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. INTRODUCTION Due 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 small- scale 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 [2-5]. 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 diodes of the VSCs. Subsequent to the fault occurrence, the control scheme of the converter turns off the main switches of the VSC (e.g. IGBTs) to protect them against the overcurrent; hence, the VSC operates as an uncontrolled full-bridge rectifier and the fault will be fed from the ac side of the VSC (through the freewheeling diodes paths) [7]. Therefore, fault currents in VSC-based dc networks will have three different components, each with its special characteristics: i) the dc link capacitors discharge current, ii) the cable inductance discharge through the freewheeling diodes, and iii) the ac-grid current [8]. Given this fault current behavior, the conventional protection devices and methods that are used in ac systems are faced with new challenges. For example, the electromechanical circuit breakers (CBs) that are used in ac networks are not fast enough to protect the vital and vulnerable components of the VSCs against the faults in dc networks [9]. This is due to the fact that an adequate protection scheme for the dc microgrids should be able to operate during the capacitor discharge period to prevent the fault current from flowing through the VSC components. In other words, the critical operating time for the protection of a VSC is the beginning of the second component of the dc fault current and before the fault current starts flowing through the freewheeling diodes. As this time is given by the size of the dc-link capacitor, it will be typically very short (in the range of a few milliseconds) [8]. Hence, a relatively faster protection device is required for dc networks [10]. Different types of DCCBs can be found; however, they are more expensive than their ac counterparts, especially in the level of medium voltage. Therefore, it is not economically feasible to use individual DCCBs for all the feeders of a microgrid. Another important aspect is that, although overcurrent relays (OCRs) can provide fast fault detection and fast tripping, coordination of theses relays and providing a selective protection is a challenging task in VSC-based dc systems. This is mainly due to the reason that the dc line reactance is fairly lower than the counterpart ac systems. Therefore, considering the small length of distribution lines, it is difficult to distinguish between faults that occur inside or outside of a protected line [11]. In addition, because of the very fast increment in the dc fault current, it is very difficult to coordinate the conventional time-inverse OCRs in dc networks. Also, the performance analysis of the OCRs operating in dc networks shown in [7] illustrates that there is not enough time interval between the operation of series OCRs to guarantee their coordinated operation. Moreover, the connection of a DG to a distribution feeder can change the power flow and the fault current direction; this may disturb the coordination of the consecutive OCRs. For these reasons, the protection of dc microgrids requires Centralized Protection Strategy for Medium Voltage DC Microgrids Mehdi Monadi, Member, IEEE, Catalin Gavriluta, Student Member, IEEE, Alvaro Luna, Member, IEEE, Jose Ignacio Candela, Member, IEEE, Pedro Rodriguez, Fellow, IEEE