Method for starting control of modular multilevel converter and flexible direct current power transmission system

Abstract

This application relates to a startup control method, apparatus, flexible DC transmission system, dielectric, and computer program product for a modular multilevel converter. The method includes: upon receiving a startup charging command, controlling each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging; upon receiving self-test completion information corresponding to each submodule in the modular multilevel converter, controlling each full-bridge submodule to enter bypass mode; when the differential voltage state between each full-bridge submodule and each half-bridge submodule reaches a preset condition, controlling each full-bridge submodule to charge in half-bridge mode; and controlling the on / off state of each full-bridge submodule and each half-bridge submodule according to the real-time sorting of capacitor voltages, until each capacitor voltage reaches a preset target voltage. This method enables balanced charging of each submodule during the startup charging process of the modular multilevel converter.

Description

Technical Field

[0001] This application relates to the field of power supply technology, and in particular to a method and apparatus for starting control of a modular multilevel converter, a flexible DC transmission system, a computer-readable storage medium, and a computer program product. Background Technology

[0002] As the core converter equipment in flexible DC transmission systems, the Modular Multilevel Converter (MMC) typically consists of a bridge arm composed of a mixture of full-bridge submodules with DC fault ride-through capability and more economical half-bridge submodules in a certain proportion.

[0003] During the uncontrolled charging phase on the AC side of a flexible DC transmission system, the response characteristics of full-bridge and half-bridge submodules to the AC-side feed current differ inherently due to their topological differences. Full-bridge submodules can charge capacitors regardless of whether the arm current is positive or negative, while half-bridge submodules can only charge when the arm current direction enables their diodes to conduct. This results in a significant voltage imbalance between the capacitors of the full-bridge and half-bridge submodules within the arm during the initial charging phase. In subsequent charging, there may be instances where the half-bridge submodule is not fully charged, but the full-bridge submodule voltage exceeds its rated value. This leads to a situation where, after the controlled charging cycle of module switching is completed, the full-bridge submodule voltage is higher than its rated value, while the half-bridge submodule voltage is lower. The subsequent voltage equalization process is then only controlled by the discharge time of the full-bridge voltage equalization resistor, thus prolonging the controlled charging time of the modules. Summary of the Invention

[0004] Based on this, it is necessary to provide a startup control method, device, flexible DC transmission system, computer-readable storage medium, and computer program product for a modular multilevel converter that enables balanced charging of each sub-module during startup charging, thereby addressing the aforementioned technical problems.

[0005] In a first aspect, this application provides a startup control method for a modular multilevel converter, the method comprising:

[0006] Upon receiving a charge start command, the system controls each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging.

[0007] Upon receiving the self-test completion information of each sub-module in the modular multilevel converter, control each full-bridge sub-module to be in bypass mode.

[0008] When the differential pressure between each full-bridge submodule and each half-bridge submodule reaches the preset condition, control each full-bridge submodule to charge in half-bridge mode.

[0009] Based on the real-time sequencing of the capacitor voltages of each full-bridge submodule and each half-bridge submodule, the on / off state of each full-bridge submodule and each half-bridge submodule is controlled until the capacitor voltage reaches the preset target voltage.

[0010] In one embodiment, after controlling each full-bridge submodule to be in bypass mode, the method further includes:

[0011] Obtain the average voltage of the half-bridge and the average voltage of the full-bridge. The average voltage of the half-bridge is the average of the current capacitor voltages of each half-bridge submodule, and the average voltage of the full-bridge is the average of the current capacitor voltages of each full-bridge submodule.

[0012] If the average voltage difference between the half-bridge average voltage and the full-bridge average voltage is less than a preset difference threshold, the differential voltage state between each full-bridge submodule and each half-bridge submodule is determined to meet the preset condition.

[0013] In one embodiment, after controlling each full-bridge submodule to be in bypass mode, the method further includes:

[0014] If the duration of each full-bridge submodule in bypass mode reaches the first preset duration, the differential pressure state between each full-bridge submodule and each half-bridge submodule is determined to meet the preset condition.

[0015] In one embodiment, the on / off state of each full-bridge submodule and each half-bridge submodule is controlled according to the real-time sorting of the capacitor voltages of each full-bridge submodule and each half-bridge submodule, including:

[0016] Within each control cycle, a preset number of target submodules are determined based on the real-time sorting of the capacitor voltages of each full-bridge submodule and each half-bridge submodule.

[0017] The target control submodule is in bypass mode.

[0018] In one embodiment, after controlling each full-bridge submodule to be in bypass mode, the method further includes:

[0019] Obtain the capacitor voltage of each full-bridge submodule;

[0020] If the capacitor voltage of a full-bridge submodule increases, the full-bridge submodule with the increased capacitor voltage is identified as a faulty module, and the faulty module is controlled to be in permanent bypass mode.

[0021] In one embodiment, after controlling each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging, the method further includes:

[0022] After a second preset delay, self-test completion information for each sub-module in the modular multilevel converter is generated.

[0023] Secondly, this application also provides a start-up control device for a modular multilevel converter, comprising:

[0024] The startup module is used to control each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging when a startup charging command is received.

[0025] The bypass switching module is used to control each full-bridge submodule to be in bypass mode when it receives the self-test completion information of each submodule in the modular multilevel converter.

