The　internal　equivalent　impedance　of　DFIG　may　change　during　the　system　fault,　which　obviously　impacts　on　the　applicability　of　distance　protection　for　the　outgoing　transmission　line　of　wind　farm.　The　power　source　characteristics　and　fault　transient　characteristics　of　DFIG-based　wind　farm　are　discussed　and　the　action　characteristics　of　phase-comparison　distance　protection　based　on　polarized　voltage　are　analyzed.　The　analytical　results　show　the　time-varying　impedance　circle　characteristic　may　cause　the　improper　action　of　distance　protection　during　the　system　fault　and　the　results　of　RTDS　simulation　prove　the　correctness　of　theoretical　analysis.　©　2016　Automation　of　Electric　Power　Systems　Press.