Photovoltaic inverter discretization small signal modeling method based on bilinear transformation method

Abstract

The invention discloses a photovoltaic inverter discretization small signal modeling method based on a bilinear transformation method, and relates to the technical field of miniature photovoltaic inverters, and the method comprises the steps: obtaining a state equation and an expression of each state detection vector according to the topology and control mode of a single-phase miniature photovoltaic inverter; determining the time scale of modeling according to the switching frequency of the single-phase miniature photovoltaic inverter, and solving each state equation by using a discrete time mapping method to obtain a discrete iteration equation of each state quantity; separating the steady state quantity and the small signal quantity of the state quantity, and converting the nonlinear large signal discrete iteration equation into a linear small signal discrete iteration equation; deriving a z-domain transfer function according to the small signal equation, and converting the z-domain transfer function into an s-domain transfer function by using a bilinear transformation method; and drawing a Bode diagram and a Nyquist curve according to the discretized small signal model, and analyzing the stability characteristics of the single-phase miniature photovoltaic inverter. According to the method, the switching process among all modes of the single-phase micro photovoltaic inverter can be completely described.

Description

Technical Field

[0001] The present invention relates to the technical field of micro photovoltaic inverters, and specifically to a discrete small-signal modeling method for photovoltaic inverters based on the bilinear transformation method. Background Art

[0002] In recent years, with the vigorous development of distributed energy generation, more and more distributed power sources are connected to the power grid, among which photovoltaic is one of the main energy sources. However, the traditional centralized photovoltaic power generation has the "short board effect", and its output power is restricted by a single photovoltaic module. Distributed photovoltaic power generation solves this problem. With the expansion of the scope of distributed photovoltaic power generation and the increase in its penetration rate in the power grid, micro photovoltaic inverters are widely used. As a bridge for transmitting energy, in order to be able to connect to the power grid safely and reliably, the stable operation of micro photovoltaic inverters is particularly important. This requires an accurate modeling method to describe the behavior of micro photovoltaic inverters as the basis for analyzing their stability.

[0003] As the most widely used model, the state-space averaging model has been widely applied in the modeling and stability analysis of micro photovoltaic inverters. The state-space averaging model is a linear time-invariant model, which is relatively simple itself and can reflect the low-frequency behavior of micro photovoltaic inverters in both the time domain and the frequency domain. However, in the modeling process, the state-space averaging model performs an averaging process according to the duty cycle, ignoring the switching process of switching devices and unable to reflect the high-frequency behavior of the system. Another commonly used model is the discrete-time mapping model, which details each switching process under the switching action of power electronic devices and continuously iterates. The discrete-time mapping model has higher accuracy than the traditional state-space averaging model, but it is a non-linear model and the model is relatively complex. Moreover, this model is often used to study the bifurcation and chaotic behavior of the system in the time domain and is difficult to analyze in the frequency domain.

[0004] For the stability study of the micro photovoltaic inverter grid-connected system, a model is often constructed in the frequency domain, and the Bode diagram or Nyquist curve is plotted based on this to analyze its stability. Therefore, a modeling method that can reflect the switching action of the inverter switching device and can construct an expression in the frequency domain is needed. Summary of the Invention

[0005] In view of the above problems, the present invention is proposed.

[0006] Therefore, the present invention provides a discrete small-signal modeling method for photovoltaic inverters based on the bilinear transformation method, which can solve the problems mentioned in the background art.

[0007] To solve the above technical problems, the present invention provides the following technical solutions: A discrete small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method, including: obtaining the state equation and the expressions of each state detection vector according to the specific topology and control method of a single-phase micro photovoltaic inverter;

[0008] Determining the time scale for modeling according to the switching frequency of the single-phase micro photovoltaic inverter, and using the discrete-time mapping method to solve each state equation to obtain the discrete iterative equations of each state variable;

[0009] Separating the steady-state quantity and the small-signal quantity of the state variable, and transforming the nonlinear large-signal discrete iterative equation into a linear small-signal discrete iterative equation;

[0010] Deriving the z-domain transfer function according to the small-signal equation, and using the bilinear transformation method to convert the z-domain transfer function into an s-domain transfer function;

[0011] Drawing the Bode diagram and Nyquist curve according to the discrete small-signal model, and analyzing the stability characteristics of the single-phase micro photovoltaic inverter.

