Risk assessment method and system for broadband oscillation of offshore wind power through flexible direct current sending-out system

Abstract

The invention relates to the technical field of power systems, and discloses a broadband oscillation risk assessment method and system for offshore wind power through a flexible direct current sending-out system, and the method comprises the steps: dividing a broadband oscillation risk assessment frequency band into a medium-high frequency band and a sub-super-synchronous frequency band; aiming at different broadband oscillation risk assessment frequency bands, respectively establishing an offshore wind power plant side impedance model and a flexible direct current converter side impedance model in a corresponding offshore wind power flexible direct current sending-out system, and determining an equivalent impedance frequency characteristic curve of the offshore wind power plant side and the flexible direct current converter side. The broadband oscillation risk of the offshore wind power through the flexible direct current sending-out system is comprehensively evaluated by using the equivalent impedance frequency characteristic curve, the topological structure and the operation condition are fully considered, and the accuracy and adaptability of evaluation are improved. And when the broadband oscillation risk occurs, the system is adjusted in a targeted manner according to different evaluation frequency bands, so that the broadband oscillation risk is effectively eliminated, and the operation stability of the offshore wind power output system through the flexible direct current is enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method and system for assessing the risk of broadband oscillation in an offshore wind power flexible direct current transmission system. Background Art

[0002] The sending end of the Flexible DC transmission system for offshore wind farms includes power electronic conversion devices such as Flexible DC converters and wind turbine inverters. These devices interact with high-frequency filtering circuits, bus cables, and other equipment, increasing the risk of resonance in the Flexible DC grid-connected offshore wind farm system. Frequent resonance incidents pose a serious threat to the safe and stable operation of the grid-connected equipment and the power system. Numerous harmonic resonance incidents have occurred in large-scale Flexible DC grid-connected renewable energy projects, posing a significant threat to the safety and stability of the system.

[0003] This indicates that the risk of broadband oscillations occurring in power electronic device access systems is high. These oscillations are characterized by a wide frequency band, diverse types, and complex mechanisms, posing a serious threat to the safe and stable operation of grid-connected equipment and the power system. Current broadband oscillation risk assessment methods for offshore wind farms connected to flexible HVDC transmission systems fail to account for various system operating conditions, resulting in inaccurate and poor adaptability. Summary of the Invention

[0004] In view of this, the present invention provides a method and system for assessing the risk of broadband oscillation of an offshore wind power transmission system via a flexible direct current transmission system, which solves the technical problem that the method for assessing the risk of broadband oscillation of an offshore wind farm via a flexible direct current transmission system is difficult to consider various operating conditions of the system, which easily leads to low accuracy and poor adaptability of the risk assessment of broadband oscillation of an offshore wind farm via a flexible direct current transmission system.

[0005] A first aspect of the present invention provides a method for assessing the broadband oscillation risk of an offshore wind power system via a flexible direct current transmission system. The method is applied to assess the broadband oscillation risk of an offshore wind power system via a flexible direct current transmission system in different broadband oscillation risk assessment frequency bands, wherein the broadband oscillation risk assessment frequency bands include medium-high frequency bands and sub-supersynchronous frequency bands; and comprises:

[0006] Establishing an offshore wind farm-side impedance model within the offshore wind power HVDC Flexible transmission system based on the topology of the system and different broadband oscillation risk assessment frequency bands;

[0007] Determining, according to different broadband oscillation risk assessment frequency bands, a flexible DC converter side impedance model corresponding to the broadband oscillation risk assessment frequency band;

[0008] Determining an equivalent impedance frequency characteristic curve on the offshore wind farm side and an equivalent impedance frequency characteristic curve on the flexible DC converter side respectively according to the offshore wind farm side impedance model and the flexible DC converter side impedance model;

[0009] Using the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible DC converter side, the broadband oscillation risk of the offshore wind power transmission system through the flexible DC converter is assessed;

[0010] When it is assessed that there is a risk of broadband oscillation in the offshore wind power transmission system via flexible direct current, the offshore wind power transmission system via flexible direct current is adjusted according to different broadband oscillation risk assessment frequency bands until the risk of broadband oscillation is eliminated.

[0011] Preferably, the process of establishing the offshore wind farm side impedance model includes:

[0012] Based on different broadband oscillation risk assessment frequency bands, the voltage and current small signal phasors at the common coupling point of a single wind turbine under three-phase symmetrical working conditions are constructed;

[0013] Based on the voltage and current small-signal phasors, a Laplace transform is performed on the control process of the single wind turbine, and combined with the primary topology of the single wind turbine, an AC-side admittance matrix of the common coupling point of the single wind turbine is obtained; wherein the AC-side admittance matrix of the sub-supersynchronous frequency band introduces a phase-locked loop control link coefficient matrix affected by different AC-side power operating levels;

[0014] Determine the impedance characteristic model of a single wind turbine according to the AC side admittance matrix of the common coupling point;

[0015] The Bergeron model is used to simulate the AC line between the two wind turbines and obtain the equivalent impedance model of the AC line.

[0016] For each feeder, based on the connection relationship between the multiple wind turbines on the feeder and the AC line, the impedance characteristic model of the single wind turbine and the equivalent impedance model of the AC line are connected in series and parallel to obtain the equivalent impedance model of the single feeder;

[0017] According to the feeder topology structure of the offshore wind power transmission system via flexible direct current, equivalent impedance models of a plurality of the single feeders are topologically connected to obtain the offshore wind farm side impedance model.

[0018] Preferably, when the broadband oscillation risk assessment frequency band is a medium or high frequency band, determining the flexible DC converter side impedance model corresponding to the broadband oscillation risk assessment frequency band according to different broadband oscillation risk assessment frequency bands includes:

[0019] Construct the voltage and current small signal phasors at the common coupling point on the flexible DC converter side;

[0020] Based on the voltage and current small-signal phasors, the impedance model of the flexible DC converter side is obtained by performing Laplace transform on the control process of the flexible DC converter side in combination with the primary topology structure of the flexible DC converter side;

[0021] When the wideband oscillation risk assessment frequency band is a sub-supersynchronous frequency band, determining the flexible DC converter side impedance model corresponding to the wideband oscillation risk assessment frequency band according to different wideband oscillation risk assessment frequency bands includes:

[0022] Construct the voltage and current small signal phasors at the common coupling point on the flexible DC converter side;

[0023] Based on the voltage and current small-signal phasors, the control process on the flexible DC converter side and the dynamic model of the bridge arm affected by different AC side power operating levels are Laplace transformed, and the impedance model of the flexible DC converter side is obtained in combination with the primary topology structure on the flexible DC converter side; wherein the dynamic model of the bridge arm is used to characterize the equivalent bridge arm structure affected by different AC side power operating levels.

[0024] Preferably, the method further comprises:

[0025] According to the interconnection relationship between the offshore wind farm side and the flexible DC converter side, the equivalent impedance of the offshore wind farm side and the equivalent impedance of the flexible DC converter side under the influence of the frequency coupling effect are determined by using the voltage-current relationship between the impedance model of the offshore wind farm side and the impedance model of the flexible DC converter side under the influence of the frequency coupling effect;

[0026] The equivalent impedance frequency characteristic curve of the offshore wind farm side and the equivalent impedance frequency characteristic curve of the flexible DC converter side are determined by utilizing the equivalent impedance of the offshore wind farm side and the equivalent impedance of the flexible DC converter side under the influence of the frequency coupling effect.

[0027] Preferably, the evaluating the broadband oscillation risk of the offshore wind power transmission system through the flexible direct current converter by using the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible direct current converter side includes:

[0028] Determining, based on the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible DC converter side, a frequency interval range in which the equivalent impedance amplitude of the impedance model on the flexible DC converter side is greater than the equivalent impedance amplitude of the impedance model on the offshore wind farm side within a wide-band oscillation risk assessment frequency band;

[0029] Determining whether a phase difference between the impedance model on the flexible DC converter side and the impedance model on the offshore wind farm side within the frequency range is greater than a preset phase difference threshold;

[0030] If it is determined that the phase difference is greater than the preset phase difference threshold, it is determined that there is a risk of broadband oscillation in the offshore wind power transmission system via the flexible direct current system;

[0031] If it is determined that the phase difference is not greater than the preset phase difference threshold, it is determined that there is no broadband oscillation risk in the offshore wind power transmission system via the flexible direct current transmission system.

[0032] Preferably, the offshore wind power corresponding to the sub-supersynchronous frequency band adopts an operation mode at different power operation levels through the flexible DC transmission system; the offshore wind power corresponding to the medium and high frequency bands adopts an operation mode at the rated operating power level through the flexible DC transmission system; wherein, the operation mode adopts one or more of the following modes, wherein the operation mode includes:

[0033] Black start, full connection of wind farm, exit of some wind turbines, exit of some feeders, exit of some AC busbars and exit of some transformers, wherein the black start includes no-load black start on the flexible DC converter side, no-load black start on the flexible DC converter side with AC busbars and no-load operation on the flexible DC converter side with offshore wind farms.

[0034] Preferably, when it is assessed that the offshore wind power transmission system via flexible direct current (HVDC) has a broadband oscillation risk, adjusting the offshore wind power transmission system via flexible direct current (HVDC) according to different broadband oscillation risk assessment frequency bands until the broadband oscillation risk is eliminated includes:

[0035] If it is assessed that the offshore wind power transmission system via flexible direct current has a broadband oscillation risk, the offshore wind power transmission system via flexible direct current is adjusted according to a preset risk adjustment plan corresponding to the broadband oscillation risk assessment frequency band until the broadband oscillation risk is eliminated;

[0036] The preset first risk adjustment scheme corresponding to the medium and high frequency bands adopts one or more of the following scheme combinations, and the preset first risk adjustment scheme includes:

[0037] Changing the primary circuit, control structure and control parameters of the wind turbine generator set on the offshore wind farm side;

[0038] Changing the primary circuit, control structure, and control parameters of the flexible DC converter on the flexible DC converter side;

[0039] The preset second risk adjustment scheme corresponding to the secondary supersynchronous frequency band adopts one or more of the following scheme combinations, and the preset second risk adjustment scheme includes:

[0040] Reducing the phase-locked loop (PI) controller parameters of the wind turbine generator set on the offshore wind farm side;

[0041] Increasing the current inner loop PI controller parameters of the wind turbine generator set on the offshore wind farm side;

[0042] A positive damping signal is added to the transmission signal of the offshore wind power through the flexible direct current transmission system in the sub-supersynchronous frequency band.

