Single-ended protection method and system for three-terminal hybrid dc transmission system

Abstract

The application relates to a single-end protection method and system for a three-terminal hybrid DC power transmission system. By obtaining pole current and pole voltage in fault data, line mode voltage and line mode current are obtained through pole mode conversion to obtain a refraction coefficient. The fault pole is selected by using the zero mode voltage amplitude, the fault area is selected by using the refraction coefficient amplitude, the three-terminal hybrid DC system is divided into multiple areas, and the expression of the refraction coefficient of each area provides technical support for subsequent fault analysis of similar DC systems.

Description

Technical Field

[0001] This invention relates to the field of power transmission protection technology, specifically to a single-end protection method and system for three-terminal hybrid DC transmission systems. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Hybrid DC transmission technology enables long-distance, high-capacity multi-terminal power transmission. During operation, it can perform traveling wave protection based on the fault characteristics of DC grid fault currents under low-resistance and high-resistance grounding conditions, or single-end ranging protection based on the wave velocity difference between the line-mode and zero-mode traveling waves after a fault, or protection based on the smoothing of inductor voltage change rate. These protection methods still rely on the rate of change of the traveling wave in the time domain. In practical applications, three-terminal hybrid DC transmission systems contain various power electronic components. Current protection methods, due to the need to consider the combined effects of these components, reduce the reliability of the protection scheme. Simultaneously, traditional single-terminal traveling wave protection has weak high-resistance and near-end fault identification capabilities. If applied to three-terminal hybrid DC transmission systems, its applicability is difficult to control, and reliability issues remain unresolved. Summary of the Invention

[0004] To address the technical problems mentioned above, this invention provides a single-end protection method and system for three-terminal hybrid DC transmission systems. By identifying the fault area through the phase of the refractive index, it can withstand fault resistance and noise interference without communication time, thus providing technical support for the safe and reliable operation of DC transmission.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] The first aspect of the present invention provides a single-ended protection method for a three-terminal hybrid DC transmission system, comprising the following steps:

[0007] The electrode current and electrode voltage in the fault data are obtained, and the line mode voltage and line mode current are obtained through electrode mode conversion. The refractive index is then obtained.

[0008] The fault pole is selected by using the zero-mode voltage amplitude. Specifically, the zero-mode voltage at the moment of protection activation is determined. If the zero-mode voltage is higher than the threshold, a positive pole fault occurs; if the zero-mode voltage is lower than the threshold, a negative pole fault occurs; otherwise, a bipolar short-circuit fault occurs.

[0009] The fault area is selected by using the amplitude of the refractive index. Specifically, the refractive index at the moment of protection activation is determined. If the amplitude of the refractive index at the first frequency is higher than the set value and the amplitude of the refractive index at the second frequency is not infinite, then a fault occurs within the area. Otherwise, a fault occurs outside the area.

[0010] Furthermore, by using the line mode voltage and line mode current, and through pole mode conversion, the line mode voltage and line mode current are obtained, and the refractive index is obtained, as shown in the following formula:

[0011]

[0012] In the formula: α is the refractive index, u1 is the line mode voltage, i1 is the line mode current, and Z... c1 This is the line-mode impedance.

[0013] Furthermore, the fault pole is selected using the zero-mode voltage amplitude, as shown in the following formula:

[0014]

[0015] In the formula: u0 is the zero-mode voltage at the measurement point, k set The threshold for fault selection.

[0016] Furthermore, the fault region can be selected using the magnitude of the refractive index, as shown in the following formula.

[0017]

[0018] Among them, M1-M4 are all set measurement points, α (x1Hz) and α (x2Hz) All are first frequencies, α (y1Hz) and α (y2Hz) This is the second frequency.

[0019] Furthermore, for the designated measurement points M1 and M4, the first frequency α (x1Hz) For frequencies higher than the set value, the second frequency α (y1Hz) The set fixed frequency point;

[0020] For the set measurement points M2 and M3, the first frequency α (x2Hz) For the set frequency range, the second frequency α (y2Hz) The frequency point is higher than the set value.

[0021] Furthermore, based on the topology of the three-terminal hybrid DC transmission system, the regions are divided, and the propagation characteristics of the refractive index of faults within and outside the region are obtained based on Peterson's rule. The refractive index of each region is determined, the amplitude-frequency characteristics corresponding to each refractive index are obtained, and the values ​​of the first and second frequencies are determined.

