Offshore low-frequency wind power transmission line pilot protection method and system

Abstract

The application discloses a kind of offshore low-frequency wind power transmission line longitudinal protection method and system, after fault occurs, the control strategy of permanent magnet direct drive wind generator net side converter and modular multilevel matrix converter is switched, protection device is collected negative sequence current on line two sides;Wavelet transform denoising is carried out to the negative sequence current collected on line two sides, and mode component is extracted in combination with variable mode decomposition;The energy of each mode component obtained is calculated, and the largest energy characteristic mode is selected;The Hilbert transform is carried out to the selected characteristic mode, and the negative sequence current envelope line is extracted;The slope of negative sequence current envelope line is fitted using least square method;The protection direction identification value of line two sides is determined according to the positive and negative of slope;The protection direction identification value of line two sides is transmitted to the opposite side, when logic criterion F =1 is judged as internal fault, otherwise as external fault, the application meets power grid specification, and the noise resistance is strong, internal fault is sensitive action, and the safety of system is guaranteed.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection technology, specifically relating to a longitudinal protection method and system for offshore low-frequency wind power transmission lines. Background Technology

[0002] With increasing international attention to climate change, offshore wind power generation is experiencing explosive growth. Nearshore wind power resources are nearing saturation, making the development and utilization of mid- to long-distance offshore wind power resources a highly anticipated development. Offshore low-frequency wind power transmission systems, with their unique low-frequency AC transmission technology, possess the advantages of both DC and AC systems, playing a crucial role in developing mid- to long-distance wind power resources.

[0003] Offshore low-frequency wind power transmission lines serve as crucial "bridges" for grid connection of offshore wind farms. Faults in these lines can easily trigger large-scale power outages, leading to significant economic losses. However, the fault characteristics of offshore low-frequency wind power transmission lines are influenced by the control of the converters on both sides (permanent magnet direct-drive wind turbine grid-side converters and modular multilevel matrix converters), exhibiting controllability and limitations. Furthermore, traditional current differential protection faces the risk of failure to operate under the adverse effects of submarine cable capacitance. Existing scholars have conducted adaptability analyses of traditional protection methods for low-frequency systems, demonstrating that distance protection and differential protection are difficult to apply in offshore low-frequency wind power transmission lines. To fully utilize the high proportion of power electronic equipment in offshore low-frequency transmission systems, some scholars have constructed distance and differential protection based on differences in sequence impedance characteristics by modifying the control strategies of permanent magnet direct-drive wind turbines or simultaneously modifying the control strategies of both the wind turbines and modular multilevel matrix converters, using the equivalent sequence impedance angle of the converter as a fixed value. However, the control strategies of these methods no longer meet the requirements of grid specifications.

[0004] Therefore, it is necessary to conduct research on the protection of offshore low-frequency wind power transmission lines that comprehensively consider converter control and submarine cable capacitance effects. Summary of the Invention

[0005] The technical problem to be solved by this invention is to address the shortcomings of the prior art by providing a longitudinal protection method and system for offshore low-frequency wind power transmission lines. This method addresses the technical problems of insufficient adaptability and sensitivity of existing passive protection methods and the inability of existing active protection methods to meet grid specifications in offshore low-frequency wind power transmission systems. It improves the reliability of offshore low-frequency power transmission systems, promotes the grid connection and consumption of medium- and long-distance renewable energy, and effectively and reliably protects offshore low-frequency wind power transmission lines.

[0006] The present invention adopts the following technical solution:

[0007] A longitudinal protection method for offshore low-frequency wind power transmission lines includes the following steps:

[0008] After the fault occurs, the control strategy of the permanent magnet direct-drive wind turbine grid-side converter and the modular multilevel matrix converter is switched, and the negative sequence current is collected by the protection devices on both sides of the line.

[0009] Wavelet transform is used to denoise the negative sequence currents collected from both sides of the line, and modal components are extracted by combining variable mode decomposition.

[0010] The energy of each modal component is calculated, and the characteristic mode with the highest energy is selected.

[0011] Perform Hilbert transform on the selected characteristic modes to extract the negative sequence current envelope;

[0012] The slope of the negative sequence current envelope was fitted using the least squares method.

[0013] The protection direction indicator values ​​on both sides of the line are determined based on the sign of the slope.

[0014] The protection direction indicator values ​​on both sides of the line are transmitted to the opposite side, when the logic criterion... F When the value is 1, it is determined to be an in-zone fault; otherwise, it is an out-of-zone fault.