[0043] In a second aspect, the present invention further provides a broadband oscillation risk assessment system for an offshore wind power transmission system via flexible direct current (HVDC) for use in assessing broadband oscillation risks in the offshore wind power transmission system via flexible direct current (HVDC) in different broadband oscillation risk assessment frequency bands, wherein the broadband oscillation risk assessment frequency bands include medium-high frequency bands and sub-supersynchronous frequency bands; and includes:

[0044] A wind farm side impedance construction module is used to establish an offshore wind farm side impedance model within the offshore wind power flexible direct current transmission system according to the topology of the offshore wind power flexible direct current transmission system;

[0045] A flexible DC converter side impedance construction module is used to determine the flexible DC converter side impedance model corresponding to different broadband oscillation risk assessment frequency bands according to the broadband oscillation risk assessment frequency bands;

[0046] An impedance curve determination module, configured to determine an equivalent impedance frequency characteristic curve on the offshore wind farm side and an equivalent impedance frequency characteristic curve on the flexible DC converter side, respectively, based on the impedance model on the offshore wind farm side and the impedance model on the flexible DC converter side;

[0047] a risk assessment module, configured to assess the broadband oscillation risk of the offshore wind power transmission system via the flexible direct current (HVDC) by using the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible direct current (HVDC) converter side;

[0048] The risk adjustment module is used to adjust the offshore wind power flexible direct current transmission system according to different broadband oscillation risk assessment frequency bands when it is assessed that the offshore wind power flexible direct current transmission system has a broadband oscillation risk until the broadband oscillation risk is eliminated.

[0049] In a third aspect, the present invention further provides an electronic device, comprising a memory and a processor, wherein a computer program is stored in the memory, and when the computer program is executed by the processor, the processor executes the steps of the method for assessing the risk of broadband oscillation of an offshore wind power flexible direct current transmission system as described in the first aspect.

[0050] In a fourth aspect, the present invention further provides a computer-readable storage medium having a computer program stored thereon, which, when executed, implements the steps of the method for assessing the risk of broadband oscillation of an offshore wind power flexible direct current transmission system as described in the first aspect.

[0051] It can be seen from the above technical solutions that the present invention subdivides the broadband oscillation risk assessment frequency band into medium and high frequency bands and sub-supersynchronous frequency bands. By constructing an offshore wind farm side impedance model in the offshore wind power transmission system via flexible direct current, and establishing corresponding flexible direct current converter side impedance models for different broadband oscillation risk assessment frequency bands, the equivalent impedance frequency characteristic curves of the offshore wind farm side and the flexible direct current converter side are determined. By using the equivalent impedance frequency characteristic curves of the two, a comprehensive assessment of the broadband oscillation risk of the offshore wind power transmission system via flexible direct current is conducted, fully considering the topological structure and operating conditions, and significantly improving the accuracy and adaptability of the assessment. When the risk of broadband oscillation occurs, the system is adjusted in a targeted manner according to different assessment frequency bands, effectively eliminating the risk of broadband oscillation and further enhancing the operational stability of the offshore wind power transmission system via flexible direct current. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0053] Figure 1 Provides an application environment for the broadband oscillation risk assessment method for offshore wind power transmission systems via flexible HVDC;

[0054] Figure 2 This is a flow chart of a method for assessing the risk of broadband oscillation in an offshore wind power HVDC flexible transmission system.

[0055] Figure 3 This is a topological diagram of the offshore wind power transmission system via flexible DC transmission;

[0056] Figure 4 Schematic diagram of the topology of a single wind turbine;

[0057] Figure 5 This is the control block diagram of a single fan;

[0058] Figure 6 Schematic diagram of the impedance scanning system structure;

[0059] Figure 7 is the equivalent diagram of the AC line;

[0060] Figure 8 is the equivalent model of a single feeder;

[0061] Figure 9 Schematic diagram of the topological structure of an offshore wind farm;

[0062] Figure 10a 、 Figure 10b These are typical control block diagrams of offshore flexible DC converter stations;

[0063] Figure 11 This is the main circuit topology diagram of the flexible DC converter station;

[0064] Figure 12 This is the voltage and current relationship diagram of the interconnection system between the offshore wind farm and the flexible DC converter station;

[0065] Figure 13a 、 Figure 13b They are respectively the positive sequence subsystem and negative sequence subsystem of the offshore wind power transmission system via flexible DC transmission;

[0066] Figure 14a 、 Figure 14b The theoretical value curve and the actual value curve of the impedance model of a single wind turbine in the medium and high frequency bands;

[0067] Figure 15a 、 Figure 15b The theoretical value curve and the actual value curve of the single feeder impedance model in the medium and high frequency bands;

[0068] Figure 16a 、 Figure 16b The amplitude curve and phase angle curve of the high-frequency impedance characteristics of the offshore wind farm;

[0069] Figure 17a 、 Figure 17b The theoretical value curve and the actual value curve of the scanning for the flexible DC converter station model in the medium and high frequency bands;

[0070] Figure 18a 、 Figure 18b Develop risk assessment curves for medium and high frequency oscillations for all feeders in offshore wind farms;

[0071] Figure 19a 、 Figure 19b The theoretical value curve of the impedance model of a single wind turbine in the sub-supersynchronous frequency band and the actual value curve of the scan;

[0072] Figure 20a-1 、 Figure 20a-2 This is the impedance characteristic curve of a single wind turbine in the sub-supersynchronous frequency band under working condition 1;

[0073] Figure 20b-1 、 Figure 20b-2 This is the impedance characteristic curve of a single wind turbine in the sub-synchronous frequency band under working condition 2;

[0074] Figure 21a 、 Figure 21b The theoretical value curve and the actual value curve of the single feeder impedance model in the sub-supersynchronous frequency band;

[0075] Figure 22a 、 Figure 22b This is the impedance characteristic curve of the offshore wind farm in the super synchronous frequency band;

[0076] Figure 23a 、 Figure 23b The theoretical value curve and the actual value curve of the impedance model at the flexible DC converter station side in the sub-supersynchronous frequency band are shown;

[0077] Figure 24a 、 Figure 24b The curves of theoretical and actual scanning values of equivalent impedance of offshore wind farm before and after frequency coupling effect;

[0078] Figure 25a 、 Figure 25b To assess the oscillation risk of offshore wind power transmission through flexible HVDC system under the influence of frequency coupling effect;

[0079] Figure 26 This is a schematic diagram of the structure of a broadband oscillation risk assessment system for offshore wind power transmission via flexible DC transmission systems;

[0080] Figure 27 The figure is a structural diagram of an electronic device. DETAILED DESCRIPTION

[0081] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0082] The broadband oscillation risk assessment method for offshore wind power through flexible direct current transmission system provided in the embodiment of the present application can be applied to Figure 1 In the application environment shown, the various device nodes of the offshore wind power flexible DC transmission system communicate with server 102 via a network. The data storage system can store data that server 102 needs to process. The data storage system can be integrated with server 102 or placed on the cloud or other network servers. Server 102 can be a standalone physical server, a server cluster or distributed system consisting of multiple physical servers, or a cloud server providing cloud computing services.

[0083] An embodiment of the present application provides a method for assessing the broadband oscillation risk of an offshore wind power transmission system via a flexible direct current (HVDC) transmission system, which is applied to assessing the broadband oscillation risk of an offshore wind power transmission system via a flexible direct current (HVDC) transmission system in different broadband oscillation risk assessment frequency bands, wherein the broadband oscillation risk assessment frequency bands include medium and high frequency bands and sub-supersynchronous frequency bands.

[0084] Among them, the embodiments of the present application respectively carry out different flexible DC converter side impedance model establishment processes for different broadband oscillation risk assessment frequency bands of offshore wind power flexible DC transmission systems. The broadband oscillation risk assessment frequency bands include medium and high frequency bands (frequency bands greater than 200 Hz) and sub-supersynchronous frequency bands (frequency bands of 1~200 Hz).

[0085] At the same time, the embodiment of the present application adaptively adjusts the impedance model on the offshore wind farm side and the impedance model on the flexible DC converter side according to different broadband oscillation risk assessment frequency bands to ensure the accuracy and effectiveness of the impedance model.

[0086] like Figure 2 As shown, the embodiment of the present application provides a method for assessing the risk of broadband oscillation of an offshore wind power flexible direct current transmission system. Figure 1 The server 102 in the example is used as an example to illustrate the method, which includes the following steps S1 to S5.

[0087] Step S1: establishing an offshore wind farm side impedance model within the offshore wind power flexible DC transmission system according to the topology of the offshore wind power flexible DC transmission system and different broadband oscillation risk assessment frequency bands.

[0088] Among them, the topology of the offshore wind power flexible DC transmission system is as follows: Figure 3 As shown in the figure, the wind turbines are connected to the machine side converter (MSC) and grid side converter (GSC) and then to the offshore converter station (wind farm voltage source converter, WFVSC) of the Modular Multilevel Converter-High Voltage Direct Current (MMC-HVDC) system via an AC submarine cable. The WFVSC converts AC power into DC power and transmits the power to the onshore converter station (grid side voltage source converter, GSVSC) via a submarine cable, where it is then integrated into the AC grid.

[0089] The offshore wind farm side impedance model represents the equivalent impedance of each wind turbine and its associated electrical equipment within an offshore wind farm. By establishing an accurate offshore wind farm side impedance model, we can reflect the electrical characteristics of the wind farm under different operating conditions and frequencies, providing a foundation for subsequent broadband oscillation risk assessments.

[0090] Step S2: Determine the flexible DC converter side impedance model corresponding to the broadband oscillation risk assessment frequency band according to different broadband oscillation risk assessment frequency bands.

[0091] The HVDC Flexible converter side impedance model represents the equivalent impedance of the HVDC Flexible converter station in different broadband oscillation risk assessment frequency bands. As a key component of offshore wind power transmission systems via HVDC Flexible, the impedance characteristics of the HVDC Flexible converter have a significant impact on the system's broadband oscillation risk. By establishing corresponding HVDC Flexible converter side impedance models for different assessment frequency bands, the converter's electrical behavior in these frequency bands can be more accurately described, thereby improving the accuracy of broadband oscillation risk assessment.

[0092] Step S3: determining an equivalent impedance frequency characteristic curve on the offshore wind farm side and an equivalent impedance frequency characteristic curve on the flexible DC converter side according to the impedance model on the offshore wind farm side and the impedance model on the flexible DC converter side, respectively.

[0093] The equivalent impedance frequency characteristic curve for the offshore wind farm side is obtained by performing a frequency sweep on the impedance model of the offshore wind farm side. This curve reflects the impedance variation of the offshore wind farm side at different frequencies. Similarly, the equivalent impedance frequency characteristic curve for the flexible DC converter side is also obtained by performing a frequency sweep on the impedance model of the flexible DC converter side. It describes the impedance characteristics of the flexible DC converter side at different frequencies.

[0094] Step S4: Evaluate the broadband oscillation risk of the offshore wind power transmission system through the flexible DC power transmission system using the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible DC power converter side.

