voltage sag and swell

1. PowerQuality
Syllabus- Introduction,Overviewof Powerquality problems,Voltageand currentquality,sourcesof poor
powerquality,Loadsthatcan causespowerquality problems,powerquality problemsin distribution
network,PowerqualityIEEE standardsand Regulations,Importanceof powerquality ,Evaluation of
powerquality, effectsof harmonicson powerquality.
Introduction to Power Quality
Powerqualityisanyabnormal behavioronapowersystemarisinginthe formof voltage orcurrent,
whichaffectsthe normal operationof electricalorelectronic equipment.
Powerqualityisanydeviationof the voltage orcurrentwaveformfromitsnormal sinusoidal wave
shape.
Powerqualityhasbeendefinedasthe parametersof the voltage thataffectthe customer’s
supersensitive equipment.
Overview of Power quality problems
o Voltage sag
o Voltage swell
o Voltage Flicker
o Harmonics
o Over voltage
o Under voltage
o Transients
Voltage sagsare consideredthe mostcommonpowerqualityproblem.Thesecan be causedby the
utilityorbycustomerloads.Whensourcedfromthe utility,they are mostcommonlycausedbyfaultson
the distributionsystem.These sagswill be from3 to 30 cyclesandcan be single orthree phase.
Dependingonthe design of the distributionsystem, agroundfaulton1 phase can cause a simultaneous
swell onanotherphase.
Powerqualityproblemsare relatedtogrounding,groundbondsandneutral to groundvoltages,ground
loops,groundcurrentor groundassociatedissues Harmonicsare distortionsinthe ACwaveform.These
distortionsare causedby loadsonthe electrical systemthatuse the electrical powerata different
frequency thanthe fundamental50 or 60 Hz.
Voltage and current quality
1.Voltage quality:Deviationsof the voltage fromasinusoidal waveform.
2.Currentquality: Deviationsof the currentfroma sinusoidal waveform.
The common term is power quality; however, it is actually the quality of the voltage that is being
addressedinmostcases.Technically, in engineering terms, power is the rate of energy delivery and is
proportional tothe productof the voltage and current.It wouldbe difficult to define the quality of this

2. quantity in any meaningful manner. The power supply system can only control the quality of the
voltage;ithas nocontrol overthe currentsthat particularloadsmightdraw.Therefore,the standards in
the power quality area are devoted to maintaining the supply voltage within certain limits
there is always a close relationship between voltage and current in any practical power system.
Althoughthe generatorsmayprovide anear-perfectsine-wave voltage,the current passing through the
impedance of the system can cause a variety of disturbances to the voltage.
For example,
1. The currentresultingfromashort circuitcausesthe voltage tosag or disappearcompletely,asthe
case maybe.
2. Currentsfrom lightningstrokespassingthroughthe powersystemcause high-impulse voltagesthat
frequentlyflashoverinsulationandleadtootherphenomena,suchasshortcircuits.
3. Distortedcurrentsfromharmonic-producingloadsalsodistortthe voltage astheypassthroughthe
systemimpedance.Thusadistortedvoltage ispresentedtootherendusers.
Therefore,while itisthe voltage withwhichwe are ultimatelyconcerned,we mustalsoaddress
phenomenainthe currenttounderstand the basisof manypowerqualityproblems.
Power quality concern
Thus,powerqualityreferstomaintainasinusoidal waveformof busvoltage atbusvoltagesat rated
voltage andfrequency. So,youshouldhave aconstantfrequencyandpropersinusoidal voltageand
currentwithconstantmagnitude,thenyousaythat we are gettingapure good qualityof power.
So,mostlywe can considerthe generationstage are purelysinusoidal andfree forany distortion,in
betweenthere are manysource of contamination(manydevicesthatdistortthe waveforms) hence
for thisreasonwe geta contaminatedpower.
Thisdistortionmaypropagate all overthe electricnetworkoritmay be restrictedorbypassor
segregated,there are manyoptions,if thispropagatesthenitcalledashazardsto the anotherperson.

3. Figure 1. containsan ideal sinusoidal wavealongwithadistortedwave.The distortionintroducedina
wave can create waveformdeformityaswell asphase shift
(REF- Electrical Power Systems Quality, Second Edition)
The ultimate reasonthatwe are interestedinpowerqualityiseconomicvalue.There are economic
impactson utilities,theircustomers, andsuppliersof loadequipment
The electricutilityisconcernedaboutpowerqualityissuesaswell. Meetingcustomerexpectationsand
maintainingcustomerconfidence are strongmotivators.Withtoday’smovementtowardderegulation
and competitionbetweenutilities,theyare more importantthanever.Thelossof adisgruntled
customerto a competingpowersuppliercan have a verysignificantimpactfinanciallyonautility.
Sources /causes of poor power qualityin Powersystems
Althoughasignificantliterature onpowerqualityisnow available,mostengineers,facilitymanagers,and
consumersremainunclearasto whatconstitutesapowerqualityproblem.Furthermore,due tothe
powersystemimpedance,anycurrent(or voltage) harmonicwillresultinthe generationand
propagationof voltage (orcurrent)harmonicsandaffectsthe entirepowersystem.Figure 1.1illustrates
the impact of currentharmonicsgeneratedbya nonlinearloadonatypical powersystemwith
linearloads

4. Figure 1.1 Propagationof harmonics(generatedbyanonlinearload) inpowersystem
The distortionsources are divided intothree categories:
 small andpredictable (e.g.,residential consumers generatingharmonics),
 large and random(e.g.,arc furnacesproducingvoltage fluctuationsandflicker),and
 large and predictable (e.g.,staticconvertersof smeltersandhigh-voltage DCtransmission
causingcharacteristicanduncharacteristicharmonicsaswell as harmonicinstability).
However,the likelyanswerstothe questionare these:unpredict-able events,the electricutility,the
customer,andthe manufacturer
Unpredictable Events
Both electricutilitiesandendusersagree thatmore than 60% of powerqualityproblems are generated
by natural and unpredictable events[6].Some of these include faults,lightningsurge propagation,
resonance,ferroresonance,andgeomagneticallyinducedcurrents(GICs) due tosolarflares.These
eventsare consideredtobe utilityrelated problems.
The ElectricUtility
There are three mainsourcesof poorpowerqualityrelatedtoutilities:
• The pointof supplygeneration.Althoughsynchronousmachinesgenerate nearlyperfectsinusoidal
voltages(harmoniccontentlessthan3%),there are powerqualityproblems originatingatgenerating
plantswhichare mainlydue to scheduling,eventsleadingtoforced outages,andloadtransferringfrom
one substationtoanother.

5. The transmissionsystem-.Relativelyfewpowerqualityproblemsoriginateinthe transmissionsystem.
Typical powerqualityproblemsoriginatinginthe transmissionsystem are galloping(underhigh-wind
conditionsresultinginsupplyinterruptionsand/or randomvoltage variations),lightning(resultingina
spike ortransientovervoltage),insulatorflashover,voltage dips(due tofaults),interruptions(due to
plannedoutages byutility),transientovervoltages(generatedbycapacitorand/orinductorswitching,
and lightning),transformerenergizing(resultingininrushcurrentsthatare rich inharmonic
components),improperoperationof voltage regulationdevices(whichcanlead tolong-durationvoltage
variations),slow voltage variations(duetoa long-termvariationof the loadcausedbythe continuous
switchingof devicesandload),flexibleAC transmissionsystem(FACTS) devices] andhigh-voltageDC
(HVDC) systems,corona[ powerline carriersignals ,broadbandpowerline (BPL) communications,and
electromagneticfields(EMFs).
• The distributionsystem.-Typical powerqualityproblemsoriginatinginthe distributionsystemare
voltage dips,spikes,andinterruptions,transientovervoltages,transformer energizing,improper
operationof voltage regulationdevices,slow voltagevariations,powerlinecarriersignals, ,broadband
powerline (BPL,) andEMFs
The Customer
Customerloadsgenerate aconsiderableportionof powerqualityproblemsintoday’s powersystems.
Some end-user/customerrelatedproblemsare harmonics(generatedbynonlinear loadssuchaspower
electronicdevicesandequipment,renewable energysources, FACTSdevices,adjustable-speeddrives,
uninterruptiblepowersupplies(UPS),fax machines,laserprinters,computers,andfluorescentlights),
poor powerfactor(due to highlyinductive loadssuchasinductionmotorsandair-conditioningunits),
flicker(generatedbyarcfurnaces , transients(mostlygeneratedinside afacilitydue todevice switching,
electrostaticdischarge,andarcing),impropergrounding(causingmost reportedcustomerproblems),
frequencyvariations(whensecondaryandbackuppower sources,suchasdiesel engine andturbine
generators,are used),misapplicationof technology,wiringregulations,andotherrelevantstandards
ManufacturingRegulations
There are twomainsourcesof poorpowerqualityrelatedtomanufacturingregulations:
• Standards.The lackof standardsfor testing,certification,sale,purchase,installation,and use of
electronicequipmentandappliancesisamajor cause of powerqualityproblems.
• Equipmentsensitivity.The productionof “sensitive”electronicequipmentand appliancesisone of the
mainreasonsforthe increase of powerqualityproblems.
(Ref -PowerQualityinPowerSystemsandElectrical Machines)
Powerqualitytherefore mustnecessarilybe tackledfromthree fronts,namely:
• The utilitymustdesign,maintain,andoperate the powersystemwhile minimizing powerquality
problems;
• The endusermust employproperwiring,systemgroundingpractices,andstate-of- the-artelectronic
devices;and
• The manufacturermustdesignelectronicdevicesthatkeepelectrical environmental disturbancestoa
minimumandthatare immune toanomaliesof the powersupply

6. Power Quality IEEE Standards
IEEE Standardsare publicationsthatprovide acceptable designpractice and proceduresandmethodsfor
performingapowerqualityanalysis.
IEEE Standardsaddressingpowerqualityinclude thosedefiningacceptablepowerquality(IEEE Standard
519) and anotherstandardrelatingtothe measurementof power-quality“events”(IEEEStandard1159).
IEEE Standard519 (denotedIEEEStd.519-1992) is titled“IEEERecommendedPracticesand
RequirementsforHarmonicControl inElectrical PowerSystems.”Thisguide appliestoall typesof static
powerconvertersusedinindustrialandcommercial facilities.The problemsinvolvedinthe harmonic
control and reactive compensationof suchconvertersare addressed,andanapplicationguideis
provided.Limitsof disturbancestothe ac powerdistributionsystemthataffectotherequipmentand
communicationsare recommended.Thisguide isnotintendedtocoverthe effectof radiofrequency
interference..The 1992 standardis a revisionof anearlierIEEEwork publishedin1981 covering
harmoniccontrol.
The basic themesof IEEE Standard519 are twofold.First,the utilityhasthe responsibilitytoproduce
goodqualityvoltage sine waves.
The basic themesof IEEE Standard519 are twofold.First,the utilityhasthe responsibilitytoproduce
goodqualityvoltage sine waves.Secondly,end-use customershave the responsibilitytolimitthe
harmoniccurrentstheircircuitsdrawfrom the line
IEEE Standard 241-1986
It dealswiththe properselection,application,andcoordinationof the componentswhichconstitute
systemprotectionforindustrialplantsandcommercial buildings.Systemprotectionandcoordination
serve tominimize damage toasystemand itscomponentsinordertolimitthe extentand durationof
any service interruptionoccurringonanyportionof the system
IEEE Standard 141-1993

