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AC and DC technology in microgrids: A review

Estefanía Planas n, Jon Andreu, José Ignacio Gárate, Iñigo Martínez de Alegría, Edorta IbarraUniversity of the Basque Country UPV/EHU, Bilbao, Spain

a r t i c l e i n f o

Article history:Received 23 June 2014Received in revised form14 October 2014Accepted 1 November 2014Available online 5 December 2014

Keywords:Distributed generationMicrogridsAC technologyDC technology

a b s t r a c t

Microgrids are a suitable, reliable and clean solution to integrate distributed generation into the mains grid.Microgrids can present both AC and DC distribution lines. The type of distribution conditions the performanceof distribution line and implies different features, advantages and disadvantages in each case. This paperanalyses, in detail, all this parameters for AC and DC microgrids in order to identify and describe the availablealternatives for building and configuring a microgrid. Elements and issues involved in the implementationand development, such as protections, power converters, economic analysis, and availability are discussed anddescribed. This analysis constitutes a tool for selecting a suitable configuration of a microgrid adapted to theneeds in each situation. In addition, the paper provides a picture of the current situation of microgrids, andidentifies and proposes future research lines.

& 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7262. Description of a microgrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7273. Transmission system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7304. Point of common coupling (PCC): interconnection between the mains grid and the microgrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7325. Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7336. Protection schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7347. Monitoring and measuring system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7358. Power converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7359. Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 735

9.1. Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7369.2. Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7369.3. Power quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7369.4. Optimisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738

10. Regulatory issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73911. Economic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74012. Future research areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74213. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746

1. Introduction

The penetration of distributed generation on the mains grid hasincreased greatly during the last years due to its several advantages.

It is mainly based on renewable resources which reduces theenvironmental impact, and allows to exploit and harvest localenergy sources. This growing presence of distributed generationrequires an analysis of its impact on the mains grid in order tominimise losses, line loadings, and reactive power requirements [1].On the other hand, it is a challenge to integrate renewable energysources directly into the mains grid because of their intermittence,randomness and the uncertainty caused by meteorological factors.

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n Corresponding author.E-mail address: [email protected] (E. Planas).

Renewable and Sustainable Energy Reviews 43 (2015) 726–749

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In this sense, as microgrids integrate distributed and renewablesources, energy storage devices and large variety of loads, they are asuitable interface between this distributed generation and themains grid [2,3].

The microgrids definition states that; they are local distributionsystems that include generation, storage and load capabilities, andthey can work isolated or connected to the mains grid [4]. This abilityfor operation both connected and disconnected from the mains gridimproves the reliability and power quality of the users connected tothem. In addition, if these microgrids are planned following anecodesign, nearly zero energy buildings (NZEB) can be obtained [5].

Microgrids can be designed to support alternating current (AC)or direct current (DC). Each alternative has distinctive features,which imply different advantages and disadvantages that need tobe pondered. Comparatives between the two types of microgridsin terms of control, protections and power losses are provided in[6–9]. Following the same line, this paper presents a detailed studyof AC and DC microgrids that provides the main characteristics ofthe components of each type of microgrid. The work also discussesand analyses technical, economic and regulation issues such asavailable power converters architectures, applicable directive,monitoring systems, and economic analysis. The main objectivesof this analysis are:

� Highlighting the advantages and drawbacks of each technology.� Identifying and classifying feasible configurations and archi-

tectures require to implement a microgrid.

The aforementioned objectives become a tool that allows tochoose the most suitable microgrid that fulfils the specificationsrequired in a given situation. In addition, this analysis constitutes apicture of the microgrids state of art, and proposes the researchlines to deal with microgrids current needs, and to furtherdevelop them.

2. Description of a microgrid

Microgrids are integrated systems in which distributed energyresources (DERs) create a grid that feeds a variable number of

distributed loads. Both elements constitute the main body of amicrogrid.

DERs, can be classified into two groups (Fig. 2):

� Distributed generators (DG). Electric microgrids include differenttypes of DGs that can be based on either renewable or non-renewable resources. This characteristic allows to adequatelyexploit the available resources in each location (wind, sun,biomass, etc.) (Fig. 2). Tables 1 and 2 summarise the main typesof distributed generators [1,10–12]. This table highlights thatnon-renewable based technologies provide yet higher efficien-cies. It also shows that wind and small hydro-power basedsystems offer the highest efficiencies among the renewablebased technologies. Ocean energy, although is a very promisingsource of energy, still requires further research and develop-ment to become profitable and reach the market with guaran-tees of success [10].

� Storage systems (SS). The use of SSs improves the stability,power quality, reliability of supply and the overall performanceof a microgrid [13,14]. Table 3 summarises some key energystorage technologies available for microgrid applications[15–17]. It is interesting to underline that, even if supercon-ducting magnetic energy storage (SMES) provides high effi-ciency, this technology is still in the demonstration stage.Table 3 also shows that there are other technologies such aspumped hydro storage (PHS) or compressed air energy storage(CAES) that, in spite of their lower efficiencies, have highercapacities with longer lifetimes. There are different proceduresto estimate the amount of power that a SS requires. Parametersand considerations for sizing the energy storage in stand alonesystems can be found in [18]. In addition, Ref. [19] providesmodels for the same purpose, that also guarantee safe energysupply and help to reach suitable power quality in the micro-grid. For the particular case of lead-acid batteries in stand-alone photovoltaic systems, some guidelines and recom-mended practices for its sizing can also be found in [20].

There is a wide range of loads (residential, industrial, etc.)(Fig. 2) that can be connected to a microgrid. Two main groups areconsidered, critical/sensitive and Noncritical loads [21]. This clas-sification is often performed by the microgrid central controller

CONNECTION WITH

DC MICROGRID

vDC vAC

5 KWINVERTER

WASHINGMACHINE DRYER

DISHWASHER

VACUUMCLEANERPC

FRIDGEOVEN

INDUCTIONBURNER TV

MICROWAVE

Fig. 1. R1.1 Home feeded by a DC microgrid with an internal AC distribution system.

E. Planas et al. / Renewable and Sustainable Energy Reviews 43 (2015) 726–749 727

Table 1Main technologies of non-renewable DGs.

Energy basedtechnologytype

Primary energy Outputtype

Modulepower(kW)

Electricalefficiency(%)

Overallefficiency(%)

Advantages Disadvantages

Reciprocatingengines

Diesel or gas AC 3–6000 30–43 �80–85Low cost Environmentally

unfriendly emissions

High efficiency

Ability to use various

inputsGas turbine Diesel or gas AC 0.5–30,000 21–40 �80–90

High efficiencies when

using with CHP

Too big for small

consumers

Environmentally friendly

Cost effective

Micro-turbine Bio-gas, propane or natural gas AC 30–1000 14–30 �80–85Small size and light

weight

Expensive technology

Easy start-up and shut-

down

Cost-effectiveness

sensitive to the price of fuel

Low maintenance costs Environmentally

unfriendly emissionsFuel cell Ethenol, H2, N2, natural gas, PEM,

phosphoric acid or propaneDC 1–20,000 05–55 �80–90

One of the most

environmentally friendlygenerator

Extracting hydrogen

is expensive

Extremely quiet Expensive

infrastructure for hydrogen

Useful for combined heat

andelectricity applications

1 Transmission 2 3 DistributionPCC Protections Monitoring Powerconverters

4 5 6 7 Control 8 Regulatoryissues

9 Economicanalysis

€

Fig. 2. A general scheme of a microgrid. Elements that differ in AC microgrids (blue) and elements that differ in DC microgrids (green). (For interpretation of the references tocolor in this figure caption, the reader is referred to the web version of this article.)

E. Planas et al. / Renewable and Sustainable Energy Reviews 43 (2015) 726–749728

(MGCC) [22], but can also be done in a decentralised manner byagents [23], both alternatives are detailed in Section 9.

Besides the aforementioned elements, microgrids require otherinfrastructures (Fig. 2):

� Point of common coupling (PCC) (Fig. 2 - ②). Microgrids canoperate connected or disconnected from the mains grid. PCCconstitutes the gateway between both grids. This connectioncan be performed through switchgears and power converters,which are detailed in Section 4.

� Distribution (Fig. 2 - ③). The main elements of a microgrid(DERs and loads) are interconnected with distribution lines.Meanwhile AC microgrids use single phase or three phase lines,the distribution in DC microgrids is monopolar, homopolar orbipolar (Section 5).

� Protections (Fig. 2 - ④). Microgrids, either connected or dis-connected to the mains grid, must have protection mechanismsthat guarantees safe operation. These protection elements mustbe designed following different principles and parameters.Besides, they must allow to be configured in different manners(Section 6).

� Monitoring (Fig. 2 - ⑤). There are many parameters in amicrogrid such as voltage, frequency, and power quality thatmust be continuously supervised by means of a monitoringsystem. Its main components and features for AC and DCmicrogrids are described in Section 7.

� Power converters (Fig. 2 - ⑥). DERs and loads are generallyconnected to the distribution lines of the microgrid throughpower converters. They adapt the currents and voltages levelsof the microgrid to the connected units. According to the type

Table 2Main technologies of renewable DGs.

Energy basedtechnologytype

Primaryenergy

Outputtype

Modulepower(kW)

Electricalefficiency(%)

Overallefficiency(%)

Advantages Disadvantages

Wind Wind AC 0.2–3000 –a �50–80Day and night power generation Still expensive

One of the most developed renewable energy

technology

Storage mechanisms

requiredPhotovoltaic

systemsSun DC 0.02–1000 –a �40–45

Emission free Storage mechanisms

required

Useful in a variety of applications High up-front cost

Biomassgasification

Biomass AC 100–20,000

15–25 �60–75Minimal environmental impact

Available throughout the world Still expensive

Alcohols and other fuels produced by biomass

are efficient, viable, and relatively clean burning

A net loss of energy in

small scaleSmall hydro

powerWater AC 5–100,000 –a �90–98

Economic and environmentally friendly Suitable site characteristics

required

Relatively low up-front investment costs and

maintenance

Difficult energy expansion

Useful for providing peak power and spinning

reserves

Environmental impact

Geothermal Hotwater

AC 5000–100,000

10–32 �35–50Extremely environmentally friendly Non-availability of

geothermal spots in the land ofinterest

Low running costs

Ocean energy Oceanwave

AC 10–1000 –a –a

High power density Lack of commercial

projects

More predictable than solar or wind Unknown operations and

maintenance costsSolar thermal Sun and

waterAC 1000–

80,00030–40 �50–75

Simple, low maintenance Unknown operations and

maintenance costs

Operating costs nearly zero Low energy density

Mature technology Limited scalability

a No data available.


of microgrid, different types of power converters are used(Section 8).

� Control (Fig. 2 - ⑦). In a microgrid, there are different sourcesand mechanisms available to gather information. This informa-tion requires further processing, which eventually implies thatcertain tasks need to be performed and coordinated, such asload sharing, voltage level control, and electric generation. BothAC and DC microgrids have a hierarchical control system thatcarries out such tasks. Section 9 explains in detail this controlsystem.

Finally, deployment and maintenance of microgrids mustponder the next practical issues (Fig. 2):

� Regulatory issues (Fig. 2 - ⑧). Microgrids need a regulatoryframework in order to provide a successful development andimplementation. Standardisation of AC and DC microgrids hasprogressed during the last years because there are currentlyunder development several projects that deal with this objec-tive (Section 10).

� Economic analysis (Fig. 2 - ⑨). An analysis of the cost andbenefits of a microgrid requires to consider multiple factors andvariables. Section 11 describes which are those parameters andmethodologies to perform economic analysis of microgrids.

The aforementioned elements and issues mainly depend on thetechnology (AC or DC) of the distribution line of the microgrid.At the same time, because microgrids can be connected to AC andDC transmission systems (Fig. 2 - ①) with different advantagesand disadvantages (Section 3). The next sections deal with all

these questions and identify the characteristics and features of thetransmission systems according to the type of technology used.

3. Transmission system

The main characteristic of microgrids is that they can workboth connected or disconnected from the mains grid (Fig. 2 - ①).Nowadays, the majority of power transmission lines use threephase AC technology [24] (Fig. 2 - ①, 1a). However, in recent years,DC high voltage direct current (HVDC) technology (Fig. 2 - ①‘, 1b)has become more popular for power transmission due to theadvantages that offers, such as high power density, controlledemergency support, no contribution to short circuit level and morestability [25–27]. Moreover, an upgrade of AC lines into DC onesimplies minor infrastructure changes and increases the transmis-sion capacity [24]. In this sense, there are several examples ofHVDC installations in different countries (Table 4) [26,28]. Theirlocations can be found in Fig. 3. There are remarkable ones like theRio Madeira in Brazil (Fig. 3, 3), which is the longest HVDCinstallation built ever (2375 km) [29]. Jinping-Sunan7800 kVultrahigh voltage direct current (UHVDC) installation, in China(Fig. 3, 33) is the most powerful transmission line in the world,with a rated capacity of 7.2–7.6 GW [29]. More examples of HVDCinstallations can be found in many other countries such as USA,India and Canada, and their number is continuously increasing dueto, among other reasons, the multiple advantages that offers HVDCfor power transmission in long distances (4 800 km for overheadlines and 450 km for cable systems).

