Transient overvoltage suppression method for large-scale wind power through extra-high voltage direct current transmission

Abstract

This invention relates to a transient overvoltage suppression method for large-scale wind power transmission via ultra-high voltage direct current (UHVDC), belonging to the field of new energy grid-connected stability control technology. The method addresses the problem of transient overvoltage at the sending end caused by reactive power control delay of wind turbines after a DC fault. It divides the low-voltage ride-through process into three stages: voltage drop, recovery, and stabilization, and sets differentiated control objectives. The core of the method lies in establishing an equivalent circuit model of the sending-end system, analytically deriving the dynamic equation of the grid-connected voltage, calculating the optimal reactive current command and compensation amount in real time, and pre-adjusting reactive power injection during the voltage recovery stage to offset the impact of control delay. Simultaneously, a multi-dimensional intelligent switching criterion integrating voltage amplitude, rate of change, and system short-circuit ratio is constructed, enabling adaptive and precise start-up and graded exit of the control strategy. This invention effectively suppresses transient overvoltage, improves system stability, requires no additional hardware, and has strong engineering feasibility.

Description

Technical Field

[0001] This invention belongs to the field of new energy grid connection stability control technology, specifically involving a transient overvoltage suppression method for large-scale wind power transmission via ultra-high voltage direct current. Background Technology

[0002] With the deepening of global energy structure transformation and the "dual carbon" strategic goal, the large-scale wind power bases in western and northern my country have achieved cross-regional clean power transmission through ultra-high voltage direct current (UHVDC) transmission projects, which has become a core technical path for optimizing energy resource allocation and enhancing the absorption capacity of new energy sources. However, UHVDC transmission systems are susceptible to multiple disturbances in actual operation, such as AC system faults at the sending and receiving ends, commutation failures, and DC blocking. When typical faults such as commutation failures occur in the DC system, the sending-end AC grid will experience a significant transient process. The voltage dynamic characteristics at the grid connection point exhibit a typical nonlinear response feature of "first a sharp drop and then a steep rise," with the transient overvoltage phenomenon during the voltage recovery phase being particularly prominent and dangerous. The physical essence of transient overvoltage stems from the instantaneous imbalance of reactive power in the system during a fault: In the initial stage of DC commutation failure, the reactive power absorbed by the converter station decreases sharply due to the sudden drop in DC power. At the same time, the sending-end wind farm injects a large amount of capacitive reactive current according to the low voltage ride-through specification to support the dropped grid connection voltage. When the DC system recovers rapidly, if the reactive power control system of the wind turbine fails to reduce reactive power output in time due to inherent response delay (usually determined by the controller sampling period, filtering link, and command generation and execution link), the excess capacitive reactive power will cause the voltage of the sending-end system to rise sharply, forming a transient overvoltage spike with an amplitude significantly exceeding the rated voltage level of the system. This overvoltage not only seriously threatens the insulation safety of key equipment such as power devices of the wind turbine converter, turbine transformer, and collector lines, but may also trigger protection interlocking actions, causing large-scale wind turbine disconnection, thereby destroying the transient stability of the entire sending-end system and causing serious socio-economic losses and the risk of energy supply interruption.

