Outdoor cable terminal tail pipe lower insulation latent fault early warning method

Abstract

The invention discloses an outdoor cable terminal tail pipe lower insulation latent fault early warning method, which fully considers the characteristics of an insulation outer shielding layer semi-conductive material caused by temperature rise after a cable terminal lead seal is completely cracked, and accurately obtains grounding current data at different cable terminal temperatures as input data. Building a terminal grounding current calculation model after the lead seal of the tail pipe of the cable terminal is completely cracked, and building a cable terminal temperature field distribution calculation model considering heat-flow coupling, so as to form an iterative algorithm for the steady-state temperature field distribution in the tail pipe of the cable terminal in a state that the lead seal is completely cracked; outdoor cable terminal tail pipe latent fault early warning based on the tail pipe abnormal temperature rise signal is achieved, the defect that steady-state temperature distribution calculation after lead sealing is completely cracked is not established in an existing system is overcome, the accuracy of terminal fault defect detection based on the temperature rise signal can be improved, and finally the purposes of reducing the maintenance cost, improving the cable line operation reliability and improving the service life are achieved. And the goal of constructing a smart power grid is promoted.

Description

Technical Field

[0001] The invention relates to the technical field of fault diagnosis and online monitoring of high-voltage cable accessories, and in particular to an early warning method for latent insulation faults under a tail tube of an outdoor cable terminal. Background Art

[0002] In recent years, the power grid has further transformed from a traditional physical grid to an intelligent grid. The new generation of intelligent grid is based on the intelligence of power equipment, which requires accurate detection and rapid diagnosis of power equipment. As one of the important equipment in the power transmission network, cable equipment will have a huge impact on the economy and operational safety once a failure occurs. The cable includes the cable body and cable accessories. The outdoor cable terminal tail tube is an important part of the cable accessories. It is installed at the end of the cable line and has certain insulation and sealing properties. It is used to connect the cable to the power grid or other electrical equipment.

[0003] As a key component of the cable line, the reliable operation of the outdoor cable terminal is related to the operational reliability of the entire cable line. However, due to the combined influence of multiple factors such as the design, installation process and actual operating environment of the outdoor cable terminal itself, it often becomes one of the weak links in the insulation of high-voltage cables, and outdoor cable terminal breakdown failures occur frequently. In existing research, the influence of the cracking of the tail tube lead seal on the development process of the outdoor cable terminal breakdown failure has been recognized. Therefore, the existing detection methods are mostly focused on detecting the degree of cracking of the lead seal to determine the possibility of latent faults in the tail tube of the outdoor cable terminal. However, in fact, after the tail tube lead seal is cracked, the terminal can still operate safely for a period of time. Considering the time and high cost of maintenance investment, it is necessary to propose a method that can more accurately reflect the operating status of the outdoor cable terminal, while ensuring the reliable operation of the cable line, reducing the problem of "insufficient maintenance" or "excessive maintenance", reducing maintenance costs, and improving the safe operation of high-voltage cables. It is of great significance.

[0004] In the existing research, many domestic and foreign scholars have conducted in-depth research on the discharge mechanism, discharge detection and prevention and improvement measures inside the cable terminal. The main method for temperature rise signals is infrared detection. Based on the principle that objects at different temperatures radiate different energy, the cable terminal is detected by infrared in real time, and the data is processed by imaging instruments and combined with computer image processing systems. Then, the specific data is analyzed to determine whether there is a fault defect based on the terminal temperature change or comparison with similar temperatures. The existing technology only focuses on the connection between abnormal temperature rise and cable terminal fault defects, but there is no outdoor cable terminal latent fault warning method based on abnormal temperature rise signals. The existing technology system lacks attention to the quantitative correlation between abnormal temperature rise signals and the insulation health status of cable terminals. It cannot answer how to determine the insulation health status of the cable terminal tail pipe and the terminal fault development stage. In addition, due to the lack of accurate temperature warning thresholds, fault judgment relies more on the experience of on-site personnel, that is, there may be problems of "over-maintenance" or "under-maintenance".

