A flexible HVDC transmission system fault ride-through system and method based on directly connected energy storage device

Abstract

The application discloses a kind of based on the flexible HVDC transmission system fault ride-through system and method of direct hanging energy storage equipment.The system includes flexible HVDC transmission system, direct hanging energy storage equipment and fault ride-through control module, direct hanging energy storage equipment is directly parallel between DC bus positive and negative, and is made of multiple cascade sub-modules.Fault ride-through control module is based on voltage threshold classification control strategy by detecting DC bus voltage and energy storage battery state of charge SOC, when DC bus voltage exceeds upper threshold, control energy storage battery absorbs unbalanced power, adjusts charging current to realize smooth transition in voltage recovery process, when voltage is below rated value, control energy storage battery releases energy to maintain system stability, and realize the equalization management of multiple sub-module energy storage batteries in combination with SOC equalization control strategy.The application realizes the rapid suppression and stable control of DC bus voltage by directly introducing energy storage regulation channel on DC side, and improves the fault ride-through capability of system.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC transmission technology, specifically relating to a fault ride-through system and method for flexible DC transmission systems based on directly connected energy storage devices. Background Technology

[0002] Flexible DC transmission system is a DC transmission technology based on voltage source converter (VSC). It has advantages such as strong long-distance power transmission capability, high power flow controllability, and the ability to connect to large-scale new energy sources. It has become an important technical means for large-scale wind power transmission.

[0003] In flexible DC transmission systems, "fault ride-through" refers to the ability of the system to maintain stable DC-side voltage and ensure the continuous grid connection of DC power sources such as wind farms without disconnection when a short circuit, voltage drop, or other transient fault occurs in the AC grid at the receiving end of the system. In other words, it suppresses abnormal rises or drastic fluctuations in DC bus voltage through rapid energy regulation and power balance control, thus avoiding system outages caused by DC overvoltage protection actions.

[0004] However, when a fault occurs in the AC grid at the receiving end, the ability of the grid-side Modular Multilevel Converter (MMC) to deliver active power to the AC grid is significantly reduced or even limited. Meanwhile, the power generated by the wind farm is difficult to adjust quickly in a short period of time, resulting in a significant power imbalance on the DC bus. This causes the DC bus voltage to rise rapidly, and in severe cases, it triggers protection actions, leading to the system shutting down.

[0005] In existing technologies, the following two methods are typically used to solve the above problems: On the one hand, by configuring a resistor-based energy-consuming device on the DC side, excess electrical energy during a fault can be consumed as heat energy, thereby suppressing the rise of DC voltage. However, this method has the problem of low energy utilization, resulting in a direct waste of wind power energy. On the other hand, the technical solution disclosed in CN117277396A introduces an energy storage system control module into the system and divides the control system into an MMC control layer, an energy control layer, and an energy storage battery control layer. When a fault occurs, the energy storage battery absorbs the power surplus, and DC voltage stability is achieved through power outer loop control. However, the above-mentioned prior art still has the following shortcomings: 1) This scheme relies on a multi-level coordinated control structure of MMC control layer, energy control layer and energy storage battery control layer. The control link is long and the parameter tuning is complicated. The response speed is limited when the system disturbance changes rapidly, and it is difficult to meet the requirement of rapid suppression of DC bus voltage. 2) The energy storage system in this scheme mainly relies on the AC side or the internal structure of the MMC to participate in energy regulation. However, the direct-connected energy storage device installed on the AC side requires a large number of components, is costly, and is difficult to control, resulting in a long power transmission path, limited energy exchange efficiency, and increased system structural complexity. 3) When the charging status of the energy storage system does not meet the operating constraints, load shedding control needs to be implemented, which reduces the continuous operation capability of the system and may affect the continuous grid connection stability of the wind farm during the fault period. 4) This type of method does not fully consider the differences in the state of charge (SOC) between energy storage batteries and lacks a fine-grained energy allocation mechanism for multiple sub-modules, resulting in uneven utilization of energy storage units and affecting the overall lifespan and availability of the system.

[0006] Therefore, how to implement a fault ride-through method for flexible DC transmission systems that can directly connect to the DC bus, has a fast response speed, can be balanced and managed by the State of Charge (SOC), and can continuously and stably suppress DC bus overvoltage under the condition of AC grid faults at the receiving end has become an urgent technical problem to be solved. Summary of the Invention

[0007] The purpose of this invention is to overcome the defects of the prior art by providing a fault ride-through system and method for a flexible DC transmission system based on direct-connected energy storage devices.

