Power-Sharing of Parallel Inverters in Micro-Grids via Droop control and Virtual Impedance

International Research Journal of Engineering and Technology (IRJET) e-ISSN:2395-0056

Volume: 09 Issue: 11 | Nov 2022 www.irjet.net p-ISSN:2395-0072

Power-Sharing of Parallel Inverters in Micro-Grids via Droop control and Virtual Impedance

1Faculty of Technology and Education, Helwan University, Egypt, 2 Electricity Teacher, Industrial Secondary School, Menoufia, 3 Engineering Department, Faculty of Engineering, Menoufia University, Egypt, 4Faculty of Technology and Education, Helwan University, Egypt. ***

Abstract – The inverter based small networks connected in parallel, the inverters can operate in connected or separate network mode, and in the connected mode, the set points for each inverter are created by processing data on the active output powers and placing all the inverters in a principal controller basedonthe neededoutput power ratios. Here,two proposed power-sharing structures are used to provide a fast and accurate dynamic response to a low circulating current between parallel inverters and the ability to adapt to the required changes in the system to make it stable and make each inverter give its full capacity to the loads. In this article, two different types of system control are used to share energy through electrical inverters. The droop control and virtual impedance are used, and each system improves the performance of the control in a better way to share the load energy. The proposed energy-sharing control systems strategies have been validated using mat-lab/Simulink simulation results

Keywords: Droop Control; Micro-grid control; Power Sharing; parallel inverter; line impedance.

1. INTRODUCTION

The integration of several distributed energy resources (DERs)thatarelinkedinparallel,suchasparallelinverters in micro-grid operation, is necessary to meet the growing needforlarge-scalepowersupplywithhighdependability [1].Advancedcontroltechniquesarenecessaryforparallel inverterstooperateproperly.Manyofthesemethodswere first presented decades ago, and they are still developing today [2]. a frequency-voltage droop approach is a wellrecognizedwidelyused,andsuccessfulmethodforoperating parallel inverters [3]. this method simulates how a largescalepowersystemworksbyusingapre-setdroopfeature that links improvements in generator speed and output activepower.Thistechnologyisknownaswirelesscontrol sincenocommunicationisnecessarybetweentheinverters makingit simpletodeploy anddependable[4].But,ithas variousdrawbacksthatmighthinderitseffectiveness.Some ofitslimitationswerejustasdescribedinthefollowing:its frequency and amplitude differences are load dependent, resulting in poor load voltage regulation performance; an inherent trade-off between voltage regulation and power

sharingbetweeninverters,andimpedancemismatchamong invertersaffectspowersharingperformance[5].

Manyimprovementshavebeenproposedinrecentyearsto increasetheeffectivenessofthedroopcontrolapproachin order to satisfy the rising needs of micro-grids. Modified droop [6-8], adaptive droop [9-11], mixed droop [12-14], and interconnected droop working principle [15-18] are someofthesuggestedchanges.Astandarddroopsystemis given a boost in transient responsiveness in [6] by the addition of power derivative-integral terms. Selecting the propercoefficientsforthederivativeterm,however,inorder toguaranteestablesystemperformance,ischallenging.The authorsof[11]suggestedcombiningstaticdroopfeatures withanadaptivetransientdroopfunctiontoenableactive dampening of power oscillations. The authors did not, however, provide experimental confirmation for this method.Theauthorsof[16]presentedanenhanceddroop method that employs web-based limited bandwidth connection to enhance load-sharing ability. This performancerealizesefficientelectricpower-sharingwith themicrogrid.Adroopcontrolwithanoptimizationsystem is presented in [13]. It uses particle cloud optimization to optimizethe(v-f)constant.Itshowsrespectableactiveand reactive power sharing in simulation, but there is no hardwareconfirmation.

Inverterslinkedinparallelhavelatelycometounderstand thatexchangingcertaininformationamongthemmayhelp accomplishgreatcurrentsharingandvoltagemanagement inaparallelsystem.Activeload-sharingmethodsareafew examples of control strategies that make use of communication between parallel inverters. These include theaveragecurrentconfiguration[21,22],themaster-slave system[19,20],andthesphericalcurrenttechnique.

Inthecircular-chaincurrentapproach,succeedinginverter modules follow the current of the preceding inverter to achieveequalcurrentcirculating.Thefundamentalflawwith thisstrategyisthatitsignificantlyreliesoncommunications, whichintroducessubstantialuncertaintiesintothesystem. Themaster/slaveapproachemploysoneinvertertocontrol theamplitudeandfrequency,whiletheremaininginverters serve as slaves that inject currents. All of the micro grid's inverters participate in the typical current-sharing mechanism,whichregulatesvoltage,frequency,andcurrent.

