An Investigation of Negative DC Partial Discharge Decomposition of SF6 Under Different Metal Materials

For determine the influence of metal materials on the negative DC partial discharge (PD) decomposition of SF
₆
and put forward for the method of SF
₆
DC gas insulated equipment condition monitoring and fault diagnosis based on decomposed component analysis, this study investigates the decomposition characteristics of SF
₆
under three common metal materials (Al, 304 stainless steel, and Cu) in gas insulated equipment by using SO
₂
, SOF
₂
, and SO
₂
F
₂
as characteristic component gases under negative DC PD. Results show that the types of metal materials have substantial effects on the negative DC PD decomposition characteristics of SF
₆
. The simulation results based on the first principles confirm the phenomena observed in the experiment: the amounts and generation rates of SOF
₂
were the most abundant, followed by SO
₂
F
₂
and SO
₂
. The calculation based on free electron cloud and finite depth particle in a box model shows that the electron current density of field emission has the relationship with metal work function γ and electrical field E: j(0, E) ≈ (BE
²
/γ) exp(-Dγ
^3/2
/E), where B and D is constants. The electron current density by field emission is inversely proportional to the work function of metal with constant electric field. The metal material with smaller work function means the greater electron current density, further promoting the decomposition of SF
₆
to generate more characteristic component gases. The results can well explain the phenomena observed in the experiment, that is, the decomposition of SF
₆
is the most serious under Al electrode with lowest work function, followed by 304 stainless steel, and Cu.