[0008] The objective of this invention can be achieved through the following technical solutions: The present invention provides a fault ride-through system for a flexible DC transmission system based on a direct-connected energy storage device, comprising a flexible DC transmission system, a direct-connected energy storage device, and a fault ride-through control module; The flexible DC transmission system includes a wind farm, a transformer, a modular multilevel converter on the wind turbine side, a DC bus, a modular multilevel converter on the grid side, and an AC grid. The direct-connected energy storage device is connected between the positive and negative terminals of the DC bus. The direct-connected energy storage device includes multiple cascaded sub-modules, each of which includes a half-bridge unit, a bidirectional DC / DC converter, and an energy storage battery. The fault ride-through control module includes a voltage detection unit, a status detection unit, and a control unit. The voltage detection unit detects the DC bus voltage; the status detection unit detects the state of charge (SOC) of the energy storage battery; and the control unit is connected to the voltage detection unit, the status detection unit, and the directly connected energy storage device. The control unit is used to control the direct-connected energy storage device to start operation according to the state of charge (SOC) of the energy storage battery when the DC bus voltage exceeds a preset upper limit threshold, so that the energy storage battery absorbs the unbalanced power in the flexible DC transmission system, reduces the DC bus voltage, and controls the direct-connected energy storage device to stop operation when the DC bus voltage recovers to the rated range, thereby realizing fault ride-through of the flexible DC transmission system.

[0009] Furthermore, the wind farm is connected to the wind turbine-side modular multilevel converter via a transformer. The DC side of the wind turbine-side modular multilevel converter is connected to the DC bus, and the DC bus is connected to the AC grid via the grid-side modular multilevel converter.

[0010] Furthermore, the direct-connected energy storage device also includes a circuit breaker and a filter inductor; the circuit breaker and the filter inductor are connected in series to the DC bus, and the multiple sub-modules are connected in series between the filter inductor and the DC bus.

[0011] Furthermore, the input terminal of the half-bridge unit is connected to the DC bus, the output terminal of the half-bridge unit is connected to the input terminal of the bidirectional DC / DC converter, and the output terminal of the bidirectional DC / DC converter is connected to the energy storage battery.

[0012] Furthermore, the number of sub-modules is determined according to the following formula: in, Number of submodules; This is the DC bus voltage; This is the minimum operating voltage of the energy storage battery; This is the redundancy coefficient; This is the floor function.

[0013] Furthermore, the control unit is used to perform the following processes: Receive the DC bus voltage detected by the voltage detection unit and compare it with the DC bus voltage rating. and upper limit threshold Comparison, among which ; when When the system is determined to be in a fault state, fault passthrough control is triggered; when At the same time, the system controls the direct-connected energy storage device to maintain operation and adjusts the charging current of the energy storage battery to bring the DC bus voltage towards the rated value. convergence; when When the state of charge (SOC) of each submodule energy storage battery is detected by the state detection unit, the submodule energy storage batteries with an SOC greater than or equal to a preset lower threshold are selected to participate in discharge control, and the corresponding energy storage batteries in the direct-connected energy storage device are controlled to release energy to the DC bus to compensate for system power fluctuations or maintain DC bus voltage stability.

[0014] Furthermore, the fault ride-through control includes the following processes: The system receives the State of Charge (SOC) of the energy storage batteries in each submodule detected by the State Detection Unit, selects energy storage batteries with an SOC not greater than a preset upper limit to participate in the control, and controls the direct-connected energy storage equipment to start operation, so that the energy storage batteries in the corresponding submodules can be charged, absorb the unbalanced power in the flexible DC transmission system, and reduce the DC bus voltage.

[0015] Furthermore, during the discharge control process involving the energy storage battery, the control unit adopts a communication-free SOC equalization discharge strategy, setting the corresponding discharge current based on the SOC of each sub-module's energy storage battery. And determined according to the following relationship: in, This is the maximum discharge current of the energy storage battery. For the first The state of charge of each sub-module energy storage battery; For the first The discharge current of the energy storage battery in each submodule; This is the preset lower threshold for SOC to participate in discharge control.

