A high-voltage cable line parameter extraction method and system based on online monitoring

Abstract

The application discloses a high-voltage cable line parameter extraction method and system based on online monitoring. The high-voltage cable line parameter extraction method comprises the following steps: based on the multi-conductor transmission line theory, a multi-conductor transmission line equation containing the relationship between voltage phasors and current phasors at both ends of the cable is written; a plurality of groups of cable operation data are collected at both ends of the cable, and each group of cable operation data is selected at a moment when the load amplitude is different; chain parameter matrices in the multi-conductor transmission line equation and voltage phasors and current phasors at the first end of the cable are reduced in order to obtain a reduced multi-conductor transmission line equation; the collected cable operation data are substituted into the reduced multi-conductor transmission line equation, and the reduced chain parameter matrices are solved through matrix operation; and the length of the cable line is solved by using the chain parameter matrices and the reduced chain parameter matrices. The application is helpful to improve the calculation accuracy of the theoretical value of the sheath circulating current, and reduce the misjudgment rate and the missed judgment rate of online diagnosis based on the sheath circulating current.

Description

Technical Field

[0001] This invention belongs to the field of cable online monitoring technology, specifically a method and system for extracting parameters of high-voltage cable lines based on online monitoring. Background Technology

[0002] Compared to overhead lines, cable lines have advantages such as better environmental tolerance and higher reliability. Cross-linked polyethylene (XLPE) single-core high-voltage cables are increasingly prevalent in urban power grids, playing a more crucial role in ensuring safe and reliable power supply. The health status of these high-volume cable lines has a significant impact on the safe and stable operation of urban power grids. Furthermore, with the continuous increase in customer electricity demand and the year-on-year improvement in power supply reliability requirements, improving the accuracy of power cable operation status monitoring has become an urgent problem to solve. Strong magnetic and electric field coupling exists between the conductor and the metal sheath of a single-core cable. The load current carried by the three-phase cable core generates an alternating magnetic field around the core, which links with the metal sheath to induce a voltage. According to power safety regulations, the metal sheath of single-core high-voltage cables must be grounded. The main grounding methods include single-end grounding, cross-interconnection grounding, and double-end grounding. Cable lines with long transmission distances generally use cross-interconnection grounding, which can effectively reduce the induced voltage and circulating current on the metal sheath. Defects in the grounding system often cause changes in the metal sheath current. Therefore, the metal sheath circulating current is one of the important state quantities used for cable line condition evaluation and fault diagnosis.

[0003] In 2013, Du Boxue, Li Zhonglei, Zhang Kai, and others studied the analytical calculation formula for induced voltage in three-circuit cables without considering sheath-to-sheath mutual inductance in their paper "Calculation and Application of Grounding Current for 220kV Cross-linked Polyethylene Power Cables," which can be extended to multi-circuit cables with arbitrary arrangements. IEEE Std 575-2014 provides the forms of grounding systems for the metallic sheath of single-core AC cables and simplified calculation formulas for the induced voltage of the metallic sheath in single-circuit and double-circuit cables. In 2019, Tu Jingyun, in her paper "Research on Fault Diagnosis and Location Technology of High-Voltage Cables Based on Sheath Circulating Current Method," considered the induced electromotive force generated on the sheath by the sheath current and ground current when calculating the sheath induced voltage using an analytical method. In 2015, Yuan Yanling, Zhou Hao, Dong Jie, and others published "Online Monitoring and Fault Diagnosis Technology for Sheath Current of High Voltage Power Cables". Based on the equivalent circuit model, they calculated the current for three types of faults: water ingress into the cross-connection box, open circuit faults caused by loose connections at the joints, and cable joint breakdown. They also formulated fault diagnosis and location standards based on the ratio of the fault current to the normal current.

