Stability analysis method and system for offshore wind power grid-connected system

Abstract

The invention provides a stability analysis method and system for an offshore wind power grid-connected system. The method comprises the following steps: acquiring real-time operation data of a wind turbine generator and a static var generator of the offshore wind power grid-connected system; inputting the real-time operation data into a preset impedance model of the offshore wind power grid-connected system, so that the impedance model of the offshore wind power grid-connected system calculates and obtains first equivalent impedance of the offshore wind power grid-connected system according to the real-time operation data, the impedance model of the offshore wind power grid-connected system is constructed according to an impedance model of each part of the offshore wind power grid-connected system; obtaining second equivalent impedance of the power grid according to a preset power grid harmonic impedance model or an actual power grid harmonic scanning value; and performing stability analysis on the offshore wind power grid-connected system according to the first equivalent impedance and the second equivalent impedance. The accuracy of stability judgment of the offshore wind power grid-connected system is improved.

Description

Technical Field

[0001] The present application relates to the technical field of power systems, and in particular to a stability analysis method and system for an offshore wind power grid-connected system. Background Art

[0002] Large-scale offshore wind power has the characteristics of large collection capacity and many power electronic devices, which puts higher requirements on the resonance stability of grid-connected transmission. Grid-connected oscillation accidents will cause economic losses and threaten the safety and stability of the system. Therefore, studying the mechanism of wind farm grid-connected oscillation is of great significance to the safe operation of the power grid.

[0003] The dominant small disturbance stability of voltage source converter can be divided into two categories according to the frequency band: low-frequency resonance and medium-high frequency resonance. Low-frequency resonance refers to the sub / supersynchronous oscillation problem near the fundamental frequency, which is mainly affected by factors such as the internal dynamic characteristics of the voltage source converter, various control links, and frequency coupling effects. It requires the use of harmonic state space or dynamic phasor methods, but the model order is high and the calculation amount is large. Medium-high frequency resonance covers the frequency band from 200Hz to several kHz, and is mainly affected by factors such as the voltage source converter delay link and filtering device. The frequency coupling effect and the control bandwidth are far lower than the medium-high frequency band can be ignored, thereby reducing the complexity of the model.

[0004] In terms of modeling and stability analysis, the existing technical solutions propose to establish a state space matrix model and use the characteristic root analysis method; to establish a frequency domain impedance model and use the impedance analysis method. Compared with the mode analysis method, the impedance analysis method only requires the input and output models of two subsystems to judge the stability of the interconnected system, with lower calculation difficulty and more suitable for large-scale offshore wind power grid-connected projects. However, in response to the resonance stability problem of large-scale offshore wind power grid-connected projects, existing studies mainly use typical simple examples for modeling, and simplify the boosting, transmission, and reactive power supplement links, which has low engineering practicality. In addition, in terms of model construction, the existing technology mainly establishes a complex harmonic state space model for the high-frequency resonance problem in the converter, and does not ignore factors with small influence, resulting in a high model order and a large amount of calculation. Summary of the invention

[0005] In response to the above technical problems, the present application provides a stability analysis method and system for an offshore wind power grid-connected system to improve the accuracy of stability judgment on the offshore wind power grid-connected system and provide data support for actual engineering assessment of system oscillation risks and suppression of harmonic resonance.

[0006] In a first aspect, the present application provides a stability analysis method for an offshore wind power grid-connected system, comprising:

[0007] Obtain real-time operating data of wind turbines and static reactive power generation devices in offshore wind power grid-connected systems;

[0008] Inputting the real-time operation data into a preset offshore wind power grid-connected system impedance model, so that the offshore wind power grid-connected system impedance model calculates and obtains a first equivalent impedance of the offshore wind power grid-connected system according to the real-time operation data, wherein the offshore wind power grid-connected system impedance model is constructed based on impedance models of various parts of the offshore wind power grid-connected system, including a wind turbine impedance model, a wind farm impedance model, a submarine cable impedance model, and a static VAR generator impedance model;

[0009] Obtaining a second equivalent impedance of the power grid according to a preset power grid harmonic impedance model or an actual power grid harmonic scanning value;

[0010] A stability analysis of the offshore wind power grid-connected system is performed based on the first equivalent impedance and the second equivalent impedance.

[0011] The embodiment of the present application provides a stability analysis method for an offshore wind power grid-connected system, which obtains the first equivalent impedance of the offshore wind power grid-connected system as a whole by acquiring the real-time operating data of the wind turbines and the static reactive power generator and inputting it into the impedance model of the offshore wind power grid-connected system. The impedance model of the offshore wind power grid-connected system is constructed based on the impedance models of the various parts of the offshore wind power grid-connected system, while considering the influence of the wind turbine impedance, wind farm impedance, submarine cable impedance and static reactive power generator impedance on the equivalent impedance of the offshore wind power grid-connected system, and modeling is performed based on the complete wind power transmission link to improve the accuracy of the model's equivalent impedance calculation. Furthermore, on the basis of accurately calculating the first equivalent impedance, the embodiment of the present application also combines the second equivalent impedance of the power grid for stability analysis, thereby improving the accuracy of the judgment of the stability of the offshore wind power grid-connected system and providing data support for the actual engineering assessment of system oscillation risks and suppression of harmonic resonance.

[0012] In a possible implementation manner, the offshore wind power grid-connected system impedance model calculates and obtains a first equivalent impedance of the offshore wind power grid-connected system according to the real-time operation data, including:

[0013] Inputting the real-time operation data of the wind turbine generator set into the impedance model of the wind turbine generator set, so that the impedance model of the wind turbine generator set outputs the high-voltage side admittance data of the wind turbine generator set;

[0014] Inputting the high-voltage side admittance data of the wind turbine group into the wind farm impedance model, so that the wind farm impedance model outputs the high-voltage side admittance data of the wind farm;

[0015] Inputting the high-voltage side admittance data of the wind farm into the submarine cable impedance model, so that the submarine cable impedance model outputs the impedance data of the submarine cable delivery system;

[0016] Inputting the impedance data of the submarine cable delivery system and the real-time operation data of the static VAR generator into the impedance model of the static VAR generator, so that the impedance model of the static VAR generator outputs the impedance data of the high-voltage side of the static VAR generator;

[0017] The first equivalent impedance of the offshore wind power grid-connected system is calculated based on the impedance data of the submarine cable delivery system and the impedance data of the high-voltage side of the static VAR generator.