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Inparticular,theinvertercurrentinjectionaveragedovera common bus is taken into account as a reference for each modulewhendeterminingtheaveragecurrentdata.

Active load sharing approach has unique benefits and drawbacks. There are many different ways to solve the problem,fromcommunicationtotechniquesthatneedless communication.Toavoidoverheatingandmaintainalonger lifespan,someapplicationshavestrictloadingrequirements for the inverters, and correct active and reactive power sharing is essential. These implementations will be made easierbyhavingtheabilitytochangethepowerreferences while maintaining precise sharing and damped responsiveness. Therefore, a control scheme that is adaptable, trustworthy, and has strong voltage stability, currentsharing,anddecreasedcurrentflowingfeaturesis preferred. It should also have minimal reduced communications and be resistant to communication overhead.

In this article, a comparison will be made between two different power-sharing technologies. In this paper, a comparisonwillbemadebetweentwootherpower-sharing techniques(droopcontrollerandvirtualimpedance),which usesfuzzylogic,andnecessarilyrequiresalow-bandwidth connectiontoacentralcontroller.Alloftheparallelinverters that are connected to the central control unit send information about their active and reactive power, and it usesthatdatatocalculateeachinverter'sactiveandreactive power references. Based on a particular ratio of output powertoeachinverter,thesereferencesarecomputed.This informationisusedbyeachinvertertomodifythevoltage andamplitudereferencephaseinrelationtothesharedAC vector in order to achieve the required output power. By adjustingthephaseratherthanthefrequency,asaresult,it ispossibletoachieveeffectivefrequencyregulationwithout affectingthenatureofthevariousfiltersLCLandLCusedto sharepowerbetweeninvertersconnectedinparallelwith the grid between power and load sources for system stability.Theoveralldesignoftheinverterisshowninfigure 1.Thevoltagecontrollertransmitsmodulatedsinewavesto itsIGBTswitchingcomponents,whichbeginreceivingthem. Thesystemcontainstwosolarcellnetworks,inwhichthe maximumpowerisobtainedthrough PerturbandObserve (o&p)[33].Thetwonetworksareconnectedfromtheside oftheinverteroutput.Theinverteriscontrolledthroughthe two techniques Below is a simplified explanation of each partofthesystem.

five sections make up the structure of this term paper Section(2and3)discussesthesystemarchitectureOneof thefundamentalproblemsofparallelinvertersistherobust designofeachinverteroperatinginparallelwiththepower gridandtheproposedpower-sharingmethod,asAstability study of the proposed technologies for virtual impedance andsystemcontrolalgorithm,whichhasamaximumpower pointtrackingMPPTphotoelectriccontroller,ispresentedin

Section (4). Analysed in a mat-lab environment Simulink simulationresultsinvariableweatherconditionsareshown inSections(5)tosupportthefeasibilityandefficacyofthe proposedmethodrespectively,theconclusionsemphasize themainresultsandtheabilitytocontributetothisarticle.

Fig-1: GeneralmicrogridStructure

1.1 Photo voltaic (P.V) System

A PV module consists of several solar cells connected in seriesandparalleltoobtainthedesiredvoltage&current outputlevelsshowninFig2.PVconsistsofaphotocurrent source��ph,diode,andinternalresistances����andRp[35,36].

Iph: Lightproducedcurrent(A).

ID:Diodesaturationcurrent(A).

Io:reversesaturationcurrentofthediode

q:electroncharge( )

K:Boltzmannconstantin1.3865×10^-23(J/K)

µ: conversionefficiency(%)

Id:currentthroughthediode

Tc: operatingtemperature(ºc)

Fig-2: Modelforasolarcell

Parasite resistivity is a component of a functional photovoltaicarray.

Due to the continued resistance of the interconnections metalgrid,pandnlayer,thereisseriesresistance

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Shunt resistivity brought on by p-n connection leakage current.

A PV system is physically described in the following equations[35,36].

Fig-3:Schematicdiagramofboostconvertercontrol.

InFig.4thephoto-voltaicoutputpowerisplottedagainst the plate voltage for specific radiation. Traditional P&O technology [27,30] uses photovoltaic panels or current disruptionstocontinueoperatingandcomparesthepower ratingofthephotovoltaiccelltothepriorperturbationcycle. Features1- InitialRestorativeDynamics

2-Analog/digitalimplementation

Thesolarmodule'sefficiencyisdeterminedbydividingthe greatest amounts of electricity it can generate by solar irradiation[28].