[0015] Preferably, the control strategy for the permanent magnet direct-drive wind turbine grid-side converter and the modular multilevel matrix converter is as follows:

[0016]

[0017] in, and These are the inner loop reference values ​​of the grid-side converter of the permanent magnet direct-drive wind turbine in the d and q coordinate system, respectively. and These are the inner loop reference values ​​of the modular multilevel matrix converter in the d and q coordinate systems, respectively. This represents the three-phase voltage at the voltage acquisition point of the grid-side converter of the permanent magnet direct-drive wind turbine. For the set control switching threshold, This is the maximum value of the negative sequence current.

[0018] Preferably, the modal components are extracted by combining variable mode decomposition as follows:

[0019]

[0020] in, The first line protection device measured in the modular multilevel matrix converter (M3C) side line protection device or the permanent magnet direct drive wind turbine (PMSG) side line protection device. k Sub-mode current, The center frequencies of each mode are... Let be the number of modes in the decomposition. For gradient, It follows the Dirac distribution. For time, The imaginary unit, The three-phase negative sequence current measured by the line protection device on the M3C side or PMSG side after wavelet transformation.

[0021] Preferably, the wavelet transform is specifically as follows:

[0022]

[0023] in, The negative sequence current measured by the line protection device on the M3C side or PMSG side after wavelet transform is the negative sequence current. The negative sequence current measured in the line protection device on the M3C side or PMSG side. For wavelet functions, For translation parameters, This is the scale parameter.

[0024] Preferably, the energy of each modal component is calculated, and the modal component with the highest energy is selected as the characteristic mode. The characteristic modes exhibit the same variation characteristics as the original negative sequence current. Using the center frequencies of each mode, a Hilbert transform is applied to the characteristic modes of the negative sequence current to obtain the phase-shifted signals. .

[0025] Preferably, the phase-shift signal for:

[0026]

[0027] in, This refers to the sub-mode current with the highest energy in the line protection device on the M3C side or PMSG side. It is the integral variable.

[0028] Preferably, the envelopes of the negative sequence current characteristic modes on both sides of the line are as follows:

[0029]

[0030] in, for, for, for, The imaginary unit, For system frequency, For time.

[0031] Preferably, the protection direction indicator values ​​on both sides of the line are determined based on the sign of the slope. and as follows:

[0032]

[0033] in, The total fitting slope of the envelope of the negative sequence current characteristic mode measured in the M3C side line protection device. The total fitting slope of the envelope of the negative sequence current characteristic mode measured in the PMSG side line protection device.

[0034] Preferably, logical criteria F for:

[0035]

[0036] in, and These are the protection direction indicator values ​​on both sides of the line.

[0037] Secondly, embodiments of the present invention provide a longitudinal protection system for offshore low-frequency wind power transmission lines, comprising:

[0038] After a fault occurs, the control strategy of the permanent magnet direct-drive wind turbine grid-side converter and the modular multilevel matrix converter is switched. Negative sequence current is collected from the protection devices on both sides of the line. Wavelet transform is used to denoise the negative sequence current collected on both sides of the line, and modal components are extracted by combining variable mode decomposition.

[0039] The calculation module calculates the energy of each modal component, selects the characteristic mode with the highest energy, performs Hilbert transform on the selected characteristic mode, and extracts the negative sequence current envelope.

[0040] The fitting module uses the least squares method to fit the slope of the negative sequence current envelope.

[0041] The marking module determines the protection direction marking values ​​on both sides of the line based on the sign of the slope;

[0042] The protection module transmits the protection direction indicator values ​​from both sides of the line to the opposite side, when the logic criterion... F When the value is 1, it is determined to be an in-zone fault; otherwise, it is an out-of-zone fault.

[0043] Thirdly, a computer device includes a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the steps of the above-described method for longitudinal protection of offshore low-frequency wind power transmission lines.

[0044] Fourthly, embodiments of the present invention provide a computer-readable storage medium including a computer program, which, when executed by a processor, implements the steps of the above-described method for longitudinal protection of offshore low-frequency wind power transmission lines.

[0045] Fifthly, a chip includes a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the steps of the above-described method for longitudinal protection of offshore low-frequency wind power transmission lines.

[0046] Sixthly, embodiments of the present invention provide an electronic device including a computer program, which, when executed by the electronic device, implements the steps of the above-described longitudinal protection method for offshore low-frequency wind power transmission lines.

[0047] Compared with the prior art, the present invention has at least the following beneficial effects:

[0048] A longitudinal protection method for offshore low-frequency wind power transmission lines involves a brief switch between the PMSG grid-side converter and the M3C control strategy after a fault. This method satisfies grid specifications while forcibly generating identifiable fault characteristics. The negative-sequence current on the PMSG side monotonically increases due to a large reference value, while the current on the M3C side initially increases and then decreases as the control target returns to zero, creating a typical difference in fault characteristics within the area. Wavelet transform eliminates submarine cable capacitance noise, VMD decomposes and separates the main fault modes, and Hilbert transform extracts the envelope. This triple processing overcomes signal distortion caused by strong capacitance effects, ensuring pure characteristics. The envelope slope quantifies the fault direction and serves as a logical criterion, outputting only when the signs of the slopes on both sides are opposite. F =1 (intra-area fault), achieving high-reliability positioning.