[0005] In the scenario where the lead seal of the outdoor cable terminal tail tube is intact or partially cracked, the grounding current flows directly from the end of the cable aluminum sheath along the lead seal into the terminal grounding system. At this time, the temperature distribution of the insulation and the outer shielding layer under the tail tube mainly depends on the thermal effect generated by the conductor load current. In the scenario where the lead seal of the outdoor cable terminal tail tube is completely cracked, the grounding current starts from the end of the cable aluminum sheath and needs to flow through the outer shielding layer under the tail tube before flowing into the terminal grounding system. At this time, the temperature distribution of the insulation and the outer shielding layer depends on the combined effect of the conductor 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 body resistance of the outer shielding layer under the tail tube. When discharge occurs in the tail tube, the outer shielding layer of the insulation will be burned by the discharge to produce a carbonized area. The appearance of the carbonized area will cause the body resistance of the outer shielding layer of the insulation to decrease and thus increase the amplitude of the grounding current. Correspondingly, the temperature of the outer shielding layer under the tail tube will be significantly increased. Therefore, the degree of damage to the outer shielding layer of the insulation can be directly reflected by monitoring the temperature of the outer shielding layer under the tail tube. Therefore, a temperature monitoring-based early warning method for insulation latent faults under the outdoor cable terminal tail tube is proposed to monitor the health status of the insulation under the terminal tail tube, so as to timely perform equipment maintenance before a fault occurs and reduce maintenance costs. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art, and provide an early warning method for latent insulation faults under the tail tube of an outdoor cable terminal. The method comprehensively considers the characteristics of the semi-conductive material of the cable outer shielding layer, obtains the grounding current data at different semi-conductive shielding layer temperatures based on a simulation model, and sets the thermal parameter data in combination with the heat calculation results of the cable conductor operation. An iterative algorithm for the steady-state temperature field distribution inside the cable terminal tail tube is proposed, and the steady-state temperature distribution of the cable terminal tail tube is obtained, thereby establishing a connection between the abnormal temperature rise of the tail tube and the healthy state of the terminal insulation, so as to achieve the effect of more accurately judging the different stages of the terminal fault.

[0007] To achieve the above object, the technical solution provided by the present invention is: a method for early warning of latent insulation failure under the tail tube of an outdoor cable terminal, comprising the following steps: S1. Based on the analysis of the grounding current path of the outdoor cable terminal after the tail tube lead seal is completely cracked, combined with the cable line information, cable load data and grounding resistance parameters, a terminal grounding current calculation model after the cable terminal tail tube lead seal is completely cracked is built, and the setting of the grounding resistance parameters in the model is changed to achieve the acquisition of terminal grounding current data under different grounding states; S2. Based on the analysis of the internal heat transfer process of the outdoor cable terminal and the heat exchange process between the cable terminal and the external environment, a cable terminal temperature field distribution calculation model considering heat-fluid coupling is established. The body heat source of the cable conductor calculated by the conductor current and the body heat source of the insulating outer shielding layer calculated by the 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 semi-conductive materials, the terminal grounding current calculation model and the cable terminal temperature field distribution calculation model are combined to form an iterative algorithm for the steady-state temperature field distribution inside the cable terminal tail tube when the lead seal is completely cracked; S4. With the monitored cable load current and ambient temperature as input, the iterative algorithm of the steady-state temperature field distribution inside the cable terminal tail tube is used to obtain the calculation result of the tail tube surface temperature distribution in the scenario where the insulation under the tail tube is not damaged. By comparing the measured results and the calculated results of the tail tube surface temperature distribution, it is possible to judge whether the insulation under the tail tube is damaged and the degree of damage.

