Method and system for judging stability of large signal of direct-current micro-grid

Abstract

The invention relates to the technical field of direct-current micro-grids, in particular to a direct-current micro-grid large-signal stability judgment method and system, and the method comprises the steps: firstly carrying out the simplification and equivalence of a direct-current micro-grid, and obtaining a simplified equivalent diagram of the direct-current micro-grid, and a correlation expression of the simplified equivalent diagram; secondly, constructing an equivalent mathematical model of the direct-current micro-grid system according to a simplified equivalent diagram correlation expression; then, according to the equivalent mathematical model of the DC micro-grid system, constructing a state equation of the DC micro-grid system; then a Lyapunov direct method is utilized to limit a state equation, and a stability constraint condition for judging the stability of the large signal of the direct-current micro-grid is deduced; and finally, according to a stability constraint condition, constant power load limit values are set in a classified manner, a judgment process is designed, and the large signal stability of the direct-current micro-grid is judged according to the constant power load limit values. The problem that the large-signal stability of the single direct-current micro-grid with the comprehensive load cannot be simply and quickly judged is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current microgrids, and in particular to a method and system for judging large signal stability of a direct current microgrid. Background Art

[0002] The loads in the DC microgrid can be divided into constant power loads and resistive loads according to whether they are connected to the busbar by power electronic converters. The constant power load is connected to the grid by the converter, tends to consume constant power, and exhibits negative impedance characteristics, thereby reducing system damping and even causing system instability. Therefore, DC microgrid systems generally have two types of loads connected: constant power loads and resistive loads. However, when performing DC microgrid stability analysis, only one of the load types, resistive loads or constant power loads, is analyzed (usually only constant power loads that have a greater impact on system stability are analyzed), and the impact of load type and size on stability when resistive loads and constant power loads coexist is not revealed, so the applicability of its conclusions is limited.

[0003] More importantly, the current DC microgrid large signal stability judgment method has the following characteristics:

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

[0005] (2) The judgment method is based on the hybrid potential function (commonly used to describe the interaction between multi-body systems) to analyze the impact of constant power load on the stability of the DC microgrid cluster.

[0006] Based on the above characteristics, the hybrid potential function usually needs to adjust the weights and parameters of multiple potential functions, which takes a lot of time and computing resources, and requires an in-depth understanding of the system. Therefore, it requires the superposition of multiple potential functions in different forms, which makes the interpretation of stability complicated; therefore, when analyzing and judging a single DC microgrid distributed in a small geographical area, the judgment method is too redundant, and if the resistive load is added for analysis, the function analysis process is difficult to adjust, and the complexity of the judgment method is further increased. Summary of the invention

[0007] In view of the above defects, the purpose of the present invention is to propose a method and system for judging the large signal stability of a DC microgrid, which solves the problem that it is impossible to simply and quickly judge the large signal stability of a single DC microgrid with a comprehensive load.

[0008] To achieve this object, the present invention adopts the following technical solutions:

[0009] A method for judging large signal stability of a DC microgrid comprises the following steps:

[0010] A1: Simplify and equalize the resistive load, busbar capacitance, constant power load, constant power load power supply and droop power supply to obtain the simplified equivalent diagram of the DC microgrid and the related relationship of the simplified equivalent diagram;

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

[0012] A3: Based on the equivalent mathematical model of the DC microgrid system, its state equation is constructed; then the Lyapunov direct method is used to limit its state equation and derive the stability constraint conditions for judging the large signal stability of the DC microgrid;

[0013] A4: According to the stability constraints, the constant power load limit values ​​are set by category, and the judgment process is designed accordingly to judge the large signal stability of the DC microgrid based on the constant power load limit values.

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

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

[0016] A12: All bus capacitors running in parallel are combined into an equivalent bus capacitor C:

[0017] Formula (1):

[0018] Among them, the number of bus capacitors in the DC microgrid is m, and the i-th bus capacitor is represented by C i ;

[0019] A13: All the resistance loads running in parallel are combined into an equivalent resistance load R:

[0020] Formula (2):

[0021] Among them, the number of resistive loads in the DC microgrid is k, and the jth resistive load is represented by R j ;

[0022] A14: All constant power sources P connected in parallel cps And constant power load P cpl Equivalent to constant power load current I cpl .

