proceedings of ncpce-8

A Direct AC/AC Converter Based onCurrent-Source Converter Modules

Mehmod Kazerani,

Abstract—The research on direct ac/ac conversion in the pasttwenty years has been following two major directions:

1) improving the performance of the original nine-bidirec-tional-switch matrix converter topology;

2) using converter topologies composed of unidirectionalswitches only.

In this paper, a direct ac/ac converter topology based on current-source converter modules is proposed. It is shown, through analysisand simulation, that the proposed topology is free of any switchingdifficulties and gives high-quality sinusoidal waveforms on bothsides. Furthermore, with the same number of semiconductor de-vices as in the original nine-bidirectional-switch matrix convertertopology, it is able to perform all aspects of frequency changing,real power flow control, and independent reactive power flow con-trol on both sides. The proposed topology finds applications in elec-tric drive industry, as an ac motor drive, and power system in-dustry, as an asynchronous link.

Index Terms—AC/AC converter, CSC, frequency changer,high-quality sinusoidal waveforms, independent reactive powerflow control, L–C filter, nine-bidirectional-switch, NPT IGBT,real power flow control, semiconductor devices, SVC, THDs,topology.

I. INTRODUCTION

I N THE conventional nine-bidirectional-switch matrix con-verter topology, inaccurate timing of the gating signals sent

to the bidirectional switches can lead to serious problems [1].A dead-time or an overlap between the on-states of the twobidirectional switches commutating an inductive load currentcan lead to hazardous Ldi/dt overvoltages or dangerously highovercurrents. To protect the matrix converter switches, some re-searchers have used snubber networks [2], while others haveimplemented multi-step switching algorithms [3]. Snubber cir-cuits are bulky and dissipative in nature, whereas multi-stepswitching techniques complicate the control circuit. Further-more, in the nine-switch topology, the fact that one and only oneof the three switches connecting phases a, b, and c on side-1 toeach phase on side-2 has to be ON at each moment of time,asks for the distribution of the ON-periods of these switchesover one switching period according to a certain pattern. TheTHDs of the side-1 and side-2 waveforms are strongly affectedby the pattern of the above distribution, and high-quality wave-forms can be realized only at a high switching frequency. Whilesome researchers have tried to perfect the nine-switch matrix

Nirma University, Gujarath, India. The project was funded by NCIC, GoI.The authors are with the Department of Electrical and Computer Engineering,

converter topology, others such as Kazerani and Ooi [4], [5]and Kim, Sul, and Lipo [6] have considered topologies that useonly unidirectional switches. In [4], [5], a direct ac/ac convertertopology based on three-phase voltage-source converter mod-ules has been introduced. In this paper, the dual topology, i.e., adirect ac/ac converter topology based on three three-phase cur-rent-source converter (CSC) modules is proposed.

In the early stages of development, current-source convertertopology was facing a lot of difficulties, mainly because of lackof understanding of its special features and shortcomings of thesemiconductor switch technology. Almost all of the original dif-ficulties associated with the CSC topology have been overcomethrough research and development. The problem of misopera-tion of CSC under bi-level switching regime has been solved bythe introduction of dynamic tri-level switching strategy [7], [8].The problem of magnification of low-order harmonics due tothe resonance of ac-side capacitors with the system inductanceshas been solved by introducing damping in the system throughproper control [9]. The requirement of connecting a diode in se-ries with each switch in the CSC topology, due to the inadequacyof switch reverse voltage withstand, has been lifted thanks to theavailability of reverse-blocking non-punch-through (NPT) insu-lated gate bipolar transistor (IGBT) switches [10]. Finally, theintroduction of super-conducting materials to magnetic energystorage has made it possible to reduce the losses in the dc-sideinductor dramatically.

In the following sections, first, the structure of the proposedac/ac converter topology will be described. The principles ofoperation of the system will be presented next, followed by theintroduction of available control levers and possible transforma-tion matrix structures. Finally, selected simulation results willbe used to verify the analytical expectations.

