Flexible direct current system alternating current and direct current fault ride-through capability assessment method based on combined empowerment

Abstract

The invention discloses a flexible DC system AC / DC fault ride-through capability assessment method based on combined weighting. The method comprises the following steps: constructing a stable calculation sample set for AC / DC fault ride-through capability assessment; the transient response process of the flexible direct current system during the fault period and the recovery capability of the flexible direct current system after the fault is cleared are comprehensively considered, the supporting capability of the flexible direct current system for the connected alternating current system is also considered, and an alternating current and direct current fault ride-through capability evaluation index system of the flexible direct current system is constructed; subjective weight determination and objective weight determination are carried out on each evaluation index to obtain a subjective weight and an objective weight of each evaluation index; for each evaluation index, determining an optimal value of a combined weight of the evaluation index according to each subjective weight and objective weight; and according to the optimal value of the combination weight of each evaluation index, carrying out flexible direct current system alternating current and direct current fault ride-through capability quantitative evaluation on the stable calculation samples in the stable calculation sample set.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method for evaluating the AC / DC fault ride-through capability of a flexible DC system based on combined weighting. Background Art

[0002] In the existing power transmission system, there is an imbalance in the development of new energy in various regions. In some areas, large-scale and long-distance transmission of new energy electricity, mainly wind and solar, to load centers has become the main way to achieve cross-regional comprehensive utilization of new energy. However, some remote areas are at the end of the power grid, and the power grid structure is weak, which cannot provide sufficient support for the grid connection of new energy. Flexible DC transmission needs to be used to dynamically compensate the reactive power of the system to adapt to the frequent fluctuations in the output of new energy. However, in the large-scale new energy transmission system through flexible DC, the occurrence of AC and DC faults may cause the shutdown of power electronic equipment in the converter station, further causing chain failures, which not only hinders the effective transmission and consumption of new energy electricity, but also poses a threat to the safe and stable operation of the power system in serious cases. How to reliably and comprehensively evaluate the AC and DC fault ride-through capability of the flexible DC transmission system is one of the key challenges faced in the actual operation of the sending-end power grid.

[0003] At present, some methods have been proposed and applied to evaluate the fault ride-through capability of flexible DC transmission systems, but most of them have certain limitations. Traditional evaluation methods usually rely on static analysis, focusing on parameters such as system power flow and node voltage after the fault occurs, which can provide certain system stability information. However, these methods ignore the dynamic behavior of the system in the early stage of the fault and the recovery process, and cannot fully reflect the fault ride-through capability of the system. In addition, some methods consider the dynamic response and transition process of the flexible DC transmission system after the fault occurs by establishing accurate mathematical models. Although these methods can simulate the short-term stability of the system and evaluate the fault ride-through capability, they often have huge computational complexity and are difficult to meet the evaluation needs of large-scale power grids or complex working conditions. In addition, some studies have also introduced sensitivity analysis and multi-objective optimization strategies to explore the influence of various factors such as converter station control parameters when system faults occur. However, these methods are difficult to correctly weigh the importance of different indicators in the context of multi-factor and complex systems, which easily leads to one-sided evaluation results. In general, most of the existing evaluation methods focus on local analysis in specific situations and lack comprehensive and systematic evaluation of fault ride-through capability. Summary of the invention

[0004] In view of this, the present invention provides a method for evaluating the AC / DC fault ride-through capability of a flexible DC system based on combined weighting, so as to at least solve the problem of limitations of the fault ride-through capability evaluation of a flexible DC transmission system in the prior art when dealing with complex systems.

