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ANALYSIS AND DESIGN OF A NEW ACTIVE POWER FILTER TOCANCEL NEUTRAL CURR ENT HARMON ICS IN THREE PHASEFOUR W IRE ELECTRIC DISTRIBUTION SYSTEMS

P. Enjeti, W . Shireen, P. Packebush I. PitelPower Electronics Laboratory Magna-Pow er ElectronicsDepartment of Electrical Engineering 135 Route 10Texas A&M U niversity

Tel: (409) 845-7466Fax: (409) 845-6259College Station, TX. 77843-3 128

Abstract - Recent surveys of 208/120V hree phase four wireelectric systems, buildings and industrial plants withcomputers and nonlinear loads, show excessive currents in theneutral These neutral currents are fundamentally thirdharmonic and their presence is tied to wiring failures, elevatingof neutral potentials, transformer overheating, etc.In response to these concerns, this paper proposes a newactive power filter schem e to cancel n eutral current harmonics.The proposed approach employs a stad delta transformer alongwith a two switch PWM ontrolled active filter. The closedloop control of the active power filter guarantees cancellationof neutral current harmonics under varying load conditions.The proposed system drastically improves the overall systemperformance and virtually elimina tes transformer overheatingdue to harmonics. Experimental results from a proto-typeactive power filter confirms the suitability of the proposedapproach.1. INTRODUCTION

A typical low voltage electric distribution system in theU.S. consists of a 208/120V three phase four wire systemand is widely employed in distributing electric energy toseveral office buildings, and manufacturing plants. In suchsystems, under normal operating conditions with the loadsreasonably balanced, the cu rrent in the neu tral is expected tobe small and not to exceed 20 percent of the normal phasecurrent magnitude. However, in view of the recentsignificant increase in the use of advanced power conversiontechnologies and com puter/data processing equipment, suchideal operating conditions no longer p revail.Typical loads connected to a low voltage three phase fourwire system include: adjustable speed heating ventilation andair-conditioning W A C ) systems; fluorescent lightingcircuits with conventional and electronic ballasts; computersfor data processing and office automation as well as manyother sensitive electronic loads. Almost all of the aboveapplications employ switched mode type power electronicconverters which draw excessive harmonic currents of w hicha significant portion is the third harmonic (180 Hz)

Whippany, NJ 07981Tel: (201) 428-1 197Fax: (201) 428-2853

component. Further, saturated iron cored inductive ballastsas well as electronic ballast in fluorescent lighting circuitsalso contribute to third harmon ic currents [l]. The thirdharmonic and odd multiples of 3rd (i.e. 9th, 15th, etc.) areco-phasal in each phase and do not cancel each other in theneutral. The result is in fact a cum ulative addition, and theprimary source for excessive neutral currents in modemthree phase fou r wire distribution systems. A recent surveyconducted by Liebert Customer Service engineers in 146computer sites across the country revealed that 22.6 percentof the sites had neutral currents in excess of 100% of thephase current [2]. These results also concur with the surveyconducted by the computer and business equipmentmanufacturers association (CBEM A). CBEMA recentlypublished a white paper waming that a shared neutralconductor in modem buildings may carry increasedharmonic currents and result in wiring failures [4]. Potentialproblems directly related to excessive harmonic currents inthe neutral con ductor are:(i.) Wiring failure due to improper sizing of the neutralconductor.(ii.) Overheating of the transformer due to harmoniccurrents, and insulation damage an d failure.(iii.) Intermittent electrical noise from connectionsloosened by thermal cycling.(iv.) Excessive neutral to ground voltage due to a voltagedrop caused by the neutral current. This commonmode potential can result in th e malfbnction ofsensitive electronic com ponents.Both Liebert Corporation and CBEMA recommend thefollowing practices and corrective measures [3,4],(i.) Derate transform ers.

(ii.) Use separate neutral conductors for nonlinear loadsand avoid shared neutral cond uctors where practical.(iii.) Use neutral over current sensors to trip phaseconductors.(iv.) Use true rmammeters and instruments withsufficient bandwidth for m easurement.0-7803-462-x/93$03.OO01993EJE 939

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Threc phaseTransformer

. _ _ _ _ _ _ ..... ..... ...

