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  • 1.
    Ahmed, Noman
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
    Haider, Arif
    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).
    M2C-BASED MTDC SYSTEM FOR HANDLING OF POWERFLUCTUATIONS FROM OFFSHORE WIND FARMS2011In: Proceedings of IET RPG-2011 / [ed] IET, 2011Conference paper (Refereed)
    Abstract [en]

    In this paper a modular multilevel converter (M2C) basedmulti-terminal direct current (MTDC) system is proposed forthe connection of offshore wind farms. Each M2C ismodelled with 36 sub-modules per arm with a total of 216sub-modules consisting of half bridges. An open-loopconverter control method is employed for the M2Cs. Powersynchronizationcontrol is used instead of a phase-locked loop(PLL) for synchronization. A voltage controller isimplemented with power-synchronization control as an innerloop. By means of numerical simulations in PSCAD, it isshown that the system is self stabilizing both at steady stateand following power fluctuations of the wind farm.

  • 2.
    Ahmed, Noman
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion. ABB Corporate Research.
    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.
    Performance of the modular multilevel converter with redundant submodules2015In: IECON 2015 - 41st Annual Conference of the IEEE Industrial Electronics Society, Institute of Electrical and Electronics Engineers (IEEE), 2015, p. 3922-3927, article id 7392712Conference paper (Refereed)
    Abstract [en]

    The modular multilevel converter (MMC) is the state-of-the-art voltage-source converter (VSC) topology used for various power-conversion applications. In the MMC, submodule failures can occur due to various reasons. Therefore, additional submodules called the redundant submodules are included in the arms of the MMC to fulfill the fault-safe operation requirement. The performance of the MMC with redundant submodules has not been widely covered in the published literature. This paper investigates the performance of the MMC with redundant submodules in the arms. Two different control strategies are used and compared for integrating redundant submodules. The response of the MMC to a submodule failure for the two strategies is also studied. Moreover, the operation of the MMC with redundant submodules is validated experimentally using the converter prototype in the laboratory.

  • 3.
    Ahmed, Noman
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Mahmood, Shahid
    Antonopoulos, Antonios
    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.
    Efficient Modeling of an MMC-Based Multiterminal DC System Employing Hybrid HVDC Breakers2015In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 30, no 4, p. 1792-1801Article in journal (Refereed)
    Abstract [en]

    The feasibility of future multiterminal dc (MTDC) systems depends largely on the capability to withstand dc-side faults. Simulation models of MTDC systems play a very important role in investigating these faults. For such studies, the test system needs to be accurate and computationally efficient. This paper proposes a detailed equivalent model of the modular multilevel converter (MMC), which is used to develop the MTDC test system. The proposed model is capable of representing the blocked-mode operation of the MMC, and can be used to study the balancing control of the capacitor voltages. In addition, the operation of the MMC when redundant submodules are included in the arms can also be studied. A simplified model of a hybrid high-voltage dc breaker is also developed. Hence, the developed test system is capable of accurately describing the behavior of the MMC-based MTDC system employing hybrid HVDC breakers, during fault conditions. Using time-domain simulations, permanent dc-side faults are studied in the MTDC system. In addition, a scheme to control the fault current through the MMC using thyristors on the ac side of the converter is proposed.

  • 4.
    Ahmed, Noman
    et al.
    KTH.
    Ängquist, Lennart
    KTH.
    Mehmood, Shahid
    Antonopoulos, Antonios
    Harnefors, Lennart
    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.
    Efficient Modeling of an MMC-Based Multiterminal DC System Employing Hybrid HVDC Breakers2016In: 2016 IEEE POWER AND ENERGY SOCIETY GENERAL MEETING (PESGM), IEEE , 2016Conference paper (Refereed)
  • 5.
    Ahmed, Noman
    et al.
    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.
    Continuous modeling of open-loop control based negative sequence current control of modular multilevel converters for HVDC transmission2013In: Eur. Conf. Power Electron. Appl., EPE, 2013Conference paper (Refereed)
    Abstract [en]

    Negative sequence currents are obtained during ac-side asymmetrical faults of converters in highvoltage direct current (HVDC) transmission systems. Consequently, second order harmonics in the dc-side voltage and current, unbalanced ac-side currents, and power oscillations can be observed. This paper presents a negative sequence current control (NSCC) scheme that eliminates second order harmonic ripples in the voltage and current of the dc-side during unbalanced grid conditions. Controllers for this purpose are investigated using a continuous model of the modular multilevel converter (M2C). The proposed scheme utilizes an open-loop controller for lower level control of the M2C. The continuous model used also has the capability to model blocking and deblocking events which may be used during protective actions. Simulation results reveal that the proposed NSCC scheme is effective in suppressing dc-side voltage and current ripples. Moreover, it keeps the ac-side phase currents balanced during asymmetrical fault conditions.

