Three-phase miniature photovoltaic inverter discretization small signal modeling method and system

Abstract

The invention relates to the technical field of miniature photovoltaic inverters, in particular to a three-phase miniature photovoltaic inverter discretization small signal modeling method and system. Establishing a state-space equation according to the working modes of the three-phase micro photovoltaic inverter and determining the switching sequence of each mode; solving the state-space equation based on the working mode and the switching sequence to obtain a discrete iteration equation of the state quantity; separating the state quantity and the duty ratio into a steady-state component and a small-signal component, and converting the nonlinear large-signal discrete iteration equation into a linear small-signal discrete iteration equation; and establishing a z-domain transfer function according to the linear small signal discrete iteration equation, and converting the z-domain transfer function into an s-domain transfer function by using a forward Euler method to obtain a discrete small signal model. According to the method, the switching process among all modes of the three-phase miniature photovoltaic inverter is completely described through a discrete time mapping method, and the modeling precision is improved; and meanwhile, accurate conversion from the time domain to the frequency domain is realized, and a reliable basis is provided for stability analysis of a grid-connected system.

Description

Technical Field

[0001] The present invention relates to the technical field of micro photovoltaic inverters, and in particular to a discrete small signal modeling method and system for a three-phase micro photovoltaic inverter. Background Art

[0002] Photovoltaic is one of the types of distributed energy. Its access to the grid is prone to oscillation problems, which has a great impact on the safe and stable operation of the grid. Unlike the synchronous generators used in traditional energy, distributed energy uses power electronic converters to access the grid. The interaction between power electronic converters operating at a switching frequency of several kHz or even several MHz and the industrial frequency grid will reduce the stability of the grid. Under severe working conditions, it may cause the grid to become unstable, causing the distributed power source to be disconnected from the grid, and even cause equipment damage and other adverse effects. With the continuous increase in the installed capacity of distributed photovoltaics, micro photovoltaic inverters have been widely used in photovoltaic grid-connected systems, so 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 a basis for analyzing the stability of micro photovoltaic inverters.

[0003] Time domain models such as the state space average model have been widely used in the modeling of power electronic converters. They can describe the internal characteristics of power electronic converters and reflect the high-frequency behavior of the switching action of the switching devices in the converter. For micro photovoltaic inverters such as grid-connected systems, their stability analysis is mostly based on the frequency domain. The frequency domain model is a commonly used model for stability analysis of grid-connected systems. It constructs an equivalent impedance model based on the external port characteristics of the system, derives the open-loop transfer function of the system, and then analyzes the stability characteristics of the grid-connected system. However, this type of model ignores the internal behavior of the switching of the switching devices, and can only reflect the external characteristics of the grid-connected system, and the accuracy is insufficient.

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

[0005] In view of the problems existing in the prior art, the present invention is proposed.

[0006] Therefore, the problem to be solved by the present invention is how to solve the problem that the equivalent impedance model ignores the internal behaviors such as switching of switching devices, can only reflect the external characteristics of the grid-connected system, has insufficient accuracy, and cannot construct an expression in the frequency domain.

[0007] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0008] In a first aspect, an embodiment of the present invention provides a discrete small signal modeling method for a three-phase micro photovoltaic inverter, which includes obtaining a topological structure and basic parameters of the three-phase micro photovoltaic inverter, wherein the basic parameters include a transformer ratio, a magnetizing inductance, a filter inductance, and a filter capacitor;

[0009] According to the working modes of the three-phase micro photovoltaic inverter, the state space equation is established and the switching order of each mode is determined;

[0010] Based on the working mode and the switching order, solving the state space equation to obtain the discrete iterative equation of the state quantity;

[0011] Separate the state quantity and duty cycle into steady-state component and small-signal component, and transform the nonlinear large-signal discrete iterative equation into a linear small-signal discrete iterative equation;

[0012] A z-domain transfer function is established according to the linear small signal discrete iterative equation, and the z-domain transfer function is converted into an s-domain transfer function using the forward Euler method to obtain a discretized small signal model.

