A method for online parameter self-optimization of grid-type converters

Abstract

This invention belongs to the field of converter control technology, specifically relating to an online parameter self-optimization method for grid-connected converters. The method includes: deriving the converter impedance equation based on the grid-connected converter control flow; performing FFT harmonic analysis on the current grid voltage to obtain the harmonic voltage amplitude at each frequency point, and extracting the multiple frequency points with the highest harmonic amplitudes; obtaining the grid equivalent impedance equation through an online identification algorithm, and adding the grid equivalent impedance to the converter impedance equation to obtain the total impedance equation; substituting each extracted frequency point into the total impedance equation to calculate the total impedance value at each frequency point; calculating the expected harmonic current based on the harmonic voltage amplitude and the total impedance value at each frequency point, and combining the expected harmonic currents at each frequency point to obtain the total expected harmonic current; using the Grey Wolf optimization algorithm to optimize parameters with the goal of minimizing the total expected harmonic current, adjusting parameters in real time to reduce the converter harmonic current response caused by grid voltage harmonics.

Description

Technical Field

[0001] This invention belongs to the field of converter control technology, specifically relating to an online parameter self-optimization method for grid-type converters. Background Technology

[0002] With the widespread application of power electronics technology in power grids, traditional grid-connected converters, as key components, bear the important responsibility of stabilizing grid operation and enabling flexible power conversion and transmission. However, a significant characteristic of traditional grid-connected converters is that their operating parameters are fixed. This fixed parameter setting also means that the converter's impedance characteristics are fixed, making it impossible to adjust according to changes in the actual grid environment. In an ideal grid environment, fixed parameters and impedance characteristics may meet basic operating requirements, but actual grid conditions are complex and variable, especially when grid voltage harmonics are present. Grid voltage harmonics interfere with the normal operation of the converter. Because the converter cannot adaptively adjust, it may generate excessive harmonic current responses, which in severe cases can even cause system oscillations, posing a serious threat to the stability and reliability of the power grid.

[0003] Given the limitations of traditional grid-connected converters in dealing with grid voltage harmonics, exploring new solutions is urgently needed. If we can break away from the traditional fixed-parameter model and dynamically and in real-time adjust the converter parameters based on real-time grid conditions, such as the frequency and amplitude of voltage harmonics, then the converter's impedance characteristics can be specifically improved. This flexible parameter adjustment method allows the converter to better adapt to grid changes and effectively reduce harmonic currents caused by voltage harmonics. Simultaneously, the improved impedance characteristics help enhance the converter's ability to suppress grid disturbances, reduce the risk of system oscillations, thereby improving the stability and power quality of the entire power system and ensuring its safe and reliable operation. Summary of the Invention

[0004] To overcome the problems in the prior art, this invention proposes an online parameter self-optimization method for grid-type converters.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: This invention provides an online parameter self-optimization method for grid-type converters, comprising the following steps: Step 100: Based on the control flow of the grid-type converter, derive the converter impedance equation; Step 200: Perform FFT harmonic analysis on the current grid voltage to obtain the harmonic voltage amplitude at each frequency point, and extract the multiple frequency points with the highest harmonic amplitude; Step 300: The equivalent impedance equation of the power grid is obtained through an online identification algorithm. The equivalent impedance of the power grid is added to the impedance equation of the converter to obtain the total impedance equation. Substitute each frequency point into the total impedance equation to calculate the total impedance value at each frequency point. Step 400: For each frequency point taken out, calculate the expected harmonic current based on its harmonic voltage amplitude and overall impedance value, and combine the expected harmonic currents of each frequency point to obtain the total expected harmonic current; Step 500: With the goal of minimizing the total expected harmonic current, use the Grey Wolf optimization algorithm to optimize the parameters and output the optimal parameter combination.

[0006] Furthermore, the converter impedance equations include positive-sequence converter impedance equations and negative-sequence converter impedance equations.

[0007] Further, step 200 specifically includes: The grid voltage is collected for a fixed period of time, and the collected grid voltage is analyzed by fast Fourier transform to decompose the voltage harmonic analysis results at each frequency point in the positive sequence and the voltage harmonic analysis results at each frequency point in the negative sequence. Based on the voltage harmonic analysis results at each positive and negative frequency point, N positive and N negative harmonic frequency points are selected by sorting the harmonic amplitudes from largest to smallest.

[0008] Furthermore, the online identification algorithm in step 300 is specifically a perturbation injection method.

