A method and system for judging large signal stability of a direct current microgrid

Abstract

The present application relates to the technical field of direct current micro-grid, especially to a method and system for judging the large signal stability of direct current micro-grid, which comprises the following steps: firstly, simplifying and equivalent the direct current micro-grid to obtain the simplified equivalent diagram of the direct current micro-grid and the related formula of the simplified equivalent diagram; secondly, constructing the equivalent mathematical model of the direct current micro-grid system according to the related formula of the simplified equivalent diagram; then, constructing the state equation of the direct current micro-grid system according to the equivalent mathematical model; then, limiting the state equation by using the Lyapunov direct method to derive the stability constraint condition for judging the large signal stability of the direct current micro-grid; finally, setting the constant power load limit value according to the stability constraint condition, and designing the judgment process to judge the large signal stability of the direct current micro-grid by the constant power load limit value; the problem that the large signal stability of a single direct current micro-grid with comprehensive load cannot be simply and quickly judged is solved.

Description

Technical Field

[0001] This invention relates to the field of DC microgrid technology, and in particular to a method and system for judging the large-signal stability of DC microgrids. Background Technology

[0002] Loads in DC microgrids can be categorized into constant power loads and resistive loads based on whether they are connected to the bus via power electronic converters. Constant power loads, connected to the grid via converters, tend to consume constant power and exhibit negative impedance characteristics, thus reducing system damping and potentially leading to system instability. Therefore, DC microgrid systems commonly have both constant power and resistive loads connected. However, current stability analyses of DC microgrids typically only analyze one type of load (either resistive or constant power) (usually focusing only on constant power loads, which have a greater impact on system stability). The analysis fails to reveal the impact of load type and size on stability when both resistive and constant power loads coexist, thus limiting the applicability of the conclusions drawn.

[0003] More importantly, the current methods for judging the stability of large signals in DC microgrids have the following characteristics:

[0004] (1) The object of judgment is a DC microgrid cluster, such as a ring network structure composed of multiple small DC microgrids connected by connecting lines.

[0005] (2) The judgment method takes the mixed potential function as the core (often used to describe the interaction between multi-body systems) and analyzes the impact of constant power load on the stability of DC microgrid cluster.

[0006] In summary, because hybrid potential functions typically require adjusting the weights and parameters of multiple potential functions, which consumes significant time and computational resources and necessitates a deep understanding of the system, they often require the superposition of multiple potential functions of different forms. This complicates the interpretation of stability. Consequently, when analyzing and judging a single DC microgrid distributed over a small geographical area, the judgment method becomes overly redundant. Furthermore, if resistive loads are added for analysis, the adjustment process during function analysis becomes more difficult, further increasing the complexity of the judgment method. Summary of the Invention

[0007] To address the aforementioned shortcomings, the present invention aims to propose a method and system for judging the large-signal stability of a DC microgrid, thereby solving the problem of the inability to easily and quickly determine the large-signal stability of a single DC microgrid with a comprehensive load.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] A method for judging the stability of large signals in a DC microgrid includes the following steps:

[0010] A1: By simplifying the equivalent circuit of the DC microgrid through resistive load, bus capacitor, constant power load, constant power load power supply and drooping power supply, a simplified equivalent diagram of the DC microgrid and related relationships of the simplified equivalent diagram are obtained.

[0011] A2: Construct an equivalent mathematical model of the DC microgrid system based on the relevant relationships in the simplified equivalent diagram;

[0012] A3: Based on the equivalent mathematical model of the DC microgrid system, construct its state equations; then use the Lyapunov direct method to constrain its state equations and derive the stability constraints for judging the large-signal stability of the DC microgrid.

[0013] A4: Based on the stability constraints, set the constant power load limit value according to the categories, and design a judgment process to judge the large signal stability of the DC microgrid by the constant power load limit value.

[0014] Furthermore, step A1 includes the following sub-steps:

[0015] A11: Combine all m parallel-operating droop-controlled energy storage devices in the droop power supply into an equivalent energy storage system;

[0016] A12: Combine all parallel-operating bus capacitors into an equivalent bus capacitor C:

[0017] Formula (1): ;

[0018] In this DC microgrid, the number of bus capacitors is m, and the i-th bus capacitor is represented as... ;

[0019] A13: Combine all parallel-operating resistive loads into an equivalent resistive load R:

[0020] Formula (2): ;

[0021] In this DC microgrid, the number of resistive loads is k, and the j-th resistive load is represented as... ;

[0022] A14: All constant power supplies operating in parallel and constant power load Equivalent to constant power load current .

