Negative Corona Current Pulses in SF6– Air Mixtures

Mechanism for negative corona current pulse in CO2 SF6mixtures

“…The observed effects of both graphite and CuI coatings on the stepped pulses were qualitatively similar for SF 6 admixtures ranging from 2.5 up to 30%. The observed sensitivity of the step height and relative insensitivity of the pulse maximum to the changing the cathode γ p -emission, which are identical with the discharge behaviour observed in SF 6 -air and SF 6 -N 2 mixtures [10,11], are also in a good agreement with the above mentioned streamer-based hypothesis for the discharge mechanism.…”

“…For the SF 6 percentage above some 20% this trend resulted in a total extinction of the step. It is interesting to note that this effect of SF 6 on the pulse shape is opposite to that observed in similar conditions in SF 6 -air and SF 6 -N 2 mixtures [10,11], where not the step on the pulse leading edge but the following current rise is quenched by increasing the SF 6 content.…”

“…The experimental arrangement and method have been described in the preceding papers [10,11] and so will only be briefly outlined here: the current waveforms of the first negative corona pulses in a short point-to-plane gap have been measured with a nanosecond time resolution in CO 2 -SF 6 mixtures and various gap voltages. The cathode curvature and point-to-plane spacing were 0.1 mm and 12 mm, respectively.…”

“…The gas pressures ranged from 6.65 to 60 kPa, and the SF 6 percentage varied from 0% to 30%. To clarify the role of γ p -emission processes, copper iodide and graphite, which have exceptionally high and exceptionally low photoelectric yields, respectively, were used as alternative cathode surface materials to copper [5,7,10,11].…”

“…In a recent series of papers [10,11] it has been concluded that in air-SF 6 and N 2 -SF 6 mixtures containing less than approximately 10% of SF 6 the negative corona pulse is associated with the formation of a cathode-directed streamerlike ionizing wave in the immediate vicinity of the cathode. However, in the mixtures containing more than 10% of SF 6 the ionizing wave was quenched and, consequently, the discharge was governed by the Townsend ionization mechanism fed by cathode γ p -emission processes.…”