Control techniques and the mathematical model of microgrids

With the ever-increasing demand of electricity, traditional power systems with centralized generation and long transmission lines are facing unprecedented challenges in their capacity and reliability. Updating the existing network may be not feasible or economically not viable, especially for remote growing communities with the costs of construction and maintenance of the transmission lines considerably high. The idea of a more flexible electric grid with Distributed Energy Resources (DERs) geographically close to the power demand led to the conceptualization of a Microgrid. Despite its recently gained popularity, the definition of Microgrid however is still somehow ambiguous. In this Chapter we define Microgrid as a geographically close cluster of power sources, loads and power devices that can function either in grid-connected or islanded mode. These power sources can be fuel-based generators or renewable energy sources, usually coupled with electronic converters and inverters. Battery Energy Storage Systems (BESSs) coupled with bi-directional inverters can also be included in a Microgrid in order to cope with surplus and shortage in electric power generation. In the first instance, the top priority of a Microgrid is to maintain voltage and frequency stability of electric power supplied to the loads, using the available resources and data; however, taking advantage of the flexibility of the DERs and BESSs, the optimal power flow problem in terms of power losses and generation costs can be addressed. The performances and the optimization objectives of a grid-connected and an islanded Microgrid are similar; in fact, an islanding procedure for a grid-connected Microgrid in case of dangerous disturbances or faults in the main grid is a self-preserving strategy that has been widely adopted. For a geographically remote Microgrid that is far from the main grid but close to other Microgrids, coupling with one or multiple Microgrids can be implemented as a healing strategy to maintain and reinforce the voltage and frequency stability of the Microgrid. Microgrid controllers, with centralized, decentralized and distributed topologies, must drive the system through the normal operation and contingency stages to ensure stability and optimality. This work is devoted to provide a critical review of fundamental knowledge and theories underpinning the formation of Microgrids, and techniques and strategies that have been proposed in recent years for the purpose of maintaining their stability. The categorization of Microgrid system topology will be detailed, which will be followed by a hierarchical classification of the control techniques for different objectives targeting different types ofMicrogrids, where the advantages and drawbacks of each control methodology will be elaborated. At the same time, a practical and simplified mathematical model of Microgrid will be derived and presented, for the purpose of simulating and comparing the performances of diverse controlmethodologies inMicrogrids integrated with renewable energy sources. Lastly, discussions of current trends and future work in Microgrid research will also be presented in this Chapter.