A flexibility evaluation method for grid-connected inverters

Abstract

The application discloses a kind of flexibility evaluation methods for grid-connected inverter, belong to grid-connected inverter evaluation field.The application makes grid-connected inverter adopt different control strategies, run at different series compensation degree and grid strength, and collect index data representing flexibility under each working condition, process data by entropy weight method and carry out comprehensive evaluation.Entropy weight method in the application is composed of standardization processing, probability calculation, entropy value calculation, information utility value calculation, objective weight calculation and linear weighting method.The application is aimed at the problem that grid-connected inverter flexibility is difficult to represent and quantify, by collecting the flexibility data of grid-connected inverter under different working conditions, intuitively displays the flexibility of grid-connected inverter based on data driving, and provides scientific and objective reference for the selection and comparison of grid-connected inverter control strategy under different scenes.

Description

Technical Field

[0001] This invention relates to the field of grid-connected inverter evaluation, and more specifically to a method for evaluating the flexibility of grid-connected inverters. Background Technology

[0002] Currently, renewable energy power generation has become a major strategic need in my country. The continuous increase in installed renewable energy capacity has led to a rising penetration rate of renewable energy power generation, posing a threat to the stability of the power system. Simultaneously, the random fluctuations on both the renewable energy generation side and the grid side necessitate a certain degree of flexibility in grid-connected inverters. As a pivotal device in the connection between renewable energy and the grid, the grid-connected inverter is required to adapt to the random fluctuations of natural resources such as wind and solar power and complex grid condition changes while maintaining efficient and stable power generation. By employing reasonable evaluation methods, the flexibility level of the grid-connected inverter under different control strategies can be accurately assessed, providing a scientific and objective reference for selecting appropriate control strategies under different operating conditions.

[0003] Numerous academic papers and patents have researched the issue of flexibility assessment, for example:

[0004] 1) The literature "Analyzing operational flexibility of electric power systems" (2015, 72: 155-164) establishes flexibility indicators in three aspects: energy, power, and ramp rate, from the perspective of power supply balance. These three physical quantities are related by derivatives and integrals, and the flexibility indicators based on them can comprehensively describe the system's response speed and regulation capability in maintaining a balance between power generation and demand.

[0005] 2) The literature “Assessment and optimization of the flexibility of power systems with a high proportion of wind power integration”, 2019, 40(9): 1-10, proposes a comprehensive flexibility assessment system that combines dynamic and static indicators. The static flexibility indicator assesses the system’s flexibility supply capacity based on the inherent technical characteristics of each power generation resource; the dynamic flexibility indicator combines the volatility and uncertainty of net load with the system’s supply capacity to assess the degree of system flexibility insufficiency under different operating conditions.

[0006] 3) The literature “Research on the evaluation method of the flexibility supply capacity of high-penetration new energy systems”, 2023, 46(12): 117-123. (“Research on the evaluation method of the flexibility supply capacity of high-penetration new energy systems”, 2023, Vol. 46, No. 12, pp. 117-123) proposes an evaluation method for the flexibility supply capacity of high-penetration new energy systems. Based on the time-series fluctuation characteristics of net load, a flexibility demand model for high-penetration new energy systems is established. The idea of ​​node operation flexibility is used to establish an evaluation index for flexibility resource supply capacity. Through the coordinated optimization of source, load and storage flexibility resources, the system can achieve the optimization of overall flexibility supply capacity while making the system more economical.

[0007] 4) Chinese patent document CN202310753684.1, published on September 19, 2023, entitled "Method, Device and Storage Medium for Determining Power System Flexibility", proposes a method, device and storage medium for determining power system flexibility. It determines the three-level flexibility index by obtaining the ramp-up index and peak-shaving index, thereby solving the problem that the existing methods for determining power system flexibility consider only one perspective.

[0008] Based on the above literature, the existing research has the following shortcomings:

[0009] 1. Existing literature research is generally based on the flexibility assessment of the power system. Although grid-connected inverters are an important part of the power system, the assessment process often focuses on the overall flexibility of the power system and ignores the flexibility of the individual grid-connected inverters.

