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Abstract
Recent advances in semiconductor switches, new multilevel converter topologies and advanced converter modulation techniques have contributed to the expansion of voltage source converters (VSCs) to higher voltage and power ratings for utility-scale and motor drive applications.
Multilevel VSC topologies extend the advantages of the fully controlled, four-quadrant, two-level converter by improving the quality of the output waveforms and minimizing filtering requirements. Additionally, the stresses across the switching devices are reduced, converter losses and electromagnetic interference are decreased and the overall efficiency is improved.
The challenges associated with the multilevel converters are well documented and include: capacitor voltage deviation and voltage balancing issues, increased complexity in the circuit configuration, control and regulation of voltages and currents. The main research efforts focus on further minimizing the losses, increasing the efficiency of the converter topologies and providing tight regulation of the voltages and currents while delivering a cost-effective solution for commercial applications.
The thesis deals with selective harmonic elimination pulse width modulation (SHE-PWM) for three-phase, two-level and multilevel converter topologies. SHE-PWM for the three-phase two-level converter is initially treated. Different formulations of SHE-PWM based on relaxing the symmetry requirements, previously imposed, are investigated. New solution sets are calculated by imposing half-wave symmetry or completely eliminating the symmetry requirements. Based on multiple harmonic performance factors, an evaluation identifies the solution sets that exhibit superior harmonic performance. The theoretical analysis and simulations are verified through experimental work on a laboratory prototype.
Multilevel SHE-PWM (MSHE-PWM) techniques are analyzed for various number of levels in the output waveforms and harmonics eliminated from the output voltage harmonic spectrum. The prime challenge of MSHE-PWM is the acquisition of the different solutions while ensuring their continuity over the modulation index range. The solutions are evaluated for their harmonic performance. The operation of hybrid multilevel converters, namely the five-level hybrid H-bridge based converter and the seven-level hybrid cascaded ANPC converter under MSHE-PWM is also analyzed. In both topologies, the regulation of the voltages in the H-bridge cells of the converter together with the application and performance of MSHE-PWM are investigated. Extended simulation and experimental results from both topologies are provided.
MSHE-PWM is further extended to include the voltage levels of the PWM waveform as variables. The proposed approach offers advantages in terms of the acquired solutions. The switchings assume constant values over the modulation index range, the voltage levels vary linearly with the increase of the modulation index and the formulation of the problem allows for elimination of additional harmonics from the output voltage harmonic spectrum. The advantages of the proposed formulations come at the cost of increased complexity in the regulation of the DC voltages, particularly in closed loop implementations. Simulation and experimental results based on laboratory setups for five and seven-level waveforms are provided.
The thesis also deals with the modulation of the modular multilevel converter (MMC) and its operation in utility-scale applications. The MMC is the state-of-the-art multilevel converter based on the cascaded connection of half-bridge sub-modules (SMs) and can be extended to provide large number of levels in the output voltage waveform. The circuit configuration and modular concept make the MMC a particularly attractive multilevel topology for medium and
high-voltage, medium and high-power utility-scale applications.
Two different modulation methods for the MMC are proposed and analyzed. The two methods, under the same switching frequency for the switches, provide different operating characteristics. MSHE-PWM techniques for the MMC are also proposed. The techniques, requiring calculation of switchings for large number of levels, are combined with a method to balance the SM capacitor voltages through sorting of the SM voltage values. The operation of the MMC under both modulation methods, based on the MSHE-PWM is verified through simulation and experimental results.
Finally, the thesis discusses the back-to-back configuration of MMC topologies. The converters are operated with sinusoidal PWM, under both the modulation techniques and the voltage balancing method presented earlier in the thesis. Based on controllers in the synchronous rotating reference frame, the system is investigated for its operation under steady state and during transients in the active and reactive power references of the converters. Extensive simulation results are provided to illustrate the system behavior.