[0022] Furthermore, the refractive index propagation characteristics include the refractive index measured by each protection device during the effective time of the Peterson law under forward fault conditions, and the refractive index being related to the back side region of the device.

[0023] When a fault occurs outside the reverse zone, the protection devices cannot measure the refractive index within the effective time of Peterson's law.

[0024] A second aspect of the present invention provides a system for implementing the above-described method, comprising:

[0025] The data acquisition module is configured to: acquire the polar current and polar voltage from the fault data, convert them into line mode voltage and line mode current through polar mode conversion, and obtain the refractive index;

[0026] The fault selection module is configured to select the fault pole based on the zero-mode voltage amplitude. Specifically, it determines the zero-mode voltage at the moment of protection activation. If the zero-mode voltage is higher than the threshold, a positive fault occurs; if the zero-mode voltage is lower than the threshold, a negative fault occurs; otherwise, a bipolar short-circuit fault occurs.

[0027] The fault identification module is configured to select the fault area using the amplitude of the refractive index. Specifically, it determines the refractive index at the moment the protection starts. If the amplitude of the refractive index at the first frequency is higher than the set value and the amplitude of the refractive index at the second frequency is not infinite, then a fault occurs within the area. Otherwise, a fault occurs outside the area.

[0028] Furthermore, when the fault selection module and the fault identification module meet the set conditions, they send an action signal to the faulty pole and end the action.

[0029] Compared with existing technologies, one or more of the above technical solutions have the following beneficial effects:

[0030] By using the zero-mode voltage amplitude to select the fault pole and the refractive index amplitude to select the fault region, only single-ended quantity information is required, and communication time is not relied upon, ensuring rapid response. By utilizing the characteristic that the refractive index is not affected by the fault resistance, the resistance withstand capability of the protection is improved, and the reliability is enhanced. The three-terminal hybrid DC system is divided into multiple regions, and the derived expression of the refractive index of each region provides technical support for the fault analysis of similar DC systems in the future. Attached Figure Description

[0031] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0032] Figure 1 This is a schematic diagram of a single-ended protection process for a three-terminal hybrid DC transmission system provided by one or more embodiments of the present invention;

[0033] Figure 2 This is a schematic diagram of an in-region fault in the refractive index propagation characteristics provided by one or more embodiments of the present invention;

[0034] Figure 3This is a schematic diagram of an out-of-region fault in the refractive index propagation characteristics provided by one or more embodiments of the present invention;

[0035] Figure 4 This is a schematic diagram of the refractive index amplitude characteristics provided by one or more embodiments of the present invention;

[0036] Figure 5 This is a schematic diagram of the refractive index amplitude characteristics provided by one or more embodiments of the present invention;

[0037] Figure 6 This is a schematic diagram of the refractive index amplitude characteristics provided by one or more embodiments of the present invention;

[0038] Figure 7 This is a schematic diagram of the topology of a hybrid DC transmission system provided in one or more embodiments of the present invention. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0040] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0041] As described in the background section, the reliability of protection schemes for three-terminal hybrid DC transmission systems is affected by several factors. First, a variety of power electronic components exist in three-terminal hybrid DC transmission systems, and current protection methods do not consider the combined effects of these components, thus reducing the reliability of the protection scheme. Second, the identification capabilities of traditional single-ended traveling wave protection for high-impedance faults and near-end faults still have problems, and its applicability in three-terminal hybrid DC transmission systems may still be unsatisfactory.

[0042] Therefore, the following embodiments provide a single-end protection method and system for three-terminal hybrid DC transmission systems. By identifying the fault area through the amplitude of the refractive index, the fault resistance withstand capability of the protection can be enhanced, providing technical support for the safe and reliable operation of DC transmission.

[0043] Example 1:

[0044] like Figures 1-7 As shown, a single-ended protection method for a three-terminal hybrid DC transmission system includes the following steps:

[0045] Step 1: Collect pole current and pole voltage fault data (referring to line current and line voltage in DC system), perform line pole mode conversion on the data to obtain line mode voltage and line mode current, and calculate the refractive index;

[0046] Step 2: Input the zero-mode voltage into the fault selection program and the refractive index into the fault identification program; the fault selection program uses the zero-mode voltage amplitude to select the fault pole, and the fault identification program uses the refractive index amplitude to select the fault region.

