Generator issues

[sjvanhecke]

I have a garbage burning generator that is having load fluctations. The plant has 3 generators, (2) 13.5MW units, and this 25MW unit. The twin 13.5MW units are stable, but the 25MW unit is constantly moving about 1 MW every minute. The 25MW unit swings even if the two 13.5MW units are offline. All 3 generators are, of course connected to the grid, and they all individually sustain their own load (their boiler feed pumps, ID & FD Fans, etc.). The govenor has been completely gone through and has been thought to not be the problem. The problem occurs no matter what mode the unit is in (droop, island, etc.) ANy help would be appreciated.

 

[powerjunx]

HI SJVANHECKE,

IN OUR PLANT, WE ENCOUNTER THAT KIND OF SCENARIO YOU HAD.
BUT MAY I KNOW WHAT KIND OF GOVERNOR, SPEED AND LOAD CONTROLLER WERE USED IN YOUR GENERATOR SET? I GUESS YOU USED WOODWARD BRAND AS WE HAD.

AND ALSO PLEASE GAVE ME THE OVERALL SCENARIO: THE SET UP, THE TIME IT FLACTUATES AFTER DELIVERING A LOAD, THE TIME/ LOAD SCHEMES, ETC.

 

[harmonicoscillation]

I've run into this any number of times. I'm only aware of two possibilities, barring anything really bizarre. It's either a governor issue or a control valve issue. If you have a (steam) control valve that is binding, it will constantly over- and under- shoot causing a significant oscillation in output. That is actually pretty common. It may even be even more common than a governor issue.

 

[harmonicoscillation]

I also want to note that the trouble spot can be pinpointed with some degree of certainty by observing the characteristic of the output. If the load swings are a nice smooth sine wave the governor is more suspect. If the swings are irregular a control valve is more suspect.

I would also note that this is not a problem the unit can live with for the long term, as it has implications for the mechanical condition of the unit.

 

[powerjunx]

harmonicosill..,

you are on the right track but still this is a problem,
since "25MW unit is constantly moving about 1 MW every minute" as it was stated by sjvanhecke.. imagine after 25 minutes the generator is unloaded! or perhaps it will trip due to overspeeding as it abruptly never holds the V/F ratio.

sjvanhecke..
can you give me any history upon maintenance or repair that was done to your 25MW generator? is there any history of governor repair?

 

[gentech]

This may seem silly. What do you mean by garbage burning. You mention bioler feed pumps so I assume steam as prime mover. If your fuel is methane from a landfill that could cause instability due to varying HHV of the off gassing.

 

[ScottyUK]

When you state that the control valve is not moving, are you relying upon the feedback transducer on the valve to determine this, or have you physically measured at the valve with a clock gauge (or even a ruler)? Measure at the valve if you have not already done so. Check the feedback transducer - is it an LVDT? - is securely fastened to the valve, and that any linkages are not worn or loose.



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If we learn from our mistakes,
I'm getting a great education!

 

[thinde]

My suggestion would be that it is not the governor / load controller, but it is the linkage from the valve actuator to the steam valve. It doesn't take much movement in the linkage system to cause a load swing.

 

[sjvanhecke]

I guess saying that the "load" fluctautes is not correct, the generator output changes by 1 MW.

 

[powercomplex]

1-Reduce its sensitivity by adjusting drop at Governer setting and observe the responce.
2-Redue integral action, if PID incorporated.
3-Check MW feed back comming to control, complete loop
4-Service or replace linkage
5-Check total span movement of valve with signal range, calibrate if required. for example signal 4-20mA , opening 0% to 100%. should be linear responce.
Regards,