[0016] Another aspect of the present invention provides a fault ride-through method for a flexible DC transmission system, comprising the following steps: The DC bus voltage is obtained through a voltage detection unit. Meanwhile, the state of charge (SOC) of each submodule's energy storage battery is obtained through the state detection unit. The DC bus voltage Respectively compared with the rated DC bus voltage and upper limit threshold Comparison, among which And determine the system operating status based on the comparison results; when When the system is determined to be in a DC overvoltage fault state, fault ride-through control is triggered, including: Select sub-module energy storage batteries with SOC less than or equal to a preset upper limit threshold to participate in control, and control the direct-connected energy storage equipment to start operation, so that the energy storage batteries can absorb the unbalanced power in the flexible DC transmission system in a charging manner, thereby reducing the DC bus voltage. when At the same time, the system controls the direct-connected energy storage device to maintain operation and adjusts the charging current of the energy storage battery to bring the DC bus voltage towards the rated value. convergence; when When the SOC is greater than or equal to the preset lower threshold, the selection criteria are: The sub-module energy storage battery participates in the discharge control, enabling the corresponding energy storage battery to release energy to the DC bus; When the DC bus voltage returns to the rated range and the AC power grid returns to normal, the directly connected energy storage device is taken out of operation, so that the flexible DC transmission system can return to normal operation.

[0017] Furthermore, during the discharge process, a communication-free SOC equalization control strategy is adopted to distribute the discharge current according to the differences in the state of charge of each sub-module energy storage battery, so that the state of charge of each sub-module energy storage battery tends to be balanced.

[0018] Compared with the prior art, the present invention has the following advantages: (1) In the prior art, when a fault occurs in the AC grid at the receiving end of a flexible DC transmission system, a DC-side resistive energy dissipation device is often used to consume the unbalanced power. This method only releases the excess electrical energy output by the wind farm in the form of heat energy, which cannot achieve effective energy utilization and results in a large waste of wind power resources. The present invention sets up a direct-connected energy storage device between the positive and negative poles of the DC bus and uses DC bus voltage detection and a SOC-based charging and discharging control strategy to absorb and store the unbalanced power when the system is overvoltaged, and releases the stored energy when the system recovers or resumes normal operation, thereby avoiding ineffective energy dissipation, improving the comprehensive utilization rate of wind power energy, and improving the energy utilization efficiency and operating economy of the flexible DC transmission system during faults.

[0019] (2) In the prior art, some solutions, such as CN117277396A, introduce a multi-level energy storage control structure into the system and rely on the AC-side MMC control system to achieve power regulation. However, this method has a long control link and a complex structure, and mainly relies on the AC side or the energy regulation path inside the MMC, resulting in a long power regulation path and limited response speed, which is not conducive to timely suppression under the condition of rapid fluctuations in DC bus voltage. This invention directly connects the direct-connected energy storage device between the positive and negative poles of the DC bus and adopts a hierarchical control strategy based on the DC bus voltage threshold (Uo and Up) to realize the rapid absorption and release of unbalanced power on the DC side by the energy storage system, shortening the energy regulation path to a direct DC side channel, thereby significantly improving the system's response speed and regulation efficiency to DC bus voltage fluctuations and enhancing the system's rapid voltage support capability under fault conditions.

[0020] (3) In the prior art, when the charging state of an energy storage system is limited during operation, it is usually necessary to avoid system instability by load shedding. This method will cause the energy storage system to shut down, reduce the system's continuous operation capability, and may affect the continuous grid connection stability of the wind farm during the fault period. The present invention sets up an energy storage battery SOC detection unit and performs graded screening of energy storage sub-modules according to the SOC threshold during the control process, so that sub-modules that meet the SOC conditions participate in charge and discharge control. This avoids overcharging and over-discharging while realizing the continuous participation of the energy storage system in regulation, avoids system shutdown due to single state constraints, and improves the continuous operation capability and grid connection stability of the flexible DC transmission system during the fault period.

[0021] (4) In the prior art, energy storage systems often do not finely control the differences in state of charge (SOC) among multiple energy storage batteries, and lack a balanced energy distribution mechanism for multiple sub-modules. This easily leads to some batteries operating at high loads for a long time while others are underutilized, thus affecting the consistency of energy storage unit lifespan and the overall system reliability. This invention adopts a communication-free SOC equalization control strategy to distribute the charging and discharging current according to the SOC of each sub-module energy storage battery. This allows batteries with higher SOC to undertake greater power output or absorption tasks, while batteries with lower SOC participate less. This achieves dynamic equalization control of multiple sub-module energy storage batteries, improves the operational consistency of the energy storage system, extends the overall battery lifespan, and improves the long-term operational reliability of the system.