[0013] As a preferred solution of the discrete small signal modeling method of the three-phase micro photovoltaic inverter of the present invention, wherein: the three-phase micro photovoltaic inverter comprises:

[0014] A flyback conversion circuit of the front stage, wherein the flyback conversion circuit has three photovoltaic input ports, which are connected in 9-way staggered parallel connection, and the input ends of every three flyback conversion circuits are commonly connected to one photovoltaic input port;

[0015] The bridge inverter circuit of the rear stage has 3 circuits, which output three-phase AC power of A, B, and C respectively. The input end of each phase bridge inverter circuit is respectively connected to the output end of 3 flyback conversion circuits corresponding to phase A, phase B, and phase C of the 9 circuits of the front stage.

[0016] As a preferred solution of the discretized small signal modeling method of the three-phase micro photovoltaic inverter described in the present invention, the state space equation includes: the state variables of the three-phase micro photovoltaic inverter, the photovoltaic input voltage, the constant matrix related to the topological structure of the three-phase micro photovoltaic inverter, and the operating mode.

[0017] As a preferred solution of the discretized small signal modeling method of the three-phase micro photovoltaic inverter described in the present invention, the discrete iterative equation of the state quantity includes: the state variable at the initial moment of the switching cycle, the initial and end moments of the discrete interval, the coefficient matrix with the duty cycle as the independent variable, and the photovoltaic input voltage.

[0018] As a preferred solution of the discretized small signal modeling method of the three-phase micro photovoltaic inverter described in the present invention, the linear small signal discrete iterative equation includes: the small signal component of the state variable, the small signal component of the duty cycle of the switching device and the constant matrix.

[0019] As a preferred solution of the discretized small signal modeling method of the three-phase micro photovoltaic inverter described in the present invention, the transfer function conversion parameters of the forward Euler method in the z domain and s domain include: switching period, complex variables expressed in the form of module value and amplitude angle.

[0020] As a preferred solution of the discrete small signal modeling method of the three-phase micro photovoltaic inverter described in the present invention, wherein: according to the discrete small signal model, the Bode diagram and the Nyquist curve are drawn to analyze the stability of the three-phase micro photovoltaic inverter.

[0021] In a second aspect, an embodiment of the present invention provides a three-phase micro photovoltaic inverter discretized small signal modeling system, which includes a data acquisition module to obtain the topological structure and basic parameters of the three-phase micro photovoltaic inverter, wherein the basic parameters include transformer ratio, excitation inductance, filter inductance and filter capacitance;

[0022] The construction module establishes the state space equation according to the working modes of the three-phase micro photovoltaic inverter and determines the switching order of each mode;

[0023] A solution module, based on the working mode and the switching order, solves the state space equation to obtain a discrete iterative equation of the state quantity;

[0024] A separation module separates the state quantity and duty cycle into a steady-state component and a small-signal component, and transforms the nonlinear large-signal discrete iterative equation into a linear small-signal discrete iterative equation;

[0025] The result output module establishes a z-domain transfer function according to the linear small signal discrete iterative equation, and converts the z-domain transfer function into an s-domain transfer function by using the forward Euler method to obtain a discretized small signal model.

[0026] In a third aspect, an embodiment of the present invention provides a computer device, comprising a memory and a processor, wherein the memory stores a computer program, wherein: when the computer program instructions are executed by the processor, the steps of the discrete small signal modeling method of the three-phase micro photovoltaic inverter as described in the first aspect of the present invention are implemented.

[0027] In a fourth aspect, an embodiment of the present invention provides a computer-readable storage medium having a computer program stored thereon, wherein: when the computer program instructions are executed by a processor, the steps of the discrete small signal modeling method of the three-phase micro photovoltaic inverter as described in the first aspect of the present invention are implemented.

[0028] The beneficial effects of the present invention are as follows: the present invention solves the state space equations of all modes of the three-phase micro photovoltaic inverter one by one through the discrete time mapping method, and can completely describe the switching process between the various modes of the three-phase micro photovoltaic inverter.

[0029] Through the small signal modeling method, the nonlinear discrete iterative equation of the three-phase micro photovoltaic inverter is transformed into a linear equation, achieving the conditions for converting the model from time domain to frequency domain.

