Method for suppressing wide frequency oscillation of doubly-fed wind turbine based on additional current impedance reshaping

Abstract

The application discloses a method for suppressing wide-frequency oscillation of a doubly-fed wind turbine based on additional current impedance remodeling, which comprises the following steps: step S1, based on a doubly-fed wind turbine grid-connected system, a grid-connected model of the coupling of the doubly-fed wind turbine and the power grid is constructed, and the total impedance of the grid-connected model is calculated; step S2, based on the total impedance, an impedance analysis method is adopted to determine the oscillation frequency band to be suppressed in the grid-connected model; step S3, based on the oscillation frequency band, the phase margin of the grid-connected model is calculated, and the oscillation suppression control parameter is set according to the phase margin; and step S4, the impedance remodeling branch based on the dq-axis current feedback is added on the basis of the traditional VSC voltage and current double-loop control, the impedance remodeling branch is configured to form an additional current control strategy, and the capacitive negative damping in the grid-connected model is offset.

Description

Technical Field

[0001] This invention relates to the field of broadband oscillation analysis and suppression technology, and in particular to a method for suppressing grid-connected broadband oscillations of doubly fed wind turbines based on additional current impedance reshaping. Background Technology

[0002] As the scale of grid connection of renewable energy sources such as wind power continues to expand, double-fed induction generators have become the mainstream model of wind power generation due to their advantages such as good economic efficiency and flexible control. However, due to the influence of resource distribution, wind farms are mostly located in remote areas and need to be connected to the grid through long-distance transmission lines, resulting in the grid exhibiting weak grid characteristics of high impedance and low inertia.

[0003] Under weak grid conditions, when doubly-fed induction generators (DFIGs) are connected to the grid via power electronic converters, they are prone to interacting with the grid, causing wide-frequency oscillations that seriously threaten the safe and stable operation of the power system. Therefore, an impedance model of the DFIG grid-connected system and an equivalent impedance model for the weak grid can be established to accurately characterize the system's wide-frequency dynamic characteristics and provide a basic model support for oscillation mechanism analysis. Based on the impedance model, the traditional grid-connected DFIG control strategy relies on phase-locked loops (PLLs) for synchronization. Its output impedance exhibits capacitive negative damping characteristics over a wide frequency range, and interaction with the inductive impedance of the weak grid easily leads to phase instability. Existing improvement methods, such as PLL parameter optimization and current loop bandwidth adjustment, can improve system stability to some extent, but parameter optimization comes at the cost of dynamic performance and is difficult to adapt to wide-range variations in grid impedance. Adding active dampers or static var compensators can suppress oscillations, but requires additional equipment, resulting in high engineering costs and poor economic efficiency. Traditional subsynchronous damping controllers are mostly designed based on specific oscillation frequencies, exhibiting poor frequency adaptability; when the oscillation frequency deviates, the suppression effect decreases significantly.

[0004] Therefore, there is an urgent need to study a broadband oscillation suppression method that is highly adaptable, economical, and does not require modification of the original doubly fed fan control structure. Summary of the Invention

[0005] In view of the above-mentioned prior art, the present invention provides a method for suppressing grid-connected broadband oscillations of doubly fed wind turbines based on additional current impedance reshaping, which mainly solves the technical problems existing in the above-mentioned background art.

[0006] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping includes the following steps: Step S1: Based on the doubly fed wind turbine grid-connected system, construct a grid-connected model of the doubly fed wind turbine coupled with the power grid, and calculate the total impedance of the grid-connected model; Step S2: Based on the total impedance, use impedance analysis to determine the oscillation frequency band that needs to be suppressed in the grid-connected model; Step S3: Based on the oscillation frequency band, calculate the phase margin of the grid-connected model, and adjust the oscillation suppression control parameters according to the phase margin; Step S4: Using the oscillation suppression control parameters, an impedance reshaping branch based on dq-axis current feedback is added to the traditional VSC voltage and current dual closed-loop control. The impedance reshaping branch is configured to form an additional current control strategy to offset the capacitive negative damping in the grid-connected model.

[0007] Optionally, step S1 includes: Construct an impedance model for the doubly-fed induction generator (DFIG) wind turbine and an impedance model for the power grid; the expression for the DFIG impedance model is as follows:

[0008] Among them, F u For voltage feedforward coupling function; F i R is the current loop control function; s L s These are the stator's resistance and inductance, respectively; R r L r , respectively, represent the rotor's resistance and inductance; s is the Laplace operator. For the mutual inductance of the stator and rotor; The equivalent impedance of the doubly-fed fan; The expression for the power grid impedance model is as follows:

[0009] Among them, Z g (s) represents the equivalent impedance on the grid side; R g L is the equivalent series resistance of the power grid. g The equivalent series inductance of the power grid is s; s is the Laplace operator; C g For series compensation capacitors; Based on the doubly-fed induction generator (DFIG) impedance model and the grid impedance model, the total impedance of the grid-connected model of the DFIG and grid coupling is obtained, and the expression for the total impedance is:

[0010] Where Z(s) is the total impedance; Z g (s) represents the equivalent impedance on the grid side; Z DFIG (s) represents the equivalent impedance of the doubly fed fan.

[0011] Optionally, step S2 includes: Step S21: Perform frequency domain transformation on the total impedance to obtain the equivalent impedance of the doubly-fed wind turbine and grid coupling after frequency domain transformation, expressed as:

[0012] Where j is the imaginary unit; Re[·] represents the real part of the impedance; Im[·] represents the imaginary part of the impedance. This represents the real part of the equivalent impedance of a doubly-fed wind turbine. This represents the imaginary part of the equivalent impedance of a doubly-fed wind turbine. Let the real part be the equivalent impedance of the power grid. This represents the imaginary part of the power grid's equivalent impedance. Equivalent impedance; Step S22: Based on the equivalent impedance of the doubly fed wind turbine and the power grid coupling, the frequency oscillation point is determined using the RLC series resonance principle within a preset wide-frequency sweep range. ; Step S23: Based on the frequency oscillation point, determine whether there is a risk of continuous divergent oscillation instability in the doubly fed wind turbine grid-connected system, and determine the oscillation frequency band that needs to be suppressed.

