Direct-current fault ride-through control method for direct-current direct-connection energy storage auxiliary flexible direct-current system and related device

Abstract

The application discloses a DC fault ride-through control method for a direct-current direct-hanging energy storage auxiliary flexible direct-current system and related devices, and relates to the technical field of power electronics. The scheme provided by the application utilizes the voltage control capability of a direct-current direct-hanging energy storage battery submodule to make the output voltage higher than the MMC direct-current side voltage when a direct-current side fault occurs in a flexible direct-current system converter station, thereby effectively inhibiting the discharge of the MMC to the fault point. As a result, the discharge current of the MMC can be attenuated to zero, thereby providing reliable breaking conditions for the fast mechanical switch of the MMC direct-current side. The control scheme does not need to configure an expensive DC circuit breaker or full-bridge submodule, thereby reducing the system investment cost, ensuring the stable operation and fast recovery of the flexible direct-current system during a direct-current fault, and improving the overall economy and reliability.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to a DC fault ride-through control method and related devices for a DC-connected energy storage auxiliary flexible DC system. Background Technology

[0002] A key characteristic of DC fault current is the absence of a zero-crossing point. Unlike the periodic alternating current following an AC fault, the current in a DC fault increases linearly. Therefore, mechanical switches alone cannot interrupt the fault arc. Existing solutions all artificially create zero-crossing points to isolate the faulty line through different approaches. One approach is a half-bridge MMC with a DC circuit breaker, and the other is a hybrid MMC based on half-bridge and full-bridge submodules. The DC circuit breaker solution uses current switching, inserting a surge arrester in series in the fault circuit, and utilizing the large equivalent resistance of the surge arrester to extinguish the fault arc. The hybrid MMC dynamically sets the DC port voltage of the converter station to zero, thus eliminating the fault current by removing the discharge source.

[0003] Since the growth pattern of DC fault current follows Ohm's law, the two aforementioned schemes can be summarized as "increasing resistance" and "reducing voltage" to clear DC faults, respectively. The surge arrester used in the circuit breaker scheme is an equivalent large resistor, but installing a large resistor in a high-voltage, high-current circuit requires significant investment. The semi-full hybrid scheme achieves arc extinguishing by outputting zero voltage from the converter valve itself, but the converter valve requires full-bridge submodules, etc., which, although lower in cost than the circuit breaker scheme, increases operating losses. In summary, the primary goal of DC fault isolation is to artificially create a zero-crossing point for the current, thus creating conditions for the mechanical circuit breaker to disconnect. However, existing DC circuit breakers or semi-full hybrid schemes all rely on additional high-cost hardware investments (such as DC circuit breakers and full-bridge submodules) to achieve DC fault ride-through, resulting in high overall system costs and limiting the further popularization of flexible DC transmission systems. Summary of the Invention

[0004] This application provides a DC fault ride-through control method and related device for a DC direct-connected energy storage-assisted flexible DC system, which solves the technical problem of high implementation cost caused by the excessive reliance on high-cost hardware such as DC circuit breakers in existing DC fault ride-through control schemes.

[0005] To address the aforementioned technical problems, the first aspect of this application provides a DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system, applied to a flexible DC system converter station including an MMC and a DC-connected energy storage battery submodule, comprising: When a DC-side fault is detected in the converter station of the flexible DC system, the preset fault ride-through control logic is triggered; According to the fault ride-through control logic, the output voltage of the DC-connected energy storage battery submodule is increased, so that the output voltage of the DC-connected energy storage battery submodule is higher than the DC side voltage of the MMC. The discharge current of the MMC is monitored in real time. When the discharge current is detected to decay to 0, a switching action trigger command is generated to trigger the fast mechanical switch on the DC side of the MMC to perform a disconnection action.

[0006] Preferably, after triggering the fast mechanical switch on the DC side of the MMC to perform the disconnection action, the method further includes: Output the IGBT turn-off signal of the energy storage submodule to control the grounding switch of the energy storage battery to open, and connect the energy-consuming resistor to the freewheeling circuit to consume the residual electromagnetic energy. Upon detecting a DC fault clearing message, the energy storage battery grounding switch is closed again.

[0007] Preferably, it further includes: When the DC side of the flexible DC converter station is in normal operating condition, the DC-connected energy storage battery submodule is output controlled according to the preset power reference value and constant power control logic.

[0008] Preferably, the output control of the DC-connected energy storage battery submodule according to a preset power reference value and constant power control logic specifically includes: The difference between the actual power value and the preset power reference value is input into the preset PI controller, so that the adjustment amount of the number of conducting sub-modules in the DC direct-connected energy storage battery sub-module can be obtained through the calculation of the PI controller. Based on the sum of the adjustment amount and the baseline amount of the number of conducting submodules, the total number of conducting submodules required in a switching cycle is determined, so as to control the energy storage submodule input of the DC direct-connected energy storage battery submodule according to the total number of conducting submodules, and to perform output control on the DC direct-connected energy storage battery submodule.

[0009] Preferably, after controlling the output of the DC-connected energy storage battery submodule by controlling the number of energy storage submodules in operation according to the total number of the conducting submodules, the method further includes: When a change in the total number of the conducting submodules is detected, the charging and discharging status of the DC direct-connected energy storage battery submodule and the SOC status of each energy storage submodule are collected. When the DC-connected energy storage battery submodule is in discharge mode, energy storage submodules are selected in descending order of SOC status, with the number equal to the total number of conducting submodules. When the DC-connected energy storage battery submodule is in discharge mode, energy storage submodules are selected in order of SOC from low to high, with the number equal to the total number of the conducting submodules.

[0010] Meanwhile, a second aspect of this application provides a DC fault ride-through control device for a DC-DC connected energy storage-assisted flexible DC system, applied to a flexible DC system converter station including an MMC and a DC-DC connected energy storage battery submodule, comprising: The fault response unit is used to trigger a preset fault ride-through control logic when a DC-side fault is detected in the converter station of the flexible DC system. The voltage regulation unit is used to increase the output voltage of the DC-connected energy storage battery submodule according to the fault ride-through control logic, so that the output voltage of the DC-connected energy storage battery submodule is higher than the DC side voltage of the MMC. The disconnection action trigger determination unit is used to monitor the discharge current of the MMC in real time. When the discharge current is detected to decay to 0, a switch action trigger command is generated to trigger the fast mechanical switch on the DC side of the MMC to perform a disconnection action.

