Generally electrical power is generated at low voltages (generation voltage will be around 11 to 33kV) depending on the power output rating of the generator and transmitted to the load centers by stepping up the generating power through step-up transformer.
In Electrical Generation plants, the generation voltage will be in the range between 11kV and 33kV. Generator designers wishes to generate the electric power at high voltages as possible to limit the stator current (for large alternators stator current will be in the order of thousands of amperes) required to achieve the desired output. But certain technical and economical problems will arise which results in designing the generator for particular voltage and current ratings.
Low voltage generation for particular power level (say 500MW) requires large size of the stator conductors as huge currents flow through the stator because of low voltage. ( Any current carrying conductor size is decided by the amount of current flowing through the conductors). With larger the stator current flowing through the higher the copper loss and the heat dissipated. Therefore requires better and larger heat removing medium. Thereby increase in the cost and size of the machine. Low voltage power generation also results in increase in the size of the generator and occupies larger space.
On the other hand, increase of the machine voltage significantly beyond the minimum necessary value results in requirement of more insulation for the generator stator windings, thus increasing its size and cost. This increase in voltage for power generation also increases the size of the machine.
Therefore an optimum value of Voltage and Current values are attained for designing the generator considering the cost and size factors of the generator.
Example:
660 MW electrical generation plant usually operate at 23.5 kV with line currents of about 19,000 A at 0.85 power factor.