[0095] Among them, through in-depth analysis of the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible DC converter side, a comprehensive assessment can be made of the broadband oscillation risks that offshore wind power may face when transmitting electricity through the flexible DC transmission system.

[0096] Step S5: When it is assessed that there is a broadband oscillation risk in the offshore wind power HVDC flexible transmission system, the offshore wind power HVDC flexible transmission system is adjusted according to different broadband oscillation risk assessment frequency bands until the broadband oscillation risk is eliminated.

[0097] Adjustments to the offshore wind power HVDC Flexible transmission system based on different broadband oscillation risk assessment frequency bands can include, but are not limited to, adjusting electrical parameters on the offshore wind farm side, optimizing the control strategy of the HVDC Flexible converter, and improving the topology of the offshore wind power HVDC Flexible transmission system. These targeted adjustments can effectively reduce or eliminate broadband oscillation risks and improve the operational stability and reliability of the offshore wind power HVDC Flexible transmission system.

[0098] It should be noted that the embodiment of the present application subdivides the broadband oscillation risk assessment frequency band into medium and high frequency bands and sub-supersynchronous frequency bands. By constructing an offshore wind farm side impedance model in the offshore wind power transmission system via flexible direct current, and establishing corresponding flexible direct current converter side impedance models for different broadband oscillation risk assessment frequency bands, the equivalent impedance frequency characteristic curves of the offshore wind farm side and the flexible direct current converter side are determined. By using the equivalent impedance frequency characteristic curves of the two, a comprehensive assessment of the broadband oscillation risk of the offshore wind power transmission system via flexible direct current is conducted, fully considering the topological structure and operating conditions, and significantly improving the accuracy and adaptability of the assessment. When the risk of broadband oscillation occurs, the system is adjusted in a targeted manner according to different assessment frequency bands, effectively eliminating the risk of broadband oscillation and further enhancing the operational stability of the offshore wind power transmission system via flexible direct current.

[0099] In some embodiments, the process of establishing the offshore wind farm side impedance model includes:

[0100] Step S101 : constructing voltage and current small-signal phasors at a common coupling point of a single wind turbine under a three-phase symmetrical working condition according to different broadband oscillation risk assessment frequency bands.

[0101] Step S102: Based on the voltage and current small signal phasors, Laplace transform is performed on the control process of a single wind turbine, and combined with the primary topology of the single wind turbine, the AC side admittance matrix of the common coupling point of the single wind turbine is obtained; wherein, the AC side admittance matrix of the sub-supersynchronous frequency band introduces a phase-locked loop control link coefficient matrix affected by different AC side power operating levels.

[0102] It should be noted that in the phase-locked loop control link of a single wind turbine, the input is the AC side voltage, and its output is the phase θ. The phase-locked loop control frequency band is located in the sub-supersynchronous frequency band. The influence of the phase-locked loop control rapidly decays in the medium and high frequency bands. Therefore, for wind turbines in the medium and high frequency bands, the impact of the system power operating level caused by the phase-locked loop on the impedance characteristics can be ignored. However, due to the phase-locked loop control, the sub-supersynchronous impedance characteristics of the wind turbine are affected by the system power level. Therefore, when conducting sub-supersynchronous frequency band oscillation risk assessment, it is necessary to consider the impact of the power level on the sub-supersynchronous impedance characteristics of a single wind turbine (see Table 1 for examples of wind turbines with different power levels).

[0103] On the premise that a single fan can operate stably, the topological structure and control block diagram of a single fan are as follows: Figure 4 and 5 As shown in Figure 1, due to the frequency coupling effect caused by the dynamic characteristics of the DC bus and the asymmetric control of the converter's dq axes, each system signal quantity must be described by a two-dimensional vector. By linearizing at the steady-state point, a small signal vector of each dynamic quantity can be constructed. The vector consists of the Fourier coefficients of the dynamic quantity at different frequencies.

[0104] Figure 4 In, Z t is the leakage reactance of the commutation transformer, Y c is the admittance of the AC side filter group, Z f is the passive branch impedance on the AC side, Z G is the AC impedance of a single fan viewed from the high-pressure side, Z g is the AC impedance of a single fan viewed from the low-pressure side, I source It is a controlled current source for parts such as the converter station on the machine side.

[0105] Figure 5 In, P in Inject active power into the DC side, u dc 、C dc are the DC side voltage, DC side equivalent capacitance, and u mabc is the three-phase voltage abc at the fan outlet, L f is the filter inductor, u gabc 、i gabc ,q g are the voltage, current and reactive power at the common coupling point, R f 、C f They are filter resistor and filter capacitor respectively, L t1 is the leakage inductance of the commutation transformer, k t1 is the commutation ratio. PLL is the phase-locked loop, θ PLL is the phase-locked loop output phase angle, abc / dq represents the Park transformation process, H f is the filtering link, i gdq ,u gdq They are the dq axis current and voltage after Park transformation, respectively, including the dq axis current i gd 、i gq and dq axis voltage u gd 、u gq , U dc ref , Q ref are the DC voltage reference value and the AC side common coupling point reactive power reference value, G V , G Q , G I , K dThey are the voltage outer loop, reactive outer loop, current inner loop PI control link and decoupling link, dq / abc is the Park inverse transformation link, H D is the system equivalent control link delay, PWM is the modulation process, and the three-phase modulation wave m is generated. abc .

[0106] The voltage and current small signal phasors constructed at the fan port are as follows. Since it is a three-phase symmetrical working condition, the voltage and current vectors at any common coupling point of one phase are recorded as 、 :

[0107] (1)

[0108] Where, 、 are the voltages at frequencies p and p-2 respectively, 、 They are currents with frequencies p and p-2 respectively.

[0109] The AC side admittance matrix of a single fan can be written as follows:

[0110] (2)

[0111] Where Y 11 、Y 22 All are wind turbine self-admittance, Y 12 、Y 21 Both are fan coupling admittance, Y G (s) represents the admittance matrix of the wind turbine. Among them, the impedance matrix of the wind turbine can be defined according to The impedance matrix can be calculated. By using the state-space average method to establish an equivalent system model and performing Laplace transforms and linearization on the voltage and current small-signal phasors and the control process, the admittance matrix of the fan looking into the AC side from the point of common coupling (PCC), including the AC-side passive branches, AC-side filters, and transformers, can be obtained. Its specific form is as follows:

[0112] (3)

[0113] Where, is the AC side admittance matrix, s is the Laplace operator, is the system control link delay matrix, Z f (s), Z t (s), Y c (s) are the 2×2 matrices related to the AC side passive branch, the converter transformer leakage inductance, and the AC test filter, , G V,i(s) are coefficient matrices related to the DC voltage outer loop voltage and current variables, is the coefficient matrix of the reactive outer loop voltage variable, , is the coefficient matrix of the voltage variable of the phase-locked loop control link, 、 are the coefficient matrices related to the DC voltage inner loop voltage and current variables, E is a 2×2 unit matrix, is the coefficient matrix related to the reactive outer loop current variable.

[0114] Among them, the 2×2 matrix related to the AC side passive branch, the leakage inductance of the converter transformer and the AC test filter is the primary topology parameter of a single wind turbine.

[0115] Among them, for the mid- and high-frequency bands can be ignored, that is =1, and the AC side admittance matrix can be further simplified. Since the input is the AC side voltage, its output reference phase θ is greatly affected by the AC side power operation level, so it is necessary to Make reservations.

[0116] Step S103: determining the impedance characteristic model of a single wind turbine according to the AC side admittance matrix of the common coupling point.

[0117] Among them, the AC side admittance matrix of the common coupling point is used as the impedance characteristic model of a single wind turbine.

[0118] In some embodiments, verification is required to provide support for the broadband oscillation risk assessment of the offshore wind power flexible DC transmission system. Considering that the present invention focuses on the impedance characteristics of the model, the present invention uses a modeling verification method based on impedance scanning to verify the correctness of the impedance characteristic model of a single wind turbine, such as Figure 6 The impedance scanning system structure shown in FIG. 1 , wherein the device to be scanned is a single fan.

[0119] Specifically, the process of verifying the correctness of the impedance characteristic model of a single wind turbine using the impedance scan modeling verification method includes:

[0120] (1) When a single wind turbine is in steady-state operation, a disturbance voltage at multiple measurement frequency points is applied to the AC side of the single wind turbine, and the grid connection point voltage and current data of the AC side of the single wind turbine are obtained;

[0121] Among them, when the device to be scanned maintains stable operation, a small disturbance voltage (or a small disturbance current) of different measurement frequencies is applied to the AC side, and an impedance scanning module is used on the AC side to obtain the grid connection point voltage and current data.

[0122] (2) Perform fast Fourier transform on the voltage and current data of the grid connection point to obtain the voltage FFT complex variables and current FFT complex variables at multiple different measurement frequency points;

[0123] (3) Determine the impedance complex value based on the voltage FFT complex variables and current FFT complex variables at multiple different measurement frequency points;

[0124] (4) Determine the impedance external characteristic curve of a single wind turbine based on the amplitude and phase of the complex impedance value;

[0125] (5) Compare the similarity between the impedance external characteristic curve and the impedance external characteristic curve determined by the impedance characteristic model of a single wind turbine;

[0126] (6) Verify whether the impedance characteristic model of a single wind turbine is correct based on the similarity comparison results;

[0127] (7) If the impedance characteristic model of a single wind turbine is verified to be incorrect, the parameters of the impedance characteristic model of the single wind turbine are updated until the impedance characteristic model of the single wind turbine is verified to be correct.

[0128] When updating the parameters of the impedance characteristic model of a single wind turbine, it is possible to consider updating parameters such as voltage and current small signal phasors, and then re-derive the impedance characteristic model of the single wind turbine.

[0129] Step S104: Use the Bergeron model to perform equivalent simulation on the AC line between the two wind turbines to obtain an equivalent impedance model of the AC line.

[0130] Among them, the equivalent diagram of the AC line is as follows Figure 7 As shown, the equivalent impedance and equivalent admittance expressions of the Bergeron model are as follows:

[0131] (4)

[0132] Where γ is the propagation constant of the line, ; Z C is the wave impedance, , Z line is the equivalent impedance of the Bergeron model, Y line is the equivalent admittance of the Bergeron model, assuming that the equivalent AC cable length is l0, and the impedance and admittance per unit length are z0 and y0 respectively.

[0133] Step S105: For each feeder, based on the connection relationship between multiple wind turbines and AC lines on the feeder, the impedance characteristic model of a single wind turbine and the equivalent impedance model of the AC line are connected in series and parallel to obtain the equivalent impedance model of the single feeder.

