| EPE 2011 - DS2e: Topic 05: High Power Multilevel Converters and Voltage Regulator Modules |
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 | A Comparison Between Two-commutation-cells Multichannel and Multilevel Buck-Derived Topologies
By Christian ROD, Alfred RUFER | [View]
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Abstract: In the frame of the development of a very high speed synchronous drive, a DC/DC converter is studied.In this context, this paper presents the comparison between two two-commutation-cell topologies.A multichannel and a multilevel converter are formally compared in terms of output current ripple reductionand control complexity. Moreover, partial experimental validations of the theoretical results arepresented.
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| | A Comparison of PWM Techniques for Three-Level Five-Phase Voltage Source Inverters
By Obrad DORDEVIC, Martin JONES, Emil LEVI | [View]
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Abstract: Carrier-based PWM (CBPWM) and space vector PWM (SVPWM) are well-known PWM techniques that have been extensively studied in conjunction with multilevel three-phase inverters. Recently, PWM algorithms for multiphase multilevel inverters have started to appear in literature. One such SVPWM technique, developed for three-level five-phase voltage source inverters (VSIs), is considered in this paper. A slight modification of this method is further elaborated, and this is the second considered SVPWM. Next, two existing three-phase CBPWM techniques are adapted to five-phase systems in a simple manner. A comparative analysis of two CBPWM techniques and two SVPWM techniques in conjunction with three-level five-phase VSI is then performed. The comparison is based on detailed simulation studies and includes output phase voltage waveform and its spectrum, total harmonic distortion (THD) of the phase voltage and harmonic distortion (HD) of the phase voltage components in mutually perpendicular planes, and HD of the common mode voltage (CMV). The main conclusion is that the differences between the four considered PWM schemes are marginal, with one of the CBPWM techniques giving identical behaviour as one of the SVPWM methods.
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| | A new 5-Level Topology allowing the balancing of the DC bus capacitors voltages
By Alexandre LEREDDE, Guillaume GATEAU | [View]
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Abstract: Multilevel structures are becoming more and more common in the industry because they can convert a DC voltage into an AC voltage for high-power and medium voltage condition. However there are limitations in some topologies where the DC Bus is divided by capacitors. With 3 or more DC bus capacitors, it is difficult to ensure the voltages balance across each capacitor. This paper proposes a 5-level structure based on the SMC converter (Stacked Multicell Converter). The study of this structure is focused on the DC-AC conversion and the balance of the capacitors voltages. The way used to accomplish these objectives are based on the transfer of energy from one to another capacitor through an inductive storage element.
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| | A New Capacitor Balancing Technique in Diode-Clamped Multilevel Converters with Active Front End for Extended Operation Range
By Mario MARCHESONI, Paolo FAZIO, Gianmarco MARAGLIANO, Luis VACCARO | [View]
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Abstract: A new PWM technique, capable of achieving a correct dc-link capacitor voltages balance in Diode-Clamped Multilevel inverters with Active Front End (AFE), beyond the limits defined by the theory for converters without AFE, is presented. A detailed explanation of the new devised PWM technique and the main results are reported with analytical computations.
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| | A New Multilevel Converter with Multi-Windings Medium-Frequency Transformer
By Zedong ZHENG, Zhigang GAO, Yongdong LI | [View]
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Abstract: In this paper, a novel multi-level converter was proposed where an isolated DC-DC converter with a multi-winding medium frequency transformer is used to realize the energy equalization within different cells. A cascaded H-bridge rectifier is directly connected to the AC grid and the normal input transformer was not necessary. Several separate DC buses can be produced by the cascaded H-bridge rectifier which can be inverted to medium frequency AC voltages and feed the primary windings of the multi-winding medium frequency transformer. On the secondary windings, several isolated DC buses will be produced from the output by the transformer. In this paper, the stability of the proposed converter and the equalization of the DC bus were analyzed. The control strategy of the whole system was studied and verified by some simulation and experiments.
