A method and system for calculating the leakage current of the main insulation of a cable based on sheath circulating current

Abstract

The present invention discloses a method and system for calculating the leakage current of the main insulation of a cable based on sheath circulating current. The method for calculating the leakage current of the main insulation of the cable according to the present invention includes: Step 1, establishing a calculation model for the resistive component and capacitive component of the leakage current of the main insulation of the cable; Step 2, obtaining a calculation formula for the sheath circulating current; Step 3, collecting the i group of current data; Step 4, according to the sheath circulating current calculation formula in Step 2 and the data obtained in Step 3, using the inverse operation of the matrix to solve the coupling ratio coefficients of the magnetic field induced current and load current and the main insulation leakage current and core voltage; Step 5, repeating Step 3 and 4, summing and averaging the coupling ratio coefficients of the magnetic field induced current and load current and the leakage current and core voltage obtained from each group of data, and then calculating the leakage current. The present invention can quickly and accurately calculate the cable leakage current and decompose it into resistive current components, which can effectively reflect the aging degree of the cable insulation.

Description

Technical Field

[0001] The 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 used in urban power grids and play an increasingly important role in ensuring the safe and reliable power supply in cities. However, the health status of a large number of cable lines cannot be ignored for the safe and stable operation of urban power grids. In addition, with the continuous increase of customers' power consumption demands and the annual improvement of 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-linked interconnected high-voltage cables are widely used in long-distance power transmission and distribution in urban power grids. Compared with single-ended grounded or double-ended grounded cable systems, the cross-linked interconnected grounding system is grounded at both ends of the main section, and the three-phase cable sheaths are cross-connected together to eliminate the influence of induced voltage in the sheath, thereby reducing the sheath circulating current.

[0003] When a cross-linked interconnected 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 the metal sheath of the cable to the grounding points at both ends of the cross-linked interconnected system. When the XLPE cable insulation is in good condition, the main current flowing through the cable main insulation is 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 cable main insulation, 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 the quality of the cable main insulation. However, in a cross-linked interconnected high-voltage cable, the sheath circulating current is composed of leakage current and induced current, and how to separate the leakage current has become a difficulty in cable main insulation monitoring.

[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-connected 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 to calculate the equivalent impedance value of the main insulation of the cable. 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 prior art 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 and with the help of the existing cross-connected 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:

[0008] Step 1, establish a calculation model for the resistive component and capacitive component of the leakage current of the main insulation of the cable;

[0009] 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;

[0010] 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 ;

[0011] 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 field induced current with the load current and the main insulation leakage current with the core voltage.

[0012] Step 5: Repeat Step 3 and Step 4. Finally, sum and average the coupling proportionality coefficients of the magnetic field induced current with the load current and the leakage current with the core voltage obtained from each group of data, separate the magnetic field induced current and the 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 respectively.

[0013] The present invention calculates the main insulation leakage current of the cable by using the existing on-line monitoring system without the need for additional installation of current transformers. Based on the on-site sheath circulating current data, the present invention performs matrix operations to separate the leakage current and the magnetic field induced current, and then takes the real part operation of the leakage current to obtain the resistive component, thereby enabling the judgment of the state of the main insulation of the cable line.

[0014] Further, in Step 1, the calculation models of the resistive component and capacitive component of the main insulation leakage current of the cable are as follows:

[0015] ,

[0016] ,

[0017] Wherein, 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.

[0018] Furthermore, the calculation formula of the equivalent capacitance C per unit length of the cable is as follows:

[0019] ,

[0020] Wherein, is the vacuum permittivity, is the relative permittivity,D C is the diameter of the cable core, is the insulator thickness.

[0021] Further, in the step 2, the calculation formula of the sheath circulating current is as follows:

[0022] , j= 1 or 2;

[0023] 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, represents the i th group and the j th data in the B phase core load current, represents the coupling ratio coefficient between the magnetic field induced current of the grounding loop and the three-phase core load current, 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 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 phase operating voltage can be used to represent the three-phase voltage.