[0134] Among them, a feeder of a wind farm is composed of multiple wind turbines and multiple AC lines connected in series and parallel. The impedance model of a single feeder of a new energy station is obtained through the series and parallel relationship of the equivalent impedance model of the wind turbines and the collecting lines.

[0135] Specifically, starting from the head end wind turbine, the wind turbine admittance is connected in parallel with the Bergeron model equivalent admittance, then the Bergeron model equivalent impedance is connected in series, and then the Bergeron model equivalent admittance is connected in parallel to obtain the equivalent impedance viewed from the next wind turbine to the head end. According to the feeder topology structure, that is, the Bergeron model parameters, the equivalent model of the entire feeder can be obtained by series and parallel connection, such as Figure 8 As shown (the process of the present invention is applicable to a single feeder topology including but not limited to Figure 8 ), assuming that the impedance of a single wind turbine is Z Gij , admittance is Y Gij (Each wind turbine can have different power operation levels, control parameters, and control structures.) The equivalent impedance of the Bergeron model of the AC submarine cable between wind turbines is Z aij , the equivalent admittance is Y aij .

[0136] according to Figure 8 The impedance characteristic model of a single feeder can be obtained as follows:

[0137] (5)

[0138] Where, the impedance at the end of a single feeder is assumed to be Z Fi , then Z Fi =1 / Y Eij That is, the impedance of a single feeder. Assume that the impedance and admittance of a single wind turbine are Z Gij 、Y Gij , the equivalent impedance and admittance of the Bergeron model of the jth section of the feeder line are Z aij 、Y bij , the equivalent impedance and admittance from the outlet of a certain fan to the head end is Z pij 、Y pij The equivalent impedance and admittance from the Bergeron line head end to the line head end are Z cij 、Y cij The equivalent impedance and admittance from the equivalent impedance at the end of the Bergeron line to the beginning of the line are Z hi j, Y hij ,according to Figure 8 By modeling the impedance characteristics of a single wind turbine and the Bergeron model and the topology of a single feeder, the impedance characteristic curve Z of a single feeder can be established through the impedance series-parallel relationship. Fi , the impedance characteristic curve of a single feeder can be verified through impedance scanning.

[0139] Step S106 : Based on the feeder topology of the offshore wind power transmission system via the flexible DC transmission system, the equivalent impedance models of multiple single feeders are topologically connected to obtain an offshore wind farm side impedance model.

[0140] Among them, according to the line-out method of the offshore wind farm, the impedance of each feeder is connected in parallel to obtain the impedance characteristic curve of multiple feeders in parallel; further considering the electrical relationship of the offshore converter platform transformer, the impedance model of the offshore wind farm side is obtained.

[0141] For example, suppose that an offshore wind farm has m feeders in operation. The topology of the offshore wind farm is as follows: Figure 9 As shown (the process of the present invention is applicable to offshore wind farms but includes but is not limited to Figure 9 The impedance model expression of the offshore wind farm side is:

[0142] (6)

[0143] Where, is the impedance model of the offshore wind farm side, is the admittance of the i-th feeder.

[0144] It should be noted that when the embodiment of the present application conducts a broadband oscillation risk assessment of an offshore wind power flexible DC transmission system, starting from a single wind turbine and feeder impedance model, the feeder topology and wind farm operation mode are considered to form a detailed impedance model of the offshore wind farm.

[0145] When the broadband oscillation risk assessment frequency band is a medium or high frequency band, the flexible DC converter side impedance model corresponding to the broadband oscillation risk assessment frequency band is determined according to different broadband oscillation risk assessment frequency bands, including:

[0146] Step S11: constructing voltage and current small signal phasors at the common coupling point on the flexible DC converter side;

[0147] Step S12: Based on the voltage and current small signal phasors, Laplace transform is performed on the control process of the flexible DC converter side, and the impedance model of the flexible DC converter side is obtained in combination with the primary topology structure of the flexible DC converter side.

[0148] A typical control block diagram of an offshore flexible DC converter station is shown in the following figure: Figure 10a ~b. Among them, Figure 10a Where θ1 is the system phase, i a 、i b 、i c They are the three-phase currents abc on the AC side, u a 、u b 、u c They are the three-phase voltages abc on the AC side, and the current and voltage on the AC side are measured after a delay of G.si , G sv Then, the abc three-phase current and voltage are transferred to the dq axis through Park transformation. The dq axis current and voltage pass through the positive and negative sequence separation link G sd Then output the positive and negative sequence voltage and current under the dq axis, such as u dP is the d-axis positive sequence voltage, i qN is the q-axis negative sequence current, and the rest of the variables are similar.

[0149] Figure 10b Middle U * dP 、U * qP 、U * dN 、U * qN are the reference values of the positive and negative sequence voltages of the dq axes, respectively, i * dP 、i * qP 、i * dN 、i * qN are the reference values of the positive and negative sequence currents of the dq axes respectively, PI is the proportional integral link of the control, K d and G fv are the decoupling coefficient and voltage feedforward coefficient respectively, ω1 is the system angular frequency, and the positive sequence voltage reference value u output by the MMC control link ref ABCP and negative sequence voltage reference value u ref ABCN After the Park inverse transformation, it changes from the dq axis to the abc axis for modulation and delay G d Rear output abc axis reference voltage u ref ABC Each signal quantity in the offshore flexible DC converter station needs to be described by a two-dimensional vector. By linearizing at the steady-state point, a small signal vector of each dynamic quantity can be constructed. The vector is composed of the Fourier coefficients of the dynamic quantity at different frequencies.

[0150] The offshore flexible DC converter adopts a fixed AC side voltage control strategy, which is a positive and negative sequence voltage and current dual-loop control structure, mainly including the dq axis conversion link, the positive sequence voltage outer loop, the negative sequence voltage outer loop, and the negative sequence current inner loop. Among them, the current inner loop includes the PI control link, the decoupling control link and the voltage feedforward link.

[0151] When modeling the medium and high frequencies of the HVDC Flexible converter station, the bridge arm dynamics (bridge arm current, submodule capacitor voltage, etc.) can be ignored. Considering the influence of factors such as the AC side equivalent inductance, the positive-sequence current controller, the positive-sequence dq-axis decoupling link, the negative-sequence current controller, the negative-sequence dq-axis decoupling link, voltage feedforward, the positive-sequence voltage outer loop, the negative-sequence voltage outer loop, system modulation, and link delay, the harmonic linearization method is used to derive the impedance of the offshore HVDC Flexible converter. The expressions for its positive and negative sequence impedances are shown as follows:

[0152] (7)

[0153] Where, 、 are positive and negative sequence impedances, respectively, L armeq is the equivalent impedance of the bridge arm reactor on the AC side; L T is the equivalent reactance of the commutation transformer, j is the imaginary function, G sd is the transfer function of the 1 / 4 power frequency period delay filter link, , T is the power frequency period of the grid voltage; K d is the current inner loop decoupling coefficient; G sv , G si are the transfer functions of voltage sampling and current sampling respectively; G d is the system modulation and link delay transfer function; G fv is the voltage feedforward low-pass filter transfer function; 、 are the transfer functions of the AC voltage outer loop and current inner loop PI controllers respectively; the superscripts “+” and “-” of the transfer functions represent different frequency offsets, where 、 .

[0154] Among them, the positive and negative sequence impedance expressions are used as the impedance model on the flexible DC converter side, and the commutation transformer equivalent reactance is the primary topology parameter on the flexible DC converter side.

[0155] In some embodiments, when the broadband oscillation risk assessment frequency band is a sub-supersynchronous frequency band, determining the flexible DC converter side impedance model corresponding to the broadband oscillation risk assessment frequency band according to different broadband oscillation risk assessment frequency bands includes:

[0156] Step S21: constructing small-signal phasors of voltage and current at the common coupling point on the flexible DC converter side.

[0157] Step S22: Based on the voltage and current small-signal phasors, Laplace transform is performed on the control process on the flexible DC converter side and the dynamic model of the bridge arm affected by different AC-side power operating levels, and the impedance model on the flexible DC converter side is obtained in combination with the primary topology structure on the flexible DC converter side; wherein the dynamic model of the bridge arm is used to characterize the equivalent bridge arm structure affected by different AC-side power operating levels.

[0158] Among them, the AC side power operating levels include: maximum capacitive reactive output at 100% active output, maximum inductive reactive output at 100% active output, 0 reactive output at 100% active output, maximum capacitive reactive output at 10% active output, maximum inductive reactive output at 10% active output, and 0 reactive output at 10% active output.

[0159] Among them, the modeling of the flexible DC converter station in the sub-supersynchronous frequency band needs to consider the influence of power level and the influence of bridge arm dynamics on system impedance characteristics. The following details the modeling of the flexible DC converter station considering bridge arm dynamics in the sub-supersynchronous frequency band. The main circuit topology structure of the flexible DC converter station is shown in the figure below. Figure 11 shown.

[0160] Figure 11 in, u j 、 i j are the AC side voltage and AC side current (j=a, b, c); u dc is the DC side voltage; L t is the leakage inductance of the commutation transformer, k t is the transformer ratio; R arm 、L arm is the bridge arm resistance and bridge arm inductance; 、 is the voltage of the upper and lower bridge arms of phase a, 、 is the current of the upper and lower bridge arms of phase a; 、 is the modulation coefficient of the upper and lower bridge arms of phase a; C is the capacitance value of the bridge arm; 、 is the sum of the capacitor voltages of the upper and lower bridge arms of phase a, and PCC is the point of common coupling (PCC).

[0161] First, consider the electrical quantities on the AC and DC sides and the electrical quantities on the bridge arms. For any phase j = a, b, or c, the KVL theorem gives:

[0162] (8)

[0163] in, is the AC neutral point voltage, and the common mode / differential mode voltage / current are defined as follows:

[0164] (9)

[0165] Substituting equation (9) into equation (8), the system main loop equation is expressed using common-mode / differential-mode voltage / current:

[0166] (10)

[0167] Where, 、 Represent common mode current and differential mode current respectively.

[0168] Harmonic linearization of the system's main loop equation near the steady-state point and transfer to the frequency domain yields:

[0169] (11)

[0170] Where, is the DC side voltage, 、 、 are the voltage differences of the bridge arms of phases a, b, and c respectively. 、 、 are the current differences of phase a, phase b, and phase c bridge arms, respectively. 、 、 are the voltage differences of phase a, phase b, and phase c respectively, 、 、 They are the small signal quantities of three-phase differential mode voltage, 、 、 They are the small signal quantities of three-phase differential mode current respectively.

