EPSR3077

This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/copyright

http://www.elsevier.com/copyright

Author's personal copy

Electric Power Systems Research 80 (2010) 1477–1487

Contents lists available at ScienceDirect

Electric Power Systems Research

journa l homepage: www.e lsev ier .com/ locate /epsr

Review

Review of control techniques for inverters parallel operation

Alaa Mohda,∗, Egon Ortjohanna, Danny Mortonb, Osama Omaric

a South Westphalia University of Applied Sciences/Division Soest, Lübecker Ring 2, 59494 Soest, Germanyb The University of Bolton, Deane Road, Bolton, UKc The Arab American University, Jenin, Palestine

a r t i c l e i n f o

Article history:Received 5 September 2008Received in revised form 10 January 2010Accepted 10 June 2010Available online 16 July 2010

Keywords:Inverter controlVoltage source invertersMaster/slave controlCurrent/power sharing control techniquesFrequency and voltage droop controlControl methods for electrical systems

a b s t r a c t

This paper presents state-of-the-art review of control methods applied currently to parallel power elec-tronic inverters. Different system architectures, their modes of operation, management and controlstrategies will be analyzed; advantages and disadvantages will be discussed. Though, it is not easy togive a general view at the state of the art for the research area since it is rapid and going in differentdirections, this paper will focus on the main streams.

This paper will start by briefly reviewing the current trends in power supply systems and the increasingimportance for including power electronic devices. Next, the different techniques to parallel inverterssuggested in the literature will be checked. These can be categorized to the following main approaches:master/slave control techniques, current/power sharing control techniques, and frequency/voltage droopcontrol techniques. Finally, based on the reviewed state of the art, the study concludes by comparing thedifferent approaches reported. In addition, their weaknesses and strengths are strained.

© 2010 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14772. Master/slave control techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14783. Current/power sharing control techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14794. Frequency and voltage droop control techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1480

4.1. Adopting conventional frequency/voltage droop control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14804.2. Opposite frequency/voltage droop control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14824.3. Droop control in combination with other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1483

5. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1485References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1485

1. Introduction

Fossil fuels are the major source of energy in the world today.However, as the world is considering more economical and envi-ronmentally friendly alternative energy generation systems, theglobal energy mix is becoming more complex. Factors forcing theseconsiderations are (a) the increasing demand for electric powerby both developed and developing countries, (b) many developingcountries lacking the resources to build power plants and distribu-tion networks, (c) some industrialized countries facing insufficientpower generation and (d) greenhouse gas emission and climate

∗ Corresponding author at: South Westphalia University of Applied Sci-ences/Division Soest, Power Supply, Lübecker Ring 2, 59494 Soest, NRW, Germany.Tel.: +49 2921 378 454; fax: +49 2921 378 433.

E-mail address: [email protected] (A. Mohd).

change concerns. Renewable energy sources such as wind turbines,photovoltaic solar systems, solar-thermo power, biomass powerplants, fuel cells, gas micro-turbines, hydropower turbines, com-bined heat and power (CHP) micro-turbines and hybrid powersystems will be part of future power generation systems [1–9].

This new trend is developing toward distributed generation(DG), which means that energy conversion systems (ECSs) are sit-uated close to energy consumers and large units are substitutedby smaller ones. For the consumer the potential lower cost, higherservice reliability, high power quality, increased energy efficiency,and energy independence are all reasons for interest in distributedenergy resources (DER). The use of renewable distributed energygeneration and “green power” such as wind turbines, photovoltaicsolar systems, solar-thermo power, biomass power plants, fuelcells, gas micro-turbines, hydropower turbines, combined heat andpower (CHP) micro-turbines and hybrid power systems can alsoprovide a significant environmental benefit [10–12]. This is also

0378-7796/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.epsr.2010.06.009


1478 A. Mohd et al. / Electric Power Systems Research 80 (2010) 1477–1487

driven by an increasingly strained transmission and distributioninfrastructure as new lines lag behind demand and to reduce over-all system losses in transmission and distribution. Further, theincreased need for reliability and security in electricity supply, highpower quality needed by an increasing number of activities requir-ing UPS like systems and to prevent or delay the expansion ofcentral generation stations by supplying the growing loads locally[13,14].

However, the exploitation of renewable energy sources (RESs),and more efficient utilization of energy sources due to local (dis-tributed generation) result in a large number of sources at theLV-grid. For most micro-turbines, wind plants, fuel cells and pho-tovoltaic cells electrical power is generated as a direct current (DC)and converted to an alternating current (AC) by means of inverters[15].

Because of that, the inverter is considered an essential com-ponent at the grid side of such systems due to the wide range offunctions it has to perform. It has to convert the DC voltage to sinu-soidal current for use by the grid in addition to act as the interfacebetween the ECSs, the local load and the grid. It also has to handlethe variations in the electricity it receives due to varying levels ofgeneration by the RESs, loads and grid voltages [16]. Inverters influ-ence the frequency and the voltage of the grid and seem to be themain universal modular building block of future smart grids mainlyat low and medium voltage.

Inverters are often paralleled to construct power systems inorder to improve performance or to achieve a high system rat-ing. Parallel operation of inverters offers also higher reliability overa single centralized source because in case one inverter fails theremained (n − 1) modules can deliver the needed power to the load.This is as well driven by the increase of renewable energy sourcessuch as photovoltaic and wind.