  • 6.
    Ahmed, Noman
    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.
    Antonopoulos, Antonios
    ABB Corporate Research Center, Sweden.
    Harnefors, Lennart
    ABB Corporate Research Center, Sweden.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    A computationally efficient continuous model for the modular multilevel converter2014In: IEEE Journal of Emerging and Selected Topics in Power Electronics, ISSN 2168-6777, Vol. 2, no 4, p. 1139-1148, article id 6840290Article in journal (Refereed)
    Abstract [en]

    Simulation models of the modular multilevel converter (MMC) play a very important role for studying the dynamic performance. Detailed modeling of the MMC in electromagnetic transient simulation programs is cumbersome, as it requires high computational effort and simulation time. Several averaged or continuous models proposed in the literature lack the capability to describe the blocked state. This paper presents a continuous model, which is capable of accurately simulating the blocked state. This feature is very important for accurate simulation of faults. The model is generally applicable, although it is particularly useful in high-voltage dc applications.

  • 7.
    Ahmed, Noman
    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.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Efficient Modeling of Modular Multilevel Converters in HVDC-Grids Under Fault Conditions2014In: 2014 IEEE PES General Meeting | Conference & Exposition, IEEE Computer Society, 2014, p. 6939166-, article id 6939166Conference paper (Refereed)
    Abstract [en]

    High-voltage direct current (HVDC) grids using modular multilevel converters (M2Cs) have strongly been considered for the integration of distant renewable energy sources and also as a backbone to the existing ac-grids. The dynamic performance of the M2C is of particular interest in these grids. For electromagnetic transient (EMT) programs, modeling of HVDC-grids using detailed M2C models is unrealistic, as it requires extremely high computational effort and simulation time. In this paper an HVDC-grid test system is developed using a continuous simulation model of the M2C. The model is also capable of describing the blocking events of the M2C. Using time-domain simulations in PSCAD/EMTDC, the dynamic performance of the M2C in HVDC-grids under fault conditions is investigated. Simulation results reveal that the continuous M2C model can efficiently be used to study the dynamic performance of the M2C in HVDC-grids with high computational speed, under different fault conditions.

  • 8.
    Ahmed, Noman
    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.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Validation of the continuous model of the modular multilevel converter with blocking/deblocking capability2012In: AC and DC Power Transmission (ACDC 2012), 10th IET International Conference on, 2012Conference paper (Refereed)
    Abstract [en]

    This paper presents the continuous model for the Modular Multilevel Converter (M2C). The model operates in two modes, either operating as a voltage source in deblocked mode or as a rectifying diode bridge in blocked mode. The model is validated by comparison with a detailed M2C model having 36 submodules per arm, using different control strategies. The comparison is based on time-domain simulations in PSCAD/EMTDC. The continuous model shows a very good agreement with the detailed model.

  • 9.
    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.

  • 10.
    Antonopoulos, Antonios
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Mörée, Gustav
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Soulard, Juliette
    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.
    Experimental evaluation of the impact of harmonics on induction motors fed by modular multilevel converters2014In: Proceedings - 2014 International Conference on Electrical Machines, ICEM 2014, 2014, p. 768-775Conference paper (Refereed)
    Abstract [en]

    Inverter-based electrical-machine drives suffer from significantly higher losses compared to sinusoidal-supply-based alternatives, fed directly from the grid. Using multilevel inverters it becomes possible to partially mitigate the effects of the switched supply waveform, while keeping the advantages of variable-speed operation. This paper aims to evaluate the increase of the losses occurring in an induction motor (IM) fed by a modular multilevel converter (M2C), when compared to grid-connected operation, in order to evaluate the impact of the inverter-generated harmonics in the machine. It is confirmed that the losses created in the motor due to the harmonic content of the inverter-generated waveforms are very low, and almost equivalent to a purely sinusoidal supply. The investigation includes an analysis of the harmonic content from experimental waveforms obtained by an 11-kW IM laboratory setup, and it is further supported by measurements of the temperature rise in the IM-stator windings. It is concluded that the M2C could create the conditions even for high-power motors to be operated without any derating.

  • 11.
    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.

     

     

  • 12.
    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.

  • 13.
    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.

  • 14.
    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.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Optimal selection of the average capacitor voltage for modular multilevel converters2013In: 2013 IEEE Energy Conversion Congress and Exposition, ECCE 2013, IEEE , 2013, p. 3368-3374Conference paper (Refereed)
    Abstract [en]

    Variable-speed drives have reduced voltage requirements when operating below the base speed. In a modular-multilevel-converter-based (M2C-based) motor drive it is then possible to operate with reduced voltage in the submodule capacitors, than at the base speed. In this sense, a greater capacitor-voltage ripple can be accommodated, without exceeding the maximum peak-capacitor voltage. This paper presents an analytical investigation for the optimal selection of the average capacitor voltage for M2Cs, when the motor is operating with rated torque, below the base speed. This method does not require any power exchange between the converter arms, so it keeps the conduction losses at the minimum level. Additionally, the method decreases the switching losses, due to the decreased capacitor-voltage level. The overall ratings of the converter remain the same as in the base-speed operation. It is shown that this method can be applied at a speed range between the base speed and down to approximately one third of it, i.e, an operating range that covers the requirements for typical pump- and fan-type applications. The results obtained from the analytical investigation are experimentally verified on a down-scaled laboratory prototype M2C.

