MMC sub-module accurate switching control method and device based on voltage real-time adjustment

Abstract

This application discloses a precise switching control method and apparatus for MMC submodules based on real-time voltage adjustment, relating to the field of MMC control technology. The proposed solution, by accurately calculating the voltage deviation and combining it with the direction of the bridge arm current to determine the number of supplementary switches, can effectively reduce the inherent error between the actual generated voltage of the bridge arm and the target modulation voltage. Therefore, when applying a novel switching strategy aimed at reducing the switching frequency, this solution can ensure the frequency reduction effect of submodule switching while also reducing DC-side voltage fluctuations, thereby improving the voltage control quality of the MMC system.

Description

Technical Field

[0001] This application relates to the field of MMC control technology, and in particular to a method and apparatus for precise switching control of MMC submodules based on real-time voltage adjustment. Background Technology

[0002] In recent years, high-voltage direct current (HVDC) transmission systems based on modular multilevel converters (MMCs) have been widely used in the field of HVDC transmission due to their advantages such as four-quadrant decoupling control of active and reactive power and easy expansion.

[0003] Traditional MMC converter submodule switching strategies typically determine the number of submodules (n) to be switched on per cycle by rounding the instantaneous modulated voltage. The switching strategy selects n submodules from N submodules for switching on, ideally with the actual output voltage of the bridge arm approximating the bridge arm reference voltage. However, an inherent error exists between the generated voltage and the target modulated voltage of the bridge arm. This error is often not noticeable in conventional switching strategies under high-frequency conditions. However, in recent years, as maintenance departments have increasingly focused on the switching losses and lifespan of critical components in MMC systems, new switching strategies aimed at reducing switching frequencies, such as those based on the hold factor method, have been proposed to reduce switching losses. While these strategies lower the switching frequency and extend the lifespan of critical components, they also amplify the inherent error between the actual generated voltage and the target voltage of the bridge arm, leading to a decline in voltage control quality caused by DC-side voltage. Summary of the Invention

[0004] This application provides a method and apparatus for precise switching control of MMC submodules based on real-time voltage adjustment, which aims to reduce the inherent error between the actual generated voltage of the bridge arm and the target modulation voltage while ensuring the frequency reduction effect of submodule switching, thereby reducing DC side voltage fluctuations.

[0005] To achieve the above-mentioned objectives, the first aspect of this application provides a method for precise switching control of MMC submodules based on real-time voltage adjustment, comprising: Obtain the bridge arm generation voltage of the target MMC system in the current cycle; The voltage deviation is determined based on the difference between the preset target modulation voltage and the generated voltage of the bridge arm; The number of submodules to be switched on or off is determined based on the voltage deviation and the preset submodule rated voltage. Based on the number of submodules to be switched on and off, and in conjunction with the direction of the bridge arm current in the target MMC system, a submodule switching control strategy is determined, and the corresponding submodule switching action is executed according to the submodule switching control strategy.

[0006] Preferably, the calculation method for the number of additional cuts by the submodule is as follows:

[0007] In the formula, The voltage deviation is the amount mentioned above. To replenish the number of switches for the submodule, The rated voltage of the submodule is , and round is the rounding function.

[0008] Preferably, determining the submodule switching control strategy based on the number of submodules to be switched on and off, combined with the direction of the bridge arm current in the target MMC system, includes: Based on the numerical polarity of the number of additional cuts by the submodule, the type of cut action in the current cycle is determined, wherein the type of cut action includes: additional input or cut-off. Based on the switching action type and the bridge arm current direction in the target MMC system, the sub-module filtering logic is determined according to the preset filtering logic correspondence. The filtering logic correspondence is the correspondence between the switching action type and the bridge arm current direction and the sub-module filtering logic. Based on the type of switching action, the number of additional switching actions in the submodule, and the filtering logic of the submodule, a submodule switching control strategy is generated.

[0009] Preferably, determining the type of switching action in the current cycle based on the numerical polarity of the number of switches added by the submodule includes: When the polarity of the number of feeds added by the submodule is positive, the feed action type for the current cycle is determined to be supplementary feed. When the polarity of the number of additional cuts added by the submodule is negative, the type of cut action in the current cycle is determined to be cut.

[0010] Preferably, the step of determining the submodule filtering logic according to the correspondence between the switching action type and the arm current direction in the target MMC system, based on a preset filtering logic, includes: When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the charging direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value in ascending order, select several sub-modules that are in the disconnected state. When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the discharge direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value from large to small, select several sub-modules that are in the disconnected state. When the switching action type is cut-off and the bridge arm current direction in the target MMC system is the charging direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value from large to small, select several sub-modules that are in the already engaged state. When the switching action type is cut-off and the bridge arm current direction in the target MMC system is the discharge direction, the sub-module screening logic is determined as follows: several sub-modules in the already engaged state are selected in order of ascending capacitor voltage values; wherein, the number of selected sub-modules is equal to the number of sub-modules to be added and switched.