[0026] The half-bridge switching module is used to control each full-bridge submodule to charge in half-bridge mode when the differential pressure between each full-bridge submodule and each half-bridge submodule reaches a preset condition.

[0027] The controllable charging module is used to control the on / off state of each full-bridge submodule and each half-bridge submodule according to the real-time sorting of the capacitor voltages of each full-bridge submodule and each half-bridge submodule, until the capacitor voltage reaches the preset target voltage.

[0028] Thirdly, this application also provides a flexible DC transmission system, including a modular multilevel converter and a main controller. The main controller stores a computer program, and when the main controller executes the computer program, it performs the following steps:

[0029] Upon receiving a charge start command, the system controls each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging.

[0030] Upon receiving the self-test completion information of each sub-module in the modular multilevel converter, control each full-bridge sub-module to be in bypass mode.

[0031] When the differential pressure between each full-bridge submodule and each half-bridge submodule reaches the preset condition, control each full-bridge submodule to charge in half-bridge mode.

[0032] Based on the real-time sequencing of the capacitor voltages of each full-bridge submodule and each half-bridge submodule, the on / off state of each full-bridge submodule and each half-bridge submodule is controlled until the capacitor voltage reaches the preset target voltage.

[0033] In one embodiment, the flexible DC transmission system further includes a charging resistor and an AC contactor. One end of the charging resistor is connected to an AC power source, and the other end is connected to a modular multilevel converter. The charging resistor is connected in parallel across the AC contactor, which is connected to the main controller.

[0034] The main controller is also used to control the AC contactor to disconnect when a start charging command is received, so that the AC power supply can supply power to the modular multilevel converter through the charging resistor.

[0035] The main controller is also used to control the AC contactor to turn on after each full-bridge submodule in the modular multilevel converter is charged in half-bridge mode.

[0036] Fourthly, this application also provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, performs the following steps:

[0037] Upon receiving a charge start command, the system controls each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging.

[0038] Upon receiving the self-test completion information of each sub-module in the modular multilevel converter, control each full-bridge sub-module to be in bypass mode.

[0039] When the differential pressure between each full-bridge submodule and each half-bridge submodule reaches the preset condition, control each full-bridge submodule to charge in half-bridge mode.

[0040] Based on the real-time sequencing of the capacitor voltages of each full-bridge submodule and each half-bridge submodule, the on / off state of each full-bridge submodule and each half-bridge submodule is controlled until the capacitor voltage reaches the preset target voltage.

[0041] Fifthly, this application also provides a computer program product, including a computer program that, when executed by a processor, performs the following steps:

[0042] Upon receiving a charge start command, the system controls each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging.

[0043] Upon receiving the self-test completion information of each sub-module in the modular multilevel converter, control each full-bridge sub-module to be in bypass mode.

[0044] When the differential pressure between each full-bridge submodule and each half-bridge submodule reaches the preset condition, control each full-bridge submodule to charge in half-bridge mode.

[0045] Based on the real-time sequencing of the capacitor voltages of each full-bridge submodule and each half-bridge submodule, the on / off state of each full-bridge submodule and each half-bridge submodule is controlled until the capacitor voltage reaches the preset target voltage.

[0046] The aforementioned startup control method, device, flexible DC transmission system, computer-readable storage medium, and computer program product for modular multilevel converters, after the modular multilevel converter completes its self-test, control the full-bridge submodules to enter bypass mode, decoupling the capacitors of the full-bridge submodules from the charging circuit, thereby pausing the voltage rise. At this time, charging energy is concentrated on the half-bridge submodules, causing their capacitor voltages to rise rapidly. When the voltage difference between the full-bridge and half-bridge submodules reaches a preset condition, the full-bridge submodules are switched to half-bridge mode, allowing all submodules to enter the subsequent charging stage with the same topology. Finally, based on the real-time ranking of the capacitor voltages of all submodules, the on / off state of each submodule is dynamically controlled, temporarily bypassing the submodules with higher voltages and allowing the submodules with lower voltages to continue charging, until the capacitor voltages of all submodules reach the preset target voltage, completing the startup of the modular multilevel converter. Throughout the process, by actively pausing the charging of the full-bridge submodules to reduce the initial voltage difference, unifying the submodule topology, and using dynamic equalization control based on real-time ranking, the charging balance of each submodule during the startup charging process of the modular multilevel converter can be achieved. Attached Figure Description

[0047] To more clearly illustrate the technical solutions in the embodiments of this application or related technologies, the drawings used in the description of the embodiments of this application or related technologies will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0048] Figure 1 This is an application environment diagram of the startup control method for a modular multilevel converter in one embodiment;

[0049] Figure 2 This is a schematic diagram of the structure of a flexible DC transmission system in one embodiment;

[0050] Figure 3 This is a flowchart illustrating the startup control method of a modular multilevel converter in one embodiment;

[0051] Figure 4 This is a flowchart illustrating the startup control method for a modular multilevel converter in another embodiment;

[0052] Figure 5 This is a detailed flowchart illustrating the steps of controlling the on / off states of each full-bridge submodule and each half-bridge submodule based on the real-time sorting of capacitor voltages of each full-bridge submodule and each half-bridge submodule in one embodiment.

