Low voltage ride through risk assessment method and system based on sliding mode theory analysis

Abstract

The application provides a repeated low voltage ride through risk assessment method and system based on a sliding mode theory analysis, and belongs to the technical field of low voltage ride through risk assessment. The application comprises the following steps: based on a grid-connected system model of a grid-following type converter with low voltage ride through control, eight-order unified state space expression is constructed in combination with a current inner loop, a phase-locked loop and a dynamic of an alternating current main circuit; a sliding mode surface function is constructed by using a difference between a grid-connected point voltage and a LVRT switching threshold value, and a direction derivative of a sub-state space vector field about the sliding mode surface function in a normal operation mode and a LVRT mode is used as a chatter criterion; and the evolution trend of the grid-connected point voltage near the low voltage ride through threshold value is judged according to the sign characteristic of the chatter criterion, so as to assess the risk of repeated low voltage ride through. The application can quantitatively reflect the influence of device side and grid side parameters on the repeated low voltage ride through of the system, and provides a novel and practical method for stability assessment and parameter optimization of a new energy grid-connected system.

Description

Technical Field

[0001] This invention belongs to the field of low voltage ride-through risk assessment technology, and particularly relates to a method and system for assessing repeated low voltage ride-through risk based on sliding mode theory analysis. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] With the vigorous development of new energy grid connection, a large number of wind power, photovoltaic and other equipment containing grid-connected converters have been connected to the grid. When a grid fault causes a voltage drop, the converter needs to enter the low voltage ride-through (LVRT) mode to provide reactive power support. However, there is discrete control switching between the normal operation mode and the LVRT mode of the converter. Under specific control parameters or weak grid conditions, the grid connection point voltage may drop again after the fault is cleared and fluctuate repeatedly near the low voltage ride-through threshold, forming a voltage oscillation phenomenon caused by repeated LVRT.

[0004] Currently, most mechanistic analyses of the recurring LVRT problem rely on electromagnetic transient time-domain simulations or traditional quasi-steady-state analysis methods such as PV curves and bifurcation theory. However, these methods often neglect the nonlinear dynamic behavior of the converter control mode switching process, and are mostly only qualitative analyses of the mechanistic explanation of the recurring LVRT problem and the influence of single parameters. They are difficult to accurately characterize the essential characteristic of the system operating point repeatedly crossing the switching threshold, and lack effective quantitative assessment methods for identifying the risk of recurring LVRT. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, this invention provides a method and system for assessing repeated low voltage ride-through risk based on sliding mode theory analysis. This method can intuitively quantify the impact of equipment-side and grid-side parameters on repeated low voltage ride-through (LVRT) of the system, providing a new and practical method for stability assessment and parameter optimization of new energy grid-connected systems.

[0006] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions: The first aspect of this invention provides a method for assessing the risk of repeated low-voltage ride-through based on sliding mode theory analysis.

[0007] The risk assessment method for repeated low-voltage ride-through based on sliding mode theory includes: Based on the grid-connected system model of the grid-connected converter with low-voltage ride-through control, and combining the dynamics of its inner current loop, phase-locked loop and AC main circuit, an eighth-order unified state-space expression is constructed. A sliding mode surface function is constructed using the difference between the grid connection point voltage and the low voltage ride-through control switching threshold, and the directional derivative of the sub-state space vector field with respect to the sliding mode surface function under normal operation mode and low voltage ride-through control mode is used as the chatter criterion. Based on the sign characteristics of the flutter criterion, the evolution trend of the grid connection point voltage near the low-voltage ride-through threshold is determined to assess the risk of repeated low-voltage ride-throughs.

[0008] Furthermore, the dynamic equation of the inner current loop is expressed as: ; in, To achieve synchronous speed with the AC power grid; , Converters d axis, q The PWM command value for the axis; and These are the proportional gain and integral gain of the PI link in the inner current loop, respectively. , Converters d axis, q The current in the shaft; , Converters d axis, q Shaft current reference value; and The inner loop PI element of the current circuit is respectively in d axis, q Integrator state variables on the axis; and These are voltage feedforward filtering stages. d axis, q Axis output.

