Abstract
People began to pay more attention to the development of renewable energy technologies because of the rapid climate changes. Therefore, micro-grids and smart-grids are substituted for the small-scale traditional power plant to reduce the dependence on oil fuel. Both are applications of the distributed generation system (DGS). Micro-grids system contains many kinds of renewable energy resources. The renewable energy resources are connected to the ac power grid through distributed interface converters to improve the reliability of energy supply. However, when the AC power grid fails, micro-grids must be disconnected from the AC power grid. When the AC power grid has been recovered, it would be reconnected. Therefore, in the process of mode transforming, the current transient must be alleviated to avoid excessive power transients. In this research, the droop control method is adopted for distributed interface converters to operate in islanded mode or grid-connected mode. For reducing the power transient, the central controller is used to monitor the grid frequency. Moreover, the central controller also estimates the differences of the voltage magnitude and voltage phase between AC power grid and Micro-girds. Then, the central controller sends estimated values through the RS232 to the controller of distributed interface converters. The latter would adjust the voltage command according to received information. In order to verify the proposed method, the simulation results and experimental results would be analyzed and discussed.