On-service cable insulation evaluation method and system fusing multivariate target optimization

Abstract

The invention belongs to the technical field of cable insulation evaluation, and provides an on-service cable insulation evaluation method and system fused with multivariate target optimization in order to solve the problem that it is difficult to comprehensively and accurately evaluate the cable insulation state in the prior art. The active service cable insulation evaluation method fused with multivariate target optimization comprises the following steps: determining a time domain incident signal and a frequency domain incident signal; variational mode decomposition is carried out on the measured time domain reflection signal to obtain a time domain actual reflection signal; designing a cable propagation filter by using an initial propagation constant, and estimating a time domain reflection signal by combining a frequency domain incident signal; determining a target function according to energy and position differences between the estimated time domain reflection signal and the actual time domain reflection signal, and optimizing a propagation constant in a set range to minimize a target function value under each frequency; and calculating a dielectric loss factor according to the optimized relationship between the propagation constant and the dielectric constant, and evaluating the cable insulativity.

Description

Technical Field

[0001] The present invention belongs to the technical field of cable insulation evaluation, and in particular relates to an in-service cable insulation evaluation method and system integrating multi-objective optimization. Background Art

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

[0003] As an important power transmission equipment, the operating reliability of cross-linked polyethylene cable directly affects the safe and stable operation of the power system. Over time, the insulation layer of the cable will gradually age under the influence of electrical, thermal, environmental factors and mechanical stress, which may cause failures, power outages and even accidents. Therefore, it is of great significance to study scientific and effective methods for evaluating the insulation status of in-service cables and provide a reasonable basis for cable operation and maintenance plans, which is of great significance to improving the reliability of the power system.

[0004] Traditional cable insulation assessment methods have many shortcomings. For example, the withstand voltage test may cause additional damage to the insulation, the insulation resistance method cannot accurately identify local insulation degradation, and ultra-low frequency testing has problems such as reduced accuracy and equipment limitations for long-distance cables. The degradation level of the cable insulation layer affects the signal propagation characteristics of the cable. Conversely, the information in the signal propagation characteristics can reflect the insulation condition of the cable. The current cable insulation assessment fails to fully consider the complex changes in the cable propagation characteristics under different insulation conditions, lacks the optimal signal propagation constant for evaluating the cable insulation status, and is difficult to comprehensively and accurately evaluate the cable insulation status. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a method and system for evaluating the insulation of in-service cables that integrates multi-objective optimization, which estimates the propagation constant and dielectric loss of the cable at each frequency by optimizing the loss function between the actual measurement signal and the reconstructed signal, and performs insulation status evaluation.

[0006] In order to achieve the above object, the present invention adopts the following technical solution: A first aspect of the present invention provides a method for evaluating insulation of in-service cables integrating multi-objective optimization.

[0007] In one or more embodiments, a method for evaluating insulation of an in-service cable integrating multi-objective optimization is provided, comprising: Determine the time domain incident signal and the frequency domain incident signal according to the length of the cable to be evaluated; When the cable end is kept open and a time domain incident signal is injected from the cable head end, a measured time domain reflection signal is obtained, and a variational mode decomposition is performed on the measured time domain reflection signal to obtain an actual time domain reflection signal; Using a preset initial propagation constant to design a cable propagation filter, and then combining the frequency domain incident signal to estimate the time domain reflection signal; wherein the propagation constant includes a signal attenuation constant and a phase constant; Determine the objective function according to the energy and position difference between the estimated time domain reflection signal and the actual time domain reflection signal, and optimize the propagation constant within the set range so that the objective function value at each frequency is minimized; The dielectric loss factor is calculated based on the optimized relationship between the propagation constant and the dielectric constant, and the cable insulation is evaluated based on the dielectric loss factor.

[0008] As an implementation method, the objective function is: ; in, Represents the time delay between the incident signal in the time domain and the reflected signal in the time domain; Indicates time; represents the preset frequency of the incident signal in the frequency domain; represents the signal attenuation constant, represents the phase constant; represents the estimated time domain reflection signal; Represents the actual time domain reflection signal; represents the duration of the incident signal in the time domain; Indicates the length of the cable to be evaluated.

[0009] As an implementation method, the expression of the cable propagation filter is:

[0010] in, Represents the preset initial propagation constant; Indicates the length of the cable to be evaluated; Indicates the preset frequency.