There are many techniques to parallel inverters which arealready suggested in the literature, they can be categorized to thefollowing main approaches: master/slave control techniques, cur-rent/power sharing control techniques and frequency and voltagedroop control techniques. This latter one includes three main cat-egories: adopting conventional frequency/voltage droop control,opposite frequency/voltage droop control and droop control incombination with other methods. These will be discussed in thefollowing sections.

2. Master/slave control techniques

The master/slave control method uses a voltage-controlledinverter as a master unit and current-controlled inverters as theslave units. The master unit maintains the output voltage sinu-soidal, and generates proper current commands for the slave units[17–19].

One of the master/slave configuration is the scheme suggested in[20,21], see Fig. 1, which is a combination of voltage-controlled andcurrent-controlled PWM inverters for parallel operation of a single-phase uninterruptible power supply (UPS). The types of PWMinverters considered are voltage-controlled (VCPI) or current-controlled (CCPI) with voltage source. The voltage-controlledinverter (master) is developed to keep a constant sinusoidal waveoutput voltage. The current-controlled inverter units are operatedas slave controlled to track the distributive current. The invertersdo not need a phased locked loop (PLL) circuit for synchroniza-tion since these units are interlinked and are communicating withthe power center (A PLL is needed when the frequency is mea-sured from the grid such as in the droop method where it is theonly way of communicating, or if the inverter have only to sup-ply power to the grid and is not part of the control scheme assolar inverters current application); this gives a good load sharing.

Fig. 1. Combined voltage- and current-controlled inverters [20].

However, the system is not redundant since it has a single point offailure.

A comparable scheme is also presented in [22] but it needseven more interconnection since it is sharing the voltage andcurrent signals. In [23] the system is redundant by extended mon-itoring of the status and the operating conditions of all powerelectronic equipment. Each block of the UPS system is monitoredby two independent microcomputers that process the same data.The microcomputers are part of a redundant distributed mon-itoring system that is separately interlinked by two serial databuses through which they communicate. They establish a hierar-chy among the participating blocks by defining one of the healthyinverter blocks as the master.

The scheme proposed in [24], see Fig. 2, is based on the mas-ter/slave configuration but is using a rotating priority windowwhich provides random selection of a new master and thereforeresults in true redundancy and increase reliability.

In [25] the system is also redundant since a status line is used todecide about the master inverter using a logical circuit (flip-flop),if the master is disconnected one slave becomes automatically themaster. The auto-master-slave control presented in [26] is designedto let the unit with highest output real power act as a master of

Fig. 2. Proposed master/slave configuration in [24].


A. Mohd et al. / Electric Power Systems Research 80 (2010) 1477–1487 1479

Fig. 3. Proposed distributed control configuration in [29].

real power and derives the reference frequency, the others have tofollow as slaves. The regulation of the reactive power is similar, thehighest output reactive power module acts as master of reactivepower and adjusts the voltage reference amplitude.

In [9,27] the paper focus on operation and behaviour of the iso-lated microgrids under different conditions and scenarios. This wasinvestigated for two main control strategies, single master opera-tion where a voltage source inverter (VSI) can be used as voltagereference (grid forming) when the main power supply is lost; allthe other inverters can then be operated in PQ mode (grid support-ing). And multi-master operation where more than one inverter areoperated as a VSI, other PQ inverters may also coexist.

In more recent papers [18,28,29] an enhanced approach is intro-duced, the master inverter is replaced by a central control blockwhich controls the output voltages and can influence the out-put current of the different units, this is sometimes called centralmode control or distributed control. This means that the voltagemagnitude, frequency and power sharing are controlled centrally(commands are distributed through a low-bandwidth communica-tion channels to the inverters) and other issues such as harmonicsuppression are done locally, see Fig. 3. In this figure we can see acontrol system with nested control loops. An inner current-control-loop is formed around the inductor and is arranged to have a fastresponse plus the outer voltage-control-loop. The current demandfor distribution to the various modules is in dq-form. The controlis distributed but relies on one control loop for the most impor-tant feature of voltage control. This leaves the system open to asingle-point failure mechanism despite its modularity.

3. Current/power sharing control techniques

In this control technique the total load current is measured anddivided by the number of units in the system to obtain the averageunit current. The actual current from each unit is measured andthe difference from the average value is calculated to generate thecontrol signal for the load sharing [17].

In the approach suggested in [30], see Fig. 4, the inverter has ahigh-speed current minor loop, and therefore there is no possibilityof output over current by limiting the current reference i*. The volt-age controller adjusts the voltage deviation and keeps the voltage

constant. The current deviation (�I) signal is measured and feed tothe current loop, and the power deviation (�P) signal controls thephase of the reference sine-wave. The system has a very good loadsharing and the transient response is very superior due to the feed-forward control signal. Such systems are used mainly for UPS appli-cations but also for solar and auxiliary systems for ships and trains.

A method of current sharing for paralleled power convertersis introduced and then explored in [31–33], in this approach, seeFig. 5. Each converter is controlled such that its average outputcurrent is directly related to its switching frequency. As a result,the frequency content of the aggregate output ripple voltage con-tains information about the individual cell output currents. Eachcell measures the output ripple voltage and uses this informationto achieve current balance with the other cells.