[0023] Furthermore, in sub-step A11, the equivalent energy storage system includes:

[0024] Formula (3): ;

[0025] Formula (4): ;

[0026] Formula (5): ;

[0027] Wherein, the equivalent droop coefficient is The equivalent inductance is L, and the equivalent output current is The rated voltage of the busbar is The bus voltage control command is In a drooping power supply, the number of droop factors or inductors is m, and the nth droop factor is represented as... The nth inductor is represented as .

[0028] Furthermore, step A2 includes the following sub-steps:

[0029] A21: Combining formulas (1) to (5), construct a reduced-order model of the DC microgrid;

[0030] A22: Define the state variables of the DC microgrid system, the steady-state operating point of the DC microgrid system, and the change in the steady-state operating point. Substitute these into the reduced-order model of the DC microgrid to construct the equivalent mathematical model of the DC microgrid system.

[0031] Furthermore, sub-step A21 includes: combining formulas (1) to (5), the simplified reduced-order model of the DC microgrid is as follows:

[0032] ;

[0033] Sub-step A22 includes: Let Let be the state variables of the DC microgrid system. Let be the steady-state operating point of the DC microgrid system. Substituting the change at the steady-state operating point into the reduced-order model of the DC microgrid, the equivalent mathematical model of the DC microgrid system can be expressed as:

[0034] ;

[0035] in, The characteristic matrix of the system is expressed as:

[0036] ;

[0037] in, It is the instantaneous value of the bus voltage. It is the effective value of the bus voltage.

[0038] Furthermore, step A3 includes the following sub-steps:

[0039] A31: Construct the state equations of a DC microgrid system based on its equivalent mathematical model;

[0040] A32: Reconstruct its state equations based on the Lyapunov function;

[0041] A33: Based on the criteria for determining the small-signal stability and large-signal stability of the system at the operating point in Lyapunov's direct method, the small-signal stability constraints of the reconstructed state equations are derived respectively.

[0042] A34: Using the small-signal stability constraint, the stability constraint for judging the large-signal stability of DC microgrids is derived.

[0043] Furthermore, step A4 includes the following sub-steps:

[0044] A41: Let the selection judgment parameter be set. Substituting the stability constraints, we obtain at least two constant power load limit values.

[0045] A42: Determine parameters based on selection Based on the constant power load limit value, a judgment process is designed, and DC microgrid parameters are collected and substituted into the selection of judgment parameters. Then, the selection and judgment parameters are determined. Select the corresponding constant power load limit value to determine the large signal stability of the DC microgrid.

[0046] Furthermore, sub-step A41 includes: setting selection judgment parameters. Substituting the stability constraints, we obtain the first constant power load limit value. Second constant power load limit value .

[0047] Furthermore, in step A42, the determination process includes the following steps:

[0048] B1: Collect parameters of resistive loads, bus capacitance, constant power loads, constant power load power sources, and drooping power sources in the DC microgrid, and calculate the equivalent droop coefficient. Equivalent bus capacitance C and equivalent resistive load R;

[0049] B2: Substitute the relevant parameters into the selection judgment parameters calculate;

[0050] B3: Judgment Does this hold true? If so, calculate the first constant power load limit value. If the condition is not met, proceed to step B4; otherwise, calculate the second constant power load limit value. And proceed to step B5;

[0051] B4: Determining a Constant Power Load Is it less than the first constant power load limit? If so, the large signal of the DC microgrid is stable; otherwise, the large signal of the DC microgrid is unstable.

[0052] B5: Determining a Constant Power Load Is it less than the second constant power load limit? If so, the large signal of the DC microgrid is stable; otherwise, the large signal of the DC microgrid is unstable.

[0053] A DC microgrid large-signal stability assessment system, applying the aforementioned DC microgrid large-signal stability assessment method, includes:

[0054] The first module is used to simplify and equivalence the data of resistive load, bus capacitor, constant power load, constant power load power supply and droop power supply to obtain the relevant relationship of simplified equivalent diagram;

[0055] The second module is used to construct the equivalent mathematical model of the DC microgrid system based on the relevant relationships of the simplified equivalent diagram;

[0056] The third module is used to construct the state equations of the DC microgrid system based on the equivalent mathematical model of the DC microgrid system; then, the Lyapunov direct method is used to constrain the state equations and derive the stability constraints for judging the large-signal stability of the DC microgrid.

[0057] The fourth module is used to classify and set constant power load limit values ​​according to stability constraints, and to design a judgment process based on the constant power load limit values ​​to determine the large-signal stability of the DC microgrid.

