Multi-terminal flexible DC system network connection and islanding switching method and system based on adaptive network construction

Abstract

The present invention discloses a multi-terminal flexible DC system networking and islanding switching method and system based on adaptive network formation, which relates to the field of power technology. Currently, there is a lack of an adaptive networking and islanding switching control method for dynamic characteristics such as grid frequency and power fluctuations, and it is unable to effectively cope with the system instability risks brought about by the increasing proportion of new energy. The present invention simulates the rotor motion equation of a synchronous generator through adaptive network formation technology to obtain the virtual rotational speed and inertia time constant, so as to adjust the grid frequency and system frequency response; when it is monitored that the grid frequency change exceeds a predetermined threshold, the system enters the temporary island control mode and switches to the corresponding control mode according to the islanding signal of the stability control system; during the entire switching process, the system dynamically adjusts the virtual rotational speed and damping coefficient. This technical solution ensures frequency stability and power balance, thereby realizing safe and stable networking and islanding switching, and improving the stability and adaptability of the system in a weak grid environment.

Description

Technical Field

[0001] The present invention relates to the field of power technology, and particularly to a multi-terminal flexible DC system networking island switching method based on adaptive network formation. Background Art

[0002] As a new power transmission technology, flexible DC transmission adopts the voltage source converter (VSC-HVDC) technology of fully controlled power electronic devices to achieve efficient DC power transmission. The traditional networking control of flexible DC systems usually adopts the vector control method based on the phase-locked loop (PLL). By sampling the grid-connected voltage phase and coordinate transformation, the output current is controlled to achieve the regulation of active and reactive power. This control method exhibits current source characteristics and is mainly used for grid-connected control with a strong grid. The control framework is usually classified as grid-following control.

[0003] However, with the gradual decrease in the proportion of synchronous generators in the power grid, the strength of the power grid has been significantly weakened. Under extremely weak system conditions, the traditional PLL method may not be able to accurately track the grid voltage due to its strong dependence on the grid frequency and phase, resulting in instability problems during small disturbances. This problem is particularly prominent in areas with a large amount of new energy connected to the grid, and the volatility and uncertainty of new energy increase the difficulty of grid control.

[0004] To address these challenges, scholars at home and abroad have proposed the concept of "converter network-forming control", aiming to improve the stability of flexible DC systems in weak grids by simulating the inertial characteristics and voltage support capabilities of the power grid. However, existing research mainly focuses on steady-state operation control strategies and does not involve how to perform adaptive networked island switching control when the system enters the networked island state. Therefore, in view of the dynamic characteristics such as grid frequency fluctuations and power fluctuations, it is urgent to propose a networked island switching method with adaptive functions to ensure the safe and stable operation of flexible DC systems under complex conditions, especially in the context of the increasing proportion of new energy generation. Summary of the Invention

[0005] The technical problem to be solved and the technical task proposed by the present invention are to improve and refine the existing technical solutions, and provide a multi-terminal flexible DC system networking island switching method based on adaptive network formation, aiming to ensure the frequency stability and power balance of the system when switching from the networked state to the island state. To this end, the present invention adopts the following technical solutions.

[0006] A multi-terminal flexible DC system networking island switching method based on adaptive network formation includes the following steps:

[0007] 1) Using the power synchronization link to simulate the rotor motion equation of the synchronous generator through adaptive network formation technology to obtain the virtual speed ω and the inertia time constant T J, where the virtual speed ω is used to adjust the grid frequency, and the inertia time constant T J is used to adjust the inertia characteristic of the system frequency response;

[0008] 2) Monitor the grid frequency in real time, and judge whether the grid enters the island state according to the change of the grid frequency;

[0009] 3) When the change of the grid frequency exceeds the predetermined threshold, enter the temporary island control mode;

[0010] 4) After a period of delay, if the flexible DC system receives the island signal from the stability control system, it is confirmed that the grid is in the island state, and switch to the island control mode, and adjust the damping coefficient of the grid-forming control algorithm;

[0011] 5) If the flexible DC system does not receive the island signal from the stability control system, it is confirmed that the grid is still in the grid-connected state, maintain the grid-connected control mode, and adjust the damping coefficient of the grid-forming control algorithm;

[0012] 6) During the entire switching process, dynamically adjust the virtual speed and damping coefficient according to the fluctuations of the grid frequency and power to ensure the frequency stability and power balance of the system in the grid-connected and island modes.

[0013] This technical solution realizes a more accurate and flexible grid-connected island switching through the adaptive grid-forming technology, ensuring the stability and security of the system in a complex grid environment, and at the same time can effectively cope with the frequency and power fluctuation problems caused by the integration of new energy; specifically:

[0014] By adopting the adaptive grid-forming technology, simulating the rotor motion equation of the synchronous generator, generating the virtual speed and the inertia time constant, effectively improving the frequency regulation and inertia response ability of the system under weak grid conditions. Significantly enhancing the stability of the flexible DC system in a low-inertia and weak grid environment, avoiding the risk of instability of traditional control methods under extremely weak operating conditions.

[0015] This method judges whether the grid enters the island state by monitoring the change of the grid frequency in real time and combining the threshold of the frequency change. With the help of the island signal of the stability control system, the system can identify the island state in a timely and accurate manner, so as to quickly adjust the control strategy. This process enables the system to respond more flexibly to the frequency fluctuations of the grid, reduces the possibility of misjudgment, and improves the reliability of the switching process.

[0016] During the switching process from grid-connected to island, a delay mechanism is adopted to ensure the stable operation of the system. According to whether the island signal of the stability control system is received, the system can flexibly switch to the island control mode or maintain the grid-connected control mode. At the same time, by adjusting the damping coefficient in the grid-forming control algorithm, the frequency and power of the grid can be effectively balanced to ensure the stability of the system in both operating modes.

[0017] During the switching process, the system dynamically adjusts the virtual speed and damping coefficient according to the fluctuations of the grid frequency and power. This adaptive adjustment enables the system to seamlessly switch between grid-connected and island modes, ensuring frequency stability and power balance, and avoiding system instability caused by the inadaptability of control parameters to the changes in grid conditions.

[0018] The switching steps consider various grid conditions, including changes in grid frequency and the response of the stability control system signals. Through precise parameter adjustment and intelligent switching, a smoother grid-connected and island mode switching can be achieved, reducing frequency fluctuations, power surges, and interference to the grid in traditional methods.

[0019] This technical solution is aimed at a multi-terminal flexible DC system and considers the coordinated operation of multiple converter stations. Through a distributed control strategy, it acts synergistically throughout the system to ensure the coordinated stability between different converter stations, adapts to the needs of large-scale new energy grid connection, and improves the reliability and flexibility of the system.

[0020] This technical solution enhances the system's ability to regulate frequency fluctuations, ensuring that the flexible DC system can effectively cope with the uncertainties and fluctuations brought by new energy grid connection, and improving the overall operation efficiency of the system.