[0047] The fault selection formula is as follows:

[0048]

[0049] The fault identification formula is as follows:

[0050]

[0051] Step 3-1: The specific implementation of the fault polarity selection procedure is as follows: Calculate the zero-mode voltage at the moment of protection startup. If the zero-mode voltage is higher than the threshold, it is determined that a positive fault has occurred; if the zero-mode voltage is lower than the threshold, it is determined that a negative fault has occurred; otherwise, it is determined that a bipolar short-circuit fault has occurred.

[0052] Step 3-2: The specific implementation of the fault identification procedure is as follows: The refractive index is calculated at the moment the protection system starts. If the amplitude of the refractive index at the first frequency is higher than the threshold, and the amplitude of the refractive index at the second frequency is not infinite, the fault is determined to occur within the fault zone; otherwise, the fault is determined to occur outside the fault zone. For measurement points M1 and M4, the first and second frequencies are higher than 2kHz and 1Hz, respectively; for measurement points M2 and M3, the first and second frequencies are (1-100)Hz and higher than 5kHz, respectively.

[0053] Step 4: When the fault selection procedure and fault identification procedure meet the conditions, an action signal is sent to the faulty pole, and the procedure ends.

[0054] by Figure 7 The hybrid DC transmission system topology shown in this embodiment illustrates the specific steps as follows:

[0055] In this embodiment, the formula for calculating the refractive index using line mode voltage and line mode current is as follows:

[0056]

[0057] In the formula: α represents the refractive index, u1 represents the line mode voltage, i1 represents the line mode current, and Z... c1 This represents the line-mode impedance.

[0058] 1) Fault-selective polarity

[0059] Three-terminal hybrid DC transmission systems have a bipolar symmetrical structure. In the case of a unipolar fault, coupling effects may cause coupling information to be generated in the healthy pole, leading to malfunction of the protection system. In a bipolar short-circuit fault, the zero-mode voltage is within the threshold range. In a unipolar ground fault, a zero-mode voltage exists at the faulty pole, but not at the healthy pole. Therefore, the fault pole selection formula is:

[0060]

[0061] In the formula: u0 represents the zero-mode voltage at the measurement point, k set The threshold representing fault selection, in this embodiment, is threshold k. set It is 0.5kV.

[0062] 2) Fault Identification

[0063] (1) Analyze the propagation characteristics of the refractive index, such as Figures 2-3 As shown.

[0064] In case of a fault within the area, such as Figure 2 As shown, the prerequisite for considering Peterson's law is the process before the reflected wave reaches the measurement point for the second time; therefore, the unconsidered traveling wave process is represented by dashed lines. Taking fault f2 as an example, the traveling wave propagates from the fault point to both measurement points. The reverse traveling wave 1 generates refracted wave 2 through the LCC, and the reverse traveling wave 3 generates refracted wave 4 through MMC1. Refracted wave 4 continues to propagate to MMC2, generating refracted wave 5. The analysis method for other forward faults is the same and will not be elaborated further. The refractive index at M1 is defined as α. lcc The refractive indices at M2 and M3 are α and α, respectively. MMC12 and α MMC13 The refractive index at M4 is α MMC2 As can be seen from the traveling wave transmission process, during a forward fault, each protection device can measure the refractive index within the effective time of Peterson's law, and this index is related to the back side region of the device.

[0065] In this embodiment, M1 to M4 are the measurement points of the protection device, which are all set at the beginning and end of the line.

[0066] When a fault occurs outside the zone, such as Figure 3 As shown, based on Peterson's law, the traveling wave that determines the refractive index is represented by the solid line, while other traveling waves are represented by the dashed lines. Taking a reverse fault f1 on line l1 as an example, the traveling wave arrives at measuring point M1 from the fault point, and then travels through line l1 to M2. At this time, only the forward traveling wave (dashed line) exists at M1, and there is no reverse traveling wave. According to the definition, the denominator of the formula for calculating the refractive index is 0 at this time, that is, there is no refractive index at M1. With the arrival of reverse wave 1 and forward traveling wave 3, the refractive index can be measured at measuring points M1 and M2 after time t2, but the refractive index at this time is not within the scope of this paper. The analysis leads to the conclusion that, during faults outside the reverse zone, the protection devices cannot measure the refractive index within the effective time of Peterson's law.