7. Recommendationsare made regardingsystemplanning;voltage considerations;surge voltage
protection;systemprotective devices;faultcalculations;grounding;powerswitching,transformation,
and motor-control apparatus;instrumentsandmeters;cable systems;busways;electrical energy
conservation;andcostestimation.
Thisguide appliestoall typesof staticpowerconvertersusedinindustrial andcommercial power
systems.The problemsinvolvedinthe harmoniccontrol andreactive compensationof suchconverters
are addressed,andanapplicationguide isprovided.Limitsof disturbancestothe ac powerdistribution
systemthataffectotherequipmentand communicationsare recommended.Thisguide isnotintended
to coverthe effectof radiofrequencyinterference.
IEEE Standard 1250-2018
Thisstandard seekstolimitthe harmonicinjectionfromindividual customerssothattheydonot create
unacceptable voltage distortionundernormal systemcharacteristicsandtolimitthe overall harmonic
distortioninthe voltage suppliedbythe utility.The voltageandcurrentdistortionlimitsshouldbe used
as systemdesignvaluesforthe worst case of normal operatingconditionslastingmore than1h. For
shorterperiods,suchasduringstart-ups,the limitsmaybe exceededby50 percent.
IEEE Standard1159-1995, RecommendedPractice forMonitoringElectricPower
Quality.Monitoringof electricpowerqualityof ACpowersystems,definitionsof power
qualityterminology,impactof poorpowerqualityonutilityandcustomerequipment,andthe
measurementof electromagneticphenomenaare covered.Keychaptersinclude:
Monitoringobjectives,Measurementinstruments,monitorapplicationtechniques,and
interpretingmonitoringresults
IEEE1250-2018,The use of some electrical equipmentattachedtotypical powersystemscreates
powerqualityconcerns.There isanincreasingawarenessthatsome equipmentisnotdesignedto
withstandthe surges,faults,distortion,andreclosingdutypresentontypical utilitydistributionsystems.
Traditional concernsaboutsteady-statevoltage levelsandlightflickerdue tovoltage fluctuationalso
remain.These concernsare addressedbythis guide bydocumentingtypical levelsof these aspectsof
powerqualityandindicatinghowtoimprove them
Overview of Power quality problems
There are differentclassificationsforpowerqualityissues,eachusingaspecificproperty tocategorize
the problem.Some of themclassifythe eventsas“steady-state”and“non- steady-state”phenomena.
In some regulations(e.g.,ANSIC84.1[22]) the mostimportant factoris the durationof the event.Other
guidelines(e.g.,IEEE-519) use the wave shape (durationandmagnitude)of eacheventtoclassifypower
qualityproblems.Otherstandards (e.g.,IEC) use the frequencyrange of the eventforthe classification.

8. The nature of the variationinthe basic componentsof the sine wave,i.e.,voltage,current,and
frequency,identifiesthe type of powerqualityproblem.
Table 1

9. Transients
Powersystemtransientsare undesirable,fast- andshort-durationeventsthatproduce distortions.Their
characteristicsandwaveformsdependonthe networkparameters(e.g.,resistance,inductance,and
capacitance)
“Surge” isoftenconsideredsynonymouswithtransient. Theyoftendissipate quickly
Transientscanbe classifiedwiththeircharacteristiccomponentssuchasamplitude,duration,rise time,
and frequencyof occurrence.
Transientsare usuallyclassifiedinto twocategories:impulsive andoscillatory
Impulsive transientcausedbya lightningstroke.switchingof lineswithpower factorcorrection
capacitor banks,poorgrounding,switchingof inductiveloads,utilityfaultclearing,disconnectionof
heavyloads,andelectrostaticdischarge..Impulsive transientscanbe veryfastevents(5ns rise time
fromsteadystate to the peak of the impulse) of short-termduration(lessthan50 ns),and mayreach
thousandsof volts,eveninlowvoltage.
Devicesare neededtopreventsdamage toelectrical equipmentcausedbyimpulsive transientsfrom
lightningstrokesUtilitiesuse lightningarrestersmountedontheirtransmissionanddistributionsystems
and intheirsubstations,while manyutilitycustomersuse transientvoltage surge suppression(TVSS)
Fig- impulse transients
Oscillatorytransientsoccurwhenswitchinginductiveorcapacitive loadssuchasmotorsor capacitor
banks.Anoscillatorytransientoccursbecause the loadresiststhe change.Lighting,utilityfaultclearing
and transformerenergizationandferroresonance couldalsocause oscillatorytransients.
Oscillatorytransientsdonotdecayquicklylike impulsivetransients. Theytendtocontinue tooscillate
for 0.5 to 3 cyclesand reach 2 timesthe nominal voltage orcurrent.Anothercause of oscillatory
transients,besideslightningstrokesgoingintoresonance,isswitching of equipmentandpowerlineson
the utility’spowersystem .

10. Figoscillatorytransients
TransientscandestroycomputerchipsandTV
Voltage sags(dips)
Voltage sagsare referredtoas voltage dipsinEurope.IEEE defines voltage sagsasa reduction involtage
for a short time.The duration of a voltage sagislessthan 1 minute butmore than 8 milliseconds (0.5
cycles).The magnitude of the reductionisbetween10 percentand 90 percentof the normal rootmean
square (rms) voltage at rated frequency.The Utilitiesandenduserscancause voltage sagson
transmissionanddistributionsystems.Forexample,atransformer failurecanbe the initiatingeventthat
causesa faulton the utilitypowersystemthatresultsinavoltage sag.These faultsdraw energyfrom
the powersystem.A voltage sagoccurs while the faultis onthe utility’spowersystem.Assoonasa
breakeror recloserclears the fault,the voltage returnstonormal.Transmissionfaultscause voltagesags
that lastabout 6 cycles,or 0.10 second.
Distributionfaults lastlongerthantransmissionfaults,while large motorloadscancause
voltage sagon utility’sandenduser’spowersystems.
Comparedtoother powerqualityproblemsvoltagesags occurmost frequently.They reduce the energy
beingdeliveredtothe enduserand cause computerstofail,adjustable-speeddrivestoshutdown,and
motorsto stall and overheat.
Solutionstovoltage sagproblemsinclude equipmentthatprotectsloadsthat are sensitivetovoltage
sags.Examplesof these typesof equipmentinclude,constantvoltage transformers; dynamicvoltage
restorers(DVRs);superconductingenergystorage devices;flywheels anduninterruptible power
supplies(UPS).

11. Voltage swells
Voltage swells,ormomentaryovervoltages,are rmsvoltage variations thatexceed 110percentof the
nominal voltage andlastforlessthan1 minute.Voltageswellsoccurlessfrequentlythanvoltage sags.
Single line togroundfaultscausesvoltage swells.
Examplesof single-linetogroundfaultsinclude lightningoratree strikinga live conductor.The
increasedenergyfromavoltage swell oftenoverheatsequipmentand reducesitslife.
Overvoltage
An overvoltage isan eventwhere the rmsvoltage risesabove 110% of the nominal rmsvoltage and
staysthere for more than one minute. Long-duration overvoltage are close cousinstovoltage swells,
excepttheylastlonger.
Like voltage swells,theyare rmsvoltage variations thatexceed110 percentof the nominal voltage.
Unlike swells,they lastlongerthana minute
Several typesof initiatingeventscause overvoltages.The majorcause of overvoltagesiscapacitor
switching.Thisisbecause acapacitoris a chargingdevice.Whenacapacitor isswitchedon,itadds
voltage tothe utility’ssystem.Anothercause of overvoltage isthe droppingof load. Lightload
conditionsinthe eveningalsocause overvoltagesonhighvoltage systems.
Anothercommoncause of overvoltage isthe missettingof voltage tapsontransformers.Extended
overvoltagesshortenthe lifeof lightingfilamentsandmotors.Solutionstoovervoltagesinclude
usinginductorsduringlightloadconditionsandcorrectlysettingtransformer taps.

12. Fig.1.2: Voltage magnitude vsdurationplot
Undervoltages
Undervoltagesoccurwhenthe voltage dropsbelow 90percentof the nominal voltage formore than1
minute.Theyare sometimesreferred toas“brownouts,”.Theyare recognizedbyenduserswhentheir
lightsdimandtheirmotorsslowdown.
Too much loadon the utility’ssystem, orthe lossof a major transmissionline servinga regioncancause
undervoltages.Overloadingdistributionsystemcancause undervoltages.
Undervoltagescancause sensitivecomputer equipmenttoreaddata incorrectlyandmotorsto stall and
operate inefficiently.Utilitiescanpreventundervoltagesbybuildingmore generationandtransmission
lines

13. Interruptions
Interruptionsare acomplete lossof voltage (adropto lessthan10 percentof nominal voltage) inone or
more phases.IEEE Recommended Practice forMonitoringElectricPowerQuality(IEEEStandard1159-
1995,) definesthree typesof interruptions.Theyare categorizedbythe time periodthatthe
interruptionsoccur:momentary, temporary,andlong-durationinterruptions.
Momentaryinterruptionsare the completelossof voltage onone or more phase conductorsfor a time
periodbetween0.5cycles,or 8 milliseconds, and3seconds.
A temporary,orshort-duration,interruptionisadrop of voltage below 10percentof the nominal
voltage fora time periodbetween3secondsand1 minute.
Long-duration,orsustained, interruptionslastlongerthan1 minute.
Anykindof interruptioncanresultinlossof productioninan office, retail market,orindustrial factory.
Notonlydoesthe lossof electrical service cause lostproduction,Some typesof processescannot“ride
through”evenshortinterruptions. (“Ride through”isthe capabilityof equipment tocontinue tooperate
duringa powerdisturbance).Forexample,inaplasticinjectionmoldingplant,forashortinterruptionof
0.5 secondittakes6 hoursto restore production.
The common methodsof reducingthe impactof costlyinterruptions include alternative sourcesof
electrical supply.An endusermayinstall battery-operateduninterruptiblepowersupplies(UPS) or
motor-generatorsets,whileautility mayprovide an source that includestwofeederswitha high-
speedswitchthatswitchestothe alternate feederwhenone feederfails.
Harmonics
Harmonics arethe major sourceof sinewaveform distortion.The increaseduse of nonlinearequipmenthave
causedharmonics to become more common. Ananalysisof the sine wave architecture provides an
understandingof the basicinvestigationof harmonics. Harmonicsare integral multiplesof the
fundamental frequencyof the sine wave showninFigure 2.1,.
Harmonicsare multiples of the 50-Hz fundamental voltage andcurrent.Theyaddto the fundamental
50-Hz waveformanddistortit.Theycan be 2, 3, 4, 5, 6, 7, etc., timesthe fundamental.Forexample,the
thirdharmonicis 50 Hz times 3, or 150 Hz, and the sixthharmonicis 50 Hz times6,or 300 Hz.

14. Figure 2.1 -
Causesof harmoniccurrents - Theyare usuallycausedbynonlinear loads,like adjustable speeddrives,
solid-state heatingcontrols,electronicballastsforfluorescentlighting,switched-mode powersuppliesin
computers,staticUPS systems,electronicandmedical testequipment, rectifiers,filters,andelectronic
office machines.
Nonlinearloadscause harmoniccurrentstochange froma sinusoidal currenttoa nonsinusoidal
currentby drawingshortburstsof currenteach cycle or interrupting the currentduringacycle.This
causesthe sinusoidal current waveformtobecome distorted.The total distortedwaveshape is
cumulative.
The resultingnonsinusoidal wave shapewillbe acombinationof the fundamental50-Hzsine wave and
the variousharmonics.
Voltage fluctuations
Voltage fluctuationsare rapidchangesinvoltage withinthe allowablelimitsof voltage magnitude of
0.95 to 1.05 of nominal voltage.
Deviceslike electricarc furnacesandweldersthathave continuous,rapidchangesinloadcurrentcause
voltage fluctuations.Voltage fluctuationscancause incandescentandfluorescentlightstoblinkrapidly.
Thisblinkingof lightsisoftenreferredtoas“flicker.”Thischange inlightintensityoccursatfrequencies
of 6 to 8 Hz and isvisible tothe humaneye.Itcan cause people tohave headachesandbecome stressed
and irritable.Itcanalsocause sensitiveequipmenttomalfunction.Inthe case of an arc furnace,this
usuallyinvolvesthe use of costlybuteffectivestaticVARcontrollers(SVCs)thatcontrol the voltage
fluctuationf bycontrollingthe amountof reactive powerbeingsuppliedtothe arc furnace.
Figure 2 showsvoltage fluctuationsthatproduce flicker.