[0021] As a preferred technical means: The rotor motion equation is:

[0022]

[0023] In the formula: ω ref and P ref are the frequency and active power reference values respectively; ω is the virtual speed; P s is the output electromagnetic power; T J is the inertia time constant; D is the damping coefficient; θ is the voltage source phase.

[0024] This technical solution uses the adaptive network-forming technology to simulate the rotor motion equation of a synchronous generator and adopts a flexible damping coefficient adjustment strategy, effectively improving the performance of the flexible DC system in frequency regulation, power balance, and system stability. It is especially suitable for power systems with a large proportion of new energy, which can significantly enhance the operation stability, reliability, and anti-disturbance ability of the system, providing effective technical support for achieving stable power supply and smart grid. Specifically:

[0025] This technical solution simulates the rotor motion equation of a synchronous generator, enabling the flexible DC system to dynamically adjust the frequency response. By adjusting the virtual speed, the system can better track the grid frequency change and enhance the system's response ability during frequency fluctuations. In traditional power electronic systems, the low inertia makes it difficult for the grid frequency to recover quickly. However, this solution effectively simulates the characteristics of grid inertia by introducing the inertia time constant (TJ), enabling the system to have stronger inertia support when facing frequency changes, thereby enhancing the system stability.

[0026] This technical solution realizes precise power regulation of the grid through power synchronization link control, combining the active power reference value (Pref) and the output electromagnetic power (Ps). Through precise control of the electromagnetic power, the system can effectively balance the active and reactive powers, reducing the unstable factors caused by frequency fluctuations and power fluctuations. This enables the system to quickly adapt to different loads and operating conditions, optimizing the power balance of the grid. Especially in the case of large uncertainties caused by new energy grid connection, the system can maintain stable operation.

[0027] By adjusting the damping coefficient in real time, the system can flexibly adjust the stability of the power system according to the grid frequency change. Through intelligent regulation control algorithms, the system can dynamically adjust the control parameters according to the actual operating conditions of the grid to ensure better stability and response ability in a frequently changing power environment.

[0028] This technical solution improves the inertia characteristics of the system by simulating the rotor motion of a synchronous generator, enhancing the response ability to frequency fluctuations. It is particularly suitable for weak grids and environments with a large amount of new energy access, helping to improve the system's tolerance to grid disturbances.

[0029] Due to the strong volatility and uncertainty of new energy generation (such as wind power, photovoltaic, etc.), traditional control methods may not be able to effectively handle these fluctuations. However, this solution can effectively respond to the grid frequency fluctuations by adjusting the virtual speed and inertia time constant in real time, ensuring that the stability of the power system is not affected when new energy is connected. Through flexible adjustment of the grid frequency, the system can reduce the frequency impact caused by new energy fluctuations and improve the reliability of the power system.

[0030] Compared with the traditional PLL (phase-locked loop) - based control method, the control method of simulating the rotor motion of a synchronous generator is more in line with the actual dynamic behavior of the grid, reducing the problem of PLL control failure when the grid voltage changes unstably. This technical solution makes the control algorithm more concise and effective, helping to reduce the hardware requirements and the complexity of the control system, while improving the control accuracy.

[0031] As a preferred technical means: The determination of whether to enter the island state includes:

[0032] Frequency change criterion, which judges whether the grid frequency exceeds a preset threshold by detecting the change amount of the grid frequency;

[0033] Stable control system signal criterion, which confirms whether it is necessary to switch to the island control mode by monitoring the island signal of the stable control system.

[0034] The frequency change criterion can detect the change amount of the grid frequency in real time and judge whether it exceeds the preset threshold. This enables the system to respond sensitively and quickly to abnormal fluctuations in the grid frequency and timely determine whether the grid enters the island state. Compared with traditional island detection methods, the frequency change criterion can more accurately identify frequency fluctuations and avoid misjudgment or missed judgment. The stable control system signal criterion confirms the island state by monitoring the island signal from the stable control system. It provides a secondary confirmation mechanism for the system, effectively reducing the probability of misjudgment and improving the reliability of the judgment result.

[0035] By combining the frequency change criterion and the stable control system signal criterion, the system can obtain a reliable judgment of the grid state in a short time. This dual-criterion mechanism enhances the real-time performance and reliability of island detection. Especially when the grid frequency fluctuates greatly or in case of emergencies, it can respond more accurately and avoid misjudgment and lag that may occur with a single criterion. In the initial stage of island occurrence, the frequency change criterion can quickly identify potential island situations and further confirm them through the stable control system signal criterion, thus ensuring the timely switching and response of the system.

[0036] Island detection helps the system to immediately switch the mode when facing the island state, thus maintaining the stability of the system. If the determination of island switching is inaccurate, it may lead to a decline in system stability and even damage to power equipment. This technical solution ensures that the system can switch the control mode according to accurate signals by adding two criteria, improving the safety and stability of the power system in the island state.

[0037] Without timely and accurate island determination, the power grid may continue to operate in an unstable state, resulting in power failures or losses. This technical solution effectively avoids misjudgment and slow response through double verification of frequency and stable control signals, minimizing the risk of system failures.

[0038] By timely confirming the island state and switching to the island control mode, the system can ensure power balance and frequency stability in the island state, avoiding damage to power equipment or interruption of power supply caused by untimely switching.

[0039] Due to the complexity and uncertainty of the power grid, various factors such as frequency fluctuations and power changes may cause the power grid to enter an island state. Through the method of multiple criteria, the system can better adapt to complex and dynamic power grid conditions, ensuring the accuracy and stability of island detection in different power grid environments.

[0040] Especially in power grids with a relatively high proportion of new energy systems connected, due to the volatility of renewable energy, island detection is more challenging. The combination of frequency change criterion and stable control system signal criterion can effectively address the challenges brought by the volatility of new energy, ensuring that the system can identify the island state in a timely and accurate manner.

[0041] Accurate island detection helps optimize the load distribution and resource management of the power grid in island mode. In the island state, the power grid must ensure the stable power supply of local loads. By correctly determining the island state and switching the control mode, the system can flexibly adjust the power output and scheduling, thus avoiding power grid frequency and power imbalance. Through precise determination, the network formation strategy and power control can be quickly adjusted when an island occurs, ensuring the load balance of the power grid and the efficient utilization of resources.

[0042] The island detection method using dual criteria simplifies the possibly complex island detection algorithm in traditional systems and enhances the accuracy of determination at the same time. Since the system has less dependence on frequency and stable control signals, in terms of hardware implementation and control logic design, it can reduce excessive redundant functions and lower the complexity of design and operation and maintenance.

[0043] As an optimal technical means: The frequency change criterion formula is:

[0044]

[0045] In the formula: is the change in the AC voltage frequency of the system, is the threshold value of the frequency difference, is the threshold value of the change in the frequency difference.