In [34] circular chain control (3C) strategy is proposed, see Fig. 6,all modules have the same circuit configuration, and each moduleincludes an inner current loop and an outer voltage loop control.With the 3C strategy, the modules are in circular chain connectionand each module has an inner current loop control to track theinductor current of its previous module, achieving an equal currentdistribution. This is used mainly for UPS application and automaticvoltage regulation (AVR).

The authors of [35] proposed an inverter current feed-forwardcompensation which makes the output impedance resistive ratherthan inductive in order to get a precise load sharing. In [36] thepaper goes further based on the approach introduced in [35] andproposes a solution to the noise problem of harmonic circulatingcurrents due to PWM non-synchronization which is affecting theload sharing precision. This is done in [37] using a digital controlalgorithm for parallel connected three-phase inverters. The digitalvoltage controller, which has high-speed current control as a minorloop, provides low voltage distortion even for nonlinear loads. Out-put current of each UPS module is controlled to share the totalload current equally and the voltage reference command of eachinverter is controlled to balance the load current. In [38–40] sim-ilar approaches are suggested. In [41] the focus is on developinga solution for the effect of DC offset between paralleled invertersand its effect on the circulating currents. In [42] the authors suggesttwo-line share bus connecting all inverters, one for current sharingcontrol and the other to adjust the voltage reference.



Fig. 4. Proposed parallel operation of inverter with current minor loop [30].

4. Frequency and voltage droop control techniques

Many methods were found in the literature and can be roughlycategorized into the following:

a. Adopting conventional frequency/voltage droop control.b. Opposite frequency/voltage droop control.c. Droop control in combination with other methods.

4.1. Adopting conventional frequency/voltage droop control

In [43] the paper proposes a control technique for operating twoor more single-phase inverter modules in parallel with no auxiliaryinterconnections. In the proposed parallel inverter system, eachmodule includes an inner current loop and an outer voltage loopcontrols, see Fig. 7. This technique is similar to the conventionalfrequency/voltage droop concept using frequency and fundamentalvoltage droop to allow all independent inverters to share the loadin proportion to their capacities. However, the paper consideredonly inductive lines.

In [44] scheme for controlling parallel connected invertersin a stand-alone ac supply system is presented, see Fig. 8. Thisscheme is suitable for control of inverters in distributed sourceenvironments such as in isolated AC systems, large UPS sys-tems, and PV systems connected to AC grids. Active and reactivepower sharing between inverters can be achieved by controllingthe power angle (by means of frequency), and the fundamentalinverter voltage magnitude. The control is done in the d-q refer-ence frame; an inverter flux vector is formed by integrating thevoltage space vector. The choice of the switching vectors is essen-tially accomplished by hysteresis comparators for the set valuesand then using a look-up table to choose the correct inverter out-put voltage vector. The considerations for developing the look-uptable are dealt with in [45]. However, the inductance connectedbetween the inverter and the load makes the output impedancehigh. Therefore, the voltage regulation as well as the voltagewaveform quality is not good under load change conditions aswell as nonlinear load condition. The authors explain the sameconcept but with focusing in control issues of UPS systems in[46].

Fig. 5. Proposed current sharing control in [31–33].



Fig. 6. The proposed circular chain control (3C) strategy [34].

In [47,48] the inverse droop equations are used to control theinverter. The inverter is able to work in parallel with a constant-voltage constant-frequency system, as well as with other invertersor also in stand-alone mode. There is no communication interfaceneeded. The different power sources can share the load also underunbalanced conditions. Very good load sharing is achieved by usingan outer control loop with active and reactive power controller, forwhich the set point variables are derived out of droops. Further-more, a relatively big inductance of 12 mH (C = 10 �F) is used inthe LC filter and a small decoupling reactance is used to decouplethe inverter from other voltage sources. The interface inductance

makes the voltage source converters (VSCs) less sensitive to dis-turbances on the load bus [49,50]. The authors address here UPSapplications and renewable energy systems (Fig. 9).

In [49] an interesting autonomous load sharing technique forparallel connected three-phase voltage source converters is pre-sented. This paper focuses on an improvement to the conventionalfrequency droop scheme for real power sharing and the devel-opment of a new reactive power sharing scheme. The improvedfrequency droop scheme computes and sets the phase angle ofthe VSC instead of its frequency. It allows the operator to tune thereal power sharing controller to achieve desired system responsewithout compromising frequency regulation by adding an inte-gral gain into the real power control. The proposed reactive powersharing scheme introduces integral control of the load bus volt-age, combined with a reference that is drooped against reactivepower output. This causes two VSCs on a common load bus to sharethe reactive load exactly in the presence of mismatched interfaceinductors if the line impedances are much smaller than the inter-face reactors (assuming short lines). Moreover, in the proposedreactive power control, the integrator gain can be varied to achievethe desired speed of response without affecting voltage regulation.

In [51] the authors are considering that large DC cross cur-rents can flow between the different inverters which is normallyneglected since only the AC cross current is normally taken intoconsideration by means of control schemes. However, this can hap-pen only if we have a considerable DC voltage offset differencebetween the inverters which is usually not the case since theseerrors are generated by the sensors and they are very small. Onemore condition to make this happen is a very small output resis-tance of each inverter in comparison to it is output impedance. Itshould be noted that this droop scheme can only make invertershave the same DC-offset voltage so as to avoid the DC cross current,whereas it cannot get rid of the DC-offset voltage.

