Capacitor voltage control method and device for flexible low-frequency power transmission modular multilevel matrix converter

Abstract

The invention discloses a capacitor voltage control method and device for a flexible low-frequency power transmission modular multi-level matrix converter. The main purpose of the invention is to control the stability of the capacitor voltage of the modular multi-level matrix converter and improve the safety of a flexible low-frequency power transmission system at the same time. According to the main technical scheme of the invention, the method comprises the steps: obtaining the actual capacitor voltage of a modular multilevel matrix converter, and converting the actual capacitor voltage through an outer-loop voltage controller, thereby obtaining a target current value; obtaining a target voltage value of each bridge arm by using an inner loop current controller according to the target current value; generating a target sub-module switching strategy of each bridge arm based on the target voltage value, wherein the target sub-module switching strategy is used for switching sub-modules accessed in the bridge arms to suppress capacitor voltage fluctuation in the bridge arms; and executing the target sub-module switching strategy of each bridge arm so as to control the capacitance voltage of the modular multilevel matrix converter to be stable. The method is used for controlling the stability of the capacitor voltage of the modular multilevel matrix converter.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and particularly to a method and device for controlling the capacitor voltage of a flexible low-frequency power transmission modular multilevel matrix converter. Background Art

[0002] In a flexible low-frequency power transmission system, a modular multilevel matrix converter (M3C) plays a key role. The M3C converter usually includes 9 bridge arms, and there are multiple full-bridge sub-modules connected in series on each bridge arm. To ensure the safety and reliability of the flexible low-frequency power transmission system, it is necessary to suppress the fluctuations generated by the capacitor voltage on each bridge arm to control the stable operation of the M3C converter.

[0003] Currently, by analyzing the internal circulating current of the M3C converter, a high-frequency circulating current is injected to reconstruct the capacitor energy exchange path, so that the high-frequency circulating current forms a symmetric charging / discharging path within the capacitor charging and discharging cycle, and further makes the average value of the capacitor voltage within the high-frequency cycle be the steady-state value, thereby canceling out the fluctuations and achieving the purpose of suppressing the sub-module capacitor voltage fluctuations of the M3C converter.

[0004] Although injecting a high-frequency circulating current can suppress the capacitor voltage fluctuations, during actual operation, the circulating current may affect the input and output currents, and further affect the power quality and operation stability of the entire system. For example, the unreasonable flow of the circulating current may cause an increase in the current harmonic content, and these harmonic currents will be transmitted in the lines of the flexible low-frequency power transmission system, reducing the safety of the flexible low-frequency power transmission system. Summary of the Invention

[0005] In view of the above problems, the present invention proposes a method and device for controlling the capacitor voltage of a flexible low-frequency power transmission modular multilevel matrix converter, and the main purpose is to control the capacitor voltage of the modular multilevel matrix converter to be stable while improving the safety of the flexible low-frequency power transmission system.

[0006] To achieve the above object, the present invention mainly provides the following technical solutions: In a first aspect, the present invention provides a method for controlling the capacitor voltage of a flexible low-frequency power transmission modular multilevel matrix converter, and the method includes: Obtain the actual capacitor voltage of the modular multilevel matrix converter, and use an outer-loop voltage controller to convert the actual capacitor voltage to obtain a target current value; According to the target current value, use an inner-loop current controller to obtain the target voltage value of each bridge arm; Generate a target sub-module switching strategy for each bridge arm based on the target voltage value, and the target sub-module switching strategy is used to switch the sub-modules connected in the bridge arm to suppress the capacitor voltage fluctuations within the bridge arm; Execute the target sub-module switching strategy for each arm to control the capacitor voltage stability of the modular multilevel matrix converter.

[0007] In a second aspect, the present invention provides a capacitor voltage control device for a flexible low-frequency power transmission modular multilevel matrix converter, the device comprising: An outer loop control unit, configured to obtain the actual capacitor voltage of the modular multilevel matrix converter, and convert the actual capacitor voltage by using an outer loop voltage controller to obtain a target current value; An inner loop control unit, configured to obtain a target voltage value for each arm by using an inner loop current controller according to the target current value obtained by the outer loop control unit; A generating unit, configured to generate a target sub-module switching strategy for each arm based on the target voltage value obtained by the inner loop control unit, the target sub-module switching strategy being used to switch the sub-modules connected in the arm to suppress the capacitor voltage fluctuation in the arm; An execution unit, configured to execute the target sub-module switching strategy for each arm generated by the generating unit to control the capacitor voltage stability of the modular multilevel matrix converter.

[0008] In a third aspect, the present invention further provides a computing device, the computing device comprising: at least one processor, and a memory, wherein the memory stores instructions executable by the processor, and when the instructions are executed by the processor, the processor is capable of executing a capacitor voltage control method for a flexible low-frequency power transmission modular multilevel matrix converter in the first aspect above.

[0009] In a fourth aspect, the present invention further provides a readable storage medium, the readable storage medium being used to store a computer program, wherein when the computer program runs, it controls the device where the storage medium is located to execute a capacitor voltage control method for a flexible low-frequency power transmission modular multilevel matrix converter in the first aspect above.

