Marine low-frequency wind power transmission line protection method and system based on transient characteristics

Abstract

The application discloses a kind of offshore low-frequency wind power transmission line protection method and system based on transient characteristics, belong to the technical field of power system relay protection.The method includes: collecting three-phase voltage and current on both sides of line, calculating voltage mutation and starting protection according to threshold value;Extract negative sequence current and convert to two-phase rotating coordinate system to obtain q axis negative sequence current;Adaptive variational modal decomposition is combined with frequency relative deviation criterion to determine the optimal modal number, and the main frequency component is extracted relying on energy criterion;Calculate the mean value of both sides of the main frequency component in the preset time window, and solve the characteristic mean value difference by combining transformer ratio and protection coefficient;According to the system and control parameter theory, set the protection action threshold value, if the mean value difference is greater than the threshold value, it is judged that the fault is in the area and tripped, otherwise it is judged that the fault is outside the area and locked out.The application can tolerate high transition resistance, has excellent speed, sensitivity and reliability, and is suitable for single-phase grounding, two-phase and two-phase grounding asymmetric fault protection.

Description

Technical Field

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

[0002] Low-frequency power transmission technology, with its advantages of long transmission distance, high transmission power, and superior voltage transformation performance compared to DC transmission, is widely used in offshore wind power transmission projects. This technology uses low-frequency AC power below 50Hz to transmit electrical energy, making it suitable for long-distance submarine cable transmission scenarios and a core technology for efficient offshore wind power transmission.

[0003] However, the unique topology of offshore low-frequency wind power transmission lines results in a situation where one side of the line connects to the grid-side inverter of an offshore direct-drive wind turbine, while the other side connects to a modular multilevel matrix inverter. The converters on both sides employ independent fault control targets, leading to fundamental differences in the electrical characteristics of the line fault compared to traditional synchronous motor grids. Traditional power frequency current differential protection relies on the stable voltage support and large short-circuit current provided by the synchronous motor. However, in low-frequency scenarios, the current amplitude and phase angle of both converters are controlled, and the fault short-circuit current amplitude is limited. Lacking the stable fault characteristics of traditional grids, protection failure is highly likely.

[0004] Meanwhile, the operating frequency of low-frequency transmission systems is much lower than that of the power frequency, and the current fundamental frequency period is extended several times. The duration of the fault transient decay process is significantly increased. Traditional protection methods that rely on the fault steady-state power frequency phasor can only operate after the transient process has finished decaying, which greatly increases the operation delay and cannot meet the speed requirement of rapid fault clearing for wind power transmission lines.

[0005] Furthermore, existing protection systems are mostly designed for conventional power frequency grids and do not consider the positive and negative sequence current control strategies and fault ride-through logic constraints of converters in low-frequency offshore scenarios. Forcibly applying these systems would require modifying the original control parameters of the wind turbine and M3C, violating grid fault ride-through specifications and affecting equipment safety. Moreover, existing protection thresholds largely rely on engineering experience for setting, lacking theoretical support from system control parameters. This results in insufficient sensitivity in high transition resistance and slightly asymmetrical fault scenarios, easily leading to false trips or failures to trip, making it difficult to adapt to the complex operation and fault conditions of low-frequency offshore wind power. This invention addresses all the above-mentioned technical pain points by constructing a longitudinal protection scheme based on the dominant frequency component of transient negative sequence current, resolving the shortcomings of existing technologies one by one from the aspects of feature extraction, fault identification, and threshold setting. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method and system for protecting offshore low-frequency wind power transmission lines based on transient characteristics, which addresses the shortcomings of the prior art and solves the technical problems of insufficient speed and sensitivity of the existing protection methods, thereby ensuring the safe and stable operation of the offshore low-frequency wind power transmission system under fault conditions.

[0007] The present invention adopts the following technical solution: A protection method for offshore low-frequency wind power transmission lines based on transient characteristics includes the following steps: S1. Collect three-phase voltage and three-phase current on both sides of the line respectively, and calculate the voltage change. When the voltage change of any phase on any side is greater than the start threshold, the protection is activated. S2. After the protection is started, the negative sequence current is extracted from both sides of the line, and the negative sequence current is transformed from the three-phase stationary coordinate system to the two-phase rotating coordinate system to obtain the q-axis negative sequence current. S3. Perform adaptive variational mode decomposition on the q-axis negative sequence current, determine the optimal preset mode number based on the frequency relative deviation under different preset mode numbers, and determine the dominant frequency component in the decomposed intrinsic mode function according to the energy criterion. S4. Within the preset protection time window, calculate the characteristic mean value of the main frequency components on both sides of the line respectively; S5. Calculate the difference in characteristic mean values ​​of the two main frequency components based on the characteristic mean values ​​and protection coefficients of the two main frequency components. S6. Compare the mean difference of the features with the protection action threshold set according to the system parameters and control parameters: when the mean difference of the features is greater than the protection action threshold, it is determined to be an intra-zone fault and tripping is performed; otherwise, it is determined to be an extra-zone fault.

[0008] Preferably, in step S1, the calculation of voltage surge and the protection activation conditions are as follows:

[0009] in, For the first Phase voltage sudden change For the first The sampling voltage at phase time t, The threshold for triggering voltage surges. T It is the fundamental frequency period of a low-frequency power transmission system.

