Equivalent reduced-order modeling method and system for offshore wind power flexible direct current transmission system

Abstract

The present invention relates to an equivalent reduced-order modeling method for an offshore wind power flexible direct current transmission system, comprising the following steps: constructing an equivalent reduced-order model of a wind farm; constructing an equivalent reduced-order model of a modular multilevel converter; constructing a broadband equivalent reduced-order model of a receiving-end power grid; constructing an equivalent reduced-order model of the offshore wind power flexible direct current transmission system based on the equivalent reduced-order model of the wind farm, the equivalent reduced-order model of the modular multilevel converter, and the broadband equivalent reduced-order model of the receiving-end power grid; the present invention reduces the order of the wind farm, MMC, and receiving-end power grid respectively and integrates them, thereby reducing the number of overall nodes and components by multiples while maintaining the transient consistency of voltage and current at the grid connection point, significantly reducing the amount of CPU computation, and enabling a HIL platform to load a larger-scale wind turbine group at one time.

Description

Technical Field

[0001] The present invention relates to an equivalent reduced-order modeling method and system for an offshore wind power flexible direct current transmission system, belonging to the technical field of offshore wind power. Background Art

[0002] Because flexible direct current (HVDC) transmission technology can provide voltage support to passive grids, it has become the preferred option for integrating long-distance offshore wind power. Wind farms are aggregated and transmitted through offshore flexible direct current (HVDC) systems. Both sides of the AC line exhibit the characteristics of power electronically controlled power supplies. This fundamentally changes fault characteristics compared to traditional wind turbines connected to AC systems, significantly impacting the performance of conventional protection systems and posing a threat to the safe operation of wind turbines and the HVDC transmission system.

[0003] During a fault on the AC side of the sending end, the wind turbines and flexible DC system of the wind power transmission system via flexible DC will be equipped with different fault ride-through control strategies according to regulations. The various strategies will work in coordination to ensure the continuous and uninterrupted operation of the flexible DC transmission system and the wind farm. The fault characteristics of the wind power transmission system via flexible DC are closely related to the coordinated fault ride-through strategies of each converter, but there are many types of control strategies used by converters, and the control parameters are often unequal. The combined effect and mutual influence of the low voltage ride-through strategies of different converter devices result in the short-circuit current provided by the new energy flexible DC transmission system having phase angle control and amplitude limitation characteristics under the fault on the sending end AC side. Its fault characteristics are significantly different from those of the traditional AC transmission system, resulting in a lack of clear understanding of the fault characteristics of the transmission line of the wind power flexible DC transmission system.

[0004] When different types of faults occur in the offshore wind power flexible direct current transmission system, the fault characteristics at the wind turbine end mainly include: three-phase voltage, three-phase current, positive-sequence, negative-sequence and zero-sequence voltage and current components at the machine-end monitoring point, phase angle jump, system frequency offset, broadband oscillation and wind turbine output power, etc.

[0005] Large-scale offshore wind farms are characterized by clustering, with typical configurations typically comprising dozens of wind turbines. When conducting simulation analysis involving multiple control strategies, multiple parameter combinations, and multiple fault scenarios, hardware-in-the-loop (HIL) simulation testing methods are often employed. By controlling the converters in the system using actual hardware controllers, the controllers of the actual turbines under test are fully consistent with the actual turbines, further improving the consistency and fidelity between simulation and actual scenarios. However, the use of conventional full-order models presents technical bottlenecks such as high demands on simulation computing resources and significant time-consuming feature extraction and operating condition traversal. Furthermore, large-scale wind farm clusters and their flexible direct current (DC) transmission systems consume significant resources, severely impacting the scale and efficiency of hardware-in-the-loop (HIL) simulation testing systems. Summary of the Invention

[0006] In order to solve the above problems existing in the prior art, the present invention proposes an equivalent reduced-order modeling method and system for an offshore wind power flexible direct current transmission system.

[0007] The technical solutions of the present invention are as follows:

[0008] In a first aspect, the present invention provides an equivalent reduced-order modeling method for an offshore wind power flexible direct current transmission system, comprising the following steps:

[0009] Construct an equivalent reduced-order model of a wind farm;

[0010] Construct an equivalent reduced-order model of a modular multilevel converter;

[0011] Construct a broadband equivalent reduced-order model of the receiving-end power grid;

[0012] An equivalent reduced-order model of the offshore wind power flexible direct current transmission system is constructed based on the equivalent reduced-order model of the wind farm, the equivalent reduced-order model of the modular multilevel converter and the broadband equivalent reduced-order model of the receiving power grid.

[0013] As a preferred embodiment, the wind farm equivalent reduced order model includes an equivalent reduced order model of a wind farm collector line, an equivalent reduced order model of a wind turbine generator set, and an equivalent reduced order model of a wind turbine generator set terminal transformer.

[0014] As a preferred embodiment, the equivalent reduced-order model of the wind farm collector line is constructed using an equal voltage loss method or an equal power loss method.

[0015] As a preferred embodiment, the equivalent reduced-order model of the wind farm collector line is modeled as an equivalent impedance model of the wind farm collector line, including:

[0016] Positive sequence equivalent impedance model of wind farm collector line, negative sequence equivalent impedance model of wind farm collector line and zero sequence equivalent impedance model of wind farm collector line.

[0017] As a preferred embodiment, the equivalent reduced-order model of the wind turbine generator set is constructed by using any one of a single-machine aggregation method, a parameter identification method, and a multi-machine equivalent method.

[0018] As a preferred embodiment, the method for constructing the equivalent reduced-order model of the wind turbine generator set terminal transformer is as follows:

[0019] The transformers at the end of all wind turbines in the wind farm are equivalent to the expanded capacity of a single transformer, and the parameters of the equivalent single transformer are calculated, including the capacity and impedance of the equivalent single transformer.

[0020] As a preferred embodiment, the equivalent reduced-order model of the modular multilevel converter is constructed based on either a half-bridge modular multilevel converter arm average value model or a full-bridge modular multilevel converter arm average value model.

[0021] As a preferred embodiment, the broadband equivalent reduced-order model of the receiving-end power grid is constructed using an RLC equivalent circuit modeling method based on full-band equivalent impedance analysis.

[0022] In a second aspect, the present invention further provides an equivalent reduced-order modeling system for an offshore wind power flexible direct current transmission system, comprising:

[0023] Wind farm model construction module, used to build equivalent reduced-order models of wind farms;

[0024] Converter model building module, used to build equivalent reduced-order models of modular multilevel converters;

[0025] The receiving-end power grid model construction module is used to construct a broadband equivalent reduced-order model of the receiving-end power grid;

[0026] The integration module is used to integrate the equivalent reduced-order model of the wind farm, the equivalent reduced-order model of the modular multilevel converter, and the broadband equivalent reduced-order model of the receiving power grid to construct an equivalent reduced-order model of the offshore wind power flexible direct current transmission system.

[0027] In a third aspect, the present invention also provides an electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor. When the program is executed on the electronic device, the equivalent reduced-order modeling method for the offshore wind power flexible direct current transmission system as described in any embodiment of the present invention can be implemented.

[0028] In a fourth aspect, the present invention further provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the equivalent reduced-order modeling method of the offshore wind power flexible direct current transmission system as described in any embodiment of the present invention.

[0029] In a fifth aspect, the present invention further provides a method for analyzing fault characteristics of an offshore wind power flexible direct current transmission system, comprising the following steps:

[0030] An equivalent reduced-order model of a target offshore wind power flexible direct current transmission system is established using the equivalent reduced-order modeling method of the offshore wind power flexible direct current transmission system as described in any embodiment of the present invention;

[0031] When a fault occurs in the target offshore wind power flexible direct current transmission system, the AC voltage at the fault point in the target offshore wind power flexible direct current transmission system is obtained based on the equivalent reduced-order model of the target offshore wind power flexible direct current transmission system;

[0032] The fault characteristics are analyzed based on the AC voltage at the fault point.

[0033] As a preferred embodiment, the step of performing fault characteristic analysis based on the acquired AC voltage at the fault point includes:

[0034] Obtain the three-phase vector of the AC voltage at the fault point;

[0035] The three-phase vector of the AC voltage at the fault point is rotated 90 degrees with a time delay to obtain an orthogonal virtual phasor;

[0036] Decoupling operation is performed based on the three-phase vector and orthogonal virtual phasor of the AC voltage at the fault point to obtain the positive sequence, negative sequence and zero sequence components of the AC voltage at the fault point;

[0037] The fault characteristics are analyzed based on the positive sequence, negative sequence and zero sequence components of the AC voltage at the fault point.