[0010] With the above technical solution, a method and device for controlling the capacitor voltage of a flexible low-frequency power transmission modular multilevel matrix converter provided by the present invention convert the actual capacitor voltage of the entire M3C converter into a target current value through an outer-loop voltage controller to ensure that the current values on each arm of the M3C converter are consistent and stable; then, the inner-loop current controller precisely adjusts the target voltage value of each arm, achieving high-precision control of the capacitor voltage of each arm; the sub-module switching strategy generated based on the target voltage value can quickly respond to system changes and ensure real-time adjustment of the capacitor voltage; through the collaborative work of the outer-loop voltage controller and the inner-loop current controller, the capacitor voltage of each sub-module in the M3C converter is ensured to be within the set range by using the target voltage value, thereby improving the stability and reliability of the system. By precisely controlling the capacitor voltage of each arm, harmonic distortion and power fluctuations are reduced, the power quality of the entire system is improved, and the safety of the flexible low-frequency power transmission system is enhanced while controlling the stability of the capacitor voltage of the modular multilevel matrix converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] By reading the following detailed description with reference to the accompanying drawings, the above and other objects, features, and advantages of the exemplary embodiments of the present invention will become readily understood. In the drawings, several embodiments of the present invention are shown by way of illustration and not limitation, and like or corresponding reference numerals indicate like or corresponding parts, wherein: Figure 1 Schematically shows a flowchart of a method for controlling the capacitor voltage of a flexible low-frequency power transmission modular multilevel matrix converter proposed by an embodiment of the present invention; Figure 2 Schematically shows a flowchart of another method for controlling the capacitor voltage of a flexible low-frequency power transmission modular multilevel matrix converter proposed by an embodiment of the present invention; Figure 3 Schematically shows a structural diagram of a device for controlling the capacitor voltage of a flexible low-frequency power transmission modular multilevel matrix converter proposed by an embodiment of the present invention; Figure 4 Schematically shows a structural diagram of another device for controlling the capacitor voltage of a flexible low-frequency power transmission modular multilevel matrix converter proposed by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0012] Hereinafter, the exemplary embodiments of the present invention will be described in more detail with reference to the accompanying drawings. Although the exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present invention can be more thoroughly understood and the scope of the present invention can be fully conveyed to those skilled in the art.

[0013] It should be noted that unless otherwise specified, the technical terms or scientific terms used in the present invention shall have the ordinary meanings understood by those skilled in the art to which the present invention pertains.

[0014] In a flexible low-frequency power transmission system, the converter is a core device. Through the modular multilevel matrix converter (M3C), direct AC / AC conversion can be achieved without a DC link, with high power quality, bidirectional power flow capability, and bilateral controllable power factor. However, there are relatively many sub-modules on each arm of the M3C converter, and the capacitor voltages of the sub-modules will fluctuate, affecting the overall voltage stability of the M3C converter, and further affecting the safety of the flexible low-frequency power transmission system. Therefore, it is necessary to control the stability of the capacitor voltages in the M3C converter. Currently, the more commonly used control method is to analyze the internal circulating current of the M3C converter and inject a high-frequency circulating current to suppress the capacitor voltage fluctuations of the sub-modules of the M3C converter. However, the method of injecting a high-frequency circulating current is likely to cause an increase in the current harmonic content, and further affect the safety of the flexible low-frequency power transmission system. On this basis, the applicant of this case obtained the mathematical model of the M3C converter through mathematical model derivation, obtained the mathematical relationships between various currents and voltages, and further designed an internal and external double-loop controller capable of controlling a three-phase system according to its mathematical relationships to obtain the target voltage values that can stabilize the capacitor voltages in each arm of the M3C converter, and realized the control of the stability of the capacitor voltages in the M3C converter by switching the sub-modules to make the capacitor voltages in each arm meet their corresponding target voltage values.

[0015] For this reason, the applicant of this case proposed a method for controlling the capacitor voltages of a flexible low-frequency power transmission modular multilevel matrix converter. This method can obtain the target voltage values of each arm by using an outer-loop voltage controller and an inner-loop current controller, and stabilize the capacitor voltages of each arm at the target voltage values through a sub-module switching strategy. A method for controlling the capacitor voltages of a flexible low-frequency power transmission modular multilevel matrix converter according to an embodiment of the present invention is specifically as Figure 1 shown and includes: 101. Obtain the actual capacitor voltages of the modular multilevel matrix converter, and use the outer-loop voltage controller to convert the actual capacitor voltages to obtain target current values.

[0016] In the process of controlling the stable operation of a modular multilevel matrix converter (M3C), it is first necessary to obtain the actual capacitor voltage of the M3C converter in real time through a voltage sensor. This actual capacitor voltage is the average value of the capacitor voltages of all sub-modules in the M3C converter. The outer-loop voltage controller is the core part of the common double-loop control structure in power electronic systems (such as converters, inverters, etc.). Its principle is based on the coordinated action of the voltage outer loop and the current inner loop, and realizes the stable output and dynamic response of the system voltage through hierarchical regulation. In this hierarchical control structure, the outer-loop voltage controller is located at the top layer of the control system and is responsible for generating the reference signal of the inner-loop current controller. The control process includes voltage feedback and error processing, etc. Voltage feedback includes real-time acquisition of the output voltage or capacitor voltage, comparing it with the set reference voltage value, and generating a voltage error signal; error processing includes processing the error through control algorithms (such as PID, fuzzy PID, etc.) and outputting the reference current value of the inner-loop current. In this step, the real-time acquired capacitor voltage is the above-mentioned actual capacitor voltage, and the reference current value of the inner-loop current is the above-mentioned target current value.

