Hybrid fault detection method for high voltage DC transmission line based on DC boundary voltage energy ratio and its application

Abstract

This application relates to the field of DC transmission protection technology, and particularly to a fault identification method for hybrid three-terminal high-voltage DC transmission lines based on the DC boundary voltage energy ratio and its application. By analyzing the boundary frequency characteristics of the DC line, rectifier side, and inverter side of the system, frequency characteristic curves of each boundary are obtained. Based on the frequency response characteristics of the overall system boundary frequency characteristic curve, highly distinguishable characteristic frequency bands are determined. When a fault occurs in the system, DC voltage signals at both ends of the rectifier / inverter side are acquired, and wavelet packet decomposition is performed on the acquired signals to extract the energy within the characteristic frequency bands. Based on the energy and characteristics of the two characteristic frequency bands, identification criteria for fault location and fault polarity are constructed. This enables rapid fault identification of hybrid three-terminal DC systems without the need for communication, thereby improving the safety and reliability of hybrid three-terminal high-voltage DC transmission lines. The aim is to solve the problem of fault type identification in hybrid three-terminal DC systems.

Description

Technical Field

[0001] This application relates to the field of DC transmission protection technology, and in particular to a method for fault identification of hybrid three-terminal high-voltage DC transmission lines based on the DC boundary voltage energy ratio and its application. Background Technology

[0002] With the continuous growth of my country's renewable energy installed capacity and the expanding demand for long-distance inter-regional power transmission, high-voltage direct current (HVDC) transmission has become an important supporting technology for new power systems due to its advantages such as large capacity, long distance, and low loss. Among them, hybrid LCC-VSC DC transmission, which combines the advantages of LCC-HVDC and VSC-HVDC, is gradually being applied to long-distance, large-capacity, and even multi-terminal DC projects, becoming a key technology direction for high-proportion renewable energy grid connection and inter-regional consumption.

[0003] However, hybrid multi-terminal DC systems involve long line distances and rapid fault development, with extremely high rise rates of fault current on the DC side. If identification and isolation cannot be completed within milliseconds, it will cause serious damage to power electronic equipment and threaten the safe and stable operation of the system. Therefore, DC line protection needs to achieve rapid identification and polarity selection of faults inside and outside the protection zone within 1ms.

[0004] However, there are currently few line protection methods for hybrid multi-terminal DC systems. Among them, longitudinal protection, which relies on communication, has high selectivity but limited operating speed; single-terminal protection based on transient quantities does not require communication, but its stability under high impedance faults or strong noise conditions still needs improvement; data-driven protection methods have high identification accuracy but rely on a large number of samples for training, making engineering implementation complex. Therefore, existing protection schemes still have certain limitations in terms of response speed, anti-interference capability, and adaptability to complex multi-terminal topologies.

[0005] Therefore, it is necessary to propose a novel fast discrimination method based on the energy ratio of the characteristic frequency band of the boundary voltage, so as to realize fast, accurate and robust fault identification of hybrid three-terminal DC systems without communication, and improve the safety and reliability of hybrid three-terminal high-voltage DC transmission lines. Summary of the Invention

[0006] The main objective of this application is to provide a method for fault identification of hybrid three-terminal high-voltage direct current transmission lines based on the DC boundary voltage energy ratio, aiming to solve the problem of how to identify fault types in hybrid three-terminal DC systems.

[0007] To achieve the above objectives, this application provides a fault identification method for hybrid three-terminal high-voltage direct current (HVDC) transmission lines based on the DC boundary voltage energy ratio. This method is applied to hybrid three-terminal HVDC transmission lines including a rectifier side, a DC line, and an inverter side. The method includes the following steps: S10, acquire the DC line boundary frequency characteristic curve and the rectifier / inverter side boundary frequency characteristic curve of the system, and determine the characteristic frequency band performance of the hybrid three-terminal high-voltage DC transmission line based on the DC line boundary frequency characteristic curve and the rectifier / inverter side boundary frequency characteristic curve. The characteristic frequency band performance includes attenuation performance and gain performance. S20, when a system fault is detected, the DC voltage signals at the left and right ends of the rectifier / inverter side are collected and wavelet packet decomposition is performed to obtain the first characteristic frequency band energy and the second characteristic frequency band energy. S30, determine the fault location based on the first characteristic frequency band energy, the second characteristic frequency band energy, and the characteristic frequency band performance, and / or determine the fault polarity based on the first characteristic frequency band energy or the second characteristic frequency band energy.

[0008] Optionally, in S30, the first characteristic frequency band energy and the second characteristic frequency band energy are obtained by collecting DC voltage signals from the left and right ends of the rectifier side. The fault location includes reverse external faults and internal faults of the transmission line. The step of determining the fault location includes: S21, Calculate the ratio of the energy of the second characteristic frequency band to the energy of the first characteristic frequency band; S221, when the ratio is less than a preset first threshold and the characteristic frequency band shows attenuation, the fault location is determined to be a reverse external fault. S222, when the ratio is less than a preset first threshold and the characteristic frequency band is a gain expression, the fault location is determined to be an internal fault of the transmission line. S223, when the ratio is greater than a preset second threshold and the characteristic frequency band shows attenuation, the fault location is determined to be an internal fault of the transmission line. S224, when the ratio is greater than a preset second threshold and the characteristic frequency band is a gain expression, the fault location is determined to be a reverse external fault.