[0009] Furthermore, the dynamic equation of the phase-locked loop is expressed as: ; in, and These represent the frequency and phase of the phase-locked loop output, respectively. and These are the proportional gain and integral gain of the PI element in the phase-locked loop, respectively. x 3 represents the output of the phase-locked loop integrator. This represents the q-axis component of the grid connection point voltage.

[0010] Furthermore, the expression of the dynamic equations of the AC main circuit includes: when neglecting the dynamics of the filter inductor, equating the current flowing through the AC mains with the current output by the converter, so as to address the current containing... d axis, qThe dynamic equations of the AC power grid in the axial coordinate system are used to represent the dynamics; otherwise, the dynamic equations of the filtering stage are used to represent the dynamics of the filtering stage, ignoring the effect of the filter capacitor.

[0011] Furthermore, the grid-connected converter system model with low voltage ride-through control represents a grid-connected converter that determines its control mode by judging whether the grid connection point voltage is lower than the switching threshold. The control mode includes normal operation mode and LVRT mode. The only difference between the normal operation mode and LVRT mode of the grid-connected converter system model is the different current reference command values. The LVRT mode is the low voltage ride-through control mode.

[0012] Furthermore, when the target system is in normal operating mode, the sliding surface function is greater than 0; when the target system is in LVRT mode, the sliding surface function is less than 0; the effect of each sub-state space vector field on the system operating point is reflected by its directional derivative.

[0013] Furthermore, based on the sign characteristics of the flutter criterion, the evolution trend of the grid connection point voltage near the low-breakdown threshold is determined, including: When both the first and second symbolic features are greater than 0, the vector field effects of the two subsystems will cause the operating point to tend towards a sliding mode surface function greater than 0, and the grid connection point voltage will return to the normal range without repeated LVRT. When both the first and second symbolic features are less than 0, the vector field effects of the two subsystems will cause the running point to tend towards a sliding surface function that is less than 0, and the system will directly enter and stabilize in LVRT mode. When the first symbolic feature is less than 0 and the second symbolic feature is greater than 0, or when the first symbolic feature is greater than 0 and the second symbolic feature is less than 0, the two vector fields act in opposite directions. It is then assumed that the trajectory of the running point repeatedly crosses the sliding surface, and the system experiences chattering. That is, voltage oscillations will occur in the system due to repeated LVRT.

[0014] The second aspect of this invention provides a risk assessment system for repeated low-voltage ride-through based on sliding mode theory analysis.

[0015] A risk assessment system for repeated low-voltage ride-through based on sliding mode theory analysis includes: The dynamic characterization module is configured to: construct an eighth-order unified state-space expression based on the grid-connected system model of the grid-connected converter with low-voltage ride-through control, combined with the dynamics of its inner current loop, phase-locked loop and AC main circuit. The flutter criterion generation module is configured to: construct a sliding mode surface function based on the difference between the grid connection point voltage and the low voltage ride-through control switching threshold, and use the directional derivative of the sub-state space vector field with respect to the sliding mode surface function under normal operation mode and low voltage ride-through control mode as the flutter criterion; The risk assessment module is configured to: determine the evolution trend of the grid connection point voltage near the low-voltage ride-through threshold based on the sign characteristics of the flutter criterion, so as to assess the risk of repeated low-voltage ride-throughs.

[0016] A third aspect of the present invention provides a computer-readable storage medium having a program stored thereon, which, when executed by a processor, implements the steps in the method for assessing repeated low-voltage ride-through risk based on sliding mode theory analysis as described in the first aspect of the present invention.

[0017] The fourth aspect of the present invention provides an electronic device including a memory, a processor, and a program stored in the memory and executable on the processor, wherein the processor executes the program to implement the steps in the repeated low voltage ride-through risk assessment method based on sliding mode theory analysis as described in the first aspect of the present invention.

