## 1 Introduction  
Microgrid (MG) systems have been a topic of interest in the electric power industry worldwide. They effectively integrate a mix of solar, wind, and other Renewable Energy Sources (RES). An MG consists of distributed energy generation units, loads and storage systems interconnected through transmission lines and transformers. It can be operated in two modes, either connected to utility grid mode or islanded mode. In the connected mode, the MG gets energy from its own generation units, but in case of power shortage, power will be provided from the main grid. The surplus power can be sold to the main grid as well. In the islanded mode, the MG operates as an independent power system. MG can be implemented in different areas, ranging from large buildings to isolated rural villages. Figure 1 presents a diagram of the MG system. The Point of Common Coupling (PCC) is the main Circuit Breaker (CB) that is used to switch the mode of operation between grid-connected and islanded modes. With several roles and purposes supported by MG, many studies have been made, with emphasis on the MG as a promising field of research [1, 2, 3]. An overview of the MG system as an essential key in smart energy networks is presented in Reference [4]. Further, an advanced and functional architecture of MG is proposed by Reference [5], with details on obstacles that may face MG, especially during the transition between MG operation modes. A list of existed MG projects over the globe are given in Reference [6]. For instance, Fort Carson in Colorado Springs is one of several MG projects underway on United States bases under the SPIDERS (Smart Power Infrastructure Demonstration for Energy Reliability and Security) program. The SPIDERS MG is composed of existing assets, a 1 MW PV array and three diesel generators with a total power of 3 MW, and five electric vehicles. The MG project is intended to keep a group of central base facilities operating without grid power as an island, in the event of grid failure. The Sendai MG project was carried out in Japan by New Energy and Industrial Technology Development Organization (NEDO) between 2005 and 2008. The MG showed excellent performance during the 2011 earthquake and tsunami through supplying the teaching hospital of Tohuku Fukushi University with both power and heat for the duration of the two-day blackout. The MG project in Mannheim-Wallstadt, Germany, was implemented by the state utility company in 2006. The main goal of this project was to develop a true MG able to swiftly and smoothly switch from grid-connected mode to island mode. The system is addressed to residential and commercial units and loads. Power systems resiliency indicates its capability to cope with the faults and any abnormal conditions that could lead to a power blackout. It also allows recovery of the system in order to retrieve its normal operation. The novelty of this paper lies in introducing the level of resilience (LoR) as a measure to compare and evaluate different MG topologies modeled using MATLAB during a fault/attack scenario. The LoR is determined by the sag voltage percentage, the reduction in load served, and the recovery time. Another merit used in determining the LoR is the time needed to reach the power balance state during the absence of the main grid (islanded mode). In such a mode, we studied the effect of the load shedding technique as a strategy to enhance the resilience of MG under attack. It is shown that the placement of MG resources can affect MG resilience, as given by LoR metric. Thus, using this metric, other designs can be investigated to obtain a best-case resilience using the given set of resources. The rest of the paper is organized as follows: Section 1.1 presents the latest related work. The topologies and the architecture of the proposed MG are illustrated in Section 2. Section 3 presents the resiliency measure formulation. Section 4 illustrates the resiliency enhancement using Load Shedding technique. Experimental results are shown in Section 5.