters. This method combines the advantages of grouping strategy, open-loop boost charging and closed-loop charging, which improves the charging speed while ensuring the safety and controllability of the pre-charging process.

[0006] Therefore, the present invention adopts the following technical solution: a hybrid fast pre-charge method for a modular multilevel matrix converter, comprising: All submodule capacitors are briefly charged using uncontrolled rectification to raise them to the power supply voltage threshold required by the submodule auxiliary circuit. Each bridge arm submodule is divided into several groups, and each group is sequentially subjected to open-loop boost charging to raise the capacitor voltage to the preset initial voltage. Based on the initial voltage, closed-loop charging control is performed on all sub-modules to enable the capacitor voltage to rise quickly and smoothly to the rated voltage. The target value of the closed-loop charging control is set as the voltage ramp target value with adjustable slope, and the voltage rise rate is adjusted according to the preset charging time. At the same time, a charging current feedforward value matching the ramp rise rate is introduced to compensate for the dynamic lag in the ramp tracking process and improve the capacitor voltage target tracking performance.

[0007] This invention divides charging into three stages: uncontrolled rectified charging, grouped open-loop boost charging, and closed-loop charging.

[0008] Furthermore, during the open-loop boost charging stage, the capacitor charging efficiency per unit time is increased by reducing the number of sub-modules participating in a single charge, thereby accelerating the overall capacitor voltage rise rate.

[0009] Furthermore, during the open-loop boost charging stage, the charging speed is controlled by adjusting the number of groups and the duty cycle of the switching signal.

[0010] Furthermore, the method is applied to an MMMC system comprising nine bridge arms, each bridge arm having N Each sub-module's overall workflow includes: Step 1: The system port resources are initialized, the drive signals of all sub-modules are blocked, the low-frequency network side circuit breaker and the power frequency network side circuit breaker are in the open state, and the current limiting resistor is connected to the power frequency network side. Step 2: Using an uncontrolled rectification charging method, the voltage of all sub-module capacitors is rapidly increased to the power supply voltage threshold required by the sub-module auxiliary circuit, so that the sub-module IGBT enters a controllable state. Step 3: Divide each bridge arm into sub-modules. G Groups, each group contains 10 sub-modules. N / G and set a counter cnt This is used to record the number of groups that have completed charging. Step four, for each bridge arm cnt The sub-module performs open-loop boost charging, causing the capacitor voltage of the sub-module to rise rapidly until it meets the initial voltage required for subsequent closed-loop charging control. At the same time, the actual value of the capacitor voltage is monitored in real time to see if it meets the requirements. During this stage, the other sub-modules are bypassed. Step 5, when the first cnt After the group completes charging, the counter is activated. cnt Accumulate the count and determine if the updated counter exceeds the total number of submodule groups. G If yes, then it is determined that all submodule groups have completed open-loop boost charging and proceed to step six; if no, then return to step four and charge the next group of submodule capacitors. Step 6: Implement a closed-loop charging control strategy for all sub-modules to rapidly and smoothly raise the capacitor voltage of the sub-modules from the initial voltage to the rated voltage, while simultaneously executing a capacitor voltage detection program in real time.

[0011] Furthermore, in step four, the MMMC is equivalently split into three star-connected sub-converters, which are considered to operate in parallel; for each bridge arm... cnt The submodules are subjected to the same trigger signal to form a consistent capacitor charging path. This submodule has two operating states: bypass and active. When the submodule is in bypass state, the grid-side power supply charges the inductor; when the submodule is in active state, the grid-side power supply and the inductor simultaneously charge the submodule capacitor. Boost charging is achieved by applying a switching signal with a specific frequency and duty cycle. D The relationship between the maximum rechargeable voltage of each submodule and the formula is:

[0012] In the formula, U boost,max This is the highest rechargeable voltage for the submodule. U r,rms This is the effective value of the phase voltage; to improve charging efficiency, the initial voltage... U C,init < U boost,max .

