Comparison of PD patterns at power and very low frequency: Is diagnostic possible?

Partial discharge phenomena from different sources (corona, internal and surface discharges) were investigated under sinusoidal and cosine-rectangular waveforms having frequencies of both 0.1 Hz and 50 Hz. The results show that testing at very low frequencies can be affected by sensitivity issues (PD magnitudes are about 50% at 0.1 Hz if compared with those obtained at 50 Hz). Using sinusoidal waveforms, PD patterns are comparable with those obtained at 50 Hz. For cosine rectangular waveforms, identification seems to be much more complicated, if not impossible.