Lecture for Reading 2

8/18/2019 Lecture for Reading 2

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Power System Quality

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Power frequencydisturbance

• The term power frequencydisturbance describes events thatare slower and loner lastincom!ared to electrical transients

• Power frequency disturbances canlast anywhere from one com!lete

cycle to several seconds or evenminutes

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• #hile the disturbance can be nothinmore than an inconvenience manifestinitself as a $ic%erin of lihts or bum!y ride

in an elevator& in other instances thee'ects can be harmful to electricalequi!ment

• The e'ects of !ower frequencydisturbances vary from one !iece ofequi!ment to another and with the ae ofthe equi!ment

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• )qui!ment that is old and has been sub*ected toharmful disturbances over a !roloned !eriod ismore susce!tible to failure than new equi!ment

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+ne of the most common !ower frequencydisturbances is voltae sa

• ,y denition& voltae sa is an event that canlast from half of a cycle to several seconds

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-oltae sas ty!ically are due to startin onlare loads& such as an electric motor or an arcfurnace

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• /nduction motors draw startincurrents ranin between 0 and3 of their nominal full loadcurrents

• The current starts at the hih valueand ta!ers o' to the normal runnin

current in about 2 to sec& based onthe motor desin and load inertia

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Motor4startin current waveform5 6 .4h! motor was started across theline5 The

motor full4load current was 0 65 The rst half4cycle !ea% reached avalue of 0 65

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• 8urin the rst half of the cycle& theasymmetrical current attains a !ea%value of 0 6

• #hen the circuit feedin the motorhas hih im!edance& a!!reciablevoltae sa can be !roduced

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9iure in ne:t slide shows a 14%-6transformer feedin the .4h! motor *ust described


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Schematic diaram

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• /f the transformer has a lea%aereactance of .53& the voltae sadue to startin this motor is calculated

as follows<• 9ull load current of the 14%-6

transformer at ( - = 12 65

• -oltae dro! due to the startin inrush= .5>0?@12>A2B = 2.5"35

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• /f the reactance of the !ower linesand the utility transformer feedinwere included in the calculations& the

voltae sa would be worse than thevalue indicated

• /t is not dicult to see that any

device that is sensitive to a voltaesa of 2.3 would be a'ected by themotor startin event

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• 6rc furnaces are another e:am!le ofloads that can !roduce lare voltaesas in electrical !ower systems

• 6rc furnaces o!erate by im!osin ashort circuit in a batch of metal andthen drawin an arc& which !roducestem!eratures in e:cess of 1&DC&

which melt the metal batch• 6rc furnaces em!loy lare inductors to

stabiliEe the current due to the arc

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6rc furnace connection andresultin current

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• Tens of thousands of am!eres are drawndurin the initial few seconds of the !rocess

• +nce the arc becomes stable& the currentdraw becomes more uniform

• 8ue to the nature of the current drawn by thearc furnace& which is e:tremely nonlinear&lare harmonic currents are also !roduced

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Severe voltae sas are common in !owerlines that su!!ly lare arc furnaces& whichare ty!ically rated in the "4 to .4M-6 raneand hiher

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• 6rc furnaces are o!erated in con*unctionwith lare ca!acitor ban%s and harmoniclters to im!rove the !ower factor and

also to lter the harmonic frequencycurrents so they do not unduly a'ectother !ower users sharin the same!ower lines

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/t is not uncommon to see arc furnacessu!!lied from dedicated utility !ower linesto minimiEe their im!act on other !owerusers

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• The !resence of lare ca!acitance in anelectrical system can result in voltaerise due to the leadin reactive !owerdemands of the ca!acitors& unless theyare adequately canceled by the lainreactive !ower required by the loads

• This is why ca!acitor ban%s& whether for!ower factor correction or harmonic

current ltration& are switched on whenthe furnace is brouht on line andswitched o' when the arc furnace is o'line5

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• Ftility faults are also res!onsible for voltaesas5 6!!ro:imately 73 of the utility4related faults occur in overhead !ower lines

•Some common causes of utility faults arelihtnin stri%es& contact with trees or birdsand animals& and failure of insulators

• The utility attem!ts to clear the fault by

o!enin and closin the faulted circuitusin reclosure& which can require from (to 0 cycles

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• The !ower line e:!eriences voltaesas or total loss of !ower for the shortduration it ta%es to clear the fault

• +bviously& if the fault !ersists& the!ower outae continues until the!roblem is corrected