[0018] The above one or more technical solutions have the following beneficial effects: This invention constructs a unified state-space expression based on the dynamic equations of the current inner loop, phase-locked loop, and AC main circuit of a grid-connected converter system model with low-voltage ride-through (LVRT). A sliding mode surface function is constructed using the difference between the grid-connected point voltage and the LVRT switching threshold. The directional derivatives of the sub-state-space vector fields with respect to the sliding mode surface function under normal operation and LVRT modes are used as chatter criteria. By constructing this explicit algebraic directional derivative criterion, the complex electromagnetic transient time-domain simulation of repeated LVRT is avoided. Simultaneously, this criterion can directly quantify and analyze the impact of equipment-side control parameters and grid-side parameters on system stability, thereby accurately characterizing the essential characteristics of repeated LVRT switching thresholds at the system operating point and achieving effective identification and quantitative assessment of repeated LVRT risks.

[0019] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0021] Figure 1 This is a flowchart of the repeated low-voltage ride-through risk assessment method based on sliding mode theory analysis in Embodiment 1 of the present invention.

[0022] Figure 2 This is a schematic diagram illustrating the influence of different line inductances on the criterion in Embodiment 1 of the present invention.

[0023] Figure 3This is a schematic diagram of the time-domain simulation results under different line inductances in Embodiment 1 of the present invention; wherein, Figure 3 (a) in the middle is Lg Schematic diagram of grid connection point voltage when =0.43pu. Figure 3 (b) in the middle is Lg A schematic diagram of the grid connection point voltage when =0.35pu. Detailed Implementation

[0024] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations of the present invention.

[0026] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0027] The overall approach of this invention is as follows: By fully considering control switching conditions and the continuous dynamic process of the system, this invention constructs a directional derivative criterion for measuring the effects of different sub-state spaces. Based on this, it proposes a method for assessing the risk of repeated low-voltage ride-through (LVRT) caused by repeated LVRT based on sliding mode theory analysis. This method involves: First, establishing an eighth-order unified state-space expression based on a grid-connected converter system model with LVRT control, considering the dynamics of its inner current loop, phase-locked loop, and AC main circuit. Then, using the difference between the grid connection point voltage and the LVRT switching threshold as the sliding mode surface function, the directional derivatives of the sub-state space vector fields of the normal operation mode and LVRT mode with respect to this sliding mode surface function are derived, i.e., the chatter criterion. Finally, based on the sign distribution of the chatter criterion, the dynamic characteristics of the system's operating trajectory on both sides of the sliding mode surface are determined, thereby achieving a quantitative assessment of the risk of repeated low-voltage ride-through under the influence of key parameters on the equipment side and the grid side.

[0028] Example 1 This embodiment discloses a method for assessing the risk of repeated low-voltage ride-through based on sliding mode theory analysis.

[0029] like Figure 1 As shown, the risk assessment method for repeated low-voltage ride-through based on sliding mode theory includes: Step S1: Based on the grid-connected system model of the grid-connected converter with low voltage ride-through control, and combining the dynamics of its inner current loop, phase-locked loop and AC main circuit, construct an eighth-order unified state-space expression. Step S2: Construct a sliding mode surface function using the difference between the grid connection point voltage and the low voltage ride-through control switching threshold, and use the directional derivative of the sub-state space vector field with respect to the sliding mode surface function under normal operation mode and low voltage ride-through control mode as the chatter criterion. Step S3: Based on the sign characteristics of the flutter criterion, determine the evolution trend of the grid connection point voltage near the low voltage ride-through threshold to assess the risk of repeated low voltage ride-throughs.

[0030] Based on the above process, this invention can intuitively quantify the impact of equipment-side and grid-side parameters on repeated LVRT of the system, providing a novel and practical method for stability assessment and parameter optimization of new energy grid-connected systems. To facilitate understanding of the technical solution of this invention, the specific implementation methods of this invention will be further explained and described below.

