harmonic analysis and filter design for medium-voltage

8/13/2019 Harmonic Analysis and Filter Design for Medium-Voltage

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Harmonic Analysis and Filter Design for Medium-VoltageMultilevel PWM InvertersWang Yi, Li Heming, Shi Xinchun, Zhu LingDepartment of Electrical Engineering, North China Electric Power Ilniversity, Ch ina

Abstract-Multilevel PW M inverters have becom e thedevelopment trends of medium-voltage and high-powermotor adjustable-speed drives (ASDs). Harmonic analysis ofmedium-voltage ASD outpu t voltage is descr ibed firstly inthis paper. Through lots of simulation and experimentalresults, the frequency spectrum characters of multilevelPWM voltage waveforms are concluded. It offers theacademic fundamentals for the filter design. Use of L R Cfilter is a simple and efficient m ethod to mitigate the vo ltagesurges effects. The filter parameters are related to theharmonic components of ASD output voltage and cablelength. Furthermore it makes some problems more seriouswith the increasing of motor capability. On the basis ofconsidering some problems synthetically such asfundamental voltage drop, active power loss, currentharmonics and resonance, this paper presents the parameterchoice formulae that can be applied to medium-voltage andhigh-power ASDs output filter design.

Keywords-ASD, multilevel, PW M, harmonics, filter

1. IN TRO D U CTIO NAs an efficient energy savings way, the applicationsof adjustable-speed drives (ASDs) for medium-voltageinduction motors are widely spread [13. Because thesingle power electronic device cant endure medium-voltage presently, multilevel PWM control is applied inmost medium-voltage ASDs. Steep-fronted surgesgenerated by PWM inverters have long been recognizedto pose a hazard to the turn insulation of motors [2], [3].For m edium-voltage motors this problem is more serious.In order to make ASDs more popular in the higher voltageranges, the harmonics problems must be dealt well with.Through lots of simulation and experimental results, thefrequency spectrum characters of multilevel PWMvoltage are concluded. On this basis, the design methodsof output filters for medium-voltage motor ASDs areprovided.

11. H A M O N I CN A LY SISF M U L T I L E V E LWM INVERTERO U TPU T O LTA G EPWM control can make inverters output voltage andcurrent more similar to sine curve. But harmoniccomponents related to carrier wave are producedinherently for modulations of pressing carrier wave onsine signal [4]. The frequency and amplitude of voltageharmonics has to be specified before filter design. Besidestwo-level PWM inverters, neutral point clamped three-

level and cells cascaded multi-level voltage-source PW Minverters are more popular in the medium-voltage ASDranges. Their output voltage is multilevel PWMwaveform. In order to analysis the harmonic problems ofmedium-voltage ASDs, the simulation models ofmultilevel PWM inverters by MT L AB/SI MUL I NK andthe experimental system are established. The followingfrequency spectrum characters of multilevel PWMvoltage are achieved from many simulation andexperimental results.1) Harmonic distribution centers on the carrier waveGequency--f,, 2f etc [ 5 ] As the common rule of thelow-voltage PWM inverters, it is also applicable to the

multilevel ones. But the definition of carrier frequency isextended. It is not always equal to carrier frequency or thepower electronic device switch frequency. It should beconsidered as:

where A. is the equivalent carrier frequency, r is themodulation frequency, np is the numbers of PWM pulsesin one output voltage period.Fig.1 shows the output voltage waveform andfrequency spectrum of three-level and three-cell cascadeinverters. And the output line voltage harmonic contentsof different multilevel PWM inverters are shown in Table1. Equivalent carrier frequency is 3kHz, and the each celldevice switch frequency of three-cell cascaded inverters isonly 500Hz. So the filter should be designed as low-passtype and its cut-off frequency is less thanf ,.2) With the increasing of the output voltage steps,harmonic amplitude becomes less, and it can reduce thefilters capability. Table I shows that the harmoniccontents are inversely proportional to the positive voltagestep numbers (or negative step numbers).3) n the course of VVVF (Variable VoltageVariable S peed), the hlarmonic distribution centers ar erelated to the frequency modulation index, and harmonicamplitude is related to amplitude modulation index. Theirdefinitions are as follows:

where N is the frequency modulation index, D is theamplitude modulation index, M , is the modulation wavepeak value, M, is the carrier wave peak value.For the synchronous modulation, N is a constant andthe harmonic distribution center is changed with thealtering of ji Therefore the filter design should takeaccount of the minimum carrier frequency in V VVF.