[0011] The third aspect of this application provides a flexible DC system based on MMC and DC-connected energy storage battery submodules. The DC-side converter station of the flexible DC system is composed of: several MMC submodules and several DC-connected energy storage battery submodules. The MMC submodule and the DC direct-connected energy storage battery submodule are respectively configured as MMC bridge arm and direct-connected energy storage unit bridge arm according to the bridge arm structure, and the MMC bridge arm and the direct-connected energy storage unit bridge arm are arranged in parallel between the AC grid and the current-limiting inductor. The DC-side converter station integrates a control device, which includes a memory and a processor. The memory is used to store program code, which corresponds to the DC fault ride-through control method for DC-connected energy storage-assisted flexible DC system provided in the first aspect of this application. The processor is used to read and execute the program code to implement the DC fault ride-through control method of the DC direct-connected energy storage assisted flexible DC system.

[0012] Preferably, the DC-connected energy storage battery submodule is composed of multiple energy storage submodules connected in series.

[0013] Preferably, each energy storage submodule includes: a bypass switch, a half-bridge submodule, and a battery connected in parallel on the capacitor side, with a current-limiting inductor and a current-limiting resistor connected in series across the battery. The bypass switch controls the connection of the current-limiting resistor.

[0014] The fourth aspect of this application provides a computer-readable storage medium storing program code, which is used to be read and executed by a processor to implement the DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system as provided in the first aspect of this application.

[0015] As can be seen from the above technical solutions, this application has the following advantages: The solution provided in this application utilizes the voltage control capability of the DC-connected energy storage battery submodule to ensure its output voltage is higher than the DC-side voltage of the MMC (Multi-Mechanical Storage Cell) when a DC-side fault occurs at the converter station of a flexible DC system. This effectively suppresses the discharge of the MMC to the fault point. Consequently, the discharge current of the MMC can decay to zero, providing a reliable breaking condition for the fast mechanical switching of the MMC's DC side. This control scheme eliminates the need for expensive DC circuit breakers or full-bridge submodules, reducing system investment costs. Simultaneously, it ensures stable operation and rapid recovery of the flexible DC system during DC faults, improving overall economy and reliability. Attached Figure Description

[0016] 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.

[0017] Figure 1 This is a flowchart illustrating an embodiment of a DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system provided in this application.

[0018] Figure 2 A schematic diagram illustrating the current flow direction at different fault stages during DC fault ride-through for DC-connected energy storage battery submodule converter stations.

[0019] Figure 3 Waveforms of various electrical parameters for DC fault ride-through in a DC-connected energy storage battery submodule converter station.

[0020] Figure 4 This is a schematic diagram of the architecture of an embodiment of a DC fault ride-through control device for a DC-connected energy storage auxiliary flexible DC system provided in this application.

[0021] Figure 5 This application provides a schematic diagram of the structure of a DC-side converter station for a flexible DC system based on MMC and DC-connected energy storage battery submodules. Detailed Implementation

[0022] This application provides a DC fault ride-through control method and related apparatus for a DC direct-connected energy storage-assisted flexible DC system, which solves the technical problem of high implementation costs caused by the excessive reliance on high-cost hardware such as DC circuit breakers in existing DC fault ride-through control schemes.

[0023] 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.

[0024] First, a detailed description of an embodiment of a DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system provided in this application is as follows: Please see Figure 1 This application proposes a DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system, applicable to a flexible DC system converter station including an MMC and a DC-connected energy storage battery submodule. The steps include: Step 101: When a DC-side fault is detected in the converter station of the flexible DC system, the preset fault ride-through control logic is triggered; Step 102: According to the fault ride-through control logic, increase the output voltage of the DC direct-connected energy storage battery submodule so that the output voltage of the DC direct-connected energy storage battery submodule is higher than the DC side voltage of the MMC. Step 103: Monitor the discharge current of the MMC in real time. When the discharge current is detected to decay to 0, generate a switch action trigger command to trigger the fast mechanical switch on the DC side of the MMC to perform a disconnection action.

[0025] For ease of understanding, the following explains some key terms in this embodiment: A flexible DC converter station is a power electronic device used to convert AC to DC power, playing a core role in flexible DC transmission systems. This converter station typically contains multiple power electronic modules, enabling flexible control of power flow and voltage levels.

[0026] MMC (Modular Multilevel Converter) is an advanced power electronic converter topology. It consists of multiple sub-modules connected in series. Each sub-module typically contains switching devices and capacitors. By controlling the connection and disconnection of sub-modules, high-voltage, high-capacity AC-DC conversion can be achieved.

[0027] A high-voltage direct-current energy storage (HDES) battery submodule refers to an energy storage battery unit that is directly connected to the DC side of a flexible DC system. This energy storage battery can provide or absorb DC power, playing roles such as energy buffering, voltage support, and fault response during system operation.

[0028] A DC-side fault refers to an abnormal situation such as a short circuit or grounding that occurs in the DC transmission line or DC bus of a flexible DC system. This fault can cause a sharp increase in DC current, threatening the stable operation of the system.

[0029] Fault-crossing control logic refers to a series of pre-set control strategies and action sequences designed to ensure system stability and rapid recovery when a system failure occurs. This logic aims to adjust system operating parameters to enable the system to withstand fault impacts and maintain critical functions.

[0030] A fast mechanical switch is a mechanical switching device that can complete the breaking or closing action in a short time. In DC systems, it is used to quickly isolate faulty lines after a fault occurs, in order to protect other parts of the system from damage.

[0031] More specifically, during the operation of a flexible DC-DC converter station, when a DC-side fault is detected, the preset fault ride-through control logic will be triggered. Specifically, fault detection can be performed in several ways. For example, voltage and current sensors can be configured at key nodes on the DC side to continuously monitor the DC bus voltage and line current. When a sudden drop in DC bus voltage or an abnormal increase in line current exceeding a preset threshold is detected, a DC-side fault can be determined. At this time, the fault detection unit will immediately send a fault signal to the main controller, thereby activating the preset fault ride-through control logic. As another implementation method, the differential protection principle can be used to accurately determine the fault location and type by comparing the current difference between the two ends of the line. Once the fault is confirmed, the corresponding control logic is triggered.