15. Figure 2. Voltage fluctuation(flicker) plot.
Loads That Cause Power Quality Problems
Most of the electrical loadshave nonlinearbehavioratthe AC mains.Astheydraw harmonic currents(
varioustypessuchas characteristicharmonics,noncharacteristicharmonics,interharmonics,
subharmonics,reactive powercomponentof current,fluctuatingcurrent,unbalancedcurrents ) from
the AC mains,these loadsare knownasnonlinearloads.
Majorityof rotatingelectricmachinesandmagnetic devicessuchastransformers,reactors,chokes,
magneticballasts,andsoon behave asnonlinearloadsdue tosaturationintheirmagneticcircuits,
geometrysuchas presence of teethandslots,windingdistribution, airgapasymmetry,andsoon.

16. Many fluctuatingloadssuchas furnaces,electrichammers,andfrequently switchingdevicesexhibit
highlynonlinearbehavioraselectrical loads.
Evennonsaturatingelectrical loadssuchaspowercapacitorsbehave asnonlinearloadsatthe AC mains
and theycreate a numberof powerqualityproblemsdue toswitchingandresonance withmagnetic
componentsinthe systemandare overloadeddue toharmoniccurrentscausedbythe presence of
harmonicvoltagesinthe supplysystem.
Moreover,the solid-statecontrol of ACpowerusingdiodes,thyristors,andothersemiconductor
switchesiswidelyusedtofeedcontrolledpowertoelectrical loadssuchaslightingdeviceswith
electronicballasts,controlledheatingelements,magnetpowersupplies,batterychargers,fans,
computers,copiers, TVs,switchedmode powersupplies(SMPS)incomputersandotherequipments,
furnaces,electroplating, electrochemical processes,adjustablespeeddrives(ASDs) inelectrictraction,
air-conditioningsystems, pumps,wastewatertreatmentplants,elevators,conveyers,cranes,andsoon.
These ACloadsconsisting of solid-state convertersdraw nonsinusoidal currentsfromthe ACmainsand
behave ina nonlinearmannerandtherefore theyare alsoknownasnonlinearloads.
These nonlinearloadsconsistingof solid state convertersdraw harmoniccurrentsandreactive power
componentof currentfromthe ACmains.
In three-phasesystems,theycouldalsocause unbalance andsometimesdraw excessiveneutral current,
especiallythe distributedsingle-phasenonlinearloadsonthree-phase four-wire supplysystem.These
solid-state convertersmaybe AC–DCconverters,ACvoltage controllers,cycloconverters,andsoon.
The injectedharmoniccurrents,reactive powerburden,unbalancedcurrents,andexcessiveneutral
currentcausedby these nonlinearloadsresultinlow systemefficiency,poorpowerfactor(PF),
maloperationof protectionsystems,ACcapacitorsoverloadingandnuisance tripping,noiseand
vibrationin electrical machines,heatingof the rotorbarsdue to negative sequence currents,deratingof
components of distributionsystem,userequipment,andsoon.
Theyalsocause distortioninthe supplyvoltage, disturbance toprotectivedevicesandotherconsumers,
and interference innearbycommunication networksanddigitalandanalogcontrol systems.
Power quality problems in distribution network
From the consumer'sperspective,powersystemfaults canbe dividedintofourcategories:sags,swells,
interruptionsandoutages.Sagsandswellsoccurduringtemporaryfaultsonthe distributionsystem
whichare notin the directpathsupplyingthe load.Interruptionsoccurwhen abreakeror recloser
whichisin the directpathfrom source to customer.interruptsatemporaryfaultand successfully
recloses.Outagesoccurwhenpermanent faultsdevelopinthe directpathfeedingthe customer.
Owingtothe nature of the overheaddistributionsystem, temporaryfaultsare more commonthan
permanentfaults,andmanyconsumerswillhave more exposure to remote faultsthantodirectfaults.
Therefore,the numberof sags,swellsandinterruptionswouldtypicallybe greaterthanthe numberof
outages.
Owingtotheirmore severe nature,however,outageshave historically receivedgreaterattentionthan
the otherpowerquality The effectof systemfaultsonsags,swells,andinterruptions,however,has
receivedlessattention.Whilesagsinparticularare more numerousthanoutages,theireffect has
historicallybeenof lesserconsequence formanyloads andformanyconsumers.
Recently,however,sagshave become increasinglyimportant,fortworeasons.First, mostmodern
electronicloadsare sensitive tothe voltage sagsandinterruptionswhichoccurdue tothese faults.
Secondly,increasinglevelsof industrial automationhas ledtoa significantnumberof installations
where the disruptioncausedbythese faultshasa cascading effectandhighcosts.

17. The electricpowerdistributionsystemwillconsistof one ormore sourcesof energy,one ormore
substation transformers,andacombinationof overheadandunder-groundlines.The mostcommon
cause of majorpowerdisturbanceson these systemsare fromfaultsonthe overheadlines.
Since the qualityof electrical power,e.g.the voltage atthe pointof commoncoupling,hasbecome an
importantfeature of consumergoodsonthe m k e t , the interestonfinding, describingand above all in
forecastingsystembehaviorgrows continuously.Additionally,anextensiveuse of power
electronicloads,especiallyindistributionnetworks,introduces new inconveniencestopropersystem
operationanddemands ananalytical methodwhichshouldforecastseriouspowerqualityproblems
before theyoccur.
Importance of Power Quality
Withthe introductionandwidespreaduse of sensitive electronicequipment,energyusershave become
much more aware and sensitivetotransientsandotherpoweranomalies.Previously,equipmentwas
fairlyimmune toshort-termpowerfluctuationsanddidnotprojectproblemsbackintothe utility’s
system.Now,withthe introductionof nonlineardevices,harmonicsare createdwhichcanaffectthe
customer’sequipmentandthe utility’sequipment.The utilityisnolongerjustprovidingpowertoturn
lightsonand start motors.Asa result,there hasbeenanincrease of problemsexperiencedbyelectrical
end-users.Todetermine the source of these issues,there are availablemeansforcustomers,utilities,
and consultantstoeasilymonitor,record,andanalyze the electricpower.Basedonthe source of the
problem,powertreatmentmethodsare available tocure orlimitthe problems.
Good powerqualitysavesmoneyand energy.Directsavingstoconsumerscome fromlowerenergycost
and reactive powertariffs.Indirectsavingsare gainedbyavoidingcircumstancessuchasdamage and
premature agingof equipment,lossof productionorlossof data andwork. Powerqualitycanaffectthe
overall companyperformance,whichisafact easilyoverlookedbythe management.
Evaluation of power quality
A complete powerqualityevaluationisanothertool thatcan be usedto maintainasafe and reliable
electrical distributionsystem.Powerqualityconcernsincrease asthe systemdesignandprocesses
become more complicated.Inanefforttoreduce energycosts, utilities canmanage energyusage
throughloadsheddingandpeakshavingoptions.However,because of these energymanagement
options,many utilities are beingfacedwithmore powerqualityissuesthanbefore.
The increasingnumberof harmonicproducingloadscandramaticallyaffectthe qualityof poweronthe
system.The operationof switch-mode powersupplies andcomputersare prime examplesof products

18. and activitiesthatcancreate the harmoniccurrents thatflow throughoutthe circuitsina powersystem.
Othersourcesof harmonicsinclude variable speeddrives(VSD),uninterruptiblepowersupplies(UPS
systems),andemergency/standbygenerators.
A complete powerqualityevaluationwill generallyinvolveone ormore of the followingpowersystems
studies:aharmonicanalysisstudy;groundinganalysis;voltageflicker;and/ortransientvoltage surge
suppression.
A harmonicanalysisstudywill accuratelydeterminethe sourcesandmagnitudesof harmoniccurrents
and voltagesthatare presentinthe electrical powersystem.Up-to-datemeasurementsare essential in
verifyingharmonicgenerationfromall significantharmonicsources.Harmonicmeasurements
demonstrate the effectof systemresonance causedbypowersystemconfigurationandcharacteristics.
Powersystemengineersuse the recordedmeasurementdatainthe analytical modelingof the system.
Followingthe study,itis imperativetoannuallymaintain,test,andupdate all equipmentwithinthe one-
line diagram.
Effects of harmonics on Electrical Equipment
1.Conductors
There are twomechanismsinwhichharmonic currentscancause heatinginconductorsthat is greater
than expectedforthe rmsvalue of the current.The first mechanismisdue tocurrent redistribution
withinthe conductorandinclude the skineffectandthe proximity effect.The skineffectisdue tothe
shieldingof the innerportionof the conductorby the outer layer.Since the currentisconcentratedin
the outerlayer,the effectiveresistance of the conductorisincreased.
Skineffectincreaseswithfrequencyandconductordiameter. The proximityeffectisdue tothe
magneticfieldof conductorsdistortingthe currentdistributionin adjacentconductors
The secondmechanismcausesabnormallyhigh currentsonthe neutral conductorof 3-phase 4-wire
distributionsystemsfeedingsingle phaseloads.Some loads,suchasswitched-mode powersupplies,
produce significantthirdharmoniccurrents.Balanced fundamentalfrequencythree-phase currentswill
resultinno neutral current.However,inthree-phase circuits, thirdharmoniccurrentsaddratherthan
cancel in the neutral andcan be as muchas 1.7 times the phase currentfor converterloads.Since the
neutral conductorisusuallysizedthe same asthe phase conductors,the neutral conductorcan be
overloaded.The problemis mostlikelytooccur incommercial buildingswhere a three-phase
distributionsystemfeedslarge single-phase electronicoffice equipmentloads.The mostcommonfix is
to size the neutral conductorto be at leasttwice the phase conductorampacity
2. ElectronicEquipment
There are several mechanismsbywhichharmonic distortion affectselectronicequipment.Itiscommon
for electroniccircuitstouse the voltage zerocrossingof the fundamental powerfrequencyfortiming
purposes.
However,harmonicdistortionthatcausesmore frequent zerocrossingsthanthe fundamental
frequencycandisruptoperationof the equipment.Anydevice thatsynchronizesto
the zero crossingshouldbe consideredvulnerable to disruptionbyharmonicdistortion.
Semiconductorsare oftenswitchedatzerovoltage crossingtoreduce electromagneticinterference and
inrushcurrent.Multiple crossingscanchange the switchingtimesof the device anddisruptoperationof
the equipment.