In [52–54] the author discusses the application of conventionaldroops for voltage source inverters and categorize the system com-ponents to form a modular AC-hybrid power system. Then in [55]by the same author an investigation of what is called opposite droop(active power/voltage and reactive power droop) control is carriedout. The focus is on the need of different droop functions for differ-

Fig. 7. Reference voltage and power calculation [43].

Fig. 8. Inverter control scheme [44].



Fig. 9. Inverter control scheme proposed in [47,48].

ent types of grids. In [55] it is found that for high voltage (mainlyinductive lines) grids the regular droop functions can be used. Forlow voltage (mainly resistive) grids, so-called opposite droop func-tions could be used instead but the regular droop functions areadvantageous since it allows connectivity to higher voltage levelsand power sharing also with rotating generators [55–58].

A microgrid control was introduced and implemented in[6,59–61], the microgrid has two critical components, the staticswitch and the micro-source. The static switch has the abilityto autonomously island the microgrid from disturbances such asfaults, IEEE 1547 events, or power quality events. After island-ing, the reconnection of the microgrid is achieved autonomouslyafter the tripping event is no longer present. This synchroniza-tion is achieved by using the frequency difference between theislanded microgrid and the utility grid insuring a transient freeoperation without having to match frequency and phase angles atthe connection point. Each micro-source can seamlessly balancethe power on the islanded microgrid using a power vs. frequencydroop controller. This frequency droop also insures that the micro-grid frequency is different from the grid to facilitate reconnectionto the utility. The introduced micro-source control is shown inFig. 10.

The authors of [19] present a scheme for controlling parallelconnected inverters using droop sharing method in a stand-aloneac system. The scheme proposes PI regulator to determine the set

points for generator angle and flux. The model of the microgridpower system is simulated in MATLAB/SIMULINK. The dynamicresponse of the system is investigated under different impedanceload conditions especially motor loads. Paper [62] analyzes the faultbehaviour of four wire paralleled inverters (in droop mode) basedon their control methodology.

4.2. Opposite frequency/voltage droop control

In [56,63] the method selected here for inverters used in dis-tributed generation systems is to modify the droop functions of thesource converters so that the regular droop functions are used inthe steady-state case and opposite droops are used in transients,see Fig. 11. Note that here ωref = ωn and vref = Vn. The steady-statedroop functions are according to:

p∗s = Kω(ωref − ω) (1)

q∗s = Kv(vref − vq) (2)

where ps and qs are the active and reactive power references. Forthe transient droop functions according to:

p∗s = Kv(vref − vq) (3)

q∗s = −Kω(ωref − ω) (4)

where ωref = ω* and vref = v∗.

Fig. 10. Inverter control scheme proposed in [6,59–61].



Fig. 11. Regular conventional droop functions (left) and transient droop functions (right) [56,63].

In this method the load sharing is acceptable for the investi-gated, highly resistive, network. Still, in the case of line inductancein the same order of magnitude as the converter output filter induc-tance there can be a considerable degradation of power qualityin terms of voltage disturbance. The origin of this degradation isthe LC-circuit formed by the line inductance and the converter ACside capacitors. Furthermore, using this approach it is not possibleto connect with the high level voltage which is using the regularconventional droop functions.

In [63–67] the authors focus on the transient behaviour ofparallel connected UPS inverters, they claim that damping andoscillatory phenomena of phase shift difference between the par-alleled inverters could cause instabilities, and a large transientcirculating current that can overload and damage the paralleledinverters. To overcome this they proposed using a method called“droop/boost” control scheme which adds integral-derivativeterms to the droop function. Stable steady-state frequency andphase and a good dynamic response are obtained. Further, vir-tual output impedance is proposed in order to reduce the lineimpedance impact and to properly share nonlinear loads, this isdone using a high pass filter, the filter gain and pole values of thismust be carefully chosen. Furthermore, the test results shown areconsidering a short resistive line, but the method is not taking intoconsideration what happens if the distance between the inverters isconsiderable. Which is normally the case in distributed generationwere an inductive impedance component appears. Nevertheless,when an inverter is connected suddenly to the common AC bus, acurrent peak appears due to the initial phase error [68].

Compatibility problems are expected because of the oppositedroop scheme. The characteristic and the scheme are shown below(Fig. 12):

E = E∗ − nP − nddP

dt(5)

ω = ω∗ − mQ − mddQ

dt(6)

As an addition in [68] a soft-start is included to avoid the initial cur-rent peak as well as a bank of band pass filters in order to share thesignificant output current harmonics. In more recent papers [69,70]the authors use the conventional droop equations for a Microgridtoo.

Fig. 12. Static droop/boost characteristics for resistive output impedance [63–67].

E = E∗ − n(Q − Q ∗) (7)

ω = ω∗ − m(P − P∗) (8)

4.3. Droop control in combination with other methods

In [14,71–73] each inverter supplies a current that is theresult of the voltage difference between a reference AC volt-age source and the grid voltage across a virtual impedance withreal and/or imaginary parts. The reference AC voltage source issynchronized with the grid, with a phase shift, depending onthe difference between nominal and real grid frequency. Thismethod behaviour is equal to the normal existing droop con-trol methods except that, short-circuit behaviour is better sinceit is controlling the active and reactive currents and not thepower. It behaves also better in case of a non-negligible line resis-tance. The emulation of the finite-output impedance is obtainedby the use of a hybrid voltage–current controller. Instead ofhaving a current-control-loop inside a voltage-control-loop (orthe other way round), both current and voltage are simultane-ously controlled to emulate the finite-output impedance. Here, thelinear quadratic Gaussian (LQG) optimal control approach is fol-lowed, using a Kalman estimator and a linear quadratic regulator(Fig. 13).

