A New Power Line Conditioner for Harmonic Compensation

8/8/2019 A New Power Line Conditioner for Harmonic Compensation

1/6

1570 IEEETransactionson Power Delivery, Vol. 10. No.3, July 1995A New Power Line Conditioner for Harmonic Compensation

in Power SystemsHirofunii Akagi, Member, IEEE Hideaki Fuj i ta , Member, IEEEDepartm ent of Elect r ica l Engineering

Okayam a Universi ty3-1-1 Tsushirnanaka, Okayama-city, 700 J A P A N

Abstract - This paper proposes a new power line conditionerconsisting of two sinall rating series active filters and a shunt passivefil ter. The power line conditioner4ms at a general filtering systemwhich will be installed at the point of common coupling in a powersystem feeding harmonic-sensitive loads and unidentified harmonic-producing loads. One of the two active filters is connected in series withthe supply, while another is in series with the shunt passive filter. Th epurpose of th e power line conditioner is to reduce voltage distortion a tthe connection point, and is to eliminate harmonic currents escapinginto the system upstream of the connection point.

A control scheme of the two series active filters which play an im-portant role is described in this paper. Its filtering characteristics arediscussed with the focus on voltage and current distortion. A pro-totype model of 2OkVA is con structed to verify the functionality andperformance of th e power line conditioner.

I. I N T R O D U C T I O NHarnionic pollution caused by nonlinear loads has been a serious prob-lem with the proliferation of diode or thyristor rectifiers and cyclocon-verters in industrial applications and tr;msinission/distribntion sys-tems. Due to a finite amount of supply impedance, voltage distortionat the point of common coupling results from harmonic currents pro-duced by the nonlinear loads.

Passive filters consisting of tuned LC filters and/or high pass filtershave traditionally been used to improve power factor and to absorbharmonics in power systems because of their simplicity, low cost andhigh efficiency. A tuned LC filter should be designed t o exhibit lowerimpedance at a tuned harmonic frequency than the supply impedance,so that almost all harmonic current at the harmonic frequency entersth e LC ilter. In principle, filtering characteristics of a passive filter ar edetermined by the impedance ratio of the su pply and the passive filter.Therefore, it is difficult for the passive filter installed in the vicinityof a harmonic-producing load, which is connected to a fairly stiff acsupply, to meet th e above design criteria. In addition, th e passive filterhas t he following drawbacks:

Th e background voltage distortion on the utility supp ly may over-load the passive filter. In th e worst case, the passive filter mayfall in series resonance with the supply impedance.At a specific frequency, the passive filter may fall in parallel res-onance with the supply impedance, so that amplification of theharmonic current occurs and the currents in the supply and thepassive filter may be excessive.

This paper was presented at the 1994 IntemationalConference on Harmonics and Power Systems held inBologna, Italy, September21-23. 1994.

Active filters, which are classified into shunt and series ones, havebeen researched to compensate for reactive power, negative-sequence,harmonics, and/or flicker in industrial power systems since their basiccomp ensatio n principles were propos ed in th e 1970's [1]-[3]. However,there was almost no advance in active filters beyond the laboratorytesting stage since at that time circuit technology was too poor topractically implement th e compen sation principles. Recent progressin voltage-current rating and switching speed of semiconductor powerdevices such as IGBTs and GTO thyris tors ha s spurred interest in thestudy of active power filters with the focus on practical applications.Sophisticated PW M inverter technology, along with the so-called "pq-theory" 141, has made i t possible to put them in to a commercial sta gein Japan [5]-[7].

In 1982, a shunt active filter of 8OOkVA, which consists of current-source PWM inverters using GTO thyristors, was put into practicaluse for harmonic compensation [5]. In 1986, a combined system ofa shunt active filter of SOOkVA, comprising voltage-source PWM in-verters using bipolar junction transistors, and a shun t passive filter of6600kVA was installed t o absorb harmonics gener ated by large capac-ity cycloconverters for steel mill drives [7]. In 1991, a shunt active fil-ter of 20MVA, consisting of voltage-source PW M inverters using GT Othyristors, was developed for flicker compensation for arc furnaces withthe help of a shunt passive filter of 2OMVA 181 Although th e researchon active filters has been done by many researchers, almost all thepublished papers have dealt with active filters installed in the vicin-ity of an identified harmonic-producing load at the end terminal in apower system 191.

