Transient stability assessment method and device for hybrid grid-connected converter systems

Abstract

This invention relates to the field of power electronic converter control technology and discloses a method and apparatus for judging the transient stability of a hybrid system with a grid-connected converter. The method includes constructing an equivalent model of the hybrid system comprising a grid-connected converter and a network-connected converter, wherein a virtual impedance is connected in series in the branch of the network-connected converter; constructing a Lyapunov energy function based on the equivalent model, wherein the independent variables include the power angle and phase-locked loop frequency of the grid-connected converter, and the power angle, frequency, and voltage amplitude of the network-connected converter; determining the stability domain boundary of the hybrid system using the Lassel invariant set principle when the derivative of the energy function with respect to time is less than or equal to 0; calculating the critical range of the virtual impedance based on the changing trend of the stability domain boundary, and determining the target value of the virtual impedance to maintain the transient stability of the hybrid system. The above method achieves quantitative judgment and parameter tuning of the transient stability of the hybrid system.

Description

Technical Field

[0001] This invention relates to the field of power electronic converter control technology, and in particular to a method, device, storage medium, and computer equipment for judging transient stability in a grid-connected converter hybrid system. Background Technology

[0002] With the integration of a high proportion of renewable energy into the power system, traditional synchronous generators are gradually being replaced by power electronic converters. Currently, grid-connected converters for new energy mainly employ two control strategies: grid-following control and grid-connecting control. Grid-following converters achieve synchronization by tracking the grid voltage through a phase-locked loop (PLL), exhibiting an external characteristic of a current source. Grid-connecting converters, on the other hand, autonomously establish voltage and frequency, exhibiting an external characteristic of a voltage source, similar to synchronous machines. In practical applications, these two types of converters are often operated in parallel, forming a hybrid grid-connected system. However, considering the fundamental differences between the two in synchronization mechanisms, response characteristics, and time scales, large disturbances such as grid faults can easily trigger complex dynamic interaction processes, even leading to system instability and seriously affecting the safe operation of the new power system.

[0003] Currently, for transient stability analysis of hybrid systems containing grid-connected and grid-connected converters, existing technologies mainly rely on adjusting fixed parameters within the system to improve stability. However, in actual engineering, the fixed parameters within the system are limited by manufacturing processes and operating conditions, making them difficult to change flexibly. Furthermore, parameter drift may cause the preset stability strategy to fail. In addition, existing analysis methods mostly use qualitative descriptions or graphical means, lacking quantitative stability criteria and failing to provide specific parameter boundaries for maintaining system transient stability, thus limiting their engineering application value.

[0004] Therefore, a transient stability determination method that can be quantitatively analyzed and does not rely on internal parameter adjustments is urgently needed. Summary of the Invention

[0005] In view of this, this application provides a method, device, storage medium and computer equipment for judging transient stability of a hybrid system with grid-connected converters. The main purpose is to solve the technical problem in the prior art that the internal fixed parameters of the hybrid system are difficult to adjust flexibly, and that there is a lack of quantitative stability criteria, and that it is impossible to give specific parameter boundaries for maintaining transient stability.

[0006] According to a first aspect of the present invention, a method for determining transient stability in a hybrid system of grid-connected converters is provided, comprising: Construct an equivalent model of a hybrid system, wherein the hybrid system includes a grid-connected converter, a grid-connected converter and a power grid, and a virtual impedance is connected in series in the branches of the grid-connected converter; The Lyapunov energy function of the hybrid system is constructed based on the equivalent model, wherein the independent variables of the Lyapunov energy function include the power angle and phase-locked loop frequency of the grid converter, as well as the power angle, frequency and voltage amplitude of the grid converter. When the derivative of the Lyapunov energy function with respect to time is less than or equal to 0, the stability domain boundary of the hybrid system is determined based on the Lyapunov energy function using the Lassell invariant set principle. The critical range of the virtual impedance is calculated based on the changing trend of the stability domain boundary, and the target value of the virtual impedance is determined within the critical range, wherein the target value is used to maintain the transient stability of the hybrid system.

[0007] According to a second aspect of the present invention, a transient stability determination device for a hybrid grid-connected converter system is provided, the device comprising: The model building module is used to build an equivalent model of the hybrid system, wherein the hybrid system includes a grid-connected converter, a grid-connected converter and a power grid, and the branches of the grid-connected converter are connected in series with virtual impedances; The function construction module is used to construct the Lyapunov energy function of the hybrid system according to the equivalent model, wherein the independent variables of the Lyapunov energy function include the power angle and phase-locked loop frequency of the grid converter, as well as the power angle, frequency and voltage amplitude of the grid converter; The boundary delineation module is used to determine the stability domain boundary of the hybrid system based on the Lyapunov energy function when the derivative of the Lyapunov energy function with respect to time is less than or equal to 0, using the Lassell invariant set principle. The value determination module is used to calculate the critical value range of the virtual impedance based on the changing trend of the stability domain boundary, and determine the target value of the virtual impedance within the critical value range, wherein the target value is used to maintain the transient stability of the hybrid system.

[0008] According to a third aspect of the present invention, a storage medium is provided having a computer program stored thereon, which, when executed by a processor, implements the above-described transient stability judgment method for a grid-connected converter hybrid system.

[0009] According to a fourth aspect of the present invention, a computer device is provided, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the above-described transient stability judgment method for a grid-connected converter hybrid system.

