The turbine shaft, or shafts if there is more than one, are coupled to a generator which converts the rotary mechanical motion into the electrical energy that the plant is designed to provide. Generators, like steam turbines, first appeared during the nineteenth century. All utilise a coil of a conducting material, usually copper moving in a magnetic field to generate electricity.
The generators used in most power stations, including coal-fired power stations, are designed to deliver an alternating current (AC) to a power grid. An AC current is preferred because it allows the voltage to be raised or lowered easily using a transformer. For transmission of power over long distances it is preferable to use a very high voltage and a low current. The voltage is then reduced with a transformer before delivery to the consumer.
The need to generate an AC voltage determines the speed at which the generator rotates. This must be an exact multiple of the grid frequency (normally grids operate at either 50 or 60 Hz). For grids operating at 50 Hz the traditional generator speed is 50 cycles per second, or 3000 rpm. The equivalent 60 Hz machine rotates at 3600 rpm. This speed, in turn, determines the operating speed of the steam turbine. Large low-pressure steam turbines may operate at half these speeds.
Generators may be as large as 2000 MW, and large generators are normally built to suit a particular project. Modern generators operate with an efficiency of greater than 95%. The remaining 5% of the mechanical input energy from the turbine is usually lost as heat within the generator windings and magnetic components. Even though the percentage is small, this still represents an enormous amount of energy; perhaps 50 MW in a 1000-MW machine. Hence generators require very efficient cooling systems in order to prevent them overheating. A variety of cooling mediums are used, including hydrogen which is extremely efficient because of its low density and high specific heat.
The broad outline of generator design has changed little over a century. However new materials have improved efficiencies. The latest developments involve the use of superconducting materials to reduce energy and increase efficiencies.
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