Harmonic Current Injection

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Paper accepted for presentation at 2003 IEEE Bologna PowerTech Conference, June 23-26, Bologna, Italy

A Novel Three-phase Full Bridge ThyristorRectifier Based on the Controlled ThirdHarmonic Current Injection

PredragBoioviC, and PredragPejovic, Member, IEEEAbslract-A novel three-phase full bridge thyristor reclifierthat applies controlled third harmonic current injection isdescribed in this paper. To compensate for the variations inthe bridge firing angle and the rectifier load current, theconcept of resistor-f reecurrent injection network is adaptedapplying controlled cnrrent injection method.This approachmakes it possible to obtain near optimal current injectionresulting in minimum input current THD, while preservinghigh efficiency of the rectifier. Optimilationof the injectedcurrenth attained through the time domain analysis, on thewaveform level. Based on this analysisthe selectionof currentinjection network parameters i s made Feasibility of theproposed concept is verified both experimentally and bysimulation.I ndex Terms-AC-DC power conversion, convertem, harmonicdistortion,power conversion harmonics, thyristors,rectifiers.

1. INTRODUCTIONO " E A R nature of rectifiers causes harmonicN ollution.n power distribution systems. Standards andrecommendations that treat this problem motivate intensive

research in harmonic reduction methods. Prominent amongthese methods is the thud harmonic current injection, anapproach that reduces the input current harmonics of three-phase rectifiers.The three-phase diode bridge rectifiers that applypassive current injection networks are proposed in [I ] and[2]. The output voltage ripple is used to generate theinjected current. The networks are characterised bysimplicity and robustness. In [31, rectifiers proposed in 111and [2] are analyzed. Optimal amplitude of the injectedcurrent as a function of the injected current phasedisplacement to the spectral components of the outputterminal voltages at the triple of the line frequency isderived. It is shown that a part of rectifier input power hasto be taken by the current injection network in order toimprove the rectifier input current total harmonic distortions(THD). In the caseof the optimal current injection method,derived for the rectifier operating in the continuousconduction mode [3], this power is equal to 8.571% of theinput power, resulting in the input currentTHD of 5.125%.

P. BoioviC is with PupinTeecornDKTS, Belgrade11080, Serbia andMontenegro (e-mil:[email protected]).P. PejoviC is with theFaculty of Elecaical Engi neeri ng. University ofBelgrade, Belgrade 11120, Serbia and M ontenegro, (e-&[email protected]).

The power is dissipated on the current injection networkresistor that should have variable resistance to adjust to theload current variations. Application of passive resistanceemulator, reported in [4], increases rectifier efficiency at theexpense of its complexity. Current injection network withthe emulated resistor contains two resonant circuits thathave tobetuned to the third hannonic of the line frequency,making the system performano: sensitive to variations ofthe resonant circuits parameters.The idea of the resistor-free current injection network,developed in [ 5 ] , increases rectifier efficiency on thepenalty of moderate degradation of the rectifier inputcurrent THD. This concept is used as an inspiration for thethree-phase thyristor bridge rectifier proposed in this paper.To handle the variations in magnitude and phaseof the thirdharmonic component of the output terminal voltages, causedby variations of the thyristor bridge firing angle, thenetwork proposed in [5] is adapted applying controlledcurrent injection method [6].In Section U, structure 01 the three-phase thyristorbridge rectifier applying simultaneous current injection inall three of the phases is presented. The spectra of therectifier output voltage termnal!; arederived and discussed.The resistor-free current injection network that appliescontrolled injection is presented in Section III. Optimizationof the injected current is attained through the time domainanalysis, on the waveform level. In that manner, influenceof the rectifier output termnal voltage harmonics, otherthan the third harmonic, on input currentTHD is included.Simulation results for the input currentTllD as function ofthe bridge fuing angle, rectifier load current and resonantcircuit parameters are given in Section IV. Powerconsumption of current injection network is also discussed.Experimental results are presented in Section V, andconclusions are given in Section VI.

