Moving water makes the turbine spin

In this generating unit, water rushes down the penstock into the scrollcase and then along the turbine runner to push the turbine blades. Drawn to the turbine axis, the water exits through the draft tube underneath. The energy of rushing water exerts a tremendous force on the turbine; this force is transmitted to the alternator, which converts the mechanical energy from the turbine into electrical energy.

Driven by the turbine, the alternator produces alternating current

The alternator is connected to the turbine drive shaft. It has a moving part–the rotor–and a fixed part–the stator. The rotor's outer surface is covered with electromagnets. The stator's inner surface, or cylinder wall, is made up of copper windings. When the rotor turns inside the stator, the electrons in the copper windings "vibrate." Their movement generates an electric current, similar to the one created by Michael Faraday in his 1831 experiment on electromagnetic induction, but on a much larger scale.

Click on the picture to see a cross-section view of a generating unit

Installed capacity

Turbines have a constant rotation speed

All the generating units in a power system must be synchronized. In other words, it's essential that they maintain an exact rotation speed. Why? To ensure adequate power quality. Equipment that runs on electricity is designed to use alternating current of a specific frequency. This frequency depends on the generating unit's rotation speed, i.e., the number of times per second that rotor magnets travel past the stator windings. This frequency is expressed in cycles per second, or hertz (Hz), named after the German physicist Heinrich Hertz, who proved the existence of radio waves.

In North America, the standard alternating-current cycle is 60 times per second, but in Europe it is 50 times per second. This means that a clock designed to work at 60 Hz will be slower when plugged into a European socket.

Rotors at La Grande-3 generating station