Prospects for Improving Unified Digitally Controlled Electric Power Systems Designed for Electric Vessels

Abstract

Marine engine packages based on electric propulsion systems (EPSs) are highly reliable and improve the safety of vessel navigation. However, the currents consumed by an EPS with a capacity close to that of a vessel electric power plant contain upper harmonics and generate voltage distortion in the vessel mains. According to the International Association of Classification Societies, the total harmonic distortion coefficient of the vessel mains voltage curve must not exceed 8%. In the widely used structure of a unified vessel electric power system designed by Siemens AG, the required quality of voltage is attained by including high-capacity transformers in the EPS. This article shows how to ensure this quality of the vessel mains voltage by including multiphase generators in a vessel electric power plant. In this case, EPSs can be used without transformers, which allows making the electric equipment lighter, smaller, and cheaper. Multiphase generators also allow using three-level inverters as parts of frequency converters and reduce the share of upper harmonics in the phase currents of the electric propulsion motor, which will weaken the torque pulsation and increase the motor’s efficiency ratio.