The　arm　parameters　of　the　upper　and　lower　arm　of　modular　multilevel　converters　(MMCs)　are　not　always　symmetric　in　the　practical　projects.　In　fact,　there　will　be　some　differences　inevitably.　Under　this　unbalanced　condition,　there　is　an　interaction　between　the　internal　circulating　current　and　the　AC　side　current.　A　theoretical　analysis　of　MMCs　under　the　asymmetric　arm　impedance　conditions　is　carried　out　to　find　the　characteristics　of　the　AC　side　output　current.　In　this　case,　a　DC　component　will　appear　on　the　AC　side　and　the　expression　for　the　bias　current　is　achieved.　The　DC　component　will　lead　to　a　DC-bias　problem　on　transformers.　A　simulation　model　is　built　with　PSCAD/EMTDC,　which　verifies　the　influence　on　the　AC　side　of　MMCs　under　the　asymmetric　arm　parameter　conditions.　©　Springer　Nature　Singapore　Pte　Ltd.　2020.