[0031] In step S1, based on the grid-connected system model of the grid-connected converter with low-voltage ride-through control, and combining the dynamics of its inner current loop, phase-locked loop and AC main circuit, an eighth-order unified state-space expression is constructed.

[0032] The dynamic equation for the inner current loop is expressed as: (1) in, To achieve synchronous speed with the AC power grid; , Converters d axis, q The PWM command value for the axis; and These are the proportional gain and integral gain of the PI link in the inner current loop, respectively. , Converters d axis, q The current in the shaft; , Converters d axis, q Shaft current reference value; and The inner loop PI element of the current circuit is respectively in d axis, q Integrator state variables on the axis. and These are voltage feedforward filtering stages. d axis, q The output of the shaft, and its corresponding dynamic equation, is expressed as: (2) In the formula, , These are the grid connection point voltages. d axis,q Axial components; T This is the time constant of the voltage feedforward filter stage.

[0033] The grid-connected converter achieves synchronization with the power grid through a phase-locked loop, and its dynamic equation is expressed as: (3) in, and These represent the frequency and phase of the phase-locked loop output, respectively. and These are the proportional gain and integral gain of the PI element in the phase-locked loop, respectively. x 3 represents the output of the phase-locked loop integrator.

[0034] Grid-connected converters with LVRT control determine whether the grid connection point voltage is lower than the switching threshold. Determine the control mode of the converter, specifically: (4) In the formula, and When in normal operating mode and LVRT mode respectively d Shaft current reference value; and When in normal operating mode and LVRT mode respectively q Shaft current reference value; This refers to the voltage amplitude at the grid connection point. To control the switching voltage threshold, it is usually set to... ; To switch the flag, when When this time, it indicates that the system is in normal operating mode. ;when When this time, it indicates that the system is in LVRT mode. .

[0035] If we ignore the dynamics of the filter inductor, we can assume that the current flowing through the AC mains is equal to the current output by the converter. dq The dynamic equations of an AC power grid in the coordinate system are: (5) In the formula, This is the frequency reference value; L g represents the line inductance; , These are the mains voltages. d axis, q The axis component, its specific expression is: (6) In the formula, EThis refers to the external power grid voltage.

[0036] If the effect of the filter capacitor is ignored, the dynamic equation of the filter circuit can be expressed as: (7) In the formula, This is a filter inductor.

[0037] This completes the establishment of the electromagnetic transient model for the grid-connected converter system. The two control modes of the grid-connected converter system with LVRT control differ only in the current command; therefore, a unified state-space expression can be established using the aforementioned dynamic equations.

[0038] Selecting the converter current , The output of the integrator controlled by the PI loop in the current loop. , Phase-locked loop output phase and PI control integrator output and the output of the voltage feedforward filter stage , For the state variables, an eighth-order state-space expression is established. Since the state-space expressions for all variables except the converter current have already appeared in the above dynamic equations, only the state-space expression for the converter current needs to be established separately. Combining equations (1) and (7), we get: (8) Equation (8) contains non-state variables. , ,as well as , Substituting equations (5) and (6) into the equations, we get: (9) In the formula, , , Because the two sub-state spaces are different, they need to be accessed through... as well as The different responses therefore necessitate the retention of non-state variables. as well as By combining equations (9), the third and fourth equations in equation (1), equation (2), and the second and third equations in equation (3), we can obtain the state-space expression of the system: (10) In the formula, It is an eight-dimensional column vector, where The specific expression is: (11) In step S2, a chatter criterion for the repetitive LVRT problem is constructed, namely: a sliding mode surface function is constructed based on the difference between the grid connection point voltage and the low voltage ride-through control switching threshold, and the directional derivative of the sub-state space vector field with respect to the sliding mode surface function under normal operation mode and low voltage ride-through control mode is used as the chatter criterion.

