Before I answer, I think if you use the Search box in the top right corner of the forum and search for "Temperature Control" you should find some relevant information, it's been discussed many times before.
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There are at least three "fields" on the left side, center (vertically) of most GE-design heavy duty gas turbine HMI displays. The three are usually: Status, Control, Sequence (if I recall correctly).
I would guess the field you saw change to "Base Load Temperature" was the 'Control' field.
The fuel which is being admitted to the turbine can be controlled by any one of several methods: Start-up, Acceleration, Droop Speed Control, Isochronous Speed Control, Shutdown Control and Exhaust Temperature Control. The last one, Exhaust Temperature Control is referred to by many different terms, including Base Load Temperature Control, Base Load, CPD_biased Exhaust Temperature Control, and so on.
When the given (average) exhaust temperature for a particular axial compressor discharge pressure (signal name CPD) reaches a certain value, then the unit is said to be on Exhaust Temperature Control, or Base Load Temperature control, or CPD-biased Exhaust Temperature Control. At this point, no more fuel can be admitted to the turbine; it is operating at its maximum allowable fuel flow-rate/power output for the given conditions at that time (axial compressor cleanliness; inlet filter cleanliness; ambient temperature and -humidity); hot gas path part condition; exhaust duct back pressure; ambient (barometric) pressure), etc.
When an operator selects Base Load operating mode, that tells the turbine control system to increase the fuel until the unit reaches the maximum allowable exhaust temperature for the axial compressor discharge pressure and then maintain that fuel flow-rate (as a function of exhaust temperature versus axial compressor discharge pressure). The unit is said to be "at Base Load" which is the maximum allowable power output possible for the current operating conditions of the machine (external (ambient) and internal (mechanical)).
The GE-design heavy duty control community here at control.com has been going strong for more than 14 years, I think. Many similar questions have been asked and answered in that time--repeatedly. All of the past questions and answers can be accessed by using the 'Search' feature cleverly hidden at the far right of the Menu bar at the top of every control.com webpage. (The search format and terms are not intuitive--you need to probably use the Search 'Help' the first couple of times.) If you don't find what you're looking for, then we can usually answer questions. If you find what you're looking for, and you like the answer(s), click on the 'Thumbs Up' icon at the top of the page. If, however, you don't like the answer, please--post your problem with the answer and we'll try to correct it.
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