Bi Directional Converter

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942 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 15, NO. 5, SEPTEMBER 2000

A Bidirectional ACDC Power Converter with PowerFactor Correction

S. Y. (Ron) Hui, Senior Member, IEEE, Henry Shu-Hung Chung, Member, IEEE, and Siu-Chung Yip

AbstractThis paper presents new operation and performanceof a thyristor-based acdc current-controlled boost-type converterthat allows bidirectional power handling capability and providesinput power factor correction and a near-sinusoidal input currentwaveform. The new converter can reduce harmonic pollution anddisturbance on the supply mains. The feature of bidirectionalpower flow allows the stored energy in loads, such as motors, toregenerate back to the supply source, leading to an increase inoverall energy efficiency and possibly a reduction in the size of thedc link capacitor. The operation is confirmed with the successfulimplementation of an experimental prototype.

Index TermsHigh power converters, power factor corrections.

I. INTRODUCTION

MANY EXISTING power converters and motor drive

systems draw nonsinusoidal input current from the AC

supply mains. The classical acdc rectification approach of

using a full-wave diode bridge followed by a bulk capacitor is

unsuitable because of the undesirable input current harmonic

content [1]. Other equipment using the same power supply

may be adversely affected by these current harmonics. Thus,

international standards, such as IEC-1000 and IEEE-519, have

imposed restrictions on the harmonic contents of the input

current. Many research efforts [2][11] have focused on the

control of harmonic emission from power electronics circuits.Harmonic control techniques such as 1) passive filtering, 2)

active filtering, and 3) power factor conditioning/correction

have been used.

One problem associated with many existing drive systems

with frequent regeneration is that the size of the dc link capac-

itor is often very large in order to limit the link voltage. Nor-

mally, a large capacitor bank of thousands of micro-Farad is re-

quired. The large capacitor bank not only increases the size and

weight of the converter equipment, but also the equipment cost.

If a braking resistor is used to dissipate the regenerative energy,

the overall efficiency of the drive system becomes low. In order

to reduce the link capacitor, a bidirectional switched-mode rec-

tifier can be used so that regenerative energy can be absorbedby the supply instead of being stored in large capacitor bank or

dissipated in a braking resistor. With the bidirectional feature,

the switched mode converter concept originally developed for

Manuscript received July 28, 1998; revised May 23, 2000. This paper wassupported by the Research Grant Council of Hong Kong under Project (CERGProject 9040355) and the Small Grant Scheme of the City University of HongKong (SRG Project 7000692). Recommended by Associate Editor P. K. Jain.

The authors are with the Department of Electronic Engineering, City Univer-sity of Hong Kong, Kowloon, Hong Kong (e-mail: [email protected]).

Publisher Item Identifier S 0885-8993(00)07323-3.

Fig. 1. Schematic of a commonly used single-phase bidirectional power flowacdc converter.

switched mode power supplies can now be employed in elec-

tronic drive systems.

Various topologies that have bidirectional flow capability

have been proposed [7][10]. For single-phase system, the

commonly used bidirectional converter topology consists

of four fully controlled switches as shown in Fig. 1. Most

of these proposals use expensive fully controlled switches

such as GTO thyristors or IGBT [7][9]. One exception is a

bidirectional converter circuit [11] that uses primarily SCR

thyristors and one fully controlled switch such as IGBT. Theuse of SCR thyristors in the acdc front rectification power

stage is advantageous because 1) SCR thyristors are low-cost

and highly robust and 2) they can be commutated naturally,

making switching control simple. The bidirectional power flow

capability of this converter, that draws rectangular currents, has

been successfully demonstrated in [7][11].