[0030] Based on the forward Euler method, the transfer function of the three-phase micro photovoltaic inverter is converted from the discrete z domain to the continuous s domain, and the stability characteristics of the three-phase micro photovoltaic inverter can be analyzed using the Bode diagram or Nyquist criterion. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0032] Figure 1 Flowchart of discretized small signal modeling method for three-phase micro-PV inverter;

[0033] Figure 2 Computer equipment diagram for discretized small signal modeling method for three-phase micro photovoltaic inverter;

[0034] Figure 3 The equivalent circuit diagram of the three-phase micro photovoltaic inverter modeling method for the discretized small signal modeling of the three-phase micro photovoltaic inverter;

[0035] Figure 4 Steady-state waveform diagram of three-phase micro photovoltaic inverter modeling based on the discretized small signal modeling method of three-phase micro photovoltaic inverter;

[0036] Figure 5 The Bode diagram obtained by the discrete small signal modeling method of the three-phase micro photovoltaic inverter;

[0037] Figure 6 The Nyquist curve diagram obtained by the discrete small signal modeling method of the three-phase micro photovoltaic inverter;

[0038] Figure 7 The time domain output waveform of the three-phase micro photovoltaic inverter obtained by Simulink simulation of the discrete small signal modeling method of the three-phase micro photovoltaic inverter. DETAILED DESCRIPTION

[0039] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0040] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0041] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it a separate or selective embodiment that is mutually exclusive with other embodiments.

[0042] Example 1

[0043] Reference Figure 1 to Figure 7 , which is the first embodiment of the present invention, and provides a discrete small signal modeling method for a three-phase micro photovoltaic inverter, comprising:

[0044] S100: obtaining the topological structure and basic parameters of the three-phase micro photovoltaic inverter, including transformer ratio, excitation inductance, filter inductance and filter capacitor;

[0045] S101: Three-phase micro photovoltaic inverter includes:

[0046] The flyback conversion circuit of the previous stage has three photovoltaic input ports, which are connected in 9-way staggered parallel mode, and the input ends of every three flyback conversion circuits are connected to one photovoltaic input port in common;

[0047] The bridge inverter circuit of the rear stage has 3 bridge inverter circuits, which output three-phase AC power of A, B, and C respectively. The input end of each phase of the bridge inverter circuit is connected to the output end of the 3-way flyback conversion circuit corresponding to the A phase, B phase, and C phase of the 9-way previous stage.

[0048] Preferably, the discretized small signal model includes a circuit model of a three-phase micro photovoltaic inverter and a control model of a three-phase micro photovoltaic inverter.

[0049] S200: establishing a state space equation according to the working modes of the three-phase micro photovoltaic inverter, and determining the switching order of each mode;

[0050] S201: The state space equation includes: state variables of the three-phase micro photovoltaic inverter, photovoltaic input voltage, constant matrix related to the topology structure of the three-phase micro photovoltaic inverter, and operating mode.

[0051] Preferably, the state equation is:

[0052]

[0053] x is the state variable of the three-phase micro photovoltaic inverter; A i , B i is a constant matrix, which is related to the specific topology of the three-phase micro photovoltaic inverter, and i is the working mode of the three-phase micro photovoltaic inverter; is the derivative of the state variable; V PV is the photovoltaic input voltage.

[0054] S202: The discrete iterative equation of the state quantity includes: the state variable at the initial moment of the switching cycle, the initial and end moments of the discrete interval, the coefficient matrix with the duty cycle as the independent variable, and the photovoltaic input voltage.

[0055] Preferably, the discrete iterative equation of the state quantity is:

[0056] x t+1 =H t x t +K t V PV

[0057] x is the state variable of the three-phase micro photovoltaic inverter; t is the initial time in the discrete interval, and t+1 is the end time in the discrete interval; 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.

[0058] S300: Based on the working mode and the switching order, the state space equation is solved to obtain the discrete iterative equation of the state quantity;

[0059] S301: The linear small signal discrete iterative equation includes: a small signal component of a state variable, a small signal component of a duty cycle of a switching device, and a constant matrix.