[0003] Existing low-voltage ride-through control strategies for wind turbines primarily focus on enhancing reactive power support during voltage dips. They generally employ a droop control strategy based on the grid connection point voltage amplitude to generate capacitive reactive current commands to support voltage recovery. However, this type of strategy lacks a proactive compensation mechanism for control delay effects during the voltage recovery phase. Its reactive power command adjustment heavily relies on real-time feedback of voltage amplitude, causing reactive power reduction to lag behind voltage change trends during rapid voltage recovery. This not only fails to effectively suppress overvoltage but also exacerbates voltage overshoot due to the delay effect. Furthermore, the phase switching logic of traditional control strategies often uses a single voltage amplitude threshold criterion, failing to comprehensively consider... Key dynamic characteristics such as voltage change rate, trend, and grid strength are prone to misjudgment of switching timing under voltage fluctuations or weak grid conditions (e.g., premature withdrawal of reactive power support leading to secondary voltage drops, or delayed reduction of reactive power causing overvoltage deterioration). While some improved solutions attempt to introduce voltage change rate as an auxiliary criterion, they lack a theoretical mapping relationship between criterion parameters (such as sensitivity coefficients) and grid electrical characteristic parameters such as system short-circuit ratio and impedance ratio. Parameter tuning heavily relies on field commissioning experience, lacking universality and adaptability, making it difficult to maintain robustness and effectiveness of control performance under different grid strength scenarios (especially weak grids with low short-circuit ratios). Therefore, there is an urgent need to propose a transient overvoltage suppression method that can deeply integrate voltage dynamic characteristics, system electrical parameters, and control delay mechanisms. This method should achieve precise timing division of control strategies, forward-looking compensation of reactive power commands, and adaptive tuning of switching criteria to comprehensively improve the transient voltage stability level of large-scale wind power transmission systems via UHVDC. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for suppressing transient overvoltages in large-scale wind power transmission via ultra-high voltage direct current. By systematically analyzing the coupling mechanism between reactive power control delay and transient voltage dynamic response, a phased adaptive control framework based on three-stage timing division and linked by intelligent criteria is constructed, thereby achieving active, precise, and robust suppression of transient overvoltages.

[0005] To achieve the above objectives, the technical solution of the present invention is: a method for suppressing transient overvoltages in large-scale wind power transmission via ultra-high voltage direct current, comprising:

[0006] The low-voltage ride-through process of wind farms after DC system faults is divided into three stages: voltage drop, voltage recovery, and voltage stabilization.

[0007] Differentiated control objectives are set to address the coupling effect of voltage change characteristics and reactive power control delay at each stage: providing maximum capacitive reactive power support to suppress voltage drop during the voltage dip stage, reducing reactive power injection in advance to offset the effect of control delay during the voltage recovery stage, and smoothly exiting fault ride-through control during the voltage stabilization stage.

[0008] Based on Thevenin's equivalent theory, an equivalent circuit model of the sending-end system is established. The quantitative relationship between the grid connection point voltage and the wind turbine current of the wind farm is analytically derived. Combining the system short-circuit ratio and impedance ratio, the optimal reactive current command during the voltage recovery stage is solved in real time. The adaptive reactive compensation amount is calculated as the difference between the optimal reactive current command and the low-voltage ride-through reactive current command, and then superimposed on the original reactive current command.

[0009] A smart switching criterion integrating voltage amplitude, voltage change rate, and system short-circuit ratio is constructed to achieve adaptive start-up and graded exit of the control strategy.

[0010] Furthermore, the specific control strategies for the three stages of voltage drop, voltage recovery, and voltage stabilization are as follows:

[0011] During the voltage sag phase, corresponding to the 0~t1 time period of rapid voltage drop at the grid connection point during the initial DC fault, the wind turbine executes the maximum reactive power support mode, calculating the reactive current command based on the voltage droop characteristics at the grid connection point. , where V PCC Where is the grid connection point voltage of the wind farm, k is the reactive current regulation coefficient, and I is the voltage at the grid connection point of the wind farm. N The rated current of the wind turbine is used, while the reactive current is limited to [value missing]. I d This refers to the active current component output by the wind turbine. This is the maximum allowable current of the converter;

[0012] During the voltage recovery phase, corresponding to the t2~t3 time period when the grid connection point voltage rapidly recovers, the reactive current command is adjusted to... , This is the reactive current command compensation amount;

[0013] During the voltage stabilization phase, corresponding to the t3~t4 time period after the grid connection point voltage has basically recovered and stabilized, the fault ride-through control is exited, and the reactive current command is gradually reduced to 0 according to the preset slope.

