Impedance measurement method for power electronic power system

Abstract

The invention discloses an impedance measurement method for a power electronic power system. The method comprises the steps of setting a frequency point set based on a frequency range of preset impedance measurement; constructing a disturbance signal set comprising a plurality of sinusoidal signals; with the initial phases of all the sinusoidal signals as independent variables and the time-domain wave crests after the superposition of the plurality of sinusoidal signals as dependent variables, constructing a target function and solving to obtain a minimum time-domain wave crest superposed sinusoidal disturbance signal; for the component to be detected, based on the sine disturbance signal corresponding to the injection position, the sampled voltage and current data are analyzed by adopting fast Fourier transform to obtain impedance frequency domain information of the component under each frequency point, and the impedance frequency domain information comprises impedance amplitude and phase information; and performing fitting based on impedance frequency domain information under a certain frequency point so as to obtain a complete impedance curve of the component in impedance measurement. The method provided by the invention can ensure that the response information of the corresponding frequency can be accurately identified in the FFT process of the subsequent impedance information solving.

Description

Technical Field

[0001] The invention belongs to the technical field of impedance measurement of power electronic converters, and in particular relates to an impedance measurement method for power electronic power systems. Background Art

[0002] With the continuous development of power systems and new energy generation technologies, more and more new energy generation and power electronic equipment such as wind power and photovoltaics are connected to the power grid, which significantly increases the degree of power electronics in the power system and forms a power electronics power system. However, due to the small inertia of power electronic equipment and the fact that many links in the power system are often subject to disturbances, the operating state of the power system will change significantly when it is subjected to small disturbances such as random load fluctuations and slow changes in system parameters. In addition, the dynamic characteristics of power electronic devices make the small disturbance stability problem of the system more prominent than that of traditional power systems. Among the many analysis methods, impedance analysis has become a common method for analyzing the instability mechanism of the system due to its clear physical concepts, high degree of visualization, and strong research ductility.

[0003] For the above impedance stability analysis method, it is necessary to accurately obtain the impedance information of the source and load converters. The traditional converter port impedance modeling technology is complex to derive, especially for multiple converters with input series output parallel (ISOP) or input parallel output series (IPOS), the modeling is very complex and cumbersome.

[0004] Academic literature Research on online measurement technology of grid impedance of high-power grid-connected inverter based on multi-sinusoidal signal injection [J / OL]. Proceedings of the Chinese Society of Electrical Engineering. A method based on multi-sinusoidal signals as voltage disturbances is disclosed, and the disturbance signal is optimized by referring to the Gerchberg-Saxton phase recovery algorithm in spatial light field theory. This method only analyzes the grid impedance information and does not mention how to obtain the corresponding impedance curve.

[0005] Patent document CN114520594A discloses a high-power impedance measurement device based on constant amplitude injection of Chirp disturbance voltage. The device injects a Chirp signal with a constant voltage amplitude into the circuit, then measures the frequency response of the voltage and current at the corresponding position, and finally calculates and obtains the impedance information of the converter. The Chirp signal can freely set the signal frequency band, and more than 90% of the energy is concentrated in the set frequency band. In the entire set frequency band, the signal amplitude is basically consistent, and the crest factor is low, which is conducive to signal identification and reduces the impact on the system to be measured. However, since a large number of disturbance signals of similar frequency bands are injected at one time, mutual interference between signals will inevitably occur, and such signals contain a large amount of energy in non-measured frequency bands, which will have a certain impact on the stability of the system, and thus the accuracy of the measurement cannot be guaranteed. Summary of the invention

[0006] The object of the present invention is to provide an impedance measurement method for a power electronic power system, which can ensure that the FFT process of subsequent impedance information acquisition accurately identifies the response information of the corresponding frequency.