[0010] This invention provides a transient stability assessment method, device, storage medium, and computer equipment for a hybrid system with integrated grid-type converters. It integrates virtual impedance into the branches of the grid-type converter as an online adjustable additional control method. By establishing an equivalent model of the hybrid system including virtual impedance and employing nodal impedance analysis to incorporate the virtual impedance into the network equations, the system impedance characteristics can be dynamically adjusted during faults, thereby optimizing the electrical coupling relationship between the integrated and hybrid grid-type converters and compensating for the inability of fixed parameters to adapt to varying operating conditions. Based on this, a Lyapunov energy function is constructed that simultaneously includes the power angle and phase-locked loop frequency of the integrated grid-type converter, as well as the power angle, frequency, and voltage amplitude of the hybrid grid-type converter, comprehensively reflecting the dynamic coupling characteristics of both. Furthermore, the Lassel invariant set principle is used to determine the stability domain boundary of the hybrid system, elevating stability assessment from traditional qualitative description to mathematical quantitative analysis. Finally, the critical range of virtual impedance is calculated based on the changing trend of the stability domain boundary, and specific target values ​​are determined, directly outputting the parameter boundaries. In summary, the above methods not only solve the technical problems of strong parameter dependence and lack of quantitative criteria in existing technologies, but also realize the quantitative judgment and parameter tuning of the transient stability of hybrid systems.

[0011] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0012] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and, together with their description, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings: Figure 1 A flowchart illustrating a transient stability determination method for a hybrid system with a grid-type converter provided by an embodiment of the present invention is shown. Figure 2 This invention provides a method for judging the transient stability of a hybrid system of grid-connected converters, which is illustrated in an embodiment of the present invention. Figure 3 The diagram shows the control block diagram of the grid converter in a transient stability judgment method for a hybrid system with a grid converter provided in an embodiment of the present invention. Figure 4 The diagram shows the control block diagram of the grid converter in a transient stability judgment method for a hybrid system with a grid converter provided by an embodiment of the present invention. Figure 5This diagram illustrates a pure reactive power injection vector diagram of a grid-type converter under fault conditions in a transient stability judgment method for a hybrid system with a grid-type converter, provided by an embodiment of the present invention. Figure 6 This diagram illustrates the structure of a transient stability judgment device for a hybrid system of grid-connected converters provided in an embodiment of the present invention. Figure 7 A schematic diagram of the device structure of a computer device provided in an embodiment of the present invention is shown. Detailed Implementation

[0013] Exemplary embodiments of the present application will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present application are shown in the drawings, it should be understood that the present application may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this application will be thorough and complete, and will fully convey the scope of the present application to those skilled in the art.

[0014] This application provides a method for determining the transient stability of a hybrid system with a grid-connected converter, such as... Figure 1 As shown, the method includes the following steps: 101. Construct an equivalent model of a hybrid system, which includes a grid-connected converter, a grid-connected converter, and a power grid. Virtual impedances are connected in series in the branches of the grid-connected converter.

[0015] In power systems, grid-connected converters are power electronic devices that achieve synchronization by tracking the grid voltage through a phase-locked loop. Their external characteristics are those of a current source, and they are typically used to control the output current to follow the grid operation. Grid-connected converters are power electronic devices that autonomously establish voltage and frequency. Their external characteristics are those of a voltage source, and they can simulate the operating characteristics of a synchronous generator to provide voltage and frequency support to the grid. Virtual impedance is an equivalent impedance simulated through a control algorithm. It does not require modification of the hardware and can be flexibly adjusted during operation to improve the electrical characteristics of the system. A hybrid system refers to a multi-converter grid-connected system composed of grid-connected and grid-connected converters connected in parallel to a common coupling point.

[0016] Specifically, this step first constructs an equivalent model of the hybrid system. The model includes three core components: a grid-connected converter, a grid-connected converter, and the power grid. A virtual impedance is introduced into the branch of the grid-connected converter. That is, an adjustable equivalent impedance is simulated in series in the output circuit of the grid-connected converter through a control algorithm. The equivalent model established based on this provides a mathematical foundation for subsequent transient stability analysis, allowing the virtual impedance to participate in the electrical equations of the system as an adjustable parameter, thereby realizing a quantitative description of the dynamic characteristics of the system.

[0017] In this embodiment, by connecting a virtual impedance in series in the branch of the grid-type converter, the virtual impedance, as an online adjustable parameter, replaces the reliance on fixed internal parameters in the traditional method, avoiding the failure of the preset stability strategy due to parameter drift or manufacturing process limitations. On the other hand, the introduction of the virtual impedance changes the equivalent impedance characteristics of the system, allowing the electrical coupling relationship between the grid-type converter and the grid-connected converter to be optimized by adjusting the virtual impedance value in subsequent steps. This lays a structural foundation for suppressing oscillations under fault conditions and improving the transient stability of the system.

[0018] 102. Construct the Lyapunov energy function of the hybrid system based on the equivalent model. The independent variables of the Lyapunov energy function include the power angle and phase-locked loop frequency of the grid converter, as well as the power angle, frequency and voltage amplitude of the grid converter.

[0019] Among them, the Lyapunov energy function is a mathematical tool used to analyze the stability of nonlinear systems. It constructs a scalar function that describes the energy changes of the system and judges whether the system tends to be stable based on the trend of the function's change over time. The power angle refers to the phase difference between the converter output voltage and the grid voltage, and is a key state variable used to reflect synchronous stability. The phase-locked loop frequency refers to the angular frequency of the phase-locked loop output in the grid-connected converter, and is used to track the phase and frequency of the grid voltage. The frequency refers to the actual output angular frequency of the grid-connected converter, which reflects the dynamic response of the power loop. The voltage amplitude refers to the magnitude of the output voltage of the grid-connected converter, which is determined by the reactive power loop control and is an important state variable reflecting the voltage support capability.

