Considering only the secondary steam turbine stage of a thermoelectric plant, if the system is shut down, and the pressure is at equilibrium throughout the system, how is a pressure differential initially created across the vanes to begin rotation? Or is it jump started somehow? Does it have to be decoupled from a load first?
first you start the boiler, which makes steam. you shut the steam valve to the turbine until the boiler reaches its operating pressure, then you open the valve. Now you have hot steam flowing through the turbine, and it begins speeding up. When it reaches its normal operating speed, you then apply the generator load by ramping up the field coil excitation until the rated voltage output is achieved.
This is a simplified picture. In actual practice there are automatic control systems that modulate the output of the whole plant by simultaneously varying boiler fuel flow, steam pressure, steam flow rate, and generator output. And the process of bringing a generating plant fully on-line from a cold start takes many hours.