[0012] As a preferred solution of the discrete small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method of the present invention, wherein: the state equation is:

[0013]

[0014] wherein, x is the state variable of the single-phase micro photovoltaic inverter; A i , B i are constant matrices, related to the specific topology of the single-phase micro photovoltaic inverter, and i is the working mode of the single-phase micro photovoltaic inverter; is the derivative function of the state variable; V PV is the photovoltaic input voltage.

[0015] As a preferred solution of the discrete small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method of the present invention, wherein: the discrete iterative equation is:

[0016] x t+1 =H t x t +K t V PV ;

[0017] wherein, x is the state variable of the single-phase micro photovoltaic inverter; t and t + 1 are respectively the starting time and the ending time of the discrete interval; H t , K t are coefficient matrices with the duty cycle d as the independent variable at time t; V PV is the photovoltaic input voltage.

[0018] As a preferred embodiment of the discrete small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method according to the present invention, wherein: the discrete small-signal iterative equation is:

[0019]

[0020] Wherein, is the small-signal quantity of the state quantity of the single-phase micro photovoltaic inverter; is the small-signal quantity of the duty cycle of the switching device of the single-phase micro photovoltaic inverter; t and t+1 are the starting and ending times of the discrete interval respectively; P t and Q t are constant matrices.

[0021] As a preferred embodiment of the discrete small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method according to the present invention, wherein: the transfer functions in the z-domain and s-domain are:

[0022]

[0023] Wherein, z is a complex variable represented in the form of modulus |z| and argument θ, in the form of |z|e jθ ; s is a complex variable represented in the form of modulus |s| and argument θ, in the form of |s|e jθ ; The conversion between the z-domain and the s-domain is carried out using the bilinear transformation method, and the relationship between the two is z = (1 + 0.5sT s ) / (1 - 0.5sT s ), T s is the switching period; a i (i = 1,..., n), b j (j = 1,..., m), c k (k = 1,..., p), d l (l = 1,..., q) are constants.

[0024] As a preferred embodiment of the discrete small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method according to the present invention, wherein: if the single-phase micro photovoltaic inverter operates in the discontinuous mode, taking the flyback switching period T S as the time scale, then the single-phase micro photovoltaic inverter contains 3 operating modes, specifically mode 1 [t 0 - t 1 , mode 2 [t 1 - t 2 and mode 3 [t 2 - t 3 .

[0025] As a preferred solution of the discrete small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method of the present invention, wherein: the discrete small-signal model includes a circuit model of a single-phase micro photovoltaic inverter and a control model of a single-phase micro photovoltaic inverter.

[0026] To further solve the above technical problems, the present invention provides the following technical solutions: A discrete small-signal modeling system for a photovoltaic inverter based on the bilinear transformation method, including: a modeling module for obtaining a state equation and an expression of a state detection vector according to the topology and control mode of a single-phase micro photovoltaic inverter;

[0027] A discretization module for determining a modeling time scale based on the switching frequency and using the discrete-time mapping method to solve the discrete iterative equation of the state quantity;

[0028] A linearization module for separating the steady-state quantity and the small-signal quantity of the state quantity and converting the non-linear large-signal discrete iterative equation into a linear small-signal discrete iterative equation;

[0029] A transformation module for deriving a z-domain transfer function according to the small-signal equation and converting it into an s-domain transfer function using the bilinear transformation method;

[0030] An analysis module for drawing a Bode diagram and a Nyquist curve according to the discrete small-signal model and analyzing the stability characteristics of the inverter.

[0031] A computer device includes a memory and a processor, the memory stores a computer program, and is characterized in that when the processor executes the computer program, the steps of the discrete small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method as described above are implemented.

[0032] A computer-readable storage medium stores a computer program thereon, and is characterized in that when the computer program is executed by a processor, the steps of the discrete small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method as described above are implemented.