[0013] Furthermore, in step six, the closed-loop charging control strategy reuses the power grid-side control module, bridge arm capacitor voltage balancing module, and modulation module during the steady-state operation of MMMC, and achieves closed-loop charging control only by switching the reference value generation method and modulation enable logic.

[0014] Furthermore, the closed-loop charging control strategy specifically includes: locking the low-frequency grid-side control module; generating a target voltage ramp value for the sub-module capacitor based on a preset charging time; tracking this target voltage ramp value through the outer voltage loop to generate a reference value for the d-axis current of the inner current loop on the power frequency grid side, and then calculating the bridge arm voltage reference value; simultaneously, using the inter-bridge arm capacitor voltage balancing module to perform real-time equalization control on the capacitor voltages of different bridge arm sub-modules to ensure consistent charging status for each bridge arm; finally, the modulation module generates a pulse signal for each sub-module based on the voltage reference value of each bridge arm.

[0015] Furthermore, the expression for the voltage outer loop control law is:

[0016] In the formula, This is the reference value for the d-axis current of the inner loop on the power frequency grid side. For the bridge arm capacitor voltage matrix via - The zero-sequence-zero-sequence component obtained by the transformation; This is the feedforward value of the charging current; This is the target value for the voltage ramp of the submodule capacitor; The proportional gain of the PI controller; This is the integral gain of the PI controller.

[0017] Furthermore, the target value of the voltage ramp The rising slope Represented as:

[0018] In the formula, T c The charging time for the closed-loop charging phase. U C,init The initial voltage, U C,ref This is the rated voltage.

[0019] Furthermore, to improve slope target tracking performance, a charging current feedforward value is introduced into the outer voltage loop. The formula for calculating the feedforward value of the charging current is:

[0020] In the formula, C sub This refers to the capacitance value of a single submodule. This represents the d-axis component of the power frequency grid side voltage.

[0021] The beneficial effects of this invention are as follows: 1. In the initial capacitor voltage establishment stage of closed-loop charging control, this invention employs a grouped open-loop boost charging strategy. This grouping strategy, when charging power is limited, can improve capacitor charging efficiency per unit time by reducing the number of sub-modules participating in a single charge, thereby accelerating the overall capacitor voltage rise. Simultaneously, this stage can be accelerated by adjusting the number of groups and the duty cycle of the switching signals. D To enable control over the charging speed.

[0022] 2. This invention employs a closed-loop charging method to rapidly boost the voltage of the submodule capacitor to its rated voltage. The target value of the closed-loop control process is set as a ramp signal with an adjustable slope, allowing for flexible adjustment of the voltage rise rate based on a preset charging time, thus improving the smoothness and controllability of the charging process. Simultaneously, a charging current feedforward value matching the ramp rise rate is introduced to compensate for dynamic lag during ramp tracking, thereby improving the capacitor voltage target tracking performance.

[0023] 3. This invention fully combines the advantages of grouping strategy, open-loop boost charging and closed-loop charging, realizing the rapid establishment of the initial capacitor voltage base of the submodule and the smooth increase of the subsequent capacitor voltage, thereby improving the stability and startup safety of the MMMC system pre-charging process. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a topology diagram of the MMMC of the present invention; Figure 2 This is a flowchart of the hybrid fast charging method for MMMC of the present invention; Figure 3 This is a flowchart illustrating the hybrid fast charging method for MMMC of the present invention. Figure 4 The equivalent circuit diagram for the open-loop boost charging of this invention is shown below. Figure 5 This is a control block diagram of the closed-loop charging method of the present invention. Detailed Implementation

[0026] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0027] Figure 1 The topology of MMMC is a three-phase structure containing nine arms, each arm consisting of... N Each submodule SM and a bridge arm inductor L armThe bridge is configured in series, with the low-frequency and power frequency power grids connected to its two ends respectively. The bridge arm inductors are primarily used to suppress circulating currents caused by power imbalances between the bridge arms and to mitigate the impact of current on circuit equipment during system faults. The submodule adopts a full-bridge structure to meet the output requirements of the bridge arm AC modulation wave.

