Date of Award
Open Access Thesis
Herbert L Ginn III
Modular multilevel converters are an emerging voltage source converter topology suitable for many applications. The increased utilization of HVDC power transmission solutions has resulted in modular multilevel converters becoming a more common converter type. Other applications include interfacing renewable energy power sources to the grid and motor drives. Modular multilevel converters are beneficial for medium voltage motor drives because the properties of this converter topology, such as low distortion, allow for an efficient motor drive design.
Modular multilevel converters are designed where instead of using 6 IGBTs as in a conventional voltage source converter, numerous low-rated IGBTs are used to produce the desired voltage. The converter is made up of a series of IGBT half-bridge circuits with a capacitor across both devices. Benefits of this converter include reduced semiconductor device costs due to the ability to use more commercially available low-rated IGBTs and reduced or potentially the elimination of filter components. The number of voltage levels which corresponds to the number of submodules is what causes the harmonic reduction allowing for this omission. Other benefits include lower operating switching frequencies which also results in reduced converter losses. A drawback to using modular multilevel converters is an increase in the complexity of the control schemes and data processing requiring many more sensors and because of this a thorough understanding of the benefits and limitations of all the control strategies is desired.
One area of control flexibility is in the pulse width modulation and voltage balancing algorithms applicable to modular multilevel converters. The pulse width modulation options are multicarrier solutions that focus on two categories: phase-shifted PWM which utilizes multiple carrier waveforms with the same frequency and amplitude but a different phase shift and level-shifted techniques which utilize multiple carrier waveforms with identical frequency and amplitude but a different DC bias. An important aspect of modular multilevel converters is that the capacitor voltages need to be as closely balanced to the desired DC voltage as possible with a typical acceptable voltage ripple of 10%. In order to achieve this, various voltage balancing algorithms have been developed for modular multilevel converters with this work focusing on two common algorithms.
This work focuses on analyzing both modulation techniques and voltage balancing algorithms using a range of metrics to better understand the most applicable strategies based on the specific application of the converter. A MATLAB/Simulink model using SimPowerSystems of a 21-level three phase modular multilevel converter has been built in order to implement and analyze the various methodologies. The result will be a comprehensive analysis of the optimal approach based on capacitor voltage ripple, converter power loss, and converter voltage THD.
Blackmon, R.(2013). Analysis of Modulation and Voltage Balancing Strategies for Modular Multilevel Converters. (Master's thesis). Retrieved from http://scholarcommons.sc.edu/etd/2404