Design and characterization of a system for loss factor measurement in MV underground cable joints under temperature variation conditions

Cable joints are considered to be the weakest components of the MV cable network, being the ones with the highest failure rate. Furthermore, from statistical analysis of different cable network failure data, a noticeably increase in cable joint breakdowns has been showed during summer months. The request for Distribution System Operators to avoid line outages as much as possible indicates a significant need for non-invasive diagnostic technologies able to monitor the health condition of MV cable junctions. Moreover, a better knowledge is required on the main causes of the detected increase in cable joint failure rate, in which temperature seems to play an important role. The present master thesis consists in the design and complete characterization of a simple and cheap laboratory setup for loss factor measurements on cable joints. Using the developed circuit, tan delta measurements are performed on four thermally cycled cable joint samples and the trend in relation to temperature variations has been investigated and widely discussed. In particular, it has been found that the loss factor of cable joint samples decreases considerably when temperature rises and vice-versa. From result analysis, some hypotheses have been made trying to explain this particular behavior of the loss factor versus temperature. In particular, temperature variations seem to cause expansions and contractions of the different dielectric layers present in cable joint, giving rise to pressure fluctuations at the interface between them. Consequently, variations in the breakdown strength of the joint insulation system and in the amount of conduction losses are expected to happen. These conclusions must be demonstrated by future studies and tests on cable junctions. It is worth saying that a conference proceeding has been published from the present work, and, currently, a journal paper is submitted, waiting to be approved.