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Excessive frequency fluctuation on marine genset

Excessive frequency fluctuation on marine genset

Excessive frequency fluctuation on marine genset

I have a strange problem where I am seeing excessive frequency fluctuation, especially when the generator is online (the other generator operates with no issues seemingly ruling out a buss or consumer issue). I am getting a slight fluctuation while the generator is offline, and as you would expect it is pronounced when it takes a load. We are not running in parallel. When starting a motor or a higher demand item we are getting the fluctuation that seems to take too long to settle out. This is a Cummins engine and as such does not utilize a traditional fuel rack system. The governor is electric and we have replaced the card and attempted to solve the problem with the gain signal, but no success. The droop of my other generator is set to 20%, so I see the new governor card the same. I have not yet tried setting the droop on the fluctuating generator to 0%, as there are short times where the ships crew will parallel (but only for changing over gens, they do not operate in parallel). This same problem has occurred on this generator before, and the AVR and fuel pump was changed with no effect. All fuel filters are new, and the tacho pick up setting has been verified. I'm not opposed to changing the fuel pump again (or the AVR but I'm less convinced the AVR is the issue) as this troubleshooting occurred before my arrival so i didn't see exactly what happened. The only remaining option that comes to mind are the injectors. Thanks in advance for any ideas.

RE: Excessive frequency fluctuation on marine genset

AC generator frequency is solely determined by its speed, AVR plays no direct role in it.

Governor, fuel lines & pump, engine internals are most likely causes.


RE: Excessive frequency fluctuation on marine genset

Quote (topes)

...The droop of my other generator is set to 20%, so I see the new governor card the same...
20% droop seems greatly excessive to me. Are you sure that is the correct figure?

RE: Excessive frequency fluctuation on marine genset

3% droop is common for islanded sets. 5% is common for grid parallel sets. The no-load speed should be set to 103% or 105% depending on the droop.
20% droop will cause excessive, load dependent, frequency drops.
On a 60 Hz set, 20% droop will cause the frequency to drop to about 48 Hz with 100% block loading. The voltage may drop to 80% to 85%. due to UFRO action.

Ohm's law
Not just a good idea;
It's the LAW!

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