[0009] Optionally, the first characteristic frequency band energy and the second characteristic frequency band energy are obtained by collecting DC voltage signals from the left and right ends of the inverter side. The fault location includes both external faults and internal faults of the transmission line. The step of determining the fault location includes: S21, Calculate the ratio of the energy of the first characteristic frequency band to the energy of the second characteristic frequency band; S231, when the ratio is less than a preset third threshold and the characteristic frequency band shows attenuation, the fault location is determined to be a positive external fault. S232, when the ratio is less than a preset third threshold and the characteristic frequency band is a gain expression, the fault location is determined to be an internal fault of the transmission line. S233, when the ratio is greater than a preset fourth threshold and the characteristic frequency band shows attenuation, the fault location is determined to be an internal fault of the transmission line. S234, when the ratio is greater than a preset fourth threshold and the characteristic frequency band is a gain expression, the fault location is determined to be a positive external fault.

[0010] Optionally, in step S30, the step of determining the fault polarity includes: S24, Calculate the second characteristic frequency band positive electrode energy E at the right end of the rectifier / inverter side. r+ Second characteristic frequency band negative electrode energy E r - ratio E r+ / E r- ; S25, when the ratio E r+ / E r- When the fault is within a preset polarity fault range, the fault polarity is determined to be a bipolar fault in the transmission line. S26, when the ratio E r+ / E r- When the fault polarity exceeds the upper limit of the preset polarity fault range, the fault polarity is determined to be a positive fault of the transmission line. S27, when the ratio E r+ / E r- When the value is less than the lower limit of the preset polarity fault range, the fault polarity is determined to be a negative pole fault of the transmission line.

[0011] Optionally, in step S10, determining the characteristic frequency band performance of the hybrid three-terminal high-voltage direct current transmission line based on the DC line boundary frequency characteristic curve and the rectifier / inverter side boundary frequency characteristic curve includes: S11, Based on the boundary frequency characteristic curve of the DC line, determine the target characteristic frequency band in the hybrid three-terminal high-voltage DC transmission line; S12, determine the numerical relationship between the frequency and amplitude of the target characteristic frequency band in the rectifier / inverter side boundary frequency characteristic curve; S13, when the numerical relationship is negatively correlated, the characteristic frequency band is determined to exhibit attenuation behavior; S14, when the numerical relationship is positively correlated, the characteristic frequency band is determined to be a gain performance.

[0012] Optionally, the expression for the boundary frequency response curve of a DC line is:

[0013] The expression for the frequency response curve of the rectifier side boundary is:

[0014] The expression for the inverter side boundary frequency response curve is:

[0015] In the formula, U1(jω) and U3(jω) are the line-side voltages of the rectifier side and the inverter side, respectively; U0(jω) and U2(jω) are the valve-side voltages of the rectifier side and the inverter side, respectively; Z1(jω) and Z0(jω) represent the complex impedances of the DC filter and the smoothing reactor on the rectifier side, respectively; γ0(jω) is the propagation coefficient of the DC transmission line; x is the distance from the DC side port of the rectifier station to the fault point of the DC transmission line; Z2(jω) and Z3(jω) represent the complex impedances of the current-limiting reactor and the equivalent ground capacitance on the inverter side, respectively.

[0016] Optionally, S20 includes: S21. The acquired DC voltage signal is decomposed into N-level wavelet packets using a preset wavelet basis function to obtain 2 N Each bandwidth is f s / (2 N+1 The frequency band E1~E2 Hz N ; S22, according to the frequency bands E1~E2 N Calculate the energy E of the first characteristic frequency band of the voltage at the left and right measuring points on the rectifier / inverter side after wavelet packet decomposition. r Second characteristic band energy E l :

[0017]

[0018] In the formula, E ri With E li represents the energy of the voltage at the left and right measuring points on the rectifier side / inverter side within the i-th frequency band, respectively; m represents the first sub-band of the selected characteristic frequency band; and n represents the last sub-band of the selected characteristic frequency band.

[0019] In addition, to achieve the above objectives, this application also provides a DC protection device for implementing the hybrid three-terminal high-voltage DC transmission line fault discrimination method based on DC boundary voltage energy ratio as described in any of the preceding claims.

[0020] In addition, to achieve the above objectives, this application also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the hybrid three-terminal high-voltage direct current transmission line fault identification method based on DC boundary voltage energy ratio as described in any of the preceding claims.