[0018] The embodiment of the present application provides a method for calculating and obtaining the first equivalent impedance of an offshore wind power grid-connected system, wherein the calculation is performed step by step from the network end of the offshore wind power grid-connected system to the grid-connected point, and the high-voltage side admittance data of the wind turbine group, the high-voltage side admittance data of the wind farm, the impedance data of the submarine cable delivery system, the impedance data of the high-voltage side of the static VAR generator, and the first equivalent impedance of the offshore wind power grid-connected system are calculated in turn according to the corresponding impedance models, wherein each model is calculated on the basis of the previous model, thereby ensuring the integrity and tightness of the wind power delivery process and the calculation accuracy of the impedance model of the offshore wind power grid-connected system.

[0019] In a possible implementation manner, the performing stability analysis on the offshore wind power grid-connected system according to the first equivalent impedance and the second equivalent impedance includes:

[0020] generating a corresponding first amplitude-phase-frequency characteristic curve according to the first equivalent impedance;

[0021] generating a corresponding second amplitude-phase-frequency characteristic curve according to the second equivalent impedance;

[0022] If there is an intersection point between the first amplitude-phase-frequency characteristic curve and the second amplitude-phase-frequency characteristic curve, obtaining an intersection point frequency corresponding to the intersection point;

[0023] If the phase difference between the first amplitude-phase-frequency characteristic curve at the intersection frequency and the second amplitude-phase-frequency characteristic curve at the intersection frequency is greater than 180 degrees, it is determined that the offshore wind power grid-connected system is unstable.

[0024] The embodiment of the present application provides a stability analysis method, which draws the corresponding first amplitude-phase frequency characteristic curve and the second amplitude-phase frequency characteristic curve according to the first equivalent impedance and the second equivalent impedance, and then compares and analyzes the first amplitude-phase frequency characteristic curve and the second amplitude-phase frequency characteristic curve. In the analysis process, the present application uses the Nyquist stability criterion. If there is an intersection between the first amplitude-phase frequency characteristic curve and the second amplitude-phase frequency characteristic curve and the phase difference between the two at the intersection frequency is greater than 180°, it can be determined that the system is unstable, thereby realizing the determination of the stability of the offshore wind power grid-connected system based on real-time operation data, and being able to detect the system oscillation risk in a timely manner, thereby improving the stability and safety of the operation of the offshore wind power grid-connected system.

[0025] Furthermore, the process of constructing the wind turbine impedance model includes:

[0026] The wind turbine impedance model is obtained by jointly constructing a frequency domain small signal main loop equation of a converter in the wind turbine and a frequency domain small signal control equation of the converter.

[0027] Furthermore, the process of constructing the wind farm impedance model includes:

[0028] According to the impedance and admittance at the feeder end of the wind farm, the impedance and admittance of the feeder port are derived to obtain the expressions of the impedance and admittance of the feeder port;

[0029] The wind farm impedance model is constructed based on the expressions of impedance and admittance of the feeder port.

[0030] Furthermore, the process of constructing the submarine cable impedance model includes:

[0031] According to the impedance and admittance at the end of the submarine cable, the impedance and admittance expressions of the submarine cable port are derived to the submarine cable port;

[0032] The submarine cable impedance model is constructed based on the expressions of impedance and admittance of the submarine cable port.

[0033] Furthermore, the construction process of the impedance model of the static VAR generator includes:

[0034] Determine the AC voltage steady-state operating point of the static VAR generator;

[0035] Superimposing a signal disturbance of a preset frequency at the AC voltage steady-state operating point;

[0036] Analyze the frequency domain distribution characteristics of the signal disturbance between various voltages, currents and modulation signals, and determine the frequency domain small signal main loop equation of the static VAR generator and the frequency domain small signal control equation of the converter of the static VAR generator;

[0037] The impedance model of the static VAR generator is obtained by jointly constructing a frequency domain small signal main loop equation of the static VAR generator and a frequency domain small signal control equation of the converter of the static VAR generator.

[0038] In a possible implementation manner, the method further includes calculating the model accuracy of the offshore wind power grid-connected system impedance model, including:

[0039] Establish electromagnetic transient model of offshore wind power grid-connected system on simulation platform;

[0040] Calculate the impedance scanning values ​​corresponding to the electromagnetic transient model at each preset oscillation frequency using a frequency scanning method;

[0041] Inputting the operation data corresponding to the electromagnetic transient model at each preset oscillation frequency into the offshore wind power grid-connected system impedance model, so that the offshore wind power grid-connected system impedance model outputs the corresponding equivalent impedances;

[0042] Calculating the absolute value of the difference between each of the impedance scan values ​​and the corresponding equivalent impedances to obtain each error value;

[0043] An error average value is calculated according to the various error values ​​to obtain the model accuracy of the offshore wind power grid-connected system impedance model.

[0044] The embodiment of the present application provides a method for calculating model accuracy, by establishing an electromagnetic transient model of an offshore wind power grid-connected system on a simulation platform, using the frequency scanning method and the offshore wind power grid-connected system impedance model provided by the present application to calculate the impedance scanning values ​​and equivalent impedance calculation values ​​corresponding to the electromagnetic transient model at each preset oscillation frequency, comparing each impedance scanning value with the equivalent impedance calculation value, and calculating the model accuracy of the offshore wind power grid-connected system impedance model to achieve quantitative description of the model accuracy. Subsequently, the model can be screened or adjusted according to the model accuracy to ensure that the model can output an accurate first equivalent impedance value during use, thereby ensuring the accuracy of the judgment of the stability of the offshore wind power grid-connected system.

[0045] In a second aspect, accordingly, the present application provides a stability analysis system for an offshore wind power grid-connected system, comprising a first acquisition module, an impedance calculation module, a second acquisition module, and an analysis module;

[0046] Wherein, the first acquisition module is used to acquire the real-time operation data of the wind turbines and static reactive power generation devices of the offshore wind power grid-connected system;

[0047] The impedance calculation module is used to input the real-time operation data into a preset offshore wind power grid-connected system impedance model, so that the offshore wind power grid-connected system impedance model calculates the first equivalent impedance of the offshore wind power grid-connected system according to the real-time operation data, wherein the offshore wind power grid-connected system impedance model is constructed based on the impedance models of various parts of the offshore wind power grid-connected system, including a wind turbine impedance model, a wind farm impedance model, a submarine cable impedance model, and a static VAR generator impedance model;

[0048] The second acquisition module is used to obtain a second equivalent impedance of the power grid according to a preset power grid harmonic impedance model or an actual power grid harmonic scanning value;

[0049] The analysis module is used to perform stability analysis on the offshore wind power grid-connected system according to the first equivalent impedance and the second equivalent impedance.

