R

D

Aa

b

a

ARRAA

KDDDPSS

C

h0

Electric Power Systems Research 119 (2015) 407–417

Contents lists available at ScienceDirect

Electric Power Systems Research

j o ur na l ho mepage: www.elsev ier .com/ locate /epsr

eview

C microgrids and distribution systems: An overview

hmed T. Elsayeda, Ahmed A. Mohamedb, Osama A. Mohammeda,∗

Energy Systems Research Laboratory, Department of Electrical and Computer Engineering, Florida International University, Miami, FL, USAGrove School of Engineering, Department of Electrical Engineering, City College of the City University of New York, NY, USA

r t i c l e i n f o

rticle history:eceived 16 August 2014eceived in revised form 15 October 2014ccepted 16 October 2014vailable online 15 November 2014

eywords:C distributionC standards

a b s t r a c t

This paper presents an overview of the most recent advances in DC distribution systems. Due to thesignificantly increasing interest that DC power systems have been gaining lately, researchers investigatedseveral issues that need to be considered during this transition interval from current conventional powersystems into modern smart grids involving DC microgrids. The efforts of these researchers were mostlydirected toward studying the feasibility of implementing DC distribution on a given application, DCdistribution design-related aspects such as the system architecture or its voltage level, or the uniquechallenges associated with DC power systems protection and stability. In this paper, these research effortswere categorized, discussed and analyzed to evaluate where we currently stand on the migration path

esignrotectiontabilitymart grid

from the overwhelming fully AC power system to a more flexible hybrid AC/DC power system. Moreover,the impediments against more deployment of DC distribution systems and some of the proposed solutionsto overcome those impediments in the literature will be discussed. One of the obstacles to increasedDC system penetration is the lack of standards. This problem will be discussed, and the most recentstandardization efforts will also be summarized and presented.

© 2014 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4082. Motivation for DC systems reconsideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408

2.1. DC loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4082.2. Renewable energy sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4082.3. Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4082.4. Data centers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4082.5. Plug-in electric vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4092.6. DC microgrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409

3. Feasibility of DC distribution systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4094. Design of DC distribution systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4105. Stability of DC distribution systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4116. Protection of DC distribution systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4127. Standardization efforts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4128. Existing DC distribution systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413

8.1. There are several power systems that typically employ DC distribution. Some of these systems include, Spacecraft . . . . . . . . . . . . . . . . . . . . 4138.2. Data centers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413

8.3. Telecommunication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.4. Traction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.5. Shipboard power systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author at: 10555 West Flagler Street, Room 3983, Miami, FL 33174, USE-mail address: mohammed@fiu.edu (O.A. Mohammed).

ttp://dx.doi.org/10.1016/j.epsr.2014.10.017378-7796/© 2014 Elsevier B.V. All rights reserved.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414

A. Tel.: +1 305 348 3040; fax: +1 305 348 3707.

408 A.T. Elsayed et al. / Electric Power Systems Research 119 (2015) 407–417

8.6. Experimental setups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4149. Conclusion and future work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414

. . . . . .

1

etWsdimtttMfiopiFh

vprswrtoattpwt

2dsaatmtedtpsw

2

eaIDt

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

The turn of the 20th century witnessed a fierce battle over howlectricity would be generated, transmitted and utilized. This bat-le, famously known as the “War of currents,” was waged by G.

estinghouse and N. Tesla supporting AC on one side, and T. Edi-on, leading proponents of DC, on the opponent side. Obviously, theebate ended by predominant implementation of AC distribution

n the vast majority of our power systems, due to reasons that madeuch sense at that time. One of these reasons was the invention of

he transformers which offered a great and simple means to step uphe voltage, and consequently widen the area covered by a distribu-ion system, while changing DC voltage levels was an impediment.

oreover, the invention of poly-phase AC machines helped peoplend an alternative to DC machines, which had remained the onlyption for some time back then. However, DC systems did not com-letely disappear from the distribution scene. For instance, there

s an old system used by Pacific Gas and Electric (PG&E) in Sanrancisco to feed variable speed DC-motored elevators in severalistoric buildings [1].

The advances achieved in power electronics, which made DColtage regulation a simple task, in addition to the increasingenetration of DC loads and sources encouraged researchers toeconsider DC distribution for at least portions of today’s powerystem to increase its overall efficiency. In this paper, the authorsill present an exhaustive literature survey and overview of the

esearch efforts conducted on several issues such as the design, con-rol, operation, stability and protection of DC systems. The objectivef the paper is to give an integrated background about what haslready been achieved in these areas, by giving details about theopics and/or guidance on where to find further information abouthem. The paper also attempts to develop a simplified conceptualath to the newly researchers in the field of DC power systems onhat the challenges of DC systems are and how their peers tackled

hem.The remainder of this paper is organized as follows; in Section

, the reasons for reconsidering DC distribution are classified andetailed. Section 3 provides some of the feasibility studies pre-ented in the literature. In Section 4, the issues and challengesssociated with the design of DC power systems are addresseds well as some of the proposed solutions and design techniqueshat can be found in the literature. Sections 5 and 6 highlight the

ost recent and significant efforts done on the stability and pro-ection of the DC systems respectively. Section 7 summarizes thexisting standards and standardization efforts done toward a well-efined standard for DC systems. A brief description of some ofhe currently existing DC power systems and their applications isresented in Section 8. Finally, Section 9 presents the main conclu-ions that can be derived from this survey and the required futureork.

. Motivation for DC systems reconsideration

Recently, dealing with DC power systems became significantlyasier due to the stunning advent of semiconductor technology,

nd the continuous developments of power electronic converters.n addition, there are several other valid reasons for rethinking ofC deployment. These reasons can be classified as reasons related

o the loads, sources and storage elements.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414

2.1. DC loads

Many of today’s consumer loads are DC supplied. Electronicbased office and home appliances, such as computers, laptops,tablets, phones, printers, TVs [2], microwave ovens [3] and lighting,consume electricity in DC form [4–7]. Newer more-efficient lightingtechnologies such as compact fluorescent fixtures and solid-statelighting involve a DC stage and hence it is more efficient to uti-lize them in a DC distribution system [8,9]. DC power is used inVariable Speed Drives (VSD) for pumps, Heating, Ventilation andAir Conditioning (HVAC) systems, fans, elevators, mills and trac-tion systems. In addition, for industrial applications, steel industryis employing more DC electric arc furnaces since they consumeless energy than their corresponding AC ones and cause less lightflicker [10]. Electrochemical industry is almost pure DC application[11,12]. Supplying these loads through the predominant AC dis-tribution systems adds conversion stages and consequently, addsinefficiencies to the delivery chain. According to [13], nearly 30%of the generated AC power passes through a power electronic con-verter before it is utilized. The amount of lost energy varies, butgenerally it lies within the range of 10–25% [14]. In another study[15], the authors mentioned that the power conversion efficiencycan be increased by about 8% if a DC-bus system is used and furthersavings of around 25% can be achieved as a result of removing onerectifier and one PFC stage.

2.2. Renewable energy sources

Motivated by environmental and economic conditions, there isa global trend toward more utilization of renewable energy sources(RES). Some of the RES are natively DC, such as photovoltaic (PV)and fuel cells (FC). In case of offshore wind turbines, which areintegrated to the AC grid through a DC link [16,17], convertingthe distribution system to DC can eliminate a conversion stage,and consequently increase the efficiency. Microturbines generatinghigh-frequency AC are also easier to connect to a DC system.

2.3. Storage

One of the great benefits of DC microgrids is their inheritedcapability of facilitating static storage integration. Most of stor-age elements are purely, DC such as batteries and ultra-capacitors.Moreover, flywheels, even though they are mechanical energystorage systems, are mostly coupled to a permanent magnet syn-chronous machine (PMSM) that is integrated to the distributionsystem through a DC link [18,19]. A study carried out by NipponTelegraph and Telephone Corporation (NTT), a Japanese telecom-munication company, to compare between an AC uninterruptiblepower supply (UPS) and a DC one, from availability perspective,shows that the reliability of DC supply is higher [20].

2.4. Data centers

The main feature that must be maintained in a data center powersystem is high reliability [20–22]. Therefore, data centers are typi-cally equipped with Uninterruptable Power Supplies (UPS), which

require multiple conversion stages to connect the batteries to aDC bus. These conversion stages create losses that can be avoidedif the power is distributed in DC form [23]. Consequently, energycost, which contributes to around 20% of the total operating cost

System

oe

sb(a(ismcrad

2

pitsotafw

asaccvntDcma

2

ebwsSeamms

tIstto

vqt

A.T. Elsayed et al. / Electric Power

f a data center, is decreased. Therefore, DC distribution is a moreconomical and efficient option for data centers [24,25].

Firstly, in 2006, the idea of utilizing DC-based power distributionystems in data centers was opposed in [26], the conclusion wasased on comparing the efficiencies of 5 distribution architecturestwo AC-based and three DC-based). In 2008, a more recent andccurate study prepared by Lawrence Berkeley National LaboratoryLBNL) revealed that converting the typical AC distribution systemsn data centers to DC-based systems can achieve up to 28% energyaving [27]. LBNL prepared a research roadmap toward high perfor-ance data center [28], they emphasized on the importance of the

onversion of the main power infeed to DC as a step to improve theeliability and efficiency of data center power supplies. In [20], theuthors implicitly promoted the idea of utilizing DC distribution forata centers.

.5. Plug-in electric vehicles

The global call for reduced CO2 emissions, the investments thationeer automotive companies have been making to advance plug-

n all electric, and hybrid electric, vehicle (EV) technologies, andhe problems inherently associated with fuel availability and pricetability will inevitably lead to a significant increase in the numbersf electric vehicles in the near future. The problem of coordinatinghe charging process of a large number of EVs has lately acquired thettention of many researchers, and is still under study [29]. A toolor assessing the impacts of the EV charging and its coordinationith the electricity tariff is presented in [30].

