Method and system for calculating critical short-circuit ratio of grid-connected system of grid-connected converter

Abstract

This application belongs to the field of transient stability assessment of new energy power systems, and specifically discloses a method and system for calculating the critical short-circuit ratio of a grid-connected converter system. Through this application, the dominant links of the grid-connected converter system, including the terminal voltage link and the synchronization link, are simplified and reduced in order, and the reactance value of the grid line is calculated according to the Routh criterion, thereby obtaining the critical short-circuit ratio for small disturbances, which can effectively evaluate the grid strength for stable operation of the system; and the difference between the critical short-circuit ratio for large disturbances and the critical short-circuit ratio for small disturbances obtained by a large number of simulations can be used to obtain the critical short-circuit ratio for large disturbances of the system, which can be used to estimate the critical short-circuit ratio for large disturbances of the system, thereby increasing the reference value for the system to be able to operate safely and stably after a large disturbance, thereby improving the grid connection stability.

Description

Technical Field

[0001] The present application belongs to the field of transient stability assessment of new energy power systems, and more specifically, to a method and system for calculating the critical short-circuit ratio of a grid-connected system of a grid-connected converter. Background Art

[0002] Renewable energy generation is generally connected to the power grid through power electronic equipment represented by voltage source converters. While this grid-connected method brings flexibility and controllability to the power system, it also brings complex problems and challenges to the large-disturbance stability of the power system due to its strong coupling and strong nonlinear characteristics.

[0003] In power system stability analysis, small disturbances typically refer to disturbances in the system small enough to be analyzed using linearized state equations. Large disturbances, in contrast to small disturbances, are events that have a greater impact on the system. Grid strength is often measured using the short-circuit ratio; generally speaking, a higher short-circuit ratio indicates a stronger grid.

[0004] Current research indicates that grid-connected converters can experience instability when connected to a strong grid. Most research focuses on small-disturbance stability analysis, with little attention paid to large-disturbance stability assessment. Furthermore, many current stability assessment methods and optimization schemes focus on the critical short-circuit ratio (SCR) corresponding to small-disturbance instability, using it as a metric for system performance under strong grid conditions. The SCR refers to the system short-circuit ratio at which a grid-connected converter experiences strong grid instability. Exceeding this value results in system instability; maintaining stability within this value.

[0005] However, studies have found that as grid strength increases, grid-connected converter systems, after experiencing small-disturbance instability, continue to experience multiple instability patterns, ultimately leading to divergent instability, at which point the system reaches the large-disturbance stability boundary. The critical short-circuit ratio corresponding to monotonic divergent instability can be used as a measure of the large-disturbance stability boundary of grid-connected converters. Therefore, the key to evaluating system stability under large-disturbance conditions lies in quantitatively estimating this critical short-circuit ratio. Summary of the Invention

[0006] In response to the defects of the existing technology, the purpose of this application is to provide a method and system for calculating the critical short-circuit ratio of a grid-connected system of a grid-connected converter, aiming to solve the shortcomings of the existing small-disturbance critical short-circuit ratio calculation that mostly relies on time domain simulation and is relatively complex to calculate, and to provide a quantitative calculation method for the large-disturbance critical short-circuit ratio.

[0007] To achieve the above objectives, in a first aspect, the present application provides a method for calculating the small-disturbance critical short-circuit ratio of a grid-connected system of a grid-connected converter, comprising:

[0008] Obtain the values ​​of various control parameters in the dominant control link of the grid-connected system of the grid-connected converter;

[0009] Substituting the control parameter values ​​in the dominant control link into the polynomial fraction of the Routh stability criterion coefficient with respect to the grid line reactance, setting it equal to 0, and calculating the grid line reactance;

[0010] Calculate the inverse of the grid line reactance to obtain the critical short-circuit ratio for small disturbances.

[0011] Preferably, the control parameter values ​​in the dominant control link include: a terminal voltage loop proportional coefficient, a terminal voltage loop integral coefficient, a synchronization loop proportional coefficient and a synchronization loop low-pass filter cutoff frequency.