[0008] Further, the step S1 comprises the following steps: Considering the change of the terminal grounding current path after the lead seal cracks, the terminal grounding current is released from the end of the cable aluminum sheath along the insulating outer shielding layer under the tail tube, the terminal tail tube and the grounding wire in sequence, which increases the terminal grounding resistance, which is equivalent to the terminal changing from direct grounding to grounding through a large resistance. Based on the key parameters of the cable line, including cable voltage level, cable structure parameters, cable line length, cable laying spacing and cable grounding method, a terminal grounding current calculation model is built in the PSCAD software. The model uses a three-phase AC power supply, and A, B, and C are set as the corresponding three-phase AC voltage sources in the software's built-in model library. C1, C 2. C3 is a cable group in the software's own model library, where A is connected to C1, B is connected to C2, and C is connected 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 concentrated resistance model is used for calculation and simulation. The modal decomposition method is used to decouple the three-phase network into three independent single-phase networks, and the voltage at any position is directly obtained. The concentrated 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 cable conductor load current Ias4, Ias5, and Ias6 are adjusted by changing the cable load resistance R1. Since the semi-conductive material of the cable insulation outer shielding layer has a positive temperature change characteristic 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 terminal grounding resistance R2, that is, the value of the aluminum sheath grounding resistance, is changed to simulate the grounding state under different temperatures; According to the user's demand for calculation time, the simulation time and step size of the terminal grounding current calculation model are set in the PSCAD software, so as to obtain the dynamic change data of the terminal grounding current under different grounding conditions.

[0009] Further, the step S2 comprises the following steps: Based on the outdoor cable terminal structure parameters, a cable terminal geometry model is built in the COMSOL simulation software. According to the real material parameters, material properties are set for each layer of the cable terminal geometry model in the 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. The calculation formula is as follows: (1); Where: is the heat loss generated by the cable conductor per unit time, Indicates the cross-sectional area of ​​the cable conductor. represents the length of the cable conductor, I represents the current passing through the conductor, Indicates the cable conductor temperature, represents the volume resistance of the cable conductor, ρ represents the conductivity of copper after ignoring the skin effect and proximity effect, Indicates the resistivity of copper at 20°C, represents the temperature coefficient of resistance of copper; When the lead seal is completely cracked, the temperature distribution of the insulation outer shielding layer in the tail pipe is affected by the thermal effects of the conductor load current and the grounding current. The calculation method of the heat loss of the insulation outer shielding layer is shown in the following formula: (2); Where: is the heat loss generated by the insulating outer shielding layer per unit time, Indicates the length of the insulating outer shield. Indicates the cross-sectional area of ​​the insulating outer shielding layer. , are the outer diameter and inner diameter of the insulating outer shield layer, represents the ground current through the insulating outer shield, Represents the bulk resistance of the insulating outer shield; Based on the heat loss calculation results of the cable conductor and the outer insulation shielding layer, the heat source calculation formula is used to obtain the body heat source of the cable conductor and the outer insulation shielding layer, and add it as a temperature field heat source to the cable terminal simulation object. The heat source solution formula is as follows: (3); Where: The body heat source of the cable conductor or the outer insulation shield. is the heat loss generated by the cable conductor or the outer insulation shielding layer per unit time, obtained by equations (1) and (2), It represents the volume of the cable conductor or the outer insulation shielding layer, l represents the length of the cable conductor or the outer insulation shielding layer, Indicates the cross-sectional area of ​​the cable conductor or the outer insulation shield; Based on the actual operating conditions of the cable terminal, boundary conditions are imposed on the outer boundaries of the cable terminal simulation object in the simulation software to achieve equivalent simulation of the heat transfer process between the cable terminal and the external environment; the boundary conditions include convection heat dissipation and thermal radiation, among which the convection heat dissipation boundary conditions need to set the air temperature and the air convection heat dissipation coefficient according to the actual operating environment monitoring parameters of the cable terminal, while the thermal radiation is determined based on the actual monitoring results of the solar radiation intensity, and finally a cable terminal temperature field distribution calculation model considering heat-fluid coupling is formed.