[0005] In order to achieve the above object, the present invention adopts the following technical solution: A method for evaluating the AC / DC fault ride-through capability of a flexible DC system based on combined weighting includes the following steps: S1. Comprehensively consider various operation modes, fault scenarios, fault durations of the sending-end power grid and different control modes of the sending-end flexible DC converter station, and construct a stable calculation sample set for AC / DC fault ride-through capability assessment; S2. Comprehensively consider the transient response process of the flexible DC system during the fault period and its recovery capability after the fault is cleared, and also consider the support capability of the flexible DC system to the connected AC system, and construct an AC / DC fault ride-through capability evaluation index system for the flexible DC system; S3. Subjectively and objectively weight each evaluation indicator in the AC / DC fault ride-through capability evaluation index system to obtain a subjective weight and an objective weight for each evaluation indicator; S4. for each evaluation indicator, determine the combined weight of the evaluation indicators according to each subjective weight and objective weight, and determine the optimal value of the combined weight by minimizing the deviation between the combined weight and the subjective weight and the objective weight; S5. According to the optimal value of the combined weight of each evaluation index, the stable calculation samples in the stable calculation sample set are quantitatively evaluated for the AC / DC fault ride-through capability of the flexible DC system.

[0006] Preferably, the specific content of S1 includes: Determine various operation modes of the sending-end power grid, including: setting different load levels, starting and stopping states of different generator sets and different power grid topologies. The operation modes are recorded as , ,in An index indicating the operation mode of the power grid at the sending end. is the total number of operation modes; The fault scenarios near the sending-end flexible DC converter station are simulated, including three-phase short circuit fault, single-phase short circuit fault, DC line short circuit and DC line disconnection. The fault scenarios are recorded as ,in The index of the fault scenario. is the total number of failure scenarios; Set different fault durations, and record the fault duration as ,in An index representing the duration of the fault, is the total number of fault durations; Different control modes of the sending-end flexible DC converter station are set, where the control modes include active power control mode and reactive power control mode. The control modes are recorded as ,in The index indicating the control mode of the converter station. is the total number of converter station control modes; Batch build stable calculation sample sets including different operation modes, fault scenarios, fault durations and control methods of the sending-end flexible DC converter station on the simulation platform , , ,in represents the index of the stable calculation sample set, Calculate the total number of samples for stability.

[0007] Preferably, the specific contents of the AC / DC fault ride-through capability evaluation index system of the flexible DC system in S2 include: an AC fault ride-through capability evaluation index and a DC fault ride-through capability evaluation index; AC Fault Ride-Through Capability Evaluation Indicators Specifically: After the fault occurs, the lowest voltage on the AC side Relative to rated AC voltage The degree of deviation: , Maximum AC voltage Relative to rated AC voltage The degree of deviation: , Any time after the fault is cleared Internal, AC side voltage Relative to rated AC voltage The integral value of the deviation is: , In the formula, It is the moment of fault clearing; After the fault occurs, the DC side voltage minimum value Relative to DC rated voltage The degree of deviation: , Maximum DC voltage Relative to DC rated voltage The degree of deviation: , Any time after the fault is cleared Internal, DC side voltage Relative to DC rated voltage The integral value of the deviation is: , During a fault, the flexible DC system delivers active power Relative to the steady-state value of active power before the fault Maintenance ratio: , In the formula, is the time when the fault occurs; DC Fault Ride-Through Capability Evaluation Index Specifically: The DC fault current value during the period from when the fault occurs to when the fault is cleared. The integral value over time and the steady-state value of DC current before the fault Ratio of the integrated values ​​over time: , After the fault occurs, the DC fault current peak : , Minimum DC voltage Relative to DC rated voltage The degree of deviation: , After the fault occurs, the maximum voltage on the DC side is Relative to DC rated voltage The degree of deviation: , Any time after the fault is cleared Internal, DC side voltage Relative to DC rated voltage The integral value of the deviation is: , During a fault, the flexible DC system delivers active power Relative to the steady-state value of active power before the fault Maintenance ratio: .