[0050] In a possible implementation, the impedance calculation module includes a wind turbine high-voltage side admittance calculation unit, a wind farm high-voltage side admittance calculation unit, a submarine cable delivery system impedance calculation unit, a static reactive power generating device high-voltage side impedance calculation unit, and a grid-connected system impedance calculation unit;

[0051] The wind turbine high-voltage side admittance calculation unit is used to input the real-time operation data of the wind turbine into the wind turbine impedance model, so that the wind turbine impedance model outputs the wind turbine high-voltage side admittance data;

[0052] The wind farm high-voltage side admittance calculation unit is used to input the high-voltage side admittance data of the wind turbine group into the wind farm impedance model, so that the wind farm impedance model outputs the wind farm high-voltage side admittance data;

[0053] The submarine cable delivery system impedance calculation unit is used to input the high-voltage side admittance data of the wind farm into the submarine cable impedance model, so that the submarine cable impedance model outputs the impedance data of the submarine cable delivery system;

[0054] The static VAR generator high-voltage side impedance calculation unit is used to input the impedance data of the submarine cable delivery system and the real-time operation data of the static VAR generator into the static VAR generator impedance model, so that the static VAR generator impedance model outputs the impedance data of the high-voltage side of the static VAR generator;

[0055] The grid-connected system impedance calculation unit is used to calculate and obtain the first equivalent impedance of the offshore wind power grid-connected system based on the impedance data of the submarine cable delivery system and the impedance data of the high-voltage side of the static VAR generator. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 : A flow chart of a stability analysis method for an offshore wind power grid-connected system provided in an embodiment of the present application.

[0057] Figure 2 : A flow chart of a method for calculating the first equivalent impedance in a stability analysis method for an offshore wind power grid-connected system provided in an embodiment of the present application.

[0058] Figure 3 :This is a power circuit and control block diagram of a wind turbine in an offshore wind power grid-connected system.

[0059] Figure 4 : A schematic diagram of the equivalent circuit of a wind farm in an offshore wind power grid-connected system.

[0060] Figure 5 : A schematic diagram of the equivalent circuit of a submarine cable in an offshore wind power grid-connected system.

[0061] Figure 6 :This is a topology and control block diagram of a static reactive power generator in an offshore wind power grid-connected system.

[0062] Figure 7 : Schematic diagram of the equivalent circuit of an offshore wind power grid-connected system.

[0063] Figure 8 : Another flow chart of a stability analysis method for an offshore wind power grid-connected system provided in an embodiment of the present application.

[0064] Fig. 9 : It is a structural schematic diagram of the offshore wind power grid-connected system of a certain offshore wind power transmission project in an embodiment of the present application.

[0065] Fig.10 : A schematic diagram of the equivalent impedance of the power grid of a certain offshore wind power transmission project in an embodiment of the present application.

[0066] Fig.11 : It is a schematic diagram comparing the analytical value of the impedance model of the offshore wind power grid-connected system of a certain offshore wind power transmission project in an embodiment of the present application and the scanning value of the electromagnetic transient model.

[0067] Fig.12 : It is a schematic diagram comparing the frequency characteristics of the offshore wind power grid-connected system impedance and the grid impedance at medium and high frequencies of a certain offshore wind power transmission project in an embodiment of the present application.

[0068] Fig.13 : Schematic diagram of the operation data changes of the simulation model built on the PSCAD / EMTDC platform according to the embodiment of the present application.

[0069] Fig.14: A structural schematic diagram of a stability analysis system for an offshore wind power grid-connected system provided in an embodiment of the present application.

[0070] Fig.15 : A structural schematic diagram of the impedance calculation module of a stability analysis system for an offshore wind power grid-connected system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0071] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0072] It should be noted that the step numbers in the text are only for the convenience of explanation of the specific embodiments and do not serve to limit the order in which the steps are executed. In the description of this application, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0073] Embodiment 1:

[0074] like Figure 1 As shown, embodiment 1 provides a stability analysis method for an offshore wind power grid-connected system, including steps S1-S4;

[0075] Step S1, obtaining real-time operating data of wind turbines and static reactive power generating devices of an offshore wind power grid-connected system;

[0076] Step S2, inputting the real-time operation data into a preset offshore wind power grid-connected system impedance model, so that the offshore wind power grid-connected system impedance model calculates and obtains a first equivalent impedance of the offshore wind power grid-connected system according to the real-time operation data, wherein the offshore wind power grid-connected system impedance model is constructed based on impedance models of various parts of the offshore wind power grid-connected system, including a wind turbine impedance model, a wind farm impedance model, a submarine cable impedance model, and a static VAR generator impedance model;

[0077] Step S3, obtaining a second equivalent impedance of the power grid according to a preset power grid harmonic impedance model or an actual power grid harmonic scanning value;

[0078] Step S4: performing stability analysis on the offshore wind power grid-connected system according to the first equivalent impedance and the second equivalent impedance.

[0079] The embodiment of the present application provides a stability analysis method for an offshore wind power grid-connected system, which obtains the first equivalent impedance of the offshore wind power grid-connected system as a whole by acquiring the real-time operating data of the wind turbines and the static reactive power generator and inputting it into the impedance model of the offshore wind power grid-connected system. The impedance model of the offshore wind power grid-connected system is constructed based on the impedance models of the various parts of the offshore wind power grid-connected system, while considering the influence of the wind turbine impedance, wind farm impedance, submarine cable impedance and static reactive power generator impedance on the equivalent impedance of the offshore wind power grid-connected system, and modeling is performed based on the complete wind power transmission link to improve the accuracy of the model's equivalent impedance calculation. Furthermore, on the basis of accurately calculating the first equivalent impedance, the embodiment of the present application also combines the second equivalent impedance of the power grid for stability analysis, thereby improving the accuracy of the judgment of the stability of the offshore wind power grid-connected system and providing data support for the actual engineering assessment of system oscillation risks and suppression of harmonic resonance.

[0080] In a possible implementation, in step S2, the offshore wind power grid-connected system impedance model calculates and obtains a first equivalent impedance of the offshore wind power grid-connected system according to the real-time operation data, such as Figure 2 As shown, steps S201-S205 are included:

[0081] Step S201, inputting the real-time operation data of the wind turbine generator set into the impedance model of the wind turbine generator set, so that the impedance model of the wind turbine generator set outputs the high-voltage side admittance data of the wind turbine generator set;

[0082] Step S202, inputting the high-voltage side admittance data of the wind turbine group into the wind farm impedance model, so that the wind farm impedance model outputs the high-voltage side admittance data of the wind farm;

[0083] Step S203, inputting the high-voltage side admittance data of the wind farm into the submarine cable impedance model, so that the submarine cable impedance model outputs the impedance data of the submarine cable delivery system;

[0084] Step S204: input the impedance data of the submarine cable delivery system and the real-time operation data of the static VAR generator into the impedance model of the static VAR generator, so that the impedance model of the static VAR generator outputs the impedance data of the high-voltage side of the static VAR generator;

[0085] Step S205: Calculate and obtain the first equivalent impedance of the offshore wind power grid-connected system based on the impedance data of the submarine cable delivery system and the impedance data of the high-voltage side of the static VAR generator.

