Fuel Savings Obtained by Replacing Traditional AC-distribution Systems onboard Vessels with DC-distribution Systems

Abstract

Today most of the diesel-electric systems on board ships are based on an AC-distribution of the generated power. The electric power is fed directly from the synchronous generators into an AC-grid, where it is distributed to the different loads on board. This system requires a constant rotational speed to provide the grid with the rated frequency.By introducing a DC-grid, the system no longer has to depend on constant speed. This opens the possibility to vary the speed during operations and thereby optimize the fuel-efficiency of the prime mover to the load-situation. This thesis has investigated the fuel savings by replacing traditional AC-distribution systems on board ships with DC-distribution systems.The DC-distribution systems that have been considered is a DC-grid supplied by three-phase, full-bridge diode rectifiers, and a DC-grid supplied by active front-end (AFE) IGBT based rectifiers.The thesis has concluded that vessels which experience frequent low load situations will benefit from introducing an AFE based DC-distribution system on board. The thesis has also drawn the conclusion that vessels which mostly experience high load situations, the extra cost of investing in AFE-rectifiers cannot be justified, as the fuel savings will be virtually zero at high loads. For these vessels it will be expedient to replace the AC-distribution with the less expensive diode based DC-distribution system.