[0020] Specifically, this step, based on the equivalent model of the hybrid system with virtual impedance established in step 101, constructs the corresponding Lyapunov energy function. This energy function is a scalar function containing five state variables: the power angle and phase-locked loop frequency of the grid-connected converter, reflecting synchronous tracking characteristics and frequency response characteristics, respectively; and the power angle, frequency, and voltage amplitude of the grid-connected converter, reflecting power angle stability, frequency dynamic characteristics, and voltage support capability, respectively. These five state variables comprehensively cover the key dynamic behaviors of the grid-connected and grid-connected converters during transient processes, enabling the constructed energy function to fully characterize the coupling relationship between the two and their impact on the overall stability of the hybrid system.

[0021] In this embodiment, the transient stability assessment of the hybrid system is upgraded from traditional qualitative analysis to mathematical quantitative analysis, providing a rigorous theoretical basis for determining the stability domain boundary in subsequent steps. In addition, the Lyapunov energy function considers the key state variables of both the grid-connected converter and the network-connected converter, fully reflecting the coupling and interaction relationship between the two under fault conditions. This avoids the limitations of ignoring coupling or only considering the dynamics of a single machine in the prior art, thus enabling a more accurate description of the transient stability characteristics of the hybrid system and laying a theoretical foundation for the quantitative tuning of the virtual impedance.

[0022] 103. When the derivative of the Lyapunov energy function with respect to time is less than or equal to 0, the stability domain boundary of the hybrid system is determined based on the Lyapunov energy function using the Lassell invariant set principle.

[0023] Among them, the derivative of the Lyapunov energy function with respect to time refers to the rate of change of the function over time, which is used to judge the trend of energy change in the system. When the rate of change of the function with respect to time is less than or equal to zero, it indicates that the energy of the system is constantly dissipating or remains constant, and the system has the characteristic of tending to be stable. The Russell invariant set principle is an important extension of the Lyapunov stability theory. It is used to analyze the final behavior of the system when the derivative of the energy function is zero. By identifying the invariant set that the system state trajectory eventually converges to, the stable region of the system can be determined. The stability region boundary refers to the maximum range of boundaries in the state space where the system can remain stable. If the system state is within the boundary, it can converge to the equilibrium point after a transient process; otherwise, it may lead to instability.

[0024] Specifically, this step first verifies whether the Lyapunov energy function constructed in step 102 satisfies the basic conditions for stability analysis, that is, whether the derivative of the energy function with respect to time is less than or equal to zero. When the condition is met, it indicates that the constructed energy function can correctly describe the energy dissipation process of the system, and the system has the characteristic of local asymptotic stability. On this basis, the Russell invariant set principle is further introduced to quantitatively analyze the stability domain of the hybrid system based on the effective energy function. Specifically, by identifying the invariant sets in the state space that satisfy the condition that the derivative of the energy function is zero, and combining the equipotential surface distribution of the energy function, the maximum range in which the system can maintain transient stability is determined, that is, the boundary of the stability domain. The obtained stability domain boundary clarifies the maximum extent to which the state variables of the system are allowed to deviate from the equilibrium point after suffering large disturbances such as faults.

[0025] In this embodiment, verifying the non-positive nature of the derivative of the energy function provides a rigorous theoretical premise for subsequent stability analysis, ensuring that the constructed energy function can correctly reflect the energy dissipation characteristics of the system and avoiding misjudgments caused by the failure of the energy function. In addition, the Lassel invariant set principle is used to determine the boundary of the stability region, transforming the transient stability of the system from an abstract mathematical description into a specific state space region. This provides a clear geometric boundary for the quantitative tuning of the virtual impedance. The boundary of the stability region is not only the basis for judging whether the system can recover stability after a fault, but also the basis for calculating the critical range of virtual impedance values ​​in subsequent steps, realizing the quantification and visualization of transient stability analysis.

[0026] 104. Calculate the critical range of virtual impedance based on the changing trend of the stability domain boundary, and determine the target value of virtual impedance within the critical range. The target value is used to maintain the transient stability of the hybrid system.

[0027] Among them, the trend of change of the stability domain boundary refers to the expansion or contraction of the system stability domain boundary as the virtual impedance value changes, reflecting the degree of influence of virtual impedance on the transient stability of the system; the critical value range refers to the range of virtual impedance values ​​that can maintain the transient stability of the system, with the lower limit and upper limit of the range corresponding to the minimum and maximum allowable impedance values ​​for the system to remain stable, respectively; the target value refers to the specific virtual impedance value finally selected within the critical value range, which is used to guide the setting of virtual impedance in actual control; transient stability refers to the ability of the system to recover to a stable operating state through a dynamic process after suffering a large disturbance such as a fault.

[0028] Specifically, this step, based on the stability region boundary determined in step 103, further analyzes the influence of virtual impedance value changes on the stability region boundary. Specifically, by gradually adjusting the value of the virtual impedance, the expansion or contraction trend of the stability region boundary is observed, thereby determining the quantitative relationship between the virtual impedance and the system's transient stability. When the virtual impedance value is too small, the stability region boundary contracts, and the system's disturbance rejection capability decreases. When the virtual impedance value is too large, it may cause the stability region boundary to disappear, and the system to lose its stable operating point. Based on this trend, this step calculates the critical value range within which the virtual impedance can maintain the system's transient stability, i.e., the interval between the minimum value greater than zero and the maximum value that guarantees the existence of the stability region. Based on this, according to actual engineering requirements, such as current limiting requirements and voltage support requirements, a specific target value for the virtual impedance is selected within the critical value range to guide subsequent control implementation.

[0029] In this embodiment, the critical range of virtual impedance is directly output, transforming the abstract stability analysis results into specific and executable parameter boundaries. This solves the technical problems of lacking quantitative stability criteria and being unable to provide specific parameter boundaries in the prior art. Then, the target value of virtual impedance is determined within the critical range, providing a clear parameter tuning basis for the actual control system. This allows the virtual impedance to be dynamically adjusted to the corresponding target value when a fault occurs, thereby effectively suppressing system oscillations and maintaining the transient stability of the hybrid system.

