Method and system for evaluating transient stability of network-following converter connected to low-inertia power grid based on Zuelbov method

Abstract

The invention discloses a method and a system for evaluating transient stability of a following grid converter connected to a low-inertia power grid based on a Zuobov method, and the method comprises the steps: (1) constructing a following grid converter-low-inertia system dynamic equation, and obtaining a series form of the dynamic equation through Taylor expansion; (2) calculating a truncation energy function of the system based on a zubov method; (3) obtaining a set WM of which the time derivative of the truncation energy function is 0; (4) calculating an energy function value of each operation point in the set WM, obtaining a minimum value # imgabs0 #, and taking the set # imgabs1 # as a system stability domain; (5) carrying out numerical integration on the fault system; and in each integration step length, the initial state after the fault is calculated, the initial state after the fault is substituted into the set V (M), and the last integration time before # imgabs2 is the estimated maximum fault clearing time. By utilizing the method, the problem of difficult solving caused by negative damping and low-inertia system dynamics can be avoided.

Description

Technical Field

[0001] The present invention belongs to the field of electrical engineering, and in particular relates to a transient stability assessment method and system for a grid-following converter connected to a low-inertia grid based on a Zubov method. Background Art

[0002] As the penetration rate of renewable energy in the power system continues to increase, the proportion of traditional synchronous generators will drop significantly, and converter-based power sources will gradually become dominant. The grid-connected converter is the main interface for renewable energy grid connection. Its synchronization mechanism based on phase-locked loop (PLL) is significantly different from that of the synchronous machine, which will cause synchronization instability under large disturbances in the power grid. Therefore, the transient stability analysis of the grid-connected converter has become an important research topic. The synchronous machine is the main stable support equipment of the traditional power grid. The reduction in its proportion will weaken the stability of the power system, which also has a significant impact on the transient stability of the grid-connected converter.

[0003] For example, Chinese patent document with publication number CN118137550A discloses a transient stability modeling and optimization method for a grid-following converter taking into account the dynamics of DC capacitors; Chinese patent document with publication number CN118572771A discloses a transient stability analysis and enhancement method for a grid-following converter.

[0004] The transient analysis method of power system based on traditional direct method is closely related to the transient characteristics of synchronous machine and other equipment. The equal area method and Lyapunov method are the most widely used methods. Studies have shown that the dynamic equation of grid-connected converter is similar to the swing equation of synchronous machine, but negative damping problem will appear within a certain phase angle range, which brings difficulties to the application of traditional methods. Traditional transient analysis methods are mainly based on the perspective of energy conservation, and the positive damping characteristics of synchronous machine make the damping term negligible. However, the negative damping that may appear in grid-connected converter may make the traditional direct method inapplicable. There are two ways to solve the negative damping problem: 1) limit the attraction domain to the positive damping area, which will lead to a large conservative error; 2) calculate or analyze the work done by the damping term, which is feasible in the case of continuous faults, but when considering fault clearance, the fault trajectory is more complicated and the damping work calculation is difficult. On the other hand, as the proportion of synchronous machines decreases, the system will present 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 the application of traditional methods (such as the equal area method) more difficult. Summary of the invention

[0005] The present invention provides a transient stability assessment method and system for a grid-following converter connected to a low-inertia grid based on the Zubov method, which can avoid the problem of difficult solutions caused by negative damping and low-inertia system dynamics.

[0006] A transient stability assessment method for a grid-following converter connected to a low-inertia grid based on the Zubov method is provided, the purpose of which is to predict how long it will take to clear the fault for a specific grid short-circuit fault, so as to ensure that the grid-following converter does not lose synchronization stability, that is, to predict the maximum clearing time; the method comprises the following steps:

[0007] (1) Connect the grid-following converter to the low-inertia grid system model, construct the dynamic equation of the grid-following converter-low-inertia system, and obtain the series form of the dynamic equation through Taylor expansion;

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

[0009] (3) Obtain the set W whose time derivative of the truncated energy function is 0 M ;

[0010] (4) Calculate the set W M The energy function value of each operating point in the equation is calculated, and the minimum value is obtained. Will gather As a stable attraction domain for the system;

[0011] (5) To predict the maximum removal time, the system differential equation after the power grid short-circuit fault is first numerically integrated; in each integration step, the initial state after the fault is calculated. in They represent the relative power angle between the converter and the grid at the moment when the fault is cleared, the angular velocity of the converter phase-locked loop output, and the angular velocity corresponding to the grid-side frequency respectively;

[0012] Will Substitute into set V (M) , The last integration time before the fault is the estimated maximum fault clearing time.