A new power line conditioner proposed in this paper is intended to beinstalled at the point of common coupling in a power system feedingharmonic-sensitive loads and unidentified harmonic-producing loads.Th e power line conditioner is characterized by th e system configurationconsisting of two small rating series active filters and a shunt passivefilter.

In this paper, a control scheme of the two series active filters isdescribed, based on the hybrid passive-series active filter which hasbeen already proposed by the authors [lo], [ll]. Some interestingexperimental results obtained from a prototype model of 2OkVA areshown to verify the functionality and performance of the proposedpower line conditioner.

11. SYSTEMC O N F I G U R A T I O NFig.1 shows an experimental power system and a circuit diagramof a new power line conditioner enclosed with a broken line. T h e

main circuit of the power line conditioner consists of two active filtersAF1 and AF2, matching transformers of turn ratio 1:20 M T l andMT2, and a passive filter PF. Active filter AF1 is connected in serieswith the supply through m atching transformer MT1, while active filterAF2 is connected in series with the passive filter through matchingtransform er MT2. Each active filter of 0.5kVA consists of three single-phase voltage-source PW M inverters using power M OSFETs. The dcterminals of the six single-phase inverters ar e connected t o each otherand to a dc capacitor of 22 00p F in parallel. Fig.2 shows a detailedcircuit diagram of active filters AF1 and AF2. Th e passive filter of8kVA consists of 11th and 13th tuned LC filters and a high pass filter.Table 1shows the circuit constants of passive filter PF.

A harmonic-sensitive load L1, and two harmonic-producing loads0885-8977/95/$04.00 @ 1994 IEEE

Authorized licensed use limited to: Walchand College of Engg. Downloaded on July 05,2010 at 07:55:49 UTC from IEEE Xplore. Restrictions apply.


2/6

1571

60Hz2 0 0 vIII

II A F 1I . .I Active FiltersF2 , . ! O F ,

Fig.1. S ystem configu ration of power line conditioner.

Fig.2. Detailed circuit of active filters.

Table 1. Circuit constants of passive filter.

11 inductance I caoacitance I 011th 11 380pH I 1 5 0 p F I2 013th 11 300uH I 140uF I 2 01IP 11 40pII I 260pF I20

Fig.3. Equivalent circuit of AFl and PF.

L2 and L3 are connected on a common bus, where the bus voltagevg is 200V. A three-phase twelve-pulse thyristor rectifier of 20kVA isan identified load L3, which dominantly produces 11th and 13th har-monic currents. On the other hand, a three-phase diode rectifier of3kVA is an ~ d e n t i f i e doad L2, which dominantly generates 5th and7th harmonic currents. Therefore, neither 5th nor 7th tuned LC filtersare connected in the specially designed passive filter for this experi-ment. A power capacitor of SkVA, with a series reactor of 5% of thecapacitor rating, is considered an hypothetical harmonic-sensitive loadbecause voltage distortion at the common bus causes a large amountof harmonic currents to flow in i ~ 1 . he supply reactance in Fig.1 is3%on 200V, 60A, 60Hz, 20kVA base.In a practical application, a passive filter consisting of 5th and 7thtuned LC filters plus a high pass filter will be used in the power lineconditioner because 5th and 7th harmonics are th e most dominant inreal power systems.