[0039] System chatter manifests as high-frequency, small-amplitude oscillations of the operating point trajectory on both sides of the sliding surface. In the iterative LVRT problem, for the two control switching subsystems of the converter, a sliding surface function can be established based on their control switching conditions. S : (12) When the system is in normal operating mode S >0, when the system is in LVRT operating mode S <0. The effect of each sub-state space vector field on the system's operating point can be reflected by its directional derivative: (13) In the formula, This indicates calculating the gradient; and These are the vector fields corresponding to the normal operation mode subsystem and the LVRT mode subsystem, respectively; and They are respectively and about S The directional derivative, the present invention will and These two directional derivatives serve as the first and second evolution criteria, respectively. If a certain sub-state space vector field is related to... S If the directional derivative is greater than 0, the system running point will follow the path that makes... S The direction of increase is developing; if a certain sub-state space vector field is about S If the direction is less than 0, the system running point will move along... S It is developing in a decreasing direction.

[0040] Substituting equation (12) into equation (13), we get: (14) In the formula, , ; for The corresponding element is specifically composed as follows: (15) To obtain the specific expression of equation (14), it is necessary to calculate the values ​​separately. and and their relationship with respect to the state variable vector The gradient. Substituting equation (5) into equation (9), we get: (16) In the formula, , Therefore, we can conclude that: (17) (18) Substituting equations (16)-(18) into equation (14) yields the result. or The specific expression: (19) and The sign of the vector field reflects the nature of its effect, that is, whether the running point will move towards S>0 or S<0 under its influence. and The magnitude of the absolute value reflects the strength of the effect of the corresponding vector field. The larger the absolute value, the stronger the effect of the vector field, and vice versa.

[0041] In step S3, a risk assessment of repeated LVRT is performed based on the flutter criterion, that is: according to the sign characteristics of the flutter criterion, the evolution trend of the grid connection point voltage near the low-voltage ride-through threshold is determined to assess the risk of repeated low-voltage ride-through.

[0042] When both the first symbolic feature and the second symbolic feature are greater than 0 (i.e.) When the vector field effect of both subsystems is applied, the operating point will tend to move towards S>0, and the grid connection point voltage will return to the normal range without repeated LVRT. When both the first symbolic feature and the second symbolic feature are less than 0 (i.e.) When the vector field effect of both subsystems causes the operating point to tend towards S<0, the system will directly enter and stabilize in LVRT mode; When the first symbolic feature is less than 0 and the second symbolic feature is greater than 0 (i.e.) ), or the first symbolic feature is greater than 0 and the second symbolic feature is less than 0 (i.e. When the operating point trajectory repeatedly crosses the sliding surface, the system exhibits obvious chattering, meaning that voltage oscillations occur in the system due to repeated LVRT.

[0043] To further illustrate the superiority of the repeated low-voltage ride-through risk assessment method based on sliding mode theory analysis provided by this invention, this embodiment uses parameter changes... For example, the calculation only The results of different proposed criteria with the same other parameters are as follows: Figure 2 As shown. It can be seen that under this working condition... Always greater than 0, and As can be seen from the criterion proposed in this invention, the process goes through a transition from positive to negative. As the voltage increases, the system changes from stable operation to exhibiting repeated LVRT problems.

[0044] To demonstrate the effectiveness of the proposed method, this embodiment also uses time-domain simulation results for comparison. An electromagnetic transient model of a grid-connected system with a grid-connected converter was built using Matlab and Simulink platforms. The model was set to simulate a fault where the external grid voltage drops by 0.8 pu at time t=1s and recovers to above 0.9 pu after 0.1s. The following parameters were set: The values ​​are 0.43 and 0.35. When it is 0.43, it is from Figure 2 It can be known at this time According to the criteria, the voltage oscillation should be caused by repeated LVRT. When it is 0.35 The system should operate stably. (Combined with...) Figure 3 The diagram shows the time-domain simulation results for different line inductances; where, Figure 3 (a) in the middle is Lg Schematic diagram of grid connection point voltage when =0.43pu. Figure 3 (b) in the middle is Lg The schematic diagram of the grid connection point voltage when =0.35pu shows that the time-domain simulation results are consistent with the results obtained by the criterion, which demonstrates the effectiveness of the method proposed in this invention.

