10.1109-JSYST.2013.2296075-Control Methods and Objectives for Electronically Coupled Distributed Energy Resources in Microgrids- A Review

7/21/2019 10.1109-JSYST.2013.2296075-Control Methods and Objectives for Electronically Coupled Distributed Energy Resour

1/13

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

IEEE SYSTEMS JOURNAL 1

Control Methods and Objectives for Electronically

Coupled Distributed Energy Resources in

Microgrids: A ReviewBilal M. Eid,Student Member, IEEE, Nasrudin Abd Rahim, Senior Member, IEEE,

Jeyraj Selvaraj, and Ahmad H. El Khateb, Member, IEEE

AbstractIncreased penetration of distributed energy re-sources into conventional power systems increases control chal-lenges. These can be suitably met by microgrids. This paperexamines the architecture of microgrids and reviews their clas-sifications and the literatures discussing their control objectivesduring islanded mode. It finds the use of microgrids enhancing theconventional power systems grid smartness. It also summarizesmicrogrid control objectives and their most common problemsand

solutions.Index TermsDistributed energy resources (DERs), droop

method, electronically coupled, load sharing (LS), microgrid con-trol, power converters.

I. INTRODUCTION

THE POSSIBILITY of increased blackouts in power sys-

tems can be due to both economical and physical reasons,

e.g., 1) increased demand for power that had to be transferred

over long distances, resulting in huge amounts of lost power;

2) continual load growth unparalleled by sufficient investment

into power generation and transmission systems; and 3) extreme

swings from one day to the next in power flow dispatch, makingconventional offline planning useless. These push power sys-

tems to their physical limits, with a possibility of compromising

grid reliability. A distributed energy resource (DER) is one

solution that can reduce the electrical and physical distances

between the load and the generator, improve reactive power

to enhance grid voltage profile and power quality, remove

bottlenecks from distribution and transmission lines, reduce

transmission and distribution losses, make better use of waste

heat, postpone the necessity to establish new transmission lines

Manuscript received July 16, 2013; revised November 4, 2013; accepted

December 16, 2013. This work was supported by the Secretariat of theUniversity of Malayas High Impact Research through the Campus NetworkSmartgrid for Energy Security under Project UM.C/HIR/MOHE/ENG/32.

B. M. Eid is with the University of Malaya Power Energy DedicatedAdvanced Centre (UMPEDAC), Wisma R&D, University of Malaya, KualaLumpur 59990, Malaysia, and also with the Department of Electrical En-gineering, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail:[email protected]).

N. A. Rahim is with the University of Malaya Power Energy DedicatedAdvanced Centre (UMPEDAC), Wisma R&D, University of Malaya, KualaLumpur 59990, Malaysia and also with the Department of Electrical andComputer Engineering, Faculty of Engineering, King Abdulaziz University,Jeddah 21589, Saudi Arabia.

J. Selvaraj and A. H. El Khateb are with the University of Malaya PowerEnergy Dedicated Advanced Centre (UMPEDAC), Wisma R&D, University ofMalaya, Kuala Lumpur 59990, Malaysia.

Digital Object Identifier 10.1109/JSYST.2013.2296075

and huge power generation plants, and keep carbon emission

levels low [1], [2][5].

The challenges to having multiple DERs include the fol-

lowing: 1) the current control strategy (CCS) is unable to

function during islanded mode because there is no dominant

source of energy [6][8]. 2) Multiple DERs have multiple

power generation characteristics and capacities; therefore, the

microgrid needs rapid regulation more in islanded mode than ingrid-connected mode [6]. 3) An increased presence of DERs,

particularly in a distribution network (whether medium or low

voltage), may cause problems such as voltage rise and unstable

network voltage and frequency (either during operation of

the DERs or upon their sudden tripping) [6], [9][11]. Such

problems can be solved by microgrids, whose many features

include the following:

1) integrating DERs without interrupting public-grid opera-

tion, i.e., a lot of DERs can be installed without reforming

or rewiring the distribution system [2], [12], [13];

2) enabling power systems to observe and control faults

more effectively and to reduce the damage caused by aDER outage, by continually feeding critical loads [4],

[11], [14];

3) allowing load shedding and automated switching through

control algorithms, to shorten outage and power restora-

tion time, and keeping the faulted section of the distri-

bution line isolated until utility crews locate the fault

location [4], [14], [15];

4) allowing to run in either (grid-connected or islanded)

mode, according to the economy or a planned disconnec-

tion, or to restore the public-grid power quality when it

falls below certain standards [12], [16][18];

5) improving system reliability and flexibility, through themany options of DERs [2], [13], [19][22];

6) using DERs waste heat to improve generation efficiency

[2], [6], [13], [20], [23][25].

The microgrid paradigm is that a group of microsources and

loads, with some form of energy storage, operate as a control-

lable system, providing heat and power to local communities

[7], [12]. The Consortium for Electric Reliability Technology

Solutions (CERTS) [13] showed that clustered DERs, storage

systems, and loads can operate in either grid-connected or

islanded mode. Factors affecting the choice of the required

control and operation strategies of a microgrid include the

following: power-quality restrictions; DER type and depth

1932-8184 2014IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.Seehttp://www.ieee.org/publications_standards/publications/rights/index.htmlfor more information.
mailto:%[email protected]://www.ieee.org/publications_standards/publications/rights/index.htmlmailto:%[email protected]://www.ieee.org/publications_standards/publications/rights/index.html


2/13


2 IEEE SYSTEMS JOURNAL

Fig. 1. Basic architecture of a microgrid.

of penetration; single-phase load, which is more common inmicrogrids; high presence of uncontrolled sources, such as

wind and photovoltaic (PV) units; short- and long-term energy

storage devices, which affect control and operation; priority

for critical load; load characteristics; and load market-sharing

requirements. Thus, significantly different control in the micro-

grid is required [7], [13], [26], [27].

This paper reviews the classification and control technique

and objectives of electronically coupled DERs in microgrids.

Section II illustrates the microgrid configuration, whereas

Section III presents the classification types. Section IV ex-

amines the control objectives, techniques, and strategies; Sec-

tions VVII discuss the control functions of microgrids; andSection VIII summarizes the control problems and their solu-

tions. Section IX concludes the paper.

II. CONFIGURATION OF A M ICROGRID

Fig. 1 shows a basic microgrid architecture. The electrical

system consists of three radial feeders that connect to the main

grid through a separation device called the point of common

coupling (PCC). A microgrid central controller (MGCC) is the

main controller of the DERs. It is responsible for stabilizing

the active and reactive powers dispatched from each DER

and also the voltage and frequency at the PCC. The circuit

breaker installed at each feeder startpoint provides the critical

feeders (which contain the DERs and the critical loads) with

the capability to supply the loads separately from the main

grid. The loads and DERs have local controllers (LCs) that

control the production and storage units and some of the local

loads. In a centralized operation, they receive set points from

the MGCC, whereas in a decentralized operation, they make

decisions locally [6], [27], [28].

III. CLASSIFICATION OF M ICROGRIDS

Microgrids can be classified in many ways as follows.

1) In terms of power type (whether ac or dc) [27], Fig. 2illustrates the following microgrid power types: dc

Fig. 2. Classification of microgrids based on power type (ac or dc).

Fig. 3. Two-stage conversion systems for PV.

microgrid [29], high-frequency ac microgrid [30], [31]

(which is used to solve power-quality problems caused by

the presence of lots of energy conversion devices and to

reduce the impact of fluctuations in renewable energy in

the microgrid), line-frequency ac (LFAC) microgrid (the

most researched since the concept of microgrid was for-

mulated [27]), and hybrid dc- and ac-coupled microgrids.

2) In terms of the application used for [21], they can be

classified into three categories subject to their application:

a) utility microgrids (a district of the city operates as a

microgrid), b) commercial and industrial microgrids, andc) isolated microgrids.

3) In terms of system structure, they can be classified into

two structures, depending on the number of power pro-

cessing stages: a) single-stage power-conversion systems

[27], [32], [33] and b) two-stage power-conversion sys-

tems [24], [34][36] (which is the most common con-

figuration for all electronically coupled DER). Two-stage

conversion systems have two converters, i.e., one on the

PV side, which is used to extract the maximum power

from the PV, and another is connected to the grid side and

controlled to follow grid requirements (refer to Fig. 3).

4) In terms of supervisory control, they can be either cen-tralized or decentralized [27]; in centralized control, the

central controller in the microgrid sends the required set

points to the LC through a two-way communication link,

but the technique has low reliability and is redundant

[37]; the decentralized technique is a multiagent system,

providing flexibility to the system and communication

between two agents through a communication language

such as Java-Jade [27], [38], [39].

5) In terms of the DERs connection to the microgrid, i.e.,

either electronically coupled (converters are used) or

conventional rotating distributed generators [7], [27].

Microgrids may also be single- or three-phase systems

or connected to low- or medium-voltage distributionnetworks [40]; their mode of operation is either islanded


3/13


EID et al.: CONTROL METHODS AND OBJECTIVES FOR ELECTRONICALLY COUPLED DERS IN MICROGRIDS 3

or grid-connected; each mode has its own operation re-

quirements and requires a distinct control strategy [26].

