In our plant, we have two frame 5GT's (12 MW each) and one H25 (25 MW) Hitachi Gas turbines. The Hitachi Gas turbine has FSRLLBIAS factor kept as 3% to restrict the GT to take sudden load. It is limited to take 4.2 MW maximum only. Why is this factor necessary? According to them it is kept to prevent sudden frequency drop. Is there any restriction on GE machines for the sudden load run up? What is the allowable sudden load run up in GE gas turbines?
I'm not familiar with the H25 machines and precisely what might be the reason for the loading rate restriction(s).
Most of the GE-design B/E heavy duty gas turbines (so not F-class machines or H-class machines) could be loaded from zero load to Base Load in as little as 30 seconds (Fast Load automatic loading rate), and when the frequency dropped the loading rate was even faster (nearly infinite--because as the error between the turbine speed reference and the actual turbine speed changes so does the speed-biased fuel control reference).
I would have to believe that there is some mechanical reason (hot gas path parts--fuel nozzles; liners; turbine nozzles; turbine buckets) that results in a maximum load value, but that doesn't make a lot of sense because you aren't defining how quickly the unit can make the load change.
As with all things combustion-related, it's really the rate of change that causes the most problem for hot gas path parts, including the exhaust diffuser and even the exhaust duct components especially if the GT exhausts into an HRSG (Heat Recovery Steam Generator--a boiler--to produce steam). Sometimes the HRSG components can't stand a fast change in exhaust heat. It's the thermal stresses resulting from a very fast load change (which results from a very fast change in firing temperature and exhaust temperature) that are most injurious to hot gas path components and what are trying to be avoided.
Why is it felt it is necessary to load the unit faster? Is it because of frequency excursions, or some other desire/need?
>Why is it felt it is necessary to load the unit faster? Is it because of frequency
>excursions, or some other desire/need?
The question arise since at two instances, Hitachi machine did not pick up load when one of the GE machine tripped resulting in low frequency. We are not connected to the grid.
Hitachi machine was on ISOCH with 8 MW load and other two GE machines (8 MW and 4 MW load) on DROOP. 8 MW loaded GE machine tripped, resulting the drop in frequency from 60 to 58.5 HZ. Reason was the Hitachi machine picked up only 5 MW in ISOCH mode.