[0171] For periodic time-varying links in the system (such as the Park transform matrix), the HSS matrix is a Toeplitz matrix, while for time-invariant linear controllers in the system (such as the current inner loop PI link), the HSS matrix is a diagonal matrix composed of frequency shifts. In the MMC average value model, the bridge arm dynamics have the following relationship:

[0172] (12)

[0173] Similarly, we can linearize the harmonics near the steady-state point and transfer it to the frequency domain to obtain:

[0174] (13)

[0175] Where subscript 0 represents the Toeplitz matrix in the steady state, Y c is the admittance matrix composed of submodule capacitances.

[0176] Combining equation (11) and equation (13) we can get:

[0177] (14)

[0178] Where, In matrix form, 、 are the upper and lower bridge arm capacitor voltages and small signal quantities of phase a respectively, 、 are the small signal quantities of the modulation coefficients of the upper and lower bridge arms of phase a respectively.

[0179] Among them, the second matrix in formula (14) is the bridge arm dynamic model. The bridge arm dynamic model represents the equivalent bridge arm structure affected by different AC side power operating levels. The operating parameters of the equivalent bridge arm structure include the capacitor voltage and small signal quantity of the upper and lower bridge arms of phase a, the modulation coefficient small signal quantity of the upper and lower bridge arms of phase a, etc.

[0180] Where, See formula (15), which is the modeling of the main circuit of the flexible DC converter station after considering the dynamics of the bridge arm:

[0181] (15)

[0182] In the sub-supersynchronous frequency band, the influence of factors such as bridge arm dynamics, AC side equivalent inductance, positive sequence current controller, positive sequence dq axis decoupling link, negative sequence current controller, negative sequence dq axis decoupling link, voltage feedforward, positive sequence voltage outer loop, negative sequence voltage outer loop, system modulation and link delay are considered. The impedance of the offshore flexible DC converter is derived by the harmonic linearization method, and its positive and negative sequence impedance expressions are obtained as shown in Equation (24).

[0183] (16)

[0184] Where, 、 are the transfer functions of the positive and negative sequence control links including control link delay and voltage-related, is the admittance matrix containing the three-phase admittance information, To express the matrix form, from Equation (24), it can be seen that it is the transfer function from the small disturbance value of the three-phase voltage on the AC side to the small disturbance value of the three-phase current on the AC side, The specific form is shown in formula (17):

[0185] (17)

[0186] In formula (17), 、 are the positive sequence and negative sequence control link transfer functions including control link delay and related to current, 、 They are the transfer functions of the positive-sequence and negative-sequence circulating current suppression links including the control link delay.

[0187] Among them, in the sub-supersynchronous frequency band, the frequency coupling effect is relatively strong. In order to conduct accurate oscillation risk assessment, it is necessary to establish equivalent impedance models of offshore wind farms and flexible DC converter stations that consider the frequency coupling effect, such as formulas (16) and (17). When conducting oscillation risk assessment, it is necessary to consider the mutual coupling relationship between the impedance of the wind turbine side and the flexible DC side.

[0188] In formula (17), in the sub-supersynchronous frequency band, due to the consideration of the bridge arm dynamics, the matrix In addition to the influence of the phase-locked loop control, there is also an AC current component related to the AC side power. , which makes the impedance characteristic curve of the flexible DC converter in the sub-supersynchronous frequency band be affected by the system power level. However, in the medium and high frequency bands, since it is not in the phase-locked loop control bandwidth frequency band and the bridge arm dynamics in the medium and high frequency bands are relatively weak, the impedance characteristic curve in the medium and high frequency bands is not affected by the power level.

[0189] In some embodiments, when the broadband oscillation risk assessment frequency band of the offshore wind power transmission system via flexible HVDC is a sub-supersynchronous frequency band, the method further includes:

[0190] Step S31: According to the interconnection relationship between the offshore wind farm side and the flexible DC converter side, the voltage-current relationship between the impedance model of the offshore wind farm side and the impedance model of the flexible DC converter side under the influence of the frequency coupling effect is used to determine the equivalent impedance of the offshore wind farm side and the equivalent impedance of the flexible DC converter side under the influence of the frequency coupling effect.

[0191] Step S32: Determine an equivalent impedance frequency characteristic curve on the offshore wind farm side and an equivalent impedance frequency characteristic curve on the flexible DC converter side by using the equivalent impedance on the offshore wind farm side and the equivalent impedance on the flexible DC converter side under the influence of the frequency coupling effect.

[0192] The above-mentioned impedance models for the offshore wind farm and the flexible DC converter station, which take into account the dynamics of the bridge arm, were established. This impedance model is a 2×2 matrix, consisting of the self-impedance on the main diagonal and the coupled impedance induced by the frequency coupling effect on the sub-diagonal. In the medium and high frequency bands, the frequency coupling effect is weak, and the impact of the coupled impedance on oscillation risk assessment can be ignored. However, in the sub-supersynchronous frequency band, where the frequency coupling effect is strong, equivalent impedance models for the offshore wind farm and the flexible DC converter station, which take into account the frequency coupling effect, are required for oscillation risk assessment.

[0193] Under the influence of frequency coupling effect, the equivalent impedances of the offshore wind farm and the flexible DC converter station are as follows:

[0194] (18)

[0195] Where, is the 2×2 impedance matrix on the AC side of the wind farm considering the frequency coupling effect, The 2×2 impedance matrix on the AC side of the flexible DC converter considering the frequency coupling effect is: is the 2×2 admittance matrix of the AC side of the wind farm considering the frequency coupling effect, The 2×2 admittance matrix of the AC side of the flexible DC converter considering the frequency coupling effect is: 、 is the self-impedance admittance, 、 is the coupling impedance admittance, 、 Both are the self-impedance admittance of the AC side of the flexible DC converter considering the frequency coupling effect. Both are the coupling impedance admittances on the AC side of the flexible DC converter considering the frequency coupling effect.

[0196] Under the influence of frequency coupling effect, the voltage and current relationship between the offshore wind farm and the flexible DC converter station interconnection system is as follows: Figure 12 shown.

[0197] in, Figure 12 In, ω P1 is the frequency f P1 The corresponding angular frequency, ω P2 is the mirror coupling frequency f P1 The angular frequency corresponding to -2f1 is 、 and 、 are the voltage and current disturbances at a pair of mirror frequencies responded to by the interconnected system PCC. Figure 12 The voltage-current relationship shown in the figure is simplified by Mason's formula. Using formula (18), the equivalent impedance of the offshore wind farm side and the flexible DC converter station side under the influence of frequency coupling effect can be obtained. That is, the equivalent impedance model of the offshore wind farm side and the equivalent impedance model of the flexible DC converter side under the strong frequency coupling in the sub-supersynchronous frequency band are:

[0198] (19)

[0199] Among them, the system frequency coupling effect in the medium and high frequency bands can be ignored, that is, the mutual impedance admittance in formula (19) is zero, and the final impedance model on the offshore wind farm side and the flexible DC converter side impedance model can be obtained.

[0200] By utilizing the equivalent impedance on the offshore wind farm side and the equivalent impedance on the flexible DC converter side under the influence of the frequency coupling effect, the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible DC converter side can be determined.

[0201] In some embodiments, the broadband oscillation risk of the offshore wind power transmission system through the flexible DC power transmission system is assessed using the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible DC power converter side, including:

[0202] Step S401: Determine, based on the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible DC converter side, a frequency range in which the equivalent impedance amplitude of the impedance model on the flexible DC converter side is greater than the equivalent impedance amplitude of the impedance model on the offshore wind farm side within a wide-band oscillation risk assessment frequency band.

[0203] The frequency range in which the impedance amplitude of the HVDC-Flex converter-side impedance model exceeds that of the offshore wind farm-side impedance model within the broadband oscillation risk assessment frequency band indicates that the HVDC-Flex converter-side impedance has a dominant influence on system oscillations within this frequency range. When the system is disturbed within this frequency range, oscillations are more likely to be induced on the HVDC-Flex converter side, posing a threat to the stable operation of the offshore wind power HVDC-Flex transmission system.

[0204] Step S402 : determining whether a phase difference between the impedance model on the flexible DC converter side and the impedance model on the offshore wind farm side within a frequency range is greater than a preset phase difference threshold.

[0205] Based on engineering experience, the phase difference threshold can be set to 180°.

[0206] Step S403: If it is determined that the phase difference is greater than a preset phase difference threshold, it is determined that there is a risk of broadband oscillation in the offshore wind power transmission system via the flexible DC transmission system;

[0207] Step S404: If it is determined that the phase difference is not greater than the preset phase difference threshold, it is determined that there is no broadband oscillation risk in the offshore wind power transmission system via the flexible DC transmission system.

[0208] Among them, such as Figure 13a-13b The positive sequence subsystem and negative sequence subsystem of the offshore wind power transmission system via flexible DC transmission are shown in the figure, where Z Rp , I Rp Respectively represent the new energy positive sequence impedance and equivalent current source; Z Mp 、V Mp Respectively represent the positive sequence impedance and equivalent voltage source of the flexible DC converter station; V outp , Ioutp Respectively represent the AC bus voltage and current of the new energy station. Rn , I Rn Respectively represent the new energy negative sequence impedance and equivalent current source; Z Mn 、V Mn They represent the negative sequence impedance and equivalent voltage source of the flexible DC converter station respectively; V outn , I outn They represent the AC bus voltage and current of the new energy station respectively.

[0209] Assuming that the offshore wind farm and the flexible DC converter station are both in a stable state when operating independently, I Wp 、V Mp Therefore, when the new energy station and the flexible DC converter station are interconnected, the stability of the positive sequence subsystem depends on the impedance ratio Z Mp (s) / Z Wp (s).

[0210] The system does not meet the stability criterion and oscillation occurs if and only if the following two conditions are met: ① There is Z Mp (s) Impedance amplitude is greater than Z Rp (s) Frequency range of impedance amplitude; ② At Z Mp (s) Impedance amplitude is greater than Z Rp (s) Impedance amplitude frequency range, Z Mp (s) and Z Rp (s) The phase-frequency characteristic curves differ by more than 180°.

[0211] In some embodiments, the offshore wind power corresponding to the sub-supersynchronous frequency band adopts an operation mode at different power operating levels through the flexible DC transmission system; the offshore wind power corresponding to the medium and high frequency bands adopts an operation mode at the rated operating power level through the flexible DC transmission system.

[0212] Understandably, the sub-supersynchronous frequency band's offshore wind power transmission system via flexible DC transmission adopts different operating modes at different power levels. This is because the system's impedance characteristics are significantly affected by the power level in the sub-supersynchronous frequency band. Therefore, impedance modeling and oscillation risk assessment are required for the system at different power levels. This can more accurately reflect the system's dynamic behavior under different operating conditions, thereby more effectively assessing the system's oscillation risk.

[0213] Specifically, in the sub-supersynchronous frequency band, as power levels increase, the impedance characteristics of offshore wind farms and flexible DC converter stations change. This change can cause impedance mismatch in certain frequency ranges, leading to oscillations. Therefore, to comprehensively assess the system's oscillation risk, it is necessary to consider the system's impedance characteristics at different power operating levels.