On the basis of these classifications and relating to the

reviewed papers, research is shown to have focused on LFAC,

industrial and commercial applications, two-stage power-

conversion microgrids, three-phase systems, and electronically

coupled DERs.

IV. MICROGRIDC ONTROL M ETHODS AND O BJECTIVES

Control of a microgrid during islanded mode is particularly

critical [7], [27], [41]. The next part of this paper examines the

system control used for the islanded operation (e.g., voltage

regulation (VR), frequency regulation (FR), and load-sharing

(LS) optimization). Since there is no synchronous machine in

most microgrids to achieve demand and supply balancing, the

inverters should be responsible for balancing. The use of a

voltage-source inverter (VSI) to provide a reference for voltage

and frequency enables microgrids to operate in islanded mode

[42], [43].Many papers have discussed the term grid-forming [7],

[26], [27], [41], and others have used islanded operation

mode to mean the same definition. The grid-forming mode

is used to regulate voltage and frequency and to achieve LS

in microgrids, when the system operates in islanded mode [7],

[27], [41]. The control strategies, layer interconnections, and

functions are discussed in the following.

A. Control Strategies

There are two main control strategies proposed for micro-

grids during islanded mode: a) single-master operation (SMO)

and b) multimaster operation (MMO) [42], [44]. Both use a

VSI to provide a reference for voltage and frequency [43],

and a convenient secondary load-frequency control must be

considered to maintain the frequency between the specified

limits and to run the DER economically [17], [42], [45].

SMO [42], [44]: This approach has one inverter acting as

VSI (the master) and others as followers (the slaves). When

the main power supply is lost, the slaves take voltage reference

from the master and operate in thePQmode. The LCs receiveset points from the MGCC to maintain generation of active and

reactive powers at the specified values. The part within the thick

dashed lines in Fig. 4 illustrates the SMO scheme.

MMO [42], [44]: Fig. 4 also illustrates MMO approach, inwhich several inverters act as VSI (the master). The VSI can

be connected to storage devices or to DERs. Other inverters

withP Q control may also coexist. The generation profile canbe modified by the MGCC, which can define new set points for

the LCs.

B. Wire and Nonwire Interconnections

Instantaneous LS in microgrids is achieved through two main

control schemes. The control schemes are classified according

to their control-wire interconnections [27], [46].

One of the two schemes is the active LS technique, which

has parallel-connected microgrid converters, including masterslave (MS) [47][58], centralized LS [59], [60], average LS

Fig. 4. MMO control, with SMO inside the dashed lines.

(ALS) [61][68], current limitation control [69], and circular

chain control (3C) [69], [70]. These control schemes critically

need intercommunication lines, which can decrease system

reliability and expandability, although they enable good VR and

accurate current sharing [71], [72].The other control scheme for parallel inverters is based

mainly on a droop method [7], [16], [26], [41], [57], [73]

[109]. This technique adjusts the output voltage and frequency

in functions of active power (P) and reactive power (Q)delivered by the inverter. The droop method uses only local

power measurements; hence, better reliability and flexibility is

achieved in the physical location of the units [71], [103], [110].

C. Control Layers

The microgrid control system has to ensure that all the con-

trol functions are accomplished (e.g., supply of electrical and/orthermal energy, continuous feeding of the critical loads, energy-

market participation, autoreconnection after failure, etc.). The

control objectives can be achieved through either centralized or

decentralized control and through three control layers, as shown

in Fig. 5. Some authors [6], [39] call the supervisory control

architecture a multiagent controller. The control levels are the

following [6], [7], [27], [58], [72], [104], [110], [111]:

1) distribution network operator (DNO) and market operator

(MO);

2) MGCC;

3) LC, which can be either the source controller or the

microload controller.

The DNO is necessary when there is more than one mi-

crogrid in the distribution system. In addition, for the market

environment of a specific area, one MO or more is responsible

for market management of the microgrid. Both the DNO and

the MO are part of the main grid; they do not belong to the

microgrid. The second level is the MGCC, which is the main

integrator of the DER clusters in a microgrid. It is responsible

in stabilizing the voltage and frequency at the PCC and also

responsible for the active and reactive powers dispatched from

each DER [26], [58]. The LCs are the lower layer of control

(sometimes called peer-to-peer, as in [6]). They control the

DERs and some of the local load and seek to balance active and

reactive powers. They have a certain intelligence level and, thus,can make decisions locally in a decentralized operation model


4/13



Fig. 5. General architecture of a hierarchical microgrid control system (mul-tiagent controller).

(whereas in a centralized model, the LCs receive set points

from the MGCC) [7], [111][113]. In [114], the intelligent

node (iNode) and intelligent socket (iSocket) performed the

same tasks as the MGCC and LC, respectively, whereas [58]

proposed four control layers, adding a layer called the main-

station layer before the DNO layer. Fig. 5 shows the DNO,

MGCC, and LC control layers.

D. Control Function

Published works on microgrids have been mostly about the

control functions and strategies [8], [42], [56], [115]. The next

part of this paper reviews the techniques and strategies for con-trol of DER converters in a microgrid system. The two modes

of operation for microgrids are equally important; however, the

islanded mode is emphasized because it is particularly more

challenging than the grid-connected mode [107]. Microgrid

grid-forming mode (islanded mode) is discussed next, demon-

strating the control techniques and objectives.

V. VOLTAGES TABILITY

Terms relating to voltage stability include voltage quality,

VR, voltage distortion, and voltage profile. Voltage stability in

a microgrid is about keeping the voltage amplitude stable at alevel required by the system. A voltage controller at each DER

unit provides local stability. Without local voltage control, sys-

tems with high penetrations of DER might experience voltage

and/or reactive power oscillations [2], [13]. Voltage stability

in islanded mode has been much studied [2], [5][7], [13],

[16], [20][22], [27], [41], [42], [44], [56], [58], [104], [106],

[107], [110], [116][123]. The concept of voltage control can

be expanded to include voltage balancing between the three

phases [119].

A. Factors Affecting Voltage Stability

1) Control is more complicated in low-voltage distributionnetworks because of their resistive nature, which may

Fig. 6. (a) Voltage set point to remove the circulating reactive current.(b) VQD controller.

cause coupling between the active power and the voltage

(instead of the frequency) [106].

2) Load characteristics can affect microgrid performance in,

e.g., voltage stability and transient stability [27].

3) There are reactive currents circulating between the DERs.

The problem is prominent in a microgrid because the

impedance between the DERs is not large enough to

prevent the circulating current. A possible solution is the

use of a voltage-versus-reactive-power controller, which

should reduce the set point of the local voltage, at capac-

itive reactive power, and should increase it, at inductive

reactive power (see Fig. 6) [2], [13], [20], [27], [122],

[124]. The PV generator can be used as a synchronous

generator to compensate the reactive power through the

inverter [125], [126].

4) Distance between the loads and the DERs [13], because

line impedance varies with it, causes an unbalanced flow

of reactive power [96], [123].The techniques and strategies of microgrid voltage control

reported in the aforementioned papers are discussed next.

B. Control Techniques and Strategies

Nested Control Loops: The term refers to two control loops

[22], [56], [119]: inner and outer. The inner loop is placed

around the inductor and the VSI to form a controlled current

source (current control loop), whereas the outer loop is a

voltage control loop, which works on the voltage error, setting

the required current for the inner loop. The voltage control

loop aims to provide better tracking of slow changes in theoutput-voltage reference signal and to minimize the output-

voltage errors caused by load output current disturbances [40],

[56], [119].

MGCC: As mentioned in Section III, there are three control

layers in microgrids, and the MGCC is the second layer, which

is responsible for keeping the voltage at the PCC between the

specific limits [4], [6], [7], [23], [26], [27], [42], [44], [111].

V/fInverter Control: This is not a new control technique.It has been used [58] to illustrate that, during islanded mode,

voltage and frequency are controlled by the DER converter.

Voltage Control Loop: Despite this terms frequent use [35],

[41], [56], [104], [107], [108], [110], [116], [117], [119], [127],

it represents neither a control technique nor strategy but onlydescribes the controller that controls voltage.


5/13



VQD Controller: The voltage-versus-reactive-power droop

(VQD) control strategy is one of the most famous and important

strategies used to control voltage during islanded mode [2],

[13], [20], [23], [27], [42], [104], [110], [122]. It mainly ensures

that the circulating reactive current between the sources does

not exceed the DER ratings. A droop controller for voltage

versus reactive power is thus required, increasing the localvoltage set point when the DER generates inductive reactive

power (conversely, the set point reduces when Q becomescapacitive) [2], [20] (see Fig. 6).

Current-Controlled MC: One of the purposes of the current-

controlled matrix converter (MC) [120] is to regulate voltage at

the load terminals. It must thus feed a specific amount of current

to the load, and therefore controls the load current, keeping the

terminal voltage at the desired level.

VPD: Unlike the VQD controller, the voltage power droop

(VPD) technique shows that microgrid voltage depends on

active power balancing and not on reactive power [116], [117].