II. STRUCTURE OF THEPROPOSEDAC/ACCONVERTERTOPOLOGY

Fig. 1 shows the schematic diagram of the proposed ac/acconverter topology. It consists of three identical current-sourceconverter modules. On the ac-side, the modules are connectedin parallel to the ac source through an L–C filter. On the dc-side,the CSC modules are connected to the three-phase load througha three-phase three-legged-core transformer with separate pri-mary and secondary windings and delta-connected secondarywindings.

Each CSC module is controlled to produce a regulated dc cur-rent superimposed by a sinusoidal current of desired amplitude,frequency, and phase angle. The dc components ensure unidi-rectional current on the dc-side. The ac components are phase

Sania Shah

54

Fig. 1. Schematic diagram of the proposed ac/ac converter topology.

Fig. 2. Three-phase three-legged-core transformer.

shifted by 120 with respect to one another. The primary wind-ings of the transformer secure a loop for the dc-side currents andprovide enough inductance for the regulation of the dc compo-nents of the dc-side currents. The dc components do not con-tribute to any flux in the transformer core and thus, do not leadto core saturation. The reason for this can be explained in thelight of the structure of the three-phase three-legged-core trans-former shown in Fig. 2.

As the primary currents , and in Fig. 2 containboth dc and ac components, the resulting fluxes , andwill be composed of both constant and time-varying compo-nents. Due to the three-legged structure of the core,

. As the time-varying components of , and con-stitute a balanced system and thus add up to zero, the sum ofthe constant flux components must be zero. Knowing that theconstant flux components are generated by equal and unidirec-tional dc current components, and are therefore equal and uni-directional, the only way for their sum to be zero is that eachconstant flux component is equal to zero. This guarantees thatthere will be no constant flux component in the transformer coreand as a result, the core will not saturate.

Before moving to the principles of operation of the proposedac/ac converter topology, a qualitative cost-benefit comparisonbetween the proposed topology and the conventional nine-bidi-rectional-switch matrix converter topology seems to be useful.The number of switching devices in the proposed topology is thesame as that in the conventional topology. In the current-sourceconverter-based ac/ac converter, there are 18 switches and 18 se-ries diodes. In the conventional matrix converter topology, thereare nine four-quadrant switch elements, each composed of twosemiconductor switches and two series diodes, adding up to 18switches and 18 diodes. The proposed topology does not haveany switching problems and as a result, does not require imple-menting multi-step switching algorithms or using bulky snub-bers to rectify the switching problems. The proposed topologyis based on the standard converter modules which have maturedthrough years from both hardware design and control techniquepoints of view. The proposed topology enjoys a high degree ofcontrollability due to the availability of a high number of controllevers, simplifying the implementation of closed-loop controlsystems for active and reactive power flow controls and wave-

International Conference on Advanced Power Engineering 201355

shaping. However, the proposed topology requires switching de-vices of higher current ratings than those used in the conven-tional matrix converter topology due to the presence of the dccomponents in the side-2 currents. Also, a three-phase three-legged-core transformer is required to ensure the elimination ofthe dc current components and maintain a high performance forthe current-source converter modules. As the transformer offersisolation and possibility of voltage level adjustment as well, itsexistence may be justified. Overall, it can be concluded that theperformance of the current-source converter-based ac/ac con-verter topology is superior to that of the conventional matrixconverter topology, but it costs more. A trade-off between theperformance and cost has always to be made based on the spe-cific application.

III. PRINCIPLES OFOPERATION OF THEPROPOSEDAC/ACCONVERTERTOPOLOGY

The nine modulating signals corresponding to the nine legsof the three converters can be presented in the form of a trans-formation matrix [H]

(1)

where

(2)

with , and

(3)

where

(4)

(5)

with and.

The dc- and ac-side currents and voltages are related in thefollowing way:

(6)

Assuming

(7)

(8)

the voltage transformation in (6) yields

(9)

where

(10)

(11)

where

(12)

(13)

The dc-side currents will be

(14)

where

(15)

(16)

where

(17)

(18)


Fig. 3. Typical side-1 voltage and current waveforms (f = 60 Hz, f = 120 Hz). Top:0:1e andi . Middle: i . Bottom:i .

