DC-DC energy storage system suitable for offshore wind power energy storage and surplus power absorption

Abstract

This invention proposes a DC-connected energy storage system suitable for offshore wind power energy storage and surplus power absorption. The energy storage system consists of multiple identical sub-modules connected in series. Each sub-module includes a battery energy storage module and a half-bridge power module. The battery energy storage module is used for storing and releasing electrical energy to stabilize the DC bus voltage. The half-bridge power module includes an inductor, capacitor, IGBT (Insulated Gate Bipolar Transistor), a high-capacity diode, and a thyristor, used to control the connection and disconnection of the battery energy storage module. The half-bridge structure of the half-bridge power module circuit uses a high-capacity diode connected in reverse parallel to the upper transistor to increase the current carrying capacity, and a thyristor connected in forward parallel to the lower transistor to facilitate the disconnection of the sub-module. This invention is beneficial for the safe and stable operation of the system and can achieve expansion to multiple voltage levels.

Description

Technical Field

[0001] This invention relates to the fields of offshore wind power and battery energy storage technology, and in particular to a DC direct-connected energy storage system suitable for offshore wind power energy storage and surplus power absorption. Background Technology

[0002] Offshore wind energy resources boast advantages such as abundant reserves, high power generation efficiency, proximity to load centers, and minimal land occupation. Offshore wind power generation represents the forefront and pinnacle of wind power technology and is widely considered the future direction of the power generation industry. However, offshore wind energy is characterized by high randomness and strong fluctuations, and large-scale grid connection of wind farms can adversely affect the operation of the power system.

[0003] Energy storage technology can resolve the contradiction between the intermittency and volatility of wind power and the need for real-time balance in the power system, improving the controllability of the output power of large-scale wind power bases. Among various energy storage technologies, battery energy storage is not limited by geographical or geological conditions, is flexible in installation, and is convenient to implement, making it the main form of energy storage application in the field of new energy power generation.

[0004] The power conversion system (PCS) is the intermediate link between DC batteries and the grid connection, and is the core of energy control in energy storage systems.

[0005] Currently, there is considerable research and application of AC energy storage converter topologies both domestically and internationally. In terms of power levels, AC energy storage converters are divided into single-stage and two-stage types; in terms of topology, current mainstream applications and research focus on two-level, three-level, and multi-level structures. Multi-level converters are further divided into Cascaded H-Bridge (CHB) and Modular Multilevel Converter (MMC). Different topologies can achieve different battery energy storage system capacity levels.

[0006] The parallel boosting scheme of low-voltage energy storage converters in AC energy storage is the most mature and widely used in engineering. The cascaded H-bridge (CHB) AC direct-connection scheme connects the energy storage system directly to the medium-voltage grid without a transformer, reducing losses and costs. This scheme is relatively mature in research and has seen breakthroughs in engineering applications. Since its initial proposal in 2002, the MMC topology has received widespread attention for its application in lightweight DC transmission. Flexible DC transmission technology using the MMC topology (MMC-HVDC) has become the preferred topology for current flexible DC transmission projects. Because half-bridge MMC requires fewer components and has lower operating losses, all MMC DC transmission projects already in operation both domestically and internationally adopt the half-bridge structure.

[0007] Currently, there are two schemes for DC-side energy storage. One is the Modular Multilevel Converter based Battery Energy Storage System (MMC-BESS), where the energy storage device is integrated inside the MMC submodule. Based on the battery arrangement, it can be divided into two types: centralized arrangement on a common DC bus and distributed arrangement within submodules. The other is a DC-connected energy storage system, where multiple energy storage submodules are connected in series and then directly connected in parallel between the positive and negative terminals of the DC bus.

[0008] MMC-BESS is a promising topology for large-scale battery energy storage, and research on it both domestically and internationally is still in its early stages. Current research mainly focuses on modulation methods, power control, battery equalization, two-stage capacitor voltage balancing, circulating current control, eliminating battery current fluctuations, and DC fault protection and handling. Currently, MMC-BESS is largely in the research and demonstration engineering phase, and further in-depth research and application are needed.