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

[0024] Formula (3):

[0025] Formula (4):

[0026] Formula (5):

[0027] Among them, the equivalent droop coefficient is k d , the equivalent inductance is L, the equivalent output current is i dc , the busbar rated voltage is U N , the bus voltage control command is u ref ; The number of droop coefficients or inductances in a droop power supply is m, and the nth droop coefficient is k n , the nth inductor is represented by L n .

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

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

[0030] A22: Assume the state variables of the DC microgrid system, the steady-state operating point of the DC microgrid system, and the change of the steady-state operating point, substitute them into the reduced-order model of the DC microgrid, and construct an equivalent mathematical model of the DC microgrid system.

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

[0032]

[0033] The sub-step A22 includes: setting x=[i s ,u bus ] T is the state variable of the DC microgrid system, let x e =[i se ,u buse ] T is the steady-state operating point of the DC microgrid system, let is the change in the steady-state operating point. Substituting it into the reduced-order model of the DC microgrid, the equivalent mathematical model of the DC microgrid system can be expressed as:

[0034]

[0035] Among them, J is the characteristic matrix of the system, expressed as:

[0036]

[0037] Among them, u bus is the instantaneous value of the bus voltage, U bus is the effective value of the bus voltage.

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

[0039] A31: Based on the equivalent mathematical model of the DC microgrid system, construct its state equation;

[0040] A32: Reconstruct its state equation according to Lyapunov function;

[0041] A33: Based on the recognition criteria of 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 equation are derived respectively;

[0042] A34: Using the small signal stability constraints, derive the stability constraints for determining the large signal stability of the DC microgrid.

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

[0044] A41: Assume the selection judgment parameter α, substitute it into the stability constraint condition, and obtain at least two constant power load limit values;

[0045] A42: Based on the selection judgment parameter α and the constant power load limit value, a judgment process is designed. After the collected DC microgrid parameters are substituted into the selection judgment parameter α, the corresponding constant power load limit value is selected by the selection judgment parameter α to judge the large signal stability of the DC microgrid.

[0046] Further, the sub-step A41 includes: setting the selection judgment parameter Substituting the stability constraint, we get the first constant power load limit value and the second constant power load limit

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

[0048] B1: Collect the parameters of the resistive load, bus capacitance, constant power load, constant power load power supply and droop power supply in the DC microgrid, and calculate the equivalent droop coefficient k d , equivalent bus capacitance C and equivalent resistance load R;

[0049] B2: Substitute the relevant parameters into the selection judgment parameter α for calculation;

[0050] B3: Determine whether α≤1 is true; if so, calculate the first constant power load limit value P cpl.limit1 , and execute step B4; otherwise calculate the second constant power load limit value P cpl.limit2 , and execute step B5;

[0051] B4: Determine the constant power load Pcpl Is it less than the first constant power load limit value P cpl.limit1 If so, the large signal of the DC microgrid is stable, otherwise the large signal of the DC microgrid is unstable;

[0052] B5: Determine the constant power load P cpl Is it less than the second constant power load limit value P cpl.limit2 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 judgment system, using the above-mentioned DC microgrid large signal stability judgment method, comprises:

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

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

[0056] The third module is used to construct the state equation according to the equivalent mathematical model of the DC microgrid system; then the state equation is limited by the Lyapunov direct method to derive the stability constraint conditions for judging the large signal stability of the DC microgrid;

[0057] The fourth module is used to classify and set the constant power load limit value according to the stability constraint conditions, thereby designing a judgment process to judge the large signal stability of the DC microgrid based on the constant power load limit value.

[0058] The technical solution provided by the present invention may include the following beneficial effects: adding a resistive load and a constant power load simultaneously in the simplified equivalent process of a DC microgrid, obtaining a simplified equivalent diagram of the DC microgrid, analyzing the equivalent mathematical model of a DC microgrid system constructed for a DC microgrid with a comprehensive load, and more comprehensively evaluating the stability of a large signal of a DC microgrid. Because a DC microgrid is a nonlinear system and a time-varying system, the Lyapunov direct method is particularly suitable for judging the stability of a single DC microgrid system. Therefore, according to the simplified equivalent diagram correlation formula, an equivalent mathematical model of a DC microgrid system is constructed to derive a state equation, which is convenient for the Lyapunov direct method to limit the stability of the state equation. After the stability constraint condition is obtained, it is easier to classify and evaluate the stability of the DC microgrid according to the actual situation, and the judgment process is simpler and more rigorous. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 It is a flow chart of a method for judging large signal stability of a DC microgrid according to one embodiment of the present invention.