It is not yet determined whether EVs will be charged casu-lly at home like any other home appliance, at a fast chargingtation, similar to a gas-fueling station for conventional vehicles, at

place where a discharged EV battery is replaced with a completelyharged one, or at a smart charging park where EVs are coordinatedentrally at a smart garage that enable vehicle-to-vehicle (V2V) andehicle-to-grid (V2G) services [31–34]. Each of these different tech-iques has supporters and opponents for reasons that are outsidehe scope of this review paper. However, the last model relates toC distribution since some of the researchers who work on the con-ept of smart charging parks believe that they should operate as DCicrogrids, with a common DC bus at which the EV batteries and

ny DG units should be integrated [35–38].

.6. DC microgrids

Microgrids are local energy networks that involve renewablenergy sources and storage systems. They have the capability toe locally controlled. Therefore, they can disconnect from the gridhen there is a blackout, or a fault at the main grid, and continue to

upply a portion of their local loads in a so called “islanded mode.”everal states in the USA invested millions to promote high pen-tration of microgrids as a part of their climate resiliency plansgainst natural disasters, especially after hurricane Sandy. Sinceicrogrids typically include renewable sources and batteries, DCicrogrids [39–48] will have the capability to increase the overall

ystem efficiency [49–57].Various papers have shown that DC microgrids can play an effec-

ive role in solving some operational issues on the main grid [58].n [59], a DC microgrid involving PV generation and hybrid energytorage (ultra-capacitors and Li-Ion batteries) was used for mitiga-ion of heavy non-linear Loads. It was shown experimentally in [60]hat a DC microgrid can be used for voltage support, by making usef its capability of injecting reactive power as an ancillary service.

In conclusion of this section, the aforementioned factors moti-ated many researchers to raise a fundamental yet essentialuestion; is AC distribution still the most efficient means to dis-ribute electrical power or it is time to reconsider deploying DC

s Research 119 (2015) 407–417 409

distribution systems? Researchers realized that DC power systemsare not outdated anymore; they are more aligned with our today’sneeds than they were 100 years ago. A realistic proof is providedlater in this paper in Section 8, where different existing DC systemsall over the world are listed.

3. Feasibility of DC distribution systems

The feasibility of using DC for power distribution has been stud-ied by several researchers in the recent years. One of the majorfactors that were used to judge the superiority of DC over AC is effi-ciency. Therefore, comparing AC to DC in terms of efficiency, lossesand economic merits received a special attention.

Hammerstrom, presented in [61] a model to compare the over-all conversion efficiencies of AC and DC distribution topologies,for residential applications. Based on the author’s assertion, eachpower conversion stage loses about 2.5% of the energy it converts.It was shown that DC systems incorporating fuel cells, or otherlocal DC generation, encounter less conversion losses. This conclu-sion was supported by the results presented in [62] by Seo et al.They presented a mathematical model to analyze the losses of thecomponents of DC distribution systems. It was also shown thatthe converter efficiency increases as the power capacity and loadincrease.

A loss comparison between AC and DC distribution systems wasconducted by the authors of [63]. The authors created two models,AC and DC, for a large distribution system consisting of 714 busesand 235 loads. The comparison results showed that for the sameconductor cross sectional area, the DC current can be 1.22 timeslarger than the AC current so that the distribution system encoun-ters the same conduction losses. Nevertheless, the comparison wasoversimplified by assuming that the resistance of the cable willremain the same in the AC and DC cases, neglecting the skin effect.The same assumption was used in the study presented in [64], inaddition to some other overestimated assumptions quoted from[65], such as considering the wire resistance as 0.069 �/m, whichis impractically large [66–68], (also see Tables 8 and 9 of reference[69]). This led to a conclusion that delivering power to residentialpremises in DC form is not recommended. Later studies showeddifferent conclusions; according to the study presented in [70], therelation between AC and DC cable resistances can be given as:

Rac = � · r2

� · r2 − � · (r − ı)2· Rdc (1)

where Rac and Rdc are the cable AC and DC resistances, respectively,r is the conductor radius and ı is the conductor skin depth, whichis dependent on the frequency. From this formula, it can be con-cluded that the cable AC resistance will be always higher than itscorresponding DC resistance. The difference between the AC and DCresistances increases as the frequency goes up, due to the increasein the skin depth. Hence, in 60 Hz or 400 Hz systems, the losses arehigher. In order to test the practicality of this conclusion, the ACand DC values of cable resistances listed in one of the large man-ufactures’ catalog [68] were compared. It was found that the ACresistance is more than the DC resistance by approximately 19%for cables with cross sectional area (CSA) ranging from 4 mm2 to95 mm2. Moreover, for larger CSA, the difference is in the range of21–37%. This difference was considered in [71] when the authorscompared AC and DC systems for off-shore distribution applica-tions. It was emphasized that the resistance increase due to skinand proximity effects has to be considered while comparing DC

to AC systems in order to derive accurate conclusions. This com-parative analysis showed that by utilizing double pole DC system,the cable loss can be reduced to 40–50% of that of an AC system.It was also concluded that DC systems have lower losses over a

4 Systems Research 119 (2015) 407–417

wdD[

b[tra

iswTla

4

gciOpIssaawbt

adiicwta

mttt(stnsimmtoasrgeos

G

SensitiveLoads

dcdc

dcdc

BatteryBank

dcac

SensitiveLoads HVAC

acdc

acdc

AC grid

(a)

G

SensitiveLoads

dcdc

dcdc

BatteryBank

SensitiveLoads HVAC

acdc

AC grid

(b)

Non-sensitiveLoads

10 A.T. Elsayed et al. / Electric Power

ide range of operating voltages, load currents and transmissionistances. Larruskain et al. proposed converting existing AC lines toC lines to increase the current carrying capacity of the these lines

72].In [73], it was shown that DC distribution systems are feasi-

le for commercial buildings with sensitive electronic loads. In74], the same conclusion was seconded. Moreover, it was shownhat DC distribution leads to advantages than those related toeduced losses, such as safety, reduction of electromagnetic fields,nd power quality improvement.

In [75], the authors studied the applicability of DC distributionn industrial applications. It was shown that DC distribution is fea-ible for industrial applications, and that the challenges associatedith DC distribution can be addressed by proper system design.

he feasibility of connecting multiple AC microgrids through DCink is investigated in [76], results show that systems’ reliabilitynd sustainability are improved.

. Design of DC distribution systems

The design of DC distribution systems has been lately investi-ated in several publications. Various factors should be taken intoonsideration while designing a DC distribution system, especiallyf the used equipment is originally designed for AC applications.ne of the basic requirements of a reliable design is to obtain sim-lified models that express the load behavior under DC operation.

n [4], the authors developed steady state and transient models forixty-three loads. It was found that heating loads can be modeled (inteady state) as a pure resistance and lighting loads can be modeleds a temperature-dependent resistance. The steady state model of

universal machine is a variable current source, I = YoU + Io, while itas found that electronic loads that use switch mode power supply

ehave as constant power loads. This means that the load consumeshe same amount of power regardless of its supply voltage changes.

Salomonsson et al. discussed in [73] the general design issuesssociated with DC power systems. They held a comparison amongifferent cable configurations. It was shown that a DC five wire

nstallation is slightly better than that of AC, while for three wirenstallations, DC is superior to AC in terms of power transferringapacity. The authors tested the performance of some typical loadshen operated with DC power, it was demonstrated that supplying

he loads with a DC supply can prevent voltage disturbance fromffecting the loads.

In [77], the authors proposed an adaptive control system for DCicrogrids installed in data centers. They compared two configura-

ions for the data center power system (shown in Fig. 1). Accordingo the authors, configuration (a) is better than (b) to avoid genera-or synchronization and achieve better power flow control, while,b) can be better than (a) in terms of power losses and converterize. It is worth mentioning that other advantages may be addedo (b) over (a), such as: (1) connecting HVAC to the same DC busot only increases the converter size but also increases the energytorage capacity and consequently increases the cost. Moreover,t increases the complexity of protection, operation and transition

odes; (2) it is not practically preferred to connect high powerachine loads to the same bus where sensitive loads are connected

o minimize voltage fluctuations. The main focus of this study wasn system operation and control [77]. The study indicated thatmong the eight possible operation modes and twenty-three tran-itions, the ones of interest were defined and discussed. Simulationesults showed that continuous supply for sensitive DC load was

uaranteed by coordinating the main two converters. The authorsmphasized on the importance of having fast detection of AC-gridutage and fast switches [77]. Another study of different operationituations, and transitions between interconnected and islanded

Fig. 1. Configurations of data center power system discussed in [77].

modes of DC microgrids, was presented in [78], but with moresimplifications and limitations.

In [74], Sannino et al. proposed a simplified scheme for DC distri-bution system, in which a lower number of converters is needed inorder to increase the overall efficiency. They studied the feasibilityof the proposed system by simulating its implementation on theirown research facility using actual parameters and conditions, andwith four different DC voltage levels: 48 V, 120 V, 230 V and 326 V.Voltage drops and power losses were calculated and compared tothose of the existing AC system. In addition, the system was eco-nomically evaluated by calculating installation and operation costs.The final conclusions were: (1) DC supply can lead to major advan-tages if a proper voltage level is chosen; results showed that 326 Vis the most suitable. It should be noticed that this conclusion wasbased on the European system, and cannot be generalized to sys-tems that use other voltage levels, such as the U.S. power system;(2) By adding a battery bank, they guaranteed emergency backuppower for their critical loads for much longer time than that guar-anteed by commercial UPS, with less costs. (3) It was shown that thecommercially available circuit breakers can be adopted to provideadequate DC protection, even at relatively high current rating andshort circuit capacity.

Amin et al. compared in [5] between low voltage distributionsystems with different voltage levels (24 V and 48 V), and 230 V ACdistribution system when feeding different household appliances.Conductor losses, and device losses were calculated and consid-ered for each system. They presented a principle for cable crosssection optimization based on comparing the investment cost ofthe cable and the cable losses. The results showed that the 48 VDC systems with optimized cable area have the lowest total energyconsumption, and the 24 V DC system has high losses. This conclu-sion was expected since the total distributed power was assumedto be around 8 kW, which is too high for such a low voltage.

In [6], Techakittiroj et al. carried out an experiment to demon-strate the possibility of using the appliances available in marketin DC distribution systems without modification. They suppliedcompact fluorescent lamp, LED lamp, television, computer andsmall motor drive with DC power. Successful results and improvedpower quality confirmed the possibility and plausibility of supply-ing appliances directly with DC voltage. The authors emphasized

on the idea of co-existence of AC and DC distribution systems foreasier migration toward DC.