[0011] A second aspect of this application provides a precise switching control device for MMC submodules based on real-time voltage adjustment, comprising: The bridge arm generation voltage acquisition unit is used to acquire the bridge arm generation voltage of the target MMC system in the current cycle. The voltage deviation calculation unit is used to determine the voltage deviation amount based on the difference between the preset target modulation voltage and the generated voltage of the bridge arm; The supplementary switching quantity determination unit is used to determine the supplementary switching quantity of the submodule based on the voltage deviation and the preset submodule rated voltage; The switching control strategy generation unit is used to determine the submodule switching control strategy based on the number of submodules to be switched and the direction of the bridge arm current in the target MMC system, so as to execute the corresponding submodule switching action according to the submodule switching control strategy.

[0012] Preferably, the calculation method for the number of additional cuts by the submodule is as follows:

[0013] In the formula, The voltage deviation is the amount mentioned above. To replenish the number of switches for the submodule, The rated voltage of the submodule is , and round is the rounding function.

[0014] Preferably, the switching control strategy generation unit is specifically used for: Based on the numerical polarity of the number of additional cuts by the submodule, the type of cut action in the current cycle is determined, wherein the type of cut action includes: additional input or cut-off. Based on the switching action type and the bridge arm current direction in the target MMC system, the sub-module filtering logic is determined according to the preset filtering logic correspondence. The filtering logic correspondence is the correspondence between the switching action type and the bridge arm current direction and the sub-module filtering logic. Based on the type of switching action, the number of additional switching actions in the submodule, and the filtering logic of the submodule, a submodule switching control strategy is generated.

[0015] Preferably, determining the type of switching action in the current cycle based on the numerical polarity of the number of switches added by the submodule includes: When the polarity of the number of feeds added by the submodule is positive, the feed action type for the current cycle is determined to be supplementary feed. When the polarity of the number of additional cuts added by the submodule is negative, the type of cut action in the current cycle is determined to be cut.

[0016] Preferably, the step of determining the submodule filtering logic according to the correspondence between the switching action type and the arm current direction in the target MMC system, based on a preset filtering logic, includes: When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the charging direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value in ascending order, select several sub-modules that are in the disconnected state. When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the discharge direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value from large to small, select several sub-modules that are in the disconnected state. When the switching action type is cut-off and the bridge arm current direction in the target MMC system is the charging direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value from large to small, select several sub-modules that are in the already engaged state. When the switching action type is cut-off and the bridge arm current direction in the target MMC system is the discharge direction, the sub-module screening logic is determined as follows: several sub-modules in the already engaged state are selected in order of ascending capacitor voltage values; wherein, the number of selected sub-modules is equal to the number of sub-modules to be added and switched.

[0017] As can be seen from the above technical solutions, this application has the following advantages: The proposed solution effectively reduces the inherent error between the actual generated voltage of the bridge arm and the target modulation voltage by accurately calculating the voltage deviation and determining the number of supplementary switches based on the direction of the bridge arm current. Therefore, when applying a novel switching strategy aimed at reducing the switching frequency, this solution can ensure the frequency reduction effect of submodule switching while also reducing DC-side voltage fluctuations, thereby improving the voltage control quality of the MMC system. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a flowchart illustrating an embodiment of a precise switching control method for MMC submodules based on real-time voltage adjustment provided in this application.

[0020] Figure 2 This is a flowchart illustrating another embodiment of the precise switching control method for MMC submodules based on real-time voltage adjustment provided in this application.

[0021] Figure 3 This is a schematic diagram of the MMC system.

[0022] Figure 4 This is a schematic diagram comparing the target voltage with the actual voltage applied to the submodule.

[0023] Figure 5 This is a schematic diagram comparing the target voltage with the actual voltage applied to the submodule under another operating condition.

[0024] Figure 6 The image shows a comparison of the effects before and after enabling the precise switching control method for MMC submodules based on real-time voltage adjustment provided in this application.

[0025] Figure 7 Another comparison diagram showing the effect before and after enabling the precise switching control method for MMC submodules based on real-time voltage adjustment provided in this application.

[0026] Figure 8 This is a schematic diagram of the architecture of an embodiment of a precise switching control device for MMC submodules based on real-time voltage adjustment provided in this application. Detailed Implementation

[0027] In conventional submodule switching strategies and current optimization strategies (such as hold factor optimization and limit method optimization), the submodule capacitor voltage is considered according to its rated value. However, the submodule voltage fluctuates, and after charging and discharging at various times, the submodule voltage deviates from its rated value, resulting in a deviation between the actual voltage of the bridge arm and the target modulation voltage. Research and analysis show that in existing modulation strategies, the number of submodules deployed in each cycle is determined by rounding the instantaneous modulation wave voltage. Specifically, assuming the total number of bridge arm submodules is N, the rated value of the submodule capacitor voltage is UC_SM, and the target modulation wave voltage uarm_ref is provided by the modulation stage, then the theoretical number of submodules deployed is:

[0028] In the formula: round is the floor function; it performs integer operations on real variables within the function.

