With the rapid development of the economy, the demand for electricity is also constantly increasing. The production and daily life of modern society are always inseparable from the demand for electricity. At the same time, every power grid operation failure will disrupt the normal production and daily life of society. Therefore, the safe operation of the power grid is of great importance. The operating temperature of the equipment is an important parameter for evaluating the safe operation of the equipment, so direct temperature monitoring of power equipment is of great value.
In the power system, current is mainly transmitted through conductors, which are the source of heat generation. Most external heat generation comes from current carrying conductors, but both low-voltage and high-voltage conductors are insulated and isolated from the outside. The higher the voltage, the thicker the insulation layer. When the voltage exceeds 3kV, a semi conductive layer will be designed inside and outside the insulation for electric field equalization. As the voltage increases, the range of power supply will expand, and the importance of the line will gradually increase. Therefore, it is crucial and important for the operation and inspection department to directly measure the operating temperature of conductors, but there are also many difficulties.
Ring main unit (RMU) has the advantages of compact structure, long electrical life, and strong breaking force. In the past decade, in the renovation of urban and rural power grids, RMU has been widely used in distribution systems to save costs, ensure safety and reliability. The T-shaped head of high-voltage cables, as an indispensable and important component in the electrical connection of ring main cabinets, has also been widely used. This type of high-voltage cable T-head is distributed in distribution stations and box type substations in residential communities, high-rise buildings, large public buildings, factories and enterprises in various cities. Due to the influence of processing technology, construction quality, operating conditions, and operating environment, the failure rate of high-voltage cable T-heads is high. The normal operation of cable heads is directly related to the normal operation of the distribution network. Among them, the T-shaped cable plug is mainly used in medium voltage ring main cabinets, medium voltage switchgear, cable branch boxes, etc., to ensure the safety of the cable system when connecting to the main circuit of the cabinet itself. However, the current T-shaped connectors only achieve the connection of the main electrical circuit at the cable connection point, and their connection quality cannot be evaluated. It still needs to rely on installation level and external equipment or manual inspection to monitor them. Due to the fact that the development speed of electrical defects and faults often falls outside of this monitoring or cannot be completed in time, more than 80% of current cable accidents are directly caused by poor installation of cable accessories. Therefore, how to monitor the operation status of the cable T-head body and avoid blind spots in line monitoring is a key focus. The T-joint of the cable is the core component in the cable branch box, which plays a role in tapping and connecting the cable, while improving the electric field at the end of the cable. Most of the problems that occur in cable branch boxes are related to T-joints.
The main factors contributing to the failure of T-shaped cable connectors are:
(1) Insufficient internal cleanliness, dust and impurities causing internal interface creep, resulting in burning and breakdown;
(2) During the installation process, due to installation errors, the position of the T-shaped plug relative to the semi conductive shielding layer of the cable was shifted, resulting in a decrease in insulation resistance between the conductive shielding layers of the T-shaped plug and causing breakdown and burning damage;
(3) The metal burrs after cable crimping were not treated, resulting in partial discharge of the shielding layer inside the T-shaped plug and forming a slow discharge arc, which ultimately led to burning and breakdown of the insulation layer;
(4) After installation, the lateral stress caused by poor cable laying position leads to the formation of a gap between the T-plug and the cable body. During operation, dust enters the internal interface, causing crawling and burning of the channel, resulting in breakdown;
Why do cable T-joints need temperature measurement
The uneven application of silicone grease between the main insulation surface and the inner surface of the stress cone and the transition zone of the main insulation results in gaps, leading to gap discharge and an increase in temperature of the T-shaped head, which burns out; Improper installation of the stress cone during installation results in the distortion of the local electric field due to the failure of the stress cone to provide a uniform electric field, leading to electric field breakdown during long-term operation; Furthermore, due to the increased moisture in the cable branch box and the lower temperature inside, the water vapor in the air condenses into water droplets, resulting in condensation. After condensation occurs, the water droplets condensed on the inner wall of the branch box drip onto the T-shaped head of the cable under the action of gravity, reducing its insulation and causing discharge. Long term operation can burn the T-shaped head and cause accidents. In actual accidents, it is likely to be the result of the combined action of these multiple factors. Therefore, while improving the quality and installation quality of T-joints, real-time monitoring of cable T-joints during operation is important. When summarizing the vast majority of accident phenomena, it can be found that various breakdown phenomena are accompanied by high-temperature ablation processes. Therefore, if it is possible to monitor the voltage, current, and other parameters of the T-shaped plug body inside the switch, and indirectly monitor the temperature changes of the cable accessories through changes in its internal insulation resistance, the operating status of the T-shaped plug body can be monitored to ensure that it is at a good level.
Temperature measurement method for cable T-head
In response to the above situation, in the field of high-voltage cable T-joints, common testing methods currently include infrared temperature measurement, active wireless temperature measurement, and other solutions. Infrared temperature measurement cannot be measured online and requires inspection. It can only measure the temperature outside the insulation layer of the joint, non direct contact temperature measurement. In addition, normal operating ring network cabinets are not allowed to open the cabinet door, and even the temperature outside the insulation layer cannot be measured; The active wireless temperature measurement scheme requires the use of batteries or CT power supply outside the T-shaped head of high-voltage cables, which poses usage risks and requires significant maintenance in the later stage. At the same time, it can only measure the temperature outside the insulation layer of the joint. Therefore, these traditional temperature measurement methods have some shortcomings and cannot meet the temperature measurement requirements of complex high-voltage cable T-head applications.
Online temperature monitoring system for T-shaped cable heads
FJINNO adopts a new intelligent fiber optic temperature measurement and monitoring system to achieve real-time and effective direct monitoring of temperature changes at the connection points of power equipment. The fiber optic sensor probe is closely attached to the surface of the measured object, accurately reflecting the real-time temperature of the equipment surface and has no impact on the operating equipment. Combined with the backend temperature monitoring system, when the operating temperature of the equipment exceeds the preset alarm temperature value, the system will automatically alarm, and the operating department will take appropriate measures in a timely manner to avoid equipment failures caused by temperature rise, ensuring the safe and reliable operation of the power grid.