[0007] In order to achieve the purpose of the present invention, the following technical solution is provided: an impedance measurement method for a power electronic power system, comprising the following steps:

[0008] A frequency point set is set based on a preset frequency range of impedance measurement, wherein frequency values ​​corresponding to the frequency points in the frequency point set are evenly distributed in a logarithmic domain in the frequency range;

[0009] All frequency points in the frequency point set are divided according to the number of disturbance signals injected at a single time to obtain injection positions of signals in different frequency bands, and different sinusoidal signal amplitudes are assigned to the disturbance signal injected at each injection position to construct a disturbance signal set containing multiple sinusoidal signals;

[0010] Taking the initial phase of all sinusoidal signals as the independent variable and the time domain peak after the superposition of multiple sinusoidal signals as the dependent variable, the objective function is constructed and solved to obtain the sinusoidal disturbance signal with the minimum time domain peak superposition;

[0011] For the components to be tested, based on the sinusoidal disturbance signal corresponding to the injection position, the sampled voltage and current data are analyzed by fast Fourier transform to obtain the impedance frequency domain information of the components at each frequency point, wherein the impedance frequency domain information includes impedance amplitude and phase information;

[0012] Fitting is performed based on the impedance frequency domain information at all frequency points to obtain the complete impedance curve of the components in the impedance measurement.

[0013] The present invention synthesizes the disturbance signal by superposing multiple sinusoidal signals of custom frequency points, ensuring targeted measurement of impedance information within the required frequency band, with little impact on system stability. At the same time, the PSO algorithm is used to select the initial phase of the superimposed disturbance signal, which can effectively reduce the time domain peak of the synthesized signal and reduce the impact on the system. Finally, all the acquired impedance information points are fitted in the frequency domain through bilinear mapping and least squares method to obtain the impedance expression in the frequency domain.

[0014] Specifically, the expression of the disturbance signal set is as follows:

[0015]

[0016] Where i is the number of characteristic harmonic frequency points in the multi-sinusoidal signal, M i , f i ,θ i are respectively the amplitude, frequency and initial phase angle of the i-th sinusoidal characteristic harmonic frequency.

[0017] Specifically, the differentiation rules of the injection positions are as follows:

[0018]

[0019] In the formula, f Kn is the frequency of the Knth injection signal, K represents the number of sinusoidal signals set for a single injection, and n represents the number of injections.

[0020] Specifically, different sinusoidal signal amplitudes are given to the injected disturbance signal by gradually increasing from low frequency to high frequency based on one percent of the preset amplitude.

[0021] Specifically, the amplitude of the sinusoidal signal is selected as follows:

[0022] M n =M ref ×(1%+n×0.2%)

[0023] Where M ref is the preset amplitude of the injection point signal, M n Represents the amplitude of the nth sine wave of a single injected disturbance signal.

[0024] Specifically, the PSO algorithm is used to solve the objective function.

[0025] Specifically, the expression of the objective function is as follows:

[0026] M obj =f(θ1,θ2,...,θ n )

[0027] Where Mobj is the peak value of the single injection signal in the time domain, θ n is the initial phase value of the nth sinusoidal wave component of the single injection signal.

[0028] Specifically, the fitting includes bilinear mapping and least squares method.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] In order to solve the problem that PRBS, Chirp and other signals contain a large amount of energy in frequency bands other than the one to be measured, the signal aliasing effect is minimized by reasonably dividing the low, medium and high frequency bands for signal synthesis. This ensures that the FFT process for obtaining the subsequent impedance information can accurately identify the response information of the corresponding frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A schematic diagram of the process of the impedance measurement method for a power electronic power system provided in this embodiment;

[0032] Figure 2 A schematic diagram of the synthesis of multiple sine waves provided in this embodiment;

[0033] Figure 3 The comparison results of the input impedance measurement and analytical model of the two-point flat Buck converter under the circuit parameters provided in this embodiment;

[0034] Figure 4 The comparison results of the two-point flat Buck converter input impedance measurement and analytical model under another circuit parameter provided in this embodiment. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0036] like Figure 1 As shown, an impedance measurement method for a power electronic power system provided by this embodiment, the specific process is as follows:

[0037] The frequency range [f1-f2] of the impedance measurement of the power electronic device is given according to the measurement frequency range requirement, and the number of frequency points K of the measurement is given according to the time and measurement accuracy requirements, wherein the corresponding frequency values ​​are evenly distributed in the logarithmic domain within the selected frequency range.

