Method and system for suppressing wide frequency oscillation of offshore wind power flexible direct current grid-connected system and medium

Abstract

The present application relates to the technical field of power system stability control, and particularly relates to a wideband oscillation suppression method and system for an offshore wind power flexible direct current grid-connected system, and a medium, comprising sequentially injecting positive and negative sequence perturbation sinusoidal harmonic signals at a wind turbine generator grid-connected point, constructing a multiple-input multiple-output impedance matrix, and reducing the multiple-input multiple-output impedance matrix to a single-input single-output equivalent positive sequence impedance; quantifying the equivalent positive sequence impedance to obtain a stability margin of the offshore wind power flexible direct current grid-connected system; when the stability margin is lower than a preset margin threshold, reconstructing a wind turbine grid-side converter into a matrix control structure by cascading a weighting coefficient matrix between inner and outer loop control circuits of the wind turbine grid-side converter; based on impedance sensitivity, iteratively optimizing the matrix control structure, and synchronously connecting a low-pass filter in series in a voltage and current feedback channel to remodel impedance, thereby completing wideband oscillation risk suppression. The present application realizes wideband oscillation suppression of the offshore wind power flexible direct current grid-connected system through impedance modeling and a matrix control strategy.

Description

Technical Field

[0001] This invention relates to the field of power system stability control technology, and in particular to a broadband oscillation suppression method, system and medium for offshore wind power flexible DC grid connection systems. Background Technology

[0002] As the penetration rate of power electronic equipment in the power grid continues to increase, its multi-timescale control characteristics and complex dynamic interactions significantly increase the risk of broadband oscillations in the power system. For example, the subsynchronous / supersynchronous oscillations and medium-to-high frequency oscillations that occur in offshore wind power transmitted through flexible DC transmission systems seriously threaten the safe and stable operation of the system. The broadband oscillation mechanism of the power grid involving such power electronic equipment is fundamentally different from the traditional low-frequency or subsynchronous oscillation mechanism of the power grid. This is mainly due to the strong coupling effect between the dynamics of multiple links of the power electronic equipment controller and the electromagnetic transients of the main circuit. At present, impedance modeling and frequency sweeping methods are widely used to evaluate the stability margin of the system. However, in actual engineering, parameter uncertainties and operating point deviations can easily lead to the deterioration of impedance characteristics, which in turn can cause broadband oscillations. It is urgent to improve the damping characteristics of the system through impedance reshaping.

[0003] Existing broadband oscillation suppression strategies are mainly divided into two categories: passive impedance reshaping and active impedance reshaping. Passive impedance reshaping mainly adjusts the electromagnetic transient characteristics of the system by optimizing the main loop parameters or connecting passive devices in series and parallel, but its design flexibility is insufficient and it is difficult to adapt to changing operating conditions. Active impedance reshaping strategies focus on controller improvement strategies such as control parameter reshaping, control loop reshaping, and control structure reconstruction. Among them, control parameter reshaping achieves the resonant peak frequency band shift by adjusting the original controller parameters. Although it is easy to implement, it can only improve the stability margin to a limited extent and is not effective for mechanistic instability problems. Control loop reshaping can effectively suppress oscillations caused by control delay or sampling error by introducing filters in the feedback or measurement links, but its design depends on the accurate identification of specific frequency bands and has poor versatility. Control structure reconstruction fundamentally adjusts the impedance characteristics by changing the control strategy, and has the best suppression effect, but existing methods are mostly limited to adjusting a single control architecture and cannot take into account the global optimization of broadband impedance characteristics.

[0004] In summary, existing methods are mostly designed for single oscillation modes and are difficult to cope with the complex multi-band oscillation coupling problem in offshore wind power transmission systems via flexible DC. Therefore, how to effectively reshape the impedance characteristics of offshore wind power transmission systems via flexible DC to suppress broadband oscillation risks has become an important engineering problem that urgently needs to be solved in the field of offshore wind power transmission via flexible DC. Summary of the Invention

[0005] To address the above technical problems, this invention provides a broadband oscillation suppression method, system, and medium for offshore wind power flexible DC grid connection systems.

[0006] In a first aspect, the present invention provides a method for suppressing broadband oscillations in an offshore wind power flexible DC grid-connected system, the method comprising the following steps:

[0007] Positive and negative sequence micro-perturbation sinusoidal harmonic signals are sequentially injected at the grid connection point of the wind turbine, and the voltage and current harmonic response signals at the grid connection point are collected simultaneously.

[0008] Fast Fourier analysis was performed on the voltage and current harmonic response signals at the grid connection point to obtain measured impedance frequency sweep data;

[0009] Based on the measured impedance sweep data, a multi-input multi-output impedance matrix considering multi-harmonic coupling is constructed, and the multi-input multi-output impedance matrix is ​​reduced to an equivalent positive-sequence impedance with single input and single output.

[0010] Based on the equivalent positive sequence impedance, a Bode plot of the source-grid side impedance characteristics is constructed, the phase difference at the intersection of the source-grid impedance amplitudes is read, and the stability margin of the offshore wind power flexible DC grid-connected system is obtained by quantization based on the phase difference.

[0011] When the stability margin is lower than the preset margin threshold, the wind turbine grid-side converter is reconstructed into a matrix control structure by cascading a weighted coefficient matrix between the inner and outer loop control loops of the wind turbine grid-side converter.

[0012] The weighting coefficient matrix in the matrix control structure is iteratively optimized based on impedance sensitivity, and a low-pass filter is simultaneously connected in series in the voltage and current feedback channels to reshape the impedance, thereby suppressing the risk of wideband oscillation.

[0013] In a further embodiment, the step of constructing a multi-input multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance sweep data includes:

[0014] Background harmonic components and abnormal frequency points are removed from the measured impedance sweep data to obtain preprocessed impedance sweep data.

[0015] Based on the preprocessed impedance sweep data, voltage harmonic vectors and current harmonic vectors are constructed in order of frequency from low to high.

[0016] The impedance value at each frequency sweep point is calculated based on the ratio between the voltage harmonic vector and the current harmonic vector, and a multi-input multi-output impedance matrix considering multi-harmonic coupling is constructed.

[0017] In a further embodiment, the step of reducing the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance of a single-input single-output impedance includes:

[0018] Based on the physical topology and electrical parameters of the offshore wind power flexible DC grid connection system, a multi-harmonic linearized theoretical impedance model is established.

[0019] The consistency verification results of the multi-harmonic linearized theoretical impedance model and the multi-input multi-output impedance matrix are obtained by performing a consistency deviation verification.

[0020] After the consistency deviation verification result is passed, the key impedance elements in the multi-input multi-output impedance matrix are extracted;

[0021] Based on the key impedance elements, the multi-input multi-output impedance matrix is ​​reduced to an equivalent positive-sequence impedance with single input and single output through the Schur complement transformation.

[0022] In a further implementation scheme, the step of establishing a multi-harmonic linearized theoretical impedance model based on the physical topology and electrical parameters of the offshore wind power flexible DC grid connection system includes:

[0023] Based on the physical topology of the offshore wind power flexible DC grid-connected system, a main loop model of the system is established, and the harmonic components of each steady-state variable at each characteristic frequency point are extracted from the main loop model of the system to obtain the multi-frequency harmonic linearized representation of all steady-state variables.

[0024] The linearized representation of multi-frequency harmonics is substituted into the main circuit model of the wind turbine grid-side converter and the main circuit model of the flexible DC system for linearization, respectively, to obtain the small-signal equation of the main circuit harmonics.

[0025] Establish the transmission relationship between grid connection point voltage disturbance and current disturbance and wind turbine modulation ratio disturbance to the wind turbine controller, and decompose the wind turbine controller transmission relationship into wind turbine harmonic linearization equations according to frequency points;

[0026] Establish the transmission relationship of the flexible DC controller from grid connection point voltage disturbance and current disturbance to flexible DC modulation ratio disturbance, and decompose the flexible DC controller transmission relationship into flexible DC harmonic linearization equations according to frequency points;

[0027] The harmonic linearization equation of the wind turbine and the harmonic linearization equation of the flexible DC harmonic are integrated to construct the multi-frequency harmonic small-signal equation of the controller.

[0028] By combining the harmonic small-signal equations of the main circuit and the multi-frequency harmonic small-signal equations of the controller, a multi-harmonic linearized theoretical impedance model is obtained.

[0029] In a further implementation, the steps of constructing a Bode plot of the source-grid side impedance characteristics based on the equivalent positive sequence impedance, reading the phase difference at the intersection of the source-grid impedance amplitudes, and quantizing the stability margin of the offshore wind power flexible DC grid-connected system based on the phase difference include:

[0030] Based on the equivalent positive sequence impedance, calculate the wind field impedance amplitude, wind field impedance phase, flexible DC impedance amplitude, and flexible DC impedance phase at each frequency sweep point to obtain impedance amplitude and phase data;

[0031] A Bode plot of the source-network impedance characteristics is constructed using the frequency sweep points as the horizontal axis and the impedance amplitude and phase data as the vertical axis.

[0032] Locate the intersection frequency point of the source-grid impedance amplitude in the source-grid side impedance characteristic Bode plot, and read the wind field impedance phase value and flexible DC impedance phase value at the intersection frequency point;

[0033] Calculate the difference between the phase value of the wind field impedance and the phase value of the flexible DC impedance to obtain the phase difference at the intersection of the source and grid impedance amplitudes;

[0034] The deviation between the phase difference and the preset stability margin threshold is calculated to obtain the stability margin of the offshore wind power flexible DC grid connection system.

[0035] In a further embodiment, the step of reconstructing the wind turbine grid-side converter into a matrix control structure by cascading a weighted coefficient matrix between the inner and outer loop control loops of the wind turbine grid-side converter includes:

[0036] The actual value of the quadrature-axis voltage at the grid connection point is collected and the reference value of the quadrature-axis voltage is compared to obtain the quadrature-axis voltage deviation value. The quadrature-axis voltage deviation value is then used to generate a quadrature-axis synchronization signal through a proportional-integral controller with saturation limiting.

[0037] In the constant DC voltage outer loop control mode, the actual value of DC voltage is acquired, and the actual value of DC voltage is subtracted from the preset DC voltage reference value to obtain the DC voltage deviation value. The DC voltage deviation value is then used to generate a direct-axis outer loop signal through a proportional-integral controller with saturation limiting.

[0038] In the constant reactive power outer loop control mode, the actual reactive power value is collected, the actual reactive power value is subtracted from the reactive power reference value to obtain the reactive power deviation value, and the reactive power deviation value is used to generate the quadrature axis outer loop signal through the proportional-integral controller with saturation limit.

[0039] The quadrature axis synchronization signal, the direct axis outer loop signal, and the quadrature axis outer loop signal are used as input column vectors. The input column vector signals are cross-linked and mapped through a pre-constructed weighting coefficient matrix to generate an output column vector containing frequency deviation signal, direct axis current reference value, and quadrature axis current reference value.

[0040] The frequency deviation signal is superimposed on the actual frequency of the power grid and integrated to obtain the synchronization phase angle. The actual value of the direct-axis current is then collected under synchronization control using the synchronization phase angle.

[0041] The direct-axis current deviation value is obtained by subtracting the actual value of the direct-axis current from the reference value of the direct-axis current. The direct-axis current deviation value is then passed through a proportional-integral circuit with saturation limiting and a decoupling signal containing the actual value of the quadrature-axis current is added to generate the direct-axis inner loop signal.

