At present

strategy on the microgrid smooth switching from distribution network is usually designed on premise of microgrid operating in the island state detected. Adopting this strategy will enforce microgrid losing frequency and voltage reference during the transient process between grid-connected and off-grid

which seriously influenced the stability of the micro power grid. In order to achieve a smooth transition from the network

the U/f Droop character of control scheme is improved

and it turns into a P/Q character of control scheme while the mirogrid is grid-connected.By introducing the traditional single droop into a tri-linear curve of variable slope droop

and reasonably combining voltage deviation and voltage change rate（frequency deviatio

n and frequency change rate as well） based on fuzzy control method which needs not detect the state of microgrid

the microgrid can switch smoothly from parallel operation to islanding. The simulation results show that the microgrid can provide positive reactive power to stabilize voltage when the large voltage deviation of the system occurs over national criterion-5%

+

7%; On-and-off grid conversion

the grid frequency fluctuation within ±0.2 Hz

voltage fluctuation within-5%

+

7%

and other control modes can be used as a micro power frequency and voltage reference.