Cable main insulation leakage current calculation method and system based on sheath ring current

Abstract

The invention discloses a cable main insulation leakage current calculation method and system based on sheath ring current. The cable main insulation leakage current calculation method comprises the following steps: step 1, establishing a calculation model of a resistive component and a capacitive component of cable main insulation leakage current; step 2, obtaining a calculation formula of the sheath ring current; step 3, collecting current data of the ith group; 4, according to the sheath loop current calculation formula in the step 2 and the data obtained in the step 3, solving coupling proportion coefficients of magnetic field induction current and load current and main insulation leakage current and wire core voltage by adopting inverse operation of a matrix; and 5, repeating the steps 3 and 4, summing the coupling proportionality coefficients of the magnetic field induction current and the load current and the coupling proportionality coefficients of the leakage current and the wire core voltage solved by each group of data, averaging, and then calculating to obtain the leakage current. According to the method, the cable leakage current can be rapidly and accurately calculated, the resistive current component is obtained through decomposition, and the aging degree of cable insulation can be effectively reflected.

Description

Technical Field

[0001] The present invention belongs to the technical field of on-line cable monitoring, and particularly relates to a method and system for calculating the leakage current of the main insulation of a cable based on sheath circulating current. Background Art

[0002] Compared with overhead lines, cable lines have the characteristics of good environmental tolerance and high reliability. Crosslinked polyethylene (XLPE) single-core high-voltage cables are increasingly occupying a higher proportion in urban power grids and playing an increasingly important role in ensuring the safe and reliable power supply of cities. However, the health status of a high proportion of cable lines cannot be ignored for the safe and stable operation of urban power grids. In addition, with the continuous increase in customer electricity demand and the annual increase in power supply reliability requirements, how to improve the accuracy of monitoring the operating status of power cables has become an urgent problem to be solved. Cross-connected high-voltage cables are widely used in long-distance power transmission and distribution in urban power grids. Compared with single-end grounded or double-end grounded cable systems, the cross-connected grounding system grounds both ends of the main section and cross-connects the three-phase cable sheaths together to eliminate the influence of induced voltage in the sheath, thereby reducing the sheath circulating current.

[0003] When a cross-connected high-voltage cable is operating, the high voltage of the three-phase cores acts on the main insulation to generate leakage current (including capacitive current and resistive current), which flows through the main insulation and metal sheath of the cable to the grounding points at both ends of the cross-connected system. When the insulation of the XLPE cable is in good condition, the current flowing through the main insulation of the cable is mainly capacitive current, and the resistive current is generally only 0.1 - 0.25 times that of the capacitive current; when there are defects or aging in the main insulation of the cable, the capacitive current remains basically unchanged, but the resistive current changes greatly. Therefore, the magnitude of the resistive current component in the leakage current can be used as a basis for judging the quality of the main insulation of the cable. However, in a cross-connected high-voltage cable, the sheath circulating current is composed of leakage current and induced current. How to separate the leakage current has become a difficult point in the monitoring of the main insulation of the cable.

[0004] Journal of Electric Machines and Control, 2023, 27(02): 79-88. Zhu Bo, Yu Xiaoyang, Tian Ligang et al. proposed an insulation monitoring and diagnosis method for cross-bonded cables based on resistive current and sheath current. Current transformers and voltage transformers are installed at the head and end of each phase. The difference between the current values measured by the head and end current transformers is used as the leakage current, and half of the sum of the measured voltage phasors at the head and end is used as the reference voltage. The equivalent impedance value of the main insulation of the cable is calculated, and then the resistive current and capacitive current components are calculated. This method requires installing 2 current transformers and 2 voltage transformers on each phase, which is complex to operate, and using half of the sum of the head and end voltage phasors as the reference voltage is not accurate enough, resulting in an error between the calculated equivalent impedance of the main insulation and the actual value. In the patent document CN105021871A, "A Method for Determining the Leakage Current of a Cable Line under Incomplete Information", it is considered that the leakage current of a certain section of the cable is the phasor difference between the head and end load currents. Based on this calculation model, the linear regression method is used to solve the unary linear equation to obtain the value of the leakage current. This method has a large amount of calculation and a slow calculation speed. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the defects of the above-mentioned existing technologies, and provide a method and system for calculating the leakage current of the main insulation of a cable based on sheath circulating current. Starting from the composition of the sheath circulating current of the cable, with the help of the existing cross-bonded equivalent circuit model of the cable, the relationship between the load current and the magnetic induction current is found; at the same time, according to the generation mechanism of the leakage current, the relationship between the core voltage and the leakage current is found to simplify the calculation of the leakage current.

