An outdoor cable terminal tail pipe down insulation incubation fault early warning method

Abstract

The application discloses a kind of outdoor cable terminal tail pipe lower insulation incubation fault early warning method, this method fully considers the insulation outer shield layer semiconductive material characteristics caused by temperature rise after cable terminal lead seal completely cracks, accurately obtains the ground current data at different cable terminal temperatures as input data, builds the terminal ground current calculation model after cable terminal tail pipe lead seal completely cracks and establishes the temperature field distribution calculation model of cable terminal considering heat-flow coupling, and then form the steady-state temperature field distribution iteration algorithm in the state of lead seal completely cracking under cable terminal tail pipe, realize the incubation fault early warning of outdoor cable terminal tail pipe based on tail pipe abnormal temperature rise signal, make up the deficiency that steady-state temperature distribution calculation has not been established after lead seal completely cracks in existing system, can improve the precision of terminal fault defect based on temperature rise signal detection, ultimately realize the goal of reducing maintenance cost, improving cable line operation reliability, and promoting the construction of intelligent power grid.

Description

Technical Field

[0001] This invention relates to the technical field of fault diagnosis and online monitoring of high-voltage cable accessories, and in particular to a method for early warning of latent insulation faults under the tailpipe of outdoor cable terminals. Background Technology

[0002] In recent years, the power grid has been transforming from a traditional physical grid to a smart grid. The new generation of smart grids is based on the intelligence of power equipment, which requires precise detection and rapid diagnosis of power equipment. Cable equipment, as one of the important components of the power transmission network, can have a significant impact on economy and operational safety if it fails. Cables include the cable body and cable accessories. Outdoor cable termination pipes are an important part of cable accessories; they are installed at the end of the cable line, have certain insulation and sealing properties, and are used to connect the cable to the power grid or other electrical equipment.

[0003] As a critical component of cable lines, the reliable operation of outdoor cable terminals is crucial to the overall reliability of the cable line. However, due to the combined effects of various factors such as the design, installation process, and actual operating environment of outdoor cable terminals, they often become one of the weak points in the insulation of high-voltage cables, leading to frequent breakdown faults. Existing research has recognized the impact of tailpipe seal cracking on the development process of outdoor cable terminal breakdown faults. Therefore, existing detection methods mostly focus on detecting the degree of seal cracking to determine the likelihood of latent faults in the tailpipe of outdoor cable terminals. However, in reality, terminals can still operate safely for a period of time after tailpipe seal cracking. Considering the high time and cost of maintenance, it is necessary to propose a method that can more accurately reflect the operating status of outdoor cable terminals, ensuring the reliable operation of cable lines while reducing the problems of "under-maintenance" or "over-maintenance," thereby lowering maintenance costs and significantly improving the safe operation of high-voltage cables.

[0004] Existing research has yielded numerous studies by domestic and international scholars on the discharge mechanism, discharge detection, and preventative improvement measures within cable terminals. For temperature rise signals, infrared detection is a primary method, based on the principle that objects at different temperatures radiate different amounts of energy. Real-time infrared detection of the cable terminal is performed, and the data is processed by imaging instruments and combined with a computer image processing system. Further analysis of the specific data allows for the determination of faults based on terminal temperature changes or comparisons with similar temperatures. Current technologies only focus on the relationship between abnormal temperature rises and cable terminal faults, but a method for early warning of latent faults in outdoor cable terminals based on abnormal temperature rise signals has not yet been developed. Existing technologies lack attention to the quantitative correlation between abnormal temperature rise signals and the insulation health status of cable terminals, failing to address how to determine the insulation health status of the cable terminal tailpipe and the stage of terminal fault development. Furthermore, due to the lack of precise temperature warning thresholds, fault judgment relies heavily on the experience of on-site personnel, potentially leading to either over-maintenance or under-maintenance.

