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  • The Essence of Three-Phase AC/AC Converter Systems

    J. W. Kolar, T. Friedli, F. Krismer, S. D. RoundPower Electronic Systems Laboratory, ETH Zurich, Zurich, Switzerland

    email: kolar@lem.ee.ethz.ch

    Abstract In this paper the well-known voltage and current DC-link converter systems, used to implement an AC/AC converter,are initially presented. Using this knowledge and their spacevector modulation methods we show their connection to the familyof Indirect Matrix Converters and then finally the connection todirect Matrix Converters. A brief discussion of extended MatrixConverter circuits is given and a new unidirectional three-levelMatrix Converter topology is proposed. This clearly shows thetopological connections of the converter circuits that directly leadto an adaptability of the modulation methods. These allow thereader who is familiar with space vector modulation of voltageand current DC-link converters to simply incorporate and identifynew modulation methods. A comparison of the converter concepts,with respect to their fundamental, topology-related characteristics,complexity, control and efficiency, then follows. Furthermore, bytaking the example of a converter that covers a typical operationregion in the torque-speed plane (incl. holding torque at standstill),the necessary silicon area of the power semiconductors is calculatedfor a maximum junction temperature. This paper concludes withproposals for subjects of further research in the area of MatrixConverters.

    Keywords AC/AC converter, matrix converter, voltage sourceconverter, current source converter.

    I. INTRODUCTION

    For conversion from a three-phase mains source to a three-phase voltage load with an arbitrary frequency and amplitude,e.g. variable speed drives, converter systems with either avoltage or current DC-link are mainly used today (Fig. 1). Inthe case of the voltage DC-link, the mains coupling can, in thesimplest case, be implemented by a diode bridge. To accomplishbraking operation of the load, a pulse-controlled braking resistormust be placed in the DC-link or an antiparallel thyristorbridge be provided on the mains side. The disadvantages arethe relatively high mains distortion and high reactive powerrequirements, especially during inverter operation.

    A mains-friendly AC/AC converter with bidirectional powerflow can be implemented by coupling a PWM rectifier and a

    a

    b

    c

    A

    B

    C

    A

    B

    C

    a)

    b)

    La,b,c

    a

    b

    c

    I > 0L

    C

    Ca,b,c CA,B,C

    U > 0

    Figure 1: Three-phase AC/AC converter with a) voltage DC-link(U-BBC) and b) current DC-link (I-BBC).

    PWM inverter to the DC-link. The DC-link quantity is thenimpressed by an energy storage element that is common to bothstages, i.e. a capacitor C for the voltage DC-link (U-BBC) or aninductor L for the current DC-link (I-BBC). The PWM rectifieris controlled such that a sinusoidal mains current is drawn that isin phase or anti-phase (energy feedback) with the correspondingmains phase voltage. For the realization of the AC/AC converter,12 power transistors and 12 diodes are required, or in thefuture 12 reverse conducting IGBTs (RC-IGBTs) or 12 reverseblocking IGBTs (RB-IGBTs), since for the I-BBC the feedbackof energy can be achieved only by the reversal of the DC-linkvoltage polarity because of the unidirectional current flow setby the power semiconductors.

    Due to the DC-link storage element, there is the advantagethat both converter stages are to a large extent decoupled forcontrol purposes. Furthermore, a constant, mains independentinput quantity exists for the PWM inverter stage, which resultsin high utilization of the converters power capability. On theother hand, the DC-link energy storage element has a relativelylarge physical volume, and when electrolytic capacitors are used,in the case of a voltage DC-link, there is potentially a reducedsystem lifetime.

    With the goal of higher power density and reliability, itis hence obvious to consider the so-called Matrix Converterconcepts that achieve three-phase AC/AC conversion withoutany intermediate energy storage element. The physical basisof these systems is the constant instantaneous power producedby a symmetrical three-phase current-voltage system, whichallows, for example, to directly provide the constant powerconsumption of an electrical machine generating constant torqueand running at constant speed (Fig 2). Conventional directMatrix Converters (CMC, Fig. 3a) carry out voltage and currentconversion in one stage. Alternatively, there is the option ofindirect conversion by means of an Indirect Matrix Converter(IMC, Fig. 3b). As with the U-BBC and I-BBC, separate stagesare again provided for voltage and current conversion, but theDC-link has no intermediate storage element. Realization of

    11

    t0

    pa pb pc

    pa,b,c

    pa+ pb+ pc

    Figure 2: Instantaneous power pa,b,c(t) of the phases a, b, c of asymmetrical three-phase current-voltage system; also shown is the

    total power of the phases p(t) = pa(t) + pb(t) + pc(t) = P .