[0021] This application has at least the following beneficial effects: 1. By analyzing the boundary frequency characteristics of the rectifier side, DC line, and inverter side of the LCC-VSC three-terminal hybrid DC system, characteristic frequency bands with significant attenuation differences are selected. A segment discrimination quantity based on the energy ratio of the characteristic frequency bands of the voltage across the smoothing reactor is constructed, enabling clear differentiation between internal and external faults in the high-frequency domain. This method can quickly distinguish between internal and external faults within approximately 1ms, significantly improving the speed and selectivity of protection actions. 2. By acquiring instantaneous DC voltage across the smoothing reactor on the rectifier side, five-layer wavelet packet decomposition is used to extract characteristic frequency band energy, and segment discrimination and polarity identification quantities are constructed respectively, so that fault characteristics are fully extracted in the high-frequency domain. This feature construction method is insensitive to system noise and changes in operating status, thereby enhancing the stability and robustness of the protection criteria and maintaining reliable identification performance under various operating conditions; 3. Using the single-ended voltage of the rectifier / inverter side as the sole input, the criterion structure is simple, with few setting parameters. It does not rely on communication links or data synchronization mechanisms, facilitating rapid integration and online setting of protection devices. This method is applicable to various topologies and operating modes of multi-terminal hybrid DC systems, exhibiting strong engineering feasibility and facilitating its widespread application in practical DC transmission projects. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating the hybrid three-terminal high-voltage direct current transmission line fault identification method based on DC boundary voltage energy ratio involved in the embodiments of this application; Figure 2 This is a schematic diagram of the hybrid three-terminal high-voltage direct current transmission system involved in the embodiments of this application; Figure 3 This is a schematic diagram of the equivalent circuit of the rectifier side boundary involved in the embodiments of this application; Figure 4 This is a schematic diagram of the equivalent circuit of the inverter side boundary involved in the embodiments of this application; Figure 5 This is a frequency response curve of the rectifier side boundary involved in the embodiments of this application; Figure 6 This is a frequency response curve of the inverter side boundary involved in the embodiments of this application; Figure 7 This is a frequency characteristic curve of a DC transmission line involved in an embodiment of this application; Figure 8 This is a graph showing the total boundary frequency response of the system involved in the embodiments of this application; Figure 9 This is a comparative schematic diagram showing the energy differences in the characteristic frequency bands at both ends of the smoothing reactor on the rectifier side under different fault conditions in the embodiments of this application.

[0023] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0024] To better understand the above technical solutions, exemplary embodiments of this disclosure will be described in more detail below with reference to the accompanying drawings. While exemplary embodiments of this disclosure are shown in the drawings, it should be understood that this disclosure can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of this disclosure to those skilled in the art.

[0025] First Embodiment Reference Figure 1 This embodiment provides a fault detection method for hybrid three-terminal high-voltage direct current (HVDC) transmission lines based on the DC boundary voltage energy ratio. This method is applied to hybrid three-terminal HVDC transmission lines including a rectifier side, a DC line, and an inverter side. The method includes the following steps: S10, acquire the DC line boundary frequency characteristic curve and the rectifier / inverter side boundary frequency characteristic curve of the system, and determine the characteristic frequency band performance of the hybrid three-terminal high-voltage DC transmission line based on the DC line boundary frequency characteristic curve and the rectifier / inverter side boundary frequency characteristic curve. The characteristic frequency band performance includes attenuation performance and gain performance. In this embodiment, the characteristic frequency band of the hybrid three-terminal high-voltage direct current transmission line to be judged is first determined. In this embodiment, the characteristic frequency band characteristics include attenuation and gain. Whether the characteristic frequency band characteristics belong to attenuation or gain is determined by two boundary frequency characteristic curves: the boundary frequency characteristic curve of the DC line and the boundary frequency characteristic curves of the rectifier side / inverter side.

[0026] It should be noted that, in this embodiment, "acquiring the DC line boundary frequency characteristic curve and the rectifier / inverter side boundary frequency characteristic curve" refers to the characteristic frequency band performance of the hybrid three-terminal high-voltage DC transmission line. This can be determined simply by using the boundary frequency characteristic curve of the DC line and the boundary frequency characteristic curve of either the rectifier or inverter side. The difference between choosing the rectifier side or the inverter side lies in the fact that the fault location determined by collecting the boundary frequency characteristic curves of different sides is different.

[0027] In this embodiment, the equivalent circuit diagrams of each boundary are obtained by combining the structural composition of each boundary of the system and the actual engineering needs. Based on the equivalent circuit diagrams, the transfer function reflecting the boundary voltage relationship, i.e., the boundary frequency characteristic expression, is constructed. The boundary frequency characteristic diagrams of each boundary are obtained through the boundary frequency characteristic expressions.

[0028] Optionally, the expression for the boundary frequency response curve of a DC line is:

[0029] The expression for the frequency response curve of the rectifier side boundary is:

[0030] The expression for the inverter side boundary frequency response curve is:

[0031] In the formula, U1(jω) and U3(jω) are the line-side voltages of the rectifier side and the inverter side, respectively; U0(jω) and U2(jω) are the valve-side voltages of the rectifier side and the inverter side, respectively; Z1(jω) and Z0(jω) represent the complex impedances of the DC filter and the smoothing reactor on the rectifier side, respectively; γ0(jω) is the propagation coefficient of the DC transmission line; x is the distance from the DC side port of the rectifier station to the fault point of the DC transmission line; Z2(jω) and Z3(jω) represent the complex impedances of the current-limiting reactor and the equivalent ground capacitance on the inverter side, respectively.