In this paper, a primarily thyristor-based bidirectional power

circuit is re-examined and modified. Its potential for power

factor correction is explored. It will be shown that, by the use

of a simple control method, this converter can provide power

factor correction in both power flow directions. The input cur-

rent can be sinusoidally shaped to follow the input voltage (i.e.,

either in phase with the input voltage in motoring operation or180 out of phase with the input voltage in the regenerating

mode). Thus, the current distortion factor approaches unity and

the new converter operation can reduce harmonic pollution

and disturbances on the power supply by minimizing the input

current harmonics. The bidirectional feature allows stored

energy in the load, such as a motor, to be recovered back to

the supply, leading to an increase in overall energy efficiency

and an reduction in the dc link capacitance. This bidirectional

converter can be applied to general industrial electronic motor

drive systems. Because of the power factor correction feature,

08858993/00$10.00 2000 IEEE
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HUI et al.: BIDIRECTIONAL ACDC POWER CONVERTER 943

Fig. 2. Proposed high-power-factor bidirectional power flow acdc converter: (a) positive half cycle of power source, (b) negative half cycle of power source,and (c) timing diagram.

the proposed circuit can meet new international harmonics

standards.

Section II describes the operating principles of the proposed

converter under motoring mode and regenerating mode. Sec-

tion III gives the theoretical analysis and design equations. Sec-

tion IV shows a design example and the experimental results.

Section V presents conclusions.

II. PRINCIPLES OF OPERATION

Fig. 2 shows the circuit configuration of the proposed high

power factor acdc bidirectional converter. It consists of three

main components. They are the power conversion stage, in-

ductor average current controller, and the synchronization cir-

cuit for triggering the thyristors. The power conversion stage

consists of four low-cost, highly robust thyristors ( ), twofully controlled switches ( and , such as IGBTs or power

mosfets), two diodes ( and ), one inductor and one ca-

pacitor . It is important to note that must be a fully con-

trolled switch. could be a thyristor as demonstrated in [10],

[11]. But is chosen to be a fully controlled switch in this

study. This point will be addressed at the end of this section.

The operating mode of the converter (i.e., either in motoring

mode or in regenerating mode) is controlled by the conduc-

tion state of , which is determined either by sensing the dc

link voltage or by a control signal generated by a motor drive

controller [11]. For motoring operation, the converter is oper-

ated as a boost converter and is kept in the blocking state.

For regeneration, is turned on and the converter is operated

as a buck converter. For both motoring and regeneration oper-

ations, is used to shape the current to follow a sinusoidal

waveform. Under normal operation, the select signal controls

and changes the triggering signal applied to through

the synchronization network. The synchronization network syn-

chronizes the conduction of the rectifying thyristors ( )

with the line voltage. The operations of the power conversion

stage and the timing diagrams of are illustrated in Fig. 3

and Fig. 4.

A. Motoring Operation

The triggering signals of are synchronized with the

line voltage by the synchronization network shown in Fig. 2.

is turned off for the entire motoring period. is forward-

biased. and are turned on in the positive half cycle of

while and are turned onin the negative half cycle. A boostconverter is formed by and [14]. The inductor

current is controlled to follow the rectified waveform of

(i.e., ) by a PWM signal generated from a current mode

controller [15]. As shown in Fig. 5, the feedback current

is compared with the reference sinusoidal waveform and

is forced to remain between the maximum and the minimum

values of .

Fig. 6 shows the operations of the motoring mode. It consists

of Topology AI and Topology AII in one switching cycle of

period . AI is operated for a time interval of (where

is the duty cycle of ) and AII is operated for a time interval

of .
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(a)

(b)

(c)

Fig. 3. Operation of theconverter in themotoringmode:(a) positive half cycleof powersource,(b) negative half cycle ofpower source, and(c) timingdiagram.

1) Topology AIFig. 6(a): is defined as

(1)

is the initial value of at the beginning of a switching

cycle. At the end of this interval

(2)

2) Topology AIIFig. 6(b):

(3)

At the end of the cycle

(4)

By using (4), the ideal quasi steady-state conversion charac-

teristic is given by

(5)

As ,

(6)

where .

3) Average Output Current : If the input current of

the converter is assumed to be perfectly sinusoidal and the

conversion efficiency is assumed to be 100%

(7)

and

(8)

where is the peak value of .

As is controlled by the current controller, the output

voltage can be varied by changing .

4) Design of the Value of : The value of is determined

by the desired ripple in . Considering the input current shown

in Fig. 6, its peak-to-peak value is given as

(9)

Differentiating with respect to and equating the

expression to zero gives

(10)

for maximum value of . The minimum value of that

limits the maximum ripple current to a value is,

(11)


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(a)

(b)

(c)

Fig. 4. Operations of the converter in the regenerating mode.

