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  • 1.
    Antonopoulos, Antonios
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    On Interaction between Internal Converter Dynamics and Current Control of High-Performance High-Power AC Motor Drives with Modular Multilevel Converters2010In: Proc. IEEE Energy Conversion Congress and Exposition (ECCE), 2010, p. 4293-4298Conference paper (Refereed)
    Abstract [en]

    The modular multilevel converter (M2C) is a promising converter technology for various high-voltage highpower applications. The reason to this is that low-distortion output quantities can be achieved with low average switching frequencies per switch and without output filters. With the M2C the output voltage has such a low harmonic content that highpower motors can be operated without any derating. However, the apparent large number of devices, requires more complex converter control techniques than a two-level counterpart. Even though there have been several ways suggested to control the converter itself, it is still a challenge to investigate the interaction of these controllers with an external motor current controller. It is shown in the paper that the anticipated interaction will not result in any problems neither for the converter nor for the motor control itself.

  • 2.
    Antonopoulos, Antonios
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Siemaszko, Daniel
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Vasiladiotis, Michail
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Evaluation of Control and Modulation Methods forModular Multilevel Converters2010In: Proc. Int. Power Electronics Conf. (IPEC), 2010, p. 746-753Conference paper (Refereed)
    Abstract [en]

    The modular multilevel converter is a promising converter technology for various high-voltage high-power applications. Despite the apparent simplicity of the circuit, the inherent dynamics of the converter and the balancing of the sub-module capacitor voltages impose high requirements on the control system, which can be implemented in quite different ways. To illustrate this, and to provide a guidance for future research on the subject, this paper presents an evaluation of four different control and modulation methods. The investigation is based on experiments on a down-scaled 10 kVA converter having 10 submodules per phase leg. The main items to be investigated are dynamics within the sub-modules, arm voltages and circulating currents. It is found that the suggested open-loop control method provides the fastest arm-voltage response and that the balancing approach based on a sorting algorithm is substantially faster and less complicated to implement than the method using a dedicated voltage controller for each sub-module.

     

     

  • 3.
    Antonopoulos, Antonios
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion. ABB, Sweden.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Global Asymptotic Stability of Modular Multilevel Converters2014In: IEEE transactions on industrial electronics (1982. Print), ISSN 0278-0046, E-ISSN 1557-9948, Vol. 61, no 2, p. 603-612Article in journal (Refereed)
    Abstract [en]

    Modular multilevel converters require that the controller is designed so that the submodule capacitor voltages are equalized and stable, independent of the loading conditions. Assuming that the individual capacitor-voltage sharing is managed effectively, an open-loop strategy has been designed to ensure that the total amount of energy stored inside the converter always will be controlled. This strategy, using the steady-state solutions of the dynamic equations for controlling the total stored energy in each converter arm, has proven to be effective. The intention of this paper is to explain in a rigorous way the mechanism behind the suggested strategy and to prove that, when this open-loop strategy is used, the system becomes globally asymptotically stable. Experimental verification on a three-phase 10-kVA prototype is presented.

  • 4.
    Antonopoulos, Antonios
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Stability Analysis of Modular Multilevel Converters With Open-Loop Control2013In: 39th Annual Conference of the IEEE Industrial Electronics Society, IECON 2013, IEEE , 2013, p. 6316-6321Conference paper (Refereed)
    Abstract [en]

    Modular multilevel converters (M2Cs) require that the controller is designed so that the submodule capacitor voltages are equalized and stable, independent of the loading conditions. Provided that the individual capacitor voltage sharing is managed effectively, an open-loop strategy can been designed to ensure that the total amount of energy stored inside the converter always will be controlled. This strategy, using the steady-state solutions of the dynamic equations for controlling the total stored energy in each converter arm, has proven to be effective. The intention of this paper is to explain in a rigorous way the mechanism behind the suggested strategy, and to prove that, when this open-loop strategy is used, the system becomes globally asymptotically stable.

  • 5.
    Antonopoulos, Antonios
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Modular multilevel converter AC motor drives with constant torque from zero to nominal speed2014In: IEEE transactions on industry applications, ISSN 0093-9994, E-ISSN 1939-9367, Vol. 50, no 3, p. 1982-1993Article in journal (Refereed)
    Abstract [en]

    Modular multilevel converters are shown to have a great potential in the area of medium-voltage drives. Low-distortion output quantities combined with low average switching frequencies for the semiconductor devices create an ideal combination for very high-efficiency drives. However, the large number of devices and capacitors that have to conduct the fundamental-frequency current require more complex converter control techniques than its two-level counterpart. Special care needs to be taken for starting and operation at low speeds, where the low-frequency current may cause significant unbalance between the submodule capacitor voltages and disturb the output waveforms. In this paper, principles for converter operation with high torque in the whole speed range are investigated. Experimental results from a down-scaled 12-kVA prototype converter running a loaded motor at various speeds between standstill and the rated speed are also provided.