Data collected about the amount of HVDC installed in last years,shows an annual linear increase in number of installations and an

Table 3Main technologies of SSs.

Technology Efficiency(%)

Capacity(MW)

Energydensity(Wh/kg)

Capital(€/kW)

Lifetime(years)

Maturity Environmentalimpact

Examples

TESa 30–60 0–300 80–250 140–220 5–40 Developed Small Solar two Central Receiver Solar Power Plant,California (USA)

PHSb 75–85 100–5.000

0.5–1.5 400–1500 40–60 Mature Negative Rocky River PHS plant, Hartford (USA)

CAESc 50–89 3–400 30–60 250–1500 20–60 Developed Negative Huntorf (Germany) and McIntosh, Alabama (USA)Flywheel 93–95 0–25 10–30 250 �15 Demonstration Almost Commercially supplied by AFS-Trinity (USA), Beacon

Power (USA), Piller (USA), etc.Pbacid batteryd 70–90 0–40 30–50 200 5–15 Mature Negative BEWAG Plant, Berlin (Germany)NiCd batterye 60–65 0–40 50–75 350–1.100 10–20 Commercial Negative Golden Valley, Alaska USANaS batteryf 80–90 0.05–8 150–240 700–2.100 10–15 Commercial Negative Tokyo Electric Power Company, JapanLi-ion batteryg 85–90 0–1 75–200 3.000 5–15 Demonstration Negative Kyushu Electric Power and Mitsubishi Heavy

Industries (Japan)Fuel cells 20–50 0–50 800–

10.000350–1.100 5–15 Developing Small Topsoe Fuel Cell, Lyngby, (Denmark)

Flow battery 75–85 0.3–15 10–50 400–1.100 5–15 Developing Negative Innogys Little Barford Power Station, (UK)Capacitors 60–65 0–0.05 0.05–5 250 �5 Developed Small Commercially supplied by SAFT (France),

NESS (Korea), ESMA (Russia) etc.Supercapacitors 90–95 0–0.3 2.5–15 200 4 20 Developed Small PowerCache (Maxwell, USA), ELIT (Russia),

PowerSystem Co. (Japan), Chubu Electric Power(Japan), etc.

SMESh 95–98 0.1–10 0.5–5 200 4 20 Demonstration Positive Wisconsin Public Service Corporation (USA)

a TES: Thermal Energy Storage.b PHS: Pumped Hydro Storage.c CAES: Compressed Air Energy Storage.d Pb-acid battery: Lead-acid battery.e Ni–Cd battery: Nickel–Cadmium battery.f Na–S battery: Sodium–sulfur battery.g Li-ion battery: Lithium-ion battery.h SMES: Superconducting Magnetic Energy Storage.


exponential growth of the installed power (Fig. 4). Table 5 com-pares the performance of the two transmission technologies [26].The table also shows that DC transmission systems are more costeffective than AC ones for long distances, and they overcome somedrawbacks presented in AC transmission systems, such as

maintenance of synchronism, need of line compensation and highground impedance. Thus, DC transmission systems are moresuitable than AC transmission systems in applications that requiretransmission of power over long distances, such as undergroundand underwater cables. However, nowadays tapping microgrids to

Table 4Principle HVDC installations.

Continent Location Fig. 2 HVDC links Continent Location Figs. 2 and 3 HVDC links

Africa DR Congo 1 Inga-Shaba Asia India 35 VindhyachalMozambique–S. Africa 2 Cahora Bassa 36 Sileru–Barsoor

America Brazil 3 Itaipu I and II 37 Rihand–Delhi4 Rio Madeira 38 Chandrapur–Ramagundum

Canada 5 Vancouver I and II 39 Chandrapur–Padghe6 Nelson River I, II and bipole II 40 Gazuwaka–Jeypore7 Eel River 41 East-South Intercon.8 Chateauguay 42 Vizag I and II9 Madawaska Japan 43 Sakuma

10 McNeill 44 Shin-Shinano11 Nicolet Tap 45 Hokkaido Honshu

Canada–USA 12 Des Cantons–Comerford Philippines 46 Leyte–Luzun13 Quebec–New England South Korea 47 Haenam–Cheju

Paraguay 14 Acaray Thailand–Malaysia 48 Thailand–MalaysiaUSA 15 Pacific Intertie and Pacific

Intertie upgradeEurope Austria 49 Duernrohr

16 Square Butte 50 Vienna-South east17 David A. Hamil Denmark–Germany 51 Kontek Interconnection18 CU project Denmark–Sweden 52 Konti–Skan I and II19 Eddy County Estonia–Finland 53 Estlink20 Oklaunion Finland–Sweden 54 Fenno–Skan21 Blackwater France–United Kingdom 55 Les Mandarins–Sellindge22 Highgate Greece–Italy 56 Galatina–Arachthos23 Miles City Ireland–Scotland 57 Moyle24 Intermountain power project Italy 58 Sardinia25 Sidney (Virginia Smith) 59 Sacoi26 WelchMonticello Norway 60 Valhall offshore27 Rapid City DC tie Norway–Denmark 61 Skagerrak I, II and III28 Lamar Norway–Netherlands 62 NorNed

Asia China 29 TSQ-Beijao Russia 63 Volgograd–Donbass30 Three Gorges–Changzhou 64 Vyborg31 Three Gorges–Quangdong Sweden 65 Gotland I, II and III32 Guizhou–Guangdong Sweden–Germany 66 Baltic Cable Project33 Three Gorges–Shanghai United Kingdom 67 Kingsnorth34 Jinping–Sunan Oceania Australia 68 Broken Hill

Australia–Tasmania 69 BasslinkNew Zealand 70 Inter-island

Fig. 3. Principle HVDC installations all over the world. Note: Numbers referred to Table 3.


HVDC or medium voltage DC (MVDC) systems is a technicalchallenge.

4. Point of common coupling (PCC): interconnection betweenthe mains grid and the microgrid

Microgrids are connected and disconnected from the mains gridat the PCC (Fig. 2 - ②). This interconnection is done through aswitchgear, that can also be combined with power electronics.Three main types of switchgears are used: circuit breakers (CB),contactors and switches [30]. In microgrids, among the aforemen-tioned types of devices, the preferred switchgear is the switch. Infact, recent researches on microgrids employ static switches withfast response or digital signal processor (DSP) [21]. These inter-connection switches are designed to meet grid interconnection

standards, reduce custom engineering and site-specific approvalprocesses, and minimise the cost [14]. A study of the grid inter-connection standards and guidelines for Europe and America can befound in [31]. Although, the study of microgrid interconnection canbe simplified in medium voltage (MV) and low voltage (LV) net-works [32]. LV switches are normally air insulated, meanwhile MVtypes are air, oil or sulfur hexafluoride (SF6) insulated [30].

Some examples of microgrids interconnected to the mains gridonly with switches (Fig. 2 - ②, 2a) can be found [14,33–37].However, this type of connection usually requires additionalelements that isolate the microgrid from the mains grid. Thosedevices are transformers (Fig. 2 - ②, 2b) [14,21,38–50] and powerconverter (Fig. 2 - ②, 2c, 2e, 2 g and 2i) [21,51–53].

Table 6 summarises most of the possible interconnectionsbetween microgrids and the mains grid as a function of the typeof current and voltage level. It also classifies the main topologiesthat can be used for PCC. Thus, Table 6 (blue text) comprisespossible topologies for DC transmission (Fig. 2 - ②, 2d, 2f and 2 h)with combinations for different voltage levels (back to backconverters and matrix converters [54,55], multilevel converters[56], modular multilevel converters [57], inverters [58] and non-isolated and isolated DC/DC converters [59].

The microgrid technology (AC or DC) is a key factor that mustbe considered in order to chose a proper interconnection switch.A mechanical switch does not open instantaneously, so there willbe a short arc until the dielectric strength is enough to hold offthe driving voltage. In an AC circuit, this interruption process isassisted by the fact that there is a natural ‘current zero’. DC circuitdo not have natural ‘current zero’, which implies that the switchmust incorporate a mechanism to set the current to zero. As aresult, the stress on the contacts are greater in a DC switchthan in an AC one for the same voltage and current. Unless,sometimes it is feasible to use an overrated AC circuit breaker inDC systems, many applications require the development of aspecific DC breakers. Nowadays, this technical drawback can beovercome through commercial products for LV [60] and MV

80

100

120

140

160

180

200

u / G

W

Number of installations

Power installed (GW)

0

20

40

60

1954

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2014

Year

Fig. 4. Evolution of the number and power of HVDC installations installed allaround the world.

Table 5Comparison between AC and DC transmission systems.

Issue AC DC

Transmission costs� Investment costs

– Right-of-way (ROW) Higher Lower–Towers Higher Simpler and cheaper–Insulators Higher Lower–Conductors Higher Lower–Terminal equipment Lower Higher

� Operation costs–Losses Higher Lower–Skin effect Yes No–Dielectric losses Higher Lower–Corona effects Higher Lower–Compensation By means of reactive power Lower

� Technical considerations–Control of transmitted power Need of reactive power control Full–Transient and dynamic stability Worse Better–Fault currents Limited Higher–Power carrying capacity Distance dependent No distance dependency–Voltage control Load dependent No reactive power (Q) control–Line compensation Yes No–Interconnection Need of synchronism and large power oscillations Asynchronous connection–Ground impedance High Low–Reliability Similar Similar–Availability Similar Similar

Applications Short distances (o 50 km) Underground and underwater cablesLong distance bulk power transmissionAsynchronous interconnections of AC systemsStability of power flows in integrated power systems


microgrids [61]. Nevertheless, solutions for AC systems are stillmore economical.

5. Distribution

Microgrids can be classified into AC and DC microgrids basedon the characteristics of the distribution line (Fig. 2 - ③). There are

also hybrid microgrids that combine AC and DC distribution linesthat are controlled separately [69–72].

AC microgrids can present different distribution types: singlephase (Fig. 2 - ③, 3a), three phase without neutral (Fig. 2 - ③, 3b)and three phase with neutral (Fig. 2 - ③, 3c).

In DC microgrids, there are three main types of distribution:monopolar (Fig. 2 - ③, 3d), bipolar (Fig. 2 - ③, 3e) and homopolar(Fig. 2 - ③, 3f) [9,52]. There are also some DC microgrids withmultiple buses in order to obtain higher reliability [73]. Due to the

Table 6Power converters topologies for the PCC (Fig. 2): topologies in real and experimental microgrids (black) and proposed topologies (blue).

GRID MG

MV LV

AC DC AC DC

HV ACa Transformer [62] a Transformer and inverter

-b -b

a Transformer and back to

back converter or matrixconverter

a Multilevel inverter and modular multilevel

inverter

-b -b

DCa Multilevel inverter and

modular multilevel inverter

aMultilevel converter and modular multilevel

converter

-b -b

a Inverter and transformer-b -b

MV ACa Transformer [63]


inverter

a Transformera Multilevel inverter and


a Back to back converter

and matrix converter

a Transformer and rectifiera Transformer and back to


a Transformer and

inverter



DCaInverter and transformer

a Multilevel converter and modular multilevel

converter and isolated and nonisolated DC DC powerconverters

a Multilevel inverter and


a Multilevel converter

and modular multilevelconverter

a Multilevel inverter and


a Inverter and transformer

LV ACa Transformer a Transformer and inverter

a No power device [14,33–

37]

a Inverter [64]




inverter

aTransformer

[38,14,21,39–50]

a Transformer and

inverter [65,52]

a Back to back converter

[51,66–68,53]

DCa Multilevel inverter and


a Multilevel converter a Inverter a No power device

a Modular multilevel converter a Inverter and transformera Isolated and

nonisolated DC DC powerconverters

a Numbers referred to Fig. 1.b Not possible combination.


fact that there is no reactive power (Q), DC distribution presentsseveral advantages, such as reduction of the power losses andvoltage drop, and an increase of capacity of the electrical lines [6].Therefore, its planning, implementation and operation is simplerand less expensive. A comparison between AC (single-phase andthree phase) and DC (monopolar and bipolar) distribution lines interms of resistances, cable sections and conductor material couldbe found in [74]. It concludes that DC bipolar transmission is thebest option. Furthermore, the studies presented in [75,76] showthat, in the same conditions, DC transmission lines can transmitmore power. Thus, the DC alternative allows a bigger extension ofthe network for the same load, and provides a reliable and high-quality power distribution [77].

Taking into account all the aforementioned facts, it can be saidthat DC distribution has more advantages compared with ACdistribution. However, DC distribution lines have only been usedfor special applications, such as telecommunication systems,vehicles, ships and traction systems [78]. Important researchefforts are being carried out in order to include DC distributionlines in buildings. USA has several examples of this type ofinstallations (Table 7) [79].

