Balancing Output to Demand in an Electrical Power Plant

The power stations on a grid are divided into several categories. Plants designated as "base loaded" produce all the power they can because they are the cheapest producers. They try to run at 100% all of the time, assuming that there is enough connected load on the grid to take all of the output from the base loaded. Obviously, it there is not enough load, at least one of the base loaded units must cut back. Nuclear power plants, hydoro-electric power plants and coal fired power plants are typical base loaded units. When natural gas is cheap, then combined cycle plants are used as base loaded units as well.

There is an entity for every grid calle the "Load Dispatcher". This organization make decisions concerning who is going to make the power. In a competitive environment, power producers bid, and the dispatcher takes the lowest bid for the next increment of power needed. Otherwise, the Dispatcher simply orders the lowest production cost unit to come on line or increase power.

One or more units on a grid are designated for load following (or load control or frequency control) duty. They run up and down in power as required to maintain the power generated equal to the power consumed. An increase in load on the grid will decrease the frequency of the grid as all machines will tend to slow down. The load control units will respond by opening their throttles to produce more power and bring the grid back up to 60 Hz (or 50 Hz).

Another important quantity is the grid voltage. The grid voltage depends on the level of field excitiation of the generators. There is reactive power in addition to real power in an AC distribution system. Both the reative power and the real power need to be controlled.

Within normal limits, the real power produced is controlled by the throttle and the reactive power "produced" by a generator is controlled by the field excitation.

The reactive load is shared between generators by adjusting the field excitiations, just as the real power is shared by adjusting the throttles. If the field excitation of a generator is increased, it increases the voltage of the grid and also causes that generator to assume more of the reactive load (and the other generators less).

Naturally, if there is only one generator on a grid (isolated grid), then the real load and reactive load that the generator sees is completely determined by the connected load. This one generator, obviously, must make all throttle and voltage regulation changes necessary to maintain the frequency and voltage of the isolated grid.