[0058] The technical solution provided by this invention can include the following beneficial effects: By simultaneously incorporating resistive and constant power loads in the simplified equivalent process of a DC microgrid, a simplified equivalent diagram of the DC microgrid is obtained. This allows for the analysis of the equivalent mathematical model of the DC microgrid system with comprehensive loads, providing a more comprehensive evaluation of the stability of large-signal DC microgrids. Furthermore, because DC microgrids are nonlinear and time-varying systems, the Lyapunov direct method is particularly suitable for judging the stability of individual DC microgrid systems. Therefore, based on the relevant relationships in the simplified equivalent diagram, an equivalent mathematical model of the DC microgrid system is constructed, thereby deriving the state equations. This facilitates the Lyapunov direct method in applying stability constraints to the state equations. After obtaining the stability constraints, it becomes easier to classify and evaluate the stability of the DC microgrid according to its actual conditions, making the judgment process simpler and more rigorous. Attached Figure Description

[0059] Figure 1 This is a flowchart of a method for judging the large signal stability of a DC microgrid, which is one embodiment of the present invention.

[0060] Figure 2 Is it like this? Figure 1 The equivalent process diagram of the DC microgrid in step A1 is shown.

[0061] Figure 3 Is it like this? Figure 2 The diagram shows the droop control principle of a single energy storage device in the equivalent energy storage system.

[0062] Figure 4 Is it like this? Figure 1 The flowchart of the judgment process in step A4 is shown.

[0063] Figure 5 This is the bus voltage waveform diagram for Experiment Case 1.

[0064] Figure 6 This is the bus voltage waveform diagram for Case 2.

[0065] Figure 7 This is a schematic diagram of a DC microgrid large signal stability judgment system according to one embodiment of the present invention. Detailed Implementation

[0066] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0067] In the description of embodiments of the present invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the stated features. In the description of embodiments of the present invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0068] In the description of the embodiments of the present invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of the present invention according to the specific circumstances.

[0069] The following is combined Figures 1 to 7This invention describes a method and system for judging the large-signal stability of a DC microgrid according to an embodiment of the present invention.

[0070] Example 1

[0071] A method for judging the stability of large signals in a DC microgrid includes the following steps:

[0072] A1: By simplifying the equivalent circuit of the DC microgrid through resistive load, bus capacitor, constant power load, constant power load power supply and drooping power supply, a simplified equivalent diagram of the DC microgrid and related relationships of the simplified equivalent diagram are obtained.

[0073] A2: Construct an equivalent mathematical model of the DC microgrid system based on the relevant relationships in the simplified equivalent diagram;

[0074] A3: Based on the equivalent mathematical model of the DC microgrid system, construct its state equations; then use the Lyapunov direct method to constrain its state equations and derive the stability constraints for judging the large-signal stability of the DC microgrid.

[0075] A4: Based on the stability constraints, set the constant power load limit value according to the categories, and design a judgment process to judge the large signal stability of the DC microgrid by the constant power load limit value.

[0076] In a preferred embodiment of the method for judging the large-signal stability of a DC microgrid proposed in this invention, such as... Figure 1 As shown, resistive loads and constant power loads are simultaneously added in the simplified equivalent process of a DC microgrid to obtain a simplified equivalent diagram of the DC microgrid. An equivalent mathematical model of the DC microgrid system with comprehensive loads is then constructed for analysis, providing a more comprehensive evaluation of the large-signal stability of the DC microgrid. Furthermore, because DC microgrids are nonlinear and time-varying systems, the Lyapunov direct method is particularly suitable for judging the stability of individual DC microgrid systems. Therefore, based on the relevant relationships in the simplified equivalent diagram, an equivalent mathematical model of the DC microgrid system is constructed, from which the state equations are derived. This facilitates the Lyapunov direct method in applying stability constraints to the state equations. After obtaining the stability constraints, it is easier to classify and evaluate the stability of the DC microgrid according to its actual situation, making the judgment process simpler and more rigorous.

[0077] Furthermore, step A1 includes the following sub-steps:

[0078] A11: Combine all m parallel-operating droop-controlled energy storage devices in the droop power supply into an equivalent energy storage system;

[0079] A12: Combine all parallel-operating bus capacitors into an equivalent bus capacitor C:

[0080] Formula (1): ;

[0081] In this DC microgrid, the number of bus capacitors is m, and the i-th bus capacitor is represented as... ;

[0082] A13: Combine all parallel-operating resistive loads into an equivalent resistive load R:

[0083] Formula (2): ;

[0084] In this DC microgrid, the number of resistive loads is k, and the j-th resistive load is represented as... ;

[0085] A14: All constant power supplies operating in parallel and constant power load Equivalent to constant power load current .

[0086] In this embodiment, the equivalent process of the DC microgrid is as follows: Figure 2 As shown, after circuit analysis, the m parallel-operating droop-controlled energy storage devices in the droop power supply are merged, and then the parallel-operating bus capacitors and resistive loads are merged respectively, based on all parallel-operating constant power supplies (power). and constant power loads (power) It can be simplified to an equivalent constant power load (power). Combined with the effective value of the bus voltage The constant power load current can be obtained. That is, the equivalent constant power load The current; thus simplified, we obtain Figure 2 The final simplified equivalent diagram facilitates the construction of an equivalent mathematical model for a DC microgrid system.