The microgrid availability is an issue directly related with thetype of distribution. There are few research papers that deal withthe availability of AC and DC microgrids. In Ref. [80] AC and DCpower systems are compared in terms of reliability, overall feedingefficiency, scalability and maintainability. The analysis in terms ofreliability is carried through the comparison of two AC and DCmodels, and it concludes that the reliability obtained in DC powersystems is about two orders higher than the one reached in ACpower systems. In the same manner, DC microgrids availability isaffected by the circuit topology design choices for the powerelectronic interfaces between the DGs and the rest of the micro-grid [81].

6. Protection schemes

The design of protection schemes for AC and DC microgrids(Fig. 2 - ④) is a challenging task because there are severalparameters involved, such as the dynamic structure of the micro-grid and different levels of current of the devices. Protections mustbe designed based on the following parameters: sensitivity,selectivity, speed of response and security level [9]. Among allthese parameters and issues, the protection scheme is basicallyderived from two: the number of the installed DGs, and theavailability of a sufficient level of short-circuit current in anislanded operating mode of the microgrid [9,82].

Several protection schemes (Fig. 2 - ④) have been developedfor AC microgrids [9,83–87], they can be classified into centralisedand decentralised schemes. On decentralised schemes each DGprovides its own relay, which is an efficient technique against line-

to-ground and line-to-line faults but limited to faults with lowimpedance [9]. On the other hand, centralised methods are basedon a voltage protection scheme and require a central protectionunit in the MGCC [9].

The protection scheme for DC microgrids (Fig. 2 - ④, 4b)requires a different approach because it faces different challenges,mainly the immaturity of standards and guidelines, and thelimited practical experience [88]. Moreover, a common objectionagainst the application of DC in power systems is that the currentin DC systems does not have any natural zero crossing [89,90],thus, short-circuit current interruption is more difficult to obtainthan in an AC system. However, low-voltage DC system can use ACbreakers if the ratings are adjusted to the correction factors thatmanufactures provide [90]. For this reason, it is interesting toidentify which are the principles of AC system protection that maybe applied, and whether it is enough a conventional AC systemprotection. Nowadays, there are several protection devices avail-able on the market for LV DC systems, that comprehend fuses, CBs,molded-case CBs (MCCB), LV power CBs, and isolated-case CBs.In [60] a protection scheme for DC microgrids that uses mostlycommercial DC protection devices is proposed. Another examplecan be found in [91], which proposes a design framework for DCmicrogrids that aims to optimise the protection scheme on thebasis of economic issues. Table 8 summarises the principalprotection schemes proposed in the literature for both AC andDC microgrids [9,60,86,91–93].

Protection schemes must also include ground fault detectionand isolation mechanisms to ensure system safety [88]. Groundingmethods for AC and DC low voltage and medium voltage drivesystems can be classified into solidly grounded, low-resistance,high-resistance and ungrounded systems [94]. Unless there is anoticeable research and practical experience with ground faultdetection and isolation in AC systems, DC systems still requirefurther research and innovation, specially for DC microgrids [88].

Table 7Examples of DC distribution systems in USA.

Country State Location

USA California CA Lightning Tech Center & UC Davis Campus (Davis)Southern Cal Edison, Utility Services Office (Irwindale)LA Community College, Trade Tech Campus (Los Angeles)Johnson Controls, Headquarters Office (Milwaukee)UC San Diego, Sustainability Center (San Diego)

Michigan Optima Engineering, MEP firm (Charlotte)Michigan Nextek Power, NextEnergy Center (Detroit)Pennsylvania PNC Financial, Headquarters Office (Pittsburgh)Texas Lauckgroup Architectural Office (Dallas)Washington US Green Building Council, Conference Rooms (Washington DC)

Table 8Examples of protection schemes for AC and DC microgrids.

Type Protection strategy Operation Communicationlink

AC Adaptive Both YesBased on communication-assisteddigital relays

Both Yes

Voltage Islanded YesHarmonic content Islanded YesCurrent travelling waves Grid-connected NoDistance Both No

DC Based on commercial DCprotections

Both No

Optimal unit and non-unitprotection

Both Yes


7. Monitoring and measuring system

Microgrid monitoring system (Fig. 2 - ⑤) must perform severaltasks and must fulfil certain requirements: distributed structure tomonitor each unit simultaneously, online definition and modifica-tion of system configuration and remote control and monitoring ofthe whole system, among others [95,96]. The available literatureproposes a wide range of monitoring schemes for microgrids[95,97–102]. The following alternatives can be highlighted:

� A framework based on the service-oriented architecture (SOA)model [98]. A SOA can be defined as a component model thatinterrelates different functional units of an application, calledservices, through well-defined interfaces and contractsbetween services.

� Installation at each component of the microgrid universal monitor-ing, protection, and control units (UMPCUs) [99]. These units aresimilar to intelligent electronic devices (IEDs). They collect dataabout connectivity, device model and measurements.

There are vendors that supply commercial solutions for mon-itoring and control of microgrids [103]. It is also possible tomonitor microgrids through phasor measurement units (PMU),as it has been proposed in [104]. These PMUs are systems whichoffer more accurate data about the power system, which allow tomanage the system more efficiently and with a higher level ofresponse [105].

Therefore, the main difference between AC and DC microgridsis that DC ones only require to monitor and control a reducednumber of variables. In fact, monitoring systems for DC microgrids(Fig. 2 - ⑤, 5b) are simpler than AC microgrids (Fig. 2 - ⑤, 5a)because they do not require to monitor the frequency nor thereactive power.

8. Power converters

The three main components of a microgrid (DGs, SSs and loads)are connected to it through power converters (Fig. 2 - ⑥). Thesepower converters rely on the type of microgrid (AC or DC), as wellas on other features of the devices (voltage levels, power flowdirection, etc.). In addition, they usually include a transformer inorder to obtain galvanic isolation.

Tables 18 and 19 summarise the main topologies of isolatedpower converters used in DGs and SSs for coupling to AC grids(Fig. 2 - ⑥, 6a and 6b) [106,107]. Both tables show that powerconverters usually require a controlled inverter, which is com-posed of insulated-gate bipolar transistors (IGBT) and capacitorson the DC bus. Matrix converters can also be used, whichguarantee bidirectional power flow by means of bidirectionalswitches and without any significant reactive component [108].

Since DC grids are not yet widespread, there are no standardtopologies of power converters to couple DERs to them. Tables 18and 19 present some feasible power converters architectures forcoupling DGs and SSs to DC microgrids (Fig. 2 - ⑥, 6c and 6d)[59,107,109–111]. Both tables show that power converters usuallyrequire fewer components in DC lines than in three-phase AClines. Some of DC topologies can also use high-frequency trans-formers, which implies that they are smaller and lighter than thelow-frequency transformers of AC microgrids.

The efficiency of the power converter is a key parameter.It depends on many variables, such as power rate, load rate, corevolume and material when galvanic isolation is needed [112]. Themajority of power converters have optimal efficiency when theywork at their nominal power. In [113] an approach for comparisonof system conversion efficiencies for residences is presented. This

work concludes that the total conversion efficiency in residentialdistribution lines increases in case of AC lines and decreases for DClines. However, if the power source of a residence is a fuel cell oranother DC generator, the total conversion efficiency within aresidential DC distribution system could be similar or better thanfor AC distribution. Another study [114] shows that AC and DCdistribution systems can have similar efficiencies if the followingconditions are met: loads are equal in ratio, and an AC sourcesupplies the AC power and a DC source supplies the DC power. Thework in [115] studies the efficiency of DC systems. Among otherconclusions, it states that, under the assumption that semicon-ductor losses decrease by half, the efficiency of a pure DC systembecomes higher than AC system. Moreover, [116] concludes thatthe current ripple (or harmonics in the AC cases) is reduced in theDC case due to two facts: the AC/DC circuit inside most devicesacts only as a reverse polarity protection, and the ripple caused bydifferent devices is not synchronised. Thus, it can be said that theefficiency of power conversion is normally higher in DC systems,and it is mainly affected by the technology of the primary sourceand the AC and DC load ratios.

9. Control

AC and DC microgrids require different control tasks (Fig. 2 -⑦)in order to guarantee a correct operation of the system. Normally,a hierarchical control carries out these tasks. There are three levelsof controls are described from the outer to the inner level ofcontrol as follows (Fig. 5) [22]:

� Grid level. At this level a distribution network operator (DNO)and a market operator (MO) are found. Active managementtechniques in microgrids allow DNO to take advantage of aseveral control variables which are not considered in theircurrent operation and planning philosophies [117]. The MO isin charge of the participation of the microgrid in energymarkets and it can follow different market policies such asserving the total demand of the microgrid using its localproduction or participating in the open market buying andselling active and reactive power to the grid [118].

� Management level. A MGCC performs those tasks related to themanagement of the microgrid. The main functions of the MGCCare the restoration of the frequency (only in AC microgrids) andvoltage, synchronism between the microgrid and the grid (onlyin AC microgrids), load shedding and optimisation of theproduction of the microgrid [119].

� Field level. One local controller (LC) is placed in each element ofthe microgrid (DGs, SSs or loads). According to the type ofelement (DG, SS or load), the corresponding LC carries outdifferent tasks:○ LC for DGs (Fig. 2 - ⑦, 7a and 7b). The best alternative for

controllable DGs, such gas turbines, small diesel generators,

LC

DG SS Load

MGCC

DNOMO

Grid level

Management level

Field levelLC LC

Fig. 5. General architecture of centralised and decentralised controls of microgrids.


ect., in which the primary energy is controllable, is thedroop control [22,117,120–125]. This control method offersa high reliability and does not require communicationsbetween DERs. Several variations of this method that over-come its drawbacks can be found in the literature [43]. Fornon-controllable DGs, in which the primary energy is notcontrollable, such as wind turbines and PV, nonlinear droopcontrol [126] and hybrid droop control with maximumpower point tracking (MPPT)can be used [127–129].

○ LC for SSs (Fig. 2 - ⑦, 7c and 7d). SSs need specific charge/discharge control strategies. In [130–132] the proposedcontrol method is the state of charge (SoC)-based adaptivedroops. In addition, for an optimal operation differentcontrol techniques as a function of the SS technology usedare proposed in [107].In this control level the islanding detection is carried out inDGs and SSs, which guarantees their good performance bothconnected or disconnected from the mains grid. There areseveral islanding detection techniques for AC lines that canbe classified into local and remote detection techniques[133–135]. All these techniques are based on measuringdifferent magnitudes (voltage amplitude, frequency, phaseand harmonics). However, in DC systems only voltage can bemeasured, so new islanding detection techniques must bedeveloped. There are few works that deal with this issue[136] so more research efforts are needed.

○ LC for Loads. If a centralised hierarchical control is used, loadshedding is controlled by the MGCC. In case of agents basedcontrol, an agent is placed in each load in order to performits control.

Both DNO and MO functions are done at the mains grid level,while MGCC is located in a central computer, and the localcontrollers are placed in power converters coupled to eachelement of the microgrid (DGs, SSs or loads). This hierarchicalcontrol can be carried out in a centralised or decentralised manner,which is commonly called multi-agent based control (MAS). Thecentralised control alternative offers the possibility of implement-ing a basic management system with low costs of installation andoperation. On the other hand, MAS control provides plug and playperformance at expense of a more complex control design [43].

DC microgrids present the advantage that there is no need forcontrolling frequency nor reactive power. In this manner, thecontrol tasks are simplified since the devices can be connectedto the microgrid directly, without any process of synchronism.Therefore the connection of the microgrid to the mains grid isdone asynchronously and only the flow of active power must becontrolled. On the other hand, in AC microgrids frequency must becontrolled and the devices must be synchronised before theirconnection with the microgrid, and the microgrid must besynchronised with the mains grid before its connection. A sum-mary of the main synchronisation methods can be found in [137],which are mostly based on phased-locked loops (PLL). Further-more, reactive power flow must be also controlled in the PCC,which adds complexity to the whole control. There are DC and ACcases that also require voltage balancing. In three-phase ACmicrogrids, voltage balancing can be carried out in several man-ners: introducing special control loops to converter based DGs andSSs, using reactive support devices such as static var compesators(SVC) or static synchronous compesators (D-STATCOM) withappropriate control and using simple switched capacitors [138].For DC microgrids with bipolar or homopolar distribution, voltagebalancing must also be carried out. For this purpose, voltagebalancers can be placed near the power converters in order tobalance the distribution lines [52]. In Table 9 a summary of the

principal tasks that must be carried on in each kind of microgrid isshown [9,43,139,140].

9.1. Stability

If the sources of a microgrid are controlled by means of localmeasurements, stability becomes a critical issue [141]. Wheninvestigating the stability of a microgrid, both the generationdynamics and the load dynamics must be considered [142]. Thelargest body of work done in microgrid stability analysis is forradial microgrids while stability studies for meshed microgrids arestill an open research area [141].