[0067] (2) Then, the expressions for each refractive index are analyzed.

[0068] The refractive index α on the LCC side lcc The expression is:

[0069]

[0070] In the formula: Z L1 Z represents the equivalent impedance of the L1 current-limiting reactor. filter Z represents the equivalent impedance of a DC filter. l1 This represents the line surge impedance.

[0071] The refractive index α on the MMC1 side MMC12 and α MMC13 The expression is:

[0072]

[0073] In the formula: Z L2 L represents the equivalent impedance of the L2 current-limiting reactor; cmmc1 R cmmc1 C cmmc1 These represent the equivalent inductance, equivalent resistance, and equivalent capacitance of the MMC1 converter, respectively. l1 and Z l2 These represent the wave impedances of lines l1 and l2, respectively.

[0074] The refractive index α on the MMC2 side MMC2 The expression is:

[0075]

[0076] In the formula: Z L3 L represents the equivalent impedance of the L3 current-limiting reactor; cmmc2 R cmmc2 C cmmc2 These represent the equivalent inductance, equivalent resistance, and equivalent capacitance of the MMC2 converter, respectively. l2 This represents the wave impedance of line l2.

[0077] (3) Next, we analyze the amplitude-frequency characteristics of the three refractive indices.

[0078] like Figures 4-6 As shown, the amplitude-frequency characteristics of the refractive index at the four measurement points can be obtained, as detailed below:

[0079] Measurement point M1: When there is a fault outside the reverse zone, the refractive index is absent, and the amplitude is much greater than 2. Note: The range of the refractive index is (0, 2); when there is a fault inside the zone, the amplitude of the refractive index above 2kHz is greater than 1.8; when there is a fault outside the forward zone, the amplitude of the refractive index at 9.8Hz is absent and close to 0.

[0080] Measurement points M2 and M3: When there is a fault outside the reverse zone, the refractive index is absent and the amplitude is much greater than 2; when there is a fault inside the zone, the amplitude of the refractive index at (1-100) Hz is greater than 0.2; when there is a fault outside the forward zone, the refractive index above 5 kHz is absent.

[0081] Measurement point M4: When there is a fault outside the reverse zone, the refractive index is absent, and the amplitude is much greater than 2. Note: The range of the refractive index is (0, 2); when there is a fault inside the zone, the amplitude of the refractive index above 2kHz is greater than 1.8; when there is a fault outside the forward zone, the amplitude of the refractive index at 9.8Hz is absent and close to 0.

[0082] (4) Based on the analysis above, a single-end protection scheme based on the magnitude of the refractive index is proposed, and the criteria are as follows:

[0083]

[0084] If the amplitude of the refractive index at the first frequency is higher than the threshold, and the amplitude of the refractive index at the second frequency is not infinite, the fault is determined to occur within the fault zone; otherwise, the fault is determined to occur outside the fault zone.

[0085] For measurement points M1 and M4, the first frequency and the second frequency are higher than 2kHz and 1Hz, respectively;

[0086] For measurement points M2 and M3, the first frequency and the second frequency are (1-100) Hz and above 5 kHz, respectively.

[0087] The above process divides the three-terminal hybrid DC system into multiple regions. Based on the detailed expression of the refractive index for each region, it provides technical support for fault analysis of similar DC systems. Existing research on three-terminal hybrid DC systems follows the conventional DC approach, focusing on the line and the inductive components on both sides. This approach does not consider various components on the boundary back side, resulting in insufficient understanding of the system and inevitably leading to unreliable judgment criteria. This proposed solution, however, divides the entire system into several regions, considering all components within the system's scope, thus further improving the reliability of the proposed judgment criteria.

[0088] The proposed protection scheme requires only single-ended data, eliminating reliance on communication time and ensuring rapid response. Existing schemes, to improve reliability, use information from both ends to identify faults, requiring signal transmission to the other end. This communication time reduces the speed of response. The proposed scheme only needs single-ended current and voltage data to identify faults inside and outside the protection zone, eliminating the need for signal transmission to the other end and thus eliminating communication time.

