The first and foremost task for shipboard power system design is to compile a table of all connected ship electrical loads along with their load factors in all operating modes of the ships. The major operating modes during a cargo ship’s port-to-port trip are:
- In-port loading the cargo
- Maneuvering in and out of the port
- At sea cruising at full speed
- In-port unloading the cargo
- Anchor and standby
- Emergency operation
Since all connected load equipment does not draw full power continuously, we must use their load factors for deriving the combined contribution to the ship generator kW capacity requirement.
That is, we must account for each wired load’s peak kW rating
and the load factor (time diversity in power demand).
The total load on generator in any particular operating mode is given by adding the (kW wired × LF) product for all equipments in that operating mode.
Calculate ship generator kW rating
The generator kW rating is the maximum of all powers calculated by Equation in all operating modes, that is,
Pgen = Maximum of {Pat-sea, Pmaneuver, Pinport-load, Pinport-unload}
Allowing a 30% margin is common for growth and for uncertainties in loads during a preliminary power system design of a new line of ship. In heritage design, a 10% margin may be adequate.
Wire ampere rating for ship system
Although each load has its own average contribution to the generator power rating, the wire ampere rating for each wired load must be based on its own peak kW rating with 30% margin as follows:
Ampere rating of each load wiring Peak watts = 1.3 each load can draw / Load voltage × Power factor
The cargo-handling machinery during in-port unloading generally consumes high power, making the Pinport-unload the maximum load on a conventional ship. But, this has to be ascertained by actual load analysis. In an electrically propelled ship, the load during cruising at sea is generally the maximum.
A load analysis for a typical midsize cargo ship finds the following electrical
loads (not including 10,000 hp mechanical power for propulsion):
- 1250 kW at sea
- 800 kW during maneuvering
- 7000 kW for in-port cargo unloading
- 700 kW for in-port cargo loading
- emergency load power of 400 kW
How we determine the ship generator kVA and the diesel engine horsepower ratings
Assume a reasonable power factor and efficiency where needed.
First, we assume a generator efficiency 95% and a pf of 90% lagging. The
ship’s generator should be rated for Maximum {1250, 800, 7000, 700} = 7000
kW that will meet the maximum power demand during in-port cargo unloading.
The generator kVA rating = 7000 ÷ 0.9 = 7778 kVA, and the generator
input = 7000 ÷ 0.95 = 7368 kW, which is the diesel engine output rating. So,
the engine horsepower rating = 7368 ÷ 0.747 = 9863 hp.
Ship shore connection power calculation
If the shore power connection were available when required, how would the generator and engine ratings change?
For this ship in particular, use of shore power during in-port cargo unloading can significantly reduce the generator kW rating from 7000 to 1250 kW. This would reduce the generator kVA rating to
1250 ÷ 0.90 = 1389 kVA,
and the diesel engine kW rating to
1250 ÷ 0.95 = 1316 kW
These are significant reductions in both the generator and the diesel engine ratings, to about 1/5th.
The subsequent capital cost reduction must be traded off with the high price of shore power compared with the self-generated power on the ship using cheap oil. However, many ships now may still favor expensive shore power due to recent emphasis on ship emissions at busy ports around the world.