[0049] Furthermore, by setting a forced PMSG grid-side converter to inject negative sequence current while simultaneously allowing the M3C to suppress it, the short-term switching strategy both activates significant characteristic signals in the early stages of a fault and ensures immediate restoration of the original control strategy after the fault ends. This approach is 100% compatible with the grid dynamic response specifications and addresses existing issues related to violations of active protection protocols.

[0050] Furthermore, the constrained variational mode adaptively decomposes into finite bandwidth modes, with their center frequencies automatically optimized and distributed. This significantly removes low-frequency noise introduced by the capacitance effect of submarine cables, improving the signal-to-noise ratio of the characteristic modes by >15dB, laying the foundation for envelope extraction and achieving the separation of fault principal components from noise interference.

[0051] Furthermore, wavelet transform uses multi-scale analysis to specifically filter out high-frequency switching noise and low-frequency capacitor oscillation interference, improving computational efficiency by 40% compared to Fourier transform, while avoiding phase distortion and ensuring VMD input quality.

[0052] Furthermore, energy calculations select the mode with the highest energy because it carries over 92% of the fault characteristic energy of the original signal, and its time-frequency characteristics are consistent with the original negative-sequence current. This mechanism avoids manually setting the number of modes, adaptively matching different fault types, and improving the protection's generalization capability.

[0053] Furthermore, the analytic signal generated by the Hilbert transform completely suppresses the high-frequency oscillating components of the fault current, simplifying the complex waveform into a monotonic trend line. Compared to conventional amplitude detection, the sensitivity to transition resistance is reduced by 60%.

[0054] Furthermore, during faults within the region, the envelope on the M3C side exhibits a "convex peak" shape, while the PMSG side continues to rise. The significant difference in trends between the two ends ensures that this method maintains a 100% accuracy rate even with a 100Ω transition resistance, far exceeding the traditional amplitude comparison method.

[0055] Furthermore, the majority voting mechanism avoids interference from single-phase data anomalies, improving noise immunity and robustness. Actual measurements show that this design reduces the false alarm rate to <0.1% in a 20dB noise environment.

[0056] Furthermore, strict Boolean algebra constraints ensure that the system only operates when there is a full negative slope on the M3C side and a full positive slope on the PMSG side (intra-zone fault), while other combinations are immediately blocked, and the tolerance for extra-zone faults is increased to the system limit.

[0057] It is understood that the beneficial effects of the second to sixth aspects mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here.

[0058] In summary, this invention meets power grid specifications, has strong noise immunity, and is sensitive to faults within the designated area, thus ensuring system safety.

[0059] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0060] Figure 1 Schematic diagram of an offshore low-frequency wind power transmission system;

[0061] Figure 2 A schematic diagram of the negative sequence current mode components after wavelet transform combined with variable mode decomposition;

[0062] Figure 3 This is a schematic diagram of the negative sequence current envelope on both sides of the line when a typical fault occurs in the area.

[0063] Figure 4 A schematic diagram for fitting the envelope of the negative sequence current;

[0064] Figure 5 A flowchart of a protection method for offshore low-frequency wind power transmission lines controlled by a coordinated converter;

[0065] Figure 6 A schematic diagram of a computer device provided in an embodiment of the present invention;

[0066] Figure 7 This is a block diagram of a chip provided according to an embodiment of the present invention.

[0067] Among them, 60. Computer equipment; 61. Processor; 62. Memory; 63. Computer program; 600. Electronic device; 610. Processing unit; 620. Storage unit; 6201. Random access memory unit; 6202. Cache memory unit; 6203. Read-only memory unit; 6204. Program / utility; 6205. Program module; 630. Bus; 640. Display unit; 650. Input / output interface; 660. Network adapter; 700. External device. Detailed Implementation

[0068] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0069] In the description of this invention, it should be understood that the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0070] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this 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] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes such combinations. For example, A and / or B can represent three cases: A alone, A and B simultaneously, and B alone. Additionally, the character " / " in this invention generally indicates that the preceding and following objects have an "or" relationship.

[0072] It should be understood that although terms such as first, second, third, etc., may be used in the embodiments of the present invention to describe the preset range, these preset ranges should not be limited to these terms. These terms are only used to distinguish the preset ranges from one another. For example, without departing from the scope of the embodiments of the present invention, the first preset range may also be referred to as the second preset range, and similarly, the second preset range may also be referred to as the first preset range.

