To guarantee a smooth transition from islanded mode to grid-connected mode

synchronization is the first issue to be considered. In this paper

a phase locked loop (PLL)-based active synchronization control approach is presented

which is implemented in the secondary control level of the microgrid hierarchical control structure by actively controlling the microgrid voltage to track the main grid. In this way

it can reduce the voltage difference to get the microgrid back into grid in minimum transient state. With the cross product of voltage vectors employed as the phase detector

the microgrid voltage frequency and phase are enabled to track the main grid at the same time. In addition

instead of fast communication to transmit signals in the time domain

the proposed method only needs a low-bandwidth communication link

which is simple and easy to implement. Finally

hardware-in-the-loop simulation and experiment results show the feasibility of the approach proposed.