[0053] Figure 6This is a flowchart illustrating the startup control method for a modular multilevel converter in yet another embodiment;

[0054] Figure 7 This is a detailed flowchart illustrating the startup control method for a modular multilevel converter in one embodiment;

[0055] Figure 8 This is a structural block diagram of the start-up control device for a modular multilevel converter in one embodiment;

[0056] Figure 9 This is an internal structure diagram of the main controller in one embodiment. Detailed Implementation

[0057] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0058] It should be noted that the terms "first," "second," etc., used in this application can be used to describe various elements, but these elements are not limited by these terms. These terms are only used to distinguish the first element from the second element. The terms "comprising" and "having," and any variations thereof, used in this application, are intended to cover non-exclusive inclusion. The term "multiple" used in this application refers to two or more. The term "and / or" used in this application refers to one of the embodiments, or any combination of multiple embodiments.

[0059] Based on the above problems, this application proposes a startup control method for a modular multilevel converter. The startup control method for a modular multilevel converter provided in this application can be applied to, for example... Figure 1 The flexible DC transmission system shown includes a modular multilevel converter 110 and a main controller 120. Upon receiving a start-charging command, the main controller 120 controls the modular multilevel converter 110 to begin charging.

[0060] In some embodiments, such as Figure 2 As shown, the modular multilevel converter 110 includes multiple full-bridge submodules 111 and multiple half-bridge submodules 112. The flexible DC transmission system also includes a charging resistor R. charge And AC contactor KM, charging resistor R charge One end is connected to an AC power source, and the other end is connected to a modular multilevel converter 110, with a charging resistor R. charge It is connected in parallel across the two ends of the AC contactor KM, and the AC contactor KM is connected to the main controller 120.

[0061] The main controller 120 is also used to control the AC contactor KM to disconnect upon receiving a start charging command, allowing AC power to flow through the charging resistor R. charge Charging the modular multilevel converter 110.

[0062] The main controller 120 is also used to control the AC contactor KM to turn on after each full-bridge submodule 111 in the modular multilevel converter 110 is charged in half-bridge mode.

[0063] Among them, the charging resistor R charge A contactor is a passive component used to limit current in a circuit. It generates a voltage drop when current flows through it, consuming some electrical energy and thus limiting the current amplitude. An AC contactor (KM) is an electrical appliance used for frequently connecting and disconnecting AC main circuits and high-capacity control circuits over long distances. Its main function is to control the on / off state of the power circuit. The AC power source can be the transformer valve side or the power grid.

[0064] In this embodiment, the charging resistor R charge One end is connected to an AC power source, and the other end is connected to a modular multilevel converter 110, with a charging resistor R. charge The resistor R is connected in parallel across the AC contactor KM, which is connected to the main controller 120. When the AC contactor KM is open, and the AC power supply charges the modular multilevel converter 110 through this resistor, its current-limiting characteristics suppress the charging current within a safe range, preventing damage to the converter equipment from inrush current caused by direct closing and ensuring a smooth pre-charging process. The function of the AC contactor KM in the flexible DC transmission system is to execute open or closed operations according to the commands issued by the main controller 120, maintaining the open state during the initial charging stage to force current to flow through the charging resistor R. charge To achieve current-limited charging, the submodule switches to the on state after partial charging to short-circuit the charging resistor R. charge Thus, the charging resistor R charge This disconnects the converter from the main circuit, allowing it to quickly charge to the preset target voltage and maintain its power transmission efficiency for normal operation.

[0065] In some embodiments, the flexible DC transmission system further includes a circuit breaker QF, which is connected between the AC power supply and the charging resistor R. charge In the process of receiving a charging start command, the main controller 120 first controls the circuit breaker QF to close. The closing action of the circuit breaker QF completes the connection of the main circuit between the AC power supply and the converter valve, allowing the power supply voltage to pass through the charging resistor R in the subsequent stage. charge Applied to the modular multilevel converter 110.

[0066] In one exemplary embodiment, such as Figure 3As shown, a startup control method for a modular multilevel converter 110 is provided, which is applied to... Figure 1 Taking the main controller 120 as an example, the explanation includes the following steps 302 to 308. Wherein:

[0067] Step 302: Upon receiving a start charging command, control each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging.

[0068] The charging start command is a control signal issued by the host computer or station control system to initialize the converter startup process, marking the beginning of the charging process. The modular multilevel converter is the target device to be charged, and each arm is composed of a mixture of full-bridge and half-bridge submodules. A full-bridge submodule is an H-bridge topology consisting of four switching devices with anti-parallel diodes and capacitors, allowing bidirectional current flow and outputting positive, zero, and negative voltage levels. A half-bridge submodule consists of two switching devices with anti-parallel diodes and capacitors, capable of outputting both positive and zero voltage levels.

[0069] In this embodiment, after receiving the start charging command, the main controller applies the grid voltage to the valve side of the modular multilevel converter via a current-limiting charging resistor by closing the AC side circuit breaker. During this stage, the switching devices controlling all full-bridge and half-bridge submodules remain off, relying solely on their anti-parallel diodes to form a rectifier circuit. At this time, each submodule uses its internal anti-parallel diodes to form a rectifier circuit, converting AC to DC, and the AC power pre-charges the energy storage capacitor of the submodule. The purpose of this process is to raise the capacitor voltage of each submodule from zero to a basic voltage level, providing operating power to the submodule controller embedded in each submodule, enabling it to complete power-on initialization and self-test. Due to the inherent differences in topology, the full-bridge submodule can charge its capacitor through its four diodes during either a positive or negative half-cycle, while the half-bridge submodule can only charge when its diodes are conducting during a half-cycle. Therefore, the capacitor voltage rise rate of the full-bridge submodule is much faster than that of the half-bridge submodule. This process continues until the main controller of each submodule completes its power-on self-test and sends a self-test completion message back to the main controller.

