A dual-time-scale latent fault modeling method and system for medium-voltage distribution cables

Abstract

The present invention proposes a dual-time-scale latent fault modeling method and system for medium-voltage distribution cables, comprising: establishing a carbonized channel resistance model with the cable latent fault as the time scale, comprising two parts: the carbonized channel resistance of the carbonized part and the solid dielectric breakdown resistance of the sound part; using a step function to describe the carbonized channel resistance value with the cable latent fault as the time scale, to obtain a first expression; based on the relationship between the current flowing through the solid dielectric and the applied voltage, using the three-stage model to describe the solid dielectric breakdown resistance of the sound part, to obtain a second expression; and obtaining a latent fault model of the medium-voltage distribution cable based on the first expression and the second expression.

Description

Technical Field

[0001] The present invention belongs to the technical field of distribution network grounding fault modeling, and in particular relates to a dual-time-scale latent fault modeling method and system for medium-voltage distribution cables. 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] Distribution networks, directly connected to loads, are crucial infrastructure that impacts people's well-being. However, their primary equipment is weak, their wiring is complex, and their operating environment is harsh. This makes them prone to failures and anomalies far more frequent than transmission networks. Latent faults in distribution networks are numerous and complex, making conventional electrical mechanism analysis and probabilistic statistical methods ineffective. If these latent faults are not effectively addressed, they often develop into permanent failures, posing even more serious risks to the distribution network.

[0004] Operating experience with medium-voltage cables shows that before a permanent fault occurs, a transient, self-recovering ground fault may occur at the same location on the cable. Due to the short duration (1 / 4 to 4 power frequency cycles) and low fault current of such faults, traditional protective devices cannot activate. This stage can be referred to as the latent fault of medium-voltage distribution cables. A significant number of cable faults not caused by external forces evolve from latent faults. The key to avoiding the serious harm caused by distribution cable faults is to seize this golden detection stage of "latent faults." The industry currently agrees that cables experience a latent fault stage before a permanent fault occurs. However, due to the lack of a clear explanation for the evolution mechanism of latent faults, a widely accepted latent fault model for medium-voltage distribution cables has not yet been established.

[0005] Currently, latent fault models used in the identification and location of latent faults in medium-voltage distribution cables are mostly "black-box" models based on thermodynamic theory, such as the Mayr model, the Cassie model, cybernetics models, and the recently widely adopted Kizilcay model. However, these models are primarily derived from the characteristics of long arcs in high-resistance faults in open environments such as air, which significantly differs from the environments in which latent faults in medium-voltage distribution cables occur. Furthermore, during the fault modeling process, the parameters of these "black-box" models are difficult to obtain from the external environment. Current research tends to set these parameters to fit nonlinear arc waveforms, thereby establishing fault models in purely mathematical forms using differential equations. This also reflects the loss of physical meaning in these "black-box" models. However, during the transient transition of a fault, the volt-ampere characteristics of these purely mathematical "black-box" models are unstable, making it difficult to accurately obtain the voltage and current waveforms of latent under-frequency cable faults. Summary of the Invention

[0006] In order to overcome the shortcomings of the above-mentioned existing technologies regarding the current lack of a latent fault model for medium-voltage distribution cables, the present invention provides a dual-time-scale latent fault modeling method for medium-voltage distribution cables, which utilizes dual-time-scale latent fault modeling of medium-voltage distribution cables based on fault mechanism and realizes fault detection based on the established model.

[0007] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0008] In a first aspect, a dual-time-scale latent fault modeling method for medium-voltage distribution cables is disclosed, comprising:

[0009] A carbonization channel resistance model based on the time scale of cable latent fault is established, which includes the carbonization channel resistance of the carbonized part and the solid dielectric breakdown resistance of the sound part.

[0010] The first expression is obtained by using a step function to describe the carbonization channel resistance value with the cable latent fault as the time scale;

[0011] Based on the relationship between the current flowing through the solid dielectric and the applied voltage, the three-stage model is used to describe the breakdown resistance of the solid dielectric in the healthy part, and the second expression is obtained;

[0012] Based on the first expression and the second expression, a latent fault model of a medium voltage distribution cable is obtained.

[0013] The fault model can be used to fit the actual measured fault waveform, and a fault simulation model that can be used for fault identification and location can be extracted and summarized.

