Serious　corona　discharges　occur　on　some　grading　rings　of　500-kV　compact　transmission　lines　in　China　owing　to　defects　in　the　structural　design.　The　noise　of　the　corona　is　heard　clearly　even　50　m　away　from　the　tower.　In　order　to　solve　this　problem,　we　have　optimized　the　structure　of　the　grading　rings　to　suppress　the　corona.　In　this　study,　the　grading　rings　that　had　serious　corona　discharges　were　identified　and　observed　with　an　ultraviolet　imager　(UVI),　and　the　electric　field　(E-field)　distributions　of　the　grading　rings　were　calculated　by　means　of　the　three-dimensional　finite　element　method.　Based　on　an　analysis　of　the　areas　on　the　grading　ring　where　excessive　high　E-field　magnitudes　were　observed,　we　optimized　the　structures　of　the　grading　rings　to　improve　the　field　distributions　on　their　surfaces.　Based　on　the　optimization　suggestions,　new　grading　rings　were　manufactured　to　replace　the　old　ones.　UVI　observations　showed　that　the　new　grading　rings　could　effectively　prevent　the　occurrence　of　corona　discharge.　(C)　2014　Institute　of　Electrical　Engineers　of　Japan.　Published　by　John　Wiley　&　Sons,　Inc.