[0030] This invention provides a transient stability assessment method, device, storage medium, and computer equipment for a hybrid system with integrated grid-type converters. It integrates virtual impedance into the branches of the grid-type converter as an online adjustable additional control method. By establishing an equivalent model of the hybrid system including virtual impedance and employing nodal impedance analysis to incorporate the virtual impedance into the network equations, the system impedance characteristics can be dynamically adjusted during faults, thereby optimizing the electrical coupling relationship between the integrated and hybrid grid-type converters and compensating for the inability of fixed parameters to adapt to varying operating conditions. Based on this, a Lyapunov energy function is constructed that simultaneously includes the power angle and phase-locked loop frequency of the integrated grid-type converter, as well as the power angle, frequency, and voltage amplitude of the hybrid grid-type converter, comprehensively reflecting the dynamic coupling characteristics of both. Furthermore, the Lassel invariant set principle is used to determine the stability domain boundary of the hybrid system, elevating stability assessment from traditional qualitative description to mathematical quantitative analysis. Finally, the critical range of virtual impedance is calculated based on the changing trend of the stability domain boundary, and specific target values ​​are determined, directly outputting the parameter boundaries. In summary, the above methods not only solve the technical problems of strong parameter dependence and lack of quantitative criteria in existing technologies, but also realize the quantitative judgment and parameter tuning of the transient stability of hybrid systems.

[0031] Specifically, in the above embodiments, the hybrid system further includes a first line impedance, a second line impedance, a common coupling point, and a controller; the grid-connected converter is connected to the common coupling point through the first line impedance; the grid-connected converter is connected to the common coupling point through the second line impedance and the virtual impedance; the controller is connected to the virtual impedance and is used to determine the target value of the virtual impedance within the critical value range of the virtual impedance.

[0032] In this embodiment, the hybrid system topology diagram of the present application, which includes grid-connected converters and grid-connected converters, is as follows: Figure 2 As shown, in the section on grid-connected (GFL) converters, GFL converter refers to the main body of the grid-connected converter; U dc Represents the DC side voltage; R1, L1, and C1 are the filter resistor, filter inductor, and filter capacitor of the GFL converter, respectively. Together, they form an LCL filter to smooth the output waveform; I GFL ∠(θ GFL +δ GFL) represents the output current of the GFL converter, where I GFL Let θ be the current amplitude. GFL δ is the phase angle difference between the output voltage and the output current. GFL U is the phase angle difference between the output voltage and the mains voltage. GFL Z represents the terminal voltage of the grid-connected converter. GFL This indicates the connection point U between the grid converter and the common coupling point. PCC The line impedance between them.

[0033] In the section on grid-connected (GFM) converters, GFM converter refers to the main body of a grid-connected converter; U dc Represents the DC side voltage; R2, L2, and C2 are the filter resistor, filter inductor, and filter capacitor of the GFM converter, respectively, which also constitute an LCL filter; U GFM ∠δ GFM U represents the output voltage of the GFM converter, where U GFM δ is the current amplitude. GFM For the output voltage phase; U GFM I represents the terminal voltage of the GFM converter. GFL Z represents the terminal current of the grid-connected converter. GFM1 Indicates the connection point U between the grid-type converter and the common coupling point. PCC The actual line impedance between; U PCC The voltage at the point of common coupling (PCC) is the connection point where two converter branches converge and connect to the power grid. The entire system is connected to the main power grid through this PCC. g U is the line impedance on the power grid side. g Let θ be the amplitude of the grid voltage. g This represents the phase angle of the grid voltage.

[0034] Specifically, in the above embodiments, constructing the equivalent model of the hybrid system includes: equating the grid-connected converter to a current source, wherein the amplitude of the current source is determined based on the output current amplitude of the grid-connected converter, and the phase angle of the current source is determined jointly based on the phase angle difference between the output voltage and output current of the grid-connected converter, and the phase angle difference between the output voltage of the grid-connected converter and the voltage of the grid; equating the grid-connected converter to a voltage source, wherein the amplitude of the voltage source is a reference voltage value, and the phase angle of the voltage source is the phase of the output voltage of the grid-connected converter; based on the current source and voltage source, establishing the equivalent model of the hybrid system using the nodal impedance analysis method, and adding the virtual impedance to the total impedance of the branch of the grid-connected converter, wherein the total impedance is obtained by superimposing the actual line impedance of the branch of the grid-connected converter with the virtual impedance.

[0035] In this embodiment, for the GFL converter, the current loop bandwidth is much larger than the phase-locked loop bandwidth. Therefore, the dynamic changes of the current loop can be ignored, and the current loop is considered to have reached a quasi-steady state. The GFL converter is analyzed on the time scale of the power loop and the phase-locked loop, and its external characteristics can be equivalent to the amplitude I. GFL Phase angle θ GFL +δ GFL The current source, θ GFL Let δ be the phase angle difference between the output voltage and output current of the GFL converter. Similarly, for the GFM converter, the bandwidth of the voltage-current loop is much larger than that of the power loop. Therefore, the dynamic changes of the voltage-current loop can be ignored in transient stability analysis, i.e., δ GFM =θ GFM Analysis is performed on the power loop time scale, assuming the external characteristic of the grid-type converter is the amplitude U. GFM =U ref Phase angle δ GFM The voltage source, and the grid reference value θ is selected. g =0.