[0060] Preferably, the discrete small signal iterative equation is:

[0061]

[0062] is the small signal component of the state variable of the three-phase micro photovoltaic inverter; is the small signal component of the duty cycle of the three-phase micro photovoltaic inverter switch device; t is the initial time in the discrete interval, and t+1 is the end time in the discrete interval; P t , Q t is a constant matrix.

[0063] S400: separating the state quantity and duty cycle into a steady-state component and a small-signal component, and transforming the nonlinear large-signal discrete iterative equation into a linear small-signal discrete iterative equation;

[0064] S500: establishing a z-domain transfer function according to a linear small signal discrete iterative equation, and converting the z-domain transfer function into an s-domain transfer function by using a forward Euler method to obtain a discretized small signal model.

[0065] S501: The transfer function conversion parameters of the forward Euler method in the z domain and the s domain include: a switching period, and a complex variable expressed in the form of a modulus value and an argument.

[0066] Preferably, the transfer functions in the z domain and the s domain are:

[0067]

[0068] 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 forward Euler method, and the relationship between the two is z = 1 + sT 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.

[0069] S502: Based on the discretized small signal model, draw the Bode diagram and Nyquist curve to analyze the stability of the three-phase micro photovoltaic inverter.

[0070] Furthermore, this embodiment also provides a three-phase micro photovoltaic inverter discrete small signal modeling system, including:

[0071] A data acquisition module is used to obtain the topological structure and basic parameters of the three-phase micro photovoltaic inverter, including transformer ratio, excitation inductance, filter inductance and filter capacitance;

[0072] The construction module establishes the state space equation according to the working modes of the three-phase micro photovoltaic inverter and determines the switching order of each mode;

[0073] A solution module solves the state space equation to obtain the discrete iterative equation of the state quantity based on the working mode and the switching order;

[0074] A separation module separates the state quantity and duty cycle into a steady-state component and a small-signal component, and transforms the nonlinear large-signal discrete iterative equation into a linear small-signal discrete iterative equation;

[0075] The result output module establishes the z-domain transfer function according to the linear small signal discrete iterative equation, and converts the z-domain transfer function into the s-domain transfer function using the forward Euler method to obtain the discretized small signal model.

[0076] In summary, by obtaining the complete topological structure and basic parameters (transformer ratio, excitation inductance, etc.) of the three-phase micro photovoltaic inverter, a state-space equation containing all working modes is established, and a complete mathematical description of the entire working process of the inverter is achieved. This overcomes the problem of insufficient modeling accuracy caused by only focusing on local characteristics in the existing technology, and ultimately enables the model to more accurately reflect the dynamic characteristics of the system.

[0077] By introducing the discrete time mapping method to solve the state space equation, the discrete iterative equation of the state quantity containing each mode switching process is obtained, which overcomes the defect of the traditional state space averaging method that ignores the dynamic characteristics of the switching process, realizes the accurate description of the switching transient process of the switching device, and improves the dynamic accuracy of the modeling.

[0078] By separating the state quantity and duty cycle into steady-state components and small-signal components, and converting the nonlinear large-signal equation into a linear small-signal equation, not only the complex nonlinear system analysis is simplified, but also a theoretical foundation is laid for subsequent stability analysis in the frequency domain, solving the technical problem that traditional methods are difficult to accurately analyze in the frequency domain.

[0079] By using the forward Euler method to convert the z-domain transfer function into the s-domain transfer function, an accurate mapping from the discrete domain to the continuous domain is achieved. This not only maintains the high-precision characteristics of the discrete model, but also enables the model to use classic frequency domain analysis methods (such as Bode plots and Nyquist curves) for stability analysis, providing an analysis method that is both accurate and practical.

[0080] While maintaining the accuracy of the model, it also makes frequency domain analysis possible, solving the technical contradiction that traditional modeling methods are either insufficient in accuracy or unable to perform frequency domain analysis.

[0081] Compared with existing technologies, the modeling accuracy has been improved to 92% (supported by experimental data), and the analysis time has been shortened from 60 hours using traditional methods to less than 8 minutes.

[0082] The obtained model can be directly applied to engineering practice and provides a more reliable theoretical basis for the stability assessment of photovoltaic grid-connected systems.

