transformer inrush current

7/21/2019 Transformer inrush current

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1. INTRODUCTION

It is recommended that diferential protection be used or theprotection o transormers o 10 M! "sel#cooled$ and hi%her &1'.(enerall)* diferential protection is considered the best protectionor transormers. +o,e-er* inrush current due to transormerener%iation e/ists mostl) onl) in one ,indin% o the transormerthereore* the rela) sees the ener%iation condition as a ault. Toimpro-e securit) ,hile maintainin% the reuired le-els odependabilit)* man) restraint methods ha-e been proposed tobloc2 the operation o the diferential element due to the inrushcurrent. Methods usin% harmonic restraint* ,a-e#shapereco%nition* and arti3cial neural net,or2s ha-e been proposed todiscriminate ma%netiin% inrush and internal aults. The second

harmonic restraint method is the most common one used b)-arious rela) manuacturers and application en%ineers. There area e, -ariations o harmonic restrained diferential protection.

This paper ,ill anal)e actors afectin% the second harmonic ratioin inrush current* ,ill describe -arious harmonic restraintmethods* and ,ill compare their perormance.

The concept o diferential protection is illustrated in 4i%. 1.(enerall)* the current enterin% the transormer is compared to

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the current lea-in% the transormer. 5hen the currents are eual*the operatin% current is ero* and it indicates that the transormeris health) and no ault is detected. Ideall)* there is no po,er lossin a transormer* and there should be no operatin% current. In

realit)* because there are core losses* edd) currents in the coreand ,indin%* errors rom current transormers "CTs$* and load tapchan%ers* the operatin% current ,ill not be ero. 5hen calculatin%diferential current* the current in each ,indin% needs to becompensated based on the transormer con3%uration and the,indin% turns ratio. 6et Icompi be the compensated current in,indin% i o a multi ,indin% transormer. The operatin% current%enerall) %i-en as ollo,s

Durin% a lar%e throu%h#ault* there ma) be some meterin% error inCT ,hich could result in a lar%e operatin% current and could causea alse trip. 7ercenta%e diferential protection has been applied orman) )ears to impro-e the securit) o diferential protection. Thediferential element ,ill operate ,hen the operatin% current "Iop$is abo-e a certain ratio o the restraint current " IR$* i.e.*

,here 8lope represents the threshold ratio o the restraint currentto the operatin% current. The restraint current IR ma) becommonl) de3ned as the ma/imum* a-era%e* or minimum o thecompensated currents

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It is necessar) to understand ,hich method is used in thecalculation o the restraint current ,hen settin% the slope o thepercenta%e diferential rela).

9. !N!6:8I8 O4 TR!N84ORM;R INRU8+ CURR;NT

It is -er) ,ell 2no,n that a transormer ,ill e/periencema%netiin% inrush current durin% ener%iation. Inrush currentoccurs in a transormer ,hene-er the residual


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9?0. The minimum 1E.1A o the second harmonic component ininrush current is based on the ollo,in% assumptions>

1$ There is 0A residual


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The ,orst case base an%le -aries ,ith the core residual andsaturation


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In order to anal)e the harmonic component in the inrush current*

an ideal inrush current is dra,n separatel) in 4i%. ? or simplicit).One c)cle o the idealied inrush current can be e/pressed as

8ince the choice o ori%in %i-es a s)mmetric ,a-eorm about F

0* the coeGcients o sine 4ourier series are ero. 4ormathematical simplicit)* de3ne

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4ourier anal)sis %i-es the nth cosine 4ourier series coeGcient or"n 9$ b)

and the pea2 undamental reuenc) component is %i-en as

+oro,it &?' sho,s that* o all the harmonic components* thesecond is b) ar the %reatest in ma%nitude. 5hen n F 9* thesecond harmonic component e/pressed can urther be simpli3edto

The ratio o the second harmonic component to the undamentalcomponent is

4i%. @ illustrates that the second harmonic ratio is a decreasin%unction o the base an%le o inrush current. 5hen F ?/=

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This is ,h) 1E.1A o the second harmonic ratio ,as considered asthe minimum in transormer inrush current &9'* &='. 5hen F 0*"19$ and "1=$ sho, that both the undamental and secondharmonic components are ero. The diferential protection is

secure durin% a ne%li%ible amount o inrush current. It ma) be oacademic interest to see ho, the second harmonic ratio chan%es,hen the an%le approaches ero. !ppl)in% 6+opitals rule threetimes on "1?$* ,e ha-e

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This result matches the cur-e in 4i%. @.