[0014] The dual time scale specifically refers to the carbonization channel resistance modeling based on the number of fault occurrences as the time scale and the dynamic modeling of the solid breakdown resistance during each latent fault process, and the actual fault waveform is fitted.

[0015] As a further technical solution, the first expression is:

[0016]

[0017] Where A is the original resistance value of the carbonized part when no carbonization occurs, i is the number of occurrences of latent faults in the medium voltage distribution cable, ranging from 1 to n within the effective monitoring data range, A i is the reduction in the resistance of the cable carbonization channel of the ith latent fault of the medium-voltage distribution cable compared to the i-1th latent fault, t i is the time when the i-th latent fault of the medium voltage distribution cable occurs, ε(t) is a step function;

[0018] t iObtained from the experiment, A i is the model parameter, where the original resistance value A of the carbonized part can be given in advance, and A1 is used to correct the carbonized channel resistance.

[0019] As a further technical solution, based on the relationship between the current flowing through the solid dielectric and the applied voltage, it is divided into three regions: region a, region b and region c;

[0020] In region a, the voltage and current have a linear relationship, which conforms to Ohm's law;

[0021] In region b, the current and voltage have an exponential relationship;

[0022] In region c, the current and voltage have an exponential relationship.

[0023] As a further technical solution, the exponential relationship between the voltage and the current in the region b is approximated by a quadratic function;

[0024] In region c, the solid dielectric resistance R s It is already very small, and the carbonized channel resistance R c It works, and it can be considered that the current and voltage of the solid dielectric in the healthy part are in a linear relationship.

[0025] As a further technical solution, in region c, the relationship between current and voltage is expressed by the following formula:

[0026]

[0027] Where u s is the voltage across the solid dielectric of the healthy part, i s is the current flowing through the healthy part of the solid medium, u1 and u2 are the voltage dividing points between area a and area b, and area b and area c, respectively, g1 g2 and k1 k2 k3 k4 are waveform fitting parameters.

[0028] As a further technical solution, by the current i s For voltage u s Taking the derivative, we can get the conductivity g s The expression is as follows:

[0029]

[0030] Among them, u s To improve the voltage across some solid dielectrics, u1 and u2 are the voltage dividing points between region a and region b, and between region b and region c, respectively. g1 g2 and k1 k2 are waveform fitting parameters.

[0031] As a further technical solution, the solid dielectric breakdown resistance of the sound part

[0032] Among them, g s For conductivity.

[0033] As a further technical solution, u s is the partial pressure of the solid medium in the latent fault model of medium voltage distribution cable, which should be expressed by the following formula:

[0034]

[0035] Carbonization channel resistance R of the carbonized part c , g s is the conductivity, u.

[0036] As a further technical solution, the waveform fitting parameters k1 k2 k3 k4 can be obtained from the continuity of current and conductance in equations (2) and (3), and the expressions are as follows:

[0037]

[0038] Among them, u1 and u2 are the voltage dividing points between area a and area b, and area b and area c respectively, and g1g2 are waveform fitting parameters.

[0039] As a further technical solution, during a single latent fault in a medium voltage distribution cable, the solid dielectric breakdown resistance R s It is determined by the model parameters u1, u2 and g2 and is used to fit the nonlinear waveform of latent faults in medium-voltage distribution cables.

[0040] In a second aspect, a dual-time-scale latent fault modeling system for medium-voltage distribution cables is disclosed, comprising:

[0041] The model building module is configured to: establish a carbonization channel resistance model based on the time scale of the cable latent fault, including the carbonization channel resistance of the carbonized part and the solid dielectric breakdown resistance of the sound part;

[0042] The carbonized channel resistance value expression module is configured to: use a step function to describe the carbonized channel resistance value with the cable latent fault as the time scale to obtain a first expression;

[0043] The solid dielectric breakdown resistance expression module is configured to: describe the solid dielectric breakdown resistance of a healthy portion using the three-stage model based on a relationship between a current flowing through the solid dielectric and an applied voltage, thereby obtaining a second expression;

[0044] The fault prediction module is configured to obtain a latent fault model of the medium-voltage distribution cable based on the first expression and the second expression.