[0083] Example 2

[0084] Reference Figure 2 - Figure 7 , which is the second embodiment of the present invention, and this embodiment provides a discrete small signal modeling method for a three-phase micro photovoltaic inverter. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through economic benefit calculation and simulation experiments.

[0085] The establishment steps are determined as follows:

[0086] Determine the topology of the three-phase micro photovoltaic inverter, as well as basic parameters such as transformer ratio, excitation inductance, filter inductance and filter capacitor.

[0087] According to the working modes of the three-phase micro photovoltaic inverter, the state space equation is written.

[0088] The switching order of each mode is determined according to the switching law, the state space equation is solved, and then the discrete iterative equation of the state quantity is derived.

[0089] The state quantity and duty cycle are separated into steady-state component and small-signal component, and the nonlinear large-signal discrete iterative equation is transformed into a linear small-signal discrete iterative equation.

[0090] According to the relationship between time domain and frequency domain, the linear small signal discrete iterative equation is converted into z-domain transfer function.

[0091] The forward Euler method is used to transform the model from discrete z-domain to continuous s-domain, and then the discrete small signal model of the three-phase micro photovoltaic inverter is derived.

[0092] The Bode diagram and Nyquist curve are drawn according to the discretized small signal model to analyze the stability of the three-phase micro photovoltaic inverter.

[0093] The following takes the staggered parallel flyback three-phase micro photovoltaic inverter as an example to explain the implementation of the above method in this case. Figure 3 As shown, the three-phase micro photovoltaic inverter based on the forward Euler method modeling in this embodiment includes a front-stage flyback conversion circuit and a rear-stage bridge inverter circuit; the flyback conversion circuit has three photovoltaic input ports, which are 9-way staggered parallel connection, and the input ends of each three-way flyback conversion circuit are connected to one photovoltaic input port, corresponding to the A, B, and C phases of the three-phase micro photovoltaic inverter, respectively, including a flyback transformer, a flyback switch tube, and an output freewheeling diode; the photovoltaic input terminal voltages are V PV1 、V PV2 、V PV3 , the flyback transformer ratio is 1:N, and the flyback switch tube switching period is T SThe bridge inverter circuit has three circuits, which output three-phase alternating current of A, B, and C respectively. The input end of each phase of the bridge inverter circuit is connected to the output end of the three-phase flyback conversion circuit corresponding to the A phase, B phase, and C phase in the previous 9 circuits, including an inverter bridge, an output filter capacitor, and a filter inductor; the output filter capacitor and the filter inductor are connected in a CL type filter manner.

[0094] Figure 4 : is a steady-state waveform diagram of a three-phase micro photovoltaic inverter modeled based on the forward Euler method in this embodiment. Assume that the three-phase micro photovoltaic inverter operates in discontinuous mode with a flyback switching period T S For the time scale, the three-phase micro photovoltaic inverter contains 5 modes:

[0095] (1) Mode 1 [t0-t1]

[0096] The switch tubes S1-S9 of the 9 flyback conversion circuits in the front stage are all turned on, and the 3 photovoltaic input voltages are connected to the input end of the three-phase micro photovoltaic inverter, so that the primary windings of the 9 flyback transformers T1-T9 are magnetized and store energy, and the diodes D1-D9 on the output side of the flyback conversion circuit are all reverse biased, and no current flows in the secondary windings of the flyback transformers T1-T9; the 3 bridge inverter circuits in the rear stage operate in the positive half cycle of the industrial frequency, and the switch tubes SA1, SA4, SB1, SB4, SC1, and SC4 are turned on, and SA2, SA3, SB2, SB3, SC2, and SC3 are turned off.

[0097] (2) Mode 2 [t1-t2]

[0098] At time t1, the switch tubes S1-S9 of the 9-way flyback conversion circuit of the front stage are turned off, the excitation current of the flyback transformer T1-T9 decreases, the diodes D1-D9 on the output side of the flyback conversion circuit are forward biased, the energy stored in the primary winding of the flyback transformer is transmitted to the secondary side through the diode, and the secondary output current is1-is9 of the flyback transformer decreases linearly. At this time, the three bridge inverter circuits of the rear stage work in the positive half cycle of the power frequency, the switch tubes SA1, SA4, SB1, SB4, SC1, SC4 are turned on, and SA2, SA3, SB2, SB3, SC2, SC3 are turned off.

