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ABSTRACT: Multilevel converters offer advantages in terms of the output waveform quality due to the increased number of levels used in the output voltage modulation. This advantage is particularly true for cascade H-bridge (CHB) converters that can be built to produce a large number of levels owing to their modular structure. Nevertheless, this advantage comes at the cost of multiple dc links supplied by independent rectifiers through the use of a multi-output transformer for inverters. This front end complicates the implementation of converters that have a high number of levels. An alternative method of using lower voltage cells with floating dc links to compensate only for the voltage distortion of a neutral-point-clamped (NPC) converter is considered for active rectifier applications. The analogy between the floating HBs and the series active filters is used to develop a strategy for the harmonic compensation of the NPC output voltage and the control of the floating dc-link voltages. This simplifies the current control scheme and increases its bandwidth. The experimental results with a low power prototype that show the good performance of the proposed modulation technique and the resulting improvement in the output waveform are provided.
IEEE Transactions on Industrial Electronics 07/2011; · 5.16 Impact Factor
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ABSTRACT: This paper is related to faults that can appear in multilevel (ML) inverters, which have a high number of components. This is a subject of increasing importance in high-power inverters. First, methods to identify a fault are classified and briefly described for each topology. In addition, a number of strategies and hardware modifications that allow for operation in faulty conditions are also presented. As a result of the analyzed works, it can be concluded that ML inverters can significantly increase their availability and are able to operate even with some faulty components.
IEEE Transactions on Industrial Electronics 08/2010; · 5.16 Impact Factor
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ABSTRACT: Multilevel converters are an interesting alternative for high power drives, due to their good quality output signals. Despite their advantages, the large number of components required increases the fault probability. Among the multilevel topologies, the cascade multicell converter presents advantages when operating under internal fault conditions, due to its high modularity. Previous works proposed to compensate the unbalanced operation due to a fault by changing the canonical fundamental output phase shift to precalculated angles, depending on the fault condition. This solution assumes that, if the maximum output phase voltage on each leg is used, the maximum line-to-line voltage will be at a maximum as well. This paper shows how this assumption is not always valid and presents the optimum angles and modulation indexes that must be used in order to obtain the maximum balanced load voltages.
IEEE Transactions on Industrial Electronics 08/2009; · 5.16 Impact Factor
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ABSTRACT: Multilevel converters use a large amount of semiconductors, allowing the reconfigurate of the converter to work even on internal fault condition. This paper presents a method to detect faulty cells in a cascaded multicell converter requiring just one voltage measurement per output phase. The method is based on high-frequency harmonic analysis, using a dynamic prediction of their behavior, avoiding erroneous detection on transients while keeping the precision under real fault events. Once the faulty cell is detected, it can be bypassed allowing the converter to keep working according to previously reported techniques. Experimental results confirm accurate and fast fault detection, with a good rejection to normal operation transients.
IEEE Transactions on Industrial Electronics 07/2009; · 5.16 Impact Factor
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ABSTRACT: Multilevel converters and, in particular, flying capacitor (FC) converters are an attractive alternative for medium-voltage applications. FC converters do not need complex transformers to obtain the DC-link voltage and also present good robustness properties, when operating under internal fault conditions. Unfortunately, with standard modulation strategies, to increase the number of output voltage levels of FC converters, it is necessary to increase the number of cells and, hence, the number of capacitors and switches. In this paper, we develop a finite-state model predictive control strategy for FC converters. Our method controls output currents and voltages and also the FC voltage ratios. This allows one to increase the number of output voltage levels, even at high power factor load conditions and without having to increase the number of capacitors and switches. Experimental results illustrate that the proposed algorithm is capable of achieving good performance, despite possible parameter mismatch.
IEEE Transactions on Industrial Electronics 07/2009; · 5.16 Impact Factor
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ABSTRACT: The use of multilevel converters is widespread in medium-voltage applications due to their inherent capability to share the voltage between the power devices. More than one multilevel topology can be combined to form a single Hybrid Converter with improved capabilities. In particular, the series connection of a neutral point clamped (NPC) as a main inverter, providing the active power, and a floating H-Bridge in each phase as conditioning inverters is analyzed in this paper. The function of these conditioning inverters is to compensate the harmonics generated by the main inverter, reducing the current distortion by delivering reactive power. The switching frequency of the main inverter is limited to reduce the switching losses by means of synchronous pulsewidth modulation. In this work, synchronous optimal pulsewidth modulation that minimize the weighted voltage THD in the NPC has been selected, while higher frequency subharmonic PWM is used for the floating H-Bridges. Design criteria for the selection of a proper value for the H-Bridge dc-link voltage and a simple control algorithm for its regulation are proposed. This scheme achieves currents with low distortion, resulting in a lower torque ripple and lower machine losses in a induction motor drive. Experimental results using a 2.5 kW converter prototype feeding a R-L load are shown to illustrate the operation of this topology and dynamic operation of an induction machine fed by this converter under V/f control is presented.
