Iterative method and system for harmonic disturbance emission of grid-connected equipment based on small signal increment

Abstract

The application discloses a grid-connected equipment harmonic disturbance emission iteration method and system based on a small signal increment, writes a voltage-current relationship equation required for calculating a harmonic current; initial steady-state working point related data variables of a typical photovoltaic grid-connected inverter structure are expressed by using a steady-state component and a small signal component; the steady-state component and the small signal component are substituted to obtain a system small signal linearization equation; the small signal component is substituted into the system small signal linearization equation to obtain a small signal linearization time-invariant equation group; background harmonic voltage in grid voltage is divided into small disturbances in multiple stages, the amplitude of V gn is determined; the result is superimposed into the steady-state component to update the steady-state working point, and a small signal linearization time-invariant equation group required for next stage solving is obtained; the steady-state working point and the equation group are continuously updated until the applied background harmonic voltage is updated to be the same as a set target; the amplitude and the phase of each time output current harmonic are obtained.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power distribution network harmonic characteristic analysis, and particularly relates to a grid-connected equipment harmonic disturbance emission iteration method and system based on small signal increment. BACKGROUND

[0002] New energy is connected to the grid on a large scale through full-controlled power electronic equipment, and the harmonic problem in the power distribution network is increasingly prominent. The harmonic source is widely distributed and has greatly increased in proportion, which has caused not small influence on the stable operation of the power grid and power quality. Various power electronic equipment generates harmonic current under the excitation of grid voltage, thereby causing grid voltage distortion. Therefore, it is necessary to calculate and study the characteristics of the harmonic current generated by power electronic equipment under the excitation of grid voltage.

[0003] For power electronic equipment using diodes or thyristors and the like uncontrolled or semi-controlled devices, the current characteristics under specific voltage excitation are easy to calculate due to the limited control of the turn-on and turn-off of the devices. At present, the characteristics of harmonic disturbance emission have been relatively mature. With the progress of device level, the application of equipment using full-controlled power electronic devices is increasingly widespread. Compared with equipment using uncontrolled or semi-controlled devices, full-controlled power electronic equipment can control the turn-on and turn-off of the devices through a digital signal controller, and the digital signal has multiple generation methods. At present, the most commonly used method is pulse width modulation technology, which generates a modulation wave by comparing a carrier signal generated by a digital control. Due to the nonlinearity of the control, the switching function has strong nonlinear characteristics, and it is difficult to directly calculate the steady-state harmonic current under voltage disturbance according to the voltage and current equation. Moreover, the calculation cost is high and the speed is slow.

[0004] The existing research focuses on the summary of the characteristics of the emission characteristics of specific equipment, and the internal emission mechanism and equivalent model are less explored. SUMMARY

[0005] The technical problem to be solved by the application is to provide a grid-connected equipment harmonic disturbance emission iteration method and system based on small signal increment to reveal the internal harmonic emission mechanism of a certain type of full-controlled power electronic equipment through fast and accurate calculation, comprehensively analyze and summarize the emission characteristics, and solve the technical problem of calculation difficulty caused by the nonlinearity and time variation of the voltage and current equation of full-controlled power electronic equipment.

[0006] The application adopts the following technical scheme:

[0007] The grid-connected equipment harmonic disturbance emission iteration method based on small signal increment comprises the following steps:

[0008] S1, obtaining initial steady-state working point related data of a typical photovoltaic grid-connected inverter structure, and writing voltage-current relationship equations required for calculating harmonic currents according to relationships between AC and DC circuits and between controls;

[0009] S2, expressing variables of the initial steady-state working point related data of the typical photovoltaic grid-connected inverter structure obtained in step S1 by using steady-state components and small-signal components;

[0010] S3, substituting the steady-state components and the small-signal components obtained in step S2 into the voltage-current relationship equations obtained in step S1, retaining small-signal component relationships, and obtaining system small-signal linearization equations;

[0011] S4, expressing each small-signal component in the small-signal linearization equations obtained in step S3 by using a sum of harmonics;

[0012] S5, substituting the small-signal components obtained in step S4 into the small-signal linearization equations obtained in step S3, converting multiplication of a periodic time-varying coefficient and a periodic time-varying variable into multiplication between harmonics, expressing as a relationship between harmonic amplitudes, and obtaining a small-signal linearization time-invariant equation set;

[0013] S6, substituting other steady-state components under a fundamental component of a grid voltage into the small-signal linearization time-invariant equation set obtained in step S5, determining a direct-current voltage V dc0 , outputting a current instruction I ref1 , outputting a fundamental current amplitude I1, a modulation wave fundamental amplitude M1, a fundamental amplitude V g1 of a grid voltage, and a grid fundamental voltage frequency ω1;

[0014] S7, dividing background harmonic voltages in the grid voltage into small perturbations in multiple stages, substituting a grid harmonic voltage amplitude in a first stage into the small-signal linearization time-invariant equation set obtained in step S5, and determining an amplitude of V gn ;

[0015] S8, solving the small-signal linearization time-invariant equation set obtained in step S5, obtaining small-perturbation responses of each state variable under the background harmonic voltage perturbation in this stage, superimposing the small-perturbation responses of each state variable obtained by solving into steady-state components, updating a steady-state working point, obtaining a small-signal linearization time-invariant equation set required for solving in a next stage, and continuously updating the steady-state working point and the equation set until the background harmonic voltage applied is updated to be the same as a set target;

[0016] S9, solving the small-signal linearization time-invariant equation set, and obtaining amplitudes and phases of each output current harmonic.

[0017] Preferably, the initial steady-state working point related data of the typical photovoltaic grid-connected inverter structure includes a fundamental amplitude V g1, DC voltage V dc0 , output current command I ref1 , output AC current amplitude I1, modulation wave amplitude M1, grid voltage phase θ.

[0018] Preferably, the voltage-current relationship equation required for calculating the harmonic current comprises:

[0019] The voltage-current relationship of the line inductance is:

[0020]

[0021] Where L is the inductance of the inverter and grid connection, i is the output AC current, t is time, v dc is the DC voltage, m is the modulation wave, v g is the AC voltage;

[0022] The DC side and AC side current relationship is

[0023]

[0024] Where C dc is the DC side capacitor, * represents the conjugate;

[0025] The current controller controls the output current to track the current command, and the relationship between the modulation wave and the output current is:

[0026] m=G PR (t)*(e jθ i ref -i)

[0027] Where G PR (t) is the current controller, θ is the grid voltage phase extracted by the phase-locked loop, i ref is the output current command;

[0028] The inverter adopts DC voltage control mode, and the active current command is generated from the DC side voltage, and the relationship between them is:

[0029] i ref =H dvc (t)*(v dc -V dcR )

[0030] Where H dvc (t) is the DC voltage controller, V dcR is the DC voltage command;

[0031] The phase-locked loop extracts the grid point voltage phase θ as:

[0032]

[0033] wherein h pll (t) is a phase-locked loop controller, v gb is the grid voltage after fundamental band-pass filtering;

[0034] The grid voltage is band-pass filtered v gb

[0035] v gb = G bd (t) * v g

[0036] wherein G bd (t) is a fundamental band-pass filter.