[0010] Further, in step S3, the iterative algorithm for the steady-state temperature field distribution inside the cable terminal tail tube includes the following steps: S31, assuming that the initial temperature of the insulating outer shielding layer is the ambient temperature, calculate the corresponding body resistance of the insulating outer shielding layer under the initial ambient temperature, obtain the value of the terminal grounding resistance R2, and then use the terminal grounding current calculation model to solve the corresponding terminal grounding current under the initial ambient temperature condition. ; S32, combined with the terminal grounding current obtained by solution The heat source of the insulation and outer shielding layer in the tail pipe is calculated by using formulas (2) and (3). The calculation results are used as the input parameters of the cable terminal temperature field distribution calculation model, and then the current terminal grounding current is obtained. The steady-state temperature distribution inside the cable terminal under the action of the current terminal grounding current is finally obtained. The corresponding insulation outer shielding layer temperature under the action ; S33, based on the obtained insulation outer shielding layer temperature Recalculate the corresponding insulation outer shield body resistance, and correct the value of the terminal grounding resistance R2, and then use the terminal grounding current calculation model to obtain the corrected terminal grounding current ; For corrected terminal ground current , according to step S32, obtain the corrected terminal grounding current The corresponding steady-state temperature distribution results of the cable terminal tail tube and the temperature of the insulation outer shielding layer under the action ; S34, Judgment and The relationship between the difference and the set threshold, where the threshold is set according to the user's requirements for solution accuracy; and When the difference between the two values ​​does not exceed the set threshold, the steady-state temperature distribution result inside the cable terminal tail tube obtained in step S33 is output as the cable terminal temperature distribution in the state where the lead seal is completely cracked; and When the difference between the two exceeds the set threshold, = And return to step S33.

[0011] Further, the step S4 comprises the following steps: S41. Three temperature measuring surfaces are evenly arranged along the axial direction of the cable terminal tail pipe, and a temperature measuring point is arranged at a radial interval of 120° on each temperature measuring surface. A thermal resistor is selected as the temperature measuring element to realize online monitoring of the surface temperature distribution of the cable terminal tail pipe in operation; S42. Under the premise of knowing the actual operating load current and operating environment conditions of the cable terminal, determine the temperature distribution result of the tail tube surface in the scenario where the insulation outer shielding layer of the cable terminal tail tube is not damaged according to the iterative algorithm of the steady-state temperature field distribution inside the cable terminal tail tube, and use it as the early warning action value; S43. When the surface of the outer shielding layer of the tail pipe insulation is damaged, the grounding resistance will decrease. Combined with formulas (2) and (3), it can be seen that the heat source of the outer shielding layer will increase, resulting in an increase in the surface temperature of the tail pipe. The larger the carbonized area of ​​the outer shielding layer, the greater the temperature increase. Therefore, the difference between the surface temperature measurement result of the tail pipe and the early warning action value in step S41 is used as the basis for judging the severity of the latent insulation fault. When the surface temperature of the tail pipe exceeds the early warning action value, it means that there is a latent fault in the insulation under the tail pipe, and the larger the difference, the more serious the fault.

[0012] Compared with the prior art, the present invention has the following advantages and beneficial effects: The present invention fully considers the semi-conductive material properties of the insulating outer shielding layer caused by the temperature rise after the cable terminal lead seal is completely cracked, accurately obtains the grounding current data at different cable terminal temperatures as input data, builds a terminal grounding current calculation model after the cable terminal tail tube lead seal is completely cracked, and establishes a cable terminal temperature field distribution calculation model considering thermal-fluid coupling, thereby forming an iterative algorithm for the steady-state temperature field distribution inside the cable terminal tail tube when the lead seal is completely cracked, and realizes the potential fault early warning of the outdoor cable terminal tail tube based on the abnormal temperature rise signal of the tail tube, making up for the deficiency that the steady-state temperature distribution calculation after the lead seal is completely cracked has not yet been established in the existing system, and can improve the accuracy of detecting terminal fault defects based on the temperature rise signal, and finally achieve the goal of reducing maintenance costs, improving the reliability of cable line operation, and promoting the construction of smart grids. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Flow chart of the method of the present invention.

[0014] Figure 2 This is the architecture diagram of the terminal grounding current (i.e., the aluminum sheath circulating current of the cable line). DETAILED DESCRIPTION