[0086] The embodiment of the present application provides a method for calculating and obtaining the first equivalent impedance of an offshore wind power grid-connected system, wherein the calculation is performed step by step from the network end of the offshore wind power grid-connected system to the grid-connected point, and the high-voltage side admittance data of the wind turbine group, the high-voltage side admittance data of the wind farm, the impedance data of the submarine cable delivery system, the impedance data of the high-voltage side of the static VAR generator, and the first equivalent impedance of the offshore wind power grid-connected system are calculated in turn according to the corresponding impedance models, wherein each model is calculated on the basis of the previous model, thereby ensuring the integrity and tightness of the wind power delivery process and the calculation accuracy of the impedance model of the offshore wind power grid-connected system.

[0087] Wherein, in step S201, the process of constructing the wind turbine impedance model includes:

[0088] The wind turbine impedance model is obtained by jointly constructing a frequency domain small signal main loop equation of a converter in the wind turbine and a frequency domain small signal control equation of the converter.

[0089] In a preferred embodiment, the wind turbine generator set is composed of several main parts such as a wind turbine, a converter, and a step-up transformer. The converter is divided into a grid-side converter and a machine-side converter. The grid-side converter and the machine-side converter are decoupled through the DC side capacitor, and the wind turbine and the machine-side converter are equivalent to a constant power current source. The grid-side converter adopts a double closed-loop control structure to control the reactive power and DC bus voltage of the wind turbine generator set.

[0090] The low-voltage side and high-voltage side of the wind turbine refer to the output port of the converter and the output port of the high-voltage side of the step-up transformer. The impedance of the high-voltage side of the wind turbine is defined as Z G , the impedance of the low voltage side of the wind turbine is Z g The typical power circuit and control block diagram of a wind turbine is as follows: Figure 3 shown. Figure 3 Where PLL is the phase-locked loop link, abc / dq link is the Park transformation link from the three-phase abc coordinate system to the two-phase rotating dq coordinate system, dq / abc link is the inverse Park transformation link from the two-phase rotating dq coordinate system to the three-phase abc coordinate system, PWM is the pulse width modulation link, P in is the equivalent current source power, C dc is the DC bus capacitance, u dc is the DC bus capacitor voltage, L f is the filter inductor, R d is the damping resistor, C f is the filter capacitor, L t1 is the transformer leakage inductance, k t1 is the transformer ratio, u gabc is the three-phase AC voltage on the grid side, u gdq for u gabc The dq component, u mabcis the three-phase modulation voltage of the converter, i gabc is the three-phase AC current on the grid side, i gdq for i gabc The dq component, q g is the grid-side reactive power, θ pll is the phase-locked loop output phase angle, s is a complex variable, H dc is the voltage outer loop, K dp is the voltage outer loop proportional coefficient, K di is the voltage outer loop integral coefficient, H dc (s) = K dp +K di / s,H q is the reactive outer loop, K qp is the reactive outer loop proportional coefficient, K qi is the reactive outer loop integral coefficient, H q (s) = K qp +K qi / s,H i is the inner current loop, K ip K is the current inner loop proportional coefficient, ii is the current inner loop integral coefficient, H i (s) = K ip +K ii / s,H f For the low-pass filter link, T in is the time constant of the low-pass filter, H f (s) = 1 / (1 + sT in ), H S is the zero-order hold link, T s is the time constant of the zero-order holding link, H D is the delay link, T d1 is the time constant of the delay link, K d is the current loop decoupling control coefficient.

[0091] The inverter of the wind turbine has a frequency coupling effect, that is, when the three-phase AC voltage u gabc There is a frequency f P When the AC voltage is disturbed, the three-phase AC current i gabc And the three-phase modulation voltage u of the converter mabc The frequency f will be generated P With a frequency of 2f 1 -f P1 The disturbance response of 1 The base frequency is 50Hz. In the medium and high frequency bands, the frequency coupling effect of wind turbines is weak and can be ignored, that is, u gabc There is a frequency f PWhen the AC voltage is disturbed, i gabc and u mabc Only the frequency f P The disturbance response.

[0092] Taking phase A as an example, the three-phase AC voltage on the grid side, the three-phase AC current on the grid side, and the three-phase modulation voltage on the grid side are defined as u ga 、i ga 、u ma ,u ga 、i ga 、u ma The Fourier coefficients of the disturbance are ω is the angular frequency, j is the imaginary unit, and the frequency domain small signal main loop equation of the converter is:

[0093]

[0094] The wind turbine impedance model can ignore the influence of phase-locked loop and DC bus voltage fluctuation in the medium and high frequency bands. In the medium and high frequency bands, the proportional link of PI control is dominant and the integral link can be ignored. Taking phase A as an example, the frequency domain small signal control equation of the converter is:

[0095]

[0096] Among them, K u1 is the ratio of the voltage reference value to the current reference value, I 0 * with U 0 * for i ga with u ga The conjugate quantity of the steady-state value. In the formula, K related to the reactive outer loop qp K ip U 0 * The term has little effect at medium and high frequencies and can be ignored. Combining the above two equations, the impedance of the wind turbine can be obtained:

[0097]

[0098] Where Z f is the impedance of the parallel filter branch, and Z f =1 / jωC f +R d The high-voltage side admittance of the wind turbine is defined as Y G , there is Y G =1 / Z G .

[0099] In step S202, the process of constructing the wind farm impedance model includes:

[0100] According to the impedance and admittance at the feeder end of the wind farm, the impedance and admittance of the feeder port are derived to obtain the expressions of the impedance and admittance of the feeder port;

[0101] The wind farm impedance model is constructed based on the expressions of impedance and admittance of the feeder port.