[0010] 2. Existing literature does not consider the flexibility differences of grid-connected inverters from the perspective of data-driven control strategies, nor does it evaluate the flexibility level of grid-connected inverters through objective assessment methods. Summary of the Invention

[0011] The technical problem to be solved by this invention is to address the issues existing in the prior art. Specifically, this invention proposes a flexibility assessment method for grid-connected inverters, selecting flexibility indicators to characterize the flexibility of grid-connected inverters and establishing a quantitative assessment model for the flexibility of new energy grid-connected inverters. By collecting flexibility indicator data of grid-connected inverters under different operating conditions, a comprehensive assessment is performed based on the entropy weight method to compare the flexibility levels of grid-connected inverters under different control strategies.

[0012] The objective of this invention is achieved as follows: This invention provides a method for evaluating the flexibility of a grid-connected inverter. The main circuit topology of the grid-connected inverter involved in this evaluation method includes a DC-side power supply, a three-phase half-bridge inverter circuit, an LCL filter, a grid impedance, and a three-phase grid. The DC-side power supply, the three-phase half-bridge inverter circuit, the LCL filter, and the grid impedance are connected in series in the aforementioned order and then connected to the three-phase grid.

[0013] The grid-connected inverters are configured to employ different control strategies and operate at different series complement K. c Under different grid strength SCR conditions, and in each operating condition, index data characterizing flexibility are collected. The index weights are determined using the entropy weight method, and then a comprehensive evaluation is performed using a linear weighted method. Specifically, the evaluation method involves the following steps:

[0014] Step 1: Collect data on five indicators under one operating condition, including the active power output range P. rang Active power regulation speed V adj Minimum reactive power compensation Q min System phase margin PM and percentage fluctuation of voltage amplitude at the point of common coupling n;

[0015] If all five indicators in this working condition are complete, then this working condition is confirmed as a valid working condition.

[0016] Step 2: Using the same method as in Step 1, collect data on five indicators under m effective working conditions;

[0017] Step 3: Calculation of objective weights based on the entropy weight method

[0018] Step 3.1: The five indicators under the m effective operating conditions collected in Step 2 are designated as flexibility indicators, and the values ​​of the flexibility indicators are recorded as flexibility indicator value x. ij Where i is the operating condition number, i = 1, 2, ..., m; j is the flexibility index number, j = 1, 2, ..., 5; where 1 corresponds to the active power output range P range 2 corresponds to the active power regulation speed V adj 3 corresponds to the minimum reactive power compensation amount Q min 4 corresponds to the system phase margin PM, and 5 corresponds to the percentage of voltage amplitude fluctuation at the common coupling point n.

[0019] The maximum value of the flexibility index under m effective operating conditions is denoted as the maximum flexibility index value x. jmax The minimum value of the flexibility index under m effective operating conditions is denoted as the minimum flexibility index value x. jmin ;

[0020] Step 3.2: Determine the attributes of each flexibility indicator and classify them into positive flexibility indicators or negative flexibility indicators.

[0021] Step 3.3: Standardize the positive or negative flexibility index using either the positive or negative standardization formula to obtain its standardized value, which is then denoted as the standardized value z. ij ;

[0022] The positive standardization formula is: z ij =(x ij -x jmin ) / (x jmax -x jmin );

[0023] The negative standardization formula is: z ij =(x jmax -x ij ) / (x jmax -x jmin );

[0024] Step 3.4, based on the standardized value z ij The probability p of each flexibility index is obtained through probability calculation formula. ij The probability calculation formula is as follows:

[0025]

[0026] Step 3.5, based on the probability p of each flexibility indicator occurring. ij The entropy value e of each flexibility index is obtained through the entropy calculation formula. j Its entropy value is calculated using the following formula:

[0027] Step 3.6, based on the entropy value e of each flexibility index j The information utility value d for each flexibility indicator is obtained through the information utility value calculation formula. j Its information utility value is calculated using the following formula: d j =1-e j ;

[0028] Step 3.7, based on the information utility value d of each flexibility indicator. j The objective weight W of each flexibility index is obtained through the entropy weight calculation formula. j The formula for calculating its entropy weight is:

[0029]

[0030] Step 4: Calculate the flexibility assessment results based on the linear weighting method.

