Longitudinal power protection method and device

Abstract

The invention relates to a pilot power protection method and device, and relates to the technical field of power system protection, and the method comprises the steps: constructing a Bergeron equivalent circuit model of a DC line according to a system topological structure, and building a voltage and current deduction matrix from a measurement point to a reference point of each converter station; performing time-frequency analysis on a typical fault by adopting S transformation, and determining an optimal analysis frequency by comprehensively considering an amplitude accumulation index and a stability index; collecting voltage and current information of each converter station in real time, deducing the voltage and current information to a reference point through a deduction matrix after phase-mode transformation, extracting voltage and current phasors under a specific frequency by using recursion DFT, and calculating a power phasor; and constructing a longitudinal power actuating quantity and a longitudinal power braking quantity based on the law of conservation of power, establishing a longitudinal power protection criterion, and sending out a tripping instruction when the criterion is met. According to the invention, the internal fault of the three-terminal DC system can be effectively identified, and an effective protection solution is provided for the three-terminal distributed diode rectification delivery system.

Description

Technical Field

[0001] This invention relates to the field of power system protection technology, and in particular to a longitudinal power protection method and device. Background Technology

[0002] With the deepening of my country's energy transition strategy, new energy power generation technology has experienced rapid development, and the installed capacity of renewable energy has continued to climb. However, the distribution of new energy resources in my country exhibits significant regional characteristics, with a spatial mismatch between resource-rich areas and load-concentrated areas. This manifests as a structural contradiction: insufficient absorption capacity in resource-rich areas, while energy supply is relatively scarce in load-center areas. This situation severely restricts the efficient utilization and large-scale development of new energy. To solve the problem of renewable energy absorption, the technology of transmitting new energy power through three-terminal distributed diode rectification has emerged, providing an economically feasible technical path for medium-distance, provincial-level transmission of new energy. This technology constructs an independent islanded power grid by deploying a grid-type energy storage system, enabling autonomous grid operation of new energy. Combined with distributed diode rectification technology, it achieves efficient power transmission, effectively alleviating the geographical mismatch between new energy absorption and load demand.

[0003] In the field of power system relay protection, various technical methods have traditionally been employed to address fault protection issues in DC systems. For DC line protection, common methods include main protection devices based on the traveling wave principle and the differential undervoltage principle. Protection devices based on the traveling wave principle utilize the traveling wave signal generated during a fault to determine the fault location and type; protection devices based on the differential undervoltage principle detect faults by monitoring the rate and amplitude of voltage change. Additionally, longitudinal protection of traditional two-terminal systems is also a common method, which determines the fault condition by comparing the electrical quantities between two measuring terminals. Furthermore, in acquiring fault data, fault waveform data from the converter station monitoring devices in the system is collected, or relevant data is obtained through simulation. For data analysis, conventional transformation methods are also used to process the data.

[0004] However, the fault mechanism of three-terminal distributed diode rectifier transmission systems is complex, and its fault characteristics differ significantly from those of traditional two-terminal DC systems. Main protection devices based on the traveling wave principle and differential undervoltage principle experience a significant decrease in sensitivity when facing high-resistance faults. This is because the traveling wave characteristics are weak during high-resistance faults, and the traveling wave signal undergoes further energy attenuation due to complex reflections and refractions at the line T-junction, affecting fault detection sensitivity. The impact of renewable energy control strategies on system protection is even more pronounced. The sending end of this system typically connects to a very high proportion of renewable energy generation units, and the diode rectifier itself lacks active control and regulation capabilities. This allows transient fault processes to be rapidly propagated into the DC system, leading to blurred fault characteristics both inside and outside the fault zone, increasing the complexity of fault identification. Furthermore, the adverse effects of synchronization errors on longitudinal protection are further exacerbated. Traditional two-terminal systems only need to consider clock synchronization errors between two measurement terminals, while three-terminal distributed systems require precise time synchronization between multiple measurement points. The accumulation of synchronization errors threatens the reliability of protection criteria. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a longitudinal power protection method and device, which aims to solve at least one of the above-mentioned technical problems.

[0006] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: Firstly, this application provides a longitudinal power protection method, which adopts the following technical solution: A longitudinal power protection method, comprising: Based on the line topology of the three-terminal distributed diode rectifier transmission system, a Berylon equivalent circuit model of the DC line is constructed, and the line intersection point of the Berylon equivalent circuit model of the DC line is selected as the reference point. Voltage and current extrapolation matrices from the measurement points of each swapping station to the reference point are established. S-transform time-frequency analysis was performed on multiple typical fault data, and the optimal analysis frequency was determined based on the frequency selection principle and the typical fault data after time-frequency analysis. Real-time acquisition of voltage and current information at each battery swapping station measurement point of the three-terminal distributed diode rectifier transmission system; and based on the voltage and current extrapolation matrix, uniform extrapolation of the voltage and current information at each battery swapping station measurement point to a reference point to obtain extrapolated voltage and current information. The amplitude and phase of the derived voltage and current information at the optimal analysis frequency are extracted based on the recursive DFT algorithm to obtain the voltage and current phasors. Based on the voltage and current phasors, the power phasors of each converter station are calculated. Based on the power phasor of each of the aforementioned battery swapping stations, the longitudinal power action quantity and longitudinal power braking quantity are determined. Based on the longitudinal power action quantity, longitudinal power braking quantity, set proportional braking coefficient, reliability coefficient, and action setpoint, a longitudinal power protection criterion is established. Based on the longitudinal power protection criterion, the protection device is controlled to protect the longitudinal power of the three-terminal distributed diode rectifier transmission system.

[0007] The beneficial effects of this invention are as follows: By constructing a Berylon equivalent circuit model of a DC line and establishing a voltage and current extrapolation matrix, accurate modeling of the complex network topology of a three-terminal system can be achieved. Measurement information from each converter station can be uniformly extrapolated to a reference point for comparison and analysis, effectively eliminating errors caused by ground capacitance current. Therefore, there is no need to set a delay to avoid system transient processes, improving the action speed of longitudinal protection. S-transform time-frequency analysis of typical fault data determines the optimal analysis frequency. Combined with the recursive DFT algorithm, voltage and current phasor information at specific frequencies is extracted, effectively immunizing the adverse effects of synchronization errors on protection criteria in three-terminal distributed diode rectifier transmission systems, compensating for the shortcomings of traditional longitudinal protection methods with excessively high synchronization requirements in three-terminal applications. By establishing protection criteria for longitudinal power action and braking quantities, the power balance relationship of the three-terminal distributed diode rectifier transmission system can be accurately reflected, faults can be reliably distinguished, and the difficulty in fault identification caused by the high proportion of new energy access and the uncontrolled characteristics of diode rectifiers can be effectively solved, thereby effectively protecting the longitudinal power of the three-terminal distributed diode rectifier transmission system.

[0008] Based on the above technical solution, the present invention can be further improved as follows.

