부산대학교 도서관

It is shown that under sinusoidal regime, at 50 Hz, measurements highlighted the inception of PD above PDIV level, while that was not the case for lowest test frequency (0.1 Hz), mostly due to the gradual reduction of repetition rate. Measurements under a cosine rectangular waveform were not able to highlight the inception of PD activity, which might indicate that this kind of waveform is unsuitable for $\tan(\delta)$ measurements. On the whole, $\Delta\tan\delta$ can be a quantity that is affected by the inception of PD mostly at industrial frequency, while one could expect significant changes of the values of this quantity under VLF only on the presence of large PD phenomena. In other words, VLF tests detect an increase of $\Delta \tan\delta$ with applied voltage, this may be likely addressable to lossy polarization mechanisms or conduction rather than PD inception.