[0031] Based on the standardized value z of each flexibility index ij and objective weight W j The flexibility assessment score s of the grid-connected inverter for each effective operating condition is obtained by using a linear weighted method. i The formula for its linear weighted method is:

[0032]

[0033] Preferably, the implementation process of step 1 is as follows:

[0034] Step 1.1: Given the active power command signal P to the grid-connected inverter. ref Given the reactive power command signal Q of the grid-connected inverter ref And by setting its initial value to 0, the maximum active power command signal P that ensures system stability is obtained. refmax and minimum active power command signal P refmin ;

[0035] Active power output range P range Determined in the following manner:

[0036] If P refmax ≤P n P range =P refmax -P refmin ;

[0037] If P refmax >P n P range =P n -P refmin ;

[0038] Among them, P n This is the rated active power command signal;

[0039] Step 1.2: Provide the active power command signal P to the grid-connected inverter. ref The maximum active power command signal P refmax The actual maximum active power P output of the grid-connected inverter is obtained. max Given the active power command signal P of the grid-connected inverter ref The maximum active power command signal P refmax 50% of the actual active power output P of the grid-connected inverter is obtained. maxh ;

[0040] Command signal P of the grid-connected inverter ref The periodic step response signal P STEP Periodic step response signal P STEP The maximum value is the maximum active power command signal P.refmax The minimum value is the maximum active power command signal P. refmax 50%;

[0041] The given rising oscillation range is the actual maximum active power P output by the grid-connected inverter. max The given decreasing oscillation range is 90% to 110% of the actual output active power P of the grid-connected inverter. maxh 90% to 110%;

[0042] Define the rise adjustment time T1 and the fall adjustment time T2, where the rise adjustment time T1 is the periodic step response signal P. STEP When a step jump occurs, the time from the start of the step jump to the actual output active power P of the grid-connected inverter is... maxh The time it takes for the signal to first enter and stabilize within the rising oscillation range, wherein the falling adjustment time T2 is the periodic step response signal P. STEP When a step jump occurs, the time from the start of the step jump to the actual maximum active power P output by the grid-connected inverter is... max The time it takes for the first entry into and stabilization within the descending oscillation range;

[0043] Based on the actual maximum active power P output by the grid-connected inverter max The actual output active power P of the grid-connected inverter maxh The active power regulation speed V is calculated from the rise adjustment time T1 and the fall adjustment time T2. adj V adj =(P max -P maxh ) / (2T1)+(P max -P maxh ) / (2T2);

[0044] Step 1.3, obtain the reactive power command signal Q from the grid-connected inverter. ref Starting from the initial value, gradually increase the reactive power command signal Q of the grid-connected inverter. ref Until the given active power command signal P of the grid-connected inverter is given. ref The rated active power command signal P n The system is stable, and at this time the reactive power command signal Q of the grid-connected inverter is... ref The value is the minimum reactive power compensation Q of the grid-connected inverter. min ;

[0045] Step 1.4: Provide the active power command signal P to the grid-connected inverter. ref The rated active power command signal P n Given the reactive power command signal Q of the grid-connected inverter refThe initial value is used. The Bode plots of the grid-connected inverter output impedance and the grid impedance are obtained by frequency sweeping. The frequency f at the amplitude intersection point of the Bode plots of the grid-connected inverter output impedance and the grid impedance is obtained, as well as the phase θ0 of the frequency f at the amplitude intersection point of the grid-connected inverter output impedance and the phase θ0 of the frequency f at the amplitude intersection point of the grid impedance. g ;

[0046] The system phase margin PM is determined as follows:

[0047] If there is only one point of intersection of amplitudes, PM = 180° - (θ) g -θ0);

[0048] If there are multiple amplitude intersection points, calculate the phase margin of each amplitude intersection point and take the phase margin with the smallest value as the system phase margin PM.

[0049] Step 1.5: Provide the active power command signal P to the grid-connected inverter. ref The maximum active power command signal P refmax 50% of the reactive power command signal Q of the grid-connected inverter ref The initial value is used to obtain the voltage amplitude V at the common coupling point. pcc By setting the grid voltage amplitude to drop by 10%, a new common coupling point voltage amplitude V is obtained. pcc1 The percentage fluctuation of the voltage amplitude at the common coupling point, n, is obtained using the formula: n = 100% × (V pcc -V pcc1 ) / V pcc .