[0022] (5) In the prior art, direct-connected energy storage systems are mostly set on the AC side, which requires the system to go through multiple energy conversion stages such as transformers and AC-side converters. This not only increases the number of power conversion stages, but also increases the number of devices and the system footprint, resulting in higher costs and more complex control. The present invention sets the direct-connected energy storage device directly on the DC bus side and achieves bidirectional energy flow control through a small number of IGBT switching devices and cascaded sub-module structures. This reduces intermediate energy conversion stages, reduces the overall number of devices and structural complexity of the system, thereby effectively reducing system construction costs and footprint, while improving system integration and engineering applicability. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the overall topology of the flexible DC transmission system according to an embodiment of the present invention; Figure 2 This is a general structural diagram of the direct-connected energy storage device according to an embodiment of the present invention; Figure 3 This is a structural diagram of a submodule in a direct-connected energy storage device according to an embodiment of the present invention. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0025] Example 1: This embodiment provides a flexible DC fault ride-through system based on a directly connected energy storage device, the overall topology of which is as follows: Figure 1 As shown, the system includes a wind farm, a transformer, a wind turbine-side modular multilevel converter (MMC), a DC bus, a grid-side MMC, and an AC grid. The electrical energy output from the wind farm is first collected at the first point of common coupling (PCC1), then stepped up by the transformer and connected to the wind turbine-side MMC. The wind turbine-side MMC maintains stable AC voltage and frequency at PCC1 through a control strategy, thus providing stable grid-connected voltage support for the wind farm. The DC side of the wind turbine-side MMC is connected to the DC bus, which is then connected to the second point of common coupling (PCC2) via the grid-side MMC, ultimately connecting to the AC grid. The grid-side MMC employs a constant DC voltage control strategy to maintain stable DC bus voltage, ensuring stable transmission of active power from the wind farm to the AC grid. In this system, the direct-connected energy storage device is directly connected in parallel between the positive and negative poles of the DC bus (when the DC bus is bipolar) or the neutral line (when the DC bus is unipolar) to quickly adjust the power imbalance on the DC side when disturbances or faults occur in the AC grid at the receiving end.

[0026] The wind turbine-side MMC uses a constant AC voltage / frequency control method to maintain the stability of AC voltage and frequency at the first common coupling point PCC1, thereby providing a stable voltage for the wind farm. The grid-side MMC uses a constant DC voltage control method to maintain the stability of the DC bus voltage, thereby enabling the active power generated by the wind farm to be transmitted to the AC grid. The direct-connected energy storage equipment uses a fault ride-through control method.

[0027] Direct-connected energy storage devices are used to achieve rapid power absorption and release on the DC side. Their overall structure is as follows: Figure 2As shown, the system consists of a circuit breaker Br, a filter inductor L1, and multiple sub-modules connected in series. The filter inductor L1 is positioned between the DC bus and the series branch of the sub-modules. Its function is to suppress high-frequency components caused by sudden current changes during charging and discharging, improving current smoothness, thereby reducing current stress on the switching devices and improving system stability. The circuit breaker Br is used to achieve reliable switching control between the directly connected energy storage device and the DC bus during system startup, fault isolation, or shutdown. The multiple sub-modules are cascaded in series. Each sub-module contains a half-bridge unit, a bidirectional DC / DC converter, and an energy storage battery unit. The half-bridge unit is used to achieve voltage support and bypass switching of the sub-module on the DC bus. The bidirectional DC / DC converter is used to achieve bidirectional energy flow control between the energy storage battery and the DC side, enabling the energy storage battery to absorb energy for charging during DC overvoltage and to release energy to the DC side for support when the DC voltage is low.

[0028] Considering that the terminal voltage of an energy storage battery changes with its state of charge (SOC) during charging and discharging, and that the voltage drops significantly when the SOC is low, insufficient submodule quantity may prevent meeting DC bus voltage support or charging / discharging control requirements under minimum voltage conditions. Therefore, the following formula is introduced to determine the number of submodules: in, Number of submodules; This is the DC bus voltage; This is the minimum operating voltage of the energy storage battery; This is the redundancy coefficient; This is the floor function.

[0029] By superimposing the voltages of the series submodules, the energy storage unit can still cover the DC bus voltage requirement under the lowest voltage conditions. At the same time, the redundancy factor α provides additional voltage margin, enabling the system to have stronger voltage support capability and control stability during fault transients, thereby avoiding energy storage control failure or shutdown due to insufficient voltage.