  • 6.
    Antonopoulos, Antonios
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Modular multilevel converter ac motor drives with constant torque form zero to nominal speed2012In: 2012 IEEE Energy Conversion Congress and Exposition, ECCE 2012, IEEE , 2012, p. 739-746Conference paper (Refereed)
    Abstract [en]

    Modular multilevel converters (M2Cs) are shown to have a great potential in the area of medium-voltage drives. Low-distortion output quantities, combined with low average switching frequencies for the semiconductor devices create the ideal combination for very high-efficiency drives, both from an electric motor and an inverter point of view. With M2Cs the output voltage has such a low harmonic content that high-power motors can be operated without any derating. However, the large number of devices and the existence of capacitors that have to conduct the fundamental frequency current, requires more complex converter control techniques than its two-level counterpart. Special care needs to be taken under starting and operation with low frequency, where the low-frequency current may cause significant unbalance between the submodule capacitor voltages, disturb the output waveforms, and eventually cause the converter to trip. In this paper, principles for converter operation with high torque in the whole speed range, from standstill to rated speed will be investigated. The converter-control method utilizes estimation of the capacitor voltage variation, based on equations describing steady-state conditions. Experimental results from a down-scaled 12 kVA prototype converter running a loaded motor from zero up to the rated speed are provided in the paper.

  • 7.
    Antonopoulos, Antonios
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Siemaszko, Daniel
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Vasiladiotis, Michail
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Inner Control of Modular Multilevel Converters - An Approach using Open-loop Estimation of Stored Energy2010In: Proc. Int. Power Electronics Conf. (IPEC), 2010, p. 1579-1585Conference paper (Refereed)
  • 8.
    Bessegato, Luca
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Ilves, Kalle
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Harnefors, Lennart
    Norrga, Staffan
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Östlund, Stefan
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Control and Admittance Modeling of an AC/AC Modular Multilevel Converter for Railway Supplies2019In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107Article in journal (Refereed)
    Abstract [en]

    Modular multilevel converters (MMCs) can be configured to perform ac/ac conversion, which makes them suitable as railway power supplies. In this paper, a hierarchical control scheme for ac/ac MMCs for railway power supplies is devised and evaluated, considering the requirements and the operating conditions specific to this application. Furthermore, admittance models of the ac/ac MMC are developed, showing how the suggested hierarchical control scheme affects the three-phase and the single-phase side admittances of the converter. These models allow for analyzing the stability of the interconnected system using the impedance-based stability criterion and the passivity-based stability assessment. Finally, the findings presented in this paper are validated experimentally, using a down-scaled MMC. 

  • 9.
    Harnefors, Lennart
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion. ABB, Sweden.
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Global asymptotic stability of current-controlled modular multilevel converters2015In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, no 1, p. 249-258, article id 6883248Article in journal (Refereed)
    Abstract [en]

    In this paper, previously developed stability results for open-loop sum-capacitor-voltage control of modular multilevel converters are extended. To give improved damping, circulating-current feedback is included in the control law. With the output-current control loop and a first-order measurement lag taken into account, global asymptotic stability is proven. Careful consideration of the on-line sum-capacitor-voltage reference computation is given, since this is the most critical part of the control system.

  • 10.
    Hassanpoor, Arman
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Tolerance-band modulation methods for modular multilevel converters2013Conference paper (Refereed)
    Abstract [en]

    Modular multilevel converters (M2C) are increasingly used in high voltage direct current (HVDC) systems. The efficiency of M2Cs is highly related to the modulation method which determines the switching frequency and capacitor voltage ripple in the converter station. A new approach to modulation of M2C is presented in this paper. Tolerance-band methods are employed to obtain switching instants, and also cell selection. The proposed methods overcome the modulation problem for converters with few numbers of cells and also reduce the sorting efforts for cell balancing purposes while maintaining the cell-capacitor voltage limits. The evaluation is done by time-domain simulation by which the performance of each method is studied in both steady-state and transient cases. It is observed that using tolerance band methods not only reduces the switching frequency but also allows for handling severe fault cases in a grid connected system. Use of this method can reduce the switching losses and also allow for reduction of the cell capacitor size.

  • 11.
    Hassanpoor, Arman
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Tolerance band modulation methods for modular multilevel converters2015In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, no 1, p. 311-326, article id 6739183Article in journal (Refereed)
    Abstract [en]

    Modular multilevel converters (M2Cs) are increasingly used in high-voltage direct current (HVDC) systems. The efficiency of M2Cs is influenced by the modulation and cell selecting methods, which determines the switching frequency and capacitor voltage ripple in the converter station. A new approach to modulation of the M2C is presented in this paper. Tolerance band methods are employed to obtain the switching instants, and also cell selection. The proposed methods overcome the modulation problem for converters with few cells on one hand and also reduce the sorting efforts for cell balancing purposes of many cells converter on the other hand. Three different algorithms are also proposed to balance the cell capacitor voltages. The evaluation is done in time-domain simulation by which the performance of each method is studied in both the steady-state and transient cases. It is observed that using tolerance band methods not only reduces the switching frequency but also allows for handling severe fault cases in a grid-connected system. Eventually, the most promising tolerance band method has been implemented and verified in a real-time digital simulator, RTDS®. The average switching frequency of 70 Hz has been achieved for the system under study, while the capacitor voltage ripple is limited to 10% of the nominal cell voltage.