[0045] Example 2 This embodiment discloses a risk assessment system for repeated low-voltage ride-through based on sliding mode theory analysis.

[0046] A risk assessment system for repeated low-voltage ride-through based on sliding mode theory analysis includes: The dynamic characterization module is configured to: construct an eighth-order unified state-space expression based on the grid-connected system model of the grid-connected converter with low-voltage ride-through control, combined with the dynamics of its inner current loop, phase-locked loop and AC main circuit. The flutter criterion generation module is configured to: construct a sliding mode surface function based on the difference between the grid connection point voltage and the low voltage ride-through control switching threshold, and use the directional derivative of the sub-state space vector field with respect to the sliding mode surface function under normal operation mode and low voltage ride-through control mode as the flutter criterion; The risk assessment module is configured to: determine the evolution trend of the grid connection point voltage near the low-voltage ride-through threshold based on the sign characteristics of the flutter criterion, so as to assess the risk of repeated low-voltage ride-throughs.

[0047] Example 3 The purpose of this embodiment is to provide a computer-readable storage medium.

[0048] A computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps in the repeated low-voltage ride-through risk assessment method based on sliding mode theory analysis as described in Embodiment 1 of this disclosure.

[0049] Example 4 The purpose of this embodiment is to provide an electronic device.

[0050] An electronic device includes a memory, a processor, and a program stored in the memory and executable on the processor, wherein the processor executes the program to implement the steps in the repeated low-voltage ride-through risk assessment method based on sliding mode theory analysis as described in Embodiment 1 of this disclosure.

[0051] The steps and methods involved in the apparatuses of Embodiments 2, 3, and 4 above correspond to those in Embodiment 1. For specific implementation details, please refer to the relevant description section of Embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood as including any medium capable of storing, encoding, or carrying an instruction set for execution by a processor and enabling the processor to perform any of the methods in this invention.

[0052] Those skilled in the art will understand that the modules or steps of the present invention described above can be implemented using general-purpose computer devices. Optionally, they can be implemented using computer-executable program code, thereby allowing them to be stored in a storage device for execution by a computer device, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. The present invention is not limited to any particular combination of hardware and software.

[0053] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for assessing the risk of repeated low-voltage ride-through based on sliding mode theory analysis, characterized in that, include: Based on the grid-connected system model of the grid-connected converter with low-voltage ride-through control, and combining the dynamics of its inner current loop, phase-locked loop and AC main circuit, an eighth-order unified state-space expression is constructed. A sliding mode surface function is constructed using the difference between the grid connection point voltage and the low voltage ride-through control switching threshold, and the directional derivative of the sub-state space vector field with respect to the sliding mode surface function under normal operation mode and low voltage ride-through control mode is used as the chatter criterion. Based on the sign characteristics of the flutter criterion, the evolution trend of the grid connection point voltage near the low-voltage ride-through threshold is determined to assess the risk of repeated low-voltage ride-throughs.

2. The method for assessing the risk of repeated low-voltage ride-through based on sliding mode theory analysis as described in claim 1, characterized in that, The dynamic equation of the inner current loop is expressed as: ； in, To achieve synchronous speed with the AC power grid; , Converters d axis, q The PWM command value for the axis; and These are the proportional gain and integral gain of the PI link in the inner current loop, respectively. , Converters d axis, q The current in the shaft; , Converters d axis, q Shaft current reference value; and The inner loop PI element of the current circuit is respectively in d axis, q Integrator state variables on the axis; and These are voltage feedforward filter stages. d axis, q Axis output.