[0070] Step 304: Upon receiving the self-test completion information corresponding to each submodule in the modular multilevel converter, control each full-bridge submodule to be in bypass mode.

[0071] Each submodule (each full-bridge submodule and each half-bridge submodule) has an embedded local control unit, called a submodule controller. The submodule controller is responsible for monitoring the capacitor voltage of its module and performing communication and protection functions. In some embodiments, the self-test completion information is a ready signal sent by each submodule controller to the main controller after it has completed its own hardware and communication function tests upon power-on.

[0072] Bypass mode refers to the operating state of a submodule. In this mode, by triggering a specific combination of switches (for example, simultaneously turning on the first switch T1 and the third switch T3, or the second switch T2 and the fourth switch T4 in a full-bridge submodule), the output terminals of the module are short-circuited, the charging current flows through the bypass path, and the module capacitor is completely decoupled from the main charging circuit, and its voltage is no longer affected by the external charging current.

[0073] In this embodiment, after continuously monitoring and confirming receipt of self-test completion information from all submodule controllers within the bridge arm, the main controller controls each full-bridge submodule to switch to bypass mode. At this time, the capacitors of all full-bridge submodules are disconnected from the charging circuit, and their voltages are maintained at the current level. The charging circuit is thus reconfigured; the AC voltage originally acting on the full-bridge and half-bridge modules is now equivalent to charging only the series circuit formed by all half-bridge submodules through the current-limiting resistor, achieving uncontrolled charging of all half-bridge submodules. Due to the current-limiting effect of the charging resistor, the charging current is controlled, but all energy is forced to be used to boost the capacitor voltage of the lower-voltage half-bridge submodules.

[0074] Step 306: When the differential pressure between each full-bridge submodule and each half-bridge submodule reaches the preset condition, control each full-bridge submodule to charge in half-bridge mode.

[0075] The preset conditions are criteria used to determine whether the voltage balancing effect has met expectations and whether the full-bridge bypass phase can be terminated. These can be preset time thresholds (such as continuous charging for 500ms), voltage difference thresholds (such as when the average voltage difference ΔV is less than 50V), or a combination of both. Half-bridge mode refers to transforming a submodule originally configured as a full-bridge submodule into an electrically equivalent half-bridge submodule capable of unidirectional conduction by controlling specific switches (such as only turning on the fourth switch and turning off the first, second, and third switches).

[0076] In this embodiment, when the main controller continuously monitors the voltage difference between the full-bridge and half-bridge submodules within the bridge arm and sets a preset equalization condition, the main controller determines that the voltage catch-up process of the half-bridge submodules is complete. Subsequently, the main controller de-bypasses all full-bridge submodules and immediately controls them to half-bridge mode. At this point, all full-bridge and half-bridge submodules, which originally had different topologies, are functionally unified into the same half-bridge mode, creating conditions for consistent starting voltage and charging characteristics across all modules in the subsequent charging phase.

[0077] Step 308: Based on the real-time sorting of capacitor voltages of each full-bridge submodule and each half-bridge submodule, control the on / off state of each full-bridge submodule and each half-bridge submodule until each capacitor voltage reaches the preset target voltage.

[0078] The real-time sorting of capacitor voltages refers to the main controller acquiring the capacitor voltage values ​​of all submodules in the current bridge arm within each control cycle (e.g., 1ms) and dynamically sorting them according to voltage level. On / off status refers to whether each submodule is in an engaged state (module output voltage, current charging or discharging the capacitor) or a bypass state (module output voltage is zero, current does not flow through the capacitor). The preset target voltage refers to the rated operating voltage value that all submodule capacitors need to reach before the converter completes startup and is ready to unlock and enter normal operation.

[0079] In this embodiment, after entering the controllable charging stage, the main controller first bypasses the current-limiting charging resistor, enabling the system to enter a fast charging state, and then initiates a sorting-based voltage equalization control algorithm. Specifically, in each control cycle, the main controller sorts the capacitor voltages of all sub-modules within the bridge arm in real time. To achieve a balanced voltage increase across all modules, the main controller adopts a "highest voltage priority bypass" strategy: while maintaining a constant total number of sub-modules in the bridge arm, it dynamically selects several modules with the highest current voltage for bypass, while allowing modules with lower voltages to continue charging. Through this high-speed, dynamic switching control performed in each control cycle, the capacitor voltages of all sub-modules remain highly consistent during fast charging and ultimately rise synchronously and smoothly to the preset target voltage value.

[0080] After the modular multilevel converter completes its self-test, the full-bridge submodule is controlled to enter bypass mode, decoupling its capacitor from the charging circuit and pausing voltage rise. Charging energy then flows to the half-bridge submodule, rapidly increasing its capacitor voltage. After the full-bridge submodule remains in bypass mode for a preset duration, it switches to half-bridge mode, allowing all submodules to enter the subsequent charging phase with the same topology. Finally, based on the real-time ranking of all submodule capacitor voltages, the switching on and off of each submodule is dynamically controlled, temporarily bypassing higher-voltage submodules while lower-voltage submodules continue charging until all submodule capacitor voltages reach the preset target voltage. At this point, the modular multilevel converter has started up. After charging, the converter switches from locked to unlocked state and connects to the DC system or performs power transmission. Since all submodule capacitor voltages have reached their rated values ​​and are almost identical, the flexible DC transmission system does not need to handle large voltage fluctuations during unlocking, avoiding overcurrent or oscillations caused by excessive voltage differences between modules. Throughout the process, by actively pausing the charging of the full-bridge submodules to reduce the initial voltage difference, unifying the submodule topology, and using dynamic equalization control based on real-time sorting, the modular multilevel converter can achieve charging equalization of each submodule during the start-up charging process.