[0046] This technical solution realizes the rapid and accurate identification of the island state of the power grid by introducing the frequency change criterion, the threshold value of the frequency difference, and the threshold value of the change in the frequency change amount. It improves the accuracy and response speed of island state judgment, and enhances the stability, reliability, and flexibility of the system. Through the optimized frequency criterion, misjudgment and over-response can be reduced, ensuring the safe, stable, and efficient operation of the power grid, especially suitable for power grid environments with a large number of new energy accesses.

[0047] Setting the frequency difference and the threshold value of the change in the amount of frequency change can effectively avoid misjudgment caused by low-frequency and high-frequency fluctuations in the power grid. Minor fluctuations and short-term disturbances in the power grid frequency will not trigger incorrect islanding switching, thus reducing the possibility of misoperation in the system. By combining the use of the frequency difference threshold value and the threshold value of the change in the amount of frequency change, this scheme provides a more fine-grained control strategy. By setting two threshold values (static threshold and dynamic change threshold), different degrees of frequency fluctuations can be accurately distinguished, avoiding over-response or slow response, thus ensuring the accuracy and effectiveness of frequency control. The combination of the frequency difference threshold value and the threshold value of the change in the amount of frequency change enables the system to more sensitively and accurately judge the islanding state when the power grid frequency fluctuates greatly, enhancing the adaptability of the system.

[0048] Using the criterion of the change in the amount of frequency change can effectively capture the changing trend of the frequency, predict potential problems in the power grid in advance, and avoid the occurrence of islanding caused by untimely response to frequency changes. The system can identify the islanding problem in advance before the frequency change reaches the predetermined threshold value and take preventive measures, thus effectively avoiding problems such as frequency imbalance and power instability. Through precise control and dynamic adjustment, the criterion of the change in the amount of frequency change can not only effectively prevent the instability of the power grid system, but also promptly initiate the switching process when the islanding state occurs, avoiding the impact of high-frequency fluctuations on the power grid load and equipment.

[0049] As a preferred technical means: the damping coefficient D takes the value of Dlw in the grid-connected mode, and this value is relatively small to ensure the system's rapid response to changes in the power grid load; in the islanding mode, it takes the value of Dgd, and this value is relatively large to ensure frequency stability.

[0050] In the grid-connected mode, the damping coefficient Dlw is relatively small, aiming to ensure that the system can quickly respond to changes in the power grid load. This helps to quickly adjust the power output when the power grid load fluctuates or is externally disturbed, thereby reducing the impact of load fluctuations on the power grid. The relatively small damping coefficient enables the system to adjust more quickly when the load changes and will not cause the system to respond slowly due to excessive inertial response.

[0051] In the islanding mode, the damping coefficient Dgd is relatively large, aiming to enhance the system's stability control of the frequency. In the islanding mode, the power grid is disconnected from the external system, the frequency fluctuates greatly, the inertia is small, and the system is vulnerable to load fluctuations. Therefore, the relatively large damping coefficient can suppress the frequency fluctuations by increasing the damping effect of the system, ensuring that the power grid frequency in the islanding mode is more stable and avoiding instability in the islanding state due to excessive frequency fluctuations.

[0052] By dynamically adjusting the damping coefficient, unnecessary over-response can be avoided in the grid-connected mode, ensuring that the system can adapt as soon as possible when the grid load changes without causing excessive system fluctuations. In the island mode, by increasing the damping coefficient, the frequency support ability of the system can be enhanced, ensuring the stable operation of the grid in the island state. This design with different values enables the system to flexibly respond to grid changes, being able to efficiently and quickly respond to external disturbances in the grid-connected state and maintain sufficient frequency stability in the island state, avoiding excessive frequency fluctuations or grid instability.

[0053] In the grid-connected state, the system frequently interacts with other power grids, and the load changes rapidly. The system needs to respond quickly. By setting a smaller damping coefficient Dlw, the system is prevented from lagging behind the load change excessively, ensuring that the grid can be adjusted rapidly and reducing the risk of grid instability. In the island mode, the power grid is no longer connected to the external system, with lower inertia and prone to frequency fluctuations. Increasing the damping coefficient Dgd can effectively enhance the anti-disturbance ability of the system and strengthen the frequency stability of the power grid in the island state. By increasing the damping coefficient, the grid collapse caused by frequency instability in the island mode can be effectively avoided, improving the operation safety of the system in the island state.

[0054] The setting of the damping coefficient takes into account the requirements of the power grid in different states, the need for rapid response in the grid-connected mode and the need for frequency stability in the island mode. This enables the system to flexibly switch according to the grid operation state and perform adaptive adjustment, enhancing the adaptability of the entire system.

[0055] In the power system, the load, disturbance sources, and network structure are constantly changing, and the system needs to be dynamically adjusted according to different operating states. By setting different damping coefficients in the grid-connected mode and the island mode, it can be ensured that the system always maintains the optimal response and stability, with strong flexibility.

[0056] By flexibly adjusting the damping coefficient, the system can be prevented from making excessive power adjustments under unnecessary circumstances, thereby reducing the energy waste of the system. For example, a smaller damping coefficient in the grid-connected mode enables the system to quickly adapt to the grid load change and avoid losses caused by frequent power adjustments; in the island mode, a larger damping coefficient ensures the continuous and stable operation of the system, avoiding energy waste caused by frequency fluctuations. The reasonable selection of the damping coefficient can optimize the control strategy of the system, balance stability and response speed, reduce unnecessary adjustments, and improve the overall operation efficiency and energy utilization efficiency of the system.

[0057] As an optimal technical measure: when operating in the grid-connected mode, the fluctuation of the active power Ps does not exceed 1%, and the value of Dlw is less than 2.5; when operating in the island mode, the deviation range of the grid frequency f does not exceed ±0.2 Hz, and the value of Dgd is greater than 500.

[0058] The active power fluctuation of this technical solution does not exceed 1%: This design goal ensures that during grid-connected operation, grid load fluctuations will not cause excessive active power fluctuations. The stability of the power grid and the accuracy of power output directly affect the reliability of power supply. Limiting the power fluctuation within 1% can effectively reduce the over-response to factors such as load changes and system disturbances, thus maintaining the stable operation of the power grid. When operating in island mode, the grid frequency deviation range does not exceed ±0.2 Hz: The stability of the grid frequency is particularly important in island mode. Once disconnected from the main grid, the grid has no inertial support and the frequency is prone to violent fluctuations. Limiting the frequency deviation range within ±0.2 Hz can ensure the stability of the grid frequency in island state, prevent equipment damage or power supply interruption caused by excessive frequency fluctuations, and improve the reliability and safety of the system.

[0059] The value of Dlw is less than 2.5: In grid-connected mode, a smaller damping coefficient enables the system to quickly respond to changes in grid load. This helps to ensure that the power grid can quickly adapt to external load changes, without being delayed in response due to excessive damping, and maintain efficient adjustment and stability under external disturbances. A smaller damping coefficient helps the system to quickly adjust the power output, reducing the risk of power overshoot and grid instability.

