starting a steam turbine involves minimizing the thermal stress as the turbine metal temperature is raised from room to main steam temp

by using low pressure/(low temperature steam, the amount of super heated steam that initial washes over the cold turbine is reduced, thus allowing the turbine metal temp to increase slower.
Also the low pres/temp main steam will have reduced work extracted in the first turbine stages and allow steam with residual super heat to flow to the latter stages to warm them.

if rated steam was used to roll the unit, the energy to establish minimun generator load would be extracted just from the first stages and the steam condense mid stage. besides the errosion from water droplets, the latter stages will "pump" them out, adding heat and actually the exiting steam will overheat the last stage. a startup phrase heard was to use reduced main steam to "cool" the last stages.


Genrator output is DIRECTLY propertioal to steam flow,

sorry...got distracted and didn't want to delete what I had started. But can't finish now
edit.... back now

as starting to say steam flow IS generator output. a lot of control schemes use a stage pressure, which is also propertional to load, to calculate steam flow through the turbine.

the details for your startup curve are not complete and thus assume one knows how a turbine operates
at your 120MW plateu. the boiler pressure can be increased by increasing firing (and slightly increasing steam flow. thus if you hold the CVs at the point where 120MW was achieved with reduce pressure, as the boiler pressure is increased, the steam flow will increase slightly alone with MWs.
Now, once at rated pressure, if you increase the CV opening to increase MWs, immediately following that sight increase in MWs, the boiler pressure will droop. thus with constant firing, the MWs (and steam flow) will be at a constant equilembrum with pressure responding inversely to CV opening.

So when that inital decrease in pressure is seen, if the boiler controls are set to maintain constant pressure, the firing rate will increase as the CVs open and thus MWs increase with opening CVs at rated pressure.

the boiler controls and turbine controls can be coordinated so that there is no initial droop.

during the discussion, the comments are that the turbine CVs can be positioned as desired (with a closing over ride if needed by the speed governor) BUT the turbine controls can be a lot more complicated with the CV positions calculated not only from desired position but have feed backs from main steam or stage pressure

It is difficult to say why the turbine start-up curve is drawn without more information about the application, such as, type of thermal cycle, rated power output, rated inlet steam conditions (pressure and temperature). However, for large turbines with high inlet steam pressure, manufacturers generally recommend reduced pressure for starting. Reduced pressure at start-up will ensure better control of a relatively low steam flow.

Best of luck!