Multi-terminal flexible DC system networking island switching method and system based on adaptive networking

Abstract

The invention discloses a multi-terminal flexible direct current system networking island switching method and system based on adaptive networking, relates to the technical field of electric power, and aims to solve the problems that an adaptive networking island switching control method for dynamic characteristics such as power grid frequency and power fluctuation is lacked at present, and the risk of system instability caused by increase of a new energy proportion cannot be effectively handled. According to the method, a synchronous generator rotor motion equation is simulated through an adaptive network construction technology, and a virtual rotating speed and an inertia time constant are obtained, so that the power grid frequency and the system frequency response are adjusted; when it is monitored that the power grid frequency change exceeds a preset threshold value, the system enters a temporary island control mode and is switched to a corresponding control mode according to an island signal of the stability control system; in the whole switching process, the system dynamically adjusts the virtual rotating speed and the damping coefficient. According to the technical scheme, frequency stability and power balance are ensured, so that safe and stable networking and island switching is realized, and the stability and adaptability of the system in a weak power grid environment are improved.

Description

Technical Field

[0001] The present invention relates to the field of electric power technology, and in particular to a method for switching interconnected islands in a multi-terminal flexible direct current system based on adaptive networking. Background Art

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

[0003] However, as the proportion of synchronous generators in the power grid gradually decreases, 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 when small disturbances occur. This problem is particularly prominent in areas where more renewable energy is connected to the grid. The volatility and uncertainty of renewable energy increase the difficulty of grid control.

[0004] In order to meet these challenges, domestic and foreign scholars have proposed the concept of "converter grid control", which aims to improve the stability of flexible DC systems in weak grids by simulating the inertial characteristics and voltage support capabilities of the grid. However, existing research has mostly focused on steady-state operation control strategies, and has not yet involved how to perform adaptive grid-connected island switching control when the system is in a grid-connected island state. Therefore, in view of the dynamic characteristics of grid frequency fluctuations, power fluctuations, etc., it is urgent to propose a grid-connected island switching method with adaptive functions to ensure the safe and stable operation of flexible DC systems under complex working conditions, especially in the context of the increasing proportion of renewable energy power 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 perfect the existing technical solutions, and provide a method for switching the networked island of a multi-terminal flexible direct current system based on adaptive networking, so as 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 method for switching an island in a multi-terminal flexible direct current system based on adaptive networking comprises the following steps: 1) The adaptive network construction technology uses the power synchronization link to simulate the rotor motion equation of the synchronous generator to obtain the virtual speed ω and inertia time constant T J, where the virtual speed ω is used to adjust the grid frequency, and the inertia time constant T J Inertial characteristics used to adjust the system frequency response; 2) Monitor the grid frequency in real time and determine whether the grid has entered an island state based on changes in the grid frequency; 3) When the grid frequency changes beyond the preset threshold, it enters the temporary island control mode; 4) After a delay period, if the flexible DC system receives the islanding signal from the stabilization control system, it confirms that the power grid is in an islanding state, switches to the islanding control mode, and adjusts the damping coefficient of the grid control algorithm; 5) If the flexible DC system does not receive the island signal from the stabilization control system, it confirms that the power grid is still in the networking state, maintains the networking control mode, and adjusts the damping coefficient of the network control algorithm; 6) During the entire switching process, the virtual speed and damping coefficient are dynamically adjusted according to the fluctuations of grid frequency and power to ensure frequency stability and power balance of the system in both interconnected and island modes.

[0007] This technical solution achieves more accurate and flexible network island switching through adaptive networking technology, ensuring the stability and security of the system in a complex power grid environment, while effectively coping with the frequency and power fluctuation problems caused by the grid connection of new energy sources; specifically: By adopting adaptive network construction technology, simulating the rotor motion equation of the synchronous generator, generating virtual speed and inertia time constant, the frequency regulation and inertia response capabilities of the system under weak grid conditions are effectively improved. The stability of the flexible DC system in low inertia and weak grid environments is significantly enhanced, avoiding the risk of instability of traditional control methods under extremely weak conditions.

[0008] This method monitors the frequency change of the power grid in real time and determines whether the power grid has entered an island state by combining the threshold of the frequency change. With the help of the island signal of the stabilization control system, the system can timely and accurately identify the island state, thereby quickly adjusting the control strategy. This process enables the system to respond to the frequency fluctuation of the power grid more flexibly, reduces the possibility of misjudgment, and improves the reliability of the switching process.

[0009] In the process of switching from networking to islanding, a delay mechanism is used to ensure stable operation of the system. Depending on whether the islanding signal of the stabilization control system is received, the system can flexibly switch to the islanding control mode or maintain the networking control mode. At the same time, by adjusting the damping coefficient in the networking control algorithm, the frequency and power of the power grid can be effectively balanced to ensure that the system can be stable in both operating modes.

[0010] During the switching process, the system dynamically adjusts the virtual speed and damping coefficient according to the fluctuations of grid frequency and power. This adaptive adjustment enables the system to seamlessly switch between networked and island modes, ensuring frequency stability and power balance, and avoiding system instability caused by control parameters not adapting to changes in grid conditions.

[0011] The switching steps take into account a variety of grid conditions, including changes in grid frequency and the response of stabilization system signals. Through precise parameter adjustment and intelligent switching, a smoother switching between networking and islanding modes can be achieved, reducing frequency fluctuations, power shocks and interference to the grid in traditional methods.

[0012] This technical solution targets the multi-terminal flexible DC system and takes into account the coordinated work of multiple converter stations. Through the distributed control strategy, the coordinated work in the entire system ensures 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.

[0013] This technical solution enhances the system's ability to regulate frequency fluctuations, ensuring that the flexible DC system can effectively cope with the uncertainty and volatility brought about by the grid connection of renewable energy, thereby improving the overall operating efficiency of the system.

[0014] As a preferred technical means: the rotor motion equation is:

[0015] Where: ω ref and P ref are frequency and active reference value respectively; ω is 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.