[0029] The submodule switching strategy determines the starting point. N Select from each submodule n The input, under ideal conditions, the actual voltage output of the bridge arm. u arm = nU C_SM Approaching u arm_ref However, there is an inherent error between the generated voltage of the bridge arm and the target modulation voltage, which is mainly caused by two factors: First, the inherent discretization error Δu 1. Since the nearest-level approximation modulation strategy itself uses discrete bridge arm sub-module input voltages to approximate the continuous target modulation voltage, the inherent discretization error is:

[0030] Analysis shows that the range of this error is [- U C_SM / 2, U C_SM [2] The more sub-modules there are, the smaller the discretization error becomes. Therefore, the discretization error of a sub-module can only be reduced by increasing the number of sub-modules.

[0031] Secondly, the error is caused by the real-time fluctuation of the submodule capacitor voltage. Δu 2. Ideally, each submodule is considered to contribute voltage as follows: U C_SM The bridge arm output voltage is nU C_SM In fact, the submodule will deviate from its rated voltage after charging and discharging, which will lead to an actual output voltage in the bridge arm. u arm_act for:

[0032] In the formula: u i ( t ()( i = 1,2,…, n )for t Time of the first i The actual voltage of each submodule; therefore Δu 2 is:

[0033] Therefore, the total primary error Δu = Δu 1 + Δu 2. This error is not obvious in conventional switching strategies due to the high sorting frequency. However, when the switching frequency is reduced, it will affect the MMC voltage control quality. In fact, the difference between the actual generated voltage of the bridge arm and the target modulation voltage is the algebraic sum of the submodule voltage deviations. This application defines this difference as the intrinsic error between the generated voltage of the bridge arm and the modulation voltage, and points out that the more aggressive the frequency reduction effect and the worse the voltage equalization effect, the larger this value will be.

[0034] To overcome the inherent error between the actual generated voltage and the target voltage of the bridge arm caused by the deviation of the factor module capacitor voltage from its rated value after charging and discharging, which leads to a decrease in voltage control quality on the DC side, and to further improve the steady-state operation performance of the system, this application provides a precise switching control method and device for MMC submodules based on real-time voltage adjustment. Considering that the aggravation of DC voltage fluctuation is essentially caused by the inherent error between the actual generated voltage and the target modulation voltage of the bridge arm, this error is compensated by controlling the input or output of a small number of submodules during the switching process, thereby reducing the aforementioned inherent error and reducing DC side voltage fluctuation. This achieves the invention objective of reducing the inherent error between the actual generated voltage and the target modulation voltage of the bridge arm and reducing DC side voltage fluctuation while ensuring the frequency reduction effect of submodule switching.

[0035] To make the inventive objectives, features, and advantages of this application more apparent and understandable, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0036] The following is a detailed description of an embodiment of a precise switching control method for MMC submodules based on real-time voltage adjustment provided in this application: Please see Figure 1 This application provides a precise switching control method for MMC submodules based on real-time voltage adjustment, the steps of which include: Step 101: Obtain the bridge arm generation voltage of the target MMC system in the current cycle; Step 102: Determine the voltage deviation based on the difference between the preset target modulation voltage and the generated voltage of the bridge arm; Step 103: Determine the number of submodules to be switched on or off based on the voltage deviation and the preset submodule rated voltage; Step 104: Based on the number of sub-modules to be switched on and off, and in conjunction with the direction of the bridge arm current in the target MMC system, determine the sub-module switching control strategy, and execute the corresponding sub-module switching action according to the sub-module switching control strategy.

[0037] To facilitate understanding of the technical solution in this embodiment, some key terms involved are explained below: MMC system refers to modular multilevel converter system, which is usually composed of multiple sub-modules connected in series to form a bridge arm, and the multiple bridge arms form a converter topology to realize the conversion of AC and DC power.

[0038] The bridge arm generated voltage refers to the actual output voltage formed by the sum of the capacitor voltages of all engaged submodules in the current cycle within an MMC system.

[0039] The target modulation voltage refers to the voltage reference value that the bridge arm should reach, as set by the MMC system control algorithm based on the system's operational requirements.

[0040] Voltage deviation refers to the difference between the target modulation voltage and the actual generated voltage of the bridge arm, reflecting the degree of deviation between the actual output voltage of the bridge arm and the expected voltage.

[0041] The rated voltage of a submodule refers to the standard voltage value of a single submodule in an MMC system under normal operating conditions.

[0042] The number of submodules to be added or removed refers to the number of submodules that need to be added or removed in order to correct voltage deviation.

[0043] The direction of current in the bridge arm refers to the direction of current flowing through the bridge arm of the MMC system. It is usually divided into charging direction and discharging direction, which affects the trend of submodule capacitor voltage change.

[0044] The submodule switching control strategy refers to the scheme that determines the specific rules and order for submodules to be switched on or off based on the system's operating status and control objectives.

[0045] Submodule deployment / switching action refers to the actual deployment or removal operation of a submodule according to the submodule deployment / switching control strategy.

