Methods, systems and media for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines

Abstract

The present invention relates to the technical field of smart grids, and discloses a wide-band impedance modeling and dominant parameter identification method, system and medium for an offshore direct-drive wind turbine. Based on the grid-side converter structure of a direct-drive permanent magnet synchronous wind turbine generator set, by establishing a unified small-signal model considering the PLL, dq current control loop, sampling and modulation delay, and LCL filter link, the positive-sequence and negative-sequence impedance analytical expressions of the unit within a wide frequency band are obtained. And a comparative analysis is carried out on the influence of typical parameters such as the PLL bandwidth, grid-connected voltage, LCL filter inductance, and filter capacitance, and their influence on the impedance response is analyzed, and it is identified that the main influence sources of the wide-band impedance of the unit are concentrated on the dynamic process inside the control system. Through the method of the present invention, accurate modeling of the wide-band impedance of a direct-drive wind turbine and identification of the dominant factors can be achieved, providing a theoretical basis and engineering reference for the analysis of the grid-connected stability of the unit, the judgment of the oscillation mechanism, and the setting of control parameters.

Description

Technical Field

[0001] This invention relates to the field of smart grid technology, particularly to the fields of new energy power generation and power electronic converter control. Specifically, it relates to a method, system, and medium for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines, which is applicable to grid-side oscillation analysis, wind turbine controller parameter tuning, and stability assessment. Background Technology

[0002] With the large-scale grid connection of new energy sources, the grid connection control and dynamic characteristics of wind turbine generators have become important factors affecting the safe and stable operation of the power grid. Direct-drive permanent magnet synchronous wind turbine generators (PMSGs) have become one of the main types of turbines in wind farms due to their advantages such as eliminating the gearbox, high reliability, and high efficiency. This type of unit connects to the grid via a full-power converter, and its grid-side impedance characteristics are directly determined by the converter control system. Therefore, it has more significant power electronic characteristics and a wider frequency response range than traditional doubly-fed wind turbine generators.

[0003] With the decrease in grid short-circuit capacity, the increase in grid impedance, and the parallel operation of multiple power electronic devices, wind turbines frequently exhibit subsynchronous oscillations, medium-frequency oscillations, low-frequency oscillations, and high-frequency resonances under weak grid conditions. Since converter impedance plays a key role in the above oscillation mechanisms, accurately obtaining the broadband impedance characteristics of the unit is a prerequisite for conducting grid-connected stability analysis, oscillation risk assessment, and control parameter tuning.

[0004] Existing technologies for impedance modeling of doubly-fed induction generator (DFIG) wind turbines to achieve oscillation identification and suppression are relatively mature in terms of mechanism and control methods. However, for direct-drive turbines using full-power converters, the impedance characteristics are affected by multiple components such as phase-locked loops (PLLs), dq current controllers, sampling and modulation delays, and LCL filters, making the modeling process more complex and covering a wider frequency band. Existing technologies struggle to form a complete impedance analysis method. On the one hand, traditional equivalent models for DFIGs mainly use local models or focus on individual sub-modules, failing to integrate key dynamics such as PLLs, current loops, sampling delays, and modulation components into a unified modeling framework for offshore direct-drive wind turbines. On the other hand, the analysis of the impact of typical parameters such as grid voltage, filter inductance, and filter capacitor lacks systematicity, hindering the identification of key factors affecting turbine impedance. The lack of a dominant parameter identification method based on a unified model makes it difficult to establish clear guidelines for controller parameter adjustments in engineering practice.

[0005] Meanwhile, given the impedance characteristics and equivalent modeling methods of existing doubly fed induction generator (DFIG) wind turbines, the wide-band impedance characteristics fail to fully correspond to the risk scenarios under actual weak power grid conditions, resulting in limitations in the engineering application of existing methods. Summary of the Invention

[0006] In view of the shortcomings of existing technologies, and in response to the problems of incomplete broadband impedance modeling, unsystematic parameter influence analysis, and unclear identification of dominant factors in existing technologies for offshore direct-drive wind turbine generators (PMSG), the purpose of this invention is to provide a method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbine generators. This method can achieve accurate modeling of broadband impedance and identification of dominant factors of direct-drive wind turbine generators, providing a theoretical basis and engineering reference for grid-connected stability analysis, oscillation mechanism judgment, and control parameter tuning.

[0007] According to a first aspect of the present invention, a method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines is proposed, comprising the following steps:

[0008] Step 1: Based on the topology of offshore direct-drive wind turbine, the system is divided into turbine-side, grid-side converter, LCL filter, and control system. The turbine-side is considered as slow dynamic and is equivalent to a controlled current source in the target frequency band. Parameters of the core dynamic links on the grid side are collected, including the grid-connected controller, L / LCL filter, PLL, current loop, and DC side parameters.

[0009] Step 2: Based on the topology of the grid-side converter, establish the linear relationship equation between inductor current, bridge arm midpoint voltage and grid connection point voltage;

[0010] Step 3: Introduce three-phase small-signal disturbances at the grid connection point, decompose the three-phase voltage and current into fundamental component + positive-sequence disturbance component + negative-sequence disturbance component, and construct time-domain expression in the form of amplitude, phase angle and frequency offset, so as to provide a unified disturbance input for the subsequent construction of impedance model;

[0011] Step 4: Based on the disturbance input, taking into account the coupling effect of PLL, sampling delay, current controller and LCL filter, and according to the frequency domain relationship of small signal voltage, current and PLL dynamics, establish the analytical expression model of positive sequence and negative sequence impedance of the grid-side converter of direct-drive wind turbine in a wide frequency band.

