Dirt on surface of GIS insulator may cause partial discharge（PD）or even flashover. However

until now there are no better methods but passively collecting PD signals to evaluate the dirt status. This study sets up a 220 kVfinite element GIS model according to the distortion characteristic of an ultrasonic signal spreading through medi-ums

and utilizes the sweep frequency method（20

5

00 kHz）to reveal the signal distortion when an ultrasonic wavespreads through the surface of GIS insulator

so as to evaluate dirt status on the insulator. Two kinds of dirt

dust andwater

are designed

and the influences of dirt type

dirt position and insulator structure on the ultrasonic signal distortion are analyzed. This study ma

y provide a theoretical reference for corresponding experiments