[0006] To this end, the present invention adopts the following technical solutions.

[0007] In the first aspect, the present invention provides a method for calculating the leakage current of the main insulation of a cable based on sheath circulating current, which includes: Step 1, establish a calculation model for the resistive component and capacitive component of the leakage current of the main insulation of the cable; Step 2, construct the relationship between the leakage current of the main insulation, the core voltage, the magnetic induction current and the core load current to obtain the calculation formula for the sheath circulating current; Step 3, collect the i th group of current data, select two moments with different load current amplitudes, and simultaneously measure the three-phase sheath circulating current, three-phase load current and core voltage at the corresponding moments, where i = 1, 2, 3, 4, …, n ; Step 4, according to the sheath circulating current calculation formula in Step 2 and the data obtained in Step 3, use the inverse operation of the matrix to solve the coupling proportionality coefficients of the magnetic induction current and the load current and the leakage current of the main insulation and the core voltage. Step 5: Repeat Step 3 and Step 4. Finally, sum up and average the coupling ratio coefficients of the magnetic induction current and load current, and the leakage current and core voltage obtained for each group of data to separate the magnetic induction current and leakage current in the sheath circulating current, calculate the leakage current, and obtain the resistive component and capacitive component of the leakage current by taking the real part and imaginary part of the leakage current respectively.

[0008] The present invention calculates the leakage current of the main insulation of the cable using the existing on-line monitoring system without the need to additionally install current transformers. The present invention performs matrix operations based on the on-site sheath circulating current data to separate the leakage current and magnetic induction current, and then performs a real part operation on the leakage current to obtain the resistive component, thereby enabling the determination of the state of the main insulation of the cable line.

[0009] Further, in the above-mentioned Step 1, the calculation models of the resistive component and capacitive component of the leakage current of the main insulation of the cable are as follows: , , where, is the resistive component of the leakage current, is the capacitive component of the leakage current, G is the conductivity of the main insulation of the cable, , and are respectively A , B , C phase core voltages; , and respectively represent the respective lengths of three small sections within a cross-bonding section of the cable, j is the imaginary unit, is the angular frequency, C is the equivalent capacitance per unit length of the cable.

[0010] Furthermore, the calculation formula for the equivalent capacitance C per unit length of the cable is as follows: , where, is the vacuum permittivity, is the relative permittivity, D C is the diameter of the cable core, is the insulator thickness.

[0011] Further, in the above-mentioned Step 2, the calculation formula for the sheath circulating current is as follows: , j= 1 or 2; where,i is the data group number, j is the number of data items in each data group, represents the sheath circulating current of the grounding loop in the i th group and the j th data item, represents the i th group and the j th data item of the B phase conductor core load current, represents the coupling ratio coefficient between the magnetic field induced current of the grounding loop and the three-phase conductor core load current, represents the coupling ratio coefficient between the leakage current of the grounding loop and the conductor core voltage . Since the three-phase load and three-phase voltage are strictly symmetric during normal operation of the cable, the magnitudes of the three-phase voltages are basically the same, and the phase angles between the three phases are basically fixed at 120 degrees. Therefore, any one of the three-phase voltages can be used to represent the three-phase voltages.

[0012] The leakage current refers to the current flowing from the cable conductor core through the main insulation to the metal sheath. Its magnitude is only related to the main insulation strength and the conductor core voltage. Since the conductor core voltage remains unchanged during operation, the change in the leakage current can reflect the main insulation strength; the magnetic field induced current is proportional to the load current when the line structure parameters remain unchanged. The sheath circulating current mentioned above includes the induced current component generated by the interaction between the conductor core load current and the sheath, and the leakage current component between the conductor core and the main insulation. Based on the generation mechanisms of the leakage current and the magnetic field induced current respectively, the complex matrix operations based on multi-conductor transmission lines are simplified to calculate the coupling ratio coefficients between the magnetic field induced current and the load, and between the leakage current and the conductor core voltage. When the cable is operating with symmetric three-phase loads, the phasor sum of the three-phase sheath circulating currents has a linear relationship with any one of the phase load currents; when the cable insulation condition remains unchanged, the leakage current is not affected by the load current and remains constant.