[0012] Step 5: Based on the established impedance analytical model, the influence of phase-locked loop bandwidth, grid voltage, LCL filter inductor, and LCL filter capacitor on impedance characteristics is analyzed using the controlled variable method. A comparative analysis is conducted based on the amplitude-frequency characteristic curve used to characterize the amplitude-frequency relationship and the phase-frequency characteristic curve used to characterize the phase-frequency relationship obtained from numerical calculation or simulation.

[0013] Step 6: Combining the impedance model structure and the comparative analysis results of the control variable method, identify the dominant influencing factors of the broadband impedance of direct-drive wind turbine units.

[0014] As an optional implementation, taking into account the coupling effects of PLL, sampling delay, current controller and LCL filter, an analytical expression model of the positive and negative sequence impedance of the grid-side converter of the direct-drive wind turbine is established over a wide frequency band based on the frequency domain relationship of small-signal voltage, current and PLL dynamics. The specific steps include:

[0015] Step 4.1: Based on the disturbance input, perform small-signal modeling of the phase-locked loop (PLL) of the grid-side converter (GSC) of the direct-drive wind turbine, establish the angle disturbance model of the PLL, and obtain the relationship between Δθ and the port voltage disturbance. Δθ is the small phase angle disturbance caused by the small-signal voltage disturbance.

[0016] Step 4.2: After obtaining the PLL phase angle disturbance, derive the influence of the phase-locked loop dynamics on the voltage and current relationship of the converter port, and obtain the frequency domain expression between the voltage disturbance and the phase angle disturbance.

[0017] Step 4.3: Transform the three-phase current to the dq coordinate system through synchronous rotation transformation, and simultaneously introduce a current sampling function Gi(s) for sampling and PWM delay to obtain the dq current i. d i q The small-signal expression, namely the frequency domain expression of the dq-axis current with respect to the small-signal voltage disturbance and the small disturbance Δθ of the PLL phase angle, transforms the input disturbance into a current path and establishes a transmission path from disturbance input to current response;

[0018] Step 4.4, based on the dq current i d i q The small-signal expression is substituted into the current loop PI and feedforward decoupling structure to obtain the frequency domain form of the current controller output modulation wave cd and cq; then the three-phase modulation ratio is obtained through inverse dq transformation, and the dq axis modulation wave is converted into a representation in the three-phase coordinate system to obtain the frequency domain expression of the bridge arm modulation ratio ma, thus establishing the transmission channel of current disturbance → modulation wave → port voltage.

[0019] Step 4.5: Input the frequency domain expression of the arm modulation ratio ma into the linear relationship equation, and combine the frequency domain relationships of small signal voltage, current and PLL dynamics. By eliminating intermediate variables, the analytical expression of the positive and negative sequence impedance of the grid-side converter of the direct-drive wind turbine is finally obtained in a wide frequency band.

[0020] According to a second aspect of the present invention, a computer system is provided, comprising:

[0021] One or more processors;

[0022] The memory stores operable instructions that, when executed by the one or more processors, cause the one or more processors to perform operations, including the process of executing the broadband impedance modeling and dominant parameter identification method for offshore direct-drive wind turbines described in the foregoing embodiments.

[0023] In a third aspect of the present invention, a computer-readable storage medium is provided for storing a computer program comprising instructions / instruction set executable by one or more processors, wherein the instructions / instruction set, when executed by the one or more processors, implements the process of the broadband impedance modeling and dominant parameter identification method for offshore direct-drive wind turbines described in the foregoing embodiments.

[0024] The broadband impedance modeling and dominant parameter identification method for offshore direct-drive wind turbines described in the above embodiments of the present invention can model impedance across the entire control link of offshore direct-drive wind turbines, systematically characterize broadband dynamic characteristics, and clearly analyze the dominant influencing factors. This method achieves broadband impedance modeling and dominant parameter identification for offshore direct-drive wind turbines. By constructing a unified small-signal model considering multi-stage coupling, and through progressive derivation, a positive-sequence / negative-sequence impedance analytical model covering a wide frequency band is obtained. Based on this model, the influence of typical parameters is compared, thereby identifying the dominant influencing factors of broadband impedance. This provides a theoretical basis and engineering reference for grid-connected stability analysis, oscillation mechanism judgment, and control parameter tuning of the turbine.

[0025] Compared with the prior art, the significant advantages of the broadband impedance modeling and dominant parameter identification method for offshore direct-drive wind turbines of the present invention are as follows:

[0026] The broadband impedance model for offshore direct-drive wind turbines established in this invention simultaneously considers key components such as PLL, current loop, sampling delay, and LCL filter, enabling it to more accurately reflect the true dynamic characteristics of the grid-side converter. Furthermore, through comparative analysis of typical control parameters such as PLL bandwidth, grid voltage, LCL filter inductance, and LCL filter capacitor, it accurately distinguishes between dominant and non-dominant factors, eliminates irrelevant parameters, improves analysis efficiency, and reveals that the broadband impedance of direct-drive wind turbines is primarily determined by the dynamics of the control system, providing a clear direction for subsequent control parameter optimization.

[0027] It should be understood that all combinations of the foregoing concepts and the additional concepts described in more detail below may be considered part of the inventive subject matter of this disclosure, provided that such concepts do not contradict each other. Furthermore, all combinations of the claimed subject matter are considered part of the inventive subject matter of this disclosure.

[0028] The foregoing and other aspects, embodiments, and features of the teachings of the present invention will be more fully understood from the following description in conjunction with the accompanying drawings. Other additional aspects of the invention, such as features and / or beneficial effects of exemplary embodiments, will become apparent from the following description or may be learned through practice of specific embodiments according to the teachings of the present invention. Attached Figure Description

[0029] The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component shown in the various figures may be denoted by the same reference numeral. For clarity, not every component is labeled in each figure. Embodiments of various aspects of the invention will now be described by way of example and with reference to the accompanying drawings.