[0087] Furthermore, in sub-step A11, the equivalent energy storage system includes:

[0088] Formula (3): ;

[0089] Formula (4): ;

[0090] Formula (5): ;

[0091] Wherein, the equivalent droop coefficient is The equivalent inductance is L, and the equivalent output current is The rated voltage of the busbar is The bus voltage control command is In a drooping power supply, the number of droop factors or inductors is m, and the nth droop factor is represented as... The nth inductor is represented as .

[0092] In this embodiment, the equivalent energy storage system is ultimately simplified to a system with a droop coefficient of... The equivalent inductance is L and the equivalent output current is The relationship between the three parameters is expressed as follows: Figure 3 The derivation process of the droop control principle diagram shown is as follows:

[0093] In a droop power supply, the bus voltage of energy storage devices (or energy storage converters) operating in parallel varies with the load power according to the droop control. Therefore, in a system with multiple energy storage devices operating in parallel, the energy storage devices can be further equivalently represented. Typically, r is much smaller than k (r is the line resistance between the energy storage device and the load point, and k is the virtual resistance of the droop control). Therefore, the line resistance is generally ignored, and only the droop factor is considered. The virtual resistance k and the line resistance r are combined into a single virtual resistance k, which is the droop factor of the energy storage device. Therefore, the mathematical model of the nth energy storage device can be set as follows:

[0094] Formula (6): ;

[0095] The current output to the busbar by the nth energy storage device is expressed as: ;

[0096] make The equivalent output current of the equivalent energy storage system can be expressed as:

[0097] Formula (7): ;

[0098] Then, from formula (7), the droop coefficient of m parallel-operated energy storage devices in the droop power source can be obtained, and the equivalent droop coefficient after merging can be obtained. for:

[0099] Formula (3): ;

[0100] When m energy storage devices using droop control are connected in parallel, to prevent the smaller capacity energy storage device from reaching an excessively high or low charge state first, thus causing the energy storage device to shut down, the droop coefficient of the grid-connected converter of the energy storage device usually needs to be set to meet the following requirements:

[0101] ;

[0102] In practice, the filter inductance and droop coefficient of a grid-connected converter typically have the following relationship:

[0103] ;

[0104] Therefore, the equivalent inductance of an energy storage device can be written as:

[0105] Formula (4): ;

[0106] In summary, combining formulas (3), (4), (6), and (7), the equivalent output current of energy storage devices operating in parallel using droop control is... It can be rewritten as:

[0107] ;

[0108] Simplifying, we get:

[0109] Formula (5): .

[0110] Furthermore, step A2 includes the following sub-steps:

[0111] A21: Combining formulas (1) to (5), construct a reduced-order model of the DC microgrid;

[0112] A22: Define the state variables of the DC microgrid system, the steady-state operating point of the DC microgrid system, and the change in the steady-state operating point. Substitute these into the reduced-order model of the DC microgrid to construct the equivalent mathematical model of the DC microgrid system.

[0113] In this embodiment, based on obtaining the simplified equivalent diagram of the DC microgrid and the relevant relationships of the simplified equivalent diagram, a reduced-order model of the DC microgrid can be constructed to simplify the analysis and improve the computational efficiency.

[0114] Meanwhile, since the reduced-order model is a model expressed using the state-space method, by setting the state variables of the DC microgrid system, the steady-state operating point of the DC microgrid system, and the change in the steady-state operating point, and substituting them into the reduced-order model of the DC microgrid, an equivalent mathematical model of the DC microgrid system can be constructed. This equivalent mathematical model of the DC microgrid system can reflect the overall working state of the DC microgrid system, so as to facilitate the judgment of its large signal stability.

[0115] Furthermore, sub-step A21 includes: combining formulas (1) to (5), the simplified reduced-order model of the DC microgrid is as follows:

[0116] ;

[0117] Sub-step A22 includes: Let Let be the state variables of the DC microgrid system. Let be the steady-state operating point of the DC microgrid system. Substituting the change at the steady-state operating point into the reduced-order model of the DC microgrid, the equivalent mathematical model of the DC microgrid system can be expressed as:

[0118] ;

[0119] in, The characteristic matrix of the system is expressed as:

[0120] ;

[0121] in, It is the instantaneous value of the bus voltage. It is the effective value of the bus voltage.