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t

6 ....... . . ~ ~ ~ ~ ,...... : ..... ........ ....... IfE y

0 ... .. Y ..... .....5 ... ....... ../ ; .... ......14v*Ilr. .; . j : :..%w . ... ...

t I I ' f I ' I i

Fig (a) Thrcc-lcvcl PW M invcrtcrs output voltagc simulation wa vcform (b) Frcqucncy spcctrum of thrcc-lcvcl PWM invcrters output voltagc (c)Thrcc-lcvcl PW M invcrtcrs output voltagc cxpcrimcntal wavcform . (d) Thrcc-ccll cascadcd PWM invcrtcrs output voltagc simulation wavcform (c)Frcqucncy spcctrum of thrcc-ccll PWM invcrtcrs output voltage. f ) Thrcc-ccll cascadcd PW M invcrtcrs output voltagc cxpcrimcntal wavcformTABLE

OUTPUT L INE VOL T AGE HAR M ONIC CONTENTS OF D I FFER EN TMULTILEVEL PWM INVE RT E RS

Circuit Form Two-lcvcl Thrcc-levcl Thrcc-ccll cascaded Fivc-ccll cascadcdLinc Voltagc Positivc Stcp Numb crs 1 2 6

Maximal Harmonic Contents Ratio Among w 32 12 6voltarrc total harmonic distortion 73 33 14

IO3Yo8

Due to these rules of multilevel P W M inverters, theharmonic contents can also be reduced through addingequivalent carrier frequency or output voltage steps.However devices sw itching loss or circuit complex degreewill be increased largely. Compare to these two methods,a reasonable design of low-pass filter may be moreeffective and econom ic for m edium-voltage ASDs.

111. FILTERESIGNORM E D I U M - V O L T A G EU LTILEV ELP W M I N V E R T ER S

Fig. 2(a) show s the LRC low-pass filter topology andFig. 2(b) shows its single-phase equivalent circuit. L isused to restrain high-order harmonics pass, C provides abypass for harmonics and R is a damper to avoidresonance [6]. In theory if the value of L and C is largeenough, the output voltage will be similar with sine wavegreatly. However the fundamental voltage drop, activepower loss, current harmonics and resonance problems

must be considered comprehensively for high powermotor ASDs.iLTb)Fig.2 (a) LRC filter topology. (b) Singlc-phasecquivalcnt circuit of LRC tiltcr.

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To simplify analysis, the distributed cable parametersare neglected, and the motor model is expressed by R-Lseries network. The transfer function of this equivalentcircuit can be derived as follows:Z = w L + Z , l l Z m (3)

(4)

where Z,, is the inverter output current without filter,Z m = R , , , + j w L , , Z . =R+-.

formula

1j w C

If the selected RLC value of filter accords with the

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voltage is filtered out. To some certain frequencyharmonics, active power loss is direct proportion withAfter considering all above factors applied formulaeWL .used to LRC value choice are as follows:

L = 0.05 - O.IL,, (14)R = 0 05 - 0. Rm . (16)

If motor winding is connected with Y type, R,,, and L,n isdetermined by

where U,v is the rated voltage, P,v is the rated power, isthe rated circular frequency, s the power factor.Besides (7) and (13), the voltage and currentharmonics content ratio and active power loss can'texceed the limitations. Their calculation formulae areRH R U , = . HRU, ,W L

where HRUOn is the output voltage harmonics contentratio before filtering, HRIon is the output currentharmonics content ratio before filtering, HRUh is thevoltage closed-by carrier wave frequency harmonicscontent ratio.Therefore the filter design is a optimize problemwhich makes (7) and (13) as the constraint condition, andthat (19)-(21) as the destination function. Its initial valuecan be presented by (14)-(16).

