Several cruises of current measurements along various cross-Taiwan Strait transects were conducted by using shipboard Acoustic Doppler Current Profiler (ADCP) during 2001-2003. The main purpose of these experiments is to obtain seasonal variations of flow structures and volume transport in the central and southern regions of the Taiwan Strait. In each cruise the semidiurnal tidal currents were eliminated from the ADCP currents by two different methods, i.e., the phase averaging method and the TSNOW calculation. The subtidal current in the Taiwan Strait generally flows in the parallel-strait direction. In summer when the southwest monsoon prevails, the water in the strait originates from the South China Sea (SCS) or the Kuroshio. This northward-flowing water is divided into two parts by the archipelago of Penghu; the majority keeps flowing northward along the Penghu Channel (PHC), the minority flows northwestward around the Penghu Island. The flows in the surface layer of the PHC reach a maximum speed of 60 cm/s or greater. In winter, strong NE winds push the fresh and cold China Coastal water southward, along the western part of the Taiwan Strait. The SCS or Kuroshio water still flows northward on the eastern part of the strait. The maximum northward current still occurs in the PHC and is around 20 cm/s or less in the winter. Our results from the phase averaging method of all six cruises indicate that the net transports along the Taiwan Strait are all flowing northward, with a maximum value of about 2.5 Sv in summer (August 2001) and a minimum value of about 0.5 Sv in winter (March 2003). The standard deviation of the volume transport is 0.3 Sv. Due to its greater depths and strong currents, the volume transport in the PHC amounts to approximately 75% of the total transport of the Taiwan Strait. Based on the phase averaging results, the transport is related to the along-strait wind by a simple regression: , the sign convention is positive for southwesterly wind and transport.

This thesis provides insight into state-of-the-art Modular Multilevel Converters (MMC) for medium and high voltage applications. Modular Multilevel Converters have increased in interest in many industrial applications, as they offer the following advantages: modularity, scalability, reliability, distributed location of capacitors, etc. In this study, the modeling, control and design considerations of modular based multilevel converters, with an emphasis on the reliability of the converter, is carried out. Both modular multilevel converters with half-bridge and full-bridge sub-modules are evaluated in order to provide a complete analysis of the converter. From among the family of modular based hybrid multilevel converters, the newly released Alternate Arm Converter (AAC) is considered for further assessment in this study. Thus, the modular multilevel converter with half-bridge and full-bridge power cells and the Alternate Arm Converter as a commercialized hybrid structure of this family are the main areas of study in this thesis. Finally, the DC fault analysis as one of the main issues related to conventional VSC converters is assessed for Modular Multilevel Converters (MMC) and the DC fault ride-through capability and DC fault current blocking ability is illustrated in both the Modular Multilevel Converter with Full-Bridge (FB) power cells and in the Alternate Arm Converter (AAC). Accordingly, the DC fault control scheme employed in the converter and the operation of the converter under the fault control scheme are explained. The main contributions of this study are as follows: The new D-Q model for the MMC is proposed for use in the design of the inner and outer loop control. The extended control scheme from the modular multilevel converter is employed to control the Alternate Arm Converters. A practical reliability-oriented sub-module capacitor bank design is described based on different reliability modeling tools. A Zero Current Switching (ZCS) scheme of the Alternate Arm Converter is presented in order to reduce the switching losses of the Director Switches (DS) and, accordingly, to implement the ZCS, a design procedure for the Arm inductor in the AAC is proposed. The capacitor voltage waveform is extracted analytically in different load power factors and the waveforms are verified by simulation results. A reliability-oriented switching frequency analysis for the modular multilevel converters is carried out to evaluate the effect of the switching frequency on the MMC's operation. For the latter, a DC fault analysis for the MMC with Full-Bridge (FB) power cells and the AAC is performed and a DC fault control scheme is employed to provide the capacitor voltage control and DC fault current limit, and is illustrated herein. / Master of Science