A method and device for evaluating insulation of a three-core cable

Abstract

The present application discloses a method and device for evaluating the insulation of a three-core cable, the method comprising: obtaining the zero-sequence voltage signals at the head and end of the three-core cable under power disturbance, and performing Fourier transform to obtain the zero-sequence voltage frequency domain signals at the head and end, thereby determining the zero-sequence voltage monitoring signal value at each disturbance frequency, constructing a frequency-variable parameter model of the three-core cable, and thereby obtaining the zero-sequence voltage ratio target function, applying the zero-sequence voltage monitoring signal value to the zero-sequence voltage ratio target function, and obtaining the relative dielectric constant. It can be seen that by using the power disturbance naturally existing in the power system as the excitation signal source, the obtained zero-sequence voltage signals at the head and end will not interfere with the power system, and by analyzing the frequency-variable parameter model to obtain the zero-sequence voltage ratio target function, applying the voltage monitoring signal value to the zero-sequence voltage ratio target function, the relative dielectric constant used to evaluate the insulation performance can be obtained, and used as a standard for insulation evaluation to ensure the safe operation of the power system.

Description

Technical Field

[0001] The present application relates to the field of cable insulation aging assessment, and more specifically, to a method and device for three-core cable insulation assessment. Background Art

[0002] With the continuous increase in electricity demand, the power system is also developing rapidly. The conductive materials used have gradually developed from overhead lines to power cables with excellent electrical properties. In particular, power cables are widely used in medium and low voltage distribution networks. Among the types of power cables, the structure of three-core cables is different from that of single-core cables. The three-phase metal sheaths of three-core cables are close together, and both ends of the sheaths are directly grounded. Under the condition of three-phase symmetry, the sheath induced voltage is 0. Under power disturbance, the three-phase cores of the three-core cable are unbalanced. In recent years, cable accidents have occurred frequently, and many cable accidents are caused by insulation problems. In order to ensure the safe operation of the power system, there are currently many methods for evaluating the insulation of power cables.

[0003] Most of the current methods for monitoring the insulation of power cables are insulation assessment methods for single-core cables. For example, the artificial injection signal method applies a special artificial disturbance excitation source that is more sensitive to the insulation parameters to the cable, and then judges the insulation status of the cable based on the cable's response signal, but it causes interference to the power system using three-core cables. Another example is the grounding wire current method, which uses the measured current of the cable grounding wire to reflect the insulation condition of the cable, but for three-core cables, there is a problem of no grounding wire current under the steady state of the power frequency. Another example is the partial discharge method, although it is the most widely used method at present, there is no unified insulation assessment standard. In general, many current cable insulation assessment methods are not applicable to three-core cables, so a three-core cable insulation assessment method is needed to ensure the safe operation of the power system. Summary of the invention

[0004] In view of the above problems, the present application is proposed to provide a method and device for evaluating the insulation of a three-core cable to ensure the safe operation of the power system.

[0005] In order to achieve the above objectives, the specific plan is proposed as follows:

[0006] A method for evaluating insulation of a three-core cable, comprising:

[0007] Obtaining a zero-sequence voltage signal at the first end and a zero-sequence voltage signal at the end of the three-core cable to be evaluated under power disturbance;

[0008] Performing Fourier transform on the first-end zero-sequence voltage signal and the terminal zero-sequence voltage signal respectively to obtain a first-end zero-sequence voltage frequency domain signal corresponding to the first-end zero-sequence voltage signal and a terminal zero-sequence voltage frequency domain signal corresponding to the terminal zero-sequence voltage signal;

[0009] At each frequency of the terminal zero-sequence voltage frequency domain signal, the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal is used as the zero-sequence voltage monitoring signal value of the frequency;

[0010] Determine the zero-sequence voltage monitoring signal value corresponding to each frequency of the terminal zero-sequence voltage frequency domain signal, wherein the zero-sequence voltage monitoring signal value corresponding to each frequency is the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal at the frequency;

[0011] Based on the cable structure of the three-core cable to be evaluated, constructing a frequency-variable parameter model of the three-core cable to be evaluated;

[0012] According to the frequency-variable parameter model of the three-core cable to be evaluated, a zero-sequence voltage ratio objective function containing a relative dielectric constant is constructed;

[0013] The zero-sequence voltage monitoring signal value of each frequency is applied to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated, so as to evaluate the insulation performance of the three-core cable to be evaluated.

[0014] Optionally, the cable structure of the three-core cable to be evaluated includes a three-phase core and a sheath wrapping the three-phase core, each phase core includes a wire core and an insulating layer wrapping the wire core, and the cores of each phase are tangent to each other.

[0015] Optionally, the frequency-variable parameter model includes a circuit of a three-phase battery cell and a circuit of a three-phase battery cell sheath, the circuit of each phase battery cell is connected to the circuit of the three-phase battery cell sheath through the admittance of each phase battery cell, the line core impedance of each phase battery cell in the circuit of each phase battery cell is connected in series with the load impedance, the load impedances in the circuits of different batteries are the same, and the circuit of the three-phase battery cell sheath includes the sheath reactance of the three-core cable to be evaluated;

[0016] According to the frequency-dependent parameter model of the three-core cable to be evaluated, a zero-sequence voltage ratio objective function containing a relative dielectric constant is constructed, including:

[0017] Based on the topological structure of the circuit of the three-phase battery core in the frequency-varying parameter model, the frequency-varying parameter model is simplified into a zero-sequence network, wherein the zero-sequence network includes a core circuit and a core sheath circuit, wherein the core circuit is connected to the core sheath circuit via a unit zero-sequence admittance, a core unit zero-sequence impedance in the core circuit is connected in series with the load impedance, and the core sheath circuit includes a sheath unit impedance;

[0018] Select any point on the core circuit in the zero-sequence network as a target point, and determine a micro-segment of the target point;

[0019] Based on Kirchhoff's voltage law and Kirchhoff's current law, using the unit zero-sequence admittance, the unit zero-sequence impedance of the core and the unit impedance of the sheath, a differential equation group of the zero-sequence network is constructed for the differential segment, and the differential equation group is solved to obtain a general solution set;

[0020] Based on the load impedance, determining boundary conditions of the first and second ends of the three-core cable to be evaluated for the general solution set;

[0021] Substituting the boundary conditions into the general solution set, a zero-sequence voltage ratio objective function including a relative dielectric constant is determined.

