Method for determining transient stability of interconnected power system based on nonlinear decoupling

Abstract

The application discloses a kind of based on nonlinear decoupling's follow network mixed system transient stability determination method, it is applicable to follow network type converter and construct network type converter hybrid parallel operation new energy power system.In the method, first, the state space model of mixed system under the condition of large disturbance is established, and the model is handled based on the main nonlinear characteristics of the system Nonlinear decoupling, high-order system is decomposed into several low-order subsystems;Then identify the low-order subsystem that plays a leading role in the transient stability of the system, and construct the corresponding attraction domain in the decoupling domain;Finally, the initial operating point of the system under the disturbance condition is mapped to the decoupling domain, and the transient stability determination is completed according to the positional relationship between the initial operating point and the attraction domain.The method can be used to analyze the transient stability state of follow network mixed system under different disturbance conditions.

Description

Technical Field

[0001] This invention relates to the field of power system operation analysis and new energy grid connection control technology, specifically to a transient stability determination method for a hybrid grid system based on nonlinear decoupling. Background Technology

[0002] With the continuous increase in the installed capacity of new energy power generation devices such as wind power and photovoltaics in the power system, the traditional power system structure dominated by synchronous generators is undergoing significant changes. New energy power generation devices are mostly connected to the grid through power electronic converters, whose dynamic characteristics differ fundamentally from those of synchronous generators. This leads to an overall decrease in the inertia and damping level of the power system, making it more prone to increased voltage and frequency fluctuations and complex dynamic response processes under large disturbances. Against this backdrop, the analysis and assessment of the transient stability of the power system under new energy grid integration conditions has become a crucial technical foundation for ensuring the safe and stable operation of the power system.

[0003] Existing grid-connected control methods for new energy sources mainly include two categories: grid-following control and grid-connecting control. Grid-following control relies on phase-locked loops (PLLs) to track the grid voltage phase, and its operating characteristics are easily affected by grid impedance and voltage fluctuations under weak grid conditions or severe disturbances. Grid-connecting control can actively support system voltage and frequency to a certain extent, but in practical applications, grid-following and grid-connecting converters are often operated in parallel in a hybrid manner, forming a hybrid system with significantly different control characteristics and complex dynamic coupling relationships. For transient stability analysis of such hybrid systems, existing methods are mostly based on system order reduction, linearization approximation, or energy function construction, which usually require significant simplification of the system model. This makes it difficult to accurately reflect the nonlinear coupling relationships between multiple control links and multiple state variables, resulting in limitations in stability assessment results under complex operating conditions.

[0004] Therefore, in power systems where grid-connected and grid-connected converters operate in parallel, how to effectively analyze and determine the transient stability of the hybrid system under large disturbance conditions, while considering the system's higher-order characteristics and nonlinear dynamic behavior, has become a major technical problem that urgently needs to be solved in the field of operation analysis of new energy grid-connected power systems. Summary of the Invention

[0005] To address the problems of existing technologies, embodiments of the present invention provide a method for determining the transient stability of a root-structured network hybrid system based on nonlinear decoupling. The technical solution is as follows: On the one hand, a method for determining the transient stability of a root-structured network hybrid system based on nonlinear decoupling is provided, including the following steps: 1) Obtain the dynamic status information of a hybrid system in which grid-connected converters and grid-connected converters operate in parallel under disturbance conditions; 2) Based on the dynamic state information, a state-space model describing the dynamic behavior of the hybrid system under large disturbances is constructed, and the state-space model is subjected to nonlinear decoupling processing to obtain several low-order subsystems. 3) Identify the subsystem that plays a dominant role in the transient stability of the hybrid system from the lower-order subsystems, and construct the attraction domain of the dominant subsystem in the decoupling domain; 4) Determine the initial operating point of the hybrid system under the disturbance condition, and map the initial operating point to the decoupling domain; 5) Determine the transient stability of the hybrid system under the disturbance condition based on the positional relationship between the mapped initial operating point and the corresponding attraction domain.

