In order to keep the frequency of power grid constant, the speed of steam turbine needs to be maintained in a certain range. The main valve opening of the steam turbine is adjusted to increase or decrease the steam flow into the steam turbine. When the steam flow is reduced, the output of the steam turbine will be reduced, so the magnetic field strength of the generator will be reduced to maintain a constant speed. Because the magnetic field strength of the generator is reduced, the power generation will be reduced.
