The backward discharge induced by residual charge has attracted considerable attention, but the complex mechanism requires further investigation. This article analyzes the surface discharge and the residual surface charge accumulation characteristics in atmospheric air under a positive impulse. The photographic figure method, the dust figure method, and the electrostatic probe method are employed, allowing the residual charge distribution to be quantitatively determined. One main forward discharge is observed at the impulse front and several subsequent backward discharges are observed at the impulse tail. The propagation length of the forward streamer is closely related to the voltage at the moment of discharge. A model illustrating the evolution process of surface forward and backward discharges as well as the corresponding residual charge accumulation characteristics is established. The residual surface charge plays a significant role in the development of the streamer discharge. The backward discharge is incurred by the positive charge that is deposited by the previous forward discharge.
C4F7N/CO2 is a promising mixture to replace SF6 in an electrical insulation field for its environment friendly property. Understanding surface discharge behaviors of the C4F7N/CO2 mixture is essential for designing an effective insulation system. In this work, surface discharge propagation characteristics in the C4F7N/CO2 mixture under positive impulse voltages are investigated. The surface discharges are detected electrically and observed by optical imaging. The residual surface charge distribution is acquired by an electrostatic probe method and a dust figure method after surface discharge. Comparative studies in CO2 and SF6 are further conducted. Results exhibit some significant differences between CO2 and C4F7N/CO2 mixtures in the propagation pattern and morphology of surface discharges. The surface discharge in the C4F7N/CO2 mixture develops in a stepwise expansion pattern, which is a result of alternative formation of streamers and leaders. A leader channel and a streamer development region are clearly distinguished in the optical image. The dust figure shows that no charge is deposited on the leader channel area after discharge, proving that the leader channel is electrically neutral and of high conductivity. The residual surface charge distribution in the C4F7N/CO2 mixture is obtained and resembles pine branches. A little difference is found in the surface discharge propagation characteristics between the C4F7N/CO2 mixture and SF6.
When metal particle is attached on spacer surface, it will distort surface electric field and may reduce surface insulation level. Therefore, it is important to research the relationship between metal particle attachment and surface flashover property. In this study, the effect of metal particle materials on surface flashover performance of alumina-filled epoxy resin spacers in SF6/N2 mixtures is investigated. Three kinds of metal particle materials including copper, iron and aluminum are considered. The results reveal that, at low gas pressure or low surface electric field, the material of metal particle has no obvious effect on flashover voltage. At high gas pressure and high surface electric field, the flashover voltage is almost the same when the iron or aluminum particle is attached, which is lower than that with copper particle attached. It is revealed that at high pressure and high electric field, metal powders and vapor of micron size can be generated from the attached metal particle surface by electric field forces and Joule Heating Effect, which may be responsible for the difference in flashover voltage for different metal particle materials.
Due to the excellent properties, C4F7N/CO2 mixture is the most concerned eco-friendly SF6 alternative gas. For a better understanding of the surface discharge characteristics, the surface discharge pattern of C4F7N/CO2 mixture under negative impulse voltages is investigated in this work. The morphology of the surface discharge is obtained by an optical method and a dust figure method. The structure of the surface discharge is established, which from the outside to the inside consists of electron cloud, streamer, streamer stem, and leader. The propagation pattern of the surface discharge in C4F7N/CO2 mixture under negative impulse follows the stepwise expansion pattern. The structure of the surface discharge in C4F7N/CO2 mixtures with different ratios and SF6 has no obvious difference, and the propagation pattern is also the same. The results of this work can help to better understand the surface discharge phenomenon and the insulation characteristics of C4F7N/CO2 mixture.
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