Method and system for evaluating low voltage ride-through transient synchronization stability of grid-following converter

Abstract

The present invention relates to the technical field of power system stability analysis, and in particular to a method and system for evaluating the transient synchronization stability of a grid-connected converter during low voltage ride-through. Obtain system parameters during low voltage ride-through, and establish a differential equation describing the transient process of the system; calculate the positive real characteristic root and the corresponding left eigenvector of the system at the unstable equilibrium point, and obtain a linear expression of the boundary of the stable domain of the system near the unstable equilibrium point according to the left eigenvector; calculate the distance from the stable equilibrium point of the system to the boundary of the stable domain, and evaluate the transient stability of the system according to the transient stability evaluation index value. The present invention can effectively compare the strength of the transient synchronization stability of the system under different parameters, provide reliable optimization suggestions for the design of the grid-connected system of the grid-connected converter, avoid the construction of complex energy functions and a large number of real-time simulation calculations, and calculate the transient stability index value of the system through the method steps of the present invention for transient stability evaluation, which has high accuracy and rapidity.

Description

Technical Field

[0001] The present invention relates to the technical field of power system stability analysis, and in particular to a method and system for evaluating low voltage ride-through transient synchronization stability of a grid-following converter. Background Art

[0002] In order to accelerate the construction of a modern energy system and achieve the dual carbon goals, new energy power generation based on non-fossil energy has been vigorously developed. New energy power generation often uses converter equipment to achieve grid-connected synchronization. Compared with the traditional synchronous machine that achieves synchronous grid connection through the rotor, the way of achieving grid-connected synchronization through control brings complex problems and challenges to the transient stability of the power system. At present, most research works focus on the transient stability analysis of grid-connected converter systems, and few involve transient stability assessment of grid-connected converter systems. In fact, how to effectively and accurately assess the transient stability of converter grid-connected systems has important guiding value for engineering practice. In traditional power systems, the transient stability index based on the transient energy function is used to assess the severity of transient disturbances in the system, and quantify the transient stability margin of the power system based on this. In order to further improve the accuracy, some studies have proposed combining the transient energy function with time domain simulation to construct a hybrid transient stability index. In recent years, some researchers have applied the transient stability index to new energy power systems to assess the transient stability margin of grid-connected converter systems. Summary of the invention

[0003] In view of the above problems existing in the prior art, the present invention is proposed.

[0004] Therefore, the present invention provides a method for evaluating the transient synchronization stability of a grid-following converter under low voltage ride-through, which can solve the problem that traditional methods are difficult to construct an energy function of a converter system suitable for high-order, strongly coupled and nonlinear characteristics. The present invention does not need to construct an energy function, and proposes a transient stability evaluation method based on the stable manifold construction of the system's unstable equilibrium point.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions, a method for evaluating the transient synchronization stability of a grid-following converter during low voltage ride-through, comprising: obtaining system parameters during low voltage ride-through, and establishing a differential equation describing the transient process of the system; calculating the positive real characteristic roots and the corresponding left eigenvectors of the system at an unstable equilibrium point, and obtaining a linear estimation expression of the stable domain boundary of the system near the unstable equilibrium point according to the left eigenvector; calculating the distance from the stable equilibrium point of the system to the stable domain boundary, and evaluating the transient stability of the system according to the transient stability evaluation index value.

[0006] As a preferred solution of the method for evaluating transient synchronization stability of low voltage ride-through of a grid-following converter described in the present invention, wherein: the system parameters during low voltage ride-through include grid parameters, device given current values ​​and phase-locked loop PLL control parameters;

[0007] The grid parameters include line reactance, line terminal voltage vector, grid-side voltage value, phase-locked loop output phase, phase-locked loop output frequency, grid synchronization phase, grid synchronization frequency and phase-locked loop integrator output variable;

[0008] The given current value of the device includes an active current reference value;

[0009] The phase-locked loop (PLL) control parameters include an integral coefficient and a proportional coefficient.