[0042] Subtracting the actual value of the quadrature-axis current from the reference value of the quadrature-axis current yields the quadrature-axis current deviation value. This deviation value is then passed through a proportional-integral controller with saturation and supplemented with a decoupling signal containing the actual value of the direct-axis current to generate the quadrature-axis inner loop signal.

[0043] By sequentially adding the actual values ​​of the direct-axis current and quadrature-axis voltage to the direct-axis inner loop signal and the quadrature-axis inner loop signal, the direct-axis modulation ratio and the quadrature-axis modulation ratio are obtained.

[0044] A sinusoidal pulse width modulation strategy is used to modulate the direct-axis modulation ratio and the quadrature-axis modulation ratio, thereby reconstructing the wind turbine grid-side converter into a matrix control structure.

[0045] In a further implementation, the step of iteratively optimizing the weighting coefficient matrix in the matrix control structure based on impedance sensitivity, and simultaneously performing impedance reshaping in a low-pass filter connected in series in the voltage and current feedback channel to complete the suppression of wideband oscillation risk includes:

[0046] The amplitude sensitivity and phase sensitivity of each element in the weighting coefficient matrix to the equivalent positive sequence impedance on the wind field side are calculated to obtain the impedance sensitivity.

[0047] Using the direction with the highest impedance sensitivity as the search direction, gradient optimization iteration is performed on the elements in the weighting coefficient matrix, and the stability margin is recalculated after each iteration.

[0048] When the stability margin is not lower than the preset margin threshold, stop the gradient optimization iteration of the elements in the weighted coefficient matrix and output the optimized weighted coefficient matrix;

[0049] A low-pass filter is selected based on the frequency characteristics and broadband oscillation suppression requirements of the offshore wind power flexible DC grid connection system, and the low-pass filter is connected in series in the voltage feedback channel and the current feedback channel respectively;

[0050] The optimized weighted coefficient matrix is ​​loaded into the wind turbine grid-side converter controller, and the low-pass filter in the voltage and current feedback channel is activated simultaneously to reshape the impedance and suppress the risk of wideband oscillation.

[0051] In a further embodiment, the step of synchronously enabling the low-pass filter in the voltage-current feedback channel for impedance reshaping includes:

[0052] The three-phase voltage and three-phase current signals at the grid connection point of the offshore wind power flexible DC grid-connected system are collected, and the three-phase voltage signals are subjected to Parker transformation to obtain the AC and DC axis vector voltage signals.

[0053] The three-phase current signals are subjected to Parker transformation to obtain the perpendicular and direct axis vector current signals;

[0054] Low-pass filters are connected in series in the feedback channels of the quadrature-direct axis vector voltage signal and the quadrature-direct axis vector current signal to dynamically reshape the control loop and obtain the quadrature-direct axis components of voltage and current.

[0055] The instantaneous power measurement value is calculated based on the AC and DC axis components of the voltage and current, and the instantaneous power measurement value is used as the feedback quantity of the matrix control structure to reshape the impedance characteristics of the offshore wind power flexible DC grid connection system.

[0056] Secondly, the present invention provides a broadband oscillation suppression system for a flexible DC grid-connected offshore wind power system, the system comprising:

[0057] The data acquisition module is used to sequentially inject positive and negative sequence micro-perturbation sinusoidal harmonic signals at the grid connection point of the wind turbine, and simultaneously acquire the voltage and current harmonic response signals at the grid connection point.

[0058] The harmonic analysis module is used to perform fast Fourier analysis on the voltage and current harmonic response signals at the grid connection point to obtain measured impedance frequency sweep data.

[0059] The impedance analysis module is used to construct a multi-input multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance sweep data, and to reduce the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance with single input and single output.

[0060] The stability analysis module is used to construct a Bode plot of the source-grid side impedance characteristics based on the equivalent positive sequence impedance, read the phase difference at the intersection of the source-grid impedance amplitudes, and quantify the stability margin of the offshore wind power flexible DC grid-connected system based on the phase difference.

[0061] The reconfiguration control module is used to reconfigure the wind turbine grid-side converter into a matrix control structure by cascading a weighted coefficient matrix between the inner and outer loop control loops of the wind turbine grid-side converter when the stability margin is lower than a preset margin threshold.

[0062] The oscillation suppression module is used to iteratively optimize the weighting coefficient matrix in the matrix control structure based on impedance sensitivity, and simultaneously perform impedance reshaping by connecting a low-pass filter in series in the voltage and current feedback channels to complete the suppression of wideband oscillation risk.

[0063] Thirdly, the present invention also provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the steps of the above-described method.

[0064] This invention provides a broadband oscillation suppression method, system, and medium for offshore wind power flexible DC grid-connected systems. The method involves sequentially injecting positive and negative sequence micro-perturbation sinusoidal harmonic signals at the grid connection point of the wind turbine, simultaneously acquiring the voltage and current harmonic response signals at the grid connection point; performing fast Fourier analysis on the voltage and current harmonic response signals to obtain measured impedance frequency sweep data; constructing a multi-input multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance frequency sweep data, and reducing the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance with a single input and single output; based on the equivalent positive... The method constructs a Bode plot of the source-grid side impedance characteristics using sequence impedance, reads the phase difference at the intersection of the source-grid impedance amplitudes, and quantifies the stability margin of the offshore wind power flexible DC grid-connected system based on the phase difference. When the stability margin is lower than a preset margin threshold, the wind turbine grid-side converter is reconstructed into a matrix control structure by cascading weighted coefficient matrices between the inner and outer loop control loops of the wind turbine grid-side converter. The weighted coefficient matrix in the matrix control structure is iteratively optimized based on impedance sensitivity, and impedance reshaping is performed simultaneously by connecting a low-pass filter in series in the voltage and current feedback channels to complete the suppression of wideband oscillation risk. Compared with existing technologies, this method accurately evaluates and optimizes the stability of the offshore wind power flexible DC grid-connected system through impedance modeling and matrix control strategies. When the stability margin is insufficient, wideband oscillation suppression of the offshore wind power flexible DC grid-connected system is achieved through weighted coefficient matrix optimization and low-pass filter configuration, ensuring the stable operation of the offshore wind power flexible DC grid-connected system under power fluctuations. Attached Figure Description

[0065] Figure 1 This is a schematic diagram of the broadband oscillation suppression method for offshore wind power flexible DC grid connection system provided in an embodiment of the present invention;

[0066] Figure 2 This is a control structure block diagram of the matrix control strategy for the grid-side converter of a full-power offshore wind turbine provided in an embodiment of the present invention;

[0067] Figure 3 This is a block diagram of a filter connected in series in the voltage, current and power sampling loop under the active suppression strategy provided in this embodiment of the invention;

[0068] Figure 4 This is a block diagram of a broadband oscillation suppression system for a flexible DC grid-connected offshore wind power system provided in an embodiment of the present invention.

[0069] Explanation of reference numerals in the attached diagram: 101, Data acquisition module; 102, Harmonic analysis module; 103, Impedance analysis module; 104, Stability analysis module; 105, Reconstruction control module; 106, Oscillation suppression module. Detailed Implementation

[0070] The embodiments of the present invention are described in detail below with reference to the accompanying drawings. The embodiments are given for illustrative purposes only and should not be construed as limiting the present invention. The accompanying drawings are for reference and illustration only and do not constitute a limitation on the scope of patent protection of the present invention, because many changes can be made to the present invention without departing from the spirit and scope of the present invention.

[0071] Figure 1 This is a schematic flowchart of a broadband oscillation suppression method for a flexible DC-connected offshore wind power system provided in an embodiment of the present invention. The present invention provides a broadband oscillation suppression method for a flexible DC-connected offshore wind power system, such as... Figure 1 As shown, the method includes the following steps:

[0072] S1. Inject positive and negative sequence micro-perturbation sinusoidal harmonic signals sequentially at the grid connection point of the wind turbine, and simultaneously collect the voltage and current harmonic response signals at the grid connection point.

[0073] S2. Perform fast Fourier analysis on the voltage and current harmonic response signals at the grid connection point to obtain measured impedance frequency sweep data.

[0074] Specifically, in the impedance characteristic analysis process, this embodiment sequentially injects positive-sequence and negative-sequence perturbation sinusoidal harmonic signals at the offshore wind farm grid connection point. For example, this embodiment sequentially injects positive-sequence perturbation sinusoidal harmonic signals at each frequency point at the offshore wind farm grid connection point, each lasting for no less than 10 signal cycles. After completing the positive-sequence frequency sweep, it switches to negative-sequence perturbation sinusoidal harmonic signals, injecting them sequentially according to the same frequency sequence, with an interval of no less than 5 seconds between each injection to ensure that the system returns to steady state. The amplitude of the positive and negative-sequence perturbation sinusoidal harmonic signals does not exceed 5% of the rated AC voltage of the offshore wind power flexible DC grid connection system to avoid significantly affecting the steady-state operating point of the system, thereby ensuring the effectiveness of the linearization analysis method. Then, this embodiment synchronously deploys voltage and current sensors on the source side (wind farm outlet) and grid side (flexible DC converter station inlet) at the grid connection point. While injecting signals, this embodiment needs to simultaneously collect data. The system response to the injected perturbation signal yields the harmonic response signals of the grid connection point voltage and current. These signals are then processed using Fast Fourier Transform (FFT) to obtain measured impedance sweep frequency data. During signal acquisition, this embodiment considers both voltage and current harmonic signals on both sides of the source grid to comprehensively acquire impedance sweep frequency data from both sides, enabling the system to reflect its dynamic behavior under different operating conditions. Furthermore, to ensure the accuracy of the obtained impedance characteristic data, this embodiment requires a reasonable selection of the sweep frequency point distribution and the harmonic signal injection time. Specifically, the sweep frequency points should be densely distributed to cover the entire frequency range of the system, while the harmonic signal injection time should be sufficiently long to ensure the system reaches a steady state in the harmonic state space and fully converges to a steady state, thus providing a reliable basis for subsequent comparative analysis with theoretical impedance curves.

[0075] S3. Construct a multi-input multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance sweep data, and reduce the order of the multi-input multi-output impedance matrix to the equivalent positive sequence impedance of single input single output.

[0076] In some implementations, the step of constructing a multi-input multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance sweep data includes:

[0077] Background harmonic components and abnormal frequency points are removed from the measured impedance sweep data to obtain preprocessed impedance sweep data.

[0078] Based on the preprocessed impedance sweep data, voltage harmonic vectors and current harmonic vectors are constructed in order of frequency from low to high.

[0079] The impedance value at each frequency sweep point is calculated based on the ratio between the voltage harmonic vector and the current harmonic vector, and a multi-input multi-output impedance matrix considering multi-harmonic coupling is constructed.