[0122] In this embodiment, combining formulas (1) to (5), the simplified reduced-order model of the DC microgrid is preferably: This facilitates subsequent classification and judgment of the large-signal stability of the DC microgrid; furthermore, by setting the state variables, steady-state operating point, and change of steady-state operating point of the DC microgrid system in coordinate form, and substituting them into the reduced-order model of the DC microgrid, the equivalent mathematical model of the DC microgrid system is obtained, which can be used to describe the system state.

[0123] Furthermore, step A3 includes the following sub-steps:

[0124] A31: Construct the state equations of a DC microgrid system based on its equivalent mathematical model;

[0125] A32: Reconstruct its state equations based on the Lyapunov function;

[0126] A33: Based on the criteria for determining the small-signal stability and large-signal stability of the system at the operating point in Lyapunov's direct method, the small-signal stability constraints of the reconstructed state equations are derived respectively.

[0127] A34: Using the small-signal stability constraint, the stability constraint for judging the large-signal stability of DC microgrids is derived.

[0128] In this embodiment, the Lyapunov direct method, for a given nonlinear system, if its corresponding Lyapunov function... If the value is positive definite and its derivative is negative definite, then the system can be considered small-signal stable at the operating point; when If the system is asymptotically stable for large signals, then it can be considered to be asymptotically stable for large signals. Based on this, by reconstructing the state equation of the equivalent mathematical model of the DC microgrid system using the Lyapunov function (i.e., limiting its state equation), we can first obtain the judgment condition for the small-signal stability of the DC microgrid, and then further derive the judgment condition for the large-signal stability (i.e., the stability constraint condition for judging the large-signal stability of the DC microgrid).

[0129] Specifically, the derivation process of the stability constraints (i.e., step A3) is as follows:

[0130] The state equation of the equivalent mathematical model of a DC microgrid system can be expressed as:

[0131] ;

[0132] ;

[0133] in, The expression is:

[0134] ;

[0135] Therefore, the Lyapunov function can be constructed as

[0136] ;

[0137] in, It is a positive definite matrix, therefore it can be guaranteed that... It is also a positive definite matrix. The total derivative can be expressed as:

[0138] ;

[0139] ;

[0140] A system with large-signal stability is necessarily asymptotically stable with small-signal stability, therefore it is necessary to ensure... The negative definiteness. Therefore, will Let it be the identity matrix, i.e. ,So The following results can be obtained:

[0141] ;

[0142] in, and They are The determinant and the trace of the matrix, It is the identity matrix; The symbolic expression is:

[0143] ;

[0144] ;

[0145] ;

[0146] Lyapunov's stability conditions require For a matrix to be positive definite, it must satisfy the following conditions.

[0147] ;

[0148] After rearranging, we can obtain the stability constraint ① (i.e., the small-signal stability constraint):

[0149] ;

[0150] According to the principle of Lyapunov's direct method, the system is asymptotically stable for small signals when condition ① is satisfied; however, to ensure the system's stability for large signals, it is necessary to prove that when... When, its Lyapunov function satisfies To further derive the large-signal stability criterion for DC microgrid systems, It can be rewritten as:

[0151] ;

[0152] From the two formulas above, we can deduce that there exists... and .therefore, It can be represented as:

[0153] ;

[0154] As can be seen from the above formula, when Sometimes, Therefore, under stability constraint ①, the DC microgrid system is large-signal stable.

[0155] However, the stability criterion derived through Lyapunov's direct method is conservative; that is, stability constraint ① can only represent a sufficient condition for system stability, not a necessary condition. Therefore, it is necessary to verify the necessity of the stability criterion. For the equivalent second-order model of the system, the necessary and sufficient condition for its small-signal stability is:

[0156] ;

[0157] ;

[0158] This can be further simplified to stability constraint ② (i.e., stability constraint):

[0159] .

[0160] It should be noted that the mathematical expressions for stability constraint ① and stability constraint ② are consistent, meaning that stability constraint ① is a necessary and sufficient condition for the small-signal stability of the system. If the large-signal stability of the DC microgrid is stable, then its small-signal stability is also necessary and sufficient; therefore, stability constraint ① is also a necessary and sufficient condition for the large-signal stability of the system.

[0161] In summary, only when the relevant data in the simplified equivalent diagram are substituted into the stability constraints for calculation, and the calculation results meet the requirements of the stability constraints, can it be said that the large signal of the DC microgrid is stable.

[0162] Furthermore, step A4 includes the following sub-steps:

[0163] A41: Let the selection judgment parameter be set. Substituting the stability constraints, we obtain at least two constant power load limit values.

[0164] A42: Determine parameters based on selection Based on the constant power load limit value, a judgment process is designed, and DC microgrid parameters are collected and substituted into the selection of judgment parameters. Then, the selection and judgment parameters are determined. Select the corresponding constant power load limit value to determine the large signal stability of the DC microgrid.

