9. Generator Cooling Systems

A generator operates with a high magnetic field and a high current density producing unwanted heat, due to copper losses in the conductors (both rotor and stator) and due to iron losses in the iron core. This heat must be removed or the insulation will be damaged and failure of the generator will occur. As the size of the machines increases so too does the amount of heat generated and requiring to be dissipated.

Industry standards define the maximum allowable temperatures for stator and rotor windings, and these standards tend to limit the size of the generator, dependent on their means of cooling.

The first generators to go into commercial service were simple air-cooled machines. As the capacity of the machines increased, however, it became necessary to produce generating sets with a high power output and a relatively compact construction, manufacturers have been forced to devise more effective cooling systems.

There are now several methods of generator cooling employed in the industry. These include:

• Totally enclosed air cooling (This is used on nominally smaller capacity units up to 160MVA, however, large capacity, air cooled generators in the order of 500MVA are currently on the market employing a sophisticated inner cooler ventilation system);
• Totally enclosed hydrogen cooling with or without supplementary stator water cooling (above 50MVA);
• Liquid cooling.

Note:  Between 50 and 160MVA the choice of cooling methods varies between manufacturers above a nominal 160MVA hydrogen cooling is the most prominent method used.

Most modern high-output generators have a combination of cooling systems with the rotor being cooled by hydrogen, which is circulated through the generator frame by fans integral with the generator rotor, and the stator being cooled by a flow of high quality demineralised cooling water, which is circulated by an external pumping system.

Some generators, even as large as 500MW capacity, however, still utilise air cooling.