[0009] Further, step 300 includes: The positive-sequence converter impedance equation and the negative-sequence converter impedance equation are added to the equivalent grid impedance to obtain the total positive-sequence impedance equation and the total negative-sequence impedance equation. Substitute the current converter parameters into the total impedance equation, substitute the selected positive sequence frequency points into the positive sequence total impedance equation, and substitute the selected negative sequence frequency points into the negative sequence total impedance equation. The total impedance equation is then transformed from the complex frequency domain to the frequency domain to obtain the total impedance value at each frequency point.

[0010] Furthermore, the variable parameters in the current converter parameters include: the proportional parameters of the voltage loop, the integral parameters of the voltage loop, the proportional parameters of the current loop, and the integral parameters of the current loop.

[0011] Further, in step 400, the expected harmonic currents at each frequency point are combined to obtain the total expected harmonic current, including: ; In the above formula, This represents the total expected harmonic current; Represents positive sequence frequency pointsf n The expected harmonic current; Represents negative sequence frequency points f n The expected harmonic current.

[0012] Further, step 500 includes: The parameters involved in the iterative optimization include the proportional parameters of the voltage loop, the integral parameters of the voltage loop, the proportional parameters of the current loop, and the integral parameters of the current loop. With the goal of minimizing the total expected harmonic current, the fitness of the individual, i.e. the total expected harmonic current, is recalculated using the optimization parameters obtained in each iteration. After reaching the maximum number of iterations, output the optimal parameter combination.

[0013] Compared with the prior art, the present invention has the following technical effects: This invention provides an online parameter self-optimization method for grid-connected converters. By combining theoretical equations, online FFT analysis, and the Grey Wolf optimization algorithm, and targeting the expected harmonic current, it performs online parameter optimization and adjusts the converter's PI parameters in real time, effectively reducing the current harmonic response caused by grid voltage harmonics. Furthermore, this invention helps reduce oscillation risk because oscillations are characterized by a gradual increase in harmonic voltage and current at specific frequencies. Therefore, according to the optimization principle of this invention, the equivalent impedance of the converter at that frequency will also gradually increase, effectively suppressing oscillations. Attached Figure Description

[0014] To more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 Electrical topology diagram of a grid-type converter; Figure 2 The VSG active power-angle calculation is used to control the algorithm flowchart; Figure 3 For the VSG reactive power-voltage calculation in the control algorithm flowchart; Figure 4 For the voltage-current inner loop of the control algorithm flowchart; Figure 5 This is a schematic diagram of the online parameter optimization algorithm. Figure 6 A schematic diagram of the algorithm optimization process for Grey Wolf. Detailed Implementation

[0016] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the specific implementation methods, structures, features, and effects of the technical solutions proposed according to the present invention are described in detail below with reference to the accompanying drawings and preferred embodiments. Specific features, structures, or characteristics in one or more embodiments may be combined in any suitable form. Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0017] Building upon traditional grid-based control, a parameter self-optimization algorithm is added. This algorithm specifically modifies the PI parameters of the voltage and current loops based on voltage harmonic characteristics to alter the internal impedance characteristics, thereby reducing harmonic current response. Combining current control parameters, converter impedance equations, and grid impedance identification, the current equivalent impedance characteristics are derived. After real-time FFT analysis of the voltage, the expected harmonic current response is obtained by dividing by the impedance characteristics. With the goal of minimizing the expected harmonic current, the Grey Wolf optimization algorithm is used for parameter optimization, adjusting parameters in real-time. This algorithm is beneficial for reducing converter harmonic current response caused by grid voltage harmonics and also helps reduce oscillation risk.

[0018] This invention provides an online parameter self-optimization method for grid-type converters, referring to... Figures 1-6 By combining theoretical equations, online FFT analysis, and the Grey Wolf optimization algorithm, the converter PI parameters are adjusted to reduce the current harmonic response caused by grid voltage harmonics and to help reduce the risk of oscillation. The method includes the following steps: Step 100: Based on the control flow of the grid-type converter, derive the converter impedance equation; Step 200: Perform FFT harmonic analysis on the current grid voltage to obtain the harmonic voltage amplitude at each frequency point, and extract the multiple frequency points with the highest harmonic amplitude; Step 300: The equivalent impedance equation of the power grid is obtained through an online identification algorithm. The equivalent impedance of the power grid is added to the impedance equation of the converter to obtain the total impedance equation. Substitute each frequency point into the total impedance equation to calculate the total impedance value at each frequency point. Step 400: For each frequency point taken out, calculate the expected harmonic current based on its harmonic voltage amplitude and overall impedance value, and combine the expected harmonic currents of each frequency point to obtain the total expected harmonic current; Step 500: With the goal of minimizing the total expected harmonic current, use the Grey Wolf optimization algorithm to optimize the parameters and output the optimal parameter combination.