[0016] This technical solution uses adaptive networking technology to simulate the rotor motion equation of the synchronous generator and adopts a flexible damping coefficient adjustment strategy to effectively improve 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 system's operating stability, reliability and anti-disturbance ability, providing effective technical support for achieving stable power supply and smart grids. Specifically: This technical solution simulates the rotor motion equation of the synchronous generator, allowing the flexible DC system to dynamically adjust the frequency response. By adjusting the virtual speed, the system can better track the changes in grid frequency and enhance the system's responsiveness to frequency fluctuations. In traditional power electronic systems, low inertia makes it difficult for the grid frequency to recover quickly. This solution effectively simulates the characteristics of grid inertia by introducing the inertia time constant (TJ), giving the system stronger inertial support when facing frequency changes, thereby improving system stability.

[0017] This technical solution realizes accurate power regulation of the power grid by controlling the power synchronization link, combining the active reference value (Pref) and the output electromagnetic power (Ps). Through the precise control of electromagnetic power, the system can effectively balance the active and reactive power, reducing the instability factors caused by frequency fluctuations and power fluctuations. It enables the system to quickly adapt to different loads and operating conditions, optimize the power balance of the power grid, and maintain stable operation of the system, especially when there is great uncertainty caused by the connection of new energy to the grid.

[0018] By adjusting the damping coefficient in real time, the system can flexibly adjust the stability of the power system according to the changes in the power grid frequency. Through the intelligent adjustment control algorithm, the system can dynamically adjust the control parameters according to the actual operation of the power grid to ensure that it maintains good stability and responsiveness in a frequently changing power environment.

[0019] This technical solution improves the inertial characteristics of the system and enhances its response to frequency fluctuations by simulating the rotor movement of the synchronous generator. It is particularly suitable for environments with weak power grids and large-scale access to new energy sources, and helps to improve the system's tolerance to power grid disturbances.

[0020] Since renewable energy generation (such as wind power, photovoltaic power, etc.) has strong volatility and uncertainty, traditional control methods may not be able to effectively cope with these fluctuations. However, this solution can effectively cope with the fluctuation of grid frequency by adjusting the virtual speed and inertia time constant in real time, ensuring that the stability of the power system is not affected when renewable energy is connected. By flexibly adjusting the grid frequency, the system can reduce the frequency shock caused by renewable energy fluctuations and improve the reliability of the power system.

[0021] Compared with the traditional PLL (phase-locked loop)-based control method, the control method that simulates the rotor motion of the synchronous generator is more in line with the actual dynamic behavior of the power grid, reducing the problem of PLL control failure when the power grid voltage changes unstably. This technical solution makes the control algorithm more concise and effective, helps reduce hardware requirements and the complexity of the control system, and improves control accuracy.

[0022] As a preferred technical means: the determination of whether to enter an isolated island state includes: Frequency change criterion: by detecting the change in the grid frequency, determine whether the grid frequency exceeds the preset threshold value; The signal judgment of the stability control system determines whether it is necessary to switch to the island control mode by monitoring the island signal of the stability control system.

[0023] The frequency change criterion can detect the change in the grid frequency in real time and determine whether it exceeds the preset threshold value. This allows the system to respond sensitively and quickly to abnormal fluctuations in the grid frequency and promptly determine whether the grid has entered an islanding state. Compared with traditional islanding detection methods, the frequency change criterion can more accurately identify frequency fluctuations and avoid misjudgment or missed judgments. The stabilization control system signal criterion confirms the islanding state by monitoring the islanding signal from the stabilization control system. This provides a secondary confirmation mechanism for the system, effectively reducing the probability of misjudgment and improving the reliability of the judgment results.

[0024] By combining the frequency change criterion and the stability control system signal criterion, the system can obtain a reliable judgment of the power grid status in a short time. This dual criterion mechanism enhances the real-time and reliability of island detection, especially when the power grid frequency fluctuates greatly or in emergencies, it can respond more accurately and avoid the misjudgment and lag that may occur with a single criterion. In the early stage of islanding, the frequency change criterion can quickly identify potential islanding situations and further confirm them through the stability control system signal criterion, thereby ensuring timely switching and response of the system.

[0025] Island detection helps the system to switch modes immediately when facing an island state, thereby maintaining system stability. If the island switching judgment is inaccurate, it may cause system stability to decline and even damage power equipment. This technical solution ensures that the system can switch control modes according to accurate signals by adding two criteria, thereby improving the safety and stability of the power system in an island state.

[0026] Without timely and accurate islanding determination, the power grid may continue to operate in an unstable state, resulting in power failure or loss. This technical solution effectively avoids misjudgment and slow response through dual verification of frequency and stabilization control signals, and minimizes the risk of system failure.

[0027] By promptly confirming the islanding state and switching to the islanding control mode, the system can ensure power balance and frequency stability in the islanding state, avoiding damage to power equipment or power supply interruption caused by untimely switching.

[0028] Due to the complexity and uncertainty of the power grid, frequency fluctuations, power changes and other factors may cause the power grid to be in an island state. Through multiple criteria, the system can better adapt to complex and dynamic power grid conditions and ensure the accuracy and stability of island detection in different power grid environments.

[0029] Especially in power grids with a high proportion of new energy systems connected, the detection of islands is more challenging due to the volatility of renewable energy. The combination of frequency change criteria and stabilization system signal criteria can effectively cope with the challenges brought by the volatility of new energy and ensure that the system can identify island states in a timely and accurate manner.

[0030] 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 stable power supply to the local load. By correctly determining the island state and switching the control mode, the system can flexibly adjust power output and scheduling to avoid frequency and power imbalance in the power grid. Through accurate determination, the network construction strategy and power control can be quickly adjusted when an island occurs to ensure load balance and efficient use of resources in the power grid.

[0031] The island detection method using dual criteria simplifies the island detection algorithm that may be complex in traditional systems, while enhancing the accuracy of the judgment. Since the system is less dependent on frequency and stabilization signals, it can reduce excessive redundant functions in hardware implementation and control logic design, reducing the complexity of design and operation and maintenance.

[0032] As the preferred technical means: the frequency change criterion formula is:

[0033] Where: is the frequency variation of the system AC voltage, is the frequency difference threshold, is the threshold value of the frequency difference change.