[0024] The leakage current refers to the current flowing from the cable core through the main insulation to the metal sheath. Its magnitude is only related to the main insulation strength and the core voltage. Since the 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 includes the induced current component generated by the interaction between the core load current and the sheath and the leakage current component between the core and the main insulation. Based on the generation mechanisms of the leakage current and the magnetic field induced current, the complex matrix operation based on the multi-conductor transmission line is simplified to calculate the coupling ratio coefficients between the magnetic field induced current and the load, and between the leakage current and the 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-phase load current; when the cable insulation condition remains unchanged, the leakage current is not affected by the load current and remains constant.

[0025] Furthermore, the calculation formula of the sheath circulating current is changed as follows:

[0026] ,

[0027] wherein, represents the iThe sheath circulating current of the grounding loop in the first piece of data of the group represents the i sheath circulating current of the grounding loop in the second piece of data of the group i and represents the B phase conductor core load current in the first piece of data of the group i and represents the B phase conductor core load current in the second piece of data of the

[0028] Furthermore, in the said 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 :

[0029] .

[0030] Furthermore, in the said step 5, the formula for taking the average value of the sum of 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 is as follows

[0031] ,

[0032] ,

[0033] wherein represents the average value of the coupling proportionality coefficient between the magnetic field induced current and the load current and

[0034] represents the average value of the coupling proportionality coefficient between the leakage current and the core voltage

[0035] ,

[0036] 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 conductor core load current

[0037] Furthermore, the formulas for obtaining the resistive component and the capacitive component in the leakage current are as follows

[0038] ,

[0039] .

[0040] 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:

[0041] Leakage current component calculation model establishment unit: used to establish calculation models for the resistive component and capacitive component of the cable main insulation leakage current;

[0042] Sheath circulating current calculation formula acquisition unit: used to construct the relationship between the main insulation leakage current, the core voltage, the magnetic field induced current and the core load current, and obtain the calculation formula for the sheath circulating current;

[0043] Data acquisition unit: 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 ;

[0044] Coupling ratio coefficient solving unit: according to the sheath circulating current calculation formula and the data obtained by the data acquisition unit, use the inverse operation of the matrix to solve the coupling ratio coefficient between the main insulation leakage current and the core voltage;

[0045] Leakage current component acquisition unit: repeat the data acquisition unit and the coupling ratio coefficient solving unit, and finally sum and average the coupling ratio coefficients of the leakage current and the core voltage obtained for each group 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 imaginary part of the leakage current respectively.

[0046] The beneficial effects of the present invention are:

[0047] 1. Compared with the traditional leakage current calculation method, the calculation method and system of the present invention use fewer current transformers, and rely on the existing sheath circulating current monitoring device to complete the calculation of the leakage current, which is convenient to operate and consumes less manpower and material resources.

[0048] 2. The calculation method and system of the present invention can quickly and accurately calculate the cable leakage current, and decompose it into resistive current components according to the model, which can effectively reflect the aging degree of the cable insulation; at the same time, it can also calculate the magnetic field induced current component in the sheath circulating current, providing an accurate judgment basis for the online monitoring of cable faults based on the sheath circulating current. Description of the Drawings

[0049] Figure 1 is the equivalent circuit diagram of a large section of cross-connected power cable in the specific embodiment of the present invention;

[0050] Figure 2Flow chart of the calculation method for the leakage current of the main insulation of the cable in the present invention;

[0051] Figure 3 Interconnected simulation model diagram of the three-phase cable in the present invention;

[0052] Figure 4 Architecture diagram of the calculation system for the leakage current of the main insulation of the cable in the present invention. Detailed implementation manners

[0053] The following describes the technology of the present invention in detail in combination with specific implementation manners. It should be known that the following specific implementation manners are only used to help those skilled in the art understand the present invention, rather than limiting the present invention.

[0054] 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.

[0055] 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 , I m3 . The lengths of each cross-connected small section are respectively , and . Z 01 , Z 02 , Z 03 are respectively the sheath impedances of each cross-connected small section. R d1 , R d2 are the grounding resistances at both ends of the large cross-connected section. R d is the earth leakage resistance. E SAi , E SBi , E SCi are the induced electromotive forces caused by the load currents flowing through the cable cores A, B, and C phases on the cable sheath of the i th cross-connected small section. E TAi , E TBi , E Tci are respectively the sheath circulating currents of phase B and phase C, the sheath circulating currents of phase A and phase C, and the sheath circulating currents of phase A and phase B in thei The induced electromotive force on the sheath of a small section of cross-connected cable, i = 1, 2, 3.