[0102] In a preferred embodiment, consider that the wind farm is composed of m feeders, and a single feeder is connected to n wind turbines. The j-th wind turbine on the i-th feeder is G ij , the jth connecting cable on the ith feeder is l ij . Connect the cable ij It is equivalent to a π-type equivalent circuit with concentrated parameters. The equivalent circuit diagram of the wind farm is as follows: Figure 4 As shown, define the connection cable l ij The equivalent impedance of the π-type equivalent circuit is Z aij , the equivalent admittance is Y bij , and its calculation formula is as follows:

[0103]

[0104] Where i = 1, 2, ..., m, j = 1, 2, ..., n, l 0 for l ij The length of z 0 and 0 are the impedance and admittance per unit length respectively, γ is the propagation constant of the line, Z C is the wave impedance,

[0105] Wind turbine G ij The admittance is Y Gij , Y Gij According to the above wind turbine high-voltage side admittance Y G The calculation method is used to find out. ij The impedance and admittance of the port are Z Eij With Y Eij . Define the initial value Y Ei0 =0, Y Fi =1 / Z Fi , derive Y from the end of the feeder forward Eij expression:

[0106] Y Eij =Y bij +1 / [1 / (Y bij +Y Ei(j-1) +Y Gij )+Z aij ]

[0107] Where i = 1, 2, ..., m, j = 1, 2, ..., n, l 0 for l ij The length of z 0 and 0 are the impedance and admittance per unit length respectively, γ is the propagation constant of the line, Z C is the wave impedance,

[0108] The impedance of the feeder port connected to n wind turbines is Z Fi , the feeder port admittance is Y Fi , there is Z Fi =Z Ein , Y Fi =1 / Z Fi The wind farm is boosted by the offshore booster station, and the equivalent impedance of the high-voltage side of the wind farm is Z W , Z W The calculation formula is:

[0109]

[0110] Among them, L t2 is the equivalent inductance of the offshore boost transformer, k t2 is the transformer ratio. The equivalent admittance on the high voltage side of the wind farm is Y W , there is Y W =1 / Z W .

[0111] In step S203, the process of constructing the submarine cable impedance model includes:

[0112] According to the impedance and admittance at the end of the submarine cable, the impedance and admittance expressions of the submarine cable port are derived to the submarine cable port;

[0113] The submarine cable impedance model is constructed based on the expressions of impedance and admittance of the submarine cable port.

[0114] In a preferred embodiment, the modeling method of the submarine cable is the same as that of the connecting cable. The submarine cable equivalent circuit is as follows: Figure 5 As shown. Assume that the equivalent impedance of the submarine cable is Z La , the equivalent admittance is Y Lb , the impedance of the equivalent admittance port of the submarine cable in the system is Z Lc The impedance of the wind farm transmission system through the submarine cable is Z L1 , then:

[0115]

[0116] Among them, L X1 With LX2 is the inductance of the high-voltage reactor, and the equivalent admittance Y on the high-voltage side of the wind farm W Obtained in step S202.

[0117] In step S204, the process of constructing the impedance model of the static VAR generator includes:

[0118] Determine the AC voltage steady-state operating point of the static VAR generator;

[0119] Superimposing a signal disturbance of a preset frequency at the AC voltage steady-state operating point;

[0120] Analyze the frequency domain distribution characteristics of the signal disturbance between various voltages, currents and modulation signals, and determine the frequency domain small signal main loop equation of the static VAR generator and the frequency domain small signal control equation of the converter of the static VAR generator;

[0121] The impedance model of the static VAR generator is obtained by jointly constructing a frequency domain small signal main loop equation of the static VAR generator and a frequency domain small signal control equation of the converter of the static VAR generator.

[0122] In a preferred embodiment, the static VAR generator adopts an angular chain topology structure, and each bridge arm is composed of n submodules connected in series with a bridge arm inductor. The static VAR generator controller consists of a capacitor voltage outer loop, a reactive current command value calculation module, a three-phase phase-locked loop, and a current inner loop.

[0123] The impedance of the high voltage side of the static VAR generator is defined as Z S2 , the low voltage side impedance of the static VAR generator is Z S The topology and control block diagram of the static VAR generator is as follows: Figure 6 As shown, the modulation voltage of the ab phase bridge arm of the static reactive power generator is u nab , the AC voltage of the bridge arm of phase ab is u ab , the AC current of the ab phase bridge arm is i ab , the bridge arm inductance is L m . G d is the delay link, T d2 is the time constant of the delay link,

[0124] G PR To control the inner loop PR link, K p1 To control the inner loop proportional coefficient, K i1 To control the inner ring resonance coefficient, ω c1 To control the inner ring resonant frequency, G PR2 K is the current feedforward PR link, i2is the current feedforward resonance coefficient, ω c2 is the current feedforward resonant frequency,

[0125] G PRi It includes the control inner loop PR link and the current feedforward PR link, G PRi =G PR +G PR2 . Where L t3 is the transformer equivalent inductance, k t3 is the transformer ratio.

[0126] A small signal disturbance of a specific frequency is superimposed on the AC voltage steady-state operating point of the static VAR generator, and then the frequency domain distribution characteristics of the small signal between the voltage, current and modulation signal are analyzed to obtain the high-frequency impedance model of the static VAR generator. Taking the ab phase bridge arm as an example, the Fourier coefficients of the disturbances of the ab phase bridge arm modulation voltage, bridge arm AC voltage, and bridge arm AC current are defined as The frequency domain small signal main loop equation of the static VAR generator is:

[0127]

[0128] The impedance characteristics of the static VAR generator in the medium and high frequency bands can ignore the influence of the phase-locked loop and capacitor voltage fluctuations. The frequency domain small signal control equation of the converter is:

[0129]

[0130] Among them, K u2 is the ratio of the voltage reference value to the current reference value, U Q0 with I Q0 is the steady-state value of the voltage and current positive sequence components of the reactive outer loop command value. In the medium and high frequency bands, the PR control link can be approximated as the PI control link, and U related to the reactive outer loop Q0 G PR The term has little effect in the medium and high frequencies and can be ignored. Combining the above frequency domain small signal main loop equation and the frequency domain small signal control equation, the impedance model of the static VAR generator can be obtained as follows:

[0131]

[0132] In a preferred embodiment, the first equivalent impedance of the offshore wind power grid-connected system is calculated and obtained according to the impedance data of the submarine cable delivery system and the impedance data of the high-voltage side of the static VAR generator in step S205, specifically:

[0133] The three-winding transformer is connected to the static reactive power generator, the submarine cable, and the onshore power grid. Its equivalent circuit is as follows: Figure 7 As shown, where L t3, L t4 , L t5 are the equivalent inductance of the static reactive power generator, submarine cable, and transformer on the land grid side, respectively, k t3 k is the transformer step-up ratio from the static reactive power generator to the onshore power grid, t4 is the transformer step-up ratio from submarine cable to onshore power grid, k t5 is the voltage step-up ratio on both sides of the transformer. The impedance of the submarine cable branch and the static reactive power generating device branch connected in parallel is Z LS , the offshore wind power grid-connected system impedance, i.e. the first equivalent impedance, is Z wind The impedance of the wind farm after the voltage is boosted by the submarine cable transmission system is Z L2 , whose expression is:

[0134]

[0135] Among them, the impedance of the wind farm transmission system through the submarine cable is Z L1 In step S203, the high-voltage side impedance Z of the static VAR generator is obtained. S2 Obtained in step S204.