[0009] Furthermore, the construction of the equivalent circuit model of the DC line Berylon based on the line topology of the three-terminal distributed diode rectifier transmission system includes: Based on the line topology of the three-terminal distributed diode rectifier transmission system, multiple uniform lossy DC transmission lines are determined. Each uniformly lossy DC transmission line segment is divided into two parts. The divided uniformly lossy DC transmission lines are placed on both sides of the resistance and conductance lossless lines. Based on the line unit resistance, unit conductance, and line wave impedance, a Berylon equivalent circuit model of the DC line is constructed.

[0010] The beneficial effects of adopting the above-mentioned further scheme are as follows: Based on the line topology, a uniform lossy DC transmission line is determined, divided, and set up. Combined with the line unit resistance, unit conductance, and line wave impedance, a Berylon equivalent circuit model of the DC line is constructed. This enables accurate modeling of the complex network topology of the three-terminal system. The measurement information of each converter station is uniformly extrapolated to the reference point for comparison and analysis, thereby effectively eliminating the error caused by the ground capacitance current.

[0011] Furthermore, establishing the voltage and current extrapolation matrix from each battery swapping station measurement point to the reference point includes: Based on the line wave impedance, line resistance, line conductance, wave propagation time from the converter station to the reference point, delay factor, the Berylon equivalent circuit model of DC line and transmission line theory corresponding to each converter station, a voltage and current derivation matrix is ​​constructed. The expression for the voltage-current derivation matrix is: ; in, ; ; ; ; ; In the formula, Here, Ri = rli is the line impedance, Gi = gli is the line resistance, and gli is the line conductance. The time it takes for a wave to travel from the converter station to the reference point. is the delay factor, and i represents each battery swapping station.

[0012] The beneficial effect of adopting the above-mentioned further scheme is that, based on the relevant parameters of each battery swapping station and the theoretical construction of voltage and current derivation matrices, accurate modeling of the complex network topology of the three-terminal system can be achieved.

[0013] Furthermore, the typical fault data includes faults within the forward region, faults outside the reverse region, or faults outside the forward region. The step of performing S-transform time-frequency analysis on the typical fault data and determining the optimal analysis frequency based on the frequency selection principle and the typical fault data after time-frequency analysis includes: For any typical fault data, perform phase-mode transformation on the typical fault data to obtain the line-mode voltage and line-mode current at the fault time. For any typical fault data, based on the preset S-transform algorithm, S-transform time-frequency analysis is performed on the line-mode voltage and line-mode current at the fault time to obtain the time-frequency matrix of the voltage and the time-frequency matrix of the current at the fault time. Based on the sampling theorem, an upper limit for the analysis frequency is set, and the number of sampling points corresponding to each period of the analysis frequency is determined to be a positive integer, thus obtaining a candidate set of integers; For any typical fault data, the voltage time-frequency matrix and the current time-frequency matrix at fault are moduloed to obtain the modulo voltage matrix and the modulo current matrix, respectively. For any typical fault data, the magnitude product matrix is ​​determined based on the product of the magnitude voltage matrix and the magnitude current matrix; For any typical fault data, calculate the cumulative amplitude index and the stability index based on the amplitude product matrix, and determine the comprehensive index based on the cumulative amplitude index and the stability index. Based on the frequency corresponding to the maximum comprehensive index, calculate the difference between the frequency corresponding to each integer in the candidate set of integers and the frequency corresponding to the maximum comprehensive index, and select the integer with the smallest difference as the target integer; The optimal analysis frequency is determined based on the target integer and the set sampling frequency.

[0014] The beneficial effects of adopting the above-mentioned further scheme are as follows: S-transform is used to perform time-frequency analysis on multi-type fault data, and a frequency selection principle that integrates the cumulative amplitude index and the stability index is proposed to determine the optimal analysis frequency, thereby enhancing the ability to extract fault features. On this basis, the voltage and current phasors at a specific frequency are extracted in real time by combining recursive DFT, and a longitudinal power criterion with proportional braking characteristics is constructed based on the power conservation law, which can accurately and quickly distinguish between internal and external faults in the region.

[0015] Furthermore, the preset S-transform algorithm is as follows: ; ; ; Where x(k) is the input signal, i.e., the line-mode voltage and line-mode current during the fault; N=2000 represents the number of sampling points for each group of input signals; m and n are both integers, representing the number of rows and columns of the matrix obtained after S-transformation; m satisfies 0≤m≤(N / 2+1) corresponding to the frequency mfs / N; n satisfies 1≤n≤N corresponding to the sampling point timing; S T The result after S-transformation is the voltage time-frequency matrix S at fault time. T(u,type) The fault current time-frequency matrix S T(i,type) , where type represents different fault types.

[0016] The beneficial effects of adopting the above-mentioned further scheme are: by using the preset S-transform algorithm to perform time-frequency analysis on the line-mode voltage and line-mode current during the fault, the time-frequency matrix of the voltage and the time-frequency matrix of the current during the fault can be obtained, which provides a data basis for determining the optimal analysis frequency based on the frequency selection principle.

[0017] Furthermore, the step of uniformly extrapolating the voltage and current information of each measurement point at the battery swapping station to a reference point based on the voltage and current extrapolation matrix to obtain extrapolated voltage and current information includes: Based on the preset phase mode transformation formula, the voltage and current information of each of the measured points of the power exchange station are transformed to obtain the line mode voltage component and line mode current component of each substation. Based on the voltage and current extrapolation matrix and the preset extrapolation formula, the line-mode voltage components and line-mode current components of each converter station measurement point are extrapolated to the reference point to obtain the extrapolated voltage and current information, which includes the line-mode voltage components and line-mode current components of the reference point. The pre-defined derivation formula is as follows: ; in, , These are the line-mode voltage component and line-mode current component at the reference point, respectively, T i For voltage and current derivation matrices, u li(t) For the line-mode voltage component, i li(t) This represents the linear current component.

[0018] The beneficial effects of adopting the above-mentioned further scheme are as follows: By performing phase-mode transformation on the voltage and current information of each converter station measurement point, the electrical quantities of the multi-terminal system can be converted into line-mode components that are easy to analyze; by using the voltage and current derivation matrix and preset derivation formula to deduce the line-mode voltage and current components to the reference point, accurate modeling of the complex network topology of the three-terminal system can be achieved, and the measurement information of each converter station can be unified to the reference point for comparison and analysis, effectively eliminating the error caused by the ground capacitance current, and laying the foundation for subsequent accurate calculation of power phasors and establishment of protection criteria.

[0019] Furthermore, the recursive DFT algorithm is as follows: ; ; ; ; Where N = fs / fa represents the number of sampling points for one period corresponding to the optimal analysis frequency, and k ≥ 0 is an integer. , Here, K represents the line-mode voltage phasor and line-mode current phasor at the reference point, respectively. K is an integer greater than or equal to 0, fs is the sampling frequency, and fa is the optimal analysis frequency.