Kakigano et al. presented in [79] a DC microgrid for residen-tial applications. The system consists of cogeneration systems

System

ccudsurtt

nspir

iswsfdIv

MFsatswattt

5

sep

ltnLtnlwss

apNsiwrnwr

A.T. Elsayed et al. / Electric Power

onnected to a DC distribution line (3 wire, ±170 V). Ultra-apacitors were used as the main energy storage. System operationnder interconnected mode and intentional islanding mode wereemonstrated. They constructed a laboratory scale experimentalystem. The system operation was tested under voltage sag on thetility grid point of common coupling, disconnection from, andeconnection to the grid. Experimental results showed that the sys-em can supply high-quality power continuously to the loads underhose conditions.

Baran et al. investigated in [75] the neutral voltage shift phe-omenon which is associated with DC/AC power systems byimulating a small-scale shipboard system. As a solution for thishenomenon they proposed using DC/DC buck converter with an

solation transformer, and grounding the transformer through highesistance (250 p.u. was used in this case).

The concept of power buffer was adopted in [80], the authorsntegrated power buffer and load shedding to enhance the tran-ient performance of DC distribution systems. The power bufferas achieved by a boost converter with a DC bus capacitor. It was

hown that power buffer is suitable for short-term transients, whileor long term transients, load shedding is mandatory. Load shed-ing was based on load classifications according to their priorities.

n [81], Logue et al. utilized power buffering to prevent voltageiolations in DC systems by controlling the input resistance directly.

As a part of the Future Renewable Electric Energy Delivery andanagement Systems Center (FREEDM), a USA National Science

oundation-sponsored project [82], an arc free DC plug for 380 V DCystems was developed. In another study within the same project,

solid-state transformer based on SiC MOSFET was developedo replace the traditional electromagnetic transformer [83]. Solid-tate transformers [45] inherently involve a DC intermediate stage,hich increases the possibility of integrating some of the DC loads

t the DC bus to avoid unnecessary conversion losses. The cost ofhose converters are still relatively high if compared to other tradi-ional equipment, however their cost is drastically declining withhe ongoing enhancement in semiconductor technologies.

. Stability of DC distribution systems

Stability has always been one of the main concerns of powerystem engineers. The stability criteria for AC systems are wellstablished and investigated. On the contrary, the stability of DCower systems is still under investigation.

One of the sources of instability in DC power systems was high-ighted by Sokal and Middlebrook early in 70s. In [84], it was shownhat DC converters can yield a negative input resistance and if thisegative resistance exceeds the positive resistance of the inputC filter of the converter, the whole system can oscillate leadingo instability. In [85,86], Middlebrook investigated the problem ofegative input resistance at low frequencies. To eliminate the oscil-

ations, the design of a switch-mode converter and its input filteras provided, in which the output impedance of the filter is kept

maller than the input impedance of the converter to preserve theystem stability.

Based on the same impedance analysis, Feng et al. [87], defined forbidden region for the impedance ratio; if the ratio of the out-ut impedance to the input impedance is kept outside this region,yquist stability criterion is not violated and the system remains

table. Based on the defined forbidden region, the impedance spec-fications for subsystems utilized in DC distributed power systems

ere proposed. It is worth noting that the proposed forbidden

egion and impedance specifications were for each individual loadot for the aggregated load [88]. These impedance requirementsere for voltage source systems. An extended study for cur-

ent source systems showed that the stability requirements are

s Research 119 (2015) 407–417 411

actually the opposite [89]. The stability analysis for a DC–DC con-verter with its input filter using Routh–Hurwitz criterion waspresented in [90]. In a summarizing statement, for voltage sourceconverters, the system is stable if the ratio of the load impedanceto source impedance is more than unity while for current sourceconverters the system is stable if the same ratio is less than unity.

In [91], an active stabilization technique was proposed to main-tain the stability of isolated microgrids in the presence of directonline induction motors (IMs), as loads. After carrying out a detailedsmall signal admittance modeling and analysis, the authors veri-fied the previous findings. It was shown that there is a source-loadadmittance mismatch between the Voltage Source Inverters (VSIs)on one side and the IMs on another side. This mismatch led tomedium frequency instabilities due to violation of Nyquist stabil-ity criterion. The stabilization technique was based on the additionof a compensation transfer function to re-map the low dampedmodes in the open loop system to higher damping locations in theclosed-loop system. The efficiency of the stabilization techniqueswas verified by simulation and experimental results. This studywas concerned with isolated AC microgrids dominated by IMs fedfrom DC sources or DGs through VSIs. Therefore, it is of interestfor this survey. Using similar procedure, the same authors of [91]presented a comprehensive assessment and active mitigation strat-egy for the interactions in hybrid AC/DC distribution systems [92]and DC microgrids [93]. It was shown that in converter-dominateddistribution systems, even if each individual converter is stable byitself, the stability of the whole system is not guaranteed due to thetight regulation of controllers. The problem was investigated forMV multi-MW droop controlled microgrid system [94]. The stabil-ity analysis of DC loads fed through Voltage Source Rectifier (VSR)was presented in [95].

Instabilities due to negative incremental input admittance in DCsystems feeding Permanent Magnet Synchronous Motors (PMSM)through VSI speed drives were investigated by Mohamed et al. in[96]. An active compensation method based on reference voltagewas proposed to stabilize the DC link. More active stabilizationalgorithms for speed drives with DC link can be found in [97,98].A method based on modifying the control structure to emulate theeffect of a capacitor was presented in [99]. Another solution basedon a passive damping circuit was proposed in [100].

In [101], Davari et al. proposed a variable structure nonlinearcontroller for a master Voltage Sources Converter (VSC) regu-lating the DC link voltage in DC distribution systems based onmulti-terminal energy pool architecture. The controller employsa sigma-delta modulation scheme. The results showed that globalstabilization of all system states has been achieved.

Small signal stability analysis of low voltage DC microgrids waspresented in [102]. Sources and loads were modeled by first-orderdifferential equations. The distribution cables were included in themodel as well. The impacts of changing the inductance and resis-tance of the cable on the stability were investigated. It was provedthat the poles move further inside the negative half of the S plane,as cable resistances increase or inductances decrease. An impor-tant note should be considered here that any increase in the cableresistance will increase the transmission losses. Hence, a tradeoffmust be considered between the system stability and transmis-sion losses. This study modeled all the loads as constant powerloads (CPLs) which partially contradicts the findings of [4], as itwas shown that significant portion of DC loads can be representedby constant impedance models.

Instabilities of current controlled DC-based distributed genera-tion units interfaced to the grid through VSIs were investigated in

[103], the main focus is on the instabilities due to grid parametersvariation, grid distortions and the instabilities associated with thecurrent control loop parameters. A solution based on a high band-width predictive current controller combined with an adaptive

4 System

iw

thtya

tgaidoScccPa

6

otit

aetavTapat

tuiDVap

Dsdv

stiis

apd

a

12 A.T. Elsayed et al. / Electric Power

nternal model for the capacitor voltage and grid current dynamicsas proposed.

A small signal stability analysis of MVAC and MVDC architec-ures of a zonal shipboard power system showed that MVDC hasigher damping, and tends to be more stable for different exciterypes [104]. Another detailed model and small signal stability anal-sis for an electric aircraft is presented in [105], the parametersffecting system’s stability are scrutinized.

Wide deployment of DC systems has some negative impacts onhe utility grid. Various research efforts have been done to miti-ate such impacts. It was found that DG-based DC microgrids have

disturbing impact on utility grids, which may lead to instabil-ty, due to the absence of mechanical inertia, or very low inertiaynamics [106]. A solution to this problem could be the utilizationf synchronverters, which were proposed by Zhong et al. [107].ynchronverter is an inverter with modified control to emulate theharacteristics of a traditional synchronous generator. Later, theontrol of the Synchronverter was improved by adding two majorhanges to make it able to synchronize itself to the grid withoutLL [108]. The idea of emulating virtual rotor characteristics wasdopted in [109] to design a nonlinear stabilizer for microgrids.

. Protection of DC distribution systems

Since DC current does not have natural zero crossing, protectionf DC systems is a challenging task. In [73], it was suggested to usehree-phase AC circuit breakers connecting the three contact pairsn series, to eliminate the spark. Several publications investigatedhe problem of DC short circuit current calculation [110].

Salomonsson et al. proposed in [111] a protection scheme for LV DC microgrid. This scheme was studied during different faultvents located at different points on the grid. The results showedhat it is possible to use commercial AC protection devices, suchs fuses and CBs, to protect batteries and loads. However, con-erters using IGBT modules are very sensitive to over-currents.herefore, they require faster protection, which can be provided byn ultra-fast hybrid DC CB. In addition, a method for coordination ofrotection devices was discussed. It was shown that problems canrise with high-impedance ground faults. Two grounding architec-ures (TN-S & IT) for DC systems were presented.

In [112], Tang et al. presented an economic handshaking methodo locate and isolate the faults on a multi-terminal DC networksing fast DC switches instead of DC circuit breakers, resulting

n significant saving. The method is based on extinguishing theC fault current by opening all the AC-circuit breakers which theSCs are already equipped with on the AC-sides. According to theuthors, through extensive testing, they concluded that their pro-osed method is reliable.

In [113], a fault detection and isolation scheme for low-voltageC microgrid systems was presented. The proposed protection

cheme divided the microgrid into segment controllers that canetect and isolate the faulted segment. Their proposed scheme waserified by simulations, and hardware experiments.

In [114], a self-healing protection approach was proposed forhipboard MVDC applications. The authors focused not only on pro-ection system’s response to faults, but also its response to failuresn the measurement system and sensor delays. They developed anntegrated validation and protection approach that proved to beensitive to communication delays.

In [115], Jeon et al. proposed a solution for the problem ofrcing during plugging/unplugging of home appliances, when sup-

lied with DC current. Their solution was based on adding a shuntiode/capacitor branch to the plug.

In [116], the utilization of the power electronic converterslready included in the system to interrupt fault currents was

s Research 119 (2015) 407–417

discussed. It was shown that by associating relays with the differentconverters and by adopting overcurrent-based protection schemesfor these relays, the faults on DC systems can be quickly detectedand localized.

