Research on Cable Head Monitoring Technology for Transmission Lines

Buried cables are located in harsh environments with high corrosion, uneven soil acidity and alkalinity, and groundwater immersion, resulting in complex structures. Especially for long-distance 10 kV transmission cables, the path planning is relatively long and cannot be connected at once, requiring a connection at the middle end of the cable. The production of cable joints is generally carried out in the foundation pit or well, with complex processes and manual production throughout the entire process, which cannot avoid manual technical deviations. Therefore, cable line faults mostly occur in the intermediate joints. Due to the uneven distribution of the temperature field caused by the accumulation of the electric field, the heat is transferred outward from the peeling chamfer point, resulting in uneven heating. In addition to the skin effect and proximity effect of current, the accumulation of electric field at the chamfer process point of the main insulation and outer semiconducting layer is intensified, and then the insulation is gradually broken down, causing a short circuit or ground fault. This article first studies the temperature and electric field distribution at the cable joint, and considers installing a small temperature sensor at the middle joint of the cable. The sensor is powered by a small electromagnetic energy coil, which can monitor abnormal temperature and electric field changes at the joint in real-time.


Introduction
With the rapid development of China's power industry, electricity consumption is increasing, and the proportion of cables in the transmission and distribution grid is increasing.As an important link connecting substations to various electricity users, the safety of cables needs to be improved.Especially for the intermediate joints of cables, which are located below the soil layer for a long time, it is easy to generate electric field accumulation and heat during the peeling and chamfering process during the joint manufacturing process.According to the investigation, over two-thirds of short circuit grounding faults in cable faults occur at the intermediate joints and terminal positions of the cables.At present, cable joint fault detection usually uses monitoring of wire core current and leakage current, with low monitoring accuracy.The current variation of the wire core is significant, and the leakage current is the target amount after the occurrence of a short circuit fault, which cannot guarantee timely detection and accurate positioning.The most obvious change of cable intermediate joint failure is that the temperature distribution is uneven, and the temperature of the insulation shielding layer rises too fast.For this reason, the intermediate joint based on temperature sensors can monitor temperature changes in real-time [3] .The energy harvesting coil uses the principle of electromagnetic induction to supply energy and is internally equipped with storage chips and wireless communication transmission chips, which are close to the accident site, enabling rapid monitoring and early warning to reduce the occurrence of accidents.1, it can be seen that the cable production process is complex and requires AC to withstand voltage testing.However, the process requirements for peeling and chamfering are strict, ensuring that the smooth fracture surface and uniform electric field performance of the joint meet the requirements [1] .Taking the 10 kV cable joint of a certain project as an example, the structures at the joint are copper core, pressure connecting pipe, stress cone, and insulation shielding layer [2] .As shown in Figure 2.During the process of peeling and cutting chamfers for the semi-conductive and main insulation of cable intermediate joints, due to the construction environment and process level, there is an uneven and uneven slope at the chamfer [3] .Compared with the tightly adhered area, the electric field accumulation rate is higher, the thermal conductivity is smaller, and the distribution is uneven.During the process of conveying a large current, the electric field will accumulate at weak points at the peeling chamfer, resulting in a temperature field.The uneven temperature field will transfer heat outward, leading to heat dissipation.It will cause the aging of the insulation layer and decay of its lifespan.Short circuit grounding faults can occur in underground high-voltage working environments, affecting the stability of transmission and distribution performance [4] .

Composition and Fault Analysis of Buried Cables
According to the Fourier law of heat transfer and the law of conservation of energy, the governing equation for the temperature field problem of cable joints can be obtained: Where, r is the material density; c is the specific heat capacity of the material; T is the temperature variable to be determined; t is time; x, y, and z are the thermal conductivity of the material along the x, y, and z directions, respectively.For isotropic materials, there is x y z l l l = = ; The heat generated per unit volume inside an object, that is v Q , the density of the heat source.

Model establishment
In this paper, COMSOL simulation software is selected to simulate the cable intermediate joint.First, a three-dimensional spatial model is established, and then the materials in the model are added in turn.
Finally, the parameter properties, dielectric constant, thermal conductivity, and so on of the materials are set.The heat source in this article is the copper core of the cable, with a potential of 10 kV.The outer sheath of the joint is cross-linked polyethylene (XLPE) in contact with seawater, which is approximately grounded [5] .The normal heat flux density on the boundary is 0. The heat transfer formula is: ( ) Where, 0 r is the resistivity of the conductor at temperature 0 T , the unit is m Ω ⋅ ,a is the temperature coefficient, the unit is 1/ K , A is the coefficient related to the material, and the value is 3.2;j is the activation energy, taking a value is 0.56eV ; T is the temperature, the unit is K ; q is the amount of electronic charge, with a value of   4,the material properties of each part, boundary conditions, heat sources, electric fields, etc., were added to the established model.To observe the changes in the internal temperature field more clearly, the first half of the model is used for observation without the need for closure.Internally, the conductor core is crimped with the connecting tube.At the semi-conductive fracture, there is a stress relief adhesive or stress tube, and the excess part serves as a filling layer [6] .The electric potential of the intermediate joint is the highest, about 10 kV, and the generated electric field intensity is relatively high.According to the skin effect, the current tends to approach the surface, so the electromotive force gradually decreases from the wire core outward.Under normal circumstances, the potential of the insulation layer outside the joint will decrease to close to the zero potential of the ground, but the uneven distribution of the electric field at the peeling chamfer can also detect the potential of the insulation layer and the semi-conductive layer.The temperature field and potential distribution correspond to Figure 5.The temperature of the copper wire core is relatively low, while the temperature of the peeling and chamfering points and other layers is high, which can easily exceed the insulation life threshold of the protective layer, causing insulation aging and becoming one of the reasons for short circuit grounding of the intermediate joint [7] .