[0009] Currently, there is limited research on DC-connected energy storage, and no engineering application cases of DC-connected devices exist. One paper employs an improved nearest-level approximation strategy to design a DC-connected energy storage device with three modules, and experimental verification has been conducted.

[0010] Offshore wind power generation often uses overhead lines for AC transmission to the onshore grid, making transient grounding faults relatively common. After a grid fault, the voltage at the grid connection point drops, resulting in a significant reduction in the power transmission capacity of the receiving-end converter station. The surplus power accumulates on the DC transmission lines, causing the DC transmission voltage to rise rapidly, triggering overvoltage protection and causing the system to shut down.

[0011] To address the issue of DC-side overvoltage caused by power surplus, researchers have proposed solutions primarily from the aspects of reducing the power transmitted from new energy sources, dissipating surplus power, and temporarily storing or transferring surplus power.

[0012] In current practical engineering, the main method used is to dissipate excess power and suppress the rise of DC voltage by configuring energy-dissipating resistors. Considering the high cost of offshore platforms, energy-dissipating devices are generally installed on the DC side of onshore receiving-end converter stations. Early DC energy-dissipating devices consisted of IGBT series valves and energy-dissipating resistors, with the switching of the energy-dissipating resistors controlled by setting upper and lower thresholds of the DC voltage. As the voltage level of DC systems has increased, the IGBT series valves have been replaced by series circuits of half-bridge or full-bridge submodules. Depending on the arrangement of the energy-dissipating resistors, there are three types: centralized MMC-DBS, distributed MMC-DBS, and hybrid MMC-DBS.

[0013] Research on reducing the power output of new energy sources, temporarily storing surplus power, and transferring surplus power is in its early stages, and the relevant theories need to be further improved.

[0014] The main measure to dissipate surplus power is to configure energy-dissipating resistors of appropriate capacity in the converter station, so that the surplus power is dissipated as heat through the resistors. Depending on the installation location and switching method of the energy-dissipating resistors, they can be divided into: energy-dissipating devices configured on the AC side of the converter station and energy-dissipating devices configured on the DC side. Based on the arrangement of the energy-dissipating resistors, DC-side energy-dissipating devices can be further divided into three types: centralized configuration, distributed configuration, and hybrid configuration.

[0015] Regarding battery power storage for surplus power, current research primarily employs a storage-type MMC topology. Its converter station topology is consistent with the traditional MMC topology, except for a change in the submodule structure: one half is a half-bridge structure, and the other half consists of a DC-DC converter and an energy storage device. The DC-DC converter is connected in parallel with the submodule capacitor, allowing control over the energy storage device's absorption or release of energy. By controlling the DC-DC converter to charge or discharge the battery, surplus or deficit power can be balanced, maintaining a stable DC voltage.

[0016] Regarding surplus power transfer, the main approach is to transfer the surplus power to a nearby AC grid or other converter stations at a distant location. The strategy for connecting renewable energy bases to the AC grid involves operating the renewable energy base in parallel with the local AC grid at the sending end. When a power surplus occurs, the surplus power generated by the renewable energy source can be transferred to the local AC grid, thus achieving surplus power absorption. Power transfer between converter stations is mainly applied to multi-terminal flexible DC systems with multiple receiving points. When a fault occurs at one receiving end, the surplus power is transferred to other receiving ends.

[0017] Methods for reducing the output power of renewable energy power plants generally suffer from drawbacks such as slow response and limited power reduction effect. Transferring surplus power requires the cooperation of a large AC power grid, which is difficult to implement as renewable energy bases are typically located in remote inland or offshore areas. Storing surplus power is costly, requires more hardware, and involves complex control algorithms; however, with the increase in battery energy storage density and the decrease in cost, energy storage methods show promising prospects.

[0018] The above three methods are still in the theoretical research stage and have not yet been applied and verified in engineering. At present, the common practice in engineering is to install energy-dissipating resistors on the AC side of the sending-end converter station to absorb surplus power.

[0019] In existing technologies, the parallel boosting scheme of low-voltage AC energy storage converters has problems such as high losses, difficulty in coordinating multiple units, and high cost.