Moreover, a study to investigate the means to achieve fast andeffective protection system operation at a minimum installationcost was presented in [117].

In [118], Mehl et al. held a comparison between electronic andmechanical breakers for 400 V DC systems. They found that elec-tronic breakers outperform mechanical ones in terms of currentlimitation, rated current controllability, trip time curve adjustment,wire break indication, remote controllability and monitoring func-tions for current and voltage. However, due to the leakage currentat the OFF state of electronic breakers, they do not provide completephysical isolation, which makes the process of protecting themagainst line induced voltage spikes challenging.

In [119], the authors developed a DC hybrid circuit breakerwith ultra-fast contact opening and Integrated Gate-CommutatedThyristors (IGCTs). Its hybrid structure comprises a high-speedmechanical switch and bi-directional IGST. The authors verifiedexperimentally (on a 4 kA, 1.5 kV prototype), that their proposedhybrid breaker can significantly decrease the current interruptiontime.

At the end of this section it is worthy to mention that greatadvancements have been achieved. Recently, wide variety of DC cir-cuit breakers and contactors have been commercially available bylarge manufacturers [120–124]. These products are characterizedby high reliability and low failures.

7. Standardization efforts

AC has been utilized for more than a century therefore AC stan-dards are much more mature than those of DC systems. One ofthe impediments against global adoption of DC systems is the lackof the required standards, as yet we do not have a comprehen-sive standard for how to generate, transmit, distribute and use DCpower. We do not even have a standard for DC voltage levels [11].Such standards are quite essential to convey the beneficial experi-ences of experts of DC systems to help improve the reliability andefficiency of such systems. In addition, DC standards will put uni-fied outlines for the design and installation methods, thereby, DCsystems will become easier to install, and more trustworthy forentrepreneurs.

However, there are some related standards, which will be high-lighted in this section. Reviewing those existing standards showsthat; in 2004 IEEE power engineering society issued a revised ver-sion of IEEE standard 946 [125], which was issued initially in 1992.This standard provides recommended practice for the design oflead acid batteries based DC auxiliary power systems for generatingstations. It provides guidelines for the selection of the number ofbatteries, battery duty cycle, battery capacity, and voltage level aswell as the battery charger. The output ripples level for the chargerwas restricted to 2% of the operating voltage without additional fil-tering and 30 mV with extra filters. The standard briefly discussesthe effect of grounding on the operation of DC auxiliary systems.It shows that the incautious grounding design can initiate opera-tion of de-energized DC loads or prevent disconnecting energizedloads. Important guidance for design, types, minimum attributesand protection of uninterruptable power supply (UPS) systems canbe found in [126]. It covers UPS systems associated with lead acidor nickel cadmium batteries. Methods for short circuit analysis and

models of DC system components are provided in [127]. The IEEE1547 [128] standard provides a set of technical specifications for,and testing of, the DG interconnection to utility Electric Power Sys-tems (EPS). For instance, the rules for islanding from the grid when

System

tll

pEFtrt

•

•

•

•

•

lddsiwba

A.T. Elsayed et al. / Electric Power

here is a fault or reclosing when the fault is cleared, and the guide-ines for power quality such as the permissible harmonic distortionimits, are included.

Some standardization effort in the subject of DC distributionower systems and standard key points are listed in [20]. Nationallectrical Code (NEC), or known as NFPA70 issued by Nationalire Protection Association (NFPA), contains articles that regulatehe utilization and installation of DC technologies. Outlines of theelated articles are selected to be summarized here. More informa-ion and detailed provisions can be found in [69].

Regarding grounding of DC system, which can be a challengingtask, important guidelines and limitations were provided in NECarticle 250, clauses160 through 169 [69].Article 393 is a new article added in the 2014 edition [129] forLow-Voltage Suspended Ceiling Power Distribution Systems. Thegrowing interest in alternative energy sources and the prolifer-ation of low-voltage, low-power devices (sensors, LED lighting,etc.), created a significant need for adequate language supportingthe practical safeguarding of circuits and electrical equipmentoperating at 30 V AC, or 60 V DC. Although this new article is cov-ering AC and DC installations, practically, most of these systemsare DC-based.Article 625 covers the charging system of electric vehicles (EV);for the purpose of rating the EV charging equipment, three charg-ing methods were defined as levels 1–3. For level 1, the maximumload is rated 1.4 kVA and the minimum overcurrent rating is 15or 20 A. In level 2, the maximum load is limited to 32 A and theminimum overcurrent rating is 40 A. Whereas, level 3 is a highspeed method which requires high power to charge the vehiclein a short period. Specifications for charging equipment are leftto be specified by the equipment manufacturer.Article 690 was added to the code in the 1984s edition and sincethen it has been subject to improvements and additions. Thisarticle covers photovoltaic (PV) systems either standalone, inter-active, with or without energy storage. Simplified guiding circuitschematics for different configurations of PV systems are pro-vided. According to the code, bipolar PV systems are permittedbut monopole sub-arrays should be separated physically. It isrequired that the PV circuit conductor is selected to withstand– at least – 125% of the sum of parallel module rated short cir-cuit currents and the overcurrent protection device (which ismandatory) should be rated at 156% of the same sum. Regardingthe sizing of the inverter in a standalone system, the code doesnot require the inverter to be sized for the multiple loads to besimultaneously loaded to it. For grounding of PV systems involv-ing both AC and DC systems, both grounding system should bebonded together. The clauses of this article provide provisionsand important guiding lines for installation, arrangement, circuitrequirements, wiring, protection and grounding of PV system.Article 692 which covers on premises fuel cell systems was addedto the code in the 2002 revision. This article requires providingsuitable means to de-energize all current carrying conductors of afuel cell based power source. In addition, bonding the DC ground-ing system to the AC grounding system (in case of using an AC/DCinverter) to single common grounding electrode.

Emerge Alliance [130], a group of over 100 companies, researchabs and universities, works on promoting DC distribution, andeveloping DC standards. They completed two standards; the stan-ards confirm the importance of converting the existing AC powerources to DC power at a local distribution level rather than at

ndividual devices. The first one is the Occupied Space standard

hich is a guide for the hybrid use of DC power in commercialuildings [131]. The standard defines a multifunctional low volt-ge DC power distribution infrastructure layer that interconnects

s Research 119 (2015) 407–417 413

sources of power to devices in the space, which draw the power.Moreover, the Standard defines the control systems necessary tomonitor and control such devices and power sources. The secondstandard [132] focuses on the data and telecom centers domain;it defines low voltage DC power distribution requirements for usein such spaces. Specifically, the standard defines nominal 380VDCinfrastructure requirements. It is mandatory to note that thisstandard is not intended to be a replacement of NEC.

MIL-STD-1399 is a military standard, section 390 is defining theelectrical interface requirements and constraints of DC equipmentutilized in shipboard power systems [133]. For instance, it limitsthe permitted frequency tolerance to ±3% in 60 Hz system. Whilepermitted frequency modulation (periodic frequency fluctuations)is limited to ±0.5%.

Current standardization efforts done by the International Tele-com Union (ITU-T) and the European Telecom Standard Institute(ESTI) to accelerate the deployment of DC power systems with volt-age less than 400 VDC in telecom and data centers are summarizedin [134]. Various DC architectures used in data centers are discussedas well.

8. Existing DC distribution systems

8.1. There are several power systems that typically employ DCdistribution. Some of these systems include, Spacecraft

Spacecraft systems involve a large number of solar pan-els, DC–DC converters, batteries, battery chargers and DC loads[135,136]. Hence, DC distribution is employed. A good example isthe NASA International Space Station (ISS) requiring over 100 kW.The ISS is composed of two relatively independent DC systemswith different voltage levels. The American system runs at 120 Vand has solar power modules with a capacity of 76 kW. Whereas,the Russian system is divided into two voltage levels; 120 V and28 V components, and it has 29 kW solar power modules. The twosystems are linked with bi-directional DC–DC converters to enablepower transfer [137,138].

8.2. Data centers

Even though most of the existing data centers use AC distribu-tion, some of them use DC. Duke Energy data center in Charlotte,NC, is employing a 380 V DC distribution system. Duke Energy andthe Electric Power Research Institute (EPRI) prepared a study show-ing that the system uses 15% less energy than a typical AC systemwith double conversion UPS [139]. Data center of the Universityof California, San Diego is a 2.8 MW DC-based data center, which ispowered through a large fuel cell stack. The data center was broughtinto service in August 2010 [23]. “Green,” which is one of the top ICTservices providers in Switzerland, announced the opening of their1 MW DC-based data center in May, 2012. HP provided the IT equip-ment supporting DC input, commercial availability of DC enabledIT equipment is a stunning and encouraging step toward a widedeployment of DC data centers. Although DC distribution is not uti-lized in Google data centers, they managed to save $30/year/serverby optimizing the power path by eliminating two AC/DC conversionstages and bringing the batteries on the server rack.

8.3. Telecommunication

Telecommunication power systems, similar to data centerpower systems, are designed to transfer tremendous amount ofdata. They also require high reliability and efficiency at a low cost.Therefore, 48 V DC distribution power system is widely used in

4 System

ti

8

lasstDctleo

8

tcibbuaatvss

8

ohtotio(ha

tsv

ARAlwVe

bai13

14 A.T. Elsayed et al. / Electric Power

elecommunication central offices. The reliability of that systems 99.999% [140–144].

.4. Traction

DC distribution is used in traction power systems, such as trol-eybuses, trams, underground railways, mainly because DC motorsre typically used in this application [145–148]. Even for tractionystems that use induction motors (IM) [149], interfacing with DCupply is much easier and reduces conversion stages. Consequently,he system efficiency and controllability increase. Moreover, usingC distribution help designers use a single conductor since the railsan be used as the return path for the current. DC distribution inraction power systems supplies the vehicles and other auxiliaryoads on them. Their supply voltage ranges among 600 V, 750 V orven up to 1 kV [150,151]. The load flow problem and descriptionf DC traction system are discussed in [152].