[0010] Preferably, in step S3, the adaptive variational mode decomposition includes: The maximum relative deviation of frequency components between decomposed signals based on different preset numbers of IMFs is:

[0011] in, For the first The first preset mode The frequency of the first-order eigenmode function. For the first The first preset mode The central angular frequency of the first-order eigenmode function. The system sampling frequency, The preset number of modes is The maximum frequency obtained by time decomposition The preset number of modes is The maximum frequency obtained by time decomposition This represents the maximum relative frequency deviation. The frequency deviation of the main frequency component between the decomposed signals based on different preset numbers of IMFs is:

[0012] in, Preset number of modes The corresponding dominant frequency, Preset number of modes The corresponding dominant frequency; The criterion for calculating the optimal preset number of decompositions is:

[0013] in, The relative deviation between the main frequency under different preset IMFs numbers. The threshold for setting the relative deviation of the dominant frequency. The threshold is set for the maximum relative frequency deviation.

[0014] Preferably, the dominant frequency component is determined based on the energy criterion, specifically as follows:

[0015] in, For the first Preset mode side Axis No. Energy of each eigenmode function For the first Preset mode side Axis No. One negative sequence current component. for side The dominant frequency component of the negative sequence current. This is the maximum time limit for scoring. This is the lower limit of the integration time.

[0016] Preferably, in step S4, the mean value of the characteristic of the main frequency components on both sides of the line is calculated as follows:

[0017]

[0018] in, For the wind farm side Characteristic mean of the dominant frequency component of the negative sequence current. This is the maximum current limit for the negative sequence current on the wind farm side. This represents the negative sequence current control coefficient on the wind farm side. This is the reference value for the negative sequence voltage on the wind farm side. The rated current of the system. Set a per-unit value for the negative sequence voltage on the wind farm side. For M3C side Characteristic mean of the dominant frequency component of the negative sequence current. To protect the number of sampling points within the time window, To protect the sampling sequence number at the end of the time window, The starting sampling sequence number at the time the fault occurred. This is the identifier for the u / v / w sub-converter of M3C. Initial time Sub-converter negative sequence current, The integral coefficients of the inner loop of the q-axis negative sequence current control of the M3C are given. For M3C side Shaft damping ratio parameter, The equivalent inductance parameters are for the M3C side. For discrete-time variables, For M3C side The initial phase of the free component of the negative sequence current. This is the proportional coefficient of the inner loop of the q-axis negative sequence current control of the M3C.

[0019] Preferably, in step S5, the feature mean difference The calculation is as follows:

[0020] in, To protect the setting coefficient, This is the maximum current limit for the negative sequence current on the wind farm side. For the transformer ratio of the wind farm box, The transformer ratio of the main transformer in the wind farm box. This represents the negative sequence current control coefficient on the wind farm side. This is the reference value for the negative sequence voltage on the wind farm side. The rated current of the system. This represents the per-unit value of the negative sequence voltage on the wind farm side. Set a per-unit value for the negative sequence voltage on the wind farm side.

[0021] Preferably, the protection action threshold is set according to the system parameters and control parameters, and the set protection action threshold is corrected using a reliability coefficient.

[0022] Preferably, in the threshold tuning, the average mean difference of the minimum dominant frequency component is determined. for:

[0023] in, To protect the setting coefficient, This represents the negative sequence current control coefficient on the wind farm side. Set a per-unit value for the negative sequence voltage on the wind farm side; This is the maximum current limiting value on the wind farm side; the protection criterion for handling faults within the zone is... .

[0024] Preferably, the method is used to identify single-phase grounding faults, two-phase faults, and two-phase grounding faults in offshore low-frequency wind power transmission lines, and outputs tripping commands when there is a fault within the zone and blocking commands when there is a fault outside the zone.

[0025] Secondly, embodiments of the present invention provide a protection system for offshore low-frequency wind power transmission lines based on transient characteristics, including protection devices on both sides of the line and a communication unit; The protection devices on both sides of the line respectively include: The data acquisition and activation module is used to acquire three-phase voltage and three-phase current, calculate voltage surges, and activate protection according to activation conditions. The coordinate transformation module is used to extract the negative sequence current and convert it to obtain the q-axis negative sequence current. The dominant frequency extraction module is used to perform adaptive variational mode decomposition on the q-axis negative sequence current and determine the dominant frequency component according to the energy criterion. The feature calculation module is used to calculate the mean value of the main frequency component features and the difference between the mean values ​​of the features on both sides; The criterion execution module is used to compare the characteristic mean difference with the protection action threshold and output the fault determination within / outside the zone and the trip / blocking control. The communication unit is used to transmit feature information for calculating the difference in feature mean between the two protection devices; wherein the system is configured to perform the method described.

[0026] 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, when executing the computer program, implements the steps of the above-described method for protecting offshore low-frequency wind power transmission lines based on transient characteristics.

[0027] 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 protecting offshore low-frequency wind power transmission lines based on transient characteristics.

[0028] 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 protecting offshore low-frequency wind power transmission lines based on transient characteristics.

[0029] In a sixth aspect, 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 method for protecting offshore low-frequency wind power transmission lines based on transient characteristics.

