Cable operation state auxiliary diagnosis method and system

Abstract

The invention discloses a cable operation state auxiliary diagnosis method and system. The method comprises the following steps: establishing a relational expression between a reference section sheath ring current and a magnetic field induction current and a leakage current in a metal sheath, and obtaining a calculation formula of the magnetic field induction current and the leakage current of the reference section; selecting load and sheath circulation monitoring data during operation according to a laying mode of a reference section line, and solving magnetic field induction current and leakage current of the reference section; according to the change rules of the magnetic field induction current and the leakage current under various influence factors, combining the magnetic field induction current and the leakage current of the reference section, and calculating a cable sheath ring current theoretical value of the target section; and comparing the theoretical value with the measured value to obtain an auxiliary diagnosis result of the cable operation state. According to the invention, cable operation state auxiliary diagnosis based on monitoring data of different positions of the cable is realized, and the diagnosis result of the cable operation state can be more accurate in cooperation with other diagnosis criteria.

Description

Technical Field

[0001] This invention belongs to the field of cable online monitoring technology, specifically a method and system for auxiliary diagnosis of cable operating status. Background Technology

[0002] In high-voltage single-core cable metallic sheath grounding systems, sheath current is a crucial indicator for defect diagnosis. Existing standards specify the following for sheath grounding current: 1) absolute value of grounding current <100A; 2) ratio of grounding current to load current <20%, with no significant change compared to historical data; 3) ratio of maximum to minimum single-phase grounding current <3. However, these standards lack sufficient precision and cannot determine whether the sheath circulating current near the critical value indicates a normal or fault condition. Therefore, some scholars have proposed using a special digital coding system to represent the sheath circulating current to distinguish between normal and fault conditions; others have reduced the impact of capacitive current on the accuracy of the criteria by eliminating capacitive current.

[0003] The above methods are based on theoretical calculations of sheath current using cable circuit parameters, or on historical normal data recorded during initial defect-free operation. However, in actual engineering projects, due to the lack of precise cable electrical and structural parameters, cable laying methods, and the length of cross-connection sections, it is difficult to accurately calculate the sheath circulating current value under defect-free conditions using theoretical formulas. Under the influence of various errors, misjudgments may occur. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the defects of the prior art and provide a method and system for auxiliary diagnosis of cable operation status. Based on the field measurement of the sheath circulation current value of some cable cross-interconnection sections, the theoretical value of sheath circulation current of the remaining cable cross-interconnection sections is inferred. The simplified process in the calculation and the influence of various factors affecting sheath circulation current are fully considered, so that the calculation results are more accurate. It can also deduce some theoretical values ​​of sheath circulation current that cannot be obtained by calculation, thereby realizing auxiliary diagnosis of cable operation status based on monitoring data of different cable locations.

[0005] Therefore, the present invention adopts the following technical solution.

[0006] In a first aspect, the present invention provides an auxiliary diagnostic method for cable operating status, comprising: Step 1: Select n cross-interconnection sections of a long-distance high-voltage cable line under normal operation as reference sections; Step 2: Establish the relationship between the sheath circulation current of the reference section and the magnetic field induced current and leakage current in the metal sheath, and obtain the calculation formulas for the magnetic field induced current and leakage current of the reference section based on the relationship. Step 3: Select the load and sheath circulation monitoring data during operation according to the laying method of the reference section line, and use the calculation formula in Step 2 to solve the magnetic field induced current and leakage current of the reference section. Step 4: Based on the variation patterns of magnetic field induced current and leakage current under various influencing factors, and combined with the magnetic field induced current and leakage current data of the reference segment obtained in Step 3, obtain the magnetic field induced current and leakage current of the target segment. Step 5: Calculate the theoretical value of the circulating current in the cable sheath of the target section using the magnetic field induced current and leakage current of the target section. Step 6: Compare the theoretical value of the circulating current in the cable sheath of the target section with the measured value to obtain the auxiliary diagnostic results of the cable's operating status.

[0007] Furthermore, in step 1, the selection of the reference section must ensure that its cable loop parameters are consistent with those of the target section.