[0012] Preferably, the polynomial fraction of the Routh stability criterion coefficient with respect to the grid line reactance is as follows:

[0013]

[0014] Where, the expressions of coefficients ρ0~ρ6 are as follows:

[0015]

[0016] Among them, k p1 Indicates the terminal voltage loop proportional coefficient, k i1 Indicates the terminal voltage loop integral coefficient, ω p represents the cut-off frequency of the synchronous loop low-pass filter, m represents the proportional coefficient of the synchronous loop, ω b Indicates the reference value of frequency, u tref Indicates the reference value of the terminal voltage, u g Indicates the grid voltage, represents the initial value of the phase angle, x g Indicates the grid line reactance.

[0017] To achieve the above objectives, in a second aspect, the present application provides a method for calculating a large disturbance critical short-circuit ratio of a grid-connected system of a grid-connected converter, comprising:

[0018] Using the calculation method described in the first aspect, a small disturbance critical short-circuit ratio is obtained;

[0019] Determine the short-circuit ratio corresponding to the chaotic attractor breaking point corresponding to the critical stability of large perturbations;

[0020] Calculate the difference between the critical short-circuit ratio for small disturbances and the short-circuit ratio corresponding to the chaotic attractor breaking point corresponding to the critical stability for large disturbances;

[0021] The sum of the critical short-circuit ratio for small disturbances and the difference between the short-circuit ratios is calculated as the estimated value of the critical short-circuit ratio for large disturbances.

[0022] Preferably, the short-circuit ratio corresponding to the chaotic attractor breaking point corresponding to the large disturbance critical stability is determined based on time domain simulation.

[0023] Preferably, the short-circuit ratio corresponding to the chaotic attractor breaking point corresponding to the large disturbance critical stability is determined based on time domain simulation in the following manner:

[0024] Gradually increase the system short-circuit ratio and observe the phase angle Whether the phase angle reaches 180° during the oscillation process, if When it reaches 180°, it is considered that this is the large disturbance short-circuit ratio corresponding to the chaotic attractor rupture point.

[0025] Preferably, the empirical value of the short circuit ratio difference is 0.35 pu.

[0026] Preferably, it also includes:

[0027] In order to improve the large disturbance stability margin of the grid-connected system of the grid-connected converter under a strong power grid, the terminal voltage loop proportional coefficient and the terminal voltage loop integral coefficient are increased, and / or the synchronization loop proportional coefficient and the synchronization loop low-pass filter cutoff frequency are reduced.

[0028] To achieve the above-mentioned objectives, in a third aspect, the present application provides a critical short-circuit ratio calculation system for a grid-connected converter system, comprising at least one processor and at least one memory; the at least one memory is used to store computer instructions; the at least one processor is used to execute at least part of the computer instructions to implement the small-disturbance critical short-circuit ratio calculation method described in the first aspect, or to implement the large-disturbance critical short-circuit ratio calculation method described in the second aspect.

[0029] To achieve the above-mentioned purpose, in the fourth aspect, the present application provides a computer-readable storage medium, which stores computer instructions. When the computer reads the computer instructions in the storage medium, the computer executes the small disturbance critical short-circuit ratio calculation method as described in the first aspect, or executes the large disturbance critical short-circuit ratio calculation method as described in the second aspect.

[0030] It can be understood that the beneficial effects of the second to fourth aspects mentioned above can be found in the relevant description of the first aspect mentioned above, and will not be repeated here.

[0031] In general, the above technical solutions conceived by this application have the following beneficial effects compared with the existing technologies:

[0032] (1) The present application provides a method and system for calculating the small-disturbance critical short-circuit ratio of a grid-connected converter system. The method and system simplify and reduce the order of the dominant links of the grid-connected converter system, including the terminal voltage link and the synchronization link, and calculate the grid line reactance value according to the Routh criterion, thereby obtaining the small-disturbance critical short-circuit ratio. Compared with the prior art, the present application can quantitatively calculate the small-disturbance critical short-circuit ratio of the grid-connected converter system through a simple explicit expression, which can effectively evaluate the grid strength for stable operation of the system.