In [74,75] a novel fast control loops that adjust the outputimpedance of the closed-loop inverters is used in order to ensureresistive behaviour with the purpose to share the harmonic cur-rent content properly. In the measurements part a notch filter isadded to remove the unwanted harmonics, it seems that withoutthis filter the voltage regulator will not work efficiently. Further-more, the control is done in the ˛ˇ-coordinates using a discretecontroller.

The author of [76] discusses the problem of inverters withvery low output impedance (such as those employing resonantcontrollers) directly connected in parallel through a near zeroimpedance cable. Low THD content and good current sharing aresimultaneously obtained by controlling the load angle through anleast mean square (LMS) estimator and by synthesizing a variableinductance in series with the output impedance of the inverter,while the harmonic current sharing is achieved by controlling thegain of the resonant controllers at the selected frequencies.

In [77,78] the proposed droop scheme for UPS inverters is shownbelow, see Fig. 14, in which the inverter connects with the loadvia series impedance (Z) like the conventional approach. However,there exist two differences. (i) The series impedance is createdby the inverter internally, no true impedance is required. As aresult, the inverters connect with the load tightly. (ii) The seriesimpedance is frequency-dependent; it exhibits a reactive charac-teristic at the fundamental frequency and a resistive characteristicat the harmonic frequency. This means that the current sharingand voltage regulation is effective for both linear load and nonlin-ear load. In addition, the performance is insensitive to parametermismatch of the inverters. The output impedance of the inverter is



Fig. 13. Overall scheme for the proposed droop control method (left) and the full scheme of the generation source (right) [14,71–73].

(Zo) and is much lower than the series impedance (Zs) no matter atthe fundamental frequency or at the harmonic frequency.

The authors of [79,80] introduced fast control loops thatadjust the output impedance of the closed-loop inverters in orderto ensure inductive behaviours with the purpose to share theharmonic current content properly. The paper presents a small-signal analysis for parallel connected inverters in stand-alone ACpower systems. The control approaches have an inherent trade-offbetween voltage regulation and power sharing [63].

The signal injection technique proposed by [17,81] is not depen-dent in the plant parameters and can share reactive power even ifthe VSCs have not perfectly matched output inductors by havingeach VSC inject a non-60-Hz (50-Hz) signal and use it as a meansof sharing a common load with other VSCs on the network. How-ever, the circuitry required to measure the small real power outputvariations due to the injected signal adds to the complexity of thecontrol [49]. Moreover, the controllers use an algorithm which istoo complicated to calculate the current harmonic content, the har-

Fig. 14. Frequency-dependent droop scheme: (a) the series impedance is created in the inverter internally; (b) the equivalent inverter circuit at the fundamental frequencyand (c) the equivalent inverter circuit at the harmonic frequency [77,78].



Fig. 15. Schematic diagram of implementing the signal injection technique [17].

monic current sharing is achieved at the expense of reducing thestability of the system [65] (Fig. 15).

In [82] the proposed control method uses low-bandwidth datacommunication signals between each generation system in addi-tion to the locally measurable feedback signals. The focus is onsystems of distributed resources that can switch from grid con-nection to island operation without causing problems for criticalloads. This is achieved by combining two control methods: droopcontrol method and average power control method. In this method,the sharing of real and reactive powers between the units is imple-mented by two independent control variables: power angle andinverter output voltage amplitude. However, adding external com-munication can be considered as a drawback. Such communicationsincrease the complexity and reduce the reliability, since the powerbalance and the system stability rely on these signals [63]. In [83,84]a communication bus is used in addition to the conventional droop,it has to trigger all inverters to measure their load sharing parame-ters at the same line period, this is used to correct the load sharingcalculation.

5. Conclusion

From the previous discussion, it can be seen that each of thesereviewed control techniques has its own characteristics, objectives,limits and appropriate uses. The master/slave control configura-tion has many good characteristics. The inverters do not need PLLcircuit for synchronization and give a good load sharing. The lineimpedance of the interconnecting lines does not affect the loadsharing and the system is also easily expandable. There are, how-ever, a few serious disadvantages. One of the major disadvantagesis that most of these systems are not truly redundant, and havea single point of failure, the master unit. Another disadvantage ofthis configuration is that the stability of the system depends uponthe number of slave units in the system. Furthermore, all thesemaster/slave techniques, need communication and control inter-

connections, so they are less reliable for a distributed power supplysystem.

On the other hand, the current/power sharing control tech-niques have excellent features. They have very good load sharing,transient response and can reduce circulating currents between theinverters. There are as well some drawbacks. It is not easily expand-able due to the need for measuring the load current and the needto know the number of inverters in the system. The needed inter-connection makes the system less reliable and not truly redundantand distributed.

Droop control methods are based on local measurements ofthe network state variables which make them truly distributedand give them an absolute redundancy as they do not depend oncables/communication for reliable operation. It has many desir-able features such as expandability, modularity, flexibility andredundancy. Nevertheless, the droop control concept has somelimitation including frequency and amplitude deviations, slowtransient response and possibility of circulating current amonginverters due to wire impedance mismatches between inverter out-put and load bus and/or voltage/current sensor measurement errormismatches.