[0030] Compared with the prior art, the present invention has at least the following beneficial effects: A protection method for offshore low-frequency wind power transmission lines based on transient characteristics first utilizes voltage surges to initiate protection, then extracts the negative-sequence current and performs coordinate transformation. The core of this method lies in extracting the dominant frequency component using adaptive Variational Mode Decomposition (VMD) and energy criteria. Finally, the fault is determined by calculating the difference between the mean values ​​of the features on both sides and comparing it with a threshold. By introducing the dominant frequency component of the transient negative-sequence current as the core criterion, this method fundamentally solves the problem of ambiguous fault characteristics caused by power electronic equipment control in offshore low-frequency systems. Its greatest advantage lies in its strong specificity and adaptability: through the adaptive variational mode decomposition (VMD) algorithm, it can accurately extract the dominant frequency component controlled by GSCs and M3C control strategies from complex transient signals, avoiding the drawbacks of traditional Fourier transforms being affected by spectral leakage. Furthermore, this invention does not rely on fault steady-state quantities; it only uses transient information from the one-cycle following the fault to complete the judgment, greatly improving the speed of protection. By constructing a longitudinal comparison mechanism based on the difference in the mean values ​​of features on both sides, the interference that single-ended protection may be affected by when there is a fault outside the protection zone is effectively avoided, ensuring the protection reliability in low-frequency, fully controlled converter access scenarios and achieving millisecond-level rapid isolation.

[0031] Furthermore, it aligns with the inherent cyclical characteristics of low-frequency power transmission, with the time window matching the system's main frequency. This allows for precise capture of the initial transient moment of a fault, enabling early activation of the protection process. Simultaneously, it clearly defines each physical quantity and quantifies the activation logic, avoiding biases from human-based settings. The activation threshold can be adaptively matched based on system operating conditions, exhibiting strong resistance to load disturbances and harmonic interference. It effectively prevents false activation of protection caused by normal power flow fluctuations and switching operations, ensuring that protection reliably activates only at the moment of a real fault, balancing sensitivity and anti-interference capabilities.

[0032] Furthermore, the maximum frequency deviation defines the over-decomposition boundary, and the dominant frequency deviation locks the minimum effective decomposition order. The dual constraints determine the optimal number of modes, adaptively adapting to different fault types and changes in negative sequence current components under transition resistance. There is no need to manually preset the number of decomposition layers, eliminating reliance on experience, accurately separating forced components and free components, providing a pure signal base for subsequent main frequency extraction, significantly improving the accuracy of fault feature extraction, and adapting to various fault conditions such as single-phase, two-phase, and phase-to-phase.

[0033] Furthermore, using energy extreme values ​​as the screening criterion, it closely matches the physical composition of transient negative sequence currents, without the need for manual filtering parameter settings; it can adaptively adapt to two different characteristic forms: DC main frequency on the fan side and attenuated main frequency on the M3C side, unaffected by fault location or changes in transition resistance, accurately retaining the core fault characteristic components and eliminating redundant interference components, providing a high signal-to-noise ratio input for characteristic mean calculation, and improving the reliability of subsequent fault identification.

[0034] Furthermore, an integral averaging method with an integer time window is adopted to smooth transient fluctuations. At the same time, it takes into account the different control parameters, damping characteristics, and current limiting constraints of the converters on both sides. The formula parameters fully cover the equipment control variables, accurately characterize the physical laws of the differences in fault characteristics on both sides within the fault zone and the convergence of fault characteristics outside the zone. It has a high degree of quantification, strong theoretical support, and avoids the discrimination error caused by single-point sampling.

[0035] Furthermore, by incorporating parameters such as the number of wind turbines, the reference capacity of wind turbines, and the reference capacity of the line, the electrical quantities on both sides are reduced to the same reference, eliminating the metering deviation caused by the topology. The protection coefficient can be flexibly adjusted according to the system operating conditions, balancing sensitivity and safety, accurately amplifying the fault characteristic difference within the area and suppressing the small disturbance difference outside the area, making the fault boundary characteristics clearer and improving the protection's ability to distinguish under high transition resistance.

[0036] Furthermore, based on the inherent electrical and control parameters of the system, it has a strict theoretical basis, and can be adapted to different wind power transmission lines simply by replacing the parameters; the reliability coefficient can withstand modeling errors, measurement errors, and operating condition fluctuations, avoid threshold critical misjudgment, and take into account both protection sensitivity and non-maloperation requirements.

[0037] Furthermore, the lower limit of the threshold is derived from physical constraints, which fits the characteristics of fault extreme conditions and ensures that minor faults and high-resistance faults can still operate reliably. The judgment logic is simple and clear, and only the mean difference and the threshold need to be compared once to output the result. The amount of calculation is small and the response speed is fast, which is suitable for the speed requirements of low frequency systems. At the same time, it prevents external disturbances from exceeding the threshold and causing false operation, thus achieving accurate boundary division of fault judgment.

[0038] Furthermore, it adapts to three typical fault types, covering most fault scenarios on the line; at the same time, it sets up standardized output logic, directly issuing trip commands to clear faults within the zone, and maintaining a lockout outside the zone to avoid false tripping, which conforms to the relay protection engineering operation specifications and can be directly connected to existing substation protection devices without modifying the secondary control logic, making it highly adaptable and practical for engineering projects.

[0039] 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.

[0040] In summary, this invention constructs longitudinal protection based on the main frequency component of transient negative sequence current, adapting to the differentiated control characteristics of equipment on both sides of offshore low-frequency wind power transmission lines without modifying the original positive sequence control strategy. Through adaptive VMD, it accurately extracts fault characteristics and theoretically sets the protection threshold, capable of withstanding high transition resistance of 100Ω and covering multiple types of asymmetrical faults. It requires only single-cycle transient data for identification, exhibiting strong speed and good anti-interference capabilities, with sufficient theoretical basis and outstanding engineering adaptability and reliability.