[0006] Further, in step 2), the hybrid system includes at least one grid-connected converter, at least one grid-connected converter, and network impedance connecting the grid-connected converter, the grid-connected converter, and the power grid. The state-space model is used to describe the dynamic response process of the hybrid system near the steady-state operating point before the disturbance, under large disturbance conditions such as grid voltage drop. The state variables in the state-space model include converter output current, node voltage, power angle, and internal state variables in the converter control loop.

[0007] Furthermore, the state-space model is established with the steady-state operating point before the system disturbance as the reference point. By performing Taylor expansion on the nonlinear dynamic equations of the system and truncating them to the quadratic terms, a large disturbance state-space model containing linear terms and quadratic nonlinear terms is constructed to preserve the main nonlinear dynamic characteristics of the hybrid system under large disturbance conditions.

[0008] Furthermore, the nonlinear decoupling process includes the following steps: A linear similarity transformation is performed on the large disturbance state-space model to convert the linear part of the system into a standard form with system eigenvalues ​​as elements. Based on the linear similarity transformation, a nonlinear coordinate transformation is introduced to reconstruct the quadratic nonlinear term, thereby weakening the nonlinear cross-coupling relationship between different state variables and mapping the system to a nonlinear decoupling domain.

[0009] Furthermore, within the nonlinear decoupling domain, the original high-order hybrid system is decomposed into multiple low-order subsystems, wherein the low-order subsystems include first-order quadratic subsystems and second-order quadratic subsystems. The first-order quadratic subsystem corresponds to an isolated state variable that has little influence on other state variables and is weakly affected by other state variables. The second-order quadratic subsystem corresponds to a pair of state variables that have significant nonlinear interactions.

[0010] Furthermore, when the system characteristic value corresponding to the lower-order subsystem is a real number, the nonlinear coupling strength between state variables is quantified by introducing a coupling factor. This coupling factor is determined based on the distribution of the quadratic nonlinear term in the nonlinear decoupling domain. Based on the magnitude of the coupling factor, the state variables are divided into state variable coupling pairs or isolated state variables.

[0011] Furthermore, for the first-order quadratic subsystem, its attraction domain is a one-dimensional stable interval formed along the direction of the corresponding state variable; For the second-order quadratic subsystem, stable and unstable points are searched outward from the origin in the nonlinear decoupling domain using the ray method, and the attraction domain boundary of the corresponding lower-order subsystem is determined based on the unstable points obtained from the search.

[0012] Further, in step 4), the initial operating point is obtained by calculating the difference between the steady-state operating point before the system disturbance and the steady-state operating point after the disturbance to obtain the initial state vector, and then the initial state vector is mapped to the nonlinear decoupling domain by sequentially passing the linear similarity transformation and the nonlinear coordinate transformation. In the nonlinear decoupling domain, the transient stability is determined according to the positional relationship between the mapped initial operating point and the attraction domain of the corresponding low-order subsystem.

[0013] On the other hand, a computer-readable storage medium is provided, on which a computer program is stored, which, when executed by a processor, is used to implement the steps of the transient stability determination method for the hybrid system based on nonlinear decoupling.

[0014] On the other hand, an electronic device is provided, including a processor, a memory, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the steps of the aforementioned method for determining the transient stability of a hybrid grid system based on nonlinear decoupling.

[0015] The beneficial effects of the technical solution provided by the embodiments of the present invention are as follows: This invention provides a transient stability determination method for a hybrid system based on nonlinear decoupling. By uniformly modeling the hybrid system in which grid-connected converters and grid-connected converters operate in parallel, and performing nonlinear decoupling processing on the system state under large disturbance conditions, the high-order complex system is transformed into several low-order subsystems. This provides a clear and complete technical path for the analysis and determination of the transient stability of hybrid systems.