B. Regenerating Mode

During the regenerating mode, switch is turned on and

is reverse-biased. The converter now acts as a buck converter

with the voltage across the bulk capacitor as the input voltage.

In this mode, are operated in anti-phase with the opera-

tion in motoring mode, so that power can be fed back to the ac

Fig. 5. Waveforms of i and i .

power supply. As shown in Fig. 4, and are turned onin the

positive half cycle while and are turned on in the negative

half cycle. The operation is simply achieved by triggering the

select switch in Fig. 2 to exchange the synchronization sig-nals applied to .

It should be noted that the thyristors are pre-triggered in their

half cycle with an advance angle of about 15 degrees. For ex-

ample, if the supply source is in the positive half cycle and

and are in theON state [Fig.4(a)], and willbe pre-trig-

gered [11] at about 15 degrees before the voltage supply source

goes to negative half cycle [Fig. 4(b)]. Pre-triggering the in-

coming thyristors and enables the outgoing thyristors

and to naturally commutated without using extra commuta-

tion circuit.

In this operating mode, the converter is operated as a buck

converter, feeding power from the regenerative load to the

supply source. The phase current is out of phase withthe supply voltage. The operation of the current mode controller

is still applicable since the inductor current is flowing in the

same direction as that in the motoring mode. Both the motoring

mode and regenerating mode have a similar inductor current

profile [Fig. 3(c) and Fig. 4(c)]. Fig. 7 shows the two stages

of operation, namely Topology B-I and Topology B-II in one

switching cycle and the inductor current waveform.

In this mode, the first topology is operating for a time interval

of (where is the duty cycle of ) and the second

topology is operating for a time interval of .

1) Topology BIFig. 7(a): In one switching cycle, is

given by

(12)

is the initial value of the inductor current at the beginning

of the switching cycle. This stage is defined by the ON time of

(i.e., ). At the end of this stage

(13)

2) Topology BIIFig. 7(b): is given by

(14)
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Fig. 6. Operation of boost converter.

Fig. 7. Operationof buck converter (a)Upper trace:line voltage v (Ch1 : 250V/div) and Lower trace: line current i (Ch4: 5 A/div). (b) Upper trace: linevoltage v (Ch1: 250 V/div) and Lower trace: inductor current i (Ch4:5A/div). (c) Upper trace: v (250 V/div) and Lower trace: v (100 V/div).

This stage is defined for the period of ).

Therefore

(15)

By using (15), the ideal quasi-steady-state conversion char-

acteristic is given by

(16)

As

(17)

where .

3) Average Output Current: Again, if is assumed to be

sinusoidal and the conversion efficiency is 100%

(18)

and

(19)

As is determined by , the reversible power from the

load to the supply side can be controlled by adjusting .

4) Design the Value of : As illustrated in Fig. 7, the ripple

current flowing through the inductor is

(20)

It gives the same expression as (9). Therefore, the minimum

inductor that limits the maximum ripple current to a value

is

(21)

In the above mathematical derivations, the output voltage is as-

sumed to be constant.

The choice of a semi-controlled switch such as thyristor ora fully-controlled switch such as IGBT for depends on the

maximum voltage ripple allowed in the dc link capacitor. In

the motoring mode, the dc link voltage is larger than the peak

voltage of the ac line voltage because the converter acts as a

boost converter. is turned on for regeneration. If a thyristor is

usedfor , willonly be turnedoffnaturally whenits current

falls below its latching value. Because the converter now acts as

a buck converter in the regenerating mode, the dc link voltage

across the bulk capacitor (input voltage of the buck converter)

must drop below the instantaneous value of the rectified ac line

voltage (output voltage of the buck converter) for the current in

to fall to zero. This means that the voltage fluctuation of the


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dc link voltage could be large. When there is a mode change

from regeneration to motoring, the boost converter must in-

crease dc link capacitor voltage above the peak value of the line

voltage before line current shaping becomes effective. If this

voltage fluctuation is to be kept small, then it is better to use a

fully controlled switch for .