[0037] Preferably, in step S2, for the steady-state component, the sum of the fundamental steady-state component and each harmonic component is expressed as follows:

[0038]

[0039] wherein ω1 is the fundamental voltage frequency, v dc is the DC voltage, V dcn is the n-th DC voltage amplitude, is the DC voltage small-signal component, i ref is the output current command, I refn is the n-th output current command amplitude, is the output current command small-signal component, i is the output AC current, I n is the n-th output current amplitude, is the output current small-signal component, m is the modulation wave, M n is the n-th modulation wave amplitude, is the modulation wave small-signal component, v g is the AC voltage, V gn is the n-th AC voltage amplitude, is the AC voltage small-signal component, v gb is the grid voltage after fundamental band-pass filtering, V g1 is the AC voltage fundamental amplitude, is the grid voltage small-signal component after fundamental band-pass filtering, θ is the grid voltage phase extracted by the phase-locked loop, is the grid voltage phase small-signal component.

[0040] Preferably, in step S3, the small-signal linearization equation of the system is as follows:

[0041]

[0042] wherein L is the inductance of the connection between the inverter and the grid, is the output current small-signal component, V​dcn is the n-th DC voltage amplitude, ω1is the fundamental voltage frequency, is the modulation wave small signal component, M n is the n-th modulation wave amplitude, is the DC voltage small signal component, is the AC voltage small signal component, C dc is the DC side capacitance, is the DC voltage small signal component, * denotes the conjugate, I n is the n-th output current amplitude, G PR (t) is the current controller, is the output current command small signal component, I refn is the n-th output current command amplitude, is the grid voltage phase small signal component, H dvc (t) is the DC voltage controller.

[0043] Preferably, in step S4, each small signal component in the small signal linearization equation is expressed as a sum of a series of harmonics, specifically as follows:

[0044]

[0045] wherein, is the DC voltage small signal component, is the output current command small signal component, is the output current small signal component, is the modulation wave small signal component, is the AC voltage small signal component, is the grid voltage small signal component after fundamental band-pass filtering, is the grid voltage phase small signal component, ω1is the fundamental voltage frequency, is the n-th DC voltage small signal component amplitude, is the n-th output current command small signal component amplitude, is the n-th output current small signal component amplitude, is the n-th modulation wave small signal component amplitude, is the n-th AC voltage small signal component amplitude, is the grid voltage small signal component after fundamental band-pass filtering, is the grid voltage phase small signal component.

[0046] Preferably, in step S5, the invariable equation set when small signal linearization is as follows:

[0047]

[0048] Wherein, A1, A2, A3 are coefficient matrixes, L is the inductance connected between the inverter and the power grid, is the amplitude of the n-th order output current small signal component, is the amplitude of the n-th order modulation wave small signal component, is the amplitude of the n-th order DC voltage small signal component, is the amplitude of the n-th order AC voltage small signal component.

[0049] Preferably, the coefficient matrixes A1, A2, A3 are respectively:

[0050]

[0051]

[0052] Wherein, n is the harmonic order, ω1 is the fundamental voltage frequency, V d,cn is the amplitude of the n-th order DC voltage, M n is the amplitude of the n-th order modulation wave.

[0053] Preferably, in step S7, the harmonic small disturbance amplitude of each stage does not exceed 3% of the fundamental voltage amplitude.

[0054] In a second aspect, the embodiment of the present application provides a grid-connected equipment harmonic disturbance emission iteration system based on small signal increment, comprising:

[0055] A data module acquires initial steady state working point related data of a typical photovoltaic grid-connected inverter structure, and writes voltage-current relationship equations required for calculating harmonic currents according to AC-DC circuit relationship and control relationship;

[0056] An expression module expresses initial steady state working point related data variables of the typical photovoltaic grid-connected inverter structure with steady state components and small signal components; substitutes the steady state component and the small signal component expression into the voltage-current relationship equation, retains the small signal component relationship, and obtains a system small signal linearization equation; expresses each small signal component in the small signal linearization equation with a harmonic sum;

[0057] An equation module substitutes the small signal component into the small signal linearization equation, multiplies the periodic time-varying coefficient with the periodic time-varying variable to convert into a product between each harmonic, expresses as a relationship between each harmonic amplitude, and obtains a small signal linearization time-invariant equation group; substitutes other steady state components under the grid voltage fundamental component into the small signal linearization time-invariant equation group, determines the DC voltage V dc0 , the output current instruction I ref1 , the output fundamental current amplitude I1, the modulation wave fundamental amplitude M1, the fundamental amplitude V g1 of the grid voltage, and the grid fundamental voltage frequency ω1.

[0058] The dividing module divides the background harmonic voltage in the grid voltage into a plurality of stages of small perturbations, substitutes the amplitude of the harmonic voltage of the first stage into the small-signal linearization time-invariant equation set to determine the amplitude of V gn ;

[0059] The iteration module solves the obtained small-signal linearization time-invariant equation set to obtain the small-perturbation response of each state variable under the background harmonic voltage perturbation of this stage, superimposes the obtained small-perturbation response of each state variable to the steady-state component to update the steady-state working point, and obtains the small-signal linearization time-invariant equation set required for the next stage of solving; the steady-state working point and the equation set are updated constantly until the applied background harmonic voltage is updated to be the same as the set target; the small-signal linearization time-invariant equation set is solved to obtain the amplitude and phase of each output current harmonic.

[0060] In a third aspect, a computer device includes a memory, a processor, and a computer program stored in the memory and executable on the processor, and the processor implements the steps of the above small-signal incremental grid-connected device harmonic perturbed emission iterative method when executing the computer program.

[0061] In a fourth aspect, an embodiment of the present application provides a computer readable storage medium including a computer program, and the computer program implements the steps of the above small-signal incremental grid-connected device harmonic perturbed emission iterative method when executed by a processor.

[0062] In a fifth aspect, a chip includes a memory, a processor, and a computer program stored in the memory and executable on the processor, and the processor implements the steps of the above small-signal incremental grid-connected device harmonic perturbed emission iterative method when executing the computer program.

[0063] In a sixth aspect, an embodiment of the present application provides an electronic device including a computer program, and the computer program implements the steps of the above small-signal incremental grid-connected device harmonic perturbed emission iterative method when executed by the electronic device.

