energies Review State of the Art of Low and Medium Voltage Direct Current (DC) Microgrids Maria Fotopoulou , Dimitrios Rakopoulos * , Dimitrios Trigkas, Fotis Stergiopoulos, Orestis Blanas and Spyros Voutetakis Chemical Process and Energy Resources Institute, Centre for Research and Technology Hellas, Thermi, GR-57001 Thessaloniki, Greece; [email protected] (M.F.); [email protected] (D.T.); [email protected] (F.S.); [email protected] (O.B.); [email protected] (S.V.) * Correspondence: [email protected]; Tel.: +30-210-6899-689 Abstract: Direct current (DC) microgrids (MG) constitute a research field that has gained great attention over the past few years, challenging the well-established dominance of their alternating current (AC) counterparts in Low Voltage (LV) (up to 1.5 kV) as well as Medium Voltage (MV) applications (up to 50 kV). The main reasons behind this change are: (i) the ascending amalgamation of Renewable Energy Sources (RES) and Battery Energy Storage Systems (BESS), which predominantly supply DC power to the energy mix that meets electrical power demand and (ii) the ascending use of electronic loads and other DC-powered devices by the end-users. In this sense, DC distribution provides a more efficient interface between the majority of Distributed Energy Resources (DER) and part of the total load of a MG. The early adopters of DC MGs include mostly buildings with high RES production, ships, data centers, electric vehicle (EV) charging stations and traction systems. However, Citation: Fotopoulou, M.; the lack of expertise and the insufficient standards’ framework inhibit their wider spread. This review Rakopoulos, D.; Trigkas, D.; paper presents the state of the art of LV and MV DC MGs in terms of advantages/disadvantages Stergiopoulos, F.; Blanas, O.; over their AC counterparts, their interface with the AC main grid, topologies, control, applications, Voutetakis, S. State of the Art of Low ancillary services and standardization issues. Overall, the aim of this review is to highlight the and Medium Voltage Direct Current possibilities provided by DC MG architectures as well as the necessity for a solid/inclusive regulatory (DC) Microgrids. Energies 2021, 14, framework, which is their main weakness. 5595. https://doi.org/10.3390/ en14185595 Keywords: DC microgrid; architectures; applications; ancillary services; standards Academic Editors: Aditya Shekhar and Laura Ramírez Elizondo Received: 4 August 2021 1. Introduction Accepted: 30 August 2021 In electrical microgrids (MG), as in all sectors of modern technology and applica- Published: 7 September 2021 tions, the need for sustainability in terms of reducing the energy footprint is considered to be a major priority. In fact, according to the European Union (EU) targets of 2020, the Publisher’s Note: MDPI stays neutral greenhouse gas emissions need to be reduced by at least 55% by 2030, compared to 1990 with regard to jurisdictional claims in levels [1]. In order for such goals to be achieved, the reduction in fossil fuel-based energy published maps and institutional affil- production is required. As an alternative, Renewable Energy Sources (RES) have proven iations. to be a solution with minimal environmental impact of vital importance. Photovoltaic (PV) systems, wind generators (WG), biomass and geothermal installations have pene- trated the market over the past few decades, improving the energy mix that covers the electricity demand [2,3]. Nevertheless, a major drawback of many RES, is the intermittent Copyright: © 2021 by the authors. production, due to the sources that they utilize. In order for the production to meet the Licensee MDPI, Basel, Switzerland. demand curve, the utilization of Energy Storage Systems (ESS) is considered to be an This article is an open access article effective solution. ESS typically include Battery Energy Storage Systems (BESS), flywheels, distributed under the terms and compressed air systems, etc. [4]. The most common, widely utilized ESS technology is conditions of the Creative Commons the BESS, with advantages such as high controllability, fast response and geographical Attribution (CC BY) license (https:// independence [5]. Subsequently, for sustainability-related reasons, the combination of creativecommons.org/licenses/by/ distributed RES (especially PV systems and WGs) with BESS has created a new field of 4.0/). Energies 2021, 14, 5595. https://doi.org/10.3390/en14185595 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 26 Energies 2021, 14, 5595 2 of 26 has created a new field of research and development, promoting the decarbonization, autonomy and cost efficiency of MGs [6]. researchHowever, and development,the increasing promoting integration the decarbonization,of RES and ESS autonomy in the current