.5. Shipboard power systems

Normally, shipboard power systems involve a mechanical sys-em for propulsion along with an electrical system for weapons,ommunication, navigation, hotel and auxiliary loads. However, inntegrated power systems (IPS), these two energy systems are com-ined seeking an increased reliability during normal sailing andattle conditions. One of the options that are likely to be commonlysed in IPS is the DC zonal distribution system [153–155], whichssures several advantages other than the increased reliability, suchs the facilitation of protection since the sources and loads are dis-ributed into different zones each with its own converters. Mediumoltage DC distribution is another architecture that is also exten-ively investigated to be implemented on future shipboard powerystems [156,157].

.6. Experimental setups

Initiated by the imperative need for more research and devel-pment of DC microgrids (or DC systems), a wide variety of studiesave been carried out during the last couple of decades. Most ofhese studies were simulation-based. However, the recent interestf some of the funding agencies in DC systems elevated a portion ofhese studies toward hardware experimentation. A DC testing grids presented by Albu et al. in [158], the established grid is a lab-ratory scale microgrid to examine the operation of low-voltage230 V) DC grids. Extended details about the system construction,ardware implementation and the developed LabVIEW monitoringpplication are provided.

A 15 kW Naval Combat Survivability (NCS) zonal DC distributionestbed setup was developed at Purdue University and the Univer-ity of Missouri–Rolla with grants from the US Navy to examine thearious aspects related to that system [159,160].

Another example of practical experimental setups is the hybridC/DC smart grid power system established in Energy Systemsesearch Laboratory (ESRL) at Florida International University. TheC side comprises 4 generators and several bus and transmission

ine models. There are two DC microgrids interfaced to the AC net-ork through bi-directional VSIs. The voltages of DC buses are 380DC and 325 VDC. Both microgrids contain PV emulators, fuel cellmulators, wind emulators and battery storage [161,162].

Power quality indicators are introduced to characterize theehavior of DC networks. In order to study the power quality issues

ssociated with DC pulsed loads, an established microgrid testbedn UTA was presented in [163], the microgrid has a single phase20 V AC–60 Hz AC bus and a 24 V DC bus with total power of around–4 kW. The microgrid is considered low voltage-low power but

s Research 119 (2015) 407–417

it is involving various renewable energy sources, and it has thecapability of performing AC studies as well.

A test bed of 380 V DC distribution system was presented in [7];the system consists of a single phase bi-directional CLLC resonantconverter for DC bus voltage control, dual active bridge converterfor controlling the charging and discharging of the battery, andLLC resonant converter for interfacing a renewable energy simu-lator. Normal home appliances such as TV, LED, washing machine,refrigerator, air conditioner and laptop were used as loads. Someof these appliances were modified by removing the AC–DC recti-fier and the power factor correction circuits to be operable with DCpower. However, these modifications and implementation stepswere not included in details.

9. Conclusion and future work

This paper presented an exhaustive survey for the efforts con-ducted on DC distribution systems and DC microgrids. In light ofthis overview, it can be concluded that the feasibility of adoptingDC systems became evident, especially with the high penetration ofDC-supplied loads, and the presence of advanced power electronicstechnologies. Voltage selection, modeling, control, stability, protec-tion and grounding of DC systems have been investigated. However,more work needs to be done on these topics to answer all the raisedquestions. A complete system design has to be comprehensivelyinvestigated with its practical aspects and impacts. A more detailedstudy needs to be done on using equipments that were designedoriginally for AC operation in DC systems. In addition, architectureand topologies for DC power systems to meet special load require-ments, such as machine drives, electric vehicles, or pulsed loads,may be of interest as a future research subject.

References

[1] P. Fairley, DC versus AC: the second war of currents has already begun [in myview], IEEE Power Energy Mag. 10 (2012) 103–104.

[2] S.-H. Ryu, J.-H. Ahn, B.-K. Lee, K.-S. Cho, Single-switch ZVZCS quasi-resonantCLL isolated DC–DC converter for low-power 32′′ LCD TV, in: IEEE EnergyConversion Congress and Exhibition (ECCE), 2013, pp. 4887–4893.

[3] C.-H. Tsai, Y.-W. Bai, M.-B. Lin, R.J.R. Jhang, C.-Y. Chung, Reduce the standbypower consumption of a microwave oven, IEEE Trans. Consum. Electron. 59(2013) 54–61.

[4] D. Salomonsson, A. Sannino, Load modelling for steady-state and transientanalysis of low-voltage DC systems, IET Electr. Power Appl. 1 (2007) 690–696.

[5] M. Amin, Y. Arafat, S. Lundberg, S. Mangold, Low voltage DC distribution sys-tem compared with 230 V AC, in: IEEE Electrical Power and Energy Conference(EPEC), 2011, pp. 340–345.

[6] K. Techakittiroj, V. Wongpaibool, Co-existence between AC-distribution andDC-distribution: in the view of appliances, in: International Conference onComputer and Electrical Engineering (ICCEE), 2009, pp. 421–425.

[7] M. Ryu, H. Kim, J. Kim, J. Baek, J. Jung, Test bed implementation of 380V DCdistribution system using isolated bidirectional power converters, in: IEEEEnergy Conversion Congress and Exposition (ECCE), 2013, pp. 2948–2954.

[8] B.A. Thomas, Edison revisited: impact of DC distribution on the cost of LEDlighting and distribution generation, in: 27th Annual IEEE Applied PowerElectronics Conference and Exposition (APEC), 2010, pp. 588–593.

[9] W. Yu, J.-S. Lai, H. Ma, C. Zheng, High-efficiency DC–DC converter withtwin bus for dimmable LED lighting, IEEE Trans. Power Electron. 26 (2011)2095–2100.

[10] G.C. Lazaroiu, D. Zaninelli, A control system for dc arc furnaces for powerquality improvements, Electr. Power Syst. Res. 80 (2010) 1498–1505.

[11] P. Wang, L. Goel, F.H. Choo, Harmonizing AC and DC: a hybrid AC/DC futuregrid solution, IEEE Power Energy Mag. 11 (2013) 76–83.

[12] P.S. Maniscalco, V. Scaini, W.E. Veerkamp, Specifying DC chopper systemsfor electrochemical applications, IEEE Trans. Ind. Appl. 37 (May–June) (2011)941–948.

[13] G.F. Reed, DC technologies: solutions to electric power system advancements,IEEE Power Energy Mag. 10 (2012) 10–17.

[14] B.T. Patterson, DC, come home: DC microgrids and the birth of the Enernet,IEEE Power Energy Mag. 10 (2012) 60–69.

[15] T. Wu, C. Chang, L. Lin, G. Yu, Y. Chang, DC-bus voltage control with a three-phase bidirectional inverter for DC distribution systems, IEEE Trans. PowerElectron. 28 (2013) 1890–1899.

System

A.T. Elsayed et al. / Electric Power

[16] C. Dierckxsens, K. Srivastava, M. Reza, S. Cole, J. Beerten, R. Belmans, A dis-tributed DC voltage control method for VSC MTDC systems, Electr. Power Syst.Res. 82 (2012) 54–58.

[17] M. Aragüés-Penalba, A. Egea-Àlvarez, S.G. Arellano, O. Gomis-Bellmunt, Droopcontrol for loss minimization in HVDC multi-terminal transmission systemsfor large offshore wind farms, Electr. Power Syst. Res. 112 (2014) 48–55.

[18] A. Al-Diab, C. Sourkounis, Integration of flywheel energy storage system inproduction lines for voltage drop compensation, in: IEEE 37th Annual Con-ference Industrial Electronics (IECON), 2011, pp. 3882–3887.

[19] B.H. Kenny, R. Jansen, P. Kascak, T. Dever, W. Santiago, Integrated power andattitude control with two flywheels, IEEE Trans. Aerosp. Electron. Syst. 41(2005) 1431–1449.

[20] D. Becker, B. Sonnenberg, DC microgrids in buildings and data centers, in: IEEE33rd International Telecommunications Energy Conference (INTELEC), 2011,pp. 1–7.

[21] P. Gross, K. Godrich, Total DC integrated data centers, in: IEEE 27th Interna-tional Telecommunication Conference (INTELEC), 2005, pp. 125–130.

[22] S. Rajagopalan, B. Fortenbery, D. Symanski, Power quality disturbances withinDC data centers, in: IEEE 32nd International Telecommunication Energy Con-ference (INTELEC), 2010, pp. 1–7.

[23] G. AILee, W. Tschudi, Edison redux: 380 V dc brings reliability and efficiencyto sustainable data centers, IEEE Power Energy Mag. 10 (2012) 50–59.

[24] D. Kim, T. Yu, H. Kim, H. Mok, K. Park, 300V DC feed system for Internet datacenter, in: IEEE 8th International Conference on Power Electronics and ECCEAsia (ICPE & ECCE), 2011, pp. 2352–2358.

[25] A. Pratt, P. Kumar, T. Aldridge, Evaluation of 400V DC distribution in telcoand data centers to improve energy efficiency, in: IEEE 29th InternationalTelecommunication Energy Conference (INTELEC), 2007, pp. 32–39.

[26] N. Rasmussen, AC vs. DC Power Distribution for Data Centers, APC White Paper# 63, 2006.

[27] DC power distribution cuts data center energy use, 2014, Available at:http://hightech.lbl.gov/documents/data centers/CEC-TB-40.PDF

[28] High-Performance Data Centers; A Research Roadmap, 2014, Available at:http://hightech.lbl.gov/documents/datacenters roadmap final.pdf

[29] L. Zhang, F. Jabbari, T. Brown, S. Samuelsen, Coordinating plug-in electric vehi-cle charging with electric grid: valley filling and target load following, J. PowerSources 267 (2014) 584–597.

[30] G. Mills, I. MacGill, Potential power system and fuel consumption impactsof plug in hybrid vehicle charging using Australian National Electricity Mar-ket load profiles and transportation survey data, Electr. Power Syst. Res. 116(2014) 1–11.

[31] C. Liu, K.T. Chau, D. Wu, S. Gao, Opportunities and challenges of vehicle-to-home, vehicle-to-vehicle, and vehicle-to-grid technologies, Proc. IEEE 101(2013) 2409–2427.

[32] Y. Ota, H. Taniguchi, J. Baba, A. Yokoyama, Implementation of autonomousdistributed V2G to electric vehicle and DC charging system, Electr. PowerSyst. Res. (2014), http://dx.doi.org/10.1016/j.epsr.2014.05.016.