  • 15.
    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.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Optimal Selection of the Average Capacitor Voltage for Variable-Speed Drives With Modular Multilevel Converters2015In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, no 1, p. 227-234Article in journal (Refereed)
    Abstract [en]

    Variable-speed drives have reduced voltage requirementswhen operating below the base speed. In a modularmultilevel-converter-based (M2C-based) motor drive it is thenpossible to operate with reduced voltage in the submodulecapacitors, than at the base speed. In this sense, a greatercapacitor-voltage ripple can be accommodated, without exceedingthe maximum peak-capacitor voltage. This paper presents ananalytical investigation for the optimal selection of the averagecapacitor voltage for M2Cs, when the motor is operating withrated torque, below the base speed. This method does not requireany power exchange between the converter arms, so it keepsthe conduction losses at the minimum level. Additionally, themethod decreases the switching losses, due to the decreasedcapacitor-voltage level. The overall ratings of the converterremain the same as in the base-speed operation. It is shownthat this method can be applied at a speed range betweenthe base speed and down to approximately one third of it,i.e, an operating range that covers the requirements for typicalpump- and fan-type applications. The results obtained from theanalytical investigation are experimentally verified on a downscaledlaboratory prototype M2C.

  • 16.
    Antonopoulos, Antonios
    et al.
    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).
    On Dynamics and Voltage Control of the Modular Multilevel Converter2009In: 2009 13th European Conference on Power Electronics and Applications, EPE '09, IEEE , 2009, p. 3353-3362Conference paper (Refereed)
    Abstract [en]

    This paper discusses the impact of modulation on stability issues of the Modular Multilevel Converter (M2C). The main idea is to describe the operation of this converter system mathematically, and suggest a control method that offers stable operation in the whole operation range. A possible approach is to assume a continuous model, where all the modules in each arm are represented by variable voltage sources. and as a result, all pulse width modulation effects are disregarded. After simulating this model and testing different control methods, useful conclusions on the operation of the M2C have been extracted. The control methods are then implemented on a model with discrete half-bridge modules, in order to compare the results and to validate continuous model approach. When assuring that this model functions as expected, the goal of this paper is to conclude into a self-stabilizing voltage controller. A controller is proposed, which eliminates circulating currents between the phase legs and balances the arm voltages regardless of the imposed alteranting current.

  • 17.
    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.

  • 18.
    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.

  • 19.
    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)
  • 20. Bongiorno, Massimo
    et al.
    Svensson, Jan
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    On control of static synchronous series compensator for SSR mitigation2008In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 23, no 2, p. 735-743Article in journal (Refereed)
    Abstract [en]

    This paper deals with the analysis and simulation of the static series compensator (SSSC) for subsynchronous resonance (SSR) mitigation. The purpose of the paper is to derive and analyze a novel control strategy for SSSC dedicated for SSR mitigation. Objective of the proposed controller is to increase the network damping only at those frequencies that are critical for the turbine-generator shaft. By using frequency scanning analysis, the effectiveness of the proposed method for mitigation of SSR due to torsional interaction effect is presented and compared with the existing control strategy. Finally, simulation results show the performance of the proposed method in mitigating SSR due to torque amplification effect.

  • 21. Bongiorno, Massimo
    et al.
    Svensson, Jan
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Online estimation of subsynchronous voltage components in power systems2008In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 23, no 1, p. 410-418Article in journal (Refereed)
    Abstract [en]

    In this paper, the problem of online estimation of subsynchronous frequency components in the measured grid voltage is treated. Two estimation methods, one based on the use of lowpass filters and a recursive least square algorithm, are investigated and compared. In particular, due to its higher degree of freedom in the design, the lowpass filters-based estimation method is found to be the most appropriate and accordingly further analyzed. The method is improved to cope with inaccurate knowledge of the subsynchronous frequency. The simulation results prove the effectiveness of the investigated method both for the ideal and the disturbed case.

  • 22. Bongiorno, Massimo
    et al.
    Svensson, Jan
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Single-phase VSC based SSSC for subsynchronous resonance damping2008In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 23, no 3, p. 1544-1552Article in journal (Refereed)
    Abstract [en]

    In this paper, a novel control strategy for subsynchronous resonance (SSR) mitigation using a static synchronous series compensator (SSSC) will be presented. The SSSC is constituted by three single-phase voltage source converters. SSR mitigation is obtained by increasing the network damping only at those frequencies that are critical for the turbine-generator shaft. This is achieved by controlling the subsynchronous component of the grid current to zero. Using the IEEE First Benchmark Model, the effectiveness of the proposed control algorithm when mitigating SSR due to torsional interaction and torque amplification effect will be shown.