[0004] In summary, for a given operating cable line, the theoretical value of the sheath circulating current under normal and fault conditions can be calculated using its cable structure and laying parameters, serving as a criterion for judgment in an online monitoring system based on sheath circulating current. To improve the accuracy of online monitoring, it is necessary to improve the calculation accuracy of the sheath circulating current under different conditions, thereby optimizing the fault diagnosis criteria. Existing research on improving the calculation accuracy of cable sheath induced voltage and induced current focuses only on the accuracy of mutual inductance calculation, multi-circuit laying, and the accuracy of the induced voltage calculation formula, neglecting the influence of uncertain factors in the actual field. For actual operating cable lines, due to the age of the engineering drawings, the line may have undergone maintenance and repairs, altering the original laying layout. This results in discrepancies between the actual lengths of the three cross-interconnected sections and those marked on the drawings, and imbalances in the lengths of the three cross-interconnected sections. If the theoretical length of the cable line in the drawings is substituted into the formulas for cable sheath induced voltage and induced current, the calculated theoretical value of the sheath circulating current will differ significantly from the actual value, leading to misjudgments or omissions in fault detection by the online monitoring system. Summary of the Invention

[0005] 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 extracting parameters of high-voltage cable lines based on online monitoring. It starts from the theory of multi-conductor transmission lines, uses the relationship between the electrical parameters at both ends of the cable to write equations to solve for the reduced-order chain parameter matrix of the line; at the same time, according to the calculation method of the chain parameter matrix in the theory of multi-conductor transmission lines, the actual length of the cable is solved simultaneously.

[0006] Therefore, the present invention adopts the following technical solution.

[0007] In a first aspect, the present invention provides a method for extracting parameters of high-voltage cable lines based on online monitoring, comprising:

[0008] Step 1: Based on the multi-conductor transmission line theory, the three-phase cable line is regarded as a multi-port network, and the multi-conductor transmission line equation containing the relationship between the voltage phasors and current phasors at both ends of the cable is written.

[0009] Step 2: Collect multiple sets of cable operation data at both ends of the cable, selecting different times for each set of cable operation data based on different load amplitudes;

[0010] Step 3: Reduce the order of the chain parameter matrix and the voltage phasor and current phasor at the cable head in the multi-conductor transmission line equation to obtain the reduced-order multi-conductor transmission line equation.

[0011] Step 4: Substitute the collected cable operation data into the reduced-order multi-conductor transmission line equation, and solve the reduced-order chain parameter matrix through matrix operations;

[0012] Step 5: Use the chain parameter matrix and the reduced-order chain parameter matrix to solve for the length of the cable line.

[0013] Furthermore, in step 1, the three-phase cable line is considered as a 6-port network, and the multi-conductor transmission line equation, which includes the relationship between the voltage phasors and current phasors at both ends of the cable, is as follows:

[0014] ,

[0015] In the formula, The chain parameter matrix, L For cable line length, V ( 0 ), I ( 0 These are the voltage column vector and current column vector at the beginning of the cable, respectively. V ( L ), I ( L ) are the voltage column vector and current column vector at the end of the cable, respectively.

[0016] Furthermore, in step 2, the cable operating data includes three-phase core voltage, three-phase load current, three-phase sheath circulating current, and three-phase sheath induced voltage.

[0017] Furthermore, in step 3, the multi-conductor transmission line equation, which includes the relationship between the voltage phasors and current phasors at both ends of the cable, is expressed as follows after order reduction:

[0018] ,

[0019] In the formula, This represents the reduced-order chain parameter matrix. This is the reduced-order voltage matrix at the cable's beginning. This is the reduced-order current matrix at the cable's head end.