[0030] Figure 1 This is a schematic diagram of the principle of a direct-drive offshore wind turbine according to an embodiment of the present invention.

[0031] Figure 2 This is a topology diagram of an offshore direct-drive wind turbine according to an embodiment of the present invention.

[0032] Figure 3 This is a logic diagram of the control system for a direct-drive offshore wind turbine according to an embodiment of the present invention.

[0033] Figure 4(a) is the Bode plot of the closed-loop transfer function of the PLL.

[0034] Figure 4(b) shows the positive sequence impedance characteristics of the grid-connected inverter when the phase-locked loop bandwidth changes.

[0035] Figure 4(c) shows the negative sequence impedance characteristics of the grid-connected inverter when the phase-locked loop bandwidth changes.

[0036] Figure 5(a) shows the positive sequence impedance characteristics of the grid-connected inverter when the grid voltage changes.

[0037] Figure 5(b) shows the negative sequence impedance characteristics of the grid-connected inverter when the grid voltage changes.

[0038] Figure 6(a) shows the positive sequence impedance characteristics of the grid-connected inverter when the filter inductance of the grid-side converter changes.

[0039] Figure 6(b) shows the negative sequence impedance characteristics of the grid-connected inverter when the filter inductance of the grid-side converter changes.

[0040] Figure 6(c) shows the positive sequence impedance characteristics of the grid-connected inverter when the filter capacitor changes.

[0041] Figure 6(d) shows the negative sequence impedance characteristics of the grid-connected inverter when the filter capacitor changes. Detailed Implementation

[0042] To better understand the technical content of the present invention, specific embodiments are described below in conjunction with the accompanying drawings.

[0043] Various aspects of the invention are described in this disclosure with reference to the accompanying drawings, which illustrate numerous illustrative embodiments. The embodiments of this disclosure are not necessarily intended to encompass all aspects of the invention. It should be understood that the various concepts and embodiments described above, as well as those described in more detail below, can be implemented in any of many ways, because the concepts and embodiments disclosed herein are not limited to any particular implementation. Furthermore, some aspects of the invention disclosed may be used alone or in any suitable combination with other aspects of the invention disclosed.

[0044] {Example 1}

[0045] Combined with appendix Figure 1-3 As shown, the method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines according to an embodiment of the present invention includes the following steps:

[0046] Step 1: Based on the topology of offshore direct-drive wind turbine, the system is divided into turbine-side, grid-side converter, LCL filter, and control system. The turbine-side is considered as slow dynamic and is equivalent to a controlled current source in the target frequency band. Parameters of the core dynamic links on the grid side are collected, including the grid-connected controller, L / LCL filter, PLL, current loop, and DC side parameters.

[0047] Step 2: Based on the topology of the grid-side converter, establish the linear relationship equation between inductor current, bridge arm midpoint voltage and grid connection point voltage;

[0048] Step 3: Introduce three-phase small-signal disturbances at the grid connection point, decompose the three-phase voltage and current into fundamental component + positive-sequence disturbance component + negative-sequence disturbance component, and construct time-domain expression in the form of amplitude, phase angle and frequency offset, so as to provide a unified disturbance input for the subsequent construction of impedance model;

[0049] Step 4: Based on the disturbance input, taking into account the coupling effect of PLL, sampling delay, current controller and LCL filter, and according to the frequency domain relationship of small signal voltage, current and PLL dynamics, establish the analytical expression model of positive sequence and negative sequence impedance of the grid-side converter of direct-drive wind turbine in a wide frequency band.

[0050] Step 5: Based on the established impedance analytical model, the influence of phase-locked loop bandwidth, grid voltage, LCL filter inductor, and LCL filter capacitor on impedance characteristics is analyzed using the controlled variable method. A comparative analysis is conducted based on the amplitude-frequency characteristic curve used to characterize the amplitude-frequency relationship and the phase-frequency characteristic curve used to characterize the phase-frequency relationship obtained from numerical calculation or simulation.

[0051] Step 6: Combining the impedance model structure and the comparative analysis results of the control variable method, identify the dominant influencing factors of the broadband impedance of direct-drive wind turbine units.

[0052] In step 1, the direct-drive wind turbine is decomposed into four main modules: the turbine side, the grid-side converter (GSC), the LCL filter, and the control system. Since the dynamic response of the turbine side is much slower than the target wideband (0.1Hz-1kHz), it is equivalent to a controlled current source, retaining only the core dynamic components of the grid side. This equivalent treatment of the turbine side simplifies the modeling complexity and focuses on the grid-side wideband dynamics.

[0053] The data collection process includes acquiring fundamental parameters of the direct-drive wind turbine and grid-side converter, such as: rated power, grid-connected voltage, fundamental frequency, DC bus voltage, LCL filter inductance and capacitance parameters (converter-side inductance, grid-side inductance, and filter capacitor), and grid-side converter current loop PI parameters, PLL bandwidth, sampling and PWM delay, and other control-related parameters. This provides boundary conditions and basic data for subsequent modeling.

[0054] As an optional implementation, in step 2, the grid-side converter adopts a three-phase full-bridge topology, and the output is connected to the grid connection point after being connected in series with an LCL filter. Its core electrical variables include: the three-phase voltage u at the midpoint of the converter arm. a u b u c LCL filter converter-side inductor current i a i b i c ; and the three-phase voltage at the grid connection point v a v b v c ;

[0055] Then, according to Kirchhoff's voltage law, voltage-current relationships are established for the three-phase circuits respectively:

[0056]

[0057] Among them, i cx This represents the current in the corresponding filter capacitor branch of the three circuits;

[0058] Therefore, a unified three-phase main circuit time-domain equation is constructed.