[0080] This embodiment processes and transforms measured impedance frequency sweep data. A multi-input multi-output (MIMO) impedance matrix considering multi-harmonic coupling is constructed through matrix operations. Within a given error limit, this matrix is ​​compared with an impedance model constructed based on theoretical methods to verify the accuracy of the theoretical model and the effectiveness of the frequency sweep data. Specifically, before impedance frequency sweeping, since the system already contains certain harmonic components, to reduce the influence of the system's initial harmonics on the monitoring results, this embodiment subtracts the harmonic components already present in the system before the sweep from the system's harmonic components obtained after impedance frequency sweeping. For example, for a 10Hz harmonic, the voltage harmonic component at that frequency after the sweep is subtracted from the voltage harmonic component at that frequency before the sweep. The current harmonic component is handled similarly. This embodiment obtains the system harmonics after removing the influence of initial harmonics through this operation. The system harmonic component data is used to effectively reduce monitoring errors caused by initial harmonics, providing a foundation for accurate subsequent analysis. Then, in this embodiment, all impedance sweep data after removing the influence of initial harmonics are processed, and obviously abnormal data points are removed. These abnormal data points may be introduced by measurement errors or external interference, which will have an adverse effect on subsequent analysis. After removing abnormal points, this embodiment further analyzes and judges the consistency of the remaining data points to ensure the reliability and stability of the data. During the data processing, the singularity of the data near the resonance peak will be significantly enhanced, which may cause numerical errors. Therefore, in order to ensure the accuracy of the analysis results, this embodiment needs to appropriately relax the comparison error limit to avoid misjudgment due to numerical errors. After completing the above data processing steps, this embodiment constructs a multi-input multi-output impedance matrix based on the preprocessed impedance sweep data.

[0081] In this embodiment, based on the preprocessed impedance sweep data, the voltage harmonic component data at different frequencies are arranged into voltage harmonic vectors in ascending order of frequency. These voltage harmonic vectors accurately reflect the voltage harmonic characteristics at each frequency point. Similarly, following the same method as constructing the voltage data matrix, current harmonic vectors are constructed based on the current harmonic component data in the preprocessed impedance sweep data. This embodiment utilizes the voltage and current harmonic vectors to construct a multi-input multi-output impedance matrix by calculating the impedance values ​​at each frequency point (i.e., the ratio of the voltage and current harmonic vectors). This matrix comprehensively reflects the impedance characteristics of the system at different frequencies and different input / output combinations. The specific transformation process of the multi-input multi-output impedance matrix in this embodiment is as follows:

[0082]

[0083]

[0084] In the formula, The order impedance matrix is ​​the impedance characteristic of the system in the positive and negative sequence coordinate systems. It is a 2×2 complex matrix. The elements of the order impedance matrix include the positive sequence self impedance, the positive sequence to negative sequence coupling impedance, the negative sequence to positive sequence coupling impedance, and the negative sequence self impedance. This is the voltage harmonic vector at the grid connection point after data processing. This is the current harmonic vector at the grid connection point after data processing; In frequency The first harmonic component of the positive sequence voltage (such as the fundamental frequency or the positive sequence voltage response at a specific injection frequency). In frequency Below, the second harmonic component of the positive sequence voltage; In frequency Below, the first harmonic component of the negative sequence voltage, where, This is the system power frequency, which reflects the coupling characteristics of negative sequence voltage and positive sequence injection frequency; In frequency The second harmonic component of the negative sequence voltage; In frequency Below, the first harmonic component of the positive sequence current; In frequency Below, the second harmonic component of the positive sequence current; In frequency The first harmonic component of the negative sequence current reflects the response of the negative sequence current. In frequency Below, the second harmonic component of the negative sequence current; the superscript -1 indicates the inverse operation of the matrix; the subscript p indicates the positive sequence component; the subscript n indicates the negative sequence component, whose frequency shift... This reflects the frequency coupling effect between sequence components; Indicates the frequency of the injected sinusoidal perturbation signal (sweep point); negative sequence component The introduction of this reflects the multi-harmonic coupling effect in the offshore wind power flexible DC system.

[0085] In this embodiment, the order impedance matrix This embodiment uses multi-frequency sweep and harmonic linearization theory to provide the raw data for positive and negative sequence impedances, transforming the discrete sequence impedance matrix... Decomposed into a continuous transfer function form, the functional expression of the multi-input multi-output impedance matrix is ​​as follows:

[0086]

[0087] In the formula, The aggregated wind farm side has four impedance components, which are based on the source grid division according to the wind farm grid connection point. The positive sequence self-impedance of the aggregated wind field side; The positive-sequence to negative-sequence coupling impedance on the aggregated wind field side; The negative-sequence to positive-sequence coupling impedance on the aggregated wind field side; The negative sequence self-impedance of the aggregated wind field side; The flexible DC converter station side has four impedance components, which are based on the source grid division according to the wind farm grid connection point. The positive sequence self-impedance of the flexible DC side; This is the positive-sequence to negative-sequence coupling impedance on the flexible DC side; This is the flexible DC-side negative-sequence to positive-sequence coupling impedance; It is the negative sequence self-impedance of the flexible DC side.

[0088] In some embodiments, the step of reducing the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance of a single-input single-output matrix includes:

[0089] Based on the physical topology and electrical parameters of the offshore wind power flexible DC grid connection system, a multi-harmonic linearized theoretical impedance model is established.

[0090] The consistency verification results of the multi-harmonic linearized theoretical impedance model and the multi-input multi-output impedance matrix are obtained by performing a consistency deviation verification.

[0091] After the consistency deviation verification result is passed, the key impedance elements in the multi-input multi-output impedance matrix are extracted;

[0092] Based on the key impedance elements, the multi-input multi-output impedance matrix is ​​reduced to an equivalent positive-sequence impedance with single input and single output through the Schur complement transformation.

[0093] Regarding the theoretical impedance modeling stage of the system, this embodiment performs multi-harmonic linearization based on the topology, main electrical parameters, and control system of the offshore wind power system connected to the flexible DC grid, in order to construct an accurate impedance model. In some implementations, the steps of establishing the multi-harmonic linearized theoretical impedance model based on the physical topology and electrical parameters of the offshore wind power system connected to the flexible DC grid include:

[0094] Based on the physical topology of the offshore wind power flexible DC grid-connected system, a main loop model of the system is established, and the harmonic components of each steady-state variable at each characteristic frequency point are extracted from the main loop model of the system to obtain the multi-frequency harmonic linearized representation of all steady-state variables.

[0095] The linearized representation of multi-frequency harmonics is substituted into the main circuit model of the wind turbine grid-side converter and the main circuit model of the flexible DC system for linearization, respectively, to obtain the small-signal equation of the main circuit harmonics.

[0096] Establish the transmission relationship between grid connection point voltage disturbance and current disturbance and wind turbine modulation ratio disturbance to the wind turbine controller, and decompose the wind turbine controller transmission relationship into wind turbine harmonic linearization equations according to frequency points;

[0097] Establish the transmission relationship of the flexible DC controller from grid connection point voltage disturbance and current disturbance to flexible DC modulation ratio disturbance, and decompose the flexible DC controller transmission relationship into flexible DC harmonic linearization equations according to frequency points;

[0098] The harmonic linearization equation of the wind turbine and the harmonic linearization equation of the flexible DC harmonic are integrated to construct the multi-frequency harmonic small-signal equation of the controller.

[0099] By combining the harmonic small-signal equations of the main circuit and the multi-frequency harmonic small-signal equations of the controller, a multi-harmonic linearized theoretical impedance model is obtained.

[0100] In the specific implementation process, the steps of multi-harmonic linearization include obtaining the mathematical model of the main loop of the system and performing harmonic linearization on the steady-state variables of the offshore wind power connected to the flexible DC grid to obtain the basic response characteristics of the system under harmonic excitation; secondly, by injecting positive and negative sequence micro-perturbation sinusoidal voltage harmonics at the grid connection point of the wind turbine, the main loop is harmonic linearized using small-signal analysis theory to obtain the harmonic response of the offshore wind power connected to the flexible DC grid under this excitation; furthermore, this embodiment considers the dynamic influence of multiple links such as the controller synchronization link, the inner current loop, and the outer voltage loop to perform control dynamics theoretical modeling, repeating the aforementioned operations to obtain the harmonic linearization equation of the controller dynamics in sequence, and selecting the grid connection during this process. Key variables such as point voltage, grid connection point current, wind turbine modulation ratio, and flexible DC converter station modulation ratio are subjected to harmonic linearization expansion. Finally, the electrical dynamic characteristic equations related to the main circuit of the system and the control dynamic characteristic equations related to the controller are simultaneously transformed to obtain the theoretical impedance model of the offshore wind power system connected to the flexible DC grid. In specific implementation, taking the offshore wind power system connected to the flexible DC transmission system as an example, this embodiment constructs the main circuit model of the wind turbine grid-side converter and the main circuit model of the flexible DC system based on the actual physical topology of the offshore wind power flexible DC grid connection system. These two models are then combined to form the system main circuit model. Specifically, the wind turbine grid-side converter main circuit model is as follows:

[0101]

[0102] In the formula, This is the grid connection point voltage phasor of the wind turbine grid-side voltage source converter at the common grid connection point; The modulation ratio of the grid-side converter of the wind turbine; This refers to the steady-state voltage on the DC side of the grid-side converter for the wind turbine. The equivalent inductance of the transformer connecting the grid-side converter of the wind turbine to the AC power grid; This refers to the current phasor flowing through the grid connection point (i.e., the output current of the wind turbine grid-side converter).

[0103] The main circuit model of the flexible DC system is as follows:

[0104]

[0105] In the formula, For the grid connection point voltage phasor of the flexible DC converter station at the common grid connection point; This refers to the steady-state voltage on the DC side of the flexible DC converter station. The modulation ratio for the flexible DC converter station; It is the sum of the capacitor voltages of the upper arm of the modular multilevel converter, which reflects the dynamic energy of the arm. The equivalent inductance of a single bridge arm of a modular multilevel converter; The current in the upper arm of the modular multilevel converter reflects the dynamic current within the arm. The equivalent inductance of the transformer connecting the flexible DC converter station and the AC power grid.

[0106] For each steady-state variable in the system's main loop model, this embodiment extracts its harmonic components at seven characteristic frequency points (including the fundamental frequency and its upper and lower third harmonic offsets). Each steady-state variable is represented as a superposition of the fundamental steady-state value and the small-signal harmonic components at the seven frequency points, obtaining a multi-frequency harmonic linearized representation of all steady-state variables. The steady-state variables include the grid-connected voltage phasor, the current phasor flowing through the grid-connected point, the modulation ratio of the wind turbine grid-side converter, the modulation ratio of the flexible DC converter station, and the upper arm current of the modular multilevel converter, etc. The harmonic linearization expansion process of the steady-state variables in this embodiment is generally represented as follows:

[0107]

[0108] In the formula, For the steady-state variable after harmonic linearization, its physical meaning is to decompose the time-domain variable into the linear superposition of the fundamental frequency and its sideband harmonics, which is used to analyze the dynamic response of the system under multi-frequency disturbances. In frequency The small-signal harmonic components reflect the dynamic characteristics of the steady-state variable x when the fundamental frequency deviates by 3 times the power frequency; In frequency The harmonic components below; In frequency The harmonic components below; To the injection frequency The fundamental frequency component is the core frequency point for frequency sweep analysis; In frequency The harmonic components below; In frequency The harmonic components below; In frequency The harmonic components below; the superscript T indicates transpose operation; Indicates the frequency of the injected sinusoidal perturbation signal (sweep point); This is the system power frequency.