Several stability studies for AC microgrids can be found in[141,143–146]. The aforementioned studies lead to conclude thatthe stability is mainly affected by the frequency-droop coefficient,and it is usually improved by means of a good design of the droopcoefficients and a droop control with adjustable dynamic beha-viour [146]. The classification of the stability issues in microgridspresented in [143] uses that of a large power system. Based on this,Table 10 summarises the criteria and different methods forstability improvement on AC microgrids. It can be said that thestability of AC microgrids is mostly influenced by the mode ofoperation (connected or disconnected from the mains grid),control topology, types of DERs and network parameters. Inaddition, the target application and the system scenarios influencethe efficacy of the stability improvement methods.

In DC microgrids the stability issues are inherently related withthe need of power electronics interfaces for integrating sources,loads, and energy storage devices [147]. There are many worksthat deal with the small signal stability in DC microgrids [131,147–151]. Taking all these works into account, it can be said thatconstant power loads introduce voltage oscillations into themicrogrids that can be overcome by means of hardware solutions(increasing system's capacitances or resistive loads, reducingsystem's inductances and load shedding) or control strategies(linear and boundary controllers).

9.2. Modelling

Modelling microgrids is a key step in order to obtain a suitablecontrol. Table 11 summarises models for AC microgrids proposedin [147,152,153]. As it is shown, most of the models employ droopcontrol, consider constant power loads and analyse the microgridboth connected and disconnected from the mains grid. Fewmodels consider three phase systems and there are some modelsinclude SS systems which helps studying in detail dynamics andpossible situations on a microgrid. According to software tools formodelling microgrids, HOMER has been the most extended energymodelling tool. There are other tools suitable for microgridsmodelling, such as EADER, DER-CAM, Matlab-Simulink and PSCAD/EMTDC [154,155].

There are few works that deal with the modelling of DCmicrogrids. Nevertheless, simple DC microgrid models can befound when analysing their stability [131,147–151]. Although,most of them do not consider control schemes and only take intoaccount RLC devices.

9.3. Power quality

Renewable energy systems have many benefits for energysafety and environmental impact. However, the integration ofintermittent power sources in the grid and its power quality is atechnical challenge that must be cope with [19]. Some solutionshave been proposed in order to improve power quality in ACmicrogrids [155]. DC distribution networks ensure a higher powerquality to the customers than in AC distribution network and


Table 9Control tasks in the hierarchical control for AC and DC microgrids.

Hierarchical level Controller Control tasks AC DC

Grid level DNO Control of multiple microgrids

Control of the active power flow between the microgrid and the grid

Control of the reactive power flow between the microgrid and the grid

MO Market functions of each specific area

Economical concerns in the optimal operation of the microgrid

Management level MGCC Connection and disconnection of loads from the microgrid

Restoration of the nominal voltage of the microgrid

Monitoring the microgrid

Smooth connection between the microgrid and the grid

Island detection and reaction

Synchronisation with the main grid

Restoration of the nominal frequency of the microgrid

Voltage balancinga b

Field level LC of DGs Sharing of active and reactive loads

Control of the voltage and frequency of the microgrid during island mode

Smooth transient from voltage to current source and viceversa

Inner control: voltage sags, harmonics, control reference matching and flicker

Sharing of non-lineal loads

Islanding detection and reaction

Operation as FACT units

Synchronisation with the microgrid

LC for SSs Storage level control

Decision of charging or discharging

Voltage regulation

Frequency regulation

Islanding detection

Synchronisation with the microgrid

LC for loads Normally no local control, load shedding managed by the MGCC or by agents

a Only in three-phase systems.b Only in bipolar and homopolar systems.


facilitates more DGs connection [9]. The results of ongoingresearch on the field of DC microgrids indicate a significantreduction in power quality problems, losses and downtime andprotection malfunctions [9].

Thus, there is ongoing research to find out the details of DCmicrogrids which result in significant reduction in power qualityproblems, losses and downtime and protection malfunctions [9].

9.4. Optimisation

Optimal control of microgrids is an active field of researchwhose most common objective is to minimise the operating cost[156]. Some of the solutions proposed only consider the electricalgeneration of microgrids and include mathematical programming,heuristics and priority rules [156]. However, there are also somemicrogrids, most of them residential microgrids, that also includethermal generation which must be also considered. In [157] thereare examples, most of them residential, of such models of micro-grids in order to achieve an optimal thermal generation.

Optimisation of AC microgrids has been widely studied andrelated works deal with different issues:

� Load demand and environmental requirements [157]. In thiswork, a model to determine the optimum operation of a

microgrid with respect to load demand and environmentalrequirement is constructed.

� Sustainability [158]. The sustainability of a microgrid is improvedthrough the minimisation of carbon emissions and at reducingproduction costs and maximising quality in this work.

� Economics [159–161]. Some works propose different solutionsin order to economically optimise different issues in micro-grids. A hierarchical control that deals with an economicoptimisation in the second level of the architecture is presentedin [159]. Another proposal can be found in [160] whereoptimisation techniques and management functions are imple-mented in a central energy management system (CEMS). ThisCEMS is aimed at optimising the economic exploitation of themicrogrid in grid-connected mode and it also improves thereliability of the microgrid in islanding operation. In [161] amethodology to design the number and capacity for eachcomponent in a microgrid with combined heat and power(CHP) system is presented that minimises the annual cost.

� Design of different controllers, filter, and power sharing coeffi-cients [162]. Linear and nonlinear models of a microgridoperating in grid-connected and autonomous modes are pre-sented in this work. Through these models, an optimisationproblem is formulated for dealing with the design of control-lers, filters and power sharing coefficients.

Table 11AC microgrid models proposed in the literature.

Model Distribution DG SS Loads Mode Control

Admittance matrix Three-phase Constant power No Constant power Both DroopDC source with non-linear equations

that are linearisedSingle-phase First order transfer functions No RLC circuit Both Droop

DC source with equations in state-space

Single-phase DC source and VSC No RLC circuit Island Droop

State space in a common referenceframe and linearisation

Single-phase DC source and VSC No Constant power Both Droop

State space in a common referenceframe and linearisation

Single-phase Synchronous generator based DG and power No Constant power Both Droop

Small signal analysis Single-phase Non-linear equations First order transferfunctions

Constant power Island –a

Multilevel type control andmanagement scheme supportedby a communicationinfrastructure

Single-phase Dynamic model based on the characteristics ofeach type of generator

Constant DC voltagesources coupled by powerelectronics devices

Two types:constantimpedance andmotor loads

Both Droop

Electrical equations in admittanceform

Three-phase Coupled by inverter droop Coupled by inverter droop Coupled by inverterdroop

Both Droop

System matrices based on fourdefined complex vectors

Single-phase Coupled by power electronics and two types:active and reactive power regulated andvoltage-frequency regulated

No No Both Droop

Based on hub model Single-phase Energy hub model Energy hub model Constant power Island –a

a No control considered.

Table 10Main stability issues and reasons and their corresponding possible improvement methods for AC microgrids.

Issues Reasons Improvement methods

Small signal stability Feedback controller Supplementary control loopsSmall load change Coordinated control of DGsSystem damping Stabilizers for DGsPower limit for DGs Energy management system (EMS)

Transient stability Islanding Control of storageLoss of DG Load shedding methodsLarge load step Protection device settingFault Control of power electronics

Voltage stability Reactive power limit/current limiters Reactive compensationLoad dynamics (induction motors) Load sheddingUnder voltage load shedding Modified current limiter for microsourcesTap changers and voltage regulations Voltage regulations with DGs


� Fuel consumption [163,164]. In [163] an algorithm that per-forms an optimisation of fuel consumption and emissions costsof microsources using a heuristic approach is presented.Another proposal can be found in [164] where the fuelconsumption rate of the system is reduced guaranteeing thelocal energy demand (both electrical and thermal) and provid-ing a certain minimum reserve power.

� Production of local DGs and power exchanges [118]. A centralcontroller aims to optimise the operation of the microgridduring interconnected operations by means of the optimisationof the production of the local DGs and power exchanges withthe main distribution grid.

There are very few examples of DC microgrids that proposeoptimal solutions for their management. In [165] both the allocation

and sizing of internal source converter storage were studied in orderto identify the topology characteristics that can be applied moregenerally to DC microgrids. Another proposal for hybrid AC/DCmicrogrids can be found in [166]. This work formulates a multi-objective optimisation problem that allows to optimally operatehybrid AC/DC microgrids, minimising their energy costs and green-house gas emissions.

10. Regulatory issues

A regulatory framework must include a set of principles, rulesand incentives, which address both technical and economic issues[167]. The regulatory environment for microgrids (Fig. 2 - ⑧)is complex since it combines multiple existing regulations that

Table 12Applicable standards to AC and DC microgrids.

Type Standard Description Scopes

AC andDC

IEEE 1547 Criteria and requirementsfor interconnection ofDERs with the main grid

1547.1 Conformance test

1547.2. Application guide1547.3. Monitoring and control1547.4. Design, operation and integration of DERs1547.5. Interconnection guidelines for electric powersources greater than 10 MVA1547.6. Interconnection with distribution secondary networks1547.7. Distribution impact studies for interconnection of DERs1547.8. Recommended practice for establishing methods and procedures

AC EN 50160 Voltage characteristics of electricity suppliedby public distribution networks

Definitions and indicative values for a number of power qualityphenomena in LV and MV networksLimits for power frequency, voltage variations, harmonics voltage,voltage unbalance, flicker and mains signalling

IEC 61000 General conditions or rules necessaryfor achieving

Safety function and integrity requirements

Electromagnetic compatibility Compatibility levelsEmission and immunity limitsMeasurement and testing techniquesInstallation guidelines, mitigation methods and devices

IEEE C37.95 Protective relaying of utility-consumerinterconnections

Establishment of consumer service requirements and supply methods

Protection system design considerations

DC ETSI EN 300 132-3-1

Environmental engineering (EE), Powersupplyinterface at the input to telecommunicationsand datacom (ICT) equipment;Part 3; Subpart 1: Direct current sourceup to 400 V

Definition of DC interface up to DC 400 V

DC bus voltage limits considerationGuidelines for testing powered equipment behaviour for abnormaloperating conditions and stress

EPRI andEmergence

Standard for 400 Vdc in buildings (underconstruction)

Interiors and occupied spaces where lighting and control loadsdominate the need for DC electricityData centres and telecom central offices with their DC-poweredinformation and communications technology equipmentOutdoor electrical uses, including electric vehicle chargingand outdoor light-emitting diode (LED) lightingBuilding services, utilities, and HVAC with variable-speed driveand electronic DC motorised equipment

IEC SG4 LVDC distribution system up to 1500 V(under construction)

Align and coordinate activities in many areas where LVDC isused such as green data centres, commercial buildings, electricity storage forall mobile products (with batteries), electric vehicles, and so forth, includingall mobile products with batteries, lighting, multimedia, ICT etc. with electronic supplyunitsMeasuring methodsArchitecture: 100 % DC or hybridGroundingOperation and life cycle of the equipmentProtective measurements for hazards for LVDC distribution systemsThe impact of DC (corrosion, insulation, etc.)


have not been specifically devised for microgrids [168]. Moreover,the most cited restriction to interconnect microgrids with thedistribution network is the high connectivity cost in the distribu-tion network due to high connection fee policies [167]. Islandingalso creates conflict with the existing grid codes and regulations.Another important issue is the metering since netmeter withbidirectional registering capability needs to be installed at thePCC by means of metering tools that support the participation ofmicrogrids in the market [167].

Therefore, it is necessary to establish a suitable framework,adapted to each country, for the development and proliferation ofmicrogrids. In Europe, for instance, the whole interconnectionframework needs a revision that must take into account specificissues such as the change of characteristics according to the modeof the microgrid (connected or islanded), protections and com-munication infrastructure [168]. In Japan, the Ministry of economy,trade, and industry (METI) has already established rules andtechnical guidelines for grid interconnection. However, it is stillnecessary to unify worldwide the standards required for inter-connecting distributed generators with information and distribu-tion networks [168]. In this line, in the United States the IEEEStandard for Interconnecting Distributed Resources with ElectricPower Systems 1547 establishes criteria and requirements for theinterconnection of distributed resources with electric powersystems. Its section 1547.4 is being treated as the fundamentalstandard for microgrid standardisation [43].

For AC microgrids some of the existing standards for DERs canbe adapted (Table 12). This is not possible for DC distribution andthere are several organisations engaged in developing standards.These organisations also support proof of concept sites to bench-mark the needs of DC distribution, such as IEC and EMerson[169,170]. There are also many groups working on the topic, whichare developing several voltage and performance standards. Amongthese standards, there are three that must be highlighted: EN 300132-3-1 of ETSI, Emergence Alliance standard for 380 Vdc and IECSG4 for LV distribution [169]. The last standard proposes a DC-powered home (Fig. 6) [171]. Table 12 provides a summary of theexisting standards for both AC and DC microgrids [43,169].

