Unidimensional modulation technique for cascaded multilevel converters

This paper presents a simple and low computational cost modulation technique for multilevel cascaded H-bridge converters. The technique is based on geometrical considerations considering an unidimensional control region to determine the switching sequence and the corresponding switching times. In addition, a simple strategy to control the dc voltage ratio between the H-bridges of the multilevel cascaded converter is presented. Examples for the two-cell topology are shown but the proposed technique can be applied to develop modulation methods for a higher number of H-bridges. Experimental results are presented to validate the proposed technique

The Essential Role and the Continuous Evolution of Modulation Techniques for Voltage-Source Inverters in the Past, Present, and Future Power Electronics

The cost reduction of power-electronic devices, the increase in their reliability, efficiency, and power capability, and lower development times, together with more demanding application requirements, has driven the development of several new inverter topologies recently introduced in the industry, particularly medium-voltage converters. New more complex inverter topologies and new application fields come along with additional control challenges, such as voltage imbalances, power-quality issues, higher efficiency needs, and fault-tolerant operation, which necessarily requires the parallel development of modulation schemes. Therefore, recently, there have been significant advances in the field of modulation of dc/ac converters, which conceptually has been dominated during the last several decades almost exclusively by classic pulse-width modulation (PWM) methods. This paper aims to concentrate and discuss the latest developments on this exciting technology, to provide insight on where the state-of-the-art stands today, and analyze the trends and challenges driving its future

Multi-dimensional modulation technique for cascaded multilevel converters

Multilevel cascaded H-bridge converters have found industrial application in the medium voltage high power range. In this paper, a generalized modulation technique for this type of converters based on a multi-dimensional control region is presented. Using the multi-dimensional control region, it is shown that all previous modulation techniques are particularized versions of the proposed method. Several possible solutions to develop a specific implementation of the modulation method are addressed in order to show the potential possibilities and the flexibility of the proposed technique. In addition, a feed-forward version of this technique is also introduced to determine the switching sequence and the switching times avoiding low harmonic distortion with unbalanced dc voltages. Experimental results are shown in order to validate the proposed concepts.Ministerio de Educación TEC2007-6187

One dimensional feed-forward modulation of a cascaded H-bridge multi-level converter including capacitor balancing with reduced switching frequency

Multi-level converters are well suited to medium and high power applications where the priority is for high quality a.c. currents with low harmonic content and for low switching losses. Suitable modulation schemes must find a good compromise between these two conflicting aims. This paper relates one-dimensional feed-forward modulation (1DFFM) to carrier based modulation. 1DFFM can produce high quality currents despite d.c. capacitor voltage ripple by accounting for measured capacitor voltages in the modulation process and can balance capacitor voltages by eliminating states that cause the capacitor voltages to diverge. It is shown in this paper that this method increases the device switching frequency compared to carrier-based modulation with the same sampling rate. An alternative modulation method is presented which balances the capacitor voltages by assigning commutations to bridges during each sampling period. This approach is less likely to cause two bridges to commutate at the same time so produces a lower device switching frequency than 1DFFM. It is shown experimentally that the two methods produce similar a.c. current distortion and both effectively balance the capacitor voltages

Two-dimensional modulation technique with dc voltage control for single-phase two-cell cascaded converters

In this paper, a simple feed-forward modulation technique for single-phase two-cell multilevel cascaded converters is presented. All the possible switching states of the power converter are taken into account applying a two dimensional control region. The proposed technique uses the actual values of the DC-Link capacitor voltages to obtain output phase voltages and currents with low harmonic distortion with any dc voltage in the H-bridges of the cascaded converter. The possible switching sequences of the converter are studied and, depending on the actual dc voltage values, their desired values are achieved. Simulation results are shown in order validate the proposed technique working as a synchronous rectifier

Active DC voltage balancing PWM technique for high-power cascaded multilevel converters

In this paper a dedicated PWM technique specifically designed for single-phase (or four wire three-phase) multilevel Cascaded H-Bridge Converters is presented. The aim of the proposed technique is to minimize the DC-Link voltage unbalance, independently from the amplitude of the DC-Link voltage reference, and compensate the switching device voltage drops and on-state resistances. Such compensation can be used to achieve an increase in the waveform quality of the converter. This is particularly useful in high-power, low supply voltage applications where a low switching frequency is used. The DC-Link voltage balancing capability of the method removes the requirement for additional control loops to actively balance the DC-Link voltage on each H-Bridge, simplifying the control structure. The proposed modulation technique has been validated through the use of simulation and extensive experimental testing to confirm its effectiveness