In (15), is the resistance of the primary windings of thetransformer, and in (17) and (18), , where

and are the resistance and inductance seen as viewedfrom the primary-side of the transformer. It can be shown that

, whereand is the transformer primary-to-secondary turns-ratio.

The current transformation in (6) can be rewritten as

(19)The simplification in (19) results from the fact that the transfor-mations of the dc and ac components of the dc-side currents aredecoupled from each other, i.e.,

(20)

The structure of , as an example for the unfiltered side-1 cur-rents, is

(21)

As seen consists of components at only, as the low-orderharmonics present in , and are cancelled out uponaddition at the junction point on side-1. The same is true for theinput currents and . As a result, the burden on the inputfilter capacitors (C) is reduced to that of filtering the switchingharmonics.

To provide a visual presentation for the mathematical equa-tions of this section, Figs. 3 and 4 show some typical waveformsfor the voltages and currents associated with side-1 and side-2of Fig. 1, respectively.

IV. A VAILABLE CONTROL LEVERS

and given by (2)–(5), offer the following controllevers.

A.

As suggested by (15), can be used as the control lever toregulate in a closed-loop control system. The magnitude of

is chosen to be at least equal to .

B.

The first term on the right-hand-side of (21) is the reflectionof on side-1 as a result of the transformation. Thesecond and the third terms are the reflections ofon side-1 asa result of the transformation. As seen from (21), the phaseangles of the second and the third terms are fixed byand . isthe side-2 displacement angle at angular frequency. is thephase angle of the side-1 capacitor voltages with respect to theac source voltages which is very small when a small input filteris used at high switching frequency. Therefore, the phase anglesof , and , and thus , and , can be controlledby to adjust the side-1 displacement power factor (DPF).

In PWM technique, the peak values of the modulating sig-nals have to be limited to within the window defined by the peakvalue of the triangular carrier signal, , in order to avoid wave-form distortion due to over-modulation. is usually verysmall, just large enough to provide the real power necessary toregulate against the converter internal losses and the powerloss incurred in . This leaves enough room for the maneuverof . The real power flow involved in the regulation of isproportional to .

C.

As seen from (16)–(18), the magnitudes of , andcan be controlled by , or and both, if

, or is chosen as the structurefor .


Fig. 4. Typical side-2 voltage and current waveforms (f = 60 Hz, f = 120 Hz). Top:i . Middle: v . Bottom:i .

Fig. 5. Block diagram of the control system for the proposed ac/ac converter topology.

D.

As seen from (16)–(18), the phase angles of , and, and thus the load currents can be controlled by. does

not have any effect on the side-1 currents, as suggested by (21).is the control lever used for vector control in the induction

motor drives.

V. SVC AND FREQUENCYCHANGER

The proposed ac/ac converter topology virtually contains twodecoupled sub-systems: a static var compensator (SVC) and anunrestricted frequency changer. The SVC part is realized by the

matrix through control levers and . As mentioned inSection IV, is used to regulate and is used to controlthe phase angles of input currents , and with respect to

, and to control the input reactive power and thus adjustthe side-1 displacement power factor. The frequency changerpart is realized by the matrix through control leversand . Due to the special structure of , the side-1 frequency

is changed to the side-2 frequency. Also, the magnitudesof side-2 currents , and are controlled by ,whereas their phase angles with respect to those of , and

are controlled by for the vector control of induction motordrives.

VI. SELECTION OF STRUCTURE

As (3)–(5) suggest, there are three possible structures for, i.e., , and . All three

structures result in successful frequency conversion. The onlydifference will be in the phase angle of the unfiltered side-1currents , and .

By looking at (21), one can see that, in frequency transfor-mation through , the side-2 displacement angle changesfrom , in (17), to , when it appears in the unfiltered side-1currents. This conjugate property is a characteristic of the struc-ture .

By examining (21), one can observe that in frequency trans-formation through , the sign of the side-2 displacementangle is kept unchanged when it appears in the side-1 unfilteredcurrents. This nonconjugate property is a characteristic of thestructure .

