Microwave-based detection of internal defects in composite insulators for outdoor application

This paper introduces a non-destructive microwave-based detection method which was developed with respect to the detection of internal defects in composite insulators. The test equipment comprised a microwave-generating Gunn oscillator (frequency 24 GHz) combined with an open-ended rectangular waveguide as emitting and receiving sensor operating in the near-field (stand-off distance <; 5 mm). The received signal was converted to a dc voltage which correlated with the reflection coefficient of the waveguide´s aperture. Measurements were performed with insulators that contained different artificial defects, i.e. strips of wood and copper and weak sheath-core adhesion. The results of measurements approved the suitability of the microwave-based test system for non-destructive detection of copper (conductive) and wood (non-conductive) defects in composite insulators. Further tests are required to evaluate the system´s suitability for detecting weak sheath-core adhesion.