Dry Air-insulated 245 kV Dead-Tank Vacuum Circuit-Breaker - Insulation Design and its Verification

While different SF6 alternative solutions have been proposed, the combination of dry air

(80% N2 and 20% O2) insulation and vacuum switching technology is considered as an ideal

SF6 alternative solution, in particular to meet EU F-gas regulation 2024/573, CARB regulation, and the seven requirements evaluating SF6 alternative technology in Japan [1]. Vacuum circuitbreakers (VCBs) using dry air insulation have been successfully applied to medium voltage switchgears around the world, and high-voltage switchgear applications are commercially available up to 420 kV class [2-5].

This paper discusses the most relevant dielectric properties of dry air and their application to the design of a 245 kV dead-tank VCB, and documents performance verification tests conducted on a prototype VCB. The insulation characteristics of dry air for switchgear designs, established through numerous test results, are also introduced. These include breakdown characteristics such as gas pressure dependence, voltage polarity effect, area effect of electrodes, impulse ratio in quasi-uniform electric field, and the characteristics of gas-insulator interface, including partial discharge properties [6-8]. The prototype VCB was tested in the closed position to confirm the dielectric strength between phase and earth. The required performance was verified against the relevant standards IEC 62271-1 / -100 and IEEE C37.04 / C37.09.