By combing the ultrasonic online monitoring and the measurement of discharge voltage value and SF6 decomposition products

four discharge failures in 550 kV GIS during withstanding AC voltage field test are located and analyzed. The main causes of the flashover on cone-type spacer surface are discussed. The relationship among surface flashover

partial discharge of defects

and surface charge transport are investigated. It is concluded that:1)the metal particles on the surface of conevtype spacer tend to initiate negative corona and accumulate negative surface charge

thus to intensify the distortion of the surface electric field and supply the required charge for discharge channel propagation; and 2)when the contacts between cone-type spacer and electrodes is insufficient

the surface discharge originates from the dielectric barrier corona at triple-junction points

while the corona propagation and the charged particles’ drift and diffusion movement become the precondition to generate surface discharge. In addition

some suggestions are offered

concerning GIS field assemblage

ultrasonic location of failure

and the related experimental and theoretical researches.