[0020] The cascaded H-bridge AC direct-connection scheme lacks a common DC bus, thus it can only be applied to AC power grids, and suffers from circulating current suppression and battery-side secondary pulsating current smoothing issues. It cannot be used to absorb surplus power on DC lines.

[0021] Research on surplus power absorption in battery storage based on MMC-BESS is still in its early stages. In the MMC-BESS topology, battery energy storage is highly coupled within the MMC submodule. A battery failure could lead to converter shutdown. Furthermore, the MMC converter itself has three phase units, each consisting of an upper arm and a lower arm. Each arm requires an energy storage device, significantly increasing the number of storage batteries and the cost of the battery and battery management system (BMS), resulting in poor economic efficiency.

[0022] The current engineering project mainly uses a power-absorbing resistor scheme to absorb surplus power, which has the following problems:

[0023] 1) At the equipment level, DC power dissipation devices significantly increase the construction cost of DC systems and have low utilization rates. Currently available MMC-DBS DC power dissipation devices all require a large number of capacitors, greatly increasing the footprint, size, and cost of the devices. Furthermore, a separate unloading station is required. Considering the additional design needed for the unloading station's location, wiring, and insulation, the construction cost of existing DC power dissipation devices is very high, sometimes accounting for up to 40% of the cost of a single converter station.

[0024] 2) At the control level, under the current control strategy of wind power-flexible DC grid-connected systems, the receiving-end converter station exhibits the characteristics of a controlled current source to the grid, while the entire wind power-flexible DC system appears as a current source without inertia. With the rapid increase in the proportion of renewable energy in the power system, the inertia-free characteristic of current-type control reduces the frequency stability of the power system, and the grid synchronization mechanism based on phase-locked loops is also difficult to adapt to weak grid operating conditions. The current wind power-flexible DC system cannot serve as a dominant power source and is insufficient to support the operation of a new type of power system dominated by new energy sources.

[0025] Transferring surplus power to nearby AC grids requires the support of a strong local AC grid, which is difficult to achieve in remote areas rich in renewable energy. Transferring surplus power to other converter stations at distant locations requires all receiving-end converter stations to have a certain power margin to accommodate the surplus power. This means that under normal circumstances, converter stations are operating at reduced capacity, resulting in a decrease in their capacity utilization. Summary of the Invention

[0026] This invention proposes a DC direct-connected energy storage system suitable for offshore wind power energy storage and surplus power absorption, which is conducive to the safe and stable operation of the system and can be expanded to multiple voltage levels.

[0027] The present invention adopts the following technical solution.

[0028] A DC-connected energy storage system suitable for offshore wind power energy storage and surplus power absorption, wherein the energy storage system is composed of multiple identical sub-modules connected in series; the sub-modules include battery energy storage modules and half-bridge power modules.

[0029] The battery energy storage module is used for storing and releasing electrical energy to stabilize the DC bus voltage;

[0030] The half-bridge power module includes an inductor, a capacitor, an insulated gate bipolar transistor (IGBT), a high-capacity diode, and a thyristor, used to control the activation and deactivation of the battery energy storage module. The half-bridge structure of the circuit of the half-bridge power module uses a high-capacity diode connected in reverse parallel with its upper transistor to increase the current carrying capacity, and a thyristor connected in forward parallel with its lower transistor to facilitate the deactivation of the sub-module.

[0031] In the half-bridge structure, multiple diodes are connected in reverse parallel to the upper diode, so that the current carrying capacity of the first parallel structure is more than five times that of the upper diode, thereby enhancing the current path when the DC bus has power surplus.

[0032] The lower tube of the half-bridge structure is connected in forward parallel with a thyristor with strong current-carrying capacity, so that the current-carrying capacity of the second parallel structure is more than five times that of the lower tube, which is used to disconnect the battery energy storage module connected in parallel with the half-bridge from the bypass.