11. Economic analysis

An economic analysis of microgrids can be carried out bymeans of the study of the relevant benefits and costs. Two mainissues are considered: on-site generation from the customerperspective and the traditional utility economics of expansionplanning from the utility perspective [168]. In this sense, the mainbenefits are an increased reliability for microgrid participants,general reliability improvements, waste heat recovery and gen-eration adequacy [167,172,173]. Some models to quantify theseeconomic benefits can be found in [174].

Besides, the costs can be divided into microgrid developmentcosts and the costs related to the DNOs (Fig. 7) [172]. The first setof costs (most relevant costs [90]) is related to microgrid devel-opment, which involves specific investment in controllers, protec-tion devices, storage systems, installation, etc., and operation andmaintenance expenditures (staff, losses on storage systems, main-tenance of the equipments, etc.) (Fig. 7) [172]. Therefore, operatingcost of microgrids under optimal control highly relies on microgridconfiguration, optimisation method and benchmark model [156].The second set of costs is related to the DNOs expenses that mayresult from potential investment requirements in order to over-come technical problems, such as excessive voltage regulation,

Fig. 6. DC-Powered home example proposed in IEC SG4. Courtesy of Electric Power Research Institute (EPRI).

Costs of microgrids

Development costs

Controllers

Protections

Storage Systems

Installation

Operation

Mantainance

DNO costs

Voltage regulation

Adaptation of fault levels

Voltage balance

Overloading

Fig. 7. Classification of the costs related to microgrids.


fault levels, voltage unbalance, and overloading (Fig. 7). Most ofthe time, these costs can be neglected since some studies showthat significant quantities of microgeneration may be connected todistribution networks without significant investments [172], thus,they are not considered in this work. The following items describethe development costs considered in AC and DC microgrids:

� Since AC technology is more mature than DC technology, thedevelopment and production costs (Fig. 7) of customisedcomponents are usually higher for DC microgrids. However, ithas to be taken into account that the design of the control andmetering system for DC microgrids is simpler than for ACmicrogrids (no control of reactive power, no synchronisationcontrol and no frequency control).

� Installation costs in AC and DC are determined by the numberof elements installed which are highly dependant on thefeatures of the connected elements (DGs, SSs and loads).Therefore, DC microgrids lead to lower installation costs in

feeding offices or data centres since they tend to have more DCloads. Offices or data centres fed with DC microgrids havelower installation costs, since they tend to present more DCloads. R1.2 DC microgrids have been also proposed for feedingtrains and electrical bicycles in a clean and reliable manner[175,176]. Besides, AC microgrids are more suitable to feedfactories or big plants, because in these installations more ACloads are presented.R1.1 According to domestic applications, a variety of both ACand DC loads are presented in a current home. In Table 14, asummary of the power stages, internal loads and the powerconsumption that usually present the main home appliancesare presented. As it is shown, a great part of these appliancespresents AC/DC and DC/AC internal power stages. However,electronic devices such as TVs and PCs only present AC/DCpower stages since they only have internal DC loads.Homes can be fed by AC or DC microgrids in order to takeadvantage of the local energy resources. If homes are fed by AC

Table 13Development costs in AC and DC microgrids.

Device Localisation AC DC Reasons

Protections in PCC Fig. 2 - ② Lower Higher Technology more mature and lower power ratings in AC systemsDistribution Fig. 2 - ③ Higher Lower Due to skin effect, reactive power etc. higher losses in AC systemsLV protections Fig. 2 - ④ Lower Higher Lower power ratings and more mature technology in AC systemsMetering Fig. 2 - ⑤ Higher Lower More variables to be monitored in AC systemsCommunication Fig. 2 - ⑤ Similar Similar Depending to the control technique usedPower converters Fig. 2 - ⑥ Higher Lower Lower efficiencies and more components in AC systemsMCC Fig. 2 - ⑦ Higher Lower More tasks (synchronism, reactive power control, etc.) in AC systemsLC Fig. 2 - ⑦ Higher Lower More tasks (synchronism, reactive power control, etc.) in AC systemsLoad controller Fig. 2 - ⑦ Similar Similar Similar tasks in AC and DC systems.

Table 14R1.1 Power characteristics of the principal home appliances.

Appliance Power stages Loads Power

Topology I:AC/DC

Topology II: DC/AC AC DC (kW)

Fridge Motor 0.1–1.0Dishwasher Motors for the pump and blades 1.0–2.2Washing machine Motors for the pump and drum 0.3–3.0Dryer Inverter Motor for the drum, motor for the fan and

resistance0.2–2.0

Oven Rectifier with Resistance and motor for the fan Control electronicsand DC bus

0.5–4.0

Induction burner power factor Inductor 1.2 to 2.2Vacuum cleaner correction Motora 0.5 to 2.0Microwave Inverter with high

voltage transformerMagnetron and motor for fan 1.0 to 1.5

TVb – – Control electronics and screen 0.02 to 0.5PCb – – Control electronics,

screen, CPU and fan0.02 to 0.4

a This motor can be an AC motor or a DC motor.b No AC loads.

Table 15R1.1 Features of some commercial available inverters with a rated power output of 5 kW.

Manufacturer Model Power output (kW) Efficiency (%) Price (€)

Sunways NT5000 5.0 97.8 1850Fronius IG TL 50 5.0 97.7 1990Ingeteam SUN 5TL 5.0 97.0 1550ABB Power-One PVI 5.0 97.0 1800Schneider Electric Xantrex GT 5.0 SP 5.0 96.0 1990


microgrids, no change at the home appliances is necessarysince they are all prepared for AC sources. In case of feedinghomes by DC microgrids, two options can be considered. The

first option is to design a DC distribution system inside thehouse (Fig. 6), obtaining a distribution system more reliablethan AC distribution system. Besides, this option allowsremoval of several power stages inside the home appliancessince the AC/DC stage would not be necessary. Removing theAC/DC power stage from all the home appliances involves ahigher efficiency and saving costs for manufacturers. Thesesaving costs rely on the characteristics of each home applianceand each manufacturer. The second option is to install aninverter at the entrance of the house in order to maintain anAC distribution line inside the house (Fig. 1). In this case, theunique cost involved would be this inverter, which rated powercorresponds to the contracted power of the house. In Table 15some examples of commercially available inverters are pre-sented considering an average power of 5 kW at home.

� Protection costs. LV and MV protection devices are available forboth AC and DC lines. AC technology has still more attractiveprices even though DC technology has progressed.

� The operation costs are the result of the activity of themicrogrid and include losses in storage systems, staff, etc.[177]. As it has been commented on Section 5, if DC lines arecompared with AC microgrids, they present lower power lossesand, thus, lower operation costs.

Table 13 summarises the aforementioned costs and provides acomparison between AC and DC microgrids.

12. Future research areas

As it has been demonstrated through this work, microgridsattract a great deal of attention and different projects aboutmicrogrids have been developed all over the world. In extensiveand highly dispersed countries, such as Canada, USA and Japan,several organisms, such as Power Systems Engineering ResearchCenter (PSERC), Consortium for Electric Reliability TechnologySolutions (CERTS), and Consortium for Electric Reliability Technol-ogy Solutions (NEDO), respectively, feature microgrids into their

Fig. 9. Antarctic Princess Elisabeth Research Station is electrically feeded by meansof an islanded microgrid based on renewable energies. Courtesy of Antarctic PrincessElizabeth Research Station.

Table 16Examples of microgrids in Africa and America.

Continent Country Place Type Fig. 9 Continent Country Place Type Fig. 9

AC DC AC DC

Africa South-Africa Lucingweni ✓ ✓ 1 America USA Dublin, California ✓ 24Morocco Akkan ✓ 2 Marin County, California ✓ 25Senegal Diaka Madina ✓ 3 Palmdale, California ✓ 26

America Brazil Campinas ✓ 4 San Diego, California ✓ 27Chico Mendes ✓ ✓ 5 Santa Cruz island, California ✓ 28Ilha Ferradura ✓ 6 29 palms, California ✓ 29

Canada Ascension island ✓ 7 Florida ✓ 30Bella Cola ✓ 8 Miami, Florida ✓ 31Boston Bar ✓ 9 Hawaii ✓ 32Hartley Bar ✓ 10 Woodstock, Minnesota ✓ 33Kasabonika Lake ✓ 11 New Jersey ✓ 34Nemiah Valley ✓ 12 Albuquerque, New Mexico ✓ 35Ramea Island ✓ 13 Los Alamos, New Mexico ✓ 36Senneterre ✓ 14 Rochester, New York ✓ 37Toronto ✓ 15 Forth Bragg, North Caroline ✓ 38

Chile Tac island ✓ 16 Columbus, Ohio ✓ 39Coyhaique ✓ 17 Arlington, Texas ✓ 40

Mexico San Juanico ✓ 18 Austin, Texas ✓ 41Xcalac ✓ 19 Colonias, Texas ✓ 42

USA Kotzebue, Alaska ✓ 20 Waitsield, Vermont ✓ 43Wales, Alaska ✓ 21 Washington DC ✓ 44California ✓ 22 Madison, Wisconsin ✓ 45Borrego Springs, California ✓ 23

Fig. 8. Sendai microgrid in Fukushima, Japan. Courtesy of New Energy and IndustrialTechnology Development Organisation (NEDO).


research programmes. As an example, thanks to the Sendaimicrogrid, the power supply of one hospital and some laboratoriesin Fukushima was guaranteed after the serious earthquake in 2011,Fig. 8. In Europe there are also projects of microgrids, although smartgrids are the preferred alternative. Off-grid microgrids require a specialmention because they play an important role on rural electrification.Several examples of remote areas can be found in which islandedmicrogrids guarantee the power supply. The Antarctic Princess Elisa-beth Research Station, for instance, is powered by means of anislanded microgrid completely based on renewable energies (Fig. 9)[178]. Tables 16 and 17 provide a summary of microgrids installed allover the world [21,43,122,179]. As they show, microgrids can be foundin all continents, specially in countries such as China, USA and Canada.Figs. 11(a) and 11(b) are obtained combining the information ofTables 16 and 17 with the technology used and the installed powerin each microgrid [179]. The data of these Figs. highlights two facts:the number of AC microgrids and their rated power has increasedlinearly, except from 2005 to 2008, when it grew exponentially (Fig. 11(a)). There are few examples of DC microgrids before 2003, theirinstalled number started to increase slowly since then, until theirnumber grow exponentially, in the last few years.

Related to the subject of further developments, both AC and DCmicrogrids leave open research areas that must be taken intoaccount:

� Decentralised control [9,139]. Most of the existing microgridsaround the globe present a centralised control [43]. This type ofcontrol is optimal for small microgrids in which all the elementsshare the same goals. As these elements present different needsand the microgrid gets more complex, the alternative of decen-tralised control becomes more suitable. Thus, additional researchefforts are needed in order to develop the decentralised control.

� Protections [180]. Microgrids are able to operate connected ordisconnected from themain grid at any time. This dynamic schemecomplicates the design of the protection scheme which mustguarantee a safe operation in any case. Although some protectionschemes have been proposed, they are customised solutions thatdo not provide universal lines for any kind of microgrid.

DC microgrids also required specific research efforts in thefollowing issues:

� Bus selection [139]. The bus selection procedure is based onswitching mechanisms that can lead to unwanted voltageoscillations. Achieving a smooth bus selection requires anappropriate control approach and implementation. Nowadays,there are few works that deal with this problem [73], thusmore research efforts are needed.

� Standards [170]. The standardisation of AC microgrids hasgreatly advanced in the last years. DC microgrids, on the otherhand, do not have yet a specific standard. Some organisms likeEPRI and EmergeAlliance have already take the first steps intostandardisation of DC distribution lines.

� Islanding detection techniques [136]. Islanding detection is a keyrequirement for electrical safety and equipment protection.Islanding detection algorithms in AC systems are based onsystems frequency and phase, parameters that are not presentin DC lines. Thus, new methods for islanding detection in DCsystems are required to guarantee the reliability of the system.

13. Conclusions

Microgrids have attracted a great deal of attention during lastyears due to the several advantages they report. Microgrids can beTa

ble

17Ex

amplesof

microgridsin

Antarctica,A

sia,

European

dOcean

ia.

Con

tinen

tCou

ntry

Place

Type

Fig.

9Con

tinen

tCou

ntry

Place

Type

Fig.

9

AC

DC

AC

DC

Antarctica

Antarctica

Ulsteinen

Nunatak

✓46

Europe

German

yKassel,Man

heim

Wallstadt

✓68

Asia

China

Hefei

✓47

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mark

Bornholm

island

✓69

Nan

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✓48

Finland

Hailuot

✓70

Xinjian

g✓

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✓71

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jin

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✓72

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✓51

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✓73

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✓52

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Mah

arashtra

✓53

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esh

✓54

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Mila

n✓

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✓55

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onia

Agria

✓77

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✓78

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✓57

Portuga

lAzo

res

✓79

Kyoto

✓58

Ilhav

o✓

80Miyak

oisland

✓59

Porto

✓81

Sendai

✓✓

60Sp

ain

Derio

✓82

Korea

Chan

gwon

✓61

Seville

✓83

Singa

pore

PulauUbin

✓62

United

Kingd

omEigg

island,

Scotland

✓84

Taiw

anLo

ngtan

✓63

Man

chester

✓85

Turkey

Anka

ra✓

64Nottingh

am✓

86Ocean

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67


built with an AC or DC distribution system which defines the mainfeatures, advantages and disadvantages of the microgrid. Thispaper has presented a full description of microgrids. AC and DCtechnology in transmission lines has been described concludingthat DC technology offers several advantages in comparison withAC technology, specially for long distances. The PCC in AC and DCmicrogrids has been also analysed. AC technology presents severaladvantages at the PCC and AC protections are more economicalthan DC protections.