[0030] Submodule structure as follows Figure 3 As shown, each submodule includes a half-bridge unit and a bidirectional DC / DC converter, which is connected to the energy storage battery. The half-bridge unit includes a first switch VT1, a second switch VT2, and a first capacitor C1, used to support and bypass the DC bus voltage of the submodule. The bidirectional DC / DC converter includes a third switch VT3, a fourth switch VT4, a second inductor Lb, and a second capacitor C2, used to realize bidirectional energy conversion and current regulation control between the energy storage battery and the DC side.

[0031] The submodule operates in three basic states. When both the first switch VT1 and the second switch VT2 are off, the submodule is in a startup or abnormal operation state, used to pre-charge the first capacitor C1 to establish initial voltage support conditions. When the first switch VT1 is on and the second switch VT2 is off, the half-bridge unit is connected to the DC circuit, and the first capacitor C1 and the energy storage battery participate in operation. At this time, the DC bus current enters the energy storage branch, and the energy storage battery is in the charging path. This state suppresses DC voltage rise by introducing energy storage to absorb the DC-side power surplus. When the first switch VT1 is off and the second switch VT2 is on, the half-bridge unit bypasses the circuit, the first capacitor C1 and the energy storage battery are isolated, and the DC current flows directly from the positive bus to the negative bus, thereby realizing voltage regulation and branch switching of the submodule.

[0032] The energy storage battery operates in two modes via a bidirectional DC / DC converter. When the third switch VT3 is on and the fourth switch VT4 is off, the converter operates in buck mode, which introduces DC-side energy into the energy storage battery for charging. When the third switch VT3 is off and the fourth switch VT4 is on, the converter operates in boost mode, where the energy storage battery releases energy to the DC side to support the DC bus voltage.

[0033] For flexible DC transmission systems, when a short-circuit fault occurs in the receiving-end AC grid, the voltage at the second point of common coupling (PCC2) drops, and the active power delivered by the grid-side modular multilevel converter to the AC grid is reduced. Consequently, the output power decreases, and the wind farm side has difficulty reducing its output power in a short period of time. This results in a power imbalance on the DC side. Unbalanced power Represented as: in, Indicates unbalanced power on the DC side. Indicates the output power of the wind farm. This indicates the power transmitted from the grid-side converter to the AC grid. When At that time, unbalanced power If the voltage is positive, this excess energy will charge the internal capacitors of the grid-side modular multilevel converter, causing the DC bus voltage to rise rapidly. Without effective control, this will trigger DC overvoltage protection, leading to system shutdown or even disconnection of the wind farm from the grid.

[0034] To improve the continuous grid connection capability of wind farms during faults, a fault ride-through control mechanism is introduced into this system. This mechanism uses directly connected energy storage devices to rapidly absorb and regulate unbalanced power, thereby suppressing DC bus voltage rise and ensuring stable system operation during faults. To achieve this, a fault ride-through control module is installed in the system. This module includes a voltage detection unit, a status detection unit, and a control unit. The voltage detection unit collects the DC bus voltage in real time, the status detection unit obtains the state of charge (SOC) of the energy storage batteries in each sub-module, and the control unit is electrically connected to the voltage detection unit, the status detection unit, and the directly connected energy storage devices to execute corresponding control strategies based on the detection results.

[0035] During the control process, the DC bus voltage is first obtained. and compare it with the rated voltage of the DC bus. and upper limit threshold Comparison, among which The reason for setting dual thresholds is to introduce an upper threshold. As a fault triggering criterion, it can quickly initiate energy storage and absorption control when the DC voltage deviates significantly from the rated value, while the rated voltage... This serves as the adjustment target value, guiding the system to return to a stable operating state, thereby forming a hierarchical control mechanism and improving control stability and anti-disturbance capability.

[0036] when When the system is determined to be in an overvoltage fault state, fault ride-through control is initiated. During the control process, based on the SOC values ​​of the energy storage batteries in each submodule obtained by the status detection unit, the energy storage units participating in the control are screened, and submodules with an SOC not exceeding the preset upper limit are selected for operation to avoid overcharging of the energy storage batteries. In this stage, circuit breaker Br closes, connecting the direct-connected energy storage equipment to the DC bus; simultaneously, the first switch VT1 is turned on and the second switch VT2 is turned off, connecting the half-bridge unit to the DC circuit; the third switch VT3 is turned on and the fourth switch VT4 is turned off, enabling the bidirectional DC / DC converter to operate in buck mode. At this time, the DC bus energy enters the energy storage battery through the submodule, achieving constant current charging, thereby quickly absorbing unbalanced power. This suppresses the rise of DC bus voltage. The reason for using constant current charging is that it can limit the rate of change of battery charging current, avoid current surges, and at the same time achieve smooth regulation of DC voltage changes, thereby improving system stability.