[0020] The beneficial effects of adopting the above-mentioned further scheme are: it can extract the frequency domain information of line mode voltage and line mode current at the reference point under the optimal analysis frequency in real time, and combined with the optimal analysis frequency determined by S-transform, it can effectively immunize the adverse effects of synchronization error in the three-terminal system on the protection criteria, and make up for the shortcomings of the traditional longitudinal protection method in the application of three terminals with excessively high synchronization requirements.

[0021] Furthermore, based on the power phasors of each of the aforementioned battery swapping stations, the longitudinal power action quantity and longitudinal power braking quantity are determined. Based on the longitudinal power action quantity, longitudinal power braking quantity, set proportional braking coefficient, reliability coefficient, and action setpoint, a longitudinal power protection criterion is established, including: Based on the law of power conservation, the vector sum of the power phasors of all converter stations is calculated, and the modulus of the vector sum is taken to obtain the longitudinal power action quantity; Based on the magnitude of the power phasor of each battery swapping station, determine the longitudinal power braking amount; Based on the longitudinal power action quantity, longitudinal power braking quantity, set proportional braking coefficient, reliability coefficient and action set value, establish longitudinal power protection criteria; The longitudinal power protection criterion is as follows: If the longitudinal power operating amount is greater than the product of the set proportional braking coefficient and the longitudinal power braking amount, and if the longitudinal power operating amount is greater than the product of the reliability coefficient and the operating setting value, then the longitudinal power protection criterion is satisfied.

[0022] The beneficial effects of adopting the above-mentioned further scheme are as follows: Based on the law of power conservation, the longitudinal power action quantity and braking quantity are constructed, which can accurately reflect the power balance relationship of the three-terminal system, reliably distinguish between internal and external faults, and effectively solve the problem of difficult fault identification caused by the high proportion of new energy access and the uncontrolled characteristics of diode rectifiers; at the same time, by setting the proportional braking coefficient, reliability coefficient and action setpoint, and establishing the longitudinal power protection criterion, the reliability of the protection can be improved, and false operation can be avoided when there are normal load changes or measurement errors.

[0023] Furthermore, the control and protection device based on the longitudinal power protection criterion includes: When the longitudinal power protection criterion is met, the control and protection device will trip and disconnect the faulty line; When the longitudinal power protection criterion is not met, the control and protection device remains inactive.

[0024] The beneficial effects of adopting the above-mentioned further scheme are: when the longitudinal power protection criterion is met, the protection device can be controlled to trip and disconnect the faulty line; when the criterion is not met, the protection device can be controlled not to operate. It can reliably distinguish between faults inside and outside the three-terminal distributed diode rectifier power transmission system, and ensure the safe and stable operation of the system under different operating conditions.

[0025] Secondly, this application provides a longitudinal power protection device, which adopts the following technical solution: A longitudinal power protection device, comprising: The voltage and current derivation matrix establishment module is used to construct the equivalent circuit model of the DC line Berylon based on the line topology of the three-terminal distributed diode rectifier transmission system, and select the line intersection point of the equivalent circuit model of the DC line Berylon as the reference point to establish the voltage and current derivation matrix from the measurement point of each swapping station to the reference point. The optimal analysis frequency establishment module is used to perform S-transform time-frequency analysis on typical fault data, and determine the optimal analysis frequency based on the frequency selection principle and the typical fault data after time-frequency analysis. The acquisition module is used to acquire the voltage and current information of each measurement point of the three-terminal distributed diode rectifier transmission system in real time, and to uniformly extrapolate the voltage and current information of each measurement point of the three-terminal distributed diode rectifier transmission system to the reference point based on the voltage and current extrapolation matrix, so as to obtain the extrapolated voltage and current information. The calculation module is used to extract the magnitude and phase of the derived voltage and current at the optimal analysis frequency based on the recursive DFT algorithm, obtain the voltage and current phasors, and calculate the power phasors of each converter station based on the voltage and current phasors. The protection module is used to determine the longitudinal power action quantity and braking quantity based on the power phasor of each of the aforementioned power swapping stations, and to establish a longitudinal power protection criterion based on the longitudinal power action quantity, braking quantity, set proportional braking coefficient, reliability coefficient, and action set value. Based on the longitudinal power protection criterion, the protection device is controlled to protect the longitudinal power of the three-terminal distributed diode rectifier transmission system.

[0026] Additional aspects and advantages of this application will be set forth in part in the description which follows, and will become apparent from the description or may be learned by practice of this application.

[0027] In summary, the beneficial effects of this invention are: By utilizing the voltage and current derivation matrix based on the Berylone equivalent circuit model proposed in this invention, accurate modeling of the complex network topology of a three-terminal system can be achieved. Measurement information from each converter station can be uniformly deduced to a reference point for comparison and analysis, effectively eliminating errors caused by ground capacitance current. This invention determines the optimal analysis frequency through S-transform time-frequency analysis technology and extracts voltage and current phasor information at specific frequencies using recursive DFT. This effectively mitigates the adverse effects of synchronization errors in the three-terminal system on protection criteria, compensating for the excessively high synchronization requirements of traditional longitudinal protection methods in three-terminal applications. Based on the power conservation law, the longitudinal power action and braking quantities constructed in this invention accurately reflect the power balance relationship of the three-terminal system, reliably distinguishing between internal and external faults, and effectively solving the difficulties in fault identification caused by the high proportion of new energy access and the uncontrolled characteristics of diode rectifiers. Attached Figure Description

[0028] Figure 1A schematic flowchart of a longitudinal power protection method provided in one embodiment of the present invention; Figure 2 This is another schematic flowchart of a longitudinal power protection method provided in one embodiment of the present invention; Figure 3 This is a schematic diagram of a three-terminal distributed diode rectifier power transmission system provided in one embodiment of the present invention; Figure 4 A schematic diagram of a three-terminal DC line Beryllon equivalent circuit model provided in an embodiment of the present invention; Figure 5 This is a typical fault voltage and current waveform diagram of converter station B provided in an embodiment of the present invention; Figure 6 This is a typical fault mode voltage waveform diagram of converter station B line under a specific fault condition, provided in one embodiment of the present invention. Figure 7 This is a schematic diagram of the S-transform result under a typical fault, provided in an embodiment of the present invention. Figure 8 A schematic diagram of the indicators used in frequency selection calculation according to one embodiment of the present invention; Figure 9 A voltage and current waveform diagram during an intra-zone fault is provided in one embodiment of the present invention; Figure 10 This invention provides a diagram of line-mode voltage and current phasor waveforms during an intra-regional fault, as shown in one embodiment of the invention. Figure 11 A waveform diagram of longitudinal power action and braking amount during an intra-zone fault is provided in one embodiment of the present invention; Figure 12 This is a voltage and current waveform diagram for a fault outside the positive region provided in one embodiment of the present invention; Figure 13 This is a diagram of line-mode voltage and current phasor waveforms during a fault outside the positive region, provided as an embodiment of the present invention. Figure 14 This is a waveform diagram of longitudinal coupling power action and braking amount during a fault outside the positive zone, provided in one embodiment of the present invention; Figure 15 This is a schematic diagram of the structure of a longitudinal power protection device for a three-terminal distributed diode rectifier power transmission system, provided as an embodiment of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0030] Furthermore, the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0031] This application provides a longitudinal power protection method, which can be executed by an electronic device, which can be a server or a mobile terminal device. The server can be an independent physical server, a server cluster or distributed system composed of multiple physical servers, or a cloud server that provides cloud computing services. The mobile terminal device can be a laptop computer, a desktop computer, etc., but is not limited to these.