  • 23. Bongiorno, Massimo
    et al.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Svensson, Jan
    A novel control strategy for subsynchronous resonance mitigation using SSSC2008In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 23, no 2, p. 1033-1041Article in journal (Refereed)
    Abstract [en]

    In this paper, a novel control strategy for subsynchronous resonance (SSR) mitigation using a static synchronous series compensator will be presented. SSR mitigation is obtained by increasing the network damping only at those frequencies that are critical for the turbine-generator shaft. This is achieved by controlling the subsynchronous component of the grid current to zero. Using the IEEE First Benchmark Model, the effectiveness of the proposed control algorithm when mitigating SSR due to torsional interaction and torque amplification effect will be shown.

  • 24. de Toledo, Paulo Fischer
    et al.
    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).
    Frequency domain model of an HVDC link with a line-commutated current-source converter. Part I: fixed overlap2009In: IET Generation, Transmission & Distribution, ISSN 1751-8687, E-ISSN 1751-8695, Vol. 3, no 8, p. 757-770Article in journal (Refereed)
    Abstract [en]

    This study presents a frequency-domain model of an high voltage direct current (HVDC) transmission link with a line-commutated current-source converter. Using space-vector transfer functions between superimposed oscillations in the control signal and the AC and DC sides, expressions for voltages and currents have been derived. The dynamic properties of the HVDC link, taking the characteristics of the networks on both the AC and the DC sides into consideration, can be studied by applying classical Bode/Nyquist/Nichols control methods. The resulting model was validated by time-domain studies in PSCAD/EMTDC. The model is described in two papers. In this paper (Part I), it has been assumed that the overlap angle during commutation remains constant. It is shown in the validation that this assumption introduces resonances that cause severe errors at certain network conditions. In the second paper (Part II), the model is extended so as to cope with the varying overlap angle in order to bring the frequency-domain model into agreement with the results obtained from time-domain simulations.

  • 25.
    de Toledo, Paulo Fischer
    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.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Frequency domain model of an HVDC link with a line-commutated current-source converter. Part II: varying overlap2009Article in journal (Refereed)
    Abstract [en]

    The second part of the description of a frequency-domain model of a high-voltage direct current (HVDC) transmission link with line-commutated current-source converters is presented in this study. The model is based on transfer functions between superimposed oscillations in the control signal and the AC- and DC-side voltages and currents. In 'Part I' of the description the overlap angle was assumed to be constant. At certain resonance frequencies severe deviations were obtained between the calculated transfer functions and the ones extracted from time-domain simulations. In this study (Part II), the model has been extended to deal with varying overlap angle. It is shown that the agreement between transfer functions calculated in frequency- and time-domain has improved substantially.

  • 26.
    Fischer de Toledo, Paulo
    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.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Frequency-domain modelling of sub-synchronous torsional interaction of synchronous machines and a high voltage direct current transmission link with line-commutated converters2010In: IET GENERATION TRANSMISSION & DISTRIBUTION, ISSN 1751-8687, Vol. 4, no 3, p. 418-431Article in journal (Refereed)
    Abstract [en]

    The authors describe a model of a system that includes a high voltage direct current (HVDC) transmission link with line-commutated current source converters (LCC) closely connected to a synchronous generator used to perform analysis of sub-synchronous torsional interaction (SSTI) in the frequency domain. The model of the HVDC transmission link adequately represents the converters in the frequency domain and includes all essential controls for the operation of the converters, including a new sub-synchronous damping control used to mitigate interaction between the recti. er LCC and the shaft of the synchronous machine. The frequency domain model has been validated against time domain simulations showing good agreement.

  • 27. Fischer, P.
    et al.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    A new control scheme for an HVDC transmission link with capacitor-commutated converters having the inverter operating with constant alternating voltage2012In: 44th International Conference on Large High Voltage Electric Systems 2012, 2012, p. 1-11Conference paper (Refereed)
    Abstract [en]