[0013] Furthermore, the calculation formula of the sheath circulating current is changed as follows: , where, represents the sheath circulating current of the grounding loop in the i th group and the 1st data item, represents the sheath circulating current of the grounding loop in the i th group and the 2nd data item, represents the i th group and the 1st data item of the B phase conductor core load current, represents the i th group and the 2nd data item of the B phase conductor core load current.

[0014] Further, in the step 4, the inverse operation of the matrix is used to solve the coupling proportionality coefficient between the leakage current and the core voltage in the following equation : .

[0015] Further, in the step 5, the coupling proportionality coefficient between the magnetic induction current and the load current and the coupling proportionality coefficient between the leakage current and the core voltage are respectively summed and averaged according to the following formulas: , , wherein, represents the average value of the coupling proportionality coefficient between the magnetic induction current and the load current, represents the average value of the coupling proportionality coefficient between the leakage current and the core voltage.

[0016] Further, the calculation formula of the leakage current in the step 5 is as follows: , wherein, is the sheath circulating current at the starting end of the grounding loop at the same moment as , and the reference direction is the same as ; represents B the phase core load current.

[0017] Further, the acquisition formulas of the resistive component and the capacitive component in the leakage current are as follows: , .

[0018] In a second aspect, the present invention provides a cable main insulation leakage current calculation system based on sheath circulating current, which is used to implement the above-mentioned cable main insulation leakage current calculation method, and includes: Leakage current component calculation model establishment unit: used to establish calculation models for the resistive component and the capacitive component of the cable main insulation leakage current; Sheath circulating current calculation formula acquisition unit: used to construct the relationships between the main insulation leakage current and the core voltage, and between the magnetic induction current and the core load current, and obtain the calculation formula of the sheath circulating current; Data acquisition unit: used to acquire the i th group of current data, select two moments with different load current amplitudes, and simultaneously measure the three-phase sheath circulating current, the three-phase load current and the core voltage at the corresponding moments, wherein i =1, 2, 3, 4,...,n ; Coupling ratio coefficient solving unit: According to the sheath circulating current calculation formula and the data obtained by the data acquisition unit, the inverse operation of the matrix is used to solve the coupling ratio coefficient of the main insulation leakage current and the core voltage; Leakage current component acquisition unit: Repeat the data acquisition unit and the coupling ratio coefficient solving unit. Finally, sum and average the coupling ratio coefficients of the leakage current and the core voltage obtained from each group of data, and then calculate the leakage current. The resistive component and capacitive component in the leakage current are obtained by taking the real part and imaginary part of the leakage current respectively.

[0019] The beneficial effects of the present invention are as follows: 1. Compared with the traditional leakage current calculation method, the calculation method and system of the present invention use fewer current transformers. Depending on the existing sheath circulating current monitoring device, the calculation of the leakage current can be completed, with convenient operation and less consumption of human and material resources.

[0020] 2. The calculation method and system of the present invention can quickly and accurately calculate the cable leakage current, and decompose it into the resistive current component according to the model, which can effectively reflect the aging degree of the cable insulation; at the same time, the magnetic field induction current component in the sheath circulating current can also be calculated, providing an accurate judgment basis for the on-line monitoring of cable faults based on the sheath circulating current. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is the equivalent circuit diagram of a large section of cross-connected power cable in the specific embodiment of the present invention; Figure 2 is the flow chart of the cable main insulation leakage current calculation method of the present invention; Figure 3 is the cross-connected simulation model diagram of the three-phase cable of the present invention; Figure 4 is the architecture diagram of the cable main insulation leakage current calculation system of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The following describes the technology of the present invention in detail in combination with specific embodiments. It should be understood that the following specific embodiments are only used to help those skilled in the art understand the present invention, rather than limiting the present invention.