[0016] Compared with existing stability analysis methods that rely on linearization or strong order reduction assumptions, this invention can effectively distinguish the coupling relationships between different state variables while maintaining the main nonlinear dynamic characteristics of the system, and identify state variables or state variable coupling pairs that play a dominant role in the transient stability of the system, so that the stability analysis results can be closer to the actual dynamic behavior of the hybrid system under large disturbance conditions.

[0017] Furthermore, by constructing an attraction domain for a low-order subsystem within the decoupling domain and completing transient stability determination based on the positional relationship between the initial operating point and the attraction domain, this invention provides a clear determination basis and consistent determination logic for the stability determination process. This makes it applicable to the transient stability analysis of hybrid systems under different grid disturbance conditions, providing reliable technical support for the operation analysis of new energy grid-connected systems. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used 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 those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a flowchart of a method for determining the transient stability of a hybrid system based on nonlinear decoupling, according to an embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram of the topology of a GFL / GFM hybrid system according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the attraction domain and initial operating point of the hybrid system under different voltage drops according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the phase trajectory of a hybrid system under different voltage drop depths according to an embodiment of the present invention; Figure 5 The waveforms of the hybrid system under different voltage drop depths according to an embodiment of the present invention are time-domain simulation waveforms. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0022] Example 1 In this embodiment, the overall execution flow of the transient stability determination method is as follows: Figure 1As shown. This process starts with the full-order dynamic model of the hybrid network system, constructs the system's large disturbance space state equation, and performs nonlinear decoupling on the high-order nonlinear system to transform the system into several low-order subsystems. Based on this, state variables or state variable coupling pairs that play a dominant role in the system's transient stability are extracted, and an attraction domain in the decoupling domain is constructed for the corresponding low-order subsystem.

[0023] like Figure 1 As shown, in the process of determining transient stability, the steady-state equilibrium point of the system before the disturbance is first determined. With the steady-state equilibrium point after the disturbance The initial variables of the system under large disturbance conditions are defined by the difference between the two. Subsequently, through linear similarity transformation and nonlinear coordinate transformation, the initial variables are mapped step by step into the decoupling domain to obtain the corresponding initial running points of the decoupling domain. .

[0024] In this embodiment, the transient stability determination uses the attraction domain of the low-order subsystem within the decoupling domain as the determination threshold: when the initial operating point When located within the attraction domain of the corresponding lower-order subsystem, the system is determined to remain transiently stable under the disturbance condition; when the initial operating point When the system is located outside the boundary of the attraction domain, transient instability is determined. The boundary of the attraction domain is uniquely determined by the dynamic characteristics of the corresponding low-order subsystem, constituting an objective technical threshold for determining transient stability.

[0025] The mathematical models, coordinate transformations, nonlinear decoupling processes, and attraction domain solution methods involved in the following embodiments are all based on the large disturbance dynamic characteristics of the hybrid network system. The relevant variable definitions, formula derivations, and analysis steps will be explained one by one in the following content with specific formulas.

[0026] The topology diagram of the hybrid system is as follows: Figure 2As shown in the figure, subscripts 1 and 2 represent GFL and GFM respectively, subscripts d and q represent electrical quantities in the dq coordinate system respectively, subscript ref represents reference quantity, U, I, and If represent node voltage, output current, and current before filtering respectively, Lf and Cf represent filter inductance and filter capacitor respectively, L and R represent line inductance and resistance respectively, Rv and Lv represent virtual resistance and virtual inductance respectively, Lg represents the inductance of the infinite power supply branch, Us represents the voltage of the infinite power supply, Vdc represents the DC bus voltage, θpll and θFM represent the phase of the phase-locked loop and the GFM control coordinate system respectively, ωn and ωFM represent the angular frequency in the reference coordinate system and the GFM control coordinate system respectively, P and Q represent the active and reactive power output of GFM respectively, Ef represents the virtual internal potential of GFM, kp and kq represent the active frequency droop coefficient and reactive voltage droop coefficient of GFM respectively, and J and D represent the virtual inertia and damping coefficient of GFM respectively.