    Experience and theoretical analysis show that the voltage and power stability becomes an important issue when high-voltage direct current (HVDC) systems using line-commutated converters (LCC) are connected to AC systems with low short-circuit capacity and that these problems become more pronounced the lower the short-circuit capacity of the connected AC system is, as compared with the rating of the HVDC converter station. Normally some additional equipment, like static or synchronous compensators, will be installed in order to improve the performance of the HVDC transmission system, when it is connected to a weak AC system. The cost of the substation thereby will be increased. The Capacitor-Commutated Converter (CCC), which has been developed by ABB, offers an alternative to the LCC in this kind of application. It reduces the reactive power interaction between the converter and the connected AC system. The CCC is, in principle, a classical converter provided with a series capacitor placed between the converter valves and the converter transformer. In this paper a new control strategy that can be used together with such a CCC is presented. In the new control system the inverter emulates the operation of a Voltage Source Converter, in the sense that, beside the control of the active power flow through the converter, the reactive power exchange with the AC network can also be managed in order to adjust the alternating voltage of the converter bus. Studies have shown that the interaction between the inverter and the connected AC system is significantly reduced, when the CCC is used together with this new control strategy. Keeping the alternating voltage constant makes it possible to control the DC-side voltage of the HVDC transmission system, allowing the rectifier converter to control the active power by means of controlling the direct current. This combination allows stable operation of the HVDC transmission system even under severe network conditions associated with low short-circuit power in the connected AC network at the inverter side. In the paper it is shown that, with the new control scheme, it is possible to operate the inverter into an almost passive AC network (a network with very few rotating generators, resulting in almost no short circuit power). Simulation results have indicated satisfactory operation of the HVDC transmission with the inverter operating into a network having a Short-Circuit Ratio as low as 0.2 (SCR ≈ 0.2). To the knowledge of the authors, operation of a line-commutated converter at such low SCR values has never been reported previously. A description of the new control scheme will be presented. The calculated performance in a practically implemented installation, the Rio Madeira Back-to-Back system, will be illustrated.

  • 28. Gregoire, Luc-Andre
    et al.
    Blanchette, Handy Fortin
    Li, Wei
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Al-Haddad, Kamal
    Modular Multilevel Converters Overvoltage Diagnosis and Remedial Strategy During Blocking Sequences2015In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, no 5, p. 2777-2785Article in journal (Refereed)
    Abstract [en]

    In this paper, the authors first highlight an existing overvoltage phenomenon that is inherent to the modular multilevel converter (MMC) topology. The latter occurs during the blocking sequences of semiconductor devices if the converter needs to be stopped due to circulating current, loss of control, or unexpected faults. An analysis based on time-domain expressions describing each operating sequence during normal and faulty blocking conditions is used to demonstrate the origin of this overvoltage. Thereafter, system behaviour is obtained when devices gating signals are withheld as well as the exact overvoltage cause. Real-time simulation, with submicrosecond time steps, and experimental results validate the overvoltage phenomena and the proposed remedial strategy to avoid uncontrolled faulty conditions.

  • 29.
    Haider, Arif
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
    Ahmed, Noman
    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).
    Open-loop approach for control of multi-terminal DC systems based on modular multilevel converters2011In: Proceedings of the 2011-14th European Conference on Power Electronics and Applications (EPE 2011) / [ed] EPE Association, 2011Conference paper (Refereed)
    Abstract [en]

    In this paper a multi-terminal direct current (MTDC) system with modular multilevel converters (M2Cs) is suggested. An open loop control method is used for the control of the converters. Each converter is modeled with 36 sub-modules per arm with a total of 216 sub-modules consisting of half bridges. Power-synchronization control is used instead of a phase-locked loop (PLL) for synchronization. Thus, the short circuit capacities of the ac systems are no longer limiting factors and the instability caused by the PLL in weak ac systems is avoided [10]. A direct voltage controller is implemented with power-synchronization control as an inner loop in one station. Several scenarios are analyzed to demonstrate control flexibility and ride-through capability for grid transients. By means of analytical calculations and time simulations in PSCAD/EMTDC, the validity of the proposed MTDC system is confirmed.

  • 30.
    Harnefors, Lennart
    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.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Dynamic Analysis of Modular Multilevel Converters2013In: IEEE transactions on industrial electronics (1982. Print), ISSN 0278-0046, E-ISSN 1557-9948, Vol. 60, no 7, p. 2526-2537Article in journal (Refereed)
    Abstract [en]

    Theory for the dynamics of modular multilevel converters is developed in this paper. It is shown that the sum capacitor voltage in each arm often can be considered instead of the individual capacitor voltages, thereby significantly reducing the complexity of the system model. Two selections of the so-called insertion indices, which both compensate for the sum-capacitor-voltage ripples, are considered. The dynamic systems which respectively result from these selections are analyzed. An effective dc-bus model, which takes into account the contribution from the submodule capacitors, is obtained. Finally, explicit formulas for the stationary sum-capacitor-voltage ripples are derived.

  • 31.
    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.

  • 32.
    Hassanpoor, Arman
    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), Electric Power Systems.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Evaluation of different carrier-based PWM methods for modular multilevel converters for HVDC application2012In: IECON 2012 - 38th Annual Conference on IEEE Industrial Electronics Society, IEEE , 2012, p. 388-393Conference paper (Refereed)
    Abstract [en]

    The outstanding features of modular multilevel converters (M2C) make it attractive for high voltage direct current (HVDC) systems. In order to achieve high efficiency in HVDC converter stations, the switching frequency and the capacitor voltage ripple of the converter should be minimized. A suitable modulation algorithm should achieve an optimal tradeoff between these two requirements. This paper evaluates different carrier-based PWM algorithms and discusses the most challenging technical aspects of an efficient M2C. It is observed that decoupling the waveform synthesis from the selection of which cell to switch at each instant has beneficial impact on operation performance. The evaluation is done by time-domain simulation considering a grid connected, three-phase M2C converter and an advanced control system. Results of this study can be used for implementing more economical HVDC converters.

