power transformer inrush

8/13/2019 Power Transformer Inrush

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i-COST ElectronicsConuuunicauouCenferenceProceeding13-15 );JIl-201 t.

Power low ontrol by Using T S ontroller in Power SystemS. D. Wadhai, .G co Amralali.

Maharashtra State India. htra Stale. India.san [email protected]

Dr. N. D. Ghawghawe'Prof ElectricalEngg. Dept ..G o AmravaliI1Malzarasghaw [email protected] .in

Abstract-Thyristor controlled series capacitor (TCSC) controller. the first generation of flexibleAC transmission system (FACTS), can control the line impedance through the' introduction of a thyristor

. controlled capacitor in series with the tra;::;;:;:ssic-f:line. TCSC is used as series compensator intransmission system. The TCSC controller can be designed to control the power flow. to increase thetransfer.limits or to improve the transient stability. The TCSC controller can provide a very fast action toincrease the synchronization power through quick changing of the equivalent capacitive reactance to thefull compensation in the first few cycles after a fault. hence subsequent oscillation are damped. TCSC.provides variable impedance, which is required for the compensation .Jn capacitive zone. Themathematical analysis of simplified TCSC circuit is deriving for computing the TCSC reactance for thefiring angle. The TCSC controller is modeled in MATLAB SIMULINK and can be used in power systemfor power flow control. . -KEYWORDS: FACTS, TCSC, Power system,Stability, Model, simulation, Mathematical Analysis, Firing Angle, -Power system flexibility. -

Power Transformer Inrush urrents and Its ffects onProtective Relays

R.M.HolmukheElectrical Engineering eprtment

Bharati VidyapeethDeemedUniversityCollegeof Enggineering, Pune, IndiaEvm ail [email protected]

K.D.DeshpandeElectrical Engineering DepartmentBharati VidyapeethDeemedUniversityCollegeof Enggineering,Pune, IndiaE-mail: rajeslunholmukh @rediffmail.com

Abstract-There area variety of protective relays using different measuring techniquesto provide reliable secure transformer protection. They include, electro-mechanical, solid state andnumerical relays. Within eachgroup, various algorithms exist. Advancements made to transformer technology anddesign over past decades,have changed the characteristics of the transformer inrush current and have oftenintroduced incorrectoperations inthe existing harmonic relay restraint relaysduring energization.Topics to be presented inciudeComparison between old newly developed moreaccurate calculations of peakvalues, magnitude of secondharmonic, andother parametersof Inrush current.Design and systemparameterswhich influence the magnitude and wave-shapeof inrush currents,e.g. winding design and connections,corematerial, core joint geometry, short circuit capacity of network, etc.Impact of transformer design performanceparameters and new developments in the transformer technology on magnitude nature of inrush current.CurrentTransformer influence on the speed, selectivity, and dependability of transformer protection.111is paperwill addressthese topics aswell asprovide. solutions for selecting selling transformer protections.Keywords-power transforme.Lnrush current.protective relays

S.S.V.P.S.'s B.S.Deore college of Engineering Polytechnic.Dhule. Page 27


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Power Transformer Inrush Currents and Its Effects on Protective Relays

RMHolmukheElectrical Engineering DepartmentBharati Vidyapeeth Deemed UniversityCollege ofEnggineering, Pune, India

E-mail: rajeshmholmukhe@hotmaiLcom

Abstract-There are a variety of protective relays using differentmeasuring techniques to provide reliable & secure transformer T q eIecIrD.f...... .,. Wile.---mc.l m ..,s . WiI.tiI --.s aisLAbta ..tie -.u kd 'Dt hip__pat J htre lite of lite i Juas6 CWI'TtW htre * iat:r:t . ;" tile e:xist iaK '-Ie reIIt.F,'t .StIWiIdreItt.Penergization:Topics to be presented include oItl & MYdopeIl-.-e.:alRlle

cwkwletWs IIfpetlk *a. -..pihuIe of .---.and other parameters of Inrush current. Design and system parameters which influence the

itr* .- ....,. t:tUrr1IIIs. e..hip__aw-nio-s, awe.-en.J. awejtDt .sIuwtcirctIit of--- . de..

JMd of tr.asf-.u & po:f pttTlII8dDs.- - ; ,. trtuasforr.er kd UIIot F -magnitude & nature of inrush current.

Current Transformer influence on the speed selectivity anddependability of transformer protection.

