Coordination control method for wind power through flexible direct current grid-connected system based on improved control structure

Abstract

The invention relates to the technical field of fan grid connection, in particular to a wind power coordinated control method through a flexible direct current grid connection system based on an improved control structure, and a dynamic negative sequence current amplitude limiting unit, a frequency additional control unit and an alternating current voltage additional control unit are added in a control unit of a sending end converter station. When an asymmetric fault occurs in a sending end line, a negative sequence current amplitude limiting value is dynamically adjusted through a dynamic negative sequence current amplitude limiting unit to optimize negative sequence current control; when a three-phase fault occurs in a sending end line, the frequency is adjusted through droop of the frequency additional control unit; during the short-circuit period of the transmitting-end line, the positive-sequence AC voltage reference value is adjusted in real time through the AC voltage additional control unit, and the outlet voltage of the transmitting-end converter station is reduced, so that the overvoltage risk of the converter is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of wind turbine grid connection, and in particular to a coordinated control method of a wind power flexible direct current grid connection system based on an improved control structure. Background Art

[0002] As the global demand for renewable energy continues to increase, wind power generation has been widely used as a clean and sustainable form of energy. However, the grid-connected technology of wind power generation systems faces many challenges, especially the problem of stable operation under grid fault conditions. Traditional wind power generation systems usually adopt fixed control strategies when connected to the grid. These strategies often show problems such as control lag, over-suppression or performance conflicts caused by a single control target when facing asymmetric grid faults, three-phase faults or short-circuit faults, affecting the stability and reliability of the system.

[0003] In the existing technology, the control strategy of the wind power grid-connected system mainly relies on fixed-parameter negative-sequence current limiting, frequency control and voltage control. However, these fixed-parameter control strategies are difficult to flexibly deal with problems such as current fluctuations, frequency instability and excessive voltage when facing complex grid faults. Especially in the case of asymmetric faults, the traditional negative-sequence current limiting strategy cannot dynamically adjust the limiting value according to the real-time current amplitude, resulting in the system being unable to effectively suppress the negative-sequence current when the current exceeds the limit, increasing the risk of converter overcurrent. At the same time, in the fault recovery stage, the fixed limiting value may cause the system to recover slowly or even fail to restore the symmetrical state.

[0004] In addition, in the case of three-phase faults, traditional frequency control strategies usually rely on fixed frequency reference values ​​and lack the ability to respond quickly to frequency fluctuations, which can easily lead to frequency instability and cause cascading failures. During short-circuit faults, traditional voltage control strategies are often unable to dynamically adjust the positive sequence voltage reference value, resulting in excessively high converter outlet voltage, increasing the risk of converter overvoltage, and thus affecting the continued operation of non-faulty wind turbines.

[0005] Therefore, the existing wind power grid-connected flexible direct current system control strategy has problems such as control lag, over-suppression, and single control target conflict when facing complex grid faults, making it difficult to effectively ensure the stable operation of the system. In order to solve these problems, a control method that can dynamically adjust control parameters according to the real-time grid status and achieve multi-dimensional coordinated optimization is urgently needed to improve the stability and reliability of the wind power grid-connected system under complex fault conditions. Summary of the invention

[0006] In view of the above-mentioned prior art, the present invention provides a coordinated control method of a wind power flexible direct current grid-connected system based on an improved control structure, mainly to solve the technical problems existing in the above-mentioned background technology.

[0007] To achieve the above object, the technical solution of the embodiment of the present invention is implemented as follows:

[0008] A coordinated control method for a wind power flexible direct current grid-connected system based on an improved control structure, wherein a dynamic negative sequence current limiting unit, a frequency additional control unit and an AC voltage additional control unit are added to the control unit of a sending end converter station; when an asymmetric fault occurs in the sending end line, the negative sequence current limiting value is dynamically adjusted by the dynamic negative sequence current limiting unit to optimize the negative sequence current control; when a three-phase fault occurs in the sending end line, the frequency is drooped and adjusted by the frequency additional control unit; during a short circuit of the sending end line, the positive sequence AC voltage reference value is adjusted in real time by the AC voltage additional control unit to reduce the outlet voltage of the sending end converter station, thereby reducing the risk of converter overvoltage.

