When microgrid connected to main power grid is operating， the breakdown at the main network side or microgrid side may cause protection acting to trip. Then the main power source will operate switching from PQ mode to V/f mode rapidly along with tripping of the breaker at interconnection point. The seamless switchover cannot achieve， because of communication speed limitation between master controller and slave controller. In order to solve this problem， adopting industrial real-time Ethernet EtherCAT bus to replace traditional Ethernet bus for increasing communication speed between controllers is proposed. A scheme to develop master controller and slave controllers with EtherCAT interfaces and using EtherCAT bus as communication bus to connect controllers in order to build up a coordinate and control system of microgrid is also given. The method proposed enhances the real-time of master-slave controlling strategy， and realizes the seamless switchover of microgrid operation mode from grid-connected mode to islanding mode when microgrid is in breakdown.