TIIisJIIIIT wiIllMItIrru IDpiaa wdl a JI f IUe- 'tiotu for se tUrg & se i t tnocs.f-.er prtJIediD-..Keywords-power transformer.inrush current.protective relays

f. INTRODUCTIONInrush current is a form of over current that occurs duringenergization of a transformer and is a large transient currentwhich is caused by part cycle saturation of the magneticcore of the transformer. For power transformers, themagnitude of the inrush current is initially 2 to 5 times therated load current but slowly decreases by the effect ofoscillation damping due to winding and magnetizingresistances of the transformer as well as the impedance ofthe system it is connected to until it reaches finally tonormal current value. This process typically takes severalminutes.

As a result inrush current could be mistaken for ashort circuit current and the transformer is erroneously takenout of service by the over-curreot Of the diffi:rential relays.The traosfurmer design and station installationparameters affect the magnitude of the inrush currentsignificantly. Therefore, it is important to have an accuratecalculated value of the magnitude and other parameters ofinrush current in order for relaying to properly differentiate

KD.DeshpandeElectrical Engineering Department

Bharati Vidyapeeth Deemed UniversityCollege ofEnggineering, Pune, IndiaE-mail: rajeshmholmukhe@rediffinaiLcom

between inrush current and short circuit incidents. Propercalculation of the minimum % of 2nd harmonic Of inrushcurrent is a very imponant parameter for this diffi:rentiatioo.Also in recent years, there have been transfunnerdesign improvements that in fact have lead to a significantimpact on magnitudes, wave shapes, andrba:rmooic ofinrush current..

II. MOREONINRUSHCURRENTFig. I shows an un-Ioaded trnnsfonner switched on ACsupply. Let the flux in transformer be

+ =+msinwtAnd the applied voltage be,v =vm sin (IDt +9)Where, ngle at switching instant

dAlso v = N --, Faraday's Lawdt

.: Vm Sin (ID t + 9) Ot

vm.. d Sin ( ro t + 8) dt

N r.ovm.. $ Cas (ro t + 8) + k ............. ( 1)N r.o

To find k, we put initial conditionAt t = 0 let = R

Where R is residual fluxvrn

., K=+-.+ -- COSONID

............. (2)

Now we put ego (2) into egn (1), we getvm vm


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= 4>R -- Cos - - - Cos (rot 0)Nro Nro

vmLet = q m

Nro. + +. +m cos 0 ,.. +m Cos (CDt 0)............ .(3)

Thus the flux in the transformer is a function ofi) Residual flux (+.)ii)1nstant of switching (0)

and ill Magnetic properties of core i.e, amount ofmagnetizing current required to produce given amount offlux.

ThusfurO =0, and if+. =+mat wt = 1(, we get fiom eqn .(3)+=J+m - (4)

Thus the flux attains an amplitude of = J+m.Now to satisfy the flux demand of 3+m, thetransfunner- primary winding draws very large magnetizingcurrent (8-30) times full load ammt with peaky non-sinusoidal wavefonn as shown in fig 1(b).

Furdlc:r-such a high current tlows only on one sideof the bansf(KUter (primary), it looks like an internal fault tothe differential scheme, cwrent flowsthrough relay coil, so that % differential relay maJ-operatesonmagnetizing iorosb.

To protect the transfurmer fiom magnetizinginrush, percentage differential relay with bannonica restraintis used

Ill OVER VOLT GEINRUSH CURRENT

Occasionally, for a short duration, voltage may increaseduring abnormal conditions. Any abrupt increase of thepower transformer terminal voltage will result in a transientcurrent that in greater than the rated current of thetransformer. This transient current is generally termed overvoltage inrush cwrent and is typically caused by Energization of the power transfurmer Voltage recovery after the clearing of a heavy short

circuit in the power system (recovery inrush). Energization of another parallel connected power

transfonner (sympathe6c inrush) Out-of-pbase syncIJroomd:ioo of a connected generator.The over voltage may cause saturation of

transformer resulting in high differential cmreot.. At (20-25) over-voltage, in a grain oriented steel material may cause(10-100) % increase in excitation cmreot..

An increase in magnetizing current due to over-voltage causes 3rd 5th harmonics. The third harmonic

gets trapped in Delta-connection, so only th or er ,/J..lPJllonicrequired to be monitored. It is found that ;th 5th orderharmonics always are more than 8 of fundamental.

IV H RMONIC RESTR IN REL Y

In order to understand the effect of Inrush current, it isnecessary to sIudy 1be working of HannonicRelay. in brief

As already seen. when an unloaded transformer isswitched on, it draws large magnetizing current, which isseveral times the rated current of the transformer. Thismagnetizing ammt is called Magnetizing Imusb Current.Since the Imusb current tlows only in the primaIy winding,the differential protection will see this inrush current as aninternal fault and give unwanted 1ripping.