[0046] More specifically, this embodiment provides a precise switching control method for MMC submodules based on real-time voltage adjustment, the specific implementation process of which is as follows: First, the bridge arm generation voltage for the current cycle in the target MMC system is obtained. This bridge arm generation voltage is defined as the sum of the capacitor voltages of all submodules engaged in the current cycle. One implementation approach is to configure a voltage sensor on each submodule of the MMC system to measure the capacitor voltage of each submodule in real time. Then, the capacitor voltage data of all engaged submodules are aggregated to a control unit, which sums these voltage values ​​to obtain the bridge arm generation voltage for the current cycle. Another implementation approach is to install a total voltage sensor on each bridge arm to directly measure the voltage across the bridge arm and use this as the bridge arm generation voltage.

[0047] Furthermore, the voltage deviation is determined based on the difference between the preset target modulation voltage and the generated voltage of the bridge arm. Specifically, the target modulation voltage can be pre-stored or calculated in real time in the control system. Subsequently, the target modulation voltage and the bridge arm generated voltage obtained in the above manner are input to a comparison module or calculation unit. This module or unit is configured to perform a subtraction operation to calculate the difference between the two and output the difference as the voltage deviation. For example, if the target modulation voltage is 1000V and the bridge arm generated voltage is 980V, the voltage deviation is determined to be 20V.

[0048] Subsequently, based on the voltage deviation and the preset submodule rated voltage, the number of submodules to be additionally switched is determined. One implementation method is to compare the calculated voltage deviation with the preset rated voltage of a single submodule in the MMC system. For example, a preliminary number of submodules to be additionally switched, Δ, can be obtained by dividing the absolute value of the voltage deviation by the submodule rated voltage and performing simple rounding or truncation on the result. n For example, if the voltage deviation is 20V and the submodule's rated voltage is 50V, the initially calculated supplementary switching quantity might be 0 or 1. Alternatively, the supplementary switching quantity can be determined based on the absolute value of the voltage deviation by consulting a pre-defined lookup table, which maps different voltage deviation ranges to corresponding supplementary switching quantities.

[0049] Finally, the number of cuts Δ is supplemented according to this submodule. n Based on the arm current direction in the target MMC system, a submodule switching control strategy is determined, and the corresponding submodule switching action is executed according to this strategy. As one implementation, after obtaining the number of submodules to be switched and the arm current direction, the control system can generate a switching control strategy based on preset simple logic rules. Subsequently, according to the generated strategy, control commands are sent to the corresponding submodules to execute the supplementary switching or disconnection operation.

[0050] The precise switching control method for MMC submodules based on real-time voltage adjustment proposed in this application effectively reduces the inherent error between the actual generated voltage of the bridge arm and the target modulation voltage by accurately calculating the voltage deviation and determining the number of supplementary switches in combination with the direction of the bridge arm current. Therefore, when applying a novel switching strategy aimed at reducing the switching frequency, this method significantly reduces DC-side voltage fluctuations while ensuring the frequency reduction effect of submodule switching, thus improving the voltage control quality of the MMC system.

[0051] Based on the above basic embodiments, this application further proposes a calculation formula for the number of additional cuts in the submodule, the specific calculation formula being as follows:

[0052] In the formula, The voltage deviation is the amount mentioned above. To replenish the number of switches for the submodule, The rated voltage of the submodule is , and round is the rounding function.

[0053] Specifically, the voltage deviation Refers to the target modulation voltage Voltage generated with bridge arm The difference between the current generated voltage of the bridge arm and the desired voltage reflects the degree of deviation between them. This deviation can be positive or negative, indicating whether the generated voltage of the bridge arm needs to be increased or decreased, respectively. The submodule's rated voltage... This refers to the nominal voltage value of a single submodule under normal operating conditions; it is the basic unit for calculating the number of submodules to be switched on. The number of submodules to be switched on is... This value is calculated based on the voltage deviation and the rated voltage of the submodule, indicating the number of submodules that need to be switched on or off to eliminate the voltage deviation. Since submodule switching is a discrete action, meaning it can only be done in integer numbers, a rounding function is introduced. The rounding function converts the calculated number of submodules, which may be a decimal, into the closest integer to match the actual physical switching operation. This rounding function can be implemented in various ways, such as rounding to the nearest integer, rounding up, or rounding down. The specific choice can be adjusted according to the control strategy's emphasis on voltage accuracy and switching frequency. For example, rounding to the nearest integer can make the calculation result closer to the actual requirements and reduce accumulated errors; while rounding up or down may provide better control performance under specific operating conditions (such as when fast response is required or overshoot should be avoided).

[0054] The above technical solution directly and quantitatively determines the number of submodules that need to be switched by dividing the continuous voltage deviation by the submodule's rated voltage and rounding it down. This calculation method ensures the accuracy and operability of the submodule switching quantity, avoiding control errors caused by fuzzy judgments or inaccurate calculations. Since the number of submodules to be switched is accurately calculated and rounded based on the actual voltage deviation and the submodule's rated voltage, it can effectively reduce the inherent error between the actual generated voltage of the bridge arm and the target modulation voltage, thereby significantly reducing DC-side voltage fluctuations and improving the operational stability of the MMC system. Simultaneously, this method provides a clear and quantifiable basis for submodule switching, simplifies the control logic, and improves the response speed and accuracy of the control system.