[0033] The beneficial effects of the present invention are as follows: 1. By using the discrete-time mapping method to solve the state-space equations of all modes of the single-phase micro photovoltaic inverter one by one, the switching process between various modes of the single-phase micro photovoltaic inverter can be completely described. 2. By using the small-signal modeling method, the non-linear discrete iterative equation of the single-phase micro photovoltaic inverter is transformed into a linear equation, meeting the condition for converting the model from the time domain to the frequency domain. 3. Based on the bilinear transformation method, the transfer function of the single-phase micro photovoltaic inverter is transformed from the discrete domain to the continuous domain, and the stability characteristics of the single-phase micro photovoltaic inverter can be analyzed using the Bode diagram or the Nyquist criterion. Description of the Drawings

[0034] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0035] Figure 1 This is the flowchart of the discretized small-signal modeling method of the photovoltaic inverter based on the bilinear transformation method of the present invention.

[0036] Figure 2 This is the equivalent circuit diagram of the single-phase micro photovoltaic inverter modeling based on the bilinear transformation method of the present invention.

[0037] Figure 3 This is the steady-state waveform diagram of the single-phase micro photovoltaic inverter modeling based on the bilinear transformation method of the present invention.

[0038] Figure 4 This is the Bode diagram obtained by the discretized small-signal modeling method of the photovoltaic inverter based on the bilinear transformation method of the present invention.

[0039] Figure 5 This is the Nyquist diagram obtained by the discretized small-signal modeling method of the photovoltaic inverter based on the bilinear transformation method of the present invention.

[0040] Figure 6 This is the time-domain output waveform diagram of the single-phase micro photovoltaic inverter obtained by Simulink simulation. Specific Embodiments

[0041] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following will make a detailed description of the specific embodiments of the present invention in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0042] In the following description, many specific details are set forth in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0043] Embodiment 1, referring to Figures 1 to 6 , which is an embodiment of the present invention, provides a discretized small-signal modeling method of a photovoltaic inverter based on the bilinear transformation method.

[0044] In the related art,

[0045] This application provides a solution that can effectively address the above-mentioned problems. Next, multiple embodiments will be combined to elaborate in detail on how to implement the discretized small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method.

[0046] Figure 1 The overall flowchart of the discretized small-signal modeling method for a photovoltaic inverter based on the bilinear transformation method is shown, including the following steps:

[0047] As Figure 1 shown, an embodiment of a discretized small-signal modeling method for a three-phase micro photovoltaic inverter based on the forward Euler method includes the following steps:

[0048] (1) Determine the topology of the single-phase micro photovoltaic inverter, as well as basic parameters such as the excitation inductance L m , filter inductance L f , filter capacitor C f and transformer turns ratio N, etc.

[0049] (2) Write the state-space equations according to the various modes of the single-phase micro photovoltaic inverter.

[0050] (3) Determine the switching sequence of each mode according to the switching law, solve the state-space equations, and then derive the discrete iterative equations of the state variables.

[0051] (4) Separate the steady-state and small-signal components from the state variables and duty cycle, and transform the non-linear large-signal discrete iterative equations into linear small-signal discrete iterative equations.

[0052] (5) According to the relationship between the time domain and the frequency domain, convert the linear small-signal discrete iterative equations into z-domain transfer functions.

[0053] (6) Use the bilinear transformation method to transform the model from the discrete z-domain to the continuous s-domain, and then derive the discretized small-signal model of the single-phase micro photovoltaic inverter.

[0054] (7) Draw the Bode diagram and Nyquist curve according to the discretized small-signal model, and analyze the stability characteristics of the three-phase micro photovoltaic inverter.

[0055] As Figure 2 shown, the single-phase micro photovoltaic inverter modeled based on the bilinear transformation method in this embodiment includes a flyback conversion circuit at the front stage and a bridge inverter circuit at the rear stage; the flyback conversion circuit includes a photovoltaic input port, a flyback transformer, a flyback switch tube, and an output freewheeling diode; the voltage of the photovoltaic input terminal is V PV , the turns ratio of the flyback transformer is 1:N, and the switching period of the flyback switch tube is T S; The bridge inverter circuit includes an inverter bridge with 4 switching tubes, an output filter capacitor, and a filter inductor; the filter capacitor and the filter inductor are connected in the form of a CL filter.

