CCGT minimum operating load

[kkanel]

Does anyone have an idea on how low one may operate continuously a combined cycle plant- or rather how low such a plant may be designed to operate without by-passing steam?
My current problem is that I am working on this 2 GTs + 1 ST configuration and I am trying to find out whether I may operate the plant with the ST at 20% its nominal load. That is, one GT running at approximately 40% load.
I run heat balance calculations and I get only warnings of steam piping loss increase, which may be countered by proper pipe design, but other than that not much else.

 

[DougMSOE]

I would suggest to look at the energy giong into the shafts of the various generators. Please remember that the stability of the prime movers/generators is dependent on a balance between the electrical energy OUT to the mechinical energy input. This whole discussion may come down to how accurate the control valves are and how much control you have over them. Take a hard look there.