[0136] The wind turbine impedance model and static reactive power generation device impedance model established in the embodiment of the present application are mainly aimed at the medium and high frequency domain, wherein the medium and high frequency refers to the frequency band from 200Hz to several kHz. In addition to ignoring the frequency coupling effect and the link whose control bandwidth is far lower than the medium and high frequency band, the established model also makes simplified approximations for PI control and PR control, especially dividing the influence objects of the reactive power outer loop into two parts: current and voltage small disturbances, and ignoring the influence of the reactive power outer loop on the current small disturbance, so that the model is simpler while ensuring a certain accuracy.

[0137] In a possible implementation, in step S3, performing stability analysis on the offshore wind power grid-connected system according to the first equivalent impedance and the second equivalent impedance includes:

[0138] generating a corresponding first amplitude-phase-frequency characteristic curve according to the first equivalent impedance;

[0139] generating a corresponding second amplitude-phase-frequency characteristic curve according to the second equivalent impedance;

[0140] If there is an intersection point between the first amplitude-phase-frequency characteristic curve and the second amplitude-phase-frequency characteristic curve, obtaining an intersection point frequency corresponding to the intersection point;

[0141] If the phase difference between the first amplitude-phase-frequency characteristic curve at the intersection frequency and the second amplitude-phase-frequency characteristic curve at the intersection frequency is greater than 180 degrees, it is determined that the offshore wind power grid-connected system is unstable.

[0142] The embodiment of the present application provides a stability analysis method, which draws the corresponding first amplitude-phase frequency characteristic curve and the second amplitude-phase frequency characteristic curve according to the first equivalent impedance and the second equivalent impedance, and then compares and analyzes the first amplitude-phase frequency characteristic curve and the second amplitude-phase frequency characteristic curve. In the analysis process, the present application uses the Nyquist stability criterion. If there is an intersection between the first amplitude-phase frequency characteristic curve and the second amplitude-phase frequency characteristic curve and the phase difference between the two at the intersection frequency is greater than 180°, it can be determined that the system is unstable, thereby realizing the determination of the stability of the offshore wind power grid-connected system based on real-time operation data, and being able to detect the system oscillation risk in a timely manner, thereby improving the stability and safety of the operation of the offshore wind power grid-connected system.

[0143] In a possible implementation manner, the method further includes calculating the model accuracy of the offshore wind power grid-connected system impedance model, including:

[0144] Establish electromagnetic transient model of offshore wind power grid-connected system on simulation platform;

[0145] Calculate the impedance scanning values ​​corresponding to the electromagnetic transient model at each preset oscillation frequency using a frequency scanning method;

[0146] Inputting the operation data corresponding to the electromagnetic transient model at each preset oscillation frequency into the offshore wind power grid-connected system impedance model, so that the offshore wind power grid-connected system impedance model outputs the corresponding equivalent impedances;

[0147] Calculating the absolute value of the difference between each of the impedance scan values ​​and the corresponding equivalent impedances to obtain each error value;

[0148] An error average value is calculated according to the various error values ​​to obtain the model accuracy of the offshore wind power grid-connected system impedance model.

[0149] In a preferred embodiment, an electromagnetic transient model of an offshore wind power grid-connected system is established on the PSCAD / EMTDC platform, and the frequency scanning method is used to obtain the model impedance scanning value. The absolute value of the difference between the impedance scanning value at each frequency and the theoretical analytical value is calculated, and then the average value of the error is taken to obtain the model average error, which quantitatively describes the model accuracy.

[0150] The embodiment of the present application provides a method for calculating model accuracy, by establishing an electromagnetic transient model of an offshore wind power grid-connected system on a simulation platform, using the frequency scanning method and the offshore wind power grid-connected system impedance model provided by the present application to calculate the impedance scanning values ​​and equivalent impedance calculation values ​​corresponding to the electromagnetic transient model at each preset oscillation frequency, comparing each impedance scanning value with the equivalent impedance calculation value, and calculating the model accuracy of the offshore wind power grid-connected system impedance model to achieve quantitative description of the model accuracy. Subsequently, the model can be screened or adjusted according to the model accuracy to ensure that the model can output an accurate first equivalent impedance value during use, thereby ensuring the accuracy of the judgment of the stability of the offshore wind power grid-connected system.

[0151] A method flow chart of an embodiment of the present application is as follows Figure 8 As shown, based on the connection structure of the offshore wind power grid-connected system, the wind turbine impedance model, wind farm impedance model, submarine cable impedance model, static VAR generator impedance model and offshore wind power grid-connected system impedance model are established in sequence, and the model accuracy of the offshore wind power grid-connected system impedance model is calculated to ensure that the model can output accurate impedance values ​​during use. Finally, according to the first equivalent impedance value output by the model and the second equivalent impedance of the power grid, the impedance analysis method is used to analyze the stability of the offshore wind power grid-connected system.

[0152] In order to verify the effectiveness of the method proposed in this application, the embodiment of this application also uses an offshore wind power transmission project for example verification. The structure of the offshore wind power grid-connected system is shown in FIG. Fig. 9 The offshore wind farm has 77 11MW direct-drive wind turbines, which are connected to a 66kV collector line. The collector line adopts a chain topology structure, with a total of 18 feeders. The length of the connecting cable in the feeder ranges from 1km to 11km, and the cross-sectional area ranges from 95mm 2 ~500mm 2 The collector line is connected to a 330kV offshore substation, where three step-up transformers and three sets of 110Mvar high-voltage reactors are arranged. After the boost, the electric energy passes through three circuits with a length of 71km and a cross-sectional area of ​​630mm. 2 The submarine cable is sent to the onshore centralized control center. The onshore centralized control center is equipped with two three-winding step-up transformers and three sets of 160Mvar high-voltage reactors. The 36kV side of the transformer is connected to two sets of 100Mvar SVG, and the 525kV side is connected to the onshore power grid. Fig. 9 The PCC point in the diagram is the grid connection point of the offshore wind farm.

[0153] Consider the equivalent impedance of the power grid Fig.10 As shown. Where C grid =1.7F, L grid =1H, R grid1 =0.01Ω, Rgrid2 =0.01Ω, V grid is the voltage source on the AC side.

[0154] Establish a medium and high frequency accurate impedance model of offshore wind power grid-connected system, and compare the impedance scanning value of PSCAD / EMTDC electromagnetic transient model, calculate the modeling accuracy, and verify the correctness of the model. Wind turbines, wind farms, wind farms sent through submarine cables, static reactive power generation devices, offshore wind power grid-connected system high frequency impedance model scanning values ​​and analytical values ​​are as follows Fig.11 As shown in the figure, the model is highly consistent. The average error between the analytical value of the model and the impedance scanning value of the electromagnetic transient model is 3.7%, and the model accuracy is high.