[0033] (2) The present application provides a method and system for calculating the critical short-circuit ratio of a grid-connected converter system for large disturbances. The above method is used to first obtain the critical short-circuit ratio of a grid-connected converter system for small disturbances. Then, the difference between the critical short-circuit ratio of large disturbances and the critical short-circuit ratio of small disturbances obtained from a large number of simulations can be used to obtain the critical short-circuit ratio of the system for large disturbances. Compared with the existing technology, the present application only needs to calculate the critical short-circuit ratio of the system for small disturbances. Combined with the conclusions obtained from a large number of simulations, the critical short-circuit ratio of the system for large disturbances can be estimated, thereby increasing the reference value for the system to be able to operate safely and stably after a large disturbance and improving the grid-connected stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a flow chart of a method for calculating the critical short-circuit ratio of a large disturbance grid-connected system of a grid-connected converter provided in this application.

[0035] Figure 2 This is a topological diagram of the grid-connected system of the grid-connected converter provided in an embodiment of the present application.

[0036] Figure 3 A topological diagram of a reduced-order model containing a dominant link provided in an embodiment of the present application.

[0037] Figure 4 Schematic diagram of the bifurcation behavior corresponding to the large disturbance critical short-circuit ratio and the small disturbance critical short-circuit ratio, which are large disturbance stability evaluation indicators provided in the embodiments of the present application.

[0038] Figure 5 This is a schematic diagram of the structure of an electronic device provided in this application. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0040] The term "and / or" as used herein describes an association between related objects, indicating that three possible relationships exist. For example, "A and / or B" can represent: A exists alone, A and B exist simultaneously, or B exists alone. The symbol " / " as used herein indicates that the related objects are in an "or" relationship, for example, A / B means either A or B.

[0041] The terms "first" and "second" in this specification and claims are used to distinguish different objects rather than to describe a specific order of objects. For example, "first response message" and "second response message" are used to distinguish different response messages rather than to describe a specific order of response messages.

[0042] In the embodiments of this application, words such as "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of this application should not be interpreted as being preferred or advantageous over other embodiments or designs. Rather, the use of words such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0043] In the description of the embodiments of the present application, unless otherwise specified, "multiple" means two or more, for example, multiple processing units means two or more processing units, etc.; multiple elements means two or more elements, etc.

[0044] Next, the technical solutions provided in the embodiments of this application are introduced.

[0045] The inventive concept for calculating the critical short-circuit ratio for large perturbations in this application is as follows: Based on bifurcation theory, as the grid strength increases, a single-machine infinite-scale system of a grid-connected converter undergoes a Hopf bifurcation, followed by a period-doubling bifurcation, leading to chaos, and ultimately, a chaotic attractor rupture. The Hopf bifurcation point represents the system's small-perturbation stability boundary, while the chaotic attractor rupture point represents the system's large-perturbation stability boundary. Therefore, the system's small-perturbation stability boundary can be first determined using the Routh criterion. Then, by combining the numerical relationship between the two bifurcation points, a quantitative estimate of the system's large-perturbation stability boundary can be made.

[0046] By analyzing the participation factors of the characteristic roots near the imaginary axis, it is found that the strong grid instability phenomenon of the grid-type converter is mainly related to the terminal voltage control and synchronization control, and a reduced-order model containing only the dominant control links (including terminal voltage control and synchronization control links) is obtained. The terminal voltage control parameter k is obtained. p1 and k i1 and synchronous control parameters m and ω p At this point, the linearized differential equation can be described as:

[0047]

[0048] in, Indicates the change in the synchronous ring output angle, t indicates time, ω b represents the reference value of the frequency, Δω represents the change in frequency; m represents the proportional coefficient of the synchronization ring, ω p represents the cutoff frequency of the synchronous ring low-pass filter, ΔP represents the change in active power, Δx tvc1 Indicates the change in the d-axis component of the terminal voltage loop output through the integral link, Δx tvc2 Indicates the change in the q-axis component of the terminal voltage loop output through the integral link, k i1 Indicates the terminal voltage loop integral coefficient, Δu td ,Δu tq They represent the changes in the d-axis component and q-axis component of the terminal voltage respectively.