Reference [117] suggests a control scheme that allows a sin-

gle voltage source converter (VSC) to operate in an inten-

tional islanding mode but not parallel with other VSCs in

the same islanded microgrid. This is a problem overcome by

VPD/frequency-reactive power boost (FQB) controllers [116]

that allow control of multiple VSCs in the same microgrid.

In all the control schemes that use voltage power or VPD,

proportionalintegral controllers are essential to many of them

[16], [44], [94], [96], [104], [116], [117], [120], [128], [129].

C. Voltage Stability Results Comparison

Performance evaluations of the voltage stability in the afore-

mentioned papers are assessed by computer simulation. In[120], the current-controlled MC is used to regulate the PCC

voltage. A sudden load is switched on then off during 0.06 s,

a 0.05-p.u. voltage drop occurs when the load connected, and

a 0.04-p.u. voltage rises when the load disconnected. The

proposed controller gets the voltage back to the desired value

within 30 ms. In [107], a unified control strategy is used, which

benefits from both the droop method and the active feedback

compensation. During islanded mode, a three-phase-to-ground

fault strikes one of the buses for 0.1 s, and after 0.5 s, a single-

phase-to-ground fault strikes the same bus, for 0.1 s. Both

faults have an impedance of 1.0 m. During the three-phase-to-

ground fault, the bus voltage was almost zero. Then, after thefault cleared, the controller gets the voltage back to the desired

voltage within 0.05 s. At the single-phase-to-ground fault, the

voltage is distorted due to the unbalanced nature of this fault.

Furthermore, the voltage became zero at the phase where the

fault occurs; then, after the fault cleared, the controller gets

the voltage back to the desired value within 0.07 s. In [110],

a hierarchical control approach is simulated, where the primary

control regulates the voltage on each DER terminal. There are

three DERs supplying the microgrid during islanded mode.

When one of the DERs disconnected from the microgrid, the

voltage dropped 0.031 p.u. and then returned to the desired

value within 0.5 s. After that, another DER is disconnected,

causing a 0.11-p.u. voltage drop, which is recovered by thecontroller within 1 s.

VI. FREQUENCY

In islanded mode, the DERs have to control the microgrid

frequency cooperatively and synchronously with each other.

Since there is no dominant source during islanded mode, fre-

quency control and synchronization is a challenge [6][8], [40].

In a conventional power system, synchronous machines play a

main role in achieving synchronization and frequency stability,

i.e., a role that microgrid inverters must now assume [42],

[44]. The line frequency range should not exceed the preset

values. The minimum and maximum ranges of the frequency

[16] are 4851 Hz and 59.360.5 Hz, for 50-Hz and 60-Hz

grids, respectively. Microgrid frequency has been much studied

[2], [3], [6][8], [11][14], [16], [20], [23], [25], [26], [40]

[42], [44], [58], [94], [103], [105], [107], [109], [110], [116],

[120], with the general aim of overcoming the frequency insta-

bility caused by the factors listed below.

A. Factors Affecting Frequency Stability

1) Battery voltage exceeding the preset value through over-

charging affects demand and supply balancing, causing

frequency disturbance at the PCC [16], [43].

2) Variation in load or generation affects line frequency

because the demand has to be the same as the supply

instantaneously [12], [25], [109].

3) Unintentional islanding may cause frequency deviation

because some imported or exported power will be lost

suddenly [26].

The aforementioned papers used various techniques and strate-

gies for microgrid frequency control.


FPD: Real power versus system frequency is the most fa-

mous and popular technique [2], [6][8], [13], [16], [20], [23],

[40], [41], [44], [58], [94], [103], [106], [107], [109], [110].

It uses microgrid frequency to balance system-generated active

power. It is proven to be robust and seamlessly adapts to power

system variations. The relation between frequency and real

power of each DER can be expressed as

f1= fo Ku(P1 Po) (1)

where Ku is the droop constant of unit output power control

mode, f1

and P1

are respectively the new frequency and theDER output power, and fo and Po are the initial operationpoints. When the load increases, the DER output power also

increases, associating with frequency reduction, as shown in

(1). Fig. 7 shows the frequency-versus-active-power droop

(FPD) characteristic. There are two DERs (a and b), operating

at f. When the power shares of any DER increase for anyreasons (e.g., intentional or unintentional islanding or a demand

hike), the new operating points for power become Pa1 and Pb1instead ofPaoand Pbo. The frequency thus sags to a new valuef1, which is below the lower limit. Consequently, then, a newoperating line for each unit forms to return the system to the

preset frequency values.

Current-Controlled MC: One of the tasks of this controller[120] is to keep the frequency constant. To do so, the MC


6/13



Fig. 7. FPD characteristics.

feeds the load a specific amount of current, i.e., current control

forces the MC current to follow the reference value, keeping the

frequency constant within the source rating.

FQB Controller: This is the proposed technique, in which

the frequency depends on the reactive power (whereas powerbalancing determines microgrid voltage) and which is in stark

contrast with a conventional power system. Because there is no

generator with rotational inertia in microgrids, the PCC capac-

itor dynamics have to control the relation between frequency

and reactive power [9], [26], [116], [117].

C. Frequency Results Comparison

Many results have been established in the aforementioned

papers to keep the microgrid frequency stable. In [120], the

current-controlled MC is used to keep the frequency constant

at 50 Hz. Although there is a sudden load switching on thenoff, the simulation shows that the controller keeps the frequency

stable. In [107], a unified control strategy is used, which bene-

fits from both the droop method and the active feedback com-

pensation. During islanded mode, a three-phase-to-ground fault

strikes one of the buses for 0.1 s, and after 0.5 s, a single-phase-

to-ground fault strikes the same bus, for 0.1 s. Both faults have

an impedance of 1.0 m. After the three-phase-to-ground faultcleared, the controller gets the frequency back to the desired

value within 0.025 s. After the single-phase-to-ground fault

cleared, the controller gets the frequency back to the desired

value within 0.05 s. In [12], the microgrid frequency is 50 Hz.

After a 100-kW load is switched on during islanded mode,

the frequency dropped to 49.5 Hz; thereafter, the frequency

regulator reacts immediately and brings the frequency back to

50 Hz, within 2 s.

VII. L SQ A ND P

An essential criterion a power management system (PMS)

considers is the LS among DERs, to minimize system power

losses [8]. LS ensures that each DER supplies its preset pro-

portion at the steady state [109]. It becomes more complicated

in microgrids since there are multiple DERs; more attention

is needed during islanded mode. Many have studied power

sharing [2], [6][8], [11], [16], [20], [22], [25][27], [40][42],[44], [56], [57], [76], [96], [103][110], [114], [116], [119],

TABLE ITYPICAL L IN E-I MPEDANCEVALUES

[130], and they have shown that the main PMS considerations

are the following:

1) minimizing system power loss during LS;

2) considering the many limits of each DER, including type,

generation cost, maintenance interval, and environmental

impact;

3) maintaining power quality, including keeping the har-

monic distortion low and maintaining the voltage profile;

4) restoring voltage/frequency during and after transients;

5) improving the dynamic response;6) maintaining system operation within the stability margin.

Among the three levels of control in microgrids, the sec-

ondary control (MGCC) is responsible for LS [6], [104], [110],

[114], [119]. Because the voltage source presents low output

impedance, an accurate synchronization system is extremely

necessary to operate many VSCs in parallel. LS among a cluster

of VSCs, operating in islanded mode, in a microgrid is a

function of the value of their output impedance [41].

A. Factors Affecting LS Accuracy

1) The resistive nature (low X/R ratio) of distributionnetworks affects accuracy of LS [27], [42], [44], [96],

[103], [105], [131]. Table I shows the line R/X ratiosfor different line voltages [132].

2) Variation of the output impedance [27], [95], [96], [103],

[130].

3) Variation of the inverter filter parameters may affect

power sharing if the voltage is not controlled by the

MGCC [119].

4) Harmonic current (which should be taken into account

when sharing nonlinear loads, to balance active and reac-

tive power) [96], [110].

5) The distance between the DERs in a microgrid can

change the inverter output and line impedance, affecting

LS [96].

Many literatures propose controlling the power sharing with-

out any wire connection between the DERs.


Droop Control Method: The droop control method is based

on the well-known correlation between the active power flows

with the frequency and the reactive power with the voltage.

The relation shows that, when active power increases through

load increase, the frequency will decrease; the opposite happens

when load decreases. The droop method allows DERs to shareactive power automatically by measuring the local variables,


7/13



ensuring system reliability and flexibility [7], [26], [75], [96],

[103], [104], [106], [109], [116], [130].

Many droop control schemes have been proposed for linear

LS [72], [75], [77], [84], [133][135]. There are, however,

still only a few controllers for sharing nonlinear loads [77],

[84]. In [77], the proposed controller achieved LS by adjusting

the output-voltage bandwidth with the distortion componentspresent. This way, the harmonic voltage components were

drooped, encouraging the DERs to share the current harmonics.

In another way [84], to produce a proportional droop in the

corresponding harmonic voltage term, every single term of the

harmonic current was used.