[0013] In step (1), after the low inertia grid system model is connected to the grid-following converter, it includes one grid-following 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 its phase angle is δ g express;

[0015] ② The voltage support for the grid-following converter is provided by the impedance Z between the synchronous machine and the grid-following 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, assuming that the terminal voltage U g is a constant;

[0016] ③ The current control loop of the grid-following converter is ignored due to its short time scale, and the grid-following converter is 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;

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

[0018] ⑤ Ignore the impact of frequency fluctuations on reactance, that is, the reactance in the system is considered to be a constant.

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

[0020] The d0-q0 coordinate system is a reference system with a constant angular velocity ω0; the dg-qg and dc-qc coordinate systems have a variable angular velocity ω g and ω c , respectively describe the dynamic variables of the low inertia system and the grid-following converter, and their phase differences with d0-q0 are δ g and δ c ; The voltage phase of the synchronous machine coincides with the dg axis; The current phase of the grid-following converter is determined by the dc-qc axis.

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

[0022]

[0023] Among them, 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:

[0024]

[0025] In the formula, Z eq1 , Z eq2 and Z eq3 represents imaginary impedance, θ1 represents, θ2 and θ3 represent 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 and the nonlinear part, and F′ is 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 = truncated at 30.

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

[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, in which all terms have an order of m; in actual calculations, 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] In the formula, a mij V m The coefficient of one of the terms in which δ, ω, and ω g The orders of are mij, j, i respectively;

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

[0041]

[0042] Where φ is a positive definite function of the variable x, and 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] In the formula, R m (x) is a polynomial of degree m, and the coefficients of each term 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 comprises a memory and one or more processors, wherein the memory stores executable codes, and when the one or more processors execute the executable codes, the assessment method is implemented.

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

[0048] The Zubov method used in the present invention is based on the Lyapunov method and provides an effective method for constructing its energy function. Unlike the traditional method of constructing the energy function from the perspective of energy conservation, Zubov can determine the energy function in polynomial form through a recursive solution method, avoiding the problem of difficult solution caused by negative damping and low inertia system dynamics. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is a flow chart of a transient stability assessment method for connecting a grid-following converter to a low-inertia grid based on the Zubov method of the present invention;

[0050] Figure 2 It is a structural diagram of a grid-following converter-low inertia system in an embodiment of the present invention;

[0051] Figure 3 is a variable phase diagram in an embodiment of the present invention;

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

[0053] Figure 5 It is the attraction domain evaluation result and key fault trajectory in the embodiment of the present invention. DETAILED DESCRIPTION

[0054] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention and do not have any limiting effect on the present invention.

[0055] like Figure 1 As shown, a transient stability assessment method for a grid-following converter connected to a low-inertia grid based on the Zubov method includes the following steps:

[0056] Step 1: Obtain the dynamic equation of the grid-following converter-low inertia system, perform Taylor expansion on 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 of the grid-connected converter-low inertia system based on the Zubov method (M) .

[0058] Step 3: Get the set W of truncated energy functions with time derivative equal to 0 M .

[0059] Step 4: Calculate the set W M The energy function value of each operating point in the equation is calculated, and the minimum value is obtained. Will gather As the attraction domain of system stability.

[0060] Step 5: Numerically integrate the fault system. In each integration step, calculate the initial state after the fault Will Substitute V (M) , The last integration time before the fault is the estimated maximum fault clearing 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 1 grid-following converter, 1 synchronous machine, and 4 impedances. The dynamic equation of the system and the assumptions of the converter connected to the low-inertia grid model are as follows:

[0062] 1) The low inertia system dynamics are represented by the swing equation of the synchronous machine. The phase angle is 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 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, that is, the reactance in the system is considered to be a constant.