[0038] In a sixth aspect, the present invention further provides a fault characteristic analysis system for an offshore wind power flexible direct current transmission system, characterized by comprising:

[0039] A modeling module, which uses the equivalent reduced-order modeling method of the offshore wind power flexible direct current transmission system as described in any embodiment of the present invention to establish an equivalent reduced-order model of the target offshore wind power flexible direct current transmission system;

[0040] A fault monitoring module is used to monitor whether a fault occurs in the target offshore wind power flexible direct current transmission system and, when a fault occurs, obtain the AC voltage at the fault point in the target offshore wind power flexible direct current transmission system based on an equivalent reduced-order model of the target offshore wind power flexible direct current transmission system;

[0041] The fault characteristic analysis module performs fault characteristic analysis based on the acquired AC voltage at the fault point.

[0042] In the seventh aspect, the present invention also provides an electronic device, including a memory, a processor, and a computer program stored in the memory and runnable on the processor. When the program is run on the electronic device, the method for analyzing fault characteristics of an offshore wind power flexible direct current transmission system as described in any embodiment of the present invention can be implemented.

[0043] In an eighth aspect, the present invention further provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the method for analyzing fault characteristics of an offshore wind power flexible direct current transmission system as described in any embodiment of the present invention.

[0044] On the other hand, the present invention also provides an electronic device, comprising a memory, a processor, and a computer program stored in the memory and runnable on the processor, characterized in that when the program is run on the electronic device, the methods, models, and calculation steps described in the present invention can be implemented.

[0045] In another aspect, the present invention further provides a computer-readable storage medium having a computer program stored thereon, which implements the method of the present invention when the program is executed by a processor.

[0046] The present invention has the following beneficial effects:

[0047] 1. By downgrading and integrating the wind farm, MMC, and receiving power grid, the present invention reduces the number of overall nodes and components by several times while maintaining the transient consistency of voltage and current at the grid connection point. This significantly reduces the CPU computation load and enables the HIL platform to load a larger wind turbine group at one time.

[0048] 2. The present invention subdivides the wind farm into three equivalent sub-models: the collector line, the wind turbine body, and the machine-end transformer. Any sub-module can be flexibly replaced while ensuring the authenticity of the internal electromagnetic coupling, realizing rapid reconstruction and comparative analysis of different wind farm layouts.

[0049] 3. The present invention adopts the "equal voltage loss / equal power loss" method to equalize the collector line impedance, so that the simplified line fully matches the original network in terms of power flow and loss, avoiding voltage drop distortion and energy balance error.

[0050] 4. After the present invention performs positive-negative-zero sequence impedance separation modeling, the reduced-order model can still accurately reproduce the sequence component characteristics of asymmetric faults such as single-phase grounding and two-phase short circuit, providing a high-fidelity basis for relay protection coordination and resonance analysis.

[0051] 5. The present invention adaptively selects wind turbine equivalent strategies through the single-machine aggregation method / parameter identification method / multi-machine aggregation method, which can simultaneously retain cluster inertia and control dynamics with the minimum state variables, ensuring small signal stability and oscillation mode evaluation accuracy.

[0052] 6. The present invention expands all the machine-end step-up transformers into one equivalent transformer and calculates a unified impedance, which not only simplifies the topology but also makes the equivalent short-circuit capacity and leakage reactance ratio consistent with the actual field, facilitating impedance-frequency scanning and excitation inrush current research.

[0053] 7. The present invention is based on the bridge arm average value model and can reproduce the modulation, blocking and continuous flow behavior of half-bridge / full-bridge MMC with only a small number of controlled sources. The simulation step size can be increased to millisecond level while maintaining waveform accuracy, significantly accelerating transient calculations.

[0054] 8. The present invention fits the full-band impedance of the receiving power grid into a multi-order RLC transfer function model, which can quickly switch between weak and strong network scenarios while retaining the resonance peak position, providing an adjustable parameter platform for broadband oscillation suppression design.

[0055] 9. The present invention proposes a fault characteristic analysis method for offshore wind power flexible direct current transmission system. The positive, negative and zero sequence components are instantaneously separated from the three-phase voltage through a delayed 90° rotation orthogonal projection algorithm. The calculation amount is reduced by about 60% compared with the traditional synchronous double DQ transformation, while avoiding the inter-sequence coupling problem caused by frequency drift. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 Schematic diagram of wind farm modeling before equivalent reduction;

[0057] Figure 2 This is a schematic diagram of wind farm modeling after equivalent reduction;

[0058] Figure 3 A schematic diagram comparing the voltage waveforms at the grid connection point before and after equivalent order reduction in the wind farm modeling;

[0059] Figure 4 Schematic diagram of the comparison of the port voltage waveforms of the terminal wind turbines in wind farm modeling before and after equivalent order reduction;

[0060] Figure 5 This is the topology diagram of the improved half-bridge MMC bridge arm average value model;

[0061] Figure 6 Schematic diagram of current flow after the improved half-bridge MMC bridge arm average value model is locked;

[0062] Figure 7 This is the control flow chart of the improved half-bridge MMC bridge arm average value model;

[0063] Figure 8 This is the topology diagram of the improved full-bridge MMC bridge arm average value model;

[0064] Figure 9 Schematic diagram of current flow after the improved full-bridge MMC bridge arm average value model is locked;

[0065] Figure 10 Flowchart for modeling broadband equivalent of receiving-end power grid;

[0066] Figure 11 is a vector diagram of positive sequence, negative sequence and zero sequence components;

[0067] Figure 12 Schematic diagram of the projection relationship between positive sequence and negative sequence components. DETAILED DESCRIPTION

[0068] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0069] It should be understood that the step numbers used herein are only for convenience of description and are not intended to limit the order in which the steps are to be executed.

[0070] It should be understood that the terms used in the present specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, the singular forms "a", "an" and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0071] The terms “include” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0072] The term "and / or" refers to and includes any and all possible combinations of one or more of the associated listed items.

[0073] The offshore wind power flexible direct current transmission system is mainly composed of an offshore wind farm, a modular multilevel converter (MMC) converter station and a receiving power grid. The use of traditional full-order models has technical bottlenecks such as high demand for simulation computing resources, significant time consumption for feature extraction and operating condition traversal, etc. However, large-scale wind farm groups and their flexible direct current transmission systems occupy too many resources, which seriously affects the scale and efficiency of the hardware-in-the-loop (HIL) simulation test system. Compared with the traditional full-order modeling method, the embodiment of the present application proposes an equivalent reduced-order modeling method for an offshore wind power flexible direct current transmission system to ensure that the established model greatly improves the simulation speed and reduces the consumption of simulation computing resources while ensuring accuracy and fidelity. It is compatible with offline simulation and HIL real-time simulation platforms, consistent with actual engineering parameters, meets the test requirements of the IEC 61400 series standards, and provides an accurate model for the simulation analysis of offshore wind turbines. The method specifically includes the following steps:

[0074] S100. Construct an equivalent reduced-order model of a wind farm; replace a complex large-scale wind farm containing dozens to hundreds of wind turbines with a small number of equivalent units (such as a single or multiple equivalent wind turbines and their equivalent collector networks). The core of this model is to significantly reduce the computational complexity of large-scale wind turbine grid-connected simulation while ensuring simulation accuracy (such as power flow distribution, fault characteristics or dynamic response).

[0075] S200, constructing an equivalent reduced-order model of a modular multilevel converter;

[0076] S300, constructing a broadband equivalent reduced-order model of the receiving-end power grid;

[0077] S400, based on the equivalent reduced-order model of the wind farm, the equivalent reduced-order model of the modular multilevel converter and the broadband equivalent reduced-order model of the receiving power grid, constructs an equivalent reduced-order model of the offshore wind power flexible direct current transmission system.

[0078] The equivalent reduced-order model of the offshore wind power flexible direct current transmission system constructed based on the embodiments of this application can effectively reduce the complexity of the system model and improve modeling efficiency. The equivalent reduced-order model of the offshore wind power flexible direct current transmission system, constructed based on the equivalent reduced-order models of the wind farm, modular multilevel converter, and receiving power grid, can be used for subsequent performance analysis and fault diagnosis of the offshore wind power flexible direct current transmission system.

[0079] In some embodiments, step S100 specifically includes:

[0080] S101, constructing an equivalent reduced-order model of the wind farm collection line;

[0081] S102, equivalent reduced-order model of wind turbine;

[0082] S103. Equivalent reduced-order model of wind turbine generator terminal transformer.

[0083] Regarding step S101: the wind farm collection line usually includes many branches. To simplify the system analysis model, an equivalent method is often used to simplify the complex radial or ring collection network into one or a few equivalent lines.

[0084] In some embodiments, an equal voltage loss method or an equal power loss method may be used to construct an equivalent reduced-order model of a wind farm collector line.

[0085] Specifically, the equal voltage loss method involves maintaining the voltage loss at the end of the collection line (usually the wind turbine connection point) consistent before and after the equivalent value is achieved. By adjusting the impedance parameters (primarily reactance) of the equivalent line, the voltage drop at the end of the equivalent line relative to the head end is equal to the voltage drop at all wind turbine connection points in the original complex network under the same total output current or power conditions.

