Multiphase Modular Multilevel Converter Based Drive System for Oil and Gas Recovery in Subsea Applications

Adjustable Speed Drives (ASDs) in subsea/offshore oil and gas applications are involved in several processes throughout production, transportation, and treatment phases, as they supply pumps and compressors. Therefore, ASDs’ in subsea applications have gone through continuous development. The currently existing systems exhibit several energy-conversion challenges that negatively impact the industry costs and green energy. In order to improve the energy-conversion efficiencies, this thesis, apparently, adopts a trend of using low-rating semiconductor devices to handle medium-/high-voltage levels along the various drive system stages. A trend that is believed, by the author, to become in the near future the foundation of flexible, redundant, and compact energy-conversion in the majority of medium-/high-voltage power applications. This thesis proposes the employment of Modular Multilevel Converters (MMCs) in MV-ASD applications in order to ease a transformerless operation while handling medium voltages using low-rating semiconductors. This, in return, reduces the system size and weight. The zero-/low-speed operation of MMC-based ASDs (a main challenge of applying MMCs in ASDs) is investigated in this thesis. The recent techniques proposed to improve operation during those intervals, implement complex hardware/software approaches. To cope with this issue, this thesis proposes multiphase machines for MMC-based AC-drives. Among several advantages regarding the power splitting, multiphase machines provide additional degrees of freedom compared to their three-phase counterparts. Novel exploitation to these additional degrees of freedom is proposed by injecting a secondary current component in the load current with specific magnitude and frequency during zero-/low-speed intervals enabling the drive system to function duly. Since the control of these secondary components is already inherited in the current controller structure of any multiphase machine, no additional algorithms or sensors will be required. Moreover, a hybrid-boost MMC converter is proposed to boost the MMC output voltage. The stepped output voltage generated by the MMC reduces or eliminates the filtering requirements. The boosting capability of the proposed architecture eliminates the need for bulky low-frequency transformers at the converter output terminals. In order to enhance the drive output torque, a Trapezoidal Phase Disposition Pulse Width Modulation Technique (TPD-PWM) is proposed in this thesis. Different slope angles of the trapezoidal modulation signal are tested and compared to other modulation techniques in order to select the angle with the best torque enhancement meanwhile maintaining good Total Harmonic Distortion (THD) levels. Moreover, the adopted modulation technique reduces the switching losses compared to the conventional Sinusoidal-based PWM. Eventually, a series-connected Half-Bridge (HB) modules are proposed as an Active Front End (AFE) rectifier for the drive system in order to generate an adequate DC-link voltage for the medium-voltage drive system using low-rating semiconductor devices. The proposed techniques through this thesis have been supported with both experimental and simulation results.