111. CONTROL S CHE ME OF ACTIVEFILTERSA . Active filter AF 1Fig.3 shows a harmonic circuit equivalent to a hybrid filter which com-bines active filter A F1 and passive filter PF; i t is represented o n a per-phas e base. Here, ZF is the equivalent impedanc e of the passive filter,an d 2s is the supply impedance. For the sake of simplicity, only aharmonic current source IL h is assumed in the system downstream ofthe hybrid filter. A harmonic voltage existing in th e system upstreamor a background harmonic voltage in the supply, VS h is ds o includedin Fig d. Active fil ter AF l is assumed as an ideal controllable voltagesource V A F ~ ,hile active filter AF2 is removed from Fig.3.Active filter AF1 is controlled in such a way as to present zeroimpedance to the external circuit at the fundamental frequency anda high resistance K1 101 at har monic frequencies. According to refer-ences (lo] and [ll], th e command of instantaneous ac voltage of activefilter AF1, u iF 1 s given by

Here, i s h is the harmonic current in the supply, and IC1 is a high gainwhich has the dimension of R. It should be noted that the resistancel i l in F ig 3 is identical to the gain K1 in (1). If K1 is 00 under an idealcontrol condition, the supply harmonic current I s h , the bus harmonicvoltage VBh, and the ac voltage of active filter AFl, i.e., VAF~reeasily obtained from Fig.3.



3/6

1572

Fig.4. E quivalent circuit of AF2 and P F. Fig.5. Equivalent circuit of AF1, A F2 and P F.No harmonic current flows in the supply because the passive filterabsorbs all the load harmonic current ILh. In addition, VS h disappearson the common bus because active filter AF1 cancels it as shown inthe first term on th e right side of (4). However, a harmonic voltggeZFILh appears due to the existence of voltage drop between ZF an dILh. Th e combined filter shown in Fig.3 ha s the following drawback:

If IL h contains harmonic components having unspecified frequen-cies other tha n tuned frequencies in the passive filter, a relativelylarge amount of harmonic voltage would occur on the bus.B. Actitre jilter AF2

Fig.4 shows a harmonic circuit equivalent to a hybrid filter whichcombines active filter AF2 and passive filter PF. Active filter AF2 isassumed as an ideal controllable voltage source V A F ~ ,hile active fil-ter AF 1 is removed from Fig.4. Active filter AF2 is controlled in sucha way as to present zero voltage to the external circuit at the fun-damental frequency and a harmonic voltage at harmonic frequmcies.The command of instantaneous ac voltage of active filter AF 2, t);lFZsgiven by

t ' i F ~= -1i.z ' VFh (5 )Itere, It 2 is a gain and t'Fh is a harmonic voltage existing in the termi-na l voltagc across the passive filter. Th e total equivalent impedan ceof active filter AF 2 and passive filter PF , which are connected to eachother in series, is obtained from Fig.4.

ZAF = (1 - i 2 ) Z F (6 )Active filter AF2 has the ability to cancel the harmonic voltage whichappears due to the non-negligible impedance of passive filter PF, thusproviding a low impedance branch of harmonic currents. Since KZ sunity under an ideal control condition, I s h , V B h , an d VAFZ re givenby

(7 )

Since 1 b h = 0, no harmon ic voltage occurs on the bus. Moreover, noharmonic current escapes from the system downstream into the systemupstream because IL h is excluded from (7). However, the combinedfilter in Fig.4 has the following drawback:

If the combined filter Is connected to a fairly stiff ac supply in-cluding a background harmonic voltage VSh, a large amount ofharmonic curr ent, which is expressed by j ' S h / z s . would flow fromthe supply into active filter AF2 and passive filter PF.