[0023] When the three-phase load of the cable operates symmetrically, there is a linear relationship between the phasor sum of the three-phase sheath circulating currents and any one-phase load current. The following is the derivation and explanation.

[0024] The equivalent circuit model of a large section of cross-connected power cable is as Figure 1 shown. The sheath circulating currents in the three sheath circulating current loops are respectively I m1 ,I m2 and I m3 , the lengths of each cross - bonding segment are respectively and and , Z 01 , Z 02 , Z 03 are respectively the sheath impedances of each cross - bonding segment, R d1 and R d2 are the grounding resistances at both ends of the cross - bonding section, R d is the earth leakage resistance. E SAi and E SBi and E SCi are the induced electromotive forces caused by the load currents flowing through the cores A, B, C of the cable on the cable sheath of the i th cross - bonding segment, E TAi and E TBi and E Tci are respectively the induced electromotive forces caused by the sheath circulating currents of phase B and C, the sheath circulating currents of phase A and C, and the sheath circulating currents of phase A and B on the cable sheath of the i th cross - bonding segment, i = 1, 2, 3.

[0025]

[0026]

[0027] In the formula, X AA and X BB and X CC are the mutual impedances between the core and the metal sheath of the same phase, X AB and X AC and X BC are the mutual impedances between the core and the metal sheaths of other phases; X ab and X ac and X bc are the mutual impedances between metal sheaths of different phases; , , They are the load currents of phase A, phase B, and phase C respectively.

[0028]

[0029] Let , then the following formula can be written:

[0030] For three-phase cables laid in parallel, at right angles, or in a triangle, the distances from the two outer phases to the middle phase are equal, and the parameters of the three-phase cables are completely symmetrical (taking the B phase cable in the middle as an example, and the other cases can be analogously written) to obtain:

[0031] And when the three-phase cable is laid in a triangle X 2 = X 3 .

[0032]

[0033]

[0034] Adding the three current phasors of I m1 , I m2 , I m3 gives:

[0035] In actual cable laying, the lengths of the three cross-connected sections are approximately equal , then it can be known that in the right side of the above formula:

[0036]

[0037] When the loads of phases A, B, and C are completely symmetrical, , at this time: It can be seen from the above formula that when the three-phase cable is operating with symmetrical loads, the phasor sum of the three-phase sheath circulating currents and any one-phase load current have a linear relationship. The magnitude of this proportionality coefficient only depends on the lengths of the three cross-connected sections, the mutual impedance between the lines, and the line impedance, and remains unchanged during the operation process when the cable structure does not change. According to existing research, when the cable insulation condition remains unchanged, the leakage current is not affected by the load current and remains constant.

[0038] Example 1 This example provides a method for calculating the leakage current of the main insulation of a cable based on sheath circulating current, as Figure 2 shown, which includes: Step 1, establish a calculation model for the resistive component and capacitive component of the leakage current of the main insulation of the cable; Step 2, construct the relationship between the main insulation leakage current, the core voltage, the magnetic field induced current, and the core load current to obtain the calculation formula for the sheath circulating current; Step 3, collect the i th group of current data, select two moments with different load current amplitudes, and simultaneously measure the three-phase sheath circulating current, three-phase load current, and core voltage at the corresponding moments, where i = 1, 2, 3, 4, …, n ; Step 4, according to the sheath circulating current calculation formula in Step 2 and the data obtained in Step 3, use the inverse operation of the matrix to solve the coupling ratio coefficients of the magnetic field induced current and the load current, and the main insulation leakage current and the core voltage; Step 5, repeat Step 3 and Step 4, and finally sum and average the coupling ratio coefficients of the magnetic field induced current and the load current, and the leakage current and the core voltage obtained from each group of data to separate the magnetic field induced current and the leakage current in the sheath circulating current, and calculate the leakage current , and obtain the resistive component and capacitive component in the leakage current by taking the real part and imaginary part of the leakage current respectively.