[0155] Use impedance analysis method to analyze the stability of the interconnected system and draw the first equivalent impedance Z of the grid-connected system grid The second equivalent impedance Z of the power grid wind In the mid-high frequency frequency characteristics such as Fig.12 shown. Fig.12 The phase difference at the intersection of medium impedance 1812Hz is 184°, which does not meet the stable phase difference condition and the system becomes unstable.

[0156] A simulation model was built on the PSCAD / EMTDC platform to verify the stability analysis results. First, the offshore wind power grid-connected system was connected to an ideal voltage source to verify that it could operate independently and stably. Then, the grid impedance was added to the voltage source side to simulate the actual grid-connected situation. The system delay was set to 0 seconds at the initial moment. After the system stabilized at t=1.5s, the delay was switched to 520 microseconds. The A-phase current I of the system grid-connected point during the simulation period of t=1.5~1.66s PCC,A The waveform is as Fig.13 (a) shows that for I PCC,A The result of fast Fourier transform is as follows Fig.13 As shown in (b), the system is unstable after the delayed switching, and the current amplitude gradually diverges. The fast Fourier transform result shows that the oscillation frequency is 1812Hz, which is consistent with the analysis result of the impedance analysis method, indicating that the method provided by this application is correct and effective.

[0157] Embodiment 2:

[0158] like Fig.14 As shown, the second embodiment provides a stability analysis system for an offshore wind power grid-connected system, comprising a first acquisition module 10, an impedance calculation module 20, a second acquisition module 30 and an analysis module 40;

[0159] Wherein, the first acquisition module 10 is used to acquire the real-time operation data of the wind turbines and static reactive power generation devices of the offshore wind power grid-connected system;

[0160] The impedance calculation module 20 is used to input the real-time operation data into a preset offshore wind power grid-connected system impedance model, so that the offshore wind power grid-connected system impedance model calculates the first equivalent impedance of the offshore wind power grid-connected system according to the real-time operation data, wherein the offshore wind power grid-connected system impedance model is constructed based on the impedance models of various parts of the offshore wind power grid-connected system, including a wind turbine impedance model, a wind farm impedance model, a submarine cable impedance model, and a static VAR generator impedance model;

[0161] The second acquisition module 30 is used to obtain a second equivalent impedance of the power grid according to a preset power grid harmonic impedance model or an actual power grid harmonic scanning value;

[0162] The analysis module 40 is used to perform stability analysis on the offshore wind power grid-connected system according to the first equivalent impedance and the second equivalent impedance.

[0163] In one possible implementation, Fig.15 As shown, the impedance calculation module 20 includes a wind turbine high-voltage side admittance calculation unit 201, a wind farm high-voltage side admittance calculation unit 202, a submarine cable delivery system impedance calculation unit 203, a static VAR generator high-voltage side impedance calculation unit 204, and a grid-connected system impedance calculation unit 205;

[0164] The wind turbine high-voltage side admittance calculation unit 201 is used to input the real-time operation data of the wind turbine into the wind turbine impedance model, so that the wind turbine impedance model outputs the wind turbine high-voltage side admittance data;

[0165] The wind farm high-voltage side admittance calculation unit 202 is used to input the wind turbine high-voltage side admittance data into the wind farm impedance model, so that the wind farm impedance model outputs the wind farm high-voltage side admittance data;

[0166] The submarine cable delivery system impedance calculation unit 203 is used to input the high-voltage side admittance data of the wind farm into the submarine cable impedance model, so that the submarine cable impedance model outputs the impedance data of the submarine cable delivery system;

[0167] The static VAR generator high-voltage side impedance calculation unit 204 is used to input the impedance data of the submarine cable delivery system and the real-time operation data of the static VAR generator into the static VAR generator impedance model, so that the static VAR generator impedance model outputs the impedance data of the high-voltage side of the static VAR generator;

[0168] The grid-connected system impedance calculation unit 205 is used to calculate and obtain the first equivalent impedance of the offshore wind power grid-connected system according to the impedance data of the submarine cable delivery system and the impedance data of the high-voltage side of the static VAR generator.

[0169] The embodiment of the present application provides a stability analysis system for an offshore wind power grid-connected system, which obtains the first equivalent impedance of the offshore wind power grid-connected system as a whole by acquiring the real-time operating data of the wind turbines and the static reactive power generator and inputting it into the impedance model of the offshore wind power grid-connected system. The impedance model of the offshore wind power grid-connected system is constructed based on the impedance models of the various parts of the offshore wind power grid-connected system, while considering the influence of the wind turbine impedance, wind farm impedance, submarine cable impedance and static reactive power generator impedance on the equivalent impedance of the offshore wind power grid-connected system, and modeling is performed based on the complete wind power transmission link to improve the accuracy of the model's equivalent impedance calculation. Furthermore, on the basis of accurately calculating the first equivalent impedance, the embodiment of the present application also combines the second equivalent impedance of the power grid for stability analysis, thereby improving the accuracy of the judgment of the stability of the offshore wind power grid-connected system and providing data support for the actual engineering assessment of system oscillation risks and suppression of harmonic resonance.

[0170] The more detailed working principle and step flow of this embodiment can refer to, but are not limited to, the relevant records of Embodiment 1.

[0171] The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present application in detail. It should be understood that the above description is only a specific embodiment of the present application and is not intended to limit the scope of protection of the present application. It is particularly pointed out that for those skilled in the art, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of protection of the present application.

Claims

1. A stability analysis method for an offshore wind power grid-connected system, characterized in that: include: Obtain real-time operating data of wind turbines and static reactive power generation devices in offshore wind power grid-connected systems; Inputting the real-time operation data into a preset offshore wind power grid-connected system impedance model, so that the offshore wind power grid-connected system impedance model calculates and obtains a first equivalent impedance of the offshore wind power grid-connected system according to the real-time operation data, wherein the offshore wind power grid-connected system impedance model is constructed based on impedance models of various parts of the offshore wind power grid-connected system, including a wind turbine impedance model, a wind farm impedance model, a submarine cable impedance model, and a static VAR generator impedance model; Obtaining a second equivalent impedance of the power grid according to a preset power grid harmonic impedance model or an actual power grid harmonic scanning value; A stability analysis of the offshore wind power grid-connected system is performed based on the first equivalent impedance and the second equivalent impedance.