[0050] Preferably, the positive flexibility index is an index where a larger value is better, including the active power output range P. range Active power regulation speed V adj The system phase margin PM; the negative flexibility index is a value that is better the smaller it is, including the minimum reactive power compensation Q. min The percentage of voltage amplitude fluctuation at the common coupling point, n.

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0052] 1. Compared to the overall flexibility assessment of the power system, this invention enables the flexibility assessment of individual grid-connected inverters.

[0053] 2. This invention solves the problem of difficulty in quantifying the flexibility of grid-connected inverters by objectively reflecting the flexibility level of grid-connected inverters using different control strategies through the degree of data change.

[0054] 3. This invention allows for a direct comparison of the flexibility levels of grid-connected inverters employing different control strategies through flexibility assessment results. Attached Figure Description

[0055] Figure 1 This is the main circuit topology diagram of the grid-connected inverter used in the embodiments of the present invention.

[0056] Figure 2 This is a flowchart illustrating the implementation of the evaluation method of the present invention.

[0057] Figure 3 A comparison of flexibility evaluation curves for grid-connected inverters employing different control strategies under series-compensated grid conditions. Detailed Implementation

[0058] The present invention will be further described below with reference to the accompanying drawings.

[0059] The grid-connected inverter main circuit topology used in this invention is as follows: Figure 1 As shown. By Figure 1 As can be seen, the grid-connected inverter main circuit topology involved in this evaluation method includes a DC-side power supply, a three-phase half-bridge inverter circuit, an LCL filter, a grid impedance, and a three-phase grid. The DC-side power supply, the three-phase half-bridge inverter circuit, the LCL filter, and the grid impedance are connected in series in the aforementioned order and then connected to the three-phase grid.

[0060] In this embodiment, the LCL filter includes a source-side filter inductor L1, a grid-side filter inductor L2, and a filter capacitor C. f and damping resistor R d .

[0061] exist Figure 1 Above, E is the DC power supply, Y is the three-phase half-bridge inverter circuit, and Z... g R is the power grid impedance. g For the grid impedance Z g The resistive component, L g For the grid impedance Z g The emotional weight, C g For the grid impedance Z g The capacitive component. The three-phase half-bridge inverter circuit Y is connected in series between the DC power supply E and the source-side filter inductor L1. The other end of the source-side filter inductor L1 is connected to the grid-side filter inductor L2, and the filter capacitor C... f and damping resistor R d Connected in parallel between the source-side filter inductor L1 and the grid-side filter inductor L2, the grid impedance Z g It is connected in series between the grid-side filter inductor L2 and the three-phase power grid to achieve grid connection.

[0062] In this embodiment, the grid-connected inverter has a rated capacity of 20kW and a DC side voltage V. dc=770V, the inductance of the source-side filter inductor L1 is 0.9mH, and the filter capacitor C f The capacitance is 11.6uF, the inductance of the grid-side filter inductor L2 is 0.2mH, and the damping resistor R... d The resistance is 2.18Ω, and the mains impedance Z g The resistive component R g The current is 0.35Q, and the grid impedance Z is... g The sensual component L g The mains impedance Z ranges from 5.8 mH to 23.1 mH. g The capacitive component C g The grid strength ranges from 731.77 μF to 0.0176 F, with grid strength SCR ranging from 1 to 4 (in steps of 0.1), totaling 31 grid strengths, and series complement K. c The series compensation degree ranges from 0.1 to 0.6 (in increments of 0.1), with a total of 6 types. The grid-connected inverters adopt two control strategies: conventional grid-type additional damping control and q-axis impedance reshaping grid-type control. In summary, there are a total of 372 operating conditions.

[0063] Figure 2 This is a flowchart illustrating the implementation of the evaluation method of the present invention. Figure 2 As can be seen, the present invention provides a flexibility assessment method for grid-connected inverters, which consists of standardization processing, probability calculation, entropy calculation, information utility value calculation, objective weight calculation, and linear weighting method.