[0089] This protection scheme utilizes the characteristic that the refractive index is unaffected by fault resistance, thereby improving the protection's resistance withstand capability and enhancing reliability. As the formula for calculating the refractive index shows, its calculation process is completely independent of the fault resistance R; that is, the phase calculation of the refractive index does not involve the fault resistance. Applying a higher fault resistance will not change the phase of the refractive index, thus not altering the reliability of the criterion. In contrast, the calculation formula for the criterion in traditional schemes includes the fault resistance R; high-resistance faults will change the criterion, thereby affecting its reliability.

[0090] Example 2:

[0091] A system for implementing the above method includes:

[0092] The data acquisition module is configured to acquire the pole current and pole voltage from the fault data and determine the fault occurrence by comparing the current amplitude.

[0093] The data processing module is configured to: after a fault occurs, the polar voltage and polar current are converted into polar modes, and the phase of the refractive index is determined based on the obtained line mode voltage and line mode current; and the correspondence between the phase of the refractive index and the frequency is determined based on the equivalent circuit.

[0094] The fault identification module is configured such that: if the phase of the refractive index corresponding to the first frequency is within the second threshold range, a fault occurs in the positive region;

[0095] The fault identification module is also configured to: if the phase of the refractive index corresponding to the second frequency is less than the first threshold, a fault occurs outside the reverse region;

[0096] The fault identification module is also configured to detect faults outside the positive region when the phase of the refractive index corresponding to the third frequency is less than the first threshold.

[0097] By using the zero-mode voltage amplitude to select the fault pole and the refractive index amplitude to select the fault region, only single-ended quantity information is required, and communication time is not relied upon, ensuring rapid response. By utilizing the characteristic that the refractive index is not affected by the fault resistance, the resistance withstand capability of the protection is improved, and the reliability is enhanced. The three-terminal hybrid DC system is divided into multiple regions, and the derived expression of the refractive index of each region provides technical support for the fault analysis of similar DC systems in the future.

[0098] Example 3:

[0099] This embodiment provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the steps in the single-ended protection method for a three-terminal hybrid DC transmission system as described in Embodiment 1 above.

[0100] Example 4:

[0101] This embodiment provides a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, it implements the steps in the single-ended protection method for a three-terminal hybrid DC transmission system as described in Embodiment 1 above.

[0102] The steps or networks involved in Embodiments 2 to 4 above correspond to those in Embodiment 1. For specific implementation details, please refer to the relevant description section of Embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood as including any medium capable of storing, encoding, or carrying an instruction set for execution by a processor and enabling the processor to perform any of the methods in this invention.

[0103] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A single-ended protection method for a three-terminal hybrid DC transmission system, characterized in that, Includes the following steps: The electrode current and electrode voltage in the fault data are obtained, and the line mode voltage and line mode current are obtained through electrode mode conversion. The refractive index is then obtained. The fault pole is selected by using the zero-mode voltage amplitude. Specifically, the zero-mode voltage at the moment of protection activation is determined. If the zero-mode voltage is higher than the threshold, a positive pole fault occurs; if the zero-mode voltage is lower than the threshold, a negative pole fault occurs; otherwise, a bipolar short-circuit fault occurs. The fault area is selected by using the amplitude of the refractive index. Specifically, the refractive index at the moment of protection activation is determined. If the amplitude of the refractive index at the first frequency is higher than the set value and the amplitude of the refractive index at the second frequency is not infinite, then a fault occurs within the area. Otherwise, a fault occurs outside the area. The fault region is selected using the magnitude of the refractive index, as shown in the following formula: ； Among them, M1-M4 are all set measurement points. and All are first frequencies. and The second frequency; For the set measurement points M1 and M4, the first frequency The second frequency is the frequency point higher than the set value. The set fixed frequency point; for the set measurement points M2 and M3, the first frequency For the set frequency range, the second frequency The frequency point is higher than the set value; Based on the topology of the three-terminal hybrid DC transmission system, the regions are divided. The propagation characteristics of the refractive index of faults inside and outside the region are obtained based on Peterson's rule. The refractive index of each region is determined, and the amplitude-frequency characteristics corresponding to each refractive index are obtained. The values ​​of the first frequency and the second frequency are determined. The refractive index propagation characteristics include the following: during a forward fault, each protection device measures the refractive index within the effective time of Peterson's law, and this index is related to the back side region of the device. The propagation characteristic of the refractive index also includes that, in the event of a fault outside the reverse region, each protection device cannot measure the refractive index within the effective time of Peterson's law.

