Seite - 52 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 1217 thechargingtimetodistinguish theirvehicles in theEVmarkethasbecomethecurrentnormamong themanufacturers, andit requireshighervoltages thanever. Therefore,mitigating theadverseeffects isnot likelybyemploying lowchargingvoltages. To avoid these effects, and to provide efficient charging with the available infrastructure, coordinatedcharging(alsocalledcontrolledorsmartcharging)has tobeadopted. In this scheme, the EVsarechargedduringthe timeperiodswhenthedemandis low, forexample,aftermidnight. Such schemesarebeneficial ina lotofways. Itnotonlypreventsadditionofextra loadduringpeakhours, butalso increases the load invalleyareasof the loadcurve, facilitatingproperuseof thepowerplants withbetter efficiency. In [116],Richardsonet al., showed that a controlled charging rate canmake highEVpenetrationpossible in thecurrent residentialpowernetworkwithonlyafewupgrades in the infrastructure.Gengetal.,proposedachargingstrategy in[120]comprisingof twostagesaimed atprovidingsatisfactorychargingforall connectedEVswhileshiftingthe loadsonthe transformers. Ontheconsumerside, it canreduce theelectricitybill as theelectricity is consumedbytheEVsduring offpeakhours,whichgenerallyhaveacheaperunit rate thanpeakhours.Accordingto [121], smart chargingsystemscanreduce the increase investmentcost indistributionsystemby60–70%.Themajor problemsthatare faced in thepowersystemsbecauseofEVscanbechartedas following: • Voltage instability:Normallypowersystemsareoperatedclose to their stability limit. Voltage instabilities in such systemscanoccurbecauseof loadcharacteristics, and that instability can leadtoblackouts. EVloadshavenonlinearcharacteristics,whicharedifferent thanthegeneral industrial ordomestic loads, anddraw largequantitiespower ina short timeperiod [81,122]. Reference[123]corroboratedtothefactthatEVscauseseriousvoltageinstabilityinpowersystems. If theEVshaveconstant impedance loadcharacteristics, then it ispossible for thegrid tosupport a lotofvehicleswithout facinganyinstability [81].However, theEVloadscannotbeassumed beforehand and thus their power consumptions stay unpredictable; addition of a lot of EVs at a time therefore can lead to violation of distribution constraints. To anticipate these loads properly, appropriatemodelingmethodsare required. Reference [124] suggested tackling the instabilitiesbydampingtheoscillationscausedbycharginganddischargingofEVbatteriesusing awideareacontrolmethod. Thesituationcanalsobehandledbychanging the tapsettingsof transformers [125], byaproperlyplannedchargingsystem,andalsobyusingcontrol systems like fuzzy logiccontrollers tocalculatevoltagesandSOCsofbatteries [81]. • Harmonics: The EV charger characteristics, being nonlinear, gives raise high frequency componentsofcurrentandvoltage,knownasharmonics. Theamountofharmonics inasystem canbeexpressedbytheparameters total currentharmonicdistortion(THDi)andtotalvoltage harmonicdistortion(THDv): THDi= √ H ∑ h=2 I2h I1 ×100% (4) THDv= √ H ∑ h=2 V2h V1 ×100% (5) Harmonicsdistort thevoltageandcurrentwaveforms, thuscanreduce thepowerquality. It also causesstress in thepowersystemequipment likecablesandfuses [122]. Thepresentcabling iscapable ofwithstanding25%EVpenetration if slowcharging isused, incaseof rapidcharging, theamount comesdownto15%[126].Voltage imbalanceandharmonicscanalsogiverise tocurrentflowinthe neutralwire [127,128].Differentapproacheshavebeenadoptedtodetermine theeffectsofharmonics duetoEVpenetration. Reference [127] simulatedtheeffectsofharmonicsusingMonteCarloanalysis to determine thepower quality. In [129] the authors showed thatTHDv can reach 11.4% if a few numberofEVsare fast charging. This is alarmingas the safety limit ofTHDv is 8%. According to 52