[0011] As an implementation method, the expression of the estimated time domain reflection signal is:

[0012] in, represents the estimated time domain reflection signal; represents the incident signal in the frequency domain; represents Fourier transform; represents inverse Fourier transform; Represents the preset initial propagation constant; Indicates the length of the cable to be evaluated; Indicates time; Represents the preset frequency of the incident signal in the frequency domain.

[0013] As an implementation method, the dielectric loss factor is expressed as:

[0014] in, Indicates dielectric loss factor; represents the dielectric constant; represents the real part of the complex dielectric constant; represents the imaginary part of the complex dielectric constant; represents electrical conductivity; represents the frequency of the incident signal in the frequency domain; represents the optimized signal attenuation constant, represents the optimized phase constant.

[0015] A second aspect of the present invention provides an in-service cable insulation evaluation system integrating multi-objective optimization.

[0016] In one or more embodiments, a system for evaluating insulation of in-service cables integrating multi-objective optimization includes: An incident signal determination module, which is used to determine a time domain incident signal and a frequency domain incident signal according to the length of the cable to be evaluated; An actual reflection signal calculation module is used to obtain a measured time domain reflection signal while keeping the cable end open and injecting a time domain incident signal from the cable head end, and to perform variational mode decomposition on the measured time domain reflection signal to obtain a time domain actual reflection signal; A reflection signal estimation module, which is used to design a cable propagation filter using a preset initial propagation constant, and then estimate a time domain reflection signal in combination with a frequency domain incident signal; wherein the propagation constant includes a signal attenuation constant and a phase constant; A propagation constant optimization module, which is used to determine the objective function according to the energy and position difference between the estimated time domain reflection signal and the actual time domain reflection signal, and optimize the propagation constant within a set range so that the objective function value at each frequency is minimized; The dielectric loss factor calculation module is used to calculate the dielectric loss factor according to the relationship between the optimized propagation constant and the dielectric constant, and evaluate the cable insulation according to the dielectric loss factor.

[0017] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention determines the objective function by the energy and position difference between the estimated time domain reflection signal and the actual time domain reflection signal, and then minimizes the objective function value at each frequency to estimate the optimal propagation constant of the cable at each frequency. The propagation constant and dielectric loss of the cable are then used to evaluate the insulation condition of the cable, thereby achieving the purpose of comprehensively and accurately evaluating the insulation status of the cable.

[0018] (2) The present invention obtains the real reflection signal by performing variational mode decomposition on the measured time domain reflection signal, thereby effectively solving the signal attenuation and noise interference problems of long cables. The hyperparameters of variational mode decomposition are determined according to the signal characteristics, and the relevant intrinsic mode functions are identified. The signal components related to the cable propagation characteristics are highlighted, and the interference of noise and distorted signals on the reflection signal is reduced, thereby improving the accuracy of cable insulation evaluation.

[0019] (3) Compared with traditional cable insulation assessment methods, such as the withstand voltage test which may cause additional damage to the insulation, the insulation resistance method which cannot accurately identify local insulation degradation, and the ultra-low frequency test which has problems such as reduced accuracy and equipment limitations for long-distance cables, this method can accurately assess the overall insulation status of long-distance cables in service by connecting only to one end of the cable and applying a high-frequency low-voltage signal as an incident signal, greatly reducing the size requirements of the diagnostic equipment and facilitating on-site applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0021] Figure 1 It is a flow chart of an in-service cable insulation evaluation method integrating multi-objective optimization according to an embodiment of the present invention; Figure 2 is a cable distributed parameter equivalent model of an embodiment of the present invention; Figure 3 is a flow chart of a method for optimizing a propagation constant according to an embodiment of the present invention; Figure 4 is a schematic diagram of an electronic device according to an embodiment of the present invention; Figure 5 It is a schematic diagram of the structure of an in-service cable insulation evaluation system integrating multi-objective optimization according to an embodiment of the present invention. DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0023] It should be noted that the following detailed descriptions are all illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0024] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0025] Figure 1 is a flow chart of a method for evaluating insulation of in-service cables integrating multi-objective optimization in an embodiment of the present invention, such as Figure 1 The in-service cable insulation assessment method integrating multi-objective optimization in the present embodiment shown may include: S101, determining a time domain incident signal and a frequency domain incident signal according to the length of the cable to be evaluated; S102, while keeping the cable end open and injecting a time domain incident signal from the cable head end, obtaining a measured time domain reflection signal, performing variational mode decomposition on the measured time domain reflection signal, and obtaining a time domain actual reflection signal; S103, using a preset initial propagation constant to design a cable propagation filter, and then combining the frequency domain incident signal to estimate the time domain reflection signal; wherein the propagation constant includes a signal attenuation constant and a phase constant; S104, determining an objective function according to energy and position differences between the estimated time domain reflection signal and the actual time domain reflection signal, and optimizing the propagation constant within a set range so that the objective function value at each frequency is minimized; S105, calculating a dielectric loss factor according to the relationship between the optimized propagation constant and the dielectric constant, and evaluating the insulation of the cable according to the dielectric loss factor.

