The research was focused on the study of the causes and dynamics of the insulating system degradation of both homogeneous and transitional joints made with a self-shrinking technique in MV cable lines, which were directly laid in the ground to increase electricity grid resilience, as well as service reliability and quality for end users.
In the project, two separate test setups were created in succession: the first one allowed analyzing the behavior of the joints in dry soil, simulated using felts, without considering the effect of humidity; in the second one joints were installed inside tanks filled with inert material, compliant with the specifications adopted by the Utilities for directly installing components in the trenches. The focus of the second part was the analysis of the effect of soil humidity (related to atmospheric precipitation) on component functioning, with daily thermal cycles simulating service conditions in summer (i.e., when the failure rate increases).
By analyzing the data from the first test setup (Partial Discharges and Tand), the conclusion was that the degradation dynamics are related to conductor temperature and, in particular, to the critical thermal conditions reached due to climate change. Acquired with both traditional and innovative techniques, these results led to the creation of a new configuration, more similar to the real operating conditions of the cables and joints.
Both configurations include various measurement campaigns carried out offline1 interspersed with periods of online operation2, in which a certain number of daily cycles were applied to the component, set in collaboration with the Unareti Utility and designed not to stress the component beyond the intervention limits of the electricity grid protections, while trying to reproduce the service conditions of the summer period.
During the online phases the following parameters were monitored: current, voltage, and temperature of the component; humidity and temperature of the ground, which allowed highlighting the triggering and extinction of specific degradation phenomena, both in the heating and cooling phases, in the sections of cable with XLPE insulation.
Thanks to the experience gained so far, it can be stated that:
• although complying with current legislation, some kits are worse than others;
• the temperature reached in the joints is a function not only of the load request but of the installation and environmental conditions;
• in the presence of dry soil, some joints exceed the temperature limit of the insulation system, triggering an internal phenomenon that is self-sustaining even when reducing the load;
• humidity in the ground favors heat dissipation, helping to maintain the temperature of the component within the limits imposed by the technology;
• humidity helps anticipate the trigger voltage of partial discharge phenomena;
• in the event that the hydrostatic barriers have partially lost their properties, an analysis of the frequency response on the insulating system shows a significant increase in Tand at frequencies lower than 0.1Hz, highlighting a decrease in dielectric properties;
• online monitoring makes it possible to acquire information on the causes of degradation in advance of offline monitoring;
• the implementation of the communication systems of the LoRA prototypes on free frequency and NBIoT on licensed frequency makes it possible to evaluate their reliability and validate new monitoring methodologies;
• the implementation of a prototype tool (demonstrator) for the acquisition of raw data regarding the phenomenon of partial discharges is the first step of an ambitious project, whose aim is to create an advanced diagnostic tool to provide valid support to the electricity distribution provider.