[0019] The following is a detailed explanation of each of the above steps: Step 100: Based on the control process of the grid-type converter, derive the converter impedance equation.

[0020] This step is performed during the design phase; the remaining steps are calculated in real time during converter operation.

[0021] Step 110: The converter impedance equation is an equation in the s-domain (complex frequency domain) that includes internal control parameters and circuit parameters, and can be derived through theoretical knowledge of automatic control principles. Step 120: Based on the network-type control flow, derive the positive-sequence impedance equation Z of the converter in the s-domain. pos_conver ; Step 130: Based on the network-type control flow, derive the converter negative-sequence impedance equation Z in the s-domain. neg_conver .

[0022] For example, a grid converter with a power of 215kW and a rated voltage of 690V has the following topology. Figure 1 As shown, the control algorithm is attached. Figures 2-4 As shown.

[0023] The main feature of this embodiment is the real-time analysis of the grid voltage, combined with the converter impedance equation and grid impedance, to calculate the harmonic current response and perform parameter self-optimization. The remaining virtual synchronous machine (VSG) part and the voltage-current inner loop part are just one common form, and various fine-tuning versions exist. Those skilled in the art should be aware that this part can also be replaced by other common control processes.

[0024] The positive sequence impedance equation is Z pos_conver :

[0025] Negative sequence impedance equation Z neg_conver for:

[0026] in, ; ; ; ; ; ; In the above formula, The transfer function of the voltage loop is used to represent the voltage loop. The transfer function representing the current loop; Represents the active power loop transfer function of the VSG; Represents the VSG reactive power loop transfer function; This represents the admittance of the filter capacitor branch; This represents the total impedance of the filter circuit; , These are the proportional (P) parameter and integral (I) parameter of the voltage loop, respectively. , These are the proportional (P) parameter and integral (I) parameter of the current loop, respectively; Indicates the fundamental angular frequency; Indicates the amplitude of the fundamental voltage; K d This represents the decoupling coefficient of the inner current loop; The filter inductance value is represented by s; s represents the complex variable in the Laplace transform. Meaning: Since the relevant transfer functions of the control loop are all below the dq axis, they are actually different from the fundamental frequency angular velocity of the disturbance injected on the abc axis, so s changes to s-jw1; The transfer function of the voltage loop is used to represent the voltage loop. The transfer function representing the current loop; Represents the active power loop transfer function of the VSG; Represents the VSG reactive power loop transfer function; Indicates the decoupling coefficient of the inner current loop; The reactive power loop integral coefficient; J is the reactive droop coefficient; J is the virtual inertia. D p The damping coefficient; L f This is the value of the filter inductance; C f This is the value of the filter capacitor; R f V1 is the damping resistance value; V1 is the fundamental voltage amplitude; I is the fundamental current amplitude, I1=Ie jφ I2=Ie j φ-π / 2 superscript The term represents conjugate.

[0027] Step 200: Perform FFT harmonic analysis on the current grid voltage to obtain the harmonic voltage amplitude at each frequency point, and extract the multiple frequency points with the highest harmonic amplitude.

[0028] As an example, step 200 specifically includes: Step 210: Collect the grid voltage for a fixed period of time.

[0029] The specific acquisition time can be adjusted, with 0.05s being the preferred value, balancing response speed with the analysis accuracy of low-frequency harmonic results.

[0030] Step 220: Perform Fast Fourier Transform analysis on the collected grid voltage to decompose the voltage harmonic analysis results at each positive sequence frequency point and the voltage harmonic analysis results at each negative sequence frequency point.

[0031] Step 230: Based on the voltage harmonic analysis results of each positive sequence frequency point and each negative sequence frequency point, sort the harmonic amplitudes from largest to smallest and select N positive sequence harmonic frequency points and N negative sequence harmonic frequency points.

[0032] The number of frequency points is determined by the controller's computing power. The 10 frequency points with the highest harmonics are selected to obtain the positive sequence harmonic frequency points: U pos_f1 U pos_f2 ...U pos_f10 Negative sequence harmonic frequency point: U neg_f1 U neg_f2 ...U neg_10 .

[0033] Step 300: The equivalent impedance equation of the power grid is obtained through an online identification algorithm. The equivalent impedance of the power grid is added to the impedance equation of the converter to obtain the total impedance equation. Substitute each frequency point into the total impedance equation to calculate the total impedance value of each frequency point.