[0081] In some embodiments, such as Figure 4 As shown, after step 304 above, the startup control method for the modular multilevel converter further includes steps 402 and 404. Wherein:

[0082] Step 402: Obtain the average voltage of the half-bridge and the average voltage of the full-bridge.

[0083] The half-bridge average voltage refers to the arithmetic mean calculated from the capacitor voltage values ​​of all half-bridge submodules in the modular multilevel converter at the current moment, used to characterize the overall voltage level of the half-bridge submodule group. The full-bridge average voltage refers to the arithmetic mean calculated from the capacitor voltage values ​​of all full-bridge submodules at the same moment, used to characterize the overall voltage level of the full-bridge submodule group. Capacitor voltage refers to the real-time voltage value across the energy storage capacitors inside the submodule, a parameter used to measure the charging state of the submodule and for voltage equalization control.

[0084] In this embodiment, after switching all full-bridge submodules to bypass mode and concentrating charging energy to supply the half-bridge submodules, the main controller periodically collects real-time capacitor voltage data for each half-bridge submodule and each full-bridge submodule. Based on this collected data, the main controller calculates the average capacitor voltage of all half-bridge submodules, i.e., the half-bridge average voltage; and simultaneously calculates the average capacitor voltage of all full-bridge submodules, i.e., the full-bridge average voltage.

[0085] Step 404: If the average voltage difference between the half-bridge average voltage and the full-bridge average voltage is less than a preset difference threshold, determine that the differential voltage state between each full-bridge submodule and each half-bridge submodule has reached the preset condition.

[0086] The average voltage difference refers to the difference between the calculated average voltage of the full-bridge submodule and the average voltage of the half-bridge submodule. This difference reflects the degree of voltage imbalance between the full-bridge submodules and the half-bridge submodules after directional charging catch-up. The preset difference threshold is a pre-set allowable voltage deviation value stored in the main controller, for example, set to 50V, as a standard for judging whether the voltage balance has reached the expected target.

[0087] In this embodiment, after each update of the half-bridge average voltage and the full-bridge average voltage, the main controller immediately calculates the difference between the two to obtain the current average voltage difference. The main controller compares the calculated real-time average voltage difference with a preset difference threshold stored internally. When the main controller determines that the real-time average voltage difference is less than the preset difference threshold, it means that after centralized charging, the voltage difference between the full-bridge submodule and the half-bridge submodule has been reduced to an acceptable range, and the voltage of the half-bridge submodule has successfully caught up with that of the full-bridge submodule. Based on this determination, the main controller confirms that the "differential voltage state between each full-bridge submodule and each half-bridge submodule" has met the conditions for proceeding to the next stage.

[0088] By calculating the voltage difference and comparing it with the threshold, it is ensured that the state switch is carried out only after the voltage deviation has been effectively eliminated, thus avoiding insufficient balancing due to premature switching or wasting time due to delayed switching.

[0089] In some embodiments, after step 304 above, the startup control method for the modular multilevel converter further includes step 406: when the duration of each full-bridge submodule in bypass mode reaches a first preset duration, determining that the differential pressure state between each full-bridge submodule and each half-bridge submodule reaches a preset condition.

[0090] The bypass mode refers to a specific operating state of the full-bridge submodule. In this state, by triggering a specific combination of switching transistors, the output of the submodule is short-circuited, and its internal energy storage capacitor is completely disconnected from the main charging circuit. The capacitor voltage is no longer affected by the external charging current. The duration refers to the time elapsed from the moment all full-bridge submodules switch to bypass mode to the current moment. The first preset duration is a pre-determined time value, determined through theoretical calculations or simulation experiments, sufficient for the half-bridge submodule capacitor voltage to catch up to approximately equal the full-bridge submodule voltage through concentrated charging.

[0091] In this embodiment, after forcibly switching all full-bridge submodules to bypass mode and using all charging energy to boost the voltage of the half-bridge submodules, the main controller starts an internal timer to time the duration of the full-bridge submodules being in bypass mode. The main controller continuously compares this timed duration with a pre-set first preset duration. During the timing process, the main controller does not intervene regardless of whether the half-bridge submodule voltage has caught up ahead of schedule. Only when the main controller determines that the duration elapsed since the full-bridge submodules switched to bypass mode has reached or exceeded the pre-set first preset duration will it consider that the differential voltage state between each full-bridge submodule and each half-bridge submodule has met the preset conditions for transitioning to the next stage. This timing control strategy complements the aforementioned closed-loop control strategy based on voltage difference and can be flexibly selected according to the focus of the engineering application.