[0060] The value of Dgd is greater than 500: In island mode, the frequency stability of the system is particularly important. A larger damping coefficient can effectively suppress frequency fluctuations. By setting a larger Dgd, the system can enhance the suppression of grid frequency fluctuations, thus ensuring more stable operation of the power grid in island mode. Effectively avoiding the risk of system instability caused by excessive frequency fluctuations during island operation.

[0061] In actual operation, grid load and external disturbances are inevitable. By limiting the active power fluctuation and controlling the damping coefficient, the system can maintain good stability when encountering emergencies (such as large load fluctuations or external disturbances). The reasonable setting of the damping coefficient enables the system to adapt to different load and disturbance requirements according to different modes (grid-connected or island), providing high anti-disturbance ability and strong adaptability. The designed smaller and larger damping coefficients are aimed at different control requirements under different working conditions, which can ensure the system's quick response in grid-connected mode and the frequency stability in island mode, improving the robustness and anti-disturbance ability of the power grid.

[0062] By limiting the frequency deviation range in island mode, the frequency instability in the island power grid can be reduced, avoiding energy losses caused by excessive frequency fluctuations or large frequency deviations. Good frequency stability helps to reduce the regulation of reactive power, reduce system losses, and improve energy utilization efficiency.

[0063] By restricting the active power fluctuation (not exceeding 1%) in the grid-connected mode and the frequency offset range (not exceeding ±0.2 Hz) in the island mode, the risk of grid faults or instability can be effectively reduced, and the reliability and security of the system in various operating states can be enhanced. When the grid fails or the load fluctuates, the system can quickly recover to ensure a smooth transition of the grid and maintain normal power supply.

[0064] In the grid-connected mode, the grid needs to quickly synchronize and coordinate with external systems, and a small damping coefficient can ensure the rapid response of the system. In the island mode, since there is no external grid support, the frequency fluctuation may be relatively large. Therefore, a larger damping coefficient can effectively ensure the stable operation of the grid in the island state. This design scheme flexibly sets parameters according to the characteristics of different working modes, improving the performance of the system in different scenarios.

[0065] As a preferred technical means: during the grid-connected to island transition process, the converter station adopts a grid-forming control strategy, including the following contents:

[0066] Unlock the converter station at 0.6 s and set the active power command to the target value;

[0067] Unlock the photovoltaic system at 2 s and complete the pushing of active power;

[0068] After the power supply switch is opened, trigger islanding determination based on the frequency change criterion and switch the damping coefficient D of the grid-forming control algorithm to the island operation value Dgd;

[0069] The islanding criterion based on the frequency change becomes effective after 3.28 s, and LANDMode is used as the flag signal for island operation;

[0070] During the grid-connected to island transition process, based on Kirchhoff's current law, the active power Ps on the power supply side gradually decreases to 0 MW after the photovoltaic system power Ppv is pushed;

[0071] During the grid-connected to island transition process, the grid-forming control algorithm of the flexible DC system dynamically adjusts the target power through the unlock signal deblk, the active power control flag PCtrl4, and the reactive power control flag QCtrl4 to achieve frequency stability and reactive power compensation during the grid switching process.

[0072] During the process of grid-connected operation transitioning to islanded operation, the converter station adopts a network-forming control strategy to control the active power delivery and the unlocking time of the photovoltaic system. The converter station is unlocked at 0.6 seconds and the active power target value is set. The photovoltaic system is unlocked at 2 seconds and the active power delivery is completed. These steps ensure that after the power grid is disconnected, the photovoltaic system can stably and effectively supply power to the power grid, avoiding sudden power outages or power mismatches. After the power switch is opened, islanding detection is triggered based on the frequency change criterion to confirm whether the power grid enters the islanded state after the power grid is disconnected. Moreover, based on the frequency change criterion, it takes effect after 3.28 seconds and the switching of the islanded state is confirmed through the LANDMode signal. Through this progressive switching, frequency and power fluctuations during the system transient process are avoided, high-frequency interference is avoided, and the stability of the system during switching is ensured.

[0073] During the entire process of grid-connected operation transitioning to islanded operation, the active power on the power supply side gradually decreases to 0 MW, which is ensured by Kirchhoff's current law. When the power grid switches to the islanded mode, the power output on the power supply side gradually decreases, and there is no sudden power drop. Through this measure, the system can avoid voltage fluctuations or frequency fluctuations caused by sudden power outages and ensure a stable transition of the system. By dynamically adjusting parameters such as the target power, unlocking signal, active power control flag, and reactive power control flag, the flexible DC system can dynamically adjust the power output to ensure frequency stability and reactive power compensation. The dynamic adjustment function of the target power effectively compensates for the reactive power shortage caused by power grid frequency fluctuations and optimizes the frequency and voltage stability of the power grid.

[0074] During the entire switching process, the damping coefficient (D) is adjusted to the islanded operation value (Dgd) through the network-forming control algorithm to effectively suppress the frequency fluctuations in the islanded operation state. Since there is no inertia support from the external power grid in the islanded mode, a higher damping coefficient is required to ensure the stable operation of the system. By adjusting the damping coefficient appropriately, the frequency instability problem in the islanded mode can be effectively prevented. Using the frequency change criterion, it is possible to quickly determine whether the power grid enters the islanded state after the power supply is disconnected, and thus promptly switch to the islanded control mode, avoiding frequency fluctuations or instability that may occur due to detection delay.

[0075] During the entire process of grid-connected operation transitioning to islanded operation, the target power is precisely controlled using the unlocking signal (deblk), active power control flag (PCtrl4), and reactive power control flag (QCtrl4) to ensure that the system can dynamically adjust the power output during the transient process, thereby balancing power and frequency. Ensure that reactive power is promptly compensated during the power grid switching process and the power grid frequency is effectively stabilized.

[0076] During the operation of the power grid, especially when facing islanding switching, the stability of frequency and power is of crucial importance. This technical solution significantly improves the system stability by gradually reducing the active power output on the power supply side, reasonably adjusting the damping coefficient, and dynamically regulating the power control parameters, ensuring that the frequency fluctuation during islanding operation is within a predetermined range and avoiding the instability phenomenon that may occur during islanding power outage.