19. 3. Rotational machines
Nonsinusoidal voltagesappliedtoelectricmachines maycause overheating,pulsatingtorques,ornoise.
In additiontoacrossthe line applications,adjustable speed drivemotorsare fedfrominvertersthatcan
produce significantvoltagedistortion.
Rotor overheatinghasbeenthe mainproblem associatedwithvoltagedistortion .Lossesin electric
machinesare dependentuponthe frequency spectrumof the appliedvoltage.Core andstraylosses
may become significantinaninductionmotorwitha skewedrotorsuppliedfromaninverterproducing
highharmonicfrequencies.
An increase in motor operating temperature will cause reduction of the motor operating life. Single
phase motors are the most affected. The temperature rise is not uniform throughout the motor; hot
spots appear near the conductors within the iron core portions. If the harmonics are time varying, the
motor can tolerate higher peak distortion levels without increasing the hot spot temperature This is
possible because the motor thermal time constant is much longer than the period of the harmonic
variation.
4.Transformers
The primaryeffectof harmonicson transformersisthe additional heatgeneratedbythe lossescaused
by the harmoniccontentof the load current.Otherproblemsincludepossibleresonance betweenthe
transformerinductance andsystem capacitance,mechanical insulationstresses(winding and
lamination) due totemperature cyclingandpossible small core vibrations.
The additional heatingcausedbysystemharmonics requiresloadcapabilityderatingtoremainwithin
the temperature ratingof the transformer oruse of specialitytransformersdesignedfornonsinusoidal
loadcurrents.Transformerlife willbe reducedasthe result of operatingabove ratedtemperatures.
The primarylosscomponentsare windingI2R losses,windingeddy-currentlossesandstraylosses
fromelectromagneticflux inareassuchas windings, core,clampassembliesandtanks.The lossesdue to
the I2
R componentwill be due toconductorheatingandthe skineffect.Lossesfromthe windingeddy-
currentwill increase withthe square of the loadcurrentandthe square of the frequency.Otherstray
losseswill also increasewithfrequencyalthoughatapowerslightlyless thantwo
5.CircuitBreakersandFuses
There issome evidence thatharmonicdistortionof the currentcan affectthe interruptioncapabilityof
circuitbreakers.Loadcurrent can be distortedandlow level faultsmaycontainhighpercentagesof
distorted loadcurrent.Highlevel faultcurrentswill notbe influencedbydistortedloadcurrents.When
loaddistortionispresent, itcanresultinhigherdi/dtatzero crossingthan for a sinusoidal waveform
makinginterruptionmore difficult.
6.Protective relay
Waveformdistortiondoesaffectthe performance of protective relaysandmaycause relaystooperate
improperlyortonot operate whenrequired.Inmost cases,the waveformdistortionof the loadcurrent
has little effectonthe faultcurrent.However,forlow magnitudefaults,the loadmayconsistof a large
part of the loadcurrentand distortioncanbecome a significant factor.Furthermore,the relaymust
functionproperly evenwithdistortedloadcurrents.
Relaysof the same type and model from one manufacturermayevenresponddifferentlytothe same
distortion.Distortionmaycause a relaytofail to tripunderfaultconditions,oritmay cause nuisance
trippingwhennofaultexists.Varyingthe phase angle betweenthe fundamental andharmonic
componentsof a voltage orcurrentwaveformmaysignificantlyaltera relay'sresponse.

20. Harmonicdistortionaffects the currentsensingabilityof thermal magneticbreakers.
Because fusesare thermallyactuated,theyare inherentrmsovercurrentdevices.The linkinsome
utilitydistributionfusesconsistsof severalribbonsthat are susceptible toskineffectheatingby
harmoniccurrents.
7.Adjustable speeddrive(ASD)
ASDsare electronicconvertersthatpermitacor dc motor operationatvariable speed.is
vulnerable toavarietyof disturbances.isvulnerable toharmonic voltage distortioninamannersimilar
to electronicequipment
8.Lighting
The incandescentlampwill have adefinite lossof life whenoperatedwithdistortedvoltagebecause
lampsare sensitive tooperatingvoltagelevel.If the operating rmsvoltage isabove the ratedvoltage
due to harmonicdistortion, the elevatedfilamenttemperature will reduce lamplife.Asidefromaudible
noise,there isnoknowneffectof harmonicvoltage distortionondischarge lighting.
9.Meters
Modernrms respondingvoltmetersandanimeters are relativelyimmune tothe influencesof waveform
distortion.Insuchmeters,the inputvoltage orcurrent isprocessedusinganelectronicmultiplier.
Commonly usedmultipliertechniquesare variable transconductance,log/antilog,time division,
thermal, anddigital sampling.All of these techniquescanbe configuredtorespondtothe rmsvalue of
the voltage or current,independentof the harmonicamplitudeor phase,aslongas the harmonicsare
withinthe operatingbandwidthof the instrument
Adverse Effects Of Harmonics On Electrical Equipment
In thisarticle we will discussthe adverseeffectsof harmonicsonelectricalequipment.The harmonics
deterioratesthe operationallifeof the electrical equipment.The harmonicscausesthe followings
adverse effectsonelectrical equipment.
1. Overheating oftransformers and rotating equipment
Motors and generatorscanbe adverselyaffectedbythe presence of harmonicvoltage andcurrentdue
to increasedheatingcausedbyironandcopperloss.Inadditiontothisharmoniccurrentcan increase an
audible noise emissionandreduce machineefficiency.
All these effectscombinedtogethertoincrease energyconsumption,andreduce machine life
considerably.Ironlossof amachine forthe fifthharmonicscanbe calculatedwiththe following formula.
Iron Loss KW5 = KW1 X (V5/ V1)2 X (f5 / f1)2
Iron lossesincrease asharmonicfrequencyincreases.

21. Calculation of Iron loss
Example :
An inductionmotorof rating275 HP,415 V, 3 Ph, 50 Hz is fedbya 1000 KVA,11/0.433 KV transformer.
No loadlossat 50 Hz is 3 KW. In the harmonicvoltage spectrumthe individual harmonicdistortion
valuesare as follows:V5= 7 % V7 = 6 % V11 = 4.2 % Calculationof ironlossat5th harmonic
KW5 = KW1 X (V5/ V1)2 X (f5 / f1)2
= 3 X (0.07) 2X (5)2
= 0.3675 KW
Similarly,the lossesforotherorderof harmonicfrequenciescanbe calculated.
KW7 = KW1 X (V7/ V1)2 X (f7 / f1)2
= 3 X (0.06)2 X (7)2
= 0.52 KW
KW11 = KW1 X (V11/ V1)2 X (f11 / f1)2
= 3 X (0.042) 2X (11)2
= 0.64 KW
The total iron loss is equal to the sum of iron losses due to the fundamental and harmonic
voltages.
KW total = 3 + 1.5367
= 4.5367 KW
Increase in iron loss due to presence of harmonics = (4.5367 – 3) /3 X 100 %
= 51.23 %
In inductionmotoradditionallossesoccurbecause of harmonicgeneratedfields.Eachharmonic hasa
sequence +, -,and 0 sequence whichindicatesthe directionof rotationthatwouldresultif itwere tobe
appliedtoan inductionmotorwithrespecttothe fundamental.
Thirdand multiplesof thirdproduce astationaryfield,butsince the harmonicfieldfrequenciesare
higherthe magneticlossesare greatlyincreasedandthe harmonicenergyisdissipatedasheat.
Negative sequence harmonicsresultinacounter-rotatingfield(withrespecttofundamental)which
causesreducedtorque.Positive sequence harmonicproducedforwardrotatingfieldthataddsto
torque.Due to the interactionof positiveandnegative sequence harmoniccomponents motorvibrates
and reducesthe service lifeof the motor.
2. Over heating ofneutral conductor
Under balancedloadconditionswithoutharmonics,the phase currentscancel eachotherinneutral and
the resultantneutral currentiszero.However,ina4 wire systemwithsingle-phasenonlinearloads,

22. odd-numberedmultiplesof the thirdharmonics( 3,6,9, etc) do not cancel,ratheradd togetherinthe
neutral conductor.
In a systemwitha substantial amountof nonlinearsingle-phase loads,the neutral currentmayrise toa
dangerouslyhighlevel.There isapossibilityof excessive heatingof the neutral conductorsince there
are no circuitbreakersinthe neutral conductorslike inthe phase conductors. Itisimportantto take
care of the size of the neutral conductorif harmonicsare prevalentinthe system.A recentcase study
foundthat the neutral currentsas150 Ampwhile the phase currentwere only100 amps.The neutral
sizingthusbecomesverycritical.
3. Nuisance Tripping ofCircuit Breakers and Blowing offuses
Several protectiverelaysseethe neutral currentandact accordingly.Since the neutral currentincreases
due to harmonicsuch relaymalfunctions.
Similarly,relaysthatsee crestvoltage/currentorvoltage zerofortheiroperationare affectedby
harmonicdistortion.Due tothe resonance effect,the currentlevelsmayrise tomanifoldlevelswhich
resultintrippingof circuitbreakersandmeltingfuses.Thissituationresultsinseriousproblemsin
industriesthatrelyonthe qualityof powerforthe continuousoperationof theirsensitiveprocesses.
4. Overstressing OfPower factor Correction Capacitors
The impedance of a capacitoris inverselyproportional to frequency sothe impedance toharmonic
frequencyisvery lowandthe capacitortendsto hog the harmoniccurrent.Thiscausesundue heating
and reducesthe service lifeof the capacitor.
The secondproblemisthatthe capacitoralongwithline andtransformerinductance canresonate at
nearor one of the harmonicfrequenciesresultinginaveryhighcurrent.In sucha case,the capacitor
will actas a harmoniccurrentamplifier.
5. HigherI2R Loss
The resistance of conductorsincreasesathigherharmonicfrequenciesdue tothe skineffect.Due tothe
phenomenon of skineffectall current-carryingconductorsexhibithigherI2Rloss.Further,due tothe
presence of harmonics,the RMScurrent getsincreaseswhichresultinafurtherincrease inI2Rlosses.
6. Overloading/decrease oflife oftransformers
Transformers are designedtodeliverpoweratnetworkfrequency(50Hz).The iron lossesare composed
of the eddycurrentloss(Whichincreaseswiththe square of the frequency) andhysteresislosses(which
increase linearlywiththe frequency).Withincreasingfrequenciesthe lossesalsoincrease,causing
additional heatinginthe transformer.
7. Losses in distribution equipment
Harmonicsinadditiontothe fundamental currentcause additional lossesinthe cable,fuses,andalsoin
the bus bars.
8. Malfunctioningof the electronic control and computers

23. Electroniccontrolsandcomputersrelyonpowerqualityfortheirreliable operation.Harmonicsresultin
a distortedwaveform, neutral currents,andvoltage whichaffectthe performanceof these gadgets.Due
to excessivecurrentinthe neutral conductorvoltage betweenneutral andgroundrisesabove 3volt.In
thisconditionreliabilityof electronicequipmentisquestionable.
9. Measurement error in the metering systems
The accuracy of the meteringsystemisaffectedby the presence of harmonics.Watt-hourmeters
accuratelyregisterthe directionof the powerflow atharmonicfrequencies,buttheyhave magnitude
error whichincreaseswithfrequency.The accuracyof demandmetersandVARmetersisevenlessin
the presence of harmonics.The solutionlieswiththe use of True RMS meters.
10. Zero crossing noise
In orderto reduce the generationof transientsandEMIwhenonthe inductive loadsmanyelectronic
controllersdetectthe pointsatwhichsupplyvoltagecrossesthe zeropoint.Due tothe presence of
harmonic,the rate of change of voltage atzerocrossingbecomesveryfastand difficulttoidentify,
leadingtothe erraticoperation.
11. Electrostatic interference with communication circuits
Higher-orderharmonics frequencyinterface withneighboringcommunicationcircuitsanditaffectsthe
performance of the communicationsystem.
12. Resonance
The resonance betweenthe inductance of the transformerwindingandthe capacitance of the Feederto
whichtheyare connected.There are twotypesof resonance.
 SeriesResonance
 Parallel Resonance
Series resonance :
Seriesresonantcircuitisformedbyaseriesconnectionof inductive andcapacitive loads.The reactance
of the inductorisproportional tothe frequency.The reactance of the capacitorisinverselyproportional
to the frequencywhichcanbe shownas below:
It isseenthat at resonantfrequencythe impedance reducestoaminimal value.Atthe resonant
frequency,the impedance isverylowresultinahighcurrent.The primaryside of the transformeralong
withthe capacitor onthe LV side acts as a seriesresonatingcircuitandprovidesalow impedance path
for harmonicsclose toresonatingfrequency.
Thus the circuitoffersverylowimpedance atthe inputsignal atthisfrequencywhichresultsina
multiple-foldincrease inthe current.The voltage droponthe individual componentincrease moving
closerto the resonantfrequency.

