In humid conditions

such as heavy fog

water particles in the air will attach to the surface of the insulator and condense gradually. The electric field in and around the insulator will be distorted by these densedroplets. These densedroplets may form several water bandsunder the influence of an electric filed force to further distortthe electric field

inducing a vicious cycle. Our simulation results show that on this occasion the electric field intensity between the metal fittings on the high-voltage side and the first umbrella skirt is high enough to trigger corona discharge

resulting in aging or even destruction of the insulator surface and reduction of the safety and reliability of the insulator in operation. In view of this situation

in order to assure operation safety of high-speed train

this paper choose the roof composite insulator of CRH-380 electric high-speed train unit as research object. Optimal grading rings are installed to the insulator to improve the electric field distribution on the high-voltage side. Finite element simulation shows that the maximum field strength is 19.7 kV/cm when the optimal parameters of the grading ring are R=125 mm

r=21 mm

and H=44 mm. This maximum field strength is less than water droplet corona onset field strength

hence the expected goal is achieved.