[0056] Figure 3 This is the steady-state waveform diagram of the single-phase micro photovoltaic inverter modeled based on the bilinear transformation method in this embodiment. Assume that the single-phase micro photovoltaic inverter operates in the discontinuous mode, with the flyback switching period T S as the time scale, the single-phase micro photovoltaic inverter has 3 operating modes:

[0057] (1) Mode 1 [t 0 -t 1

[0058] The switching tube S of the front-stage flyback conversion circuit 0 is turned on, the photovoltaic input voltage magnetizes the primary winding of the flyback transformer and stores energy, the diode D on the output side of the flyback conversion circuit is reverse-biased, and no current flows through the secondary winding of the flyback transformer; the rear-stage bridge inverter circuit operates in the positive half cycle of the power frequency, and the switching tubes S 1 , S 4 are turned on, and S 2 , S 3 are turned off.

[0059] (2) Mode 2 [t 1 -t 2

[0060] The switching tube S of the front-stage flyback conversion circuit 0 is turned off, the diode D on the output side of the flyback conversion circuit is forward-biased, the energy stored in the primary winding of the flyback transformer is transmitted to the secondary side through the diode D, and the output current of the secondary side of the flyback transformer linearly decreases. At this time, the rear-stage bridge inverter circuit operates in the positive half cycle of the power frequency, and the switching tubes S 1 , S 4 are turned on, and S 2 , S 3 are turned off.

[0061] (3) Mode 3 [t 2 -t 3

[0062] The switching tube S of the front-stage flyback conversion circuit 0 remains off, and the magnetizing current of the flyback transformer drops to 0 at time t 2 ; the rear-stage bridge inverter circuit operates in the positive half cycle of the power frequency, and the switching tubes S 1 , S 4 are turned on, and S 2 , S 3 are turned off.

[0063] ​​​When the post-stage bridge-type inverter circuit operates in the negative half-cycle of the power frequency, the switching tubes S 2 and S 3 conduct, S 1 and S 4 turn off, and the filter inductor L f maintains the negative flow of the output current i g . The working principle of the remaining part is the same as that of modes 1-3.

[0064] According to the modal analysis, the state equation of the single-phase micro photovoltaic inverter is:

[0065]

[0066] x = [i m i L v C T is the state variable of the single-phase micro photovoltaic inverter; A i and B i are constant matrices, i is the working mode, V PV is the photovoltaic input voltage; R g is the equivalent resistance of the power grid; L m is the excitation inductor; L f is the filter inductor; C f is the filter capacitor.

[0067] Solving the state equation, the discrete iteration relationship of the state variables is obtained:

[0068]

[0069] where x n is the state variable at the initial moment of the nth switching period, which is x n ; x n,1 is the state variable at the end of mode 1 within this switching period; x n,2 is the state variable at the end of mode 2; x n,3 is the state variable at the end of mode 3, that is, the state variable x n+1 at the initial moment of the (n + 1)th switching period.

[0070] According to the modal analysis and the state equation, the discrete iteration relationships of the duty ratios of the 3 modes are solved:

[0071]

[0072] where f is the parameter matrix, f = [0 0 1 / NL m .

[0073] ​Separate the steady-state and small-signal components of the state variable and the duty cycle, then we have:

[0074]

[0075] Where X is the steady-state component of the state variable; is the small-signal component of the state variable; D is the steady-state component of the duty cycle; is the small-signal component of the duty cycle.

[0076] Substitute the small perturbation components of the state variable and the duty cycle into the discrete iteration equation to obtain the small-signal linear iteration equation:

[0077]

[0078]

[0079] After arrangement, the linear iteration model of the single-phase micro photovoltaic inverter can be obtained:

[0080]

[0081] A n,1 ,A n,2 ,A n,3a A n,3b ,B n,1 ,B n,2 ,B n,3 are parameter matrices,

[0082] Perform Z-transform on the above linear iteration model to obtain the iteration equation of the single-phase micro photovoltaic inverter in the z-domain:

[0083] zX(z) = (A n,3a A n,2 A n,1 +A n,3b A n,1 )X(z)+(B n,3 +A n,3a B n,2 +A n,3a A n,2 B n,1 +A n,3b B n,1 )D(z); D(z) is the function of the duty cycle in the z-domain, and X(z) is the function of the state variable in the z-domain.