[0041] 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

[0042] Figure 1 Schematic diagram of an offshore low-frequency wind power transmission system; Figure 2 This is a flowchart of the protection method of the present invention; Figure 3 This is a schematic diagram of the main frequency components and their characteristic mean differences under typical fault conditions. Figure 4 A schematic diagram of a computer device provided in an embodiment of the present invention; Figure 5 This is a block diagram of a chip provided according to an embodiment of the present invention.

[0043] 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

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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)."

[0050] 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.

[0051] This invention provides a protection method for offshore low-frequency wind power transmission lines based on transient characteristics, without altering the positive-sequence control strategy of the grid-side inverter and modular multilevel matrix inverter of the offshore direct-drive wind turbine. During asymmetrical faults, the modular multilevel matrix inverter employs a positive-sequence voltage support and negative-sequence current suppression strategy. The grid-side inverter of the offshore direct-drive wind turbine adopts a positive and negative-sequence current injection strategy to meet the grid specifications for wind turbine ride-through, ensuring the safety of the system and equipment. Fault information within one cycle after the fault can be used to determine the fault, greatly improving the speed of low-frequency wind power transmission line protection. The protection threshold is calculated and set based on system parameters and control parameters, providing a theoretical basis and high reliability.

[0052] Please see Figure 2 The present invention discloses a protection method for offshore low-frequency wind power transmission lines based on transient characteristics, comprising the following steps: S1. Collect three-phase voltage and current from the protection devices on both sides of the line and calculate the voltage surge. If the voltage surge exceeds the activation threshold, activate the protection; otherwise, return and continue collecting signals. Please see Figure 1 Typical offshore low-frequency wind power transmission lines generally use submarine cables, with grid-side converters (GSCs) and modular multilevel matrix converters (M3Cs) for the offshore direct-drive wind turbines connected to both ends. Low-frequency electrical energy is generated directly by the wind turbines, transmitted through the submarine cables, and then converted into industrial frequency electrical energy by the M3C before being transmitted to the main power grid.

[0053] The fault ride-through control strategy for GSCs adopted in this system is shown in equation (1). Wherein, and It is GSCs d shaft and q Reference value for the inner loop of positive sequence current; and It is GSCs d shaft and q Reference value for the inner loop of negative sequence current; This is the maximum current limit value for GSCs; and These are the rated active power and rated current of GSCs, respectively. and These are the per-unit values ​​of the positive and negative sequence voltages at PCC2, respectively. and These are the positive-sequence and negative-sequence current ratio coefficients, respectively. It sets the per-unit value of the negative sequence voltage.

[0054] (1) The control strategy for the low-frequency side of a typical AC-AC inverter used in this system is shown in equation (2). Wherein, and It's from M3C. d shaft and q outer loop reference value for positive sequence voltage; and It's from M3C. d shaft and q Positive sequence current limit value; and It's from M3C. d shaft and q Reference value for the inner loop of negative sequence current; It is the rated voltage on the low-frequency side; It is the maximum bridge arm current of the converter.

[0055] (2) During system operation, the protection devices on both sides of the line continuously collect phase voltages and calculate phase voltage fluctuations. When the voltage fluctuation on any side and in any phase exceeds the protection activation threshold, the protection is activated, as shown in equation (3). T This is the fundamental frequency period of the low-frequency transmission system. To ensure a certain margin, this paper sets... .

[0056] (3) S2. When step S1 is satisfied, negative sequence current is collected in the protection devices on both sides of the line, and the negative sequence current in the three-phase stationary coordinate system is converted to the two-phase rotating coordinate system. When the protection is activated, the three-phase negative sequence current is extracted from the protection devices on both sides of the line, and the negative sequence current is transformed from the three-phase stationary coordinate system to the two-phase rotating coordinate system.

[0057] In a two-phase rotating coordinate system, GSCs and M3C provide q The expressions for the shaft transient fault current are shown in equations (4.1) and (4.2).

[0058] (4.1) (4.1) in, This is the initial value of the negative sequence current on the q-axis of the GSCs; It is the initial phase of the q-axis negative sequence current free component of GSCs; It is the initial phase of the q-axis negative sequence current free component of GSCs; These are the filter inductance parameters of the wind farm; It is the negative sequence current reference value of GSCs in equation (1); The integral coefficient of the inner loop of the q-axis negative sequence current control of GSCs; The proportional coefficient of the inner loop of the q-axis negative sequence current control of GSCs; It is the damping ratio related to the M3C control parameters; This is the initial value of the q-axis negative sequence current of the M3C's y-sub-converter; It is the initial phase of the q-axis negative sequence current free component of M3C; These are the negative order initial conditions for GSCs. The integral coefficient of the inner loop of the q-axis negative sequence current control of M3C; The proportional coefficient of the inner loop of the q-axis negative sequence current control of M3C; These are the parameters of the bridge arm inductance and transformer cable in the M3C; t It's time.

[0059] The q-axis transient fault current provided by GSCs consists of two parts: a forced component and a free component. The amplitude of the free component is much smaller than that of the forced component, and the free component gradually decays to 0 with time. Therefore, for GSCs, the dominant frequency component is the forced component that exhibits DC characteristics, and the expression of the dominant frequency component is shown in equation (5).

[0060] (5) The characteristic mean value of the main frequency component provided by GSCs is obtained, as shown in equation (6). Wherein, N 0= t 0 / T s ; N f = N 0+ n f ; t 0 represents the time when the fault occurred; T s The sampling period; n f To protect the number of samples within the time window.