[0038] All frequency points in the frequency point set are divided according to the number of disturbance signals injected at a single time to obtain injection positions of signals in different frequency bands, and different sinusoidal signal amplitudes are assigned to the disturbance signal injected at each injection position to construct a disturbance signal set containing multiple sinusoidal signals, i.e., a multi-sinusoidal signal D signal It has the ability to simultaneously inject any finite number of frequency points of harmonic disturbance within a unit time. The expression is:

[0039]

[0040] Where i is the number of characteristic harmonic frequency points in the multi-sinusoidal signal, M i , f i ,θ i are respectively the amplitude, frequency and initial phase angle of the i-th sinusoidal characteristic harmonic frequency.

[0041] According to the acquired multi-sinusoidal signal D signal ,, K sinusoidal waves are injected at a time. The sinusoidal waves injected at a time are divided into sinusoidal waves of different frequency bands, and the division rule is:

[0042]

[0043] This avoids mutual interference caused by similar frequencies of single injected sine waves, which may result in incorrect port impedance information being obtained.

[0044] Given composition D signal The sine signal has different amplitudes, and the amplitude is taken as 1% of the reference value as the standard, and gradually increases from low frequency to high frequency.

[0045] Its amplitude can be obtained by the following formula:

[0046] M n =M ref ×(1%+n×0.2%)

[0047] Where M ref is the reference value of the injection point signal, which can be voltage, current, etc. n Represents the amplitude of the nth sinusoidal wave of a single injected disturbance signal.

[0048] like Figure 2As shown in the figure, the initial phase of all sinusoidal signals is taken as the independent variable, and the time domain peak after the superposition of multiple sinusoidal signals is taken as the dependent variable. The objective function is constructed and solved to obtain the sinusoidal disturbance signal with the minimum time domain peak superposition. That is, according to the disturbance signal injected once, the objective function with the initial phase of all sinusoidal signals as the independent variable and the time domain peak of the superimposed sinusoidal signal as the dependent variable is formed. The objective function expression is:

[0049] M obj =f(θ1,θ2,...,θ n )

[0050] Where M obj is the peak value of the single injection signal in the time domain, θ n is the initial phase value of the nth sinusoidal wave component of the single injection signal.

[0051] The objective function is solved by the PSO algorithm to obtain the sinusoidal disturbance signal with the minimum time domain peak superposition. The specific process is as follows:

[0052] (1) Parameter initialization: Determine the particle dimension D, population size N, number of iterations M, learning factors c1 and c2, and inertia weight w.

[0053] (2) Initialize the particle’s velocity v and position x, and randomly generate a population of particles x = (θ1, θ2, …, θ n ).

[0054] (3) Take the objective function obtained in step 6 as the fitness value of the particle, and calculate the initial individual extreme value p according to the fitness value of the particle ij and the global extreme value p gi :

[0055] fitness=M obj =f(θ1,θ2,...,θ n ).

[0056] (4) According to formula (3), update the particle position (x ij ) and speed (v ij ), then calculate the fitness value according to the new position, and update the individual extreme value and the global extreme value:

[0057]

[0058] Where r1, r2 are random numbers between [0, 1], and the last iteration result and the particle speed jointly determine the update of x.

[0059] Determine whether the termination condition is met. If it is met, output the optimization result x best =(θ1,θ2,…,θ n); otherwise return to (3).

[0060] The disturbance of the superposition of multiple sinusoidal signals is injected into the system, and the voltage and current are collected at the corresponding positions. The sampled voltage and current data are decomposed by fast Fourier transform, and the impedance at each frequency is calculated based on the following formula:

[0061]

[0062] In the formula, is the voltage value corresponding to the disturbance frequency, is the current value at the corresponding disturbance frequency.

