A high performance Si&SiC based hybrid asymmetrical multilevel cascaded H-bridge converter and its advanced control

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Medium voltage and high power drives are widely used in various applications, such as in petrochemical or process industries etc. However most of the installed MV motors are running with a constant speed. Thus introducing more efficient variable-speed drives with higher power quality is highly demanded. The design of controlled MV drives is faced with a number of challenges that relate to the topologies and control of converters, as well as power semiconductor devices. By synthesizing a desired voltage from several levels of DC voltages, multilevel converters can easily provide the medium voltage level and high power requirements. Nowadays, three topologies of the multilevel converter are available as commercial industrial products. They are neutral point clamped (NPC), flying capacitors (FCs) and cascaded H-bridge (CHB). Among these converter topologies, CHB converters can easily reach higher voltage level and lager power rating by standard mature technology components because of its series connection structure of several single phase converters. Unlike the conventional CHB converters cascading identical single phase inverters with equal dc-link voltage, the hybrid asymmetrical CHB converters combine various different topologies of inverters with unequal DC source voltages. By doing this, the converter can combine the advantages of different topologies, and achieve higher voltage levels with less circuit and control. The proposed project is devoted to investigate a hybrid asymmetrical CHB converter system for MV applications. Each phase of the system is composed of one Si-based 5-level NPC H-bridge (HV cell) and one SiC-based 3-level H-bridge (LV cell). A 15-level phase output voltage waveform will be synthesized by configuring the dc-bus voltages of two cells in the ratio of 6:1. The resent fast developed SiC switches are considered as ideal candidates for fast switching applications. However due to the complexity of manufacturing, the cost of SiC-based power devices is still high. The proposed hybrid topology replaces only the low voltage and fast switching cells by using SiC power switches, which increases the performance of the whole system with a reasonable price. The project aims at investigating hybrid converter systems and implementation of their advanced control methodologies.