The barmonic contents in the inrush current arediffaent than in usual fault current, theharmonic (30-70)-/., while 3' barmonic (10-30) -.. The secondbannonic is dominant in inrush ammt than infault current .This feature can be used to distinguish between a fault andan inmsh current.

Fig 2 (a) shows high-speed biased differentialscheme using barmooic n:straint feature, while fig 2 (b)shows it's concepbJa.I rqwesentation.. The relay of thisscheme is made insensitive to the inrush cum:oL

The operating principle is to fitter out theharmonics from the differential current rectify them and addto the percentage resttaint.

The tImed circuit Xc, x allows only current offimdameotaI fu:quency to flow through operating coil. Thebannonics mostly order are diverted into restraining coil

The relay is adjusted so that not to operate when barmonic exceeds 16% of fimdameotal cmreot.. Theharmonic restraint relay fails to operate on occurrence ofinternal fault, which contains considerable harmonics due toan arc or saturation of CT, at the time of switching. Toovercome this difficulty, an instantaneous over-current relayis also incorporated in the harmonic restraint scheme. It willoperate on heavy internal faults in less than cycle.

V C LCUL TION OF INRUSH CURRENT

The simplified equation often used to calcuIate the peak:value of the first cycle of inrush current in Amps is asfollows:

2U 2u.,+B,,-BsI r =[Amps]

(WLY+R'............... Eqn Jwhere:

ULHenry.ROhms.

Applied voJtagt; Volts.Air core inductance of the transformer,Total DC resistance of the transformer,


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BR Remnant flux density of the transformercore, TeslaBs Saturation flux density of the corematerial, TeslaBN Normal Rated flux density of thetransformer care, Tesla.In reality, the above equation does not give

sufficient accuracy since a number of transformer andsystem parameters, which affect the magnitude of inrushcurrent significantly, are not included in the calculations.As well, this equation does not provide information on thesubsequent oscillations throughout the duration of the inrushcurrent transient.

An improved inrush calculation has beendeveloped by ABB which provide the magnitude of inrushcurrent versus time it; hence the entire wave-shape of theinrush current can be determined. The calculation alsoincorporates the following important transformer andsystem parameters which can have as much as 60% impacton the magnitude of inrush current. The inductance of the air-core circuit adjusted for the

transient nature of the inrush current phenomenon. Impedaoce and short cin:uit capacity of the system. Core geometJy and winding configuration A

coooecbons, e.g., I-vs 3-pbase Y-vs Delta windingsconnections. Grounded Vs non grounded Y connectionsetc.

The fig (3) is shows the first 5 cycles of the inrushcurrent wave-shape for a large power transformer calculatedusing the ABB method of calcuJatiOlL

A COIIIpdisonbetween magnitude of the first cycleof inrush current as calcu1ated by the old funnula inequation (J) above versus that calculated using the rigorousABB calculation, including the above parameters. providesmuch lower magnitude of the first peak of inrush currentcompared to that calculated using the old formulacommonly used in the industry.

In fig(3) in shown the first 5 cycles of the inrushcurrent wave-shape for a large power transformer calculatedusing ABB method of calculation.

A comparison between magnitude of the first cycleof inrush current as calculated by the old formula inequation (I) above versus that calculated using the rigorousABB calculation, including the above mentionedparameters, provides much lower magnitudes of the firstpeak of inrush current compared to that calculated using theold formula commonly used by the industry.

YI EFFECTOFTR NSFORMERDESIGNP R METERS osr HRMONIC OF INRUSH

CURRENT

Effed ofDesip 8u DetasityThe mioimmo bannonic peak inrush

current ratio decreases with induction as shown in fig3.Modem transformers generally operate at higher fluxdensity value since higher grain oriented steels are usedmore more. It thus results that modem, transformers will

have lower minimum 2nd harmonic / peak: inrush currentratio.

EFFECT OF CORE MATERIALAnother new feature of modem transformer is the

use of Hi-B electrical steel type materials which have highersaturation flux densities, a larger linear portion ofmagnetizing curve, a lower remanence flux density valvescompared to Regular Grain Oriented (RGO) type materials.Thus, there higher grain orientation materials are associatedwith high her minimum 2nd harmonic / peak inrushcurrent ratios. For the same flux density, the Hi-B andDomain refined materials have an appreciably greaterminimum 2nd harmonic / peak inrush current ratio thanRGO material.

EFFECT OF CORE JOINT TYPEUntil a decade or two ago, the non step-lap joint

was commonly used in transformer cores, however modem,transformers use the step-lap type joint. Because of the highreluctance of the core joints, the remanance flux densitybuds of a transformer core is significantly lower than that ofthe core material it.sd As the non step-Iap joint bas agreater reluctance than a step-Iap joint, it fullows that a carewith the step-Iap joint would have a much lower minimumof 2 harmonic/peak current ratio than those of a corewith a non step-Iap joint..