[0033] The series connection method between each submodule is as follows: a first connection point (1) is provided between the upper and lower tubes of the half-bridge structure of the submodule, which is connected to the upper and lower tubes. The first connection point is the series input terminal of the submodule. A second connection point (2) is provided at the lower tube of the half-bridge structure of the submodule. The second connection point is the series output terminal of the submodule. The output terminal of the upper submodule is electrically connected to the input terminal of the lower submodule.

[0034] The battery energy storage module includes a battery, switch K1, switch K2, and a resistor; the circuit formed by the series connection of switch K2 and the resistor is connected in parallel with switch K1, and the two ends of the parallel circuit are respectively connected to the battery and the half-bridge power module.

[0035] In the half-bridge power module, the circuit after the capacitor and the half-bridge structure circuit are connected in parallel is connected to the battery energy storage module through an inductor.

[0036] The energy storage system operates in two modes: normal operation and surplus power operation. Before operation, K2 is closed to charge the capacitor via the energy storage battery. The resistor connected in series in branch K2 limits the current. After the capacitor is fully charged, K1 is closed and K2 is opened.

[0037] The specific operation method of the normal operation mode is as follows: In the half-bridge structure, only one of the upper and lower transistors can be turned on at any given time; when the upper transistor is turned on, the battery energy storage module is in the activated state, and the output voltage of the submodule is equal to the battery voltage of the submodule; when the lower transistor is turned on, the battery energy storage module is in the deactivated state, and the battery of the submodule is bypassed, and the output voltage of the submodule is zero; when both transistors are turned off, the submodule is in the locked state, and the locked state is used for submodule capacitor charging or fault state; in the normal operation mode, in order to control the charging and discharging current, the current reference value is calculated based on the power and DC voltage, and then the duty cycle D is generated by PI control to adjust the conduction frequency of the switching transistor.

[0038] The specific operation method of surplus power operation mode is as follows: When the DC bus power is surplus, current needs to be injected into the sub-module, that is, the DC bus charges the battery; at this time, the current path is the diode connected in parallel with the upper tube. The current injected in surplus power operation mode requires that the current carrying capacity of the diode connected in parallel with the upper tube should be more than 5 times that of the upper tube to be able to carry it; when the bus power is surplus and the voltage rises to the set threshold, the number of sub-modules to be put into operation x is calculated according to the set voltage threshold and the current sub-module voltage. The number of sub-modules x is not greater than the total number of sub-modules n; x sub-modules are selected to be put into operation according to the size of the battery's rechargeable current; for the sub-modules to be put into operation, the upper and lower tubes of the sub-module half-bridge structure are both blocked, and the bus current charges the battery through the diode connected in parallel with the upper tube; for the sub-modules that do not need to be put into operation, the lower tube and thyristor of the sub-module are turned on, and the sub-module is bypassed.

[0039] The surplus power operation mode is used for handling transient faults in the AC main grid. After the transient fault in the AC main grid is eliminated, the DC direct-connected energy storage system implements a strategy to exit surplus power protection based on the thyristor current flow direction in order to safely exit the surplus power operation mode. Specifically, it turns off the thyristors connected in the forward parallel of the lower tube; in the discharge mode, the thyristors are subjected to reverse voltage, and the system naturally exits the surplus power operation mode; in the charging mode, it waits for the AC side fault to be eliminated, or for the DC side to naturally exit the charging mode, and the current flowing through the thyristors drops below the holding current, and the system resumes normal operation mode.

[0040] The upper tube of the half-bridge structure is the upper bridge arm, and the lower tube is the lower bridge arm. The DC-connected energy storage system achieves multi-voltage level expansion through modular series connection. The energy storage device of the DC-connected energy storage system is installed on the DC side to avoid the adverse effects of energy storage battery failure on the modular multilevel converter in the connected line. The DC-connected energy storage system uses the DC-connected energy storage device to perform surplus power absorption during offshore wind power operations, forming a high-voltage DC-connected energy storage system with a half-bridge cascaded topology that can absorb surplus power.