[0032] For example, the construction principle of the three boundary frequency response curves is explained below: Reference Figure 2 The diagram shows the structure of the Wudongde LCC-VSC hybrid three-terminal high-voltage direct current transmission system. The LCC is a dual 12-pulse grid-commutated converter, while VSC1 and VSC2 are both full- and half-bridge hybrid voltage source converters. u r and u l These represent the voltages measured at the right and left ends of the smoothing reactor, respectively. S 1. S 2. S 3 and L a , L b , L c These are the AC voltages on the grid side and the smoothing reactors on the line side for LCC, VSC1, and VSC2, respectively. Z filter For the DC filter on the LCC line side; l 1. l 2 represents the system's DC transmission lines; f 1. f 2 represents the internal fault and the reverse external fault of the DC transmission line in the system, respectively. Based on the system's boundary structure and practical engineering requirements, the equivalent circuits of the rectifier and inverter sides are obtained as follows: Figure 3 and Figure 4 As shown.

[0033] Figure 3 In the middle, the rectifier side boundary consists of a smoothing reactor and two sets of three-tuned DC filters, among which... L 0 represents the parameter of the smoothing reactor. C 1- C 3. L 1- L 3 represents the parameters of the DC filter. U 0 represents the voltage output by the rectifier, i.e., the valve-side voltage of the rectifier station. U 1 represents the voltage across the rectifier side boundary, i.e., the line-side voltage.

[0034] according to Figure 3 The frequency domain expressions for the relevant parameters on the rectifier side can be obtained as follows: (1) In the formula, Z 1( jω )and Z 0( jω The impedances of the rectifier-side DC filter and the smoothing reactor are respectively represented by their complex impedances. L 0 represents the parameter of the smoothing reactor on the rectifier side. C 1- C 3. L 1- L 3 represents the parameters of the DC filter.

[0035] By constructing the ratio of the voltages on both sides of the rectifier side boundary as a transfer function, the expression for its boundary frequency characteristic is obtained as follows: (2) in, U 1( jω )and U 0( jω These are the line-side voltage and valve-side voltage on the rectifier side, respectively. Substituting the parameters of the Wudongde project into equation (2), we can obtain the frequency response curve of the rectifier side boundary of the system as follows: Figure 5 As shown.

[0036] Figure 4 In the middle, the inverter side boundary is the equivalent capacitance of the overhead DC transmission line to ground. C a and current-limiting reactors L a composition, U 2 represents the voltage output by the inverter, i.e., the valve-side voltage of the inverter station. U 3 represents the voltage across the inverter side boundary, i.e., the line side voltage.

[0037] according to Figure 4 The frequency domain expressions for the inverter-side relevant parameters can be obtained: (3) In the formula, Z 2( jω )and Z 3( jω The numbers ) represent the complex impedances of the inverter-side current-limiting reactor and the equivalent capacitance to ground, respectively. C a and L a These are the parameters for the equivalent capacitor and the current-limiting reactor, respectively.

[0038] By constructing the ratio of the voltages on both sides of the inverter side boundary as a transfer function, the expression for its boundary frequency characteristic is obtained as follows: (4) in, U 3( jω )and U 2( jω These are the line-side voltage and valve-side voltage on the inverter side, respectively. Substituting the parameters of the Wudongde project into equation (4), we can obtain the frequency response curve of the inverter side boundary of the system as follows: Figure 6 As shown.

[0039] Based on the property that DC transmission lines in symmetrical bipolar systems are susceptible to electromagnetic coupling, we consider using equation (5) to decouple fault information and obtain line mode components to analyze the frequency characteristics of DC transmission lines: (5) According to the propagation theory of traveling waves, the traveling wave voltage at the DC side port of the rectifier station can be expressed as: (6) In the formula, x This refers to the distance from the DC-side port of the rectifier station to the fault point on the DC transmission line. U 0( x , s () represents the traveling wave voltage at the fault point. U 0(0, s ) is the traveling wave voltage at the DC side port of the rectifier station, γ0( jω Let be the propagation coefficient of a DC transmission line, and its expression is as follows: (7) In the formula, r 0、 l 0、 g 0、 c 0 represents the line-mode resistance, inductance, conductance, and capacitance per unit length of the line, respectively.

[0040] The transfer function is constructed by setting the ratio of the voltage at the DC side port of the rectifier station to the voltage at the fault location, and letting... s= jω The frequency response expression for a DC transmission line is obtained as follows: (8) Substituting the parameters of the Wudongde project into equation (8), we can obtain the frequency characteristic curve of the system's DC transmission line as follows: Figure 7 As shown.

