Lightning　impulse　withstand　voltage　test　is　conducted　to　examine　the　dielectric　strength　of　turn-To-Turn　insulation　under　the　circumstance　of　uneven　voltage　distribution,　but　large　entrance　capacitance　of　ultrahigh-voltage　transformers　makes　it　difficult　to　generate　standard　lightning　impulse　(SLI)　in　the　on-site　test,　especially　for　the　front　time.　In　this　paper,　to　evaluate　the　feasibility　of　substituting　test　impulses　with　longer　front　time　for　SLI,　the　transient　voltages　of　the　outmost　turn　of　each　disc　of　the　1000kV　transformer　windings　under　impulses　with　different　front　time　were　measured　by　the　coupling　capacitive　sensor.　The　results　showed　that,　for　the　low-voltage　winding　and　the　high-voltage　winding,　whose　high　voltage　leads　were　located　at　the　top　and　middle,　respectively,　the　maximum　voltage　distributions　were　almost　independent　of　the　front　time;　while　for　the　mid-voltage　winding,　whose　lead　was　located　at　the　bottom,　the　maximum　voltage　distribution　changed　as　the　front　time　varied.　For　all　three　windings,　the　front　time　of　applied　impulse　had　a　little　effect　on　the　frequency　components　of　the　transient　voltages,　which　could　significantly　influence　the　dielectric　behavior　of　oil-paper　insulation.　Based　on　those　results,　the　equivalence　between　the　SLI　and　impulse　with　longer　wavefront　was　discussed　and　so　was　the　effect　of　very　fast　transient　overvoltage　on　transformers.　©　2018　IEEE.