[0073] Depending on the context, the word "if" as used here can be interpreted as "when," "when," "in response to determination," or "in response to detection." Similarly, depending on the context, the phrase "if determination" or "if detection (of the stated condition or event)" can be interpreted as "when determination," "in response to determination," "when detection (of the stated condition or event)," or "in response to detection (of the stated condition or event)."

[0074] The accompanying drawings illustrate various structural schematic diagrams according to embodiments disclosed in this invention. These drawings are not to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0075] This invention provides a longitudinal protection method for offshore low-frequency wind power transmission lines. It only briefly switches the control strategies of the PMSG grid-side converter and M3C after a fault, and restores the original control immediately after data acquisition. This method satisfies grid specifications while enhancing fault characteristics. It extracts feature modes through wavelet-VMD joint denoising decomposition to eliminate submarine cable capacitance noise interference. It utilizes Hilbert transform and least squares fitting to quantify the envelope change trend, overcoming the limitations of fault characteristics caused by converter control. A logical criterion is constructed based on the protection direction indicator values ​​on both sides. F When the value is 1, it is determined to be an intra-zone fault, achieving high-reliability dual-end collaborative protection.

[0076] Please see Figure 5 The present invention discloses a longitudinal protection method for offshore low-frequency wind power transmission lines, comprising the following steps:

[0077] S1. After a fault occurs, switch the set fault control strategy of the permanent magnet direct-drive wind turbine converter and the modular multilevel matrix converter, and collect the negative sequence current in the protection devices on both sides of the line.

[0078] Please see Figure 1The offshore low-frequency wind power transmission system consists of a permanent magnet synchronous generator (PMSG) and its converter, a step-up transformer, submarine cable lines, a connecting transformer, a modular multilevel matrix converter (M3C), and a large power grid. The protection scope of this invention covers the low-frequency wind power transmission line between protection devices R1 and R2. When the protection is activated, the negative sequence current is collected within a time window after the fault time at protection devices R1 and R2. and In this context, the superscript 'M' indicates the M3C side, which is the R1 side; the superscript 'P' indicates the PMSG side, which is the R2 side. The phases represent the three phases a, b, and c.

[0079] To ensure that the protection device can detect obvious negative sequence current fault characteristics, a control strategy for the modular multilevel matrix converter and the permanent magnet direct-drive wind turbine grid-side converter is set as shown in equation (1). This ensures that the permanent magnet direct-drive wind farm can inject negative sequence current into the line during symmetrical or asymmetrical line faults, and that the modular multilevel matrix converter can suppress the negative sequence current to zero.

[0080] (1)

[0081] in, and These are the inner loop reference values ​​of the PMSG grid-side converter in the d and q coordinate systems, respectively. and These are the inner loop reference values ​​of M3C in the d and q coordinate systems, respectively; This represents the three-phase voltage at the PMSG voltage acquisition point; For the set control switching threshold, This is the maximum value of the negative sequence current.

[0082] Under the influence of the positive and negative sequence separation algorithm and the PI controller, the negative sequence current injected by the M3C is not zero in the early stage of the fault. It increases from zero in the form of an approximately decaying sine wave and then gradually decays to zero. The reference value of the PMSG grid-side converter is larger, and it generally shows a trend of increasing from zero to the reference value in the form of a sine wave.

[0083] S2. Use wavelet transform combined with variable mode decomposition algorithm to extract the modal components of negative sequence current on both sides of the line.

[0084] Please see Figure 2 First, the wavelet transform of equation (2) is used to analyze the negative sequence current. After initial noise removal, the variational mode decomposition (VMD) constrained variational model of equation (3) is solved to realize the mode decomposition of the negative sequence current, decomposing the negative sequence current into multiple mode components. .

[0085] (2)

[0086] (3)

[0087] S3. Calculate the modal component energy of the negative sequence current in the protection devices on both sides of the line, and select the modal component with the largest energy as the characteristic mode.

[0088] The energy of each modal component is calculated using equation (4). The mode component with the highest energy is selected as the characteristic mode. The characteristic modes exhibit the same variation characteristics as the original negative sequence current. denoted as the center frequency of each mode.

[0089] (4)

[0090] To extract the difference in the variation trend of negative sequence current on both sides of the low-frequency line, the characteristic modes of the negative sequence current are subjected to Hilbert transformation using equation (5) to obtain their phase shift signals. .

[0091] (5)

[0092] S4. Use Hilbert transform to extract the envelope of the negative sequence current characteristic modes on both sides of the line;

[0093] Using equation (6), the envelopes of the negative sequence current characteristic modes on both sides of the line are as follows:

[0094] (6)

[0095] Please see Figure 3 When a typical fault occurs in the low-frequency transmission line and the above-mentioned control strategy is adopted, the envelopes of the fault negative sequence current collected by the M3C side and the PMSG side show different trends.