[0039] Specifically, in the above-mentioned Step 1, the calculation models for the resistive component and capacitive component of the leakage current of the main insulation of the cable are as follows: , , where is the resistive component of the leakage current, is the capacitive component of the leakage current, G is the conductivity of the main insulation of the cable, , and are respectively A , B , C phase core voltages; , and respectively represent the respective lengths of the three small sections within a cross-bonding section of the cable, j is the imaginary unit, is the angular frequency, C is the equivalent capacitance per unit length of the cable.

[0040] The equivalent capacitance of the unit - length cable C has the following calculation formula: , wherein, is the vacuum permittivity, is the relative permittivity, D C is the diameter of the cable core, is the thickness of the insulator.

[0041] Specifically, in step 2, the calculation formula of the sheath circulating current is as follows: , j= 1 or 2; wherein, i is the data - group number, j is the number of data bars in each data group, represents the sheath circulating current of the grounding loop in the i th group and the j th data bar, represents the load current of the i th group and the j th data bar in the B phase - core, represents the coupling ratio coefficient between the magnetic - induction current of the grounding loop and the load current of the three - phase core, represents the coupling ratio coefficient between the leakage current of the grounding loop and the core voltage . Since the three - phase load and three - phase voltage are strictly symmetric during the normal operation of the cable, the amplitudes of the three - phase voltages are basically the same, and the phase angles between the three phases are basically fixed at 120 degrees. Therefore, any one - phase operating voltage can represent the three - phase voltages.

[0042] Select two moments with different load - current amplitudes, and simultaneously measure the three - phase sheath circulating currents, three - phase load currents, and core voltages at the corresponding moments. Take the data at one moment as one data bar, and every two data bars as a group; the equations can be written for each group of data as follows:

[0043] For an operating cable line, its three - phase load changes with the power - supply demand, but the core voltage remains unchanged. Therefore, the matrix in the equation is non - singular. Taking the inverse of this matrix, the equations can be written as follows: , wherein, represents the sheath circulating current of the grounding loop in the i th group and the 1st data bar, represents the iThe sheath circulating current of the grounding loop in the second piece of data of the group represents the i phase conductor core load current in the first piece of data of the B group and represents the i phase conductor core load current in the second piece of data of the B group

[0044] The coupling proportionality coefficient between the magnetic field induced current and the load current and the coupling proportionality coefficient between the leakage current and the core voltage can be solved through the above equations and the coupling proportionality coefficient between the leakage current and the core voltage are as follows Specifically, in step 5, the coupling proportionality coefficient between the magnetic field induced current and the load current and the coupling proportionality coefficient between the leakage current and the core voltage are respectively summed and averaged as follows , .

[0045] In the formula represents the average value of the coupling proportionality coefficient between the magnetic field induced current and the load current represents the average value of the coupling proportionality coefficient between the leakage current and the core voltage

[0046] The leakage current is calculated as follows , where is the sheath circulating current at the starting end of the grounding loop at the same moment as , and the reference direction is the same as ; represents B the phase conductor core load current

[0047] According to the model in step 1, the resistive current and capacitive current are obtained by taking the real part and imaginary part of the leakage current

[0048] , .

[0049] The following simulation is carried out using the above cable main insulation leakage current calculation method

[0050] This simulation uses PSCAD to establish a cross-bonding model of a three-phase cable. The length of each cross-bonding section is 1500m, and the length of each small cross-bonding section is 500m. The cable uses 64 / 110YJLW02-630mm 2 ​Simulate the structural parameters; the power supply uses three 110 kV single-phase AC voltage source models, and the phase angles of the three phases differ by 120 degrees in sequence. The simulation model is as shown in Figure 3 Take the calculation of the leakage current of the A1-B2-C3 loop as an example.

[0051] Adjust the load and measure the load current, sheath circulating current, and core voltage to obtain two sets of data, and calculate the average value of the coupling ratio coefficient between the magnetic induction current and the load current and the average value of the coupling ratio coefficient between the leakage current and the core voltage .

[0052] First, according to Step 3, collect the first group and the second group of current data. For each group, select two moments with different load current amplitudes, and simultaneously measure the sheath circulating current, load current, and core voltage at the corresponding moments. The data statistics are shown in Table 1.