[0013] Optionally, step S23 includes: Calculate the phase angle difference at the frequency oscillation point; If the phase angle difference is greater than 180°, then at the frequency oscillation point, extract the real part of the equivalent impedance of the doubly fed wind turbine and the real part of the equivalent impedance of the power grid respectively, and calculate the sum of their real parts; If the sum of the real parts of the two is less than zero, it is determined that the doubly fed wind turbine grid-connected system has capacitive negative damping characteristics, and there is a risk of continuous divergent oscillation instability. The oscillation frequency band that needs to be suppressed is then determined.

[0014] Optionally, step S3 includes: Based on the oscillation frequency band, calculate the phase margin of the grid-connected model at the oscillation point of the frequency; The frequency band sweep traversal calculation is performed with the goal of maximizing the phase margin at the frequency oscillation point and the constraint that the phase margin at the oscillation frequency is greater than 30 degrees. The oscillation suppression control parameters that meet the constraints and have the largest phase margin are then selected.

[0015] Optionally, the oscillation suppression control parameters include: cutoff frequency, gain coefficient, current inner loop proportional coefficient and integral coefficient, and voltage outer loop proportional coefficient and integral coefficient; The cutoff frequency is 2Hz, the gain coefficient is 0.95, the current inner loop proportional coefficient is 10, the current inner loop integral coefficient is 50, the voltage outer loop proportional coefficient is 0.5, and the voltage outer loop integral coefficient is 20.

[0016] Optionally, step S4 includes: Step S41: Obtain the DC bus voltage reference value and reactive power reference value, and obtain the d-axis current reference value and q-axis current reference value; Step S42: Collect the three-phase current at the VSC grid connection point and obtain the d-axis current component and q-axis current component through phase loop synchronous Park transformation; Step S43: Construct an impedance reshaping branch based on dq-axis current feedback, and obtain additional current reference values ​​according to the impedance reshaping branch, the cutoff frequency, and the gain coefficient. The additional current reference values ​​include d-axis and q-axis additional current reference values. Step S44: Based on the d-axis and q-axis current reference values, the d-axis current component and the q-axis current component, the additional current reference value, the current inner loop proportional coefficient and integral coefficient, generate the d-axis adjustable voltage component and the q-axis adjustable voltage component using the current inner loop PI controller; Step S45: After superimposing the wl cross-decoupling term and the grid connection point voltage feedforward term on the d-axis adjustable voltage component and the q-axis adjustable voltage component respectively, the d-axis voltage reference value and the q-axis voltage reference value are generated according to the voltage outer loop proportional coefficient and integral coefficient. Step S46: Send the d-axis voltage reference value and q-axis voltage reference value into SVPWM respectively, drive VSC to output a compensation current that is opposite to the oscillation to the PCC port, so as to counteract the capacitive negative damping.

[0017] Optionally, step S43 includes: The impedance reshaping branch includes a symmetrical d-axis damping branch and a q-axis damping branch; both the d-axis damping branch and the q-axis damping branch are equipped with an adder, a low-pass filter, and a proportional amplifier. The d-axis current component and q-axis current component are respectively passed through the low-pass filter to remove harmonic components, resulting in the filtered d-axis DC component and q-axis DC component. The d-axis oscillating current component and the q-axis oscillating current component are obtained by calculating the d-axis current component, the d-axis DC component, the q-axis current component, and the q-axis DC component using the adder, respectively. The d-axis oscillating current component and the q-axis oscillating current component are respectively input into the proportional amplifier. The d-axis oscillating current component and the q-axis oscillating current component are multiplied by the gain coefficient to generate the d-axis additional current reference value and the q-axis additional current reference value.

[0018] Optionally, the expressions for the d-axis oscillating current component and the q-axis oscillating current component obtained in step S42 are as follows:

[0019] in, , , For the three-phase current at the grid connection point, , These are the d-axis and q-axis current components obtained after Park transformation of the three-phase current at the grid connection point;

[0020] in, , These are the filtered d-axis DC component and q-axis DC component, respectively. This represents the convolution operator. Let be the time-domain impulse response function of the low-pass filter;

[0021] in, , Let be the d-axis oscillating current component and the q-axis oscillating current component, respectively;

[0022]

[0023] in, Add a reference value for the d-axis current. Add a current reference value to the q-axis. This is the gain coefficient.

[0024] The beneficial effects of this invention are as follows: Compared with the traditional subsynchronous damping controller that needs to be designed for a specific frequency, the doubly fed wind turbine grid-connected broadband oscillation suppression method based on additional current impedance reshaping provided by this invention does not require a phase compensation stage and can adaptively extract oscillation components within the subsynchronous broadband band to achieve broadband impedance reshaping; at the same time, the parameter tuning range is clear, the engineering applicability is strong, and the adaptability under a wide range of grid impedance changes is significantly improved.

[0025] This invention first establishes a sequence impedance model and a weak grid equivalent impedance model for a doubly-fed induction generator (DFIG) grid-connected system. Using impedance analysis, it reveals the mechanism and risk of broadband oscillations caused by capacitive negative damping and interaction with grid impedance in the DFIG grid-connected system over a wide frequency band due to phase-locked loop (PLL) control, thus clarifying the root cause of the oscillations. Furthermore, by sweeping the frequency through the cutoff frequency, gain coefficient, and inner and outer loop PI parameters of the VSC impedance reshaping branch, the system phase margin under each set of parameters is calculated. The influence of each parameter on stability is quantitatively analyzed, significantly clarifying the reasonable range of parameter tuning values ​​and solving the problem of traditional parameter tuning relying on empirical trial and error.

[0026] This invention adds an additional current impedance reshaping branch to the traditional VSC voltage and current dual closed-loop control, forming an additional current control strategy. This increases the positive damping of the system, effectively suppressing wideband oscillations and avoiding the instability risk caused by wideband oscillations under weak grid conditions. It significantly improves the adaptability under wide range of grid impedance changes, thereby improving the operational reliability of the doubly fed wind turbine grid-connected system. Ultimately, it achieves wideband oscillation suppression and stable operation of the doubly fed wind turbine grid-connected system under weak grid conditions. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating the method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping provided in an embodiment of the present invention. Figure 2 This is a VSC control structure diagram of the impedance reshaping branch provided in an embodiment of the present invention; Figure 3 This is a graph showing the variation of the amplitude and phase characteristics of the reshaped admittance with the cutoff frequency in an embodiment of the present invention. Figure 4 This is a graph showing the variation of the amplitude and phase characteristics of the reshaped admittance with the gain coefficient in an embodiment of the present invention. Figure 5 The impedance sweep frequency test results of the machine-side converter after impedance reshaping provided in the embodiments of the present invention; Figure 6 These are experimental waveforms before and after adding the impedance reshaping branch, as provided in this embodiment of the invention. Detailed Implementation

[0028] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. In the following description, the expression "some embodiments" refers to a subset of all possible embodiments; however, it should be understood that "some embodiments" can be the same subset or different subsets of all possible embodiments and can be combined with each other without conflict.

