High Power Microwave Breakdown of C4F7N (Novec 4710) Gas Mixtures

Sulfur Hexafluoride’s (SF6) high dielectric strength makes it heavily relied upon as a high voltage insulating medium. However, its atmospheric lifetime of 3,200 years and overall high global warming potential has motivated a search for alternatives. One possible alternative, C4F7N (NOVEC 4710) features a dielectric strength more than twice that of SF6 yet features an atmospheric lifetime one-hundredth that of SF6. Previous research concerning breakdown characteristics of C4F7N has been primarily limited to long-timescale DC and 60 Hz AC conditions. In this study, the breakdown strength of gas mixtures comprised of 5%, 10%, and 20% C4F7N in a N2 or CO2 background were tested under pulsed, microsecond high power microwave (HPM) conditions over a range of 16 psia to 32 psia at room temperature. To achieve this, a 3 MW S-band (2.85 GHz) high-power magnetron is used to excite a traveling-wave ring resonator, which provides a nominal effective HPM power level of ~20 MW. A stepped impedance transformer is used to further increase the electric field to approximately 160 kV/cm RMS. For comparison, the reported dielectric strength of C4F7N is compared against the dielectric strength of pure SF6.

I. Introduction

Sulfur hexafluoride (SF₆) features many desirable characteristics, namely very high dielectric strength and reasonable materials compatibility, which make it the preferred gas in high voltage, high electric field environments. However, SF₆ also features an exceptionally high global warming potential, which has resulted in numerous restrictions on its use and plans to totally phase out the use of SF₆ over the next decade or so. However, many systems are reliant on SF₆ for operation, and without an alternative, will be totally unable to operate. Recently, novel carbon fluoro-nitriles, namely C₄F₇N (Novec 4710™, henceforth “Novec”) has shown promise as an SF₆ alternative, offering very high dielectric strength in DC and low frequency (~60 Hz) applications (e.g. [1, 2]). However, to the knowledge of the authors, the dielectric strength of C₄F₇N has not been tested at microwave frequencies. In this proceeding, we report the experimental dielectric strength of C₄F₇N in binary mixtures with N₂ or CO₂ (up to 20% C₄F₇N molar concentration), tested using a high power microwave (HPM) system, operating at 2.85 GHz, for multi-microsecond pulse durations.