1V . FILTERINGH A R A C TER ISTIC SFig.5 shows an equivalent circuit to harmonics of the power lineconditioner proposed in this paper. Th e comm ands for instantaneous

ac voltages of active filters AF 1 and AF2 are given by (1) and (5),respectively. As a result, the filtering characteristics of the power lineconditioner are obtained from Figs.3 and 4.1

2s + i l + (1 - i 2 ) Z FvsShSh =( 1 - ~ z ) ~ F

2s + K l + ( 1 - c 2 ) z F IL h( 1 - r'2)ZF2s + Kl + ( 1 - K z ) z FBh =

- (ZS+ K I ) ( l - K 2 ) z F2s + K l t 1 - K Z ) Z F Z L hIil

2s + Ic1 + ( 1 - K l ) ZF VShI.'aFl =t

If the ideal control conditions ( Z i l = 00 an d Ziz = 1) are assumed,equations (10) N (13) are changed into the followings.I s h = 0 (14)

VB h = 0 (15)VAFl = VS h (16)VAFZ = Z F I L h (17)

T h e ac voltage of active filter AF2 cancels the harmonic voltageappearing across passive filter PF, thus providing a harmonic currentbranch with zero impedance. Because all the harmonic currents pro-duced downstream enter passive filter PF, no harmonic current escapesupstream. Qn t he other ha nd, th e ac. voltage of active fil ter AF l com-pensates for the.background harmonic voltage, thus blocking the flowof harmonic currents from the supply into the passive fil ter. Accord-ingly, no harmonic voltage occurs on the bus as shown in (15). It isconcluded that the power line conditioner proposed in this paper ha sthe following functionality.

Active filters AF1 and AF2 are controlled so as to actively shapeboth the bus voltage OB and the supply current is into sinusoidwith t he help of passive filter PF.The power line conditioner, therefore, is useful for harmonic conipen-satioa in a power system feeding harmonic-sensitive loads and uniden-tified harmon ic-producing loads.



4/6

1573

Fig.6. Control circuit.v. CONTROL CIRCUIT

Fig.6 shows a control circuit which is developed for the following ex-periment. Three-pha se supply currents and terminal voltages acrosspassive filter P F in Fig.1 are changed into two-phase ones on the nbcoordinates.

The dq-transformation is nchieved in terms of two-phase sinusoidalsignals cos wt and sinw t which are generated by the phase locked loopcircuit. Here, w is the angular frequency of the supply.

For instance, dc components in id an d i, correspond to positive-sequence fundamental components in i an d ig because the dq-transformation is considered a kind of frequency changer. By intro-ducing four 1st-order high-pass filters HPFs, their ac components ;sa,isq , 6Fdr an d 6~ ~ are extracted from isd, s, , V F ~ ,nd UF,, respec-tively. Th e cut-off frequency of high-pass filters HPF s in Fig.6 is se tto he 1Hz in this experim ent. Th e inverse-transformation of (18) -(21) gives three-phase ha rmonic cu rrents and voltages as shown in thefollowing equations;

(22)

Amplification of the calculated harmonic currents and voltages bythe gain of l i l an d IC2 prodtices the commands for instantaneous acvoltages of the voltage-source PWM inverters. Comparison of eachcommand with a carrier signal of a riangular waveform determines theswitching pattern of power MOSF ETs. Th e control circuit designedand fabricated in this paper is based on a hybrid analog-digital circuit.

VI . E X P E R I M E N T A LESULTSAs discussed earlier, the ideal gain of active filter AF l is #I = 00 an dtha t of active filter AF2 is K2 = 1. In this experiment, however, ZCl isset to 2.251 (= 100% on 200V 60A 2OkVA base ), a nd IC2 to 0.8 in orderto avoid the instability of operation which may be caused by a t imedelay in detection and calculation by the control circuit. Althoughthe gains are experimentally decided in this paper, the relationshipbetween the gains and the impedances will be ma de clearer in the nextstage. The dc voltage of the capacitor connected to th e dc terminalsof six voltage-source PWM inverters is 120V, and the frequency ofthe carrier signal in the control circuit is 15kHz. No small capacitypassive filter is connected as shown in Fi g9 because an amoun t ofleakage inductance in matching transformers MT 1 and MT 2 plays animportant role in reducing the higher frequency harmonic componentsd u e t o t h e PWM of the active filters.