[0010] As a preferred solution of the method for evaluating the transient synchronization stability of the low voltage ride-through of the grid-following converter described in the present invention, wherein: the establishment of the differential equation describing the transient process of the system includes real-time detection of the grid-side voltage value U of the new energy power system in the fault detection link g , if U g ≥γ, the system has not entered the low voltage ride-through process. g <γ, the system enters the low voltage ride-through process, and the system active current reference value i during the low voltage ride-through period is detected. dref , get the integral coefficient k of the phase-locked loop ipll , proportionality coefficient k ppll and line reactance L g , the transient voltage equation during low voltage ride-through is obtained from the network relationship, and the formula is expressed as:

[0011]

[0012]

[0013] Where γ is the low voltage threshold, u tq is the terminal voltage u t The q-axis component in the phase-locked loop dq coordinate system, is the phase difference between the phase-locked loop coordinate system and the grid synchronization coordinate system, θ pll ,ω pll are the phase-locked loop output phase and frequency, θ 0 ,ω 0 are the grid synchronization phase and frequency respectively, t is time, and sin is the sine function;

[0014] The system enters the low voltage ride-through mode, the outer loop power control is locked, and considering that the inner loop current control speed is greater than the phase-locked synchronous control, the second-order differential dynamic equation group describing the system dynamics is expressed as follows:

[0015]

[0016] Among them, x pll is the phase-locked loop integrator output variable;

[0017] Substituting the differential dynamic equations into the transient voltage equation during low voltage ride-through, we get the differential equation of the system, which is expressed as:

[0018]

[0019] Where d is the differential operator.

[0020] As a preferred solution of the method for evaluating the transient synchronization stability of low voltage ride-through of a grid-type converter according to the present invention, wherein: the positive real characteristic root of the computing system at the unstable equilibrium point includes the state space matrix of the stable equilibrium point, the unstable equilibrium point and the unstable equilibrium point of the computing system;

[0021] Based on the differential equation of the system, the left side of the differential equation is set to zero to obtain the stable equilibrium point of the system. The formula is expressed as:

[0022]

[0023] in, is the phase angle at the stable equilibrium point, is the state variable of the phase-locked loop at the stable equilibrium point, arcsin is the inverse sine function;

[0024] The unstable equilibrium point formula is expressed as:

[0025]

[0026] in, is the phase angle at the unstable equilibrium point, is the state variable of the phase-locked loop at the unstable equilibrium point.

[0027] As a preferred solution of the method for evaluating the transient synchronization stability of low voltage ride-through of a grid-type converter according to the present invention, the state space matrix formula of the unstable equilibrium point is expressed as follows:

[0028]

[0029] Among them, J u is the state space matrix of the unstable equilibrium point, K 1 , K 2 are different matrix coefficients, and cos is the cosine function.

[0030] As a preferred solution of the method for evaluating the transient synchronization stability of the low voltage ride-through of the grid-type converter described in the present invention, wherein: the positive real characteristic root includes the positive real eigenvalue of the state space matrix of the unstable equilibrium point, which is expressed as follows:

[0031]

[0032] Among them, λ u is the positive real eigenvalue of the state space matrix of the unstable equilibrium point;

[0033] The left eigenvector corresponding to the eigenvalue is denoted by ν T =[ν 1 ,ν 2 ] indicates that the left eigenvector calculation formula is obtained.

[0034]

[0035] Among them, ν 1 , ν 2 are the components of the left eigenvector on different coordinate axes respectively;

[0036] According to the relevant theory of nonlinear dynamics, the linear expression of the boundary of the stable domain of the system near the unstable equilibrium point is:

[0037]

[0038] in, For The boundary of the system stability region on the phase plane, k is the slope of the stability region boundary.

[0039] As a preferred solution of the method for evaluating the transient synchronization stability of the low voltage ride-through of the grid-type converter described in the present invention, wherein: the transient stability of the evaluation system includes calculating the distance from the stable equilibrium point of the system to the boundary of the stable domain as an evaluation index, and the formula is expressed as follows:

[0040]

[0041] Among them, I TS is the evaluation indicator;

[0042] I TS The value range is from 0 to positive infinity. The larger the index value, the farther the system's stable equilibrium point is from the boundary of the stable domain, and the stronger the transient stability of the system.