[0036] According to the nodal impedance analysis method, we can obtain: (1) This matrix equation describes the current-voltage relationship between the point of common coupling, the generator terminal node of the grid-type converter, and the grid node. Each element in the matrix represents the admittance between the corresponding nodes, which is the reciprocal of the impedance. The diagonal elements are the self-admittance of each node, and the off-diagonal elements are the mutual admittance between nodes. The left side of the equation is the product of the admittance matrix and the node voltage vector, and the right side is the current vector injected into each node, which reflects the mathematical expression of Kirchhoff's current law at the three nodes.

[0037] Furthermore, the total impedance Z of the grid-type converter branch is... GFM It is composed of both the actual line impedance and the virtual impedance, and its definition is shown in the following formula: Z GFM =Z GFM1 +Z v In the formula, Z GFM1 Z represents the actual line impedance of the branch of the grid-type converter, which is an inherent physical parameter of the system. v Virtual impedance is an adjustable parameter simulated through a control algorithm. The introduction of virtual impedance means that the total impedance of the branch of the grid-type converter is no longer a fixed value, but can be dynamically adjusted according to the system operating status.

[0038] In addition, the terminal voltage U of the grid converter GFL The following formula can be used to further calculate the solution from the given results: U GFL =UPCC +I GFL Z GFL In the formula, Z GFL To determine the actual line impedance of the grid converter branch, in the nodal impedance analysis method, the grid converter is treated as an equivalent current source, and its voltage needs to be obtained by post-processing the relationship after calculating the voltage at the point of common coupling.

[0039] Furthermore, the control method of the GFL converter is as follows: Figure 3 As shown, the network control specifically includes a PLL loop, a power loop, and a current loop, where U GFL_d U GFL_q I represents the output voltage in the dq coordinate system. GFL_d I GFL_q The output current in the dq coordinate system; k p k i These are the proportional gain and integral gain of the PLL component, respectively; ω g ω PLL ω b These are the mains angular frequency, PLL output angular frequency, and angular frequency base value, respectively; θ PLL P is the output phase of the PLL; GFL_ref Q GFL_ref These are the reference values ​​for active power and reactive power, respectively; P GFL Q GFL These are the active and reactive power outputs of the converter, respectively; the current loop tracks the reference value to ensure that the output amplitude is constant and the frequency follows the grid current. In essence, it is a current source with constant current amplitude and frequency following the grid.

[0040] The dynamic equation of the PLL in the GFL converter is: (2) In the formula, ω0 is the rated angular frequency of the system, which is the reference synchronous angular velocity of the power grid; δGFL is the phase of the GFL output voltage and the power grid voltage.

[0041] Therefore, the output voltage of the grid converter is: (3) Based on this, the amplitude and phase angle of the output voltage of the grid-connected converter can be further obtained as follows: (4) In the formula, θ1, θ2, θ3, and θ4 are equivalent impedance angles, and the coefficients A1, A2, A3, and A4 are respectively: (5) In the formula, B1, B2, B3, and B4 are the real parts of each phasor, corresponding to the resistive components; C1, C2, C3, and C4 are the imaginary parts of each phasor, corresponding to the reactive components; j is the imaginary unit, used to describe phase information; Furthermore, by performing a Park coordinate transformation, the q-axis component of the grid-type converter output voltage is obtained as follows: (6) In the formula, the first term is the self-impedance voltage drop generated by the interaction between the GFL converter's self-injected current and the line impedance of the entire system; the second term is the voltage drop of the mutual coupling between the GFL converter and the GFM converter in the transmission network; and the third term is the impact of the grid operating state on the GFL converter, which is the voltage coupling voltage drop of the grid.

[0042] GFM converter control method as follows Figure 4 As shown, the network-type control includes a power loop and a voltage and current dual closed loop, where U GFM_d U GFM_q I represents the output voltage in the dq coordinate system. GFM_d I GFM_q P represents the output current in the dq coordinate system. GFM_ref Q GFM_ref These are the reference values ​​for active power and reactive power, respectively; P GFM Q GFM The active and reactive power outputs of the converter are represented by D; D is the damping coefficient; J s For virtual inertia; K q D is the voltage loop integral coefficient; q U is the reactive power loop droop factor. gref The grid voltage command value; ω GFM θ is the actual angular frequency. GFM For the power loop output phase; U ref This represents the output voltage amplitude of the power loop.

[0043] The power loop equation in the GFM converter is as follows: (7) For grid-type converters, the power loop dynamic equation has been given, and the output active power and reactive power in the equation can be obtained from the complex power calculation formula: (8) as well as (9) In the formula, P GFM With Q GFM The first and second items are combined to represent the active and reactive power output of a single GFM unit connected to the grid; P GFM With Q GFMThe third term is the power coupling term, which characterizes the effect of GFL on GFM, where B 23 C 23 The equivalent admittance characteristic between the GFM AC device and the power grid is reflected in the following expression: (10) In the formula, A 23 θ is the magnitude of the equivalent admittance. 23 The phase angle is the equivalent admittance.

[0044] Specifically, in the above embodiments, the Lyapunov energy function of the hybrid system is constructed according to the equivalent model, including: defining the first partial derivative of the Lyapunov energy function with respect to the power angle of the grid-connected converter and the second partial derivative of the Lyapunov energy function with respect to the frequency of the grid-connected converter's phase-locked loop, based on the phase-locked loop dynamic equation of the grid-connected converter and the quadrature-axis component expression of the grid-connected converter's output voltage; defining the third partial derivative of the Lyapunov energy function with respect to the power angle of the grid-connected converter and the fourth partial derivative of the Lyapunov energy function with respect to the frequency of the grid-connected converter, based on the active power equation of the grid-connected converter's power loop; defining the fifth partial derivative of the Lyapunov energy function with respect to the voltage amplitude of the grid-connected converter, based on the reactive power equation of the grid-connected converter's power loop; and integrating the first, second, third, fourth, and fifth partial derivatives using the undetermined gradient method to obtain the expression for the Lyapunov energy function.