[0041] Further and optionally, in step S10, the characteristic frequency band performance of the hybrid three-terminal high-voltage direct current transmission line is determined based on the boundary frequency characteristic curve of the DC line and the boundary frequency characteristic curve of the rectifier side / inverter side, including: S11, Based on the boundary frequency characteristic curve of the DC line, determine the target characteristic frequency band in the hybrid three-terminal high-voltage DC transmission line; S12, determine the numerical relationship between the frequency and amplitude of the target characteristic frequency band in the rectifier / inverter side boundary frequency characteristic curve; S13, when the numerical relationship is negatively correlated, the characteristic frequency band is determined to exhibit attenuation behavior; S14, when the numerical relationship is positively correlated, the characteristic frequency band is determined to be a gain performance.

[0042] Based on the power architecture in the above example, the determination of characteristic frequency band performance based on boundary frequency response curves is explained: For example, refer to Figure 8 The system's total boundary frequency response curve shown is derived from... Figure 8 Therefore: the rectifier side boundary is at 2500Hz < f At frequencies < 10000Hz, it has an attenuating effect on the signal; DC transmission lines in f The signal propagation attenuation is weak and can be approximately ignored at frequencies below 10kHz; the inverter-side boundary is at 2500Hz. f When the frequency is < 8345Hz, it exhibits a gain effect on the signal, while when 8345Hz < f At frequencies below 10000Hz, it exhibits attenuation, and its attenuation is weaker than that at the rectifier side boundary.

[0043] When a fault occurs within the DC transmission line zone and a fault occurs outside the reverse zone, the transient signals of the fault propagate in opposite directions across the rectifier side boundary. Combining the frequency response characteristics of the rectifier side boundary to transient signals in different frequency bands, it can be found that in the frequency band where the frequency response of the rectifier side boundary is more significant within 2500Hz < f < 10000Hz, the amplitude or energy relationship of the transient signals on both sides of the rectifier side boundary under the two fault conditions is opposite. Based on this, a frequency band that can make the above differences more significant, thereby improving the distinguishability, is selected as the characteristic frequency band. Therefore, in this example, the frequency band from 2500Hz to 5000Hz is selected as the target characteristic frequency band for subsequent fault identification.

[0044] Similarly, refer to Figure 8 Taking the rectifier side as an example (the same applies to the inverter side), when a fault occurs at the rectifier side boundary within the 2500Hz~5000Hz range, it significantly attenuates the signal. Furthermore, the transient signal propagation direction across the rectifier side boundary is opposite to that of an in-zone fault and a reverse out-of-zone fault. Therefore, the valve-side signal is significantly weaker than the line-side signal when an in-zone fault occurs within this specific frequency band, while the line-side signal is significantly weaker than the valve-side signal when a reverse out-of-zone fault occurs. This leads to the following: When the numerical relationship is negatively correlated, the characteristic frequency band is determined to exhibit attenuation behavior; when the numerical relationship is positively correlated, the characteristic frequency band is determined to exhibit gain behavior.

[0045] S20, when a system fault is detected, the DC voltage signals at the left and right ends of the rectifier / inverter side are collected and wavelet packet decomposition is performed to obtain the first characteristic frequency band energy and the second characteristic frequency band energy. In this embodiment, after determining that the characteristic frequency band of the system is characterized by gain or attenuation, fault criteria are collected. When a fault is detected in the system, the DC voltage signals at the left and right ends of either the rectifier side or the inverter side are collected and wavelet packet decomposition is performed to obtain the first characteristic frequency band energy corresponding to the left DC voltage signal and the second characteristic frequency band energy corresponding to the right DC voltage signal.

[0046] In some optional implementations, voltage measurement units are installed at both ends of the smoothing reactor on the rectifier side; the DC voltage signal at the measuring points at both ends of the smoothing reactor is acquired within a 1ms time window after fault triggering; a preset wavelet basis function is selected to perform N-level wavelet packet decomposition on the acquired DC voltage signal, where N is the preset number of decomposition levels; the characteristic frequency band energy of the voltage at the measuring points at both ends of the smoothing reactor on the rectifier side after wavelet packet decomposition is calculated and obtained respectively. E r and E l .

[0047] In some alternative implementations, voltage measurement units are installed at both ends of the DC reactor on the inverter side, and the rest is the same as the smoothing reactor on the rectifier side.

[0048] S30, determine the fault location based on the first characteristic frequency band energy, the second characteristic frequency band energy, and the characteristic frequency band performance, and / or determine the fault polarity based on the first characteristic frequency band energy or the second characteristic frequency band energy.

[0049] In this embodiment, based on the energy of the first characteristic frequency band and / or the energy of the second characteristic frequency band, and the characteristic frequency band performance of the system determined in step S10, the fault type of the system is determined. The fault type includes the fault location and / or the fault polarity.

[0050] The specific determination method will be elaborated in subsequent embodiments, and will not be repeated in this embodiment.