[0061] (6) The q-axis transient fault current provided by M3C only contains the AC attenuation component related to the fault initial conditions, system parameters, and control parameters. Therefore, for M3C, its dominant frequency component is a free component exhibiting attenuation characteristics, as shown in equation (7).

[0062] (7) The characteristic mean value of the main frequency component provided by M3C is obtained as shown in Equation (8).

[0063] (8) S3. Extract the dominant frequency component of the negative sequence current in the two-phase rotating coordinate system using the adaptive VMD algorithm and energy-based criteria; calculate the characteristic mean of the dominant frequency component of each protection device on both sides; calculate the difference in characteristic mean between the two protection devices. To accurately extract the dominant frequency component, the adaptive variable mode decomposition (VMD) algorithm is used. q The negative sequence current is decomposed into intrinsic mode functions (IMFs) with an optimal preset number of decompositions. VMD solves for the IMFs and their center frequencies iteratively by minimizing the Lagrange multiplier problem. The Lagrange multiplier problem is shown in equation (9), and the update formulas for the IMFs and their center frequencies are shown in equation (10). Indicates protection device R m and R g Measured and calculated q Negative sequence current. express Based on N The improved VMD decomposition obtained by the preset number of decompositions yields the first... k Intrinsic Mode Functions (IMFs); It is the gradient; It is a Dirac distribution; It is a secondary penalty item; Based on preset values N The k The central angular frequency of each sub-mode; Indicates the number of iterations.

[0064] (9) (10) To accurately extract the Intrinsic Functions (IMFs) of negative-sequence currents, the optimal number of submode components needs to be preset, and the IMFs are extracted using an adaptive VMD algorithm. The process of the adaptive VMD algorithm is as follows: First, the relative deviations between the maximum frequencies obtained by VMD decomposition using different preset numbers of IMFs were evaluated. The calculation formula is shown in equation (11). f s It is the sampling rate.

[0065] (11) When the preset number of IMFs is N If equation (12) is satisfied at this time, then it is considered that at this time... N The maximum number of preset IMFs that will not cause excessive decomposition, i.e., the maximum critical number of preset IMFs.

[0066] (12) definition N Among the IMFs, the IMF with the highest energy is the dominant frequency component, denoted as . And its frequency is denoted as The dominant frequency components and their frequencies can be adaptively solved using an energy-based criterion, as shown in (13). yes Energy.

[0067] (13) Secondly, by evaluating the relative deviations between dominant frequencies under different preset numbers of IMFs. The calculation formula is shown in equation (14).

[0068] (14) When the preset number of IMFs is N If equation (15) is satisfied at this time, then it is considered that at this time... N This is the minimum preset number of IMFs, i.e., the minimum critical preset number of IMFs.

[0069] (15) Subsequently, the optimal preset number of IMFs ( N A value is determined to be one that satisfies the following condition: the relative deviation of the maximum frequency is greater than a threshold. And the relative deviation of the dominant frequency is less than the threshold. As shown in formula (16).

[0070] (16) Finally, when equation (16) holds, the optimal preset number of IMFs is:N Then, using equations (9) and (10), the preset quantity is solved iteratively. N The problem of minimizing Lagrange multipliers is used to obtain the results of each protection device. q The first negative sequence current of the axis k Secondary IMF. Reuse formula (13) to select N The IMF with the highest energy among the IMFs is the dominant frequency. and .

[0071] Calculate the mean difference of the main frequency component characteristics between protection devices. As shown in equation (17). k 1 is the protection factor; , and These are the turns ratios of the isolation transformer at the M3C substation, the box transformer at the wind farm, and the main transformer. and They are R m and R g The dominant frequency component provided; and They are and The characteristic mean.

[0072] (17) When an external fault occurs, the protection devices on both sides of the line measure... q The negative sequence current of the shaft is determined by the control target of the single-sided converter, and its characteristic mean difference of the main frequency component is significant. Approximately equal to 0; when a fault occurs within the zone, the protection devices on both sides of the line measure... q The negative sequence currents on both sides are determined by the control targets of different converters, and their characteristic mean differences in the main frequency component are significant. It is a relatively large real number.

[0073] S4. Set the protection action threshold according to the system and control parameters; if the difference in the characteristic mean is greater than the protection action threshold, determine that a fault has occurred within the zone; otherwise, determine that a fault has occurred outside the zone.

[0074] when and hour, minimum value depending on The minimum characteristic mean is called And exist .

[0075] After converting the units using the line reference values... As shown in (18), hereS G,base and S base These are the base power of a single wind turbine and the power line, respectively. N WF It refers to the number of wind turbines in the wind farm.

[0076] (18) If the line reference power is set as the product of the number of wind turbines and the reference power of a single wind turbine, then... The mean difference of the minimum dominant frequency component As shown in equation (19).

[0077] (19) when or hour, Depends on the smallest At this point, the fault severity is high, and the fault current reaches the limit value. Based on the adopted control strategy, the following is obtained: d shaft and q The vector sum of the positive sequence currents is equal to the limiting value, i.e. Furthermore, the vector sum of the positive-sequence current and the negative-sequence current is also equal to the limiting value, i.e. From this, we can deduce the smallest As shown in equation (20). Approximately greater than 0.2247 times .

