Early stages of negative PD development in dielectric liquids

The pulse shapes in the early stages of partial discharge (PD) development, well before liquid streamer onset, and obtained with a 500 MHz wide band detection system using needle-to-metallic plane electrodes submerged in a mineral oil, were compared to those measured with needle to mineral oil plane electrodes in air under negative dc potential. All measurements were carried out ≈50% below the breakdown field value, using gap separations between 3 and 25 mm. Whereas the needle to plane discharge behavior in the mineral oil was characterised by discrete sporadic pulses or burst of several pulses of increasing negative amplitude and oscillatory form, the needle to mineral oil plane gap in air gave rise to primarily dense bursts of unidirectional negative pulses with sequentially decreasing amplitude. The pulses had rise times typically of the order of 1 to 2 ns, indicating the involvement of rapid PD mechanisms