[0060] Figure 2 Yes Figure 1 The DC microgrid equivalent process diagram of step A1 is shown.

[0061] Figure 3 Yes Figure 2 Schematic diagram of droop control for a single energy storage device in the equivalent energy storage system shown.

[0062] Figure 4 Yes Figure 1 The flowchart of the judgment process in step A4 is shown.

[0063] Figure 5 This is the bus voltage experimental waveform of experimental case Case 1.

[0064] Figure 6 This is the bus voltage experimental waveform of experimental case Case 2.

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

[0066] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0067] In the description of the embodiments of the present invention, the terms "first" and "second" are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the embodiments of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0068] In the description of the embodiments of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected or indirectly connected through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the embodiments of the present invention can be understood according to specific circumstances.

[0069] Combine the following Figures 1 to 7 , describing a DC microgrid large signal stability judgment method and system according to an embodiment of the present invention.

[0070] Example 1

[0071] A method for judging large signal stability of a DC microgrid comprises the following steps:

[0072] A1: Simplify and equalize the resistive load, busbar capacitance, constant power load, constant power load power supply and droop power supply to obtain the simplified equivalent diagram of the DC microgrid and the related relationship of the simplified equivalent diagram;

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

[0074] A3: Based on the equivalent mathematical model of the DC microgrid system, its state equation is constructed; then the Lyapunov direct method is used to limit its state equation and derive the stability constraint conditions for judging the large signal stability of the DC microgrid;

[0075] A4: According to the stability constraints, the constant power load limit values ​​are set by category, and the judgment process is designed accordingly to judge the large signal stability of the DC microgrid based on the constant power load limit values.

[0076] The present invention proposes a preferred embodiment of a method for judging the stability of a large signal of a DC microgrid, such as Figure 1 As shown in the figure, in the simplified equivalent process of the DC microgrid, a resistive load and a constant power load are added at the same time to obtain a simplified equivalent diagram of the DC microgrid, and an equivalent mathematical model of the DC microgrid system is constructed for the DC microgrid with a comprehensive load to analyze the stability of the DC microgrid with large signals more comprehensively. Because the DC microgrid is a nonlinear system and a time-varying system, the Lyapunov direct method is particularly suitable for judging the stability of a single DC microgrid system. Therefore, according to the simplified equivalent diagram correlation formula, an equivalent mathematical model of the DC microgrid system is constructed to derive the state equation, which is convenient for the Lyapunov direct method to limit the stability of the state equation. After the stability constraint conditions are obtained, it is easier to classify and evaluate the stability of the DC microgrid according to the actual situation, and the judgment process is simpler and more rigorous.

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

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

[0079] A12: All bus capacitors running in parallel are combined into an equivalent bus capacitor C:

[0080] Formula (1):

[0081] Among them, the number of bus capacitors in the DC microgrid is m, and the i-th bus capacitor is represented by C i ;

[0082] A13: All the resistance loads running in parallel are combined into an equivalent resistance load R:

[0083] Formula (2):

[0084] Among them, the number of resistive loads in the DC microgrid is k, and the jth resistive load is represented by R j ;

[0085] A14: All constant power sources P connected in parallel cps And constant power load P cpl Equivalent to constant power load current I cpl .

[0086] In this embodiment, the equivalent process of the DC microgrid is as follows: Figure 2 As shown, after circuit analysis, the m parallel-operated droop-controlled energy storage devices in the droop power supply are merged, and then the bus capacitors and resistive loads running in parallel are merged respectively, and based on the constant power supply (power) P of all parallel-operated cps and constant power load (power) P cpl It can be simplified to a constant power load (power) P cpl , combined with the effective value of the bus voltage U bus , we can get the constant power load current That is, the equivalent constant power load P cpl The current is thus simplified to Figure 2 The final simplified equivalent diagram makes it easier to construct an equivalent mathematical model of the 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] Among them, the equivalent droop coefficient is k d , the equivalent inductance is L, the equivalent output current is i dc , the busbar rated voltage is U N , the bus voltage control command is u ref ; The number of droop coefficients or inductances in a droop power supply is m, and the nth droop coefficient is k n, the nth inductor is represented by L n .