[0109] To address the broadband oscillation problem caused by multi-timescale coupling in power electronic equipment, this embodiment decomposes variables into multi-frequency components, enabling accurate modeling of the interactive dynamics between the wind turbine and the flexible DC system. This embodiment substitutes the obtained linearized representation of multi-frequency harmonics into the main circuit model of the wind turbine grid-side converter, eliminating steady-state terms and retaining small-signal disturbance terms to obtain the small-signal equations for the wind turbine main circuit. Simultaneously, the obtained linearized representation of multi-frequency harmonics is substituted into the main circuit model of the flexible DC system, retaining small-signal disturbance terms and introducing a modulation ratio-capacitor voltage conversion matrix to derive the small-signal harmonic equations for the flexible DC side main circuit. These two equations describe the dynamic responses of the wind turbine side and the flexible DC side under harmonic disturbances, respectively. This embodiment constructs the main circuit harmonic small-signal equations of the wind turbine side and the flexible DC side main circuit as the main circuit harmonic small-signal equations. Specifically, the wind turbine side main circuit harmonic small-signal equation is as follows:

[0110]

[0111] The specific equation for the harmonic small-signal signal of the main circuit on the flexible straight side is as follows:

[0112]

[0113] In the formula, This refers to the small-signal disturbance component of the grid connection voltage on the wind turbine side. This refers to the steady-state voltage on the DC side of the grid-side converter for the wind turbine. Small-signal disturbances to the modulation ratio of the grid-side converter of the wind turbine; The connection impedance for the transformer on the wind turbine side; For small-signal disturbances in the current flowing through the grid connection point; This refers to the small-signal disturbance component of the grid connection voltage at the flexible DC converter station side. The modulation ratio for the flexible DC converter station; The Toeplitz matrix represents the modulation ratio of the flexible DC converter station, reflecting the multi-harmonic coupling effect. For small-signal disturbances in the upper arm capacitor voltage of the modular multilevel converter; Let be the Toeplitz matrix of the sum of capacitor voltages; Small-signal disturbances to the modulation ratio of the flexible DC converter station; The equivalent impedance of a single bridge arm of a modular multilevel converter; For small-signal disturbances in the upper arm current of modular multilevel converters; This refers to the connection impedance of the transformer on the flexible DC converter station side.

[0114] Meanwhile, this embodiment analyzes the impact of grid connection point voltage and current disturbances on the wind turbine modulation ratio. Using the grid connection point voltage and current disturbances as inputs and the wind turbine modulation ratio disturbance as the output, a frequency domain transfer function is obtained. This frequency domain transfer function is used as the transfer relationship of the wind turbine controller, and this transfer relationship is decomposed according to characteristic frequency points to obtain the wind turbine harmonic linearization equation. This wind turbine harmonic linearization equation reflects the dynamic characteristics of the wind turbine controller at different frequency points. The specific wind turbine harmonic linearization equation is as follows:

[0115]

[0116] In the formula, Small-signal disturbances to the modulation ratio of the grid-side converter of the wind turbine; This is the transfer function from the grid connection point voltage disturbance to the modulation ratio in the wind turbine controller; This is the transfer function from the grid connection point current disturbance to the modulation ratio in the wind turbine controller.

[0117] Similarly, this embodiment uses grid connection point voltage disturbance and current disturbance as input, and flexible DC modulation ratio disturbance as output, to establish the flexible DC controller transmission relationship from grid connection point voltage disturbance and current disturbance to flexible DC modulation ratio disturbance, and decomposes it into flexible DC harmonic linearization equations according to characteristic frequency points. The flexible DC harmonic linearization equations are as follows:

[0118]

[0119] In the formula, Small-signal disturbances to the modulation ratio of the flexible DC converter station; This is the transfer function from the grid connection point voltage disturbance to the modulation ratio in the flexible DC controller; This is the transfer function from the grid-connected point current disturbance to the modulation ratio in a flexible DC controller.

[0120] This embodiment integrates the wind turbine harmonic linearization equation and the flexible DC harmonic linearization equation to construct the controller's multi-frequency harmonic small-signal equation. This equation combines the dynamic characteristics of the wind turbine and the flexible DC controller, providing crucial information for the subsequent impedance model establishment. This embodiment solves the main circuit harmonic small-signal equation (including the wind turbine-side main circuit harmonic small-signal equation and the flexible DC-side main circuit harmonic small-signal equation) simultaneously with the controller's multi-frequency harmonic small-signal equation. Specifically, the wind turbine-side main circuit harmonic small-signal equation is combined with the wind turbine harmonic linearization equation, and decoupled through matrix inversion to obtain the multi-frequency harmonic small-signal equation on the wind turbine side. A linearized theoretical impedance model for the flexible DC-DC power grid connection was developed. This model combines the small-signal harmonic equations of the main circuit on the flexible DC side with the linearized harmonic equations of the flexible DC side. By integrating the modulation ratio-capacitor voltage conversion matrix and decoupling through matrix operations, a linearized theoretical impedance model for the flexible DC side was obtained. This model, with grid connection point voltage and current as port variables, comprehensively reflects the impedance characteristics of the offshore wind power flexible DC grid-connected system at different frequency points, providing a theoretical basis for subsequent stability margin analysis and broadband oscillation suppression. Specifically, the linearized theoretical impedance model for the wind turbine side is as follows:

[0121]

[0122] In the formula, I is the linearized theoretical impedance model for multi-harmonics on the wind turbine side, reflecting the impedance characteristics of the wind turbine grid-side converter at the grid connection point; I is the identity matrix.

[0123] The specific theoretical impedance model for the multi-harmonic linearization of the flexible DC side is as follows:

[0124]

[0125] In the formula, A linearized theoretical impedance model for the multi-harmonics of the flexible DC side is used to describe the impedance characteristics of the modular multilevel converter at the grid connection point. This is the conversion matrix from capacitor voltage to modulation ratio for a modular multilevel converter, which reflects the influence of capacitor voltage on modulation ratio. This is a conversion matrix from capacitor voltage to current for a modular multilevel converter, which reflects the influence of capacitor voltage on arm current.

[0126] After obtaining the multi-harmonic linearized theoretical impedance model, this embodiment performs a consistency check between the constructed multi-harmonic linearized theoretical impedance model and the multi-input multi-output impedance matrix. The specific operation involves comparing the amplitude and phase impedance characteristics of the two within a certain error limit. If the deviation is within the preset error limit, the consistency is considered good, and the consistency deviation check passes. Otherwise, further analysis of the deviation is required. Specifically, this embodiment compares the amplitude and phase of the corresponding elements in the multi-input multi-output impedance matrix and the multi-harmonic linearized theoretical impedance model, calculating the amplitude and phase deviations of each element. When both the amplitude and phase deviations are within the preset deviation range, the multi-input multi-output impedance matrix and the multi-harmonic linearized theoretical impedance model pass the consistency check. This indicates good consistency between the measured data and the theoretical model, providing a reliable basis for subsequent impedance reshaping and broadband oscillation suppression. If there are elements exceeding the preset deviation range, further analysis of the cause is required. The cause may include errors in the measured data, inappropriate simplification assumptions in the theoretical model, etc., and corrections and improvements are made accordingly.

[0127] After the consistency deviation verification result is passed, this embodiment performs order reduction processing on the verified multi-input multi-output impedance matrix in the coupling impedance reduction stage, transforming it into a single-input single-output (SISO) impedance form to more intuitively analyze the system stability. Specifically, this embodiment extracts key impedance elements from the multi-input multi-output impedance matrix. These key impedance elements include the positive-sequence self-impedance component on the converged wind farm side, the positive-to-negative-sequence coupling impedance component on the converged wind farm side, the negative-to-positive-sequence coupling impedance component on the converged wind farm side, the negative-sequence self-impedance component on the converged wind farm side, the positive-sequence self-impedance component on the flexible DC converter station side, the positive-to-negative-sequence coupling impedance component on the flexible DC converter station side, the negative-to-positive-sequence coupling impedance component on the flexible DC converter station side, and the negative-sequence self-impedance component on the flexible DC converter station side. Based on these key impedance elements, this embodiment reduces the multi-input multi-output impedance matrix to a single-input single-output impedance form through Schur complement transformation. This process is achieved through Schur complement transformation, a matrix theory technique. The order reduction method described in this embodiment is based on the Schur complement transformation principle in matrix theory. While ensuring that the block diagonal matrix of the original multi-input multi-output impedance matrix is ​​a non-singular matrix, the extracted key impedance elements are processed. For the converged wind field side, the multi-input multi-output impedance is reduced to an equivalent positive-sequence impedance of single input single output through the Schur complement transformation. Specifically, the impedance components of the converged wind field side are combined according to the Schur complement transformation to eliminate the influence of negative-sequence correlated impedance, thus obtaining the equivalent positive-sequence impedance of single input single output on the converged wind field side. Similarly, the impedance components of the flexible DC side are subjected to the Schur complement transformation to obtain the equivalent positive-sequence impedance of single input single output on the flexible DC side. Through this step, the complex multi-input multi-output impedance matrix is ​​reduced to a single-input single-output impedance form that is easier to analyze, resulting in the equivalent positive-sequence impedance of single input single output. This allows the complex multi-input multi-output system to be presented in a simpler form, and the reduced impedance form facilitates further analysis and evaluation of the system's stability.

[0128] It should be noted that when both the source and network impedances are multi-input multi-output impedances, the generalized Nyquist criterion needs to be used for stability determination. However, this method has poor practicality in engineering applications. Therefore, this embodiment uses the Schur complement transformation in matrix theory for the order reduction process, requiring that the block diagonal matrix of the original matrix be a non-singular matrix, thereby ensuring the accuracy and effectiveness of the order reduction. After the order reduction process, the equivalent single-input single-output positive-sequence impedance of the source and network is obtained as follows:

[0129]

[0130]

[0131] In the formula, This is the equivalent positive sequence impedance for a single input and single output on the aggregated wind field side; This is the negative sequence self-impedance of the flexible DC side, i.e., the element of the multi-input multi-output impedance matrix; This is the negative-sequence admittance on the wind farm side, which is the reciprocal of the impedance and reflects the negative-sequence current response characteristics. For the negative-sequence to positive-sequence coupling impedance on the flexible straight side; This is the positive-sequence to negative-sequence admittance on the wind farm side, which reflects the effect of positive-sequence voltage on negative-sequence current. This is the positive-sequence admittance on the wind farm side, which represents the response of the positive-sequence current to the positive-sequence voltage; This represents the negative-sequence to positive-sequence admittance from the wind field side. The equivalent positive sequence impedance for a single input and single output on the flexible DC side; The negative sequence self-impedance of the wind field side; For flexible straight-side negative order admittance; The negative-sequence to positive-sequence coupling impedance on the wind farm side; For flexible straight-side positive-sequence to negative-sequence admittance; For flexible straight-side positive-sequence admittance; The admittance is for the flexible straight side from negative to positive order.

[0132] S4. Construct a Bode plot of the source-grid side impedance characteristics based on the equivalent positive sequence impedance, read the phase difference at the intersection of the source-grid impedance amplitudes, and quantize the stability margin of the offshore wind power flexible DC grid-connected system according to the phase difference.