[0059] Among them, combined with the appendix Figure 2As shown, the direct-drive wind turbine topology includes: three-phase bridge arms of the grid-side converter, LCL filter inductors, capacitors, and variables such as grid connection point (PCC) voltage, inductor current, and bridge arm midpoint voltage. Based on this, the relationship equation between inductor current and bridge arm midpoint voltage and PCC voltage is determined, and the linear relationship between grid-side converter port voltage, current, and filter elements is defined, laying the foundation for subsequent derivation of current, small-signal disturbance, and impedance expressions in a synchronous rotating coordinate system.

[0060] As an example, the relationship between inductor current and the midpoint voltage of the bridge arm and the voltage at point PCC is as follows:

[0061]

[0062] As an optional implementation, in step 3, a three-phase small-signal disturbance is introduced at the grid connection point to construct the fundamental and disturbance component expressions of the PCC voltage and current. The three-phase voltage and current are decomposed into a fundamental component + a positive-sequence disturbance component + a negative-sequence disturbance component. The time-domain expression is constructed in the form of amplitude, phase angle, and frequency offset to provide a unified disturbance input for the subsequent construction of the impedance model.

[0063] As an example, the process of constructing the fundamental and disturbance component expressions for the PCC voltage and current in step 3 includes the following steps:

[0064] Step 3.1: Inject a small-signal disturbance voltage at the grid connection point, with an amplitude not exceeding 10% of the power frequency voltage; wherein, taking the expressions for the voltage and current at phase A port as a unified starting point, the voltage and current of phase A are:

[0065]

[0066] Among them, V1, V up and V un These represent the amplitudes of the fundamental voltage, positive-sequence perturbation voltage, and negative-sequence perturbation voltage, respectively; I1, I ip and I in These represent the amplitudes of the fundamental current, the positive-sequence disturbance current response, and the negative-sequence disturbance current response, respectively; f1, f p and f n These are the fundamental frequency, positive-sequence perturbation frequency, and negative-sequence perturbation frequency, respectively. and These are the initial phase angles of the positive-sequence and negative-sequence disturbance voltages, respectively. and These are the initial phase angles of the fundamental current, the positive-sequence disturbance current response, and the negative-sequence disturbance current response, respectively.

[0067] Step 3.2: Based on three-phase symmetry, the phase expression for phase B lags phase A by 120°, and phase C lags phase A by 240°:

[0068]

[0069] Thus, a complete three-phase disturbance expression system is formed, which explicitly decomposes the broadband problem into the coupling relationship between the fundamental frequency and the disturbance frequency, and between the positive sequence and the negative sequence, which is the starting point for constructing the sequence impedance model.

[0070] As an optional implementation, in step 4, based on the disturbance input, taking into account the coupling effect of PLL, sampling delay, current controller and LCL filter, an analytical expression model of positive and negative sequence impedance of the grid-side converter of the direct-drive wind turbine in a wide frequency band is established according to the frequency domain relationship of small signal voltage, current and PLL dynamics.

[0071] As an optional example, the complete process of establishing an analytical expression model of the positive and negative sequence impedances of a direct-drive wind turbine grid-side converter over a wide bandwidth, based on disturbance input, includes the following steps:

[0072] Step 4.1: Based on the disturbance input, perform small-signal modeling of the phase-locked loop (PLL) of the grid-side converter (GSC) of the direct-drive wind turbine, establish the angle disturbance model of the PLL, and obtain the relationship between Δθ and the port voltage disturbance. Δθ is the small phase angle disturbance caused by the small-signal voltage disturbance.

[0073] Step 4.2: After obtaining the PLL phase angle disturbance, derive the influence of the phase-locked loop dynamics on the voltage and current relationship of the converter port, and obtain the frequency domain expression between the voltage disturbance and the phase angle disturbance.

[0074] Step 4.3: Transform the three-phase current to the dq coordinate system through synchronous rotation transformation, and simultaneously introduce a current sampling function Gi(s) for sampling and PWM delay to obtain the dq current i. d i q The small-signal expression, namely the frequency domain expression of the dq-axis current with respect to the small-signal voltage disturbance and the small disturbance Δθ of the PLL phase angle, transforms the input disturbance into a current path and establishes a transmission path from disturbance input to current response;

[0075] Step 4.4, based on the dq current i d i q The small-signal expression is substituted into the current loop PI and feedforward decoupling structure to obtain the frequency domain form of the current controller output modulation wave cd and cq; then the three-phase modulation ratio is obtained through inverse dq transformation, and the dq axis modulation wave is converted into a representation in the three-phase coordinate system to obtain the frequency domain expression of the bridge arm modulation ratio ma, thus establishing the transmission channel of current disturbance → modulation wave → port voltage.

[0076] Step 4.5: Input the frequency domain expression of the arm modulation ratio ma into the linear relationship equation, and combine the frequency domain relationships of small signal voltage, current and PLL dynamics. By eliminating intermediate variables, the analytical expression of the positive and negative sequence impedance of the grid-side converter of the direct-drive wind turbine is finally obtained in a wide frequency band.