II. STRUCTURE. OFTHE THYRISTOR BRIDGERECTIFIERStructureof the proposed rectifier is presented in Fig. 1.The rectifier is supplied by a balanced three-phase voltagesystem

(1 )

where i E #,2,3}. It is assumedthat i, >0 and i, >0 inevery time point during the line period, causingthe thyristor

2n3vi =V, cos(q,t+(i-l)-),

0-7803-7967-5/03/$17.00Cl2003IEEE


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bridge to operate in the continuous conduction mode. The harmonics at the triples of the line frequency. In the case ofrectifier consists of a thyristor bridge, a current injection thyristor bridge rectifier that operates in continuousdevice, and a current injection network. The current conduction mode, the magnitude and phase of vAinjection is achieved applyingamagnetic current injectiondevice realizedas azigzag autotransformer. harmonics aregiven by

I I -

(5)a 9k 2 -1

for 0'


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the injected current is associated not only to the variationsof the rectifier load current, but also to the variations ofmagnitude of the third harmonic driving voltage. From (6) itcan be concluded that phase of the third harmonic drivingvoltage changes faster than the bridge firing angle,deteriorating the optimal phase condition [3]. The currentinjection network presented in [5 ] should be modified tocompensate the variations in thebridge fxing angle and theload current.

I l l . STRUCTUREANDOPERATION OF THERECTIFIER

Fi g. 3. Rectifier.The three-phase ac-to-dc rectifier proposed in this

paper is presented in Fig. 3.The current injection networkconsists of a transformer with the 1:l turns ratio, aninductor, two capacitors and one pair of antiparallelconnected thyristors. The capacitors and the inductor areapplied to filter spectral components of the thyristor bridgeoutput termnal voltages v A and vBat the triple of the linefrequency. The resonant frequencyof the current injectionnetwork is determined by

1OR=30, =E ' (9 )Although the harmonic components of i,A and i,B at eventriples of the line frequency do not affect injected currenti , , they increase the input curentTHD [7]. The path forthese harmonic components is blocked applying the 1:ltransformer. Control over magnitude and phaseof injectedcurrent is achieved by insertion of antiparallel thyristors,Q, and Q, , n series with filter. In that way variations inbridge fuing angle and load current could be compensatedfor. Typical waveforms of the proposed rectifier arepresented in Fig. 4. The output termnal voltages v A andv areshown for the phase voltages specified by (1) and

Ld60 120 180 240 300 360L Oo+A0 120 180 240 300 36Cs0b-10 60 120 180 240 3W 360

-1.ot vFig.4.Typical waveformsof thethyristor renifier..


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the bridge thyristor firing angle of a=30' .The voltage effectively applied across current injectionnetwork is presented in the second diagram of Fig. 4.According to (7) and (8) this voltage is periodic with theperiod equal to the one third of the line period, containingspectral components at the odd triples of the line frequency.ThyristorsQ, and Q8 firing angles are measured startingfrom the zero crossingsof the voltagec.othof thethyristors are switched at the triple of the line frequency.The corresponding thyristors gate drive signals are given inthe thud diagram of Fig. 4.Since the thyristors conductsymmetrically with the on time varying between zero andthe one sixth of the line period, the injected current i, isdiscontinuous, as depicted in the fourth diagram. Gaps inthe input current that exist in the case the current injection isnot applied, could be observed in the f i f t h diagramof Fig.4.Patching these gaps by injectingthecurrent i, hack tothe supply lines, results in the rectifier input current with themnimumof the harmonic distortion, as depicted in the sixthdiagram.Since one thud of the inductor current i, is injectedback to each of the rectifier inputs, its steady-state timewaveform has to be constructed in order to determine theinput current THD. Due to the symmetry of the currentinjection network at the odd triples of the line frequency,spectral components of the i,A and i,, have the sameamplitude and phase, resulting in

v +v2

(10). 1 .I I A =1,11 = - I y .2

v1+yB2

Fig. 5. Equivalent Circuitof current injection network for odd hiples of linekequency.For odd triples componentsof output termnal voltages thevoltage developed across the windings of current injectionnetwork transformer iszero.Hence, the equivalent circuitofcurrent injection network presented in Fig. 3 could besimplified and reduced to a circuit of Fig. 5.A resistance ofthe equivalent circuit, denoted as R , s inherent to reactiveelementsof the current injection network and the currentinjection device, modelling their l osses.