[0214] In the mid- and high-frequency bands, because frequency coupling is weaker and the impact of bridge arm dynamics on impedance characteristics is also smaller, impedance modeling and oscillation risk assessment can be performed using full-power operation. This simplifies the assessment process while still ensuring the accuracy of the results.

[0215] In summary, impedance modeling and oscillation risk assessment based on operating modes at different power levels in different frequency bands can more comprehensively reflect the dynamic behavior of the system and provide stronger guarantees for the stable operation of the system.

[0216] The operation mode adopts one or more of the following modes, wherein the operation mode includes:

[0217] Black start, full connection of wind farm, exit of some wind turbines, exit of some feeders, exit of some AC busbars and exit of some transformers. The black start includes no-load black start on the flexible DC converter side, no-load black start with AC busbar on the flexible DC converter side and no-load operation with offshore wind farm on the flexible DC converter side.

[0218] Among them, black start refers to the restart of the generator sets with self-starting capabilities in the system after the entire system has been completely cut off due to a fault or other reasons, without relying on other network assistance, and then driving the generator sets without self-starting capabilities, gradually expanding the system recovery range, and ultimately achieving the restoration of power supply for the entire power system.

[0219] A fully wired wind farm means that all wind turbines, feeders, AC busbars and transformers in the offshore wind farm are in operation, and the system is operating at full load or near full load.

[0220] Partial wind turbine outage means that some wind turbines stop operating due to failure, maintenance or other reasons, while other wind turbines, feeders, AC busbars and transformers are still in operation.

[0221] Partial feeder outage means that part of the feeder stops operating due to fault, maintenance or other reasons, while other feeders, wind turbines, AC busbars and transformers are still in operation.

[0222] Partial AC busbar withdrawal means that part of the AC busbar stops operating due to fault, maintenance or other reasons, while other AC buses, wind turbines, feeders and transformers are still in operation.

[0223] Partial transformer withdrawal means that some transformers stop operating due to failure, maintenance or other reasons, while other transformers, wind turbines, feeders and AC busbars are still in operation.

[0224] Using these different operating modes, it is possible to simulate the operating state of the offshore wind power transmission system through the Flexible HVDC under different operating conditions, thereby more comprehensively assessing the system's oscillation risk. For example, during a black start, as the system is in the initial recovery phase, the impedance characteristics of the wind farm and the Flexible HVDC converter station may differ from those under normal operation. Therefore, specialized impedance modeling and oscillation risk assessment are required for this black start process. Similarly, if some wind turbines, feeders, AC buses, or transformers are out of service, the system's impedance characteristics and oscillation risk may also change, thus requiring corresponding assessment.

[0225] In some embodiments, when it is assessed that there is a broadband oscillation risk in the offshore wind power HVDC flexible transmission system, the offshore wind power HVDC flexible transmission system is adjusted according to different broadband oscillation risk assessment frequency bands until the broadband oscillation risk is eliminated, including:

[0226] When it is assessed that there is a risk of broadband oscillation in the offshore wind power transmission system via flexible direct current, the offshore wind power transmission system via flexible direct current is adjusted according to a preset risk adjustment plan corresponding to the broadband oscillation risk assessment frequency band until the broadband oscillation risk is eliminated.

[0227] The offshore wind power transmission system via HVDC Flexible can be adjusted using a pre-set risk adjustment plan corresponding to the broadband oscillation risk assessment frequency band. This can include, but is not limited to, adjusting the output power of the offshore wind farm, adjusting the control parameters of the HVDC Flexible converter, and optimizing the system's operating mode. During implementation, an appropriate adjustment plan can be selected based on the risk assessment results and the actual system conditions, and system parameters can be gradually adjusted until the broadband oscillation risk is eliminated.

[0228] Specifically, the preset first risk adjustment scheme corresponding to the medium and high frequency bands adopts one or more combinations of the following schemes. The preset first risk adjustment scheme includes:

[0229] (1) Change the primary circuit, control structure and control parameters of the wind turbine on the offshore wind farm side.

[0230] (2) Changing the primary circuit, control structure and control parameters of the flexible DC converter on the flexible DC converter side.

[0231] For example, during the primary loop adjustment process, an AC-side filter bank was installed; in terms of control structure adjustments, an additional filter was added to the voltage feedforward link; and in terms of control parameter adjustments, the focus was on reducing the delay of the system control link. For the first risk adjustment solution, measures were taken to reduce the negative damping region of the phase-frequency characteristic curve in the medium and high frequency bands on the offshore wind farm side and the flexible DC converter side. At the same time, the phase difference between the impedance amplitude of the impedance model on the flexible DC converter side and the impedance amplitude of the offshore wind farm side was reduced, thereby eliminating the risk of resonance in the medium and high frequency bands.

[0232] The preset second risk adjustment scheme corresponding to the sub-supersynchronous frequency band adopts one or more of the following scheme combinations. The preset second risk adjustment scheme includes:

[0233] (1) Reduce the phase-locked loop PI controller parameters of the wind turbines on the offshore wind farm side;

[0234] (2) Increase the current inner loop PI controller parameters of the wind turbines on the offshore wind farm side;

[0235] Exemplarily, the proportional parameter of the phase-locked loop PI controller of the wind turbine generator set on the offshore wind farm side is reduced, or the proportional parameter of the current inner loop PI controller of the wind turbine generator set on the offshore wind farm side is increased.

[0236] (3) Add a positive damping signal to the transmission signal of the offshore wind power in the sub-supersynchronous frequency band through the flexible DC transmission system.

[0237] Adding a positive damping signal to the sub-supersynchronous frequency band of the offshore wind power transmission system via the flexible DC transmission system can suppress the system's oscillation tendency within this frequency band and improve system stability. Specifically, this can be achieved by introducing additional damping elements in the control system or adding damping resistors to the filter design. These measures can effectively increase system damping, thereby reducing the risk of oscillation.

[0238] In summary, the second risk adjustment scheme reduces the negative damping area of the sub-supersynchronous evaluation phase-frequency characteristic curve on the offshore wind farm side and the flexible DC converter side, increases the phase difference between the impedance amplitude of the impedance model on the flexible DC converter side and the impedance amplitude of the impedance model on the offshore wind farm side, and eliminates the resonance risk in the sub-supersynchronous frequency band.

[0239] Table 1 Examples of wind turbine operating conditions at different power levels (applicable operating conditions of the invention include but are not limited to this table)

[0240]

[0241] The following is an example of the wide-band oscillation risk assessment method for offshore wind power through flexible DC transmission system proposed in the embodiment of this application in the oscillation risk assessment of medium and high frequency bands.

[0242] 1) Establish and verify the impedance model of a single fan, such as Figures 14a-14b As shown in the figure, the theoretical value of the impedance model of a single wind turbine is in good agreement with the actual value of the scan, and can be used for medium and high frequency oscillation risk assessment.

[0243] 2) Establish a single feeder impedance model and verify it, such as Figures 15a-15b As shown in the figure, due to the different types of wind turbines and control parameters on the wind farm feeders, and the different feeder topologies, the AC submarine cable modeling and verification results after the Bergeron model is adopted are given. After the impedance model of a single wind turbine and an AC submarine cable is verified, the impedance characteristic curve model of a single feeder can be obtained through the series-parallel relationship of a single feeder topology.

[0244] 3) Modeling of impedance characteristics of offshore wind farms, considering the high-frequency impedance characteristic curves of offshore wind farms fed by multiple feeders, such as Figures 16a-16b shown.

[0245] 4) Modeling and verification of impedance characteristics of flexible DC converter stations, based on Figures 17a-17b It can be seen that the theoretical values of the established flexible DC converter station model are in good agreement with the actual values of the scan, and can be used for medium and high frequency oscillation risk assessment.

[0246] 5) Risk assessment of medium and high frequency oscillations in offshore wind farms via flexible DC transmission. Taking the operating condition of all feeders in offshore wind farms as an example, Figures 18a-18b As shown in the figure, when all feeders are put into operation, the impedance phase angle difference between the offshore wind farm and the flexible DC is less than 180° in the entire frequency band, indicating that the system can operate stably. However, in the range of 1000-1500 Hz, the impedance phase angle difference is around 170°, and the system stability margin is low.

[0247] The following is an example of the broadband oscillation risk assessment method for offshore wind power through flexible DC transmission system proposed in the embodiment of the present application in the sub-supersynchronous frequency band oscillation risk assessment.

[0248] 1) Establish and verify the impedance model of a single wind turbine:

[0249] Taking the rated power as an example to verify the modeling, the theoretical value of the impedance model of a single wind turbine in the sub-supersynchronous frequency band is compared with the actual frequency scanning value. Figures 19a-19b As shown in the figure, the modeling theoretical values are in good agreement with the actual frequency sweep values, and the established model can be used to evaluate the oscillation risk in the sub-supersynchronous frequency band.

[0250] Consider the sub-synchronous frequency band of a single wind turbine under different power levels in Table 1. Figure 20a-1 、20a-2 , 20b-1, and 20b-2 (taking the first two operating conditions as examples) show the sub-supersynchronous frequency band impedance characteristic curves of a single wind turbine under different power operating levels. It can be seen that the power level has an obvious influence on the sub-supersynchronous frequency band impedance of power electronic devices.

[0251] 2) Establish and verify the impedance model of a single feeder. Due to the different types of wind turbines and control parameters on the wind farm feeders, as well as the different feeder topologies, the AC submarine cable modeling and verification results after using the Bergeron model are given here. Figure 21a 、 21b As shown in Figure 1, after the impedance model of a single wind turbine and an AC submarine cable has been verified, the impedance characteristic curve model of a single feeder can be obtained through the series-parallel relationship of a single feeder topology.

[0252] 3) Modeling of impedance characteristics of offshore wind farms, considering the super-synchronous frequency band impedance characteristic curve of offshore wind farms fed by multiple feeders, as shown in Figure 22a 、 22b shown.

[0253] 4) Modeling and verification of impedance characteristics of flexible DC converter stations: Based on Figure 23a 、 23b It can be seen that the theoretical values of the established flexible DC converter station model are in good agreement with the actual values of the scan, and can be used for medium and high frequency oscillation risk assessment.

[0254] 5) Establish an equivalent impedance model of the sub-supersynchronous frequency band of offshore wind farms and flexible DC converter stations considering the frequency coupling effect. Taking the offshore wind farm as an example, the theoretical value and actual scan value of the equivalent impedance of the offshore wind farm before and after considering the frequency coupling effect are shown in Figure 5. Figure 24a 、 24b As shown, according to Figure 24a 、 24b It can be seen that the frequency coupling effect has a significant impact on the equivalent impedance in the sub-supersynchronous frequency band, but has almost no effect in the mid- and high-frequency bands. The equivalent impedance established after considering the frequency coupling effect is in good agreement with the actual frequency sweep value and can be used for resonant stability analysis in the sub-supersynchronous frequency band.