[0084] Then the z-domain transfer function with the duty cycle as the input and the state variable as the output is:

[0085]

[0086] Regarding the relationship between the z-domain and the s-domain in the frequency domain, the present invention performs the conversion based on the bilinear transformation method. Then, the following relationship exists between the z-domain and

[0087] the s-domain:

[0088]

[0089] Based on the bilinear transformation method, the s-domain transfer function of the single-phase micro photovoltaic inverter is:

[0090]

[0091] The above equation is the discretized small-signal model of the single-phase micro photovoltaic inverter.

[0092] According to the above process, a model is built and numerical simulation is carried out. The Bode diagram and Nyquist diagram are plotted, as shown in Figure 4 and Figure 5 respectively, to analyze the stability characteristics of the single-phase micro photovoltaic inverter. The specific parameters used in the simulation are as follows: Photovoltaic input voltage V PV = 35V; Grid equivalent resistance R g = 311Ω; Excitation inductance L m = 30μH; Filter inductance L f = 10mH; Filter capacitor C f = 1μF; Flyback switch tube switching frequency f S = 10kHz; Inverter bridge switch tube switching frequency f = 50Hz.

[0093] With the same simulation parameters set, circuit simulation is carried out using Simulink software. The output waveform of the single-phase micro photovoltaic inverter is as shown in Figure 6 . Through the analysis of the output waveform, which is consistent with the stability analysis of the numerical simulation, the correctness of the photovoltaic inverter discretized small-signal modeling method based on the bilinear transformation method of the present invention is verified.

[0094] Embodiment 2, which is an embodiment of the present invention, provides a photovoltaic inverter discretized small-signal modeling system based on the bilinear transformation method, including:

[0095] A modeling module, configured to obtain a state equation and a state detection vector expression according to the topology and control mode of the single-phase micro photovoltaic inverter;

[0096] A discretization module, configured to determine the modeling time scale based on the switching frequency and solve the discrete iterative equation of the state quantity using the discrete time mapping method;

[0097] A linearization module, configured to separate the steady-state quantity and the small-signal quantity of the state quantity, and convert the non-linear large-signal discrete iterative equation into a linear small-signal discrete iterative equation;

[0098] A transformation module, configured to derive a z-domain transfer function according to a small-signal equation and convert it into an s-domain transfer function using the bilinear transformation method;

[0099] An analysis module, configured to draw a Bode plot and a Nyquist curve according to a discretized small-signal model and analyze the stability characteristics of the inverter.

[0100] Embodiment 3 is an embodiment of the present invention. The difference from the previous embodiment is that if the function is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on such an understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or a part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present invention. The aforementioned storage medium includes: various media such as USB flash drives, mobile hard disks, read-only memories (ROM, Read-Only Memory), random access memories (RAM, Random Access Memory), magnetic disks, or optical discs that can store program codes.

[0101] The logic and / or steps represented in the flowchart or described in other ways herein, for example, can be considered as a definite sequence list of executable instructions for implementing logical functions, and can be specifically implemented in any computer-readable medium for use by an instruction execution system, apparatus, or device (such as a computer-based system, a system including a processor, or other systems that can fetch instructions from the instruction execution system, apparatus, or device and execute the instructions), or used in combination with these instruction execution systems, apparatus, or devices. For the purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transmit a program for use by or in connection with an instruction execution system, apparatus, or device.

[0102] More specific examples (a non-exhaustive list) of computer-readable media include the following: an electrical connection (electronic device) having one or more wirings, a portable computer diskette (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disc read-only memory (CDROM). Additionally, the computer-readable media can even be paper or other suitable media on which the program can be printed, since the program can be obtained electronically, for example, by optically scanning the paper or other media, followed by editing, interpretation, or otherwise processing it in a suitable manner if necessary, and then storing it in a computer memory.

[0103] It should be understood that various parts of the present invention can be implemented by hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, any one or a combination of the following techniques well known in the art can be used: discrete logic circuits having logic gate circuits for implementing logical functions on data signals, application specific integrated circuits having appropriate combinational logic gate circuits, programmable gate arrays (PGAs), field programmable gate arrays (FPGAs), and the like.