[0032] like Figure 1 and Figure 2 As shown, a longitudinal power protection method mainly includes: S1. Based on the line topology of the three-terminal distributed diode rectifier transmission system, construct the equivalent circuit model of the DC line Berylon, and select the line intersection point of the equivalent circuit model of the DC line Berylon as the reference point to establish the voltage and current derivation matrix from the measurement point of each swapping station to the reference point. In this embodiment of the application, the construction of the equivalent circuit model of the DC line Berylon based on the line topology of the three-terminal distributed diode rectifier transmission system includes: Based on the line topology of the three-terminal distributed diode rectifier transmission system, multiple uniform lossy DC transmission lines are determined. Each uniformly lossy DC transmission line segment is divided into two parts. The divided uniformly lossy DC transmission lines are placed on both sides of the resistance and conductance lossless lines. Based on the line unit resistance, unit conductance, and line wave impedance, a Berylon equivalent circuit model of the DC line is constructed.

[0033] In this embodiment of the application, firstly, the line topology of the three-terminal distributed diode rectifier transmission system is obtained, and then... Figure 3Taking the three-terminal system shown as an example, a symmetrical single-pole connection is adopted, which includes converter station A, converter station B and converter station C. The three stations are connected by DC lines to form a three-terminal network topology. Converter station A and converter station B use diode uncontrolled rectifier converters and are the sending-end stations, while converter station C uses an LCC converter and is the receiving-end station.

[0034] Secondly, construct the equivalent circuit model of the DC line using the Berylon circuit. For example... Figure 4 As shown, each uniform lossy DC transmission line is divided into two parts, with the resistance and conductance concentrated on both sides of the lossless transmission line in each part. This yields the equivalent circuit model of the DC line of the three-terminal distributed diode rectifier transmission system.

[0035] Figure 4 In the diagram, r represents the unit resistance, g represents the unit conductance (obtained by measurement), lA, lB, and lC represent the line lengths (lA = 15km, lB = 13km, lC = 146km), and P represents the line intersection.

[0036] In this embodiment of the application, the line intersection point P is selected as the reference point in the three-terminal distributed diode rectifier power transmission system.

[0037] In this embodiment of the application, a voltage and current extrapolation matrix from each battery swapping station measurement point to the reference point is established, including: Based on the line wave impedance, line resistance, line conductance, wave propagation time from the converter station to the reference point, delay factor, the Berylon equivalent circuit model of DC line and transmission line theory corresponding to each converter station, a voltage and current derivation matrix is ​​constructed. The expression for the voltage-current derivation matrix is: ; in, ; ; ; ; ; In the formula, Here, Ri = rli is the line impedance, Gi = gli is the line resistance, and gli is the line conductance. The time it takes for a wave to travel from the converter station to the reference point. is the delay factor, and i represents each battery swapping station.

[0038] The following simulation experiment using PSCAD / EMTDC simulation software illustrates a longitudinal power protection method proposed in this paper.

[0039] The specific parameters of the model are as follows: Based on the constructed simulation model of the three-terminal DC transmission system, the parameters of each converter station are set as follows: Sending-end stations A and B both use 12-pulse diode rectifiers with a rated DC voltage of ±200kV and a rated power of 300MW; receiving-end station C uses a 12-pulse LCC inverter with a rated DC voltage of ±200kV, a rated power of 600MW, and a minimum shutdown angle of 15°. The DC line adopts an overhead line model with a unit resistance of 0.02Ω / km, a unit inductance of 1.2mH / km, and a unit capacitance of 0.015μF / km. The AC system of each station is at a voltage level of 220kV. The sampling frequency of the longitudinal power protection system is set to 10kHz, and the power protection start value is set to 15% of the rated power. The simulation step size is set to 50μs, and the total simulation time is 4s. Different types of faults are set at t=2s to verify the effectiveness of the proposed protection method.

[0040] Based on the expression for the voltage-current derivation matrix, the derivation matrix T is calculated. i They are respectively: ; ; .

[0041] S2, performs S-transform time-frequency analysis on multiple typical fault data, and determines the optimal analysis frequency based on the frequency selection principle and the typical fault data after time-frequency analysis; In this embodiment of the application, typical fault data are faults within the forward region, faults outside the reverse region, or faults outside the forward region.

[0042] Specifically, S2 includes the following sub-steps: S21, For any typical fault data, perform phase-mode transformation on the typical fault data to obtain the line-mode voltage and line-mode current at the fault time; S22. For any typical fault data, based on the preset S-transform algorithm, perform S-transform time-frequency analysis on the line-mode voltage and line-mode current at the fault time to obtain the time-frequency matrix of the voltage and the time-frequency matrix of the current at the fault time. S23. Based on the sampling theorem, an upper limit for the analysis frequency is set, and the number of sampling points corresponding to each cycle of the analysis frequency is determined to be a positive integer, thus obtaining a candidate set of integers. S24. For any typical fault data, take the modulus of the fault voltage time-frequency matrix and the fault current time-frequency matrix to obtain the modulus voltage matrix and the modulus current matrix. S25. For any typical fault data, determine the magnitude product matrix based on the product of the magnitude voltage matrix and the magnitude current matrix. S26. For any typical fault data, calculate the cumulative amplitude index and the stability index based on the amplitude product matrix, and determine the comprehensive index based on the cumulative amplitude index and the stability index. S27. Based on the frequency corresponding to the maximum comprehensive index, calculate the difference between the frequency corresponding to each integer in the candidate set of integers and the frequency corresponding to the maximum comprehensive index, and select the integer with the smallest difference as the target integer. S28, determine the optimal analysis frequency based on the target integer and the set sampling frequency.

[0043] In this embodiment of the application, the preset S-transform algorithm is: ; ; ; Where x(k) is the input signal, i.e., the line-mode voltage and line-mode current during the fault; N=2000 represents the number of sampling points for each group of input signals; m and n are both integers, representing the number of rows and columns of the matrix obtained after S-transformation; m satisfies 0≤m≤(N / 2+1) corresponding to the frequency mfs / N; n satisfies 1≤n≤N corresponding to the sampling point timing; S T The result after S-transformation is the voltage time-frequency matrix S at fault time. T(u,type) The fault current time-frequency matrix S T(i,type) , where type represents different fault types.