[0049] Where, the expression of ΔP is as follows:

[0050] ΔP=i d0 Δu td +u td0 Δi d +i q0 Δu tq

[0051] Among them, i d0 ,i q0 They represent the initial steady-state values ​​of the d-axis component and the q-axis component of the grid current, u td0 represents the initial steady-state value of the d-axis component of the terminal voltage, Δi d Indicates the change in the d-axis component of the grid current.

[0052]

[0053] Among them, u g Indicates the grid voltage, represents the initial value of the phase angle, x g Indicates the reactance value of the power grid line, u tref Indicates the reference value of the terminal voltage.

[0054] Its algebraic equation is:

[0055]

[0056] Among them, Δi q ,Δi d They represent the changes of the d-axis component and q-axis component of the grid current, respectively, and k p1 Represents the terminal voltage loop proportional coefficient.

[0057] Based on the above differential algebraic equations, the state matrix A of the reduced-order model can be obtained:

[0058]

[0059] Among them, γ1, γ2, and γ3 are the coefficients of the state matrix respectively.

[0060]

[0061] According to the classical automatic control principle, the stability of the fourth-order model can be analyzed with the help of Routh stability criterion.

[0062] First, based on the state matrix A of the fourth-order model, the corresponding characteristic equation is derived as follows:

[0063]

[0064] Where λ represents the system eigenvalue, I represents the identity matrix, and α0 to α4 represent the coefficients preceding the eigenvalue order.

[0065] α0=1

[0066]

[0067] Arrange the coefficients into a Routh array as shown in Table 1.

[0068] Table 1

[0069]

[0070] The expressions of coefficients β1, β2, χ1, and ε1 in Table 1 are as follows:

[0071]

[0072] According to the Routh criterion, a necessary condition for the stability of a fourth-order system is that all coefficients of the characteristic equation and the first column of the Routh array are positive, that is, the above coefficients should satisfy the following relationship:

[0073] α0,α1,α2,α3,α4>0

[0074] β1>0,χ1>0,ε1>0

[0075] Substituting the specific expressions of each coefficient, the above four stability conditions are further analyzed as follows:

[0076] 1) α0 to α4 > 0. According to the expressions of α0 to α4, except for α2, the other coefficients are always greater than 0, so the first condition can be simplified to: α2 > 0.

[0077] 2) β1 > 0. Since α1 is always greater than 0, the second condition can be simplified to: α1α2-α0α3>0.

[0078] 3)χ1>0. Therefore the third condition can be simplified to:

[0079] 4) ε1>0. ε1=α4, so ε1 is always greater than 0.

[0080] In summary, the Routh criterion can be simplified into the following three conditions:

[0081] α2>0

[0082] α1α2-α0α3>0

[0083]

[0084] Further analysis shows that the inequality in the brackets in the last line of the above formula is:

[0085]

[0086] It is a necessary and sufficient condition for the above inequality group to hold. Let η=0, and combine the expressions of α0~α4 to get the Routh stability coefficient ζ, which can be sorted into the reactance x of the power grid line. g The form of a polynomial fraction is:

[0087]

[0088] Where, the expressions of coefficients ρ0~ρ6 are as follows:

[0089]

[0090] It can calculate the system's small disturbance critical short-circuit ratio (in the per-unit system, the short-circuit ratio SCR is numerically equal to the grid line reactance x g the inverse of ).

