Development, modulation, and voltage balancing of multi-level converters and modular multilevel converters (MMCs)

Abstract

Multilevel converter (MLC) topologies with lower components counts along with higher power quality are still a major concern for industrial applications. Although in recent years, many MLCs have been proposed in the research area to increase the voltage levels, isolated DC sources and number of switching devices are the limiting factors for their applications in industries. Isolated DC sources not only, increase the weight, volume, and cost of the inverters due to the phase-shift transformer, but also, increase the power losses because of the rectifiers. Modular multilevel converters (MMCs) are the solution to replace the isolated DC source with a single DC source; however, multitudes of semiconductor devices must be utilized in the conventional and popular HB-MMCs to generate a given voltage level. DC fault current handling, voltage balancing in flying capacitors, and circulating currents are controversial issues for MMCs applications. In the first part of this thesis, a single DC source reduced components counts topology of the multilevel converter is proposed for single-phase and threephase systems. This unipolar topology called Z-Packed U-Cell (ZPUC) is used as a submodule of MMC (ZPUC-MMC) to increase the voltage levels over its counterparts. Voltage balancing integrated with phase-shift pulse width modulation (PS-PWM) is implemented on this converter to regulate the voltages of the capacitors. Higher voltage levels waveform generation with a lower total harmonic distortion (THD) makes this converter an appropriate alternative for high power and medium voltage applications such as motor drives. In the second part of this thesis, a deep-learning-based voltage balancing integrated with hybrid level-shift pulse width modulation (LS-PWM) and PS-PWM with a single reference voltage is implemented on ZPUC-MMC to increase the voltage levels. Simulation results in steady states are employed to train the deep-learning voltage balancer and it is implemented on ZPUC-MMC in the lab on both steady and transient states. In the final part of this thesis, PUC topology is developed as a single DC source three-phase Y-PUC converter. PUC converter requires three isolated DC sources for a three-phase system which limits its application. Y-PUC topology that is a modular multilevel converter (PUC-MMC) with PUC submodule is proposed to address this problem. Modular Y-PUC can be used in HVDC applications due that PUC is a bipolar converter, and it is capable to block DC fault current. Voltage balancing integrated with modulation technique is implemented in Y-PUC to illustrates the performance of the proposed converter in steady and transient states.

Date	13 Dec 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Kamal Al-Haddad (Supervisor)