Broadband oscillation suppression method and system for offshore wind power flexible direct current grid-connected system and medium

Abstract

The invention relates to the technical field of power system stability control, in particular to a broadband oscillation suppression method and system for an offshore wind power flexible direct current grid-connected system and a medium, and the method comprises the steps: sequentially injecting positive and negative sequence perturbation sine harmonic signals at a grid-connected point of a wind turbine generator, constructing a multi-input multi-output impedance matrix, reducing the order of the multi-input and multi-output impedance matrix into single-input and single-output equivalent positive sequence impedance; the stability margin of the offshore wind power flexible direct current grid-connected system is obtained based on equivalent positive sequence impedance quantification; when the stability margin is lower than a preset margin threshold value, reconstructing the fan grid-side converter into a matrix control structure by cascading a weighting coefficient matrix between the inner and outer ring control loops of the fan grid-side converter; and iteratively optimizing the matrix type control structure based on impedance sensitivity, and synchronously connecting a low-pass filter in series in a voltage and current feedback channel to carry out impedance remodeling so as to finish broadband oscillation risk suppression. According to the invention, broadband oscillation suppression of the offshore wind power flexible direct current grid-connected system is realized through impedance modeling and a matrix type control strategy.

Description

Technical Field

[0001] The present invention relates to the technical field of power system stability control, and in particular to a method, system and medium for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system. Background Art

[0002] As the penetration rate of power electronic equipment in the power grid continues to increase, its multi-time-scale control characteristics and complex dynamic interactions have significantly increased the risk of broadband oscillations in the power system. For example, sub / supersynchronous oscillations, medium and high frequency oscillations, and other problems that arise in offshore wind power 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 essentially different from the low-frequency or subsynchronous oscillation mechanism of the traditional power grid, which is mainly due to the strong coupling between the multi-link dynamics of the power electronic equipment controller and the electromagnetic transients of the main circuit. At present, impedance modeling and sweep frequency methods are widely used to evaluate the system stability margin. However, in actual engineering, parameter uncertainty and operating point offset can easily lead to the deterioration of impedance characteristics, which in turn trigger broadband oscillations. It is urgent to improve the system damping characteristics 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 system's electromagnetic transient characteristics by optimizing main loop parameters or series-parallel passive devices, 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 resonant peak frequency band shift by adjusting the original controller parameters. Although 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 relies 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 to achieve the best suppression effect, but existing methods are mostly limited to adjusting a single control architecture and it is difficult to take into account the global optimization of broadband impedance characteristics.

[0004] In summary, existing methods are mostly designed for a single oscillation mode, which makes it difficult to cope with the complex multi-band oscillation coupling problem in the offshore wind power flexible DC transmission system. Therefore, how to effectively reshape the impedance characteristics of the offshore wind power flexible DC transmission grid-connected system to suppress the risk of broadband oscillation has become an important engineering problem that needs to be urgently solved in the field of offshore wind power flexible DC transmission. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention provides a method, system and medium for suppressing broadband oscillation of an offshore wind power flexible direct current grid-connected system.

[0006] In a first aspect, the present invention provides a method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system, the method comprising the following steps: Positive and negative sequence perturbation sinusoidal harmonic signals are injected sequentially at the wind turbine grid connection point, and the voltage and current harmonic response signals of the grid connection point are collected synchronously; Performing fast Fourier analysis on the voltage and current harmonic response signals of the grid connection point to obtain measured impedance sweep frequency data; Constructing a multi-input multi-output impedance matrix considering multi-harmonic coupling according to the measured impedance sweep data, and reducing the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance with a single input and a single output; Based on the equivalent positive-sequence impedance, a Bode diagram of the source-grid side impedance characteristic is constructed, a phase difference at the intersection of the source-grid impedance amplitude is read, and a stability margin of the offshore wind power flexible direct current grid-connected system is quantified according to 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 a weighted coefficient matrix between inner and outer 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 synchronously connected in series with the voltage and current feedback channels to perform impedance reshaping, thereby achieving broadband oscillation risk suppression.

[0007] 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: Eliminating background harmonic components and abnormal frequency point data from the measured impedance sweep frequency data to obtain preprocessed impedance sweep frequency data; Based on the pre-processed impedance sweep data, constructing a voltage harmonic vector and a current harmonic vector in order of frequency from low to high; The impedance value at each frequency sweep point is calculated according to the ratio between the voltage harmonic vector and the current harmonic vector, and a multi-input and multi-output impedance matrix considering multi-harmonic coupling is constructed.

[0008] In a further embodiment, the step of reducing the multi-input multi-output impedance matrix to a single-input single-output equivalent positive-sequence impedance comprises: Based on the physical topology and electrical parameters of the offshore wind power flexible direct current grid-connected system, a multi-harmonic linearization theoretical impedance model is established; Performing consistency check on the multi-harmonic linearized theoretical impedance model and the multi-input multi-output impedance matrix to obtain a consistency deviation check result; After the consistency deviation check result is passed, extracting key impedance elements in the multi-input multi-output impedance matrix; Based on the key impedance elements, the multi-input multi-output impedance matrix is ​​reduced to an equivalent positive-sequence impedance of a single-input single-output through Schur complement transformation.

[0009] In a further embodiment, the step of establishing a multi-harmonic linearization theoretical impedance model based on the physical topology and electrical parameters of the offshore wind power flexible direct current grid-connected system includes: Based on the physical topology of the offshore wind power flexible direct current grid-connected system, the system main loop model is established, and the harmonic components of each steady-state variable in the system main loop model at each characteristic frequency point are extracted to obtain the multi-frequency harmonic linearization representation of all steady-state variables; Substitute the multi-frequency harmonic linearization representation into the main circuit model of the wind turbine grid-side converter and the main circuit model of the flexible DC system for linearization processing to obtain the main circuit harmonic small signal equation; Establish the wind turbine controller transfer relationship from the grid connection point voltage and current disturbances to the wind turbine modulation ratio disturbance, and decompose the wind turbine controller transfer relationship into the wind turbine harmonic linearization equation according to the frequency point; Establish the flexible DC controller transfer relationship from the grid connection point voltage and current disturbances to the flexible DC modulation ratio disturbance, and decompose the flexible DC controller transfer relationship into the flexible DC harmonic linearization equation according to the frequency point; The wind turbine harmonic linearization equation and the flexible direct current harmonic linearization equation are integrated to construct a controller multi-frequency harmonic small signal equation; The main circuit harmonic small signal equation and the controller multi-frequency harmonic small signal equation are combined to obtain the multi-harmonic linearization theoretical impedance model.

[0010] In a further embodiment, the steps of constructing a Bode diagram 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 quantifying the stability margin of the offshore wind power flexible direct current grid-connected system according to the phase difference include: Calculating the wind farm impedance amplitude, wind farm impedance phase, flexible DC impedance amplitude and flexible DC impedance phase at each sweep frequency point according to the equivalent positive sequence impedance to obtain impedance amplitude and phase data; Constructing a Bode diagram of the impedance characteristics of the source and network sides with the frequency points of the frequency sweep as the horizontal coordinate and the impedance amplitude and phase data as the vertical coordinate; Finding the intersection frequency point of the source network impedance amplitude in the source network side impedance characteristic Bode diagram, and reading the wind farm impedance phase value and the flexible direct current impedance phase value at the intersection frequency point; Calculating the difference between the wind farm impedance phase value and the flexible DC impedance phase value to obtain the phase difference at the intersection of the source and grid impedance amplitudes; The deviation value between the phase difference and the preset stability margin threshold is calculated to obtain the stability margin of the offshore wind power flexible direct current grid-connected system.

[0011] In a further embodiment, the step of reconfiguring the wind turbine grid-side converter into a matrix control structure by cascading weighted coefficient matrices between inner and outer control loops of the wind turbine grid-side converter comprises: The actual value of the collected grid-connected point quadrature-axis voltage is subtracted from the quadrature-axis voltage reference value to obtain a quadrature-axis voltage deviation value, and the quadrature-axis voltage deviation value is 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 DC voltage value is collected, the DC voltage actual value is subtracted from the preset DC voltage reference value to obtain the DC voltage deviation value, and the DC voltage deviation value is passed through a proportional-integral controller with saturation limiting to generate a direct axis outer loop signal; In the fixed 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 passed through the proportional-integral controller with saturation limiting to generate the quadrature axis outer loop signal; The quadrature-axis synchronization signal, the direct-axis outer loop signal, and the quadrature-axis outer loop signal are used as input column vectors, and the input column vector signals are cross-linked and mapped using a pre-constructed weighting coefficient matrix to generate an output column vector including a frequency deviation signal, a direct-axis current reference value, and a quadrature-axis current reference value; The frequency deviation signal is superimposed on the actual frequency of the power grid and then integrated to obtain a synchronous phase angle, and the actual value of the direct-axis current is collected under synchronous control using the synchronous phase angle; The direct-axis current actual value is subtracted from the direct-axis current reference value to obtain the direct-axis current deviation value. The direct-axis current deviation value is passed through a proportional-integral link with saturation limiting and a decoupling signal containing the quadrature-axis current actual value is added to generate a direct-axis inner loop signal. The quadrature-axis current actual value is subtracted from the quadrature-axis current reference value to obtain a quadrature-axis current deviation value, and the quadrature-axis current deviation value is passed through a proportional-integral controller with saturation and added with a decoupling signal containing the direct-axis current actual value to generate a quadrature-axis inner loop signal; The direct-axis inner loop signal and the quadrature-axis inner loop signal are sequentially added with the direct-axis current actual value and the quadrature-axis voltage actual value to obtain the direct-axis modulation ratio and the quadrature-axis modulation ratio; The direct-axis modulation ratio and the quadrature-axis modulation ratio are modulated by a sinusoidal pulse width modulation strategy, and the grid-side converter of the wind turbine is reconstructed into a matrix control structure.