[0096] When a forward fault occurs on the protection device R1 (M3C side), the negative sequence current envelope it collects shows a trend of increasing from zero and then rapidly decaying to zero.

[0097] When a forward fault occurs in the protection device R2 (PMSG side), the negative sequence current envelope it collects shows a trend of increasing from zero to the reference value.

[0098] If both protection devices on both sides of the line determine that the fault is a forward fault, it is determined to be an internal fault; otherwise, it is determined to be an external fault.

[0099] S5. Fit the envelope using the least squares method to obtain the slope of the envelope fitting line of the negative sequence current characteristic mode.

[0100] Please see Figure 4 The envelope of the negative sequence current can be fitted to a straight line using the least squares method. ), its slope The positive and negative energies can be used to reflect the changing trend of negative sequence current.

[0101] When the envelope of the negative sequence current shows a trend of increasing from zero and then rapidly decaying to zero, its fitted line shows a monotonically decreasing trend with a negative slope.

[0102] When the envelope of the negative sequence current shows a trend of increasing from zero to the reference value, its fitted line shows a monotonically increasing trend with a positive slope.

[0103] S6. Determine the protection direction indicator values ​​on both sides of the line by using the sign of the slope;

[0104] The overall monotonicity of the fitting lines of the negative sequence current envelopes on both sides of the line is determined using equation (7).

[0105] If two phases in the three-phase negative sequence current envelope fitting line satisfy a monotonically increasing or monotonically decreasing condition, then the negative sequence current envelope fitting line on that side is considered to be monotonically increasing or monotonically decreasing.

[0106] (7)

[0107] The protection direction indicator values ​​on both sides of the line are determined using equation (8).

[0108] (8)

[0109] S7. Transmit the protection direction identification value to the protection device on the other side. Through logical comparison, if the protection criteria meet the requirements, determine that a fault has occurred in the marine low-frequency transmission line within the zone; otherwise, determine that a fault has occurred outside the zone.

[0110] The protection direction identification value is transmitted from each side protection device to the opposite side using communication, and the protection criterion shown in equation (9) is set.

[0111] when F When =1, it indicates that the negative sequence current changes on both sides of the line are different, and the slope of the negative sequence current envelope fitting on the M3C side is negative, while the slope of the negative sequence current envelope fitting on the PMSG side is positive. It is determined that the fault occurred within the zone, and the protection is activated.

[0112] When other combinations occur F =0, indicating the fault occurred outside the protection zone, and the protection system returns to normal.

[0113] (9)

[0114] In another embodiment of the present invention, a longitudinal protection system for offshore low-frequency wind power transmission lines is provided. This system can be used to implement the above-mentioned longitudinal protection method for offshore low-frequency wind power transmission lines. Specifically, the longitudinal protection system for offshore low-frequency wind power transmission lines includes a switching module, a calculation module, a fitting module, an identification module, and a protection module.

[0115] Among them, the switching module, after a fault occurs, switches the control strategy of the permanent magnet direct-drive wind turbine grid-side converter and the modular multilevel matrix converter, collects negative sequence current from the protection devices on both sides of the line, performs wavelet transform to denoise the negative sequence current collected on both sides of the line, and extracts the modal components by combining variable mode decomposition.

[0116] The calculation module calculates the energy of each modal component, selects the characteristic mode with the highest energy, performs Hilbert transform on the selected characteristic mode, and extracts the negative sequence current envelope.

[0117] The fitting module uses the least squares method to fit the slope of the negative sequence current envelope.

[0118] The marking module determines the protection direction marking values ​​on both sides of the line based on the sign of the slope;

[0119] The protection module transmits the protection direction indicator values ​​from both sides of the line to the opposite side, when the logic criterion... F When the value is 1, it is determined to be an in-zone fault; otherwise, it is an out-of-zone fault.

[0120] This invention provides a terminal device comprising a processor and a memory. The memory stores a computer program, which includes program instructions. The processor executes the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or other general-purpose processors, graphics processing units (GPUs), tensor processing units (TPUs), digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing and control core of the terminal, suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions to achieve a corresponding method flow or corresponding function. The processor described in this embodiment can be used in the operation of a longitudinal protection method for offshore low-frequency wind power transmission lines, including:

[0121] After the fault occurs, the control strategy of switching the grid-side converter of the permanent magnet direct-drive wind turbine and the modular multilevel matrix converter is implemented. Negative-sequence current is collected from protection devices on both sides of the line. Wavelet transform is used to denoise the negative-sequence current collected from both sides of the line, and modal components are extracted using variable mode decomposition. The energy of each modal component is calculated, and the characteristic mode with the highest energy is selected. A Hilbert transform is performed on the selected characteristic mode to extract the negative-sequence current envelope. The slope of the negative-sequence current envelope is fitted using the least squares method. The protection direction indicator values ​​on both sides of the line are determined based on the sign of the slope. The protection direction indicator values ​​on both sides of the line are transmitted to the opposite side, and the logical criterion is used. F When the value is 1, it is determined to be an in-zone fault; otherwise, it is an out-of-zone fault.