[0043] Another object of the present invention is to provide a system for evaluating the transient synchronous stability of grid-following converters during low voltage crossing, evaluate the synchronous stable operation capability of new energy power generation equipment during low voltage crossing, and provide optimization suggestions for system parameter design. The method of the present invention includes obtaining grid parameters, equipment given current values ​​and phase-locked loop control parameters, calculating the stable equilibrium point and unstable equilibrium point of the system; obtaining the boundary of the system stability domain through the left eigenvector, and quantifying the transient stability of the system through the evaluation index TSI. The method of the present invention can provide quantitative stability evaluation results, and help optimize the phase-locked loop control parameters and system parameters, thereby improving the transient synchronous stability of the system during low voltage crossing.

[0044] As a preferred solution of the system for evaluating the transient synchronous stability of low voltage ride-through of a grid-type converter described in the present invention, it includes: a dynamic equation establishment module, an eigenvalue and stability domain calculation module and a transient stability evaluation module;

[0045] The dynamic equation building module obtains system parameters during low voltage ride-through and builds differential equations describing the transient process of the system;

[0046] The eigenvalue and stability domain calculation module calculates the positive real eigenroot and the corresponding left eigenvector of the system at the unstable equilibrium point, and calculates the stability domain boundary of the system according to the eigenvalue and the eigenvector;

[0047] The transient stability evaluation module calculates the distance from the stable equilibrium point of the system to the boundary of the stable region and quantifies the transient stability of the system using an evaluation index.

[0048] A computer device comprises a memory and a processor, wherein the memory stores a computer program, and is characterized in that when the processor executes the computer program, the steps of any one of the methods for evaluating the transient synchronization stability of a grid-following converter under low voltage ride-through are implemented.

[0049] A computer-readable storage medium having a computer program stored thereon, characterized in that when the computer program is executed by a processor, the steps of any one of the methods for evaluating the transient synchronization stability of a low voltage ride-through of a grid-following converter are implemented.

[0050] Beneficial effects of the present invention: The evaluation method of transient synchronization stability during low voltage ride-through of the grid-connected system of the grid-connected converter proposed in the present invention takes the stable manifold of the unstable equilibrium point of the system as the theoretical basis to constitute the stable domain boundary of the system, and uses the relevant knowledge of nonlinear dynamics to first obtain the linear estimation expression of the stable manifold of the unstable equilibrium point of the system, and use this as the estimated system stable domain boundary, and further calculate the distance from the stable equilibrium point of the system to the estimated stable domain boundary as the transient stability evaluation index of the present invention. The larger the index value, the farther the stable equilibrium point of the system is from the stable domain boundary, corresponding to the larger the transient stability domain of the system, the stronger the transient stability of the system. The present invention can effectively compare the strength of the transient synchronization stability of the system under different parameters, thereby providing reliable optimization suggestions for the design of the grid-connected system of the grid-connected converter. At the same time, compared with the methods of constructing energy function to evaluate transient stability and time domain simulation to evaluate transient stability, it avoids the construction of complex energy functions and a large number of real-time simulation calculations. After obtaining the parameters and operating conditions of the system, the transient stability index value of the system can be calculated by the method steps of the present invention to perform transient stability evaluation, which has high accuracy and rapidity. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0052] Figure 1 A topology diagram of a grid-connected converter system during a low voltage ride-through period of a method for evaluating the transient synchronization stability of a grid-connected converter during low voltage ride-through provided by an embodiment of the present invention.

[0053] Figure 2 A schematic diagram of transient stability evaluation indicators and estimated stability region boundaries on a phase plane for a method for evaluating low voltage ride-through transient synchronization stability of a grid-connected converter provided by an embodiment of the present invention.

