Very Low SF6 Insulation Gases for Medium Voltage Switchgear

Due to global warming concerns, switchgear with lower global warming potential (GWP) may become desirable. This work investigates using very low (<10%) levels of sulfur hexafluoride (SF₆) along with common gases for insulating switchgear. Design of experiments (DOE) is used to isolate significant factors and to generate a model, an empirical equation for predictive use. For calculating GWP of gas mixtures, a weighted average calculation is made. Results show that mixtures of 0-5% SF₆ and carbon dioxide (CO₂) may be sufficient for 15 kV class equipment, and 3-8.5 % SF₆ with CO₂ may be sufficient for 25 kV. In this work, CO₂ was more effective than nitrogen. Using the weighted average calculation for GWP, these blends compare favorably with the automotive air conditioner refrigerant (R-134a, GWP = 1,300) in common use today. R-134a is approved by international agreement for automotive use to 2030(1). The resulting DOE model proved to have excellent predictive power.