A Three-phasteh Ree-switcthw O-leveplw m Rectifier

8/8/2019 A Three-phasteh Ree-switcthw O-leveplw m Rectifier

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A THREE-PHASEHREE-SWITCHWO-LEVELWMRECTIFIER

Deivis Borgono vo, Yales RBmulo de Novaes, Ivo Barbi (Senior Mem ber, IEEE)Federal University of Santa Catarina - Department of Electrical Engineering - Power Electronic InstituteE-mail: [email protected] [email protected] [email protected] - This paper presents a new topology for a highpower factor three-phase P W M rectifier system, without aneutral wire, with output voltage control and high efliciency.The control is simple and was implemented using commercialsingle phase modulators with independent current loops and avoltage loop. The power circuit is also very simple, permittingthe use of low cost power devices. A mathematical analysis forthe converter wil l be presented, including th e determination ofall the power devices, the current and voltage transfer functionsand a project procedure. Finally, results obtained throughdigital simulations and the experimental results obtained from a

6kW prototype will he presented.1 INTRODUCTTON

Three-phase AC-DC converters without neutral wires areused in many applications, such as telecommunicationspower supplies, UPSS and electric drives. Conven tionalcircuits, using thyristors and diodes with passive filters,despite their simplicity and reliability, do not comply withintemational current hamo nic standards.Therefore, efforts have been made by engineers todevelop the so-called PWM rectifiers, capable of drawingpractically sinusoidal current from the mains. Many differentPWM three-phase topologies featuring low input currentTHD, most of them belonging to the family of three-levelconverters, have recently been proposed in the literature.

These converters require a complex strategy to balance thevoltage across the o utput filter capacitors.The circuit introduced in this paper does not need a outputcapacitor mid-point connection to work properly. Therefore,it is much simpler to design and control than its three-levelcounterpart, However, all the remaining desirable features arepreserved, presenting unity power factor with low TH D andoutput voltage control.11 STRUCTURE AND CHARACTERlSTlCS

The proposed circuit for the high power factor PWMthree-phase rectifier is presented as a natural evolution of themost widely used topology to control the input current ofsingle-phase rectifiers, the boost converter in continuousconduction mode (CCM), presented in the top left-handcomer of Fig.2. The development of the three-phase PWMrectifiers basic structure is presented in F i g 2Fig2 presents three independent single-phase rectifierswith output capacitors large enough to maintain the outputvoltage practically constant. However, the presence of aneutral point is undesirable. The neutral wire can be removedand the converter will still work properly, even though thetopological stag es are different.The proposed circuit for the three-phase PWM rectifier ispresented in Fig.1:

6 - -Fig.1:Proposed circuit for the three-phase P W M ectifier.

0-7803-7754-0/03/$17.0002003 IEEE 1075


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Now, gathering threesingle-phase rectifierssupplying the sameload, it can beobtained the circuitpresented below:

From the original shucture, theboost inductor can he moved to

the input, adding yet a new boostdiode, with no substantialmodificationon the converter.

Then, maintaining the samecircuit, but drawing it in adifferent way.

,I t f r I I tIr I t I

I I

It can be noted that the three-phase converter presents aninteresting sym metry. The input current period can be dividedinto 6 s ectors, and in each sector .the converter workssymmetrically. So, the analysis can he performed for achosen sector and be then easily extended to the others. Eachsector is defined by the line that presents the highest absolutevalue of the input current, defined by the sum of the absolutevalues of the other two input currents.Therefore, the topological stages and all the theoreticalanalysis for the converter will be presented for one sector. So ,initially, the input vo ltages are given by:V, (1 ) = Vp . in(w .t )V,(t)= Vp .sin(w.r-120) (1)I ( t ) = V p . in(w, + 120O)

Considering that the input currents are images ofthe inputvoltages, thus the sector defined by: 6Oa


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6IhCase - &=Off; S2=On;S,=On

Fig. 3: State ofswitches and current flow far the analyzed sec tor.I I

Fig. 4: Equivalent CirCUit for the convener presented in the Fig. IIv ~IATIIEMATTCALNALYSIS RESULTS Dynamic Transfer Functions:

Only the results of the mathematical analysis of the _ = _(s) vo (2)- 3.VP.V" s. R, .CO]+ (3)converter are presented because the whole of the analysis istoo extensive to be presented here. It should be said that the

circuit presented in Fig.4.