[0032] According to the triggered fault ride-through control logic, the output voltage of the DC-connected energy storage battery submodule will be increased to be higher than the DC-side voltage of the MMC. For example, after the fault ride-through control logic is activated, the control system can send a command to the power conversion unit of the DC-connected energy storage battery submodule to adjust its operating mode. The power conversion unit can increase the output voltage of the energy storage battery in a short time by increasing the number of energy storage submodules connected in series inside it or adjusting the boost ratio of its DC / DC converter. The increased voltage is precisely controlled to be slightly higher than the transient voltage on the DC side of the MMC to form a back electromotive force, thereby effectively suppressing the injection of current from the MMC to the fault point.

[0033] Subsequently, the system will monitor the discharge current of the MMC in real time. When the discharge current is detected to have decayed to zero, a switching action trigger command will be generated to trigger the fast mechanical switch on the DC side of the MMC to perform an interruption action. Specifically, a current sensor is placed at the DC side outlet of the MMC to continuously collect the discharge current data of the MMC. This data is transmitted to a current monitoring and judgment unit. This unit continuously analyzes the real-time current value and compares it with a very small preset threshold. Once the discharge current remains below this threshold, it is determined that the current has effectively decayed to zero. At this time, the unit will immediately generate an electrical signal as a switching action trigger command and send it to the fast mechanical switch on the DC side of the MMC. After receiving the command, the fast mechanical switch will quickly perform an interruption action, thereby isolating the faulty line.

[0034] It should be noted that the current of MMC and HDES... I MMC and I HDES Both point to DC lines. I fault This is the total current of the faulty line. On the DC side of the MMC, a Fast Mechanical Switch (FMS) is installed, which has rapid arc-breaking capability to isolate the faulty line when the current crosses zero. A Mechanical Switch (MS) is installed at the grounding electrode of the HDES; its function is to connect the energy-dissipating resistor to the fault circuit during the energy dissipation phase, consuming the fault energy. Fault clearing is divided into the following three stages: 1) Natural Discharge: In steady state, the DC voltages of MMC and HDES are equal. After a DC fault occurs at the output of the sending-end converter station, both MMC and HDES release fault current to the fault point through the DC reactance. Before the system detects the fault, the sending end will maintain a natural discharge state, and the DC-side fault current will rise rapidly, with both MMC and HDES releasing fault current to the fault point. At this time, MMC, HDES, and the total fault current form KCL, resulting in:

[0035] 2) DC Fault Clearing: To promptly isolate the converter station from the fault point after a fault occurs, it is necessary to... I MMC Create an artificial zero-crossing point. At this point, simply controlling the HDES voltage higher than the MMC voltage will reduce the MMC discharge current according to Ohm's law, while the system's fault current will be borne by the HDES. Only dynamic tracking of the HDES is required. I MMC Reaching zero creates the conditions for FMS segmentation, namely:

[0036] 3) Fault Energy Dissipation: After the DC fault is cleared, the MMC is isolated from the fault circuit. At this time, the energy storage submodule is locked, and the fault current will freewheel through the diode below the half-bridge submodule. Energy dissipation must be achieved. An energy dissipation resistor and a bypass switch MS are designed below the energy storage battery. By disconnecting the MS, the fault current can flow through the energy dissipation circuit, thus dissipating the fault energy. The current flow direction and electrical parameter waveforms corresponding to different fault stages throughout the process are shown below. Figure 2 and Figure 3 As shown.

[0037] This embodiment utilizes the voltage control capability of the DC-connected energy storage battery submodule to ensure its output voltage is higher than the DC-side voltage of the MMC (Multi-Mechanical Storage Cell) when a DC-side fault occurs at the converter station of the flexible DC system. This effectively suppresses the discharge of the MMC to the fault point. Consequently, the discharge current of the MMC can decay to zero, providing a reliable breaking condition for the fast mechanical switching of the MMC's DC side. This solution eliminates the need for expensive DC circuit breakers or full-bridge submodules, significantly reducing system investment costs and achieving economical and reliable DC fault ride-through.

[0038] In some embodiments described above, after a DC-side fault is detected in the converter station of the flexible DC system, a preset fault ride-through control logic is triggered. Based on this logic, the output voltage of the DC-connected energy storage battery submodule is increased to be higher than the DC-side voltage of the MMC, thereby suppressing the MMC from discharging to the fault point. When the discharge current of the MMC is monitored to decay to zero in real time, a switching action trigger command is generated to trigger the fast mechanical switch on the DC side of the MMC to perform an disconnection action, thereby achieving fault isolation. However, after the fast mechanical switch disconnects, residual electromagnetic energy may still exist in the system, which may damage the equipment or affect the system's rapid recovery. Furthermore, how to safely and effectively restore the system to normal operation after the fault is cleared is also a problem that needs to be considered.

[0039] In response, this application further proposes that after the fast mechanical switch on the DC side of the MMC is triggered to perform the disconnection action, the energy storage submodule IGBT is locked and the energy storage battery grounding switch is controlled to be disconnected, and the energy dissipation resistor is connected to the freewheeling circuit to consume the residual electromagnetic energy; and when the DC fault clearance information is detected, the energy storage battery grounding switch is closed again.

[0040] Specifically, controlling the grounding switch of the energy storage battery to open means that after the fast mechanical switch on the DC side of the MMC completes its disconnection action, the system control unit will immediately issue a command to lock out the IGBTs of the energy storage submodule. This means turning off the two IGBTs in the half-bridge section, allowing the current flow path to pass through the freewheeling diode of the lower IGBT, thus keeping the grounding switch connecting the energy storage battery and the system grounding circuit in an open state. Its function is to further isolate the energy storage battery, preventing it from forming a new discharge path through the grounding circuit after fault isolation, or generating unnecessary current in subsequent operations, thereby ensuring the safety of the energy storage battery.