[0034] This technical solution achieves rapid and accurate identification of the islanding state of the power grid by introducing the frequency change criterion, the frequency difference threshold value, and the frequency change change threshold value. It improves the accuracy and response speed of islanding state judgment and enhances the stability, reliability, and flexibility of the system. Through the optimized frequency criterion, it can reduce misjudgment and over-response, ensure the safe, stable, and efficient operation of the power grid, and is particularly suitable for power grid environments with a large number of new energy sources.

[0035] Setting the frequency difference and frequency change thresholds can effectively avoid misjudgments caused by low-frequency and high-frequency fluctuations in the power grid. Small fluctuations and short-term disturbances in the power grid frequency will not cause erroneous island switching, thereby reducing the possibility of system misoperation. This solution provides a more fine-grained control strategy by combining the frequency difference threshold and the frequency change threshold. By setting two thresholds (static threshold and dynamic change threshold), different degrees of frequency fluctuations can be accurately distinguished to avoid over-response or slow response, thereby ensuring the accuracy and effectiveness of frequency control. The combination of the frequency difference threshold and the frequency change threshold enables the system to more sensitively and accurately judge the island state when the power grid frequency fluctuates greatly, thereby enhancing the adaptability of the system.

[0036] The frequency change criterion can effectively capture the frequency change trend, predict potential problems of the power grid in advance, and avoid the occurrence of islanding caused by untimely response to frequency changes. The system can identify islanding problems in advance before the frequency change reaches the predetermined threshold value and take preventive measures, thereby effectively avoiding problems such as frequency imbalance and power instability. Through precise control and dynamic adjustment, the frequency change criterion can not only effectively prevent the instability of the power grid system, but also start the switching process in time when the islanding state occurs, avoiding the impact of high-frequency fluctuations on power grid loads and equipment.

[0037] As a preferred technical means: the damping coefficient D is Dlw in the networking mode, and the value is small to ensure that the system responds quickly to changes in the grid load; in the island mode, the value is Dgd, and the value is large to ensure frequency stability.

[0038] In the grid-connected mode, the damping coefficient Dlw is small to ensure that the system can respond quickly to grid load changes. This helps to quickly adjust the power output when the grid load fluctuates or there is an external disturbance, thereby reducing the impact of load fluctuations on the grid. A smaller damping coefficient allows the system to adjust more quickly when the load changes, and will not cause the system to react slowly due to excessive inertial response.

[0039] In the island mode, the damping coefficient Dgd is larger, the purpose is to enhance the system's control over the stability of the frequency. In the island mode, the power grid is disconnected from the external system, the frequency fluctuation is large, the inertia is small, and the system is susceptible to load fluctuations. Therefore, a larger damping coefficient can suppress frequency fluctuations by increasing the damping effect of the system, ensuring that the power grid frequency in the island mode is more stable and avoiding instability in the island state due to excessive frequency fluctuations.

[0040] By dynamically adjusting the damping coefficient, unnecessary over-response can be avoided in the networked mode, ensuring that the grid can adapt as quickly as possible when the load changes without causing excessive system fluctuations. In the islanded mode, by increasing the damping coefficient, the frequency support capability of the system can be enhanced to ensure the smooth operation of the grid in the islanded state. This design with different values ​​enables the system to flexibly respond to changes in the grid, and can respond to external disturbances efficiently and quickly in the networked state, and maintain sufficient frequency stability in the islanded state to avoid excessive frequency fluctuations or grid instability.

[0041] In the networked state, the system frequently interacts with other power grids, the load changes rapidly, and the system needs to respond quickly. By setting a smaller damping coefficient Dlw, the system is prevented from lagging behind the load changes excessively, ensuring that the power grid can be adjusted quickly and reducing the risk of power grid instability. In the island state, the power grid is no longer connected to the external system, has low inertia, and is prone to frequency fluctuations. Increasing the damping coefficient Dgd can effectively improve the system's anti-disturbance capability and enhance the frequency stability of the power grid in the island state. By increasing the damping coefficient, the collapse of the power grid due to frequency instability in the island mode can be effectively avoided, and the operation safety of the system in the island state can be improved.

[0042] The setting of the damping coefficient takes into account the needs of the power grid in different states, the need for rapid response in the interconnected mode and the need for frequency stability in the island mode. This enables the system to switch flexibly according to the operating status of the power grid and perform adaptive adjustments, thereby improving the adaptability of the entire system.

[0043] In the power system, load changes, disturbance sources and network structures are constantly changing, and the system needs to be dynamically adjusted according to different operating conditions. By setting different damping coefficients in the network mode and island mode, it can ensure that the system always maintains the optimal response and stability, with strong flexibility.

[0044] By flexibly adjusting the damping coefficient, the system can avoid making excessive power adjustments when it is not necessary, thereby reducing the energy waste of the system. For example, in the networked mode, a smaller damping coefficient allows the system to quickly adapt to changes in the grid load and avoid losses caused by frequent power adjustments; in the island mode, a larger damping coefficient ensures that the system can continue to operate stably and avoid energy waste caused by frequency fluctuations. Reasonable selection of the damping coefficient can optimize the system's control strategy, balance stability and response speed, reduce unnecessary adjustments, and improve the overall operating efficiency and energy efficiency of the system.

[0045] As the preferred technical means: when operating in a grid-connected mode, the active power Ps fluctuation shall not exceed 1%, and the Dlw value shall be less than 2.5; when operating in an island mode, the grid frequency f deviation range shall not exceed ±0.2Hz, and the Dgd value shall be greater than 500.

[0046] The active power fluctuation of this technical solution does not exceed 1%: This design goal ensures that when the system is connected to the grid, the grid load fluctuation will not cause excessive active power fluctuations. The stability of the grid and the accuracy of power output directly affect the reliability of the power supply. Limiting power fluctuations to less than 1% can effectively reduce excessive responses to factors such as load changes and system disturbances, thereby maintaining stable operation of the grid. The frequency offset range of the grid does not exceed ±0.2Hz during island operation: The frequency stability of the grid is particularly important in island mode, because once disconnected from the main grid, the grid has no inertial support and the frequency is prone to violent fluctuations. Limiting the frequency offset range to within ±0.2Hz can ensure the stability of the grid frequency in the island state, prevent equipment damage or power outages caused by excessive frequency fluctuations, and improve the reliability and safety of the system.