[0014] Furthermore, the quantitative relationship between the grid connection point voltage of the wind farm and the wind turbine current is as follows:

[0015]

[0016] Among them, V PCC V is the voltage at the grid connection point of the wind farm. g Let I be the system's equivalent potential, r and x be the system's equivalent resistance and equivalent reactance, respectively. d and I q These are the active current component and reactive current component output by the wind turbine, respectively.

[0017] Furthermore, the optimal reactive current command The solution process is as follows:

[0018] Define system short-circuit ratio and impedance ratio , Let r be the magnitude of the system's equivalent impedance, and x be the system's equivalent resistance and equivalent reactance, respectively. Rewrite the analytical expression for the grid connection point voltage in a form that incorporates system characteristics. V g Let V be the system's equivalent potential, and let the control target be V. PCC =V ref =1.0 per unit value, defining the simplification factor. The reactive current component I of the wind turbine output is obtained by sorting. q The quadratic equation of The coefficients are respectively , , The optimal reactive current is obtained by selecting the effective positive root that satisfies the physical constraints. .

[0019] Furthermore, reactive current command compensation amount The converter current amplitude constraint must be met. ,in, For reactive current command, I d I represents the active current component output by the wind turbine. max The maximum allowable current of the converter is used. When the calculated reactive current command compensation exceeds the constraint conditions, the boundary value is taken. ,in, The reactive current command compensation amount is calculated based on the difference between the optimal reactive current command and the low-voltage ride-through reactive current command.

[0020] Furthermore, the intelligent switching criteria include criteria for switching from the normal phase to the voltage dip phase, rising edge criteria for switching from the voltage dip phase to the voltage recovery phase, and exit criteria for switching from the voltage recovery phase to the voltage stabilization phase; wherein,

[0021] The criterion for switching from the normal phase to the voltage sag phase is: V PCC <0.8 and <0;

[0022] The rising edge criterion for switching from the voltage dip phase to the voltage recovery phase must simultaneously meet two necessary conditions: (1) the grid connection point voltage. The preset low voltage threshold Take 0.8~0.85pu; (2) Voltage change rate ,in This is the voltage recovery sensitivity coefficient;

[0023] The criterion for switching from the voltage recovery phase to the voltage stabilization phase is: the grid connection point voltage satisfies 0.9 ≤ V. PCC ≤1.1 and the stable state lasts for more than the holding time threshold. .

[0024] Furthermore, the voltage recovery sensitivity coefficient Adopting system short-circuit ratio The adaptive tuning method has the following tuning formula: ,in The reference sensitivity coefficient, This is the gain coefficient.

[0025] Furthermore, the optimal reactive current command during the voltage recovery phase and reactive current command compensation amount Real-time calculations and updates are performed within each control sampling period to adapt to dynamic changes in the system state.

[0026] Furthermore, the method also includes a timing coordination mechanism, which ensures the orderly coordination of switching criteria for each stage through priority sorting and state latching; wherein, the priorities from high to low are: the criterion for switching from the normal stage to the voltage drop stage, the rising edge criterion for switching from the voltage drop stage to the voltage recovery stage, and the exit criterion for switching from the voltage recovery stage to the voltage stabilization stage; when a certain criterion is triggered and switches to the corresponding control stage, the current stage state is latched, and the trigger signals of other low-priority criteria are blocked until the exit condition of the corresponding stage is met.

[0027] The present invention also provides a computer-readable storage medium having stored thereon computer program instructions that can be executed by a processor, wherein when the processor executes the computer program instructions, it can implement the steps of the method described above.