[0008] Preferably, the specific contents of subjective weighting of each evaluation indicator in S3 include: The interval analytic hierarchy process (IAHP) is used to subjectively weight the evaluation indicators: For each pair of evaluation indicators, a judgment interval is constructed, and an interval judgment matrix is ​​constructed based on the judgment interval, where the judgment interval Indicates The evaluation index is relative to The importance range of the evaluation indicators is Indicates The evaluation index is relative to The lower limit of the importance of the evaluation index is Indicates The evaluation index is relative to The upper limit of the importance of the evaluation indicators; Perform consistency check on the constructed interval judgment matrix to ensure the rationality of the interval judgment matrix: All the interval judgment matrix Form the lower bound matrix, all Form an upper bound matrix and calculate the upper bound matrix consistency test coefficients respectively and the lower bound matrix consistency test coefficient ,like and This indicates good consistency, where: , , In the formula, Indicates the total number of evaluation indicators; To check the consistency of the upper and lower bounds of the interval judgment matrix. If they are inconsistent, it means that there is a logical problem in the constructed interval judgment matrix, and the interval should be reconstructed or adjusted; Based on the constructed interval judgment matrix, the subjective weight interval of each evaluation indicator is solved and the midpoint of the interval is selected as the subjective weight .

[0009] Preferably, the specific contents of objectively weighting each evaluation indicator in S3 include: Use the CRITIC objective weighting method to objectively assign values ​​to evaluation indicators: Perform time-domain simulation on the stable calculation sample set to obtain data on various evaluation indicators of the flexible DC system under multiple scenarios, and standardize them to eliminate the dimensional effect; Calculate the standard deviation of each evaluation indicator based on the standardized data: , In the formula, For the The standard deviation of the evaluation index, To stabilize the total number of samples, For the In the sample The value of the evaluation indicator, For the The average value of the evaluation index in all samples; Correlation coefficients between evaluation indicators: , In the formula, For the The evaluation indicators and The correlation coefficient of the evaluation indicators is For the and The covariance of the evaluation indicators, For the The set of evaluation indicators in all samples; The larger the standard deviation, the greater the difference in the evaluation indicator in different scenarios, and the more information it contains; The larger the correlation coefficient, the better the evaluation index And evaluation indicators The more repetitive the information contained, the less useful information there is; Calculate the evaluation index by combining the standard deviation and correlation coefficient Amount of information: , In the formula, For the The amount of information of the evaluation index, is the total number of evaluation indicators; The more information, the more useful information it contains. Then, the objective weight corresponding to each evaluation indicator is obtained by combining the information contained in the evaluation indicator: , In the formula, For the The objective weight of each evaluation indicator.

[0010] Preferably, the specific content of S4 includes: Combined weight and subjective weight and objective weight Deviation The minimum is used as the objective function, and the optimal value of the combined weight is obtained through the optimization algorithm. The objective function is: , In the formula, represents the index of the evaluation metric, is the total number of evaluation indicators, To obtain the optimal value of the combined weight through the optimization algorithm.

[0011] Preferably, the specific content of S5 includes: The comprehensive evaluation scores of AC fault ride-through capability and DC fault ride-through capability calculated for the stable calculation samples are as follows: , , In the formula, is the comprehensive evaluation score of AC fault ride-through capability. Comprehensive evaluation score of DC fault ride-through capability; is the index of AC fault ride-through capability evaluation index, Index of DC fault ride-through capability evaluation indicators; is the total number of AC fault ride-through capability evaluation indicators, is the total number of DC fault ride-through capability evaluation indicators; The comprehensive scores under different control modes of the sending-end converter station are compared, and the influence of the control mode on the AC / DC fault ride-through capability of the flexible DC system is further analyzed.