This review shows how difficult it is to adapt one control schemefor all applications. However, deep understanding of these controltechniques will help in enhancing them and though will improvethe design and implementation of future distributed modular gridarchitectures where various approaches are needed to fit each sys-tem (customer) exact specifications.

References

[1] P.C. Ghosh, et al., Ten years of operational experience with a hydrogen-based renewable energy supply system, Solar Energy 75 (2003)469–478.

[2] A. Bilodeau, K. Agbossou, Control analysis of renewable energy system withhydrogen storage for residential applications, Journal of Power Sources (2005).

[3] N. Hamsic, A. Schmelter, A. Mohd, E.Ortjohann, E.Schultze, A.Tuckey, J. Zim-mermann, Stabilising the grid voltage and frequency in isolated power systems



using a flywheel energy storage system, in: The Great Wall World RenewableEnergy Forum, October 2006, Beijing, China, 2006.

[4] Y. Rebours, D. Kirschen, What is Spinning Reserve? The University of Manch-ester, 2005.

[5] ERE Council, Renewable Energy in Europe: Building Markets and Capacity,James and James Science Publishers, 2004.

[6] P. Piagi, R.H. Lasseter, Autonomous control of microgrids, in: IEEE PES Meeting,June 2006, Montreal, 2006.

[7] A.I. Estanqueiro, et al., Barriers (and solutions. . .) to very high wind penetrationin power systems, in: IEEE Power Engineering Society General Meeting, Tampy,FL, 2007.

[8] J. Driesen, et al., Active user participation in energy markets through activationof distributed energy resources, in: IEEE Power Engineering Society GeneralMeeting, 2007.

[9] J.A.P. Lopes, C.L. Moreira, A.G. Madureira, Defining control strategies for Micro-Grids islanded operation IEEE Transactions on Power Systems 21 (2) (2006)916–924.

[10] G. Koeppel, Distributed Generation, Literature Review and Outline of the SwissSituation, 2003, Zurich.

[11] D. Bohn, Decentralised energy systems: state of the art and potential, Interna-tional Journal of Energy Technology and Policy 3 (2005).

[12] E Agency, FP7 Research Priorities for the Renewable Energy Sector, 2005, March,Bruxelles.

[13] J.A. McDowall, Status and outlook of the energy storage market, in: PES 2007,Tampa, 2007.

[14] K.D. Brabandere, Voltage and Frequency Droop Control in Low Voltage Gridsby Distributed Generators with Inverter Frond-End, Departement Elektrotech-niek, Katholieke Universiteit Leuven, Leuven, 2006, October.

[15] O. Osika, Stability of Micro-Grids and Inverter-dominated Grids with High Shareof Decentralised Sources, Kassel University, 2005.

[16] D. Munro, Inside inverters, Renewable Energy World Magazine (2006).[17] A. Tuladhar, Advanced Control Techniques for Parallel Inverter Operation With-

out Control Interconnections, Dept of Electrical and Computer Engineering, TheUniversity of British Columbia, 2000.

[18] M. Prodanovic, T.C. Green, H. Mansir, A survey of control methods for three-phase inverters in parallel connection, in: Eighth International Conference onPower Electronics and Variable Speed Drives, 2000.

[19] R. Majumder, et al., Load sharing with parallel inverters in distributed genera-tion and power system stability, in: Smart Systems 2007 “Technology, Systemsand Innovation”, 2007.

[20] J.-F. Chen, C.-L. Chu, Combination voltage-controlled and current-controlledPWM inverters for UPS parallel operation, IEEE Transactions on Power Elec-tronics 10 (5) (1995) 547–558.

[21] J.-F. Chen, C.-L. Chu, Y.-C. Lieu, Modular parallel three-phase inverter system,in: Proceedings of the IEEE International Symposium on Industrial Electronics,1995.

[22] K. Siri, C.Q. Lee, T.-F. Wu, Current distribution control for parallel connected con-verters, IEEE Transactions on Aerospace and Electronic Systems 28 (3) (1992)841–851.

[23] J. Holtz, K.-H. Werner, Multi-inverter UPS system with redundant load sharingcontrol, IEEE Transactions on Industrial Electronics Industrial Electronics 37 (6)(1990) 506–513.

[24] Z. Petruzziello, G. Joos, A novel approach to paralleling of power converterunits with true redundancy, in: 21st Annual IEEE Power Electronics SpecialistsConference, San Antonio, TX, USA, 1990.

[25] H. Van Der Broeck, U. Boeke, A simple method for parallel operation of inverters,in: Twentieth International Telecommunications Energy Conference, INTELEC,1998.

[26] Y. Pei, et al., Auto-master-slave control technique of parallel inverters in dis-tributed AC power systems and UPS, in: IEEE 35th Annual Power ElectronicsSpecialists Conference, 2004.

[27] J.P. Lopes, MICROGRIDS Large Scale Integration of Microgeneration to Low Volt-age Grids-Emergency Strategies and Algorithms, 2004.

[28] M. Prodanovic, T.C. Green, High-quality power generation through distributedcontrol of a power park microgrid, IEEE Transactions on Industrial Electronics53 (October (5)) (2006) 1471–1482.

[29] T.C. Green, M. Prodanovic, Control of inverter-based micro-grids, Electric PowerSystems Research 77 (9) (2007) 1204–1213.

[30] T. Kawabata, S. Higashino, Parallel operation of voltage source inverters, IEEETransactions on Industrial Electronics Industry Applications 24 (2) (1988)281–287.