[0092] In this embodiment, the equivalent energy storage system is finally simplified to a system with a droop coefficient k d , the equivalent inductance is L and the equivalent output current is i dc The three parameter relations are expressed as follows: Figure 3 The droop control principle diagram shown in the figure is as follows:

[0093] The bus voltage of the energy storage device (or energy storage converter) running in parallel in the droop power supply changes with the change of load power according to the droop control. Therefore, in the system where multiple energy storage devices are running in parallel, the energy storage devices can be further equivalent. Usually, 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 coefficient is considered. The virtual resistance k and the line resistance r are combined into a virtual resistance k, that is, the droop coefficient k of the energy storage device n , so the mathematical model of the nth energy storage device can be set as:

[0094] Formula (6):

[0095] Among them, the current output from the nth energy storage device to the bus is represented by i n ;

[0096] Let i eq is the equivalent output current of the equivalent energy storage system, which can be expressed as:

[0097] Formula (7):

[0098] Then, the droop coefficient of m energy storage devices running in parallel in the droop power supply can be obtained from formula (7), and the equivalent droop coefficient k after equivalent combination is d for:

[0099] Formula (3):

[0100] When m energy storage devices using droop control are operated in parallel, in order to prevent the energy storage device with small capacity from reaching a state of excessively high or low charge first and causing the energy storage device to exit operation, the droop coefficient of the grid-connected converter of the energy storage device is usually set to meet the following requirements:

[0101] k 1 i 1 =k 2 i 2 =…=k n i n ;

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

[0103]

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

[0105] Formula (4):

[0106] In summary, based on formulas (3), (4), (6), and (7), the equivalent output current i of energy storage devices in parallel operation with droop control is dc can be rewritten as:

[0107]

[0108] Simplified:

[0109] Formula (5):

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

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

[0112] A22: Assume the state variables of the DC microgrid system, the steady-state operating point of the DC microgrid system, and the change of the steady-state operating point, substitute them into the reduced-order model of the DC microgrid, and construct an equivalent mathematical model of the DC microgrid system.

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

[0114] At the same time, based on the fact that the reduced-order model is a model expressed by the state space method, the state variables of the DC microgrid system, the steady-state operating point of the DC microgrid system, and the change of the steady-state operating point are substituted into the reduced-order model of the DC microgrid to construct an equivalent mathematical model of the DC microgrid system. The equivalent mathematical model of the DC microgrid system can reflect the comprehensive working state of the DC microgrid system, so as to judge the stability of its large signal.

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

[0116]

[0117] Sub-step A22 includes: setting x=[i s ,u bus ]T is the state variable of the DC microgrid system, let x e =[i se ,u buse ] T is the steady-state operating point of the DC microgrid system, let is the change in the steady-state operating point. Substituting it into the reduced-order model of the DC microgrid, the equivalent mathematical model of the DC microgrid system can be expressed as:

[0118]

[0119] Among them, J is the characteristic matrix of the system, expressed as:

[0120]

[0121] Among them, u bus is the instantaneous value of the bus voltage, U bus is the effective value of the bus voltage.

[0122] In this embodiment, in combination with formulas (1) to (5), the reduced-order model of the simplified DC microgrid is preferably: In order to facilitate the subsequent classification and judgment of the large signal stability of the DC microgrid; then, after setting the state variables of the DC microgrid system, the steady-state operating point of the DC microgrid system and the change of the steady-state operating point in the form of coordinates, the equivalent mathematical model of the DC microgrid system obtained by substituting it into the reduced-order model of the DC microgrid can be used to describe the system state.

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

[0124] A31: Based on the equivalent mathematical model of the DC microgrid system, construct its state equation;

[0125] A32: Reconstruct its state equation according to Lyapunov function;

[0126] A33: Based on the recognition criteria of 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 equation are derived respectively;

[0127] A34: Using the small signal stability constraints, derive the stability constraints for determining the large signal stability of the DC microgrid.

[0128] In this embodiment, for a given nonlinear system, according to the Lyapunov direct method, if the corresponding Lyapunov function V(x) is positive definite and its derivative is negative definite, the system can be considered to be small-signal stable at the operating point; when ‖x‖→∞, the system can be considered to be large-signal asymptotically stable; based on this, the state equation of the equivalent mathematical model of the DC microgrid system is reconstructed according to the Lyapunov function (i.e., its state equation is limited), then the judgment conditions for judging the small-signal stability of the DC microgrid can be obtained first, and then the judgment conditions for the large-signal stability can be further derived (i.e., the stability constraint conditions for judging the large-signal stability of the DC microgrid).

