Nowadays Distributed Energy Resources (DERs) are used extensively. They have eliminated lots of power system problems. However, high penetration of DERs can cause other problems like increasing power system complexities and malfunction of protection schemes. Traditional approaches for integrating DERs focus on the impacts on grid performance of one, two, or a relatively small number of DERs. Also these approaches concern ensuring that interconnected DERs will shut down automatically if problems arise on the grid. By contrast, the Microgrid concept represents an entirely new approach to integrating DERs. A Microgrid would be designed to seamlessly separate or island from the grid, and reconnect to the grid once its problems are resolved. The Microgrid concept assumes a cluster of loads and Distributed Energy Resources operating as a single controllable system that provides both power and heat to its local area. In this thesis, various Microgrids with different structures that have been implemented in the world are briefly described. One of these Microgrids is selected as the test grid of this thesis. Operation of Microgrids consisting of converter based DERs is also analyzed and their modes of operation is discussed. Properties of a grid-connected Microgrid are described. Specifically, it is explained why energy resources in a grid-connected Microgrids are preferred to operate in PQ mode. Time-domain simulations of a grid-connected Microgrid are used to analyze the operation of DERs controllers. An important mode of Microgrid is islanded mode. This mode acts as a stimulus to reliability increasing. In this mode, control system has some new responsibilities. These responsibilities are voltage control, frequency control and load sharing. The concept of load sharing and frequency synchronization is explained. Two droop characteristics used to share the active/reactive power between DERs are described and compared, and the requirements for implementing each method are discussed. Time-domain simulations are used to verify the operation of islanded Microgrids with different droop characteristics. This thesis is concerned with the modeling and analysis of islanded Microgrids and the effect of different parameters on their performance. The small signal analysis of an islanded, converter-based, Microgrid is presented. In doing so, the Microgrid is divided to some sections and different sections of it, including static converter and filter, controllers and the network are modeled separately. Finally these models are connected together and the aggregated model of the Microgrid is obtained. By introducing some perturbation in the small signal model, it is shown that the results are very close to thords: Distributed Energy Resources, Microgrid, static converter, load sharing, conventional droop characteristic, inverse droop characteristic.