[0045] One or more of the above technical solutions have the following beneficial effects:

[0046] Based on the carbonization process of medium-voltage distribution cables, the fault model is decomposed into two components: the carbonization channel resistance and the solid dielectric breakdown resistance. The carbonization channel development is a relatively long-term process. It is assumed that during a single latent cable fault, the carbonization degree of the cable remains constant, and the carbonization channel resistance is constant. However, as the number of latent cable faults increases, the carbonization channel resistance gradually decreases. A step function is used to describe the carbonization channel resistance value based on the time scale of the cable latent fault. To address the hysteresis of the volt-ampere characteristic and the instability of transient characteristics in the "black-box" arc model, a dynamic solid dielectric breakdown resistance is derived by simplifying the three-stage model of solid dielectric breakdown to describe the breakdown process of the healthy part. Finally, a dual-time-scale latent fault model for medium-voltage distribution cables based on the fault mechanism is established and simulated using simulation. The black-box model is applied in the long-air-gap arc breakdown environment. This model, based on the solid dielectric breakdown principle, does not have significant thermal effects and is more suitable for the environment in which cable latent faults occur.

[0047] Advantages of additional aspects of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The accompanying drawings, 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.

[0049] Figure 1 is a cable radial schematic diagram of the latent fault development of a medium voltage distribution cable proposed in the present disclosure;

[0050] Figure 2 is a radial schematic diagram of a cable with a permanent fault that has finally evolved as proposed by the present disclosure;

[0051] Figure 3 Schematic diagram of the cable latent fault equivalent model proposed in the present disclosure;

[0052] Figure 4 is a schematic diagram of the relationship between the current and the applied voltage of the solid dielectric used in the present disclosure;

[0053] Figure 5 It is the Simulink fault module designed by the present disclosure;

[0054] Figure 6 This is a 10kV medium-voltage distribution network model with a neutral point grounded via a small resistor, which is simulated and constructed in this disclosure.

[0055] Figure 7 (a)-(c) are respectively the voltage waveform, current waveform and fault equivalent impedance of the latent fault of the medium voltage distribution cable obtained by simulation of the present disclosure;

[0056] Figure 8 It is the volt-ampere characteristic curve of various latent fault models of medium-voltage distribution cables provided in this disclosure. DETAILED DESCRIPTION

[0057] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0058] It should be noted that the terms used herein are for describing particular embodiments only and are not intended to limit the exemplary embodiments according to the present invention.

[0059] In the absence of conflict, the embodiments of the present invention and the features thereof may be combined with each other.

[0060] Example 1

[0061] This embodiment discloses a dual-time-scale latent fault modeling method for medium-voltage distribution cables, including:

[0062] Analyze the development mechanism of latent faults in medium voltage distribution cables;

[0063] A carbonization channel resistance model is established with the cable latent fault as the time scale, and a step function is used to describe the carbonization channel development process.

[0064] A solid dielectric breakdown resistance model is established to describe the dynamic time-varying characteristics, and the nonlinear fault characteristics are described by simplifying the three-stage model of solid dielectric breakdown.

[0065] Among them, the development mechanism of latent faults in medium voltage distribution cables is analyzed as follows:

[0066] The latent fault process in distribution cables is a state between partial discharge and permanent ground fault. Defects such as air gaps and burrs in the cable and its accessories cause localized electric field concentration, inducing partial discharge and dendrite growth, which gradually degrades the insulation. As insulation degradation progresses, the defective location gradually carbonizes, forming discrete carbonized spots in the insulation layer. The resistance of the carbonized area gradually decreases.

[0067] like Figure 1As shown in the figure, as the degree of dendrite and carbonization deepens, the part of the cross-linked polyethylene (XLPE) cable insulation layer that maintains sound insulation capacity gradually becomes thinner. At the same time, since the sound part bears the higher power frequency operating voltage, when the cable insulation capacity is reduced to a certain extent, the solid dielectric breakdown occurs in the sound part, that is, the cable latent fault occurs.

[0068] As the number of discharges increases, carbonized materials adhere to the surface of the discharge channel and accumulate, such as Figure 2 As shown in the figure, when the carbonized material completely penetrates the cable core and the grounded outer shield, a carbonized channel is formed that penetrates the two poles, causing a permanent grounding fault in the cable. Therefore, the latent fault model of the medium-voltage distribution cable can be decomposed into the carbonized channel resistance R of the carbonized part. c and the solid dielectric breakdown resistance R of the sound part s Two parts, fault model as Figure 3 shown.