[0009] Preferably, when an asymmetric fault occurs in the sending-end line, the negative-sequence current limit value is dynamically adjusted by a dynamic negative-sequence current limit unit to optimize the negative-sequence current control, specifically including:

[0010] Set an initial parameter group, which includes a first-order low-pass filter time constant, an AC current amplitude i corresponding to when the negative sequence current limit value starts to decrease, and max1 , the corresponding AC current amplitude i when the negative sequence current limit value ends decreasing max2 , the corresponding AC current amplitude i when the negative sequence current limit value ends rising min1 , the corresponding AC current amplitude i when the negative sequence current limit value starts to rise min2 , Negative sequence current limit maximum value Negative sequence current limit minimum value

[0011] When an asymmetric fault occurs, the AC current amplitude i is collected mag signal and the i mag The signal is subjected to first-order low-pass filtering;

[0012] According to the filtered i mag The signal determines whether the negative sequence current limit value needs to be adjusted in the current state.

[0013] Preferably, according to the filtered i mag The signal determines whether the negative sequence current limit value needs to be adjusted in the current state, including:

[0014] when i mag >i max1 When the negative sequence current limit value is gradually reduced to the minimum value Reduce the risk of converter overcurrent by suppressing negative sequence current;

[0015] when i mag <i min2When the negative sequence current limit value is gradually increased to the maximum value Enhance the ability to suppress asymmetric components of the AC voltage at the sending end and help restore the AC voltage to a symmetrical state.

[0016] Preferably, the original i mag Signal multiplied by After that, we get the filtered i mag Signal.

[0017] Preferably, according to the filtered i mag The process of judging whether the current state needs to adjust the negative sequence current limit value by using the signal also includes:

[0018] During an asymmetric fault, if i mag Rapidly increases and exceeds i max1 When the negative sequence current limit value changes from Gradually down to

[0019] After the asymmetric failure recovery, as i mag Gradually decreases and falls below i max2 When the negative sequence current limit value changes from Gradually rise to

[0020] Preferably, when a three-phase fault occurs in the sending-end line, the frequency is drooped and adjusted by the frequency additional control unit, specifically including: obtaining the actual frequency f of the sending-end line, using Droop adjustment frequency, where Where f ref is the ideal frequency, k f is the droop coefficient, is the reference value of the q-axis positive sequence voltage in the sending-end converter station, and the obtained Applied in the control logic of the sending-end converter station, the frequency response is adjusted by changing the q-axis voltage.

[0021] Preferably, during the short circuit of the sending-end line, the positive-sequence AC voltage reference value is adjusted in real time by the AC voltage additional control unit, specifically including:

[0022] Calculated by the following formula

[0023] The d-axis positive sequence voltage reference value during the reduced fault period is calculated by the following formula:

[0024]

[0025] in, is the negative sequence AC voltage effective value, is the positive sequence AC voltage reference value.