[0055] Based on the above basic embodiments, this application further proposes specific steps for determining the submodule switching control strategy according to the number of submodules to be switched on and off, combined with the direction of the bridge arm current in the target MMC system. These steps specifically include: Based on the numerical polarity of the number of additional cuts by the submodule, the type of cut action in the current cycle is determined, wherein the type of cut action includes: additional input or cut-off. Based on the switching action type and the bridge arm current direction in the target MMC system, the sub-module filtering logic is determined according to the preset filtering logic correspondence. The filtering logic correspondence is the correspondence between the switching action type and the bridge arm current direction and the sub-module filtering logic. Based on the type of switching action, the number of additional switching actions in the submodule, and the filtering logic of the submodule, a submodule switching control strategy is generated.

[0056] Specifically, when determining the submodule switching control strategy, the switching action type for the current cycle needs to be clarified based on the polarity of the number of submodules added for switching. When the number of submodules added for switching is positive, it indicates that the actual voltage of the bridge arm is lower than the target modulation voltage, and the number of submodules added needs to be increased to raise the voltage. In this case, the switching action type is determined as "additional addition". Conversely, when the number of submodules added for switching is negative, it indicates that the actual voltage of the bridge arm is higher than the target modulation voltage, and the number of submodules added needs to be reduced to lower the voltage. In this case, the switching action type is determined as "cutoff". This judgment based on the polarity of the quantity provides a basic directional guide for the subsequent formulation of refined switching strategies, ensuring the consistency between the switching action and the voltage deviation correction direction.

[0057] Building upon this foundation, to further optimize submodule switching selection, this application introduces the direction of the bridge arm current as a key factor influencing submodule selection. The direction of the bridge arm current (charging or discharging) directly affects the changing trend of the submodule capacitor voltage. Through a pre-defined filtering logic correspondence, the most favorable submodule selection logic for capacitor voltage balancing or system stability can be dynamically determined based on the determined switching action type (engagement or disconnection) and the current bridge arm current direction. This filtering logic correspondence can be pre-stored in a lookup table or implemented through logical judgment, ensuring intelligent and precise switching decisions.

[0058] Finally, after determining the type of switching action, the number of sub-modules to be switched (i.e., the number of sub-modules to be switched), and the specific rules for filtering sub-modules (i.e., the sub-module filtering logic), the system can select a specific number of sub-modules from the sub-modules currently in the corresponding state according to the filtering logic, and generate corresponding switching instructions, thereby forming a complete sub-module switching control strategy.

[0059] Based on the above embodiments, this application further proposes that when determining the submodule screening logic, the submodule screening logic is determined according to the switching action type and the bridge arm current direction in the target MMC system, according to a preset screening logic correspondence, specifically including: When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the charging direction, the submodule selection logic is determined as follows: select several submodules in the disconnected state according to the capacitor voltage value in ascending order; when the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the discharging direction, the submodule selection logic is determined as follows: select several submodules in the disconnected state according to the capacitor voltage value in descending order; when the switching action type is disconnection and the bridge arm current direction in the target MMC system is the charging direction, the submodule selection logic is determined as follows: select several submodules in the input state according to the capacitor voltage value in descending order; when the switching action type is disconnection and the bridge arm current direction in the target MMC system is the discharging direction, the submodule selection logic is determined as follows: select several submodules in the input state according to the capacitor voltage value in ascending order; the number of selected submodules is equal to the number of submodules supplemented in the switching action.

[0060] For example, such as Figure 2 As shown, the actual voltage generated by the bridge arm submodule in the current cycle is calculated according to the initial switching strategy. Based on the modulation voltage of the current cycle, calculate the number Δ of submodules that need to be added or removed. n ; By deploying or removing corresponding sub-modules, the constraint on the number of upper and lower bridge arm sub-modules is broken, resulting in the deployment of a total of [number missing]. n + Δ n Each submodule; A trigger signal is applied to the submodule according to the switching rules.

[0061] More specifically, trigger signals are applied to submodules according to the switching rules to enable or disable the corresponding submodules. The submodule enabling rules for supplementary switching are as follows: 1) If Δ n > 0, I arm If the target modulation voltage is greater than 0, meaning the actual voltage of the bridge arm is greater than the target modulation voltage, then the submodule with the lowest capacitor voltage Δ should be selected from the submodules currently in the "cut-off" state. n Each sub-module requires additional investment; 2) If Δ n > 0, I arm If the target modulation voltage is less than 0, meaning the target modulation voltage is greater than the actual voltage of the bridge arm, then the submodule with the highest capacitor voltage Δ should be selected from the submodules currently in the "cut-off" state. n Each sub-module requires additional investment; 3) If Δ n < 0, I arm If the target modulation voltage is greater than 0, meaning the actual voltage of the bridge arm is less than the target modulation voltage, then the submodule with the highest capacitor voltage Δ should be selected from the submodules currently in the "engaged" state. n Each submodule was removed; 4) If Δ n < 0, I arm If the target modulation voltage is less than 0, meaning the actual voltage of the bridge arm is less than 0, then the submodule with the lowest capacitor voltage Δ should be selected from the submodules currently in the "engaged" state. n Sub-modules were removed.

