Design and Optimization of a Subsea Drive System with a Long step-out

Abstract

Abstract Subsea power transmission systems are essential for enabling offshore oil and gas operations, particularly as exploration extends into ultra-deep waters requiring long-distance electrical supply. The increasing demand for reliable and efficient subsea drive systems over extended cable lengths introduces several electrical and control challenges, including voltage drop, power losses, resonance effects, and harmonic distortion. This thesis develops a comprehensive model of a 150 km subsea AC transmission system using DIgSILENT PowerFactory to analyze steady-state voltage stability, power quality, and dynamic system behavior. The results demonstrate that proper cable sizing and reactive power compensation are crucial for maintaining voltage regulation along the transmission path. An open-loop scalar control strategy is applied to drive an induction motor, successfully achieving the desired steadystate speeds. However, transient oscillations observed during step frequency changes indicate the need for damping or filtering to improve dynamic response. A sinusoidal pulse width modulation (SPWM) technique is integrated into the inverter model, resulting in balanced, sinusoidal current waveforms at the motor terminals and stable performance during steady-state operation. While some distortion remains at the inverter output, the long subsea cable provides a natural filtering effect before reaching downstream components. In conclusion, the study confirms that a 150 km subsea AC drive system is technically feasible when supported by appropriate component selection, control design, and harmonic filtering. The findings underscore the importance of coordinated system optimization in achieving reliable and stable long-distance subsea power delivery.