[0017] 102. According to the target current value, use the inner-loop current controller to obtain the target voltage value of each arm.

[0018] In the hierarchical control structure, the output of the outer-loop voltage controller is the input of the inner-loop current controller, that is, the target current value in step 101 is the input of the inner-loop current controller in this step. In the process of using the inner-loop current controller to determine the target voltage value, it also includes using a current sensor to measure the actual current of each arm in real time, comparing the target current value with the actual current value, calculating the current error, and then through control algorithms (such as PI, PID, etc.), converting the current error into the target voltage value of each arm.

[0019] 103. Generate the target sub-module switching strategy for each arm based on the target voltage value.

[0020] Since the number of sub-modules on each arm of the M3C converter may vary according to the switching strategy in the historical period, it is necessary to determine the target sub-module switching strategy for each arm. The target sub-module switching strategy is the strategy for inserting or removing sub-modules in the arm in the current period, which is used to switch the sub-modules connected in the arm to suppress the capacitance voltage fluctuation generated on the corresponding arm. Different sub-module capacitance voltages will affect the selection of the switching strategy. Therefore, the capacitance voltage status of the sub-modules in each arm is monitored in real time through voltage sensors. According to the target voltage value and the current sub-module capacitance voltage, a suitable sub-module switching strategy is determined, mainly considering two cases: inserting more sub-modules and removing some sub-modules. Among them, inserting more sub-modules includes: when the target voltage value is higher than the average value of the current sub-module capacitance voltage, select to insert more sub-modules to increase the arm voltage. Specifically, sub-modules with lower capacitance voltage can be selected for insertion to balance the capacitance voltage. Removing some sub-modules includes: when the target voltage value is lower than the average value of the current sub-module capacitance voltage, select to remove some sub-modules to reduce the arm voltage. Specifically, sub-modules with higher capacitance voltage can be selected for removal to avoid overcharging.

[0021] 104. Execute the target sub-module switching strategy for each arm.

[0022] According to the sub-module switching strategy generated in step 103, specific sub-module switching instructions are generated. The sub-module switching instructions include insertion instructions and removal instructions. The insertion instructions specify which sub-modules need to be inserted in the current period, and the removal instructions specify which sub-modules need to be removed in the current period. To further improve the control accuracy, the system can also be commanded to monitor the capacitance voltage changes of each sub-module in real time and feedback the monitoring results to the control system. When it is found that the capacitance voltage exceeds the allowable range or fails to reach the target voltage value, the control system can immediately adjust the switching strategy to ensure the stability and reliability of the system. By executing the target sub-module switching strategy for each arm, the capacitance voltage of the M3C converter can be controlled stably.

[0023] Based on the above Figure 1From the implementation method, it can be seen that the actual capacitor voltage of the M3C converter is obtained in real time through a voltage sensor and used as the input of the outer-loop voltage controller. This actual capacitor voltage is the average value of the capacitor voltages of all sub-modules, ensuring the comprehensiveness and accuracy of voltage measurement. The outer-loop voltage controller generates a target current value, and the inner-loop current controller further adjusts the target voltage value of each arm according to the target current value. This double-loop control structure significantly improves the accuracy of voltage control, enabling the system to maintain a stable voltage output under complex operating conditions and also achieving a fast dynamic response of the system. Based on the target voltage value and the current state of the sub-module capacitor voltage, a suitable sub-module switching strategy is determined. When the target voltage value is higher than the average value of the current sub-module capacitor voltage, more sub-modules with lower capacitor voltages are selected to be connected; conversely, sub-modules with higher capacitor voltages are disconnected. This strategy effectively avoids overcharging or under-voltage problems caused by uneven capacitor voltages and ensures the balanced distribution of the capacitor voltages of each sub-module.

[0024] Furthermore, according to the above Figure 1 embodiment of the present invention shown, for a more specific method of controlling the capacitor voltage stability of the flexible low-frequency power transmission modular multilevel matrix converter, the embodiment of the present invention will be described in more detail. The specific steps are as Figure 2 shown and include: 201. Obtain the actual capacitor voltage of the modular multilevel matrix converter and use the outer-loop voltage controller to convert the actual capacitor voltage to obtain a target current value.

[0025] Since the purpose of the outer-loop voltage controller is to eliminate the error between the desired voltage value and the actual capacitor voltage, it is necessary to first obtain the desired voltage value and the actual capacitor voltage of the M3C converter. The actual capacitor voltage is determined according to the sub-module capacitor voltage, specifically including detecting the capacitor voltages of multiple sub-modules in the modular multilevel matrix converter and calculating the average value of the capacitor voltages of the multiple sub-modules as the actual capacitor voltage. Input the desired voltage value and the actual capacitor voltage into the outer-loop voltage controller, and use the PI regulator in the outer-loop controller to obtain a target current value that can meet the desired voltage value.