[0129] Specifically, the derivation process of the stability constraint condition (i.e., the process of step A3) is:

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

[0131]

[0132]

[0133] Among them, the expression of Ψ(x) is:

[0134]

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

[0136]

[0137] Among them, H is a positive definite matrix, so it can be guaranteed that V(x) is also a positive definite matrix, and the total derivative of V(x) can be expressed as:

[0138]

[0139] Q(x)=-[J T H+HJ];

[0140] A system with large signal stability must also be asymptotically stable with small signals, so it is necessary to ensure Therefore, if Q(x) is set as the identity matrix, that is, Q(x) = I, then H can get the following result:

[0141]

[0142] Where det(J) and Tr(J) are the determinant and trace of J, respectively, and I is the identity matrix. The symbolic expression of H is:

[0143]

[0144] Lyapunov's stability condition requires H to be a positive definite matrix and needs to satisfy the following conditions.

[0145]

[0146] After sorting out, we can get stability constraint ① (i.e. small signal stability constraint):

[0147]

[0148] According to the principle of Lyapunov direct method, when condition ① is met, the system is asymptotically stable with small signals; if the system is to satisfy the large signal stability, it is also necessary to prove that when ‖x‖→∞, its Lyapunov function satisfies V(x)→∞. In order to further derive the large signal stability criterion of the DC microgrid system, H can be rewritten as:

[0149]

[0150] From the above two formulas, we can conclude that there is and min{h a0 ,h a1 ,h c0 ,h c1}≥0. Therefore, V(x) can be expressed as:

[0151]

[0152] It can be seen from the above formula that when ‖x‖→∞, V(x)→∞. Therefore, under the stability constraint ①, the DC microgrid system is large signal stable.

[0153] However, the criterion derived from the stability criterion of the Lyapunov direct method is conservative, that is, the stability constraint condition ① can only represent the sufficient condition for system stability, but cannot explain the necessary condition for system stability; therefore, it is necessary to verify the necessity of the stability criterion. For the second-order model equivalent to the system, the necessary and sufficient conditions for small signal stability are:

[0154]

[0155] It can be further simplified into stability constraint ② (i.e. stability constraint):

[0156] Formula (8):

[0157] It should be noted that the mathematical expressions of stability constraint ① and stability constraint ② are consistent, that is, stability constraint ① is a necessary and sufficient condition for the stability of the system's small signal. If the large signal of the DC microgrid is stable, then its small signal must also be stable, and stability constraint ① is also a necessary and sufficient condition for the stability of the system's large signal.

[0158] In summary, only when the relevant data in the simplified equivalent diagram are substituted into the stability constraint conditions for calculation, so that the calculation results meet the requirements of the stability constraint conditions, can it represent that the large signal of the DC microgrid is stable.

[0159] Furthermore, step A4 includes the following sub-steps:

[0160] A41: Assume the selection judgment parameter α, substitute it into the stability constraint condition, and obtain at least two constant power load limit values;

[0161] A42: Based on the selection judgment parameter α and the constant power load limit value, a judgment process is designed. After the collected DC microgrid parameters are substituted into the selection judgment parameter α, the corresponding constant power load limit value is selected by the selection judgment parameter α to judge the large signal stability of the DC microgrid.

[0162] In this embodiment, by setting the selection judgment parameter α, according to the stability constraint condition [Formula (8)], the constant power load limit value is classified and set, and it can be known that which constant power load limit value the DC microgrid needs to adopt can be more strictly and accurately judged the stability of the DC microgrid large signal according to the selection judgment parameter α (obtained after substituting the collected DC microgrid data into the selection judgment parameter α), thereby improving the flexibility of the application of the stability constraint condition.

[0163] Further, sub-step A41 includes: setting the selection judgment parameter Substituting the stability constraint, we get the first constant power load limit value and the second constant power load limit

[0164] In this embodiment, based on the stability constraint [Formula (8)] contains two inequalities, the large signal stability of the DC microgrid should be determined according to the most stringent one according to the actual situation of the DC microgrid, and since U in the stability constraint bus is a variable, which is not easy to compare. bus Use system parameters and loads to eliminate equivalent; therefore, the selection judgment parameters can be set Substituting the stability constraint, we get the first constant power load limit value and the second constant power load limit To eliminate U bus and inequalities for selecting stability constraints.

