Improvement in sensitivity of broadband impedance spectroscopy for locating degradation in cable insulation by ascending the measurement frequency

In order to develop a truly reliable method for locating a degraded portion in a cable, we are developing a method based on the frequency domain reflectometry and inverse fast Fourier transform (IFFT). The locating ability of this method was examined for three types of degradation given to three kinds of cables, each with a length ranging from 24.4 to 29.0 m. Namely, a vented water tree was simulated in a coaxial cable insulated with cross-linked polyethylene (XLPE) by making a tiny hole filled with a NaCl solution, and γ-ray-induced aging was given for a length of 80 cm of an XLPE-insulated triplex cable, while the mutual distance between the two conductors was expanded for 10cm in a cord insulated with polyvinyl chloride. Analyses by IFFT using the data obtained from 1 MHz to 100 or 110 MHz can locate all three types of degradation, but the resolution is poor. If the highest measurement frequency is ascended to 200 to 400 MHz, the resolution becomes better. Moreover, the spectrum indicating the degraded portion shows quite a good reproducibility unless the cable is moved. This means that this method can be a very sensitive degradation locating tool usable for real cable systems.