[0026] In this embodiment, before using the outer-loop voltage controller to convert the actual capacitor voltage to obtain a target current value, it also includes determining the mathematical relationship between the desired voltage value, the actual capacitor voltage, and the target current value, and designing the outer-loop voltage controller according to the mathematical relationship between the desired voltage value, the actual capacitor voltage, and the target current value.

[0027] The mathematical relationship between the desired voltage value, the actual capacitor voltage, and the target current value is: Among them, Udc_ave is the actual capacitor voltage, U*dc_ave is the desired voltage value, and i*d0 is the target current value.

[0028] 202. According to the target current value, use the inner-loop current controller to obtain the target voltage value of each bridge arm.

[0029] In this step, the inner-loop current controller includes an input-side inner-loop current controller and an output-side inner-loop current controller, and each of the three phases U, V, and W has its corresponding input-side inner-loop current controller and output-side inner-loop current controller, namely the first input-side inner-loop current controller, the second input-side inner-loop current controller, the third input-side inner-loop current controller, the first output-side inner-loop current controller, the second output-side inner-loop current controller, and the third output-side inner-loop current controller. Input the target current value into the first output-side inner-loop current controller, the second output-side inner-loop current controller, and the third output-side inner-loop current controller corresponding to the U phase, V phase, and W phase respectively to obtain the first output-side capacitor voltage, the second output-side capacitor voltage, and the third output-side capacitor voltage; input the target current value into the first input-side inner-loop current controller, the second input-side inner-loop current controller, and the third input-side inner-loop current controller corresponding to the U phase, V phase, and W phase respectively to obtain the first input-side capacitor voltage, the second input-side capacitor voltage, and the third input-side capacitor voltage.

[0030] In this embodiment, before using the inner-loop current controller to obtain the target voltage value, it further includes determining the mathematical relationship between the target current value on the input side and the capacitor voltage, and designing the input-side inner-loop current controller according to the mathematical relationship between the target current value on the input side and the capacitor voltage; determining the mathematical relationship between the target current value on the output side and the capacitor voltage, and designing the output-side inner-loop current controller according to the mathematical relationship between the target current value on the output side and the capacitor voltage. Among them, the input-side inner-loop current controller and the output-side inner-loop current controller may include a d-axis and a q-axis, the target current value may include the target current value of the d-axis and the target current value of the q-axis, and the output target voltage value may also include the target voltage value of the d-axis and the target voltage value of the q-axis.

[0031] Taking the U phase as an example (the structures of the other two phases are the same), the mathematical relationship between the target current value on the input side and the capacitor voltage is determined as: where, i * dU is the target current value of the d-axis on the input side, i * qU is the target current value of the q-axis on the input side, u dU is the first input-side capacitor voltage of the d-axis, u qU is the first input-side capacitor voltage of the q-axis.

[0032] The mathematical relationship between the target current value on the output side and the capacitor voltage is determined as follows: where, i * 0d is the target current value on the output side of the d-axis, and i * 0q is the target current value on the output side of the q-axis, u 0d is the first output-side capacitor voltage of the d-axis, and u 0q is the first output-side capacitor voltage of the q-axis.

[0033] In a three-phase power system, the zero-sequence component is used to describe the unbalanced state of the three-phase system. When the three-phase voltage or current is asymmetric, the zero-sequence component will appear. In this step, the zero-sequence component is calculated using the first output-side capacitor voltage, the second output-side capacitor voltage, and the third output-side capacitor voltage. The formula for calculating the zero-sequence component is as follows: Using the zero-sequence component and the first input-side capacitor voltage, the second input-side capacitor voltage, and the third input-side capacitor voltage, the target voltage value of each bridge arm is calculated. The formula for calculating the target voltage value is as follows: 203. Determine the target sub-module switching strategy for each bridge arm according to the number of the first total switched sub-modules and the total number of switched sub-modules in the previous cycle of each bridge arm.

[0034] In order to achieve the stability of the capacitor voltage in real time as much as possible, it is necessary to determine the sub-module switching strategy as real-time as possible. A switching period for determining the sub-module switching strategy can be preset. When the duration of the switching period is short, it may occur that the sub-module switching strategy in the previous cycle is the same as that in the current cycle. Therefore, first calculate the total number of switched sub-modules in the current cycle, and obtain the number of the first total switched sub-modules in the previous cycle. If the two are the same, the first sub-module switching strategy in the previous cycle can be maintained, and there is no need to generate the same sub-module switching strategy for the current cycle again.

[0035] When calculating the total number of switched sub-modules, first obtain the first capacitor voltage of each bridge arm, that is, the average value of the capacitor voltages of multiple sub-modules in each bridge arm. According to the first capacitor voltage of each bridge arm and the target voltage value of each bridge arm, calculate the total number of switched sub-modules of each bridge arm. The formula for calculating the total number of switched sub-modules is as follows: where, n is the total number of switched sub-modules, round() is to take the integer value, u ij_ref is the target voltage value, and Uc is the first capacitor voltage.