[0133] In some implementations, the steps of constructing a Bode plot of the source-grid side impedance characteristics based on the equivalent positive sequence impedance, reading the phase difference at the intersection of the source-grid impedance amplitudes, and quantizing the stability margin of the offshore wind power flexible DC grid-connected system based on the phase difference include:

[0134] Based on the equivalent positive sequence impedance, calculate the wind field impedance amplitude, wind field impedance phase, flexible DC impedance amplitude, and flexible DC impedance phase at each frequency sweep point to obtain impedance amplitude and phase data;

[0135] A Bode plot of the source-network impedance characteristics is constructed using the frequency sweep points as the horizontal axis and the impedance amplitude and phase data as the vertical axis.

[0136] Locate the intersection frequency point of the source-grid impedance amplitude in the source-grid side impedance characteristic Bode plot, and read the wind field impedance phase value and flexible DC impedance phase value at the intersection frequency point;

[0137] Calculate the difference between the phase value of the wind field impedance and the phase value of the flexible DC impedance to obtain the phase difference at the intersection of the source and grid impedance amplitudes;

[0138] The deviation between the phase difference and the preset stability margin threshold is calculated to obtain the stability margin of the offshore wind power flexible DC grid connection system.

[0139] Specifically, in this embodiment, the equivalent positive sequence impedance amplitude and phase on the wind farm side and the equivalent positive sequence impedance amplitude and phase on the flexible DC converter station side are calculated point by point according to the frequency sweep of the equivalent positive sequence impedance. Then, this embodiment constructs a Bode plot of the source-grid side impedance characteristics with the frequency sweep as the abscissa and the equivalent positive sequence impedance amplitude and phase on the wind farm side and the flexible DC converter station side as the ordinate, respectively. The source-grid side impedance characteristic Bode plot includes the wind farm impedance amplitude-frequency curve, the flexible DC impedance amplitude-frequency curve, the wind farm impedance phase-frequency curve, and the flexible DC impedance phase-frequency curve. The function of the single-input single-output source-grid side impedance characteristic Bode plot is as follows:

[0140]

[0141] In the formula, It is a function of the source-network side impedance characteristic Bode plot.

[0142] In this embodiment, the intersection point of the wind field impedance amplitude curve and the flexible DC impedance amplitude curve is searched and identified in the Bode plot of the source-grid impedance characteristics to obtain the intersection frequency point. The corresponding phase value at the intersection frequency point is read, and the following formula can be derived based on the corresponding phase value at the intersection frequency point:

[0143]

[0144] The mathematical expression for stability margin is:

[0145]

[0146] In the formula, The phase angle difference is the impedance of the source network. The phase angle of the equivalent positive sequence impedance for a flexible DC-side single-input single-output circuit; The phase angle is the equivalent positive sequence impedance of the single-input single-output converter on the aggregated wind field side; This represents the stability margin of the system.

[0147] This embodiment analyzes the Bode plot of the source-network impedance characteristics to extract the impedance stability criterion. The approach is to read the deviation of the source-network impedance phase angle curve at the intersection of the source-network impedance amplitude curves and use this deviation as the system stability criterion; and then... The difference between the source-network impedance phase angle curve and the above-mentioned source-network impedance phase angle curve is used as a stability margin criterion. According to classical control theory, when the impedance phase angle characteristic curve is located at... to Within the specified range, the system exhibits good stability; however, if the impedance phase angle characteristic curve is lower than... This indicates that the impedance characteristic of this part exhibits capacitive negative damping characteristics; if the impedance phase angle characteristic curve is higher than... This indicates that the impedance characteristics of this part exhibit inductive negative damping characteristics. In this embodiment, the stability of the system can be accurately assessed in this way, providing strong guidance for subsequent broadband oscillation suppression measures.

[0148] In summary, the broadband oscillation suppression method for offshore wind power connected to a flexible DC grid system based on matrix control proposed in this embodiment increases parameter degrees of freedom through a matrix control structure, flexibly adjusting system impedance to achieve more precise and effective impedance reshaping. This effectively reduces the negative damping effect of the system, thereby ensuring the continuous and stable operation of the offshore wind power system under power fluctuations and significantly reducing the potential risk of broadband oscillations. Specifically, the offshore wind power connected to a flexible DC grid system proposed in this embodiment includes an offshore AC wind farm, an offshore wind farm collection booster station, an offshore wind farm collection bus, an offshore flexible DC converter station, a high-voltage DC transmission submarine cable, and a receiving-end onshore flexible DC converter station. The broadband oscillation suppression method performs multi-harmonic linearization and impedance modeling based on the system topology and electrical master parameters. It then performs impedance frequency sweep by injecting positive-sequence perturbation sinusoidal harmonic signals and negative-sequence perturbation sinusoidal harmonic signals, collects voltage and current signals at the offshore wind farm grid connection point for fast Fourier harmonic analysis, and then uses the obtained measured impedance frequency sweep numbers. Further transformation processing is performed to construct a multi-input multi-output system impedance matrix considering multi-harmonic coupling effects. The consistency of this matrix with the theoretical impedance modeling results is verified. When the deviation between the two is within the specified error limit, the system impedance characteristics are considered to have been accurately obtained. Next, key impedance matrix elements are extracted and subjected to SISO order reduction processing to display the source-network impedance characteristics in the form of a Bode plot. According to classical control theory, the phase angle deviation at the intersection of the source-network impedance amplitude curves is used as a stability criterion. The difference between 180° and this phase angle deviation is used as a stability margin criterion, thus effectively characterizing the system's stability margin. For example, based on control theory and practical engineering experience, if the stability margin criterion is between 20° and 30°, the system is considered to have excellent impedance characteristics and no risk of broadband oscillation. If the stability margin criterion is less than 10°, the system may have potential broadband oscillation risks due to temperature drift of engineering parameters and random shifts in power flow state. In this case, appropriate impedance reshaping strategies are needed to suppress the system's broadband oscillations.

[0149] S5. When the stability margin is lower than the preset margin threshold, the wind turbine grid-side converter is reconstructed into a matrix control structure by cascading a weighted coefficient matrix between the inner and outer loop control loops of the wind turbine grid-side converter.

[0150] In some embodiments, the step of reconstructing the wind turbine grid-side converter into a matrix control structure by cascading weighted coefficient matrices between the inner and outer loop control loops of the wind turbine grid-side converter includes:

[0151] The actual value of the quadrature-axis voltage at the grid connection point is collected and the reference value of the quadrature-axis voltage is compared to obtain the quadrature-axis voltage deviation value. The quadrature-axis voltage deviation value is then used to generate a quadrature-axis synchronization signal through a proportional-integral controller with saturation limiting.

[0152] In the constant DC voltage outer loop control mode, the actual value of DC voltage is acquired, and the actual value of DC voltage is subtracted from the preset DC voltage reference value to obtain the DC voltage deviation value. The DC voltage deviation value is then used to generate a direct-axis outer loop signal through a proportional-integral controller with saturation limiting.

[0153] In the constant reactive power outer loop control mode, the actual reactive power value is collected, the actual reactive power value is subtracted from the reactive power reference value to obtain the reactive power deviation value, and the reactive power deviation value is used to generate the quadrature axis outer loop signal through the proportional-integral controller with saturation limit.

[0154] The quadrature axis synchronization signal, the direct axis outer loop signal, and the quadrature axis outer loop signal are used as input column vectors. The input column vector signals are cross-linked and mapped through a pre-constructed weighting coefficient matrix to generate an output column vector containing frequency deviation signal, direct axis current reference value, and quadrature axis current reference value.

[0155] The frequency deviation signal is superimposed on the actual frequency of the power grid and integrated to obtain the synchronization phase angle. The actual value of the direct-axis current is then collected under synchronization control using the synchronization phase angle.

[0156] The direct-axis current deviation value is obtained by subtracting the actual value of the direct-axis current from the reference value of the direct-axis current. The direct-axis current deviation value is then passed through a proportional-integral circuit with saturation limiting and a decoupling signal containing the actual value of the quadrature-axis current is added to generate the direct-axis inner loop signal.

[0157] Subtracting the actual value of the quadrature-axis current from the reference value of the quadrature-axis current yields the quadrature-axis current deviation value. This deviation value is then passed through a proportional-integral controller with saturation and supplemented with a decoupling signal containing the actual value of the direct-axis current to generate the quadrature-axis inner loop signal.

[0158] By sequentially adding the actual values ​​of the direct-axis current and quadrature-axis voltage to the direct-axis inner loop signal and the quadrature-axis inner loop signal, the direct-axis modulation ratio and the quadrature-axis modulation ratio are obtained.

[0159] A sinusoidal pulse width modulation strategy is used to modulate the direct-axis modulation ratio and the quadrature-axis modulation ratio, thereby reconstructing the wind turbine grid-side converter into a matrix control structure.

[0160] In this embodiment, when the stability margin is lower than a preset safety threshold, a wideband oscillation active suppression strategy is implemented. This embodiment expands the shaping range of the system impedance characteristics by introducing an inter-loop weighting coefficient matrix, providing a control basis for the active oscillation suppression strategy. Its controller mainly includes a q-axis voltage phase-locked loop, a constant DC voltage outer loop, a constant reactive power outer loop, a weighting coefficient matrix cross-linking loop, a phase angle generation loop, a dq-axis vector current inner loop and its decoupling loop, and a dq-axis voltage feedforward loop. The specific implementation process of each loop is as follows:

[0161] In the q-axis grid-connected point voltage phase-locked loop (PLL) stage, this embodiment sets the q-axis voltage reference value of the grid-connected point to 0. The actual value of the grid-connected point q-axis voltage after acquisition, filtering, and per-unit processing is subtracted from the q-axis voltage reference value. The difference is used to generate a quadrature-axis synchronization signal through a proportional-integral (PI) stage with saturation limiting. The mathematical form of this quadrature-axis synchronization signal is as follows:

[0162]

[0163] In the formula, This is the quadrature-axis synchronization signal output by the phase-locked loop; The proportional gain of the phase-locked loop PI controller; S is the integral gain of the phase-locked loop PI controller; S is the Laplace operator; This is the reference value for the q-axis component of the grid connection point voltage, i.e., the q-axis voltage reference value; The measured value of the q-axis voltage inside the controller after filtering and per-unit processing; The equivalent time delay introduced by signal sampling, filtering, and normalization; This is the actual measured value of the q-axis component of the grid connection point voltage, that is, the actual q-axis voltage value obtained after the three-phase voltage is transformed by Parker transformation.

[0164] In the constant DC voltage outer loop control mode, this embodiment subtracts the actual DC voltage value after sampling, filtering, and per-unit processing from the set DC voltage reference value. The difference is used to generate a direct-axis outer loop signal through a proportional-integral circuit with saturation limiting. The mathematical form of this direct-axis outer loop signal is as follows:

[0165]

[0166] In the formula, This is the d-axis current reference signal output from the outer loop of the DC voltage; The proportional gain of the DC voltage outer loop PI controller; The integral gain of the DC voltage outer-loop PI controller; This is a reference value for DC voltage; This is the DC voltage measurement value inside the controller after filtering and per-unit processing, i.e., the DC voltage filtered value; This is the actual measured value of the DC voltage, i.e., the actual value of the DC voltage.

[0167] In the constant reactive power outer loop control mode, this embodiment subtracts the actual reactive power value after sampling, filtering, and per-unit processing from the set reactive power reference value. The difference is used to generate a quadrature-axis outer loop signal through a proportional-integral circuit with saturation limiting. The mathematical form of this quadrature-axis outer loop signal is as follows:

[0168]

[0169]

[0170] In the formula, This is the q-axis current reference signal output from the reactive power outer loop; The proportional gain of the reactive power outer loop PI controller; The integral gain of the reactive power outer loop PI controller; This is a reference value for reactive power; The measured reactive power value inside the controller is after filtering and per-unit processing. This represents the actual value of reactive power.