IV. EXP ERIMENTALESULTSThe test system is a three-cell cascaded and 3kHzcarrier PWM ASD supplying a 38OV/175W/1360rpminduction motor. Fig. 4shows the ASD's output voltageand current without filter. The Total Harmonic Distortion(THD) of the output line voltage is 14 .

L,,=5 16mH. The filter parameters calculated from(14)-( 16) areIn formula (17)-( IS), let 1=0.85, then Rm=596Rt

L=SOmH, C=OSpF, R=30i2.Through validation, the selected values meet with 7) and( 13) accords. Fig. 5presents experimental curves afterfiltering with these parameters. And the THD of outputvoltage is 9 .

5 2l

'0 1000 MO0 3000 4000 5000 6000 7000 8000 9000 10000Frequency (Hz)(b)Fig. 4 (a) Output line voltage and current without filter(b) Frequency spcctrum of the output line voltage

r

Frequency (Hz)(b)Fig. 5 (a) Output line voltage and current after filtering whe n L=SOmH,

C=OSpF, R=30R. (b) Frequency spectrumof the output line voltage.

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4Obo 5MO 6000 70bo IO S0bo 101430Frequency (Hz)(b)Fig. 6 (a) Output linc voltage and currcnt atkr filtcring whcn L=lOOmH,

C=l yF R=30Q. (b) Frequency spectrum of thc o utput linc voltagc.The further optimization result is L=lOOmH, C=lpF,R=30R. Fig. 6shows the experimental curves using thisfilter. The TH D of output voltage is 5 .

V. CONCLUSIONThrough reasonable design o f inverters output filter,the insulation demands on electric motors can be putdown, even power factor and efficiency can be increased.Thus frequency converters are well used to commonindustry motors. In this paper, multilevel PWM invertersoutput harmonics characters is illustrated firstly. On thisbasis and considering some important problems, theapplied formulae for selecting RLC low-pass filterparameters are induced. The filter design by this way hasbeen used into a 6kV l3 15kW mortarpump ASD in powerplant successfully.

REFERENCESJ. S Lai and F . Z. Peng, Multilcvel convcrtcrs-A ncw brecdof powcr converters, in Conf:Rec.IEEE-IASAnnu. Meting ,1995, pp. 2348-2356.A.Mbayc, F.Grigorcscu, T.Lebcy and Bui Ai, Existcnce ofpartial dischargcs in low-voltagc induction machincs supplicdby PWM drivcs, IEEE Trans. on Dielectrics and ElectricalInsulation, vol. 3, pp. 554-560, Aug. 1996.Mikc Mclti, Jason Sung, Sidncy Bcll and Gary L. Skibinski,Effect of surgc voltagc risctime on thc insulation of low-voltagc machincs fcd by PWM convcrtcrs, IEEE Trans. onInd. Applica., vol. 34, pp. 766-775, JuUAug . 1998.D. G. Holmcs and B. P. McGrath, Opportunitics forHarmonic Cancellation with Carricr-Bascd PWM for Two-Lcvcl and Multilcvcl Cascaded Invcrtcrs, IEEE Trsns. on IndApplica., vo1.37, pp. 574-582, Mar./Apr. 2001.

[5] W. M. Tong, K. J L I , G . F. Zhai and H. M. Xu HarmonicAnalysis of SPWM VSI in proccss of VVVF, PowerElectronics, vol. 29, pp. 47-5 I Aug. 1995Yilmaz Sozcr, David A. Torrcy and Suhan Rcva, Ncwinvcrtcr output tiltcr 1.0pology or PWM motor drivcs, IEEETrans. On Power l

harmonic analysis and filter design for medium-voltage

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