[0036] Determine the target sub-module switching strategy for each arm according to the number of the first total switching sub-modules and the number of total switching sub-modules in the previous cycle of each arm, which specifically includes: subtract the number of the first total switching sub-modules from the number of total switching sub-modules to obtain the total switching difference; if the total switching difference is 0, it means that there is no need to change the first sub-module switching strategy in the previous cycle, that is, maintain the first sub-module switching strategy in the previous cycle; if the total switching difference is less than 0, it indicates that switching needs to be reduced, that is, sub-modules are removed. At this time, determine the absolute value of the total switching difference as the target removal quantity, and determine the target sub-module switching strategy according to the charge and discharge states of the sub-modules; if the total switching difference is greater than 0, it indicates that switching needs to be increased, that is, positive / negative sub-modules are inserted. At this time, determine the total switching difference as the target insertion quantity, and determine the target sub-module switching strategy according to the charge and discharge states of the sub-modules and the insertion parameters.

[0037] The formula for calculating the total switching difference is as follows: where Δn is the total switching difference, n is the number of total switching sub-modules, and n past is the number of the first total switching sub-modules.

[0038] Since the charge and discharge states of the sub-modules can determine whether to switch the sub-modules with the highest or lowest voltage, when the total switching difference is less than 0, first judge whether the sub-module is discharging; if so, determine the target sub-module switching strategy as removing multiple sub-modules corresponding to the target removal quantity with the highest voltage; if not, determine the target sub-module switching strategy as removing multiple sub-modules corresponding to the target removal quantity with the lowest voltage. When sorting the voltage values of the sub-modules in the arm, the sub-modules that have been inserted / removed are not sorted.

[0039] The formula for determining whether the sub-module is charging is as follows: where when S _state and D arm have the same sign, C charge is 1, indicating that the sub-module is charging; when S _state and D arm have different signs, C charge is -1, indicating that the sub-module is discharging.

[0040] When the total switching difference is greater than 0, the switching strategy of the target sub-module is affected not only by the charge and discharge states of the sub-modules but also by the positive and negative values of the input parameters. Therefore, first obtain the charge and discharge states of the sub-modules and the positive and negative values of the input parameters. When the sub-module is in the charging state and the input parameter is positive, determine that the switching strategy of the target sub-module is to negatively input multiple sub-modules with the lowest voltages corresponding to the target input quantity. When the sub-module is in the charging state and the input parameter is negative, determine that the switching strategy of the target sub-module is to positively input multiple sub-modules with the lowest voltages corresponding to the target input quantity. When the sub-module is in the discharging state and the input parameter is positive, determine that the switching strategy of the target sub-module is to negatively input multiple sub-modules with the highest voltages corresponding to the target input quantity. When the sub-module is in the discharging state and the input parameter is negative, determine that the switching strategy of the target sub-module is to positively input multiple sub-modules with the highest voltages corresponding to the target input quantity. Among them, the above S _state is the input parameter.

[0041] 204. Execute the switching strategy of the target sub-module for each arm.

[0042] This step is the same as the aforementioned step 104 and will not be elaborated here.

[0043] In some embodiments, before determining the target current value using the outer-loop voltage controller, it further includes deriving the mathematical model of the M3C converter, and the specific steps are as follows: According to Kirchhoff's voltage law: The Clarke transformation matrix is given as: Multiplying equation (1) on the left by equation (2) gives: Decomposing equation (3) gives the analytical expressions of the αβ components of the voltages and currents of the three sub-converters U, V, and W as: At the same time, the analytical expression of the 0 component is obtained as: Continuing to multiply equation (5) on the left by equation (2) gives: For a three-phase symmetric input-output system, there are ui0 = 0, uj0 = 0, u00 = 0, and i00 = 0, uO’O = 0, then equation (6) can be reduced to: In summary, equations (4) and (7) constitute the mathematical model of M3C in the αβ0 coordinate system.

[0044] During the derivation process, the expressions of some αβ0 components are given by Equation (8), and the remaining expressions are similar to it, so they will not be elaborated here. Among them, the variable f represents the current i or the voltage u.

[0045] For the convenience of subsequent controller design, the mathematical model in αβ0 coordinates is further transformed into dq0 coordinates below.

[0046] The Park transformation matrices of the input and output sides are given respectively: Left-multiplying (4) by (9) gives: Left-multiplying (7) by (10) gives: Then, Equation (11) and Equation (12) together constitute the mathematical model of M3C in dq0 coordinates.

[0047] The expressions of the input and output side voltages and currents in abc coordinates are given as: Combined with Equation (8), it is easy to obtain the expressions of the input and output side voltages and currents in αβ0 coordinates as: After performing the Park transformation, the expressions of the input and output side voltages and currents in dq0 coordinates can be obtained as: Based on the above Figure 2 implementation method, it can be seen that this scheme takes the average value of the capacitor voltages of multiple sub-modules as the actual capacitor voltage, and uses the PI regulator in the outer-loop voltage controller to process the error between the desired voltage value and the actual capacitor voltage to generate an accurate target current value. Each of the three phases U, V, and W has corresponding inner-loop current controllers on the input and output sides. By measuring the actual current of each bridge arm in real time, calculating the current error and compensating it, it ensures that the system can respond quickly to load changes or grid disturbances in a short time. By judging the charge and discharge states of the sub-modules, the sub-modules to be inserted or removed are reasonably selected to avoid overcharging or under-voltage problems caused by uneven voltages. Through precise control strategies, the capacitor voltage fluctuations are reduced, the failure risk caused by uneven voltages is lowered, and the service life of the equipment is extended. At the same time, the capacitor voltage changes of each sub-module are monitored in real time, and the results are fed back to the control system to ensure the stability and reliability of the system.