[0165] In this embodiment, selection judgment parameters are set. Based on the stability constraints [Formula (8)], the limit values ​​of constant power loads are set according to categories, and the selection of judgment parameters can be used to determine the appropriate parameters. (Acquire DC microgrid data and substitute it into the selection and judgment parameters) (It was later learned) that the constant power load limit value required for the DC microgrid can more rigorously and accurately determine the stability of large signals in the DC microgrid, thus improving the flexibility of applying stability constraints.

[0166] Furthermore, sub-step A41 includes: setting selection judgment parameters. Substituting the stability constraints, we obtain the first constant power load limit value. Second constant power load limit value .

[0167] In this embodiment, based on the stability constraint condition [Equation (8)] which contains two inequalities, the large-signal stability of the DC microgrid should be judged according to the most stringent inequality, based on the actual situation of the DC microgrid. Furthermore, due to the stability constraint condition... It's a variable, making comparisons difficult; it should be... The system parameters and load can be used to eliminate the problem equivalently; therefore, selection and judgment parameters can be set. Substituting the stability constraints, we obtain the first constant power load limit value. Second constant power load limit value To eliminate And the inequalities for choosing stability constraints.

[0168] Specifically, based on the reduced-order model of the equivalent DC microgrid, the steady-state operating point of the system can be obtained. Let [the equation be inserted here]. The effective value of the bus voltage at the steady-state operating point and equivalent output current for:

[0169] ;

[0170] make and the above Substituting the expression into the stability constraints, we get:

[0171] ;

[0172] when When the stability constraint condition is met, the first inequality is a more stringent condition. The stability of the large-signal DC microgrid should be determined according to the first inequality, thus yielding the first constant power load limit value. ;

[0173] when At that time, the second inequality of the stability constraint is a more stringent judgment condition. The stability of the large signal of the DC microgrid should be judged according to the second inequality, and then the second constant power load limit value is obtained. .

[0174] Furthermore, in step A42, the judgment process includes the following steps:

[0175] B1: Collect parameters of resistive loads, bus capacitance, constant power loads, constant power load power sources, and drooping power sources in the DC microgrid, and calculate the equivalent droop coefficient. Equivalent bus capacitance C and equivalent resistive load R;

[0176] B2: Substitute the relevant parameters into the selection judgment parameters calculate;

[0177] B3: Judgment Does this hold true? If so, calculate the first constant power load limit value. If the condition is not met, proceed to step B4; otherwise, calculate the second constant power load limit value. And proceed to step B5;

[0178] B4: Determining a Constant Power Load Is it less than the first constant power load limit? If so, the large signal of the DC microgrid is stable; otherwise, the large signal of the DC microgrid is unstable.

[0179] B5: Determining a Constant Power Load Is it less than the second constant power load limit? If so, the large signal of the DC microgrid is stable; otherwise, the large signal of the DC microgrid is unstable.

[0180] In this embodiment, as Figure 4 As shown, based on this judgment process, judgment parameters can be selected quickly. Select the constant power load limit value and determine whether the large signal of the DC microgrid is stable.

[0181] To verify the accuracy of the above-mentioned method for judging the large-signal stability of DC microgrids, a hardware-in-the-loop system (simulating a DC microgrid) was built using the PLECS RT Box experimental platform and an STM32G474RE controller for experimental verification. The experimental platform consists of a main circuit and a controller.

[0182] The main circuit section is built using the PLECS simulation platform, and the RT Box simulates the power stage of the main circuit system. The complete controller can be tested without a real power stage. The analog signals of electrical quantities such as the output current of the dispatchable power supply of the DC microgrid, the bus voltage, the DC / DC converter (energy storage device), the inductance, and the current are output to the STM32G474RE controller through the analog output channel of the RT Box.

[0183] The control section of the DC microgrid mainly consists of a droop control algorithm for the dispatchable power supply. The control algorithm and parameters are compiled into an STM32G474RE controller. The controller generates PWM digital signals to control the semiconductor devices and outputs them to the RT Box via a digital signal channel. The PWM signals can be captured through the digital input of the RT Box with a time resolution of less than 10 nanoseconds. Using the captured input data, the RT Box simulates the power stage and provides simulation results through its analog output a few microseconds later.

[0184] The DC microgrid system used as an experimental case includes two parallel DC / DC converters, an LC filter, a constant power load, and a resistive load. The system component parameters and control parameters are set according to Table 1.

[0185] Table 1 Case Parameter Settings

[0186]

[0187] The constant power load adopts a controlled current source equivalent constant power load, and the resistive load is set to 5Ω. The constant power load commands for the two experimental cases, Case 1 and Case 2, and the bus voltage and the steady state of the system operation under different load commands are shown in the table below.