[0026] The beneficial effects of the present invention are as follows: through the synergistic effect of dynamic negative sequence current limiting, additional frequency control and additional AC voltage control, the technical performance of the wind power flexible direct current grid-connected system is significantly improved; in an asymmetric fault scenario, the dynamic negative sequence current limiting unit flexibly adjusts the limiting value according to the real-time AC current amplitude, which can not only reduce the negative sequence component when the current exceeds the limit to suppress the overcurrent risk of the converter, but also actively enhance the suppression ability of the voltage asymmetry component in the current recovery stage, accelerate the system to restore the symmetrical state, and avoid the control lag or excessive suppression problem caused by traditional fixed limiting; for three-phase faults, the additional frequency control unit converts the deviation between the actual frequency and the ideal frequency into the adjustment amount of the q-axis positive sequence voltage reference value through the droop adjustment strategy, quickly responds to frequency fluctuations, improves the system frequency stability, and reduces the probability of chain failures caused by frequency instability; during a short circuit fault, the AC voltage additional control unit dynamically corrects the positive sequence voltage reference value through the negative sequence voltage effective value, reduces the d-axis voltage component, maintains the converter station outlet voltage within a safe range, effectively alleviates the converter overvoltage risk, and ensures the continuous operation of non-faulty wind turbines. Compared with traditional methods, this method achieves multi-dimensional coordinated optimization in multiple fault scenarios: the linkage of dynamic negative sequence current limiting and voltage control can balance the current suppression and voltage recovery requirements, the frequency droop regulation enhances the system inertial response capability, and the voltage dynamic adjustment mechanism avoids performance conflicts under a single control target. In practical applications, this method can significantly reduce the probability of wind turbines being disconnected from the grid during faults, improve the adaptability of flexible direct current systems to grid disturbances, enhance the resilience of the grid under high-proportion new energy access, and provide technical guarantees for safe and stable operation under complex working conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic diagram of a control structure in an embodiment of the present application;

[0028] Figure 2 This is a negative sequence voltage control structure diagram in an embodiment of the present application;

[0029] Figure 3 Schematic diagram of the frequency additional control unit in the embodiment of the present application. DETAILED DESCRIPTION

[0030] The technical solution of the present invention is further elaborated in detail below in conjunction with the drawings and specific embodiments of the specification. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by technicians in the technical field of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention. In the following description, the expression "some embodiments" is related to a subset of all possible embodiments, but it should be understood that "some embodiments" can be the same subset or different subsets of all possible embodiments, and can be combined with each other without conflict.

[0031] In the following description, a large number of specific details are provided to provide a more thorough understanding of the present invention. However, it is apparent to those skilled in the art that the present invention can be implemented without one or more of these details. In other examples, in order to avoid confusion with the present invention, some technical features well known in the art are not described.

[0032] It should be understood that the present invention can be implemented in different forms and should not be interpreted as being limited to the embodiments proposed herein. On the contrary, providing these embodiments will make the disclosure thorough and complete, and the scope of the present invention will be fully conveyed to those skilled in the art. And the purpose of the terms used herein is only to describe specific embodiments and is not intended to be a limitation of the present invention. When used herein, the singular forms of "one", "one" and "said / the" are also intended to include plural forms, unless the context clearly indicates another way. It should also be understood that the terms "compose" and / or "include" when used in this specification determine the presence of the features, integers, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups. When used herein, the term "and / or" includes any and all combinations of the relevant listed items.

[0033] It should also be noted that when an element is referred to as being "fixed to" another element, it may be directly on the other element or there may be a central element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be a central element at the same time. The terms "vertical", "horizontal", "inside", "outside", "left", "right" and similar expressions used herein are for illustrative purposes only and are not intended to be the only implementation method.

[0034] In order to fully understand the present invention, a detailed structure will be proposed in the following description to illustrate the technical solution proposed by the present invention. The optional embodiments of the present invention are described in detail as follows, but in addition to these detailed descriptions, the present invention may also have other implementations.

[0035] Please refer to the attached Figure 1 The present application provides a coordinated control method for a wind power flexible direct current grid-connected system based on an improved control structure, in which a dynamic negative-sequence current limiting unit, a frequency additional control unit and an AC voltage additional control unit are added to the control unit of the sending-end converter station. When an asymmetric fault occurs in the sending-end line, the negative-sequence current limiting value is dynamically adjusted by the dynamic negative-sequence current limiting unit to optimize the negative-sequence current control; when a three-phase fault occurs in the sending-end line, the frequency is drooped and adjusted by the frequency additional control unit; during the short circuit of the sending-end line, the positive-sequence AC voltage reference value is adjusted in real time by the AC voltage additional control unit to reduce the outlet voltage of the sending-end converter station, thereby reducing the risk of converter overvoltage.

