Several multilevel converter topologies have stimulated widespread interests amongst researchers because they have the capabilities of (i) reaching high voltage ratings without magnetics and transformers and (ii) attaining low Total Harmonic Distortions (THD), using the low switching frequencies of gate-turn-off thyristors (GTOS). This thesis focuses on the diode-clamped multilevel converter topology as the basic module for realizing the high kV voltage rating and the high MVA rating in controllers for the Flexible AC Transmission Systems (FACTS). The thesis has extended the topology configurations to include: (i) multiple modules of multilevel converters and (ii) multimodules of multilevel converters in a back-to-back rectifier/inverter link. These configurations form the backbones for the following multilevel FACTS controllers: (a) the Shunt STAT(ic)COM(pensator), (b) the Series STATCOM, (c) the Unified Power Flow Controller (UPFC) and (d) the Asynchronous Link. A number of key technical innovations have been assembled together to make the new multilevel FACTS controllers possible. On the sides of the ac lines, the Fundamental Frequency Switching strategy has been extended so that it succeeds in simultaneously (a) lowering the Total Harmonic Distortions in the voltage and current waveforms, (b) controlling directly the ac voltage magnitudes, and (c) sharing the current loading in the parallel modules. On the side of the dc links of the STATCOMs, feedback controls have succeeded in: (a) regulating the total dc link voltages and (b) equalizing the dc capacitor voltages in all the levels. The voltage equalization by feedbacks cannot be accomplished in the case of the Unified Power Flow Controller (UPFC) and the Asynchronous Link. A system of locally controlled class-B choppers which transfer the charges from dc capacitors with the higher voltages to their contiguous neighbours with the lower voltages succeeds in equalizing the dc capacitor voltages. The proofs of performance of the multilevel FACTS controllers are substantiated by digital simulations. The equalization of the multilevel rectifier/inverter using the system of class-B choppers has been proven experimentally.

Ooi, Boon-Teck (Supervisor)