[0069] Among them, the carbonization channel resistance model with the cable latent fault as the time scale is established as follows:

[0070] Considering that the carbonization process is a long process, it can be considered that the carbonization degree of the cable remains unchanged during a single cable latent fault, and the carbonization channel resistance is constant. As the number of latent faults of the medium-voltage distribution cable increases, the carbonization channel resistance gradually decreases. Therefore, a step function can be used to describe the carbonization channel resistance value with the cable latent fault as the time scale. Then the carbonization channel resistance R c The expression is as follows:

[0071]

[0072] Where A is the original resistance value of the carbonized part when no carbonization occurs, i is the number of occurrences of latent faults in the medium voltage distribution cable, ranging from 1 to n within the effective monitoring data range, A i is the reduction in the resistance of the cable carbonization channel of the ith latent fault of the medium-voltage distribution cable compared to the i-1th latent fault, t i is the time when the i-th latent fault of the medium voltage distribution cable occurs, and ε(t) is a step function. i Obtained from the experiment, A i is the model parameter, where the original resistance value A of the carbonized part can be given in advance, and A1 is used to correct the carbonized channel resistance, which is specifically adjusted by fitting the actual fault waveform.

[0073] Among them, the solid dielectric breakdown resistance model describing the dynamic time-varying characteristics is established as follows:

[0074] The relationship between the current flowing through a solid dielectric and the applied voltage is as follows Figure 4As shown, when the voltage is within the range of region a, the electric field in the dielectric is low, the low electric field conductivity zone is dominated by ionic conductivity, and the voltage and current are linearly related, which conforms to Ohm's law; when the voltage is within the range of region b, the electric field in the dielectric increases, the electronic conductivity and ionic conductivity work together, and the current and voltage are exponentially related; when the voltage is within the range of region c, the dielectric presents a high electric field, the conductivity zone is dominated by electronic conductivity, and the current and voltage are exponentially related, which is the general sense of solid dielectric breakdown.

[0075] During the latent fault of the cable, the solid dielectric breakdown resistance R s The three-stage model can be used to describe it. For the convenience of calculation, the following simplifications can be made: the exponential relationship between voltage and current in region b can be approximated by a quadratic function, and the solid dielectric resistance R in region c is s Already very small, such as Figure 3 As shown, the carbonized channel resistance R c It can be considered that the current and voltage of the solid medium in the healthy part are in a linear relationship, and the relationship between current and voltage can be expressed by the following formula:

[0076]

[0077] Where u s is the voltage across the solid dielectric of the healthy part, i s is the current flowing through the healthy part of the solid medium, u1 and u2 are the voltage dividing points between area a and area b, and area b and area c respectively, g1 g2 and k1 k2 k3 k4 are waveform fitting parameters, and the current i s For voltage u s Taking the derivative, we can get the conductivity g s The expression is as follows:

[0078]

[0079] The breakdown resistance of the solid dielectric in the healthy part is Please note s It is the partial pressure of the solid medium in the sound part of the latent fault model of the medium voltage distribution cable. It cannot be measured alone and needs to be expressed by the following formula:

[0080]

[0081] The waveform fitting parameters k1 k2 k3 k4 can be obtained from the continuity of current and conductance in equations (2) and (3), and the expressions are as follows:

[0082]

[0083] Since region 3 is mainly composed of carbonized channel resistance R cThe parameter g2 can be given in advance and used with R c The complete fault model is modified. During a single latent fault of a medium voltage distribution cable, the breakdown resistance of the solid dielectric of the healthy part R s It is determined by the model parameters u1, u2, and g2 and is used to fit the nonlinear waveform of latent faults in medium-voltage distribution cables. The dielectric breakdown resistance is the reciprocal of gs in formula (5), and the relationship between gs and other parameters is given in formula (5).

[0084] In this implementation example, the fault model is decomposed into two parts based on the carbonization process of medium-voltage distribution cables: carbonization channel resistance and solid dielectric breakdown resistance. It is assumed that the carbonization channel resistance is constant during a single cable latent fault, and a step function is used to describe the carbonization channel resistance value with the cable latent fault as the time scale. A simplified three-stage model of solid dielectric breakdown is established to describe the dynamic solid dielectric breakdown resistance of the sound part breakdown process. Finally, a dual-time-scale latent fault model of medium-voltage distribution cables based on the fault mechanism is established, and the fault model is simulated based on the Matlab / Simulink system.