Neutral-point-clamped DC-AC power converters

This article reviews the fundamentals of multilevel multiphase neutral-point-clamped DC–AC power converters. These converters are configured with one or more legs and a common DC-link. Each leg is functionally equivalent to a single-pole multiple-throw switch (n¿=¿3 positions) and it is implemented with a combination of only power semiconductor devices. The main leg topologies are initially presented, both active(transistor) clamped and passive(diode) clamped, for any number of levels. The leg switching states enabling all possible leg positions are subsequently discussed. Then, the set of all possible converter switching states and their standard representation in the converter space vector diagram is systematically derived, starting from the simplest converter (single-phase single-leg) up to the most complex converter with an arbitrary number of phases. The space vector diagram is illustrated for three-, four-, and n-levels in the usual single-phase and three-phase cases. Once the converter states have been characterized, the most common converter control approaches are introduced on the basis of the space vector diagram, including space vector control, space vector modulation, programmed pulse width modulation, hysteresis control, and predictive control. These control strategies are illustrated in simple and conventional DC–AC converter configurations. Finally, the article discusses the DC-link capacitor voltage balance problem, which is inherent to these topologies whenever the DC-link is configured with a capacitive voltage divider. The basics of the different solutions to guarantee the balancing are presented, both through the inclusion of additional hardware and through the application of a suitable converter control strategyPeer ReviewedPostprint (published version

Investigation on Cascade Multilevel inverter for Medium and High-Power Applications

It is hard to connect a single power semiconductor switch directly to medium voltage grids (2.3, 3.3, 4.16, or 6.9 kV). For these reasons, a new family of multilevel inverters has emerged as the solution for working with higher voltage levels. Multilevel inverters have received more attention in industrial application, such as motor drives, static VAR compensators and renewable energy systems, etc. Primarily multilevel inverters are known to have output voltages with more than two levels. As a result, the inverter output voltages have reduced harmonic distortions and high quality of waveforms. Additionally, the devices are confined to fraction of dc-link voltage. These characteristics make multilevel inverter to adopt for high-power and high-voltage applications. A good number of multilevel inverter topologies have been proposed during the last two decades. Contemporary research has engaged novel converter topologies and unique modulation schemes. Moreover, four major multilevel inverter structures have been reported in the literature these are as follows: cascaded H-bridges inverter (CHB) with separate dc sources, diode clamped (neutral-clamped), and flying capacitors (capacitor clamped), P2 Multilevel inverters. Although different multilevel inverter exists, Cascade Multilevel Inverter (CMI) is one of the productive topology from multilevel family. In reality, on comparing with other multilevel based topologies, CMI feature a high modularity degree because each inverter can be seen as a module with similar circuit topology, control structure, and modulation. Therefore, in the case of a fault in one of these modules, it is possible to replace it quickly and easily. Moreover, with an appropriated control strategy, it is possible to bypass the faulty module without stopping the load, bringing an almost continuous overall availability. All this features make CMI an outstanding power converter. However, one of the greatest limitations of CMI is utilization of separate DC source for each H-Bridge cell. This not only increases cost but also affects the reliability of the system. This is the key motivation for this dissertation. In the present work, we have investigated different CMI based topologies with separate and single DC sources and finally proposed a new CMI based configuration with single dc source by using three-phase transformers. The proposed CMI based inverter presented in this thesis is well defined with logical and mathematical approach. Additionally to illustrate the merits, it is compared with traditional multilevel inverters. The feasibility of proposed inverter is demonstrated with different illustrations and confirmed by experimental results. The proposed CMI is well suited for grid / photovoltaic and FACTS systems. To elevate the application of proposed CMI a shunt active power filter (APF) design is demonstrated. In this case, the goal is to inject, in parallel with the load, compensation current to get a sinusoidal source current. The proposed APF is verified through Matlabsimulation. Finally, Opal-RT verifications are performed to verify the final design

Design and Control of Power Converters 2020

In this book, nine papers focusing on different fields of power electronics are gathered, all of which are in line with the present trends in research and industry. Given the generality of the Special Issue, the covered topics range from electrothermal models and losses models in semiconductors and magnetics to converters used in high-power applications. In this last case, the papers address specific problems such as the distortion due to zero-current detection or fault investigation using the fast Fourier transform, all being focused on analyzing the topologies of high-power high-density applications, such as the dual active bridge or the H-bridge multilevel inverter. All the papers provide enough insight in the analyzed issues to be used as the starting point of any research. Experimental or simulation results are presented to validate and help with the understanding of the proposed ideas. To summarize, this book will help the reader to solve specific problems in industrial equipment or to increase their knowledge in specific fields