[0077] Another object of the present invention is to provide a multi-terminal flexible DC system interconnected islanding switching system, which includes:

[0078] An adaptive network-forming module, which is used to simulate the rotor motion equation of a synchronous generator through a power synchronization link to obtain a virtual speed and an inertia time constant, so as to adjust the inertia characteristics of the grid frequency and the system frequency response;

[0079] A real-time monitoring module, which is used to monitor the grid frequency in real time and judge whether the grid enters the islanding state according to the fluctuation of the grid frequency;

[0080] A control mode switching module, which is used to enter the temporary islanding control mode when the grid frequency change exceeds a predetermined threshold, switch to the corresponding control mode after confirming the grid state according to the islanding signal of the stability control system, and at the same time adjust the damping coefficient of the network-forming control algorithm;

[0081] A dynamic adjustment module, which is used to dynamically adjust the virtual speed and the damping coefficient according to the fluctuations of the grid frequency and power during the entire switching process to ensure the system frequency stability and power balance.

[0082] The adaptive network-forming module obtains the virtual speed and the inertia time constant by simulating the rotor motion equation of the synchronous generator to adjust the grid frequency and the inertia characteristics of the system. This method effectively improves the system's response ability to frequency fluctuations, enabling the system to adjust smoothly when subjected to load fluctuations or grid faults and reducing the impact during the system transient process.

[0083] During the system switching process, the dynamic adjustment module dynamically adjusts the virtual speed and the damping coefficient according to the fluctuations of the grid frequency and power to ensure the system frequency stability and power balance. This precise control can effectively avoid system instability caused by frequency or power imbalance during the switching process.

[0084] The real-time monitoring module tracks the changes in the grid frequency in real time and determines whether the grid has entered the islanding state. This monitoring function enables the system to quickly detect the islanding phenomenon when the grid experiences anomalies and make a timely response for switching. When the grid frequency changes beyond the predetermined threshold, the system automatically enters the temporary islanding control mode. The grid state is confirmed through the islanding signal of the stability control system, and then it is decided whether to switch to the islanding control mode. This mechanism ensures the accuracy and timeliness of the switching operation, effectively avoiding the adverse effects of frequency fluctuations and power imbalances on the system.

[0085] The control mode switching module adapts to the changes in the grid by automatically switching the control mode. When the grid frequency changes beyond the set value, it first enters the temporary islanding mode, and then determines whether to switch to the islanding control mode according to the islanding signal. This automatic switching enables the system to respond to the changes in the grid within a short time, reducing manual intervention and improving the flexibility and adaptability of the system. By adjusting the damping coefficient of the network-forming control algorithm, the system can adaptively adjust the parameters in different modes, thereby optimizing the frequency response of the grid and ensuring high stability of the grid both in the grid-connected and islanding modes.

[0086] The system can dynamically adjust the virtual speed and damping coefficient to keep the grid frequency stable, especially when the grid switches to the islanding mode, ensuring the inertia and frequency response ability of the system. This adjustment avoids excessive frequency fluctuations or frequency offset problems and can reduce the stability risks during islanding operation. Due to the dynamic regulation of the virtual speed and inertia time constant, the system can achieve more refined control, accurately respond to the changes in the grid state, and further improve the reliability and flexibility of the grid operation.

[0087] The dynamic adjustment module ensures the balance of frequency and power by real-time tracking the changes in the grid frequency and power during the switching process. It not only ensures the stability during grid switching but also effectively avoids problems such as high-frequency interference, voltage surges, or power fluctuations that may occur during the switching process. The automatic control enables the system to quickly adapt when the grid state changes and timely switch the control mode. It significantly improves the response speed and adaptability of the system, especially in the case of frequent switching and large load fluctuations, and can maintain good system stability.

[0088] As a preferred technical means: The adaptive network-forming module simulates the rotor motion equation of the synchronous generator through the power synchronization link and adjusts the virtual speed ω and the inertia time constant T J ; The rotor motion equation is:

[0089]

[0090] Where: ω ref and Pref are the frequency and active power reference values respectively; ω is the virtual rotational speed; P s is the output electromagnetic power; T J is the inertia time constant; D is the damping coefficient; θ is the phase of the voltage source.

[0091] This technical solution uses the adaptive network-forming technology to simulate the rotor motion equation of a synchronous generator and adopts a flexible damping coefficient adjustment strategy, effectively improving the performance of the flexible DC system in frequency regulation, power balance, and system stability. This solution has significant advantages, especially in power systems with a large proportion of new energy, and can significantly enhance the operation stability, reliability, and disturbance resistance of the system, providing effective technical support for realizing stable power supply and smart grid.

[0092] As a preferred technical means: The frequency change criterion formula adopted by the control mode switching module is:

[0093]

[0094] In the formula: is the change in the AC voltage frequency of the system, is the threshold value of the frequency difference, is the threshold value of the change in the frequency difference;

[0095] The dynamic adjustment module sets different damping coefficient values in the grid-connected mode and the island mode respectively, and dynamically adjusts the damping coefficient according to the change of the grid state; the damping coefficient D takes the value of Dlw in the grid-connected mode, and this value is relatively small to ensure the system's rapid response to grid load changes; in the island mode, it takes the value of Dgd, and this value is relatively large to ensure frequency stability.

[0096] This technical solution ensures the stability, reliability, and efficiency of the power grid under different working modes by combining accurate frequency change criteria, dynamically adjusting the damping coefficient, and flexibly switching control modes. Thus, it can quickly respond to grid load changes, optimize frequency stability, improve the system's adaptability and disturbance resistance, and ensure no impact and instability during system switching, enhancing the overall operation efficiency and economy of the power grid.

[0097] Beneficial effects: This technical solution realizes more accurate and flexible grid-connected island switching through the adaptive network-forming technology, ensuring the stability and security of the system in a complex grid environment, and at the same time can effectively cope with the frequency and power fluctuation problems caused by the grid connection of new energy; specifically:

[0098] 1. By adopting the adaptive network construction technology, simulating the rotor motion equation of a synchronous generator, generating virtual speed and inertia time constant, the frequency regulation and inertia response capabilities of the system are effectively improved. The stability of the flexible DC system in a low-inertia and weak-grid environment is significantly enhanced, avoiding the risk of instability in extremely weak operating conditions by traditional control methods.

[0099] 2. This method monitors the change of the grid frequency in real time and determines whether the grid enters the island state by combining with the threshold of the frequency change. With the help of the island signal of the stability control system, the system can identify the island state in a timely and accurate manner, so as to quickly adjust the control strategy. This process enables the system to respond more flexibly to the frequency fluctuations of the grid, reduces the possibility of misjudgment, and improves the reliability of the switching process.

[0100] 3. During the switching process from grid-connected to islanded operation, a delay mechanism is adopted to ensure the stable operation of the system. According to whether the island signal of the stability control system is received, the system can flexibly switch to the island control mode or maintain the grid-connected control mode. At the same time, by adjusting the damping coefficient in the network construction control algorithm, the frequency and power of the grid can be effectively balanced, ensuring the stability of the system in both operating modes.

[0101] 4. During the switching process, the system dynamically adjusts the virtual speed and damping coefficient according to the fluctuations of the grid frequency and power. This adaptive adjustment enables the system to seamlessly switch between the grid-connected and islanded modes, ensuring frequency stability and power balance, and avoiding system instability caused by the inadaptability of control parameters to the changes of grid operating conditions.