[0028] In the initial stage of offshore low-frequency power transmission system startup, the low-frequency side wind turbines are not yet officially operational and cannot provide a stable charging power supply. Therefore, during the system startup phase, the power frequency grid is typically used to pre-charge the converters, and the low-frequency grid-side circuit breaker is disconnected. K s To avoid inrush current, a current-limiting resistor is connected in series in the charging circuit. R c After charging is complete, the circuit breaker will trip. K r Bypass it to reduce additional overhead during system operation.

[0029] This embodiment provides a hybrid fast pre-charge method for a modular multilevel matrix converter, the main process of which is as follows: Figure 2 As shown. The core idea of ​​this method is: First, all submodule capacitors are briefly charged using uncontrolled rectification to raise them to the power supply voltage threshold required by the submodule auxiliary circuit. Next, each bridge arm submodule is divided into several groups, and each group is sequentially subjected to open-loop boost charging to raise the capacitor voltage to the preset initial voltage. Finally, based on the initial voltage, closed-loop charging control is performed on all sub-modules to enable the capacitor voltage to rise quickly and smoothly to the rated voltage. The target value of the closed-loop charging control is set to a voltage ramp target value with an adjustable slope, and the voltage rise rate is adjusted according to the preset charging time. At the same time, a charging current feedforward value matching the slope rise rate is introduced to compensate for the dynamic lag in the ramp tracking process and improve the capacitor voltage target tracking performance.

[0030] The hybrid fast pre-charging method of the present invention will be described in detail below, such as... Figure 3 As shown.

[0031] (1) Step 1: System Initialization The system completes the initialization of resources such as the sampling port, PWM port, and GPIO port, blocks the drive signals of all submodules, and activates the circuit breakers on both sides. K s and K r When in the disconnected state, a current-limiting resistor is connected to the power frequency grid side. R c This provides the initial operating conditions for the pre-charging of the MMMC.

[0032] (2) Step 2: Submodule power supply voltage establishment Typically, IGBT drive circuits in submodules generally employ a self-supply method. When the submodule capacitor voltage has not yet reached its minimum operating threshold, the IGBT device is uncontrollable. Therefore, during the initial pre-charging phase, the MMMC needs to establish the submodule supply voltage through an uncontrolled rectification charging process. During this process, the IGBT device is in a latched-off state, and its corresponding anti-parallel diode... D 1~ D 4. A single-phase uncontrolled rectifier bridge is naturally formed, which rectifies the power frequency line voltage and provides an initial charging path for the submodule capacitors. At this time, the voltage rise process of each submodule capacitor is basically the same.

[0033] Because this charging method has limited capacity and a slow charging speed, it is difficult to reach the rated capacitor voltage using only this method. Therefore, this stage primarily provides operating voltage for the submodule's auxiliary circuitry. Based on the power supply requirements of the submodule's auxiliary circuitry, the power supply voltage threshold for this stage is set. U C,p When the submodule capacitor voltage reaches this threshold, the system will switch to the next charging phase. Because... U C,p The smaller the value, the shorter the duration of this phase.

[0034] (3) Step 3: Submodule grouping In power transmission systems, each arm of the MMMC (Multi-Module Charging System) has a large number of submodules. When charging power is limited, simultaneous charging of the entire system restricts the capacitor charging speed. Therefore, this invention employs a group charging strategy. While maintaining a constant charging power, it reduces the number of submodules participating in each charging cycle, thereby increasing the capacitor charging efficiency per unit time and accelerating the overall capacitor voltage rise. Specifically, the submodules of each arm are divided into... G Groups, each group contains 10 sub-modules. N / G and set a counter cnt This is used to record the number of groups that have completed charging.

