Under a long-term unipolar DC voltage

insulation failures at the gas-solid interface remain a critical limiting factor for the development of high-voltage

large-capacity DC GIS. Focusing on the gas-solid interface insulation issues of ±550 kV DC GIS basin spacers

we constructed a surface charge measurement platform for real-sized insulators and a charge-induced surface flashover experimental platform. The charge accumulation characteristics of gas-solid interface and the charge-induced surface discharge were investigated. The material modification and structural optimization studies of the insulator were conducted

followed by type test verification. The results reveal that adopting shielding structures

reducing the surface roughness of central conductor

or applying insulating paint can significantly decrease both density and quantity of accumulated charge spots. Homo-charges suppress the surface discharge

whereas Hetero-charges promote it

with a threshold for charge-induced surface discharge identified as 15 μC/m2. The resistivity of the epoxy formulation at 120 ℃ is 7.45×1012 Ω·m

representing an 87% increase compared to that of conventional commercial epoxy resin. With refined design criteria for ±550 kV DC GIS

the tangential electric field under DC superimposed lightning impulse voltage is ensured to be less than 11.1 kV/mm.