[0026] This embodiment determines the objective function by the energy and position difference between the estimated time domain reflection signal and the actual time domain reflection signal, and then minimizes the objective function value at each frequency to estimate the optimal propagation constant of the cable at each frequency, and then uses the propagation constant and dielectric loss of the cable to evaluate the insulation condition of the cable, thereby achieving the purpose of comprehensively and accurately evaluating the insulation status of the cable.

[0027] In step S101, the relationship of the RLGC parameters determines the propagation constant of the cable. Figure 2 The model is shown by the telegraph equation: ; Knowing the frequency-dependent RLGC value of the cable, the propagation constant in the TEM mode can be obtained. and characteristic impedance : ; ; The length of the cable should be greater than the wavelength of the signal. Considering that the signal propagation process is accompanied by attenuation and deviation, the injected signal must have sufficient energy and a reasonable duration to ensure that the high-frequency signal propagates to the end and its energy is concentrated on each frequency, while avoiding duration too long, which may cause signal overlap.

[0028] The reflection coefficient at the end of the cable is defined as: ; In the formula, is the impedance at the end of the cable. ), the reflection coefficient is 1, at this time total reflection occurs at the end of the cable; The time domain incident signal at each frequency is:

[0029] In the formula, is the amplitude, represents the duration of the incident signal in the time domain; represents the rectangular function, which is defined as: , and the formula The role of the sine wave is to Limited to a specific time range, when t is in the interval The value of the rectangular function is 1, otherwise it is 0, so that the voltage incident signal is in the time interval The value inside is .

[0030] Considering that the reflected signal is affected by different propagation constants at different frequencies, the incident signal is given by The N sinusoidal signals are composed of: represents the frequency increment; .

[0031] In step S102, the original measurement signal is composed of K signals concentrated at a specific frequency. In the time domain, it is necessary to filter out the high-frequency noise and low-frequency distortion of the reflected signal. Regardless of the frequency of the incident signal, K is fixed to 9. Through frequency bandpass filtering, the signal noise is reduced by combining the frequency information to ensure that the center frequency of the IMF is included and the signal distortion is minimized. In the actual measurement signal, the IMFs are 5~8 as the target frequency range. The actual measured reflected signal is:

[0032] in, represents the kth IMF component; represents the magnitude of the kth IMF; represents the phase of the kth IMF.

[0033] In step S103, the initial signal propagation constant is: ; represents the signal attenuation constant, represents the phase constant; The expression of the cable propagation filter is:

[0034] in, Represents the preset initial propagation constant; Indicates the length of the cable to be evaluated; Indicates the preset frequency.

[0035] The expression of the estimated time domain reflection signal is:

[0036] in, represents the estimated time domain reflection signal; represents the incident signal in the frequency domain; represents Fourier transform; represents inverse Fourier transform; Represents the preset initial propagation constant; Indicates the length of the cable to be evaluated; Indicates time; Represents the preset frequency of the incident signal in the frequency domain.

[0037] In step S104, the objective function is: ; in, Represents the time delay between the incident signal in the time domain and the reflected signal in the time domain; Indicates time; represents the preset frequency of the incident signal in the frequency domain; represents the signal attenuation constant, represents the phase constant; represents the estimated time domain reflection signal; Represents the actual time domain reflection signal; represents the duration of the incident signal in the time domain; Indicates the length of the cable to be evaluated.

