Page - 53 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 1217 Meloetal. [130],THDi alsobecomeshigh, in therangeof12%to14%, incaseof fast charging, though it remains in thesafe limitduringtimesofslowcharging. Studiesconducted in[131]showthemodern EVsgenerate lessTHDi thantheconventionalones, thoughtheirTHDvvaluesarehigher.However, with increasednumberofEVs, therearechancesofharmonicscancellationbecauseofdifferent load patterns [132,133].DifferentEVchargerscanproducedifferentphaseanglesandmagnitudeswhich canleadtosuchcancellations [133]. It isalsopossible toreduce,eveneliminateharmonicsbyapplying pulsewidthmodulation in the EV chargers [132]. HighTHDi can be avoided by using filtering equipmentat thesupplysystem[134]. • Voltagesag:Adecrease in theRMSvalueofvoltage forhalfacycleor1min isdenotedasvoltage sag. It canbecausedbyoverloadorduring thestartingofelectricmachines. Simulationmodeled withanEVcharger andapower converter in [135] stated20%EVpenetrationcanexceed the voltagesag limit. Reference [136] stated that60%EVpenetration ispossiblewithoutanynegative impact ispossible if controlledcharging isemployed. Theamount,however,plummets to10%in caseofuncontrolledcharging. Leemputetal., conductedatestemployingvoltagedroopcharging andpeakshavingbyEVcharging[137]. Thisstudyexhibitedconsiderabledecrease involtagesag withapplicationofvoltagedroopcharging.Applicationofsmartgridcanhelp ingreatextents in mitigatingthesag[138]. • Power loss: Theextra lossofpowercausedbyEVchargingcanbeformulatedas: PLE=PLEV−PLoriginal (6) PLoriginal is the lossoccurredwhen theEVsarenot connected to thegridandPLEVis the loss withEVsconnected.Reference [121]chartedthe increasedpower lossashighas40%inoffpeakhours considering60%of theUKPEVs tobeconnected todistribution system. Uncoordinatedcharging, therefore,canincreasetheamountoflossfurthermore. Takingthatintoaccount,acoordinatedcharging scheme,basedonobjectivefunction, tomitigatethelosseswasproposedin[139].Coordinatedcharging isalso favoredby[140,141] toreducepower lossessignificantly. Powergenerated in thenearvicinity can also helpminimizing the losses [142], anddistributed generation can be quite helpful in this prospect,with thevehicleownersusingenergygeneratedat theirhome(byPVcells,CHPplants, etc.) tocharge thevehicles. • Overloadingof transformers: EVchargingdirectlyaffects thedistributiontransformers [81]. The extra heat generatedbyEV loads can lead to increased aging rate of the transformers, but it alsodependsontheambient temperature. Inplaceswithgenerallycoldweather likeVermont, the agingdue to temperature is negligible [81]. Estimationof the lifetimeof a transformer is done in [143], where factors taken into account are the rate of EVpenetration, starting time of charging and the ambient temperature. It stated that transformers canwithstand 10%EV penetrationwithoutgettinganydecrease in lifetime. Theeffectof level1charging, is in fact,has negligibleeffectonthis lifetime,butsignificant increase in level2chargingcan leadto the failure of transformers [144]. Elnozahyetal., stated thatoverloadingof transformercanhappenwith 20%PHEVpenetrationfor level1charging,whereas level2does itwith10%penetration[145]. Accordingto[122], chargingthat takesplacerightafteranEVbeingpluggedincanbedetrimental to the transformers. • Powerqualitydegradation: The increasedamountofharmonicsandimbalance involtagewill degradethepowerquality incaseofmassivescaleEVpenetrationto thegrid. 8.1.2. Positive Impacts Ontheplusside,EVscanprovetobequiteuseful to thepowersystems inanumberofways: • Smart grid: In the smart grid system, intelligent communication and decision making is incorporatedwith thegridarchitecture. Smartgrid ishighly regardedas the futureofpower 53