[0005] In scenarios where the lead seal of an outdoor cable terminal pipe is intact or partially cracked, the grounding current flows directly from the end of the cable's aluminum sheath along the lead seal into the terminal grounding system. In this case, the temperature distribution of the insulation and outer shielding layer under the pipe mainly depends on the thermal effect generated by the conductor's load current. However, in scenarios where the lead seal of the outdoor cable terminal pipe is completely cracked, the grounding current originates from the end of the cable's aluminum sheath, flows through the outer shielding layer under the pipe, and then enters the terminal grounding system. In this case, the temperature distribution of the insulation and outer shielding layer depends on the combined effect of the conductor's load current and the thermal effect of the grounding current. After the lead seal is completely cracked, the amplitude of the grounding current mainly depends on the volume resistance of the outer shielding layer under the pipe. When a discharge occurs within the pipe, the outer shielding layer will be burned by the discharge, creating carbonized areas. The appearance of these carbonized areas leads to a decrease in the volume resistance of the outer shielding layer, thereby increasing the amplitude of the grounding current. Consequently, the temperature of the outer shielding layer under the pipe will be significantly raised. Therefore, monitoring the temperature of the outer shielding layer under the pipe can directly reflect the degree of damage to the outer shielding layer. Therefore, a method for early warning of latent insulation faults under outdoor cable terminal tailpipe based on temperature monitoring is proposed. This method enables the monitoring of the insulation health status under the terminal tailpipe, allowing for timely equipment maintenance before a fault occurs and reducing maintenance costs. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings and deficiencies of the prior art and provide a method for early warning of latent insulation faults in outdoor cable terminal tailpipes. Taking into account the characteristics of the semiconductive material of the cable outer shielding layer, the invention obtains grounding current data at different semiconductive shielding layer temperatures based on simulation models, and sets thermal parameter data in combination with the calculation results of the operating heat of the cable conductor. An iterative algorithm for the steady-state temperature field distribution inside the cable terminal tailpipe is proposed to obtain the steady-state temperature distribution of the cable terminal tailpipe, thereby establishing a connection between abnormal temperature rise in the tailpipe and the health status of the terminal insulation, achieving a more accurate judgment of different stages of terminal faults.

[0007] To achieve the above objectives, the technical solution provided by this invention is: a method for early warning of latent insulation faults under the tailpipe of outdoor cables, comprising the following steps:

[0008] S1. Based on the analysis of the grounding current path of outdoor cable terminals after the tailpipe seal is completely cracked, combined with cable line information, cable load data and grounding resistance parameters, a calculation model of terminal grounding current after the tailpipe seal is completely cracked is built. By changing the setting of grounding resistance parameters in the model, the terminal grounding current data under different grounding conditions can be obtained.

[0009] S2. Based on the analysis of the internal heat transfer process of outdoor cable terminals and the heat exchange process between cable terminals and the external environment, a calculation model for the temperature field distribution of cable terminals considering heat-flow coupling is established. The volume heat source of the cable conductor calculated by conductor current and the volume heat source of the insulation outer shielding layer calculated by grounding current are used as input parameters to solve the internal temperature distribution of the terminal.

[0010] S3. Taking into account the positive correlation between the resistivity and temperature of semiconductive materials, and combining the terminal grounding current calculation model and the cable terminal temperature field distribution calculation model, an iterative algorithm for the steady-state temperature field distribution inside the cable terminal tail pipe under the condition of complete lead seal cracking is formed.

[0011] S4. Using the monitored cable load current and ambient temperature as inputs, the steady-state temperature field distribution inside the cable terminal tail pipe is iterated using an algorithm to obtain the calculated results of the surface temperature distribution of the tail pipe under the scenario of no damage to the insulation under the tail pipe. By comparing the measured results and the calculated results of the surface temperature distribution of the tail pipe, the determination of whether there is damage to the insulation under the tail pipe and the degree of damage is realized.

[0012] Furthermore, step S1 includes the following steps:

[0013] Considering the change in the terminal grounding current path after the lead seal cracks, the terminal grounding current is released sequentially from the end of the cable aluminum sheath along the lower insulation outer shield layer of the tail pipe, the terminal tail pipe, and the grounding wire, resulting in an increase in the terminal grounding resistance. This is equivalent to the terminal changing from direct grounding to grounding through a large resistance. Based on key cable line parameters, including cable voltage level, cable structural parameters, cable line length, cable laying spacing, and cable grounding method, a terminal grounding current calculation model is built in PSCAD software. This model uses a three-phase AC power supply, setting A, B, and C as the corresponding three-phase AC voltage sources in the software's built-in model library, and C1 and C2 as the three-phase AC voltage sources. 2. C3 is a cable group in the software's built-in model library, where A connects to C1, B connects to C2, and C connects to C3. R1 is set as the cable load resistance, R2 is set as the terminal grounding resistance, Ias4, Ias5, and Ias6 are the cable conductor load currents, and Ias3, Ibs3, and Ics3 are the cable terminal grounding currents. In PSCAD software, there are multiple transmission line models. Here, the Bergeron lumped resistance model is used for calculation and simulation. The mode decomposition method is used to decouple the three-phase network into three independent single-phase networks, directly obtaining the voltage at any location. Moreover, the lumped parameter model is suitable for short-distance transmission lines.

[0014] In the terminal grounding current calculation model, in order to simulate the actual load current change, the load currents Ias4, Ias5, and Ias6 of the cable conductor are adjusted by changing the value of the cable load resistance R1.

