Progressing towards DC electrical systems for marine vessels

The commercial electrification of marine vessels started in the 1880s with a direct current (dc) system onboard SS Columbia. However, the popularity of the ac electrical distribution system resulted in the move towards an alternating current (ac) system onboard marine vessels. At present, there are marine vessels that operate based on a diesel-electric system. The on-board diesel engines generate ac power to a common ac bus, from which the electrical propulsion drives source their electrical power. Recently, there is an interest in a shift back to a dc-based diesel-electric system for marine vessels, due to the potential improvement in electrical performance and fuel savings. This paper evaluates the improvement obtained by moving to a dc system for marine vessels in terms of electrical performance and generator scheduling. An overview of a typical ac electrical system currently in use is provided, and modification of the system to a dc-based system is presented. The issue of multiple generator synchronization and harmonic elimination are also discussed. A discussion on the potential benefits of a shift to a dc system is provided, as well as the potential challenges that may arise. Both symmetrical and asymmetrical generator scheduling are examined and the potential fuel savings for an example vessel is calculated.