Hybrid Topologies of Multilevel Converter for Current Waveform Improvement
N. Susheela, P. Kumar, and B.Sirisha.
International Journal of Inventive Engineering and Sciences (IJIES) 1 (4): 29-37 (March 2013)

This paper presents different multilevel converter topologies that includes NPC, NPC-CHB, FC, FC-CHB converter. The operating principle of each topology and a review of the most relevant modulation methods are discussed. Multilevel converters offer advantages in terms of the output waveform quality due to the increased number of levels used in the output voltage modulation. This advantage is particularly true for cascade H-bridge (CHB) converters that can be built to produce a large number of levels owing to their modular structure. Nevertheless, this advantage comes at the cost of multiple dc links supplied by independent rectifiers through the use of a multi-output transformer for inverters. This front end complicates the implementation of converters that have a high number of levels. An alternative method of using lower voltage cells with floating dc links to compensate only for the voltage distortion of a neutral-point-clamped and flying capacitor converter is considered for active rectifier applications. The analogy between the floating HBs and the series active filters is used to develop a strategy for the harmonic compensation of the NPC output voltage and the control of the floating dc-link voltages. This simplifies the current control scheme and increases its bandwidth. The proposed topologies have been verified using MATLAB/Simulink. The results show the improvement in the output current waveforms.
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