[0020] Furthermore, in step 3, the chain parameter matrix is ​​represented as follows:

[0021] ,

[0022] The voltage column vector and current column vector are represented as follows:

[0023] , , , ,

[0024] In the formula, V 0CA , V 0CB , V 0CC These represent the voltages of phase A, B, and C conductors at the beginning of the cable. V0SA , V 0SB , V 0SC These are the induced voltages in the sheaths of phases A, B, and C at the beginning of the cable, respectively. I 0CA , I 0CB , I 0CC These are the load currents of phases A, B, and C at the beginning of the cable, respectively. I 0SA , I 0SB , I 0SC These are the circulating currents of the A, B, and C phase sheaths at the beginning of the cable; V LCA , V LCB , V LCC These represent the voltages of phase A, B, and C conductors at the end of the cable. V LSA , V LSB , V LSC These are the induced voltages in the sheaths of phases A, B, and C at the ends of the cable, respectively. I LCA , I LCB , I LCC These represent the load currents of phases A, B, and C at the ends of the cable. I LSA , I LSB , I LSC These are the circulating currents of the A, B, and C phase sheaths at the ends of the cable, respectively.

[0025] Furthermore, in step 3, the reduced-order cable head-end voltage matrix and current matrix Listed as follows:

[0026] , .

[0027] Furthermore, in step 3, the reduced-order chain parameter matrix Listed as follows:

[0028] ,

[0029] in, Multiply the corresponding elements in the chain parameter matrix by the transformation matrix. The result, Multiply the corresponding elements in the chain parameter matrix by the transformation matrix. ,by For example: .

[0030] Furthermore, in step 4, the reduced-order chain parameter matrix The calculation formula is as follows:

[0031] ,

[0032] In the formula, , ... This refers to the voltage column vector at the cable head end from the six sets of collected cable operation data. , ... This is the column vector of current at the cable head in the 6 sets of collected cable operation data; , ... This is the voltage column vector at the cable end from the six sets of collected cable operation data. , ... This is the current column vector at the cable end from the 6 sets of cable operation data collected.

[0033] Furthermore, in the chain parameter matrix of step 5, , , , All are 6th order square matrices, and are represented as follows:

[0034]

[0035] in, Y This is the unit admittance matrix of the cable line. γ 2 For matrix YZ eigenvalues, Z This is the unit impedance matrix of the cable line; T I The column vectors are matrices YZ The eigenvectors, unit impedance, and admittance matrices are calculated using electromagnetic field methods based on the geometry and medium parameters of the three-phase cable.

[0036] Will , and Each matrix is ​​written as a block matrix consisting of four 3x3 square matrices, and the matrices are written out through block matrix operations. , The expression, solving the matrix by solving the simultaneous equations. The length of the cable line was calculated. L .

[0037] Secondly, the present invention provides a high-voltage cable line parameter extraction system based on online monitoring, used to implement the above-mentioned high-voltage cable line parameter extraction method, comprising:

[0038] Equation writing unit: Based on the multi-conductor transmission line theory, the three-phase cable line is regarded as a multi-port network, and the multi-conductor transmission line equations containing the relationship between the voltage phasors and current phasors at both ends of the cable are written.

[0039] Cable operation data acquisition unit: Collects multiple sets of cable operation data at both ends of the cable, with each set of cable operation data selected at different times of load amplitude;

[0040] Unit for obtaining reduced-order equations: Reduces the order of the chain parameter matrix and the voltage phasor and current phasor at the cable head end in the multi-conductor transmission line equation to obtain the reduced-order multi-conductor transmission line equation.

[0041] Reduced-order chain parameter matrix solving unit: Substitute the collected cable operation data into the reduced-order multi-conductor transmission line equation, and solve the reduced-order chain parameter matrix through matrix operations;

[0042] Cable line length calculation unit: The length of the cable line is calculated using the chain parameter matrix and the reduced-order chain parameter matrix.

[0043] The beneficial effects of this invention are: it can use the voltage and current data at both ends of the cable to infer the actual parameters of the high-voltage cable line, effectively calculate the actual length of the three interconnected sections of the cable, eliminate the influence of uncertain factors on site, improve the accuracy of sheath circulation current calculation, optimize fault judgment criteria, and thus improve the accuracy of fault diagnosis. Attached Figure Description

[0044] 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.