[0037] When the DC bus voltage drops to During the interval, the system enters the voltage regulation phase. At this time, the directly connected energy storage device remains operational, and the DC bus voltage is gradually adjusted towards the rated value by regulating the charging current of the energy storage battery. Convergence. The focus of control in this stage is to achieve smooth voltage recovery by continuously adjusting the current, avoiding voltage oscillations caused by over-control, thereby improving the dynamic performance and stability of the system.

[0038] when At this point, the system enters the normal operation support phase. During this phase, the system is filtered based on the SOC value of each submodule's energy storage battery, selecting modules with an SOC greater than or equal to a preset lower threshold. The submodules participate in discharge control to prevent over-discharge of the energy storage battery. The third switch VT3 is turned off and the fourth switch VT4 is turned on, enabling the bidirectional DC / DC converter to operate in boost mode. The energy storage battery releases energy to the DC bus to compensate for system power fluctuations or maintain DC bus voltage stability.

[0039] During the discharge process, a communication-free SOC equalization control strategy is introduced, which allocates different discharge currents according to the SOC value of each sub-module's energy storage battery, as follows: in, This is the maximum discharge current of the energy storage battery. For the first The state of charge of each sub-module energy storage battery; For the first The discharge current of the energy storage battery in each submodule; A preset lower threshold is set for the State of Charge (SOC) to participate in discharge control. By allowing energy storage batteries with higher SOC to bear larger discharge currents and those with lower SOC to bear smaller discharge currents, adaptive balancing adjustment among energy storage units is achieved, enabling balanced energy distribution among multiple sub-modules without additional communication. This method not only reduces the complexity of the control system but also improves system reliability and effectively extends the lifespan of energy storage batteries.

[0040] Throughout the control process, the fault status on the AC grid side also needs to be monitored in real time. When the AC grid fault is detected to have been cleared and the AC voltage has returned to normal, the circuit breaker Br is opened, and the first switch VT1 is turned off while the second switch VT2 is turned on, causing the submodule to exit operation, thus achieving a smooth exit of the direct-connected energy storage device. If the fault has not been completely cleared, the charging current of the energy storage battery is reduced to stabilize the DC bus voltage at the rated value. Nearby, until the system returns to normal operation.

[0041] Through the above control process, the direct-connected energy storage equipment can realize the three functions of energy absorption, regulation and energy release in different operating stages, thereby effectively suppressing DC bus voltage fluctuations, improving the fault ride-through capability of the flexible DC transmission system under AC side fault conditions, and ensuring the continuous grid-connected operation of the wind farm.

[0042] Example 2: This embodiment provides a fault ride-through method for a flexible DC transmission system based on directly connected energy storage devices, including the following steps: Obtain DC bus voltage The system also includes the State of Charge (SOC) of each submodule's energy storage battery. The DC bus voltage reflects the system's power balance; when there is a power imbalance on the DC side, the voltage will shift. The SOC reflects the current remaining capacity of the energy storage battery, providing a basis for subsequent charge and discharge control. By simultaneously acquiring voltage and SOC information, coordinated control of voltage regulation and energy storage state management can be achieved.

[0043] DC bus voltage With DC bus rated voltage and upper limit threshold Comparison, among which The purpose of setting dual thresholds is to construct a hierarchical control mechanism. When the voltage exceeds the upper threshold, fast response control is triggered, and when the voltage is near the rated value, fine adjustment is performed, thereby avoiding frequent switching of control states and improving system stability.

[0044] when When the system is deemed to be in an overvoltage fault state, an imbalance occurs because the AC power output capacity decreases while the wind power output fails to decrease in time. By selecting sub-module energy storage batteries with a State of Charge (SOC) not exceeding a preset upper threshold for control, the energy storage units absorb excess energy through charging, thereby reducing the DC bus voltage. The upper limit of the SOC is set to prevent overcharging of the energy storage batteries and improve the operational safety of the energy storage system. During this process, by controlling the conduction state of the sub-modules and operating the bidirectional DC / DC converter in buck mode, DC-side energy flows into the energy storage batteries, enabling rapid absorption of the imbalanced power and effectively suppressing DC voltage rise.