  • 33.
    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.

  • 34.
    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.

  • 35.
    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%.

  • 36.
    Johansson, Nicklas
    et al.
    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.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    An adaptive model predictive approach to power oscillation damping utilizing variable series reactance FACTS devices2006In: PROCEEDINGS OF THE 41ST INTERNATIONAL UNIVERSITIES POWER ENGINEERING CONFERENCE, 2006, p. 790-794Conference paper (Refereed)
    Abstract [en]

    This paper describes an adaptive method of controlling FACTS devices for power oscillation damping. The method is based on step-wise series reactance modulation. Here, a reduced model of the power system with only two rotating masses is used as a basis for the control design. The model parameters are updated using local measurements of the active power on the controlled line. An adaptive closed loop controller is developed based on the principle that it is possible to stabilize an oscillation in a power system which is characterized by one major mode of oscillation by switching a reactance in series with one transmission line in a small number of steps. The reduced model parameters are recomputed when new information of the system response is known making the control scheme an adaptive one. The paper also includes the derivation of a damping controller with a power flow control feature and a verification of the controllers using digital simulations of power system models of different complexities.

  • 37.
    Johansson, Nicklas
    et al.
    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.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Discrete open loop control for power oscillation damping utilizing variable series reactance FACTS devices2006In: PROCEEDINGS OF THE 41ST INTERNATIONAL UNIVERSITIES POWER ENGINEERING CONFERENCE, 2006, p. 785-789Conference paper (Refereed)
    Abstract [en]

    This paper describes an open loop method of controlling FACTS devices for power oscillation damping. The method is based on step-wise series reactance modulation. The principle of the method is to stabilize an oscillation in a power system which is characterized by one major mode of oscillation by switching a reactance in series with one transmission line, thereby changing the total reactance between the areas participating in the oscillation. In order to stabilize the system during an oscillation, the stationary voltage angle difference between the areas is changed to coincide with the present angle at a point where the speed of the lumped machine representations of the areas is nominal. This is the case at the peaks of the oscillation. To determine the required size of the reactance step, a reduced model of the power system is used. The model parameters are continuously updated using local measurements of the active power on the reactance controlled line. Several approaches for damping with different numbers of steps are presented and verified using digital simulations of power system models.

  • 38.
    Johansson, Nicklas
    et al.
    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.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Estimation of grid parameters for the control of variable series reactance FACTS devices2006In: 2006 POWER ENGINEERING SOCIETY GENERAL MEETING, VOLS 1-9, 2006, p. 105-111Conference paper (Refereed)
    Abstract [en]

    For high performance control of Flexible AC Transmission System (FACTS) devices with controllable reactances, a representation of the surrounding grid is essential. Using such a model, an adaptive control strategy can be developed which optimizes the control in real time as the grid parameters change. This paper proposes such a generic grid model and derives the theory of how to estimate the main parameters using measurements of the line active power response from small step reactance changes. The estimation methods are verified using simple grid models in PSCAD simulations and more advanced grid models using SIMPOW simulations of a modified version of the CIGRE Nordic 32 grid. This work should be thought of as a foundation for developing control systems for variable series reactance FACTS devices.

  • 39.
    Johansson, Nicklas
    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.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    A Comparison of Different Frequency Scanning Methods for Study of Subsynchronous Resonance2011In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 26, no 1, p. 356-363Article in journal (Refereed)
    Abstract [en]

    This paper compares four different methods for determining the electrical damping of a power system seen from one generator as a function of frequency. This information is useful when the risk for subsynchronous resonance (SSR) in the system is evaluated. The study compares one frequency scanning method which is implemented in a time-domain digital simulation program with three methods of different complexity based on analytical calculations. The time-domain simulation method is easily implemented with a detailed model of the power system including complex load and generator models, whereas the analytical methods are based on simpler models of the power system. The computational effort is much larger for the time-domain method than for the analytical methods. In the study, all methods were used to determine the damping characteristics of a four-machine power system in different configurations. The study shows that fast analytical methods may provide results which closely agree with the detailed method of time-domain simulation. However, the study also shows that the level of accuracy in the analytical model is very important.

  • 40.
    Johansson, Nicklas
    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.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Adaptive control of controlled series compensators for power system stability improvement2007In: 2007 IEEE LAUSANNE POWERTECH, 2007, p. 355-360Conference paper (Refereed)
    Abstract [en]

    This paper describes the design and verification of a time-discrete adaptive controller for damping of inter-area power oscillations, power flow control, and transient stability improvement Only locally measured signals are used as inputs to the controller. The controller may be used with any FACTS device which operates as a variable series reactance in the power grid, such as for example the TCSC. The controller is based on a reduced system model which relies on the assumption of one dominating inter-area oscillation mode in the power system where the FACTS device is placed. Verification of the controller is performed by means of digital simulations of a four-machine system commonly used to study inter-area oscillations.