[0041] Simultaneously, a power-dissipating resistor is connected to the freewheeling circuit to dissipate residual electromagnetic energy. After the fast mechanical switch on the DC side of the MMC disconnects, the inductive components in the flexible DC system (such as current-limiting inductors, MMC arm inductors, etc.) will store a certain amount of electromagnetic energy. If this energy cannot be released effectively and in a timely manner, it may generate overvoltage at the moment of disconnection, damaging other electrical equipment in the system. The power-dissipating resistor is connected to a specific freewheeling circuit through a fast-switching switch (such as an IGBT or thyristor), providing an energy release path for these inductive components. By converting electromagnetic energy into heat energy through the resistor, the generation of overvoltage can be effectively suppressed, equipment can be protected, and the system's energy dissipation process can be accelerated.

[0042] Furthermore, upon detecting a DC fault clearance message, the system will reclose the energy storage battery grounding switch. The DC fault clearance message is a signal issued by the system control center or protection device, indicating that the DC-side fault has disappeared or been isolated, and the system has returned to a safe state. This information can be detected by continuously monitoring key parameters such as DC voltage, current, and insulation status, combined with the reset signal from the protection device. Once the fault clearance is confirmed, the control unit sends a closing command to the energy storage battery grounding switch, reconnecting it to the system. This allows the energy storage battery to promptly and safely restore its connection to the system, providing the necessary energy support for subsequent normal system operation or rapid reclosing operations.

[0043] Through the above technical solution, after the fast mechanical switch on the DC side of the MMC performs the disconnection action, the energy storage battery grounding switch is further controlled to open, and an energy-dissipating resistor is connected to consume the residual electromagnetic energy. This application can effectively avoid the potential damage to the equipment caused by the residual energy inside the system after the mechanical switch disconnection, suppress the generation of overvoltage, and thus improve the safety and reliability of the system. In addition, after detecting the DC fault clearance information, the energy storage battery grounding switch is reclosed, enabling the energy storage battery to restore its connection with the system in a timely and safe manner, providing the necessary energy support for the subsequent normal operation or rapid reclosing operation of the system, significantly shortening the fault recovery time and improving the availability of the system.

[0044] In some embodiments described above, it is proposed that when a DC-side fault occurs at the converter station of a flexible DC system, the output voltage of the DC-connected energy storage battery submodule be increased to be higher than the DC-side voltage of the MMC, thereby suppressing the discharge of the MMC to the fault point and providing conditions for the high-speed mechanical switch to open. However, when the system is under normal operating conditions, how to effectively manage and control the power output of the DC-connected energy storage battery submodule to ensure stable system operation and fully utilize the regulation capability of the energy storage battery is a problem that needs further consideration.

[0045] In this regard, this application further proposes that when the DC side of the flexible DC system converter station is in normal operating condition, the output control of the DC direct-connected energy storage battery submodule is performed according to the preset power reference value and constant power control logic.

[0046] Specifically, the DC side of a flexible DC-DC converter station is in normal operating condition, meaning that no short circuits, grounding faults, or other faults have occurred on the DC side of the converter station, and all operating parameters such as system voltage and current are operating stably within the design limits. In this state, the system primarily focuses on routine operating objectives such as power transmission, voltage support, and frequency regulation, rather than fault ride-through. The preset power reference value refers to the target power that the DC-connected energy storage battery submodule should output or absorb during normal system operation, based on factors such as grid dispatch instructions, system operation optimization objectives, or the energy storage battery's own management strategies. This reference value can be a constant value or a curve that varies over time, used to guide the charging and discharging behavior of the energy storage battery. Constant power control logic is a control strategy designed to ensure that the actual output power of the DC-connected energy storage battery submodule accurately tracks the preset power reference value. This logic monitors the output power of the energy storage battery in real time and compares it with the power reference value, adjusting the charging and discharging behavior of the energy storage battery according to the deviation to achieve stable power output. The implementation methods may include, but are not limited to, adjusting the number of conducting sub-modules inside the energy storage battery, changing the output voltage or current of the sub-modules, etc., to achieve the purpose of power control.

[0047] Through the above technical solution, when the DC side of the flexible DC converter station is under normal operating conditions, the DC-connected energy storage battery submodules can be precisely controlled for constant power output based on a preset power reference value. This allows the energy storage batteries to stably participate in system power regulation during non-fault periods, such as peak shaving and valley filling, smoothing fluctuations in renewable energy output, or providing ancillary services, thereby fully utilizing their energy storage and power regulation capabilities. This control strategy avoids disordered or passive charging and discharging of the energy storage batteries under normal operating conditions, ensuring the economy and stability of system operation, and providing flexible power support for the entire flexible DC system, thus improving the overall operating efficiency and reliability of the system.

[0048] In some embodiments described above in this application, it is proposed that when the DC side of the flexible DC system converter station is under normal operating conditions, the output of the DC-connected energy storage battery submodule is controlled according to a preset power reference value and constant power control logic. However, in its implementation, how to accurately and dynamically adjust the output power of the DC-connected energy storage battery submodule to stably track the preset power reference value is a technical problem that needs to be solved, especially when the DC-connected energy storage battery submodule consists of multiple energy storage submodules, due to the discreteness of the submodules.

[0049] In response, this application further proposes the above-mentioned output control method for DC-connected energy storage battery submodules, which specifically includes: inputting the difference between the actual power value and a preset power reference value into a preset PI controller, so as to obtain the adjustment amount of the number of conducting submodules in the DC-connected energy storage battery submodule through the calculation of the PI controller; determining the total number of conducting submodules required in a switching cycle based on the sum of the adjustment amount and the reference amount of the number of conducting submodules, so as to control the energy storage submodule input of the DC-connected energy storage battery submodule according to the total number of conducting submodules, and performing output control on the DC-connected energy storage battery submodule.

[0050] Specifically, the phrase "inputting the difference between the actual power value and a preset power reference value into a preset PI controller, so as to obtain the adjustment amount of the number of conducting submodules in the DC-connected energy storage battery submodule through the calculation of the PI controller" means that the system monitors the actual output power of the DC-connected energy storage battery submodule in real time and compares it with a preset power reference value to obtain a power error signal. This error signal is input into a preset proportional-integral (PI) controller. The PI controller calculates based on the current error and the accumulated error to generate a control output, which is the adjustment amount of the number of conducting submodules in the DC-connected energy storage battery submodule. This adjustment amount indicates how many conducting energy storage submodules need to be added or removed to eliminate the power error. The PI controller can effectively eliminate steady-state errors and respond quickly to dynamic changes, ensuring the accuracy of power output.

