By taking the peer-to-peer structure microgrid as the research object

a seamless transfer control method for primary control and secondary control is proposed. In the primary control part

the microgrid inverters are controlled in droop mode in both microgrid island mode and grid connected mode. The transient response can be reduced since there's no need to switch the control algorithm during the microgrid mode transfer period. In the secondary control part

a grid synchronization method based on hierarchical control is proposed

which allows droop-controlled inverters to adjust the frequency

phase

and amplitude of their output voltages to synchronize with the grid

realizing smooth transition from islanded mode to grid connected mode. The inverter control algorithm

frequency restoration and pre-synchronization control algorithm are analyzed in detail

experimental test results presented to validate the effectiveness of the proposed method.This work is supported by National Natural Science Foundation of China(No. 51307042

No. 51407057)and Anhui Provincial Natural Science Foundation of China(No. 1408085QE97).