[0056]

[0057]

[0058] Wherein, X AA , X BB , X CC are the mutual impedances between the conductor core and the metal sheath of the same phase, X AB , X AC , X BC are the mutual impedances between the conductor core and the metal sheaths of other phases; X ab , X ac , X bc are the mutual impedances between the metal sheaths of different phases; , , are the load currents of phase A, phase B, and phase C respectively.

[0059]

[0060] Let , then the following formula can be written:

[0061]

[0062] 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 rest of the cases can be analogously written) to obtain:

[0063]

[0064] And when the three-phase cable is laid in a triangle X 2 = X 3.

[0065]

[0066]

[0067] Substitute I m1 , Im2 , I m3 The addition of three current phasors gives:

[0068]

[0069] During 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:

[0070]

[0071]

[0072] When the three-phase loads of A, B, and C are completely symmetrical, , at this time:

[0073] It can be seen from the above formula that when the cable is operating with symmetrical three-phase loads, there is a linear relationship between the phasor sum of the three-phase sheath circulating currents and any one-phase load current. The magnitude of this proportionality coefficient is only related to 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 has not changed. According to existing research, when the cable insulation condition remains unchanged, the leakage current is not affected by the load current and remains constant.

[0074] Embodiment 1

[0075] This embodiment provides a method for calculating the leakage current of the main insulation of a cable based on the sheath circulating current, as Figure 2 shown, which includes:

[0076] Step 1, establish a calculation model for the resistive component and capacitive component of the leakage current of the main insulation of the cable;

[0077] Step 2, construct the relationship between the leakage current of the main insulation, the core voltage, the magnetic field induced current, and the core load current, and obtain the calculation formula for the sheath circulating current;

[0078] 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 ;

[0079] 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 field induced current and the load current, and the leakage current of the main insulation and the core voltage;

[0080] 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, and calculate the leakage current. , by taking the real part and imaginary part of the leakage current respectively, the resistive component and capacitive component in the leakage current are obtained.

[0081] Specifically, in the above-mentioned Step 1, the calculation models of the resistive component and capacitive component of the cable main insulation leakage current are as follows:

[0082] ,

[0083] ,

[0084] where is the resistive component of the leakage current, is the capacitive component of the leakage current, G is the conductivity of the cable main insulation, , and are respectively A , B , C phase core voltages; , and respectively represent the respective lengths of three small segments 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.

[0085] The formula for calculating the equivalent capacitance C per unit length of the cable is as follows:

[0086] ,

[0087] where is the vacuum permittivity, is the relative permittivity, D C is the diameter of the cable core, is the insulator thickness.

[0088] Specifically, in the above-mentioned Step 2, the formula for calculating the sheath circulating current is as follows:

[0089] , j= 1 or 2;

[0090] where i is the data group number,j is the number of the data bar in each data group, indicating the sheath circulating current of the grounding loop in the i th j group and the th i group and the j th data; B is the phase conductor core load current in the th data; represents the coupling ratio coefficient between the magnetic field induced current of the grounding loop and the three-phase conductor core load current, and 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, so any phase operating voltage can be used to represent the three-phase voltage.

[0091] Select two moments with different magnitudes of load current, and simultaneously measure the three-phase sheath circulating current, three-phase load current, and conductor core voltage at the corresponding moments. Take the data at one moment as one data, and every two data as a group; the equations that can be written for each group of data are as follows:

[0092]

[0093] For an operating cable line, its three-phase load will change with the power supply demand, but the conductor core voltage remains unchanged. Therefore, the matrix in the equation is non-singular, and the inverse of this matrix can be calculated to write the equation as follows:

[0094] ,

[0095] where is the sheath circulating current of the grounding loop in the i th group and the 1st data, is the sheath circulating current of the grounding loop in the i th group and the 2nd data, is the i phase conductor core load current in the B th group and the 1st data, is the i phase conductor core load current in the B th group and the 2nd data.

