I have had a call out to a ship that is having problems with a reverse power alarm, causing a breaker to trip. The ship in question has just had its shaft generators overhauled away from the ship, and I have overhauled the breakers and Primary injected them, The breakers are Merlin Gerin Masterpact M breakers, they have a standard overurrent relay and are closed and tripped Via a PLC except in an overcurrent situation. when the shaft generators are run i am told they start fine but after a minute one trips on reverse power, the breaker does not trip only opens on command, now my question what would cause a reverse power fault? I have not been out to the ship yet so any ideas would be great, I don't usually deal with Generators just MCC'S and PLC systems so this is kinda new to me and college was a long time ago

 

Ships often have an assortment of configurations, shaft generators, aux generators, clutches, low/high speed shafts, prop shaft speed control, propeller pitch control. What is your system topology, any other generators on a common bus in droop or isoch? can you clutch out the prop and just run the generator, does the problem still occur?

I would say it is been driven to reverse power by another generator on the bus which is running at a higher frequency with no load sharing scheme.

My knowledge and experience with shaft generators is the engine/prop/generator shaft is held at constant speed and ship speed is controlled by varying the propeller pitch. There must be another generator some where on the bus otherwise the shaft generator would just support the islanded load.

If an auxiliary generator is closed onto the bus then it needs a load control scheme, typically droop, to control generator loads.

A ship i'm familiar with had two independent prop shafts each with a shaft generator. Normal mode at sea was constant shaft speed control with the bus tie upon and the lconsumer loads split evenly. It two completely independent electrical systems. When in port load could be transferred to auxiliary generators which ran in droop before stopping the main engines. One prop shaft could also be clutched out and both shaft generators connected onto the same bus.

Is only your generator reverse powering? or is the main engine also motoring? Do you have two main engines feeding into a common gearbox with a common output shaft?