STABILITY ANALYSIS AND VOLTAGE-SAG MITIGATION OF POWER SYSTEM IN OFFSHORE OIL RIG PLATFORM

Abstract

Today¿s semi-submersible offshore oil rig platform uses variable-speed induction motors for station¿keeping during drilling and sailing. During drilling, the platform needs to be maintained within its intended location to prevent interruption of operation and damage to the drills. Thus, it is crucially important that the induction-motor drive and the power supply feeding the drive are stable. This master thesis aims at analyzing and proposing analytical solutions to these two issues. The main contributions of the thesis are: For the first time, a Matlab Simulink model is built to simulate the detailed dynamics of the induction-motor drive in offshore oil-rig platform. The model and parameters used for the drive are verified by on-site tests of the Keppel FELS motors. Genetic algorithm is developed to the small-signal model of the induction-motor drive to identify, within the full range of speed and load, the instability region of the induction motor fed from ideal variable-frequency supply or a converter. The source of instability is also investigated from the two different supply configurations of the induction-motor drive. It is shown by eigenvalue analysis that instability exists in the induction motor itself but not in the other parts of the drive i.e. the converter supplying the motor. Accordingly, speed feedback comprising a proportional regulator is added to the drive to eliminate the instability region intrinsic to the induction motor plotted using genetic algorithm Voltage sag is known to be the most common disturbance in offshore power system, which has led to blackout in severe cases. A new design of dynamic voltage restorer (DVR) is modeled in Simulink, which deals with the influence of voltage sag on the induction-motor drive within strict harmonic limits imposed by marine standards. The new design outperforms the conventional DVR design by effectively increasing the accuracy of voltage compensation and significantly reducing the level of total voltage harmonic distortion.