[0051] In the technical solution provided in this embodiment, the frequency characteristic curves of each boundary of the system are obtained by analyzing the boundary frequency characteristics of the DC line, rectifier side and inverter side of the system; the characteristic frequency band performance with high discrimination is determined according to the frequency response characteristics of the total boundary frequency characteristic curve of the system; when a fault occurs in the system, the DC voltage signal at both ends of the rectifier side / inverter side is collected, and the collected signal is decomposed by wavelet packet to extract the energy in the characteristic frequency band; the identification criteria of fault location and fault polarity are constructed based on the energy of the two characteristic frequency bands and the characteristic frequency band performance, so as to realize fast, accurate and robust fault identification of the hybrid three-terminal DC system without communication, thereby improving the safety and reliability of the hybrid three-terminal high-voltage DC transmission line.

[0052] Second Embodiment Based on the first embodiment, in this embodiment, when selecting to collect the DC voltage signals from the left and right ends of the rectifier side to obtain the first characteristic frequency band energy and the second characteristic frequency band energy, the determined fault location includes reverse external faults and internal faults of the transmission line. Specifically, it includes the following steps: S21, Calculate the ratio of the energy of the second characteristic frequency band to the energy of the first characteristic frequency band; S221, when the ratio is less than a preset first threshold and the characteristic frequency band shows attenuation, the fault location is determined to be a reverse external fault. S222, when the ratio is less than a preset first threshold and the characteristic frequency band is a gain expression, the fault location is determined to be an internal fault of the transmission line. S223, when the ratio is greater than a preset second threshold and the characteristic frequency band shows attenuation, the fault location is determined to be an internal fault of the transmission line. S224, when the ratio is greater than a preset second threshold and the characteristic frequency band is a gain expression, the fault location is determined to be a reverse external fault.

[0053] In some optional implementations, the first threshold is preset to 0.9; In some alternative implementations, the second threshold is preset to 1.1.

[0054] For example, the DC voltage signal at the measuring points across the smoothing reactor is acquired within a 1ms time window after the fault is triggered; the acquired DC voltage signal after the fault is decomposed into five layers of wavelet packets using the db4 wavelet function, resulting in 32 frequency bands E1~E with a bandwidth of 312.5Hz. 32 As shown in Table 1, the energy of the first characteristic frequency band of the voltage at the measuring points at the right and left ends of the smoothing reactor after wavelet packet decomposition is calculated respectively. E r With the energy of the second characteristic frequency band E l .

[0055] Table 1. Correspondence between Energy and Frequency Band

[0056] The specific characteristic frequency band energy is: (9) (10) Where E ri With E li These represent the energy of the voltage at the measurement points on the left and right sides of the rectifier side boundary within the i-th frequency band.

[0057] Based on the characteristic frequency band energy of the voltage measured at both ends of the smoothing reactor, the characteristic frequency band energy ratio E is calculated. r / E l And denoted as F1, that is:

[0058] The magnitudes or energies of the signals on both sides of the rectifier boundary exhibit opposite characteristics. Taking a 10% margin on each side, the following criterion can be obtained: When F1 < 0.9, the fault is determined to be a reverse external fault of the system; when F1 > 1.1, the fault is determined to be an internal fault of the system transmission line.

[0059] Third Embodiment Based on any embodiment, in this embodiment, when selecting to collect the DC voltage signals from the left and right ends of the reverse side to obtain the first characteristic frequency band energy and the second characteristic frequency band energy, the determined fault location includes both forward external faults and internal faults of the transmission line. Specifically, it includes the following steps: S21, Calculate the ratio of the energy of the first characteristic frequency band to the energy of the second characteristic frequency band; S231, when the ratio is less than a preset third threshold and the characteristic frequency band shows attenuation, the fault location is determined to be a positive external fault. S232, when the ratio is less than a preset third threshold and the characteristic frequency band is a gain expression, the fault location is determined to be an internal fault of the transmission line. S233, when the ratio is greater than a preset fourth threshold and the characteristic frequency band shows attenuation, the fault location is determined to be an internal fault of the transmission line. S234, when the ratio is greater than a preset fourth threshold and the characteristic frequency band is a gain expression, the fault location is determined to be a positive external fault.

[0060] In some optional implementations, the preset third threshold can be 0.9; In some alternative implementations, the preset fourth threshold can be 1.1.

[0061] Fourth embodiment Based on any of the above embodiments, this embodiment determines the fault polarity. In this embodiment, the selected acquisition side can be either the rectifier side or the inverter side, wherein the rectifier side acquires the right end, and the inverter side acquires the left end, and the corresponding characteristic frequency band positive electrode energy E is calculated. r+ and characteristic frequency band negative electrode energy E r - Specifically, it includes the following steps: S24, Calculate the characteristic frequency band positive electrode energy E at the right end of the rectifier side / the left end of the inverter side. r+ and characteristic frequency band negative electrode energy E r - ratio E r+ / E r- ; S25, when the ratio E r+ / E r- When the fault is within a preset polarity fault range, the fault polarity is determined to be a bipolar fault in the transmission line. S26, when the ratio E r+ / E r- When the fault polarity exceeds the upper limit of the preset polarity fault range, the fault polarity is determined to be a positive fault of the transmission line. S27, when the ratio E r+ / E r- When the value is less than the lower limit of the preset polarity fault range, the fault polarity is determined to be a negative pole fault of the transmission line.