Despite the many advantages of the droop method, its draw-

backs limit its applications. Its accurate power sharing affects

frequency and voltage, it has slow transient response (as it

requires low-pass filters), its sharing of harmonic current is un-

balanced, and it is highly affected by inverter output impedance

and line impedance [71], [88], [103], [110]. The drawbacks are

overcome as follows.

Improvements to the Conventional Droop Method: Because

dependence on the droop method increased in microgrids,

research produced the following solutions.

1) Virtual output impedance is widely used to achieve

accurate LS in microgrids [41], [72], [96], [100], [103],

[104], [108], [110], [130], [136]. It emulates lossless re-

sistors of reactors to fix the output impedance of the units,

eliminating the effect of the output and line impedances

on LS and improving the steady-state and transient re-

sponses of the parallel-connected inverters. Other, virtual

reactors and resistors [18], [137] have been included into

the droop method, with the additional purpose of sharingthe harmonic-current content properly [110].

2) D ddroop characteristicpresents droop character-

istics between the different frequency components of the

controlled signal (the ac voltage) and the active power,

the reactive power, and the distorted power (D) [27],

[72], [138][140] The use of VQD is avoided and the

line-impedance effect eliminated.D ddroop can alsocompensate the distorted powers that result from nonlin-

ear and unbalanced loads. d is the drooping frequencycomponent caused by the distorted power.

3) Proper design of the output impedance; this can reduce

the impact caused by changes in the line impedance[95], [103]. Although the inverter output impedance can

be designed well, the line impedance is still unknown,

causing unbalanced flow of reactive power [77].

4) Injection of high-frequency signals: unbalanced flow of

reactive power in the system can be solved by injecting

high-frequency signals into the power lines [72]. Such an

injection, however, limits the power rating of the DER

units and increases voltage distortion at their outputs [27],

[72], [96].

5) A central voltage controller regulates the voltage pro-

file, preventing the system from being affected by both

variations in the inverter filter parameters, or voltage

controller gains neither the steady-state voltage controlnor the power sharing [119].

Fig. 8. Equivalent circuit of distribution network.

Fig. 9. Distribution network, in phasor.

Fig. 10. Resistive impedance curves for (a) voltage versus active power and(b) frequency versus reactive power.

TABLE IIDROOP M ETHODS TRATEGY D EPENDING ONO UTPUT I MPEDANCE

6) Overcoming the problem of high R/X ratio: the re-sistivity of the low-voltage networks makes precise LS

unachievable [42], [44], [105], [108], [131]. In terms of

voltage harmonic mitigation, handling of short circuits,

and effectiveness of frequency and voltage control, the

approach proposed in [96] and [108] is superior than

existing methods because it takes into account the R-to-Xdistribution-line ratio.

7) Sharing of harmonic current, as proposed in [96] and

[110] prevents power from circulating during sharing of

nonlinear loads. The harmonic currents are considered, so

that the active and reactive powers can be balanced moreaccurately than they are in the conventional droop method.

8) Soft-start operationis necessary to avoid the initial cur-

rent peak andto createa seamless hot-swap operation[72].

9) Load voltage controlis a new LS strategy used, instead

of a conventional load-frequency-based power dispatch-

ing scheme, when a system uses a fixed frequency [109].

The output voltage of each DER is adjusted to keep the

load voltage stable. It makes for a simple and effective

system.

FPD/VQD Control: VPD/FQB control has been reported in

many papers [2], [7], [13], [20], [25], [26], [40], [104], [106],

[109], [110], [116]. Figs. 6 and 7 illustrate the VQD controllerand the FPD characteristic, respectively. The droop method uses


8/13



TABLE IIISUMMARY OF M ICROGRIDC ONTROLO BJECTIVES, PROBLEMS, A ND S OLUTIONS

the conventional power flow; (2) and (3) are derived in Fig. 8,

as in [41] and [104]; this is achieved with the two important

assumptions explained as follows:

P =VA

Z cos V

A VBZ

cos( + A B) (2)

Q=VAZ

sin VA VB

Z sin( + A B) (3)

= A B. (4)

Assumption I: In medium- and high-voltage systems, theinductive components of the line impedance are very high


9/13



TABLE III(Continued.) S UMMARY OFM ICROGRIDC ONTROLO BJECTIVES, PROBLEMS, A ND S OLUTIONS

compared with the resistive components [132]; hence, the re-

sistance can be neglected and the system considered as having

pure inductive output impedance (R= 0, Z= X, and =90). The active and reactive powers are thus

P =VA VB

X sin (5)

Q=VA VB

X cos

VB2

X (6)

where is the phase of the output impedance, and is the phaseangle between the inverter output and the Microgrid voltages.

The large filter inductor of the inverter justifies the assump-

tion. The line impedance, however, is mainly resistive in low-voltage networks, invalidating the assumption. An additional

output inductor, creating an LCL output filter, can solve this.Another solution is by programming a virtual output impedance

into the control loop in Section VII (in improvements to the

conventional droop method).

Assumption II: The angle is small; hence, sin almostequals and cos 1. Consequently

P =VA VB

X (7)

Q=VB(VA VB)

X (8)

where P and Q are the active and reactive powers, respectively,and VA and VB are the voltages in buses A and B, respectively.Fig. 9 shows a simplified system, in a phasor diagram.

VPD/FQB Controller: Low-voltage microgrids are known

to be resistive. If this is considered, a better control can be

achieved. The distance between the DERs makes wire in-

terconnections difficult; hence, droop control is used. In the

VPD/FQB controller, there is a direct relation between voltage

and active power and between frequency and reactive power

[16], [103], [116]. Fig. 10(a) shows that, when power increases,

voltage reduces; hence, to get the voltage in any operating

time, the power variation (nP) is subtracted from the voltage

at no load (E). Fig. 10(b) relates the reactive power withfrequency. The reactive power ranges from positiveQmaxat the

inductive load to Qmaxat the capacitive load. The controllercan measure the frequency by adding the frequency at no load

()plus the change in reactive power (mQ).Table II summarizes the equations used for the droop

method, for different output impedances (inductive and resis-

tive) [96].

C. LS Results Comparison

In [107], two DERs are simulated to feed the load during

islanded mode. A three-phase-to-ground fault strikes one of

the buses for a 0.1 s, and after 0.5 s, a single-phase-to-ground

fault strikes the same bus, also for 0.1 s. Both faults have animpedance of 1.0 m. The proposed unified controller revertsthe active and reactive power to their predisturbance values

once the faults are cleared. In [130], two cases are simulated,

i.e., pure resistive and resistiveinductive load, to investigate

the proper shear between two DERs during islanded mode. The

simulation shows that, after the compensation is started, the two

DERs properly shared the active and reactive power. In [104],

after the two DERs disconnected from the grid, both of them

shared the overall load. Then, when one of the DERs discon-

nected, the other supplied all the power to the microgrid with a

proper transient response, as well as the fine power regulation

of the system. In [22], after transiting to islanded mode, thetwo DERs shared the increased active power demand propor-

tionately, while the reactive power sharing is slightly dispro-

portional because the presence of the line impedance between

the two DERs. In [110], three DERs produced microgrid power

during islanded mode. The DERs switched off one by one,

keeping only one DER supplying the whole microgrid load. The

controller ensured smooth active power sharing during DER

disconnections.

VIII. SUMMARY

As this paper illustrates, there are three microgrid control

functions: VR, FR, and LS. Table III summarizes the problemsof microgrid control and their solutions.


10/13



IX. CONCLUSION

The microgrids have many attractive features that make its

research promising and boost its competitiveness in penetrating

renewable energy. Microgrid architecture and classifications

have been reviewed here, along with the control techniques

and strategies for islanded operation mode. Challenges to

microgrids during this mode include maintaining the voltage

amplitude and frequency, and optimal power sharing which

ensures minimum power losses, high power quality hence low

harmonic distortion, and operating the DERs on their preset

values after any disturbance occurs. A robustly controlled mi-

crogrid ensures seamless import/export of active and reactive

powers by the main grid and continuous supply of the critical

load during islanded mode. These lead to a flexible and smart

power system.

ACKNOWLEDGMENT

The authors would like to thank the University of Malaya

Power Energy Dedicated Advanced Centre (UMPEDAC),

where this work was carried out.

REFERENCES

[1] R. K. Khosrow and Moslehi, Vision for a self-healing power grid,ABBReview, pp. 2125, 2006.

[2] P. Piagi and R. H. Lasseter, Autonomous control of microgrids, inProc. IEEE Power Eng. Soc. Gen. Meet. , 2006, pp. 18.

[3] C. Sontidpanya, G. Radman, and R. Craven, An island detection demon-stration on a laboratory sized power grid (LabGrid), in Proc. IEEESoutheastcon, 2011, pp. 158163.

[4] M. Shamshiri, G. Chin Kim, and T. Chee Wei, A review of recent

development in smart grid and micro-grid laboratories, in Proc. IEEEInt. PEDCO, Melaka, Malaysia, 2012, pp. 367372.[5] V. Knazkins, Stability of power systems with large amounts of dis-

tributed generation, Ph.D. dissertation, KTH, Stockholm, Sweden,2004.