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| | A Versatile DC-DC Converter for Energy Collection and Distribution using the Modular Multilevel Converter
By Stephan KENZELMANN, Alfred RUFER, Michail VASILADIOTIS, Drazen DUJIC, Francisco CANALES, Yales DE NOVAES | [View]
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Abstract: The drawbacks of conventional Modular Multilevel Converters (MMC) are described in terms of short-circuits. The configuration of MMC-transformer-MMC proves to handle short-circuits at the input and output without the need for additional DC or AC circuit breakers for protection. The transformer can be operated with a higher frequency, reducing therefore the size of the transformer and the capacitors considerably. A multitude of input and output configurations are possible, AC/AC, AC/DC and DC/DC. In particular a DC/DC structure is analysed, serving as DC circuit breaker and voltage elevation interface at the same time. The control strategy of the DC/DC converter is described and an experimental prototype validates the proposed structure and control.
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| | AC power source based on series-connection between cascaded PWM multilevel inverter and linear power amplifier
By Rafael Concatto BELTRAME, Cassiano RECH, Mario Lucio da Silva MARTINS, Helio Leaes HEY | [View]
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Abstract: This paper proposes a hybrid ac power source by associating a symmetrical cascaded PWM multilevel inverter (main amplifier) with a low-voltage and low-power linear power amplifier (correction amplifier) with the aim of synthesizing high-fidelity voltage waveforms without penalizing the system efficiency. The main amplifier operates with phase-shift pulse-width modulation, enabling to reduce the output low-pass filter requirements. An accurate analysis of the main amplifier parameters, such as the number of cells and the output filter devices, which have impact on the current and voltage ripples, phase displacement and under/overshoots caused by load variation, is carried out through the paper, since the correction amplifier must be designed to compensate for any distortion on main amplifier voltage waveform. As a result, the compensation capability of the correction amplifier is defined. Thereafter, a design methodology for both main and correction amplifiers is introduced as well as a control circuit is proposed. Experimental results are presented to validate the proposed approach.
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| | Braking Chopper Solutions for Modular Multilevel Converters
By Silvia SCHOENING, Peter STEIMER, Johann Walter KOLAR | [View]
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Abstract: In many converter applications, the usage of a braking chopper system is required for energy dissipation. Thus, braking chopper solutions need to be found for Modular Multilevel Converters (MMLC). This paper describes braking chopper solutions for the indirect and the direct MMLC. For each converter topology a solution utilizing cells is analyzed. Additionally, taking the direct MMLC as an example, a solution which utilizes thyristors is shown. All chopper solutions have been simulated; this paper shows some of these simulation results.
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| | Circuit for reducing devices voltage stress due to DC-link capacitor voltage ripple in a Neutral-Point-Clamped inverter
By Georgios ORFANOUDAKIS, Suleiman SHARKH, Michael YURATICH | [View]
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Abstract: The paper presents a circuit that reduces the voltage stress caused by DC-link capacitor voltage ripple on the switching devices of a three-level Neutral-Point-Clamped (NPC) inverter. The circuit is capable of halving the amplitude of the voltage ripple seen by the inverter devices. It is proposed as an alternative to over-sizing the inverter’s DC-link capacitors, or using capacitor-balancing PWM strategies that increase the switching frequency and associated losses. The structure and operation of the circuit are described and the ratings of its components are determined. The benefit it offers is compared to that of existing solutions, based on an example NPC inverter design. The results, verified using MATLAB-Simulink, indicate that the circuit can outperform other proposed balancing circuits, halve the DC-link capacitance and cover cases where the switching frequency cannot be increased.
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| | DC-link Voltage Oscillations Reduction During Unbalanced Grid Faults for High Power Wind Turbines
By Hernan MIRANDA, Remus TEODORESCU, Pedro RODRIGUEZ, Lars HELLE | [View]
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Abstract: During unbalanced grid voltage faults the Power injected to the grid experiences 100Hz oscillations as a result of interactions between positive and negative sequence components of three-phase voltages and currents. These oscillations can become as high as \%50 percent of the rated power. In this article an improved controller is proposed which present different behavior during normal operation and faults to keep track of non-sinusoidal current reference signals. The reference signals are calculated to obtain zero power oscillations. Reconfigurable resonant controllers are used for this purpose applied to 3-level Neutral-point-clamped converter in the megawatt range.