[0012] It can be seen from the above technical solutions that, compared with the prior art, the present invention discloses a method for evaluating the AC / DC fault ride-through capability of a flexible DC system based on combined weighting, which has the following beneficial effects: By constructing an AC / DC fault ride-through capability evaluation index system and adopting a combined weighting method, key performance indicators such as the system's steady-state characteristics, transient response, and fault recovery capability are quantified and comprehensively analyzed; compared with the traditional method of weighting that only relies on subjective experience or numerical simulation, firstly, the present invention combines the professionalism of subjective experience weighting with the scientificity of objective information weighting, and makes the index weights more reasonable through the optimization algorithm, which significantly improves the accuracy and applicability of the evaluation results; and the evaluation index system of the present invention not only covers the voltage stability, dynamic response, and fault recovery capability of the AC side, but also considers the power maintenance capability and recovery time of the DC side, which fully reflects the comprehensive performance of the flexible DC transmission system under AC / DC faults; At the same time, the present invention uses the combined weight optimization model based on the minimum information identification principle, which effectively avoids the information loss problem that may be caused by single weighting, fully utilizes the experience and knowledge of experts, and quantifies the actual impact of each indicator on the fault ride-through capability; Through the present invention, power system operators can more comprehensively evaluate the fault adaptability of the flexible DC system, thereby optimizing the operation strategy, improving the safety and reliability of the system, and providing important technical support for long-distance flexible DC transmission systems with a high proportion of new energy access. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0014] Figure 1A flow chart of a method for evaluating the AC / DC fault ride-through capability of a flexible DC system based on combined weighting provided by the present invention; Figure 2 A subjective-objective-combination weight comparison radar chart provided by an embodiment of the present invention, wherein Figure 2 (a) is the comparison chart of AC fault ride-through capability evaluation index weights. Figure 2 (b) is a comparison chart of DC fault ride-through capability evaluation index weights; Figure 3 This is a scatter plot of fault ride-through capabilities under different control modes generated by an embodiment of the present invention, where Figure 3 (a) is the scatter diagram of AC fault ride-through capability evaluation under different control modes. Figure 3 (b) is a scatter plot of DC fault ride-through capability evaluation under different control modes. DETAILED DESCRIPTION

[0015] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0016] The present invention provides a method for evaluating the AC / DC fault ride-through capability of a flexible DC system based on combined weighting. Figure 1 As shown, the following steps are included: S1. Comprehensively consider various operation modes, fault scenarios, fault durations of the sending-end power grid and different control modes of the sending-end flexible DC converter station, and construct a stable calculation sample set for AC / DC fault ride-through capability assessment; S2. Comprehensively consider the transient response process of the flexible DC system during the fault period and its recovery capability after the fault is cleared, and also consider the support capability of the flexible DC system to the connected AC system, and construct an AC / DC fault ride-through capability evaluation index system for the flexible DC system; S3. Subjectively and objectively weight each evaluation indicator in the AC / DC fault ride-through capability evaluation index system to obtain a subjective weight and an objective weight for each evaluation indicator; S4. For each evaluation indicator, determine the combined weight of the evaluation indicators according to each subjective weight and objective weight, and determine the optimal value of the combined weight by minimizing the deviation between the combined weight and the subjective weight and the objective weight; S5. According to the optimal value of the combined weight of each evaluation index, the stable calculation samples in the stable calculation sample set are quantitatively evaluated for the AC / DC fault ride-through capability of the flexible DC system.

[0017] In order to further implement the above technical solution, the specific contents of S1 include: Determine various operation modes of the sending-end power grid, including: setting different load levels, starting and stopping states of different generator sets and different power grid topologies. The operation modes are recorded as , ,in An index indicating the operation mode of the power grid at the sending end. is the total number of operation modes; The fault scenarios near the sending-end flexible DC converter station are simulated, including three-phase short circuit fault, single-phase short circuit fault, DC line short circuit and DC line disconnection. The fault scenarios are recorded as , ,in The index of the fault scenario. is the total number of failure scenarios; Set different fault durations, and record the fault duration as , ,in An index representing the duration of the fault, is the total number of fault durations; Different control modes of the sending-end flexible DC converter station are set, where the control modes include active power control mode and reactive power control mode. The control modes are recorded as , ,in The index indicating the control mode of the converter station. is the total number of converter station control modes; Batch build stable calculation sample sets including different operation modes, fault scenarios, fault durations and control methods of the sending-end flexible DC converter station on the simulation platform , , ,in represents the index of the stable calculation sample set, Calculate the total number of samples for stability.

[0018] It should be noted that: In this embodiment, 20 operating modes are constructed based on the PSD-BPA platform, which simulate the occurrence of three-phase AC short-circuit faults on three AC lines near the sending-end converter station, three-phase short-circuit faults on the AC side bus of the converter station, and single-pole grounding short-circuit faults on the flexible straight line. The fault duration is set in 6 groups [0.1s, 0.14s, 0.18s, 0.22s, 0.26s, 0.3s], and there are 3 control modes for the converter station [PQ control, PV control, and VF control].