[31] D.J. Perreault, J.G. Kassakian, Distributed interleaving of paralleled power con-verters, IEEE Transactions on Circuits and Systems 44 (8) (1997) 728–734.

[32] D.J. Perreault, R.L. Selders, J.G. Kassakian, Frequency-based current-sharingtechniques for paralleled power converters, IEEE Transactions on Power Elec-tronics 13 (4) (1998) 626–634.

[33] D.J. Perreault, K. Sato, R.L. Selders, J.G. Kassakian, Switching-ripple-based cur-rent sharing for paralleled power converters, IEEE Transactions on Circuits andSystems 46 (10) (1999) 1264–1274.

[34] T.-F. Wu, Y.-K. Chen, Y.-H. Huang, 3C strategy for inverters in parallel opera-tion achieving an equal current distribution, IEEE Transactions on IndustrialElectronics 47 (2) (2000).

[35] H. Hanaoka, M. Nagai, M. Yanagisawa, Development of a novel parallel redun-dant UPS, in: The 25th International Telecommunications Energy Conference,2003.

[36] K.-H. Kim, D.-S. Hyun, A high performance DSP voltage controller with PWMsynchronization for parallel operation of UPS systems, in: 37th IEEE PowerElectronics Specialists Conference, 2006.

[37] S. Tamai, M. Kinoshita, Parallel operation of digital controlled UPS system, in:International Conference on Industrial Electronics, Control and Instrumenta-tion, 1991.

[38] H. Oshima, Y. Miyazaya, A. Hirata, Parallel redundant UPS with instantaneousPWM control, in: 13th International Telecommunications Energy Conference,1991.

[39] W. Hoffmann, R. Bugyi, P. Szumowski, PWM inverter for parallel operation ashigh quality AC source in telecommunication, in: 15th International Telecom-munications Energy Conference, 1993. INTELEC ’93, Paris, France, 1993.

[40] C.S. Lee, et al., Parallel UPS with a instantaneous current sharing control, in:Proceedings of the 24th Annual Conference of the IEEE Industrial ElectronicsSociety, Aachen, Germany, 1998.

[41] Z. Qinglin, C. Zhongying, W. Weiyang, Improved control for parallel inverterwith current-sharing control scheme, in: CES/IEEE 5th International PowerElectronics and Motion Control Conference, 2006.

[42] Y. Xing, L.P. Huang, Y.G. Yan, A decoupling control method for inverters inparallel operation, in: International Conference on Power System Technology,PowerCon 2002, 2002.

[43] C.-C. Hua, K.-A.L., J.-R. Lin, Parallel operation of inverters for distributed pho-tovoltaic power supply system, in: IEEE Annual Power Electronics SpecialistsConference.

[44] M.C. Chandorkar, D.M. Divan, R. Adapa, Control of parallel connected invertersin standalone AC supply systems, IEEE Transactions on Industry Applications29 (1) (1993).

[45] I. Takahashi, T. Noguchi, A new quick-response and high-efficiency controlstrategy of an induction motor, IEEE Transactions on Industry Applications IA(22) (1986) 820–827.

[46] M.C. Chandorkar, D.M. Divan, B. Banerjee, Control of distributed ups systems,in: IEEE Annual Power Electronics Specialists Conference, 1994.

[47] H. Matthias, S. Helmut, Control of a three-phase inverter feeding an unbal-anced load and working in parallel with other power sources, in: EPE-PEMC,Dubrovnik, 2002.

[48] S.H. Hauck Matthias, Wechselrichter problemlos parallel betreiben ElektronikH.12/2000, 2000, pp. 120–124.

[49] C.K. Sao, P.W. Lehn, Autonomous load sharing of voltage source converters, IEEETransactions on Power Delivery 20 (2) (2005) 1009–1016.

[50] M. Chandorkar, D.M. Divan, Y. Hu, B. Banerjee, Novel architectures and con-trol for distributed UPS systems, in: Applied Power Electronics Conference andExposition, Orlando, FL, USA, 1994.

[51] M. Xie, YL, K. Cai, P. Wang, X. Sheng, A novel controller for parallel operationof inverters based on decomposing of output current, in: Industry ApplicationsConference 2005: Conference Record of the 2005 Fortieth IAS Annual Meeting,2005.

[52] A. Engler, Control of parallel operating battery inverters, in: Photovoltaic HybridPower Systems Conference, Aix-en-Provence, 2000.

[53] A. Engler, M. Meinh, M. Rothert, New V/f-statics controlled battery inverter:Sunny Island—the key component for AC-coupled hybrid systems and minigrids, in: 2nd European PV Hybrid and Mini-Grid Conference, Kassel, 2003.

[54] A. Engler, et al., Pure AC-coupling —the concept for simplified design of scalablePV-hybrid systems using voltage/frequency statics controlled battery invert-ers, in: 14th International Photovoltaic Science and Engineering Conference(PVSEC-14), Bangkok, 2004.

[55] A. Engler, Applicability of droops in low voltage grids, in: 3rd European PV-Hybrid and Mini-Grid Conference, Aix-en-Provence, France, 2006.

[56] P. Karlsson, J. Björnstedt, M. Ström, Stability of voltage and frequency con-trol in distributed generation based on parallel-connected converters feedingconstant power loads, in: EPE, Dresden, Germany, 2005.

