High Voltage System on Ships

As the ship’s size and capacity increases, bigger machinery/ equipment are installed to ensure its operational efficiency; however high voltage is used only for few important machinery systems.

Usually a 3phase, 60Hz, 440 Volts supply is generated and distributed on board ships. As the ship size increases, there is a need to install more powerful engines and other machinery systems. This increase in size of equipment demands more electrical power and higher voltages.

Any voltage used on ship, if less than 1kV (1000 V) is called as LV (Low Voltage) system and any voltage above 1kV is termed as high voltage system. Typical Marine HV systems usually operate at 3.3 kV or 6.6 kV. Passenger liners such as QE2 operate at 10kV.

Why High Voltage on Ships?

Let us assume that a ship generates 8MW of power at 440V, from 4 diesel generator 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 * 10(power 6)/ (√3 * 440 * 0.8)
I = 3280.4 Amps (Approximately, 3300 Amps)
The protection devices such as circuit breaker should be rated at approximately 90kA for each feeder cable. Let us now calculate the current if the generated voltage is 6600Volts.

Image Credit: Wilhelmsen

I = 2 * 10(power 6) / (√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 - the current carried by the conductor, R - the resistance of the conductor.

The 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. Also, it is always efficient to transmit power at a higher voltage. Conversely, reducing the resistance of the conductor can reduce the power loss.

By increasing the cross-sectional area of the conductor (diameter), the resistance of the conductor and the power loss can be reduced. But this involves huge increase in costs and size/ weight of cables, requiring additional supports. This method is therefore not used to reduce the power loss during transmission and utilization.

Also, it is not necessary to a have bigger size motor for high voltage systems. This means that the motor can be of a smaller size even if it’s designed for 6600 Volts as compared to that of 440Volts. Thus, most of the new ships are fitted with high voltage systems.