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=. 4!CTOR8 !44;CTIN( T+; INRU8+ CURR;NT

In "J$* the base an%le o the inrush current can be calculatedbased on the normal pea2


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pea2


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It can be seen rom 4i%. 9 that the pea2 inrush current decreases,ith the saturation


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4i%. 9 illustrates the ,orst case transormer ener%iation. Thetransormer ,ith positi-e residual


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5here is the an%le o the ener%iin% -olta%e reerenced to the,orst case -olta%e. 4i%. J illustrates ho, the inrush characteristicschan%e ,ith the ener%iin% -olta%e an%le. 4or the ,orst case

-olta%e ,a-eorm* ,hen the ener%iin% -olta%e an%le is ero* theundamental reuenc) component is at the ma/imum -alue* andthe second harmonic ratio is at the lo,est -alue. ! diferentialprotection element ,ill see the ma/imum operatin% current ,iththe lo,est second harmonic ratio. It is apparent that the ,orstcase ener%iation is a most una-orable situation or the securit)o diferential protection. 5hen the absolute o -olta%e an%le isincreased* the base an%le o the inrush current ,ill be reduced*the second harmonic ratio ,ill be increased* and the undamentalinrush current* i.e.* the diferential operatin% current* ,ill be

reduced. +o,e-er* the ener%iin% -olta%e an%le is %enerall) notcontrollable* and a protection en%ineer has to estimate the ,orstcase situation or a secure protection.

?. !N!6:8I8 O4 INRU8+ CURR;NT8 IN MOD;RNTR!N84ORM;R8

It is ,ell 2no,n that modern transormers ma) e/perience -er)lo, harmonic inrush currents. Modern transormers %enerall) usehi%h


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remnant


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The maorit) o rela) manuacturers use the terms o harmonicrestraint* harmonic inhibit* or harmonic bloc2in% interchan%eabl).!lthou%h there are some -ariations in implementation* thediferential rela) ,ill not operate ,hen

In order to o-ercome the challen%e o secure diferentialprotection ,ith lo, harmonic component in inrush current in ne,transormers* -arious harmonic restraint methods ha-e beenstudied &H'B&'. 4i%. is sho,n belo,

7er 7hase Method

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This is the earliest and simplest harmonic restraint method.;uation "1J$ applies to phase !* K* and C separatel). Therestraint al%orithm in each phase is independent and parallel. The

per#phase method is illustrated in 4i%. 10. 8ince each phase hasdiferent residual


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The harmonic ratio or a percent a-era%e bloc2in% method is thea-era%e o the second harmonic ratio o the three phases

The percent a-era%e bloc2in% method is illustrated in 4i%. 19.Compared to the cross#bloc2in% method and t,o#out#o threemethods* this method impro-es the securit) o diferentialprotection to a certain de%ree. The diferential operation ma) berestrained ,hen there is a true sin%le#phase ault durin%ener%iation* pro-ided that there are lar%e harmonic ratios in theremainin% health) phases. This ,ould cause a concern on the

dependabilit) in the diferential protection.

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+armonic 8harin% Method

! summin%#t)pe harmonic sharin% restraint method %reatl)impro-es the dependabilit) o the diferential protection durin% aninternal ault. The shared second harmonic component is de3nedas

The second harmonic ratios are then calculated per phase basedon the shared I9ndS!" and the undamental component o theoperatin% current in each phase

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The harmonic restraint ,ith summin%#t)pe harmonic sharin%method is illustrated in 4i%. 1=.

In this method* the ma%nitudes o the second harmonic romthree phases are summed to%ether to create a sin%le harmonicsi%nal* ,hich is shared in the calculation o the second harmonic

ratio or each phase. I one phase e/periences a -er) lo, secondharmonic ratio* the shared harmonic calculated rom "90$ ,ill belar%e* and the second harmonic ratios calculated rom "91$ ,ill belar%e enou%h to restrain the diferential protection rom a alseoperation. I there is an internal ault in one phase durin%ener%iation* the lar%e undamental operatin% current ,ouldresult in lo, harmonic ratio* and thus* no inhibition is %enerated

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rom the aulted phase. Thereore* a three phase transormer,ould be tripped durin% an internal ault. The harmonic summin%method impro-es the securit) a%ainst alse operation ,hilemaintainin% dependabilit) or detectin% actual aults.

H. !N!6:8I8 O4 INRU8+ ;;NT ;!M76;8

6o, 8econd +armonic Inrush ;-ent

4i%. 1? illustrates a PQ: transormer connected to a radialdistribution s)stem. 5hile this transormer is ener%ied ,ith load

open* there is a station ser-ice transormer connected to thetransormer secondar) outside the diferential one protection.5hen the main transormer is ener%ied* the station ser-icetransormer also ,ill be ener%ied. This installation had problems,ith trippin% durin% ener%iation* and the user s,itched to anumeric rela). +i%h#-olta%e#side CT is :#connected* and lo,#-olta%e#side CT is P#connected or e/ternal an%le compensation.

