We all know about the voltages used on board a ship. It is usually a 3phase, 60Hz, 440 Volts supply being generated and distributed on board. Every day the owners and designers aim for bigger ships for more profitability. As the ship size increases, there is a need to install more powerful engines and other machineries.
This increase in size of machineries and other equipment demands more electrical power and thus it is required to use higher voltages on board a ship.
Any Voltage used on board a ship if less than 1kV (1000 V) then it is called as LV (Low Voltage) system and any voltage above 1kV is termed as High Voltage.
Typical Marine HV systems operate usually at 3.3kV or 6.6kV. Passenger Liners like QE2 operate at 10kV.
Why High Voltage?
Let us assume a ship generating 8MW of power at 440V, from 4 diesel generating sets of 2MW, 0.8 power factors each.
Each generator feeder cable and circuit breaker has to handle a full-load current of:
I = 2 * 106/ (√3 * 440 * 0.8)
I = 3280.4 Amps i.e. Approximately 3300 Amps.
The protection devices like circuit breaker should be rated at approximately 90kA for each feeder cable.
Let us now calculate the same if the generated voltage is 6600Volts.
I = 2 * 106 / (√3 * 6600 * 0.8)
I = 218.69 Amps, Approximately 220 Amps. Thus the protection devices can be rated as low as 9 k Amps.
Also Power Loss = I2 * r.
Where I is the current carried by the conductor,
R is the resistance of the conductor.
Thus power loss varies square of the current carried by the conductor. If the supply voltage is 440V, then the current carried by the conductor is 0.002P, and if the voltage is raised to 6600V, then the current carried for the same power is (1.515 *( 10^-4)) * P
Thus it implies that the power loss is reduced by a greater extent if the voltage is stepped up. Thus it is always efficient to transmit power at a higher voltage.
Conversely, the power loss can be reduced by reducing the resistance of the conductor.
r = ρ * l/a.
Thus by increasing the cross-sectional area of the conductor (diameter), the resistance of the conductor can be reduced and thus the power loss. But this involves huge increase in cost and heavy cables with supports. Thus this idea was not used to reduce the power loss during transmission and utilization.
Also a motor (let us assume a bow thruster), may be of a smaller size if it designed to operate on 6600 Volts. For the same power, the motor would be of a smaller size if it is designed for 6600Volts when compared to 440Volts.
Thus these are the major reasons why recent ships have shifted towards high voltage systems.