Economic Dispatch:

The change in generation in steady state after a load change (due to governor and AGC action), may not be the most optimal one from an economic standpoint. Note that different generators may have different cost of generating power. A power plant or system operator may wish to re-adjust the generated power in various units for maximum economic benefit. This adjustment of generation is generally done manually and is often called "tertiary control".

Of course, how this re-adjustment is done is dependent on generation cost, power -pricing mechanism and the ownership of generation. Interestingly, if power pricing itself is made a function of frequency, then tertiary control also may contribute to (slow) frequency control ! This idea (called Availability Based Tariff) is implemented in some regions of our country.

Limitations of prime mover systems:

Generation reserves available for control of frequency are typically only a fraction of the existing generation. Moreover, there are practical constraints like limits on the rate of rise of prime mover power (in a steam turbine) to avoid rapid heating. While an initial sudden change of about 10% can be tolerated, subsequently, rate of rise is limited to 2% (of plant MW rating) per minute. The boiler in a steam prime mover is relatively slow in maintaining the steam pressure by increasing fuel input. Thus as control valves open, restoration of steam pressure is slow. For some types of hydro-turbines, there are forbidden operating zones due to cavitation effects in turbines.

In countries like ours, where substantial generation shortage exists, load shedding may be done to keep frequency within bounds.