[0019] In order to further implement the above technical solution, the specific contents of the AC / DC fault ride-through capability evaluation index system of the flexible DC system in S2 include: AC fault ride-through capability evaluation index and DC fault ride-through capability evaluation index; AC Fault Ride-Through Capability Evaluation Indicators Specifically: After the fault occurs, the lowest voltage on the AC side Relative to rated AC voltage The degree of deviation: , Maximum AC voltage Relative to rated AC voltage The degree of deviation: , Any time after the fault is cleared Internal, AC side voltage Relative to rated AC voltage The integral value of the deviation is: , In the formula, It is the moment of fault clearing; After the fault occurs, the DC side voltage minimum value Relative to DC rated voltage The degree of deviation: , Maximum DC voltage Relative to DC rated voltage The degree of deviation: , Any time after the fault is cleared Internal, DC side voltage Relative to DC rated voltage The integral value of the deviation is: , During a fault, the flexible DC system delivers active power Relative to the steady-state value of active power before the fault Maintenance ratio: , DC Fault Ride-Through Capability Evaluation Index Specifically: The DC fault current value during the period from when the fault occurs to when the fault is cleared. The integral value over time and the steady-state value of DC current before the fault Ratio of the integrated values ​​over time: , After the fault occurs, the DC fault current peak : , Minimum DC voltage Relative to DC rated voltage The degree of deviation: , After the fault occurs, the maximum voltage on the DC side is Relative to DC rated voltage The degree of deviation: , Any time after the fault is cleared Internal, DC side voltage Relative to DC rated voltage The integral value of the deviation is: , During a fault, the flexible DC system delivers active power Relative to the steady-state value of active power before the fault Maintenance ratio: .

[0020] It should be noted that: In this embodiment, The t in is 0.5s. The middle t is 0.1s.

[0021] In order to further implement the above technical solution, the specific contents of subjective weighting of each evaluation indicator in S3 include: The interval analytic hierarchy process (IAHP) is used to subjectively weight the evaluation indicators: For each pair of evaluation indicators, a judgment interval is constructed, and an interval judgment matrix is ​​constructed based on the judgment interval, where the judgment interval Indicates The evaluation index is relative to The importance range of the evaluation indicators is Indicates The evaluation index is relative to The lower limit of the importance of the evaluation index is Indicates The evaluation index is relative to The upper limit of the importance of the evaluation indicators; Perform consistency check on the constructed interval judgment matrix to ensure the rationality of the interval judgment matrix: All the interval judgment matrix Form the lower bound matrix, all Form an upper bound matrix and calculate the upper bound matrix consistency test coefficients respectively and the lower bound matrix consistency test coefficient ,like and This indicates good consistency, where: , , In the formula, Indicates the total number of evaluation indicators; To check the consistency of the upper and lower bounds of the interval judgment matrix. If they are inconsistent, it means that there is a logical problem in the constructed interval judgment matrix, and the interval should be reconstructed or adjusted; Based on the constructed interval judgment matrix, the subjective weight interval of each evaluation indicator is solved and the midpoint of the interval is selected as the subjective weight .

[0022] It should be noted that: In this embodiment, an interval judgment matrix is ​​first constructed based on expert opinions, and then the subsequent subjective weighting is completed.