  • a

    b

    c

    A

    B

    C

    a)

    b)

    a

    b

    c

    p

    n

    A

    B

    C

    Figure 3: Basic Matrix Converters: a) Conventional (Direct) MatrixConverter (CMC), b) conventional Indirect Matrix Converter (IMC).

    both converter systems requires in the basic configuration 18IGBTs and 18 diodes, or 18 RB-IGBTs (CMC) or 12 RB-IGBTsand 6 RC-IGBTs (IMC): thus the storage element in the DC-linkis eliminated at the cost of more semiconductors.

    Matrix Converters are frequently seen as a future concept forvariable speed drives technology, but despite intensive researchover the decades they have until now only achieved low in-dustrial penetration. The reason for this could be, apart fromtechnical aspects, the more complex modulation and dimension-ing calculations compared with DC-link converters and the hightopological variations, especially with the introduction of thegroup of the Sparse Matrix Converters (SMC) and the HybridMatrix Converters (HMC), which is a mixture between matrix

    and DC-link converters. A classification of DC-link and MatrixConverter concepts presented up to now in the literature is shown(limited to forced commutated systems) in Fig. 4.

    In this paper, starting from the well-known converter sys-tems with voltage and current DC-links and their space vectormodulation methods (Section II), a connection is made to theIndirect Matrix Converters (Section III) and finally to directMatrix Converters (Section IV). A brief discussion of extendedMatrix Converter circuits is given and a new unidirectionalthree-level Matrix Converter topology is proposed (Section V).This clearly shows the topological connections of the convertercircuits that directly lead to an adaption of the modulationmethods and allows the readers, who are familiar with spacevector modulation of the voltage and current DC-link con-verters, to easily incorporate and identify the new modulationmethods. Then a comparison of the converter concepts withrespect to their fundamental, topology-related characteristics,complexity, control requirements and efficiency (Section VI)follows. Furthermore, taking the example of a converter thatcovers a typical operating region in the torque-speed plane (incl.holding torque at standstill), the necessary silicon area of thepower semiconductor is calculated for maintaining a maximumjunction temperature. This paper concludes with suggestions ofsubjects for further research in the field of Matrix Converters(Section VII).

    II. AC/AC CONVERTERS WITH DC-LINK

    In the following, the fundamentals of space vector modulationof PWM DC-link converters are briefly discussed and theirfunctional equivalent circuit diagrams shown. For the U-BBC,the PWM inverter stage and for the I-BBC the PWM rectifierstage are considered. This allows the development of the IMCtopology by simply combining the two subsystems, and thecontrol of the IMC by connection and coordination of thesubsystems modulation methods.

    A. Voltage DC-link PWM Inverter

    The PWM inverter stage of the voltage DC-link AC/ACconverter system shown in Fig. 1a is composed of three bridgelegs where each exhibits the function of a switch that connectsthe output to either the positive or the negative DC bus, p and n.

    AC/DC-DC/ACConverter

    with VoltageDC-link(U-BBC)

    AC/DC-DC/ACConverter

    with CurrentDC-link(I-BBC)

    Hybrid DirectMatrix Converter

    (HCMC)

    Hybrid IndirectMatrix Converter

    (HIMC)

    Conventional Matrix

    Converter(CMC)

    Full BridgeMatrix

    Converter (Open Motor-

    windings)

    AC/DC-DC/AC

    ConverterwithoutDC-link

    Capacitor

    IndirectMatrix

    Converter(IMC)

    SparseMatrix

    Converter(SMC)

    (VSMC)(USMC)

    Three-levelMatrix

    Converter

    DirectMatrix Converter

    IndirectMatrix Converter

    HybridMatrix Converter Matrix Converter

    Converter withDC-link

    AC/AC Converter

    [1] [2],[3]

    [4],[5]

    [6]

    [7],[8]

    [9],[10],[11]

    [12],[13],[14],[15]

    [16],[17],[18]

    [19],[19],[20],

    [22]

    [21],[21], [23],

    [24]

    Figure 4: Classification of three-phase AC/AC converter circuits with chronologically ordered references to the technical literature.

  • U23Re, A

    i2

    u2,(ppn)

    u2,(pnn)

    Im

    u2,(ppp)u2,(nnn)

    i22

    u2

    i2,(ppn) = - iC = i(ppn)

    i2,(pnn) = iA = i(pnn)*

    u2u2 = *

    - C

    Figure 5: Voltage space vector sector for the U-BBC PWM inverter(output) stage. The DC-link current i (input current of the PWMinverter stage) can be determined by projection of the current space

    vector i2 onto the instantaneous voltage space vector u2,j

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