[0034] As an example, step 300 specifically includes: Step 310: Identify the equivalent impedance equation Z of the power grid using the perturbation injection method. grid .

[0035] Ignoring the resistive component and approximating the grid impedance as pure inductance, a reactive current of 10A with an amplitude of 0.02s is periodically injected during stable operation of the converter. i q Record before and after injection u d The amplitude change, the equivalent inductance of the power grid It can be estimated using the following formula: ; In the formula, u d0 Before injection d shaft voltage, u d1 For the injection d shaft voltage, i q_disturb The magnitude of the injected disturbance current.

[0036] Based on the equivalent inductance of the power grid, the equivalent power grid impedance equation in the S-domain is further derived as follows: ; In the above formula, This represents the equivalent grid impedance.

[0037] Step 320: Based on the theoretical impedance characteristic equation and extracting multiple frequency points with the highest harmonic amplitude, the converter impedance characteristics are obtained; the positive sequence converter impedance characteristics and negative sequence converter impedance characteristics are added to the equivalent grid impedance respectively to obtain the total positive sequence impedance and the total negative sequence impedance.

[0038] Positive sequence total impedance equation: Z pos =Z pos_conver +Z grid ; Negative-sequence total impedance equation: Z neg =Z neg_conver +Z grid ; Substitute the current converter parameters into the total impedance equation, where the proportional parameters of the voltage loop, the integral parameters of the voltage loop, the proportional parameters of the current loop, and the integral parameters of the current loop are variable parameters, and the rest are fixed parameters; substitute the selected positive sequence frequency points into the positive sequence total impedance equation Z. pos Substitute the selected negative sequence frequency points into the negative sequence total impedance equation Z. neg ; When substituting the selected frequency point into the equation, replace s with j2 in the total impedance equation. πf ,Right now The impedance equation is transformed from the complex frequency domain (s-domain) to the frequency domain (s-domain). f (domain), to obtain the specific total impedance value at each frequency point. Z pos_f1 ，Z pos_f2 …… and Z neg_f1 , Z neg_f2 ... Z neg_fn .

[0039] Step 400: For each frequency point taken out, calculate the expected harmonic current based on its harmonic voltage amplitude and overall impedance value, and combine the expected harmonic currents of each frequency point to obtain the total expected harmonic current.

[0040] As an example, step 400 specifically includes: For each selected frequency point, the expected harmonic current is calculated based on its harmonic voltage amplitude and overall impedance value, using the positive sequence frequency points. f Taking 1 as an example, its expected harmonic current is: ; The formula for calculating the total expected harmonic current is: ; In the above formula, This represents the total expected harmonic current.

[0041] Step 500: With the goal of minimizing the total expected harmonic current, the Grey Wolf optimization algorithm is used to optimize the parameters. The fitness of the parameters obtained in each iteration is recalculated. After a fixed number of iterations, the optimal parameter combination is output.

[0042] As an example, step 500 specifically includes: As attached Figure 6 As shown, parameter optimization using the Grey Wolf optimization algorithm includes the following sub-steps: The parameters involved in the iterative optimization are: the proportional parameters of the voltage loop. Integral parameters of voltage loop Proportional parameters of the current loop Integral parameters of the current loop ; Initialize the wolf pack: Generate 12 sets of random parameters. To accelerate convergence, generate an additional 3 sets of parameters, with values ​​from the last iteration during the previous optimization. α Wolf 、β Wolf δ Wolves, 15 sets of parameters, representing a pack of 15 wolves; Individual fitness: The goal is to minimize the total harmonic current, i.e., the total harmonic current... I total The smaller the value, the higher the individual fitness. The parameters obtained in each iteration are substituted back into the formula for calculating the total expected harmonic current in step 400 to calculate the individual fitness. Update the position of the gray wolf: The position of the gray wolf is the proportional parameter of the voltage loop. Integral parameters of voltage loop Proportional parameters of the current loop Integral parameters of the current loop ; For each wolf, update its position as follows: ; ; Where X(t) is the th t The position of the gray wolf in the nth iteration; X(t+1) is the position of the gray wolf in the nth iteration. t+ The position of the gray wolf in the first iteration; D α D β D δ These represent the distances between the current gray wolf and the α wolf, β wolf, and δ wolf, respectively. This represents the position of wolf α in the t-th iteration; This indicates the position of wolf β in the t-th iteration; This represents the position of δ wolf in the t-th iteration; , , These are random weights used to adjust the calculation of the distance between the gray wolf and the corresponding alpha wolf; ; ; in, A and C These are random coefficients in the algorithm, used to increase the randomness and diversity of the search. a The convergence factor decreases linearly from 2 to 0 with the number of iterations; r 1 and r 2 is a random number between 0 and 1. A The value in [− a , a Variations within the range, when | A When |≤1, the gray wolf approaches the prey (optimal solution) to search; when | A When |>1, the gray wolf moves away from its current location, expanding its search area. C Using random weights helps to discover new solution regions during the search process and prevents the algorithm from converging prematurely.