[0041] The advantages of this invention are:

[0042] 1. Through modular series design, expansion to multiple voltage levels can be achieved;

[0043] 2. The energy storage device is directly installed on the DC side, avoiding the adverse effects of energy storage battery failure on the modular multilevel converter;

[0044] 3. The number of energy storage batteries required for DC direct-connected energy storage is only 1 / 6 that of MMC topology, greatly reducing the cost of energy storage batteries and BMS (Battery Management System);

[0045] 4. The use of DC-connected energy storage devices for surplus power absorption eliminates the need for dedicated surplus power absorption devices, saving investment costs, reducing the footprint, and avoiding resource waste.

[0046] 5. Connecting diodes and thyristors with strong current-carrying capacity in parallel is beneficial to the safe and stable operation of the system. Attached Figure Description

[0047] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0048] Appendix Figure 1 This is a schematic diagram illustrating how the present invention achieves multi-voltage level expansion through a modular series design;

[0049] Appendix Figure 2 This is a schematic diagram of the serial connection method of the sub-modules of the present invention;

[0050] Appendix Figure 3 This is a schematic diagram of the circuit structure of a submodule of the present invention. Detailed Implementation

[0051] A DC-connected energy storage system suitable for offshore wind power energy storage and surplus power absorption, wherein the energy storage system is composed of multiple identical sub-modules connected in series; the sub-modules include battery energy storage modules and half-bridge power modules.

[0052] The battery energy storage module is used for storing and releasing electrical energy to stabilize the DC bus voltage;

[0053] The half-bridge power module includes an inductor, a capacitor, an insulated gate bipolar transistor (IGBT), a high-capacity diode, and a thyristor, used to control the activation and deactivation of the battery energy storage module. The half-bridge structure of the circuit of the half-bridge power module uses a high-capacity diode connected in reverse parallel with its upper transistor to increase the current carrying capacity, and a thyristor connected in forward parallel with its lower transistor to facilitate the deactivation of the sub-module.

[0054] In the half-bridge structure, multiple diodes are connected in reverse parallel to the upper diode, so that the current carrying capacity of the first parallel structure is more than five times that of the upper diode, thereby enhancing the current path when the DC bus has power surplus.

[0055] The lower tube of the half-bridge structure is connected in forward parallel with a thyristor with strong current-carrying capacity, so that the current-carrying capacity of the second parallel structure is more than five times that of the lower tube, which is used to disconnect the battery energy storage module connected in parallel with the half-bridge from the bypass.

[0056] The series connection method between the sub-modules is as follows: the upper and lower tubes of the half-bridge structure of the sub-module are provided with a first connection point 1 connected to the upper and lower tubes, and the first connection point is the series input terminal of the sub-module; the lower tube of the half-bridge structure of the sub-module is provided with a second connection point 2, and the second connection point is the series output terminal of the sub-module; the output terminal of the upper sub-module is electrically connected to the input terminal of the lower sub-module.

[0057] The battery energy storage module includes a battery, switch K1, switch K2, and a resistor; the circuit formed by the series connection of switch K2 and the resistor is connected in parallel with switch K1, and the two ends of the parallel circuit are respectively connected to the battery and the half-bridge power module.

[0058] In the half-bridge power module, the circuit after the capacitor and the half-bridge structure circuit are connected in parallel is connected to the battery energy storage module through an inductor.

[0059] The energy storage system operates in two modes: normal operation and surplus power operation. Before operation, K2 is closed to charge the capacitor via the energy storage battery. The resistor connected in series in branch K2 limits the current. After the capacitor is fully charged, K1 is closed and K2 is opened.

[0060] The specific operation method of the normal operation mode is as follows: In the half-bridge structure, only one of the upper and lower transistors can be turned on at any given time; when the upper transistor is turned on, the battery energy storage module is in the activated state, and the output voltage of the submodule is equal to the battery voltage of the submodule; when the lower transistor is turned on, the battery energy storage module is in the deactivated state, and the battery of the submodule is bypassed, and the output voltage of the submodule is zero; when both transistors are turned off, the submodule is in the locked state, and the locked state is used for submodule capacitor charging or fault state; in the normal operation mode, in order to control the charging and discharging current, the current reference value is calculated based on the power and DC voltage, and then the duty cycle D is generated by PI control to adjust the conduction frequency of the switching transistor.

