Dielectric and thermal challenges for next generation ring main units (RMU)

Environmental concerns related to the greenhouse effect of SF₆ have driven changes to take place in the power distribution industry. This paper discusses the main challenges for next generation medium voltage (MV) ring main units (RMU) using new technologies and materials with reduced environmental impact. Market requirements are going in the direction of equal or even higher technical ratings for new products. Replacing SF₆ with any other insulation gas in RMU requires innovative solutions to be implemented. A key challenge is to be able to maintain the outer physical dimensions of the unit, as this imposes strict conditions on the dielectric and thermal performance. Dielectric design of SF₆ free RMU targets the distribution of electrical fields within the unit, aiming to reduce the field strength of weak points to compensate for the reduced dielectric strength of alternative insulating gases. Key parameters for optimization include choice of insulating materials, geometrical shape of conducting surfaces and definition of conductor/insulator interfaces. Thermal design is further critical due to the lower thermal properties of alternative insulating gases. Computational Fluid Dynamics (CFD) analysis is used to understand and optimize the temperature distributions inside the switchgear. Simulation results are validated by temperature rise tests in full scale prototypes. The main functionality of next generation RMU relies on optimized dielectric and thermal design in order to provide a cost efficient and reliable unit. In this paper a selection of techniques are discussed with references to both simulations and full scale tests based on the challenging boundary condition of keeping the same physical dimensions as an existing SF₆ product.