[33] M.A. López, S. Martín, J.A. Aguado, S. de la Torre, V2G strategies for congestionmanagement in microgrids with high penetration of electric vehicles, Electr.Power Syst. Res. 104 (2013) 28–34.

[34] L. Drude, L.C. Pereira Junior, R. Rüther, Photovoltaics (PV) and electric vehicle-to-grid (V2G) strategies for peak demand reduction in urban regions in Brazilin a smart grid environment, Renew. Energy 68 (2014) 443–451.

[35] M. Tabari, A. Yazdani, A DC distribution system for power system integrationof plug-in hybrid electric vehicles, in: IEEE Power and Energy Society GeneralMeeting (PES), 2013, pp. 1–5.

[36] A. Mohamed, V. Salehi, T. Ma, O. Mohammed, Real-time energy manage-ment algorithm for plug-in hybrid electric vehicle charging parks involvingsustainable energy, IEEE Trans. Sustain. Energy 5 (2014) 577–586.

[37] B.E. Noriega, R.T. Pinto, P. Bauer, Sustainable DC-microgrid control system forelectric-vehicle charging stations, in: 15th European Conference on PowerElectronics and Applications (EPE), 2013, pp. 1–10.

[38] L. Roggia, C. Rech, L. Schuch, J.E. Baggio, H.L. Hey, J.R. Pinheiro, Design of asustainable residential microgrid system including PHEV and energy storagedevice, in: 14th European Conference on Power Electronics and Applications(EPE), 2011, pp. 1–9.

[39] D. Chen, XuF L., Autonomous DC voltage control of a DC microgrid with mul-tiple slack terminals, IEEE Trans. Power Syst. 27 (2012) 1897–1905.

[40] M. Farhadi, O. Mohammed, Realtime operation and harmonic analysis of iso-lated and non-isolated hybrid DC microgrid, IEEE Trans. Ind. Appl. 50 (4)(2014) 2900–2909.

[41] Y. Gu, X. Xiang, W. Li, X. He, Mode-adaptive decentralized control for renew-able DC microgrid with enhanced reliability and flexibility, IEEE Trans. PowerElectron. 29 (2014) 5072–5080.

[42] L. Xu, D. Chen, Control and operation of a DC microgrid with variable gener-ation and energy storage, IEEE Trans. Power Deliv. 26 (2011) 2513–2522.

[43] X. Yu, SheF x., X. Zhou, A.Q. Huang, Power management for DC micro-grid enabled by solid-state transformer, IEEE Trans. Smart Grid 5 (2014)954–965.

[44] T. Dragicevic, J.M. Guerrero, J.C. Vasquez, D. Skrlec, Supervisory control of anadaptive-droop regulated DC microgrid with battery management capability,

IEEE Trans. Power Electron. 29 (2014) 695–706.

[45] X. She, A.Q. Huang, S. Lukic, M.E. Baran, On integration of solid-state trans-former with zonal DC microgrid, IEEE Trans. Smart Grid 3 (2012) 975–985.

[46] K. Strunz, E. Abbasi, D.N. Huu, DC microgrid for wind and solar power inte-gration, IEEE J. Emerg. Sel. Top. Power Electron. 2 (2014) 115–126.

s Research 119 (2015) 407–417 415

[47] B. Wang, M. Sechilariu, F. Locment, Intelligent DC microgrid with smartgrid communications: control strategy consideration and design, IEEE Trans.Smart Grid 3 (2012) 2148–2156.

[48] M. Simonov, Dynamic partitioning of DC microgrid in resilient clusters usingevent-driven approach, IEEE Trans. Smart Grid 5 (5) (2014) 2618–2625.

[49] X. Lu, K. Sun, J.M. Guerrero, J.C. Vasquez, L. Huang, State-of-charge balanceusing adaptive droop control for distributed energy storage systems in DCmicrogrid applications, IEEE Trans. Ind. Electron. 61 (2014) 2804–2815.

[50] Lu Xiaonan, J.M. Guerrero, Sun Kai, J.C. Vasquez, An improved droop controlmethod for DC microgrids based on low bandwidth communication with DCbus voltage restoration and enhanced current sharing accuracy, IEEE Trans.Power Electron. 29 (2014) 1800–1812.

[51] C. Jin, P. Wang, J. Xiao, Y. Tang, F.H. Choo, Implementation of hierarchicalcontrol in DC microgrids, IEEE Trans. Ind. Electron. 61 (2014) 4032–4042.

[52] D. Chen, L. Xu, L. Yao, DC voltage variation based autonomous control of DCmicrogrids, IEEE Trans. Power Deliv. 28 (2013) 637–648.

[53] S. Anand, B.G. Fernandes, M. Guerrero, Distributed control to ensure pro-portional load sharing and improve voltage regulation in low-voltage DCmicrogrids, IEEE Trans. Power Electron. 28 (2013) 1900–1913.

[54] A. Kwasinski, Quantitative evaluation of DC microgrids availability: effects ofsystem architecture and converter topology design choices, IEEE Trans. PowerElectron. 26 (2011) 835–851.

[55] H. Kakigano, Y. Miura, T. Ise, Distribution voltage control for DC microgridsusing fuzzy control and gain-scheduling technique, IEEE Trans. Power Elec-tron. 28 (2013) 2246–2258.

[56] N. Eghtedarpour, E. Farjah, Distributed charge/discharge control of energystorages in a renewable-energy-based DC micro-grid, IET Renew. PowerGener. 8 (2014) 45–57.

[57] S.-M. Chen, T.-J. Liang, K.-R. Hu, Design, analysis, and implementation of solarpower optimizer for DC distribution system, IEEE Trans. Power Electron. 28(2013) 1764–1772.

[58] J.J. Justo, F. Mwasilu, J. Lee, J. Jung, AC-microgrids versus DC-microgrids withdistributed energy sources: a review, Renew. Sustain. Energy Rev. 24 (2013)387–405.

[59] A. Mohamed, V. Salehi, O. Mohammed, Real-time energy management algo-rithm for mitigation of pulse loads in hybrid microgrids, IEEE Trans. SmartGrid 3 (2012) 1911–1922.

[60] A. Mohamed, A. Ghareeb, T. Youssef, O.A. Mohammed, Wide area monitoringand control for voltage assessment in smart grids with distributed generation,in: IEEE Innovative Smart Grid Technologies Conference (ISGT), 2013, pp. 1–6.

[61] D.J. Hammerstrom, AC versus DC distribution systems: did we get it right? in:IEEE Power and Energy Society General Meeting, 2007, pp. 1–5.

[62] G. Seo, J. Baek, K. Choi, H. Bae, B. Cho, Modeling and analysis of DC distributionsystems, in: IEEE 8th International Conference on Power Electronics and ECCEAsia (ICPE & ECCE), 2011, pp. 223–227.

[63] M.R. Starke, L.M. Tolbert, B. Ozpineci, AC vs. DC distribution: a loss compar-ison, in: IEEE PES Transmission and Distribution Conference and Exposition(T&D), 2008, pp. 1–7.

[64] K. Engelen, E. Leung Shun, P. Vermeyen, I. Pardon, R. D’hulst, J. Driesen, R.Belmans, The feasibility of small-scale residential DC distribution systems,in: IEEE 32nd Annual Conference on Industrial Electronics (IECON), 2006, pp.2618–2623.

[65] J. Pellis, The dc Low-voltage House (Master’s thesis), TU Eindhoven, 1997.[66] Available on (2014): http://www.nexans.us/eservice/USen US/fileLibrary/

Download 540190061/US/files/CORFLEX%20USA%202011%20(2) 1.pdf[67] Available on (2014): http://www.nexans.co.nz/NewZealand/2013/Power

Cable Catalogue Full version 2012.pdf[68] Available on (2014): http://www.elsewedyelectric.com/FE/Common/

ProductCategoryDetails.aspx?/13/2/Low%20Voltage%20Cables/[69] M.W. Earley, J.S. Sargent, C.D. Coache, R.J. Roux, National Electrical Code Hand-

book, Twelfth ed., NFPA, 2011.[70] A.W. Cirino, H. de Paula, R.C. Mesquita, E. Saraiva, Cable parameter determi-

nation focusing on proximity effect inclusion using finite element analysis,in: Brazilian Power Electronics Conference (COBEP), 2009, pp. 402–409.

[71] F. Wang, Y. Pei, D. Boroyevich, R. Burgos, K. Ngo, AC vs. DC distribution for off-shore power delivery, in: IEEE 34th Annual Conference of IEEE on IndustrialElectronics (IECON), 2008, pp. 2113–2118.

[72] D.M. Larruskain, I. Zamora, O. Abarrategui, Z. Aginako, Conversion of AC dis-tribution lines into DC lines to upgrade transmission capacity Electr. PowerSyst. Res. 81 (2011) 1341–1348.

[73] D. Salomonsson, A. Sannino, Low voltage DC distribution systems for commer-cial power systems with sensitive electronic loads, IEEE Trans. Power Deliv.22 (2007) 1620–1627.

[74] A. Sannino, G. Postiglione, M.H.J. Bollen, Feasibility of a DC network for com-mercial facilities, IEEE Trans. Ind. Appl. 39 (2003) 1499–1507.

[75] M.E. Baran, N.R. Mahajan, DC distribution for industrial systems: opportuni-ties and challenges, IEEE Trans. Ind. Appl. 39 (2003) 1596–1601.

[76] R. Majumder, Aggregation of microgrids with DC system, Electr. Power Syst.Res. 108 (2014) 134–143.

[77] D. Salomonsson, L. Söder, A. Sannino, An adaptive control system for a DCmicrogrid for data centers, IEEE Trans. Ind. Appl. 44 (2008) 1910–1917.

[78] H. Kakigano, Y. Miura, T. Ise, R. Uchida, DC micro-grid for super high qualitydistribution – system configuration and control of distributed generationsand energy storage devices, Proc. IEEE PESC (2006) 3148–3154.

[79] H. Kakigano, Y. Miura, T. Ise, Low-voltage bipolar-type DC microgrid for superhigh quality distribution, IEEE Trans. Power Electron. 25 (2010) 3066–3075.

4 System

16 A.T. Elsayed et al. / Electric Power

[80] R.S. Balog, W.W. Weaver, P.T. Krein, The load as an energy asset in a distribu-tion dc smart grid architecture, IEEE Trans. Smart Grid 3 (2012) 253–260.