[0102] 5. The switching steps consider various grid operating conditions, including the change of grid frequency and the response of the stability control system signal. Through precise parameter adjustment and intelligent switching, a smoother switching between the grid-connected and islanded modes can be achieved, reducing the frequency fluctuations, power surges and interference to the grid in traditional methods.

[0103] 6. This technical solution is aimed at a multi-terminal flexible DC system and considers the coordinated operation of multiple converter stations. Through the distributed control strategy, it acts synergistically throughout the system, ensuring the coordinated stability between different converter stations, adapting to the needs of large-scale new energy grid connection, and improving the reliability and flexibility of the system.

[0104] 7. This technical solution ensures that the flexible DC system can effectively cope with the uncertainty and volatility brought by new energy grid connection by enhancing the system's ability to regulate frequency fluctuations, improving the comprehensive operation efficiency of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0105] Figure 1 is the flowchart of the present invention.

[0106] Figure 2 It is the main wiring diagram of the simulation model of the present invention.

[0107] Figure 3 It is the simulation waveform diagram of the present invention. Specific embodiments

[0108] The technical solution of the present invention will be further described in detail below in conjunction with the accompanying drawings of the specification.

[0109] Embodiment 1:

[0110] As Figure 1 shown,

[0111] The present invention includes the following steps:

[0112] S1: By using the adaptive network construction technology, the rotor motion equation of the synchronous generator is simulated by adopting a power synchronization link to obtain the virtual speed ω and the inertia time constant T J , where the virtual speed ω is used to adjust the grid frequency, and the inertia time constant T J is used to adjust the inertia characteristics of the system frequency response;

[0113] The rotor motion equation is:

[0114]

[0115] In the formula: ω ref and P ref are the frequency and active power reference values respectively; ω is the virtual speed; P s is the output electromagnetic power; T J is the inertia time constant; D is the damping coefficient; θ is the voltage source phase.

[0116] Through the adaptive network construction technology, this technical solution simulates the rotor motion equation of the synchronous generator and adopts a flexible damping coefficient adjustment strategy, effectively improving the performance of the flexible DC system in frequency regulation, power balance and system stability. It is especially suitable for power systems with a large proportion of new energy, and can significantly improve the operation stability, reliability and anti-disturbance ability of the system, providing effective technical support for realizing stable power supply and smart grid.

[0117] S2: Monitor the grid frequency in real time and judge whether the grid enters the island state according to the change of the grid frequency;

[0118] Judging whether to enter the island state includes:

[0119] Frequency change criterion, by detecting the change amount of the grid frequency, judge whether the grid frequency exceeds the preset threshold value;

[0120] The formula for the frequency change criterion is as follows:

[0121]

[0122] Where: is the change in the system AC voltage frequency, is the threshold value of the frequency difference, is the threshold value of the change in the frequency difference.

[0123] By introducing the frequency change criterion, the threshold value of the frequency difference, and the threshold value of the change in the frequency change amount, the rapid and accurate identification of the power grid island state is achieved. Its advantages lie in improving the accuracy and response speed of the island state judgment, enhancing the stability, reliability, and flexibility of the system. Through the optimized frequency criterion, misjudgment and over-response can be reduced, ensuring the safe, stable, and efficient operation of the power grid, especially suitable for the power grid environment with more new energy access.

[0124] The signal criterion of the stability control system confirms whether to switch to the island control mode by monitoring the island signal of the stability control system.

[0125] By combining the frequency change criterion and the signal criterion of the stability control system, accurate and reliable island state judgment is achieved, improving the real-time response ability and stability of the system. This technology can not only effectively cope with the complex working conditions of the power grid, reduce misjudgment and delay in island switching, but also optimize the operation of the power grid in the island state, ensuring the safe, reliable, and efficient operation of the power grid.

[0126] S3: When the power grid frequency change exceeds the predetermined threshold, enter the temporary island control mode.

[0127] S4: After a certain delay, if the flexible DC system receives the island signal of the stability control system, it is confirmed that the power grid is in the island state, and switch to the island control mode, adjusting the damping coefficient of the network-forming control algorithm.

[0128] S5: If the flexible DC system does not receive the island signal of the stability control system, it is confirmed that the power grid is still in the grid-connected state, maintain the grid-connected control mode, and adjust the damping coefficient of the network-forming control algorithm;

[0129] The damping coefficient D takes the value of Dlw in the grid-connected mode, and this value is relatively small to ensure the rapid response of the system to the power grid load change; in the island mode, it takes the value of Dgd, and this value is relatively large to ensure frequency stability.

[0130] By setting different damping coefficients, using a smaller Dlw in the grid-connected mode to improve the response speed and a larger Dgd in the island mode to enhance the frequency stability, this technical solution can improve the response speed of the system, enhance the frequency stability, and improve the system security and reliability. This design fully considers the complexity and uncertainty of power grid operation, and has high adaptability, flexibility and customizability. It is an effective technical solution for the stability and intelligent requirements of future power systems.

[0131] S6: During the entire switching process, dynamically adjust the virtual speed and damping coefficient according to the fluctuations of the power grid frequency and power to ensure the frequency stability and power balance of the system in the grid-connected and island modes.

[0132] When operating in grid-connected mode, the fluctuation of the active power Ps does not exceed 1%, and the value of Dlw is less than 2.5; when operating in island mode, the deviation range of the power grid frequency f does not exceed ±0.2 Hz, and the value of Dgd is greater than 500.

[0133] By restricting the fluctuation of active power and precisely controlling the damping coefficient, not only the frequency stability and fast response ability of the system under different working conditions are ensured, but also the anti-disturbance ability, reliability and security of the system are effectively improved. Optimize the performance of the power grid in grid-connected and island states to ensure that the power system operates efficiently, stably and safely in a complex operating environment, and has strong adaptability and flexibility.

[0134] This technical solution realizes a more accurate and flexible grid-connected island switching through the adaptive network-forming technology, ensuring the stability and security of the system in a complex power grid environment, and at the same time can effectively cope with the frequency and power fluctuation problems caused by new energy grid connection; specifically:

[0135] By adopting the adaptive network-forming technology, simulating the rotor motion equation of the synchronous generator, generating the virtual speed and inertia time constant, effectively improving the frequency regulation and inertia response ability of the system under weak grid conditions. Significantly enhances the stability of the flexible DC system in a low-inertia and weak grid environment, and avoids the risk of instability of traditional control methods under extremely weak conditions.

[0136] This method determines whether the power grid enters the island state by real-time monitoring of the power grid frequency change and combining with the threshold of the frequency change. With the island signal of the stability control system, the system can identify the island state in a timely and accurate manner, so as to quickly adjust the control strategy. This process enables the system to respond more flexibly to the frequency fluctuations of the power grid, reduces the possibility of misjudgment, and improves the reliability of the switching process.

