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MODEL AND APPLICATIONS FOR HARMO NIC ANALYSIS OF AC/DCPOWER SYSTEM S COUPLED BY POWER ELECTRONIC CONVERTERS

IN HIGH-POWER INDUSTRIAL DRIVESZhao Yulin and Jiang Jianguo Member, ZEEE

China Univers i ty o f M in ing and TechnologyXuzhou, Jiangsu 221008, P. R. China

Telephone:86-5 16-3884479, Fax:86-5 16-3888682E-mail:jiang@cumt edu.cn

ABSTRACTLarge converters in applications of high-powerindustrial drives are main harmonic sources in powersystem. In this paper, the model for harmonic analysisof AC/DC power systems coupled by power electronicconverters is proposed. The basic principles are that theoperation cycling of converter and sampling switchingcharacteristics of thyristors can be represented by logicswitching and its eign equations. Two instances ofharmonic calculation for 6-pulse bridge converter and6/12-pulse converter are taken. Th e computed resultsare much in agreement with the measuring results. It isproven that the suggested method is more effective andaccurate.Keywords: harmonic analysis, harmonic model, powerconverter.

The project supported by NSFC1. INTRODUCTION

The high power motor drives have been used inindustrial area. At the end of the 1960s, when highpower thyristors for large converters were put into use,large phase-controlled converters could be built to meetthe requirements for large drives with adjustable speed.This resulted in a first breakthrough for large drivesindustrial applications during the 1970s. The powerratings of large drives reach from just 1MW up tolOMW and more in special cases. In applications of

high-power industrial drives, large converters used ashigh-power motor drives are main power loads inindustrial power networks or distributions. They aremain harmonic sources. Harmonics generated fromlarge converters are injected into power network andcaused the electric wave distortions (as shown in Fig. 1).The problem of harmonic pollution, harnessing andlimitation become an important issue,

For harmonic analysis and calculation in powersystems, one must have thought Fourier transform ofwaveforms in time domain. For harmonic analysis ofAC/DC power systems coupled by thyristor converters,it is conventionally carried out by the method of Fouriertransform, but it is quite complicated and tedious.Certainly, this work can be done by FFT techniquewithout exception. However, a new model of harmonicanalysis for converters is proposed in this paper. Thetheoretical fundamentals of this model are that theoperation processes of converter feature thecharacteristics of discrete sample and modulation, andalso are cycling, and that the dynamic characteristics ofconverter system can be represented by several eigencurrent equations.

According to the new model suggested, recording thesampling data at switching points, computing theharmonics of eigen current equations, and synthesizingthose harmonics, the harmonics or side harmonics of theintegrated system containing ac and dc sides, or acnetwork side and ac motor side can be obtained. Themodel in applications for harmonic ananlysis has theadvantages of less memory capacity needed, fastcomputation and high accuracy. This model is very

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useful and effective in estimating the harmonic sources,predicting the harmonic flow and propagation andresearching the harmonic suppression and elimination.

di, l d r = (i)i, +F 4(i)enhc+F,(i)(-EM-2V,)di, l t =-D l l ( k ) i , -D2 , (k ) i l +D& ,(k )eabc

-41 k ) - E M - ve)diV D ( t ) = D - + il RD + E M

i A B C = s D i ~ ~ ) + S K i k ~ ) >ahc - Z a h c P ) i A B C t ) -dt

2. HARMONIC MODEL OFCONVERTERS

' 1)

1 .

V

.

Fig. 1 Th e ac current waveforms and their spectrums ofharmonic so urce (mine winder fe by 6/12-pulseconverrters) and power capacitor bank

Knhc =

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Fig. 2 The circuit form of 6/12-pulse converterSupposing supply voltages containing harmoniccomponents, such as, eLl= CV,, sin rz tff q,, , and bythe core equation of converter system, the fully staticform of LSM of converter can be easily got,

N ,n I

parameters related with logic-switching variables,, VD vK and Sv, are the switching matrixes

related with param eters.By the analogy method, the model of 6/12, or 12-pulses converter in series with double 6-pulses

converters is gotten, which circuit form of 6/12-pulseconverter is as show n in Fig. 2. In DC side,

LDpii RDti EM = V D ~ V D ~ 3)

3. HARMONIC CALCULATION FORCONVERTERSAccording to the harmonic model of converters, thesteps of harmonic calculation for converters is asfollows, 1 ) recording the sampling data at switchingpoints, (2) computing the harmonics of eigen currentequations, and then 3) synthesizing those harm onics, sothat the harmonics of the integrated system containingac and dc sides can be obtained. In doing so, forexample, abou t 1 2 related po ints data are m erely neededand the harmonics of two currents (a dc current and acommutation current) are only computed for 6-pulsebridge converter, and about 24 related points data aremerely needed and the harmonics of three currents a dccurrent and two commutation currents) are onlycomputed for 6/12 -puIse and 12-pulse bridge converters,so that the overall harmonics of ac and dc sides ofthose converters can be determined. The suggestedmethod has the advantages of less memory capacityneeded, fast computation and high accuracy. Thismethod is very useful and effective in estimating the

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