[0045] Figure 1 This is a flowchart of a method for extracting parameters of high-voltage cable lines based on online monitoring, according to the present invention.

[0046] Figure 2 This is a schematic diagram of a cross-interconnection model of a three-phase cable used for simulation in this invention;

[0047] Figure 3This is a diagram illustrating the composition of a high-voltage cable line parameter extraction system based on online monitoring, as described in this invention. Detailed Implementation

[0048] 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.

[0049] Example 1

[0050] This embodiment provides a method for extracting parameters of high-voltage cable lines based on online monitoring, such as... Figure 1 As shown, the steps are as follows:

[0051] Step 1: Based on the multi-conductor transmission line theory, the three-phase cable line is regarded as a multi-port network, and the multi-conductor transmission line equation containing the relationship between the voltage phasors and current phasors at both ends of the cable is written.

[0052] Step 2: Collect multiple sets of cable operation data at both ends of the cable, selecting different times for each set of cable operation data based on different load amplitudes;

[0053] Step 3: Reduce the order of the chain parameter matrix and the voltage phasor and current phasor at the cable head in the multi-conductor transmission line equation to obtain the reduced-order multi-conductor transmission line equation.

[0054] Step 4: Substitute the collected cable operation data into the reduced-order multi-conductor transmission line equation, and solve the reduced-order chain parameter matrix through matrix operations;

[0055] Step 5: Use the chain parameter matrix and the reduced-order chain parameter matrix to solve for the length of the cable line.

[0056] The following is a detailed explanation of the above-mentioned high-voltage cable line parameter extraction method, which treats the three-phase cable line as a 6-port network.

[0057] For a three-phase cable line, based on the multi-conductor transmission line theory, the core and sheath of a three-phase single-core high-voltage cable can each form a loop with the ground. Therefore, a single-circuit three-phase cable can be regarded as 6 transmission lines with the ground as the loop. The voltage and current of the parallel three-phase cables satisfy the following equation:

[0058]

[0059] In the formula, V , I These are the voltage and current column vectors at point z of a multi-conductor transmission line composed of three-phase cables. Z、YThese are the unit length impedance matrix and admittance matrix of the transmission line, respectively. Accurate calculation of the impedance and admittance parameters of a cable is fundamental to calculating voltage and current using a multi-conductor transmission line model. The impedance and admittance parameters can be calculated using electromagnetic field methods based on the geometry and medium parameters of a three-phase cable, which will not be elaborated upon here.

[0060] The voltage and current at the beginning and end of a three-phase cable line can be represented by a chain parameter matrix:

[0061]

[0062] In the formula, L For cable line length, V ( 0 ), I ( 0 These are the voltage column vector and current column vector at the beginning of the cable, respectively. V ( L ), I ( L These represent the voltage and current column vectors at the cable ends, respectively. The block matrix in the chain parameter matrix... , , , Both are sixth-order square matrices, which can be represented as:

[0063]

[0064] In the formula, T I The current-mode transformation matrix has the following column vectors: YZ eigenvectors of a matrix; yes YZ The eigenvalues ​​of the matrix, where , , , , , , , , , , , , , , , All are third-order square matrices.

[0065] At the beginning and end of the cable line, the three-phase conductor voltage, three-phase sheath induced voltage, three-phase load current, and three-phase sheath induced current are measured respectively.

[0066] First, reduce the order of the chain parameter matrix, and then divide the first column of the chain parameter matrix into blocks. , , , and the block matrix of the third column , , , Multiply by column vectors respectively The block matrix in the second column , , , Multiply by column vectors respectively The block matrix in the fourth column , , , All remain unchanged. Substitute the measured data from both ends of the cable into the chain parameter matrix equation, where only the data measured at the beginning of the cable are selected. A Phase core voltage, three-phase sheath induced voltage, A The load current of each phase and the induced current in the three-phase sheath are given by the following equation:

[0067]