[0008] Furthermore, in step 2, the relationship between the sheath circulation current, the magnetic field induced current, and the leakage current in the reference section is as follows: , in, I Ri , I MRi For reference i Sheath circulating current and magnetic field induced current under load current I LRi For reference i The load current, I Rj , I MRj For another reference segment j Sheath circulating current and magnetic field induced current under load current I LRj For reference j The load current, of which, i =1,2,3,4,…, n, j= 1,2,3,4,…, n , n It is a positive integer, and , ; I PR The leakage current of the reference section is represented by the dots above the symbol, which represent the values ​​of the phasors; the sheath circulation current is the value measured at the end of the reference section.

[0009] Furthermore, in step 2, the calculation formulas for the magnetic field induced current and leakage current of the reference segment are obtained according to the aforementioned relationship: .

[0010] Furthermore, in step 3, when the reference section line is operating normally, for the delta-layout configuration, the load currents of two reference sections of different lengths are selected. I LR1 、I LR2 Underlying protective layer circulation I R1 、I R2 The magnetic field induced current in each sheath is calculated using the formula from step 2. I MR1 、I MR2 and leakage current I PR For horizontal or right-angle installations, select the load current of three reference sections of different lengths. I LR1 、I LR2 、I LR3 Underlying protective layer circulation I R1 、I R2 、I R3 The magnetic field induced current in each sheath is calculated using the formula from step 2. I MR1 、I MR2 、I MR3 and leakage current I PR .

[0011] Furthermore, in step 4, under the triangular laying method, the calculation formulas for the magnetic field induced current and leakage current of the target section are as follows: The magnetic field induced current in the target segment is obtained by the following formula: , , The leakage current of the target section is obtained by the following formula: , , in, I MN The magnetic field induced current in the target segment, I PN The leakage current of the target section; k 1. k Both 2 are proportionality coefficients; I LO , , These are the load current of the target section, the length imbalance, and the average length of the small section, respectively. , These are the length imbalance of the corresponding reference segment and the average segment length of the small segment, respectively. Length imbalance = ,in, , , These represent the maximum, minimum, and average values ​​of the three sub-segments of the cross-connection segment.

[0012] Furthermore, in step 4, the formulas for calculating the magnetic field induced current and leakage current of the target section under horizontal and right-angle laying methods are as follows: The magnetic field induced current in the target segment is obtained by the following formula: , in, k 3. k 4. k 5 are all coefficients, obtained from the following formula: , , in, k b This represents the exponential part of the equation relating resistance to sheath circulation current. R O The grounding resistance of the target section. R R This is the grounding resistance of the reference section.

[0013] Furthermore, in horizontal and right-angle laying methods, the leakage current is proportional to the average length of the short segment, and its calculation method is consistent with that of the triangular laying method.

[0014] Furthermore, in step 5, the magnetic field induced current of the target segment is added to the leakage current of the target segment to obtain the theoretical value of the circulating current of the cable sheath in the target segment. In step 6, the cable operating status is diagnosed based on the error between the measured value and the theoretical value of the circulating current in the cable sheath of the target section, and information on normal cable operation, fault or warning is given. Warnings need to be diagnosed in conjunction with other diagnostic criteria.

[0015] Secondly, the present invention provides a cable operating status auxiliary diagnostic system for implementing the aforementioned cable operating status auxiliary diagnostic method, comprising: Reference segment selection unit: used to select n cross-interconnected segments of a long-distance high-voltage cable line as reference segments under normal operation; Calculation formula acquisition unit: Establish the relationship between the sheath circulation current of the reference segment and the magnetic field induced current and leakage current in the metal sheath, and obtain the calculation formulas for the magnetic field induced current and leakage current of the reference segment based on the relationship. Reference section current calculation unit: Select the load and sheath circulation current monitoring data during operation according to the laying method of the reference section line, and use the calculation formula to solve the magnetic field induced current and leakage current of the reference section. Target segment current acquisition unit: Based on the variation law of magnetic field induced current and leakage current under various influencing factors, and combined with the magnetic field induced current and leakage current data of the reference segment, the magnetic field induced current and leakage current of the target segment are obtained. Sheath Circulation Current Theoretical Value Calculation Unit: Calculates the theoretical value of cable sheath circulation current in the target section using the magnetic field induced current and leakage current of the target section.

[0016] Auxiliary diagnostic criterion unit: used to compare the theoretical value of the circulating current in the cable sheath of the target section with the measured value to obtain auxiliary diagnostic results of the cable's operating status.