[0027] Figure 2 The GFL control section employs per-unit control and constant DC voltage control, locking the grid voltage phase via a PLL to achieve phase synchronization between the converter and the grid. Since the fault scenario studied in this invention is the system transient stability under large disturbances, it is assumed that the GFL will enter a low voltage ride-through (LVRT) state after a fault occurs, exiting constant DC voltage control, i.e., from... Figure 2 The control mode 1 in the model is switched to mode 2, so the control strategy in control mode 2 is used when establishing the mathematical model.

[0028] GFM uses virtual synchronous generator (VSG) control. Its control structure consists of an active / reactive outer loop, an additional virtual impedance, and a voltage and current dual inner loop. All control parts use per-unit control.

[0029] The nonlinear decoupling of the above system mainly includes the following steps: I. Mathematical Modeling of Each Component of the System In this embodiment, the established mathematical model of the hybrid grid system is used to describe the dynamic evolution of the system after being subjected to a large disturbance near the steady-state operating point. The voltage, current, power angle, and internal variables of the controller involved in the system are all represented in a synchronous rotating coordinate system, with the steady-state operating point before the disturbance as the reference point.

[0030] In the following mathematical model, the symbol "Δ" represents the increment of the corresponding state variable relative to the steady-state operating point, used to characterize the shift in system state under large disturbance conditions. The modeling process does not involve changes to the system topology and only analyzes the dynamic response under large disturbance conditions such as grid voltage drops.

[0031] according to Figure 2 The topology of GFL is shown in equation (1). In the formula: βid1 and βiq1 represent the state variables introduced by the current loop integral element.

[0032] Equation (1) above describes the dynamic behavior of the grid converter under low voltage ride-through conditions. To fully characterize the dynamic coupling relationship between different types of converters in a hybrid system, a mathematical model of the grid converter is further established.

[0033] according to Figure 2 The topological structure of GFM and the mathematical model of GFM are shown in Equation (2). In the formula: βud2, βuq2, βid2, and βiq2 represent the state variables introduced by the integral links of the voltage loop and the current loop, respectively.

[0034] In addition to establishing models for both grid-connected and grid-connected converters, it is also necessary to model the network impedance between the two and the infinite power grid to reflect the dynamic changes of electrical quantities in the network branches.

[0035] according to Figure 2 The topology of the network impedance is shown in equation (3). in II. Nonlinear Decoupling Since this invention addresses the transient stability problem of hybrid grid systems under large disturbance conditions such as grid voltage dips, the dynamic response process of the system exhibits significant nonlinear characteristics. To control model complexity while ensuring analytical accuracy, this embodiment retains the system's nonlinear terms during state-space modeling and truncates the nonlinear equations to quadratic terms after Taylor expansion, thereby constructing a large-disturbance state-space model that includes the main nonlinear features.

[0036] This invention addresses the transient stability of hybrid systems under large disturbances. Therefore, the state-space equations need to include nonlinear terms. Thus, the Taylor expansion of the hybrid system's mathematical model is performed, truncating the quadratic terms to preserve the system's nonlinear characteristics. The established 24th-order state-space equations are shown in equation (5). In the formula: A is the Jacobian matrix of the state-space model; Hi is the Hessian matrix corresponding to the i-th function. is the first derivative of X; ,Δud2,Δuq2,Δβud2,Δβuq2,Δβid2,Δβiq2,ΔωFM,ΔθFM,Δid1,Δiq1,Δid2,Δiq2,Δigd,Δigq]T. Among them, Δ represents the large disturbance increment.

[0037] The state variables in the aforementioned state vector originate from the control loops of the grid-connected converter, the control loops of the network-connected converter, and the network branch current variables, respectively, and can comprehensively reflect the main dynamic characteristics of the hybrid system under large disturbance conditions. This unified state-space modeling approach provides a foundation for subsequent nonlinear decoupling and stability analysis.