[0015] The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0016] This embodiment discloses a method for early warning of latent insulation failure under the tail tube of an outdoor cable terminal, such as Figure 1 As shown, the following steps are included: S1. Based on the analysis of the grounding current path of the outdoor cable terminal after the tail tube seal is completely cracked, combined with the cable line information, cable load data and grounding resistance parameters, a terminal grounding current (i.e., cable line aluminum sleeve circulating current) calculation model is built after the cable terminal tail tube seal is completely cracked. The setting of the grounding resistance parameters in the model is changed to obtain the terminal grounding current data under different grounding conditions. The specific situation is as follows: Considering the change of the terminal grounding current path after the lead seal cracks, the terminal grounding current is released from the end of the cable aluminum sheath along the insulating outer shielding layer under the tail tube, the terminal tail tube and the grounding wire in sequence, which increases the terminal grounding resistance, which is equivalent to the terminal changing from direct grounding to grounding through a large resistance; based on the key parameters of the cable line, including cable voltage level, cable structure 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, the model uses a three-phase AC power supply, and A, B, and C are set as the corresponding three-phase AC voltage sources in the software's own model library. C1, C2, and C3 are cable groups in the software's own model library, where A is connected to C1, B is connected to C2, and C is connected 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 concentrated resistance model is used for calculation and simulation. The modal decomposition method can be used to decouple the three-phase network into three independent single-phase networks, and the voltage at any position can be directly obtained. The concentrated 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 cable conductor load current Ias4, Ias5, and Ias6 are adjusted by changing the cable load resistance R1. Since the semi-conductive material of the cable insulation outer shielding layer has a positive temperature change characteristic 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 terminal grounding resistance R2, that is, the value of the aluminum sheath grounding resistance, is changed to simulate the grounding state under different temperatures; According to the user's demand for calculation time, the simulation time and step size of the terminal grounding current calculation model are set in the PSCAD software, so as to obtain the dynamic change data of the terminal grounding current under different grounding conditions.

[0017] S2. Based on the analysis of the internal heat transfer process of the outdoor cable terminal and the heat exchange process between the cable terminal and the external environment, a cable terminal temperature field distribution calculation model considering heat-fluid coupling is established. The body heat source of the cable conductor calculated by the conductor current and the body heat source of the insulating outer shielding layer calculated by the grounding current are used as input parameters to solve the internal temperature distribution of the terminal. The specific situation is as follows: Based on the outdoor cable terminal structure parameters, a cable terminal geometry model is built in the COMSOL simulation software. According to the real material parameters, material properties are set for each layer of the cable terminal geometry model in the 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. The calculation formula is as follows: (1); Where: is the heat loss generated by the cable conductor per unit time, Indicates the cross-sectional area of ​​the cable conductor. represents the length of the cable conductor, I represents the current passing through the conductor, Indicates the cable conductor temperature, represents the volume resistance of the cable conductor, ρ represents the conductivity of copper after ignoring the skin effect and proximity effect, Indicates the resistivity of copper at 20°C, represents the temperature coefficient of resistance of copper; When the lead seal is completely cracked, the temperature distribution of the insulation outer shielding layer in the tail pipe is affected by the thermal effects of the conductor load current and the grounding current. The calculation method of the heat loss of the insulation outer shielding layer is shown in the following formula: (2); Where: is the heat loss generated by the insulating outer shielding layer per unit time, Indicates the length of the insulating outer shield. Indicates the cross-sectional area of ​​the insulating outer shielding layer. , are the outer diameter and inner diameter of the insulating outer shield layer, represents the ground current through the insulating outer shield, Represents the bulk resistance of the insulating outer shield; Based on the heat loss calculation results of the cable conductor and the outer insulation shielding layer, the heat source calculation formula is used to obtain the body heat source of the cable conductor and the outer insulation shielding layer, and add it as a temperature field heat source to the cable terminal simulation object. The heat source solution formula is as follows: (3); Where: The body heat source of the cable conductor or the outer insulation shield. is the heat loss generated by the cable conductor or the outer insulation shielding layer per unit time, obtained by equations (1) and (2), It represents the volume of the cable conductor or the outer insulation shielding layer, l represents the length of the cable conductor or the outer insulation shielding layer, Indicates the cross-sectional area of ​​the cable conductor or the outer insulation shield; Based on the actual operating conditions of the cable terminal, boundary conditions are imposed on the outer boundaries of the cable terminal simulation object in the simulation software to achieve equivalent simulation of the heat transfer process between the cable terminal and the external environment; the boundary conditions include convection heat dissipation and thermal radiation, among which the convection heat dissipation boundary conditions need to set the air temperature and the air convection heat dissipation coefficient according to the actual operating environment monitoring parameters of the cable terminal, while the thermal radiation is determined based on the actual monitoring results of the solar radiation intensity, and finally a cable terminal temperature field distribution calculation model considering heat-fluid coupling is formed.