[0091] The system's critical short-circuit ratios for small and large disturbances are compared under multiple sets of different control parameter scenarios, as shown in Table 2. CSCR-S is the estimated value of the critical short-circuit ratio for small disturbances obtained based on the Routh stability coefficient η, and CSCR-L is the actual value of the critical short-circuit ratio for large disturbances obtained based on time-domain simulation.

[0092] By comparing the values ​​of the critical short-circuit ratios for small and large disturbances, we find that within the conventional parameter range, the difference between the two is roughly 0.35 pu. The critical short-circuit ratio for small disturbances can be approximately solved using the Routh criterion, so based on the relationship between the two, we can also quantitatively estimate the critical short-circuit ratio for large disturbances.

[0093] Table 2

[0094]

[0095] Based on the above analysis, if Figure 1 As shown, the present application proposes a method for calculating the critical short-circuit ratio of a large disturbance of a grid-connected converter system, comprising the following steps:

[0096] (1) The control parameters and command values ​​of the system terminal voltage link and the synchronization link are used to establish the state space equation of the fourth-order model including the dominant link and obtain the corresponding state matrix;

[0097] (2) The corresponding characteristic equation is derived from the state matrix of the fourth-order model, and the Routh array is written according to the coefficients of the characteristic equation to obtain the Routh criterion of the fourth-order model;

[0098] (3) Based on the relationship between the coefficients of the characteristic equation, the above criteria are simplified to obtain the Routh stability coefficient, and the small perturbation critical short-circuit ratio is further obtained as the small perturbation stability boundary of the system;

[0099] (4) Based on bifurcation analysis and time-domain simulation, the short-circuit ratio corresponding to the chaotic attractor rupture point (which manifests as divergent instability in the time domain) is taken as the large-disturbance stability boundary of the system, denoted as the large-disturbance critical short-circuit ratio;

[0100] (5) Based on the numerical relationship between the bifurcation points corresponding to the stability boundaries of small and large disturbances, a quantitative estimation of the critical short-circuit ratio for large disturbances is performed.

[0101] Example

[0102] The topology diagram of the grid-connected system of the grid-connected converter of this embodiment is as follows: Figure 2 As shown in Figure 2. Since the terminal voltage link and the synchronization link are closely related to the instability of the strong power grid, a reduced-order model containing only these two dominant links is constructed for the convenience of analysis, as shown in Figure 2. Figure 3 This embodiment calculates the large disturbance stability boundary of the system based on the reduced-order model. Figure 4 A schematic diagram shows the bifurcation behavior of a grid-connected converter system as grid strength changes. The solid line represents a stable equilibrium point, and the dashed line represents an unstable equilibrium point. As the grid strength increases, the system undergoes a Hopf bifurcation at SCR = 3.8 pu. This bifurcation behavior is manifested in the characteristic root locus as a pair of conjugate characteristic roots cross the imaginary axis and enter the right half plane, indicating a critical state for small perturbation stability. Therefore, the short-circuit ratio (SCR = 3.8 pu) corresponding to the Hopf bifurcation point is the critical short-circuit ratio for small perturbations. Further increasing the grid strength causes the system to undergo a period-doubling bifurcation and enter chaos. The chaotic attractor continues to expand and eventually collides with the unstable equilibrium point at SCR = 4.33 pu, disappearing, and the system undergoes divergent instability. The short-circuit ratio (SCR = 4.33 pu) corresponding to this bifurcation point is the critical short-circuit ratio for large perturbations.

[0103] The calculation of the large disturbance critical short circuit ratio CSCR-L in this embodiment includes:

[0104] Firstly, the control parameters and command values ​​of the system terminal voltage link and the synchronization link are obtained, and the state space equation of the fourth-order model including the dominant link is established.

[0105] Secondly, based on the state matrix of the fourth-order model, the Routh criterion is used to obtain the Routh stability coefficient η, and then the small disturbance critical short-circuit ratio of the system is calculated based on this coefficient.

[0106] Finally, based on the numerical relationship that the difference between the Hopf bifurcation point corresponding to the critical stability of small perturbations and the chaotic attractor breaking point corresponding to the critical stability of large perturbations is about 0.35 pu, the estimated value of the critical short-circuit ratio for large perturbations is calculated.

