Paralleling Generators

cac86,

>My friend told me that, after the governor of the engine
>went to 100% they have a blackout,they didn't realize
>exactly the sequence of events, because everything was too
>fast.

>Anyway they found reverse power on the other 2 running DG.
>They found over frequency on users breakers, and they found
>overspeed on the faulty DG.

All of this is consistent with what I would expect would happen for a very simple system consisting of one DG operating in Isochronous Speed Control Mode, and two DG sets operating in Droop Speed Control Mode at a low(er) load. The Isoch unit burning excess fuel for the conditions--and going to, essentially, max fuel flow-rate--would cause the frequency of the grid to increase. The Droop machines would then start to unload as the frequency increased (REMEMBER: Droop Speed Control is all about the error between the speed reference (which isn't changing in this case!) and the actual speed (which is increasing in this case)--which, in this case, would be driving the fuel flow-rate, and the power outputs of the Droop units, down--to reverse power.) The Isoch unit would go to overspeed at some point, and the overall result would be a black-out. All of which is consistent with what your shipboard friend reported happened.

The Droop units are not going to contribute to frequency control--that's the job of the Isoch unit, and if it's governor has malfunctioned resulting in 100% fuel flow-rate then the frequency (speed) of the grid is going to increase (since the load on the system isn't very high). The decreasing speed error between the speed reference (let's say it was at 101.33% speed) and the actual speed (which was increasing and approaching 101.33%) would be driving the speed error to zero as the actual speed got closer and closer to the speed reference, and that would be driving the fuel flow-rate lower and lower.

Fuel flow-rate--when the unit is synchronized and running at rated speed (frequency)--is directly proportional to the amperes being produced by the generator. In the same ways motor convert amperes to mechanical torque, generators convert mechanical power (torque) to amperes. The torque the generators convert to amperes comes from the prime mover driving the generators--diesel engines in this case. If the amount of fuel is not sufficient to make the generator have positive amperes (flowing out of the generator on to the grid) the reverse power protective relay is going to trip the generator breaker to protect the diesel(s). Now all of the power being produced is coming from the Isoch unit--whose fuel flow-rate is "uncontrolled" and much more than is required to supply the loads AND maintain rated speed (frequency). And, so the protection functions are going to shut down the Isoch unit. Probably initially because of a high frequency (speed) which may have been close or at some point exceeded the overspeed setting of the governor.

Hope this helps!
 
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