[0028] Compared with existing technologies, this invention has the following advantages: By deeply analyzing the coupling mechanism between reactive power control delay and transient voltage dynamics, this invention constructs a three-stage adaptive control framework of "voltage drop-recovery-stability," breaking through the limitations of traditional strategies that only focus on the voltage drop stage, and achieving precise control of the entire fault process. It designs a multi-dimensional intelligent switching criterion integrating voltage amplitude, rate of change, and system short-circuit ratio, establishing an adaptive tuning relationship between the criterion parameters and the system short-circuit ratio, avoiding the limitations of a single threshold criterion, achieving precise judgment of switching timing under different grid strength conditions, and improving the robustness of the control strategy. This method is entirely based on the existing control system software logic of wind turbine units, requiring no additional external hardware or changes to the system topology. It has low engineering implementation costs and strong compatibility, and has significant theoretical value and engineering application prospects for improving the safe and stable operation of my country's large-scale new energy bases via ultra-high voltage DC transmission channels. Attached Figure Description

[0029] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0030] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0031] This invention provides a method for suppressing transient overvoltages in large-scale wind power transmission via ultra-high voltage direct current (UHVDC), comprising:

[0032] The low-voltage ride-through process of wind farms after DC system faults is divided into three stages: voltage drop, voltage recovery, and voltage stabilization.

[0033] Differentiated control objectives are set to address the coupling effect of voltage change characteristics and reactive power control delay at each stage: providing maximum capacitive reactive power support to suppress voltage drop during the voltage dip stage, reducing reactive power injection in advance to offset the effect of control delay during the voltage recovery stage, and smoothly exiting fault ride-through control during the voltage stabilization stage.

[0034] Based on Thevenin's equivalent theory, an equivalent circuit model of the sending-end system is established. The quantitative relationship between the grid connection point voltage and the wind turbine current of the wind farm is analytically derived. Combining the system short-circuit ratio and impedance ratio, the optimal reactive current command during the voltage recovery stage is solved in real time. The adaptive reactive compensation amount is calculated as the difference between the optimal reactive current command and the low-voltage ride-through reactive current command, and then superimposed on the original reactive current command.

[0035] A smart switching criterion integrating voltage amplitude, voltage change rate, and system short-circuit ratio is constructed to achieve adaptive start-up and graded exit of the control strategy.

[0036] The following is a detailed implementation process of the present invention.

[0037] The core steps of this invention's transient overvoltage suppression method for large-scale wind power transmission via ultra-high voltage direct current (UHVDC) include: First, the entire process of low-voltage ride-through of wind farms caused by DC system fault disturbances is scientifically divided into three continuous and mutually exclusive time intervals based on voltage dynamic characteristics and differences in control objectives: voltage drop stage (Stage I), voltage recovery stage (Stage II), and voltage stabilization stage (Stage III). Differentiated control objectives are set for each stage—Stage I corresponds to the time interval from the initial moment of the DC fault to the voltage drop at the grid connection point reaching its lowest point [0 ~ t1], focusing on providing maximum capacitive reactive power support to suppress the voltage drop depth; Stage II corresponds to the transition interval [t2 ~ t3] where the grid connection point voltage begins to recover from its lowest point to near its rated value but is not yet fully stable, aiming to proactively reduce reactive power injection to offset control delay effects and avoid excessive voltage recovery; Stage III corresponds to the subsequent interval [t3 ~ t3] where the grid connection point voltage has recovered to its normal operating fluctuation range and remains stable. Phase III focuses on smoothly exiting the fault ride-through control logic to restore the system to normal operation. t1 represents the critical moment when the grid connection point voltage reaches its minimum dynamic value. t2 is physically closely connected to t1 (t2≈t1). t3 represents the moment when the voltage recovery process is basically completed but the system still needs to maintain fault ride-through control. t4 represents the moment when the fault ride-through control logic completely exits and the system returns to normal operation control mode. Secondly, a simplified equivalent circuit model of the sending-end AC system is constructed based on Thevenin's equivalent theory. The nonlinear quantitative relationship between the wind farm grid connection point voltage and the wind turbine output current component is rigorously derived analytically, and the system short-circuit ratio is introduced. With impedance ratio As a key parameter characterizing grid strength, it is used to solve in real time the optimal reactive current command required to maintain the grid connection point voltage at a predetermined reference value during the voltage recovery phase. And define the adaptive reactive power compensation quantity. This optimal command differs from the traditional low-voltage ride-through reactive current command. The algebraic difference is used to superimpose the compensation amount onto the original reactive current command, thereby achieving precise control of the grid connection point voltage during the voltage recovery phase; finally, a fused grid connection point voltage amplitude is constructed. Voltage change rate and system short-circuit ratio A multi-dimensional intelligent switching criterion system is established, and key criterion parameters (such as voltage recovery sensitivity coefficient) are defined. The quantitative mapping relationship between the control strategy and the system electrical characteristics enables adaptive and precise switching and hierarchical orderly exit of the control strategy from the normal operation stage to stage I, stage I to stage II, and stage II to stage III, ensuring the timing rigor and logical closed-loop nature of the entire control process.

