Fernanda Carnielutti

Generalized Carrier-Based Modulation Strategy for Cascaded Multilevel Converters Operating Under Fault Conditions

This paper proposes a simple modulation approach for symmetrical and asymmetrical cascaded multilevel converters, suitable for operation under normal and fault conditions. Initially, the set of all possible converter common-mode voltages for linear operation is derived, considering the status of each converter cell. It is demonstrated that the methods previously described in the literature belong to the derived set. By properly selecting the common-mode voltage, the entire converter voltage synthesis capability can be achieved under any fault condition. The proposed modulation approach has been applied for both symmetrical and asymmetrical cascaded multilevel converters. Simulation and experimental results show the effectiveness of the proposed modulation approach and validate the theoretical analysis.

Hybrid Modulation Strategy for Asymmetrical Cascaded Multilevel Converters Under Normal and Fault Conditions

This paper proposes a hybrid modulation strategy for asymmetrical cascaded multilevel converters operating under normal and faulty conditions on power cells. The proposed algorithm explores the advantages of both space vector (SV) and carrier-based modulation approaches. An SV is used for higher voltage cells to properly choose their voltage vectors in a way such as to avoid converter saturation, particularly under fault conditions. A generic algorithm to identify the domains of higher voltage vectors in which a reference lies, without the need for separation lines, is proposed. Then, to simplify the implementation of the algorithm, carrier-based modulation is employed for lower voltage cells, which switch with pulsewidth modulation. This suppresses the need for defining multiple switching sequences for different operational conditions, and it avoids converter saturation as much as possible, particularly under fault conditions on power cells. Simulation and experimental results for normal and faulty operational conditions are given, validating the theoretical analysis and demonstrating the good performance of the proposed hybrid modulation approach.

Space vector modulation for asymmetrical cascaded multilevel converters

This paper describes a new Space Vector modulation (SVM) strategy for asymmetrical cascaded multilevel converters. A seven-level asymmetrical multilevel cascaded converter is considered. The modulation is carried out in a new coordinate system and is divided into two steps. First, the nearest switching-state vector to the reference vector is chosen for the high-voltage-low-frequency cells, and the synthesized voltage vector is subtracted from the reference, resulting in the reference for the low-voltage-high-frequency cells. Afterwards, the three-nearest low-voltage switching-state vectors are selected, and, for each sector of the SV diagram, a switching sequence is defined, aiming, for instance, to minimize the number of commutations. Simulation results are given to validate the proposed method, which is compared to the phase-shift and phase disposition carrier-based approaches, showing its superior performance.

Extending the Operation of Asymmetrical Cascaded Multilevel Converters Under Fault Conditions on the Converter Power Cells

This paper proposes a generalized hybrid modulation strategy to extend the operation of asymmetrical cascaded multilevel converters into the overmodulation region during fault conditions on the power cells. The proposed strategy makes use of the advantages of both space vector and carrier-based geometrical modulations to allow the converter to operate with a wide range of modulation indexes and fault conditions. Also, it does not need to precalculate various trajectories for the voltage reference, which would be a very complex task, as the shape of the converter space vector diagram is different for each type of fault condition. It is shown that different kinds of overmodulation can happen during faults on the power cells, and solutions are presented for each mode of operation. Finally, simulation and experimental results are shown to demonstrate the good performance of the proposed modulation strategy.

New modulation strategy for asymmetrical cascaded multilevel converters under fault conditions

This paper proposes a new and simple hybrid modulation strategy for asymmetrical cascaded multilevel converters (ACMCs) operating under fault conditions in the low-voltage-high-frequency (LVHF) cells. The theoretical analysis is explained through an example, an ACMC with one 1pu and one 2pu cell per phase. The modulation strategy integrates the benefits of both geometrical and space vector approaches. The last one is used to define the voltages for the high-voltage-low-frequency (HVLF) cells, such as to avoid the overmodulation of the LVHF cells. The carrier-based geometrical approach is employed for the LVHF cells to simplify the implementation of the algorithm, since the PWM signals are obtained via the comparison of modulating voltages to triangular carriers, with no need to define switching sequences for the low voltage vectors. As a result, the proposed algorithm is considerably simplified when compared to the space vector. Simulation and experimental results are given to validate the proposed method and to demonstrate its good performance.