[0068] The reduced-order chain parameter matrix is ​​a 12×6 matrix, where , , , , , , , , , , , All are 3×1 matrices. Substituting six sets of data at different times of the collected load amplitude into the above equations, the reduced-order chain parameter matrix is ​​solved. The equations are as follows:

[0069]

[0070]

[0071] In the formula, V 0CA , V 0CB , V 0CC These represent the voltages of phase A, B, and C conductors at the beginning of the cable. V 0SA , V 0SB , V 0SC These are the induced voltages in the sheaths of phases A, B, and C at the beginning of the cable, respectively.I 0CA , I 0CB , I 0CC These are the load currents of phases A, B, and C at the beginning of the cable, respectively. I 0SA , I 0SB , I 0SC These are the circulating currents of the A, B, and C phase sheaths at the beginning of the cable; V LCA , V LCB , V LCC These represent the voltages of phase A, B, and C conductors at the end of the cable. V LSA , V LSB , V LSC These are the induced voltages in the sheaths of phases A, B, and C at the ends of the cable, respectively. I LCA , I LCB , I LCC These represent the load currents of phases A, B, and C at the ends of the cable. I LSA , I LSB , I LSC These are the circulating currents of the A, B, and C phase sheaths at the ends of the cable, respectively.

[0072] Next, starting with the reduced-order chain parameter matrix, we will use the chain parameter matrix calculation formula to calculate the cable length of the first segment of the cross-connection. L The impedance matrix of the cable Z and admittance matrix Y It can be found that:

[0073]

[0074]

[0075]

[0076]

[0077] in, γ 2 For matrix YZ eigenvalues, The column vectors are matrices YZ eigenvectors, where , and All are 6th order square matrices, and As a diagonal matrix, , and Each can be written as a block matrix consisting of four 3x3 square matrices.

[0078] ,

[0079] ,

[0080] ,

[0081] Substitute the above three block matrices into the matrix. :

[0082]

[0083] In , It can be obtained from the calculated reduced-order chain parameter matrix, by combining... , The expression:

[0084]

[0085]

[0086] Subtract the two equations and make the following changes to transform the matrix. K Using matrices A , B , C , D , and express:

[0087]

[0088] The matrix can be obtained from this formula. K， Simultaneous matrix K It can also be written as the following diagonal matrix.

[0089]

[0090] For functions Its inverse function is ,Will K Substituting any element of the matrix into the inverse function yields the corresponding... Substitute the corresponding eigenvalues The length of the cable can be obtained. L .

[0091] The following simulation was performed using the above-mentioned high-voltage cable line parameter extraction method:

[0092] This simulation uses PSCAD to create a cross-connection model of a three-phase cable, such as... Figure 2 As shown, the main cross-connection section is 1500m long, and each smaller cross-connection section is 500m long. The cable used is 64 / 110YJLW02-630mm². 2 The structural parameters were simulated; the power supply adopted three 110kV single-phase AC voltage source models, with the three phase angles differing by 120°.

[0093] Adjusting the load and measuring the load current, sheath circulating current, core voltage, and sheath voltage yielded six sets of data, which were then analyzed according to... and The format is written as follows:

[0094] ,

[0095] , ,

[0096] , , , ,

[0097] ,

[0098] , ,

[0099] ,

[0100] ,

[0101] ,

[0102] ,

[0103] The solution can be obtained using the two matrices above. ,in

[0104]

[0105]

[0106] The impedance matrix Z and admittance matrix Y per unit length of the cable can be used to obtain:

[0107]

[0108]

[0109]

[0110]

[0111]

[0112]

[0113] Substituting the above matrix into the formula, we can calculate the matrix. K ,

[0114]

[0115]

[0116] in, Calculations show that:

[0117]

[0118] The calculated length is the same as the cable length set in the simulation, which verifies the calculation accuracy of the high-voltage cable line parameter extraction method of the present invention.