  • 12.
    Heinig, Stefanie
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Ilves, Kalle
    KTH.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    On Energy Storage Requirements in Alternate Arm Converters and Modular Multilevel Converters2016In: 2016 18TH EUROPEAN CONFERENCE ON POWER ELECTRONICS AND APPLICATIONS (EPE'16 ECCE EUROPE), IEEE, 2016Conference paper (Refereed)
    Abstract [en]

    In this paper, a comparison of the energy storage requirements is performed for the modular multilevel converter (MMC) with half-bridge and full-bridge submodules as well as for the alternate arm converter (AAC). Concerning the AAC, the operational mode with overlap period is taken into account and an analytical relation between the overlap angle and the modulation index is presented. This ensures that the net energy exchange for the converter arms is zero over each half cycle.

  • 13.
    Heinig, Stefanie
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Jacobs, Keijo
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Ilves, Kalle
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Bessegato, Luca
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Bakas, Panagiotis
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Norrga, Staffan
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Implications of Capacitor Voltage Imbalance on the Operation of the Semi-Full-Bridge Submodule2019In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 34, no 10, p. 9520-9535, article id 8598807Article in journal (Refereed)
    Abstract [en]

    Future meshed high-voltage direct current grids require modular multilevel converters with extended functionality. One of the most interesting new submodule topologies is the semi-full-bridge because it enables efficient handling of DC-side short circuits while having reduced power losses compared to an implementation with full-bridge submodules. However, the semi-full-bridge submodule requires the parallel connection of capacitors during normal operation which can cause a high redistribution current in case the voltages of the two submodule capacitors are not equal. The maximum voltage difference and resulting redistribution current have been studied analytically, by means of simulations and in a full-scale standalone submodule laboratory setup. The most critical parameter is the capacitance mismatch between the two capacitors. The experimental results from the full-scale prototype show that the redistribution current peaks at 500A if the voltage difference is 10V before paralleling and increases to 2500A if the difference is 40V. However, neglecting very unlikely cases, the maximum voltage difference predicted by simulations is not higher than 20-30V for the considered case. Among other measures, a balancing controller is proposed which reduces the voltage difference safely if a certain maximum value is surpassed. The operating principle of the controller is described in detail and verified experimentally on a down-scaled submodule within a modular multilevel converter prototype. It can be concluded that excessively high redistribution currents can be prevented. Consequently, they are no obstacle for using the semi-full-bridge submodule in future HVDC converters.

  • 14.
    Heinig, Stefanie
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Jacobs, Keijo
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems. ABB Corporate Research.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Implications of Capacitor Voltage Imbalance on the Operation of the Semi-Full-Bridge Submodule2017In: 2017 19th European Conference on Power Electronics andApplications (EPE'17 ECCE Europe), Warsaw, Poland, 2017Conference paper (Refereed)
    Abstract [en]

    An investigation of the voltage imbalance of the two capacitors of the semi-full-bridge submodule is performed. Since the capacitances are not exactly the same, there may be a difference between the capacitor voltages. The resulting current-spike when they are connected in parallel has been analyzed in a full-scale laboratory experiment.

  • 15.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Modeling and Design of Modular Multilevel Converters for Grid Applications2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis aims to bring clarity to the dimensioning aspects and limiting factors of the modular multilevel converter (MMC). Special consideration is given to the dc capacitors in the submodules as they are a driving factor for the size and weight of the converter. It is found that if the capacitor voltages are allowed to increase by 10% the stored energy must be 21 kJ/MW in order to compensate the capacitor voltage ripple. The maximum possible output power can, however, be increased by injecting a second-order harmonic in the circulating current.

    A great advantage of cascaded converters is the possibility to achieve excellent harmonic performance at low switching frequencies. Therefore, this thesis also considers the relation between switching harmonics, capacitor voltage ripple, and arm quantities. It is shown that despite subharmonics in the capacitor voltages, it is still possible to achieve periodic arm quantities. The balancing of the capacitor voltages is also considered in further detail. It is found that it is possible to balance the capacitor voltages even at fundamental switching frequency although this will lead to a comparably large capacitor voltage ripple. Therefore, in order to limit the peak-to-peak voltage ripple, it is shown that a predictive algorithm can be used in which the resulting switching frequency is approximately 2–3 times the fundamental frequency.

    This thesis also presents two new submodule concepts. The first submodule simply improves the trade-off between the switching frequency and capacitor voltage balancing. The second submodule includes the possibility to insert negative voltages which allows higher modulation indices compared to half-bridge submodules.

    A brief comparison of cascaded converters for ac-ac applications is also presented. It is concluded that the MMC appears to be well suited for ac-ac applications where input and output frequencies are close or equal, such as in interconnection of ac grids. In low-frequency applications such as low-speed drives, however, the difficulties with handling the energy variations in the converter arms are much more severe in the MMC compared to the other considered topologies.

  • 16.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Modeling and Design of Modular MultilevelConverters for Grid Applications2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Grid-connected high-power converters are found in high-voltage direct current transmission (HVDC), static compensators (STATCOMs), and supplies for electric railways. Such power converters should have a high reliability, high efficiency, good harmonic performance, low cost, and a small footprint. Cascaded converters are promising solutions for high-voltage high-power converters since they allow the combination of excellent harmonic performance and low switching frequencies. A high reliability can also be achieved by including redundant submodules in the chain of cascaded converters.