µ = (5)

1.2 Maximum Power Point Tracking (MPPT)

Tracking the PV array's greatest peak power in order to produce the most electrical power is the crucial step for photovoltaicarrays.Thebestprocedureshouldbefollowed during the design of the PV system, and various MPPT methods can be used to ensure this [23]. It depends on irradianceandtemperature.Therearevariouslayersineach strategy.Applicationcharacteristics,includingthoseofthe mosticonicone,thehillclimbingtechniqueontheresilience oftwopoints,canhaveasignificantimpactonthechoiceof MPPT control systems, including complexity sensors, amount of digital or analogue applications, rapid convergencetraceability,andfinancialimpact.P(k),p(k-1) StartcomparingMPPTandMPPsidebyside.

The primary benefit of strategic is that it is inexpensive, simpletoimplement,doesnotrequireacontrolschemeor micro-controller,andonlyrequiresonevoltagesensor[17]. Aslongastheradiationfromthesundoesnotfluctuatetoo muchthroughouttheday,thismethodworkswell[28].

As shown in Fig.3the suggested MPPT algorithm is perturbative and controls the dc - dc converter using feedback.AboostconverterItincreasesthevoltagevalueat theexpenseofthecurrentvalueanddealswiththeconstant currentonlyandmakesacomparisonprocessbetweenthe voltageofasolarcellVpv andthevoltageinsidetheinverter

Vref

3-Minimalsoftware/hardwarerequirements.Theoperating voltage in between the PV array and the converter is perturbedbytheP&omethod.

Fig -4:P&OBasicIdeaAlgorithm

P&OItconsistsoftwomainparts,PointA: dP/dV>0Ifthe operatingpointontheleftMPPand PointB:dP/dV<0 If the operating point on the right MPP, dP/dv = 0 At the operatingpointontheMPPTurbulentisemployedinvoltage and power measurement methods, and the high value utilizationoftheturmoildirectioniscalculatedby

P . If the polarization of a power output is positive,anextdisturbancewilltravelinsamedirectionsas the prior one. If the depolarization of a power output is negative,anextdisturbanceisinthereversedirectionasthe priorone.WhenMPPisattained,theoperationisrepeated.

1.3 DC/AC Inverter and LCL Filter

the inverter as shown in the image(5) converts dc to ac current and must be included in any micro-network that uses electrical power electronics to serve single or threephase loads consisting of high frequency solid-state electronicsand anL - LCL low-passfilteritis incharge of smoothingtheoutputwaveinordertoachieveasinusoidal signal free of harmonics the inverter has a steady current

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input and is linked to a capacitor which stabilizes and regulatesthevoltageenergyfromsolarcellsandraisesitto the inverter the output is the ac current produced at the invertersoutputsignalsaresentbythevoltagecontrollerto the switching devices which are isolated gated bipolar transistors(IGBT) Vref andP.W.Msignalsaregeneratedin addition when the A.C signal of the power electronic switcheschangesharmonicsignalsarecreatedtheLCLfilter is widely used in conjunction with inverse networks to enhancecurrentexcellenceandsupplyoptimumsinusoidal powertoapowergridwhileminimizingharmonics.

Where istheresistanceofthevirtualimpedance,orit canbeinductiveas Where istheinductanceofthevirtualimpedanceandis the time constant of the high pass filter used to approximate the derivative in the transfer function of the idealvirtualinductance [25].

Fig -5:inverterandLCLfilter

2. Virtual impedance technology

Avoltagecontrollerloopisusedinbasicvoltagesource invertersVSItotrackthedesiredinputsignalandreduceits error and the measured output voltage. A proportional controller, Kv, is utilized in this study, backed by a feedforward loop. The feed-forward loop reduces steady-state errorwhileallowingforabroadercontrolband[1,3].

Fig.4depictsoneinverterphase.Theirinnerloopcontrollers are shown in this block diagram [12,14]. In this paper, (“VirtualImpedanceImpactonInverterControlTopologies”) This impedance mimics the behavior of an inductor or resistorintheprogram.Usingaprogrammableimpedance rather than a physical one reduces the losses and costs [19,22].Inaddition,beingprogrammablepresentsadaptive operation and increases the inverter’ s robustness against network impedance variations [23,27]. Fig. 6 shows the block diagram of the voltage controller with the virtual impedanceZv(s).

Fig -6:Themodelofbasicdouble-loopvoltagecontroller

3. Droop control technology

In this paper, a micro-grid consisting of two inverters as shown in Fig.1 is considered. The circuit diagram of each inverteranditsLCLfilterandcontrollerisillustratedinFig. 7.Thisisthebasisfortheconventionaldroopcharacteristic, asrepresentedin(8)and(9)[6].