[6] M. Mao, C. Liuchen, and M. Ding, Integration and intelligent controlof micro-grids with multi-energy generations: A review, inProc. IEEE

ICSET, 2008, pp. 777780.[7] F. Katiraei, R. Iravani, N. Hatziargyriou, and A. Dimeas, Microgrids

management, IEEE Power Energy Mag., vol. 6, no. 3, pp. 5465,May 2008.

[8] F. Katiraei and M. R. Iravani, Power management strategies for a mi-crogrid with multiple distributed generation units, IEEE Trans. PowerSyst., vol. 21, no. 4, pp. 18211831, Nov. 2006.

[9] P. M. Carvalho, P. F. Correia, and L. Ferreira, Distributed reactive powergeneration control for voltage rise mitigation in distribution networks,

IEEE Trans. Power Syst., vol. 23, no. 2, pp. 766772, May 2008.

[10] C. Masters, Voltage rise: The big issue when connecting embeddedgeneration to long 11 kV overhead lines,Power Eng. J., vol. 16, no. 1,pp. 512, Feb. 2002.

[11] N. D. Hatziargyriou and A. P. Sakis Meliopoulos, Distributed energysources: Technical challenges, in Proc. IEEE Power Eng. Soc. Winter

Meet., 2002, vol. 2, pp. 10171022.[12] Y. Duan, Y. Gong, Q. Li, and H. Wang, Modelling and simulation

of the microsources within a microgrid, in Proc. ICEMS, 2008,pp. 26672671.

[13] R. H. Lasseter, Smart distribution: Coupled microgrids,Proc. IEEE,vol. 99, no. 6, pp. 10741082, Jun. 2011.

[14] J. Lopes, N. Hatziargyriou, J. Mutale, P. Djapic, and N. Jenkins, Inte-grating distributed generation into electric power systems: A review ofdrivers, challenges and opportunities, Electr. Power Syst. Res., vol. 77,no. 9, pp. 11891203, Jul. 2007.

[15] N. R. Canada, Self-healing grid project, Natl. Res. Canada, Calgary,

AB, Canada, 2012.[16] E. Serban and H. Serban, A control strategy for a distributed power gen-eration microgrid application with voltage- and current-controlled source

converter,IEEE Trans. Power Electron., vol. 25, no. 12, pp. 29812992,Dec. 2010.

[17] R. M. Kamel, A. Chaouachi, and K. Nagasaka, Enhancement of micro-

grid performance during islanding mode using storage batteries and newfuzzy logic pitch angle controller, Energy Convers. Manage., vol. 52,no. 5, pp. 22042216, May 2011.

[18] O. Alsayegh, S. Alhajraf, and H. Albusairi, Grid-connected renewableenergysource systems: Challenges and proposed management schemes,

Energy Convers. Manage., vol. 51, no. 8, pp. 16901693, Aug. 2010.[19] M. Silva, H. Morais, and Z. Vale, An integrated approach for distributed

energy resource short-term scheduling in smart grids considering re-alistic power system simulation, Energy Co nvers. Manage., vol. 64,pp. 273288, Dec. 2012.

[20] R. H. Lasseter and P. Piagi, Control and design of microgrid compo-nents, Univ. Wisconsin, Madison, WI, USA, PSERC Publication 06,2006, vol. 3.

[21] J. Driesen and F. Katiraei, Design for distributed energy resources,IEEE Power Energy M ag., vol. 6, no. 3, pp. 3040, May/Jun. 2008.

[22] L. Yunwei, D. M. Vilathgamuwa, and L. Poh Chiang, Design, analysis,and real-time testing of a controller for multibus microgrid system,

IEEE Trans. Power Electron., vol. 19, no. 5, pp. 11951204, Sep. 2004.[23] B. Kroposki, R. Lasseter, T. Ise, S. Morozumi, S. Papatlianassiou, and

N. Hatziargyriou, Making microgrids work,IEEE Power Energy Mag.,vol. 6, no. 3, pp. 4053, May/Jun. 2008.

[24] F. Blaabjerg, Z. Chen, and S. B. Kjaer, Power electronics as efficient

interface in dispersed power generation systems, IEEE Trans. PowerElectron., vol. 19, no. 5, pp. 11841194, Sep. 2004.

[25] S. J. Ahn, J. W. Park, I. Y. Chung, S. I. Moon, S. H. Kang, and S. R. Nam,

Power-sharing method of multiple distributed generators consideringcontrol modes and configurations of a microgrid, IEEE Trans. Power

Del., vol. 25, no. 3, pp. 20072016, Jul. 2010.[26] T. S. Ustun, C. Ozansoy, and A. Zayegh, Recent developments in micro-

grids and example cases around the worldA review, Renew. Sustain.Energy Rev., vol. 15, no. 8, pp. 40304041, Oct. 2011.

[27] Z. Chen, A review of power electronics based microgrids, J. PowerElectron., vol. 12, no. 1, pp. 181192, Jan. 2012.

[28] H. Jiayi, J. Chuanwen, and X. Rong, A review on distributed energyresources and MicroGrid, Renew. Sustain. Energy Rev., vol. 12, no. 9,pp. 24722483, Dec. 2008.

[29] X. Xu and X. Zha, Overview of the researches on distributed generationand microgrid, in Proc. IPEC, 2007, pp. 966971.

[30] S. Chakraborty, M. D. Weiss, and M. G. Simes, Distributed intel-ligent energy management system for a single-phase high-frequencyAC microgrid, IEEE Trans. Ind. Electron., vol. 54, no. 1, pp. 97109,Feb. 2007.

[31] L. Xiaofei, A. Xin, and W. Yonggang, Study of single-phase HFACmicrogrid based on Matlab/Simulink, in Proc. 4th Int. Conf. DRPT,2011, pp. 11041108.

[32] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, A review of single-phasegrid-connected inverters for photovoltaic modules, IEEE Trans. Ind.

Appl., vol. 41, no. 5, pp. 12921306, Sep./Oct. 2005.[33] N. A. Rahim and A. M. Omar, Three-phase single-stage high-voltage

DC converter, Proc. Inst. Elect. Eng.Gen., Transmiss. Distrib.,vol. 149, no. 5, pp. 505509, Sep. 2002.

[34] S. Ponnaluri, G. O. Linhofer, J. K. Steinke, and P. K. Steimer, Compar-ison of single and two stage topologies for interface of BESS or fuel cellsystem using the ABB standard power electronics building blocks, inProc. Eur. Conf. Power Electron. Appl., 2005, pp. 19.

[35] Y. J. Song, S. K. Chung, and P. N. Enjeti, A current-fed HFlink direct DC/AC converter with active harmonic filter for fuel cellpower systems, in Conf. Rec. IEEE 39th IAS Annu. Meeting, 2004,pp. 123128.

[36] S. Inoue and H. Akagi, A bi-directional isolated DC/DC converter asa core circuit of the next-generation medium-voltage power conversionsystem, inProc. 37th IEEE PESC, 2006, pp. 17.

[37] A. G. Tsikalakis and N. D. Hatziargyriou, Centralized control for op-timizing microgrids operation, in Proc. IEEE Power Energy Soc. Gen.

Meet., 2011, pp. 18.[38] A. D. N. Hatziargyriou, S. Hatzivasiliadis, J. Jimeno, and J. Oyarzabal,

DB3: Decentralized control concepts, EU More Microgrid, Sch. Elect.Comput. Eng., Greece, 2008.

[39] A. L. Dimeas and N. D. Hatziargyriou, Operation of a multiagentsystem for microgrid control, IEEE Trans. Power Syst., vol. 20, no. 3,pp. 14471455, Aug. 2005.

[40] F. Z. Peng, Y. W. Li, and L. M. Tolbert, Control and protection of p owerelectronics interfaced distributed generation systems in a customer-driven microgrid, in Proc. IEEE PES Gen. Meet., 2009, pp. 18.


11/13



[41] J. Rocabert, A. Luna, F. Blaabjerg, and P. Rodriguez, Control of powerconverters in AC microgrids, IEEE Trans. Power Electron., vol. 27,

no. 11, pp. 47344749, Nov. 2012.[42] J. A. P. Lopes, C. L. Moreira, and A. G. Madureira, Defining control

strategies for MicroGrids islanded operation, IEEE Trans. Power Syst.,vol. 21, no. 2, pp. 916924, May 2006.

[43] D. Georgakis, S. Papathanassiou, N. Hatziargyriou, A. Engler, andC. Hardt, Operation of a prototype microgrid system based on micro-

sources quipped with fast-acting power electronics interfaces, in Proc.IEEE 35th Annu. PESC/IEEE Power Electron. Spec. Conf., 2004, vol. 4,pp. 25212526.

[44] C. Moreira, F. Resende, and J. A. P. Lopes, Using low voltage micro-grids for service restoration, IEEE Trans. Power Syst., vol. 22, no. 1,pp. 395403, Feb. 2007.