[0022] Optionally, based on Kirchhoff's voltage law and Kirchhoff's current law, using the unit zero-sequence admittance, the core unit zero-sequence impedance and the sheath unit impedance, constructing the differential equation group of the zero-sequence network for the differential segment, and solving the differential equation group to obtain a general solution set includes:

[0023] The differential equations of the zero-sequence network are constructed for the differential segment using the following formula:

[0024]

[0025] Wherein, x is the target location, dx is the micro-segment, is the zero-sequence current of the line core at the target, Y (0) is the unit zero-sequence admittance, is the line core zero-sequence voltage at the target, is the sheath zero-sequence voltage at the target, is the sheath zero-sequence current at the target, Z (0) is the unit zero-sequence impedance of the line core, Z s is the unit impedance of the sheath;

[0026] The differential equations are solved to obtain a general solution set, which is:

[0027]

[0028] in,

[0029] Optionally, the boundary conditions of the first and second ends of the three-core cable to be evaluated for the general solution set are:

[0030]

[0031] in, is the zero-sequence voltage at the head end of the three-core cable to be evaluated, is the sheath voltage at the head end of the three-core cable to be evaluated, is the zero-sequence voltage at the end of the three-core cable to be evaluated, is the zero-sequence current at the end of the three-core cable to be evaluated, Z Load is the load impedance, is the sheath voltage at the end of the three-core cable to be evaluated, and l is the length of the three-core cable to be evaluated.

[0032] Optionally, the zero-sequence voltage ratio objective function including the relative dielectric constant is:

[0033]

[0034] Among them, ε r is the relative dielectric constant, F(ε r ) is the zero-sequence voltage ratio objective function containing the relative dielectric constant, ε 0 =8.86×10 -12 F / m,r i.out is the average outer radius of the insulation layer of each phase core in the three-core cable to be evaluated, r i.in r i.out is the average inner radius of the insulation layer of each phase core in the three-core cable to be evaluated, and f is the frequency.

[0035] Optionally, applying the zero-sequence voltage monitoring signal value of each frequency to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated includes:

[0036] For each zero-sequence voltage monitoring signal value at each frequency, substituting the zero-sequence voltage monitoring signal value into the zero-sequence voltage ratio objective function to obtain a relative dielectric constant reference value at the frequency;

[0037] The lsqcurvefit function of the MATLAB software is used to perform nonlinear least square fitting on each relative dielectric constant reference value to obtain the relative dielectric constant of the three-core cable to be evaluated.

[0038] A cable insulation evaluation device, comprising:

[0039] A zero-sequence voltage monitoring unit, used to obtain a zero-sequence voltage signal at the head end and a zero-sequence voltage signal at the end of the three-core cable to be evaluated under power disturbance;

[0040] A Fourier transform unit, used to perform Fourier transform on the first-end zero-sequence voltage signal and the terminal zero-sequence voltage signal respectively, to obtain a first-end zero-sequence voltage frequency domain signal corresponding to the first-end zero-sequence voltage signal and a terminal zero-sequence voltage frequency domain signal corresponding to the terminal zero-sequence voltage signal;

[0041] A monitoring signal determination unit is used to use the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal at each frequency of the terminal zero-sequence voltage frequency domain signal as the zero-sequence voltage monitoring signal value of the frequency, and determine the zero-sequence voltage monitoring signal value corresponding to each frequency of the terminal zero-sequence voltage frequency domain signal, and the zero-sequence voltage monitoring signal value corresponding to each frequency is the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal at the frequency;

[0042] A frequency-variable parameter model building unit, used to build a frequency-variable parameter model of the three-core cable to be evaluated based on the cable structure of the three-core cable to be evaluated;

[0043] An objective function construction unit, used to construct a zero-sequence voltage ratio objective function containing a relative dielectric constant according to the frequency-variable parameter model of the three-core cable to be evaluated;

[0044] The relative dielectric constant determination unit is used to apply the zero-sequence voltage monitoring signal value of each frequency to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated, so as to evaluate the insulation performance of the three-core cable to be evaluated.

[0045] Optionally, the cable structure of the three-core cable to be evaluated based on the objective function construction unit includes a three-phase core and a sheath wrapping the three-phase core, each phase core includes a wire core and an insulating layer wrapping the wire core, and the cores of each phase are tangent to each other.

[0046] Optionally, the frequency-variable parameter model includes a circuit of a three-phase battery cell and a circuit of a three-phase battery cell sheath, the circuit of each phase battery cell is connected to the circuit of the three-phase battery cell sheath through the admittance of each phase battery cell, the line core impedance of each phase battery cell in the circuit of each phase battery cell is connected in series with the load impedance, the load impedances in the circuits of different batteries are the same, and the circuit of the three-phase battery cell sheath includes the sheath reactance of the three-core cable to be evaluated;

[0047] The objective function construction unit comprises:

[0048] A model simplification unit, for simplifying the frequency-varying parameter model into a zero-sequence network based on the topological structure of the circuit of the three-phase battery core in the frequency-varying parameter model, wherein the zero-sequence network includes a core circuit and a core sheath circuit, wherein the core circuit is connected to the core sheath circuit via a unit zero-sequence admittance, a core unit zero-sequence impedance in the core circuit is connected in series with the load impedance, and the core sheath circuit includes a sheath unit impedance;

[0049] A micro-segment determination unit, configured to select any location on a core circuit in the zero-sequence network as a target location, and determine a micro-segment at the target location;

[0050] An equation construction unit is used to construct a differential equation group of the zero-sequence network for the differential segment based on Kirchhoff's voltage law and Kirchhoff's current law, using the unit zero-sequence admittance, the core unit zero-sequence impedance and the sheath unit impedance, and solve the differential equation group to obtain a general solution set;

[0051] A boundary condition determination unit, used to determine the boundary conditions of the first and second ends of the three-core cable to be evaluated for the general solution set based on the load impedance;

[0052] The objective function determination unit is used to substitute the boundary conditions into the general solution set to determine the zero-sequence voltage ratio objective function containing the relative dielectric constant.