[0044] In the above implementation, typical fault data is first acquired. Fault waveform data or simulation data of faults within the forward zone, outside the reverse zone, and outside the forward zone of a converter station monitoring device in a three-terminal distributed diode rectifier power transmission system are collected. The sampling frequency is set to 10kHz, and voltage and current data are recorded 100ms before and after the fault to obtain typical fault data u. (p,type) (t), u (n,type) (t), i (p,type) (t), i (n,type) (t), where number 1 represents a fault within the forward region, number 2 represents a fault outside the reverse region, and number 3 represents a fault outside the forward region, i.e., a fault within the forward region u (p,1) (t), u (n,1) (t), i (p,1) (t), i (n,1) (t), reverse region external fault u (p,2) (t), u (n,2) (t), i (p,2) (t), i (n,2) (t), positive out-of-region fault u (p,3) (t), u (n,3) (t), i (p,3)(t), i (n,3) (t).

[0045] Then, typical fault data u (p,type) (t), u (n,type) (t), i (p,type) (t), i (n,type) (t) Perform phase-mode transformation to obtain the line-mode voltage u during the fault. (l,type) (t) and the line-mode current i at fault (l,type) (t), tpye=1, 2, 3, representing different fault types.

[0046] The optimal analysis frequency fa is then determined based on the frequency selection principle. Because voltage and current change abruptly during a fault, exhibiting step characteristics, it possesses full-frequency domain information. The optimal analysis frequency fa is determined by the following frequency selection principle: (1) According to the sampling theorem, the upper limit of the optimal analysis frequency should be less than half of the sampling frequency fs. In the embodiments of this application, fa ≤ 5kHz.

[0047] (2) The number of sampling points corresponding to each cycle of the analysis frequency fa is an integer, i.e., fs / fa = K, where K is a positive integer and K≤N.

[0048] (3) Calculate the cumulative amplitude index ρ according to the principle that the amount of motion is large enough. m The fault voltage time-frequency matrix S obtained after S-transformation T(u,type) With the fault current time-frequency matrix S T(i,type) S is obtained by taking the modulus. M(u,type) and S M(i,type) and S M(u,type) and S M(i,type) Multiplying the corresponding elements yields S type Summing all elements gives s total(m) Sum of rows yields s (type,m) The cumulative amplitude index ρ is calculated using the following formula. m : ; Select the frequency range where the cumulative integral of the product of the voltage and current amplitudes after the fault is larger.

[0049] (4) Calculate the stationarity index η according to the principle of immune synchronization error. m The fault voltage time-frequency matrix S obtained after S-transformation T(u,type) With the fault current time-frequency matrix S T(i,type) S is obtained by taking the modulus. M(u,type) and S M(i,type) and S M(u,type) and S M(i,type) Multiplying the corresponding elements yields S type Calculate the standard deviation σ by row.(type,m) and mean μ (type,m) The stability index η is calculated using the following formula. m : ; (5) Based on the above requirements and indicators, select the optimal analysis frequency fa. First, calculate the cumulative amplitude index ρ. m and the stability index η m The product δ m δ m The maximum number of rows m max The corresponding frequency is m maxfs For / N, calculate the integer K corresponding to the minimum error as follows: ; Obtain the error e K The minimum integer K corresponds to the integer K a Select the optimal analysis frequency f a = .

[0050] The following example uses PSCAD / EMTDC simulation software to illustrate how to determine the optimal analysis frequency.

[0051] The first step, based on PSCAD / EMTDC simulation, is to acquire inter-electrode fault waveform data for converter station B under forward fault, external forward fault, and external reverse fault conditions. The sampling frequency is 10kHz, and voltage and current data are recorded 100ms before and after each fault. (Forward fault data is also included.) u (p,1) ( t ), u (n,1) ( t ), i (p,1) ( t ), i (n,1) ( t (See attached) Figure 5 As shown.

[0052] The second step is to perform phase-mode transformation. This applies to data during a fault. u (p,type) ( t ), u (n,type) ( t ), i (p,type) ( t ), i (n,type) ( t Perform phase-mode transformation to obtain the line-mode voltage during the fault. u (l,type) (t ) and line-mode current during fault i (l,type) ( t (See attached) Figure 6 As shown.

[0053] The third step is to analyze the fault-causing line-mode voltage after phase-mode transformation. u (l,type) ( t ) and line-mode current during fault i (l,type) ( t Perform an S-transform; the result of the S-transform is shown in the appendix. Figure 7 As shown, it is presented here in the form of a time-frequency graph of its magnitude.

[0054] The fourth step is to determine the analysis frequency based on the frequency selection principles. f a .

[0055] (1) Sampling frequency f s =10kHz, here f a ≤5kHz; (2) f s / f a = K, where K is a positive integer and K≤2000; (3) Divide the voltage magnitude matrix S of each fault type M(u,type) With current magnitude matrix S M(i,type) Multiplying the corresponding elements yields three magnitude product matrices S1, S2, and S3. The cumulative magnitude index is then calculated using the following formula. ρ m : ; Cumulative Amplitude Indicator ρ m The calculation results are attached. Figure 8 As shown in (a).

[0056] (4) Calculate the standard deviation of the amplitude product matrix for each fault type row by row. σ (type,m) and mean μ (type,m) The stability index is calculated using the following formula. η m : ; Cumulative Amplitude Indicator ρ m The calculation results are attached. Figure 8 As shown in (b).

[0057] (5) Calculate the cumulative amplitude index ρ m Stability index η m product δ m As attached Figure 8 As shown in (c), δ m The maximum value corresponds to the number of rows m. max =39, corresponding to a frequency of m. max f s / N=39×10000 / 2000=195.00Hz, calculate the error according to the following formula. e K : ; error e K The calculation results are attached. Figure 8 As shown in (d). When the error is minimal. K Corresponding integer K a =51, final analysis frequency f a = f s / K a =10000 / 51=196.08Hz.

[0058] S3, real-time acquisition of voltage and current information of each measurement point of the three-terminal distributed diode rectifier transmission system at each battery swapping station, and based on the voltage and current extrapolation matrix, uniformly extrapolate the voltage and current information of each measurement point of the battery swapping station to the reference point to obtain the extrapolated voltage and current information; In this embodiment of the application, the voltage and current information of each of the measurement points of the battery swapping station is uniformly extrapolated to a reference point based on the voltage and current extrapolation matrix to obtain the extrapolated voltage and current information, including: Based on the preset phase mode transformation formula, the voltage and current information of each of the measured points of the power exchange station are transformed to obtain the line mode voltage component and line mode current component of each substation. Based on the voltage and current extrapolation matrix and the preset extrapolation formula, the line-mode voltage components and line-mode current components of each converter station measurement point are extrapolated to the reference point to obtain extrapolated voltage and current information, which includes the line-mode voltage components and line-mode current components of the reference point. The pre-defined derivation formula is as follows: ; in, , These are the line-mode voltage component and line-mode current component at the reference point, respectively, T i For voltage and current derivation matrices, u li(t) For the line-mode voltage component, i li(t) This represents the linear current component.