[0165] Specifically, based on the reduced-order model of the equivalent DC microgrid, the steady-state operating point of the system can be obtained. and Then the effective value of the bus voltage at the steady-state operating point is U bus And the equivalent output current i dc for:

[0166]

[0167] make And the above U bus Substituting the expression into the stability constraint, we can get:

[0168]

[0169] When α≤1, the first inequality of the stability constraint is a stricter judgment condition. The stability of the DC microgrid large signal should be judged according to the first inequality, then the first constant power load limit value is

[0170] When α>1, the second inequality of the stability constraint is a stricter judgment condition. The stability of the DC microgrid large signal should be judged according to the second inequality, and the second constant power load limit value is

[0171] Furthermore, in step A42, the determination process includes the following steps:

[0172] B1: Collect the parameters of the resistive load, bus capacitance, constant power load, constant power load power supply and droop power supply in the DC microgrid, and calculate the equivalent droop coefficient k d , equivalent bus capacitance C and equivalent resistance load R;

[0173] B2: Substitute the relevant parameters into the selection judgment parameter α for calculation;

[0174] B3: Determine whether α≤1 is true; if so, calculate the first constant power load limit value P cpl.limit1 , and execute step B4; otherwise calculate the second constant power load limit value P cpl.limit2 , and execute step B5;

[0175] B4: Determine the constant power load P cpl Is it less than the first constant power load limit value P cpl.limit1 If so, the large signal of the DC microgrid is stable, otherwise the large signal of the DC microgrid is unstable;

[0176] B5: Determine the constant power load P cpl Is it less than the second constant power load limit value Pcpl.limit2 If so, the large signal of the DC microgrid is stable, otherwise the large signal of the DC microgrid is unstable.

[0177] In this embodiment, Figure 4 As shown, based on this judgment process, the constant power load limit value can be quickly selected by selecting the judgment parameter α, and whether the large signal of the DC microgrid is stable can be judged.

[0178] For the above-mentioned DC microgrid large signal stability judgment method, in order to verify its accuracy, a hardware-in-the-loop system (simulating DC microgrid) was built using the PLECS RT Box experimental platform and the STM32G474RE controller for experimental verification. The experimental platform consists of a main circuit part and a controller part.

[0179] The main circuit is built by the PLECS simulation platform, and the RT Box simulates the power level of the main circuit system. The complete controller can be tested without a real power level. The analog signals of electrical quantities such as the output current, bus voltage, DC / DC converter (energy storage device), inductance, current, etc. of the dispatchable power supply of the DC microgrid are output to the STM32G474RE controller through the analog output channel of the RT Box.

[0180] The control part of the DC microgrid is mainly the droop control algorithm of the dispatchable power source. The control algorithm and control parameters are compiled into the STM32G474RE controller by using a program. The controller generates PWM digital signals to control semiconductor devices and outputs them to the RT Box through digital signal channels; the PWM signals can be captured with a time resolution of less than 10 nanoseconds through the digital input of the RT Box; using the captured input data, the RT Box simulates the power level and provides simulation results through its analog output after a few microseconds.

[0181] 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.

[0182] Table 1 Case parameter settings

[0183]

[0184]

[0185] The constant power load adopts a controlled current source equivalent constant power load, and the resistance load is set to 5Ω. The constant power load instructions of the two experimental cases Case 1 and Case 2, as well as the stable state of the bus voltage and system operation under different load instructions are shown in the following table.

[0186] Table 2 Experimental load instructions and operation status table

[0187]

[0188] Among them, P cpl is the power of the constant power load, P R is the power of the resistive load.

[0189] The bus voltage in the bus DC microgrid is an important indicator of the system operation status. It can not only reflect the droop control principle, but also characterize the system voltage deviation and stability. Therefore, we focus on the bus voltage performance. The bus voltage waveforms of experimental cases Case 1 and Case 2 are as follows: Figure 5 and 6 shown.

[0190] It can be seen from the experimental waveform that in Case 1 and Case 2, the bus voltage value gradually decreases as the load power gradually increases. In addition, the load instructions in the first three time stages of Case 1 and Case 2 are consistent, and the droop coefficients of the two groups of cases are consistent. The bus voltage in the experimental figure decreases with the increase of load, and the bus voltage change law conforms to the droop control principle. The bus voltage value in the experiment is close to the theoretical value, and the error is mainly caused by the converter switching loss.

