Frequency Swings on Gas Engine Generator

[Phlipper]

We have a situation where two gas engine generators, 800 kW, 600 VAC had been running in parallel when one of these units started exhibiting significant frequency swings to the point where operations have taken it off line.

Each of these have Woodward 721 digital governor controllers with load sharing compensation and reactive power compensation features, both incorporated into the design of controls for these generators.

A similar incident occured two years ago; it turned out that the vent line on the I/P converter on the main fuel valve was plugged,not allowing the I/P to regulate the flow of fuel as called upon by the governor. This problem was rectified and tubing re-run to minimize this occurence.

Other than the above as a possible culprit, do you know of any other possibility that could present this type of frequency swing (when in parallel with another unit). Any good troubleshooting ideas? Operations do not believe that this problem could be fuel-related, and have focussed their cross-hairs on the engine controls.

Thanks,

Phlipper

 

[peterb]

Have you checked the power transducer? This is a prime candidate.

 

[DLwalsh]

Not being a gas engine expert but logically thinking about it and since you ruled out gas flow, I would check the electronics that controls RPMs. If you are to regulate frequency then there needs to be a device that measures RPMs and then develops a signal to increase or decrease RPMs to keep the frequency stable. If the RPMs are stable I would re-investigate fuel flow.
Good Luck Dave Walsh

 

[dpc]

Are you saying that the frequency swing occurs in only one generator when operating in parallel? If they are running in parallel, I would assume both are operating at the same frequency. Do these operate in parallel with the utility?

If the two units operate only in parallel with each other, then you might want to review the setup of the governors. Are they both in droop mode or is one in ischocronous mode and the other in droop.

If somehow both are set to isochronous, then I think you could have some big power swings as the units fight each other to control the frequency.

If both are in droop, they should be stable as long as the droop settings are the same.

There are generally auxiliary contacts from the generator breakers that are used to switch the governors between droop and isochronous. Maybe some problem has developed with this wiring.

If the unit can hold frequency when running isolated, then I think you can eliminate the fuel system as a probably cause and focus on the governors and the load sharing system.

As mentioned above, you might check out the speed sensor.

 

[peterb]

Good points, dpc. I was focussing on the load sharing operation mentioned by Phlipper, and I had assumed that the swings were manifesting themselves as load swings. Obviously, the two units can't operate at different frequencies when paralleled.
If the governors are set up for load sharing operation, then one of the primary inputs to the governor, in addition to the speed sensor, is a load signal derived from a power transducer. Failure or intermittent operation of this transducer could lead to erratic governor operation and consequent load swings.

 

[Phlipper]

peterb/dlwalsh/dpc:

Thanks for your replies. I will include these on my list-to-check.

I'll let you know how this all turns out.

Phlipper

 

[jbartos]

Suggestion: It appears that an analysis of the two-generator in parallel system, regarding their system stability and associated potential resonances, is needed.