[0015] Because the semiconductive material of the outer shielding layer of cable insulation has positive temperature change characteristics within the preset temperature range, its volume resistivity gradually increases with the increase of temperature. In order to realize the analysis of the terminal grounding current state quantity under different temperature distributions, the value of the terminal grounding resistance R2, i.e. the aluminum sheath grounding resistance value, is changed to realize the simulation of the grounding state at different temperatures.

[0016] Based on the user's requirements for calculation time, the simulation time and step size of the terminal grounding current calculation model are set in the PSCAD software, thereby enabling the acquisition of dynamic changes in terminal grounding current data under different grounding conditions.

[0017] Furthermore, step S2 includes the following steps:

[0018] Based on the structural parameters of outdoor cable terminals, a geometric model of the cable terminal is built in COMSOL simulation software. According to the actual material parameters, the material properties of each layer of the cable terminal geometric model are set in COMSOL simulation software, and finally a cable terminal simulation object is formed in the simulation software.

[0019] Based on the geometric parameters of the cable conductor cross-section, the heat loss of the cable conductor is calculated using the AC resistance calculation formula, as shown below:

[0020] (1);

[0021] In the formula: This refers to the heat loss generated by the cable conductor per unit time. Indicates the cross-sectional area of ​​the cable conductor. I represents the length of the cable conductor, and I represents the current flowing through the conductor. Indicates the temperature of the cable conductor. The value ρ represents the volume resistance of the cable conductor, and ρ represents the conductivity of copper after neglecting the skin effect and proximity effect. This indicates the resistivity of copper at 20°C. Indicates the temperature coefficient of copper resistance;

[0022] When the lead seal is completely cracked, the temperature distribution of the outer insulating shield layer inside the tailpipe is simultaneously affected by the thermal effects of the conductor load current and the grounding current. The calculation method for the heat loss of the outer insulating shield layer is shown in the following formula:

[0023] (2);

[0024] In the formula: This refers to the heat loss generated by the insulating outer shielding layer per unit time. Indicates the length of the outer insulating shielding layer. This indicates the cross-sectional area of ​​the outer insulating shielding layer. , These are the outer diameter and inner diameter of the insulating outer shielding layer, respectively. This indicates the grounding current passing through the insulating outer shielding layer. This indicates the volume resistance of the insulating outer shielding layer;

[0025] Based on the calculated heat loss of the cable conductor and outer insulation shield, the volume heat source of the cable conductor and outer insulation shield is obtained using the heat source calculation formula, and then added as a temperature field heat source to the cable terminal simulation object. The heat source solution formula is shown below:

[0026] (3);

[0027] In the formula: It serves as a heat source for the cable conductor or the outer shielding layer of insulation. The heat loss generated per unit time by the cable conductor or the outer shielding layer of insulation is obtained from equations (1) and (2). The value of l represents the volume of the cable conductor or the outer shielding layer of insulation, while l represents the length of the cable conductor or the outer shielding layer of insulation. This indicates the cross-sectional area of ​​the cable conductor or the outer shielding layer of the insulation.

[0028] Based on the actual operating conditions of the cable terminal, boundary conditions are applied to the outer boundary of the simulated object of the cable terminal in the simulation software to realize the equivalent simulation of the heat transfer process between the cable terminal and the external environment. The boundary conditions include convective heat dissipation and thermal radiation. The convective heat dissipation boundary conditions need to be set according to the monitoring parameters of the actual operating environment of the cable terminal, and the air temperature and air convective heat dissipation coefficient need to be set. The thermal radiation boundary conditions are determined based on the actual monitoring results of solar radiation intensity. Finally, a calculation model of the temperature field distribution of the cable terminal considering heat-flow coupling is formed.

[0029] Furthermore, in step S3, the iterative algorithm for the steady-state temperature field distribution inside the cable terminal tailpipe includes the following steps:

[0030] S31. Assuming the initial temperature of the outer insulating shield is the ambient temperature, calculate the volume resistance of the outer insulating shield at the initial ambient temperature to obtain the value of the terminal grounding resistance R2. Then, use the terminal grounding current calculation model to solve for the terminal grounding current under the initial ambient temperature condition. ;

[0031] S32. Combined with the terminal grounding current obtained from the solution. Formulas (2) and (3) are used to calculate the heat source of the outer shielding layer of the insulation inside the tailpipe. The calculation results are used as input parameters for the cable terminal temperature field distribution calculation model, and then the grounding current at the current terminal is obtained. The steady-state temperature distribution inside the cable terminal under the influence of the action is used to obtain the current grounding current of the terminal. Temperature of the corresponding insulating outer shielding layer under action ;