[0137] During the process of switching from grid-connected to islanded operation, a delay mechanism is adopted to ensure the stable operation of the system. According to whether the islanding signal of the stability control system is received, the system can flexibly switch to the island control mode or maintain the grid-connected control mode. At the same time, by adjusting the damping coefficient in the network-forming control algorithm, the frequency and power of the power grid can be effectively balanced, ensuring the stability of the system in both operating modes.

[0138] During the switching process, the system dynamically adjusts the virtual speed and damping coefficient according to the fluctuations of the power grid frequency and power. This adaptive adjustment enables the system to seamlessly switch between grid-connected and islanded modes, ensuring frequency stability and power balance, and avoiding system instability caused by the inadaptability of control parameters to the changes in the power grid operating conditions.

[0139] The switching steps consider various power grid operating conditions, including the change of power grid frequency and the response of the stability control system signal. Through precise parameter adjustment and intelligent switching, a smoother switching between grid-connected and islanded modes can be achieved, reducing the frequency fluctuations, power surges and interference to the power grid in traditional methods.

[0140] This technical solution is aimed at a multi-terminal flexible DC system and considers the coordinated operation of multiple converter stations. Through a distributed control strategy, it acts synergistically throughout the system to ensure the coordinated stability between different converter stations, adapts to the needs of large-scale new energy grid connection, and improves the reliability and flexibility of the system.

[0141] This technical solution enhances the system's ability to regulate frequency fluctuations, ensuring that the flexible DC system can effectively cope with the uncertainties and fluctuations brought by new energy grid connection, and improving the comprehensive operation efficiency of the system.

[0142] In this embodiment, during the entire grid-connected to islanded switching process, the grid-connected and islanded states are identified through a control algorithm, and the damping coefficient D of the network-forming control algorithm is switched to achieve the grid-connected to islanded switching of the multi-terminal flexible DC system based on the adaptive network-forming technology.

[0143] As Figure 2 shown, a detailed model is built in the power system transient simulation software platform PSCAD / EMTDC. The converter station is connected to the bus through QF1, the photovoltaic system is connected to the bus through QF2, and the AC power grid is connected to the bus through QF3. The load is connected to the bus.

[0144] The grid-connected to islanded waveform is obtained through simulation, as Figure 3As shown. Pvalve is the active power on the valve side, Ps is the active power on the grid side, and Ppv is the active power of the photovoltaic system. Us(pu) is the marked value of the grid-side AC voltage, Is(pu) is the marked value of the grid-side AC current, ipa4, ipb4, ipc4, ina4, inb4, inc4 are the arm current values, Ud4(pu) is the per-unit value of the DC voltage, deblk is the unlocking signal, PCtrl4 is the active power control flag bit, QCtrl4 is the reactive power control flag bit, QF is Figure 2 the position of the grid-side switch QF3 in Figure 2 , and LANDMode is the grid-connected island flag bit.

[0145] The converter station adopts a grid-forming control strategy. The converter station unlocks (deblk) at 0.6 s, with an active power command of 100 MW (Pvalve), and stabilizes to the target value at 1 s. The 100 MW photovoltaic system unlocks at 2 s and completes the push of 100 MW (Ppv) active power at 2.5 s. According to Kirchhoff's current law, the active power (Ps) on the power supply side starts to drop to 0 MW at 2 s. The power supply switch QF3 (QF) is opened at 3.2 s, and the islanding criterion (LANDMODE) based on the frequency change amount is enabled at 3.28 s, and the damping coefficient D of the grid-forming control algorithm of the VSC-HVDC system is switched to Dgd. During the entire grid-connected to islanding process, no high-frequency components appear, the photovoltaic system does not lose power, and the AC voltage has no impact.

[0146] Embodiment 2:

[0147] A grid-connected islanding switching system for a multi-terminal VSC-HVDC system includes:

[0148] I. An adaptive grid-forming module, which is used to simulate the rotor motion equation of a synchronous generator through a power synchronization link to obtain a virtual speed and an inertia time constant, so as to adjust the grid frequency and the inertial characteristics of the system frequency response;

[0149] The adaptive grid-forming module obtains a virtual speed and an inertia time constant by simulating the rotor motion equation of a synchronous generator, so as to adjust the grid frequency and the inertial characteristics of the system. It effectively improves the system's response ability to frequency fluctuations, enabling the system to adjust smoothly when subjected to load fluctuations or grid faults, and reducing the impact during the system transient process.

[0150] II. A real-time monitoring module, which is used to monitor the grid frequency in real time and judge whether the grid enters the islanding state according to the fluctuation of the grid frequency;

[0151] The real-time monitoring module tracks the changes in the grid frequency in real time and determines whether the power grid has entered the islanding state. This monitoring function enables the system to quickly detect the islanding phenomenon when the power grid experiences anomalies and make a timely response for switching. When the grid frequency changes beyond a predetermined threshold, the system automatically enters the temporary islanding control mode. The grid status is confirmed through the islanding signal of the stability control system, and then it is decided whether to switch to the islanding control mode. This ensures the accuracy and timeliness of the switching operation, effectively avoiding the adverse effects of frequency fluctuations and power imbalances on the system.

[0152] Third, a control mode switching module is used to enter the temporary islanding control mode when the grid frequency changes beyond a predetermined threshold, confirm the grid status according to the islanding signal of the stability control system and then switch to the corresponding control mode, and at the same time adjust the damping coefficient of the grid-forming control algorithm;

[0153] The control mode switching module adapts to the changes in the power grid by automatically switching the control mode. When the grid frequency changes beyond the set value, it first enters the temporary islanding mode, and then determines whether to switch to the islanding control mode according to the islanding signal. This automatic switching enables the system to respond to the changes in the power grid within a short time, reduces manual intervention, and improves the flexibility and adaptability of the system. By adjusting the damping coefficient of the grid-forming control algorithm, the system can adaptively adjust the parameters in different modes, thereby optimizing the frequency response of the power grid and ensuring that the power grid can maintain a high level of stability whether in the grid-connected or islanding mode.

[0154] Fourth, a dynamic adjustment module is used to dynamically adjust the virtual speed and damping coefficient according to the fluctuations of the grid frequency and power during the entire switching process to ensure system frequency stability and power balance.

[0155] During the system switching process, the dynamic adjustment module dynamically adjusts the virtual speed and damping coefficient according to the grid frequency and power fluctuations to ensure system frequency stability and power balance. This effectively avoids system instability caused by frequency or power imbalance during the switching process.

[0156] The system can dynamically adjust the virtual speed and damping coefficient to keep the grid frequency stable. Especially when the power grid switches to the islanding mode, it ensures the inertia and frequency response ability of the system. This adjustment avoids excessive frequency fluctuations or frequency offset problems and can reduce the stability risk during islanding operation. Due to the dynamic adjustment of the virtual speed and inertia time constant, the system can achieve more precise control, accurately respond to the changes in the grid status, and further improve the reliability and flexibility of the power grid operation.