[0171] In the weighted coefficient matrix crosslinking stage, the quadrature-axis synchronization signal, the direct-axis outer loop signal, and the quadrature-axis outer loop signal are used as input column vectors of the matrix, and then... , , , , , , , and The weighted coefficient matrix cross-maps the above input column vector signals into an output column vector composed of the frequency deviation signal, the d-axis current reference value, and the q-axis current reference value. The structure of this output column vector can be expressed in the following mathematical form:

[0172]

[0173] In the formula, This is a frequency deviation signal, which is used to correct the output frequency of the phase-locked loop and enhance the system's synchronization stability. This is the reference value for the d-axis current. This is the reference value for the q-axis current. The elements of the weighted coefficient matrix are used to achieve dynamic coupling between multiple control loops, where the subscripts i = 1, 2, 3; and the subscript e = 1, 2, 3; the weighted coefficient matrix. It offers nine degrees of freedom, allowing for flexible reshaping of the system's impedance characteristics.

[0174] In this embodiment, the phase angle is generated by superimposing the frequency deviation signal onto the actual power grid frequency and then integrating the result. The specific mathematical expression is as follows:

[0175]

[0176] In the formula, The synchronous phase angle generated for the grid-side converter is obtained by integrating the frequency signal in this embodiment. Generate synchronous phase angle , used for Park transform.

[0177] In the dq-axis vector current inner loop and its decoupling stage, this embodiment subtracts the set d-axis current reference value from the actual value of the d-axis current after sampling, filtering, and per-unit processing. The difference is then passed through a PI circuit with saturation limiting, and a decoupling signal containing the actual value of the q-axis current is added to generate the direct-axis inner loop signal. Simultaneously, this embodiment subtracts the set q-axis current reference value from the actual value of the q-axis current after sampling, filtering, and per-unit processing. The difference is then passed through a PI circuit with saturation, and a decoupling signal containing the actual value of the d-axis current is added to generate the quadrature-axis inner loop signal. The mathematical forms of the direct-axis inner loop signal and the quadrature-axis inner loop signal are as follows:

[0178]

[0179] In the formula, This is the direct-axis inner loop signal, i.e., the d-axis current inner loop control signal; This is the proportionality coefficient of the inner loop of the d-axis current; The inner loop integral coefficient of the d-axis current; This is the d-axis current reference value, which comes from the outer loop of DC voltage. This represents the actual value of the d-axis current. The angular frequency of the power grid; This is the quadrature axis inner loop signal, i.e., the q-axis current inner loop control signal; This is the proportionality coefficient of the inner loop of the q-axis current; The integral coefficient of the inner loop of the q-axis current; This is the q-axis current reference value, which comes from the reactive power outer loop; is the actual value of the q-axis current; L is the equivalent inductance.

[0180] In this embodiment, the actual values ​​of the d-axis current and q-axis voltage, after sequential sampling, filtering, and per-unit processing, are added to the aforementioned direct-axis inner loop signal and quadrature-axis inner loop signal to obtain the d-axis modulation ratio and q-axis modulation ratio. Subsequently, a classic sinusoidal pulse width modulation strategy is used for modulation, reconstructing the wind turbine grid-side converter into a matrix control structure. Figure 2 This is a control structure block diagram of the matrix control strategy for the grid-side converter of a full-power offshore wind turbine provided in an embodiment of the present invention. Figure 2In the diagram, number 1 represents the q-axis voltage phase-locked loop; number 2 represents the constant DC voltage outer loop; number 3 represents the constant reactive power outer loop; number 4 represents the weighted coefficient matrix cross-linking link; number 5 represents the phase angle generation link; number 6 represents the dq-axis vector current inner loop and its decoupling link; and number 7 represents the dq-axis voltage feedforward link. This is the reference value for the mains voltage. The voltage is the actual measured value of the power grid; PI stands for proportional-integral controller. This is the reference value for the DC bus voltage; This is the actual value of the DC bus voltage; Q is the reference value for reactive power; Q is the actual measured value of reactive power. w is the rated angular frequency of the power grid; w is the actual angular frequency of the power grid. Let be the d-axis component of the current, which is the actual d-axis value of the current obtained through the Parker transformation; is the q-axis component of the current, which is the actual q-axis value of the current obtained through the Parker transformation; The synchronous phase angle generated for the grid-side converter is the synchronous rotation angle used for coordinate transformation (such as Parker transformation); The system voltage d-axis component; This refers to the q-axis component of the system voltage. The modulation ratio is the d-axis. This is the q-axis modulation ratio.

[0181] S6. Based on impedance sensitivity, iteratively optimize the weighting coefficient matrix in the matrix control structure, and simultaneously perform impedance reshaping by connecting a low-pass filter in series in the voltage and current feedback channels to complete the suppression of wideband oscillation risk.

[0182] In some implementations, the step of iteratively optimizing the weighting coefficient matrix in the matrix control structure based on impedance sensitivity and simultaneously performing impedance reshaping in series with a low-pass filter in the voltage and current feedback channel to achieve broadband oscillation risk suppression includes:

[0183] The amplitude sensitivity and phase sensitivity of each element in the weighting coefficient matrix to the equivalent positive sequence impedance on the wind field side are calculated to obtain the impedance sensitivity.

[0184] Using the direction with the highest impedance sensitivity as the search direction, gradient optimization iteration is performed on the elements in the weighting coefficient matrix, and the stability margin is recalculated after each iteration.

[0185] When the stability margin is not lower than the preset margin threshold, stop the gradient optimization iteration of the elements in the weighted coefficient matrix and output the optimized weighted coefficient matrix;

[0186] A low-pass filter is selected based on the frequency characteristics and broadband oscillation suppression requirements of the offshore wind power flexible DC grid connection system, and the low-pass filter is connected in series in the voltage feedback channel and the current feedback channel respectively;

[0187] The optimized weighted coefficient matrix is ​​loaded into the wind turbine grid-side converter controller, and the low-pass filter in the voltage and current feedback channel is activated simultaneously to reshape the impedance and suppress the risk of wideband oscillation.

[0188] In the impedance reshaping and margin optimization stage, this embodiment assigns initial parameter values ​​to the weighting coefficient matrix. Starting from the initial parameter values ​​and the current system operating point, it calculates the sensitivity of the weighting coefficient matrix elements to the SISO equivalent positive-sequence impedance on the wind farm side. Based on the sensitivity information and the stability margin improvement requirements, iteratively optimizes the weighting coefficient matrix parameters to achieve system impedance reshaping and stability margin improvement. Simultaneously, in the control loop reshaping stage, a low-pass filter is connected in series in the voltage and current dq-axis signal channels after Parker transformation and in the power calculation stage to reshape the control loop dynamics, suppress the resonance risk introduced by sampling errors and modulation delays, thereby effectively suppressing the broadband oscillation risk of the offshore wind power flexible DC grid-connected system. In some embodiments, the step of synchronously activating the low-pass filter in the voltage and current feedback channel for impedance reshaping includes:

[0189] The three-phase voltage and three-phase current signals at the grid connection point of the offshore wind power flexible DC grid-connected system are collected, and the three-phase voltage signals are subjected to Parker transformation to obtain the AC and DC axis vector voltage signals.

[0190] The three-phase current signals are subjected to Parker transformation to obtain the perpendicular and direct axis vector current signals;

[0191] Low-pass filters are connected in series in the feedback channels of the quadrature-direct axis vector voltage signal and the quadrature-direct axis vector current signal to dynamically reshape the control loop, thereby obtaining the voltage and current quadrature-direct axis components; the voltage and current quadrature-direct axis components include the system current direct axis component, the system current quadrature axis component, the system voltage direct axis component, and the system voltage quadrature axis component;

[0192] The instantaneous power measurement value is calculated based on the AC and DC axis components of the voltage and current, and the instantaneous power measurement value is used as the feedback quantity of the matrix control structure to reshape the impedance characteristics of the offshore wind power flexible DC grid connection system.

[0193] Specifically, in this embodiment, the three-phase voltage sine wave is converted into a constant steady-state value in the dq coordinate system to obtain the quadrature-direct axis vector voltage signal; the three-phase current sine wave is converted into a constant steady-state value in the dq coordinate system to obtain the quadrature-direct axis vector current signal. The function of the dq axis vector voltage and current signal series filter is to implement dynamic reshaping of the control loop and avoid system instability induced by controller sampling and modulation delay. The instantaneous power theoretical calculation module involving voltage and current vectors is used to generate power measurement values ​​at a very small time scale that are beneficial to controller feedback. Based on the calculated instantaneous power measurement values, this embodiment reshapes the system impedance. By adjusting the parameters of the low-pass filter and the control strategy, the impedance characteristics of the system are optimized, thereby suppressing the risk of broadband oscillation and improving the stability of the system. The mathematical form of the instantaneous power measurement value is as follows:

[0194]

[0195] In the formula, P is the instantaneous active power; Q is the instantaneous reactive power. The system voltage d-axis component; This refers to the q-axis component of the system current. This refers to the q-axis component of the system voltage. This represents the d-axis component of the system current.

[0196] Figure 3 This is a block diagram of a filter connected in series in the voltage, current, and power sampling loop under the active suppression strategy provided in this embodiment of the invention. Figure 3 In this context, P represents instantaneous active power; Q represents instantaneous reactive power. The system voltage d-axis component; This refers to the q-axis component of the system current. This refers to the q-axis component of the system voltage. The system current is represented by its d-axis component. This is the actual measured value of the voltage in phase A of the system; This is the actual measured value of the voltage in phase B of the system; This is the actual measured value of the C-phase system voltage; This is the actual measured value of the current in phase A of the system; This is the actual measured value of the current in phase B of the system; This represents the actual measured value of the current in the C-phase system. This represents the zero-sequence component of the system voltage. The zero-sequence component of the system current; PARK transform is the Park transform, which is used to convert variables in a three-phase stationary coordinate system (abc coordinate system) into variables in a two-phase rotating coordinate system (dq coordinate system); LPF is a low-pass filter.

[0197] This embodiment proposes a wideband oscillation suppression method for offshore wind power systems connected to flexible DC grids based on matrix control. It optimizes the control loop of the grid-side converter of offshore wind turbines, specifically including several key aspects such as the grid connection point q-axis voltage synchronization phase-locked loop, constant DC voltage outer loop control, constant reactive power outer loop control, weighted coefficient matrix cross-linking loop, vector current inner loop control, current decoupling loop, and grid connection point vector voltage feedforward. Specifically, this embodiment, based on the traditional control strategy of the grid-side converter for full-power offshore wind turbines, reconstructs the traditional control strategy into a matrix control structure by cascading a weighted coefficient matrix cross-linking loop at the connection between the inner and outer loops. This increases the degrees of freedom of control parameters, thereby reshaping the impedance characteristics of the offshore wind turbine. In this way, the system impedance characteristics can be flexibly adjusted. To provide a foundation for suppressing broadband oscillations, it should be noted that in the specific implementation of this embodiment, conventional parameters can be assigned to the weighting coefficient matrix based on engineering experience, and the impedance stability margin of the system can be obtained using the above method. This serves as the basis for determining whether the system needs impedance reshaping. If the system impedance stability margin is insufficient, iterative optimization is performed using the conventional parameters of the weighting coefficient matrix as initial values, based on the system impedance stability margin requirements and optimization direction, to gradually improve the impedance characteristics of the wind farm, thereby reducing the negative damping characteristics of the system. In addition, this embodiment combines the loop reshaping strategy in the active suppression strategy, connects a low-pass filter in series in the key control link, and precisely configures the filter parameters according to the impedance change trend, thereby effectively reducing the resonance risk introduced by sampling error and modulation delay, and thus more comprehensively and completely suppressing the risk of broadband oscillations in the system.

