Abstract-1 This paper presents a centralized protection strategy for medium voltage dc (MVDC) microgrids. The proposed strategy consists of a communication-assisted fault detection method with a centralized protection coordinator and a fault isolation technique that provides an economic, fast, and selective protection by using the minimum number of dc circuit breakers (DCCBs). The proposed method is also supported by a backup protection which is activated if communication fails. The paper also introduces a centralized self-healing strategy that guarantees successful operation of zones that are separated from the main grid after the operation of the protection devices. Furthermore, to provide a more reliable protection, thresholds of the protection devices are adapted according to the operational modes of the microgrid and the status of distributed generators (DGs). The effectiveness of the proposed protection strategy is validated through real-time simulation studies based on the hardware in the loop (HIL) approach.Index Terms-Adaptive protection, centralized protection, smart dc microgrids. I. INTRODUCTIONDue to the increasing penetration of DGs, especially in the form of renewable energy systems (RES), the concept of microgrids has been proposed as a method for DG integration into the electrical grids. Microgrid is a common concept in both ac and dc systems and is defined as a smallscale low or medium voltage grid consisting of loads and DGs. Such a system is capable of operating in both islanded and grid-connected modes [1]. Because of the advantages of the dc networks over the ac grids, and also because of the new developments in the technology of voltage source converters (VSCs), nowadays there is a major interest in dc grids in both research and industrial realms [2][3][4][5].At the present moment, protection is one of the most important challenges in the development of dc microgrids. Protection issues mainly arise due to the particular behavior of the fault current in VSC-based networks [6]. When a fault occurs in a dc grid, firstly, the dc-link capacitor is discharged causing the voltage of the main dc bus to drop precipitously. Then, the energy stored in the cable This work was supported in part by the Spanish Ministry of Economy and Competitiveness under Project ENE2013-48428-C2-2-R. The work of M. Monadi was supported by the Ministry of Science, Research, and Technology, Iran.M. Monadi is with Technical University of Catalonia (UPC) Barcelona, Spain and Shahid Chamran University of Ahvaz, Ahvaz, Iran (e-mail: meh_monadi@yahoo.com).C. Gavriluta is with the Grenoble Electrical Engineering Laboratory (G2ELab), France (email: catalin.gavriluta@g2elab.grenoble-inp.fr).A. Luna, J. I. Candela are with Technical University of Catalonia (UPC) Barcelona, Spain. (e-mails: luna@ee.upc.edu, candela@ee.upc.edu) P. Rodriguez is with Technical University of Catalonia (UPC) Barcelona, Spain and Abengoa research, Sevilla, Spain (e-mail: prodriguez@ee.upc.edu).inductance is also discharged through the freewheeling dio...

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Voltage source converters (VSCs) are highly vulnerable to DC fault current; thus, protection is one of the most important concerns associated with the implementation of multi-terminal VSC-based DC networks. This paper proposes a protection strategy for medium voltage DC (MVDC) distribution systems. The strategy consists of a communication-assisted fault location method and a fault isolation scheme that provides an economic, fast and selective protection by means of using the minimum number of DC circuit breakers (DCCBs). This paper also introduces a backup protection which is activated if communication network fails. The effectiveness of the proposed protection strategy is analyzed through real-time simulation studies by use of the hardware in the loop (HIL) approach. Furthermore, the effects of fault isolation process on the connected loads are also investigated. The results show that the proposed strategy can effectively protect multi-terminal DC distribution networks and VSC stations against different types of faults.