24. Parallel resonance:
The LV side of the transformeralongwiththe powerfactorcorrectioncapacitorbehavesasa parallel
resonatingcircuitatthe resonatingfrequencythe impedance offeredisveryhighconsequentlythe
harmoniccurrentcausesan increasedharmonicdropwhichmaybe accompaniedbydistortionof the
fundamental.Transformersandcapacitorsare additionallyloaded.
UNIT2
Sources of sags and interruptions , Estimatingvoltage sagperformance , fundamental principleof protection
,solution atthe end user level, evaluatingthe economics of different ride through al ternatives ,motor startingsags
utility systems faultclearingissues ,sources of transientovervoltage , utility capacitor switching,transient
problems with loads.
Sources of sags and interruptions:
Voltage sags and interruptions aregenerally caused by faults (short circuits) on the utility system.4 Consider a
customer that is supplied fromthe feeder supplied by circuitbreaker 1 on the diagramshown in Fig. .1. If there is a
faulton the same feeder, the customer will experiencea voltage sagduringthe faultfollowed by an interruption
when the breaker opens to clear the fault. If the faultis temporary in nature, a reclosingoperation on the breaker
should be successful and theinterruption will only be temporary. It will usually requireabout5 or 6 cycles for the
breaker to operate, duringwhich time a voltage sagoccurs.The breaker will remain open for typically a minimum
of 12 cycles up to 5 s depending on utility reclosingpractices. Sensitiveequipment will almostsurely trip during
this interruption.

25. Fig. 1
A much more common event would be a faulton one of the other feeders from the substation,i.e., a faulton a
parallel feeder, or a faultsome-where on the transmission system(see the faultlocations shown in Fig. 1). In either
of these cases,the customer will experiencea voltage sag duringthe period that the faultis actually on the system.
As soon as breakers open to clear the fault,normal voltage will berestored at the customer.
Note that to clear the faultshown on the transmission system,both breakers A and B must operate. Transmission
breakers will typically clear a faultin 5 or 6 cycles.
In this casethere are two lines supplying thedistribution substation and only onehas a fault. Therefore,
customers supplied fromthe substation should expect to see only a sagand not an interruption.
The distribution faulton feeder 4 may be cleared either by the lateral fuseor the breaker, depending on the
utility’s fuse-savingpractice.Any of these faultlocationscan causeequipment to misoperate in customer facilities.
The relativeimportance of faults on the transmission systemand the distribution systemwill depend on the
specific characteristics of the systems (underground versus overhead distribution,lightningflash densities,
overhead exposure, etc.) and the sensitivity of the equipment to voltage sags.Figure.2 shows an example of
the breakdown of the events that caused equipment misoperation for one industrial customer.Note that faults on
the customer feeder only accounted for 23 percent of the events that resulted in equipment
misoperation.This illustrates the importanceof understandingthe voltage sagperformance of the system and the
equipment sensitivity to these events.
Figure.2 shows an example of the breakdown of the events that caused equipment misoperation for one industrial
customer

26. Estimating voltage sag performance
1. Introduction:
It is importantto understand the expected voltage sagperformance of the supply system so that facilities can be
designed and equipment specificationsdeveloped to assurethe optimum operation of production facilities.The
followingis a general procedure for workingwith industrial customers to assurecompatibility between the supply
system characteristicsand the facility operation:
 Determine the number and characteristics of voltagesags thatresultfrom transmission systemfaults.
 Determine the number and characteristics of voltagesags thatresultfrom distribution systemfaults (for
facilities thataresupplied fromdistribution systems).
 Determine the equipment sensitivity to voltage sags.This will determine the actual performance of the
production process based on voltage sagperformance calculated in steps 1 and 2.
 Evaluate the economics of different solutions thatcould improve the performance, either on the supply
system or within the customer facility.
When a line-to-ground faultoccurs,there will be voltage saguntil the protective switch gear operates.
Some accidents in power lines such as lightningor fallingan objectcan be a causeof line-to-ground faultand
voltage sagas a result.
Sudden load changes or excessiveloads can causevoltagesag.
Depending on the transformer connections,transformers energizingcould be another reason for happening
voltage sags.
Voltage sags can arrivefromthe utility butmost arecaused by in-buildingequipment. In residential homes,we
usually seevoltagesags when the refrigerator,air-conditioner or furnacefan starts up.
Area of vulnerability
The concept of an area of vulnerability has been developed to help evaluate the likelihood of sensitive
equipment being subjected to voltage lower than its minimum voltage sag ride-through capability.5 The
latter term is defined as the minimum voltage magnitude a piece of equipment can withstand or tolerate
without misoperation or failure. This is also known as the equipment voltage sag immunity or
susceptibility limit. An area of vulnerability is determined by the total circuit miles of exposure to faults
that can cause voltage magnitudes at an end-user facility to drop below the equipment minimum voltage
sag ride-through capability. Figure 2.5 shows an example of an area of vulnerability diagram for motor
contactor and adjustable-speed-drive loads at an end-user facility served from the distribution system. The
loads will be subject to faults on both the transmission system and the distribution system.

27. Equipment sensitivity to voltage sags
Equipment within an end-user facility may havedifferent sensitivity to voltage sags.Equipment sensitivity to
voltage sags is very dependent on the specific load type, control settings,and applications.
Consequently, itis often difficultto identify which characteristicsof a given voltage sagare most likely to cause
equipment to misoperate.
The most commonly used characteristicsarethe duration and magnitude of the sag. Other less commonly used
characteristicsinclude
phaseshiftand unbalance,missingvoltage,three-phase voltageunbalanceduringthe sagevent, and the point-in-
the-wave at which the sag initiates and terminates.Generally, equipment sensitivity to voltage
sags can be divided into three categories:
■ Equipment sensitiveto only the magnitude of a voltage sag.This group includes devices such as undervoltage
relays,process controls, motor drivecontrols,6 and many types of automated machines (e.g., semiconductor
manufacturingequipment). Devices in this group are sensitiveto the minimum (or maximum) voltage magnitude
experienced duringa sag(or swell).The duration of the disturbanceis usually of secondary importancefor these
devices.
■ Equipment sensitiveto both the magnitude and duration of a voltage sag.This group includes virtually all
equipment that uses electronic power supplies.Such equipment misoperates or fails when the power supply
output voltage drops below specified values.Thus,the important characteristic for this type of equipment is the
duration that the rms voltage is belowa specified threshold atwhich the equipment trips.
■ Equipment sensitiveto characteristicsother than magnitude and duration.Some devices are affected by other
sagcharacteristics such as thephaseunbalanceduringthe sagevent, the point-in-the- wave at which the sagis
initiated,or any transientoscillationsoccurringduringthedisturbance.These characteristics aremoresubtle
than magnitude and duration,and their impacts aremuch more difficultto generalize. As a result,the rms
variation performanceindices defined here arefocused on the more common magnitude and
duration characteristics.
For end users with sensitiveprocesses,the voltage sagride-through capability is usually the most important
characteristic to consider Theseloads can generally beimpacted by very shortduration events,
and virtually all voltagesagconditionslastatleast4 or 5 cycles (unless the faultis cleared by a current-limiting
fuse). Thus, one of the most common methods to quantify equipment susceptibility to voltage sags
is usinga magnitude-duration plotas shown in Fig. 3. It shows the voltage sagmagnitude that will cause
equipment to misoperate as a function of the sagduration.

28. Figure 3. Typical equipment voltage sag ride-through capability curves.
The curve labeled CBEMA represents typical equipment sensitivity characteristics.Thecurve was developed by the
CBEMA and was adopted in IEEE 446 (Orange Book). Typical loads will likely trip off when the voltage is below the
CBEMA, or ITI,(information Technology Industry Council (ITI), curve.
The curve labeled ASD represents an example ASD voltage sagride-through capability for a device that is very
sensitiveto voltage sags.It trips for sags below 0.9 pu that lastfor only 4 cycles.The contactor curve represents
typical contactor sag ride-through characteristics.It trips for voltagesags below 0.5 pu that lastfor more than 1
cycle.
The area of vulnerability for motor contactors shown in Fig. 3 indicates thatfaults within this area will causethe
end-user voltage to drop below 0.5 pu. Motor contactors havinga minimum voltagesag ride-through capability of
0.5 pu would have tripped out when a fault causinga voltagesagwith duration of more than 1 cycleoccurs within
the area of vulnerability.However, faults outsidethis area will notcausethe voltage to drop below 0.5 pu. The
same discussion applies to the area of vulnerability for ASD loads.The less sensitivethe equipment, the smaller the
area of vulnerability will be(and the fewer times sags will causethe equipment to misoperate).
causinga voltagesagwith duration of more than 1 cycleoccurs within the area of vulnerability.However, faults
outsidethis area will not causethe voltage to drop below 0.5 pu. The samediscussion applies to the area of
vulnerability for ASD loads.The less sensitivethe equipment, the smaller the area of vulnerability will be(and the
fewer times sags will causetheequipment to misoperate).
Transmission system sag performance evaluation
The voltage sagperformance for a given customer facility will depend on whether the customer is supplied from
the transmission systemor from the distribution system.For a customer supplied fromthe transmission system,
the voltage sagperformance will depend on only the transmission systemfaultperformance. On the other hand,
for a customer supplied fromthe distribution system,the voltage sagperformance will depend on the fault
performance on both the transmission and distribution systems.
This section discusses procedures to estimate the transmission systemcontribution to the overall voltagesag
performance at a facility.
Transmission linefaults and the subsequent opening of the protective devices rarely causean interruption for any
customer because of the interconnected nature of most modern-day transmission networks. These faults do,

29. however, causevoltage sags.Depending on the equipment sensitivity,theunit may trip off, resultingin substantial
monetary losses.The ability to estimate the expected voltage sags atan end-user location is thereforevery
important.
The area of vulnerability describes all the faultlocations thatcan causeequipment to misoperate. The type of fault
must also be considered in this analysis.Single-line-to-ground faults will notresultin the same voltage sag atthe
customer equipment as a three-phase fault.
Table 1 illustratethe factthat a single-lineto-ground faulton the primary of a delta-wye grounded transformer
does not result in zero voltage on any of the phase-to-ground or phase-to-phasevoltages on the secondary of the
transformer. The magnitude of the lowest secondary voltagedepends on how the equipment is connected:
■ Equipment connected line-to-linewould experience a minimum voltage of 33 percent.
■ Equipment connected line-to-neutral would experience a minimum voltage of 58 percent.
This illustrates theimportance of both transformer connections and the equipment connections in determining the
actual voltagethat equipment will experienceduringa faulton the supply system.
TABLE 1 TransformerSecondaryVoltageswithaSingle-Line-to-GroundFaultonthe Primary
2. Utility distribution system sag performance evaluation
Customers that are supplied atdistribution voltagelevels areimpacted by faults on both the transmission system
and the distribution system.
The analysis atthe distribution level mustalso includemomentary interruptions caused by the operation of
protective devices to clear the faults.These interruptions will mostlikely trip outsensitiveequipment. The overall
voltage sagperformance at an end-user facility is thetotal of the expected voltage sagperformance from the
transmission and distribution systems.
Figure 2 shows a typical distribution systemwith multiplefeeders and fused branches,and protective devices. The
utility protection scheme plays an importantrolein the voltagesagand momentary interruption performance. The
critical information needed to compute voltage sagperformance can be summarized as follows:
■ Number of feeders supplied fromthe substation.
■ Average feeder length.

