Date of Award

May 2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Robert Cuzner

Second Advisor

Ranga Tallam

Committee Members

Adel Nasiri, David Yu, Deyang Qu

Keywords

Active Front End, Drives, Filter, Optimization, SiC, Thermal

Abstract

Electric drives with Active Front End (AFE) converters can provide benefits such as lower harmonic current injections to the grid, smaller size filters, lower THD values and cost saving for injection of power to the grid in brake situations. SiC-MOSFET based two-level converters can be a promising topology for Active Front End (AFE) application in electric drives. The possibility of high switching frequency will make the grid filters smaller. Grid filters are used for EMC and power quality issues. However, there are practical limitations for increasing the switching frequency such as dead time in the gating signals, sampling requirements, and electro-magnetic interference (EMI) considerations, besides the need for high frequency magnetic material for the LCL line filter. However, three-level converters provide the opportunity to switch at a lower frequency and also reduce the filter size compared to a two-level IGBT converter. Three-level converters can be built using low voltage rated modules with lower switching losses and reduced cost compared to SiC based two-level converters. In this work, a comparison between three-level converters and two-level converters is presented focusing on power loss, filter size and application benefits. This comparison is based on an optimization algorithm with the objective function of weight, volume and cost.

The topologies and modulation techniques for multilevel converter are categorized at first by a thorough literature survey. The pros and cons for various multilevel topologies and modulation techniques are discussed. The 3-level neutral point clamped (NPC) topology is selected to build a 25 hp, 480V power conversion system. LCL filter design for comparability with grid requirements has been done and the optimal size of the LCL filter is derived considering thermal limitations.\par

To make the comparison between different topologies and switch types possible, it is necessary to consider the maximum junction temperature relation to the switching frequency. In this work, a new modulation method is proposed to improve the performance of three-level converters considering losses and thermal performance. Also, a thermal model is derived for SiC MOSFET power modules that takes the effect of MOSFET channel conduction into consideration.

Losses for different modulation methods is analysed and compared for two-level and three-level converters. For a specific application of drives, low speed operation is investigated and the comparison between three-level and two-level converters is considered. The methods for calculating losses are considered carefully to ensure maximizing the utilization of the power semiconductors (for highest power density designs). A novel modulation method is developed for low speed operation of power converters.

Finally, an optimization is done for finding minimum volume, highest efficiency, minimum common mode pulses and complying with EMI constraints. This optimization has been broken into multiple steps for reducing the problem size. This will enable us to validate the results more efficiently. Some parts of this optimization are done automatically such as the inductor magnetic and thermal design.

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