Since 1960s, SF6 gas has been significantly used as an insulating medium in power apparatus. It is, however, one of the greenhouse gases and its global warming potential(GWP) is 23,900 times higher than that of CO2. For this reason, the use of SF6 gas is being restricted across the world and studies on substitute gases for reducing SF6 gas has been in progress. Until now, the insulation performance of mixture gases has been mainly evaluated by breakdown voltages, while studies in aspect of partial discharge(PD) have not been carried out.
This dissertation dealt with the PD characteristics depending on defect types in SF6-N2 mixture gases for insulation design and risk assessment of gas insulated switchgears(GISs). Related parameters such as discharge inception voltage(DIV), discharge extinction voltage(DEV) as well as discharge magnitude(), pulse count(), phase distribution(), and polarity ratio at the DIV and 1.25 times of the DIV were analyzed in SF6-N2 mixture gases.
Three electrode systems such as a free particle(FP), a protrusion on conductor(POC), and a protrusion on enclosure(POE) were fabricated to simulate major insulation defects in GISs. PD signal generated by defect was measured according to IEC60270 at 0.4MPa and 0.5MPa. In addition, an algorithm was designed and applied to phase resolved partial discharge(PRPD) analysis.
DIV and were higher in the FP than other defects. Parameters were not affected by the ratios of SF6-N2 mixture gases since they were dependent on the particle moving. PD pulses were distributed in all of the phases.
In POC, DIV, DEV, as well as maximum and average magnitude of PD pulse decreased as an increase of N2 ratio. In case of phase distribution, PD pulses over 95% were distributed in the negative half(230˚∼310˚) in pure N2 but they were distributed in the positive half(40˚∼130˚) as SF6 ratio was more than 20%. Parameters measured at DIV×1.25 showed that both the maximum and average pulse magnitude increased and PD pulses over 90% were distributed in the positive half(40˚∼130˚) in all ratios.
In POE, each DIV and DEV in N2 ratio below 50% was the same as those in pure SF6 and it decreased rapidly when N2 ratio was over than 80%. The average pulse magnitude was the highest in pure SF6 and almost same in other ratios. The similar tendency was observed at 0.4MPa and 0.5MPa. As applied voltage increased to DIV×1.25, breakdown occurred in SF650%-N250% and SF620%-N280%. The pulse count increased with the ratio of N2. The phase distributions appeared opposite to POC. PD pulses over 95% were distributed in the positive half in pure N2 and they were distributed in the negative half as ratio of the SF6 gas increased. However, the phase distributions in POE were not affected by the applied voltage and they appeared identically at DIV×1.25.
DIV of mixture gases, which are related with insulation breakdown was compared with pure SF6 in three defects. PD in FP was almost incepted at the same voltage without the influence of gas ratio. In POE, DIV in N2 ratio below 50% were the same as those in SF6100%. However, it decreased rapidly when the N2 ratio was over than 80% and DIV of N2100% decreased 60%. In POC, DIV decreased with the increase of N2 ratio. In SF680%-N220%, SF650%-N250%, and pure N2, DIV decreased 6%, 18%, and 70% comparing with pure SF6, respectively.
From the experimental results, although DIV in POC decreased below 6% in SF680%-N220%, the use of SF6 could be reduced to maintain insulation performance. Therefore, pure SF6 could be replaced with SF680%-N220%. All of the parameters presented in this dissertation can be used in aspect of design, manufacturing and operation of GIS.