[0053] Table 1 Measured values of sheath circulating current, load current, and core voltage

[0054] Use the first group of data obtained in Step 3 and solve the coupling ratio coefficient between the main insulation leakage current and the core voltage by using the inverse operation of the matrix: , Use the second group of data to calculate the coupling ratio coefficient between the main insulation leakage current and the core voltage: , Calculate the average value of the coupling ratio coefficients between the main insulation leakage current and the core voltage obtained from the above two groups of data. The results are shown in Table 2.

[0055] Table 2 Coupling ratio coefficients of each group and their average values

[0056] Use the formula to calculate the leakage current at the right grounding point of the cable line:

[0057]

[0058] For the leakage current Take the real part and the imaginary part to obtain the resistive current and the capacitive current.

[0059] , .

[0060] Example 2 This embodiment provides a cable main insulation leakage current calculation system based on sheath circulating current, which is used to implement the cable main insulation leakage current calculation method described in Embodiment 1, as Figure 4 shown. It consists of a leakage current component calculation model establishment unit, a sheath circulating current calculation formula acquisition unit, a data acquisition unit, a coupling ratio coefficient solving unit, and a leakage current component acquisition unit.

[0061] The leakage current component calculation model establishment unit: is used to establish the calculation models of the resistive component and capacitive component of the cable main insulation leakage current.

[0062] The sheath circulating current calculation formula acquisition unit: is used to construct the relationships between the main insulation leakage current, the core voltage, the magnetic field induced current, and the core load current, and obtain the calculation formula of the sheath circulating current.

[0063] The data acquisition unit: is used to collect the i th group of current data, select two moments with different load current amplitudes, and simultaneously measure the three-phase sheath circulating current, three-phase load current, and core voltage at the corresponding moments, where i = 1, 2, 3, 4, …, n .

[0064] The coupling ratio coefficient solving unit: According to the sheath circulating current calculation formula and the data obtained by the data acquisition unit, uses the inverse operation of the matrix to solve the coupling ratio coefficient between the main insulation leakage current and the core voltage.

[0065] The leakage current component acquisition unit: Repeats the data acquisition unit and the coupling ratio coefficient solving unit, and finally sums and averages the coupling ratio coefficients of the leakage current and the core voltage obtained for each group of data, and then calculates the leakage current . By taking the real part and imaginary part of the leakage current , the resistive component and capacitive component in the leakage current are obtained respectively.

[0066] It should be noted that each module in the above cable main insulation leakage current calculation system based on sheath circulating current can be implemented in whole or in part through software, hardware, and their combinations. The above-mentioned modules can be embedded in the processor of the computer device in hardware form or be independent of it, or be stored in the memory of the computer device in software form, so that the processor can call and execute the operations corresponding to the above-mentioned modules. For the specific limitations of the cable main insulation leakage current calculation system based on sheath circulating current, refer to the limitations of the cable main insulation leakage current calculation method based on sheath circulating current in the above text. The two have the same functions and effects, and will not be elaborated here.

[0067] The specific embodiments described in the present invention are merely illustrative of the spirit of the present invention. Those skilled in the art to which the present invention pertains may make various modifications or supplements to the described specific embodiments or use similar means for substitution, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A method for calculating cable main insulation leakage current based on sheath circulation, characterized in that: include: Step 1, establishing a calculation model for the resistive component and capacitive component of the cable main insulation leakage current; Step 2, constructing the relationship between the main insulation leakage current and the core voltage, the magnetic field induced current and the core load current, and obtaining the calculation formula of the sheath circulating current; Step 3: Collect i To set the current data, select two moments with different load current amplitudes, and measure the three-phase sheath circulating current, three-phase load current and line core voltage at the corresponding moments. i =1,2,3,4,…, n ; Step 4, according to the sheath circulating current calculation formula of step 2 and the data obtained in step 3, the coupling proportionality coefficients of the magnetic field induced current and the load current and the main insulation leakage current and the core voltage are solved by using the inverse operation of the matrix; Step 5, repeat steps 3 and 4, and finally sum and average the coupling proportional coefficients of the magnetic field induced current and load current, leakage current and core voltage obtained for each set of data, separate the magnetic field induced current and leakage current in the sheath circulating current, calculate the leakage current, and obtain the resistive component and capacitive component in the leakage current by taking the real part and imaginary part of the leakage current.