[0092] In addition, in some embodiments, a method based on voltage difference detection and customized control can be used simultaneously. Specifically, within the first time period after each full-bridge submodule is in bypass mode, if the average voltage difference between the average voltage of the half-bridge and the average voltage of the full-bridge is less than a preset difference threshold, then it is determined that the differential voltage state between each full-bridge submodule and each half-bridge submodule has reached the preset condition. If, within the first time period, the average voltage difference between the average voltage of the half-bridge and the average voltage of the full-bridge is always greater than or equal to the preset difference threshold, but the duration of the full-bridge submodule being in bypass mode reaches the first preset time period, then it is also determined that the differential voltage state between each full-bridge submodule and each half-bridge submodule has reached the preset condition.

[0093] In some embodiments, such as Figure 5 As shown, step 308 above includes steps 502 and 504. Wherein:

[0094] Step 502: In each control cycle, a preset number of target submodules are determined based on the real-time sorting of the capacitor voltages of each full-bridge submodule and each half-bridge submodule.

[0095] The control cycle refers to the fixed time interval during which the main controller executes a complete control operation and outputs a command. Real-time capacitor voltage sorting refers to the process at the beginning of each control cycle where the main controller collects the capacitor voltage values ​​of all submodules in the current bridge arm and dynamically arranges them according to the voltage values ​​from high to low or from low to high. The preset quantity is a fixed value pre-set based on the system design, the total number of bridge arm submodules, and the control strategy. The target submodules refer to those submodules selected to perform specific operations within the current control cycle, based on the sorting results and the preset quantity.

[0096] In this embodiment, after entering the controllable charging phase, the main controller first performs a data acquisition and processing task at the beginning of each control cycle. The main controller acquires the current capacitor voltage value of each submodule within the bridge arm and sorts these data to form a real-time sequence from the highest voltage to the lowest voltage. Based on this sorting result, the main controller selects a corresponding number of submodules from the top of the sequence (i.e., the highest voltage position) according to a pre-set number (e.g., 10), marks these submodules, and identifies them as target submodules that require special processing in this control cycle.

[0097] Step 504: Control the target submodule to be in bypass mode.

[0098] In this embodiment, after identifying the target submodule for the current control cycle, the main controller controls each module marked as the target submodule to switch to bypass mode. This temporarily bypasses the submodules with the highest voltage during the current control cycle, stopping their capacitor charging. Simultaneously, the remaining unselected submodules with relatively low voltage within the bridge arm remain active, continuing to receive charging energy and raising their capacitor voltage. By repeating this closed-loop process in each control cycle, the voltages of all submodules within the bridge arm are dynamically and continuously brought closer together.

[0099] By precisely identifying and temporarily bypassing several sub-modules with the highest voltage in each control cycle, charging energy is directed towards modules with lower voltage. This ensures that all sub-modules maintain voltage consistency throughout the charging process, reducing the risk of overvoltage in individual modules and enabling safe startup of the MMC system.

[0100] In some embodiments, such as Figure 6 As shown, after step 304 above, the startup control method for the modular multilevel converter further includes steps 602 and 604. Wherein:

[0101] Step 602: Obtain the capacitor voltage of each full-bridge submodule.

[0102] In this embodiment, after the main controller controls all full-bridge submodules to switch to bypass mode, to ensure the circuit state meets expectations, the main controller collects the capacitor voltage value of each full-bridge submodule. This is to obtain the actual response state of the full-bridge submodules after receiving the bypass command, and to confirm whether they have successfully switched from normal charging mode to the expected capacitor voltage state.

[0103] Step 604: If the capacitor voltage of a full-bridge submodule increases, the full-bridge submodule with the increased capacitor voltage is identified as a faulty module, and the faulty module is controlled to be in permanent bypass mode.

[0104] In this embodiment, the main controller continuously monitors the capacitor voltage of each full-bridge submodule. When the main controller detects that the capacitor voltage of one or more full-bridge submodules is still rising, it determines that the bypass operation of these modules has failed, indicating a possible fault in the full-bridge submodule. These modules are then identified as faulty modules, and the controller puts them into a permanent bypass state. This reduces the risk of the entire voltage equalization strategy failing due to the bypass failure of individual submodules.

[0105] In some embodiments, after step 302 above, the startup control method for the modular multilevel converter further includes: after a second preset time delay, generating self-test completion information corresponding to each sub-module in the modular multilevel converter.

[0106] In this embodiment, at the same moment uncontrolled charging begins and AC voltage is applied to the converter, an independent timer inside the main controller is triggered and starts counting. The main controller can wait for the count of this built-in timer to reach a pre-stored second preset duration (e.g., 3 seconds) instead of relying on real-time monitoring of whether each submodule controller returns a self-test success flag. When the count reaches this preset value, the main controller logically defaults to and generates a comprehensive message representing "all submodule controllers have completed self-tests," confirming that all submodule controllers have completed their self-tests. By using a fixed time window with sufficient margin, instead of real-time, parallel monitoring of the self-test status of multiple submodule controllers, the control logic and communication interaction in the initial startup phase are simplified, avoiding startup process delays caused by communication delays or instantaneous message loss in individual submodules.