[0064] The evaluation method allows the grid-connected inverter to adopt different control strategies and operate at different series complement K. C Under different grid strength SCR conditions, and in each operating condition, index data characterizing flexibility are collected. The index weights are determined using the entropy weight method, and then a comprehensive evaluation is performed using a linear weighted method. Specifically, the evaluation method involves the following steps:

[0065] Step 1: Collect data on five indicators under one operating condition, including the active power output range P. range Active power regulation speed V adj Minimum reactive power compensation Q min System phase margin PM and percentage fluctuation of voltage amplitude at the point of common coupling n;

[0066] If all five indicators in the above working condition are complete, then the working condition is confirmed as a valid working condition.

[0067] Step 2: Using the same method as in Step 1, collect data on five indicators under m effective operating conditions. In this embodiment, m = 350.

[0068] Simulations and index data were collected using the MATLAB / Simulink platform. Some index data are shown in Table 1.

[0069] Table 1

[0070]

[0071] Step 3: Calculation of objective weights based on the entropy weight method

[0072] Step 3.1: The five indicators under the m effective operating conditions collected in Step 2 are designated as flexibility indicators, and the values ​​of the flexibility indicators are recorded as flexibility indicator value x. ij Where i is the operating condition number, i = 1, 2, ..., m; j is the flexibility index number, j = 1, 2, ..., 5; where 1 corresponds to the active power output range P range 2 corresponds to the active power regulation speed V adj 3 corresponds to the minimum reactive power compensation amount Q min 4 corresponds to the system phase margin PM, and 5 corresponds to the percentage of voltage amplitude fluctuation at the common coupling point n.

[0073] The maximum value of the flexibility index under m effective operating conditions is denoted as the maximum flexibility index value x. jmax The minimum value of the flexibility index under m effective operating conditions is denoted as the minimum flexibility index value x. jmin .

[0074] Step 3.2: Determine the attributes of each flexibility indicator and classify them into positive flexibility indicators or negative flexibility indicators.

[0075] In this embodiment, the positive flexibility index is an index where a larger value is better, including the active power output range P. range Active power regulation speed V adj The system phase margin PM; the negative flexibility index is a value that is better the smaller it is, including the minimum reactive power compensation Q. min The percentage of voltage amplitude fluctuation at the common coupling point, n.

[0076] Step 3.3: Standardize the positive or negative flexibility index using either the positive or negative standardization formula to obtain its standardized value, which is then denoted as the standardized value z. ij ;

[0077] The positive standardization formula is: z ij =(x ij -x jmin ) / (x jmax -x jmin );

[0078] The negative standardization formula is: z ij =(x jmax -x ij ) / (x jmax -x jmin ).

[0079] Step 3.4, based on the standardized value z ij The probability p of each flexibility index is obtained through probability calculation formula. ij The probability calculation formula is as follows:

[0080]

[0081] Step 3.5, based on the probability p of each flexibility indicator appearing. ij The entropy value e of each flexibility index is obtained through the entropy calculation formula. j Its entropy value is calculated using the following formula:

[0082] Step 3.6, based on the entropy value e of each flexibility index j The information utility value d for each flexibility index is obtained through the information utility value calculation formula. j Its information utility value is calculated using the following formula: d j =1-e j .

[0083] Step 3.7, based on the information utility value d of each flexibility indicator. j The objective weight W of each flexibility index is obtained through the entropy weight calculation formula. j The formula for calculating its entropy weight is:

[0084]

[0085] Step 4: Calculate the flexibility assessment results based on the linear weighting method.

[0086] Based on the standardized value z of each flexibility index ij and objective weight W j The flexibility assessment score s of the grid-connected inverter for each effective operating condition is obtained by using a linear weighted method. i The formula for its linear weighted method is:

[0087]

[0088] In this example, the objective weight W for each flexibility index is calculated. j As shown in Table 2, the flexibility assessment scores are shown in Table 3.

