|The field current used in the rotor of the generator is produced by an excitation system. The excitation systems could be classified into two categories regarding the constructive principle:
Main demands of an excitation system are:
- Excitation systems with electrical rotating machines (either AC or DC), and
- Static excitation systems with controlled (or not) semiconductors (thirystors or diodes).
Choosing an excitation system for a hydro generator connected to a power network depends on:
- to maintain a high excitation range,
- fast voltage increasing during transient state.
- rated power,
- nominal speed,
- some network demands...
In this picture one can see a three phased rectifier schematic. By using it, one can obtain from a three phased voltage (terminals A, B, and C) a direct voltage (at "+" and "-" bars).
The alternative voltage is either supplied by an ancillary generator, or obtained directly from the generator's terminals.
The direct voltage will supply the rotor winding of the hydro generator.