“…Nanoparticles were treated in references [15,78,79], others were untreated. The sizes of samples in [75,77,78,79] There are several possible mechanisms to explain the improvement on the PD resistance of nanocomposites, such as the Multi-core Model [13,83], the Skeleton Model (fillers like skeleton dividing matrix into small parts), the Size Effect of Nanoparticles [48], Pile and Rearrangement of Fillers at Surface [77,82], Aggregative State of Space Charge [12,[84][85][86], the Effect of the Field Homogenization and High Thermal Conductivity and Insulation Effect of Fillers [87]. However, the characteristic and function of the region between matrix and fillers are still hard to describe precisely.…”