• Subsequent slide shows sa due toutility fault near renery

• The sa lasted for 21 cycles

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• 8e!endin on the instant at whichthe voltae is a!!lied to the motor&the current can be hihly

asymmetrical

• 9iure in !revious slide contains thewaveform of the startin current of a

.4h! induction motor with a ratedfull4load current of 0 6 at (0 -6C

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8enitions

• Transient is dened as a subcycledisturbance in the 6C waveform thatis discernible as a shar! discontinuity

of the waveform• The denition states that transients

are subcycle events& lastin less than

one cycle of the 6C waveform

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• Goutinely we see transients that s!anseveral cycles

• To satisfy the absolute denition& the

transient occurrin in the ne:t cycleis not considered an e:tension of thetransient in the !revious cycle

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This a!!roach allows us to isolate thedisturbance on a cycle4bycycle basisfor ease of analysis and treatment

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• Subcycle transients are some of the mostdicult anomalies to detect and treat

• Their occurrence can be random& and they

can vary in deree de!endin on theo!eratin environment at the time ofoccurrence

• Their e'ect on devices varies de!endin onthe device itself and its location in anelectrical system

• Transients are dicult to detect because oftheir short duration

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• Conventional meters are not able to detector measure the transients due to theirlimited frequency res!onse or sam!lin rate

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9or e:am!le& if a transient occurs for 2 msecand is characteriEed by a frequency contentof 2 %HE& the measurin instrument musthave a frequency res!onse or sam!lin rateof at least 1 times 2 %HE& or 2 %HE& in

order to fairly describe the characteristics ofthe transient as hiher sam!lin rates arerequired for faster transients

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• Many di'erent terms are associated withtransients& such as spikes & bumps& power

pulses & impulses & and surges

• 6!!ro!riate understandin of !ower transient

is im!ortant to understand as lareelectromanetic devices such as transformersand motors are !ractically resistant to thee'ects of transients

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Problems arise because of the sensitivity of themicroelectronic devices and circuits that ma%eu! the control elements of the !ower system

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• The micro!rocess controller is thenerve center of every !resent4daymanufacturin or commercial facility

• Medical electronic instruments usedin healthcare facilities are becominmore so!histicated and at the same

time increasinly susce!tible to)lectrical Transients

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Transient system Model

• Steady4state systems are the o!!osite oftransient systems

• /n steady state& the o!eration of a !owersystem is characteriEed by thefundamental frequency or by some low4frequency harmonic of the fundamentalfrequency

• The three !assive !arameters of the

system I resistance @ R B& inductance @ L B&and ca!acitance @ C B I determine howthe steady4state system will res!ond whenunder the in$uence of an a!!lied voltae

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• The circuit model of a transientelectrical system will a!!earconsiderably di'erent from the

steady4state model• Passive !arameters & L & and C are still

ma*or determinants of the transient

res!onse& but their e'ect on thetransient can chane with theduration of the transient

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• /n an electrical system& inductance andca!acitance are the enery4storinelements that contribute to the oscillatorynature of the transient

• Gesistance is the enery4dissi!atinelement that allows the transient to dam!enout and decay to the steady4state condition

• 9iure in ne:t slide illustrates an electrical!ower source feedin a resistiveJinductiveload @e55& a motorB via circuit brea%er S andtransformer T

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)lectrical System Model

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Low frequency re!resentationof circuit

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Transient Model of thecircuit

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):!lanation

• /n the transient model& the ca!acitanceacross the !oles of the circuit brea%er& theca!acitance of the !ower lines feedin themotor& and the ca!acitance of the sourceand the motor windins become sinicant

• +nce the transient model is created& someof the elements may be systematicallyeliminated de!endin on their manitude&the transient duration& and the relevanceof a s!ecic element to the !roblem beinaddressed

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• This brins u! some im!ortant !oints toconsider while solvin electricaltransient4related !roblems

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9irst& determine the total transientmodel& then remove elements in themodel that are not relevant to the!roblem at hand

• 6lso& develo! a mathematical model ofthe transient circuit& and then derive asolution for the needed !arameter

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Transient Models andres!onse

• Ke:t slide shows suddenly a!!lied8C voltae to GC circuit and itscorres!ondin transient res!onse

• The voltae and current res!onse isthe risin and decayin e:!onentialfunction as in subsequent slide

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• #here RC is the time constant @T B of theresistanceJca!acitance circuit and ise:!ressed in seconds

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The time constant is the time it would ta%efor an e:!onentially decayin !arameter toreach a value equal to "057;3 of the initialvalue

• This is e:!lained by notin that the!arameter would be reduced to a valueiven by 1e or 5"07; of the initial value

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6!!lication of 8C voltae toan inductor

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Lecture for Reading 2

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