[0038] This invention discloses a transient overvoltage suppression method for large-scale wind power transmission via ultra-high voltage direct current (UHVDC), specifically comprising:

[0039] (a) Control implementation during the voltage sag phase (phase I: 0 ~ t1)

[0040] Phase I spans from the moment the DC fault occurs (t=0) to the moment the grid connection voltage drops to its lowest value (t=t1). The core challenge in this phase is the rapid voltage drop caused by the system's reactive power deficit. The control objective is to inject capacitive reactive power to the maximum extent possible while ensuring the safety of the converter, thereby suppressing the depth and rate of voltage drop. The specific implementation steps are as follows:

[0041] The first step, fault monitoring and criterion triggering: the system collects the grid connection point voltage in real time. and voltage change rate When both conditions are met and When the system is determined to enter the voltage drop phase, the Phase I control logic is immediately triggered, and the trigger time is recorded as t=0 to start the phase timing.

[0042] The second step is the calculation of the reactive current command: Calculating the traditional low-voltage ride-through reactive current command based on the grid connection point voltage droop characteristics. ,in This is the reactive current regulation coefficient; This refers to the rated current of the wind turbine generator. This refers to the per-unit value of the grid connection point voltage collected in real time.

[0043] The third step is to handle the converter capacity constraint: To prevent the converter from being damaged by overcurrent, the calculated capacity constraint needs to be adjusted. Amplitude limiting is applied, with the limiting threshold set according to... Calculation, where This is the maximum allowable current of the converter, determined by the converter's design parameters, and is typically 1.1 to 1.2 times the rated current. The active current component output by the wind turbine is acquired in real time from the converter controller. Its value will adjust with changes in DC power during faults, but to ensure reactive power support capability, it needs to be maintained. Within a reasonable range, avoid due to Too large If it's too small, it will affect the reactive power support effect. The limiting logic is: if... Then directly adopt As the final reactive current command; if Then As the final instruction, ensure that control commands do not exceed equipment capacity constraints.

[0044] The fourth step, command issuance and real-time adjustment: The reactive current command, after being limited, is issued to the wind turbine converter controller. The converter adjusts the output current using pulse width modulation (PWM) technology to achieve precise injection of capacitive reactive power. Simultaneously, during Phase I operation, continuous monitoring... , , Key parameters are recalculated every sampling period (usually 1ms). and The reactive current command is dynamically adjusted to ensure a continuous response to voltage drop trends until the voltage drops to its lowest value, thus preparing for phase switching.

[0045] (II) Control Implementation during the Voltage Recovery Phase (Phase II: t2 ~ t3)

[0046] Phase II spans from the moment the voltage begins to stabilize and rise (t= t2) to the moment the voltage approaches its rated value (t=t3). The core challenge in this phase is the excessive reactive power injection caused by control delay. The key control strategy is to preemptively reduce reactive power injection through proactive compensation control, thereby offsetting the impact of control delay and suppressing voltage overshoot. The specific implementation steps are as follows:

[0047] Step 1, Phase Switching Criterion Monitoring and Triggering: During Phase I operation, continuously monitor the grid connection point voltage. and voltage change rate The transition from Phase I to Phase II is triggered when two necessary conditions are met simultaneously: one is... (Preset low voltage threshold) Take 0.8~0.85 pu), ensuring the voltage has dropped to the low voltage range and the impact of the fault has fully manifested; secondly, ,in The voltage is calculated using an adaptive tuning formula to ensure it has entered a stable recovery trend. The switching trigger time is recorded as t = t2, initiating the Phase II control logic.