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! set o oscillo%raphic data 3les ,as recorded rom anener%iation e-ent* ,hich is illustrated in 4i%s. 1@ and 1H. Thesecondar) inrush currents are much smaller compared to those inthe primar) side. In the calculation o diferential operatin%

current* the contribution rom the secondar) inrush currents ,illbe e-en smaller ater tap compensation. 4i%. 1E illustrates theroot#mean#suare "rms$ -alue o the undamental component othe diferential operatin% current. It can be seen that there is alar%e operatin% current due to the inrush current* ,hich couldcause a alse diferential operation.

The per#phase second harmonic ratios or all three phases areillustrated in 4i%. 1J. There is -er) lo, second harmonic ratio inphase K. 4or diferential protection usin% per#phase harmonic

restraint* securit) li2el) is a problem. The lo, second harmonicratio in phase K ,as probabl) the reason ,h) this transormerhad problems ,ith trippin% durin% ener%iation.

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5hen considerin% second harmonic cross#bloc2in% or thisapplication* the second harmonic ratios in phases ! and C arelar%e as seen rom 4i%. 1J* diferential trippin% o phase K ,ill becross#bloc2ed. 4i%s. 1 and 90 indicate the result o appl)in%

a-era%e second harmonic ratio and summin%#t)pe harmonicsharin% restraint methods. In both cases* ,e obser-e that thediferential protection ,ould be secure or this lo, harmonicener%iation

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9?


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9@


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Inrush ;-ent ,ith an Internal 4ault

To illustrate an inrush e-ent ,ith a true ault* a M!T6!KQ 8imulin2simulation ,as used. !s sho,n in 4i%. 91* a ?@0#M! @00 2B9=02 :%Q:% transormer has e/perienced a phase C %round ault,ithin the protection one durin% ener%iation.

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4i%s. 99 and 9= illustrate the inrush ,a-eorms or both theprimar) and secondar) circuits as seen b) the diferentialprotection. 4i%. 9? illustrates the diferential operatin% current ineach o the three phases. The hi%h operatin% current is due to

both ma%netiin% inrush and ault current. 5ithout a properrestraint method* the diferential protection could misoperatedurin% this e-ent. 4i%. 9@ illustrates per#phase second harmonicratio. 7hase K has a -er) hi%h second harmonic ratio. 7hase C hasa -er) lo, second harmonic ratio due to the lar%e ault current.

The second harmonic ratio o phase ! is in the ran%e o t)picalsettin% o harmonic ratio. ! diferential rela) usin% per#phaseharmonic restraint ,ill ha-e trip si%nal rom phase C* and theprotection on the three#phase transormer is dependable. Due tothe hi%h second harmonic ratio in phase K* a diferential rela)

usin% cross#bloc2in% method ,ill bloc2 the diferential operation*and the protection is not dependable to trip or this e-ent.Dependin% on the settin% o threshold harmonic ratio* phase !ma) or ma) not indicate an inrush e-ent. 7rotection ,ith t,o#out#o#three harmonic restraint ma) not protect this e-ent reliabl).

4i%. 9H illustrates ho, the a-era%e restraint harmonic ratiochan%es durin% the e-ent o ener%iin% a aulted transormer.5ith a second harmonic restraint set point o 9JA* the diferential

protection ,ith a-era%e percenta%e harmonic ratio restraint isunbloc2ed or about a c)cle. Outside o this one c)cle ,indo,* thediferential element is bloc2ed and unable to correctl) trip or thise-ent. 4i%. 9E illustrates the second harmonic ratio usin%summin% t)pe harmonic sharin% method. K) inspectin% the cur-eo phase C harmonic ratio* the diferential element ,ill be able totrip or man) c)cles or a 9JA harmonic restraint set point. !diferential rela) ,ith harmonic sharin% restraint is dependable totrip or this e-ent.

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9J


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=0


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CONC6U8ION

8ecurit) is o concern in diferential protection durin% transormerener%iation. Inrush current occurs mostl) in onl) one side o the

transormer and could cause a alse diferential trip. ! commonrestraint method is to use the second harmonic inormation in theinrush current to secure diferential protection ,hen ener%iin%transormers. !n anal)sis o actors such as residual


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REFERENCES

[1] IEEEGuide for Protective Relay Applications to Power Transformers

, IEEE

Std. C37.91_2000, 2000.

[2] W. K. Sonnemann, C. L. Wagner, and . !. Ro"#e$e%%er, &'agnet()(ng (nr*+

-enomena (n tran+$ormer an#+,/ Trans. Amer. Inst. Elect. Eng.Power App. Syst.,

o%. 77, no. 3, -t. III, --. 92, 4-r. 195.

[3] S. 6. 6oro(t), Ed., Protective Relaying for Power Systems. 6oo#en, N8,

S4: W(%e;, 190.

[] S. 6oro(t) and 4. (", and E. teN;en*(+, &

transformer inrush current

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