A method to characterize the insulating capacity of new insulating gases

New ecologically friendly insulating gases and gas mixtures for gas insulated substations (GIS) must be characterized by defined methods. A suitable method to characterize the insulating capacity under AC voltage stress of gas mixtures based on C
₄
-Fluoronitrile (C
₄
-FN) and CO
₂
compared to SF
₆
and synthetic air is presented with respect to the degree of uniformity and the gas pressure of typical GIS applications, while the AC test voltage does not exceed 100 kV
_rms
. The gas mixtures are produced by the manometric mixing method and the development of the gas components' concentration in a GIS-compartment by natural diffusion and forced flow are determined by gas analyses and electric strength. For different insulating gases and gas pressures, the AC breakdown voltage is measured in an electrode arrangement according to ASTM D2477-07. To eliminate the effect of statistical time lag to discharge inception on the breakdown voltage, the provision of initial electrons at the electrode surface is facilitated by UV-C irradiation. Applying the manometric mixing procedure in a real GIS gas compartment, the establishment of a homogeneous gas mixture by diffusion only could last up to 100 h, depending on gas volume and existing barriers. Ventilation is needed to shorten the necessary conditioning time. The insulating capacity of C
₄
-FN/CO
₂
gas mixtures, SF
₆
and synthetic air under different pressures is determined. When the influence of the statistical time lag is eliminated, the measured AC breakdown voltages are in close agreement to the calculated streamer inception voltages.