[0012] In a further embodiment, the steps of iteratively optimizing the weighting coefficient matrix in the matrix control structure based on impedance sensitivity and simultaneously connecting a low-pass filter in series with the voltage and current feedback channels to perform impedance reshaping to achieve broadband oscillation risk suppression include: Calculate the amplitude sensitivity and phase sensitivity of each element in the weighted coefficient matrix to the equivalent positive sequence impedance on the wind farm side to obtain the impedance sensitivity; Taking the direction with the highest impedance sensitivity as the search direction, the elements in the weighted coefficient matrix are subjected to gradient optimization iteration, and the stability margin is recalculated after each iterative optimization; When the stability margin is not lower than a preset margin threshold, stopping the gradient optimization iteration of the elements in the weight coefficient matrix and outputting the optimized weight coefficient matrix; A low-pass filter is selected according to the frequency characteristics and broadband oscillation suppression requirements of the offshore wind power flexible direct current grid-connected 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 simultaneously enabled to reshape the impedance and suppress the risk of broadband oscillation.

[0013] In a further embodiment, the step of synchronously enabling the low-pass filter in the voltage and current feedback channel to perform impedance reshaping includes: Collecting three-phase voltage signals and three-phase current signals of the grid connection point of the offshore wind power flexible direct current grid-connected system, and performing Park transformation on the three-phase voltage signals to obtain quadrature and direct axis vector voltage signals; Performing a Park transformation on the three-phase current signal to obtain a quadrature and direct axis vector current signal; Connecting low-pass filters in series in the feedback channels of the AC and DC axis vector voltage signals and the AC and DC axis vector current signals respectively to dynamically reshape the control loop to obtain the AC and DC axis components of the voltage and current; The instantaneous power measurement value is calculated according to the voltage and current quadrature and direct axis components, 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 direct current grid-connected system.

[0014] In a second aspect, the present invention provides a broadband oscillation suppression system for an offshore wind power flexible direct current grid-connected system, the system comprising: The data acquisition module is used to sequentially inject positive and negative sequence perturbation sinusoidal harmonic signals at the wind turbine grid connection point and synchronously collect the voltage and current harmonic response signals at the grid connection point; A harmonic analysis module, configured to perform fast Fourier analysis on the voltage and current harmonic response signals of the grid connection point to obtain measured impedance sweep frequency data; An 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 reduce the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance of a single input and a single output; A stability analysis module is used to construct a Bode diagram 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 amplitude, and quantify the stability margin of the offshore wind power flexible direct current grid-connected system based on the phase difference; a reconstruction control module, configured to reconstruct the wind turbine grid-side converter into a matrix control structure by cascading a weighted coefficient matrix between inner and outer 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 weighted coefficient matrix in the matrix control structure based on impedance sensitivity, and simultaneously connect a low-pass filter in series with the voltage and current feedback channels to perform impedance reshaping to achieve broadband oscillation risk suppression.

[0015] In a third aspect, the present invention further provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of the above method are implemented.

[0016] The present invention provides a method, system and medium for suppressing broadband oscillation of an offshore wind power flexible direct current grid-connected system. The method sequentially injects positive and negative sequence perturbation sinusoidal harmonic signals at the grid-connected point of a wind turbine generator set, synchronously collects the voltage and current harmonic response signals of the grid-connected point; performs fast Fourier analysis on the voltage and current harmonic response signals of the grid-connected point to obtain measured impedance sweep frequency data; constructs a multi-input and multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance sweep frequency data, and reduces the multi-input and multi-output impedance matrix to an equivalent positive-sequence impedance with a single input and a single output; based on the equivalent positive The Bode plot of the source-grid impedance characteristic is constructed based on the sequence impedance. The phase difference at the intersection of the source-grid impedance amplitude is read, and the stability margin of the offshore wind power flexible direct current grid-connected system is quantified 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 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 with the voltage and current feedback channels to perform impedance reshaping and complete broadband oscillation risk suppression. Compared with existing technologies, this method accurately evaluates and optimizes the stability of the offshore wind power flexible direct current grid-connected system through impedance modeling and matrix control strategy. When the stability margin is insufficient, the offshore wind power flexible direct current grid-connected system can achieve broadband oscillation suppression through weighted coefficient matrix optimization and low-pass filter configuration, ensuring the stable operation of the offshore wind power flexible direct current grid-connected system under power fluctuations. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a flow chart of a method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system provided by an embodiment of the present invention; Figure 2 This is a control structure block diagram of a matrix control strategy for a grid-side converter of a full-power offshore wind turbine provided by an embodiment of the present invention; Figure 3 This is a block diagram of a structure in which a filter is connected in series in a voltage, current and power sampling loop under an active suppression strategy provided by an embodiment of the present invention; Figure 4 This is a block diagram of a broadband oscillation suppression system for an offshore wind power flexible direct current grid-connected system provided by an embodiment of the present invention.

[0018] Explanation of the accompanying symbols: 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 DESCRIPTION

[0019] The following describes the embodiments of the present invention in detail with reference to the accompanying drawings. The embodiments are provided for illustrative purposes only and are not to be construed as limiting the present invention. The accompanying drawings are provided for reference and illustration only and do not constitute a limitation on the scope of protection of the present invention. Many changes may be made to the present invention without departing from the spirit and scope of the present invention.

[0020] Figure 1 The embodiment of the present invention provides a method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system. Figure 1 As shown, the method includes the following steps: S1. Inject positive and negative sequence perturbation sinusoidal harmonic signals in sequence at the wind turbine grid connection point, and synchronously collect the voltage and current harmonic response signals at the grid connection point.

[0021] S2. Perform fast Fourier analysis on the voltage and current harmonic response signals of the grid connection point to obtain measured impedance sweep frequency data.

[0022] Specifically, in the process of impedance characteristic analysis, this embodiment sequentially injects positive-sequence perturbation and negative-sequence perturbation sinusoidal harmonic signals at the offshore wind power 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 time lasting not less than 10 signal cycles. After the positive-sequence frequency sweep is completed, it switches to the negative-sequence perturbation sinusoidal harmonic signal, which is injected sequentially according to the same frequency sequence. Each injection interval is not less than 5 seconds to ensure that the system recovers to a steady state. The amplitude of the positive and negative sequence perturbation sinusoidal harmonic signal does not exceed 5% of the rated AC voltage of the offshore wind power flexible direct current grid connection system to avoid a significant impact on the steady-state operating point of the system, thereby ensuring the effectiveness of the linearization analysis method. Then, this embodiment synchronously deploys voltage sensors and current sensors on the grid connection point source side (wind farm outlet) and the grid side (flexible direct current converter station entrance). While injecting the signal, this embodiment needs to synchronously collect The system response when the perturbation signal is injected obtains the grid connection point voltage and current harmonic response signal, and the grid connection point voltage and current harmonic response signal is processed using the fast Fourier analysis method to obtain the measured impedance sweep data. In the process of collecting signals, this embodiment needs to take into account the voltage and current harmonic signals on both sides of the source network to comprehensively obtain the impedance sweep data on both sides of the source network so that it can reflect the dynamic behavior of the system under different operating conditions. In addition, to ensure the accuracy of the obtained impedance characteristic data, this embodiment needs to reasonably select the distribution of the sweep points and the injection time of the harmonic signal. Specifically, the distribution of the sweep points should be dense enough to cover the entire frequency range of the system, and the injection time of the harmonic signal should be long enough to ensure that the system reaches a steady state in the harmonic state space and ensure that the system can fully converge to a steady state in the harmonic state space, thereby providing a reliable basis for subsequent comparative analysis with the theoretical impedance curve.

[0023] S3. 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 of single input and single output.

[0024] In some embodiments, the step of constructing a multi-input multi-output impedance matrix considering multi-harmonic coupling according to the measured impedance sweep data includes: Eliminating background harmonic components and abnormal frequency point data from the measured impedance sweep frequency data to obtain preprocessed impedance sweep frequency data; Based on the pre-processed impedance sweep data, constructing a voltage harmonic vector and a current harmonic vector in order of frequency from low to high; The impedance value at each frequency sweep point is calculated according to the ratio between the voltage harmonic vector and the current harmonic vector, and a multi-input and multi-output impedance matrix considering multi-harmonic coupling is constructed.

[0025] This embodiment processes and transforms the measured impedance sweep data, constructs a multi-input multi-output (MIMO) impedance matrix considering multi-harmonic coupling through matrix operations, and performs consistency analysis on it with the impedance model constructed based on the theoretical method within a given error limit, so as to verify the accuracy of the theoretical model and the validity of the sweep data. Specifically, before performing the impedance sweep, since the system itself already has certain harmonic components, in order to reduce the impact of the system's own initial harmonics on the monitoring results, this embodiment needs to subtract the system harmonic components obtained after the impedance sweep from the harmonic components contained in the system itself before the sweep. For example, for a harmonic with a frequency of 10 Hz, the voltage harmonic components at this frequency after the sweep are subtracted from the voltage harmonic components at this frequency before the sweep, and the current harmonic components are operated in the same way. Through this operation, this embodiment can obtain the system harmonic components after removing the influence of the initial harmonics. System harmonic component data, thereby effectively reducing the monitoring error caused by the initial harmonics of the system, and providing a basis for subsequent accurate analysis. Then, this embodiment removes obviously abnormal data points from all impedance sweep data after removing the influence of the initial harmonics. These abnormal data points may be introduced due to measurement errors or external interference and other factors, which will have an adverse effect on subsequent analysis. After removing the abnormal points, this embodiment further analyzes and determines the consistency of the remaining data points to ensure the reliability and stability of the data. During the data processing process, 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 and multi-output impedance matrix based on the preprocessed impedance sweep data.

