The Modular Multilevel Matrix Converter (M3C) is an emerging topology, especially suitable for
						high torque low speed drive applications in the medium voltage range. One special benefit is
						the overload capability of the output currents. Up to 200% of the nominal values can be reached
						near standstill to overcome breakaway torques without additional efforts. This contribution gives
						an experimental verification of the operating performance by using a low voltage laboratory
						prototype with 5 cells per arm. In addition, different energy balancing methods, depending on
					the operating point are presented. Finally, possible applications for the M3C are discussed

Braun, Michael

Bräckle, Dennis

Gommeringer, Mario

Kammerer, Felix

Schnarrenberger, Mathias

KITopen

          KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association Elektrotechnisches Institut (ETI) Prof. Dr.-Ing. Michael Braun Prof. Dr.-Ing. Martin Doppelbauer Prof. Dr.-Ing. Marc Hiller  Kaiserstr.12. 76131 Karlsruhe  www.kit.edu Title: Operating Performance of the Modular Multilevel Matrix Converter in Drive Applications Authors: Felix Kammerer, Dennis Bräckle, Mario Gommeringer, Mathias Schnarrenberger, Michael Braun Institute: Karlsruhe Institute of Technology (KIT)  Elektrotechnisches Institut (ETI)  Type: Conference Proceedings of the PCIM Europe 2015 (www.pcim-europe.com) Published at: Proceedings 2015 PCIM Europe, International Conference and Exhibition for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management, Nuremberg, Germany, May 19-21, 2015 Publisher: VDE Verlag  Year: 2015 ISBN: 978-3-8007-3924-0 Pages: 549-556 Hyperlinks: http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=7149076  Operating Performance of theModular Multilevel Matrix Converter in Drive ApplicationsFelix Kammerer, Dennis Bra¨ckle, Mario Gommeringer, Mathias Schnarrenberger, Michael BraunElektrotechnisches Institut (ETI) - Electrical Drives and Power ElectronicsKarlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131 Karlsruhe, GermanyTel.: +49 721 608 42461, E-mail: felix.kammerer@kit.eduAbstractThe Modular Multilevel Matrix Converter (M3C) is an emerging topology, especially suitable forhigh torque low speed drive applications in the medium voltage range. One special benefit isthe overload capability of the output currents. Up to 200% of the nominal values can be reachednear standstill to overcome breakaway torques without additional efforts. This contribution givesan experimental verification of the operating performance by using a low voltage laboratoryprototype with 5 cells per arm. In addition, different energy balancing methods, depending onthe operating point are presented. Finally, possible applications for the M3C are discussed.subconverter 1mains subconverter 2 subconverter 3uL3uL1uL2ie3ie1ie2i 11 i 21 i 31 i 22 i 32 i 13 i 23 i 33Li 12u0i a3i a2i a1LeLeLeu 11u 21u 31u 12u 22u 32u 13u 23u 33arm currentszero sequence voltageAC machineua1 ua2 ua3La LaLaRa RaRaoutput currentsinputcurrentsthree phase z-winding arm inductorua12L Lua23ua31ui1 ui2 ui3structure of one converter arm:uxycell xyNixybypass switchcell xy1cell xy2u Cxy1Cxy1u Cxy2Cxy2u CxyNCxyNarm voltagesNormalized arm current composition:a)b)ÎxyÎa,max/3Îe/3frequency transfer ratio ν  νn -νn Îa,n/3Fig. 1: a) Modular Multilevel Matrix Converter (M3C) for feeding high torque variable speed drives. Thenine M3C converter arms consist of N series connected H-bridges with a DC-capacitor Cxyz.b) Normalized arm current composition for an M3C feeding a 3AC machine (cos(ϕa) = 1) with a nominaloutput current amplitude of Iˆa,n = 1. The maximum arm current Iˆxy is held constant in the whole speedrange. At low speeds the output current amplitude Iˆa,max can be increased up to 200% of the nominalvalue Iˆa,n to generate a higher torque [1].1. IntroductionThe Modular Multilevel Matrix Converter (M3C), shown in Fig. 1a), performs a direct 3AC to3AC power conversion. It is particularly suitable for high voltage drive applications that requirelow nominal frequencies. The