    One of the emerging cascaded converter topologies is the modular multilevel converter (M2C). This thesis aims to bring clarity to the dimensioning aspects and limiting factors of M2Cs. The dc-capacitor in each submodule is a driving factor for the size and weight of the converter. It is found that the voltage variations across the submodule capacitors will distort the voltage waveforms and also induce alternating components in the current that is circulating between the phase-legs. It is, however, shown that it is possible to control the alternating voltage by feed-forward control. It is also shown that if the circulating current is controlled, the injection of a second-order harmonic component can extend the operating region of the converter. The reason for this is that when the converter is operating close to the boundary of overmodulation the phase and amplitude of the second-order harmonic is chosen such that the capacitors are charged prior to the time when a high voltage should be inserted by the submodules.

    The controller that is used must be able to balance the sbmodule capacitor voltages. Typically, an increased switching frequency will enhance the performance of the balancing control scheme. In this thesis it is shown that the capacitor voltages can be balanced with programmed modulation, even if fundamental switching frequency is used. This will, however, increase the voltage ripple across the aforementioned capacitors. In order to quantify the requirements on the dc-capacitors a general analysis is provided in this thesis which is based on the assumption that the capacitor voltages are well balanced. It is found that for active power transfer, with a 50 Hz sinusoidal voltage reference, the capacitors must be rated for a combined energy storage of 21 kJ/MW if the capacitor voltages are allowed to increase by 10% above their nominal values.

  • 17.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Hans-Peter, Nee
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Circulating current control in modular multilevel converters with fundamental switching frequency2012In: Conference Proceedings - 2012 IEEE 7th International Power Electronics and Motion Control Conference - ECCE Asia, IPEMC 2012, IEEE , 2012, p. 249-256Conference paper (Refereed)
    Abstract [en]

    The modular multilevel converter is a suitable topology for high-voltage applications as it combines very low switching frequency and excellent harmonic performance. In fact, it has been shown that the modular multilevel converter can even be operated at the fundamental switching frequency. If the circulating current is not controlled, a second-order harmonic component will appear. This component increases the resistive losses and the capacitor voltage ripple. Different control methods have been developed for eliminating this component in the circulating current. These are, however, based on continuous representations of the system and no control method suitable for fundamental switching frequency have yet been proposed. This paper presents a control method that combines a fundamental switching frequency scheme with an active control of the circulating current. The controller is verified experimentally on a 10-kVA laboratory prototype with five submodules per arm. The experimental validation is performed in both inverter and rectifier modes.

  • 18.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Capacitor Voltage Ripple Shaping in Modular Multilevel Converters Allowing for Operating Region Extension2011In: IECON 2011: 37TH ANNUAL CONFERENCE ON IEEE INDUSTRIAL ELECTRONICS SOCIETY, New York: IEEE , 2011, p. 4403-4408Conference paper (Refereed)
    Abstract [en]

    The second-order harmonic in the circulating current of a modular multilevel converter (M2C) influences the capacitor voltage ripple. If no measures are taken to control it, it is not possible to operate the converter with unity modulation index. An open-loop method that precalculates the instantaneous values of the circulating current and the capacitor voltages is used, in order to control the circulating current. A desired second-order harmonic is intentionally induced in the circulating current in order to make the peak of the capacitor voltage coincide with the maximum requested voltage, aiming either to extend the limits of the instantaneous available voltage or avoid unnecessarily high capacitor voltages. A method for numerical estimation of the appropriate amplitude and phase of the induced second-order harmonic is described. The method is experimentally evaluated on a three-phase down-scaled laboratory prototype. From the experiments it was found that significantly improved operating conditions could be obtained.

  • 19.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES).
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES).
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES). ABB, Sweden.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES).
    A new modulation method for the modular multilevel converter allowing fundamental switching frequency2011In: IEEE 8th International Conference on Power Electronics and ECCE Asia (ICPE & ECCE), 2011: 'Green World with Power Electronics' / [ed] IEEE, IEEE , 2011, p. 991-998Conference paper (Refereed)
    Abstract [en]

    This paper presents a new modulation method for the modular multilevel converter. The proposed method is based on a fixed pulse pattern where harmonic elimination methods can be applied. Modulation methods with harmonic elimination based on calculated pulse patterns have been presented for other multilevel topologies. However, similar modulation schemes have not yet been presented for the modular multilevel topology. In the proposed modulation method, the pulse pattern is chosen in such a way that the stored energy in each submodule remains stable. It is shown that this can be done at the fundamental switching frequency without measuring the capacitor voltages or using any other form of feedback control. Such a modulation scheme has not been presented before. The theoretical results are verified by both simulations and experimental results. The simulation results show successful operation at the fundamental switching frequency with a larger number of submodules. When a smaller number of submodules are used, harmonic elimination methods may be applied. This is verified experimentally on a converter with eight submodules per phase leg. The experimental results verify that stable operation can be maintained at the fundamental switching frequency while successfully eliminating the fifth harmonic in the ac-side voltage.