[0062] The theoretical basis for the above criteria is that when a bipolar short-circuit fault occurs, the fault circuit is symmetrical and the voltage at the right end of the positive and negative smoothing reactors is equal; when a unipolar ground fault occurs, the voltage at the right end of the fault pole is greater than the voltage at the right end of the healthy pole.

[0063] In some alternative implementations, the preset polarity fault range is [0.9, 1.1].

[0064] Verification Example 1 In this embodiment, to verify the attenuation characteristics of the rectifier side boundary when different faults occur in the system obtained in the first embodiment, the boundary voltages ul and ur are extracted and wavelet packet decomposition is performed for fault f1 in the system occurrence region and fault f2 outside the reverse region, respectively, to obtain the characteristic frequency band energies Er and El, and then placed in... Figure 9 The results are consistent with those described in the first embodiment.

[0065] Verification Example 2 In this embodiment, in order to fully verify the impact of different fault types, fault distances, and transition resistances on the action results, an intra-zone fault f1 was set at 50km, 100km, 300km, 600km, 800km, and 1205km of the DC transmission line, and an extra-zone fault f2 was set at 0km outside the system reverse zone. The transition resistances were set to 0.01Ω, 100Ω, 500Ω, and 1000Ω, respectively. The action results were verified based on the simulation results, and the verification results are shown in Table 2 below.

[0066] Table 2. Influence of different fault types, fault distances, and transition resistance on the operating results.

[0067] Verification Example 3 In this embodiment, to fully verify the impact of different fault poles on the action results, faults f1 in the positive, negative, and bipolar regions were set at a distance of 200km along the DC transmission line, with transition resistances set to 0.01Ω, 100Ω, 300Ω, 500Ω, 700Ω, and 1000Ω, respectively. The action results were verified based on simulation results. The verification results are shown in Table 3 below.

[0068] Table 3. Influence of different fault poles on the action results

[0069] Furthermore, as an implementation scheme, the present application embodiment also relates to a DC protection device for implementing the above-described hybrid three-terminal high-voltage DC transmission line fault discrimination method based on DC boundary voltage energy ratio.

[0070] Furthermore, those skilled in the art will understand that all or part of the processes in the methods of the above embodiments can be implemented by a computer program instructing related hardware. The computer program includes program instructions and can be stored in a storage medium, which is a computer-readable storage medium. The program instructions are executed by at least one processor in a computer system to implement the process steps of the embodiments of the above methods.

[0071] Therefore, this application also provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the various steps of the hybrid three-terminal high-voltage direct current transmission line fault identification method based on DC boundary voltage energy ratio as described in the above embodiments.

[0072] The computer-readable storage medium can be any computer-readable storage medium capable of storing program code, such as a USB flash drive, portable hard drive, read-only memory (ROM), magnetic disk, or optical disk.

[0073] It should be noted that, since the storage medium provided in the embodiments of this application is the storage medium used to implement the methods of the embodiments of this application, those skilled in the art can understand the specific structure and variations of the storage medium based on the methods described in the embodiments of this application, and therefore will not be repeated here. All storage media used in the methods of the embodiments of this application fall within the scope of protection of this application.

[0074] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0075] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), 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 One One or more processes and / or boxes Figure One A device that provides the functions specified in one or more boxes.

[0076] 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 One One or more processes and / or boxes Figure One The function specified in one or more boxes.

[0077] 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 One One or more processes and / or boxes Figure One The steps of the function specified in one or more boxes.

[0078] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0079] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for fault identification of hybrid three-terminal high-voltage direct current transmission lines based on DC boundary voltage energy ratio, characterized in that, The method, applicable to hybrid three-terminal high-voltage direct current transmission lines including a rectifier side, a DC line, and an inverter side, comprises the following steps: S10, acquire the DC line boundary frequency characteristic curve and the rectifier / inverter side boundary frequency characteristic curve of the system, and determine the characteristic frequency band performance of the hybrid three-terminal high-voltage DC transmission line based on the DC line boundary frequency characteristic curve and the rectifier / inverter side boundary frequency characteristic curve. The characteristic frequency band performance includes attenuation performance and gain performance. S20, when a system fault is detected, the DC voltage signals at the left and right ends of the rectifier / inverter side are collected and wavelet packet decomposition is performed to obtain the first characteristic frequency band energy and the second characteristic frequency band energy. S30, determine the fault location based on the first characteristic frequency band energy, the second characteristic frequency band energy, and the characteristic frequency band performance, and / or determine the fault polarity based on the first characteristic frequency band energy or the second characteristic frequency band energy.

