In a thermoelectric power plant, the steam turbine unit is composed of the coal/gas-fired boiler, which produces the steam; the turbine, which is driven by the steam flowing from the boiler; and by the generator, which is propelled by the turbine to generate alternating current. The AC electricity generated by this set is linked up to the electrical power grid and to the auxiliary power system in the plant.
The gross electrical output is divided into the net output, which is connected to the electrical power grid outside of the plant, and the auxiliary power system output within the plant. On the other hand, the gross input is represented by tons of coal or volume of gas, measured in cubic feet, needed to produce steam in the boiler per hour.
Large steam turbine generator units have a series of steam admission valves, which are opened in sequence to obtain an ever increasing output of the unit. As the unit loading increases, the input to the unit also increases as the incremental heat rate decreases between the opening points for any two valves.
The common header plant is another type of steam unit which one can encounter. It consists of a number of different boilers which are connected to a common steam line, which is called a common header. In this type of power plant, there is not only a number of boilers and turbines, with each one of them connected to the common header, but there is also a “topping turbine” connected to the common header. A topping turbine is one in which steam is exhausted from the turbine and it is not fed to a condenser but to the common steam header.
Below, a diagram of the steam-turbine-generator unit, where the steam is represented by the boiler.
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