Although today's large turbines have efficiencies between 98.5 percent and 99 percent, turbine-generator losses tend to be some 1.5 percent of the rated output at the machine terminals. Such losses are reflected through the turbine power into the heat rate-the primary limiting factor in the generator's rating.
The heat transfer problem is more critical with today's turbine-generators because generator ratings have increased by more than 12 times during the past three decades, while generator weights have increased less than 3 times during the same time period. Thus increasing, by a factor of 4 times, the net heat to be removed from a given physical-size machine.
Generator output is determined by the temperatures that various components reach and the ability of conductors to remove the heat from the generator. (Industry standards define the temperatures different components may reach.) For the most part, the heating action is a function of the load and power factor with losses occurring through a number of components, including:
- Stator and rotor windings
- Leakage field of the stator
- Leads and connections
- Laminated iron of the stator core
- Surface of the rotor
- End supports of the stator core
- Ventilating paths of the rotor
- Magnetic induction in nearby metallic parts subjected to leakage fields of the stator end windings and leads
- Generator blower
- Bearings and gland seals
Generator rating may be increased, and the physical size of the components made smaller, by providing forced cooling of the generator's rotating and stationary components, and transferring those losses to the external environment. Coolers are also required to dissipate heat from the bearing oil, gland seal oil, and coolant circulating systems (in those machines using liquid cooling).
Stator & Rotor Heat
Stator and rotor windings may be cooled indirectly or directly:
- Indirect Cooling - directly cooled (also known as conventional) windings originally were air-cooled. However, hydrogen is now used extensively to cool turbine generators because of its low density and good thermal characteristics. The generator's stator and rotor windings are completely encapsulated in electrical insulation, with the cooling medium drawn through channels in the stator core and rotor steels. Heat generated by current flowing in the windings travels by thermal conduction through the electrical insulation and through the surrounding steel to the cooling medium.
- Direct Cooling - stator windings may be cooled with either hydrogen or demineralized water:
- Hydrogen-Cooled System - ventilating tubes pass the gas through the coil and thus effectively cool the turbine generator. The tubes are in intimate contact with the current-carrying strands, but insulated from them so as not to carry current.
- Water-Cooled System - demineralized water circulates through hollow copper strands comprising about one-half of the conductor's sectional area. The remainder of the conductor consists of solid stranding, with insulating hoses connecting the coil sides to inlet and discharge water manifolds at the ends of the generator. Because cooling water is more efficient than hydrogen, larger rated units generally are water-cooled.
Hydrogen Cooling Systems
Hydrogen gas is typically used for cooling large generators, since hydrogen has thermal properties superior to those of air and allows for reduced windage and better cooling. And, because of hydrogen's low density, windage and ventilating losses are lower. In addition, the gas's high specific heat capacity, thermal conductivity, and heat transfer coefficients, enable the building of smaller generators. Hydrogen gas, however, must be supplied to the generator under carefully controlled conditions of purity and pressure.
