General components

The gas turbine is the single most important component in a combined cycle power plant. The fact that this type of electric power generation plants has become a competitive thermal process is due essentially to the rapid technological development experienced by gas turbines, especially as regards the improvement in performance of the corresponding thermodynamic cycle.

A gas turbine is a drive turbomachine capable of converting the thermodynamic energy of a gas into a useful pressure on a shaft. In open cycle turbines, the gas is generated in the unit itself and at the time of use. This gas will be the product of the combustion together with compressed air, in a combustion chamber situated before the turbine itself.

The steam turbine transforms the thermodynamic energy from the steam generated in the reheat boiler into mechanical energy at the shaft of the machine.

The steam turbines used in combined cycle facilities are of simple design, with relatively low design parameters for fresh steam (corresponding to the level of high pressure in the reheat boiler): temperatures in the 420°C-550°C range and pressures in the 60 bar-130 bar range. In any case, work is taking place in order to increase the pressure, and even supercritical pressures can be reached (higher than 220 bar).

The figure that follows illustrates a simplified diagram of a steam turbine used in advanced combined cycle power plants. It corresponds to a single shaft, double body steam turbine, with intermediate reheating, in the 130 MW-260 MW power output range.

During the normal operation of the plant, the electrical power generated in the heat cycle will permit the plant’s own consumption to be supplied and the rest of the production to be exported to the grid. In this lies the difference between the gross and the net power output of the plant: the first will be the total power output obtained at the alternator connection terminals, and the second is the result of subtracting the part required for the plant’s own consumption.

The main component in electrical facilities is the alternator that transforms the mechanical energy at the shaft into electrical energy. The configuration used in the majority of combined cycle facilities is that of direct connection to the turbine, using twin pole alternators in these cases. In those systems whose power output is smaller than 50 MW, the habitual use of high speed turbines leads to systems of indirect connection, through a reduction gear device. In these alternatives, four pole alternators are more economical.