Why shaft will still rotate when there is no generation in steam power plant?

Answer 1

The shaft still rotates but the generator takes only enough mechanical power from the turbine to overcome friction. Thus very little steam supply is required for the turbine and fuel is used at a very slow rate.

In this condition the generator still runs at synchronous speed but it is a neutral load on the grid, drawing no power and supplying no power.

If the steam were turned off, the generator would continue to run at the same synchronous speed, but as a motor, drawing enough power from the grid connection to overcome friction. In this state the mechanical phase of the rotor would lag slightly (as a load its power factor could be controlled via the field current).

Now if the steam supply were turned up, the phase of the rotor would advance, while the generator would continue running synchronously, but it would now start pumping power into the grid and fuel would be used at a rate proportional to the power supplied.

Thus a turbo-generator set quickly adjusts its rate of fuel consumption according to the power it supplies to the grid. In this way the quantity of fuel used and CO2 emitted (if carbon fuel is used) are closely tied to the amount of energy produced.