[0086] Equal Power Loss Method: Maintain the total active power loss on the collector circuit consistent before and after the equivalent value. By adjusting the resistance parameters of the equivalent circuit, the total copper loss on the equivalent circuit is equal to the sum of the copper losses of all branches in the original complex network under the same total output current or power conditions.

[0087] In a specific embodiment, in order to help those skilled in the art quickly understand the technical principles of the present application, an equivalent reduced-order model of a wind farm collector line is constructed using the equal voltage loss method as an example. The specific steps include:

[0088] Constructing a wind farm collector line equivalent impedance model including a wind farm collector line positive sequence equivalent impedance model, a wind farm collector line negative sequence equivalent impedance model and a wind farm collector line zero sequence equivalent impedance model;

[0089] The wind farm collector line is equivalent to the corresponding positive-sequence, negative-sequence and zero-sequence equivalent impedances through the above impedance model, and then the equivalent reduced-order modeling of the wind farm is performed.

[0090] Specifically, the positive sequence equivalent impedance model of the wind farm collector line is constructed, and the specific steps for calculating the positive sequence equivalent impedance of the wind farm collector line are as follows:

[0091] Calculate the voltage difference between the terminal voltage of each wind turbine in the wind farm and the voltage at the grid connection point of the offshore wind power flexible direct current transmission system, as shown in the following formula:

[0092] ;

[0093] in: Indicates the first The voltage difference between the wind turbine generator terminal and the grid connection point of the offshore wind power flexible direct current transmission system; Indicates the first The impedance value of the line corresponding to the wind turbine generator end; represents the total number of wind turbines in the wind farm; Indicates the first Active power of wind turbines; Indicates the voltage value of the grid connection point of the offshore wind power flexible direct current transmission system;

[0094] The total voltage loss of the wind farm is calculated based on the voltage difference between each wind turbine terminal and the grid connection point of the offshore wind power flexible direct current transmission system, as shown in the following formula:

[0095] ;

[0096] in: represents the total voltage loss of the wind farm; Indicates the first Active power of wind turbines;

[0097] The positive sequence equivalent impedance of the wind farm collector line is calculated based on the total voltage loss of the wind farm, as shown in the following formula:

[0098] ;

[0099] in: It represents the positive sequence equivalent impedance of the wind farm collector line.

[0100] In a specific embodiment, assuming that there are five wind turbines in the wind farm, the steps for calculating the positive sequence equivalent impedance of the wind farm collector line are as follows:

[0101] Calculate the voltage difference between the terminals of each wind turbine in the wind farm and the grid connection point of the offshore wind power flexible direct current transmission system, as shown in the following formula:

[0102] ;

[0103] The total voltage loss of the wind farm is calculated based on the voltage difference between each wind turbine terminal and the grid connection point of the offshore wind power flexible direct current transmission system, as shown in the following formula:

[0104] ;

[0105] in: represents the total voltage loss of the wind farm; Indicates the first Active power of wind turbines;

[0106] The positive sequence equivalent impedance of the wind farm collector line is calculated based on the total voltage loss of the wind farm, as shown in the following formula:

[0107] ;

[0108] in: Indicates the positive sequence equivalent impedance of the wind farm collector line;

[0109] In the above equivalent calculation, only the resistive and inductive parameters are considered, and the capacitive parameters are calculated separately. The voltage difference within the wind farm is ignored. The capacitance value after equivalent calculation is equal to the sum of the capacitance values ​​of all lines before equivalent calculation. At this time, the resistance effect is no longer considered again.

[0110] Since the collector line is a static element, its resistance and reactance have the same positive and negative sequence parameters. Therefore, the negative sequence equivalent impedance model of the wind farm collector line is the same as the positive sequence equivalent impedance model of the wind farm collector line. However, under the action of the negative sequence current suppression module in the converter control, when a fault occurs, the negative sequence current component of the AC side of the grid-side converter is zero. At this time, the negative sequence equivalent impedance of the wind farm collector line is infinite.

[0111] Specifically, a negative-sequence equivalent impedance model of a wind farm collector line is constructed, and the specific steps for calculating the negative-sequence equivalent impedance of a wind farm collector line are as follows:

[0112] Calculate the voltage difference between the terminals of each wind turbine in the wind farm and the grid connection point of the offshore wind power flexible direct current transmission system, as shown in the following formula:

[0113] ;

[0114] in: Indicates the first The voltage difference between the wind turbine generator terminal and the grid connection point of the offshore wind power flexible direct current transmission system; Indicates the first The impedance value of the line corresponding to the wind turbine generator end; represents the total number of wind turbines in the wind farm; Indicates the first Active power of wind turbines; Indicates the voltage value of the grid-connected access point of the offshore wind power flexible direct current transmission system.

[0115] The total voltage loss of the wind farm is calculated based on the voltage difference between each wind turbine terminal and the grid connection point of the offshore wind power flexible direct current transmission system, as shown in the following formula:

[0116] ;

[0117] in: represents the total voltage loss of the wind farm; Indicates the first Active power of wind turbines.

[0118] The negative sequence equivalent impedance of the wind farm collector line is calculated based on the total voltage loss of the wind farm, as shown in the following formula:

[0119] ;

[0120] in: It represents the negative sequence equivalent impedance of the wind farm collector line.

[0121] Specifically, the zero-sequence equivalent impedance model of the wind farm collector line is constructed, and the specific steps for calculating the zero-sequence equivalent impedance of the wind farm collector line are as follows:

[0122] Calculate the fundamental zero-sequence component of the voltage phasor and current phasor at each wind turbine terminal in the wind farm, and calculate the active power of the fundamental zero-sequence component based on the fundamental zero-sequence component of the voltage phasor and current phasor, as shown in the following formula:

[0123] ;

[0124] in: Indicates the Active power of fundamental wave zero sequence component at the wind turbine generator terminal; Indicates the The real part of the impedance of the fundamental zero-sequence component of the voltage phasor at the wind turbine terminal; Indicates the The real part of the impedance of the fundamental zero-sequence component of the current phasor at the wind turbine terminal; Indicates the The impedance imaginary part of the fundamental zero-sequence component of the voltage phasor at the wind turbine terminal; Indicates the The impedance imaginary part of the fundamental zero-sequence component of the current phasor at the wind turbine terminal.

[0125] The phase voltage effective value of the fundamental zero-sequence component is calculated based on the fundamental zero-sequence component of the voltage phasor, as shown in the following formula:

[0126] ;

[0127] in: Indicates the The effective value of the phase voltage of the fundamental zero-sequence component at the wind turbine generator terminal.

[0128] The zero-sequence equivalent impedance of the wind farm collector line is calculated based on the active power of the fundamental zero-sequence component at each wind turbine generator end and the effective value of the phase voltage, as shown in the following formula:

[0129] ;

[0130] in: Indicates the zero-sequence equivalent impedance of the wind farm collector line; Represents the fundamental zero-sequence component of the voltage phasor at the grid connection point of the offshore wind power flexible direct current transmission system; Indicates the first Active power of fundamental wave zero sequence component at the wind turbine generator terminal; Indicates the first The effective value of the phase voltage of the fundamental zero sequence component at the wind turbine generator terminal and voltage difference.

[0131] Regarding step S102: the dynamic equivalent method of the wind turbine generator system is currently a key research object that has attracted widespread attention.

[0132] In some embodiments, the equivalent reduced-order model of the wind turbine generator is constructed using any one of a single-machine aggregation method, a parameter identification method, and a multi-machine equivalent method.

[0133] In a specific embodiment, a single-machine aggregation method based on capacity-weighted average is used to construct an equivalent reduced-order model of a wind turbine generator set. The specific steps include:

[0134] A wind farm consisting of wind turbines of the same model, capacity and similar control parameters is considered equivalent to the expansion of a single wind turbine (reference [1]).

[0135] The capacity of a single unit after expansion is the same as the overall capacity of the wind farm. The weight of each unit is obtained by calculating the ratio of the capacity of each wind turbine unit to the total capacity of the wind farm.

[0136] The parameters of the equivalent wind turbine are determined according to the weighted average of all wind turbine parameters (reference [2]). Reference [3] takes a doubly fed wind turbine as an example and provides a method for equivalent reduced-order modeling of wind turbines based on a single-machine aggregation method based on capacity-weighted average.

[0137] From the above analysis, it can be seen that the capacity-weighted average method has a simple calculation process for calculating the physical parameters of the equivalent wind turbine according to the capacity weight, and is more suitable for the study of steady-state output of wind farms.

[0138] In a specific embodiment, a simplified model parameter identification method matching dynamic response is used to construct an equivalent reduced-order model of a wind turbine generator, specifically:

[0139] The parameter identification method regards the equivalent wind farm (selecting one wind turbine for expansion) as a gray box model, but some parameters are unknown. The parameters of the equivalent wind farm in the gray box model are identified by comparing the measured data or simulation data of the equivalent wind farm with the original wind farm at the grid connection point (references [4-5]).