[0038] Adjust the signal attenuation constant within the specified range and phase constant To meet the objective function threshold, to minimize the value of the objective function at each frequency, the lower limit threshold of the objective function change is 10 -6, if the change is below the threshold, the optimization process is considered complete, thereby estimating the specific frequency The optimal signal propagation constant is: ; The propagation constant corresponding to each frequency is extracted by applying this process to all reflected signals.

[0039] In step S105, the dielectric loss factor is calculated using the relationship between the propagation constant and the dielectric constant. Enables cable insulation assessment: ; represents the dielectric constant; represents magnetic permeability; Indicates conductivity.

[0040] Decay Constant: ; Phase constant: ; Calculate the dielectric loss factor:

[0041] in, Indicates dielectric loss factor; represents the dielectric constant; represents the real part of the complex dielectric constant; represents the imaginary part of the complex dielectric constant; represents electrical conductivity; represents the frequency of the incident signal in the frequency domain; represents the optimized signal attenuation constant, represents the optimized phase constant.

[0042] Since more energy is lost in the form of heat when the signal propagates along the cable, when the cable insulation material deteriorates in long-term operation, the tangent value of the dielectric loss angle will increase. The relationship between the change in the tangent value of the loss angle and the propagation constant is: ; In terms of dielectric properties, the partial derivative of the dielectric loss tangent with respect to α is always positive. As thermal aging increases, the signal attenuation during propagation increases. Therefore, the loss tangent also increases accordingly. In contrast, the partial derivative of the dielectric loss tangent with respect to β is always negative.

[0043] The insulation performance of the cables in service is evaluated by examining the changes in the tangent value of the dielectric loss factor with the attenuation constant and the phase constant. From the perspective of the dielectric properties of the insulation, the influence of insulation aging on the signal propagation characteristics is studied, and the signal propagation characteristics reflect the insulation changes, which reflects the complex interaction of different physical properties in signal propagation.

[0044] Reference Figure 4 , a schematic diagram of an electronic device is given. It should be noted that, Figure 4 The electronic device 400 shown is only an example and should not bring any limitation to the functions and scope of use of the embodiments of the present invention.

[0045] like Figure 4 As shown, the electronic device 400 includes a central processing unit (CPU) 401, which can perform various appropriate actions and processes according to the program stored in the read-only memory (ROM) 402 or the program loaded from the storage part 408 to the random access memory (RAM) 403. In the RAM 403, various programs and data required for system operation are also stored. The central processing unit 401, the ROM 402 and the RAM 403 are connected to each other through a bus 404. An input / output (I / O) interface 405 is also connected to the bus 404.

[0046] The following components are connected to the I / O interface 405: an input section 406 including a keyboard, a mouse, etc.; an output section 407 including a cathode ray tube (CRT), a liquid crystal display (LCD), etc., and a speaker, etc.; a storage section 408 including a hard disk, etc.; and a communication section 409 including a network interface card such as a local area network (LAN) card, a modem, etc. The communication section 409 performs communication processing via a network such as the Internet. A drive 410 is also connected to the I / O interface 405 as needed. A removable medium 411, such as a magnetic disk, an optical disk, a magneto-optical disk, a semiconductor memory, etc., is installed on the drive 410 as needed, so that a computer program read therefrom is installed into the storage section 408 as needed.

[0047] When the central processing unit 401 in the electronic device of this embodiment executes the program, the following is achieved: Figure 1 The steps in the in-service cable insulation assessment method integrating multi-objective optimization are shown.

[0048] In particular, according to an embodiment of the present application, the process described above with reference to the flowchart can be implemented as a computer software program. For example, an embodiment of the present application includes a computer program product, which includes a computer program carried on a computer readable medium, the computer program including a computer program for executing Figure 1 In such an embodiment, the computer program can be downloaded and installed from the network through the communication part 409, and / or installed from the removable medium 411. When the computer program is executed by the central processing unit 401, various functions defined in the apparatus of the present application are executed.