[0045] Furthermore, the method also includes: during the transient process after a fault occurs, the grid-type converter adopts a pure reactive current injection mode to improve the transient stability of the grid-type converter.

[0046] In this embodiment, to quantitatively analyze the transient stability of the GFL and GFM hybrid system, the Lyapunov energy function is used to obtain the boundary virtual impedance value that can improve the transient stability of the hybrid system. Based on the constructed equivalent model of the hybrid system, the time derivative of the energy function is obtained as follows: (11) Substituting the q-axis component of the grid converter output voltage obtained from equation (6) into the phase-locked loop dynamic equation, we get: (12) Rearranging the above equation, we define the energy function V with respect to δ. GFL With ω PLL The partial derivatives are: (13) Substituting equation (13) into equation (12), we get: (14) (15) During a fault, the GFL has two modes: injecting reactive current into the grid and injecting active current into the grid. Injecting pure active current not only deteriorates its own transient characteristics but also affects the transient stability of the GFM in the hybrid system. Therefore, for the GFL to use pure reactive current injection (θ) under fault conditions... GFL =-π / 2), which can improve the transient stability of GFM, at which point θ2 and θ3 are close to 0, such as Figure 5 As shown, the GFL output frequency ω PLL Deceleration leads to a power angle δ GFL The value is negative and gradually decreases; while the GFM, due to its input power being greater than its output power, has an output frequency ω. GFM This will accelerate, causing the work angle δ GFM Increase, therefore δ GFM +θ2-δ GFL Increase, and taking δ into account GFM +θ2-δ GFL ∈(0,π), θ3-δ GFL ∈(0,π / 2); Additionally, θ GFL +θ1∈(-π / 2,0); θ GFL +θ4∈(-π / 2,0).

[0047] For the energy function of a grid-type converter, since dδ GFM / dt=ω GFM -ω g Multiply both sides of the work formula in equation (7) by dδ GFM / dt gets: (16) Rearranging equation (16), we define the energy function V for δ GFM With ω GFL Partial derivatives: (17) Substituting equation (17) into equation (16), we get: (18) Meanwhile, for the unknown variable U in the network structure GFM Multiply both sides of the reactive power loop formula in equation (7) by dU. GFM / dt gets: (19) Then we can define the energy function V for U GFM The partial derivative is: (20) Substituting equation (20) into equation (19), we get: (twenty one) Combining equations (14), (15), (18), and (21), we can obtain that the time derivative of the energy function of the hybrid system, dV / dt, is ≤0. This indicates that the time derivative of the constructed energy function is less than or equal to zero, and all partial derivative functions are continuous, satisfying the constraint condition for the energy function to be effective. The energy function V(δ) GFL , ω PLL , δ GFM , ω GFM U GFM This can be viewed as the energy dissipation of the system, and the system has a local asymptotically stable point.

[0048] According to the method of undetermined gradients, by integrating equations (13), (17), and (20), the final complete energy function is: (twenty two) Specifically, in the above embodiments, calculating the critical range of virtual impedance based on the changing trend of the stability domain boundary includes: using the gradient method to solve for the maximum stability domain of the Lyapunov energy function based on the changing trend of the stability domain boundary, and obtaining the upper limit of the virtual impedance; and determining the critical range of virtual impedance based on the upper limit of the virtual impedance.

[0049] Furthermore, the target value of the virtual impedance is greater than zero and less than or equal to the upper limit value.

[0050] In this embodiment, the gradient method is used to obtain the upper limit of the virtual impedance that maximizes the area of ​​the stable domain of the hybrid system. The specific formula is as follows: (twenty three) Furthermore, as Figure 1 In a specific implementation of the method, this application provides a transient stability judgment device for a hybrid system with a grid-connected converter, such as... Figure 6 As shown, the device includes: a model building module 301, a function building module 302, a boundary delineation module 303, and a value determination module 304.

[0051] Model building module 301 is used to build an equivalent model of a hybrid system, wherein the hybrid system includes a grid-connected converter, a grid-connected converter and a power grid, and virtual impedances are connected in series in the branches of the grid-connected converter; The function construction module 302 is used to construct the Lyapunov energy function of the hybrid system based on the equivalent model. The independent variables of the Lyapunov energy function include the power angle and phase-locked loop frequency of the grid converter, as well as the power angle, frequency and voltage amplitude of the grid converter. Boundary delineation module 303 is used to determine the stability domain boundary of the hybrid system based on the Lyapunov energy function when the derivative of the Lyapunov energy function with respect to time is less than or equal to 0, using the Lassell invariant set principle. The value determination module 304 is used to calculate the critical value range of the virtual impedance based on the changing trend of the stability domain boundary, and to determine the target value of the virtual impedance within the critical value range. The target value is used to maintain the transient stability of the hybrid system.

[0052] In specific application scenarios, the model building module 301 is used to equate the grid-connected converter to a current source. The amplitude of the current source is determined based on the output current amplitude of the grid-connected converter, and the phase angle of the current source is determined by the phase angle difference between the output voltage and output current of the grid-connected converter, as well as the phase angle difference between the output voltage of the grid-connected converter and the voltage of the grid. The grid-connected converter is equated to a voltage source. The amplitude of the voltage source is the reference voltage value, and the phase angle of the voltage source is the phase of the output voltage of the grid-connected converter. Based on the current source and voltage source, an equivalent model of the hybrid system is established using the nodal impedance analysis method, and the virtual impedance is added to the total impedance of the branch of the grid-connected converter. The total impedance is obtained by superimposing the actual line impedance and the virtual impedance of the branch of the grid-connected converter.