[0067] All dynamics (including the voltage and current phase of the grid-following converter connected to the low-inertia system) are described by 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 system with a constant angular velocity ω0. The dg-qg and dc-qc systems have variable angular velocities ω g and ω c , respectively describe the dynamic variables of the low inertia system and the grid-following converter. 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-connected 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] In the formula, 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 The voltage of the middle load node. 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, 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] The asterisk * indicates 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 are determined by the phase-locked loop dynamics. Figure 4 It is a typical phase-locked loop structure, which adjusts the current angle through the qc axis voltage at the point of common coupling (PCC). The qc axis voltage can be calculated by the following formula:

[0080]

[0081] in:

[0082]

[0083]

[0084] Where U c is the voltage at PCC point, U cq For U c The qc axis component of Z eq3 is another imaginary impedance and θ3 is its phase angle.

[0085] Through U cq , the power angle dynamics of the grid-following converter based on the phase-locked loop can be expressed as:

[0086]

[0087] Where K i and K p are the integral and proportional coefficients of the phase-locked loop.

[0088] Combining equations (1), (4) and (8) gives the system dynamics. To ensure that the equilibrium point is fixed, simplify the equation and take the dg axis as the reference phase. Since the reference phase has changed, the variable δ c Replaced by the relative angle δ between the dg axis and the dc axis = δ c -δ g The system dynamic equation can be expressed as an ordinary differential equation (ODE) model:

[0089]

[0090] Among them, 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 dynamic equation is as follows:

[0093] For the grid-following converter-low inertia system, the dynamic equation F in equation (9) contains non-monomial terms such as trigonometric terms. Therefore, only F1 and F3 need to be Taylor expanded:

[0094]

[0095]

[0096] Let F' denote the Taylor expansion form of F. In this paper, F' is T = 30. Dividing the linearized part from the nonlinear part, F' can also be written as:

[0097]

[0098] Where b ni F n The linearization term x in ′ i The coefficient of . Contains F n All m in ′ T Second item.

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

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

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

[0102] Where V m is a polynomial in which all terms have an order of m. In practical calculations, V is truncated at a certain order. Let V (M) represents V truncated at the Mth order.

[0103] There are three variables in the grid-following converter-low inertia system model: V m It can be expressed as:

[0104]

[0105] In the formula, a mji V m The coefficient of one of the terms in which δ, ω, and ω g The orders are mij, j, and i respectively.

[0106] In the Zubov method, the energy function is obtained by the following partial differential equation:

[0107]

[0108] In the formula, 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 represents 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] In the formula, 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 parameters of the system are shown in Table 1. c , The corresponding active output of the current 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 the present 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 of Figure 5 This set can be regarded as a conservative estimate of the attraction domain. In order to test the error of this method, Figure 5 The transient trajectories under the actual CCT and estimated CCT are given. The trajectories are obtained by the ODE model (9). The occurrence of faults and the frequency mutation of ω can be observed on the trajectories. If the initial state after the fault is cleared and the frequency mutation is If the system is within the predicted attraction domain, it is considered to be stable. It can be seen that the estimated overflow point is close to the actual overflow point. The estimated CCT is 0.2295s, which is 2.95% error compared with the actual 0.2365s. This is acceptable in practical engineering. The post-fault trajectories under the estimated CCT are all within the predicted attraction domain, the definition of the load attraction domain.

[0121] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A transient stability assessment method for a grid-following converter connected to a low-inertia grid based on the Zubov method, used to predict the maximum clearing time of a grid short-circuit fault; characterized in that: The following steps are involved: (1) Connect the grid-following converter to the low-inertia grid system model, construct 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 whose time derivative of the truncated energy function is 0 M ; (4) Calculate the set W M The energy function value of each operating point in the equation is calculated, and the minimum value is obtained. Will gather As a stable attraction domain for the system; (5) To predict the maximum removal time, the system differential equation after the power grid short-circuit fault is first numerically integrated; in each integration step, the initial state after the fault is calculated. in They represent the relative power angle between the converter and the grid at the moment when the fault is cleared, the angular velocity of the converter phase-locked loop output, and the angular velocity corresponding to the grid-side frequency respectively; Will Substitute into set V (M) , The last integration time before the fault is the estimated maximum fault clearing time.