[0085] A dual-time-scale latent fault model of medium-voltage distribution cables based on fault mechanism was established using the Matlab / Simulink system.

[0086] A 10kV medium-voltage low-resistance grounded distribution network model was built in the MATLAB / Simulink environment, and the voltage waveform, fault current waveform and fault equivalent impedance of the fault model were obtained through simulation.

[0087] The volt-ampere characteristics of the proposed dual-time-scale latent fault model for medium-voltage distribution cables based on fault mechanism are compared with other "black box" models.

[0088] A dual-time-scale latent fault model for medium-voltage distribution cables based on fault mechanism is proposed and compared with other models under the same working conditions.

[0089] The following steps are involved:

[0090] Step S01: Construct a dual-time-scale latent fault model of medium-voltage distribution cable based on fault mechanism in the MATLAB / Simulink environment. Simulate a single latent fault of medium-voltage distribution cable. Given the carbonized channel resistance R c =500Ω, the resistance module is used to represent the carbonized channel resistance, the fault parameters g1 = 0.0002S, g2 = 0.01S, u1 = 500V, u2 = 8000V, the model parameters are calculated using formula (2), formula (3) and formula (5), and the controlled current source is used to represent the nonlinear sound part solid dielectric breakdown resistance. The designed Simulink fault module is as follows: Figure 5 shown.

[0091] Figure 5 In the simulink module of the fault model, ports 1 and 2 on the left are connected to the cable core and ground respectively. The resistance module is used to represent the carbonization channel resistance, and the controlled current source controlled by the function with voltage and starting criterion as input represents the dynamic resistance of dielectric breakdown. Considering the uncertainty and randomness of arc occurrence, the starting criterion is set by the impact function generator, and the fault module is controlled by the logical relationship and circuit breaker to determine whether it is connected to the circuit.

[0092] Step S02: Build the following in Matlab / Simulink environment: Figure 6 The 10kV medium-voltage low-resistance grounded distribution network model shown has a system grounding resistance of 10Ω. Feeders L1 and L2 are 10km and 8km long, respectively, with the following line parameters: positive-sequence impedance Z1 = 0.27 + j0.08Ω / km; positive-sequence admittance b1 = j118.12μS / km; zero-sequence impedance Z0 = 2.7 + j0.35Ω / km; and zero-sequence admittance b0 = j86.71μS / km. The fault parameters for a latent distribution cable fault are set as in step S01. The cable latent fault is assumed to occur on feeder L1 4km from the busbar, at the peak operating voltage, and last for four cycles. The measured fault voltage waveform, fault current waveform, and fault equivalent impedance are shown in Figure 1. Figure 7 As shown in (a)-(c) in the figure, it can be found that the fault phase voltage changes slightly, the fault current has a good zero-break characteristic, and the voltage and current amplitudes are consistent with the latent fault characteristics of the medium-voltage distribution cable.

[0093] Step S03: Compare the volt-ampere characteristics of the proposed dual-time-scale medium-voltage distribution cable latent fault model based on the fault mechanism with other “black box” models. Most “black box” models are derived based on the arc energy balance theory in gas media. The principle is shown in Equation (6):

[0094]

[0095] Among them, τ is the arc time constant, p is the arc dissipation power, u, i, g are the voltage at both ends of the arc, arc current and arc conductivity respectively. By introducing various restrictions on the parameters τ and p, the Cassie model, Mayr model, cybernetic model, etc. are deduced in turn. The Cassie model and Mayr model are the two most important arc models. The Kizilcay arc model is developed from the cybernetic model and has been widely used in the field of latent fault modeling in recent years. The volt-ampere characteristic curves of the Cassie model, Mayr model, Kizilcay model and the dual-time-scale latent fault model of medium-voltage distribution cables based on fault mechanism disclosed in this disclosure are shown in Figure 2. Figure 8As shown, since the "black box" model is mostly derived from the energy balance theory of gas dielectric arc breakdown, its arcing and arc extinction are obviously affected by the heat dissipation factor of the gas environment, which is manifested as the current change lags behind the voltage change during the arcing and arc extinction processes, and the volt-ampere characteristic curves of the arcing and arc extinction processes do not overlap. Since the heat dissipation conditions of solid media are much better than those of gas media, the volt-ampere characteristic curves of the arcing and arc extinction processes should be similar. The dual-time-scale latent fault model replacement of medium-voltage distribution cables based on fault mechanism proposed in this disclosure is in line with actual conditions; at the same time, Figure 8 It can be seen from the figure that the "black box" model has large distortion in fault transients and large errors in transient volt-ampere characteristics. The purely mathematical "black box" model is difficult to accurately simulate latent faults of under-cycle cables. The dual-time-scale medium-voltage distribution cable latent fault model based on fault mechanism proposed in this paper has no transient distortion in its volt-ampere characteristics and can effectively model latent faults of under-cycle cables from the perspective of fault mechanism.