[0032] S33, Based on the obtained temperature of the outer insulating shielding layer The corresponding insulation outer shield layer resistance is recalculated, and the value of the terminal grounding resistance R2 is corrected. Then, the corrected terminal grounding current is obtained using the terminal grounding current calculation model. Regarding the corrected terminal grounding current Obtain the corrected terminal grounding current according to step S32. The steady-state temperature distribution inside the cable terminal tailpipe and the temperature of the outer insulation shielding layer under the action of the corresponding action. ;

[0033] S34, Judgment and The relationship between the inter-value difference and a set threshold, where the threshold is set according to the user's required solution accuracy; when and When the temperature difference does not exceed the set threshold, the steady-state temperature distribution inside the cable terminal tailpipe obtained in step S33 is output as the temperature distribution of the cable terminal under the condition that the lead seal is completely cracked; when and When the difference exceeds the set threshold, then... = Then return to step S33.

[0034] Furthermore, step S4 includes the following steps:

[0035] S41. Three temperature measuring surfaces are evenly set along the axial direction of the cable terminal tail pipe. A temperature measuring point is arranged at a radial interval of 120° on each temperature measuring surface. Thermistors are selected as the temperature measuring elements to realize online monitoring of the surface temperature distribution of the cable terminal tail pipe in operation.

[0036] S42. Given the actual operating load current and operating environment conditions of the cable terminal, the temperature distribution on the surface of the tail tube is determined by the iterative algorithm of the steady-state temperature field distribution inside the tail tube of the cable terminal under the scenario where the outer shielding layer of the cable terminal insulation is undamaged, and this is used as the early warning action value.

[0037] S43. When the surface of the outer shielding layer of the tailpipe insulation is damaged, the grounding resistance will decrease. According to formulas (2) and (3), the heat source of the outer shielding layer will increase, which will lead to the increase of the tailpipe surface temperature. The larger the carbonized area of ​​the outer shielding layer, the greater the temperature rise. Therefore, the difference between the tailpipe surface temperature measurement result and the warning action value in step S41 is used as the basis for judging the severity of the latent insulation fault. When the tailpipe surface temperature exceeds the warning action value, it means that there is a latent fault in the tailpipe insulation. The larger the difference, the more serious the fault.

[0038] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0039] This invention fully considers the characteristics of the semiconductive material of the outer shielding layer of the cable terminal after the lead seal is completely cracked, caused by the temperature rise. It accurately obtains grounding current data at different cable terminal temperatures as input data, builds a terminal grounding current calculation model after the cable terminal tailpipe lead seal is completely cracked, and establishes a cable terminal temperature field distribution calculation model considering thermal-current coupling. This leads to the formation of an iterative algorithm for the steady-state temperature field distribution inside the cable terminal tailpipe under the condition of completely cracked lead seal. This enables early warning of latent faults in outdoor cable terminal tailpipes based on abnormal temperature rise signals, making up for the lack of a steady-state temperature distribution calculation after the lead seal is completely cracked in the existing system. It can improve the accuracy of terminal fault detection based on temperature rise signals, ultimately achieving the goals of reducing maintenance costs, improving the reliability of cable line operation, and promoting the construction of smart grids. Attached Figure Description

[0040] Figure 1 This is a flowchart of the method of the present invention.

[0041] Figure 2A diagram illustrating the architecture of the calculation model for terminal grounding current (i.e., circulating current in the aluminum sheath of the cable line). Detailed Implementation

[0042] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0043] This embodiment discloses a method for early warning of latent insulation faults under the tailpipe of outdoor cable terminals, such as... Figure 1 As shown, it includes the following steps:

[0044] S1. Based on the analysis of the grounding current path of outdoor cable terminals after the lead seal of the tailpipe is completely cracked, and combined with cable line information, cable load data, and grounding resistance parameters, a calculation model for the terminal grounding current (i.e., the circulating current in the aluminum sheath of the cable line) after the lead seal of the tailpipe of the cable terminal is completely cracked is built. By changing the setting of the grounding resistance parameter in the model, the terminal grounding current data under different grounding conditions can be obtained. The specific situation is as follows:

[0045] Considering the change in the terminal grounding current path after the lead seal cracks, the terminal grounding current is released sequentially from the end of the cable aluminum sheath along the lower insulation outer shield layer of the tail pipe, the terminal tail pipe, and the grounding wire, resulting in an increase in the terminal grounding resistance. This is equivalent to the terminal changing from direct grounding to grounding through a large resistance. Based on key cable line parameters, including cable voltage level, cable structural parameters, cable line length, cable laying spacing, and cable grounding method, a terminal grounding current calculation model is built in PSCAD software, such as... Figure 2 As shown, this model uses a three-phase AC power supply. A, B, and C are the corresponding three-phase AC voltage sources in the software's built-in model library, and C1, C2, and C3 are the cable groups in the software's built-in model library. A is connected to C1, B to C2, and C to C3. R1 is set as the cable load resistance, R2 as the terminal grounding resistance, Ias4, Ias5, and Ias6 as the cable conductor load currents, and Ias3, Ibs3, and Ics3 as the cable terminal grounding currents. In PSCAD software, there are multiple transmission line models. Here, the Bergeron lumped resistance model is used for calculation and simulation. The mode decomposition method can be used to decouple the three-phase network into three independent single-phase networks, directly obtain the voltage at any location, and the lumped parameter model is suitable for short-distance transmission lines.

