How can we detect internal compression defects in elbow-type cable joints of ring-main units (RMUs) in advance by monitoring the temperature difference of the outer sheath of the cable joints?

In the daily operation and maintenance of power distribution systems, cable accessories are often the most frequently faulty parts, especially the elbow-type cable joints in ring-main units (RMUs). If the compression process is not standardized, the internal contact resistance will increase, and long-term heating may lead to insulation aging, equipment damage, and even power outages. To address this issue, we have developed a simple and reliable method through physical simulation and temperature rise tests:

By monitoring the temperature difference of the outer sheath, one can accurately assess the internal compression quality without disassembling the equipment.​ Below, I will explain this from three dimensions: principle, verification, and practical application.

First, why does an internal compression defect cause a temperature difference in the outer sheath? This is related to the physical laws of heat conduction. When the cable joint’s internal conductor carries current, if the compression is not proper, the contact resistance will increase significantly, generating additional heat. This heat will gradually spread from the inside to the outer sheath. However, due to the insulation effect of multiple layers of insulating materials, the temperature change of the outer sheath has a significant delay. We established a high-precision 3D simulation model (simulating the actual physical characteristics of conductors, insulating layers, and cabinets, etc.) and calibrated it through multiple on-site temperature rise tests. The error between the simulation and the actual measurement was controlled within 1%. The results show that the worse the compression quality, the higher the internal resistance ratio, and the more obvious the temperature difference in the outer sheath.​ For example, the resistance ratio of qualified compression in industry standards is usually ≤1.2, while for non-qualified joints, it may reach above 1.5, resulting in a significant temperature difference between the outer sheath and becoming a “window” for detecting internal defects.

Second, will environmental temperature and load fluctuations affect the reliability of the detection? This is a common concern in traditional maintenance. However, our comparative tests provide a clear answer. First, we simulated two extreme environments: high summer temperatures and normal temperatures. We found that the environmental temperature only raises or lowers the equipment temperature overall, but it does not change the temperature difference between defective joints and normal joints.​ In other words, the fault detection standard is not affected by the weather and can be implemented for 24/7 detection.​ Secondly, the fluctuations in daily power loads (such as peak hours during the day and low hours at night) have a negligible impact on the temperature difference, causing only a few percent of minor fluctuations, which can be ignored. Only in extreme peak loads, the temperature difference will slightly increase, but this actually helps expose potential defects and does not lead to misjudgment. Therefore, this method has strong on-site adaptability.

Third, how can this technology be implemented in actual maintenance?

Its core advantage lies in “non-invasive” detection.​ Traditional methods require disassembling multiple layers of insulation to access the internal conductors, which is time-consuming and dangerous, and many hidden problems can only be discovered after a failure occurs. Our solution only requires installing temperature sensors on the equipment’s exterior to collect the temperatures at various points of the outer sheath and automatically calculate the temperature difference.​ Once the temperature difference exceeds the preset threshold (based on simulation and test results), the system will immediately issue an alert, allowing maintenance personnel to arrange maintenance in advance to avoid equipment damage or unplanned power outages. During a power distribution network renovation project I participated in, I used this method to identify two suspected defective joints in three RMUs. Subsequent disassembly and verification were completely consistent, successfully avoiding a possible summer peak power outage accident.

Finally, to summarize the practical value.

For EPC contractors, switch cabinet manufacturers, or power grid maintenance personnel, this temperature monitoring-based solution does not require modifying the equipment structure.​ It can upgrade the maintenance level simply by using external sensors and data analysis.​ It not only extends the service life of cable accessories but also significantly improves the power supply reliability of urban distribution networks.​ If you are facing a similar problem of joint detection, you might consider starting with a small-scale pilot project to verify the temperature difference threshold, and then expand it. This will be an extremely cost-effective maintenance strategy.​ Of course, any technology has its limitations. For extremely old equipment or severely overloaded scenarios, it is recommended to combine other detection methods (such as infrared imaging or partial discharge testing) for comprehensive judgment​ to ensure absolute safety.