A method and system for evaluating transient stability of a low-inertia power grid with a grid-following converter based on the Zobov method

Abstract

The application discloses a kind of based on the method and system for evaluating transient stability of follow network converter access low-inertia power grid of zubov method, comprising: (1) construct follow network converter-low-inertia system dynamic equation, and obtain the series form of dynamic equation by Taylor expansion;(2) calculate the truncated energy function of system based on zubov method;(3) the set W M Where the derivative of truncated energy function to time is 0;(4) calculate the energy function value of each operating point in set W M , and obtain the minimum value Set as the system stability domain;(5) numerical integration is carried out on the fault system;In each integration step, calculate the initial state after fault, and substitute the initial state after fault into set V (M) , and the last integration time is the estimated maximum removal time of fault. By using the application, the difficulty problem caused by negative damping and low-inertia system dynamics can be avoided.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of electrical engineering, and particularly relates to a method and system for evaluating transient stability of a grid-connected converter connected to a low-inertia power grid based on the Zobov method. BACKGROUND

[0002] With the increasing penetration of renewable energy in power systems, the proportion of traditional synchronous generators will decrease significantly, and power sources based on converters will gradually become dominant. The grid-connected converter is the main interface for renewable energy to be connected to the grid, and its synchronization mechanism based on a phase-locked loop (PLL) is significantly different from that of a synchronous generator. Under large disturbances of the power grid, the grid-connected converter may lose synchronization. Therefore, the analysis of the transient stability of the grid-connected converter has become an important research topic. The synchronous generator is the main stable support device of the traditional power grid, and its proportion will decrease, which will weaken the stability of the power system and have a greater impact on the transient stability of the grid-connected converter.

[0003] A method for modeling and optimizing the transient stability of a grid-connected converter considering the dynamics of a direct-current capacitor is disclosed in Chinese patent document CN118137550A, and a method for analyzing and enhancing the transient stability of a grid-connected converter is disclosed in Chinese patent document CN118572771A.

[0004] The traditional direct method for power system transient analysis is closely related to the transient characteristics of devices such as synchronous generators. The equal-area method and the Lyapunov method are the most widely used methods. Studies have shown that the dynamic equation of the grid-connected converter is similar in form to the swing equation of the synchronous generator, but negative damping may occur within a certain phase angle range, which brings difficulties to the application of traditional methods. The traditional transient analysis method is mainly based on the perspective of energy conservation, and the positive damping characteristic of the synchronous generator makes the damping term negligible. However, the negative damping that may occur in the grid-connected converter may make the traditional direct method inapplicable. There are two methods to solve the negative damping problem: 1) limiting the attraction domain to the positive damping region, which will result in a large conservative error; 2) calculating or analyzing the damping term work, which is feasible in the case of continuous faults, but in the case of considering fault clearance, the fault trajectory is more complex and it is difficult to calculate the damping work. On the other hand, with the decrease in the proportion of synchronous generators, the system will exhibit low-inertia characteristics, and the introduction of system frequency dynamics will lead to an increase in the order of the dynamic equation of the entire system, making it more difficult to apply traditional methods such as the equal-area method. SUMMARY

[0005] The application provides a method and system for evaluating the transient stability of a grid-connected converter connected to a low-inertia power grid based on the Zobov method, which can avoid the problem of difficulty in solving caused by negative damping and low-inertia system dynamics.

[0006] The application discloses a method for evaluating transient stability of a grid-connected converter accessing a low-inertia power grid based on a Zubov method, which aims at predicting how long the fault is removed to ensure that the grid-connected converter does not lose synchronization, that is, predicting the maximum removal time, and comprises the following steps.

[0007] (1) accessing the grid-connected converter to a low-inertia power grid system model, constructing a dynamic equation of the grid-connected converter-low-inertia system, and obtaining a series form of the dynamic equation through Taylor expansion;

[0008] (2) calculating a truncated energy function V (M) of the system based on the Zubov method;

[0009] (3) obtaining a set W M in which a derivative of the truncated energy function with respect to time is 0;

[0010] (4) calculating energy function values of each operating point in the set W M , and obtaining a minimum value ; and taking the set W as an attractor domain of system stability;

[0011] (5) in order to predict the maximum removal time, firstly, performing numerical integration on a system differential equation after a short-circuit fault of the power grid; in each integration step, calculating an initial state after the fault, wherein respectively represent a relative power angle of the converter and the power grid at the moment when the fault is removed, an angular velocity output by a phase-locked loop of the converter and an angular velocity corresponding to a frequency of the power grid side;

[0012] substituting the initial state into the set V (M) , and taking a last integration time as an estimated maximum removal time of the fault.