[0096] Example 2

[0097] The purpose of this embodiment is to provide a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the above method when executing the program.

[0098] Example 3

[0099] The purpose of this embodiment is to provide a computer-readable storage medium.

[0100] A computer-readable storage medium stores a computer program, which, when executed by a processor, performs the steps of the above method.

[0101] Example 4

[0102] The purpose of this embodiment is to provide a dual-time-scale latent fault modeling system for medium-voltage distribution cables, including:

[0103] The model building module is configured to: establish a carbonization channel resistance model based on the time scale of the cable latent fault, including the carbonization channel resistance of the carbonized part and the solid dielectric breakdown resistance of the sound part;

[0104] The carbonized channel resistance value expression module is configured to: use a step function to describe the carbonized channel resistance value with the cable latent fault as the time scale to obtain a first expression;

[0105] The solid dielectric breakdown resistance expression module is configured to: describe the solid dielectric breakdown resistance of a healthy portion using the three-stage model based on a relationship between a current flowing through the solid dielectric and an applied voltage, thereby obtaining a second expression;

[0106] The fault prediction module is configured to obtain a latent fault model of the medium-voltage distribution cable based on the first expression and the second expression.

[0107] Example 5

[0108] The purpose of this embodiment is to provide a computer program product containing instructions, which, when running on a computer, enables the computer to execute the methods and functions involved in any of the above embodiments.

[0109] The steps involved in the apparatus of the above embodiment correspond to those of the method embodiment 1. For detailed implementation, please refer to the relevant description of embodiment 1. The term "computer-readable storage medium" should be understood to mean a single medium or multiple media containing one or more instruction sets; it should also be understood to include any medium capable of storing, encoding, or carrying an instruction set for execution by a processor and causing the processor to perform any method of the present invention.

[0110] Those skilled in the art will appreciate that the modules or steps of the present invention described above can be implemented using a general-purpose computer device. Alternatively, they can be implemented using program code executable by a computing device, which can then be stored in a storage device and executed by the computing device. Alternatively, they can be fabricated into separate integrated circuit modules, or multiple modules or steps can be fabricated into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0111] Although the above describes the specific embodiments of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without any creative work are still within the scope of protection of the present invention.

Claims

1. A dual-time-scale latent fault modeling method for medium-voltage distribution cables, characterized by: include: A carbonization channel resistance model based on the time scale of cable latent fault is established, which includes the carbonization channel resistance of the carbonized part and the solid dielectric breakdown resistance of the sound part. The first expression is obtained by using a step function to describe the carbonization channel resistance value with the cable latent fault as the time scale; Based on the relationship between the current flowing through the solid dielectric and the applied voltage, a three-stage model is used to describe the breakdown resistance of the solid dielectric in the healthy part, and the second expression is obtained; Based on the first expression and the second expression, a latent fault model of a medium voltage distribution cable is obtained; Based on the relationship between the current flowing through the solid dielectric and the applied voltage, it is divided into three regions: region a, region b and region c; In region c, the solid dielectric resistance of the healthy part It is already very small, and the resistance of the carbonized channel is mainly It works, and it can be considered that the current and voltage of the solid dielectric in the healthy part are in a linear relationship; In region c, the relationship between current and voltage is expressed by the following equation: （1） Where, For the voltage across the solid dielectric part, is the current flowing through the sound part of the solid medium, and are the voltage dividing points between area a and area b, and area b and area c, respectively. and All are waveform fitting parameters; Through current Voltage Taking the derivative, we can get the conductivity The expression is as follows: （2） in, For the voltage across the solid dielectric part, and are the voltage dividing points between area a and area b, and area b and area c, respectively. and All are waveform fitting parameters.