Figs.7, 8, and 9 show experimental waveforms in a case of the dis-connection of harmonic-producing load s L2 and L3 on th e common busin Fig.1. Table 2 shows the experimental values of harmonics of volt-age and current with respect to their fundamental components. Dueto the existence of 5th and 7th backgronnd harmonic voltages in thesupply, 5th and 5th harmon ic currents of 3.3% and 9.1% ar e present inthe supply, respectively, as shown in Fig.7. Since neither active filtersAF1 nor AF2 a re operating, an a mount of harmonic voltage appearson the bus, so t h a t a 5th harmonic current of 12% is flowing into thecapacitor, i.e., harmonic-sensitive load L1. After active filter AF1 isswitched on, the 5th harmonic current is reduced 0.7%, as shown inFig.8 because.active filter AF 1 acts as a blocking high resistor at the5th harmonic frequency. As a result , the harmonics in OB an d i ~ 1rereduced by two-thirds. Even if active filters AF1 and AF 2 are op-erating, AF2 does not generate any voltage, as shown in Fig.9. Th ereason is that almost no harmonic voltage appears on the bus due tothe effect of active filter AF1.

Figs.10, 11, and 12 show experimental waveforms where harmonic-producing loads L2 and L3 are connected. Table 3shows the experi-mental values of harmonics, corresponding t o Table 1. Neither activefil ters AF1 nor AF2 are operating in Fig.10. Since neither the 5thnor the 7th tuned LC filter is connected t o passive filter PF in Fig.1,5th and 7th harmonic currents of 3.8% and l .8%, which are generatedby the diode rectifier, i.e. unidentified harmon ic-producing load L2,

Table 2. Experim ental values in case of disconnection of L2 and LB.

condition



5/6

1574

VAT10 ' -

Fig.7. Experimental waveforms (PF).

i,T""IFig.8. Experimental waveforms (PF t AF1).

Fig.10. Experimental waveforms (PF).

Fig.11. Experimental waveforms (PF + AF1).

Table 3. Experimental values in case of connection of L2 and L3 .

Fig.9. Experimental waveforms (PF + AF 1 + AF2) . P F + AF 1 I] 1. 1 I 0. 2 1 20 . 1 2. 1 1 1.4 I 0. 8P F + AF 1 + AF 2 11 1.1 I 0. 2 I 13. I 2.1 I 1.0 I 0.8



6/6

1575[3] N. Mohan, et al, "Active filters for ac harmonic suppression",

[4] H. Akagi, Y. Kanazawa, A. Nabae, "Instantaneous reactivepower compensators comprising switching devices without energystorage components", IEEE Trans. Ind. Appl., vol.IA-20, no.3,

[5] H. Kawahira, T. Nakamura, S.Nakazawa, "Active power filters",

[SI H. Akagi, A. Nabae, S.Atoh, "C ontrol st rateg y of active power fil-ters using multiple voltage-source PWM converters", IE EE Trans.Ind. Appl., vol.IA-22, no.3, pp.460-465, 1986.

IEE E/P ES Winter Meeting, 1977, A77026-8.

pp.625-630, 1984.

IEEJ IPEC-Tokyo, pp.981-992, 1983.

[7] M. Takeda, K. keda, Y. Tominaga, "Harmonic current compensa-tion with active filter", IEEE/IAS, Annual Meeting, pp.808-815,1987.

[8] H. Akagi, "Trends in active power line conditioners", IEE E/IE S,IECON, pp.19-24, 1992.[9] S. Moran, "A line voltage regdator/conditioner for harmonic-sensitive load isolation", IE EE/IAS , Annual Meeting, pp.947-951,1989.

[lo] F. 2.Peng, H. Akagi, A. Nabae, "A new approach to harmoniccompensation in power systems - A combined system of shuntpassive and Series Active Filters", IEEE/IAS, Annual Meeting,~ ~ 8 7 4 - 8 8 0 ,988.