[0035] (4) Step 4: Group open-loop boost charging stage At this point, the IGBT devices of the submodule are in a controllable state, and the MMMC enters the grouped open-loop boost charging stage. Its equivalent charging circuit is as follows: Figure 4 As shown. Among them, R c For current limiting resistor, L r For output-side filter inductance, L arm For bridge arm inductance, C eq The equivalent capacitance of the submodules participating in charging, i.e. , C sub This refers to the capacitance value of a single submodule. Since the low-frequency grid-side circuit breaker has been disconnected, the converter can be equivalently split into three star-connected sub-converters, which can be considered to operate in parallel.

[0036] During this charging phase, the first arm of each bridge... cnt The group submodules execute an open-loop boost charging strategy, with all submodules within the group receiving the same trigger signal for synchronous activation or bypass; simultaneously, the remaining group submodules remain in bypass mode. During this stage, the submodules participating in charging primarily operate in two states: ① Bypass mode: IGBT devices in the submodule... S 1 and S 3 (or S 2 and S 4) Trigger conduction, the submodule enters the disconnected state. At this time, the grid-side power supply charges the inductor. ② Connected state: Depending on the direction of the bridge arm current, the submodule is connected to the circuit in different ways, and the capacitor enters the charging state. When the direction of the bridge arm current is positive, it conducts. S 1 and S 4; When the bridge arm current is negative, conduction occurs. S 2 and S 3. This enables capacitor charging control.

[0037] Therefore, boost charging can be achieved by applying a switching signal with a certain frequency and duty cycle. For example... Figure 4 As shown, during the boost charging process of a single sub-converter, the charging voltage mainly originates from the grid-side line voltage, and each charging circuit contains two equivalent sub-modules. Let the duty cycle of the switching signal be... D Under ideal conditions, the maximum rechargeable voltage of each submodule is:

[0038] In the formula, U boost,max This is the highest rechargeable voltage for the submodule. U r,rms This is the effective value of the phase voltage.

[0039] However, open-loop boost charging still has certain limitations. As the capacitor voltage increases, the inductor's ability to release energy gradually weakens, causing the charging current to decrease and resulting in a significant reduction in charging speed later on. Furthermore, this method relies heavily on a preset duty cycle for energy transfer, leading to poor controllability during the charging process. Therefore, this method is more suitable as a voltage boost stage before closed-loop charging, raising the submodule capacitor voltage to the initial voltage. U C,init This provides the necessary voltage foundation for subsequent closed-loop charging. To improve charging efficiency, an initial voltage is typically set for this stage. UC,init < U boost,max .

[0040] (5) Step 5: Determination of completion of group open-loop boost charging When the cnt After the group completes open-loop boost charging, the counter is... cnt Accumulate the count and determine if the updated counter exceeds the total number of submodule groups. G If yes, then all submodule groups are determined to have completed the charging phase, and the charging procedure proceeds to step six; otherwise, return to step four and execute the charging procedure for the next group of submodule capacitors.

[0041] (6) Step Six: Closed-Loop Charging Control Stage When the capacitor voltages of all submodules rise to the initial voltage U C,init When the time is right, the system can enter the closed-loop control charging stage. In this stage, the system reuses the output control module, bridge arm capacitor voltage balancing module, and modulation module that are running in steady state with the MMMC. Closed-loop charging can be achieved simply by switching the reference value generation method and the modulation enable logic. Figure 5 This is a block diagram for closed-loop charging control. Figure 5 middle, This is the target value for the voltage ramp. For the bridge arm capacitor voltage matrix via - The zero-order-zero-order components obtained by the transformation This is the feedforward value of the charging current. This is the reference value for the d-axis current of the inner loop on the power frequency grid side. This is the reference value for the q-axis current of the inner loop on the power frequency grid side. , These represent the d-axis and q-axis components of the power grid-side current, respectively. This is the reference value for the dq axis of the power frequency side voltage. This is the reference value for the low-frequency side voltage dq axis; its main control strategy is: to lock out the low-frequency grid-side control module; and to generate the target voltage ramp value of the submodule capacitor based on the preset charging time. By tracking the target value of the voltage ramp through the outer voltage loop, a reference value for the d-axis current of the inner current loop on the power frequency grid side is generated. Then, the reference value of the dq axis of the power frequency side voltage is calculated. Simultaneously, the bridge arm capacitor voltage balance controller generates bridge arm voltage compensation based on the voltage deviation between submodule capacitors in the bridge arms. The voltage of the capacitors in different bridge arm submodules is balanced in real time to ensure that the charging state of each bridge arm is consistent. Finally, the modulation module generates a bridge arm voltage modulation signal based on the voltage reference and compensation values, and then generates a PWM signal for each submodule.