[0198] It should be noted that in this embodiment, the control strategy of the wind turbine grid-side converter in the offshore wind power system with flexible DC grid connection is reconstructed into matrix control. A set of stable parameters is obtained based on engineering experience and model debugging as the initial values ​​for optimization. Since the matrix control structure introduces more degrees of freedom for parameter adjustment, it has a larger range of stable parameters compared to the original control structure. When the following equation is satisfied, the offshore aggregated wind farm controller belongs to the classic grid-following DC voltage control, which generally satisfies the stability condition:

[0199]

[0200] When the output of offshore wind power is low or the submarine cable is long, the following parameters can be selected:

[0201]

[0202] At this time, the wind turbine grid-side converter control is in grid matching control mode, which is suitable for maintaining system stability in scenarios with long submarine cables. Therefore, in this embodiment, the above two typical weighting coefficient matrix values ​​can be selected as the initial values ​​for matrix controller parameter optimization. In this embodiment, the steady-state operating point and steady-state parameters are linearized operating points. The impedance sensitivity of the key parameters in the weighting coefficient matrix to the impedance characteristics is calculated. At the same time, combined with the corresponding impedance optimization direction corresponding to the impedance sensitivity, the impedance stability margin of the system is adjusted and optimized by iteratively adjusting the key parameters in the weighting coefficient matrix. The specific impedance sensitivity calculation method is as follows:

[0203]

[0204] In the formula, This is the equivalent positive sequence impedance for a single input and single output on the aggregated wind field side; These are elements of a weighted coefficient matrix, enabling dynamic coupling between multiple control loops. The frequency domain effect function of the weighting coefficient matrix on the impedance amplitude; j is the imaginary unit; This is the frequency domain effect function of the weighting coefficient matrix on the impedance phase; This represents the impedance sensitivity of the equivalent impedance on the wind field side to the elements of the weighted coefficient matrix.

[0205] It should be noted that the optimization process of impedance stability margin is an iterative method of dynamic optimization. The specific steps are as follows: taking the determined stable operating point as the initial value, calculate the weight of the controller parameter in the direction with the fastest improvement of impedance stability margin at this point, calculate the new controller parameters, and then analyze whether the system impedance stability margin at this point meets the requirements. If it does not meet the requirements, further analyze the impedance sensitivity of the parameters at this point and repeat the above steps; if the required stability margin is reached, the optimization stops.

[0206] This embodiment modifies the control strategy of the grid-side converter of the full-power wind turbine to matrix control, which belongs to the control reconfiguration technique in the broadband oscillation suppression strategy. This method can significantly change the system impedance characteristics from a mechanistic perspective. This embodiment adaptively finds a better weighting coefficient matrix through dynamic optimization to make the system impedance stability margin reach the set target. In essence, it belongs to the control parameter reshaping technique in the broadband oscillation suppression strategy. It should be noted that the method of connecting a low-pass filter in series in the corresponding link of the existing control loop in this embodiment belongs to the control loop reshaping technique in the broadband oscillation suppression strategy. This method configures parameters according to the impedance change trend, reduces the resonance risk introduced by sampling error and modulation delay, and thus more comprehensively and completely suppresses the broadband oscillation risk of the system. Through the above steps, the present invention can effectively suppress the broadband oscillation risk of offshore wind power connected to the flexible DC grid system and ensure that the system always maintains stable operation under power fluctuations.

[0207] This invention provides a method for suppressing broadband oscillations in a flexible DC-DC grid-connected offshore wind power system. The method involves sequentially injecting positive and negative sequence micro-perturbation sinusoidal harmonic signals at the grid connection point of the wind turbine, while simultaneously acquiring the voltage and current harmonic response signals at the grid connection point. Fast Fourier analysis is performed on the voltage and current harmonic response signals to obtain measured impedance sweep data. A multi-input multi-output impedance matrix considering multi-harmonic coupling is constructed based on the measured impedance sweep data, and this matrix is ​​reduced to an equivalent positive-sequence impedance with a single input and single output. A Bode plot of the source-grid side impedance characteristics is constructed based on the equivalent positive-sequence impedance. The phase difference at the intersection of the source-grid impedance amplitudes is read, and the stability margin of the flexible DC-DC grid-connected offshore wind power system is quantized based on the phase difference. When the stability margin is lower than a preset margin threshold, the wind turbine grid-side converter is reconstructed into a matrix control structure by cascading weighted coefficient matrices between the inner and outer loop control circuits of the grid-side converter. The weighted coefficient matrix in the matrix control structure is iteratively optimized based on impedance sensitivity, and a low-pass filter is simultaneously connected in series in the voltage and current feedback channels for impedance reshaping, thus completing the suppression of broadband oscillation risks. Compared with existing technologies, this method accurately evaluates and optimizes the stability of offshore wind power flexible DC grid-connected systems through impedance modeling and matrix control strategies. When the stability margin is insufficient, it achieves broadband oscillation suppression of offshore wind power flexible DC grid-connected systems through weighted coefficient matrix optimization and low-pass filter configuration, ensuring the stable operation of offshore wind power flexible DC grid-connected systems under power fluctuations.

[0208] It should be noted that the sequence number of each process does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0209] In one embodiment, such as Figure 4 As shown, this embodiment of the invention provides a broadband oscillation suppression system for an offshore wind power flexible DC grid connection system, the system comprising:

[0210] Data acquisition module 101 is used to sequentially inject positive and negative sequence micro-perturbation sinusoidal harmonic signals at the grid connection point of the wind turbine, and simultaneously acquire the voltage and current harmonic response signals at the grid connection point.

[0211] Harmonic analysis module 102 is used to perform fast Fourier analysis on the voltage and current harmonic response signals of the grid connection point to obtain measured impedance frequency sweep data.

[0212] Impedance analysis module 103 is used to construct a multi-input multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance sweep data, and reduce the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance with single input and single output.

[0213] The stability analysis module 104 is used to construct a Bode plot of the source-grid side impedance characteristics based on the equivalent positive sequence impedance, read the phase difference at the intersection of the source-grid impedance amplitudes, and quantize the stability margin of the offshore wind power flexible DC grid-connected system based on the phase difference.

[0214] The reconfiguration control module 105 is used to reconfigure the wind turbine grid-side converter into a matrix control structure by cascading a weighted coefficient matrix between the inner and outer loop control loops of the wind turbine grid-side converter when the stability margin is lower than a preset margin threshold.

[0215] The oscillation suppression module 106 is used to iteratively optimize the weighting coefficient matrix in the matrix control structure based on impedance sensitivity, and simultaneously perform impedance reshaping by connecting a low-pass filter in series in the voltage and current feedback channel to complete the suppression of wideband oscillation risk.

[0216] For specific limitations regarding the broadband oscillation suppression system for a flexible DC-DC grid-connected offshore wind power system, please refer to the above-described limitations regarding the broadband oscillation suppression method for a flexible DC-DC grid-connected offshore wind power system, which will not be repeated here. Those skilled in the art will recognize that the various modules and steps described in conjunction with the embodiments disclosed in this application can be implemented in hardware, software, or a combination of both. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0217] This invention provides a broadband oscillation suppression system for a flexible DC-DC grid-connected offshore wind power system. The system uses a data acquisition module to sequentially inject positive and negative sequence micro-perturbation sinusoidal harmonic signals at the grid connection point of the wind turbine, simultaneously acquiring the voltage and current harmonic response signals at the grid connection point. A harmonic analysis module performs fast Fourier analysis on the voltage and current harmonic response signals at the grid connection point to obtain measured impedance frequency sweep data. The impedance analysis module constructs a multi-input multi-output impedance matrix considering multiple harmonic coupling based on the measured impedance frequency sweep data, and reduces the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance with a single input and single output. A stability analysis module is also included. Based on the equivalent positive sequence impedance, a Bode plot of the source-grid side impedance characteristics is constructed. The phase difference at the intersection of the source-grid impedance amplitudes is read, and the stability margin of the offshore wind power flexible DC grid-connected system is obtained by quantization based on the phase difference. When the stability margin is lower than the preset margin threshold, the reconfiguration control module reconfigures the wind turbine grid-side converter into a matrix control structure by cascading weighted coefficient matrices between the inner and outer loop control loops of the wind turbine grid-side converter. The oscillation suppression module iteratively optimizes the weighted coefficient matrix in the matrix control structure based on impedance sensitivity and simultaneously performs impedance reshaping by connecting a low-pass filter in series in the voltage and current feedback channels to complete the suppression of wideband oscillation risk. Compared with existing technologies, this system accurately evaluates and optimizes the stability of the offshore wind power flexible DC grid-connected system through impedance modeling and matrix control strategies. When the stability margin is insufficient, it achieves wideband oscillation suppression of the offshore wind power flexible DC grid-connected system through weighted coefficient matrix optimization and low-pass filter configuration, ensuring the stable operation of the offshore wind power flexible DC grid-connected system under power fluctuations.

[0218] In one embodiment, the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the above-described method.

[0219] In the above embodiments, implementation can be achieved entirely or partially through software, hardware, firmware, or any combination thereof. When implemented using software, it can be implemented entirely or partially as a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, all or part of the processes or functions described in the embodiments of the present invention are generated. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another. For example, the computer instructions can be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wired (e.g., coaxial cable, fiber optic, digital subscriber line) or wireless (e.g., infrared, wireless, microwave, etc.) means. The computer-readable storage medium can be any available medium that a computer can access or a data storage device such as a server or data center that integrates one or more available media. The available medium can be a magnetic medium (e.g., floppy disk, hard disk, magnetic tape), an optical medium (e.g., DVD), or a semiconductor medium (e.g., SSD), etc.

[0220] Those skilled in the art will understand that all or part of the processes in the methods of the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium, and when the computer program is executed, it can include the processes of the embodiments of the above methods.

[0221] The embodiments described above are merely preferred embodiments of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various improvements and substitutions without departing from the technical principles of this invention, and these improvements and substitutions should also be considered within the scope of protection of this application. Therefore, the scope of protection of this patent application should be determined by the scope of the claims.

