Essentially all of the world’s electric power is generated by synchronous machines. The synchronous generator has proven to be a reliable and efficientdevice for converting mechanical power to electric power.
Since the typical power system uses alternating current (60 Hz in the United States), the chief requirement of such a device is that it produces power at a controllable voltage at a constant frequency.
A typical synchronous machine consists of a rotor with a field winding and a stator with a three-phase ac winding. The rotor has a dc power supply and the stator is connected to the power system through a generator step-up transformer. The turbine rotates the field at a constant speed, often as high as 3,600rpm.
If the stator windings are connected to a load, current flows through the windings and the load. As the electrical load increases, the prime mover (turbine) must expend more mechanical energy to keep the rotor turning at a constant speed.
Thus mechanical energy input by the turbine is being transformed into electrical energy. Generators in hydro plants are also synchronous machines, but rotate at lower speeds than steam unit or gas turbines.
The electrical power produced by a synchronous generator is almost equal to the mechanical power input, the efficiency being in the range of 98%. The division of electric load among a number of generators is determined by a number of factors, including economics.
At a given operating point each turbine generator has an incremental cost, which is the cost per kWh to generate an additional small amount of power. Maximum system economy results when all generators are operating at the same incremental production cost.
The control of the real power and regulation of the speed (which must be held constant to provide a constant frequency) is done with the speed governor and automatic generator controls (AGC) and interaction with the system control center.