The selection of power system architecture for a ship primarily depends on the mission over the service life and the reliability desired. In discussing ship power system architecture, we often use the term electrical bus, which means the parallel conductor bars of a heavy cross section to which all ship generators and loads are connected.
The ship generators feed the power to the bus, and the loads draw power from the bus. The ship bus bars are typically made of thin rectangular copper bars, solid or sometimes hollow to circulate cooling water in high-power systems. These bars at ship are often called bus bars, or just the bus, or lines, or rails for their rail track look.
Modern power systems on ship widely use power electronics converters, which inherently draw non sinusoidal currents that cause high-frequency harmonics in the bus voltage.
These harmonics requires harmonic filters. Some navy ships require very clean harmonic-free power for harmonic-sensitive loads. In that case, a separate clean ship power bus is provided using a motor-generator set, which also provides electrical isolation for the loads.
Basic Conventional Ship single line power diagram
Basic architecture shown by a single-line diagram for a conventional
ship with mechanical propulsion. It shows two main 460 V generators and
one 460 V emergency generator with a battery for start-up and to power essential loads that must be kept on all the time. It has a receptacle for connecting to the shore power when the ship is anchored at a port and generators are turned off.
The loads are in two basic groups, heavy 450 V loads, and 115 V loads for small equipments and lighting. The refrigeration loads have their own control panel. The battery chargers draw power from the 120 V emergency bus.
Large cargo ship single line power diagram
A large cargo ship with conventional mechanical propulsion generally has a power system architecture with main generator’s power loads on the main switchboard, and the emergency generator switchboard serves the essential
loads, lighting, internal communication, and electronics circuits.
The ship emergency switchboard is normally tied with the ship main bus, with an emergency power source from an ship emergency generator or a battery via an automatic bus transfer switch that has two interlocks with electrically operated power circuit breakers.
With an automatically started emergency generator, the transfer occurs when the generator acquires a rated voltage at a rated frequency. With a manually started emergency generator, a temporary emergency power source battery or uninterruptible power source (UPS)—is installed.
Automatic bus transfer on ship
All automatic bus transfer switches have two interlocks with electrically operated power circuit breakers.
Only units that are marked for shipboard use are acceptable as these are ungrounded and equipped with switching to break both lines
L1 and L2.