Design of virtual inductance for droop-controlled inverter with seamless transition between islanded and grid-connected operations

A seamless transition method has been presented for a droop-controlled inverter to transfer between islanded and grid-connected operations. A so-called virtual inductance and local phase-locked-loop (PLL) were proposed to realize this transition with no grid synchronization. This paper presents designing of variable virtual inductance to suppress inrush current, including exponentially-decaying, ramp-decaying, and constant virtual inductances. In addition, phase error of the PLL and the transient of the droop control are included. Simulations and preliminary experiments are provided.