[0038] To reduce the complexity of direct analysis of high-order nonlinear systems and to distinguish between the linear and nonlinear coupling characteristics of the system, this implementation first performs a similarity transformation on the large disturbance state-space model, converting the linear part of the system to Jordan canonical form, thereby creating conditions for subsequent nonlinear decoupling processing.

[0039] Using similarity transformation, the hybrid system shown in equation (4) can be transformed to Jordan canonical form in the Y domain and discard higher-order terms of the third order and above. The similarity transformation formula is shown in equation (5), and the resulting hybrid system in the Y domain is shown in equation (6).

[0040] In the formula: P is the similarity transformation matrix; Ni is the quadratic form matrix corresponding to the i-th function in the Y domain; Λ is the first derivative of Y; Λ is a diagonal matrix composed of the eigenvalues ​​of the hybrid system.

[0041] Even after the linear similarity transformation, nonlinear coupling relationships caused by quadratic terms still exist in the system. To further mitigate the nonlinear cross-influence between different state variables, this implementation introduces a nonlinear coordinate transformation to map the system to a nonlinear decoupling domain, thereby achieving nonlinear decoupling of the hybrid system.

[0042] After decoupling the state variables, the hybrid system in the Y domain will be transformed to the Z domain (i.e., the nonlinear decoupling domain). The transformation method is shown in Equation (7), and the system space state model in the Z domain is shown in Equation (8).

[0043] In the formula: Z = [z1, z2, ..., z24]T is the state space vector in the Z domain; is the first derivative of Z; Mi is the nonlinear transformation matrix of the hybrid system from the Y domain to the Z domain; Ki is the quadratic form matrix of the hybrid system in the Z domain after decoupling.

[0044] The coupled hybrid system within the Z-domain has been decomposed into multiple first- or second-order subsystems. The state variables within these subsystems are referred to as isolated variables and coupled-pair variables, respectively. An isolated variable *zi* refers to a variable whose influence on other state variables and the influence of other state variables on it are small and can be ignored, thus forming a first-order quadratic subsystem, as shown in equation (9). Coupled-pair variables *zi* and *zj*, on the other hand, mean that they have a strong mutual influence and cannot be ignored. They should be analyzed as a whole, thus forming a second-order quadratic subsystem, as shown in equation (10).

[0045] Nonlinear transformations do not change the eigenvalues ​​of a hybrid system. Therefore, if the eigenvalues ​​λi and λj corresponding to the state variables yi and yj in the Y domain are conjugate complex pairs, then the linear terms corresponding to the state variables zi and zj in the Z domain are also conjugate. This conjugate relationship physically reveals the existence of tightly coupled oscillatory modes in the hybrid system. Therefore, zi and zj should be analyzed as a whole, referred to as a conjugate coupling pair.

[0046] In the decoupled domain, the degree of coupling between different state variables varies. To quantitatively distinguish the interaction relationships between state variables, this implementation introduces a coupling factor to measure the nonlinear coupling strength between different state variables, and thereby identify the state variables or state variable coupling pairs that play a dominant role in the transient stability of the system.

[0047] In a hybrid system, when λi and λj are real numbers, the degree of coupling between variables needs to be quantified by introducing a coupling factor. The definition of the coupling factor is shown in equation (11).

[0048] In the formula: n is an element in the quadratic form matrix N in the Y field.

[0049] The coupling factor comprehensively considers the mutual influence between zi and zj, quantifies the degree of coupling between state variables, and effectively identifies the coupling relationships in a hybrid system dominated by the interaction between control loops or circuit parameters. A larger coupling factor indicates a higher degree of coupling, and these should be selected as coupling pairs, called real-valued coupling pairs. The remaining state variables are then selected as isolated variables.

[0050] For the isolated variable zi, then: For the coupled pair of variables zi and zj, then: In the formula: k is an element in the quadratic form matrix K in the Z-domain.

[0051] Substituting equation (9) into equation (8), we get: Substituting equation (10) into equation (14), we get: Expanding and rearranging equation (15), we get: In the formula: mj is an element in the quadratic matrix M.