[0047] Dlw value is less than 2.5: In the interconnected mode, a small damping coefficient enables the system to respond quickly to changes in grid load. This helps ensure that the grid can quickly adapt to external load changes, does not delay response due to excessive damping, and maintains efficient adjustment and stability of the grid under external disturbances. A small damping coefficient helps the system quickly adjust power output and reduce the risk of power overshoot and grid instability.

[0048] Dgd value is greater than 500: In island mode, the frequency stability of the system is particularly important, and a larger damping coefficient can effectively suppress frequency fluctuations. By setting a larger Dgd, the system can enhance the suppression of grid frequency fluctuations, thereby ensuring more stable grid operation in island mode. This effectively avoids the risk of system instability caused by excessive frequency fluctuations during island operation.

[0049] In actual operation, grid load and external disturbances are inevitable. By limiting active power fluctuations and controlling the damping coefficient, the system can maintain good stability when encountering emergencies (such as large load fluctuations or external disturbances). Reasonable setting of the damping coefficient enables the system to adapt to different load and disturbance requirements according to different modes (networked or isolated), providing efficient anti-disturbance capability and strong adaptability. The designed smaller damping coefficient and larger damping coefficient are respectively aimed at the control requirements under different working conditions, which can ensure the rapid response of the system in the networked mode, ensure the stability of the frequency in the isolated island mode, and improve the robustness and anti-disturbance capability of the power grid.

[0050] By limiting the frequency deviation range in island mode, the frequency instability in the island power grid can be reduced, and energy loss caused by excessive frequency fluctuations or excessive frequency deviations can be avoided. Good frequency stability helps reduce reactive power regulation, reduce system losses, and improve energy efficiency.

[0051] By limiting the active power fluctuation in the interconnected mode (no more than 1%) and the frequency deviation range in the island mode (no more than ±0.2Hz), the risk of grid failure or instability can be effectively reduced, and the reliability and safety of the system under various operating conditions can be enhanced. When a grid failure or load fluctuation occurs, the system can recover quickly to ensure a smooth transition of the grid and maintain normal power supply.

[0052] In the interconnected mode, the power grid needs to quickly synchronize and coordinate with the external system, and a small damping coefficient can ensure the rapid response of the system. In the island mode, since there is no external power grid support, the frequency fluctuation may be large, so a large damping coefficient can effectively ensure the stable operation of the power 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.

[0053] As a preferred technical means: during the networking to islanding process, the converter station adopts a networking control strategy, including the following contents: At 0.6 seconds, the converter station is unlocked and the active power command is set to the target value; Unlock the photovoltaic system at 2 seconds and complete the push of active power; After the power switch is turned on, the islanding judgment is triggered based on the frequency change criterion, and the damping coefficient D of the network control algorithm is switched to the islanding operation value Dgd; The islanding criterion based on frequency variation takes effect after 3.28 seconds, and LANDMode is used as the sign signal of islanding operation; In the process of connecting to an island, 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; During the process of switching from interconnected to isolated island, the network control algorithm of the flexible DC system dynamically adjusts the target power through the unlocking 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.

[0054] In the process of switching from interconnected to islanded state, the converter station adopts the network control strategy to control the push of active power 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 push of active power is completed. These steps ensure that the photovoltaic system can smoothly and effectively provide power to the grid after the grid is disconnected, avoiding sudden power outages or power mismatches. After the power switch is turned off, the islanding judgment is triggered based on the frequency change criterion to ensure that the grid enters the islanding state through the frequency criterion after the grid is disconnected. In addition, based on the frequency change criterion, it takes effect after 3.28 seconds and confirms the switching of the islanding state through the LANDMode signal. Through this gradual switching, the frequency and power fluctuations during the system transition process are avoided, high-frequency interference is avoided, and the stability of the system during switching is guaranteed.

[0055] During the entire process of switching from network to island, the active power on the power supply side gradually decreases to 0 MW, which is guaranteed by Kirchhoff's current law. When the power grid switches to island mode, the power supply side gradually reduces the output power, and there will be no 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 target power, unlocking signal, power control flag, and reactive power control flag, the flexible DC system can dynamically adjust power output to ensure frequency stability and reactive power compensation. The dynamic adjustment function of target power effectively compensates for the reactive power loss caused by grid frequency fluctuations and optimizes the frequency and voltage stability of the grid.

[0056] During the entire switching process, the damping coefficient (D) is adjusted to the island operation value (Dgd) through the network control algorithm to effectively suppress the frequency fluctuations in the island operation state. Since there is no inertial support from the external power grid in the island mode, a higher damping coefficient is required to ensure stable operation of the system. By adjusting the damping coefficient appropriately, the frequency instability problem in the island mode can be effectively prevented. Using the frequency change criterion, it is possible to quickly determine whether the power grid has entered the island state after the power is disconnected, so as to switch to the island control mode in time, avoiding frequency fluctuations or instability that may be caused by judgment delays.

[0057] During the entire process of switching from network to island, the unlock signal (deblk), active power control flag (PCtrl4), and reactive power control flag (QCtrl4) are used to accurately control the target power, ensuring that the system can dynamically adjust the power output during the transition process to balance the power and frequency. This ensures that the reactive power of the power grid is compensated in time during the switching process and that the power grid frequency is effectively stabilized.

[0058] During the operation of the power grid, especially when facing island switching, the stability of frequency and power is crucial. This technical solution significantly improves the stability of the system by gradually reducing the active power output on the power supply side, reasonably adjusting the damping coefficient, and dynamically adjusting the power control parameters, ensuring that the frequency fluctuation during island operation is within the predetermined range, and avoiding the instability that may occur when the island is powered off.