To obtain i, , the second order differential equation forLCR resonant circuit

has to be solved, for the presumed value of the filterthyristor fving angle p .Thevoltage appliedacrosscurrentinjection resonant circuit is

:os(o,1-60~), 0


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IV. SIMULATION RESULTSApplying the analysis decsribed in the previous section,a numerical simulation is performed for a rectifier that

operates with the phase voltage amplitude of V, =140V ,at the line frequency of fa =50Hz , Thecurrent injectionnetwork parasitic resistance is R =2.8Q ,and the resonantconstraint of 2LC =1.12.10"s2 is satisfied. Dependence ofthe optimal filter firing angle Po , input current THD andrectifier efficiency q on I , , for two different valuesof thecurrent injection network inductor L and a=3V arepresented in Fig.6, from top to bottom.It can be concluded that improvements in the inputcurrentTHDcould be obtained by increasing the value ofL . However, this results in significant increase of thecurrent injection network capacitor voltages. Due to theimposed condition that rectifier operates in the continuous

P O

conduction mode, for lower load currents and currentinjection network inductance of L =5.12mH the on timeof thyristor Q, and Q, are much shorter ul an in the caseofL =80mH , resulting in highor value of optimal currentinjection filter iiring angle P o . that means lower rmsvalueof the injected current, and thus lower power consumed bythe current injection network, yielding higher rectifierefficiency, that could be observed in the third diagramofFig. 6. A current injected waveforms with higher crossoverare insufficient to fully patch the gaps in the rectifier inputcurrents, causing higherTHD. A.s load current increases, thecurrent injection network with 1,=5.12mH becomes mucheffective in input currentTHD reduction, at the expense ofreduction in rectifier efficiency. It can 1x concluded thatalmost for all values of rectifiar load current the rectifierefficiency is significantly higho: than 9010 due to thefactthat the injected current is discontinuous.Dependence of the optimal filter firing angle Po andinput current THD on a , or two different values of thecurrent injection network inductor L and I , =5A ispresented in Fig. 7. In thecaseof L =5.12mH ,asthyristorbridge fKing angle increases, the magnitude of thirdharmonic driving function also increases, causing highercurrent injection filter tiring angle, and thus deterioratinginput currentTHD.

Po 115O 10 20 30 40 50 60

L.'2 3 4 5 6 7

L =5.12& ...., ....

....~........ .i............... L =80 mH .............. ............... Lt3 4 5 6 7

2 3 4 5 6 7l o AI

Fig.6.Optimal current nj ection filter firing angle, input currentTHDand rectifier effificiency as a function of rectifier load current andbridge firing angleasa parameter.

4O 10 20 30 40 50 60

a [.IFig. 7. opti mal current injection filter Si ng angle2nd input current THDas a f uncti onof thyriaor bridge f ~ n gngleand rectif i er load cu(Tent asparameter.


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V. EXPERIMENTAL RESULTSTo verify the analysis of the current injection network arectifier presented inFig. 3is built and tested. 'lhe rectifieris designed to operate with the phase voltages whose rms

values are equal to IOOV, at the line frequency off o =50Hz. The constant current load is realized applying aseries combination of a huge inductor and a resistor. Thecomponent values of the current injection network areC =l l O g , L=5.21mH and R=9.2C2. ThyistorsTlC126M are applied for QI to QS. The phase control ofthe thyristors is realized applyingTCA 785circuits. For thethyristor bridge tiring angle of a=12', output current ofI , =3A and the set of specified current injection networkparameters, according to simulation the optimum currentinjection filter f i ng angle is bo =12', resulting in theinput current THD of 9.42%. Waveforms of the inputvoltage and the input current at the output power of 670Ware presented in Fig. 8, for the case before and after thecurrent injection network is applied.

. .. . . . . . . . .. . . . . . . . .. . . . . . . .. . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . .. . . . . . . . .................................................................................... . . . . . . .. . . . . . .. . .. .. .................................................................................. .. ... . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . .. . . . .-~.....1 . .~ ..............