[0188] Table 2 Experimental Load Commands and Operating Status Table

[0189]

[0190] in, P is the power of a constant power load. R This represents the power of the resistive load.

[0191] In a DC microgrid, the bus voltage is a crucial indicator of the system's operating status, reflecting both the droop control principle and the system's voltage deviation and stability. Therefore, the bus voltage performance is a key focus. The bus voltage waveforms in experimental cases Case 1 and Case 2 are shown below. Figure 5 and 6 As shown.

[0192] As shown in the experimental waveforms, in Case 1 and Case 2, the bus voltage gradually decreases as the load power increases. Furthermore, the load commands are consistent in the first three time stages of both Case 1 and Case 2, and the droop coefficients are identical in both cases. The decrease in bus voltage with increasing load is consistent in the experimental graphs, indicating that the bus voltage variation conforms to the droop control principle. The experimental bus voltage values ​​are close to the theoretical values; the errors are mainly caused by converter switching losses.

[0193] The component parameter values ​​of the DC microgrids in Case 1 and Case 2 are different, therefore the stability limits of the constant power load in the system are different. Based on the stability calculation process (i.e., the judgment process), Figure 4 The constant power load stability limit for Case 1 is 30kW, and for Case 2 it is 24.3kW. Therefore, 25kW is still within the stable range for Case 1, but it is sufficient to cause the system to become unstable in Case 2. This is reflected in the experimental waveforms: in stage ④ of Case 1 and stage ⑤ of Case 2, the constant power load command is 25kW, but the system in Case 1 can still maintain stable operation, while the system in Case 2 becomes unstable. Thus, the experimental waveforms demonstrate the feasibility of the above method for judging the large-signal stability of DC microgrids.

[0194] Example 2

[0195] A DC microgrid large-signal stability assessment system, applying the aforementioned DC microgrid large-signal stability assessment method, includes:

[0196] The first module is used to simplify and equivalence the data of resistive load, bus capacitor, constant power load, constant power load power supply and droop power supply to obtain the relevant relationship of simplified equivalent diagram;

[0197] The second module is used to construct the equivalent mathematical model of the DC microgrid system based on the relevant relationships of the simplified equivalent diagram;

[0198] The third module is used to construct the state equations of the DC microgrid system based on the equivalent mathematical model of the DC microgrid system; then, the Lyapunov direct method is used to constrain the state equations and derive the stability constraints for judging the large-signal stability of the DC microgrid.

[0199] The fourth module is used to classify and set constant power load limit values ​​according to stability constraints, and to design a judgment process based on the constant power load limit values ​​to determine the large-signal stability of the DC microgrid.

[0200] In this embodiment, a preferred embodiment of a DC microgrid large-signal stability judgment system is also proposed, such as... Figure 7 As shown, the system integrates the aforementioned method for judging the stability of large signals in a DC microgrid, enabling it to directly determine the stability of the large signals of the DC microgrid after collecting relevant data and inputting it into the system. Specifically, the system mainly allocates the first to fourth modules to handle the logic and calculations of each step according to steps A1 to A4 of the method for judging the stability of large signals in a DC microgrid. It should be noted that the sub-steps in each step can be further subdivided into sub-modules according to the actual situation, or they can be executed uniformly by the main module without sub-modules, which is not limited here.

[0201] The other components and operation of the method for determining the large-signal stability of a DC microgrid according to embodiments of the present invention are known to those skilled in the art and will not be described in detail here.

[0202] In the description of this specification, references to terms such as "embodiment," "example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0203] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for judging large signal stability of a direct current microgrid, characterized in that: Includes the following steps: A1: By simplifying the equivalent circuit of the DC microgrid through resistive load, bus capacitor, constant power load, constant power load power supply and drooping power supply, a simplified equivalent diagram of the DC microgrid and related relationships of the simplified equivalent diagram are obtained. A2: Construct an equivalent mathematical model of the DC microgrid system based on the relevant relationships in the simplified equivalent diagram; A3: Based on the equivalent mathematical model of the DC microgrid system, construct its state equations; then use the Lyapunov direct method to constrain its state equations and derive the stability constraints for judging the large-signal stability of the DC microgrid. A4: Based on the stability constraints, the constant power load limit values ​​are set according to categories, and a judgment process is designed accordingly to judge the large-signal stability of the DC microgrid by the constant power load limit values. Step A1 includes the following sub-steps: A11: Combine all m parallel-operating droop-controlled energy storage devices in the droop power supply into an equivalent energy storage system; A12: Combine all parallel-operating bus capacitors into an equivalent bus capacitor C: Equation (1): ; Wherein, the number of bus capacitors in the direct current micro-grid is m, and the i-th bus capacitor is represented as ; A13: Combine all parallel-operating resistive loads into an equivalent resistive load R: Equation (2): ; Wherein, the number of resistance load in the direct current micro grid is k, and the jth resistance load is represented as ; A14: All constant power supplies operating in parallel and constant power load Equivalent to constant power load current ; In sub-step A11, the equivalent energy storage system includes: Official (3): ; Official (4): ; Official (5): ; Wherein, the equivalent droop coefficient is The equivalent inductance is L, and the equivalent output current is The rated voltage of the busbar is The bus voltage control command is In a drooping power supply, the number of droop factors or inductors is m, and the nth droop factor is represented as... The nth inductor is represented as ; Step A4 includes the following sub-steps: A41: Let the selection judgment parameter be set. Substituting the stability constraints, we obtain at least two constant power load limit values. A42: Determine parameters based on selection Based on the constant power load limit value, a judgment process is designed, and DC microgrid parameters are collected and substituted into the selection of judgment parameters. Then, the selection and judgment parameters are determined. Select the corresponding constant power load limit value to determine the large signal stability of the DC microgrid; Sub-step A41 includes: setting selection judgment parameters Substituting the stability constraints: ； Obtain the first constant power load limit value Second constant power load limit value .

