In　order　to　further　develop　the　detection　and　diagnosis　measures　of　partial　discharges　(PDs)　in　gas　insulated　switchgear(GIS),　we　proposed　to　adopt　the　inner　grading　electrode　inside　insulting　spacer　as　an　ultra-high　frequency(UHF)　sensor　for　PDs.　Using　the　finite　difference　time　domain　(FDTD)　method,　we　studied　the　main　waveguide　modes　of　GIS　and　the　frequency　response　characteristics　of　the　inner　grading　electrode.　Then,　we　used　the　proposed　UHF　sensor　to　test　the　PD　characteristics　of　three　typical　defects,　and　compared　the　sensor　with　commercial　UHF　sensors　in　aspects　of　detection　characteristics.　The　simulation　results　show　that　the　TEM　mode　component　of　the　GIS　model　is　the　largest　and　the　TE11　follows　when　there　is　PD　in　the　GIS.　The　cutoff　frequencies　of　high-order　waveguide　modes　distribute　around　250　MHz~1　GHz,　and　there　are　a　number　of　resonance　frequencies　below　3　GHz　for　the　grading　electrode　UHF　sensor,　while　the　largest　resonance　frequency　appears　at　about　140　MHz.　The　experimental　results　show　that　the　PD　detection　sensitivity　of　the　grading　electrode　UHF　sensor　is　much　higher　than　that　of　external　sensors,　however,　it　is　slightly　lower　than　that　of　interior　sensors.　The　main　frequency　spectra　of　both　metal　protrusion　on　high　voltage　conductor　and　floating　potential　defects　distribute　below　1.5　GHz,　which　is　consistent　with　the　simulation　of　the　cutoff　frequencies　of　high-order　waveguide　modes.　According　to　the　experiments,　the　largest　resonance　frequency　of　grading　electrode　UHF　sensor　is　about　115　MHz,　close　to　the　simulation　value,　140　MHz.　©,　2015,　Science　Press.　All　right　reserved.