[0191] The component parameter values ​​of the DC microgrids in Case 1 and Case 2 are different, so the stability limit values ​​of the system constant power load are different. According to the stability calculation process (i.e., the judgment process, Figure 4 ), the constant power load stability limit of Case 1 is 30kW, and the stability limit of Case 2 is 24.3kW. Therefore, 25kW is still within the stable range for Case 1, but it is enough to make the system of Case 2 unstable. In the experimental waveform, it is reflected that in stage ④ of Case 1 and stage ⑤ of Case 2, the constant power load instruction is 25kW, but the system in Case 1 can still maintain stable operation, and the system in Case 2 is unstable. Therefore, the experimental waveform proves that the above-mentioned DC microgrid large signal stability judgment method is feasible.

[0192] Example 2

[0193] A DC microgrid large signal stability judgment system, applying the DC microgrid large signal stability judgment method according to any one of claims 1 to 9, comprising:

[0194] The first module is used to simplify and equate the data of the resistive load, bus capacitance, constant power load, constant power load power supply and droop power supply to obtain the simplified equivalent diagram related relationship;

[0195] The second module is used to construct an equivalent mathematical model of the DC microgrid system based on the simplified equivalent diagram related relationship;

[0196] The third module is used to construct the state equation according to the equivalent mathematical model of the DC microgrid system; then the state equation is limited by the Lyapunov direct method to derive the stability constraint conditions for judging the large signal stability of the DC microgrid;

[0197] The fourth module is used to classify and set the constant power load limit value according to the stability constraint conditions, thereby designing a judgment process to judge the large signal stability of the DC microgrid based on the constant power load limit value.

[0198] In this embodiment, a preferred embodiment of a DC microgrid large signal stability judgment system is also proposed, such as Figure 7 As shown, it is used to integrate the above-mentioned DC microgrid large signal stability judgment method, so that after collecting the relevant data of the DC microgrid and inputting it into the system, it can directly judge whether the large signal of the DC microgrid is stable. Specifically, the system mainly allocates the first to fourth modules to be responsible for the logic and operation of each step according to steps A1 to A4 of the DC microgrid large signal stability judgment method; it should be noted that the sub-steps in each step can also be further refined and assigned to sub-modules according to actual conditions, or no sub-modules are assigned and uniformly executed by the main module, which is not limited here.

[0199] A method for determining large signal stability of a DC microgrid according to an embodiment of the present invention and other components and operations of the system are well known to those skilled in the art and will not be described in detail herein.

[0200] In the description of this specification, the description with reference to the terms "embodiment", "example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0201] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

Claims

1. A method for judging large signal stability of a DC microgrid, characterized by: The following steps are involved: A1: Simplify and equalize the resistive load, bus capacitance, constant power load, constant power load power supply and droop power supply to obtain the simplified equivalent diagram of the DC microgrid and the related relationship of the simplified equivalent diagram; A2: Construct an equivalent mathematical model of the DC microgrid system based on the simplified equivalent diagram related relationship; A3: Based on the equivalent mathematical model of the DC microgrid system, its state equation is constructed; then the state equation is limited by the Lyapunov direct method, and the stability constraint conditions for judging the large signal stability of the DC microgrid are derived; A4: According to the stability constraints, the constant power load limit values ​​are set by category, and the judgment process is designed accordingly to judge the large signal stability of the DC microgrid based on the constant power load limit values.

2. A method for determining large signal stability of a DC microgrid according to claim 1, characterized in that: The step A1 comprises the following sub-steps: A11: All m droop-controlled energy storage devices operating in parallel in the droop power supply are combined into an equivalent energy storage system; A12: All bus capacitors running in parallel are combined into an equivalent bus capacitor C: Formula (1): Among them, the number of bus capacitors in the DC microgrid is m, and the i-th bus capacitor is represented by C i ; A13: All the resistance loads running in parallel are combined into an equivalent resistance load R: Formula (2): Among them, the number of resistive loads in the DC microgrid is k, and the jth resistive load is represented by R j ; A14: All constant power sources P connected in parallel cps And constant power load P cpl Equivalent to constant power load current I cpl .

3. A method for determining large signal stability of a DC microgrid according to claim 2, characterized in that: In the sub-step A11, the equivalent energy storage system includes: Formula (3): Formula (4): Formula (5): Among them, the equivalent droop coefficient is k d , the equivalent inductance is L, the equivalent output current is i dc , the busbar rated voltage is U N , the bus voltage control command is u ref ; The number of droop coefficients or inductances in a droop power supply is m, and the nth droop coefficient is k n , the nth inductor is represented by L n .