[l l ] H. Fuji ta, H. Akagi, "A practical approach t o harmonic compen-sation in power systems - Series connection of passive and activefilters", IEEE/IAS, Annual Meeting, pp.1107-1112, 1990.

iF d l

Fig.12. Experimental waveforms (P F t AFL t AF2).

escape into the supply. A 5th harmonic current of 21% enters capac-itor L1 because of the existence of a harmonic voltage of 1.4% in UB.After active filter AF1 is switched on, th e 5th harmo nic current in thesupply are reduced by three-fourths, so that the current waveform ofthe supply is almost sinusoidal as shown in Fig.11. However, the 5 thharmonic current generated by unidentified harmonic-producing loadL2 enters passive filter PF, so tha t a 5th har monic voltage of 1.4% ap-pears on the common bus. Th e 5th harmonic voltage in UB and the 5 thharmonic current in iL1 are nearly equal to those using only passivefilter PF. This means that harmonic interference still exists betweenharmonic-sensitive load L1 and unidentified harmonic-producing loadL3. After active filters AF1 and A F2 are switched on, the 5th har-monic voltage in vg and the 5th harmonic current in it1 are reduced1.0% and 13%, respectively, as shown in Table 3.

VII. C O N C L U S I O N SA new power line conditioner consisting of two sm all rating series activefilters and a shunt passive filter has been proposed in this paper. Itis capable of compensating for harmonics of supply currents and busvoltages, so that no harmonic interference occurs between harmonic-producing loads and harmonic-sensitive loads which are connected onthe common bus. The LC filters used in this experiment are tuned a tthe 11th and 13th harmonic frequencies in order to verify the effect ofreduction of 5th and 7th harmonics.

In a practical application, 5th and 7th tuned LC filters plus a highpass filter will be adopted as a passive filter because 5th and 7th har-monic voltages and currents are the most dominant in real power sys-tems. A power line conditioner compo sed of th e passive filter and twoactive filters can greatly reduce not only 5th and 7th harmonics butalso non-canonical harm onics or non-characteristic harmonics such as4th harmonics.

R E F E R E N C E S[I ] H. Sasaki, T. Machida, "A new method to eliminate ac harmoniccurrents by magnetic compensation - Considerations on basic de-sign", IE EE Trans. Power Appr. Sys t., ~01.90, o.5, pp.2009-2019,1971.[2] L. Gyugyi, E. C. Strycnla, "Active ac power filters", IEEE/IAS,Annual Meeting, pp.529-535, 1976.

Hirofumi Akagi (M'87) was born inOkayama city, Japa n, on August 19,1951. He received the B.S. degree fromNagoya Institute of Technology in 1974and the M.S. and Ph.D. degrees fromTokyo Institute of Technology in 1976and 1979, all in electrical engineering.In 1979, he joined Nagaoka Universityof Technology as an Assistant and thenAssociate Professor in the department ofelectrical engineering. In 1987, he wasa visiting scientist at Masachnsetts In-stit ute of Technology for ten m onths. Since 1991, he has been aProfessor in the department of electrical engineering at OkayamaUniversity. His research interests a re applications of power elec-

tronics to power systems such as active power filters, static varcompensators, and FACTS: He is a recipient of four IEEE prizepaper awards including t he first prize pap er award in IE EE Trans-actions on Industry Applications for 1991, and th e best trans actionspaper awards from IEE of Japan in 1985 and 1991.

H id eak i F u j i ta (M'91) was born inToyama Prefecture, Japan, on Septem-be r 10, 1965. He received the B.S.and M.S. degrees in electrical engineer-ing from Nagaoka University of Tech-nology in 1988 and 1990. Since 1991,he has been a Research Associate in thedepa rtm ent of electrical engineering a tOkay ama University. His research in-terests are active power filters, reactivepower compensators and resonant con-verters. He is a recipient of the first

comm ittee prize paper award of the industrial power converter com -mittee in IEE E Industr y Applications Society in 1990.

C C

Date post:	09-Apr-2018
Category:	Documents
Upload:	seemapdi
View:	217 times
Download:	0 times

A New Power Line Conditioner for Harmonic Compensation

Documents