[0051] The phrase "determining the total number of conducting submodules required within a switching cycle based on the sum of the adjustment amount and the baseline number of conducting submodules" refers to summing the adjustment amount of the number of conducting submodules output by the PI controller with a preset baseline number of conducting submodules. This baseline number can be set based on the current operating status of the system, total power demand, or empirical values. The summation result represents the total number of energy storage submodules that need to be put into operation in the current switching cycle. The switching cycle is a preset time interval used to regulate the switching frequency of energy storage submodules, avoiding frequent switching that could cause equipment damage and ensuring control stability.

[0052] The phrase "controlling the output of the DC-connected energy storage battery submodule by controlling the number of energy storage submodules in operation according to the total number of conducting submodules" refers to the control system selecting a corresponding number of energy storage submodules to be put into operation within the DC-connected energy storage battery submodule based on the total number of conducting submodules required within the aforementioned switching cycle. Each energy storage submodule typically contains power electronic devices such as bypass switches. By controlling the on / off state of these switches, the energy storage submodule can be put into or removed. By precisely controlling the number of energy storage submodules in operation, the output voltage and current of the DC-connected energy storage battery submodule can be finely adjusted, thereby achieving precise control of its actual output power and enabling it to stably track the preset power reference value.

[0053] Through the above technical solutions, the flexible DC-DC converter station can achieve precise and dynamic control of the output power of the DC-connected energy storage battery submodules under normal operating conditions. By introducing a PI controller to process power errors and converting its output into discrete adjustments to the number of conducting submodules, the system can overcome the control accuracy problems caused by the discreteness of the energy storage submodules. This power regulation method based on submodule switching enables the DC-connected energy storage battery submodules to respond stably and quickly to changes in the power reference value, effectively suppressing power fluctuations and ensuring the stable operation of the flexible DC-DC converter station. Simultaneously, this method fully utilizes the modular characteristics of the DC-connected energy storage battery submodules, achieving refined power management, improving the utilization efficiency and control flexibility of the energy storage system, thereby enhancing the overall reliability and economy of the flexible DC-DC system.

[0054] In some of the embodiments described above in this application, although a method is proposed to determine the total number of DC-connected energy storage battery sub-modules to be turned on based on a power reference value using a PI controller in order to achieve constant power output control, if the switching is performed only based on the calculated total number of turned-on sub-modules without considering the specific state of each energy storage sub-module within the DC-connected energy storage battery sub-module, some sub-modules may be in a high-load or low-load state for a long time, resulting in uneven charge and discharge depth, which in turn accelerates the aging of the sub-modules and affects the overall lifespan, performance consistency and energy utilization efficiency of the battery pack.

[0055] Specifically, when the system detects a change in the total number of active submodules, it means that the number of energy storage submodules that need to be put into operation, calculated based on the power reference value and the PI controller, has changed, triggering subsequent module selection logic. This change may be caused by adjustments to the power reference value, fluctuations in actual power, or changes in the adjustment amount of the PI controller output. Simultaneously, the system collects the charge / discharge status of the DC-connected energy storage battery submodules, i.e., whether the current DC-connected energy storage battery submodule is in charging or discharging mode. This is typically determined by monitoring the total current direction or power flow direction of the battery pack. Different charging / discharging modes may require different selection strategies for energy storage submodules to optimize battery operation and lifespan. Furthermore, the system collects the SOC (State of Charge) of each energy storage submodule, i.e., the state of charge of each independent energy storage submodule within the DC-connected energy storage battery submodule. SOC is a key indicator of remaining battery capacity, typically estimated by the battery management system (BMS) based on parameters such as voltage, current, and temperature of the submodules. Accurate SOC information is fundamental for achieving balanced submodule management and optimized switching.

[0056] When a DC-connected energy storage battery submodule needs to supply power (discharge), the system can employ a strategy of selecting energy storage submodules in descending order of State of Charge (SOC) to be put into operation, with the number equal to the total number of active submodules. This strategy helps to prioritize the consumption of submodules with higher charge, thereby achieving a certain degree of balance in the state of charge among submodules and preventing some submodules from remaining in a high SOC state for extended periods. It also ensures sufficient energy reserves when high power output is required. As another optional submodule selection strategy under the discharge mode, the system can also select energy storage submodules in ascending order of SOC to be equal to the total number of active submodules. This strategy may be suitable for "activating" low-SOC submodules or, under specific balancing strategies, for preventing performance degradation due to prolonged inactivity of certain submodules. It may also be useful in multi-objective optimization to prepare for the balance of subsequent charging stages, for example, when the system goal is to maximize the utilization rate of low-SOC modules as quickly as possible.

[0057] Through the above technical solution, when the total number of active submodules changes, the system can dynamically and intelligently select the energy storage submodules to be put into operation based on their charge / discharge status and SOC (State of Charge) status. Specifically, in discharge mode, by prioritizing the discharge of submodules with higher SOC, the system can effectively prevent some submodules from aging faster due to prolonged high SOC, promote the balance of charge status among submodules, thereby extending the service life of the entire DC-connected energy storage battery submodule and improving its operational reliability and stability. Simultaneously, a strategy of prioritizing the discharge of submodules with lower SOC is provided, offering the battery management system a more flexible means of balancing control. This allows for refined management of the charge / discharge behavior of each energy storage submodule based on actual operational needs and battery health, further optimizing the overall performance and energy utilization efficiency of the battery pack. This refined submodule management mechanism effectively solves the problems of uneven submodule lifespan and performance degradation caused by switching based solely on the total number of modules, significantly improving the operational economy and sustainability of DC-connected energy storage battery submodules in flexible DC converter stations.