[0053] Optionally, the equation building unit includes:

[0054] The first equation construction subunit is used to construct the differential equation group of the zero-sequence network for the differential segment using the following formula:

[0055]

[0056] Wherein, x is the target location, dx is the micro-segment, is the zero-sequence current of the line core at the target, Y (0) is the unit zero-sequence admittance, is the line core zero-sequence voltage at the target, is the sheath zero-sequence voltage at the target, is the sheath zero-sequence current at the target, Z (0) is the unit zero-sequence impedance of the line core, Z s is the unit impedance of the sheath;

[0057] The second equation construction subunit is used to solve the differential equation system to obtain a general solution set, and the general solution set is:

[0058]

[0059] in,

[0060] Optionally, the boundary conditions of the first and second ends of the three-core cable to be evaluated for the general solution set determined by the boundary condition determination unit are:

[0061]

[0062] in, is the zero-sequence voltage at the head end of the three-core cable to be evaluated, is the sheath voltage at the head end of the three-core cable to be evaluated, is the zero-sequence voltage at the end of the three-core cable to be evaluated, is the zero-sequence current at the end of the three-core cable to be evaluated, Z Load is the load impedance, is the sheath voltage at the end of the three-core cable to be evaluated, and l is the length of the three-core cable to be evaluated.

[0063] Optionally, the zero-sequence voltage ratio objective function containing the relative dielectric constant determined by the objective function determination unit is:

[0064]

[0065] Among them, ε r is the relative dielectric constant, F(ε r ) is the zero-sequence voltage ratio objective function containing the relative dielectric constant, ε 0 =8.86×10 -12 F / m,r i.out is the average outer radius of the insulation layer of each phase core in the three-core cable to be evaluated, r i.in is the average inner radius of the insulation layer of each phase core in the three-core cable to be evaluated, and f is the frequency.

[0066] Optionally, the relative dielectric constant determining unit includes:

[0067] A monitoring signal substitution unit, for substituting the zero-sequence voltage monitoring signal value at each frequency into the zero-sequence voltage ratio objective function to obtain a relative dielectric constant reference value at the frequency;

[0068] The relative dielectric constant fitting unit is used to perform nonlinear least square fitting on each relative dielectric constant reference value using the lsqcurvefit function of the matlab software to obtain the relative dielectric constant of the three-core cable to be evaluated.

[0069] By means of the above technical scheme, the present application obtains the head-end zero-sequence voltage signal and the terminal zero-sequence voltage signal of the three-core cable to be evaluated under power disturbance, and respectively performs Fourier transform on the head-end zero-sequence voltage signal and the terminal zero-sequence voltage signal to obtain the head-end zero-sequence voltage frequency domain signal corresponding to the head-end zero-sequence voltage signal and the terminal zero-sequence voltage frequency domain signal corresponding to the terminal zero-sequence voltage signal. At each frequency of the terminal zero-sequence voltage frequency domain signal, the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal is used as the zero-sequence voltage monitoring signal value of the frequency, and each frequency of the terminal zero-sequence voltage frequency domain signal is determined. The zero-sequence voltage monitoring signal value corresponding to the frequency, each zero-sequence voltage monitoring signal value corresponding to the frequency is the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal at the frequency. Furthermore, based on the cable structure of the three-core cable to be evaluated, a frequency-varying parameter model of the three-core cable to be evaluated is constructed. According to the frequency-varying parameter model of the three-core cable to be evaluated, a zero-sequence voltage ratio objective function containing a relative dielectric constant is constructed. The zero-sequence voltage monitoring signal value of each frequency is applied to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated, so as to evaluate the insulation performance of the three-core cable to be evaluated. It can be seen that by using the power disturbance naturally existing in the power system as the excitation signal source to obtain the zero-sequence voltage signal at the head end and the zero-sequence voltage signal at the end of the three-core cable to be evaluated, there is no need to ground the three-core cable. By analyzing the frequency-variable parameter model to obtain the zero-sequence voltage ratio objective function, the voltage monitoring signal value is applied to the zero-sequence voltage ratio objective function, and the relative dielectric constant used to evaluate the insulation performance can be obtained. This can be used as a standard for insulation evaluation, thereby effectively ensuring the safe operation of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present application. Also, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0071] Figure 1 A schematic diagram of a process for insulation evaluation of a three-core cable provided in an embodiment of the present application;

[0072] Figure 2 A schematic diagram of monitoring a zero-sequence voltage signal of a three-core cable provided in an embodiment of the present application;

[0073] Figure 3 A schematic cross-sectional view of a three-core cable provided in an embodiment of the present application;

[0074] Figure 4 A schematic diagram of the structure of a device for evaluating the insulation of a three-core cable provided in an embodiment of the present application;

[0075] Figure 5 A schematic diagram of the structure of a three-core cable insulation evaluation device provided in an embodiment of the present application. DETAILED DESCRIPTION

[0076] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0077] The present application solution can be implemented based on a terminal with data processing capabilities, which can be a computer, server, cloud, etc.

[0078] Next, combine Figure 1 The three-core cable insulation evaluation method of the present application may include the following steps:

[0079] Step S110, obtaining a head-end zero-sequence voltage signal and a terminal zero-sequence voltage signal of the three-core cable to be evaluated under power disturbance.

[0080] It is understandable that the power system contains rich frequency components when power disturbance occurs. Under power disturbance, the zero-sequence voltage signal at the head end and the zero-sequence voltage signal at the end of the three-core cable to be evaluated measured have multiple frequencies.

[0081] like Figure 2 As shown, when a power disturbance is monitored, a head-end zero-sequence voltage signal and a terminal zero-sequence voltage signal are obtained at the head end and the terminal end of the three-core cable respectively.

