Control strategy for an icebreaker propulsion system

The latest series of Canadian Coast Guard ice-breakers features a diesel electric propulsion system using synchronous motors and synchronous generators. The synchronous generators supply power to a constant-voltage 60 Hz bus that feeds the ship´s hotel load and the propulsion system´s cycloconverters. The cycloconverters convert 60 Hz AC power to variable frequency 0-18 Hz AC power for the synchronous propulsion motors. The propulsion motors are directly connected to the ship´s propellers by means of shafting. This AC-AC propulsion system realizes the `power plant´ concept, which optimizes the number of generators that are online in order to match the generated electrical power to the total electrical demand of the ship. The propulsion control system has two control loops that limit the power delivered by the propulsion motors to prevent the diesel engines, which drive the generators, from stalling during instantaneous or sustained overloads