[0013] In step (1), after the low-inertia power grid system model accesses the grid-connected converter, the model contains one grid-connected converter, one synchronous machine and four impedances, and the model satisfies the following five assumptions:

[0014] ① the swing equation of the synchronous machine is used to represent the dynamics of the low-inertia system, and a phase angle is represented by δ g ;

[0015] ② the voltage support of the grid-connected converter is reflected by impedances Z c and Z g between the synchronous machine and the grid-connected converter, and the low voltage support corresponds to a larger impedance value, therefore, the machine terminal voltage U g is considered as a constant when the power angle dynamics of the system is simulated;

[0016] ③ The current control loop of grid-connected converter is ignored due to its short time scale, and the grid-connected converter is regarded as a controlled current source with fixed current amplitude I c and phase angle δ c determined by phase-locked loop (PLL);

[0017] ④ The load is constant impedance Z l , and when a three-phase short-circuit fault occurs, the fault resistance R f is connected in parallel with Z l ;

[0018] ⑤ The influence of frequency fluctuation on reactance is ignored, i.e., the reactance in the system is regarded as constant.

[0019] The voltage and current phase of the grid-connected converter connected to the low-inertia system are described by DQ coordinate system, and the variables of the grid-connected converter-low-inertia system are described by three DQ coordinate systems d0-q0, dg-qg and dc-qc;

[0020] The d0-q0 coordinate system has a constant angular velocity ω0reference system; the dg-qg and dc-qc coordinate systems have varying angular velocities ω g and ω c , respectively, which describe the dynamic variables of the low-inertia system and the grid-connected converter, and their phase difference with d0-q0is δ g and δ c , respectively; the voltage phase of the synchronous machine coincides with the dg axis; and the current phase of the grid-connected converter is determined by the dc-qc axis.

[0021] The dynamic equation of the grid-connected converter-low-inertia system is expressed as a constant differential equation model, and the formula is as follows:

[0022]

[0023] wherein δ represents the relative angle between dg axis and dc axis, δ = δ c - δ g , P′ ma,g , P′ el,g , D′ g , P′ ma,c , P′ el,c , D′ c , and are intermediate parameters, which are expressed as follows:

[0024]

[0025] In the formula, Z eq1 , Z eq2 and Z eq3 represent virtual impedance, θ1represents, θ2and θ3represent Z eq1 , Z eq2 and Z eq3The phase angle of J g and D g Represents the damping and inertia of the system equivalent synchronous machine; P ma,g and U g Indicates the mechanical power of the system equivalent synchronous machine and the terminal voltage setting value; K i and K p Indicates the integral coefficient and proportional coefficient of the converter phase-locked loop; I c and Indicates the current amplitude and phase angle setting value of the converter.

[0026] In step (1), the series form of the dynamic equation is obtained by Taylor expansion, as follows:

[0027] For the grid-following converter-low inertia system, only F1 and F3 of the dynamic equation F need to be Taylor expanded:

[0028]

[0029] Let F′ represent the Taylor expansion form of F. Divide the linearized part from the nonlinear part, and F′ can be written as:

[0030]

[0031] Where b ni F n The linearization term x in ′ i The coefficient of Contains F n All m in ′ T The second term, x is the system variable, N is the dimension; in the grid-following converter-low inertia system, (x1,x2,x3)=(δ,ω,ω g ), N=3.

[0032] In the present invention, the Taylor expansion form F′ of F is T = 30 is truncated.

[0033] The specific process of step (2) is:

[0034] In the Zubov method, the energy function V is in the form of an infinite series, expressed as:

[0035] V=V2+V3+...+V m +...