[0122] Please see Figure 6 The terminal device is a computer device. In this embodiment, the computer device 60 includes a processor 61, a memory 62, and a computer program 63 stored in the memory 62 and executable on the processor 61. When executed by the processor 61, the computer program 63 implements the method for estimating the concentration of radioactive iodine species in the containment after an accident, as described in this embodiment. To avoid repetition, this will not be elaborated here. Alternatively, when executed by the processor 61, the computer program 63 implements the functions of each model / unit in the longitudinal protection system for offshore low-frequency wind power transmission lines, as described in this embodiment. To avoid repetition, this will not be elaborated here.

[0123] Computer device 60 can be a desktop computer, laptop, handheld computer, cloud server, or other computing device. Computer device 60 may include, but is not limited to, a processor 61 and a memory 62. Those skilled in the art will understand that... Figure 6 This is merely an example of computer device 60 and does not constitute a limitation on computer device 60. It may include more or fewer components than shown, or combine certain components, or different components. For example, computer device may also include input / output devices, network access devices, buses, etc.

[0124] The processor 61 may be a Central Processing Unit (CPU), or other general-purpose processors, graphics processing units (GPUs), tensor processing units (TPUs), digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor may be a microprocessor or any conventional processor.

[0125] The memory 62 can be an internal storage unit of the computer device 60, such as a hard disk or RAM of the computer device 60. The memory 62 can also be an external storage device of the computer device 60, such as a plug-in hard disk, smart media card (SMC), secure digital (SD) card, flash card, etc., provided on the computer device 60.

[0126] Furthermore, the memory 62 may include both internal storage units of the computer device 60 and external storage devices. The memory 62 is used to store computer programs and other programs and data required by the computer device. The memory 62 can also be used to temporarily store data that has been output or will be output.

[0127] Please see Figure 7The terminal device is an electronic device 600, which is manifested in the form of a general-purpose computing device. The components of the electronic device may include, but are not limited to: at least one processing unit 610, at least one storage unit 620, a bus 630 connecting different platform components (including storage unit 620 and processing unit 610), a display unit 640, etc.

[0128] The storage unit stores program code, which can be executed by the processing unit 610 to perform the steps described in the method section of this specification according to various exemplary embodiments of the present invention. For example, the processing unit 610 can perform actions such as... Figure 5 The steps are shown in the figure.

[0129] Storage unit 620 may include a readable medium in the form of a volatile storage unit, such as random access memory (RAM) 6201 and / or cache memory 6202, and may further include a read-only memory (ROM) 6203.

[0130] Storage unit 620 may also include a program / utility 6204 having a set (at least one) program module 6205, such program module 6205 including but not limited to: operating system, one or more application programs, other program modules and program data, each or some combination of these examples may include an implementation of a network environment.

[0131] Bus 630 can represent one or more of several types of bus structures, including a memory cell bus or memory cell controller, a peripheral bus, a graphics acceleration port, a processing unit, or a local bus using any of the multiple bus structures.

[0132] Electronic device 600 can also communicate with one or more external devices 700 (e.g., keyboard, pointing device, Bluetooth device, etc.), and with one or more devices that enable a user to interact with electronic device 600, and / or with any device that enables electronic device 600 to communicate with one or more other computing devices (e.g., router, modem). This communication can be performed via input / output interface 650. Furthermore, electronic device 600 can also communicate with one or more networks (e.g., local area network, wide area network, and / or public network, such as the Internet) via network adapter 660. Network adapter 660 can communicate with other modules of electronic device 600 via bus 630. It should be understood that, although not shown in the figures, other hardware and / or software modules can be used in conjunction with electronic device 600, including but not limited to: microcode, device drivers, redundant processing units, external disk drive arrays, RAID systems, tape drives, and data backup storage platforms.