[0062] Specifically, when the switching action type is supplementary input, and the bridge arm current direction in the target MMC system is the charging direction, that is... I arm When the voltage is >0, the bridge arm voltage needs to be increased, and the bridge arm current is charging the submodule capacitors. To better balance the submodule capacitor voltages, priority should be given to connecting those previously disconnected submodules with lower capacitor voltages. This way, these low-voltage submodules can be charged to normal levels more quickly after being connected, preventing further overcharging of high-voltage submodules and thus promoting overall voltage balance.

[0063] When the switching action type is supplementary input, and the arm current direction in the target MMC system is the discharge direction, i.e. Iarm When the voltage is less than 0, the bridge arm voltage needs to be increased, and the bridge arm current is discharging the submodule capacitors. To better balance the submodule capacitor voltages, priority should be given to connecting those disconnected submodules with higher capacitor voltages. These high-voltage submodules can then perform the discharge task, causing their voltages to drop, thereby preventing further over-discharge of low-voltage submodules and helping to maintain the stability of the submodule voltages.

[0064] When the switching action type is "cut-off" and the bridge arm current direction in the target MMC system is the charging direction, the bridge arm voltage needs to be reduced, and the bridge arm current is charging the submodule capacitors. To better balance the submodule capacitor voltages, those already connected submodules with higher capacitor voltages should be prioritized for cutting off. By cutting off high-voltage submodules, it is possible to prevent them from continuing to be charged and causing overvoltage, while allowing other low-voltage submodules to continue charging, thereby achieving voltage rebalancing.

[0065] When the switching action type is "cut-off" and the bridge arm current direction in the target MMC system is in the discharge direction, it is necessary to reduce the bridge arm voltage, as the bridge arm current is discharging the submodule capacitors. To better balance the submodule capacitor voltages, priority should be given to cutting off those connected submodules with lower capacitor voltages. Cutting off low-voltage submodules prevents them from continuing to discharge and causing over-discharge, thus protecting the submodules and allowing other high-voltage submodules to continue discharging, which helps maintain voltage balance.

[0066] It should be noted that the number of sub-modules selected above is equal to the number of sub-modules to be switched on, ensuring that the actual number of sub-modules switched on is consistent with the required number calculated based on the voltage deviation, thereby accurately compensating for the voltage deviation.

[0067] Through the above technical solution, this application introduces a refined screening logic based on the submodule capacitor voltage state when determining the submodule switching control strategy. This screening mechanism can intelligently select the most suitable submodule for activation or deactivation based on the bridge arm current direction and switching action type, thereby effectively avoiding excessive imbalance of submodule capacitor voltage. For example, when a submodule needs to be activated, selecting a high-voltage or low-voltage submodule for activation based on the current direction can accelerate the voltage equalization process; when a submodule needs to be deactivated, selecting a high-voltage or low-voltage submodule for deactivation can prevent submodule overcharging or over-discharging. Furthermore, this application is applicable to various submodule switching strategy algorithms, and by controlling the switching of a small number of submodules, it breaks the limitation of the total number of bridge arm submodules activated. n The constraints of each component reduce the inherent deviation between the actual generated voltage of the bridge arm and the target voltage, thus compensating for the problem of increased DC voltage fluctuations that may be caused by conventional submodule optimization strategies during frequency reduction and maintaining DC voltage stability.

[0068] To further demonstrate the technical effectiveness of this solution, a more specific example is provided below for a more in-depth explanation: by Figure 3 Taking the conventional MMC system structure diagram shown as an example, the MMC system in the diagram consists of a three-phase 6-arm bridge, with each arm consisting of... N The bridge consists of a cascaded half-bridge submodule and a bridge arm inductor. The half-bridge submodule comprises two IGBTs, two anti-parallel diodes, and one capacitor. The upper and lower bridge arm currents are... i pj , i nj (j = a,b,c), the DC side voltage is U dc The reference voltage is shown in the diagram.

[0069] based on Figure 3 The system shown typically considers the submodule voltage as its rated value in the submodule switching strategy. However, in reality, the submodule voltage fluctuates due to the charging and discharging of the bridge arm current. Each cycle, the submodule voltage deviates from its rated value after charging and discharging, causing the generated bridge arm voltage to be unequal to the target modulation voltage, resulting in an inherent error. This error is not noticeable in conventional switching strategies due to the high sorting frequency. However, when using submodule optimization strategies such as the hold factor method and limit method, the switching frequency decreases, leading to increased DC-side voltage ripple, further affecting the MMC voltage control quality. A comparison chart of the target voltage and the actual generated bridge arm voltage can be found in the [reference needed]. Figure 4 and Figure 5 .

[0070] A comparison chart showing the changes before and after adopting the submodule switching strategy correction method, as shown below. Figure 6 As shown, after implementing the proposed strategy at point 1.5, the actual voltage of the bridge arm further approaches the target modulation voltage, and can ensure long-term stable operation without instability.