[0078] (20) because Greater than 1, generally in the range of 1.2 to 2 p.u. It must be greater than 0.2247 pu. Therefore, As shown in equation (21).

[0079] (twenty one) In summary, Depends on the maximum current limit Or it depends on the negative sequence voltage setpoint. With negative sequence current control coefficient The product of is shown in equation (22).

[0080] (twenty two) Therefore, the proposed protection threshold is calculated as follows: ,in k 2 is the reliability coefficient of the protection threshold. The protection criterion when an intra-zone fault occurs is shown in equation (23).

[0081] (twenty three) In another embodiment of the present invention, a protection system for offshore low-frequency wind power transmission lines based on transient characteristics is provided. This system can be used to implement the above-mentioned protection method for offshore low-frequency wind power transmission lines based on transient characteristics. Specifically, the protection system for offshore low-frequency wind power transmission lines based on transient characteristics includes protection devices on both sides of the line and a communication unit.

[0082] The protection devices on both sides of the line respectively include: The data acquisition and activation module is used to acquire three-phase voltage and three-phase current, calculate voltage surges, and activate protection according to activation conditions. The coordinate transformation module is used to extract the negative sequence current and convert it to obtain the q-axis negative sequence current. The dominant frequency extraction module is used to perform adaptive variational mode decomposition on the q-axis negative sequence current and determine the dominant frequency component according to the energy criterion. The feature calculation module is used to calculate the mean value of the main frequency component features and the difference between the mean values ​​of the features on both sides; The criterion execution module is used to compare the characteristic mean difference with the protection action threshold and output the fault determination within / outside the zone and the trip / blocking control. The communication unit is used to transmit feature information for calculating the difference in feature mean between the two protection devices.

[0083] 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 function. The processor described in this embodiment can be used in the operation of a protection method for offshore low-frequency wind power transmission lines based on transient characteristics, including: Three-phase voltage and three-phase current are collected on both sides of the line, and voltage surge is calculated. When the voltage surge of any phase on either side exceeds the activation threshold, protection is activated. After protection is activated, negative sequence current is extracted on both sides of the line, and the negative sequence current is transformed from a three-phase stationary coordinate system to a two-phase rotating coordinate system to obtain the q-axis negative sequence current. Adaptive variational mode decomposition is performed on the q-axis negative sequence current, and the optimal preset mode number is determined based on the frequency relative deviation under different preset mode numbers. The dominant frequency component is determined in the intrinsic mode function obtained by decomposition according to the energy criterion. Within the preset protection time window, the characteristic mean of the dominant frequency component on both sides of the line is calculated. Based on the characteristic mean of the dominant frequency component on both sides and the protection coefficient, the characteristic mean difference of the dominant frequency component on both sides is calculated. The characteristic mean difference is compared with the protection action threshold set according to system parameters and control parameters: when the characteristic mean difference is greater than the protection action threshold, it is determined to be an intra-zone fault and tripping is performed; otherwise, it is determined to be an extra-zone fault.

[0084] Please see Figure 4The 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 transient feature-based offshore low-frequency wind power transmission line protection method described in this embodiment. To avoid repetition, these details are not elaborated here. Alternatively, when executed by the processor 61, the computer program 63 implements the functions of each model / unit in the transient feature-based offshore low-frequency wind power transmission line protection system described in this embodiment. To avoid repetition, these details are not elaborated here.

[0085] 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 4 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.

[0086] 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.

[0087] The memory 62 can be an internal storage unit of the computer device 60, such as a hard disk or memory 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. equipped on the computer device 60.

[0088] 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.

[0089] Please see Figure 5 The 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.

[0090] 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 2 The steps are shown in the figure.

[0091] Storage unit 620 may include readable media in the form of volatile storage units, such as random access memory (RAM) 6201 and / or cache memory 6202, and may further include read-only memory (ROM) 6203.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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 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).

[0098] 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 offshore low-frequency wind power transmission line protection method based on transient characteristics 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: Three-phase voltage and three-phase current are collected on both sides of the line, and voltage surge is calculated. When the voltage surge of any phase on either side exceeds the activation threshold, protection is activated. After protection is activated, negative sequence current is extracted on both sides of the line, and the negative sequence current is transformed from a three-phase stationary coordinate system to a two-phase rotating coordinate system to obtain the q-axis negative sequence current. Adaptive variational mode decomposition is performed on the q-axis negative sequence current, and the optimal preset mode number is determined based on the frequency relative deviation under different preset mode numbers. The dominant frequency component is determined in the intrinsic mode function obtained by decomposition according to the energy criterion. Within the preset protection time window, the characteristic mean of the dominant frequency component on both sides of the line is calculated. Based on the characteristic mean of the dominant frequency component on both sides and the protection coefficient, the characteristic mean difference of the dominant frequency component on both sides is calculated. The characteristic mean difference is compared with the protection action threshold set according to system parameters and control parameters: when the characteristic mean difference is greater than the protection action threshold, it is determined to be an intra-zone fault and tripping is performed; otherwise, it is determined to be an extra-zone fault.

[0099] 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.

[0100] 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.

[0101] in accordance with Figure 1 An electromagnetic transient simulation model of an offshore low-frequency wind power transmission system was built in PSCAD / EMTDC. The system's rated frequency is 50 / 3 Hz. The sampling frequency of the protection device is 10 kHz. The transmission line is a 100 km, 220 kV cross-linked polyethylene insulated cable (cross-sectional area 630 mm²). Protection coefficient k 1=15, sensitivity coefficient k 2 = 1.5. The calculated protection threshold for this simulation test system is... S set =1.