[0157] During the switching process, the dynamic adjustment module ensures the balance of frequency and power by tracking the changes in grid frequency and power in real time. It not only ensures the stability during grid switching but also effectively avoids problems such as high-frequency interference, voltage surges, or power fluctuations that may occur during the switching process. The automated control enables the system to quickly adapt when the grid state changes and promptly switch the control mode. This significantly improves the system's response speed and adaptability, especially in the case of frequent switching and large load fluctuations, and can maintain good system stability.

[0158] The functions of the specific modules are not elaborated here as they are repetitive with the method.

[0159] The above-described multi-terminal flexible DC system networking island switching method and system based on adaptive network construction are specific embodiments of the present invention, which have already reflected the substantial features and progress of the present invention. According to actual usage needs, equivalent modifications can be made under the inspiration of the present invention, and they are all within the protection scope of this solution.

Claims

1. A multi-terminal flexible DC system networking islanding switching method based on adaptive network construction, characterized in that It includes the following steps: 1) By using the adaptive network construction technology, the power synchronization link is adopted to simulate the rotor motion equation of the synchronous generator, and the virtual speed ω and the inertia time constant T are obtained J , Among them, the virtual rotational speed ω is used to adjust the grid frequency, and the inertia time constant T J is used to adjust the inertia characteristics of the system frequency response; 2) Monitor the grid frequency in real time and determine whether the grid enters the island state according to the change of the grid frequency; 3) When the change of the grid frequency exceeds the predetermined threshold, enter the temporary island control mode; 4) After a delay period, if the flexible DC system receives the island signal from the stability control system, confirm that the grid is in the island state, switch to the island control mode, and adjust the damping coefficient of the grid-forming control algorithm; 5) If the flexible DC system does not receive the island signal from the stability control system, confirm that the grid is still in the grid-connected state, maintain the grid-connected control mode, and adjust the damping coefficient of the grid-forming control algorithm; 6) During the entire switching process, dynamically adjust the virtual speed and damping coefficient according to the fluctuations of the grid frequency and power to ensure the frequency stability and power balance of the system in the grid-connected and island modes; The judgment of whether to enter the island state includes: Frequency change criterion, by detecting the change amount of the grid frequency, judge whether the grid frequency exceeds the preset threshold value; Stability control system signal criterion, by monitoring the island signal of the stability control system, confirm whether to switch to the island control mode; The frequency change criterion formula is: Wherein: is the change amount of the system AC voltage frequency, is the threshold value of the frequency difference, is the threshold value of the change amount of the frequency difference.

2. The multi-terminal flexible DC system networking island switching method based on adaptive network construction according to claim 1, wherein: The rotor motion equation is: Where: ω ref and P ref are the frequency and active power reference values respectively; ω is the virtual speed; P s is the output electromagnetic power; T J is the inertia time constant; D is the damping coefficient; θ is the voltage source phase.

3. A multi-terminal flexible DC system networking islanding switching method based on adaptive network construction according to claim 2, characterized in that: The damping coefficient D takes the value of Dlw in the grid-connected mode, and this value is relatively small to ensure the rapid response of the system to the change of the grid load; it takes the value of Dgd in the island mode, and this value is relatively large to ensure the frequency stability.

4. A multi-terminal flexible DC system networking islanding switching method based on adaptive network formation according to claim 3, characterized in that: During grid-connected operation, the fluctuation of the active power Ps does not exceed 1%, and the value of Dlw is less than 2.5; during island operation, the deviation range of the grid frequency f does not exceed ±0.2 Hz, and the value of Dgd is greater than 500.

5. A multi-terminal flexible DC system networking islanding switching method based on adaptive network construction according to claim 4, characterized in that: During the process of grid-connected to island, the converter station adopts the grid-forming control strategy, including the following contents: Unlock the converter station at 0.6 s and set the active power command to the target value; Unlock the photovoltaic system at 2 s and complete the push of the active power; After the power supply switch is opened, trigger the island determination based on the frequency change amount criterion and switch the damping coefficient D of the grid-forming control algorithm to the island operation value Dgd; The island criterion based on the frequency change amount takes effect after 3.28 s, and LANDMode is used as the flag signal for island operation; During the process of grid-connected to island, based on Kirchhoff's current law, the active power Ps on the power supply side gradually drops to 0 MW after the power of the photovoltaic system Ppv is pushed; During the process of grid-connected to island, the grid-forming control algorithm of the flexible DC system dynamically adjusts the target power through the unlock signal deblk, the active power control flag bit PCtrl4, and the reactive power control flag bit QCtrl4 to achieve the frequency stability and reactive power compensation during the grid switching process.

6. A multi-terminal flexible DC system networking islanding switching system adopting a multi-terminal flexible DC system networking islanding switching method according to any one of claims 1-5, characterized in that It includes: An adaptive grid-forming module, which is used to simulate the rotor motion equation of a synchronous generator through a power synchronization link, obtain the virtual speed and the inertia time constant, so as to adjust the grid frequency and the inertial characteristics of the system frequency response; A real-time monitoring module, which is used to monitor the grid frequency in real time and determine whether the grid enters the island state according to the fluctuation of the grid frequency; The control mode switching module is used to enter the temporary island control mode when the grid frequency change exceeds a predetermined threshold, switch to the corresponding control mode after confirming the grid state according to the islanding signal of the stability control system, and at the same time adjust the damping coefficient of the network-forming control algorithm; The dynamic adjustment module is used to dynamically adjust the virtual speed and damping coefficient according to the fluctuations of the grid frequency and power during the entire switching process to ensure system frequency stability and power balance.

7. The multi-terminal flexible DC system networking island switching system according to claim 6, wherein: The adaptive network construction module simulates the rotor motion equation of a synchronous generator through the power synchronization link to adjust the virtual speed ω and the inertia time constant T J ; The rotor motion equation is as follows: In the formula: ω ref and P ref are the frequency and active power reference values respectively; ω is the virtual speed; P s is the output electromagnetic power; T J is the inertia time constant; D is the damping coefficient; θ is the voltage source phase.

8. The multi-terminal flexible DC system networking and islanding switching system according to claim 7, characterized in that: The frequency change criterion formula adopted by the control mode switching module is: Where: is the change in the system AC voltage frequency, is the threshold value of the frequency difference, is the threshold value of the change in the frequency difference; The dynamic adjustment module sets different damping coefficient values in the grid-connected mode and the island mode respectively, and dynamically adjusts the damping coefficient according to the change of the grid state; The damping coefficient D takes the value of Dlw in the grid-connected mode, and this value is relatively small to ensure the system's rapid response to grid load changes; it takes the value of Dgd in the island mode, and this value is relatively large to ensure frequency stability.

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