[0107] To better understand the above embodiments, an optional embodiment will be explained in detail below. Please refer to... Figure 7In one embodiment, the main controller receives a charging start command. At this time, the AC side circuit breaker closes, and the grid voltage is applied to the converter valve through the current-limiting charging resistor. All submodules are in a locked state, relying on anti-parallel diodes for uncontrolled rectified charging. During this stage, because the full-bridge submodules can charge the capacitors in both positive and negative half-cycles, while the half-bridge submodules only charge in a single half-cycle, the average capacitor voltage of the full-bridge submodules is much higher than that of the half-bridge submodules after the typical power-on self-test time (e.g., 3 seconds) of the submodule controller, resulting in a significant initial voltage difference. After a 3-second delay to confirm the completion of the submodule self-test, the main controller forcibly switches all full-bridge submodules to bypass mode, decoupling their capacitors from the charging circuit. At this time, the AC power is concentrated on charging the series circuit consisting only of half-bridge submodules, directionally raising the voltage of the half-bridge submodules until the difference between the average voltage of the full-bridge and the average voltage of the half-bridge is less than 100V. When the voltages of the full-bridge and half-bridge submodules are brought to approximately equal levels, the main controller releases the bypass state of the full-bridge submodules and controls them in an equivalent half-bridge mode. Subsequently, the main controller closes the AC contactor, bypassing the current-limiting charging resistor, and the system enters the fast charging phase. The main controller initiates a controllable charging logic based on real-time sorting, sorting the capacitor voltages of all submodules within the bridge arm in each control cycle, and dynamically bypassing the submodules with the highest voltages, thus tilting energy towards the modules with lower voltages. This causes the capacitor voltages of all submodules to rise synchronously, and finally, when all capacitor voltages are greater than the preset target voltage, the entire start-up charging process is completed.

[0108] It should be understood that although the steps in the flowcharts of the above embodiments are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the above embodiments may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the steps or stages in other steps. It is understood that the steps in different embodiments can be freely combined as needed, and all non-contradictory solutions formed by such combinations are within the scope of protection of this application.

[0109] Based on the same inventive concept, this application also provides a startup control device for a modular multilevel converter to implement the startup control method of the modular multilevel converter described above. The solution provided by this device is similar to the solution described in the above method. Therefore, the specific limitations of one or more startup control device embodiments of the modular multilevel converter provided below can be found in the limitations of the startup control method of the modular multilevel converter described above, and will not be repeated here.

[0110] In one exemplary embodiment, such as Figure 8 As shown, a start-up control device for a modular multilevel converter is provided, comprising:

[0111] The startup module 801 is used to control each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging when a startup charging command is received.

[0112] The bypass switching module 802 is used to control each full-bridge submodule to be in bypass mode when it receives the self-test completion information of each submodule in the modular multilevel converter.

[0113] The half-bridge switching module 803 is used to control each full-bridge submodule to charge in half-bridge mode when the differential pressure between each full-bridge submodule and each half-bridge submodule reaches a preset condition.

[0114] The controllable charging module 804 is used to control the on / off state of each full-bridge submodule and each half-bridge submodule according to the real-time sorting of the capacitor voltages of each full-bridge submodule and each half-bridge submodule, until the capacitor voltage reaches the preset target voltage.

[0115] In one embodiment, the half-bridge switching module 803 is further configured to obtain the half-bridge average voltage and the full-bridge average voltage, wherein the half-bridge average voltage is the average of the current capacitor voltages of each half-bridge submodule, and the full-bridge average voltage is the average of the current capacitor voltages of each full-bridge submodule; and if the average voltage difference between the half-bridge average voltage and the full-bridge average voltage is less than a preset difference threshold, it is determined that the differential voltage state between each full-bridge submodule and each half-bridge submodule has reached a preset condition.

[0116] In one embodiment, the half-bridge switching module 803 is further configured to determine that the differential pressure state between each full-bridge submodule and each half-bridge submodule reaches a preset condition when the duration of each full-bridge submodule in bypass mode reaches a first preset duration.

[0117] In one embodiment, the controllable charging module 804 is further configured to determine a preset number of target sub-modules based on the real-time sorting of the capacitor voltages of each full-bridge sub-module and each half-bridge sub-module in each control cycle; and control the target sub-modules to be in bypass mode.

[0118] In one embodiment, the bypass switching module 802 is further configured to acquire the capacitor voltage of each full-bridge submodule; if the capacitor voltage of a full-bridge submodule increases, the full-bridge submodule with the increased capacitor voltage is identified as a faulty module, and the faulty module is controlled to be in permanent bypass mode.

[0119] In one embodiment, the startup module 801 is further configured to generate self-test completion information corresponding to each sub-module in the modular multilevel converter after a second preset time delay.

[0120] Each module in the startup control device of the aforementioned modular multilevel converter can be implemented entirely or partially through software, hardware, or a combination thereof. These modules can be embedded in hardware within or independently of the processor in the flexible DC transmission system, or stored in software within the memory of the flexible DC transmission system, so that the processor can call and execute the corresponding operations of each module.

[0121] In one exemplary embodiment, a main controller is provided, the internal structure of which can be as follows: Figure 9 As shown, the main controller includes a processor, memory, input / output (I / O) interfaces, and a communication interface. The processor, memory, and I / O interfaces are connected via a system bus, and the communication interface is also connected to the system bus via the I / O interfaces. The processor provides computational and control capabilities. The memory includes non-volatile storage media and internal memory. The non-volatile storage media stores the operating system, computer programs, and a database. The internal memory provides the environment for the operating system and computer programs stored in the non-volatile storage media. The database stores the controller's control data. The I / O interfaces are used for information exchange between the processor and external devices. The communication interface is used for communication with external terminals via a network connection. When executed by the processor, the computer program implements a startup control method for a modular multilevel converter.

