A　universal　controller　is　proposed　in　this　article　to　operate　parallel　inverters　in　both　grid-connected　(GC)　state　and　standalone　(SA)　state　and　ensure　seamless　transfer　between　them　without　reconfiguring　the　control　structure.　The　universal　controller　is　mainly　composed　of　frequency-locked-loop　and　three　cascaded　control　loops:　a　grid　current　loop,　capacitor　voltage　loop,　and　inductor　current　loop.　A　proportional-integral　regulator　is　adopted　in　the　grid　current　loop,　and　a　limiter　is　inserted　after　the　integrator.　In　the　GC　state,　the　proposed　controller　accurately　regulates　the　grid　current　of　an　individual　inverter.　When　islanding　occurs,　the　proposed　controller　can　automatically　convert　from　grid　current　control　to　v_{C}\hbox{　-　}　i_{g}-based　droop　control;　critical　islanding　detection　is　not　needed.　This　universal　controller　has　several　advantages.　First,　it　can　realize　seamless　transfer　of　parallel　inverters;　microgrid　reliability　can　be　guaranteed　in　the　SA　state,　and　power　sharing　can　be　achieved　among　parallel　inverters　without　communication　lines.　Second,　compared　with　droop　control,　it　can　regulate　the　grid　current　accurately　and　output　constant　power　when　the　grid　voltage　fluctuates　in　the　GC　state.　Third,　the　grid　current　harmonics　in　the　GC　state　and　capacitor　voltage　harmonics　in　the　SA　state　can　be　mitigated.　Simulation　and　experimental　results　verify　the　effectiveness　of　the　controller.　©　1986-2012　IEEE.