Development of an SF6-free AC HV Dead-Tank Circuit Breaker Rated 145 kV, 63 kA

Design and verification testing performed during the development of a mixed gas-insulated AC HV dead-tank power circuit breaker rated $145 \mathrm{kV}, 63 \mathrm{kA}, 4000 \mathrm{~A}, 60 \mathrm{~Hz}$ are described. The circuit breaker utilizes a mixture consisting of carbon dioxide $\left(\mathrm{CO}_{2}\right)$, oxygen $\left(\mathrm{O}_{2}\right)$ and fluoronitrile $(\mathrm{C} 4-\mathrm{FN})$ gases. The development of this breaker required the adaptation of existing gas-insulated technology to the interrupting and insulating characteristics of this gas mixture. The gas mass in the circuit breaker is $40 \%$ of that of an SF6-insulated breaker and the GWP of the gas mixture is $1.3 \%$ of SF6 GWP. Breaking tests were performed under terminal, transformer limited, short-line, and out-of-phase fault conditions. Breaking service capability was also demonstrated. Capacitive switching tests were performed for a C2 Class with voltage factor of 1.7. Dielectric tests included power frequency withstand, BIL, chopped wave and AC withstand at atmospheric pressure. Mechanical endurance tests achieved M2 class with 10,000 operations. Current carrying capability tests were performed at 3000 and $4000 \mathrm{~A}$. The circuit breaker main current path resistance in new condition and after reaching the electrical endurance was stable. This technology demonstrated similar performance to SF6 with nearly the same material content, dimensional characteristics, and without requiring excessive operating pressure. Material content has significant implications for the equipment’s climate change impact, compensating for the favorable emission impact of competing technologies using technical air insulation with zero GWP.