[0029] To fully understand this invention, a detailed structure will be presented in the following description to illustrate the technical solution proposed by this invention. Optional embodiments of the invention are described in detail below; however, in addition to these detailed descriptions, the invention may have other embodiments.

[0030] Example Please refer to the attached document. Figure 1 This application provides a method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping, including the following steps: Step S1: Based on the doubly fed wind turbine grid-connected system, construct a grid-connected model of the doubly fed wind turbine coupled with the power grid, and calculate the total impedance of the grid-connected model; Step S2: Based on the total impedance, use impedance analysis to determine the oscillation frequency band that needs to be suppressed in the grid-connected model; Step S3: Based on the oscillation frequency band, calculate the phase margin of the grid-connected model, and adjust the oscillation suppression control parameters according to the phase margin; Step S4: Using the oscillation suppression control parameters, an impedance reshaping branch based on dq-axis current feedback is added to the traditional VSC voltage and current dual closed-loop control. The impedance reshaping branch is configured to form an additional current control strategy to offset the capacitive negative damping in the grid-connected model.

[0031] Specifically, the grid-connected model of the doubly-fed induction generator (DFIG) wind turbine and its grid connection in this invention is constructed based on the DFIG grid-connected system. The topology of the DFIG grid-connected system includes an equivalent grid-side unit, a DFIG power generation unit, and a VSC converter unit. These three are electrically coupled through a common PCC grid connection point. The specific topology connection relationship is as follows: the AC grid is equivalent to an ideal voltage source U. grid The power grid is connected in series, and the grid is equivalent to the grid-side equivalent impedance Zg(s) before being connected to the PCC common bus. Two independent branches are simultaneously connected in parallel on this PCC common bus. The first branch is the equivalent branch of the doubly-fed induction generator (DFIG) at the entire unit port, equivalent to the DFIG's equivalent impedance ZDFIG(s). The second branch is the VSC converter branch, equivalent to the VSC's equivalent admittance Y. VSC (s); The two ends of the ZDFIG(s) branch and the YVSC(s) branch are the same PCC bus voltage Upcc. After the currents of the two branches converge at the PCC node, they flow to the AC grid side through the grid impedance Zg(s). Therefore, in step S1, based on this topological relationship, a grid-connected model of the doubly fed wind turbine and the power grid is constructed, and the equivalent impedance of the doubly fed wind turbine and the equivalent impedance of the power grid side are calculated to obtain the total impedance of the grid-connected model of the doubly fed wind turbine and the power grid, which provides a basis for subsequent judgment of broadband oscillation. In step S2, based on the total impedance, frequency domain decomposition and full-band frequency sweep are performed to locate the inherent oscillation frequency band of the doubly-fed induction generator (DFIG) exhibiting negative damping, providing a basis for the design of impedance reshaping branch parameters in subsequent steps. Therefore, in order to change the reshaping component in the VSC admittance, step S3 finds the optimal combination of oscillation suppression control parameters. In step S4, the optimal oscillation suppression control parameters are applied to the added impedance reshaping branch based on dq-axis current feedback. The additional current control strategy is implemented through the impedance reshaping branch to obtain a compensation current that is opposite to the vibration. The compensation current can offset the capacitive negative damping of the DFIG, thereby reshaping the system impedance and achieving the effect of suppressing grid-connected broadband oscillation, making the DFIG grid-connected system more stable and reliable under weak grid conditions.

[0032] As an optional implementation, step S1 includes: Construct an impedance model for the doubly-fed induction generator (DFIG) wind turbine and an impedance model for the power grid; the expression for the DFIG impedance model is as follows:

[0033] Among them, F u For voltage feedforward coupling function; F i R is the current loop control function; s L s These are the stator's resistance and inductance, respectively; R r L r , respectively, represent the rotor's resistance and inductance; s is the Laplace operator. For the mutual inductance of the stator and rotor; The equivalent impedance of the doubly-fed fan; The expression for the power grid impedance model is as follows:

[0034] Among them, Z g (s) represents the equivalent impedance on the grid side; R g L is the equivalent series resistance of the power grid. g The equivalent series inductance of the power grid is s; s is the Laplace operator; C g For series compensation capacitors; Based on the doubly-fed induction generator (DFIG) impedance model and the grid impedance model, the total impedance of the grid-connected model of the DFIG and grid coupling is obtained, and the expression for the total impedance is:

[0035] Where Z(s) is the total impedance; Z g (s) represents the equivalent impedance on the grid side; Z DFIG (s) represents the equivalent impedance of the doubly fed fan.

[0036] Specifically, the impedance characteristics of a doubly-fed induction generator (DFIG) are mainly determined by the turbine-side converter, from which the stator and rotor voltage and current relationships can be obtained. The expression for the stator and rotor voltage and current relationships is as follows:

[0037] Among them, u s i s For the stator voltage and current; R s L s For stator resistance and inductance; R r L r For rotor resistance and inductance; u r1 i r1 The rotor voltage and current are calculated based on frequency; K is the turns ratio of the stator and rotor; σ is the slip; and s is the Laplace operator. For the mutual inductance of the stator and rotor; Based on the current loop control structure, the d-axis modulation signal m rd It can be represented as:

[0038] Among them, i rd,ref This is the reference value for the rotor current; i rd K is the rotor current of the d-axis. rd H is the rotor current decoupling coefficient; i (s) is the transfer function of the current loop PI controller; The expression for the frequency offset relationship is:

[0039] in, It is the rest frequency. It is the rest frequency. For the fundamental frequency; By simultaneously solving and eliminating equations based on the stator and rotor voltage and current relationships, the current loop control equations, and the frequency offset relationship, the impedance model of the doubly-fed induction generator (DFIG) can be obtained. The expression for the DFIG impedance model is as follows:

[0040] Among them, F u For voltage feedforward coupling function; F i R is the current loop control function; s L s These are the stator's resistance and inductance, respectively; R r L r , respectively, represent the rotor's resistance and inductance; s is the Laplace operator. For the mutual inductance of the stator and rotor; The equivalent impedance of the doubly-fed fan; Next, a power grid impedance model is constructed. A power grid typically includes R... g L g With respect to the effective value C of the grid capacitance g , will R g L g After being connected in series with C g By connecting in parallel, we obtain the power grid impedance model, which is expressed as:

[0041] Among them, Z g (s) represents the equivalent impedance on the grid side; R g L is the equivalent series resistance of the power grid. g The equivalent series inductance of the power grid is s; s is the Laplace operator; C g For series compensation capacitors;

[0042] Where δ is the short-circuit ratio; This refers to the rated capacity of the doubly fed fan system. For the fundamental frequency; This is the effective value of the grid voltage; Based on the doubly-fed induction generator (DFIG) impedance model and the grid impedance model, the total impedance of the grid-connected model of the DFIG and the grid coupling is obtained. The total impedance can be expressed as:

[0043] Where Z(s) is the total impedance; Z g (s) represents the equivalent impedance on the grid side; Z DFIG (s) represents the equivalent impedance of the doubly fed fan.

[0044] As an optional implementation, step S2 includes: Step S21: Perform frequency domain transformation on the total impedance to obtain the equivalent impedance of the doubly-fed wind turbine and grid coupling after frequency domain transformation, expressed as:

[0045] Where j is the imaginary unit; Re[·] represents the real part of the impedance; Im[·] represents the imaginary part of the impedance. This represents the real part of the equivalent impedance of a doubly-fed wind turbine. This represents the imaginary part of the equivalent impedance of a doubly-fed wind turbine. Let the real part be the equivalent impedance of the power grid. This represents the imaginary part of the power grid's equivalent impedance. Equivalent impedance; Step S22: Based on the equivalent impedance of the doubly fed wind turbine and the power grid coupling, the frequency oscillation point is determined using the RLC series resonance principle within a preset wide-frequency sweep range. ; Step S23: Based on the frequency oscillation point, determine whether there is a risk of continuous divergent oscillation instability in the doubly fed wind turbine grid-connected system, and determine the oscillation frequency band that needs to be suppressed.

[0046] Step S23 includes: Calculate the phase angle difference at the frequency oscillation point; If the phase angle difference is greater than 180°, then at the frequency oscillation point, extract the real part of the equivalent impedance of the doubly fed wind turbine and the real part of the equivalent impedance of the power grid respectively, and calculate the sum of their real parts; If the sum of the real parts of the two is less than zero, it is determined that the doubly fed wind turbine grid-connected system has capacitive negative damping characteristics, and there is a risk of continuous divergent oscillation instability. The oscillation frequency band that needs to be suppressed is then determined.

[0047] Specifically, first, the total impedance obtained in step 1 is transformed in the frequency domain. Let s = j2πf, and extract the real and imaginary parts respectively to obtain the equivalent impedance after frequency domain transformation. The equivalent impedance includes the real part of the doubly-fed wind turbine equivalent impedance, the imaginary part of the doubly-fed wind turbine equivalent impedance, the real part of the power grid equivalent impedance, and the imaginary part of the power grid equivalent impedance. According to the RLC series resonance principle, when the imaginary part of the equivalent impedance of the doubly-fed induction generator (DFIG) and the imaginary part of the equivalent impedance of the grid are both zero, the DFIG grid-connected system is in a resonant state, and the frequency at which it is in the resonant state is the frequency oscillation point. When the equivalent impedance of the doubly-fed induction generator (DFIG) interacts with the equivalent impedance of the power grid, capacitive negative damping occurs in the mid-to-high frequency range, leading to instability with a phase difference exceeding 180°. At the frequency oscillation point, the equivalent impedance of the DFIG and the equivalent impedance of the power grid satisfy the following relationship:

[0048] in , These represent the frequency oscillation points, respectively. The real part of the equivalent impedance of the doubly fed wind turbine and the real part of the equivalent impedance of the power grid are given. In other words, at the frequency oscillation point, if the sum of the real part of the equivalent impedance of the doubly-fed induction generator (DFIG) and the real part of the equivalent impedance of the grid is less than zero, that is, within this frequency band, the real part of the grid's equivalent impedance is insufficient to offset the capacitive negative resistance presented by the DFIG. The DFIG grid-connected system as a whole exhibits negative damping and is at risk of oscillation divergence. If the capacitive negative resistance of the DFIG grid-connected system is not offset, it will lead to broadband oscillation divergence, triggering the turbine to disconnect from the grid and limiting the wind power transmission capacity under weak grid conditions.

[0049] To find the frequency oscillation point, specifically: the preset wideband sweep range is [ , ]( This represents the minimum value within the sweep frequency range; (The maximum value of the sweep frequency range), the amplitude difference at each frequency is calculated point by point with a fixed step size Δf. :

[0050] Traverse all scan frequencies and find the one that satisfies the requirement. For adjacent frequency points (where k is the discrete sweep point index), if the amplitude difference changes from positive to negative or from negative to positive within this interval, then there exists an intersection frequency point. The intersection frequency point can be solved using interpolation formulas. :

[0051] The intersection frequency point obtained by solving is the frequency oscillation point. That is, satisfying Frequency points; Next, calculate the phase angle difference at the oscillation point of that frequency. The calculation formula is:

[0052] in, The phase angle difference at the frequency oscillation point, where arg represents the impedance phase angle; when A phase angle greater than 180° indicates that the phase condition for oscillatory instability is met; Finally, the real part of the impedance at the oscillation point of that frequency is extracted, and the criteria are applied. This allows for a quantitative determination of whether a frequency band exhibits capacitive negative damping characteristics. If such characteristics are found, then the frequency band is an oscillation band that needs to be suppressed.

[0053] As an optional implementation, step S3 includes: Based on the oscillation frequency band, calculate the phase margin of the grid-connected model at the oscillation point of the frequency; The frequency band sweep traversal calculation is performed with the goal of maximizing the phase margin at the frequency oscillation point and the constraint that the phase margin at the oscillation frequency is greater than 30 degrees. The oscillation suppression control parameters that meet the constraints and have the largest phase margin are then selected.

