AC Volume Breakdown and Surface Flashover of a 4% NovecTM 4710/96% CO2 Gas Mixture Compared to CO2 in Highly Nonhomogeneous Fields

Abstract

AC pre-discharge currents, breakdown, and flashover voltage measurements are reported in a 10 mm needle-plane arrangement in a 4% NovecTM 4710/96% CO2 gas mixture and compared with CO2 for pressures up to 8.8 bar abs. Flashover measurements were performed on different solid dielectrics (Al2O3_filled epoxy resins, PTFE (Polytetrafluoroethylene) and PE (polyethylene)) for different roughness surface finishes. The effect of fixed conducting needles at various positions on electrical strength is reported. A strong nonlinear behaviour as function of gas pressure was observed for all the studied parameters (gas, needle position, solid insulator, insulator roughness). The non-linear behaviour is attributed to the inception and quenching of glow corona, in the interval between inception and breakdown or flashover voltages. It is found that a 4% concentration of NovecTM 4710 in CO2 has a breakdown/flashover voltage ≈ 1.14 higher than CO2. The glow corona-induced stabilization effect is seen for pressures between 2 and 5 bar abs for all the studied parameters. The peak flashover voltage and its associated pressure of the different insulators are strongly dependent on surface roughness. At 8.8 bar abs, the flashover voltage level obtained with various materials was ordered as follows: PTFE > PE-UHMW > Epoxy > HDPE(High-density polyethylene).

Clarifications on the Behavior of Alternative Gases to SF6 in Divergent Electric Field Distributions under AC Voltage

“…The breakdown voltage of N2 showed a linear dependence on increasing gas pressure in contrast to electronegative gas/gas mixtures that present nonlinear tendency for the same needle-plane configuration, such as CO2 and 4% C4F7N-96% CO2 [6]. The breakdown occurred exclusively under a negative AC voltage peak for the pressure range.…”

“…To study pure gas/gas mixture pre-breakdown and breakdown characteristics under non-uniform fields, a needle/plane defect model was utilized as described in previous work [6]. The needle was placed between two plane electrodes of Bruce profile to provide homogeneous background electric field conditions.…”

“…On the other hand, for C4F7N/CO2 mixtures [6], the PDs activity under AC voltage varies as a function of pressure and the magnitude of the electric field, including breakdown polarity reversal (PR), when the AC breakdown polarity changes from negative half-cycle to positive half-cycle with the increase of pressure. At the reversal critical pressure (PR pressure), breakdowns occur on positive or negative half-cycles.…”

“…The linear increase is characterized by a constant polarity reversal electric field EPR calculated according to Equation (1), where Vi is the positive PDIV for divergent electric fields and the breakdown voltage for quasi-uniform electric fields, d is the electrode gap. Table 1 reports EPR for different gas/gas mixtures [6][7][8].…”

“…Gas/Gas Mixture EPR (kV/mm) Pressure (bar abs) η SF6 [7,8] 86 7.5 0.17 CO2 [6] 64 2.5 0.0385 3.7% C4F7N-96.3% CO2 [8] 47 9.5 0.33 10% C4F7N-90% CO2 [7] 66 5.5 0.22…”