[0017] This invention provides a theoretical calculation of sheath circulation current based on measured sheath circulation current values, which can take into account a variety of influencing factors that are often ignored or oversimplified in theoretical analysis, including changes in soil resistivity along the cable route, electromagnetic interference from adjacent cable lines, and irregularities in physical laying.

[0018] The beneficial effects of this invention are: 1. Compared with the traditional calculation of the theoretical value of sheath circulation current, the present invention can infer the theoretical value of sheath circulation current of the target cable segment based on the actual operation of similar cables in the absence of accurate electrical and structural parameters and layout of the cable. It can realize auxiliary diagnosis of the operation status based on monitoring data of different locations of the cable, and the diagnosis results can be more accurate when combined with other diagnostic criteria.

[0019] 2. This invention takes into account a number of influencing factors that are often ignored or oversimplified in theoretical analysis, including changes in soil resistivity along cable routes, electromagnetic interference from adjacent cable routes, and irregularities in physical laying, making the calculation results more accurate and the diagnostic criteria more precise. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 This is a simplified lumped parameter equivalent circuit diagram of the magnetic field induced current in the metal sheath circuit of the present invention; Figure 2 This is a simplified lumped parameter equivalent circuit diagram of the leakage current in the metal sheath circuit of the present invention; Figure 3 This is a flowchart of an auxiliary diagnostic method for cable operating status according to the present invention; Figure 4 This is a diagram showing the composition of a cable operation status auxiliary diagnostic system according to the present invention. Detailed Implementation

[0022] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1 The following will discuss the protective layer circulation ( I R ) is broken down into magnetic field induced current ( I MR and leakage current (I PR ) The influence of grounding resistance, average length of small segments in cross-interconnection sections, unbalance in cross-interconnection section length, and load current on two types of currents were analyzed using calculation models.

[0024] like Figure 1 As shown, the magnetic field induced currents in the three sheath circulation loops are respectively I M1 、I M2 、I M3 The lengths of each cross-connection segment are respectively 、 、 , R The grounding resistance at both ends of the cross-connection segment. L Ai 、L Bi 、L Ci These are the self-inductances of each small segment of the cross-connection segment. E Ai 、E Bi 、E CiThe load current flowing through the cable core in the first... i The induced electromotive force induced on the sheath of a cross-connected short cable segment. i =1,2,3.

[0025] Formula (1) is the method for calculating the magnetic field induced current. Taking the sheath circulation loop 1 as an example, the total potential ( E 1) is E A1 、E B1 , E C1 The sum of three individual potentials; the rest of the loops are similar. j Represents the imaginary unit; Indicates angular frequency; L A 、L B 、L C These are the unit length self-inductances of the three-phase metallic sheath; M A 、M B 、M C These represent the mutual inductance per unit length between each phase conductor and its corresponding metal sheath. The dots above the symbols represent phasors. All physical quantities involved in the calculations of this invention are phasors.

[0026] (1) like Figure 2 As shown, the leakage currents in the three sheath circulation loops are respectively I p1 、I p2 、I p3 ，U A 、U B 、U C This indicates the three-phase core wire voltage of the cable. C Ai 、C Bi 、C Ci ( i =1,2,3) represents the capacitance of each cross-connection segment, I CA1 、I CB1 、I CC1 They represent passing through C A1 、CB1 、C C1 The current in the capacitor.

[0027] The following formula is the solution method for leakage current. , , express I CA1 , I CB1 , I CC1 The proportion of leftward flow; L A , L B , L C The self-inductance coefficient per unit length of the three-phase metallic sheath; C 0 represents the capacitance per unit length of the cable core to the sheath.

[0028] (2) The following factors are taken into consideration: (1) Grounding resistance 1) Magnetic field induced current The influence of grounding resistance on magnetic field induced current is fitted by function, and the fitting relationship is shown in equation (3). Grounding resistance and magnetic field induced current show an inverse correlation.

[0029] (3) in, k a 、k b All are coefficients.

[0030] 2) Leakage current In all cable laying methods, the leakage current of the cable remains almost constant as the resistance increases. Therefore, the effect of grounding resistance on the sheath circulation current is mainly magnetic field-induced current.

[0031] (2) Length imbalance of cross-connection segments This invention defines length imbalance ( As shown in equation (4): (4) in, , , They are respectively 、 、 The maximum, minimum and average values.