[0036] Where V m is a polynomial, where the order of all terms is m; in actual calculation, V is truncated at a certain order, let V (M) represents V truncated at the Mth order;

[0037] There are three variables in the grid-following converter-low inertia system model: V m Expressed as:

[0038]

[0039] Where a mij V m The coefficient of one of the terms δ, ω, and ω g The orders are mij, j, i respectively;

[0040] In the Zubov method, the energy function is given by the following partial differential equation:

[0041]

[0042] Where φ is a positive definite function of the variable x, F n represents the nth equation in F;

[0043] The formula Substitution Compare The coefficients of the terms on both sides of the formula; V2, V3, ..., V m The calculation formula is as follows:

[0044]

[0045] Where R m (x) is a polynomial of degree m, and the coefficients are given by the formula and From V2, V3, ..., V m-1 and F′ are calculated; therefore, V m The calculation of V is a recursive process. For each m, V m It is obtained from the result of the previous step.

[0046] A transient stability assessment system for a grid-following converter connected to a low-inertia power grid based on the Zubov method includes a memory and one or more processors. The memory stores executable code, and the one or more processors implement the above-mentioned assessment method when executing the executable code.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] The Zubov method employed in this paper is based on the Lyapunov method and provides an effective approach for constructing its energy function. Unlike traditional methods that construct energy functions from the perspective of energy conservation, the Zubov method can determine the energy function in polynomial form through recursive solution, avoiding the difficulties associated with negative damping and low-inertia system dynamics. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 A flow chart of a method for evaluating transient stability of a grid-following converter connected to a low-inertia power grid based on the Zubov method;

[0050] Figure 2 A structure diagram of a grid-following converter-low-inertia system in an embodiment of the present application;

[0051] Figure 3 A variable phase diagram in an embodiment of the present application;

[0052] Figure 4 A typical phase-locked loop structure diagram in an embodiment of the present application;

[0053] Figure 5 Attractive domain evaluation results and key fault trajectories in an embodiment of the present application. DETAILED DESCRIPTION

[0054] The present application will be further described in detail below with reference to the accompanying drawings and embodiments, which are intended to facilitate the understanding of the present application and do not impose any limitation on the present application.

[0055] As shown in the drawings, Figure 1 a method for evaluating transient stability of a grid-following converter connected to a low-inertia power grid based on the Zubov method, comprising the following steps:

[0056] Step 1: Obtain the dynamic equation of the grid-following converter-low-inertia system, Taylor expand the dynamic equation of the grid-following converter-low-inertia system, and obtain the series form of the dynamic equation through Taylor expansion.

[0057] Step 2: Calculate the truncated energy function V (M) of the grid-following converter-low-inertia system based on the Zubov method.

[0058] Step 3: Obtain the set W M of which the derivative of the truncated energy function with respect to time is 0.

[0059] Step 4: Calculate the energy function values of each operating point in the set W M , and obtain the minimum value The set is taken as the attractive domain of system stability.

[0060] Step 5: Numerically integrate the fault system. In each integration step, calculate the initial state after the fault. Substitute (M) into V , and the last integration time is the estimated maximum fault clearance time.

[0061] In step 1, the structure diagram of the grid-following converter-low inertia system is as follows: Figure 2 As shown, it consists of a grid-following converter, a synchronous machine, and four impedances. The system dynamic equations and the assumptions of the converter connected to the low-inertia grid model are as follows:

[0062] 1) The low inertia system dynamics is represented by the swing equation of the synchronous machine. The phase angle is expressed by δ g express.

[0063] 2) The voltage support for the grid-following converter is provided by the impedance Z between the synchronous machine and the converter. c and Z g Reflection. Low voltage support corresponds to a larger impedance value. Therefore, the synchronous machine only needs to simulate the power angle dynamics of the system, and the terminal voltage U g is a constant.

[0064] 3) The current control loop of the grid-following converter is ignored due to its short time scale. The grid-following converter can be regarded as a fixed current amplitude I c The controlled current source with a phase angle δ c Determined by the phase-locked loop (PLL).

[0065] 4) The load is a constant impedance Z l Load. When a three-fault short circuit fault occurs, the fault resistance R f With Z l in parallel.

[0066] 5) In actual large power grids, frequency fluctuations during transient processes are generally small; the impact of frequency fluctuations on reactance is ignored, and the reactance in the system is considered to be constant.

[0067] All dynamics (including the voltage and current phase of the grid-following converter connected to the low-inertia system) are described using the DQ coordinate system. The three DQ coordinate systems d0-q0, dg-qg, and dc-qc are used to describe the variables of the grid-following converter-low-inertia system, such as Figure 3 As shown. The d0-q0 system has a reference frame with a constant angular velocity ω0. The dg-qg and dc-qc systems have varying angular velocities ω g and ω c , respectively describe the low inertia system and the grid-following converter dynamic variables. Their phase differences with d0-q0 are δ g and δ c The synchronous machine voltage phase coincides with the dg axis; the grid-following converter current phase is determined by the dc-qc axis. is the angle between the grid-following converter current and the DC axis. In the present invention, it is a fixed parameter.