[0026] In this embodiment, based on the pre-processed impedance sweep data, the voltage harmonic component data at different frequencies are arranged in ascending order of frequency into a voltage harmonic vector. The voltage harmonic vector can accurately reflect the voltage harmonic characteristics at each frequency point. Similarly, according to the same method as for constructing the voltage data matrix, a current harmonic vector is constructed based on the current harmonic component data in the pre-processed impedance sweep data. In this embodiment, the voltage harmonic vector and the current harmonic vector are used to calculate the impedance value at each frequency point (i.e., the ratio of the voltage harmonic vector to the current harmonic vector) to construct a multi-input and multi-output impedance matrix. This matrix comprehensively reflects the impedance characteristics of the system at different frequencies and different input-output combinations. In this embodiment, the specific transformation process of the multi-input and multi-output impedance matrix is ​​as follows: Where, is the sequence impedance matrix, which is the impedance characteristic of the system in the positive-sequence and negative-sequence coordinate systems. It is a 2×2 complex matrix. The elements of the sequence impedance matrix include positive-sequence self-impedance, positive-sequence to negative-sequence coupling impedance, negative-sequence to positive-sequence coupling impedance, and negative-sequence self-impedance; is the voltage harmonic vector of the grid connection point after data processing; is the current harmonic vector of the grid-connected point after data processing; For the frequency The first harmonic component of the positive sequence voltage (such as the positive sequence voltage response of the fundamental or a specific injection frequency) under For the frequency Next, the second harmonic component of the positive sequence voltage; For the frequency The first harmonic component of the negative sequence voltage is is the system power frequency, which reflects the coupling characteristics of negative sequence voltage and positive sequence injection frequency; For the frequency Next, the second harmonic component of the negative sequence voltage; For the frequency Next, the first harmonic component of the positive sequence current; For the frequency Next, the second harmonic component of the positive sequence current; For the frequency Under , the first harmonic component of the negative sequence current, which reflects the negative sequence current response; For the 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, and its frequency offset Reflect the frequency coupling effect between sequence components; Indicates the frequency of the injected sinusoidal perturbation signal (sweep frequency point); negative sequence component The introduction of reflects the multi-harmonic coupling effect in offshore wind power flexible direct current system.

[0027] In this embodiment, the sequence impedance matrix Used to provide the original data of positive and negative sequence impedance, this embodiment uses multi-frequency sweep and harmonic linearization theory to convert the discrete sequence impedance matrix Decomposed into a continuous transfer function form, the functional expression of the multi-input and multi-output impedance matrix is: Where, The multi-input and multi-output impedance of the wind farm side is aggregated based on the wind farm grid connection point as the source network division basis. It has four impedance components. is the positive sequence self-impedance on the aggregated wind farm side; is the positive-sequence to negative-sequence coupling impedance on the aggregated wind farm side; is the negative-sequence to positive-sequence coupling impedance on the aggregated wind farm side; is the negative sequence self-impedance on the aggregated wind farm side; The multi-input and multi-output impedance on the flexible DC converter station side is divided based on the wind farm grid connection point, which has four impedance components; is the positive sequence self-impedance on the flexible DC side; is the coupling impedance from positive sequence to negative sequence on the flexible DC side; is the negative-sequence to positive-sequence coupling impedance on the flexible DC side; is the negative sequence self-impedance of the flexible DC side.

[0028] In some embodiments, the step of reducing the multi-input multi-output impedance matrix to a single-input single-output equivalent positive-sequence impedance includes: Based on the physical topology and electrical parameters of the offshore wind power flexible direct current grid-connected system, a multi-harmonic linearization theoretical impedance model is established; Performing consistency check on the multi-harmonic linearized theoretical impedance model and the multi-input multi-output impedance matrix to obtain a consistency deviation check result; After the consistency deviation check result is passed, extracting key impedance elements in the multi-input multi-output impedance matrix; Based on the key impedance elements, the multi-input multi-output impedance matrix is ​​reduced to an equivalent positive-sequence impedance of a single-input single-output through Schur complement transformation.

[0029] Regarding the theoretical impedance modeling of the system, this embodiment performs multi-harmonic linearization based on the topology, main electrical parameters, and control system of the offshore wind power flexible DC grid-connected system to construct an accurate impedance model. In some embodiments, the steps of establishing the multi-harmonic linearization theoretical impedance model based on the physical topology and electrical parameters of the offshore wind power flexible DC grid-connected system include: Based on the physical topology of the offshore wind power flexible direct current grid-connected system, the system main loop model is established, and the harmonic components of each steady-state variable in the system main loop model at each characteristic frequency point are extracted to obtain the multi-frequency harmonic linearization representation of all steady-state variables; Substitute the multi-frequency harmonic linearization representation into the main circuit model of the wind turbine grid-side converter and the main circuit model of the flexible DC system for linearization processing to obtain the main circuit harmonic small signal equation; Establish the wind turbine controller transfer relationship from the grid connection point voltage and current disturbances to the wind turbine modulation ratio disturbance, and decompose the wind turbine controller transfer relationship into the wind turbine harmonic linearization equation according to the frequency point; Establish the flexible DC controller transfer relationship from the grid connection point voltage and current disturbances to the flexible DC modulation ratio disturbance, and decompose the flexible DC controller transfer relationship into the flexible DC harmonic linearization equation according to the frequency point; The wind turbine harmonic linearization equation and the flexible direct current harmonic linearization equation are integrated to construct a controller multi-frequency harmonic small signal equation; The main circuit harmonic small signal equation and the controller multi-frequency harmonic small signal equation are combined to obtain the multi-harmonic linearization theoretical impedance model.

[0030] In the specific implementation process, the specific steps of multi-harmonic linearization include obtaining the mathematical model of the main circuit of the system, and performing harmonic linearization processing on the steady-state variables of the offshore wind power grid-connected system via flexible direct current to obtain the basic response characteristics of the system under harmonic excitation; secondly, by injecting positive and negative sequence perturbation sinusoidal voltage harmonics at the wind turbine grid-connected point, the main circuit is harmonically linearized using the small signal analysis theory to obtain the harmonic response of the offshore wind power grid-connected system via flexible direct current under this excitation; furthermore, this embodiment considers the dynamic influence of multiple links such as the controller synchronization link, the current inner loop and the voltage outer loop to perform control dynamic theoretical modeling, repeats the above operations, and obtains the harmonic linearization equations of the controller dynamics in turn. In this process, the grid-connected Key variables such as the point voltage, grid connection point current, wind turbine modulation ratio, and flexible DC converter station modulation ratio are harmonically linearized and expanded. Finally, the electrical dynamic characteristic equations related to the main circuit of the above system and the control dynamic characteristic equations related to the controller are transformed simultaneously to obtain the theoretical impedance model of the offshore wind power system connected to the flexible DC grid. In the specific implementation process, taking the offshore wind power transmission system through the flexible DC as an example, this embodiment constructs the wind turbine grid-side converter main circuit model and the flexible DC system main circuit model according to the actual physical topology of the offshore wind power flexible DC grid-connected system, and constructs the wind turbine grid-side converter main circuit model and the flexible DC system main circuit model into the system main circuit model. Among them, the wind turbine grid-side converter main circuit model is specifically: Where, is the grid connection point voltage phasor of the wind turbine grid-side voltage source converter at the public grid connection point; is the modulation ratio of the wind turbine grid-side converter; is the steady-state voltage of the DC side of the wind turbine grid-side converter; The equivalent inductance of the transformer connecting the wind turbine grid-side converter and the AC grid; is the current phasor flowing through the grid connection point (i.e. the output current of the wind turbine grid-side converter).

[0031] The main loop model of the flexible DC system is as follows: Where, The grid connection point voltage phasor of the flexible DC converter station at the public grid connection point; is the steady-state voltage on the DC side of the flexible DC converter station; is the modulation ratio of the flexible DC converter station; is the sum of the bridge arm capacitor voltages of the modular multilevel converter, which reflects the energy dynamics of the bridge arm; is the equivalent inductance of a single bridge arm of the modular multilevel converter; is the upper arm current of the modular multilevel converter, which reflects the internal current dynamics of the bridge arm; is the equivalent inductance of the transformer connecting the flexible DC converter station and the AC grid.

[0032] For each steady-state variable in the system 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 harmonic small signal components of the seven frequency points. This obtains a multi-frequency harmonic linearized representation of all steady-state variables. The steady-state variables include the grid connection point voltage phasor, the current phasor flowing through the grid connection 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. This embodiment generally expresses the harmonic linearization expansion process of the steady-state variables as follows: Where, It is 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. For the frequency The small signal harmonic component under , reflects the dynamic characteristics of the steady-state variable x when the fundamental frequency shifts by 3 times the power frequency; For the frequency The harmonic components under For the frequency The harmonic components under The injection frequency The fundamental component under is the core frequency point of the sweep frequency analysis; For the frequency The harmonic components under For the frequency The harmonic components under For the frequency The harmonic components under ; the superscript T is the transpose operation; Indicates the frequency of the injected sinusoidal perturbation signal (sweep frequency point); is the system operating frequency.