D ( s ) s . Lmathematical analysis was developed based on the equivalent I,@) 2.e

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Input Inductors: . Output power +P0=6kW;Performance + q=90%:Peak voltage of the mains 9 p=180V(4)

( 5 ) . Input current ripple +dllN=lO%;z.4I L L - =__3 . v p . q . Output voltage ripple +AV0=5%;Output Capacitor: . Switchingfrequency fs=50KHz.CO 23 .v, . 2. v, - 3 . v,]

L l ~ q > f s . AZLI , % . 4 . p , . VU . Output voltage + V,=450V;p0 . 2. V, 3 , q . , )2 . q .V, ,fs AV, ( 6 ) Thus, the obtained parameters are: Lm = 650uH andCO= 8,2uF. However, the output capacitor will be defined by6 0 , 6 1 3 ~ V o - V p (7 ) its RMS current, so an equivalent capacitance of 3mF wasused. Using the mathematical analysis, the obtained resultsare presented in Table 1.I C o - - =-.,L-Observation: In reality, the output capacitor is determined by its

Switrhrw Devices definition:Table IMS current.

VI SIMULATION AND EXPERIMENTAL ESULTS

Dkm Diodes (see Fig.2):-

Di.gs Diodes (see Fig.2):ID,.,,., (14)

V DESIGN EXAMPLEBased on the mathematical analysis, an AC-DC converterwas designed for an output power of 6kW, using the IC

U C 3 8 5 4 B in the current control loop (for each phase) andonly one voltage loop (feedback and feedforward). In thismanner, classical control theory was used to project the PIDcontrollers for the current loops and a PI controller for thevoltage loop. The project of these controllers is classical, so itwill not be presented here.The snubbers and other auxiliaries circuits will not bepresented, either. The project input parameters are:

--ig. i: Input cumnts obtained through simulation, using he software Pspice.

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Fig. 3: 1 oftheprototype::: -2.3% I

h-sL.-.-l-l---2 4 6 8 o 12 14 16 i a 20 22 24 26 28 30 32 34 36 38 40Fig 4 Harmonic magnitude as a % of the h d a m e n t a lamplitude of the current9 8 195 1

93 ,90 %88.

85 .83180 1

81 0 1 6 1 0 IS20 3560 4460 5 2 6 0 5670 5960 6110Fig. 5 : Efficiency curve obtainedf he prototype (efficiency X outputpower).

A THD of 3% was obtained with a power factor of0.999. The input currents THD was not lower due to thecurrent reference THD , caused by d istorted input currents.In spite of this fact, the THD and the power factorcomply with the telecommunications standards. The

output voltage was not presented, because it was regulatedat 450V, atisfying the ripple limits. An efficiency of 96%was m easured at rated conditions.VI1CONCLUSION

Based on the single-phase AC-DC boost rectifierwith unity power factor, this paper presented a topologyfor a three-phase PWM rectifier. A project example,simulations and experimental results were also presented.In this manner, the following characteristics wereobserved in the proposed converter:Very simple power circuit, using only three switches;Simple control system, using conventional devicesnormally employed in single-phase AC-DC PW M boostrectifiers;High power factor and low input current THD;High efficiency, with low weight, volume and cost.

VI11 REFERENCES[ I ] G. Spiazzi, and F. C. Lee, implementation of single-phase boostpower factor correction circuits in three-phase applications,Switching Rectifiers for Power Factor Correction, Volume V, VPECPublication Senes.[2] A. C. C. Neto, Retificador PW M Trif4sico de 26 kW, TrSs Niveis,Unidirecianal, Fator de Potencia Unithio e Alto Rendimento paraAplicaph em Centrais de TelecomunicagHo, Masters DegeeDissertation, INEPIEEUUFSC, ApriW2002.[3] Y. R. Navaes, Unidade Retificadora Tnfisisica de 6 kW paraTelecomunicapPles, Internal Report, INEPIEEUUFSC, January2002.[4] A. Nabae, I. Takahashi, and H. Akagi, A ew neutral-pointGclampedPWM inverter, IEEE Trans. nd. Appl., vol 17, no. 5, pp.518-523,Septemberloctober, 1981.[SI Y . Zhao, Y. i , and T. A. Lipa, Force Commutated Three LevelBoast Type Rectifier, E E E Trans. hd. Apd., vol. 31 , no. 1,January/February 1995.[6] . W. olar, and F. C. Zach, A novel three-phase three-switch three-

level unity power factor PW M rectifier, Pmeedings of the 28hPower Conversion Conference, Niremberg, Germany, June 28-30,1994,pp. 125-138.[7] J. W. Kolar, and F. C. Zach, A novel three-phase three-switch three-level unity power factor PWM rectifier, Proceedings of the 2gChPower Conversion Conference, Niiremberg, Germany, June 28-30,1994,pp. 125-138.181 U. Borganovo, Madelagem e Controle de Retificadores PW MTrifisisicos Empregando a Transformaph de Park, Masters DegreeDissertation, INEPIEEUUFSC, November/2001

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A Three-phasteh Ree-switcthw O-leveplw m Rectifier