[0032] 1) Magnetic field induced current Of all the laying methods, except In very small amounts, this effect depends on the grounding resistance, and therefore a non-linear relationship may occur. Beyond this range, the magnetic field-induced current... I M Overall length imbalance rate The relationship is linear, as shown in equation (5): (5) in k c 、k d is a coefficient.

[0033] 2) Leakage current Length imbalance rate With leakage current I P The linear relationship is shown by the fitting function in equation (6): (6) in k e 、k f is a coefficient.

[0034] (3) Average length of cross-connected segments 1) Magnetic field induced current In a delta-layout configuration, the induced current is approximately zero because the magnetic field-induced voltage is... E 1 、E 2 and E The vector sums of 3 cancel each other out. For horizontal and right-angle installations, E 1 、E 2 and E The sum of 3 and 4 varies linearly with the length of the short segment. However, due to the grounding resistance... R The impact, I M and The relationship between them becomes nonlinear. The relationship is shown in equation (7): (7) in k g 、k h is a coefficient.

[0035] 2) Leakage current Of all cable laying methods, leakage current is the most common. I P and They exhibit a linear proportional relationship. This relationship is due to the capacitance. C A1 , C B1 and C C1 It is directly proportional to the length of the short cable segment, while the conductivity is... , , Keeping the length constant, the average segment length is related to the leakage current. I P The linear relationship between them can be expressed by the fitting function of equation (8): (8) in k i 、k j is a coefficient.

[0036] (4) Load current 1) Magnetic field induced current In the triangular laying method, when the short segment length 、 、 When they are equal, the induced current is zero. I M The correlation with the load current is negligible; for horizontal and right-angle installations, since the three-phase induced voltages are equal, therefore... I M and I L The relationship is linear, as shown in equation (9): (9) in k k is a coefficient.

[0037] When the cable circuit parameters are known, the magnetic field induced current and leakage current can be accurately calculated using equations (1) and (2), and then superimposed to obtain the theoretical value of the sheath circulation current. I N If the cable layout is known and the circuit parameters are the same as those of this invention, the fitted formula can be used to estimate the magnetic field induced current and leakage current. When the circuit parameters are unknown, and the cable inductance and capacitance parameters are unclear, the theoretical value of the sheath circulating current can be calculated using one of the following cable operating conditions as an auxiliary diagnostic tool.

[0038] A method for auxiliary diagnosis of cable operating status, such as Figure 3 As shown, the steps are as follows: Step 1: Select n cross-interconnection sections of a long-distance high-voltage cable line under normal operation as reference sections; Step 2: Establish the relationship between the sheath circulation current of the reference section and the magnetic field induced current and leakage current in the metal sheath, and obtain the calculation formulas for the magnetic field induced current and leakage current of the reference section based on the relationship. Step 3: Select the load and sheath circulation monitoring data during operation according to the laying method of the reference section line, and use the calculation formula in Step 2 to solve the magnetic field induced current and leakage current of the reference section. Step 4: Based on the variation patterns of magnetic field induced current and leakage current under various influencing factors, and combined with the magnetic field induced current and leakage current data of the reference segment obtained in Step 3, obtain the magnetic field induced current and leakage current of the target segment. Step 5: Calculate the theoretical value of the circulating current in the cable sheath of the target section using the magnetic field induced current and leakage current of the target section. Step 6: Compare the theoretical value of the circulating current in the cable sheath of the target section with the measured value to obtain the auxiliary diagnostic results of the cable's operating status.

[0039] The above method is based on two characteristics: the similarity between the reference and target segments of the cable crossover sections, and the quantifiable relationship between sheath circulation current and its influencing factors. Unlike traditional methods, the method of this invention completely bypasses the need to calculate cable inductance and induced voltage, and takes into account various influencing factors that are often ignored or oversimplified in theoretical analysis, including variations in soil resistivity along the cable route, electromagnetic interference from adjacent cable lines, and irregularities in physical laying.

[0040] Specifically, in step 1, the selection of the reference segment must ensure that its cable loop parameters are consistent with those of the target segment. In practical applications, the reference segment can be selected from other segments of the same cable line as the target segment, or from the corresponding segment of an adjacent parallel line of the target segment.