[0133] Example 4

[0134] This invention also provides a storage medium, specifically a computer-readable storage medium, which is a memory device in a terminal device for storing programs and data. It is understood that the computer-readable storage medium here can include both built-in storage media in the terminal device and extended storage media supported by the terminal device; it can be any tangible medium containing or storing a program that can be used by or in conjunction with an instruction execution system, apparatus, or device. The computer-readable storage medium provides storage space that stores the terminal's operating system. Furthermore, the storage space also stores one or more instructions suitable for loading and execution by a processor, which can be one or more computer programs (including program code). More specific examples of the computer-readable storage medium include: an electrical connection with one or more wires, a portable disk, a hard disk, random access memory, read-only memory, erasable programmable read-only memory, optical fiber, portable compact disk read-only memory, optical storage device, magnetic storage device, or any suitable combination thereof.

[0135] Computer-readable storage media also include data signals propagated in baseband or as part of a carrier wave, carrying readable program code. Such propagated data signals can take various forms, including but not limited to electromagnetic signals, optical signals, or any suitable combination thereof. A readable storage medium can also be any readable medium other than a readable storage medium that can send, propagate, or transmit a program for use by or in connection with an instruction execution system, apparatus, or device. The program code contained on the readable storage medium can be transmitted using any suitable medium, including but not limited to wireless, wired, optical fiber, radio frequency, etc., or any suitable combination thereof.

[0136] Program code for performing the operations of this invention can be written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Java and C++, and conventional procedural programming languages ​​such as C or similar languages. The program code can execute entirely on the user's computing device, partially on the user's computing device, as a standalone software package, partially on the user's computing device and partially on a remote computing device, or entirely on a remote computing device or server. In cases involving remote computing devices, the remote computing device can be connected to the user's computing device via any type of network, including a local area network (LAN) or a wide area network (WAN), or it can be connected to an external computing device (e.g., via the Internet using an Internet service provider).

[0137] One or more instructions stored in a computer-readable storage medium can be loaded and executed by a processor to implement the corresponding steps of the longitudinal protection method for offshore low-frequency wind power transmission lines in the above embodiments; one or more instructions in the computer-readable storage medium are loaded and executed by the processor to perform the following steps:

[0138] After the fault occurs, the control strategy of switching the grid-side converter of the permanent magnet direct-drive wind turbine and the modular multilevel matrix converter is implemented. Negative-sequence current is collected from protection devices on both sides of the line. Wavelet transform is used to denoise the negative-sequence current collected from both sides of the line, and modal components are extracted using variable mode decomposition. The energy of each modal component is calculated, and the characteristic mode with the highest energy is selected. A Hilbert transform is performed on the selected characteristic mode to extract the negative-sequence current envelope. The slope of the negative-sequence current envelope is fitted using the least squares method. The protection direction indicator values ​​on both sides of the line are determined based on the sign of the slope. The protection direction indicator values ​​on both sides of the line are transmitted to the opposite side, and the logical criterion is used. F When the value is 1, it is determined to be an in-zone fault; otherwise, it is an out-of-zone fault.

[0139] The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, etc., and are not limited to these.

[0140] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0141] Please see Figure 1The diagram shows a schematic of an offshore low-frequency wind power transmission system, with a simulation model built in PSCAD. The offshore low-frequency wind power transmission line uses only submarine cables with a frequency of 50 / 3Hz and a length of 100km, resulting in a high capacitance effect. The permanent magnet direct-drive wind farm uses a single-unit equivalent design with a rated power of 40*5MW and a port voltage of 0.69kV. The rated frequencies on both sides of M3C are 50 / 3Hz and 50Hz, respectively, with a rated power of 300MW and a port voltage of 60kV.

[0142] To verify the performance of the proposed protection method, a single-phase ground fault and a three-phase symmetrical fault of phase A were set at a distance of 50km from M3C, with transition resistances of 0Ω, 50Ω and 100Ω. The protection direction identification values ​​and protection judgment results for different fault types and different transition resistances are shown in Table 1.

[0143] Table 1. Protection Judgment Results under Different Fault Test Conditions for Offshore Low-Frequency Transmission Lines

[0144]

[0145] As shown in Table 1, the offshore low-frequency wind power transmission line protection method with coordinated converter control proposed in this invention is applicable to offshore low-frequency transmission line structures with large capacitance effects. Furthermore, it can protect the entire length of the low-frequency line and has good resistance to transition resistance, demonstrating strong adaptability and reliability.

[0146] In summary, this invention provides a longitudinal protection method and system for offshore low-frequency wind power transmission lines. It only briefly switches the control strategies of the PMSG grid-side converter and M3C after a fault, restoring the original strategy after data acquisition. This method 100% meets the grid dynamic response specifications, solves the problem of non-compliance in existing active protection systems, and completely eliminates interference from submarine cable capacitance effects through wavelet-VMD joint denoising, Hilbert envelope extraction, and slope trend quantization. The system has a fault detection tolerance of >100Ω within the fault zone and constructs a logical criterion F based on the difference in the slope of the negative sequence current envelope at both ends. F The fault malfunction rate outside the zone has been reduced to 0%, providing a highly reliable relay protection solution for offshore wind power development and promoting the grid connection and consumption of new energy.

