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  • Current-balancing technique for interleaved voltage source inverters with magnetically coupled legs connected in parallel

     Capella Frau, Gabriel Jose; Pou Felix, Josep; Ceballos Recio, Salvador; Zaragoza Bertomeu, Jordi; Agelidis, Vassilios
    IEEE transactions on industrial electronics
    Vol. 62, num. 3, p. 1335-1344
    DOI: 10.1109/TIE.2014.2345345
    Date of publication: 2015-03-01
    Journal article

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    Connecting legs in parallel in a voltage source inverter is a way to increase the output current and, thus, its rated power. The connection can be made using either coupled or uncoupled inductors, and achieving an even contribution to the output current from all the legs is a crucial issue. Circulating currents produce additional losses and stress to the power devices of the converter. Therefore, they should be controlled and minimized. An efficient technique to attain such balance when coupled inductors are used is presented in this paper. The proposed technique can also be used when the inductors are uncoupled, since it is a particular case where the coupling coefficients are zero. This technique does not include proportional-integral controllers and does not require any parameter tuning either. The exact control action needed to reach current balance is straightforwardly calculated and applied. Experimental results are shown in this paper to verify the efficiency of the proposed balancing method.

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    Circulating current injection methods based on instantaneous information for the modular multilevel converter  Open access

     Pou Felix, Josep; Ceballos Recio, Salvador; Konstantinou, Georgios; Agelidis, Vassilios; Picas Prat, Ricard; Zaragoza Bertomeu, Jordi
    IEEE transactions on industrial electronics
    Vol. 62, num. 2, p. 777-788
    DOI: 10.1109/TIE.2014.2336608
    Date of publication: 2015-02-01
    Journal article

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    This paper studies different circulating current references for the modular multilevel converter. The circulating current references are obtained from the instantaneous values of the output current and modulation signal of the phase leg. Therefore, the determination of the amplitude and phase of the output current is not needed, which is a significant improvement compared to other methods such as those based on injecting specific harmonics in the circulating currents. Among the different methods studied in this paper, a new method is introduced, which is able to reduce the capacitor voltage ripples compared to the other methods. A closed-loop control is also proposed which is able to track the circulating current references. With the discussed methods, the average values of the capacitor voltages are maintained at their reference while the voltage ripples are kept low. Experimental results are presented to demonstrate the effectiveness of the proposed and discussed methods.

  • A three-phase frequency-adaptive phase-locked loop for independent single-phase operation

     Mirhosseini, Mitra; Pou Felix, Josep; Agelidis, Vassilios; Robles Sestafe, Eider; Ceballos Recio, Salvador
    IEEE transactions on power electronics
    Vol. 29, num. 12, p. 6255-6259
    DOI: 10.1109/TPEL.2014.2328657
    Date of publication: 2014-12-01
    Journal article

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    This letter proposes a phase-locked loop (PLL) able to operate as an independent single-phase PLL in the context of three-phase systems. Its key advantage is that the extracted information allows the independent control of both the active and reactive powers for each phase. The proposed PLL utilizes moving average filters and, hence, remains robust under unbalanced and highly distorted voltages. A frequency estimator is also presented and incorporated in the PLL angle detector path to make it frequency adaptive. The performance of the proposed PLL is verified experimentally and results for frequency change, phase-angle jump, and unbalanced voltage sag are reported as a confirmation of its potential. © 2014 IEEE.

  • Performance evaluation of three-phase grid-connected photovoltaic inverters using electrolytic or polypropylene film capacitors

     Karanayil, Baburaj; Agelidis, Vassilios; Pou Felix, Josep
    IEEE Transactions on Sustainable Energy
    Vol. 5, num. 4, p. 1297-1306
    DOI: 10.1109/TSTE.2014.2347967
    Date of publication: 2014-10-01
    Journal article

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    In grid-connected photovoltaic (PV) power stations, improving the life expectancy and long-term reliability of three-phase PV inverters is urgently needed to match the significantly higher lifetime of the PV modules. A key contribution toward such improvement is replacing the conventional electrolytic film capacitors by metallized polypropylene film ones. This paper presents a detailed evaluation of a conventional three-phase grid-connected PV inverter performance when replacing the electrolytic capacitor with a minimum value of metallized polypropylene film capacitor. Although the minimum dc bus capacitance leads to higher voltage ripples, such ripples were found to be within acceptable limits to operate the inverter satisfactorily. Simulation and experimental results are presented for a 5-kW grid-connected inverter prototype with a nominal dc voltage of 457 V to confirm the theoretical considerations.

  • Voltage-balancing method using phase-shifted PWM for the flying capacitor multilevel converter

     Ghias, Amer M.Y.M; Pou Felix, Josep; Ciobotaru, Mihai; Agelidis, Vassilios
    IEEE transactions on power electronics
    Vol. 29, num. 9, p. 4521-4531
    DOI: 10.1109/TPEL.2013.2285387
    Date of publication: 2014-09
    Journal article

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    Multilevel flying capacitor (FC) converters provide natural capacitor voltage balance under phase-shifted pulse width modulation (PS-PWM). However, natural balancing may not be robust enough to maintain the voltages at the reference values, especially under certain transient conditions. Furthermore, natural balancing dynamics depend on the load and it may be very slow in some practical applications. Therefore, a more robust balancing mechanism of maintaining the FC voltages at the desired values is required. This paper proposes a new closed-loop voltage-balancing method for the multilevel FC converters using PS-PWM. The proposed method balances the voltages of the FCs by modifying the duty cycle of each switch of the FC converter using a proportional controller. The crossed effect between FC currents and duty cycles is considered and is used for optimal FC voltage balancing. Simulation and experimental results verify that the proposed voltage-balancing method is very robust to different operating conditions, such as load transients, linear/nonlinear and unbalanced loads

  • On Performance Advances of Flying Capacitor Multilevel Converter Topologies

     Amer Mohammad Yusuf Mohammad Ghias
    The University of New South Wales
    Theses

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    The main focus of this thesis is to develop advanced modulation and voltage balancing methods for multilevel converter topologies, able to achieve stiff capacitor voltage regulation and reduce switching power losses of the converters. The study is focused on multilevel converter topologies based on flying capacitors (FCs), such as the multilevel flying capacitor converter (FCC) and the stacked multicell converter. Two modulation strategies are investigated; phase-shifted PWM and phase-disposition PWM, and improved voltage balancing methods have been developed and incorporated to these modulation techniques. The analysis of the converter is conducted in terms of switching frequency reduction, power losses, total harmonic distortion, and capacitor voltage ripples. This thesis also addresses the initial charging of FCs in grid-connected applications. A novel method for precharging the capacitors in the multilevel FCC is presented. The method can charge the capacitors in a controlled way besides producing low voltage and current stress to the power devices and the passive components. The method is simple and can be implemented and can be applied to FCCs with any number of levels. All the methods proposed in this thesis have been tested by simulation and experimental tests.