[0102] To verify the performance of the proposed protection method, single-phase B-phase grounding fault, two-phase AB-phase fault, and two-phase ABG-phase grounding fault were set up at different locations on the transmission line with transition resistances of 0 Ω, 50 Ω, and 100 Ω. The protection parameters and judgment results for different fault scenarios are shown in the table below.

[0103] Table 1. Protection Judgment Results under Different Fault Test Conditions for Offshore Low-Frequency Transmission Lines

[0104] As can be seen, the longitudinal protection system for offshore low-frequency wind power transmission lines based on transient negative sequence current proposed in this invention can effectively identify internal and external faults, demonstrating excellent performance under various fault types and locations. Furthermore, this solution can continue to operate even under fault conditions with a transition resistance as high as 100Ω, ensuring the sensitivity and reliability of fault identification.

[0105] To verify the speed of operation of the method of the present invention, a metallic ABG two-phase ground fault was simulated at the midpoint of the transmission line, and the protection device was obtained. R g and R m Calculated main frequency components and like Figure 3 As shown, the horizontal axis represents time (in seconds), and the vertical axis represents the normalized characteristic amplitude and the difference between characteristic means. The figure includes the fault initiation time, the protection time window interval, the attenuation curve of the main frequency component, the characteristic mean curves on both sides, and the characteristic mean difference curve. The effective protection time window is 0-60ms after the fault occurs; characteristic calculation and fault identification can be completed using only this single-cycle transient data. The main frequency component exhibits attenuation characteristics on the M3C side and stable DC characteristics on the wind turbine side, with significant differences between the two sides. The characteristic mean difference remains consistently above 6.0, far exceeding the set protection threshold of 1, clearly indicating an in-zone fault; a trip command is output immediately upon the end of the protection time window. The figure visually demonstrates that this invention only requires a short-term time window to achieve fault identification; the characteristic difference shows no attenuation even under high transition resistance, and the boundary distinction is clear, fully verifying the technical advantages of this invention: excellent speed of operation, strong adaptability to high-resistance faults, and high fault characteristic identification. This invention's protection method uses only data from one cycle after the fault for protection calculations.

[0106] It can be seen that the present invention can make good use of the characteristic information of the transient stage of the fault. In a system with a system frequency of 50 / 3Hz, it is only necessary to collect data for 60ms after the fault to accurately identify the fault in the area. Compared with the protection based on power frequency phasors and steady-state quantities, the method of the present invention has better speed.

[0107] In summary, this invention provides a protection method and system for offshore low-frequency wind power transmission lines based on transient characteristics. It eliminates the need to modify the existing fault sequence control strategies of the grid-side inverters and M3C frequency converters of offshore direct-drive wind turbines, ensuring full compatibility with existing equipment operating logic. The system employs an adaptive VMD algorithm to automatically match the optimal decomposition mode, eliminating reliance on manual experience and accurately extracting the transient main frequency characteristics of the fault. Protection thresholds are theoretically set based on the system's electrical and control parameters, abandoning traditional trial-and-error methods, resulting in rigorous and reliable threshold setting. Fault identification can be completed using only a single low-frequency cycle transient information after a fault, with an action delay as low as 60ms, significantly outperforming traditional steady-state quantity protection. It is adaptable to three typical asymmetrical fault types: single-phase grounding, two-phase, and two-phase grounding, and can withstand a maximum transition resistance of 100Ω. It reliably trips faults within the protection zone and strictly blocks faults outside the zone, exhibiting strong anti-interference capabilities and high identification accuracy. Its engineering adaptability and operational reliability are greatly improved, making it highly valuable for offshore low-frequency wind power engineering applications.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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 of the present invention 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, a recording medium, a USB flash drive, a portable hard drive, a magnetic disk, an optical disk, a computer memory, a read-only memory (ROM), a random-access memory (RAM), an electrical carrier signal, a telecommunication signal, and a software distribution medium, etc.

[0115] 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.

[0116] 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 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0117] 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.

[0118] 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 method for offshore low frequency wind farm transmission line protection based on transient characteristics, characterized in that, Includes the following steps: S1. Collect three-phase voltage and three-phase current on both sides of the line respectively, and calculate the voltage change. When the voltage change of any phase on any side is greater than the start threshold, the protection is activated. S2. After the protection is started, the negative sequence current is extracted from both sides of the line, and the negative sequence current is transformed from the three-phase stationary coordinate system to the two-phase rotating coordinate system to obtain the q-axis negative sequence current. S3. Perform adaptive variational mode decomposition on the q-axis negative sequence current, determine the optimal preset mode number based on the frequency relative deviation under different preset mode numbers, and determine the dominant frequency component in the decomposed intrinsic mode function according to the energy criterion. S4. Within the preset protection time window, calculate the characteristic mean value of the main frequency components on both sides of the line respectively; S5. Calculate the difference in characteristic mean values ​​of the two main frequency components based on the characteristic mean values ​​and protection coefficients of the two main frequency components. S6. Compare the mean difference of the features with the protection action threshold set according to the system parameters and control parameters: when the mean difference of the features is greater than the protection action threshold, it is determined to be an intra-zone fault and tripping is performed; otherwise, it is determined to be an extra-zone fault.