[45] A. Madureira, C. Moreira, and J. P. Lopes, Secondary load fre-quency control for microgrids in islanded operation, in Proc. ICREPQ,

Crdoba, Spain, 2005, pp. 15.[46] J. Gurrero, L. G. De Vicuna, and J. Uceda, Uninterruptible power

supply systems provide protection, IEEE Ind. Electron. Mag., vol. 1,no. 1, pp. 2838, Spring 2007.

[47] J. Holtz, W. Lotzkat, and K.-H. Werner, A high-power multitransistor-inverter uninterruptable power supply system,IEEE Trans. Power Elec-tron., vol. 3, no. 3, pp. 278285, Jul. 1988.

[48] J. Holtz and K.-H. Werner, Multi-inverter UPS system with redundantload sharing control,IEEE Trans. Ind. Electron., vol. 37, no. 6, pp. 506

513, Dec. 1990.[49] H. Van Der Broeck and U. Boeke, A simple method for parallel opera-

tion of inverters, inProc. 20th INTELEC, 1998, pp. 143150.

[50] C. Lee, S. Kim, C. Kim, S. Hong, J. Yoo, S. Kim, C. Kim, H. Woo, andH. Sun, Parallel UPS with a instantaneous current sharing control, inProc. 24th Annu. Conf. IEEE IECON, 1998, pp. 568573.

[51] Y. Pei, G. Jiang, X. Yang, and Z. Wang, Auto-master-slave control tech-nique of parallel inverters in distributed AC power systems and UPS, inProc. IEEE 35th Annu. PESC, 2004, pp. 20502053.

[52] W.-C. Lee, T.-K. Lee, S.-H. Lee, K.-H. Kim, D.-S. Hyun, and I.- Y. Suh,A master and slave control strategy for parallel operation of three-phaseUPS systems with different ratings, in Proc. 19th Annu. IEEE APEC,2004, pp. 456462.

[53] J.-F. Chen and C.-L. Chu, Combination voltage-controlled and current-controlled PWM inverters for UPS parallel operation, IEEE Trans.Power Electron., vol. 10, no. 5, pp. 547558, Sep. 1995.

[54] S. Tamai and M. Kinoshita, Parallel operation of digital controlled UPSsystem, inProc. IEEE IECON/IECON, 1991, pp. 326331.[55] Y. J. Cheng and E. K. K. Sng, A novel communication strategy for

decentralized control of paralleled multi-inverter systems,IEEE Trans.Power Electron., vol. 21, no. 1, pp. 148156, Jan. 2006.

[56] T. C. Green and M. Prodanovi, Control of inverter-based micro-grids,Elect. Power Syst. Res., vol. 77, no. 9, pp. 12041213, Jul. 2007.

[57] S. K. Mazumder, M. Tahir, and K. Acharya, Masterslave current-sharing control of a parallel DCDC converter system over an RFcommunication interface, IEEE Trans. Ind. Electron., vol. 55, no. 1,pp. 5966, Jan. 2008.

[58] Z. Bo, Z. Xuesong, and C. Jian, Integrated microgrid laboratorysystem, IEEE Trans. Power Syst., vol. 27, no. 4, pp. 21752185,

Nov. 2012.[59] T. Iwade, S. Komiyama, Y. Tanimura, M. Yamanaka, M. Sakane, and

K. Hirachi, A novel small-scale UPS using a parallel redundant opera-tion system, inProc. 25th INTELEC, 2003, pp. 480484.

[60] A. P. Martins, A. S. Carvalho, and A. Araujo, Design and implementa-tion of a current controller for the parallel operation of standard UPSs,inProc. IEEE 21st IECON/IECON, 1995, pp. 584589.

[61] X. Sun, Y.-S. Lee, and D. Xu, Modeling, analysis, and implementationof parallel multi-inverter systems with instantaneous average-current-sharing scheme,IEEE Trans. Power Electron., vol. 18, no. 3, pp. 844856, May 2003.

[62] Y.-K. Chen, T.-F. Wu, Y.-E. Wu, and C .-P. Ku, A current-sharing con-

trol strategy for paralleled multi-inverter systems using microprocessor-based robust control, in Proc. IEEE TENCON, 2001, pp. 647653.

[63] Y.-K. Chen, Y.-E. Wu, T.-F. Wu, and C.-P. Ku, ACSS for paralleledmulti-inverter systems with DSP-based robust controls, IEEE Trans.

Aerosp. Electron. Syst., vol. 39, no. 3, pp. 10021015, Jul. 2003.[64] X. Sun, L.-K. Wong, Y.-S. Lee, and D. Xu, Design and analysis of an

optimal controller for parallel multi-inverter systems, IEEE Trans. Circ.Syst. II, Exp. Briefs, vol. 53, no. 1, pp. 5661, Jan. 2006.

[65] Z. He, Y. Xing, and Y. Hu, Low cost compound current sharing controlfor inverters in parallel operation, in Proc. IEEE 35th Annu. PESC/IEEE

Power Electron. Spec. Conf., 2004, pp. 222227.

[66] H.-M. Hsieh, T.-F. Wu, Y.-E. Wu, and H.-S. Nien, A compensationstrategy for parallel inverters to achieve precise weighting current distri-

bution, inConf. Rec. IEEE 14th IAS Annu. Meeting, 2005, pp. 954960.[67] H. Oshima, Y. Miyazaya, and A. Hirata, Parallel redundant UPS

with instantaneous PWM control, in Proc. 13th INTELEC, 1991,pp. 436442.

[68] J. Tan, H. Lin, J. Zhang, and J. Ying, A novel load sharing control tech-nique for paralleled inverters, in Proc. IEEE 34th Annu. PESC/IEEE

Power Electron. Spec. Conf., 2003, pp. 14321437.[69] S. Chiang, C. Lin, and C. Yen, Current limitation control technique

for parallel operation of UPS inverters, in Proc. IEEE 35th Annu.PESC/IEEE Power Electron. Spec. Conf., 2004, pp. 19221926.

[70] T.-F. Wu, Y.-K. Chen, and Y.-H. Huang, 3C strategy for inverters inparallel operation achieving an equal current distribution,IEEE Trans.

Ind. Electron., vol. 47, no. 2, pp. 273281, Apr. 2000.[71] J. M. Guerrero, L. Hang, and J. Uceda, Control of distributed unin-

terruptible power supply systems, IEEE Trans. Ind. Electron., vol. 55,no. 8, pp. 28452859, Aug. 2008.

[72] A. Tuladhar, H. Jin, T. Unger, and K. Mauch, Control of parallelinverters in distributed AC power systems with consideration of lineimpedance effect, IEEE Trans. Ind. Appl., vol. 36, no. 1, pp. 131138,Jan/Feb. 2000.

[73] L. Mihalache, Paralleling control technique with no intercommunica-tion signals for resonant controller-based inverters, in Conf. Rec. IEEE

38th IAS Annu. Meeting, 2003, pp. 18821889.

[74] M. Chandorkar, D. Divan, Y. Hu, and B. Banerjee, Novel architecturesand control for distributed UPS systems, in Proc. 9th Annu. APEC,1994, pp. 683689.

[75] M. C. Chandorkar, D. M. Divan, and R. Adapa, Control of parallelconnected inverters in standalone ac supply systems, IEEE Trans. Ind.

Appl., vol. 29, no. 1, pp. 136143, Jan./Feb. 1993.[76] M. Chandorkar and D. Divan, Decentralized operation of distributed

UPS systems, in Proc. Int. Conf. Power Electron., Drives, Energy Syst.Ind. Growth, 1996, pp. 565571.

[77] A. Tuladhar, H. Jin, T. Unger, and K. Mauch, Parallel operation ofsingle phase inverter modules with no control interconnections, inProc.12th Annu. APEC, 1997, pp. 94100.

[78] F. Zanxuan and Q. Wenlong, A current share method for parallel single-phase DC/AC inverter without control interconnection, in Proc. 4th Int.

IPEMC Con f., 2004, pp. 953955.[79] X. Chen, Y. Kang, and J. Chen, Operation, control technique of parallel

connected high power three-phase inverters, in Proc. 4th Int. IPEMCConf., 2004, pp. 956959.[80] M. Xie, Y. Li, K. Cai, P. Wang, and X. Sheng, A novel controller for

parallel operation of inverters based on decomposing of output current,inConf. Rec. IEEE 14th IAS Annu. Meeting, 2005, pp. 16711676.

[81] Y. Byun, T. Koo, K. Joe, E. Kim, J. Seo, and D. Kim, Parallel operationof three-phase UPS inverters by wireless load sharing control, in Proc.22nd INTELEC, 2000, pp. 526532.

[82] A. Daneshpooy, Dead-beat control of parallel connected UPS, inProc.

17th Annu. IEEE APEC, 2002, pp. 580583.[83] C. Mak and L. Bolster, Bus-tie synchronization and load share tech-

nique in a ring bus system with multiple power inverters, in Proc. 20thAnnu. IEEE APEC, 2005, pp. 871874.

[84] U. Borup, F. Blaabjerg, and P. N. Enjeti, Sharing of nonlinear loadin parallel-connected three-phase converters, IEEE Trans. Ind. Appl.,vol. 37, no. 6, pp. 18171823, Nov./Dec. 2001.