[0082] Step S120, performing Fourier transform on the first-end zero-sequence voltage signal and the terminal zero-sequence voltage signal respectively to obtain a first-end zero-sequence voltage frequency domain signal corresponding to the first-end zero-sequence voltage signal and a terminal zero-sequence voltage frequency domain signal corresponding to the terminal zero-sequence voltage signal.

[0083] It can be understood that the zero-sequence voltage signal obtained at both ends of the cable to be evaluated is a time-sequence signal. Since the zero-sequence voltage signal during power disturbance contains multiple frequencies, the zero-sequence voltage signal can be Fourier transformed to obtain the head-end zero-sequence voltage frequency domain signal and the end-end zero-sequence voltage frequency domain signal in the frequency domain.

[0084] Step S130: at each frequency of the terminal zero-sequence voltage frequency domain signal, taking the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal as the zero-sequence voltage monitoring signal value of the frequency.

[0085] Specifically, the zero-sequence voltage monitoring signal value can represent the frequency change of the system frequency from the head end through the three-core cable body to be evaluated to the end, reflecting the conductive performance of the three-core cable.

[0086] Step S140: Determine a zero-sequence voltage monitoring signal value corresponding to each frequency of the terminal zero-sequence voltage frequency domain signal.

[0087] Specifically, the zero-sequence voltage monitoring signal values ​​at all frequencies may be determined.

[0088] The zero-sequence voltage monitoring signal value corresponding to each frequency is the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal at the frequency.

[0089] Step S150: constructing a frequency-variable parameter model of the three-core cable to be evaluated based on the cable structure of the three-core cable to be evaluated.

[0090] Specifically, the cable structure of the three-core cable to be evaluated is as follows: Figure 3 As shown, the three-core cable to be evaluated may include a three-phase core and a sheath wrapping the three-phase core, each phase core may include a wire core and an insulating layer wrapping the wire core, wherein the insulating layer and the wire core may be isolated by an inner semi-conductive shield, the outer semi-conductive shield wraps the insulating layer, the metal shield wraps the outer semi-conductive shield, the cores of each phase are mutually circumscribed, the sheath may be composed of an inner protective layer, a metal armor and an outer protective layer, the outer protective layer wraps the metal armor, the metal armor wraps the three-phase core, and the filler may be used to fill the three-core cable to be evaluated so that there is no air inside the three-core cable to be evaluated.

[0091] Step S160: construct a zero-sequence voltage ratio objective function including a relative dielectric constant according to the frequency-dependent parameter model of the three-core cable to be evaluated.

[0092] It can be understood that the relative dielectric constant is related to the admittance of the three-core cable. The frequency-varying parameter model of the three-core cable to be evaluated can be analyzed, and the relationship between the admittance of the three-core cable and the zero-sequence voltage monitoring signal value can be established to obtain the zero-sequence voltage ratio objective function containing the relative dielectric constant.

[0093] Step S170: Apply the zero-sequence voltage monitoring signal value of each frequency to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated, so as to evaluate the insulation performance of the three-core cable to be evaluated.

[0094] Specifically, the relative dielectric constant can be obtained from the zero-sequence voltage monitoring signal value through the zero-sequence voltage ratio objective function. The relative dielectric constant can represent the insulation performance of the three-core cable to be evaluated. The larger the relative dielectric constant, the better the insulation performance of the three-core cable to be evaluated, and vice versa.

[0095] The three-core cable insulation evaluation method provided in this embodiment uses the power disturbance naturally existing in the power system as the excitation signal source to obtain the head-end zero-sequence voltage signal and the end-end zero-sequence voltage signal of the three-core cable to be evaluated, without causing interference to the power system. Without the need to ground the three-core cable, the zero-sequence voltage ratio target function is obtained by analyzing the frequency-variable parameter model, and the voltage monitoring signal value is applied to the zero-sequence voltage ratio target function. The relative dielectric constant used to evaluate the insulation performance can be obtained and used as a standard for insulation evaluation, thereby effectively ensuring the safe operation of the power system.

[0096] In some embodiments of the present application, the cable structure of the three-core cable to be evaluated mentioned in the above embodiment is introduced, and the cable structure may include:

[0097] A three-phase battery core and a sheath wrapping the three-phase battery core.

[0098] Each phase of the battery core includes a wire core and an insulating layer wrapping the wire core, and the battery cores of each phase are circumscribed to each other.

[0099] Based on this, the frequency-variant parameter model of the three-core cable to be evaluated in the process of constructing the frequency-variant parameter model of the three-core cable to be evaluated based on the cable structure of the three-core cable to be evaluated in step S150 of the above embodiment is introduced. The frequency-variant parameter model may include:

[0100] The circuit of the three-phase battery cell and the circuit of the three-phase battery cell sheath, the circuit of each phase battery cell is connected to the circuit of the three-phase battery cell sheath through the admittance of each phase battery cell, the line core impedance of each phase battery cell in the circuit of each phase battery cell is connected in series with the load impedance, the load impedances in the circuits of different batteries are the same, and the circuit of the three-phase battery cell sheath includes the sheath reactance of the three-core cable to be evaluated.

[0101] Based on this, the process of constructing a zero-sequence voltage ratio objective function containing a relative dielectric constant according to the frequency-dependent parameter model of the three-core cable to be evaluated in step S160 of the above embodiment is introduced. The process may include:

[0102] S1. Based on the topological structure of the circuit of the three-phase battery cell in the frequency-variable parameter model, simplify the frequency-variable parameter model into a zero-sequence network.

[0103] Specifically, the zero-sequence network includes a core circuit and a core sheath circuit, the core circuit is connected to the core sheath circuit via a unit zero-sequence admittance, the core unit zero-sequence impedance in the core circuit is connected in series with the load impedance, and the core sheath circuit includes a sheath unit impedance.

[0104] It can be understood that the topological structure of the circuit of each phase of the three-phase battery cell is equipotential, so one phase of the three-phase battery cell can be taken as the research object, wherein the corresponding circuit as the research object is the line core circuit representing the three-phase battery cell.

[0105] S2. Select any point on the core circuit in the zero-sequence network as a target point, and determine a micro-segment of the target point.