[0107] Furthermore, Table 3 shows the actual values ​​of the system's large-disturbance critical short-circuit ratio and the quantitatively estimated values ​​calculated by this application under different terminal voltage link and synchronization link control parameters. The results show that the actual values ​​are consistent with the estimated values. Therefore, the evaluation method of this application can accurately predict the large-disturbance stability boundary of the system under strong power grid. In addition, it can be found that the system's large-disturbance critical short-circuit ratio CSCR-L increases with k p1 and k i1 The table increases with the increase of n and ω p Therefore, according to the evaluation method and index proposed in this application, when designing parameters, the proportional coefficient k of the terminal voltage control can be appropriately increased. p1 and the integral coefficient k i1 , reduce the droop coefficient m of the synchronization link and the low-pass filter cutoff frequency ω p , to improve the system's large disturbance stability margin under a strong power grid.

[0108] Table 3

[0109]

[0110] It is understandable that the detailed functional implementation of each of the above units / modules can be found in the introduction of the aforementioned method embodiment, and will not be repeated here.

[0111] It should be understood that the above-mentioned device is used to execute the method in the above-mentioned embodiment. The implementation principle and technical effect of the corresponding program module in the device are similar to those described in the above-mentioned method. The working process of the device can refer to the corresponding process in the above-mentioned method and will not be repeated here.

[0112] Based on the method in the above embodiment, the embodiment of the present application provides an electronic device, such as Figure 5As shown, the electronic device may include: a processor, a communications interface, a memory, and a communications bus, wherein the processor, the communications interface, and the memory communicate with each other via the communications bus. The processor may call logic instructions in the memory to execute the method of the above embodiment.

[0113] In addition, the logical instructions in the above-mentioned memory can be implemented in the form of a software functional unit and can be stored in a computer-readable storage medium when sold or used as an independent product. Based on this understanding, the technical solution of the present application, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product, which is stored in a storage medium and includes a number of instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the method described in each embodiment of the present application.

[0114] Based on the method in the above embodiment, an embodiment of the present application provides a computer-readable storage medium, which stores a computer program. When the computer program runs on a processor, the processor executes the method in the above embodiment.

[0115] Based on the method in the above embodiment, an embodiment of the present application provides a computer program product. When the computer program product runs on a processor, the processor executes the method in the above embodiment.

[0116] It is understood that the processor in the embodiments of the present application may be a central processing unit (CPU), or may be other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field programmable gate arrays (FPGA), or other programmable logic devices, transistor logic devices, hardware components, or any combination thereof. The general-purpose processor may be a microprocessor or any conventional processor.

[0117] The method steps in the embodiments of the present application can be implemented by hardware or by a processor executing software instructions. The software instructions can be composed of corresponding software modules, which can be stored in random access memory (RAM), flash memory, read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), registers, hard disks, mobile hard disks, CD-ROMs or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor so that the processor can read information from the storage medium and write information to the storage medium. Of course, the storage medium can also be a component of the processor. The processor and the storage medium can be located in an ASIC.

[0118] In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware or any combination thereof. When implemented using software, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the process or function described in the embodiment of the present application is generated in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted via the computer-readable storage medium. The computer instructions can be transmitted from one website, computer, server or data center to another website, computer, server or data center via a wired (e.g., coaxial cable, optical fiber, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) method. The computer-readable storage medium can be any available medium that a computer can access or a data storage device such as a server or data center that includes one or more available media integrated. The available medium can be a magnetic medium (e.g., a floppy disk, a hard disk, a tape), an optical medium (e.g., a DVD), or a semiconductor medium (e.g., a solid state drive (SSD)).

[0119] It will be understood that the various numerical numbers involved in the embodiments of the present application are merely distinctions for the convenience of description and are not intended to limit the scope of the embodiments of the present application.