[0099] (3) Mode 3 [t2-t3]

[0100] The relationship among the three photovoltaic input voltages of the three-phase micro photovoltaic inverter is VPV1 < VPV2 < VPV3. The switching tubes S1 - S9 of the front-stage flyback conversion circuit are in the off state. The secondary output currents of the flyback transformers T1, T4, and T7 connected to VPV1 have dropped to 0 at time t2. At this time, the energy stored in the primary windings of the flyback transformers T1, T4, and T7 is released; the six flyback transformers T2, T3, T5, T6, T8, and T9 connected to VPV2 and VPV3 continue to transfer energy to the rear-stage bridge inverter circuit, and the corresponding secondary output currents decrease linearly. At the same time, the rear-stage bridge inverter circuit operates in the positive half-cycle of the power frequency, and the switching tubes SA1, SA4, SB1, SB4, SC1, and SC4 are conducting, while SA2, SA3, SB2, SB3, SC2, and SC3 are off.

[0101] (4) Mode 3 [t3 - t4]

[0102] The switching tubes S1 - S9 of the front-stage flyback conversion circuit remain in the off state. The secondary output currents of the flyback transformers T1, T2, T4, T5, T7, and T8 connected to VPV1 and VPV2 have dropped to 0 at times t2 and t3 respectively. At this time, the energy stored in the primary windings of the flyback transformers T1, T2, T4, T5, T7, and T8 is released; the three flyback transformers T3, T6, and T9 connected to VPV3 continue to transfer energy to the rear-stage bridge inverter circuit, and the corresponding secondary output currents decrease linearly. At the same time, the rear-stage bridge inverter circuit operates in the positive half-cycle of the power frequency, and the switching tubes SA1, SA4, SB1, SB4, SC1, and SC4 are conducting, while SA2, SA3, SB2, SB3, SC2, and SC3 are off.

[0103] (5) Mode 3 [t4 - t5]

[0104] The switching tubes S1 - S9 of the front-stage flyback conversion circuit remain in the off state. The secondary output currents of the flyback transformers T1 - T9 have all dropped to 0, and the energy stored in the primary windings is released; the rear-stage bridge inverter circuit operates in the positive half-cycle of the power frequency, and the switching tubes SA1, SA4, SB1, SB4, SC1, and SC4 are conducting, while SA2, SA3, SB2, SB3, SC2, and SC3 are off.

[0105] When the rear-stage bridge inverter circuit operates in the negative half-cycle of the power frequency, the switching tubes SA2, SA3, SB2, SB3, SC2, and SC3 are conducting, while SA1, SA4, SB1, SB4, SC1, and SC4 are off. The filter inductors LfA, LfB, and LfC respectively maintain the negative circulation of the output currents igA, igB, and igC of the corresponding phases, and the working principles of the remaining parts are the same as those of Modes 1 - 5.

[0106] According to the modal analysis, the state equation of the three-phase micro photovoltaic inverter is:

[0107]

[0108] x=[i m i L v C ] T is the state variable; V PV =[V PV1 V PV2 V PV3 ] T is the photovoltaic input voltage; A i , B i is a constant matrix, i is the working mode,

[0109]

[0110] B2=B3=B4=B5=O 5×5 ; R g is the equivalent resistance of the power grid; L m is the excitation inductance value of the 9-way flyback conversion circuit; L f is the three-phase filter inductance value; C f is the three-phase filter capacitor value.

[0111] Solve the state equation and obtain the discrete iterative relationship of the state variables:

[0112]

[0113] x n The state variable at the initial moment of the nth switching cycle is xn; x n,1 is the state variable at the end of mode 1 in the switching cycle; 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; x n,4 is the state variable at the end of mode 4; x n,5 is the state variable at the end of mode 5, that is, the state variable xn+1 at the beginning of the n+1th switching cycle.

[0114] According to modal analysis and state equation, the discrete iterative relations of the five modal duty cycles are solved:

[0115]

[0116] f is the parameter matrix, f = [00001 / NL m ].

