Global　offshore　wind　farm　is　developing　towards　large-scale,　clustered　and　long-distance　deep　offshore.　The　transmission　and　grid　connection　of　large-scale　offshore　wind　farm　with　a　distance　greater　than　100　kilometers　has　become　a　major　focus　of　offshore　wind　energy　development.　A　technical　and　economic　evaluation　method　for　systematically　evaluating　various　grid-connected　methods　is　established　by　analyzing　transmission　limit　and　loss　of　cable　and　other　electrical　equipment,using　the　uniform　annual　value　method　and　considering　equipment　investment　cost,　maintenance　cost,　loss　cost　and　cable　selection.　Taking　a　400　MW　offshore　wind　farm　as　an　example,　the　technical　and　economic　comparison　of　high　voltage　alternating　current　(HVAC),　high　voltage　direct　current　(HVDC)　and　fractional　frequency　transmission　system　(FFTS)　is　carried　out.　The　demonstration　shows　that　FFTS　by　reducing　the　frequency　can　not　only　realize　the　growth　of　the　ampacity　of　cable,　but　also　significantly　reduce　the　charging　current　in　the　AC　cable.　Therefore,　the　transmission　distance　of　FFTS　increases　remarkably,　which　indicates　that　FFTS　has　both　technical　and　economic　advantages　on　large-scale　and　long-distance　offshore　wind　farm　transmission　and　connection　system.　©　2019　Automation　of　Electric　Power　Systems　Press.