[0048] The second step is to acquire the system's equivalent parameters and real-time status variables: After starting Phase II control, the equivalent parameters and real-time status variables of the sending-end system are acquired first, including the system's equivalent potential. System equivalent resistance r, equivalent reactance x, and active current of the wind turbine generator. Grid connection point voltage wait.

[0049] The third step is to calculate the system characteristic parameters: based on the collected equivalent system parameters, calculate the system short-circuit ratio. ( and impedance ratio ,in Characterizes the strength of the system. The inductive characteristics that characterize the equivalent impedance of a system.

[0050] Step 4: Optimal reactive current command Solution: First, define the simplification coefficients. Transform the analytical expression of the grid connection point voltage into a function of... The quadratic equation of The coefficients a, b, and c are calculated according to their respective formulas. After solving the quadratic equation, two roots are obtained. These roots are then selected based on physical constraints: the positive root is prioritized (corresponding to capacitive reactive power injection, which meets the control requirements of the voltage recovery phase), while ensuring that the reactive current corresponding to this root does not exceed the converter capacity constraint. Finally, the optimal reactive current command is determined. .

[0051] Step 5: Reactive current compensation Calculation and constraint handling: by Calculate the compensation amount, where The reactive current command value is used for the last sampling cycle of Phase I to ensure the continuity of the compensation. Subsequently, a converter current amplitude constraint check is performed on the compensation, i.e., verification. Is it true? If true, then use the calculated result. If not, then take the boundary value. This ensures that the total reactive current after compensation does not exceed the converter's capacity limit.

[0052] Step 6, Final Reactive Current Command Issuance and Dynamic Adjustment: The compensation amount is superimposed on the original low-voltage ride-through reactive current command to obtain the final reactive current command for Stage II. This information is then sent to the converter controller. During Phase II operation, the calculation process described in steps three through six is ​​repeated for each sampling period, dynamically updating... , , , and final instructions This ensures that the compensation amount is adapted to the system status and voltage change trend in real time, continuously suppressing voltage overshoot until the voltage approaches the rated value range.

[0053] (III) Control Implementation during the Voltage Stabilization Phase (Phase III: t3 ~ t4)

[0054] Phase III spans from the moment the voltage enters the normal fluctuation range (t = t3) to the moment the reactive current command drops to 0 and the system returns to normal operation (t = t4). The core objective of this phase is to smoothly exit fault ride-through control, avoid voltage oscillations caused by sudden changes in reactive current command, and ensure a stable transition of the system to normal operation. The specific implementation steps are as follows:

[0055] Step 1, Phase Switching Criterion Monitoring and Triggering: During Phase II operation, continuously monitor the grid connection point voltage. and stable duration, when Enter The normal fluctuation range of the (per unit value) and the duration of this state exceeds the holding time threshold. When the system enters the voltage stabilization phase, it is determined that the system has entered the phase II to phase III phase. The switching time is recorded as t = t3. The phase II timing is stopped, and the phase III control logic is started.

[0056] The second step is to gradually exit the reactive current command control: after starting the Phase III control, the compensation calculation logic of Phase II is stopped, and the reactive current command gradual reduction control is started to gradually reduce the current reactive current command to 0 according to the preset slope.

[0057] The third step is to exit the fault ride-through control logic: when the reactive current command drops to 0, continuous monitoring is performed. Whether it remains within the normal fluctuation range, if it operates stably for more than If the fault-crossing control logic is completely exited, including the three-stage control module, the multi-dimensional switching criterion module, and the adaptive compensation calculation module, the wind turbine will return to the conventional reactive power control strategy under normal operating conditions, and the conventional controller will maintain the grid connection point voltage stability.