[0033] In order to solve the problem of broadband oscillation caused by multi-time scale coupling of power electronic equipment, this embodiment decomposes variables into multi-frequency components to accurately model the interactive dynamics of the wind turbine and the flexible DC system. In this embodiment, the obtained multi-frequency harmonic linearization representation is substituted into the main loop model of the wind turbine grid-side converter, the steady-state term is eliminated, and the small signal disturbance term is retained to obtain the small signal equation of the wind turbine main loop; at the same time, the obtained multi-frequency harmonic linearization representation is substituted into the main loop model of the flexible DC system, the small signal disturbance term is retained, and the modulation ratio-capacitor voltage conversion matrix is ​​introduced to derive the harmonic small signal equation of the flexible DC side main loop. The wind turbine side main loop harmonic small signal equation and the flexible DC side main loop harmonic small signal equation respectively describe the dynamic responses of the wind turbine side and the flexible DC side under harmonic disturbance. In this embodiment, the wind turbine side main loop harmonic small signal equation and the flexible DC side main loop harmonic small signal equation are constructed as the main loop harmonic small signal equation, where the wind turbine side main loop harmonic small signal equation is specifically: The harmonic small signal equation of the main circuit on the flexible DC side is specifically: Where, is the small signal disturbance component of the grid connection point voltage on the wind turbine side; is the steady-state voltage of the DC side of the wind turbine grid-side converter; is the small signal disturbance of the modulation ratio of the wind turbine grid-side converter; The transformer connection impedance on the wind turbine side; is the small signal disturbance of the current flowing through the grid connection point; is the small signal disturbance component of the grid connection point voltage on the flexible DC converter station side; is the modulation ratio of the flexible DC converter station; is the Toeplitz matrix of the flexible DC converter station modulation ratio, which reflects the multi-harmonic coupling effect; is the small signal disturbance of the upper arm capacitor voltage and the sum of the modular multilevel converter; is the Toeplitz matrix of the capacitor voltage sum; is the small signal disturbance of the modulation ratio of the flexible DC converter station; is the equivalent impedance of a single bridge arm of the modular multilevel converter; It is the small signal disturbance of the upper arm current of the modular multilevel converter; The transformer connection impedance at the flexible DC converter station side.

[0034] At the same time, this embodiment analyzes the impact of the grid-connected point voltage disturbance and current disturbance on the wind turbine modulation ratio, takes the grid-connected point voltage disturbance and current disturbance as input, and the wind turbine modulation ratio disturbance as output, obtains the frequency domain transfer function, uses the frequency domain transfer function as the transfer relationship of the wind turbine controller, and decomposes the wind turbine controller transfer relationship according to the characteristic frequency point to obtain the wind turbine harmonic linearization equation. The wind turbine harmonic linearization equation reflects the dynamic characteristics of the wind turbine controller at different frequency points. The wind turbine harmonic linearization equation is specifically: Where, is the small signal disturbance of the modulation ratio of the wind turbine grid-side converter; is the transfer function from the grid connection point voltage disturbance to the modulation ratio in the wind turbine controller; is the transfer function from the grid-connected point current disturbance to the modulation ratio in the wind turbine controller.

[0035] Similarly, this embodiment uses the grid-connected point voltage and current disturbances as inputs and the flexible DC modulation ratio disturbance as output. A flexible DC controller transfer relationship is established from the grid-connected point voltage and current disturbances to the flexible DC modulation ratio disturbance. The flexible DC harmonic linearization equation is decomposed into the flexible DC harmonic linearization equation according to the characteristic frequency point. The flexible DC harmonic linearization equation is specifically: Where, is the small signal disturbance of the modulation ratio of the flexible DC converter station; is the transfer function from the grid-connected point voltage disturbance to the modulation ratio in the flexible DC controller; is the transfer function from the grid-connected point current disturbance to the modulation ratio in the flexible DC controller.

[0036] This embodiment integrates the wind turbine harmonic linearization equation and the flexible direct current harmonic linearization equation to construct a controller multi-frequency harmonic small signal equation. This equation combines the dynamic characteristics of the wind turbine and the flexible direct current controller, and provides key information for the establishment of the subsequent impedance model. This embodiment solves the main circuit harmonic small signal equation (including the wind turbine side main circuit harmonic small signal equation and the flexible direct current side main circuit harmonic small signal equation) and the controller multi-frequency harmonic small signal equation. Specifically, the wind turbine side main circuit harmonic small signal equation is solved together with the wind turbine harmonic linearization equation, and decoupled by matrix inversion operation to obtain the wind turbine side multi-harmonic small signal equation. The multi-harmonic linearization theoretical impedance model is obtained. The harmonic small signal equation of the main circuit of the flexible DC side is combined with the flexible DC harmonic linearization equation, and the modulation ratio-capacitance voltage conversion matrix is ​​combined. The multi-harmonic linearization theoretical impedance model of the flexible DC side is obtained through matrix operation decoupling. Finally, the multi-harmonic linearization theoretical impedance model with the grid connection point voltage and current as the port variables is obtained. The multi-harmonic linearization theoretical impedance model fully reflects the impedance characteristics of the offshore wind power flexible DC grid-connected system at different frequencies, providing a theoretical basis for subsequent stability margin analysis and broadband oscillation suppression. Among them, the multi-harmonic linearization theoretical impedance model of the wind turbine side is specifically as follows: Where, is the multi-harmonic linearized theoretical impedance model on the wind turbine side, reflecting the impedance characteristics of the wind turbine grid-side converter at the grid connection point; I is the unit matrix.

[0037] The multi-harmonic linearization theoretical impedance model of the flexible DC side is specifically: Where, A multi-harmonic linearized theoretical impedance model for the flexible DC side is developed to describe the impedance characteristics of the modular multilevel converter at the grid connection point. is the conversion matrix from capacitor voltage to modulation ratio of modular multi-level converter, which reflects the influence of capacitor voltage on modulation ratio; It is the conversion matrix from capacitor voltage to current of the modular multi-level converter, which reflects the influence of capacitor voltage on bridge arm current.

[0038] After obtaining the multi-harmonic linearization theoretical impedance model, this embodiment performs a consistency check on the constructed multi-harmonic linearization theoretical impedance model and the multi-input and multi-output impedance matrix. The specific operation of the check is to compare the impedance characteristics such as amplitude and phase of the two within a certain error limit. If the deviation between the two is within the preset error limit, it is considered that the consistency is good and the consistency deviation check passes; otherwise, it is necessary to further analyze the cause of the deviation. Specifically, this embodiment compares the amplitude and phase of the elements at corresponding positions in the multi-input and multi-output impedance matrix and the multi-harmonic linearization theoretical impedance model, and calculates the amplitude deviation and phase deviation of each element. When the amplitude deviation and the phase deviation are both within the preset deviation range, it is determined that the multi-input and multi-output impedance matrix and the multi-harmonic linearization theoretical impedance model pass the consistency check, which indicates that there is 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 that exceed the preset deviation range, it is necessary to further analyze the cause, which may include measured data errors, improper simplified assumptions of the theoretical model, etc., and corrections and improvements are made accordingly.

[0039] After the consistency deviation check result is passed, this embodiment reduces the order of the verified multi-input multi-output impedance matrix in the coupling impedance reduction link and converts it into a single-input single-output (SISO) impedance form, so as to more intuitively analyze the stability of the system. Specifically, this embodiment extracts key impedance elements from the multi-input multi-output impedance matrix. The key impedance elements include the positive-sequence self-impedance component on the aggregated wind farm side, the positive-sequence to negative-sequence coupling impedance component on the aggregated wind farm side, the negative-sequence to positive-sequence coupling impedance component on the aggregated wind farm side, the negative-sequence self-impedance component on the aggregated wind farm side, the positive-sequence self-impedance component on the flexible DC converter station side, the positive-sequence to negative-sequence coupling impedance component on the flexible DC converter station side, the negative-sequence 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 the 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. Schur complement transformation is a matrix theory. In the reduction method, this embodiment processes the extracted key impedance elements based on the Shure complement transformation principle in matrix theory, while ensuring that the block diagonal matrix of the original multi-input multi-output impedance matrix is ​​non-singular. For the aggregated wind farm side, the multi-input multi-output impedance is reduced to a single-input single-output equivalent positive-sequence impedance through the Shure complement transformation. Specifically, the impedance components on the aggregated wind farm side are combined according to the Shure complement transformation to eliminate the influence of the negative-sequence related impedance, thereby obtaining the single-input single-output equivalent positive-sequence impedance of the aggregated wind farm side. Similarly, the impedance components on the flexible direct current side are subjected to the Shure complement transformation to obtain the single-input single-output equivalent positive-sequence impedance of the flexible direct current side. Through this step, the complex multi-input multi-output impedance matrix is ​​reduced to a single-input single-output impedance form that is easy to analyze, thereby obtaining the single-input single-output equivalent positive-sequence impedance. This allows the complex multi-input multi-output system to be presented in a more concise form, and the reduced-order impedance form facilitates further analysis and evaluation of the system stability.

[0040] It should be noted that when the source network impedance adopts multi-input and multi-output impedance, the generalized Nyquist criterion needs to be used for stability determination. However, this method has poor practicality in engineering applications. Therefore, in the order reduction process of this embodiment, based on the Schur complement transformation in matrix theory, it is required that the block diagonal matrix of the original matrix is ​​a non-singular matrix to ensure the accuracy and effectiveness of the order reduction. After the order reduction process, the equivalent single-input and single-output positive-sequence impedance of the source network is obtained as follows: Where, is the equivalent positive sequence impedance of single input and single output on the aggregated wind farm side; is the negative-sequence self-impedance on the flexible DC side, i.e., an element of the multi-input and multi-output impedance matrix; is the negative sequence admittance on the wind farm side, which is the inverse of the impedance and reflects the negative sequence current response characteristics; is the negative-sequence to positive-sequence coupling impedance on the flexible DC side; is the positive-sequence to negative-sequence admittance on the wind farm side, which reflects the impact of the positive-sequence voltage on the negative-sequence current; is the positive sequence admittance on the wind farm side, which represents the response of the positive sequence current to the positive sequence voltage; is the negative-sequence to positive-sequence admittance on the wind farm side; is the equivalent positive sequence impedance of the flexible DC side with single input and single output; is the negative sequence self-impedance on the wind farm side; is the negative sequence admittance on the flexible DC side; is the negative-sequence to positive-sequence coupling impedance on the wind farm side; It is the positive-sequence to negative-sequence admittance on the flexible DC side; is the positive sequence admittance on the flexible direct current side; It is the negative-sequence to positive-sequence admittance on the flexible DC side.