[0054] Figure 3 A corresponding relationship diagram between a transient stability evaluation index of a method for evaluating the transient synchronization stability of a grid-type converter low voltage ride-through provided by an embodiment of the present invention and a transient stability domain area of ​​the system.

[0055] Figure 4 A trend diagram of changes in transient stability evaluation indicators of a method for evaluating low voltage ride-through transient synchronization stability of a grid-type converter provided by an embodiment of the present invention as a function of system control parameters and line parameters. DETAILED DESCRIPTION

[0056] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in the art without creative work should fall within the scope of protection of the present invention.

[0057] Example 1, reference Figure 1 , which is the first embodiment of the present invention, provides a method for evaluating the transient synchronization stability of a low voltage ride-through of a grid-following converter, comprising:

[0058] S1: Obtain system parameters during low voltage ride-through and establish differential equations to describe the system transient process.

[0059] It should be noted that if Figure 1 As shown in FIG. 1 , an application scenario of the present invention is given, namely, a topological diagram of a grid-type converter system during low voltage ride-through. f and L g are the filter reactance and line reactance of the system respectively, u t is the terminal voltage vector of the line, and I is the line current vector.

[0060] During low voltage ride-through, the system outer loop is locked, and the speed of the current inner loop is much faster than the phase-locked link. Figure 1 The VSC in the figure is regarded as a current source with a constant amplitude, i dref is a given value. The control block diagram of the phase-locked loop (PLL) is given in the dotted box, where θ pll ,ω pll are the phase and frequency of the phase-locked loop output, x pll is the phase-locked loop integrator output variable, k ipll and k ppll are the integral system and proportional coefficient of the phase-locked link, u tq That is, the terminal voltage u t The q-axis component in the phase-locked loop dq coordinate system represents the input variable of the phase-locked loop link.

[0061] Assume that the grid synchronous rotating coordinate system is xy, and its phase and frequency are θ 0 and ω 0 . Real-time detection of the grid-side voltage value U of the new energy power system in the fault detection link g , if U g ≥γ, the system has not entered the low voltage ride-through process. g <γ, the system enters the low voltage ride-through process, and the system active current reference value i during the low voltage ride-through period is detected. dref, get the integral coefficient k of the phase-locked loop ipll , proportionality coefficient k ppll and line reactance L g , the transient voltage equation during low voltage ride-through is obtained from the network relationship, and the formula is expressed as:

[0062]

[0063]

[0064] Among them, γ is the low voltage threshold, u tq is the terminal voltage u t The q-axis component in the phase-locked loop dq coordinate system, is the phase difference between the phase-locked loop coordinate system and the grid synchronization coordinate system, θ pll ,ω pll are the phase-locked loop output phase and frequency, θ 0 ,ω 0 are the grid synchronization phase and frequency respectively, that is, ω 0 =100π, t is time, sin is sine function;

[0065] When a transient disturbance occurs, the system enters the low voltage ride-through mode. At this time, the outer loop power control is locked. At the same time, because the inner loop current control speed is much faster than the phase-locked synchronous control, in this scenario, the equations describing the system dynamics can be described by the following second-order differential dynamic equations:

[0066]

[0067] Among them, x pll is the phase-locked loop integrator output variable;

[0068] Substituting the differential dynamic equations into the transient voltage equation during low voltage ride-through, we get the differential equation of the system, which is expressed as:

[0069]

[0070] Where d is the differential operator.

[0071] S2: Calculate the positive real eigenvalues ​​and the corresponding left eigenvalues ​​of the system at the unstable equilibrium point. According to the left eigenvalues, obtain the linear expression of the boundary of the stable region of the system near the unstable equilibrium point.

[0072] It should be noted that the positive real characteristic roots of the computing system at the unstable equilibrium point include the stable equilibrium point, the unstable equilibrium point and the state space matrix of the unstable equilibrium point of the computing system;

[0073] Based on the differential equation of the system, the left side of the differential equation is set to zero to obtain the stable equilibrium point of the system. The formula is expressed as:

[0074]

[0075] in, is the phase angle at the stable equilibrium point, is the state variable of the phase-locked loop at the stable equilibrium point, arcsin is the inverse sine function;

[0076] The unstable equilibrium point formula is expressed as:

[0077]

[0078] in, is the phase angle at the unstable equilibrium point, is the state variable of the phase-locked loop at the unstable equilibrium point.