[0042] The voltage outer loop control law is:

[0043] In the formula, This is the feedforward value of the charging current; The proportional gain of the PI controller; This is the integral gain of the PI controller.

[0044] To suppress the bridge arm current surge during closed-loop charging switching, the target value is... Set a ramp target value that increases over time. By controlling the capacitor voltage to follow the ramp target value, it can be smoothly and stably increased to the rated voltage, thereby improving the stability and controllability of the charging process. The ramp target value's ramp slope can be expressed as:

[0045] In the formula, T c The charging time for the closed-loop charging phase. U C,init The initial voltage, U C,ref This is the rated voltage.

[0046] To improve slope target tracking performance, a charging current feedforward value is introduced into the outer voltage loop. The formula for calculating the feedforward value of the charging current is:

[0047] In the formula, C sub This refers to the capacitance value of a single submodule. This represents the d-axis component of the power frequency grid side voltage.

[0048] As the above analysis shows, the charging time in the closed-loop charging stage can be set by adjusting the rising slope of the capacitor voltage ramp target value, thereby improving the controllability of the charging process. Simultaneously, introducing a charging current feedforward can compensate for the dynamic lag in the ramp tracking process, reduce tracking errors, and improve the capacitor voltage's ability to follow the target trajectory.

[0049] The embodiments disclosed herein are merely preferred embodiments of the present invention. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the present invention, and are not intended to limit the present invention. Any modifications and variations made by those skilled in the art within the scope of this specification should fall within the protection scope of the present invention.

Claims

1. A hybrid fast pre-charge method for a modular multilevel matrix converter, characterized in that, include: All submodule capacitors are briefly charged using uncontrolled rectification to raise them to the power supply voltage threshold required by the submodule auxiliary circuit. Each bridge arm submodule is divided into several groups, and each group is sequentially subjected to open-loop boost charging to raise the capacitor voltage to the preset initial voltage. Based on the initial voltage, closed-loop charging control is performed on all sub-modules to enable the capacitor voltage to rise quickly and smoothly to the rated voltage. The target value of the closed-loop charging control is set to a voltage ramp target value with an adjustable slope, and the voltage rise rate is adjusted according to the preset charging time. At the same time, a charging current feedforward value matching the slope rise rate is introduced to compensate for the dynamic lag in the ramp tracking process.

2. The hybrid fast pre-charging method according to claim 1, characterized in that, During the open-loop boost charging stage, the capacitor charging efficiency per unit time is increased by reducing the number of sub-modules participating in a single charge, thereby accelerating the overall capacitor voltage rise rate.

3. The hybrid fast pre-charging method according to claim 1, characterized in that, During the open-loop boost charging stage, the charging speed is controlled by adjusting the number of groups and the duty cycle of the switching signal.