[0054] Specifically, the phase margin is the phase margin of the loop gain of a doubly-fed induction generator (DFIG) grid-connected system at the amplitude crossover frequency, used to characterize the system's stability. A phase margin greater than 0° indicates system stability; a phase margin less than 0° indicates system instability and a tendency to oscillate and diverge; a larger phase margin value indicates a stronger ability of the system to resist disturbances. Based on the oscillation frequency band determined in step S2 and the negative damping characteristics within that band, the frequency sweep range is determined to be 0.1. ~10 The phase margin at the frequency oscillation point is calculated under various combinations of oscillation suppression control parameters, and the expression is:

[0055] in, For phase margin, Zg is the equivalent impedance of the doubly fed wind turbine; Zg is the equivalent impedance of the grid side; arg[·] is the phase angle taken as a complex number; After sweeping through all oscillation suppression control parameters, the objective is to maximize the phase margin, with the constraint condition as follows: With a stable phase margin greater than 30°, the optimal oscillation suppression control parameters are selected.

[0056] As an optional implementation, the oscillation suppression control parameters include: cutoff frequency, gain coefficient, current inner loop proportional coefficient and integral coefficient, and voltage outer loop proportional coefficient and integral coefficient. The cutoff frequency is 2Hz, the gain coefficient is 0.95, the current inner loop proportional coefficient is 10, the current inner loop integral coefficient is 50, the voltage outer loop proportional coefficient is 0.5, and the voltage outer loop integral coefficient is 20.

[0057] Specifically, the cutoff frequency mainly affects the low-frequency and subsynchronous frequency characteristics of the reshaped admittance. As the cutoff frequency increases, the amplitude of the reshaped admittance gradually decreases, and it exhibits negative resistance characteristics in the varying region. To minimize the negative resistance region, the cutoff frequency is selected... At this point, the doubly fed wind turbine grid-connected system exhibits optimal stability and the largest phase margin, thus meeting the stability margin requirements.

[0058] Please refer to the attached document. Figure 3-4 Gain coefficient It has almost no effect on the additional admittance, but it will change the magnitude of the reshaping admittance; as the gain factor increases, the magnitude of the reshaping admittance increases, so a smaller value needs to be selected to reduce the influence of the negative resistance; too small a value is not conducive to impedance reshaping; therefore, selecting... =0.95, ensuring sufficient phase margin while avoiding the influence of excessive negative resistance; In power electronic converter control, the tuning of inner and outer loop PI parameters follows the principle of tuning the inner current loop first, followed by the outer voltage loop. The outer loop bandwidth should be much lower than the inner loop bandwidth to avoid loop coupling leading to oscillation. The proportional coefficient of the inner current loop is set to [0.1, 10], and the integral coefficient is set to [10, 500]. The proportional coefficient of the outer voltage loop is set to [0.1, 2], and the integral coefficient is set to [5, 100]. The inner current loop PI parameters are tuned according to the typical current loop bandwidth design principle. In this embodiment, the proportional coefficient of the inner current loop is set to 10, and the integral coefficient is set to 50 to ensure fast tracking performance. The outer voltage loop parameters are set according to the bandwidth separation principle, with a proportional coefficient of 0.5 and an integral coefficient of 20 to maintain DC voltage stability.

[0059] As an optional implementation method, please refer to the appendix. Figure 2 Step S4 includes: Step S41: Obtain the DC bus voltage reference value and reactive power reference value, and obtain the d-axis current reference value and q-axis current reference value; Step S42: Collect the three-phase current at the VSC grid connection point and obtain the d-axis current component and q-axis current component through phase loop synchronous Park transformation; Step S43: Construct an impedance reshaping branch based on dq-axis current feedback, and obtain additional current reference values ​​according to the impedance reshaping branch, the cutoff frequency, and the gain coefficient. The additional current reference values ​​include d-axis and q-axis additional current reference values. Step S44: Based on the d-axis and q-axis current reference values, the d-axis current component and the q-axis current component, the additional current reference value, the current inner loop proportional coefficient and integral coefficient, generate the d-axis adjustable voltage component and the q-axis adjustable voltage component using the current inner loop PI controller; Step S45: After superimposing the wl cross-decoupling term and the grid connection point voltage feedforward term on the d-axis adjustable voltage component and the q-axis adjustable voltage component respectively, the d-axis voltage reference value and the q-axis voltage reference value are generated according to the voltage outer loop proportional coefficient and integral coefficient. Step S46: Send the d-axis voltage reference value and q-axis voltage reference value into SVPWM respectively, drive VSC to output a compensation current that is opposite to the oscillation to the PCC port, so as to counteract the capacitive negative damping.

[0060] Specifically, the additional current control strategy is implemented through steps S41-S46, as follows: First, the d-axis control loop uses DC voltage outer loop control, with the DC bus voltage reference value as the reference value. The outer loop input obtains the d-axis current reference value to stabilize the DC bus voltage; the q-axis control loop uses reactive power outer loop control, with the reactive power reference value... The outer loop input is used to obtain the q-axis current reference value to achieve grid-connected reactive power regulation; the three-phase current is collected at the VSC grid connection point, and the three-phase current is transformed by coordinate transformation to obtain the d-axis current component and the q-axis current component; Subsequently, an impedance reshaping branch based on dq-axis current feedback is constructed. Both the d-axis and q-axis current components are split into two paths. One path is fed into the inner current loop for fundamental frequency control, while the other path is fed into a second-order Butterworth low-pass filter with a cutoff frequency of 2Hz. Utilizing the characteristic that the fundamental dq-axis component is DC and the wideband oscillation component is AC, the fundamental dq-axis component is filtered out to extract the wideband oscillation component. This wideband oscillation component is multiplied by a gain factor of 0.95 to generate additional current reference values ​​for the d and q axes. Based on the proportional and integral coefficients of the inner current loop, a PI controller in the inner current loop generates d-axis and q-axis adjustable voltage components. The d-axis and q-axis adjustable voltage components are then superimposed with a wl cross-decoupling term and a grid-connected point voltage feedforward term, respectively. Combined with the proportional and integral coefficients of the outer voltage loop, a PI controller in the outer voltage loop generates d-axis and q-axis voltage reference values. Finally, these d-axis and q-axis voltage reference values ​​are fed into the SVPWM module to drive the VSC. The output is a compensation current that is out of phase with the oscillation; this compensation current is equivalent to a parallel positive resistance at the system port, which cancels the capacitive negative damping of the doubly fed wind turbine, thereby achieving wideband impedance reshaping. The small-signal change in the additional current reference value output by the impedance reshaping branch (the small-signal change in the additional current reference values ​​of the d-axis and q-axis), after being input into the inner current loop, drives the VSC to output a physical compensation current that is out of phase with the oscillation. The expression for the small-signal change in the compensation current reference value is:

[0061] in, For use as a low-pass filter in the impedance reshaping branch; The transformer turns ratio at the VSC output; For the current measured at the grid connection point; This refers to the small-signal disturbance of the current measured at the grid connection point. This is the gain coefficient; This is the small-signal change in the additional current reference value output by the impedance reshaping branch.