[85] S. Chiang and J. Chang, Parallel operation of series-connected PWM

voltage regulators without control interconnection, in Proc. Inst. Elect.Eng.Elect. Power Appl., 2001, pp. 141147.

[86] L. Chen, L. Xiao, C. Gong, and Y. Yan, Circulating currents character-istics analysis and the control strategy of parallel system based on doubleclose-loop controlled VSI, in Proc.IEEE 35th Annu. PESC/IEEE Power

Electron. Spec. C onf., 2004, pp. 47914797.[87] E. A. A. Coelho, P. C. Cortizo, and P. F. D. Garcia, Small-signal stabil-

ity for parallel-connected inverters in stand-alone AC supply systems,IEEE Trans. Ind. Appl., vol. 38, no. 2, pp. 533542, Mar./Apr. 2002.

[88] E. Coelho, P. Cortizo, and P. Garcia, Small signal stability for singlephase inverter connected to stiff AC system, in Conf. Rec. IEEE 34th

IAS Annu. Meeting, 1999, pp. 21802187.[89] J. Guerrero, L. G. de Vicua, J. Miret, J. Matas, and J. Cruz, Output

impedance performance for parallel operation of UPS inverters usingwireless and average current-sharing controllers, in Proc. IEEE 35th

Annu. PESC/IEEE Power Electron. Spec. Co nf., 2004, pp. 24822488.

[90] L. Xinchun, F. Feng, D. Shanxu, K. Yong, and C. Jian, Modeling andstability analysis for two paralleled UPS with no control interconnec-tion, inProc. IEEE IEMDC, 2003, pp. 17721776.


12/13



[91] L. Xinchun, F. Feng, D. Shanxu, K. Yong, and C. Jian, The droopcharacteristic decoupling control of parallel connected UPS with no

control interconnection, in Proc. IEEE IEMDC, 2003, pp. 17771780.[92] E. Sato, A. Kawamura, and R. Fujii, Theoretical and experimental

verification of independent control for parallel-connected multi-UPSs,inProc. 25th INTELEC, 2003, pp. 485492.

[93] Y. Sun, W. Qu, and B. Liang, An advanced synchronization and current-sharing method for paralleled DC/AC inverterswithout interconnection,

inProc. 8th ICEMS, 2005, pp. 10131016.[94] J. M. Guerrero, L. G. de Vicua, J. Matas, M. Castilla, and J. Miret, A

wireless controller to enhance dynamic performance of parallel invertersin distributed generation systems,IEEE Trans. Power Electron., vol. 19,no. 5, pp. 12051213, Sep. 2004.

[95] J. M. Guerrero, L. GarciadeVicuna, J. Matas, M. Castilla, and J. Miret,Output impedance design of parallel-connected UPS inverters withwireless load-sharing control,IEEE Trans. Ind. Electron., vol. 52, no. 4,

pp. 11261135, Aug. 2005.[96] J. M. Guerrero, J. Matas, L. G. De Vicuna, N. Berbel, and J. Sosa,

Wireless-control strategy for parallel operation of distributed generationinverters, inProc. IEEE ISIE, 2005, pp. 845850.

[97] Y. Ito and O. Iyama, Parallel redundant operation of UPS with robustcurrent minor loop, in Proc. Power Convers. Conf., Nagaoka, Japan,1997, pp. 489494.

[98] K. Wallace and G. Mantov, Wireless load sharing of single phase tele-com inverters, inProc. 21st INTELEC, 1999, pp. 17.

[99] H.-P. Glauser, M. Keller, A. Pluss, M. Schwab, and R. Scherwey, Newinverter module with digital control for parallel operation, inProc. 3rd

Int. TELESCON, 2000, pp. 265269.

[100] S. Chiang, C. Yen, and K. Chang, A multimodule parallelable series-connected PWM voltage regulator,IEEE Trans. Ind. Electron., vol. 48,no. 3, pp. 506516, Jun. 2001.

[101] T. Skjellnes, A. Skjellnes, and L. E. Norum, Load sharing for par-allel inverters without communication, in Proc. NORPIE, Stockholm,Sweden, 2002, pp. 14.

[102] H. Hanaoka, M. Nagai, and M. Yanagisawa, Development of a novelparallel redundant UPS, inProc. 25th INTELEC, 2003, pp. 493498.

[103] J. M. Guerrero, J. Matas, L. G. de Vicua, M. Castilla, and J. Miret,Decentralized control for parallel operation of distributed generationinverters using resistive output impedance,IEEE Trans. Ind. Electron.,vol. 54, no. 2, pp. 9941004, Apr. 2007.

[104] J. M. Guerrero, J. C. Vasquez, J. Matas, M. Castilla, and L. G. de Vicuna,

Control strategy for flexible microgrid based on parallel line-interactiveUPS systems,IEEE Trans. Ind. Electron., vol. 56, no. 3, pp. 726736,Mar. 2009.

[105] E. Barklund, N. Pogaku, M. Prodanovic, C. Hernandez-Aramburo, andT. C. Green, Energy management in autonomous microgrid usingstability-constrained droop control of inverters, IEEE Trans. Power

Electron., vol. 23, no. 5, pp. 23462352, Sep. 2008.[106] N. Pogaku, M. Prodanovic, and T. C. Green, Modeling, analysis and

testing of autonomous operation of an inverter-based microgrid, IEEE

Trans. Power Electron., vol. 22, no. 2, pp. 613625, Mar. 2007.[107] M. B. Delghavi and A. Yazdani, A unified control strategy for electron-

ically interfaced distributed energy resources, IEEE Trans. Power Del.,vol. 27, no. 2, pp. 803812, Apr. 2012.

[108] K. De Brabandere, B. Bolsens, J. Van den Keybus, A. Woyte, J. Driesen,and R. Belmans, A voltage and frequency droop control method for par-allel inverters, IEEE Trans. Power Electron., vol. 22, no. 4, pp. 11071115, Jul. 2007.

[109] Z. Zhang, X. Huang, J. Jiang, and B. Wu, A load-sharing control schemefor a microgrid with a fixed frequency inverter, Electr. Power Syst. Res.,vol. 80, no. 3, pp. 311317, Mar. 2010.

[110] J. M. Guerrero, J. C. Vasquez, J. Matas, L. G. de Vicua, and M. Castilla,Hierarchical control of droop-controlled AC and DC microgridsAgeneral approach toward standardization, IEEE Trans. Ind. Electron.,vol. 58, no. 1, pp. 158172, Jan. 2011.

[111] N. D. Hatziargyriou, A. Dimeas, A. G. Tsikalakis, J. A. P. Lopes,

G. Karniotakis, and J. Oyarzabal, Management of microgrids in marketenvironment, inProc. Int. Conf. Future Power Syst., 2005, pp. 17.

[112] R. H. Lasseter and P. Piagi, Control of small distributed energy re-sources, U.S. Patent 7 116 010 B2, 2006.

[113] A. Engler and N. Soultanis, Droop control in LV-grids, in Proc. Int.Conf. Future Power Syst., 2005, pp. 16.

[114] A. Colet-Subirachs, A. Ruiz-Alvarez, O. Gomis-Bellmunt,F. Alvarez-Cuevas-Figuerola, and A. Sudria-Andreu, Centralized

and distributed active and reactive power control of a utility connectedmicrogrid using IEC61850, IEEE Syst. J., vol. 6, no. 1, pp. 5867,Mar. 2012.

[115] B. Eid, Microgird system with multi distributed energy resources usingMatalb/Simulink, inProc. PECS, 2012, pp. 194196.

[116] C. K. Sao and P. W. Lehn, Control and power management of converterfed microgrids,IEEE Trans. Power Syst., vol. 23, no. 3, pp. 10881098,Aug. 2008.

[117] C. K. Sao and P. W. Lehn, Intentional islanded operation of con-verter fed microgrids, inProc. IEEE Power Eng. Soc. Gen. Meet., 2006,pp. 16.

[118] F. Katiraei, M. R. Iravani, and P. Lehn, Micro-grid autonomous oper-ation during and subsequent to islanding process, IEEE Trans. Power

Del., vol. 20, no. 1, pp. 248257, Jan. 2005.[119] M. Prodanovic and T. C. Green, High-quality power generation through

distributed control of a power park microgrid, IEEE Trans. Ind. Elec-tron., vol. 53, no. 5, pp. 14711482, Oct. 2006.

[120] M. I. Marei, A unified control strategy based on phase angle estimationfor matrix converter interface system, IEEE Syst. J., vol. 6, no. 2,

pp. 278286, Jun. 2012.[121] A. K. Srivastava, A. A. Kumar, and N. N. Schulz, Impact of distributed

generations with energy storage devices on the electric grid,IEEE Syst.J., vol. 6, no. 1, pp. 110117, Mar. 2012.

[122] N. C. Hien, N. Mithulananthan, and R. Bansal, Location and sizingof distributed generation units for loadabilty enhancement in primaryfeeder,IEEE Syst. J., vol. 7, no. 4, pp. 797806, 2013.