30. ■ Average feeder reactance.
■ Short-circuitequivalentreactanceat the substation.
Figure 2 Typical distributionsystemillustratingprotectiondevices.
■ Feederreactors,if any.
■ Average feederfaultperformancewhichincludesthree-phase-lineto-ground(3LG) faultsandsingle-
line-to-ground(SLG) faultsinfaultspermile permonth.
The feederperformance datamaybe available fromprotectionlogs.However,dataforfaultsthat are
clearedbydownline fusesordownline protectivedevicesmaybe difficulttoobtainandthisinformation
may have to be estimated.There are twopossiblelocationsforfaultsonthe distributionsystems,i.e.,
on the same feederandonparallel feeders.Anareaof vulnerabilitydefiningthe total circuitmilesof
faultexposuresthatcancause voltage sagsbelow equipmentsagride-throughcapabilityataspecific
customerneedstobe defined.
The computationof the expectedvoltage sagperformancecanbe performedasfollows:
1.Faults on parallel feeders. Voltage experiencedatthe end-userfacilityfollowingafaultonparallel
feederscanbe estimatedbycalculatingthe expectedvoltage magnitudeatthe substation.The voltage
magnitude atthe substationisimpactedbythe faultimpedance andlocation,the configurationof the
powersystem,andthe systemprotectionscheme.
2.Faults on the same feeder.
In thisstepthe expectedvoltagesagmagnitude atthe end-userlocationiscomputedasafunction of
faultlocationonthe same feeder.Note that,however,the computationisperformedonlyforfault
locationsthatwill resultinasag but will notresultina momentaryinterruption,whichwill be computed
separately.Examplesof suchfaultlocations include faultsbeyondadownlinerecloserora branched
fuse thatis coordinatedtoclearbefore the substationrecloser.
Fundamental Principles of Protection
Several thingscanbe done bythe utility,enduser,andequipmentmanufacturertoreduce the number
and severityof voltage sagsandtoreduce the sensitivityof equipmenttovoltage sags.
Figure 3 illustratesvoltagesagsolutionalternativesandtheirrelativecosts.

31. As thischart indicates,itisgenerallylesscostlytotackle the problem atitslowestlevel,close tothe
load.The bestansweristo incorporate ride throughcapabilityintothe equipmentspecifications
themselves.Thisessentiallymeanskeepingproblemequipmentoutof the plant,or at leastidentifying
aheadof time powerconditioningrequirements.Several ideas,outlinedhere,couldeasilybe
incorporatedintoanycompany’sequipmentprocurementspecificationstohelpalleviate problems
associatedwithvoltage sag
1. Equipmentmanufacturersshouldhave voltage sagride-throughcapabilitycurves(similartothe ones
shownpreviously) available totheircustomerssothatan initial evaluationof the equipmentcanbe
performed.Customersshouldbegintodemandthatthese typesof curvesbe made availablesothat
theycan properlyevaluate equipment.
2. The companyprocuringnewequipmentshouldestablishaprocedure thatratesthe importance of the
equipment.If the equipmentiscritical innature,the companymustmake sure thatadequate
ride-throughcapabilityisincludedwhenthe equipmentispurchased.If the equipmentisnotimportant
or doesnot cause majordisruptionsinmanufacturingorjeopardize plantandpersonnel safety,voltage
sag protectionmaynotbe justified.
3. Equipmentshouldatleastbe able toride throughvoltage sagswitha minimumvoltage of 70 percent
(ITIcurve).The relative probabilityof experiencingavoltage sagto70 percentor lessof nominal ismuch
lessthanexperiencingasagto 90 percentor lessof nominal.A more ideal ride-throughcapabilityfor
short-durationvoltage sagswouldbe 50percent,asspecifiedbythe semiconductorindustryinStandard
SEMI F-47.17
As we entertain solutions at higher levels of available power, the solutions generally become more
costly.If the requiredride-throughcannot be obtained at the specification stage, it may be possible to
apply an uninterruptible power supply (UPS) system or some other type of power conditioning to the
machine control. This is applicable when the machines themselves can withstand the sag or
interruption, but the controls would automatically shut them down.
At level 3inFig.3., some sort of backup powersupplywiththe capability to support the load for a brief
period is required.
Level 4 represents alterations made to the utility power system to significantly reduce the number of
sags and interruptions

32. Solutions at the End-User Level
Solutions to improve the reliability and performance of a process or facility can be applied at many
different levels. The different technologies available should be evaluated based on the specific
requirementsof the process to determine the optimum solution for improving the overall voltage sag
performance.AsillustratedinFig. 3, the solutions can be discussed at the following different levels of
application:
1. Protectionforsmall loads[e.g.,lessthan5kilovoltamperes(kVA)].Thisusuallyinvolvesprotection for
equipmentcontrolsorsmall,individualmachines.Manytimes,these are single-phase loadsthatneedto
be protected.
2. Protection for individual equipment or groups of equipment up to about 300 kVA. This usually
represents applying power conditioning technologies within the facility for protection of critical
equipmentthatcanbe groupedtogetherconveniently. Since usually not all the loads in a facility need
protection,thiscanbe a veryeconomical methodof dealingwiththe critical loads,especiallyif the need
for protection of these loads is addressed at the facility design stage.
3. Protection for large groups of loads or whole facilities at the low-voltage level. Sometimes such a
large portion of the facility is critical or needs protection that it is reasonable to consider protecting
large groups of loads at a convenient location (usually the service entrance). New technologies are
available for consideration when large groups of loads need protection.
4. Protectionatthe medium-voltagelevel oronthe supplysystem.If the whole facilityneedsprotection
or improvedpowerquality,solutionsatthe medium-voltagelevel canbe considered. The size ranges in
these categoriesare quite arbitrary,andmanyof the technologiescan be applied over a wider range of
sizes.The followingsectionsdescribe the majortechnologiesavailable andthe levelswhere they can be
applied
Major technologies available and the levels where they can be applied are described as follows
1.Ferroresonant transformers
Ferroresonanttransformers,alsocalledconstant-voltagetransformers (CVTs), can handle most voltage
sag conditions. CVTsare especiallyattractiveforconstant,low-powerloads.Ferroresonanttransformers
are basically 1:1 transformers which are excited high on their saturation curves, thereby providing an
output voltage which is not significantly affected by input voltage variations. A typical ferroresonant
transformer schematic circuit diagram is shown in Fig. 4.

33. Fig. 4. ferroresonant transformer schematic
Figure 5 Voltage sag improvement with ferroresonant transformer
Figure 5 shows the voltage sag ride-through improvement of a process controller fed from a 120-VA
ferroresonanttransformer.Withthe CVT,the processcontrollercan ride through a voltage sag down to
30 percentof nominal,asopposedto82 percentwithout one.Notice how the ride-through capability is
held constant at a certain level. The reason for this is the small power requirement of the process
2. Magnetic synthesizers
Magneticsynthesizersuse asimilaroperatingprincipletoCVTsexcepttheyare three-phasedevices and
take advantage of the three-phase magnetics to provide improved voltage sag support and regulation
for three-phaseloads.They are applicable over a size range from about 15 to 200 kVA and are typically
applied for process loads of larger computer systems where voltage sags or steady-state voltage
variations are important issues.
A block diagram of the process is shown in Fig. 7. Energy transfer and line isolation are accomplished
through the use of nonlinear chokes. This eliminates problems such as line noise. The ac output
waveformsare built by combining distinct voltage pulses from saturated transformers. The waveform
energy is stored in the saturated transformers and capacitors as current and voltage. This energy
storage enables the output of a clean waveform with little harmonic distortion.

34. Fig. 7
Finally, three-phase power is supplied through a zigzag transformer.
3 Active series compensators
Advancesinpowerelectronictechnologiesand new topologies for these devices have resulted in new
options for providing voltage sag ride through support to critical loads. One of the important new
optionsisa device thatcan boostthe voltage byinjectingavoltage inseries with the remaining voltage
during a voltage sag condition. These are referred to as active series compensation devices. They are
available in size ranges from small single-phase devices (1 to 5 kVA) to very large devices that can be
applied on the medium-voltage systems (2 MVA and larger).
Figure 9 is an example of a small single-phase compensator that can be used to provide ride-through
support for single-phase loads
Figure 9 Topology illustrating the operation of the active series compensator
4 On-line UPS

35. Figure 10 shows a typical configuration of an on-line UPS
Figure 10 showsa typical configurationof an on-line UPS. In this design, the load is always fed through
the UPS. The incoming ac power is rectified into dc power, which charges a bank of batteries. This dc
poweristheninvertedbackintoac power, tofeedthe load. If the incoming ac power fails, the inverter
is fed from the batteries and continues to supply the load. In addition to providing ride-through for
power outages, an on-line UPS provides very high isolation of the critical load from all power line
disturbances. However, the on-line operation increases the losses and may be unnecessary for
protection of many loads.
5 Standby UPS A standby power supply
Figure 11 Standby UPS
It (Fig. 11) is sometimes termed off-line UPS since the normal line power is used to power the
equipmentuntil adisturbance isdetectedanda switchtransfersthe loadtothe batterybackedinverter.
The transfertime fromthe normal source to the battery-backedinverterisimportant.The CBEMA curve
shows that 8 ms is the lower limit on interruption through for power-conscious manufacturers.
Therefore a transfer time of 4 ms would ensure continuity of operation for the critical load. A standby
powersupplydoesnottypicallyprovide anytransientprotectionorvoltage regulationasdoesanon-line
UPS. This is the most common configuration for commodity UPS units available at retail stores for
protection of small computer loads. UPS specifications include kilovoltampere capacity, dynamic and
staticvoltage regulation,harmonicdistortionof the inputcurrentand output voltage, surge protection,
and noise attenuation. The specifications should indicate, or the supplier should furnish, the test
conditions under which the specifications are valid

36. 6 Hybrid UPS Similar in design to the standby UPS,
hybridUPS (Fig12) utilizesa voltage regulator on the UPS output to provide regulation to the load and
momentary ride-through when the transfer from normal to UPS supply is made
Figure 12. Hybrid UPS
7 Motor-generator sets
Motor-generator (M-G) sets come in a wide variety of sizes and configurations. This is a mature
technologythatisstill useful forisolatingcritical loadsfromsagsandinterruptionsonthe powersystem.
The concept isverysimple,asillustratedinFig.13.A motor poweredby the line drives a generator that
powers the load. Flywheels on the same shaft provide greater inertia to increase ride-through time.
Whenthe line suffersadisturbance,the inertiaof the machinesandthe flywheels maintains the power
supplyfor several seconds. This arrangement may also be used to separate sensitive loads from other
classesof disturbances such as harmonic distortion and switching transients. While simple in concept,
M-G sets have disadvantages for some types of loads:
1. There are lossesassociatedwiththe machines,althoughtheyare notnecessarilylarger than those in
other technologies described here.
2. Noise and maintenance may be issues with some installations
Fig. 13 Block diagram of typical M-G set with flywheel.
3. The frequency and voltage drop during interruptions as the machine slows. This may not work well
with some loads.