[0036] The control method proposed in this application realizes coordinated optimization under different fault scenarios by introducing three core modules, namely dynamic negative-sequence current limiting, additional frequency control and additional AC voltage control, into the control unit of the sending-end converter station. When an asymmetric fault occurs in the sending-end line, the dynamic negative-sequence current limiting unit collects the AC current amplitude signal in real time, and after first-order low-pass filtering, dynamically adjusts the negative-sequence current limiting value according to a preset threshold. For example, if the current amplitude exceeds the set upper limit, the limiting value is gradually reduced to suppress the negative-sequence current and reduce the risk of overcurrent in the converter; conversely, if the current amplitude is lower than the lower limit, the limiting value is gradually increased to enhance the ability to suppress the asymmetric components of the AC voltage and assist the system in restoring a symmetrical state. This dynamic adjustment mechanism avoids the rigidity of the traditional fixed limiting value, allowing the system to more flexibly adapt to current fluctuations during faults;

[0037] For three-phase fault scenarios, the additional frequency control unit improves the frequency response through a droop adjustment strategy. Specifically, by measuring the deviation between the actual frequency and the ideal frequency, the q-axis positive-sequence voltage reference value is generated in combination with the droop coefficient, and directly acts on the converter station control logic. By adjusting the q-axis voltage component, the system can respond quickly to frequency changes, such as increasing the voltage output when the frequency drops to stabilize the grid frequency, thereby reducing the risk of cascading failures caused by frequency instability. During a short-circuit fault, the AC voltage additional control unit dynamically adjusts the positive-sequence voltage reference value through the effective value of the negative-sequence voltage, reduces the d-axis positive-sequence voltage component, and maintains the output voltage of the sending-end converter station at a low level, effectively alleviating the converter overvoltage problem. For example, when the negative-sequence voltage increases, the positive-sequence reference voltage decreases accordingly. The orthogonal decomposition of the voltage vector ensures that the total voltage amplitude is stable, avoiding non-faulty wind turbines from being disconnected from the grid due to overvoltage;

[0038] The synergy of the three modules is reflected in the multi-dimensional optimization of the entire fault process. The dynamic negative-sequence current limiting unit focuses on suppressing abnormal current fluctuations, the frequency additional control unit ensures frequency stability, and the AC voltage additional control unit focuses on voltage amplitude control. Compared with traditional methods, this method can more accurately balance the negative-sequence current suppression and voltage recovery requirements during asymmetric faults, improve the system inertial response through frequency droop adjustment during three-phase faults, and reduce overvoltage risks through dynamic voltage adjustment during short circuits. For example, in a compound fault scenario, the three modules can synchronously adjust the limit value, frequency reference, and voltage vector to avoid performance conflicts caused by a single control target, significantly improving the fault ride-through capability and operational stability of the wind power grid-connected system. In practical applications, this method can reduce the probability of wind turbines being disconnected from the grid due to faults, enhance the adaptability of the flexible direct current system to grid disturbances, and provide reliable technical support for the access of a high proportion of new energy.

[0039] In an optional implementation, when an asymmetric fault occurs in the sending-end line, the negative-sequence current limit value is dynamically adjusted by a dynamic negative-sequence current limit unit to optimize the negative-sequence current control, specifically including:

[0040] Set an initial parameter group, which includes a first-order low-pass filter time constant, an AC current amplitude i corresponding to when the negative sequence current limit value starts to decrease, and max1 , the corresponding AC current amplitude i when the negative sequence current limit value ends decreasing max2 , the corresponding AC current amplitude i when the negative sequence current limit value ends rising min1 , the corresponding AC current amplitude i when the negative sequence current limit value starts to rise min2 , Negative sequence current limit maximum value Negative sequence current limit minimum value

[0041] When an asymmetric fault occurs, the AC current amplitude i is collected mag signal and the i mag The signal is subjected to first-order low-pass filtering;

[0042] According to the filtered i mag The signal determines whether the negative sequence current limit value needs to be adjusted in the current state.