[0041] Specifically, in step 2, the relationship between the sheath circulation current, magnetic field induced current, and leakage current of the reference section is as follows:

[0042] in, I Ri , I MRi For reference i Sheath circulating current and magnetic field induced current under load current I LRi For reference i The load current, I Rj , I MRj For another reference segment jSheath circulating current and magnetic field induced current under load current I LRj For reference j The load current, of which, i =1,2,3,4,…, n, j= 1,2,3,4,…, n , n It is a positive integer, and , ; I PR The leakage current of the reference section is represented by the dots above the symbol, which represent the values ​​of the phasors; the sheath circulation current is the value measured at the end of the reference section.

[0043] Based on the above relationships, the formulas for calculating the magnetic field induced current and leakage current of the reference segment are obtained as follows: .

[0044] Specifically, in step 3, when the reference section line is operating normally, for the delta-layout configuration, the load currents of two reference sections of different lengths are selected. I LR1 、I LR2 Underlying protective layer circulation I R1 、I R2 The magnetic field induced current in each sheath is calculated using the formula from step 2. I MR1 、I MR2 and leakage current I PR For horizontal or right-angle installations, select the load current of three reference sections of different lengths. I LR1 、I LR2 、I LR3 Underlying protective layer circulation I R1 、I R2 、I R3 The magnetic field induced current in each sheath is calculated using the formula from step 2. I MR1 、I MR2 、I MR3 and leakage current I PR .

[0045] Specifically, in step 4, under the triangular laying method, the calculation formulas for the magnetic field induced current and leakage current of the target section are as follows: The magnetic field induced current in the target segment is obtained by the following formula:

[0046]

[0047] The leakage current of the target section is obtained by the following formula:

[0048]

[0049] in, I MN The magnetic field induced current in the target segment, I PN The leakage current of the target section; k 1. k Both 2 are proportionality coefficients; I LO , , These are the load current of the target section, the length imbalance, and the average length of the small section, respectively. , These are the length imbalance of the corresponding reference segment and the average segment length of the small segment, respectively. Length imbalance = ,in, , , These represent the maximum, minimum, and average values ​​of the three sub-segments of the cross-connection segment.

[0050] The formulas for calculating the magnetic field induced current and leakage current of the target section under horizontal and right-angle laying methods are as follows: The magnetic field induced current in the target segment is obtained by the following formula:

[0051] in, k 3. k 4. k 5 are all coefficients, obtained from the following formula:

[0052]

[0053] in, k b This represents the exponential part of the equation relating resistance to sheath circulation current. R O The grounding resistance of the target section. RR This is the grounding resistance of the reference section.

[0054] In horizontal and right-angle laying methods, the leakage current is proportional to the average length of the short segment, and its calculation method is the same as that for triangular laying method.

[0055] In step 5, the magnetic field induced current of the target section is added to the leakage current of the target section to obtain the theoretical value of the circulating current of the cable sheath in the target section.

[0056]

[0057] Among them, I N The protective layer circulation for the target section.

[0058] In step 6, the cable operating status is diagnosed based on the error between the measured value and the theoretical value of the circulating current in the cable sheath of the target section, and information on normal cable operation, fault or warning is given. Warnings need to be diagnosed in conjunction with other diagnostic criteria.

[0059] The following simulation was performed using the method described above: Taking a triangular installation as an example, two reference segments are selected, and the values ​​of the reference segments are shown as R1 in Table 1. By solving for the magnetic field induced current and leakage current, the magnetic field induced current and leakage current under a 500A load current are calculated as I. MR =5.152、I PR =2.172. T1 is the target segment number, followed by the parameters of the target segment. Substitute the parameters of the reference segment into the calculation formula of the proportional coefficient k to obtain the proportional coefficient. Then, substitute the parameters of the target segment to obtain the magnetic field induced current and leakage current of the target segment. Finally, synthesize the sheath circulation current of the target segment. The calculation results are shown in Table 1.

[0060] Table 1. Estimation results for the triangular laying method

[0061] Eight representative cross-interconnection reference segments were selected. The normal sheath current I of the target reference segments was calculated using the method of this invention. N The results show a strong consistency with the simulated values, with all errors remaining below 4%.

