Voltage Sag Mitigation in Offshore Oil Rig Power System by Dynamic Voltage Restorer

Voltage sag is a common power-quality problem in offshore power systems. In contrast with land power systems, offshore oil-rig platforms are "weak grid" and "finite bus" systems characterized by limited number of diesel generators supplying power to large induction motors. Thus, voltage sag due to power supply faults or trips can severely impact their performance. Dynamic voltage restorer provides a solution to voltage sag. When voltage sag is detected, ac voltage obtained from dc power supply is injected at the point of common coupling (PCC) through a injection transformer to compensate the sag. However, during this process, undesirable harmonics is injected to the system. Given the strict regulation on power quality of offshore power systems, the total harmonic voltage distortion (THDv) needs to be within 5% at PCC. In this paper, an easy-to-implement proportional-integral (PI) controlled DVR design is proposed. In addition to instantaneous detection and accurate compensation of voltage sag, it successfully limits voltage harmonics to 0.52% by using a three-level inverter and a line-side filter capacitor. The proposed DVR design is modeled with offshore oil rig power system and simulated in the SIMULINK/MATLAB environment. Time domain simulations verify the effectiveness of the design.