[0023] In order to further implement the above technical solution, the specific contents of objectively weighting each evaluation indicator in S3 include: Use the CRITIC objective weighting method to objectively assign values ​​to evaluation indicators: Perform time-domain simulation on the stable calculation sample set to obtain data on various evaluation indicators of the flexible DC system under multiple scenarios, and standardize them to eliminate the dimensional effect; Calculate the standard deviation of each evaluation indicator based on the standardized data: , In the formula, For the The standard deviation of the evaluation index, To stabilize the total number of samples, For the In the sample The value of the evaluation indicator, For the The average value of the evaluation indicators in all samples; Correlation coefficients among evaluation indicators: , In the formula, For the The evaluation indicators and The correlation coefficient of the evaluation indicators is For the and The covariance of the evaluation indicators, For the The set of evaluation indicators in all samples; The larger the standard deviation, the greater the difference in the evaluation indicator in different scenarios, and the more information it contains; The larger the correlation coefficient, the better the evaluation index And evaluation indicators The more repetitive the information contained, the less useful information there is; Calculate the evaluation index by combining the standard deviation and correlation coefficient Amount of information: , In the formula, For the The amount of information of the evaluation index, is the total number of evaluation indicators; The more information, the more useful information it contains. Then, the objective weight corresponding to each evaluation indicator is obtained by combining the information contained in the evaluation indicator: , In the formula, For the The objective weight of each evaluation indicator.

[0024] In order to further implement the above technical solution, the specific contents of S4 include: Combined weight and subjective weight and objective weight Deviation The minimum is used as the objective function, and the optimal value of the combined weight is obtained through the optimization algorithm. The objective function is: , In the formula, represents the index of the evaluation metric, is the total number of evaluation indicators, To obtain the optimal value of the combined weight through the optimization algorithm.

[0025] It should be noted that: The subjective-objective-combination weight comparison radar chart generated in this embodiment is as follows: Figure 2 As shown, Figure 2 The three weights of the same set of evaluation indicators (subjective weight obtained by interval analytic hierarchy process, objective weight obtained by CRITIC method, and combined weight obtained by optimization algorithm with the goal of minimizing the deviation between subjective and objective weights) are plotted on the radar chart to reflect the differences under the three weight allocation methods. Figure 2 (a) is the comparison chart of AC fault ride-through capability evaluation index weights. Figure 2(b) is a comparison chart of the weights of DC fault ride-through capability evaluation indicators; it can be seen from the figure that the combined weight is basically between the subjective and objective weights, effectively balancing the subjective and objective limitations when assigning weights.

[0026] In order to further implement the above technical solution, the specific contents of S5 include: The comprehensive evaluation scores of AC fault ride-through capability and DC fault ride-through capability calculated for the stable calculation samples are as follows: , , In the formula, is the comprehensive evaluation score of AC fault ride-through capability. Comprehensive evaluation score of DC fault ride-through capability; is the index of AC fault ride-through capability evaluation index, Index of DC fault ride-through capability evaluation indicators; is the total number of AC fault ride-through capability evaluation indicators, is the total number of DC fault ride-through capability evaluation indicators; The comprehensive scores under different control modes of the sending-end converter station are compared, and the impact of the control mode on the AC and DC fault ride-through capability of the flexible DC system is further analyzed, so as to enhance the rapid response and recovery capability of the flexible DC system under faults, and provide reliable technical support for the access of a high proportion of new energy and cross-regional long-distance power transmission.

[0027] It should be noted that: The scatter diagram of fault ride-through capability under different control modes obtained in this embodiment is as follows: Figure 3 As shown, Figure 3 The fault ride-through capability score of each stable calculation sample is calculated based on the combined weight obtained above, and each sample is classified according to the control method adopted by the flexible DC converter station (PQ control: fixed active power, fixed reactive power; PV control: fixed active power, fixed AC voltage; VF control: fixed AC voltage, frequency), and the advantages and disadvantages of the control method are reflected in the form of a scatter plot. From the results, it can be seen that in an embodiment of the present invention, when the PQ control method is adopted, the AC and DC fault ride-through capability of the flexible DC system is relatively higher.