[0140] The parameter identification process is essentially an optimization problem. The optimization goal is to minimize the variance of the dynamic response of the equivalent wind field and the original wind field within a certain period of time at the grid connection point. The quantities to be determined are the physical parameters of the wind turbine. The optimization process involves nonlinear calculations and solving differential equations, and generally requires the use of intelligent optimization algorithms (reference [6]).

[0141] In a specific embodiment, a multi-machine equivalent method based on clustering of wind turbine characteristics or operating states is used to construct an equivalent reduced-order model of a wind turbine, specifically:

[0142] Multi-machine equivalence mainly includes two steps: clustering and equivalence parameter calculation.

[0143] Regarding the research on clustering methods, references [7-8] proposed to divide the wind turbine group according to factors such as wind speed and wake effect. Reference [9] used the multi-scale entropy of active power, reactive power, voltage and current as clustering indicators, and further combined the multi-view migration fuzzy C-means algorithm to achieve clustering. Reference

[10] proposed to use the wind turbine operating state variables after dimensionality reduction using principal component analysis as clustering indicators. References [11-12] clustered based on the similarity of the dynamic response trajectory of wind turbines, using the short-circuit current envelope trajectory and active power time series data as clustering indicators.

[0144] In terms of equivalent parameter calculation, existing studies often use the capacity weighting method (reference

[13] ). Reference

[14] uses an improved genetic learning particle swarm hybrid algorithm to perform parameter identification on the equivalent model of a single unit in a direct-drive wind farm. Reference

[15] proposes a classification and key identification strategy for a doubly fed wind farm model with different control strategies.

[0145] Regarding step S103, in some embodiments, all wind turbine end transformers in the wind farm are equivalent to the expanded capacity of a single transformer, and the parameters of the equivalent single transformer are calculated, including the capacity and impedance of the equivalent single transformer.

[0146] In a specific embodiment, the specific steps of equivalent reduced-order modeling of the wind turbine generator set terminal transformer are as follows:

[0147] The transformers at the end of all wind turbines in the wind farm are equivalent to the expanded capacity of a single transformer, and the parameters of the equivalent single transformer are calculated. The parameters include the capacity and impedance of the equivalent single transformer. The specific calculation formula is:

[0148] ;

[0149] ;

[0150] in, and are the capacity and impedance of an equivalent single transformer respectively; and The wind farm The capacity and impedance of typhoon turbines.

[0151] Figure 1 and Figure 2This is a calculation model built based on MATLAB / Simulink, which respectively represents the wind farm modeling before and after equivalent order reduction. The voltage comparison at the grid connection point PCC is as follows: Figure 3 As shown, V2 and V5 represent the terminal voltages of the terminal fans before and after equal order reduction, and their comparison waveforms are shown in Figure 4 As shown in the figure, it can be seen that within the allowable error range, the equivalent method is effective and feasible. To verify the accuracy of the model, the same simulation system was built on PSCAD, and the equivalent comparison results were consistent with those of Simulink.

[0152] The equivalent reduced-order modeling of the wind farm in this embodiment can greatly reduce the amount of simulation calculations, significantly reduce simulation resources without reducing simulation accuracy, and improve simulation efficiency.

[0153] In some embodiments, the equivalent reduced-order model of the modular multilevel converter in S200 is constructed based on either a half-bridge modular multilevel converter arm average value model or a full-bridge modular multilevel converter arm average value model.

[0154] In a specific embodiment, an equivalent reduced-order model of a modular multilevel converter is constructed based on an average value model of a half-bridge modular multilevel converter bridge arm, specifically:

[0155] See also Figure 5 In the AC side equivalent circuit of the average value model of the half-bridge modular multilevel converter bridge arm, an equivalent resistor is connected in series with the controlled voltage source, and three controllable unidirectional conductive power electronic devices are added at the position shown in the figure to connect the DC side equivalent circuit to be equivalent to the IGBT anti-parallel diode in the locked state of the MMC bridge arm. The specific construction steps are as follows:

[0156] The controlled voltage source in the AC side equivalent circuit of the original modular multilevel converter bridge arm average value model is Negative electrode and equivalent resistance One end is connected;

[0157] The controlled voltage source Positive electrode and controllable unidirectional conductive power electronic device The input terminal is connected to the controllable unidirectional conductive power electronic device Equivalent capacitance between output terminal and bridge arm One end is connected;

[0158] The equivalent capacitance of the bridge arm The other end is connected to a controllable unidirectional conductive power electronic device The input terminal is connected to the controllable unidirectional conductive power electronic device Output terminal and equivalent resistance The other end is connected;

[0159] Controllable unidirectional conductive power electronic devices Output end and controllable unidirectional conductive power electronic device The input terminal is connected to the controllable unidirectional conductive power electronic device Output end and controllable unidirectional conductive power electronic device Input connection;

[0160] The bridge arm equivalent capacitance It is obtained by simplifying the equivalent relationship of the series sub-module capacitance of the MMC, representing the average value of the capacitance characteristics of the entire bridge arm, and is taken as the total equivalent capacitance value. , Indicates the total number of submodules; It reflects the equivalent current source of the bridge arm capacitance current dynamics, and its value is related to the proportion of the conducting submodules and the bridge arm current;

[0161] In normal operation, the controlled voltage source It is calculated by the DC side accompanying circuit capacitor voltage and the number of conducting submodules from the modulation module, and the equivalent resistance Assume that the switch tube conduction resistance (approximately 0), and the controlled current source current of the accompanying circuit is Calculated by the bridge arm current and the number of conducting submodules;

[0162] In the locked state, the controller sets the number of conducting submodules to 0, and the equivalent resistance Set as the switch tube off resistance (MΩ level), and give three controllable unidirectional conductive power electronic devices at the same time 、 、 Continuous conduction signal, at this time, the voltage of the controlled voltage source and the current of the controlled current source are calculated to be 0, and the bridge arm current can flow through the controllable unidirectional conductive power electronic device 、 、 and equivalent resistance Implement the freewheeling process to simulate the transient process of the MMC bridge arm detailed model after blocking;

[0163] Since the direction of the current flowing into the submodule is uncertain when the MMC is locked, the current loop in the average value model of the bridge arm of the improved half-bridge modular multilevel converter is also uncertain. There are two cases. If the current flowing into the submodule at the locking moment is greater than zero, the current Flow direction Figure 6 As shown in (a), during the blocking period, the current will charge the capacitor through the upper and lower controllable unidirectional conductive power electronic devices; if the current flowing into the submodule at the blocking moment is less than zero, the current will continue to flow through the middle controllable unidirectional conductive power electronic device, and the current flow direction is shown in 6 (b), where Indicates the The switching tube current of the phase bridge arm.

[0164] In a specific embodiment, see Figure 7 The overall control process of the equivalent reduced-order model of the modular multilevel converter constructed based on the average value model of the bridge arm of the half-bridge modular multilevel converter is as follows:

[0165] Step 1: Initialize data and check whether the MMC is running normally;

[0166] Step 2: If the MMC operates normally, control the controllable unidirectional conductive power electronic device 、 、 When the bridge arm is turned off, the equivalent resistance of the bridge arm is 0, and the number of conducting submodules in each bridge arm and the equivalent controlled voltage source and current source are calculated. If the MMC operates abnormally, a blocking signal is sent to control the controllable unidirectional conductive power electronic device. 、 、 When the bridge arm is conducting, the equivalent resistance of the bridge arm is infinite. Calculate the equivalent on-resistance of the anti-parallel diode and the equivalent controlled voltage source and current source.

[0167] Step 3: Solve the system network equations using an electromagnetic transient simulation program based on the data obtained in step 2;

[0168] Step 4: Determine whether the MMC is operating normally at the next moment based on the solution of the network equation, and repeat steps 1 to 3 until the simulation ends.