[0046] In the terminal grounding current calculation model, in order to simulate the actual load current change, the load currents Ias4, Ias5, and Ias6 of the cable conductor are adjusted by changing the value of the cable load resistance R1.

[0047] Because the semiconductive material of the outer shielding layer of cable insulation has positive temperature change characteristics within the preset temperature range, its volume resistivity gradually increases with the increase of temperature. In order to realize the analysis of the terminal grounding current state quantity under different temperature distributions, the value of the terminal grounding resistance R2, i.e. the aluminum sheath grounding resistance value, is changed to realize the simulation of the grounding state at different temperatures.

[0048] Based on the user's requirements for calculation time, the simulation time and step size of the terminal grounding current calculation model are set in the PSCAD software, thereby enabling the acquisition of dynamic changes in terminal grounding current data under different grounding conditions.

[0049] S2. Based on the analysis of the internal heat transfer process of outdoor cable terminals and the heat exchange process between the cable terminals and the external environment, a calculation model for the temperature field distribution of cable terminals considering heat-flow coupling is established. The model uses the volume heat source of the cable conductor calculated from the conductor current and the volume heat source of the insulation outer shield layer calculated from the grounding current as input parameters to solve for the internal temperature distribution of the terminal. The specific details are as follows:

[0050] Based on the structural parameters of outdoor cable terminals, a geometric model of the cable terminal is built in COMSOL simulation software. According to the actual material parameters, the material properties of each layer of the cable terminal geometric model are set in COMSOL simulation software, and finally a cable terminal simulation object is formed in the simulation software.

[0051] Based on the geometric parameters of the cable conductor cross-section, the heat loss of the cable conductor is calculated using the AC resistance calculation formula, as shown below:

[0052] (1);

[0053] In the formula: This refers to the heat loss generated by the cable conductor per unit time. Indicates the cross-sectional area of ​​the cable conductor. I represents the length of the cable conductor, and I represents the current flowing through the conductor. Indicates the temperature of the cable conductor. The value ρ represents the volume resistance of the cable conductor, and ρ represents the conductivity of copper after neglecting the skin effect and proximity effect. This indicates the resistivity of copper at 20°C. Indicates the temperature coefficient of copper resistance;

[0054] When the lead seal is completely cracked, the temperature distribution of the outer insulating shield layer inside the tailpipe is simultaneously affected by the thermal effects of the conductor load current and the grounding current. The calculation method for the heat loss of the outer insulating shield layer is shown in the following formula:

[0055] (2);

[0056] In the formula: This refers to the heat loss generated by the insulating outer shielding layer per unit time. Indicates the length of the outer insulating shielding layer. This indicates the cross-sectional area of ​​the outer insulating shielding layer. , These are the outer diameter and inner diameter of the insulating outer shielding layer, respectively. This indicates the grounding current passing through the insulating outer shielding layer. This indicates the volume resistance of the insulating outer shielding layer;

[0057] Based on the calculated heat loss of the cable conductor and outer insulation shield, the volume heat source of the cable conductor and outer insulation shield is obtained using the heat source calculation formula, and then added as a temperature field heat source to the cable terminal simulation object. The heat source solution formula is shown below:

[0058] (3);

[0059] In the formula: It serves as a heat source for the cable conductor or the outer shielding layer of insulation. The heat loss generated per unit time by the cable conductor or the outer shielding layer of insulation is obtained from equations (1) and (2). The value of l represents the volume of the cable conductor or the outer shielding layer of insulation, while l represents the length of the cable conductor or the outer shielding layer of insulation. This indicates the cross-sectional area of ​​the cable conductor or the outer shielding layer of the insulation.

[0060] Based on the actual operating conditions of the cable terminal, boundary conditions are applied to the outer boundary of the simulated object of the cable terminal in the simulation software to realize the equivalent simulation of the heat transfer process between the cable terminal and the external environment. The boundary conditions include convective heat dissipation and thermal radiation. The convective heat dissipation boundary conditions need to be set according to the monitoring parameters of the actual operating environment of the cable terminal, and the air temperature and air convective heat dissipation coefficient need to be set. The thermal radiation boundary conditions are determined based on the actual monitoring results of solar radiation intensity. Finally, a calculation model of the temperature field distribution of the cable terminal considering heat-flow coupling is formed.

