Recent studies have demonstrated that fluoronitrile (FN) NovecTM 4710 and fluoroketone (FK) NovecTM 5110 show higher dielectric strength than SF6. These gases can be mixed with a buffer gas such as CO2 and technical air to have suitable dielectric properties for high voltage insulation applications. With the rapid growth of HVDC networks, these gases may replace SF6 in future HVDC apparatus such as GIS/GIL. This paper focuses on the study of the partial discharge (PD) characteristics of SF6 and SF6 alternative gases with a protrusion defect on the high voltage conductor with DC voltage. The results show that, for calculated equivalent SF6 dielectric strength under homogeneous electric field, a difference in terms of partial discharge inception voltage was noted for gas mixtures compared to SF6. This difference may be due to the influence of gas pressure under highly inhomogeneous electric field. FN-CO2 and FK-Air mixtures showed the highest PD amplitude under positive polarity while it was higher under negative polarity for SF6. Moreover, PD repetition frequency is higher for SF6 investigated alternative gases. The results can help for the design of SF6 free HVDC GIS partial discharge monitoring system.

C. Toigo, T. Vu-Cong, F. Jacquier and A. Girodet