\

____ ,

bi 1 i

@>Fig. 1 (a) Ropasedactivepower filter topologyto cancelneutralcurrentharmonics(b) Three phase transformer corewith star connectedprimary(delta connectedsecondary not

show). Thezerosequence lu x 4o generatedby thethi rdharmonic current IN/3 isin eachlimb windingThe above recom mended practices are effective temporarymeasures and h ave the following serious drawbacks.(i.) Derating a transform er is a temporary fix and oftentranslates nto lower efficiency operation andincreased heat for losses. Derated transformersalso run he risk of being perceived to be partiallyloaded and future load additions are possible.

(ii.) Separate neutral conductors for computer loads isalmost impossible to implem ent, due to a widescattering of data processing equipm ent all over thebuilding.Other suggested methods to reduce neutral currentharmonics include the use of nonstandard zigzag

transform er connections [ 5 ] . These m ethods also contributeto the elevation of the neutral potential and are noteconom ical to implement. A recent paper [9] proposes aneight switch active filter topology which is complicated toimplement and control.In response to these concerns, this paper proposes a newactive power filter topology (Fig. 1) designed to cancelneutral current harmonics in a three phase four wire system.The proposed topology con sists of a stadd elta transformer, adiode rectifier and a sim ple half bridge PWM inverter. Theactive power filter generates compensating currents thatresult in effective cancellation of the harmonic currentsflowing in the system n eutral du ring closed loop operation.940

. .. . -

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It should be noted that the control scheme does not monitorthe fundamental (6OHz)current flowing in the ne utral due toload unbalance, but only accounts for the harmonicscurrents. Also, he staddelta transformer in Fig. l(b) drawsnegligible 60Hz urrents from the three phase ac mains asthe active power filter does not consume any real powerother than that required for losses.

2. P R O P O SED ACTIVE OWER FILTER TO CANCEL NEUTRALCURRENT HARMONICSFig. l(a ) illustra tes the proposed active filter topology.The Ycon necte d transformer primary, as shown in Fig. I@),provides a relative neutral point, 'N', between the activepower filter and distribution system n eutral, 'n'. The deltaconnected secondary provides a path for zero sequencecurrents to circulate, thereby m aintaining s t a r point 'N'at the

same potential as the supply neutral 'd. Further, thesecondary of the transformer supplies a three phase diodebridge rectifier that provides the small amount of powerrequired to maintain th e dc voltage across the capacitors.The operation of the active power filter is as follows; theneutral current, h, is sensed via a current sensor and is

processed th rough a 60Hz otch filter in ord er to remove anyhndam ental current component in I,,. The filtered currentsignal is then comp ared with Jd seeFig. 2) which is set tozero. The resulting error signal is fed to the PWM controllogic in order to inject an equal and opposite current IN,thereby a chieving cancellation in closed loop. This currentinjection technique neutralizes any harmo nic current that isflowing in the neutral, and thus protects the upstreamdistribution system and transformer. Hence, if the filter iscanceling 1W /o of the neutral current, then IN =I,, an d I,,, =0. It should be noted that the injected harmonic current IN,from the active power filter, is essentially zero sequence(3rd, 9th, etc). The zero sequence current IN is injected andequally divided between the three primary windings of thetransformer such th at $=IN/3 (seeFig. l(b)).In this system, three limb core con struction of thetransform er is preferred for the following reasons. In thecore type construction (Fig. I@) ) the zero sequ ence flux inthe three legs do no add to zero as in the positive sequencecase. Instead the sum 3$0 must seek a path throu gh the airor through the transformer tank, either of which presents ahigh reluctance. The result is a low zero sequence excitationimpedance, so low that it can be neglected. Therefore thezero sequence currents encounter only a small leakage

Three phaseTransformer

1... L f

NONLINEARLOAD

(A combinationof single phase arthree phase t ypeloads.)'ICTIVEPOWERFILTER

ControllerotchFilter

Fig.2 (a) Closed loop control oftheneutralcurrent cancellation schemeby proposed active power filter941

