Distribution Power System

Integrated electric MVDC systems

The project analyzed the possibility of building a large ship equipped with a medium voltage direct current power system, dealing with all constraints on distribution system issues.
First of all, several types of distribution and network topologies are considered. There is then a preliminary selection of the one best suited to the vessel (cargo, cruise, navy, etc.). This preliminary engineering selection is not identical to the one needed for a traditional alternating current ship, although it is not culturally very different. The analysis of the possible distribution system configurations allows the engineers to identify the pros and cons of the various solutions depending on the particular application. For instance, it is possible to choose between a more economy-oriented distribution system, or one more geared to ensure service continuity, and so on.

One of the several possible MVDC distribution systems : two poles with insulated conductors and the middle point connected to the hull.

In the MVDC Large Ship project, the phenomenon of short circuit network currents was studied in detail, highlighting the temporal trend and providing the designer with the knowledge necessary to select the proper protections. The study of fault currents in the DC system also highlighted the decisive role of the converters. Unlike with the traditional alternating current ships, with MVDC the converters play an important  role in fault discrimination and circuit interruption . However, it should be noted that the interruption-protection devices currently available on the market do not yet fully guarantee the completely selective detection of fault location and current circuit interruption suitable for every possible type of DC distribution network.

In the future it is thus important to continue investigating and identifying new techniques for fault detection and interruption.  Rapid progress is being made in this field, and we can say that no insurmountable obstacles to the design and construction of MVDC ships in the very near future are expected.

Low voltage DC users subsystems

In the MVDC Large Ship project, the low voltage DC distribution system was also considered, with special reference to the sub-distribution system for passenger and crew cabins. An analysis was made of the loads directly powered by DC (LED lighting, thermal loads, various services), as well as of the management of any load still maintaining AC power supply.

Possible faults affecting a medium voltage DC distribution system.

Possible faults affecting a medium voltage DC distribution system

Special attention was paid to electrical safety issues.
The differences between the traditional AC and the novel DC solution, in terms of efficiency and weight- space saving, were also studied.
Finally, the use of a power line carrier as a data transfer bus, and the related net automation, was also considered.
All the studies show with the benefits of a full DC solution, which almost completely eliminates the alternating current supply to the final loads. This vision is not too futuristic, considering the wider diffusion of mobile DC devices.

As far as communications are concerned, the possibility of a relatively simple signal acquisition and command transmission is also a very important goal and a significant advantage, especially for the ships load power control.