[0079] The state space matrix formula of the unstable equilibrium point is expressed as:

[0080]

[0081] Among them, J u is the state space matrix of the unstable equilibrium point, K 1 , K 2 are different matrix coefficients, and cos is the cosine function.

[0082] The positive real eigenvalues ​​include the positive real eigenvalues ​​of the state space matrix of the unstable equilibrium point, which can be expressed as:

[0083]

[0084] Among them, λ u is the positive real eigenvalue of the state space matrix of the unstable equilibrium point;

[0085] The left eigenvector corresponding to the eigenvalue is denoted by ν T =[ν 1 ,ν 2 ] indicates that the left eigenvector calculation formula is obtained.

[0086]

[0087] Among them, ν 1 , ν 2 are the components of the left eigenvector on different coordinate axes respectively;

[0088] According to the relevant theory of nonlinear dynamics, the linear expression of the boundary of the stable domain of the system near the unstable equilibrium point is:

[0089]

[0090] in, For The boundary of the system stability region on the phase plane, k is the slope of the stability region boundary.

[0091] S3: Calculate the distance from the system's stable equilibrium point to the boundary of the stable region, and evaluate the system's transient stability based on the transient stability evaluation index value.

[0092] It should be noted that the transient stability of the evaluation system includes calculating the distance from the stable equilibrium point of the system to the boundary of the stable domain as an evaluation index, which is expressed as follows:

[0093]

[0094] Among them, I TS is the evaluation indicator;

[0095] I TS The value range is from 0 to positive infinity. The larger the index value, the farther the system's stable equilibrium point is from the boundary of the stable domain, and the stronger the transient stability of the system.

[0096] The above is a schematic scheme of a method for evaluating the transient synchronization stability of a grid-type converter under low voltage crossing in this embodiment. It should be noted that the technical scheme of the system for evaluating the transient synchronization stability of a grid-type converter under low voltage crossing is the same as the technical scheme of the method for evaluating the transient synchronization stability of a grid-type converter under low voltage crossing in this embodiment. The details not described in detail in the technical scheme of the system for evaluating the transient synchronization stability of a grid-type converter under low voltage crossing in this embodiment can be referred to the description of the technical scheme of the method for evaluating the transient synchronization stability of a grid-type converter under low voltage crossing in this embodiment.

[0097] In this embodiment, the system for evaluating the transient synchronous stability of low voltage ride-through of a grid-following converter includes: a dynamic equation establishment module, an eigenvalue and stability domain calculation module, and a transient stability evaluation module;

[0098] The dynamic equation building module obtains system parameters during low voltage ride-through and builds differential equations describing the transient process of the system;

[0099] The eigenvalue and stability domain calculation module calculates the positive real eigenroot and the corresponding left eigenvector of the system at the unstable equilibrium point, and calculates the stability domain boundary of the system according to the eigenvalue and the eigenvector;

[0100] The transient stability evaluation module calculates the distance from the system stable equilibrium point to the boundary of the stable region and uses the evaluation index I TS Quantify the transient stability of a system.

[0101] This embodiment further provides a computing device, which is suitable for evaluating the transient synchronization stability of a low voltage ride-through of a grid-following converter, and includes:

[0102] If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, 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. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the methods described in each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, etc., which can store program codes.

[0103] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute instructions), or in conjunction with such instruction execution systems, devices or apparatuses. For the purposes of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in conjunction with such instruction execution systems, devices or apparatuses.

[0104] More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be a paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering or, if necessary, processing in another suitable manner, and then stored in a computer memory.

[0105] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, a plurality of steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0106] Example 2, reference Figure 2-Figure 4 , which is the second embodiment of the present invention, provides a method for evaluating the transient synchronization stability of a grid-following converter during low voltage ride-through. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through experiments.

