Multilevel converters are considered today as the state of the art power conversion systems for high power and power quality demanding applications. This paper presents a tutorial on this technology, covering the operating principles, the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in depth and self-contained information, while recent advances and state of the art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.
In high power applications the maximum switching frequency is limited due to thermal losses. This leads to highly distorted output waveforms. In such applications, it is necessary to filter the output waveforms using bulky passive filtering systems. The recently presented selective harmonic mitigation technique (SHMPWM) produces output waveforms where the harmonic distortion is limited fulfilling specific grid codes when the number of switching angles is high enough. The related technique has been previously presented using a switching frequency equal to 750Hz. In this paper, a special implementation of the SHMPWM technique optimized for very low switching frequency is studied. Experimental results obtained applying SHMPWM to a three-level neutral point clamped converter using a switching frequency equal to 350Hz are presented. The obtained results show that the SHMPWM technique improves the results of previous selective harmonic elimination (SHEPWM) techniques for very low switching frequencies. This fact highlights that the SHMPWM technique is very useful in high power applications leading its use an important reduction of the bulky and expensive filtering elements.
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