[0061] The specific operation method of surplus power operation mode is as follows: When the DC bus power is surplus, current needs to be injected into the sub-module, that is, the DC bus charges the battery; at this time, the current path is the diode connected in parallel with the upper tube. The current injected in surplus power operation mode requires that the current carrying capacity of the diode connected in parallel with the upper tube should be more than 5 times that of the upper tube to be able to carry it; when the bus power is surplus and the voltage rises to the set threshold, the number of sub-modules to be put into operation x is calculated according to the set voltage threshold and the current sub-module voltage. The number of sub-modules x is not greater than the total number of sub-modules n; x sub-modules are selected to be put into operation according to the size of the battery's rechargeable current; for the sub-modules to be put into operation, the upper and lower tubes of the sub-module half-bridge structure are both blocked, and the bus current charges the battery through the diode connected in parallel with the upper tube; for the sub-modules that do not need to be put into operation, the lower tube and thyristor of the sub-module are turned on, and the sub-module is bypassed.

[0062] The surplus power operation mode is used for handling transient faults in the AC main grid. After the transient fault in the AC main grid is eliminated, the DC direct-connected energy storage system implements a strategy to exit surplus power protection based on the thyristor current flow direction in order to safely exit the surplus power operation mode. Specifically, it turns off the thyristors connected in the forward parallel of the lower tube; in the discharge mode, the thyristors are subjected to reverse voltage, and the system naturally exits the surplus power operation mode; in the charging mode, it waits for the AC side fault to be eliminated, or for the DC side to naturally exit the charging mode, and the current flowing through the thyristors drops below the holding current, and the system resumes normal operation mode.

[0063] The upper tube of the half-bridge structure is the upper bridge arm, and the lower tube is the lower bridge arm. The DC-connected energy storage system achieves multi-voltage level expansion through modular series connection. The energy storage device of the DC-connected energy storage system is installed on the DC side to avoid the adverse effects of energy storage battery failure on the modular multilevel converter in the connected line. The DC-connected energy storage system uses the DC-connected energy storage device to perform surplus power absorption during offshore wind power operations, forming a high-voltage DC-connected energy storage system with a half-bridge cascaded topology that can absorb surplus power.

[0064] In energy storage projects, the rechargeable current of a battery is provided by the battery management system (BMS) via communication. The BMS calculates the rechargeable current based on the battery's state of charge (SOC) information, combined with battery temperature and the highest and lowest individual cell voltages.