[0059] Another object of the present invention is to provide a multi-terminal flexible direct current system networking island switching system, comprising: The adaptive network construction module is used to simulate the rotor motion equation of the synchronous generator through the power synchronization link, obtain the virtual speed and inertia time constant, and adjust the inertia characteristics of the grid frequency and system frequency response; Real-time monitoring module, used to monitor the grid frequency in real time and determine whether the grid has entered an island state based on the fluctuation of the grid frequency; The control mode switching module is used to enter the temporary island control mode when the grid frequency changes beyond the preset threshold, and switch to the corresponding control mode after confirming the grid state according to the island signal of the stabilization control system, and adjust the damping coefficient of the network control algorithm at the same time; The dynamic adjustment module is used to dynamically adjust the virtual speed and damping coefficient according to the fluctuations of grid frequency and power during the entire switching process to ensure system frequency stability and power balance.

[0060] The adaptive network module simulates the rotor motion equation of the synchronous generator to obtain the virtual speed and inertia time constant to adjust the grid frequency and the inertia characteristics of the system. This method effectively improves the system's ability to respond to frequency fluctuations, allowing the system to adjust smoothly when subjected to load fluctuations or grid failures, reducing the impact of the system transition process.

[0061] 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 the frequency stability and power balance of the system. This precise control can effectively avoid system instability caused by frequency or power imbalance during the switching process.

[0062] The real-time monitoring module tracks the changes in the grid frequency in real time and determines whether the grid has entered an island state. This monitoring function enables the system to quickly detect the island phenomenon when an abnormality occurs in the grid and respond promptly to the switch. When the grid frequency changes beyond the predetermined threshold, the system automatically enters the temporary island control mode. The grid status is confirmed by the island signal of the stabilization system, and then a decision is made whether to switch to the island control mode. This mechanism ensures the accuracy and timeliness of the switching operation, and effectively avoids the adverse effects of frequency fluctuations and power imbalance on the system.

[0063] The control mode switching module adapts to changes in the power grid by automatically switching the control mode. When the power grid frequency changes beyond the set value, it first enters the temporary island mode, and then determines whether to switch to the island control mode based on the island signal. This automatic switching enables the system to respond to changes in the power grid in a short time, reduces manual intervention, and improves the flexibility and adaptability of the system. By adjusting the damping coefficient of the network control algorithm, the system can adaptively adjust parameters in different modes, thereby optimizing the frequency response of the power grid and ensuring that the power grid can maintain a high stability whether in networking or island mode.

[0064] The system can dynamically adjust the virtual speed and damping coefficient to keep the grid frequency stable, especially when the grid switches to island mode, to ensure the system's inertia and frequency response capabilities. This adjustment avoids excessive frequency fluctuations or frequency offsets, and can reduce stability risks during island operation. Due to the dynamic adjustment of the virtual speed and inertia time constant, the system can achieve more sophisticated control and accurately respond to changes in the grid state, further improving the reliability and flexibility of grid operation.

[0065] The dynamic adjustment module ensures the balance of frequency and power by tracking the changes in grid frequency and power in real time during the switching process. It not only ensures the stability of the grid during switching, but also effectively avoids high-frequency interference, voltage shock or power fluctuation problems that may be caused during the switching process. Automatic control enables the system to adapt quickly when the grid status changes and switch the control mode in time. 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.

[0066] As a preferred technical means: the adaptive network building module simulates the rotor motion equation of the synchronous generator through the power synchronization link, adjusts the virtual speed ω and the inertia time constant T J ; The rotor motion equation is:

[0067] Where: ω ref and P ref are frequency and active reference value respectively; ω is 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.

[0068] This technical solution uses adaptive networking technology to simulate the rotor motion equation of the synchronous generator and adopts a flexible damping coefficient adjustment strategy to effectively improve the performance of the flexible DC system in frequency regulation, power balance and system stability. This solution has significant advantages, especially suitable for power systems with a large proportion of new energy, which can significantly improve the system's operating stability, reliability and anti-disturbance ability, and provide effective technical support for achieving stable power supply and smart grids.

[0069] As a preferred technical means: the frequency change criterion formula used by the control mode switching module is:

[0070] Where: is the frequency variation of the system AC voltage, is the frequency difference threshold, is the threshold value of the frequency difference change; The dynamic adjustment module sets different damping coefficient values ​​in the networking mode and the island mode, and dynamically adjusts the damping coefficient according to the change of the grid state; the damping coefficient D is Dlw in the networking mode, and this value is small to ensure that the system responds quickly to the change of grid load; in the island mode, it is Dgd, and this value is large to ensure frequency stability.

[0071] This technical solution ensures the stability, reliability and efficiency of the power grid in different working modes by combining accurate frequency change criteria, dynamically adjusting the damping coefficient and flexibly switching the control mode. It can quickly respond to changes in power grid load, optimize frequency stability, improve the adaptability and anti-disturbance ability of the system, and ensure that there is no impact or instability during the system switching process, thus enhancing the overall operation efficiency and economy of the power grid.

[0072] Beneficial effects: This technical solution achieves more accurate and flexible network island switching through adaptive networking technology, ensuring the stability and security of the system in a complex power grid environment, and can effectively deal with the frequency and power fluctuation problems caused by the grid connection of new energy; specifically: 1. By using adaptive networking technology to simulate the rotor motion equation of the synchronous generator, generate virtual speed and inertia time constant, and effectively improve the frequency regulation and inertia response capabilities of the system under weak grid conditions. The stability of the flexible DC system in low inertia and weak grid environments is significantly enhanced, avoiding the risk of instability of traditional control methods under extremely weak conditions.

[0073] 2. This method monitors the frequency changes of the power grid in real time and determines whether the power grid has entered an island state by combining the threshold of the frequency change. With the help of the island signal of the stabilization control system, the system can timely and accurately identify the island state, thereby quickly adjusting the control strategy. This process enables the system to respond to frequency fluctuations of the power grid more flexibly, reduces the possibility of misjudgment, and improves the reliability of the switching process.

[0074] 3. In the process of switching from networking to islanding, a delay mechanism is used to ensure stable operation of the system. Depending on whether the islanding signal of the stabilization control system is received, the system can flexibly switch to the islanding control mode or maintain the networking control mode. At the same time, by adjusting the damping coefficient in the networking control algorithm, the frequency and power of the power grid can be effectively balanced to ensure that the system can be stable in both operating modes.