[0122] Those skilled in the art will understand that Figure 9 The structure shown is merely a block diagram of a portion of the structure related to the present application and does not constitute a limitation on the main controller to which the present application is applied. The specific main controller may include more or fewer components than shown in the figure, or combine certain components, or have different component arrangements.

[0123] In one exemplary embodiment, a flexible DC transmission system is provided, including a memory and a processor. The memory stores a computer program, and the processor executes the computer program to implement the steps in the above-described embodiment of the modular multilevel converter startup control method.

[0124] In one embodiment, a computer-readable storage medium is provided having a computer program stored thereon, which, when executed by a processor, implements the steps in the above-described embodiment of the startup control method for a modular multilevel converter.

[0125] In one embodiment, a computer program product is provided, including a computer program that, when executed by a processor, implements the steps in the above-described embodiment of the startup control method for a modular multilevel converter.

[0126] Those skilled in the art will understand that all or part of the processes in the methods of the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium, and when executed, it can include the processes of the embodiments of the above methods. Any references to memory, databases, or other media used in the embodiments provided in this application can include at least one of non-volatile memory and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM). The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, artificial intelligence (AI) processors, etc., and are not limited to these.

[0127] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this application.

[0128] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A startup control method for a modular multilevel converter, characterized in that, The method includes: Upon receiving a charge start command, the system controls each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging. Upon receiving the self-test completion information corresponding to each sub-module in the modular multilevel converter, control each full-bridge sub-module to be in bypass mode; When the differential pressure between each full-bridge submodule and each half-bridge submodule reaches a preset condition, each full-bridge submodule is controlled to charge in half-bridge mode. Based on the real-time sorting of the capacitor voltages of each full-bridge submodule and each half-bridge submodule, the on / off state of each full-bridge submodule and each half-bridge submodule is controlled until the capacitor voltage reaches the preset target voltage.

2. The method according to claim 1, characterized in that, After controlling each of the full-bridge submodules to be in bypass mode, the method further includes: Obtain the half-bridge average voltage and the full-bridge average voltage, wherein the half-bridge average voltage is the average of the current capacitor voltages of each half-bridge submodule, and the full-bridge average voltage is the average of the current capacitor voltages of each full-bridge submodule. If the average voltage difference between the half-bridge average voltage and the full-bridge average voltage is less than a preset difference threshold, it is determined that the differential voltage state between each full-bridge submodule and each half-bridge submodule has reached a preset condition.

3. The method according to claim 1, characterized in that, After controlling each of the full-bridge submodules to be in bypass mode, the method further includes: If the duration of each full-bridge submodule in bypass mode reaches a first preset duration, it is determined that the differential pressure state between each full-bridge submodule and each half-bridge submodule reaches a preset condition.

4. The method according to claim 1, characterized in that, The step of controlling the on / off state of each full-bridge submodule and each half-bridge submodule based on the real-time sorting of the capacitor voltages of each full-bridge submodule and each half-bridge submodule includes: Within each control cycle, a preset number of target submodules are determined based on the real-time sorting of the capacitor voltages of each full-bridge submodule and each half-bridge submodule. The target submodule is controlled to be in bypass mode.

5. The method according to claim 1, characterized in that, After controlling each of the full-bridge submodules to be in bypass mode, the method further includes: Obtain the capacitor voltage of each of the full-bridge submodules; If the capacitor voltage of the full-bridge submodule increases, the full-bridge submodule with the increased capacitor voltage is identified as a faulty module, and the faulty module is controlled to enter permanent bypass mode.

6. The method according to claim 1, characterized in that, After the control of each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging is completed, the method further includes: After a second preset time delay, self-test completion information corresponding to each sub-module in the modular multilevel converter is generated.

7. A start-up control device for a modular multilevel converter, characterized in that, The device includes: The startup module is used to control each full-bridge submodule and each half-bridge submodule of the modular multilevel converter to start charging upon receiving a start charging command. The bypass switching module is used to control each of the full-bridge sub-modules to be in bypass mode when it receives the self-test completion information corresponding to each sub-module in the modular multilevel converter. The half-bridge switching module is used to control each full-bridge submodule to charge in half-bridge mode when the differential pressure state between each full-bridge submodule and each half-bridge submodule reaches a preset condition. The controllable charging module is used to control the on / off state of each full-bridge submodule and each half-bridge submodule according to the real-time sorting of the capacitor voltages of each full-bridge submodule and each half-bridge submodule, until the capacitor voltage reaches the preset target voltage.

8. A flexible DC transmission system, characterized in that, It includes a modular multilevel converter and a main controller, the main controller being used to control the startup of the modular multilevel converter according to any one of claims 1 to 6.

9. The flexible DC transmission system according to claim 8, characterized in that, The flexible DC transmission system also includes a charging resistor and an AC contactor. One end of the charging resistor is connected to an AC power source, and the other end is connected to the modular multilevel converter. The charging resistor is connected in parallel across the AC contactor, and the AC contactor is connected to the main controller. The main controller is also used to control the AC contactor to disconnect when a charging start command is received, so that the AC power supply can supply power to the modular multilevel converter through the charging resistor; The main controller is also used to control the AC contactor to turn on after controlling each full-bridge submodule in the modular multilevel converter to charge in half-bridge mode.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, it implements the steps of the method according to any one of claims 1 to 6.

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