[0049] in, Figure 1The computer program instructions corresponding to the method shown may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce a product including an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0050] Figure 5 is a schematic diagram of the structure of an in-service cable insulation evaluation system integrating multi-objective optimization in an embodiment of the present invention. Figure 1 The corresponding method of insulation evaluation of in-service cables based on the fusion of multi-objective optimization is as follows: Figure 5 As shown, the in-service cable insulation evaluation system integrating multi-objective optimization in this embodiment may include: An incident signal determination module 501, which is used to determine a time domain incident signal and a frequency domain incident signal according to the length of the cable to be evaluated; The actual reflection signal calculation module 502 is used to obtain the measured time domain reflection signal under the condition that the cable end is kept open and the time domain incident signal is injected from the cable head end, and perform variational mode decomposition on the measured time domain reflection signal to obtain the time domain actual reflection signal; A reflection signal estimation module 503, which is used to design a cable propagation filter using a preset initial propagation constant, and then estimate a time domain reflection signal in combination with a frequency domain incident signal; wherein the propagation constant includes a signal attenuation constant and a phase constant; A propagation constant optimization module 504 is used to determine an objective function according to the energy and position difference between the estimated time domain reflection signal and the actual time domain reflection signal, and optimize the propagation constant within a set range so that the objective function value at each frequency is minimized; The dielectric loss factor calculation module 505 is used to calculate the dielectric loss factor according to the relationship between the optimized propagation constant and the dielectric constant, and evaluate the cable insulation according to the dielectric loss factor.

[0051] Specifically, in the propagation constant optimization module 504, the objective function is: ; in, Represents the time delay between the incident signal in the time domain and the reflected signal in the time domain; Indicates time; represents the preset frequency of the incident signal in the frequency domain; represents the signal attenuation constant, represents the phase constant; represents the estimated time domain reflection signal; Represents the actual time domain reflection signal; represents the duration of the incident signal in the time domain; Indicates the length of the cable to be evaluated.

[0052] Specifically, in the reflection signal estimation module 503, the expression of the cable propagation filter is:

[0053] in, Represents the preset initial propagation constant; Indicates the length of the cable to be evaluated; Indicates the preset frequency.

[0054] In the actual reflection signal calculation module 502, the expression of the estimated time domain reflection signal is:

[0055] in, represents the estimated time domain reflection signal; represents the incident signal in the frequency domain; represents Fourier transform; represents inverse Fourier transform; Represents the preset initial propagation constant; Indicates the length of the cable to be evaluated; Indicates time; Represents the preset frequency of the incident signal in the frequency domain.

[0056] Specifically, in the dielectric loss factor calculation module 505, the expression of the dielectric loss factor is:

[0057] in, Indicates dielectric loss factor; represents the dielectric constant; represents the real part of the complex dielectric constant; represents the imaginary part of the complex dielectric constant; represents electrical conductivity; represents the frequency of the incident signal in the frequency domain; represents the optimized signal attenuation constant, represents the optimized phase constant.

[0058] It should be noted here that Figure 5 The modules in the current cable insulation evaluation system integrated with multi-objective optimization are Figure 1 The various steps in the in-service cable insulation evaluation method integrating multi-objective optimization correspond to each other, and the specific implementation process is the same, which will not be repeated here.

[0059] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium, and when the program is executed, it can include the processes of the embodiments of the above-mentioned methods. The storage medium can be a disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM), etc.

[0060] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for evaluating the insulation of in-service cables integrating multi-objective optimization, characterized in that: include: Determine the time domain incident signal and the frequency domain incident signal according to the length of the cable to be evaluated; When the cable end is kept open and a time domain incident signal is injected from the cable head end, a measured time domain reflection signal is obtained, and a variational mode decomposition is performed on the measured time domain reflection signal to obtain an actual time domain reflection signal; Using a preset initial propagation constant to design a cable propagation filter, and then combining the frequency domain incident signal to estimate the time domain reflection signal; wherein the propagation constant includes a signal attenuation constant and a phase constant; Determine the objective function according to the energy and position difference between the estimated time domain reflection signal and the actual time domain reflection signal, and optimize the propagation constant within the set range so that the objective function value at each frequency is minimized; The dielectric loss factor is calculated based on the optimized relationship between the propagation constant and the dielectric constant, and the cable insulation is evaluated based on the dielectric loss factor.