[0075] 4. During the switching process, the system dynamically adjusts the virtual speed and damping coefficient according to the fluctuations of grid frequency and power. This adaptive adjustment enables the system to switch seamlessly between networking and island modes, ensuring frequency stability and power balance, and avoiding system instability caused by control parameters not adapting to changes in grid conditions.

[0076] 5. The switching procedure takes into account a variety of grid conditions, including changes in grid frequency and the response of the stabilization system signal. Through precise parameter adjustment and intelligent switching, a smoother switching between networking and island mode can be achieved, reducing frequency fluctuations, power shocks and interference to the grid in traditional methods.

[0077] 6. This technical solution takes into account the coordinated work of multiple converter stations for multi-terminal flexible DC systems. Through distributed control strategies, the coordinated work in the entire system ensures 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.

[0078] 7. This technical solution enhances the system's ability to regulate frequency fluctuations, ensuring that the flexible DC system can effectively cope with the uncertainty and volatility brought about by the grid connection of renewable energy, thereby improving the overall operating efficiency of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Figure 1 It is a flow chart of the present invention.

[0080] Figure 2 It is the mainframe line diagram of the simulation model of the present invention.

[0081] Figure 3 It is a simulation waveform diagram of the present invention. DETAILED DESCRIPTION

[0082] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings.

[0083] Embodiment 1: like Figure 1 As shown, The present invention comprises the following steps: S1: The power synchronization link is used to simulate the rotor motion equation of the synchronous generator through adaptive networking technology to obtain the virtual speed ω and inertia time constant T J , where the virtual speed ω is used to adjust the grid frequency, and the inertia time constant T J Inertial characteristics used to adjust the system frequency response; The rotor motion equation is:

[0084] Where: ω ref and P ref are frequency and active reference value respectively; ω is 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.

[0085] This technical solution uses adaptive networking technology to simulate the rotor motion equation of the synchronous generator and adopts a flexible damping coefficient adjustment strategy to effectively improve 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 system's operating stability, reliability and anti-disturbance ability, providing effective technical support for achieving stable power supply and smart grids.

[0086] S2: monitor the grid frequency in real time and determine whether the grid has entered an island state based on changes in the grid frequency; Determining whether an island state has been entered includes: Frequency change criterion: by detecting the change in the grid frequency, determine whether the grid frequency exceeds the preset threshold value; The frequency change criterion formula is:

[0087] Where: is the frequency variation of the system AC voltage, is the frequency difference threshold, is the threshold value of the frequency difference change.

[0088] By introducing the frequency change criterion, the frequency difference threshold and the frequency change change threshold, the rapid and accurate identification of the island state of the power grid is achieved. Its advantage is that 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, it can reduce misjudgment and excessive response, ensure the safe, stable and efficient operation of the power grid, and is particularly suitable for power grid environments with more access to new energy.

[0089] The signal judgment of the stability control system determines whether it is necessary to switch to the island control mode by monitoring the island signal of the stability control system.

[0090] By combining the frequency change criterion and the stabilization system signal criterion, accurate and reliable island state judgment is achieved, improving the real-time response capability and stability of the system. This technology can not only effectively cope with complex grid conditions and reduce misjudgment and delay in island switching, but also optimize the operation of the grid in the island state, ensuring the safe, reliable and efficient operation of the grid.

[0091] S3: When the grid frequency changes beyond a preset threshold, it enters temporary island control mode.

[0092] S4: After a delay period, if the flexible DC system receives the islanding signal from the stabilization control system, it confirms that the power grid is in an islanding state, switches to the islanding control mode, and adjusts the damping coefficient of the network control algorithm.

[0093] S5: If the flexible DC system does not receive the island signal from the stabilization control system, it confirms that the power grid is still in the networking state, maintains the networking control mode, and adjusts the damping coefficient of the network control algorithm; The damping coefficient D is Dlw in the interconnected mode, and this value is small to ensure that the system responds quickly to changes in the grid load; it is Dgd in the island mode, and this value is large to ensure frequency stability.

[0094] By setting different damping coefficients, using a smaller Dlw in the networking mode to improve the response speed, and using a larger Dgd in the island mode to enhance the frequency stability, this technical solution can improve the system's response speed, enhance frequency stability, and improve system safety and reliability. This design fully considers the complexity and uncertainty of power grid operation, has high adaptability, flexibility and customizability, and is an effective technical solution for the stability and intelligence needs of future power systems.

[0095] S6: During the entire switching process, the virtual speed and damping coefficient are dynamically adjusted according to the fluctuations of grid frequency and power to ensure the frequency stability and power balance of the system in both interconnected and island modes.

[0096] When operating in a grid-connected mode, the active power Ps fluctuation does not exceed 1%, and the Dlw value is less than 2.5; when operating in an island mode, the grid frequency f deviation range does not exceed ±0.2Hz, and the Dgd value is greater than 500.

[0097] By limiting active power fluctuations and finely controlling the damping coefficient, not only the frequency stability and rapid response capability of the system under different working conditions are guaranteed, but also the anti-disturbance capability, reliability and safety of the system are effectively improved. The performance of the power grid in the interconnected and islanded states is optimized to ensure that the power system operates efficiently, stably and safely in a complex operating environment, with strong adaptability and flexibility.

[0098] This technical solution achieves more accurate and flexible network island switching through adaptive networking technology, ensuring the stability and security of the system in a complex power grid environment, while effectively coping with the frequency and power fluctuation problems caused by the grid connection of new energy sources; specifically: By adopting adaptive network construction technology, simulating the rotor motion equation of the synchronous generator, generating virtual speed and inertia time constant, the frequency regulation and inertia response capabilities of the system under weak grid conditions are effectively improved. The stability of the flexible DC system in low inertia and weak grid environments is significantly enhanced, avoiding the risk of instability of traditional control methods under extremely weak conditions.

[0099] This method monitors the frequency change of the power grid in real time and determines whether the power grid has entered an island state by combining the threshold of the frequency change. With the help of the island signal of the stabilization control system, the system can timely and accurately identify the island state, thereby quickly adjusting the control strategy. This process enables the system to respond to the frequency fluctuation of the power grid more flexibly, reduces the possibility of misjudgment, and improves the reliability of the switching process.