The output impedance with virtual impedance can be derivedas,(“VirtualImpedanceImpactonInverterControl Topologies”)

ThenatureofZvcouldbechosentoberesistiveas,

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Where �� ∗ , �� ∗ are the nominal frequency and nominal voltagereferences,mandnarethefrequency-proportional drooping coefficient and voltage-proportional drooping coefficient,respectively.The droopslopesaredetermined accordingtothepowerratingoftheinverterandaccording to the maximum allowable variations in output frequency andvoltage[11,13].Ingrid-connectedmode,theactiveand reactivepower set-points and areadjustedtobeequal to the reference power values. The conventional droop characteristic is summarized in Fig.8. In this figure, the loaded active- and reactive power is denoted along the xaxes.Asshown,anincreasedactiveloadleadstoareduction in the frequency, while the output voltage is reduced if a morereactiveloadisaddedtothesystem.Thischaracteristic can be utilized in the conventional droop control method, wheretheactivepowerfedintothelineiscontrolledbased onfrequencydeviations,whiledeviationsinvoltagecontrol thereactivepowersupply.Thedeviationsaredeterminedby

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the droop coefficients, which are given based on (10) and (11)[36,26].

(13)

∆ωmaxisthemaximumallowedfrequencydeviationduetoa loadchangeinactivepower,Pchange.Themaximumallowed voltagechange, ∆Vmax,isin a similarwayrelatedtoa load changeinreactivepower,Qchange.Often,∆ωmaxand∆Vmaxare decided based on the maximum deviations allowed in the grid. The droop coefficients also affect the power sharing among inverters, where larger droop coefficients lead to betterpowersharing.However,thecoefficientshaveupper limits,whereincreasingthemwouldleadtoinstabilityinthe system.Withinthestabilitylimits, thechoiceofdroopcoefficientsisatrade-offbetweenthe power sharing performance and the deviations in voltage andfrequency.[15,17]

Numberofparallel-connectedcells:39cell

Numberofseries-connectedcells:5cell

shows Fig.9 shows the power-voltage characteristic of PV moduleinfourcases,andFig.01showsthesun'sradiationof value.

To verify the performance of the simulation that was conductedonMATLAB/Simulink,tocomparetwodifferent technologies, namely, the virtual impedance and the traditional droop control to control the electric power outputtosharetheelectricloadswiththemainnetwork.The micro-grid contains two electrically identical stations conditionedbyvariablesolarradiationrangingbetween250 and1000W/ ataconstanttemperatureof25°C.

Fig -7:Implementationoftheconventionaldroopcontrol

Fig -8:Thecharacteristicoftheconventionaldroop control

4. SIMULATION RESULTS AND DISCUSSION

ThissectionillustratedtheperformanceofPVmoduleand performance of perturb and observe algorithm to track maximumpowerpointofPVmoduleundervariousweather conditions. The PV module simulation is implemented in MATLAB Simulink. The parameter of the PV module (330 SunPowermodules). isdisplayedforonemoduleis:

Fig-9: Characterizesofphotovoltaic

The efficiency of the P.V system in various weather conditions and turbulence is also asa monitoring method used to track the highest energy point. The photovoltaic modulecharacteristicshowsthe(60)kWphotovoltaiccellin theproposedmodelconsistingof(330)solarmodules.The PV module parameters for a single module are shown as follows:

ItisclearfromFig.10thatthenumberofpanelsconnected inseries(5)successivestages

•T=(0-0.5)sec:witharadiationintensityof(200)W/ ataconstanttemperature(25°C),thentheintensityof solarradiationincreasesto1000W/ gradually

•T=(0.5-2)sec:theradiationintensityremainsconstant,

• T=(2-4)sec:the radiation intensitygraduallydecreases untilitreaches(300)W/

• T=(4-5)sec:the radiation intensitygraduallydecreases from300tozero.

•T=(5-6)sec:theradiationintensitybecomes(0),meaning that the two stations exit the system and give electrical energy=0,andtheloadderivesitsenergyfromtheelectrical grid.

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Fig -01:IrradianceinputtothePVarray

Theactivepoweroutputcurveofthephotovoltaiccellsover timewithavariablesolarirradiancevaluefrom200to1000 W/ asinFigure11andataconstant25°C.Themaximum valueofthecapacityofthesolarcellsis60kW,andwiththe changeofsolarradiationovertime,whenthesystemstarts inthefirst0.5sec,asuddenchangeinthevalueofelectric energy occurred when using the virtual impedance technologytosharetheelectricpoweroftheloadswiththe microgrid.