[81] D.L. Logue, P.T. Krein, Preventing instability in DC distribution systems byusing power buffering, in: Rec. IEEE Power Electron. Specialists Conf., 2001,pp. 33–37.

[82] FREEDM System, 2014, Available at http://www.freedm.ncsu.edu/[83] X. She, X. Yu, F. Wang, A.Q. Huang, Design and demonstration of a

3.6KV–120V/10KV. A solid-state transformer for smart grid application, IEEETrans. Power Electron. 29 (2014) 3982–3996.

[84] N.O. Sokal, System oscillations from negative input resistance at power inputport of switching-mode regulator, amplifier, DC/DC converter, or DC/ACinverter, in: IEEE Power Electronics Specialists Conference PESC, 1973, pp.138–140.

[85] R.D. Middlebrook, Input filter considerations in design and application ofswitching regulators, in: IEEE Industrial Applications Society Annual Meeting,1976.

[86] R.D. Middlebrook, Design techniques for preventing input-filter oscillationsin switched-mode regulators, in: Proceedings of the Fifth National Solid StatePower Conversion Conference, 1978.

[87] Feng Xiaogang, Liu Jinjun, F.C. Lee, Impedance specifications for stable DCdistributed power systems, IEEE Trans. Power Electron. 17 (March) (2002).

[88] X. Feng, Z. Ye, K. Xing, F.C. Lee, D. Borojevic, Individual load impedance spec-ification for a stable DC distributed power system, in: 14th Applied PowerElectronics Conference and Exposition, APEC ′99, vol. 2, 1999, pp. 923–929.

[89] J. Sun, Impedance-based stability criterion for grid-connected inverters, IEEETrans. Power Electron. 26 (2011) 3075–3078.

[90] X. Yu, M. Salato, An optimal minimum-component DC–DC converter inputfilter design and its stability analysis, IEEE Trans. Power Electron. 29 (2014)829–840.

[91] A.A. Amr, Radwan, A.-R.I. Yasser, Mohamed, Stabilization of medium-frequency modes in isolated microgrids supplying direct online inductionmotor loads, IEEE Trans. Smart Grid 5 (2014) 358–370.

[92] A.A.A. Radwan, Y.A.-R.I. Mohamed, Assessment and mitigation of interactiondynamics in hybrid AC/DC distribution generation systems, IEEE Trans. onSmart Grid 3 (2012) 1382–1393.

[93] A.A.A. Radwan, Y.A.-R.I. Mohamed, Linear active stabilization of converter-dominated DC microgrids, IEEE Trans. Smart Grid 3 (2012) 203–216.

[94] A. Kahrobaeian, A.-R. Yasser, I. Mohamed, Analysis and mitigation of low-frequency instabilities in autonomous medium-voltage converter-basedmicrogrids with dynamic loads, IEEE Trans. Ind. Electron. 61 (2014)1643–1658.

[95] A.A.A. Radwan, Y.A.-R.I. Mohamed, Analysis and active-impedance-based sta-bilization of voltage-source-rectifier loads in grid-connected and isolatedmicrogrid applications, IEEE Trans. Sustain. Energy 4 (2013) 563–576.

[96] Y.A.-R.I. Mohamed, A.A.A. Radwan, T.K. Lee, Decoupled reference-voltage-based active DC-link stabilization for PMSM drives with tight-speedregulation, IEEE Trans. Ind. Electron. 59 (2012) 4523–4536.

[97] R. Maheshwari, S. Munk-Nielsen, K. Lu, An active damping technique forsmall DC-link capacitor based drive system, IEEE Trans. Ind. Inform. 9 (2013)848–858.

[98] W.-J. Lee, S.-K. Sul, DC-link voltage stabilization for reduced DC-link capacitorinverter, IEEE Trans. Ind. Appl. 50 (2014) 404–414.

[99] P. Magne, D. Marx, B. Nahid-Mobarakeh, S. Pierfederici, Large-signal stabiliza-tion of a DC-link supplying a constant power load using a virtual capacitor:impact on the domain of attraction, IEEE Trans. Ind. Appl. 48 (2012) 878–887.

[100] Z. Bing, J. Sun, Line-frequency rectifier DC-bus voltage instability analysisand mitigation, in: IEEE 12th Workshop on Control and Modeling for PowerElectronics (COMPEL), 2010.

[101] M. Davari, Y. Mohamed, Variable-structure-based nonlinear control for themaster vsc in DC-energy-pool multiterminal grids, IEEE Trans. Power Elec-tron. 29 (2014) 6196–6213.

[102] S. Anand, B.G. Fernandes, Reduced-order model and stability analysis of low-voltage DC microgrid, IEEE Trans. Ind. Electron. 60 (2013) 5040–5049.

[103] Y.A-R.I. Mohamed, Mitigation of converter-grid resonance, grid-induceddistortion, and parametric instabilities in converter-based distributed gen-eration, IEEE Trans. Power Electron. 26 (2011) 983–996.

[104] S.R. Rudraraju, A.K. Srivastava, S.C. Srivastava, N.N. Schulz, Small signal sta-bility analysis of a shipboard MVDC power system, in: IEEE Electric ShipTechnologies Symposium, ESTS 2009, 2009, pp. 135–141.

[105] A. Griffo, J. Wang, Modeling and stability analysis of hybrid power systemsfor the more electric aircraft, Electr. Power Syst. Res. 82 (2012) 59–67.

[106] M. Benidris, S. Elsaiah, S. Sulaeman, J. Mitra, Transient stability of distributedgenerators in the presence of energy storage devices, in: North AmericanPower Symposium (NAPS), 2012.

[107] Q.-C. Zhong, G. Weiss, Synchronverters: inverters that mimic synchronousgenerators, IEEE Trans. Ind. Electron. 58 (2011) 1259–1267.

[108] Q.-C. Zhong, P.-L. Nguyen, Z. Ma, W. Sheng, Self-synchronized synchronvert-ers: inverters without a dedicated synchronization unit, IEEE Trans. PowerElectron. 29 (2014) 617–630.

[109] S.M. Ashabani, Y.A.I. Mohamed, A flexible control strategy for grid-connectedand islanded microgrids with enhanced stability using nonlinear microgrid

stabilizer, IEEE Trans. Smart Grid 3 (2012) 1291–1301.

[110] A. Berizzi, A. Silvestri, D. Zaninelli, M. Massucco, Short-circuit current calcu-lations for DC systems, IEEE Trans. Ind. Appl. 32 (1996) 990–997.

[111] D. Salomonsson, L. Söder, A. Sannino, Protection of low-voltage DC microgrids,IEEE Trans. Power Deliv. 24 (2009) 1045–1053.

s Research 119 (2015) 407–417

[112] L. Tang, B.-T. Ooi, Locating and isolating DC faults in multi-terminal DC sys-tems, IEEE Trans. Power Deliv. 22 (2007) 1877–1884.

[113] J.-D. Park, J. Candelaria, Fault detection and isolation in low-voltage DC-busmicrogrid system, IEEE Trans. Power Deliv. 28 (2013) 779–787.

[114] Li Huimin, Li Weilin, Luo Min, A. Monti, F. Ponci, Design of smart MVDC powergrid protection, IEEE Trans. Instrum. Meas. 60 (2011) 3035–3046.

[115] J. Jeon, J. Kim, J. Hur, H. Jin, Design guideline of DC distribution systems forhome appliances: issues and solution, in: IEEE International Electric Machinesand Drives Conference (IEMDC), 2011, pp. 657–662.

[116] M.E. Baran, N.R. Mahajan, Overcurrent protection on voltage-source-converter-based multiterminal DC distribution systems, IEEE Trans. PowerDeliv. 22 (2007) 406–412.

[117] S.D.A. Fletcher, P.J. Norman, S.J. Galloway, P. Crolla, G.M. Burt, Optimizing theroles of unit and non-unit protection methods within DC microgrids, IEEETrans. Smart Grid 3 (2012) 2079–2087.

[118] R. Mehl, P. Meckler, Comparison of advantages and disadvantages of elec-tronic and mechanical protection systems for higher voltage DC 400 V, in:Proceedings of the 35th International Telecommunication Energy Conference(INTELEC), 2013, pp. 1–7.

[119] J.-M. Meyer, A. Rufer, A DC hybrid circuit breaker with ultra-fast contact open-ing and integrated gate-commutated thyristors (IGCTs), IEEE Trans. PowerDeliv. 21 (2006) 646–651.

[120] VL Circuit Breakers (Information Guide), 2014, Available at: https://extranet.w3.siemens.com/us/internet-dms/btlv/CircuitProtection/MoldedCase-Breakers/docs MoldedCaseBreakers/SIE TA VL Info Guide.pdf

[121] ABB Circuit Breakers for Direct Current Applications, 2014, Available at:http://www05.abb.com/global/scot/scot260.nsf/veritydisplay/de4ebee4798-b6724852576be007b74d4/$file/1sxu210206g0201.pdf

[122] General DC Circuit Breakers, 2014, Available at: http://www.eaton.com/Eaton/ProductsServices/Electrical/ProductsandServices/CircuitProtection/MoldedCaseCircuitBreakers/DCBreakers/General/index.htm-tabs-2

[123] SACE Emax DC, 2014, Available at: http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/297dcb6f49ac0ecbc12576d4005b5b32/$file/1sdc200012d0202.pdf

[124] Dual Rated AC/DC Circuit Breakers, 2014, Available at: http://www.schneider-electric.com/products/us/en/50300-circuit-breakers/50370-direct-current-dc-rated-circuit-breakers/7218-dual-rated-ac-dc-circuit-breakers/

[125] IEEE Recommended Practice for the Design of DC Auxiliary Power Systemsfor Generating Stations, IEEE Std. 946-2004, 2004.

[126] IEEE Recommended Practice for Emergency and Standby Power Sys-tems for Industrial and Commercial Applications, IEEE Std. 446-1995,1995.

[127] IEC 61660, Short-circuit Currents in dc Auxiliary Installations in Power Plantsand Substations, 1997.

[128] IEEE Recommended Practice for Interconnecting Distributed Resources withElectric Power Systems Distribution Secondary Networks, in: IEEE Std 1547.6-2011, 2011.