[0061] S3. Considering the positive correlation between the resistivity and temperature of semiconductive materials, and combining the terminal grounding current calculation model and the cable terminal temperature field distribution calculation model, an iterative algorithm for the steady-state temperature field distribution inside the cable terminal tailpipe under the condition of complete lead seal cracking is formed; wherein, the iterative algorithm for the steady-state temperature field distribution inside the cable terminal tailpipe includes the following steps:

[0062] S31. Assuming the initial temperature of the outer insulating shield is the ambient temperature, calculate the volume resistance of the outer insulating shield at the initial ambient temperature to obtain the value of the terminal grounding resistance R2. Then, use the terminal grounding current calculation model to solve for the terminal grounding current under the initial ambient temperature condition. ;

[0063] S32. Combined with the terminal grounding current obtained from the solution. Formulas (2) and (3) are used to calculate the heat source of the outer shielding layer of the insulation inside the tailpipe. The calculation results are used as input parameters for the cable terminal temperature field distribution calculation model, and then the grounding current at the current terminal is obtained. The steady-state temperature distribution inside the cable terminal under the influence of the action is used to obtain the current grounding current of the terminal. Temperature of the corresponding insulating outer shielding layer under action ;

[0064] S33, Based on the obtained temperature of the outer insulating shielding layer The corresponding insulation outer shield layer resistance is recalculated, and the value of the terminal grounding resistance R2 is corrected. Then, the corrected terminal grounding current is obtained using the terminal grounding current calculation model. Regarding the corrected terminal grounding current Obtain the corrected terminal grounding current according to step S32. The steady-state temperature distribution inside the cable terminal tailpipe and the temperature of the outer insulation shielding layer under the action of the corresponding action. ;

[0065] S34, Judgment and The relationship between the inter-value difference and a set threshold, where the threshold is set according to the user's required solution accuracy; when and When the temperature difference does not exceed the set threshold, the steady-state temperature distribution inside the cable terminal tailpipe obtained in step S33 is output as the temperature distribution of the cable terminal under the condition that the lead seal is completely cracked; when and When the difference exceeds the set threshold, then... = Then return to step S33.

[0066] S4. Using the monitored cable load current and ambient temperature as inputs, the steady-state temperature field distribution inside the cable terminal tailpipe is iterated using an algorithm to obtain the calculated temperature distribution on the surface of the tailpipe under a scenario where the insulation below the tailpipe is undamaged. By comparing the measured and calculated results of the temperature distribution on the tailpipe surface, the presence and degree of damage to the insulation below the tailpipe are determined. This includes the following steps:

[0067] S41. Three temperature measuring surfaces are evenly set along the axial direction of the cable terminal tail pipe. A temperature measuring point is arranged at a radial interval of 120° on each temperature measuring surface. Thermistors are selected as the temperature measuring elements to realize online monitoring of the surface temperature distribution of the cable terminal tail pipe in operation.

[0068] S42. Given the actual operating load current and operating environment conditions of the cable terminal, the temperature distribution on the surface of the tail tube is determined by the iterative algorithm of the steady-state temperature field distribution inside the tail tube of the cable terminal under the scenario where the outer shielding layer of the cable terminal insulation is undamaged, and this is used as the early warning action value.

[0069] S43. When the surface of the outer shielding layer of the tailpipe insulation is damaged, the grounding resistance will decrease. According to formulas (2) and (3), the heat source of the outer shielding layer will increase, which will lead to the increase of the tailpipe surface temperature. The larger the carbonized area of ​​the outer shielding layer, the greater the temperature rise. Therefore, the difference between the tailpipe surface temperature measurement result and the warning action value in step S41 is used as the basis for judging the severity of the latent insulation fault. When the tailpipe surface temperature exceeds the warning action value, it means that there is a latent fault in the tailpipe insulation. The larger the difference, the more serious the fault.