When the frequency transformation takes place through, the unfiltered side-1 currents expe-

rience both conjugate and nonconjugate effects. The effect ofside-2 displacement angle on side-1 displacement angle will bedetermined by the relative magnitudes of and . Inconventional nine-bidirectional-switch matrix converter,and are adjusted to vary the amplitude of the side-2


Fig. 6. Simulation results: Frequency sweep. Top:f (Hz). Middle:0:1e (V) & i (A). Bottom: i ; i ;&i (A).

Fig. 7. Simulation results: UDPF atf = 30 Hz. Top:0:1e (V) & i (A). Bottom: i (A).

currents and adjust the side-1 displacement angle to the desiredvalue or achieve unity displacement power factor (UDPF) onside-1. In the topology proposed in this paper, the two extradegrees of freedom provided by , i.e., and , canbe used to regulate the magnitude of the dc component of thedc-side currents and adjust the side-1 displacement angle. Inthis case, one can choose . As a result,

, given by (21), becomes

(22)

As seen from (22), the conjugate and nonconjugate effects havebeen cancelled out as a result of choosing asthe structure for .

The choice between , and hasto be made based on the advantage that one can offer over theothers in side-1 displacement power factor correction. In CSCtopology, the ac-side L–C filter might be capacitive or inductiveat the fundamental frequency. If the input L–C filter is inductive,

can be used to reflect the inductive load on side-2 as acapacitive impedance on side-1. This helps in neutralizing theinductive part of the input filter impedance. If the input L–Cfilter is capacitive, can be used to reflect the inductive




load on side-2 as an inductive impedance on side-1. This helpsin neutralizing the capacitive part of the input filter impedance.Fine adjustments toward the desired DPF (usually UDPF), atany value of , can be then made by proper choice ofin .

When the structure or is used, and the load isheavily inductive, the reflected displacement angle from side-2on side-1 will be large. As a result, a largewill be required forpower factor correction. A largeasks for a large for pro-viding the necessary active power for side-2 dc currents regula-tion. This results in saturation in the PWM process due to over-modulation and hence waveform distortion. When using thethird structure, i.e., , with ,the effect of side-2 displacement angle on that of side-1 is can-celled and a very small effort from results in successfulpower factor correction by-control. Therefore, the third struc-ture is regarded as the favorite structure in the particular case ofheavily-inductive load as well as any other case. The rest of thestudy will thus concentrate on the third structure.

VII. CLOSED-LOOPCONTROL

Fig. 5 shows the block diagram of the closed-loop controlsystem for the proposed ac/ac converter topology. The magni-tudes of the dc and ac components of the dc-side currents of theCSCs are controlled by and , respectively. Controllingthe ac components of the side-2 currents of the CSCs is pre-ferred over the direct control of load currents, since controllingthe load currents requires three additional current sensors. Thedesired magnitude and phase of the ac components of the side-2currents can be easily obtained from those of the load currentsusing the following phasor relation obtained from Fig. 1:

(23)

Input DPF control through and load current phase angle con-trol through are performed in open loop.


VIII. SIMULATION RESULTS

The power circuit of Fig. 1 together with the control systemof Fig. 5 was simulated using the structure

. The following parameters were used:

Transformer mH, mH,kHz.

The side-1 L–C filter was designed for the resonant frequencyof 2002 Hz and damping ratio of 0.283. On the base of 3.0 kVAand 208 V, pu and pu. The input L–Cfilter was capacitive at 60 Hz. The design ensures thatdoes not result in more than 10% voltage drop at full load, thefundamental current component in the filter capacitor is smalland the switching frequency current components are filtered ef-fectively by the capacitor filter.

Fig. 6 shows the simulation results for A,A, , and , as assumes different

values of 30, 60, and 120 Hz. As seen, due to the proper choiceof structure, for all values of , very close to UDPFexists on side-1, without any effort to correct the power factorusing -control. Also, the transition from one frequency toanother frequency, on side-2, is very fast and smooth, withoutany effect on the amplitude of the side-2 currents.

Figs. 7–9 show the simulation results for A,A, , and Hz, respectively,

and , with UDPF on side-1.

