Since 1994 there is a lot of research effort put into the investigation of nanocomposites and their applications in various fields of high voltage engineering. Among the possible most interesting applications is the field of underground cable insulation. Given the fact that XLPE was proven to be a reliable insulating material in the past decades, modern efforts concentrate into the possible applications of the said material enriched with small wt% of nanoparticles. The samples used were polymer nanocomposite insulating materials. The polymeric material was XLPE, while SiO2 and Al2O3 were used as nanoparticles. The samples were XLPE with 0 wt% nanoparticles and XLPE with 0.2 wt%, 0.5 wt% and 1 wt% of SiO2 nanoparticles. Furthermore XLPE with 0.2 wt%, 0.5 wt% and 1 wt% of Al2O3 nanoparticles were investigated, as well. In the context of the present work, surface discharges as well as flashover voltages were investigated with a variety of XLPE nanocomposites, namely XLPE with various wt% of SiO2 and Al2O3 nanoparticles. A uniform electrode arrangement was used for the experiments, in which AC voltage was applied. Since surface discharges and flashover voltages are strongly influenced by humidity, various water droplet arrangements of different conductivities were positioned on the sample surface. The influence of water droplet positioning, droplet conductivity and number of droplets is investigated. It seems that the above parameters play a decisive role on surface discharges and flashover voltages and that the samples of XLPE with SiO2 nanoparticles give the most encouraging results regarding the flashover voltage. © 2020 Institute of Electrical Engineers of Japan. All rights reserved.