After the PCC analysis, the AC and DC distribution lines havebeen studied. DC distribution lines have definitely more advan-tages than AC lines due to lower loses, lower cable sections andhigher transmittable power. Therefore, although the vast majorityof distribution lines use AC technology, the interest on DCdistribution lines is increasing and nowadays several examplesof DC distribution lines can be found. Microgrids also need aprotection scheme in order to guarantee a safe operation. In thissense, several protection schemes for AC microgrids and a fewfor DC microgrids have been presented. Moreover, commercial

solutions for DC protections are still more expensive than ACprotections. Besides, there is considerable experience in groundfault detection and isolation in AC systems which is not the case inDC systems. Thus, research efforts are needed in terms of thedesign of the protection scheme in DC microgrids andprotection units.

Monitoring AC and DC microgrids has also been analysed inthis paper. From this perspective, the tools and schemes for ACmicrogrids can be simplified for their use in DC microgrids. DCmicrogrids present two main advantages it terms of monitoring:generally simpler topologies of power converters for couplingunits to DC microgrids and normally a higher efficiency of thepower conversion in DC systems. According to the control,centralised or decentralised hierarchical control is normally usedfor AC and DC microgrids. Most of the installed microgrids usecentralised control since its design simpler and easier for smallmicrogrids. Although, there is a growing interest in decentralisedcontrol since it offers a plug and play performance and it is moresuitable in case of bigger microgrids. The stability is another issue

Table 18Main power converters topologies for photovoltaic panels and wind turbines.


that has been commented and has been widely studied for AC andDC microgrids. Similar control and hardware solutions have beenproposed for both AC and DC microgrids. Besides, islandingdetection methods for AC systems are widely studied but veryfew works deal with this issue in DC systems.

Different proposals for modelling AC microgrids have beenfound that generally consider single-phase distribution lines andboth modes of operation (connected and disconnected fromthe main grid). Research efforts are needed in modelling DC

microgrids. The power quality has been also studied in AC andDC microgrids, concluding that DC systems offer higher powerquality. Several proposals of optimisation methods for AC micro-grids have been found that deal with different issues such assustainability, fuel consumption and design of controllers. DCmicrogrids optimisation, although, requires research efforts.

Efforts have been made in the last years in defining standardsand guidelines for building AC and DC microgrids. This studyshows that the normative for AC microgrids is more mature than

Table 19Main power converters topologies for batteries and flywheels.

1

3

2

4

5

6

7

8 910 1112

131415

1617

18 19

2021

2223

242526

2728 293031

32

333435

36

37

3839

404142

43

4445

46

4748

49 50

51

525354

5556

57

5859

6061

62

63

64

65

66 67

6869

70

7271

73

7475

76 77

78

798081 82

83

848586

Fig. 10. AC (blue), DC (green) and hybrid AC/DC (black) microgrids all around the world. (For interpretation of the references to color in this figure caption, the reader isreferred to the web version of this article.)


for DC microgrids, but there are several companies and organismscurrently dealing with this subject. Economic analysis of AC andDC microgrids is also an important point to be considered. Theconclusion of this study is that the costs derived from customisingunits and protections are lower for AC microgrids. The costs ofcontrollers and metering systems are lower for DC microgrids.In order to reduce the installation costs, AC microgrids are moresuitable for feeding installations with a high number of AC loads(factories, big plants, etc.) and DC microgrids more appropriate foroffices and data centres which include more DC loads. Finally, thecurrent situation of microgrids has been presented together with asummary of examples of installed microgrids all over the world.As results of the work carried out, some research areas have beenidentified which require further research and development, suchas the decentralised control and protections for both AC and DCmicrogrids (Fig. 10).

The present research shows a tendency to DC systems due tothe several advantages they report. In fact, the presence of HVDCinstallations all over the world is permanently increasing (Fig. 4)and several projects about DC distribution lines in buildings can befound (Table 7). For the particular case of microgrids, it has beenshown that although the number of DC microgrids has beengrowing during the last years (Fig. 11(b)), still most of them useAC distribution lines (Tables 16 and 17). This is because DCmicrogris need more research efforts in standardisation, busselection and islanding control techniques are needed. Thus,before DC microgrids are settled, hybrid AC/DC microgrids couldbe the intermediate step.

Acknowledgments

This work has been carried out inside the Research andEducation Unit UFI11/16 of the UPV/EHU and supported by theDepartment of Education, Universities and Research of the Basque

Government within the fund for research groups of the Basqueuniversity system IT394-10 and by the Government of the BasqueCountry within the research program ETORTEK as the projectENERGIGUNE12 (IE12-335).

References

[1] Viral R, Khatod D. Optimal planning of distributed generation systems indistribution system: a review. Renew Sustain Energy Rev 2012;16(7):5146–65.

[2] Zeng Z, Yang H, Zhao R. Study on small signal stability of microgrids: areview and a new approach. Renew Sustain Energy Rev 2011;15(9):4818–28.

[3] Venkataramanan G, Marnay C. A larger role for microgrids. IEEE PowerEnergy Mag 2008;6(3):78–82.

[4] Chicco G, Mancarella P. Distributed multi-generation: a comprehensive view.Renew Sustain Energy Rev 2009;13(3):535–51.

[5] Parise G, Martirano L, Parise L. Ecodesign of ever net-load microgrids. IEEETrans Ind Appl 2014;50(1):10–6.

[6] Asad R, Kazemi A. A quantitative analysis of effects of transition from ac to dcsystem, on storage and distribution systems: smart grid technologies. In:IEEE PES Asia-Pacific power and energy engineering conference (APPEEC);2012. p. 1–5.

[7] Guerrero J, Loh PC, Lee T-L, Chandorkar M. Advanced control architectures forintelligent microgrids-part II: power quality, energy storage, and ac/dcmicrogrids. IEEE Trans Ind Electron 2013;60(4):1263–70.

[8] Ali S, Babar M, Maqbool S, Al-Ammar E. Comparative analysis of ac dcmicrogrids for the saudi arabian distribution system. In: IEEE/PES transmis-sion and distribution conference and exposition (T&D); 2012. p. 1–8.

[9] Justo JJ, Mwasilu F, Lee J, Jung J. AC-microgrids versus DC-microgrids withdistributed energy resources: a review. Renew Sustain Energy Rev2013;24:387–405.

[10] Lopez I, Andreu J, Ceballos S, de Alegria IM, Kortabarria I. Review of waveenergy technologies and the necessary power-equipment. Renew SustainEnergy Rev 2013;27(0):413–34.

[11] Akorede MF, Hizam H, Pouresmaeil E. Distributed energy resources andbenefits to the environment. Renew Sustain Energy Rev 2010;14(2):724–34.

[12] Thornton A, Monroy CR. Distributed power generation in the United States.Renew Sustain Energy Rev 2011;15(9):4809–17.

[13] Ribeiro P, Johnson B, Crow M, Arsoy A, Liu Y. Energy storage systems foradvanced power applications. In: Proceedings of the IEEE, vol. 89, no. (12);2001. p. 1744–56.

[14] Kroposki B, Lasseter R, Ise T, Morozumi S, Papatlianassiou S, Hatziargyriou N.Making microgrids work. IEEE Power Energy Mag 2008;6(3):40–53.

[15] Kousksou T, Bruel P, Jamil A, Rhafiki TE, Zeraouli Y. Energy storage:applications and challenges. Solar Energy Mater Solar Cells 120, Part A2014(0):59–80.

[16] Ferreira HL, Garde R, Fulli G, Kling W, Lopes JP. Characterisation of electricalenergy storage technologies. Energy 2013;53(0):288–98.

[17] Chen H, Cong TN, Yang W, Tan C, Li Y, Ding Y. Progress in electrical energystorage system: a critical review. Progr Nat Sci 2009;19(3):291–312.

[18] Kaldellis J, Zafirakis D, Kavadias K. Techno-economic comparison of energystorage systems for island autonomous electrical networks. Renew SustainEnergy Rev 2009;13(2):378–92.

[19] Fu Q, Montoya L, Solanki A, Nasiri A, Bhavaraju V, Abdallah T, et al. Microgridgeneration capacity design with renewables and energy storage addressingpower quality and surety. IEEE Trans Smart Grid 2012;3(4):2019–27.

[20] IEEE recommended practice for sizing lead-acid batteries for stand-alonephotovoltaic (PV) systems, IEEE Std 1013-2007 (Revision of IEEE Std 1013-2000); 2007. p. 1–54.

[21] Lidula N, Rajapakse A. Microgrids research: a review of experimentalmicrogrids and test systems. Renew Sustain Energy Rev 2011;15(1):186–202.

[22] Katiraei F, Iravani R, Hatziargyriou N, Dimeas A. Microgrids management.IEEE Power Energy Mag 2011;9(5):54–65.

[23] Jimeno J, Anduaga J, Oyarzabal J, de Muro A. Architecture of a microgridenergy management system. Eur Trans Electr Power 2011;21(2):1142–58.

[24] Larruskain D, Zamora I, Abarrategui O, Aginako Z. Conversion of ac distribu-tion lines into dc lines to upgrade transmission capacity. Electr Power SystRes 2011;81(7):1341–8.

[25] Clerici A, Paris L, Danfors P. HVDC conversion of HVAC lines to providesubstantial power upgrading. IEEE Trans Power Deliv 1991;6(1):324–33.

[26] Rashid M. Power electronics handbook. USA: Elsevier; 2011.[27] Ackermann T, Negra N, Todorovic J. Loss evaluation of HVAC and HVDC

transmission solutions for large offshore wind farms. Electr Power Syst Res2006;76(11):916–27.

[28] Bahrman M, Kirby N, Koropatnick R. HVDC projects listing, HVDC andFlexible AC Transmission Subcommittee of the IEEE Transmission andDistribution Committee; 2012. p. 1–2.

[29] ⟨http://new.abb.com/⟩.[30] Bayliss C, Hardy B. Transmission and distribution electrical engineering.

Oxford, United Kingdom, Boston, USA: Newnes; 2007.[31] Stadler I. Study about international standards for the connection of small

distributed generators to the power grid. Technical report, Cologne Uni-versity of Applied Science; 2011.

100

150

200

250

300

u/M

W

Total number of installationsTotal power

0

50

4

6

8

10

12

14

u/M

W

Total number of installations

Total power

0

2

4

1992

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

1996

1999

2001

2003

2005

2007

2009

2011

2013

Year

Year

Fig. 11. Quantity and total power installed in AC and DC microgrids.


http://refhub.elsevier.com/S1364-0321(14)01006-5/sbref1






















































http://new.abb.com/



[32] Dvorsky E, Hejtmankova P. Microgrid interconnection to distribution powernetworks. In: IEEE/PES transmission and distribution conference and exhibi-tion; 2006. p. 1–3.

[33] Bilbao E, Gaztanaga H, Mir L, Etxeberria-Otadui I, Milo A. Design anddevelopment of a supercapacitor-based microgrid dynamic support system.In: European conference on power electronics and applications (EPE); 2009.p. 1–10.

[34] Cornforth D. Microgrid research in Australia. In: JEJU symposium on micro-grids; 2011. p. 1–23.

[35] SDG&E borrego springs microgrid demonstration project. DOE Peer Review;2010. p. 1–26.

[36] Russell M. The microgrid solution at 29 palms. In: JEJU symposium onmicrogrids; 2011. p. 1–21.

[37] Li Y, Vilathgamuwa D, Loh PC. Design, analysis, and real-time testing of acontroller for multibus microgrid system. IEEE Trans Power Electron 2004;19(5):1195–204.

[38] Barnes M, Dimeas A, Engler A, Fitzer C, Hatziargyriou N, Jones C, et al.Microgrid laboratory facilities. In: International conference on future powersystems; 2005. p. 1–6.

[39] Buchholz B, Erge T, Hatziargyriou N. Long term european field tests formicrogrids. In; Power conversion conference; 2007. p. 643–45.

[40] Mariam L, Basu M, Conlon M. A review of existing microgrid architectures.J Eng 2013:1–8.

[41] Khattabi M, Drenkard S, Dietschmann H, Dimeas P, Moutis A. Advancedarchitectures and control concepts for more microgrids. Technical report,More microgrids project; 2009.

[42] Taleski R. Microgrids: the agria test location. Thermal Sci 2010;14(3):747–58.[43] Planas E, Gil-de Muro A, Andreu J, Kortabarria I, Martinez de Alegria I.