[0053] In specific application scenarios, the function construction module 302 is specifically used to define the first partial derivative of the Lyapunov energy function with respect to the power angle of the grid-connected converter, and the second partial derivative of the Lyapunov energy function with respect to the frequency of the grid-connected converter's phase-locked loop, based on the phase-locked loop dynamic equation of the grid-connected converter and the quadrature-axis component expression of the output voltage of the grid-connected converter; to define the third partial derivative of the Lyapunov energy function with respect to the power angle of the grid-connected converter, and the fourth partial derivative of the Lyapunov energy function with respect to the frequency of the grid-connected converter, based on the active power equation of the power loop of the grid-connected converter; to define the fifth partial derivative of the Lyapunov energy function with respect to the voltage amplitude of the grid-connected converter, based on the reactive power equation of the power loop of the grid-connected converter; and to perform integration operations on the first, second, third, fourth, and fifth partial derivatives using the undetermined gradient method to obtain the expression of the Lyapunov energy function.

[0054] In specific application scenarios, the function construction module 302 is also used to improve the transient stability of the grid-type converter by adopting a pure reactive current injection mode during the transient process after a fault occurs.

[0055] In specific application scenarios, the value determination module 304 can be used to solve the maximum stability region of the Lyapunov energy function by using the gradient method based on the changing trend of the stability region boundary, and obtain the upper limit value of the virtual impedance; and determine the critical value range of the virtual impedance based on the upper limit value of the virtual impedance.

[0056] In specific application scenarios, the value determination module 304 is also used to ensure that the target value of the virtual impedance is greater than zero and less than or equal to the upper limit value.

[0057] In specific application scenarios, the hybrid system in model building module 301 also includes a first line impedance, a second line impedance, a common connection point, and a controller; the grid-connected converter is connected to the common connection point through the first line impedance; the grid-connected converter is connected to the common connection point through the second line impedance and the virtual impedance; the controller is connected to the virtual impedance and is used to determine the target value of the virtual impedance within the critical value range of the virtual impedance.

[0058] It should be noted that other corresponding descriptions of the functional units involved in the transient stability judgment device for a hybrid grid-connected converter system provided in this embodiment can be found in [reference needed]. Figure 1 The corresponding description in [the document] will not be repeated here.

[0059] Based on the above, Figure 1 Accordingly, this embodiment also provides a storage medium storing a computer program, which, when executed by a processor, implements the above-described transient stability judgment method for a hybrid grid-type converter system.

[0060] Based on this understanding, the technical solution of this application can be embodied in the form of a software product. The software product to be identified can be stored in a non-volatile storage medium (such as CD-ROM, USB flash drive, mobile hard drive, etc.) and includes several instructions to enable a computer device (such as a personal computer, server, or network device, etc.) to execute the transient stability judgment method of the hybrid system of grid-connected converters in various implementation scenarios of this application.

[0061] Based on the above, Figure 1 The method shown, and Figure 6 The illustrated embodiment of the transient stability judgment device for a hybrid system with a grid-connected converter is designed to achieve the above objectives, such as... Figure 7 As shown, this embodiment also provides a physical device for the transient stability judgment method of a grid-connected converter hybrid system. This device includes a communication bus, a processor, a memory, and a communication interface. It may also include input / output interfaces and a display device. The various functional units can communicate with each other via the bus. The memory stores a computer program, and the processor executes the program stored in the memory to perform the transient stability judgment method of the grid-connected converter hybrid system described in the above embodiment.

[0062] Optionally, the physical device may also include a user interface, a network interface, a camera, radio frequency (RF) circuitry, sensors, audio circuitry, a Wi-Fi module, etc. The user interface may include a display screen, input units such as a keyboard, etc., and optional user interfaces may also include USB interfaces, card reader interfaces, etc. The network interface may optionally include standard wired interfaces, wireless interfaces (such as Wi-Fi interfaces), etc.

[0063] Those skilled in the art will understand that the physical equipment structure of the transient stability judgment method for a grid-connected converter hybrid system provided in this embodiment does not constitute a limitation on the physical equipment, and may include more or fewer components, or combine certain components, or have different component arrangements.

[0064] The storage medium may also include an operating system and a network communication module. The operating system is a program that manages the hardware and software resources of the aforementioned physical device, supporting the operation of information processing programs and other software and / or programs to be identified. The network communication module is used to enable communication between the various components within the storage medium, as well as communication with other hardware and software in the information processing physical device.

[0065] Through the above description of the implementation methods, those skilled in the art can clearly understand that this application can be implemented using software plus necessary general-purpose hardware platforms, or it can be implemented in hardware. By applying the technical solution of this application, virtual impedance is integrated into the branch of the grid-type converter as an additional control means that can be adjusted online. By establishing an equivalent model of the hybrid system including virtual impedance, and using the nodal impedance analysis method to incorporate virtual impedance into the network equation, the system impedance characteristics can be dynamically adjusted during faults, thereby optimizing the electrical coupling relationship between the grid-type and the hybrid converter, and making up for the defect that fixed parameters cannot adapt to variable operating conditions. On this basis, a Lyapunov energy function is constructed that simultaneously includes the power angle and phase-locked loop frequency of the grid-type converter, as well as the power angle, frequency, and voltage amplitude of the hybrid converter, to comprehensively reflect the dynamic coupling characteristics of the two. Then, the Lassel invariant set principle is used to determine the stability domain boundary of the hybrid system, elevating the stability judgment from a traditional qualitative description to a mathematical quantitative analysis. Finally, the critical value range of virtual impedance is calculated based on the changing trend of the stability domain boundary, and the specific target value is determined, directly outputting the parameter boundary. In summary, the above methods not only solve the technical problems of strong parameter dependence and lack of quantitative criteria in existing technologies, but also realize the quantitative judgment and parameter tuning of the transient stability of hybrid systems.