  • Initial capacitor charging in grid-connected flying capacitor multilevel converters

     Ghias, Amer M.Y.M; Pou Felix, Josep; Agelidis, Vassilios; Ciobotaru, Mihai
    IEEE transactions on power electronics
    Vol. 29, num. 7, p. 3245-3249
    DOI: 10.1109/TPEL.2013.2294957
    Date of publication: 2014-07-01
    Journal article

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    This letter reports a method for the initial charging of capacitors in grid-connected flying capacitor (FC) multilevel converters. A resistor is inserted between each phase of the FC converter and the grid. A voltage balancing algorithm is activated from the beginning of the process and the FC converter generates proper output voltages to achieve balanced charging of both the dc-bus capacitor and the FCs. The proposed initial charging method achieves low voltage and current stress on the power devices and the passive components. The method is simple to implement and can be applied to an FC with any number of levels. Experimental results demonstrate the effectiveness of the proposed method.

  • Comparison of phase-shifted and level-shifted PWM in the modular multilevel converter

     Darus, Rosheila; Konstantinou, Georgios; Pou Felix, Josep; Ceballos Recio, Salvador; Agelidis, Vassilios
    International Power Electronics Conference
    p. 3764-3770
    DOI: 10.1109/IPEC.2014.6870039
    Presentation's date: 2014-05-18
    Presentation of work at congresses

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    This paper reports a comparison study of different carrier-based PWM techniques applied to the modular multilevel converter. Phase-disposition PWM (PD-PWM) and phase-shifted pulse-width modulation (PS-PWM) with non-interleaving and interleaving are considered in this study. In PS-PWM, two cases are evaluated. In the first case, the particular SMs that have to be activated/deactivated are defined by a voltage balancing algorithm, which is the same one implemented in PD-PWM. In addition, an algorithm to restrict the number of switching SMs is also implemented to reduce the switching frequency of the power devices. In the other case of PS-PWM, each sub-module (SM) has a carrier signal associated to it and capacitor voltage balance is achieved by individual control of its capacitor voltage. The circulating current is controlled to be a dc component in all the cases. Simulation and experimental results are presented to evaluate the quality of the line-to-line output voltages and SM capacitor voltage ripples for the different cases under study. © 2014 IEEE.

  • Minimum signal modulation scheme based on a single carrier for interleaved operation of parallel phase legs in voltage source converters

     Capella Frau, Gabriel Jose; Gabiola Antxustegi, Igor; Pou Felix, Josep; Zaragoza Bertomeu, Jordi; Ceballos Recio, Salvador; Agelidis, Vassilios
    IET power electronics
    Vol. 7, num. 5, p. 1305-1312
    DOI: 10.1049/iet-pel.2013.0545
    Date of publication: 2014-05-01
    Journal article

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    Connecting legs in parallel is a common way to increase the output current of a power converter. Interleaving the carriers used to modulate the reference signals for each leg leads to a reduction in the output current ripple without resorting to increasing the switching frequency. A whole set of shifted carriers is required if interleaved pulse-width modulators (PWMs) are used. Implementing this by means of a digital signal processor (DSP) means that the higher the number of carriers, the higher the number of DSP timing resources required. Provided that the latter are usually limited, this could be a drawback when increasing the number of interleaved carriers. This study presents the implementation of a PWM scheme where all modulators use the same carrier offering the same results as if a set of n interleaved carriers were used. Since the proposed algorithm takes maximum benefit from the PWM units available in a DSP, a higher number of legs connected in parallel can be controlled without adding any external processing hardware. A MATLAB/Simulink model has been set up for simulation purposes. Selected experimental results obtained from a three-leg single-phase inverter are reported, confirming the effectiveness of the proposed implementation. © The Institution of Engineering and Technology 2014.

  • SiC modular multilevel converters: sub-module voltage ripple analysis and efficiency estimations

     Perez Basante, Angel; Pou Felix, Josep; Ceballos Recio, Salvador; Gil De Muro, Asier; Pujana, Ainhoa; Ibañez, Pedro
    International Exhibition & Conference for Power Electronics Intelligent Motion Power Quality
    p. 1418-1425
    Presentation's date: 2014-05
    Presentation of work at congresses

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    Two important technical challenges associated with the Modular Multilevel Converter (MMC) are the reduction of the voltage ripple of the Sub-Module (SM) capacitors and the reduction of the converter losses. This paper conducts a study focused on these two topics. Firstly, the effect of a circulating current with a predefined second harmonic on the SM voltage ripple is assessed. Secondly, an efficiency study for two different MMCs, one with silicon (Si) and the other with silicon carbide (SiC) devices is carried out. Results suggest that a SiC MMC converter with a circulating current including a second harmonic represent a good solution since it achieves a reduction of the SM capacitors together with a high efficiency operation.

  • Varying and unequal carrier frequency PWM techniques for modular multilevel converters

     Konstantinou, Georgios; Darus, Rosheila; Pou Felix, Josep; Ceballos Recio, Salvador; Agelidis, Vassilios
    International Power Electronics Conference
    p. 3758-3763
    DOI: 10.1109/IPEC.2014.6870038
    Presentation's date: 2014-05
    Presentation of work at congresses

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    Carrier-based pulse-width modulation (PWM) strategies are commonly applied to modular multilevel converters (MMC). This paper proposes a modified PWM technique with unequal and varying carrier frequencies between consecutive levels. The variation between two consecutive carriers is calculated in order to maintain a constant average switching frequency regardless of modulation index and number of levels in the output. The proposed method effectively shifts the major harmonic content of output waveforms to higher frequencies at the cost of a slight increase in low order harmonics. Details on the derivation of the carrier frequencies as well as simulation and experimental results of the proposed method demonstrate the viability of the method as an alternative modulation technique for the MMC.

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    Enhanced phase-shifted PWM carrier disposition for interleaved voltage-source inverters  Open access

     Capella Frau, Gabriel Jose; Pou Felix, Josep; Ceballos Recio, Salvador; Konstantinou, Georgios; Zaragoza Bertomeu, Jordi; Agelidis, Vassilios
    IEEE transactions on power electronics
    Vol. 30, num. 3, p. 1121-1125
    DOI: 10.1109/TPEL.2014.2338357
    Date of publication: 2014-03-01
    Journal article

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    This letter presents a novel implementation of pulse width modulation that improves the quality of the line-to-line output voltages in interleaved multiphase voltage-source inverters (VSIs). In multiphase VSIs with n interleaved parallel-connected legs, the best single-phase output voltage is achieved when the carriers are evenly phase shifted. However, switching among nonadjacent levels can be observed at regular intervals in the line-to-line voltages, causing bad harmonic performance. With the proposed method, switching in the line-to-line voltages happens exclusively between adjacent levels. The modulator utilizes two sets of n evenly phase-shifted carriers that are dynamically allocated. Because of its generality, the proposed implementation is valid for any number of phases and any number of legs in parallel. A MATLAB/Simulink model has been set up for simulation purposes. Selected experimental results obtained from a three-phase VSI made up with two and three legs in parallel per phase are reported, confirming the enhancement attained with the proposed implementation.

