I. Introduction
Because of excellent insulation and mechanical properties, silicone rubber (SR) has become a common insulating material in high voltage electrical equipment. The high hydrophobicity and unique hydrophobic recovery of SR prevent the formation of a water film on its surface, thereby reducing the occurrence of pollution flashover. HVDC transmission has been widely used in the long-distance transmission of electric energy and access to new energy because of its advantages, such as low loss, large transmission capacity, and easy realization of grid interconnection. However, HVDC systems have to meet many challenges and problems at present, compared with HVAC systems. In the outdoor environment, electrostatic adsorption is more likely to occur under DC high voltage, which makes it easier for pollutants in the air to be deposited on the surface of the insulator, so flashover is more likely to occur. The accumulated charge on the surface of the insulator will lead to the distortion of the surface electric field and reduce the flashover voltage, which is affected by the polarity and distribution of the accumulated charge [1].