[0096] The coupling ratio coefficient between the magnetic field induced current and the load current and the coupling ratio coefficient between the leakage current and the conductor core voltage can be solved through the above equations:

[0097] Specifically, in step 5 described above, the coupling ratio coefficient and the coupling ratio coefficient of the leakage current and the core voltage The formulas for separately summing and taking the average are as follows:

[0098] ,

[0099] .

[0100] In the formula, represents the average value of the coupling ratio coefficient of the magnetic field induced current and the load current, represents the average value of the coupling ratio coefficient of the leakage current and the core voltage.

[0101] Leakage current The calculation formula is as follows:

[0102] ,

[0103] 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 core load current.

[0104] As can be seen from the model in step 1, for the leakage current the real part and the imaginary part are taken to obtain the resistive current and the capacitive current.

[0105] ,

[0106] .

[0107] The following simulation is carried out using the above cable main insulation leakage current calculation method.

[0108] In this simulation, a cross-bonding model of a three-phase cable is established using PSCAD. The length of each cross-bonding section is 1500 m, and the length of each small cross-bonding section is 500 m. The cable uses the structural parameters of 64 / 110 YJLW02 - 630 mm 2 for simulation; 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 Figure 3 shown, taking the leakage current calculation of the A1 - B2 - C3 loop as an example.

[0109] 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 of the magnetic field induced current and the load current and the average value of the coupling ratio coefficient of the leakage current and the core voltage .

[0110] 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.

[0111] Table 1 Measured values of sheath circulating current, load current, and core voltage

[0112]

[0113] Utilize the first group of data obtained in Step 3 and adopt the inverse operation of the matrix to solve the coupling proportionality coefficient between the main insulation leakage current and the core voltage:

[0114] ,

[0115] Utilize the second group of data to calculate the coupling proportionality coefficient between the main insulation leakage current and the core voltage:

[0116] ,

[0117] Calculate the average value of the coupling proportionality 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.

[0118] Table 2 Coupling proportionality coefficients of each group and their average values

[0119]

[0120] Calculate the leakage current at the right grounding point of the cable line by using the formula:

[0121]

[0122]

[0123] For the leakage current Take the real part and the imaginary part to obtain the resistive current and the capacitive current.

[0124] ,

[0125] .

[0126] Embodiment 2

[0127] 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 proportionality coefficient solving unit, and a leakage current component acquisition unit.

[0128] The leakage current component calculation model establishing unit: for establishing the calculation models of the resistive component and the capacitive component of the leakage current of the main insulation of the cable.

[0129] The sheath circulating current calculation formula obtaining unit: for constructing the relationships between the main insulation leakage current and the core voltage, and between the magnetic field induced current and the core load current, to obtain the calculation formula of the sheath circulating current.

[0130] The data acquisition unit: for acquiring the i th group of current data, selecting two moments with different load current amplitudes, and simultaneously measuring the three-phase sheath circulating current, three-phase load current, and core voltage at the corresponding moments, where i = 1, 2, 3, 4, …, n .

[0131] The coupling ratio coefficient solving unit: according to the sheath circulating current calculation formula and the data obtained by the data acquisition unit, using the inverse operation of the matrix to solve the coupling ratio coefficient between the main insulation leakage current and the core voltage.

[0132] The leakage current component obtaining unit: repeating the data acquisition unit and the coupling ratio coefficient solving unit, finally summing and averaging the coupling ratio coefficients of the leakage current and the core voltage obtained for each group of data, and then calculating the leakage current , and obtaining the resistive component and the capacitive component in the leakage current by taking the real part and the imaginary part of the leakage current respectively.

[0133] 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 by software, hardware, and their combination. The above-mentioned modules can be embedded in the processor of the computer device in the form of hardware or be independent of it, or can be stored in the memory of the computer device in the form of software, so that the processor can call and execute the operations corresponding to the above-mentioned modules. For the specific limitations of a cable main insulation leakage current calculation system based on sheath circulating current, refer to the limitations of a 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.