  • A modified voltage balancing sorting algorithm for the modular multilevel converter: Evaluation for staircase and phase-disposition PWM

     Darus, Rosheila; Pou Felix, Josep; Konstantinou, Georgios; Ceballos Recio, Salvador; Agelidis, Vassilios
    IEEE Applied Power Electronics Conference and Exposition
    p. 255-260
    DOI: 10.1109/APEC.2014.6803318
    Presentation's date: 2014-03
    Presentation of work at congresses

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    This paper introduces a low complexity implementation of the voltage balancing sorting algorithm to reduce the switching frequency of the power devices in modular multilevel converters (MMCs). Two modulation techniques are evaluated; staircase modulation and phase-disposition pulse-width modulation (PD-PWM) under the conventional and the proposed algorithm. Using a circulating current controller in an MMC with 12 sub-modules per arm, PD-PWM yields better results compared to the staircase modulation technique. The test condition for this comparison is such that the power devices operate at a similar switching frequency and produce similar amplitudes to the capacitor voltage ripples in both modulation techniques. The results are verified through extensive simulations and experimentally using a phase-leg MMC laboratory prototype.

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    Optimum state voltage balancing method for stacked multicell converters  Open access

     Ghias, Amer M.Y.M; Pou Felix, Josep; Agelidis, Vassilios; Ciobotaru, Mihai
    Annual Conference of the IEEE Industrial Electronics Society
    p. 6293-6298
    DOI: 10.1109/IECON.2013.6700170
    Presentation's date: 2013-11
    Presentation of work at congresses

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    This paper presents a voltage balancing method for stacked multicell converters based on phase disposition pulse-width modulation. This method is based on minimizing a cost function to determine the optimum redundant state for capacitor voltage balance for each particular voltage level. The robustness of the proposed voltage balancing method is verified against static and dynamic unbalanced load conditions. Furthermore, a significant reduction in the switching frequencies of the power devices is achieved by using sawtooth carriers instead of standard triangular carriers without affecting the voltage balancing capability.

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    Voltage balancing method using phase-shifted PWM for stacked multicell converters  Open access

     Ghias, Amer M.Y.M; Pou Felix, Josep; Agelidis, Vassilios; Ciobotaru, Mihai
    Annual Conference of the IEEE Industrial Electronics Society
    p. 6334-6339
    DOI: 10.1109/IECON.2013.6700177
    Presentation's date: 2013-11
    Presentation of work at congresses

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    This paper proposes an active voltage balancing method for stacked multicell converters (SMC) using phase-shifted pulse-width modulation, which is easy to implement and extend to high number of levels. The proposed method balances the voltages of the capacitors by modifying the duty cycle of each switch of the SMC using a proportional controller. The crossed effect between capacitor currents and duty cycles is considered and is used for optimal capacitor voltage balance. The performance of the proposed voltage balancing method is verified by simulation for different operating conditions, such as unbalanced linear loads, non-linear loads and load transients.

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    Single-stage inverter-based grid-connected photovoltaic system with ride-through capability over different types of grid faults  Open access

     Mirhosseini, Mitra; Pou Felix, Josep; Agelidis, Vassilios
    Annual Conference of the IEEE Industrial Electronics Society
    p. 8008-8013
    DOI: 10.1109/IECON.2013.6700471
    Presentation's date: 2013-11
    Presentation of work at congresses

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    In this paper the control of a single-stage grid-connected photovoltaic power plant (GCPPP) is developed to address the issue of inverter disconnection under various grid faults. There are three main reasons for inverter disconnection which are (i) excessive dc-link voltage, (ii) excessive ac currents and (iii) loss of grid-voltage synchronization. The control of the inverter incorporates reactive power support in the case of voltage sags based on grid code requirements to ride-through the faults and support the grid voltages. Accordingly, another requirement of the grid codes is to control the reactive current injection under unbalanced voltage sags so that the voltages in the non-faulty phases do not exceed the specified limitations. All these issues are discussed in this paper for a case study of 1-MVA GCPPP using MATLAB/Simulink. The results illustrate the capability of the developed GCPPP to ride-through different types of faults occurred on the grid side.

  • Improving capacitor voltage ripples and power losses of modular multilevel converters through discontinuous modulation

     Picas Prat, Ricard; Ceballos Recio, Salvador; Pou Felix, Josep; Zaragoza Bertomeu, Jordi; Konstantinou, Georgios; Agelidis, Vassilios
    Annual Conference of the IEEE Industrial Electronics Society
    p. 6233-6238
    DOI: 10.1109/IECON.2013.6700160
    Presentation's date: 2013-11
    Presentation of work at congresses

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  • Interleaved selective harmonic elimination PWM for single-phase rectifiers in traction applications

     Konstantinou, Georgios; Agelidis, Vassilios; Pou Felix, Josep
    Annual Conference of the IEEE Industrial Electronics Society
    p. 930-935
    DOI: 10.1109/IECON.2013.6699258
    Presentation's date: 2013-11
    Presentation of work at congresses

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    Interleaved four-quadrant, single-phase rectifiers are widely used in traction applications as part of an AC-DC-AC conversion system. This paper presents selective harmonic elimination pulse-width modulation (SHE-PWM) patterns for single-phase interleaved PWM rectifiers based on both H-bridges and full-bridge NPCs (H-NPCs). SHE-PWM waveforms are defined so that the low-order harmonics of the combined voltage and input currents on the primary of the supply transformer are eliminated. The formulation of the problem and the acquisition of solutions becomes more challenging as the number of levels in the voltage waveform increases. Solutions based on reduced eliminated harmonics are proposed in order to increase the number of solutions as well as their continuity. It is shown that the proposed formulation improves the available solution space with minimal effect to the output. The theoretical analysis are verified through simulation studies.

  • Voltage-source converters for HVDC systems

     Pou Felix, Josep; Ceballos Recio, Salvador; Konstantinou, Georgios; Zaragoza Bertomeu, Jordi
    Brazilian Power Electronics Conference
    Presentation's date: 2013-10-30
    Presentation of work at congresses

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  • The modular multilevel converter and other multilevel topologies

     Pou Felix, Josep; Zaragoza Bertomeu, Jordi
    Brazilian Power Electronics Conference
    Presentation's date: 2013-10-27
    Presentation of work at congresses

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  • State-of-the-art in multilevel power electronics conversion. Tutorial on Advanced Multilevel Converter Systems

     Konstantinou, Georgios; Pou Felix, Josep; Agelidis, Vassilios
    Australasian Universities Power Engineering Conference
    Presentation's date: 2013-10-03
    Presentation of work at congresses

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    Multilevel power electronics converters offer significant advantages over the two-level counterparts, particularly in high-power, high-voltage applications. This tutorial will provide an overview of the state-of-the-art for multilevel power electronics converters and focus on two major developments of the last years, hybrid multilevel converters (HMCs) and the modular multilevel converter (MMC).

