The distributed power generation system simulating the characteristics of synchronous generators is grid-friendly to the large power grid. However

a large dash current may be generated at the instant the virtual synchronous generator(VSG)is connected to the grid. This paper proposes a pre-synchronization unit based on the virtual power and the secondary frequency and voltage regulation to achieve seamless transform between the off-disconnection and grid-connection of VSG. Accordingly

the influence of inertia

damping and droop coefficients on the frequency regulation characteristics is analyzed in detail. Simulation and experimental results show that the proposed pre-synchronization control is able to achieve seamless transform between different VSG modes

which is simpler than the conventional pre-synchronization control

while having a good active and reactive power tracking performance. This work is supported by National Natural Science Foundation of China(No. 51277150)

Industrial Research Project of Shaanxi Province(No. 2013K07-05)

and Industrial Cultivation Project of Shaanxi Provincial Department of Education(No. 14JF020).