  • 41.
    Johansson, Nicklas
    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.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    An Adaptive Controller for Power System Stability Improvement and Power Flow Control by Means of a Thyristor Switched Series Capacitor (TSSC)2010In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 25, no 1, p. 381-391Article in journal (Refereed)
    Abstract [en]

    In this paper, a controller for a thyristor switched series capacitor (TSSC) is presented. The controller aims to stabilize the power system by damping interarea power oscillations and by improving the transient stability of the system. In addition to this, a power flow control feature is included in the controller. The power oscillation damping controller is designed based on a nonlinear control law, while the transient stability improvement feature works in open loop. The damping controller is adaptive and estimates the power system parameters according to a simplified generic model of a two-area power system. It is designed for systems where one poorly damped dominant mode of power oscillation exists. In the paper, a verification of the controller by means of digital simulations of one two-area, four-machine power system, and one 23-machine power system is presented. The results show that the controller improves the stability of both test systems significantly in a number of fault cases at different levels of interarea power flow.

  • 42.
    Johansson, Nicklas
    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.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    An Adaptive Controller for Power System Stability Improvement and Power Flow Control by Means of a Thyristor Switched Series Capacitor (TSSC) (vol 25, pg 381, 2010)2010In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 25, no 2, p. 1200-1200Article in journal (Refereed)
  • 43.
    Johansson, Nicklas
    et al.
    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).
    Aspects on Power Flow Control by means of a Dynamic Power Flow ControllerArticle in journal (Other academic)
    Abstract [en]

    This paper, which is focused on steady-state power flow control, investigates the capability of three different Power Flow Controllers (PFC) to control power flows in two test systems. The studied devices are the Dynamic Power Flow Controller (DPFC), the Phase Shifting Transformer (PST) and the Thyristor Switched Series Capacitor (TSSC). The DPFC is actually a combination of the PST and the TSSC and the study thus attempts to solve the power flow control problems either by a PST, a TSSC or a combination of these devices. The different PFC:s are designed using the Power controller plane where it is possible to decouple the surrounding grid characteristics from the PFC characteristics in a convenient way. It is found that the installation of a PFC can improve the power transfer capacity significantly in the studied test systems. In some of the studied cases, it is found that the power flow control problem can only be solved by a PST or a DPFC. One main conclusion of the study is that a DPFC may be an attractive alternative to a traditional PST installation in some cases due to a lower required rating, a better voltage control capability and a higher speed of control.

  • 44. Johansson, Nicklas
    et al.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Preliminary Design of Power Controller Devices Using the Power-Flow Control and the Ideal Phase-Shifter Methods2012In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 27, no 3, p. 1268-1275Article in journal (Refereed)
    Abstract [en]

    This paper introduces a new method for the preliminary design of power controllers (PCs) in the electric power grid. The method, which is denoted the ideal phase-shifter (IPS) method, utilizes the concept of the power controller plane where the active power of the PC line is plotted versus the difference in voltage angle between the PC terminals. The power controller plane makes it possible to graphically visualize the working area of a PC in a power grid and thus determine the grid situations which are dimensioning for the PC. The IPS method offers the possibility of plotting the grid characteristics in the power controller plane which are unbiased with respect to the reactive properties of the PC. This makes the method suitable for comparison and preliminary design of PCs of different types and with different characteristics by simple geometrical considerations. In this process, the IPS method uses the power-flow control method for deriving the PC characteristics. This paper includes an application example of the method where it is used for dimensioning of two different PCs in a 26-bus test system.

  • 45.
    Johansson, Nicklas
    et al.
    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).
    Berggren, Bertil
    ABB AB, Corporate research.
    A dynamic power flow controller for power system stability improvement and loss reduction2008In: The 16th Power Systems Computation Conference, Power Systems Computation Conference ( PSCC ) , 2008Conference paper (Refereed)
    Abstract [en]

    In this paper, a novel FACTS device denoted Dynamic Power Flow Controller (DPFC) is described. The device consists of a Phase-Shifting Transformer (PST) connected in series with a set of thyristor-switched capacitive and/or inductive elements. When compared to a normal PST, this device has faster dynamic properties and in addition to the normal PST functions, it also allows for power oscillation damping, transient stability improvement, and voltage stability improvement in a power grid. The DPFC is believed to be less costly than other FACTS devices with the same functionalities. In this paper, the benefits and functions of the DPFC are discussed. Additionally, an adaptive controller for DPFC power oscillation damping, transient stability improvement, and power flow control is presented and verified by means of digital simulations.

