1949-3053 (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TSG.2017.2737595, IEEE Transactions on Smart Grid > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—In this paper a seamless interchange method between interconnected and islanded operational mode of a DC Microgrid is presented. The DC Microgrid examined consists of photovoltaic generation, batteries and loads. A novel islanding detection method is presented ensuring fast detection of the islanding condition. The proposed detection method is based on the insertion of a controllable load in parallel with the DC Microgrid central switch. To achieve seamless interconnection to the utility grid, a proportional-integral (PI) regulator is installed in the Microgrid central controller (MGCC). The PI regulator sends to the voltage forming unit (batteries) the appropriate setpoint, in order to align the voltage of the DC Microgrid to the utility network voltage during reconnection. The performance of the proposed control strategy is evaluated through simulations and Control Hardware-in-the-loop experiments performed at the Electric Energy Systems Laboratory of National Technical University of Athens. Index Terms—Control Hardware-in-the-loop (CHIL), DC Microgrids, islanding detection method, seamless transition. I. INTRODUCTION n the last few years, the question on whether to use AC or DC electrical power systems attracts significant interest. This is shown by the efforts of different committees and standardization bodies (e.g. Emerge Aliance, International Electrotechnical Commission, IEEE Standards Association, etc.) on identifying new areas of standardization to be undertaken concerning DC electrical systems [1], [2]. Many of the factors that made AC technologies in transmission and distribution power systems more advantageous than DC are now debatable [3]. For instance, in the past decades, DC power at low voltage could not be transmitted over longer distances without increased losses. Nowadays, due to significant developments in the power- electronics domain, this issue is not a major problem. Moreover, DC-based power systems offer relative advantages compared with AC [3], such as efficiency improvement and cost reduction due to fewer DC-AC and AC-DC conversions for the connection of DC energy resources. Although, Low Voltage DC (LVDC) power systems have been widely used in domains like marine, aerospace and automobility [4], [5], only recently the concept of LVDC in distribution power systems has been investigated. Two of the main challenges impeding the widespread use of LVDC in distribution power systems are the lack of standards as well as safety/protection issues [6], [7]. Although LVDC distribution systems have not reached the state of maturity for their wide commercialization however, significant research is done in many institutes around the world [8], [9], [10], [11], [12]. Towards this direction, a number of pilot sites employing LVDC distribution networks have been built. For example, in Finland a 1,7 km long terrain- isolated ±750 V DC network has been constructed feeding actual electricity end-users, in order to investigate the operation of a fully functional LVDC system [13]. In Japan, a demonstration facility has been built in Fukuoka City consisting of a hybrid AC-DC distribution system with a 380V DC bus interconnecting loads, batteries, photovoltaics and a wind turbine [14]. In recent years, the application of DC technologies in Microgrids has attracted a lot of research interest [15], [16], [17], [18] as well as, due to the DC nature of the interconnected distributed energy resources (DER), like batteries, PVs, etc. and the high penetration of power electronic interfaces. According to their definition Microgrids are electricity distribution systems containing loads and distributed energy resources, (such as distributed generators, storage devices, or controllable loads) that can be operated in a controlled, coordinated way either while connected to the main power network or while islanded [19]. Microgrids differ from traditional distribution networks in terms of control and coordination of loads and all the available generation and storage resources, so that they appear to the upstream network as a single coordinated unit. Microgrids control is often organized in a hierarchical structure comprising three main control levels, primary, secondary and tertiary control [15], [18], [20], [21], [22], [23]. DER droop control based on local measurements is mostly applied as the first level of control, in order to stabilize the system during the first periods after a power mismatch. During the islanded mode of operation voltage and frequency stability of an AC Microgrid is achieved by the power dispatched by DERs. In interconnected mode, DERs deliver their power as dictated by their Maximum Power Point (MPP) curve, while power mismatches are mainly covered by the utility grid [15], [18], [20], [21], [22], [23]. The second level of the hierarchical control is responsible for voltage regulation and frequency regulation (applicable to AC) and can be coordinated by the Microgrid central controller (MGCC) or performed decentralized by local DER controllers. It is slower than the primary control and normally Seamless transition between interconnected and islanded operation of DC Microgrids Vasilis A. Kleftakis, Dimitris T. Lagos, Christina N. Papadimitriou, and Nikos D. Hatziargyriou, Senior Member, IEEE Ι