[0064] Compared with the prior art, the present application has at least the following beneficial effects:

[0065] A small-signal incremental grid-connected device harmonic perturbed emission iterative method, which is used for solving the small-signal linearization time-invariant equation set obtained by substituting the amplitude of the harmonic voltage of the first stage into the small-signal linearization time-invariant equation set to determine the amplitude of V Figure 1The harmonic calculation of the middle photovoltaic inverter is to take the grid voltage as input and calculate the harmonic characteristics in the output current under the excitation of the grid voltage. The formula obtained in step S1 has typical nonlinear characteristics such as multiplication between variables and exponential operation, and the AC voltage and current in the equation are time-varying variables, so it is difficult to directly calculate the harmonics. In order to solve the nonlinear problem, step S2 represents the variables in step S1 with steady-state components and small-signal components. Then, step S3 adopts the small-signal linearization method to convert the nonlinear equation in step S1 into a linear equation near the operating point. Since the steady-state quantity is a known quantity and the small-signal quantity is a to-be-solved quantity, there is no multiplication between variables in the equation, and there is no exponential equation either. The equation is linear, and obviously the linear equation is easier to calculate than the nonlinear equation, and the calculation amount is reduced exponentially. Since the steady-state quantity coefficient in step S3 changes with time and changes periodically with the fundamental period, in order to convert the coefficient period time-varying equation into a coefficient period time-invariant equation, step S4 also splits the small-signal variable into a series of harmonic sums, and this series of harmonics has the same reference frequency as the period time-varying coefficient. Steps S5 and S4 together use the harmonic transfer function method to convert the multiplication of the period time-varying coefficient (steady-state quantity coefficient) and the period time-varying variable (small-signal quantity) into the product of each harmonic, and further simplify it to the relationship between the amplitudes of each harmonic. Through the above steps, the nonlinear time-varying equation is converted into a linear time-invariant equation set, and a series of coefficients can also be represented by a matrix with obvious characteristics, so it is very convenient to use matrix calculation for linear solution. The process of dynamic iterative calculation is given, that is, the "large disturbance" of the background harmonic voltage is dispersed into multiple stages of "small disturbance", the disturbance response under the excitation of each stage "small disturbance" is superimposed on the steady-state component to update the steady-state operating point, and then the next stage of iteration is more accurately performed until the harmonic voltage reaches the predetermined target. In this process, the calculation result of the last stage is superimposed on the steady-state working quantity of the last stage to obtain the steady-state working quantity of the next stage, so the coefficients of the to-be-solved variables in the equation of each stage are known, and the calculation relationship between harmonics and fundamental waves and harmonics can be considered at the same time, and finally the steady-state result of the harmonic disturbance emission is accurately and quickly calculated.

[0066] Further, according to the main circuit structure and control structure of the typical photovoltaic grid-connected inverter, the grid voltage, output current, DC side current, DC side voltage, modulation wave, grid voltage phase will interact and be related to the current controller, DC voltage controller, phase-locked loop controller and other control links, so the voltage and current relationship equation is set to quantitatively calculate the harmonic current emission of the converter under the excitation of the background harmonic voltage.

[0067] Further, since the typical photovoltaic grid-connected inverter adopts full-controlled power electronic devices, the switching function has strong nonlinearity, which will produce current harmonic response different from the voltage excitation harmonic number. By expressing the steady-state component with the fundamental and each harmonic component, the harmonic disturbance emission can be more clearly expressed. At the same time, considering that the present application divides the harmonic large disturbance into multiple stages of harmonic small disturbance, the current equation of each stage is updated, i.e. the steady-state operating point is updated, and the harmonic component can be used to describe the influence of different harmonic excitation.

[0068] Further, the original nonlinear equation has variables multiplied by variables and exponential operations, which is complex to calculate. By using small signal decomposition, the nonlinear equation is converted into a linearized equation near the steady-state operating point. Since the steady-state quantity is a known quantity and the small signal quantity is a to-be-solved quantity, there is no longer a case of variables multiplied by variables in the equation, and there is no longer an exponential equation. The equation is linear, and obviously the linearized equation is easier to calculate than the nonlinear equation, and the calculation amount is exponentially reduced.

[0069] Further, since the steady-state quantity coefficient in the above small signal linearized equation is periodically time-varying, in order to convert the coefficient periodically time-varying equation into a coefficient periodically time-invariant equation, the harmonic transfer function method is adopted, i.e. each small signal component in the small signal linearized equation is expressed as a series of harmonic sum. After the series of harmonic small signal components are substituted into the small signal linearized equation, the multiplication of the periodically time-varying coefficient (steady-state quantity coefficient) and the periodically time-varying variable (small signal quantity) can be converted into the product between each harmonic, and can be expressed as the relationship between the amplitudes of each harmonic. This conversion solves the problem of difficult calculation of the coefficient time-varying equation.

[0070] Further, when the harmonic component in the external grid voltage belongs to "large disturbance", it is very likely that the "small disturbance" characteristic is not met, at which time the small signal equation cannot describe the nonlinear characteristics of the harmonic "large disturbance". Therefore, multiple stages of small signal increment iterative calculation are used to approximate the nonlinear process of "large disturbance", and the method of dividing stages is that the harmonic small disturbance amplitude of each stage does not exceed 3% of the fundamental voltage amplitude, so as to ensure that the small signal equation can effectively calculate the change of each stage.

[0071] It can be understood that the beneficial effects of the above-mentioned second aspect to sixth aspect can be referred to the related description in the above-mentioned first aspect, which will not be repeated here.

[0072] In summary, the application solves the calculation and analysis problem of harmonic emission of full-controlled power electronic equipment under the background voltage 'large disturbance' excitation. By dispersing the 'large disturbance' into multiple stages of'small disturbance', using small signal equation to solve the'small disturbance' response and superimposing it to the steady-state component, updating and iteratively calculating the new small signal equation, the steady-state harmonic response under 'large disturbance' can be accurately quantitatively solved, and the calculation precision is high. At the same time, the application uses small signal equation linearization and harmonic transfer function transformation to convert the periodic time-varying nonlinear equation set into a linear time-invariant equation set, which can be linearly solved by matrix operation, greatly reducing the calculation complexity.

[0073] The technical solutions of the application will be further described in detail below with the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0074] Figure 1 It is a typical photovoltaic grid-connected inverter control structure diagram;

[0075] Figure 2 It is a harmonic disturbance emission iterative calculation flowchart based on small signal increment;

[0076] Figure 3 is a comparison diagram of the theoretical calculation value and the simulation value of the harmonic current of a typical photovoltaic grid-connected inverter, wherein (a) is a harmonic current amplitude comparison diagram, and (b) is a harmonic current phase comparison diagram;

[0077] Figure 4 It is a schematic diagram of a computer device provided by an embodiment of the application;

[0078] Figure 5 It is a block diagram of a chip provided by an embodiment of the application. DETAILED DESCRIPTION

[0079] The technical solutions in the embodiments of the application will be described clearly and completely below with the drawings in the embodiments of the application. Obviously, the described embodiments are part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0080] In the description of the application, it should be understood that the terms 'include' and 'contain' indicate the existence of described features, whole, steps, operations, elements and / or components, but do not exclude the existence or addition of one or more other features, whole, steps, operations, elements, components and / or sets thereof.

[0081] It should also be understood that the terms used in the present specification are only for the purpose of describing particular embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, the singular forms "a", "an", and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0082] It should be further understood that the term "and / or" as used in the present specification and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items. For example, A and / or B may represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in the present invention generally indicates that the associated objects are in an "or" relationship.

[0083] It should be understood that although the terms "first," "second," and "third" may be used to describe preset ranges in embodiments of the present invention, these preset ranges should not be limited to these terms. These terms are merely used to distinguish one preset range from another. For example, without departing from the scope of embodiments of the present invention, the first preset range may also be referred to as the second preset range, and similarly, the second preset range may also be referred to as the first preset range.