[0119] This invention, starting from actual field needs, utilizes online monitoring data to eliminate the influence of uncertain factors on-site and calculates accurate parameters for cable lines. This provides a solid foundation for subsequent calculations of theoretical values ​​of sheath circulation current. The high-precision calculated values ​​of sheath circulation current can provide accurate judgment criteria for online monitoring of cable faults based on sheath circulation current.

[0120] This invention can be calculated using the existing online monitoring system for sheath circulation, eliminating the need for additional monitoring equipment and greatly reducing hardware costs.

[0121] Example 2

[0122] This embodiment provides a high-voltage cable line parameter extraction system based on online monitoring, used to implement the high-voltage cable line parameter extraction method described in Embodiment 1, such as... Figure 3 As shown, it consists of an equation writing unit, a cable operation data acquisition unit, a reduced-order equation acquisition unit, a reduced-order chain parameter moment solution unit, and a cable line length solution unit.

[0123] Equation writing unit: Based on the multi-conductor transmission line theory, the three-phase cable line is regarded as a multi-port network, and the multi-conductor transmission line equations containing the relationship between the voltage phasors and current phasors at both ends of the cable are written.

[0124] Cable operation data acquisition unit: Collects multiple sets of cable operation data at both ends of the cable, with each set of cable operation data selected at different times of load amplitude;

[0125] Unit for obtaining reduced-order equations: Reduces the order of the chain parameter matrix and the voltage phasor and current phasor at the cable head end in the multi-conductor transmission line equation to obtain the reduced-order multi-conductor transmission line equation.

[0126] Reduced-order chain parameter matrix solving unit: Substitute the collected cable operation data into the reduced-order multi-conductor transmission line equation, and solve the reduced-order chain parameter matrix through matrix operations;

[0127] Cable line length calculation unit: The length of the cable line is calculated using the chain parameter matrix and the reduced-order chain parameter matrix.

[0128] It should be noted that each unit in the above-mentioned online monitoring-based high-voltage cable line parameter extraction 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 online monitoring-based high-voltage cable line parameter extraction system, please refer to the limitations of the online monitoring-based high-voltage cable line parameter extraction method (i.e., Example 1) above; both have the same function and role, and will not be repeated here.

[0129] 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 online monitoring based extraction of high voltage cable line parameters, characterized in that, The method comprises the following steps: Step 1, based on the multi-conductor transmission line theory, a three-phase cable line is regarded as a multi-port network, and a multi-conductor transmission line equation containing the relationship between the voltage vectors and the current vectors at both ends of the cable is written, in which the voltage and current vectors at the end of the cable are obtained by multiplying the voltage and current vectors at the head of the cable by a chain parameter matrix; Step 2, a plurality of sets of cable operation data are collected at both ends of the cable, and each set of cable operation data is selected at a time point at which the load amplitude is different; Step 3, the chain parameter matrix in the multi-conductor transmission line equation and the voltage and current vectors at the head of the cable are reduced in order to obtain a reduced multi-conductor transmission line equation; Step 4, the collected cable operation data are substituted into the reduced multi-conductor transmission line equation, and the reduced chain parameter matrix is solved through matrix operation; Step 5, the length of the cable line is solved by using the chain parameter matrix and the reduced chain parameter matrix. In step 3, the reduced chain parameter matrix The column write is as follows: ， wherein, is the chain parameter matrix multiplied by the transformation matrix resulting in, is the chain parameter matrix multiplied by the transformation matrix ; The chain parameter matrix in Step 5 is , , , are 6x6 matrices and are given by ， wherein Y is the unit admittance matrix of the cable line, YZ 2 is the eigenvalue of the matrix YZ Z is the unit impedance matrix of the cable line; T I is the eigenvector of the matrix In step 1, the three-phase cable line is regarded as a 6-port network, and the multi-conductor transmission line equation containing the relationship between the voltage vectors and the current vectors at both ends of the cable is as follows: The unit impedance and admittance matrices are calculated by electromagnetic field methods from the geometrical structure and media parameters of the three-phase cable.​ Write , and as block matrices composed of 4 3x3 matrices, respectively, substitute the block matrices into the matrix , and obtain the matrix K， The matrix K is written as a diagonal matrix as follows: For the function its inverse function is , we have K Substitute any element in the matrix into the inverse function, find the corresponding , substitute the corresponding eigenvalue get the length of the cable L .