M3C can be used to replace line commutated cycloconvertersdue to its modular and scalable structure. Its high voltage quality on the input and output sideavoids expensive filters. High reliability requirements can be fulfilled with the M3C by installingbypass switches (see Fig. 1a)). In case of cell failures the corresponding bypass switches areclosed and the operation continues with a reduced number of cells.In the first part of this paper the output current overload capability and the energy balancingof the M3C is explained. Subsequently the low voltage laboratory prototype is presented andmeasurement results show the operating performance of the M3C for feeding 3AC machines.In the last part possible applications for the M3C are discussed.2. Output current overload capability of the M3CThe overall system including the three-phase AC grid, the Modular Multilevel Matrix Converter(M3C) and the three-phase machine is shown in Fig. 1a). The nine converter arms are modeledas controllable voltage sources and consist of N series connected cells. They are realized as H-bridges with a capacitor Cxyz (x = input phase number, y = output phase number, x, y ∈ {1, 2, 3},z = cell number, z ∈ {N|1 ≤ z ≤ N}). By switching the transistors of the cells, each arm is ableto generate an adjustable arm voltage uxy with 2N +1 voltage steps. The nine arm currents ixycan be described as four space vectors with their corresponding αβ-components. The detailedcalculations are given in [2]. Fig. 2 shows the four space vectors with the corresponding phasecomponents marked in red, blue and green. The distribution of the phase components alongthe nine converter arms is given in the lower part of Fig. 2. The nine arm currents ixy canbe calculated by adding the phase components of the four space vectors according to thecolors given in Fig. 2. For example, the arm current i11 contains the following space vectorcomponents:i11 =ie13+ia13+ id11 + id21 (1)The maximum arm current Iˆxy is the dimensioning criteria for the current capability of the semi-conductors and cell capacitors Cxyz. With (1) and assuming sinusoidal currents, it can becalculated to [1]:Iˆxy =Iˆe3+Iˆa3+ Iˆd1 + Iˆd2 (2)Equation (2) contains four different components. This feature can be used in drive applicationsto generate a higher output current and therefore a higher torque at low speeds [1]. This ispossible due to the fact that the active output power Pa and therefore the input currents iex arelow at low speeds. The remaining arm current capability can be used to generate a higheroutput current in this operation area. The maximum output current amplitude Iˆa,max can becalculated for a given arm current Iˆxy. Here the losses and the internal diagonal currents id1and id2 are neglected due to their low values at steady state operation [1]:Iˆa,max =3 · Iˆxy1 + Uˆa·cos(ϕa)Uˆe·cos(ϕe)(3)321/α γa+φa ia1=iaα  Iaia2 ia3 iaβ β  321/α γe+φe ie1 =ieα Ieie2 ie3 ieβ β  321/α γd1 Id1id12 id13 id1β β  id11=id1α 321/α γd2 Id2id22 id23 id2β β  id21=id2α   verticalhorizontalie1  ie2ie3 ie1/3  ie2/3ie3/3 ia1/3  ia2/3 ia3/3 ia1  ia2 ia3 id11  id12 id13 id21  id22 id23 a) b) c) d)Îa=|Ia| Îe=|Ie| Îd1=|Id1| Îd2=|Id2|Fig. 2: Four space vectors calculated from the nine arm currents ixy [2] with their distribution along thenine converter arms: a) output current space vector Ia, b) input current space vector Ie, c) diagonalcurrent 1 space vector Id1, d) diagonal current 2 space vector Id2Fig. 1b) shows the normalized arm current composition Iˆxy for an M3C feeding a 3AC ma-chine that is operating with cos(ϕa) = 1. The output current component Iˆa,max is doubled atstandstill, compared to the nominal output current Iˆa,n reached at the nominal operation point|νn| = 0.4 =|ωa,n|ωe(ν = frequency transfer ratio, ωa = output angular frequency, ωe = input angu-lar frequency) [1]. This special feature of the M3C can be used to overcome high breakawaytorques without over-sizing of the cell semiconductors and capacitors Cxyz. Experimental re-sults using the output current overload capability are shown in Section 