Design of an innovative SF6 free ring main unit with direct connection to transformer

This paper presents a design of an innovative and compact SF6 free Ring Main Unit with direct connection to transformer. A compact solution using pure air as insulation medium and vacuum interrupter for current breaking has been developed. To optimise the cost and to keep the operator habit of 3 position switch combining vacuum breaking and disconnector has been designed. This technology is called Shunt Vacuum Interruption (SVI)™. RMU architecture in United Kingdom is standardised for long time with direct connection of transformer at the rear of RMU and cable connection on lateral side enabling a very compact footprint of the complete secondary distribution substation. There is a huge installed base of RMU’s in substations which will have to be replaced in future without changing the dimensions of the substation room. This solution using pure air at 1 Bar relative for 12kV / 95 kV BIL enables to keep the dimensions of SF6 RMU while reaching the same performances and functionalities. Retrofit will be then easy to do in current substations without modifications to the existing civil works. In this paper, a design study including simulation will be presented. This allows us to achieve dielectric performance 95kV BIL in a compact space and 13.1kA −1sec internal arc withstand in kiosk dimensions of 3m × 2.5m × 2.1m. This RMU has passed qualification requirements as mandated by ENA 41-41 and IEC 62271-200. In particular, internal arc performance in very compact kiosk without basement has been passed.