Schema of a steam turbine of a typical 3000MWth PWR.
A generator, or electric generator is a device that converts mechanical energy of the steam turbine to electrical energy. Since any AC electric generator can be called an alternator, for large power plants engineers also use the term turbo-alternator, which refers to alternators driven by steam turbines. Since the turbo-alternator usually consists of two parts, the larger of the two sections is the main generator and the smaller one is the exciter. In thermal power plants, there is usually one main generator, which provides all of the power for electric power grids. A steam turbine is usually on the same shaft with a main generator and an exciter.
In general, a main generator consists of a rotating part and a stationary part:
- Stator. Stator is the stationary part of an electric generator, which surrounds the rotor. The stator has a wire winding in which the changing field induces an electric current
- Rotor. Rotor is the rotating part of an electric generator and generates a magnetic field.
A typical turbo-alternator in nuclear power plants consists of:
- Main generator
- 4-pole hydrogen-cooled rotor. The rotor winding is made up of hollow conductors through which the hydrogen gas flows. The hydrogen is cooled in the hydrogen/water heat exchangers.
- Stator with water-cooled winding (demineralized water).
- Brushless excitation system. Shaft-driven, air-cooled brushless exciter. The exciter keeps a current going through the wires of the rotor. When this rotor turns, it induces a voltage in the stator.
Typically the main generator operates at speeds about:
- 3000 RPM for 50 Hz systems for 2-pole generator (or 1500RPM for 4-pole generator),
- 1800 RPM for 60 Hz systems for 4-pole generator (or 3600 RPM for 2-pole generator).
with an output voltage of 24,000 volts (i.e. 24kV), nominal rating – 1111 MVA, efective power – 1000 MWel, power factor – 0,9 and efficiency – 99%.