[0048] Furthermore, as for the above Figure 1 、 2For the implementation of the method embodiments shown, embodiments of the present invention provide a capacitor voltage control device for a flexible low-frequency power transmission modular multilevel matrix converter, which is used to control the stability of the capacitor voltage of the modular multilevel matrix converter. The embodiments of this device correspond to the foregoing method embodiments. For the convenience of reading, the details in the foregoing method embodiments will not be elaborated one by one in this embodiment. However, it should be clear that the device in this embodiment can correspondingly implement all the contents in the foregoing method embodiments. Specifically, as Figure 3 shown, the device includes: An outer loop control unit 31, configured to obtain the actual capacitor voltage of the modular multilevel matrix converter, and convert the actual capacitor voltage by using an outer loop voltage controller to obtain a target current value; An inner loop control unit 32, configured to obtain the target voltage value of each arm by using an inner loop current controller according to the target current value obtained by the outer loop control unit 31; A generating unit 33, configured to generate a target sub-module switching strategy for each arm based on the target voltage value obtained by the inner loop control unit 32, and the target sub-module switching strategy is used to switch the sub-modules connected in the arm to suppress the capacitor voltage fluctuation in the arm; An execution unit 34, configured to execute the target sub-module switching strategy for each arm generated by the generating unit 33 to control the stability of the capacitor voltage of the modular multilevel matrix converter.

[0049] Further, as Figure 4 shown, the outer loop control unit 31 includes: A detection module 311, configured to detect the capacitor voltages of multiple sub-modules in the modular multilevel matrix converter; A calculation module 312, configured to calculate the average value of the capacitor voltages of the multiple sub-modules detected by the detection module 311 as the actual capacitor voltage; An input module 313, configured to input the desired voltage value and the actual capacitor voltage obtained by the calculation module 312 into the outer loop voltage controller, and use the PI regulator in the outer loop controller to obtain a target current value that can meet the desired voltage value.

[0050] Further, as Figure 4 shown, the inner loop control unit 32 includes: A first input module 321, configured to input the target current value into the first output side inner loop current controller, the second output side inner loop current controller, and the third output side inner loop current controller corresponding to the U phase, the V phase, and the W phase respectively, to obtain the first output side capacitor voltage, the second output side capacitor voltage, and the third output side capacitor voltage; A first calculation module 322, configured to calculate the zero-sequence component by using the first output side capacitor voltage, the second output side capacitor voltage, and the third output side capacitor voltage obtained by the first input module 321; The second input module 323 is configured to input the target current value into the first input - side inner - loop current controller, the second input - side inner - loop current controller, and the third input - side inner - loop current controller corresponding to the U - phase, V - phase, and W - phase respectively, so as to obtain the first input - side capacitor voltage, the second input - side capacitor voltage, and the third input - side capacitor voltage; The second calculation module 324 is configured to calculate the target voltage value of each bridge arm by using the zero - sequence component obtained by the first calculation module 322 and the first input - side capacitor voltage, the second input - side capacitor voltage, and the third input - side capacitor voltage obtained by the second input module 323.

[0051] Further, as Figure 4 shown, the generating unit 33 includes: An acquisition module 331, configured to acquire the first capacitor voltage of each bridge arm; A calculation module 332, configured to calculate the total number of switching sub - modules of each bridge arm according to the first capacitor voltage of each bridge arm acquired by the acquisition module 331 and the target voltage value of each bridge arm; A determination module 333, configured to determine the target sub - module switching strategy of each bridge arm according to the first total number of switching sub - modules in the previous cycle of each bridge arm and the total number of switching sub - modules obtained by the calculation module 332.

[0052] Further, as Figure 4 shown, the determination module 333 includes: A calculation sub - module 33301, configured to subtract the first total number of switching sub - modules from the total number of switching sub - modules to obtain the total switching difference; A control sub - module 33302, configured to maintain the first sub - module switching strategy of the previous cycle if the total switching difference obtained by the calculation sub - module 33301 is 0; A first determination sub - module 33303, configured to determine the absolute value of the total switching difference as the target cut - off quantity if the total switching difference obtained by the calculation sub - module 33301 is less than 0, and determine the target sub - module switching strategy according to the charge - discharge state of the sub - modules; A second determination sub - module 33304, configured to determine the total switching difference as the target input quantity if the total switching difference obtained by the calculation sub - module 33301 is greater than 0, and determine the target sub - module switching strategy according to the charge - discharge state and input parameters of the sub - modules.

[0053] Further, as Figure 4 shown, the determination module 333 further includes: A judgment sub - module 33305, configured to judge whether the sub - module is discharging; The third determination sub-module 33306 is configured to, if the determination sub-module 33305 determines that the sub-module is discharging, determine that the target sub-module switching strategy is to remove a plurality of sub-modules with the highest voltages corresponding to the target removal quantity; The fourth determination sub-module 33307 is configured to, if the determination sub-module 33305 determines that the sub-module is charging, determine that the target sub-module switching strategy is to remove a plurality of sub-modules with the lowest voltages corresponding to the target removal quantity.