[129] NFPA 70®, National Electric Code®, 2014 ed., 2014.[130] Available on (2014): http://www.emergealliance.org/[131] Public Overview of the Emerge, Public Overview of the Emerge Alliance Occu-

pied Space Standard Version 1.1.[132] Public Overview of the Emerge, Public overview of the Emerge Alliance

Data/Telecom Center Standard Version 1.0.[133] MIL-STD-1399/390, Military Standard: Interface Standard for Shipboard Sys-

tems (Section 390) Electric Power, Direct Current, 1987.[134] D. Marquet, T. Tanaka, K. Murai, T. Toru, T. Babasaki, DC power wide spread in

telecom/data center and in home/office with renewable energy and energyautonomy, in: Proceedings of the 35th International TelecommunicationEnergy Conference (INTELEC), 2013.

[135] F. Grassi, S. Pignari, J. Wolf, Channel characterization and EMC assessment ofa PLC system for spacecraft DC differential power buses, IEEE Trans. Electro-magn. Compat. 53 (2011) 664–675.

[136] S.S. Gerber, R. Patterson, B. Ray, C. Stell, Performance of a spacecraftDC–DC converter breadboard modified for low temperature operation, in:Proceedings of the 31st Intersociety Energy Conversion Engineering Confer-ence, IECEC 96, vol. 1, 1996, pp. 592–598.

[137] R. Nelms, L. Grigsby, Simulation of DC spacecraft power systems, IEEE Trans.Aerosp. Electron. Syst. 25 (1989) 90–95.

[138] O. Mourra, A. Fernandez, F. Tonicello, S. Landstroem, Multiple port DC–DCconverter for spacecraft power conditioning unit, in: 27th Annual IEEEApplied Power Electronics Conference and Exposition (APEC), 2012, pp.1278–1285.

[139] D. Kintner, Duke Energy – EPRI DC Powered Data Center Demonstration:Executive Summary, Electric Power Research Institute (EPRI), 2011.

[140] D. Thompson, DC voltage stabilization control in telecommunications DCdistribution systems, in: 24th International Telecommunications Energy Con-ference (INTELEC), 2002, pp. 74–78.

[141] T. Gruzs, J. Hall, AC, DC or hybrid power solutions for today’s telecom-munications facilities, in: 22nd International Telecommunications EnergyConference (INTELEC), 2000, pp. 361–368.

[142] C. Foster, M. Dickinson, High voltage DC power distribution for telecommu-

nications facilities, in: IEEE 30th International Telecommunication EnergyConference (INTELEC), 2008, pp. 1–4.

[143] I. Barbi, R. Gules, Isolated DC–DC converters with high-output voltage forTWTA telecommunication satellite applications, IEEE Trans. Power Electron.18 (2003) 975–984.

System

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

A.T. Elsayed et al. / Electric Power

144] Sangsun Kim, M.H. Todorovic, P.N. Enjeti, Three-phase active harmonic recti-fier (AHR) to improve utility input current THD in telecommunication powerdistribution system, IEEE Trans. Ind. Appl. 39 (2003) 1414–1421.

145] Kongwei, Qinlijun, Yangqixun, Dingfuhua, DC side short circuit transient sim-ulation of DC traction power supply system, in: International Conference onPower Systems Technology (PowerCon), vol. 1, 2004, pp. 182–186.

146] Y. Cai, M. Irving, S. Case, Iterative techniques for the solution of complexDC-rail-traction systems including regenerative braking, IEEE Proc. – Gener.Transm. Distrib. 142 (1995) 445–452.

147] S. Joshi, R. Pathak, A. Jain, Modeling and analysis of DC traction system in lightof recent innovations from HPC and virtual reality, in: International Confer-ence on Advances in Recent Technologies in Communication and Computing(ARTCom ′09), 2009, pp. 525–527.

148] Z. Panfeng, L. Yongli, An adaptive protection scheme in subway DC tractionsupply system, in: International Conference on Power System Technology,vol. 2, 2002, pp. 716–719.

149] H. Ouyang, K. Zhang, P. Zhang, Y. Kang, J. Xiong, Repetitive compensationof fluctuating DC link voltage for railway traction drives, IEEE Trans. PowerElectron. 26 (2011) 2160–2171.

150] W. Liu, Q. Li, M. Chen, Study of the simulation of DC traction power supplysystem based on AC/DC unified Newton–Raphson method, in: InternationalConference on Sustainable Power Generation and Supply (SUPERGEN ′09),2009, pp. 1–4.

151] D. Paul, DC traction power system grounding, IEEE Trans. Ind. Appl. 38 (2002)818–824.

152] P. Arboleya, G. Diaz, M. Coto, Unified AC/DC power flow for traction systems:a new concept, IEEE Trans. Veh. Technol. 61 (2012) 2421–2430.

153] J. Ciezki, R. Ashton, Selection and stability issues associated with a navy ship-board DC zonal electric distribution system, IEEE Trans. Power Deliv. 15 (2000)665–669.

154] P. Kankanala, S. Srivastava, A. Srivastava, N. Schulz, Optimal control of voltageand power in a multi-zonal MVDC shipboard power system, IEEE Trans. PowerSyst. 27 (2012) 642–650.

155] Y. Pan, P.M. Silveira, M. Steurer, T.L. Baldwin, P.F. Ribeiro, A fault locationapproach for high-impedance grounded DC shipboard power distributionsystems, in: IEEE Power and Energy Society General Meeting, 2008, pp. 1–6.

156] G. Sulligoi, A. Tessarolo, V. Benucci, M. Baret, A. Rebora, A. Taffone, Mod-eling, simulation, and experimental validation of a generation system formedium-voltage dc integrated power systems, IEEE Trans. Ind Appl. 46 (2010)1304–1310.

157] I.-Y. Chung, W. Liu, K. Schoder, D.A. Cartes, Integration of a bi-directionalDC–DC converter model into a real-time system simulation of a shipboardmedium voltage DC system, Electr. Power Syst. Res. 81 (2011) 1051–1059.

158] M. Albu, E. Kyriakides, G. Chicco, M. Popa, A. Nechifor, Online monitoring ofthe power transfer in a DC test grid, IEEE Trans. Instrum. Meas. 59 (2010)1104–1118.

159] S.D. Sudhoff, S. Pekarek, B. Kuhn, S. Glover, J. Sauer, D. Delisle, Naval combatsurvivability testbeds for investigation of issues in shipboard power elec-tronics based power and propulsion systems, in: Power Engineering SocietySummer Meeting, 2002, pp. 347–350.

160] B. Cassimere, C.R. Valdez, S. Sudhoff, S. Pekarek, B. Kuhn, D. Delisle, E. Zivi,System impact of pulsed power loads on a laboratory scale integrated fightthrough power (IFTP) system, in: IEEE Electric Ship Technologies Symposium,2005, pp. 176–183.

161] V. Salehi, A. Mohamed, A. Mazloomzadeh, O.A. Mohammed, Laboratory-based

smart power system, part I: design and system development, IEEE Trans.Smart Grid 3 (2012) 1394–1404.

162] V. Salehi, A. Mohamed, A. Mazloomzadeh, O.A. Mohammed, Laboratory-basedsmart power system, part II: control, monitoring, and protection, IEEE Trans.Smart Grid 3 (2012) 1405–1417.

s Research 119 (2015) 407–417 417

[163] J.P. Kelley, D.A. Wetz, J.A. Reed, I.J. Cohen, G.K. Turner, W. Lee, The impact ofpower quality when high power pulsed DC and continuous AC loads are simul-taneously operated on a microGrid testbed, in: IEEE Electric Ship TechnologiesSymposium (ESTS), 2013, pp. 6–12.

Ahmed T. Elsayed (GS’2012) was born in Qaluobia, Egyptin 1984. He received his B.Sc. and M.Sc. degrees fromthe Faculty of Engineering, Benha University, Egypt in2006 and 2010, respectively. From 2006 to 2012, he wasa research/teaching assistant in the Faculty of Engineer-ing, Benha University. Currently, he is a PhD candidateand a research assistant at the Electrical and ComputerEngineering Department, College of Engineering and Com-puting, Florida International University, Miami, Florida,USA. His current research interests are DC distributionarchitectures, Flywheel energy storage and Energy Man-agement of Power Systems. aghar002@fiu.edu.

Ahmed A. Mohamed (El-Tallawy) (GS’2009, M’2013) is anAssistant Professor of Electrical Engineering at the GroveSchool of Engineering, City College of the City Univer-sity of New York (CCNY). He received his Ph.D. degreefrom Florida International University, Miami, Florida in2013, and then worked as a post-doctoral research fel-low at the Energy Systems Research Laboratory, FloridaInternational University before joining CCNY. His currentresearch interests include AC and DC microgrids, renew-able energy utilization and distributed control of powersystems. amohamed@ccny.cuny.edu.

Osama A. Mohammed (S’79, SM’84, F’94) is a Professor ofElectrical Engineering and is the Director of the EnergySystems Research Laboratory at Florida InternationalUniversity, Miami, Florida. He received his Master andDoctoral degrees in Electrical Engineering from VirginiaTech in 1981 and 1983, respectively. He has performedresearch on various topics in power and energy systemsin addition to computational electromagnetics and designoptimization in electric machines, electric drive systemsand other low frequency environments. He performedmultiple research projects for several Federal agenciessince 1990s dealing with; power system analysis, physicsbased modeling, electromagnetic signature, sensorless

control, electric machinery, high frequency switching, electromagnetic interferenceand ship power systems modeling and analysis. He has currently active researchprograms in a number of these areas funded by DoD, the US Department of Energyand several industries. He is a world renowned leader in electrical energy systemsand computational electromagnetics. He has published more than 400 articles inrefereed journals and other IEEE refereed International conference records. He alsoauthored a book and several book chapters. Professor Mohammed is an elected Fel-low of IEEE and is an elected Fellow of the Applied Computational ElectromagneticSociety. Professor Mohammed is the recipient of the prestigious IEEE Power and

Energy Society Cyril Veinott electromechanical energy conversion award and the2012 outstanding research award from Florida International University. He servesas editor of several IEEE Transactions including the IEEE Transactions on EnergyConversion, the IEEE Transactions on Smart Grid, IEEE Transactions on IndustryApplications and COMPEL.