[0106] S3. Based on Kirchhoff's voltage law and Kirchhoff's current law, using the unit zero-sequence admittance, the core unit zero-sequence impedance and the sheath unit impedance, construct a differential equation group of the zero-sequence network for the differential segment, and solve the differential equation group to obtain a general solution set.

[0107] Specifically, the differential equation group of the zero-sequence network can be constructed for the differential segment using the following formula:

[0108]

[0109] Wherein, x is the target location, dx is the micro-segment, is the zero-sequence current of the line core at the target, Y (0) is the unit zero-sequence admittance, is the line core zero-sequence voltage at the target, is the sheath zero-sequence voltage at the target, is the sheath zero-sequence current at the target, Z (0) is the unit zero-sequence impedance of the line core, Z s is the unit impedance of the sheath;

[0110] The differential equations are solved to obtain a general solution set, which is:

[0111]

[0112] in,

[0113] S4. Based on the load impedance, determine the boundary conditions of the first and second ends of the three-core cable to be evaluated for the general solution set.

[0114] The boundary conditions of the first and last ends of the three-core cable to be evaluated for the general solution set are:

[0115]

[0116] in, is the zero-sequence voltage at the head end of the three-core cable to be evaluated, is the sheath voltage at the head end of the three-core cable to be evaluated, is the zero-sequence voltage at the end of the three-core cable to be evaluated, is the zero-sequence current at the end of the three-core cable to be evaluated, Z Load is the load impedance, is the sheath voltage at the end of the three-core cable to be evaluated, and l is the length of the three-core cable to be evaluated.

[0117] S5. Substitute the boundary conditions into the general solution set to determine a zero-sequence voltage ratio objective function including a relative dielectric constant.

[0118] Specifically, after substituting the boundary conditions into the general solution set, the zero-sequence voltage ratio objective function is simplified.

[0119] It can be understood that the zero-sequence voltage ratio objective function is related to the relative dielectric constant and can be expressed as:

[0120]

[0121] Among them, ε r is the relative dielectric constant, F(ε r ) is the zero-sequence voltage ratio objective function containing the relative dielectric constant, ε 0 =8.86×10 -12 F / m,r i.out is the average outer radius of the insulation layer of each phase core in the three-core cable to be evaluated, r i.in is the average inner radius of the insulation layer of each phase core in the three-core cable to be evaluated, and f is the frequency.

[0122] The three-core cable insulation evaluation method provided in this embodiment constructs an equivalent frequency-varying parameter model through the cable structure of the three-core cable to be evaluated, thereby obtaining a simplified zero-sequence network, analyzing the topological relationship between the core circuit and the core sheath circuit from the zero-sequence network, constructing a differential equation group of the zero-sequence network and solving it to obtain a general solution group, and using the boundary conditions of the two ends of the three-core cable to be evaluated for the general solution set to determine the zero-sequence voltage ratio objective function containing the relative dielectric constant, thereby establishing the relationship between the relative dielectric constant and the zero-sequence voltage monitoring signal.

[0123] In some embodiments of the present application, the process of applying the zero-sequence voltage monitoring signal value of each frequency to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated to evaluate the insulation performance of the three-core cable to be evaluated is introduced, and the process may include:

[0124] S1. For the zero-sequence voltage monitoring signal value at each frequency, substitute the zero-sequence voltage monitoring signal value into the zero-sequence voltage ratio objective function to obtain a relative dielectric constant reference value at the frequency.

[0125] It can be understood that the zero-sequence voltage monitoring signal under power disturbance has multiple frequency components. Therefore, multiple zero-sequence voltage monitoring signal values ​​can be obtained based on the frequency. By substituting each zero-sequence voltage monitoring signal value into the zero-sequence voltage ratio objective function related to the relative dielectric constant, each relative dielectric constant reference value can be obtained. The relative dielectric constant reference values ​​corresponding to different frequencies may be different.

[0126] S2. Use the lsqcurvefit function of the matlab software to perform nonlinear least square fitting on each relative dielectric constant reference value to obtain the relative dielectric constant of the three-core cable to be evaluated.

[0127] It is understandable that, since the relative dielectric constant reference values ​​may be different, the lsqcurvefit function of the MATLAB software can be used to perform nonlinear least squares fitting on the relative dielectric constant reference values ​​to obtain a more objective relative dielectric constant of the three-core cable to be evaluated.

[0128] The three-core cable insulation evaluation method provided in this embodiment obtains multiple relative dielectric constant reference values ​​by substituting the multiple zero-sequence voltage monitoring signal values ​​obtained by monitoring into the zero-sequence voltage ratio objective function containing the relative dielectric constant, and performs nonlinear least squares fitting on the multiple relative dielectric constant reference values ​​obtained, thereby obtaining a more objective relative dielectric constant of the three-core cable to be evaluated, so as to evaluate the insulation performance of the three-core cable to be evaluated.

[0129] The following is a description of an apparatus for implementing insulation evaluation of a three-core cable provided in an embodiment of the present application. The apparatus for implementing insulation evaluation of a three-core cable described below and the method for implementing insulation evaluation of a three-core cable described above can be referenced to each other.

[0130] See also Figure 4 , Figure 4 A schematic diagram of the structure of a device for implementing insulation evaluation of a three-core cable disclosed in an embodiment of the present application.

[0131] like Figure 4 As shown, the device may include:

[0132] A zero-sequence voltage monitoring unit 11 is used to obtain a zero-sequence voltage signal at the head end and a zero-sequence voltage signal at the end of the three-core cable to be evaluated under power disturbance;

[0133] A Fourier transform unit 12 is used to perform Fourier transform on the first-end zero-sequence voltage signal and the terminal zero-sequence voltage signal respectively to obtain a first-end zero-sequence voltage frequency domain signal corresponding to the first-end zero-sequence voltage signal and a terminal zero-sequence voltage frequency domain signal corresponding to the terminal zero-sequence voltage signal;

[0134] A monitoring signal determination unit 13 is used to use the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal at each frequency of the terminal zero-sequence voltage frequency domain signal as the zero-sequence voltage monitoring signal value of the frequency, and determine the zero-sequence voltage monitoring signal value corresponding to each frequency of the terminal zero-sequence voltage frequency domain signal, and the zero-sequence voltage monitoring signal value corresponding to each frequency is the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal at the frequency;

[0135] A frequency-variable parameter model building unit 14, configured to build a frequency-variable parameter model of the three-core cable to be evaluated based on the cable structure of the three-core cable to be evaluated;

[0136] An objective function construction unit 15 is used to construct a zero-sequence voltage ratio objective function containing a relative dielectric constant according to the frequency-dependent parameter model of the three-core cable to be evaluated;

[0137] The relative dielectric constant determination unit 16 is used to apply the zero-sequence voltage monitoring signal value of each frequency to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated, so as to evaluate the insulation performance of the three-core cable to be evaluated.