[0089] Table 2

[0090]

[0091] Table 3

[0092]

[0093] In this embodiment, the implementation process of step 1 is as follows:

[0094] Step 1.1: Given the active power command signal P to the grid-connected inverter. ref Given the reactive power command signal Q of the grid-connected inverter ref And by setting its initial value to 0, the maximum active power command signal P that ensures system stability is obtained. refmax and minimum active power command signal P refmin ;

[0095] Active power output range P range Determined in the following manner:

[0096] If P refmax ≤P n P range =P refmax -P refmin ;

[0097] If P refmax >P n P range =P n -P refmin ;

[0098] Among them, P n This is the rated active power command signal;

[0099] Step 1.2: Provide the active power command signal P to the grid-connected inverter. ref The maximum active power command signal P refmax The actual maximum active power P output of the grid-connected inverter is obtained. max Given the active power command signal P of the grid-connected inverter ref The maximum active power command signal P refmax 50% of the actual active power output P of the grid-connected inverter is obtained. maxh ;

[0100] Command signal P of the grid-connected inverter ref The periodic step response signal P STEP Periodic step response signal P STEP The maximum value is the maximum active power command signal P. refmax The minimum value is the maximum active power command signal P. refmax 50%;

[0101] The given rising oscillation range is the actual maximum active power P output by the grid-connected inverter. max The given decreasing oscillation range is 90% to 110% of the actual output active power P of the grid-connected inverter. maxh 90% to 110%;

[0102] Define the rise adjustment time T1 and the fall adjustment time T2, where the rise adjustment time T1 is the periodic step response signal P. STEP When a step jump occurs, the time from the start of the step jump to the actual output active power P of the grid-connected inverter is... maxh The time it takes for the signal to first enter and stabilize within the rising oscillation range, wherein the falling adjustment time T2 is the periodic step response signal P. STEP When a step jump occurs, the time from the start of the step jump to the actual maximum active power P output by the grid-connected inverter is... max The time it takes for the first entry into and stabilization within the descending oscillation range;

[0103] Based on the actual maximum active power P output by the grid-connected inverter max The actual output active power P of the grid-connected inverter maxh The active power regulation speed V is calculated from the rise adjustment time T1 and the fall adjustment time T2. adj V adj =(P max -P maxh ) / (2T1)+(P max -P maxh ) / (2T2);

[0104] Step 1.3, obtain the reactive power command signal Q from the grid-connected inverter. ref Starting from the initial value, gradually increase the reactive power command signal Q of the grid-connected inverter. ref Until the given active power command signal P of the grid-connected inverter is given. ref The rated active power command signal P n The system is stable, and at this time the reactive power command signal Q of the grid-connected inverter is... ref The value is the minimum reactive power compensation Q of the grid-connected inverter. min ;

[0105] Step 1.4: Provide the active power command signal P to the grid-connected inverter. ref The rated active power command signal P n Given the reactive power command signal Q of the grid-connected inverter refThe initial value is used. The Bode plots of the grid-connected inverter output impedance and the grid impedance are obtained by frequency sweeping. The frequency f at the amplitude intersection point of the Bode plots of the grid-connected inverter output impedance and the grid impedance is obtained, as well as the phase θ0 of the frequency f at the amplitude intersection point of the grid-connected inverter output impedance and the phase θ0 of the frequency f at the amplitude intersection point of the grid impedance. g ;

[0106] The system phase margin PM is determined as follows:

[0107] If there is only one point of intersection of amplitudes, PM = 180° - (θ) g -θ0);

[0108] If there are multiple amplitude intersection points, calculate the phase margin of each amplitude intersection point and take the phase margin with the smallest value as the system phase margin PM.

[0109] Step 1.5: Provide the active power command signal P to the grid-connected inverter. ref The maximum active power command signal P refmax 50% of the reactive power command signal Q of the grid-connected inverter ref The initial value is used to obtain the voltage amplitude V at the common coupling point. pcc By setting the grid voltage amplitude to drop by 10%, a new common coupling point voltage amplitude V is obtained. pce1 The percentage fluctuation of the voltage amplitude at the common coupling point, n, is obtained using the formula: n = 100% × (V pcc -V pcci ) / V pcc .

[0110] To visually compare the flexibility of grid-connected inverters under different operating conditions, MATLAB plots were created for this invention.

[0111] Figure 3 The flexibility evaluation curves for grid-connected inverters using different control strategies under series-compensated grids include those for grid-connected inverters using conventional grid-following additional damping control under series-compensated grids and those using q-axis impedance reshaping grid-following control under series-compensated grids.