[0070] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for early warning of latent insulation faults under the tailpipe of outdoor cable terminals, characterized in that, Includes the following steps: S1. Based on the analysis of the grounding current path of outdoor cable terminals after the tailpipe seal is completely cracked, combined with cable line information, cable load data and grounding resistance parameters, a calculation model of terminal grounding current after the tailpipe seal is completely cracked is built. By changing the setting of grounding resistance parameters in the model, the terminal grounding current data under different grounding conditions can be obtained. S2. Based on the analysis of the internal heat transfer process of outdoor cable terminals and the heat exchange process between cable terminals and the external environment, a calculation model for the temperature field distribution of cable terminals considering heat-flow coupling is established. The volume heat source of the cable conductor calculated by conductor current and the volume heat source of the insulation outer shielding layer calculated by grounding current are used as input parameters to solve the internal temperature distribution of the terminal. S3. Taking into account the positive correlation between the resistivity and temperature of semiconductive materials, and combining the terminal grounding current calculation model and the cable terminal temperature field distribution calculation model, an iterative algorithm for the steady-state temperature field distribution inside the cable terminal tail pipe under the condition of complete lead seal cracking is formed. S4. Using the monitored cable load current and ambient temperature as inputs, the steady-state temperature field distribution inside the cable terminal tail pipe is iterated using an algorithm to obtain the calculated results of the surface temperature distribution of the tail pipe under the scenario of no damage to the insulation under the tail pipe. By comparing the measured results and the calculated results of the surface temperature distribution of the tail pipe, the determination of whether there is damage to the insulation under the tail pipe and the degree of damage is realized.

2. The method for early warning of latent insulation faults under the tailpipe of an outdoor cable terminal as described in claim 1, characterized in that, Step S1 includes the following steps: Considering the change in the terminal grounding current path after the lead seal cracks, the terminal grounding current is released sequentially from the end of the cable aluminum sheath along the lower insulation outer shield layer of the tail pipe, the terminal tail pipe, and the grounding wire, resulting in an increase in the terminal grounding resistance. This is equivalent to the terminal changing from direct grounding to grounding through a large resistance. Based on key cable line parameters, including cable voltage level, cable structural parameters, cable line length, cable laying spacing, and cable grounding method, a terminal grounding current calculation model is built in PSCAD software. This model uses a three-phase AC power supply, setting A, B, and C as the corresponding three-phase AC voltage sources in the software's built-in model library, and C1 and C2 as the three-phase AC voltage sources.

2. C3 is a cable group in the software's built-in model library, where A connects to C1, B connects to C2, and C connects to C3. R1 is set as the cable load resistance, R2 is set as the terminal grounding resistance, Ias4, Ias5, and Ias6 are the cable conductor load currents, and Ias3, Ibs3, and Ics3 are the cable terminal grounding currents. In PSCAD software, there are multiple transmission line models. Here, the Bergeron lumped resistance model is used for calculation and simulation. The mode decomposition method is used to decouple the three-phase network into three independent single-phase networks, directly obtaining the voltage at any location. Moreover, the lumped parameter model is suitable for short-distance transmission lines. In the terminal grounding current calculation model, in order to simulate the actual load current change, the load currents Ias4, Ias5, and Ias6 of the cable conductor are adjusted by changing the value of the cable load resistance R1. Because the semiconductive material of the outer shielding layer of cable insulation has positive temperature change characteristics within the preset temperature range, its volume resistivity gradually increases with the increase of temperature. In order to realize the analysis of the terminal grounding current state quantity under different temperature distributions, the value of the terminal grounding resistance R2, i.e. the aluminum sheath grounding resistance value, is changed to realize the simulation of the grounding state at different temperatures. Based on the user's requirements for calculation time, the simulation time and step size of the terminal grounding current calculation model are set in the PSCAD software, thereby enabling the acquisition of dynamic changes in terminal grounding current data under different grounding conditions.

3. The method for early warning of latent insulation faults under the tailpipe of an outdoor cable terminal as described in claim 2, characterized in that, Step S2 includes the following steps: Based on the structural parameters of outdoor cable terminals, a geometric model of the cable terminal is built in COMSOL simulation software. According to the actual material parameters, the material properties of each layer of the cable terminal geometric model are set in COMSOL simulation software, and finally a cable terminal simulation object is formed in the simulation software. Based on the geometric parameters of the cable conductor cross-section, the heat loss of the cable conductor is calculated using the AC resistance calculation formula, as shown below: （1）； In the formula: This refers to the heat loss generated by the cable conductor per unit time. Indicates the cross-sectional area of ​​the cable conductor. I represents the length of the cable conductor, and I represents the current flowing through the conductor. Indicates the temperature of the cable conductor. The value ρ represents the volume resistance of the cable conductor, and ρ represents the conductivity of copper after neglecting the skin effect and proximity effect. This indicates the resistivity of copper at 20°C. Indicates the temperature coefficient of copper resistance; When the lead seal is completely cracked, the temperature distribution of the outer insulating shield layer inside the tailpipe is simultaneously affected by the thermal effects of the conductor load current and the grounding current. The calculation method for the heat loss of the outer insulating shield layer is shown in the following formula: （2）； In the formula: This refers to the heat loss generated by the insulating outer shielding layer per unit time. Indicates the length of the outer insulating shielding layer. This indicates the cross-sectional area of ​​the outer insulating shielding layer. , These are the outer diameter and inner diameter of the insulating outer shielding layer, respectively. This indicates the grounding current passing through the insulating outer shielding layer. This indicates the volume resistance of the insulating outer shielding layer; Based on the calculated heat loss of the cable conductor and outer insulation shield, the volume heat source of the cable conductor and outer insulation shield is obtained using the heat source calculation formula, and then added as a temperature field heat source to the cable terminal simulation object. The heat source solution formula is shown below: （3）； In the formula: It serves as a heat source for the cable conductor or the outer shielding layer of insulation. The heat loss generated per unit time by the cable conductor or the outer shielding layer of insulation is obtained from equations (1) and (2). The value of l represents the volume of the cable conductor or the outer shielding layer of insulation, while l represents the length of the cable conductor or the outer shielding layer of insulation. This indicates the cross-sectional area of ​​the cable conductor or the outer shielding layer of the insulation. Based on the actual operating conditions of the cable terminal, boundary conditions are applied to the outer boundary of the simulated object of the cable terminal in the simulation software to realize the equivalent simulation of the heat transfer process between the cable terminal and the external environment. The boundary conditions include convective heat dissipation and thermal radiation. The convective heat dissipation boundary conditions need to be set according to the monitoring parameters of the actual operating environment of the cable terminal, and the air temperature and air convective heat dissipation coefficient need to be set. The thermal radiation boundary conditions are determined based on the actual monitoring results of solar radiation intensity. Finally, a calculation model of the temperature field distribution of the cable terminal considering heat-flow coupling is formed.