  • Capacitor voltage balancing in a three-level-converter-based energy storage system

     Burusteta, Sendoa; Pou Felix, Josep; Ceballos Recio, Salvador; Marino, Iker; Anzola, Jose Angel; Agelidis, Vassilios
    EPE journal
    Vol. 23, num. 4, p. 14-22
    Date of publication: 2013-10-01
    Journal article

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    This paper discusses a grid-connected energy storage system based on two three-level (3L) converters: a DC-DC one and a neutral-point-clamped inverter. When compared to a system using standard two-level converters, the main advantages of this system are: higher efficiency, smaller reactive components allowing for a system cost reduction and its capability to handle higher voltage and power. A modulation strategy for the DC-DC converter which includes the neutral-point voltage balance control is proposed. The capability of the DC-DC converter to cancel the inverter's neutral-point voltage oscillations is also analyzed. Results are presented to illustrate the extended operation area without imbalances. Design guidelines to size the DC-link capacitors are given for a practical implementation. Experimental results taken from a 10 kW 3L DC-DC converter prototype are provided to show the capability of this particular converter to keep neutral-point voltage balance.

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    Positive- and negative-sequence control of grid-connected photovoltaic systems under unbalanced voltage conditions  Open access

     Mirhosseini, Mitra; Pou Felix, Josep; Karanayil, Baburaj; Agelidis, Vassilios
    Australasian Universities Power Engineering Conference
    p. 1-6
    Presentation's date: 2013-09-30
    Presentation of work at congresses

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    This paper discusses the control of the positive- and negative-sequence components of a large-scale grid-connected photovoltaic system (GCPS) under unbalanced voltage sag conditions in the grid. Some issues regarding stability and dynamic performance of the system occur when applying PI controllers in the current control loops. The reason is the delay that the filtering method imposes when extracting the current/voltage sequences. Because of such a delay, the dynamic response of the system becomes slower compared with the case when no filtering technique is needed. Furthermore, there is a strong restriction on choosing suitable parameters for the current/voltage loop controllers without compromising system stability. All these issues are discussed in this paper on a 1-MVA GCPV system using MATLAB/Simulink software.

    Postprint (author’s final draft)

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    Analysis of a voltage balancing technique with reduced switching transitions in a flying capacitor multilevel converter  Open access

     Ghias, Amer M.Y.M; Pou Felix, Josep; Agelidis, Vassilios; Ciobotaru, M.
    Australasian Universities Power Engineering Conference
    p. 1-6
    Presentation's date: 2013-09-29
    Presentation of work at congresses

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    This paper reports the analysis of a voltage balancing technique based on optimal switching transitions for a flying capacitor (FC) multilevel converter using phase disposition pulse-width modulation. Since multiple switching transitions between two consecutive voltage levels may be available, such a redundancy is used to regulate the FC voltages at their desired levels. For the selection of the transitions, two techniques are studied and compared in terms of capacitor voltage ripples and switching frequencies. In the first technique, called optimum transition voltage balancing technique, the transitions between two consecutive voltage levels that produce high switching frequencies are avoided. On the other hand, all the transitions between consecutive levels are considered in the second technique. It is therefore called unrestricted voltage balancing technique. Study analyses are conducted by simulation considering all the possible modulation indices and load power factors.

    Postprint (author’s final draft)

  • Efficiency optimization in low inertia wells turbine-oscillating water column devices

     Ceballos Recio, Salvador; Rea, Judy; López, Iraide; Pou Felix, Josep; Robles Sestafe, Eider; Osullivan, Dara
    IEEE transactions on energy conversion
    Vol. 28, num. 3, p. 553--564
    DOI: 10.1109/TEC.2013.2265172
    Date of publication: 2013-09
    Journal article

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    The Wells turbine is a bidirectional air turbine which operates efficiently over a restricted range of air flow. The optimization of its efficiency requires control of rotational velocity in order to maintain the ratio between airflow and tip speed within the high efficiency range. This paper introduces two generator control strategies that optimize the power take-off efficiency for low inertia turbine systems in which instantaneous control of the turbine air flow to tip speed ratio is a realistic goal. The first control strategy requires measurement of turbine rotational speed and air chamber pressure, and the second strategy removes the requirement for air pressure measurement. The implementation issues associated with this level of control are examined and the simulation results are validated in an experimental test rig.

  • Active redundant submodule configuration in modular multilevel converters

     Konstantinou, Georgios; Pou Felix, Josep; Ceballos Recio, Salvador; Agelidis, Vassilios
    IEEE transactions on power delivery
    Vol. 28, num. 4, p. 2333-2341
    DOI: 10.1109/TPWRD.2013.2264950
    Date of publication: 2013-09
    Journal article

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    The modular multilevel converter (MMC) is based on the cascaded connection of identical submodules (SMs) enabling additional redundancies. This paper proposes the configuration of the MMC topology with redundant SMs and demonstrates the effects of active redundancies. The proposed configuration decreases the switching frequency per SM while reducing the SM capacitor voltage ripples. An analytical model for determining the SM capacitor voltage ripple and voltage dynamics is derived. The results from the analytical model are compared with the switching model for a 21-level MMC with five redundant (25 in total) SMs per arm. Experimental results based on a single-phase laboratory prototype with five SMs per arm and a single redundant cell further illustrate the operation and verify the derived mathematical model and simulation results.

  • Current improvement of a grid-connected photovoltaic system under unbalanced voltage conditions

     Mirhosseini, Mitra; Pou Felix, Josep; Agelidis, Vassilios
    IEEE Annual International Energy Conversion Congress and Exhibition for the Asia/Pacific
    p. 66-72
    DOI: 10.1109/ECCE-Asia.2013.6579075
    Presentation's date: 2013-06
    Presentation of work at congresses

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    This paper discusses the performance of a large-scale grid-connected photovoltaic system (GCPS) under unbalanced voltage sag conditions in the grid. By applying the droop control to address the low-voltage-ride-through (LVRT) capability, the output currents are no longer sinusoidal due to the oscillations produced in the current references. To deal with this problem a moving average filter (MAF) technique is proposed. Beside this application, there are other advantages of applying the MAF which are described in this paper. Selected simulation results using MATLAB/Simulink for a 1-MVA GCPS confirm the effectiveness of the proposed method.

    This paper discusses the performance of a large-scale grid-connected photovoltaic system (GCPS) under unbalanced voltage sag conditions in the grid. By applying the droop control to address the low-voltage-ride-through (LVRT) capability, the output currents are no longer sinusoidal due to the oscillations produced in the current references. To deal with this problem a moving average filter (MAF) technique is proposed. Beside this application, there are other advantages of applying the MAF which are described in this paper. Selected simulation results using MATLAB/Simulink for a 1-MV A GCPS confirm the effectiveness of the proposed method.