EFFECTS ON RELAYINGEffects oa RdayiJlg TraasfonDer diffeJ elltiaiprotcrlioaAny abrupt increase of the power transformer

terminal voltage wilJ result in a transient current that isgreater than the ratio current of the tran.sfonoer_ Thistransient current in generally tenned as inrush current and istypically caused by : Energization of the power transformer. Voltage recovery after clearing of a heavy short circuit

the power system (recovery inrush) Energization of another parallel connected power

transformer (sympathetic inrush) Out-of-phase synchronization of a connected generator.

VII INRUSH DETECTION BY H RMONIC N LYSISOF INST NT NEOUS DIFFERENTI L CURRENT

Referring to fig.(3), it can be seen that the minimum 2ndharmonic content is in the range of 20% - 25% at low fluxdensities.. Tr.msfunner protection relays that use the secondhannonic as the restraint criteria have setting values for-this15% - 2oo range_ This was correct for-tnmsfonner designsoperating at low flux densities, The second harmoniccontent fur present day transfunoer designs are significantlylower, in the range of 5-100 /0 , as indicated in fig(3)_

This will affect the performance of relays that usethe second harmonic setting set to the range of 15-20 /0 harmonic restraint relays may not restrain correctly duringthe energization of power transformer that have a higherrated flux density 1.5 - 1.75 T


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For transformers which have the higher rated fluxdensity, it is recommended that the lower (5-12 ) 2ndharmonic restraint relay setting be used. Inrush detection bywaveform analysis of Instantaneous differential current.

It can be observed from fig.(2) of the inrush currentwaveform that there is a period of time in each powersystem cycle during which very low magnetizing currentsflow. From thri, an inrush condition can be identified wherea low rate of change of the instantaneous differential currentexists for at least a quarter of the fundamental powersystem cycle.

This criterion can be mathematically expressed forPhase A as:

lildiff-a< : C,

where Idiff - a is instantaneous differential current inPhase A, t is time and C, is a constant fixed in the relayalgorithm.

Practice bas shown that aI1hougbusing tbe secondharmonic restrainlblocking approach may prevent falsetripping dming iorosb current conditions, it may somdimesincrease fault clearance time foe heavy internal fuultsfollowed yr saturation.. On tbe positive side, tbe secondhannonic restrainJblocking approach will increase securityof tbe differential relay for-a heavy external fault with rsaturation..INRUSH DETECTION BY ADAPTIVE TECHNIQUES

The combination of the 2nd harmonic/fundamental inrushcurrent ratios (I 2 / I I) and waveform analysis methods,allow the relay designer to take advantage of both methods,while at the same time avoid there drawbacks. Twopossible ways to combine these methods are. :Conditional (recommended) In this mode of operation thesetwo criteria's are used as follows: Employ both the 12/1 Iand the waveform criteria to

detect initial inrush conditions. Disable the 12/11criterion one minute after power

transformer energizing in order to avoid long clearancetime foe a heavy internal faults and let tbe wavefunncriterion alone take care of tbe sympathetic andrecovery iorosb.

Temporarily enable 12 /11 for-6 seconds when a heavyexternal fault has been detected to gain additionalsecurity foe external faults,

Always (traditioaal appl' Ollld)-This option is similar to tbe usual 121I1 criterion.. The 121I1 is active at aUtimes, and in addition, tbe waveformcriterion works in parallel. This has no benefits in tams ofspeed for-heavy internal faults,

VIII INTERNL F ULTS FOLLOWEDBY THE CT S TUR TION

For heavy internal transformer faults followed byCT saturation, the distorted CT secondary current maycontain the high level of second harmonic. As aconsequence, delayed operation f the restrained differentialprotection will occur as indicated in fig. 4.

For an external fault, the fictitious negativesequence source will be located outside the differentialprotection zone at the fault point. Thus, the megativesequence currents will enter the healthy power transformeron the fault side, and exit on the opposite side, properlytransformed The negative sequence currents on tberespective power transfOl1llelsides will have oppositedirections. In other words. the internal / external faultdiscriminat.or sees these RUTeDts as having a relative phasedifference of exactly 180 electrical degrees, as shown in fig.

For an internal fault (with fictitious megativesequence source within protected power transformer), themegative sequence currents will flow out of the faultypower transformer on both sides. The megative sequencecurrents OIl tbe respective power transfunner sideswill havethe same direction.