  • 46.
    Johansson, Niklas
    et al.
    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).
    Designing power flow control devices using the power controller plane: An improved methodArticle in journal (Other academic)
    Abstract [en]

    This paper introduces an improved method for design of Power Flow Controllers (PFC) in the power controller plane where the active power of the PFC line is plotted versus the difference in voltage angle between the PFC terminals. The power controller plane makes it possible to graphically visualize the working area of a PFC in a power grid and thus determine the grid situations which are dimensioning for the PFC. The paper extends the existing methods for design of PFC:s in the power controller plane by introducing a method to decouple the power grid- and PFC characteristics in a power system of arbitrary size. With this approach, it is possible to graphically compare and design PFC:s of different types and with different characteristics in the same plot by simple geometrical considerations. The method is intended to provide additional insight into how the power flow control capabilities of different PFC:s depend on the selected PFC characteristics and the grid conditions. Additionally, the method has the potential to simplify the process for selection and design of a PFC. The paper includes a study of the accuracy of the proposed method in two test systems of different complexity.

  • 47. Leterme, W.
    et al.
    Ahmed, Noman
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Beerten, J.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Van Hertem, D.
    Norrga, Staffan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    A new HVDC grid test system for HVDC grid dynamics and protection studies in EMT-type software2015In: IET Seminar Digest, Institution of Engineering and Technology, 2015, no CP654Conference paper (Refereed)
    Abstract [en]

    This paper proposes a new HVDC grid test system for electro-magnetic transient analysis, suitable for HVDC power system studies ranging from protection to dynamic studies investigating converter behaviour and interactions. In the recent past research interest in HVDC grids has increased, leading to a multitude of studies concerning dc power flow and optimal power flow, dynamics and HVDC grid protection. However, each of these studies makes use of different grid topologies, configurations and transmission line parameters. In this paper, a standard HVDC grid test system is proposed and an implementation in EMT-type software is provided. The implementation in EMT-type software makes use of a frequency dependent cable model, continuous converter model and a reduced dc breaker model. By means of a protection study, the effectiveness and computational efficiency of the proposed HVDC grid test system is demonstrated. The model with its parameters will be made publicly available.

  • 48.
    Monge, Mauro
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.
    Johansson, Nicklas
    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.
    Adaptive control for power oscillation damping by means of a Thyristor Controlled Series Capacitor (TCSC)2008In: 2008 PROCEEDINGS OF THE 43RD INTERNATIONAL UNIVERSITIES POWER ENGINEERING CONFERENCE, NEW YORK: IEEE , 2008, p. 37-41Conference paper (Refereed)
    Abstract [en]

    This paper describes an adaptive method for control of a TCSC device for power oscillation damping. The aim is to mitigate power oscillations in power systems with one dominating oscillation mode. To design the controller, a simple generic model of the power system with only a few variables is used. Initially, a fixed-parameter controller which is based on a pole-placement technique is designed. The controller is then implemented in a digital simulator and tested in a four-machine power system. The fixed-parameter controller performs well in most cases but in some contingencies, a performance decay is seen. To improve the controller performance, an adaptive control design has been investigated. The investigated controller is a Self-Tuning Regulator and a Recursive Least-Squares method is used to estimate the system model parameters. An open-loop controller for transient stability improvement and a closed-loop PI-controller for power flow control are also presented. The adaptive controller has been validated with good results by means of simulations of a number of contingencies in different operating conditions of the four-machine system.

  • 49.
    Nee, Hans-Peter
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Rabkowski, Jacek
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Peftitsis, Dimosthenis
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Tolstoy, Georg
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Colmenares, Juan
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Sadik, Diane
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Bakowski, Mietek
    Acreo Swedish ICT AB, Sweden.
    Lim, Jang-Kwon
    Acreo AB, Kista.
    Antonopoulos, Antonios
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Zdanowski, Mariusz
    Warsaw University of Technology.
    High-Efficiency Power Conversion Using Silicon Carbide Power Electronics2013In: Proc. of International Conference on silicon carbide and related materials (ICSCRM) 2013, Miyazaki, Japan, Sept. 29–Oct. 4, 2013, Trans Tech Publications Inc., 2013, p. 1083-1088Conference paper (Refereed)
    Abstract [en]

    The message of this paper is that the silicon carbide power transistors of today are good enough to design converters with efficiencies and switching speeds beyond comparison with corresponding technology in silicon. This is the time to act. Only in the highest power range the devices are missing. Another important step towards high powers is to find new solutions for multi-chip circuit designs that are adapted to the high possible switching speeds of unipolar silicon carbide power transistors.

  • 50.
    Norrga, Staffan
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Li, Xiaoqian
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Converter Topologies for HVDC Grids2014In: ENERGYCON 2014 - IEEE International Energy Conference, IEEE Computer Society, 2014, p. 1554-1561Conference paper (Refereed)
    Abstract [en]

    HVDC grids have significant benefits and will likely play an important role in the future energy system. However, protection and fault handling aspects will be different from those of current ac transmission grids and point-to-point HVDC connections. Therefore, new power electronic solutions will be required. Different fault handling strategies are being discussed and the required converter hardware to implement these is identified. Also different converter topologies with the needed properties, mainly dc-side short-circuit handling capability, are described and compared.

12 1 - 50 of 74
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