[0077] Further, in step 4.1, the aim is to establish a synchronous rotating coordinate transformation matrix and perform small-signal linearization, derive the expression for the dq-axis voltage perturbation, and obtain the relationship between Δθ and the port voltage perturbation. This couples the PLL phase angle perturbation Δθ with the dq-axis voltage, serving as a key input for subsequently establishing the PLL small-signal model. An example process includes:

[0078] Based on the PLL synchronous control of the grid-side converter of the direct-drive wind turbine, current control is performed in the dq coordinate system, and the rotation reference angle θ output by the PLL is... PLL Positive rotation angle θ generated by the fundamental voltage l It consists of the small signal perturbation Δθ, i.e., θ PLL =θ l +Δθ;θ1=2πf1t;

[0079] Construct with θ PLL The synchronous rotation coordinate transformation matrix T(θ) is the angle. PLL ):

[0080]

[0081] Based on Δθ in θ PLL The disturbance is caused by a small-signal voltage fluctuation, which is a small disturbance and has a very small value. Based on this, we can further use the trigonometric function approximation formula to approximate T(θ) under the premise of Δθ. PLL Performing a first-order approximate expansion, we obtain cosθ in the transformation matrix. PLL and sinθ PLL Approximate expression:

[0082] cosθ PLL =cos(θ1+Δθ)cosθ1-Δθsinθ1;

[0083] sinθ PLL =sin(θ1+Δθ)≈sinθ1+Δθcosθ1;

[0084] Substitute into the synchronous rotation coordinate transformation matrix T(θ) PLL The linearized matrix T'(θ) is obtained. PLL Furthermore, the PCC three-phase voltage v containing the fundamental component and the disturbance component is further... α v b v c Input T'(θ) PLLThe grid-connected inverter's dq-axis voltage at the grid connection point is obtained through matrix multiplication projection.

[0085]

[0086] As an optional implementation, in step 4.2, after obtaining the PLL phase angle disturbance, the influence of the phase-locked loop dynamics on the voltage and current relationship of the converter port is derived, and the frequency domain expression between the voltage disturbance and the phase angle disturbance is obtained.

[0087] Specifically, based on the PLL output angle θ PLL With q-axis voltage v q The closed-loop relationship between them, introducing a small-signal voltage disturbance and the transfer function G between Δθ in the frequency domain. PLL (s), and using the given v d v q By combining the expression with the PLLPI controller structure, the transfer function between the disturbance voltage and Δθ is derived. Furthermore, substituting this transfer function into the aforementioned coordinate transformation relationship, cosθ is obtained. PLL and sinθ PLL The expression for small-signal perturbation in the frequency domain.

[0088] As an example, the process includes:

[0089] Based on the control topology of the grid-side converter, determine the PLL output angle θ. PLL With q-axis voltage v q The closed-loop relationship between them is:

[0090] Δθ[f)=H PLL (s)V q [f];

[0091] Control the topology to determine the PLL output angle θ PLL With q-axis voltage v q The closed-loop relationship between them is as follows: Assume that the transfer function between the small-signal voltage disturbance and Δθ in the frequency domain is:

[0092]

[0093] According to v d v q The expression yields v d and v q The expression in the frequency domain is:

[0094]

[0095] Furthermore, in the combined phase-locked loop controller, θ PLL With v q The relationship, and v dand v q From the frequency domain expression, the transfer function between the small-signal voltage disturbance and Δθ can be obtained as follows:

[0096] G n (s)=±jH PLL (s) / [1+V1H PLL (s)];

[0097]

[0098] Furthermore, we can obtain cosθ PLL and sinθ PLL The expression for the small-signal perturbation in the frequency domain is:

[0099]

[0100] As an optional implementation, in step 4.3, the three-phase current is transformed to the dq coordinate system through synchronous rotation transformation, and a current sampling function Gi(s) is introduced for sampling and PWM delay to obtain the dq current i. d i q The small-signal expression, i.e., the frequency domain expression of the dq-axis current with respect to the small-signal voltage disturbance and the small disturbance Δθ of the PLL phase angle, transforms the input disturbance into a current path, establishing a transmission path from disturbance input to current response, including:

[0101] According to the synchronous rotation coordinate transformation matrix T(θ) PLL ), will the grid-connected inverter inductor current i a i b i c Convert to i d i q The conversion formula is as follows:

[0102]

[0103] cosθ PLL and sinθ PLL Substituting the expression in the frequency domain into the above equation, we get i d i q The frequency domain expression is:

[0104]

[0105] Among them, G i (s) is the current sampling function, used to simulate sampling delay, PWM delay and sampling low-pass filter.

[0106] Furthermore, based on the current feedforward decoupling control structure of the grid-side converter of the direct-drive wind turbine, a current PI controller H is established in the dq coordinate system. i(s) and the inductor current feedforward path. Using the i obtained in step 6 d i q The expression is used to derive the modulated wave c output by the current controller. d c q The frequency domain expression is then obtained. Subsequently, the representation of the modulated wave in the three-phase coordinate system is obtained through inverse dq transform, and the bridge arm modulation ratio m is further derived. a The frequency domain expression.

[0107] As an optional implementation, in step 4.4, based on the dq current i d i q The small-signal expression is substituted into the current loop PI and feedforward decoupling structure to obtain the frequency domain form of the current controller output modulation waves cd and cq; then, the three-phase modulation ratio is obtained through inverse dq transformation, and the dq-axis modulation wave is converted into a representation in the three-phase coordinate system to obtain the frequency domain expression of the bridge arm modulation ratio ma. This establishes the transmission path from current disturbance to modulation wave to port voltage, including:

[0108] Define the modulation wave c of the grid-connected inverter in the synchronous rotating coordinate system. d and c q Expressed as:

[0109]

[0110] Among them, H i (s) is a current PI controller, H i (s)=k p +k i / s,k p k i These represent the PI control parameters of the current PI controller;

[0111] For c d and c q Perform a Laplace transform, combining the i from the previous steps. d i q The frequency domain expression is used to obtain the modulation wave c. d and c q Frequency domain expression:

[0112]

[0113] From the inverse dq coordinate transformation, we can obtain:

[0114]

[0115] Combination Figure 3 m a =c a +K f v aFurthermore, by converting the dq-axis modulated wave into a representation in a three-phase coordinate system, the frequency domain expression of the bridge arm modulation ratio ma is obtained as follows:

[0116]

[0117] Therefore, the modulation stage links the dq axis control quantity with the actual three-phase bridge arm drive signal, thereby coupling the control system with the main circuit topology.