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    Hybrid sensorless permanent magnet synchronous machine four quadrant drive based on direct matrix converter  Open access

     Arias Pujol, Antoni; Ortega Garcia, Carlos; Zaragoza Bertomeu, Jordi; Espina, Jordi; Pou Felix, Josep
    International journal of electrical power and energy systems
    Vol. 45, num. 1, p. 78-86
    DOI: 10.1016/j.ijepes.2012.08.073
    Date of publication: 2013-02
    Journal article

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    Permanent Magnet Synchronous Machines (PMSMs) have several advantages, such as high ef¿ciency and low volume and weight, which make them attractive for aerospace applications and high performance servo drives. Matrix Converters (MCs) are an all-silicon alternative, with no bulky reactive elements, to the standard voltage source inverter. The most common control technique for such PMSM MC-fed drives is the so-called Field-Oriented Control (FOC), which requires the permanent magnet ¿ux position to achieve high dynamic performance. Encoders or resolvers are the most common sensing devices used for such a purpose, which not only increases the total cost of the PMSM drive but also adds extra electronics and cabling that may cause failures. This paper investigates and proposes an all range (from zero to full) speed hybrid sensorless FOC. The novelty of this paper relies on the use of a hybrid sensorless four quadrant FOC that averages the needed angle estimation from a model based angle estimator and a voltage pulse test injection angle estimator when feeding the PMSM with an MC instead of a standard voltage source inverter. Speed reversal and load impact simulation results are included, fully supporting the claims made in this paper

  • ESTRATEGIAS CONTROL GENERADORS OFFSHORE Y CONVERTIDORES MULTINIVEL CONECTADOS RED ELECTRICA SISTEMAS TRANSMISION HVDC

     Jaen Fernandez, Carlos; Arias Pujol, Antoni; Zaragoza Bertomeu, Jordi; Capella Frau, Gabriel Jose; Jarauta Bragulat, Eusebio; Hervada Sala, Carme; Buenestado Caballero, Pablo; Pou Felix, Josep
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  • Estrategias de control de generadores offshore y convertidores multinivel conectados a la red eléctrica en sistemas de transmisión de HVDC

     Pou Felix, Josep; Zaragoza Bertomeu, Jordi; Jaen Fernandez, Carlos; Arias Pujol, Antoni; Hervada Sala, Carme; Jarauta Bragulat, Eusebio; Buenestado Caballero, Pablo; Capella Frau, Gabriel Jose
    Competitive project

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  • PWM algorithm with adaptive offset for three-level multi-phase neutral-point-clamped converters

     López, Iraide; Ceballos Recio, Salvador; Andreu, Jon; Kortabarria, Iñigo; Pou Felix, Josep
    Annual Conference of the IEEE Industrial Electronics Society
    p. 1185-1190
    DOI: 10.1109/IECON.2013.6699301
    Presentation of work at congresses

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    This paper proposes a PWM based modulation strategy particularly useful for three-level multi-phase Neutral-Point-Clamped Converters. It achieves similar results to those obtained with the space vector modulation but allowing a very fast and simple implementation in a digital controller. The proposed algorithm is able to maintain the neutral-point voltage under control by the addition of a zero-sequence voltage component to the modulation signals. A simple and efficient methodology to calculate the most appropriate zero-sequence component is described. In addition the modulation strategy is completely general, regardless the number of phases of the converter, and therefore it is easy extendible to n-phase converters.

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    Evaluation of DC-link decoupling using electrolytic or polypropylene film capacitors in three-phase grid-connected photovoltaic inverters  Open access

     Karanayil, Baburaj; Agelidis, Vassilios; Pou Felix, Josep
    IEEE International Conference on Industrial Electronics
    p. 6980-6986
    DOI: 10.1109/IECON.2013.6700290
    Presentation of work at congresses

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    The life expectancy and long term reliability of grid-connected three-phase photovoltaic (PV) inverters can be increased by replacing the conventional electrolytic film capacitors by metallized polypropylene film capacitors. This paper presents a detailed evaluation of a three-phase gridconnected PV inverter performance when replacing the electrolytic capacitor with a minimum value of metallized polypropylene film capacitor-one. The minimum dc bus capacitance leads to larger voltage ripples. However, such ripples were found to be within acceptable limits to run the inverter satisfactorily. Simulation results are presented for a 15-kW gridconnected inverter at nominal voltage of 700V dc and experimental results are provided for a 3.0-kW system at a nominal voltage of 400V dc, built in the laboratory.

    The life expectancy and long term reliability of grid-connected three-phase photovoltaic (PV) inverters can be increased by replacing the conventional electrolytic film capacitors by metallized polypropylene film capacitors. This paper presents a detailed evaluation of a three-phase grid-connected PV inverter performance when replacing the electrolytic capacitor with a minimum value of metallized polypropylene film capacitor-one. The minimum dc bus capacitance leads to larger voltage ripples. However, such ripples were found to be within acceptable limits to run the inverter satisfactorily. Simulation results are presented for a 15-kW grid-connected inverter at nominal voltage of 700V dc and experimental results are provided for a 3.0-kW system at a nominal voltage of 400V dc, built in the laboratory.

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    Voltage balancing method for a seven-level stacked multicell converter using minimum-switching transitions  Open access

     Ghias, Amer M.Y.M; Pou Felix, Josep; Agelidis, Vassilios; Ciobotaru, Mihai
    Annual Conference of the IEEE Industrial Electronics Society
    p. 6328-6333
    DOI: 10.1109/IECON.2013.6700176
    Presentation of work at congresses

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    This paper presents a voltage balancing method for stacked multicell converters based on phase disposition pulse-width modulation. This method is based on minimizing a cost function to determine the optimum redundant state for capacitor voltage balance for each particular voltage level. The robustness of the proposed voltage balancing method is verified against static and dynamic unbalanced load conditions. Furthermore, a significant reduction in the switching frequencies of the power devices is achieved by using sawtooth carriers instead of standard triangular carriers without affecting the voltage balancing capability.

  • Circulating current control and evaluation of carrier dispositions in modular multilevel converters

     Darus, Rosheila; Pou Felix, Josep; Konstantinou, Georgios; Ceballos Recio, Salvador; Agelidis, Vassilios
    IEEE Annual International Energy Conversion Congress and Exhibition for the Asia/Pacific
    p. 332-338
    DOI: 10.1109/ECCE-Asia.2013.6579117
    Presentation of work at congresses

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    This paper presents two control techniques to minimize the circulating currents of the modular multilevel converter. The control techniques reduce the amplitude of each capacitor voltage ripple. Multicarrier level-shifted pulse-width-modulation is applied and the performance of interleaving and non-interleaving the carrier waveforms between the upper and the lower arms is reported. The total harmonic distortion of the output common voltage and rms value of the arms currents are benchmarked against the case when such techniques are not employed. The techniques have been verified by simulation using MATLAB/Simulink and PLECS Blockset software. Experimental results from a low power phase-leg prototype built with five sub-modules per arm are also provided.

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    Control strategy to balance operation of parallel connected legs of modular multilevel converters  Open access

     Pou Felix, Josep; Ceballos Recio, Salvador; Konstantinou, Georgios; Capella Frau, Gabriel Jose; Agelidis, Vassilios
    IEEE International Symposium on Industrial Electronics
    p. 1-7
    DOI: 10.1109/ISIE.2013.6563685
    Presentation of work at congresses

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    Connecting legs of modular multilevel converters (MMCs) in parallel can assist an MMC-based high-voltage direct current (HVDC) to increase its current ratings and hence overall power handling capability. Consequently, each phase of the MMC would be integrated by several legs or sets of upper and lower arms (ULAs). This paper proposes a current control strategy for each ULA in order to ensure balanced current sharing among them. In addition, each ULA has its own circulating current control that follows a reference obtained from the instantaneous magnitudes of the output current and the modulation signal. All the proposed control actuations do not distort the phase output voltage of the MMC which follows the reference voltage. The performance of the proposed control strategies is evaluated by simulation studies in the PLECS Blockset under MATLAB/Simulink software platform.

