Negative and positive partial discharge inception voltages and breakdown measurements are reported in a needle-plane electrode system as a function of pressure under AC voltage for natural gases (N₂, CO₂, and O₂/CO₂), pure Novec^TM gases (C₄F₇N and C₅F₁₀O) and Novec^TM in different natural gas admixtures. For compressed 4% C₄F₇N–96% CO₂ and 6% C₅F₁₀O–12% O₂–82% CO₂ gas mixtures, the positive-streamer mode is identified as the breakdown mechanism. Breakdown and negative partial discharge inception voltages of 6% C₅F₁₀O–12% O₂–82% CO₂ are higher than those of 4% C₄F₇N–96% CO₂. At 8.8 bar abs, the breakdown voltage of 6% C₅F₁₀O–12% O₂–82% CO₂ is equal to that of 12.77% O₂–87.23% CO₂ (buffer gas). Synergism in negative partial discharge inception voltage/electric field fits with the mean value and the sum of each partial pressure individually component for a 20% C₄F₇N–80% CO₂ and 6% C₅F₁₀O–12% O₂–82% CO₂, respectively. In 9% C₄F₇N–91% CO₂, the comparison of partial discharge inception electric fields is E_max (CO₂) = E_max(C₄F₇N), and E_max (12.77% O₂–87.23% CO₂) = E_max(C₅F₁₀O) in 19% C₅F₁₀O–81%(12.77% O₂–87.23% CO₂). Polarity reversal occurs under AC voltage when the breakdown polarity changes from negative to positive cycle. Polarity reversal electric field E_PR was quantified. Fitting results show that E_PR (CO₂) = E_PR(9% C₄F₇N–91% CO₂) and E_PR(SF₆) = E_PR (22% C₄F₇N–78% CO₂). E_PR (4% C₄F₇N–96% CO₂) = E_PR (12.77% O₂–87.23% CO₂) and E_PR (6% C₅F₁₀O–12% O₂–82% CO₂) < E_PR (4% C₄F₇N–96% CO₂) < E_PR (CO₂).

AC pre-discharge currents, breakdown, and flashover voltage measurements are reported in a 10 mm needle-plane arrangement in a 4% Novec^TM 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 Novec^TM 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).