[0255] 6) Risk assessment of sub-synchronous frequency band oscillation of offshore wind farms via flexible DC transmission: Taking the rated power level as an example, the risk assessment of sub-synchronous frequency band oscillation of offshore wind farms via flexible DC transmission system is carried out. The AC equivalent impedance of the offshore wind farm and the flexible DC side considering the frequency coupling effect established and verified in this application is as follows: Figure 25a 、 25bAccording to the impedance analysis method, the impedance amplitude-frequency characteristic curve of the interconnected system between the offshore wind farm and the flexible DC converter station intersects at 79Hz within a wide frequency band, and the phase-frequency characteristic curve differs by 209.4°. Based on the system stability criterion, it can be determined that the system does not have a positive phase margin and is unstable at this time.

[0256] Based on the same inventive concept, an embodiment of the present application also provides an offshore wind power flexible direct current transmission system broadband oscillation risk assessment system for implementing the above-mentioned offshore wind power flexible direct current transmission system broadband oscillation risk assessment method.

[0257] The implementation solution to the problem provided by this system is similar to the implementation solution described in the above method. Therefore, the specific limitations in one or more embodiments of the offshore wind power flexible direct current transmission system broadband oscillation risk assessment system provided below can be referred to the above limitations on the offshore wind power flexible direct current transmission system broadband oscillation risk assessment method, which will not be repeated here.

[0258] like Figure 26 As shown, an embodiment of the present application provides a broadband oscillation risk assessment system for an offshore wind power transmission system via flexible direct current (HVDC), which is applied to assess the broadband oscillation risk of the offshore wind power transmission system via flexible direct current (HVDC) in different broadband oscillation risk assessment frequency bands. The broadband oscillation risk assessment frequency bands include medium-high frequency bands and sub-supersynchronous frequency bands; and includes:

[0259] The wind farm side impedance construction module 100 is used to establish an offshore wind farm side impedance model within the offshore wind power flexible direct current transmission system according to the topology of the offshore wind power flexible direct current transmission system;

[0260] The flexible DC converter side impedance construction module 200 is used to determine the flexible DC converter side impedance model corresponding to the broadband oscillation risk assessment frequency band according to different broadband oscillation risk assessment frequency bands;

[0261] An impedance curve determination module 300 is used to determine an equivalent impedance frequency characteristic curve on the offshore wind farm side and an equivalent impedance frequency characteristic curve on the flexible DC converter side according to an impedance model on the offshore wind farm side and an impedance model on the flexible DC converter side, respectively;

[0262] The risk assessment module 400 is used to assess the broadband oscillation risk of the offshore wind power transmission system through the flexible DC transmission system by using the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible DC converter side;

[0263] The risk adjustment module 500 is used to adjust the offshore wind power transmission system through the flexible direct current transmission system according to different broadband oscillation risk assessment frequency bands when assessing the existence of broadband oscillation risk in the offshore wind power transmission system through the flexible direct current transmission system until the broadband oscillation risk is eliminated.

[0264] In some embodiments, the wind farm side impedance construction module 100 is used to:

[0265] Based on different broadband oscillation risk assessment frequency bands, the voltage and current small signal phasors at the common coupling point of a single wind turbine under three-phase symmetrical working conditions are constructed;

[0266] Based on the voltage and current small-signal phasors, the control process of a single wind turbine is Laplace transformed and combined with the primary topology of the single wind turbine to obtain the AC-side admittance matrix of the common coupling point of the single wind turbine. The AC-side admittance matrix in the sub-supersynchronous frequency band introduces the phase-locked loop control link coefficient matrix affected by different AC-side power operating levels.

[0267] Determine the impedance characteristic model of a single wind turbine based on the AC side admittance matrix of the common coupling point;

[0268] The Bergeron model is used to simulate the AC line between the two wind turbines and obtain the equivalent impedance model of the AC line.

[0269] For each feeder, based on the connection relationship between multiple wind turbines and AC lines on the feeder, the impedance characteristic model of a single wind turbine and the equivalent impedance model of the AC line are connected in series and parallel to obtain the equivalent impedance model of the single feeder;

[0270] According to the feeder topology of the offshore wind power transmission system through flexible DC, the equivalent impedance models of multiple single feeders are topologically connected to obtain the impedance model of the offshore wind farm side.

[0271] In some embodiments, when the broadband oscillation risk assessment frequency band is a medium or high frequency band, determining the flexible DC converter side impedance model corresponding to the broadband oscillation risk assessment frequency band according to different broadband oscillation risk assessment frequency bands includes:

[0272] Construct the voltage and current small signal phasors at the common coupling point on the flexible DC converter side;

[0273] Based on the voltage and current small signal phasors, the control process of the flexible DC converter side is Laplace transformed, and the impedance model of the flexible DC converter side is obtained in combination with the primary topology structure of the flexible DC converter side.

[0274] When the broadband oscillation risk assessment frequency band is a sub-supersynchronous frequency band, the flexible DC converter side impedance model corresponding to the broadband oscillation risk assessment frequency band is determined according to different broadband oscillation risk assessment frequency bands, including:

[0275] Construct the voltage and current small signal phasors at the common coupling point on the flexible DC converter side;

[0276] Based on the voltage and current small-signal phasors, the control process on the flexible DC converter side and the dynamic model of the bridge arm affected by different AC side power operating levels are Laplace transformed, and the impedance model of the flexible DC converter side is obtained in combination with the primary topology structure on the flexible DC converter side. Among them, the dynamic model of the bridge arm is used to characterize the equivalent bridge arm structure affected by different AC side power operating levels.

[0277] In some embodiments, the system further comprises: an equivalent impedance building module for:

[0278] Based on the interconnection relationship between the offshore wind farm side and the flexible DC converter side, the voltage and current relationship between the impedance model of the offshore wind farm side and the impedance model of the flexible DC converter side under the influence of the frequency coupling effect is used to determine the equivalent impedance of the offshore wind farm side and the equivalent impedance of the flexible DC converter side under the influence of the frequency coupling effect;

[0279] The equivalent impedance frequency characteristic curve of the offshore wind farm side and the equivalent impedance frequency characteristic curve of the flexible DC converter side are determined by utilizing the equivalent impedance of the offshore wind farm side and the equivalent impedance of the flexible DC converter side under the influence of the frequency coupling effect.

[0280] In some embodiments, the risk assessment module 400 is configured to:

[0281] Based on the equivalent impedance frequency characteristic curves on the offshore wind farm side and the equivalent impedance frequency characteristic curves on the flexible DC converter side, determine the frequency range within which the equivalent impedance amplitude of the impedance model on the flexible DC converter side is greater than the equivalent impedance amplitude of the impedance model on the offshore wind farm side within the wide-band oscillation risk assessment frequency band;

[0282] Determine whether the phase difference between the impedance model on the flexible DC converter side and the impedance model on the offshore wind farm side within the frequency range is greater than a preset phase difference threshold;

[0283] If the phase difference is greater than the preset phase difference threshold, it is determined that there is a risk of broadband oscillation in the offshore wind power transmission system via the flexible DC transmission system;

[0284] If it is determined that the phase difference is not greater than the preset phase difference threshold, it is determined that there is no broadband oscillation risk in the offshore wind power transmission system through the flexible direct current transmission system.

[0285] In some embodiments, the offshore wind power corresponding to the sub-supersynchronous frequency band is transmitted through the flexible DC transmission system in different operating modes at different power operating levels; the offshore wind power corresponding to the medium and high frequency bands is transmitted through the flexible DC transmission system in a mode at the rated operating power level; wherein the operating mode adopts one or a combination of the following modes, wherein the operating mode includes:

[0286] Black start, full connection of wind farm, exit of some wind turbines, exit of some feeders, exit of some AC busbars and exit of some transformers. The black start includes no-load black start on the flexible DC converter side, no-load black start with AC busbar on the flexible DC converter side and no-load operation with offshore wind farm on the flexible DC converter side.

[0287] In some embodiments, the risk adjustment module 500 is configured to:

[0288] If it is assessed that there is a risk of broadband oscillation in the offshore wind power transmission system via flexible DC, the offshore wind power transmission system via flexible DC is adjusted according to the preset risk adjustment plan corresponding to the broadband oscillation risk assessment frequency band until the broadband oscillation risk is eliminated;

[0289] The preset first risk adjustment scheme corresponding to the medium and high frequency bands adopts one or more of the following scheme combinations. The preset first risk adjustment scheme includes:

[0290] Changing the primary circuit, control structure and control parameters of the wind turbines on the offshore wind farm side;

[0291] Changing the primary circuit, control structure, and control parameters of the flexible DC converter on the flexible DC converter side;

[0292] The preset second risk adjustment scheme corresponding to the sub-supersynchronous frequency band adopts one or more of the following scheme combinations. The preset second risk adjustment scheme includes:

[0293] Reduce the phase-locked loop PI controller parameters of wind turbines on the offshore wind farm side;

[0294] Increase the current inner loop PI controller parameters of the wind turbines on the offshore wind farm side;

[0295] A positive damping signal is added to the transmission signal of the offshore wind power in the sub-supersynchronous frequency band through the flexible DC transmission system.

[0296] like Figure 27 As shown, an embodiment of the present application also provides an electronic device, the electronic device 10 includes a memory 20 and a processor 30, the memory 20 stores a computer program, and when the computer program is executed by the processor 30, the processor 30 executes the steps of the method for assessing the risk of broadband oscillation of an offshore wind power flexible direct current transmission system as described in the above embodiment.

[0297] An embodiment of the present application also provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed, the steps of the method for assessing the risk of broadband oscillation of an offshore wind power flexible direct current transmission system in the above embodiment are implemented.

[0298] Those skilled in the art will clearly understand that, for the convenience and brevity of description, the specific working processes of the above-described systems, electronic devices, computer storage media and computer-readable storage media can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0299] It should be noted that the terms "including" and "having" and any variations thereof in the specification and claims of the present invention and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or are inherent to these processes, methods, products or apparatuses.

[0300] In several embodiments provided by the present invention, it is understood that each box in the flow chart or block diagram can represent a module, program segment or part of the code, and the module, program segment or part of the code contains one or more executable instructions for realizing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the box can also occur in a different order than that marked in the accompanying drawings. For example, two consecutive boxes can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved.