5.3. Energy balancing of the Modular Multilevel Matrix ConverterOne main task for the control of the M3C is to ensure that the average values of the nine armcapacitor voltages uCxy can be maintained at their reference values [2, 3, 4, 5]. The actual armcapacitor voltages uCxy =∑Nz=1 uCxyz contain a time-variant part u˜Cxy which depends on theenergy pulsation w˜Cxy caused by the actual arm power pxy = uxy · ixy [2]:uCxy = u˜Cxy + uCxy ≈NCxyz · uCxy∫pxydt+ uCxy =NCxyz · uCxy∫uxyixydt+ uCxy (4)To maintain a constant average arm capacitor voltage uCxy, the control system has to ensurethat the average arm power pxy is zero for all nine converter arms. For this purpose a cascadedvector control system can be used [2, 3, 4, 5].One special challenge is the operation of the M3C in cases where the input and output fre-quency are close together or equal |ωa| ≈ |ωe|. Here some converter arms will be continuouslycharged and others continuously discharged. To allow a continuous operation, the energy mustbe redistributed between the nine converter arms. For this purpose a zero sequence voltageu0 together with the internal diagonal currents id1 or id2 can be used [2, 4, 5]. Experimentalresults which include the switch over to normal operation with |ωa| 6= |ωe| are shown in Section5.4. Low voltage M3C laboratory prototype with 5 cells per arma) b)  9xSemiconductor switches: - 4x MOSFET IPP110N20N3, 200V,   10mΩ, heat sinkCapacitors:- 2x EETUQ, 2200µF, 160V- Local cell power supply with flyback   converter: 3x 15V, 1x 5VMicrocontroller: - dsPIC 30F3010- Gate signal generation- Cell capacitor voltage measurement- 2 fiber optic cables per cell- RS-232 protocol (2MBaud/s)Fig. 3: a) M3C low voltage laboratory prototype with a rated power of Pn = 15 kW. b) Arm circuit boardwith N = 5 cells and corresponding technical data.Fig. 3 shows the constructed M3C low voltage laboratory prototype with a rated power ofPn = 15 kW. It contains 9 · 5 = 45 cells consisting of a MOSFET H-bridge and an electrolyticcapacitor. A dsPIC 30F3010 microcontroller from Microchip is used in each cell to generatethe gate signals and to measure the cell capacitor voltage uCxyz. The required voltages forthe cell control system are generated locally from the cell capacitor with a flyback converter.A Cyclone III Field Programmable Gate Array (FPGA) from Altera and a TMS320C6748 digitalsignal processor (DSP) from Texas Instruments are used as central control system. The FPGAcommunicates with the cell microcontrollers via 2 fiber optic cables and a RS-232 protocol.Additionally, it calculates the nine arm capacitor voltages uCxy from the measured cell capacitorvoltages uCxyz and performs the cell balancing inside of the nine converter arms [6]. The pulsewidth modulation frequency is selected to fPWM = 8kHz. Due to the cell balancing, an averagecell switching frequency of fs ≈ 2 kHz is achieved. The cascaded vector control scheme [2, 3, 6]is calculated in real time on the DSP in less than tA =1fPWM= 125µs.5. Experimental results with a low voltage laboratory prototype0200400600800speed nn/min−1600625650675700arm capacitor voltages uC11, uC21, uC31, uC00 (black)u/V0153045output currents iaq (red), iad (blue)i/A−20020arm currents i11, i21, i31, maximum Îxy (black)i/A0 0.5 1 1.50153045input currents ied (red), ieq (blue)i/At/s0 0.5 1 1.5−404diagonal currents id1α, id1β, id2α, id2βi/At/st≈ 1,5sFig. 4: Measurement of the M3C control variables during the run-up of an induction machine ton = 680min−1 with iaq ≈ 32A, short time averaged values at tA =1fPWM= 18 kHz.Fig. 4 shows the start up performance of the M3C feeding an induction machine up to its ratedspeed of n = 680min−1 which corresponds to an electrical output frequency of fa ≈ 24Hz. Theload torque ML = 120Nm, generated by a DC-machine, is constant and corresponds to therated torque of the induction machine and a torque generating current of iaq ≈ 28, 5A. Themagnetization current iad = 10A and the torque generating current iaq ≈ 33A are constantduring the run-up. The run-up time is t ≈ 1, 5 s and the maximum arm current