Claims

1. A DC-connected energy storage system suitable for offshore wind power energy storage and surplus power absorption, characterized in that: The energy storage system is composed of multiple identical sub-modules connected in series; the sub-modules include battery energy storage modules and half-bridge power modules. The battery energy storage module is used for storing and releasing electrical energy to stabilize the DC bus voltage; The half-bridge power module includes an inductor, a capacitor, an insulated gate bipolar transistor (IGBT), a high-capacity diode, and a thyristor, used to control the activation and deactivation of the battery energy storage module. The half-bridge structure of the circuit of the half-bridge power module has a high-capacity diode connected in reverse parallel with its upper transistor to increase the current carrying capacity, and a thyristor connected in forward parallel with its lower transistor to facilitate the deactivation of the sub-module. The series connection method between each submodule is as follows: a first connection point (1) is provided between the upper and lower tubes of the half-bridge structure of the submodule, which is connected to the upper and lower tubes. The first connection point is the series input terminal of the submodule; a second connection point (2) is provided at the lower tube of the half-bridge structure of the submodule, which is the series output terminal of the submodule. The output terminal of the upper submodule is electrically connected to the input terminal of the lower submodule. The energy storage system operates in two modes: normal operation mode and surplus power operation mode. Before the energy storage system is put into operation, K2 is closed first, and the capacitor is charged through the energy storage battery. The resistor connected in series in the K2 branch plays a current limiting role. After the capacitor is fully charged, close K1 and open K2. The specific operation method of the normal operation mode is as follows: In the half-bridge structure, only one of the upper and lower transistors can be turned on at any given time; when the upper transistor is turned on, the battery energy storage module is in the activated state, and the output voltage of the submodule is equal to the battery voltage of the submodule; when the lower transistor is turned on, the battery energy storage module is in the deactivated state, and the battery of the submodule is bypassed, and the output voltage of the submodule is zero; when both transistors are turned off, the submodule is in the locked state, and the locked state is used for submodule capacitor charging or fault state; in the normal operation mode, in order to control the charging and discharging current, the current reference value is calculated based on the power and DC voltage, and then the duty cycle D is generated by PI control to adjust the conduction frequency of the switching transistor; The specific operation method of surplus power operation mode is as follows: When the DC bus power is surplus, current needs to be injected into the sub-module, that is, the DC bus charges the battery; at this time, the current path is the diode connected in parallel with the upper tube. The current injected in surplus power operation mode requires that the current carrying capacity of the diode connected in parallel with the upper tube should be more than 5 times that of the upper tube to be able to carry it; when the bus power is surplus and the voltage rises to the set threshold, the number of sub-modules to be put into operation x is calculated according to the set voltage threshold and the current sub-module voltage. The number of sub-modules x is not greater than the total number of sub-modules n; sorted according to the rechargeable current of the battery, x sub-modules are selected for operation; for the sub-modules to be put into operation, the upper and lower tubes of the sub-module half-bridge structure are locked, and the bus current charges the battery through the diode connected in parallel with the upper tube; for the sub-modules that do not need to be put into operation, the lower tube and thyristor of the sub-module are turned on, and the sub-module is bypassed. The upper tube of the half-bridge structure is the upper bridge arm, and the lower tube is the lower bridge arm. The DC-connected energy storage system achieves multi-voltage level expansion through modular series connection. The energy storage device of the DC-connected energy storage system is installed on the DC side to avoid the adverse effects of energy storage battery failure on the modular multilevel converter in the connected line. The DC-connected energy storage system uses the DC-connected energy storage device to perform surplus power absorption during offshore wind power operations, forming a high-voltage DC-connected energy storage system with a half-bridge cascaded topology that can absorb surplus power.

2. The DC-connected energy storage system suitable for offshore wind power energy storage and surplus power absorption according to claim 1, characterized in that: In the half-bridge structure, multiple diodes are connected in reverse parallel to the upper diode, so that the current carrying capacity of the first parallel structure is more than five times that of the upper diode, thereby enhancing the current path when the DC bus has power surplus.

3. The DC direct-connected energy storage system suitable for offshore wind power energy storage and surplus power absorption according to claim 1, characterized in that: The lower tube of the half-bridge structure is connected in forward parallel with a thyristor with strong current-carrying capacity, so that the current-carrying capacity of the second parallel structure is more than five times that of the lower tube, which is used to disconnect the battery energy storage module connected in parallel with the half-bridge from the bypass.

4. The DC-connected energy storage system suitable for offshore wind power energy storage and surplus power absorption according to claim 1, characterized in that: The battery energy storage module includes a battery, switch K1, switch K2, and a resistor; the circuit formed by the series connection of switch K2 and the resistor is connected in parallel with switch K1, and the two ends of the parallel circuit are respectively connected to the battery and the half-bridge power module. In the half-bridge power module, the circuit after the capacitor and the half-bridge structure circuit are connected in parallel is connected to the battery energy storage module through an inductor.

5. The DC-connected energy storage system suitable for offshore wind power energy storage and surplus power absorption according to claim 1, characterized in that: The surplus power operation mode is used for handling transient faults in the AC main grid. After the transient fault in the AC main grid is eliminated, the DC direct-connected energy storage system implements a strategy to exit surplus power protection based on the thyristor current flow direction in order to safely exit the surplus power operation mode. Specifically, it turns off the thyristors connected in the forward parallel of the lower tube; in the discharge mode, the thyristors are subjected to reverse voltage, and the system naturally exits the surplus power operation mode; in the charging mode, it waits for the AC side fault to be eliminated, or for the DC side to naturally exit the charging mode, and the current flowing through the thyristors drops below the holding current, and the system resumes normal operation mode.

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