title: Detection of low residual current flow through gas during the arcing period
Creators: name: Harry, Inye Hamilton
Publisher: name: University of Liverpool Repository
Year: 2014

description: Within the switching technology and switchgear industry, the insulation properties of gases are extensively utilized as excellent methods of arc quenching. To optimize the efficiency and effectiveness of switchgear performance in interruption of the fault current through extinguishing the arc as well as to withstand the rate of rise of recovery voltage (rrrv) after current zero, the properties of these gases have been utilized. The present research is to develop a technique to identify the probe current flow during the weakened dielectric strength of the gases. Through the probe current flow during the weakened dielectric strength of the gases, the behaviour of the gases are described and compared since gases are most used in the circuit breaker for fault current extinction. In particular, the properties of sulphur hexafluoride (SF6) gas have been widely utilized due to its outstanding properties, but, given the connection of SF6 with global warming, there is a drive to find alternatives (National Electrical Manufacturers Association). Although the exposed SF6 gas is recycled back into the cylinder after exposure, there may be some leakages into the atmosphere. More so, the decomposed SF6 gas into sulphur-fluoride and metal fluoride powdery substances when exposed to the environment after the gas might have been recycled may contribute to global warming at a later period. Hence, this research is focused to develop a technique used to determine the behavior of alternative gases which are potential candidates to replace SF6. The work is an experimental assessment of gases and their reaction to the presence of an arc discharge and recovery from that exposure. The investigation of these gases is achieved by using a negative DC voltage-biased dielectric probe and monitoring small current flow through this probe. The primary gases considered are dry air, nitrogen (N2) and sulphur hexafluoride (SF6) as a comparison. The probe current flowing through these gases are investigated and identified respectively. The research identifies the variation in leakage current in compressed dry air in a changing electric field and pressure rise, typical examples of gas behaviour in an electric field. The search indicates that nitrogen gas is more susceptible to the fault current as compared to dry air, followed by sulphur hexafluoride SF6 gas. These gases were examined from the probe current flow responses obtained from the negative DC voltage-biased dielectric probe when the fault current was passed in the gases’ respective vicinity. Consequently, the research identified that a SF6 dielectric medium has better insulation property regarding the fault current as compared to dry air and N2 gas mediums under the same experimental conditions. The probe current flow in N2 gas was experimentally determined and calculated, and the direction of current flow before and during the arcing period of the arc discharge was also identified. The direction of current depended on the circuit condition; in this case, the dielectric probe operates below its breakdown voltage in the gas, meaning the probe tips and the surrounding gas may have with positive ions, thus applying the positive half cycle of fault current will repel the positive ions while recombining with the negative ions and electrons. Moreover, the polarities of the probe voltages (input V1 and output V2) may also influence the direction of the probe current flow during the arcing period, since current flow is conventionally from negative to positive. The probe current flowing during the arc was identified as more when compared to no arc discharge. The gap resistance for nitrogen gas at various increases in pressure before and during the arcing era were also calculated. The result demonstrates that the gap resistance of nitrogen gas is greater when no arc was present, though both show some forms of variation over time.