[0066] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of a preferred embodiment, and the modules or processes shown in the drawings are not necessarily essential for implementing this application. Those skilled in the art will understand that the modules in the apparatus of the embodiment can be distributed within the apparatus of the embodiment as described, or can be modified to be located in one or more apparatuses different from this embodiment. The modules of the above-described embodiment can be combined into one module, or further divided into multiple sub-modules.

[0067] The serial numbers in this application are for descriptive purposes only and do not represent the superiority or inferiority of any particular implementation scenario. The above disclosures are merely a few specific implementation scenarios of this application; however, this application is not limited thereto, and any variations conceived by those skilled in the art should fall within the protection scope of this application.

Claims

1. A method for determining the transient stability of a hybrid system with a grid-connected converter, characterized in that, include: Construct an equivalent model of a hybrid system, wherein the hybrid system includes a grid-connected converter, a grid-connected converter and a power grid, and a virtual impedance is connected in series in the branches of the grid-connected converter; The Lyapunov energy function of the hybrid system is constructed based on the equivalent model, wherein the independent variables of the Lyapunov energy function include the power angle and phase-locked loop frequency of the grid converter, as well as the power angle, frequency and voltage amplitude of the grid converter. When the derivative of the Lyapunov energy function with respect to time is less than or equal to 0, the stability domain boundary of the hybrid system is determined based on the Lyapunov energy function using the Lassell invariant set principle. The critical range of the virtual impedance is calculated based on the changing trend of the stability domain boundary, and the target value of the virtual impedance is determined within the critical range, wherein the target value is used to maintain the transient stability of the hybrid system.

2. The method according to claim 1, characterized in that, The equivalent model for constructing the hybrid system includes: The grid-connected converter is equivalent to a current source, wherein the amplitude of the current source is determined based on the output current amplitude of the grid-connected converter, and the phase angle of the current source is determined based on the phase angle difference between the output voltage and the output current of the grid-connected converter, and the phase angle difference between the output voltage of the grid-connected converter and the voltage of the grid. The grid-type converter is equivalent to a voltage source, wherein the amplitude of the voltage source is a reference voltage value, and the phase angle of the voltage source is the phase of the output voltage of the grid-type converter; Based on the current source and the voltage source, an equivalent model of the hybrid system is established using the nodal impedance analysis method, and the virtual impedance is added to the total impedance of the branch of the grid converter. The total impedance is obtained by superimposing the actual line impedance of the branch of the grid converter with the virtual impedance.

3. The method according to claim 1, characterized in that, The construction of the Lyapunov energy function of the hybrid system based on the equivalent model includes: Based on the phase-locked loop dynamic equation of the grid converter and the quadrature-axis component expression of the output voltage of the grid converter, the first partial derivative of the Lyapunov energy function with respect to the power angle of the grid converter is defined, and the second partial derivative of the Lyapunov energy function with respect to the phase-locked loop frequency of the grid converter is defined. Based on the active power equation of the power loop of the grid converter, the third partial derivative of the Lyapunov energy function with respect to the power angle of the grid converter is defined, and the fourth partial derivative of the Lyapunov energy function with respect to the frequency of the grid converter is defined. Based on the power loop reactive power equation of the grid converter, the fifth partial derivative of the Lyapunov energy function with respect to the voltage amplitude of the grid converter is defined. The expression for the Lyapunov energy function is obtained by integrating the first, second, third, fourth, and fifth partial derivatives using the method of undetermined gradients.

4. The method according to claim 3, characterized in that, The method further includes: During the transient process following a fault, the grid-connected converter adopts a pure reactive current injection mode to improve the transient stability of the grid-connected converter.

5. The method according to claim 1, characterized in that, The calculation of the critical range of the virtual impedance based on the changing trend of the stability region boundary includes: Based on the changing trend of the stability region boundary, the maximum stability region of the Lyapunov energy function is solved using the gradient method, and the upper limit value of the virtual impedance is obtained. The critical range of values ​​for the virtual impedance is determined based on the upper limit of the virtual impedance.

6. The method according to claim 5, characterized in that, The target value of the virtual impedance is greater than zero and less than or equal to the upper limit value.

7. The method according to any one of claims 1 to 6, characterized in that, The hybrid system also includes a first line impedance, a second line impedance, a common connection point, and a controller; The grid-connected converter is connected to the common connection point through the first line impedance. The grid-type converter is connected to the common connection point through the second line impedance and the virtual impedance; The controller is connected to the virtual impedance and is used to determine the target value of the virtual impedance within the critical range of the virtual impedance.

8. A transient stability judgment device for a hybrid system of grid-connected converters, characterized in that, The device includes: The model building module is used to build an equivalent model of the hybrid system, wherein the hybrid system includes a grid-connected converter, a grid-connected converter and a power grid, and the branches of the grid-connected converter are connected in series with virtual impedances; The function construction module is used to construct the Lyapunov energy function of the hybrid system according to the equivalent model, wherein the independent variables of the Lyapunov energy function include the power angle and phase-locked loop frequency of the grid converter, as well as the power angle, frequency and voltage amplitude of the grid converter; The boundary delineation module is used to determine the stability domain boundary of the hybrid system based on the Lyapunov energy function when the derivative of the Lyapunov energy function with respect to time is less than or equal to 0, using the Lassell invariant set principle. The value determination module is used to calculate the critical value range of the virtual impedance based on the changing trend of the stability domain boundary, and determine the target value of the virtual impedance within the critical value range, wherein the target value is used to maintain the transient stability of the hybrid system.

9. A storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, it implements the steps of the method according to any one of claims 1 to 7.

10. A computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the computer program is executed by a processor, it implements the steps of the method according to any one of claims 1 to 7.

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