ANALYSIS, IMPLEMENTATION AND EXPERIMENTAL EVALUATION OF A PHASE SHIFTED PWM CONTROL SYSTEM FOR A MODULAR MULTILEVEL CONVERTER

Abstract: Nowadays, it is problematic to connect only one power semiconductor switch directly to the grid due to the high voltage range. In order to solve this difficulty, a new type of power converter has been introduced as a solution in high power applications. Multilevel Converters use high speed switching components, avoiding the problem of linking them directly to the grid by connecting single devices among multiple DC levels. Differents Multilevel topologies have been developed in the last few years. Multilevel Converters are more complex to modulate than the two level traditional converters because of the number of switching alternatives that are available. The latest and most promising such topology for high power applications is the Modular Multilevel Converter (M2C). Several control and modulation methods have been suggested for this topology. The aim of this master thesis project is to deeply investigate and evaluate one of them, based on a carrier phase-shifted Pulse Width Modulation (PWM) techniques. Four different control topologies using phase shift PWM techniques on M2C are studied and explored in this work. These topologies include the following loops of control: Averaging Control based on the currents inside the converter, Individual Balancing Control based on the output current and capacitors voltages, and Arm Balancing Control based on the voltage difference between the arms of the converter. The operation principle of an M2C is presented. This project proposes a switching frequency that meets the two required criteria: low enough to maintain cost feasibility, and high enough to reach a harmonic performance target. Additionally, this work proposes an analytic expression for the output voltage spectrum of the converter, which enables prediction of harmonic performance. Three distinct simulations were performed each one using different control topologies and switching frequencies. The first controller simulated took into account the Averaging Control topology, based on the circulating current. Within this topology both individual and arm balancing techniques are also explored. A second controller is also simulated using Averaging Control, based on the arms currents, as well as the other control loops. For the last case an Averaging Control, based on the arm currents, without the Arm Balancing is simulated. The results of each simulation are discussed and compared. Finally, these topologies are implemented and verified experimentally on a 10-KVA M2C prototype. The experiments are performed using only one phase and 11-level modulation methods. The controller efficiency is studied and verified through step response analysis.