[0052] If λj≠λk+λl, j,k,l=1,2,…,24, then the nonlinear transformation matrix Mj can be calculated according to equation (17).

[0053] Thus, the decoupling expression for the hybrid system in the Z domain is obtained.

[0054] The attraction domain of a first-order quadratic system is an interval, which can be solved using formula (18).

[0055] Solving for the attraction domain of a second-order quadratic system is quite complex, but several mature methods exist. This invention employs the ray casting method to solve for its attraction domain. The basic principle is to explore stable and unstable points sequentially outwards from the origin. Once an unstable point is found, the search stops and is recorded. Connecting these unstable points sequentially forms the attraction domain of the dominant unstable variable subsystem. Although this method is relatively slow, it provides a more accurate attraction domain that fully reflects the system's stability.

[0056] After completing the nonlinear decoupling and obtaining the dynamic expressions of each low-order subsystem, this implementation method performs the transient stability determination process in the following order: First, determine the low-order subsystem corresponding to the dominant instability state variable; second, solve the attraction domain of the low-order subsystem in the decoupling domain; then calculate the initial operating point of the system under the disturbance condition and map it to the decoupling domain; finally, complete the transient stability determination based on the positional relationship between the initial operating point and the attraction domain.

[0057] After obtaining the attraction region, the transient stability of the hybrid system can be determined by the location of the initial operating point. If the initial operating point is within the attraction region, the system is stable; if it is outside the attraction region, the system is unstable. Since the attraction region is solved in the Z-domain, the solution for the initial operating point should also be transformed to the Z-domain. The specific method is as follows: Solve for X0: X0 = Xs - Xe, where Xs is the operating point of the hybrid system before the disturbance, and Xe is the steady-state operating point after the disturbance; Solve for Y0: Y0 = P-1X0; Solving for Z0: Z0 can be obtained by solving the nonlinear algebraic equation system Z + M(Z) - Y0 = 0, which can be solved using the Newton-Raphson algorithm with the initial value Z = Y0. The operating point furthest from the origin in Z0, i.e., the point where the hybrid system is most likely to be outside the attraction domain leading to transient instability, is selected as the primary initial operating point for this study.

[0058] The following simulation analysis constitutes a specific embodiment of the method of the present invention, used to illustrate the execution process and determination result of the transient stability determination method in a real hybrid system, and does not constitute a limitation on the range of system parameter values.

[0059] To verify the effectiveness of the transient stability determination method proposed in this invention, a system was built in Matlab / Simulink. Figure 2 The simulation model of the hybrid system shown is illustrated in Table 1, and the main simulation parameters are as follows.

[0060] Table 1 Simulation Model Parameters of Hybrid System I. Drawing of the attraction region and determination of transient stability under different grid voltage sag conditions For a GFL / GFM hybrid system, the 24th-order state-space model established based on a nonlinear decoupling method can be decomposed into a series of low-order subsystems in the Z-domain. Using the method proposed in this invention, the low-order subsystems corresponding to the dominant instability state variables and their corresponding initial operating points are extracted. The resulting attraction domains and corresponding initial operating points under different voltage drop conditions are as follows: Figure 3 As shown. The phase trajectory diagrams corresponding to the four operating conditions are as follows. Figure 4 As shown.

[0061] Depend on Figure 3 It can be seen that when Uf = 0.8Us, 0.5Us, and 0.3Us, the initial operating point is within the attraction region, and the system remains stable. The initial operating point will eventually converge to the stable equilibrium point. However, as the voltage drop depth increases, the initial operating point gradually approaches the boundary of the attraction region, and the stability margin of the system decreases. When Uf = 0.3Us, the system is already in a critical stable state; when Uf = 0.2Us, the initial operating point is outside the attraction region, the system is transiently unstable, and cannot converge to the stable equilibrium point. Figure 4 It can be seen that when Uf=0.8Us, 0.5Us, and 0.3Us, the initial running point can converge to the stable equilibrium point, proving that the system can maintain transient stability at this time. When Uf=0.2Us, the corresponding phase trajectory diverges, and the system becomes transiently unstable.