[0169] In a specific embodiment, an equivalent reduced-order model of a modular multilevel converter is constructed based on an average value model of a half-bridge modular multilevel converter bridge arm, specifically:

[0170] See also Figure 8 In the AC side equivalent circuit, the average value model of the full-bridge modular multilevel converter bridge arm is connected in series with an equivalent resistor to the controlled voltage source. Four controllable unidirectional conductive power electronic devices are added at the positions shown in the figure to connect the equivalent circuit on the DC side. This is used to represent the IGBT anti-parallel diode in the locked state of the full-bridge MMC bridge arm. The specific construction steps are as follows:

[0171] The controlled voltage source in the AC side equivalent circuit of the original modular multilevel converter bridge arm average value model is Negative electrode and equivalent resistance One end is connected;

[0172] The controlled voltage source Positive electrode and controllable unidirectional conductive power electronic device Input terminal and controllable unidirectional conductive power electronic device Output connection;

[0173] Controllable unidirectional conductive power electronic devices The output end is respectively connected to the bridge arm equivalent capacitance One-end and controllable unidirectional conductive power electronic device Output connection;

[0174] Controllable unidirectional conductive power electronic devices The input end and the bridge arm equivalent capacitance The other end and the controllable unidirectional conductive power electronic device The input terminal is connected to the controllable unidirectional conductive power electronic device The output ends are connected to controllable unidirectional conductive power electronic devices Input terminal and equivalent resistance The other end is connected;

[0175] In the normal working state of the full-bridge MMC, all controllable unidirectional conductive power electronic devices will remain in the closed state, there is no connection between the capacitor circuit and the bridge arm circuit, and the equivalent resistance The resistance value is zero. At this time, the improved full-bridge modular multi-level converter bridge arm average value model has the same function as the traditional model. When the full-bridge MMC switches to the locked state, all controllable unidirectional conductive power electronic devices will be turned on, and the capacitor circuit and the bridge arm circuit are connected through the corresponding controllable unidirectional conductive power electronic devices. The equivalent resistance The resistance value is set to infinity, and the voltage of the controlled voltage source and the current of the controlled current source are controlled to zero, which is equivalent to putting zero submodules into the traditional model. In the locked state, the actual current flow direction in the average value model of the bridge arm of the improved full-bridge modular multilevel converter is as follows: Figure 9 As shown, when the direction of the bridge arm current is from bottom to top, as shown in Figure 9 As shown in (a), the equivalent capacitance of the bridge arm Through controllable unidirectional conductive power electronic devices 、 Charging; when the bridge arm current direction is from top to bottom, such as Figure 9 As shown in (b), the equivalent capacitance of the bridge arm pass 、 Charge;

[0176] The full-bridge type is slightly less economical than the half-bridge type, but it can isolate DC faults without the need for an additional DC circuit breaker;

[0177] Through the above improvements and control strategies, the improved bridge arm average value model proposed by the present invention for MMC flexible DC can accurately reflect the locking process characteristics of the flexible DC under disturbance, and realize the accurate extraction of fault characteristics.

[0178] In some embodiments, the broadband equivalent reduced-order model of the receiving-end power grid in S300 is constructed using an RLC equivalent circuit modeling method based on full-band equivalent impedance analysis.

[0179] In a specific embodiment, the broadband equivalent reduced-order modeling of the receiving-end power grid of the offshore wind power flexible direct current transmission system is performed by using the RLC equivalent circuit modeling method based on full-band equivalent impedance analysis, specifically:

[0180] Collect the full-band equivalent impedance (DQ axis) of the receiving-end power grid, average the full-band equivalent impedance, and extract the equivalent impedance and corresponding resonant frequency of the single-input single-output (SISO) receiving-end power grid;

[0181] The equivalent impedance of a single-input single-output with a preset bandwidth and the corresponding resonant frequency are intercepted and fitted into a multi-order transfer function. The parameters of the multi-order transfer function are optimized by an optimization algorithm (such as a least squares method, a genetic algorithm, etc.). The optimization is stopped after the fitting accuracy of the multi-order transfer function reaches a preset fitting accuracy threshold, thereby obtaining a successfully fitted multi-order transfer function.

[0182] Optionally, the equivalent impedance of a single-input single-output and the corresponding resonant frequency are fitted by a direct impedance fitting method;

[0183] The parameters of the RLC components (resistance R, inductance L, capacitance C) in the RLC circuit are extracted by fitting the denominator coefficient and numerator coefficient of the successful multi-order transfer function, and then the corresponding RLC equivalent circuit is constructed.

[0184] Generally, by analyzing the pole and zero formulas of the transfer function, the resistance R, inductance L, and capacitance C corresponding to each order are derived in series or parallel combination with an ideal voltage source to form a broadband equivalent reduced-order model of the receiving end power grid;

[0185] The RLC circuit obtained above exhibits equivalent short-circuit capacity at power frequency (reflected by the equivalent real part of impedance) and reproduces the resonance characteristics of the power grid at high frequencies. Therefore, it is more accurate than the simple ideal voltage source + fixed impedance model. By adjusting the parameters of the RLC elements, it is convenient to simulate the conditions of weak power grids (large impedance, low short-circuit ratio) or strong power grids (small impedance, high short-circuit ratio).

[0186] When the receiving grid's dynamic characteristics are complex, encompassing multiple resonant frequencies and dynamic processes with varying time constants, multi-order transfer functions can more accurately describe the system's full-frequency characteristics. In applications where computational complexity is critical, or where only preliminary analysis and design of the system's key dynamic characteristics are required, reducing the multi-order transfer function to a second-order transfer function can greatly simplify the calculation process and improve analysis efficiency.

[0187] In a specific embodiment, the equivalent reduced-order modeling of the receiving-end power grid of the offshore wind power flexible direct current transmission system is performed using the ideal grid equivalent method, specifically as follows:

[0188] The receiving-end power grid is equivalent to an ideal voltage source and an infinite bus, and the internal impedance and dynamic characteristics of the power grid are ignored.

[0189] In a specific embodiment, the equivalent reduced-order modeling of the receiving-end power grid of the offshore wind power flexible direct current transmission system is performed using the RL grid equivalent method (based on the short-circuit ratio equivalent), specifically as follows:

[0190] According to the short-circuit ratio of the receiving-end power grid, the receiving-end power grid is equivalent to a resistor or an inductor.

[0191] The present application also proposes an equivalent reduced-order modeling system for an offshore wind power flexible direct current transmission system. The equivalent reduced-order modeling system is constructed based on the equivalent reduced-order modeling method for an offshore wind power flexible direct current transmission system proposed in the present application. The equivalent reduced-order modeling system for an offshore wind power flexible direct current transmission system includes:

[0192] Wind farm model construction module, used to build equivalent reduced-order models of wind farms;

[0193] Converter model building module, used to build equivalent reduced-order models of modular multilevel converters;

[0194] The receiving-end power grid model construction module is used to construct a broadband equivalent reduced-order model of the receiving-end power grid;

[0195] The integration module is used to integrate the equivalent reduced-order model of the wind farm, the equivalent reduced-order model of the modular multilevel converter, and the broadband equivalent reduced-order model of the receiving power grid to construct an equivalent reduced-order model of the offshore wind power flexible direct current transmission system.

[0196] In offshore wind power flexible direct current transmission systems, the precise separation and resolution of the positive-sequence, negative-sequence, and zero-sequence components of various electrical quantities is crucial for analyzing and controlling system fault characteristics. Due to factors such as zero-sequence path nonlinearity caused by frequency-varying submarine cable parameters, transient converter overcurrents under bipolar short-circuit faults, and background harmonic interference from weak power grids and wind farms, a rational and effective method for separating the sequence components is required to accurately capture the phase and amplitude characteristics of each transient component of the fault. This provides critical data support for relay protection setting, fault ride-through control, oscillation suppression strategies, and accurate simulation of test conditions.

[0197] The zero-sequence component is directly calculated by the algebraic sum of the three-phase quantities in the three-phase four-wire system. It is always zero in the three-phase three-wire system, so the zero-sequence is usually processed independently, and the separate solution of the positive-sequence and negative-sequence components becomes the key and difficulty.

[0198] The traditional symmetrical component method, based on the assumption of a linear time-invariant system, decouples three-phase asymmetry into mutually orthogonal positive-sequence, negative-sequence, and zero-sequence components via a complex transformation matrix. Its core approach is to construct orthogonal rotating basis vectors based on the power frequency fundamental to achieve frequency-domain decoupling. This method exhibits significant limitations in offshore wind power flexible direct current (DC) grid-connected systems. The transient processes induced by bipolar short-circuit faults in modular multilevel converters exhibit strong nonlinear and time-varying characteristics, undermining the applicability of the linear superposition principle.

[0199] The traditional symmetrical component method, based on the assumption of a linear time-invariant system, decouples three-phase asymmetry into mutually orthogonal positive-sequence, negative-sequence, and zero-sequence components via a complex transformation matrix. Its core approach is to construct orthogonal rotating basis vectors based on the power frequency fundamental to achieve frequency-domain decoupling. This method exhibits significant limitations in offshore wind power flexible direct current (DC) grid-connected systems. The transient processes induced by bipolar short-circuit faults in modular multilevel converters exhibit strong nonlinear and time-varying characteristics, undermining the applicability of the linear superposition principle.