Electric Machines and Drives Conference, 2009. IEMDC '09. IEEE International; 06/2009
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ABSTRACT: Harmonics are a very important issue in the design and operation of medium-voltage inverter drives, due to restrictions imposed by regulations. This paper studies the harmonics generated by a three-phase multicell cascade inverter, with single-phase pulsewidth modulation rectifiers at the input side of each cell; thus, the entire topology is able to regenerate power to the net. This paper identifies accurately the harmonic generation process in the dc-link voltage, the input current of each cell, and, therefore, at the input side of the inverter. Moreover, this paper proposes a method to reduce and, in some cases, eliminate the input current harmonics, reaching a very high input power factor for any operation condition. Simulation and experimental results are provided to validate the theoretical analysis.
IEEE Transactions on Industrial Electronics 03/2009; · 5.16 Impact Factor
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ABSTRACT: Cascaded multilevel inverters provide high quality voltages at the load, reducing pulsating torques and common mode voltage. Also, the input current has a low harmonic content due to the multipulse transformer, providing a nearly sinusoidal current. This paper introduces a topology with an active-front-end rectifier at the input of each cell to achieve four quadrant operation. Thus, the main features of the drive are: sinusoidal input and output currents, unity power factor and regeneration of power to the power supply. This work also studies the complete drive behavior of a synchronous machine using this topology.
Industry Applications Society Annual Meeting, 2008. IAS '08. IEEE; 11/2008
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ABSTRACT: Reliability and fault tolerant operation are key aspects in high power drives, specially when these drives are used to operate critical loads, as downhill conveyors. This paper presents a solution for the operation under internal fault of high power regenerative cascade multicell converter, which allows to operate temporally in normal conditions or to turn off the equipment in a controlled way. Previous works have shown that it is possible to reduce the effects on the load due to internal faults by modifying the phase- shift between the output voltages. The proposed method uses the additional degree of freedom given by the controlled dc-link voltage of the PWM rectifiers, to increase the operation range, even reaching non-fault operation.
Control and Modeling for Power Electronics, 2008. COMPEL 2008. 11th Workshop on; 09/2008
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ABSTRACT: The use of multilevel converters is widespread in medium voltage applications due to their inherent voltage sharing among the devices. Also, the multilevel offers advantages regarding the waveform quality due to the multiple steps of the output voltage waveform. This advantage is specially true for cascaded H-bridge inverters that can produce a large number of levels due to their modular structure. Nevertheless, this advantage comes at the cost of multiple dc-links that are normally supplied by independent rectifiers coupled though a multipulse input transformer. This multiple dc sources complicates the implementation of converters with high number of levels. The alternative of using the lower voltage cells with floating dc-links to supply only reactive power, compensating for voltage distortion, has been proposed in literature. In this work, this idea is applied to an neutral point clamped (NPC) inverter operating at low switching frequency. Here, the analogy between the floating dc-link converters and series active filters is used to develop a novel control strategy for these dc-links and the three phase output voltage.
Industrial Electronics, 2008. ISIE 2008. IEEE International Symposium on; 08/2008
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ABSTRACT: Multilevel converters are a very interesting alternative for medium and high power drives. One of the more flexible topologies of this type is the cascaded multicell converter. This paper proposes the use of a single-phase reduced cell suitable for cascaded multilevel converters. This cell uses a reduced single-phase active rectifier at the input and an H-bridge inverter at the output side. This topology presents a very good performance, effectively controlling the waveform of the input current and of the output voltage and allowing operation in the motoring and regenerative mode. The results presented in this paper confirm that this medium voltage inverter effectively eliminates low frequency input current harmonics at the primary side of the transformer and operates without problems in regenerative mode.
IEEE Transactions on Industrial Electronics 04/2008; · 5.16 Impact Factor
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ABSTRACT: Like most power converter topologies, multilevel inverters are controlled with modulation techniques that are conceptually based on nonlinear waveform synthesis assuming constant dc-link voltages. However, real applications have load and supply dependent dc-links that usually present important low frequency ripple, which is also modulated and transmitted to the load, generating undesirable low frequency voltage and current distortion. This paper introduces a simple but effective dc-link ripple feedforward strategy into traditional carrier-based modulation techniques. The dc-link ripples are measured and used to modify the carriers or the reference directly in the modulation stage. Simulation and experimental results show the accuracy of the proposed method, eliminating low order harmonics in the load current.
IEEE Transactions on Power Electronics 02/2008; · 4.65 Impact Factor
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ABSTRACT: A new control strategy for a Matrix Converter feeding an induction machine is presented. Simple discrete models of the machine and the converter are used to predict the behavior of torque and flux. The switching state that optimizes the value of a quality function is selected. The proposed strategy considers the discrete nature of control processors and power converters, presenting a different approach to achieve control. Results confirm a very good motor performance with an extremely simple and versatile control method.
