Fractional　frequency　transmission　is　an　emerging　technology　for　the　delivery　of　large-scale,　far　and　deep-sea　offshore　wind　power,　but　the　reliability　of　an　offshore　wind　power　fractional　frequency　delivery　system　has　not　yet　been　fully　studied.　This　paper　analyzes　the　reliability　of　offshore　wind　power　systems　based　on　three　aspects:　the　offshore　wind　farms,　the　long-distance　power　transmission　submarine　cables,　and　the　converter　stations.　First,　the　protection　zone　model　and　the　enumeration　calculation　process　are　established　for　the　collector　system　to　perform　the　reliability　analysis　in　the　wind　farm　level.　Second,　the　aging　of　the　fractional-frequency　cables　is　analyzed　through　the　electromagnetic　thermal　coupling　simulation,　and　a　method　for　evaluating　the　reliability　of　the　long-distance　power　transmission　submarine　cables　considering　the　operating　status　is　proposed.　At　the　same　time,　the　modularized　layered　method　is　used　to　model　the　reliability　of　the　frequency　conversion　station,　and　the　reliability　parameters　of　the　frequency　conversion　station　system　are　obtained.　By　integrating　the　reliability　analytic　results　of　the　wind　farms,　the　frequency　conversion　stations,　and　the　submarine　cables,　the　outage　table　and　grid-connected　output　distribution　of　the　test　case　fractional　frequency　system　is　constructed.　Finally,　the　reliability　weaknesses　of　the　system　and　measures　to　improve　the　reliability　are　analyzed,　and　two　reliability　reference　indicators　are　proposed.　The　evaluation　results　show　that　the　reliability　model　and　the　method　proposed　in　this　paper　are　effective　and　feasible,　and　they　are　suitable　for　the　reliability　evaluation　of　the　offshore　wind　power　fractional　frequency　systems.　©　2022　Power　System　Technology　Press.　All　rights　reserved.