[0062] Horizontal or right-angled laying method: Taking a horizontal laying of 0.2m as an example, three reference sections of different lengths are selected. The values ​​of the reference sections are shown as R1, R2, and R3 in Table 2. By solving for the magnetic field induced current and leakage current, the magnetic field induced current and leakage current under each parameter are calculated. T1 is the target section number, followed by the parameters of the target section. Substituting the parameters of the reference sections into the proportionality coefficient k, the proportionality coefficient is obtained. Then, substituting the parameters of the target sections, the magnetic field induced current and leakage current of the target sections are obtained, and finally, the sheath circulation current of the target section is synthesized.

[0063] Table 2. Estimation results for horizontal laying method

[0064] Sixteen representative cross-connection segments were selected. The normal sheath current I of these cross-connection segments was calculated. N All errors remained below 6%.

[0065] Example 2 This embodiment provides an auxiliary diagnostic system for cable operating status, used to implement the method described in Embodiment 1, such as... Figure 4 As shown, it consists of a reference segment selection unit, a calculation formula acquisition unit, a reference segment current solution unit, a target segment current acquisition unit, a sheath circulation theoretical value calculation unit, and an auxiliary diagnostic criterion unit.

[0066] Reference segment selection unit: used to select n cross-interconnected segments of a long-distance high-voltage cable line as reference segments under normal operation.

[0067] Calculation formula acquisition unit: Establish the relationship between the sheath circulation current of the reference segment and the magnetic field induced current and leakage current in the metal sheath, and obtain the calculation formulas for the magnetic field induced current and leakage current of the reference segment based on the relationship.

[0068] Reference section current calculation unit: Select the load and sheath circulation current monitoring data during operation based on the laying method of the reference section line, and use the calculation formula to solve the magnetic field induced current and leakage current of the reference section.

[0069] Target segment current acquisition unit: Based on the variation law of magnetic field induced current and leakage current under various influencing factors, and combined with the magnetic field induced current and leakage current data of the reference segment, the magnetic field induced current and leakage current of the target segment are obtained.

[0070] Sheath Circulation Current Theoretical Value Calculation Unit: Calculates the theoretical value of cable sheath circulation current in the target section using the magnetic field induced current and leakage current of the target section; Auxiliary diagnostic criterion unit: used to compare the theoretical value of the circulating current in the cable sheath of the target section with the measured value to obtain auxiliary diagnostic results of the cable's operating status.

[0071] It should be noted that each unit in the aforementioned cable operation status auxiliary diagnostic system can be implemented entirely or partially through software, hardware, or a combination thereof. These units can be embedded in or independent of the processor in a computer device in hardware form, or stored in the memory of a computer device in software form, so that the processor can call and execute the operations corresponding to each unit. For specific limitations regarding the cable operation status auxiliary diagnostic system, please refer to the limitations of the cable operation status auxiliary diagnostic method (i.e., Embodiment 1) above; both have the same function and role, and will not be repeated here.

[0072] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to the above embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A method for auxiliary diagnosis of cable operating status, characterized in that, include: Step 1: Select n cross-interconnection sections of a long-distance high-voltage cable line under normal operation as reference sections; Step 2: Establish the relationship between the sheath circulation current of the reference section and the magnetic field induced current and leakage current in the metal sheath, and obtain the calculation formulas for the magnetic field induced current and leakage current of the reference section based on the relationship. Step 3: Select the load and sheath circulation monitoring data during operation according to the laying method of the reference section line, and use the calculation formula in Step 2 to solve the magnetic field induced current and leakage current of the reference section. Step 4: Based on the variation patterns of magnetic field induced current and leakage current under various influencing factors, and combined with the magnetic field induced current and leakage current data of the reference segment obtained in Step 3, obtain the magnetic field induced current and leakage current of the target segment. Step 5: Calculate the theoretical value of the circulating current in the cable sheath of the target section using the magnetic field induced current and leakage current of the target section. Step 6: Compare the theoretical value of the circulating current in the cable sheath of the target section with the measured value to obtain the auxiliary diagnostic results of the cable's operating status.

2. The method according to claim 1, characterized in that, In step 1, the selection of the reference section must ensure that its cable loop parameters are consistent with those of the target section.

