Composite insulators for High-Voltage overhead lines have better performances and are lighter than traditional designs, especially in heavily polluted areas. However, since it is a relatively recent technology, reliable methods to perform live-line diagnostics are still under development, especially in relation to internal defects, which provide few external symptoms.

Thermal cameras can be employed but their use is not always straightforward as the sun radiation can hide the thermal footprint of internal degenerative effects. In this work, an optical Efield sensor has been used to diagnose internal defects of a set of composite insulators. Moreover, a modelling activity using Finite Elements has been carried out to identify the possible nature of the defects by com-paring the experimental E-field profiles to the ones simulated assuming a specific defect geometry.

The results show that the sensor can detect the presence of an internal defect, whose presence distorts the E-field profile when compared to the profile of a sound insulator. Moreover, the measured E-field profiles are compatible with the corresponding simulated ones when a conductive defect is considered. However, it was observed that a defect whose conductivity is not at least two orders of magnitude the conductivity of the surroundings remains undetected.