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e Controllergainstage-

I KC KAh4P Ih - I NC I -. R + L p

(b)Fig.2. @) Cl& loop block diagram

impedance (nearly same as the fundamental positive switching block KmF The control signal ,V,, obtainedsequence impedance) in the path through the three legged after the controller gain stage K,, is used as the modulatingcore type transformer [8]. However, the delta connected signal and is compared with a high frequency (20 KHz)secondary carries circulating third harmonic current to triangular wave in order to get the gating commands or thebalance the primary limb IN/3. A more detailed inverter switches. Hence, if th e peak amplitude of thecharacterization of a three phase transformer under zero triangular wave is AT then,(1)In the design, the two capacitors that maintain the dc busvoltage are chosen to provide a ripple free dc voltage andhave to be suitably rated to circulate th e neutral current Finally, the current injected by the active power filter I,, is

harmonics caused by the pwM operation of the two error produces a new neutral curren t \-IN that is againcompared with kf n order to generate an error signal. Inswitches. closed loop, the m easured error is reduced to near zero, andThe advantages of the proposed approach are;(i.) continuous measurement and cancellation of the neutral current harmonics are effectively canceled by theactive power filter. Furth er, the closed loop continues tocurrent harmonics. respond to changes in load conditions an d suitably provides(ii.) Active power filters du not consume any real powerother than that required to account for internal continuous cancellation.losses. The proposed active power filter is expectedto be over 90% efficient.conditions.bandwidth to can cel several zero sequence harmon icsappearing in the neutral. The proposed active powerfilter operates at a high frequency (~ 20 kH z) nd aninput filte r stage can be designed to bypass thewith the l ine . 0 ) R+L,s(V.) Emp loys state Of the art power semicondu ctor devicesand therefore is compact, light in weight andoccupies less space.

sequence excitation is detailed in [SI. ' 'ABK M p =--T 2

harmonics. The inductor Lf is selected to filter the switching "pared with the current In. Th e

2.2 ANALYSISFTH E CLOSED W P SYSTEM(iii.) The proposed system can adap t to chan ging load(iv.) Has fast response characteristics and sufficient Approximate modeling of the closed loop control systemsuggests that it is a first order system . From Fig. 2(b), theopen loop transfer fun ction between the controlled output

and erro r signal is given by,(2)(1, -IN)(s) - Kc*KAMpswitching harmo nics, thereby avoiding interference GI s)= -

The corresponding closed loop transfer function can beexpressed as,

K c KAM P (3 )(I , - N )(s) --2.1 CLOSEDOOP CURRENT CANCELLATION SYSTEM Ircf(SI L f s+R +K, K M pwhere,ONFIGURATIONThe closed loop system configuration is illustrated in Fig .2. Following the block diagram shown in Fig. 2(b), the

system operates as follows: The curren t flowing in theneutral, In-IN, s sensed, passed through a 60 Hz notch filter,and then compared with a reference level I The referencelevel is set to zero as the desired neutral current is zero. Theerror resulting from the comparison is then am plified in thecontroller gain stage K,, which in turn controls the powero,

K, = Controller gainK m p = Gain in the half bridge PWM inverterLf = Filter inductorR = resistance in the current p athThe closed loop transfer function between theutput I,, - IN and input I, is given by,(I , - IN)(S) - R+L, s

IN (s ) L ~ s R +K, * K m p

stage

controlled- (4)

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The value of the gains K, and K m P determine the amountof corrective effort which is applied for a given m agnitude oferror, e. For low values of controller gain the correctiveeffort is small and hence the response is likely to be slow.As gain is increased the response of the system for the samemagnitude of error increases. On the other hand if K, is toolarge, instability is likely to result. Therefore, the m agnitudeof the steady state error, the value to which th e error sig naltends as he transient disturbance from any input change diesout, is of importance since it is a measure of systemaccuracy.The steady state error ess, due to a step change in input isgiven by, 1- -

ss s j o i+K,K,, l + K pe = Lt R +L,swhere,

Thus the steady state error of the first order system isfinite. Suitable values of the loop gain Kp are selected inorder to obtain a low steady state error.