[0045] when At this point, the system enters the voltage regulation phase. Although the DC bus voltage is higher than the rated value, it has not reached a severe overvoltage level. By adjusting the charging current of the energy storage battery, the absorbed power is gradually reduced, thereby causing the DC bus voltage to smoothly decrease and converge to the rated value. This stage, by continuously adjusting the current instead of using on / off control, avoids voltage oscillations and improves the system's dynamic performance and stability.

[0046] when At this point, the system enters the normal operation support phase. During this phase, the system is filtered based on the SOC value of each submodule's energy storage battery, selecting modules with an SOC greater than or equal to a preset lower threshold. The energy storage battery participates in discharge control to avoid over-discharge. During this stage, by controlling the bidirectional DC / DC converter to operate in boost mode, the energy storage battery releases energy to the DC bus, thereby compensating for system power fluctuations or maintaining DC bus voltage stability.

[0047] During the discharge process, a communication-free SOC equalization control strategy is adopted, allocating different discharge currents according to the state of charge of each sub-module's energy storage battery. The relationship is as follows: in, Indicates the first The discharge current of the energy storage battery in each sub-module Indicates the first The state of charge of each sub-module energy storage battery Indicates the maximum permissible discharge current. This represents the lower limit threshold of the State of Charge (SOC) for participating in discharge control. The principle behind this strategy is to allow energy storage batteries with higher SOCs to bear larger discharge currents, while those with lower SOCs bear smaller discharge currents. This enables adaptive balancing and adjustment among energy storage units without the need for communication, reducing the complexity of the control system and extending the lifespan of the energy storage batteries.

[0048] The system continuously monitors the AC grid's operating status. When the AC grid fault is cleared and the voltage returns to normal, the charging and discharging control of the energy storage device is stopped, allowing the system to return to normal operation. If the fault is not completely cleared, the energy storage regulation process is maintained, and the DC bus voltage is stabilized near the rated value by adjusting the charging and discharging current until the system returns to stability.

[0049] Through the above steps, rapid suppression and stable control of DC bus voltage under AC side fault conditions are achieved, while taking into account the safe operation and balanced management of energy storage system, thereby improving the fault ride-through capability and overall operational reliability of flexible DC transmission system.

[0050] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this invention, or the part that contributes to the prior art, or a 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 this 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.

[0051] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A fault ride-through system for a flexible DC transmission system based on directly connected energy storage equipment, characterized in that, This includes flexible DC transmission systems, direct-connected energy storage devices, and fault ride-through control modules; The flexible DC transmission system includes a wind farm, a transformer, a modular multilevel converter on the wind turbine side, a DC bus, a modular multilevel converter on the grid side, and an AC grid. The direct-connected energy storage device is connected between the positive and negative terminals of the DC bus. The direct-connected energy storage device includes multiple cascaded sub-modules, each of which includes a half-bridge unit, a bidirectional DC / DC converter, and an energy storage battery. The fault ride-through control module includes a voltage detection unit, a status detection unit, and a control unit. The voltage detection unit detects the DC bus voltage; the status detection unit detects the state of charge (SOC) of the energy storage battery; and the control unit is connected to the voltage detection unit, the status detection unit, and the directly connected energy storage device. The control unit is used to control the direct-connected energy storage device to start operation according to the state of charge (SOC) of the energy storage battery when the DC bus voltage exceeds a preset upper limit threshold, so that the energy storage battery absorbs the unbalanced power in the flexible DC transmission system, reduces the DC bus voltage, and controls the direct-connected energy storage device to stop operation when the DC bus voltage recovers to the rated range, thereby realizing fault ride-through of the flexible DC transmission system.

2. The fault ride-through system for a flexible DC transmission system based on a directly connected energy storage device according to claim 1, characterized in that, The wind farm is connected to the wind turbine-side modular multilevel converter via a transformer. The DC side of the wind turbine-side modular multilevel converter is connected to the DC bus, and the DC bus is connected to the AC grid via the grid-side modular multilevel converter.

3. The fault ride-through system for a flexible DC transmission system based on a directly connected energy storage device according to claim 1, characterized in that, The direct-connected energy storage device also includes a circuit breaker and a filter inductor; the circuit breaker and the filter inductor are connected in series to the DC bus, and the multiple sub-modules are connected in series between the filter inductor and the DC bus.

