Motors, generators and frequency converters electromechanical and system optimization for oil & gas plants

In recent years, the Oil & Gas industry has been subjected to a progressive electrification process aiming to comply with global environmental requirements emissions of Greenhouse Gas (GHG) and avoiding potential systems failures. This dissertation, developed in collaboration with the electrical department of Baker Hughes - Nuovo Pignone, focuses in particular on the Liquefied Natural Gas (LNG) plants. Throughout the production, extraction, storage and transport, the gas-powered mechanical drives are superseded by electrical drives. The electrical machines are employed in the Turbine Generator (TG) units and in the motor-compressor trains. The electrical machines can be directly connected to the power network or they can be fed by Variable Frequency Drives (VFDs) achieving soft-start operation and higher steady-state performance. This present thesis summarizes the research activities focused on the electrical machines and the power conversion stages which compose a typical LNG plant. Firstly, the compression train start-up is examined, pointing out the challenges implied by a Direct-on-Line (DOL) start-up and by a soft-start. Subsequently, the interactions among the TG units and the power conversion stages based on Thyristor Variable Frequency Drives (TVFDs) are analyzed. These interactions are denoted as Sub-Synchronous Torsional Interactions (SSTIs) and they can lead to high torsional vibrations of power generations units. Then, the SSTI phenomena can determine unexpected downturns of the TG units and in some cases of the overall plant. In the dissertation it is demonstrated that the SSTI phenomena are associated to instability of the overall electromechanical system when the TG units exhibit negative torsional damping. In this scenario the development of detailed small-signal models of the electrical equipment is fundamental in order to obtain optimized design of the turbomachinery shaft line and to ensure the torsional stability. In particular, it is discussed how the damping of the TG units is affected by the action of the power conversion stages and of their control system. It is verified that fine tuning of the power conversion stage control system can reduce the risk related to torsional instability. However, there is also highlighted how, in some peculiar plant configurations, the SSTI risk cannot be avoided acting just on the control system. For these cases a dedicated equipment, based on power converters, can be included in the electromechanical system to solve instability. Hence a Sub-Synchronous Damper (SSD) based on a thyristor rectifier can be designed to mitigate the torsional oscillations of the TG units in any plant configurations. The feasibility of the proposed solutions is validated by a complete simulation platform and an intensive test campaign on a real LNG plant. It is pointed out that the provided theoretical models and the developed simulation platform can be directly applied also to different island-operated power systems with different power levels and applications but based on similar power converters topologies.