Iˆxy ≈ 23A isreached at the end of the run-up (see red circle in Fig. 4). The active output power at that pointis Pa ≈ 12 kW. The internal diagonal currents id1α/β and id2α/β are small and the arm capacitorvoltages uCxy are pulsating around the actual average arm capacitor voltage uC00 ≈ 675V.The run-up time can be shortened to t ≈ 0, 6 s by using the maximum arm current Iˆxy ≈ 23Aduring the entire run-up sequence, see Fig. 5. Here the torque generating current can beincreased up to iaq ≈ 48A in the low speed range resulting in a higher starting torque.In addition it is possible to use the higher torque generating current iaq in the low speed rangeto overcome breakaway torques. This is shown in Fig. 6. Here the initial load torque isML0 = 170Nm which is decreased to ML = 120Nm at a speed of n = 200min−1. The M3Cis able to run-up to n = 680min−1 in about t ≈ 0, 9 s against the initial torque ML0. The initialtorque generating current at standstill is iaq,0 ≈ 38A (blue circle). It is higher than the torquegenerating current at the end of the run-up iaq ≈ 33A (green circle). In this case the run-up ton = 680min−1 is only possible by using the entire arm current Iˆxy ≈ 23A to generate a highertorque generating current iaq at low speeds.Fig. 7 shows an induction machine run-up from n = 1100min−1 to n = 1650min−1. In the0200400600800speed nn/min−1600625650675700arm capacitor voltages uC11, uC21, uC31, uC00 (black)u/V0153045output currents iaq (red), iad (blue)i/A−20020arm currents i11, i21, i31, maximum Îxy (black)i/A0 0.5 1 1.50153045input currents ied (red), ieq (blue)i/At/s0 0.5 1 1.5−404diagonal currents id1α, id1β, id2α, id2βi/At/st≈ 0,6sFig. 5: Measurement of the M3C control variables during the run-up of an induction machine ton = 680min−1 with Iˆxy ≈ 23A, short time averaged values at tA =1fPWM= 18 kHz.0200400600800speed nn/min−1600625650675700arm capacitor voltages uC11, uC21, uC31, uC00 (black)u/V0153045output currents iaq (red), iad (blue)i/A−20020arm currents i11, i21, i31, maximum Îxy (black)i/A0 0.5 1 1.50153045input currents ied (red), ieq (blue)i/At/s0 0.5 1 1.5−404diagonal currents id1α, id1β, id2α, id2βi/At/sML=120 NmML0=170 NmFig. 6: Measurement of the M3C control variables during the run-up of an induction machine ton = 680min−1 with Iˆxy ≈ 23A and a initial load torque of ML0 = 170Nm, short time averaged values attA =1fPWM= 18 kHz.frequency range fa = 44..56Hz, a DC zero sequence voltage u0 = 90V is used together withid1α/β and id2α/β to balance the nine arm capacitor voltages uCxy [2, 4, 5]. The maximum armcurrent Iˆxy is significantly increased and reduces the maximum output currents and thereforethe maximum torque. With the control strategies presented in [2, 4, 5] it is possible to achieve anactive output power of Pa ≈ 8.5 kW at fa ≈ 50Hz. This is more than 50% of the nominal valuePn = 15 kW. Therefore the M3C is suitable for applications with nominal output frequenciesgreater than the input frequencies |fa,n| > |fe| and a quadratic load characteristic. Here asuccessful run-up can be performed by using the control strategies presented in [2, 4, 5].10001200140016001800speed nn/min−1620645670695720arm capacitor voltages uC11, uC21, uC31, uC00 (black)u/V0153045output currents iaq (red), iad (blue)i/A−40−2002040arm currents i11, i21, i31, maximum Îxy (black)i/A0153045input currents ied (red), ieq (blue)i/A−10010diagonal currents id1α, id1βi/A0 0.5 1 1.5−1000100zero sequence voltage u0u/Vt/s0 0.5 1 1.5−10010diagonal currents id2α, id2βi/At/sfa=44..56HzFig. 7: Measurement of the M3C control variables during the run-up of an induction machine fromn = 1100min−1 to n = 1650min−1, short time averaged values at tA =1fPWM= 18 kHz.6. Applications for the Modular Multilevel Matrix ConverterThe M3C is suitable for medium and high voltage applications with different input and outputfrequencies. It can be used for:• Replacement of line commutated cycloconverters or conventional two or three level con-verters in low speed applications like direct driven wind generators or mills. In mills theoutput current overload capability can be used to overcome breakaway torques [1].