[0058] Step 4, System Status Monitoring and Reset: After the control logic exits, continuously monitor key system parameters, including... , , The system short-circuit ratio and other parameters are used to ensure stable system operation. At the same time, the timers, criterion status, compensation calculation results and other parameters of each stage are reset to prepare for the next fault and complete the closed loop of the entire fault ride-through control process.

[0059] In the timing coordination mechanism of the entire control flow, to achieve orderly coordination of switching criteria at each stage and avoid conflicts between different criteria, a timing coordination mechanism of "priority sorting + state latching" is constructed. The priority of each criterion from high to low is as follows: voltage drop stage start criterion > voltage recovery stage rising edge criterion > voltage stabilization stage exit criterion. This ensures that low voltage support is activated first when a fault occurs, overvoltage suppression is executed first when the fault recovers, and control is exited in an orderly manner after the system stabilizes. The state latching mechanism is as follows: when a certain criterion is triggered and switches to the corresponding control stage, the current stage state is latched, while the trigger signals of other low-priority criteria are blocked until the exit condition of the stage is met or the fault state undergoes a fundamental change.

[0060] This invention significantly improves the transient voltage stability and safety margin of large-scale wind power transmission systems via ultra-high voltage direct current (UHVDC) under various disturbance scenarios, and provides a new approach to solving the common technical problem of transient overvoltage at the DC transmission end in the context of high proportion of new energy access.

[0061] The present invention also provides a computer-readable storage medium having stored thereon computer program instructions that can be executed by a processor, wherein when the processor executes the computer program instructions, it can implement the steps of the method described above.

[0062] The above are preferred embodiments of the present invention. Any changes made to the technical solution of the present invention that do not exceed the scope of the technical solution of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for suppressing transient overvoltages in large-scale wind power transmission via ultra-high voltage direct current (UHVDC), characterized in that, include: The low-voltage ride-through process of wind farms after DC system faults is divided into three stages: voltage drop, voltage recovery, and voltage stabilization. Differentiated control objectives are set to address the coupling effect of voltage change characteristics and reactive power control delay at each stage: providing maximum capacitive reactive power support to suppress voltage drop during the voltage dip stage, reducing reactive power injection in advance to offset the effect of control delay during the voltage recovery stage, and smoothly exiting fault ride-through control during the voltage stabilization stage. Based on Thevenin's equivalent theory, an equivalent circuit model of the sending-end system is established. The quantitative relationship between the grid connection point voltage and the wind turbine current of the wind farm is analytically derived. Combining the system short-circuit ratio and impedance ratio, the optimal reactive current command during the voltage recovery stage is solved in real time. The adaptive reactive current command compensation is calculated as the difference between the optimal reactive current command and the low-voltage ride-through reactive current command, and then added to the original reactive current command. Construct an intelligent switching criterion that integrates voltage amplitude, voltage change rate, and system short-circuit ratio to achieve adaptive start-up and graded exit of the control strategy; The optimal reactive current command The solution process is as follows: Define system short-circuit ratio and impedance ratio , Let r be the magnitude of the system's equivalent impedance, and x be the system's equivalent resistance and equivalent reactance, respectively. Rewrite the analytical expression for the grid connection point voltage in a form that incorporates system characteristics. V PCC V is the voltage at the grid connection point of the wind farm. g I is the equivalent potential of the system. d and I q Let V be the active current component and reactive current component output by the wind turbine, respectively, and let the control target V be... PCC =V ref =1.0 per unit value, defining the simplification factor. The reactive current component I of the wind turbine output is obtained by sorting. q The quadratic equation of The coefficients are respectively , , The optimal reactive current is obtained by selecting the effective positive root that satisfies the physical constraints. ; Reactive current command compensation amount The converter current amplitude constraint must be met. ,in, For reactive current command, I max The maximum allowable current of the converter is used. When the calculated reactive current command compensation exceeds the constraint conditions, the boundary value is taken. ,in, The reactive current command compensation amount is calculated based on the difference between the optimal reactive current command and the low-voltage ride-through reactive current command.