[0068] In step 1, the system dynamic equation is as follows:

[0069] Based on the above assumptions, the low-inertia system is a controllable voltage source. The voltage amplitude is fixed, and the power angle dynamics are described by the synchronous machine swing equation, which can be expressed as:

[0070]

[0071] Where, J g and D g are the inertia and damping coefficient of the synchronous machine respectively. ma,g is the input mechanical power. Assuming that the output of the synchronous machine speed regulator is constant during the transient process, then P ma,g is a constant. el,g is the electromagnetic power output by the synchronous machine, which can be calculated by the power flow equation:

[0072]

[0073] in:

[0074]

[0075] Where U l for Figure 2 Medium load node voltage. I g is the current output of the synchronous machine. g is the impedance between the synchronous machine and the load node. Z′ l is the imaginary impedance. Before and after the fault, Z′ l =Z l , during the fault period, Z′ l =Z l / / R f To simplify the equation, Z eq1 and Z eq2 As two imaginary impedances, the expressions are given. θ1 and θ2 are Z eq1 and Z eq2 The dot symbol indicates that the variable is a vector, and the variable without a dot symbol indicates the magnitude.

[0076] Taking the dg axis as the reference phase, P el,g It can be calculated by the following formula:

[0077]

[0078] where the asterisk * denotes the conjugation of the variable.

[0079] The grid-following converter can be regarded as a controllable current source with a fixed current amplitude, and its power angle dynamics is determined by the phase-locked loop dynamics. Figure 4 This is a typical phase-locked loop structure that adjusts the current angle through the QC axis voltage at the point of common coupling (PCC). The QC axis voltage can be calculated as follows:

[0080]

[0081] where:

[0082]

[0083]

[0084] where U c is the PCC point voltage, U cq is the dc-axis component of U c . Z eq3 is another imaginary impedance and θ3is its phase angle.

[0085] The PLL-based grid-following converter power angle dynamics can be expressed as: cq

[0086]

[0087] where K i and K p are the integral and proportional coefficients of the PLL.

[0088] The system dynamics are obtained by combining eqs. (1), (4), and (8). To ensure the equilibrium point is fixed, the equation is simplified by taking the dg-axis as the reference phase. Since the reference phase has changed, the variable δ c is replaced by the relative angle between the dg-axis and the dc-axis, δ = δ c - δ g . The system dynamics equation can be expressed as an ordinary differential equation (ODE) model:

[0089]

[0090] where P' ma,g , P' el,g , D' g , P' ma,c , P' el,c , D' c , and are intermediate parameters, which can be expressed as:

[0091]

[0092] The Taylor expansion form of the system dynamics equation is as follows:

[0093] For the grid-following converter-low-inertia system, the dynamics equation F in eq. (9) contains non-monomial terms such as the trigonometric function term. Therefore, only F1and F3need to be Taylor expanded:

[0094] ​

[0095]

[0096] Let F' denote the Taylor expansion of F, in this paper, F' is truncated at M T = 30. Dividing the linearized part and the non-linearized part, F' can also be written as:

[0097]

[0098] where b ni is the coefficient of the linearized term x n in F i '. contains all the m n th order terms in F T '.

[0099] In step 2, the energy function is calculated as follows:

[0100] In Zubov's method, the energy function V is in the form of an infinite series, which is expressed as:

[0101] V = V2+ V3+... + V m +... (14)

[0102] where V m is a polynomial, in which all the terms are of order m. In practical calculation, V is truncated at a certain order. Let V (M) denote V truncated at order M.

[0103] In the model of the grid-following converter-low inertia system, there are three variables, V m , which can be expressed as:

[0104]

[0105] where a mji is the coefficient of a term in V m , in which the orders of δ, ω, ω g are m-i-j, j, i, respectively.

[0106] In Zubov's method, the energy function is obtained from the partial differential equation as follows:

[0107]

[0108] where x is the system variable, and N is the dimension. In the grid-following converter-low inertia system, (x1, x2, x3) = (δ, ω, ω g ), N = 3. φ is a positive definite function of the variable x. F n denotes the nth equation in F.