A robust PI current controller for grid-connected renewable energy systems

This paper presents a robust PI current controller for three-phase grid-connected converters with LCL-filter for renewable energy systems. The controller is designed in synchronous reference frame and the grid impedance uncertainty is considered in the design. The controller consists of a PI current controller and state-feedback, where the gains of the PI controller and the gains of the state-feedback are designed together in a one-step procedure. The robustness of the current controller is ensured by an LMI approach, which guarantees closed-loop stability in the whole range for the considered grid impedance uncertainty.

Sorting algorithm for a pd modulation for a cascaded multilevel converter

This paper proposes a simple phase disposition modulation with sorting algorithm for three-phase cascaded multilevel converters. The sorting algorithm can be implemented entirely in software. The main features of the proposed modulation is a better distribution of PWM pulses, switching losses and DC bus current when compared with the conventional phase disposition (PD) modulation. In addition, it results in the nearest level PWM in the line-to-line output voltages leading to a smaller THD if compared with phase-shifted (PS) modulation. Simulation results are presented to demonstrate the good performance of the proposed algorithm.

Algorithm for on-line definition of switching sequences for space vector modulation of asymmetrical cascaded multilevel converters

This paper proposes an algorithm for the on-line definition of switching sequences for Space Vector Modulation (SVM) of the low voltage power cells of asymmetrical multilevel cascaded converters. Initially, an algorithm to define on-line the redundancies of the three nearest low voltage vectors is described in details. The first vector of the new sequence is chosen so as to minimize the switching losses between two consecutive reference samples. Then, the remaining vectors are organized within the switching period in a way such as to have the least number of commutations. As a result, minimum switching losses is achieved, even in the transition between sectors. Furthermore, the algorithm can also define sequences for any fault condition, avoiding the use of different look-up tables. Finally, experimental results are shown to validate the proposed algorithm.

Space vector modulation for cascaded asymmetrical multilevel converters under fault conditions

This paper proposes a space vector modulation (SVM) strategy for asymmetrical cascaded multilevel converters operating under fault conditions on the power cells. During each switching interval, one voltage vector is chosen to be implemented by the high voltage cells. It is then subtracted from the reference, resulting in a new reference for the low voltage cells. However, under fault conditions, if the high voltage vectors are not properly chosen, the converter may saturate. This paper presents a comparison between the choice of the high voltage vector as the nearest one to the reference, and as the one whose domain encompasses it. It is shown that the second choice is better, as it avoids saturation of the converter as much as possible. The proposed SVM is performed in the output line-to-line voltages coordinate system, in which the converter voltage vectors have only integer entries. The algorithm is presented considering as an example a converter with two cells per phase: one of 1 pu that switches with PWM, and one of 2 pu, switching at the fundamental frequency, respectively called low-voltage-high-frequency and high-voltage-low-frequency cells. Finally, experimental results are given to demonstrate and validate the good performance of the proposed SVM.

Hybrid modulation for asymmetrical cascaded multilevel converters under fault conditions

This paper proposes a simple hybrid modulation strategy for asymmetrical cascaded multilevel converters (ACMCs) with fault conditions. The algorithm is described by means of an example, considering a converter with one 1pu cell and one 2pu cell per phase. The modulation integrates the benefits of both the geometrical and space vector approaches. The last one is used to define the voltages to be synthesized by the high-voltage-low-frequency (HVLF) cells, avoiding the overmodulation of the low-voltage-high-frequency LVHF cells. The carrier-based geometrical approach is used for the LVHF cells to simplify the implementation of the algorithm. As a result, this new modulation is considerably simplified when compared to the space vector. Finally, experimental results are shown to validate the proposed modulation strategy.