Power Electronics and Applications, 2007 European Conference on; 10/2007
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ABSTRACT: Cascaded H-bridge multilevel inverters are commonly controlled using multicarrier Phase-Shifted PWM, since it allows an even power distribution among the converter power cells, which can lead to lower harmonic distortion in the input current if an adequate transformer is used. However it is known that other multicarrier methods, like Phase Disposition PWM (PD- PWM) have better output voltage quality with lower distortion. Nevertheless, when used with cascaded inverters, this method produces uneven usage of the converter cells, that disables the harmonic cancelations at the transformer input. This paper presents a modified PD-PWM technique, that combines the benefits of both modulation methods, achieving good output voltage and input current quality.
Power Electronics Specialists Conference, 2007. PESC 2007. IEEE; 07/2007
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ABSTRACT: Multilevel converters are a very interesting alternative for medium and high power drives. Among these topologies the Multicell group present some advantages due to its modularity and scalability. This paper propose a new topology for Multilevel Multicell Inverters. This inverter is based on a new cell obtained from the the mixture of the two most popular multicell topologies, flying capacitor and cascaded multicell inverter. The new cell provide a high number of output levels, high modularity, low number of components and low losses. The results presented in this paper confirm that this new medium voltage inverter topology is able to reach high quality output voltages, allowing further advances in modulation techniques and other issues.
Industrial Electronics, 2007. ISIE 2007. IEEE International Symposium on; 07/2007
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ABSTRACT: Multicell converters are one of the alternative topologies for medium-voltage industrial drives. For an application requiring regenerative capability, each power cell must be constructed with a three- or single-phase pulsewidth-modulation (PWM) rectifier as front end. The choice of single-phase PWM rectifiers for the input of the cells results in a reduced number of power switches and a simpler input transformer than the three-phase equivalent. However, its control is not as straightforward. This paper proposes the use of higher order resonant controllers in the classical control structure of the single-phase PWM rectifier. This ensures zero steady-state tracking error of the reference current at fundamental frequency. A detailed description of the design criteria for the position of the zeros and poles of the controller is given. Experimental results showing the good performance of the single-phase input cells and its proposed control are included
IEEE Transactions on Industrial Electronics 05/2007; · 5.16 Impact Factor
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ABSTRACT: Multicell converters are one of the alternative topologies for medium voltage industrial drives. For application requiring regenerative capability each power cell must be constructed with a three- or single-phase PWM rectifier as a front end. The choice of single-phase PWM rectifiers for the input of the cells results in a reduced number of power switches and a simpler input transformer than the three-phase equivalent. However, its control is not as straight forward. This paper proposes the use of higher order resonant controllers in the classical control structure of the single-phase PWM rectifier. This ensures a zero steady-state tracking error of the reference current at fundamental frequency. A detailed description of the design criteria for the position of the zeros and poles of the controller is given. Experimental results showing the good performance of the single-phase input cells and its proposed control are included.
Industrial Electronics Society, 2005. IECON 2005. 31st Annual Conference of IEEE; 12/2005
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ABSTRACT: Multilevel converters are a very interesting alternative for medium and high power drives. One of the more flexible topologies of this type is the multicell converter. This paper presents a new regenerative cell suitable for cascaded multilevel inverters. The cell uses a reduced single-phase active rectifier at the input and an H-bridge inverter at the output side. The cell has a reduced switch count and presents a very good performance, controlling effectively the waveform of the input current and of the output voltage. The results presented in this paper confirm that this medium voltage inverter effectively eliminates low frequency input current harmonics at primary side of the transformer and operates without problem in regenerative mode
Industrial Electronics Society, 2005. IECON 2005. 31st Annual Conference of IEEE; 12/2005
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ABSTRACT: This paper presents a new control scheme for a regenerative AC/DC/AC converter using model based predictive control. The control strategy minimizes quality functions, which represent the desired behavior of the converter. At the inverter side, the load current error is minimized, while at the input side, the active and reactive powers are controlled directly. The main advantages of this method are: no need of linear current controllers, coordinates transformations or modulators for converter and inverter. The rectifier operates with sinusoidal input currents and unity power factor, while sinusoidal currents are generated out at the inverter load.
Industry Applications Conference, 2005. Fourtieth IAS Annual Meeting. Conference Record of the 2005; 11/2005
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ABSTRACT: This paper presents a method for operating cascaded multilevel inverters when one or more power cells are damaged. The method is based on the use of additional switches in the power circuit to bypass the faulty cell. To control the cells, the angle of phase shifting in the carrier signals is modified according to the number of operating cells, to minimize the load voltage distortion, when the inverter operates in failure mode. The reference signals of the pulsewidth-modulation modulators are also modified to increase the output voltage. Simulation and experimental results show the effectiveness of this method, which significantly increases the reliability of the drive.
IEEE Transactions on Industrial Electronics 09/2005; · 5.16 Impact Factor
