Question

why thermal power plants have minimum and maximum output

Answer 1

For thermal power plants input is coal. By burning the coal heat energy will be produced. This will turn water into steam and given to steam turbines. Now this steam turbines will rotate generator rotor and electrical energy will be generated.

Now even generator is not connected to grid, means there is no output from generator. In this case the energy required to burn coal, producing steam and everything is wasted because the output is zero.

Power plant organization will be in loss. Because they are not getting any money from the generator. So every generator will be fixed with some minimum output power considering economic constraints of the generator.

Whenever the load in the system increases, the generator will pickup and supply that load. Output power of the generator will increase. We know that from P-delta curve, to generate more power the delta (load angle ) of generator should increase. Increase in delta value results decrease in stability margin of generator. So we normally fix the amount of maximum power one generator can generate.

The windings of the stator and rotor of an generator are designed for specific amperes and insulation is designed for specific voltage. So generator should not generator more than these rated voltages and rated currents . So the maximum power output of a thermal plant depends on these factors also.

The minimum output power of thermal plant des on on economic constraints and maximum power output depends on thermal rating of generators and stability of the system.