Secondary reverse streamer observed in an ester insulating liquid under negative impulse voltage

The so-called 'secondary reverse streamer (SRS)' in this study depicts a newly observed streamer-in-liquid phenomenon which subsequently occurs well after the extinction of the primary streamer (PS) propagation within a single shot of negative impulse voltage and has reverse polarity to the PS. Tests were carried out in a synthetic ester liquid at a 50 mm needle-plane gap under negative impulse voltages up to 140 kV. Based on the current and light waveforms as well as the photographic images, two types of SRS were found: one has a bright trunk with a high velocity of 9.79 km s(-1) belonging to the 3rd mode streamer; and the other has a weak tree-like channel with a low velocity of 1.28 km s(-1) resembling the 2nd mode streamer. It should be emphasized that SRS under negative impulse voltages has positive polarity being opposite to the negative PS, which is believed to be due to the reverse electric field induced by the residual space charges left by the PS. This has been confirmed by further verification tests under tail-chopped impulse voltages, for SRSs are significantly enhanced in both axial and lateral directions after the early removal of the external applied field.