[0120] It is easy for those skilled in the art to understand that the above is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A method for calculating the critical short-circuit ratio of a grid-connected system with a grid-connected converter, characterized in that: include: Obtain the values ​​of various control parameters in the dominant control link of the grid-connected system of the grid-connected converter; Substituting the control parameter values ​​in the dominant control link into the polynomial fraction of the Routh stability criterion coefficient with respect to the grid line reactance, setting it equal to 0, and calculating the grid line reactance; Calculate the inverse of the grid line reactance to obtain the critical short-circuit ratio for small disturbances.

2. The calculation method according to claim 1, wherein: The control parameter values ​​in the dominant control link include: a terminal voltage loop proportional coefficient, a terminal voltage loop integral coefficient, a synchronization loop proportional coefficient, and a synchronization loop low-pass filter cutoff frequency.

3. The calculation method according to claim 1, wherein: The polynomial fraction of the Routh stability criterion coefficient with respect to the grid line reactance is as follows: Where, the expressions of coefficients ρ0~ρ6 are as follows: Among them, k p1 Indicates the terminal voltage loop proportional coefficient, k i1 Indicates the terminal voltage loop integral coefficient, ω p represents the cut-off frequency of the synchronous loop low-pass filter, m represents the proportional coefficient of the synchronous loop, ω b Indicates the reference value of frequency, u tref Indicates the reference value of the terminal voltage, u g Indicates the grid voltage, represents the initial value of the phase angle, x g Indicates the grid line reactance.

4. A method for calculating the critical short-circuit ratio of a grid-connected system with a grid-connected converter under large disturbances, characterized in that: include: Using the calculation method according to any one of claims 1 to 3, a small disturbance critical short-circuit ratio is obtained; Determine the short-circuit ratio corresponding to the chaotic attractor breaking point corresponding to the critical stability of large perturbations; Calculate the difference between the critical short-circuit ratio for small disturbances and the short-circuit ratio corresponding to the chaotic attractor breaking point corresponding to the critical stability for large disturbances; The sum of the critical short-circuit ratio for small disturbances and the difference between the short-circuit ratios is calculated as the estimated value of the critical short-circuit ratio for large disturbances.

5. The calculation method according to claim 4, wherein: The short-circuit ratio corresponding to the chaotic attractor breaking point of critical stability under large disturbance is determined based on time domain simulation.

6. The calculation method according to claim 5, wherein: The method for determining the short-circuit ratio of the chaotic attractor rupture point corresponding to the critical stability of large disturbance based on time domain simulation is as follows: Gradually increase the system short-circuit ratio and observe the phase angle Whether the phase angle reaches 180° during the oscillation process, if When it reaches 180°, it is considered that the point at which the chaotic attractor breaks down corresponds to the large disturbance short-circuit ratio.

7. The calculation method according to claim 4, wherein: The empirical value of the short-circuit ratio difference is 0.35 pu.

8. The calculation method according to any one of claims 4 to 7, characterized in that: Also includes: In order to improve the large disturbance stability margin of the grid-connected system of the grid-connected converter under a strong power grid, the terminal voltage loop proportional coefficient and the terminal voltage loop integral coefficient are increased, and / or the synchronization loop proportional coefficient and the synchronization loop low-pass filter cutoff frequency are reduced.

9. A critical short-circuit ratio calculation system for a grid-connected converter system, characterized in that: comprising at least one processor and at least one memory; The at least one memory is for storing computer instructions; The at least one processor is used to execute at least part of the computer instructions to implement the small disturbance critical short circuit ratio calculation method described in any one of claims 1 to 3, or to implement the large disturbance critical short circuit ratio calculation method described in any one of claims 4 to 8.

10. A computer-readable storage medium, characterized in that The storage medium stores computer instructions. When the computer reads the computer instructions in the storage medium, the computer executes the small disturbance critical short circuit ratio calculation method as described in any one of claims 1 to 3, or executes the large disturbance critical short circuit ratio calculation method as described in any one of claims 4 to 8.

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