[0084] The word "if," as used herein, may be interpreted as "at the time of" or "when" or "in response to determining" or "in response to detecting," depending on the context. Similarly, the phrases "if it is determined" or "if (stated condition or event) is detected" may be interpreted as "when it is determined" or "in response to the determination" or "when detecting (stated condition or event)" or "in response to detecting (stated condition or event)," depending on the context.

[0085] The accompanying drawings illustrate various schematic diagrams of structures according to embodiments disclosed herein. These figures are not drawn to scale; for clarity, some details are exaggerated and some details may be omitted. The shapes of the various regions and layers shown in the figures, as well as their relative sizes and positional relationships, are merely exemplary and may deviate in practice due to manufacturing tolerances or technical limitations. Those skilled in the art may design regions / layers with different shapes, sizes, and relative positions as needed.

[0086] The application provides a small-signal increment-based grid-connected equipment harmonic disturbance emission iteration method, which can be used for calculating harmonic current emission under excitation of three-phase unbalanced negative sequence voltage and inter-harmonic voltage, and can be used for calculating harmonic current emission under excitation of three-phase unbalanced negative sequence voltage and inter-harmonic voltage. For three-phase unbalanced negative sequence voltage, the frequency of the harmonic voltage can be regarded as -ω1. For inter-harmonic voltage, the reference frequency can be reduced, such as being reduced from 50Hz interval to 10Hz interval, so that the inter-harmonic component can be calculated.

[0087] Please refer to Figure 2 The application provides a small-signal increment-based grid-connected equipment harmonic disturbance emission iteration method, which comprises the following steps:

[0088] S1, for a typical photovoltaic grid-connected inverter structure, obtaining initial steady-state working point related data, i.e. the fundamental wave amplitude V g1 of the grid voltage, the DC voltage V dc0 , the output current instruction I ref1 , the output AC current amplitude I1, the modulation wave amplitude M1 and the grid voltage phase θ; according to the relationship between the AC-DC circuit and the control, the voltage-current relationship equation required for calculating the harmonic current is written;

[0089] The voltage-current relationship of the line inductance is

[0090]

[0091] Wherein, L is the inductance connected between the inverter and the grid, i is the output AC current, t is time, V dc is the DC voltage, m is the modulation wave, V g is the AC voltage.

[0092] The current relationship between the DC side and the AC side is

[0093]

[0094] Wherein, C dc is the DC side capacitor, * represents conjugate.

[0095] The current controller controls the output current to track the current instruction, and the relationship between the modulation wave and the output current is

[0096] m=G PR (t)*(e jθ i ref -i) (3)

[0097] Wherein, G PR (t) is the current controller, θ is the grid voltage phase extracted by the phase-locked loop, and i ref is the output current instruction.

[0098] The inverter adopts a DC voltage control mode, and active current commands are generated from the DC side voltage, and the relationship is:

[0099] i ref =H dvc (t)*(v dc -V dcR ) (4)

[0100] Where H dvc (t) is a DC voltage controller, and V dcR is a DC voltage command.

[0101] The phase-locked loop extracts the grid voltage phase, and the calculation formula is

[0102]

[0103] Where h pll (t) is a phase-locked loop controller, and v gb is the grid voltage after band-pass filtering.

[0104] The calculation formula of the grid voltage after band-pass filtering is

[0105] v gb =G bd (t)*v g (6)

[0106] Where G bd (t) is a fundamental band-pass filter.

[0107] S2, the variables such as voltage, current, modulation wave, phase and the like appearing in step S1 are expressed by steady-state components and small-signal components, that is, the expressions of the seven variables of DC voltage, output current command, output AC current, modulation wave, AC voltage, grid voltage after band-pass filtering, and grid voltage phase; for the steady-state component, the sum of the fundamental steady-state component and each harmonic component is expressed as follows:

[0108]

[0109] Where ω1 is the fundamental voltage frequency, v dc is the DC voltage, V dcn is the n-th DC voltage amplitude, is the DC voltage small-signal component, i ref is the output current command, I refn is the n-th output current command amplitude, is the output current command small-signal component, i is the output AC current, I n is the n-th output current amplitude, is the output current small-signal component, m is the modulation wave, Mn is the n-th order modulation wave amplitude, is the modulation wave small signal component, v g is the AC voltage, V gn is the n-th order AC voltage amplitude, is the AC voltage small signal component, v gb is the grid voltage after fundamental band-pass filtering, V g1 is the AC voltage fundamental amplitude, is the grid voltage small signal component after fundamental band-pass filtering, θ is the grid voltage phase extracted by the phase-locked loop, is the grid voltage phase small signal component.

[0110] S3, the steady-state component and the small signal component formula (7) obtained in step S2 are substituted into the voltage and current relationship equation formula (1)-(6) obtained in step S1, and the small signal component relationship therein is retained to obtain the small signal linearization equation of the system, see formula (8)-(12);

[0111]

[0112]

[0113] wherein L is the inductance connected between the inverter and the grid, is the output current small signal component, V dcn is the n-th order DC voltage amplitude, ω1 is the fundamental voltage frequency, is the modulation wave small signal component, M n is the n-th order modulation wave amplitude, is the DC voltage small signal component, is the AC voltage small signal component, C dc is the DC side capacitor, is the DC voltage small signal component, * denotes conjugate, I n is the n-th order output current amplitude, G PR (t) is the current controller, is the output current command small signal component, I refn is the n-th order output current command amplitude, is the grid voltage phase small signal component, H dvc (t) is the DC voltage controller.

[0114] S4, for the small signal linearization equation obtained in step S3, formula (8)-(12), each small signal component therein is expressed as a series of harmonic wave sums, i.e., the seven small signal components of the DC voltage, the output current command, the output AC current, the modulation wave, the AC voltage, the grid voltage after fundamental band-pass filtering, and the grid voltage phase;

[0115]

[0116] wherein, is a direct voltage small signal component, is an output current command small signal component, is an output current small signal component, is a modulation wave small signal component, is an alternating voltage small signal component, is a fundamental wave band-pass filtered grid voltage small signal component, is a grid voltage phase small signal component, and ω1 is a fundamental voltage frequency, is an n-th direct voltage small signal component amplitude, is an n-th output current command small signal component amplitude, is an n-th output current small signal component amplitude, is an n-th modulation wave small signal component amplitude, is an n-th alternating voltage small signal component amplitude, is a fundamental wave band-pass filtered grid voltage small signal component, is a grid voltage phase small signal component.

[0117] S5, the small signal component formula (13) obtained in step S4 is substituted into the small signal linearization equation formula (8)-(12) obtained in step S3, the periodic time-varying coefficient (steady-state quantity coefficient) is multiplied by the periodic time-varying variable (small signal quantity) to be converted into the product between each harmonic, and can be expressed as the relationship between the amplitudes of each harmonic;

[0118] The following only lists the conversion of formula (8), and the obtained formula is shown in formula (14). The coefficients of the equation set are time-invariant, and the equation set is a linear equation set.