III. DESIGN EXAMPLE

The induction motor is designed for 110 V operation. A

step-down transformer was used to step down the 220 V ( )

into 110 V ( ). An experimental ac/dc converter was tested

under the following conditions:

(This frequency is arbitrarily chosen.)

of

The output power is 500 W. Therefore, the value of is

(22)

The values of from (6) and from (17) are

(23)

Under motoring mode, the current ripple is

(24)

By using (11) and (21), the minimum value of the inductor

should be based on the ripple current in (24)

(25)

The prototype bidirectional converter has been tested under

motoring and regenerating conditions. During the motoring test,the converter was loaded with an electronic load. The regener-

ative test was carried out by connecting a dc voltage source to

the converter output. Experimental results for motoring and re-

generating operations are shown in Figs. 8 and 9, respectively.

Figs. 8(a) and 9(a) show the supply voltage and input current

of the bidirectional converter . is in phase with during

the motoring mode and is 180 out of phase during the regen-

erating mode. Some slight distortion can be observed in the re-

generative current near the zero crossing region. This current

distortion is due to the pre-triggering of the incoming thyristors

as explained earlier. Fig. 8(b) shows the waveforms of and

inductor current in the motoring operation. Fig. 8(c) shows

(a)

(b)

(c)

Fig. 8. Experimental waveforms in the motoring mode.

the waveforms of and . These experimental results con-

firm the bidirectional power flow and power factor correction

capability of the new operation of the converter. Further work is

being carried out to a complete electronic AC drive system.


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Fig. 9. Experimental waveforms in the regenerating mode. Upper trace: v(250 V/div) and Lower trace: i (5 A/div).

IV. CONCLUSIONS

A new converter switching operation of a bidirectional acdc

power converter is presented and practically demonstrated. It

has the following advantages:

1) The converter system can be operated with unity power

factor for bidirectional power flow conditions and thus

can reduce harmonic pollution of ac drives.

2) The power-factor-corrected ac/dc converter uses a phase

controlled thyristor rectifier (with two additional con-

trolled switches), that are low-cost and robust.

3) Since the converter has bidirectional power flow capa-

bility, the energy storage requirement and therefore the

size of the dc link capacitor can be reduced.4) The direction of the current flow in the inductor is the

same under both motoring and regenerating conditions.

This makes the current control simple.

The proposed operation provides a low-cost and reliable op-

tion for AC motor drive systems that require frequent regenera-

tion. The control strategy adopted in the demonstration is simple

and commonly used in current-controlled power converters. The

power factor correction feature of the new converter operation

therefore does not require a complicated control system.

REFERENCES

[1] D. Simonotti, J. Sebastn, and J. Uceda, The discontinuous conductionmode sepic and cuk power factor preregulators: Analysis and design,

IEEE Trans. Ind. Electron., vol. 44, pp. 630637, Oct. 1997.[2] J. Holtz and L. Springob, Reduced harmonics PWM controlled

line-side converter for electric drivers, in Proc. IEEE-IAS Annu.Meeting Seattle, WA, Oct. 712, 1990, vol. II, pp. 959964.

[3] M. K. Nalbant and J. Klein, Design of a 1 kW power factor correctorcircuits, in Proc. Power Conv., Oct. 1989, pp. 121135.

[4] R. Itoh and Ishizaka, Single phase sinusoidal rectifier with stepup/down characteristics, IEE Proc. Part B, vol. 138, no. 6, pp.338344, Nov. 1991.

[5] N. Mohan, T. Undeland, and R. J. Ferraro, Sinusoidal line currentrectification with a 100 kHz BSIT step-up converter, in Proc. IEEEPESC84, 1984, pp. 9298.

[6] M. Kazerani, P. D. Ziogas, and G. Joos, A novel active current wave-shaping technique for solid state input power factor conditioners, IEEETrans. Ind. Electron., vol. 38, pp. 7278, Feb. 1991.

[7] J. T. Boys and A. W. Green, Current forced single phase reversiblerectifier, IEE Proc. Part B, vol. 136, no. 5, pp. 205212, 1989.