2. The method for protecting offshore low-frequency wind power transmission lines based on transient characteristics according to claim 1, characterized in that, In step S1, the calculation of voltage surge and the protection activation conditions are as follows: in, For the first Phase voltage sudden change For the first The sampling voltage at phase time t, The threshold for triggering voltage surges. T It is the fundamental frequency period of a low-frequency power transmission system.

3. The method for protecting offshore low-frequency wind power transmission lines based on transient characteristics according to claim 1, characterized in that, In step S3, the adaptive variational mode decomposition includes: The maximum relative deviation of frequency components between decomposed signals based on different preset numbers of IMFs is: in, For the first The first preset mode The frequency of the first-order eigenmode function. For the first The first preset mode The central angular frequency of the first-order eigenmode function. The system sampling frequency, The preset number of modes is The maximum frequency obtained by time decomposition The preset number of modes is The maximum frequency obtained by time decomposition This represents the maximum relative frequency deviation. The frequency deviation of the main frequency component between the decomposed signals based on different preset numbers of IMFs is: in, Preset number of modes The corresponding dominant frequency, Preset number of modes The corresponding dominant frequency; The criterion for calculating the optimal preset number of decompositions is: in, The relative deviation between the main frequency under different preset IMFs numbers. The threshold for setting the relative deviation of the dominant frequency. The threshold is set for the maximum relative frequency deviation.

4. The method for protecting offshore low-frequency wind power transmission lines based on transient characteristics according to claim 1 or 3, characterized in that, The dominant frequency component is determined based on the energy criterion, specifically as follows: in, For the first Preset mode side Axis No. Energy of each eigenmode function For the first Preset mode side Axis No. One negative sequence current component. for side The dominant frequency component of the negative sequence current. This is the maximum time limit for scoring. This is the lower limit of the integration time.

5. The method for protecting offshore low-frequency wind power transmission lines based on transient characteristics according to claim 1, characterized in that, In step S4, the mean values ​​of the dominant frequency components on both sides of the line are calculated as follows: in, For the wind farm side Characteristic mean of the dominant frequency component of the negative sequence current. This is the maximum current limit for the negative sequence current on the wind farm side. This represents the negative sequence current control coefficient on the wind farm side. This is the reference value for the negative sequence voltage on the wind farm side. The rated current of the system. Set a per-unit value for the negative sequence voltage on the wind farm side. For M3C side Characteristic mean of the dominant frequency component of the negative sequence current. To protect the number of sampling points within the time window, To protect the sampling sequence number at the end of the time window, The starting sampling sequence number at the time the fault occurred. This is the identifier for the u / v / w sub-converter of M3C. Initial time Sub-converter negative sequence current, The integral coefficients of the inner loop of the q-axis negative sequence current control of the M3C are given. For M3C side Shaft damping ratio parameter, The equivalent inductance parameters are for the M3C side. For discrete-time variables, For M3C side The initial phase of the free component of the negative sequence current. This is the proportional coefficient of the inner loop of the q-axis negative sequence current control of the M3C.

6. The method for protecting offshore low-frequency wind power transmission lines based on transient characteristics according to claim 1, characterized in that, In step S5, the difference in feature means The calculation is as follows: in, To protect the setting coefficient, This is the maximum current limit for the negative sequence current on the wind farm side. For the transformer ratio of the wind farm box, The transformer ratio of the main transformer in the wind farm box. This represents the negative sequence current control coefficient on the wind farm side. This is the reference value for the negative sequence voltage on the wind farm side. The rated current of the system. This represents the per-unit value of the negative sequence voltage on the wind farm side. Set a per-unit value for the negative sequence voltage on the wind farm side.

7. The method for protecting offshore low-frequency wind power transmission lines based on transient characteristics according to claim 1, characterized in that, The protection action threshold is set according to the system parameters and control parameters, and the set protection action threshold is corrected using a reliability coefficient.

8. The method for protecting offshore low-frequency wind power transmission lines based on transient characteristics according to claim 7, characterized in that, In the threshold tuning, the mean difference of the minimum dominant frequency component is determined. for: in, To protect the setting coefficient, This represents the negative sequence current control coefficient on the wind farm side. Set a per-unit value for the negative sequence voltage on the wind farm side; the protection criterion for when an intra-zone fault occurs is... .

9. The method for protecting offshore low-frequency wind power transmission lines based on transient characteristics according to claim 1, characterized in that, The method is used to identify single-phase grounding faults, two-phase faults, and two-phase grounding faults in offshore low-frequency wind power transmission lines, and outputs tripping commands when the fault is within the zone and blocking commands when the fault is outside the zone.

10. A protection system for offshore low-frequency wind power transmission lines based on transient characteristics, characterized in that, Includes protection devices on both sides of the line and communication units; The protection devices on both sides of the line respectively include: The data acquisition and activation module is used to acquire three-phase voltage and three-phase current, calculate voltage surges, and activate protection according to activation conditions. The coordinate transformation module is used to extract the negative sequence current and convert it to obtain the q-axis negative sequence current. The dominant frequency extraction module is used to perform adaptive variational mode decomposition on the q-axis negative sequence current and determine the dominant frequency component according to the energy criterion. The feature calculation module is used to calculate the mean value of the main frequency component features and the difference between the mean values ​​of the features on both sides; The criterion execution module is used to compare the characteristic mean difference with the protection action threshold and output the fault determination within / outside the zone and the trip / blocking control. The communication unit is used to transmit feature information for calculating the difference in feature mean between the two protection devices; wherein the system is configured to perform the method according to any one of claims 1-9.

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