[0041] S4. Construct a Bode diagram 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 amplitude, and quantify the stability margin of the offshore wind power flexible direct current grid-connected system based on the phase difference.

[0042] In some embodiments, the steps of constructing a Bode diagram of the source-grid side impedance characteristic based on the equivalent positive-sequence impedance, reading the phase difference at the intersection of the source-grid impedance amplitudes, and quantifying the stability margin of the offshore wind power flexible direct current grid-connected system according to the phase difference include: Calculating the wind farm impedance amplitude, wind farm impedance phase, flexible DC impedance amplitude and flexible DC impedance phase at each sweep frequency point according to the equivalent positive sequence impedance to obtain impedance amplitude and phase data; Constructing a Bode diagram of the impedance characteristics of the source and network sides with the frequency points of the frequency sweep as the horizontal coordinate and the impedance amplitude and phase data as the vertical coordinate; Finding the intersection frequency point of the source network impedance amplitude in the source network side impedance characteristic Bode diagram, and reading the wind farm impedance phase value and the flexible direct current impedance phase value at the intersection frequency point; Calculating the difference between the wind farm impedance phase value and the flexible DC impedance phase value to obtain the phase difference at the intersection of the source and grid impedance amplitudes; The deviation value between the phase difference and the preset stability margin threshold is calculated to obtain the stability margin of the offshore wind power flexible direct current grid-connected system.

[0043] Specifically, this embodiment calculates 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, point by point according to the equivalent positive-sequence impedance and the frequency sweep point. Then, this embodiment uses the frequency sweep point as the horizontal coordinate and 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 as the vertical coordinate to construct a Bode diagram of the source-grid side impedance characteristic. The Bode diagram of the source-grid side impedance characteristic includes a wind farm impedance amplitude-frequency curve, a flexible DC impedance amplitude-frequency curve, a wind farm impedance phase-frequency curve, and a flexible DC impedance phase-frequency curve. The function of the single-input and single-output source-grid side impedance characteristic Bode diagram is as follows: Where, It is a function of the Bode plot of the source-grid side impedance characteristics.

[0044] In this embodiment, the intersection of the wind farm impedance amplitude curve and the flexible direct impedance amplitude curve is searched and identified in the Bode diagram of the impedance characteristic on the source-grid side to obtain the intersection frequency point. The phase value corresponding to the intersection frequency point is read. Based on the phase value corresponding to the intersection frequency point, the following formula can be obtained: The mathematical expression of the stability margin is: Where, is the source-network impedance phase angle difference; is the phase angle of the equivalent positive sequence impedance of the flexible DC side with single input and single output; is the phase angle of the equivalent positive sequence impedance of the single-input and single-output aggregated wind farm side; is the stability margin of the system.

[0045] The idea of ​​analyzing the Bode diagram of the source network impedance characteristics to extract the impedance stability criterion in this embodiment is to read the source network impedance phase angle curve deviation at the intersection of the source network impedance amplitude curve and use it as the system stability criterion; The difference between the deviation of the source-grid impedance phase angle curve and the above-mentioned source-grid impedance phase angle curve is used as the stability margin criterion. According to classical control theory, when the impedance phase angle characteristic curve is located at to When the impedance phase angle characteristic curve is lower than , it indicates that the impedance characteristics of this part present capacitive negative damping characteristics; if the impedance phase angle characteristic curve is higher than , it indicates that the impedance characteristics of this part present an inductive negative damping characteristic. In this way, this embodiment can accurately evaluate the stability of the system and provide powerful guidance for subsequent broadband oscillation suppression measures.

[0046] In summary, the broadband oscillation suppression method of the offshore wind power flexible direct current grid-connected system based on matrix control proposed in this embodiment uses a matrix control structure to increase the parameter freedom and flexibly adjust the system impedance to achieve more accurate and effective impedance reshaping, effectively reducing the negative damping effect of the system, thereby ensuring that the offshore wind power system can continue to operate stably under power fluctuations and significantly reducing the potential risk of broadband oscillations. Specifically, the offshore wind power flexible direct current grid-connected system proposed in this embodiment includes an offshore AC wind farm, an offshore wind farm collection boosting station, an offshore wind farm collection bus, an offshore flexible direct current converter station, a high-voltage direct current transmission submarine cable and an onshore flexible direct current converter station at the receiving end. The broadband oscillation suppression method performs multi-harmonic linearization and impedance modeling based on the system topology and electrical main parameters, and performs impedance sweeping 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-connected point for fast Fourier harmonic analysis, and then obtains the measured impedance sweep data. Through further transformation processing, an impedance matrix for a multi-input, multi-output system considering multiharmonic coupling effects is constructed and verified for consistency with the theoretical impedance modeling results. When the deviation between the two is within the specified error limit, the system's impedance characteristics are considered to have been accurately obtained. Next, SISO-order reduction is performed by extracting key impedance matrix elements, and the source-network impedance characteristics are displayed 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 curve is read as the stability criterion, and the difference between 180° and this phase angle deviation is used as the stability margin criterion, effectively characterizing the system's stability margin. For example, based on control theory and actual engineering experience, if the stability margin criterion is between 20° and 30°, the system is considered to have excellent impedance characteristics and is not at risk of broadband oscillation. If the stability margin criterion is less than 10°, the system may be at risk of broadband oscillation due to temperature drift of engineering parameters and random shifts in power flow conditions. In this case, a corresponding impedance reshaping strategy is needed to suppress the system's broadband oscillation.

[0047] S5. 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 a weighted coefficient matrix between inner and outer control loops of the wind turbine grid-side converter.

[0048] In some embodiments, the step of reconfiguring the wind turbine grid-side converter into a matrix control structure by cascading weighted coefficient matrices between inner and outer control loops of the wind turbine grid-side converter includes: The actual value of the collected grid-connected point quadrature-axis voltage is subtracted from the quadrature-axis voltage reference value to obtain a quadrature-axis voltage deviation value, and the quadrature-axis voltage deviation value is 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 DC voltage value is collected, the DC voltage actual value is subtracted from the preset DC voltage reference value to obtain the DC voltage deviation value, and the DC voltage deviation value is passed through a proportional-integral controller with saturation limiting to generate a direct axis outer loop signal; In the fixed 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 passed through the proportional-integral controller with saturation limiting to generate the quadrature axis outer loop signal; The quadrature-axis synchronization signal, the direct-axis outer loop signal, and the quadrature-axis outer loop signal are used as input column vectors, and the input column vector signals are cross-linked and mapped using a pre-constructed weighting coefficient matrix to generate an output column vector including a frequency deviation signal, a direct-axis current reference value, and a quadrature-axis current reference value; The frequency deviation signal is superimposed on the actual frequency of the power grid and then integrated to obtain a synchronous phase angle, and the actual value of the direct-axis current is collected under synchronous control using the synchronous phase angle; The direct-axis current actual value is subtracted from the direct-axis current reference value to obtain the direct-axis current deviation value. The direct-axis current deviation value is passed through a proportional-integral link with saturation limiting and a decoupling signal containing the quadrature-axis current actual value is added to generate a direct-axis inner loop signal. The quadrature-axis current actual value is subtracted from the quadrature-axis current reference value to obtain a quadrature-axis current deviation value, and the quadrature-axis current deviation value is passed through a proportional-integral controller with saturation and added with a decoupling signal containing the direct-axis current actual value to generate a quadrature-axis inner loop signal; The direct-axis inner loop signal and the quadrature-axis inner loop signal are sequentially added with the direct-axis current actual value and the quadrature-axis voltage actual value to obtain the direct-axis modulation ratio and the quadrature-axis modulation ratio; The direct-axis modulation ratio and the quadrature-axis modulation ratio are modulated by a sinusoidal pulse width modulation strategy, and the grid-side converter of the wind turbine is reconstructed into a matrix control structure.

[0049] This embodiment implements a broadband oscillation active suppression strategy when the stability margin is lower than a preset safety threshold. 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. The controller mainly includes a q-axis voltage phase-locked link, a constant DC voltage outer loop, a constant reactive power outer loop, a weighting coefficient matrix cross-linking link, a phase angle generation link, a dq-axis vector current inner loop and its decoupling link, and a dq-axis voltage feedforward link. The specific implementation process of each link is as follows: In the q-axis grid-connected point voltage phase-locked link, this embodiment sets the q-axis voltage reference value of the grid-connected point to 0. The actual value of the q-axis voltage at the grid-connected point after acquisition, filtering, and per-unit processing is subtracted from the q-axis voltage reference value. The difference is passed through a proportional-integral (PI) link with saturation limiting to generate a quadrature-axis synchronization signal. The mathematical form of the quadrature-axis synchronization signal is as follows: Where, It is the quadrature axis synchronization signal output by the synchronous phase-locked loop; is the proportional gain of the phase-locked loop PI controller; is the integral gain of the phase-locked loop PI controller; S is the Laplace operator; is the reference value of the q-axis component of the grid connection point voltage, that is, the q-axis voltage reference value; is the measured value of the controller's internal q-axis voltage after filtering and standard processing; The equivalent time delay introduced for signal sampling, filtering and normalization; is the actual measured value of the q-axis component of the grid-connected point voltage, that is, the actual value of the q-axis voltage obtained after the three-phase voltage is transformed by Parker.

[0050] 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 passed through a proportional-integral link with saturation limiting to generate a direct-axis outer loop signal. The mathematical form of the direct-axis outer loop signal is as follows: Where, It is the d-axis current reference value signal output by the DC voltage outer loop; is the proportional gain of the DC voltage outer loop PI controller; is the integral gain of the DC voltage outer loop PI controller; is the reference value of DC voltage; is the DC voltage measurement value inside the controller after filtering and standard processing, that is, the DC voltage filtering value; is the actual measured value of the DC voltage, that is, the actual value of the DC voltage.