  • 20.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    A New Modulation Method for the Modular Multilevel Converter Allowing Fundamental Switching Frequency2012In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 27, no 8, p. 3482-3494Article in journal (Refereed)
    Abstract [en]

    This paper presents a new modulation method for the modular multilevel converter. The proposed method is based on a fixed pulse pattern where harmonic elimination methods can be applied. In the proposed modulation method, the pulse pattern is chosen in such a way that the stored energy in each submodule remains stable. It is shown that this can be done at the fundamental switching frequency without measuring the capacitor voltages or using any other form of feedback control. Such a modulation scheme has not been presented before. The theoretical results are verified by both simulations and experimental results. The simulation results show successful operation at the fundamental switching frequency with a larger number of submodules. When a smaller number of submodules are used, harmonic elimination methods may be applied. This is verified experimentally on a converter with eight submodules per phase leg. The experimental results verify that stable operation can be maintained at the fundamental switching frequency while successfully eliminating the fifth harmonic in the ac-side voltage.

  • 21.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Steady-State Analysis of Interaction Between Harmonic Components of Arm and Line Quantities of Modular Multilevel Converters2012In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 27, no 1, p. 57-68Article in journal (Refereed)
    Abstract [en]

    The fundamental frequency component in the arm currents of a modular multilevel converter is a necessity for the operation of the converter, as is the connection and bypassing of the submodules. Inevitably, this will cause alternating components in the capacitor voltages. This paper investigates how the arm currents and capacitor voltages interact when the submodules are connected and bypassed in a sinusoidal manner. Equations that describe the circulating current that is caused by the variations in the total inserted voltage are derived. Resonant frequencies are identified and the resonant behaviour is verified by experimental results. It is also found that the effective values of the arm resistance and submodule capacitances can be extracted from the measurements by least square fitting of the analytical expressions to the measured values. Finally, the analytical expression for the arm currents is verified by experimental results.

  • 22.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
    Controlling the ac-side voltage waveform in a modular multilevel converter with low energy-storage capability2011In: Proceedings of the 2011-14th European Conference on Power Electronics and Applications (EPE 2011) / [ed] EPE Association, 2011, p. 1-8Conference paper (Refereed)
    Abstract [en]

    During nominal operation of a modular multilevel converter the stored energy in the submodule capacitors will vary with time. If the energy storage capability of the capacitors is relatively small compared to the energy variations, this will give large variations in the capacitor voltages. These voltage variations will distort the ac-side voltage waveform and induce harmonic components in the current that is circulating between the dc terminals. The adverse effects on the ac-side voltage can be compensated for by identifying the factors that cause the distortion. It is shown that the compensation can be done by means of feed forward control while maintaining stable operating conditions and thus eliminating the need of additional stabilizing controllers. It is also shown that the voltage controller can be combined with a circulating current controller that removes the harmonics in the current that is circulating between the dc terminals. The functionality of the proposed controller is verified by both simulations and experimental results from a 10 kVA laboratory prototype. The simulations illustrate how the proposed controller successfully removes the distortion from the ac-side voltage waveform. The experimental results demonstrate stable operation during a step transient when the output power is increased by 125%.

  • 23.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Bessegato, Luca
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Semi-Full-Bridge Submodule for Modular Multilevel ConvertersManuscript (preprint) (Other academic)
  • 24.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Bessegato, Luca
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Comparison of Cascaded Multilevel Converter Topologies for AC/ AC Conversion2014In: 2014 International Power Electronics Conference, IPEC-Hiroshima - ECCE Asia 2014, IEEE Computer Society, 2014, p. 1087-1094Conference paper (Refereed)
    Abstract [en]

    This paper presents a simplified qualitative comparison of previously presented cascaded multilevel converter topologies for ac-ac conversion with particular emphasis on motor drive applications. Performance criteria such as the pulsation of the stored energy in the cell capacitors and the total required semiconductor rating are derived by analytical methods. The main conclusion is that the back-to-back connected modular multilevel converter operates best at synchronous speed, whereas the modular matrix converter and Hexverter are better suited for low-frequency output. However, by injecting circulating currents in the phase arms the operating region can be extended for all of the studied topologies.

  • 25.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Analysis and operation of modular multilevel converters with phase-shifted carrier PWM2013In: 2013 IEEE Energy Conversion Congress and Exposition, ECCE 2013, IEEE , 2013, p. 396-403Conference paper (Refereed)
    Abstract [en]

    The modular multilevel converter is a suitable topology for high-voltage high-power applications since the cascaded submodules can generate high-voltage waveforms with excellent harmonic performance at low switching frequencies. Many publications have been presented on the modulation and control of this converter type, some of which are based on phase-shifted carrier modulation. This paper presents an analysis of how the switching frequency affects the capacitor voltages, circulating current and alternating voltage at phase-shifted carrier modulation. It is found that integer multiples of the fundamental frequency should be avoided as they can cause the capacitor voltages to diverge. Suitable switching frequencies are then defined for which the arm and line quantities will be periodic and symmetric in the upper and lower arms. The theoretical results are then validated by both simulations and experimental results.

  • 26.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Analysis and Operation of Modular Multilevel Converters With Phase-Shifted Carrier PWM2015In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, no 1, p. 268-283Article in journal (Refereed)
    Abstract [en]

    Many publications have been presented on the modulation and control of the modular multilevel converter, some of which are based on phase-shifted carrier modulation. This paper presents an analysis of how the switching frequency affects the capacitor voltages, circulating currents, and alternating voltages using phase-shifted carrier modulation. It is found that switching frequencies that are integer multiples of the fundamental frequency should be avoided as they can cause the capacitor voltages to diverge. Suitable switching frequencies are derived for which the arm and line quantities will be periodic with symmetric operating conditions in the upper and lower arms. Thus, the practical outcome of this paper is a detailed description of how the switching frequency should be chosen in order to achieve advantageous operating conditions. The theoretical results from the analysis are validated by both simulations and experimental results.