37. Another type of M-G set uses a special synchronous generator called a written-pole motor that can
produce a constant 60-Hz frequency as the machine slows. It is able to supply a constant output by
continually changing the polarity of the rotor’s field poles. Thus, each revolution can have a different
number of poles than the last one. Constant output is maintained as long as the rotor is spinning at
speeds between 3150 and 3600 revolutions per minute (rpm). Flywheel inertia allows the generator
rotor to keep rotating at speeds above 3150 rpm once power shuts off. The rotor weight typically
generates enough inertia to keep it spinning fast enough to produce 60 Hz for 15 s under full load.
Anothermeansof compensatingforthe frequencyandvoltage dropwhile energyisbeingextractedisto
rectify the output of the generator and feed it back into an inverter. This allows more energy to be
extracted, but also introduces losses and cost.
8. Flywheel energy storage systems
Motor-generatorsetsare onlyone meanstoexploitthe energystoredinflywheels. A modern flywheel
energysystemuses high-speed flywheels and power electronics to achieve sag and interruption ride-
through from 10 s to 2 min.
While M-G sets typically operate in the open and are subject to aerodynamic friction losses, these
flywheels operate in a vacuum and employ magnetic bearings to substantially reduce standby losses.
Designswithsteel rotorsmayspinatapproximately10,000 rpm, while those withcomposite rotors may
spin at much higher speeds. Since the amount of energy stored is proportional to the square of the
speed, a great amount of energy can be stored in a small space.
9 Superconducting magnetic energy storage (SMES) devices
An SMES device canbe usedto alleviate voltage sags and brief interruptions. The energy storage in an
SMES-basedsystemisprovidedbythe electricenergystoredinthe currentflowinginasuperconducting
magnet.Since the coil is lossless, the energy can be released almost instantaneously. Through voltage
regulatorandinverterbanks,thisenergycanbe injectedintothe protectedelectrical systeminlessthan
1 cycle to compensate for the missing voltage during a voltage sag event.
The SMES-based system has several advantages over battery-based UPS systems:
1. SMES-based systems have a much smaller footprint than batteries for the same energy storage and
power delivery capability.
2. The stored energy can be delivered to the protected system more quickly.
3. The SMES system has virtually unlimited discharge and charge duty cycles. The discharge and
recharge cyclescan be performedthousandsof times without any degradation to the superconducting
magnet. The recharge cycle is typically less than 90 s from full discharge.

38. Figure 14 Typical power quality–voltage regulator (PQ-VR) functional block diagram
Figure 14 shows the functional block diagram of a common system. It consists of a superconducting
magnet, voltage regulators, capacitor banks, a dc-to-dc converter, dc breakers, inverter modules,
sensing and control equipment, and a series-injection transformer. The superconducting magnet is
constructed of a niobium titanium (NbTi) conductor and is cooled to approximately 4.2 kelvin (K) by
liquid helium. The cryogenic refrigeration system is based on a two-stage recondenser. The magnet
electrical leads use high-temperature superconductor (HTS) connections to the voltage regulator and
controls. The magnet might typically store about 3 megajoules (MJ).
In the example system shown, energy released from the SMES passes through a current-to-voltage
converter to charge a 14-microfarad (mF) dc capacitor bank to 2500 Vdc. The voltage regulator keeps
the dc voltage at its nominal value and also provides protection control to the SMES. The dc-to-dc
converterreducesthe dcvoltage downto750 Vdc.The invertersubsystemmodule consistsof six single-
phase inverter bridges. Two IGBT inverter bridges rated 450 amperes (A) rms are paralleled in each
phase to provide atotal rating of 900 A perphase.The switchingscheme forthe inverterisbasedon the
pulse-widthmodulation(PWM) approachwhere the carriersignal isasine-triangle withafrequencyof 4
kHz.15 A typical SMES system can protect loads of up to 8 MVA for voltage sags as low as 0.25 pu. It can
provide up to 10 s of voltage sag ridethrough depending on load size. Figure 3.27 shows an example
where the gridvoltage experiencesavoltage sagof 0.6 pu forapproximately7cycles.The voltage at the
protected load remains virtually unchanged at its prefault value
10 Static transfer switches and fast transfer switches
There are a number of alternatives for protection of an entire facility that may be sensitive to voltage
sags.
Anotheralternativethat can be applied at either the low-voltage level or the medium-voltage level is
the automatictransferswitch.Automatictransferswitchescanbe of varioustechnologies, ranging from
conventional breakers to static switches. Conventional transfer switches will switch from the primary
supply to a backup supply in seconds. Fast transfer switches that use vacuum breaker technology are

39. available thatcantransferinabout 2 electrical cycles.Thiscanbe fastenoughto protect many sensitive
loads.Staticswitchesuse powerelectronicswitchestoaccomplishthe transferwithinaboutaquarterof
an electrical cycle. The transfer switch configuration is shown in Fig. 15.
Figure 15- Configuration of a static transfer switch used to switch between a primary supply and a
backup supply in the event of a disturbance.
Evaluating the Economics of Different Ride-Through Alternatives
The economic evaluation procedure to find the best option for improving voltage sag performance
consists of the following steps:
1. Characterize the system power quality performance.
2. Estimate the costs associated with the power quality variations.
3. Characterize the solution alternatives in terms of costs and effectiveness.
4. Perform the comparative economic analysis.
we have outlined the major technologies that can be used to improve the performance of the facility.
Now, we will focus on evaluating the economics of the different options
1 Estimating the costs for the voltage sag events
The costs associatedwithsageventscanvary significantly per event. The cost will vary not only among
different industry types and individual facilities but also with market conditions. Higher costs are
typicallyexperiencedif the endproductisinshortsupplyand there is limited ability to make up for the
lost production. Not all costs are easily quantified or truly reflect the urgency of avoiding the
consequencesof avoltage sagevent.The costof a powerqualitydisturbance can be captured primarily
through three major categories: ■ Product-related losses, such as loss of product and materials, lost
production capacity, disposal charges, and increased inventory requirements.

40. ■ Labor-related losses, such as idled employees, overtime, cleanup, and repair.
■ Ancillary costs such as damaged equipment, lost opportunity cost, and penalties due to shipping
delays.
Costs will typically vary with the severity (both magnitude and duration) of the power quality
disturbance.. This relationship can often be defined by a matrix of weighting factors. The weighting
factors are developed using the cost of a momentary interruption as the base
Voltage sagsandotherpowerquality variations will always have an impact that is some portion of this
total shutdown. If a voltage sag to 40 percent causes 80 percent of the economic impact that a
momentaryinterruptioncauses,thenthe weightingfactorfora 40 percentsag wouldbe 0.8. Similarly,if
a sag to 75 percent only results in 10 percent of the costs that an interruption causes, then the
weighting factor is 0.1.
Table 3 providesan example of weighting factors that were used for one investigation. The weighting
factors can be furtherexpandedtodifferentiatebetweensags that affect all three phases and sags that
only affect one or two phases
TABLE 3 Example of Weighting Factors for Different Voltage Sag Magnitude
TABLE 4 Example Costs for Different Types of Power Quality Improvement Technologies
More commonly,the solutionwouldbe implemented in the facility and either a dynamic sag corrector
or flywheel-basedstandbypowersupplymight make sense for protecting the 2 MW of sensitive loads.

41. In this case, protecting just the controls with CVTs does not provide the best solution because the
machines themselves are sensitive to voltage sags.
Motor-Starting Sags
Motors have the undesirable effect of drawing several times their full load current while starting. This
large current will, by flowing through system impedances, cause a voltage sag which may dim lights,
cause contactors to drop out, and disrupt sensitive equipment. The situation is made worse by an
extremely poor starting displacement factor—usually in the range of 15 to 30 percent. The time
required for the motor to accelerate to rated speed increases with the magnitude of the sag, and an
excessive sag may prevent the motor from starting successfully.
Motor starting sags can persist for many seconds, as illustrated in Fig. 16
Fig. 16Typical motor-starting voltage sag.
Motor-starting methods
Energizingthe motorina single step(full-voltagestarting) provides low cost and allows the most rapid
acceleration. It is the preferred method unless the resulting voltage sag or mechanical stress is
excessive.Autotransformerstartershave twoautotransformersconnectedinopendelta.Tapsprovide a
motor voltage of 80, 65, or 50 percent of system voltage during start-up. Line current and starting
torque vary withthe square of the voltage applied to the motor, so the 50 percent tap will deliver only
25 percentof the full-voltagestartingcurrentandtorque.The lowesttapwhichwill supply the required
starting torque is selected.
Resistance and reactance starters initially insert an impedance in series with the motor. After a time
delay,thisimpedance is shorted out. Starting resistors may be shorted out over several steps; starting
reactorsare shortedoutin a single step. Line current and starting torque vary directly with the voltage
appliedtothe motor,so fora givenstartingvoltage,these startersdraw more currentfromthe line than
withautotransformerstarters,butprovide higher starting torque. Reactors are typically provided with

42. 50, 45, and 37.5 percent taps. Part-winding starters are attractive for use with dual-rated motors
(220/440 V or 230/460 V). The stator of a dual-rated motor consists of two windings connected in
parallel atthe lowervoltage rating,orinseriesatthe highervoltage rating. When operated with a part-
winding starter at the lower voltage rating, only one winding is energized initially, limiting starting
current and starting torque to 50 percent of the values seen when both windings are energized
simultaneously. Delta-wye starters connect the stator in wye for starting and then, after a time delay,
reconnectthe windings in delta. The wye connection reduces the starting voltage to 57 percent of the
systemline-line voltage; starting current and starting torque are reduced to 33 percent of their values
for full-voltage start.
Utility System Fault-Clearing Issues
Utility feeder design and fault-clearing practices have a great influence on the voltage sag and
interruption performance at a distribution-connected load
Utilitieshave twobasicoptionstocontinue toreduce the numberandseverityof faultsontheirsystem:
1. Prevent faults.
2. Modify fault-clearing practices.
Utilitiesderive importantbenefitsfromactivities that prevent faults. These activities not only result in
improved customer satisfaction but prevent costly damage to power system equipment. Fault
preventionactivitiesinclude tree trimming,addingline arresters, insulator washing, and adding animal
guards. Insulation on utility lines cannot be expected to withstand all lightning strokes.
However,anyline thatshowsa highsusceptibilityto lightning-inducedfaultsshouldbe investigated.On
transmission lines, shielding can be analyzed for its effectiveness in reducing direct lightning strokes.
Towerfootingresistance isanimportantfactorinback flashoversfromstaticwire toa phase wire. If the
tower footing resistance is high, the surge energy from a lightning stroke will not be absorbed by the
ground as quickly.
On distributionfeeders,shieldingmayalsobe anoptionas isplacingarrestersalongthe line frequently.
Of course,one of the mainproblemswithoverheaddistribution feeders is that storms blow tree limbs
intothe lines.Inareaswhere the vegetationgrowsquickly,itisaformidable tasktokeep trees properly
trimmed.Improvedfault-clearingpracticesmayincludeadding line reclosers, eliminating fast tripping,
adding loop schemes, and modifying feeder design. These practices may reduce the number and/or
durationof momentaryinterruptionsandvoltage sags,bututilitysystemfaultscanneverbe eliminated
completely.
Sources of Transient Overvoltages
There are two main sources of transient overvoltages on utility systems: capacitor switching and
lightning.