[0059] In the above implementation, firstly, the positive-to-ground voltage is collected in real time at converter stations A, B, and C of the three-terminal DC system. u pi ( t negative electrode to ground voltage u ni ( t ), positive current i pi ( t ), negative electrode current i ni ( t The sampling frequency is set to 10kHz, where i = A, B, C, representing the converter station.

[0060] The collected positive and negative voltage and current of each converter station u pi ( t ), u ni ( t ), i pi ( t ), i ni ( t Perform phase-mode transformation to obtain the linear mode components: ; ; In the formula, u li(t) For the line-mode voltage component, i li(t) This represents the linear current component.

[0061] Finally, based on the voltage-current derivation matrix, the line-mode voltage components u of each converter station are... li(t) Linear current component i li(t) Extend the derivation to the reference point P.

[0062] S4. Based on the recursive DFT algorithm, extract the amplitude and phase of the derived voltage and current information at the optimal analysis frequency to obtain the voltage and current phasors, and calculate the power phasors of each converter station based on the voltage and current phasors. In this embodiment of the application, the recursive DFT algorithm is: ; ; ; ; Where N = fs / fa represents the number of sampling points for one period corresponding to the optimal analysis frequency, and k ≥ 0 is an integer. , Here, K represents the line-mode voltage phasor and line-mode current phasor at the reference point, respectively. K is an integer greater than or equal to 0, fs is the sampling frequency, and fa is the optimal analysis frequency.

[0063] Using the line-mode voltage phasor at the reference point of each converter station and line mode current components Calculate the power phasor using the following formula. S i .

[0064] ; in, Linear current component The conjugates of i, i = A, B, C.

[0065] S5. Based on the power phasor of each of the aforementioned battery swapping stations, determine the longitudinal power action quantity and the longitudinal power braking quantity. Based on the longitudinal power action quantity, the longitudinal power braking quantity, the set proportional braking coefficient, the reliability coefficient, and the action setpoint, establish a longitudinal power protection criterion. Based on the longitudinal power protection criterion, control the protection device to protect the longitudinal power of the three-terminal distributed diode rectifier transmission system.

[0066] In this embodiment of the application, the step of determining the longitudinal power action quantity and the longitudinal power braking quantity based on the power phasor of each of the aforementioned power swapping stations, and establishing a longitudinal power protection criterion based on the longitudinal power action quantity, the longitudinal power braking quantity, the set proportional braking coefficient, the reliability coefficient, and the action setpoint, includes: Based on the law of power conservation, the vector sum of the power phasors of all converter stations is calculated, and the modulus of the vector sum is taken to obtain the longitudinal power action quantity; Based on the magnitude of the power phasor of each battery swapping station, determine the longitudinal power braking amount; Based on the longitudinal power action quantity, longitudinal power braking quantity, set proportional braking coefficient, reliability coefficient and action set value, establish longitudinal power protection criteria; The longitudinal power protection criterion is as follows: If the longitudinal power operating amount is greater than the product of the set proportional braking coefficient and the longitudinal power braking amount, and if the longitudinal power operating amount is greater than the product of the reliability coefficient and the operating setting value, then the longitudinal power protection criterion is satisfied.

[0067] In the above implementation method, the formula for calculating the longitudinal power action is: .

[0068] The formula for calculating the longitudinal braking force is: .

[0069] The reliability of the protection is improved by using longitudinal power braking, and the braking force is constructed to avoid malfunctions during normal load changes or measurement errors.

[0070] In this embodiment of the application, a proportional braking coefficient is set. K res =0.05, reliability coefficient K rel =1.2, constant value S set =12MVA, and the above coefficients can be adjusted based on actual operating experience.

[0071] In this embodiment of the application, the step of controlling the protection device based on the longitudinal power protection criterion includes: When the longitudinal power protection criterion is met, the control and protection device will trip and disconnect the faulty line; When the longitudinal power protection criterion is not met, the control and protection device remains inactive. The system continues to operate normally.

[0072] The following examples illustrate different typical faults, specifically S3 to S5.

[0073] When a single-pole grounding fault occurs within the zone, and the transition resistance is 100Ω, S3 includes: Step 1: Real-time acquisition of positive and negative electrode voltage and current data at each converter station, with a sampling frequency set to 10kHz. Acquisition parameters include: positive electrode voltage to ground. u pi ( t negative electrode to ground voltage u ni ( t ), positive current i pi ( t ), negative electrode current i ni ( t The specific waveform is shown in the attached image. Figure 9 As shown in (a).

[0074] The second step is to perform phase-mode conversion processing. This involves processing the positive and negative voltage and current signals acquired from each converter station. u pi ( t ), u ni ( t ), i pi (t ), i ni ( t Perform phase mode transformation calculations, extract line mode components, and the transformation results are shown in the attached figure. Figure 9 As shown in (b).

[0075] The third step is to build upon the derivation matrix constructed in Step 1. T i The line-mode voltage components of each converter station u li ( t ), Linear current component i li ( t The derivation is extended to reference point P, and the line-mode voltage component at the reference point is obtained. The derivation results are attached. Figure 9 As shown in (c).

[0076] S4 includes: The first step is to analyze the line-mode voltage components at the reference point obtained in Step 3. and line mode current components Perform recursive DFT transformation to extract and analyze frequencies in real time. f a The frequency domain information below, including the line-mode voltage phasors and line-mode current phasors at the reference point for each converter station, is shown in the attached figure. Figure 10 As shown.

[0077] The second step is to calculate the power phasor using the following formula. S i : .

[0078] S5 includes: The longitudinal power operating and braking quantities were calculated, and fault diagnosis and protection decisions were made based on the longitudinal power protection criterion. The results are attached. Figure 11 As shown, the protection criterion condition is met within 2ms after the fault occurs, and the protection device can operate reliably without being affected by the 100Ω transition resistance, which verifies the effectiveness and reliability of the protection scheme.

[0079] When a three-phase ground fault occurs outside the forward zone, and the transition resistance is 10Ω, S3 includes: Step 1: Real-time acquisition of positive and negative electrode voltage and current data at each converter station, with a sampling frequency set to 10kHz. Acquisition parameters include: positive electrode voltage to ground. u pi ( t negative electrode to ground voltage u ni ( t ), positive current ipi ( t ), negative electrode current i ni ( t The specific waveform is shown in the attached image. Figure 12 As shown in (a).

[0080] The second step is to perform phase-mode conversion processing. This involves processing the positive and negative voltage and current signals acquired from each converter station. u pi ( t ), u ni ( t ), i pi ( t ), i ni ( t Perform phase mode transformation calculations, extract line mode components, and the transformation results are shown in the attached figure. Figure 12 As shown in (b).

[0081] The third step is to build upon the derivation matrix constructed in Step 1. T i The line-mode voltage components of each converter station u li ( t ), Linear current component i li ( t The derivation is extended to reference point P, and the line-mode voltage component at the reference point is obtained. The derivation results are attached. Figure 12 As shown in (c).

