Multi-level Matrix Converters

Abstract

AC to AC drives have been the corner stone of industrial drives control. These drives convert input voltage of fixed magnitude and frequency to output voltage of variable magnitude and/or variable frequency. These are classified as indirect or direct converter based on the presence or absence of an intermediate DC stage. The indirect converter consists of an AC to DC front end converter and a DC to AC voltage source inverter (VSI). The front end can be implemented by means of diode bridge rectifier, thyristor based 6 pulse converter or an active VSI with the latter having the best performance in terms of input current harmonics and filter requirement. The DC link voltage of these converters are usually maintained by means of large and unreliable Al capacitors. In addition to reliability issues, the DC link capacitors suffer from poor thermal performance and with advent of wide-band gap devices which can operate at much higher temperatures, these capacitors pose a serious limitation on their capability. Matrix converters fall under the other category, namely the direct AC to AC converters. These converters do not have an intermediate DC link capacitor making them more reliable and compact in comparison to indirect converters. Despite this advantage, matrix converter adoption into industrial drives has been limited due to its shortcomings such as high switch count, limited voltage transfer ratio, no ride-through capability etc. Over the past two decade, various modifications to the conventional matrix converters and its modulation algorithm have been proposed to fix each of these issues. The sparse matrix converters were developed to reduce switch count, matrix converter with an auxiliary capacitor to enable zero voltage ride through and so on. These were modified further in open-end winding matrix converter to eliminate common mode voltage which would be theoretically impossible in the two stage converter. The output voltage and input currents of an AC to AC converter must have low harmonic content. In general, higher the levels in the output voltage, lower is the harmonic content. In case of indirect converters this is achieved by synthesizing multiple DC voltage levels from a single DC source and they are classified into diode clamped, flying capacitor and cascaded H-bridge inverter based on how these DC voltages are synthesized. This multi-level DC voltage is in turn is used to generate multi-level output voltage. This dissertation presents new modulation algorithms and three new matrix converters topologies that address this issue of being able to generate multi-level output voltage in direct converters, to reduce output voltage THD. The proposed modulation algorithm for the direct three level matrix converter, reduces number of switching transitions by up to 25% compared to the conventional methodology. The first two proposed topologies enable three level output voltage by generating additional AC voltage levels from input three phase while the last one enables generating up to five level output voltage. In addition, the modulation schemes for these converters was developed. The functioning of these converters and the modulation algorithm was verified in simulation by means of Matlab as well as a custom developed model based numerical simulation software. The developed platform has inbuilt model to code translation and auto programming feature allowing rapid prototyping without need for any gruesome programming from the user end. A hardware prototype of direct three level matrix converter was developed and its functioning was validated with simulation results on both platforms.