• Feeding the rotor circuit of Doubly Fed Induction Generators (DFIG) used for wind turbinesor variable speed pump storage power plants. Here the dynamics and part load efficiencycan be increased compared to fixed speed synchronous generators [7].• Connection of offshore wind-farms via a low frequency AC grid (e.g. 16, 7Hz) to theconventional 50/60Hz public grid instead of a expensive HVDC connection [8].• High speed drives with a quadratic load characteristic, e.g. pumps, fans, etc. Here theM3C requires smaller cell capacitors Cxyz compared to the Modular Multilevel Converter[9].7. ConclusionThis paper demonstrates the operating performance of the Modular Multilevel Matrix Converter(M3C) in drive applications by using a low voltage laboratory prototype. Measurement resultsshow the excellent run-up performance against a high breakaway torque near standstill. There-fore the M3C is a favorable solution for applications with high torque requirements and lownominal output frequencies like mills. For applications that require higher output frequenciesthe run-up must be guaranteed. For this purpose the performance with equal input and outputfrequency is demonstrated, here more than 50% of the nominal output power can be achieved.This makes the M3C a favorable solution for medium and high voltage applications with differentinput and output frequencies.AcknowledgmentThe authors would like to thank the DFG (German Research Foundation) which finances thisresearch project under grant BR 1780/8-1.8. References[1] F. Kammerer, M. Gommeringer, J. Kolb, and M. Braun. Overload capability of the modularmultilevel matrix converter for feeding high torque low speed drives. In Proceedings of PCIMSouth America , Sao Paulo, Brasil, volume 1, pages 20–27, October 2014.[2] F. Kammerer, M. Gommeringer, J. Kolb, and M. Braun. Energy balancing of the modularmultilevel matrix converter based on a new transformed arm power analysis. In PowerElectronics and Applications (EPE’14-ECCE Europe), 2014 16th European Conference on,pages 1–10, August 2014.[3] F. Kammerer, J. Kolb, and M. Braun. Fully decoupled current control and energy balancingof the modular multilevel matrix converter. In Power Electronics and Motion Control Confer-ence (EPE/PEMC), 2012 15th International, pages LS2a.3–1–LS2a.3–8, September 2012.[4] W. Kawamura, M. Hagiwara, and H. Akagi. Control and experiment of a modular multilevelcascade converter based on triple-star bridge cells (mmcc-tsbc). Industry Applications,IEEE Transactions on, PP(99):1–1, 2014.[5] W. Kawamura, M. Hagiwara, and H. Akagi. Control and experiment of a 380-v, 15-kwmotor drive using modular multilevel cascade converter based on triple-star bridge cells(mmcc-tsbc). In Power Electronics Conference (IPEC-Hiroshima 2014 - ECCE-ASIA), 2014International, pages 3742–3749, May 2014.[6] F. Kammerer, J. Kolb, and M. Braun. A novel cascaded vector control scheme for themodular multilevel matrix converter. In IECON 2011 - 37th Annual Conference on IEEEIndustrial Electronics Society, pages 1097–1102, November 2011.[7] F. Kammerer, M. Gommeringer, J. Kolb, and M. Braun. Benefits of operating doubly fedinduction generators by modular multilevel matrix converters. PCIM Europe, Nuremberg,Germany, 2013.[8] W. Fischer, R. Braun, and I. Erlich. Low frequency high voltage offshore grid for transmissionof renewable power. In Innovative Smart Grid Technologies (ISGT Europe), 2012 3rd IEEEPES International Conference and Exhibition on, pages 1–6, October 2012.[9] K. Ilves, L. Bessegato, and S. Norrga. Comparison of cascaded multilevel converter topolo-gies for ac/ac conversion. In Power Electronics Conference (IPEC-Hiroshima 2014 - ECCE-ASIA), 2014 International, pages 1087–1094, May 2014.

Operating Performance of the Modular Multilevel Matrix Converter in Drive Applications

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Operating Performance of the Modular Multilevel Matrix Converter in Drive Applications

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