Surface coating affecting charge distribution and flashover voltage of cone-type insulator under DC stress

In order to scale down the dimensions of gas insulated line (GIL) insulators and to improve their reliability at ultra high voltage (UHV), considerable research has been conducted to raise the flashover voltage of the insulators. Usually, a non-uniform electric field distribution is an important factor triggering flashovers. Under AC and impulse voltages, the application of functionally graded materials (FGM) with spatial distribution of dielectric permittivity (ε-FGM) has been shown to be an effective solution. But at DC, the electric field distribution depends not only on the conductivity of the dielectrics but also the charge distribution on the surface. Based on the cone-type insulator, this paper reports on the surface charge accumulation and DC flashover of epoxy (EP)/graphene (GR) coated insulators in air. The EP/GR coated insulators with different filler amounts were prepared to test their surface charge accumulation and flashover characteristics. The results show that the uniformly distributed homopolar surface charge is helpful in reducing the peak electric field thereby raising the flashover voltage. The 0.1% EP/GR composite has the slowest surface potential decay process. Accordingly, the 0.1% coated insulator has the highest flashover voltage among the other EP/GR coated insulators and uncoated insulator. The bipolar surface charge regions caused by wire-type metal particles distort the electric field distribution. Flashovers of the 0.1% coated insulator occurs at lower voltage than the uncoated insulator when a wire-type particle attaches to the surface.