[0109] Substitute equation (13) into equation (16) and compare the coefficients of each term on both sides of equation (16). m The calculation formula is as follows:

[0110]

[0111]

[0112] Where R m (x) is a polynomial of degree m, and the coefficients of each term are obtained from V2, V3, ..., V m-1 and F' are calculated. Specifically, R m (x) consists of two parts: φV calculated from the right side of (16) m-2 and from the left side of (16) Therefore, V m The calculation of V is a recursive process. For each m, V m It is obtained from the result of the previous step.

[0113] In the embodiment of the present invention, corresponding to Figure 2 The system parameters are shown in Table 1 below. c 、 The corresponding active output of the shunt transformer is 300MW and the reactive output is 50MVar respectively.

[0114] Table 1 System parameters

[0115]

[0116] In order to verify the correctness of the method proposed in this invention, a transient analysis process is implemented. The numerical integration of the fault system in step 5 of the transient analysis is first based on the ODE model in (9). The Taylor expansion form of the dynamic equation F' is in M T =30, and the energy function V is truncated at M=16. (16) There are many items in V 16 There are 153 items in ; only the coefficients of V2 and V3 are given in Table 2 below. The coefficients are named according to formula (15).

[0117] Table 2 Energy function coefficients

[0118]

[0119]

[0120] Get V (16) and back, and The collection ofFigure 5 The set can be considered as a conservative estimate of the attraction region. To check the error of the method, the transient trajectory under the actual CCT is given. The trajectory is obtained from the ODE model (9). The occurrence of the fault and the ω frequency jump can be observed on the trajectory. If the fault is cleared, the post-fault initial state Figure 5 The transient trajectory under the actual CCT is given. The trajectory is obtained from the ODE model (9). The occurrence of the fault and the ω frequency jump can be observed on the trajectory. If the fault is cleared, the post-fault initial state The system is considered stable if the post-fault trajectory under the estimated CCT is within the predicted attraction region. It can be seen that the estimated escape point is close to the actual escape point. The estimated CCT is 0.2295 s, compared with the actual 0.2365 s, the error is 2.95%. This is acceptable in practical engineering. The post-fault trajectory under the estimated CCT is all within the predicted attraction region, the definition of the load attraction region.

[0121] The above-described embodiments of the present application have been described in detail, it should be understood that the above-described only for the specific embodiments of the present application, and is not intended to limit the present application, any modifications, supplements and equivalent replacements made within the scope of the principles of the present application, should be included within the scope of the present application.

Claims

1. A method for evaluating the transient stability of a low-inertia power grid with a grid-following converter access based on the Zobov method, for predicting the maximum clearing time of a power grid short-circuit fault; characterized in that, Comprising the following steps: (1) access the low-inertia power grid system model to the grid-following converter, build the dynamic equation of the grid-following converter-low-inertia system, and obtain the series form of the dynamic equation through Taylor expansion; (2) Calculate the truncated energy function V of the system based on the Zubov method (M) ; (3) obtain the set W for which the time derivative of the truncated energy function is zero M ; (4) Calculate the energy function value of each operating point in set W M and obtain the minimum value Set W is the attraction domain of system stability; (5) To predict the maximum cut-off time, the system differential equations after the short-circuit fault of the power grid are first numerically integrated; in each integration step, the initial state after the fault is calculated wherein and respectively represent the relative phase angle of the momentary converter and the power grid after the fault is cleared, the angular velocity corresponding to the angular velocity output by the converter phase-locked loop and the grid side frequency; Substitute into the set V (M) , The last integration time is the estimated maximum trip time for the fault.

2. The method for low-inertia grid transient stability assessment of a Z-source inverter based grid-tie converter according to claim 1, wherein, In step (1), after the low-inertia power grid system model is accessed to the grid-following converter, it contains one grid-following converter, one synchronous machine and four impedances, and the model satisfies the following five assumptions: ① The low-inertia system dynamics is expressed by the swing equation of a synchronous machine, whose phase angle is denoted by δ g ; (ii) The voltage support to the grid-following converter is reflected by the impedance Z between the synchronous machine and the grid-following converter c and Z g The low voltage support corresponds to a large impedance value, so the synchronous machine only needs to simulate the power angle dynamics of the system, considering the terminal voltage U g as a constant; ③The current control loop of grid-connected converter is ignored due to its short time scale. The grid-connected converter is regarded as a controlled current source with fixed current amplitude I c and phase angle δ c determined by phase-locked loop (PLL). (4) Load is constant impedance Z l Load, when a three-fault short-circuit fault occurs, fault resistance R f Z l Parallel; ⑤ ignore the influence of frequency fluctuation on the reactance, that is, consider the reactance in the system as a constant.