3. The method according to claim 1, characterized in that, In step 2, the relationship between the sheath circulation current, magnetic field induced current, and leakage current in the reference section is as follows: ， in, I Ri , I MRi For reference i Sheath circulating current and magnetic field induced current under load current I LRi For reference i The load current, I Rj , I MRj For another reference segment j Sheath circulating current and magnetic field induced current under load current I LRj For reference j The load current, of which, i =1,2,3,4,…, n, j= 1,2,3,4,…, n , n It is a positive integer, and , ; I PR The leakage current of the reference section is represented by the dots above the symbol, which represent the values ​​of the phasors; the sheath circulation current is the value measured at the end of the reference section.

4. The method according to claim 3, characterized in that, In step 2, the calculation formulas for the magnetic field induced current and leakage current of the reference segment are obtained according to the aforementioned relationship: 。 5. The method according to claim 4, characterized in that, In step 3, when the reference section line is operating normally, for the delta-layout configuration, the load currents of two reference sections of different lengths are selected. I LR1 、I LR2 Underlying protective layer circulation I R1 、I R2 The magnetic field induced current in each sheath is calculated using the formula from step 2. I MR1 、I MR2 and leakage current I PR For horizontal or right-angle installations, select the load current of three reference sections of different lengths. I LR1 、I LR2 、I LR3 Underlying protective layer circulation I R1 、I R2 、 I R3 The magnetic field induced current in each sheath is calculated using the formula from step 2. I MR1 、I MR2 、I MR3 and leakage current I PR .

6. The method according to claim 5, characterized in that, In step 4, under the triangular laying method, the calculation formulas for the magnetic field induced current and leakage current of the target section are as follows: The magnetic field induced current in the target segment is obtained by the following formula: ， ， The leakage current of the target section is obtained by the following formula: ， ， in, I MN The magnetic field induced current in the target segment, I PN The leakage current of the target section; k 1. k Both 2 are proportionality coefficients; I LO , , These are the load current of the target section, the length imbalance, and the average length of the small section, respectively. , These are the length imbalance of the corresponding reference segment and the average segment length of the small segment, respectively. Length imbalance = ,in, , , These represent the maximum, minimum, and average values ​​of the three sub-segments of the cross-connection segment.

7. The method according to claim 6, characterized in that, In step 4, the formulas for calculating the magnetic field induced current and leakage current of the target section under horizontal and right-angle laying methods are as follows: The magnetic field induced current in the target segment is obtained by the following formula: ， in, k 3. k 4. k 5 are all coefficients, obtained from the following formula: ， ， in, k b This represents the exponential part of the equation relating resistance to sheath circulation current. R O The grounding resistance of the target section. R R This is the grounding resistance of the reference section.

8. The method according to claim 1, characterized in that, In horizontal and right-angle laying methods, the leakage current is proportional to the average length of the short segment, and its calculation method is the same as that for triangular laying method.

9. The method according to claim 1, characterized in that, In step 5, the magnetic field induced current of the target section is added to the leakage current of the target section to obtain the theoretical value of the circulating current of the cable sheath in the target section. In step 6, the cable operating status is diagnosed based on the error between the measured value and the theoretical value of the circulating current in the cable sheath of the target section, and information on normal cable operation, fault, or warning is given.

10. A cable operating status auxiliary diagnostic system, used to implement the method according to any one of claims 1-9, characterized in that, include: Reference segment selection unit: used to select n cross-interconnected segments of a long-distance high-voltage cable line as reference segments under normal operation; Calculation formula acquisition unit: Establish the relationship between the sheath circulation current of the reference segment and the magnetic field induced current and leakage current in the metal sheath, and obtain the calculation formulas for the magnetic field induced current and leakage current of the reference segment based on the relationship. Reference section current calculation unit: Select the load and sheath circulation current monitoring data during operation according to the laying method of the reference section line, and use the calculation formula to solve the magnetic field induced current and leakage current of the reference section. Target segment current acquisition unit: Based on the variation law of magnetic field induced current and leakage current under various influencing factors, and combined with the magnetic field induced current and leakage current data of the reference segment, the magnetic field induced current and leakage current of the target segment are obtained. Sheath Circulation Current Theoretical Value Calculation Unit: Calculates the theoretical value of cable sheath circulation current in the target section using the magnetic field induced current and leakage current of the target section; Auxiliary diagnostic criterion unit: used to compare the theoretical value of the circulating current in the cable sheath of the target section with the measured value to obtain auxiliary diagnostic results of the cable's operating status.

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