[0051] In the fixed 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 passed through a proportional-integral link with saturation limiting to generate a quadrature-axis outer loop signal. The mathematical form of the quadrature-axis outer loop signal is as follows: Where, It is the q-axis current reference value signal output by the reactive power outer loop; is the proportional gain of the reactive power outer loop PI controller; is the integral gain of the reactive power outer loop PI controller; is the reference value of reactive power; is the reactive power measurement value inside the controller after filtering and standardization; is the actual value of reactive power.

[0052] In the cross-linking link of the weighted coefficient matrix, the quadrature axis synchronization signal, the direct axis outer ring signal and the quadrature axis outer ring signal are used as the input column vectors of the matrix. 、 、 、 、 、 、 、 and The weighted coefficient matrix constructed by cross-linking maps the above input column vector signal 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 the output column vector can be expressed as the following mathematical form: Where, It is the frequency deviation signal, which is used to correct the phase-locked loop output frequency and enhance the system synchronization stability; is the d-axis current reference value; is the q-axis current reference value; is the weighted coefficient matrix element to realize the dynamic coupling between multiple control loops, where subscript i=1, 2, 3; subscript e=1, 2, 3; weighted coefficient matrix Provides nine degrees of freedom to flexibly reshape the system impedance characteristics.

[0053] In this embodiment, the frequency deviation signal is superimposed on the actual frequency of the power grid and then integrated to generate the phase angle. The specific mathematical expression is: Where, The synchronous phase angle generated by the grid-side converter is obtained by integrating the frequency signal Generate synchronous phase angle , used for Parker transform.

[0054] In the dq-axis vector current inner loop and its decoupling link, this embodiment subtracts the actual d-axis current value after sampling, filtering, and per-unit processing from the set d-axis current reference value. The difference is passed through a PI link with saturation limiting, and the decoupling signal containing the actual q-axis current value is added to generate a direct-axis inner loop signal. Simultaneously, this embodiment subtracts the actual q-axis current value after sampling, filtering, and per-unit processing from the set q-axis current reference value. The difference is passed through a PI link with saturation, and the decoupling signal containing the actual d-axis current value is added to generate a 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: Where, is the direct axis inner loop signal, i.e. the d-axis current inner loop control signal; is the inner loop proportional coefficient of the d-axis current; is the d-axis current inner loop integral coefficient; is the d-axis current reference value, which comes from the DC voltage outer loop; is the actual value of the d-axis current; is the grid angular frequency; is the quadrature axis inner loop signal, i.e. the q-axis current inner loop control signal; is the q-axis current inner loop proportional coefficient; is the q-axis current inner loop integral coefficient; is the q-axis current reference value, which comes from the reactive power outer loop; is the actual value of q-axis current; L is the equivalent inductance.

[0055] In this embodiment, the d-axis current actual value and the q-axis voltage actual value after the direct-axis inner loop signal and the quadrature-axis inner loop signal are sampled, filtered, and calibrated in order to obtain the d-axis modulation ratio and the q-axis modulation ratio. Subsequently, the classic sinusoidal pulse width modulation strategy is used for modulation to reconstruct 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 the offshore full-power wind turbine provided by the embodiment of the present invention. Figure 2 In the equation, number 1 represents the q-axis voltage phase-locked link; 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; number 7 represents the dq-axis voltage feedforward link, where: is the grid voltage reference value; is the actual measured value of the grid voltage; PI is the proportional-integral controller; is the DC bus voltage reference value; is the actual value of DC bus voltage; is the reactive power reference value; Q is the actual measured value of reactive power; is the rated angular frequency of the power grid; w is the actual angular frequency of the power grid; is the d-axis component of the current, which is the actual value of the d-axis current obtained by Park transformation; is the q-axis component of the current, which is the actual value of the q-axis current obtained by Park transformation; The synchronization phase angle generated for the grid-side converter is the synchronization rotation angle used for coordinate transformation (such as Park transformation); is the d-axis component of the system voltage; is the q-axis component of the system voltage; is the d-axis modulation ratio; is the q-axis modulation ratio.

[0056] S6. Iteratively optimize the weighting coefficient matrix in the matrix control structure based on impedance sensitivity, and simultaneously connect a low-pass filter in series with the voltage and current feedback channels to perform impedance reshaping to achieve broadband oscillation risk suppression.

[0057] In some embodiments, the step of iteratively optimizing the weighting coefficient matrix in the matrix control structure based on impedance sensitivity and simultaneously connecting a low-pass filter in series with the voltage and current feedback channels to perform impedance reshaping to achieve broadband oscillation risk suppression includes: Calculate the amplitude sensitivity and phase sensitivity of each element in the weighted coefficient matrix to the equivalent positive sequence impedance on the wind farm side to obtain the impedance sensitivity; Taking the direction with the highest impedance sensitivity as the search direction, the elements in the weighted coefficient matrix are subjected to gradient optimization iteration, and the stability margin is recalculated after each iterative optimization; When the stability margin is not lower than a preset margin threshold, stopping the gradient optimization iteration of the elements in the weight coefficient matrix and outputting the optimized weight coefficient matrix; A low-pass filter is selected according to the frequency characteristics and broadband oscillation suppression requirements of the offshore wind power flexible direct current grid-connected 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 simultaneously enabled to reshape the impedance and suppress the risk of broadband oscillation.

[0058] In the impedance reshaping and margin optimization link, this embodiment assigns initial parameter values ​​to the weighting coefficient matrix, and uses the initial parameter values ​​and the current system operating point as the starting point to calculate the sensitivity of the weighting coefficient matrix elements to the SISO equivalent positive sequence impedance on the wind farm side, and iteratively optimizes the weighting coefficient matrix parameters according to the sensitivity information and the stability margin improvement requirements to achieve system impedance characteristic reshaping and stability margin improvement; at the same time, in the control loop reshaping link, a low-pass filter is connected in series in the voltage and current dq-axis signal channels and the power calculation link after the Park transformation to reshape the control loop dynamics and suppress the resonance risk introduced by sampling errors and modulation delays, thereby effectively suppressing the broadband oscillation risk of the offshore wind power flexible direct current grid-connected system; in some embodiments, the step of synchronously enabling the low-pass filter in the voltage and current feedback channel to perform impedance reshaping includes: Collecting three-phase voltage signals and three-phase current signals of the grid connection point of the offshore wind power flexible direct current grid-connected system, and performing Park transformation on the three-phase voltage signals to obtain quadrature and direct axis vector voltage signals; Performing a Park transformation on the three-phase current signal to obtain a quadrature and direct axis vector current signal; Low-pass filters are connected in series in the feedback channels of the AC-axis vector voltage signal and the AC-axis vector current signal to dynamically reshape the control loop to obtain voltage and current AC-axis components; the voltage and current AC-axis components include a system current direct-axis component, a system current quadrature-axis component, a system voltage direct-axis component, and a system voltage quadrature-axis component; The instantaneous power measurement value is calculated according to the voltage and current quadrature and direct axis components, 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 direct current grid-connected system.

[0059] Specifically, this embodiment converts the three-phase voltage sine wave into a constant steady-state value in the dq coordinate system to obtain a direct-axis vector voltage signal; converts the three-phase current sine wave into a constant steady-state value in the dq coordinate system to obtain a 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 to avoid system instability caused by controller sampling and modulation delays. The instantaneous power theoretical calculation module involving the voltage and current vectors is to generate power measurement values ​​at an extremely small time scale that is conducive to controller feedback. This embodiment reshapes the system impedance based on the calculated instantaneous power measurement value. 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: Where, P is the instantaneous active power; Q is the instantaneous reactive power; is the d-axis component of the system voltage; is the q-axis component of the system current; is the q-axis component of the system voltage; is the d-axis component of the system current.

[0060] Figure 3 This is a block diagram of a structure in which a filter is connected in series in a voltage, current and power sampling loop under an active suppression strategy provided by an embodiment of the present invention. Figure 3 In the equation, P is the instantaneous active power; Q is the instantaneous reactive power; is the d-axis component of the system voltage; is the q-axis component of the system current; is the q-axis component of the system voltage; is the d-axis component of the system current; is the actual measured value of the A-phase system voltage; is the actual measured value of the B-phase system voltage; is the actual measured value of the C-phase system voltage; is the actual measured value of the A-phase system current; is the actual measured value of the B-phase system current; is the actual measured value of the C-phase system current; is the zero-sequence component of the system voltage; is the zero-sequence component of the system current; PARK transformation is Park transformation, which is used to convert variables in the three-phase stationary coordinate system (abc coordinate system) into variables in the two-phase rotating coordinate system (dq coordinate system); LPF is a low-pass filter.

[0061] The wide-band oscillation suppression method of the offshore wind power flexible DC grid-connected system based on matrix control proposed in this embodiment optimizes the control link of the grid-side converter of the offshore wind turbine, specifically including the grid-connected point q-axis voltage synchronous phase-locked link, the fixed DC voltage outer loop control, the fixed reactive power outer loop control, the weighted coefficient matrix cross-linking link, the vector current inner loop control, the current decoupling link and the grid-connected point vector voltage feedforward and other key links. Specifically, this embodiment is based on the traditional control strategy of the grid-side converter of the offshore full-power wind turbine. By cascading the weighted coefficient matrix cross-linking link at the connection between the inner loop and the outer loop, the traditional control strategy of the grid-side converter of the offshore full-power wind turbine is reconstructed into a matrix control structure to increase the freedom of the control parameters, thereby realizing the reshaping of the impedance characteristics of the offshore wind turbine. In this way, the system impedance characteristics can be flexibly adjusted. , providing a basis for suppressing broadband oscillations. It should be noted that, in the specific implementation process of this embodiment, conventional parameters can be assigned to the weighting coefficient matrix based on engineering experience, and the above method can be used to obtain the impedance stability margin of the system, which can be used as the basis for determining whether the system needs impedance reshaping. If the system impedance stability margin is insufficient, according to the system impedance stability margin requirements and optimization direction, iterative optimization is performed with the conventional parameters of the weighting coefficient matrix as the initial value to gradually improve the wind farm impedance characteristics, thereby weakening 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 accurately 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 system broadband oscillation risk.