[0043] renew a , A and C Decrease linearly with the number of iterations a Regenerate random numbers to recalculate A and C .

[0044] Calculate the total fitness: Substitute the updated parameters back into the formula for calculating the total expected harmonic current in step 400 to calculate the fitness. renew α, β, δ Select the three wolves with the highest fitness from the wolf pack.

[0045] Maximum number of iterations: After 20 iterations, select the optimal solution that minimizes the total harmonic distortion from the iteration results, and use the iteration results as the proportional parameters of the voltage loop. Integral parameters of voltage loop Proportional parameters of the current loop Integral parameters of the current loop Application in control algorithms.

[0046] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A method for online parameter self-optimization of a grid-type converter, characterized in that, Includes the following steps: Step 100: Based on the control flow of the grid-type converter, derive the converter impedance equation; Step 200: Perform FFT harmonic analysis on the current grid voltage to obtain the harmonic voltage amplitude at each frequency point, and extract the multiple frequency points with the highest harmonic amplitude; Step 300: The equivalent impedance equation of the power grid is obtained through an online identification algorithm. The equivalent impedance of the power grid is added to the impedance equation of the converter to obtain the total impedance equation. Substitute each selected frequency point into the total impedance equation to calculate the total impedance value at each frequency point; Step 400: For each frequency point taken out, calculate the expected harmonic current based on its harmonic voltage amplitude and overall impedance value, and combine the expected harmonic currents of each frequency point to obtain the total expected harmonic current; Step 500: With the goal of minimizing the total expected harmonic current, use the Grey Wolf optimization algorithm to optimize the parameters and output the optimal parameter combination, including: The parameters involved in the iterative optimization include the proportional parameters of the voltage loop, the integral parameters of the voltage loop, the proportional parameters of the current loop, and the integral parameters of the current loop. With the goal of minimizing the total expected harmonic current, the fitness of the individual, i.e. the total expected harmonic current, is recalculated using the optimization parameters obtained in each iteration. After reaching the maximum number of iterations, output the optimal parameter combination.

2. The online parameter self-optimization method for a grid-type converter according to claim 1, characterized in that, The converter impedance equations include positive-sequence converter impedance equations and negative-sequence converter impedance equations.

3. The online parameter self-optimization method for a grid-type converter according to claim 1, characterized in that, Step 200 specifically includes: The grid voltage is collected for a fixed period of time, and the collected grid voltage is analyzed by fast Fourier transform to decompose the voltage harmonic analysis results at each frequency point in the positive sequence and the voltage harmonic analysis results at each frequency point in the negative sequence. Based on the voltage harmonic analysis results at each positive and negative frequency point, N positive and N negative harmonic frequency points are selected by sorting the harmonic amplitudes from largest to smallest.

4. The online parameter self-optimization method for a grid-type converter according to claim 1, characterized in that, The online identification algorithm in step 300 is specifically the perturbation injection method.

5. The online parameter self-optimization method for a grid-type converter according to claim 4, characterized in that, Step 300 includes: The positive-sequence converter impedance equation and the negative-sequence converter impedance equation are added to the equivalent grid impedance to obtain the total positive-sequence impedance equation and the total negative-sequence impedance equation. Substitute the current converter parameters into the total impedance equation, substitute the selected positive sequence frequency points into the positive sequence total impedance equation, and substitute the selected negative sequence frequency points into the negative sequence total impedance equation. The total impedance equation is then transformed from the complex frequency domain to the frequency domain to obtain the total impedance value at each frequency point.

6. The online parameter self-optimization method for a grid-type converter according to claim 5, characterized in that, The variable parameters in the current converter parameters include: the proportional parameters of the voltage loop, the integral parameters of the voltage loop, the proportional parameters of the current loop, and the integral parameters of the current loop.

7. The online parameter self-optimization method for a grid-type converter according to claim 1, characterized in that, In step 400, the expected harmonic currents at each frequency point are combined to obtain the total expected harmonic current, including: ； In the above formula, This represents the total expected harmonic current; Represents positive sequence frequency points f n The expected harmonic current; Represents negative sequence frequency points f n The expected harmonic current.

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