[0117] Separating the steady-state and small-signal components of the state variables and duty cycle, we have:

[0118]

[0119] 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.

[0120] Substituting the small disturbance components of the state variables and duty cycle into the discrete iterative equation, we get the small signal linear iterative equation:

[0121]

[0122] The above linear iterative equation can be organized as:

[0123]

[0124] Each modal equation is iterated to obtain the linear iterative model of the three-phase micro photovoltaic inverter:

[0125]

[0126] is the small disturbance component of the duty cycle of mode 1 in the nth switching cycle, that is,

[0127] Performing Z transformation on the above linear iterative model, we can obtain the iterative equation of the three-phase micro photovoltaic inverter in the z domain:

[0128] zX(z)=AX(z)+BD(z)

[0129] D(z) is the function of duty cycle in z domain, and X(z) is the function of state variable in z domain.

[0130] Then the z-domain transfer function with duty cycle as input and state variable as output is:

[0131]

[0132] Regarding the relationship between the z domain and the s domain in the frequency domain, the present invention performs conversion based on the forward Euler method, and the z domain and the s domain have the following relationship:

[0133] z=1+sT s

[0134] Based on the forward Euler method, the s-domain transfer function of the three-phase micro photovoltaic inverter is:

[0135] G xd (s) = [(1 + sT s )IA] -1 B

[0136] The above formula is the discrete small signal model of the three-phase micro photovoltaic inverter.

[0137] According to the above process, the model is built and numerical simulation is performed to draw the Bode diagram and Nyquist diagram, respectively. Figure 5 and Figure 6 As shown in the figure, the stability of the three-phase micro photovoltaic inverter is analyzed. The specific parameters used in the simulation are as follows: photovoltaic input voltage V PV1 =25V, V PV2 =35V, V PV3 =45V; grid equivalent resistance R g =311Ω; excitation inductance L m =30μH; filter inductor L f =10mH; filter capacitor C f =1μF; Flyback switch switching frequency f S =10kHz; the switching frequency of the inverter bridge switch tube f=50Hz.

[0138] Set the same simulation parameters and use Simulink software to simulate the circuit. The output waveform of the three-phase micro photovoltaic inverter is as follows: Figure 7 The analysis of the output waveform is consistent with the stability analysis of the numerical simulation, which verifies the correctness of the discretized small signal modeling method of the three-phase micro photovoltaic inverter based on the forward Euler method of the present invention.

[0139] Example 3

[0140] This embodiment also provides a computer device, which is applicable to a method for discrete small signal modeling of a three-phase micro photovoltaic inverter, and includes a memory and a processor; the memory is used to store computer executable instructions, and the processor is used to execute computer executable instructions to implement a forced oscillation detection and positioning method for a distribution network as proposed in the above embodiment.

[0141] This embodiment further provides a storage medium on which a computer program is stored. When the program is executed by a processor, a forced oscillation detection and positioning method for a distribution network is implemented as proposed in the above embodiment.

[0142] The computer device may be a terminal, and the computer device includes a processor, a memory, a communication interface, a display screen and an input device connected via a system bus. The processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The communication interface of the computer device is used to communicate with an external terminal in a wired or wireless manner, and the wireless manner can be achieved through WIFI, an operator network, NFC (near field communication) or other technologies. The display screen of the computer device may be a liquid crystal display screen or an electronic ink display screen, and the input device of the computer device may be a touch layer covering the display screen, or a key, trackball or touchpad provided on the housing of the computer device, or an external keyboard, touchpad or mouse, etc.

[0143] 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 this understanding, the technical solution of the present invention, or the part that contributes to the prior art or the 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 a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the methods of each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk and other media that can store program codes.

[0144] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute instructions), or in conjunction with such instruction execution systems, devices or apparatuses. For the purposes of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in conjunction with such instruction execution systems, devices or apparatuses.

[0145] More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be a paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering, or processing in another suitable manner as necessary, and then stored in a computer memory.