[0058] The above is a detailed description of an embodiment of a DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system provided in this application. The following is a detailed description of related embodiments of a DC fault ride-through control device, flexible DC system, and computer-readable storage medium provided in this application: Please see Figure 4 The second aspect of this application provides a DC fault ride-through control device for a DC-DC connected energy storage-assisted flexible DC system, applied to a flexible DC system converter station including an MMC and a DC-DC connected energy storage battery submodule, comprising: The fault response unit 201 is used to trigger a preset fault ride-through control logic when a DC-side fault is detected in the converter station of the flexible DC system. The voltage regulation unit 202 is used to increase the output voltage of the DC direct-connected energy storage battery submodule according to the fault ride-through control logic, so that the output voltage of the DC direct-connected energy storage battery submodule is higher than the DC side voltage of the MMC. The disconnection action trigger determination unit 203 is used to monitor the discharge current of the MMC in real time. When the discharge current is detected to decay to 0, a switch action trigger command is generated to trigger the fast mechanical switch on the DC side of the MMC to perform a disconnection action.

[0059] This solution combines a fault response unit, a voltage regulation unit, and a tripping action triggering unit in a collaborative manner. During a DC fault, it controls the output voltage of the DC-connected energy storage battery submodule to be slightly higher than the DC-side voltage of the MMC, effectively suppressing the MMC's discharge to the fault point and reducing the discharge current to zero. This achieves fault ride-through without the need for expensive DC circuit breakers, significantly reducing system investment costs. Specifically, when a DC-side fault occurs, the fault response unit triggers the control logic by real-time monitoring of a sudden drop in DC bus voltage or an abnormal increase in line current. The voltage regulation unit then adjusts the operating parameters of the power conversion unit of the DC-connected energy storage battery submodule, precisely increasing its output voltage to generate a back electromotive force, blocking the MMC discharge path. Furthermore, the tripping action triggering unit continuously collects the MMC DC-side outlet current data. When the discharge current remains below a preset threshold, it determines that the current has returned to zero and promptly generates a switching action trigger command. The above technical solution fully utilizes the transient DC current control capability of the DC-connected energy storage battery submodule, achieving reliable suppression of DC fault current and precise creation of mechanical switch disconnection conditions without requiring additional high-cost hardware investment, thus providing technical support for the economical and reliable operation of flexible DC transmission systems.

[0060] Furthermore, embodiments of this application also provide a flexible DC system based on MMC and DC-connected energy storage battery submodules, such as... Figure 5 As shown, the DC-side converter station of the flexible DC system consists of several MMC sub-modules and several DC-connected energy storage battery sub-modules. The MMC sub-modules and the DC-connected energy storage battery sub-modules are respectively configured as MMC bridge arms and direct-connected energy storage unit bridge arms, and are connected in parallel between the AC grid and the current-limiting inductor. The DC-side converter station integrates a control device (not shown), which includes a memory and a processor. The memory stores program code corresponding to the aforementioned DC fault ride-through control method for the DC-connected energy storage-assisted flexible DC system. The processor reads and executes the program code to implement the DC fault ride-through control method for the DC-connected energy storage-assisted flexible DC system.

[0061] This embodiment integrates the DC-connected energy storage battery submodule and the MMC submodule in a parallel bridge arm structure in the flexible DC system converter station. By executing fault ride-through control logic through a control device, the transient DC current control capability of the DC-connected energy storage battery submodule is used to suppress the discharge of the MMC to the fault point during DC faults, so that the fault current decays to zero. This achieves the effect of economical and reliable DC fault ride-through without the need for expensive DC circuit breakers or full-bridge submodules.

[0062] In its implementation, the DC-side converter station of this flexible DC system adopts a modular design. Several MMC sub-modules are connected in series to form an MMC bridge arm, used for energy conversion between AC and DC. Simultaneously, several DC-connected energy storage battery sub-modules are connected in series to form a direct-connected energy storage unit bridge arm. This energy storage battery is directly connected to the DC side and can provide or absorb DC power. The MMC bridge arm and the direct-connected energy storage unit bridge arm are connected in parallel between the AC grid and the current-limiting inductor. This topology allows the DC-connected energy storage battery sub-modules to quickly intervene in system control during faults. Since the growth law of DC fault current conforms to Ohm's law, this embodiment, through the above parallel structure, provides a physical path for the DC-connected energy storage battery sub-modules to directly act on the fault circuit, thereby avoiding the introduction of additional hardware.

[0063] The control unit, as the core of the system, is integrated into the DC-side converter station. This control unit includes a memory and a processor. The memory stores program code corresponding to the aforementioned DC fault ride-through control method for the DC-connected energy storage-assisted flexible DC system. The processor reads and executes this program code to achieve real-time monitoring of the system's operating status and the execution of control strategies. When the system detects a DC-side fault, the processor triggers preset fault ride-through control logic. Specifically, the processor controls the DC-connected energy storage battery submodule to increase its output voltage, making it slightly higher than the MMC DC-side voltage, thereby generating a back electromotive force and effectively suppressing the MMC from discharging to the fault point. During this process, the system monitors the MMC's discharge current in real time. When the discharge current is detected to have decayed to zero, a switching action trigger command is generated to trigger the fast mechanical switch on the MMC DC side to perform a disconnection action, thereby isolating the faulty line.

[0064] Through the above technical solution, this embodiment fully utilizes the transient control capability of the DC-connected energy storage battery submodule, achieving DC fault ride-through without the need for additional high-cost hardware. Compared to traditional solutions, this embodiment significantly reduces system investment costs while ensuring the reliability and economy of fault ride-through, providing technical support for the widespread adoption of flexible DC transmission systems. Overall, this solution, based on the optimization of existing topology and control strategies, effectively solves the technical challenge of no zero-crossing point in DC fault current through the dynamic voltage regulation capability of the DC-connected energy storage battery submodule, achieving a balance between the economy and reliability of fault ride-through.

[0065] In some of the above embodiments, the DC side of the flexible DC system converter station integrates a DC-connected energy storage battery submodule to provide transient DC current control during DC faults and power control under normal operating conditions. However, if the DC-connected energy storage battery submodule is directly connected to the system as a single unit, it may face technical challenges in achieving the high DC voltage level, large capacity configuration, and precise and rapid adjustment of the output voltage required for flexible DC systems during fault ride-through control. Furthermore, internal fault isolation of individual battery cells and the overall operational reliability of the system may also be limited.

