Normal and Droop Mode on an "Island" Grid

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Thread Starter

Mikey15

Hello,

I am very new to this industry and I had a few questions regarding the equipment we have onboard here. I work on a steam ship that is equipped with five GE steam turbine generators and two emergency diesel generators made by ALCO. all generators are equipped with Woodward 2301 governors that are designed to operate with each other in both isochronous and droop mode. Part of my job onboard is to operate the distribution system via an analog board. The plant isn't very big in comparison with some of the examples that I've been reading about here, but I suppose it is a good start for me.

The turbine generators are rated at 4000 amps, 2500 Kw, and produce 450 volts at 60Hz. The diesels are slightly less at 3200 Amps, 2000 Kw, with 450 volts at 60Hz. All I do is maintain power on half the bus, which is split forward and aft. I know droop mode and isochronous mode have been beaten to death here, and I apologize for bringing it up again, but curiosity has gotten the better of me.

Our generators have the option of being operated in four modes "normal" and "droop" on the voltage regulator, and "normal" and "droop" on the governor. I've only seen droop mode discussed as it relates to speed and the governor, but I am assuming the same concept applies to the voltage regulator. Also, given my understanding of isochronous mode, I know that the generators are not operated in true isochronous mode on either the governor or voltage regulator. We configure the plant with two generators on each bus with one more on stand by. We operate these generators in "normal" on both the VR and the governor, and they have no problem sharing the load. Is this mode a type of droop mode? I know the circuit samples the load on the bus via current transformers and the signal gets sent to the governor via an amplifier that also takes input from frequency sensors. Thus 60Hz and 450 volt are maintained automatically regardless of loads put on and taken off the bus.

We use droop mode when we are taking a generator offline, in which case we put only the governor on "droop" and give the load to the other generators. Also when shifting from ship to shore power and vice versa in which case both the VR and the governor are placed in droop mode. I understand why we can't maintain "normal" mode while paralleling to the grid, but I still don't quite understand exactly what "normal" mode is.

While in "normal" mode we can share the load, both Kw's and amps by raising on one governor while simultaneously lowering on the other one, the same applies to the voltage regulator for the amps. From my understanding, we are just changing the reference point the generators are working with and changing how much steam is going through each governor thereby changing how much load each one has while not actually changing the speed. if we want to change the speed, we raise or lower on both governors at the same time and the load remains the same on each but the frequency is raised or lowered accordingly. The same process works for the voltage regulator by controlling the excitation applied to the field windings.

Seeing as how the generators are designed to produce 450 volts and 60 Hz, "normal" mode will compensate for any load changes that are put on the generators, but we can still manually change the voltage with the raise and lower switches which actuate motor operated potentiometers which controls the signal sent to the governor and voltage regulator respectively.

I am just unclear why our system seems to differ so much from all of the others that I read about. Even other "island" grids like ours seem to run one generator in isochronous with any others in droop as standard practice. The only answer I can think of is that our "normal" is some sort of droop mode, but then what does that make our "droop" mode?

Sorry if I wasn't clear, I will be happy to answer any questions you might have, and I appreciate in advance any and all help in clarifying this for me.

Thanks,

Mikey
 
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Peter Macalua

I sympathize with your predicaments on the splits between normal, droop and isochronous. Since your on a ship, iso should be your normal mode for your steam turbine -generator. And islanding. Good luck.
 
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