Setting of temperature sensor
Small temperature sensors are generally installed at cable terminals to monitor wire core temperature.The temperature sensor in this article is placed in the filling layer of the middle joint of the cable [9] .According to simulation analysis, the temperature of structural layers with high conductivity and thermal conductivity increases rapidly in the joint, and the temperature can easily reach a high temperature that damages the insulation during faults.So small temperature sensors are installed in the filling layer, which has lower conductivity and thermal conductivity compared to other layers and relatively lower extrusion strength, which can reduce the pressure of the temperature sensor when the joint is displaced.The temperature sensor can be powered by an energy-harvesting coil, as shown in Figure 6.There is an alternating magnetic field around the high-voltage copper wire core, and the magnetic field around the filling layer changes with the current in the high-voltage wire core.Then, the induction voltage is generated by the induction energy harvesting coil.The induction voltage is transformed and processed by energy-harvesting power modules such as rectification, filtering, and voltage stabilization, ultimately achieving the power supply function for the equipment.Once the alarm threshold is reached, an early warning signal is issued to reduce the power supply at both ends until the early warning temperature returns to normal [8] .Due to the proximity of the temperature sensor to the fault point, in the event of a short circuit breakdown, the temperature sensor can also be damaged, the temperature sensor monitoring system is shown in Figure 7, which occurs in extreme cases.In most cases, the temperature will show a downward trend after the early warning, reducing losses.When intermediate connectors accumulate faults for a long time, a short circuit to ground or phase-to-phase occurs.At this time, the temperature sensor has been damaged and cannot be used as a sensing monitoring element anymore.Therefore, during maintenance, it is necessary to add a temperature sensor again and connect it to the energy harvesting coil.Overall, adding a temperature sensor to the intermediate joint can monitor the occurrence of multiple faults in advance, reducing the number of line repairs and inspections.

Conclusion
In this article, the study of cable intermediate joints first analyzes the manufacturing process and structure inside the joints and points out that the internal peeling and chamfering process is the weak link in heating.Then, a temperature field simulation was conducted on the joint, and it was analyzed that the structure with high thermal conductivity increases temperature quickly, which is prone to line short circuit faults.Finally, it is pointed out that the temperature sensor is installed in a filling layer with low thermal conductivity and powered by an energy-harvesting coil.This system can improve the frequent occurrence of short circuits and ground faults in the middle joint of the cable and provide early warning for monitoring by the upper computer to avoid expanding losses.

Figure 2 .
Figure 2. Structural diagram of buried cable intermediate joint

;
b is the electric field coefficient, and the value is

7 2 .Figure 3 .
Figure 3.A sectional view of cable intermediate joint As shown in Figure4,the material properties of each part, boundary conditions, heat sources, electric fields, etc., were added to the established model.To observe the changes in the internal temperature field more clearly, the first half of the model is used for observation without the need for closure.Internally, the conductor core is crimped with the connecting tube.At the semi-conductive fracture, there is a stress relief adhesive or stress tube, and the excess part serves as a filling layer[6] .

Figure 4 .Figure 5 .
Figure 4. Electric potential diagram of cable intermediate joint

Figure 6 .
Figure 6.Schematic diagram of the energy-taking coil of the filling layerAccording to the law of electromagnetic induction, the alternating magnetic field in the energy-taking coil:V N t f Δ = − Δ, N is the number of coils, f the magnetic flux, V is the induced electromotive force, and generates energy to power the temperature sensor.The temperature sensor is close to the intermediate connector, making it easy to monitor the rate of temperature change and temperature rise.Once the alarm threshold is reached, an early warning signal is issued to reduce the power supply at both ends until the early warning temperature returns to normal[8] .Due to the proximity of the temperature sensor to the fault point, in the event of a short circuit breakdown, the temperature sensor can also be damaged, the temperature sensor monitoring system is shown in Figure7, which occurs in extreme cases.In most cases, the temperature will show a downward trend after the early warning, reducing losses.When intermediate connectors accumulate faults for a long time, a short circuit to ground or phase-to-phase occurs.At this time, the temperature sensor has been damaged and cannot be used as a sensing monitoring element anymore.Therefore, during maintenance, it is necessary to add a temperature sensor again and connect it to the energy harvesting coil.Overall, adding a temperature sensor to the intermediate joint can monitor the occurrence of multiple faults in advance, reducing the number of line repairs and inspections.

Figure 7 .
Figure 7. Temperature Sensor Monitoring System for Cable Intermediate Joints

Table 1 .
flux density, h is the heat transfer coefficient, ext T is the ambient temperature, T is the temperature at this time.The parameters of each part of the intermediate joint are shown in table 1: Margins and print area specifications.