[0043] Specifically, unlike onshore converter stations and wind turbine grid-side converters, the sending-end converter station adopts a control strategy of negative sequence current of 0, which cannot cope with the voltage recovery after an asymmetric fault at the sending end. After the fault line is removed from the sending-end converter station, due to the AC voltage fluctuation, θ1 in the wind turbine grid-side converter will also fluctuate, causing the wind turbine grid-side converter to detect a certain degree of negative sequence current. Since the wind turbine grid-side converter adopts a control strategy of negative sequence current of 0. Therefore, the negative sequence current control of the wind turbine grid-side converter will generate a corresponding negative sequence voltage to suppress the negative sequence current, resulting in an increase in the negative sequence voltage of the sending-end AC system. To cope with the increase in negative sequence voltage at the sending end after the fault line is removed, refer to Figure 2 , the sending-end converter station needs to adopt a control strategy with a negative sequence voltage of 0. By outputting the corresponding negative sequence current, the negative sequence voltage at the output of the sending-end converter station is controlled to be 0. The negative sequence current limit value in the sequence voltage control determines the negative sequence voltage control capability of the sending-end converter station. The larger the negative sequence current limit value, the stronger the negative sequence voltage control capability of the sending-end converter station, and the faster the AC voltage recovery speed after the fault line is removed. Without considering the zero sequence current, the output current i of the sending-end converter station is determined by the positive sequence current amplitude i + and the negative sequence current amplitude i - It is jointly determined that the higher the negative-sequence current limit value of the sending-end converter station is, the higher the output negative-sequence current amplitude i of the sending-end converter station is, and the greater the possibility of increasing the output current amplitude is.

[0044] On the contrary, the smaller the negative-sequence current limit value, the weaker the negative-sequence voltage control capability of the sending-end converter station, and the slower the recovery speed of the AC voltage after the fault line is removed. During the fault recovery process, the wind turbine grid-side converter outputs a negative-sequence AC voltage, and the weak negative-sequence voltage control capability of the sending-end converter station will also cause the AC voltage to be unable to recover, and the system cannot resume stable operation. Therefore, the larger the negative-sequence current limit value, the stronger the negative-sequence voltage control capability of the sending-end converter station, and the higher the short-circuit current amplitude. The smaller the negative-sequence current limit value, the lower the short-circuit current amplitude, the weaker the negative-sequence voltage control capability of the sending-end converter station, and the greater the possibility that the AC voltage cannot be restored.

[0045] In order to reduce the short-circuit current and ensure that the AC voltage can be restored after a fault, the dynamic negative-sequence current limiting unit proposed in this application uses a hysteresis link, first passes through a first-order low-pass filter link, and then according to the current amplitude i mag The negative sequence current limit value is adjusted dynamically, the first order low pass filter link avoids the frequent jitter of the current amplitude signal, and the hysteresis link avoids the repeated oscillation of the negative sequence current limit value.

[0046] Among them, according to the filtered i mag The signal determines whether the negative sequence current limit value needs to be adjusted in the current state, including:

[0047] when i mag >i max1When the negative sequence current limit value is gradually reduced to the minimum value Reduce the risk of converter overcurrent by suppressing negative sequence current;

[0048] when i mag <i min2 When the negative sequence current limit value is gradually increased to the maximum value Enhance the ability to suppress asymmetric components of the AC voltage at the sending end and help restore the AC voltage to a symmetrical state.

[0049] Furthermore, in the hysteresis link, if during an asymmetric fault, if i mag Rapidly increases and exceeds i max1 When the negative sequence current limit value changes from Gradually down to

[0050] After the asymmetric failure recovery, as i mag Gradually decreases and falls below i max2 When the negative sequence current limit value changes from Gradually rise to

[0051] i max1 、i min2 、i min1 、i min2 , The negative sequence current limit value under the current state is determined, thereby affecting the control ability of the negative sequence voltage and negative sequence current of the sending end converter station, i max1 、i min2 , The smaller it is, the faster the negative sequence current limit value decreases during the fault period, and the better the negative sequence current suppression effect is. min1 、i min2 , The larger the value, the faster the negative sequence current limit value rises after the fault line is removed, and the faster the AC voltage recovers. However, considering the response speed of the current inner loop and avoiding repeated switching of the dynamic negative sequence current limit value, i max1 、i min2 , Can't be too small, min1 、i min2 , It can't be too big either.