[0118] As an optional implementation, in step 4.5, m a Substituting the frequency domain expression into the main circuit equation, we can obtain the positive and negative sequence impedance expressions for the grid-side converter of the direct-drive wind turbine as follows:

[0119]

[0120] Thus, the positive and negative sequence impedance modeling of the grid-side converter of the offshore direct-drive wind turbine has been completed over a wide frequency band.

[0121] Furthermore, the influence of different control parameters and operating conditions on broadband impedance characteristics is analyzed to identify the dominant parameters that have the most significant impact on impedance characteristics. This embodiment mainly considers factors such as PLL bandwidth, grid-connected voltage, and LCL filter inductor and LCL filter capacitor.

[0122] Impedance frequency response obtained based on analytical model and simulation results: Based on the established analytical impedance model of the grid-side converter of the direct-drive wind turbine, and combined with specific control and analysis parameters, the positive-sequence and negative-sequence impedance frequency response curves over a wide frequency band are calculated for the target direct-drive wind turbine. Through numerical solutions or simulation tools, the amplitude-frequency curves and phase-frequency curves within the selected frequency range are obtained and plotted as the Bode plot and sequence impedance characteristic diagram in the attached figures.

[0123] Specifically, based on the set variables and benchmarks, the impedance curves of grid-connected voltage, PLL bandwidth, grid-connected voltage, LCL filter inductor, and LCL filter capacitor are compared under their respective changing conditions (three changes for each parameter are analyzed) to obtain the degree of influence of their changes on the unit's impedance characteristics.

[0124] As an example, compare the impedance curves under different phase-locked loop (PLL) bandwidth conditions. Keeping other parameters constant, change the PLL bandwidth (by adjusting the PLL's PI controller parameters), for example, PLL bandwidths of 27Hz, 43Hz, and 81Hz. Calculate and compare the corresponding positive-sequence and negative-sequence impedance curves: take k... pllp =0.048, k plli =0.659, corresponding to bandwidth f pll =27Hz; k pllp =0.06, k plli=2.1, corresponding to bandwidth f ppl =43Hz; k pllp =0.082, k plli =8.1, corresponding to bandwidth f pll =81Hz.

[0125] The impact of changes in phase-locked loop bandwidth, such as Figure 4(a) , 4(b) As shown in Figures 4(b) and 4(c), the effect of PLL bandwidth variation on the positive sequence impedance of the grid-side converter of the direct-drive wind turbine is mainly concentrated in the 40–60 Hz frequency band. Within this frequency band, the change in PLL bandwidth mainly causes a change in the peak value of the positive sequence impedance amplitude resonance and expands the frequency range where the phase shift is significant, mainly affecting the peak value of the resonance. The effect of PLL bandwidth variation on the negative sequence impedance of the grid-connected converter is not significant, and the negative sequence impedance curve changes little under different bandwidths.

[0126] As an example, compare the impedance curves under different grid-connected voltages: change the grid-connected voltage conditions, obtain the corresponding negative sequence impedance curves, and compare them. While keeping the PLL bandwidth and other control parameters constant, change the voltage amplitude at the grid connection point.

[0127] For example, by changing the grid-connected voltage of the grid-connected converter to 0.9V... nom V nom 1.1V nom The impedance characteristics of the grid-connected inverter change are shown in Figure 5(a). When the grid voltage changes, the impact on the positive sequence impedance of the grid-connected inverter is mainly concentrated around the 50Hz fundamental frequency, and the impact is relatively small. Specifically, the higher the voltage, the larger the corresponding positive sequence impedance amplitude, and the stronger the inductive or capacitive characteristics. As shown in Figure 5(b), the grid voltage change has a weak impact on the negative sequence impedance characteristics, and the negative sequence impedance curve remains basically unchanged with voltage changes.

[0128] As an example, the impedance curves under different LCL filter inductors and capacitors are compared. While keeping the control parameters and grid voltage constant, the values ​​of the converter-side inductor and filter capacitor in the LCL filter are changed respectively. The corresponding positive-sequence and negative-sequence impedance curves are calculated and compared numerically.

[0129] Taking inductors as an example, typical values ​​such as 0.08mH, 0.16mH, and 0.30mH can be selected, and their corresponding positive-sequence and negative-sequence impedance characteristics can be calculated, as shown in Figure 6(a) and Figure 6(b).

[0130] Taking capacitors as an example, typical values ​​such as 15μF, 17μF, and 20μF can be selected, and the corresponding positive-sequence and negative-sequence impedance characteristics can be calculated, as shown in Figure 6(c) and Figure 6(d).

[0131] Combining 6(a), 6(b), 6(c), and 6(d), it can be concluded that changes in the filter inductance and filter capacitor have little impact on the broadband impedance characteristics of the direct-drive fan, and the shapes of the positive-sequence and negative-sequence impedance curves and the resonance characteristics do not change significantly. In this model, changes in the filter inductance and capacitor have little effect on the impedance characteristics of the direct-drive fan.