[0062] It should be noted that this embodiment reconstructs the wind turbine grid-side converter control strategy of the offshore wind power flexible DC grid-connected system under study into matrix control. Based on engineering experience and model debugging, a set of stable parameters is obtained as the initial optimization value. Since the matrix control structure introduces more parameters adjustment freedom, it has a larger stable parameter selection range than the original control structure. When the following equation is satisfied, the offshore aggregated wind farm controller belongs to the classic grid-following constant DC voltage control and generally meets the stability condition: When the offshore wind power output is low or the collection cable is long, the following parameters can be selected: At this time, the wind turbine grid-side converter control is in the network matching control mode, which is suitable for maintaining system stability in scenarios where long submarine cables are gathered. Therefore, this embodiment can select the above two typical weighting coefficient matrix values ​​as the initial values ​​for matrix controller parameter optimization. This embodiment uses the steady-state operating point and the steady-state parameters as linearized operating points, calculates the impedance sensitivity of the key parameters in the weighting coefficient matrix to the impedance characteristics, and at the same time, combined with the corresponding impedance optimization direction corresponding to the impedance sensitivity, adjusts and optimizes the impedance stability margin of the system by continuously iterating the key parameters in the weighting coefficient matrix. The specific impedance sensitivity calculation method is as follows: Where, is the equivalent positive sequence impedance of single input and single output on the aggregated wind farm side; is the weighted coefficient matrix element to achieve dynamic coupling between multiple control loops; is the frequency domain influence function of the weighted coefficient matrix on the impedance amplitude; j is the imaginary unit; is the frequency domain influence function of the weighted coefficient matrix on the impedance phase; is the impedance sensitivity of the equivalent impedance on the wind farm side to the elements of the weighting coefficient matrix.

[0063] It should be noted that the optimization process of the impedance stability margin belongs to the iterative method of dynamic optimization. The specific steps are to take the determined stable operating point as the initial value, calculate the controller parameter weight for the fastest improvement of the impedance stability margin at this time, calculate the new controller parameters, and then analyze whether the system impedance stability margin at this time meets the requirements. If it does not meet the requirements, further analyze the parameter impedance sensitivity at this time and repeat the above steps; if the required stability margin is reached, stop the optimization.

[0064] The method of changing the grid-side converter control strategy of the full-power wind turbine to matrix control in this embodiment belongs to the control reconstruction technology route in the broadband oscillation suppression strategy. This method can significantly change the system impedance characteristics from a mechanistic perspective. The method of adaptively finding a better weighting coefficient matrix through a dynamic optimization method in this embodiment so that the impedance stability margin of the system reaches the set target is essentially a control parameter reshaping technology route in the broadband oscillation suppression strategy. It should be noted that the method of connecting a low-pass filter in series with the corresponding link in the existing control loop in this embodiment belongs to the control loop reshaping technology route 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 system broadband oscillation risk. Through the above steps, the present invention can effectively suppress the broadband oscillation risk of the offshore wind power flexible DC grid-connected system, ensuring that the system always maintains stable operation under power fluctuations.

[0065] An embodiment of the present invention provides a method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system. The method sequentially injects positive and negative sequence perturbation sinusoidal harmonic signals at the wind turbine grid-connected point, and synchronously collects voltage and current harmonic response signals at the grid-connected point; performs fast Fourier analysis on the voltage and current harmonic response signals at the grid-connected point to obtain measured impedance sweep data; constructs a multi-input and multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance sweep data, and reduces the multi-input and multi-output impedance matrix to an equivalent positive-sequence impedance with a single input and a single output; constructs a Bode diagram of the source-grid side impedance characteristic based on the equivalent positive-sequence impedance, reads the phase difference at the intersection of the source-grid impedance amplitude, and quantifies the stability margin of the offshore wind power flexible direct current grid-connected system based on the phase difference; when the stability margin is lower than a preset margin threshold, reconstructs the wind turbine grid-side converter into a matrix control structure by cascading a weighting coefficient matrix between the inner and outer loop control loops of the wind turbine grid-side converter; iteratively optimizes the weighting coefficient matrix in the matrix control structure based on impedance sensitivity, and synchronously connects a low-pass filter in series in the voltage and current feedback channel to perform impedance reshaping to complete broadband oscillation risk suppression. Compared with the existing technology, this method accurately evaluates and optimizes the stability of the offshore wind power flexible direct current grid-connected system through impedance modeling and matrix control strategy. When the stability margin is insufficient, it achieves broadband oscillation suppression of the offshore wind power flexible direct current grid-connected system through weighted coefficient matrix optimization and low-pass filter configuration, thereby ensuring the stable operation of the offshore wind power flexible direct current grid-connected system under power fluctuations.

[0066] It should be noted that the size of the serial numbers of the above-mentioned processes does not mean 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 embodiment of this application.

[0067] In one embodiment, Figure 4 As shown, an embodiment of the present invention provides a broadband oscillation suppression system for an offshore wind power flexible direct current grid-connected system, the system comprising: The data acquisition module 101 is used to sequentially inject positive and negative sequence perturbation sinusoidal harmonic signals at the wind turbine grid connection point and synchronously acquire the voltage and current harmonic response signals at the grid connection point; The 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 sweep frequency data; An impedance analysis module 103 is configured to construct a multi-input multi-output impedance matrix taking into account 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 of a single input and single output; The stability analysis module 104 is configured to construct a Bode diagram of the source-grid side impedance characteristic based on the equivalent positive-sequence impedance, read the phase difference at the intersection of the source-grid impedance amplitude, and quantify the stability margin of the offshore wind power flexible direct current grid-connected system based on the phase difference; a reconstruction control module 105, configured to reconstruct the wind turbine grid-side converter into a matrix control structure by cascading a weighted coefficient matrix between inner and outer control loops of the wind turbine grid-side converter when the stability margin is lower than a preset margin threshold; The oscillation suppression module 106 is used to iteratively optimize the weighted coefficient matrix in the matrix control structure based on impedance sensitivity, and simultaneously connect a low-pass filter in series with the voltage and current feedback channels to perform impedance reshaping to achieve broadband oscillation risk suppression.

[0068] For the specific definition of a broadband oscillation suppression system for an offshore wind power flexible direct current grid-connected system, please refer to the above-mentioned definition of a broadband oscillation suppression method for an offshore wind power flexible direct current grid-connected system, which will not be repeated here. A person of ordinary skill in the art will appreciate 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 performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0069] An embodiment of the present invention provides a broadband oscillation suppression system for an offshore wind power flexible direct current grid-connected system. The system sequentially injects positive and negative sequence perturbation sinusoidal harmonic signals at the grid-connected point of a wind turbine through a data acquisition module, and synchronously acquires the voltage and current harmonic response signals of the grid-connected point; the harmonic analysis module performs fast Fourier analysis on the voltage and current harmonic response signals of the grid-connected point to obtain measured impedance sweep frequency data; the impedance analysis module constructs a multi-input and multi-output impedance matrix considering multi-harmonic coupling based on the measured impedance sweep frequency data, and reduces the multi-input and multi-output impedance matrix to an equivalent positive sequence impedance with a single input and a single output; the stability analysis module Based on the equivalent positive-sequence impedance, a Bode plot of the source-grid impedance characteristic is constructed, the phase difference at the intersection of the source-grid impedance amplitude is read, and the stability margin of the offshore wind power flexible direct current grid-connected system is quantified based on the phase difference. When the stability margin is lower than the preset margin threshold, the reconstruction control module reconstructs 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. The oscillation suppression module iteratively optimizes the weighted coefficient matrix in the matrix control structure based on impedance sensitivity and simultaneously connects a low-pass filter in series with the voltage and current feedback channels to perform impedance reshaping to achieve broadband oscillation risk suppression. Compared with existing technologies, this system accurately evaluates and optimizes the stability of the offshore wind power flexible direct current grid-connected system through impedance modeling and matrix control strategy. When the stability margin is insufficient, the system achieves broadband oscillation suppression of the offshore wind power flexible direct current grid-connected system through weighted coefficient matrix optimization and low-pass filter configuration, ensuring the stable operation of the offshore wind power flexible direct current grid-connected system under power fluctuations.

[0070] In one embodiment, the present invention provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the steps of the above method are implemented.

[0071] In the above embodiments, all or part of the embodiments can be implemented by software, hardware, firmware, or any combination thereof. When implemented using software, all or part of the embodiments can be implemented in the form of 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 computer-readable storage medium. 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 a wired (e.g., coaxial cable, optical fiber, digital subscriber line) or wireless (e.g., infrared, wireless, microwave, etc.) method. The computer-readable storage medium can be any available medium that can be accessed by a computer or a data storage device such as a server or data center that includes one or more available media. The available medium can be a magnetic medium (e.g., a floppy disk, a hard disk, a tape), an optical medium (e.g., a DVD), or a semiconductor medium (e.g., an SSD).

[0072] Those skilled in the art will understand that all or part of the processes in the above-mentioned embodiment methods can be implemented by instructing related hardware through a computer program. The computer program can be stored in a computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods.

[0073] The above-described embodiments merely represent several preferred implementations of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art could make several improvements and substitutions without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be based on the scope of protection of the claims.

