For　further　understanding　creepage　flashover　in　converter　transformers,　we　studied　the　creepage　flashover　under　oil-paper　insulation　of　two　electrode　structures　which　were　in　the　presence　and　absence　of　the　back　electrode,　and　adopted　a　power　supply　for　composite　AC　and　DC　voltage　to　provide　an　adjustable　percentage　of　DC　voltage.　The　results　showed　that　the　flashover　voltage　in　the　presence　of　the　back　electrode　was　lower　than　that　in　the　absence　of　the　back　electrode.　Furthermore,　for　both　electrode　structures,　there　was　the　lowest　flashover　voltage　under　pure　AC　voltage,　and　the　flashover　voltage　increased　with　DC　component　of　the　composite　voltage.　Besides,　to　find　out　the　effect　of　interface　of　oil-paper　insulation,　flashover　voltages　of　pure　oil　under　the　same　electrode　structures　were　also　studied.　The　breakdown　voltage　of　pure　oil　was　compared　with　that　of　oil-paper　insulation.　The　comparison　showed　that,　when　DC　component　was　relatively　low,　the　breakdown　voltage　was　decreased　by　the　interface　of　oil-paper,　yet　it　would　be　increased　by　the　interface　if　the　DC　voltage　component　was　large　enough.　From　composite　AC　and　DC　electric　field　point　of　view,　when　DC　component　was　low,　there　would　be　larger　electric　filed　around　the　electrode,　leading　to　the　oil　molecules　ionizing　that　produced　a　large　amount　of　current　carriers,　and　then　promoted　the　development　of　creepage　flashover.　However,　with　the　increase　of　DC　component,　the　electric　field　distribution　would　change　and　the　creepage　flashover　voltage　would　increase.　At　the　same　time,　the　electric　field　distribution　mentioned　above　also　caused　the　fact　that　the　flashover　voltage　was　lower　than　breakdown　voltage　when　DC　component　was　relatively　low.