[0100] In the process of switching from networking to islanding, a delay mechanism is used to ensure stable operation of the system. Depending on whether the islanding signal of the stabilization control system is received, the system can flexibly switch to the islanding control mode or maintain the networking control mode. At the same time, by adjusting the damping coefficient in the networking control algorithm, the frequency and power of the power grid can be effectively balanced to ensure that the system can be stable in both operating modes.

[0101] During the switching process, the system dynamically adjusts the virtual speed and damping coefficient according to the fluctuations of grid frequency and power. This adaptive adjustment enables the system to seamlessly switch between networked and island modes, ensuring frequency stability and power balance, and avoiding system instability caused by control parameters not adapting to changes in grid conditions.

[0102] The switching steps take into account a variety of grid conditions, including changes in grid frequency and the response of stabilization system signals. Through precise parameter adjustment and intelligent switching, a smoother switching between networking and islanding modes can be achieved, reducing frequency fluctuations, power shocks and interference to the grid in traditional methods.

[0103] This technical solution targets the multi-terminal flexible DC system and takes into account the coordinated work of multiple converter stations. Through the distributed control strategy, the coordinated work in the entire system ensures 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.

[0104] This technical solution enhances the system's ability to regulate frequency fluctuations, ensuring that the flexible DC system can effectively cope with the uncertainty and volatility brought about by the grid connection of renewable energy, thereby improving the overall operating efficiency of the system.

[0105] In the entire networking island switching process, this embodiment identifies the networking island state through the control algorithm, switches the damping coefficient D of the networking control algorithm, and realizes the networking island switching of the multi-terminal flexible direct current system based on the adaptive networking technology.

[0106] like Figure 2 As shown in the figure, a detailed model is built in the power system transient simulation software platform PSCAD / EMTDC. The converter station is connected to the busbar through QF1, the photovoltaic system is connected to the busbar through QF2, and the AC grid is connected to the busbar through QF3. The load is connected to the busbar.

[0107] The network-to-island waveform is obtained through simulation, such as Figure 3 As shown. Pvalve is the valve side active power, Ps is the grid side active power, and Ppv is the PV system active power. Us(pu) is the grid side AC voltage mark value, Is(pu) is the grid side AC current mark value, ipa4, ipb4, ipc4, ina4, inb4, inc4 are the bridge arm current values, Ud4(pu) is the DC voltage per unit value, deblk is the unlock signal, PCtrl4 is the active power control flag, QCtrl4 is the reactive power control flag, and QF is Figure 2 In the switch QF3 position on the China power grid side, LANDMode is the network island flag.

[0108] The converter station adopts a grid control strategy. The converter station is unlocked (deblk) at 0.6s, the active power command is 100MW (Pvalve), and it stabilizes to the target value in 1s. The 100MW photovoltaic system is unlocked at 2s and pushes 100MW (Ppv) active power in 2.5s. According to Kirchhoff's current law, the active power (Ps) on the power supply side begins to drop to 0 MW at 2s. At 3.2s, the power switch QF3 (QF) is opened, and at 3.28s, the island criterion (LANDMODE) based on the frequency change is enabled and the damping coefficient D of the flexible direct current system grid control algorithm is switched to Dgd. During the entire process of networking to islanding, no high-frequency components appear, the photovoltaic system does not lose power, and the AC voltage has no impact.

[0109] Embodiment 2: A multi-terminal flexible direct current system networking island switching system comprises: 1. Adaptive network construction module, which is used to simulate the rotor motion equation of the synchronous generator through the power synchronization link, obtain the virtual speed and inertia time constant, so as to adjust the grid frequency and the inertia characteristics of the system frequency response; The adaptive network construction module simulates the rotor motion equation of the synchronous generator to obtain the virtual speed and inertia time constant to adjust the grid frequency and the inertia characteristics of the system. It effectively improves the system's response to frequency fluctuations, enables the system to adjust smoothly when subjected to load fluctuations or grid failures, and reduces the impact of the system transition process.

[0110] 2. Real-time monitoring module, used to monitor the grid frequency in real time and determine whether the grid has entered an island state based on the fluctuation of the grid frequency; The real-time monitoring module tracks the changes in the grid frequency in real time and determines whether the grid has entered an island state. This monitoring function enables the system to quickly detect the island phenomenon when an abnormality occurs in the grid and respond promptly to the switch. When the grid frequency changes beyond the predetermined threshold, the system automatically enters the temporary island control mode. The grid status is confirmed by the island signal of the stabilization system, and then it is decided whether to switch to the island control mode. The accuracy and timeliness of the switching operation are ensured, and the adverse effects of frequency fluctuations and power imbalance on the system are effectively avoided.

[0111] 3. Control mode switching module, which is used to enter the temporary island control mode when the grid frequency changes beyond the preset threshold, and switch to the corresponding control mode after confirming the grid state according to the island signal of the stability control system, and adjust the damping coefficient of the network control algorithm at the same time; The control mode switching module adapts to changes in the power grid by automatically switching the control mode. When the power grid frequency changes beyond the set value, it first enters the temporary island mode, and then determines whether to switch to the island control mode based on the island signal. This automatic switching enables the system to respond to changes in the power grid in a short time, reduces manual intervention, and improves the flexibility and adaptability of the system. By adjusting the damping coefficient of the network control algorithm, the system can adaptively adjust parameters in different modes, thereby optimizing the frequency response of the power grid and ensuring that the power grid can maintain a high stability whether in networking or island mode.

[0112] 4. Dynamic adjustment module, which is used to dynamically adjust the virtual speed and damping coefficient according to the fluctuations of grid frequency and power during the entire switching process to ensure system frequency stability and power balance.

[0113] 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 the frequency stability and power balance of the system, effectively avoiding system instability caused by frequency or power imbalance during the switching process.

[0114] The system can dynamically adjust the virtual speed and damping coefficient to keep the grid frequency stable, especially when the grid switches to island mode, to ensure the system's inertia and frequency response capabilities. This adjustment avoids excessive frequency fluctuations or frequency offsets, and can reduce stability risks during island operation. Due to the dynamic adjustment of the virtual speed and inertia time constant, the system can achieve more sophisticated control and accurately respond to changes in the grid state, further improving the reliability and flexibility of grid operation.