2. The in-service cable insulation evaluation method integrating multi-objective optimization as claimed in claim 1 is characterized in that: The objective function is: ； in, Represents the time delay between the incident signal in the time domain and the reflected signal in the time domain; Indicates time; represents the preset frequency of the incident signal in the frequency domain; represents the signal attenuation constant, represents the phase constant; represents the estimated time domain reflection signal; Represents the actual time domain reflection signal; represents the duration of the incident signal in the time domain; Indicates the length of the cable to be evaluated.

3. The in-service cable insulation evaluation method integrating multi-objective optimization as claimed in claim 1 is characterized in that: The expression of the cable propagation filter is: in, Represents the preset initial propagation constant; Indicates the length of the cable to be evaluated; Indicates the preset frequency.

4. The in-service cable insulation evaluation method integrating multi-objective optimization as claimed in claim 1 is characterized in that: The expression of the estimated time domain reflection signal is: in, represents the estimated time domain reflection signal; represents the incident signal in the frequency domain; represents Fourier transform; represents inverse Fourier transform; Represents the preset initial propagation constant; Indicates the length of the cable to be evaluated; Indicates time; Represents the preset frequency of the incident signal in the frequency domain.

5. The in-service cable insulation evaluation method integrating multi-objective optimization as claimed in claim 1 is characterized in that: The expression of dielectric loss factor is: in, Indicates dielectric loss factor; represents the dielectric constant; represents the real part of the complex dielectric constant; represents the imaginary part of the complex dielectric constant; represents electrical conductivity; represents the frequency of the incident signal in the frequency domain; represents the optimized signal attenuation constant, represents the optimized phase constant.

6. An in-service cable insulation evaluation system integrating multi-objective optimization, characterized in that: include: An incident signal determination module, which is used to determine a time domain incident signal and a frequency domain incident signal according to the length of the cable to be evaluated; An actual reflection signal calculation module is used to obtain a measured time domain reflection signal while keeping the cable end open and injecting a time domain incident signal from the cable head end, and to perform variational mode decomposition on the measured time domain reflection signal to obtain a time domain actual reflection signal; A reflection signal estimation module, which is used to design a cable propagation filter using a preset initial propagation constant, and then estimate a time domain reflection signal in combination with a frequency domain incident signal; wherein the propagation constant includes a signal attenuation constant and a phase constant; A propagation constant optimization module, which is used to determine the objective function according to the energy and position difference between the estimated time domain reflection signal and the actual time domain reflection signal, and optimize the propagation constant within a set range so that the objective function value at each frequency is minimized; The dielectric loss factor calculation module is used to calculate the dielectric loss factor according to the relationship between the optimized propagation constant and the dielectric constant, and evaluate the cable insulation according to the dielectric loss factor.

7. The in-service cable insulation evaluation system integrating multi-objective optimization as claimed in claim 6 is characterized in that: The objective function is: ； in, Represents the time delay between the incident signal in the time domain and the reflected signal in the time domain; Indicates time; represents the preset frequency of the incident signal in the frequency domain; represents the signal attenuation constant, represents the phase constant; represents the estimated time domain reflection signal; Represents the actual time domain reflection signal; represents the duration of the incident signal in the time domain; Indicates the length of the cable to be evaluated.

8. The in-service cable insulation evaluation system integrating multi-objective optimization as claimed in claim 6 is characterized in that: The expression of the cable propagation filter is: in, Represents the preset initial propagation constant; Indicates the length of the cable to be evaluated; Indicates the preset frequency.

9. The in-service cable insulation evaluation system integrating multi-objective optimization as claimed in claim 6, characterized in that: The expression of the estimated time domain reflection signal is: in, represents the estimated time domain reflection signal; represents the incident signal in the frequency domain; represents Fourier transform; represents inverse Fourier transform; Represents the preset initial propagation constant; Indicates the length of the cable to be evaluated; Indicates time; Represents the preset frequency of the incident signal in the frequency domain.

10. The in-service cable insulation evaluation system integrating multi-objective optimization as claimed in claim 6, characterized in that: The expression of dielectric loss factor is: in, Indicates dielectric loss factor; represents the dielectric constant; represents the real part of the complex dielectric constant; represents the imaginary part of the complex dielectric constant; represents electrical conductivity; represents the frequency of the incident signal in the frequency domain; represents the optimized signal attenuation constant, represents the optimized phase constant.

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