[0200] The transformation first projects the three-phase quantities into a stationary orthogonal coordinate system for decoupling and obtains the zero-sequence component. Then, the fundamental positive / negative sequence components are converted into DC quantities respectively through positive / negative synchronous rotating dq transformation for separation. This combined method has essential defects in offshore wind power flexible DC systems. The double dq transformation structure causes positive and negative sequence cross-coupling oscillations due to rotation angle mismatch when the grid frequency fluctuates, and The transformation lacks the ability to suppress the high-order harmonics introduced by the converter switches, resulting in spectral pollution of the sequence components; more importantly, the frequency-varying parameters of the submarine cable make the zero-sequence path exhibit nonlinear impedance characteristics, the zero-sequence decoupling assumption in the stationary coordinate system fails, and the joint transformation cannot characterize its dynamic coupling mechanism. In the event of a bipolar fault, amplitude-phase solution deviations will occur, seriously affecting the reliability of transient protection.

[0201] The discrete Fourier transform extracts the amplitude and phase information of the fundamental positive, negative, and zero-sequence components through spectral analysis of the time-domain sampling sequence. Essentially, it uses orthogonal basis vectors to project the signal into the frequency domain for frequency-domain decoupling. This method has fundamental flaws in offshore wind power flexible grid-connected systems. The full-cycle sampling requirement results in dynamic response delays that cannot meet the needs for rapid sequence separation of converter transient faults. Spectral leakage effects under asynchronous sampling cause fundamental energy to diffuse into sidebands, leading to deviations in the calculated amplitude and phase of positive and negative sequences. The high-order characteristic harmonics generated during the switching process of modular multilevel converters further lead to spectral aliasing and cross-contamination of sequence components. More critically, the nonlinear characteristics of the zero-sequence path caused by the frequency-varying parameters of the submarine cable cannot be accurately modeled using fixed frequency-domain basis vectors. This results in transient sequence component decoupling failure during bipolar short-circuit faults, severely limiting the reliability of protection operation and the accuracy of fault ride-through control.

[0202] To solve the above problems, the present invention proposes a method for analyzing fault characteristics of an offshore wind power flexible direct current transmission system, which specifically includes the following steps:

[0203] S01. Using the equivalent reduced-order modeling method for the offshore wind power flexible direct current transmission system proposed in the above embodiment, an equivalent reduced-order model of the target offshore wind power flexible direct current transmission system is established;

[0204] S02. When a fault occurs in the target offshore wind power flexible direct current transmission system, obtaining the AC voltage at the fault point in the target offshore wind power flexible direct current transmission system based on an equivalent reduced-order model of the target offshore wind power flexible direct current transmission system;

[0205] S03. Perform fault characteristic analysis based on the acquired AC voltage at the fault point.

[0206] In some embodiments, the specific steps of S03 are:

[0207] Obtain the three-phase vector of the AC voltage at the fault point;

[0208] The three-phase vector of the AC voltage at the fault point is rotated 90 degrees with a time delay to obtain an orthogonal virtual phasor;

[0209] Decoupling operation is performed based on the three-phase vector and orthogonal virtual phasor of the AC voltage at the fault point to obtain the positive sequence, negative sequence and zero sequence components of the AC voltage at the fault point;

[0210] The fault characteristics are analyzed based on the positive sequence, negative sequence and zero sequence components of the AC voltage at the fault point.

[0211] In a specific embodiment, the step of performing fault characteristic analysis on the acquired AC voltage at the fault point includes:

[0212] Obtain the three-phase vector of the AC voltage at the fault point;

[0213] Eliminate the zero-sequence component of the AC voltage at the fault point, as shown in the following formula:

[0214] ;

[0215] ;

[0216] in: A phase vector representing the AC voltage at the fault point; B-phase vector representing the AC voltage at the fault point; C-phase vector representing the AC voltage at the fault point; Indicates the three-phase zero-sequence component of the fault point; 、 They represent the positive sequence and negative sequence components of the AC voltage phase A vector at the fault point respectively; 、 They represent the positive sequence and negative sequence components of the AC voltage phase B vector at the fault point respectively; 、 They represent the positive sequence and negative sequence components of the AC voltage C phase vector at the fault point respectively;

[0217] The three-phase vector of the AC voltage at the fault point is rotated 90 degrees with a time delay, as shown in Figure 11 As shown, a new three-phase vector is obtained, and then the zero-sequence component of the new three-phase vector is eliminated, as shown in the following formula:

[0218] ;

[0219] ;

[0220] in: 、 、 The new A-phase vector, new B-phase vector, and new C-phase vector represent the AC voltage at the fault point respectively; Represents the three-phase zero-sequence component of the three-phase vector of the AC voltage at the fault point after a 90-degree delay rotation; 、 Respectively represent the positive sequence and negative sequence components of the new A phase vector of the AC voltage at the fault point; 、 Respectively represent the positive sequence and negative sequence components of the new B-phase vector of the AC voltage at the fault point; 、 Represent the positive sequence and negative sequence components of the new C-phase vector of the AC voltage at the fault point respectively;

[0221] like Figure 12 As shown, at this time, Direction and The direction is the same and the magnitude is of times, Direction and The direction is the same and the magnitude is of times;

[0222] Eliminate the negative sequence component in the above formula and extract the positive sequence component of the three-phase AC voltage vector at the fault point, as shown in the following formula:

[0223] ;

[0224] ;

[0225] ;

[0226] Similarly, after eliminating the positive sequence component, the negative sequence component of the three-phase AC voltage vector at the fault point is obtained, as shown in the following formula:

[0227] ;

[0228] ;

[0229] ;

[0230] The zero-sequence component of the three-phase AC voltage vector at the fault point is directly calculated by the algebraic sum of the three-phase quantities in the three-phase four-wire system, as shown in the following formula:

[0231] ;

[0232] in: 、 、 They represent the zero-sequence components of the A, B, and C phase vectors of the AC voltage at the fault point respectively.

[0233] The method for extracting the positive-sequence, negative-sequence and zero-sequence components of the AC voltage simplifies the matrix operations containing complex numbers of the symmetrical component method into algebraic operations, simplifies the calculation method, and greatly improves the efficiency of fault feature extraction in the case of wind turbine faults.

[0234] The present application also provides a fault characteristic analysis system for an offshore wind power flexible direct current transmission system. The system is constructed based on the fault characteristic analysis method for an offshore wind power flexible direct current transmission system provided in the present application. The system includes:

[0235] A modeling module, which uses the equivalent reduced-order modeling method of the offshore wind power flexible direct current transmission system described in the embodiment to establish an equivalent reduced-order model of the target offshore wind power flexible direct current transmission system;

[0236] A fault monitoring module is used to monitor whether a fault occurs in the target offshore wind power flexible direct current transmission system and, when a fault occurs, obtain the AC voltage at the fault point in the target offshore wind power flexible direct current transmission system based on an equivalent reduced-order model of the target offshore wind power flexible direct current transmission system;

[0237] The fault characteristic analysis module performs fault characteristic analysis based on the acquired AC voltage at the fault point.

[0238] An embodiment of the present application further provides an electronic device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the program, the method described in any embodiment of the present invention is implemented.

[0239] The embodiments of the present application further provide a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the method described in any embodiment of the present invention.

[0240] References:

[0241] [1]Huang PH, Moursi ME, Xiao W, et al. Subsynchronous ResonanceMitigation for Series-Compensated DFIG-Based Wind Farm by Using Two-Degree-of-Freedom Control Strategy[J]. IEEE Transactions on Power Systems, 2015,30(3):1442-1454.

[0242] [2] Chao Pupu, Li Weixing, Jin Xiaoming, et al. Practical equivalent method of doubly fed wind farm based on active power response [J]. Proceedings of the CSEE, 2018, 38(6):8.

[0243] [3]Fang R, Shang R, Wu M, et al. Application of gray relational analysis to k-means clustering for dynamic equivalent modeling of wind farm[J]. International Journal of Hydrogen Energy, 2017, 42(31).

[0244] [4] Du Xiaoyan, Yin Huajie, Ye Chao, et al. Study on wind farm equivalence based on improved fuzzy C-means clustering and MPSO [J]. Guangdong Electric Power, 2016, 9(8):7.

[0245] [5]Zhan Z, Jie C, Jing H, et al. Wind farm modeling based on runningdata and SVM algorithm[J]. Renewable Energy Resources, 2015.

[0246] [6]Zhou Y, Zhao L, Lee WJ. Robustness Analysis of Dynamic EquivalentModel of DFIG Wind Farm for Stability Study[J]. IEEE Transactions onIndustryApplications,2018,54(6):5682-5690.

[0247] [7]ALI M, ILIE I, MILANOVIC JV, et al. Wind farm model aggregationusing probabilistic clustering[J]. IEEE Transactions on Power Systems, 2013,28(1): 309-316.

[0248] [8] Yu Hao, Li Canbing, Ye Zhiliang, et al. Dynamic equivalent modeling of short-circuit fault in wind farm considering wake effect[J]. Journal of Shanghai Jiaotong University, 2024.58(6):798-805.

[0249] [9] Han Ji, Miao Shihong, Li Lixing, et al. Wind farm cluster division and comprehensive optimization of equivalent wind farm parameters based on multi-perspective transfer learning [J]. Proceedings of the CSEE, 2020, 40(15): 4866-4881.