Claims

1. A method for suppressing broadband oscillations in an offshore wind power flexible DC grid-connected system, characterized in that, Includes the following steps: Positive and negative sequence micro-perturbation sinusoidal harmonic signals are sequentially injected at the grid connection point of the wind turbine, and the voltage and current harmonic response signals at the grid connection point are collected simultaneously. Fast Fourier analysis was performed on the voltage and current harmonic response signals at the grid connection point to obtain measured impedance frequency sweep data; Based on the measured impedance sweep data, a multi-input multi-output impedance matrix considering multi-harmonic coupling is constructed, and the multi-input multi-output impedance matrix is ​​reduced to an equivalent positive-sequence impedance with single input and single output. Based on the equivalent positive sequence impedance, a Bode plot of the source-grid side impedance characteristics is constructed, the phase difference at the intersection of the source-grid impedance amplitudes is read, and the stability margin of the offshore wind power flexible DC grid-connected system is obtained by quantization based on the phase difference. When the stability margin is lower than the preset margin threshold, the wind turbine grid-side converter is reconstructed into a matrix control structure by cascading a weighted coefficient matrix between the inner and outer loop control loops of the wind turbine grid-side converter. The weighting coefficient matrix in the matrix control structure is iteratively optimized based on impedance sensitivity, and a low-pass filter is simultaneously connected in series in the voltage and current feedback channels to reshape the impedance, thereby suppressing the risk of wideband oscillation.

2. The broadband oscillation suppression method for a flexible DC-DC grid-connected offshore wind power system as described in claim 1, characterized in that, The step of constructing a multi-input multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance sweep data includes: Background harmonic components and abnormal frequency points are removed from the measured impedance sweep data to obtain preprocessed impedance sweep data. Based on the preprocessed impedance sweep data, voltage harmonic vectors and current harmonic vectors are constructed in order of frequency from low to high. The impedance value at each frequency sweep point is calculated based on the ratio between the voltage harmonic vector and the current harmonic vector, and a multi-input multi-output impedance matrix considering multi-harmonic coupling is constructed.

3. The broadband oscillation suppression method for a flexible DC-DC grid-connected offshore wind power system as described in claim 1, characterized in that, The step of reducing the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance with single input and single output includes: Based on the physical topology and electrical parameters of the offshore wind power flexible DC grid connection system, a multi-harmonic linearized theoretical impedance model is established. The consistency verification results of the linearized multi-harmonic theoretical impedance model and the multi-input multi-output impedance matrix are obtained by performing a consistency deviation verification. After the consistency deviation verification result is passed, the key impedance elements in the multi-input multi-output impedance matrix are extracted; Based on the key impedance elements, the multi-input multi-output impedance matrix is ​​reduced to an equivalent positive-sequence impedance with single input and single output through the Schur complement transformation.

4. The broadband oscillation suppression method for a flexible DC-DC grid-connected offshore wind power system as described in claim 3, characterized in that, The steps for establishing a multi-harmonic linearization theoretical impedance model based on the physical topology and electrical parameters of an offshore wind power flexible DC grid-connected system include: Based on the physical topology of the offshore wind power flexible DC grid-connected system, a main loop model of the system is established, and the harmonic components of each steady-state variable at each characteristic frequency point are extracted from the main loop model of the system to obtain the multi-frequency harmonic linearized representation of all steady-state variables. The linearized representation of multi-frequency harmonics is substituted into the main circuit model of the wind turbine grid-side converter and the main circuit model of the flexible DC system for linearization, respectively, to obtain the small-signal equation of the main circuit harmonics. Establish the transmission relationship between grid connection point voltage disturbance and current disturbance and wind turbine modulation ratio disturbance to the wind turbine controller, and decompose the wind turbine controller transmission relationship into wind turbine harmonic linearization equations according to frequency points; Establish the transmission relationship of the flexible DC controller from grid connection point voltage disturbance and current disturbance to flexible DC modulation ratio disturbance, and decompose the flexible DC controller transmission relationship into flexible DC harmonic linearization equations according to frequency points; The harmonic linearization equations of the wind turbine and the harmonic linearization equations of the flexible DC circuit are integrated to construct the multi-frequency harmonic small-signal equations of the controller. By combining the harmonic small-signal equations of the main circuit and the multi-frequency harmonic small-signal equations of the controller, a multi-harmonic linearized theoretical impedance model is obtained.

5. The broadband oscillation suppression method for a flexible DC-DC grid-connected offshore wind power system as described in claim 1, characterized in that, The steps of constructing a Bode plot of the source-grid side impedance characteristics based on the equivalent positive sequence impedance, reading the phase difference at the intersection of the source-grid impedance amplitudes, and quantizing the stability margin of the offshore wind power flexible DC grid-connected system based on the phase difference include: Based on the equivalent positive sequence impedance, calculate the wind field impedance amplitude, wind field impedance phase, flexible DC impedance amplitude, and flexible DC impedance phase at each frequency sweep point to obtain impedance amplitude and phase data; A Bode plot of the source-network impedance characteristics is constructed using the frequency sweep points as the horizontal axis and the impedance amplitude and phase data as the vertical axis. Locate the intersection frequency point of the source-grid impedance amplitude in the source-grid side impedance characteristic Bode plot, and read the wind field impedance phase value and flexible DC impedance phase value at the intersection frequency point; Calculate the difference between the phase value of the wind field impedance and the phase value of the flexible DC impedance to obtain the phase difference at the intersection of the source and grid impedance amplitudes; The deviation between the phase difference and the preset stability margin threshold is calculated to obtain the stability margin of the offshore wind power flexible DC grid connection system.

6. The broadband oscillation suppression method for a flexible DC-DC grid-connected offshore wind power system as described in claim 1, characterized in that, The step of reconstructing the wind turbine grid-side converter into a matrix control structure by cascading weighted coefficient matrices between the inner and outer loop control loops of the wind turbine grid-side converter includes: The actual value of the quadrature-axis voltage at the grid connection point is collected and the reference value of the quadrature-axis voltage is compared to obtain the quadrature-axis voltage deviation value. The quadrature-axis voltage deviation value is then used to generate a quadrature-axis synchronization signal through a proportional-integral controller with saturation limiting. In the constant DC voltage outer loop control mode, the actual value of DC voltage is acquired, and the actual value of DC voltage is subtracted from the preset DC voltage reference value to obtain the DC voltage deviation value. The DC voltage deviation value is then used to generate a direct-axis outer loop signal through a proportional-integral controller with saturation limiting. In the constant reactive power outer loop control mode, the actual reactive power value is collected, the actual reactive power value is subtracted from the reactive power reference value to obtain the reactive power deviation value, and the reactive power deviation value is used to generate the quadrature axis outer loop signal through the proportional-integral controller with saturation limit. The quadrature axis synchronization signal, the direct axis outer loop signal, and the quadrature axis outer loop signal are used as input column vectors. The input column vector signals are cross-linked and mapped through a pre-constructed weighting coefficient matrix to generate an output column vector containing frequency deviation signal, direct axis current reference value, and quadrature axis current reference value. The frequency deviation signal is superimposed on the actual frequency of the power grid and integrated to obtain the synchronization phase angle. The actual value of the direct-axis current is then collected under synchronization control using the synchronization phase angle. The direct-axis current deviation value is obtained by subtracting the actual value of the direct-axis current from the reference value of the direct-axis current. The direct-axis current deviation value is then passed through a proportional-integral circuit with saturation limiting and a decoupling signal containing the actual value of the quadrature-axis current is added to generate the direct-axis inner loop signal. Subtracting the actual value of the quadrature-axis current from the reference value of the quadrature-axis current yields the quadrature-axis current deviation value. This deviation value is then passed through a proportional-integral controller with saturation and supplemented with a decoupling signal containing the actual value of the direct-axis current to generate the quadrature-axis inner loop signal. By sequentially adding the actual values ​​of the direct-axis current and quadrature-axis voltage to the direct-axis inner loop signal and the quadrature-axis inner loop signal, the direct-axis modulation ratio and the quadrature-axis modulation ratio are obtained. A sinusoidal pulse width modulation strategy is used to modulate the direct-axis modulation ratio and the quadrature-axis modulation ratio, thereby reconstructing the wind turbine grid-side converter into a matrix control structure.

7. The broadband oscillation suppression method for a flexible DC-DC grid-connected offshore wind power system as described in claim 1, characterized in that, The steps for suppressing broadband oscillation risk include iteratively optimizing the weighting coefficient matrix in the matrix control structure based on impedance sensitivity and simultaneously reshaping the impedance using a low-pass filter connected in series in the voltage and current feedback channels. The amplitude sensitivity and phase sensitivity of each element in the weighting coefficient matrix to the equivalent positive sequence impedance on the wind field side are calculated to obtain the impedance sensitivity. Using the direction with the highest impedance sensitivity as the search direction, gradient optimization iteration is performed on the elements in the weighting coefficient matrix, and the stability margin is recalculated after each iteration. When the stability margin is not lower than the preset margin threshold, stop the gradient optimization iteration of the elements in the weighted coefficient matrix and output the optimized weighted coefficient matrix; A low-pass filter is selected based on the frequency characteristics and broadband oscillation suppression requirements of the offshore wind power flexible DC grid connection system, and the low-pass filter is connected in series in the voltage feedback channel and the current feedback channel respectively; The optimized weighted coefficient matrix is ​​loaded into the wind turbine grid-side converter controller, and the low-pass filter in the voltage and current feedback channel is activated simultaneously to reshape the impedance and suppress the risk of wideband oscillation.

8. The broadband oscillation suppression method for a flexible DC-DC grid-connected offshore wind power system as described in claim 7, characterized in that, The step of synchronously enabling the impedance reshaping of the low-pass filter in the voltage and current feedback channel includes: The three-phase voltage and three-phase current signals at the grid connection point of the offshore wind power flexible DC grid-connected system are collected, and the three-phase voltage signals are subjected to Parker transformation to obtain the AC and DC axis vector voltage signals. The three-phase current signals are subjected to Parker transformation to obtain the perpendicular and direct axis vector current signals; Low-pass filters are connected in series in the feedback channels of the quadrature-direct axis vector voltage signal and the quadrature-direct axis vector current signal to dynamically reshape the control loop and obtain the quadrature-direct axis components of voltage and current. The instantaneous power measurement value is calculated based on the AC and DC axis components of the voltage and current, and the instantaneous power measurement value is used as the feedback quantity of the matrix control structure to reshape the impedance characteristics of the offshore wind power flexible DC grid connection system.

9. A broadband oscillation suppression system for a flexible DC grid-connected offshore wind power system, characterized in that, The system includes: The data acquisition module is used to sequentially inject positive and negative sequence micro-perturbation sinusoidal harmonic signals at the grid connection point of the wind turbine, and simultaneously acquire the voltage and current harmonic response signals at the grid connection point. The harmonic analysis module is used to perform fast Fourier analysis on the voltage and current harmonic response signals at the grid connection point to obtain measured impedance frequency sweep data. The impedance analysis module is used to construct a multi-input multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance sweep data, and to reduce the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance with single input and single output. The stability analysis module is used to construct a Bode plot of the source-grid side impedance characteristics based on the equivalent positive sequence impedance, read the phase difference at the intersection of the source-grid impedance amplitudes, and quantify the stability margin of the offshore wind power flexible DC grid-connected system based on the phase difference. The reconfiguration control module is used to reconfigure the wind turbine grid-side converter into a matrix control structure by cascading a weighted coefficient matrix between the inner and outer loop control loops of the wind turbine grid-side converter when the stability margin is lower than a preset margin threshold. The oscillation suppression module is used to iteratively optimize the weighting coefficient matrix in the matrix control structure based on impedance sensitivity, and simultaneously perform impedance reshaping by connecting a low-pass filter in series in the voltage and current feedback channels to complete the suppression of wideband oscillation risk.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program that, when executed by a processor, implements the method as described in any one of claims 1 to 8.

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