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Multilevel inverters have recently been received more attention in the power industry. Two traditional types of multilevel inverters are multilevel voltage-source inverter and multilevel current-source inverter. Despite this widespread use, multilevel Z-source inverter (ML-ZSI) has not been regarded enough, because a unique approach to voltage control of ML-ZSI has not yet been presented. Besides, the phase voltage and its total harmonic distortion (THD) have not been calculated for ML-ZSI so far. This study proposes two new methods for the formulation of phase voltage in ML-ZSI. The first method is based on Fourier series analysis of the phase-voltage waveform, and the second utilises the staircase waveform of the phase voltage. In addition, two new methods are introduced for computing the phasevoltage THD in ML-ZSI: approximate method and waveform-integration method. Although the waveform-integration method leads to the accurate answer, using this method is more difficult than an approximate method. The detailed description of suggested methods and comparison with each other are presented in this study. Simulation and experimental results are also given to demonstrate the new features of the proposed methods. [ABSTRACT FROM AUTHOR]

This paper proposes a harmonic elimination technique for online reducing harmonics in voltage source inverters. In the presented method, in order to provide required fundamental voltage and eliminate specified harmonics simultaneously, the value of the switching angles are determined using a new efficient optimization algorithm named Enhanced Teaching-learning-based Optimization (ETLBO). The efficiency of the presented strategy is validated on three-phase seven-level diode-clamped inverter while other structures of multi-level inverters (MLIs) can be employed. The aim is to reduce the Total Harmonic Distortion (THD) in multilevel converter. In this paper, a new optimization algorithm named ETLBO has been successfully implemented to achieve optimal switching angles in order to harmonic elimination in MLIs. A three-phase seven-level diode-clamped MLI has been selected as a test case to validate the effectiveness of the proposed method. The simulation results demonstrated the ETLBO method not only is in preference to the Genetic Algorithm and Particle Swarm Optimization for including no user-defined parameter and simple implementation, but also for providing higher quality solutions. Moreover, in order to verify the superiority of the ETLBO algorithm over conventional methods for harmonic elimination, the Newton-Raphson (NR) method was implemented to find optimal switching angles. The results obtained by the NR method compared with those of the ETLBO and the superiority of the ETLBO in terms of the both computational time and the resulted %THD was concluded. As a whole, the results show that the ETLBO algorithm is expected to become widespread in power systems where on-line updating harmonics is needed since it provide accurate and high-quality solution in extremely short time. [ABSTRACT FROM AUTHOR]