[0112] like Figure 3 As shown, within the range of series complement of 0.1 to 0.6 and SCR of 1 to 3.5, grid-connected inverters employing q-axis impedance reshaping and grid-type control exhibit higher flexibility than those employing conventional grid-type control with additional damping. Within the range of series complement of 0.2 to 0.4 and SCR of 3.5 to 4, grid-connected inverters employing conventional grid-type control with additional damping exhibit higher flexibility than those employing q-axis impedance reshaping and grid-type control.

Claims

1. A method for evaluating the flexibility of a grid-connected inverter, wherein the main circuit topology of the grid-connected inverter involved in the evaluation method includes a DC-side power supply, a three-phase half-bridge inverter circuit, an LCL filter, a grid impedance, and a three-phase grid, wherein the DC-side power supply, the three-phase half-bridge inverter circuit, the LCL filter, and the grid impedance are connected in series in the aforementioned order and then connected to the three-phase grid. Its features are, The grid-connected inverters are configured to employ different control strategies and operate at different series complement K. C Under different grid strength SCR conditions, and in each operating condition, index data characterizing flexibility are collected. The index weights are determined using the entropy weight method, and then a comprehensive evaluation is performed using a linear weighted method. Specifically, the evaluation method involves the following steps: Step 1: Collect data on five indicators under one operating condition, including the active power output range P. range Active power regulation speed V adj Minimum reactive power compensation Q min System phase margin PM and percentage fluctuation of voltage amplitude at the point of common coupling n; If all five indicators in the above working condition are complete, then the working condition is confirmed as a valid working condition. Step 2: Using the same method as in Step 1, collect data on five indicators under m effective working conditions; Step 3: Calculation of objective weights based on the entropy weight method Step 3.1: The five indicators under the m effective operating conditions collected in Step 2 are designated as flexibility indicators, and the values ​​of the flexibility indicators are recorded as flexibility indicator value x. ij Where i is the operating condition number, i = 1, 2, ..., m; j is the flexibility index number, j = 1, 2, ..., 5; where 1 corresponds to the active power output range P range 2 corresponds to the active power regulation speed V adj 3 corresponds to the minimum reactive power compensation amount Q min 4 corresponds to the system phase margin PM, and 5 corresponds to the percentage of voltage amplitude fluctuation at the common coupling point n. The maximum value of the flexibility index under m effective operating conditions is denoted as the maximum flexibility index value x. jmax The minimum value of the flexibility index under m effective operating conditions is denoted as the minimum flexibility index value x. jmin ; Step 3.2: Determine the attributes of each flexibility indicator and classify them into positive flexibility indicators or negative flexibility indicators. Step 3.3: Standardize the positive or negative flexibility index using either the positive or negative standardization formula to obtain its standardized value, which is then denoted as the standardized value z. ij , The positive standardization formula is: z ij =(x ij -x jmin ) / (x jmax -x jmin ); The negative standardization formula is: z ij =(x jmax -x ij ) / (x jmax -x jmin ); Step 3.4, based on the standardized value z ij The probability p of each flexibility index is obtained through probability calculation formula. ij The probability calculation formula is as follows: Step 3.5, based on the probability p of each flexibility indicator occurring. ij The entropy value e of each flexibility index is obtained through the entropy calculation formula. j Its entropy value is calculated using the following formula: Step 3.6, based on the entropy value e of each flexibility index j The information utility value d for each flexibility indicator is obtained through the information utility value calculation formula. j Its information utility value is calculated using the following formula: d j =1-e j ; Step 3.7, based on the information utility value d of each flexibility indicator. j The objective weight W of each flexibility index is obtained through the entropy weight calculation formula. j The formula for calculating its entropy weight is: Step 4: Calculate the flexibility assessment results based on the linear weighting method. Based on the standardized value z of each flexibility index ij and objective weight W j The flexibility assessment score s of the grid-connected inverter for each effective operating condition is obtained by using a linear weighted method. i The formula for its linear weighted method is:

2. The method for evaluating the flexibility of grid-connected inverters according to claim 1, characterized in that, The implementation process of step 1 is as follows: Step 1.1: Given the active power command signal P to the grid-connected inverter. ref Given the reactive power command signal Q of the grid-connected inverter ref And by setting its initial value to 0, the maximum active power command signal P that ensures system stability is obtained. refmax and minimum active power command signal P refmin ; Active power output range P range Determined in the following manner: If P refmax ≤P n P range =P refmax -P refmin ; If P refmax >P n P range =P n -P refmin ; Among them, P n This is the rated active power command signal; Step 1.2: Provide the active power command signal P to the grid-connected inverter. ref The maximum active power command signal P refmax The actual maximum active power P output of the grid-connected inverter is obtained. max Given the active power command signal P of the grid-connected inverter ref The maximum active power command signal P refmax 50% of the actual active power output P of the grid-connected inverter is obtained. maxh ; Command signal P of the grid-connected inverter ref The periodic step response signal P STBP Periodic step response signal P STEP The maximum value is the maximum active power command signal P. refmax The minimum value is the maximum active power command signal P. refmax 50%; The given rising oscillation range is the actual maximum active power P output by the grid-connected inverter. max The given decreasing oscillation range is 90% to 110% of the actual output active power P of the grid-connected inverter. maxh 90% to 110%; Define the rise adjustment time T1 and the fall adjustment time T2, where the rise adjustment time T1 is the periodic step response signal P. STEP When a step jump occurs, the time from the start of the step jump to the actual output active power P of the grid-connected inverter is... maxh The time it takes for the signal to first enter and stabilize within the rising oscillation range, wherein the falling adjustment time T2 is the periodic step response signal P. STEP When a step jump occurs, the time from the start of the step jump to the actual maximum active power P output by the grid-connected inverter is... max The time it takes for the first entry into and stabilization within the descending oscillation range; Based on the actual maximum active power P output by the grid-connected inverter max The actual output active power P of the grid-connected inverter maxh The active power regulation speed V is calculated from the rise adjustment time T1 and the fall adjustment time T2. adj V adj =(P max -P maxh ) / (2T1)+(P max -P maxh ) / (2T2); Step 1.3, obtain the reactive power command signal Q from the grid-connected inverter. ref Starting from the initial value, gradually increase the reactive power command signal Q of the grid-connected inverter. ref Until the given active power command signal P of the grid-connected inverter is given. ref The rated active power command signal P n The system is stable, and at this time the reactive power command signal Q of the grid-connected inverter is... ref The value is the minimum reactive power compensation Q of the grid-connected inverter. min ; Step 1.4: Provide the active power command signal P to the grid-connected inverter. ref The rated active power command signal P n Given the reactive power command signal Q of the grid-connected inverter ref The initial value is used. The Bode plots of the grid-connected inverter output impedance and the grid impedance are obtained by frequency sweeping. The frequency f at the amplitude intersection point of the Bode plots of the grid-connected inverter output impedance and the grid impedance is obtained, as well as the phase θ0 of the frequency f at the amplitude intersection point of the grid-connected inverter output impedance and the phase θ0 of the frequency f at the amplitude intersection point of the grid impedance. g ; The system phase margin PM is determined as follows: If there is only one point of intersection of amplitudes, PM = 180° - (θ) g -θ0); If there are multiple amplitude intersection points, calculate the phase margin of each amplitude intersection point and take the phase margin with the smallest value as the system phase margin PM. Step 1.5: Provide the active power command signal P to the grid-connected inverter. ref The maximum active power command signal P rofmax 50% of the reactive power command signal Q of the grid-connected inverter ref The initial value is used to obtain the voltage amplitude V at the common coupling point. pcc By setting the grid voltage amplitude to drop by 10%, a new common coupling point voltage amplitude V is obtained. pcc1 The percentage fluctuation of the voltage amplitude at the common coupling point, n, is obtained using the formula: n = 100% × (V pcc -V pcc1 ) / V pcc .

3. The method for evaluating the flexibility of grid-connected inverters according to claim 1, characterized in that, The positive flexibility index is a higher value is better, including the active power output range P. range Active power regulation speed V adj The system phase margin PM; the negative flexibility index is a value that is better the smaller it is, including the minimum reactive power compensation Q. min The percentage of voltage amplitude fluctuation at the common coupling point, n.

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