4. The hybrid fast pre-charging method according to any one of claims 1-3, characterized in that, The method is applied to an MMMC system comprising nine bridge arms, each bridge arm having... N Each sub-module's overall workflow includes: Step 1: The system port resources are initialized, the drive signals of all sub-modules are blocked, the low-frequency network side circuit breaker and the power frequency network side circuit breaker are in the open state, and the current limiting resistor is connected to the power frequency network side. Step 2: Using an uncontrolled rectification charging method, the voltage of all sub-module capacitors is rapidly increased to the power supply voltage threshold required by the sub-module auxiliary circuit, so that the sub-module IGBT enters a controllable state. Step 3: Divide each bridge arm into sub-modules. G Groups, each group contains 10 sub-modules. N / G and set a counter cnt This is used to record the number of groups that have completed charging. Step four, for each bridge arm cnt The sub-module performs open-loop boost charging, causing the capacitor voltage of the sub-module to rise rapidly until it meets the initial voltage required for subsequent closed-loop charging control; during this stage, the other sub-modules are bypassed. Step 5, when the first cnt After the group completes charging, the counter is activated. cnt Accumulate the count and determine if the updated counter exceeds the total number of submodule groups. G If yes, then it is determined that all submodule groups have completed open-loop boost charging and proceed to step six; if no, then return to step four and charge the next group of submodule capacitors. Step 6: Implement a closed-loop charging control strategy for all sub-modules to rapidly and smoothly raise the capacitor voltage of the sub-modules from the initial voltage to the rated voltage, while simultaneously executing a capacitor voltage detection program in real time.

5. The hybrid fast pre-charging method according to claim 4, characterized in that, In step four, the MMMC is equivalently split into three star-connected sub-converters, which are considered to operate in parallel; for each bridge arm... cnt The submodules are subjected to the same trigger signal to form a consistent capacitor charging path. This submodule has two operating states: bypass and active. When the submodule is in bypass state, the grid-side power supply charges the inductor; when the submodule is in active state, the grid-side power supply and the inductor simultaneously charge the submodule capacitor. Boost charging is achieved by applying a switching signal with a specific frequency and duty cycle. D The relationship between the maximum rechargeable voltage of each submodule and the formula is: In the formula, U boost,max This is the highest rechargeable voltage for the submodule. U r,rms This is the effective value of the phase voltage; to improve charging efficiency, the initial voltage... U C,init < U boost,max .

6. The hybrid fast pre-charging method according to claim 4, characterized in that, In step six, the closed-loop charging control strategy reuses the power grid-side control module, bridge arm capacitor voltage balancing module, and modulation module from the steady-state operation of MMMC, and achieves closed-loop charging control only by switching the reference value generation method and modulation enable logic.

7. The hybrid fast pre-charging method according to claim 6, characterized in that, The closed-loop charging control strategy is as follows: The low-frequency grid-side control module is locked; a target voltage ramp value for the sub-module capacitor is generated based on a preset charging time; this target voltage ramp value is tracked through the outer voltage loop to generate a reference value for the d-axis current of the power frequency grid-side inner current loop, thereby calculating the bridge arm voltage reference value; simultaneously, the inter-bridge arm capacitor voltage balancing module is used to perform real-time equalization control of the capacitor voltages of different bridge arm sub-modules to ensure consistent charging status for each bridge arm; finally, the modulation module generates a pulse signal for each sub-module based on the voltage reference values ​​of each bridge arm.

8. The hybrid fast pre-charging method according to claim 7, characterized in that, The expression for the voltage outer loop control law is: In the formula, This is the reference value for the d-axis current of the inner loop on the power frequency grid side. For the bridge arm capacitor voltage matrix via - The zero-sequence-zero-sequence component obtained by the transformation; This is the feedforward value of the charging current; This is the target value for the voltage ramp of the submodule capacitor; The proportional gain of the PI controller; This is the integral gain of the PI controller.

9. The hybrid fast pre-charging method according to claim 8, characterized in that, Voltage ramp target value The rising slope Represented as: In the formula, T c The charging time for the closed-loop charging phase. U C,init The initial voltage, U C,ref This is the rated voltage.

10. The hybrid fast pre-charging method according to claim 9, characterized in that, To improve slope target tracking performance, a charging current feedforward value is introduced into the outer voltage loop. The formula for calculating the feedforward value of the charging current is: In the formula, C sub This refers to the capacitance value of a single submodule. This represents the d-axis component of the power frequency grid side voltage.

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