[0052] Furthermore, the original i mag Signal multiplied by After that, we get the filtered i mag Signal.

[0053] In an optional implementation, according to the filtered i magThe process of judging whether the current state needs to adjust the negative sequence current limit value by using the signal also includes:

[0054] During an asymmetric fault, if i mag Rapidly increases and exceeds i max1 When the negative sequence current limit value changes from Gradually down to

[0055] After the asymmetric failure recovery, as i mag Gradually decreases and falls below i max2 When the negative sequence current limit value changes from Gradually rise to

[0056] In an optional embodiment, when a three-phase fault occurs in the sending-end line, the frequency is drooped and adjusted by the frequency additional control unit. Stable frequency control is a prerequisite for wind energy transmission at the sending end. In the traditional control method, the electrical angle θ=ω0t of the sending-end converter station is generated by the controller. When the electrical angle is fixed, the traditional control indirectly realizes us=1 (per unit value) and f=f by controlling the dq axis component of the AC voltage at the common connection point. ref The control objectives are to take into account The influence of f is based on the fact that the electrical angle θ of the sending-end converter station is still generated by the controller. Droop adjustment frequency, where Where f ref is the ideal frequency, k f is the droop coefficient, where the unit of the droop coefficient is V / Hz, is the reference value of the q-axis positive sequence voltage in the sending-end converter station, and the obtained In the control logic applied to the sending-end converter station, the frequency response is adjusted by changing the q-axis voltage. In the formula, if the frequency is less than the reference value, will be greater than 0. Under the control of the q-axis, is also greater than 0, its frequency will increase accordingly.

[0057] Figure 3 Schematic diagram of the frequency additional control unit, u v The frequency additional control unit makes full use of the flexibility of the q-axis voltage to introduce the frequency into A frequency feedback link is added to the control link, making the frequency control of the sending-end converter station more direct, thereby stabilizing the sending-end voltage and improving the frequency response of the system.

[0058] In an optional embodiment, during the short circuit of the fan side at the delivery end, due to the action of the frequency additional control unit, the AC voltage at the delivery end It will change with the frequency and will no longer be 0. Considering that during an asymmetric fault, there is also a certain proportion of negative-sequence voltage and zero-sequence voltage components in the AC power grid. Under the action of the dynamic negative-sequence current limiting unit, the sending-end converter station will also output a certain proportion of negative-sequence voltage to suppress the negative-sequence current.

[0059] If the D-axis positive sequence voltage of the sending-end converter station controller is still set The positive-sequence AC voltage reference value is the rated value. The superposition of the d-axis AC voltage component, q-axis voltage component, and negative-sequence voltage component output by the inner loop may cause over-regulation of the converter, resulting in severe overvoltage and disconnection of the wind turbine, which seriously affects the stability of the wind power sent by the flexible direct current. Therefore, during the short circuit of the sending-end line, the positive-sequence AC voltage reference value is adjusted in real time through the AC voltage additional control unit, including:

[0060] Calculated by the following formula

[0061] The d-axis positive sequence voltage reference value during the reduced fault period is calculated by the following formula:

[0062]

[0063] in, is the negative sequence AC voltage effective value, is the positive sequence AC voltage reference value.

[0064] During a fault, the negative sequence voltage unbalance component of the AC grid is actively reduced. Then by get By actively reducing Lower the output voltage of the sending-end converter station, thereby reducing the risk of converter overvoltage and avoiding non-faulty wind turbines from being disconnected from the grid.

