With　a　large　amount　of　distributed　generation　connected　to　the　medium　and　low　voltage　distribution　network,　three-phase　power　flow　calculation　is　the　basic　means　to　analyze　their　influence.　The　existing　research　on　three-phase　power　flow　for　a　medium-voltage　ungrounded　distribution　network　mainly　adopts　the　method　of　manually　setting　the　reference　neutral　point　or　zero-sequence　voltage.　This　makes　it　difficult　to　simulate　and　analyze　the　neutral　point　or　zero-sequence　voltage　offset,　and　the　zero-sequence　power　flow　and　the　imbalance　of　phase　to　ground　voltage.　Thus　this　paper　proposes　a　new　three-phase　power　flow　algorithm　for　a　medium-voltage　ungrounded　distribution　network.　First,　the　particularity　of　three-phase　power　flow　for　this　network　in　terms　of　node　variables　and　admittance　matrix　is　analyzed.　Second,　based　on　a　power　balance　equation　and　the　Newton-Raphson　method,　a　three-phase　power　flow　algorithm　for　an　ungrounded　distribution　network　with　constrained　zero-sequence　current　is　designed　by　using　admittance　matrix　reversible　processing　and　matrix　replacement　technology.　Finally,　the　power　flow　for　the　ungrounded　distribution　network　is　simulated　based　on　the　6-bus　system　and　the　modified　IEEE123　test　case.　The　accuracy　of　the　proposed　method　in　different　scenarios　and　its　applicability　and　convergence　in　large　power　grids　are　verified　by　PSCAD,　and　a　practical　application　is　carried　out　in　local　power　grids.　The　example　analysis　shows　that　the　proposed　algorithm　gives　accurate　calculation　results　and　good　engineering　practicability.　©　2023　Power　System　Protection　and　Control　Press.　All　rights　reserved.