Two or more electrical power sources on ship are often connected in parallel to increase the load capability or reliability of the ship power system. In the case of two sources, both sources must meet the following conditions before they can be connected in parallel to share the load:
- Voltage magnitudes must be equal within a few percents.
- Voltage polarities must be the same (+ connected to + and − connected to −).
- The phase sequence of 3-phase sources must be the same.
- Frequencies of ac sources must be equal (else voltages will get out of phase).
The load shared by two static sources—such as two ship transformers, ship batteries, fuel cells, etc.—is determined by their terminal voltage droop lines.
With two static sources 1 and 2 the load is shared in parallel.
By parallel connection at a ship common bus, we force both sources to share the load such that they have the same terminal voltage equal to the bus voltage.
The load sharing on ship power system can be adjusted by raising or lowering the voltage droop line of one or both sources working in parallel. This is generally done by changing the no-load voltage, which in turn raises or lowers the terminal voltage versus load line while maintaining the droop rate.
Voltage sharing on ship transformers
In transformers, this is easily done by changing the taps on the primary or secondary side.
Voltage sharing on ship batteries
In batteries and fuel cells, it is done by adding or subtracting one or more cells in one stack. For two sources in parallel to share an equal load, it is important to have their no-load voltages and the droop rates equal or
as close as possible.
Voltage sharing on ship generators
For two ship dc generators in parallel, the foregoing analysis for static sources also applies. For two ship motors in parallel sharing a torque load on a common shaft, the analysis presented here applies equally well after substituting the voltages in Equation with individual motor speeds versus horsepower load. The horsepower sharing between two locomotive engines hauling a long train can also be determined by using their respective speed versus horsepower droop lines.
The load sharing among ac generators is complex, as both the real and the reactive powers are shared.