[0071] In addition, please see Figure 7 By adopting the switching strategy proposed in this application, the DC side voltage ripple is reduced and the voltage control quality is improved by temporarily adjusting the total number of sub-modules connected to the bridge arm by controlling a small number of sub-modules.

[0072] The above is a detailed description of an embodiment of a precise switching control method for MMC submodules based on real-time voltage adjustment provided in this application. The following is a detailed description of an embodiment of a precise switching control device for MMC submodules based on real-time voltage adjustment provided in this application.

[0073] Please see Figure 8 This application provides a precise switching control device for MMC submodules based on real-time voltage adjustment, comprising: The bridge arm generation voltage acquisition unit 201 is used to acquire the bridge arm generation voltage of the current cycle in the target MMC system; The voltage deviation calculation unit 202 is used to determine the voltage deviation amount based on the difference between the preset target modulation voltage and the generated voltage of the bridge arm. The supplementary switching quantity determination unit 203 is used to determine the supplementary switching quantity of the submodule based on the voltage deviation and the preset submodule rated voltage; The switching control strategy generation unit 204 is used to determine the switching control strategy of the sub-module based on the number of sub-modules to be switched and combined with the direction of the bridge arm current in the target MMC system, so as to execute the corresponding sub-module switching action according to the switching control strategy of the sub-module.

[0074] Furthermore, the calculation method for the number of additional cuts in the submodule is as follows:

[0075] In the formula, The voltage deviation is the amount mentioned above. To replenish the number of switches for the submodule, The rated voltage of the submodule is , and round is the rounding function.

[0076] Furthermore, the switching control strategy generation unit 204 is specifically used for: Based on the numerical polarity of the number of additional cuts by the submodule, the type of cut action in the current cycle is determined, wherein the type of cut action includes: additional input or cut-off. Based on the switching action type and the bridge arm current direction in the target MMC system, the sub-module filtering logic is determined according to the preset filtering logic correspondence. The filtering logic correspondence is the correspondence between the switching action type and the bridge arm current direction and the sub-module filtering logic. Based on the type of switching action, the number of additional switching actions in the submodule, and the filtering logic of the submodule, a submodule switching control strategy is generated.

[0077] Furthermore, determining the type of switching action in the current cycle based on the numerical polarity of the number of switches supplemented by the submodule includes: When the polarity of the number of feeds added by the submodule is positive, the feed action type for the current cycle is determined to be supplementary feed. When the polarity of the number of additional cuts added by the submodule is negative, the type of cut action in the current cycle is determined to be cut.

[0078] Furthermore, the step of determining the submodule filtering logic based on the switching action type and the arm current direction in the target MMC system, according to a preset filtering logic correspondence, includes: When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the charging direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value in ascending order, select several sub-modules that are in the disconnected state. When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the discharge direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value from large to small, select several sub-modules that are in the disconnected state. When the switching action type is cut-off and the bridge arm current direction in the target MMC system is the charging direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value from large to small, select several sub-modules that are in the already engaged state. When the switching action type is cut-off and the bridge arm current direction in the target MMC system is the discharge direction, the sub-module screening logic is determined as follows: several sub-modules in the already engaged state are selected in order of ascending capacitor voltage values; wherein, the number of selected sub-modules is equal to the number of sub-modules to be added and switched.

[0079] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the terminals, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0080] In the several embodiments provided in this application, it should be understood that the disclosed terminals, devices, and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be an indirect coupling or communication connection between devices or units through some interfaces, and may be electrical, mechanical, or other forms.

[0081] The terms “first,” “second,” “third,” “fourth,” etc. (if present) in the specification and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a particular order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the application described herein can be implemented, for example, in orders other than those illustrated or described herein. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0082] It should be understood that in this application, "at least one (item)" means one or more, and "more than" means two or more. "And / or" is used to describe the relationship between related objects, indicating that three relationships can exist. For example, "A and / or B" can represent three cases: only A exists, only B exists, and both A and B exist simultaneously, where A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects are in an "or" relationship. "At least one (item) of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, at least one (item) of a, b, or c can represent: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, and c can be single or multiple.

[0083] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0084] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0085] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0086] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A precise switching control method for MMC submodules based on real-time voltage adjustment, characterized in that, include: Obtain the bridge arm generation voltage of the target MMC system in the current cycle; The voltage deviation is determined based on the difference between the preset target modulation voltage and the generated voltage of the bridge arm; The number of submodules to be switched on or off is determined based on the voltage deviation and the preset submodule rated voltage. Based on the number of submodules to be switched on and off, and in conjunction with the direction of the bridge arm current in the target MMC system, a submodule switching control strategy is determined, and the corresponding submodule switching action is executed according to the submodule switching control strategy.

2. The precise switching control method for MMC submodules based on real-time voltage adjustment according to claim 1, characterized in that, The calculation method for the number of additional cuts in the submodule is as follows: In the formula, The voltage deviation is the amount mentioned above. To replenish the number of switches for the submodule, The rated voltage of the submodule is , and round is the rounding function.