[0062] II. Simulation Verification of Transient Stability under Different Voltage Drop Conditions Simulation conditions: At 5 seconds, the voltage of the infinite grid in the hybrid system drops to different depths. The voltage at the PCC point, the system output active power, and the power angle of the grid-connected units are measured to verify that the system experiences synchronous instability.

[0063] Simulation results: such as Figure 5 (a)-(c) When Uf=0.8Us, 0.5Us, and 0.3Us, the system output power and power angle can recover to stability. However, as the fault deepens, the oscillation amplitude of the transient regulation after the fault occurs increases, the convergence time becomes longer, and the power angle of both types of units increases, indicating that the system is approaching the stability boundary and the stability margin is reduced. When Uf=0.3Us, the system is already in a critical instability state. When Uf=0.2Us, the system experiences transient instability, and all measurements show periodic oscillations and cannot converge to the equilibrium point.

[0064] As demonstrated by the simulation analysis above, the nonlinear decoupling method used in this invention achieves transient stability analysis results under different operating conditions that are consistent with the corresponding time-domain simulation conclusions, accurately predicting system instability or stabilization phenomena. Furthermore, the nonlinear decoupling method used in this invention accurately characterizes the irregular shape of the attraction domain boundary obtained from the analysis of the hybrid system, exhibiting lower conservatism and providing instructive analytical conclusions.

[0065] In other embodiments, a computer-readable storage medium is also provided for implementing the transient stability determination method for a hybrid grid system based on nonlinear decoupling described in the above embodiments.

[0066] In this embodiment, the computer-readable storage medium stores computer program instructions. When the computer program instructions are executed by the processor, they perform the following functions: acquiring dynamic state information of the hybrid parallel operation system of grid-connected and grid-connected converters under disturbance conditions; constructing a large disturbance state-space model of the hybrid system based on the dynamic state information, and performing nonlinear decoupling processing on the state-space model to obtain several low-order subsystems; identifying the subsystem that plays a dominant role in the transient stability of the system from the low-order subsystems, and constructing a corresponding attraction domain within the decoupling domain; determining the initial operating point of the system under disturbance conditions, and mapping the initial operating point to the decoupling domain; and determining the transient stability of the hybrid system based on the positional relationship between the mapped initial operating point and the corresponding attraction domain.

[0067] In this embodiment, the computer-readable storage medium may be a read-only memory, random access memory, disk, optical disk, solid-state memory, or other storage medium capable of storing program instructions, and its specific physical form does not constitute a limitation on the scope of protection of this invention.

[0068] In other embodiments, an electronic device is provided for implementing the transient stability determination method for a hybrid grid system based on nonlinear decoupling described in the above embodiments.

[0069] As shown in this embodiment, the electronic device includes a processor and a memory, wherein the memory stores a computer program, and the processor executes the computer program. When the processor executes the computer program, it performs the following functions: based on the collected or acquired dynamic state information of the hybrid parallel operation system of grid-connected and grid-connected converters, it constructs a state-space model describing the dynamic behavior of the hybrid system under large disturbances; it performs nonlinear decoupling processing on the state-space model to obtain the corresponding low-order subsystems; it constructs an attraction domain that dominates the low-order subsystems within the decoupling domain; and it determines the transient stability state of the hybrid system based on the positional relationship between the initial operating point of the system under disturbance conditions and the attraction domain.

[0070] In this embodiment, the electronic device may be a server, industrial control computer, embedded control device, or other computing device with data processing capabilities. Its specific hardware structure and deployment form may be configured according to the actual needs of the operation analysis of the new energy power system, and shall not be construed as a limitation on the scope of protection of this invention.