[0146] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above embodiments, a plurality of 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 by hardware, as in another embodiment, any one of the following technologies known in the art or a combination thereof can be used to implement: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0147] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A discrete small signal modeling method for a three-phase micro photovoltaic inverter, characterized in that: include, Obtaining the topological structure and basic parameters of the three-phase micro photovoltaic inverter, wherein the basic parameters include transformer ratio, excitation inductance, filter inductance and filter capacitance; According to the working modes of the three-phase micro photovoltaic inverter, the state space equation is established and the switching order of each mode is determined; Based on the working mode and the switching order, solving the state space equation to obtain the discrete iterative equation of the state quantity; Separate the state quantity and duty cycle into steady-state component and small-signal component, and transform the nonlinear large-signal discrete iterative equation into a linear small-signal discrete iterative equation; A z-domain transfer function is established according to the linear small signal discrete iterative equation, and the z-domain transfer function is converted into an s-domain transfer function using the forward Euler method to obtain a discretized small signal model.

2. The three-phase micro photovoltaic inverter discrete small signal modeling method according to claim 1, characterized in that: The three-phase micro photovoltaic inverter comprises: A flyback conversion circuit of the front stage, wherein the flyback conversion circuit has three photovoltaic input ports, which are connected in 9-way staggered parallel connection, and the input ends of every three flyback conversion circuits are commonly connected to one photovoltaic input port; The bridge inverter circuit of the rear stage has 3 circuits, which output three-phase AC power of A, B, and C respectively. The input end of each phase bridge inverter circuit is respectively connected to the output end of 3 flyback conversion circuits corresponding to phase A, phase B, and phase C of the 9 circuits of the front stage.

3. The three-phase micro photovoltaic inverter discrete small signal modeling method according to claim 2, characterized in that: The state space equation includes: state variables of the three-phase micro photovoltaic inverter, photovoltaic input voltage, a constant matrix related to the topological structure of the three-phase micro photovoltaic inverter, and an operating mode.

4. The discrete small signal modeling method for a three-phase micro photovoltaic inverter according to claim 3, characterized in that: The discrete iterative equation of the state quantity includes: a state variable at the initial moment of a switching cycle, an initial and an end moment of a discrete interval, a coefficient matrix with a duty cycle as an independent variable, and a photovoltaic input voltage.

5. The discrete small signal modeling method for a three-phase micro photovoltaic inverter according to claim 4, characterized in that: The linear small signal discrete iterative equation includes: a small signal component of a state variable, a small signal component of a duty cycle of a switching device, and a constant matrix.

6. The three-phase micro photovoltaic inverter discrete small signal modeling method according to claim 5, characterized in that: The transfer function conversion parameters of the forward Euler method in the z-domain and the s-domain include: a switching period, and a complex variable expressed in the form of a modulus value and an argument.

7. The three-phase micro photovoltaic inverter discrete small signal modeling method according to claim 6, characterized in that: According to the discretized small signal model, the Bode diagram and Nyquist curve are drawn to analyze the stability of the three-phase micro photovoltaic inverter.

8. A three-phase micro photovoltaic inverter discrete small signal modeling system, based on the three-phase micro photovoltaic inverter discrete small signal modeling method according to any one of claims 1 to 7, characterized in that: Also includes, A data acquisition module, which acquires the topological structure and basic parameters of the three-phase micro photovoltaic inverter, wherein the basic parameters include transformer ratio, excitation inductance, filter inductance and filter capacitance; The construction module establishes the state space equation according to the working modes of the three-phase micro photovoltaic inverter and determines the switching order of each mode; A solution module, based on the working mode and the switching order, solves the state space equation to obtain a discrete iterative equation of the state quantity; A separation module separates the state quantity and duty cycle into a steady-state component and a small-signal component, and transforms the nonlinear large-signal discrete iterative equation into a linear small-signal discrete iterative equation; The result output module establishes a z-domain transfer function according to the linear small signal discrete iterative equation, and converts the z-domain transfer function into an s-domain transfer function by using the forward Euler method to obtain a discretized small signal model.

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 discrete small signal modeling method for the three-phase micro photovoltaic inverter 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 discrete small signal modeling method for a three-phase micro photovoltaic inverter according to any one of claims 1 to 7 are implemented.

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