[0132] After excluding the influence of filters and grid connection, and based on the derivation structure of the impedance model, it can be seen that the main source of influence of the broadband impedance of the direct-drive wind turbine comes from the dynamic process inside the control system, including: the inner loop of dq current and the DC-side power coupling term.

[0133] Through modeling and comparative analysis of the above embodiments, it can be seen that this invention establishes a wideband impedance analytical model for direct-drive wind turbines, taking into account the phase-locked loop (PLL), dq current control loop, sampling and modulation delay, and LCL filter coupling effects. Based on this model, impedance characteristics of typical parameters such as PLL bandwidth, grid voltage, LCL filter inductance, and filter capacitor are compared and analyzed. The analysis results show that changes in grid voltage have a relatively small impact on the wideband impedance of the direct-drive wind turbine; changes in LCL filter inductance and capacitor within common value ranges also have no significant impact on the positive-sequence and negative-sequence impedance curves. Therefore, under the grid-side converter structure of the direct-drive wind turbine, grid voltage and LCL filter parameters can be considered non-dominant factors, while the main source of influence on impedance characteristics lies in the dynamic processes within the control system.

[0134] Based on the broadband impedance modeling and dominant parameter identification method for offshore direct-drive wind turbines described in the above embodiments, this invention also proposes a computer system, comprising: one or more processors; and a memory for storing operable instructions.

[0135] When the instruction is executed by the one or more processors, it causes the one or more processors to perform an operation, which includes the process of performing the broadband impedance modeling and dominant parameter identification method for offshore direct-drive wind turbines described in the foregoing embodiments.

[0136] Based on the broadband impedance modeling and dominant parameter identification method for offshore direct-drive wind turbines described in the above embodiments, this invention also proposes a computer-readable storage medium for storing a computer program, wherein the computer program includes instructions / instruction sets executable by one or more processors.

[0137] When the instructions / instruction set are executed by the one or more processors, they implement the process of broadband impedance modeling and dominant parameter identification method for offshore direct-drive wind turbines as described in the foregoing embodiments.

[0138] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbine units, characterized in that, Includes the following steps: Step 1: Based on the topology of offshore direct-drive wind turbine, the system is divided into turbine-side, grid-side converter, LCL filter, and control system. The turbine-side is considered as slow dynamic and is equivalent to a controlled current source in the target frequency band. Parameters of the core dynamic links on the grid side are collected, including the grid-connected controller, L / LCL filter, PLL, current loop, and DC side parameters. Step 2: Based on the topology of the grid-side converter, establish the linear relationship equation between inductor current, bridge arm midpoint voltage and grid connection point voltage; Step 3: Introduce three-phase small-signal disturbances at the grid connection point, decompose the three-phase voltage and current into fundamental component + positive-sequence disturbance component + negative-sequence disturbance component, and construct time-domain expression in the form of amplitude, phase angle and frequency offset, so as to provide a unified disturbance input for the subsequent construction of impedance model; Step 4: Based on the disturbance input, taking into account the coupling effect of PLL, sampling delay, current controller and LCL filter, and according to the frequency domain relationship of small signal voltage, current and PLL dynamics, establish the analytical expression model of positive sequence and negative sequence impedance of the grid-side converter of direct-drive wind turbine in a wide frequency band. Step 5: Based on the established impedance analytical model, the influence of phase-locked loop bandwidth, grid voltage, LCL filter inductor, and LCL filter capacitor on impedance characteristics is analyzed using the controlled variable method. A comparative analysis is conducted based on the amplitude-frequency characteristic curve used to characterize the amplitude-frequency relationship and the phase-frequency characteristic curve used to characterize the phase-frequency relationship obtained from numerical calculation or simulation. Step 6: Combining the impedance model structure and the comparative analysis results of the control variable method, identify the dominant influencing factors of the broadband impedance of direct-drive wind turbine units.

2. The method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines according to claim 1, characterized in that, In step 2, the grid-side converter adopts a three-phase full-bridge topology, and the output is connected to the grid connection point after being connected in series with an LCL filter. Its core electrical variables include: the three-phase voltage u at the midpoint of the converter arm. a u b u c LCL filter converter-side inductor current i a i b i c ; and the three-phase voltage at the grid connection point v a v b v c ; Then, according to Kirchhoff's voltage law, voltage-current relationships are established for the three-phase circuits respectively: Among them, i cx This represents the current in the corresponding filter capacitor branch of the three circuits; Therefore, a unified three-phase main circuit time-domain equation is constructed.

3. The method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines according to claim 1, characterized in that, In step 3, a three-phase small-signal disturbance is introduced at the grid connection point. The three-phase voltage and current are decomposed into a fundamental component + a positive-sequence disturbance component + a negative-sequence disturbance component. A time-domain representation is constructed using amplitude, phase angle, and frequency offset to provide a unified disturbance input for the subsequent impedance model construction, including: Step 3.1: Inject a small-signal disturbance voltage at the grid connection point, with an amplitude not exceeding 10% of the power frequency voltage; wherein, taking the expressions for the voltage and current at phase A port as a unified starting point, the voltage and current of phase A are: Among them, V1, V up and V un These represent the amplitudes of the fundamental voltage, positive-sequence perturbation voltage, and negative-sequence perturbation voltage, respectively; I1, I ip and I in These represent the amplitudes of the fundamental current, the positive-sequence disturbance current response, and the negative-sequence disturbance current response, respectively; f1, f p and f n These are the fundamental frequency, positive-sequence perturbation frequency, and negative-sequence perturbation frequency, respectively. and These are the initial phase angles of the positive-sequence and negative-sequence disturbance voltages, respectively. and These are the initial phase angles of the fundamental current, the positive-sequence disturbance current response, and the negative-sequence disturbance current response, respectively. Step 3.2: Based on three-phase symmetry, the phase expression for phase B lags phase A by 120°, and phase C lags phase A by 240°: Thus, a complete system of three-phase disturbance expressions is formed.