43. Some powerelectronicdevicesgenerate significanttransientswhentheyswitch.transientovervoltages
can be generated at high frequency (load switching and lightning), medium frequency (capacitor
energizing), or low frequency.
1 Capacitor switching
It is one of the most common switching events on utility systems. Capacitors are used to provide
reactive power (in units of vars) to correct the power factor, which reduces losses and supports the
voltage on the system. Alternative methods such as the use of rotating machines and electronic var
compensators are much more costly or have high maintenance costs. Thus, the use of capacitors on
power systems is quite common and will continue to be.
One drawback to the use of capacitors is that they yield oscillatory transients when switched. Some
capacitorsare energizedall the time (afixed bank), while others are switched according to load levels.
Variouscontrol means,including time, temperature, voltage, current, and reactive power, are used to
determine whenthe capacitorsare switched.Itiscommonforcontrolsto combine twoor more of these
functions, such as temperature with voltage override
One of the common symptoms of power quality problems related to utility capacitor switching
overvoltagesisthatthe problemsappearatnearlythe same time eachday.On distributionfeederswith
industrial loads, capacitors are frequently switched by time clock in anticipation of an increase in load
with the beginning of the working day. Common problems are adjustable-speed-drive trips and
malfunctionsof otherelectronicallycontrolledloadequipmentthatoccur without a noticeable blinking
of the lights or impact on other, more conventional loads.
Figure 17 shows the one-line diagram of a typical utility feeder capacitor-switching

44. Fig. 18 - Typical utility capacitor-switching transient reaching 134 percent voltage,
Whenthe switchis closed,atransient isas shown inFig. 18, maybe observed.Inthiscase,the capacitor
switch contacts close at a point near the system voltage peak. This causes the insulation across the
switch contacts tends to break down when the voltage across the switch is at a maximum value. The
voltage across the capacitor at this instant is zero. Since the capacitor voltage cannot change
instantaneously, and rises as the capacitor begins to charge toward the system voltage. Because the
powersystem source isinductive,the capacitorvoltage overshootsandringsatthe natural frequencyof
the system. The overshoot will generate a transient between 1.0 and 2.0 pu depending on system
damping. In this case the transient observed at the monitoring location is about 1.34 pu.
Utilitycapacitor-switching transients are commonly in the 1.3- to 1.4-pu range. The transient shown in
the oscillogram propagates into the local power system and will generally pass through distribution
transformers into customer load facilities by nearly the amount related to the turns ratio of the
transformer.If there are capacitorson the secondarysystem, the voltage may actually be magnified on
the load side of the transformer if the natural frequencies of the systems are properly aligned .While
such brief transients up to 2.0 pu are not generally damaging to the system insulation, they can often
cause misoperationof electronicpowerconversiondevices. Controllers may interpret the high voltage
as a sign that there is an impending dangerous situation and subsequently disconnect the load to be
safe. The transient may also interfere with the gating of thyristors
Lightning
Lightning is a potent source of impulsive transients.

45. Fig. 18
Figure 18 illustrates the places where lightning can strike that results in lightning currents being
conductedfromthe powersysteminto loads. The most obvious conduction path occurs during a direct
strike toa phase wire,eitheronthe primaryor the secondaryside of the transformer.This can generate
very high overvoltages. similar transient overvoltages can be generated by lightning currents flowing
along ground conductor paths.
Note that there can be numerouspaths for lightning currents to enter the grounding system. Common
ones,indicatedbythe dottedlinesinclude the primaryground,the secondaryground,andthe structure
of the load facilities. Note also that strikes to the primary phase are conducted to the ground circuits
throughthe arresterson the service transformer.Thus,manymore lightningimpulses may be observed
at loads.mostof the surge current may eventually be dissipated into the ground connection closest to
the strike,there will be substantial surge currentsflowing in other connected ground conductors in the
first few microseconds of the strike.
A directstrike toa phase conductor generallycausesline flashover near the strike point. Not only does
this generate an impulsive transient, but it causes a fault with the accompanying voltage sags and
interruptions.The lightningsurge canbe conducteda considerabledistance alongutilitylines and cause
multiple flashovers at pole and tower structures as it passes. Depending on the effectiveness of the
grounds along the surge current path, some of the current may find its way into load apparatus.
Arrestersnearthe strike maynot survive because of the severeduty(mostlightningstrokes are actually
many strokes in rapid-fire sequence). Lightning does not have to actually strike a conductor to inject
impulsesintothe powersystem.Lightningmaysimplystrike nearthe line and induce an impulse by the
collapse of the electricfield.Lightningmayalsosimplystrikethe ground near a facility causing the local
groundreference torise considerably.Thismayforce currentsalonggroundedconductorsintoaremote
ground, possibly passing near sensitive load apparatus.
lightningsurgesenterloadsfromthe utilitysystemthroughthe interwinding capacitance of the service
transformerasshowninFig. 19. The lightning impulse is so fast that the inductance of the transformer
windingsblocksthe firstpartof the wave frompassingthrough.However,the interwinding capacitance
may offera ready path for the high-frequency surge. This can permit the existence of a voltage on the
secondary terminals that is much higher than what the turns ratio of the windings would suggest. The
degree to which capacitive coupling occurs is greatly dependent on the design of the transformer.
resulting transient is a very short single impulse, or train of impulses, because the interwinding
capacitance chargesquickly.Arrestersonthe secondarywindingshouldhave nodifficultydissipatingthe
energy in such a surge, but the rates of rise can be high. Thus, lead length becomes very important to
the successof an arresterinkeepingthisimpulse outof loadequipment.Manytimes, a longer impulse,

46. whichissometimesoscillatory,isobservedonthe secondarywhenthere isa strike to a utility’s primary
distribution system.
Fig. 19 Coupling of impulses through the interwinding capacitance of transformers
The chief power quality problems with lightning stroke currents entering the ground system are
1. They raise the potential of the local ground above other grounds in the vicinity by several kilovolts.
Sensitiveelectronicequipment that is connected between two ground references, such as a computer
connected to the telephone system through a modem, can fail when subjected to the lightning surge
voltages.
2. They induce high voltages in phase conductors as they pass through cables on the way to a better
ground.
Other switching transients
Line energizationtransientsoccur,whenaswitchisclosed connecting a line to the power system. They
generally involve higher-frequency content than capacitor energizing transients. The transients are a
result of a combination of traveling-wave effects and the interaction of the line capacitance and the
system equivalent source inductance. Traveling waves are caused by the distributed nature of the
capacitance and inductance of the transmissionordistribution line. Line energizing transients typically
result in rather benign overvoltages at distribution voltage levels and generally do not cause any
concern. Line energizing transients usually die out in about 0.5 cycle.
Anothersource forovervoltagesthatis relatedtoswitchingisthe single-line-to-groundfault,the sound
phase will experience a voltage rise during the fault. The typical voltage rise on effectively grounded
four-wire is generally no more than 15 to 20 percent.
Summary- the actual impact of this overvoltage on the secondary side of the system depends heavily
on the service transformer connection. While the common grounded wye-wye connection will
transform the voltages directly, transformers with a delta connection will help protect the load from
seeing overvoltages due to these faults.
Principles of Overvoltage Protection
The fundamental principles of overvoltage protection of load equipment are

47. 1. Limit the voltage across sensitive insulation.
2. Divert the surge current away from the load.
3. Block the surge current from entering the load.
4. Bond grounds together at the equipment.
5. Reduce, or prevent, surge current from flowing between grounds.
6. Create a low-pass filter using limiting and blocking principles.
Figure 20
Figure 20 illustrates these principles, which are applied to protect from a lightning strike. The main
function of surge arresters and transient voltage surge suppressors (TVSSs) is to limit the voltage that
can appear between two points in the circuit. the foremost concern in arrester application is to place
the arrestersdirectlyacross the sensitive insulation that is to be protected so that the voltage seen by
the insulation is limited to a safe value. Surge currents, just like power currents, must obey Kirchoff’s
laws. They must flow in a complete circuit, and they cause a voltage drop in every conductor through
whichtheyflow.One of the pointstowhicharresters,orsurge suppressors,are connectedis frequently
the local ground,but thisneednotbe the case. Howeverlocal ground may not remain at zero potential
duringtransientimpulseevents. Surge suppression devices should be located as closely as possible to
the critical insulation with a minimum of lead length on all terminals.
Arrestersappliedatthe pointwhere the power line enters the load equipment are generally the most
effective in protecting that particular load. In some cases, the best location is actually inside the load
device.Forexample,manyelectroniccontrolsmade forservice inthe power system environment have
protectors[metal-oxide varistor(MOV) arresters,gaps,zener diodes, or surge capacitors] on every line
that leaves the cabinet.
In Fig. 20 the first arrester is connected from the line to the neutral-ground at the service entrance. It
limits the line voltage V1 from rising too high relative to the neutral and ground voltage . When it
performs its voltage-limiting action, it provides a low impedance path for the surge current to travel
onto the ground lead. Note that the ground lead and the ground connection itself have significant
impedance. Therefore, the potential of the whole power system is raised with respect to that of the
remote ground by the voltage drop across the ground impedance.
surge arrestercalledas surge diverterbecause itsvoltage-limiting action offers a low-impedance path
aroundthe loadbeingprotected.However,itcanonlybe a diverterif there isasuitable path into which
the current can be diverted.

48. In this figure, there is another possible path for the surge current— the signal cable indicated by the
dottedline andbondedtothe safetyground.If thisisconnectedtoanotherdevice that is referenced to
ground elsewhere, there will be some amount of surge current flowing down the safety ground
conductor. Damaging voltages can be impressed across the load as a result.
Note that the signal cable is bonded to the local ground reference at the creates an unwanted ground
loop. However, it is essential to achieving protection of the load and the low-voltage signal circuits.
Otherwise, the power components can rise in potential with respect to the signal circuit reference by
several kilovolts. Many loads have multiple power and signal cables connected to them.
The firstarresterat the service entrance iselectricallytooremote to provide adequate load protection.
Therefore,asecondarresterisappliedatthe load—again,directlyacrossthe insulationtobe protected.
It is connected “line to neutral” so that it only protects against normal mode transients.
This illustrates the principles without complicating the diagram but should be considered as the
minimum protection one would apply to protect the load. Frequently, surge suppressors will have
suppression on all lines to ground, all lines to neutral, and neutral to ground.
In cases where surge currents are diverted into other load circuits, arresters must be applied at each
load along the path to ensure protection
Also, a load may be in an environment where it is close to another load and operators or sensitive
equipmentare routinelyincontactwithbothloads.Thisraisesthe possibilitythat a lightning strike may
raise the potential of one ground much higher than the others. This can cause a flashover across the
insulation that is between the two ground references or cause physical harm to operators. Thus, all
groundreference conductors(safetygrounds,cable shields, cabinets, etc.) should be bonded together
at the loadequipment.the principle isto tie the references together so that all power and signal cable
references in the vicinity rise together
This phenomenon is a common reason for failure of electronic devices. The situation occurs in TV
receivers connected to cables, computers connected to modems, computers with widespread
peripherals powered from various sources, and in manufacturing facilities with networked machines.
Since a few feet of conductor make a significant difference at lightning surge frequencies, it is
sometimesnecessarytocreate aspecial low-inductance,groundreference planeforsensitiveelectronic
equipment such as mainframe computers that occupy large spaces. Efforts to block the surge current
are mosteffectiveforhigh-frequencysurge currentssuchasthose originatingwithlightningstrokes and
capacitor-switching events. Since power frequency currents must pass through the surge suppressor
with minimal additional impedance, it is difficult and expensive to build filters that are capable of
discriminating between low-frequency surges and power frequency currents. Blocking can be done
relativelyeasilyfor high-frequency transients by placing an inductor, or choke, in series with the load.
The highsurge voltage will dropacrossthe inductor.One mustcarefullyconsiderthathighvoltage could
damage the insulationof boththe inductorandthe loads. However, a line choke alone is frequently an
effective means to block such high-frequency transients as line-notching transients from adjustable-
speeddrives.The blockingfunctionisfrequentlycombinedwith the voltage-limiting function to form a
low-passfilterin which there is a shunt-connected voltage-limiting device on either side of the series
choke.
Figure 20 illustrateshowsucha circuitnaturallyoccurswhenthere are arresters on bothendsof the line
feeding the load. The line provides the blocking function in proportion to its length. Such a circuit has
very beneficial overvoltage protection characteristics. The inductance forces the bulk of fast-rising
surges into the first arrester. The second arrester then simply has to accommodate what little surge
energy gets through.
Many surge-protection problems occur because the surge current travels between two, or more,
separate connectionstoground.Thisisa particularproblemwithlightningprotectionbecause lightning
currentsare seekinggroundandbasicallydivideaccordingtothe ratiosof the impedancesof the ground