Parallelling of GTG with different ratings

[raptor6458]

Hi guys, just wanted to ask if my new gas turbine generator rated 11500kW, 6.6kV, 3phase, 50Hz, with an NGR can be paralleled with the existing GTG of 10800kW 6.6kV, 50Hz, 3phase, 50Hz also with an NGR. These gen set are use on offshore platforms.
Instrumentation, control and synchro of the GTG's are all in place. What will be the possible problems we may encounter?
Thanks for any suggestion and help you can share.

 

[slavag]

Hi.
Possible only one "problem", check level of earth fault in case of paralleling NGR and option of circualation current between generators NGR ( 3-rd harmonic current)
Of course you have check paralleling work of AVR's and governors, id all ready to this work, share of reactive and active power.
Best Regards.
Slava

 

[waross]

Droop on both governors should be set the same. The droop setting determines the load sharing. Isochronous mode doesn't work well with paralleled generators. Isochronous may be used on one set, but it will require constant operator supervision (check and adjust every 15 minutes or less).
If you have a load control panel controlling the generators it may be set in isochronous mode.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nawao]

waross,
Advanced electronic governors like Woodward 2301A load share & speed control unit, allows correct load sharing in isoch mode. In the other hand, in an islanded system the only way to safely control constant frequency is operating in isoch mode.

 

[raptor6458]

Slavag, waross & nawao appreciate your help!!!!
Thanks guys!

 

[waross]

Actually, I consulted for a number of years for an islanded system on an island. We ran multiple paralleled generators. We found that droop mode gave adequate frequency control and was trouble free. We discussed going to iso mode but rejected it because of the possible real problems that may arise with parallel generators trying to run iso without a load control panel. Some years ago a new generator was purchased. Citing warranty issues, the vendor forbad me to touch the controls and then spent over a year proving what problems could arise trying to run multiple sets in iso without a load control panel. And at that, they couldn't get the system stable enough long enough to discover all the possible problems with ISO. I had other things to do and stayed away from the circus. By the way, A rock solid frequency is nice but not always necessary. Our frequency drifted one or two percent and never caused an issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nawao]

I agree with waross that is impossible to run iso without load control. Load sharing lines must be interconnected between all the digital governors, and among other signals, the Woodward 2301A load share & speed control unit needs the imputs from the kW transducer of each generator panel in order to achieve correct speed control and load share.
I agree that “a rock solid frequency is nice but not always necessary” but when is necessary like Airports where very sensitive equipment coexist there is not other alternative than running isochronous.

 

[waross]

And even with some isochronous schemes, the response to a block load change is a droop response with the attendant frequency shift. The isochronous feature then corrects the frequency. It may take a few seconds. The UPS sets may beep and chirp and flash their lights when this happens.

Bill
--------------------
"Why not the best?"
Jimmy Carter