[8] M. Morimoto, K. Oshitani, K. Sumito, and S. Okuma, New singlephase unity power factor PWM converter inverter system, in Proc.

IEEE PESC89, 1989, pp. 585589.[9] F. Rahman, L. Zhong, and S. Y. R. Hui, A single-phase, regenerative

variable speedinductionmotor drive, inEPE95, 1995, pp. 37773780.[10] R. M. Davis and C. G. Alexander, A bidirectional ACDC power con-

verter for fixed polarity dc loads, in Proc. IEE PEVD88, 1988, pp.

142145.[11] P. Weeler, J. Clare, and M. Sumner, The integration of a bidirectionalrectifier and a voltage fed inverter in an optimized high performancevariable speed ac motor drive system, in Proc. IEE PEVD96, 1996,pp. 396400.

[12] L. Morln, L. Fernndez, J. Dixon, and R. Wallace, A simple and lowcost control strategy for active power filters connected in cascade,IEEETrans. Ind. Electron., vol. 44, pp. 621629, Oct. 1997.

[13] W. M. Grady, M. J. Samotyi, and A. H. Noyola, Survey of active powerline conditioning methodologies, IEEE Trans. Power Delivery, vol. 5,pp. 15361542, July 1990.

[14] M. H. Rashid, Power Electronics: Circuits, Devices, and Applications,2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 1993.

[15] R. Mammano and R. Neidorff, Improving power factorA new activecontroller simplifies the task, in Proc. Power Conversion, Oct. 1989,pp. 100109.

S. Y. (Ron) Hui (SM94) was born in Hong Kongin 1961. He received the B.Sc. degree (with honors)from the University of Birmingham, Birmingham,U.K., in 1984, and the D.I.C. and Ph.D. degreesfrom the Imperial College of Science, Technologyand Medicine, London, U.K., in 1987.

He was a Lecturer in power electronics at theUniversity of Nottingham, U.K., from 1987 to1990. In 1990, he went to Australia and took up alectureship at the University of Technology, Sydney,where he became a Senior Lecturer in 1991. Later,

he joined the University of Sydney and was promoted to Reader of ElectricalEngineering and Director of Power Electronics and Drives Research Group in1996. Presently, he is a Chair Professor of Electronic Engineering and Asso-ciate Dean of the Faculty of Science and Engineering at the City Universityof Hong Kong. He has published over 150 technical papers, including about80 refereed journal publications. His research interests include all aspects ofpower electronics.

Dr. Hui received the Teaching Excellence Award from the City University ofHongKongin 1999. Heis a Fellow oftheIEE,theIEAust,and the HKIE. Hehasbeen an Associate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICSsince 1997.

Henry Shu-Hung Chung (S92M95) received theB.Eng. degree (with first class honors) in electricalengineering and the Ph.D. degree from The HongKong Polytechnic University, Kowloon, in 1991 and

1994, respectively.Since 1995, he has been with the City University

of Hong Kong. He is currently an AssociateProfessor in the Department of Electronic Engi-neering. His research interests include time- andfrequency-domain analysis of power electronicscircuits, switched-capacitor-based converters,

random-switching techniques, digital audio amplifiers, fuzzy-logic control, andsoft-switching converters. He has authored over 105 technical papers includingover 47 refereed journal publications.

Dr. Chung received the China Light and Power Prize and the Scholarshipand Fellowship of the Sir Edward Youde Memorial Fund in 1991 and 1993, re-spectively. He is currently Chairman of the Council of the Sir Edward YoudeScholars Association and IEEE student branch counselor. He was Track Chairof the Technical Committee on Power Electronics Circuits and Power Systems,IEEE Circuits andSystemsSociety, from 1997 from 1998.He is currentlyan As-sociateEditor of theIEEE TRANSACTIONS ON CIRCUITS AND SYSTEMSPART I.


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Siu-Chung Yip received the B.Eng. degree (withhonors) in electronic engineering from the CityUniversity of Hong Kong, Kowloon, Hong Kong, in1997 where he is currently pursuing the PhD. degree.

His research interests include design of acdcpower converters, analysis of power factor correctiontechniques, and design of bidirectional powerconverters.

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