Fig-01:Activepoweroutputofthetwoinverters (withoutpowersharing)

Case study (2): Simulation results when using the virtual impedance control techniqueto share theactivepower of twoIdenticalinvertersconnectedtothemicro-gridhavethe sameoutputpower.

Fig.02 shows the output power of each inverter. It is assumedthatbothinvertersworktogetherinparallelasthe inverteriscontrolledbycontrollingthevoltagesourceusing the visual impedance. Where it was found from the operation that the output capacity of the first inverter (whichoutputisconnectedtoafilteroftypeLCL)doesnot reachitsmaximumpower,butisloweredfromitbyasimple value

Fig -01:ActivepowermeanofthetwoPV

Case study (1): Simulation results if no droop control technique or no virtual impedance is applied to share the outputpoweroftwoidenticalmicro-gridinverters.

Figure01illustratestheoutputpowerofeachinverter.Itis assumedthatbothinvertersarealreadyoperatinginparallel andthatthewithoutpowersharing.Wenotethattheoutput ofthesecondreflector(Ppv2)givesitsfullvalue,whilethe first (Ppv1) reflector gives a much lower value with the change in solar radiation. the power responses lost the stability

Fig-13:Activepoweroutputofthetwoinverters (withvirtualimpedance)

Case study (3): Simulation results when using the droop controltechniquetosharetheoutputpoweroftwoIdentical invertersconnectedtothemicro-grid.

Fig.14 shows the output power of each inverter. It is assumedthatbothinvertersworktogetherinparallelasthe inverteriscontrolledbyvoltagesourcecontrolusingdroop control. The process shows that the output capacitance of thefirstinverter(whichoutputisconnectedtoaLCL-type filter)hasafastresponseandachievescooperationwiththe changeinthevalueoftheoutputpowerofthesolarcells.

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balancedandstableineffectiveforcesharingwhentheline resistanceisactuallychangedfromthedefaultimpedance technology,plusitdoesnotneedhumanintervention.

Table -1: ComparisonbetweenDroopcontrolandVirtual impedance

Comparison scheme

Capabilitytoset powerratio

(v/f)Droop control technique

Fig -14:Activepoweroutputofthetwoinverters (withdroopcontrol)

5. Conclusion

In this article, two methods of output power sharing betweeninvertersconnectedinparallelwiththemicro-grid, namelydroopcontrolandvirtualimpedance,areproposed tobeappliedtoACnetworkstoensurehighaccuracyand flexibilityinelectricpowersharing.Andweconcludefrom the results obtained from simulations in the Mat-lab / Simulink environment for two parallel inverters can be painted:

•Whenvirtualimpedancecontrolisappliedinasmallgrid systemasatechniqueforsharingelectricalenergybetween inverterswithfuzzyusedtoimprovetheoutputshapeofthe effectivepower,itdoesnotworkwellunderthesechanging conditions,astheeffectivepowersharingisaffectedbythe virtualimpedancesinthesmallgridbetweentheinverters. TransformersandPCCduringthesharingprocess.

• To overcome this problem, the proposed droop control technique is applied to adjust the line resistance value to properly share the effective power between the inverters andworkbetterunderthesameconditionsevenwhenthere aredifferencesinthelineresistanceinasmallnetwork.

•Thevoltagedroopcanbereducedintheproposeddroop controltechnique.

•SwingcontrolTheabilitytoquicklyswitchaccordingtothe proportionsoftheactiveelectricalpoweraccurately.

When implementing a proposed micro-grid with VSI transformersparalleltotheelectricalnetworkforsupplying connectedelectricalloads,considerationshouldbegivento: DC sources with galvanic isolation; Determines the total electrical capacity of the load. In addition, the controller controls each of the VSI switches connected to the micronetwork so that no interference occurs and the system is balanced,stable,andworkswell.

Ithasbeenscientificallyprovenfromthesimulationresults that the proposed droop control method is superior and worksbetterandmoreaccuratelywhilemakingthesystem

Advantage

Disadvantage

Virtual impedance scheme

Yes, without the needforfuzzy Yes,butneedsa fuzzy

- Fast dynamic response at the start of the operation

- Very flexible and has the abilitytoexpand. -Systemstability and power sharingaccuracy -Itisnotaffected by any fundamental change in the system -Ease of implementation intheabsenceof contact between theinverters

-Inaccurate voltage and frequency regulation

-Can be used with linear and non-linearloads when electric powerisshared - Reduces harmonic voltage -Decoupled active and reactive power controls

-Low dynamic response -It is necessary to know the physical variablesfirst -Inaccurate voltage and frequency regulation -Affectedbyany physical change inthesystem

- The system is unstable, especially when starting, and less accurate in sharing electric power

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