  • 27.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    ABB Corporate Research, Västerås, Sweden .
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Predictive sorting algorithm for modular multilevel converters minimizing the spread in the submodule capacitor voltages2013In: 2013 IEEE ECCE Asia Downunder - 5th IEEE Annual International Energy Conversion Congress and Exhibition, IEEE ECCE Asia 2013, IEEE , 2013, p. 325-331Conference paper (Refereed)
    Abstract [en]

    The modular multilevel converter is a suitable topology for bidirectional ac-dc conversion in high-voltage high-power applications. By connecting submodule circuits in series, a high-voltage waveform with excellent harmonic performance can be achieved with a very high efficiency and low switching frequency. The balancing of the capacitor voltages will, however, become increasingly difficult as the switching frequency is reduced. Although the capacitor voltages can be kept balanced over time even at the fundamental switching frequency, the spread and thus also the peak variation in the capacitor voltages will typically increase at lower switching frequencies. This paper presents a capacitor voltage balancing strategy which aims to combine a low switching frequency with a low capacitor voltage ripple. This is done by a predictive algorithm that calculates the amount of charge that must be stored in the submodule capacitors during the following fundamental frequency period. The converter is then controlled in such a way that the stored charge in the submodule capacitors is evenly distributed among all the submodules when the capacitor voltages reach their maximum values. In this way, it is possible to limit the peak voltage in the capacitor at switching frequencies as low as 2-3 times the fundamental frequency. The capacitor voltage balancing strategy is first validated by simulation results at 110 Hz switching frequency. It is observeved that when the proposed method is used, the capacitor voltage ripple is 35% lower compared to the case when a conventional sorting algorithm is used. The capacitor voltage balancing strategy is also validated experimentally at 130 Hz switching frequency. The experimental results show that it is possible to combine the proposed method with previously presented circulating-current control methods.

  • 28.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    ABB Corporate Research, Västerås, Sweden .
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Predictive Sorting Algorithm for Modular Multilevel Converters Minimizing the Spread in the Submodule Capacitor Voltages2015In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, no 1, p. 440-449Article in journal (Refereed)
    Abstract [en]

    The balancing of the capacitor voltages in modular multilevel converters becomes increasingly difficult when the switching frequency is reduced. Typically, a reduced switching frequency will increase the spread in the capacitor voltages and, thus, the voltage ripple in the individual submodules. This paper presents a capacitor voltage balancing strategy which aims to combine a low switching frequency with a low capacitor-voltage ripple. This is done by a predictive algorithm that controls the converter in such a way that the stored charge in the submodule capacitors is evenly distributed among all the submodules when the capacitor voltages reach their maximum values. In this way, it is possible to limit the peak voltages in the submodule capacitors at switching frequencies as low as 2-3 times the fundamental switching frequency. The proposed capacitor voltage balancing strategy is validated by both simulations and experimental results with 130-Hz and 140-Hz switching frequency. In the simulations, the capacitor voltage ripple was reduced by 24% compared to the case when a conventional sorting algorithm is used, and the experimental results show that it is possible to combine the proposed voltage balancing strategy with a circulating-current controller.

  • 29.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Analysis of arm current harmonics in modular multilevel converters with main-circuit filters2012In: International Multi-Conference on Systems, Signals and Devices, SSD 2012 - Summary Proceedings, IEEE , 2012Conference paper (Refereed)
    Abstract [en]

    In a modular multilevel converter the circulating current that flows through each phase leg can affect the performance and efficiency of the converter. If measures are not taken to control the circulating current, it will inevitably contain a second-order harmonic. There are various solutions for eliminating this second-order harmonic. One of the proposed solutions includes a main-circuit filter that is tuned to block the second-order harmonic in the circulating current. This paper presents an analytical relation between the ac-side current and the higher-order harmonics in the circulating current when such a filter is used. It is found that when third-order harmonic injection is used, a large fourth-order harmonic component may appear in the circulating current. This is verified by simulating a 32-MVA converter designed for grid connected applications. The simulation results support the conclusion that it is essential to take this effect into consideration when designing the main-circuit filter.

  • 30.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    On Energy Storage Requirements in Modular Multilevel Converters2014In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 29, no 1, p. 77-88Article in journal (Refereed)
    Abstract [en]

    The modular multilevel converter is a promising topology for high-voltage and high-power applications. By using submodules equipped with dc-capacitors excellent output voltage waveforms can be obtained at low switching frequencies. The rated energy storage of the submodule capacitors is a driving factor of the size, cost, and weight of the submodules. Although the modular multilevel converter has been thoroughly investigated in the literature, a more detailed analysis of the energy-storage requirements will provide an important contribution for dimensioning and analysis of modular multilevel converters. Such an analysis is presented in this paper. The analysis relates the power transfer capability to the stored energy in the converter and the findings are validated by both simulations and experimental results. The required size of the submodule capacitors in a 4.5 MW grid-connected converter is first calculated and the calculated operating range is then compared with simulation results. The experimental results show that if the average capacitor voltage is allowed to increase 10% above the nominal value an energy storage to power transfer ratio of 21 J/kW can be achieved. It is concluded that the presented theory can relate the power transfer capability to the energy storage in the converter and is thus a valuable tool in the design and analysis of modular multilevel converters.