3. The method for assessing the risk of repeated low-voltage ride-through based on sliding mode theory analysis as described in claim 1, characterized in that, The dynamic equation of the phase-locked loop is expressed as: ； in, and These represent the frequency and phase of the phase-locked loop output, respectively. and These are the proportional gain and integral gain of the PI element in the phase-locked loop, respectively. x 3 represents the output of the phase-locked loop integrator. This represents the q-axis component of the grid connection point voltage.

4. The method for assessing the risk of repeated low-voltage ride-through based on sliding mode theory analysis as described in claim 1, characterized in that, The dynamic equation representation of the AC main circuit includes: when neglecting the dynamics of the filter inductor, equating the current flowing through the AC mains with the current output by the converter, in order to address the current containing... d axis, q The dynamic equations of the AC power grid in the axial coordinate system are used to represent the dynamics; otherwise, the dynamic equations of the filtering stage are used to represent the dynamics of the filtering stage, ignoring the effect of the filter capacitor.

5. The method for assessing the risk of repeated low-voltage ride-through based on sliding mode theory analysis as described in claim 1, characterized in that, The grid-connected converter system model with low voltage ride-through control represents a grid-connected converter that determines its control mode by judging whether the grid connection point voltage is lower than the switching threshold. The control modes include normal operation mode and LVRT mode. The only difference between the normal operation mode and LVRT mode of the grid-connected converter system model is the different current reference command values. The LVRT mode is the low voltage ride-through control mode.

6. The method for assessing the risk of repeated low-voltage ride-through based on sliding mode theory analysis as described in claim 1, characterized in that, When the target system is in normal operating mode, the sliding surface function is greater than 0; when the target system is in LVRT mode, the sliding surface function is less than 0; the effect of each sub-state space vector field on the system operating point is reflected by its directional derivative.

7. The method for assessing the risk of repeated low-voltage ride-through based on sliding mode theory analysis as described in claim 1, characterized in that, Based on the sign characteristics of the flutter criterion, the evolution trend of the grid connection point voltage near the low-break-through threshold is determined, including: When both the first and second symbolic features are greater than 0, the vector field effects of the two subsystems will cause the operating point to tend towards a sliding mode surface function greater than 0, and the grid connection point voltage will return to the normal range without repeated LVRT. When both the first and second symbolic features are less than 0, the vector field effects of the two subsystems will cause the running point to tend towards a sliding surface function that is less than 0, and the system will directly enter and stabilize in LVRT mode. When the first symbolic feature is less than 0 and the second symbolic feature is greater than 0, or when the first symbolic feature is greater than 0 and the second symbolic feature is less than 0, the two vector fields act in opposite directions. It is then assumed that the trajectory of the running point repeatedly crosses the sliding surface, and the system experiences chattering. That is, voltage oscillations will occur in the system due to repeated LVRT.

8. A risk assessment system for repeated low-voltage ride-through based on sliding mode theory analysis, characterized in that, include: The dynamic characterization module is configured to: construct an eighth-order unified state-space expression based on the grid-connected system model of the grid-connected converter with low-voltage ride-through control, combined with the dynamics of its inner current loop, phase-locked loop and AC main circuit. The flutter criterion generation module is configured to: construct a sliding mode surface function based on the difference between the grid connection point voltage and the low voltage ride-through control switching threshold, and use the directional derivative of the sub-state space vector field with respect to the sliding mode surface function under normal operation mode and low voltage ride-through control mode as the flutter criterion; The risk assessment module is configured to: determine the evolution trend of the grid connection point voltage near the low-voltage ride-through threshold based on the sign characteristics of the flutter criterion, so as to assess the risk of repeated low-voltage ride-throughs.

9. A computer-readable storage medium having a program stored thereon, characterized in that, When executed by the processor, the program implements the steps in the repeated low-voltage ride-through risk assessment method based on sliding mode theory analysis as described in any one of claims 1-7.

10. An electronic device, comprising a memory, a processor, and a program stored in the memory and executable on the processor, characterized in that, When the processor executes the program, it implements the steps in the repeated low-voltage ride-through risk assessment method based on sliding mode theory analysis as described in any one of claims 1-7.

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