The　solid-state　transformer　(SST)　has　the　ability　to　operate　with　only　two　input　phases　which　can　be　called　single-phase　open-circuit　(SPO)　operation.　With　this　ability,　the　SST　can　maintain　operation　under　permanent　single-phase-to-ground　faults　where　the　fault　phase　must　be　isolated,　providing　a　better　power　supply　sustainability　than　that　of　the　conventional　low　frequency　transformer　(LFT).　In　the　absence　of　research　on　the　SPO　operation　of　an　SST,　this　paper　presents　a　study　of　it.　The　influence　of　SPO　operation　on　the　grid　and　the　SST　itself　is　analyzed.　Under　SPO　state,　the　SST　will　inject　negative-sequence　current　to　the　grid　meanwhile　the　maximum　load　power　will　decline,　thus　the　optimized　power　factor　under　SPO　state　is　unity.　The　maximum　power　of　Δ-SST　is　higher　than　that　of　the　Y-SST　under　the　SPO　state　owing　to　the　Δ　connection.　A　control　strategy　for　the　SST　medium-voltage　cascaded　H-bridge　stage　is　proposed.　SPO　current　control　can　be　achieved　by　controlling　the　positive-sequence　current.　A　proper　negative-sequence　voltage　can　be　injected　to　synchronize　the　cluster　voltages　to　obtain　a　lower　current　total　harmonic　distortion　and　smaller　DC　voltage　ripple.　In　the　Δ-SST,　a　proper　zero-sequence　current　can　be　injected　to　fully　use　the　power　rating　of　the　double-cluster　branch.　IEEE