[0063] The transfer function is fitted for all the acquired impedance information points. The transfer function is fitted by using bilinear mapping and least squares method to estimate parameters, and the optimization algorithm is used to select the best model structure and parameters, and finally an estimated continuous transfer function model is returned. The specific process is as follows:

[0064] (1) Perform a bilinear mapping to transform the domain (frequency grid) of the transfer function. For continuous-time models, the imaginary axis is transformed to a unit circle.

[0065] (2) Perform SK iteration to solve the nonlinear least squares problem - Consider a multi-input single-output system. The nonlinear least squares problem is to minimize the following loss function:

[0066]

[0067] Where W is the specified frequency-dependent weight. D is the denominator of the transfer function model to be estimated, and N i is the numerator corresponding to the i-th input. Y and u are the measured output and input data respectively. f and n u is the frequency and the number of inputs, and ω is the frequency. Rearranging the above equation gives the following:

[0068]

[0069] To perform the SK iteration, the algorithm iteratively solves:

[0070]

[0071] Where m is the current iteration number and Dm-1(ω) is the denominator response determined in the previous iteration. Each step of the iteration is now a linear least squares problem, where the determined parameters capture the responses Dm(ω) and Ni,m(ω) for i = 1, 2, …nu. The iteration is initialized by choosing D0(ω) = 1.

[0072] The first iteration of the algorithm determines the D1(ω) denominator response. The D1(ω) and Ni,1(ω) polynomials are expressed in terms of monomial basis functions.

[0073] The second and subsequent iterations use orthogonal rational basis functions on the unit circle to represent the polynomials Dm(ω) and Ni,m(ω). These rational basis functions are as follows:

[0074]

[0075] where λj,m-1 is the jth pole determined in the previous iteration m-1 of iteration m. (λj,m-1)* is the complex conjugate of λj,m-1, and q is a frequency domain variable on the unit circle.

[0076] The algorithm is run for a maximum of 20 iterations. If the relative change in the loss function value is less than 0.001 in the last three iterations, the iteration is terminated early.

[0077] Linear optimizations (SK iterations) are performed, but even if they converge, they do not always produce local optimal solutions. A second set of iterations is performed to find a critical point in the optimization problem that produces a local optimal solution. A critical point is a solution to a set of nonlinear equations, and the algorithm searches for a critical point by successively constructing linear approximations to the nonlinear equations and solving the resulting linear equations in the least squares sense. The equation is as follows:

[0078] The jth denominator parameter equation:

[0079]

[0080] Enter the jth molecular parameter equation corresponding to l:

[0081]

[0082] The first iteration starts with the optimal solution for the numerator Ni and denominator D parameters from the SK iteration. Unlike the SK iteration, the basis functions Bj(ω) do not change in each iteration; the iteration is performed with the basis functions that produced the optimal solution in the SK iteration. As before, the algorithm is run for a maximum of 20 iterations. The iteration is terminated early if the relative change in the loss function value is less than 0.001 in the last three iterations.

[0083] If bounds on the transfer function coefficients are specified, these bounds are incorporated into the necessary optimality conditions using generalized Lagrange multipliers. The resulting constrained linear least squares problem is solved using the same method explained in the SK iteration step.

[0084] Returns the transfer function parameters corresponding to the optimal solution.

[0085] In order to better illustrate the effect of the method provided in this embodiment, the input impedance measurement and analytical model comparison results of the two-level Buck converter under different circuit parameters are shown in FIG. Figure 3 and Figure 4 shown.

[0086] in Figure 3 L = 2.45μH, and Figure 4 L = 1.45μH.

[0087] like Figure 3 It can be seen that the measurement results of the method provided in this embodiment are basically consistent with the results of the existing analytical model, and this method can avoid the complicated impedance modeling derivation process in the stability analysis process, can realize the online measurement of the converter port impedance characteristics, and has been verified by experiments to have high accuracy. This method can provide an important standardized tool and research background for system impedance acquisition and stability analysis.