2. The method for calculating cable main insulation leakage current according to claim 1, characterized in that: In step 1, the calculation model of the resistive component and the capacitive component of the cable main insulation leakage current is as follows: ， ， in, is the resistive component of the leakage current, is the capacitive component of the leakage current, is the main insulation conductivity of the cable, , and They are A , B , C Phase core voltage; , and They represent the lengths of the three small sections in a cross-connected section of the cable. j is an imaginary unit, is the angular frequency, C is the equivalent capacitance per unit length of cable.

3. The method for calculating cable main insulation leakage current according to claim 1, characterized in that: The equivalent capacitance of the cable per unit length C The calculation formula is as follows: ， in, is the dielectric constant of vacuum, is the relative dielectric constant, is the diameter of the cable core, is the thickness of the insulator.

4. The method for calculating cable main insulation leakage current according to claim 1, characterized in that: In the step 2, the calculation formula of the sheath circulation is as follows: , j= 1 or 2; in, i is the data group number, j is the number of the data bar in each data group, Indicates i Group j The sheath circulation at the end of the ground loop in the data, Indicates i Group j In the data B Phase core load current, It represents the coupling proportionality coefficient between the magnetic field induced current of the ground loop and the three-phase core load current, Indicates the leakage current and core voltage of the ground loop The coupling proportionality coefficient.

5. The method for calculating cable main insulation leakage current according to claim 4, characterized in that: The calculation formula of the sheath circulation is changed as follows: ， in, Indicates i The sheath current of the grounding loop in the first data of group, Indicates i The sheath current of the grounding loop in the second data group, Indicates i In the first data of group B Phase core load current, Indicates i Group 2 data B Phase core load current.

6. The method for calculating cable main insulation leakage current according to claim 5, characterized in that: In step 4, the inverse operation of the matrix is ​​used to solve the magnetic field induced current and the load current in the following equation: The coupling proportionality coefficient of leakage current and line core voltage : 。 7. The method for calculating cable main insulation leakage current according to claim 6, characterized in that: In step 5, the coupling ratio coefficient between the magnetic field induced current and the load current is The coupling proportionality coefficient of leakage current and line core voltage The formula for summing and averaging is as follows: ， ， In the formula, It represents the average value of the coupling proportionality coefficient between the magnetic field induced current and the load current. It represents the average value of the coupling proportionality coefficient between leakage current and core voltage.

8. The method for calculating cable main insulation leakage current according to claim 7, characterized in that: The leakage current in step 5 is The calculation formula is as follows: ， in, For The sheath current at the beginning of the ground loop at the same time, the reference direction is same; express B Phase core load current.

9. The method for calculating cable main insulation leakage current according to claim 8, characterized in that: The formula for obtaining the resistive and capacitive components in the leakage current is as follows: ， 。 10. A cable main insulation leakage current calculation system based on sheath circulation, used to implement the cable main insulation leakage current calculation method according to any one of claims 1 to 9, characterized in that: include: Leakage current component calculation model establishment unit: used to establish the calculation model of the resistive component and capacitive component of the cable main insulation leakage current; Sheath circulating current calculation formula acquisition unit: used to construct the relationship between the main insulation leakage current and the line core voltage, the magnetic field induced current and the line core load current, and obtain the calculation formula of the sheath circulating current; Data acquisition unit: used to collect i To set the current data, select two moments with different load current amplitudes, and measure the three-phase sheath circulating current, three-phase load current and line core voltage at the corresponding moments. i =1,2,3,4,…, n ; Coupling proportional coefficient solving unit: according to the sheath circulating current calculation formula and the data obtained by the data acquisition unit, the coupling proportional coefficient of the main insulation leakage current and the core voltage is solved by using the inverse operation of the matrix; Leakage current component acquisition unit: repeat the data acquisition unit and the coupling proportional coefficient solving unit, and finally sum and average the coupling proportional coefficient of the leakage current and the core voltage obtained for each set of data, and then calculate the leakage current, and obtain the resistive component and capacitive component in the leakage current by taking the real part and the imaginary part of the leakage current respectively.

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