This　article　proposes　a　systematic　control　strategy　for　solid-state　transformer　(SST)　composed　of　dual　active　bridge　(DAB)　and　cascaded　H-bridge,　which　increases　SST　efficiency　over　a　wide　operating　range　in　PV　application.　In　real　practice,　the　PV　panel　presents　a　wide　voltage　range,　which　could　significantly　affect　SST　efficiency,　mainly　for　DAB.　To　improve　the　efficiency,　a　universal　loss　model　for　DAB　under　frequency　domain　analysis　is　established.　Based　on　this,　the　optimum　efficiency　modulation　scheme　is　derived.　Besides　the　efficiency　improvement　of　DAB　from　the　modulation　scheme,　this　article　fully　explores　extra　control　freedom,　which　is　an　intermediate　voltage　between　DAB　and　H-bridge.　Then,　the　variable　intermediate　voltage　control　scheme　is　proposed,　which　reduces　the　conduction　loss　and　switching　loss　of　DAB.　Furthermore,　zero-sequence　modulation　signal　injection　and　the　proportional-resonant　controller　is　implemented　in　the　proposed　control　strategy,　which　expands　the　range　for　further　efficiency　improvement.　Finally,　to　verify　the　effectiveness　of　the　proposed　method,　the　experiment　is　carried　out　on　the　submodule　and　10-kV/1-MW　SST　real　scale　prototype.　Under　the　proposed　method,　the　SST　system　achieves　0.9%　efficiency　improvement　compared　with　the　conventional　method.　©　1986-2012　IEEE.