[57] A. Engler, N. Soultanis, Droop control in LV-grids, in: International ConferenceFuture Power Systems, Amsterdam, Niederlande, 2005.

[58] A. Engler, Applicability of droops in low voltage grids, International Journal ofDistributed Energy Resources 1 (1) (2005).

[59] R.H. Lasseter, MicroGrids, in: Power Engineering Society Winter Meeting, 2002.[60] R. Lasseter, P. Piagi, Control and Design of Microgrid Components, University

of Wisconsin-Madison, 2006, P.P. 06-03.[61] R.H. Lasseter, Microgrids and distributed generation, Journal of Energy Engi-

neering (2007).[62] M. Brucoli, T.C. Green, Fault response of inverter dominated microgrids, in: 2nd

International Conference on Integration of Renewable and Distributed EnergyResources, Napa, CA, USA, 2006.

[63] J.M. Guerrero, et al., Drop control method for the parallel operation of onlineuninterruptible power systems using resistive output impedance, in: AppliedPower Electronics Conference and Exposition, 2006: APEC’06, Twenty-FirstAnnual IEEE, 2006.

[64] J.M. Guerrero, et al., A wireless controller to enhance dynamic performanceof parallel inverters in distributed generation systems, IEEE Transactions onPower Electronics 19 (5) (2004) 1205–1212.

[65] J.M. Guerrero, et al., A wireless controller for parallel inverters in distributedonline UPS systems, in: The 29th Annual Conference of the IEEE IndustrialElectronics Society, 2003.

[66] J.M. Guerrero, J. Matas, L.G. de Vicuna, N. Berbel, J. Sosa, Wireless-control strat-egy for parallel operation of distributed generation inverters, in: IndustrialElectronics, 2005, ISIE 2005, 2005.



[67] J.M. Guerrero, N. Berbel, J. Matas, L.G. de Vicuna, J. Miret, Decentralized controlfor parallel operation of distributed generation inverters in microgrids usingresistive output impedance, in: 32nd Annual Conference on IEEE IndustrialElectronics, IECON 2006, Paris, France, 2006.

[68] J.M. Guerrero, J. Matas, L.G. De Vicuna, M. Castilla, J. Miret, Wireless-controlstrategy for parallel operation of distributed-generation inverters, IEEE Trans-actions on Industrial Electronics 53 (5) (2006) 1461–1470.

[69] J.M. Guerrero, et al., Control of line-interactive UPS connected in parallel form-ing a microgrid, in: IEEE International Symposium on Industrial Electronics,Vigo, Spain, 2007.

[70] J.M. Guerrero, et al., Parallel operation of uninterruptible power supply systemsin MicroGrids, in: EPE 2007, Aalborg, Denmark, 2007.

[71] K.D. Brabandere, et al., A voltage and frequency droop control method for par-allel inverters, in: The 35th IEEE PESC Conference, Aachen, Germany, 2004.

[72] K. De Brabandere, et al., Prevention of inverter voltage tripping in high densityPV grids, in: 19th EU-PVSEC, Paris, France, 2004.

[73] K. De Brabandere, et al., Control of microgrids, in: IEEE Power EngineeringSociety General Meeting, Tampa, FL, 2007.

[74] T. Skjellnes, A. Skjellnes, L. Norum, Load sharing for parallel inverters with-out communication, in: Nordic Workshop on Power and Industrial Electronics(Norpie 2002), Stockholm, Sweden, 2002.

[75] E. Hoff, T. Skjellnes, L. Norum, Paralleled three-phase inverters, in: NORPIE’04,Nordic Workshop on Power and Industrial Electronics, 2004.

[76] L. Mihalache, Paralleling control technique with no intercommunication signalsfor resonant controller-based inverters, in: 38th IAS Annual Meeting IndustryApplications Conference, 2003.

[77] S.J. Chiang, J.M. Chang, Parallel control of the ups inverters with frequency-dependent drop scheme, in: IEEE 32nd Annual Power Electronics SpecialistsConference, 2001, PESC Record.

[78] H.-C. Chiang, A frequency-dependent droop scheme for parallel controlof UPS inverters, Journal of Chinese Institute of Engineers 24 (2001)699–708.

[79] E.A.A. Coelho, et al., Small signal stability for parallel connected inverters instand-alone AC supply systems, in: The IEEE Industry Applications Confer-ence, 2000: Conference Record of the IEEE Industry Applications Conference,2000.

[80] E.A.A. Coelho, P.C. Cortizo, P.F.D. Garcia, Small-signal stability for parallel-connected inverters in stand-alone AC supply systems, IEEE Transactions onIndustry Applications 38 (2) (2002) 533–542.

[81] A. Tuladhar, H. Jin, T. Unger, K. Mauch, Control of parallel inverters in distributedac power systems with consideration of the line impedance effect, in: APEC,1998.

[82] M.N. Marwali, J.-W. Jung, A. Keyhani, Control of distributed generation systems.Part II. Load sharing control, IEEE Transactions on Power Electronics 19 (6)(2004).

[83] H.-P. Glauser, et al., New inverter module with digital control for paralleloperation, in: The Third International Telecommunications Energy Special Con-ference, 2000.

[84] X. Chen, Y. Kang, J. Chen, Operation, control technique of parallel connectedhigh power three-phase inverters, in: The 4th International Power Electronicsand Motion Control Conference, 2004.

Documents

EPSR3077