An Integrated Diagnostic Framework of Electrochemical Analysis for Assessing Silicone Oil Health in HV Cable Outdoor Terminations

Up to now, Terna, the Italian Transmission System Operator (TSO) has installed a total of 4305 km of High Voltage insulated cable lines. A key component of an insulated cable system is the termination, which comes in a variety of designs depending on the characteristics of the insulator, cable screen connection, dielectric material and other factors. In the following, dielectric oil insulated outdoor terminations are examined in detail. Cable terminations are typically engineered to be maintenance free; nevertheless, a variety of operational stresses or anomalies, such as temperature cycling, mechanical loads, water ingress, or simply time dependent degradation, can compromise their long-term electric performance. For this reason, targeted diagnostic checks are essential to ensure insulation integrity and prevent unplanned service interruptions. To this aim and among available techniques, silicone oil analysis is a reasonable trade-off between the accuracy of the health status assessment of the terminations and the ease of related in-field operation. On the one hand, the assessment of transformers oil health status has been extensively studied and debated in literature and standards; on the other hand, the reliability and dielectric performance of silicone-based insulating oils in outdoor terminations remains poorly addressed. In particular, no standards or acceptance criteria currently govern their in-service evaluation. This study presents a comprehensive analysis of the different aspects of the water contamination phenomenon of the dielectric oil, from studying the behaviour of water into an outdoor termination to trial experiment on dielectric fluid. In this research, statistical analysis was conducted to establish correlations among the various measured parameters and thoroughly examine the intrinsic weaknesses and inherent limitations associated with commonly employed chemical analytical techniques. During field service operations, oil samples have been collected from a total of 678 outdoor terminations supplied by different manufacturers, each exhibiting comparable physicochemical profiles. In particular 511 were sampled from 2017 to 2019 (Campaign 1) and 167 from 2023 to 2025 (Campaign 2).