[0018] S3. Taking into account the positive correlation between the resistivity and temperature of the semi-conductive material, 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 tube under the state of complete lead seal cracking is formed; wherein the iterative algorithm for the steady-state temperature field distribution inside the cable terminal tail tube includes the following steps: S31, assuming that the initial temperature of the insulating outer shielding layer is the ambient temperature, calculate the corresponding body resistance of the insulating outer shielding layer under the initial ambient temperature, obtain the value of the terminal grounding resistance R2, and then use the terminal grounding current calculation model to solve the corresponding terminal grounding current under the initial ambient temperature condition. ; S32, combined with the terminal grounding current obtained by solution The heat source of the insulation and outer shielding layer in the tail pipe is calculated by using formulas (2) and (3). The calculation results are used as the input parameters of the cable terminal temperature field distribution calculation model, and then the current terminal grounding current is obtained. The steady-state temperature distribution inside the cable terminal under the action of the current terminal grounding current is finally obtained. The corresponding insulation outer shielding layer temperature under the action ; S33, based on the obtained insulation outer shielding layer temperature Recalculate the corresponding insulation outer shield body resistance, and correct the value of the terminal grounding resistance R2, and then use the terminal grounding current calculation model to obtain the corrected terminal grounding current ; For corrected terminal ground current , according to step S32, obtain the corrected terminal grounding current The corresponding steady-state temperature distribution results of the cable terminal tail tube and the temperature of the insulation outer shielding layer under the action ; S34, Judgment and The relationship between the difference and the set threshold, where the threshold is set according to the user's requirements for solution accuracy; and When the difference between the two values ​​does not exceed the set threshold, the steady-state temperature distribution result inside the cable terminal tail tube obtained in step S33 is output as the cable terminal temperature distribution in the state where the lead seal is completely cracked; and When the difference between the two exceeds the set threshold, = And return to step S33.

[0019] S4. Using the monitored cable load current and ambient temperature as input, the cable terminal tail tube internal steady-state temperature field distribution iterative algorithm is used to obtain the tail tube surface temperature distribution calculation result in the scenario where the insulation under the tail tube is not damaged. By comparing the measured results of the tail tube surface temperature distribution with the calculated results, it is possible to determine whether the insulation under the tail tube is damaged and the degree of damage, including the following steps: S41. Three temperature measuring surfaces are evenly arranged along the axial direction of the cable terminal tail pipe, and a temperature measuring point is arranged at a radial interval of 120° on each temperature measuring surface. A thermal resistor is selected as the temperature measuring element to realize online monitoring of the surface temperature distribution of the cable terminal tail pipe in operation; S42. Under the premise of knowing the actual operating load current and operating environment conditions of the cable terminal, determine the temperature distribution result of the tail tube surface in the scenario where the insulation outer shielding layer of the cable terminal tail tube is not damaged according to the iterative algorithm of the steady-state temperature field distribution inside the cable terminal tail tube, and use it as the early warning action value; S43. When the surface of the outer shielding layer of the tail pipe insulation is damaged, the grounding resistance will decrease. Combined with formulas (2) and (3), it can be seen that the heat source of the outer shielding layer will increase, resulting in an increase in the surface temperature of the tail pipe. The larger the carbonized area of ​​the outer shielding layer, the greater the temperature increase. Therefore, the difference between the surface temperature measurement result of the tail pipe and the early warning action value in step S41 is used as the basis for judging the severity of the latent insulation fault. When the surface temperature of the tail pipe exceeds the early warning action value, it means that there is a latent fault in the insulation under the tail pipe, and the larger the difference, the more serious the fault.