[0134] The specific embodiments described in the present invention are only examples to illustrate the spirit of the present invention. Those skilled in the technical field to which the present invention belongs can make various modifications or supplements to the described specific embodiments, or use similar ways to replace them, 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 the leakage current of the main insulation of a cable based on sheath circulating current, characterized in that, Including: Step 1: Establish a calculation model for the resistive and capacitive components of the leakage current of the main insulation of the cable; Step 2: Construct the relationships between the main insulation leakage current and the core voltage, and between 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 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 coupling ratio coefficients of the main insulation leakage current and the core voltage; Step 5: Repeat Step 3 and Step 4. Finally, sum and average the coupling ratio coefficients of the magnetic field induced current and the load current, and the coupling ratio coefficients of the main insulation leakage current and the core voltage obtained for each set of data, separate the magnetic field induced current and the leakage current in the sheath circulating current, calculate the leakage current, and obtain the resistive and capacitive components in the leakage current by taking the real and imaginary parts of the leakage current respectively; In the said Step 2, the calculation formula for the sheath circulating current is as follows: , j= 1 or 2; Among them, i is the data group number, j is the number of data items in each data group, represents the sheath circulating current at the end of the grounding loop in the i th j data item of the th group, i represents the j th B phase conductor core load current in the data item of the th group, represents the coupling ratio coefficient of the magnetic field induced current and the load current, represents the coupling ratio coefficient of the main insulation leakage current and the core voltage; According to the sheath circulating current calculation formula in Step 2 and the data obtained in Step 3, list the equation as follows: ， Among them, represents the sheath circulating current of the grounding loop in the first piece of data of the i th group, represents the sheath circulating current of the grounding loop in the second piece of data of the i th group, represents the i phase conductor core load current in the first piece of data of the B th group, represents the i phase conductor core load current in the second piece of data of the B th group; The leakage current in step 5 described above has the following calculation formula: ， Among them, is the sheath circulating current at the starting end of the grounding loop at the same moment, and the reference direction is the same as ; Same; Indicates B the load current of the phase conductor core, represents the average value of the coupling ratio coefficient of the magnetic field induced current and the load current, represents the average value of the coupling ratio coefficient of the main insulation leakage current and the conductor core voltage.

2. The method for calculating the leakage current of the main insulation of a cable according to claim 1, characterized in that In the said Step 1, the calculation model for the resistive and capacitive components of the leakage current of the main insulation of the cable is as follows: ， ， Among them, is the resistive component of the leakage current, is the capacitive component of the leakage current, is the conductivity of the main insulation of the cable, 、 and are respectively A 、 B 、 C phase line 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.

3. The method for calculating the leakage current of the main insulation of a cable according to claim 2, characterized in that, The equivalent capacitance of the unit-length cable C The calculation formula is as follows: ， Among them, is the vacuum permittivity, is the relative permittivity, is the diameter of the cable core, is the insulator thickness.

4. The method for calculating the leakage current of the main insulation of a cable according to claim 1, characterized in that In the said step 4, the inverse operation of the matrix is used to solve the coupling proportionality coefficients of the magnetic induction current and the load current and the coupling proportionality coefficient of the main insulation leakage current and the core voltage in the following equation: 。 5. The method for calculating the leakage current of the main insulation of the cable according to claim 4, characterized in that In the said step 5, the coupling proportionality coefficient between the magnetic induction current and the load current and the coupling proportionality coefficient between the main insulation leakage current and the core voltage The formulas for respectively summing and taking the average are as follows: ， 。 6. The method for calculating the leakage current of the main insulation of a cable according to claim 5, wherein, The acquisition formulas for the resistive and capacitive components in the leakage current are as follows: ， 。 7. 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 any one of claims 1-6, characterized in that, Including: Leakage current component calculation model establishment unit: used to establish a calculation model for the resistive and capacitive components of the leakage current of the main insulation of the cable; 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 field induced current and the core load current, to obtain the calculation formula for the sheath circulating current; Data acquisition unit: used to collect the i 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 ; Coupling ratio coefficient solving unit: according to the sheath circulating current calculation formula and the data obtained by the data acquisition unit, 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 coupling ratio coefficients 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 magnetic field induced current and the load current, and the coupling ratio coefficients of the main insulation leakage current and the core voltage obtained for each set of data, separate the magnetic field induced current and the leakage current in the sheath circulating current, then calculate the leakage current, and obtain the resistive and capacitive components in the leakage current by taking the real and imaginary parts of the leakage current respectively.

Images