This report describes a research activity aimed at increasing the resilience and improving the reliability of the electricity system, with particular reference to cable Transmission and Distribution lines.

Today, transmission and distribution of electricity are mainly ensured by means of two distinct carriers, namely overhead lines and cable lines. The former, much more widespread, have the disadvantage of a significant environmental impact, while the latter allow energy to be transmitted with no substantial modification of the territory and have other criticalities, the major of which is the vulnerability of cables to mechanical stresses. One of the most critical elements within a cable connection system is the connection between the cables themselves, i.e., the joints between the cables. Use of joints is necessary because cables can be only created within a maximum length, due to both manufacturing and installation reasons. The creation of joints is a very delicate operation and is carried out on site by highly qualified workers. These workers often have to work in narrow environments and in the presence of powdery material, and therefore, joints present impurities or imperfections that lead, once the line is energised, to a more or less rapid deterioration of the insulating material.

The presence of ‘defects’ originates Partial Discharges (PDs) which, over time, erode the insulating material between the conductive core and the external metal shield. This erosion is known as ‘treeing’. Once it has crossed the entire insulation, the cable undergoes a destructive discharge that inevitably leads to the line being put out of service.

The use of current diagnostic techniques, which include both sensors and algorithms, do not allow for the localisation of defects nor consequently their evolution.

Therefore, this work aims to better and more accurate localisation of defects inside joints and to reconstruct their evolution over time. This is crucial to estimate the health status of joints with greater accuracy in order to prevent untimely failures and consequent line disruptions.

Starting from a totally analytical physical-mathematical model developed in the previous three years, three minimisation algorithms are analysed that are capable of identifying, through the measurement of physical quantities such as charges, currents and external effects, the spatial coordinates of a defect in cylindrical symmetries such as those of joints. These algorithms are then mutually compared in order to evaluate the strengths and weaknesses of each of them. In order to evaluate the robustness and the quality of the methodology, a sensitivity analysis was also performed by introducing artificial noise in the starting data.

Finally, these methodologies were extended to complex three-dimensional configurations, using numerical techniques such as FEM analysis.

The results showed that these models, combined with the localisation algorithms, are able to localise with high accuracy the radial and angular position of the defects with PDs inside the joints.

They also detect their evolution over time, making the estimate of the degradation of the joint very precise and accurate. Unlike conventional methods that are based on an equivalent model that leads to determine so-called ‘apparent charges, this methodology also allows us to obtain the real charge generated during the discharges’.

Determining the real electric charge involved in the phenomenon is essential for estimating with good precision the energy of the discharge and therefore its physical-chemical impact on the polymeric material. Report 19012956 Page 6/39 The entire activity therefore showed an important potential for application in the predictive diagnostics of ageing and degradation phenomena inside the joints of medium- and high-voltage cables. In the near future; the goal is the experimental validation of these results.