General aspects, hierarchical controls and droop methods in microgrids: areview. Renew Sustain Energy Rev 2013;17:147–59.

[44] Meiqin M, Ming D, Jianhui S, Chang L, Min S, Guorong Z. Testbed formicrogrid with multi-energy generators. In: Canadian conference on elec-trical and computer engineering (CCECE); 2008. p. 637–40.

[45] Majumder R, Ghosh A, Ledwich G, Zare F. Enhancing stability of anautonomous microgrid using a gain scheduled angle droop controller withderivative feedback. Int J Emerg Electr Power Syst 2009;10(5):1–30.

[46] Nam K, Ahn J, Choi H, Kim S, Kim J, Cho C, et al. Establishment of a pilot plantfor KERI microgrid system based on power IT development program inKorea. In: Transmission and distribution conference and exposition: Asia andPacific (T&D Asia); 2009. p. 1–6.

[47] Shahidehpour M. IIT microgrid. A new hub in energy infrastructure, IIT WindConsortium Project Plenary Session; 2011. p. 1–15.

[48] Saka H. Survey on microgrid R&D in Japan. In: JEJU symposium on Micro-grids; 2011.

[49] Cleveland F. California DR integration projects: San Diego and Marin county.Xanthus Consulting International; 2008. p. 1–24.

[50] Rocabert J, Azevedo G, Luna A, Guerrero J, Candela J, Rodriguez P. Intelligentconnection agent for three-phase grid-connected microgrids. IEEE TransPower Electron 2011;26(10):2993–3005.

[51] Majumder R, Dewadasa M, Ghosh A, Ledwich G, Zare F. Control andprotection of a microgrid connected to utility through back-to-back con-verters. Electr Power Syst Res 2011;81(7):1424–35.

[52] Kakigano H, Miura Y, Ise T. Low-voltage bipolar-type DC microgrid for superhigh quality distribution. IEEE Trans Power Electron 2010;25(12):3066–75.

[53] Deng N, Zhang X-P, Wang P, Gu X, Wu M. A converter-based generalinterface for AC microgrid integrating to the grid. IEEE/PES Innov SmartGrid Technol Europe (ISGT) 2013:1–5.

[54] Kolar JW, Friedli T. The essence of matrix converters. Swiss Federal Instituteof Technology (ETH) Zurich Power Electronic Systems Laboratory; 2008. p. 1–113.

[55] Andreu J, Kortabarria I, Ormaetxea E, Ibarra E, Martin J, Apiñaniz S. A stepforward towards the development of reliable matrix converters. IEEE TransInd Electron 2012;59(1):167–83.

[56] Rodriguez J, Franquelo L, Kouro S, Leon J, Portillo R, Prats M, et al. Multilevelconverters: an enabling technology for high-power applications. In: Pro-ceedings of the IEEE, vol. 97, no. 11; 2009. p. 1786–817.

[57] Kenzelmann S. Modular DC/DC converter for DC distribution and collectionnetworks [Ph.D. thesis]. École Polythecnique Federale de Lausanne; 2012.

[58] Bellar M, Wu T, Tchamdjou A, Mahdavi J, Ehsani M. A review of soft-switchedDC–AC converters. IEEE Trans Ind Appl 1998;34(4):847–60.

[59] Onederra O, Odriozola H, Planas E, Lopez I, Lopez V. Overview of DCtechnology—energy conversion. In: International conference on renewableenergy and power quality (ICREPQ); 2013. p. 1–6.

[60] Salomonsson D, Soder L, Sannino A. Protection of low-voltage DC microgrids.IEEE Trans Power Deliv 2009;24(3):1045–53.

[61] Kempkes M, Roth I, Gaudreau M. Solid-state circuit breakers for mediumvoltage dc power. In: IEEE electric ship technologies symposium (ESTS);2011. p. 254–57.

[62] Katiraei F, Abbey C, Tang S, Gauthier M. Planned islanding on rural feeders—utility perspective. In: IEEE power & energy society general meeting; 2008.p. 1–6.

[63] Abbey C, Katiraei F, Brothers C, Dignard-Bailey L, Joos G. Integration ofdistributed generation and wind energy in Canada. In: IEEE power engineer-ing society general meeting; 2006. p. 1–7.

[64] Ito Y, Zhongqing Y, Akagi H. DC microgrid based distribution powergeneration system, vol. 3; 2004, p. 1740–5.

[65] Martini L, Tornelli C, Bossi C, Tironi E, Superti-Furga G. Design and develop-ment of a LV test facility for DC active distribution system. In: Internationalconference and exhibition on electricity distribution (CIRED); 2009. p. 1–4.

[66] Morozumi S, Nakama H, Inoue N. Demonstration projects for grid-connection issues in Japan. Elektrotech Informationstechnik 2008;125(12):426–31.

[67] Majumder R. Aggregation of microgrids with DC system. Electr Power SystRes 2014;108:134–43.

[68] Majumder R. A hybrid microgrid with DC connection at back to backconverters. IEEE Trans Smart Grid 2014;5(1):251–9.

[69] Jiang Z, Yu X. Power electronics interfaces for hybrid DC and AC-linkedmicrogrids. In: IEEE international power electronics and motion controlconference; 2009. p. 730–6.

[70] Jiang Z, Yu X. Hybrid DC- and AC-linked microgrids: towards integration ofdistributed energy resources. In: IEEE energy 2030 conference; 2008. p. 1–8.

[71] Liu X, Wang P, Loh PC. A hybrid AC/DC microgrid and its coordinationcontrol. IEEE Trans Smart Grid 2011;2(2):278–86.

[72] Loh P, Li D, Chai YK, Blaabjerg F. Autonomous operation of hybrid microgridwith AC and DC subgrids. IEEE Trans Power Electron 2013;28(5):2214–23.

[73] Balog R, Krein P. Bus selection in multibus DC microgrids. IEEE Trans PowerElectron 2011;26(3):860–7.

[74] Andreu J, Fernandez-Navamuel A, Kortabarria I, Llaria A, Alzola JA. Definicionde una microrred para la recarga de vehiculos electricos. Semin Anual deAutomática y Electrón Ind (SAAEI) 2011;1:281–6.

[75] Starke M, Li F, Tolbert L, Ozpineci B. AC vs. DC distribution: maximumtransfer capability. In: IEEE power and energy society general meeting; 2008.p. 1–6.

[76] Borioli E, Brenna M, Faranda R, Simioli G. Comparison between the electricalcapabilities of the cables used in LV AC and DC power lines. In: Internationalconference on harmonics and quality of power; 2004. p. 408–13.

[77] Baran M, Mahajan N. DC distribution for industrial systems: opportunitiesand challenges. IEEE Trans Ind Appl 2003;39(6):1596–601.

[78] Nilsson D. DC distribution systems [Master's thesis]. Department of Energyand Environment. Chalmers University of Technology; 2005.

[79] Patterson BT. DC—the power to change the world. From data centers towhole buildings. In: Microgrids RD & D workshop; 2012. p. 1–29.

[80] Ikebe H. Power systems for telecommunications in the it age. Int Telecom-mun Energy (INTELEC); 2003. p. 1–8.

[81] Kwasinski A. Quantitative evaluation of DC microgrids availability: effects ofsystem architecture and converter topology design choices. IEEE TransPower Electron 2011;26(3):835–51.

[82] Jiayi H, Chuanwen J, Rong X. A review on distributed energy resources andmicrogrid. Renew Sustain Energy Rev 2008;12(9):2465–76.

[83] Sortomme E, Mapes G, Foster B, Venkata S. Fault analysis and protection of amicrogrid. In: North American Power symposium (NAPS); 2008. p. 1–6.

[84] Nikkhajoei H, Lasseter R. Microgrid protection. In: IEEE power engineeringsociety general meeting; 2007. p. 1–6.

[85] Islam M, Gabbar H. Analysis of microgrid protection strategies. In: IEEEinternational conference on smart grid engineering (SGE); 2012. p. 1–6.

[86] Haron A, Mohamed A, Shareef H. A review on protection schemes andcoordination techniques in microgrid system. J Appl Sci 2012;12(2):101–12.

[87] Voima S, Kauhaniemi K, Laaksonen H. Novel protection approach for MVmicrogrid. In: International conference on electricity distribution (CIRED);2011. p. 1–4.

[88] Cuzner R, Venkataramanan G. The status of DC micro-grid protection. In:IEEE industry applications society annual meeting (IAS); 2008. p. 1–8.

[89] Salomonsson D, Sannino A. Low-voltage DC distribution system for com-mercial power systems with sensitive electronic loads. IEEE Trans PowerDeliv 2007;22(3):1620–7.

[90] Sannino A, Postiglione G, Bollen M. Feasibility of a DC network forcommercial facilities. IEEE Trans Ind Appl 2003;39(5):1499–507.

[91] Fletcher S, Norman P, Galloway S, Crolla P, Burt G. Optimizing the roles ofunit and non-unit protection methods within DC microgrids. IEEE TransSmart Grid 2012;3(4):2079–87.

[92] Sortomme E, Venkata S, Mitra J. Microgrid protection using communication-assisted digital relays. IEEE Trans Power Deliv 2010;25(4):2789–96.

[93] Gopalan SA, Sreeram V, Iu HH. A review of coordination strategies andprotection schemes for microgrids. Renew Sustain Energy Rev 2014;32(0):222–8.

[94] Das J, Osman R. Grounding of AC and DC low-voltage and medium-voltagedrive systems. IEEE Trans Ind Appl 1998;34(1):205–16.

[95] Jingding R, Yanbo C, Lihua Z. Discussion on monitoring scheme of distributedgeneration and micro-grid system. In: International conference on powerelectronics systems and applications (PESA); 2011. p. 1–6.

[96] Zhichun Y, Jian L, Kaipei L, Wei C. Preliminary study on the technicalrequirements of the grid-connected microgrid. In: International conferenceon electric utility deregulation and restructuring and power technologies(DRPT); 2011. p. 1656–62.

[97] Hai-xuan L. Studies on the monitoring and control platform of microgrids.In: China international conference on electricity distribution (CICED); 2012.p. 1–5.

[98] Vaccaro A, Popov M, Villacci D, Terzija V. An integrated framework for smartmicrogrids modeling, monitoring, control, communication, and verification.In: Proceedings of the IEEE, vol. 99, no. 1; 2011. p. 119–32.








































































[99] Shamshiri M, Gan CK, Tan CW. A review of recent development in smart gridand micro-grid laboratories. In: 2012 IEEE international power engineeringand optimization conference; 2012. p. 367–72.

[100] Ruiz-Alvarez A, Colet-Subirachs A, Gomis-Bellmunt O, Fernandez-Mola J,Alvarez-Cuevas-Figuerola F, Lopez-Mestre J, et al. Design, management andcomissioning of a utility connected microgrid based on IEC 61850. In: IEEEPES innovative smart grid technologies conference Europe (ISGT); 2010.p. 1–7.

[101] Islam M, Lee H-H. IEC61850 based operation, control and management ofutility connected microgrid using wireless technology. Intell Comput TheorAppl (ICIC); 2012. p. 258–65.

[102] Mohamed A, Ghareeb A, Youssef T, Mohammed O. Wide area monitoring andcontrol for voltage assessment in smart grids with distributed generation. In:IEEE PES innovative smart grid technologies conference (ISGT); 2013. p. 1–6.

[103] CERTS, Microgrid energy management system. Technical report, CaliforniaEnergy Commission; 2003.

[104] Wells C. Solar microgrids to accommodate renewable intermittency. In: IEEE/PES transmission & distribution conference & exposition (T & D); 2010.p. 1–9.

[105] Chenine M, Nordstrom L. Modeling and simulation of wide-area commu-nication for centralized PMU-based applications. IEEE Trans Power Deliv2011;26(3):1372–80.

[106] Chakraborty S, Kramer B, Kroposki B. A review of power electronicsinterfaces for distributes energy systems towards achieving low-cost mod-ular design. Renew Sustain Energy Rev Elsevier 2009;13:2323–35.

[107] Tan X, Li Q, Wang H. Advances and trends of energy storage technology inmicrogrid. Electr Power Energy Syst 2013;44(1):179–91.

[108] Andreu J, de Diego JM, de Alegría IM, Kortabarria I, Martín JL, Ceballos S. Newprotection circuit for high speed switching and start-up of a practical matrixconverter. IEEE Trans Ind Electron 2008;55(8):3100–14.

[109] Venmathi M, Ramaprabha R. A comprehensive survey on multi-port bidirec-tional DC–DC converters for renewable energy systems. ARPN J Eng Appl Sci2013;8(5):348–56.

[110] Taghvaee M, Radzi M, Moosavain S, Hizam H, Hamiruce Marhaban M.A current and future study on non-isolated DC–DC converters for photo-voltaic applications. Renew Sustain Energy Rev 2013;17:216–27.

[111] Du Y, Lukic S, Jacobson B, Huang A. Review of high power isolated bi-directional DC-DC converters for PHEV/EV DC charging infrastructure. IEEEEnergy Convers Congr Expo (ECCE) 2011:553–60.