2. A stability analysis method for an offshore wind power grid-connected system according to claim 1, characterized in that: The offshore wind power grid-connected system impedance model calculates and obtains a first equivalent impedance of the offshore wind power grid-connected system according to the real-time operation data, including: Inputting the real-time operation data of the wind turbine generator set into the impedance model of the wind turbine generator set, so that the impedance model of the wind turbine generator set outputs the high-voltage side admittance data of the wind turbine generator set; Inputting the high-voltage side admittance data of the wind turbine group into the wind farm impedance model, so that the wind farm impedance model outputs the high-voltage side admittance data of the wind farm; Inputting the high-voltage side admittance data of the wind farm into the submarine cable impedance model, so that the submarine cable impedance model outputs the impedance data of the submarine cable delivery system; Inputting the impedance data of the submarine cable delivery system and the real-time operation data of the static VAR generator into the impedance model of the static VAR generator, so that the impedance model of the static VAR generator outputs the impedance data of the high-voltage side of the static VAR generator; The first equivalent impedance of the offshore wind power grid-connected system is calculated based on the impedance data of the submarine cable delivery system and the impedance data of the high-voltage side of the static VAR generator.

3. A stability analysis method for an offshore wind power grid-connected system according to claim 1, characterized in that: The performing stability analysis on the offshore wind power grid-connected system according to the first equivalent impedance and the second equivalent impedance comprises: generating a corresponding first amplitude-phase-frequency characteristic curve according to the first equivalent impedance; generating a corresponding second amplitude-phase-frequency characteristic curve according to the second equivalent impedance; If there is an intersection point between the first amplitude-phase-frequency characteristic curve and the second amplitude-phase-frequency characteristic curve, obtaining an intersection point frequency corresponding to the intersection point; If the phase difference between the first amplitude-phase-frequency characteristic curve at the intersection frequency and the second amplitude-phase-frequency characteristic curve at the intersection frequency is greater than 180 degrees, it is determined that the offshore wind power grid-connected system is unstable.

4. A stability analysis method for an offshore wind power grid-connected system according to claim 1, characterized in that: The construction process of the wind turbine impedance model includes: The wind turbine impedance model is obtained by jointly constructing a frequency domain small signal main loop equation of a converter in the wind turbine and a frequency domain small signal control equation of the converter.

5. The stability analysis method of an offshore wind power grid-connected system according to claim 1, characterized in that: The construction process of the wind farm impedance model includes: According to the impedance and admittance at the feeder end of the wind farm, the impedance and admittance of the feeder port are derived to obtain the expressions of the impedance and admittance of the feeder port; The wind farm impedance model is constructed based on the expressions of impedance and admittance of the feeder port.

6. A stability analysis method for an offshore wind power grid-connected system according to claim 1, characterized in that: The construction process of the submarine cable impedance model includes: According to the impedance and admittance at the end of the submarine cable, the impedance and admittance expressions of the submarine cable port are derived to the submarine cable port; The submarine cable impedance model is constructed based on the expressions of impedance and admittance of the submarine cable port.

7. A stability analysis method for an offshore wind power grid-connected system according to claim 1, characterized in that: The construction process of the impedance model of the static VAR generator includes: Determine the AC voltage steady-state operating point of the static VAR generator; Superimposing a signal disturbance of a preset frequency at the AC voltage steady-state operating point; Analyze the frequency domain distribution characteristics of the signal disturbance between various voltages, currents and modulation signals, and determine the frequency domain small signal main loop equation of the static VAR generator and the frequency domain small signal control equation of the converter of the static VAR generator; The impedance model of the static VAR generator is obtained by jointly constructing a frequency domain small signal main loop equation of the static VAR generator and a frequency domain small signal control equation of the converter of the static VAR generator.

8. A stability analysis method for an offshore wind power grid-connected system according to any one of claims 1 to 7, characterized in that: The method further includes calculating the model accuracy of the offshore wind power grid-connected system impedance model, including: Establish electromagnetic transient model of offshore wind power grid-connected system on simulation platform; Calculate the impedance scanning values ​​corresponding to the electromagnetic transient model at each preset oscillation frequency using a frequency scanning method; Inputting the operation data corresponding to the electromagnetic transient model at each preset oscillation frequency into the offshore wind power grid-connected system impedance model, so that the offshore wind power grid-connected system impedance model outputs the corresponding equivalent impedances; Calculating the absolute value of the difference between each of the impedance scan values ​​and the corresponding equivalent impedances to obtain each error value; An error average value is calculated according to the various error values ​​to obtain the model accuracy of the offshore wind power grid-connected system impedance model.

9. A stability analysis system for an offshore wind power grid-connected system, characterized in that: It includes a first acquisition module, an impedance calculation module, a second acquisition module and an analysis module; Wherein, the first acquisition module is used to acquire the real-time operation data of the wind turbines and static reactive power generation devices of the offshore wind power grid-connected system; The impedance calculation module is used to input the real-time operation data into a preset offshore wind power grid-connected system impedance model, so that the offshore wind power grid-connected system impedance model calculates the first equivalent impedance of the offshore wind power grid-connected system according to the real-time operation data, wherein the offshore wind power grid-connected system impedance model is constructed based on the impedance models of various parts of the offshore wind power grid-connected system, including a wind turbine impedance model, a wind farm impedance model, a submarine cable impedance model, and a static VAR generator impedance model; The second acquisition module is used to obtain a second equivalent impedance of the power grid according to a preset power grid harmonic impedance model or an actual power grid harmonic scanning value; The analysis module is used to perform stability analysis on the offshore wind power grid-connected system according to the first equivalent impedance and the second equivalent impedance.

10. A stability analysis system for an offshore wind power grid-connected system according to claim 9, characterized in that: The impedance calculation module includes a wind turbine high-voltage side admittance calculation unit, a wind farm high-voltage side admittance calculation unit, a submarine cable delivery system impedance calculation unit, a static reactive power generating device high-voltage side impedance calculation unit, and a grid-connected system impedance calculation unit; The wind turbine high-voltage side admittance calculation unit is used to input the real-time operation data of the wind turbine into the wind turbine impedance model, so that the wind turbine impedance model outputs the wind turbine high-voltage side admittance data; The wind farm high-voltage side admittance calculation unit is used to input the high-voltage side admittance data of the wind turbine group into the wind farm impedance model, so that the wind farm impedance model outputs the wind farm high-voltage side admittance data; The submarine cable delivery system impedance calculation unit is used to input the high-voltage side admittance data of the wind farm into the submarine cable impedance model, so that the submarine cable impedance model outputs the impedance data of the submarine cable delivery system; The static VAR generator high-voltage side impedance calculation unit is used to input the impedance data of the submarine cable delivery system and the real-time operation data of the static VAR generator into the static VAR generator impedance model, so that the static VAR generator impedance model outputs the impedance data of the high-voltage side of the static VAR generator; The grid-connected system impedance calculation unit is used to calculate and obtain the first equivalent impedance of the offshore wind power grid-connected system based on the impedance data of the submarine cable delivery system and the impedance data of the high-voltage side of the static VAR generator.

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