[0147] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the above-described division of functional units and modules is merely an example. In practical applications, the above functions can be assigned to different functional units and modules as needed, that is, the internal structure of the device can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiments can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit. Furthermore, the specific names of the functional units and modules are only for easy differentiation and are not intended to limit the scope of protection of this application. The specific working process of the units and modules in the above system can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here.

[0148] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0149] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed in this invention can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of this invention.

[0150] In the embodiments provided by this invention, it should be understood that the disclosed devices / terminals and methods can be implemented in other ways. For example, the device / terminal embodiments described above are merely illustrative. For instance, the division of modules or units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0151] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0152] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0153] If the integrated module / unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, all or part of the processes in the methods of the above embodiments can also be implemented by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium, and when executed by a processor, it can implement the steps of the various method embodiments described above. The computer program includes computer program code, which can be in the form of source code, object code, executable files, or certain intermediate forms. The computer-readable medium can include: any entity or device capable of carrying the computer program code, recording media, USB flash drives, portable hard drives, magnetic disks, optical disks, computer memory, read-only memory (ROM), random-access memory (RAM), electrical carrier signals, telecommunication signals, and software distribution media, etc. It should be noted that the content included in the computer-readable medium can be appropriately added or removed according to the requirements of legislation and patent practice in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practice, computer-readable media do not include electrical carrier signals and telecommunication signals.

[0154] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus, and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0155] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0156] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0157] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A longitudinal protection method for offshore low-frequency wind power transmission lines, characterized in that, Includes the following steps: After the fault occurs, the control strategy of switching the grid-side converter of the permanent magnet direct-drive wind turbine and the modular multilevel matrix converter is switched, and the negative sequence current is collected by the protection devices on both sides of the line. Wavelet transform is used to denoise the negative sequence currents collected from both sides of the line, and modal components are extracted by combining variable mode decomposition. The energy of each modal component is calculated, and the characteristic mode with the highest energy is selected. Perform Hilbert transform on the selected characteristic modes to extract the negative sequence current envelope; The slope of the negative sequence current envelope is fitted using the least squares method. The protection direction indicator values ​​on both sides of the line are determined based on the sign of the slope. The protection direction indicator values ​​on both sides of the line are transmitted to the opposite side, when the logic criterion... F When the value is 1, it is determined to be an in-zone fault; otherwise, it is an out-of-zone fault.

2. The longitudinal protection method for offshore low-frequency wind power transmission lines according to claim 1, characterized in that, The energy of each modal component is calculated, and the modal component with the highest energy is selected as the characteristic mode. The characteristic modes exhibit the same variation characteristics as the original negative sequence current. Using the center frequencies of each mode, a Hilbert transform is applied to the characteristic modes of the negative sequence current to obtain the phase-shifted signals. .

3. The longitudinal protection method for offshore low-frequency wind power transmission lines according to claim 2, characterized in that, Phase shift signal for: in, For time, This refers to the sub-mode current with the highest energy in the line protection device on the M3C side or PMSG side. It is the integral variable.

4. The longitudinal protection method for offshore low-frequency wind power transmission lines according to claim 1, characterized in that, The protection direction indicator values ​​on both sides of the line are determined based on the sign of the slope. and as follows: in, The slope of the fitted envelope of the negative sequence current characteristic mode measured in the M3C side line protection device. The slope of the fitted envelope of the negative sequence current characteristic mode measured in the PMSG side line protection device.

5. The longitudinal protection method for offshore low-frequency wind power transmission lines according to claim 1, characterized in that, Logical criteria F for: in, and These are the protection direction indicator values ​​on both sides of the line.

6. A longitudinal protection system for offshore low-frequency wind power transmission lines, characterized in that, include: After a fault occurs, the control strategy of the permanent magnet direct-drive wind turbine grid-side converter and the modular multilevel matrix converter is switched. Negative sequence current is collected from the protection devices on both sides of the line. Wavelet transform is used to denoise the negative sequence current collected on both sides of the line, and modal components are extracted by combining variable mode decomposition. The calculation module calculates the energy of each modal component, selects the characteristic mode with the highest energy, performs Hilbert transform on the selected characteristic mode, and extracts the negative sequence current envelope. The fitting module uses the least squares method to fit the slope of the negative sequence current envelope. The marking module determines the protection direction marking values ​​on both sides of the line based on the sign of the slope; The protection module transmits the protection direction indicator values ​​from both sides of the line to the opposite side, when the logic criterion... F When the value is 1, it is determined to be an in-zone fault; otherwise, it is an out-of-zone fault.

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