2. The dual-time-scale latent fault modeling method for medium-voltage distribution cables according to claim 1, wherein: The first expression is: （3） Where A is the original resistance value of the carbonized part when no carbonization occurs, i is the number of occurrences of latent faults in medium voltage distribution cables, ranging from 1 to n within the valid monitoring data range. For the i Compared with the latent fault of medium voltage distribution cable i -1 reduction in the resistance of the cable carbonization channel for a latent fault, For the i When a latent fault of a medium voltage distribution cable occurs, is a step function; Obtained from experiments, The model parameters are given in advance, where the original resistance value A of the carbonized part is given in advance and Correction of carbonized channel resistance.

3. The dual-time-scale latent fault modeling method for medium-voltage distribution cables according to claim 1, wherein: In region a, the voltage and current have a linear relationship, which conforms to Ohm's law; In region b, the current and voltage have an exponential relationship; In region c, the current and voltage have an exponential relationship.

4. A dual-time-scale latent fault modeling method for medium-voltage distribution cables according to claim 3, characterized in that: The exponential relationship between the voltage and the current in the region b is approximated by a quadratic function.

5. The dual-time-scale latent fault modeling method for medium-voltage distribution cables according to claim 1, wherein: Through current Voltage Taking the derivative, we can get the conductivity The expression is as follows: （4） in, For the voltage across the solid dielectric part, and are the voltage dividing points between area a and area b, and area b and area c, respectively. and All are waveform fitting parameters; Solid dielectric breakdown resistance of the healthy part , the complete expression is as follows: （5） is the partial pressure of the solid medium in the latent fault model of medium voltage distribution cable, which should be expressed by the following formula: Carbonization channel resistance of the carbonized part , is the conductance, u is the voltage across the fault model; Waveform fitting parameters It can be obtained from the continuity of current and conductance in equations (1) and (2), and the expression is as follows: in, and are the voltage dividing points between area a and area b, and area b and area c, respectively. All are waveform fitting parameters; During a single latent fault in a medium voltage distribution cable, the breakdown resistance of the solid dielectric in the healthy part By model parameters and The nonlinear waveform of latent fault in medium voltage distribution cable is determined and used to fit the waveform.

6. A dual-time-scale latent fault modeling system for medium-voltage distribution cables, characterized by: include: The model building module is configured to: establish a carbonization channel resistance model based on the time scale of the cable latent fault, including two parts: the carbonization channel resistance of the carbonized part and the solid dielectric breakdown resistance of the sound part; The carbonized channel resistance value expression module is configured to: use a step function to describe the carbonized channel resistance value with the cable latent fault as the time scale to obtain a first expression; The solid dielectric breakdown resistance expression module is configured to: describe the solid dielectric breakdown resistance of a healthy portion using a three-stage model based on a relationship between a current flowing through the solid dielectric and an applied voltage, thereby obtaining a second expression; The fault prediction module is configured to: obtain a latent fault model of the medium voltage distribution cable based on the first expression and the second expression; Based on the relationship between the current flowing through the solid dielectric and the applied voltage, it is divided into three regions: region a, region b and region c; In region c, the solid dielectric resistance of the healthy part It is already very small, and the resistance of the carbonized channel is mainly It works, and it can be considered that the current and voltage of the solid dielectric in the healthy part are in a linear relationship; In region c, the relationship between current and voltage is expressed by the following equation: （1） Where, For the voltage across the solid dielectric part, is the current flowing through the sound part of the solid medium, and are the voltage dividing points between area a and area b, and area b and area c, respectively. and All are waveform fitting parameters; Through current Voltage Taking the derivative, we can get the conductivity The expression is as follows: （2） in, For the voltage across the solid dielectric part, and are the voltage dividing points between area a and area b, and area b and area c, respectively. and All are waveform fitting parameters.

7. A computer program product comprising a computer program, characterized in that When the computer program is executed by a processor, the method according to any one of claims 1 to 5 is implemented.

8. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the program, the steps of the method described in any one of claims 1 to 5 are implemented.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method according to any one of claims 1 to 5 are performed.

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