3. DESIGN XAMPLEIn this section a design example is given to facilitate theunde rstanding of the design appro ach for the proposed activepower filter. 1pu voltage =120V1pu kVA =3.6kVA1pu current =30A1 pu im pedance =4 n1 pu frequen cy =60Hz1 pu induc tance=10.61mH1 pu cap acitance =663.14uFLe t us assume the active filter is required to cancel 1 pu ofthird harmonic (180 Hz) urrent flowing in the neutral. Thisrequires filte r curren t, I, =1pu at 180Hz.

Three phase transformer:The active filter requires two magnetic components, adeld wy e transformer (Fig. l(b)) and a filter inductor LPThe KVA rating of the transformer is determined by theproduct of the voltage supported at the m ains frequency andthe currents circulated in the windings. These currents arecomprised of harmonics produced by the inverter in the wyewindings, triplen harmonic currents circulating in the lowimpedance delta windings, and the fundamental currentsrequired to feed inverter losses. Assuming reasonableinverter efficiencies, the burden of the inverter can be

ignored. Since the transformer is excited at utilityfrequencies, core m aterial can be conventional gra in orientedsteel. Primary voltage =1pu =120VPrimary current =IN/3=.33 pu =1OATurn ratio (sec/pri)=.5Secondary voltage = .5 pu =60 VSecondary curren t =.66 pu =20A

DC ide capacitors:dc v oltage.The dc side capacitors are designed to provide a ripple freeDC bus voltage Vm =1.35 * .5 =.67 puVoltage across each capacitor, V, =.335 pu =40VRipple current through the capacitors, = lpu at180Hz.

Ripplevoltage, A V ~ , ~ ~ ~ ~-w cIn order to limit AV,,,,,,, =0.05 pu

= 13,262pF0C = 3 - 3 7 7 4 O a . 0 5

(7 )

A capacitor with a very low equivalent series resistance(ESR) has to be selected to obtain m inimum losses.Filter inductor:The inductor L, must circulate energy at harm onics of thepower mains an d support voltages at th e inv erter's switchingfrequency. The preferred mag netic core should exhibitrelatively low losses at ultrasonic frequ encies under high dcmagn etization. Molypermalloy powder cores (MPP),composed of nickel, iron, and molybdenum, are ideallysuited for this application. These cores saturate at typically8000 Gauss and offer losses comparable to that of ferrite.The value of the filter inductor L, is selected to limit theswitching harmo nics. Mo reove r, L, should be small enoughto allow the flow of 1 pu 180Hz curre nt.

(8 )c A,Filter curre nt, I, =-z Lf.3where A, =modu lation index, Xu,3 =30LfFor 1 =1pu and A, =.8.335 .8 = . 1 9 p u = .76a

JzL f , 3 =Lf= .67 mHFilter current at the switching frequency (20 KHz)

= . 4 8 AVC2 ~ .0 . 1 0 ~ . L,N.20KHz -943

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so, L,,= .67mH limits the switching harmonics to 1.6%which iswithin the acceptable range.

4. EXPERIMENTALESULTSA laboratory prototype of the proposed active power filter

has been constructed in the power electronics laboratory ofTexas A&M University. The results obtained are discussedin this section.Fig. 3(a) shows the neutral current In prior to anycancellation, and Fig. 3(b) is the frequency spectrum of thiscurrent. As can be seen from the figures, the neutral currentconsists of the triplen harmonics, with the 3rd beingdominant. Fig. 4(a) shows the harmonic current IN beinginjected by the active filter in order to cancel harmonics inthe neutral line, and Fig. 4(b) is the frequency spectrum ofthe filter current I,. The overall effect of the active filter onthe neutral line current In*,(see Fig. 1) ca n be seen in Fig. 5 . From Fig. 5(a) it is shown that the amount of harmoniccurrent flowing in th e neutral ha s been significantly reduced,and Fig. 5(b) shows the frequency spectrum of In,,confirming the drastic reduction of neutral current due to theactive power filter action. Fig. 6(a) shows the current 1,flowing through on e leg of h e transformer primary, whileFig. 6(b ) shows the frequency spectrum of I As expected,1, is one third of I,. The current circulating in the deltaconnected secondary of the three phase transformer is shownin Fig. 7(a). Th e corresponding frequency spectrum in Fig.7(b) shows a dominant third harmonic and other triplenharmonics. A small amount of 60Hz component in thiscurrent is a representation of the real power drawn toaccount for losses in the active filter action. Fig. 8 shows theinput current of the three phase diode rectifier whichmaintains the voltage across the dc capacitors. As expected,this current is of a small magnitude and is required tocompensate for the system losses.