[0104] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A photovoltaic inverter discretization small signal modeling method based on bilinear transformation method, characterized in that: include: According to the specific topology and control method of the single-phase micro photovoltaic inverter, the state equation and the expression of each state detection vector are obtained; According to the switching frequency of the single-phase micro photovoltaic inverter, the time scale of modeling is determined, and the discrete time mapping method is used to solve each state equation to obtain the discrete iterative equation of each state quantity; Separate the steady-state quantity and small-signal quantity of the state quantity, and transform the nonlinear large-signal discrete iterative equation into a linear small-signal discrete iterative equation; The z-domain transfer function is derived from the small signal equation and converted into the s-domain transfer function using the bilinear transformation method; The Bode diagram and Nyquist curve are drawn according to the discretized small signal model, and the stability characteristics of the single-phase micro photovoltaic inverter are analyzed.

2. The photovoltaic inverter discretization small signal modeling method based on the bilinear transformation method according to claim 1, characterized in that: The state equation is: Among them, x is the state variable of the single-phase micro photovoltaic inverter; A i , B i is a constant matrix, which is related to the specific topology of the single-phase micro photovoltaic inverter, and i is the working mode of the single-phase micro photovoltaic inverter; is the derivative of the state variable; V PV is the photovoltaic input voltage.

3. The photovoltaic inverter discretization small signal modeling method based on the bilinear transformation method according to claim 2, characterized in that: The discrete iterative equation is: x t+1 =H t x t +K t V PV ; Where x is the state variable of the single-phase micro photovoltaic inverter; t and t+1 are the starting and ending times of the discrete interval respectively; H t , K t is the coefficient matrix with duty cycle d as the independent variable at time t; V PV is the photovoltaic input voltage.

4. The photovoltaic inverter discretization small signal modeling method based on the bilinear transformation method as claimed in claim 3, characterized in that: The discrete small signal iterative equation is: in, It is a small signal quantity of the state quantity of the single-phase micro photovoltaic inverter; is the small signal quantity of the duty cycle of the switching device of the single-phase micro photovoltaic inverter; t and t+1 are the starting time and the ending time of the discrete interval respectively; P t , Q t is a constant matrix.

5. The photovoltaic inverter discretization small signal modeling method based on the bilinear transformation method as claimed in claim 4, characterized in that: The z-domain and s-domain transfer functions are: Where z is a complex variable expressed in the form of modulus |z| and argument θ, such as |z|e jθ ; s is a complex variable expressed in the form of modulus |s| and argument θ, such as |s|e jθ ; The z domain and s domain are converted using the bilinear transformation method, and the relationship between the two is z = (1 + 0.5sT s ) / (1-0.5sT s ), T s is the switching cycle; a i (i=1,…,n), b j (j=1,…,m), c k (k=1,…,p), d l (l=1,…,q) is a constant.

6. The photovoltaic inverter discretization small signal modeling method based on the bilinear transformation method according to claim 5, characterized in that: If the single-phase micro photovoltaic inverter works in discontinuous mode, the flyback switching period T S As the time scale, the single-phase micro photovoltaic inverter contains three working modes, namely mode 1 [t0-t1], mode 2 [t1-t2] and mode 3 [t2-t3].

7. The photovoltaic inverter discretization small signal modeling method based on the bilinear transformation method according to claim 6, characterized in that: The discretized small signal models include the circuit model of the single-phase micro photovoltaic inverter and the control model of the single-phase micro photovoltaic inverter.

8. A photovoltaic inverter discrete small signal modeling system based on a bilinear transformation method, based on the photovoltaic inverter discrete small signal modeling method based on a bilinear transformation method according to any one of claims 1 to 7, characterized in that: include, A modeling module, used to obtain state equations and state detection vector expressions according to the topology and control method of the single-phase micro photovoltaic inverter; A discretization module, used to determine the modeling time scale based on the switching frequency and solve the discrete iterative equations of the state quantity using a discrete time mapping method; Linearization module, used to separate the steady-state quantity and small signal quantity of the state quantity, and transform the nonlinear large-signal discrete iterative equation into a linear small-signal discrete iterative equation; A transformation module, used for deriving a z-domain transfer function according to a small signal equation and converting it into an s-domain transfer function using a bilinear transformation method; The analysis module is used to draw Bode diagrams and Nyquist curves based on the discretized small signal model and analyze the inverter stability characteristics.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the photovoltaic inverter discretization small signal modeling method based on the bilinear transformation method described in any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the photovoltaic inverter discretization small signal modeling method based on the bilinear transformation method described in any one of claims 1 to 7 are implemented.

Images