[0054] Further, as Figure 4 shown, the determination module 333 further includes: The sub-module acquisition module 33308 is configured to acquire the charge and discharge state of the sub-module and the positive or negative of the input parameter; The fifth determination sub-module 33309 is configured to, when the sub-module acquisition module 33308 obtains that the sub-module is in a charging state and the input parameter is positive, determine that the target sub-module switching strategy is to negatively input a plurality of sub-modules with the lowest voltages corresponding to the target input quantity; The sixth determination sub-module 33310 is configured to, when the sub-module acquisition module 33308 obtains that the sub-module is in a charging state and the input parameter is negative, determine that the target sub-module switching strategy is to positively input a plurality of sub-modules with the lowest voltages corresponding to the target input quantity; The seventh determination sub-module 33311 is configured to, when the sub-module acquisition module 33308 obtains that the sub-module is in a discharging state and the input parameter is positive, determine that the target sub-module switching strategy is to negatively input a plurality of sub-modules with the highest voltages corresponding to the target input quantity; The eighth determination sub-module 33312 is configured to, when the sub-module acquisition module 33308 obtains that the sub-module is in a discharging state and the input parameter is negative, determine that the target sub-module switching strategy is to positively input a plurality of sub-modules with the highest voltages corresponding to the target input quantity.

[0055] Further, an embodiment of the present invention further provides a computing device, where the computing device includes: at least one processor, and a memory, where the memory stores instructions executable by the processor, and when the instructions are executed by the processor, the processor can execute a capacitance voltage control method for a flexible low-frequency power transmission modular multilevel matrix converter as described in the above Figure 1 、 2 .

[0056] Further, an embodiment of the present invention further provides a readable storage medium, where the readable storage medium is used to store a computer program, where when the computer program runs, it controls the device where the storage medium is located to execute a capacitance voltage control method for a flexible low-frequency power transmission modular multilevel matrix converter as described in the above Figure 1 、 2 .

[0057] In the above embodiments, the descriptions of the various embodiments each have their own emphasis. For parts not detailed in a certain embodiment, reference may be made to the relevant descriptions of other embodiments.

[0058] It can be understood that the relevant features in the above methods and devices can be referred to each other. In addition, the "first", "second", etc. in the above embodiments are used to distinguish the various embodiments, and do not represent the advantages or disadvantages of the various embodiments.

[0059] Those skilled in the art can clearly understand that for the convenience and brevity of description, the specific working processes of the systems, devices, and units described above can refer to the corresponding processes in the foregoing method embodiments, and will not be elaborated herein.

[0060] The algorithms and displays provided herein are not inherently related to any particular computer, virtual system, or other device. Various general-purpose systems can also be used in conjunction with the teachings herein. The structure required to construct such a system will be apparent from the above description. In addition, the present invention is not directed to any particular programming language. It should be understood that the content of the present invention described herein can be implemented using various programming languages, and the description of the specific language above is for the purpose of disclosing the best mode of the present invention.

[0061] In addition, the memory may include non-permanent memory in a computer-readable medium, random access memory (RAM), and / or non-volatile memory in the form of, for example, read-only memory (ROM) or flash memory (flash RAM), and the memory includes at least one storage chip.

[0062] Those skilled in the art should understand that the embodiments of the present invention can be provided as a method, a system, or a computer program product. Therefore, the present invention can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present invention can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0063] The present invention is described with reference to the flowcharts and / or block diagrams of methods, apparatuses (systems), and computer program products according to embodiments of the present invention. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of flows and / or blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices generate for implementing in the process Figure 1 one process or multiple processes and / or blocksFigure 1 means for the functions specified in one or more boxes.

[0064] These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to work in a particular manner, such that the instructions stored in the computer-readable memory produce a manufacture including an instruction means that implements the functions specified in one Figure 1 one or more processes and / or boxes Figure 1 means for the functions specified in one or more boxes.

Claims

1. A capacitor voltage control method for a flexible low-frequency power transmission modular multi-level matrix converter, characterized in that: The method comprises: Acquiring an actual capacitor voltage of a modular multi-level matrix converter, and converting the actual capacitor voltage using an outer loop voltage controller to obtain a target current value; According to the target current value, a target voltage value of each bridge arm is obtained by using an inner loop current controller; Generating a target submodule switching strategy for each bridge arm based on the target voltage value, wherein the target submodule switching strategy is used to switch the submodules connected to the bridge arm to suppress the voltage fluctuation of the capacitor in the bridge arm; The target submodule switching strategy of each bridge arm is executed to control the capacitor voltage stability of the modular multi-level matrix converter.

2. The method according to claim 1, characterized in that Acquiring an actual capacitor voltage of a modular multi-level matrix converter, and converting the actual capacitor voltage using an outer loop voltage controller to obtain a target current value, including: detecting a plurality of submodule capacitor voltages in the modular multi-level matrix converter; Calculate the average value of the capacitor voltages of the plurality of submodules as the actual capacitor voltage; The expected voltage value and the actual capacitor voltage are input into the outer loop voltage controller, and the target current value that can meet the expected voltage value is obtained by using the PI regulator in the outer loop controller.