and cost energy efficiency mix does notof only MGs result [6]. to the rise of eco-friendly energy supply, but also to the rise of proliferation of DC systems,However, i.e., the DC increasing generation integration and DC of RESstorage and units. ESS in In the fact, current some energy of the mix most does widely not only result to the rise of eco-friendly energy supply, but also to the rise of proliferation utilized RES and ESS, such as PV and BESS, originally produce DC power (either as of DC systems, i.e., DC generation and DC storage units. In fact, some of the most widely currentutilized or voltage RES and sources), ESS, such which as PV is and then BESS, converted originally to AC produce power DC through power DC/AC (either as power electronicscurrent or converters voltage sources), in order which to be is theninjected converted to the toAC AC distribution power through grid. DC/AC Additionally, power the sameelectronics phenomenon converters is observed in order on to bethe injected side of toenergy the AC demand. distribution More grid. specifically, Additionally, DC loads includingthe same electric phenomenon vehicles is observed(EVs), Light on the Emitting side of energy Diode demand. (LED) systems, More specifically, DC motors, DC data centersloads includingand other electric battery-based vehicles (EVs), devices Light Emittinghave penetrated Diode (LED) the systems, market DC following motors, an ascendingdata centers curve and [7,8]. other Yet, battery-based these devices devices too haveincorporate penetrated special the market converters following that anconvert ACascending into DC power curve [ 7in,8]. order Yet, theseto function. devices tooAlso, incorporate one should special also convertershave in mind that convertthat most of AC into DC power in order to function. Also, one should also have in mind that most the electronics loads and devices are based on DC power. Taking the above facts into of the electronics loads and devices are based on DC power. Taking the above facts into consideration,consideration, it itis is evident evident that traditionaltraditional AC AC distribution distribution needs needs to copeto cope with with the new the new developments,developments, and and innovative innovative DC distributiondistribution challenges challenges its dominance,its dominance, as presented as presented in in FigureFigure 1.1 . Figure 1. The transition from AC MGs to DC MGs. Figure 1. The transition from AC MGs to DC MGs. Up until recently, DC power systems have been utilized mostly in High Voltage (HV) applications,Up until recently, predominantly DC power for the systems purpose have of powerbeen utilized transmission mostly over in longHigh distances, Voltage (HV) applications,due to the low-power predominantly losses, high-powerfor the purpose quality of and power cost efficiencytransmission that theover High long Voltage distances, dueDirect to the Current low-power (HVDC) losses, transmission high-power has qualit to offery and in such cost applications. efficiency that This the has High been Voltage a Directmilestone Current of DC (HVDC) power systems,transmission as presented has to in offer the work in such of [9– 13applications.]. Nevertheless, This since has the been a milestonenecessary of high-voltage, DC power systems, high-power as andpresented efficient in power the work electronics of [9–13]. and DCNevertheless, cables have since only been developed during the past decades, the advantages of DC power systems have the necessary high-voltage, high-power and efficient power electronics and DC cables not been fully exploited in most sectors, especially at the Low Voltage (LV) (indicatively haveup only to 1.5 been kV) developed and Medium during Voltage the (MV) past levels decades, (indicatively the advantages up to 50 of kV), DC leaving power thesystems havedominance not been of powerfully systemsexploited to ACin architectures.most sectors, Yet, especially the modern at challenges the Low for Voltage efficient (LV) (indicativelyintegration ofup RES to 1.5 in the kV) LV and and Medium MV levels Voltage have brought (MV) DC levels power (indicatively to the spotlight. up to Low 50 kV), leavingVoltage the Direct dominance Current of (LVDC) power as systems well as Mediumto AC architectures. Voltage Direct Yet, Current the modern (MVDC) challenges MGs forconstitute efficient aintegration modern field of ofRES research in the and LV development and MV levels and have foundbrought applications DC power in ato the spotlight.variety ofLow fields. Voltage However, Direct there Current are still (LVDC) obstacles as thatwell the as DCMedium MGs have Voltage to overcome Direct Current in (MVDC)order to MGs be widely constitute adopted, a modern most significant field of of research