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| | Design Optimization of a Five-level Active NPC
By Yugo KASHIHARA, Jun-ichi ITOH | [View]
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Abstract: This paper discusses the optimization designing method for a 5-level ANPC inverter. The point of the mathematical expression is to observe the losses of a converter based on the study of the device condition. Selection of the capacitor is also discussed accordingly based on the design method. The performance of the 5-level ANPC inverter is compared with a conventional 2-level inverter. The validity of the proposed calculation method for the power loss is confirmed with the experimental results. The 5-level ANPC inverter can achieve higher efficiency than the 2-level inverter.
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| | Evaluation and Proposal of MMC-HVDC Control Strategies under Transient and Steady State Conditions
By Gilbert BERGNA, Maialen BOYRA, José VIVAS | [View]
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Abstract: The present article evaluates the performance of different MMC-HVDC converter control strategiesfor both internal and external control, and analyzes the performance and compatibility with the currentvector control method. A full analytical expression of the system dynamics including the gridconnection is presented and two novel inner control methods are exposed.
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| | Fault Detection in multilevel cascaded inverter using Parks Vector approach with balanced battery power usage
By Rui VENTURA, Andre MENDES, Antonio CARDOSO | [View]
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Abstract: Among the multilevel converter topologies the cascaded H-bridge converter (MCHBC) differs from other topologies by having series connections of several power cells, producing a better voltage quality and higher voltage output range, and using the same voltage power supply levels. This characteristic adds several benefits as it allows a modular system, the use of low voltage power supplies, and output voltage waveforms with low level of harmonic distortion. Having more semiconductors than a conventional 2-level voltage source inverter (VSI) the probability of a fault occurrence is bigger, and therefore the importance of having a system that can pinpoint the malfunctioning device is higher. This paper presents an integrated method to control a 3-phase MCHBC capable of distributing the energy uniformly by all batteries and simultaneously diagnose isolated gate bipolar transistors (IGBTs) open-circuit faults in the converter cells, using the phase voltage Park’s Vector components of the star connected MCHBC. Simulation and experimental results of the proposed control are presented for a three-phase induction motor load.
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| | Harmonic Optimization for Multilevel Converters Using Space Vector Fundamental Frequency Modulation
By Gert MEHLMANN, Moritz VON ZIMMERMANN, Bernhard PIEPENBREIER, Gerhard HEROLD | [View]
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Abstract: Multilevel converters feature a staircase output voltage, resulting in a lower number of switching op-erations and better spectral characteristics. Switching every element only once a load cycle (funda-mental frequency switching), losses can be minimized. By calculating optimum switching angles, the emergence of lower harmonics can be avoided. Derived from conventional space vector modulation (SVPWM), a novel method overcoming the complexity of selective harmonic elimination (SHE) is presented. It optimizes the spectrum while being suitable for real-time calculation. A simple and easy to implement routing algorithm to choose appropriate switching states is given, which works for an arbitrary number of levels. Finally, results from both space vector fundamental frequency modulation (SVFFM) and selective harmonic elimination are compared.
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| | Improving the Performance of Modular Multilevel Converter by Reducing the Dead Time Effect
By Zixin LI, Yaohua LI, Ping WANG, Haibin ZHU, Zunfang CHU, Song WANG | [View]
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Abstract: Modular multilevel converter (MMC) is a highly attractive and promising topology especially for high voltage applications. In this paper, the dead time effect introduced by the real power switches on the performance of MMC is analyzed. The results show that the dead time will deteriorate the output voltage and is very harmful to the buffer inductors. To alleviate the effect of the dead time, an improved method for voltage sampling of the submodules is proposed. Simulation and experimental results show substantially reduced switching frequency with the proposed method. Meantime, the narrow pulses in the output voltage and on the buffer inductors together with the harmonics in the output voltage can all be suppressed remarkably.
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| | Multilevel Converters for 10 MW Wind Turbines
By Ke MA, Frede BLAABJERG | [View]
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Abstract: Several promising multi-level converter configurations for 10 MW Wind Turbines both with direct drive and one-stage gear box drive using Permanent Magnet Synchronous Generator (PMSG) are proposed, designed and compared. Reliability is a crucial indicator for large scale wind power converters, therefore the evaluations are mainly focused on the power device thermal performances, which are closely related to the life time and cost of the converter. Simulation results of different converter candidates regarding the loss and junction temperature are presented and analyzed. It is concluded that the three-level and five-level H-bridge converter topologies both have potential to achieve improved thermal performances compared to the three-level Neutral-Point-Clamped converter topology in the wind power application.