In other wards, the internal external faultdiscriminator sees these currents as having a relativedisplacements of zero electrical degrees, as shown in fig. 6.

In reality, for-an internal f3ult, there might be somesmall phase shift between these two currents due to possibledifferent megative sequence impedance angles of the soun:eequivalent circuits on the two power transformer sides.

It operation is based on the relative position of thetwo phasors representing HV L.V. negative sequencecurrent contributions. It practically performs directionalcomparison between these two phasors. First, the L V sidephasor is positioned along the zero degree time. Next, therelevant position of the H V side phasor in the complexplain is determined. The overall directional characteristicsof the internal/external fault discriminator is shown in fig.7. Using the negative sequence internal/external faultdiscriminator, the second harmonic blocking feature can beby passed and faster operation of transformer diff.protection will be achieved as shown in fig. 8.

IX RESTRICTED E lUH F ULT PROTECTJONBreakdown of the insulation between a pease

conducto.- and ground in an effectively grounded or-lowimpedance grounded power system reseals in a large faultcurrent.. A breakdown of the insulation between atransformer winding and the core or-the tank may result in alarge fault current which in tmn causes severe damage to tbewindings and the transformer core. A high gas pressuremay develop, rupturing the transformer tank..

Fast and sensitive detection of ground faults in apower transformer winding can be obtained in solidlygrounded or low impedance grounded networks by the


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K-FadDICaIn fil a Tvoia N


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included. Some use upto the 15th harmonic, others the 25thharmonic and still others include up to the s harmonic.For the same load, each of these calculations canfield significantly different K-Factors because even verysmall current levels associated with higher harmonics whenmultiplied by harmonic nwnber squared (e.g. 502 2500),can add significantly to the K-Factor. Based on theunderlying acceptations of C 57.110, it seems reasonable tolimit the K-Factor calculations to harmonic currents lessthan 25th harmonic. .

Sample calculations are shown as under:In establishing standard transformer K-Factorratings, UL chose ratings of 1, 4, 9, 13, 20, 30, 40 50.From practical viewpoint individual loads with K-Factorsgrater than 20 are infrequent at best official areas with somenon-linear loads and large computer rooms normally havedeserved K-Factors of 4 to 9. areas with high concentrationof single phase computers & terminals have, observedK-Factors of 13 to 17.

ON LUSIONSThe old formula for calculation of peak inrush

current of power transformer provides magnitudes as muchas twice that calculated using the more accurate equationrecadIy developed by ABB. This calculation accounts for aomnber of important traosfunner and system parameters thatsignificantly impact the magnitude and wave fono of theimusb cum:nt.

Accurately predicting the peak inrush cum:otshould be very criticaI in designing and. detennioiog thesettings of the over-current reJaying used with powerIlansfunner. Also the new calculation providing thefunction of inrush current versus time throughout theduration of the transient as well as the magnitude of the 2ndharmonic of the inrush current. This is the parametercommonly used today to differentiate between short circuit

and inrush current occurrences, hence preventing a powertransformer of being erroneously removed from service bythe over current or the differential relays.Inrush current parameters (peak, 2 harmonic, andduration) of today's power transformer differ from those ofolder designs due to the use of higher grain oriented coresteels, the step-lap core joint type, and higher rated designcore induction values. This should have a significant impacton selecting the proper relaying protection of thetransformer.

KNOWLEDGMENTBharati Vidyapeeth Deemed University college ofEngineering, Pune

R F R N1) Elmore WA - 1995, Protective Relaying Theory and

Application, ABB Power T &D.2) RET 670 Technical reference manual, LMRK 504048 -

UEN.3) Electrical Transmission and Distribution reference Book,Westing house Electric Corpn.4) F-Mekic, Z-Gajic, S-Ganeshan A tive feature onNumberical Differential Relays, (29 Annual Westen

Protective relay conference, Spokane, WA October 22-27,2002)5 Z. Gajic, B-HiUsbum, F-Mekic V Sham: reactosecrets fix Pratedion F.ogioea, (3{f' AnouaI Wc:sImProtective relay ronfi:n:nre, Spokane WA Odoba 2123,2003)

6 Z. Gajic, R Hillstrom, L Bracis, L lvaokorie ScositiveTurn-to-Turn Fault Protection, (32dl Annual WestanPratedioo relay aJOfen:nre., Spokane. W.A., Oct. 21-232003).7 F-Mekic, Ram Sis Girgi's, Zor.mGajic.,Ed IeNyamousPower Transformer Characteristics their effect onProtective relays (33 Annual western protection relayconference, Spokane, W.A. Oct. 1-19,2006).

power transformer inrush

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