[0119]

[0120] wherein, A1, A2, A3 are all matrices. In order to express the relationship between the harmonics, A2 and A3 are in the form of Toeplitz matrix (the elements on the main diagonal are equal, and the elements on the lines parallel to the main diagonal are also equal), and the specific expression is as follows:

[0121]

[0122]

[0123] S6, other steady-state components under the fundamental component of the grid voltage obtained in step S1 are substituted into the small signal linearization time-invariant equation set obtained in step 6, that is, V dc0 , I ref1 , I1, M1, Vg1 , the value of ω1;

[0124] S7, divide the background harmonic voltage in the grid voltage into multiple stages of small disturbances, and the amplitude of the harmonic small disturbance in each stage does not exceed 3% of the fundamental voltage amplitude. Substitute the first stage grid harmonic voltage amplitude into the small signal linearization time-invariant equation group obtained in step S5, and determine V gn The amplitude of , the other undetermined steady-state components in the equations are all zero;

[0125] S8. Solve the small-signal linearized time-invariant equations obtained in step S5 to obtain the small-disturbance responses of each state variable under the background harmonic voltage disturbance of this stage; superimpose the solved small-disturbance responses of each state variable into the steady-state component, update the steady-state operating point, and thus obtain the small-signal linearized time-invariant equations required for solving the next stage; continuously update the steady-state operating point and the equations until the applied background harmonic voltage is updated to be the same as the set target;

[0126] S9. Solve the small signal linearization time-invariant equations to obtain the amplitude and phase of each output current harmonic.

[0127] Those skilled in the art will appreciate that various aspects of the present invention may be implemented as systems, methods, or program products. Accordingly, various aspects of the present invention may be implemented in the following forms: entirely in hardware, entirely in software (including firmware, microcode, etc.), or in a combination of hardware and software, collectively referred to herein as "circuits," "modules," or "platforms."

[0128] In another embodiment of the present invention, a small-signal-increment-based iterative system for harmonic interference emission of grid-connected equipment is provided. The system can be used to implement the above-mentioned small-signal-increment-based iterative method for harmonic interference emission of grid-connected equipment. Specifically, the small-signal-increment-based iterative system for harmonic interference emission of grid-connected equipment includes a data module, a representation module, an equation module, a division module, and an iteration module.

[0129] The data module obtains the initial steady-state operating point data of a typical photovoltaic grid-connected inverter structure and writes the voltage-current relationship equation required to calculate the harmonic current based on the relationship between the AC and DC circuits and the control.

[0130] A representation module is used to represent the data variables related to the initial steady-state operating point of a typical photovoltaic grid-connected inverter structure using steady-state components and small-signal components. The steady-state components and small-signal components are substituted into the voltage-current relationship equation, retaining the small-signal component relationship to obtain the system small-signal linearization equation. Each small-signal component in the small-signal linearization equation is represented by the sum of harmonics.

[0131] The equation module substitutes the small signal component into the small signal linearization equation, converts the multiplication of the periodic time-varying coefficient and the periodic time-varying variable into the product between the harmonics, represents the relationship between the amplitudes of the harmonics, and obtains the small signal linearization time-invariant equation group. The other steady-state components under the fundamental component of the grid voltage are substituted into the small signal linearization time-invariant equation group, the DC voltage V dc0 , the output current instruction I ref1 , the output fundamental current amplitude I1, the modulation wave fundamental amplitude M1, the fundamental amplitude V g1 of the grid voltage, and the value of the grid fundamental voltage frequency ω1 are determined.

[0132] The division module divides the background harmonic voltage in the grid voltage into small perturbations in multiple stages, substitutes the amplitude of the grid harmonic voltage in the first stage into the small signal linearization time-invariant equation group, and determines the amplitude of V gn .

[0133] The iteration module solves the small signal linearization time-invariant equation group obtained to obtain the small perturbation response of each state variable under the background harmonic voltage perturbation in this stage. The small perturbation response of each state variable obtained by solving is superimposed on the steady-state component to update the steady-state operating point, and the small signal linearization time-invariant equation group required for solving in the next stage is obtained. The steady-state operating point and the equation group are constantly updated until the applied background harmonic voltage is updated to be the same as the set target. The small signal linearization time-invariant equation group is solved to obtain the amplitude and phase of each output current harmonic.

[0134] In another embodiment of the present application, a terminal device is provided, which comprises a processor and a memory, the memory being configured to store a computer program, the computer program comprising program instructions, and the processor being configured to execute the program instructions stored in the computer storage medium. The processor can be a central processing unit (CPU), and can also be other general-purpose processors, graphics processing units (GPUs), tensor processing units (TPUs), digital signal processors (DSPs), application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc., which are the computing core and control core of the terminal, and are suitable for implementing one or more instructions, and are particularly suitable for loading and executing one or more instructions to implement a corresponding method flow or a corresponding function; the processor in the embodiments of the present application can be used for the operation of the small-signal-increment-based grid-connected device harmonic disturbance emission iteration method, including:

[0135] Obtaining initial steady-state working point related data of a typical photovoltaic grid-connected inverter structure, writing voltage-current relationship equations required for calculating harmonic currents according to AC-DC circuit relationship and control relationship; representing initial steady-state working point related data variables of the typical photovoltaic grid-connected inverter structure with steady-state components and small-signal components; substituting the steady-state components and the small-signal components into the voltage-current relationship equations, retaining small-signal component relationships, and obtaining system small-signal linearization equations; representing each small-signal component in the small-signal linearization equations with a sum of harmonics; substituting the small-signal components into the small-signal linearization equations, converting the multiplication of the periodic time-varying coefficients and the periodic time-varying variables into the multiplication between each harmonic, representing the relationship between the amplitudes of each harmonic, and obtaining a small-signal linearization time-invariant equation set; substituting other steady-state components under the fundamental component of the grid voltage into the small-signal linearization time-invariant equation set, determining the DC voltage V dc0 , output current instruction I ref1 , output fundamental current amplitude I1, modulation wave fundamental amplitude M1, fundamental amplitude V g1 of the grid voltage, and the value of the grid fundamental voltage frequency ω1; dividing the background harmonic voltage in the grid voltage into small perturbations in multiple stages, substituting the grid harmonic voltage amplitude in the first stage into the small-signal linearization time-invariant equation set, and determining V gnThe amplitude of the small-signal linearization time-invariant equation group obtained by solving is obtained. The small disturbance response of each state variable under the background harmonic voltage disturbance of this stage is obtained. The small disturbance response of each state variable obtained by solving is superimposed on the steady-state component to update the steady-state working point, and the small-signal linearization time-invariant equation group required for the next stage of solving is obtained. The steady-state working point and the equation group are updated constantly until the applied background harmonic voltage is updated to be the same as the set target. The small-signal linearization time-invariant equation group is solved to obtain the amplitude and phase of each output current harmonic.