[0138] Optionally, the cable structure of the three-core cable to be evaluated, on which the objective function construction unit 15 is based, includes a sheath and three-phase cores, each phase core includes a wire core and an insulating layer wrapping the wire core, and the cores of each phase are tangent to each other.

[0139] Optionally, the frequency-variable parameter model includes a circuit of a three-phase battery cell and a circuit of a three-phase battery cell sheath, the circuit of each phase battery cell is connected to the circuit of the three-phase battery cell sheath through the admittance of each phase battery cell, the line core impedance of each phase battery cell in the circuit of each phase battery cell is connected in series with the load impedance, the load impedances in the circuits of different batteries are the same, and the circuit of the three-phase battery cell sheath includes the sheath reactance of the three-core cable to be evaluated;

[0140] The objective function construction unit 15 comprises:

[0141] A model simplification unit, for simplifying the frequency-varying parameter model into a zero-sequence network based on the topological structure of the circuit of the three-phase battery core in the frequency-varying parameter model, wherein the zero-sequence network includes a core circuit and a core sheath circuit, wherein the core circuit is connected to the core sheath circuit via a unit zero-sequence admittance, a core unit zero-sequence impedance in the core circuit is connected in series with the load impedance, and the core sheath circuit includes a sheath unit impedance;

[0142] A micro-segment determination unit, configured to select any location on a core circuit in the zero-sequence network as a target location, and determine a micro-segment at the target location;

[0143] An equation construction unit is used to construct a differential equation group of the zero-sequence network for the differential segment based on Kirchhoff's voltage law and Kirchhoff's current law, using the unit zero-sequence admittance, the core unit zero-sequence impedance and the sheath unit impedance, and solve the differential equation group to obtain a general solution set;

[0144] A boundary condition determination unit, used to determine the boundary conditions of the first and second ends of the three-core cable to be evaluated for the general solution set based on the load impedance;

[0145] The objective function determination unit is used to substitute the boundary conditions into the general solution set to determine the zero-sequence voltage ratio objective function containing the relative dielectric constant.

[0146] Optionally, the equation building unit includes:

[0147] The first equation construction subunit is used to construct the differential equation group of the zero-sequence network for the differential segment using the following formula:

[0148]

[0149] Wherein, x is the target location, dx is the micro-segment, is the zero-sequence current of the line core at the target, Y (0) is the unit zero-sequence admittance, is the line core zero-sequence voltage at the target, is the sheath zero-sequence voltage at the target, is the sheath zero-sequence current at the target, Z (0) is the unit zero-sequence impedance of the line core, Z s is the unit impedance of the sheath;

[0150] The second equation construction subunit is used to solve the differential equation system to obtain a general solution set, and the general solution set is:

[0151]

[0152] in,

[0153] Optionally, the boundary conditions of the first and second ends of the three-core cable to be evaluated for the general solution set determined by the boundary condition determination unit are:

[0154]

[0155] in, is the zero-sequence voltage at the head end of the three-core cable to be evaluated, is the sheath voltage at the head end of the three-core cable to be evaluated, is the zero-sequence voltage at the end of the three-core cable to be evaluated, is the zero-sequence current at the end of the three-core cable to be evaluated, Z Load is the load impedance, is the sheath voltage at the end of the three-core cable to be evaluated, and l is the length of the three-core cable to be evaluated.

[0156] Optionally, the zero-sequence voltage ratio objective function containing the relative dielectric constant determined by the objective function determination unit is:

[0157]

[0158] Among them, ε r is the relative dielectric constant, F(ε r ) is the zero-sequence voltage ratio objective function containing the relative dielectric constant, ε 0 =8.86×10 -12 F / m,r i.out is the average outer radius of the insulation layer of each phase core in the three-core cable to be evaluated, r i.in is the average inner radius of the insulation layer of each phase core in the three-core cable to be evaluated, and f is the frequency.

[0159] Optionally, the relative dielectric constant determining unit 16 includes:

[0160] A monitoring signal substitution unit, for substituting the zero-sequence voltage monitoring signal value at each frequency into the zero-sequence voltage ratio objective function to obtain a relative dielectric constant reference value at the frequency;

[0161] The relative dielectric constant fitting unit is used to perform nonlinear least square fitting on each relative dielectric constant reference value using the lsqcurvefit function of the matlab software to obtain the relative dielectric constant of the three-core cable to be evaluated.

[0162] The three-core cable insulation evaluation device provided in the embodiment of the present application can be applied to three-core cable insulation evaluation equipment, such as terminals: mobile phones, computers, etc. Optionally, Figure 5 The hardware structure diagram of the three-core cable insulation evaluation equipment is shown in FIG. Figure 5 ,The hardware structure of the three-core cable insulation evaluation device may include: at least one processor 1, at least one communication interface 2, at least one memory 3 and at least one communication bus 4;

[0163] In the embodiment of the present application, the number of the processor 1, the communication interface 2, the memory 3, and the communication bus 4 is at least one, and the processor 1, the communication interface 2, and the memory 3 communicate with each other through the communication bus 4;

[0164] The processor 1 may be a central processing unit CPU, or an application-specific integrated circuit ASIC (Application Specific Integrated Circuit), or one or more integrated circuits configured to implement the embodiments of the present invention, etc.;

[0165] The memory 3 may include a high-speed RAM memory, and may also include a non-volatile memory, such as at least one disk memory;

[0166] The memory stores a program, and the processor can call the program stored in the memory, wherein the program is used to:

[0167] Obtaining a zero-sequence voltage signal at the first end and a zero-sequence voltage signal at the end of the three-core cable to be evaluated under power disturbance;

[0168] Performing Fourier transform on the first-end zero-sequence voltage signal and the terminal zero-sequence voltage signal respectively to obtain a first-end zero-sequence voltage frequency domain signal corresponding to the first-end zero-sequence voltage signal and a terminal zero-sequence voltage frequency domain signal corresponding to the terminal zero-sequence voltage signal;

[0169] At each frequency of the terminal zero-sequence voltage frequency domain signal, the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal is used as the zero-sequence voltage monitoring signal value of the frequency;

[0170] Determine the zero-sequence voltage monitoring signal value corresponding to each frequency of the terminal zero-sequence voltage frequency domain signal, wherein the zero-sequence voltage monitoring signal value corresponding to each frequency is the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal at the frequency;

[0171] Based on the cable structure of the three-core cable to be evaluated, constructing a frequency-variable parameter model of the three-core cable to be evaluated;

[0172] According to the frequency-variable parameter model of the three-core cable to be evaluated, a zero-sequence voltage ratio objective function containing a relative dielectric constant is constructed;

[0173] The zero-sequence voltage monitoring signal value of each frequency is applied to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated, so as to evaluate the insulation performance of the three-core cable to be evaluated.

[0174] Optionally, the detailed functions and extended functions of the program may refer to the above description.

[0175] The embodiment of the present application further provides a storage medium, which may store a program suitable for execution by a processor, wherein the program is used to:

[0176] Obtaining a zero-sequence voltage signal at the first end and a zero-sequence voltage signal at the end of the three-core cable to be evaluated under power disturbance;

[0177] Performing Fourier transform on the first-end zero-sequence voltage signal and the terminal zero-sequence voltage signal respectively to obtain a first-end zero-sequence voltage frequency domain signal corresponding to the first-end zero-sequence voltage signal and a terminal zero-sequence voltage frequency domain signal corresponding to the terminal zero-sequence voltage signal;

[0178] At each frequency of the terminal zero-sequence voltage frequency domain signal, the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal is used as the zero-sequence voltage monitoring signal value of the frequency;

[0179] Determine the zero-sequence voltage monitoring signal value corresponding to each frequency of the terminal zero-sequence voltage frequency domain signal, wherein the zero-sequence voltage monitoring signal value corresponding to each frequency is the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal at the frequency;

[0180] Based on the cable structure of the three-core cable to be evaluated, constructing a frequency-variable parameter model of the three-core cable to be evaluated;

[0181] According to the frequency-variable parameter model of the three-core cable to be evaluated, a zero-sequence voltage ratio objective function containing a relative dielectric constant is constructed;

[0182] The zero-sequence voltage monitoring signal value of each frequency is applied to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated, so as to evaluate the insulation performance of the three-core cable to be evaluated.

[0183] Optionally, the detailed functions and extended functions of the program may refer to the above description.

[0184] Finally, it should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the presence of other identical elements in the process, method, article or device including the elements.

[0185] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The various embodiments can be combined as needed, and the same or similar parts can refer to each other.

[0186] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for evaluating the insulation of a cable, characterized in that: include: Obtaining a zero-sequence voltage signal at the first end and a zero-sequence voltage signal at the end of the three-core cable to be evaluated under power disturbance; Performing Fourier transform on the first-end zero-sequence voltage signal and the terminal zero-sequence voltage signal respectively to obtain a first-end zero-sequence voltage frequency domain signal corresponding to the first-end zero-sequence voltage signal and a terminal zero-sequence voltage frequency domain signal corresponding to the terminal zero-sequence voltage signal; At each frequency of the terminal zero-sequence voltage frequency domain signal, the ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal is used as the zero-sequence voltage monitoring signal value of the frequency; Based on the cable structure of the three-core cable to be evaluated, constructing a frequency-variable parameter model of the three-core cable to be evaluated; According to the frequency-variable parameter model of the three-core cable to be evaluated, a zero-sequence voltage ratio objective function containing a relative dielectric constant is constructed; Applying the zero-sequence voltage monitoring signal value of each frequency to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated, so as to evaluate the insulation performance of the three-core cable to be evaluated; The frequency-variable parameter model includes a circuit of a three-phase battery cell and a circuit of a three-phase battery cell sheath, the circuit of each phase battery cell is connected to the circuit of the three-phase battery cell sheath through the admittance of each phase battery cell, the line core impedance of each phase battery cell in the circuit of each phase battery cell is connected in series with the load impedance, the load impedances in the circuits of different batteries are the same, and the circuit of the three-phase battery cell sheath includes the sheath reactance of the three-core cable to be evaluated; According to the frequency-dependent parameter model of the three-core cable to be evaluated, a zero-sequence voltage ratio objective function containing a relative dielectric constant is constructed, including: Based on the topological structure of the circuit of the three-phase battery core in the frequency-varying parameter model, the frequency-varying parameter model is simplified into a zero-sequence network, wherein the zero-sequence network includes a core circuit and a core sheath circuit, wherein the core circuit is connected to the core sheath circuit via a unit zero-sequence admittance, a core unit zero-sequence impedance in the core circuit is connected in series with the load impedance, and the core sheath circuit includes a sheath unit impedance; Select any point on the core circuit in the zero-sequence network as a target point, and determine a micro-segment of the target point; Based on Kirchhoff's voltage law and Kirchhoff's current law, using the unit zero-sequence admittance, the unit zero-sequence impedance of the core and the unit impedance of the sheath, a differential equation group of the zero-sequence network is constructed for the differential segment, and the differential equation group is solved to obtain a general solution set; Based on the load impedance, determining boundary conditions of the first and second ends of the three-core cable to be evaluated for the general solution set; Substituting the boundary conditions into the general solution set, a zero-sequence voltage ratio objective function including a relative dielectric constant is determined.

2. The method according to claim 1, characterized in that: The cable structure of the three-core cable to be evaluated includes a three-phase electric core and a sheath wrapping the three-phase electric core, each phase electric core includes a wire core and an insulating layer wrapping the wire core, and the electric cores of each phase are mutually circumscribed.