[0301] In the several embodiments provided by the present invention, it should be understood that the disclosed systems, electronic devices, computer storage media and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of the units is merely a logical function division. In actual implementation, there may be other division methods, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be an indirect coupling or communication connection through some interfaces, devices or units, which can be electrical, mechanical or other forms.

[0302] The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units. Some or all of these units may be selected to achieve the purpose of this embodiment according to actual needs.

[0303] In addition, the functional units in the various embodiments of the present invention may be integrated into a single processing unit, each unit may exist physically separately, or two or more units may be integrated into a single unit. The aforementioned integrated units may be implemented in the form of hardware or software functional units.

[0304] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the portion that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for executing all or part of the steps of the method described in each embodiment of the present invention via a computer device (which can be a personal computer, server, or network device, etc.). The aforementioned storage medium includes various media that can store program code, such as a USB flash drive, a mobile hard drive, a read-only memory (ROM), a random access memory (RAM), a magnetic disk, or an optical disk.

[0305] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for assessing the broadband oscillation risk of an offshore wind power system via a flexible direct current transmission system, which is applied to assess the broadband oscillation risk of an offshore wind power system via a flexible direct current transmission system in different broadband oscillation risk assessment frequency bands, including medium-high frequency bands and sub-supersynchronous frequency bands; It is characterized by: The method comprises: Establishing an offshore wind farm-side impedance model within the offshore wind power HVDC Flexible transmission system based on the topology of the system and different broadband oscillation risk assessment frequency bands; Determining, according to different broadband oscillation risk assessment frequency bands, a flexible DC converter side impedance model corresponding to the broadband oscillation risk assessment frequency band; Determining an equivalent impedance frequency characteristic curve on the offshore wind farm side and an equivalent impedance frequency characteristic curve on the flexible DC converter side respectively according to the offshore wind farm side impedance model and the flexible DC converter side impedance model; Using the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible DC converter side, the broadband oscillation risk of the offshore wind power transmission system through the flexible DC converter is assessed; When it is assessed that there is a risk of broadband oscillation in the offshore wind power transmission system via flexible direct current, the offshore wind power transmission system via flexible direct current is adjusted according to different broadband oscillation risk assessment frequency bands until the risk of broadband oscillation is eliminated.

2. The method for assessing the risk of broadband oscillation of an offshore wind power system via a flexible direct current transmission system according to claim 1, characterized in that: The process of establishing the offshore wind farm side impedance model includes: Based on different broadband oscillation risk assessment frequency bands, the voltage and current small signal phasors at the common coupling point of a single wind turbine under three-phase symmetrical working conditions are constructed; Based on the voltage and current small-signal phasors, a Laplace transform is performed on the control process of the single wind turbine, and combined with the primary topology of the single wind turbine, an AC-side admittance matrix of the common coupling point of the single wind turbine is obtained; wherein the AC-side admittance matrix of the sub-supersynchronous frequency band introduces a phase-locked loop control link coefficient matrix affected by different AC-side power operating levels; Determine the impedance characteristic model of a single wind turbine according to the AC side admittance matrix of the common coupling point; The Bergeron model is used to simulate the AC line between the two wind turbines and obtain the equivalent impedance model of the AC line. For each feeder, based on the connection relationship between the multiple wind turbines on the feeder and the AC line, the impedance characteristic model of the single wind turbine and the equivalent impedance model of the AC line are connected in series and parallel to obtain the equivalent impedance model of the single feeder; According to the feeder topology structure of the offshore wind power transmission system via flexible direct current, equivalent impedance models of a plurality of the single feeders are topologically connected to obtain the offshore wind farm side impedance model.

3. The method for assessing the risk of broadband oscillation of an offshore wind power HVDC Flexible transmission system according to claim 1, characterized in that: When the broadband oscillation risk assessment frequency band is a medium or high frequency band, determining the flexible DC converter side impedance model corresponding to the broadband oscillation risk assessment frequency band according to different broadband oscillation risk assessment frequency bands includes: Construct the voltage and current small signal phasors at the common coupling point on the flexible DC converter side; Based on the voltage and current small-signal phasors, the impedance model of the flexible DC converter side is obtained by performing Laplace transform on the control process of the flexible DC converter side in combination with the primary topology structure of the flexible DC converter side; When the wideband oscillation risk assessment frequency band is a sub-supersynchronous frequency band, determining the flexible DC converter side impedance model corresponding to the wideband oscillation risk assessment frequency band according to different wideband oscillation risk assessment frequency bands includes: Construct the voltage and current small signal phasors at the common coupling point on the flexible DC converter side; Based on the voltage and current small-signal phasors, the control process on the flexible DC converter side and the dynamic model of the bridge arm affected by different AC side power operating levels are Laplace transformed, and the impedance model of the flexible DC converter side is obtained in combination with the primary topology structure on the flexible DC converter side; wherein the dynamic model of the bridge arm is used to characterize the equivalent bridge arm structure affected by different AC side power operating levels.

4. The method for assessing the risk of broadband oscillation of an offshore wind power system via a flexible direct current transmission system according to any one of claims 1 to 3, characterized in that: Also includes: According to the interconnection relationship between the offshore wind farm side and the flexible DC converter side, the equivalent impedance of the offshore wind farm side and the equivalent impedance of the flexible DC converter side under the influence of the frequency coupling effect are determined by using the voltage-current relationship between the impedance model of the offshore wind farm side and the impedance model of the flexible DC converter side under the influence of the frequency coupling effect; The equivalent impedance frequency characteristic curve of the offshore wind farm side and the equivalent impedance frequency characteristic curve of the flexible DC converter side are determined by utilizing the equivalent impedance of the offshore wind farm side and the equivalent impedance of the flexible DC converter side under the influence of the frequency coupling effect.

5. The method for assessing the risk of broadband oscillation of an offshore wind power HVDC Flexible transmission system according to claim 1, characterized in that: The method of evaluating the broadband oscillation risk of the offshore wind power transmission system through the flexible direct current converter by using the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible direct current converter side includes: Determining, based on the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible DC converter side, a frequency interval range in which the equivalent impedance amplitude of the impedance model on the flexible DC converter side is greater than the equivalent impedance amplitude of the impedance model on the offshore wind farm side within a wide-band oscillation risk assessment frequency band; Determining whether a phase difference between the impedance model on the flexible DC converter side and the impedance model on the offshore wind farm side within the frequency range is greater than a preset phase difference threshold; If it is determined that the phase difference is greater than the preset phase difference threshold, it is determined that there is a risk of broadband oscillation in the offshore wind power transmission system via the flexible direct current system; If it is determined that the phase difference is not greater than the preset phase difference threshold, it is determined that there is no broadband oscillation risk in the offshore wind power transmission system via the flexible direct current transmission system.

6. The method for assessing the risk of broadband oscillation of an offshore wind power HVDC Flexible transmission system according to claim 1, characterized in that: The offshore wind power corresponding to the sub-supersynchronous frequency band adopts an operation mode at different power operation levels through the flexible DC transmission system; the offshore wind power corresponding to the medium and high frequency bands adopts an operation mode at the rated operating power level through the flexible DC transmission system; wherein, the operation mode adopts one or more of the following modes, wherein the operation mode includes: Black start, full connection of wind farm, exit of some wind turbines, exit of some feeders, exit of some AC busbars and exit of some transformers, wherein the black start includes no-load black start on the flexible DC converter side, no-load black start on the flexible DC converter side with AC busbars and no-load operation on the flexible DC converter side with offshore wind farms.

7. The method for assessing the risk of broadband oscillation of an offshore wind power system via a flexible DC transmission system according to claim 1, characterized in that: When it is assessed that the offshore wind power transmission system via flexible direct current (HVDC) has a broadband oscillation risk, adjusting the offshore wind power transmission system via flexible direct current (HVDC) according to different broadband oscillation risk assessment frequency bands until the broadband oscillation risk is eliminated includes: If it is assessed that the offshore wind power transmission system via flexible direct current has a broadband oscillation risk, the offshore wind power transmission system via flexible direct current is adjusted according to a preset risk adjustment plan corresponding to the broadband oscillation risk assessment frequency band until the broadband oscillation risk is eliminated; The preset first risk adjustment scheme corresponding to the medium and high frequency bands adopts one or more of the following scheme combinations, and the preset first risk adjustment scheme includes: Changing the primary circuit, control structure and control parameters of the wind turbine generator set on the offshore wind farm side; Changing the primary circuit, control structure, and control parameters of the flexible DC converter on the flexible DC converter side; The preset second risk adjustment scheme corresponding to the secondary supersynchronous frequency band adopts one or more of the following scheme combinations, and the preset second risk adjustment scheme includes: Reducing the phase-locked loop (PI) controller parameters of the wind turbine generator set on the offshore wind farm side; Increasing the current inner loop PI controller parameters of the wind turbine generator set on the offshore wind farm side; A positive damping signal is added to the transmission signal of the offshore wind power through the flexible direct current transmission system in the sub-supersynchronous frequency band.

8. A broadband oscillation risk assessment system for offshore wind power transmission via flexible direct current transmission system, characterized in that: The method is used to assess the broadband oscillation risk of offshore wind power through a flexible DC transmission system in different broadband oscillation risk assessment frequency bands, including medium and high frequency bands and sub-supersynchronous frequency bands; including: A wind farm side impedance construction module is used to establish an offshore wind farm side impedance model within the offshore wind power flexible direct current transmission system according to the topology of the offshore wind power flexible direct current transmission system; A flexible DC converter side impedance construction module is used to determine the flexible DC converter side impedance model corresponding to different broadband oscillation risk assessment frequency bands according to the broadband oscillation risk assessment frequency bands; An impedance curve determination module, configured to determine an equivalent impedance frequency characteristic curve on the offshore wind farm side and an equivalent impedance frequency characteristic curve on the flexible DC converter side, respectively, based on the impedance model on the offshore wind farm side and the impedance model on the flexible DC converter side; a risk assessment module, configured to assess the broadband oscillation risk of the offshore wind power transmission system via the flexible direct current (HVDC) by using the equivalent impedance frequency characteristic curve on the offshore wind farm side and the equivalent impedance frequency characteristic curve on the flexible direct current (HVDC) converter side; The risk adjustment module is used to adjust the offshore wind power flexible direct current transmission system according to different broadband oscillation risk assessment frequency bands when it is assessed that the offshore wind power flexible direct current transmission system has a broadband oscillation risk until the broadband oscillation risk is eliminated.

9. An electronic device, characterized in that: The electronic device includes a memory and a processor, wherein a computer program is stored in the memory. When the computer program is executed by the processor, the processor performs the steps of the method for assessing the risk of broadband oscillation of an offshore wind power flexible direct current transmission system as described in any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed, the steps of the method for assessing the risk of broadband oscillation of an offshore wind power flexible direct current transmission system according to any one of claims 1 to 7 are implemented.

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