[0020] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for early warning of latent insulation failure under the tail tube of an outdoor cable terminal, characterized in that: The following steps are involved: S1. Based on the analysis of the grounding current path of the outdoor cable terminal after the tail tube lead seal is completely cracked, combined with the cable line information, cable load data and grounding resistance parameters, a terminal grounding current calculation model after the cable terminal tail tube lead seal is completely cracked is built, and the setting of the grounding resistance parameters in the model is changed to achieve the acquisition of terminal grounding current data under different grounding states; S2. Based on the analysis of the internal heat transfer process of the outdoor cable terminal and the heat exchange process between the cable terminal and the external environment, a cable terminal temperature field distribution calculation model considering heat-fluid coupling is established. The body heat source of the cable conductor calculated by the conductor current and the body heat source of the insulating outer shielding layer calculated by the 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 semi-conductive materials, the terminal grounding current calculation model and the cable terminal temperature field distribution calculation model are combined to form an iterative algorithm for the steady-state temperature field distribution inside the cable terminal tail tube when the lead seal is completely cracked; S4. With the monitored cable load current and ambient temperature as input, the iterative algorithm of the steady-state temperature field distribution inside the cable terminal tail tube is used to obtain the calculation result of the tail tube surface temperature distribution in the scenario where the insulation under the tail tube is not damaged. By comparing the measured results and the calculated results of the tail tube surface temperature distribution, it is possible to judge whether the insulation under the tail tube is damaged and the degree of damage.

2. The method for early warning of latent insulation failure under the tail tube of an outdoor cable terminal according to claim 1 is characterized in that: The step S1 comprises the following steps: Considering the change of the terminal grounding current path after the lead seal cracks, the terminal grounding current is released from the end of the cable aluminum sheath along the insulating outer shielding layer under the tail tube, the terminal tail tube and the grounding wire in sequence, which increases the terminal grounding resistance, which is equivalent to the terminal changing from direct grounding to grounding through a large resistance. Based on the key parameters of the cable line, including cable voltage level, cable structure parameters, cable line length, cable laying spacing and cable grounding method, a terminal grounding current calculation model is built in the PSCAD software. The model uses a three-phase AC power supply, and A, B, and C are set as the corresponding three-phase AC voltage sources in the software's built-in model library. C1, C 2. C3 is a cable group in the software's own model library, where A is connected to C1, B is connected to C2, and C is connected 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 concentrated resistance model is used for calculation and simulation. The modal decomposition method is used to decouple the three-phase network into three independent single-phase networks, and the voltage at any position is directly obtained. The concentrated 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 cable conductor load current Ias4, Ias5, and Ias6 are adjusted by changing the cable load resistance R1. Since the semi-conductive material of the cable insulation outer shielding layer has a positive temperature change characteristic 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 terminal grounding resistance R2, that is, the value of the aluminum sheath grounding resistance, is changed to simulate the grounding state under different temperatures; According to the user's demand for calculation time, the simulation time and step size of the terminal grounding current calculation model are set in the PSCAD software, so as to obtain the dynamic change data of the terminal grounding current under different grounding conditions.

3. The method for early warning of latent insulation failure under the tail tube of an outdoor cable terminal according to claim 2 is characterized in that: The step S2 comprises the following steps: Based on the outdoor cable terminal structure parameters, a cable terminal geometry model is built in the COMSOL simulation software. According to the real material parameters, material properties are set for each layer of the cable terminal geometry model in the 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. The calculation formula is as follows: （1）； Where: is the heat loss generated by the cable conductor per unit time, Indicates the cross-sectional area of ​​the cable conductor. represents the length of the cable conductor, I represents the current passing through the conductor, Indicates the cable conductor temperature, represents the volume resistance of the cable conductor, ρ represents the conductivity of copper after ignoring the skin effect and proximity effect, represents the resistivity of copper at 20°C, represents the temperature coefficient of resistance of copper; When the lead seal is completely cracked, the temperature distribution of the insulation outer shielding layer in the tail pipe is affected by the thermal effects of the conductor load current and the grounding current. The calculation method of the heat loss of the insulation outer shielding layer is shown in the following formula: （2）； Where: is the heat loss generated by the insulating outer shielding layer per unit time, Indicates the length of the insulating outer shield. Indicates the cross-sectional area of ​​the insulating outer shielding layer. , are the outer diameter and inner diameter of the insulating outer shield layer, represents the ground current through the insulating outer shield, Represents the bulk resistance of the insulating outer shield; Based on the heat loss calculation results of the cable conductor and the outer insulation shielding layer, the heat source calculation formula is used to obtain the body heat source of the cable conductor and the outer insulation shielding layer, and add it as a temperature field heat source to the cable terminal simulation object. The heat source solution formula is as follows: （3）； Where: The body heat source of the cable conductor or the outer insulation shield. is the heat loss generated by the cable conductor or the outer insulation shielding layer per unit time, obtained by equations (1) and (2), It represents the volume of the cable conductor or the outer insulation shielding layer, l represents the length of the cable conductor or the outer insulation shielding layer, Indicates the cross-sectional area of ​​the cable conductor or the outer insulation shield; Based on the actual operating conditions of the cable terminal, boundary conditions are imposed on the outer boundaries of the cable terminal simulation object in the simulation software to achieve equivalent simulation of the heat transfer process between the cable terminal and the external environment; the boundary conditions include convection heat dissipation and thermal radiation, among which the convection heat dissipation boundary conditions need to set the air temperature and the air convection heat dissipation coefficient according to the actual operating environment monitoring parameters of the cable terminal, while the thermal radiation is determined based on the actual monitoring results of the solar radiation intensity, and finally a cable terminal temperature field distribution calculation model considering heat-fluid coupling is formed.

