Insulation characteristics of gas mixtures including perfluorocarbon gas

This paper describes discharge properties of N
₂
and CO
₂
-based gas mixtures including a perfluorocarbon (PFC) gas such as CF
₄
, C
₃
F
₈
and c-C
₄
F
₈
under non-uniform field. The mixture ratio between a base gas of N
₂
or CO
₂
and the additive PFC gas was fixed as 9:1; namely, 90%N
₂
/10%PFC or 90%CO
₂
/10%PFC gas mixture. The PFC gases have even smaller global warming potential (GWP) than SF
₆
gas and have good insulation properties as SF
₆
gas. Thus, PFC gas mixture is expected to be a SF
₆
substitute without highly pressurizing the gas over the conventional pressure of 0.5 to 0.6 MPa. In this study, in order to compare the partial discharge (PD) inception voltage V
_PD
and breakdown voltage V
_B
properties between N
₂
and CO
₂
-based gas mixtures, as well as between the additive gas of PFC and SF
₆
gas, we investigated these properties of the gas mixtures with a needle to plane electrode under ac high voltage application. The gas pressure was changed from 0.1 to 0.6 MPa. As a result, it was found that V
_PD
and V
_B
characteristics of N
₂
and CO
₂
-based gas mixtures differed considerably, especially the gas pressure dependence of V
_B
(so-called the N shape characteristics). V
_B
characteristics of N
₂
-based gas mixture including c-C
₄
F
₈
proved to be excellent within the test conditions over the wide gas pressure region, showing the maximum breakdown voltage. In terms of V
_PD
properties, CO
₂
-based gas mixture had an advantage over N
₂
-based gas mixture due to higher V
_PD
. Furthermore, we discussed the synergy effects of V
_PD
and V
_B
for N
₂
and CO
₂
-based gas mixtures using the index R
_n
which was defined to quantify the degree of the effect. R
_n
for CO
₂
-based gas mixture was higher than that of N
₂
-based gas mixture.