Design of Fault Tolerant Power Converters
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- Institutt for elkraftteknikk 
Abstract In recent years, multilevel converters have seen an increasing popularity in medium and high voltage applications. Reliability and fault tolerant are some of the significant aspects of multilevel converters. The main topics of thesis were to investigate the operations of the Cascaded H Bridge and the Modular Multilevel Converters with emphasis on their fault tolerant aspect. The other part of the thesis was to do a functional and reliability comparison for the two converters.Models for the Cascaded H bridge and Modular Multilevel Converters were created using Simulink. Cascaded multilevel inverters synthesize a voltage output based on the series connection of power cells which use standard low-voltage components configurations. This characteristic allows to achieve high quality output voltages and input currents, and also an outstanding availability due to theirs intrinsic components redundancy. Due to these features the cascaded multilevel inverter has been recognized as an important alternative in the inverter market. The Modular multilevel Converter was proposed to meet some of the shortfalls of the Cascaded H bridge Converter. A very smooth and nearly ideal sinus waveform can thus be generated with MMC converters, for each submodule is switched individually. It is for this reason that the requirements to filter circuits are much less severe. Additionally, the submodules can be switched at a significantly lower frequency which, in its turn, leads to lower operational losses of the converter. The MMC converter has a modular design resulting in its high flexibility.Results show that the proposed fault correction allows to provide nominal power to the load even under faulty operation. This guarantees the continuous operation of the load or allows to perform a controlled shut-down, increasing the reliability of the converter and making the two converters suitable for driving high power critical loads.A functional and reliability comparison was done for the two topologies. The Cascaded H-Bridge has higher reliability than the two MMC topologies, though not much more than that of the MMC with two switches. This is because of the fewer components as compared to the MMC with four switches. If the failure rate of the gate and control drive are also taken into account, the reliability of the Cascaded H-Bridge is even higher than that of the two MMC topologies.