[0250]

[10] Yan Xiangwu, Li Junyan. Direct-drive wind farm clustering method based on principal component analysis[.Power System Protection and Control, 2020, 48(5): 127-133.

[0251]

[11] Ouyang Jinxin, Diao Yanbo, Zheng Di, et al. Electromagnetic transient coherence clustering method for doubly fed wind turbines based on current trajectory similarity [J]. Proceedings of the CSEE, 2017, 37(10): 2896-2904.

[0252]

[12] Li Longyuan. Research on dynamic equivalent modeling method of direct-drive wind farm[D]. Chengdu: Southwest Jiaotong University, 2022.

[0253]

[13] Sun Huadong, Li Jiahao, Li Wenfeng, et al. Research on the structure and modeling method of renewable energy station model for large-scale power system simulation (Part II): electromechanical transient model [J]. Proceedings of the CSEE, 2023, 43(6): 2190-2202.

[0254]

[14] Zhang Jian, He Yigang. Parameter identification of equivalent model of permanent magnet direct-drive wind farm based on trajectory sensitivity analysis[J]. Transactions of the Chinese Society of Electrotechnical Engineering, 2020, 35(15): 3303-3313.

[0255]

[15] Pan Xueping, Qi Xiangwei, Liang Wei, et al. Wind farm multi-machine equivalent and parameter overall identification based on integrated model aggregation and parameter identification [J]. Electric Power Automation Equipment, 2022, 42(1): 124-132.

[0256] In the embodiments of the present application, "at least one" refers to one or more, and "more" refers to two or more. "And / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent the existence of A alone, the existence of A and B at the same time, and the existence of B alone. Among them, A and B can be singular or plural. The character " / " generally indicates that the previous and next associated objects are in an "or" relationship. "At least one of the following" and similar expressions refer to any combination of these items, including any combination of single or plural items. For example, at least one of a, b and c can represent: a, b, c, a and b, a and c, b and c or a and b and c, where a, b, c can be single or multiple.

[0257] Those skilled in the art will appreciate that the various units and algorithm steps described in the embodiments disclosed herein can be implemented using a combination of electronic hardware, computer software, and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professionals and technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0258] Those skilled in the art will clearly understand that, for the convenience and brevity of description, the specific working processes of the systems, devices and units described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0259] In the several embodiments provided in this application, if any function is implemented in the form of 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 this application, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for enabling a computer device (which can be a personal computer, server, or network device, etc.) to execute all or part of the steps of the method described in each embodiment of this application. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (Read-Only Memory; hereinafter referred to as: ROM), random access memory (Random Access Memory; hereinafter referred to as: RAM), magnetic disk or optical disk, and other media that can store program code.

[0260] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention's description and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. An equivalent reduced-order modeling method for an offshore wind power flexible direct current transmission system, characterized in that: The following steps are involved: Construct an equivalent reduced-order model of a wind farm; The equivalent reduced-order model of the wind farm includes an equivalent reduced-order model of the wind farm collector line, an equivalent reduced-order model of the wind turbine generator set, and an equivalent reduced-order model of the wind turbine generator set terminal transformer; The equivalent reduced-order model of the wind farm collector line is constructed using an equal voltage loss method or an equal power loss method; The equivalent reduced-order model of the wind farm collector line is modeled as an equivalent impedance model of the wind farm collector line, including: Positive-sequence equivalent impedance model of wind farm collector lines, negative-sequence equivalent impedance model of wind farm collector lines, and zero-sequence equivalent impedance model of wind farm collector lines; The specific steps of constructing the equivalent reduced-order model of the wind farm collector line using the equal voltage loss method are as follows: Constructing a wind farm collector line equivalent impedance model including a wind farm collector line positive sequence equivalent impedance model, a wind farm collector line negative sequence equivalent impedance model and a wind farm collector line zero sequence equivalent impedance model; The wind farm collection line is equivalent to the corresponding positive sequence, negative sequence and zero sequence equivalent impedances through the above impedance model; The positive sequence equivalent impedance model of the wind farm collector line is constructed. The specific steps for calculating the positive sequence equivalent impedance of the wind farm collector line are as follows: Calculate the voltage difference between the terminal voltage of each wind turbine in the wind farm and the voltage at the grid connection point of the offshore wind power flexible direct current transmission system, as shown in the following formula: ； in: Indicates the first The voltage difference between the wind turbine generator terminal and the grid connection point of the offshore wind power flexible direct current transmission system; Indicates the first The impedance value of the line corresponding to the wind turbine generator end; represents the total number of wind turbines in the wind farm; Indicates the first Active power of wind turbines; Indicates the voltage value of the grid connection point of the offshore wind power flexible direct current transmission system; The total voltage loss of the wind farm is calculated based on the voltage difference between each wind turbine terminal and the grid connection point of the offshore wind power flexible direct current transmission system, as shown in the following formula: ； in: represents the total voltage loss of the wind farm; Indicates the first Active power of wind turbines; The positive sequence equivalent impedance of the wind farm collector line is calculated based on the total voltage loss of the wind farm, as shown in the following formula: ； in: Indicates the positive sequence equivalent impedance of the wind farm collector line; The specific steps for constructing the negative-sequence equivalent impedance model of the wind farm collector line and calculating the negative-sequence equivalent impedance of the wind farm collector line are as follows: Calculate the voltage difference between the terminals of each wind turbine in the wind farm and the grid connection point of the offshore wind power flexible direct current transmission system, as shown in the following formula: ； in: Indicates the first The voltage difference between the wind turbine generator terminal and the grid connection point of the offshore wind power flexible direct current transmission system; Indicates the first The impedance value of the line corresponding to the wind turbine generator end; represents the total number of wind turbines in the wind farm; Indicates the first Active power of wind turbines; Indicates the voltage value of the grid connection point of the offshore wind power flexible direct current transmission system; The total voltage loss of the wind farm is calculated based on the voltage difference between each wind turbine terminal and the grid connection point of the offshore wind power flexible direct current transmission system, as shown in the following formula: ； in: represents the total voltage loss of the wind farm; Indicates the first Active power of wind turbines; The negative sequence equivalent impedance of the wind farm collector line is calculated based on the total voltage loss of the wind farm, as shown in the following formula: ； in: It represents the negative sequence equivalent impedance of the wind farm collector line; The specific steps for constructing the zero-sequence equivalent impedance model of the wind farm collector line and calculating the zero-sequence equivalent impedance of the wind farm collector line are as follows: Calculate the fundamental zero-sequence component of the voltage phasor and current phasor at each wind turbine terminal in the wind farm, and calculate the active power of the fundamental zero-sequence component based on the fundamental zero-sequence component of the voltage phasor and current phasor, as shown in the following formula: ； in: Indicates the Active power of fundamental wave zero sequence component at the wind turbine generator terminal; Indicates the The real part of the impedance of the fundamental zero-sequence component of the voltage phasor at the wind turbine terminal; Indicates the The real part of the impedance of the fundamental zero-sequence component of the current phasor at the wind turbine terminal; Indicates the The impedance imaginary part of the fundamental zero-sequence component of the voltage phasor at the wind turbine terminal; Indicates the The impedance imaginary part of the fundamental zero-sequence component of the current phasor at the wind turbine terminal; The phase voltage effective value of the fundamental zero-sequence component is calculated based on the fundamental zero-sequence component of the voltage phasor, as shown in the following formula: ； in: Indicates the The effective value of the phase voltage of the fundamental zero-sequence component at the wind turbine generator terminal; The zero-sequence equivalent impedance of the wind farm collector line is calculated based on the active power of the fundamental zero-sequence component at each wind turbine generator end and the effective value of the phase voltage, as shown in the following formula: ； in: Indicates the zero-sequence equivalent impedance of the wind farm collector line; Represents the fundamental zero-sequence component of the voltage phasor at the grid connection point of the offshore wind power flexible direct current transmission system; Indicates the first Active power of fundamental wave zero sequence component at the wind turbine generator terminal; Indicates the first The effective value of the phase voltage of the fundamental zero sequence component at the wind turbine generator terminal and The voltage difference; Construct an equivalent reduced-order model of a modular multilevel converter; Construct a broadband equivalent reduced-order model of the receiving-end power grid; An equivalent reduced-order model of the offshore wind power flexible direct current transmission system is constructed based on the equivalent reduced-order model of the wind farm, the equivalent reduced-order model of the modular multilevel converter and the broadband equivalent reduced-order model of the receiving power grid.

2. The equivalent reduced-order modeling method for an offshore wind power flexible direct current transmission system according to claim 1 is characterized by: The equivalent reduced-order model of the wind turbine generator set is constructed by using any one of a single-machine aggregation method, a parameter identification method, and a multi-machine equivalent method.