[0115] The dynamic adjustment module ensures the balance of frequency and power by tracking the changes in grid frequency and power in real time during the switching process. It not only ensures the stability of the grid during switching, but also effectively avoids high-frequency interference, voltage shock or power fluctuation problems that may be caused during the switching process. Automatic control enables the system to adapt quickly when the grid status changes and switch the control mode in time. 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.

[0116] The specific functions of each module are repeated in the method and will not be repeated here.

[0117] The above-mentioned method and system for networked island switching of a multi-terminal flexible direct current system based on adaptive networking is a specific embodiment of the present invention, which has embodied the essential characteristics and progress of the present invention. It can be equivalently modified according to actual usage needs under the guidance of the present invention, and all are within the protection scope of this scheme.

Claims

1. A method for switching islands in a multi-terminal flexible direct current system based on adaptive networking, characterized in that The following steps are involved: 1) Through the adaptive network construction technology, the power synchronization link is used to simulate the rotor motion equation of the synchronous generator to obtain the virtual speed ω and inertia time constant T J , Among them, the virtual speed ω is used to adjust the grid frequency, and the inertia time constant T J Inertial characteristics used to adjust the system frequency response; 2) Monitor the grid frequency in real time and determine whether the grid has entered an island state based on changes in the grid frequency; 3) When the grid frequency changes beyond the preset threshold, it enters the temporary island control mode; 4) After a delay period, if the flexible DC system receives the islanding signal from the stabilization control system, it confirms that the power grid is in an islanding state, switches to the islanding control mode, and adjusts the damping coefficient of the grid control algorithm; 5) If the flexible DC system does not receive the island signal from the stabilization control system, it confirms that the power grid is still in the networking state, maintains the networking control mode, and adjusts the damping coefficient of the network control algorithm; 6) During the entire switching process, the virtual speed and damping coefficient are dynamically adjusted according to the fluctuations of grid frequency and power to ensure frequency stability and power balance of the system in both interconnected and island modes.

2. According to the method of claim 1, a multi-terminal flexible direct current system networking island switching method based on adaptive networking is characterized by: The rotor motion equation is: Where: ω ref and P ref are frequency and active reference value respectively; ω is 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. According to claim 2, a method for switching multi-terminal flexible direct current system network islands based on adaptive networking, characterized in that: The determining whether to enter an isolated island state comprises: Frequency change criterion: by detecting the change in the grid frequency, determine whether the grid frequency exceeds the preset threshold value; The signal judgment of the stability control system determines whether it is necessary to switch to the island control mode by monitoring the island signal of the stability control system.

4. According to claim 3, a method for switching multi-terminal flexible direct current system network islands based on adaptive networking, characterized in that: The frequency change criterion formula is: Where: is the frequency variation of the system AC voltage, is the frequency difference threshold, is the threshold value of the frequency difference change.

5. According to the method of claim 2, a multi-terminal flexible direct current system networking island switching method based on adaptive networking is characterized in that: The damping coefficient D is Dlw in the networking mode, and the value is small to ensure that the system responds quickly to changes in the grid load; in the island mode, the value is Dgd, and the value is large to ensure frequency stability.

6. According to claim 5, a method for switching multi-terminal flexible direct current system network islands based on adaptive networking, characterized in that: When operating in a grid-connected mode, the active power Ps fluctuation does not exceed 1%, and the Dlw value is less than 2.5; when operating in an island mode, the grid frequency f deviation range does not exceed ±0.2Hz, and the Dgd value is greater than 500.

7. The method for switching islands in a multi-terminal flexible direct current system based on adaptive networking according to claim 6 is characterized by: During the above-mentioned networking and islanding process, the converter station adopts a networking control strategy, including the following contents: At 0.6 seconds, the converter station is unlocked and the active power command is set to the target value; Unlock the photovoltaic system at 2 seconds and complete the push of active power; After the power switch is turned on, the islanding judgment is triggered based on the frequency change criterion, and the damping coefficient D of the network control algorithm is switched to the islanding operation value Dgd; The islanding criterion based on frequency variation takes effect after 3.28 seconds, and LANDMode is used as the sign signal of islanding operation; In the process of connecting to an island, 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; During the process of switching from interconnected to isolated island, the network control algorithm of the flexible DC system dynamically adjusts the target power through the unlocking 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.

8. A multi-terminal flexible direct current system networking island switching system adopting a multi-terminal flexible direct current system networking island switching method based on adaptive networking as described in any one of claims 1 to 7, characterized in that include: The adaptive network construction module is used to simulate the rotor motion equation of the synchronous generator through the power synchronization link, obtain the virtual speed and inertia time constant, and adjust the inertia characteristics of the grid frequency and system frequency response; Real-time monitoring module, used to monitor the grid frequency in real time and determine whether the grid has entered an island state based on the fluctuation of the grid frequency; The control mode switching module is used to enter the temporary island control mode when the grid frequency changes beyond the preset threshold, and switch to the corresponding control mode after confirming the grid state according to the island signal of the stabilization control system, and adjust the damping coefficient of the network control algorithm at the same time; The dynamic adjustment module is used to dynamically adjust the virtual speed and damping coefficient according to the fluctuations of grid frequency and power during the entire switching process to ensure system frequency stability and power balance.

9. The multi-terminal flexible direct current system networking island switching system according to claim 8, characterized in that: The adaptive network 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: Where: ω ref and P ref are frequency and active reference value respectively; ω is 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.

10. The multi-terminal flexible direct current system networking island switching system according to claim 9, characterized in that: The frequency change criterion formula used by the control mode switching module is: Where: is the frequency variation of the system AC voltage, is the frequency difference threshold, is the threshold value of the frequency difference change; The dynamic adjustment module sets different damping coefficient values ​​in the network mode and the island mode, and dynamically adjusts the damping coefficient according to the change of the power grid state; The damping coefficient D is Dlw in the interconnected mode, and this value is small to ensure that the system responds quickly to changes in the grid load; it is Dgd in the island mode, and this value is large to ensure frequency stability.

Images