  • 31.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    On energy variations in modular multilevel converters with full-bridge submodules for Ac-Dc and Ac-Ac applications2013In: Power Electronics and Applications (EPE), 2013 15th European Conference on, 2013Conference paper (Refereed)
    Abstract [en]

    The modular multilevel converter is a promising topology for high-voltage high-power applications. By the series-connection of submodules it can generate high-quality voltage waveforms with low harmonic distortion at low switching frequencies. The submodules are low-voltage converters with capacitive energy storages. These capacitive energy storages are a driving factor of the size, weight, and cost of the converter. For this reason it is important to ensure that the stored energy in the converter is distributed as evenly among the submodules as possible. In this paper it is found that during nominal operating conditions, a large amount of energy is moved back and forth between the upper and lower arms in the converter. These energy oscillations can, however, be reduced or even eliminated if an appropriate modulation index or voltage ratio is used. It is also found that an appropriate modulation index or voltage ratio can reduce the power rating of the semiconductors as well. The theoretical findings are validated by simulating two systems with different voltage ratios for ac-ac conversion.

  • 32.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Taffner, Franz
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    A submodule implementation for parallel connection of capacitors in modular multilevel converters2013In: Power Electronics and Applications (EPE), 2013 15th European Conference on, 2013Conference paper (Refereed)
    Abstract [en]

    The modular multilevel converter is a suitable converter topology for high-voltage high-power applications and consists of series-connected submodules. Typically, these submodules are half-bridges with dc capacitors. A voltage ripple in the submodule capacitors is inevitable due to the current flowing in the arms. The converter should therefore be controlled in such a way that the capacitor voltages are kept balanced and close to their nominal values over time. This paper presents a new submodule circuit which alleviates the balancing of the capacitor voltages. The proposed submodule circuit consists of two capacitors and eight switches, forming a three-level submodule. Ideally, the voltage and current rating of the switches can be chosen such that the combined power rating of the semiconductors is the same as for equivalent half-bridge submodules. The proposed submodule circuit provides the possibility of connecting the two capacitors in parallel when the intermediate voltage level is used. This will reduce the capacitor voltage ripple, especially at low switching frequencies and thus allow for a reduction of the size, weight, and cost of the submodule capacitors. The proposed submodule circuit is validated by both simulation results and experiments on a laboratory prototype. It is found that the parallel connection of the submodule capacitors will, in fact, significantly improve the balancing of the capacitor voltages.

  • 33.
    Ilves, Kalle
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Taffner, Franz
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    A Submodule Implementation for Parallel Connection of Capacitors in Modular Multilevel Converters2015In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, no 7, p. 3518-3527Article in journal (Refereed)
    Abstract [en]

    In modular multilevel converters there is a trade-off between the switching frequency and the voltage ripple in the submodule capacitors. The reason for this is that it becomes increasingly difficult to balance the capacitor voltages when the switching frequency is reduced. This paper presents a new submodule circuit which improves the balancing of the capacitor voltages at low switching frequencies. The proposed submodule circuit consists of two capacitors and eight switches, forming a three-level submodule. Ideally, the voltage and current ratings of the switches can be chosen such that the combined power rating of the semiconductors is the same as for the equivalent solution with conventional half-bridge submodules. The proposed submodule circuit provides the possibility of connecting the two capacitors in parallel when the intermediate voltage level is used. This will reduce the capacitor voltage ripple, especially at low switching frequencies and thus allow for a reduction of the size, weight, and cost of the submodule capacitors. The proposed submodule circuit is validated by both simulations and experimental results. It is found that the parallel connection of the submodule capacitors will, in fact, significantly improve the balancing of the capacitor voltages.

  • 34.
    Kjellqvist, Tommy
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930). KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group.
    Norrga, Staffan
    ABB AB, Sweden .
    Östlund, Stefan
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
    Thermal Evaluation of a Medium Frequency Transformer in a Line Sider Conversion System2009In: Proceedings of the 13th International European Electronics Conference and Exhibition, EPE '09, IEEE , 2009, p. 2517-2526Conference paper (Refereed)
    Abstract [en]

    A central part of a medium frequency conversion system is the transformer. A medium frequency transformer for railway propulsion may be stressed beyond 30kV with voltage slopes around 50kV/mu s depending on topology while the geometric dimensions are small. A 170kVA transformer prototype operated at 4 kHz has been developed and tested.

    A transient thermal model of the transformer is developed and verified experimentally. The transient thermal behavior is important in propulsion applications where the conversion system is often heavily overloaded. Simulations indicate that the overloading capability of the transformer is comparable to a conventional traction transformer. Experimental results are in good agreement with the model.