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| | Novel High Efficiency Multilevel DC-DC Boost Converter Topologies and Modulation Strategies
By Gianluca BUTTI, Jürgen BIELA | [View]
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Abstract: In this paper new topologies and interleaving modulation concepts for multilevel DC-DC boost converter enabling a significantly less loss and a reduced chip size of the power semiconductors are proposed. Moreover, a new soft switching concept is presented. The achievable efficiency, chip area and boost inductance reduction are compared with existing solutions for an input voltage of 800 V, an output voltage of 3 kV and an output power of 150 kW. A 30\% loss reduction and a chip area decrease by 50\% is achieved.
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| | One Dimensional Feed-Forward Modulation of a Cascaded H-Bridge Multi-Level Converter including Capacitor Balancing with Reduced Switching Frequency
By John VODDEN, Pat WHEELER, Leopoldo G. FRANQUELO, Jose I. LEON, Sergio VAZQUEZ | [View]
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Abstract: Multi-level converters are well suited to medium and high power applications where the priority is forhigh quality a.c. currents with low harmonic content and for low switching losses. Suitable modulationschemes must find a good compromise between these two conflicting aims.This paper relates one-dimensional feed-forward modulation (1DFFM) to carrier based modulation.1DFFM can produce high quality currents despite d.c. capacitor voltage ripple by accounting for mea-sured capacitor voltages in the modulation process and can balance capacitor voltages by eliminatingstates that cause the capacitor voltages to diverge. It is shown in this paper that this method increases thedevice switching frequency compared to carrier-based modulation with the same sampling rate.An alternative modulation method is presented which balances the capacitor voltages by assigning com-mutations to bridges during each sampling period. This approach is less likely to cause two bridges tocommutate at the same time so produces a lower device switching frequency than 1DFFM. It is shownexperimentally that the two methods produce similar a.c. current distortion and both effectively balancethe capacitor voltages.
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| | Open-loop Approach to Control a Modular Multilevel Frequency Converter
By Lennart ANGQUIST, Arif HAIDER, Hongbo JIANG, Hans-Peter NEE | [View]
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Abstract: Frequency converters are required to supply power from the public three-phase network to railways using 16.7 Hz single-phase in their rolling stock. The emerging Modular Multilevel Converter (M2C) technology offers the possibility to make such power conversion in one step using Modular Multilevel Frequency Converters (MMFC) with full-bridge sub-modules. A detailed study of the modulation and control of such converters is presented in this paper. The voltage inserted by each arm of the MMFC is a mixture of the three-phase 50 Hz voltage and the single phase 16.7 Hz voltage. It is important that the voltage reference given to the modulator will be carefully reproduced by the converter because any deviation can introduce undesired frequency components from the single-phase side to the three-phase side and vice versa. The fact that the low frequency, single-phase load causes low-frequency ripple in the total capacitor voltage complicates the problem to generate the correct modulation pattern. In this paper it is proposed to solve that difficulty by estimating the instaneous total capacitor voltages in each arm and provide that information to the modulator. It is shown that such estimations can be performed using measured currents on the single- and three-phase sides. Stable single-phase voltage, symmetrical undistorted three-phase currents and stable converter capacitor voltages are achieved under varying loading conditions. Models in Matlab/Simulink (continuous) and EMTDC/PSCAD (discrete sub-modules) have been developed in this study.