[0062] As an optional implementation, step S43 includes: The impedance reshaping branch includes a symmetrical d-axis damping branch and a q-axis damping branch; both the d-axis damping branch and the q-axis damping branch are equipped with an adder, a low-pass filter, and a proportional amplifier. The d-axis current component and q-axis current component are respectively passed through the low-pass filter to remove harmonic components, resulting in the filtered d-axis DC component and q-axis DC component. The d-axis oscillating current component and the q-axis oscillating current component are obtained by calculating the d-axis current component, the d-axis DC component, the q-axis current component, and the q-axis DC component using the adder, respectively. The d-axis oscillating current component and the q-axis oscillating current component are respectively input into the proportional amplifier. The d-axis oscillating current component and the q-axis oscillating current component are multiplied by the gain coefficient to generate the d-axis additional current reference value and the q-axis additional current reference value.

[0063] Specifically, to quantitatively characterize the dynamic coupling relationship between various electrical quantities and control quantities, a small-signal equation for the VSC control system is established based on the decoupled dq-axis current feedback form, and its expression is:

[0064]

[0065]

[0066]

[0067]

[0068]

[0069]

[0070] in, , These represent the small-signal changes in the voltage and current reference values, respectively, in the dq coordinate system. ΔU is the small-signal change in the output current of the VSC AC side; ΔU is the small-signal change in the grid connection point voltage. This refers to the small-signal voltage disturbance at the output of the VSC inverter. The small-signal change in current output from the impedance reshaping branch; The transfer function matrix for the voltage outer loop PI controller; The transfer function matrix for the inner current loop PI controller; The coupling coefficient matrix is ​​formed by the steady-state voltages at the grid connection point; The coupling coefficient matrix is ​​composed of the steady-state currents at the grid connection point; This is the dq-domain cross-coupling matrix corresponding to the filter inductor; is the transfer function matrix corresponding to the dynamic characteristics of the DC side; s is the Laplace operator; , These are the proportional and integral coefficients of the voltage loop PI controller, respectively. , These are the proportional and integral coefficients of the current loop PI controller, respectively. , These are the steady-state values ​​of the VSC inverter output voltage along the d-axis and q-axis in the dq coordinate system, respectively. , These are the steady-state values ​​of the output current of the VSC inverter along the d-axis and q-axis in the dq coordinate system, respectively. Angular frequency; This is the steady-state voltage value on the DC side; Inductor for VSC grid connection point filter; This is the actual value of the DC-side capacitor; This invention, based on the traditional small-signal model, introduces additional compensation currents (reference values ​​of additional currents on the d and q axes) generated by the impedance reshaping branch as new control variables to construct a small-signal equation set adapted to the impedance reshaping strategy. Based on the small-signal equation set, the equivalent impedance of the VSC port is analytically derived: by constructing a compensation amount that is out of phase with the oscillation through the additional damping current on the d and q axes, an equivalent positive resistance is generated in the wide-frequency oscillation band to offset the negative damping characteristics of the doubly-fed wind turbine, thus overcoming the problems of phase distortion and inability to suppress the subsynchronous interactive oscillation between the wind turbine and the weak grid in existing active damping technology solutions.

[0071] The specific steps of step S42 are as follows: The d-axis current component... and q-axis current component Each current component is divided into two paths. One path sends the d-axis and q-axis current components to the negative feedback input of the inner current loop to achieve fundamental active and reactive current control. The other path inputs the d-axis and q-axis current components to the impedance reshaping branch. After passing through a second-order Butterworth low-pass filter, the difference between the input and output is calculated with the original d-axis and q-axis current components to extract the current oscillation components, resulting in the d-axis and q-axis oscillating current components. These components are then passed through a gain circuit to obtain the additional current reference value (d-axis additional current reference value) output by the impedance reshaping branch. and q-axis additional current reference value The d-axis and q-axis additional current reference values ​​are respectively compared with the current reference command output by the outer loop. and The corrected current reference command is obtained by superposition and synthesis. The corrected current reference command is sent to the inner loop PI controller. The inner loop PI controller performs negative feedback closed-loop calculation on the corrected current reference command and the measured d-axis current component and q-axis current component to obtain the current error signal. After the current error signal is adjusted by the inner loop proportional and integral parameters, the d-axis voltage reference value and q-axis voltage reference value are output to realize the fundamental active and reactive current tracking control. When the system oscillates, the impedance reshaping branch generates an additional current reference value in real time according to the oscillation component, dynamically corrects the current reference command, and then reshapes the grid-connected impedance of the converter, adding damping to the system and suppressing subsynchronous oscillation.