2. The method for judging the large-signal stability of a DC microgrid according to claim 1, characterized in that: Step A2 includes the following sub-steps: A21: Combining formulas (1) to (5), construct a reduced-order model of the DC microgrid; A22: Define the state variables of the DC microgrid system, the steady-state operating point of the DC microgrid system, and the change in the steady-state operating point. Substitute these into the reduced-order model of the DC microgrid to construct the equivalent mathematical model of the DC microgrid system.

3. The method for judging the large-signal stability of a DC microgrid according to claim 2, characterized in that: Sub-step A21 includes: combining formulas (1) to (5), the simplified reduced-order model of the DC microgrid is as follows: ； Sub-step A22 includes: Let Let be the state variables of the DC microgrid system. Let be the steady-state operating point of the DC microgrid system. Substituting the change at the steady-state operating point into the reduced-order model of the DC microgrid, the equivalent mathematical model of the DC microgrid system can be expressed as: ； in, The characteristic matrix of the system is expressed as: ； in, It is the instantaneous value of the bus voltage. It is the effective value of the bus voltage.

4. The method for judging the large-signal stability of a DC microgrid according to claim 3, characterized in that: Step A3 includes the following sub-steps: A31: Construct the state equations of a DC microgrid system based on its equivalent mathematical model; A32: Reconstruct its state equations based on the Lyapunov function; A33: Based on the criteria for determining the small-signal stability and large-signal stability of the system at the operating point in Lyapunov's direct method, the small-signal stability constraints of the reconstructed state equations are derived respectively. A34: Using the small-signal stability constraint, the stability constraint for judging the large-signal stability of DC microgrids is derived.

5. The method for judging the large-signal stability of a DC microgrid according to claim 1, characterized in that: In step A42, the determination process includes the following steps: B1: Collect parameters of resistive loads, bus capacitance, constant power loads, constant power load power sources, and drooping power sources in the DC microgrid, and calculate the equivalent droop coefficient. Equivalent bus capacitance C and equivalent resistive load R; B2: Substitute the relevant parameters into the selection judgment parameters calculate; B3: Judgment Does this hold true? If so, calculate the first constant power load limit value. If the condition is not met, proceed to step B4; otherwise, calculate the second constant power load limit value. And proceed to step B5; B4: Determining a Constant Power Load Is it less than the first constant power load limit? If so, the large signal of the DC microgrid is stable; otherwise, the large signal of the DC microgrid is unstable. B5: Determining a Constant Power Load Is it less than the second constant power load limit? If so, the large signal of the DC microgrid is stable; otherwise, the large signal of the DC microgrid is unstable.

6. A system for judging the stability of large signals in a DC microgrid, characterized in that: The method for determining the large-signal stability of a DC microgrid according to any one of claims 1 to 5 includes: The first module is used to simplify and equivalence the data of resistive load, bus capacitor, constant power load, constant power load power supply and droop power supply to obtain the relevant relationship of simplified equivalent diagram; The second module is used to construct the equivalent mathematical model of the DC microgrid system based on the relevant relationships of the simplified equivalent diagram; The third module is used to construct the state equations of the DC microgrid system based on the equivalent mathematical model of the DC microgrid system; then, the Lyapunov direct method is used to constrain the state equations and derive the stability constraints for judging the large-signal stability of the DC microgrid. The fourth module is used to classify and set constant power load limit values ​​according to stability constraints, and to design a judgment process based on the constant power load limit values ​​to determine the large-signal stability of the DC microgrid.

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