[0065] The above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. The protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A coordinated control method for wind power connected to the grid via flexible direct current based on an improved control structure, characterized in that: A dynamic negative-sequence current limiting unit, a frequency additional control unit and an AC voltage additional control unit are added to the control unit of the sending-end converter station. When an asymmetric fault occurs in the sending-end line, the dynamic negative-sequence current limiting unit is used to dynamically adjust the negative-sequence current limiting value to optimize the negative-sequence current control. When a three-phase fault occurs in the sending-end line, the frequency is drooped and adjusted through the frequency additional control unit. During the short circuit of the sending-end line, the positive-sequence AC voltage reference value is adjusted in real time through the AC voltage additional control unit to reduce the outlet voltage of the sending-end converter station, thereby reducing the risk of converter overvoltage.

2. A coordinated control method for wind power connected to the grid based on an improved control structure according to claim 1, characterized in that: When an asymmetric fault occurs in the sending-end line, the negative-sequence current limit value is dynamically adjusted through the dynamic negative-sequence current limit unit to optimize the negative-sequence current control, specifically including: Set an initial parameter group, which includes a first-order low-pass filter time constant, an AC current amplitude i corresponding to when the negative sequence current limit value starts to decrease, and max1 , the corresponding AC current amplitude i when the negative sequence current limit value ends decreasing max2 , the corresponding AC current amplitude i when the negative sequence current limit value ends rising min1 , the corresponding AC current amplitude i when the negative sequence current limit value starts to rise min2 , Negative sequence current limit maximum value Negative sequence current limit minimum value When an asymmetric fault occurs, the AC current amplitude i is collected mag signal and the i mag The signal is subjected to a first-order low-pass filter; According to the filtered i mag The signal determines whether the negative sequence current limit value needs to be adjusted in the current state.

3. A coordinated control method for wind power connected to the grid based on an improved control structure according to claim 2, characterized in that: According to the filtered i mag The signal determines whether the negative sequence current limit value needs to be adjusted in the current state, including: when i mag >i max1 When the negative sequence current limit value is gradually reduced to the minimum value Reduce the risk of converter overcurrent by suppressing negative sequence current; when i mag <i min2 When the negative sequence current limit value is gradually increased to the maximum value Enhance the ability to suppress asymmetric components of the AC voltage at the sending end and help restore the AC voltage to a symmetrical state.

4. The method for coordinated control of wind power through flexible direct current grid-connected system based on improved control structure according to claim 2 is characterized in that: The original i mag Signal multiplied by After that, we get the filtered i mag Signal.

5. A coordinated control method for wind power connected to the grid based on an improved control structure according to claim 4, characterized in that: According to the filtered i mag The process of judging whether the current state needs to adjust the negative sequence current limit value by using the signal also includes: During an asymmetric fault, if i mag Rapidly increases and exceeds i max1 When the negative sequence current limit value changes from Gradually down to After the asymmetric failure recovery, as i mag Gradually decreases and falls below i max2 When the negative sequence current limit value changes from Gradually rise to 6. The method for coordinated control of wind power connected to the grid through flexible direct current system based on improved control structure according to claim 1, characterized in that: When a three-phase fault occurs in the sending-end line, the frequency is drooped and adjusted by the frequency additional control unit, specifically including: obtaining the actual frequency f of the sending-end line, using Droop adjustment frequency, where Where f ref is the ideal frequency, k f is the droop coefficient, is the reference value of the q-axis positive sequence voltage in the sending-end converter station, and the obtained Applied in the control logic of the sending-end converter station, the frequency response is adjusted by changing the q-axis voltage.

7. The method for coordinated control of wind power through flexible direct current grid-connected system based on improved control structure according to claim 5 is characterized in that: During the short circuit of the sending end line, the positive sequence AC voltage reference value is adjusted in real time through the AC voltage additional control unit, including: Calculated by the following formula The d-axis positive sequence voltage reference value during the reduced fault period is calculated by the following formula: in, is the negative sequence AC voltage effective value, is the positive sequence AC voltage reference value.

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