Claims

1. A method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system, characterized in that: The following steps are involved: Positive and negative sequence perturbation sinusoidal harmonic signals are injected sequentially at the wind turbine grid connection point, and the voltage and current harmonic response signals of the grid connection point are collected synchronously; Performing fast Fourier analysis on the voltage and current harmonic response signals of the grid connection point to obtain measured impedance sweep frequency data; Constructing a multi-input multi-output impedance matrix considering multi-harmonic coupling according to the measured impedance sweep data, and reducing the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance with a single input and a single output; A Bode diagram of the impedance characteristic of the source-grid side is constructed based on the equivalent positive-sequence impedance, a phase difference at the intersection of the source-grid impedance amplitude is read, and a stability margin of the offshore wind power flexible direct current grid-connected system is quantified 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 a weighted coefficient matrix between inner and outer 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 synchronously connected in series with the voltage and current feedback channels to perform impedance reshaping, thereby achieving broadband oscillation risk suppression.

2. A method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system according to claim 1, characterized in that: The step of constructing a multi-input and multi-output impedance matrix considering multi-harmonic coupling according to the measured impedance sweep data includes: Eliminating background harmonic components and abnormal frequency point data from the measured impedance sweep frequency data to obtain preprocessed impedance sweep frequency data; Based on the pre-processed impedance sweep data, constructing a voltage harmonic vector and a current harmonic vector in order of frequency from low to high; The impedance value at each frequency sweep point is calculated according to the ratio between the voltage harmonic vector and the current harmonic vector, and a multi-input and multi-output impedance matrix considering multi-harmonic coupling is constructed.

3. A method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system according to claim 1, characterized in that: The step of reducing the multi-input multi-output impedance matrix to a single-input single-output equivalent positive-sequence impedance comprises: Based on the physical topology and electrical parameters of the offshore wind power flexible direct current grid-connected system, a multi-harmonic linearization theoretical impedance model is established; Performing consistency check on the multi-harmonic linearized theoretical impedance model and the multi-input multi-output impedance matrix to obtain a consistency deviation check result; After the consistency deviation check result is passed, extracting key impedance elements in the multi-input multi-output impedance matrix; Based on the key impedance elements, the multi-input multi-output impedance matrix is ​​reduced to an equivalent positive-sequence impedance of a single-input single-output through Schur complement transformation.

4. A method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system according to claim 3, characterized in that: The steps of establishing a multi-harmonic linearization theoretical impedance model based on the physical topology and electrical parameters of the offshore wind power flexible direct current grid-connected system include: Based on the physical topology of the offshore wind power flexible direct current grid-connected system, the system main loop model is established, and the harmonic components of each steady-state variable in the system main loop model at each characteristic frequency point are extracted to obtain the multi-frequency harmonic linearization representation of all steady-state variables; Substitute the multi-frequency harmonic linearization representation into the main circuit model of the wind turbine grid-side converter and the main circuit model of the flexible DC system for linearization processing to obtain the main circuit harmonic small signal equation; Establish the wind turbine controller transfer relationship from the grid connection point voltage and current disturbances to the wind turbine modulation ratio disturbance, and decompose the wind turbine controller transfer relationship into the wind turbine harmonic linearization equation according to the frequency point; Establish the flexible DC controller transfer relationship from the grid connection point voltage and current disturbances to the flexible DC modulation ratio disturbance, and decompose the flexible DC controller transfer relationship into the flexible DC harmonic linearization equation according to the frequency point; The wind turbine harmonic linearization equation and the flexible direct current harmonic linearization equation are integrated to construct a controller multi-frequency harmonic small signal equation; The main circuit harmonic small signal equation and the controller multi-frequency harmonic small signal equation are combined to obtain the multi-harmonic linearization theoretical impedance model.

5. The method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system according to claim 1, characterized in that: The steps of constructing a source-grid side impedance characteristic Bode diagram based on the equivalent positive-sequence impedance, reading the phase difference at the intersection of the source-grid impedance amplitude, and quantifying the stability margin of the offshore wind power flexible direct current grid-connected system according to the phase difference include: Calculating the wind farm impedance amplitude, wind farm impedance phase, flexible DC impedance amplitude and flexible DC impedance phase at each sweep frequency point according to the equivalent positive sequence impedance to obtain impedance amplitude and phase data; Constructing a Bode diagram of the impedance characteristics of the source and network sides with the frequency points of the frequency sweep as the horizontal coordinate and the impedance amplitude and phase data as the vertical coordinate; Finding the intersection frequency point of the source network impedance amplitude in the source network side impedance characteristic Bode diagram, and reading the wind farm impedance phase value and the flexible direct current impedance phase value at the intersection frequency point; Calculating the difference between the wind farm impedance phase value and the flexible DC impedance phase value to obtain the phase difference at the intersection of the source and grid impedance amplitudes; The deviation value between the phase difference and the preset stability margin threshold is calculated to obtain the stability margin of the offshore wind power flexible direct current grid-connected system.

6. A method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system according to 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 inner and outer control loops of the wind turbine grid-side converter comprises: The actual value of the collected grid-connected point quadrature-axis voltage is subtracted from the quadrature-axis voltage reference value to obtain a quadrature-axis voltage deviation value, and the quadrature-axis voltage deviation value is 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 DC voltage value is collected, the DC voltage actual value is subtracted from the preset DC voltage reference value to obtain the DC voltage deviation value, and the DC voltage deviation value is passed through a proportional-integral controller with saturation limiting to generate a direct axis outer loop signal; In the fixed 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 passed through the proportional-integral controller with saturation limiting to generate the quadrature axis outer loop signal; The quadrature-axis synchronization signal, the direct-axis outer loop signal, and the quadrature-axis outer loop signal are used as input column vectors, and the input column vector signals are cross-linked and mapped using a pre-constructed weighting coefficient matrix to generate an output column vector including a frequency deviation signal, a direct-axis current reference value, and a quadrature-axis current reference value; The frequency deviation signal is superimposed on the actual frequency of the power grid and then integrated to obtain a synchronous phase angle, and the actual value of the direct-axis current is collected under synchronous control using the synchronous phase angle; The direct-axis current actual value is subtracted from the direct-axis current reference value to obtain the direct-axis current deviation value. The direct-axis current deviation value is passed through a proportional-integral link with saturation limiting and a decoupling signal containing the quadrature-axis current actual value is added to generate a direct-axis inner loop signal. The quadrature-axis current actual value is subtracted from the quadrature-axis current reference value to obtain a quadrature-axis current deviation value, and the quadrature-axis current deviation value is passed through a proportional-integral controller with saturation and added with a decoupling signal containing the direct-axis current actual value to generate a quadrature-axis inner loop signal; The direct-axis inner loop signal and the quadrature-axis inner loop signal are sequentially added with the direct-axis current actual value and the quadrature-axis voltage actual value to obtain the direct-axis modulation ratio and the quadrature-axis modulation ratio; The direct-axis modulation ratio and the quadrature-axis modulation ratio are modulated by a sinusoidal pulse width modulation strategy, and the grid-side converter of the wind turbine is reconstructed into a matrix control structure.

7. A method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system according to claim 1, characterized in that: The steps of iteratively optimizing the weighting coefficient matrix in the matrix control structure based on impedance sensitivity and synchronously connecting a low-pass filter in series with the voltage and current feedback channels to perform impedance reshaping to achieve broadband oscillation risk suppression include: Calculate the amplitude sensitivity and phase sensitivity of each element in the weighted coefficient matrix to the equivalent positive sequence impedance on the wind farm side to obtain the impedance sensitivity; Taking the direction with the highest impedance sensitivity as the search direction, the elements in the weighted coefficient matrix are subjected to gradient optimization iteration, and the stability margin is recalculated after each iterative optimization; When the stability margin is not lower than a preset margin threshold, stopping the gradient optimization iteration of the elements in the weight coefficient matrix and outputting the optimized weight coefficient matrix; A low-pass filter is selected according to the frequency characteristics and broadband oscillation suppression requirements of the offshore wind power flexible direct current grid-connected 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 simultaneously enabled to reshape the impedance and suppress the risk of broadband oscillation.

8. A method for suppressing broadband oscillations in an offshore wind power flexible direct current grid-connected system according to claim 7, characterized in that: The step of synchronously enabling the low-pass filter in the voltage and current feedback channel to perform impedance reshaping includes: Collecting three-phase voltage signals and three-phase current signals of the grid connection point of the offshore wind power flexible direct current grid-connected system, and performing Park transformation on the three-phase voltage signals to obtain quadrature and direct axis vector voltage signals; Performing a Park transformation on the three-phase current signal to obtain a quadrature and direct axis vector current signal; Connecting low-pass filters in series in the feedback channels of the AC and DC axis vector voltage signals and the AC and DC axis vector current signals respectively to dynamically reshape the control loop to obtain the AC and DC axis components of the voltage and current; The instantaneous power measurement value is calculated according to the voltage and current quadrature and direct axis components, 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 direct current grid-connected system.

9. A broadband oscillation suppression system for an offshore wind power flexible direct current grid-connected system, characterized in that: The system comprises: The data acquisition module is used to sequentially inject positive and negative sequence perturbation sinusoidal harmonic signals at the wind turbine grid connection point and synchronously collect the voltage and current harmonic response signals at the grid connection point; A harmonic analysis module, configured to perform fast Fourier analysis on the voltage and current harmonic response signals of the grid connection point to obtain measured impedance sweep frequency data; An 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 reduce the multi-input multi-output impedance matrix to an equivalent positive-sequence impedance of a single input and a single output; A stability analysis module is used to construct a Bode diagram 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 amplitude, and quantify the stability margin of the offshore wind power flexible direct current grid-connected system based on the phase difference; a reconstruction control module, configured to reconstruct the wind turbine grid-side converter into a matrix control structure by cascading a weighted coefficient matrix between inner and outer 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 weighted coefficient matrix in the matrix control structure based on impedance sensitivity, and simultaneously connect a low-pass filter in series with the voltage and current feedback channels to perform impedance reshaping to achieve broadband oscillation risk suppression.

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

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