3. The precise switching control method for MMC submodules based on real-time voltage adjustment according to claim 1, characterized in that, The step of determining the submodule switching control strategy based on the number of submodules to be switched on and off, combined with the direction of the bridge arm current in the target MMC system, includes: Based on the numerical polarity of the number of additional cuts by the submodule, the type of cut action in the current cycle is determined, wherein the type of cut action includes: additional input or cut-off. Based on the switching action type and the bridge arm current direction in the target MMC system, the sub-module filtering logic is determined according to the preset filtering logic correspondence. The filtering logic correspondence is the correspondence between the switching action type and the bridge arm current direction and the sub-module filtering logic. Based on the type of switching action, the number of additional switching actions in the submodule, and the filtering logic of the submodule, a submodule switching control strategy is generated.

4. The precise switching control method for MMC submodules based on real-time voltage adjustment according to claim 3, characterized in that, The step of determining the type of throwing action in the current cycle based on the numerical polarity of the number of throws supplemented by the submodule includes: When the polarity of the number of feeds added by the submodule is positive, the feed action type for the current cycle is determined to be supplementary feed. When the polarity of the number of additional cuts added by the submodule is negative, the type of cut action in the current cycle is determined to be cut.

5. The precise switching control method for MMC submodules based on real-time voltage adjustment according to claim 3, characterized in that, The step of determining the sub-module filtering logic based on the switching action type and the arm current direction in the target MMC system, according to a preset filtering logic correspondence, includes: When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the charging direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value in ascending order, select several sub-modules that are in the disconnected state. When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the discharge direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value from large to small, select several sub-modules that are in the disconnected state. When the switching action type is cut-off and the bridge arm current direction in the target MMC system is the charging direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value from large to small, select several sub-modules that are in the already engaged state. When the switching action type is cut-off and the bridge arm current direction in the target MMC system is the discharge direction, the sub-module screening logic is determined as follows: several sub-modules in the already engaged state are selected in order of ascending capacitor voltage values; wherein, the number of selected sub-modules is equal to the number of sub-modules to be added and switched.

6. A precise switching control device for MMC submodules based on real-time voltage adjustment, characterized in that, include: The bridge arm generation voltage acquisition unit is used to acquire the bridge arm generation voltage of the target MMC system in the current cycle. The voltage deviation calculation unit is used to determine the voltage deviation amount based on the difference between the preset target modulation voltage and the generated voltage of the bridge arm; The supplementary switching quantity determination unit is used to determine the supplementary switching quantity of the submodule based on the voltage deviation and the preset submodule rated voltage; The switching control strategy generation unit is used to determine the submodule switching control strategy based on the number of submodules to be switched and the direction of the bridge arm current in the target MMC system, so as to execute the corresponding submodule switching action according to the submodule switching control strategy.

7. The precise switching control device for MMC submodules based on real-time voltage adjustment according to claim 6, characterized in that, The calculation method for the number of additional cuts in the submodule is as follows: In the formula, The voltage deviation is the amount mentioned above. To replenish the number of switches for the submodule, The rated voltage of the submodule is , and round is the rounding function.

8. The precise switching control device for MMC submodules based on real-time voltage adjustment according to claim 6, characterized in that, The switching control strategy generation unit is specifically used for: Based on the numerical polarity of the number of additional cuts by the submodule, the type of cut action in the current cycle is determined, wherein the type of cut action includes: additional input or cut-off. Based on the switching action type and the bridge arm current direction in the target MMC system, the sub-module filtering logic is determined according to the preset filtering logic correspondence. The filtering logic correspondence is the correspondence between the switching action type and the bridge arm current direction and the sub-module filtering logic. Based on the type of switching action, the number of additional switching actions in the submodule, and the filtering logic of the submodule, a submodule switching control strategy is generated.

9. The precise switching control device for MMC submodules based on real-time voltage adjustment according to claim 8, characterized in that, The step of determining the type of throwing action in the current cycle based on the numerical polarity of the number of throws supplemented by the submodule includes: When the polarity of the number of feeds added by the submodule is positive, the feed action type for the current cycle is determined to be supplementary feed. When the polarity of the number of additional cuts added by the submodule is negative, the type of cut action in the current cycle is determined to be cut.

10. The precise switching control device for MMC submodules based on real-time voltage adjustment according to claim 8, characterized in that, The step of determining the sub-module filtering logic based on the switching action type and the arm current direction in the target MMC system, according to a preset filtering logic correspondence, includes: When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the charging direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value in ascending order, select several sub-modules that are in the disconnected state. When the switching action type is supplementary input and the bridge arm current direction in the target MMC system is the discharge direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value from large to small, select several sub-modules that are in the disconnected state. When the switching action type is cut-off and the bridge arm current direction in the target MMC system is the charging direction, the sub-module screening logic is determined as follows: according to the capacitor voltage value from large to small, select several sub-modules that are in the already engaged state. When the switching action type is cut-off and the bridge arm current direction in the target MMC system is the discharge direction, the sub-module screening logic is determined as follows: several sub-modules in the already engaged state are selected in order of ascending capacitor voltage values; wherein, the number of selected sub-modules is equal to the number of sub-modules to be added and switched.

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