2. The method as described in claim 1, characterized in that, In S30, the first characteristic frequency band energy and the second characteristic frequency band energy are obtained by collecting the DC voltage signals from the left and right ends of the rectifier side. The fault location includes reverse external faults and internal faults of the transmission line. The step of determining the fault location includes: S21, Calculate the ratio of the energy of the second characteristic frequency band to the energy of the first characteristic frequency band; S221, when the ratio is less than a preset first threshold and the characteristic frequency band shows attenuation, the fault location is determined to be a reverse external fault. S222, when the ratio is less than a preset first threshold and the characteristic frequency band is a gain expression, the fault location is determined to be an internal fault of the transmission line. S223, when the ratio is greater than a preset second threshold and the characteristic frequency band shows attenuation, the fault location is determined to be an internal fault of the transmission line. S224, when the ratio is greater than a preset second threshold and the characteristic frequency band is a gain expression, the fault location is determined to be a reverse external fault.

3. The method as described in claim 1, characterized in that, The first and second characteristic frequency band energies are obtained by collecting DC voltage signals from the left and right ends of the inverter side. The fault location includes both external faults and internal faults of the transmission line. The steps for determining the fault location include: S21, Calculate the ratio of the energy of the first characteristic frequency band to the energy of the second characteristic frequency band; S231, when the ratio is less than a preset third threshold and the characteristic frequency band shows attenuation, the fault location is determined to be a positive external fault. S232, when the ratio is less than a preset third threshold and the characteristic frequency band is a gain expression, the fault location is determined to be an internal fault of the transmission line. S233, when the ratio is greater than a preset fourth threshold and the characteristic frequency band shows attenuation, the fault location is determined to be an internal fault of the transmission line. S234, when the ratio is greater than a preset fourth threshold and the characteristic frequency band is a gain expression, the fault location is determined to be a positive external fault.

4. The method as described in claim 2 or 3, characterized in that, In step S30, the step of determining the fault polarity includes: S24, Calculate the characteristic frequency band positive electrode energy E at the right end of the rectifier side / the left end of the inverter side. r+ and characteristic frequency band negative electrode energy E r - ratio E r+ / E r- ; S25, when the ratio E r+ / E r- When the fault is within a preset polarity fault range, the fault polarity is determined to be a bipolar fault in the transmission line. S26, when the ratio E r+ / E r- When the fault polarity exceeds the upper limit of the preset polarity fault range, the fault polarity is determined to be a positive fault of the transmission line. S27, when the ratio E r+ / E r- When the value is less than the lower limit of the preset polarity fault range, the fault polarity is determined to be a negative pole fault of the transmission line.

5. The method as described in claim 1, characterized in that, In step S10, the characteristic frequency band performance of the hybrid three-terminal high-voltage direct current transmission line is determined based on the boundary frequency characteristic curve of the DC line and the boundary frequency characteristic curves of the rectifier side / inverter side, including: S11, Based on the boundary frequency characteristic curve of the DC line, determine the target characteristic frequency band in the hybrid three-terminal high-voltage DC transmission line; S12, determine the numerical relationship between the frequency and amplitude of the target characteristic frequency band in the rectifier / inverter side boundary frequency characteristic curve; S13, when the numerical relationship is negatively correlated, the characteristic frequency band is determined to exhibit attenuation behavior; S14, when the numerical relationship is positively correlated, the characteristic frequency band is determined to be a gain performance.

6. The method as described in claim 5, characterized in that, The expression for the boundary frequency response curve of a DC line is: ； The expression for the frequency response curve of the rectifier side boundary is: ； The expression for the inverter side boundary frequency response curve is: ； In the formula, U1(j ω ) and U3(j ω ) represent the line-side voltages of the rectifier side and the inverter side, respectively; U0(j ω ) and U2(j ω Z1(j) represents the valve-side voltage on the rectifier side and the inverter side, respectively; ω ) and Z0(j ω ) represent the complex impedances of the rectifier-side DC filter and the smoothing reactor, respectively; γ0(j ω Z2(j) represents the propagation coefficient of the DC transmission line; x represents the distance from the DC side port of the rectifier station to the fault point of the DC transmission line; ω ) and Z3(j ω ) represent the complex impedances of the inverter-side current-limiting reactor and the equivalent capacitance to ground, respectively.

7. The method as described in claim 1, characterized in that, S20 includes: S21. The acquired DC voltage signal is decomposed into N-level wavelet packets using a preset wavelet basis function to obtain 2 N Each bandwidth is f s / (2 N+1 The frequency band E1~E2 Hz N ; S22, according to the frequency bands E1~E2 N Calculate the energy E of the first characteristic frequency band of the voltage at the left and right measuring points on the rectifier / inverter side after wavelet packet decomposition. r Second characteristic band energy E l : ； ； In the formula, E ri With E li represents the energy of the voltage at the left and right measuring points on the rectifier side / inverter side within the i-th frequency band, respectively; m represents the first sub-band of the selected characteristic frequency band; and n represents the last sub-band of the selected characteristic frequency band.

8. A DC protection device for implementing the hybrid three-terminal high-voltage DC transmission line fault discrimination method based on DC boundary voltage energy ratio as described in any one of claims 1 to 7.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that, when executed by a processor, implements the steps of the hybrid three-terminal high-voltage direct current transmission line fault identification method based on DC boundary voltage energy ratio as described in any one of claims 1 to 7.

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