The valve side windings of converter transformer withstand complex voltages

including AC and DC combined voltage. Highly damped oil-paper insulation system is respected as the predominant factor of surface dischargephenomena. The present study focuses on the surface discharge characteristics of oil-immersed pressboards with dif-ferent moisture content

whereas different AC and DC components are applied to the sphere-plane defect model. Dis-charge parameters and patterns are acquired by detector using pulse current method. Results show that the inceptionand flashover voltage decrease with the increase of moisture content

and gradually increase under higher DC compo-nent voltage. As the damped state in oil-paper sample gets more serious

discharge tolerance time is shorten sharplyand higher DC component has a more obvious accelerated effect on the surface discharge. The whole developing pro-cess can be divided into four stages. The amplitude Q of surface discharge increases while the time interval Δt decreases at high moistened state.