3. The method according to claim 1, characterized in that According to the target current value, the target voltage value of each bridge arm is obtained by using the inner loop current controller, including: Input the target current value to the first output side inner loop current controller, the second output side inner loop current controller, and the third output side inner loop current controller corresponding to the U phase, the V phase, and the W phase, respectively, to obtain the first output side capacitor voltage, the second output side capacitor voltage, and the third output side capacitor voltage; Calculating a 0-sequence component using the first output side capacitor voltage, the second output side capacitor voltage, and the third output side capacitor voltage; Input the target current value to the first input side inner loop current controller, the second input side inner loop current controller, and the third input side inner loop current controller corresponding to the U phase, the V phase, and the W phase, respectively, to obtain the first input side capacitor voltage, the second input side capacitor voltage, and the third input side capacitor voltage; The target voltage value of each bridge arm is calculated using the 0-sequence component and the first input side capacitor voltage, the second input side capacitor voltage, and the third input side capacitor voltage.

4. The method according to any one of claims 1 to 3, characterized in that: Generating a target submodule switching strategy for each bridge arm based on the target voltage value includes: Obtaining a first capacitor voltage of each bridge arm; Calculate the total number of switched sub-modules of each bridge arm according to the first capacitor voltage of each bridge arm and the target voltage value of each bridge arm; The target submodule switching strategy of each bridge arm is determined according to the first total switched submodule quantity of the previous cycle of each bridge arm and the total switched submodule quantity.

5. The method according to claim 4, characterized in that The step of determining the target submodule switching strategy of each bridge arm according to the first total number of switched submodules in the previous cycle of each bridge arm and the total number of switched submodules includes: Subtract the first total number of switched submodules from the total number of switched submodules to obtain a total switching difference; If the total switching difference is 0, the first submodule switching strategy of the previous cycle is maintained; If the total switching difference is less than 0, the absolute value of the total switching difference is determined as the target switching quantity, and the target submodule switching strategy is determined according to the charging and discharging state of the submodule; If the total switching difference is greater than 0, the total switching difference is determined as the target switching quantity, and the target submodule switching strategy is determined according to the charging and discharging state and the switching parameters of the submodule.

6. The method according to claim 5, characterized in that If the total switching difference is less than 0, the absolute value of the total switching difference is determined as the target switching quantity, and the target submodule switching strategy is determined according to the charging and discharging state of the submodule, including: Determining whether the submodule is discharging; If yes, then determining the target submodule switching strategy is to switch off a plurality of submodules with the highest voltage corresponding to the target switching quantity; If not, then the target submodule switching strategy is determined to be to switch off a plurality of submodules with the lowest voltage corresponding to the target switching quantity.

7. The method according to claim 5, characterized in that If the total switching difference is greater than 0, the total switching difference is determined as the target input quantity, and the target submodule switching strategy is determined according to the charge and discharge state and input parameters of the submodule, including: Obtaining the charging and discharging status of the submodule and the positive and negative values ​​of the input parameters; When the submodule is in a charging state and the input parameter is a positive number, determining that the target submodule switching strategy is to negatively input a plurality of submodules with the lowest voltage corresponding to the target input quantity; When the submodule is in a charging state and the input parameter is a negative number, determining the target submodule switching strategy to be positively inputting a plurality of submodules with the lowest voltage corresponding to the target input quantity; When the submodule is in a discharge state and the input parameter is a positive number, determining the target submodule switching strategy to negatively input a plurality of submodules with the highest voltage corresponding to the target input quantity; When the submodule is in a discharging state and the input parameter is a negative number, the target submodule switching strategy is determined to be to positively input a plurality of submodules with the highest voltage corresponding to the target input quantity.

8. A capacitor voltage control device for a flexible low-frequency power transmission modular multi-level matrix converter, characterized in that: The device comprises: An outer loop control unit, used for obtaining an actual capacitor voltage of the modular multi-level matrix converter, and converting the actual capacitor voltage using an outer loop voltage controller to obtain a target current value; An inner loop control unit, used to obtain a target voltage value of each bridge arm using an inner loop current controller according to the target current value obtained by the outer loop control unit; A generating unit, used for generating a target submodule switching strategy for each bridge arm based on the target voltage value obtained by the inner loop control unit, wherein the target submodule switching strategy is used for switching the submodule connected to the bridge arm to suppress the voltage fluctuation of the capacitor in the bridge arm; An execution unit is used to execute the target submodule switching strategy of each bridge arm generated by the generation unit to control the capacitor voltage stability of the modular multi-level matrix converter.

9. A computing device, characterized in that The computing device includes: at least one processor, and a memory, wherein the memory stores instructions that can be executed by the processor, and the instructions are executed by the processor, so that the processor can execute the capacitor voltage control method of a flexible low-frequency power transmission modular multi-level matrix converter as described in any one of claims 1-7.

10. A readable storage medium, characterized in that: The readable storage medium is used to store a computer program, wherein when the computer program is running, it controls the device where the storage medium is located to execute a capacitor voltage control method for a flexible low-frequency power transmission modular multi-level matrix converter as described in any one of claims 1 to 7.

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