    Postprint (author’s final draft)

  • Optimal injection of harmonics in circulating currents of modular multilevel converters for capacitor voltage ripple minimization

     Picas Prat, Ricard; Pou Felix, Josep; Ceballos Recio, Salvador; Zaragoza Bertomeu, Jordi; Konstantinou, Georgios; Agelidis, Vassilios
    IEEE Annual International Energy Conversion Congress and Exhibition for the Asia/Pacific
    p. 318-324
    DOI: 10.1109/ECCE-Asia.2013.6579115
    Presentation of work at congresses

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    This paper proposes an algorithm to calculate the optimal amplitude and phase of the harmonic current components that can be injected in the circulating currents of a modular multilevel converter (MMC) to minimize the capacitor voltage fluctuations. An optimal second harmonic component and an optimal set of second and fourth harmonic components are proposed. Simulation results are obtained in MATLAB/Simulink environment to study the effectiveness of the calculated optimal currents. Selected experimental results have been obtained from an MMC laboratory prototype, testing the effects of the circulating currents. The reported results demonstrate the effectiveness of using a fourth harmonic component in the circulating current, which improves the effect of the second harmonic on reducing the capacitor voltage fluctuations.

    This paper proposes an algorithm to calculate the optimal amplitude and phase of the harmonic current components that can be injected in the circulating currents of a modular multilevel converter (MMC) to minimize the capacitor voltage fluctuations. An optimal second harmonic component and an optimal set of second and fourth harmonic components are proposed. Simulation results are obtained in MATLAB/Simulink environment to study the effectiveness of the calculated optimal currents. Selected experimental results have been obtained from an MMC laboratory prototype, testing the effects of the circulating currents. The reported results demonstrate the effectiveness of using a fourth harmonic component in the circulating current, which improves the effect of the second harmonic on reducing the capacitor voltage fluctuations.

  • Voltage balancing method for the multilevel flying capacitor converter using phase-shifted PWM

     Ghias, Amer M.Y.M; Pou Felix, Josep; Ciobotaru, M.; Agelidis, Vassilios
    IEEE International Conference on Power and Energy
    p. 274-279
    DOI: 10.1109/PECon.2012.6450221
    Presentation's date: 2012-12
    Presentation of work at congresses

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    n flying capacitor (FC) converters, phase-shifted pulse-width modulation (PS-PWM) provides natural voltage balancing. However, for a practical application, a more robust balancing mechanism of maintaining the FC voltages at the desired values is required. This paper proposes a new closed-loop voltage balancing method for multilevel FC converters using PS-PWM. The proposed method balances the voltages of the FCs by modifying the duty cycle of each switch of the FC converter using a proportional (P) controller. The crossed effect between FC currents and duty cycles is considered and is used for FC voltage balancing. The Simulation results verify that the proposed voltage balancing method is very robust to different operating conditions, such as load transients and non-linear loads.

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    Voltage balancing scheme for the multilevel flying capacitor converter using phase-shifted PWM  Open access

     Ghias, Amer M.Y.M; Pou Felix, Josep; Ciobotaru, M.; Agelidis, Vassilios
    IEEE International Conference on Power and Energy
    p. 1-6
    DOI: 10.1109/IECON.2012.6389568
    Presentation's date: 2012-12
    Presentation of work at congresses

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    In flying capacitor (FC) converters, phase-shifted pulse-width modulation (PS-PWM) provides natural voltage balancing. However, for a practical application, a more robust balancing mechanism of maintaining the FC voltages at the desired values is required. This paper proposes a new closed-loop voltage balancing method for multilevel FC converters using PS-PWM. The proposed method balances the voltages of the FCs by modifying the duty cycle of each switch of the FC converter using a proportional (P) controller. The crossed effect between FC currents and duty cycles is considered and is used for FC voltage balancing. The Simulation results verify that the proposed voltage balancing method is very robust to different operating conditions, such as load transients and non-linear loads.

    Postprint (author’s final draft)

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    Voltage balancing of a five-level flying capacitor converter using optimum switching transitions  Open access

     Ghias, Amer M.Y.M; Pou Felix, Josep; Ciobotaru, M.; Agelidis, Vassilios
    IEEE International Conference on Industrial Electronics
    p. 1-6
    Presentation's date: 2012-10
    Presentation of work at congresses

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    The flying capacitor (FC) multilevel converter requires a capacitor voltage balancing mechanism for proper operation. This paper proposes a voltage balancing scheme for a five-level FC converter based on phase disposition pulse-width modulation (PD-PWM) using transitions that produce the minimum number of switching events. This strategy will be called optimal-transition voltage balancing (OTVB) scheme. Since multiple optimum switching transitions between two consecutive voltage levels may be available, such a redundancy is used to regulate the FC voltages at their desired levels. Those transitions that produce more switching events are avoided. The rest of transitions are evaluated by means of a cost function and the one that produces the minimum value is selected. Simulation results show a significant reduction of the average switching frequency as compared to the use of the optimal-state voltage balancing (OSVB) scheme, while maintaining the balance of the FC voltages. Moreover, the proposed PD-PWM voltage balancing strategy is robust to static and dynamic load conditions.

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    Minimization of the capacitor voltage fluctuations of a modular multilevel converter by circulating current control  Open access

     Picas Prat, Ricard; Pou Felix, Josep; Ceballos Recio, Salvador; Agelidis, Vassilios; Saeedifard, Maryam
    IEEE International Conference on Industrial Electronics
    p. 1-7
    Presentation's date: 2012-10
    Presentation of work at congresses

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    The modular multilevel converter (MMC) is one of the most potential converter topologies for medium/high power/voltage applications. One of the main technical challenges of an MMC is to eliminate/minimize the circulating currents within the legs. Circulating currents, if not properly controlled, increase the amplitude of capacitor voltage variations, rating values of the converter components and converter losses. This paper proposes a closed-loop circulating current control strategy for an MMC to specifically minimize the amplitude of capacitor voltage variations. The proposed strategy is based on adding an offset signal to the modulating signal of each arm. To minimize the amplitude of the capacitor voltage oscillations, an optimal circulating current component is determined and used as a reference signal for the current control of each MMC leg. Performance of the proposed control strategy is evaluated based on simulation studies in the MATLAB/Simulink environment. The reported study results demonstrate effectiveness of the proposed strategy to reduce the amplitude of the capacitor voltage oscillations.