4. A method for determining large signal stability of a DC microgrid according to claim 3, characterized in that: The step A2 comprises the following sub-steps: A21: Combining formulas (1) to (5), a reduced-order model of the DC microgrid is constructed; A22: Assume the state variables of the DC microgrid system, the steady-state operating point of the DC microgrid system, and the change of the steady-state operating point, substitute them into the reduced-order model of the DC microgrid, and construct an equivalent mathematical model of the DC microgrid system.

5. A method for determining large signal stability of a DC microgrid according to claim 4, characterized in that: The sub-step A21 includes: combining formulas (1) to (5), the simplified reduced-order model of the DC microgrid is: The sub-step A22 includes: setting x=[i s ,u bus ] T is the state variable of the DC microgrid system, let x e =[i se ,u buse ] T is the steady-state operating point of the DC microgrid system, let is the change in the steady-state operating point. Substituting it into the reduced-order model of the DC microgrid, the equivalent mathematical model of the DC microgrid system can be expressed as: Among them, J is the characteristic matrix of the system, expressed as: Among them, u bus is the instantaneous value of the bus voltage, U bus is the effective value of the bus voltage.

6. A method for determining large signal stability of a DC microgrid according to claim 5, characterized in that: The step A3 comprises the following sub-steps: A31: Based on the equivalent mathematical model of the DC microgrid system, construct its state equation; A32: Reconstruct its state equation according to Lyapunov function; A33: Based on the recognition criteria of 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 equation are derived respectively; A34: Using the small signal stability constraints, derive the stability constraints for determining the large signal stability of the DC microgrid.

7. A method for determining large signal stability of a DC microgrid according to claim 6, characterized in that: The step A4 includes the following sub-steps: A41: Assume the selection judgment parameter α, substitute it into the stability constraint condition, and obtain at least two constant power load limit values; A42: Based on the selection judgment parameter α and the constant power load limit value, a judgment process is designed. After the collected DC microgrid parameters are substituted into the selection judgment parameter α, the corresponding constant power load limit value is selected by the selection judgment parameter α to judge the large signal stability of the DC microgrid.

8. A method for determining large signal stability of a DC microgrid according to claim 7, characterized in that: The sub-step A41 includes: setting the selection judgment parameter Substituting the stability constraint condition, we get the first constant power load limit value and the second constant power load limit 9. A method for determining large signal stability of a DC microgrid according to claim 8, characterized in that: In step A42, the judgment process includes the following steps: B1: Collect the parameters of the resistive load, bus capacitance, constant power load, constant power load power supply and droop power supply in the DC microgrid, and calculate the equivalent droop coefficient k d , equivalent bus capacitance C and equivalent resistance load R; B2: Substitute the relevant parameters into the selection judgment parameter α for calculation; B3: Determine whether α≤1 is true; if so, calculate the first constant power load limit value P cpl.limit1 , and execute step B4; otherwise calculate the second constant power load limit value P cpl.limit2 , and execute step B5; B4: Determine the constant power load P cpl Is it less than the first constant power load limit value P cpl.limit1 If so, the large signal of the DC microgrid is stable, otherwise the large signal of the DC microgrid is unstable; B5: Determine the constant power load P cpl Is it less than the second constant power load limit value P cpl.limit2 If so, the large signal of the DC microgrid is stable, otherwise the large signal of the DC microgrid is unstable.

10. A DC microgrid large signal stability judgment system, characterized by: The method for judging large signal stability of a DC microgrid according to any one of claims 1 to 9 comprises: The first module is used to simplify and equate the data of the resistive load, bus capacitance, constant power load, constant power load power supply and droop power supply to obtain the simplified equivalent diagram related relationship; The second module is used to construct an equivalent mathematical model of the DC microgrid system based on the simplified equivalent diagram related relationship; The third module is used to construct the state equation according to the equivalent mathematical model of the DC microgrid system; then the state equation is limited by the Lyapunov direct method to derive the stability constraint conditions for judging the large signal stability of the DC microgrid; The fourth module is used to classify and set the constant power load limit value according to the stability constraint conditions, thereby designing a judgment process to judge the large signal stability of the DC microgrid based on the constant power load limit value.

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