Medium-Voltage Switchgear Low Temperature Dielectric Performance when Using Alternative Insulating Gas to SF6

The use and emissions of SF6 are increasingly being reduced and regulated. There are green designs for S&C Vista® Underground Distribution Switchgear (Vista) that use a mixture of C4 Fluoronitrile gas and Carbon Dioxide (CO2) instead of SF6. These designs were tested for Power Frequency Withstand (PFWS) and Basic Impulse Level (BIL) at temperatures at and below $\mathbf{-30^{\circ}C}$, the prevailing standard for underground distribution switchgear. The calculated Dew Point of the mixture is $\mathbf{-37.2^{\circ}C}$. The results indicate that the gear meets PFWS at $\mathbf{-50^{\circ}C}$ for all voltages, and BIL at $\mathbf{-50^{\circ}C}$ for $\mathbf{15 kV}$ class and at $\mathbf{-30^{\circ}C}$ for $\mathbf{25kV}$ and $\mathbf{35 kV}$ class. BIL at $\mathbf{-50^{\circ}C}$ for 27 and 38 KV classes require a 10% derating.