4. The method for early warning of latent insulation failure under the tail tube of an outdoor cable terminal according to claim 3 is characterized in that: In step S3, the iterative algorithm for the steady-state temperature field distribution inside the cable terminal tail tube includes the following steps: S31, assuming that the initial temperature of the insulating outer shielding layer is the ambient temperature, calculate the corresponding body resistance of the insulating outer shielding layer under the initial ambient temperature, obtain the value of the terminal grounding resistance R2, and then use the terminal grounding current calculation model to solve the corresponding terminal grounding current under the initial ambient temperature condition. ; S32, combined with the terminal grounding current obtained by solution The heat source of the insulation and outer shielding layer in the tail pipe is calculated by using formulas (2) and (3). The calculation results are used as the input parameters of the cable terminal temperature field distribution calculation model, and then the current terminal grounding current is obtained. The steady-state temperature distribution inside the cable terminal under the action of the current terminal grounding current is finally obtained. The corresponding insulation outer shielding layer temperature under the action ; S33, based on the obtained insulation outer shielding layer temperature Recalculate the corresponding insulation outer shield body resistance, and correct the value of the terminal grounding resistance R2, and then use the terminal grounding current calculation model to obtain the corrected terminal grounding current ; For corrected terminal ground current , according to step S32, obtain the corrected terminal grounding current The corresponding steady-state temperature distribution results of the cable terminal tail tube and the temperature of the insulation outer shielding layer under the action ; S34, Judgment and The relationship between the difference and the set threshold, where the threshold is set according to the user's requirements for solution accuracy; and When the difference between the two values ​​does not exceed the set threshold, the steady-state temperature distribution result inside the cable terminal tail tube obtained in step S33 is output as the cable terminal temperature distribution in the state where the lead seal is completely cracked; and When the difference between the two exceeds the set threshold, = And return to step S33.

5. The method for early warning of latent insulation failure under the tail tube of an outdoor cable terminal according to claim 4 is characterized in that: The step S4 comprises the following steps: S41. Three temperature measuring surfaces are evenly arranged along the axial direction of the cable terminal tail pipe, and a temperature measuring point is arranged at a radial interval of 120° on each temperature measuring surface. A thermal resistor is selected as the temperature measuring element to realize online monitoring of the surface temperature distribution of the cable terminal tail pipe in operation; S42. Under the premise of knowing the actual operating load current and operating environment conditions of the cable terminal, determine the temperature distribution result of the tail tube surface in the scenario where the insulation outer shielding layer of the cable terminal tail tube is not damaged according to the iterative algorithm of the steady-state temperature field distribution inside the cable terminal tail tube, and use it as the early warning action value; S43. When the surface of the outer shielding layer of the tail pipe insulation is damaged, the grounding resistance will decrease. Combined with formulas (2) and (3), it can be seen that the heat source of the outer shielding layer will increase, resulting in an increase in the surface temperature of the tail pipe. The larger the carbonized area of ​​the outer shielding layer, the greater the temperature increase. Therefore, the difference between the surface temperature measurement result of the tail pipe and the early warning action value in step S41 is used as the basis for judging the severity of the latent insulation fault. When the surface temperature of the tail pipe exceeds the early warning action value, it means that there is a latent fault in the insulation under the tail pipe, and the larger the difference, the more serious the fault.

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