3. The method according to claim 1, characterized in that Based on Kirchhoff's voltage law and Kirchhoff's current law, the unit zero-sequence admittance, the unit zero-sequence impedance of the core and the unit impedance of the sheath are used to construct the differential equation group of the zero-sequence network for the differential segment, and the general solution set is obtained by solving the differential equation group, including: The differential equations of the zero-sequence network are constructed for the differential segment using the following formula: Wherein, x is the target location, dx is the micro-segment, is the zero-sequence current of the line core at the target, Y (0) is the unit zero-sequence admittance, is the line core zero-sequence voltage at the target, is the sheath zero-sequence voltage at the target, is the sheath zero-sequence current at the target, Z (0) is the unit zero-sequence impedance of the line core, Z s is the unit impedance of the sheath; The differential equations are solved to obtain a general solution set, which is: in, 4. The method according to claim 3, characterized in that The boundary conditions of the first and last ends of the three-core cable to be evaluated for the general solution set are: in, is the zero-sequence voltage at the head end of the three-core cable to be evaluated, is the sheath voltage at the head end of the three-core cable to be evaluated, is the zero-sequence voltage at the end of the three-core cable to be evaluated, is the zero-sequence current at the end of the three-core cable to be evaluated, Z Load is the load impedance, is the sheath voltage at the end of the three-core cable to be evaluated, and l is the length of the three-core cable to be evaluated.

5. The method according to claim 4, characterized in that The zero-sequence voltage ratio objective function containing the relative dielectric constant is: Among them, ε r is the relative dielectric constant, F(ε r ) is the zero-sequence voltage ratio objective function containing the relative dielectric constant, ε0=8.86×10 -12 F / m,r i.out is the average outer radius of the insulation layer of each phase core in the three-core cable to be evaluated, r i.in is the average inner radius of the insulation layer of each phase core in the three-core cable to be evaluated, and f is the frequency.

6. The method according to claim 1, characterized in that The step of applying the zero-sequence voltage monitoring signal value of each frequency to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated includes: For each zero-sequence voltage monitoring signal value at each frequency, substituting the zero-sequence voltage monitoring signal value into the zero-sequence voltage ratio objective function to obtain a relative dielectric constant reference value at the frequency; The lsqcurvefit function of the MATLAB software is used to perform nonlinear least square fitting on each relative dielectric constant reference value to obtain the relative dielectric constant of the three-core cable to be evaluated.

7. A cable insulation evaluation device, characterized in that: The insulation evaluation method for a cable as claimed in claim 1, the device comprising: A zero-sequence voltage monitoring unit, used to obtain a zero-sequence voltage signal at the head end and a zero-sequence voltage signal at the end of the three-core cable to be evaluated under power disturbance; A Fourier transform unit, used to perform Fourier transform on the first-end zero-sequence voltage signal and the terminal zero-sequence voltage signal respectively, to obtain a first-end zero-sequence voltage frequency domain signal corresponding to the first-end zero-sequence voltage signal and a terminal zero-sequence voltage frequency domain signal corresponding to the terminal zero-sequence voltage signal; A monitoring signal determination unit, configured to use, at each frequency of the terminal zero-sequence voltage frequency domain signal, a ratio of the amplitude of the head-end zero-sequence voltage frequency domain signal to the amplitude of the terminal zero-sequence voltage frequency domain signal as a zero-sequence voltage monitoring signal value of the frequency; A frequency-variable parameter model building unit, used to build a frequency-variable parameter model of the three-core cable to be evaluated based on the cable structure of the three-core cable to be evaluated; An objective function construction unit, used to construct a zero-sequence voltage ratio objective function containing a relative dielectric constant according to the frequency-variable parameter model of the three-core cable to be evaluated; The relative dielectric constant determination unit is used to apply the zero-sequence voltage monitoring signal value of each frequency to the zero-sequence voltage ratio objective function to obtain the relative dielectric constant of the three-core cable to be evaluated, so as to evaluate the insulation performance of the three-core cable to be evaluated.

8. The device according to claim 7, characterized in that The cable structure of the three-core cable to be evaluated, on which the objective function construction unit is based, includes three-phase cores and sheaths wrapping the three-phase cores, each phase core includes a wire core and an insulating layer wrapping the wire core, and the cores of each phase are mutually tangent.

9. The device according to claim 7, characterized in that The frequency-variable parameter model includes a circuit of a three-phase battery cell and a circuit of a three-phase battery cell sheath, the circuit of each phase battery cell is connected to the circuit of the three-phase battery cell sheath through the admittance of each phase battery cell, the line core impedance of each phase battery cell in the circuit of each phase battery cell is connected in series with the load impedance, the load impedances in the circuits of different batteries are the same, and the circuit of the three-phase battery cell sheath includes the sheath reactance of the three-core cable to be evaluated; The objective function construction unit comprises: A model simplification unit, for simplifying the frequency-varying parameter model into a zero-sequence network based on the topological structure of the circuit of the three-phase battery core in the frequency-varying parameter model, wherein the zero-sequence network includes a core circuit and a core sheath circuit, wherein the core circuit is connected to the core sheath circuit via a unit zero-sequence admittance, a core unit zero-sequence impedance in the core circuit is connected in series with the load impedance, and the core sheath circuit includes a sheath unit impedance; A micro-segment determination unit, configured to select any location on a core circuit in the zero-sequence network as a target location, and determine a micro-segment at the target location; An equation construction unit is used to construct a differential equation group of the zero-sequence network for the differential segment based on Kirchhoff's voltage law and Kirchhoff's current law, using the unit zero-sequence admittance, the core unit zero-sequence impedance and the sheath unit impedance, and solve the differential equation group to obtain a general solution set; A boundary condition determination unit, used to determine the boundary conditions of the first and second ends of the three-core cable to be evaluated for the general solution set based on the load impedance; The objective function determination unit is used to substitute the boundary conditions into the general solution set to determine the zero-sequence voltage ratio objective function containing the relative dielectric constant.

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