4. The method for early warning of latent insulation faults under the tailpipe of an outdoor cable terminal as described in claim 3, characterized in that, In step S3, the iterative algorithm for the steady-state temperature field distribution inside the cable terminal tailpipe includes the following steps: S31. Assuming the initial temperature of the outer insulating shield is the ambient temperature, calculate the volume resistance of the outer insulating shield at the initial ambient temperature to obtain the value of the terminal grounding resistance R2. Then, use the terminal grounding current calculation model to solve for the terminal grounding current under the initial ambient temperature condition. ; S32. Combined with the terminal grounding current obtained from the solution. Formulas (2) and (3) are used to calculate the heat source of the outer shielding layer of the insulation inside the tailpipe. The calculation results are used as input parameters for the cable terminal temperature field distribution calculation model, and then the grounding current at the current terminal is obtained. The steady-state temperature distribution inside the cable terminal under the influence of the action is used to obtain the current grounding current of the terminal. Temperature of the corresponding insulating outer shielding layer under action ; S33, Based on the obtained temperature of the outer insulating shielding layer The corresponding insulation outer shield layer resistance is recalculated, and the value of the terminal grounding resistance R2 is corrected. Then, the corrected terminal grounding current is obtained using the terminal grounding current calculation model. Regarding the corrected terminal grounding current Obtain the corrected terminal grounding current according to step S32. The steady-state temperature distribution inside the cable terminal tailpipe and the temperature of the outer insulation shielding layer under the action of the corresponding action. ; S34, Judgment and The relationship between the inter-value difference and a set threshold, where the threshold is set according to the user's required solution accuracy; when and When the temperature difference does not exceed the set threshold, the steady-state temperature distribution inside the cable terminal tailpipe obtained in step S33 is output as the temperature distribution of the cable terminal under the condition that the lead seal is completely cracked; when and When the difference exceeds the set threshold, then... = Then return to step S33.

5. The method for early warning of latent insulation faults under the tailpipe of an outdoor cable terminal as described in claim 4, characterized in that, Step S4 includes the following steps: S41. Three temperature measuring surfaces are evenly set along the axial direction of the cable terminal tail pipe. A temperature measuring point is arranged at a radial interval of 120° on each temperature measuring surface. Thermistors are selected as the temperature measuring elements to realize online monitoring of the surface temperature distribution of the cable terminal tail pipe in operation. S42. Given the actual operating load current and operating environment conditions of the cable terminal, the temperature distribution on the surface of the tail tube is determined by the iterative algorithm of the steady-state temperature field distribution inside the tail tube of the cable terminal under the scenario where the outer shielding layer of the cable terminal insulation is undamaged, and this is used as the early warning action value. S43. When the surface of the outer shielding layer of the tailpipe insulation is damaged, the grounding resistance will decrease. According to formulas (2) and (3), the heat source of the outer shielding layer will increase, which will lead to the increase of the tailpipe surface temperature. The larger the carbonized area of ​​the outer shielding layer, the greater the temperature rise. Therefore, the difference between the tailpipe surface temperature measurement result and the warning action value in step S41 is used as the basis for judging the severity of the latent insulation fault. When the tailpipe surface temperature exceeds the warning action value, it means that there is a latent fault in the tailpipe insulation. The larger the difference, the more serious the fault.

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