2. The method of claim 1, wherein, In step 2, the cable operation data include three-phase core voltages, three-phase load currents, three-phase sheath circulating currents and three-phase sheath induced voltages. ， wherein is a chain parameter matrix, L is a cable line length, V 0 I 0 are a voltage column vector and a current column vector at the cable head end, respectively, V L I L are a voltage column vector and a current column vector at the cable end, respectively.​​​​​​ 3. The method of claim 1, wherein the method is characterized by, In step 3, the multi-conductor transmission line equation containing the relationship between the voltage vectors and the current vectors at both ends of the cable is expressed as follows after being reduced in order:

4. The method of claim 2, wherein the method is characterized by, In step 3, the chain parameter matrix is expressed as follows: ， wherein denotes the reduced chain parameter matrix, is the reduced cable head-end voltage matrix, is the reduced cable head-end current matrix.

5. A method for extracting parameters of a high voltage cable line based on online monitoring according to claim 4, characterized in that, The voltage column vector and the current column vector are expressed as follows: ， The method comprises the following steps: ， ， ， ， wherein, V 0CA , V 0CB , V 0CC are the A, B, C phase core voltages at the cable head end, respectively, V 0SA , V 0SB , V 0SC are the A, B, C phase sheath induced voltages at the cable head end, respectively, I 0CA , I 0CB , I 0CC are the A, B, C phase load currents at the cable head end, respectively, I 0SA , I 0SB , I 0SC are the A, B, C phase sheath circulating currents at the cable head end, respectively; V LCA , V LCB , V LCC are the A, B, C phase core voltages at the cable end, respectively, V LSA , V LSB , V LSC are the A, B, C phase sheath induced voltages at the cable end, respectively, I LCA , I LCB , I LCC are the A, B, C phase load currents at the cable end, respectively, I LSA , I LSB , I LSC are the A, B, C phase sheath circulating currents at the cable end, respectively.

6. A method for extracting parameters of a high voltage cable line based on online monitoring according to claim 5, characterized in that, In step 3, the reduced order cable head-end voltage matrix and current matrix is written as follows: ， 。 7. A method for extracting parameters of a high voltage cable line based on online monitoring according to claim 6, characterized in that, In step 4, the reduced chain parameter matrix is calculated as follows: ， wherein , , is the voltage column vector of the cable head end in the 6 sets of collected cable operation data, , , is the current column vector of the cable head end in the 6 sets of collected cable operation data; , , is the voltage column vector of the cable tail end in the 6 sets of collected cable operation data, , , is the current column vector of the cable tail end in the 6 sets of collected cable operation data.

8. A system for online monitoring based high voltage cable line parameter extraction for implementing the method of any of claims 1-7, characterized by An equation writing unit: based on the multi-conductor transmission line theory, a three-phase cable line is regarded as a multi-port network, and a multi-conductor transmission line equation containing the relationship between the voltage vectors and the current vectors at both ends of the cable is written; A cable operation data collecting unit: a plurality of sets of cable operation data are collected at both ends of the cable, and each set of cable operation data is selected at a time point at which the load amplitude is different; A reduced equation obtaining unit: the chain parameter matrix in the multi-conductor transmission line equation and the voltage and current vectors at the head of the cable are reduced in order to obtain a reduced multi-conductor transmission line equation; A reduced chain parameter matrix solving unit: the collected cable operation data are substituted into the reduced multi-conductor transmission line equation, and the reduced chain parameter matrix is solved through matrix operation; A cable line length solving unit: the length of the cable line is solved by using the chain parameter matrix and the reduced chain parameter matrix. ​

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