[0028] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A method for evaluating the AC and DC fault ride-through capability of a flexible DC system based on combined weighting, characterized in that: The following steps are involved: S1. Comprehensively consider various operation modes, fault scenarios, fault durations of the sending-end power grid and different control modes of the sending-end flexible DC converter station, and construct a stable calculation sample set for AC / DC fault ride-through capability assessment; S2. Comprehensively consider the transient response process of the flexible DC system during the fault period and its recovery capability after the fault is cleared, and also consider the support capability of the flexible DC system to the connected AC system, and construct an AC / DC fault ride-through capability evaluation index system for the flexible DC system; S3. Subjectively and objectively weight each evaluation indicator in the AC / DC fault ride-through capability evaluation index system to obtain a subjective weight and an objective weight for each evaluation indicator; S4. For each evaluation indicator, determine the combined weight of the evaluation indicators according to each subjective weight and objective weight, and determine the optimal value of the combined weight by minimizing the deviation between the combined weight and the subjective weight and the objective weight; S5. According to the optimal value of the combined weight of each evaluation index, the stable calculation samples in the stable calculation sample set are quantitatively evaluated for the AC / DC fault ride-through capability of the flexible DC system.

2. According to the combined weighted method for evaluating the AC / DC fault ride-through capability of a flexible DC system according to claim 1, it is characterized in that: The specific contents of S1 include: Determine various operation modes of the sending-end power grid, including: setting different load levels, starting and stopping states of different generator sets and different power grid topologies. The operation modes are recorded as , ,in An index indicating the operation mode of the power grid at the sending end. is the total number of operating modes; The fault scenarios near the sending-end flexible DC converter station are simulated, including three-phase short circuit fault, single-phase short circuit fault, DC line short circuit and DC line disconnection. The fault scenarios are recorded as ,in The index of the fault scenario. is the total number of failure scenarios; Set different fault durations, and record the fault duration as ,in An index representing the duration of the fault, is the total number of fault durations; Different control modes of the sending-end flexible DC converter station are set, where the control modes include active power control mode and reactive power control mode. The control modes are recorded as ,in The index indicating the control mode of the converter station. is the total number of converter station control modes; Batch build stable calculation sample sets including different operation modes, fault scenarios, fault durations and control methods of the sending-end flexible DC converter station on the simulation platform , , ,in represents the index of the stable calculation sample set, Calculate the total number of samples for stability.

3. The method for evaluating the AC / DC fault ride-through capability of a flexible DC system based on combined weighting according to claim 1 is characterized in that: The specific contents of the AC / DC fault ride-through capability evaluation index system of the flexible DC system in S2 include: AC fault ride-through capability evaluation index and DC fault ride-through capability evaluation index; AC Fault Ride-Through Capability Evaluation Indicators Specifically: After the fault occurs, the lowest voltage on the AC side Relative to rated AC voltage The degree of deviation: ， Maximum AC voltage Relative to rated AC voltage The degree of deviation: ， Any time after the fault is cleared Internal, AC side voltage Relative to rated AC voltage The integral value of the deviation is: ， In the formula, It is the moment of fault clearing; After the fault occurs, the DC side voltage minimum value Relative to DC rated voltage The degree of deviation: ， Maximum DC voltage Relative to DC rated voltage The degree of deviation: ， Any time after the fault is cleared Internal, DC side voltage Relative to DC rated voltage The integral value of the deviation is: ， During a fault, the flexible DC system delivers active power Relative to the steady-state value of active power before the fault Maintenance ratio: ， In the formula, is the time when the fault occurs; DC Fault Ride-Through Capability Evaluation Index Specifically: The DC fault current value during the period from when the fault occurs to when the fault is cleared. The integral value over time and the steady-state value of DC current before the fault Ratio of the integrated values ​​over time: ， After the fault occurs, the DC fault current peak : ， Minimum DC voltage Relative to DC rated voltage The degree of deviation: ， After the fault occurs, the maximum voltage on the DC side is Relative to DC rated voltage The degree of deviation: ， Any time after the fault is cleared Internal, DC side voltage Relative to DC rated voltage The integral value of the deviation is: ， During a fault, the flexible DC system delivers active power Relative to the steady-state value of active power before the fault Maintenance ratio: 。 4. The method for evaluating the AC / DC fault ride-through capability of a flexible DC system based on combined weighting according to claim 3 is characterized in that: The specific contents of subjective weighting of each evaluation indicator in S3 include: The interval analytic hierarchy process (IAHP) is used to subjectively weight the evaluation indicators: Select any two evaluation indicators from the AC fault ride-through capability evaluation indicators or the DC fault ride-through capability evaluation indicators as a pair of evaluation indicators; For each pair of evaluation indicators, a judgment interval is constructed, and an interval judgment matrix is ​​constructed based on the judgment interval, where the judgment interval Indicates The evaluation index is relative to The importance range of the evaluation indicators is Indicates The evaluation index is relative to The lower limit of the importance of the evaluation index is Indicates The evaluation index is relative to The upper limit of the importance of the evaluation indicators; Perform consistency check on the constructed interval judgment matrix to ensure the rationality of the interval judgment matrix: All the interval judgment matrix Form the lower bound matrix, all Form an upper bound matrix and calculate the upper bound matrix consistency test coefficients respectively and the lower bound matrix consistency test coefficient ,like and This indicates good consistency, where: ， ， In the formula, Indicates the total number of evaluation indicators; To check the consistency of the upper and lower bounds of the interval judgment matrix. If they are inconsistent, it means that there is a logical problem in the constructed interval judgment matrix, and the interval should be reconstructed or adjusted; Based on the constructed interval judgment matrix, the subjective weight interval of each evaluation indicator is solved and the midpoint of the interval is selected as the subjective weight .