[0107] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0108] In the low voltage ride-through scenario of the grid-connected system of the grid-following converter considered by the present invention, the active and reactive current reference values ​​are directly given without the need for output through the outer loop control. Since the current inner loop control speed is very fast, the embodiment of the present invention adopts a second-order differential equation considering the dynamics of the phase-locked loop to describe the transient process of the system during the low voltage ride-through.

[0109] like Figure 2 As shown, given System Evaluation Index on Plane I TS and the estimated stability region boundary The black dashed line represents the boundary of the system's attraction domain obtained by numerical calculation, the black solid point represents the system's stable equilibrium point, the black hollow point represents the system's unstable equilibrium point, and the estimated stable domain boundary Represented by the black solid line passing through the unstable equilibrium point, the evaluation index I TS The geometric meaning of is the perpendicular line from the stable equilibrium point to the estimated boundary of the stable region ( Figure 2 The black dotted line in the figure describes the

[0110] like Figure 3As shown in the figure, the corresponding relationship between the transient stability domain and the evaluation index of the system when the line reactance is 0.4pu, 0.6pu and 0.8pu respectively is given. It can be seen from the figure that when the transient stability domain area is larger, the distance from the system stable equilibrium point to the estimated stability domain boundary is farther, and the evaluation index is larger. Further, Table 1 gives the transient stability domain area and transient stability index I of the system under different line reactances, different phase-locked loop integral coefficients and proportional coefficients. TS The results show that I TS There is a positive correlation between the value and the transient stability domain area. Therefore, it can be obtained that the system's I TS The larger it is, the larger the transient stability domain is, and thus the stronger the transient stability is.

[0111] Table 1. Calculation results

[0112]

[0113] like Figure 4 As shown, I TS With the phase-locked loop control parameters (k ipll ,k ppll ) and line parameters (L g ,i dref The results show that I TS Follow k ipll and k ppll increases with the increase of L g and i dref Therefore, according to the evaluation method and index proposed by the present invention, when designing parameters, the integral coefficient k of the phase-locked loop can be appropriately increased. ipll and the proportionality factor k ppll , thereby obtaining a larger transient stability domain to improve the system's ability to maintain transient synchronous stability during low voltage ride-through, especially under conditions of high active power output and extremely weak power grid.

[0114] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for evaluating the transient synchronization stability of a grid-following converter under low voltage ride-through, characterized in that: include, Obtain system parameters during low voltage ride-through and establish differential equations describing the system transient process; Calculate the positive real eigenvalues ​​and the corresponding left eigenvectors of the system at the unstable equilibrium point, and obtain the linear expression of the boundary of the stable region of the system near the unstable equilibrium point according to the left eigenvector; Calculate the distance from the system's stable equilibrium point to the boundary of the stable region, and evaluate the system's transient stability based on the transient stability evaluation index value; The positive real characteristic roots of the computing system at the unstable equilibrium point include the state space matrices of the stable equilibrium point, the unstable equilibrium point and the unstable equilibrium point of the computing system; Based on the differential equation of the system, the left side of the differential equation is set to zero to obtain the stable equilibrium point of the system. The formula is expressed as: in, is the phase angle at the stable equilibrium point, is the state variable of the phase-locked loop at the stable equilibrium point, arcsin is the inverse sine function, ω0 is the grid frequency, i dref is the system active current reference value, L g is the line reactance, U g Indicates the grid-side voltage value of the new energy power system; The unstable equilibrium point formula is expressed as: in, is the phase angle at the unstable equilibrium point, is the state variable of the phase-locked loop at the unstable equilibrium point; The state space matrix of the unstable equilibrium point is expressed as, Among them, J u is the state space matrix of the unstable equilibrium point, K1 and K2 are different matrix coefficients, cos is the cosine function, k ppll is the proportional coefficient of the phase-locked link, k ipll It is the integral system of the phase-locked link; The positive real eigenvalues ​​include the positive real eigenvalues ​​of the state space matrix of the unstable equilibrium point, which can be expressed as: Among them, λ u is the positive real eigenvalue of the state space matrix of the unstable equilibrium point; The left eigenvector corresponding to the eigenvalue is denoted by ν T =[ν1,ν2], we get the left eigenvector calculation formula, Among them, ν1 and ν2 are the components of the left eigenvector on different coordinate axes; According to the theory of nonlinear dynamics, the linear expression of the boundary of the stable region of the system near the unstable equilibrium point is: in, For The system stability region boundary on the phase plane, k is the slope of the stability region boundary, x pll is the phase-locked loop integrator output variable, is the phase difference between the phase-locked loop coordinate system and the grid synchronization coordinate system.