3. The method for low-inertia grid transient stability assessment of a Z-source inverter based grid-tie converter according to claim 2, wherein, The voltage and current phase of the grid-following converter accessed to the low-inertia system are described through the DQ coordinate system, and the variables of the grid-following converter-low-inertia system are described through three DQ coordinate systems d0-q0, dg-qg and dc-qc; The d0-q0 coordinate system has a constant angular velocity ω0; the dg-qg and dc-qc coordinate systems have varying angular velocities ω g and ω c , respectively, which describe the low-inertia system and the grid-following converter dynamic variables, respectively, which are separated from the d0-q0 phase difference by δ g and δ c , respectively; the synchronous machine voltage phase coincides with the dg axis; the grid-following converter current phase is determined by the dc-qc axis.

4. The method for low-inertia grid transient stability assessment of a Z-source inverter based grid-tie converter according to claim 3, wherein, The dynamic equation of the grid-following converter-low-inertia system is expressed as a constant differential equation model, and the formula is as follows: where δ represents the relative angle between the dg axis and the dc axis, δ = δ c - δ g ; P' ma,g , P' el,g , D' g , P' ma,c , P' el,c , D' c , and are intermediate parameters, respectively expressed as: where Z eq1 , Z eq2 and Z eq3 represent virtual impedance, θ1, θ2 and θ3 represent phase angles of Z eq1 , Z eq2 and Z eq3 ; J g and D g represent damping and inertia of the equivalent synchronous machine of the system; P ma,g and U g represent mechanical power and terminal voltage setting value of the equivalent synchronous machine of the system; K i and K p represent integral coefficient and proportional coefficient of the phase-locked loop of the converter; I c and represent current amplitude and phase angle setting value of the converter.

5. The method for low-inertia grid transient stability assessment of a Z-source inverter based grid-tie converter according to claim 4, wherein, In step (1), the series form of the dynamic equation is obtained through Taylor expansion, which is as follows: For the grid-following converter-low-inertia system, only F1 and F3 of the dynamic equation F need to be Taylor expanded: Let F' represent the Taylor expansion of F, dividing the linearized part and the non-linearized part, F n ' is written as: where b ni is F n ′ in the linearization term x i , the coefficient of contains all m n times of F T ′ in the linearization term x, and N is the dimension; in the grid-connected VSC-low-inertia system, (x1, x2, x3) = (δ, ω, ω g ), N = 3.

6. The method for low-inertia grid transient stability assessment of a Z-source inverter based grid-tie converter according to claim 5, wherein, The Taylor expansion of F, F', is truncated at M T = 30.

7. The method for low-inertia grid transient stability assessment of a Z-source inverter based grid-tie converter according to claim 5, wherein, The specific process of step (2) is as follows: In the Zubov method, the energy function V is in the form of an infinite series, which is expressed as: V = V2+ V3+... + V m +... where V m is a polynomial of degree m, and in practical calculations V is truncated at some degree, say M, denoted by V (M) . In the grid-connected converter-low inertia system model there are three variables, V m is expressed as: wherein a mij is a coefficient of one of the terms δ, ω, ω m of orders m-i-j, j, i, respectively, in the term g of one of the terms δ, ω, ω In the Zubov method, the energy function is obtained from the following partial differential equation: where φ is a positive definite function of the variable x, F n denotes the nth equation in F; Substitute the formula into Compare the coefficients of the terms on each side of the formula; V2, V3, …, V The formula for calculating V is as follows: m ​ where R m (x) is a polynomial of degree m, and the coefficients of the terms are given by the formula and V2, V3,..., V m-1 and F' are calculated; thus, V m The calculation of V m is a recursive process, for each m, V m is obtained from the result of the previous step.

8. A transient stability assessment system for grid-connected converters connected to low-inertia power grids based on the Zubov method, characterized in that: Comprising a memory and one or more processors, the memory storing executable code, the one or more processors executing the executable code to implement the evaluation method of any one of claims 1-7.

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