5. The method for evaluating the AC and DC fault ride-through capability of a flexible DC system based on combined weighting according to claim 1 is characterized in that: The specific contents of objectively weighting each evaluation indicator in S3 include: Use the CRITIC objective weighting method to objectively assign values ​​to evaluation indicators: Perform time-domain simulation on the stable calculation sample set to obtain data on various evaluation indicators of the flexible DC system under multiple scenarios, and standardize them to eliminate the dimensional effect; Calculate the standard deviation of each evaluation indicator based on the standardized data: ， In the formula, For the The standard deviation of the evaluation index, To stabilize the total number of samples, For the In the sample The value of the evaluation indicator, For the The average value of the evaluation indicators in all samples; Correlation coefficients between evaluation indicators: ， In the formula, For the The evaluation indicators and The correlation coefficient of the evaluation indicators is For the and The covariance of the evaluation indicators, For the The set of evaluation indicators in all samples; The larger the standard deviation, the greater the difference in the evaluation indicator in different scenarios, and the more information it contains; The larger the correlation coefficient, the better the evaluation index And evaluation indicators The more repetitive the information contained, the less useful information there is; Calculate the evaluation index by combining the standard deviation and correlation coefficient Amount of information: ， In the formula, For the The amount of information of the evaluation index, is the total number of evaluation indicators; The more information, the more useful information it contains. Then, the objective weight corresponding to each evaluation indicator is obtained by combining the information contained in the evaluation indicator: ， In the formula, For the The objective weight of each evaluation indicator.

6. The method for evaluating the AC and DC fault ride-through capability of a flexible DC system based on combined weighting according to claim 1 is characterized in that: The specific contents of S4 include: Combined weight and subjective weight and objective weight Deviation The minimum is used as the objective function, and the optimal value of the combined weight is obtained through the optimization algorithm. The objective function is: ， In the formula, represents the index of the evaluation metric, is the total number of evaluation indicators, To obtain the optimal value of the combined weight through the optimization algorithm.

7. The method for evaluating the AC and DC fault ride-through capability of a flexible DC system based on combined weighting according to claim 1 is characterized in that: The specific contents of S5 include: The comprehensive evaluation scores of AC fault ride-through capability and DC fault ride-through capability calculated for the stable calculation samples are as follows: ， ， In the formula, is the comprehensive evaluation score of AC fault ride-through capability. Comprehensive evaluation score of DC fault ride-through capability; is the index of AC fault ride-through capability evaluation index, Index of DC fault ride-through capability evaluation indicators; is the total number of AC fault ride-through capability evaluation indicators, is the total number of DC fault ride-through capability evaluation indicators; The comprehensive scores under different control modes of the sending-end converter station are compared, and the influence of the control mode on the AC / DC fault ride-through capability of the flexible DC system is further analyzed.

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