TABLE OF CONTENTS The following sections describe the excitation, cooling, and lubricating systems associated with the generator.
The excitation system controls the output voltage and the reactive power (VAR) delivered by the generator. Brushless excitation relies on ac machines with diode bridges to convert the ac power to dc. The diode bridges can be stationary or rotating on the shaft. Self-excitation is also used by supplying power through a transformer connected to the terminals of the generator. The output of the transformer supplies either a diode or a thyristor bridge.
The following are the ranges of air and hydrogen cooling in generators:
Generators having a rating lower than 20 MW are air-cooled.
Generators having a rating between 20 and 200 MW are either air- or hydrogen-cooled. However, the majority of generators having a rating over 100 MW are hydrogen-cooled.
Generators having a rating over 200 MW are hydrogen-cooled.
Hydrogen has proved to be an excellent cooling medium for generators. Air-cooled generators having a rating higher than 100 MW had frequent forced outages and shorter lifetime due to poor cooling.
The hydrogen pressure inside the generator is 4 to 5 bar. It cools the stator and the rotor. The purity of the hydrogen should be maintained within specifications. Carbon dioxide and nitrogen have been used as a purging gas in generators. Note that carbon dioxide is not inert. It should not be stored in the generator for long periods.
The hydrogen cooling capability decreases with the pressure. If the...