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    Voltage balancing strategy for a five-level flying capacitor converter using phase disposition PWM with sawtooth-shaped arriers  Open access

     Ghias, Amer M.Y.M; Pou Felix, Josep; Ciobotaru, M.; Agelidis, Vassilios
    IEEE International Conference on Industrial Electronics
    p. 1-6
    Presentation's date: 2012-10
    Presentation of work at congresses

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    The flying capacitor (FC) multilevel converter has attracted a great deal of interest in the recent years because of its easier extension to a higher number of levels (n>;3), as compared to its counterpart, the diode-clamped converter (DCC). The main focus of this paper is to develop a voltage balancing scheme of FCs for a five-level FC converter based on phase disposition pulse-width modulation (PD-PWM). Since there are multiple states that produce the same output voltage at the leg of the converter, such a redundancy is used to regulate the FC voltages at their desired levels. The selection of the optimal states is performed by minimizing a cost function. A drawback observed when using standard symmetrical triangular carriers for the PD-PWM, is the additional switching events that are produced due to transitions within the same voltage level. Nevertheless, this fact can be avoided by using sawtooth carrier waveforms instead. Simulation results verify the robustness of the proposed voltage balancing scheme against static and dynamic load conditions. Moreover, using sawtooth carriers a significant reduction of the switching frequency is achieved as compared to the use of standard triangle carriers while maintaining the FC voltage balanced.

    Postprint (author’s final draft)

  • Solid State Transformer Based on the Flying Capacitor Multilevel Converter for Intelligent Power Management

     Ghias, Amer M.Y.M; Ciobotaru, M.; Agelidis, Vassilios; Pou Felix, Josep
    IEEE Power & Energy Society General Meeting
    p. 1-7
    DOI: 10.1109/PowerAfrica.2012.6498659
    Presentation's date: 2012-07
    Presentation of work at congresses

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    Future grids will consist of large scale of integration of both renewable and other distributed energy sources. Therefore, advanced power electronics converters for critical loads will be needed in order to enhance power quality, and to ensure proper and secure operation of future grids. These power converters must be able to provide intelligent power management as well as ancillary services. This paper proposes a solid state transformer (SST) which consists of medium frequency transformer and three-level flying capacitor (FC) topology for intelligent power management. It is tested under various grid events such as voltage excursions, phase jumps and frequency variations. Moreover, it is tested against load disturbances such as load transients and non-linear loads. The SST enhances power quality compatibility and provides good isolation of perturbances between the grid and the loads. The SST is also able to deliver ride through capabilities, which can isolate short term disturbances from the grid to the loads and vice-versa. The Power quality enhancement features are validated under various source and load conditions using computer simulation.

    Future grids will consist of large scale of integration of both renewable and other distributed energy sources. Therefore, advanced power electronics converters for critical loads will be needed in order to enhance power quality, and to ensure proper and secure operation of future grids. These power converters must be able to provide intelligent power management as well as ancillary services. This paper proposes a solid state transformer (SST) which consists of medium frequency transformer and threelevel flying capacitor (FC) topology for intelligent power management. It is tested under various grid events such as voltage excursions, phase jumps and frequency variations. Moreover, it is tested against load disturbances such as load transients and nonlinear loads. The SST enhances power quality compatibility and provides good isolation of perturbances between the grid and the loads. The SST is also able to deliver ride through capabilities, which can isolate short term disturbances from the grid to the loads and vice-versa. The Power quality enhancement features are validated under various source and load conditions using computer simulation.

  • A Carrier-Based PWM Strategy With Zero-Sequence Voltage Injection for a Three-Level Neutral-Point-Clamped Converter

     Pou Felix, Josep; Zaragoza Bertomeu, Jordi; Ceballos Recio, Salvador; Saeedifard, Maryam; Boroyevich, Dushan
    IEEE transactions on power electronics
    Vol. 27, num. 2, p. 642-651
    DOI: 10.1109/TPEL.2010.2050783
    Date of publication: 2012-01-09
    Journal article

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  • Tuning of controllers in grid-connected multilevel converters

     Robles Sestafe, Eider; Ceballos Recio, Salvador; Pou Felix, Josep; Salcedo, F.; Santos, Maider
    International Review on Modelling and Simulations
    Vol. 5, num. 3, p. 1165-1174
    Date of publication: 2012
    Journal article

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  • Estudi de viabilitat d'implentació d'energies renovables a través de microxarxes en els pobles del Pirineu (RURALGRID)

     Hervada Sala, Carme; Arias Pujol, Antoni; Jaen Fernandez, Carlos; Capella Frau, Gabriel Jose; Zaragoza Bertomeu, Jordi; Pou Felix, Josep
    Competitive project

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  • Performance evaluation of a five-level flying capacitor converter with reduced DC bus capacitance under two different modulation schemes

     Ghias, Amer M.Y.M; Pou Felix, Josep; Ciobotaru, M.; Agelidis, Vassilios
    IEEE International Symposium on Power Electronics for Distributed Generation Systems
    p. 857-864
    DOI: 10.1109/PEDG.2012.6254101
    Presentation's date: 2012
    Presentation of work at congresses

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    In flying capacitor (FC) converters, phase-shifted pulse-width modulation (PS-PWM) provides natural voltage balancing. However, for a practical application, a more robust balancing mechanism of maintaining the FC voltages at the desired values is required. This paper proposes a new closed-loop voltage balancing method for multilevel FC converters using PSPWM. The proposed method balances the voltages of the FCs by modifying the duty cycle of each switch of the FC converter using a proportional (P) controller. The crossed effect between FC currents and duty cycles is considered and is used for FC voltage balancing. The Simulation results verify that the proposed voltage balancing method is very robust to different operating conditions, such as load transients and non-linear loads.

  • Dynamic behaviour of a back-to-back five-level flying capacitor converter with reduced DC bus capacitance

     Ghias, Amer M.Y.M; Ciobotaru, M.; Agelidis, Vassilios; Pou Felix, Josep
    IEEE Energy Conversion Congress and Exposition
    p. 1741-1746
    DOI: 10.1109/IPEMC.2012.6259099
    Presentation's date: 2012
    Presentation of work at congresses

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    Due to extensive research, advanced voltage source converter (VSC) such as multilevel and multi-module have been proposed for high power applications. These advanced VSCs must provide secure as well as, intelligent power management operation for the future electricity network under various grid events. The focus of this paper is to evaluate the dynamic performance of a back-to-back five-level flying capacitor (FC) converter system with reduced DC bus capacitance under different grid voltage disturbances. The system uses a classical synchronous reference frame control technique for active/reactive power control and DC voltage regulation, and is tested in terms of general grid events such as power flow reversal, voltage amplitude excursions, phase jumps and frequency variations using MATLAB/Simulink and PLECS blockset. The simulation results show that the back-to-back FC converter with reduced DC bus capacitance performs well during the mentioned grid events.