[0072] It should be noted that during normal steady-state operation of a doubly-fed induction generator (DFIG) grid-connected system, there are no oscillation components, and the additional current reference value is... , At this point, the corrected current reference command is approximately equal to the current reference command output by the outer loop, and does not affect the steady-state regulation of active and reactive power. When the power grid oscillates and an oscillating current component appears, the impedance reshaping branch detects the oscillating current in real time and generates a corresponding additional current reference value. , The corrected current reference command is obtained by superimposing it onto the original current reference command output from the outer loop. This corrected current reference command is then sent to the inner current loop PI controller, where it forms a negative feedback closed-loop operation with the measured d-axis current component and the measured q-axis current component of the actual output from the VSC to obtain the current error signal. The current error signal is then superimposed through the proportional-integral regulation of the inner current loop PI controller, the cross-coupling compensation of the dq-axis decoupling term, and the superposition of the grid connection point voltage feedforward term to generate the voltage reference command (d-axis voltage reference value and q-axis voltage reference value). This voltage reference command is then transformed by Park to obtain a sinusoidal modulated wave in the three-phase stationary coordinate system. This wave is then modulated by PWM and compared with a triangular carrier wave to generate a switching signal, which drives the IGBT in the VSC to turn on and off, causing the VSC to output the actual three-phase voltage. This actual three-phase voltage is filtered by an inductor to generate an actual current at the grid connection point. This current is injected into the grid on one hand and fed back to the input of the inner current loop after Park transformation on the other hand, forming a complete closed-loop control. Therefore, by dynamically changing the tracking target value of the inner current loop, the converter can follow the updated corrected current reference command and actively adjust the output current under the action of the voltage reference command, injecting a current into the external grid that is opposite to the oscillation trend. The grid-connected impedance of the converter is determined by its own control loop. Under conventional control, the converter exhibits weak damping or even negative damping characteristics in the subsynchronous frequency band, which is prone to oscillation. This invention uses the oscillation signal to drive the additional current command, which effectively changes the external characteristics of the converter and can construct a positive damping impedance in the subsynchronous oscillation frequency band to offset the original negative damping of the system, thereby suppressing oscillation divergence.

[0073] The expressions for the d-axis oscillating current component and the q-axis oscillating current component obtained in step S42 are as follows:

[0074] in, , , For the three-phase current at the grid connection point, , These are the d-axis and q-axis current components obtained after Park transformation of the three-phase current at the grid connection point;

[0075] in, , These are the filtered d-axis DC component and q-axis DC component, respectively. This represents the convolution operator. Let be the time-domain impulse response function of the low-pass filter;

[0076] in, , Let be the d-axis oscillating current component and the q-axis oscillating current component, respectively;

[0077]

[0078] in, Add a reference value for the d-axis current. Add a current reference value to the q-axis. This is the gain coefficient.

[0079] Specifically, three-phase current is collected at the VSC grid connection point, and the d-axis and q-axis current components are obtained through coordinate transformation. These components are then input into a Butterworth second-order low-pass filter. The transfer function of the Butterworth second-order low-pass filter is as follows:

[0080] in, For transfer functions, s is the cutoff frequency of the low-pass filter; s is the Laplace operator; Since the fundamental current dq is transformed into a DC quantity, while the harmonic components are transformed into AC quantities, the d-axis and q-axis current components are passed through a low-pass filter to remove the harmonic components, thus obtaining the filtered d-axis DC component. and q-axis DC component d-axis current component and q-axis current component DC component of the d-axis and q-axis DC component Subtraction effectively filters out the DC fundamental frequency (DC component), extracting the oscillating current component to obtain the d-axis and q-axis oscillating current components. Then, these components are multiplied by a gain coefficient to generate additional d-axis and q-axis current reference values. These additional current reference values ​​are superimposed on the current reference command of the inner current loop. The PI controller in the inner current loop tracks the new reference value and outputs a modulated wave. This wave, driven by PWM, drives the VSC to output a compensation current that is out of phase with the oscillation, thus effectively acting as a parallel positive resistor at the system port—the compensation current output by the VSC. With grid connection point voltage The relationship can be represented as a parallel resistor, expressed as:

[0081] in, For parallel resistors, The voltage at the grid connection point. To compensate for the current; According to the formula Determine if there is a risk of oscillation; if so, increase... The capacitive negative damping of the doubly fed wind turbine is offset by a method that reduces the total impedance to no less than 0, thus achieving impedance reshaping. Therefore, by changing the equivalent impedance of the system port through the additional current control of the VSC, the system impedance is reshaped, and wideband oscillations are suppressed.

[0082] Please refer to the attached document. Figure 5-6 To verify the effectiveness of the proposed method, impedance sweep frequency verification and wideband oscillation effect verification experiments were carried out on the established doubly fed wind turbine grid-connected system model to verify the accuracy and effectiveness of the model and method.

[0083] A doubly fed wind turbine grid-connected system model was built and experiments were conducted in the MATLAB / Simulink simulation environment. The impedance sweep frequency test results of the machine-side converter after impedance reshaping are as follows: Figure 5 As shown. Figure 5 (a) The solid line represents the theoretical positive sequence impedance characteristic curve, and the dot marks represent the positive sequence impedance data points obtained by frequency sweep measurement. Figure 5 (b) The solid line represents the theoretical negative-sequence impedance characteristic curve, and the dotted points represent the negative-sequence impedance data points obtained through frequency sweep measurement. As can be seen from the figure, the marked points obtained by impedance frequency sweep basically coincide with the solid line of the theoretical model, verifying the feasibility of the present invention.

[0084] To visually verify the effectiveness of this invention in suppressing broadband oscillations in a doubly-fed wind turbine grid-connected system, such as... Figure 6As shown, the dynamic response waveforms of the system without and with the impedance reshaping branch proposed in this invention are compared. Wideband oscillations occur under weak grid conditions. However, after using the VSC-based additional current impedance reshaping method proposed in this invention, the oscillations are quickly suppressed, and the waveform returns to a sinusoidal state. This fully demonstrates the significant effect of this invention in suppressing wideband oscillations and improving the adaptability of weak grids.

[0085] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. The scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping, characterized in that, Includes the following steps: Step S1: Based on the doubly fed wind turbine grid-connected system, construct a grid-connected model of the doubly fed wind turbine coupled with the power grid, and calculate the total impedance of the grid-connected model; Step S2: Based on the total impedance, use impedance analysis to determine the oscillation frequency band that needs to be suppressed in the grid-connected model; Step S3: Based on the oscillation frequency band, calculate the phase margin of the grid-connected model, and adjust the oscillation suppression control parameters according to the phase margin; Step S4: Using the oscillation suppression control parameters, an impedance reshaping branch based on dq-axis current feedback is added to the traditional VSC voltage and current dual closed-loop control. The impedance reshaping branch is configured to form an additional current control strategy to offset the capacitive negative damping in the grid-connected model.