[0066] In this regard, this application further proposes that the DC-DC connected energy storage battery submodule is composed of multiple energy storage submodules connected in series. The energy storage submodule is the basic functional unit constituting the DC-DC connected energy storage battery submodule. Each energy storage submodule typically includes one or more battery cells, a battery management system (BMS), and a power electronic converter for voltage regulation and power control. These submodules are designed as standardized, modular units capable of independent charge / discharge management and state monitoring. By integrating the BMS, each energy storage submodule can achieve overcharge, over-discharge, overcurrent, and over-temperature protection for battery cells, as well as state of charge (SOC) balancing management, ensuring the safe and stable operation of the battery. Multiple energy storage submodules are connected in series to form the DC-DC connected energy storage battery submodule. The purpose of this series connection is to superimpose the voltages of individual energy storage submodules to achieve the high DC voltage level required by the flexible DC system. By controlling the on (conduction) or off (bypass) state of each series submodule, fine-grained, step-wise adjustment of the output voltage of the entire DC-DC connected energy storage battery submodule can be achieved.

[0067] By designing the DC-connected energy storage battery submodule as a series-connected module, this application effectively addresses the limitations of a single battery cell in terms of voltage level, power capacity, voltage regulation accuracy, and system reliability. This modular series structure allows for flexible configuration of the DC-connected energy storage battery submodule to meet the high DC voltage and large capacity requirements of flexible DC systems. More importantly, independent switching control of the series-connected energy storage submodules enables precise and rapid adjustment of the output voltage, which is crucial for accurately boosting the battery output voltage to suppress MMC discharge to the fault point during DC fault ride-through control. Furthermore, the modular design enhances system redundancy and maintainability. When a submodule fails, it can be bypassed or isolated without affecting the normal operation of the entire system, significantly improving the operational reliability and availability of the flexible DC system. This structure also provides a foundation for subsequent submodule switching strategies (such as SOC-based switching), further optimizing battery life and performance.

[0068] In some embodiments described above in this application, the DC-side converter station of the flexible DC system integrates an MMC submodule and a DC-connected energy storage battery submodule, wherein the DC-connected energy storage battery submodule is composed of multiple energy storage submodules connected in series. However, in actual operation, a single energy storage submodule may face overcurrent, overvoltage, or internal faults. Without effective protection and control mechanisms, these faults may propagate, affecting the stability and reliability of the entire energy storage battery pack and even the flexible DC system. Simply connecting the energy storage submodules in series makes it difficult to achieve refined management and fault isolation for each submodule, thus limiting the system's adaptability and safety under complex operating conditions.

[0069] In this regard, this application further proposes that each energy storage submodule includes: a bypass switch, a half-bridge submodule and a battery connected in parallel on the capacitor side, and a current-limiting inductor and a current-limiting resistor are connected in series across the battery terminals; the bypass switch controls the connection of the current-limiting resistor.

[0070] Among them, the bypass switch is used to control the switching of the current path, such as Figure 5 As shown K 1, K2. It can be a mechanical switch or an electronic switch based on semiconductor devices (such as IGBTs, MOSFETs, etc.). Under normal operating conditions, the bypass switch can remain open, allowing current to flow through other paths; under specific circumstances, such as when a current-limiting resistor needs to be connected to the circuit or when isolating a faulty submodule, the bypass switch can close, thereby changing the current flow path and achieving protection or functional adjustment of the submodule. The half-bridge submodule is the basic unit of power electronic conversion, usually consisting of two series-connected switching devices (such as IGBTs) and corresponding anti-parallel diodes, with a DC capacitor connected in parallel. Its main functions are to realize DC voltage step-up / step-down conversion, current control, and bidirectional energy flow. By controlling the switching state of the half-bridge submodule, its output voltage and current can be precisely adjusted, thereby controlling the charging and discharging process of the energy storage submodule and participating in system voltage support and power regulation. The battery, as the core energy storage element of the energy storage submodule, is connected in parallel to the DC capacitor side of the half-bridge submodule. This connection method allows the battery to charge and discharge through the half-bridge sub-module, maintaining a relatively stable DC bus voltage with the half-bridge sub-module, thus providing a stable DC power supply to the half-bridge sub-module or absorbing its output energy. A current-limiting inductor and a current-limiting resistor are connected in series across the battery, primarily used to limit the current flowing through the battery. The current-limiting inductor suppresses rapid current changes and reduces current surges, while the current-limiting resistor, in the event of a short circuit or overcurrent fault, limits the amplitude of the fault current by consuming energy, thereby protecting the battery from damage by excessive current and improving system safety. The bypass switch determines whether the current-limiting resistor is connected to the battery's current loop based on its open / closed state. For example, during normal operation or when a rapid response is required, the bypass switch can be open, preventing the current-limiting resistor from participating in current limiting to reduce energy loss; while when an overcurrent or fault condition is detected, the bypass switch can be closed, connecting the current-limiting resistor to the loop, effectively limiting the current flowing through the battery, preventing battery damage, and providing a buffer time for fault handling.

[0071] Through the above technical solution, each energy storage submodule integrates a bypass switch, a half-bridge submodule, a battery, and a series-connected current-limiting inductor and resistor. The bypass switch can control the connection of the current-limiting resistor. This design enables each energy storage submodule to have independent and flexible current limiting and protection capabilities. When an internal fault occurs in the energy storage submodule or an overcurrent condition occurs in the system, the bypass switch can quickly activate, connecting the current-limiting resistor to the battery circuit, effectively suppressing the fault current flowing through the battery and preventing battery damage due to overcurrent. This significantly improves the fault tolerance and safety of a single energy storage submodule. Simultaneously, the presence of the half-bridge submodule ensures precise voltage and current control, achieving effective energy management. This refined submodule structure not only isolates local faults, preventing them from spreading to the entire DC-connected energy storage battery submodule, but also, when combined with the aforementioned DC fault ride-through control method, more reliably supports the system's voltage and current control during faults, creating conditions for the rapid mechanical switching action on the MMC DC side, further enhancing the stability and reliability of the flexible DC system during DC fault ride-through.