[0082] S4 includes: The first step is to process the line-mode voltage component at the reference point obtained in S3. and line mode current components Perform recursive DFT transformation to extract and analyze frequencies in real time. f a The frequency domain information below, including the line-mode voltage phasors and line-mode current phasors at the reference point for each converter station, is shown in the attached figure. Figure 13 As shown.

[0083] The second step is to calculate the power phasor using the following formula. S i : .

[0084] S5 includes: The longitudinal power operating and braking quantities were calculated, and fault diagnosis and protection decisions were made based on the longitudinal power protection criterion. The analysis results are attached. Figure 14As shown, the longitudinal power protection criterion did not meet the operating conditions, and the protection device correctly identified it as an external fault and reliably did not operate, verifying the good selectivity of the protection scheme for external faults.

[0085] This invention achieves accurate modeling of the complex network topology of a three-terminal system by constructing a Berylon equivalent circuit model of a DC line and establishing a voltage and current extrapolation matrix. It unifies the extrapolation of measurement information from each converter station to a reference point for comparison and analysis, effectively eliminating errors caused by ground capacitance current. By performing S-transform time-frequency analysis on typical fault data to determine the optimal analysis frequency, and combining this with a recursive DFT algorithm to extract voltage and current phasor information at specific frequencies, it effectively mitigates the adverse effects of synchronization errors on protection criteria in three-terminal distributed diode rectifier transmission systems, compensating for the excessively high synchronization requirements of traditional longitudinal protection methods in three-terminal applications. By establishing protection criteria for longitudinal power action and braking quantities, it accurately reflects the power balance relationship of the three-terminal distributed diode rectifier transmission system, reliably distinguishes faults, and effectively solves the difficulties in fault identification caused by the high proportion of new energy access and the uncontrolled characteristics of diode rectifiers, thereby effectively protecting the longitudinal power of the three-terminal distributed diode rectifier transmission system.

[0086] Figure 15 A schematic diagram of the structure of a longitudinal power protection device 200 for a three-terminal distributed diode rectifier power transmission system is shown.

[0087] like Figure 15 As shown, a longitudinal power protection device 200 for a three-terminal distributed diode rectifier power transmission system mainly includes: The voltage and current derivation matrix establishment module 201 is used to construct the equivalent circuit model of the DC line Berylon based on the line topology of the three-terminal distributed diode rectifier transmission system, and select the line intersection point of the equivalent circuit model of the DC line Berylon as the reference point to establish the voltage and current derivation matrix from the measurement point of each swapping station to the reference point. The optimal analysis frequency establishment module 202 is used to perform S-transform time-frequency analysis on typical fault data, and determine the optimal analysis frequency based on the frequency selection principle and the typical fault data after time-frequency analysis. The acquisition module 203 is used to acquire the voltage and current information of each measurement point of the three-terminal distributed diode rectifier transmission system in real time, and to uniformly extrapolate the voltage and current information of each measurement point of the three-terminal distributed diode rectifier transmission system to the reference point based on the voltage and current extrapolation matrix, so as to obtain the extrapolated voltage and current information. The calculation module 204 is used to extract the magnitude and phase of the derived voltage and current at the optimal analysis frequency based on the recursive DFT algorithm, obtain the voltage and current phasors, and calculate the power phasors of each converter station based on the voltage and current phasors. The protection module 205 is used to determine the longitudinal power action quantity and braking quantity based on the power phasor of each of the aforementioned power swapping stations, and to establish a longitudinal power protection criterion based on the longitudinal power action quantity, braking quantity, set proportional braking coefficient, reliability coefficient, and action set value. Based on the longitudinal power protection criterion, the protection device is controlled to protect the longitudinal power of the three-terminal distributed diode rectifier transmission system.

[0088] In one example, the module in any of the above devices may be one or more integrated circuits configured to implement the above methods, such as one or more application-specific integrated circuits (ASICs), or one or more digital signal processors (DSPs), or one or more field-programmable gate arrays (FPGAs), or a combination of at least two of these integrated circuit forms.

[0089] For example, when modules in a device can be implemented via a processing element scheduler, the processing element can be a general-purpose processor, such as a central processing unit (CPU) or other processor capable of calling programs. Alternatively, these modules can be integrated together as a system-on-a-chip (SOC).

[0090] In this application, various objects such as messages / information / devices / network elements / systems / apparatus / actions / operations / processes / concepts may be named. It is understood that these specific names do not constitute a limitation on the relevant objects. The names may be changed depending on the scenario, context, or usage habits. The understanding of the technical meaning of the technical terms in this application should be mainly determined from their functions and technical effects embodied / performed in the technical solution.

[0091] Those skilled in the art will understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and modules described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0092] Those skilled in the art will recognize that the modules and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein 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 implementation should not be considered beyond the scope of this application.

[0093] The terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0094] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the foregoing application concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions claimed in this application.

Claims

1. A longitudinal power protection method, characterized in that, include: Based on the line topology of the three-terminal distributed diode rectifier transmission system, a Berylon equivalent circuit model of the DC line is constructed, and the line intersection point of the Berylon equivalent circuit model of the DC line is selected as the reference point. Voltage and current extrapolation matrices from the measurement points of each swapping station to the reference point are established. S-transform time-frequency analysis was performed on multiple typical fault data, and the optimal analysis frequency was determined based on the frequency selection principle and the typical fault data after time-frequency analysis. Real-time acquisition of voltage and current information at each battery swapping station measurement point of the three-terminal distributed diode rectifier transmission system; and based on the voltage and current extrapolation matrix, uniform extrapolation of the voltage and current information at each battery swapping station measurement point to a reference point to obtain extrapolated voltage and current information. The amplitude and phase of the derived voltage and current information at the optimal analysis frequency are extracted based on the recursive DFT algorithm to obtain the voltage and current phasors. Based on the voltage and current phasors, the power phasors of each converter station are calculated. Based on the power phasor of each of the aforementioned battery swapping stations, the longitudinal power action quantity and longitudinal power braking quantity are determined. Based on the longitudinal power action quantity, longitudinal power braking quantity, set proportional braking coefficient, reliability coefficient, and action setpoint, a longitudinal power protection criterion is established. Based on the longitudinal power protection criterion, the protection device is controlled to protect the longitudinal power of the three-terminal distributed diode rectifier transmission system.

2. The longitudinal power protection method according to claim 1, characterized in that, The line topology based on the three-terminal distributed diode rectifier transmission system is used to construct a Berylon equivalent circuit model for the DC line, including: Based on the line topology of the three-terminal distributed diode rectifier transmission system, multiple uniform lossy DC transmission lines are determined. Each uniformly lossy DC transmission line segment is divided into two parts. The divided uniformly lossy DC transmission lines are placed on both sides of the resistance and conductance lossless lines. Based on the line unit resistance, unit conductance, and line wave impedance, a Berylon equivalent circuit model of the DC line is constructed.