IX. CONCLUSION

In this paper, a direct ac/ac converter topology based on threethree-phase current-source converter modules is proposed.Thanks to the well-established technology of the current-sourceconverter and because of using only unidirectional switches, noswitching difficulties are observed. The special power-circuitstructure and transformation-matrix design, give the proposedac/ac converter topology two distinct and decoupled features:static var compensator (SVC) and frequency changer. The SVCoperation provides input displacement power factor control(i.e., input reactive power flow control), while the frequencychanger operation allows for unrestricted frequency conversion,as well as active and reactive power flow control on side-2.Three different frequency changer matrix structures have

been introduced. The structure requiring the minimum controleffort to achieve unity displacement power factor on side-1with no risk of over-modulation is identified and regarded asthe favorite structure. The theoretical expectations have beenverified using simulation results.

REFERENCES

[1] N. Burany, “Safe control of four-quadrant switches,” inProc. 1989 IEEEInd. Applicat. Soc. Annu. Meeting (IAS’89), San Diego, CA, Oct. 1–5,1989, pp. 1190–1194.

[2] P. D. Ziogas, S. I. Khan, and M. H. Rashid, “Analysis and design offorced commutated cycloconverter structures with improved transfercharacteristics,”IEEE Trans. Ind. Electron., vol. IE-33, pp. 271–280,Aug. 1986.

[3] L. Huber and D. Borojevic, “Space vector modulation with unityinput power factor for forced commutated cycloconverters,” inProc.1991 IEEE Ind. Applicat. Soc. Annu. Meeting (IAS’91), Dearborn, MI,Sept./Oct. 1991, pp. 1032–1041.

[4] M. Kazerani and B. T. Ooi, “Direct AC-AC matrix converter based onthree-phase voltage-source converter modules,” inProc. 1993 IEEEInd. Electron. Conf. (IECON’93), Lahaina, HI, Nov. 15–19, 1993, pp.812–817.

[5] , “Feasibility of both vector control and displacement factor correc-tion by voltage source type AC-AC matrix converter,”IEEE Trans. Ind.Electron., vol. 42, pp. 524–530, Oct. 1995.

[6] S. Kim, S. K. Sul, and T. A. Lipo, “AC to AC power conversion basedon matrix converter topology with unidirectional switches,” inProc.1998 IEEE Appl. Power Electron. Conf. (APEC’98), Anaheim, CA, Feb.15–19, 1998, pp. 301–307.

[7] X. Wang and B. T. Ooi, “Unity PF current-source rectifier based ondynamic tri-logic PWM,” IEEE Trans. Power Electron., vol. 8, pp.288–294, July 1993.

[8] M. Kazerani and B. T. Ooi, “Linearly controllable boost voltages fromtri-level PWM current-source inverter,”IEEE Trans. Ind. Electron., vol.42, pp. 72–77, Feb. 1995.

[9] X. Wang, “Advances in pulse width modulation techniques,” Ph.D. dis-sertation, McGill Univ., Montreal, QC, Canada, Mar. 1993.

[10] S. Bernet, T. Matsuo, and T. A. Lipo, “A matrix converter using reverseblocking NPT-IGBTs and optimized pulse patterns,” inProc. 1996 IEEEPower Electron. Spec. Conf. (PESC’96). Milan, Italy, June 23–27, 1996,pp. 107–113.

Mehrdad Kazerani (S’88–M’96–SM’02) received the B.Sc. degree fromShiraz University, Iran, in 1980, the M.Eng. degree from Concordia University,Montreal, QC, Canada, in 1990, and the Ph.D. degree from McGill University,Montreal, in 1995.

From 1982 to 1987, he was with the Energy Ministry of Iran. He is presentlyan Associate Professor with the Department of Electrical and Computer Engi-neering, University of Waterloo, Waterloo, ON, Canada. His research interestsare in the areas of power electronic circuits and systems design, active powerfilters, matrix converters, distributed power generation, utility interface of alter-native energy sources, and FACTS.

Dr. Kazerani is a Registered Professional Engineer in the province of Ontario,Canada.


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