2. The single-ended protection method for a three-terminal hybrid DC transmission system as described in claim 1, characterized in that, The refractive index is obtained by using the line-mode voltage and line-mode current, through pole-mode conversion, as shown in the following formula: ； In the formula: α The refractive index is 1. u 1 represents the line-mode voltage. i 1 represents the line-mode current. Z c1 This is the line-mode impedance.

3. The single-ended protection method for a three-terminal hybrid DC transmission system as described in claim 1, characterized in that, The fault pole is selected using the zero-mode voltage amplitude, as shown in the following formula: ； In the formula: u 0 represents the zero-mode voltage at the measurement point. k set The threshold for fault selection.

4. The single-ended protection method for a three-terminal hybrid DC transmission system as described in claim 1, characterized in that, The refractive index at measurement point M1 α lcc As shown in the following formula: ； In the formula: Z L1 represent L 1. Equivalent impedance of current-limiting reactor Z filter Represents the equivalent impedance of a DC filter. Z l1 This represents the line surge impedance.

5. The single-ended protection method for a three-terminal hybrid DC transmission system as described in claim 1, characterized in that, The refractive indices at measurement points M2 and M3 are respectively and As shown in the following formula: ； In the formula: Z L2 represent L 2. Equivalent impedance of the current-limiting reactor; L cmmc1 , R cmmc1 , C cmmc1 These represent the equivalent inductance, equivalent resistance, and equivalent capacitance of the MMC1 converter, respectively. Z l1 and Z l2 These represent the wave impedances of lines l1 and l2, respectively.

6. The single-ended protection method for a three-terminal hybrid DC transmission system as described in claim 1, characterized in that, The refractive index at measurement point M4 As shown in the following formula: ； In the formula: Z L3 represent L 3. Equivalent impedance of the current-limiting reactor; L cmmc2 , R cmmc2 , C cmmc2 These represent the equivalent inductance, equivalent resistance, and equivalent capacitance of the MMC2 converter, respectively. Z l2 This represents the wave impedance of line l2.

7. A single-ended protection system for a three-terminal hybrid DC transmission system, characterized in that, include: The data acquisition module is configured to: acquire the polar current and polar voltage from the fault data, convert them into line mode voltage and line mode current through polar mode conversion, and obtain the refractive index; The fault selection module is configured to select the fault pole based on the zero-mode voltage amplitude. Specifically, it determines the zero-mode voltage at the moment of protection activation. If the zero-mode voltage is higher than the threshold, a positive fault occurs; if the zero-mode voltage is lower than the threshold, a negative fault occurs; otherwise, a bipolar short-circuit fault occurs. The fault identification module is configured to select the fault area using the amplitude of the refractive index. Specifically, it determines the refractive index at the moment of protection activation. If the amplitude of the refractive index at the first frequency is higher than the set value and the amplitude of the refractive index at the second frequency is not infinite, then a fault occurs within the area; otherwise, a fault occurs outside the area. The fault region is selected using the magnitude of the refractive index, as shown in the following formula: ； Among them, M1-M4 are all set measurement points. and All are first frequencies. and The second frequency; For the set measurement points M1 and M4, the first frequency The second frequency is the frequency point higher than the set value. The set fixed frequency point; for the set measurement points M2 and M3, the first frequency For the set frequency range, the second frequency The frequency point is higher than the set value; Based on the topology of the three-terminal hybrid DC transmission system, the regions are divided. The propagation characteristics of the refractive index of faults inside and outside the region are obtained based on Peterson's rule. The refractive index of each region is determined, and the amplitude-frequency characteristics corresponding to each refractive index are obtained. The values ​​of the first frequency and the second frequency are determined. The propagation characteristics of the refractive index include the following: during a forward fault, each protection device measures the refractive index within the effective time of Peterson's law, and this index is related to the back side region of the device. The propagation characteristic of the refractive index also includes that, in the event of a fault outside the reverse region, each protection device cannot measure the refractive index within the effective time of Peterson's law.

8. The single-ended protection system for a three-terminal hybrid DC transmission system as described in claim 7, characterized in that, When the fault selection module and the fault identification module meet the set conditions, they send an action signal to the faulty pole and end the action.

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