8.1.3   On board
The power supply on board is provided by the electric power generation system of the ship. The generators are either powered by auxiliary diesel motors with capacities of up 4 MW and/or using a shaft generator driven by the main engine. Since the main engine runs at different speeds depending on the current speed of the ship, it will be necessary to use a frequency converter when using a shaft generator in order to ensure that the power supply frequency remains constant. This may lead to high-frequency harmonics in the power supply frequency which may in turn lead to problems with electronic components. In particular, operating a power line monitoring system (see below) is often made more difficult as a result.
On ships with a relatively small number of slots for refrigerated containers, the generators usually run at the on-board voltage/frequency of 460 V / 60 Hz, although other voltage/frequency combinations are also found. On the other hand, if there are large numbers of refrigerated container slots, a medium voltage busbar is used (2 to 4 kV) because otherwise the current levels are too high. In order to supply the refrigerated containers with power, this voltage must be transformed down to the operating voltage used by the container.
Some ships do not have sufficient generator capacity to power the refrigerated containers, and thus, during peak times, mobile diesel generators housed in 20' containers are made available on deck. These can either supply the refrigerated containers directly or the power they generate flows into the onboard.
The onboard power supply network is usually rather erratic and is subject to frequent fluctuations in frequency. Since these networks are always small isolated networks, any special considerations need to be taken into account.

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