4. The method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines according to claim 1, characterized in that, In step 4, based on the disturbance input, and considering the coupling effects of the PLL, sampling delay, current controller, and LCL filter, an analytical expression model of the positive and negative sequence impedance of the direct-drive wind turbine grid-side converter is established over a wide bandwidth according to the frequency domain relationship of small-signal voltage, current, and PLL dynamics. This model includes: Step 4.1: Based on the disturbance input, perform small-signal modeling of the phase-locked loop (PLL) of the grid-side converter (GSC) of the direct-drive wind turbine, establish the angle disturbance model of the PLL, and obtain the relationship between Δθ and the port voltage disturbance. Δθ is the small phase angle disturbance caused by the small-signal voltage disturbance. Step 4.2: After obtaining the PLL phase angle disturbance, derive the influence of the phase-locked loop dynamics on the voltage and current relationship of the converter port, and obtain the frequency domain expression between the voltage disturbance and the phase angle disturbance. Step 4.3: Introduce the current sampling function Gi(s) to obtain the dq current i. d i q The small signal expression; Step 4.4, based on the dq current i d i q The small-signal expression is used to obtain the frequency domain form of the current controller output modulation wave cd and cq, and the frequency domain expression of the bridge arm modulation ratio ma is established accordingly. Step 4.5: Input the frequency domain expression of the arm modulation ratio ma into the linear relationship equation, and simultaneously establish the frequency domain relationships of small-signal voltage, current, and PLL dynamics. By eliminating intermediate variables, the positive-sequence and negative-sequence impedance analytical expressions of the grid-side converter of the direct-drive wind turbine over a wide bandwidth are finally obtained, where: The analytical expression for positive sequence impedance is the frequency domain ratio of positive sequence disturbance voltage to positive sequence disturbance current; The analytical expression for negative sequence impedance is the frequency domain ratio of negative sequence disturbance voltage to negative sequence disturbance current.

5. The method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines according to claim 4, characterized in that, In step 4.3, according to the synchronous rotation coordinate transformation matrix T(θ) PLL The three-phase current is transformed to the dq coordinate system through synchronous rotation transformation. Simultaneously, a current sampling function Gi(s) is introduced for sampling and PWM delay. Substituting the frequency domain expression between voltage disturbance and phase angle disturbance, the dq current i is obtained. d i q The small-signal expression, namely the frequency domain expression of the dq-axis current with respect to the small-signal voltage disturbance and the small disturbance Δθ of the PLL phase angle, transforms the input disturbance into a current path, establishing a transmission path from disturbance input to current response.

6. The method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines according to claim 4, characterized in that, In step 4.4, based on the dq current i d i q The small-signal expression is substituted into the current loop PI controller and feedforward decoupling structure, and the frequency domain form of the current controller output modulation wave cd and cq is obtained through Laplace transform. Then, the three-phase modulation ratio is obtained through inverse dq transformation. The dq-axis modulation wave is converted into a representation in the three-phase coordinate system to obtain the frequency domain expression of the bridge arm modulation ratio ma, and the transmission channel of current disturbance → modulation wave → port voltage is established.

7. The method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines according to any one of claims 1-6, characterized in that, In step 5, for the comparative analysis of each control variable, only the phase-locked loop bandwidth, grid-connected three-phase voltage, LCL filter inductor, and LCL filter capacitor are changed. Substituting these into the analytical expressions for positive and negative sequence impedance, the amplitude-frequency characteristic curves and phase-frequency characteristic curves of positive / negative sequence impedance over a wide bandwidth are obtained through numerical solutions or simulation tools. The positive and negative sequence impedance curves under different control variables are compared and analyzed to identify the influence of phase-locked loop bandwidth, grid-connected three-phase voltage, LCL filter inductor, and LCL filter capacitor on the impedance characteristics of offshore direct-drive wind turbines.

8. The method for broadband impedance modeling and dominant parameter identification of offshore direct-drive wind turbines according to claim 7, characterized in that, In step 6, combining the impedance model structure and the comparative analysis results of the control variable method, the dominant influencing factors of the broadband impedance of direct-drive wind turbine units are identified, including: Based on the influence of phase-locked loop bandwidth, three-phase voltage at grid connection point, LCL filter inductor, and LCL filter capacitor on the impedance characteristics of offshore direct-drive wind turbines, factors that do not have a broadband dominant effect are identified. The dominant influencing factor of the broadband impedance of direct-drive wind turbine generators is determined to be the dynamic process within the control system, specifically including: the PI control dynamics of the dq current inner loop, the phase tracking dynamics of the PLL, the sampling and modulation delay dynamics, and the DC-side power coupling dynamics.

9. A computer system, characterized in that, include: One or more processors; The memory stores operable instructions that, when executed by the one or more processors, cause the one or more processors to perform operations, including the process of performing the broadband impedance modeling and dominant parameter identification method for offshore direct-drive wind turbines as described in any one of claims 1-8.

10. A computer-readable storage medium for storing a computer program, characterized in that, The computer program includes instructions / instruction sets that can be executed by one or more processors. When executed by the one or more processors, the instructions / instruction sets implement the process of broadband impedance modeling and dominant parameter identification method for offshore direct-drive wind turbines as described in any one of claims 1-8.

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