[0088] In addition, the terms "upper", "lower", "inner", "outer", "front", "rear" are used for descriptive purposes only and should not be understood as indicating or implying relative importance. Unless otherwise specifically stated, the relative steps, numerical expressions and values ​​of the components and steps described in these embodiments do not limit the scope of the present invention.

[0089] Of course, the above description is only a specific embodiment of the present invention and is not intended to limit the scope of implementation of the present invention. All equivalent changes or modifications made according to the structure, characteristics and principles described in the patent application scope of the present invention should be included in the patent application scope of the present invention.

[0090] Finally, it should be noted that the above-described embodiments are only specific implementations of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The protection scope of the present invention is not limited thereto. Although the present invention is described in detail with reference to the above-described embodiments, ordinary technicians in the field should understand that any technician familiar with the technical field can still modify the technical solutions recorded in the above-described embodiments within the technical scope disclosed by the present invention, or can easily think of changes, or make equivalent replacements for some of the technical features therein; and these modifications, changes or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. An impedance measurement method for a power electronic power system, characterized in that: The following steps are involved: A frequency point set is set based on a preset frequency range of impedance measurement, wherein frequency values ​​corresponding to the frequency points in the frequency point set are evenly distributed in a logarithmic domain in the frequency range; All frequency points in the frequency point set are divided according to the number of disturbance signals injected at a single time to obtain injection positions of signals in different frequency bands, and different sinusoidal signal amplitudes are assigned to the disturbance signal injected at each injection position to construct a disturbance signal set containing multiple sinusoidal signals; Taking the initial phase of all sinusoidal signals as the independent variable and the time domain peak after the superposition of multiple sinusoidal signals as the dependent variable, the objective function is constructed and solved to obtain the sinusoidal disturbance signal with the minimum time domain peak superposition; For the components to be tested, based on the sinusoidal disturbance signal corresponding to the injection position, the sampled voltage and current data are analyzed by fast Fourier transform to obtain the impedance frequency domain information of the components at each frequency point, wherein the impedance frequency domain information includes impedance amplitude and phase information; Fitting is performed based on the impedance frequency domain information at all frequency points to obtain the complete impedance curve of the components in the impedance measurement.

2. The impedance measurement method for power electronic power system according to claim 1, characterized in that: The expression of the disturbance signal set is as follows: Where i is the number of characteristic harmonic frequency points in the multi-sinusoidal signal, M i , f i ,θ i are respectively the amplitude, frequency and initial phase angle of the i-th sinusoidal characteristic harmonic frequency.

3. The impedance measurement method for power electronic power system according to claim 1, characterized in that: The differentiation rules for the injection sites are as follows: In the formula, f Kn is the frequency of the Knth injection signal, K represents the number of sinusoidal signals set for a single injection, and n represents the number of injections.

4. The impedance measurement method for power electronic power system according to claim 1, characterized in that: The injected disturbance signal is given different sinusoidal signal amplitudes by gradually increasing from low frequency to high frequency based on one percent of the preset amplitude.

5. The impedance measurement method for power electronic power system according to claim 4, characterized in that: The amplitude of the sinusoidal signal is selected as follows: M n =M ref ×(1%+n×0.2%) Where M ref is the preset amplitude of the injection point signal, M n Represents the amplitude of the nth sine wave of a single injected disturbance signal.

6. The impedance measurement method for power electronic power system according to claim 1, characterized in that: The PSO algorithm is used to solve the objective function.

7. The impedance measurement method for power electronic power system according to claim 1 or 6, characterized in that: The expression of the objective function is as follows: M obj =f(θ1,θ2,...,θ n ) Where M obj is the peak value of the single injection signal in the time domain, θ n is the initial phase value of the nth sinusoidal wave component of the single injection signal.

8. The impedance measurement method for power electronic power system according to claim 1, characterized in that: The fitting includes bilinear mapping and least square method.

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