[0071] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for determining the transient stability of a hybrid grid system based on nonlinear decoupling, characterized in that, Includes the following steps: 1) Obtain the dynamic status information of a hybrid system in which grid-connected converters and grid-connected converters operate in parallel under disturbance conditions; 2) Based on the dynamic state information, a state-space model describing the dynamic behavior of the hybrid system under large disturbances is constructed, and the state-space model is subjected to nonlinear decoupling processing to obtain several low-order subsystems. 3) Identify the subsystem that plays a dominant role in the transient stability of the hybrid system from the lower-order subsystems, and construct the attraction domain of the dominant subsystem in the decoupling domain; 4) Determine the initial operating point of the hybrid system under the disturbance condition, and map the initial operating point to the decoupling domain; 5) Determine the transient stability of the hybrid system under the disturbance condition based on the positional relationship between the mapped initial operating point and the corresponding attraction domain.

2. The method according to claim 1, characterized in that, In step 2), the hybrid system includes at least one grid-connected converter, at least one grid-connected converter, and network impedance connecting the grid-connected converter, the grid-connected converter, and the power grid. The state-space model is used to describe the dynamic response process of the hybrid system near the steady-state operating point before the disturbance, under large disturbance conditions such as grid voltage drop. The state variables in the state-space model include converter output current, node voltage, power angle, and internal state variables in the converter control loop.

3. The method according to claim 2, characterized in that, The state-space model is established with the steady-state operating point before the system disturbance as the reference point. By performing Taylor expansion on the nonlinear dynamic equations of the system and truncating them to the quadratic terms, a large disturbance state-space model containing linear terms and quadratic nonlinear terms is constructed to preserve the main nonlinear dynamic characteristics of the hybrid system under large disturbance conditions.

4. The method according to claim 3, characterized in that, The nonlinear decoupling process includes the following steps: A linear similarity transformation is performed on the large disturbance state-space model to convert the linear part of the system into a standard form with system eigenvalues ​​as elements. Based on the linear similarity transformation, a nonlinear coordinate transformation is introduced to reconstruct the quadratic nonlinear term, thereby weakening the nonlinear cross-coupling relationship between different state variables and mapping the system to a nonlinear decoupling domain.

5. The method according to claim 4, characterized in that, Within the nonlinear decoupling domain, the original high-order hybrid system is decomposed into multiple low-order subsystems. The low-order subsystems include first-order quadratic subsystems and second-order quadratic subsystems. The first-order quadratic subsystems correspond to isolated state variables that have little influence on other state variables and are weakly affected by other state variables. The second-order quadratic subsystems correspond to state variable coupling pairs that have significant nonlinear interactions between state variables.

6. The method according to claim 5, characterized in that, When the system characteristic value corresponding to the lower-order subsystem is a real number, the nonlinear coupling strength between state variables is quantified by introducing a coupling factor. The coupling factor is determined based on the distribution of the quadratic nonlinear term in the nonlinear decoupling domain. Based on the magnitude of the coupling factor, the state variables are divided into state variable coupling pairs or isolated state variables.

7. The method according to claim 5, characterized in that, For the first-order quadratic subsystem, its attraction domain is a one-dimensional stable interval formed along the direction of the corresponding state variable; For the second-order quadratic subsystem, stable and unstable points are searched outward from the origin in the nonlinear decoupling domain using the ray method, and the attraction domain boundary of the corresponding lower-order subsystem is determined based on the unstable points obtained from the search.

8. The method according to claim 1, characterized in that, In step 4), the initial operating point is obtained by calculating the difference between the steady-state operating point before the system disturbance and the steady-state operating point after the disturbance. The initial state vector is then mapped to the nonlinear decoupling domain through the linear similarity transformation and the nonlinear coordinate transformation. In the nonlinear decoupling domain, the transient stability is determined based on the positional relationship between the mapped initial operating point and the attraction domain of the corresponding low-order subsystem.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it is used to implement the steps of the transient stability determination method for the hybrid interconnected network system based on nonlinear decoupling as described in any one of claims 1 to 8.

10. An electronic device comprising a processor, a memory, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the steps of the transient stability determination method for a hybrid grid system based on nonlinear decoupling as described in any one of claims 1 to 8.

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