Frequency Control Of a Power System
It can’t unless each generator has room to move up and down in its allowed power output band. The power output band is defined as its maximum capacity minus its minimum capacity. Each generator has a maximum power output based on the unit size and a minimum power output based on both economics and engineering.
The frequency is determined by the ratio of load to generation. If there is too much generation then the frequency will rise. If there is too little generation then the frequency will drop. Assuming the load is fixed, the generation output has to be changed to adjust the frequency. If the balance of generation or the share of total output is to remain the same, then each generator has to be able to move up or down in its power output band to adjust the system frequency. Now this will never happen due to economics.
Each generator has a cost curve which dictates the price at each output range the generator will bid at. In the electric market the system will use the cost curves to find the most economical dispatch. This means some units with a lower price will be 100% committed to their maximum output while other more expensive units may be committed at their lowest output or below maximum.
The cost is dictated by several factors such as startup costs, fuel costs, and operations and maintenance (O&M) costs among others. At the Pmin - minimum power output the cost will be comprised of a startup cost plus fuel costs etc... As the units output rises, the startup cost stays fixed but the fuel and O&M costs rise.
The system frequency will be maintained by bringing generators online and offline in an economical order. As load rises the higher costs units will be brought online as all the lower units are already committed and generating.
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