2. The transient stability assessment method for connecting a grid-connected converter to a low-inertia grid based on the Zubov method according to claim 1 is characterized in that: In step (1), after the low inertia grid system model is connected to the grid-following converter, it includes one grid-following converter, one synchronous machine and four impedances, and the model satisfies the following five assumptions: ① The swing equation of the synchronous machine is used to represent the dynamics of the low inertia system, and its phase angle is δ g express; ② The voltage support for the grid-following converter is provided by the impedance Z between the synchronous machine and the grid-following 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, assuming that the terminal voltage U g is a constant; ③ The current control loop of the grid-following converter is ignored due to its short time scale, and the grid-following converter is 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; ④The load is constant impedance Z l Load, when a three-fault short circuit fault occurs, the fault resistance R f With Z l in parallel; ⑤ Ignore the impact of frequency fluctuations on reactance, that is, the reactance in the system is considered to be a constant.

3. The transient stability assessment method for connecting a grid-connected converter to a low-inertia grid based on the Zubov method according to claim 2 is characterized in that: The voltage and current phases of the grid-following converter connected to the low-inertia system are described by the DQ coordinate system. The variables of the grid-following converter-low-inertia system are described by the three DQ coordinate systems d0-q0, dg-qg and dc-qc. The d0-q0 coordinate system is a reference system with a constant angular velocity ω0; the dg-qg and dc-qc coordinate systems have a variable angular velocity ω g and ω c , respectively describe the dynamic variables of the low inertia system and the grid-following converter, and their phase differences with d0-q0 are δ g and δ c ; The voltage phase of the synchronous machine coincides with the dg axis; The current phase of the grid-following converter is determined by the dc-qc axis.

4. The transient stability assessment method for connecting a grid-connected converter to a low-inertia grid based on the Zubov method according to claim 3 is characterized in that: The dynamic equation of the grid-following converter-low inertia system is expressed as an ordinary differential equation model, and the formula is as follows: Among them, 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: In the formula, Z eq1 , Z eq2 and Z eq3 represents imaginary impedance, θ1 represents, θ2 and θ3 represent Z eq1 , Z eq2 and Z eq3 The 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.

5. The transient stability assessment method for connecting a grid-connected converter to a low-inertia grid based on the Zubov method according to claim 4 is characterized in that: In step (1), the series form of the dynamic equation is obtained by Taylor expansion, 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 form of F, divide the linearized part and the nonlinear part, and F′ is written as: 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.

6. The transient stability assessment method for connecting a grid-connected converter to a low-inertia grid based on the Zubov method according to claim 5 is characterized in that: The Taylor expansion form F′ of F in M T = 30 is truncated.

7. The transient stability assessment method for connecting a grid-connected converter to a low-inertia grid based on the Zubov method according to claim 5 is characterized in that: 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, expressed as: V=V2+V3+...+V m +... Where V m is a polynomial, in which all terms have an order of m; in actual calculations, V is truncated at a certain order, let V (M) represents V truncated at the Mth order; There are three variables in the grid-following converter-low inertia system model: V m Expressed as: In the formula, a mij V m The coefficient of one of the terms in which δ, ω, and ω g The orders of are mij, j, i respectively; In the Zubov method, the energy function is obtained by the following partial differential equation: Where φ is a positive definite function of the variable x, and F n represents the nth equation in F; 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: In the formula, R m (x) is a polynomial of degree m, and the coefficients of each term 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.

8. A transient stability assessment system for grid-connected converters connected to low-inertia power grids based on the Zubov method, characterized in that: The system comprises a memory and one or more processors, wherein the memory stores executable codes, and when the one or more processors execute the executable codes, the system is used to implement the evaluation method according to any one of claims 1 to 7.

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