The activity was developed to identify the causes and degradation dynamics of the insulating system in joints—both homogeneous and transition joints—constructed with the self-restraining technique in buried medium-voltage power lines. This contributes effectively to providing utilities with guidance to enhance the quality and reliability of service to users.

This work concludes the initial experimental phase, focusing on analyzing components under dry soil conditions. After analyzing the obtained data, a new circuit configuration was developed, incorporating innovations and better reflecting the actual operating conditions of cables and joints.

Regarding the custom-built test circuit at the “Medium Voltage Cable Diagnostics” laboratory, the experimental work, including the latest measurement campaign, has been completed. The results show that degradation dynamics are related to the conductor’s temperature, particularly the critical thermal conditions reached due to ongoing climate changes and the resulting alternation between droughts and heavy rainfall, sometimes characterized by extreme weather events. The decision to use various measurement methodologies (continuous or online monitoring and offline measurement campaigns) has proven correct. Specifically, online measurements have allowed for monitoring and highlighting the onset and extinction of localized degradation phenomena in XLPE-insulated cable sections during both heating and cooling phases.

Frequency response analysis of Tan δ conducted on the insulating system provides sometimes more reliable evaluations compared to traditional specific frequency measurement methods such as 50 Hz and 0.1 Hz. Additionally, this methodology anticipates degradation phenomena better than traditional systems.

Although soil moisture has not been considered so far, the following conclusions can be drawn:

Under applied voltage, some joints reach operating temperatures exceeding the limits imposed by the technology, given the same load conditions.
Innovative testing methodologies provide complementary information to traditional methods and are more sensitive to degenerative effects.
Online monitoring allows for the early acquisition of information about degradation causes compared to offline monitoring.
The new circuit setup will also evaluate the effect of soil moisture due to several introduced innovations:

Transitioning from direct burial of cable sections in the ground to a “Cloverleaf” laying configuration.
Creating a custom system to reproduce various types of precipitation.
Implementing prototype LoRa communication systems on unlicensed frequencies and NB-IoT on licensed frequencies to assess their reliability and validate new field monitoring methodologies.
Collecting data to determine possible correlations between fault phenomena, component operating conditions, and meteorological situations.
To better correlate laboratory results with the environmental operating conditions of buried medium-voltage cables, it is desirable to integrate information from installed systems, which is currently unavailable.