2. The method for evaluating the transient synchronization stability of a grid-connected converter under low voltage ride-through as claimed in claim 1, characterized in that: The system parameters during the low voltage ride-through period include power grid parameters, equipment given current values ​​and phase-locked loop (PLL) control parameters; The grid parameters include line reactance, line terminal voltage vector, grid-side voltage value, phase-locked loop output phase, phase-locked loop output frequency, grid synchronization phase, grid synchronization frequency and phase-locked loop integrator output variable; The given current value of the device includes an active current reference value; The phase-locked loop (PLL) control parameters include an integral coefficient and a proportional coefficient.

3. The method for evaluating the low voltage ride through transient synchronization stability of a grid-connected converter according to claim 2, characterized in that: The establishment of the differential equation describing the transient process of the system includes real-time detection of the grid-side voltage value U of the new energy power system in the fault detection link. g , if U g ≥γ, the system has not entered the low voltage ride-through process. g <γ, the system enters the low voltage ride-through process, and the system active current reference value i during the low voltage ride-through period is detected. dref , get the integral coefficient k of the phase-locked loop ipll , proportionality coefficient k ppll and line reactance L g , the transient voltage equation during low voltage ride-through is obtained from the network relationship, and the formula is expressed as, Where γ is the low voltage threshold, u tq is the terminal voltage u t The q-axis component in the phase-locked loop dq coordinate system, θ pll ,ω pll are the phase and frequency of the phase-locked loop output, θ0 is the grid synchronization phase, t is the time, and sin is the sine function; The system enters the low voltage ride-through mode, the outer loop power control is locked, and considering that the inner loop current control speed is greater than the phase-locked synchronous control, the second-order differential dynamic equation group describing the system dynamics is expressed as follows: Substituting the differential dynamic equations into the transient voltage equation during low voltage ride-through, we get the differential equation of the system, which is expressed as: Where d is the differential operator.

4. The method for evaluating the transient synchronization stability of a grid-connected converter under low voltage ride-through as claimed in claim 3, characterized in that: The transient stability of the evaluation system includes calculating the distance from the stable equilibrium point of the system to the boundary of the stable domain as an evaluation index, and the formula is expressed as follows: Among them, I TS is the evaluation indicator; I TS The value range is from 0 to positive infinity. The larger the index value, the farther the system's stable equilibrium point is from the boundary of the stable domain, and the stronger the transient stability of the system.

5. A system for evaluating the transient synchronization stability of low voltage ride-through of a grid-connected converter based on any one of claims 1 to 4, characterized in that: It includes dynamic equation building module, eigenvalue and stability domain calculation module and transient stability assessment module; The dynamic equation building module obtains system parameters during low voltage ride-through and builds differential equations describing the transient process of the system; The eigenvalue and stability domain calculation module calculates the positive real eigenroot and the corresponding left eigenvector of the system at the unstable equilibrium point, and calculates the stability domain boundary of the system according to the eigenvalue and the eigenvector; The transient stability evaluation module calculates the distance from the stable equilibrium point of the system to the boundary of the stable region and quantifies the transient stability of the system using an evaluation index.

6. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: When the processor executes the computer program, the steps of the method according to any one of claims 1 to 4 are implemented.

7. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method according to any one of claims 1 to 4 are implemented.

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