## 1 Introduction  
Security, reliability, and economy are the main basic requirements for the operation of the electric power system. With economic growth driving a gradual increase in electricity demand, electric power systems have recently introduced advanced grid or new energy technologies to satisfy these demands. In many advanced electric grid technologies, the microgrid (_u_grid) system is one of most important applications, acting as a controllable localized electricity supplier for providing reliable energy to area demand facilities, promoting energy savings, minimizing carbon emissions, and reducing electricity bills for electricity users [1; 2]. The major components of _u_grid systems include distributed/renewable energy resources, different types of energy storage systems (ESSs), grid-connected and islanding operation mechanisms, and various real-time monitoring and management/control methods. According to developed _u_grid technologies and market segments, the types of state-of-art _u_grid systems can be categorized as follows. The first category refers to the campus _u_grid. The University of California, San Diego (UCSD) and Center of Illinois Institute of Technology (IIT) have developed _u_grids in this category [3; 4]. Secondly, there is the military _u_grid. Sandia National Laboratories [5] have led a Smart Power Infrastructure Demonstration for Energy Reliability and Security (SPIDERS) Joint Capability Technology Demonstration (JCD) program that plans a three-phase _u_grid demonstration site at military bases in Hickam, Fort Carson, and Camp H.M. Smith, respectively. Thirdly, there are commercial and industrial (C&I) _u_grids. Mesa del Sol in NewMexico has created a commercial-scale _u_grid through a Sandia National Laboratories and Japan New Energy and Industrial Technology Development Organization (NEDO) collaboration to offer a clean slate for testing and demonstrating the latest _u_grid technologies [6]. Fourthly, remote (off-grid) and island _u_grids are shown for example by the University of Chile, which developed Chile's first _u_grid project in a remote area called Huatacondo [6]. This _u_grid system uses a social supervisory control and data acquisition (Social-SCADA) approach to assist the information exchange among electric system users, and then further improve the operation and maintenance of the system. Another category is comprised of community and utility _u_grids, seen in three practical applications in Taiwan:  
* Xinglong Public Housing in [7] was constructed by the Taipei City Government using various energy management system designs, electric vehicle (EV) charging station integrations, demand response control, and advanced metering and monitoring technologies to realize a smart house scenario in a community _u_grid (C-_u_grid). * The Kaohsiung Xiaolin Village C-_u_grid in [8] was supported by a project of the Department of Industrial Technology (DoIT) of the Ministry of Economy Affairs (MOEA), Taiwan. This village suffered from Typhoon Morakot in 2009, and most of electric grid infrastructures were severely damaged. To raise the electrification in this remote village, a C-_u_grid system was thus planned and installed. It integrates C-_u_grid management control, and uses large amounts of solar power and retired-battery ESSs to provide power supply to the electricity users in village. Even if the utility electric grid fails to supply power, electricity in village continues to be provided and will not be interrupted due the capability of C-_u_grid system operating in islanding mode. * The Kinmen Dongkeng C-_u_grid [9], as shown in Figure 1, is an upgrade of Taiwan's first ESS-based _u_grid system at Kinmen Kinshui elementary school, extending from one _u_grid user scenario in the original system to a scenario with fifteen community users. The system is also supported by the DoIT project. The operational purpose of this system is to demonstrate C-_u_grid integration technology in a low-voltage distribution system. The seventh category comprises other _u_grids.