Power Generation

Any ac power system begins with a generating source. Electric generators are devices that convert energy from a mechanical form into an electrical form. This process, known as electromechanical energy conversion , involves magnetic fields that act as an intermediate medium. The input to the generating machine can be

derived from a number of energy sources. For example, in the generation of large-scale electric power, coal can produce steam that drives the shaft of the machine. Typically, for such a thermal process, only about 1/3 of the raw energy (i.e., from coal) is converted into mechanical energy. The final step of the energy conversion is quite efficient, with efficiency close to 100 percent.
A simplified diagram of a three-phase generator is shown in Figure 1.25. Note that poles A ', B ', and C ' represent the start of each of the phase windings, while poles A , B , and C  represent the ends of each of the windings. As with transformers, the windings of the generator can be connected in either of two ways:
-    Wye configuration . A circuit arrangement in which the schematic diagram of the windings form            a Y.
-    Delta configuration . A circuit arrangement in which the schematic diagram of the windings form a delta.

Figure 1.26 illustrates the connection arrangements. The generator shown in Figure 1.25 is a rotating-field type of device. A magnetic

field is developed by an external dc voltage. Through electromagnetic induction, a current is induced into each of the stationary ( stator ) coils of the generator. Because each of the phase windings is separated by 120°, the output voltage of the generator also is offset for each phase by 120° (Figure 1.27). Three-phase power is used almost exclusively for power distribution because it is an efficient method of transporting electrical energy.