P' .

Fig. 3 (a) Neutral current, I,, before cancellation (see Fig. ! (a))

180Hz 540HzFig. 3(b) Frequency spectruni of 3(a)

.2A

4.2A

Fig. 4 (a) Active Filter current IN operating in closed loop

18OHz 540HzFig. 4 (b) Frequency spectrum of 4(a)

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Fig. 5 (a) The resulting neutral current, In, =In - IN aftercancellation

1SOH2 540HzFig . 5 (b ) Frequency spectrum of 5(a)Notice the near c ancellation and d rastic reduction in neutral current ham1onics

due to the active filter action

PFig. 6 (a) Current through one leg of the transformer primary, I

180Hz 540HzFig.6 (b) Frequency spectrum of6(a)

1OA

- lO AFig 7 (a! Current circulating in the delta connected secondary

Fig.7 (b ) Frequency spectrum of 7(a)

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[SI P.P. Chorea, "ApplicationofZigzag T I W K & " for educing Harmonicsin the NeutralConductor ofLow Voltage DisfributicmSystem", IEEE IASW.Rec. 1990.Filter usingCurrmt Sourceconvater", EE E IASM. cc.1990, p.%5-970.

[6] S. ukuda nd M. Yamagi, "Designandch"W'csofActivePowa

[7] H. Aka@,A Nabae and S. Atoh, "ControlStratqgofAdive P o w ilterusingMultiple Voltagesource PWM Coavaters", IEEE T m s , on 4Vol. IA-22, NO.3,M . y / J w 1 98 6[8] Paul Anderson,"AnalysisofFaultedPowa yrrtans", text book, Iowa StateUniversity Press, 1973, pp. 251-257[9] C. k Quinn,N. Mohm . "Active FiltaingofH&c cun#mi n"e-

phase, F O L U - W ~ystanswitht hteephaeeand Sn&-* Noa-LinearLoads",APEC 1992, pp. 829-835

Fig. 8 Cwrent flowinginto one egofthreephaserectifier. Ii

4. CONCLUSIONIn this paper an active power filter to cancel harmoniccurrents in the neutral of a three phase four wire system hasbeen proposed. Th e proposed active power filter displays theability to effectively cancel undesirable excessive harmonicsflowing in the ne utral line of a three phase four wire system,and is highly efficient and low in cost. The proposedtopology drastically improves the system performance,contributes to efficient use of electric energy and virtuallyeliminates excessive h eating of distribution transformers dueto harmonic currents. Finally, experimental results on a

prototype system validate the proposed approach and itsdesign fundamentals.

ACKNOWLEDGMENTThis research is funded by a gra nt under the AdvancedTechnology program by he state of Texas.

REFERENCES[l ] A Liew, "Excessive Neutral Currents in Three Phase Fluorescent LightingCircuits", IEEE Tram on A, Vol. 25, No. 4, July/August 1989,pp. 776-782.[2] T.M. GNZS,"A SurveyofNeutral Cu rrents inThreePhase Canp uterPower Systems", EE E Tram on IA, Vol. 26 ,No. , July/Augusl 1990,

p ~ .19-725.[3] "Nonlinear Lard0mean rouble", EW M , March 1988,pp. 83-90.[4] "CBEMA Idomration Letter", CBEMA, ESC-3 Conunittee, C BEMA -311 First Street, N. W. uite 500, Washinglon,DC 0001

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