4. The fault ride-through system for a flexible DC transmission system based on directly connected energy storage equipment according to claim 1, characterized in that, The input terminal of the half-bridge unit is connected to the DC bus, the output terminal of the half-bridge unit is connected to the input terminal of the bidirectional DC / DC converter, and the output terminal of the bidirectional DC / DC converter is connected to the energy storage battery.

5. A fault ride-through system for a flexible DC transmission system based on directly connected energy storage equipment according to claim 1, characterized in that, The number of submodules is determined according to the following formula: in, Number of submodules; This is the DC bus voltage; This is the minimum operating voltage of the energy storage battery; This is the redundancy coefficient; This is the floor function.

6. The fault ride-through system for a flexible DC transmission system based on a directly connected energy storage device according to claim 1, characterized in that, The control unit is used to perform the following processes: Receive the DC bus voltage detected by the voltage detection unit and compare it with the DC bus voltage rating. and upper limit threshold Comparison, among which ; when When the system is determined to be in a fault state, fault passthrough control is triggered; when At the same time, the system controls the direct-connected energy storage device to maintain operation and adjusts the charging current of the energy storage battery to bring the DC bus voltage towards the rated value. convergence; when When the state of charge (SOC) of each submodule energy storage battery is detected by the state detection unit, the submodule energy storage batteries with an SOC greater than or equal to a preset lower threshold are selected to participate in discharge control, and the corresponding energy storage batteries in the direct-connected energy storage device are controlled to release energy to the DC bus to compensate for system power fluctuations or maintain DC bus voltage stability.

7. A fault ride-through system for a flexible DC transmission system based on a directly connected energy storage device according to claim 6, characterized in that, The fault ride-through control The process includes the following: The system receives the State of Charge (SOC) of the energy storage batteries in each submodule detected by the State Detection Unit, selects energy storage batteries with an SOC not greater than a preset upper limit to participate in the control, and controls the direct-connected energy storage equipment to start operation, so that the energy storage batteries in the corresponding submodules can be charged, absorb the unbalanced power in the flexible DC transmission system, and reduce the DC bus voltage.

8. A fault ride-through system for a flexible DC transmission system based on a directly connected energy storage device according to claim 6, characterized in that, During the discharge control process involving the energy storage battery, the control unit adopts a communication-free SOC equalization discharge strategy, setting the corresponding discharge current based on the SOC of each sub-module's energy storage battery. And determined according to the following relationship: in, This is the maximum discharge current of the energy storage battery. For the first The state of charge of each sub-module energy storage battery; For the first Discharge current of each sub-module energy storage battery; This is the preset lower threshold for SOC to participate in discharge control.

9. A method for fault ride-through of a flexible DC transmission system based on any one of the flexible DC transmission system fault ride-through systems described in claims 1-8, characterized in that, Includes the following steps: The DC bus voltage is obtained through a voltage detection unit. Meanwhile, the state of charge (SOC) of each submodule's energy storage battery is obtained through the state detection unit. The DC bus voltage Respectively compared with the rated DC bus voltage and upper limit threshold Comparison, among which And determine the system operating status based on the comparison results; when When the system is determined to be in a DC overvoltage fault state, fault ride-through control is triggered, including: Select sub-module energy storage batteries with SOC less than or equal to a preset upper limit threshold to participate in control, and control the direct-connected energy storage equipment to start operation, so that the energy storage batteries can absorb the unbalanced power in the flexible DC transmission system in a charging manner, thereby reducing the DC bus voltage. when At the same time, the system controls the direct-connected energy storage device to maintain operation and adjusts the charging current of the energy storage battery to bring the DC bus voltage towards the rated value. convergence; when When the SOC is greater than or equal to the preset lower threshold, the selection criteria are: The sub-module energy storage battery participates in the discharge control, enabling the corresponding energy storage battery to release energy to the DC bus; When the DC bus voltage returns to the rated range and the AC power grid returns to normal, the directly connected energy storage device is taken out of operation, so that the flexible DC transmission system can return to normal operation.

10. A fault ride-through method for a flexible DC transmission system according to claim 9, characterized in that, During the discharge process, a communication-free SOC equalization control strategy is adopted to distribute the discharge current according to the differences in the state of charge of each sub-module energy storage battery, so that the state of charge of each sub-module energy storage battery tends to be balanced.

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