2. The method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping according to claim 1, characterized in that, Step S1 includes: Construct an impedance model for the doubly-fed induction generator (DFIG) wind turbine and an impedance model for the power grid; the expression for the DFIG impedance model is as follows: Among them, F u F is the voltage feedforward coupling function; i R is the current loop control function; s L s These are the stator's resistance and inductance, respectively; R r L r , respectively, represent the rotor's resistance and inductance; s is the Laplace operator. For the mutual inductance of the stator and rotor; The equivalent impedance of the doubly-fed fan; The expression for the power grid impedance model is as follows: Among them, Z g (s) represents the equivalent impedance on the grid side; R g L is the equivalent series resistance of the power grid. g The equivalent series inductance of the power grid is s; s is the Laplace operator; C g For series compensation capacitors; Based on the doubly-fed induction generator (DFIG) impedance model and the grid impedance model, the total impedance of the grid-connected model of the DFIG and grid coupling is obtained, and the expression for the total impedance is: Where Z(s) is the total impedance; Z g (s) represents the equivalent impedance on the grid side; Z DFIG (s) represents the equivalent impedance of the doubly fed fan.

3. The method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping according to claim 1, characterized in that, Step S2 includes: Step S21: Perform frequency domain transformation on the total impedance to obtain the equivalent impedance of the doubly-fed wind turbine and grid coupling after frequency domain transformation, expressed as: Where j is the imaginary unit; Re[·] represents the real part of the impedance; Im[·] represents the imaginary part of the impedance. This represents the real part of the equivalent impedance of a doubly-fed wind turbine. This represents the imaginary part of the equivalent impedance of a doubly-fed wind turbine. Let the real part be the equivalent impedance of the power grid. This represents the imaginary part of the power grid's equivalent impedance. Equivalent impedance; Step S22: Based on the equivalent impedance of the doubly fed wind turbine and the power grid coupling, the frequency oscillation point is determined using the RLC series resonance principle within a preset wide-frequency sweep range. ; Step S23: Based on the frequency oscillation point, determine whether there is a risk of continuous divergent oscillation instability in the doubly fed wind turbine grid-connected system, and determine the oscillation frequency band that needs to be suppressed.

4. The method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping according to claim 3, characterized in that, Step S23 includes: Calculate the phase angle difference at the frequency oscillation point; If the phase angle difference is greater than 180°, then at the frequency oscillation point, extract the real part of the equivalent impedance of the doubly fed wind turbine and the real part of the equivalent impedance of the power grid respectively, and calculate the sum of their real parts; If the sum of the real parts of the two is less than zero, it is determined that the doubly fed wind turbine grid-connected system has capacitive negative damping characteristics, and there is a risk of continuous divergent oscillation instability. The oscillation frequency band that needs to be suppressed is then determined.

5. The method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping according to claim 1, characterized in that, Step S3 includes: Based on the oscillation frequency band, calculate the phase margin of the grid-connected model at the oscillation point of the frequency; The frequency band sweep traversal calculation is performed with the goal of maximizing the phase margin at the frequency oscillation point and the constraint that the phase margin at the oscillation frequency is greater than 30 degrees. The oscillation suppression control parameters that meet the constraints and have the largest phase margin are then selected.

6. The method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping according to claim 5, characterized in that, The oscillation suppression control parameters include: cutoff frequency, gain coefficient, current inner loop proportional coefficient and integral coefficient, and voltage outer loop proportional coefficient and integral coefficient; The cutoff frequency is 2Hz, the gain coefficient is 0.95, the current inner loop proportional coefficient is 10, the current inner loop integral coefficient is 50, the voltage outer loop proportional coefficient is 0.5, and the voltage outer loop integral coefficient is 20.

7. The method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping according to claim 1, characterized in that, Step S4 includes: Step S41: Obtain the DC bus voltage reference value and reactive power reference value, and obtain the d-axis current reference value and q-axis current reference value; Step S42: Collect the three-phase current at the VSC grid connection point and obtain the d-axis current component and q-axis current component through phase loop synchronous Park transformation; Step S43: Construct an impedance reshaping branch based on dq-axis current feedback, and obtain additional current reference values ​​according to the impedance reshaping branch, the cutoff frequency, and the gain coefficient. The additional current reference values ​​include d-axis and q-axis additional current reference values. Step S44: Based on the d-axis and q-axis current reference values, the d-axis current component and the q-axis current component, the additional current reference value, the current inner loop proportional coefficient and integral coefficient, generate the d-axis adjustable voltage component and the q-axis adjustable voltage component using the current inner loop PI controller; Step S45: After superimposing the wl cross-decoupling term and the grid connection point voltage feedforward term on the d-axis adjustable voltage component and the q-axis adjustable voltage component respectively, the d-axis voltage reference value and the q-axis voltage reference value are generated according to the voltage outer loop proportional coefficient and integral coefficient. Step S46: Send the d-axis voltage reference value and q-axis voltage reference value into SVPWM respectively, drive VSC to output a compensation current that is opposite to the oscillation to the PCC port, so as to counteract the capacitive negative damping.

8. The method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping according to claim 7, characterized in that, Step S43 includes: The impedance reshaping branch includes a symmetrical d-axis damping branch and a q-axis damping branch; both the d-axis damping branch and the q-axis damping branch are equipped with an adder, a low-pass filter, and a proportional amplifier. The d-axis current component and q-axis current component are respectively passed through the low-pass filter to remove harmonic components, resulting in the filtered d-axis DC component and q-axis DC component. The d-axis oscillating current component and the q-axis oscillating current component are obtained by calculating the d-axis current component, the d-axis DC component, the q-axis current component, and the q-axis DC component using the adder, respectively. The d-axis oscillating current component and the q-axis oscillating current component are respectively input into the proportional amplifier. The d-axis oscillating current component and the q-axis oscillating current component are multiplied by the gain coefficient to generate the d-axis additional current reference value and the q-axis additional current reference value.

9. The method for suppressing grid-connected broadband oscillations of doubly-fed wind turbines based on additional current impedance reshaping according to claim 8, characterized in that, The expressions for the d-axis oscillating current component and the q-axis oscillating current component obtained in step S42 are as follows: in, , , For the three-phase current at the grid connection point, , These are the d-axis and q-axis current components obtained after Park transformation of the three-phase current at the grid connection point; in, , These are the filtered d-axis DC component and q-axis DC component, respectively. This represents the convolution operator. Let be the time-domain impulse response function of the low-pass filter; in, , Let be the d-axis oscillating current component and the q-axis oscillating current component, respectively; in, Add a reference value for the d-axis current. Add a current reference value to the q-axis. This is the gain coefficient.

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