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| | Power and DC Link Voltage Control Considerations for Indirect AC/AC Modular Multilevel Converters
By Michail VASILADIOTIS, Stephan KENZELMANN, Nicolas CHERIX, Alfred RUFER | [View]
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Abstract: A highly interesting application field of the Modular Multilevel Converter is the interconnection of theEuropean three-phase 50 Hz industrial grid with the single-phase 16 2/3 Hz railway supply. Contraryto existing technologies, the utilization of a bulky and costly transformer on the low frequency side aswell as additional filtering components on the AC lines or DC link are avoided. However, the inherentconverter dynamics still pose challenges in the design of control systems, which should ensure safeoperation as well as high performance in steady state and transient conditions. This paper investigates an effective way to control the power as well as the DC link voltage in such a Modular Multilevel Converter-based network interface. The additional control objectives, such as branch energy stabilization as well as sub-module capacitor voltage balancing are maintained, regardless of the aforementioned externally controlled magnitudes. The DC link second harmonic power fluctuation, which is a property of the single-phase side, is also compensated by means of proper circulating current control. The analyzed concepts are evaluated by means of simulation results, using a discrete-time model of a real power application.
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| | Power Losses Evaluation of Three Multilevel Converter Topologies for Direct Interface with Medium Voltage Grids
By Mohamed RASHED, Christian KLUMPNER, Greg ASHER | [View]
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Abstract: This paper presents the power losses evaluation and comparison of three different Mega Watt MW scale multilevel converter topologies proposed for interfacing energy storage systems to medium voltage grids using Medium Frequency Transformers MFTs to provide the galvanic isolation. The candidates are the Hybrid Cascaded H Bridge HB Multilevel Converter HCHB-MLC topology, the second is an Unfold Cyclo-converter cell based Multilevel Converter UFCYC-MLC topology and the third is a proposed Folded Cyclo-converter based Multilevel Converter FCYC-MLC topology. Detailed electrical and dynamic thermal models of the converters’ power electronics circuits are implemented and simulated using the PSIM. The semiconductor power losses of the various converters components and the power losses of the DC-link capacitors and the MFTs are evaluated using the simulation waveforms obtained at the nominal voltage of the converters and at different loading currents for the full range of the power factor. The results show that the HCHB-MLC topology gives the highest efficiency for a wide range of loading condition and the proposed FCYC-MLC topology is found more efficient than the conventional UFCYC-MLC.
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| | PWM-Based Control of a Cascaded Three-Phase Multilevel Inverter
By António MARTINS, Emanuel MEIRELES, Adriano CARVALHO | [View]
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Abstract: The modular structure of the multilevel inverter and the Field-Programmable Gate Array (FPGA) architecture make a perfect match. Three-phase cascaded H-bridge multilevel inverters require a large number of control signals and the flexibility of the FPGA appears as the right solution to generate the control signals for the multilevel inverter. This paper presents the analysis and design of a FPGA-based multi-carrier PWM for a five-level three-phase inverter and shows PSIM-based simulation results and experimental data demonstrating the expected theoretical results.
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| | Research on three-phase PWM Hybrid-Clamped five-level AC-DC converter with high power factor
By Kai YIN, Mingyao MA, Cheng TAN, Quanwei LIU, Yan DENG, Xiangning HE | [View]
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Abstract: A three-phase PWM AC-DC converter with high power factor (PF) using hybrid-clamped five-level topology is researched in this paper. The mathematical model and control strategy are analyzed. In addition, a novel method to determine ac-side inductance according to required current ripple is proposed. Simulation and experimental results show the dc-link capacitor voltages imbalance extensively existing in multilevel converters can be solved, and the power factor correction is realized on the grid side.
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| | Voltage balancing control in 3-Level Neutral-Point Clamped Converters using triangular carrier PWM modulation for FACTS applications
By Javier CHIVITE-ZABALZA, Pedro IZURZA, Gorka CALVO, Miguel RODRIGUEZ-VIDAL | [View]
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Abstract: In this paper, a novel technique to balance the voltage of the two split dc capacitors of a 3-Level Neutral-Point-Clamped inverter using triangular carrier PWM modulation is presented. This technique, suitable for reactive power compensation and for inverters operating with a relatively low switching frequency, consists in adding a square wave at six times the output frequency. Subsequently, the paper presents a comparison with two already known strategies in which a sinusoidal waveform at two and six times the output frequency are injected. The current contribution to the mid-point of the dc bus is then analysed for different modulation indexes and operating conditions. Based on this analysis, a small-signal averaged model, suitable for control design purposes is presented. Finally, simulation and experimental results on a 690 V AC, 120 kVA test bench that validate the theory are shown.
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