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in injected current waveform can be observed. Thewaveform of rectifier current i , in Fig. 10 proves thatrectifier operates in continuous conduction mode.Efficiencyof 90.8% s measured at the specified load.

VI . CONCLUSIONSA new three-phase thyristor bridge rectifier is proposedin this paper. The rectifier consists of athyristor bridge anda resistance free current shaping network. The currentshaping network consistsof two capacitors, one inductor,alow volt-ampere rated transformer, andapair of antiparallelconnected thyristors, that enables controlled currentinjection. The influenceof abridge fuing angle on spectraof the output terminal voltages, used to drive currentshaping network, is discussed. It is shown that the harmoniccomponents at the triple of the l ine frequency change inmagnitude and phase with thebridge firing angle variations.To include influence of these effects on the input currentTHD, the operation of the rectifier is analyzed in time

domain. Application of this analysis makes it possible toobtain near optimal third harmonic current injectionregardless variationsof the thyristor bridge firing angle andthe load current, preserving high efficiency. To verify theanalytical results, the current injection method is tested on670W experimental rectifier. Experimental results are inexcellent agreement with thetheoretical predictions.

REFEENCES[I ] S. Ki m P.Enjeti, P. Packebush, and I. Pitel, "A new approach toimprove power factor and reduce harmonics in a three-phase dioderectifier type utility interface," IEEE Trans. Ind. Applicot.. vol. 30,

W. B. Lawr ence andW. Mielczarski, "Harmonic current reduction ina three-phase di ode bridgeredtier," EEE Trans. Ind. Electron., vol.39,pp. 571-576.Dec. 1992.

pp. 15~7.1564,ovmec. 1994.[21

131 P. Pejavif, 2. J an& "An analysis of three-phase low-harmonicreaifiers applying the third harmordc arrent injection," IEEE Trans.PowerElectron., vol. 14,pp. 397407, May 1999.141 P. Pejorib, "Three-phasehigh power factor rectifier based on theuli rd harmonic Current injection with passive resistance emulation,"PESCZWOConferenceRecord,pp 1342.1347.P. Boiovib and P. PejoriC, "Three-phase high power faetor rec t i f i erbased on the third harmonic airrent injection in discont i nuousconductionmode," PCIMZOOZConferenceRecord,pp. 501-506.W. Mielczarski, W. B. Lawrence, R. Novacki, and D. G. Holmes,"Harmonic current reduction in ttuee-phase bridgwectifier circuitsusing conmlled current injection," IEEE Trans. Ind. Electron., vol.44,pp. 604411,Oct.1997.P. Pejovib, "An improved current injection network for three-phasehigh-power-factor rectifiers that ;apply the ttdrd harmonic currentinjection," IEEE Trans. Ind. Electron.. vol. 47, pp. 497-499. Apr.

[SI

161

[7l2000.

Predrag BoZoviC was born in Ri jeka, Y ugoslavia,in 1970. He received the 8.6:. and M.S. degrees inelectrical engineering from the University ofBelgrade, Belgrade, Yugoslavia, in 1994and 2000.respectively. He is currently working towards thePh.D. degreeat the UniversityofB elgrade.Since 1994, he has been research anddevelopment engineer in Pupin Telecom DKTS,l eadi ng Serbian manufacturer of telephoneswitching publii: systems. His research intereStsinclude three-phase low harmonic rectifiers, and power semiconductordevices modelling.

Predrag PeoviC ( S91- M96) was born inBelgrade, Y ugoslavia, in 1966. He received theB.S. and M.S. degrees in eectrical engineeringt om the University of Belgrade, Belgrade,Y ugoslavia, in 1990 and 1992, respectively. andthe Ph.D. degm: from the LMniversity of Colorado,Boulder, in 199%In 1995.he wjained the University of Belgrade,where he is preiently an Associate Rofessor. Hisresearch interests ar e in dynamics of nonlinearsystems, analog circuit design, three-phase high power factor rectifiers,demoni c measurements, and techniques for computer-aided analysis anddesign of power elecuonicsystems.

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