2. The transient overvoltage suppression method for large-scale wind power transmission via ultra-high voltage direct current as described in claim 1, characterized in that, The specific control strategies for the three stages of voltage dip, voltage recovery, and voltage stabilization are as follows: During the voltage sag phase, corresponding to the 0~t1 time period of rapid voltage drop at the grid connection point during the initial DC fault, the wind turbine executes the maximum reactive power support mode, calculating the reactive current command based on the voltage droop characteristics at the grid connection point. Where k is the reactive current regulation coefficient, I N The rated current of the wind turbine is used, while the reactive current is limited to [value missing]. ; During the voltage recovery phase, corresponding to the t2~t3 time period when the grid connection point voltage rapidly recovers, the reactive current command is adjusted to... ; During the voltage stabilization phase, corresponding to the t3~t4 time period after the grid connection point voltage has basically recovered and stabilized, the fault ride-through control is exited, and the reactive current command is gradually reduced to 0 according to the preset slope.

3. The transient overvoltage suppression method for large-scale wind power transmission via ultra-high voltage direct current as described in claim 1, characterized in that, The quantitative relationship between the grid connection point voltage and the wind turbine current of the wind farm is as follows: 。 4. The transient overvoltage suppression method for large-scale wind power transmission via ultra-high voltage direct current as described in claim 1, characterized in that, The intelligent switching criteria include criteria for switching from the normal phase to the voltage dip phase, rising edge criteria for switching from the voltage dip phase to the voltage recovery phase, and exit criteria for switching from the voltage recovery phase to the voltage stabilization phase; wherein, The criterion for switching from the normal phase to the voltage sag phase is: V PCC <0.8 and <0; The rising edge criterion for switching from the voltage dip phase to the voltage recovery phase must simultaneously meet two necessary conditions: (1) the grid connection point voltage. The preset low voltage threshold Take 0.8~0.85pu; (2) Voltage change rate ,in This is the voltage recovery sensitivity coefficient; The criterion for switching from the voltage recovery phase to the voltage stabilization phase is: the grid connection point voltage satisfies 0.9 ≤ V. PCC ≤1.1 and the stable state lasts for more than the holding time threshold. .

5. The transient overvoltage suppression method for large-scale wind power transmission via ultra-high voltage direct current as described in claim 4, characterized in that, The voltage recovery sensitivity coefficient Adopting system short-circuit ratio The adaptive tuning method has the following tuning formula: ,in The reference sensitivity coefficient, This is the gain coefficient.

6. The transient overvoltage suppression method for large-scale wind power transmission via ultra-high voltage direct current as described in claim 1, characterized in that, The optimal reactive current command during the voltage recovery phase and reactive current command compensation amount Real-time calculations and updates are performed within each control sampling period to adapt to dynamic changes in the system state.

7. The transient overvoltage suppression method for large-scale wind power transmission via ultra-high voltage direct current as described in claim 4, characterized in that, It also includes a timing coordination mechanism, which ensures the orderly coordination of switching criteria for each stage through priority sorting and state latching. The priorities from high to low are: the criterion for switching from the normal stage to the voltage drop stage, the rising edge criterion for switching from the voltage drop stage to the voltage recovery stage, and the exit criterion for switching from the voltage recovery stage to the voltage stabilization stage. When a certain criterion is triggered and the corresponding control stage is switched, the current stage state is latched, and the trigger signals of other low-priority criteria are blocked until the exit condition of the corresponding stage is met.

8. A computer-readable storage medium, characterized in that, It stores computer program instructions that can be executed by a processor, and when the processor executes the computer program instructions, it can implement the steps of the method as described in any one of claims 1-7.

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