3. The longitudinal power protection method according to claim 2, wherein establishing the voltage and current extrapolation matrix from each battery swapping station measurement point to the reference point includes: Based on the line wave impedance, line resistance, line conductance, wave propagation time from the converter station to the reference point, delay factor, the Berylon equivalent circuit model of DC line and transmission line theory corresponding to each converter station, a voltage and current derivation matrix is ​​constructed. The expression for the voltage-current derivation matrix is: ； in, ； ； ； ； ； In the formula, For the line wave impedance, Ri = rl i For the line resistance, Gi = gl i For line conductance, The time it takes for a wave to travel from the converter station to the reference point. is the delay factor, and i represents each battery swapping station.

4. The longitudinal power protection method according to claim 1, characterized in that, The typical fault data are faults within the forward region, faults outside the reverse region, or faults outside the forward region. The step of performing S-transform time-frequency analysis on the typical fault data and determining the optimal analysis frequency based on the frequency selection principle and the typical fault data after time-frequency analysis includes: For any typical fault data, perform phase-mode transformation on the typical fault data to obtain the line-mode voltage and line-mode current at the fault time. For any typical fault data, based on the preset S-transform algorithm, S-transform time-frequency analysis is performed on the line-mode voltage and line-mode current at the fault time to obtain the time-frequency matrix of the voltage and the time-frequency matrix of the current at the fault time. Based on the sampling theorem, an upper limit for the analysis frequency is set, and the number of sampling points corresponding to each period of the analysis frequency is determined to be a positive integer, thus obtaining a candidate set of integers; For any typical fault data, the voltage time-frequency matrix and the current time-frequency matrix at fault are moduloed to obtain the modulo voltage matrix and the modulo current matrix, respectively. For any typical fault data, the magnitude product matrix is ​​determined based on the product of the magnitude voltage matrix and the magnitude current matrix; For any typical fault data, calculate the cumulative amplitude index and the stability index based on the amplitude product matrix, and determine the comprehensive index based on the cumulative amplitude index and the stability index. Based on the frequency corresponding to the maximum comprehensive index, calculate the difference between the frequency corresponding to each integer in the candidate set of integers and the frequency corresponding to the maximum comprehensive index, and select the integer with the smallest difference as the target integer; The optimal analysis frequency is determined based on the target integer and the set sampling frequency.

5. The longitudinal power protection method according to claim 4, wherein the preset S-transform algorithm is: ； ； ； in, x(k) represents the input signals, namely the line-mode voltage and line-mode current during the fault. N=2000 indicates the number of sampling points for each input signal group. m and n are both integers, representing the number of rows and columns of the matrix obtained after the S-transform. m satisfies 0≤m≤(N / 2+1) corresponding to the frequency mf. s / N; n satisfies 1≤n≤N corresponding sampling point timing; S T The result after S-transformation is the voltage time-frequency matrix S at fault time. T(u,type) The fault current time-frequency matrix S T(i,type) , where type represents different fault types.

6. The longitudinal power protection method according to claim 1, characterized in that, The voltage and current information of each measurement point in the battery swapping station is uniformly extrapolated to a reference point based on the voltage and current extrapolation matrix to obtain extrapolated voltage and current information, including: Based on the preset phase mode transformation formula, the voltage and current information of each of the measured points of the power exchange station are transformed into phase mode to obtain the line mode voltage component and line mode current component of each substation. Based on the voltage and current extrapolation matrix and the preset extrapolation formula, the line-mode voltage components and line-mode current components of each converter station measurement point are extrapolated to the reference point to obtain the extrapolated voltage and current information, which includes the line-mode voltage components and line-mode current components of the reference point. The pre-defined derivation formula is as follows: ； in, , These are the line-mode voltage component and line-mode current component at the reference point, respectively, T i For voltage and current derivation matrices, u li(t) For the line-mode voltage component, i li(t) This represents the linear current component.

7. A longitudinal power protection method according to claim 6, characterized in that, The recursive DFT algorithm is as follows: ； ； ； ； Where N = fs / fa represents the number of sampling points for one period corresponding to the optimal analysis frequency, and k ≥ 0 is an integer. , Here, K represents the line-mode voltage phasor and line-mode current phasor at the reference point, respectively. K is an integer greater than or equal to 0, fs is the sampling frequency, and fa is the optimal analysis frequency.

8. The longitudinal power protection method according to claim 1, characterized in that, Based on the power phasors of each of the aforementioned battery swapping stations, the longitudinal power action quantity and longitudinal power braking quantity are determined. Based on the longitudinal power action quantity, longitudinal power braking quantity, set proportional braking coefficient, reliability coefficient, and action setpoint, a longitudinal power protection criterion is established, including: Based on the law of power conservation, the vector sum of the power phasors of all converter stations is calculated, and the modulus of the vector sum is taken to obtain the longitudinal power action quantity; Based on the magnitude of the power phasor of each battery swapping station, determine the longitudinal power braking amount; Based on the longitudinal power action quantity, longitudinal power braking quantity, set proportional braking coefficient, reliability coefficient and action set value, establish longitudinal power protection criteria; The longitudinal power protection criterion is as follows: If the longitudinal power operating amount is greater than the product of the set proportional braking coefficient and the longitudinal power braking amount, and if the longitudinal power operating amount is greater than the product of the reliability coefficient and the operating setting value, then the longitudinal power protection criterion is satisfied.

9. A longitudinal power protection method according to claim 8, characterized in that, The control and protection device based on the longitudinal power protection criterion includes: When the longitudinal power protection criterion is met, the control and protection device will trip and disconnect the faulty line; When the longitudinal power protection criterion is not met, the control and protection device remains inactive.

10. A longitudinal power protection device, characterized in that, include: The voltage and current derivation matrix establishment module is used to construct the equivalent circuit model of the DC line Berylon based on the line topology of the three-terminal distributed diode rectifier transmission system, and select the line intersection point of the equivalent circuit model of the DC line Berylon as the reference point to establish the voltage and current derivation matrix from the measurement point of each swapping station to the reference point. The optimal analysis frequency establishment module is used to perform S-transform time-frequency analysis on typical fault data, and determine the optimal analysis frequency based on the frequency selection principle and the typical fault data after time-frequency analysis. The acquisition module is used to acquire the voltage and current information of each measurement point of the three-terminal distributed diode rectifier transmission system in real time, and to uniformly extrapolate the voltage and current information of each measurement point of the three-terminal distributed diode rectifier transmission system to the reference point based on the voltage and current extrapolation matrix, so as to obtain the extrapolated voltage and current information. The calculation module is used to extract the magnitude and phase of the derived voltage and current at the optimal analysis frequency based on the recursive DFT algorithm, obtain the voltage and current phasors, and calculate the power phasors of each converter station based on the voltage and current phasors. The protection module is used to determine the longitudinal power action quantity and braking quantity based on the power phasor of each of the aforementioned power swapping stations, and to establish a longitudinal power protection criterion based on the longitudinal power action quantity, braking quantity, set proportional braking coefficient, reliability coefficient, and action set value. Based on the longitudinal power protection criterion, the protection device is controlled to protect the longitudinal power of the three-terminal distributed diode rectifier transmission system.

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