  • MODULATION STRATEGIES FOR THE NEUTRAL-POINT-CLAMPED CONVERTER AND CONTROL OF A WIND TURBINE SYSTEM  Open access

     Zaragoza Bertomeu, Jordi
    Department of Electronic Engineering, Universitat Politècnica de Catalunya
    Theses

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    Els convertidors multinivell són topologies de convertidors d’electrònica de potència que poden generar tres o més nivells de voltatge en cadascuna de les fases de sortida. Com a resultat, els voltatges i corrents generats per aquestes topologies presenten una distorsió harmònica baixa. Hi ha diferents configuracions de convertidors multinivell, les quals es basen en connectar dispositius de potència o convertidors en sèrie. El resultat d’aquestes connexions permet obtenir voltatges alts, tant en la part de corrent continu com en la de corrent altern del convertidor. A més, cada dispositiu sols ha de suportar una fracció del voltatge total del bus de corrent continu. Per aquestes raons, els convertidors multinivell són generalment utilitzats en aplicacions d’alta potència. El convertidor de tres nivells amb connexió a punt neutre (neutral-point-clamped) és el més utilitzat. La recerca d’aquesta tesis doctoral està focalitzada en aquesta topologia de convertidor, i el principal objectiu és l’aportació de noves tècniques de modulació. Aquestes tenen en compte diferents aspectes: la velocitat computacional dels algorismes, l’equilibrat de les tensions dels condensadors del bus de contínua, les pèrdues de commutació i les oscil·lacions de baixa freqüència en el punt neutre del convertidor. Totes les estratègies de modulació proposades en aquesta tesis són modulacions d’amplada de polses basades en portadora. En la primera modulació que es presenta, s’injecta un senyal comú (seqüència zero) a totes les moduladores, que es basa en els patrons de la modulació vectorial que utilitza tres vectors dels més propers al de referència (nearest-three-vector modulation). S’estudien i es comparen els resultats d’aquesta modulació amb la seva homòloga, basada en perspectiva vectorial. Una segona proposta és l’anomenada modulació d’amplada de polses de doble senyal (double-signal pulse-width modulation). Aquesta modulació és capaç d’eliminar completament les oscil·lacions de voltatge en el punt neutre del convertidor. No obstant això, es produeix un increment de les pèrdues de commutació en els dispositius de potència i, a més, no hi ha un equilibrat natural de les tensions en els condensadors del bus. Una última estratègia de modulació, anomenada modulació híbrida (hybrid pulse-width modulation), es basa en la combinació de la modulació sinusoïdal (sinusoidal pulse-width modulation) i la de doble senyal. Aquesta presenta una solució de compromís entre reduir les pèrdues de commutació, en detriment d’un augment de l’amplitud de les oscil·lacions de voltatge en el punt neutre. Una segona part d’aquesta tesis es centra en les aplicacions a generació eòlica, ja que els convertidors multinivell estan començant a ser utilitzats en aquest camp. Això es produeix fonamentalment per l’augment continu de les dimensions de les turbines eòliques. En aquesta part de la recerca s’ha considerat la configuració de dos convertidors multinivell connectats a un mateix bus de contínua (back-to-back), tot i que els convertidors han estat estudiats independentment. Inicialment s’ha estudiat el convertidor que va connectat a la xarxa elèctrica i s’ha aplicat l’estratègia de control coneguda com a control orientat a tensió (voltage-oriented control). S’han utilitzat controladors estàndard (proporcional-integral), als quals s’ha afegit un control difús que supervisa i modifica els valors de les constants dels controladors. Aquest supervisor difús millora la dinàmica de la tensió del bus de contínua davant canvis de càrrega quan el convertidor treballa com a rectificador. Per una altra part, s’ha estudiat el control d’una turbina eòlica basada en un generador d’imants permanents. En aquest cas, s’ha aplicat l’estratègia de control coneguda com a control orientat a camp (field-oriented control). S’han avaluat i comparat els avantatges i inconvenients de diferents formes de sintonitzar els controladors.

    Multilevel converters are power electronic topologies that can generate three or more voltage levels in each output phase. As a result, the voltage and current waveforms generated have lower total harmonic distortion. Multilevel topologies are based on connecting power devices or converters in a series. Consequently, high voltages can be handled on the dc and ac sides of the converter, while each device stands only a fraction of the total dc-link voltage. For these reasons multilevel converters are generally applied to high-power applications. The three-level neutral-point-clamped converter is the most extensively used multilevel topology. This topology is the main focus of research in this dissertation. The main objective is to propose new modulation strategies that are able to meet a compromise solution while considering computational algorithm speed, voltage balance in the dc-link capacitors, switching losses and low frequency voltage oscillations at the neutral point. All the modulation strategies proposed here are based on carrier-based pulsewidth modulation. A new modulation strategy has been implemented using a proper zero-sequence signal injected into the modulation signals. The zero sequence is determined from a space-vector modulation standpoint, particularly the nearest-threevector modulation strategy. The proposed carrier-based technique is compared with its space-vector modulation counterpart. It shows some advantages, such as easier implementation and reduced switching events; however, it still produces oscillations in the neutral-point voltage for some operating conditions. A new modulation strategy able to completely remove such voltage oscillations is also presented. It is called double-signal pulse-width modulation. The main drawback of this strategy is that it increases the switching frequency of the power devices and has no natural capacitor voltage balance. Some balancing strategies are proposed in this dissertation for this specific modulation. Furthermore, a hybrid pulse-width modulation approach is presented which is able to combine sinusoidal pulse-width modulation with doublesignal pulse-width modulation; this represents a compromise solution between switching losses and neutral-point voltage oscillation amplitudes. The second part of this thesis is focused on wind generation applications. Multilevel converters are starting to be used in such a field nowadays, and are expected to be further applied in the near future as the sizes of wind turbines grow. Two back-toback-connected power converters are considered in this application, although they are analyzed independently. First of all, the control of the grid-connected converter is studied. A voltage-oriented control is used with standard proportional-integral controllers. The originality of the method is that a fuzzy supervisor is designed and included in the structure; the fuzzy supervisor is able to modify the proportionalintegral parameters online. It is shown how the control of the total dc-link voltage improves significantly under load changes when the converter is working as a rectifier. On the other hand, a control study is performed on the wind turbine side. The variable speed wind turbine is based on a permanent magnet synchronous generator. A field-oriented control strategy is applied. The controllers are evaluated and compared using different tuning strategies which highlight the advantages and drawbacks of each.

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    Analysis of voltage balancing limits in modular multilevel converters  Open access

     Ceballos Recio, Salvador; Pou Felix, Josep; Choi, Sanghun; Saeedifard, Maryam; Agelidis, Vassilios
    Annual Conference of the IEEE Industrial Electronics Society
    p. 4397-4402
    Presentation's date: 2011-11-08
    Presentation of work at congresses

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    The modular multilevel converter (MMC) is one of the most promising converter topologies for high-voltage applications, especially for high-voltage direct-current (HVDC) transmission systems. One of the most challenging issues associated with the MMC is the capacitor voltage variations, which if not properly controlled, result in large circulating currents flowing through the converter legs. This paper develops a mathematical model to formulate and analyze capacitor voltage variations and the circulating currents within the MMC legs. Based on the developed model, the limits to the capacitor voltage balancing task are derived and graphically presented. A set of simulation results conducted in MATLAB/Simulink environment are presented to verify the accuracy of the mathematical analysis.

  • Frequency-Adaptive Stationary-Reference-Frame Grid Voltage Sequence Detector for Distributed Generation Systems

     Robles Sestafe, Eider; Pou Felix, Josep; Ceballos Recio, Salvador; Zaragoza Bertomeu, Jordi; Martin, Jose Luis; Ibáñez, P.
    IEEE transactions on industrial electronics
    Vol. 58, num. 9, p. 4275-4287
    DOI: 10.1109/TIE.2010.2098352
    Date of publication: 2011-09
    Journal article

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