This paper proposes a novel method based on pulse width modulation techniques toreduce and control the common-mode voltage in three-phase multilevel inverters. Besides controllingthe common-mode voltage, this method is capable of controlling capacitors voltages and load currentswith low switching losses and harmonic contents. In fact, due to the existence of different pulsepatterns and the possibility of choosing the optimum patterns based on the most important criteria, thecontrollability of this method has risen properly. Furthermore, the controller randomly selects one ofthe optimum pulse patterns, which has another advantage of spreading the spectrum contents of theoutput voltage for low electromagnetic interferences and mechanical vibration. This method can beapplied to a three-phase, three-level inverter with the flying capacitor topology. Another advantage ofthis method is that the technique can be applied to more voltage levels without significantly changingthe control algorithm. The simulation results of a five-level inverter in this paper indicate that theproposed technique can be used to implement a multilevel inverter.

This paper presents a single-phase topology for multilevel inverters with minimum number of switching devices. The proposed topology significantly reduces the number of DC voltage sources, switches, and power diodes as the number of output voltage levels increases. The proposed multilevel inverter is constructed using series-connection of multilevel strings. Suggested multilevel string is composed of multiple basic switching units. The proposed multilevel inverter has extendable configuration that increases the number of output voltage levels more and more by adding more stages. The proposed multilevel inverter would be implemented in both symmetric and asymmetric configurations. Two different algorithms are introduced for determination of magnitude of DC voltage sources to reach the maximum number of output voltage levels with minimum number of semiconductor devices. Important characteristics of both symmetric and asymmetric configurations are extracted and compared with similar multilevel inverter topologies. Finally, a prototype of the proposed multilevel inverter is simulated and implemented experimentally to verify operation of the proposed multilevel inverter.

Background and Objectives: Increasing environmental problems and challenges have led to increased use of renewable energy sources such as photovoltaic or PV system. One of the attractive research fields is power electronic converters as interfaces for renewable energy sources. Multilevel inverters can operate as such interfaces. This paper introduces modified topologies of switched-capacitor multilevel inverters, designed to overcome constraints of low voltage renewable energy sources such as PV. Methods: Configuration of topologies utilize a single DC source with series or parallel connection of capacitors to produce 7-level, 9-level, and 11-level voltage in the converter load side. The paper presents the converter operation principle, elements voltage stress analysis, and capacitor sizing calculations. Also, operation analysis of suggested inverter topologies is validated using implemented set up. Results: Comprehensive comparative analysis reveals that the proposed topologies have merits and superior performance compared to existing solutions regarding component number, voltage boost factor, and voltage stress. The experimental measurement results confirm the accuracy of multilevel output voltage waveforms and the self-balancing of capacitor voltages, as predicted by theoretical analysis.Conclusion: The suggested switched-capacitor multilevel inverters, moreover the superiority over previously presented topologies, show great potential for application in photovoltaic systems and electric vehicle battery banks.

In practical applications of inverters, unbalanced conditions may be occurred. For instance, semiconductor switches of a converter may not be exactly the same or switching circuit may be unbalanced. This unbalance leads to additional harmonics in the output of converter. The additional harmonics usually are not considered in the design of circuit, so cause many problems. For precise investigation of these harmonics, a new model of inverter based on switching functions is presented. With this model, analytical equations of harmonics in unbalanced switching are calculated. These analytical equations can be used for designing process such as investigation of optimal cases or elimination of undesired harmonics. The accuracy of the analytical calculations is verified by simulation results. As well, an experimental prototype is constructed to verify analytical results.

In this paper, two new cascaded inverters are proposed, by using the series connection of new Submultilevel inverters. Each of the proposed Submultilevel inverters consists of three batteries and eight power switches. Four algorithms are presented to determine the voltages of these batteries for each of the proposed structures. In this study the comparison between the proposed structures with conventional structures has been done. At first, the proposed algorithms of new structures are compared with each other and after that comparisons between proposed structures based on selected algorithms and the traditional structures are performed. This comparison shows that the proposed inverters can produce high number of output voltage levels due to determined number of power electronic switches. Also blocked voltage of the proposed structures is smaller than other compared structures which leads to reduce size and weight of the proposed inverters. Other advantages of these structures are reduction of voltage sources number, DC sources variety, the conduction losses and the number of power diodes. In order to demonstrate the correct operation of the proposed structures and applied algorithms, simulation results by using PSCAD/EMTDC software are shown.

The multilevel cascade inverter is one of the most widely used power-electronics based interfaces in electrical distribution systems. Due to high losses and harmonics, the switching frequency of the inverter should be low in medium and high power applications. For this reason, the conventional carrier wave-based sinusoidal pulse modulation (PWM) and space vector PWM that have high switching frequencies cannot be used in these applications. The optimal PWM methods for inverters with step modulation result in lower total harmonic distortion (THD) in output voltage than other common modulation methods. However, one of the major disadvantages of these methods is that the optimal switching angles should be determined using the switching table, limiting the application of the optimal PWM. This paper proposes a method for determination of switching angles by using the iterative quadratic programming method. In each iteration, the proposed method calculates the switching angles by solving the quadratic sub equations with equality constraints and linear equations. Also in the appropriate conditions, global and asymmetric convergences are faster, more accurate, and more efficient, and there is no need for much time and memory for switching angles determination. The optimum switching angles minimize the switching frequency, switching losses, and THD in voltage and current of a three-phase cascade multilevel inverter with step modulation. Also, the power circuit breakers are switched on and off only once in each period. The effectiveness of the proposed method is evaluated through simulation case studies in MATLAB environment.

The conventional space vector pulse-width modulation (SVPWM) for cascaded H-bridge inverters (CHBIs) has problems of computational complexity and memory requirements. Operation in overmodulation mode is the other reason for the complexity in SVPWM. This paper proposes a novel modulation method, named as level vector pulse-width modulation (LVPWM), for voltage control of CHBIs. The concept of the proposed method is similar to the SVPWM but with different vector diagram and dwell times calculations. Unlike the SVPWM, the α and β axes and also their variables are considered separately without gathering in complex variables. The vector diagram has two separated α and β axes each of which contains individual switching vectors and reference vectors. The selection of the vectors to synthesize the reference vectors depends only on the amplitudes of the reference vectors. The computational overhead and memory requirement are independent of the number of cascaded H-bridges. Lower computational overhead and easy and continuous extension to overmodulation region are the advantages of the proposed method compared with the SVPWM-based methods. Moreover, the switching algorithm achieves improved efficiency for the inverter. Simulation and experimental results verify the effectiveness of the proposed algorithm.

In this paper, a novel approach for comprehensive state-space modelling of the grid connected Multi-level inverters is proposed. Details of the developed method is presented using cascaded H-bridge converters, however it can be applied to other topologies of the grid connected inverters as well. In Multi-level converters, due to their nonlinear characteristic, application of the nonlinear controllers is more beneficial to ensure stability of the system in a wide range of operation. Hence, the state-space model is required to design a nonlinear controller. To achieve converter model, it is divided into some sub-circuits considering different operational intervals in a switching cycle. To verify accuracy and effectiveness of the obtained state-space model, a laboratory setup of a Multi-level. Converter with two H-bridges has been designed and implemented. Also, results of the developed state-space model has been compared with the simulation/experimental results of the grid-connected converter. According to the simulation and experimental result, accuracy of the model is verified. It should be noted that all of the simulations have been performed by EMTDC/PSCAD toolbox.