To meet the requirements of large capacity and long distance transmissions

in recent years

the processof UHV DC transmission projects accelerates in China

the demand for gas-insulated metal-enclosed transmission line（GIL）becomes urgent. But one of the key barriers is the insulator surface charge accumulation may reduce the sur-face flashover voltage

so it is necessary to study the surface charge accumulation effect on UHV DC GIL insulator.Therefore

based

on the positive and negative ions transport equations in SF

6

gas

a simulation model is establishedto investigate the surface charge accumulation characteristics of insulator in UHV DC GIL test unit of Chinese EPRI.From the simulation results

both positive and negative charges accumulate on the inner surface and the outer surfaceof the cone-type spacer

but the charge distribution of each surface is different. And the surface charge density is close-ly related to the magnitude of the applied voltage

when the -800 kV voltage is applied

the maximum positive chargedensity +19.64 μC·m

-2

and the maximum negative charge density-22.93 μC·m

-2

are on the outer surface and inner sur-face of the insulator

respectively. Results also show that the negative withstand voltage of the insulator is lower.