3. The equivalent reduced-order modeling method for an offshore wind power flexible direct current transmission system according to claim 1 is characterized in that: The method for constructing the equivalent reduced-order model of the wind turbine generator set terminal transformer is as follows: The transformers at the end of all wind turbines in the wind farm are equivalent to the expanded capacity of a single transformer, and the parameters of the equivalent single transformer are calculated, including the capacity and impedance of the equivalent single transformer.

4. The equivalent reduced-order modeling method for an offshore wind power flexible direct current transmission system according to claim 1 is characterized by: The equivalent reduced-order model of the modular multilevel converter is constructed based on either a half-bridge modular multilevel converter bridge arm average value model or a full-bridge modular multilevel converter bridge arm average value model; Based on the average value model of the half-bridge modular multilevel converter bridge arm, an equivalent reduced-order model of the modular multilevel converter is constructed, specifically: The controlled voltage source in the AC side equivalent circuit of the original half-bridge modular multilevel converter bridge arm average value model is converted into Negative electrode and equivalent resistance One end is connected; The controlled voltage source Positive electrode and controllable unidirectional conductive power electronic device The input terminal is connected to the controllable unidirectional conductive power electronic device Equivalent capacitance between output terminal and bridge arm One end is connected; The equivalent capacitance of the bridge arm The other end is connected to a controllable unidirectional conductive power electronic device The input terminal is connected to the controllable unidirectional conductive power electronic device Output terminal and equivalent resistance The other end is connected; Controllable unidirectional conductive power electronic devices Output end and controllable unidirectional conductive power electronic device The input terminal is connected to the controllable unidirectional conductive power electronic device Output end and controllable unidirectional conductive power electronic device Input connection; Based on the average value model of the bridge arm of the full-bridge modular multilevel converter, an equivalent reduced-order model of the modular multilevel converter is constructed, specifically: The controlled voltage source in the AC side equivalent circuit of the original full-bridge modular multilevel converter bridge arm average value model is converted into Negative electrode and equivalent resistance One end is connected; The controlled voltage source Positive electrode and controllable unidirectional conductive power electronic device Input terminal and controllable unidirectional conductive power electronic device Output connection; Controllable unidirectional conductive power electronic devices The output end is respectively connected to the bridge arm equivalent capacitance One-end and controllable unidirectional conductive power electronic device Output connection; Controllable unidirectional conductive power electronic devices The input end and the bridge arm equivalent capacitance The other end and the controllable unidirectional conductive power electronic device The input terminal is connected to the controllable unidirectional conductive power electronic device The output ends are connected to controllable unidirectional conductive power electronic devices Input terminal and equivalent resistance Connect the other end.

5. The equivalent reduced-order modeling method for an offshore wind power flexible direct current transmission system according to claim 1 is characterized by: The broadband equivalent reduced-order model of the receiving-end power grid is constructed using the RLC equivalent circuit modeling method based on full-band equivalent impedance analysis, specifically: Collect the full-band equivalent impedance of the receiving-end power grid, average the full-band equivalent impedance, and extract the equivalent impedance of the receiving-end power grid with single input and single output and the corresponding resonant frequency; The equivalent impedance of a single-input single-output with a preset bandwidth and the corresponding resonant frequency are intercepted and fitted into a multi-order transfer function. The parameters of the multi-order transfer function are optimized by an optimization algorithm until the fitting accuracy of the multi-order transfer function reaches a preset fitting accuracy threshold, and the optimization is stopped to obtain a successfully fitted multi-order transfer function. The parameters of the RLC components in the RLC circuit are extracted by fitting the denominator coefficient and numerator coefficient of the successful multi-order transfer function, and then the corresponding RLC equivalent circuit is constructed. An ideal voltage source is set on the basis of the RLC equivalent circuit to realize the broadband equivalent reduced-order model of the receiving-end power grid.

6. An equivalent reduced-order modeling system for an offshore wind power flexible direct current transmission system, characterized in that: The equivalent reduced-order modeling method for the offshore wind power flexible direct current transmission system according to any one of claims 1 to 5 comprises: Wind farm model construction module, used to build equivalent reduced-order models of wind farms; Converter model building module, used to build equivalent reduced-order models of modular multilevel converters; The receiving-end power grid model construction module is used to construct a broadband equivalent reduced-order model of the receiving-end power grid; The integration module is used to integrate the equivalent reduced-order model of the wind farm, the equivalent reduced-order model of the modular multilevel converter, and the broadband equivalent reduced-order model of the receiving power grid to construct an equivalent reduced-order model of the offshore wind power flexible direct current transmission system.

7. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the program is run on the electronic device, the equivalent reduced-order modeling method for the offshore wind power flexible direct current transmission system as described in any one of claims 1 to 5 can be implemented.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the equivalent reduced-order modeling method of the offshore wind power flexible direct current transmission system as described in any one of claims 1 to 5 is implemented.

9. A method for analyzing fault characteristics of an offshore wind power flexible direct current transmission system, characterized in that: The following steps are involved: An equivalent reduced-order model of a target offshore wind power flexible direct current transmission system is established by using the equivalent reduced-order modeling method of the offshore wind power flexible direct current transmission system according to any one of claims 1 to 5; When a fault occurs in the target offshore wind power flexible direct current transmission system, the AC voltage at the fault point in the target offshore wind power flexible direct current transmission system is obtained based on the equivalent reduced-order model of the target offshore wind power flexible direct current transmission system; The fault characteristics are analyzed based on the AC voltage at the fault point.

10. A fault characteristic analysis method for an offshore wind power flexible direct current transmission system according to claim 9, characterized in that: The step of performing fault characteristic analysis based on the acquired AC voltage at the fault point includes: Obtain the three-phase vector of the AC voltage at the fault point; Eliminate the zero-sequence component of the AC voltage at the fault point, as shown in the following formula: ； ； in: A phase vector representing the AC voltage at the fault point; B-phase vector representing the AC voltage at the fault point; C-phase vector representing the AC voltage at the fault point; Indicates the three-phase zero-sequence component of the fault point; 、 They represent the positive sequence and negative sequence components of the AC voltage phase A vector at the fault point respectively; 、 They represent the positive sequence and negative sequence components of the AC voltage phase B vector at the fault point respectively; 、 They represent the positive sequence and negative sequence components of the AC voltage C phase vector at the fault point respectively; The three-phase vector of the AC voltage at the fault point is rotated 90 degrees with a time delay to obtain an orthogonal virtual phasor, i.e., a new three-phase vector. The zero-sequence component of the new three-phase vector is then eliminated, as shown in the following formula: ； ； in: 、 、 The new A-phase vector, new B-phase vector, and new C-phase vector represent the AC voltage at the fault point respectively; Represents the three-phase zero-sequence component of the three-phase vector of the AC voltage at the fault point after a 90-degree delay rotation; 、 Respectively represent the positive sequence and negative sequence components of the new A phase vector of the AC voltage at the fault point; 、 Respectively represent the positive sequence and negative sequence components of the new B-phase vector of the AC voltage at the fault point; 、 Represent the positive sequence and negative sequence components of the new C-phase vector of the AC voltage at the fault point respectively; Decoupling operation is performed based on the three-phase vector and orthogonal virtual phasor of the AC voltage at the fault point to obtain the positive sequence, negative sequence and zero sequence components of the AC voltage at the fault point, specifically: Eliminate the negative sequence component in the above formula and extract the positive sequence component of the AC voltage at the fault point, as shown in the following formula: ； ； ； Similarly, after eliminating the positive sequence component, the negative sequence component of the AC voltage at the fault point is obtained, as shown in the following formula: ； ； ； The zero-sequence component of the AC voltage at the fault point is directly calculated by the algebraic sum of the three-phase quantities in the three-phase four-wire system, as shown in the following formula: ； in: 、 、 Respectively represent the zero-sequence components of the AC voltage phase A, B, and C at the fault point; The fault characteristics are analyzed based on the positive sequence, negative sequence and zero sequence components of the AC voltage at the fault point.

11. A fault characteristic analysis system for an offshore wind power flexible direct current transmission system, characterized in that: include: A modeling module, which uses the equivalent reduced-order modeling method of the offshore wind power flexible direct current transmission system according to any one of claims 1 to 5 to establish an equivalent reduced-order model of the target offshore wind power flexible direct current transmission system; A fault monitoring module is used to monitor whether a fault occurs in the target offshore wind power flexible direct current transmission system and, when a fault occurs, obtain the AC voltage at the fault point in the target offshore wind power flexible direct current transmission system based on an equivalent reduced-order model of the target offshore wind power flexible direct current transmission system; The fault characteristic analysis module performs fault characteristic analysis based on the acquired AC voltage at the fault point.

12. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the program is run on the electronic device, the fault characteristic analysis method of the offshore wind power flexible direct current transmission system as described in any one of claims 9 to 10 can be implemented.

13. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the method for analyzing fault characteristics of an offshore wind power flexible direct current transmission system as described in any one of claims 9 to 10 is implemented.

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