Gas　insulated　switchgear　(GIS)　is　widely　used　in　power　system.　To　improve　its　reliability,　field　impulse　voltage　withstand　test　is　an　important　step.　Standard　lightning　impulse　(SLI)　is　normally　adopted　in　factory　withstand　test　for　GIS　equipment.　However,　oscillating　lightning　impulse　(OLI)　is　often　used　in　field.　Differences　exist　between　OLI　and　OSI　concerning　their　capability　of　insulation　assessment　and　defect　exposition.　Sharp　protrusions　or　particles　on　HV　conductor　are　typical　defects　in　GIS　and　are　critical　at　lightning　impulse　voltages.　This　paper　therefore　aims　at　breakdown　characteristics　study　of　protrusion　on　HV　conductor　under　SLI　and　OLI.　Firstly,　the　impulse　voltage　generator　was　established　to　generate　SLI　and　OLI.　They　had　the　same　front　time　and　time　to　half　value.　Secondly,　a　25mm-long　protrusion　defect　on　HV　conductor　was　set　up　in　a　330kV　GIS　chamber.　And　then　the　50%　probability　breakdown　voltage　(BDV50)　and　Voltage-time　characteristics　(V-t　characteristics)　were　experimentally　studied.　The　results　show　that　for　positive　and　negative　polarity,　BDV50　under　OLI　is　0.4%　and　10.9%　higher　than　that　under　SLI　respectively.　V-t　plots　under　SLI　show　a　steeper　downward　slope　than　that　under　OLI.　Breakdowns　under　positive　OLI　concentrate　in　the　vicinity　of　the　first　wave　peak,　while　under　negative　OLI,　breakdowns　concentrate　in　the　first　three　peaks　and　minimal　breakdown　voltage　(V-min)　occurs　in　the　third　peak.