[0136] Please refer to Figure 4 The terminal device is a computer device. The computer device 60 of the embodiment includes a processor 61, a memory 62, and a computer program 63 stored in the memory 62 and executable on the processor 61. The computer program 63 implements the small-signal incremental based grid-connected device harmonic disturbed emission iterative method in the embodiment when executed by the processor 61. To avoid repetition, details are not described here. Alternatively, the computer program 63 implements the functions of each model / unit in the small-signal incremental based grid-connected device harmonic disturbed emission iterative system of the embodiment when executed by the processor 61. To avoid repetition, details are not described here.

[0137] The computer device 60 can be a desktop computer, a notebook computer, a palm computer, and a cloud server, etc. The computer device 60 can include, but is not limited to, the processor 61 and the memory 62. Those skilled in the art can understand that Figure 4 The computer device 60 is only an example and does not constitute a limitation on the computer device 60, and can include more or fewer components than shown, or combine certain components, or different components, for example, the computer device can also include an input / output device, a network access device, a bus, etc.

[0138] The processor 61 can be a central processing unit (CPU), and can also be other general-purpose processors, graphics processing units (GPUs), tensor processing units (TPUs), digital signal processors (DSPs), application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or the processor can also be any conventional processor.

[0139] The memory 62 can be an internal storage unit of the computer device 60, such as a hard disk or a memory of the computer device 60. The memory 62 can also be an external storage device of the computer device 60, such as a plug-in hard disk, a smart media card (SMC), a secure digital (SD) card, a flash card, and the like.

[0140] Further, the memory 62 can include both an internal storage unit and an external storage device of the computer device 60. The memory 62 is used to store computer programs and other programs and data required by the computer device. The memory 62 can also be used to temporarily store data that has been output or will be output.

[0141] Referring to Figure 5 The terminal device is a chip, and the chip 600 of the embodiment includes one or more processors 622 and a memory 632 for storing computer programs executable by the processor 622. The computer programs stored in the memory 632 can include one or more than one module each corresponding to a set of instructions. In addition, the processor 622 can be configured to execute the computer programs to perform the small-signal increment-based iteration method for harmonic disturbed emission of grid-connected equipment described above.

[0142] In addition, the chip 600 can further include a power supply component 626 configured to perform power management of the chip 600 and a communication component 650 configured to implement communication of the chip 600, such as wired or wireless communication. In addition, the chip 600 can further include an input / output interface 658. The chip 600 can operate based on an operating system stored in the memory 632.

[0143] In another embodiment of the present application, the present application also provides a storage medium, specifically a computer readable storage medium, which is a memory device in a terminal device and is used to store programs and data. It can be understood that the computer readable storage medium herein can include an internal storage medium in the terminal device, and of course can also include an expansion storage medium supported by the terminal device. The computer readable storage medium provides a storage space, and the storage space stores an operating system of the terminal. In addition, one or more than one instruction suitable for being loaded and executed by a processor is also stored in the storage space, and the instruction can be one or more than one computer program. It should be noted that the computer readable storage medium herein can be a high-speed RAM memory or a non-volatile memory, such as at least one disk memory.

[0144] The one or more instructions stored in the computer readable storage medium can be loaded and executed by the processor to implement the corresponding steps of the small-signal increment-based iteration method for harmonic-disturbed emission of grid-connected equipment in the above embodiments; the one or more instructions stored in the computer readable storage medium are loaded and executed by the processor to implement the following steps:

[0145] The initial steady-state working point related data of a typical photovoltaic grid-connected inverter structure is obtained, and a voltage-current relationship equation required for calculating harmonic current is written according to the relationship between AC and DC circuits and the relationship between controls; the initial steady-state working point related data variables of the typical photovoltaic grid-connected inverter structure are expressed by steady-state components and small-signal components; the steady-state components and the small-signal components are substituted into the voltage-current relationship equation, and the small-signal component relationship is retained to obtain a system small-signal linearization equation; each small-signal component in the small-signal linearization equation is expressed by a sum of harmonics; the small-signal components are substituted into the small-signal linearization equation, and a multiplication of a periodic time-varying coefficient and a periodic time-varying variable is converted into a multiplication between harmonics, which is expressed as a relationship between harmonic amplitudes to obtain a small-signal linearization time-invariant equation set; other steady-state components under the grid voltage fundamental component are substituted into the small-signal linearization time-invariant equation set to determine the DC voltage V dc0 , the output current instruction I ref1 , the output fundamental current amplitude I1, the modulation wave fundamental amplitude M1, the fundamental amplitude V g1 of the grid voltage, and the grid fundamental voltage frequency ω1; the background harmonic voltage in the grid voltage is divided into small perturbations in multiple stages, the grid harmonic voltage amplitude of the first stage is substituted into the small-signal linearization time-invariant equation set to determine the amplitude of V gn ; the small-signal linearization time-invariant equation set obtained by solving is used to obtain the small-perturbation response of each state variable under the background harmonic voltage disturbance in this stage; the small-perturbation response of each state variable obtained by solving is superimposed on the steady-state component to update the steady-state working point, and the small-signal linearization time-invariant equation set required for solving the next stage is obtained; the steady-state working point and the equation set are constantly updated until the applied background harmonic voltage is updated to be the same as the set target; the small-signal linearization time-invariant equation set is solved to obtain the amplitude and phase of each output current harmonic.

[0146] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0147] The simulation results verify the accuracy and reliability of the present application. A typical photovoltaic inverter grid-connected simulation model is built as shown in the following table: Figure 1

[0148] The harmonic content of the output current is measured in the simulation. At the same time, the small-signal step method proposed in the present application is used to calculate the harmonic current content under the same simulation parameters. In order to compare, the large-signal equation which ignores the product between harmonics is also used to solve, and the comparison is shown in Figure 3.

[0149] From the comparison results in Figure 3, firstly, for the fully controlled power electronic equipment, under the excitation of harmonic voltage, not only the harmonic current of the same frequency component will be generated, but also the current of other harmonic frequencies will be coupled. Secondly, the harmonic current content of the same frequency component is much larger than the coupled harmonic frequency current. Finally, the calculation results of the small-signal step method proposed in the present application are very close to the simulation, and the calculation accuracy of the amplitude and phase of the harmonic current is better than the method of solving the steady-state equation.

[0150] In summary, the present application is an iterative method and system for harmonic disturbance emission of grid-connected equipment based on small-signal increment. The present application solves the problem of harmonic emission calculation and analysis of fully controlled power electronic equipment under the excitation of background voltage "large disturbance". By dispersing the "large disturbance" into multiple stages of "small disturbance", using the small-signal equation to solve the "small disturbance" response and superimposing it to the steady-state component, updating and iteratively calculating the new small-signal equation, the steady-state harmonic response under "large disturbance" can be accurately quantitatively solved, and the calculation accuracy is high. At the same time, the present application uses small-signal equation linearization and harmonic transfer function transformation to convert the periodic time-varying nonlinear equation set into a linear time-invariant equation set, which can be linearly solved by matrix operation, greatly reducing the calculation complexity.

