The valve-side bushing of ultra-high voltage (UHV) converter transformers is a key component in DC power transmission systems

playing a critical role in ensuring the safe and stable operation of the power grid. However

in recent years

surface flashover failures have frequently occurred at the SF₆-solid insulation interface of valve-side bushings

and the surface charge distribution at this interface directly affects insulation performance. Therefore

understanding the charge accumulation characteristics at the SF₆-solid interface is essential for maintaining the safe and stable operation of valve-side bushings. In this study

a charge measurement system was developed to assess the gas-solid interface of a scaled epoxy-impregnated paper model of a valve-side bushing. Surface charge distributions were measured under DC voltage in an SF₆ environment

including both short-term and long-term behaviors. Experimental results reveal that two charge distribution patterns appear under DC voltage

namely

a fundamental pattern determined by the bushing structure

and a unipolar spot pattern influenced by surface conditions. Finally

simulation analysis was conducted to investigate the effects of solid current

gas current

and surface current components on surface charge accumulation.It is concluded that the surface current components is the main influencing factor of the core model surface charge accumulation.