I. Introduction
Direct Current (DC) microgrids are drawing continually increasing attention over Alternating Current (AC) microgrids due to their advantages in higher efficiency, high reliability, reduced losses, and natural integration with renewable energy sources and energy storage resources [1]. With the recent advances in power electronics technologies, converter-interfaced DC microgrids are becoming more prevalent while posing a number of significant different control challenges. The challenges include stability issues due to the low-inertia characteristics of power electronics-based generations as well as uncertainties affecting the microgrid topology regarding system expandability, flexibility, and plug-and-play (PnP) functionality of distributed generation (DG) units. The plug-and-play operation of DG units affects the topology and global model of DC microgrids. However, the local loads in DC microgrids have to be stabilized without retuning the microgrid control system, in the absence of any communication link. Under the PnP functionality of DG units, classical control approaches which rely on a global model of the microgrids need to retune their control system in order to guarantee the stability of DC microgrid with a new topology. Therefore, due to importance of the plug-and-play problem in flexible and expandable microgrids, it is required to develop scalable control strategies for microgrids, where the design of a local controller for a DG unit is not based on the knowledge of the whole microgrid and the complexity of local control design is independent of the microgrid size. In a plug-and-play control system, the flexibility in the structure of the microgrids is reflected in the control architectures. As a result, when the microgrid topology changes, the control structure can be updated while ensuring the stable operation of the microgrids. A significant challenge in DC microgrids under the plug-and-play operation of DG units is to ensure voltage stability by means of the decentralized control of each DG unit [2].