[0151] ​Those skilled in the art can clearly understand that, for the convenience and brevity of description, only the above-mentioned division of each functional unit and module is exemplified, and in actual application, the above-mentioned functions can be completed by different functional units and modules according to needs, that is, the internal structure of the device is divided into different functional units or modules to complete all or part of the functions described above. Each functional unit and module in the embodiment can be integrated in one processing unit, or each unit can be physically present separately, or two or more units can be integrated in one unit. The above-mentioned integrated unit can be realized in the form of hardware or software. In addition, the specific names of each functional unit and module are only for the convenience of mutual distinction, and do not limit the protection scope of the present application. The specific working process of the units and modules in the above system can refer to the corresponding process in the foregoing method embodiments, which will not be described here.

[0152] In the above embodiments, the description of each embodiment has its own emphasis, and the parts not described or recorded in detail in a certain embodiment can be referred to the related description of other embodiments.

[0153] Those of ordinary skill in the art can realize that the units and algorithm steps of each example described in combination with the embodiments disclosed in the present application can be realized by electronic hardware or a combination of computer software and electronic hardware. Whether the functions are executed in hardware or software depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of the present application.

[0154] In the embodiments provided by the present application, it should be understood that the disclosed devices / terminals and methods can be implemented by other ways. For example, the device / terminal embodiments described above are only schematic, and the division of the modules or units is only a logical function division, and there can be another division way in actual implementation, for example, a plurality of units or components can be combined or integrated into another system, or some features can be ignored or not executed. In addition, the coupling or direct coupling or communication connection between the shown or discussed units can be indirect coupling or communication connection through some interface, device or unit, and can be electrical, mechanical or other forms.

[0155] The units described as separate components can or can not be physically separated, and the components shown as units can or can not be physical units, that is, they can be located in one place, or can be distributed on multiple network units. Part or all of the units can be selected according to actual needs to achieve the purpose of the embodiment.

[0156] In addition, each functional unit in each embodiment of the present application can be integrated in one processing unit, or each unit can exist physically, or two or more units can be integrated in one unit. The integrated unit can be implemented in the form of hardware or in the form of a software functional unit.

[0157] The integrated module / unit, if implemented in the form of a software functional unit and sold or used as an independent product, can be stored in a computer readable storage medium. Based on such understanding, all or part of the processes in the above-mentioned embodiment methods can be completed by a computer program instructing related hardware, and the computer program can be stored in a computer readable storage medium. The computer program can implement the steps of each method embodiment when executed by a processor. The computer program includes computer program code, which can be in the form of source code, object code, executable files, or some intermediate forms. The computer readable medium can include any entity or device capable of carrying the computer program code, recording medium, U disk, mobile hard disk, magnetic disk, optical disk, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signal, telecommunication signal, and software distribution medium, etc. It should be noted that the computer readable medium can include or exclude contents according to the requirements of legislation and patent practice in the jurisdiction, for example, in some jurisdictions, according to legislation and patent practice, the computer readable medium does not include electrical carrier signals and telecommunication signals.

[0158] The present application is described with reference to flowcharts and / or block diagrams according to the methods, devices, and computer program products of embodiments of the present application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of flows and / or blocks in the flowcharts and / or block diagrams can be implemented by computer program instructions. These computer program instructions can be provided to a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to produce a machine, so that the instructions executed by the computer or other programmable data processing devices produce a device that implements the functions specified in the flowcharts and / or block diagrams. Figure 1 The device that implements the functions specified in one flow or multiple flows and / or blocks Figure 1 The device that implements the functions specified in one flow or multiple flows and / or blocks

[0159] These computer program instructions can also be stored in a computer readable storage medium that can guide the computer or other programmable data processing devices to work in a specific way, so that the instructions stored in the computer readable storage medium produce a product including instruction devices that implement the functions specified in the flowcharts and / or block diagrams. Figure 1one or more processes and / or blocks Figure 1 the function specified in the one or more blocks.

[0160] These computer program instructions can also be loaded into computer or other programmable data processing devices, so that a series of operational steps are performed on the computer or other programmable data processing devices to generate a computer-implemented process, so that the instructions executed on the computer or other programmable data processing devices provide a process for implementing the flowchart Figure 1 one or more processes and / or blocks Figure 1 the steps of the function specified in the one or more blocks.

[0161] The above is only to illustrate the technical idea of the present application, and cannot limit the protection scope of the present application. Any modification made according to the technical idea of the present application on the basis of the technical scheme falls within the protection scope of the claims of the present application.

Claims

1. An iterative method for harmonic interference emission of grid-connected equipment based on small signal increments, characterized in that: The following steps are involved: S1. Obtain the initial steady-state operating point data of a typical photovoltaic grid-connected inverter structure, and write the voltage-current relationship equation required to calculate the harmonic current based on the relationship between the AC and DC circuits and the control. S2. Representing the initial steady-state operating point-related data variables of the typical photovoltaic grid-connected inverter structure obtained in step S1 using steady-state components and small-signal components; S3. Substitute the steady-state component and small-signal component obtained in step S2 into the voltage-current relationship equation obtained in step S1, retaining the small-signal component relationship, and obtain the system small-signal linearization equation, which is as follows: in, L is the inductance connecting the inverter and the grid, is the small signal component of the output current, V dcn is the nth DC voltage amplitude, ω 1 is the fundamental voltage frequency, is the small signal component of the modulation wave, M n is the amplitude of the n-th modulation wave, is the small signal component of DC voltage, is the small signal component of the AC voltage, C dc is the DC side capacitance, is the small signal component of DC voltage, * represents conjugation, I n is the n-times output current amplitude, G PR ( t ) is the current controller, is the small signal component of the output current command, I refn is the n-times output current command amplitude, is the small signal component of the grid voltage phase, H dvc ( t ) is a DC voltage controller; S4, expressing each small signal component in the small signal linearization equation obtained in step S3 by the sum of harmonics; S5. Substituting the small signal component obtained in step S4 into the small signal linearization equation obtained in step S3, multiplying the periodic time-varying coefficient and the periodic time-varying variable to convert them into products between harmonics, expressed as the relationship between the amplitudes of the harmonics, and obtaining a small signal linearization time-invariant equation group; S6, substituting other steady-state components under the fundamental component of the grid voltage into the small signal linearization time-invariant equation group obtained in step S5, to determine the DC voltage , output current instruction , output fundamental current amplitude , modulation wave fundamental amplitude , fundamental amplitude of grid voltage , grid fundamental voltage frequency The value of S7, divide the background harmonic voltage in the grid voltage into multiple stages of small disturbances, substitute the grid harmonic voltage amplitude of the first stage into the small signal linearization time-invariant equation group obtained in step S5, and determine The amplitude of S8. Solve the small-signal linearized time-invariant equations obtained in step S5 to obtain the small-disturbance responses of each state variable under the background harmonic voltage disturbance of this stage; superimpose the solved small-disturbance responses of each state variable into the steady-state component, update the steady-state operating point, and obtain the small-signal linearized time-invariant equations required for solving the next stage; continuously update the steady-state operating point and the equations until the applied background harmonic voltage is updated to be the same as the set target; S9. Solve the small signal linearization time-invariant equations to obtain the amplitude and phase of each output current harmonic.