[123] M. M. Aman, G. B. Jasmon, H. Mokhlis, and A. H. A. Bakar, Optimalplacement and sizing of a DG based on a new power stability index and

line losses, Int. J. Elect. Power Energy Syst., vol. 43, no. 1, pp. 12961304, Dec. 2012.

[124] R. H. Lasseter, Microgrids, in Proc. IEEE Power Eng. Soc. WinterMeet., 2002, pp. 305308.

[125] S. A. Azmi, G. P. Adam, K. H. Ahmed, S. J. Finney, and B. W. Williams,Grid interfacing of multimegawatt photovoltaic inverters, IEEE Trans.Power Electron., vol. 28, no. 6, pp. 27702784, Jun. 2013.

[126] K. Turitsyn, P. Sulc, S. Backhaus, and M. Chertkov, Options for controlof reactive power by distributed photovoltaic generators, Proc. IEEE,vol. 99, no. 6, pp. 10631073, Jun. 2011.

[127] H. Kim, T. Yu, and S. Choi, Indirect current control algorithm for util-ity interactive inverters in distributed generation systems, IEEE Trans.

Power Electron., vol. 23, no. 3, pp. 13421347, May 2008.[128] J. M. Guerrero, P. C. Loh, T.-L. Lee, and M. Chandorkar, Advanced

control architectures for intelligent microgridsPart II: Power quality,energy storage, and AC/DC microgrids, IEEE Trans. Ind. Electron.,

vol. 60, no. 4, pp. 12631270, Apr. 2012.[129] A. H. El Khateb, N. A. Rahim, and J. Selvaraj, Novel uk-Buck MPPTbattery charger for standalone PV-inverter applications, Int. Rev. Elect.

Eng. (IREE), vol. 7, no. 2, pp. 37493758, Apr. 2012.[130] M. Savaghebi, A. Jalilian, J. C. Vasquez, and J. M. Guerrero, Au-

tonomous voltage unbalance compensation in an islanded droop-controlled microgrid, IEEE Trans. Ind. Electron., vol. 60, no. 4,pp. 13901402, Apr. 2013.

[131] N. Pogaku, Analysis, control and testing of inverter-based distributedgeneration in standalone and grid-connected applications, Ph.D. disser-tation, Imperial College, London, U.K., 2006.

[132] A. Engler, Applicability of droops in low voltage grids,Int. J. Distrib.Energy Res., vol. 1, pp. 315, 2005.

[133] C.-C. Hua, K.-A. Liao, and J.-R. Lin, Parallel operation of invertersfor distributed photovoltaic power supply system, in Proc. IEEE 33rd

Annu. PESC /IEEE Power Electron. Spec. Conf., 2002, pp. 19791983.[134] S. Barsali, M. Ceraolo, P. Pelacchi, and D. Poli, Control techniques of

dispersed generators to improve the continuity of electricity supply, inProc. IEEE Power Eng. Soc. Winter Meet., 2002, pp. 789794.

[135] I.-Y. Chung, S.-W. Park, H.-J. Kim, S.-I. Moon, B.-M. Han, J.-E. Kim,and J.-H. Choi, Operating strategy and control scheme of premiumpower supply interconnected with electric power systems,IEEE Trans.Power Del., vol. 20, no. 3, pp. 22812288, Jul. 2005.

[136] A. Engler, Control of parallel operating battery inverters, in Proc.Photovoltaic Hybrid Power Syst. Conf., 2000, pp. 14.

[137] H. Kakigano, Y. Miura, T. Ise, and R. Uchida, DC micro-grid forsuper high quality distributionSystem configuration and control ofdistributed generations and energy storage devices, in Proc. 37th

IEEE PESC/IEEE Power Electron. Spec. Con f., 2006, pp. 17.[138] M. N. Marwali, J.-W. Jung, and A. Keyhani, Control of distributed

generation systemsPart II: Load sharing control,IEEE Trans. PowerElectron., vol. 19, no. 6, pp. 15511561, Nov. 2004.

[139] P.-T. Cheng, C.-A. Chen, T.-L. Lee, and S.-Y. Kuo, A cooperative

imbalance compensation method for distributed-generation interfaceconverters, IEEE Trans. Ind. Appl., vol. 45, no. 2, pp. 805815,Mar./Apr. 2009.


13/13



[140] T.-L. Lee and P.-T. Cheng, Design of a new cooperative harmonic filter-ing strategy for distributed generation interfaceconverters in an islanding

network,IEEE Trans. Power Electron., vol. 22, no. 5, pp. 19191927,Sep. 2007.

[141] R. A. Walling and N. W. Miller, Distributed generation islandingImplications on power system dynamic performance, in Proc. IEEEPower Eng. Soc. Summer Meet., 2002, vol. 1, pp. 9296.

[142] J. A. Laghari, H. Mokhlis, A. B. H. Abu Bakar, M. Karimi, and

A. Shahriari, An intelligent under frequency load shedding schemefor islanded distribution network, in Proc. IEEE Int. PEDCO, Melaka,Malaysia, 2012, pp. 4045.

[143] M. Karimi, H. Mohamad, H. Mokhlis, and A. H. A. Bakar, Under-Frequency Load Shedding scheme for islanded distribution network con-nected with mini hydro,Int. J. Elect. Power Energy Syst., vol. 42, no. 1,pp. 127138, Nov. 2012.

[144] D. De and V. Ramanarayanan, Decentralized parallel operation of in-

verters sharing unbalanced and nonlinear loads, IEEE Trans. PowerElectron., vol. 25, no. 12, pp. 30153025, Dec. 2010.

[145] J. Matas, M. Castilla, L. G. de Vicua, J. Miret, and J. C . Vasquez, Vir-tual impedance loop for droop-controlled single-phase parallel invertersusing a second-order general-integrator scheme, IEEE Trans. Power

Electron., vol. 25, no. 12, pp. 29933002, Dec. 2010.[146] A. El Khateb, N. A. Rahim, J. Selvaraj, and M. N. Uddin, Maxi-

mum power point tracking of single-ended primary-inductor converteremploying a novel optimisation technique for proportional-integral-

derivative controller,IET Power Electron., vol. 6, no. 6, pp. 11111121,Jul. 2013.

Bilal M. Eid (S11) received the B.Sc. degree fromSanaa University, Sanaa, Yemen, in 2008 and theM.Sc. degree in electrical energy and power sys-

tems in 2011 from the University of Malaya, KualaLumpur, Malaysia, where he is currently workingtoward the Ph.D. degree in the field of microgrids

and renewable energy integration at the Faculty ofEngineering.

Since 2011, he has been a Research Assistantwith the Power Energy Dedicated Advanced Cen-tre (UMPEDAC), University of Malaya. His main

research interests include microgrids, smart grids, power conversion systems,renewable energy, photovoltaic inverters, and dcdc converters.

Nasrudin Abd Rahim(M89SM08) was born inJohor, Malaysia, in 1960. He received the B.Sc.

(Hons.) and M.Sc. degrees from the University ofStrathclyde, Glasgow, U.K., in 1985 and 1988, re-spectively, and the Ph.D. degree from Heriot-WattUniversity, Edinburgh, U.K., in 1995.

He is currently a Professor with the University ofMalaya, where he is the Director of the Power En-

ergy Dedicated Advanced Centre (UMPEDAC) andthe Chairman of the University of Malaya AdvancedEngineering and Technology Research Cluster. His

research interests include power electronics, real-time control systems, electri-cal drives, and renewable energy systems.

Prof. Rahim is a Fellow of the Academy of Sciences Malaysia and theInstitution of Engineering and Technology, U.K. He is a Chartered Engineer.

Jeyraj Selvaraj received the B.Eng.(Hons.) degreefrom Multimedia University, Cyberjaya, Malaysia,in 2002, the M.Sc. degree in power electron-ics and drives jointly from the University ofBirmingham, Birmingham, U.K., and the Universityof Nottingham, Nottingham, U.K., in 2004, and thePh.D. degree from the University of Malaya, KualaLumpur, Malaysia, in 2009.

He is currently with the Power Energy Dedi-

cated Advanced Centre (UMPEDAC), University ofMalaya.

His research interests include single- and three-phase multilevel inverters,digital current-control techniques, photovoltaic inverters, and dcdc converters.

Ahmad H. El Khateb (M10) received the B.Sc. andM.Sc. degrees from the Islamic University of Gaza,Gaza, Palestine, in 2005 and 2007, respectively, andthe Ph.D. degree in electrical engineering from theUniversity of Malaya, Kuala Lumpur, Malaysia, in2013.

He was a Lecturer with the Department of Electri-cal Engineering, Islamic University of Gaza, and hehad spills with Al-Azhar University, Cairo, Egypt,

and the University College of Applied Sciences,Gaza, Palestine. He is currently with the Power

Energy Dedicated Advanced Centre (UMPEDAC), University of Malaya. Hismain research interests include power electronics, digital control, dc-to-dcconverters, photovoltaic power generation, and grid integration.

Dr. Ahmad is a Reviewer for I EEE TRANSACTIONS, IET TRANSACTIONS,and several conferences.

10.1109-JSYST.2013.2296075-Control Methods and Objectives for Electronically Coupled Distributed Energy Resources in Microgrids- A Review

Documents