  • 35.
    Norrga, Staffan
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Jin, Lebing
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Wallmark, Oskar
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Mayer, Anna
    Universität der Bundeswehr München.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    A Novel Inverter Topology for Compact EV and HEV Drive Systems2013In: 39th Annual Conference of the IEEE Industrial Electronics Society, IECON 2013, IEEE , 2013, p. 6590-6595Conference paper (Refereed)
    Abstract [en]

    This paper describes a new modular inverter topol­ogy suitable for compact drive systems in electric road vehicles. The topology is based on dc-side cascading of three-phase two ­level inverter bridges, each feeding a set of three-phase windings of a motor. This topology is compared by calculation to the known modular high-frequency (MHF) converter, and is found to have similar losses and semiconductor expenditure requirements. Simulations verify the dynamic and static properties of the converter in a realistic application.

  • 36.
    Norrga, Staffan
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Operating region extension for multilevel converters in HVDC applications by optimisation methods2012In: IET Conference Publications: 10th IET International Conference on AC and DC Power Transmission (ACDC 2012) Volume 2012, Issue 610 CP, 2012, 2012Conference paper (Refereed)
    Abstract [en]

    Modular multilevel converters, based on cascading of halfbridge converter cells, can combine low switching frequency with low harmonic interference. They can be designed for high operating voltages without direct series connection of semiconductor elements. This has led to a rapid adoption within high-power applications such as high voltage direct current transmission. However, the overall hardware expenditure is generally greater than for conventional converters. Improving the operating range in the PQ-plane of the converter is thus desirable. In this work it is shown that optimisation of the internal variables of the converter can increase the power handling capability significantly for a given set of rating constraints. In this process optimal values for the circulating currents, the zero-sequence ac-side voltages and the average stored energy are sought. Also, a control algorithm is demonstrated by which the converter can be operated in the optimised operating point. Simulation results clearly indicate the feasibility of the proposed methodology.

  • 37.
    Norrga, Staffan
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    ABB AB.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Decoupled steady-state model of the modular multilevel converter with half-bridge cells2012Conference paper (Refereed)
    Abstract [en]

    Modular multilevel converters, based on cascading of halfbridge converter cells, can combine low switching frequency with low harmonic interference. They can be designed for high operating voltages without direct series connection of semiconductor elements. This has led to a rapid adoption within high-power applications such as HVDC, STATCOM and railway interties. Analysing the operation of these converters in the frequency domain poses a few challenges due to the presence of significant low-order harmonic voltages in the cell capacitors. This paper presents a frequency-domain model of the MMC converter with halfbridge cells, based on a two-stage approach. First, the circuit equations are decoupled by a simple linear transformation, whereby the circuit schematic can be separated into a dc-side and an ac-side part. Second, the switching operation within the phase arms is modelled in the frequency domain by iterated convolution. The model is verified against a timedomain simulation of a converter with ratings valid for HVDC applications. It is shown that the proposed methodology, where all calculations are made in the frequency domain, can accurately reproduce the results from the simulation.

  • 38.
    Norrga, Staffan
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Harnefors, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Frequency-Domain Modeling of Modular Multilevel Converters With Application to Maximizing the Operating Region2012In: IECON 2012 - 38th Annual Conference on IEEE Industrial Electronics Society, IEEE , 2012, p. 4967-4972Conference paper (Refereed)
    Abstract [en]

    Modular multilevel converters (MMC), based on cascading of half-bridge converter cells, can combine low switching frequency with low harmonic interference. They can be designed for high operating voltages without direct series connection of semiconductor elements. This has led to a rapid adoption within high-power applications such as high voltage direct current transmission, railway interties and medium voltage industrial motor drives. Analyzing the operation of these converters in the frequency domain poses a few challenges due to the presence of significant low-order harmonic voltages in the cell capacitors. This paper treats a frequency-domain methodology for computing inner variables of the MMC with half-bridge cells, based on a two-stage approach. First, the circuit equations are decoupled by a simple linear transformation, whereby the circuit schematic can be separated into a dc-side and an ac-side part. Second, the variables of the cell strings are computed in the frequency domain by iterated convolution. It is shown that the proposed methodology, where all calculations are made in the frequency domain, can accurately reproduce the results from a PSCAD simulation. Furthermore, the model is successfully employed as part of an optimization algorithm for maximizing the power handling capability of the converter by appropriately controlling the circulating current and zero-sequence ac-side voltage. Results from this work point to significant possibilities for improvement.

  • 39.
    Ängquist, Lennart
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Siemaszko, Daniel
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ilves, Kalle
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Vasiladiotis, Michail
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Open-Loop Control of Modular Multilevel Converters Using Estimation of Stored Energy2011In: IEEE transactions on industry applications, ISSN 0093-9994, E-ISSN 1939-9367, Vol. 47, no 6, p. 2516-2524Article in journal (Refereed)
    Abstract [en]

    The internal control of a modular multilevel converter aims to equalize and stabilize the submodule capacitor voltages independent of the loading conditions. It has been shown that a submodule selection mechanism, included in the modulator, can provide voltage sharing inside the converter arm. Several procedures for controlling the total stored energy in each converter arm exist. A new approach is described in this paper. It is based on estimation of the stored energy in the arms by combining the converter electromotive force reference, the measured alternating output current, and the known direct voltage. No feedback controllers are used. Experimental verification on a three-phase 10 kVA prototype is presented along with the description of the new procedure.

1 - 39 of 39
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Output format
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  • asciidoc
  • rtf