2. The iterative method for harmonic interference emission of grid-connected equipment based on small signal increment according to claim 1, characterized in that: The initial steady-state operating point data of a typical photovoltaic grid-connected inverter structure include the fundamental amplitude of the grid voltage V g1 , DC voltage V dc0 , output current instruction I ref1 , output AC current amplitude I 1. Modulation wave amplitude M 1. Grid voltage phase θ .

3. The iterative method for harmonic interference emission of grid-connected equipment based on small signal increment according to claim 1, characterized in that: The voltage-current relationship equations required to calculate harmonic currents include: The voltage-current relationship of line inductance is: in, L is the inductance connecting the inverter and the grid, i is the output AC current, t For time, v dc is the DC voltage, m is the modulated wave, v g is the AC voltage; The relationship between the DC side and AC side current is in, C dc is the DC side capacitance, * indicates conjugation; The current controller controls the output current to track the current command, and the relationship between the modulation wave and the output current is: in, G PR ( t ) is the current controller, θ is the grid voltage phase extracted by the phase-locked loop, i ref is the output current instruction; The inverter adopts DC voltage control mode, and the active current command is generated by the DC side voltage. The relationship between the two is: in, H dvc ( t ) is the DC voltage controller, V dcR is the DC voltage instruction; Phase-locked loop extracts grid voltage phase for: in, h pll ( t ) is the phase-locked loop controller, v gb is the grid voltage after fundamental wave bandpass filtering; The grid voltage is bandpass filtered for: in, G bd ( t ) is a fundamental bandpass filter.

4. The iterative method for harmonic interference emission of grid-connected equipment based on small signal increment according to claim 1, characterized in that: In step S2, the steady-state component is expressed as the sum of the fundamental steady-state component and each harmonic subcomponent as follows: in, ω 1 is the fundamental voltage frequency, v dc is the DC voltage, V dcn is the nth DC voltage amplitude, is the small signal component of DC voltage, i ref is the output current instruction, I refn is the n-times output current command amplitude, is the small signal component of the output current command, i is the output AC current, I n is the n-times output current amplitude, is the small signal component of the output current, m is the modulated wave, M n is the amplitude of the n-th modulation wave, is the small signal component of the modulation wave, v g is the AC voltage, V gn is the nth AC voltage amplitude, is the small signal component of the AC voltage, v gb is the grid voltage after fundamental bandpass filtering, V g1 is the fundamental amplitude of the AC voltage, is the small signal component of the grid voltage after fundamental bandpass filtering, θ is the grid voltage phase extracted by the phase-locked loop, is the small signal component of the grid voltage phase.

5. The iterative method for harmonic interference emission of grid-connected equipment based on small signal increment according to claim 1, characterized in that: In step S4, each small signal component in the small signal linearization equation is represented by the sum of a series of harmonics, as follows: in, is the small signal component of DC voltage, is the small signal component of the output current command, is the small signal component of the output current, is the small signal component of the modulation wave, is the small signal component of the AC voltage, is the small signal component of the grid voltage after fundamental bandpass filtering, is the small signal component of the grid voltage phase, ω 1 is the fundamental voltage frequency, is the amplitude of the nth DC voltage small signal component, is the amplitude of the small signal component of the n-th output current instruction, is the amplitude of the n-th output current small signal component, is the amplitude of the small signal component of the nth modulation wave, is the amplitude of the small signal component of the nth AC voltage, is the small signal component of the grid voltage after fundamental bandpass filtering, is the small signal component of the grid voltage phase.

6. The iterative method for harmonic interference emission of grid-connected equipment based on small signal increment according to claim 1, characterized in that: In step S5, the small signal linearization time-invariant equations are: in, A 1, A 2, A 3 are coefficient matrices, L is the inductance connecting the inverter and the grid, is the amplitude of the n-th output current small signal component, is the amplitude of the small signal component of the nth modulation wave, is the amplitude of the nth DC voltage small signal component, is the amplitude of the small signal component of the nth AC voltage.

7. The iterative method for harmonic interference emission of grid-connected equipment based on small signal increment according to claim 6, characterized in that: Coefficient matrix A 1, A 2, A 3 are: Where n is the harmonic order, ω 1 is the fundamental voltage frequency, V d,cn is the nth DC voltage amplitude, M n is the amplitude of the n-th modulation wave.

8. The iterative method for harmonic interference emission of grid-connected equipment based on small signal increment according to claim 1, characterized in that: In step S7, the amplitude of the harmonic small disturbance in each stage does not exceed 3% of the fundamental voltage amplitude.

9. A small signal increment based iterative system for harmonic interference emission of grid-connected equipment, characterized in that: include: The data module obtains the initial steady-state operating point data of a typical photovoltaic grid-connected inverter structure and writes the voltage-current relationship equation required to calculate the harmonic current based on the relationship between the AC and DC circuits and the control. A representation module is used to represent the data variables related to the initial steady-state operating point of a typical photovoltaic grid-connected inverter structure using steady-state components and small-signal components. The steady-state components and small-signal components are substituted into the voltage-current relationship equation, retaining the small-signal component relationship to obtain the system small-signal linearization equation. Each small-signal component in the small-signal linearization equation is represented by the sum of harmonics. In the equation module, the small signal component is substituted into the small signal linearization equation. The periodic time-varying coefficient is multiplied by the periodic time-varying variable to convert it into the product between each harmonic, which is expressed as the relationship between the amplitudes of each harmonic. The small signal linearization time-invariant equation group is obtained as follows: in, L is the inductance connecting the inverter and the grid, is the small signal component of the output current, V dcn is the nth DC voltage amplitude, ω 1 is the fundamental voltage frequency, is the small signal component of the modulation wave, M n is the amplitude of the n-th modulation wave, is the small signal component of DC voltage, is the small signal component of the AC voltage, C dc is the DC side capacitance, is the small signal component of DC voltage, * represents conjugation, I n is the n-times output current amplitude, G PR ( t ) is the current controller, is the small signal component of the output current command, I refn is the n-times output current command amplitude, is the small signal component of the grid voltage phase, H dvc ( t ) is the DC voltage controller; Substitute the other steady-state components under the grid voltage fundamental component into the small signal linearization time-invariant equation group to determine the DC voltage , output current instruction , output fundamental current amplitude , modulation wave fundamental amplitude , fundamental amplitude of grid voltage , grid fundamental voltage frequency The value of The background harmonic voltage in the grid voltage is divided into multiple stages of small disturbances. The amplitude of the grid harmonic voltage in the first stage is substituted into the small signal linearization time-invariant equation group to determine The amplitude of The iterative module solves the small-signal linearized time-invariant equations to obtain the small-disturbance responses of each state variable under the background harmonic voltage disturbance in this stage; the small-disturbance responses of each state variable are superimposed on the steady-state components, the steady-state operating point is updated, and the small-signal linearized time-invariant equations required for the next stage are obtained; the steady-state operating point and the equations are continuously updated until the applied background harmonic voltage is updated to the same as the set target; the small-signal linearized time-invariant equations are solved to obtain the amplitude and phase of each output current harmonic.

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