[0072] In some embodiments, this application also proposes a computer-readable storage medium storing program code for a processor to read and execute, thereby implementing the aforementioned DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system. The core innovation of this embodiment lies in storing the program code in a computer-readable storage medium, enabling the processor to execute the DC fault ride-through control logic of the DC-connected energy storage-assisted flexible DC system. This eliminates the need for additional high-cost hardware (such as DC circuit breakers or full-bridge submodules), directly utilizing the existing DC-connected energy storage battery submodules and the MMC for fault ride-through. Specifically, when the processor reads and executes the program code, it can trigger preset control logic based on the DC-side fault detection signal, increasing the output voltage of the DC-connected energy storage battery submodule to be higher than the DC-side voltage of the MMC, monitoring the MMC discharge current in real time, and generating a switching action trigger command to disconnect the fast mechanical switch when the current decays to zero.

[0073] The above technical solution effectively suppresses the discharge process of MMC to the fault point, causing the fault current to rapidly decay to zero, thus creating the necessary conditions for reliable disconnection of the mechanical switch. Compared with traditional solutions, this embodiment avoids the need for expensive hardware configuration, significantly reducing system investment costs, while ensuring stable operation and rapid recovery of the flexible DC system during DC faults, improving overall economy and reliability.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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 DC fault ride-through control method for a DC-DC connected energy storage-assisted flexible DC system, applied to a flexible DC system converter station including an MMC and a DC-DC connected energy storage battery submodule, characterized in that, include: When a DC-side fault is detected in the converter station of the flexible DC system, the preset fault ride-through control logic is triggered; According to the fault ride-through control logic, the output voltage of the DC-connected energy storage battery submodule is increased, so that the output voltage of the DC-connected energy storage battery submodule is higher than the DC side voltage of the MMC. The discharge current of the MMC is monitored in real time. When the discharge current is detected to decay to 0, a switching action trigger command is generated to trigger the fast mechanical switch on the DC side of the MMC to perform a disconnection action.

2. The DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system according to claim 1, characterized in that, After triggering the fast mechanical switch on the DC side of the MMC to perform the breaking action, the following also applies: The two IGBTs in the energy storage submodule are locked out, so that the current flows through the diode corresponding to the lower IGBT. Then, the grounding switch of the energy storage battery is opened, and the energy-consuming resistor is connected to the freewheeling circuit to consume the residual electromagnetic energy. Once the DC fault clearance information is detected, the energy storage battery grounding switch is closed again.

3. The DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system according to claim 1, characterized in that, Also includes: When the DC side of the flexible DC converter station is in normal operating condition, the DC-connected energy storage battery submodule is output controlled according to the preset power reference value and constant power control logic.

4. The DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system according to claim 3, characterized in that, Based on a preset power reference value, the output control of the DC-connected energy storage battery submodule according to the constant power control logic specifically includes: The difference between the actual power value and the preset power reference value is input into the preset PI controller, so that the adjustment amount of the number of conducting sub-modules in the DC direct-connected energy storage battery sub-module can be obtained through the calculation of the PI controller. Based on the sum of the adjustment amount and the baseline amount of the number of conducting submodules, the total number of conducting submodules required in a switching cycle is determined, so as to control the energy storage submodule input of the DC direct-connected energy storage battery submodule according to the total number of conducting submodules, and to perform output control on the DC direct-connected energy storage battery submodule.

5. The DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system according to claim 4, characterized in that, After controlling the output of the DC-connected energy storage battery submodule by controlling the number of energy storage submodules in operation according to the total number of the conducting submodules, the process further includes: When a change in the total number of the conducting submodules is detected, the charging and discharging status of the DC direct-connected energy storage battery submodule and the SOC status of each energy storage submodule are collected. When the DC-connected energy storage battery submodule is in discharge mode, energy storage submodules are selected in descending order of SOC status, with the number equal to the total number of conducting submodules. When the DC-connected energy storage battery submodule is in discharge mode, energy storage submodules are selected in order of SOC from low to high, with the number equal to the total number of the conducting submodules.

6. A DC fault ride-through control device for a DC-DC connected energy storage-assisted flexible DC system, applied to a converter station of a flexible DC system including an MMC and a DC-DC connected energy storage battery submodule, characterized in that, include: The fault response unit is used to trigger a preset fault ride-through control logic when a DC-side fault is detected in the converter station of the flexible DC system. The voltage regulation unit is used to increase the output voltage of the DC-connected energy storage battery submodule according to the fault ride-through control logic, so that the output voltage of the DC-connected energy storage battery submodule is higher than the DC side voltage of the MMC. The disconnection action trigger determination unit is used to monitor the discharge current of the MMC in real time. When the discharge current is detected to decay to 0, a switch action trigger command is generated to trigger the fast mechanical switch on the DC side of the MMC to perform a disconnection action.

7. A flexible DC system based on MMC and DC-connected energy storage battery submodules, characterized in that, The DC-side converter station of the flexible DC system consists of several MMC sub-modules and several DC-connected energy storage battery sub-modules. The MMC submodule and the DC direct-connected energy storage battery submodule are respectively configured as MMC bridge arm and direct-connected energy storage unit bridge arm according to the bridge arm structure, and the MMC bridge arm and the direct-connected energy storage unit bridge arm are arranged in parallel between the AC grid and the current-limiting inductor. The DC-side converter station integrates a control device, which includes a memory and a processor. The memory is used to store program code, which corresponds to the DC fault ride-through control method of DC direct-connected energy storage assisted flexible DC system according to any one of claims 1 to 5; The processor is used to read and execute the program code to implement the DC fault ride-through control method of the DC direct-connected energy storage assisted flexible DC system.

8. A flexible DC system based on MMC and DC-DC connected energy storage battery submodules according to claim 7, characterized in that, The DC-connected energy storage battery submodule is composed of multiple energy storage submodules connected in series.

9. A flexible DC system based on MMC and DC-DC connected energy storage battery submodules according to claim 8, characterized in that, Each energy storage submodule includes: a bypass switch, a half-bridge submodule, and a battery connected in parallel on the capacitor side, with a current-limiting inductor and a current-limiting resistor connected in series across the battery terminals; The bypass switch controls the connection of the current-limiting resistor.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium contains program code that is read and executed by a processor to implement the DC fault ride-through control method for a DC-connected energy storage-assisted flexible DC system as described in any one of claims 1 to 5.

Images