Number of wires on ship power system
Modem merchant vessels usually use alternating current systems classified as three-phase three-wire insulated neutral systems, of three-phase four-wire systems.
However, direct current systems can also be found.
Cruise ship power requirements
Large passenger ships have three or four large generators rated at 2MW or more to supply die extensive hotel services on board.
However and exception to this rule can be noticed in modem passenger vessels where the power requirements exceed 1OOMW!
Superheated steam at high-pressure, produced from exhaust gases can also be used to drive steam-turbine generator sets.
What is passenger ship?
A passenger ship is a ship that carries more than twelve passengers.
A passenger is every person other than the master and the members of the crew or other persons employed or engaged in any capacity on board a ship on the business of that ship and a child less than one year of age.
Cargo ship power requirements
A cargo ship may have two or more main generators typically rated from 350 kW to a few megawatts, which are sufficient to supply the engine room auxiliaries while at sea, and the winches or cranes for handling cargo while in port.
The limited load required during an emergency demands that the emergency generators be rated from about lOkW for a small coaster to about 70OkW or more for a cargo vessel.
The shipbuilder must estimate the number and power rating of the required generators by assessing the power demand of the load for all situations whether at sea or in a port.
Type of power on ship
Electrical power onboard a ship is commonly generated at 440V, 60Hz (sometimes 380V, 50Hz).
These values have been adopted because they are standard shore supplies in the American and European continents. Ships with a very large electrical power demand may be designed to operate at 3.3kV and even 6.6kV or higher (up to a maximum of 15kV).
Normally high- voltage (HV) generators supply power to propulsion systems, bow thrusters, air conditioning compressors, and similar heavy duty equipment.
The British Standard and International Electrotechnical Commission definition of low voltage is that voltage which is between 50V a.c. and 1000V a.c. (the IEC defines this to harmonise British and European standards).
Lighting and other domestic supplies usually operate at 110, 115V or 220V a.c. (single-phase).
Transformers are used to reduce the generated voltage to this safer voltage level. Where portable equipment is to be used in dangerous, hot and damp locations, it is advisable to operate at 55V or even 25V, supplied again by a step-down transformer.
Occasionally, transformers are also used to step up voltages, to say, 3.3kV for a large bow thruster motor from a 440V switchboard supply.
Batteries for various services operate at 12V or 24V but sometimes, higher voltages are used.