[112] Paajanen P, Kaipia T, Partanen J. DC supply of low-voltage electricityappliances in residential buildings. In: International conference and exhibi-tion on electricity distribution (CIRED); 2009. p. 1–4.

[113] Hammerstrom D. AC versus DC distribution systems. Did we get it right?.IEEE Power Eng Soc Gen Meet; 2007, p. 1–5.

[114] Starke M, Tolbert L, Ozpineci B. AC vs. DC distribution: a loss comparison.IEEE/PES transmission and distribution conference and exposition; 2008. p.1367–73.

[115] Nilsson D, Sannino A. Efficiency analysis of low- and mediumvoltage DCdistribution systems. IEEE Power Eng Soc Gen Meet 2004;2:2315–21.

[116] Techakittiroj K, Wongpaibool V. Co-existence between AC-distribution andDC-distribution: in the view of appliances. In: International conference oncomputer and electrical engineering (ICCEE), vol. 1; 2009, p. 421–5.

[117] Keyhani A, Marwali M. Smart power grids 2011. Germany: Springer; 2011.[118] Tsikalakis A, Hatziargyriou N. Centralized control for optimizing microgrids

operation. IEEE Trans Energy Convers 2008;23(1):241–8.[119] Hatziargyriou N, Dimeas A, Tsikalakis A, Lopes J, Karniotakis G, Oyarzabal J.

Management of microgrids in market environment. In: International con-ference on future power systems; 2005. p. 1–7.

[120] Lopes J, Moreira C, Madureira A. Defining control strategies for microgridsislanded operation. IEEE Trans Power Syst 2006;21(2):916–24.

[121] Zamora R, Srivastava A. Controls for microgrids with storage: review,challenges, and research needs. Renew Sustain Energy Rev 2010;14(7):2009–18.

[122] Ustun T, Ozansoy C, Zayegh A. Recent developments in microgrids andexample cases around the world—a review. Renew Sustain Energy Rev2011;15(8):4030–41.

[123] Lasseter R, Akhil A, Marnay K, Stevens J, Dagle J, Guttromson R, et al. Whitepaper on integration of distributed energy resources the microgrid concept.Technical Report, CERTS; 2002.

[124] Vandoorn T, Meersman B, De Kooning J, Vandevelde L. Analogy betweenconventional grid control and islanded microgrid control based on a globalDC-link voltage droop. IEEE Trans Power Deliv 2012;27(3):1405–14.

[125] Abusara M, Guerrero J, Sharkh S. Line-interactive UPS for microgrids. IEEETrans Ind Electron 2014;61(3):1292–300.

[126] Bryan J, Duke R, Round S. Decentralized generator scheduling in a nanogridusing DC bus signaling. IEEE Power Eng Soc Gen Meet 2004;1:977–82.

[127] Schonberger J, Duke R, Round S. DC-bus signaling: a distributed controlstrategy for a hybrid renewable nanogrid. IEEE Trans Ind Electron 2006;53(5):1453–60.

[128] Schonberger J, Round S, Duke R. Autonomous load shedding in a nanogridusing DC bus signalling. In: Conference on IEEE industrial electronics(IECON); 2006. p. 5155–60.

[129] Schonberger J, Duke R, Round S. Decentralised source scheduling in a modelnanogrid using DC bus signalling. Aust J Electr Electron Eng 2005;2(3):183–90.

[130] Lu X, Sun K, Guerrero J, Vasquez J, Huang L. State-of-Charge balance usingadaptive droop control for distributed energy storage systems in DC micro-grid applications. IEEE Trans Ind Electron 2014;61(6):2804–15.

[131] Lu X, Sun K, Guerrero JM, Vasquez JC, Huang L, Teodorescu R. SoC-baseddroop method for distributed energy storage in DC microgrid applications.In: Proceedings of the IEEE international symposium on industrial electronics(ISIE); 2012. p. 1640–5.

[132] Dragicevic T, Guerrero J, Vasquez J, Skrlec D. Supervisory control of anadaptive-droop regulated DC microgrid with battery management capability.IEEE Trans Power Electron 2014;29(2):695–706.

[133] Llaria A, Curea O, Jimenez J, Camblong H. Survey on microgrids: unplannedislanding and related inverter control techniques. Renew Energy 2011;36(8):2052–61.

[134] Velasco D, Trujillo C, Garcera G, Figueres E. Review of anti-islandingtechniques in distributed generators. Renew Sustain Energy Rev 2010;14(6):1608–14.

[135] Khamis A, Shareef H, Bizkevelci E, Khatib T. A review of islanding detectiontechniques for renewable distributed generation systems. Renew SustainEnergy Rev 2013;28(0):483–93.

[136] Seo G-S, Cho B-H, Lee K-C. DC islanding detection algorithm using injectioncurrent perturbation technique for photovoltaic converters in DC distribu-tion. IEEE Energy Convers Congr Expo (ECCE) 2012:3722–6.

[137] Singh RR, Chelliah TR, Agarwal P. Power electronics in hydro electric energysystems—a review. Renew Sustain Energy Rev 2014;32(0):944–59.

[138] Lidula N, Rajapakse A. Voltage balancing and synchronization of microgridswith highly unbalanced loads. Renew Sustain Energy Rev 2014;31(0):907–20.

[139] Bidram A, Davoudi A. Hierarchical structure of microgrids control system.IEEE Trans Smart Grid 2012;3:1963–76.

[140] Chen D, Xu L. DC microgrid with variable generations and energy storage. IETConf Renew Power Gener (RPG) 2011:6.

[141] Guerrero J, Chandorkar M, Lee T, Loh P. Advanced control architectures forintelligent microgrids. Part I: decentralized and hierarchical control. IEEETrans Ind Electron 2013;60(4):1254–62.

[142] Bottrell N, Prodanovic M, Green T. Dynamic stability of a microgrid with anactive load. IEEE Trans Power Electron 2013;28(11):5107–19.

[143] Majumder R. Some aspects of stability in microgrids. IEEE Trans Power Syst2013;28(3):3243–52.

[144] Iyer S, Belur M, Chandorkar M. A generalized computational method todetermine stability of a multi-inverter microgrid. IEEE Trans Power Electron2010;25(9):2420–32.

[145] IEEE guide for design, operation, and integration of distributed resourceisland systems with electric power systems. IEEE Std 1547.4-2011; 2011.p. 1–54.

[146] Planas E, Gil-de Muro A, Andreu J, Kortabarria I, Martinez de Alegria I. Designand implementation of a droop control in d–q frame for islanded microgrids.IET Renew Power Gener 2013;7(5):458–74.

[147] Kwasinski A, Onwuchekwa C. Dynamic behavior and stabilization of DCmicrogrids with instantaneous constant-power loads. IEEE Trans PowerElectron 2011;26(3):822–34.

[148] Boroyevich D, Cvetkovic I, Dong D, Burgos R, Wang F, Lee F. Future electronicpower distribution systems: a contemplative view. In: International con-ference on optimization of electrical and electronic equipment (OPTIM);2010. p. 1369–80.

[149] Radwan A, Mohamed Y-R. Linear active stabilization of converter-dominatedDC microgrids. IEEE Trans Smart Grid 2012;3(1):203–16.

[150] Acevedo SS, Molinas M. Identifying unstable region of operation in a micro-grid system. Energy Proced 2012;20:237–46.

[151] Sanchez S, Molinas M, Degano M, Zanchetta P. Stability evaluation of a DCmicro-grid and future interconnection to an AC system. Renew Energy2014;62(0):649–56.

[152] Dobakhshari A, Azizi S, Ranjbar A. Control of microgrids: aspects andprospects. In: International conference on networking, sensing and control(ICNSC); 2011. p. 38–43.

[153] Mahmoud M, Hussain SA, Abido M. Modeling and control of microgrid: anoverview. J Frankl Inst 2014(0):1–38.

[154] Mendes G, Ioakimidis C, Ferrao P. On the planning and analysis of integratedcommunity energy systems: a review and survey of available tools. RenewSustain Energy Rev 2011;15(9):4836–54.

[155] Basak P, Chowdhury S, Halder nee Dey S, Chowdhury S. A literature reviewon integration of distributed energy resources in the perspective of control,protection and stability of microgrid. Renew Sustain Energy Rev 2012;16(8):5545–56.

[156] Kriett P, Salani M. Optimal control of a residential microgrid. Energy 2012;42(1):321–30.

[157] Mohamed F, Koivo H. Power management strategy for solving powerdispatch problems in microgrid for residential applications. In: IEEE inter-national energy conference (ENERGYCON); 2010. p. 746–51.

[158] Sanseverino E, Di Silvestre M, Ippolito M, De Paola A, Lo Re G. An execution,monitoring and replanning approach for optimal energy management inmicrogrids. Energy 2011;36(5):3429–36.

[159] Vandoorn T, Zwaenepoel B, De Kooning J, Meersman B, Vandevelde L. Smartmicrogrids and virtual power plants in a hierarchical control structure. In:IEEE PES international conference and exhibition on innovative smart gridtechnologies (ISGT Europe); 2011. p. 1–7.















































































































[160] Milo A, Gaztanaga H, Etxeberria-Otadui I, Bilbao E, Rodriguez P. Optimizationof an experimental hybrid microgrid operation: reliability and economicissues. In: IEEE Bucharest PowerTech (POWERTECH); 2009. p. 1–6.

[161] Asano H, Bando S. Economic evaluation of microgrids. IEEE Power Energy SocGen Meet 2008:1–6.

[162] Hassan M, Abido M. Optimal design of microgrids in autonomous and grid-connected modes using particle swarm optimization. IEEE Trans PowerElectron 2011;26(3):755–69.

[163] Alvarez E, Campos A, Arboleya P, Gutierrez A. Microgrid management with aquick response optimization algorithm for active power dispatch. ElectrPower Energy Syst 2012;43:465–73.

[164] Hernandez-Aramburo C, Green T, Mugniot N. Fuel consumption minimiza-tion of a microgrid. IEEE Trans Ind Appl 2005;41(3):673–81.

[165] Heath M, Vosters G, Parker G, Weaver W, Wilson D, Robinett I, RD. DCmicrogrid optimal storage distribution using a conductance and energy statemodeling approach. In: International symposium on power electronics,electrical drives, automation and motion (SPEEDAM); 2012. p. 170–4.

[166] Bozchalui M, Sharma R. Optimal operation of commercial building micro-grids using multi-objective optimization to achieve emissions and efficiencytargets. IEEE Power Energy Soc Gen Meet 2012:1–8.

[167] Basu A, Chowdhury S, Chowdhury S, Paul S. Microgrids: energy managementby strategic deployment of DERs—a comprehensive survey. Renew SustainEnergy Rev 2011;15(9):4348–56.

[168] Xiarnay C, Asano H, Papathanassiou S, Strbac G. Policy making for microgrids.IEEE Power Energy Mag 2008;6(3):66–77.

[169] Becker D, Sonnenberg B. DC microgrids in buildings and data centers. In:International telecommunications energy conference (INTELEC); 2011, p. 1–7.

[170] Patterson B. DC, come home: DC microgrids and the birth of the “Enernet”.IEEE Power Energy Mag 2012;10(6):60–9.

[171] Symanski DP. DC distribution in buildings including homes, commercialbuildings, data centers and telecom central offices. In: IEC SG4 low voltageDC workshop; 2011. p. 1–32.

[172] Costa P, Matos M, Lopes J. Regulation of microgeneration and microgrids.Energy Policy 2008;36(10):3893–904.

[173] Pudjianto D, Strbac G, van Oberbeeke F, Androutsos A, Larrabe Z, Saraiva J.Investigation of regulatory, commercial, economic and environmental issuesin microgrids. In: International conference on future power systems; 2005.p. 1–6.

[174] Gil H, Joos G. Models for quantifying the economic benefits of distributedgeneration. IEEE Trans Power Syst 2008;23(2):327–35.

[175] Katsuma H, Kameya T, Suzuki G, Asai H. Soft energy will make light railvehicle run in the stricken area without a caternary system. In: Proceedingsof American solar energy society; 2013. p. 1–3.

[176] Jaffery SHI, Khan M, Ali L, Khan HA, Mufti RA, Khan A, et al. The potential ofsolar powered transportation and the case for solar powered railway inPakistan. Renew Sustain Energy Rev 2014;39(0):270–6.

[177] Costa P, Matos M. Economic analysis of microgrids including reliabilityaspects. International conference on probabilistic methods applied to powersystems, vol. 1; 2006. p. 312–9.

[178] ⟨http://www.antarcticstation.org/⟩.[179] Hossain E, Kabalci E, Bayindir R, Perez R. Microgrid testbeds around the

world: state of art. Energy Convers Manag 2014;86(0):132–53.[180] Ping J, Xin ZX, ShouYuan W. Review on sustainable development of island

microgrid. In: IEEE international conference on advanced power systemautomation and protection (APAP), vol. 3; 2011. p. 1806–13.





























http://www.antarcticstation.org/



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